Raman scattering from Zn/ZnO core-shell nanoparticles

Abstract

ABSTRACT We have synthesized Zn/ZnO core-shell nanoparticles by pulsed laser ablation in liquid (PLAL) media using nanosecond pulsed Nd:YAG laser. The formation of crystalline core-shell nanoparticles of varying core and shell thickness with varying SDS concentration is confirmed by HRTEM images. The optical absorption shows distinct features corresponding to ZnO exciton and Zn surface plasmon. Raman spectrum from Zn/ZnO core-shell nanoparticles shows E 2 (high) phonon modes of the bulk which are insensitive to the size and modes unique to the core-shell structures. Moreover, the surface optical mode is dominant feature of the nonresonant spectrum. We have also examined the wavelength dependence of the phonon modes in Zn/ZnO core-shell structure. Keywords: Raman Scattering, Zn/ZnO core-shell, laser ablation. 1. INTRODUCTION ZnO is one of the promising semiconductor materials applied to ultraviolet (UV) emitting devices, transparent electrodes and sensing devices. As a direct wide band gap (3.4 eV) semiconductor material, ZnO has unique electrical and optical properties such as low dielectric constant, high chemical stability, good photoelectric and piezoelectric behaviors and the extreme stability of excitons due to large exciton binding energy (60 meV). Further the nanocrystallization can enhance the optical and electrical properties of ZnO by the quantum confinement effect. The Zn/ZnO core shell nanostructures are of great technological interest as the Zn/ZnO interface in the core-shell nanostructure acts as natural metal/semiconductor junction. The synthesis and understanding of the growth of ZnO nanostructures and their applications are at the leading edge of todays research in nanotechnology. Furthermore, Zn/ZnO core shell nanoparticles are ideal candidate for functional building blocks in nanoelectronics/optoelectronics. The core-shell structured nanoparticles have the advantage of tuning and tailoring their physical properties by designing the chemical compositions as well as sizes of the core and the shell. Various ZnO nanostructures, such as nanobelts, nanoribbons, nanowires, nanotubes, nanohelixes and nanorods, have been fabricated by vapor phase techniques, such as metalorganic vapor phase epitaxy (MOVPE), metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and by solution phase processes such as solvothermal process and impregnation method [1]. However, these preparation methods involve complex procedures, sophisticated equipment, and rigorous experimental conditions. Moreover, a metal catalyst is necessary for crystalline growth in some vapor phase techniques. In recent years Pulsed Laser Ablation in Liquid media (PLAL) has emerged as a powerful technique in material science for fabricating metal [2-3] and metal-oxide nanoparticles [2, 4 -11]. PLAL is a versatile technique for preparing many kinds of nano-scaled materials which include metal nanoparticles, metal oxide nanoparticles, nano-composites etc.The PLAL technique requires inexpensive equipment and no vacuum system. It is capable of generating large amount of NPs in the form of colloidal without the formation of by-products. The entire product can be completely collected in solutions, and the obtained colloid solution is very easy to handle. PLAL is capable of producing NP without any surface-active agents or counter-ions, which is indispensable for some applications, e.g. for medical applications. This technique require minimum amount of chemical and hence it is chemically clean and simple technique. The most important advantage of PLAL is the ability to directly produce crystalline nanoparticles without any subsequent heat treatment, which cannot be achieved by other fabrication methods such as solid-state or chemical methods [2-11]. *ravisoni@physics.iitd.ac.in; phone 91 11 26591304; fax 91 11 26591114