Synthesis and Optical Properties of Oriented Cu Nanoparticles
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Synthesis and Optical Properties of Oriented Cu Nanoparticles Om Parkash and P. Sen School of Physical Sciences, Jawaharlal Nehru University New Delhi – 110067, India. ABSTRACT Crystallographic orientation of atomic planes makes a metal amenable to specific properties as desired in catalysis and environment sensing. Oriented metal nanoparticles are however less known. Here we present X-ray diffraction, transmission electron microscopy and atomic force microscopy data to provide evidence of copper nanoparticles with preferred orientation, achieved by electro-explosion of copper wires in an aqueous medium. The optical absorption in these particles is devoid of the usual Mie resonance at ∼ 580 nm, while maintaining bulk-like lattice periodicity. Under the highest reorientation (largest orientation index), absorption in the ultraviolet region is characterized by distinct and sharp resonant peaks, which are correlated with occupied valence band density of state (DOS) from copper clusters, making these truly crystalline particles with atom-like electronic properties. INTRODUCTION Metallic nanoparticles have been a source of immense interest due to their novel properties, different from those of metal atoms and bulk metal. They have remarkable physical and chemical properties. This make them candidates with significant potential in a wide range of applications such as catalysis, magnetic recording, optoelectronics, magnetic fluids, composite materials, fuel cells, pigments and sensors [1-3]. Uniqueness in properties of nanoparticles arises due to their high surface area to volume ratio, resulting from their reduced size. Crystallographic orientation of atomic planes makes a material further tuned to specific properties as desired in catalysis and environment sensing [4, 5]. Metallic nanoparticles absorb in the visible range of the optical spectrum due to surface plasmons [6]; surface plasmons are collective oscillations of the conduction electrons. In small nanoparticles (< 10 nm) the energy levels are discrete in nature and are widely spaced; so due to quantum confinement effects, this significantly reduces the number of electrons that can be transferred to the surface plasmon states during optical absorption, passing over to single electron excitations between quantized levels, broadening the plasmon absorption peak [7]. Here we focus attention on copper nanoparticles, with the prospect of various applications as catalysts, lubricants, environment sensors and electronic materials to name a few [8, 9]. Several techniques have been employed in their preparation, such as, chemical reduction, laser irradiation, flow levitation, reverse micelles, and hydrothermal method [10-12]. In this work, we employ an adaptation of the electro-explosion of wires (EEW) technique for preparation of copper nanoparticles, a process in which high
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currents are passed through a needle - plate geometry [13, 14]. Forces rupturing the metallic wire and plate arise from the imbalance of the self induced electromotive
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