Paper EN-ThP11
Study of Optical and Thermal Properties Of CdTe Quantum Dots Using Photoacoustic Spectroscopy
Thursday, October 21, 2010, 6:00 pm, Room Southwest Exhibit Hall
| Session: |
Energy Frontiers Topical Conference Poster Session |
| Presenter: |
N. Al-Hosiny, Taif University, Saudi Arabia |
| Authors: |
A. Badawi, Taif University, Saudi Arabia
N. Al-Hosiny, Taif University, Saudi Arabia
S. Abdallah, Taif University, Saudi Arabia and Ain Shams University, Egypt
S. Negm, Ain Shams University, Egypt
H. Talaat, Ain Shams University, Egypt
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| Correspondent: |
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Semiconductor nanoparticles are currently of great interest for their industrial applications in a variety of optoelectronic devices specially solar cells. Recently, CdTe semiconductor quantum dots (QDs) have become one of the promising materials for high efficiency photovoltiac solar cell. Therefore, massive attention has been devoted to investigate their optical and thermal properties in order to improve the performance of the solar cell. In this work, CdTe QDs were fabricated by the chemical solution deposition (CD) technique [1]. Four samples of increasing time of growth leading to different sizes labeled (a–d) were obtained The sizes of these samples were estimated using transmission electron microscope (TEM); as an sample (d) is shown in Fig. (1)
NOTE: WE ARE UNABLE TO INCLUDE FIGURES
Photoacoustic spectroscopy (PA) was employed to study both the optical and thermal properties of the samples at room temperature and modulation frequency f = 15Hz. Fig ( 2) show the PA spectra for the four different particle sizes samples (a–d) in the wavelength range 400–700 nm. The absorption edges shift towards lower energy region with increasing size , from (a) at 540 nm to (d) at 595 nm. The PA spectra give typical results with those obtained by regular UV-Vis absorption, though the samples were in colloidal form for UV-Vis and in powder form for the PA. The sizes of the CdTe nanocrystals were calculated using the effective mass approximation (EMA) model [2].
NOTE: WE ARE UNABLE TO INCLUDE FORMULAS
where m* is the reduced electron-hole mass, Eg is the bulk crystal band gap, R is the average radius of the nanocrystal, Eg(nano) is the lowest energy for electronic transition and h is the Planck’s constant. The calculated average diameters of CdTe nanocrystal show an increase from 2.13 nm for (a) to 2.43 nm for (d), in agreement with the values determined by TEM.
NOTE: WE ARE UNABLE TO INCLUDE FIGURES
The PA technique was also employed to investigate the thermal properties of the CdTe QDs which are of great importance to solar energy conversion. The powder of each sample size was compressed into a disk. The PA signal amplitude was recorded at various chopping frequencies for each sample (depth profile analysis) using the Argon laser at wavelength 514 nm. The plot of ln PA amplitude versus the ln f. (Fig. 3) shows a distinct change in slope, at the characteristic frequency (fc ) where the sample changes from being thermally thick to thermally thin. The thermal diffusivity (D), was then calculated using the relation [4].
NOTE: WE ARE UNABLE TO INCLUDE FORMULAS
where L is the thickness of the sample. The calculated diffusivity varies from 0.65 cm2/s for (a) to 0.38 cm2/s for (d). These values of the thermal diffusivity are at least one order of magnitude larger than the bulk value (0.05 cm2/s) [4]. Our results are in agreement with the results of other authors [4,5], where the decrease in thermal diffusivity with increasing the particle size is attributed to the decrease in the population of phonons.
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Huan-Tsung Chang , J. Mater. Chem., 2009, 19, 2349–2355
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[ 3.] Al.L. Efros, A.L. Efros, Sov. Phys. Semicond. 16, 772 (1982)
[ 4] P. Raji, C. Sanjeeviraja, K. Ramachandran, Cryst. Res. Technol. 39, 617 (2004) [5] T. El- Brolossy, S. Abdallah,, S. Negm and H. Talaa Eur. Phys. J. 153, 361–
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