Light-Intensity Switching of Graphene/WSe2 Synaptic Devices

2D van der Waals heterojunctions (vdWH) have emerged as an attractive platform for the realization of optoelectronic synaptic devices, which are critical for energy-efficient computing systems. Photogating induced by charge traps at the interfaces indeed results in ultrahigh responsivity and tunable photoconductance. Yet, optical potentiation and depression remain mostly modulated by gate bias, requiring relatively high energy inputs. Thus, advanced all-optical synapse switching strategies are still needed. In this work, a reversible switching between positive photoconductivity (PPC) and negative photoconductivity (NPC) is achieved in graphene/WSe2 vdWH solely through light-intensity modulation. Consequently, the graphene/WSe2 synaptic device shows tunable optical potentiation and depression behavior with an ultralow power consumption of 127 aJ. The study further unravels the complex interplay of gate bias and incident light power in determining the sign and magnitude of the photocurrent, showing the critical role of charge trapping and photogating at interfaces. Interestingly, it is found that switching between PPC to NPC can be also obtained at 0 mV drain-source voltage. Overall, the reversible potentiation/depression effect based on light intensity modulation and its combination with additional gate bias tunability is very appealing for the development of energy-efficient optical communications and neuromorphic computing.

Enhanced LWIR response of InP/InAsP quantum discs-in-nanowire array photodetectors by photogating and ultra-thin ITO contacts

Here we report on an experimental and theoretical investigation of the long-wavelength infrared (LWIR) photoresponse of photodetectors based on arrays of three million InP nanowires with axially embedded InAsP quantum discs. An ultra-thin top indium tin oxide contact combined with a novel photogating mechanism facilitates an improved LWIR normal incidence sensitivity in contrast to traditional planar quantum well photodetectors. The electronic structure of the quantum discs, including strain and defect-induced photogating effects, and optical transition matrix elements were calculated by an 8-bandk·psimulation along with solving drift-diffusion equations to unravel the physics behind the generation of narrow linewidth intersubband signals observed from the quantum discs.

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.

Photogating Effect-Driven Photodetectors and Their Emerging Applications

Rather than generating a photocurrent through photo-excited carriers by the photoelectric effect, the photogating effect enables us to detect sub-bandgap rays. The photogating effect is caused by trapped photo-induced charges that modulate the potential energy of the semiconductor/dielectric interface, where these trapped charges contribute an additional electrical gating-field, resulting in a shift in the threshold voltage. This approach clearly separates the drain current in dark versus bright exposures. In this review, we discuss the photogating effect-driven photodetectors with respect to emerging optoelectrical materials, device structures, and mechanisms. Representative examples that reported the photogating effect-based sub-bandgap photodetection are revisited. Furthermore, emerging applications using these photogating effects are highlighted. The potential and challenging aspects of next-generation photodetector devices are presented with an emphasis on the photogating effect.

Inorganic-Organic Hybrid Phototransistor Array with Enhanced Photogating Effect for Dynamic Near-Infrared Light Sensing and Image Preprocessing

Narrow-band-gap organic semiconductors have emerged as appealing near-infrared (NIR) sensing materials by virtue of their unique optoelectronic properties. However, their limited carrier mobility impedes the implementation of large-area, dynamic NIR sensor arrays. In this work, high-performance inorganic-organic hybrid phototransistor arrays are achieved for NIR sensing, by taking advantage of the high electron mobility of In₂O₃ and the strong NIR absorption of a BTPV-4F:PTB7-Th bulk heterojunction (BHJ) with an enhanced photogating effect. As a result, the hybrid phototransistors reach a high responsivity of 1393.0 A W^-1, a high specific detectivity of 4.8 × 10¹² jones, and a fast response of 0.72 ms to NIR light (900 nm). Meanwhile, an integrated 16 × 16 phototransistor array with a one-transistor-one-phototransistor (1T1PT) architecture is achieved. On the basis of the enhanced photogating effect, the phototransistor array can not only achieve real-time, dynamic NIR light mapping but also implement image preprocessing, which is promising for advanced NIR image sensors.

Twisted Light-Induced Photocurrent in a Silicon Nanowire Field-Effect Transistor

Light can possess orbital angular momentum (OAM), in addition to spin angular momentum (SAM), which offers nearly infinite possible values of momentum states, allowing a wider degree of freedom for information processing and communications. The OAM of light induces a selection rule that obeys the law of conservation of angular momentum as it interacts with a material, affecting the material's optical and electrical properties. In this work, silicon nanowire field-effect transistors are subjected to light with OAM, also known as twisted light. Electrical measurements on the devices consequently reveal photocurrent enhancements after incrementing the OAM of the incident light from 0ℏ (fundamental mode) to 5ℏ. Such a phenomenon is attributed to the enhancements of the photogating and the photoconductive effects under the influence of the OAM of light, the underlying mechanism of which is proposed and discussed using energy band diagrams. With these observations, a strategy for controlling photocurrent has been introduced, which can be a realization of the application in the field of optoelectronics technology.

Imaging Spatial Distribution of Photogenerated Carriers in Monolayer MoS2 with Kelvin Probe Force Microscopy

The spatial distribution of photogenerated carriers in atomically thin MoS₂ flakes is investigated by measuring surface potential changes under light illumination using Kelvin probe force microscopy (KPFM). It is demonstrated that the vertical redistribution of photogenerated carriers, which is responsible for photocurrent generation in MoS₂ photodetectors, can be imaged as surface potential changes with KPFM. The polarity of surface potential changes points to the trapping of photogenerated holes at the interface between MoS₂ and the substrate as a major mechanism for the photoresponse in monolayer MoS₂. The temporal response of the surface potential changes is compatible with the time constant of MoS₂ photodetectors. The spatial inhomogeneity in the surface potential changes at the low light intensity that is related to the defect distribution in MoS₂ is also investigated.

Emerging Single-Photon Detectors Based on Low-Dimensional Materials

Single-photon detectors (SPDs) that can sense individual photons are the most sensitive instruments for photodetection. Established SPDs such as conventional silicon or III-V compound semiconductor avalanche diodes and photomultiplier tubes have been used in a wide range of time-correlated photon-counting applications, including quantum information technologies, in vivo biomedical imaging, time-of-flight 3D scanners, and deep-space optical communications. However, further development of these fields requires more sophisticated detectors with high detection efficiency, fast response, and photon-number-resolving ability, etc. Thereby, significant efforts have been made to improve the performance of conventional SPDs and to develop new photon-counting technologies. In this review, the working mechanisms and key performance metrics of conventional SPDs are first summarized. Then emerging photon-counting detectors (in the visible to infrared range) based on 0D quantum dots, 1D quantum nanowires, and 2D layered materials are discussed. These low-dimensional materials exhibit many exotic properties due to the quantum confinement effect. And photodetectors built from these nD-materials (n = 0, 1, 2) can potentially be used for ultra-weak light detection. By reviewing the status and discussing the challenges faced by SPDs, this review aims to provide future perspectives on the research directions of emerging photon-counting technologies.

Orbital Gating Driven by Giant Stark Effect in Tunneling Phototransistors

Conventional gating in transistors uses electric fields through external dielectrics that require complex fabrication processes. Various optoelectronic devices deploy photogating by electric fields from trapped charges in neighbor nanoparticles or dielectrics under light illumination. Orbital gating driven by giant Stark effect is demonstrated in tunneling phototransistors based on 2H-MoTe₂ without using external gating bias or slow charge trapping dynamics in photogating. The original self-gating by light illumination modulates the interlayer potential gradient by switching on and off the giant Stark effect where the d_z 2-orbitals of molybdenum atoms play the dominant role. The orbital gating shifts the electronic bands of the top atomic layer of the MoTe₂ by up to 100 meV, which is equivalent to modulation of a carrier density of 7.3 × 10¹¹ cm^-2 by electrical gating. Suppressing conventional photoconductivity, the orbital gating in tunneling phototransistors achieves low dark current, practical photoresponsivity (3357 AW^-1 ), and fast switching time (0.5 ms) simultaneously.

Robust Visible-Blind Wearable Infrared Sensor Based on IrP2 Nanoparticle-Embedded Few-Layer Graphene and the Effect of Photogating

It is challenging to realize a visible-blind infrared photodetector as the materials that absorb infrared light also absorb visible light. Here, we report the synthesis of IrP₂ nanoparticle-embedded few-layer graphene by one-step solid-state pyrolysis and its application in visible-blind infrared sensing. A linear photodetector device was fabricated by drop casting IrP₂ nanoparticle-embedded few-layer graphene onto a flexible PET substrate with two gold electrodes separated by ∼16 μm. The photoconductive gain was found to be as high as ∼145% with response and decay times of ∼0.4 and ∼2.8 s, respectively, under 1550 nm irradiation of 800 mW cm^-2. The room-temperature responsivity was ∼1.81 A W^-1 at 80 mW cm^-2 and ∼0.54 A W^-1 at a high incident power of ∼2200 mW cm^-2 under a bias of 1 V. Interestingly, the device showed response even in the long-wavelength infrared region, but no response was found under visible light. The embedded IrP₂ nanoparticles act as trap centers inducing photogating in the device, and the average trap state energy was estimated to be ∼16.5 ± 1.5 meV from the temperature-dependent photocurrent studies. The device was found to be immune to air exposure and bending, suggestive of use a a wearable sensor.

Charge-Transfer Effect and Enhanced Photoresponsivity of WS2- and MoSe2-Based Field Effect Transistors with π-Conjugated Polyelectrolyte

The characteristics of field effect transistors (FETs) fabricated using two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be modulated by surface treatment of the active layers. In this study, an ionic π-conjugated polyelectrolyte, poly(9,9-bis(4'-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was used for the surface treatment of MoSe₂ and WS₂ FETs. The photoluminescence (PL) intensities of monolayer (1L)-MoSe₂ and 1L-WS₂ clearly decreased, and the PL peaks were red-shifted after FPS-K treatment, suggesting a charge-transfer effect. In addition, the n-channel current of both the MoSe₂ and WS₂ FETs increased and the threshold voltage (V_th) shifted negatively after FPS-K treatment owing to the charge-transfer effect. The photoresponsivity of the MoSe₂ FET under light irradiation (λ_ex = 455 nm) increased considerably, from 5300 A W^-1 to approximately 10 000 A W^-1, after FPS-K treatment, and similar behavior was observed in the WS₂ FET. The results can be explained in terms of the increase in electron concentration due to photogating. The external quantum efficiency and photodetectivity of both FETs were also enhanced by the charge-transfer effect resulting from surface treatment with FPS-K containing mobile cations (K⁺) and fixed anions (SO₃^-), as well as by the photogating effect. The variation in charge-carrier density due to the photogating and charge-transfer effects is estimated to be approximately 2 × 10¹² cm^-2. The results suggest that π-conjugated polyelectrolytes such as FPS-K can be a promising candidate for the passivation of TMDC-based FETs and obtaining enhanced photoresponsivity.

An Ultrasensitive Gateless Photodetector Based on the 2D Bilayer MoS2-1D Si Nanowire-0D Ag Nanoparticle Hybrid Structure

Atomically thin transition metal dichalcogenides (TMDC) have received much attention due to their wide variety of optical and electronic properties. Among various TMDC materials, molybdenum disulfide (MoS₂) has been intensely studied owing to its potential applications in nanoelectronics and optoelectronics. However, two-dimensional MoS₂ photodetectors suffer from low responsivity due to low optical cross section. Combining MoS₂ with plasmonic nanostructures can drastically increase scattering cross section and enhance local light-matter interaction. Moreover, suspended MoS₂ has been shown to exhibit higher photoluminescence intensity and strong photogating effect, which can be employed in photodetectors. Herein, we propose an approach to utilize plasmonic nanostructures and physical suspension for 2D MoS₂ photosensing enhancement by hybridizing 2D bilayer MoS₂, 1D silicon nanowires, and 0D silver nanoparticles. The hybrid structure shows a gateless responsivity of 402.4 A/W at a wavelength of 532 nm, which represents the highest value among the ever reported gateless plasmonic MoS₂ photodetector. The great responsivity and large active area results in an exceptional detectivity of 2.34 × 10¹² Jones. This study provides a new approach for designing high-performance 2D TMDC-based optoelectronic devices.

Trion-Mediated Förster Resonance Energy Transfer and Optical Gating Effect in WS2/hBN/MoSe2 Heterojunction

van der Waals two-dimensional layered heterostructures have recently emerged as a platform, where the interlayer couplings give rise to interesting physics and multifunctionalities in optoelectronics. Such couplings can be rationally controlled by dielectric, separation, and stacking angles, which affect the overall charge or energy-transfer processes, and emergent potential landscape for twistronics. Herein, we report the efficient Förster resonance energy transfer (FRET) in WS₂/hBN/MoSe₂ heterostructure, probed by both steady-state and time-resolved optical spectroscopy. We clarified the evolution behavior of the electron-hole pairs and free electrons from the trions, that is, ∼59.9% of the electron-hole pairs could transfer into MoSe₂ by FRET channels (∼38 ps) while the free electrons accumulate at the WS₂/hBN interface to photogate MoSe₂. This study presents a clear picture of the FRET process in two-dimensional transition-metal dichalcogenides' heterojunctions, which establishes the scientific foundation for developing the related heterojunction optoelectronic devices.

Ultralow-Transition-Energy Organic Complex on Graphene for High-Performance Shortwave Infrared Photodetection

Room-temperature, high-sensitivity, and broadband photodetection up to the shortwave infrared (SWIR) region is extremely significant for a wide variety of optoelectronic applications, including contamination identification, thermal imaging, night vision, agricultural inspection, and atmospheric remote sensing. Small-bandgap semiconductor-based SWIR photodetectors generally require deep cooling to suppress thermally generated charge carriers to achieve increased sensitivity. Meanwhile, the photogating effect can provide an alternative way to achieve superior photosensitivity without the need for cooling. The optical photogating effect originates from charge trapping of photoinduced carriers at defects or interfaces, resulting in an extremely high photogain (10⁶ or higher). Here, a highly sensitive SWIR hybrid photodetector, fabricated by integrating an organic charge transfer complex on a graphene transistor, is reported. The organic charge transfer complex (tetrathiafulvalene-chloranil) has an exceptional low-energy intermolecular electronic transition down to 0.5 eV, with the aim of achieving efficient SWIR absorption for wavelengths greater than 2 µm. The photogating effect at the organic complex and graphene interface enables an extremely high photogain and a high detectivity of ≈10¹³ Jones, along with a response time of 8 ms, at room temperature for a wavelength of 2 µm.

Gate- and Light-Tunable Negative Differential Resistance with High Peak Current Density in 1T-TaS2/2H-MoS2 T-Junction

Metal-based electronics is attractive for fast and radiation-hard electronic circuits and remains one of the long-standing goals for researchers. The emergence of 1T-TaS₂, a layered material exhibiting strong charge density wave (CDW)-driven resistivity switching that can be controlled by an external stimulus such as electric field and optical pulses, has triggered a renewed interest in metal-based electronics. Here we demonstrate a negative differential resistor (NDR) using electrically driven CDW phase transition in an asymmetrically designed T-junction made up of 1T-TaS₂/2H-MoS₂ van der Waals heterojunction. The principle of operation of the proposed device is governed by majority carrier transport and is distinct from usual NDR devices employing tunneling of carriers; thus it avoids the bottleneck of weak tunneling efficiency in van der Waals heterojunctions. Consequently, we achieve a peak current density in excess of 10⁵ nA μm^-2, which is about 2 orders of magnitude higher than that obtained in typical layered material based NDR implementations. The peak current density can be effectively tuned by an external gate voltage as well as photogating. The device is robust against ambiance-induced degradation, and the characteristics repeat in multiple measurements over a period of more than a month. The findings are attractive for the implementation of active metal-based functional circuits.

Photosensitive n-Type Doping Using Perovskite CsPbX3 Quantum Dots for Two-Dimensional MSe2 (M = Mo and W) Field-Effect Transistors

Perovskite CsPbX₃ (X = Br, Cl, and I) nanostructures have been intensively studied as they are luminescent, photovoltaic, and photosensitizing active materials. Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) with MX₂ (M = Mo, W; X = S, Se, Te, etc.) structures have been used in flexible optoelectronic devices. In this study, perovskite green-light-emitting CsPbBr₂I₁ quantum dots (QDs) and blue-light-emitting CsPb(Cl/Br)₃-QDs are utilized to enhance the photoresponsive characteristics of 2D MSe₂ (M = Mo and W)-based field-effect transistors (FETs). From laser confocal microscopy photoluminescence (PL) experiments, PL quenching of the perovskite CsPb(Cl/Br)₃-QDs and CsPbBr₂I₁-QDs is observed after hybridization with MoSe₂ and WSe₂ layers, respectively, which reflects the charge-transfer effect. According to the characteristics of the FETs based on the WSe₂, MoSe₂, WSe₂/CsPbBr₂I₁-QDs hybrid, and MoSe₂/CsPb(Cl/Br)₃-QDs hybrid, the p-channel current (with hole mobility) is considerably decreased after the hybridization with the QDs. Notably, under incident light, the n-channel photocurrent and photoresponsivity of the FET are substantially increased, and the threshold voltage is negatively shifted owing to the hybridization with the perovskite QDs. The results show that the photosensitive n-type doping effect on the 2D MoSe₂ and WSe₂ nanosystems originates from the photogating effect by the trap states after the hybridization with various perovskite CsPbX₃-QDs.

Optically Stimulated Synaptic Devices Based on the Hybrid Structure of Silicon Nanomembrane and Perovskite

Optoelectronic synaptic devices have been attracting increasing attention due to their critical role in the development of neuromorphic computing based on optoelectronic integration. Here we start with silicon nanomembrane (Si NM) to fabricate optoelectronic synaptic devices. Organolead halide perovskite (MAPbI₃) is exploited to form a hybrid structure with Si NM. We demonstrate that synaptic transistors based on the hybrid structure are very sensitive to optical stimulation with low energy consumption. Synaptic functionalities such as excitatory post-synaptic current (EPSC), paired-pulse facilitation, and transition from short-term memory to long-term memory (LTM) are all successfully mimicked by using these optically stimulated synaptic transistors. The backgate-enabled tunability of the EPSC of these devices further leads to the LTM-based mimicking of visual learning and memory processes under different mood states. This work contributes to the development of Si-based optoelectronic synaptic devices for neuromorphic computing.

Photogating WS2 Photodetectors Using Embedded WSe2 Charge Puddles

Performance of 2D photodetectors is often predominated by charge traps that offer an effective photogating effect. The device features an ultrahigh gain and responsivity, but at the cost of a retarded temporal response due to the nature of long-lived trap states. In this work, we devise a gain mechanism that originates from massive charge puddles formed in the type-II 2D lateral heterostructures. This concept is demonstrated using graphene-contacted WS₂ photodetectors embedded with WSe₂ nanodots. Upon light illumination, photoexcited carriers are separated by the built-in field at the WSe₂/WS₂ heterojunctions (HJs), with holes trapped in the WSe₂ nanodots. The resulting WSe₂ hole puddles provide a photoconductive gain, as electrons are recirculating during the lifetime of holes that remain trapped in the puddles. The WSe₂/WS₂ HJ photodetectors exhibit a responsivity of 3 × 10² A/W with a gain of 7 × 10² electrons per photon. Meanwhile, the zero-gate response time is reduced by 5 orders of magnitude as compared to the prior reports for the graphene-contacted pristine WS₂ monolayer and WS₂/MoS₂ heterobilayer photodetectors due to the ultrafast intralayer excitonic dynamics in the WSe₂/WS₂ HJs.

High Responsivity of InP/InAsP Nanowire Array Broadband Photodetectors Enhanced by Optical Gating

High-performance photodetectors operating in the near-infrared (0.75-1.4 μm) and short-wave infrared (1.4-3.0 μm) portion of the electromagnetic spectrum are key components in many optical systems. Here, we report on a combined experimental and theoretical study of square millimeter array infrared photodetectors comprising 3 million n⁺-i-n⁺ InP nanowires grown by MOVPE from periodically ordered Au seed particles. The nominal i-segment, comprising 20 InAs_0.40P_0.60 quantum discs, was grown by use of an optimized Zn doping to compensate the nonintentional n-doping. The photodetectors exhibit bias- and power-dependent responsivities reaching record-high values of 250 A/W at 980 nm/20 nW and 990 A/W at 532 nm/60 nW, both at 3.5 V bias. Moreover, due to the embedded quantum discs, the photoresponse covers a broad spectral range from about 0.70 to 2.5 eV, in effect outperforming conventional single InGaAs detectors and dual Si/Ge detectors. The high responsivity, and related gain, results from a novel proposed photogating mechanism, induced by the complex charge carrier dynamics involving optical excitation and recombination in the quantum discs and interface traps, which reduces the electron transport barrier between the highly doped n⁺ contact and the i-segment. The experimental results obtained are in perfect agreement with the proposed theoretical model and represent a significant step forward toward understanding gain in nanoscale photodetectors and realization of commercially viable broadband photon detectors with ultrahigh gain.

Photogating in the Graphene-Dye-Graphene Sandwich Heterostructure

In this work, we developed an atomically thin (∼2.5 nm) heterostructure consisting of a monolayer rhodamine 6G (R6G) film as a photoactive layer that was sandwiched between graphene films functioning as channels (graphene-R6G-graphene, G-R-G). Through a comparison of results of both photocurrent measurements and chemically enhanced Raman scattering (CERS) experiments, we found that our G-R-G heterostructure exhibited ∼7 and ∼30 times better performance than R6G-attached single-graphene (R6G-graphene, R-G) and MoS₂ devices, respectively; here, the CERS enhancement factor was highly correlated with the relative photoinduced Dirac voltage change. Furthermore, the photocurrent of the G-R-G device was found to be ∼40 times better than that of the R-G photodetector. The top graphene was highly operative in the monolayer, of which the performance is significantly deteriorated by fluorescence and tailored charge transfer efficiency with the increment of R6G film thickness. Overall, the responsivity of the G-R-G photodetector was ∼40 times higher than that of the R-G photodetector because of the more efficient carrier transfer between the organic dye and graphene induced by weaker π-π interactions between the top and bottom graphene channels in the former device. This atomically thin (∼2.5 nm) and highly photosensitive photodetector can be employed for post-Si-photodiode (PD) image sensors, single-photon detection devices, and optical communications.

High-Performance, Room Temperature, Ultra-Broadband Photodetectors Based on Air-Stable PdSe2

Photodetection over a broad spectral range is crucial for optoelectronic applications such as sensing, imaging, and communication. Herein, a high-performance ultra-broadband photodetector based on PdSe₂ with unique pentagonal atomic structure is reported. The photodetector responds from visible to mid-infrared range (up to ≈4.05 µm), and operates stably in ambient and at room temperature. It promises improved applications compared to conventional mid-infrared photodetectors. The highest responsivity and external quantum efficiency achieved are 708 A W^-1 and 82 700%, respectively, at the wavelength of 1064 nm. Efficient optical absorption beyond 8 µm is observed, indicating that the photodetection range can extend to longer than 4.05 µm. Owing to the low crystalline symmetry of layered PdSe₂ , anisotropic properties of the photodetectors are observed. This emerging material shows potential for future infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Optoelectronic Properties of Printed Photogating Carbon Nanotube Thin Film Transistors and Their Application for Light-Stimulated Neuromorphic Devices

Artificial synapses/neurons based on electronic/ionic hybrid devices have attracted wide attention for brain-inspired neuromorphic systems since it is possible to overcome the von Neumann bottleneck of the neuromorphic computing paradigm. Here, we report a novel photoneuromorphic device based on printed photogating single-walled carbon nanotube (SWCNT) thin film transistors (TFTs) using lightly n-doped Si as the gate electrode. The drain currents of the printed SWCNT TFTs can gradually increase to over 3000 times of their starting value after being pulsed with light stimulation, and the electrical signals can maintain for over 10 min. These characteristics are similar to the learning and memory functions of brain-inspired neuromorphic systems. The working mechanism of the light-stimulated neuromorphic devices is investigated and described here in detail. Important synaptic characteristics, such as low-pass filtering characteristics and nonvolatile memory ability, are successfully emulated in the printed light-stimulated artificial synapses. It demonstrates that the printed SWCNT TFT photoneuromorphic devices can act as the nonvolatile memory units and perform photoneuromorphic computing, which exhibits potential for future neuromorphic system applications.

MoS2-Based Optoelectronic Gas Sensor with Sub-parts-per-billion Limit of NO2 Gas Detection

Red light illumination with photon energy matching the direct band gap of chemical vapor deposition grown single-layer MoS₂ with Au metal electrodes was used to induce a photocurrent which was employed instead of dark current for NO₂ gas sensing. The resulting Au/MoS₂/Au optoelectronic gas sensor showed a significant enhancement of the device sensitivity S toward ppb level of NO₂ gas exposure reaching S = 4.9%/ppb (4900%/ppm), where S is a slope of dependence of relative change of the sensor resistance on NO₂ concentration. Further optimization of the MoS₂-based optoelectronic gas sensor by using graphene (Gr) with a work function lower than that of Au for the electrical contacts to the MoS₂ channel allowed an increase of photocurrent. The limit of detection of NO₂ gas at the level of 0.1 ppb was obtained for the MoS₂ channel with graphene electrodes coated by Au. This value was calculated using experimentally obtained sensitivity and noise values and exceeds the U.S. Environment Protection Agency requirement for NO₂ gas detection at ppb level.

Ultrascaled Germanium Nanowires for Highly Sensitive Photodetection at the Quantum Ballistic Limit

We report an experimental study on quasi-one-dimensional Al-Ge-Al nanowire (NW) heterostructures featuring unmatched photoconductive gains exceeding 10⁷ and responsivities as high as 10 A/μW in the visible wavelength regime. Our observations are attributed to the presence of GeO _x related hole-trapping states at the NW surface and can be described by a photogating effect in accordance with previous studies on low-dimensional nanostructures. Utilizing an ultrascaled photodetector device operating in the quantum ballistic transport regime at room temperature we demonstrate for the first time that individual current channels can be addressed directly by laser irradiation. The resulting quantization of the photocurrent represents the ultimate limit of photodetectors, allowing for advanced concepts including highly resolved imaging, light effect transistors and single photon detectors with practically zero off-state current.

Photogating in Low Dimensional Photodetectors

Low dimensional materials including quantum dots, nanowires, 2D materials, and so forth have attracted increasing research interests for electronic and optoelectronic devices in recent years. Photogating, which is usually observed in photodetectors based on low dimensional materials and their hybrid structures, is demonstrated to play an important role. Photogating is considered as a way of conductance modulation through photoinduced gate voltage instead of simply and totally attributing it to trap states. This review first focuses on the gain of photogating and reveals the distinction from conventional photoconductive effect. The trap- and hybrid-induced photogating including their origins, formations, and characteristics are subsequently discussed. Then, the recent progress on trap- and hybrid-induced photogating in low dimensional photodetectors is elaborated. Though a high gain bandwidth product as high as 109 Hz is reported in several cases, a trade-off between gain and bandwidth has to be made for this type of photogating. The general photogating is put forward according to another three reported studies very recently. General photogating may enable simultaneous high gain and high bandwidth, paving the way to explore novel high-performance photodetectors.

Simultaneous Thermoelectric and Optoelectronic Characterization of Individual Nanowires

Semiconducting nanowires have been explored for a number of applications in optoelectronics such as photodetectors and solar cells. Currently, there is ample interest in identifying the mechanisms that lead to photoresponse in nanowires in order to improve and optimize performance. However, distinguishing among the different mechanisms, including photovoltaic, photothermoelectric, photoemission, bolometric, and photoconductive, is often difficult using purely optoelectronic measurements. In this work, we present an approach for performing combined and simultaneous thermoelectric and optoelectronic measurements on the same individual nanowire. We apply the approach to GaN/AlGaN core/shell and GaN/AlGaN/GaN core/shell/shell nanowires and demonstrate the photothermoelectric nature of the photocurrent observed at the electrical contacts at zero bias, for above- and below-bandgap illumination. Furthermore, the approach allows for the experimental determination of the temperature rise due to laser illumination, which is often obtained indirectly through modeling. We also show that under bias, both above- and below-bandgap illumination leads to a photoresponse in the channel with signatures of persistent photoconductivity due to photogating. Finally, we reveal the concomitant presence of photothermoelectric and photogating phenomena at the contacts in scanning photocurrent microscopy under bias by using their different temporal response. Our approach is applicable to a broad range of nanomaterials to elucidate their fundamental optoelectronic and thermoelectric properties.

Multifunctional graphene optoelectronic devices capable of detecting and storing photonic signals

The advantages of graphene photodetectors were utilized to design a new multifunctional graphene optoelectronic device. Organic semiconductors, gold nanoparticles (AuNPs), and graphene were combined to fabricate a photodetecting device with a nonvolatile memory function for storing photonic signals. A pentacene organic semiconductor acted as a light absorption layer in the device and provided a high hole photocurrent to the graphene channel. The AuNPs, positioned between the tunneling and blocking dielectric layers, acted as both a charge trap layer and a plasmonic light scatterer, which enable storing of the information about the incident light. The proposed pentacene-graphene-AuNP hybrid photodetector not only performed well as a photodetector in the visible light range, it also was able to store the photonic signal in the form of persistent current. The good photodetection performance resulted from the plasmonics-enabled enhancement of the optical absorption and from the photogating mechanisms in the pentacene. The device provided a photoresponse that depended on the wavelength of incident light; therefore, the signal information (both the wavelength and intensity) of the incident light was effectively committed to memory. The simple process of applying a negative pulse gate voltage could then erase the programmed information. The proposed photodetector with the capacity to store a photonic signal in memory represents a significant step toward the use of graphene in optoelectronic devices.