Author(s): Linke Xu
Mentor(s): Yun Yu, chemistry and biochemistry department
Abstract
Rhenium disulfide (ReS₂) is a unique Transition Metal Dichalcogenide (TMD) known for its structural asymmetry, which dictates an intrinsic optical and electronic anisotropy. We investigated the influence of doping environment on this anisotropy. While intrinsic ReS₂ exhibits ideal anisotropic behavior, we found that p-type doping causes the angle-dependent properties to vanish. By engineering a van der Waals heterostructure and utilizing charge transfer doping from an ITO substrate, we successfully controlled and restored the material’s anisotropy. Our results demonstrate that the mechanism of photoelectrochemical response in ReS₂ is fundamentally coupled to the Fermi level position, allowing for rational design of specific-use optoelectronic devices.
Audio Transcript
Hello everyone, my name is Linke Xu, and today I am presenting my project,
“Tuning the Photoelectrochemical Properties of ReS₂ via van der Waals Heterostructures.” This work was completed in the Chemistry and Biochemistry Department at George Mason University.
ReS₂ is a fascinating two-dimensional material because its distorted lattice creates strong in-plane anisotropy—meaning its photoresponse depends on the angle of incoming light.
When I began this project, my goal was to explore whether light polarization could be used to influence photoelectrochemical reactions on ReS₂, especially hydrogen evolution. 53However, very early in the process we observed something surprising: the anisotropy did not behave consistently. Instead, it changed depending on the doping environment of the material.
This unexpected behavior prompted us to investigate a more fundamental question: What actually determines anisotropy in ReS₂—its crystal structure, or the electronic occupation of its band-edge states?
Based on these early observations, we hypothesized that anisotropy requires electrons in the Re d-orbital band-edge states. If the material become mes p-doped and those states are empty, the directional response should disappear. But if we raise the Fermi level again through charge-transfer doping, anisotropy should return.
1.56To test this idea, we exfoliated ReS₂ nanosheets onto two different types of substrates. Placing the flake directly on ITO induces n-doping, while transferring it onto a graphene/hBN stack isolates the flake and makes it p-type.
We then measured photoelectrochemical current and used scanning electrochemical cell microscopy, or SECCM under polarized illumination to map photocurrent anisotropy with spatial resolution.
Our results revealed a clear and consistent trend.
2.43First, when ReS₂ was n-doped on ITO, we observed strong anisotropy. Both the absorption and photocurrent showed the expected sinusoidal angle dependence, confirming that directional excitation was present.
3.01Second, when the same flake was placed onto the graphene/hBN heterostructure and became p-doped, the anisotropy almost completely disappeared. The photocurrent became nearly circular, indicating that the response no longer depended on angle.
3.22Finally, when we returned the flake to ITO, charge-transfer doping raised the Fermi level again—and the anisotropy reappeared.
3.34Because the crystal structure never changed, this reversible switching demonstrates that anisotropy is controlled electronically rather than purely structurally.
Overall, our findings show that the anisotropy of ReS₂ is not a fixed property of its lattice. Instead, it depends on whether the directional Re d-orbital band-edge states are occupied. By adjusting the doping environment, we can effectively turn anisotropy on and off, offering a simple and powerful strategy for designing polarization-dependent optoelectronic and photoelectrochemical systems.
4.25I would like to thank the OSCAR Undergraduate Research Scholars Program, my mentor Dr. Yu, my research partner Anna, and all members of our lab for their support throughout this project.
Thank you for listening.