Synthesis of Semiconducting 2H-Phase WTe2 Nanosheets with Large Positive Magnetoresistance

Gate and Temperature Driven Phase Transitions in Few-Layer MoTe2

MoTe₂ has a stable hexagonal semiconducting phase (2H) as well as two semimetallic phases with monoclinic (1T') and orthorhombic (T_d) structures. A structural change can thus be accompanied by a significant change in electronic transport properties. The two semimetallic phases are connected by a temperature driven transition and could exhibit topological properties. Here we make extensive Raman measurements as a function of layer thickness, temperature, and electrostatic doping on few layer 2H-MoTe₂ and also on 1T'-MoTe₂ and T_d-WTe₂. Recent work in MoTe₂ has raised the possibility of a 2H-1T' transition through technology compatible pathways. It has been claimed that such a transition, of promise for device applications, is activated by electrostatic gating. We investigate this claim and find that few-layer tellurides are characterized by high mobility of Te ions, even in ambient conditions and especially through the variation of external parameters like electric field or temperature. These can generate Te clusters, vacancies at crystalline sites, and facilitate structural transitions. We however find that the purported 2H-1T' transition in MoTe₂ cannot be obtained by a pure electrostatic field.

Boosting Electrocatalytic Reduction of CO2 to HCOOH on Ni Single Atom Anchored WTe2 Monolayer

Achieving efficient conversion of carbon dioxide (CO₂ ) to formic acid (HCOOH) at mild conditions is a promising means to reduce greenhouse gas emission and mitigate the energy crisis. Herein, spin-polarized density functional theory calculations with van der Waals corrections (DFT+D3) are performed to analyze the catalytic activity of seven metals (Ti, Fe, Ni, Cu, Zn, In, and Sn) anchored on a tungsten ditelluride monolayer (M@WTe₂ ) and screen favorable CO₂ reduction pathways. These results demonstrate that Ni single atoms strongly bind to the WTe₂ monolayer and exist in isolated form due to the high diffusion barriers. Also, Ni-anchored WTe₂ monolayer (Ni@WTe₂ ) possesses a considerably low limiting-potential (-0.11 V vs reversible hydrogen electrode) to convert CO₂ to HCOOH due to moderate OCHO adsorption energy and a suppressed competing hydrogen evolution reaction (HER). Therefore, Ni@WTe₂ monolayer is a promising electrocatalytic material for the CO₂ reduction reaction (CO₂ RR). This study sheds light on strategies of designing single metal atom anchored WTe₂ catalysts for improved CO₂ RR performances.

Light-Tunable Charge Density Wave Orders in MoTe_{2} and WTe_{2} Single Layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure, and band gap. Moreover, optical pumping of the semimetallic 1T^{'} phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

Accelerated discovery of superoxide-dismutase nanozymes via high-throughput computational screening

The activity of nanomaterials (NMs) in catalytically scavenging superoxide anions mimics that of superoxide dismutase (SOD). Although dozens of NMs have been demonstrated to possess such activity, the underlying principles are unclear, hindering the discovery of NMs as the novel SOD mimics. In this work, we use density functional theory calculations to study the thermodynamics and kinetics of the catalytic processes, and we develop two principles, namely, an energy level principle and an adsorption energy principle, for the activity. The first principle quantitatively describes the role of the intermediate frontier molecular orbital in transferring electrons for catalysis. The second one quantitatively describes the competition between the desired catalytic reaction and undesired side reactions. The ability of the principles to predict the SOD-like activities of metal-organic frameworks were verified by experiments. Both principles can be easily implemented in computer programs to computationally screen NMs with the intrinsic SOD-like activity.

Crossover of positive and negative magnetoconductance in composites of nanosilica glass containing dual transition metal oxides

A facile sol-gel approach to prepare composites of nanosilica glass containing dual transition metal oxides with compositions xCoO·(20 - x)NiO·80SiO2 comprising x values 5 (NC-1), 10 (NC-2) and 15 (NC-3) within hexagonal pores of SBA-15 template has been demonstrated. The synergistic effect of dual transition metal oxide ions on MD properties and crossover of positive and negative magnetoconductance phenomena were observed in these nanocomposite systems. The physical origin of magnetoconductance switching is explained based on the factors: nanoconfinement effect, wave-function shrinkage and spin polarized electron hopping. DFT calculations were performed to understand the structural correlation of the nanoconfined system. The static (dc) and dynamic (ac) responses of magnetization revealed the spin-glass behaviour of the investigated samples. Both scaling law and Vogel-Fulcher law provide a satisfactory fit to our experimental results which are considered as a salient feature of the spin-glass system. Our studies indicate the possibilities of fabricating magnetically controlled multifunctional devices.