Xingxing Zhang

New residents of 2D group VI materials, the rhenium dichalcogenides (ReS₂), are explored by two different transient spectroscopies. Ultrafast direct exciton relaxation (<1 ps) and indirect exciton relaxation (≈100 ps) assisted by the one‐phonon emission process in the indirect bandgap of ReS₂ are revealed.

Abstract

Atomically thin‐layered ReS₂ with a distorted 1T structure has attracted attention because of its intriguing optical and electronic properties. Here, the direct and indirect exciton dynamics of a three‐layered ReS₂ is investigated by polarization‐resolved transient photoluminescence (PL) and ultrafast pump‐probe spectroscopy. The various time scales of the decay signals of the time‐resolved PL (<10 ps), with monitoring of the populations of electron–hole pairs (exciton), and the transient differential reflectance (≈1 and 100 ps), with monitoring of the populations of electrons and/or holes in the excited states, are observed. These results reveal the characteristic exciton dynamics: rapid relaxation of direct excitons (electron–hole pairs) and slow relaxation of the momentum‐mismatched indirect excitons accompanied by a one‐phonon emission process. These findings provide important information regarding the indirect bandgap nature of few‐layered ReS₂ and its characteristic exciton dynamics, boosting the understanding of the novel electronic and optical properties of atomically thin‐layered ReS₂.

In article number 1804470, Du Xiang, Wei Chen, and co‐workers report photoinduced nonvolatile and programmable doping in MoTe₂ based on a heterostructure of MoTe₂ and h‐BN. By spatially controlling the photodoping region, high‐performance photoresist‐free p–n junctions and inverters in the MoTe₂ homostructure are achieved, illustrating the great potential of applying this photodoping technique in 2D logic electronics.

In this work, a MoS₂‐graphene lateral heterostructure is synthesized by means of chemical vapor deposition, investigating the effect of MoS₂ growth temperature on vertical/lateral heterostructure. The field effect mobility of field effect transistors (FETs) based on the fabricated heterostructure exceeds that of FETs based on only MoS₂, which is ascribed to the decreased contact resistance resulting from the use of the graphene as source/drain.

Abstract

2D materials have been extensively investigated in view of their excellent electrical/optical properties, with particular attention directed at the fabrication of vertical or lateral heterostructures. Although such heterostructures exhibit unexpected or enhanced properties compared to those of singly used 2D materials, their fabrication is challenged by the difficulty of realizing spatial control and large area integration. Herein, MoS₂ is grown on patterned graphene at variable temperatures, combining the concept of lateral heterostructure with chemical vapor deposition to realize large area growth with precise spatial control, and probe the spatial distribution of graphene and MoS₂ by a number of instrumental techniques. The prepared MoS₂‐graphene lateral heterostructure is employed to construct field effect transistors with graphene as the source/drain and MoS₂ as the channel, and the performance of these transistors (on/off ratio ≈10⁹, maximum field effect mobility = 8.5 cm² V⁻¹ s⁻¹) is shown to exceed that of their MoS₂‐only counterparts.

Ultimate limit in size and performance of WSe₂ vertical diodes

Ultimate limit in size and performance of WSe₂ vertical diodes, Published online: 18 December 2018; doi:10.1038/s41467-018-07820-8

Vertical charge transport through homogeneous WSe2 layers can be effectively tuned by the layer number and contacting metals deposited. Here, the authors report WSe2 vertical diodes with superior device performance characteristics based on variable WSe2 thickness and gadolinium and platinum contact metals.

A density functional theory study concerning the origin of the recently reported ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015027/tdmaaf20bieqn005.gif] charge density wave (CDW) instability in single-layer TiTe ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015027/tdmaaf20bieqn006.gif] is reported. It is shown that, whereas calculations employing the semi-local functional PBE favor the undistorted structure, the hybrid functional HSE06 correctly predicts a ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015027/tdmaaf20bieqn007.gif] distortion. The study suggests that the magnitude of the semi-metallic overlap between the valence band top at ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015027/tdmaaf20bieqn008.gif] and the conduction band bottom at ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015027/tdmaaf20bieqn009.gif] is a key factor controlli...

In article 1805188, Xining Zang, Jiajun Gu, Liwei Lin, and co‐workers develop a self‐assembly process to synthesize two‐dimensional transition metal carbides (≈10 nm thickness and ≈100 µm lateral size). The metal ions (Mo, Co, W) self‐organize within a gelatin template into a lamellar nanostructure (metallo‐hydrogel). Subsequent carbonization at moderate temperatures in a reducing atmosphere (600 °C) yields ultrathin 2D‐TMC sheets with high conductivity and rich active sites ideal for the hydrogen evolution reaction (HER).

h‐BN/MoTe₂/graphene/SnS₂/h‐BN van der Waals heterostructure photodetectors present an extraordinary broadband responsivity exceeding 2.6 × 10³ A W⁻¹ and detectivity up to ≈10¹³ Jones in a wide spectrum, which is attributed to the enhanced light absorption and highly effective exciton dissociation originating from the vertical built‐in electric field and multiple photoactive layers in the unique heterostructures.

Abstract

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next‐generation optoelectronics since they can be stacked layer‐by‐layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice‐mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h‐BN/p‐MoTe₂/graphene/n‐SnS₂/h‐BN p–g–n junction, fabricated by a layer‐by‐layer dry transfer, demonstrates high‐sensitivity, broadband photodetection at room temperature. The combination of the MoTe₂ and SnS₂ of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built‐in electric field for high‐efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5−7‐layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W⁻¹ with fast photoresponse and specific detectivity up to ≈10¹³ Jones in the ultraviolet–visible–near‐infrared spectrum. This result suggests that the vdW p–g–n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh‐sensitivity and broadband photonic detectors.

In situ transmission electron microscopy is a powerful tool to capture spatiotemporal evolution of dynamic phenomena. In article number 1804925, Vinayak P. Dravid, Kai He, and co‐workers implement this method to uncover atomic structural evolution of electrochemically reversible phase transformations during lithiation and delithiation of SnSe₂, which is promising for lithium‐ion batteries.

Electrolyte‐gated transistors based on air‐brushed WS₂ nanosheet networks as an n‐type semiconductor are presented. The transistors exhibit current modulations of >10⁴ with electron mobilities around 0.01 cm² V⁻¹ s⁻¹ and high volumetric capacitance of 30 F cm⁻³. Charge transport within the network is found to depend significantly on the lateral electric field and to be thermally activated.

Abstract

Solution‐processed, low cost thin films of layered semiconductors such as transition metal dichalcogenides (TMDs) are potential candidates for future printed electronics. Here, n‐type electrolyte‐gated transistors (EGTs) based on porous WS₂ nanosheet networks as the semiconductor are demonstrated. The WS₂ nanosheets are liquid phase exfoliated to form aqueous/surfactant stabilized inks, and deposited at low temperatures (T < 120 °C) in ambient atmosphere by airbrushing. No solvent exchange, further additives, or complicated processing steps are required. While the EGTs are primarily n‐type (electron accumulation), some hole transport is also observable. The EGTs show current modulations > 10⁴ with low hysteresis, channel width‐normalized on‐conductances of up to 0.27 µS µm⁻¹ and estimated electron mobilities around 0.01 cm² V⁻¹ s⁻¹. In addition, the WS₂ nanosheet networks exhibit relatively high volumetric capacitance values of 30 F cm⁻³. Charge transport within the network depends significantly on the applied lateral electric field and is thermally activated, which supports the notion that hopping between nanosheets is a major limiting factor for these networks and their future application.