Physica E 31 (2006) 93–98 Optical properties of CdS nanoparticles upon annealing V. Sivasubramanian à , A.K. Arora, M. Premila, C.S. Sundar, V.S. Sastry Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 20 May 2005; received in revised form 12 September 2005; accepted 10 October 2005 Available online 15 November 2005 Abstract The metastable cubic phase of CdS has been found to be stabilized in the form of nanoparticles. Zinc-blende to Wurtzite structural transformation of CdS nanoparticles, synthesized using chemical precipitation, was investigated using X-ray diffraction (XRD), Raman, photoluminescence (PL) and infrared (IR) absorption spectroscopy. The nanocrystalline powder was annealed in argon atmosphere in the temperature range 473–773 K for 2 h at each temperature. The hexagonal fraction increased monotonically during annealing and the shape of the particle becomes anisotropic. PL spectra exhibited a marginal decrease in peak position for annealing up to 573 K and then an increase. In Raman spectra, the intensity of 1-LO phonon decreases while that of 2-LO phonon increases indicating an increase in electron–phonon interaction with increase in particle size. In addition to the Frohlich surface optical phonon mode and TO phonon mode, a new mode at 195 cm 1 is found in IR spectra, which is attributed to a defect-activated zone-boundary phonon. The changes in the optical properties are attributed to those arising from particle growth and structural transformation. r 2005 Elsevier B.V. All rights reserved. PACS: 61.46.+w; 78.30.j; 78.55.m Keywords: Nanoparticles; Electron–phonon interaction; Frohlich mode and photoluminescence 1. Introduction The electronic and optical properties of II–VI compound semiconductor nanoparticles have been extensively inves- tigated in view of a wide variety of applications. With decrease in the particle size, dramatic modification of their electronic and optical properties takes place due to the three-dimensional quantum confinement of electrons and holes when the size of the particle approaches the Bohr radius of exciton [1,2]. In addition to the change in the electronic and optical properties, the structural behaviour also exhibits changes with reduction in the size of the particle. It is well known that bulk CdS has stable wurtzite (hexagonal) structure from room temperature to melting point. However, metastable cubic phase has been found in thin films and nanocrystalline powders [3,4]. Although the coexistence of cubic and hexagonal phases has been reported for CdS [5], a quantitative analysis of the volume fractions and growth characteristics upon annealing was not carried out. Furthermore, nanoparticles of CdS have been reported to melt at a substantially lower temperature [6]. Although among semiconductor nanoparticles CdS has been most extensively studied, there is considerable diversity in the results that are reported. For example, the structural phase transition in CdS nanocrystalline thin films prepared by chemical bath deposition has been investigated using Raman [7], photoluminescence (PL) [2] and optical absorption [8]. A decrease in the band gap [8] and also the energy of the green PL [2] has been found upon annealing within the cubic phase. Bon et al. [8] tentatively attribute it to structural disorder prior to the cubic–wurtzite transition. Several PL studies have been reported on CdS nanoparticles prepared by various methods. For example, surface capped CdS nanoparticles [9] and those dispersed on GeO 2 [10] glass exhibit emission from recombination from defects occurring at the energies lower than the bulk band-edge. Also, certain aspects of the metastable cubic to the wurtzite phase transition upon annealing have not been well understood. Yellow-green ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.10.001 à Corresponding author. Tel.: +914427480347; fax: +914427480081. E-mail address: shiva@igcar.ernet.in (V. Sivasubramanian).