Study of phase transitions and stability of V_xP_1-x alloys (V=As, Bi, Sb)


Tue, 27 Jul 2021, 11:00 am - 12:00 pm
Gradute Thesis Defense Spring 2021
Dinushika Vithanage
University of Lousiville


Phosphorene, which is the isolated single layer of Black Phosphorous (BP), having unique layer structure, wide bandgap tunability, strong in-plane anisotropy and high carrier mobility, has been alluring the attention of copious disciplines. Despite its exquisite properties, phosphorene suffers from limited air-stability. The experimental evidence of the existence of predicted blue phosphorene (A7) structure, which is predicted to be more stable, is vague. Alloying Phosphorene with other group V elements (As, Sb, Bi) in a controllable manner, is expected to retain some of the most exquisite properties akin to BP and blue phosphorene. This project will focus on,

1. Studying the underlying mechanism in phase transition from black (A17) to blue (A7) structures in VxP1-x alloys. It is proposed that critical parameters which can trigger structural transitions are, (i) composition of alloys, (ii) degree of intercalation and (iii) high-pressure among other things.

2. Understanding the degradation mechanism and stability of VxP1-x alloys. It is expected to study the mechanism in depth by systematic control of oxygen and moisture concentrations and visible light.

In addition, preliminary results obtained for As2P3 alloy so far by lithium intercalation and degradation will be addressed. As2P3 alloy was synthesized by means of chemical vapor transport growth method and characterized with Raman, TEM, SEM. A systematic study on electrochemical charge transfer in Li-intercalated As2P3 was carried out by in-situ Raman scattering and ex-situ X- ray diffraction (XRD). Galvanostatic discharge of dedicated in-situ electrochemical cell for Raman spectroscopy was used to study the time evolution of vibrational modes under lithiation. Abrupt shift in characteristic Raman peak positions during lithiation, provides evidence for intercalation induced phase transition at a specific degree of lithiation. In addition, evidence from ex-situ XRD spectra reinforce the aforementioned claim. It can be concluded that, Raman shifts indicate competing effects of strain and charge transfer at the initial intercalation levels followed by structural reorganization before the phase transition. In the final stages of the intercalation, the entire process is governed by charge transfer.

Phosphorene is known to degrade under ambient conditions. But VxP1-x alloys are expected to exhibit higher durability and enhanced resistance against degradation. It is proposed to conduct the systematic environmental degradation studies of VxP1-x alloys as a function of composition. In- situ transport measurements will be utilized to monitor the degradation of these samples. Preliminary results of degradation experiment conducted on As2P3 alloy will be presented. Moreover, High-pressure studies will be done on VxP1-x alloys, using diamond anvil cell and by in-situ Raman spectroscopy.