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High-performance visible and near-infrared photodetection and single-pixel imaging can be achieved by our γ-InSe/Ge photodiode at zero bias. Meanwhile, a maximum polarization sensitivity of 3.01 is obtained under 635 nm among the broadband spectrum.

Two-dimensional (2D) polarization-sensitive detection as a new photoelectric application technology is extensively investigated. However, most devices are mainly based on individual anisotropic materials, which suffer from large dark current and relatively low anisotropic ratio, limiting the practical application in polarized imaging system. Herein, we design a van der Waals (vdWs) p-type SnS/n-type InSe vertical heterojunction with proposed type-II band alignment

A solution-fabricated Te/Si mixed-dimensional vdWH can function well as a high-performance broadband self-powered photodetector, achieving an ultrahigh responsivity of 6.49 A W−1 at 0 V.

Two-dimensional (2D) bismuth oxyselenide (Bi2O2Se) has attracted increasing attention due to its high mobility, tunable band gap, and air stability. The surface reconstruction of cleaved Bi2O2Se due to the electrostatic interlayer interactions can lead to the in-plane anisotropic structure and physics. In this work, we first discovered the strong anisotropy in phonon modes through the angle-resolved polarized Raman (ARPR) spectra. Benefiting from the anisotropic feature, a high-performance polarization-sensitive photodetector has been achieved by constructing a heterostructure composed of the multilayer Bi2O2Se as polarized-light sensitizers and 2D WSe2 as a photocarrier transport channel. The detectors exhibit broadband response spectra from 405 to 1064 nm along with high responsivity, fast speed, and high sensitivity owing to the photogating effect in this device architecture. More importantly, the photocurrent shows strong light polarization dependence with the maximum dichroism ratio of 4.9, and a reversal is observed for the angle-dependent photocurrent excited by polarized 405 and 635 nm light. This work provides new insight in terms of optical and photocurrent anisotropy of exfoliated Bi2O2Se and expands its applications in angle-resolved electronics and optoelectronics.

Polarization-sensitive photodetectors based on van der Waals heterojunctions (vdWH) have excellent polarization-resolved optoelectronic properties that can enable the applications in polarized light identification and imaging. With the development of optical microcomputer control systems (OMCS), it is crucial and energy efficient to adopt the self-powered and polarization-resolved signal-generators to optimize the circuit design of OMCS. In this work, the selenium (Se) flakes with in-plane anisotropy and p-type character are grown and incorporated with n-type tungsten disulfide (WS

van der Waals (vdW) heterostructures by artificially stacking different two-dimensional (2D) materials are a promising platform to achieve diverse physical phenomena and device functions. In general, the gate voltage is utilized to modulate the carrier concentration and transport, thus realizing novel electronic and optoelectronic devices such as logic circuit and ternary inverters. Here, we present a novel out-of-plane heterostructure stacked by the layered p-type MoTe2 and n-type InSe. An anti-ambipolar transfer character was found in the heterojunction with a large work window (>60 V) and a high peak-to-valley current ratio (>103). The underlying transport mechanism of the anti-ambipolar behavior was investigated through the energy band structure, and we then proposed a basic design principle for constructing anti-ambipolar devices based on a 2D vdW heterostructure with type-II band alignment. In addition, under 405 nm laser irradiation, the heterojunction exhibits a high self-driven photodetection performance with a photoresponsivity of 15.4 mA W−1 and a specific detectivity up to ∼3.02 × 1014 jones in the absence of external bias. This work not only demonstrates high-performance multifunctional optoelectronics based on InSe/MoTe2 heterostructures, but also provides a new understanding of the anti-ambipolar band alignment and its applications in multivalued logic circuits and flexible devices.

Two-dimensional (2D) piezoelectric materials are mechanically flexible and atomically thin, enabling new emerging applications, such as wearable electronics, human–machine interfacing and strain sensors. The main obstacles for 2D piezoelectric materials are their low in-plane polarization and weak piezoelectric effect. In this study, we found piezoelectric effect in monolayer WS2 for the first time. Further, the in-plane uniaxial deformation was applied along the polarized armchair direction of WS2, giving rise to an enlarged in-plane polarization as evidenced from the higher Raman anisotropy of angle-resolved polarized Raman spectra. As a result, engineered WS2 exhibited a significantly improved dynamic piezoelectric signal by almost one order of magnitude. This study demonstrates a new strategy to artificially engineer monolayer WS2 by applying in-plane deformation for the larger anisotropy, higher polarization and more superior piezoelectricity, which can open up a new way for applications in 2D-based piezotronics and angle-dependent electronics.

Because of the in-gap defect levels, high deep ultraviolet (DUV) light absorption and low leakage current, the incorporation of a 4H-silicon carbide (SiC) substrate has been confirmed to enhance the optoelectrical properties of two-dimensional (2D) layered materials in recent years. Tin disulfide (SnS2) is a post-transition metal dichalcogenide (PTMD) with high light absorption coefficient and thickness-dependent band gap modulation, leading to potential applications in solar cells, DUV-visible photodetectors (PDs), and flexible wearable devices. Here, we first transferred SnS2 nanosheets onto a SiO2 layer and epitaxial n−-type 4H-SiC substrate, respectively. The optoelectrical performance of the SnS2/4H-SiC structure is improved under the UV and visible light. In particular, under 325 nm illumination, the photocurrent density significantly increased by ∼44 times, the response time is shortened to 17 ms, and the specific detectivity (D*) increases to 7.3 × 1013 Jones. Moreover, the responsivity (R325 nm) of 2.42 × 104 A W−1 is increased by one order of magnitude, which outperforms most of 2D materials DUV PDs reported up to date. In particular, the enhancement of mobility, decreased trap density, and suppression of dark current can be also obtained in the SnS2/SiC structure compared with the SnS2/SiO2 structure. This work provides a facile route toward 2D materials/SiC-based PDs with high performance, facile processing, and simple architecture for future applications in industrial, environmental, and even biological fields.

One-dimensional (1D)–two-dimensional (2D) van der Waals (vdWs) mixed-dimensional heterostructures with advantages of an atomically sharp interface, high quality and good compatibility have attracted tremendous attention in recent years. Herein, a mixed-dimensional vertical heterostructure is constructed by transferring mechanically exfoliated 2D WS2 nanosheets on epitaxially grown 1D tellurium (Te) microwires. According to the theoretical type-II band alignment, the device exhibits a photovoltaic effect and serves as an excellent self-powered photodetector with a maximum open-circuit voltage (Voc) up to ∼0.2 V. Upon 635 nm light illumination, the photoresponsivity, external quantum efficiency and detectivity of the self-powered photodetector (SPPD) are calculated to be 471 mA W−1, 91% and 1.24 × 1012 Jones, respectively. Moreover, the dark current of the SPPD is highly suppressed to the sub-pA level due to the large lateral built-in electric field, which leads to a high Ilight/Idark ratio of 104 with a rise time of 25 ms and decay time of 14.7 ms. The abovementioned properties can be further enhanced under a negative bias of −2 V. In brief, the 1D Te–2D WS2 mixed-dimensional heterostructures have great application potential in high performance photodetectors and photovoltaics.

Polarization-sensitive photodetectors in the infrared range have attracted considerable attention because of their unique and wide application prospects in polarization sensors and remote sensing. However, it is challenging to achieve short-wave infrared polarization detection as most polarization-sensitive photodetectors are based on transition-metal dichalcogenide (TMD) materials with in-plane symmetric crystal structure and sizable band gap (1-2 eV). In this work, we design a type-II GeAs/WS

Background charges or defects can substantially lower the photoluminescence (PL) in two-dimensional (2D) transition-metal dichalcogenides (TMDCs). Previous technologies such as chemical passivation...

Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells (PSCs). Interface engineering is a promising route for further improving the performance of PSCs. Here we perform first-principles calculations to explore the effect of four candidate buffer materials (MACl, MAI, PbCl2 and PbI2) on the electronic structures of the interface between MAPbI3 absorber and TiO2. We find that MAX (X = Cl, I) as buffer layers will introduce a high electron barrier and enhance the electron-hole recombination. Additionally, MAX does not passivate the surface states well. The conduction band minimum of PbI2 is much lower than that of MAPbI3 absorber, which significantly limits the band bending of the absorber and open-circuit voltage of solar cells. On the other side, suitable bandedge energy level positions, small lattice mismatch with TiO2 surfaces, and excellent surface passivation make PbCl2 a promising buffer material for absorber/electron-transport-layer interface engineering in PSCs. Our results in this work thus provide deep understanding on the effects of interface engineering with a buffer layer, which shall be useful for improving the performance of PSCs and related optoelectronics.

Two-dimensional (2D) materials have undergone a rapid development toward real applications since the discovery of graphene. At first, graphene is a star material because of the ultrahigh mobility and novel physics, but it always suffered from zero bandgap and limited device application. Then, 2D binary compounds such as transition-metal chalcogenides emerged as complementary materials for graphene due to their sizable bandgap and moderate electrical properties. Recently, research interests have turned to monoelemental and ternary 2D materials. Among them, monoelemental 2D materials such as arsenic (As), antimony (Sb), bismuth (Bi), tellurium (Te), etc., have been the focus. For example, bismuthene can act as a 2D topological insulator with nontrivial topological edge states and high bulk gap, providing the novel platforms to realize the quantum spin-Hall systems. Meanwhile, ternary 2D materials such as Bi2O2Se, BiOX and CrOX (X=Cl, Br, I) have also emerged as promising candidates in optoelectronics and spintronics due to their extraordinary mobility, favorable band structures and intrinsic ferromagnetism with high Curie temperature. In this review, we will discuss the recent works and future prospects on the emerging monoelemental and ternary materials in terms of their structure, growth, physics and device applications.

Atomically thin two-dimensional (2D) materials make it possible to create a variety of van der Waals (vdW) heterostructures with different physical features and attributes, which enables the growth of innovative electronics and optoelectronics applications. The band alignment and charge transfer play a crucial role in the physical and optoelectrical properties of the vdW heterostructure. Here, we design a vdW heterojunction device comprising low-symmetric CrOCl to induce a stable anti-ambipolar behavior and polarization-sensitive photodetection performance. 2D CrOCl exhibits strong in-plane anisotropy and linear dichroism, and an anti-ambipolar transport behavior is observed in a MoTe2 channel due to the gate-tunable band bending and charge transfer at MoTe2/CrOCl interface. The devices also exhibit well photodetection performance with a responsivity of 1.05 A/W and a temporal response of 970 μs. Owing to the anisotropic CrOCl serving as a photosensitizing layer, the device achieves the capability of polarization-sensitive photodetection with a photocurrent dichroic ratio up to ∼6. This work offers a valid device model and design strategy to realize the versatile optoelectronics, including the anti-ambipolar transistor and polarimetric photodetectors.

Polarization-sensitive photodetection is highly appealing considering its great important applications. However, the inherent in-plane symmetry of a material and the single structure of a detector hinder the further development of polarization detectors with high anisotropic ratios. Herein, we design a p-WSe

Since the discovery of Dirac semimetal graphene, two-dimensional (2D) Weyl semimetals (WSMs) have been widely used in low-energy photon detection, polarization imaging, and other systems due to their rich physical characteristics, such as unique nonlinear optical structure, topological nontrivial electronic structure, thickness-tunable bandgap, high electric conductivity, and so on. However, it is difficult to detect the photocurrent signal at room temperature because of its large intrinsic background current. Fortunately, the fabrication of a van der Waals (vdW) heterojunction based on WSM can effectively suppress the background current, greatly extend the detection range, improve the light absorption efficiency, and increase the response speed. Herein, the 2D type-II WSM 1T′-WTe2/bulk GaAs vdW vertical Schottky diode is investigated. Benefiting from the lateral built-in electric field of 260 meV and zero-bandgap structure of 52 nm 1T′-WTe2, it delivers a rectifying ratio over 103 and can respond to the wavelength range of 400–1100 nm. Particularly, when the light power density is 0.02 mW/cm2, the maximum photoresponsivity (R) and specific detectivity (D*) under 808 nm are 298 mA/W and 1.70 × 1012 Jones, respectively. Meanwhile, the Ilight/Idark ratio and response time are 103 and 520/540 μs, respectively. Moreover, an abnormal negative response behavior can be observed with thin WTe2 (11 nm) under 1064 nm illumination because of the open surface bandgap. It is suggested that such 2D WTe2/GaAs mixed-dimensional vdW structure can be extended to other WSM/3D semiconductor junctions and used in fast response and wide broadband spectrum photodetectors' arrays.

Stimuli-responsive crystals capable of energy conversion have emerged as promising materials for smart sensors, actuators, wearable devices, and robotics. Here, a novel ferrocene-based organic molecule crystal (Fc-Cz) that possesses anisotropic piezoelectric, optical, and mechanical properties is reported. It is demonstrated that the new crystal Fc-Cz can be used as an ultrasensitive piezoelectric material in fabricating strain sensors. The flexible sensor made of crystal Fc-Cz can detect small strains/deformations and motions with a fast response speed. Analysis based on density functional theory (DFT) indicates that an external pressure can affect the dipole moment by changing the molecular configuration of the asymmetric single crystal Fc-Cz in the crystalline state, leading to a change of polarity, and thereby an enhanced dielectric constant. This work demonstrates a new artificial organic small molecule for high-performance tactile sensors, indicating its great potential for developing low-cost flexible wearable sensors.

Discovery of topological materials associated with an exotic phenomenon has attracted increasing attention in modern condensed matter physics. A typical example is the chiral anomaly proposed in the Dirac or Weyl semimetals. In addition to the well-known topological semimetals, such as TaAs and Na3Bi, recently, group IV GeSn alloys were also proposed to be Dirac semimetals in theory, demonstrating potential applications compatible with current Si-based technology. Here, we report the observation of large negative magnetoresistance (MR) that is sensitive to the orientation of the magnetic and electric field in the GeSn strip. This negative MR emerges only when the applied magnetic field is parallel to the electric field, which is consistent with the chiral anomaly in topological semimetals. This work paves a new way toward exploring the negative MR behavior and underlying mechanism in a new class of Dirac semimetals.

Two-dimensional (2D) bismuth triiodide (BiI3) has been emerging as a potential layered material for optoelectronic applications due to its air stability and high atomic density. Although much effort has been devoted to improvements of carrier mobility, conductivity and photoelectric response, performance is still very limited. Here, we report a simple and scalable strategy for greatly improving the electrical and optical properties of 2D BiI3 through high-κ dielectric engineering. Upon covering with a high-κ dielectric oxide (Al2O3), air isolation and dielectric screening effects can lead to the reduction of the contact barrier, passivation of trap states, and suppression of Coulomb scattering. As a result, BiI3-based phototransistors can increase carrier mobility by three orders of magnitude and improve photoresponsivity by three orders of magnitude up to 8.05 × 103 A W−1. This work develops a new 2D BiI3 material and efficient dielectric engineering for improving mobility and photoelectrical performance, expanding the family of 2D materials and offering a promising strategy for potential nano-device applications.

One-dimensional (1D)-two-dimensional (2D) van der Waals (vdWs) mixed-dimensional heterostructures with advantages of an atomically sharp interface, high quality and good compatibility have attracted tremendous attention in recent years. Herein, a mixed-dimensional vertical heterostructure is constructed by transferring mechanically exfoliated 2D WS

Non-centrosymmetric crystals with piezoelectric properties have emerged as promising materials for smart wearable systems and biomimetic robots. Here we present a novel small ferrocene-based organic molecule crystal (Fc-Cz) possessing high anisotropic-dependent optical and electronic properties, which has been utilized as an ultrasensitive piezoelectric material for the development of a strain sensor. The flexible piezoelectric sensor can distinguish subtle strain or deformations (such as wrist motion) with fast response time (< 40 ms) via detectable piezoelectric signals (Imax = 580 pA). Density functional theory (DFT) indicated that the external pressure can affect the dipole moment by changing the molecular configuration of the asymmetric single crystal Fc-Cz in the crystalline state, leading to a change of polarity, as well as an enhanced dielectric constant. Based on our knowledge, this work is the first example verifying that artificial organic small molecules can serve as simple, stable, high-performance tactile sensors, and this has the potential to open the door to low-cost flexible wearable devices and energy harvesting applications.

Mobility engineering is one of the most important challenges that determine the optoelectronic performance of two-dimensional (2D) materials. So far, charged-impurity scattering and electrical-contact barriers have been suppressed through high-κ dielectrics and seamless contact engineering, giving rise to carrier-mobility improvement in exfoliated 2D semiconducting MoS2. Here we demonstrate a facile and scalable technique to effectively suppress both Coulomb scattering and electron–phonon scattering via the HfO2 overlayer, resulting in a large mobility improvement in CVD-grown monolayer MoS2, in excess of 60 cm2 V−1 s−1. Surface passivation and suppression of Coulomb scattering can partially contribute to the mobility increase. Interestingly, we correlate the mobility increase with phonon quenching through Raman and temperature-dependent mobility measurements. The experimental method is facile, industrially scalable, and renders phonon engineering an additional leverage towards further improvements in 2D semiconductor mobility and device performance.

Hybrids composed of two-dimensional (2D) and zero-dimensional (0D) materials have demonstrated great application potentials in electronics and optoelectronics. The CdSe@Zn1−XCdXS (CSZCS) quantum dots (QDs) possess unique gradient band structure with a continuously increasing energy level along the radial direction from the center to the surface, which is favorable for light-harvesting, photocarrier transfer and promising for photovoltaic and photodetection applications. Here, the gradient alloyed CSZCS QDs, acting as a photosensitive layer, have been first integrated with 2D InSe as a carrier transport layer. The new 2D–0D hybrids exhibit a 300-fold improvement in responsivity compared with that of pristine InSe, which is much more superior to hybrids composed of core–shell CdSe@ZnS QDs/InSe. Thanks to the low dark current and large photo-gain induced by the photo-gating effect, the responsivity and detectivity of the CSZCS QDs/InSe photodetector can reach up to 30.16 A/W and 1.69 × 1012 Jones, respectively. This work provides a new kind of promising QDs with the gradient alloyed structure that can be explored into 2D–0D hybrids for further development of high-performance photodetectors.

Carrier mobility is one of most important figures of merit for materials that can determine to a large extent the corresponding device performances. So far, extensive efforts have been devoted to the mobility improvement of two-dimensional (2D) materials regarded as promising candidates to complement the conventional semiconductors. Graphene has amazing mobility but suffers from zero bandgap. Subsequently, 2D transition-metal dichalcogenides benefit from their sizable bandgap while the mobility is limited. Recently, the 2D elemental materials such as the representative black phosphorus can combine the high mobility with moderate bandgap; however the air-stability is a challenge. Here, we report air-stable tellurium flakes and wires using the facile and scalable physical vapour deposition (PVD) method. The prototype field-effect transistors were fabricated to exhibit high hole mobility up to 1485 cm 2 V −1 s −1 at room temperature and 3500 cm 2 V −1 s −1 at low temperature (2 K). This work can attract numerous attentions on this new emerging 2D tellurium and open up a new way for exploring high-performance optoelectronics based on the PVD-grown p-type tellurium.

Metal halide perovskite nanocrystals (NCs) possess high photoluminescence (PL) quantum yield (PL) and color tunability. Yet, until now, it has been difficult to maintain the high PL observed in solution in spin-coated films. Here, we report a novel CsPbBr3 NCs room-temperature synthesis based on Cs acetate, which displays near-unity PL in solid-state films upon post-synthetic treatment with lead bromide. The as-synthesized NCs show a PL of 80% in spin-coated films but the post-synthesis treatment further enhances the efficiency >95%. The high PL is further confirmed by the monomolecular decay of the PL (5.8 ns) indicating a nearly complete suppression of non-radiative channels. The obtained films demonstrate high stability in air and require around 3 weeks to decrease to half the initial PL value.

Two-dimensional transition metal dichalcogenide-based photodetectors have demonstrated potential for the next generation of 2-dimensional optoelectronics. However, to date, their sensitivity has not been superior to that of other technologies. Here we report an ultrasensitive two-dimensional photodetector employing an in-plane phototransistor with an out-of-plane vertical MoS2 p–n junction as a sensitizing scheme. The vertical built-in field is introduced for the first time in the transport channel of MoS2 phototransistors by facile chemical surface doping, which separates the photo-excited carriers efficiently and produces a photoconductive gain of >105 electrons per photon, external quantum efficiency greater than 10%, responsivity of 7 × 104 A W−1, and a time response on the order of tens of ms. This taken together with a very low noise power density yields a record sensitivity with specific detectivity $$D^*$$ D * of 3.5 × 1014 Jones in the visible and a broadband response up to 1000 nm.

Van der Waals heterojunctions, stacked with different two-dimensional materials, have enabled novel optoelectronic functionalities with high performances for various applications such as in photodetectors, solar cells and light-emitting diodes. Herein, we fabricated MoSe2 and CVD-grown MoS2 heterojunctions with significant gate modulated photovoltaic effect with an open-circuit voltage of 0.16 V and power conversion efficiency of ∼0.5% due to the type-II band alignment and gate tunable band slope. Moreover, the photo-responsivity was dramatically improved to ∼350 A W−1, which is two orders of magnitude larger than that of the isolated components (MoSe2 or MoS2), and the temporal response was as fast as ∼10 ms at negative back gate. All these results indicate the vast potential applications of heterojunctions, with facile procedure, in photovoltaic cells and photodetectors.

Two-dimensional (2D) material based photovoltaic devices have attracted attention due to their atomically thin profile, strong light–matter interaction and mechanical flexibility. van der Waals heterojunctions, assembled with different 2D materials and a homojunction, are achieved with a doping technique and have been demonstrated to have considerable photovoltaic properties. Here, a more facile approach is proposed to realise an in-plane MoSe2 homojunction with a gate tunable and highly efficient photovoltaic effect. By applying a negative back gate, the device can exhibit a current rectification ratio as high as 105 and a significant Voc of 0.24 V under illumination. The high power conversion efficiency of 1.9% and high photo-responsivity of 550 A W−1 with a fast temporal response of 10 ms indicate that our MoSe2 homojunction based device is a promising candidate in photovoltaic and photodetection applications.