Two-dimensional (2D) layered materials, transition metal dichalcogenides and black phosphorus, have attracted much interest from the viewpoints of fundamental physics and device applications. The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier, owing to their remarkable optical properties and prospects for new devices. Here, we report the anisotropic optical properties of layered 2D monochalcogenide of germanium sulfide (GeS). Three Raman scattering peaks corresponding to the B3g, A 1 g , and A 2 g modes with strong polarization dependence are demonstrated in the GeS flakes, which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at around 1.66 eV that originates from the direct optical transition in GeS at room temperature. Moreover, determination of the polarization dependent characteristics of the PL and absorption reveals an anisotropic optical transition near the band edge of GeS, which is also supported by the density functional theory calculations. This anisotropic layered GeS presents the opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties.

Carbon nanotubes have long been described as rolled-up graphene sheets. It is only fairly recently observed that longitudinal cleavage of carbon nanotubes, using chemical, catalytical and electrical approaches, unzips them into thin graphene strips of various widths, the so-called graphene nanoribbons. In contrast, rolling up these flimsy ribbons into tubes in a real experiment has not been possible. Theoretical studies conducted by Kit et al. recently demonstrated the tube formation through twisting of graphene nanoribbon, an idea very different from the rolling-up postulation. Here we report the first experimental evidence of a thermally induced self-intertwining of graphene nanoribbons for the preferential synthesis of (7, 2) and (8, 1) tubes within parent-tube templates. Through the tailoring of ribbon’s width and edge, the present finding adds a radically new aspect to the understanding of carbon nanotube formation, shedding much light on not only the future chirality tuning, but also contemporary nanomaterials engineering.

Two‐dimensional (2D) transition metal dichalcogenides (TMDCs) with unique electrical properties are fascinating materials used for future electronics. However, the strong Fermi level pinning effect at the interface of TMDCs and metal electrodes always leads to high contact resistance, which seriously hinders their application in 2D electronics. One effective way to overcome this is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW) contacts. Alternatively, using highly conductive doped TMDCs will have a profound impact on the contact engineering of 2D electronics. Here, a novel chemical vapor deposition (CVD) using mixed molten salts is established for vapor–liquid–solid growth of high‐quality rhenium (Re) and vanadium (V) doped TMDC monolayers with high controllability and reproducibility. A tunable semiconductor to metal transition is observed in the Re‐ and V‐doped TMDCs. Electrical conductivity increases up to a factor of 108 in the degenerate V‐doped WS2 and WSe2. Using V‐doped WSe2 as vdW contact, the on‐state current and on/off ratio of WSe2‐based field‐effect transistors have been substantially improved (from ≈10–8 to 10–5 A; ≈104 to 108), compared to metal contacts. Future studies on lateral contacts and interconnects using doped TMDCs will pave the way for 2D integrated circuits and flexible electronics.

Artificial van der Waals heterostructures of 2D layered materials are attractive from the viewpoint of the possible discovery of new physics together with improved functionalities. Stacking various combinations of atomically thin semiconducting transition metal dichalcogenides, MX 2 (M = Mo, W; X = S, Se, Te) with a hexagonal crystal structure, typically leads to the formation of a staggered Type II band alignment in the heterostructure, where electrons and holes are confined in different layers. Here, the comprehensive studies are performed on heterostructures prepared from monolayers of WSe 2 and MoTe 2 using differential reflectance, photoluminescence (PL), and PL excitation spectroscopy. The MoTe 2 /WSe 2 heterostructure shows strong PL from the MoTe 2 layer at ≈1.1 eV, which is different from the quenched PL from the WSe 2 layer. Moreover, enhancement of PL intensity from the MoTe 2 layer is observed because of the near-unity highly efficient photocarrier transfer from WSe 2 to MoTe 2 . These experimental results suggest that the MoTe 2 /WSe 2 heterostructure has a Type I band alignment where electrons and holes are confined in the MoTe 2 layer. The findings extend the diversity and usefulness of ultrathin layered heterostructures based on transition metal dichalcogenides, leading to possibilities toward future optoelectronic applications.

The development of bulk synthetic processes to prepare functional nanomaterials is crucial to achieve progress in fundamental and applied science. Transition-metal chalcogenide (TMC) nanowires, which are one-dimensional (1D) structures having three-atom diameters and van der Waals surfaces, have been reported to possess a 1D metallic nature with great potential in electronics and energy devices. However, their mass production remains challenging. Here, a wafer-scale synthesis of highly crystalline transition-metal telluride nanowires is demonstrated by chemical vapor deposition. The present technique enables formation of either aligned, atomically thin two-dimensional (2D) sheets or random networks of three-dimensional (3D) bundles, both composed of individual nanowires. These nanowires exhibit an anisotropic 1D optical response and superior conducting properties. The findings not only shed light on the controlled and large-scale synthesis of conductive thin films but also provide a platform for the study on physics and device applications of nanowire-based 2D and 3D crystals.

3.8% when first reported [1] to 22.1% [2] within a few years only. The striking rapid advances in the photovoltaic performance of PSCs are mainly attributable to the development of perovskite photoactive layer materials with superior properties, including strong light absorption from the visible to the infrared region, [3] a small exciton binding energy, [4][5][6] micrometerscale charge carrier diffusion lengths, [7,8] and high charge collection ability. [9] The PCE of PSCs based on methylammonium lead halide (CH 3 NH 3 PbX 3 ) as a model PSC device has reached 20.1% and 19.5% in inverted (p-i-n) and normal (n-i-p) architectures, [10,11] respectively. Some endeavors have been invested to improve quality of perovskite layer, such as N,Ndimethylformamide (DMF) used as a fumed source of perovskite, [12] CH 3 NH 3 I vapor and CH 3 NH 3 I employed in mixed solvents, [13][14][15] however, some urgent challenges are still to be solved for the photovoltaic performance of CH 3 NH 3 PbX 3 to be further improved. Specifically, (i) compact, void-free, and fully covered perovskite layers should be developed to reduce the leakage of photocurrent and increase the voltage in PSCs, (ii) a method of depositing perovskite layers with a flat surface is needed to improve adhesiveness to the hole transport layer, and (iii) the resistance of the perovskite phases to humidity, oxygen, and ambient Perovskite solar cells (PSCs) have attracted intensive attention as the most promising next-generation photovoltaic technology because they both enable accelerated development of photovoltaic performance and are compatible with low-cost fabrication methods. The strategy of interface engineering of the perovskite layer in PSCs is expected to result in further enhancement of the power conversion efficiency (PCE) of PSCs via minimizing the charge recombination loss. Here, a high current-voltage (stabilized power output) PCE of 20.4% (19.9%) in CH 3 NH 3 PbI 3 PSCs under reverse scanning conditions is demonstrated by incorporating a solution-processed polymer layerof poly(methyl methacrylate) (PMMA) between the perovskite photoactive layer and the hole transport layer. Moreover, steady-state and time-resolved photoluminescence spectroscopy and impedance spectroscopy are used to reveal the mechanism of the enhancement of the photovoltaic performance and its stability by the PMMA layer in a CH 3 NH 3 PbI 3 PSC device. The morphology modification, surface passivation, and protection of the perovskite layer by the insulating PMMA layer substantially contribute to the enhancement of photovoltaic performance and its stability, despite a slight reduction of the charge extraction efficiency.

Atomically thin-layered ReS 2 with a distorted 1T structure has attracted attention because of its intriguing optical and electronic properties. Here, we investigated the direct and indirect exciton dynamics of a three-layered ReS 2 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, were 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. Our findings provide important information regarding the indirect band gap nature of few-layered ReS 2 and its characteristic exciton dynamics, boosting our understanding of the novel electronic and optical properties of atomically thin-layered ReS 2 .

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