Journal of Non-Crystalline Solids 356 (2010) 2299–2301

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids

j ourna l homepage: www.e lsev ie r.com/ locate / jnoncryso l

Optical properties of CdSe quantum dots in silicate glasses

Kai Xu a, Chao Liu a, Woon Jin Chung b, Jong Heo a,⁎a Center for Information Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang,Gyeongbuk 790-784, Republic of Koreab Division of Advanced Materials Engineering, Kongju National University, 275, Budae-dong, Cheonan, Chungnam 330-717, Republic of Korea

⁎ Corresponding author. Tel.: +82 54 279 2147; fax:E-mail address: jheo@postech.ac.kr (J. Heo).

0022-3093/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.jnoncrysol.2010.05.097

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 September 2009Received in revised form 6 April 2010

Keywords:Quantum dots;Quantum confinement;Photoluminescence;Glasses

CdSe quantum dots (QDs) were precipitated in silicate glasses under various durations and temperatures ofheat-treatment. Absorption and photoluminescence (PL) spectra of the QDs showed the quantumconfinement effect from CdSe QDs. When the glass was heat-treated for 10 h, estimated average QD sizesincreased from 5.2 nm when the glass was heat-treated at 490 °C and to 6.9 nm when it was treated at500 °C. When the glass was heat-treated at 480 °C, estimated average QD sizes were 4.9 nm after 20 h and5.3 nm after 30 h. The PL peak position was independent of the excitation wavelengths; this result indicatesthat the size distribution of CdSe QDs was narrow.

+82 54 279 8653.

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor nanocrystals (quantum dots or QDs) have beenextensively investigated because of their unique optical and electronicproperties compared to bulk materials [1]. QDs with discrete electronand hole states show the quantum confinement effect and exhibitsize-tunable optical transitions [2]. These QDs can be used to makenew optical devices if QDs can be incorporated into the appropriate,stable matrices such as glasses. II–VI QDs embedded in glass matriceshave also been for use as sharp-cut filters, and for applications such asRaman amplifiers [3] and active optical emitters [4].

The early studies of CdSe nanocrystals in glasses focused onnucleation and growth [5], and the optical properties were studied atcryogenic temperatures [6]. At room temperature, the emissionaround 800 nm from glasses containing CdSe QDs mostly originatesfrom surface trap states [7,8], which are surface vacancies or foreignspecies adsorbed on the surface of QDs. As temperature increases,holes can move to the surface and finally recombine into the excitonpairs, producing a broad emission spectrum [9]. QDs in glass matricesare formed by thermally-driven nucleation and growth processes.Therefore, obtaining perfect nanocrystals is difficult, and as a result,photoluminescence (PL) from glasses show strong influence from thesurface traps.

This work reports the optical properties of CdSe QDs in silicateglasses with ZnO. ZnO has been frequently used to facilitate the

precipitation of chalcogenide nanocrystals in glasses [10]. Strong PL ofCdSe QDs was observed at room temperature and wavelengths of thePL were successfully controlled using careful thermal treatment.

2. Experimental procedures

The host glass with the nominal composition (in mol%) of 50SiO2–

35Na2O–5Al2O3–10ZnO with additional 0.4CdO–0.4ZnSe was pre-pared. After mixing, the batch was melted in an alumina crucible at1300 °C for 40 min under the ambient atmosphere. The melt wasquenched by pouring onto a brass mold and the glass was annealed at350 °C for 3 h. Subsequent thermal treatments were performed at480~500 °C for 10~30 h to precipitate CdSe QDs.

Glasseswere optically polished into sections of ~1.5 mm thickness.Absorption spectra of the glasses from 300–850 nm wavelengthswere recorded using a UV/VIS/NIR spectrophotometer (Perkin ElmerLambda 750 S). PL spectra were recorded using excitation at 325 and442 nm from an He-Cd laser, and at 532 nm using a mode-locked Nd:YVO4 laser. A combination of a mechanical chopper, a 1/4 mmonochromator, a photomultiplier tube (PMT) detector and a lock-in amplifier system was used to collect and amplify the signal. Allexperiments were performed at room temperature. A high-resolutiontransmission electron microscope (HR-TEM, JEOL JEM-2100F) wasused to identify the crystal structure of QDs under an acceleratingvoltage of 200 kV.

3. Results

A glass heat-treated at 490 °C for 10 h was examined using HR-TEM (Fig. 1). The diffraction pattern was almost identical to that of abulk CdSe crystal which has a hexagonal structure (wurtzite) with

Fig. 1. TEM image of one CdSe nanocrystal and Fast Fourier Transform pattern (inset)from the area in the circle. The specimen was heat-treated at 490 °C for 10 h.

Table 1Peak positions of absorption (λabs) and photoluminescence (λPL), calculated averagediameter⁎, Stokes shift and full width at half maximum (FWHM) of the PL band fromCdSe QDs in glass matrices precipitated by different heat-treatments.

Heat-treatmentcondition

λabs

(±2 nm)λPL

(±2 nm)Calculatedaveragediameter (nm)

Stokes shift(±2 nm)

FWHM(±4 nm)

490 °C—10 h 524 553 5.2 29 71500 °C—10 h 585 611 6.9 26 54480 °C—20 h 505 547 4.9 42 83480 °C—30 h 535 571 5.4 36 69

⁎ Average diameter of CdSe QDs was estimated using Eq. (1) with uncertainty of 5%.

2300 K. Xu et al. / Journal of Non-Crystalline Solids 356 (2010) 2299–2301

lattice constants of 4.30 and 7.01 Å. This result confirms that CdSe QDsformed in the glass.

Light absorption by QDs is due to the generation of electron andhole pairs induced by the excitation photon, and emission by QDsis due to the recombination of the excitons, similar to thephenomena in bulk semiconductors [11]. In room temperatureabsorption spectra (Fig. 2) of glass heat-treated under the differentcombinations of temperature and time, the absorption bandsappeared at the wavelengths of 524 and 585 nm when glasseswere heat-treated at 490 and 500 °C for 10 h, respectively. Whenthe duration of heat-treatment at 480 °C increased, absorptionbands shifted from 505 nm when treated for 20 h to 535 nm whentreated for 30 h. These characteristic absorption bands are due tothe first exciton transition of CdSe QDs from the ground state.Observed blue-shift from the bulk CdSe band gap energy(~716 nm) demonstrated the quantum confinement effect in theCdSe QDs-doped glass. The peak positions moved to the longerwavelengths due to the increased size of QDs as temperature orduration of heat-treatment increased.

In normalized PL spectra of the CdSe QDs, the center emissionwavelength became longer as heat-treatment temperature or dura-tion increased (Table 1). In glasses excited at 442 nm (Fig. 3), thecenter wavelengths of the PL shifted from 553 to 611 nm as the heat-treatment temperature was increased from 490 to 500 °C while

Fig. 2. Absorption spectra of glasses as-prepared and with heat-treatment underdifferent thermal regimes. All spectra were recorded at room temperature.

duration was kept constant at 10 h. Similarly, the center wavelengthsincreased from 547 to 571 nm with increasing heat-treatmentduration from 20 to 30 h at 480 °C. They showed the tendency similarto the absorption spectra such that the center wavelength of PLspectra shifted to the longer wavelength as heat-treatment temper-ature or duration increased.

To obtain information on the size distribution of QDs, PL spectra ofglasses were obtained by using different excitation wavelengths. ThePL spectra of glass treated at 480 °C for 30 h became broader whenexcited by higher energy sources (532 nm→442 nm→325 nm) butthe wavelengths of the emission peaks remained unchanged (Fig. 4).

4. Discussion

Average diameters (D, nm) of CdSe QDs can be calculated from thefirst excitonic absorption peak and calculated using the followingempirical equation [12].

D = 1:6122 × 10−9� �

λ4− 2:6575 × 10−6� �

λ3

+ 1:6242 × 10−3� �

λ2−0:4227λ + 41:57;ð1Þ

where λ (nm) is the peak wavelength of the first excitonic absorption.The calculated average diameters using Eq. (1) were 4.9 nm whenglass was heat-treated at 480 °C for 20 h and 5.4 nmwhen it was heat-treated at 480 °C for 30 h. The estimated diameters of CdSe QDs inglasses under the various conditions (Table 1) increased as temper-ature or duration of the heat-treatment increased.

Stokes shifts and full width at half maximum (FWHM) intensitiesof PL from CdSe QDs were estimated (Table 1). The Stokes shifts inglasses were smaller than 40 nm and this result implies that the directelectron-hole recombination is responsible for the observed lumines-cence. When QDs were precipitated in glass heat-treated at lowtemperature for a short duration, FWHM values were relatively large

Fig. 3. Photoluminescence (PL) spectra from CdSe QDs precipitated in glasses heat-treated at different thermal schedules. All spectra of glasses were recorded at roomtemperature with the excitation at 442 nm.

Fig. 4. Absorption and PL spectra of glasses heat-treated at 480 °C for 30 h pumped by325, 442 and 532 nm laser at room temperature.

2301K. Xu et al. / Journal of Non-Crystalline Solids 356 (2010) 2299–2301

due to the broad size distribution. However, when precipitated inglass heat-treated at high temperature under the controlled manner,FWHM values of PL decreased [5,13].

When the size distribution of QDs is small, the emission peak willappear at the same position regardless of the excitation wavelength[14]. The peak wavelengths of PL spectra from glasses excited by thedifferent excitation wavelengths in Fig. 4 clearly appeared at the samepositions. This result indicates the narrow size distribution of CdSeQDs in glass heat-treated at 480 °C for 30 h. The small hump in all PLspectra at 700 nm is an artifact induced by the detector.

5. Conclusions

CdSe QDs were fabricated in SiO2–Na2O–Al2O3–ZnO glass systemwith the addition of CdO and ZnSe. HR-TEM images with its FFTpattern identified the formation of the CdSe QDs through the heat-treatment. The PL of CdSe QDs was tuned via thermal treatment at480~500 °C for 10~30 h in wavelength range of 550~610 nm withthe average diameter of 4.9~6.9 nm.

Acknowledgements

This work was supported by the Korea Research Foundation Grantfunded by the Korean Government (KRF-2008-005-J00501).

References

[1] V.I. Klimov, A.A. Mikhailovsky, S. Xu, A. Malko, J.A. Hollingsworth, C.A. Leatherdale,H.-J. Eisler, M.G. Bawendi, Science 290 (2000) 314.

[2] A.P. Alivisatos, Science 271 (1996) 933.[3] A. Céreyon, A.M. Jurdyc, V. Martinez, E. Burov, A. Pastouret, B. Champagnon,

J. Non-Cryst. Solids 354 (2008) 3458.[4] R. Jia, D.S. Jiang, P.H. Tan, B.Q. Sun, Appl. Phys. Lett. 79 (2001) 153.[5] L.C. Liu, S.H. Risbud, J. Appl. Phys. 68 (1990) 28.[6] M. Chamarro, C. Gourdon, P. Lavallard, Phys. Rev. B 53 (1996) 1336.[7] N.F. Borreli, D.W. Hall, H.J. Holland, D.W. Smith, J. Appl. Phys. 61 (1987) 5399.[8] Z. Su, P.A.M. Rodrigues, P.Y. Yu, S.H. Risbud, J. Appl. Phys. 80 (1996) 1054.[9] M.G. Bawendi, P.J. Carroll, W.L. Wilson, L.E. Brus, J. Chem. Phys. 96 (1992) 946.

[10] C. Liu, J. Heo, X.H. Zhang, J.L. Adam, J. Non-Cryst. Solids 354 (2008) 618.[11] A.D. Yoffe, Adv. Phys. 42 (1993) 173.[12] W.W. Yu, L. Qu, X. Peng, Chem. Mater. 15 (2003) 2854.[13] A. Ekimov, J. Lumin. 70 (1996) 1.[14] U. Woggon, O. Wind, F. Gindele, E. Tsitsishvili, M. Müller, J. Lumin. 70 (1996) 269.