Optical, mechanical, and electrical properties of nano-structured materials

This thesis includes three main topics covering a broad range of nanoscience: mechanical and optical properties of single walled carbon nanotubes (SWCNTs), optical manipulation and characterization of the crystal structure of silicon nanowires, and a systematic study of minority carrier dynamics in surface-passivated GaAs nanostructures, including nanowires and nanosheets. The physical properties of these materials and related phenomena are briefly reviewed in Chapter 1, including physical, electronic, and phonon structures of SWCNTs, light absorption and phonon confinement in nanocrystals, and surface states in semiconductors.;In Chapter 2, we study the ultimate breaking strain of SWCNTs with in-situ Raman spectroscopy. Chemical vapor deposition (CVD) with a high flow rate is used to synthesize ultra-long suspended SWCNTs across an extendible slit. The G band Raman frequency downshifts linearly with strain and its downshifting rate spans a wide range from -6.2 to -23.6 cm -1/%. This observation corroborates the theoretical prediction that the downshifting rate is strongly chiral dependent. A threshold downshift of 75 cm-1 is observed for the G band lineshape broadening. In this experiment, we achieve strains up to 13.7% without slippage, breakage, and defect formation.;In Chapter 3, we observe emergence of the D band Raman mode in SWCNTs under axial strain. The D to G mode Raman intensity ratio (ID/IG) is seen to increase with strain quadratically by more than a factor of 100. However, the ratio returns to its original pre-strain value, indicating that there is no permanent defect formation. Strain-induced symmetry lowering is proposed to explain the appearance of the D band without a real defect in the lattice. Another possible scenario is a strain-induced reversible transition from sp2 to sp3 bond configuration in a twisted nanotube bundle.;In Chapter 4, we develop a method of locally tailoring crystal structure of individual crystalline-amorphous core-shell silicon nanowires with a polarized laser spot. We are able to control the crystallinity of the silicon nanowires from 0 to 0.93 by controlling the incident laser power. Raman spectroscopy is used to determine the annealing temperature while annealing the nanowires and to characterize the crystal structure before and after annealing. High resolution transmission electron microscopy (HRTEM) is used to confirm the laser-induced crystallization. Due to the one-dimensional nature of nanowires, a strong polarization dependent heating/annealing is observed. The most efficient annealing occurs when the laser polarization is aligned along the axis of the nanowires.;In Chapter 5, we focus on the minority carrier dynamics in AlxGa 1-xAs-passivated GaAs nanowires. With passivation, the minority carrier diffusion length (Ldiff) increases from 30 nm to 180 nm, as measured by electron beam induced current (EBIC) mapping, and the photoluminescence (PL) lifetime increases from sub-60 ps to 1.3 ns. A 48-fold enhancement in the continuous-wave PL intensity is observed on the same individual nanowire with and without the AlxGa 1-xAs passivation layer, indicating a significant reduction in surface recombination. These results indicate that the minority carrier lifetime is not limited by twin stacking faults in passivated nanowires. From the PL lifetime and minority carrier diffusion length, we estimate the surface recombination velocity (SRV) to range from 1.7x103 to 1.1x10 4 cm/s, and the minority carrier mobility is estimated to lie in the range from 10.3 to 67.5 cm2/V.s for the passivated nanowires.;In Chapter 6, we explore the interesting phenomena rising from the over-growth of GaAs nanosheets. Due to the shape anisotropy, we are able to grow stacking fault free nanosheets in the initial growth region. The twin-free crystal structure is confirmed by HRTEM images. However, clear boundaries appear at the interface between initial growth and over-growth regions in both EBIC and SEM contrast images. Possible scenarios, such as sudden changes in the crystal structure, doping, and surface quality, are discussed.