Electric-field-tunable molecular adsorption on germanane

Two-Dimensional Layered Heterojunctions for Photoelectrocatalysis

Two-dimensional (2D) layered nanomaterial heterostructures, arising from the combination of 2D materials with other low-dimensional species, feature a large surface area to volume ratio, which provides a high density of active sites for catalytic applications and for (photo)electrocatalysis (PEC). Meanwhile, their electronic band structure and high electrical conductivity enable efficient charge transfer (CT) between the active material and the substrate, which is essential for catalytic activity. In recent years, researchers have demonstrated the potential of a range of 2D material interfaces, such as graphene, graphitic carbon nitride (g-C3N4), metal chalcogenides (MCs), and MXenes, for (photo)electrocatalytic applications. For instance, MCs such as MoS2 and WS2 have shown excellent catalytic activity for hydrogen evolution, while graphene and MXenes have been used for the reduction of carbon dioxide to higher value chemicals. However, despite their great potential, there are still major challenges that need to be addressed to fully realize the potential of 2D materials for PEC. For example, their stability under harsh reaction conditions, as well as their scalability for large-scale production are important factors to be considered. Generating heterojunctions (HJs) by combining 2D layered structures with other nanomaterials is a promising method to improve the photoelectrocatalytic properties of the former. In this review, we inspect thoroughly the recent literature, to demonstrate the significant potential that arises from utilizing 2D layered heterostructures in PEC processes across a broad spectrum of applications, from energy conversion and storage to environmental remediation. With the ongoing research and development, it is likely that the potential of these materials will be fully expressed in the near future.

Emulating synaptic behavior in surface-functionalized MoS2 through modulation of interfacial charge transfer via external stimuli

Neuromorphic computing requires materials able to yield electronic switching behavior in response to external stimuli. Transition-metal dichalcogenides surfaces covered by partial or full monolayers of molecular species have shown promise due to their potential for tunable interfacial charge transfer. Here, we demonstrate a class of molecules able to position MoS₂ surfaces on the cusp of electronic instabilities. Density functional theory (DFT) calculations and ab initio molecular dynamics simulations are used to study the interaction of four reduced pyridinium-derived pi-conjugated molecules with the pristine basal planes of MoS₂, by exploring the dynamical evolution of the system at room temperature with regards to the effective band gap, radius of gyration (rog), and charge transfer. Computed rog profiles show that low concentrations of small reduced methyl viologen molecules have high mobilities on top of the surface of the basal plane at room temperature leading to unstable surface deposition, whereas a full monolayer of larger fused-ring molecules deposited on the basal surface shows greater thermal stability. DFT analyses show these larger reduced pyridinium derivatives promote n-type doping on the basal plane due to a built-in electric field, which can be systematically tuned to induce a switching effect, opening and closing a bandgap and providing a fundamental means of driving electronic instabilities needed for emulating neuronal functionality.

Van der Waals heterostructure of graphene and germanane: tuning the ohmic contact by electrostatic gating and mechanical strain

Recent exciting developments in synthesis and properties study of the Germanane (GeH) monolayer have inspired us to investigate the structural and electronic properties of the van der Waals GeH/Graphene (Gr) heterostructure by the first-principle approach. The stability of the GeH/Gr heterostructure is verified by calculating the phonon dispersion curves as well as by thermodynamic binding energy calculations. According to the band structure calculation, the GeH/Gr interface is n-type Ohmic. The effects of different interlayer distances and strains between the layers and the applied electric field on the interface have been investigated to gain insight into the van der Waals heterostructure modifications. An interlayer distance of 2.11 Å and compressive strain of 6% alter the contact from Ohmic to Schottky status, while the electric field can tune the GeH/Gr contact as p- or n-type, Ohmic, or Schottky. The average electrostatic potential of GeH/Gr and the Bader charge analysis have been used to explain the results obtained. Our theoretical study could provide a promising approach for improving the electronic performance of GeH/Gr-based nano-rectifiers.

Effects of Strain and Electric Field on Molecular Doping in MoSSe

Recently, synthesized Janus MoSSe monolayers have attracted tremendous attention in science and technology due to their novel properties and promising applications. In this work, we investigate their molecular adsorption-induced structural and electronic properties and tunable doping effects under biaxial strain and external electric field by first-principles calculations. We find an effective n-type or p-type doping in the MoSSe monolayer caused by noncovalent tetrathiafulvalene (TTF) or tetracyanoquinodimethane (TCNQ) molecular adsorption. Moreover, the concentration of doping carrier with respect to the S or Se side also exhibits Janus characteristics because of the electronegativity difference between S and Se atoms and the intrinsic dipole moment in the MoSSe monolayer. In particular, this n-type or p-type molecular doping effect can be flexibly tuned by biaxial strain or under external electric field. By analyzing the valence band maximum (VBM) and conduction band minimum (CBM) in the band structure of MoSSe/TTF under strain, the strain-tunable band gap of MoSSe and the n-type molecular doping effect is revealed. Further explanation of charge transfer between TTF or TCNQ and the MoSSe monolayer by an equivalent capacitor model shows that the superimposition of external electric field and molecular adsorption-induced internal electric field plays a crucial role in achieving a controllable doping concentration in the MoSSe monolayer.

Electrostatic Field-Induced Oscillator Strength Focusing in Molecules

The development of light-harvesting devices based on molecular materials depends critically on the ability to focus the electronic oscillator strength of molecules into the UV-vis spectral window. Typical molecular chromophores have only about 1% of their total electronic oscillator strength in this spectral region and thus perform at a small fraction of their possible effectiveness. This theoretical study finds that the electronic oscillator strength of polyenes in the UV-vis region may be enhanced by 1 order of magnitude using electrostatic fields, motivating specific experimental studies of oscillator strength focusing. We find scaling relationships between the polyene length, the intensity of the applied field, and the field-induced increase in oscillator strength that are useful for the implementation of light-harvesting strategies based on polyenes.