Absorption and photoluminescence of PbS QDs in glasses

Journal of Non-Crystalline Solids 355 (2009) 1880–1883

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Absorption and photoluminescence of PbS QDs in glasses Chao Liu, Yong Kon Kwon, Jong Heo * Center for Information Materials, Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, Republic of Korea

a r t i c l e

i n f o

Article history: Available online 18 July 2009 PACS: 78.40.Hc 78.55.m 78.55.Qr 78.67.7

a b s t r a c t Optical properties of PbS quantum dots (QDs) precipitated inside the oxide glass matrix were investigated. Photoluminescence (PL) from the PbS QDs showed peak wavelengths located at 1170–1680 nm with widths of 150–550 nm. Radii of QDs in glasses were 2.3–4.7 nm depending upon the thermal treatment. Peak wavelengths of PL bands shifted as much as 70 nm as the temperatures and excitation irradiances increased. Calculated effective local temperatures indicated that these shifts of PL spectra were associated with local heating induced by the temperatures and laser beam. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Glasses Laser–matter interactions Measurement techniques Atomic absorption Optical spectroscopy STEM/TEM Nanoparticles, colloids and quantum structures Nanocrystals Quantum wells, wires and dots Optical properties Absorption Luminescence Oxide glasses Borosilicates

1. Introduction Broadband fiber-optic amplifiers have been developed as important components in the telecommunication network [1]. Rare-earth doped fiber amplifiers, especially erbium doped fiber amplifiers (EDFAs) were successfully commercialized [2]. New host materials such as non-oxide glasses containing various rare-earth ions were also investigated to cover the bandwidths where the conventional EDFA cannot [3]. Semiconductor and Raman optical amplifiers can provide a wide choice of operation wavelength with high power output and low cost. However, they suffer from the low pumping efficiency, high coupling loss, polarization dependence and high noise figures. Semiconductor nano-crystals (quantum dots or QDs) have been extensively investigated because of their unique optical * Corresponding author. Address: Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hoyja-dong, Pohang, Gyeongbuk 790-784, Republic of Korea. Tel.: +82 54 279 2147; fax: +82 54 279 8653. E-mail address: [email protected] (J. Heo). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.04.045

and electronic properties compared to their bulk counterparts [4,5]. Near IR luminescence that covers the important wavelength region for the fiber-optic communication can be obtained by tuning the band gap energy of the QDs through the control of their particle sizes. In particular, narrow band gap IV–VI materials such as PbS and PbSe with band gap energies of 0.41 eV and 0.26 eV, respectively, have attracted much attention recently [6]. Furthermore, due to the large bulk exciton Bohr radii of PbS (18 nm) and PbSe (46 nm), respectively, quantum confinement effect can be realized over the wide range of the particle size. It is well known that PbS nanoparticles can provide the luminescence over the whole transmission window (1.2–1.7 lm) of silica optical fibers and therefore, can be potentially used for the universal optical amplifiers [7]. Current work reports the near-infrared photoluminescence (PL) from the PbS QDs precipitated in the glasses through thermal treatment. Near-infrared PL that cover the entire transmission window of silica fibers were successfully achieved. The effects of excitation intensities and temperatures on the PL properties of PbS QDs were investigated.

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2. Experimental procedures Two host glasses with nominal compositions (mol%) of 66SiO2– 8B2O3–18K2O–4BaO–4ZnO (abbreviated as BZ glass, hereafter) with additional 1.0 PbS and 55SiO2–35Na2O–5Al2O3–8ZnO–2ZnS with additional 1.0PbO (Z glass) were prepared. Starting powders were melted in an alumina crucible at 1350 °C for 90 min and 60 min for BZ and Z glass, respectively. After melting under the ambient atmosphere, the melt was quenched by pouring onto a brass mold at room temperature. Glasses thus obtained were heat-treated at various temperatures to precipitate PbS QDs. Transmission electron microscope (TEM) images and diffraction patterns were used to identify the size and shape of PbS QDs. Absorption spectra of the glasses were recorded using an UV/Vis/NIR spectrophotometer over the wavelength region of 300–2400 nm. Photoluminescence (PL) spectra and the changes of PL intensities with time were recorded by pumping the specimens with an 800 nm beam from a continuous wave Ti: sapphire laser. A combination of the monochromator, silicon PIN detector, lock-in amplifier and digital oscilloscope was used for detection. Low temperature PL spectra were recorded with an additional cryostat system cooled by compressed helium gas. 3. Results

wavelengths of 1050, 1180, 1450 and 1690 nm for those heat-treated at 600 °C/20 h, 600 °C/50 h, 620 °C/20 h and 640 °C/h, respectively [9]. These broad absorption peaks were directly related to the formation of PbS nano-crystals. Large blue-shift of the absorption bands from the bulk band gap energy of PbS (0.41 eV) clearly indicated the quantum confinement effect. Average radii of PbS QDs estimated from the absorption spectra [9] were 2.3–4.7 nm and they increased as heat-treatment temperatures and durations increased. Accordingly, PL spectra of heat-treated glasses also changed with thermal treatment [9]. As the size of PbS QDs increased, peak of the PL bands shifted from 1166, 1318, 1620 to 1680 nm. They also showed Stokes shifts of approximately 100 meV compared the peak position of the corresponding absorption spectra. These large Stokes shifts were most probably induced by the defects on the surface of PbS QDs. In addition, the PL band showed full width at half maximum PL intensity values of 150–500 nm and they are considerably larger than 40 nm usually observed from the 1.54 lm emission of Er3+. The large FWHM was probably induced by the large size dispersion of PbS QDs in glasses. It is important to mention that wavelengths of PL from PbS QDs can be tuned through the thermal treatment to achieve the amplification over the entire transmission window of silica fiber. 3.2. Effect of excitation intensities and temperatures on the PL spectra Fig. 1(a) shows the PL spectra of 2.6 nm PbS QDs formed in BZ glasses recorded at various temperatures with an excitation power of 140 W/cm2. PL bands shifted 70 nm to the high energy side as the temperature increased from 50 to 294 K. This temperature-in-

PL intensity (a.u.)

-1

Formation of PbS QDs in the glass was confirmed by the high resolution transmission electron microscopy [8]. Upon the formation the PbS QDs in the glass, color of the glass changed from yellow to black. The broad absorption bands were formed at the

Absorption coeffcient (cm )

3.1. Absorption and PL spectra of PbS QDs in BZ glasses

1.0

(a)

294K

50K

Excitation power: 2 140 W/cm

0.5

0.0 1000 1100 1200 1300 1400 1500 1600 1700

20

(a)

o

(1) 460 C-10hr o (2) 460 C-20hr o (3) 460 C-30hr o (4) 480 C-10hr

15

10

5 (1) (2)

0

1000

(b)

850W/cm

2

70W/cm

Temperature: 100K

0.5

0.0 1000 1100 1200 1300 1400 1500 1600 1700

Wavelength (nm) Fig. 1. Photoluminescence spectra of PbS QDs (radius 2.6 nm) recorded at (a) various temperatures with the excitation power of 140 W/cm2 and (b) various excitation intensities at the temperature of 100 K.

PL Intensity (a.u.)

PL intensity (a.u.)

1.0

(4)

1500

2000

Wavelength (nm)

Wavelength (nm) 2

(3)

1.0

(b)

(1) (2)

(3) (4)

0.8

o

(1) 460 C-10hr o (2) 460 C-20hr o (3) 460 C-30hr o (4) 480 C-10hr

0.6 0.4 0.2 0.0 1000

1200

1400

1600

1800

2000

2200

Wavelength (nm) Fig. 2. (a) Room temperature absorption spectra of Z glass heat-treated under different conditions. (b) Room temperature PL spectra of PbS QDs-doped glasses under the 800 nm laser excitation. The thermal treatment conditions are indicated in the figure.

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duced shift in the PL band was consistent with the results reported previously [10]. In addition, PL bands also blue-shifted approximately 70 nm in magnitude when the excitation power increased from 70 to 850 W/cm2 at the fixed temperature of 100 K (Fig. 1(b)). It is apparent that temperatures and excitation intensities exerted similar effects on the energies of the PL bands. Peak wavelengths of PL energies converged to values within a small range when temperature or excitation intensities became high. In addition, full width at half maximum (FWHM) intensities of PL also increased with temperature and excitation intensities. Energies of PL peaks and values of FWHM intensities of PL measured with different excitation irradiances and experimental temperatures were shown in our last report [11]. PL from the PbS QDs with the different average radii of 2.3 and 3.6 nm also showed similar dependences on temperatures and intensities.

3.3. Effect of host glass composition on the optical properties Lead chalcogenide compounds have maximum solubilities of approximately 0.1 and 1.0 at.% in the borosilicate and phosphate glasses, respectively [12,13]. In the BZ glasses, the maximum absorption coefficient due to the PbS QDS was 4.3 cm1 in glass when heat-treated at 600 °C for 50 h [9]. Therefore, it is necessary to increase the concentration of PbS QDs to enhance the luminescence of PbS QDs in the near-infrared wavelength region. Fig. 2(a) shows the absorption spectra of Z glasses where absorption coefficients increased to 6.4, 8.9, 8.4 and 7.6 cm1 when heat-treated at 460 °C/10 h, 460 °C/20 h, 460 °C/30 h and 480 °C/10 h, respectively. Fig. 2(b) shows PL spectra of Z glasses heat-treated under various conditions and detailed information was summarized in Table 1.

Effective temperature (K)

4. Discussion 350

4.1. Excitation intensities and temperature-dependences of photoluminescence

(a)

300 250 200

2

70 W/cm 2 140 W/cm 2 280 W/cm 2 565 W/cm 2 850 W/cm

150 100 50

50

100

150

200

250

300

Experimental temperature (K) 120

FWHM (meV)

115

(b)

Texp:

110

50 K 100 K 150 K 200 K 250 K 294 K

105 100 95 90

 1 WðT Eff Þ ¼ W 0 þ a expðhx=jB T Eff Þ  1 :

85 80 75 0.002

Since excitation intensities and experimental temperatures (Texp) had similar effects on the photoluminescence of PbS QDs (Fig. 1), it is reasonable to expect that the increase in excitation intensity led to the increase of the local temperature around the QDs. The effective local temperatures (TEff) have been calculated using the temperature dependence of the band gap energies of PbS QDs [11]. The calculated effective local temperatures in Fig. 3 show several important points. First, TEff increased as the excitation intensities increased, a direct indication of laser-induced local heating effect. Second, TEff was similar to Texp when the excitation intensity was low but the difference between them increased with increasing excitation intensity. In addition, at high excitation intensity of 850 W/cm2, all calculated values of TEff became similar to each other regardless of the experimental temperature. All these intensity dependencies of TEff suggested that laser beam used for the excitation induced the local heating and it was responsible for the shift of the PL bands. These calculated effective temperatures were used to simulate the full width at half maximum intensity of photoluminescence using the following equation [10]:

0.004

0.006

0.008

-1

0.010

0.012

1/TEff (K ) Fig. 3. (a) Effective local temperatures calculated for different measurement temperatures (Texp) and excitation intensities. (b) Fitting results of the values of FWHM intensities using Eq. (1). The solid line represents the fitting results, and dots are measured values of FWHM intensities at various conditions.

ð1Þ

Here, W(TEff) is the measured FWHM values at the effective temperatures, W0 is the value due to the size dispersion only. a is the electron–phonon coupling strength,  hx is the energy of the optical phonon (215 cm1) and jB is the Boltzmann’s constant. The results of fitting using Eq. (1) were shown in Fig. 3(b) with values of W0 and a of 75.47 ± 0.16 meV and 52.65 ± 0.43 meV, respectively. The value of W0 was comparable to that obtained from PbS QDs capped with thiol (W0 = 90 ± 10 meV) [10]. However, calculated value of a was smaller than the reported value of 110 ± 20 meV, most probably

Table 1 Calculated average radii [9], peak absorption coefficient, peak wavelength of PL and full width at half maximum (FWHM) luminescence intensity of PbS QDs embedded in different glass matrices. Glass

Heat-treatment condition

Calculated average radii (nm)

Absorption coefficient (±0.1 cm1)

Peak wavelength of PL (±2 nm)

FWHM (±4 nm)

BZ

600 °C–20 h 600 °C–50 h 620 °C–20 h 640 °C–20 h

2.3 2.6 3.6 4.7

2.1 4.3 3.0 3.1

1166 1318 1620 1680

160 180 200 500

Z

460 °C–10 h 460 °C–20 h 460 °C–30 h 480 °C–10 h

2.4 2.6 3.2 4.1

6.4 8.9 8.4 7.6

1300 1400 1560 1660

190 220 170 350

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due to the different surface states and sizes of QDs. In addition, fitting of the FWHM values were difficult when the values of experimental temperatures Texp were used instead of the effective local temperatures TEff, and this was especially true when the excitation intensities were high.

and excitation intensities changed. These shifts were due to the changes in the band gap energies induced by the temperatures and heating.

4.2. Effect of glass composition on the optical properties of PbS QDs

This work was financially supported by the SRC/ERC and STAR programs of MOST/KOSEF (R11-2003-006 and M6-0501-000083), Korean Research Foundation Grant funded by the Korea Government (MOEHRD) (KRF-2005-005-J13101) and BK21 Korea.

Glass Z was prepared by adding ZnS and PbO as sources of PbS. PbS QDs precipitated in this glass showed larger absorption coefficients compared to the values observed from BZ glass (Fig. 2). Introducing sulfur in the form of ZnS instead of PbS has several advantages. First, ZnS has a higher melting temperature than PbS and therefore, can reduce the sulfur volatilization during glass melting. Second, it is easier to compensate the evaporation of sulfur and keep the stoichiometry between lead and sulfur in the glass. On the other had, lead and sulfur was introduced in the form of PbS in BZ glass and the final glass usually became lead rich. Optimization of the host glass composition seems to provide a clue to the improvement of absorption coefficients of PbS QDs. 5. Conclusions Absorption and photoluminescence properties of PbS QDs in glasses were investigated. Absorption and PL of PbS QDs in the 1–2 lm wavelength region were realized by controlling the sizes of the QDs through thermal treatment. Peak wavelengths of the photoluminescence shifted as much as 70 nm when temperatures

Acknowledgment

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