INTERCALATION AND HIGH-PRESSURE INVESTIGATIONS OF BLACK ARSENIC PHOSPHORUS: UNRAVELING MATERIAL TRANSFORMATIONS

Speaker:Dinushika Vithanage (PhD Defense)

Abstract: Black arsenic phosphorus (b-As_yP_1-y) alloys have emerged as intriguing materials within the realm of two-dimensional (2D) materials, following the discovery of black phosphorus (BP). These alloys possess capability to overcome major limitations of BP and exhibit potential for tunability and enhancement of properties making them promising materials for a wide range of applications. Inspired by the intriguing findings obtained for BP, this research focuses on understanding the structural modifications that can be achieved in b-As_yP_1-y alloys through the application of stimuli such as intercalation and high pressure.
In the initial phase, b-As_yP_1-y alloys were synthesized using the chemical vapor transport (CVT) method and thoroughly characterized. The subsequent investigation focused on studying the structural evolution of b-As_yP_1-y alloys during lithium (Li) intercalation, with varying As concentration (y). In-situ Raman spectroscopy, facilitated by a dedicated in-situ electrochemical cell, was employed to analyze the real-time vibrational modes of the alloys during Li intercalation. The vibrational modes of b-As_yP_1-y alloys encompass eight distinct modes, representing P-P bonds (A¹_g, A²_g, 𝐵_2𝑔), As-As bonds (A¹_g, A²_g, 𝐵_2𝑔), and As-P bonds (two modes). During the initial stages of the intercalation process, a monotonic redshift was observed in all vibrational modes of b-AsyP1-y samples due to the softening of each mode caused by the intercalation-driven donor-type charge-transfer from Li to b-As_yP_1-y. Above a specific intercalation threshold, the emergence of a new peak, identified as the E_g mode of gray As, indicated the presence of an intercalation-driven structural phase segregation process. Furthermore, the A_1g mode of gray As emerged after this intercalation threshold and closely overlapped with the A²_g Raman mode of b-As_yP_1-y. Beyond the intercalation threshold, all peaks exhibited an upshift due to the co-existence of gray As with b-As_yP_1-y alloys, causing strain and phonon mode hardening. In the sample with the highest As concentration, phase segregation occurred during the synthesis process. Computational modeling revealed the co-existence of gray As in b-As_yP_1-y alloys with high As concentrations and the occurrence of local structural segregation during the intercalation process at a critical Li concentration.
In the final stage, the structural evolution of b-As_yP_1-y alloys under hydrostatic pressure was investigated using in-situ Raman spectroscopy with a Diamond Anvil Cell (DAC). The experiments revealed pressure-induced changes in vibrational modes, leading to the observation of two distinct pressure regimes. In Region I, all vibrational modes showed a monotonic upshift, indicating phonon hardening due to hydrostatic pressure. In Region II, As_0.4P_0.6 and As0.6P0.4 alloys displayed a linear blueshift at a reduced rate, suggesting local structural reorganization with less bond compression. Notably, As_0.8P_0.2 exhibited anomalous behavior in Region II. Interestingly, the pressure range also revealed the emergence of new peaks corresponding to the E_g mode and A_1g mode of the gray-As phase, indicating compressive strain-induced structural changes. The anomalous changes in Region II confirmed the formation of a new local structure, characterized by elongation of the P-P, As-As, and As-P bonds along the zigzag direction within the b-As_yP_1-y phase, possibly near the grain boundary. Additionally, the gray-As phase underwent compressive structural changes. This study highlights the significance of intercalation and pressure in inducing structural transformations and exploring novel phases in two-dimensional (2D) materials, including b-As_yP_1-y alloys.