Liquid Exfibration and Optoelectronic Devices of Fibrous Phosphorus.

Quasi-one-dimensional (Q1D) semiconductor materials, such as carbon nanotubes, SbSI, MP15 (M = Li, Na, K), and selenium and tellurium nanowires, show amazing potential for applications in future nanoelectronic and optoelectronic devices. However, intricate chirality in the structure of carbon nanotubes limits their applications. Also, the performance of MP15 in optoelectronics has yet to be extensively explored. One new Q1D semiconductor material, fibrous phosphorus (FP), has recently received attention because its raw material is less toxic. However, the ability to characterize FP by phase identification is limited in the assessment of micro/nano-thickness, such as exfibrated FP. So, identifying a precise Raman spectrum will allow for much better characterization. Here, a sufficiently sharp Raman spectrum of FP was obtained and analyzed. Moreover, we demonstrated that high-quality, few-layer FP fibers with thicknesses as low as 5.55 nm can be produced by liquid-phase exfibration under ambient conditions in solvents. More importantly, an optoelectronic detector based on a single FP fiber field-effect-transistor configuration was investigated. A rise time as short as about 40 ms was obtained for the FP transistors, illustrating the potential of FP single bundle crystals as a new one-dimensional material for optoelectronic device applications.

High-Performance Photodiode Based on Atomically Thin WSe2 /MoS2 Nanoscroll Integration.

Self-assembled structures of 2D materials with novel physical and chemical properties, such as the good electrical and optoelectrical performance in nanoscrolls, have attracted a lot of attention. However, high photoresponse speed as well as high responsivity cannot be achieved simultaneously in the nanoscrolls. Here, a photodiode consisting of single MoS2 nanoscrolls and a p-type WSe2 is demonstrated and shows excellent photovoltaic characteristics with a large open-circuit voltage of 0.18 V and high current intensity. Benefiting from the heterostructure, the dark current is suppressed resulting in an increased ratio of photocurrent to dark current (two orders of magnitude higher than the single MoS2 nanoscroll device). Furthermore, it yields high responsivity of 0.3 A W-1 (corresponding high external quantum efficiency of ≈75%) and fast response time of 5 ms, simultaneously. The response speed is increased by three orders of magnitude over the single MoS2 nanoscroll device. In addition, broadband photoresponse up to near-infrared could be achieved. This atomically thin WSe2 /MoS2 nanoscroll integration not only overcomes the disadvantage of MoS2 nanoscrolls, but also demonstrates a single nanoscroll-based heterostructure with high performance, promising its potential in the future optoelectronic applications.

A High-Performance In-Memory Photodetector Realized by Charge Storage in a van der Waals MISFET.

The emerging data-intensive applications in optoelectronics are driving innovation toward the fused integration of sensing, memory, and computing to break through the restrictions of the von Neumann architecture. However, the present photodetectors with only optoelectronic conversion functions cannot satisfy the growing demands of the multifunctions required in single devices. Here, a novel route for the integration of non-volatile memory into a photodetector is proposed, with a WSe2 /h-BN van der Waals heterostructure on a Si/SiO2 substrate to realize in-memory photodetection. This photodetector exhibits an ultrahigh readout photocurrent of 3.4 µA and photoresponsivity of 337.8 A W-1 in the solar-blind wavelength region, together with an extended retention time of more than 10 years. Furthermore, the charge-storage-based non-volatile mechanism of h-BN/SiO2 is successfully proven through a novel analysis of in situ optoelectronic electron energy-loss spectroscopy. These results represent a leap forward to future applications and insightful mechanisms of in-memory photodetection.

Switchable Unipolar-Barrier Van der Waals Heterostructures with Natural Anisotropy for Full Linear Polarimetry Detection.

Polarization-resolved photodetection in a compact footprint is of great interest for ultraminiaturized polarimeters to be used in a wide range of applications. However, probing the states of polarization (SOP) in materials with natural anisotropy are usually weak, limited by the material's natural dichroism or diattenuation. Here, a twisted unipolar-barrier van der Waals heterostructure (vdWH) to construct a bias-switchable polarization detection for retrieval of full SOP (from 0 to 180°) for linear polarized incident light is reported. As a demonstration example, this study realizes the concept in a b-AsP/WS2 /b-AsP vdWH relying on the natural anisotropic properties of the materials without using additional plasmonic/metasurface nanostructures to realize linear polarimetry in the mid-infrared range. Polarimetric imaging is further demonstrated with the developed linear polarimetry by directly displaying the Jones-vector-described SOP distribution of certain target object. This method, with the capabilities of detecting full linear SOP, is promising for the next-generation on-chip miniaturized polarimeters.

Electrically tunable two-dimensional heterojunctions for miniaturized near-infrared spectrometers.

Miniaturized spectrometers are of considerable interest for their portability. Most designs to date employ a photodetector array with distinct spectral responses or require elaborated integration of micro & nano optic modules, typically with a centimeter-scale footprint. Here, we report a design of a micron-sized near-infrared ultra-miniaturized spectrometer based on two-dimensional van der Waals heterostructure (2D-vdWH). By introducing heavy metal atoms with delocalized electronic orbitals between 2D-vdWHs, we greatly enhance the interlayer coupling and realize electrically tunable infrared photoresponse (1.15 to 1.47 µm). Combining the gate-tunable photoresponse and regression algorithm, we achieve spectral reconstruction and spectral imaging in a device with an active footprint < 10 µm. Considering the ultra-small footprint and simple fabrication process, the 2D-vdWHs with designable bandgap energy and enhanced photoresponse offer an attractive solution for on-chip infrared spectroscopy.

High performance sub-bandgap photodetection via internal photoemission based on ideal metal/2D-material van der Waals Schottky interface.

Two-dimensional (2D) materials have been demonstrated to be promising candidates to design high performance photodetectors owing to their strong light-matter interaction. However, the performance of 2D material photodetectors is still unsatisfactory, such as slow response speed due to defects and vulnerable contact interface, which impede their rapid development in the field of optoelectronics. In this paper, we obtained the ideal and large photosensitive van der Waals Schottky interface by the laminating-flipping method. Hence, a fast response speed (<1 ms) and high detectivity (>1012 Jones) are observed on the van der Waals Schottky junction photodiode. More importantly, benefiting from the flat Schottky interface (the roughness ∼0.6 nm), a sub-bandgap light response modulated by the Schottky barrier height (cut-off edge at 1050 nm) has been detected based on the large Au/MoSe2 sensitive Schottky interface internal photoemission. As a result, a universal strategy for the sub-bandgap near-infrared van der Waals Schottky junction detector of 2D materials was obtained.

Controlled synthesis of few-layer SnSe2 by chemical vapor deposition.

Few-layer SnSe2 has intrinsic low thermal conductivity and unique phase transition from amorphous to crystalline state under laser irradiation. It has been extensively used in the fields of thermoelectric conversion and information storage. However, the traditional precursors like tin oxide and organic compounds have either high melting points or complex compositions, and the improper deposition temperature of the substrate may lead to mixed products, which impedes controllable synthesis of high-quality few-layer SnSe2. Here, we propose a chemical vapor deposition (CVD) method, in which the precursor evaporation and deposition have been controlled via the adjustment of precursors/substrate positions, which effectively avoided mixed product growth, thus achieving the growth of high-quality few-layer SnSe2. The calculated first-order temperature coefficient of the A1g module is -0.01549 cm-1 K-1, which is superior to other two-dimensional (2D) materials. Meanwhile, two exciton emissions from few-layer SnSe2 have been found, for which the higher energy one (1.74 eV) has been assigned to near-band-gap emission, while the lower one (1.61 eV) may have roots in the surface state of SnSe2. The few-layer SnSe2 also exhibits large exciton binding energies (0.195 and 0.177 eV), which are greater than those of common semiconductors and may contribute to stability of excitons, showing broad application prospects in the field of optoelectronics.

Strain Effect Enhanced Ultrasensitive MoS2 Nanoscroll Avalanche Photodetector.

Two-dimensional (2D) materials and their derived quasi one-dimensional structure provide incredible possibilities for the field of photoelectric detection due to their intrinsic optical and electrical properties. However, the photogenerated carriers in atomically thin media are poor due to the low optical absorption, which greatly limits their performance. Here, in the MoS2 nanoscroll photodetector, we meticulously investigated the avalanche multiplication effect. The results show that by employing the nanoscroll structure, the required threshold electrical field for triggering avalanche multiplication is significantly lower than that of MoS2 flake due to the modulation of the energy band and intervalley scattering through the strain effect. Consequently, avalanche multiplication could efficiently enhance the photoresponsivity to >104 A/W. Furthermore, enhanced avalanche multiplication could be generalized to other TMDCs through theoretical prediction. The results not only are significant for the understanding of the intrinsic nature of 2D materials but also reveal meaningful advances in high-performance and low-power consumption photodetection.