In-sensor computing using a MoS2 photodetector with programmable spectral responsivity.

Optical spectroscopy is an indispensable technique in almost all areas of scientific research and industrial applications. After its acquisition, an optical spectrum is usually further processed using a mathematical algorithm to classify or quantify the measurement results. Here we present the design and realization of a smart photodetector that provides such information directly without the need to explicitly record a spectrum. This is achieved by tailoring the spectral responsivity of the device to a specific purpose. In-sensor computation is performed at the lowest possible level of the sensor system hierarchy - the physical level of photon detection - and does not require any external processing of the measurement data. The device can be programmed to cover different types of spectral regression or classification tasks. We present the analysis of spectral mixtures as an example, but the scheme can also be applied to any other algorithm that can be represented by a linear operator. Our prototype physical implementation utilizes an ensemble of optical cavity-enhanced MoS2 photodetectors with different center wavelengths and individually adjustable peak responsivities. This spectroscopy method represents a significant advance in miniaturized and energy-efficient optical sensing.

A photosensor employing data-driven binning for ultrafast image recognition.

Pixel binning is a technique, widely used in optical image acquisition and spectroscopy, in which adjacent detector elements of an image sensor are combined into larger pixels. This reduces the amount of data to be processed as well as the impact of noise, but comes at the cost of a loss of information. Here, we push the concept of binning to its limit by combining a large fraction of the sensor elements into a single "superpixel" that extends over the whole face of the chip. For a given pattern recognition task, its optimal shape is determined from training data using a machine learning algorithm. We demonstrate the classification of optically projected images from the MNIST dataset on a nanosecond timescale, with enhanced dynamic range and without loss of classification accuracy. Our concept is not limited to imaging alone but can also be applied in optical spectroscopy or other sensing applications.

Sparse pixel image sensor.

As conventional frame-based cameras suffer from high energy consumption and latency, several new types of image sensors have been devised, with some of them exploiting the sparsity of natural images in some transform domain. Instead of sampling the full image, those devices capture only the coefficients of the most relevant spatial frequencies. The number of samples can be even sparser if a signal only needs to be classified rather than being fully reconstructed. Based on the corresponding mathematical framework, we developed an image sensor that can be trained to classify optically projected images by reading out the few most relevant pixels. The device is based on a two-dimensional array of metal-semiconductor-metal photodetectors with individually tunable photoresponsivity values. We demonstrate its use for the classification of handwritten digits with an accuracy comparable to that achieved by readout of the full image, but with lower delay and energy consumption.

Multilayer WSe2 /MoS2 Heterojunction Phototransistors through Periodically Arrayed Nanopore Structures for Bandgap Engineering.

While 2D transition metal dichalcogenides (TMDs) are promising building blocks for various optoelectronic applications, limitations remain for multilayered TMD-based photodetectors: an indirect bandgap and a short carrier lifetime by strongly bound excitons. Accordingly, multilayered TMDs with a direct bandgap and an enhanced carrier lifetime are required for the development of various optoelectronic devices. Here, periodically arrayed nanopore structures (PANS) are proposed for improving the efficiency of multilayered p-WSe2 /n-MoS2 phototransistors. Density functional theory calculations as well as photoluminescence and time-resolved photoluminescence measurements are performed to characterize the photodetector figures of merit of multilayered p-WSe2 /n-MoS2 heterostructures with PANS. The characteristics of the heterojunction devices with PANS reveal an enhanced responsivity and detectivity measured under 405 nm laser excitation, which at 1.7 × 104 A W-1 and 1.7 × 1013 Jones are almost two orders of magnitude higher than those of pristine devices, 3.6 × 102 A W-1 and 3.6 × 1011 Jones, respectively. Such enhanced optical properties of WSe2 /MoS2 heterojunctions with PANS represent a significant step toward next-generation optoelectronic applications.

High-Performance Hybrid InP QDs/Black Phosphorus Photodetector.

Zero-dimensional-two-dimensional (0D-2D) hybrid optoelectronic devices have demonstrated high sensitivity and high performance due to the high absorption coefficient of 0D materials with a tunable detection range and a high carrier transport property of 2D materials. However, the reported 0D-2D hybrid devices employ toxic nanomaterials as sensitizing layers, which can limit the practical applications. In this study, we first fabricated the 0D-2D hybrid photodetector using nontoxic InP quantum dots (QDs) as a light-absorbing layer and black phosphorus (BP) as a transport layer. The surface treatment using 1,2-ethanedithiol and thermal treatment were carried out to remove the surface long ligands of colloidal QDs, which can accelerate the charge injection of the photogenerated carriers through the interfaces between InP QDs and BP. The InP QDs/BP hybrid photodetector demonstrates a high responsivity of 1 × 109 A/W and detectivity of 4.5 × 1016 Jones at 0.05 µW cm-2 under 405 nm illumination. The results show that 0D-2D hybrid photodetectors based on III-V semiconducting QD materials can be optimized for high-performance photodetectors.

Realizing Long-Term Stability and Thickness Control of Black Phosphorus by Ambient Thermal Treatment.

Few-layer black phosphorus (BP) has shown great potential for next-generation electronics with tunable band gap and high carrier mobility. For the electronic applications, the thickness modulation of a BP flake is essential due to its thickness-dependent electronic properties. However, controlling the precise thickness of few-layer BP is a challenge for the high-performance device applications. In this study, we demonstrate that thermal treatment under ambient condition precisely controls the thickness of BP flake. The thermal etching method utilizes the chemical reactivity of BP surface with oxygen and water molecules by the repeated formation and evaporation of phosphoric acid during thermal annealing. Field-effect transistor of the thickness-modulated BP sheet by thermal etching method shows a high hole mobility of ∼576 cm2 V-1 s-1 and a high on-off ratio of ∼105. The stability of the BP devices remained for 1 month under ambient condition without an additional protecting layer, resulting from the preservation of active BP layers below native surface phosphorus oxide.

Hybrid Black Phosphorus/Zero-Dimensional Quantum Dot Phototransistors: Tunable Photodoping and Enhanced Photoresponsivity.

Recently, black phosphorus (BP) with direct band gap exhibited excellent potential for optoelectronic applications because of its high charge carrier mobility and low dark current as well as the variable band gap of 0.3-1.5 eV depending on the number of layers. However, few-layer BP-based phototransistors (photo-FETs) have been limited in sensitivity and wavelength selectivity. To overcome the drawback of these photo-FETs, we studied hybrid photo-FETs combined with the novel properties of the two materials between the channel and sensitizer layers. By combining a strong absorbance of a quantum dot (QD) layer and a two-dimensional layer material with high carrier mobility, the hybrid photo-FETs are expected to produce high-performance photodetectors that can effectively control the responsivity, detectivity, and response time. In this study, we demonstrate that the photogenerated carriers formed from QD sensitizer layers migrate to the BP transport layer with high charge mobility and not only improve the photodetector performance but also enhance the photodoping effect of the BP transport layer with an ambipolar characteristic by electrons transferred from n-type CdSe QDs or holes injected from p-type PbS QDs. The responsivity and detectivity of hybrid BP/0D photo-FETs exhibit 1.16 × 109 A W-1 and 7.53 × 1016 Jones for the BP/CdSe QD photo-FET and 5.36 × 108 A W-1 and 1.89 × 1016 Jones for the BP/PbS QD photo-FET, respectively. The photocurrent rise (τrise) and decay (τdecay) times were τrise = 0.406 s and τdecay = 0.815 s for BP/CdSe QD photo-FET and τrise = 0.576 s and τdecay = 0.773 s for BP/PbS QD photo-FET, respectively.

Dual-Gate Black Phosphorus Field-Effect Transistors with Hexagonal Boron Nitride as Dielectric and Passivation Layers.

Two-dimensional black phosphorus (BP) has attracted much attention recently because of its applicability in high-performance electronic and optoelectronic devices. BP field-effect transistors (FETs) with a tunable band gap (0.3-1.5 eV) have demonstrated a high on-off current ratio and a high hole mobility with an ambipolar behavior in global-gated devices. However, local-gated BP FETs for integrated circuits have been reported with only p-type behaviors and a low on-current compared with global-gated BP FETs. Furthermore, BP, which is not stable in air, forms sharp spikes on its surface when exposed to humid air. This phenomenon plays a role in accelerating the degradation of the electrical properties of BP devices, which can occur even within a day. In this paper, we first demonstrate the origin of transport limitations of local-gated BP FETs by comparing the transport properties of hexagonal boron nitride (h-BN)-based device architectures with those of a bottom-gated BP FET on a Si/SiO2 substrate. By using h-BN as passivation and dielectric layers, BP FETs with a low gate operating voltage were fabricated with two different transistor geometries: top-gated and bottom-gated FETs. The highest mobility extracted from the global-gated BP FETs was 249 cm2 V-1 s-1 with a subthreshold swing of 848 mV dec-1.

Recovery Mechanism of Degraded Black Phosphorus Field-Effect Transistors by 1,2-Ethanedithiol Chemistry and Extended Device Stability.

Black phosphorus (BP) has drawn enormous attention for both intriguing material characteristics and electronic and optoelectronic applications. In spite of excellent advantages for semiconductor device applications, the performance of BP devices is hampered by the formation of phosphorus oxide on the BP surface under ambient conditions. It is thus necessary to resolve the oxygen-induced degradation on the surface of BP to recover the characteristics and stability of the devices. To solve this problem, it is demonstrated that a 1,2-ethanedithiol (EDT) treatment is a simple and effective way to remove the bubbles formed on the BP surface. The device characteristics of the degraded BP field-effect transistor (FET) are completely recovered to the level of the pristine cases by the EDT treatment. The underlying principle of bubble elimination on the BP surface by the EDT treatment is systematically analyzed by density functional theory calculation, atomic force microscopy, and X-ray photoelectron spectroscopy analysis. In addition, the performance of the hexagonal boron nitride-protected BP FET is completely retained without changing device characteristics even when exposed to 30 d or more in air. The EDT-induced recovering effect will allow a new route for the optimization of electronic and optoelectronic devices based on BP.

Solution-phase synthesis of rubidium lead iodide orthorhombic perovskite nanowires.

Recently, metal halide perovskite nanocrystals have demonstrated outstanding properties in various optoelectronic applications. Cesium lead halides (CsPbX3) are the most studied perovskites in nanoscale dimensions. However, halide perovskite nanocrystals with other cations have rarely been reported. It is important to develop new perovskite compositions to further expand their application in various fields. In this paper, we first report the synthesis of colloidal rubidium lead iodide (RbPbI3) nanowires (NWs). RbPbI3 NWs have an orthorhombic crystal structure and are single-crystalline in nature. The diameter of the NWs is around 32 nm with lengths up to several tens of micrometers. RbPbI3 NWs absorb strongly below 450 nm. RbPbI3 devices exhibited good photoresponsive behavior, suggesting a potential use in optoelectronics.

A hybrid MoS2 nanosheet-CdSe nanocrystal phototransistor with a fast photoresponse.

2-Dimensional (2D) and 0-dimensional (0D) hybrid nanostructures have been reported as promising new systems for highly-sensitive and wavelength-tunable photodetectors. Although the performance of hybrid photodetectors was enhanced by charge injection from 0D nanocrystals (NCs) to 2D nanosheets (NSs), the response time of hybrid photodetectors is still very slow due to the trapping and leakage of residual carriers at the interfaces of the hybrid materials. Here, we demonstrate a MoS2/CdSe hybrid phototransistor with enhanced responsivity of 2.5 × 105 A W-1 and detectivity of 1.24 × 1014 Jones. In addition, the device exhibited a fast rise (τrise) and decay time (τdecay) of 60 ms, respectively. The mechanism for the improved photoresponse time has been discussed using a charge injection model in an n-n type heterojunction energy band diagram of hybrid materials.