Date on Master's Thesis/Doctoral Dissertation

8-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Sumanasekera, Gamini

Committee Co-Chair (if applicable)

Jasinski, Jacek

Committee Member

Jasinski, Jacek

Committee Member

Yu, Ming

Committee Member

Jayanthi, Chakram

Author's Keywords

black arsenic phosphorus; intercalation; high-pressure; diamond anvil cell; Raman; XRD

Abstract

Black arsenic phosphorus (b-AsyP1-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, including lithium-ion batteries. Inspired by the intriguing findings obtained for BP, this research focuses on understanding the structural modifications that can be achieved in b-AsyP1-y alloys through the application of intercalation and high pressure. The initial phase of our investigation was dedicated to synthesizing b-AsyP1-y alloys under controlled laboratory conditions using the chemical vapor transport (CVT) method. The characterization techniques revealed a structural expansion correlated with increasing arsenic (As) concentration. For the highest As concentration, segregation of a new phase (identified as g-As) from the b-AsyP1-y phase was observed. The subsequent phase focused on investigating the structural evolution of b-AsyP1-y alloys during lithium (Li) intercalation, with varying As concentration (y). In-situ Raman spectroscopy was utilized to analyze the real-time vibrational modes of the alloys during Li intercalation, employing a dedicated in-situ electrochemical cell. The vibrational modes of b-AsyP1-y alloys encompass a total of eight distinct modes, corresponding to P-P bonds (A1g, A2g, ), As-As bonds (A1g, A2g, ), and As-P bonds (two modes). A monotonic redshift of all the vibrational modes of all b-AsyP1-y samples was observed during the initial stages of the intercalation process due to the softening of each mode caused by the intercalation driven donor-type charge-transfer from Li to b-AsyP1-y. The emergence of a new peak identified as the Eg mode of gray As, above an intercalation threshold, is indicative of the presence of an intercalation-driven structural phase segregation process. Further, A1g mode of gray As emerging after this intercalation threshold lies in close proximity of A2g Raman mode of As-As bonds in b-AsyP1-y. All the peaks beyond the intercalation threshold show an upshift due to the co-existence of gray As with b-AsyP1-y alloys causing strain and thus hardening of the phonon modes. In the sample with the highest As concentration phase segregation takes place during the synthesis process. Computational modeling reveals the co-existence of gray As in the b-AsyP1-y alloys with high As concentrations. It also confirms the existence of a local structural segregation taking place at a critical Li concentration during the intercalation process.

The final stage of this project was focused on the structural evolution of b-AsyP1-y alloys under hydrostatic pressure using in-situ Raman spectroscopy. High-pressure experiments were conducted using a Diamond Anvil Cell (DAC), which revealed pressure-induced shifts in vibrational modes in b-AsyP1-y alloys. Two distinct pressure regimes were observed. In the first regime (Region I), all vibrational modes exhibited a monotonic upshift, indicating phonon hardening due to hydrostatic pressure. In the second regime (Region II), As0.4P0.6 and As0.6P0.4 alloys displayed a linear blueshift (or negligible change in some modes) at a reduced rate, suggesting local structural reorganization with less compression on the bonds. Notably, the alloy with the highest As concentration, As0.8P0.2, exhibited anomalous behavior in the second pressure regime, with a downward shift observed in all As-As and As-P Raman modes (and some P-P modes). Interestingly, the emergence of new peaks corresponding to the Eg mode and A1g mode of the g-As phase was observed in this pressure range, indicating compressive strain-induced structural changes. The anomalous change in Region II confirms 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-AsyP1-y phase, possibly near the grain boundary. Additionally, a g-As phase undergoes compressive structural changes. This study underscores the significance of intercalation and pressure in inducing structural transformations and exploring novel phases in two-dimensional (2D) materials, including b-AsyP1-y alloys.

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