Fabrication, photoluminescence and applications of quantum dots embedded glass ceramics

Fabrication, photoluminescence and applications of quantum dots embedded glass ceramics

Journal Pre-proofs Review Fabrication, photoluminescence and applications of quantum dots embedded glass ceramics Junpeng Xue, Xiangfu Wang, Jung Hyun...

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Journal Pre-proofs Review Fabrication, photoluminescence and applications of quantum dots embedded glass ceramics Junpeng Xue, Xiangfu Wang, Jung Hyun Jeong, Xiaohong Yan PII: DOI: Reference:

S1385-8947(19)32494-5 https://doi.org/10.1016/j.cej.2019.123082 CEJ 123082

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

29 May 2019 1 October 2019 4 October 2019

Please cite this article as: J. Xue, X. Wang, J.H. Jeong, X. Yan, Fabrication, photoluminescence and applications of quantum dots embedded glass ceramics, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.123082

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Fabrication, photoluminescence and applications of quantum dots embedded glass ceramics Junpeng Xuea,b, Xiangfu Wanga,c*, Jung Hyun Jeongb*, Xiaohong Yana,c,d* aCollege of Electronic and Optical Engineering & College of Microelectronics, Nanjing University of Posts and Telecommunications, Nanjing, 210046, People’s Republic of China, Email: [email protected] bDepartment of Physics, Pukyong National University, Busan 608-737, Republic of Korea, Email: [email protected] cKey

Laboratory of Radio Frequency and Micro-Nano Electronics of Jiangsu Province, Nanjing 201146, Jiangsu, People’s Republic of China dSchool of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China, Email: [email protected] ABSTRACT Quantum dots (QDs) embedded glass ceramics have been widely studied in laser crystals, LEDs, optical fiber amplifiers, optoelectronic devices, photocatalysts and sensors due to the size dependent multicolor fluorescence emissions, crystal field controllability, high transparency and stability, and high temperature sensitivity. The rapid advancements of QDs embedded glass ceramics are reviewed in detail. The synthetic methods, crystallization theory, spectral modulation, nonlinear optical property, photoluminescence mechanism, and applications of QDs embedded glass ceramics are exhibited. The most important results obtained in each case are summarized, and the key challenges in market viability are also discussed.

Keywords: Quantum dots embedded glass ceramics; Synthesis methods; Mechanism and properties; Wide application

1

1. Introduction Since Ekimov and Onushchenko [1] observed quantum confinement effects in CuClembedded silicate glass ceramics in 1981, a large number of QDs have been extensively studied, such as CdTe [2-4], CdS [5-6], CuBr [7], and CdSe [8] embedded glass ceramics. QDs embedded glass ceramics have attracted much attention because they have great potential in various applications for our lives, such as optoelectronics devices, photocatalyst and sensors, as displayed in fig. 1. In particular, their optoelectronic properties play an essential role in color filters and saturable absorbers for pulse laser generation, etc [9-14]. Generally, QDs are defined as approximately spherical nanomaterials with a three-dimensional dimension in the range of 1-20 nm, which have significant quantum effects arising from Bohr radii. As new optical materials, QDs have the advantages of tunable photoluminescence (PL), high

quantum

efficiency,

broad

absorption,

excellent

resistance

toward

photobleaching. These advantages make them potential applications in many areas, including light emitting diodes (LEDs), displays, solar cells, photovoltaic cells, telecommunication, photocatalysis and gas sensors [15-22]. One knows that the term glass-ceramics were introduced by S.D. Stookey in 1957 and were defined as inorganic, non-metallic materials prepared by controlled crystallization of glasses via different processing methods [23]. They contain at least one type of functional crystalline phase and a residual glass with the volume fraction crystallized varying from ppm to almost 100% [23]. Glass ceramics are widely used in cookware, bakeware, cooktops and radomes owning to their high impact resistance, high strength, low co-efficient of thermal expansion and good resistance to thermal shock [24]. It is well known that QDs are unstable and most QDs are synthesized by wet chemistry, which makes them sensitive to the atmosphere [25]. In practical applications, QDs embedded glass ceramics are an excellent choice because glass ceramics have chemically and physical stability, such as high mechanical strength, high-temperature resistance and chemical corrosion resistance [25-33]. Furthermore, the high density 2

and uniform composition make QDs not be eroded by the surrounding environment. Meanwhile, glass ceramics can prevent QDs particles from aggregating or dissolving and it is relatively easy to control the size and distribution of QDs embedded glass ceramics relatively [34]. Compared with glass ceramics, other host materials, such as polymers, sol-gel thin films, etc, are chemically and mechanically unstable, and inconvenient for application in optoelectronic devices [34]. Therefore, QDs embedded glass ceramics can fully use the advantages of QDs and glass ceramics, which are very promising optical materials. Based on the definition of QDs, they contain carbon nanodots, graphene QDs, perovskite QDs, and semiconductor QDs such as group II-VI, IV-VI elements (CdSe, CdS, CdTe, PbSe, PbS, ZnS, ZnTe, CdSe/ZnS core/shell, etc.) [35-42]. However, only the semiconductor QDs of group II-VI, IV-VI elements, and perovskite QDs have been reported to be embedded in glass ceramics [8,25,43-49].

Fig. 1 QDs embedded glass ceramics with unique properties have great potential applications in optoelectronics, photocatalysis and sensor.

3

Fig. 2 Dependence of fluorescence emission wavelengths of QDs on their chemical composition. Reproduced with permission [35]. Copyright 2013, Elsevier. For QDs embedded glass ceramics, the luminescent properties, including spectral range and intensity, are the same as the QDs, as shown in fig. 2. The emission of the QDs has a fixed range, which relates to the composition and size of the QDs. The size of QDs is controlled by the heating temperature and time, which involves the growth mechanism of QDs embedded glass ceramics [12,25,50]. Besides, it is possible to regulate the luminescence intensity and emission wavelength by co-embedded ions, such as energy transfer between QDs and rare earth ions [51]. In this review, we summarize the preparation methods, crystallization behavior, luminescence mechanism, radius calculation models, the properties of QDs embedded glass ceramics as well as their potential application in optoelectronic devices, photocatalysts and sensors. Besides, we also describe the luminescent property, stability and potential application of perovskite (CsPbBr3) QDs embedded glass ceramics. Finally, we look forward to providing a new perspective for the development and expansion of its application range by comprehensively describing the current research status of QDs embedded glass ceramics. 2. Preparation methods There are many methods to prepare QDs embedded glass ceramics, such as the co-melting method, sol-gel method, ion-implantation method, ion exchange method 4

and so on [52]. Among them, the co-melting method and the sol-gel method are commonly used methods. Through different preparation methods, the performance of QDs

embedded

glass-ceramics

can

be

studied

more

systematically

and

comprehensively.

Fig. 3 (a) The schematic of QDs embedded glass ceramics preparation by the co-melting method. (b) The color of the glass changed from pale brown to brownish black with an increase in the heat-treatment time at 530 °C. (c) TEM image of the sample. (d) HR-TEM images of the sample. (b) Reproduced with permission [54]. Copyright 2017, Elsevier. (c) Reproduced with permission [57]. Copyright 2014, Elsevier. (d) Reproduced with permission [45]. Copyright 2011, American Ceramic Society and Wiley Periodicals, Inc. 2.1 Co-melting method Table 1 The feature of the different synthesis methods used for the preparation of QDs

Synthetic Methods

Advantages

Disadvantages

Ref.

Co-melting

A simple process, various sizes, and shapes controllable reaction

Disordered distribution, clustering and aggregation

[53-57]

5

Sol-gel

Mild synthesis technology, high purity, good dispersion

The costly and toxic organic solvent, long reaction time

[58-61]

Ion-implantation

The precisely controlled, freely selected position

Complicated equipment, unsatisfactory size, distribution, and damage to parent glass

[62-69]

Ion exchange

A simple process, low production cost, good stability

Inaccurate micron-scale distribution

[70-79]

Femtosecond laser beam

Controllable spatial distribution

A wide size distribution, damage to parent glass

[80-86]

The co-melting method is to mix the necessary raw materials for forming QDs with glass materials and then get the glass by co-melting. Finally, corresponding QDs embedded glass ceramics are obtained by different nucleation and crystallization treatment, as shown in fig. 3a [53,54]. Xu et al. used the conventional co-melting technique to produce the PbSe QDs embedded borosilicate glass ceramics with the composition of SiO2-B2O3-Al2O3-ZnO-AlF3-Na2O [54]. At the temperature of 530°C, the color of the glass ceramics changed from pale brown to brownish-black as increased the heat-treatment time, as shown in Fig. 3b. Borrelli et al. obtained a series of PbS(Se) QDs embedded glass ceramics by co-doping S, Se, and PbO in silicate glass ceramics at different temperatures from 500 -725°C [55]. Apte et al. also used the co-melting method to get CdS0.5Se0.5 /CdSe embedded glass ceramics and their size varied within 4-10 nm [44]. Furthermore, as shown in Fig. 3c and d, the QDs embedded glass ceramics prepared by the co-melting method usually display spherical nanoparticles. This phenomenon is mainly due to the mutual solubility of the environmental medium and the QDs so that the composition of the QDs changes continuously or discontinuously from the center to the radial direction, which is called graded material [10,56]. The composition of the QDs is generally non-uniform but varies in the radial direction to form spherical gradient QDs [10]. Dong et al. also 6

discovered similar phenomena and explained to them using the first principle method and the anisotropic differential effective dipole approximation theory [56]. The co-melting method is a simple and controllable reaction process for preparing glass ceramics of various sizes and shapes. Single-stage and double-stage heat treatment also affect the crystalline quality and PL properties of QDs embedded glass ceramics, as illustrated in Fig. 4 [57]. Ma et al. explained that the double-stage thermal treatment process was beneficial for the crystallization of PbSe QDs in silicate glass ceramics [55]. In a single-stage process, the nucleation and growth of PbSe crystals conducted in the same heat-treatment operation, causing the first nucleated crystal to grow earlier than the latter embryo. Thus, the initially formed embryo grew into a large size, while the latter had a smaller one. Undoubtedly, continuous nucleation and growth resulted in a broad distribution of crystal sizes. For the double-stage procedure, the PbSe embryos produced inside the glass ceramics in the nucleation stage and the formed larger embryos began to grow faster while the smaller ones tended to redissolve due to an elevated temperature during the growth stage. Therefore, compared with the single-stage process, the degree of PbSe crystal growth was enhanced and the PbSe crystals precipitated uniformly in the glass ceramics. However, there are still some disadvantages, such as disorderly distribution in glass ceramics, low production efficiency and high production cost. Besides, the co-melting method is prone to clustering and aggregation, which influence the properties of QDs embedded glass ceramics, as displayed in Table 1.

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Fig. 4 Crystallisation of a glass to form a glass-ceramic by two-stages heat treatment. (a) Temperature dependence of the nucleation and growth rates with negligible overlap and (b) two-stage heat treatment. Crystallisation of a glass to form a glass-ceramic by a single-stage heat treatment. (c) Temperature dependence of the nucleation and growth rates with significant overlap and (d) single-stage heat treatment. Reproduced with permission [24]. Copyright 2006, Springer Science + Business Media, Inc. 2.2 Sol-gel method The sol-gel method is a mild synthesis technology, as displayed in Fig. 5a. The basic principle is to use high purity compounds as precursors and uniformly mix these raw materials in liquid phases, performing hydrolysis and condensation chemical reaction and then obtaining a stable, transparent sol system in solution. Then the sol ages and forms a three-dimensional network with non-flowing solvent. Finally, the gel is dried and sintered to prepare material of molecular or even nanoscale structure. Nogami et al. dissolved Cd(CH3COO)2.2H2O in a methanol solution to form CdO-doped SiO4 gel, heat-treated at 500℃ in H2S gas and the gel was converted into glass ceramics, while partially CdS microcrystals were formed and controlled the growth of crystallites by changing the temperature of the gas and the aeration time, as displayed in Fig. 5b [58]. From transmission electron micrographs and electron 8

diffraction patterns (see Fig .6a and b), there was a smaller sized granular structure and the electron diffraction pattern displayed a faint ring, indicating the glassy state of the heated product. On the contrary, in the glass reacted with H2S gas, precipitated crystallites were seen as dark spheres, smaller than 10 nm diameter, among the background granular structures. Fig. 6c shows the absorption spectra of glasses heated for 2 h at 500°C and exposed to H2S gas for various periods at room temperature. It was apparent that the absorption edges were blue shifted compared with bulk CdS crystals with diameters of 520 nm. Meanwhile, as shown in fig. 6d, the gap energies of CdS crystals were calculated by extrapolating the absorbance to zero. At 1, 2, 5 and 20 h exposure times, they were 2.89 (429), 2.71(458), 2.58 (481) and 2.43 eV (510 nm), respectively. Compared to bulk CdS, it was equivalent to the blue shift of to 0.05 to 0.52 eV. Manickaraj et al. used (3-Aminopropyl) trimethoxysilane (APS) as the silica precursor to prepared APS glass-CdTe QD nanocomposite sample by the sol-gel method [59]. Wang et al. and Kang et al. also reported the sol-gel approach to obtain highly fluorescent [email protected] QDs embedded in silica monoliths and PbS/TiO2 composite materials, respectively [60,61]. The sol-gel method can obtain fine particles with uniform size and excellent dispersion, which suitable for preparing various materials, such as glass ceramics, powders, films, and fibers. Meanwhile, the sol-gel method has some disadvantages. Generally, organic solvents are toxic and have a relatively long reaction time, sometimes as long as 1-2 months. Besides, it is easy to crack during the drying process and difficult to achieve large-scale industrial production. .

9

Fig. 5 (a) Schematic of QDs embedded glass ceramics preparation by Sol-gel method. (b) The schematic of CdS QDs embedded silica glass preparation by Sol-gel method. The insets are the samples under daylight and 365nm lamp. (a) Reproduced with permission [59,60]. Copyright 2013, SPIE and Copyright 2005, American Chemical Society.

Fig. 6 Transmission electron micrographs and (inset) electron diffraction patterns of glasses (a) heated for 2 h at 500°C and (b) followed by reacting with H2S gas for 20 h at room temperature. (c) Absorption spectra of glasses heated for 2 h at 500°C and reacted with H2S gas for 1, 2, 5 and 20 h at room temperature. (d) The relationship between the absorption edge and the reciprocal square size of CdS crystals. Reproduced with permission [58]. Elsevier Science Publishers B.V. (North-Holland). 10

2.3 Ion-implantation method Ion implantation is a method of accelerating ionized dopants to become high-energy plasmas in electric fields and changing the direction of motion to inject them into solid surfaces [62-66]. The method can be used for surface modification and optimization of materials such as metals, plastics, ceramics, etc., as shown in fig. 7a [62]. Mokkapati et al. selectively grew InGaAs semiconductor QDs in the matrix by ion implantation and achieved satisfactory results [67]. Carder et al. successfully fabricated PbSe embedded silicon dioxide glass ceramics via the ion-implantation method, as exhibited in Fig. 7b

and c [68]. From the TEM images, it was observed

that PbSe QDs successfully embedded in silicon dioxide glass ceramics and there was a band of nanoclusters at about 50-70 nm beneath the surface. Furthermore, the synthesis of nanocrystals by sequentially injecting lead sulfur in silica glass ceramics was also reported [69]. As displayed in Table 1, the ion implantation method has distinct characteristics. For example, it can precisely control the concentration of the injected elements and the implantation depth. It can perform the implantation process at normal temperature without adjusting the size and freely select the position of ion implantation. Moreover, the plasmas are distributed in the longitudinal direction like Gaussian function [66]. However, there are also some problems. This method only increases the concentration of QDs within a thin layer of several hundred nanometers from the surface of the glass. This increases the distribution of QDs and makes it difficult to control the distribution along with the thickness of the glass ceramics [33]. In addition, this method causes serious damage to glass ceramics and its equipment is complicated and expensive.

11

Fig. 7 (a) The schematic of conventional ion implantation technique. (b) HAADF-STEM cross sectional image of the Pb + Se implanted and annealed sample. (c) Cross-sectional TEM showing a number of PbSe clusters in the near surface region. (a) Reproduced with permission [62]. Copyright 1987, American Institute of Physics. (b and c) Reproduced with permission [68]. Copyright 2013, Elsevier. 2.4 Ion exchange method The ion exchange method is a reversible chemical reaction between ions in the liquid phase and ions in the solid phase. In general, the exchange of ion species is vibrant, mainly including Na+, K+, Ag+, Cu+ monovalent ions, which can be used in material modification technology [70-73]. Auxier et al. presented the ion-exchanged waveguides of PbS QDs embedded glass ceramics, as shown in Fig. 8a [70]. The phase-contrast micrograph of PbS QDs embedded channel waveguides after K+-Na+ ion exchange. Xu et al. also reported the PbS QDs embedded glass ceramics controlled by silver diffusion [73]. After the diffusion of Ag+ ions, the crystal growth behavior was different such that the glass ceramics affected by diffusion became black after heat treatment, while the interior remained light brown, just like the 12

schematics of the glass ceramics cross-section, as shown in Fig. 8b and c. Heo et al. studied the effect of Ag+ ions on the PbS QDs embedded glass ceramics by ion exchange method [74]. The glass ceramics were ion-exchanged in AgNO3 salt melt at 260°C for the 60s, followed by heat treatment 440-460°C for 10 hours to precipitate Ag nanoparticles (NP) and PbS QDs. As displayed in Fig. 8d-f, it was observed that the average diameter D of PbS QDs was larger in Ag+ ion-exchanged regions (5.2 ≤ D ≤ 5.5 nm) than in Ag+-free regions (2.4 ≤ D ≤3.3 nm). From Fig. 8f, the photoluminescence wavelength λPL of PbS QDs was longer (1525 ≤ λPL ≤ 1580 nm) in Ag+ ion-exchanged regions than in Ag+-free regions (1030 ≤ λPL ≤ 1200 nm). They explained that the Ag NPs that formed during heat treatment provided the sites for heterogeneous nucleation of PbS QDs and promoted the precipitation of PbS QDs in glass ceramics. Karthikeyan group often used this method to prepare glass ceramics [75,76]. Cao et al. synthesized PbS/CdS core/shell QDs by ion exchange method and exhibited higher PL strength than bare PbS QDs [77]. Table 1 shows the advantages of ion exchange technology, such as simple process, low production cost and excellent stability. Although the precipitation of QDs can be limited to a narrow range of 5 mm, the exchange area is still too large and cannot be accurate to the micron level in three dimensions [33]. Besides, plasmon and electric field can assist precipitation of QDs in glass ceramics by ion exchange method. Heo et al. introduced the two methods in detail, taking PbS QDs embedded glass ceramics as an example [78,79]. After 5 minutes of irradiation with a 1.5 W CW laser, PbS QDs precipitate with a diameter of 5 nm were obtained, and the PL peak was observed at 1490 nm. Moreover, with increasing the Ag+ ion exchange and laser irradiation duration increased, the PL moved to a longer wavelength. Under heat-treatment temperature of 430 °C, with increasing the applied voltage from 2 to 10 kV/cm, the peak wavelengths of the PL bands shifted from 1175 to 1270 nm and the penetration depth of Ag+ was about 20 μm. The results demonstrated that the plasmon and electric field could help precipitation of QDs and could control spatial distribution, thereby changing the peak wavelengths of the PL bands. 13

Fig. 8 (a) Ion-exchange process and a phase-contrast micrograph of two PbS QDs-doped channel waveguides after K+-Na+ ion exchange. (b) Photograph and (c) schematics of the cross-section of the glass after Ag+ ion diffusion. TEM images of PbS QDs in (d) glasses ion-exchanged at 260°C for 60 s and (e) Ag+-free glasses. (f) Peak positions of absorption (λAbs.) and PL (λPL) with the calculated average diameters (D) of PbS QDs in ion-exchanged glasses. (a) Reproduced with permission [70]. Copyright 2004, American Institute of Physics. (b and c) Reproduced with permission [73]. Copyright 2012, Elsevier. (d-f) Reproduced with permission [74]. Copyright 2012, The American Ceramic Society. 2.5 Femtosecond laser beam method The femtosecond laser (fs) beam is a selective growth method. Firstly, the QDs embedded glass ceramics are directly heated by laser irradiation to form nucleation centers and promote the growth of local QDs during annealing. Firstly, using laser irradiation heats QDs embedded glass-ceramics to form nucleation centers and then promotes the growth of local QDs during annealing [16]. The process is controlled by laser beam parameters, such as the temperature and duration of the heat treatment. This

14

Fig. 9 (a) Black curves-absorption spectra of sample S1 in regions 1 and 2. The circles indicate the regions from which spectra 1 and 2 were taken. (b) Raman spectra of QDs grown in glass by fs laser beam heating (sample S1). The spectra were taken in the regions indicated by circles 1 and 2 in the inset in (a). (c) Illustration of the end-face coupling experiment set-up. (d) Beam cross-section of the output 632 nm signals from a 10 mm long waveguide. (a and b) Reproduced with permission [16]. Copyright 2016, AIP Publishing LLC. (c and d) Reproduced with permission [33]. Copyright 2015, The Royal Society of Chemistry. method has been successfully used in other systems and growth mechanisms for QDs selective growth, including nanoparticles of silver [80], copper [81], SbSi [82], Si [83] and InGaAs [84]. Bell et al. used this method to embed CdSe QDs in glass ceramics, as shown in Fig. 9a [16]. The inset in Fig. 9a exhibits the QDs inlaid area of glass ceramics sample with the result of focused, high-repetition-rate fs laser beam and heat treatment. The tapered shape of the brown region displayed the focal volume of the beam, while the focal volume of the beam was prevented by the temporal dispersion in the glass ceramics, resulting in a uniform spatial distribution of coloration. From Fig. 9b, it was found that the CdSe QDs with an average radius of about 2.0 nm grown embedded glass ceramics by the fs laser beam method, which opened the way for an adjustable selective growth model. Fan et al. used fs pulsation to selectively control the formation of microdomains and PbS QDs embedded glass ceramics, as exhibited in Fig. 9c and d [33]. Liu et al. reported that the nucleation and growth of 15

the PbS QDs embedded glass ceramics were promoted by fs radiation [85]. Firstly, the crystals preferentially formed in the irradiation region and the subsequent heat treatment enhanced the precipitation of QDs in the radiation region, thereby realizing the spatial control of PbS QDs precipitation. The fs irradiation can control the spatial distribution of PbS QDs embedded glass ceramics, but it also leads to the large size distribution of QDs and damages to the parent glass ceramics [85,86]. 3. Crystallization behavior, luminescence mechanism and Radius calculation 3.1 Crystallization behavior According to previous reports, the formation of QDs is accompanied by the following stages: phase separation, nucleus formation, crystal growth, and sometimes emerges Ostwald ripening stage [52]. The so-called crystallization is a combination of two processes: nucleation and crystal growth. For QDs embedded glass ceramics, the temperature is a critical factor, especially between the glass transition temperature (Tg) and the crystallization temperature (Tc). Generally speaking, crystallization can be described as the thermal and kinetic processes which allow the formation from a structurally disordered phase, of a stable solid phase with regularly ordered geometry [87]. At a certain temperature above Tg, the nucleation rate is the largest. After that, as the temperature rises, the nucleation rate decreases sharply, and the maximum value of the crystal growth rate gradually appears. This temperature is far greater than the maximum temperature of the nucleation rate. In the above, we have compared the effects of single-stage process and the double-stage procedure on QDs embedded glass ceramics. Besides, the solubility, diffusion, composition of the QDs embedded glass ceramics, and nucleating agents influence the optical properties of QDs. 3.1.1 Crystallization behavior theory In common glass ceramics, phase separation generally occurs within the softening range of glass ceramics before nucleus formation and crystal growth. At low temperatures, they are subjected to kinetic or thermodynamical inhibition, while the separated phases dissolve mutually at elevated temperatures [52]. In theory, the QDs phase would be separated from the host glass ceramics due to the Gibbs free energy 16

difference (G) between the equilibrium crystalline phases and QDs undercooled liquid and then crystallization would be formed in the glass ceramics. In general, the Turnbull method uses to estimate the G value as the Gibbs free energy difference between the QDs undercooled liquid and the equilibrium crystal phase [88]. The thermodynamic driving force required for the change is provided by the Gibbs free energy difference (G) between the QDs undercooled liquid and the equilibrium crystalline phases. The greater the driving force for the transformation, the greater the free energy difference. One knows that nucleation in glass ceramics involves two types of mechanisms, namely: homogeneous nucleation and heterogeneous nucleation [87]. Homogeneous nucleation refers to the nucleation probability of overall host is the same, without external involvement and independent of phase boundaries, structural defects, etc., also known as spontaneous nucleation [87]. Homogeneous nucleation is difficult and rare in glass ceramics, but it can occur in Li2O-SiO2 glass ceramics with high Li2O content because the Li+ ions have high field intensity [89]. The nucleation rate ( I ) depends on the probability of cluster formation for creating a critical nucleus or free energy barrier (Gmax) as well as on the diffusion activation energy (ED) according to the general equation: 𝐼 = 𝐴 exp

(

―𝐺𝑚𝑎𝑥 + 𝐸𝐷 𝑘𝑇

)

(1)

here, A, Gmax, ED, k, T are the preexponential factor, free energy change, diffusion energy, Boltzmann constant and temperature, respectively. 𝑘𝑇

𝜎 1/2



𝑘𝑇

( )( )

𝐴 = 2𝑛𝑣𝑉1/3

(2)

where nv is the number of atoms of crystallizing component phase per unit volume of the liquid, V is the volume per formula unit, σ is the interfacial free energy per unit area between crystal and liquid, h is Planck’s constant. The free energy change Gmax is derived from the net free energy of a spherical nucleus, as follows [90]: 4

3 2 𝐺 = 3𝜋𝑟 ∆𝐺𝑉 +4𝜋𝑟 𝜎

(3)

here, the first one stands for the change in volume free energy and the second term is the change in surface energy. Gv is the free energy of the liquid phase and r is 17

nucleus radius. When r is at the critical radius, the nucleus is just stable and tends to grow up. The value of Gmax can be obtained by the following expression [90]: 𝐺𝑚𝑎𝑥 =

16𝜋𝜎3

(4)

3∆𝐺2𝑉

Based on the previous report, one can evaluate σ with the following expression [52]: 𝜎=α

(

)

∆𝐻𝑚

(5)

2 1 𝑉𝑚3𝑁𝐴 3

among them, Hm, Vm, and NA are the melting enthalpy, the molar volume, Avogadro’s number, respectively. α is a dimensionless empirical constant. The diffusion energy ED can be expressed in terms of an effective diffusion coefficient given by 𝐷 = 𝑘𝑇𝜆2 ℎ𝑒𝑥𝑝( ―𝐸𝐷 𝑘𝑇)

(6)

D can be related also with the viscosity by using the Stokes-Einstein diffusion relation 𝑘𝑇

(7)

𝐷 = 3𝜋𝜆𝜂

Thus, by substituting the former equation on the nucleation rate (I), the following expression is obtained. 𝑛𝑣𝑘𝑇

I = 3𝜋𝜆3𝜂𝑒 ―Δ𝐺𝑚𝑎𝑥/𝑘𝑇

(8)

Heterogeneous nucleation refers to the process of nucleation through heterogeneous sites such as phase boundaries, grain boundaries or structural defects, which is a common nucleation process. The colloidal precipitates, solid impurities, microbubbles in the supercooled liquid reduce the maximum free energy more than in the homogeneous nucleation by reducing the surface energy σ. Therefore, in the nucleation rate, the Gmax is modified by a factor f(θ) that is expressed by:

[

𝐼ℎ𝑒𝑡 = 𝐴 𝑒𝑥𝑝

―𝐺𝑚𝑎𝑥𝑓(𝜃) + 𝐸𝐷 𝑘𝑇

]

(9)

The factor f(θ) is related with the contact angle between the matrix and the germe, nucleus, impurity, or interface: 𝑓(𝜃) =

(2 + 𝑐𝑜𝑠𝑣)(1 ― 𝑐𝑜𝑠𝜃)2 4

(10)

The different contact angles in heterogeneous nucleation are shown in Fig. 10. If θ = 180°, then cos θ = -1 and f(θ) = 1, in which case its nucleation energy is the same as 18

the homogeneous nucleation. If the wetting between phases is complete, thus θ = 0° and therefore f(θ) = 0, and the nucleation energy is null. Therefore, adjusting the wetting angle (θ) can control heterogeneous nucleation. Furthermore, the free energy change G is also related to the wetting angle (θ), as follows: ∆G =

16𝜋𝜎3 3∆𝐺2𝑉

×

(2 + 𝑐𝑜𝑠𝜃)(1 ― 𝑐𝑜𝑠𝜃)2 4

(11)

When the wetting angle θ < 180°, the free energy barrier of the heterogeneous nucleation is smaller than homogeneous nucleation. When θ = 60°, the barrier is about one-sixth of the homogeneous nucleation, so the heterogeneous nucleation is easy to take place than homogeneous nucleation.

Fig. 10 Variation with the contact angle of wetting on a germe-glass interface: (a) when there is welting and (b) where there is no wetting. Reproduced with permission [87]. Copyright 1992, Taylor & Francis. Crystallization is a stage in which the size and number of crystals increase at the same time. Based on Turnbull and Cohen [91], the following equation can be used to represent crystallization: 𝑈=

𝑓𝐷 𝑎

[1 ― 𝑒( ―∆𝐺 𝑅𝑇)]

(12)

Here, based on Stokes-Einstein, D is the diffusion constant, thus, the U can be expressed:

19

𝑓𝑅𝑇

𝑈 = 3𝑅𝑎0𝑁0𝜂[1 ― 𝑒

( ―Δ𝐺 𝑅𝑇)

]

(13)

Considering the relationship between undercooling (T) enthalpy (H) and the entropy change (S), G can be expressed as: Δ𝐺≅∆𝑆 * ∆𝑇≅

∆𝐻 ∗ ∆𝑇

(14)

𝑇𝐿

Where TL is the melting temperature of the consideration phase. Generally, the crystallization rate can be expressed as: 𝑓𝑅𝑇

― (Δ𝐻 ∗ Δ𝑇)

[

𝑈 = 3𝑅𝑎0𝑁0𝜂 1 ― 𝑒

(𝑅𝑇 𝑇𝐿)

]

(15)

Three models have been proposed to define the crystal growth mechanism as shown below [92]: a. The normal growth model. It is a simple activated process, in which a rough interface in the atomic order of magnitude and no large number of centers for accumulation and migration of atoms. In this model, there is a small deviation from the state of equilibrium and a linear relationship between the growth rate and the undercooling. Thus:

[

𝑈 = 𝑣𝑎 1 ― 𝑒

― (∆𝐻𝑀𝑇) (𝑅𝑇 𝑇 ) 𝐿

]

(16)

Here, U, v, a, HM, TL, T are the growth rate per unit of interface, frequency factor for transport at crystal-liquid interface, growth step of interface, fusion heat per g/atom, melting point and the undercooling, respectively. b. The screw dislocation growth model. This model begins with screw dislocation and atoms can be added to the surface. In this case, the former growth rate (U) expression is multiplied by an f factor or fraction for preferred growth sites on the interface: 𝑎∆𝐻𝑀∆𝑇

∆𝑇

𝑓 = 4𝜋𝜎𝑇𝐿𝑉𝑀≅2𝜋𝑇𝐿

(17)

Where M and σ are the gram-atom-volume and specific surface energy, respectively. In this case, the interface is smooth in the atomistic range. c. The surface nucleation growth model. This model often occurs in different composition glasses and the growth is provided by two-dimensional nuclei at an interface. The growth rate is as follows: 20

𝑈 = 𝐴𝑣𝑒

( ―𝜋𝑎0𝑉𝑀𝑇𝐿𝜎2𝐿) (3𝑘∆𝐻𝑀𝑇∆𝑇)

(18)

Among them, σL is the specific surface energy of the nucleus. In this type of model, the growth rate usually changes with undercooling. Besides, the final radius (r) is related to the nucleus radius (rN) and the volume fraction of crystals by: 4𝑟𝑁

𝑟 = 3𝑉𝑓

(19)

Hence, large Vf values decrease the r, which is the most convenient situation for the mechanical properties of controlled devitrified glasses such as glass ceramics [89]. For the relationship between average QDs radius and heating time, the diffusion-limited growth mode conforms to the initial crystal growth stage, that is, the average nanocrystal radius increases with the square root of the heat treatment time (Rave α t1/2). At the Ostwald ripening stage, larger particles engulf smaller particles, where average nanocrystal radius is proportional to the cubic root of heat-treatment time (Rave α t1/3), following the classical Lifshitz-Slyozov-Wagner (LSW) theory [93]. Here, we take the crystallization behavior of PbSe QDs embedded silicate glass ceramics as an example, as displayed in fig. 11 [52]. Apparently, the nucleation rate of PbSe QDs depended on the empirical constant α, which judged from the peak temperatures of nucleation rates. The peak temperature dropped from 607°C to 531°C, while α increased from 0.300 to 0.409. In summary, the nucleation rates of PbSe QDs were determined by the free surface energy between the PbSe nuclei and the host glass ceramics. The surface free energy α had a smaller effect on the growth rates of PbSe QDs and the growth rates and the maximum U temperature change provided in Fig. 11b. Compared to the above, the change of growth rate peak and the maximum U temperature were small, as α increased from 0.300 to 0.409. Taking CdTe QDs embedded glass ceramics as an example, as shown in Fig. 11c and d, the average particle radius was described concerning the square root and cubic root of the heat treatment time, respectively. In Fig. 11c, the straight line was a fitting result of heat-treatment time up to 16 h, describing the linear relationship between the average radius and the square root of heat-treatment time, which was identical with 21

diffusion-limited growth. The line (see Fig. 11d) was the fitting result of heat-treatment time greater than 38.5h, describing the linear relationship between the cubic roots of the heat treatment time, the same as Ostwald ripening [94]. Furthermore, the average radius of QDs in the normal growth stage fitted the Gaussian distribution, while in the competitive arena, it is the Lifshitz–Slyozov distribution [2,95].

Fig. 11 (a) Temperature dependence of the steady-state nucleation rates of PbSe QDs predicted by CNT. (b) Temperature dependence of the growth rates of PbSe QDs. (c) Average nanocrystal radius is plotted against the square root of heat-treatment time in (c) and the cubic root of heat-treatment time in (d). The straight line in (c) is the linear fit for the heat-treatment times up to 16 h and in (d) the linear fit for heat-treatment times greater than 38.5 h. (a and b) Reproduced with permission [52]. Copyright 2013, The American Ceramic Society. (c and d) Reproduced with permission [94]. Copyright 2008, Elsevier. 3.1.2 Influencing crystalline behavior One knows that the addition of nucleating agents can promote nucleation in glass ceramics, especially for heterogeneous nucleation. According to the previous reports, a brief introduction to the heterogeneous nuclear agents, as listed in Fig. 12 [87]. It was observed that the nucleating agents can be divided into four categories, namely, metallic colloids, oxides, halides, glass-in-glass phase separation. Generally, the 22

temperatures and durations of thermal treatment directly influence the crystallization behavior of QDs embedded glass ceramics, whereas some components also affect the nucleation and growth of QDs [96,97]. ZnO plays an important role in glass preparations and has been widely used as an intermediate in the glass ceramics industry. It can prevent the excessive evaporation of selenium and affect the crystalline behavior of QDs embedded glass ceramics from both experimental and theoretical aspects [96,98]. ZnO plays an

Fig. 12 The classification of nucleating agents. Reproduced with permission [87]. Copyright 1992, Taylor & Francis. important role in glass preparations and has been widely used as an intermediate in the glass ceramics industry. It can prevent the excessive evaporation of selenium and affect the crystalline behavior of QDs embedded glass ceramics from both experimental and theoretical aspects [96,98]. Ma et al. described that the prepared glass ceramics exhibited the black to transparent straw-yellow colors as increasing the amount of ZnO, further indicating that ZnO influenced on the crystallization behavior of PbSe QDs

23

Fig. 13 (a) Glass samples with the upper row denoting the as-prepared glass and the underneath denoting the annealed samples. (b) The Mean Sizes, Degree of Crystallization, and Volume Fraction of PbSe QDs. (c) Temperature dependence of the steady-state nucleation rates of PbSe QDs. (d) Temperature dependence of the growth rates of PbSe QDs. Reproduced with permission [96]. Copyright 2014, The American Ceramic Society. embedded silicate glass ceramics, as demonstrated in fig. 13a [96]. From fig. 13b, the average size of PbSe QDs increased from 5.1 to 8.5 nm and the corresponding crystallinity degree from 3.9 to 10.4 to 3.0 with increasing ZnO content, demonstrating that ZnO content affected the crystalline behavior of PbSe QDs embedded glass ceramics. Fig. 13b and c show that the nucleation rate of the PbSe QDs gradually decreased with increasing ZnO content in glass ceramics, while the growth rate of the PbSe QDs increased. This was mainly because ZnO contributed to lowering the viscosity of the glass ceramics, thereby reducing the kinetic barrier of growth, while the rate of nucleation for PbSe QD played a key role in the free surface energy between the PbSe nuclei and the host glass ceramics. Embedding ZnO into silicate glass ceramics improved the surface tension of the melts as well as the interfacial free energy σ between PbSe and the glass ceramics [98]. Using equations (2) and (4), assuming that the nucleation kinetic barrier of the PbSe QDs remained unchanged, it was considered that the nucleation rate of the PbSe QDs was reduced 24

from sample G1 to G5. Compared with the thermodynamic barrier, the kinetic barrier played a minor role in the QD nucleation rate, which was consistent with previous studies [52].

Fig. 14 (a) TEM image of one PbS nanocrystal and fast Fourier transform pattern obtained from the area in the circle. Absorption spectra of PbS QDs in glass (b) without silver and (c) with 20 ppm of Ag2O. (d) Absorption peak position (λabs), absorption coefficient (α), and calculated average radius (R) of PbS QDs precipitated in glass with 0, 10 or 20 ppm Ag2O by heat-treatment for 10 h at different temperatures. (e) Absorption and PL spectra of PbS QDs in glass containing different contents of Ag2O after heat-treatment at 460 °C for 10 h. Reproduced with permission [100]. Copyright 2011, Elsevier. In glass ceramics, Ag nanoclusters also act as nuclei and promote the formation of oxide nanocrystals [99]. Xu et al. reported that silver nanoclusters had an impact on the formation of PbS QDs in the glass ceramics [100]. Fig. 14a is the HR-TEM image of PbS nanocrystals that containing 20ppm Ag2O precipitated in the glass. The QDs were almost spherical with about 3.3 nm average radius, indicating that the PbS QDs formed in the glass ceramics. As shown in Fig. 14b-d, the position of bands shifted to higher λ with increasing the temperature and the absorption coefficients of glass containing 20 ppm Ag2O were much higher than without Ag. With increasing Ag2O concentration, the absorption coefficients and the PL intensity were significantly enhanced, showing that the absorption coefficient was directly associated with the 25

PbS QDs concentration, as illustrated in Fig. 14e [101]. Firstly, increasing the concentration of silver was equivalent to increasing the number of Ag clusters, which provided sites for nucleation of QDs. These nucleation sites formed a large amount of PbS QDs in the glass ceramics, thereby enhancing the absorption spectra coefficient and PL intensity. The results indicated that Ag clusters promoted the formation of QDs in the glass ceramics and used the Ag concentration to control the number of nucleation sites [102].

Fig. 15 Normalized absorption spectra of glass ceramics with different Er2O3 concentrations heat treated at (a) 490°C for 20 h and (b) 500°C for 20 h. (c) Peak emission wavelengths of absorption and photoluminescence (PL) and mean radius (r) of QDs in glass ceramics with different Er2O3 concentration treated at different temperatures for 20 h. Normalized PL spectra of PbS quantum dots formed in glasses with different Er2O3 concentration heat-treated at (d) 490°C for 20 h and (e) 500°C for 20 h. Reproduced with permission [103]. Copyright 2010, The American Ceramic Society. Heo group also studied the erbium ions influenced PbS QDs embedded glass ceramics [103]. They reported that the Er2O3 concentration affected on nucleation and optical properties of PbS QDs embedded silicate glass ceramics under heat treatment at 490°C or 500°C, as shown in Fig.15. In glass ceramics treated at 490°C (see Fig. 15a-c), as Er2O3 concentration increased from 0.1 to 0.4 mol%, the center 26

wavelengths of the absorption bands decreased from 1817 nm to 1546 nm, the radius of QDs decreased from 4.5 nm to 3.6 nm and the center wavelengths of the PL spectra decreased from 1972 nm to 1600 nm. From Fig. 15c-e, a similar phenomenon occurred in glass ceramics treated at 500°C. With increasing the Er2O3 concentration, the center wavelengths of the absorption bands decreased from 2039 nm to 1910 nm, the radius of QDs decreased from 5.4 nm to 4.8 nm and the center wavelengths of the PL spectra decreased from 2084 nm to 1893 nm. They attributed the phenomenon to bridging the oxygen-linked Er3+ ions clusters as the nucleation centers of PbS QDs.

Fig. 16 (a) TEM image of a 50SiO2-5Al2O3-25NaO-10BaO-8ZnO-2ZnS with an additional 0.8 mol% PbO glass containing 0.1 mol% Nd3+ ions after heat-treatment at 500°C for 30 h. (b) XRD patterns measured for 50SiO2-5Al2O3-25NaO-10BaO-8ZnO-2ZnS with an additional 0.8PbO and xNd2O3 (x = 0.0–0.4 mol%) glasses heat treated in two steps, at 500°C for 30 h and then at 590°C for 30 h. (c) Calculated full width at half maxima (FWHM) of PL and stoke shifts between absorption and PL for the PbS QDs in glasses with different Nd2O3 concentration. Absorption spectra (d) and (e) normalized photoluminescence spectra of PbS QDs formed in 50SiO2-5Al2O3-25NaO-10BaO-8ZnO-2ZnS with an additional 0.8PbO and xNd2O3 (x = 0.0-0.6 mol%) glasses heat treated at 500°C for 15 h. Reproduced with permission [104]. Copyright 2015, The American Ceramic Society. Furthermore, Heo et al. also investigated that Nd3+ ions influenced the precipitation and optical properties of PbS QDs embedded silicate glass ceramics, as shown in Fig. 16 [104]. The TEM and XRD confirmed that the PbS QDs with Nd3+ ions were 27

precipitated in silicate glass ceramics by heat treatment (see Fig. 16a and b). With increasing the Nd3+ content from 0.0 to 0.6 mol%, the diameters of PbS QDs decreased from 4.4 to 3.7 nm. Meanwhile, the absorption and photoluminescence peaks shifted to shorter wavelengths with increasing Nd3+ concentrations, as displayed in Fig. 16c-e. Finally, they proved that the Nd3+ ions preferentially existed inside the PbS nanocrystals and the Nd-O cluster acted as a nucleation site for the PbS embedded silicate glass ceramics.

Fig. 17 (a) Room temperature absorption spectra and (b) photoluminescence spectra of B glass ceramics (0.99(66SiO2-8B2O3-18K2O-8BaO)-0.01PbS) containing PbS QDs. (c) Room temperature absorption spectra and (d) photoluminescence spectra of BZ glass ceramics (0.99(66SiO2-8B2O3-18K2O-4BaO-4ZnO)-0.01PbS) containing PbS QDs. (e) Radius, effective bandgap energy, peak wavelength of photoluminescence (PL) and full width at half maximum (FWHM) luminescence intensity of PbS QDs embedded in different glass matrices. Reproduced with permission [46]. Copyright 2007, Springer. Besides, Heo et al. also demonstrated that modification of host glass ceramics composition improved the ability to control the sizes of QDs [46]. Fig. 17 show that a small change in host glass ceramics composition did have an effect on the precipitation and optical properties of PbS QDs. When the glass heat-treated at high temperatures, the absorption in the BZ glass ceramics occurred at a longer wavelength than that from the B glass ceramics. Under the same heat treatment conditions, the average radius of PbS QDs was larger in the glass containing ZnO because ZnO could alleviate the volatilization of chalcogenide during melting. Also, Zn could weaken the 28

S bonding with the glass ceramics network and cause S to diffuse more easily. Therefore, the size of QDs could also be controlled by the modification of host glass ceramics composition. 3.2 Luminescence mechanism

Fig. 18 Schematic energy diagram for QDs embedded glass ceramics. The luminescence of QDs is caused by the interaction of electrons, holes and their surroundings and the schematic energy diagram of QDs embedded glass ceramics is shown in Fig. 18 [102]. When the QDs are excited, the transitions marked (1) is that the electron directly transitions from the valence band to the conduction band. The transitions marked (2) is that the most electrons on the conduction band convert back to the valence band to emit photons. The remaining portion of the electrons falls into the trap of the QDs embedded glass ceramics. When electrons fall into the shallow trap, electrons transition (3) back to the valence band in the form of photons. When electrons fall into a deep trap, most of the electrons undergo non-radiative transitions marked (4). Only a tiny number of electrons absorb a certain amount of energy and then take place reverse transition (5). The transition strength (5) is small because the electron density is low in the deep trap or the wave function of the corresponding state has a small overlap. Moreover, the probability of the inverse transformation of (5) is much smaller than the transformation (3). Surface states, lattice, compositional fluctuations, foreign atoms or vacancies can cause traps in QDs embedded glass 29

ceramics, which is often difficult to clarify because the glass matrix and preparation conditions largely determine the presence of traps [102,105]. 3.3 Radius calculation It is known from the above that the size of QDs is related to the nucleus radius (rN) and the volume fraction of crystals. For the size of QDs embedded glass ceramics, two techniques, X-ray broadening and electron microscopy, are used to determine the average particle sizes. Besides, the theoretical equations based on different models can also calculate the QDs sizes. 3.3.1 Calculating radius using XRD data The Debye-Scherrer formula can calculate the average QDs size [106,107]: D = Kλ/β cosθ

(20)

where D is the QDs diameter; K is a shape and size constant; β is the full width at half maximum (FWHM) of the measured diffraction peak; θ is the diffracting angle, and λ is

the

X-ray

wavelength.

Moreover,

considering

the

Stokes

correction,

Debye-Scherrer’s equation can be written as follows [107]: D=

0.89𝜆 𝛽2 ― 𝛽20𝑐𝑜𝑠𝜃

(21)

where β0 is the Stokes correction of the FWHM. However, for particles smaller than 25Å, their size cannot currently be reliably determined. Furthermore, it can only determine the average particle size because the size distribution is unknown for this technique [106]. 3.3.2 The theoretical equations based on different models Based on previous reported, using the molecular orbital approach determine several qualitative conclusions: as the size of semiconductor crystallite is reduced, the band gap (the HOMO-LUMO gap) should open up; in very small size regime (the molecular limit), discrete absorption bands can appear [108]. Wang et al. used the phenomenological model to account for the size dependence of the band gap, using PbS as an example, as shown in Fig. 19.

30

Fig. 19 Band gap of PbS as a function of particle size.□ represents experimental data, -□ indicates 13Å is the upper limit. Dotted line is a theoretical calculation, representing the electron-hole-in-a-box model with effective mass approximation. Solid line is a theoretical calculation the hyperbolic band model. + represents the calculated band gap from the cluster model. Reproduced with permission [106]. Copyright 1987, American Institute of Physics. a. Electron hole in a box- the effective mass approximation Previous workers used the elementary electron-hole-in-a-box model with different variations to account for the optical properties of small colloidal particles and thin films [109-111]. According to the reported, a confined Wannier exciton Hamiltonian was used in the Schrödinger equation for the crystallite excited state. However, because the absolute position of the electron and hole determined some parameters, the problem can not be converted to the center of mass coordinate and therefore cannot be solved accurately [110]. An approximate solution can be obtained by taking the solution for the first excited state of the particle-in-a-box problem and also assuming that the electron and hole are uncorrelated, as follows [106]: ∆𝐸 =

ℎ2𝜋2 2𝑅2

(1 𝑚𝑒 + 1 𝑚ℎ) ―

1.8𝑒2 𝜖𝑅

31

+ polarization term

(22)

where ∆E, R, ϵ, h, me, mh are the energy of the first excited state, the radius of the particle, the dielectric constant, the constant of Planck, and the effective masses of electron and hole, respectively. The first term is the kinetic energy of the electron and hole, which increases as the particle size decreases, the second term is the screened Coulomb interaction, which stabilizes the electron-hole pair, and the third term is the polarization energy, which is generally small. The dotted line was a theoretical calculation from Eq. (22), representing the electron-hole-in-a-box model with effective mass approximation, as shown in Fig. 19. In the report on the QDs embedded glass ceramics, the researchers often used a different version of the formula to solve QDs radius, as follows [112]: 𝐸𝑛 = 𝐸𝑔 +

𝑛2ℎ2 8𝑅

2

(

1

𝑚𝑒∗

1

)

+ 𝑚∗ ― ℎ

1.8𝑒2 𝜖𝑅

(23)

here, En and Eg are the exciton transition energy and the band gap energy of bulk semiconductor. However, it was reported that this simple model can not adequately explain the observed size dependence. b. Hyperbolic band model-the breakdown of effective mass approximation The breakdown of the effective mass approximation and the size-dependent coulomb interaction can be used to explain the inadequacy of the simple model, taking PbS particles as an example. Although the optical dielectric constant of bulk PbS was large (ϵ∞ = 17.2), the contribution to the Coulomb and polarization terms in Eq. (23) was small. The effective dielectric constant became reduced for sufficiently small particles, because of the inability of the lattice polarization to follow the more rapid electron and hole motion associated with a smaller radius [113]. The coulomb effect of high-ionic solid PbS was still low even to the nearest neighbor level, because the generation of exciton states responsible for the band gap mainly involved the transfer of electrons from S to Pb atoms, resulting in the rearrangement of ionic charges, but essentially no new charge separation [113,114]. The most important effect of modifying the PbS simple model may be the destruction of the effective mass approximation. PbS had a very small band gap and small effective masses for both holes and electrons. Wang et al. proposed a simple effective mass approximation 32

that provided a good description of the bandgap of all PbS particles except the smallest diameter without significant coulomb correction [106]. This approximation was based on the idea that the lowest excitation of the PbS lattice involved a simple electron transfer from S- to Pb+ with an energy cost equaled to the bulk band gap Eg. Only two bands, the highest occupied valence band and the lowest unoccupied conduction band at the L point, were important for calculating the band gap. The following expression shows the hyperbolic band model in Fig. 19 [106]: ∆E = (𝐸2𝑔 + 4𝜆)

12

2

= [𝐸2𝑔 + 2ℎ2𝐸𝑔(𝜋 𝑅) /𝑚 ∗ ]

12

(24)

The ∆E, R, ϵ, h, me, mh are the same with Eq. (22). The Eq. (23) adequately described the observed band gap for all but the smallest particles. Coulomb corrections, believed to be small, would lower the curve shown. It can be seen that effective mass approximation starts to break down for PbS particles smaller than about 100 Å. Currently, the same formula is often used in QDs embedded glass ceramics, as follows [112,115]:

(𝐸𝑔(𝑅))2 = 𝐸2𝑔 +

(

2ℎ2𝐸𝑔 𝑚∗

𝜋 2

)( ) 𝑅

(25)

c. Cluster model The previous model, where E(k) was approximated as a hyperbola, was effective in treating larger particles. Wang et al. developed a model accurate model for handing smaller clusters [106]. The process was moderately approximated using a vastly simplified Hamiltonian function to fit the calculated band structure [106]. The clusters containing more than 4000 atoms can be easily solved with this scheme, which was basically the same as the result of Eq. (23). The model began with the dual-band model from the previous section. It was believed that the local states of the PbS lattice near the L-point bandgap were primarily p-like, localized mainly on S for the highest valence band state, and on Pb for the lowest conduction band state. Thus, a model was proposed in which electrons were allowed to be in the p-state only on S or Pb. Fig. 19 shows the calculated band gap for clusters chosen in the form of crystals bounded by spheres centered in the center of a cubic PbS building block. For particle sizes larger 33

than about 40 Å, the cluster model gave the same result as that of Eq. (23). Below that size, the cluster model still quite close to that of Eq. (23). In the very small cluster size regime (<20 Å), the assumption that only two bands are involved can break down, hence, mixing from all other bands had to be considered. Lent et al. gave the tight-binding model that the cluster model gave the gap in the monomolecular limit [106]: ∆𝐸 = [𝐸2𝑔 + 4𝛽2]

12

(26)

where β is the nearest-neighbor transfer integral. Taking the PbS as an example, the ∆E = 2.51 eV. The result compared closely with the measured 0-0 transition of the first allowed the excited state of PbS in the gas phase, 2.33eV. Besides, some empirical equations came from colloidal QDs, but they were also used in QDs embedded glass ceramics, we just listed them as follows [116-121]: 1

(27)

𝐸𝑔(𝐷) = 𝐸𝑔(∞) + 0.0105𝐷2 + 0.2655𝐷 + 0.0667

Here, Eg(D) and Eg(∞) are the band gap of QDs (eV) and bulk semiconductor, respectively. Among them, Eg(D) can estimate from the absorption peak position. 𝐷 = (1.6122 × 10 ―9)𝜆4 ― (2.6575 × 10 ―6) + (1.6242 × 10 ―3)𝜆2 ―0.4227𝜆 + 41.57

(28)

where D is the average diameters (D, nm) of QDs that based on the first excitonic absorption peak and λ (nm) is the peak wavelength of the first excitonic absorption. 𝐷=

𝜆 ― 143.75 281.25

(29)

where λ (nm) stands for the PL peak position and D (nm) is the average diameter of QDs. These values are slightly larger, which ascribe to the fact that the PL peak position is larger than the first absorption peak position due to the Stokes effect, resulting in the overestimation of the QDs size. 4. The Properties of QDs embedded glass ceramics As important optical materials, QDs embedded glass ceramics have attracted attention due to their unique optical and electrical properties derived from the quantum confinement effect [12]. 4.1 Spectral variety of QDs embedded glass ceramics 34

4.1.1 Tunable absorption and photoluminescence spectra Changing the size of QDs embedded glass ceramics adjust the absorption and photoluminescence spectra which is related to the preparation conditions, especially heat treatment temperature and time. As displayed in Fig. 20a, the absorption spectra of PbS QDs embedded glass ceramics under different temperatures (500, 525, 550, 575°C) for 24 h. It was apparent that as the temperature increased, the absorption edge moved to the longer wavelength and the intensity of the absorption band increased. As well known, the electron and hole pairs caused by excitation photons lead to absorption band, indicating that PbS QDs emerged to form at 525 °C in the glass matrix.

Fig. 20 (a) Absorption spectra of glass ceramics heated at different temperatures for 24 h. (b) PL spectra of glass heated at different temperatures for 24 h. An 808 nm LD was used as an excitation source. (c) TEM and HR-TEM image of glass heated at 550 °C for 24 h. (d) Room temperature absorption spectra of precursor BC glass and thermally treated glass. (e) Room temperature photoluminescence spectra of PbS embedded glass ceramics under the 800 nm laser excitation. (f) TEM image of one PbS QDs. (a-c) Reproduced with permission [12]. Copyright 2014, Elsevier. (d-f) Reproduced with permission [50]. Copyright 2007, Springer. Meanwhile, PbS QDs formed in the glass ceramics exhibited broadband PL at 808 nm excitation, as shown in Fig. 20b. Moreover, the full width at half maximum (FWHM) also extended from 220 nm to 300 nm and the size distribution of PbS QDs 35

embedded glass ceramics broadened obviously. The average radii of PbS QDs embedded glass ceramic were calculated by Eq. (10) to be 1.4 nm, 3.2 nm and 5.8 nm, respectively. The obtained results of radii were consistent with that in Fig. 20c [106]. After heat treatment at 600 °C for 20, 30 and 70 h, the glass ceramics exhibited a broad absorption band at 1070, 1200 and 1200 nm, as shown in Fig. 20d. These absorption peaks were blue-shifted from the band gap energy of the bulk PbS (0.41 eV), which demonstrated the quantum confinement effect. From Fig. 20e, the center wavelength of PbS QDs photoluminescence spectra were 1200, 1340, 1450 nm when the heat-treatment duration was 20, 30 to 70 h, respectively. Similarly, the average radius of QDs were 2.5, 2.7 and 3.0 nm when thermally treated for 20, 30 and 70 h, respectively, which also coincided with the average diameter of PbS QDs, as exhibited in Fig. 20f. Based on the above results, it can be inferred that the absorption and PL spectrum of the QDs embedded glass ceramics can be widely adjusted by controlling the heat treatment and time conditions [50,122]. 4.1.2 Luminescence enhancement and quenching

Fig. 21 (a) Schematic diagrams of fabrication process of CdS QDs and Ag NPs in glass. (b) Confocal PL images and corresponding intensities of CdS QDs in glass after ion exchange (IE) at 260°C for (1) 10 s, (2) 1 min, (3) 10 min and (4) 30 min and then heat treatment at 400°C for 10 h. (c) Enhancement of the PL intensities from CdS 36

QDs versus ion-exchange durations. Reproduced with permission [123]. Copyright 2013, The American Ceramic Society. The luminescence intensity of QDs embedded glass ceramics can also vary, including enhancement and quenching. Xu et al. enhanced and quenched the luminescence intensity of CdS QDs embedded glass ceramics by Ag+ ion exchange Na+ ions [86,123]. As shown in Fig. 21a, they used a four-step method to prepare CdS QDs and Ag nanoparticles (NPs) embedded glass ceramics. It was worth noting that Ag NPs precipitated only in the ion-exchanged region of the CdS QDs surface. Fig. 21b is the confocal PL images and relational intensities of CdS QDs embedded glass for Ag+ ion exchange under different times. Fig. 21c summarized the PL intensities from CdS QDs versus ion exchange duration. From the above results, the amount of Ag+ ions diffused into the glass and the PL of CdS QDs depended on the ion exchange duration. When the ion exchange duration was 1 minute, the PL intensity increased by about three times. However, when the duration extended to 30 minutes, severe quenching took place. Therefore, doping Ag+ enhanced and quenched the PL intensity of CdS QDs since increasing the amount of Ag+ ions was equivalent to increasing the number of Ag NPs and reducing the average distance between Ag NPs and CdS QD, which caused the variation of luminescence intensity [123]. 4.1.3 Energy transfer between QDs and rare earth ions Many groups have studied the rare earth (RE) ions and QDs co-embedded glass ceramics. The luminescence intensity of RE ions significantly enhanced due to the energy transfer between QDs and RE ions [51,124-126]. Serqueira et al. described the luminescence of Nd3+ ions excited by CdSe QDs embedded glass ceramics [51]. Under different excitation sources, the luminescence and absorption spectra of the Nd3+ ions in CdSe QDs embedded glass ceramics, as displayed in Fig. 22a. A band centered at about 882 nm that attributed to the electronic transition from the 4F3/2 state to the 4I9/2 state of the Nd3+ ions. Meanwhile, there was no luminescence at 882 nm under 409 nm excitation, which meant that there were no photon emissions at the 4F3/2 → 4I9/2 transition of the Nd3+ ions. Fig. 22b shows the luminescence spectrum of CdSe 37

QD in SNAB matrix heat treated at 560 °C for 0, 1, 2, 4, 10, 24 and 36 hours. At 409 nm excitation, the CdSe QDs had an emission range between 500 and 900 nm with increasing the heating time. Therefore, the 409 nm excitation wavelength used to study the energy transfer between CdSe QDs and Nd3+ ions. The absorption spectra and luminescence of CdSe QDs and Nd3+ ions in the SNAB glass system at 560°C for 0, 1, 2, 10, 24, and 36 h, as presented in Fig. 22c and d. At 409 nm excitation, the characteristic spectrum peaks (4F3/2 → 4I9/2 transition) and the redshift of emission phenomenon with increasing heating time were observed. The redshift increased the overlap between the broadband of CdSe QDs and the absorption bands of the Nd3+ ions. The greater the overlap, the enhanced photon absorption of the Nd3+ ions embedded in the SNAB glass system, which demonstrated the energy transfer from the CdSe QDs to the Nd3+ ions. Furthermore, the 4F3/2 → 4I9/2 transition intensity of Nd3+ ions increased with redshift under 409 nm excitation, which provided additional evidence of energy transfer [51]. Chen et al. also reported the existence of energy transfer from the CdS QDs to Er3+ ions [124]. Planelles-Aragó et al. reported the phenomenon of energy transfer existed in Eu3+ and Mn2+ ions doped ZnS QDs embedded in silica glass [127]. Falci et al. explained the energy transfer behavior between ZnTe QDs and Yb3+ ions in phosphate glass ceramics [46].

38

Fig. 22 (a) Luminescence spectra of Nd3+ ions embedded in the SNAB system. The electronic transition luminescence is strongly dependent on excitation source. Note that luminescence is not produced from the 409 nm. (b) Luminescence spectra of CdSe QDs in this glass system heat-treated at 560° C for 0 at 36 h. (c) Absorption and (d) luminescence of CdSe QDs and Nd3+ ions in the SNAB glass system heat-treated at 560°C for 0, 1, 2, 10, 24, and 36 h. Reproduced with permission [51]. Copyright 2014, Optical Society of America. 4.1.4 Increasing PL emission wavelength Lead chalcogenide (PbS, PbSe, and PbTe) QDs have strong quantum confinement effect, which allows them to emit infrared photoluminescence (PL). However, the quantum confinement effect rapidly decreases as their size increases, such that the wavelengths (λ) of PL from lead chalcogenide QDs are limited to λ < 2 μm and difficult to prepare QDs sufficient to emit PL of λ > 2 μm. Although their band gaps are narrow, the mid-infrared PL can not be obtained by simply increasing their size. Additionally, several semiconductors are toxic or have limited solubility in conventional glass ceramics. Therefore, it is necessary to find a suitable method to 39

develop QDs with mid-infrared PL. Zhang et al. obtained a mid-infrared PL emission of λ~2.6 μm by doping a small amount of the third element (Sn) in silicate glass ceramics [122]. Fig. 23a and b present the absorption spectra of QDs embedded silicate glass ceramics without SnO and with SnO under heat treatment at different temperatures for 10 h. It was observed that the longest absorption peak caused by PbSe QD was at λ = 1850 nm in the SnO-free glass and the peak was at λ = 2580 nm in the glass ceramics containing SnO. Meanwhile, the intensity of absorption peaks increased with raising the temperature. Fig. 23c and d show the PL spectra of QDs embedded silicate glass ceramics without SnO and with SnO under heat treatment at different temperatures for 10h at 800 nm excitation. The longest emission band was at λ = 1940 nm in the silicate glass ceramics without SnO and the PL band moved to 2650 nm for the glass ceramics containing SnO. Meanwhile, the PL peak red-shifted with increasing the temperature. The above results indicated that the incorporation of Sn resulted in the photoluminescence to be converted to the mid-infrared wavelengths [122]. Moreover, Wang et al. reported that the incorporation of Ge increased the mid-infrared wavelength in silicate glass ceramics because GeO2 had strong diffusion capacity, which formed large PbSe QDs and caused red-shift [128].

40

Fig. 23 Absorption spectra of silicate glass (a) without SnO addition and (b) with SnO addition. PL spectra of QDs in glass (c) without SnO and (d) with SnO. Excitation wavelength for the measurement was 800 nm. Reproduced with permission [122]. Copyright 2016, Elsevier. 4.2 Nonlinear optical property QDs embedded glass ceramics are widely used in all-optical data processing due to ultra-fast electronic response and stronger nonlinear optical performance in a few picoseconds [129,130]. Du et al. studied the nonlinear optical property of ZnTe QDs embedded in ZnO–TeO2–P2O5 (ZTP) glass ceramics [45]. As presented in Fig. 24a, when the glass ceramics without TeO2, they were colorless and had no absorption band in the wavelength range of 425-725 nm. As the content of TeO2 increased, the color of glass ceramics changed from a pale color to dark red and a distinct absorption band at 532 nm. Based on the previous equations, the third-order nonlinear optical susceptibility χ(3) were calculated, as shown in Fig. 24b and c [131]. Obviously, these glass ceramics exhibited a larger third-order nonlinear optical coefficient χ(3) up to 10-11 esu, which was about 103 times that of silica glass. With increasing TeO2 content, the magnitude of χ(3) increased from 10 to 30, which was proportional to the content 41

of ZnTe QDs in the glass ceramics [47,131]. The results indicated that ZnTe QDs embedded glass ceramics could possess large third-order optical nonlinearities. Amore et al. also reported nonlinear optical characteristics of CdS QDs embedded glass ceramics and the χ(3) dot (p = 3%) of CdS sample was 7×10-12 e.s.u for λ between 1500 and 1700 nm, as illustrated in Fig. 24d [132]. The above results demonstrated that these glass ceramics had high nonlinear optical properties, which used in applications such as all-optical modulator and a saturable absorber [13,132].

Fig. 24 (a) Optical absorption spectra of ZnO–TeO2–P2O5-x glass. The inset is the images of ZnO–TeO2–P2O5-x glass. Typical Z-scan normalized transmittance curves for closed (b) and open (c) aperture measurements of ZnO–TeO2–P2O5-10 glass sample. (d) Dispersion of the χ(3) for two CdS doped silica glass (10% and 3% filling factor), compared to the χ(3) for CdS bulk sample. (a-c) Reproduced with permission [47]. Copyright 2013, The American Ceramic Society. (d) Reproduced with permission [132]. Copyright 2004, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 4.3 Stable characteristics

42

Fig. 25 (a) Thermal lens signal at 55 K and (b) thermal diffusivity behavior as a function of the temperature for the samples PZABP1Te0Yb and PZABP1Te5Yb. (c) Photocatalytic activity for hydrogen evolution of CdS0.5Se0.5 dot–glass nanosystem. (d) Photocatalytic activity for hydrogen evolution of CdSe quantum dot–glass nanosystem. (a and b) Reproduced with permission [46]. Copyright 2016, Elsevier. (c and d) Reproduced with permission [44]. Copyright 2014, The Royal Society of Chemistry. The QDs embedded glass ceramics have high stability in practical applications, including not only thermal stability but also repeated use. High thermal diffusivity is an essential feature of active media for high power lasers and requires operation over a wide temperature range. Falci et al. found that the thermal diffusivity of Yb3+-ZnTe embedded phosphate glass ceramics was insensitive to temperature variations, including intervals from room temperature to a low temperature by the Gaussian excitation laser [46]. Based on the behavior of the graph, the lens formed by the glass ceramics was a convergent one. As displayed in Fig. 25a and b, the thermal diffusivity of the two samples was relatively constant with temperature in consideration of the overall behavior of temperature and was near D = (2.47±0.2) х 10-3cm2s-1. The results 43

suggested that the thermal diffusivity of the Yb3+ -ZnTe embedded phosphate glass ceramics was very stable, which used in high power photonic devices. Furthermore, Apte et al. described CdS0.5Se0.5 and CdSe embedded germanate glass ceramics had the photocatalytic activity for hydrogen evolution and reused multiple times, as shown in Fig. 25c and d [44]. All the CdS0.5Se0.5 and CdSe embedded germanate glass ceramics exhibited excellent repeatability and reusability under identical experimental conditions for hydrogen evolution. 5. A wide range of applications In recent years, QDs embedded glass ceramics have a wide range of potential applications because they are composite materials with many advantages of QDs and glass ceramics. Fabrication of QDs embedded glass matrices has several advantages, such as chemical and high mechanical stability as well as adaptability to the device fabrication process. Fig.1 shows QDs embedded glass ceramics with unique properties have great potential applications in optoelectronic devices, photocatalysts and sensors. QDs embedded glass ceramics are nanomaterials with unique optical properties, excellent size and controlled distribution, which are essential for the fabrication of devices in the form of planar and optical fibers, especially laser crystals, amplifiers, waveguides, etc [133,134]. 5.1 Optoelectronic devices 5.1.1 Light emitting diodes (LEDs) At present, the commercial WLED fabricated by blue InGaN chip and Y3Al5O12:Ce3+ yellow phosphor exposes problems, such as high color correction temperature, poor color rendering index and relatively poor stability, limiting its full application indoor illumination. In order to solve the above problems, Wang et al. described the phosphors embedded glass ceramics to overcome problems and work on them. Besides, some researchers have reported that QDs embedded glass ceramics to improve the above problems [135]. One knows that the photoluminescence of QDs easily adjusted depending on the size and composition of the QDs and their spectral ranges are from UV to the IR region, as exhibited in Fig.2. Fortunately, even in the 44

glass ceramics, QDs retain this feature, which makes them suitable for light emitting diodes (LEDs), display, etc. Han et al. reported that CdS and CdSe QDs embedded silicate glass ceramics could be used for the LED color converter [25]. From Fig. 26a, under 455 nm LED excitation, two broad emission bands at 640 and 680 nm appeared in PL spectrum of CdS QDs embedded glass ceramics due to the direct recombination of the exciton pairs and recombination of the exciton pairs through trap surface states [136]. As shown in Fig. 26b, CdSe QDs embedded glass ceramics achieved spectral vary and color conversion,

Fig. 26 (a) EL + PL spectrum of the LED incorporating silicate glass with embedded CdS QDs. The insets are the PL spectra of the glasses when pumped at 455-nm LED source, and a photograph of the LED with the mounted glass. (b) Absorption spectra of silicate glass with CdO and ZnSe for varying heat treatment duration times at 520°C. The inset photographs show the glass before and after the heat treatment at varying duration times. (c) (1) CIE color coordination of the LEDs with silicate glass embedded CdSe QDs heat-treated at 520°C for varying duration times. The actual photographs of the LEDs were displayed on the right side. The insets (2) and (3) show EL + PL spectra of the LEDs and spectral overlap between the absorption and emission spectra. Reproduced with permission [25]. Copyright 2015, The American Ceramic Society and Wiley Periodicals, Inc. such as from yellow, red to dark brown, under heat treatment at different durations. PL spectrum detected by 450 nm excitation source and the emission peak caused by 45

exciton recombination exhibited redshift with increasing QDs size. The CdSe QDs embedded glass ceramics and the 450 nm blue LED chip constituted WLED, as shown in Fig. 26c. It was found that the CIE chromaticity coordinates changed at different heating times, that was when the heat treatment time was increased to 15 h, the chromaticity coordinates changed from blue to red, over 20 h, the color returned to blue. Besides, the actual photographs of the LEDs, PL and EL emission exhibited in Fig. 26c, indicating that CdSe QDs embedded glass ceramics as a color converter for WLED had the practical potential and feasibility. Han et al. also reported that Cd-S-Se QDs embedded silicate glass ceramics for white LEDs as color conversion [137]. QD-embedded glass ceramics have been exhibited reasonable color conversion and have the potential to achieve white LEDs. However, the color tunability and conversion efficiency of those LEDs were limited mainly due to QD reabsorption and rapid nonradiative recombination of exciton pairs by trap surface states [25]. Moreover, quantum yield (QY), CRI and luminous efficacy cannot satisfy commercial requirements. Hence, the QD size, glass thickness and core/shell structure need to be carefully adjusted to improve efficiency. 5.1.2 Optical fiber amplifier

46

Fig. 27 (a) Schematic diagram of optical amplification experimental system. (b) PL spectra of P1.5, P1 and Z2 glass ceramics heat treated at different temperatures for 24 h. An 808 nm LD was using as an excitation source. (c) Optical gain (I/I0) of PbS QDs doped glass collected as a function of pumping power. The insert shows the amplified signals of P1 glass ceramics heat treated at 580 ◦C for 24 h. (d) Fiber fabrication and the insert is the sodium-aluminum-borosilicate glass fiber containing PbSe QDs. (a-c) Reproduced with permission [134]. Copyright 2011, Elsevier. (d) Reproduced with permission [9]. Copyright 2011, Elsevier. Optical fiber amplifier is an all-optical amplifier used in fiber-optical communication lines to achieve signal amplification. QDs embedded glass ceramics have the potential for optical fiber amplifier. For the practical application as the gain medium of fiber amplifier and fiber laser, the optical amplification of QDs embedded glass ceramics is the most fundamental factor. Dong et al. studied the optical amplification property of PbS QDs embedded glass ceramics, as shown in Fig. 27a [134]. For P1.5 glass ceramics heat treated for 24 h, the central wavelength of emission band shifted (1370 → 1705 → 1795 nm) as temperature increased as well as the full width at half-maximum (FWHM), as displayed Fig. 27b. Meanwhile, the red shifting and broadening of the emission band appeared in P1 glass. For Z2 glass ceramics prepared with ZnS + PbO as the source of Pb and S, the red shifting and broadening of emission band were also found. Furthermore, compared to P1.5 and P1 glass ceramics, Z2 glass ceramics had a narrower FWHM, namely, 220 nm, 260 nm and 285 nm under the same conditions. Fig. 27c presents the optical amplified signals at 1.55 m of PbS QDs embedded glass ceramics measured by the two-wave mixing configuration. The P1 glass ceramics heat-treated at 580 ◦C for 24 h was selected as the representative sample for 1.55 m optical amplification measurement due to the excellent peak matching and intensive PL, as displayed in the inset of Fig. 27c. As the excitation power increased, the g value increased linearly in logarithmic form. When the excitation power was directed as 730 mW, the optical gain g was about 1.26 and the optical gain was as high as 1.97 at 830 mW. For Z2 glass ceramics, the corresponding optical gain was approximately 1.34 and 2.89 at an excitation power of 730 mW and 830 mW, respectively, which 47

was significantly larger than P1 glass ceramics. The above results indicated that the optical amplification of PbS QDs embedded glass ceramics detected at 1.55 μm and 1.33 μm. Furthermore, the optical amplification performance could be further improved by improving the glass ceramics and the synthesis parameters, which may replace the conventional erbium-doped fiber amplifier [134]. The wiredrawing method of fiber preparation was described by Cheng et al., as displayed in Fig. 27d [9]. The insert of Fig. 26d is a photo of the wiredrawn fiber in curvature radius of 2 cm, exhibiting similar flexibility and ductility to SiO2 fibers. Dong et al. also described multi-wavelength optical amplification of PbS QDs embedded silicate glass ceramics [12]. QDs embedded glass ceramics provide the advantages of operation wavelength, low cost and high power output. Of course, they also have disadvantages such as high noise figures, polarization dependence and high coupling loss. By the improvement of the formation and PL properties of QDs embedded glass ceramics, the QDs embedded glass ceramics can be potentially applied in broadband fiber amplifiers and tunable fiber lasers. 5.1.3 Saturable absorber (SA) for mode-locking Researchers have paid considerable attention to pulse in the infrared wavelength range because lasers emitting have been widely used in light detection, ranging, telecommunications, therapy, and surgery [135]. All of these applications need to be based on reliable, simple and compact high power pulsed laser sources. The SA put in the laser cavity without any electronic equipment to meet the requirements of passive laser Q-switching and mode locking for the compactness and simplicity of the laser [133]. The main requirements for SA are the narrow size distribution function of QDs (R/R
cost associated with the complexity of implementing corresponding technology. Many groups have demonstrated that QDs embedded glass ceramics could be used as SA for passive mode locking in solid-state lasers [27,138-141]. Gaponenko et al. presented a passive mode locking of diode-pumped Tm: KYW laser with PbS QDs embedded glass as SA, as exhibited in Fig. 28a and b [138]. The PbS QDs displayed significant absorption saturation in the range of the first exciton absorption band, as shown in Fig. 28c. From Fig. 28d, the Q-switched pulse sequence was very stable with a jitter of fewer than 10 μs. As shown in Fig. 28e and f, a series of mode-locking pulses made up each Q-switched pulse, separated by 5.4 ns cavity round-trip time, corresponding to the frequency of 185 MHz. In the Q-switched mode locking operation, the conversion efficiency of output power in free-running mode was about 30%. The results suggested that PbS QDs embedded glass ceramics was used as a candidate for SAs in ultrashort-pulse solid-state lasers within the emission range of 2m [138]. Dantas et al. found that PbSe QDs embedded oxide-glass ceramics and fluorophosphate glass ceramics had the potential to become SAs for passive mode-locking in solid-state lasers [142]. Malyarevich et al. and Lagatsky et al. also demonstrated a new SA based on PbS QDs embedded glass ceramics for the passive mode-locking [143,144]. Compared with typical absorbers based on solid-state bulk or quantum well structures, the novel semiconductor QDs embedded glass ceramics, which offered low saturation intensity and a fast time response as well as the advantages of simpler and less expensive [138]. However, the SAs of QDs embedded glass ceramics also have disadvantages, such as a very short time (an important factor), which requires a good narrow size distribution of QDs and uniform distribution in the glass ceramics by changing the preparation conditions.

49

Fig. 28 (a) Schematic of the Tm:KYW laser. (b) Schematic of the final SAM. (c) Transmission spectrum of the SA structure on a glass substrate. (d) A single Q-switched pulse consisted of a train of mode-locking pulses. (e) A train of Q-switched pulses with the repetition rate of 16.7 kHz. (f) A train of modelocking pulses at the frequency of 185 MHz within a Q-switched pulse envelope. Reproduced with permission [138]. Copyright 2010, WILEY-VCH Verlag GmbH & Co. KGaA. 5.1.4 Q-switching in near-IR lasers Passively Q-switched diode-pumped thulium lasers with near-IR wavelengths emission pulse are widely used for communication, environmental atmosphere monitoring and medicine [145]. The passive Q switching with SA is simpler, more compact than the active Q switching and does not require the advantage of an auxiliary electronic device. Gaponenko et al. used the PbS embedded silicate glass ceramics as SA and achieved passive Q switching of a diode-pumped 1.9 m Tm: KYW laser [145]. Fig. 29a is the schematic diagram of the diode-pumped Tm: KYW laser passively Q-switched with the PbS QDs embedded glass ceramics SA. Fig. 29b shows the average output power of Q-switched Tm: KYW laser, the relationship between the pulse energy, pulse repetition rate and the absorbed pump power. As shown in Fig. 29c, an energy output pulses of 44 μJ got at the repetition rate of 2.5 kHz, the average output power of 110 mW, the corresponding Q-switching conversion efficiency was 33% and the Q-switched laser slope efficiency was 20%. 50

Malyarevich et al. used the PbS QDs embedded phosphate glass ceramics as an intracavity saturable absorber and achieved passive Q switching of 1.54-m Er: Yb: glass laser [141]. The above results indicated that the PbS QDs embedded glass ceramics had the potential for high power diode-pumped thulium lasers passively Q-switched. In practical applications, the laser Q-switch output is further improved by optimizing the cavity parameters and the initial transmission of the saturable absorber to achieve the high-power diode-pumped thulium lasers passively Q-switched with QDs embedded glass ceramics.

Fig. 29 (a) Schematics of the diode-pumped Tm:KYW laser passively Q-switched with the PbS QDs embedded glass saturable absorber. (b) Average output power, pulse energy, and pulse repetition rate of the Q-switched Tm:KYW laser versus absorbed pump power. (c) Output parameters of the diode-pumped Tm:KYW laser passively Q-switched with the PbS QDs embedded glass SA measured at 1240 mW of the absorbed pump power. Reproduced with permission [145]. Copyright 2008, Springer. 5.1.5 All-optical modulators (AOMs) Optical modulators, the especially all-optical modulator (AOM), can transmit information to light signals as they zip via kilometers of optical fibers at the center of data-sharing networks, which play a key role in the era of high-speed communication [146-148]. The modulator is typically evaluated using standards such as modulation 51

depth (MD), device size, switching energy, modulation frequency, etc [13]. Since 2004, researchers have made significant progress in AOMs research. Balaghi et al. described CdSe QDs embedded silica glass ceramics as AOM materials, as shown in Fig. 30a [13]. A planar waveguide was seen which included a Si substrate, an amorphous SiO2 film and a CdSe QDs embedded thick SiO2 film. Fig. 30b displays the relaxation process, including Auger recombination process, trapped electron-hole recombination, fast carrier trapping in surface states and recombination from deep trap states in CdSe QDs.

Fig. 30 (a) (1) Designed active planar waveguide. (2) Band structure of the CdSe QDs in glass barrier with the pulsed pump energy of 2.6 eV and the CW probe of 0.8147 eV(Left). (b) Calculated output modulated probe power by injecting pump pulse train at frequency of 71 GHz. The input CW probe power is 5 mW. (c) Calculated MD versus different input pump power density value at the frequency of 71 GHz and at the probe power of 5 mW. (d) Calculated MD versus different input probe power values at the fix pump power density of 5.6 MW/m2 and frequency of 71 GHz. (e) Calculated MD versus modulation frequency at the pump power level of 5.6 MW/m2 and probe power level of 5 mW. Reproduced with permission [13]. Copyright 2013, IEEE. The pump and probe transmit information from one wavelength to another with a carrier full recovery time of 14 ps. Fig. 30c is calculated MD versus different input pump power density value at the frequency of 71 GHz and the probe power of 5 mW. 52

The MD became larger with increasing the pump power density owing to the increase of the electron population at the GS. Fig. 30d depicts the relationship between the calculated MD and 5.6 MW/m2 fixed pump power density and 71 GHz frequency. With increasing the probe power, the number of electrons in ES increased, continuing until the state of electron saturation. Subsequently, due to the reduction of the electronic transition to the upper state, the MD began to decrease very slowly by increasing the probe power. The derived values of MD in response to different pump signal frequencies with a power density of 5.6 MW/m2, as displayed in Fig. 30e. It can be observed that the higher the pump frequency, the lower the MD and the moderate MD was about 45%, clearly indicating the possibility of achieving a high bit rate in terabits per second [13]. Pacifici et al. also reported all-optical modulation by plasmonic excitation of CdSe QDs [147]. QDs embedded glass ceramics have a stable and tunable wavelength. However, in the huge market demand, researchers should continue to find optical modulators with wider bandwidth and better flat gain characteristics by optimizing amplifier parameters, etc.

5.1.6 Photovoltaic application (PV)

Fig. 31 (a) Transmission spectra of oxide-layer-embedded CdSe QDs. (b) Photoluminescence spectra of the same structures. (c) Reflection of incident photons on silicon after forming CdSe QDs embedded glass layer. Reproduced with permission [14]. Copyright 2008, SPIE Digital Library. As well known, photovoltaic has a wide range of applications, from communication satellites to terrestrial power grids. Generally, luminescent concentrators attached to 53

the solar cell to achieve the purpose of spectral photon conversion. Among these concentrators, organic dye molecules are the most comprehensive candidate. In recent years, QDs has often been proposed to replace organic dye molecules as luminescent concentrators [149]. Compared with organic dye molecules, QDs has the advantages of high brightness, stability and quantum efficiency as well as all light having a wavelength less than the absorption maximum. Therefore, the size and spacing of the QDs and the host material must be adequately selected to optimize optical performance [150]. Sadeghimakki et al. reported the CdSe QDs embedded glass ceramics as part of the conversion layer for photovoltaic applications [14]. As presented in Fig. 31a and b, the results of transmission and photoluminescence indicated that spin cast CdSe QDs introduced in an oxide layer converted the incident high-energy photons to lower energy photons, which can be more efficiently absorbed with underneath crystalline silicon solar cells. From Fig. 31c, after the formation of CdSe QDs embedded glass ceramics layer, reducing the reflection of incident photons on the silicon glass ceramics passivated the solar cell and served as a useful converter layer. The above results demonstrated that CdSe QDs embedded glass ceramics can be used for potential photovoltaic applications [14]. Compared with traditional dye molecules, QDs embedded glass ceramics have excellent light stability, high quantum yield and adjustable emission and absorption spectra, which greatly improve the performance of luminescent concentrators. However, the self-absorption and spectral overlap of quantum dots result in light transmission loss. Therefore, adjusting the size, distribution and optimization process of the QDs embedded glass ceramics to improve the light output, which can improve the application in the solar concentrator. 5.2 Photocatalyst

54

Fig. 32 (a) Schematic diagram for the photogenerated excitons transfer processes of proton reduction and sacrifcial reagent oxidation in bare CdSe QDs. (b) (1) Photograph of CdS0.5Se0.5 embedded germanate glass. (2) Photograph of CdSe embedded germanate glass. (c) Photocatalytic activities for hydrogen evolution of CdS0.5Se0.5 embedded germanate glass. (d) Photocatalytic activities for hydrogen evolution of CdSe embedded germanate glass. (a) Reproduced with permission [156]. Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b-d) Reproduced with permission [44]. Copyright 2014, The Royal Society of Chemistry. Converting solar energy into chemical energy is an ideal way to solve future energy and

environmental

crises.

Hydrogen

generation

from

the

photocatalytic

decomposition of water is an effective way to achieve this goal. Therefore, the search for efficient, inexpensive and stable photocatalytic decomposition of water to produce hydrogen has received extensive attention. In recent years, QDs have been favored in the field of photocatalytic hydrogen production because of their good visible light response, the suitable position of the valence band, adjustable band gap, abundant surface active sites and quantum confinement effect. Some groups have achieved good photocatalytic hydrogen production performance by using photocatalytic hydrogen production systems constructed with QDs as light absorbing units, such as CdSe QDs and the schematic diagram is shown in Fig. 32a [151-154]. However, the naked

QDs

have

the

disadvantages 55

of

photo-corrosion,

instability

and

non-recyclability, which limits the using of QDs. As shown in Fig. 32b, Apte, et al. prepared a series of CdS0.5Se0.5/CdSe QDs embedded germanate glass ceramics [44]. The results demonstrated that the hydrogen evolution for glass ceramics was much higher than the powders, as shown in Fig. 32c and d [44]. The results manifested that the photocatalysts of CdS0.5Se0.5/CdSe QDs embedded germanate glass ceramics exhibited excellent repeatability and reusability under the same conditions of hydrogen evolution experiments, as displayed in Fig. 25c and d [44].

Fig. 33 (a) Photocatalytic activities for H2 evolution of Ge03 and Ge04 glass. The inset is photocatalytic activities for H2 evolution from H2S. UV-Vis absorption spectra of degraded MB solution (10 ppm) of Ge03 (b) and Ge04 (c) glass. (d) Ge03 and Ge04 glass decompose MB. Reproduced with permission [155]. Copyright 2012, The Royal Society of Chemistry. Besides, using solar energy to split H2S to produce hydrogen is also an attractive environmentally friendly method. Apte et al. also reported that orthorhombic CdS QDs were stably embedded germanate glass ceramics to split H2S and generated H2 [155]. Fig. 33a exhibits the time-dependent H2 evolution rate and amount of H2 of CdS QDs embedded germanate glass ceramics. Besides, CdS QDs embedded glass ceramics exhibited excellent methylene blue (dye) degradation under visible light illumination. Fig. 33b-d show decomposition rate of MB degradation and UV-Vis 56

absorption spectra of degraded MB (10 ppm) solution. The samples presented a higher degradation rate, namely, the Ge03 sample displayed 80.5% degradation of MB whereas the Ge04 sample exhibited only 71% of degradation in 100 min [155]. Obviously, the advantages of QDs embedded glass ceramics possess excellent stability that can be used multiple times. Improving the photocatalyst efficiency of QDs embedded glass ceramics is critical because humans are facing some of the most serious environmental and energy problems. The specific surface area can be varied by size adjustment, thereby changing the catalytically active site. If the size is not suitable, it is easy to cause many surface defects, leading to photogenerated excitons easy to be captured. Moreover, morphology and crystal form are equally important in a photocatalyst. Besides, the inherent toxicity of cadmium chalcogenide semiconductors (e.g., CdS) poses a serious risk to human health and the environment, hence, searching for non-toxic QDs embedded glass ceramics is also a very urgent arrangement. 5.3 Sensor Development of QDs fiber is particularly interesting because they combine the advantages of QD and fiber technology to achieve optimal sensor system design. In an external physical or chemical environment, monitoring the emission intensity of QDs embedded glass ceramics can detect the temperature or ammonia content, etc [43,156]. Agafonova et al. found that the shapes and spectral positions of CdS and CdSxSe1–x QDs in oxyfluoride glass ceramics fibers remained unchanged, while the intensity was constantly changing in the 25-250°C range [156]. As shown in Fig. 34a, the luminescence intensity exhibited a four- to fivefold declining in the range of 25 to 225°C. After cooling, the intensity returned to the initial level and the temperature dependence reproduced multiple times. Fig. 34b lists the average sensitivity of glass ceramics fibers in the temperature range of 25-250°C, which compared with some analogs. The temperature sensitivity of CdS and CdSxSe1–x QDs fibers was comparable with other materials and even had a wide operating temperature range and reversible characteristics. Besides, Orlova et al. proposed that CdSe/ZnS QDs 57

embedded glass ceramics used as a highly sensitive element for the quantitative vapor detection of nitrogen-containing compounds [43]. The principle was to form QDs/ammonia complexes on the surface of CdSe/ZnS QDs, which then resulted in an effective quenching of QDs PL and a reduction in average PL decay time. From Fig. 34c, as the exposure time of CdSe/ZnS QDs samples in glass ceramics increased, the QDs PL intensity decreased and the average decay time declined. At present, the application of QDs embedded glass ceramics in sensors is not very mature, but the QDs possess a narrow emission spectrum and a wide range of absorbable spectra, which provides sufficient advantages. By changing the composition or size of the QDs, the position of the emission spectrum can be changed to suit the practical application. In addition, the development of non-toxic QDs embedded glass ceramics has received extensive attention.

Fig. 34 (a) The temperature dependences of the integral luminescence intensity in optical fibers of oxyfluoride glass with CdS (1) and CdSxSe1–x (2). (b) Temperature sensitivity of the luminescence intensity for fiber temperature sensors and QDs. (c) Typical dependences of the QD PL intensity (1) and the average QD PL decay time (2) on exposure time for a sample of CdSe/ZnS QDs in porous glass exposed to ammonia vapor. (a and b) Reproduced with permission [156]. Copyright 2013, Pleiades Publishing, Ltd. (c) Reproduced with permission [43]. Copyright 2013, IOP Publishing Ltd.

58

6. New materials - perovskite quantum dots embedded glass ceramics In recent years, inorganic CsPbX3 (X = Cl, Br, I) QDs have attracted widespread attention due to their remarkable optoelectronic properties. They possess the advantages of tunable photoluminescence, high photoluminescence quantum yield (PLQY, up to 90%), narrow full width at half-maximum (FWHM, ∼20 nm) and large optical gains [157-164]. Because of these advantages, CsPbX3 QDs have become potential materials in many fields, including light-emitting diodes (LEDs) [165,166], photovoltaic devices [167,168], solar cells [169,170], lasers [171], and displays [172]. Like CdS, PbS, PbSe, and ZnS QDs et al, CsPbX3 QDs also have poor stability under wet conditions, which influence their photostability and thermal stability. For applications, the CsPbX3 QDs incorporate into some robust hosts, such as polymers or glass ceramics, to enhance their stability. As a representative of new materials, CsPbBr3 QDs embedded glass ceramics are usually obtained by conventional melt quenching and heat treatment method. 6.1 Solid-state lighting and random upconverted lasing Ai et al. successfully prepared CsPbBr3 QDs embedded phosphate glass ceramics by heat treatment and a very intense green emission was observed under 365 nm light source, as shown in Fig. 35a and b [173]. Furthermore, the emission peak reached a maximum under 470 nm excitation. Di and co-workers reported that CsPbBr3 QDs embedded phosphosilicate glass ceramics possessed high stability, which used in highly efficient white LEDs [174]. Fig. 35c and d exhibit the images of the CsPbBr3 QDs embedded glass ceramics that heat-treated at different temperatures under ambient light and UV light. Under the irradiation of 365 nm light source, the samples emitted intense green emissions. Meanwhile, the red-shifting was observed in photoluminescence and absorption spectra of the glass ceramics samples, demonstrating that the growth of CsPbBr3 QDs with increasing the heat-treatment temperature. Besides, the InGaN blue chip, commercial red-emitting CaAlSiN3:Eu2+ phosphor and CsPbBr3 QDs embedded glass ceramics samples constructed WLED devices, as shown in Fig. 35e and f. They emitted bright white light with a high CRI 59

of 83.4, a low CCT of 3674 K and a LE of 50.5 lm W-1. Besides, after a few hours, the EL spectrum, the corresponding CRI and CCT remained almost unchanged, which demonstrated the stability of the constructed WLED and the potential application of the CsPbBr3 QDs embedded glass ceramics in solid-state lighting [174].

Fig. 35 (a) excitation spectra and (b) emission spectra under various excitation wavelength of glass. Inset is the powders upon 365 nm light excitation. (c) Photoluminescence and (d) optical absorption spectra of the glass specimens. Insets are the photographs of the glasses under room light and UV light. (e) EL spectra of a InGaN blue chip, a green-LED, a red-LED and a WLED. (f) CIE color coordinates of the LED device with different ratios of the green CsPbBr3 QD glass to the red CaAlSiN3:Eu2+ phosphor. T (a and b) Reproduced with permission [173]. Copyright 2016, The American Ceramic Society. (c-f) Reproduced with permission [174]. Copyright 2017, The Royal Society of Chemistry. Yuan et al. also successfully synthesized CsPbBr3 QDs embedded TeO2 glass ceramics and their stability, including photostability, water resistance and thermal stability, was systematically studied, as illustrated in Fig. 36a [173]. Under the UV lamp, the PL intensity and FWHM of QDs embedded glass ceramics did not significantly vary due to the good protection of glass ceramics, as shown in Fig. 36b. Fig. 36c exhibits the water resistance test of CsPbBr3 QDs embedded glass ceramics with different immersing time. The remaining PL intensity only decreased with 60

prolonging the immersion time in the water and still maintained an initial emission intensity of about 90% after 120 hours and even about 60% of the intensity when the time reached 45 days. The CsPbBr3 QDs embedded glass ceramics retained intense green emission in water, as displayed in Fig. 36d. The colloidal CsPbBr3 QDs decomposed rapidly after immersing in water for 2 h and its emission intensity was only 5% of the initial intensity. The above results indicated that the glass ceramics was beneficial for protecting the CsPbBr3 QDs from the external environment and greatly improving their moisture resistance. As illustrated in Fig. 36e and f, the heating/cooling

cycle

program

evaluated

the

thermal

stability

for

the

temperature-dependent PL intensity of QDs embedded glass ceramics and colloidal QDs. After three heating/cooling cycles at 100, 150 and 200 °C, the PL intensity of CsPbBr3 QDs embedded glass ceramics retained at about 60-70% of the original temperature, which was much better than that of colloidal QDs, indicating that the inorganic glass ceramics body played an indispensable role in improving the thermal resistance of CsPbBr3 QDs. To verify their application, the commercial CaAlSiN3 red phosphor and green CsPbBr3 QDs embedded glass ceramics powder were excited with an InGaN blue chip to construct WLED, as shown in Fig. 36g. A bright white-light emission with a color coordinate of (0.33, 0.35), a CRI of 92, a CCT of 5600 K and luminous efficiency of 50-60 lm W-1 was obtained, which further proved their promising application in the solid-state lighting. Besides, perovskite QDs have received significant attention in random laser applications due to their low threshold and ultra-stable stimulated emission under atmospheric conditions. The uniform distribution and stability of CsPbBr3 QDs embedded glass ceramics made them suitable as disordered gain media for random upconversion lasers. To demonstrate the application in random upconversion lasers, the samples were pumped with an 800 nm femtosecond laser. CsPbBr3 QDs embedded glass ceramics emitted green UC emissions band centered at 530 nm. There were no additional laser resonators in QDs embedded glass ceramics and the corresponding threshold was ∼200 μJ cm−2 at 77 K. 61

The UC emission spectrum varied with different angles, which attributed to the random distribution of CsPbBr3 QDs inside the glass ceramics and the θ dependence of the corresponding scattering intensity, confirming the potential application of CsPbBr3 QDs embedded glass ceramics in random UC lasers [175]. Xiang group also reported a new luminescent material family of CsPbBr3:xEu3+ QDs embedded borosilicate glass ceramics [176].

Fig. 36 (a) Glass crystallization strategy to fabricate QDs-embedded glass ceramics. (b) Photostability test of QDs embedded glass ceramics illuminated with 365 nm UV lamp. (c) Water resistance test by directly immersing QDs embedded glass ceramics in aqueous solution. (d) Luminescent photographs of QDs embedded glass ceramics in water with storing time. Temperature-dependent PL intensities for (e) QDs embedded glass ceramics and (f) colloidal QDs via three heating/cooling cycles, respectively. (g) EL spectra of the LEDs by coupling InGaN blue chip with the mixtures of green-emitting QDs embedded glass ceramics powder and red-emitting CaAlSiN3 phosphor. (h) UC emission spectra of CsPbBr3 QDs embedded glass ceramics vs the pump power of 800 nm femtosecond laser. Reproduced with permission [175]. Copyright 2018 American Chemical Society. 6.2 Backlight displays for liquid crystal displays (LCDs) LEDs are widely used as backlights for LCDs, and their color gamut is determined by the color coordinates of the red/green/blue (RGB) emissions emitted by the LEDs. One knows that candidates for backlight display include β-SiAlON:Eu2+, Mn4+-activated fluorides, RbLi(Li3SiO4)2:Eu2+ phosphor and halide perovskite QDs et 62

al, which exhibit excellent spectral properties [177-182]. However, they suffer from degradation under high temperature or moisture environment after long-term use. Furthermore, some candidates exhibit a wider full width half-peak (FWHM), which limits the maximum accessible color gamut of LCD [178]. Therefore, it is crucial to develop candidates with appropriate peak positions, narrow emission bands, high photoluminescence quantum efficiency (PLQY) and excellent thermal stability to meet the application. Lin et al. reported CsPbBr3 embedded glass ceramics could be used for wide-color gamut liquid crystal display (LCD) [183]. They successfully synthesized CsPbBr3 embedded glass ceramics with wide size distribution under the Ostwald Ripening mechanism. CsPbBr3 embedded glass ceramics exhibited excellent physical and chemical stability due to the robust inorganic glass ceramics host. It was found that the luminescence intensity of prepared CsPbBr3 embedded glass ceramics could be recovered after undergoing a 25-300 °C heating-cooling cycle and the luminescence still retained about 55% of the original one after immersing into the water for 42 days, as shown in Fig. 37a and b. To prove the application of backlight display,

the

prepared

CsPbBr3

embedded

glass

ceramic,

K2SiF6:Mn4+

phosphor-in-glass (PiG) were ground into powders, based on 455 nm emitting InGaN blue chip, to build white LEDs. The CsPbBr3 embedded glass ceramic powder and K2SiF6:Mn4+ PiG powder were weighed in different ratios (from 1:2 to 1:10), dispersed into silicones, and then coupled with the blue-emitting chip, as displayed in Fig. 37c. It was observed that the electroluminescent (EL) spectra exhibited the isolated RGB emissive components. As the weight ratio of K2SiF6:Mn4+ increased, the white light got warmer and the color gamut of the constructed w-LEDs could cover 99.0-103.1% NTSC (73.9-77.0% Rec. 2020) in CIE 1931 color space, as illustrated in Fig. 37d. The results demonstrated that CsPbBr3 embedded glass ceramic had the potential for backlights for liquid crystal displays. Researchers need to continue to work hard to find efficient QDs embedded glass ceramics to meet practical applications.

63

Fig. 37 (a) Temperature dependent integrated PL intensity of the CsPbBr3 QDs embedded glass ceramics. (b) Water resistance test by directly immersing the CsPbBr3 QDs embedded glass ceramics in aqueous solution for 42 days; The insets in (b) show the corresponding luminescent photographs as processing time prolongs under (λem = 365 nm). (c) EL spectra of the fabricated w-LEDs using CsPbBr3 QDs embedded glass ceramics (G) and K2SiF6:Mn4+ PiG (R) as color converters, where the G/R weight ratios varies from 1:2 to 1:10; the insets show the corresponding luminescent photos of w-LEDs on state. (d) Color gamut of the NTSC standard, the Rec.2020 standard and the constructed wLEDs based on CsPbBr3 QDs embedded glass ceramics and K2SiF6:Mn4+ without color filtering; the CIE chromaticity coordinates of G/R weight ratio dependent w-LEDs are also marked out. Reproduced with permission [183]. Copyright 2019 Elsevier. 6.3 Force sensors and Pb2+ detection. Lin and Wang’s group broadened the range of applications for CsPbBr3 embedded glass ceramics [184]. They explained the mechanism of stress-induced glass crystallization and its application in force sensors and Pb2+ detection based on the unique mechanoluminescence (ML) mechanism. They used a self-made dynamometry device to establish the relationship between friction force (f) and luminescent intensity 64

(I) and green light with different brightness were generated at 365 nm excitation, as shown in Fig. 38a and b. From fig. 38c, it was found that the relationship between f and I followed a linear relationship well, suggesting the potential of glass ceramics to quantitatively visualize the dynamical force. Furthermore, based on the same stress-induced glass crystallization mechanism, an equivalent amount of NaBr was used instead of PbBr2 as the application for Pb2+ detection. Fig. 38d shows the ML spectra of the mixtures of PNCS glass and PbF2 after grinding under the same condition. It was observed that the ML intensity increased as the Pb2+ content elevating. Fig. 38e plots the relationship between relative luminescence intensity (I/I0) and the Pb2+ content, where the limit of detection (LOD) was determined as 5.3 ppm. Compared to those ML responses from the other environmentally relevant metal ions, the glass ceramics had high selectivity for Pb2+ ions, as exhibited in Fig. 38f. The results indicated that the obtained CsPbBr3 embedded glass ceramics had potential application in force sensors and Pb2+ detection. This is a new research application that requires researchers to explore and use it in real life.

Fig. 38 (a) Schematical illustration of the self-made dynamometry device to measure the friction force. (b) Luminescent photographs of the glass ceramics and their moving trails upon different friction force after travelling the same distance along straight line at constant velocity (λex=365 nm). (c) The plot of measured ML intensity versus friction force. (d) ML spectra of the mixtures of glass ceramics and PbF2 at various content after grinding under the same condition. (e) The dependence of the relative ML intensity (I/I0) versus the Pb2+ content (λex = 365 nm). (f) ML responses 65

of the glass ceramics probe to different metal ions, demonstrating its high selectivity to Pb2+. Reproduced with permission [184]. Copyright 2019 Tsinghua University Press and Springer-Verlag GmbH Germany. 7. Conclusion and outlook In this review, we have summarized the research progress of QDs embedded glass ceramics, focusing on their synthesis methods, crystallization behavior, luminescence mechanism, radius calculation, PL properties, and applications in optoelectronic devices, photocatalyst and sensor. The tunable PL spectra of QDs embedded glass ceramics merely adjust via changing their size and composition, which emits light from the ultraviolet region to the infrared region. Furthermore, co-embedded of ions and QDs into glass ceramics can improve the luminescence performance, such as enhancing luminescence, energy transfer, increasing PL emission wavelength. Moreover, the QDs embedded glass ceramics are chemically stable and thermally stable. In the past, many groups have studied the growth mechanism of QDs embedded glass ceramics and the fundamental mechanisms of nucleation and growth of QDs are well understood and described, but there are still many ambiguous problems. Generally, the study of nucleation and growth kinetics of the crystalline phase is mainly limited to glass-ceramic systems, mostly the chemical component undergoes crystallization during heat treatment and little know much about the nucleation and growth of minor or traces elements in glass ceramics hosts. Furthermore, QDs embedded glass ceramics have inherent problems of high activation energy, which relate to the composition, solubility of the semiconductor particles in the glass ceramics [97]. As well known, the stable glass ceramics not only prevent the agglomeration of QDs but also easily adapt to various shapes, making the QDs embedded glass ceramics become an ideal precursor for all-solid devices. Therefore, it is very critical to choose suitable glass ceramics and QDs. For instance, if choosing phosphate glass ceramics, the thermal stability and chemical stability are relatively weak due to the significant coefficient of thermal expansion, which affects their 66

application in some cases. In the case of silicate glass ceramics, the melting temperature is relatively high (over 1200 °C), which makes it difficult to raise the concentration of certain volatile elements and quantum components in the glass ceramics to a satisfactory level, resulting in low PL efficiency [135]. Besides, it is easy to form vacancies, substitutions and dangling bonds at the interface between QDs and glass ceramics, which greatly reduces the PL efficiency of QDs and influences its applications in optoelectronic devices [185,186]. For instance, the defect of QDs embedded glass ceramics resulted in its non-ideal growth, which slowed down the response time of the device [25]. Although scholars have proposed several methods for synthesizing QDs embedded glass ceramics, it is difficult to obtain a well-defined structure and precise size of QDs. In the above, we know that CsPbBr3 QDs embedded glass ceramics provide with many advantages, such as narrow emission bands, thermal stability and water resistance, which can be potentially used in LEDs, random lasers, backlight display, force sensors and Pb2+ detection. Researchers should do their utmost to expand the application of CsPbX3 (X = Cl, Br, I) QDs embedded glass ceramics in the direction of optoelectronic devices, sensors and photocatalyst, etc. Among the application of optoelectronic devices, the backlight display should receive sufficient attention, because CsPbX3 (X = Cl, Br, I) QDs embedded glass ceramics meet the conditions of backlight displays, promising become industrial production candidate of backlights for liquid crystal displays. Besides, QDs containing green element Bi3+ ion have received extensive attention, especially MA3Bi2X9 (MA=CH3NH3, X = Cl, Br, I) QDs, whose photoluminescence quantum yield (PLQY) has reached the level that compares to that of inorganic lead-based QDs [187]. However, the report of Bi3+ ion-based QDs embedded glass ceramics is rare, which is very worthy of attention. In addition, the replacement of toxic lead with manganese (Mn) has received widespread attention since it could enrich color emission and improve thermal stability [188]. QDs embedded glass ceramics are used as substitutes for traditional heavy metal ions, but in terms of photovoltaic technology and catalysts have not been fully utilized. In the 67

future, we look forward to finding a synthetic route that mild, green, beneficial to industrial production and emerging creative applications to realize the potential of QDs embedded glass ceramics.

Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (11404171), Jiangsu Natural Science Foundation for Excellent Young Scholar (BK20170101), and the Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY218015). This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded

by

the

Ministry

of

Science,

ICT

and

Future

Planning

(No.

2018R1A2B6005179) and the Functional Phosphor Bank at Pukyong National University (No.2017 M3A9B8069470).

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Photograph and biography

Junpeng Xue Junpeng Xue obtained his Bachelor’s degree (BSc) from Qingdao Agricultural University in 2015. He is presently a Ph.D. student under the supervision of Prof. Jung Hyun Jeong at Pukyong National University (PKNU). His current research concentrates on the photoluminescence of rare-earth ion doped materials and quantum dots embedded glass ceramics.

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Xiangfu Wang Xiangfu Wang received his Ph.D. degree from Nanjing University of Aeronautics and Astronautics, in 2012. From 2012 to present, he works as an associate professor at the Nanjing University of Posts and Telecommunications, China. His current research concentrates on photoluminescence and optical temperature sensing of rare-earth ions doped nano-materials and transparent glass ceramics.

Jung Hyun Jeong Jung Hyun Jeong received his Ph.D. degree from the Tsukuba University of Physics in 1990. From 1982 to present, he has worked as a professor in the Department of Physics at Pukyong National University. His current research concentrates on the photoluminescence of rare-earth ion and transition metal doped materials.

Xiaohong Yan Xiaohong Yan received his Ph.D. degree from Institute of Solid State Physics, Chinese Academy of Science, in 1997. From 2010 to present, he works as a full professor in the College of Electronic Science and Engineering at Nanjing University of Posts and Telecommunications. His current research concentrates on transport and manipulation of the quantum system and new energy materials. 88

Highlights: (1) The preparation methods of quantum dots embedded glass ceramics are summarized. (2) Crystallization behavior of quantum dots embedded glass ceramics is summarized. (3) The luminescence mechanism of QDs embedded glass ceramics is summarized. (4) Applications of quantum dots embedded glass ceramics are summarized. (5) The future development of quantum dots embedded glass ceramics is predicted.

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service or company that could be construed as influencing the position presented in this manuscript submitted.

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