Waveguide-integrated van der Waals heterostructure photodetector at telecom wavelengths with high speed and high responsivity

Balancing Carrier Dynamics in Oxygen-Vacancy-Tuned Amorphous Ga2O3 Thin-Film Self-Powered Photoelectrochemical-Type Solar-Blind Photodetector Arrays for Underwater Imaging

Underwater imaging technology plays a pivotal role in marine exploration and reconnaissance, necessitating photodetectors (PDs) with high responsivity, fast response speed, and low preparation costs. This study presents the synergistic optimization of responsivity and response speed in self-powered photoelectrochemical (PEC)-type photodetector arrays based on oxygen-vacancy-tuned amorphous gallium oxide (a-Ga2O3) thin films, specifically designed for solar-blind underwater detection. Utilizing a low-cost one-step sputtering process with controlled oxygen flow, a-Ga2O3 thin films with varying oxygen vacancy (VO) concentrations are fabricated. By balancing the trade-offs among electrocatalytic reactions, charge transfer, carrier recombination, and trapping, both the responsivity and response speed of a-Ga2O3-based self-powered PEC-PDs are simultaneously improved. Consequently, the optimized PEC-PDs demonstrated exceptional performance, achieving a responsivity of 33.75 mA W-1 and response times of 12.8 ms (rise) and 31.3 ms (decay), outperforming the vast majority of similar devices. Furthermore, a pronounced positive correlation between anomalous transient photocurrent spikes and the concentration of VO defects is observed, offering compelling evidence for VO-mediated indirect recombination. Finally, the proof-of-concept solar-blind underwater imaging system, utilizing an array of self-powered PEC-PDs, demonstrated clear imaging capabilities in seawater. This work provides valuable insight into the potential for developing cost-effective, high-performance a-Ga2O3 thin-film-based PEC-PDs for advanced underwater imaging technology.

Contact Resistance Engineering in WS2-Based FET with MoS2 Under-Contact Interlayer: A Statistical Approach

One of the primary factors hindering the development of 2D material-based devices is the difficulty of overcoming fabrication processes, which pose a challenge in achieving low-resistance contacts. Widely used metal deposition methods lead to unfavorable Fermi level pinning effect (FLP), which prevents control over the Schottky barrier height at the metal/2D material junction. We propose to harness the FLP effect to lower contact resistance in field-effect transistors (FETs) by using an additional 2D interlayer at the conducting channel and metallic contact interface (under-contact interlayer). To do so, we developed a new approach using the gold-assisted transfer method, which enables the fabrication of heterostructures consisting of TMDs monolayers with complex shapes, prepatterned using e-beam lithography, with lateral dimensions even down to 100 nm. We designed and demonstrated tungsten disulfide (WS2) monolayer-based devices in which the molybdenum disulfide (MoS2) monolayer is placed only in the contact area of the FET, creating an Au/MoS2/WS2 junction, which effectively reduces contact resistance by over 60% and improves the Ion/Ioff ratio 10 times in comparison to WS2-based devices without MoS2 under-contact interlayer. The enhancement in the device operation arises from the FLP effect occurring only at the interface between the metal and the first layer of the MoS2/WS2 heterostructure. This results in favorable band alignment, which enhances the current flow through the junction. To ensure the reproducibility of our devices, we systematically analyzed 160 FET devices fabricated with under-contact interlayer and without it. Statistical analysis shows a consistent improvement in the operation of the device and reveals the impact of contact resistance on key FET performance indicators.

Directional self-assembly of organic vertically superposed nanowires

Organic crystal-based superimposed heterostructures with inherent multichannel characteristics demonstrate superior potential for manipulating excitons/photons at the micro/nanoscale for integrated optoelectronics. However, the precise construction of organic superimposed heterostructures with fixed superimposed sites remains challenging because of the random molecular nucleation process. Here, organic vertically superimposed heterostructures (OSHs) with fixed superimposed positions are constructed via semi-wrapped core/shell heterostructures with partially exposed cores, which provide preferential nucleation sites for further molecular epitaxial growth processes. Furthermore, the relative length ratio from 21.7% to 95.3% between interlayers is accurately adjusted by regulating the exposed area of the semi-wrapped core/shell heterostructures. Significantly, these OSHs with anisotropic optical characteristics demonstrate well regulation of excitation position-dependent waveguide behaviors and can function as photonic barcodes for information encryption. This strategy provides a facile approach for controlling the nucleation sites for the controllable preparation of organic heterostructures and advanced applications for integrated optoelectronics.

Atomic-Thin WS2 Kirigami for Bidirectional Polarization Detection

The assembly and patterning engineering in two-dimensional (2D) materials hold importance for chip-level designs incorporating multifunctional detectors. At present, the patterning and stacking methods of 2D materials inevitably introduce impurity instability and functional limitations. Here, the space-confined chemical vapor deposition method is employed to achieve state-of-the-art kirigami structures of self-assembled WS2, featuring various layer combinations and stacking configurations. With this technique as a foundation, the WS2 nano-kirigami is integrated with metasurface design, achieving a photodetector with bidirectional polarization-sensitive detection capability in the infrared spectrum. Nano-kirigami can eliminate some of the uncontrollable factors in the processing of 2D material devices, providing a freely designed platform for chip-level multifunctional detection across multiple modules.

Efficient Charge Transfer in Graphene/CrOCl Heterostructures by van der Waals Interfacial Coupling

Due to the large volume of exposed atoms and electrons at the surface of two-dimensional materials, interfacial charge coupling has been proven as an efficient strategy to engineer the electronic structures of two-dimensional materials assembled in van der Waals heterostructures. Recently, heterostructures formed by graphene stacked with CrOCl have demonstrated intriguing quantum states, including a distorted quantum Hall phase in the monolayer graphene and the unconventional correlated insulator in the bilayer graphene. Yet, the understanding of the interlayer charge coupling in the heterostructure remains challenging. Here, we demonstrate clear evidences of efficient hole doping in the interfacial-coupled graphene/CrOCl heterostructure by detailed Raman spectroscopy and electrical transport measurements. The observation of significant blue shifts and stiffness of graphene Raman modes quantitatively determines the concentration of hole injection of about 1.2 × 1013 cm-2 from CrOCl to graphene, which is highly consistent with the enhanced conductivity of graphene. First-principles calculations based on density functional theory reveal that due to the large work function difference and the electronegativity of Cl atoms in CrOCl, the electrons are efficiently transferred from graphene to CrOCl, leading to hole doping in graphene. Our findings provide clues for understanding the exotic physical properties of graphene/CrOCl heterostructures, paving the way for further engineering of quantum electronic states by efficient interfacial charge coupling in van der Waals heterostructures.

Two-dimensional layered material photodetectors: what could be the upcoming downstream applications beyond prototype devices?

With distinctive advantages spanning excellent flexibility, rich physical properties, strong electrostatic tunability, dangling-bond-free surface, and ease of integration, 2D layered materials (2DLMs) have demonstrated tremendous potential for photodetection. However, to date, most of the research enthusiasm has been merely focused on developing novel prototype devices. In the past few years, researchers have also been devoted to developing various downstream applications based on 2DLM photodetectors to contribute to promoting them from fundamental research to practical commercialization, and extensive accomplishments have been realized. In spite of the remarkable advancements, these fascinating research findings are relatively scattered. To date, there is still a lack of a systematic and profound summarization regarding this fast-evolving domain. This is not beneficial to researchers, especially researchers just entering this research field, who want to have a quick, timely, and comprehensive inspection of this fascinating domain. To address this issue, in this review, the emerging downstream applications of 2DLM photodetectors in extensive fields, including imaging, health monitoring, target tracking, optoelectronic logic operation, ultraviolet monitoring, optical communications, automatic driving, and acoustic signal detection, have been systematically summarized, with the focus on the underlying working mechanisms. At the end, the ongoing challenges of this rapidly progressing domain are identified, and the potential schemes to address them are envisioned, which aim at navigating the future exploration as well as fully exerting the pivotal roles of 2DLMs towards the practical optoelectronic industry.

Anisotropic van der Waals Tellurene-Based Multifunctional, Polarization-Sensitive, In-Line Optical Device

Polarization plays a paramount role in scaling the optical network capacity. Anisotropic two-dimensional (2D) materials offer opportunities to exploit optical polarization-sensitive responses in various photonic and optoelectronic applications. However, the exploration of optical anisotropy in fiber in-line devices, critical for ultrafast pulse generation and modulation, remains limited. In this study, we present a fiber-integrated device based on a single-crystalline tellurene nanosheet. Benefiting from the chiral-chain crystal lattice and distinct optical dichroism of tellurene, multifunctional optical devices possessing diverse excellent properties can be achieved. By inserting the in-line device into a 1.5 μm fiber laser cavity, we generated both linearly polarized and dual-wavelength mode-locking pulses with a degree of polarization of 98% and exceptional long-term stability. Through a twisted configuration of two tellurene nanosheets, we realized an all-optical switching operation with a fast response. The multifunctional device also serves as a broadband photodetector. Notably, bipolar polarization encoding communication at 1550 nm can be achieved without any external voltage. The device's multifunctionality and stability in ambient environments established a promising prototype for integrating polarization as an additional physical dimension in fiber optical networks, encompassing diverse applications in light generation, modulation, and detection.

A simple 230 MHz photodetector based on exfoliated WSe2 multilayers

We demonstrate 230 MHz photodetection and a switching energy of merely 27 fJ using WSe2 multilayers and a very simple device architecture. This improvement over previous, slower WSe2 devices is enabled by systematically reducing the RC constant of devices through decreasing the photoresistance and capacitance. In contrast to MoS2, reducing the WSe2 thickness toward a monolayer only weakly decreases the response time, highlighting that ultrafast photodetection is also possible with atomically thin WSe2. Our work provides new insights into the temporal limits of pure transition metal dichalcogenide photodetectors and suggests that gigahertz photodetection with these materials should be feasible.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Graphene photodetectors integrated with silicon and perovskite quantum dots

Photodetectors (PDs) play a crucial role in imaging, sensing, communication systems, etc. Graphene (Gr), a leading two-dimensional material, has demonstrated significant potential for photodetection in recent years. However, its relatively weak interaction with light poses challenges for practical applications. The integration of silicon (Si) and perovskite quantum dots (PQDs) has opened new avenues for Gr in the realm of next-generation optoelectronics. This review provides a comprehensive investigation of Gr/Si Schottky junction PDs and Gr/PQD hybrid PDs as well as their heterostructures. The operating principles, design, fabrication, optimization strategies, and typical applications of these devices are studied and summarized. Through these discussions, we aim to illuminate the current challenges and offer insights into future directions in this rapidly evolving field.

High-speed carbon nanotube photodetector based on a planarized silicon waveguide

The integration of silicon waveguides with low-dimensional materials with excellent optoelectronic properties can enable compact and highly integrated optical devices with multiple advantages for multiple fields. A carbon nanotube (CNT) photodetector integrated on the silicon waveguide has the potential to meet on-chip high-speed optical interconnection systems, based on the outstanding properties of CNTs such as picosecond-level intrinsic photoresponse time, high charge carrier mobility, broad spectral response, high absorption coefficient, and so on. However, the thermal stability of the device may be compromised due to the local suspension in the channel for the height difference between the WG and the substrate. Here, we report a low-cost and low-optical-loss method to achieve the planarized silicon waveguide. After that, the CNT photodetectors integrated on the original and planarized waveguide with asymmetric palladium (Pd)-hafnium (Hf) metal contacts are fabricated. The influence of this planarization method on the performance of devices is analyzed via comparing the dark leakage current, the leakage current rectification ratio (CRR), the series resistances (R S), and the photoelectric response. Finally, a CNT photodetector based on the planarized waveguide with a photocurrent (I p h ) ∼510.84n A, a photoresponsivity (R I) ∼51.04m A/W, the dark current ∼0.389µA, as well as a 3 dB bandwidth ∼34G H z at the large reverse voltage -3V is shown.

Toward Gigahertz Photodetection with Transition Metal Dichalcogenides

ConspectusTransition metal dichalcogenides (TMDCs) exhibit favorable properties for optical communication in the gigahertz (GHz) regime, such as large mobilities, high extinction coefficients, cheap fabrication, and silicon compatibility. While impressive improvements in their sensitivity have been realized over the past decade, the bandwidths of these devices have been mostly limited to a few megahertz. We argue that this shortcoming originates in the relatively large RC constants of TMDC-based photodetectors, which suffer from high surface defect densities, inefficient charge carrier injection at the electrode/TMDC interface, and long charging times. However, we show in a series of papers that rather simple adjustments in the device architecture afford TMDC-based photodetectors with bandwidths of several hundreds of megahertz. We rationalize the success of these adjustments in terms of the specific physical-chemical properties of TMDCs, namely their anisotropic in-plane/out-of-plane carrier behavior, large optical absorption, and chalcogenide-dependent surface chemistry. Just one surprisingly simple yet effective pathway to fast TMDC photodetection is the reduction of the photoresistance by using light-focusing optics, which enables bandwidths of 0.23 GHz with an energy consumption of only 27 fJ/bit.By reflecting on the ultrafast intrinsic photoresponse times of a few picoseconds in TMDC heterostructures, we motivate the application of more demanding chemical strategies to exploit such ultrafast intrinsic properties for true GHz operation in real devices. A key aspect in this regard is the management of surface defects, which we discuss in terms of its dependence on the layer thickness, its tunability by molecular adlayers, and the prospects of replacing thermally evaporated metal contacts by laser-printed electrodes fabricated with inks of metalloid clusters. We highlight the benefits of combining TMDCs with graphene to heterostructures that exhibit the ultrafast photoresponse and large spectral range of Dirac materials with the low dark currents and high responsivities of semiconductors. We introduce the bulk photovoltaic effect in TMDC-based materials with broken inversion symmetry as well as a combination of TMDCs with plasmonic nanostructures as means for increasing the bandwidth and responsivity simultaneously. Finally, we describe the prospects of embedding TMDC photodetectors into optical cavities with the objective of tuning the lifetime of the photoexcited state and increasing the carrier mobility in the photoactive layer.The findings and concepts detailed in this Account demonstrate that GHz photodetection with TMDCs is feasible, and we hope that these bright prospects for their application as next-generation optoelectronic materials motivate more chemists and material scientists to actively pursue the development of the more complicated material combinations outlined here.

Gate-Tunable Multiband van der Waals Photodetector and Polarization Sensor

A single photodetector with tunable detection wavelengths and polarization sensitivity can potentially be harnessed for diverse optical applications ranging from imaging and sensing to telecommunications. Such a device will require the combination of multiple material systems with different structures, band gaps, and photoelectrical responses, which is extremely difficult to engineer using traditional epitaxial films. Here, we develop a multifunctional and high-performance photosensor using all van der Waals materials. The device features a gate-tunable spectral response that is switchable between near-infrared/visible and short-/midwave infrared, as well as broad-band operation, at room temperature. The linear polarization sensitivity in the telecommunication O-band can also be directly modulated between horizontal, vertical, and nonpolarizing modes. These effects originate from the balance of photocurrent generation in two of the active layers that can be manipulated by an electric field. The photodetector features high detectivity (>109 cmHz1/2W-1) together with fast operation speed (∼1 MHz) and can be further exploited for dual visible and infrared imaging.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

Optical Coupling in Atomic Waveguide for Vertically Integrated Photonics

Integrated 2-dimensional (2D) photonic devices such as monolayer waveguide has generated exceptional interest because of their ultimate thinness. In particular, they potentially permit stereo photonic architecture through bond-free van der Waals integration. However, little is known about the coupling and controlling of the single-atom guided wave to its photonic environment, which governs the design and application of integrated system. Here, we report the optical coupling of atomically guided waves to other photonic modes. We directly probe the mode beating between evanescent waves in a monolayer 2D waveguide and a silicon photonic waveguide, which constitutes a vertically integrated interferometer. The mode-coupling measures the dispersion relation of the guided wave inside the atomic waveguide and unveils it strongly modifies matter's electronic states, manifesting by the formation of a propagating polariton. We also demonstrated light modulating and spectral detecting in this compact nonplanar interferometer. These findings provide a generalizable and versatile platform toward monolithic 3-dimensional integrated photonics.

Two-dimensional molybdenum ditelluride waveguide-integrated near-infrared photodetector

Low-cost, small-sized, and easy integrated high-performance photodetectors for photonics are still the bottleneck of photonic integrated circuits applications and have attracted increasing attention. The tunable narrow bandgap of two-dimensional (2D) layered molybdenum ditelluride (MoTe2) from ∼0.83 to ∼1.1 eV makes it one of the ideal candidates for near-infrared (NIR) photodetectors. Herein, we demonstrate an excellent waveguide-integrated NIR photodetector by transferring mechanically exfoliated 2D MoTe2onto a silicon nitride (Si3N4) waveguide. The photoconductive photodetector exhibits excellent responsivity (R), detectivity (D*), and external quantum efficiency at 1550 nm and 50 mV, which are 41.9 A W-1, 16.2 × 1010Jones, and 3360%, respectively. These optoelectronic performances are 10.2 times higher than those of the free-space device, revealing that the photoresponse of photodetectors can be enhanced due to the presence of waveguide. Moreover, the photodetector also exhibits competitive performances over a broad wavelength range from 800 to 1000 nm with a highRof 15.4 A W-1and a largeD* of 59.6 × 109Jones. Overall, these results provide an alternative and prospective strategy for high-performance on-chip broadband NIR photodetectors.

Frequency-selective perovskite photodetector for anti-interference optical communications

Free-space coupling, essential for various communication applications, often faces significant signal loss and interference from ambient light. Traditional methods rely on integrating complex optical and electronic systems, leading to bulkier and costlier communication equipment. Here, we show an asymmetric 2D-3D-2D perovskite structure device to achieve a frequency-selective photoresponse in a single device. By combining two electromotive forces of equal magnitude in the opposite directions, the device output is attenuated to zero under constant light illumination. Because these reverse photodiodes have different response speeds, the device only responds near a certain frequency, which can be tuned by manipulating the 2D perovskite components. The target device achieves an ultrafast response of 19.7/18.3 ns in the frequency-selective photoresponse range 0.8-9.7 MHz. This anti-interference photodetector can accurately transmit character and video data under strong light interference with a source intensity of up to 454 mW cm-2.

Photodetectors integrating waveguides and semiconductor materials

Photodetectors integrating substrates and semiconductor materials are increasingly attractive for applications in optical communication, optical sensing, optical computing, and military owing to the unique optoelectronic properties of semiconductor materials. However, it is still a challenge to realize high-performance photodetectors by only integrating substrates and semiconductor materials because of the limitation of incident light in contact with sensitive materials. In recent years, waveguides such as silicon (Si) and silicon nitride (Si3N4) have attracted extensive attention owing to their unique optical properties. Waveguides can be easily hetero-integrated with semiconductor materials, thus providing a promising approach for realizing high-performance photodetectors. Herein, we review recent advances in photodetectors integrating waveguides in two parts. The first involves the waveguide types and semiconductor materials commonly used to fabricate photodetectors, including Si, Si3N4, gallium nitride, organic waveguides, graphene, and MoTe2. The second involves the photodetectors of different wavelengths that integrate waveguides, ranging from ultraviolet to infrared. These hybrid photodetectors integrating waveguides and semiconductor materials provide an alternative way to realize multifunctional and high-performance photonic integrated chips and circuits.

Photo-oxidative Crack Propagation in Transition Metal Dichalcogenides

Monolayered transition-metal dichalcogenides (TMDs) are easily exposed to air, and their crystal quality can often be degraded via oxidation, leading to poor electronic and optical device performance. The degradation becomes more severe in the presence of defects, grain boundaries, and residues. Here, we report crack propagation in pristine TMD monolayers grown by chemical vapor deposition under ambient conditions and light illumination. Under a high relative humidity (RH) of ∼60% and white light illumination, the cracks appear randomly. Photo-oxidative cracks gradually propagated along the grain boundaries of the TMD monolayers. In contrast, under low RH conditions of ∼2%, cracks were scarcely observed. Crack propagation is predominantly attributed to the accumulation of water underneath the TMD monolayers, which is preferentially absorbed by hygroscopic alkali metal-based precursor residues. Crack propagation is further accelerated by the cyclic process of photo-oxidation in a basic medium, leading to localized tensile strain. We also found that such crack propagation is prevented after the removal of alkali metals via the transfer of the sample to other substrates.

Strain modulation in crumpled Si nanomembranes: Light detection beyond the Si absorption limit

Although Si is extensively used in micro-nano electronics, its inherent optical absorption cutoff at 1100-nm limits its photonic and optoelectronic applications in visible to partly near infrared (NIR) spectral range. Recently, strain engineering has emerged as a promising approach for extending device functionality via tuning the material properties, including change in optical bandgap. In this study, the reduction in bandgap with applied strain was used for extending the absorption limit of crystalline Si up to 1310 nm beyond its intrinsic bandgap, which was achieved by creating the crumpled structures in Si nanomembranes (NMs). The concept was used to develop a prototype NIR image sensor by organizing metal-semiconductor-metal-configured crumpled Si NM photosensing pixels in 6 × 6 array. The geometry-controlled, self-sustained strain induction in Si NMs provided an exclusive photon management with shortening of optical bandgap and enhanced photoresponse beyond the conventional Si absorption limit.

Photogating-assisted tunneling boosts the responsivity and speed of heterogeneous WSe2/Ta2NiSe5 photodetectors

Photogating effect is the dominant mechanism of most high-responsivity two-dimensional (2D) material photodetectors. However, the ultrahigh responsivities in those devices are intrinsically at the cost of very slow response speed. In this work, we report a WSe2/Ta2NiSe5 heterostructure detector whose photodetection gain and response speed can be enhanced simultaneously, overcoming the trade-off between responsivity and speed. We reveal that photogating-assisted tunneling synergistically allows photocarrier multiplication and carrier acceleration through tunneling under an electrical field. The photogating effect in our device features low-power consumption (in the order of nW) and shows a dependence on the polarization states of incident light, which can be further tuned by source-drain voltages, allowing for wavelength discrimination with just a two-electrode planar structure. Our findings offer more opportunities for the long-sought next-generation photodetectors with high responsivity, fast speed, polarization detection, and multi-color sensing, simultaneously.

Advances in white light-emitting organic crystals

In past decades, organic crystals have presented considerable potential in the field of optoelectronics due to their rich tunable physical and chemical properties and excellent optoelectronic characteristics. White-light emission, as a special application, has received widespread attention and has been applied in various fields, generating significant interest in the scientific community. By preparing white light-emitting organic crystals, a series of applications for future white-light sources can be realized. This article reviews the research progress on the molecular design and synthesis, preparation, and application of white light-emitting organic crystals in recent years. We hope that this review will help to understand and facilitate the development of white light-emitting organic crystals.

On-water surface synthesis of electronically coupled 2D polyimide-MoS2 van der Waals heterostructure

The water surface provides a highly effective platform for the synthesis of two-dimensional polymers (2DP). In this study, we present an efficient on-water surface synthesis of crystalline monolayer 2D polyimide (2DPI) through the imidization reaction between tetra (4-aminophenyl) porphyrin (M1) and perylenetracarboxylic dianhydride (M2), resulting in excellent stability and coverage over a large area (tens of cm2). We further fabricate innovative organic-inorganic hybrid van der Waals heterostructures (vdWHs) by combining with exfoliated few-layer molybdenum sulfide (MoS2). High-resolution transmission electron microscopy (HRTEM) reveals face-to-face stacking between MoS2 and 2DPI within the vdWH. This stacking configuration facilitates remarkable charge transfer and noticeable n-type doping effects from monolayer 2DPI to MoS2, as corroborated by Raman spectroscopy, photoluminescence measurements, and field-effect transistor (FET) characterizations. Notably, the 2DPI-MoS2 vdWH exhibits an impressive electron mobility of 50 cm2/V·s, signifying a substantial improvement over pristine MoS2 (8 cm2/V·s). This study unveils the immense potential of integrating 2D polymers to enhance semiconductor device functionality through tailored vdWHs, thereby opening up exciting new avenues for exploring unique interfacial physical phenomena.

Organic Bilayer Heterostructures with Built-In Exciton Conversion for 2D Photonic Encryption

Organic multilayer heterostructures with accurate spatial organization demonstrate strong light-matter interaction from excitonic responses and efficient carrier transfer across heterojunction interfaces, which are considered as promising candidates toward advanced optoelectronics. However, the precise regulation of the heterojunction surface area for finely adjusting exciton conversion and energy transfer is still formidable. Herein, organic bilayer heterostructures (OBHs) with controlled face-to-face heterojunction via a stepwise seeded growth strategy, which is favorable for efficient exciton propagation and conversion of optical interconnects are designed and synthesized. Notably, the relative position and overlap length ratio of component microwires (LDSA /LBPEA = 0.39-1.15) in OBHs are accurately regulated by modulating the crystallization time of seeded crystals, resulting into a tailored heterojunction surface area (R = Loverlap /LBPEA = 37.6%-65.3%). These as-prepared OBHs present the excitation position-dependent waveguide behaviors for optical outcoupling characteristics with tunable emission colors and intensities, which are applied into two-dimensional (2D) photonic barcodes. This strategy opens a versatile avenue to purposely design OBHs with tailored heterojunctions for efficient energy transfer and exciton conversion, facilitating the application possibilities of advanced integrated optoelectronics.

Polymer-Waveguide-Integrated 2D Semiconductor Heterostructures for Optical Communications

The demand for high-speed and low-loss interconnects in modern computer architectures is difficult to satisfy by using traditional Si-based electronics. Although optical interconnects offer a promising solution owing to their high bandwidth, low energy dissipation, and high-speed processing, integrating elements such as a light source, detector, and modulator, comprising different materials with optical waveguides, presents many challenges in an integrated platform. Two-dimensional (2D) van der Waals (vdW) semiconductors have attracted considerable attention in vertically stackable optoelectronics and advanced flexible photonics. In this study, optoelectronic components for exciton-based photonic circuits are demonstrated by integrating lithographically patterned poly(methyl methacrylate) (PMMA) waveguides on 2D vdW devices. The excitonic signals generated from the 2D materials by using laser excitation were transmitted through patterned PMMA waveguides. By introducing an external electric field and combining vdW heterostructures, an excitonic switch, phototransistor, and guided-light photovoltaic device on SiO2/Si substrates were demonstrated.

A mixed-dimensional quasi-1D BiSeI nanowire-2D GaSe nanosheet p-n heterojunction for fast response optoelectronic devices

Due to the unique combination configuration and the formation of a built-in electric field, mixed-dimensional heterojunctions present fruitful possibilities for improving the optoelectronic performances of low-dimensional optoelectronic devices. However, the response times of most photodetectors built from mixed-dimensional heterojunctions are within the millisecond range, limiting their applications in fast response optoelectronic devices. Herein, a mixed-dimensional BiSeI/GaSe van der Waals heterostructure is designed, which exhibits visible light detection ability and competitive photoresponsivity of 750 A W-1 and specific detectivity of 2.25 × 1012 Jones under 520 nm laser excitation. Excitingly, the device displays a very fast response time, e.g., the rise time and decay time under 520 nm laser excitation are 65 μs and 190 μs, respectively. Our findings provide a prospective approach to mixed-dimensional heterojunction photodetection devices with rapid switching capabilities.

All-dielectric scale invariant waveguide

Total internal reflection (TIR) governs the guiding mechanisms of almost all dielectric waveguides and therefore constrains most of the light in the material with the highest refractive index. The few options available to access the properties of lower-index materials include designs that are either lossy, periodic, exhibit limited optical bandwidth or are restricted to subwavelength modal volumes. Here, we propose and demonstrate a guiding mechanism that leverages symmetry in multilayer dielectric waveguides as well as evanescent fields to strongly confine light in low-index materials. The proposed waveguide structures exhibit unusual light properties, such as uniform field distribution with a non-Gaussian spatial profile and scale invariance of the optical mode. This guiding mechanism is general and can be further extended to various optical structures, employed for different polarizations, and in different spectral regions. Therefore, our results can have huge implications for integrated photonics and related technologies.

A waveguide-integrated self-powered van der Waals heterostructure photodetector with high performance at the telecom wavelength

Two-dimensional (2D) materials are attractive candidates for high-performance photodetectors due to their wide operating wavelength and potential to integrate with silicon photonics. However, due to their limited atomic thickness and short carrier lifetime, they suffer from high driving source-drain voltages, weak light-matter interactions and low carrier collection efficiency. Here, we present a high-performance van der Waals (vdWs) heterostructure-based photodetector integrated on a silicon nitride photonic platform combining p-type black phosphorus (BP) and n-type molybdenum disulfide (MoS2). Owing to the efficient carrier separation process and dark current suppression at the junction interface of the vdWs heterostructure, high photodetectivity and a fast response speed can be achieved. A fast response time (∼2.08/3.54 μs), high responsivity (11.26 mA W-1), and a high light on/off ratio (104) operating in the near-infrared telecom band are obtained at zero bias. Our research highlights the great potential of the high-efficiency waveguide-integrated vdWs heterojunction photodetector for integrated optoelectronic systems, such as high-data-rate interconnects operated at standardized telecom wavelengths.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Toward High-Peak-to-Valley-Ratio Graphene Resonant Tunneling Diodes

The resonant tunneling diode (RTD) is one of the very few room-temperature-operating quantum devices to date that is able to exhibit negative differential resistance. However, the reported key figure of merit, the current peak-to-valley ratio (PVR), of graphene RTDs has been up to only 3.9 at room temperature thus far. This remains very puzzling, given the atomically flat interfaces of the 2D materials. By varying the active area and perimeter of RTDs based on a graphene/hexagonal boron nitride/graphene heterostructure, we discovered that the edge doping can play a dominant role in determining the resonant tunneling, and a large area-to-perimeter ratio is necessary to obtain a high PVR. The understanding enables establishing a novel design rule and results in a PVR of 14.9, which is at least a factor of 3.8 higher than previously reported graphene RTDs. Furthermore, a theory is developed allowing extraction of the edge doping depth for the first time.

2D Ferroionics: Conductive Switching Mechanisms and Transition Boundaries in Van der Waals Layered Material CuInP2 S6

The recently unfolded ferroionic phenomena in 2D van der Waals (vdW) copper-indium-thiophosphate (CuInP2 S6 or CIPS) have received widespread interest as they allow for dynamic control of conductive switching properties, which are appealing in the paradigm-shift computing. The intricate couplings between ferroelectric polarization and ionic conduction in 2D vdW CIPS facilitate the manipulation and dynamic control of conductive behaviors. However, the complex interplays and underlying mechanisms are not yet fully explored and understood. Here, by investigating polarization switching and ionic conduction in the temperature and applied electric field domains, it is discovered that the conducting mechanisms of CIPS can be divided into four distinctive states (or modes) with transitional boundaries, depending on the dynamics of Cu ions in the material. Further, it demonstrates that dynamically-tunable synaptic responsive behavior can be well implemented by governing the working-state transition. This research provides an in-depth, quantitative understanding of the complex phenomena of conductive switching in 2D vdW CIPS with coexisting ferroelectric order and ionic disorder. The developed insights in this work lay the ground for implementing high-performance, function-enriched devices for information processing, data storage, and neuromorphic computing based on the 2D ferroionic material systems.

Quantum tailoring for polarization-discriminating Bi2S3 nanowire photodetectors and their multiplexing optical communication and imaging applications

In this study, cost-efficient atmospheric pressure chemical vapor deposition has been successfully developed to produce well-aligned high-quality monocrystalline Bi2S3 nanowires. By virtue of surface strain-induced energy band reconstruction, the Bi2S3 photodetectors demonstrate a broadband photoresponse across 370.6 to 1310 nm. Upon a gate voltage of 30 V, the responsivity, external quantum efficiency, and detectivity reach 23 760 A W-1, 5.55 × 106%, and 3.68 × 1013 Jones, respectively. The outstanding photosensitivity is ascribed to the high-efficiency spacial separation of photocarriers, enabled by synergy of the axial built-in electric field and type-II band alignment, as well as the pronounced photogating effect. Moreover, a polarization-discriminating photoresponse has been unveiled. For the first time, the correlation between quantum confinement and dichroic ratio is systematically explored. The optoelectronic dichroism is established to be negatively correlated with the cross dimension (i.e., width and height) of the channel. Specifically, upon 405 nm illumination, the optimized dichroic ratio reaches 2.4, the highest value among the reported Bi2S3 photodetectors. In the end, proof-of-concept multiplexing optical communications and broadband lensless polarimetric imaging have been implemented by exploiting the Bi2S3 nanowire photodetectors as light-sensing functional units. This study develops a quantum tailoring strategy for tailoring the polarization properties of (quasi-)1D material photodetectors whilst depicting new horizons for the next-generation opto-electronics industry.

High-responsivity and high-speed black phosphorus photodetectors integrated with proton exchanged thin-film lithium niobate waveguides

We present a high-performance broadband (450-1550 nm) black phosphorus photodetector based on a thin-film lithium niobate waveguide. The waveguides are fabricated by the proton exchange method with flat surfaces, which reduces the stress and deformation of two-dimensional materials. At a wavelength of 1550 nm, the photodetector simultaneously achieves a high responsivity and wide bandwidth, with a responsivity as high as 147 A/W (at an optical power of 17 nW), a 3-dB bandwidth of 0.86 GHz, and a detectivity of 3.04 × 1013 Jones. Our photodetector exhibits one of the highest responsivity values among 2D material-integrated waveguide photodetectors.

Metamaterial graphene photodetector with bandwidth exceeding 500 gigahertz

Although graphene has met many of its initially predicted optoelectronic, thermal, and mechanical properties, photodetectors with large spectral bandwidths and extremely high frequency responses remain outstanding. In this work, we demonstrate a >500 gigahertz, flat-frequency response, graphene-based photodetector that operates under ambient conditions across a 200-nanometer-wide spectral band with center wavelengths adaptable from <1400 to >4200 nanometers. Our detector combines graphene with metamaterial perfect absorbers with direct illumination from a single-mode fiber, which breaks with the conventional miniaturization of photodetectors on an integrated photonic platform. This design allows for much higher optical powers while still allowing record-high bandwidths and data rates. Our results demonstrate that graphene photodetectors can outperform conventional technologies in terms of speed, bandwidth, and operation across a large spectral range.

Integration of Self-Passivated Topological Electrodes for Advanced 2D Optoelectronic Devices

With the rapid development of two-dimensional semiconductor technology, the inevitable chemical disorder at a typical metal-semiconductor interface has become an increasingly serious problem that degrades the performance of 2D semiconductor optoelectronic devices. Herein, defect-free van der Waals contacts have been achieved by utilizing topological Bi₂ Se₃ as the electrodes. Such clean and atomically sharp contacts avoid the consumption of photogenerated carriers at the interface, enabling a markedly boosted sensitivity as compared to counterpart devices with directly deposited metal electrodes. Typically, the device with 2D WSe₂ channel realizes a high responsivity of 20.5 A W^-1 , an excellent detectivity of 2.18 × 10¹²  Jones, and a fast rise/decay time of 41.66/38.81 ms. Furthermore, high-resolution visible-light imaging capability of the WSe₂ device is demonstrated, indicating its promising application prospect in future optoelectronic systems. More inspiringly, the topological electrodes are universally applicable to other 2D semiconductor channels, including WS₂ and InSe, suggesting its broad applicability. These results open fascinating opportunities for the development of high-performance electronics and optoelectronics.

WSe2 Light-Emitting Device Coupled to an h-BN Waveguide

Optical information processing using photonic integrated circuits is a key goal in the field of nanophotonics. Extensive research efforts have led to remarkable progress in integrating active and passive device functionalities within one single photonic circuit. Still, to date, one of the central components, i.e., light sources, remain a challenge to be integrated. Here, we focus on a photonic platform that is solely based on two-dimensional materials to enable the integration of electrically contacted optoelectronic devices inside the light-confining dielectric of photonic structures. We combine light-emitting devices, based on exciton recombination in transition metal dichalcogenides, with hexagonal boron nitride photonic waveguides in a single van der Waals heterostructure. Waveguide-coupled light emission is achieved by sandwiching the light-emitting device between two hexagonal boron nitride slabs and patterning the complete van der Waals stack into a photonic structure. Our demonstration of on-chip light generation and waveguiding is a key component for future integrated van der Waals optoelectronics.

Two-dimensional semiconducting SnP2Se6 with giant second-harmonic-generation for monolithic on-chip electronic-photonic integration

Two-dimensional (2D) layered semiconductors with nonlinear optical (NLO) properties hold great promise to address the growing demand of multifunction integration in electronic-photonic integrated circuits (EPICs). However, electronic-photonic co-design with 2D NLO semiconductors for on-chip telecommunication is limited by their essential shortcomings in terms of unsatisfactory optoelectronic properties, odd-even layer-dependent NLO activity and low NLO susceptibility in telecom band. Here we report the synthesis of 2D SnP₂Se₆, a van der Waals NLO semiconductor exhibiting strong odd-even layer-independent second harmonic generation (SHG) activity at 1550 nm and pronounced photosensitivity under visible light. The combination of 2D SnP₂Se₆ with a SiN photonic platform enables the chip-level multifunction integration for EPICs. The hybrid device not only features efficient on-chip SHG process for optical modulation, but also allows the telecom-band photodetection relying on the upconversion of wavelength from 1560 to 780 nm. Our finding offers alternative opportunities for the collaborative design of EPICs.

Ultrabroadband Imaging Based on Wafer-Scale Tellurene

High-resolution imaging is at the heart of the revolutionary breakthroughs of intelligent technologies, and it is established as an important approach toward high-sensitivity information extraction/storage. However, due to the incompatibility between non-silicon optoelectronic materials and traditional integrated circuits as well as the lack of competent photosensitive semiconductors in the infrared region, the development of ultrabroadband imaging is severely impeded. Herein, the monolithic integration of wafer-scale tellurene photoelectric functional units by exploiting room-temperature pulsed-laser deposition is realized. Taking advantage of the surface plasmon polaritons of tellurene, which results in the thermal perturbation promoted exciton separation, in situ formation of out-of-plane homojunction and negative expansion promoted carrier transport, as well as the band bending promoted electron-hole pair separation enabled by the unique interconnected nanostrip morphology, the tellurene photodetectors demonstrate wide-spectrum photoresponse from 370.6 to 2240 nm and unprecedented photosensitivity with the optimized responsivity, external quantum efficiency and detectivity of 2.7 × 10⁷  A W^-1 , 8.2 × 10⁹ % and 4.5 × 10¹⁵  Jones. An ultrabroadband imager is demonstrated and high-resolution photoelectric imaging is realized. The proof-of-concept wafer-scale tellurene-based ultrabroadband photoelectric imaging system depicts a fascinating paradigm for the development of an advanced 2D imaging platform toward next-generation intelligent equipment.

Silicon Waveguide-Integrated Carbon Nanotube Photodetector with Low Dark Current and 48 GHz Bandwidth

Low-dimensional materials with excellent optoelectronic properties and complementary metal-oxide-semiconductor (CMOS) process compatibility have the potential to construct high-performance photodetectors used in a cost-efficient monolithic or hybrid integrated optical communication system. Carbon nanotubes (CNTs) have attracted a lot of attention due to special geometric structure and broad band response, high optical absorption coefficient, ps-level intrinsic light response, high carrier mobility and wafer-scaled production process. Here, we demonstrated a high-performance waveguide-integrated CNT photodetector with asymmetric palladium (Pd) and hafnium (Hf) contact electrodes. The ideal photodetector structure was realized via comparing with simulation and experimental results, where the optimized device achieved a high 3 dB bandwidth ∼48 GHz at 0 V, as well as a responsivity ∼73.62 mA/W and dark current ∼0.157 μA at -2 V bias voltage. This waveguide-integrated CNT photodetector with low dark current and high bandwidth is helpful for next-generation optical communication and high-speed optical interconnects.

A two-dimensional mid-infrared optoelectronic retina enabling simultaneous perception and encoding

Infrared machine vision system for object perception and recognition is becoming increasingly important in the Internet of Things era. However, the current system suffers from bulkiness and inefficiency as compared to the human retina with the intelligent and compact neural architecture. Here, we present a retina-inspired mid-infrared (MIR) optoelectronic device based on a two-dimensional (2D) heterostructure for simultaneous data perception and encoding. A single device can perceive the illumination intensity of a MIR stimulus signal, while encoding the intensity into a spike train based on a rate encoding algorithm for subsequent neuromorphic computing with the assistance of an all-optical excitation mechanism, a stochastic near-infrared (NIR) sampling terminal. The device features wide dynamic working range, high encoding precision, and flexible adaption ability to the MIR intensity. Moreover, an inference accuracy more than 96% to MIR MNIST data set encoded by the device is achieved using a trained spiking neural network (SNN).

High-Performance and Polarization-Sensitive Imaging Photodetector Based on WS2 /Te Tunneling Heterostructure

Next-generation imaging systems require photodetectors with high sensitivity, polarization sensitivity, miniaturization, and integration. By virtue of their intriguing attributes, emerging 2D materials offer innovative avenues to meet these requirements. However, the current performance of 2D photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, the lack of photoconductive gain, and insufficient photocarrier collection. Here, a tunneling dominant imaging photodetector based on WS₂ /Te heterostructure is reported. This device demonstrates competitive performance, including a remarkable responsivity of 402 A W^-1 , an outstanding detectivity of 9.28 × 10¹³ Jones, a fast rise/decay time of 1.7/3.2 ms, and a high photocurrent anisotropic ratio of 2.5. These outstanding performances can be attributed to the type-I band alignment with carrier transmission barriers and photoinduced tunneling mechanism, allowing reduced interfacial trapping effect, effective photoconductive gains, and anisotropic collection of photocarriers. Significantly, the constructed photodetector is successfully integrated into a polarized light imaging system and an ultra-weak light imaging system to illustrate the imaging capability. These results suggest the promising application prospect of the device in future imaging systems.

Large-Area Black Phosphorus/PtSe2 Schottky Junction for High Operating Temperature Broadband Photodetectors

High operating temperature (HOT) broadband photodetectors are urgently necessary for extreme condition applications in infrared-guided missiles, infrared night vision, fire safety imaging, and space exploration sensing. However, conventional photodetectors show dramatic carrier mobility decreases and carrier losses with low photoresponsivity at HOT due to the increased carrier scattering in channels at high temperatures. Herein, the HOT broadband photodetectors from room temperature to 470 K are developed for the first time by large-area black phosphorus (BP)/PtSe₂ films device arrays via a depletion-enhanced photocarrier dynamics strategy. Attributed to the 2D Schottky junction at BP/PtSe₂ interface and resulting in full depleted working channels, the BP/PtSe₂ photodetector arrays exhibit high tolerance to carrier mobility decrease during the increasing operating temperature in a wide wavelength range from 532 to 2200 nm. Thus, the photodetector shows a state-of-the-art operating temperature at 470 K with the photo-responsivity (R) and specific detectivity (D*) of 25 A W^-1 and 6.4 × 10¹¹ Jones under 1850 nm illumination, respectively. Moreover, BP/PtSe₂ photodetector arrays show high-uniformity photo-response in a large area. This work provides new strategies for high-performance broadband photodetector arrays with HOT by Schottky junction of large-area BP/PtSe₂ films.

Homogeneous in-plane WSe2 P-N junctions for advanced optoelectronic devices

Conventional doping schemes of silicon (Si) microelectronics are incompatible with atomically thick two-dimensional (2D) transition metal dichalcogenides (TMDCs), which makes it challenging to construct high-quality 2D homogeneous p-n junctions. Herein, we adopt a simple yet effective plasma-treated doping method to seamlessly construct a lateral 2D WSe₂ p-n homojunction. WSe₂ with ambipolar transport properties was exposed to O₂ plasma to form WO_x on the surface in a self-limiting process that induces hole doping in the underlying WSe₂via electron transfer. Different electrical behaviors were observed between the as-exfoliated (ambipolar) region and the O₂ plasma-treated (p-doped) region under electrostatic modulation of the back-gate bias (V_BG), which produces a p-n in-plane homojunction. More importantly, a small contact resistance of 710 Ω μm with a p-doped region transistor mobility of ∼157 cm² V^-1 s^-1 was achieved due to the transformation of Schottky contact into Ohmic contact after plasma treatment. This effectively avoids Fermi-level pinning and significantly improves the performance of photodetectors. The resultant WSe₂ p-n junction device thus exhibits a high photoresponsivity of ∼7.1 × 10⁴ mA W^-1 and a superior external quantum efficiency of ∼228%. Also, the physical mechanism of charge transfer in the WSe₂ p-n homojunction was analyzed. Our proposed strategy offers a powerful route to realize low contact resistance and high photoresponsivity in 2D TMDC-based optoelectronic devices, paving the way for next-generation atomic-thickness optoelectronics.

Reversible Electrical Control of Interfacial Charge Flow across van der Waals Interfaces

Bond-free integration of two-dimensional (2D) materials yields van der Waals (vdW) heterostructures with exotic optical and electronic properties. Manipulating the splitting and recombination of photogenerated electron-hole pairs across the vdW interface is essential for optoelectronic applications. Previous studies have unveiled the critical role of defects in trapping photogenerated charge carriers to modulate the photoconductive gain for photodetection. However, the nature and role of defects in tuning interfacial charge carrier dynamics have remained elusive. Here, we investigate the nonequilibrium charge dynamics at the graphene-WS₂ vdW interface under electrochemical gating by operando optical-pump terahertz-probe spectroscopy. We report full control over charge separation states and thus photogating field direction by electrically tuning the defect occupancy. Our results show that electron occupancy of the two in-gap states, presumably originating from sulfur vacancies, can account for the observed rich interfacial charge transfer dynamics and electrically tunable photogating fields, providing microscopic insights for optimizing optoelectronic devices.

MoSe2/WS2 heterojunction photodiode integrated with a silicon nitride waveguide for near infrared light detection with high responsivity

We demonstrate experimentally the realization and the characterization of a chip-scale integrated photodetector for the near-infrared spectral regime based on the integration of a MoSe₂/WS₂ heterojunction on top of a silicon nitride waveguide. This configuration achieves high responsivity of ~1 A W^-1 at the wavelength of 780 nm (indicating an internal gain mechanism) while suppressing the dark current to the level of ~50 pA, much lower as compared to a reference sample of just MoSe₂ without WS₂. We have measured the power spectral density of the dark current to be as low as ~1 × 10^-12 A Hz^-0.5, from which we extract the noise equivalent power (NEP) to be ~1 × 10^-12 W Hz^-0.5. To demonstrate the usefulness of the device, we use it for the characterization of the transfer function of a microring resonator that is integrated on the same chip as the photodetector. The ability to integrate local photodetectors on a chip and to operate such devices with high performance at the near-infrared regime is expected to play a critical role in future integrated devices in the field of optical communications, quantum photonics, biochemical sensing, and more.

Research Advances in Amorphous-Crystalline Heterostructures Toward Efficient Electrochemical Applications

Interface engineering of heterostructures has proven a promising strategy to effectively modulate their physicochemical properties and further improve the electrochemical performance for various applications. In this context related research of the newly proposed amorphous-crystalline heterostructures have lately surged since they combine the superior advantages of amorphous- and crystalline-phase structures, showing unusual atomic arrangements in heterointerfaces. Nonetheless, there has been much less efforts in systematic analysis and summary of the amorphous-crystalline heterostructures to examine their complicated interfacial interactions and elusory active sites. The critical structure-activity correlation and electrocatalytic mechanism remain rather elusive. In this review, the recent advances of amorphous-crystalline heterostructures in electrochemical energy conversion and storage fields are amply discussed and presented, along with remarks on the challenges and perspectives. Initially, the fundamental characteristics of amorphous-crystalline heterostructures are introduced to provide scientific viewpoints for structural understanding. Subsequently, the superiorities and current achievements of amorphous-crystalline heterostructures as highly efficient electrocatalysts/electrodes for hydrogen evolution reaction, oxygen evolution reaction, supercapacitor, lithium-ion battery, and lithium-sulfur battery applications are elaborated. At the end of this review, future outlooks and opportunities on amorphous-crystalline heterostructures are also put forward to promote their further development and application in the field of clean energy.

Van der Waals Nonlinear Photodetector with Quadratic Photoresponse

With unique electronic and optical attributes and dangling-bond-free surface, two-dimensional (2D) materials have broadened the functionalities of photodetectors. Here, we report a quadratically nonlinear photodetector (QNPD) composed of a van der Waals (vdW) stacked GaSe/InSe heterostructure. Compared with the reported 2D material-based photodetectors, the extra second-harmonic generation (SHG) process in GaSe/InSe leads to the quadratically nonlinear function between photocurrent and optical intensity, extending the photodetection wavelength from 900 to 1750 nm. The QNPD is highly sensitive to the variation of optical intensity with improved spatial resolution. With the light-light interaction in SHG converted into electrical signal directly, we also demonstrate the QNPD as an autocorrelator for measuring ultrafast pulse widths and an optoelectronic mixer of two modulated pulses for signal processings. The simultaneous involvement of light-light interaction and photoelectric conversion in the vdW stacked QNPD promises its potential to simplify the optoelectronic systems.

Investigating active area dependent high performing photoresponse through thin films of Weyl Semimetal WTe2

WTe₂ is one of the wonder layered materials, displays interesting overlapping of electron-hole pairs, opening of the surface bandgap, anisotropy in its crystal structure and very much sought appealing material for room temperature broadband photodection applications. Here we report the photoresponse of WTe₂ thin films and microchannel devices fabricated on silicon nitride substrates. A clear sharp rise in photocurrent observed under the illumination of visible (532 nm) and NIR wavelengths (1064 nm). The observed phoresponse is very convincing and repetitive for ON /OFF cycles of laser light illumination. The channel length dependence of photocurrent is noticed for few hundred nanometers to micrometers. The photocurrent, rise & decay times, responsivity and detectivity are studied using different channel lengths. Strikingly microchannel gives few orders of greater responsivity compared to larger active area investigated here. The responsivity and detectivity are observed as large as 29 A/W and 3.6 × 10⁸ Jones respectively. The high performing photodetection properties indicate that WTe₂ can be used as a broad band material for future optoelectronic applications.

Planar Optical Cavities Hybridized with Low-Dimensional Light-Emitting Materials

Low-dimensional light-emitting materials have been actively investigated due to their unprecedented optical and optoelectronic properties that are not observed in their bulk forms. However, the emission from low-dimensional light-emitting materials is generally weak and difficult to use in nanophotonic devices without being amplified and engineered by optical cavities. Along with studies on various planar optical cavities over the last decade, the physics of cavity-emitter interactions as well as various integration methods are investigated deeply. These integrations not only enhance the light-matter interaction of the emitters, but also provide opportunities for realizing nanophotonic devices based on the new physics allowed by low-dimensional emitters. In this review, the fundamentals, strengths and weaknesses of various planar optical resonators are first provided. Then, commonly used low-dimensional light-emitting materials such as 0D emitters (quantum dots and upconversion nanoparticles) and 2D emitters (transition-metal dichalcogenide and hexagonal boron nitride) are discussed. The integration of these emitters and cavities and the expect interplay between them are explained in the following chapters. Finally, a comprehensive discussion and outlook of nanoscale cavity-emitter integrated systems is provided.

Waveguide-Integrated MoTe2 p-i-n Homojunction Photodetector

Two-dimensional (2D) materials, featuring distinctive electronic and optical properties and dangling-bond-free surfaces, are promising for developing high-performance on-chip photodetectors in photonic integrated circuits. However, most of the previously reported devices operating in the photoconductive mode suffer from a high dark current or a low responsivity. Here, we demonstrate a MoTe₂ p-i-n homojunction fabricated directly on a silicon photonic crystal (PC) waveguide, which enables on-chip photodetection with ultralow dark current, high responsivity, and fast response speed. The adopted silicon PC waveguide is electrically split into two individual back gates to selectively dope the top regions of the MoTe₂ channel in p- or n-types. High-quality reconfigurable MoTe₂ (p-i-n, n-i-p, n-i-n, p-i-p) homojunctions are realized successfully, presenting rectification behaviors with ideality factors approaching 1.0 and ultralow dark currents less than 90 pA. Waveguide-assisted MoTe₂ absorption promises a sensitive photodetection in the telecommunication O-band from 1260 to 1340 nm, though it is close to MoTe₂'s absorption band-edge. A competitive photoresponsivity of 0.4 A/W is realized with a light on/off current ratio exceeding 10⁴ and a record-high normalized photocurrent-to-dark-current ratio of 10⁶ mW^-1. The ultrasmall capacitance of p-i-n homojunction and high carrier mobility of MoTe₂ promise a high dynamic response bandwidth close to 34.0 GHz. The proposed device geometry has the advantages of employing a silicon PC waveguide as the back gates to build a 2D material p-i-n homojunction directly and simultaneously to enhance light-2D material interaction. It provides a potential pathway to develop 2D material-based photodetectors, laser diodes, and electro-optic modulators on silicon photonic chips.