Tue, 20 Jul 2021, 10:00 am - 11:00 am
Gradute Thesis Defense Spring 2021
Manthila Chathurange Rajapakse
University of Lousiville


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 was 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.

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-intercalated  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 the 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 shifting about 1.6 times faster than out-of-plane mode. Further characterizations using 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.

Next, the focus was on the high-pressure response of pristine and Li intercalated BP. Structural evolution of Li-intercalated and pristine BP under high-pressure (up to ∼ 8 GPa) was studied using a diamond anvil cell and in-situ Raman spectroscopy. Though both materials showed a monotonic blueshift of the out-of-plane vibrational mode with pressure, Li-intercalated BP did not show a blueshift until a threshold pressure (2.4 GPa) was reached to compensate the structural expansion caused by intercalation. However, the in-plane modes in each sample responded differently. In the mid-pressure region, they both showed redshifts which in Li-intercalated BP was also followed by abrupt blueshifts. Such behavior indicated pressure-induced structural reorganizations inside the material. Computational modeling revealed the existence of a process of P-P bond breaking and reforming in the system due to the redistribution of intercalated Li atoms under pressure. This work shows the significance of combined effect of pressure and intercalation on structural changes in the search for new phases of BP and other 2D materials.