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Jianzhong Li SETI Institute, NASA Ames Research Center, Moffett Field, California 94035

Qibing Pei Department of Materials Science and Engineering, University of California, Los Angeles, California 90095

Bin Chena) SETI Institute, NASA Ames Research Center, Moffett Field, California 94035 (Received 3 April 2006; accepted 27 July 2006)

Electrochemically grown cadmium sulfide (CdS) nanorod arrays were studied with Raman spectroscopy. The resonant Raman spectroscopy unravels the enhanced electron-phonon interaction up to the fifth-order multiphonon process in the vertically aligned CdS nanorods after annealing. Resonant Raman scattering at room temperature reveals a surface phonon mode at 253 cm−1 in the annealed nanorod. This unprecedented observation is accounted for by the lateral confinement in the nanorod whose average aspect ratio is approximately 5. An intersubband transition near 3000 cm−1 is also observed. These results point to important optoelectronic applications of this material. I. INTRODUCTION

Cadmium sulfide (CdS) is an important semiconductor material for optoelectronic, photovoltaic, and photodetection applications. For example, it has been recently demonstrated that single CdS nanorod has excellent light-emitting quantum efficiency as a nanorod laser.1 Future applications with CdS rely upon a good understanding of its electronic, optical, and thermal characteristics. To this end, the behaviors and effects of electrons and phonons must be investigated and clarified. In polar semiconductors, which CdS belongs to, optical phonons interact with electrons mainly through the macroscopic electric field that is caused by the relative displacement of the anions and cations. This Fröhlich interaction leads to the formation of polarons composite particles of electrons and phonons, thermalization of photo-excited electrons and holes, and broadening of optical resonances, and so forth. Either normal optical phonons or surface optical (SO) phonons can couple with electrons and excitons via Fröhlich interaction, hence affect the optical properties. SO phonons exist in finite crystals because the periodicity therein is lost. These

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0360 J. Mater. Res., Vol. 21, No. 11, Nov 2006

http://journals.cambridge.org

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vibrational modes involve ions of the top few layers in the proximity of the surface. They are localized at the surface as a consequence of their mode frequencies lying between those of the bulk longitudinal optical (LO) and transverse optical (TO) phonons. Normally, SO phonons are not Raman active as the conservation of momentum cannot be satisfied for a material with perfect crystallinity and surface. It is however proposed that they could be activated as a result of nanorod growth instability.2 Another possible SO phonon origin comes from the finite size and high aspect ratio,3 which was demonstrated by previous Raman studies on nanocrystals4,5 and nanoribbons6 of CdS, curved and