Tunable photoluminescence of polymer doped with PbSe quantum dots

Tunable photoluminescence of polymer doped with PbSe quantum dots

Materials Science and Engineering C 27 (2007) 1078 – 1081 www.elsevier.com/locate/msec Tunable photoluminescence of polymer doped with PbSe quantum d...

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Materials Science and Engineering C 27 (2007) 1078 – 1081 www.elsevier.com/locate/msec

Tunable photoluminescence of polymer doped with PbSe quantum dots Igor Voitenko a , J.F. Muth a,⁎, Michael Gerhold b , Dehu Cui c , Jian Xu c a

ECE Department, North Carolina State University, USA b Electronics Division, Army Research Office, USA c Pennsylvania State University, USA

Received 7 May 2006; received in revised form 13 September 2006; accepted 13 September 2006 Available online 24 October 2006

Abstract Semiconductor nanoparticle/polymer composites potentially allow the design of photonic materials for optoelectronic devices. The optical characteristics of PbSe quantum dots placed in polymer matrices are investigated for potential applications in electrically controlled absorption modulators and integrated optical circuit components. The photoluminescence yield, shape of absorption band, and effects of size variation of PbSe quantum dots on spectral features are analyzed near the absorption edge of host polymers for the different concentrations of nanocrystals in the composite. It was found that for quantum dots of nominally the same size, there is a strong dependence of position of the absorption peaks in the spectrum depending on the concentration of quantum dots. This results in the emission in the 1500–1600 nm range being tunable with quantum dot concentration. A second emission in the 1200–1300 nm range was also observed and energy transfer from the polymer matrix to the quantum dot appears to mediate the strength of this photoluminescence. © 2006 Elsevier B.V. All rights reserved. Keywords: Polymer composite; Semiconductor quantum dots; Photoluminescence

1. Introduction There is substantial interest in developing nanocomposite materials for optoelectronic devices. Specifically, inorganic crystallites like semiconductor quantum dots of PbSe, CdS, or PbS, can be combined with organic polymers and spin cast into thin films [1–4]. The nanocomposite films can then be used for waveguide devices such as modulators and optical amplifiers. The emission of quantum dots is primarily defined by the physical size of the crystallites [5]. However, one must consider that the quantum dots are randomly distributed in the host polymer matrix and local variations of the environment can significantly influence energy transfer mechanisms between the matrix and the quantum dot and, thus, influence the emission process. Several different types of energy transfer can occur depending on relative interaction strengths between the polymer

⁎ Corresponding author. E-mail address: [email protected] (J.F. Muth). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.09.018

and quantum dots including direct injection that results in charging the dot, or by energy transfer from the excited state of the polymer molecule to the dot. The binding energy of excitons has been widely discussed in the literature [6–10] and modification of the Coulomb potential has been discussed by Brus [11–13]. Various regimes of quantum confinement have been investigated with respect to the ratio of the crystallite radius R to the exciton Bohr radius (aB = h¯2ε2/μ · e2), where μ = memh(me + mh)− 1 with the effective mass of the electron (me) and hole (mh), and ε is the dielectric constant of the semiconductor material. However, little work has been done on emission from PbSe/ polymer complexes and investigating the interactions between the polymer and quantum dots and how the emission varies when the quantum dot concentration is increased. This paper endeavors to answer some of these questions. We present the experimental results and detailed analysis of the luminescent properties of PbSe quantum dots incorporated in the poly (methyl methacrylate) (PMMA) mixed with MEH-PPV (poly [2-methoxy,5-(2-ethyl-hexyloxy)-p-phenylene-vinylene]) dissolved in toluene. Specifically, the emission of PbSe/polymer

I. Voitenko et al. / Materials Science and Engineering C 27 (2007) 1078–1081

Fig. 1. The absorption spectra of PMMA, and PMMA:MEH-PPV. Note the blue shift to shorter wavelengths with the addition of MEH-PPV and also the appearance of additional structure in the absorption spectrum.

nanocomposites is investigated for different quantum dot concentrations and excitation conditions. 2. Experimental Crystalline PMMA having a maximum molecular weight of 996,000 and a glass transition temperature of Tg = 125 °C was obtained from Aldrich and master solution of PMMA in toluene was prepared for the average viscosity of 200 cP. The mixtures with PMMA-in-toluene and MEH-PPV-in-toluene were mixed in the ultrasonic bath and a solvent was evaporated until viscosity of around 180 cP at 25 °C was reached. It was found that a blend ratio of PMMA to MEH-PPV of 4:1 limited the agglomeration of quantum dots, with the intent of isolating each dot from neighboring dots. The polymers were miscible with this blend ratio and also had suitable adhesion properties for spin casting and dip-coating thin films on glass substrates. The PbSe quantum dots were prepared using standard colloidal chemistry methods and ranged in size from 5 to 6 nm. The PL spectra were taken using a conventional arrangement consisting of a grating scanning monochromator and a chopped PbS detector. The tungsten halogen light source spanning the range from 1000 to 2300 nm and light-emitting diode (LED) with the central wavelength at 970 nm were used as the pumping sources. The fluorescent properties and the photo-oxidation of the polymer blends with the different ratio of components were quantified using photoluminescence and absorption spectroscopy with and without quantum dots to ensure that luminescence and self-absorption from the polymer matrix were not interfering with the measurement. All measurements were performed at room temperature.

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small [9,11]. The small bulk band gap energy of PbSe quantum dots allows an experimental tuning of the ground excited state transition throughout the near-infrared spectral region. In our case, the size of the dot is chosen to produce emission near 1550–1600 nm. It is our observation that depending on the surrounding polymer matrix and the density of quantum dots, tuning of the fundamental transition can occur. Furthermore, depending on the means of excitation and density of quantum dots, another transition occurring near 1240 nm can be excited by energy transfer from the polymer to the quantum dot. In the case of higher concentrations of quantum dots, dot-to-dot interactions may help promote emission from this higher energy transition. The absorption spectra of PMMA taken at room temperature as shown in Fig. 1 consist of strong absorption peaks between 1100 and 1200 nm, a broad peak near 1300–1400 nm and a strong absorption for wavelengths longer than 1630 nm. With the addition of MEH-PEV, the absorption structure shifted 67 nm to shorter wavelengths but is not otherwise significantly altered. The absorption spectrum of these quantum dots and photoluminescence (in chloroform) at 1550 nm excited by a 980-nm laser is shown in Fig. 2. Note that in addition to the fundamental transition, there is a second absorption peak near 1200 nm. This absorption band is coincidentally near the 1100– 1200 nm absorption band of the PMMA:MEH-PVV blend. When the nanocrystals were mixed with the polymer, it was found that the FWHM of each PL peak was dependent on the specifics of the polymer blend and quantum dot concentration. This variation in FWHM was attributed to the random placement of nanodots in the host matrix and the dispersion in quantum dot size. Since the structure of energy levels in the quantum dots are strongly dependent on their sizes and surrounding dielectric, the shape of the absorption lines will be varied and, hence, the emission characteristics. One can also imagine that as the quantum dot concentration increases, interaction between dots can occur. Nevertheless, the main features of the spectra are repeatable and behave consistently. We consider three cases to compare.

3. Results The PbSe quantum dots studied here have diameters approximately ranging from 5 to 6 nm which are significantly smaller than the bulk exciton Bohr radius aB. This case corresponds to a strong confinement regime (R ≪ aB) in which a Coulomb attraction of the electron and the hole becomes very

Fig. 2. The absorption spectra of PbSe quantum dots similar to those used in the films (solid line), and the emission spectra of the PbSe (dashed line).

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Fig. 3 shows the emission from two nanocomposite films with the same polymer blend but a different concentration of quantum dots. Fig. 4 shows the lower concentration film, and Fig. 5 shows the higher concentration film, but with each film studied using two different excitation conditions, the broadband incandescent source and the narrower, higher intensity LED source. In Fig. 3, where the excitation source is a broadband light source, we observe two distinct peaks, the fundamental transition at 1616 nm for the 18 mg/ml sample and 1555 nm for the higher concentration 25 mg/ml sample. These data show that the fundamental emission can be tuned in the 1500–1600 nm region, not just by adjusting the quantum dot size, but also by changing the concentration. Furthermore, we see a second feature at 1280 and 1230 nm for the 18 and 25 mg samples that correspond to the second peak in the absorption spectra shown in Fig. 2. These second peaks in the 1230–1280 nm region are not seen when the quantum dots are excited while in solvents such as chloroform or toluene and are believed to be the result of energy transfer from the 1100–1200 nm absorption band shown in Fig. 1. In Figs. 4 and 5, the spectral range from 1000 to 1700 nm was examined, and it was found that the response of the two films with different concentrations is distinctly different if a narrowband LED source at 970 nm is used or the broad-band incandescent source is used. In Fig. 3 (the 18 mg/ml nanocomposite film), when pumped with the broadband incandescent source, one finds two peaks, one at 1227 nm and one at 1534 nm. However, under LED excitation, the peak near 1227 nm is essentially absent, and the peak at 1534 nm is shifted to the red by 14 nm to 1548 nm. In this case, we believe that the broad-band light source is exciting the quantum dots both directly by absorption near the fundamental transition of the quantum dot and also by exciting the polymer host in the 1100– 1200 nm absorption band that is resonant with the second absorption peak seen in Fig. 2. The reason the peak does not appear under LED excitation is that that the LED wavelength

Fig. 3. Photoluminescence excited by the broad band light source with (1) the 25 mg/ml concentration sample, and (2) the 18 mg/ml sample.

Fig. 4. Photoluminescence of sample with concentration of 18 mg/ml. Peaks labeled (1) are excited by broadband light source. Peaks labeled (2) are excited by LED.

does not efficiently excite the polymer since this wavelength is at an absorption minimum of the polymer. However, the above band gap absorption of the 970 nm LED by the quantum dot still results in the excitation of the nanodot which de-excites to the ground state of the exciton and emits near 1548 nm. Thus, we postulate that the excitation of the polymer may play a significant role in the emission dynamics of the quantum dot. However, the situation becomes more complicated when the concentration of quantum dots is increased. In Fig. 5 (the 25 mg/ml thin film), while the incandescent source still shows two peaks at 1230 and 1555 nm, the LED excited spectra now also exhibits emission at 1282 nm. In this case, it is suspected that the interaction between the nanoparticles is permitting energy transfer between smaller diameter particles and the larger diameter particles perhaps with the polymer in an excited state mediating the interaction. Thus, the 970 nm light is absorbed in the continuum by the smaller of the 5–6 nm size of particles, and this excited state is transferred to the larger of the dots resulting a red shift and emission at 1280 nm.

Fig. 5. Photoluminescence of sample with concentration of 25 mg/ml. Peaks labeled (1) are excited by broadband light source. Peaks labeled (2) are excited by LED.

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4. Conclusions In conclusion, we have reported on the systematic study of the PL spectra from polymer blends doped with PbSe quantum dots. In this study, we show that the concentration of quantum dots can have significant effects on the wavelength of the photoluminescence, and that understanding the absorption spectra of the polymer matrix is important. The results indicate the possibility of being able to tune the wavelength of emission of PbSe nanodots in the technologically important telecommunications window. In addition to the fundamental emission of the quantum dots in the 1550–1600 nm region, we show that emission can be obtained in the 1240–1280 nm region, but this emission is strongly dependent of the type of wavelength of the excitation source, which indicate that energy transfer mechanisms play an important role. At low concentrations of quantum dots, the energy transfer appears to be from excitation of the polymer which is transferred to the quantum dot. At higher concentrations, it appears that quantum dot to quantum dot interactions are important and are mediated by the polymer matrix.

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