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Effect of size confinement on CdSe nanocrystals in a GeO2 glass matrix characterized by photoacoustic spectroscopy
Physica B 316–317 (2002) 476–478
Effect of size confinement on CdSe nanocrystals in a GeO2 glass matrix characterized by photoacoustic spectroscopy T. Toyoda*, D. Arae, Q. Shen Department of Applied Physics and Chemistry, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
Abstract Change in the electronic states due to quantum confinement effect was studied for CdSe nanocrystals in a GeO2 glass matrix by photoacoustic (PA) spectroscopy. The PA technique is a photothermal method which is useful for optical absorption characterization of opaque samples. In the PA method, one can detect an acoustic energy directly proportional to the thermal energy induced by nonradiative processes. The lowest excited energy of CdSe nanocrystals in a GeO2 glass by quantum confinement effect shifts to lower energy region with the increase of annealing time and temperature, indicating the increase of nanocrystal size. The nanocrystal growing is a diffusion process and the thermal activation energy of ion diffusion during nanocrystal growth is determined to be 1.7 eV. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.40. q; 78.40.Fy; 78.67.Bj Keywords: Photoacoustic spectroscopy; CdSe nanocrystals; Quantum confinement effect
1. Introduction Semiconductor particles with sizes of a few nanometers in a glass matrix show very attractive properties completely different from those of bulk materials [1]. They have potentials both for basic study of the three-dimensional quantum confinement effect in semiconductors and for applications in the field of optoelectronic devices. The study of optical absorption is very important to understand the behavior of semiconductor nanocrystals. However, samples should be sufficiently thin and have good-quality surfaces by pretreatments. The photoacoustic (PA) technique is a photothermal *Corresponding author. Tel.: +81-424-435-464; fax: +81424-435-501. E-mail address: [email protected] (T. Toyoda).
detection technique and it is proved to be useful for investigating the optical absorption of opaque samples [2]. Recently, it was observed that the excitonic transitions of semiconductor nanocrystals were well resolved in PA spectra compared with the corresponding optical absorption spectra [3]. In the present paper, we report the annealing time and temperature dependence of the PA spectra for CdSe nanocrystals in a GeO2 glass matrix to investigate the effect of size confinement on their electronic states.
2. Experimental procedure A mixture of GeO2 (92 mol%), Na2GeO3 (8 mol%), and CdSe (1 wt%) powders was melted
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 5 4 7 - 1
T. Toyoda et al. / Physica B 316–317 (2002) 476–478
at 11801C for 4 h in air. After that, it was quenched in water. The samples of suitable dimensions for the PA measurements were annealed at 4881C in a time range from 1 to 11 h to investigate the annealing time dependence. Also, the samples were annealed for 30 min in a temperature range from 4801C to 6001C to investigate the annealing temperature dependence. The PA cell was composed of an aluminum cylinder with a small channel at its periphery in which a microphone was inserted (gas-microphone method). The light source was a 300 W xenon arc lamp, and the sample holder could be easily removed from the cell to maintain the optical configuration. The modulation frequency was 33 Hz for the measurements using a mechanical chopper. Measurements of the PA spectra were carried out at room temperature in the wavelength range 320–800 nm. The PA spectra were normalized (light intensity normalization) using PA signals from a carbon black sheet.
3. Results and discussion Fig. 1 shows the PA spectra for the samples with different annealing times. The spectra shift to lower energy region with the increase of annealing time and the absorption terrace (k) can be observed. We assume that the lowest excited energy due to quantum confinement effect corre-
sponds to the terrace in the spectrum. Therefore, the lowest excited energy of CdSe nanocrystals in a GeO2 glass shifts to lower energy region with the increase of annealing time. The nanocrystal size can be calculated by the effective mass approximation [4]. The obtained average diameter of CdSe nanocrystal increases from 4.3 to 5.8 nm as annealing time increases from 1 to 11 h. These values are smaller than the twice of the Bohr radius (B5 nm for CdSe bulk material) [4], indicating the possibility of the regime of individual electron and hole confinement. The average diameters of CdSe nanocrystals are shown in Fig. 2 as a function of cubic root of annealing time. Fig. 2 shows that the average diameter of CdSe nanocrystals increase proportionally to cubic root of annealing time within the experimental accuracy. Hence, CdSe nanocrystals in a GeO2 glass grow by a diffusion process in which the diameter depends on the cubic root of annealing time [5]. Fig. 3 shows the PA spectra for the samples with different annealing temperatures. The spectra shift to lower energy region with the increase of annealing temperature and the absorption terrace (k) can be observed. The lowest excited energy of CdSe nanocrystals in a GeO2 glass shifts to lower energy region with the increase of annealing temperature. The obtained average diameter of CdSe nanocrystal increases from 3.7 to 10 nm as the annealing temperature increases from 4881C to
6.0
ANNEALING TIME (h) 11 8 3.5 1
AVERAGE DIAMETER (nm)
PAINTENSITY (ARB. UNITS)
0.8
0.6
0.4
0.2
0.0 1.5
2.0
2.5
477
3.0
3.5
PHOTON ENERGY (eV) Fig. 1. PA spectra for CdSe nanocrystals in a GeO2 glass for different annealing times.
5.5
5.0
4.5
4.0
1.0
1.2
1.4
1.6
1.8 1/3
(ANNEALING TIME)
2.0
2.2
2.4
(HOURS)
Fig. 2. Dependence of the average diameters on cubic root of annealing times.
T. Toyoda et al. / Physica B 316–317 (2002) 476–478
PAINTENSITY (ARB. UNITS)
478
ANNEALING 1.5 TEMPERATURE(˚C) 600 537 513 1.0 488
0.5
0.0 1.5
2.5
2.0
3.0
3.5
PHOTON ENERGY (eV) Fig. 3. PA spectra for CdSe nanocrystals in a GeO2 glass for different annealing temperatures.
6001C. The average diameters of CdSe nanocrystals are shown in Fig. 4 as a function of reciprocal number of annealing temperature (the Arrhenius plot). Fig. 4 shows that the diameter of CdSe nanocrystals decreases in proportion to the inverse of annealing temperature within the experimental accuracy. Therefore, the thermal activation energy of ion diffusion resulting in nanocrystals growth is determined to be 1.67 eV from the Arrhenius plot (Fig. 4). In the future, ultrafast photothermal and photoluminescence measurements will be necessary to provide information regarding the comparison between the radiative and nonradiative processes in CdSe nanocrystals in a GeO2 glass matrix.
(AVERAGE DIAMETER)3 (nm3)
Acknowledgements 103
This work was partially supported by a Grantin-Aid for Scientific Research (No. 09650887) from the Ministry of Education, Science, Sports and Culture.
102
References 11.5
12.0
12.5
13.0
(ANNEALING TEMPERATURE)−1 (×10−4 K−1) Fig. 4. Dependence of the average diameters on inverse of annealing temperature.
[1] N.F. Borrelli, et al., J. Appl. Phys. 61 (1987) 5399. [2] A. Rosencwaig, A. Gersho, J. Appl. Phys. 47 (1977) 64. [3] P. Nandakumar, et al., Solid State Commun. 106 (1998) 193. [4] Y.-S. Yuang, et al., J. Appl. Phys. 76 (1994) 3041. [5] A.I. Ekimov, et al., Solid State Commun. 56 (1985) 921.
Effect of size confinement on CdSe nanocrystals in a GeO2 glass matrix characterized by photoacoustic spectroscopy T. Toyoda*, D. Arae, Q. Shen Department of Applied Physics and Chemistry, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
Abstract Change in the electronic states due to quantum confinement effect was studied for CdSe nanocrystals in a GeO2 glass matrix by photoacoustic (PA) spectroscopy. The PA technique is a photothermal method which is useful for optical absorption characterization of opaque samples. In the PA method, one can detect an acoustic energy directly proportional to the thermal energy induced by nonradiative processes. The lowest excited energy of CdSe nanocrystals in a GeO2 glass by quantum confinement effect shifts to lower energy region with the increase of annealing time and temperature, indicating the increase of nanocrystal size. The nanocrystal growing is a diffusion process and the thermal activation energy of ion diffusion during nanocrystal growth is determined to be 1.7 eV. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.40. q; 78.40.Fy; 78.67.Bj Keywords: Photoacoustic spectroscopy; CdSe nanocrystals; Quantum confinement effect
1. Introduction Semiconductor particles with sizes of a few nanometers in a glass matrix show very attractive properties completely different from those of bulk materials [1]. They have potentials both for basic study of the three-dimensional quantum confinement effect in semiconductors and for applications in the field of optoelectronic devices. The study of optical absorption is very important to understand the behavior of semiconductor nanocrystals. However, samples should be sufficiently thin and have good-quality surfaces by pretreatments. The photoacoustic (PA) technique is a photothermal *Corresponding author. Tel.: +81-424-435-464; fax: +81424-435-501. E-mail address: [email protected] (T. Toyoda).
detection technique and it is proved to be useful for investigating the optical absorption of opaque samples [2]. Recently, it was observed that the excitonic transitions of semiconductor nanocrystals were well resolved in PA spectra compared with the corresponding optical absorption spectra [3]. In the present paper, we report the annealing time and temperature dependence of the PA spectra for CdSe nanocrystals in a GeO2 glass matrix to investigate the effect of size confinement on their electronic states.
2. Experimental procedure A mixture of GeO2 (92 mol%), Na2GeO3 (8 mol%), and CdSe (1 wt%) powders was melted
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 5 4 7 - 1
T. Toyoda et al. / Physica B 316–317 (2002) 476–478
at 11801C for 4 h in air. After that, it was quenched in water. The samples of suitable dimensions for the PA measurements were annealed at 4881C in a time range from 1 to 11 h to investigate the annealing time dependence. Also, the samples were annealed for 30 min in a temperature range from 4801C to 6001C to investigate the annealing temperature dependence. The PA cell was composed of an aluminum cylinder with a small channel at its periphery in which a microphone was inserted (gas-microphone method). The light source was a 300 W xenon arc lamp, and the sample holder could be easily removed from the cell to maintain the optical configuration. The modulation frequency was 33 Hz for the measurements using a mechanical chopper. Measurements of the PA spectra were carried out at room temperature in the wavelength range 320–800 nm. The PA spectra were normalized (light intensity normalization) using PA signals from a carbon black sheet.
3. Results and discussion Fig. 1 shows the PA spectra for the samples with different annealing times. The spectra shift to lower energy region with the increase of annealing time and the absorption terrace (k) can be observed. We assume that the lowest excited energy due to quantum confinement effect corre-
sponds to the terrace in the spectrum. Therefore, the lowest excited energy of CdSe nanocrystals in a GeO2 glass shifts to lower energy region with the increase of annealing time. The nanocrystal size can be calculated by the effective mass approximation [4]. The obtained average diameter of CdSe nanocrystal increases from 4.3 to 5.8 nm as annealing time increases from 1 to 11 h. These values are smaller than the twice of the Bohr radius (B5 nm for CdSe bulk material) [4], indicating the possibility of the regime of individual electron and hole confinement. The average diameters of CdSe nanocrystals are shown in Fig. 2 as a function of cubic root of annealing time. Fig. 2 shows that the average diameter of CdSe nanocrystals increase proportionally to cubic root of annealing time within the experimental accuracy. Hence, CdSe nanocrystals in a GeO2 glass grow by a diffusion process in which the diameter depends on the cubic root of annealing time [5]. Fig. 3 shows the PA spectra for the samples with different annealing temperatures. The spectra shift to lower energy region with the increase of annealing temperature and the absorption terrace (k) can be observed. The lowest excited energy of CdSe nanocrystals in a GeO2 glass shifts to lower energy region with the increase of annealing temperature. The obtained average diameter of CdSe nanocrystal increases from 3.7 to 10 nm as the annealing temperature increases from 4881C to
6.0
ANNEALING TIME (h) 11 8 3.5 1
AVERAGE DIAMETER (nm)
PAINTENSITY (ARB. UNITS)
0.8
0.6
0.4
0.2
0.0 1.5
2.0
2.5
477
3.0
3.5
PHOTON ENERGY (eV) Fig. 1. PA spectra for CdSe nanocrystals in a GeO2 glass for different annealing times.
5.5
5.0
4.5
4.0
1.0
1.2
1.4
1.6
1.8 1/3
(ANNEALING TIME)
2.0
2.2
2.4
(HOURS)
Fig. 2. Dependence of the average diameters on cubic root of annealing times.
T. Toyoda et al. / Physica B 316–317 (2002) 476–478
PAINTENSITY (ARB. UNITS)
478
ANNEALING 1.5 TEMPERATURE(˚C) 600 537 513 1.0 488
0.5
0.0 1.5
2.5
2.0
3.0
3.5
PHOTON ENERGY (eV) Fig. 3. PA spectra for CdSe nanocrystals in a GeO2 glass for different annealing temperatures.
6001C. The average diameters of CdSe nanocrystals are shown in Fig. 4 as a function of reciprocal number of annealing temperature (the Arrhenius plot). Fig. 4 shows that the diameter of CdSe nanocrystals decreases in proportion to the inverse of annealing temperature within the experimental accuracy. Therefore, the thermal activation energy of ion diffusion resulting in nanocrystals growth is determined to be 1.67 eV from the Arrhenius plot (Fig. 4). In the future, ultrafast photothermal and photoluminescence measurements will be necessary to provide information regarding the comparison between the radiative and nonradiative processes in CdSe nanocrystals in a GeO2 glass matrix.
(AVERAGE DIAMETER)3 (nm3)
Acknowledgements 103
This work was partially supported by a Grantin-Aid for Scientific Research (No. 09650887) from the Ministry of Education, Science, Sports and Culture.
102
References 11.5
12.0
12.5
13.0
(ANNEALING TEMPERATURE)−1 (×10−4 K−1) Fig. 4. Dependence of the average diameters on inverse of annealing temperature.
[1] N.F. Borrelli, et al., J. Appl. Phys. 61 (1987) 5399. [2] A. Rosencwaig, A. Gersho, J. Appl. Phys. 47 (1977) 64. [3] P. Nandakumar, et al., Solid State Commun. 106 (1998) 193. [4] Y.-S. Yuang, et al., J. Appl. Phys. 76 (1994) 3041. [5] A.I. Ekimov, et al., Solid State Commun. 56 (1985) 921.