Intercalation and high pressure studies of black phosphorous - pathways to novel materials and physics.

Abstract

Discovery of graphene in 2004 initiated a new trend of materials known as two-dimensional (2D) materials which have exciting surface properties and anisotropies than their bulk counterparts. Phosphorene, which is the layered version of black phosphorous (BP) is one of the top 2D materials in terms of research interests and applications of the present day. Moving a step further, our interest is to understand the possibilities for structural modifications of phosphorene, by means of stimuli such as intercalation and high-pressure. It has been predicted by theoretical studies that these stimuli may lead to the formation of new structures and phases which widens the applications of these materials. The initial phase of this work was dedicated to synthesize BP in laboratory conditions. In today’s market, 1g of pure black phosphorous crystals costs as high as $800. Thus, high quality BP crystals were grown in our lab using chemical vapor transport technique, and characterized for its quality using several characterization techniques. In the next phase, A systematic study on electrochemical charge transfer in Li-doped black phosphorus (BP) was carried out by both in-situ and ex-situ Raman scattering. Galvanostatic discharge of dedicated in-situ electrochemical cell for Raman spectroscopy was used to study time evolution of vibrational modes under lithiation. In addition to the peak broadening, which is a result of structural expansion, peaks corresponding to all three Raman-active atomic vibrational modes were found to redshift as a result of lithiation. Peaks corresponding to in-plane modes were red-shifted about 1.6 times faster than the out-of-plane mode. Further characterizations using optical and electron microscopy showed that the intercalation of BP is highly anisotropic, where channels along the zigzag direction were found to be the easy direction for intercalation. X-ray diffraction on intercalated samples confirmed a reduction of thickness as lithiation weakens interlayer bonding, thus resulting in a partial exfoliation of BP flakes. Furthermore, first principle studies using density functional theory were used (performed by DR. Ming Yu and Md. Rajib Khan Musa) to develop a theoretical model for the intercalation mechanism. The discrepancy between the experimental and the theoretical results was also addressed. Next, the focus was on the high-pressure response of pristine and Li intercalated BP. Structural evolution of Li-intercalated and pristine black phosphorous (BP) under high-pressure (up to ∼ 8 GPa) was studied using in-situ Raman spectroscopy. Even though both materials showed a monotonic blueshift of the out-of-plane vibrational mode with pressure, Li-intercalated