Prevalence of oxygen defects in an in-plane anisotropic transition metal dichalcogenide
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-11-09 |
| Journal | Physical review. B./Physical review. B |
| Authors | Ryan Plumadore, Mehmet BaĆkurt, Justin Boddison-Chouinard, Gregory P. Lopinski, M. Modarresi |
| Institutions | National Research Council Canada, Izmir Institute of Technology |
| Citations | 14 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study provides an atomic-scale characterization of defects in Rhenium Disulfide (ReS2), an in-plane anisotropic transition metal dichalcogenide (TMD), using combined Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Density Functional Theory (DFT).
- Defect Identification: The most common atomic-scale defect (Type A) is definitively identified as oxygen atoms adsorbed at sulfur vacancy sites (O absorbed by S-vacancy), confirming that ambient exposure leads to oxygenation of intrinsic lattice defects.
- Anisotropy Confirmation: The intrinsic in-plane anisotropy of ReS2 was directly visualized via STM, resolving chains of Rhenium atoms forming diamond-shaped clusters.
- Electronic Properties: The material is confirmed to be a semiconductor with a measured energy gap (Egap) of 1.35 ± 0.1 eV, which aligns closely with ab-initio DFT calculations (1.3 eV).
- Doping State: The ReS2 crystal samples were found to be n-doped, with the Fermi level (EF) positioned near the bottom of the conduction band (Ec).
- Spectroscopic Signature: Unlike a pure sulfur vacancy (which creates detrimental mid-gap states), the oxygenated vacancy defect maintains the semiconducting gap, explaining the absence of in-gap states in the measured STS spectra of the common defect.
- Methodological Advance: This work establishes a critical pathway for understanding and engineering the properties of 2D anisotropic semiconductors by correlating atomic-scale topographic and spectroscopic signatures with theoretical models.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Measured Energy Gap (Egap) | 1.35 ± 0.1 | eV | Scanning Tunneling Spectroscopy (STS) average. |
| Calculated DFT Energy Gap | 1.3 | eV | 3-layer ReS2 slab (ABA stacking, Kohn-Sham gap at Î point). |
| Calculated GOW0 Bandgap | 2.3 | eV | Quasiparticle level correction (Note: Optical gap is reduced by excitonic effects). |
| Crystal Doping Type | n-doped | N/A | Fermi level (EF) found close to the conduction band (Ec). |
| STM Operating Temperature | 80 - 300 | K | Ultrahigh vacuum (UHV) system operating range. |
| STM Bias Voltage (Vb) Example | -1.60 | V | Topographic map of step edges (Figure 1c). |
| STM Tunneling Current (IT) Example | 450 | pA | Topographic map of step edges (Figure 1c). |
| Re 4f Core Level Position | 42.6 (7/2), 45 (5/2) | eV | XPS analysis, consistent with 1T-ReS2 structure. |
| XPS AlKα Radiation Energy | 1486.69 | eV | X-ray source energy for XPS data collection. |
| DFT Plane-wave Basis Cut-off | 400 | eV | VASP calculations for structural optimization/LDOS. |
| DFT Gaussian Broadening Width | 0.05 | eV | Used for structural optimization and LDOS calculations. |
Key Methodologies
Section titled âKey Methodologiesâ- Sample Preparation: Commercial bulk ReS2 crystals (HQ Graphene) were cleaved in air and immediately transferred into an ultrahigh vacuum (UHV) environment for measurement.
- Scanning Tunneling Microscopy/Spectroscopy (STM/STS):
- Measurements were performed using a commercial RHK Pan Freedom system under UHV conditions at temperatures between 80 K and 300 K.
- STS (dI/dV) was used to measure the local density of states (LDOS) and determine the semiconducting gap and Fermi level position.
- Contrast changes in STM images were observed by varying the bias voltage (e.g., -0.80 V vs. +0.80 V) to characterize defect electronic signatures.
- X-ray Photoelectron Spectroscopy (XPS):
- XPS spectra were collected using a Kratos Axis Nova spectrometer with AlKα radiation (1486.69 eV).
- XPS confirmed the elemental presence of Rhenium, Sulfur, and, crucially, Oxygen, supporting the defect identification.
- Ab-initio Calculations (DFT):
- Calculations for energy bands, bandgap, and density of states were performed using the Quantum Espresso and VASP codes.
- The Generalized Gradient Approximation (GGA) form of Perdew-Burke-Ernzerhof (PBE) was used for exchange-correlation functional.
- Simulated STM images were generated by calculating partial charge densities across relevant energy ranges (e.g., [-2, 0] eV for valence band, [-2, 2] eV for conduction band).
- Defect models (S-vacancy, O adsorbed on S-vacancy, Re-antisite) were simulated to match experimental LDOS and topographic signatures.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings regarding defect prevalence and electronic structure in ReS2 are critical for the development and reliability of next-generation 2D electronic and photonic devices:
- Anisotropic Opto-Electronics: ReS2âs in-plane anisotropy is leveraged for devices requiring directional response, such as highly sensitive polarized light detectors and anisotropic field-effect transistors (FETs).
- 2D Semiconductor Manufacturing: The ability to identify and model oxygenated vacancies provides a quality control metric for ReS2 growth and processing, ensuring that defects do not introduce detrimental mid-gap states that degrade device performance.
- Quantum Emitter Engineering: Defects in TMDs are candidates for single-photon sources. Understanding how ambient oxygen passivates sulfur vacancies is essential for engineering stable, high-purity quantum emitters by controlling the local chemical environment.
- Catalysis and Energy Storage: Defects influence the catalytic activity (e.g., Hydrogen Evolution Reaction). Controlling the n-doping and the presence of oxygenated defects allows for targeted charge engineering to optimize surface reactivity.
- High-Density Memory and Logic: The use of ReS2 in integrated digital inverters and negative differential resistance devices (due to its unique band structure) benefits directly from precise defect control to ensure consistent electronic transport.
View Original Abstract
Atomic scale defects in semiconductors enable their technological\napplications and realization of novel quantum states. Using scanning tunneling\nmicroscopy and spectroscopy complemented by ab-initio calculations we determine\nthe nature of defects in the anisotropic van der Waals layered semiconductor\nReS$_2$. We demonstrate the in-plane anisotropy of the lattice by directly\nvisualizing chains of rhenium atoms forming diamond-shaped clusters. Using\nscanning tunneling spectroscopy we measure the semiconducting gap in the\ndensity of states. We reveal the presence of lattice defects and by comparison\nof their topographic and spectroscopic signatures with ab initio calculations\nwe determine their origin as oxygen atoms absorbed at lattice point defect\nsites. These results provide an atomic-scale view into the semiconducting\ntransition metal dichalcogenides, paving the way toward understanding and\nengineering their properties.\n