Revisiting the problem of crystallisation and melting of selenium.

In this paper, we study the structure of the solid selenium (Se) formed by the vapor deposition method. We provide direct visual evidence that faceted crystal-like shapes obtained from vapor phase deposition are a self-assembly of linear strands that have a persistence length of 10µm. These strands are held together by weak forces and can easily be separated. These chains occasionally get entangled to form chiral structures and often meander about destroying long range orientation and translation order in a continuous manner. Moreover, it is easy for the long strands of linear chains to slide past the neighboring ones, and hence the system has a large concentration of disinclination like defects in addition to the defects caused by the entanglement of the chains. Like organic polymers, the obtained Se structures also exhibit a spread in the melting temperature. This spread is closely related to the density of the sub-structures present in the system. The infrared imaging shows that these structures heat up in an inhomogeneous manner and the cross polarized images show that the process of melting initiates in the bulk.

Growth, structural and optical characterization of wurtzite GaP nanowires.

Bulk gallium phosphide (GaP) crystallizes in the zinc-blende (ZB) structure and has an indirect bandgap. However, GaP nanowires (NWs) can be synthesized in the wurtzite (WZ) phase as well. The contradictory theoretical predictions and experimental reports on the band structure of WZ GaP suggest a direct or a pseudo-direct bandgap. There are only a few reports of the growth and luminescence from WZ and ZB GaP NWs. We first present a comprehensive study of the gold-catalyzed growth of GaP NWs via metalorganic vapor phase epitaxy on various crystalline and amorphous substrates. We optimized the growth parameters like temperature, pressure and reactant flow rates to grow WZ GaP NWs with minimal taper. These wires were characterized using electron microscopy, x-ray diffraction, Raman scattering and photoluminescence spectroscopy. The luminescence studies of bare GaP NWs and GaP/AlGaP core-shell heterostructures with WZ- and ZB-phase GaP cores suggest that the WZ-phase GaP has a pseudo-direct bandgap with weak near-band-edge luminescence intensity.

Understanding the effect of nanowire orientation on time evolution of Raman spectra from laser irradiated InAs nanowire surface.

We investigate differences observed in the time evolution of Raman spectra for differently oriented (in plane) InAs nanowires (NWs), using polarized Raman spectroscopy. Specially designed polarized Raman spectroscopy experiments elucidate that laser irradiation leads to the formation of an oriented crystalline oxide film on the InAs NW surface. Both the formation of oriented crystalline oxides and Raman selection rules leading to the presence/absence of oxide peaks in the unpolarized Raman spectra are uncommon occurrences and can lead to incorrect interpretations of the oxidation process, if not looked into carefully. Further, the specially designed heating and cooling experiments for a mixed phase (wurtzite + zinc blende) InAs NW revealed the formation of specific allotropes of elemental As, i.e. gray-As (rhombohedral) and black-As (orthorhombic: metastable) at low (700-950 K) and high simulated temperatures (1000-1300 K) on the InAs NW surface, respectively. Both have high electrical conductivity due to a layered structure and control over the growth of only a few layers using laser irradiation envisages properties similar to graphene. This kind of surface of InAs NWs has the potential for novel device applications, where a semiconductor-insulator-metal heterostructure is required.