Quantum dots - Polymer composites and the influence of gold nanoparticles on photoluminescence of polymer composite films

Quantum dots - Polymer composites and the influence of gold nanoparticles on photoluminescence of polymer composite films

Journal Pre-proof Quantum dots - Polymer composites and the influence of gold nanoparticles on photoluminescence of polymer composite films A.A. Ezhov...

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Journal Pre-proof Quantum dots - Polymer composites and the influence of gold nanoparticles on photoluminescence of polymer composite films A.A. Ezhov, O.N. Karpov, A.S. Merekalov, S.S. Abramchuk, G.N. Bondarenko, R.V. Talroze PII:

S0022-2313(19)31311-0

DOI:

https://doi.org/10.1016/j.jlumin.2019.116992

Reference:

LUMIN 116992

To appear in:

Journal of Luminescence

Received Date: 6 July 2019 Revised Date:

27 November 2019

Accepted Date: 22 December 2019

Please cite this article as: A.A. Ezhov, O.N. Karpov, A.S. Merekalov, S.S. Abramchuk, G.N. Bondarenko, R.V. Talroze, Quantum dots - Polymer composites and the influence of gold nanoparticles on photoluminescence of polymer composite films, Journal of Luminescence (2020), doi: https:// doi.org/10.1016/j.jlumin.2019.116992. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

QUANTUM DOTS - POLYMER COMPOSITES AND THE INFLUENCE OF GOLD NANOPARTICLES ON PHOTOLUMINESCENCE OF POLYMER COMPOSITE FILMS A.A. Ezhova,b,c, O.N. Karpova, A.S. Merekalova, S.S. Abramchukd, G.N. Bondarenkoa, R.V. Talrozea a

A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences,

Leninsky prospect 29, 119991 Moscow, Russia. b

M.V. Lomonosov Moscow State University, Faculty of Physics, Leninskiye Gory 1-2,

119991 Moscow, Russia. c

M.V. Lomonosov Moscow State University, Quantum Technologies Center, Leninskiye

Gory 1-35, 119991, Moscow, Russia. d

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences

Vavilova Street 28, 119334 Moscow, Russia.

1

Abstract We report on the synthesis of composites based on poly(styrene) (PS) and CdSe/ZnS quantum dots (QDs). The reduction of terminal functional group of RAFT polymerized PS to thiol group allows for a partial substitution of low molecular weight ligands at the surface of CdSe/ZnS QDs by a polymer. The combination of TEM and FTIR data results in the model of the composite formation with PS macromolecules wrapped around QDs. The photoluminescence (PL) of QDs in PSCdSe/ZnS composites decreases with an additional embedding of Au nanoparticles (NPs). The reason of the PL drop observed is analyzed in terms of combined inner filter effects and the influence of dielectric properties of the medium surrounding Au NPs embedded in PS-CdSe/ZnS composites. The calculations of the radial distribution function of QDs regarding the position of Au NPs are carried out and compared with the experimental data.

Keywords:

quantum

dots;

nanoparticles;

poly(styrene)

2

photoluminescence;

polymer

composite;

1.

Introduction.

Modern science and engineering enable the compositions of nanoparticles (NPs) of different kinds with various matrices in which NPs are embedded. Functionalized nanocomposites which contain NPs embedded into polymer hosts [1-4] represent new materials with unusual properties such as size-dependent photoluminescence (PL) of semiconductor quantum dots (QDs) or localized surface plasmon resonance characteristic for noble metal NPs. Polymers are normally transparent in a wide spectral range and can be easily processed as a flexible platform for devices based on optical properties of QDs and metallic NPs. Some semiconductor- and metal-based functional nanocomposites with block copolymer matrices have been extensively studied [5, 6]. Depending on the NP size, the length of macromolecular blocks, and the interactions between NP and blocks, NPs are located either in the center or at the interfaces of lamellar or spherical domains after the microphase separation. One of the most attractive ways for creating stable NP-polymer composites involves the use of so-called ligand exchange when pre-synthesized polymer macromolecules containing functional groups may replace low molecular weight ligands at the surface of NPs. In such a case, specific binding of macromolecules via electrostatic and hydrophobic interactions of functional groups with NP surface are responsible for the stability of nanocomposites [4, 6-9]. This approach has advantages because one can use pre-synthesized macromolecules carrying the required functional groups capable of interaction with NP surface. The enhancement of the PL of QDs by plasmon NPs was observed in lithographic arrays [10], layer-by-layer assemblies [11] and even in solutions [12]. The degree of PL enhancement may be influenced by the inter particle distance [13, 14], compositions of both NPs and semiconductor QDs [15, 16] as well as well PL and absorption maxima [17, 18]. Chan et al, [19] measured the PL enhancement in layer-by-layer assemblies of CdSe QDs and plasmon NPs with the separating polymer layers of different thickness. Two-fold enhancement of PL close to Au NPs was observed at 11 nm distance between QD and NP whereas the maximal enhancement of PL was achieved at 8 nm. Contrary to the data mentioned above Kulakovich et all. [20] detected the maximal five-fold PL enhancement of CdSe/ZnS QDs at the 11 nm distance from Au NPs, when silica dioxide was used for layers separation. The difference in those results shows that additional factors exist that influence PL increase. For example, the composition of a NP influences the degree of PL enhancement [21, 22-24]. Viste with coworkers [21] discovered that the increase in the size of Au NPs from 80 up to 160 nm induced the 3

transformation of the PL quenching process to PL enhancement even if all other parameters were the same. It happened due to the increase in both the local field created by NPs and the distance to the QD surface as well as in the growth of the radiation effectivity. Geddes and Lakowicz [25] presented a theory of fluorescence enhancement in which the effects result from at least three different mechanisms. The first mechanism considers quenching energy transfer near the metallic surface that can increase the intrinsic nonradiative decay rate of fluorophore [26]. It implies the modification of the rate at which a fluorophore emits a phonon. The second mechanism addresses an increase in the emission intensity due to the increase in the local incident field provided by metal particles [27-29]. Finally, the third mechanism results from the amplification of the rate of the radiative decay of QDs induced by metallic NPs [30]. Preparing the composites containing both fluorophores and plasmonic NPs in the same matrix, one has to keep in mind the probability of the influence of both on the optical properties of the hybrid system as a whole. Note that there are no publications on the interaction between QDs and NPs in one and the same polymer matrix though Quach et al. [31] reported on the histidine modified QDs (QD−His) covalently attached to carboxyl-modified polystyrene (PS) microspheres to form highly emitting PS microspheres (QD−PS). Gold nanoparticles (Au NPs) were then covalently attached to the QD−PS surface to form Au NP−QD−PS composite microspheres that were supposed to be used in FRET probes. However, the attachment of Au NPs to QD−PS quenched the QD emission through FRET interactions. If using the polymer matrix chemically linked to QDs and NPs just embedded in that matrix one can assume that if the majority of macromolecules are attached to NPs the distance between QDs NPs in a solid polymer film will depend on the size of macromolecules. Even in the case of larger distances it may not be possible to exclude the influence of plasmonic NPs on the emission intensity of a fluorophore. One can consider the effect of concentrations of fluorophore and plasmonic NPs on the total absorption of the exciting light and the re-absorption of the emitted light by fluorophore and plasmonic NPs called as inner filter effect. In addition, there is a potential change in the integrative refraction index of the medium due to the change in the concentration of NPs. In our first paper [32] we have described the peculiarities of the photoluminescence and photostability of CdSe QDs in composites with PS in sols in the absence and presence of Au NPs. The main task of the current research is to create solid films of composite systems containing CdSe/ZnS QDs as fluorophores chemically attached to the polymer matrix and plasmonic Au NPs and to determine the role of that combination in the optical properties of the hybrid systems. The 4

synthetic approach includes the preparation of the narrow size distribution polymer, containing terminal functional group capable of interacting with pre-synthesized core-shell CdSe/ZnS QDs. The choice of the latter was dictated by stable systems characterized by high emission intensity. PS is chosen as a polymer matrix for composite systems based on the consideration of their potential applications in optoelectronics. This choice is dictated by its reliable properties as a host material for semiconductor and metallic NPs, good film forming properties, moderate chemical resistance, and optical transparency. We expect that systems developed in that paper may serve as media, which instead of creating closely packed QD / NP solids may be considered as potential for novel electronics, optical and sensing applications if the optimal particle sized and inter particle distance are found.

2.

Materials and methods.

Reagents and solvents. Styrene was supplied by “Reachem” and distilled under reduced pressure before use. RAFT agent, S-(2-Cyanoprop-2-yl)-S-dodecyltrithiocarbonate (CPDTC), nbutylamine (both from “Sigma-Aldrich”), sodium borohydride (“Acros Organics”), chloroauric acid (“Aurat”), tetraoctylammonium bromide (TOAB, “Acros Organics”), cadmium oxide (“Fluka”, ≥99,99 %), oleic acid (“Sigma-Aldrich”), zink acetate, selenium, sulphur, oleic acid (“Strem Chemicals”), trioctylphosphine (TOP), octadecene (“Sigma-Aldrich”) and sodium borohydride (“Sigma-Aldrich”) were used as received. Solvents, toluene, CH2Cl2, ethanol, acetone, (“ChemMed”), methanol (“Labscan”), hexane (“Component-reactiv”) were distilled before use. Argon gas of the highest purity (“PGS Servis”) was used. Polymer synthesis. RAFT polymerization at high temperature has been used for the synthesis of poly(styrene). No radical initiator has been used. RAFT agent, CPDTC, was dissolved in styrene (3.6x102 mol/l). Then, the solution was poured into an ampoule, degassed and sealed. After heating at 110oC during 72 h, the reaction mixture was cooled, the polymer was dissolved in CH2Cl2 and dried by lyophilization. Aminolysis was used for thiocarbonylthio group reduction. RAFT PS (0.01 mol/ml) was dissolved in 10 mL of THF, solution was poured in an ampoule and the reaction mixture was bubbled with argon on stirring during 20 min. Afterwards, 10X excess of n-butylamine was added on stirring and again bubbled with argon within 15 min, and ampoule was sealed. The reaction mixture was kept on stirring for 3-4 days in the inert atmosphere. Then the ampoule was opened, the 5

polymer was precipitated with ethanol, centrifuged during 20 min. at 6000 rpm. The residual polymer was dissolved in toluene and re-precipitated again. The polymer was separated and dried. Quantum dots synthesis. CdSe quantum dots were synthesized as described in [32]. CdSe/ZnS core/shell QDs with composition gradient the synthesis was done in accordance with [33] with the use of cadmium oxide (0.4 mmol), zink acetate (4 mmol), and oleic acid (17.6 mmol). The above mentioned reagents were mixed with 20 ml of 1-octadecene. The reaction mixture was gradually heated in a flow of argon under intense stirring till the total dissolution. After that, the mixture was heated up to 280oC in argon atmosphere and the solution of 0.4 mmol of selenium and 4 mmol of sulfur powder dissolved in 3 ml of TOP was injected. Then, the reaction mixture was maintained at the high temperature for 5 min and rapidly cooled to room temperature to stop the growth of QDs. The reaction mixture was divided in 5 equal parts and 15 ml of acetone was added in each part. Each part was centrifuged during 10 minutes at 6000 rpm. After that step the precipitates were separated, brought together and dissolved in 5 ml of toluene. To purify QDs, the precipitation was done three times by adding 15 ml of acetone. As it comes from our selected area electron diffraction (SAED) experimental data (Figure S11) the total bulk of CdSe/ZnS core/shell QDs has the same Wurtzite crystal structure with a hexagonal symmetry. Synthesis of Au NPs. NPs were synthesized according to the following protocol. 0.03 mol/l solution of chloroauric acid (9.29 µmole) in water was mixed with 0.036 mol/l solution of TOAB (0.1858 mmol) in toluene upon vigorous stirring during 30 min. Water phase was discarded when chloroauric acid was transferred into organic phase. 0.4 M water solution of NaBH4 (0.103 mmol) was added dropwise (one drop every 15 sec) upon stirring. When the mixture became colorless, the remainder of NaBH4 solution was added at once. After 24 hours of stirring, the aqueous layer was discarded and a concentrated solution of 1-decanethiol in toluene was added. After another 1 h of stirring, NPs were isolated by repeated precipitation with ethanol. The weight of the organic part of Au NPs was about 10% of the total mass. This value obtained was based on the TEM results and data on the densities of gold and decantiol taken from the literature. The geometrical parameters, namely, the Au NP diameter and the gap between adjacent close-packed Au NPs, were determined based on TEM images of Au NPs sols after the complete removal of the dispersion medium. PS-QDs synthesis. Polymer containing thiol end group was dissolved in 2 ml toluene (polymer concentration in solution was 50 mg/ml) and mixed with 1 ml of QDs sol of 25 mg/ml concentration. The obtained homogeneous mixture of red color was left for 3-4 days under stirring. 6

To separate the composite from non-reacted QDs, the composite was precipitated from toluene in hexane (5 ml of hexane per 1 ml of the sol in toluene) for three times and centrifuged at 6000 rpm during 10 minutes. It is evident that in the process of the composite formation a part of the polymer could remain unreacted. To isolate the composite product from the unreacted polymer 50 mg of the composite were dissolved in 1 ml of toluene. Fivefold volume of hexane was added and the system was centrifuged during 10 minutes at 6000 rpm. The non-bound polymer precipitated. After that the sol was decanted and cooled down to -20oC. After cooling the sol was again centrifuged at room temperature during 20 minutes at 6000 rpm. The cooling and centrifuging were repeated several times until the composite precipitated in full. The content of QDs in the composite was measured by spectrophotometric method with the use of the calibration curve because QDs absorb light in the range between 500 and 600 nm. The calibration curve was built in accordance with the absorption of the sol as depended on the amount (concentration) of QDs stabilized by oleic acid. As shown before [34] the mass fraction of organic part in CdSe QDs is about 30 wt. %. Thus, the amount of the inorganic part was calculated along the calibration curve and because the initial concentration of the composite in the sol is known in advance the ratio between the inorganic and organic parts of PS-QDs compositions is calculated. Preparation of Au NPs-decanethiol + PS-QDs composites. Sols with gold NPs in toluene (1 mg/ml) was added to the sols of PS-QD in toluene (1 mg/ml) in the amounts mentioned in Table 1. We vacuumed the solvent and the dry blend was dissolved in 10 ml of chloroform. The mixture in chloroform was deposited on substrate. We dried films in the air until the constant weight. The content of components is given in Table 1. TEM images done for the solutions indicated homogeneous distribution of QDs and Au NPs (Fig.S1-2) Table1. Content of components in composite samples Au NPs-decanethiol + PS-QDs.

Composite samples

PSCdSe/ZnS QDs, weight, mg

Weight fraction of CdSe/ZnS QDs in PSCdSe/ZnS QDs, %

Au NPsdecanethiol weight, mg

Weight fraction of Au NPs in Au NPsdecanethiol, %

F1_1

1.24±0.02

30±0.6

_

_

F1_2

1.24±0.02

30±0.6

F1_3

1.24±0.02

30±0.6

F1_4

1.24±0.02

30±0.6

0.033 ±0.002 0.065 ±0.004 0.130 7

90±3 90±3 90±3

Weight of inorganic part, mg CdSe/ZnS QDs 0.372 ±0.013 0.372 ±0.013 0.372 ±0.013 0.372

Au NPs

_ 0.029 ±0.003 0.059 ±0.006 0.117

±0.007 F2_1

1.26±0.02

14±0.3

F2_2

1.26±0.02

14±0.3

F2_3

1.26±0.02

14±0.3

F2_4

1.26±0.02

14±0.3

_ 0.063 ±0.004 0.083 ±0.005 0.125 ±0.007

_ 90±3 90±3 90±3

±0.013 0.176 ±0.006 0.176 ±0.006 0.176 ±0.006 0.176 ±0.006

±0.012 _ 0.056 ±0.006 0.075 ±0.008 0.113 ±0.011

Instrumentation The average molecular weights and dispersity (Ð) of PS were determined by gel permeation chromatography (GPC) conducted in THF at 25°C using high-pressure modular liquid chromatograph equipped with LabAlliance Series 1500 Constant Flow Pump and Refractive Index Detector 2142 (“LKB, Bromma”) and linear polystyrene standards (“Agilent”) for calibration. Waters WAT054460 and TOSOH Biosep G3000HHR columns filled with the crosslinked polystyrene were connected in series. THF was used as an eluent. The software package Multi Chrome (Ampersand) was used for the analysis of chromatograms. UV-vis spectra of the polymers and RAFT agents were recorded in solutions using Ultrospec 1100 pro UV-vis spectrophotometer (“Amersham Biosciences”) and Specord M-400 UV-vis spectrophotometer (“Carl Zeiss Jena”) using quartz glass cuvettes. The UV spectrophotometric studies of the polymers were performed in chloroform solution The absorption spectrum of individual chloroform was used as a reference. TEM images and small area electron diffraction (SAED) patterns were obtained on transmission electron microscope LEO 912 AB OMEGA (“Carl Zeiss”) operating at 100 kV accelerating voltage. Samples were drop cast from toluene sol onto mesh polyvinyl formal coated copper grids. Over 200 QDs and Au NPs were sized for each sample using ImageJ 1.50b analysis software (“National Institutes of Health”). FTIR spectra were recorded with IFS 66 v/s (“Bruker”) (30 scans, resolution 1 cm-1, wavelength range 400 – 4000 cm-1). Samples were prepared in the form of thin layers located between two KBr optical glasses. Thin layers were organized from toluene sols after the solvent evaporation. Hettich ROTOFIX 32A centrifuge was used for separation. The PL spectra of sols were registered by USB 2000 spectrometer (“Ocean Optics”) using hermetic glass optical cuvettes in the geometry close to 90°. The error of the PL intensity 8

measurements is about 0.5 – 1.5 %. Optical resolution ±2 nm. The solutions of QDs and composites in “dry” toluene were purged with dry argon before measurements. Excitation sources were laser moduli with emission wavelengths of 403 or 520 nm and optical output power 5–20 mW. The management software was SpectraSuite (“Ocean Optics”). All components of the measuring system were isolated from light. XR fluorescence was measured with ARL™ PERFORM'X Sequential X-Ray Fluorescence Spectrometer (“Thermo Scientific”) for advanced materials characterization integrates bulk elemental analysis with mapping and small spot analysis to create a solution that evaluates up to 90 elements in nearly any solid or liquid sample. Polymer samples were measured in toluene solution whereas QDs - in toluene sol. The accuracy of the measurements depended on the element and was measured to be about 10%. Measurements of PL of films of CdSe/ZnS QDs composites with or without of Au NPs were carried out using a confocal laser scanning microscope (CLSM) FV1000 (“Olympus”). The power of lasers used for excitation of the photoluminescence at wavelengths 405, 488, and 515 nm was in the range of 1.6–3.3 µW. The wavelength range at which the PL intensity was detected was the same for all cases (525–575 nm) with the exception of spectral measurements. CLSM was assembled on the base of the IX81 inverted microscope from “Olympus”. CLSM detection block was comprised of two spectral type detectors and with the detector supplied with bandpass-filters having fixed spectral transmission width. One of the spectral type detectors was used for recording fluorescence spectra. In addition, the barrier dichroic filters were applied as beam splitters. The laser combine used as a part of CLSM allowed measurements at different excitation. Olympus UPLSAO 10× objective with numerical aperture (NA) 0.40 was used. The size of the confocal diaphragm was established to be equal to a standard one. The automation control of CLSM and the primary data processing were carried out with FV10-ASW software. The quantitative measurements of PL were carried out in the “reflection geometry” with the help of DM405/488 or DM458/515 beam splitter depended on the excitation wavelength. To avoid the influence of the chromatic aberration and drift before each measurement one found such a position of the objective along Z axis at which PL signal would be maximal. For the measurements of PL spectra, the size of the spectrometer slit was 5 nm, whereas the step size was 1 nm.

3.

Results and discussion

3.1. Synthesis of functionalized PS. 9

The scheme of the styrene polymerization in the presence of the RAFT agent is given in Scheme 1.

Scheme 1. RAFT polymerization of styrene. According to literature data S-(2-Cyanoprop-2-yl)-S-dodecyltrithiocarbonate (CPDTC) with sterically hindered leaving groups –C(CH3)2(CN) [35-37] was successfully used as a RAFT agent for PS synthesis. High efficiency of that RAFT agent in styrene polymerization is responsible for the formation of low polydispersity PS (Ð = 1.06) with number average molecular weight Mn equal to 2.2 kDa. To calculate the yield of the RAFT polymerization that results in the formation of the functionalized living chains, the number of living chains was determined by UV spectrophotometry. The UV spectra of CPDTC and PS containing dodecyl trithiocarbonate group were measured in chloroform. CPDTC shows a characteristic absorption band with a maximum at 309 nm with the molar extinction coefficient of about 19.1 l/mmol*cm. Our calculations showed that the majority of macromolecules were in the form of living chains. At the next step the terminal trithiocarbonate group in the synthesized RAFT-based PS sample was subjected to the modification. According to the literature data, the most popular approaches are to cleave thiocarbonylthio moiety via reaction with either aqueous solution of NaBH4 or primary aliphatic amines [38]. We have used n-butylamine following the reaction Scheme 2.

Scheme 2. Conversion of dodecyl trithiocarbonate group of PS to thiol group.

Our UV absorbance spectra revealed that the content of dodecyl trithiocarbonate group of PS (Fig.1, spectrum 1) decreased to nearly zero after aminolysis (Fig.1, spectrum 2).

10

Figure 1. Absorbance spectra of the RAFT PS before (1) and after (2) reduction.

To find out the amount of SH-containing PS macromolecules, we have used XR fluorescence method. The analysis of the XR fluorescence spectrum of PS in toluene solution proves the presence of the sulfur atoms in the amount of 1.44 - 1.47 wt. %. The ratio between the experimental S - atoms amount measured and the theoretical calculation (1.45 wt. %) of the amount of S atoms in PS having 2.2 kDa is close to 1. This correlation implies that the synthesized functional macromolecules contain S atoms in nearly every macromolecule after the reduction of trithiocarbonate group.

3.2.

Synthesis and characterization of CdSe/ZnS QDs.

To prepare QDs considered as a part of composites with PS, we have chosen single step approach described by Bae et.al [33], which provides the formation of QDs with the chemical composition gradient. These QDs are known to be much more stable and possess higher quantum efficiency than QDs based on pure CdSe. Figure 2 presents the absorbance and photoluminescence of CdSe/ZnS in toluene with the exciton maximum of the absorption wavelength at about 540 nm and PL maximum at 550 nm. If one changes the toluene sol of QDs by solid QDs (dried sol), the position of PL maximum shifts up to 560 nm.

11

Figure 2. Absorption and PL (1, 2) spectra of CdSe/ZnS QDs in toluene sol (2) and in the solid state (3). (λex = 520 nm (2) and 515 nm (3))

In accordance with the PL data published in [39], the size of QDs core in our CdSe/ZnS QDs has to be estimated as about 3nm. To verify the chemical structure of CdSe/ZnS QDs stabilized by low molecular weight ligands, we have also used the XR fluorescence method. The presence of all elements constituting QDs is proved and the relative ratio of components is determined as Cd : Zn : Se : S = 1 : 1.6 : 1.3 : 0.9. We did not carry out a precise determination of the core diameter because the synthesis technique we used led to a continuous change in the composition of the quantum dot from the center to the surface. Recently published results on the element distributions in quantum dots with a gradient composition show that the characteristic scale of changes in the concentration of elements is of nanometer size. [40]. Given that the outer diameter of the quantum dots we synthesized is 4.5±0.5 nanometers, we used some effective core diameter, which was determined based on the results of PL emission spectroscopy. It looks reasonable because in accordance with [41] the relationship between the maximum in the photoluminescence emission spectrum and the size of the core the band gap in gradient core/shell QDs is shown to change inside every QD. The structure and the presence of the organic ligands on the surface of QDs are shown with the FTIR-spectroscopy. The IR spectrum of CdSe/ZnS QDs (Fig.3a) shows the band splitting at 1547 and 1530 cm-1. The analysis of FTIR-spectra given in Figure 3b in the wavelength range corresponding to the absorbance of carboxylate anion in Cd oleate (1), Zn oleate (2) and CdSe QDs (3) incomplete sentence. In the spectrum of carboxylate anions with Cd2+ exists fairly broad band at 1547 cm-1(1), whereas the spectrum of the carboxylate anion with Zn2+ (2) contains two bands at 1531 and 1547 cm-1 with equal intensities. 12

The same two bands are presented in the spectrum of CdSe/ZnS QDs (3) indicating the presence of Zn atoms at the surface, which are ionically bonded to oleic acid ligands. The other bands (Fig.3a) are related to the aliphatic CH2 chains in oleate fragments (720, 1457, 2852 и 2921 сm-1). Cis-substituted double bond in oleic acid fragment is clearly seen with 740, 1405, 1650 and 3005 cm-1 spectral bands.

Figure 3. FTIR spectra: a - CdSe/ZnS QDs stabilized by oleic acid; b - Cd oleate (1), Zn oleate (2) and CdSe/ZnS QDs stabilized by oleic acid (3); c and d CdSe/ZnS QDs stabilized by oleic acid (1), PS-SH (2), CdSe/ZnS stabilized by oleic acid and PS-SH (3).

3.3.

Composites of PS with CdSe/ZnS QDs.

The composite of PS and CdSe/ZnS QDs was formed under mixing of PS bearing SH- end group and QDs stabilized by low molecular weight ligands. The FTIR spectra in Figure 3, c and d show the comparison of the FTIR spectra of CdSe/ZnS QDs (1), PS-2200-SH (2) and the product of their interaction (3). The spectrum of PS-SH contains very weak bands related to the absorption of S-H thiol group at 2577 and 2637 cm-1 but they completely disappear in the spectrum of the reaction 13

product CdSe/ZnS QDs and PS-SH (Fig.3c). At the same time, the relative intensity of carboxylate anions drops down and the C=O band of non-ionized oleic acid at 1730 cm-1 appears (Fig.3d). The combination of above data shows that the interaction of PS with CdSe/ZnS occurs due to the partial substitution of oleic acid molecules by PS. The intensity of the spectral bands in the range of 1000 – 1200 cm-1 attributed to C-C bond stretch in the PS backbone changes dramatically. It may indicate the change of the PS polymer chain conformation due to the interaction of PS with QDs (Fig.3d). The spectral band at 697 cm-1 corresponding to the out-of-plane deformation vibration of H–C C in the aromatic core is characterized by the certain shape specificity. The second band of the vibrations of the same type at 755 cm-1 is not as characteristic as 697 cm-1 and includes also the skeletal C-C vibration in the side group (benzene ring), but it is very sensitive to the backbone conformation [42]. As seen in Figure 3d, this band becomes less intense in the case of the composite structure. It means that the backbone conformation changes significantly due to PS interaction with QDs. Note that the reaction yield is very low, and for instance, the addition of 20 wt. % of QDs results in the system with a polymer containing 1.5 – 3 wt. % (0.3 – 0.7 vol. %) of QDs. To isolate the composite system from unreacted polymer, the procedure described in details in Experimental part was applied. The amount of CdSe/ZnS QDs in a composite system was about 3.4 and 8.5 vol. %. As far as the binding of PS with QDs which is expected via the formation of QD–S-PS link [43], the spectral bands corresponding to these groups are not manifested in the FTIR spectrum. In our opinion, it is related to a very small amount of end groups of PS macromolecules. However, the indirect argument in favor of the chemical binding of macromolecules with QDs is the total phase separation that occurs between conventional non-functionalized PS and the same QDs. The combination of above spectral data also proves the simultaneous presence of both low molecular weight and polymer ligands at the surface of the QD. TEM images of CdSe/ZnS with low molecular weight ligands are presented in Figure 4a. The average size of QD is about 4.5±0.5 nm (Fig. S2-1). Thus the effective thickness of the ZnS shell may be estimated as 0.75±0.25 nm. QDs are densely packed, and we have estimated the average distance between centers of nearby QDs to be 6.4±0.5 nm (Fig.5a, Fig. S2-2a). It means that the thickness of the organic shell is 0.95±0.25 nm. The latter is in good agreement with the length of the oleic acid fragment at the surface of QDs and may be presented with the structural model as a dense brush described in [9]. After the partial replacement of oleic acid fragments with PS-S ligands, one may expect the distance between neighboring QDs containing PS macromolecules to depend on the content of QDs 14

in a polymer matrix and on the length of the macromolecule, although in this report we only discuss the low molecular weight polymer (2.2 kDa).

Figure 4. TEM images of CdSe/ZnS QDs stabilized by oleic acid (a) and composites of PS-QD with CdSe/ZnS QDs containing about 1 vol. % (b) and 8.5 vol. % of QDs (c).

Figure 5. Schematic representation of the introduction of PS as a ligand for PSCdSe/ZnS composite.

The measurements of the average distance between the centers of neighboring QDs with the help of the TEM image of the composite film containing about 1 vol.% of QDs show that QDs are located far (about 9.5±0.5 nm) apart from each other (Fig.4b, Fig. S2-2b). The embedding of the higher amount of QDs (about 8 vol. %) results in the decrease in the inter-QDs distance down to 2.2±0.5 nm (Fig.4c, Fig. S2-2c) which is very close to that between QDs with oleic acid molecules as ligands (Fig.4a). Moreover, the packing of 8.5 vol.% of QDs modified by PS becomes fairly dense (Fig.4c). Considering two known models of NPs covered with polymer chains [9], namely, the brush (extended macromolecular chains) or the mushroom (random coil) structures depending 15

on polymer coverage we can conclude that neither of those models work in our case. On the one hand, one cannot expect the coiling of the short PS chains consisting of three Kuhn segments only. On the other hand, the measured average distance between QDs (2.2 nm) suggests that PS chains most likely twine QDs as shown in Figure 5b. Thus, the analysis of the dependence of the intercenter distance between QDs in the composites on the volume fraction of QDs indicates the location of PS macromolecules around QDs.

3.3.

Optical properties of CdSe/ZnS QDs.

PL spectra of CdSe QDs (a) and CdSe/ZnS QDs (b) in the solid composite stabilized by PS (30 wt. % of QDs) are given in Figure 6.

Figure 6. Change of PL spectrum of PS-CdSe QDs (a) and PS-CdSe/ZnS QDs (b) in the solid composite under continues irradiation (λex = 520 nm) with time of irradiation: 1-0, 2-30, 3-70, 4-150, 5-260 min.

The PL bands of the QDs in PS correspond to their exciton band. As we have reported in [32], CdSe QDs containing low molecular weight ligands and polymer chains in toluene sols show the growth of photoluminescence of quantum dots during continuous irradiation, known as photoactivation. The origin of the photoactivation effect was described as photoannealing [44-46] resulted from the rearrangement of QDs surfaces due to the light-induced heating contributed in the defects removal. Note that the effect of photoactivation in CdSe/ZnS is much less pronounced. If the intensity of PL of PS-CdSe QDs – increases 4.7 times during 260 minutes of irradiation, the PSCdSe/ZnS QDs system PL intensity changes only slightly up to 1.1 times.

16

As we have expected core-shell CdSe/ZnS QDs are much more stable and their PL is of much higher intensity than that of CdSe QDs. The latter made us to make a choice for CdSe/ZnS QDs for our measurements. The influence of the Au NPs stabilized by 1-decanethiol on the PL is measured for solid PS films containing CdSe/ZnS QDs in the amount of 0.034 and 0.085 vol. fractions correspondingly at three excitation wavelengths, 405, 488 and 515 nm. Au NPs stabilized by 1-decanethiol having about 4.5 nm in diameter were used as plasmonic dopant (Figs. S2-3a,b). Two sets of composite samples (F1 and F2) and their compositions are presented in Table 2.

Table 2. Volume fractions of components in two sets of composite films.

F1_1

Volume fraction CdSe/ZnS PS Au NPs QDs 0 0.915±0.045 0.085±0.005

F1_2

0.910±0.095 0.085±0.009 0.0017±0.0004 0.0043±0.0009

F1_3

0.905±0.095 0.084±0.009 0.0031±0.0006 0.0079±0.0016

F1_4

0.895±0.095 0.083±0.009 0.0061±0.0012 0.0160±0.0032

F2_1

0.966±0.045 0.034±0.002

F2_2

0.956±0.095 0.034±0.004 0.0028±0.0004 0.0072±0.0009

F2_3

0.954±0.095 0.034±0.004 0.0034±0.0007 0.0086±0.0017

F2_4

0.948±0.095 0.034±0.004 0.0051±0.0010 0.0129±0.0026

Composite samples

0

Decanethiol ligands 0

0

To prove that the whole film exhibits similar PL behavior, the measurements of PL emission spectra were carried out at different areas of the sample having different PL intensity. An example of the set of PL spectra obtained in the areas of the different size and PL intensity is given in Figures S3 and S4. The measurements show that the PL intensity changes, but the shape of the spectra does not depend on the position and size of the area under investigation. The latter provides the opportunity to assess the relationship between the PL intensity and the volume fractions of QDs and Au NPs. The experimental results are given in the 3D diagram in which the intensity of PL is presented as a function of QDs and NPs volume fractions (Fig.7).

17

Figure 7. 3D diagram of emission intensity of composite films versus CdSe/ZnS QDs and Au NPs fractions. The results given in Figure 7 allow us to conclude that the intensity of PL increases with the increase in the CdSe/ZnS QDs volume fraction and decreases with the increase in the Au NPs volume fraction. We have assumed that the absorbed photons induce the PL and that is why its intensity depends on the intensity of excitation, absorbance and quantum yield. The analysis of the correlation between the PL intensity and volume fractions of CdSe/ZnS QDs and Au NPs is described in the ESI in detail (equations S1 – S3). An influence of the absorptions of the exciting and emission light to the PL intensity may be calculated with the well-known equation [47] (1) where: Iem - PL intensity; Iex – intensity of the exiting light; κ – quantum yield; optical density of the sample at the excitation wavelength;

- cumulative

- cumulative optical density of the

– (partial) optical density of QDs at the

sample at the wavelength of the emission; wavelength of excitation.

If there are no additional interactions between CdSe/ZnS QDs and Au NPs besides the ones related to emission and absorption, the optical densities of a complex system may be presented as a sum and

of

optical

densities

of , where

components. ,

That ,

is

why and

are optical densities of

CdSe/ZnS QDs and Au NPs at excitation and emission wavelengths, respectively. At the same time, optical densities may be expressed via the absorption cross-sections of CdSe/ZnS QDs and Au NPs at excitation (σQD,ex, σAu,ex,) and emission wavelengths 18

, which are calculated taking into account the sample thickness t, number concentration of CdSe/ZnS QDs (

) and number concentration of Au NPs (

) (number concentration is the

amount of NPs per one volume unit): , , . By means of the equation S4, relating the molar extinction coefficient ( CdSe/ZnS QD absorption cross-section (

), we have calculated

) and

at different wavelengths

from the experimental absorption spectra of CdSe/ZnS QDs (Fig.S5a). In line with calculations of CdSe/ZnS QD absorption cross-section as a function of wavelength we have computed the absorption cross-sections

of Au NP on the basis of absorbance spectra of Au NPs sols

(Fig.S5b). The values of

and

are calculated on the basis of absorption coefficients

and , , where NA – is Avogadro’s number. The curves corresponding to

as a function of wavelength are given in Figure 8.

Figure 8. Absorption cross-section of CdSe/ZnS QD (1) and Au NP (2) as a function of wavelength.

19

The results of calculations for CdSe/ZnS QDs are in good agreement with the published data [36]. It allows one to estimate the dependence of PL intensity (Iem), on the volume fraction of QDs (Fig.9) under the assumption that the quantum yield value k is equal 0.3. It is known that core/shell type CdSe/ZnS QDs have a far greater quantum yield than core type CdSe QDs [48] In assessing the effect of the effects of the internal filter, we assumed that the quantum yield of quantum dots does not change with a change in the composition of the samples. The thickness of the sample layer is chosen to be 6 µm as it is evaluated on the base of the point spread function calculated by ImageJ 1.20b software (NIH, USA)) for the objective used for the measurements (Fig. S6).

Figure 9. PL intensity as a function of CdSe/ZnS QDs volume fraction: dependences over the entire range (a) and enlarged regions (b). Table 3. PL intensity of composite samples at different radiation wavelength. Volume fraction Composite samples F1_1 F1_2 F1_3 F1_4 F2_1 F2_2

CdSe/ZnS QDs 0.085± 0.005 0.085± 0.009 0.084± 0.009 0.083± 0.009 0.034± 0.002 0.034± 0.004

Au NPs 0

PL intensity (experimental) λ ex, nm 405 488 515

PL intensity (simulation) λ ex, nm 405 488 515

1570±310

1845±355

1650±355

1570

1845

1650

1630±300

1760±340

1545±325

1365

1556

1348

1450±275

1645±330

1355±290

1201

1331

1121

1195±310

1230±325

990±280

956

1009

815

0

680±210

670±205

575±190

680

670

575

0.0028± 0.0004

525±175

480±150

390±130

530

502

412

0.0017± 0.0004 0.0031± 0.0006 0.0061± 0.0012

20

0.034± 0.004 0.034± 0.004

F2_3 F2_4

0.0034± 0.0007 0.0051± 0.0010

385±125

335±120

275±95

490

460

373

325±110

260±90

210±80

424

390

309

The analysis of data in Figure 9 and Table 3 shows that the PL intensity versus CdSe/ZnS volume fraction seems linear in small fraction regions only though there is no total linearity in the whole fraction range studied. The straight lines show the piecewise linear approximation of the dependence of PL intensity on the volume fraction of CdSe/ZnS QDs. The third line corresponding to λ ex = 405 nm excitation wave length overlaps partially with the one corresponding to λ ex = 488 nm. It is necessary to take into account that the medium surrounding Au NPs may affect the values of

λ) [49] in accordance with the equation 2 ,

(2)

where RAu – radius of Au NPs, εm – dielectric permittivity of the medium containing Au NPs, εAu,r and εAu,i – real and imaginary parts of the complex dielectric permittivity of the metallic Au . The mainstreaming of the influence of dielectric properties of a matrix surrounding Au NPs is carried out by the following approach. Based on the experimental data on information about

, the published

[50] and on the dependence of

we have estimated the dispersions of the real and imaginary parts of approximated with

. Those dispersions are

taken from [51] with a third order polynomial with the initial published in [51].

coefficients values, which are obtained by the approximation of data on Thereafter,

the

dispersions

и

are

with the change in

by

used

in

the

or by

formula

for

calculated for

PS containing CdSe/ZnS QDs [52]. Effective dielectric constant of PS containing CdSe/ZnS QDs is calculated via application of the Maxwell Garnett approximation for the corresponding regularities for PS and QDs. The latter is approximated by a constant value. The reason for that choice is shown in Figure S7, which shows the dispersion of dielectric constants. The spectra

obtained on the basis of the above calculations are given in Figure 10.

21

Figure 10. Calculated spectra of absorption cross section of Au NPs localized in different media.

We show the position of the excitation wavelengths and the spectral range 525-575 nm, which is used for measuring the intensity of the signal for the quantitative estimates in Figure S4. Thus, on the basis of data about absorption cross sections of Au NPs we have carried out the calculations of the dependence of PL of CdSe/ZnS QDs on the volume fraction of CdSe/ZnS QDs, Au NPs, excitation wavelength and dielectric properties of composites. PL intensity is integrated across the spectral range 520 – 575 nm. The calculated values of PL of F1 and F2 samples are normalized to experimentally measured PL of the samples, which do not contain Au NPs (at corresponding

. The results are shown in Figure 11 and Table 3.

Figure 11. PL intensity as a function of Au NPs volume fraction: lines represent the results of simulation and points – experimental results (

=405, 488 and 515

nm). Upper group of lines and points is related to the composites of F1 group and the lower one – to the composites of F2 group.

22

The experimental PL data are in line with the results of the numerical simulation which is considering inner filter effects causing the non‐linearity between PL intensity and CdSe/ZnS QDs concentration, as well as the influence of the dielectric constant of the medium on the optical properties of Au NPs (Fig.S8 and Fig.S9). One can conclude that the observed correlation is sufficient to interpret the above data as computationally accurate explanation of the effects resulted from the radiative interaction between fluorescent QDs and plasmonic NPs. However, non-radiative interactions can occur if the distance between CdSe/ZnS QDs and Au NPs reaches a critical value comparable to the size of NPs [21]. That is why we have carried out the numerical simulation for estimating the distances between QDs and Au NPs to analyze the probability of non-radiative effects in the polymer composite films containing chemically bound CdSe/ZnS QDs and Au NPs under consideration. It allows for calculating the radial distribution function of the gap between CdSe/ZnS QDs surface and the surface of the Au NP. We have used Wigner-Seitz sphere with the radius related to the reciprocal number concentration of Au NPs. Every sphere contains one Au NP in the center of the spherical coordinate system and a number of CdSe/ZnS QDs are placed in the sphere in accordance with the random distribution along the homogeneous distribution law with the correction on the distances between the surfaces of CdSe/ZnS QDs and between surfaces of CdSe/ZnS QDs and Au NPs to be not smaller than 2 nm. The distance from the CdSe/ZnS QD center to the center of the spherical cell could not exceed the Wigner – Seitz radius. Several examples of Wigner-Seitz spheres analyzed are shown in Figure 12. The total set of Maxwell Garnett cells is shown in ESI (Fig.S10).

Figure 12. Typical distributions of CdSe/ZnS QDs used for the calculation of radial functions of gaps distribution. The images correspond to the samples F1_2 (a) and F2_2 (b). The neighborhood of Au NP is shown having diameter equal to triple diameter of Au NP.

23

The distributions of QDs and Au NPs within the composite films of different compositions studied are illustrated with the cells of 50nm × 50nm × 50nm size in Figure S11. The integral radial distribution functions are built via determination of the fraction of QDs with the gap between of CdSe/ZnS QDs and Au NPs less or equal than the specific value normalized by the radius of Au NP (2.25 nm). To reach the statistical validity of results the big set of distributions (about 300) is analyzed. Figure 13 represents the integral radial distribution functions.

Figure 13. Integral radial distribution functions of gaps between the surfaces of Au NP and CdSe/ZnS QDs for polymer films of different compositions: The gap size is normalized to the Au NP radius.

Radial distribution function of gaps between NP and QD surfaces for polymer films depends on the composite concentration. At the highest Au NP content under investigation the fraction of Au NPs localized at the distances equal to the diameter of the NP where non radiative interactions may occur reaches 40%. It means that the experimental data are reasonably described by inner filter effects and dielectric constant change although we should not exclude non radiation interactions. One can suppose that the concurrent non-radiative interactions that tend to the PL enhancing on one hand and to PL quenching on the other hand, counteract and compensate each other.

4.

Conclusions

RAFT polymerized PS with thiol functional group at the end of macromolecule is capable of surface modification of CdSe/ZnS QDs due to the formation of Zn-S-PS bond. As shown with FTIR spectroscopy the bond formation of PS with QD surface happens due to the partial exchange of the oleic acid molecules by PS. The coexistence of low molecular weight and polymer ligands at the 24

CdSe/ZnS QD surface is confirmed by FTIR. TEM results based dependence of the inter-particle distance between CdSe/ZnS QDs in the composites on the volume fraction of QDs indicates the presence of PS macromolecules around QDs. One of the possible reasons for such packing is the interaction between PS chains and QD surface proved by FTIR. The composites of PS chemically attached to CdSe/ZnS QDs show PL which is reasonably high. An addition of Au NPs with 1-decanethiol ligands affects the PL resulting in its decrease. The analysis of the experimental data shows good correlation with the model considering both the inner filters effects and the influence of the change of effective dielectric constant of composite to the absorbance of Au NPs, but does not exclude the possible compensation effects of non-radiative interactions. The latter follows from the analysis of the model integral radial distribution functions of the gap between CdSe/ZnS QDs and Au NPs.

Acknowledgement: This work was carried out within the State Program of TIPS RAS. TEM and PL measurements were performed on equipment from the Centers for Collective Use “Transmission Electron Microscopy” and “Technology of Obtaining New Nanostructured Materials and Their Complex Study” at M.V. Lomonosov Moscow State University. The authors are grateful to Dr. Sorokin S.F. (TIPS, RAS) for XR fluorescence measurements

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Monochelic polystyrene forms a composite with CdSe/ZnS QDs by replacement of ligands Polystyrene molecules wrapped around QD’s composites form highly fluorescent films Au NPs addition to polystyrene-QDs composites changes their fluorescence efficiency Inner filter effects must be considered for evaluation of the composites fluorescence QD-NP gap distributions indicates the presence of non-radiative interactions

Author Statement A.A. Ezhov: Conceptualization, Methodology, Investigation. O.N. Karpov::Investigation, Resources, Writing - Original Draft. A,S. Merekalov: Investigation, project administration. S.S. Abramchuk: TEM analysis, Interpretation. G.N. Bondarenko: IR spectroscopy Investigation and Interpretation. R.V. Talroze: Supervision, Conceptualization, WritingReviewing and Editing, Editing,

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: