Influence of the Particle Size on Acoustic Phonon Modes of ZnO Nanocrystals
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Influence of the Particle Size on Acoustic Phonon Modes of ZnO Nanocrystals Harish Kumar Yadav1, K. Sreenivas1, Vinay Gupta1, S. P. Singh2, B. Sundarakannan3, and R. S. Katiyar3 1 Department of Physics and Astrophysics, University of Delhi, Delhi, 110007, India 2 National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi, 110012, India 3 Department of Physics, University of Puerto Rico, San Juan, 00931-3343, Puerto Rico ABSTRACT
Size dependence of the low frequency vibrational spectra of ZnO nanocrystals prepared using chemical method has been investigated. Optical transmission spectra of the ZnO colloid solution exhibit a shift in the onset of the absorption band edge from 332 to 350 nm due to particle growth. X-ray diffraction analysis of the prepared ZnO nanocrystals exhibit peaks corresponding to the hexagonal wurtzite structure. Two peaks with unusually very high intensity were observed in the low frequency (~ 10- 25 cm-1) Raman spectra of these nanocrystals. The peak position of these phonon modes shifted towards lower frequencies as the size of the nanocrystals increases and assigned to the confinement of acoustic phonons in ZnO nanocrystals. INTRODUCTION Zinc oxide (ZnO) nanostructures have received a broad attraction due to their distinguished performance in the fields of electronics, optics and photonics [1]. With the reduction in the size of these nanostructures novel electrical, mechanical, chemical and optical properties are introduced that are different from the related bulk crystals [2]. These properties are usually originated from the surface and quantum confinement effects because of the small size of these systems [3]. These nanocrystals are the ideal system for studying the transport process in the confinement limit which is of benefit in understanding the fundamental phenomena in low dimensional systems [1-3]. Size dependent shift in the optical phonons of ZnO nanoparticles were reported by Rajalakshmi et al. and Alim et al. using Raman spectroscopy [3,4]. However, no one reported the confinement of acoustic phonons in ZnO nanocrystals. The low frequency Raman phonon modes are due to the vibrational motion of the nano-crystallites and play a crucial role in the optical properties of these nanocrystals. Acoustic phonons in metal or semiconductor nanocrystals embedded in glass matrix are reported by other groups [5-12]. The peak frequency of these acoustic modes was shown to disappear or shift towards lower frequencies as the particle size increases. In the present work, we investigate the size dependence of the low frequency Raman spectra of the ZnO nanocrystals. The observed size dependence of the low frequency acoustic modes in the polarized Raman spectra were compared to the theoretical calculations based on the Lamb’s theory [14] and is shown to be originated from the confinement of acoustic phonons in spherical ZnO nanocrystals.
EXPERIMENT The synthesis of ZnO nanocrystals was performed by modified sol-gel method reported by Spanhel and Anderson under ambient temperature
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