Xingxing Zhang

Here, it is demonstrated that the surface‐enhanced Raman scattering performance of the WSe₂ monolayer can be enhanced via tailoring the atomic ratio of WSe₂, which is correlated to the exciton and charge‐transfer resonances. The amplitude of the exciton and charge‐transfer resonances is estimated by the femtosecond optical pump‐probe measurement and a bipolar junction transistor consisting of the probe molecules and WSe₂.

Abstract

Recently, 2D transition‐metal dichalcogenides (2D TMDCs) are identified as ideal substrates for surface‐enhanced Raman scattering (SERS). However, the effect of enhancement factor (EF) on TMDCs is lower than metal‐based SERS substrates. Here, it is demonstrated that the SERS performance of WSe₂ monolayer can be enhanced via tailoring the atomic ratio of WSe₂; this correlates to the exciton and charge‐transfer resonances. CuPc molecules are adsorbed onto WSe₂ monolayers as the probe molecules, and the atomic ratio (Se:W) of WSe₂ is tailored from 2 to 1.92. For an atomic ratio of 1.96, the maximum EF on irradiated WSe₂ is more than 120; this is enhanced by more than 40 times compared with pristine WSe₂. The amplitude of exciton and charge‐transfer resonances is estimated by femtosecond optical pump‐probe measurement and a bipolar junction transistor (BJT) consisting of CuPc film and 2D materials. It is found that the intensity of resonances in the CuPc–WSe₂ system is tailored by the atomic ratio of WSe₂. This is closely correlated to the SERS performance of WSe₂. This study shows that the SERS performance of WSe₂ is enhanced by tuning the atomic ratio of WSe₂ and provides qualitative theoretical explanations for the mechanism of enhanced SERS.

Relativistic quantum chemical calculations show that the uranium molecule U₂ has a quadruple bond

Relativistic quantum chemical calculations show that the uranium molecule U₂ has a quadruple bond, Published online: 29 October 2018; doi:10.1038/s41557-018-0158-9

Ultrathin 2D hexagonal‐W₂N₃ (h‐W₂N₃) flakes are synthesized at atmospheric pressure via a salt‐templated method. The formation energy of h‐W₂N₃ can be dramatically decreased owing to the strong interaction and domain matching epitaxy between KCl and h‐W₂N₃. 2D h‐W₂N₃ demonstrates an excellent catalytic activity for the cathodic hydrogen evolution reaction.

Abstract

2D transition metal nitrides, especially nitrogen‐rich tungsten nitrides (W _x N _y , y > x), such as W₃N₄ and W₂N₃, have a great potential for the hydrogen evolution reaction (HER) since the catalytic activity is largely enhanced by the abundant WN bonding. However, the rational synthesis of 2D nitrogen‐rich tungsten nitrides is challenging due to the large formation energy of WN bonding. Herein, ultrathin 2D hexagonal‐W₂N₃ (h‐W₂N₃) flakes are synthesized at atmospheric pressure via a salt‐templated method. The formation energy of h‐W₂N₃ can be dramatically decreased owing to the strong interaction and domain matching epitaxy between KCl and h‐W₂N₃. 2D h‐W₂N₃ demonstrates an excellent catalytic activity for cathodic HER with an onset potential of −30.8 mV as well as an overpotential of −98.2 mV for 10 mA cm⁻².

Re‐doped MoS₂ atomic layers in the distorted tetragonal structure show excellent activity and stability for electrocatalytic hydrogen production. Atomic‐level scanning transmission electron microscopy combined with density functional theory calculations reveal active local Mo‐rich structures and explain the best performance in Re_0.55Mo_0.45S₂. The study provides a new catalyst design strategy through chemical doping.

Abstract

The development of stable and efficient hydrogen evolution reaction (HER) catalysts is essential for the production of hydrogen as a clean energy resource. A combination of experiment and theory demonstrates that the normally inert basal planes of 2D layers of MoS₂ can be made highly catalytically active for the HER when alloyed with rhenium (Re). The presence of Re at the ≈50% level converts the material to a stable distorted tetragonal (DT) structure that shows enhanced HER activity as compared to most of the MoS₂‐based catalysts reported in the literature. More importantly, this new alloy catalyst shows much better stability over time and cycling than lithiated 1T‐MoS₂. Density functional theory calculations find that the role of Re is only to stabilize the DT structure, while catalysis occurs primarily in local Mo‐rich DT configurations, where the HER catalytic activity is very close to that in Pt. The study provides a new strategy to improve the overall HER performance of MoS₂‐based materials via chemical doping.

The electrochemically reversible phase transformations during lithiation and delithiation of SnSe₂ single crystals are visualized atomically in real time using in situ high‐resolution transmission electron microscopy. Combined with density functional theory calculation, the atomic structures of the intermediate phases are identified and the reversible reaction mechanism is confirmed, which provides valuable implications to 2D metal chalcogenides as electrodes for lithium‐ion batteries.

Abstract

2D materials have shown great promise to advance next‐generation lithium‐ion battery technology. Specifically, tin‐based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions—intercalation, conversion, and alloying—but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real‐time and atomic‐scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe₂ single crystals, using in situ high‐resolution transmission electron microscopy complemented by first‐principles calculations. Lithiation proceeds sequentially through intercalation, conversion, and alloying reactions (SnSe₂ → Li _x SnSe₂ → Li₂Se + Sn → Li₂Se + Li₁₇Sn₄) in a manner that maintains structural and crystallographic integrity, whereas delithiation forms numerous well‐aligned SnSe₂ nanodomains via a homogeneous deconversion process, but gradually loses the coherent orientation in subsequent cycling. Furthermore, alloying and dealloying reactions cause dramatic structural reorganization and thereby consequently reduce structural stability and electrochemical cyclability, which implies that deep discharge for Sn chalcogenide electrodes should be avoided. Overall, the findings elucidate atomistic lithiation and delithiation mechanisms in SnSe₂ with potential implications for the broader class of 2D metal chalcogenides.

Surface modulation at the atomic level has been an important approach for boosting the performance of electrocatalysts. Here, a combined theoretical and experimental study on Ni atom decorated hierarchical MoS₂ nanosheets supported on a multichannel carbon matrix (MCM) is presented. The obtained hybrid MCM@MoS₂–Ni electrocatalyst with activated S sites exhibits high performance in electrocatalytic hydrogen evolution.

Abstract

Surface modulation at the atomic level is an important approach for tuning surface chemistry and boosting the catalytic performance. Here, a surface modulation strategy is demonstrated through the decoration of isolated Ni atoms onto the basal plane of hierarchical MoS₂ nanosheets supported on multichannel carbon nanofibers for boosted hydrogen evolution activity. X‐ray absorption fine structure investigation and density functional theory (DFT) calculation reveal that the MoS₂ surface decorated with isolated Ni atoms displays highly strengthened H binding. Benefiting from the unique tubular structure and basal plane modulation, the newly developed MoS₂ catalyst exhibits excellent hydrogen evolution activity and stability. This single‐atom modification strategy opens up new avenues for tuning the intrinsic catalytic activity toward electrocatalytic water splitting and other energy‐related processes.

Employing the organic charge transfer material 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane, the electrical properties of MoS₂ field‐effect transistors including V _on's and subthreshold swings are successfully modulated with no degradation of mobility. The charge transfer mechanism is investigated by first‐principles calculation and scanning Kelvin probe microscopy. A high‐performance NH₃ gas sensor fabricated from this transistor reaches more than 1000% sensitivity at 100 ppm.

Abstract

The development of van der Waals heterostructures in 2D materials systems has attracted considerable interests for exploring new insights of (opto‐) electrical characteristics, device physics, and novel functional applications. Utilizing organic molecular material with strong electron withdrawing ability, charge transfer van der Waals interfaces are formed between 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) and MoS₂, via which the modulation of the onset voltages and optimization of subthreshold swing values in MoS₂‐based field effect transistors are realized. Charge transfer process and its functionality mechanisms are further verified and investigated with first‐principles calculation, scanning Kelvin probe microscope characterization, and temperature‐dependent electrical characterization. With the charge transfer effect between reducing gas molecules and F₄TCNQ, NH₃ gas sensor is proposed and fabricated with the sensitivity reaching higher than 1000% at 100 ppm, much more outstanding performance than those of any reported MoS₂‐based NH₃ gas sensors. The F₄TCNQ‐MoS₂ hybrid strategy might open up a pathway for tuning and optimizing the electrical properties, in addition to novel functional units designing and fabrications in electric devices based on low‐dimensional semiconducting systems.