Polarity-Tunable Field Effect Phototransistors.

Field-effect phototransistors feature gate voltage modulation, allowing dynamic performance control and significant signal amplification. A field-effect phototransistor can be designed to be inherently either unipolar or ambipolar in its response. However, conventionally, once a field-effect phototransistor has been fabricated, its polarity cannot be changed. Herein, a polarity-tunable field-effect phototransistor based on a graphene/ultrathin Al2O3/Si structure is demonstrated. Light can modulate the gating effect of the device and change the transfer characteristic curve from unipolar to ambipolar. This photoswitching in turn produces a significantly improved photocurrent signal. The introduction of an ultrathin Al2O3 interlayer also enables the phototransistor to achieve a responsivity in excess of 105 A/W, a 3 dB bandwidth of 100 kHz, a gain-bandwidth product of 9.14 × 1010 s-1, and a specific detectivity of 1.91 × 1013 Jones. This device architecture enables the gain-bandwidth trade-off in current field-effect phototransistors to be overcome, demonstrating the feasibility of simultaneous high-gain and fast-response photodetection.

Intermolecular 3D-MoRSE Descriptors for Fast and Accurate Prediction of Electronic Couplings in Organic Semiconductors.

The theoretical rational design of organic semiconductors faces an obstacle in that the performance of organic semiconductors depends very much on their stacking and local morphology (for example, phase domains), which involves numerous molecules. Simulation becomes computationally expensive as intermolecular electronic couplings have to be calculated from density functional theory. Therefore, developing fast and accurate methods for intermolecular electronic coupling estimation is essential. In this work, by developing a series of new intermolecular 3D descriptors, we achieved fast and accurate prediction of electronic couplings in both crystalline and amorphous thin films. Three groups of developed descriptors could perform faster and higher accuracy prediction on electronic couplings than the most advanced state-of-the-art descriptors. This work paves the way for large-scale simulations, high-throughput calculations, and screening of organic semiconductors.

Gradual funnel photon trapping enhanced InAs/GaSb type-II superlattice infrared detector.

InAs/GaSb type-II superlattice materials have attracted in the field of infrared detection due to their high quality, uniformity and stability. The performance of InAs/GaSb type-II superlattice detector is limited by dark noise and light response. This work reports a gradual funnel photon trapping (GFPT) structure enabling the light trapping in the T2SL detector absorption area. The GFPT detector exhibits an efficient broadband responsivity enhancement of 30% and a darker current noise reduction of 3 times. It has excellent passivated by atomic layer deposition and achieves a high detectivity of 1.51 × 1011 cm Hz1/2 at 78 K.

Dynamical manipulation of a dual-polarization plasmon-induced transparency employing an anisotropic graphene-black phosphorus heterostructure.

Dynamical tunable plasmon-induced transparency (PIT) possesses the unique characteristics of controlling light propagation states, which promises numerous potential applications in efficient optical signal processing chips and nonlinear optical devices. However, previously reported configurations are sensitive to polarization and can merely operate under specific single polarization. In this work we propose an anisotropic PIT metamaterial device based on a graphene-black phosphorus (G-BP) heterostructure to realize a dual-polarization tunable PIT effect. The destructive interference coupling between the bright mode and dark modes under the orthogonal polarization state pronounced anisotropic PIT phenomenon. The coupling strength of the PIT system can be modulated by dynamically manipulating the Fermi energy of the graphene via the external electric field voltage. Moreover, the three-level plasmonic system and the coupled oscillator model are employed to explain the underlying mechanism of the PIT effect, and the analytical results show good consistency with the numerical calculations. Compared to the single-polarization PIT devices, the proposed device offers additional degrees of freedom in realizing universal tunable functionalities, which could significantly promote the development of next-generation integrated optical processing chips, optical modulation and slow light devices.

Electrically tunable graphene metamaterial with strong broadband absorption.

The coupling system with dynamic manipulation characteristics is of great importance for the field of active plasmonics and tunable metamaterials. However, the traditional metal-based architectures suffer from a lack of electrical tunability. In this study, a metamaterial composed of perpendicular or parallel graphene-Al2O3-graphene stacks is proposed and demonstrated, which allows for the electric modulation of both graphene layers simultaneously. The resultant absorption of hybridized modes can be modulated to more than 50% by applying an external voltage, and the absorption bandwidth can reach 3.55 µm, which is 1.7 times enhanced than the counterpart of single-layer graphene. The modeling results demonstrate that the small relaxation time of graphene is of great importance to realize the broadband absorption. Moreover, the optical behaviors of the tunable metamaterial can be influenced by the incident polarization, the dielectric thickness, and especially by the Fermi energy of graphene. This work is of a crucial role in the design and fabrication of graphene-based broadband optical and optoelectronic devices.

Anomalous redshift of graphene absorption induced by plasmon-cavity competition.

Anomalous redshift of the absorption peak of graphene in the cavity system is numerically and experimentally demonstrated. It is observed that the absorption peak exhibits a redshift as the Fermi level of graphene increases, which is contrary to the ordinary trend of graphene plasmons. The influencing factors, including the electron mobility of graphene, the cavity length, and the ribbon width, are comprehensively analyzed. Such anomalous redshift can be explained by the competition between the graphene plasmon mode and the optical cavity mode. The study herein could be beneficial for the design of graphene-based plasmonic devices.

Mechanism of propagating graphene plasmons excitation for tunable infrared photonic devices.

The mechanism of propagating graphene plasmons excitation using a nano-grating and a Fabry-Pérot cavity as the optical coupling components is studied. It is demonstrated that the system could be well described within the temporal coupled mode theory using two phenomenological parameters, namely, the intrinsic loss rate and the coupling rate of a graphene plasmonic mode, and their analytical expressions are derived. It is found that the intrinsic loss rate is solely determined by the electron relaxation time of graphene, while independent of the field distributions of the modes. Such result originates from the negligible magnetic field energy of the graphene plasmonic mode. The coupling rate is governed by the optical coupling components parameters, and varies periodically with the Fabry-Pérot cavity length. By modulating the two rates, quality factors and absorption rates can be adjusted. Furthermore, it is revealed that low refractive index of the Fabry-Pérot cavity material is vital to the enlargement of tunable band, and the underlying physics is discussed. Such plasmon excitation configuration is insensitive to light incident angle and could serve as a platform for many tunable infrared photonic device, such as surface-enhanced infrared absorption spectroscopies, infrared detectors and modulators.

Tunable spin Hall effect of light with graphene at a telecommunication wavelength.

The spin Hall effect of light (SHEL) has been widely studied for manipulating spin-polarized photons. In this Letter, we present a mechanism to tune the spin shift of the SHEL electrically at 1550 nm by means of introducing a graphene layer. The spin shift is quite sensitive to a graphene layer near the Brewster angle for horizontal polarization incidence and can be dynamically tuned by varying the Fermi energy of graphene. We find that the position of the Brewster angle and the value of the spin shift are decided by the real and imaginary parts of graphene conductivity, respectively. In addition, two different tuned regions have been revealed: one is the "step-like switch" region where the spin shift switches between two values, and the other is the "negative modulation" region where the spin shift declines gradually as the Fermi energy increases. These findings may provide a new paradigm for a tunable spin photonic device.

Ultrafast growth of large-area monolayer MoS2 film via gold foil assistant CVD for a highly sensitive photodetector.

Two-dimensional molybdenum disulfide (MoS2) is a promising material for ultrasensitive photodetectors owing to its tunable band gap and high absorption coefficient. However, controlled synthesis of high-quality, large-area monolayer molybdenum disulfide (MoS2) is still a challenge in practical application. In this work, we report a gold foil assistant chemical vapor deposition method for the synthesis of large-size (>400 µm) single-crystal MoS2 film on a silicon dioxide (SiO2) substrate. The influence of Au foil in enlarging the size of single-crystal MoS2 is investigated systemically using thermal simulation in Ansys workbench 16.0, including thermal conductivity, temperature difference and thermal relaxation time of the interface of SiO2 substrate and Au foil, which indicate that Au foil can increase the temperature of the SiO2 substrate rapidly and decrease the temperature difference between the oven and substrate. Finally, the properties of the monolayer MoS2 film are further confirmed using back-gated field-effect transistors: a high photoresponse of 15.6 A W-1 and a fast photoresponse time of 100 ms. The growth techniques described in this study could be beneficial for the development of other atomically thin two-dimensional transition metal dichalcogenide materials.

Single Gold Nanorod Charge Modulation in an Ion Gel Device.

A reliable and reproducible method to rapidly charge single gold nanocrystals in a solid-state device is reported. Gold nanorods (Au NRs) were integrated into an ion gel capacitor, enabling them to be charged in a transparent and highly capacitive device, ideal for optical transmission. Changes in the electron concentration of a single Au NR were observed with dark-field imaging spectroscopy via localized surface plasmon resonance (LSPR) shifts in the scattering spectrum. A time-resolved, laser-illuminated, dark-field system was developed to enable direct measurement of single particle charging rates with time resolution below one millisecond. The added sensitivity of this new approach has enabled the optical detection of fewer than 110 electrons on a single Au NR. Single wavelength resonance shifts provide a much faster, more sensitive method for all surface plasmon-based sensing applications.

The Electrical Behaviors of Grain Boundaries in Polycrystalline Optoelectronic Materials.

Polycrystalline optoelectronic materials are widely used for photoelectric signal conversion and energy harvesting and play an irreplaceable role in the semiconductor field. As an important factor in determining the optoelectronic properties of polycrystalline materials, grain boundaries (GBs) are the focus of research. Particular emphases are placed on the generation and height of GB barriers, how carriers move at GBs, whether GBs act as carrier transport channels or recombination sites, and how to change the device performance by altering the electrical behaviors of GBs. This review introduces the evolution of GB theory and experimental observation history, classifies GB electrical behaviors from the perspective of carrier dynamics, and summarizes carrier transport state under external conditions such as bias and illumination and the related band bending. Then the carrier scattering at GBs and the electrical differences between GBs and twin boundaries are discussed. Last, the review describes how the electrical behaviors of GBs can be influenced and modified by treatments such as passivation or by consciously adjusting the distribution of grain boundary elements. By studying the carrier dynamics and the relevant electrical behaviors of GBs in polycrystalline materials, researchers can develop optoelectronics with higher performance.

Bionic visual-audio photodetectors with in-sensor perception and preprocessing.

Serving as the "eyes" and "ears" of the Internet of Things, optical and acoustic sensors are the fundamental components in hardware systems. Nowadays, mainstream hardware systems, often comprising numerous discrete sensors, conversion modules, and processing units, tend to result in complex architectures that are less efficient compared to human sensory pathways. Here, a visual-audio photodetector inspired by the human perception system is proposed to enable all-in-one visual and acoustic signal detection with computing capability. This device not only captures light but also optically records sound waves, thus achieving "watching" and "listening" within a single unit. The gate-tunable positive, negative, and zero photoresponses lead to highly programmable responsivities. This programmability enables the execution of diverse functions, including visual feature extraction, object classification, and sound wave manipulation. These results showcase the potential of expanding perception approaches in neuromorphic devices, opening up new possibilities to craft intelligent and compact hardware systems.

Uncooled Broadband Photodetection via Light Trapping in Conformal PtTe2-Silicon Nanopillar Heterostructures.

Dirac semimetals have demonstrated significant attraction in the field of optoelectronics due to their unique bandgap structure and high carrier mobility. Combining them with classical semiconductor materials to form heterojunctions enables broadband optoelectronic conversion at room temperature. However, the low light absorption of layered Dirac semimetals substantially limits the device's responsivity in the infrared band. Herein, a three-dimensional (3D) heterostructure, composed of silicon nanopillars (SiNPs) and a conformal PtTe2 film, is proposed and demonstrated to enhance the photoresponsivity for uncooled broadband detection. The light trapping effect in the 3D heterostructure efficiently promotes the interaction between light and PtTe2, while also enhancing the light absorption efficiency of silicon, which enables the enhancement of the device responsivity across a broadband spectrum. Experimentally, the PtTe2-SiNPs heterojunction device demonstrates excellent photoelectric conversion behavior across the visible, near-infrared, and long-wave infrared (LWIR) bands, with its responsivity demonstrating an order-of-magnitude improvement compared to the counterparts with planar silicon heterojunctions. Under 11 µm laser irradiation, the noise equivalent power (NEP) can reach 1.76 nW·Hz-1/2 (@1 kHz). These findings offer a strategic approach to the design and fabrication of high-performance broadband photodetectors based on Dirac semimetals.

Schottky infrared detectors with optically tunable barriers beyond the internal photoemission limit.

Internal photoemission is a prominent branch of the photoelectric effect and has emerged as a viable method for detecting photons with energies below the semiconductor bandgap. This breakthrough has played a significant role in accelerating the development of infrared imaging in one chip with state-of-the-art silicon techniques. However, the performance of these Schottky infrared detectors is currently hindered by the limit of internal photoemission; specifically, a low Schottky barrier height is inevitable for the detection of low-energy infrared photons. Herein, a distinct paradigm of Schottky infrared detectors is proposed to overcome the internal photoemission limit by introducing an optically tunable barrier. This device uses an infrared absorbing material-sensitized Schottky diode, assisted by the highly adjustable Fermi level of graphene, which subtly decouples the photon energy from the Schottky barrier height. Correspondingly, a broadband photoresponse spanning from ultraviolet to mid-wave infrared is achieved, with a high specific detectivity of 9.83 × 1010 cm Hz1/2 W-1 at 2,700 nm and an excellent specific detectivity of 7.2 × 109 cm Hz1/2 W-1 at room temperature under blackbody radiation. These results address a key challenge in internal photoemission and hold great promise for the development of the Schottky infrared detector with high sensitivity and room temperature operation.

Ultralow-Noise MoS2/Type II Superlattice Mixed-Dimensional van der Waals Barrier Long-Wave Infrared Detector.

Low-noise, high-performance long-wave infrared detectors play a crucial role in diverse applications, including in the industrial, security, and medical fields. However, the current performance of long-wave detectors is constrained by the noise associated with narrow bandgaps. Therefore, exploring novel heterostructures for long-wavelength infrared detection is advantageous for the development of compact and high-performance infrared sensing. In this investigation, we present a MoS2/type II superlattice mixed-dimensional van der Waals barrier long-wave infrared detector (Mixed-vdWH). Through the design of the valence band barrier, substantial suppression of device dark noise is achieved, resulting in 2 orders of magnitude reduction in dark current. The device exhibits outstanding performance, with D* reaching 4 × 1010 Jones. This integration approach synergizes the distinctive properties of two-dimensional layered materials (2DLM) with the well-established processing techniques of traditional three-dimensional semiconductor materials, offering a compelling avenue for the large-scale integration of 2DLM.

Synergistic-potential engineering enables high-efficiency graphene photodetectors for near- to mid-infrared light.

High quantum efficiency and wide-band detection capability are the major thrusts of infrared sensing technology. However, bulk materials with high efficiency have consistently encountered challenges in integration and operational complexity. Meanwhile, two-dimensional (2D) semimetal materials with unique zero-bandgap structures are constrained by the bottleneck of intrinsic quantum efficiency. Here, we report a near-mid infrared ultra-miniaturized graphene photodetector with configurable 2D potential well. The 2D potential well constructed by dielectric structures can spatially (laterally and vertically) produce a strong trapping force on the photogenerated carriers in graphene and inhibit their recombination, thereby improving the external quantum efficiency (EQE) and photogain of the device with wavelength-immunity, which enable a high responsivity of 0.2 A/W-38 A/W across a broad infrared detection band from 1.55 to 11 µm. Thereafter, a room-temperature detectivity approaching 1 × 109 cm Hz1/2 W-1 is obtained under blackbody radiation. Furthermore, a synergistic effect of electric and light field in the 2D potential well enables high-efficiency polarization-sensitive detection at tunable wavelengths. Our strategy opens up alternative possibilities for easy fabrication, high-performance and multifunctional infrared photodetectors.

Surface plasmon coupling in end-to-end linked gold nanorod dimers and trimers.

Colloidal gold nanorods were aligned end-to-end via dithiol coupling. The scattering properties of the resultant nanostructures were investigated at the single particle level by combining dark-field microscopy and high resolution scanning electron microscopy. The longitudinal surface plasmon resonance of end-to-end coupled Au nanorods exhibited a red-shift as the number of rods in the chain increased. The nanostructures exhibited polarization-dependent optical properties, due to selective excitation of collective bonding and anti-bonding modes. The surface plasmon peak energy was not strongly dependent on the angle of rod-sphere-rod trimers. The experimental scattering spectra were compared with the results obtained from theoretical calculations using the Finite Element Method (FEM) and found to be in good agreement.

Graphene nanowalls in photodetectors.

Graphene nanowalls (GNWs) have emerged as a promising material in the field of photodetection, thanks to their exceptional optical, electrical, mechanical, and thermodynamic properties. However, the lack of a comprehensive review in this domain hinders the understanding of GNWs' development and potential applications. This review aims to provide a systematic summary and analysis of the current research status and challenges in GNW-based photodetectors. We begin by outlining the growth mechanisms and methods of GNWs, followed by a discussion on their physical properties. Next, we categorize and analyze the latest research progress in GNW photodetectors, focusing on photovoltaic, photoconductive, and photothermal detectors. Lastly, we offer a summary and outlook, identifying potential challenges and outlining industry development directions. This review serves as a valuable reference for researchers and industry professionals in understanding and exploring the opportunities of GNW materials in photodetection.

Enhancing and Broadening the Photoresponse of Monolayer MoS2 Based on Au Nanoslit Array.

Two-dimensional molybdenum disulfide (MoS2), featuring unique optoelectronic properties, has attracted tremendous interest in developing novel photodetection devices. However, the limited light absorption and small carrier transport rate of the monolayer MoS2 result in low photoresponse, and the large band gap limits its detection range in the visible region. In this study, we propose a nanoslit array-MoS2 hybrid device architecture with enhanced and broadened photoresponse. The nanoslit array can localize free-space light to achieve strong interactions with MoS2, and acts as the channel to improve charge transport. As a result, the Au nanoslit array-MoS2 hybrid detector exhibits a nearly 100-fold increase in photocurrent compared to the pure MoS2 device. More importantly, the hybrid device can broaden the photoresponse to the optical communication band of 1550 nm which is lower than the band gap of MoS2, by efficiently utilizing the hot carriers generated by the Au nanoslits. The experimental results are supported by both theoretical analysis and numerical simulation. Since our demonstration leverages the engineering of the hybrid photodetectors with metal nanostructures rather than semiconductor materials, it should be universal and applicable to other devices for broadband, high-efficiency photoelectric conversion.

Enhanced Photogating Effect in Graphene Photodetectors via Potential Fluctuation Engineering.

The photogating effect in hybrid structures has manifested itself as a reliable and promising approach for photodetectors with ultrahigh responsivity. A crucial factor of the photogating effect is the built-in potential at the interface, which controls the separation and harvesting of photogenerated carriers. So far, the primary efforts of designing the built-in potential rely on discovering different materials and developing multilayer structures, which may raise problems in the compatibility with the standard semiconductor production line. Here, we report an enhanced photogating effect in a monolayer graphene photodetector based on a structured substrate, where the built-in potential is established by the mechanism of potential fluctuation engineering. We find that the enhancement factor of device responsivity is related to a newly defined parameter, namely, fluctuation period rate (Pf). Compared to the device without a nanostructured substrate, the responsivity of the device with an optimized Pf is enhanced by 100 times, reaching a responsivity of 240 A/W and a specific detectivity, D*, of 3.4 × 1012 Jones at 1550 nm wavelength and room temperature. Our experimental results are supported by both theoretical analysis and numerical simulation. Since our demonstration of the graphene photodetectors leverages the engineering of structures with monolayer graphene rather than materials with a multilayer complex structure. it should be universal and applicable to other hybrid photodetectors.