We report depressurization amorphization of single-crystal boron carbide (B4C) investigated by in situ high-pressure Raman spectroscopy. It was found that localized amorphization of B4C takes place during unloading from high pressures, and nonhydrostatic stresses play a critical role in the high-pressure phase transition. First-principles molecular dynamics simulations reveal that the depressurization amorphization results from pressure-induced irreversible bending of C-B-C atomic chains cross-linking 12 atom icosahedra at the rhombohedral vertices.
The practical applications of high-capacitance polypyrrole (PPy) as an active material for electrochemical supercapacitors are mainly limited by their poor cycling stability. By utilizing nanoporous gold enhanced Raman scattering of PPy, we investigated ex situ and in situ the structural origins of the low electrochemical stability of PPy. This study provided direct evidence that the stability degradation of PPy is caused by the irreversible transition between oxidation and reduction states of PPy, which is associated with the permanent deformation of PPy molecular chains subjected to applied potentials.
The molecular beam epitaxy growth of topological insulator Bi 2 Te 3 thin films on Si(111) substrates has been investigated in situ by low-energy electron microscopy. The crystal structure and surface morphology during growth were directly revealed, which enables us to identify the optimal growth conditions for single crystalline Bi 2 Te 3 films. The formation of thin films is preceded by several surface structures, including a wetting layer and a Te/Bi-terminated Si(111)-1Â1 reconstruction. Raman scattering spectra and AFM measurements indicate that, under Te-rich conditions, single crystalline films of Bi 2 Te 3 grow along the [111] direction in a layer-by-layer mode. Transport measurements prove the insulating behavior of the films grown in this way.
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