Fabrication, Characterization and Thermal Properties of Nanowires

Abstract

The physical properties of nanomaterials are novel and completely different from those in conventional bulk materials. The new discovered properties of nanomaterials not only reflected in electronic transports but also exhibited in optics and magnetism. For past decades, intensive and broad studies on nanoscience and nanotechnology were initialized all over the world. Nanoparticles (or quantum dots) are the first candidate of nanomaterials for such a study. In past the most important findings in nanoparticles are phonon softening, electronic energy level splitting and red shift etc. However due to their small size and dispersed characteristic, only few limited applications have been developed, the examples are the quantum dots in matrix for opto-electronics, dispersed nanoparticles for drug carries and thermal therapy of cancers etc. For above reasons, the study of nanowires (NWs) stimulates a great interest in their fundamental scientific researches and potential applications. The size effects, in general, have two aspects: one is the Quantum Size Effect (QSE), which originates from the variation of electronic density-of-states in nanostructure. QSE can thus derive a drastic change of transport and thermodynamic properties from those of the bulks. In contrast to Quantum Size Effect (QSE), the physical properties changed due to vast surface atoms, grain boundaries and structural deficiency are categorized to NonQuantum Size Effect (NQSE), with no doubt this also inevitably plays an important role in the novel properties of nanowires. For examples, thermal and electrical conductivity of nanowires are significantly reduced as a result of electron/phonon scatterings occurring at grain boundaries of nanoscale crystallization. One of the most important characteristics of nanowire is its substantial small thermal conductivity. Thus, from manipulated materials to low-dimensional nanowires, it provides an innovative strategy to improve heat-electricity conversion efficiency for thermoelectric materials. In the content of this chapter, two main sections will be included, the first section is the fabrication and characterization of nanowires, and the second section is the measurement techniques and setups for Seebeck coefficient, electrical conductivity and thermal conductivity. For nanowire fabrication, two categories of fabrication method will be included. The first category is called the Bottom-up method, in which the nanowires were formed selfassembly. The second category of nanowire fabrication is the Top-down method, in which many patterning and etching processes are required for forming individual nanowires.