Photoinduced Tuning of Schottky Barrier Height in Graphene/MoS2 Heterojunction for Ultrahigh Performance Short Channel Phototransistor

Two-dimensional (2D) layered materials with properties such as a large surface-to-volume ratio, strong light interaction, and transparency are expected to be used in future optoelectronic applications. Many studies have focused on ways to increase absorption of 2D-layered materials for use in photodetectors. In this work, we demonstrate another strategy for improving photodetector performance using a graphene/MoS₂ heterojunction phototransistor with a short channel length and a tunable Schottky barrier. The channel length of sub-30 nm, shorter than the diffusion length, decreases carrier recombination and carrier transit time in the channel and improves phototransistor performance. Furthermore, our graphene/MoS₂ heterojunction phototransistor employed a tunable Schottky barrier that is only controlled by light and gate bias. It maintains a low dark current and an increased photocurrent. As a result, our graphene/MoS₂ heterojunction phototransistor showed ultrahigh responsivity and detectivity of 2.2 × 10⁵ A/W and 3.5 × 10¹³ Jones, respectively. This is a considerable improvement compared to previous pristine MoS₂ phototransistors. We confirmed an effective method to develop phototransistors based on 2D materials and obtained ultrahigh performance of our phototransistor, which is promising for high-performance optoelectronic applications.

A Graphene/Polycrystalline Silicon Photodiode and Its Integration in a Photodiode-Oxide-Semiconductor Field Effect Transistor

In recent years, the characteristics of the graphene/crystalline silicon junction have been frequently discussed in the literature, but study of the graphene/polycrystalline silicon junction and its potential applications is hardly found. The present work reports the observation of the electrical and optoelectronic characteristics of a graphene/polycrystalline silicon junction and explores one possible usage of the junction. The current–voltage curve of the junction was measured to show the typical exponential behavior that can be seen in a forward biased diode, and the photovoltage of the junction showed a logarithmic dependence on light intensity. A new phototransistor named the “photodiode–oxide–semiconductor field effect transistor (PDOSFET)” was further proposed and verified in this work. In the PDOSFET, a graphene/polycrystalline silicon photodiode was directly merged on top of the gate oxide of a conventional metal–oxide–semiconductor field effect transistor (MOSFET). The magnitude of the channel current of this phototransistor showed a logarithmic dependence on the illumination level. It is shown in this work that the PDOSFET facilitates a better pixel design in a complementary metal–oxide–semiconductor (CMOS) image sensor, especially beneficial for high dynamic range (HDR) image detection.

Unsymmetrical Small Molecules for Broad-Band Photoresponse and Efficient Charge Transport in Organic Phototransistors

Organic photosensitizers have been investigated as effective light-sensing elements that can promote strong absorption with high field-effect mobility in organic phototransistors (OPTs). In this study, a novel organic photosensitizer is synthesized to demonstrate broad-band photoresponse with enhanced electrical performance. An unsymmetrical small molecule of a solubilizing donor (D_sol)-acceptor (A)-dye donor (D_dye) type connected with a twisted conjugation system is designed for broad-band detection (ranging from 250 to 700 nm). This molecule has high solubility, thereby facilitating the formation of uniformly dispersed nanoparticles in an insulating polymer matrix, which is deposited on top of OPT semiconductors by a simple solution process. The broad-band photodetection shown by the organic photosensitizer is realized with improved mobility close to an order of magnitude and high on/off current ratio (∼10⁵) of the organic semiconductor. Furthermore, p-type charge transport behavior in the channel of the OPT is enhanced through the intrinsic electron-accepting ability of the organic photosensitizer caused by the unique molecular configuration. These structural properties of organic photosensitizers contribute to an improvement in broad-band photosensing systems with new optoelectronic properties and functionalities.

Organic Transistor Nonvolatile Memory with Three-Level Information Storage and Optical Detection Functions

By the current processing technology, it is a challenge to obtain ultrahigh-density information storage in the conventional binary floating-gate-based organic field-effect transistor (FG-OFET) nonvolatile memories (NVMs). To develop a multilevel memory in one cell is a feasible solution. In this work, we demonstrate FG-OFET NVMs with an integrated polymer floating-gate/tunneling (I-FG/T) layer consisting of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and polystyrene. The photoelectric effect of organic/polymer semiconductors is used to improve the controllability of the polarity and the number of the charges stored in the floating-gate. The FG-OFET NVMs integrate light sensitivity and nonvolatile information storage functions. By selecting suitable optical and electrical programming/erasing conditions, three-level information storage states, corresponding to electron storage, approximate neutrality, and hole storage in the floating-gate, are achieved and freely switched to each other. The memory mechanism and the dependence of the memory performances on the F8BT contents in I-FG/T layers are investigated. As a result, good memory performances, with mobility larger than 1.0 cm² V^-1 s^-1, reliable three-level switching endurance over 100 cycles, and stable three-level retention capability over 20 000 s, are achieved in our memory. Furthermore, an imaging system with a nonvolatile information storage function is demonstrated in a 16 × 5 array of FG-OFET NVMs.

Highly Efficient Photo-Induced Charge Separation Enabled by Metal-Chalcogenide Interfaces in Quantum-Dot/Metal-Oxide Hybrid Phototransistors

Quantum dot (QD)-based optoelectronics have received great interest for versatile applications because of their excellent photosensitivity, facile solution processability, and the wide range of band gap tunability. In addition, QD-based hybrid devices, which are combined with various high-mobility semiconductors, have been actively researched to enhance the optoelectronic characteristics and maximize the zero-dimensional structural advantages, such as tunable band gap and high light absorption. However, the difficulty of highly efficient charge transfer between QDs and the semiconductors and the lack of systematic analysis for the interfaces have impeded the fidelity of this platform, resulting in complex device architectures and unsatisfactory device performance. Here, we report ultrahigh detective phototransistors with highly efficient photo-induced charge separation using a Sn₂S₆^4--capped CdSe QD/amorphous oxide semiconductor (AOS) hybrid structure. The photo-induced electron transfer characteristics at the interface of the two materials were comprehensively investigated with an array of electrochemical and spectroscopic analyses. In particular, photocurrent imaging microscopy revealed that interface engineering in QD/AOS with chelating chalcometallate ligands causes efficient charge transfer, resulting in photovoltaic-dominated responses over the whole channel area. On the other hand, monodentate ligand-incorporated QD/AOS-based devices typically exhibit limited charge transfer with atomic vibration, showing photo-thermoelectric-dominated responses in the drain electrode area.

Structure-Property Relation in Organic-Metal Oxide Hybrid Phototransistors

We report an optoelectronic device consisting of a solution-processed indium gallium zinc oxide (IGZO) thin-film transistor and vacuum-deposited small organic molecules. Depending on the configurations of the organic materials, either bulk heterojunction or planar heterojunction (PHJ), the device assumes the functionality of either a photosensor or a photoinduced memory, respectively. Under λ = 625 nm light illumination, the photosensor shows response and recovery time of ∼50 ms, responsivity of ∼5 mA/W, sensitivity above 10⁴, and a linear response. The mechanism of the photoinduced memory is studied experimentally and verified using a device simulation. We find that the memory is due to long charge retention time at the organic PHJ interface which is stable for over 9 days. It is correlated with the low leakage current found in ordered organic junctions having low subgap tail states. The presented integration of the PHJ with the transistor constitutes a new design of write-once-read-many-times memory device that is likely to be attractive for low-cost applications.

One-Step Coating Processed Phototransistors Enabled by Phase Separation of Semiconductor and Dielectric Blend Film

Fabrication of organic thin-film transistors (OTFTs) via high throughput solution process routes have attracted extensive attention. Herein, we report a simple one-step coating method for vertical phase separation of the poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(methyl methacrylate) (PMMA) blends as semiconducting and dielectric layers in OTFTs. These OTFTs can be used as phototransistors for ultraviolet (UV) light detection, where the phototransistors exhibited great photosensitivity of 597.6 mA/W and detectivity of 4.25 × 10¹⁰ Jones under 1 mW/cm² UV light intensity. Studies of the electrical properties in these phototransistors suggested that optimized P3HT contents in the blend film can facilitate the improvement of film morphology, and therefore form optimized vertical phase separation of the PMMA and P3HT. These results indicate that the simple one-step fabrication method creates possibilities for realizing high throughput phototransistors with great photosensitivity.

Solution-Processed Phototransistors Combining Organic Absorber and Charge Transporting Oxide for Visible to Infrared Light Detection

This report demonstrates high-performance infrared phototransistors that use a broad-band absorbing organic bulk heterojunction (BHJ) layer responsive from the visible to the shortwave infrared, from 500 to 1400 nm. The device structure is based on a bilayer transistor channel that decouples charge photogeneration and transport, enabling independent optimization of each process. The organic BHJ layer is improved by incorporating camphor, a highly polarizable additive that increases carrier lifetime. An indium zinc oxide transport layer with high electron mobility is employed for rapid charge transport. As a result, the phototransistors achieve a dynamic range of 127 dB and reach a specific detectivity of 5 × 10¹² Jones under a low power illumination of 20 nW/cm², outperforming commercial germanium photodiodes in the spectral range below 1300 nm. The photodetector metrics are measured with respect to the applied voltage, incident light power, and temporal bandwidth, demonstrating operation at a video-frame rate of 50 Hz. In particular, the frequency and light dependence of the phototransistor characteristics are analyzed to understand the change in photoconductive gain under different working conditions.

SnSe2 Quantum Dots: Facile Fabrication and Application in Highly Responsive UV-Detectors

Synthesizing quantum dots (QDs) using simple methods and utilizing them in optoelectronic devices are active areas of research. In this paper, we fabricated SnSe₂ QDs via sonication and a laser ablation process. Deionized water was used as a solvent, and there were no organic chemicals introduced in the process. It was a facile and environmentally-friendly method. We demonstrated an ultraviolet (UV)-detector based on monolayer graphene and SnSe₂ QDs. The photoresponsivity of the detector was up to 7.5 × 10⁶ mAW⁻¹, and the photoresponse time was ~0.31 s. The n–n heterostructures between monolayer graphene and SnSe₂ QDs improved the light absorption and the transportation of photocarriers, which could greatly increase the photoresponsivity of the device.

Intrinsic Optoelectronic Characteristics of MoS2 Phototransistors via a Fully Transparent van der Waals Heterostructure

In the past decade, intensive studies on monolayer MoS₂-based phototransistors have been carried out to achieve further enhanced optoelectronic characteristics. However, the intrinsic optoelectronic characteristics of monolayer MoS₂ have still not been explored until now because of unintended interferences, such as multiple reflections of incident light originating from commonly used opaque substrates. This leads to overestimated photoresponsive characteristics inevitably due to the enhanced photogating and photoconductive effects. Here, we reveal the intrinsic photoresponsive characteristics of monolayer MoS₂, including its internal responsivity and quantum efficiency, in fully transparent monolayer MoS₂ phototransistors employing a van der Waals heterostructure. Interestingly, as opposed to the previous reports, the internal photoresponsive characteristics do not significantly depend on the wavelength of the incident light as long as the electron-hole pairs are generated in the same k-space. This study provides a deeper understanding of the photoresponsive characteristics of MoS₂ and lays the foundation for two-dimensional materials-based transparent phototransistors.

Sn-Doping Enhanced Ultrahigh Mobility In1-xSnxSe Phototransistor

Two-dimensional ternary materials are attracting widespread interest because of the additional degree of freedom available to tailor the material property for a specific application. An In_1-xSn_xSe phototransistor possessing tunable ultrahigh mobility by Sn-doping engineering is demonstrated in this study. A striking feature of In_1-xSn_xSe flakes is the reduction in the oxide phase compared to undoped InSe, which is validated by spectroscopic analyses. Moreover, first-principles density functional calculations performed for the In_1-xSn_xSe crystal system reveal the same effective mass when doped with Sn atoms. Hence, because of an increased lifetime owing to the enhanced crystal quality, the carriers in In_1-xSn_xSe have higher mobility than in InSe. The internally boosted electrical properties of In_1-xSn_xSe exhibit ultrahigh mobility of 2560 ± 240 cm² V^-1 s^-1 by suppressing the interfacial traps with substrate modification and channel encapsulation. As a phototransistor, the ultrathin In_1-xSn_xSe flakes are highly sensitive with a detectivity of 10¹⁴ Jones. It possesses a large photoresponsivity and photogain (V_g = 40 V) as high as 3 × 10⁵ A W^-1 and 0.5 × 10⁶, respectively. The obtained results outperform all previously reported performances of InSe-based devices. Thus, the doping-engineered In_1-xSn_xSe-layered semiconductor finds a potential application in optoelectronics and meets the demand for faster electronic technology.

Salt-Assisted Growth of Ultrathin GeSe Rectangular Flakes for Phototransistors with Ultrahigh Responsivity

Two-dimensional (2D) GeSe is an important IVA-VIA semiconductor for future applications in electronics and optoelectronics because of its high absorption coefficient, mobility, and photoresponsivity. However, the controllable synthesis of 2D GeSe flakes is still a huge problem. Here, high-quality single-crystalline ultrathin 2D GeSe flakes are synthesized by a salt-assisted chemical vapor deposition method. The flakes tend to grow along the [010] crystal orientation presenting a rectangular shape with a thickness down to 5 nm. Then, the electrical and optoelectronic properties have been systematically investigated. A thickness-dependent Schottky barrier is shown in GeSe field-effect transistors. The p-type conductivity of GeSe is mainly caused by the Ge deficiency, which is proven by a variable-temperature experiment and theoretical calculations. In addition, the phototransistors based on as-grown GeSe flakes present an ultrahigh responsivity of 1.8 × 10⁴ A/W and an excellent external quantum efficiency of 4.2 × 10⁶%.

Nanostructured High-Performance Thin-Film Transistors and Phototransistors Fabricated by a High-Yield and Versatile Near-Field Nanolithography Strategy

Thin-film transistors (TFTs) and field-effect transistors (FETs) are basic units to build functional electronic circuits and investigate transport physics. In conventional TFTs or FETs, performance in terms of current level, on-off ratio, and the sensitivity of detection is limited by homogeneous semiconducting layers. In this paper, we develop TFTs with submicron heterostructures by using a strategy based on near-field photolithography. We use an array of total-reflective polydimethylsiloxane pyramids or trenches as a soft photomask in photolithography to induce multiple reflections and diffractions to focus the light. The textured feature enables the generation of gaps, dots, and grids at the nanoscale, with dimensions as small as sub-100 nm on substrates at the centimeter scale. We demonstrated the very high performance oxide TFTs on the nanoscale and periodic degenerately doped heterojunctions, and they yielded a nearly 20-fold increase in transconductance and apparent device mobility. The on-off ratio was higher than 10⁹, with notably enhanced output current and clear scaling effect with channel length. We also built nanostructured wide-gap/narrow-gap heterojunctions to balance the high on-off ratio and sensitive photoresponse in a unidirectional phototransistor. This study shows the viability of programming a variety of nanoscale submicron patterns or interfaces in TFTs and FETs to significantly enlarge the scope of research on multifunctional TFTs and FETs.

One-Step Growth of Spatially Graded Mo1- xW xS2 Monolayers with a Wide Span in Composition (from x = 0 to 1) at a Large Scale

Alloying is an effective way to modulate material's properties. In particular, graded alloying within a single domain of two-dimensional transition-metal chalcogenide (2D-TMC) is of great technological importance, for example, for achieving band gap modulations. Here, we report a facile method to grow gradient alloying of Mo_{1- x}W _xS₂ monolayers with large domain sizes and high crystal qualities via the chemical vapor deposition technique. The as-grown Mo_{1- x}W _xS₂ monolayers have a gradient composition of W from x = ∼0 to ∼1 in a single domain with a lateral dimension up to 300 μm, and the span in band gap can be readily tuned. Owing to the grading in band offsets, the compositionally graded Mo_{1- x}W _xS₂ alloy monolayer demonstrates an excellent rectifying effect with the ratio of forward to reverse current up to ∼10⁴. Moreover, phototransistors based on the compositionally graded Mo_{1- x}W _xS₂ monolayers exhibit a high responsivity up to 298.4 A/W in the visible light regime, and particularly a decent responsivity of 28.7 A/W in the near-infrared regime. The control of band gap offset gradient and span in alloyed 2D-TMC semiconductors provides an additional degree of freedom in designing fascinating applications in achieving multifunctional optoelectronic devices on individual substrates.

Enhanced Detectivity and Suppressed Dark Current of Perovskite-InGaZnO Phototransistor via a PCBM Interlayer

Hybrid phototransistors based on InGaZnO (IGZO) metal oxide thin-film transistors (TFT) and a photoabsorbing capping layer such as perovskite (MAPbI₃) are a promising low-cost device for developing advanced X-ray and UV flat-panel imagers. However, it is found that the introduction of MAPbI₃ inevitably damages the IGZO channel layer during fabrication, leading to deteriorated TFT characteristics such as off-current rising and threshold voltage shift. Here, we report an effective approach for improving the performance of the perovskite-IGZO phototransistor by inserting a [6,6]-phenyl C61-butyric acid methyl ester (PCBM) or PCBM:PMMA interlayer between the patterned MAPbI₃ and IGZO. The interlayer effectively prevents the IGZO from damage by the perovskite fabrication process, while allowing efficient charge transfer for photosensing. In this configuration, we have achieved a high-detectivity (1.35 × 10¹² Jones) perovskite-IGZO phototransistor with suppressed off-state drain current (∼10 pA) in the dark. This work points out the importance of interface engineering for realizing higher performance and reliable heterogeneous phototransistors.

Flexible, Low-Voltage, and n-Type Infrared Organic Phototransistors with Enhanced Photosensitivity via Interface Trapping Effect

Flexible and low-voltage near-infrared organic phototransistors (NIR OPTs) were prepared with a low-band gap donor-acceptor conjugated polymer as the semiconductor layer and n-octadecyl phosphonic acid modified anodic alumina (AlO _x/ODPA) as the insulating layer. The phototransistors exhibit the typical n-type transistor characteristics at a voltage below 5 V. The photosensitivity of phototransistors can be enhanced by regulating the packing densities of the ODPA self-assembled monolayers and forming different trap states. The enhanced OPTs exhibit good photosensitivity to 808-980 nm NIR with the photocurrent/dark current ratio and photoresponsivity as high as 5 × 10³ and 20 mA W^-1, respectively, benefiting from the charge-trapping effect at the AlO _x/ODPA interface. The OPTs also present a fast optical switching speed of 20/30 ms and an excellent mechanical flexibility. The outstanding performance of the NIR OPTs indicates that the development of wearable electronics is, indeed, possible.

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 (τ_rise) and decay (τ_decay) times were τ_rise = 0.406 s and τ_decay = 0.815 s for BP/CdSe QD photo-FET and τ_rise = 0.576 s and τ_decay = 0.773 s for BP/PbS QD photo-FET, respectively.

High Speed Ultraviolet Phototransistors Based on an Ambipolar Fullerene Derivative

Combining high charge carrier mobility with ambipolar transport in light-absorbing organic semiconductors is highly desirable as it leads to enhanced charge photogeneration, and hence improved performance, in various optoelectronic devices including solar cells and photodetectors. Here we report the development of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM)-based ultraviolet (UV) phototransistors with balanced electron and hole transport characteristics. The latter is achieved by fine-tuning the source-drain electrode work function using a self-assembled monolayer. Opto/electrical characterization of as-prepared ambipolar PC₆₁BM phototransistors reveals promising photoresponse, particularly in the UV-A region (315-400 nm), with a maximum photosensitivity and responsivity of 9 × 10³ and 3 × 10³ A/W, respectively. Finally, the temporal response of the PC₆₁BM phototransistors is found to be high despite the long channel length (10 s of μm) with typical switching times of <2 ms.

Interface Engineering via Photopolymerization-Induced Phase Separation for Flexible UV-Responsive Phototransistors

Interface engineering has been recognized to be substantially critical for achieving efficient charge separation, charge carrier transport, and enhanced device performance in emerging optoelectronics. Nevertheless, precise control of the interface structure using current techniques remains a formidable challenge. Herein, we demonstrate a facile and versatile protocol wherein in situ thiol-ene click photopolymerization-induced phase separation is implemented for constructing heterojunction semiconductor interfaces. This approach generates continuous mountainlike heterojunction interfaces that favor efficient exciton dissociation at the interface while providing a continuous conductive area for hole transport above the interface. This facile low-temperature paradigm presents good adaptability to both rigid and flexible substrates, offering high-performance UV-responsive phototransistors with a normalized detectivity up to 6.3 × 10¹⁴ cm Hz^1/2 W^-1 (also called jones). Control experiments based on ex situ photopolymerization and in situ thermal polymerization are also implemented to demonstrate the superiority of this novel paradigm.

Simple Hydrogen Plasma Doping Process of Amorphous Indium Gallium Zinc Oxide-Based Phototransistors for Visible Light Detection

A homojunction-structured amorphous indium gallium zinc oxide (a-IGZO) phototransistor that can detect visible light is reported. The key element of this technology is an absorption layer composed of hydrogen-doped a-IGZO. This absorption layer is fabricated by simple hydrogen plasma doping, and subgap states are induced by increasing the amount of hydrogen impurities. These subgap states, which lead to a higher number of photoexcited carriers and aggravate the instability under negative bias illumination stress, enabled the detection of a wide range of visible light (400-700 nm). The optimal condition of the hydrogen-doped absorption layer (HAL) is fabricated at a hydrogen partial pressure ratio of 2%. As a result, the optimized a-IGZO phototransistor with the HAL exhibits a high photoresponsivity of 1932.6 A/W, a photosensitivity of 3.85 × 10⁶, and a detectivity of 6.93 × 10¹¹ Jones under 635 nm light illumination.

Ultrahigh-Gain and Fast Photodetectors Built on Atomically Thin Bilayer Tungsten Disulfide Grown by Chemical Vapor Deposition

The low responsivity observed in photodetectors based on monolayer transition-metal dichalcogenides has encouraged the pursuit of approaches that can efficiently enhance the external quantum efficiency, which relies predominantly on the light absorption, the lifetime of the excess carriers, and the charge collection efficiency. Here, we demonstrate that phototransistors fabricated on large-area bilayer tungsten disulfide (WS₂) grown by chemical vapor deposition exhibit remarkable performance with photoresponsivity, photogain, and detectivity of up to ∼3 × 10³ A/W, 1.4 × 10⁴, and ∼5 × 10¹² Jones, respectively. These figures of merit of bilayer WS₂ provide a significant advantage over monolayer WS₂ due to the greatly improved carrier mobility and significantly reduced contact resistance. The photoresponsivity of bilayer WS₂ phototransistor can be further improved to up to 1 × 10⁴ A/W upon biasing a gate voltage of 60 V, without evident reduction in detectivity. Moreover, the bilayer WS₂ phototransistor exhibits a high response speed of less than 100 μs, large bandwidth of 4 kHz, high cycling reliability of over 10⁵ cycles, and spatially homogeneous photoresponse. These outstanding figures of merit make WS₂ bilayer a highly promising candidate for the design of high-performance optoelectronics in the visible regime.

Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors

Highly sensitive photodetection even approaching the single-photon level is critical to many important applications. Graphene-based hybrid phototransistors are particularly promising for high-sensitivity photodetection because they have high photoconductive gain due to the high mobility of graphene. Given their remarkable optoelectronic properties and solution-based processing, colloidal quantum dots (QDs) have been preferentially used to fabricate graphene-based hybrid phototransistors. However, the resulting QD/graphene hybrid phototransistors face the challenge of extending the photodetection into the technologically important mid-infrared (MIR) region. Here, we demonstrate the highly sensitive MIR photodetection of QD/graphene hybrid phototransistors by using plasmonic silicon (Si) QDs doped with boron (B). The localized surface plasmon resonance (LSPR) of B-doped Si QDs enhances the MIR absorption of graphene. The electron-transition-based optical absorption of B-doped Si QDs in the ultraviolet (UV) to near-infrared (NIR) region additionally leads to photogating for graphene. The resulting UV-to-MIR ultrabroadband photodetection of our QD/graphene hybrid phototransistors features ultrahigh responsivity (up to ∼10⁹ A/W), gain (up to ∼10¹²), and specific detectivity (up to ∼10¹³ Jones).

Transparent Large-Area MoS2 Phototransistors with Inkjet-Printed Components on Flexible Platforms

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have gained considerable attention as an emerging semiconductor due to their promising atomically thin film characteristics with good field-effect mobility and a tunable band gap energy. However, their electronic applications have been generally realized with conventional inorganic electrodes and dielectrics implemented using conventional photolithography or transferring processes that are not compatible with large-area and flexible device applications. To facilitate the advantages of 2D TMDCs in practical applications, strategies for realizing flexible and transparent 2D electronics using low-temperature, large-area, and low-cost processes should be developed. Motivated by this challenge, we report fully printed transparent chemical vapor deposition (CVD)-synthesized monolayer molybdenum disulfide (MoS₂) phototransistor arrays on flexible polymer substrates. All the electronic components, including dielectric and electrodes, were directly deposited with mechanically tolerable organic materials by inkjet-printing technology onto transferred monolayer MoS₂, and their annealing temperature of <180 °C allows the direct fabrication on commercial flexible substrates without additional assisted-structures. By integrating the soft organic components with ultrathin MoS₂, the fully printed MoS₂ phototransistors exhibit excellent transparency and mechanically stable operation.

Infrared Black Phosphorus Phototransistor with Tunable Responsivity and Low Noise Equivalent Power

The narrow band gap property of black phosphorus (BP) that bridges the energy gap between graphene and transition metal dichalcogenides holds great promise for enabling broadband optical detection from ultraviolet to infrared wavelengths. Despite its rich potential as an intriguing building block for optoelectronic applications, however, very little progress has been made in realizing BP-based infrared photodetectors. Here, we demonstrate a high sensitivity BP phototransistor that operates at a short-wavelength infrared (SWIR) of 2 μm under room temperature. Excellent tunability of responsivity and photoconductive gain are acquired by utilizing the electrostatic gating effect, which controls the dominant photocurrent generation mechanism via adjusting the band alignment in the phototransistor. Under a nanowatt-level illumination, a peak responsivity of 8.5 A/W and a low noise equivalent power (NEP) of less than 1 pW/Hz^1/2 are achieved at a small operating source-drain bias of -1 V. Our phototransistor demonstrates a simple and effective approach to continuously tune the detection capability of BP photodetectors, paving the way to exploit BP to numerous low-light-level detection applications such as biomolecular sensing, meteorological data collection, and thermal imaging.

Mercury Telluride Quantum Dot Based Phototransistor Enabling High-Sensitivity Room-Temperature Photodetection at 2000 nm

Near-to-mid-infrared photodetection technologies could be widely deployed to advance the infrastructures of surveillance, environmental monitoring, and manufacturing, if the detection devices are low-cost, in compact format, and with high performance. For such application requirements, colloidal quantum dot (QD) based photodetectors stand out as particularly promising due to the solution processability and ease of integration with silicon technologies; unfortunately, the detectivity of the QD photodetectors toward longer wavelengths has so far been low. Here we overcome this performance bottleneck through synergistic efforts between synthetic chemistry and device engineering. First, we developed a fully automated aprotic solvent, gas-injection synthesis method that allows scalable fabrication of large sized HgTe QDs with high quality, exhibiting a record high photoluminescence quantum yield of 17% at the photoluminescence peak close to 2.1 μm. Second, through gating a phototransistor structure we demonstrate room-temperature device response to reach >2 × 10¹⁰ cm Hz^1/2 W^-1 (at 2 kHz modulation frequency) specific detectivity beyond the 2 μm wavelength range, which is comparable to commercial epitaxial-grown photodetectors. To demonstrate the practical application of the QD phototransistor, we incorporated the device in a carbon monoxide gas sensing system and demonstrated reliable measurement of gas concentration. This work represents an important step forward in commercializing QD-based infrared detection technologies.