Exploring novel physical properties of nanowires and nanocrystals: Part I. Optical properties and device applications of nanowires. Part II. Mechanical and tribological properties of nanocrystals

Nanometer scale structures are promising candidates for use in nanoelectronics and nanoelectromechanical systems. Developing an understanding of the fundamental physical properties of these nanostructures is therefore crucial for realizing their potential applications. In this thesis, we report studies of the optical properties of semiconductor nanowires and the mechanical properties of nanocrystals.; In the first part of this thesis, we discuss the synthesis, optical characterization, and preliminary device applications of semiconductor nanowires. We developed a general synthetic method to grow semiconductor nanowires with both diameter and length control. The ability to control the physical size of semiconductor nanowires allowed us to undertake size-dependent optical studies on individual nanowires. We focused on indium phosphide (InP) nanowires because InP is a direct band gap semiconductor with strong photoluminescence. We found that photoluminescence emission of InP nanowires systematically blue shifts with decreasing nanowire diameter and quantitative analysis with an effective mass model showed that the data are in agreement with radial quantum confinement. Moreover, we observed a giant photoluminescence polarization anisotropy from individual InP nanowires. We further exploited this striking polarization anisotropy to fabricate light-emitting diodes with highly polarized light emission and polarization-sensitive photodetectors using individual InP nanowires.; In the second part of this thesis, we discuss tribological and mechanical properties of molybdenum trioxide (MoO₃) nanocrystals. First, we characterized the tribological behavior of the model lubricant molybdenum disulphide (MoS₂) as a function of thermal oxidation with scanning tunneling microscopy and atomic force microscopy (AFM) in ultrahigh vacuum. We found a direct relationship between friction and defect density caused by thermal oxidation. After longer oxidation times when well-defined MoO ₃ nanocrystals were present, we observed load-independent friction. Second, we measured friction forces both for sliding MoO₃ nanocrystals on atomically smooth MoS₂ terraces and for moving nanocrystals down MoS₂ surface steps and found that the friction forces for moving down steps are about five times larger than those for sliding on terraces. Lastly, we used AFM to bend MoO₃ nanocrystal nanoplates and determined that the Young's moduli of these MoO₃ nanocrystals are significantly smaller than that of bulk MoO₃. This novel behavior was further substantiated in subsequent experiments where MoO₃ nanocrystals showed enormous flexibility when slid over multilayer MoS₂ steps.