Wannier Koopmans Method Calculations of 2D Material Band Gaps

A major drawback of the widely successful density functional theory is its underestimation of the material band gap. Various methods have been proposed to correct its band gap predictions. Wannier Koopmans method (WKM) is recently developed for this purpose to predict the band gap of extended 3D bulk systems. While the WKM has also been shown to be successful for isolated molecules, it is still a question whether it will work for 2D materials that are in between the 0D molecules and 3D bulk systems. We apply the WKM to 16 commonly known well studied 2D materials and find that the WKM predicted band gaps are on par with their GW calculated results

Interfacial Properties of Monolayer SnS–Metal Contacts

Two-dimensional semiconducting SnS is expected to have great potential for application in nanoelectronics. By using both ab initio electronic structure calculations and more reliable quantum transport simulations, we systematically explored the interfacial properties of monolayer (ML) SnS in contact with a series of metals (Ag, Al, Au, Pd, Cu, and Ni) for the first time. According to the adsorption level, three categories are found: strong adsorption is found in ML SnS–Pd and Ni contacts; medium adsorption is found in ML SnS–Cu contacts; and weak adsorption is found in ML SnS–Ag, Al, and Au contacts. Because the band structure of ML SnS is destroyed in all of the contact systems, a vertical Schottky barrier at the ML SnS–metal interface is absent. However, at the metalized-SnS/uncontacted-SnS interface in a transistor configuration, a lateral Schottky contact is always formed as a result of strong Fermi level pinning (with a pinning factor of 0.17–0.28) according to the quantum transport simulations. This work provides guidelines to design ML SnS-based devices with optimized electrode contact for high performance

First-Principles Study of Cu₉S₅: A Novel p‑Type Conductive Semiconductor

Cu₉S₅ (digenite) is a p-type semiconductor with excellent electrical conductivity, high mobility of copper ions, and high work function. When used as the back electrode of CdTe solar cells, a high power conversion efficiency (PCE) is obtained. Density functional theory (DFT) method was used to study the structural and electronic properties of Cu₉S₅ in this work. From the calculated band structures, we find that the Fermi level of the Cu₉S₅ slightly crosses the valence band by about 0.08 eV below the valence band maximum (VBM), indicating a high hole concentration and potential high electrical conductivity as a p-type semiconductor. It is also found that the crystal structure of Cu₉S₅ remained stable with a few Cu atoms diffused away, which introduces a p-type doping effect. Finally, we give a quantitative discussion on why CdTe solar cells with Cu₉S₅ as the back electrode shows the high PCE

Few-Layer Fe₃(PO₄)₂·8H₂O: Novel H‑Bonded 2D Material and Its Abnormal Electronic Properties

Using first-principles calculations, we study the structural and electronic properties of a new layered hydrogen-bonded 2D material Fe₃(PO₄)₂·8H₂O. Interestingly, unlike other common 2D materials, such as layered van der Waals 2D materials, the band gap of 2D Fe₃(PO₄)₂·8H₂O-(010)-(1 × 1) is smaller than bulk Fe₃(PO₄)₂·8H₂O, which does not obey the normal quantum confinement effect and can be attributed to the edge states and the hydrogen bonds between the layers. We also find that the band-gap variation with the reduced layers depends on the length of the interlayer hydrogen bond and the stronger interlayer hydrogen bond leads to the larger band gap