Self-Rolled-Up WSe2 One-Dimensional/Two-Dimensional Homojunctions: Enabling High-Performance Self-Powered Polarization-Sensitive Photodetectors.

Mixed-dimensional heterostructures integrate materials of diverse dimensions with unique electronic functionalities, providing a new platform for research in electron transport and optoelectronic detection. Here, we report a novel covalently bonded one-dimensional/two-dimensional (1D/2D) homojunction structure with robust junction contacts, which exhibits wide-spectrum (from the visible to near-infrared regions), self-driven photodetection, and polarization-sensitive photodetection capabilities. Benefiting from the ultralow dark current at zero bias voltage, the on/off ratio and detectivity of the device reach 1.5 × 103 and 3.24 × 109 Jones, respectively. Furthermore, the pronounced anisotropy of the WSe2 1D/2D homojunction is attributed to its low symmetry, enabling polarization-sensitive detection. In the absence of any external bias voltage, the device exhibits strong linear dichroism for wavelengths of 638 and 808 nm, with anisotropy ratios of 2.06 and 1.96, respectively. These results indicate that such mixed-dimensional structures can serve as attractive building blocks for novel optoelectronic detectors.

Broad -band nonlinear optical response in Bi2Te0.6S2.4 alloys based on alloy engineering.

Alloy engineering plays an important role in regulating the optoelectronic properties of materials. This work demonstrates that Bi2Te0.6S2.4 alloys can extend nonlinear optical response to the near-infrared range. Te alloying at S sites can narrow the band gap, as proved by density functional theory (DFT) calculations, leading to a broadband saturable absorption response ranging from ultraviolet (350 nm) to near-infrared (1100 nm) wavelength with negative nonlinear optical absorption coefficient ranging from -0.12 cm GW-1 to -1.28 cm GW-1. Moreover, the broadband carrier dynamic of Bi2Te0.6S2.4 alloys was investigated via femtosecond transient absorption (TA) at an excitation of 325 nm. A faster carrier dynamic at near-infrared wavelength was observed because of an increase in electron density at the conduction band minimum due to the additional Bi-Te interaction, which was corroborated by DFT calculations. These results suggest that alloy engineering provides an effective way for the development of broadband nonlinear optical devices.

Spectrometer-Less Remote Sensing Image Classification Based on Gate-Tunable van der Waals Heterostructures.

Remote sensing technology, which conventionally employs spectrometers to capture hyperspectral images, allowing for the classification and unmixing based on the reflectance spectrum, has been extensively applied in diverse fields, including environmental monitoring, land resource management, and agriculture. However, miniaturization of remote sensing systems remains a challenge due to the complicated and dispersive optical components of spectrometers. Here, m-phase GaTe0.5Se0.5 with wide-spectral photoresponses (250-1064 nm) and stack it with WSe2 are utilizes to construct a two-dimensional van der Waals heterojunction (2D-vdWH), enabling the design of a gate-tunable wide-spectral photodetector. By utilizing the multi-photoresponses under varying gate voltages, high accuracy recognition can be achieved aided by deep learning algorithms without the original hyperspectral reflectance data. The proof-of-concept device, featuring dozens of tunable gate voltages, achieves an average classification accuracy of 87.00% on 6 prevalent hyperspectral datasets, which is competitive with the accuracy of 250-1000 nm hyperspectral data (88.72%) and far superior to the accuracy of non-tunable photoresponse (71.17%). Artificially designed gate-tunable wide-spectral 2D-vdWHs GaTe0.5Se0.5/WSe2-based photodetector present a promising pathway for the development of miniaturized and cost-effective remote sensing classification technology.

Centimeter-Scale PdS2 Ultrathin Films with High Mobility and Broadband Photoresponse.

2D materials with mixed crystal phase will lead to the nonuniformity of performance and go against the practical application. Therefore, it is of great significance to develop a valid method to synthesize 2D materials with typical stoichiometry. Here, 2D palladium sulfides with centimeter scale and uniform stoichiometric ratio are synthesized via controlling the sulfurization temperature of palladium thin films. The relationship between sulfurization temperature and products is investigated in depth. Besides, the high-quality 2D PdS2 films are synthesized via sulfurization at the temperature of 450-550 °C, which would be compatible with back-end-of-line processes in semiconductor industry with considering of process temperature. The PdS2 films show an n-type semiconducting behavior with high mobility of 10.4 cm2 V-1 s-1 . The PdS2 photodetector presents a broadband photoresponse from 450 to 1550 nm. These findings provide a reliable way to synthesizing high-quality and large-area 2D materials with uniform crystal phase. The result suggests that 2D PdS2 has significant potential in future nanoelectronics and optoelectronic applications.

Doping Engineering in the MoS2 /SnSe2 Heterostructure toward High-Rejection-Ratio Solar-Blind UV Photodetection.

The intentionally designed band alignment of heterostructures and doping engineering are keys to implement device structure design and device performance optimization. According to the theoretical prediction of several typical materials among the transition metal dichalcogenides (TMDs) and group-IV metal chalcogenides, MoS2 and SnSe2 present the largest staggered band offset. The large band offset is conducive to the separation of photogenerated carriers, thus MoS2 /SnSe2 is a theoretically ideal candidate for fabricating photodetector, which is also verified in the experiment. Furthermore, in order to extend the photoresponse spectrum to solar-blind ultraviolet (SBUV), doping engineering is adopted to form an additional electron state, which provides an extra carrier transition channel. In this work, pure MoS2 /SnSe2 and doped MoS2 /SnSe2 heterostructures are both fabricated. In terms of the photoelectric performance evaluation, the rejection ratio R254 /R532 of the photodetector based on doped MoS2 /SnSe2 is five orders of magnitude higher than that of pure MoS2 /SnSe2 , while the response time is obviously optimized by 3 orders. The results demonstrate that the combination of band alignment and doping engineering provides a new pathway for constructing SBUV photodetectors.

Gate-controlled ambipolar transport in b-AsP crystals and their VIS-NIF photodetection.

As a new two-dimensional elemental layered semiconductor, black phosphorus (b-P) has received tremendous attention due to its excellent physical and chemical properties and has potential applications in the fields of catalysis, energy, and micro/nano-optoelectronic devices. However, studies have found that b-P is very unstable and will decompose within a few minutes under humid air conditions. Element doping is an effective method for adjusting the physical and chemical properties of crystals. Theoretical and experimental studies have confirmed that the stability of b-P crystals is significantly improved after arsenic doping, and the crystals also exhibit excellent photoresponse and electrical transport performances. In this work, we investigate the physical properties of a component of black arsenic phosphorus crystals (b-As0.084P0.916) and the potential applications in field effect transistors (FETs) and broadband photodetectors. An obvious ambipolar behavior is observed in the transfer characteristics of b-As0.084P0.916 based FETs, with drain current modulation on the order of 105 and the highest charge-carrier mobility of up to 147 cm2 V-1 s-1. The physisorption of atmospheric species on the surface of the FETs is the main factor for the formation of Schottky contacts between the Au electrodes and the b-As0.084P0.916 crystal. Temperature-dependent electrical characteristics show that the Fermi level shifts from the valence band to the middle level between the conduction band and valence band as the temperature decreases. In addition, the FETs also exhibit excellent photoresponse properties from the visible to near-infrared region (450-2200 nm), with a responsivity of 37 A W-1, a specific detectivity of 7.18 × 1010 Jones, and a fast response speed (τrise ≈ 0.04 s and τdecay ≈ 0.14 s). These results suggest that b-As0.084P0.916 crystals are a promising candidate for future electronic and optoelectronic devices.

Rashba valleys and quantum Hall states in few-layer black arsenic.

Exciting phenomena may emerge in non-centrosymmetric two-dimensional electronic systems when spin-orbit coupling (SOC)1 interplays dynamically with Coulomb interactions2,3, band topology4,5 and external modulating forces6-8. Here we report synergetic effects between SOC and the Stark effect in centrosymmetric few-layer black arsenic, which manifest as particle-hole asymmetric Rashba valley formation and exotic quantum Hall states that are reversibly controlled by electrostatic gating. The unusual findings are rooted in the puckering square lattice of black arsenic, in which heavy 4p orbitals form a Brillouin zone-centred Γ valley with pz symmetry, coexisting with doubly degenerate D valleys of px origin near the time-reversal-invariant momenta of the X points. When a perpendicular electric field breaks the structure inversion symmetry, strong Rashba SOC is activated for the px bands, which produces spin-valley-flavoured D± valleys paired by time-reversal symmetry, whereas Rashba splitting of the Γ valley is constrained by the pz symmetry. Intriguingly, the giant Stark effect shows the same px-orbital selectiveness, collectively shifting the valence band maximum of the D± Rashba valleys to exceed the Γ Rashba top. Such an orchestrating effect allows us to realize gate-tunable Rashba valley manipulations for two-dimensional hole gases, hallmarked by unconventional even-to-odd transitions in quantum Hall states due to the formation of a flavour-dependent Landau level spectrum. For two-dimensional electron gases, the quantization of the Γ Rashba valley is characterized by peculiar density-dependent transitions in the band topology from trivial parabolic pockets to helical Dirac fermions.

In-Plane Optical and Electrical Anisotropy of 2D Black Arsenic.

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

All-Inorganic Perovskite CsPb2Br5 Nanosheets for Photodetector Application Based on Rapid Growth in Aqueous Phase.

All-inorganic cesium lead-halide perovskites exhibit a great development prospect in optoelectronic devices owing to their stability and remarkable optoelectronic properties. Herein, we investigate the solution-processed synthesis of perovskite CsPb2Br5 nanosheets by using aqueous and ethanol as solvents. The results show that the aqueous environment ensures the phase formation of CsPb2Br5 and that the supersaturated solution in ethanol boosts nucleation of the nanosheets. The substrate temperature is the key factor for the evolution of morphology and the variation of the thickness of CsPb2Br5 nanosheets. Lower substrate temperature (<35 °C) is conducive to the formation of evenly distributed nanosheets with less stacking. The spatial and time-resolved fluorescence spectra indicate the heterogeneity of the defect density and the recombination process in different nanosheet regions. The photodetector based on the prepared CsPb2Br5 nanosheet displays an excellent switching current ratio (9 × 102), a short rise and decay time (43 and 83 ms, respectively), and good stability (75% of the initial current after 90 days in air). In addition, the mechanical stability and flexibility of the photodetector on the flexible substrate are investigated for 500 bending cycles.

Temperature dependence of charge transport in solid-state molecular junctions based on oligo(phenylene ethynylene)s.

The ultimate goal of molecular electronics is to achieve practical applications. For approaching the target, we have successfully fabricated solid-state junctions based on oligo(phenylene ethynylene)s (OPEs) and cruciform OPEs with extended tetrathiafulvalene (TTF) (OPE3 and OPE3-TTF) self-assembled monolayers (SAMs) with a diamine anchoring group. SAMs were confined in micropores with gold substrates to ensure well-defined device surface areas. The transport properties were conducted on a double-junction layout, which the rGO films used for top contacts and interconnects between adjacent SAMs. The solid-state devices based on OPE3-TTF SAMs showed the expected higher conductance under ambient conditions because of the incorporation of a TTF moiety. The two devices displayed varying degrees of temperature dependence with decreasing temperature, which resulted from the cross-conjugated OPE3-TTF molecule exhibiting quantum interference while the linear-conjugated OPE3 molecule did not. This study shows the temperature dependence of the electrical properties of molecular devices based on cruciform OPEs, further enriching the research results of functional molecular devices.

Stability and Phase Transition of Metastable Black Arsenic under High Pressure.

Black arsenic (bAs) is a metastable phase of arsenic that has attracted increasing interest owing to its layered structure, tunable band gap, high carrier mobility, and large on/off ratio. Here, we systematically investigated the high-pressure behaviors of bAs up to ∼14 GPa. A phase transition from bAs to gray arsenic (gAs) occurred at critical pressure of 3.48 GPa, and the bAs and gAs coexisted between 3.48 and 5.37 GPa before bAs completely converted to gAs above 5.37 GPa. The structure was reversible for bAs after pressure was released from about 1-3 GPa, indicating the stability of bAs at pressures less than the critical pressure. At pressures above 5.37 GPa, bAs transformed to gAs and remained gAs after pressure was released. Molecular dynamics (MD) simulation was performed to explain the phase transition mechanism. This work provides insights into the phase stability and phase transition of metastable bAs under high pressure.

Enhanced Photoresponse of Indium-Doped Tin Disulfide Nanosheets.

Doping of tin disulfide (SnS2) is an effective strategy to regulate its physical and chemical properties. In this work, In doping was used to manipulate the photoresponse behavior of SnS2-based photodetectors. In-doped SnS2 nanosheets were synthesized via a facile hydrothermal method. It was found that the In doping concentration plays an important role in the size of the fabricated SnS2 nanosheets. With the increase in the In doping concentration, the lateral size of samples increased from ∼210 to ∼420 nm, but the crystallinity became poor at higher concentrations. Energy dispersive X-ray-mapping results show that the In was homogeneously distributed in the samples. In addition, a red shift was observed in the binding energy of Sn and S with increased In doping concentration, which may be due to the p-type doping of In in SnS2. After In doping, the performance of SnS2-based photodetectors was significantly improved. The photoresponse speed of In-doped SnS2-based photodetectors was faster than that of pristine SnS2-based devices under the illumination of 532 and 405 nm lasers. This work develops an effective approach of In doping to enhance the photoresponse characteristics of SnS2-based photodetectors and proves that In-doped SnS2 has a vast potential in optoelectronic applications.

Band-like transport in small-molecule thin films toward high mobility and ultrahigh detectivity phototransistor arrays.

With the fast development of organic electronics, organic semiconductors have been extensively studied for various optoelectronic applications, among which organic phototransistors recently emerged as one of the most promising light signal detectors. However, it is still a big challenge to endow organic phototransistors with both high mobility and high light-sensitivity because the low mobility of most organic photoresponsive materials limits the efficiency of transporting and collecting charge carriers. We herein report band-like charge transport in vacuum-deposited small-molecule thin films for organic phototransistor arrays which can be operated at very low dark currents (~10-12 A). Both high mobility and excellent optical figures of merit including photosensitivity, photoresponsivity and detectivity are achieved, wherein, unprecedentedly, a detectivity greater than 1017 cm Hz1/2 W-1 is obtained. All these key parameters are superior to state-of-the-art organic phototransistors, implying a great potential in optoelectronic applications.

Perpendicular Optical Reversal of the Linear Dichroism and Polarized Photodetection in 2D GeAs.

The ability to detect linearly polarized light is central to practical applications in polarized optical and optoelectronic fields and has been successfully demonstrated with polarized photodetection of in-plane anisotropic two-dimensional (2D) materials. Here, we report the anisotropic optical characterization of a group IV-V compound-2D germanium arsenic (GeAs) with anisotropic monoclinic structures. High-quality 2D GeAs crystals show the representative angle-resolved Raman property. The in-plane anisotropic optical nature of the GeAs crystal is further investigated by polarization-resolved absorption spectra (400-2000 nm) and polarization-sensitive photodetectors. From the visible to the near-infrared range, 2D GeAs nanoflakes demonstrate the distinct perpendicular optical reversal with a 75-80° angle on both the linear dichroism and polarization-sensitive photodetection. Obvious anisotropic features and the high dichroic ratio of Ipmax /Ipmin ∼ 1.49 at 520 nm and Ipmax /Ipmin ∼ 4.4 at 830 nm are achieved by the polarization-sensitive photodetection. The polarization-dependent photocurrent mapping implied that the polarized photocurrent mainly occurred at the Schottky photodiodes between electrode/GeAs interface. These experimental results are consistent with the theoretical calculation of band structure and band realignment. Besides the excellent polarization-sensitive photoresponse properties, GeAs-based photodetectors also exhibit rapid on/off response. These results demonstrate that the 2D GeAs crystals have promising potential for polarization optical applications.

Large tunneling magnetoresistance in magnetic tunneling junctions based on two-dimensional CrX3 (X = Br, I) monolayers.

Magnetic tunneling junctions (MTJs) have atomic thickness due to the use of two-dimensional (2D) materials. Combining density functional theory with non-equilibrium Green's function formalism, we systematically investigate the structural and magnetic properties of CrX3/h-BN/CrX3 (X = Br, I) MTJs, as well as their spin-dependent transport characteristics. Through calculation of the transmission spectrum, the large tunneling magnetoresistance (TMR) effect was observed in these MTJs. Moreover, their conductance based on two-dimensional materials was greatly improved over that of traditional MTJs. The transmission mechanism was analyzed using the symmetry of the orbit and the eigenstates of the transmitted electrons. We also discuss the problem of Schottky contact between different metal electrodes and devices. Our results suggest that MTJs based on two-dimensional ferromagnets are feasible.

Two-dimensional plumbum-doped tin diselenide monolayer transistor with high on/off ratio.

Doping can effectively regulate the electrical and optical properties of two-dimensional semiconductors. Here, we present high-quality Pb-doped SnSe2 monolayer exfoliated using a micromechanical cleavage method. X-ray photoelectron spectroscopy measurement demonstrates that Pb content of the doped sample is ∼3.6% and doping induces the downward shift of the Fermi level with respect to the pure SnSe2. Transmission electron microscopy characterization exhibits that Pb0.036Sn0.964Se2 nanosheets have a high-quality hexagonal symmetry structure and Pb element is uniformly distributed in the nanosheets. The current of the SnSe2 field effect transistors (FETs) was found to be very difficult to turn off due to the high electron density. The FETs based on the Pb0.036Sn0.964Se2 monolayer show n-type behavior with a high on/off ratio of 106 which is higher than any values of SnSe2 FETs reported at the moment. The estimated carrier concentration of Pb0.036Sn0.964Se2 is approximately six times lower than that of SnSe2. The results suggest that the method of reducing carrier concentration by doping to achieve high on/off ratio is effective, and Pb-doped SnSe2 monolayer has significant potential in future nanoelectronic and optoelectronic applications.

Black Arsenic: A Layered Semiconductor with Extreme In-Plane Anisotropy.

2D layered materials have emerged in recent years as a new platform to host novel electronic, optical, or excitonic physics and develop unprecedented nanoelectronic and energy applications. By definition, these materials are strongly anisotropic between the basal plane and cross the plane. The structural and property anisotropies inside their basal plane, however, are much less investigated. Black phosphorus, for example, is a 2D material that has such in-plane anisotropy. Here, a rare chemical form of arsenic, called black-arsenic (b-As), is reported as a cousin of black phosphorus, as an extremely anisotropic layered semiconductor. Systematic characterization of the structural, electronic, thermal, and electrical properties of b-As single crystals is performed, with particular focus on its anisotropies along two in-plane principle axes, armchair (AC) and zigzag (ZZ). The analysis shows that b-As exhibits higher or comparable electronic, thermal, and electric transport anisotropies between the AC and ZZ directions than any other known 2D crystals. Such extreme in-plane anisotropies can potentially implement novel ideas for scientific research and device applications.

Highly polarization sensitive photodetectors based on quasi-1D titanium trisulfide (TiS3).

Photodetectors with high polarization sensitivity are in great demand in advanced optical communication. Here, we demonstrate that photodetectors based on titanium trisulfide (TiS3) are extremely sensitive to polarized light (from visible to the infrared), due to its reduced in-plane structural symmetry. By density functional theory calculation, TiS3 has a direct bandgap of 1.13 eV. The highest photoresponsivity reaches 2500 A W-1. What is more, in-plane optical selection caused by strong anisotropy leads to the photoresponsivity ratio for different directions of polarization that can reach 4:1. The angle-dependent photocurrents of TiS3 clearly display strong linear dichroism. Moreover, the Raman peak at 370 cm-1 is also very sensitive to the polarization direction. The theoretical optical absorption of TiS3 is calculated by using the HSE06 hybrid functional method, in qualitative agreement with the observed experimental photoresponsivity.

A two-dimensional Fe-doped SnS2 magnetic semiconductor.

Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. In this study, we present a high-quality Fe-doped SnS2 monolayer exfoliated using a micromechanical cleavage method. Fe atoms were doped at the Sn atom sites, and the Fe contents are ∼2.1%, 1.5%, and 1.1%. The field-effect transistors based on the Fe0.021Sn0.979S2 monolayer show n-type behavior and exhibit high optoelectronic performance. Magnetic measurements show that pure SnS2 is diamagnetic, whereas Fe0.021Sn0.979S2 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ~31 K. Density functional theory calculations show that long-range ferromagnetic ordering in the Fe-doped SnS2 monolayer is energetically stable, and the estimated Curie temperature agrees well with the results of our experiment. The results suggest that Fe-doped SnS2 has significant potential in future nanoelectronic, magnetic, and optoelectronic applications.

Heterostructured ZnS/InP nanowires for rigid/flexible ultraviolet photodetectors with enhanced performance.

Heterostructured ZnS/InP nanowires, composed of single-crystalline ZnS nanowires coated with a layer of InP shell, were synthesized via a one-step chemical vapor deposition process. As-grown heterostructured ZnS/InP nanowires exhibited an ultrahigh Ion/Ioff ratio of 4.91 × 103, a high photoconductive gain of 1.10 × 103, a high detectivity of 1.65 × 1013 Jones and high response speed even in the case of very weak ultraviolet light illumination (1.87 µW cm-2). The values are much higher than those of previously reported bare ZnS nanowires owing to the formation of core/shell heterostructures. Flexible ultraviolet photodetectors were also fabricated with the heterostructured ZnS/InP nanowires, which showed excellent mechanical flexibility, electrical stability and folding endurance besides excellent photoresponse properties. The results elucidated that the heterostructured ZnS/InP nanowires could find good applications in next generation flexible optoelectronic devices.