仲云雷

The epitaxial growth of single crystal MoS₂ monolayer is achieved on Au (111) thin film substrate by chemical vapor deposition. High growth temperature can rotate the MoS₂ seeds to a favorable orientation, and finally achieve a unidirectional ratio of over 99%. Moreover, the unidirectional MoS₂ domains seamlessly stitched into single crystal monolayer without grain boundaries formation.

Abstract

Monolayer transition metal dichalcogenides (TMDCs) with high crystalline quality are important channel materials for next-generation electronics. Researches on TMDCs have been accelerated by the development of chemical vapor deposition (CVD). However, antiparallel domains and twin grain boundaries (GBs) usually form in CVD synthesis due to the special threefold symmetry of TMDCs lattices. The existence of GBs severely reduces the electrical and photoelectrical properties of TMDCs, thus restricting their practical applications. Herein, the epitaxial growth of single crystal MoS₂ (SC-MoS₂) monolayer is reported on Au (111) film across a two-inch c-plane sapphire wafer by CVD. The MoS₂ domains obtained on Au (111) film exhibit unidirectional alignment with zigzag edges parallel to the <110> direction of Au (111). Experimental results indicated that the unidirectional growth of MoS₂ domains on Au (111) is a temperature-guided epitaxial growth mode. The high growth temperature provides enough energy for the rotation of the MoS₂ seeds to find the most favorable orientation on Au (111) to achieve a unidirectional ratio of over 99%. Moreover, the unidirectional MoS₂ domains seamlessly stitched into single crystal monolayer without GBs formation. The progress achieved in this work will promote the practical applications of TMDCs in microelectronics.

A photosensitive nanodrug An-NP that allows sustained ¹O₂ generation and sufficient oxygen supply during the entire phototherapy is engineered by alternatively applying PDT and PTT controlled by dual light. In PDT, An-NP generates ¹O₂ with extra ¹O₂ being trapped via the conversion into EPO-NP. In subsequent PTT, EPO-NP undergoes thermolysis to liberate the captured ¹O₂ and regenerates An-NP.

Abstract

Photodynamic therapy (PDT), which utilizes photosensitizer to convert molecular oxygen into singlet oxygen (¹O₂) upon laser irradiation to ablate tumors, will exacerbate the already oxygen shortage of most solid tumors and is thus self-limiting. Herein, a sophisticated photosensitive polymeric material (An-NP) that allows sustained ¹O₂ generation and sufficient oxygen supply during the entire phototherapy is engineered by alternatively applying PDT and photothermal therapy (PTT) controlled by two NIR laser beams. In addition to a photosensitizer that generates ¹O₂, An-NP consists of two other key components: a molecularly designed anthracene derivative capable of trapping/releasing ¹O₂ with superior reversibility and a dye J-aggregate with superb photothermal performance. Thus, in 655 nm laser-triggered PDT process, An-NP generates abundant ¹O₂ with extra ¹O₂ being trapped via the conversion into EPO-NP; while in the subsequent 785 nm laser-driven PTT process, the converted EPO-NP undergoes thermolysis to liberate the captured ¹O₂ and regenerates An-NP. The intratumoral oxygen level can be replenished during the PTT cycle for the next round of PDT to generate ¹O₂. The working principle and phototherapy efficacy are preliminarily demonstrated in living cells and tumor-bearing mice, respectively.

A low-cost post-processing method to etch the defects in phase-transition-assisted CVD-grown 2H-MoTe₂ by using the triiodide ion solution is reported. The etching mechanism is discussed based on the high Te-vacancy densities in defects and 1T′ phase. The etching results are confirmed by electrical measurements and chemical analysis.

Abstract

2D molybdenum ditelluride (MoTe₂) with polymorphism is a promising candidate to developing phase-change memory, high-performance transistors and spintronic devices. The phase-transition-assisted chemical vapor deposition (CVD) process has been used to prepare large-scale 2H-MoTe₂ with large grain size and low density of grain boundary. However, because of the lack of precise control of the growth condition, some defects including the amorphous regions and grain boundaries in 2H-MoTe₂ are hardly avoidable. Here, a facile method of selectively etching defects in large-scale CVD-grown 2H-MoTe₂ by triiodide ion (I₃ ⁻) solution is reported. The defect etching is attributed to the reduced lattice symmetry, high chemisorption activity and high conductivity of the defects due to the high density of Te vacancies. The treated 2H-MoTe₂ shows the suppressed hysteresis in the electrical transfer curve, enhances hole mobility and the higher effective barrier height on the metal contact, suggesting the decreased density of defects. Further chemical analysis indicates that the 2H-MoTe₂ is not damaged or doped by I₃ ⁻ solution during the etching process. This simple and low-cost post-processing method is effective for etching the defects in large-area 2H-MoTe₂ for high-performance device applications.

A ReS₂/In₂ZnS₄ 2D/2D van der Waals heterojunction (vdWH) exhibits a highly promoted visible-light photocatalytic H₂-evolution rate of 2515 µmol h⁻¹ g⁻¹. This is 22.66 times that for pristine In₂ZnS₄. The outstanding performance is attributed to efficient interfacial electron–hole dissociation/transportation, together with substantial atomic-level S active sites on ReS₂. Results will be of immediate benefit in the rational design/synthesis of vdWHs for catalysis/(opto)electronics.

Abstract

Owing to dwindling fossil fuels reserves, the development of alternative renewable energy sources is globally important. Photocatalytic hydrogen (H₂) evolution represents a practical and affordable alternative to convert sunlight into carbon-free H₂ fuel. Recently, 2D/2D van der Waals heterostructures (vdWHs) have attracted significant research attention for photocatalysis. Here, for the first time a ReS₂/In₂ZnS₄ 2D/2D vdWH synthesized via a facile physical mixing is reported. It exhibits a highly promoted photocatalytic H₂-evolution rate of 2515 µmol h⁻¹ g⁻¹. Importantly, this exceeds that for pristine In₂ZnS₄ by about 22.66 times. This, therefore, makes ReS₂/In₂ZnS₄ one of the most efficient In₂ZnS₄-based photocatalysts without noble-metal cocatalysts. Advanced characterizations and theoretical computations results show that interlayer electronic interaction within ReS₂/In₂ZnS₄ vdWH and atomic-level S active centers along the edges of ReS₂ NSs work collaboratively to result in the boosted light-induced H₂ evolution. Results will be of immediate benefit in the rational design and preparation of vdWHs for applications in catalysis/(opto)electronics.

An atomic-switch-type artificial synapse fabricated on Ti₃C₂T_X MXene nanosheets with lots of surface functional groups successfully mimics the dynamics of biological synapse. The synaptic dynamics originate from the gradual formation and annihilation of the conductive metallic filaments on the MXene surface with distributed functional groups. Subsequently, its feasibility for an HW-NN with learning ability is demonstrated using a convolutional neural network composed MXene synapse devices.

Abstract

MXenes, an emerging class of two-dimensional (2D) transition metal carbides and nitrides, have attracted wide attention because of their fascinating properties required in functional electronics. Here, an atomic-switch-type artificial synapse fabricated on Ti₃C₂T_x MXene nanosheets with lots of surface functional groups, which successfully mimics the dynamics of biological synapses, is reported. Through in-depth analysis by X-ray photoelectron spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy, it is found that the synaptic dynamics originated from the gradual formation and annihilation of the conductive metallic filaments on the MXene surface with distributed functional groups. Subsequently, via training and inference tasks using a convolutional neural network for the Canadian-Institute-For-Advanced-Research-10 dataset, the applicability of the artificial MXene synapse to hardware neural networks is demonstrated.

Sequential chemical-vapor growth of stacked 2D materials: the thermal decomposition of molecular precursors—ammonia borane and ethylene—on catalytically active Pt(111) and a temporarily intermediate Pt film, which is removed via intercalation, enables the fabrication of a bilayer stacking comprising hexagonal boron nitride and graphene in a surface science approach.

Abstract

In the studies presented here, the subsequent growth of graphene on hexagonal boron nitride (h-BN) is achieved by the thermal decomposition of molecular precursors and the catalytic assistance of metal substrates. The epitaxial growth of h-BN on Pt(111) is followed by the deposition of a temporary Pt film that acts as a catalyst for the fabrication of the graphene sheet. After intercalation of the intermediate Pt film underneath the boron-nitride mesh, graphene resides on top of h-BN. Scanning tunneling microscopy and density functional calculations reveal that the moiré pattern of the van-der-Waals-coupled double layer is due to the interface of h-BN and Pt(111). While on Pt(111) the graphene honeycomb unit cells uniformly appear as depressions using a clean metal tip for imaging, on h-BN they are arranged in a honeycomb lattice where six protruding unit cells enframe a topographically dark cell. This superstructure is most clearly observed at small probe–surface distances. Spatially resolved inelastic electron tunneling spectroscopy enables the detection of a previously predicted acoustic hybrid phonon of the stacked materials. Its’ spectroscopic signature is visible in surface regions where the single graphene sheet on Pt(111) transitions into the top layer of the stacking.

Ultrasensitive photodetectors with defect tolerance are demonstrated by interfacing layered-perovskite thin films and MoS₂ grown by chemical vapor deposition. With the pure crystallographic orientation and type-II band alignment, efficient light absorption and exciton transport in layered perovskites, and ultrafast charge transfer sustain a high responsivity of 2.5 × 10⁴ A W⁻¹, and a specific detectivity of 4.1 × 10¹⁴ Jones.

Abstract

Heterostructures for charge-carrier manipulation have laid the foundation of modern optoelectronic devices, such as photovoltaics and photodetectors. High-performance heterostructure devices usually impose stringent requirements on the material quality to sustain efficient carrier transport and charge transfer, thus leading to sophisticated fabrication processes. Here, a simple yet efficient strategy is proposed to develop ultrasensitive photodetectors based on heterostructures of chemical vapor deposition-grown MoS₂ and polycrystalline-layered perovskites. The layered perovskites possess pure crystallographic orientation with conductive edges in contact with MoS₂, which gives rise to efficient light absorption, exciton diffusion, and interfacial charge transfer. In dark state, the mismatch of work functions of two materials facilitates low dark currents by the depletion of electrons in MoS₂. Under light irradiation, efficient exciton diffusion and interfacial charge transfer are realized in the heterostructures with type-II band alignment, which produces drifting electrons in MoS₂ and leaves trapped holes in layered perovskites. The photodetectors present suppress noises and boost photocurrents, yielding a champion device with a responsivity of 2.5 × 10⁴ A W⁻¹, and a specific detectivity of 4.1 × 10¹⁴ Jones. The results demonstrate a scalable approach for the integration of high-performance devices with high tolerance to defects.

Nature Communications, Published online: 14 July 2021; doi:10.1038/s41467-021-24233-2

Nature Nanotechnology, Published online: 08 July 2021; doi:10.1038/s41565-021-00933-0