MoS2 Phototransistors Research Articles

Thermally stable photosensing using poly(methyl methacrylate)-coated MoS2 phototransistor for improved imaging reliability

In this paper, we report thermally stable photosensing using MoS2 phototransistor with a poly(methyl methacrylate) (PMMA) coating. The increase in the OFF current of the PMMA-coated MoS2 phototransistor degraded to less than 87.72% of that of the pristine MoS2 phototransistor under harsh temperature conditions (250 °C). PMMA coating on the pristine MoS2 phototransistor improved the photosensitivity and drain current stability as a function of time by 315.71% at 250 °C and 91.26% under intense negative bias temperature illumination stress (NBTIS) test (V gs = −30 V, V ds = 10 V, λ ex = 638 nm, P inc = 1.0 mW, and T = 250 °C), respectively. This simple and useful method provides valuable insight for improving the reliability of photodetectors and image sensor systems under harsh temperature.

Highly responsive broadband Si-based MoS2 phototransistor on high-k dielectric

Highly responsive broadband Si-based MoS2 phototransistor on high-k dielectric

Vanadium Metal Doping of Monolayer MoS2 for p-Type Transistors and Fast-Speed Phototransistors.

Modulating the electrical properties of two-dimensional (2D) materials is a fundamental prerequisite for their development to advanced electronic and optoelectronic devices. Substitutional doping has been demonstrated as an effective method for tuning the band structure in monolayer 2D materials. Here, we demonstrate a facile selective-area growth of vanadium-doped molybdenum disulfide (V-doped MoS2) flakes via pre-patterned vanadium-metal-assisted chemical vapor deposition (CVD). Optical microscopy characterization revealed the presence of flake arrays. Transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were employed to identify the chemical composition and crystalline structure of as-grown flakes. Electrical measurements indicated a light p-type conduction behavior in monolayer V-doped MoS2. Furthermore, the response time of phototransistors based on V-doped MoS2 monolayers exhibited a remarkable capability of 3 ms, representing approximately 3 orders of magnitude faster response than that observed in pure MoS2 phototransistors. This work hereby provides a feasible approach to doping of 2D materials, promising a scalable pathway for the integration of these materials into emerging electronic and optoelectronic devices.

Integrating Image Perception and Time‐to‐First‐Spike Coding in MoS2 Phototransistors for Spiking Neural Network

AbstractHuman vision system remains alert for dangers and adopts high‐speed and low‐power coding methods to convert the image information to spike signals. To meet the demand for danger alert in machine vision, it is important to design intelligent sensors to integrate the functions of image perception and high‐efficiency coding for high‐priority analysis. Inspired by the human visual system, a MoS2 phototransistor is introduced on SiNx substrate enabling simultaneous image perception and time‐to‐first‐spike (TTFS) coding. The device demonstrates exceptional performance in encoding 3‐bit grayscale images, achieving a low mean squared error of 0.008 and a high structural similarity index of 0.9784. Spiking neural networks (SNN) with TTFS coding achieve high recognition accuracy (98.86%) while reducing spike count by 75%. The device array also perceives motion direction and object states by converting data temporally. This work establishes a hardware foundation to promote the performance of SNNs in efficiently identifying crucial information.

Ambipolar MoS2 Field Effect Transistors with Negative Photoconductivity and High Responsivity Using an Ultrathin Epitaxial Ferroelectric Gate

AbstractFerroelectric field effect transistors (FeFETs), characterized by their low power consumption and polarization effect, can be employed in photodetectors based on 2D materials. In this paper, a MoS2 phototransistor with epitaxial ferroelectric (Hf0.5Zr0.5)O2 (HZO) is reported as a gate dielectric layer. Gate‐dielectric‐polarization‐dependent ambipolar behavior is observed in the FET, and relatively low power consumption and hysteresis‐free loop are achieved in the FeFET. The anomalous negative photoconductivity (NPC) is observed as well. Possible reasons for such phenomenon are clarified including the photogating effect originating from the interface traps and the polarization‐dependent electric‐field control through ferroelectric gating. The high responsivity of −8.44 × 103 A W−1 in the negative photoconductivity as well as the response time of 500 ms are reported. The demonstrated Molybdenum disulfide (MoS2) FeFET photodetectors show great potential in the on‐chip complementary metal‐oxide semiconductor (CMOS)‐compatible circuits for multifunctional devices.

High-performance MoS2 phototransistors with Hf1–x Al x O back-gate dielectric layer grown by plasma enhanced atomic layer deposition

Molybdenum sulfide (MoS2) as an emerging optoelectronic material, shows great potential for phototransistors owing to its atomic thickness, adjustable band gap, and low cost. However, the phototransistors based on MoS2 have been shown to have some issues such as large gate leakage current, and interfacial scattering, resulting in suboptimal optoelectronic performance. Thus, Al-doped hafnium oxide (Hf1–x Al x ) is proposed to be a dielectric layer of the MoS2-based phototransistor to solve this problem because of the relatively higher crystallization temperature and dielectric constant. Here, a high-performance MoS2 phototransistor with Hf1–x Al x O gate dielectric layer grown by plasma-enhanced atomic layer deposition has been fabricated and studied. The results show that the phototransistor exhibits a high responsivity of 2.2 × 104 A W−1, a large detectivity of 1.7 × 1017 Jones, a great photo-to-dark current ratio of 2.2 × 106%, and a high external quantum efficiency of 4.4 × 106%. The energy band alignment and operating mechanism were further used to clarify the reason for the enhanced MoS2 phototransistor. The suggested MoS2 phototransistors could provide promising strategies in further optoelectronic applications.

Polarizable Nonvolatile Ferroelectric Gating in Monolayer MoS2 Phototransistors.

Given the requirements for power and dimension scaling, modulating channel transport properties using high gate bias is unfavorable due to the introduction of severe leakages and large power dissipation. Hence, this work presents an ultrathin phototransistor with chemical-vapor-deposition-grown monolayer MoS2 as the channel and a 10.2 nm thick Al:HfO2 ferroelectric film as the dielectric. The proposed device is meticulously modulated utilizing an Al:HfO2 nanofilm, which passivates traps and suppresses charge Coulomb scattering with Al doping, efficiently improving carrier transport and inhibiting leakage current. Furthermore, a bipolar pulses excitable polarization method is developed to induce a nonvolatile electrostatic field. The MoS2 channel is fully depleted by the switchable and stable floating gate originating from remanent polarization, leading to a high detectivity of 2.05 × 1011 Jones per nanometer of gating layer (Jones nm-1) and photocurrent on/off ratio >104 nm-1, which are superior to the state-of-the-art phototransistors based on two-dimensional (2D) materials and ferroelectrics. The proposed polarizable nonvolatile ferroelectric gating in a monolayer MoS2 phototransistor promises a potential route toward ultrasensitive photodetectors with low power consumption that boast of high levels of integration.

Enhanced performance of MoS2 phototransistors with ferroelectric HfLaON as gate dielectric through NH3-plasma treatment followed by rapid thermal annealing

A novel processing method of NH3-plasma treatment followed by rapid thermal annealing (RTA) is used to prepare HfLaON ferroelectric thin films, and obviously enhanced remanent polarization (Pr) and largely decreased gate leakage are obtained. The relevant HfLaON MoS2 phototransistor exhibits a subthreshold swing of 31 mV/dec and remarkable responsivity/detectivity of 941.37 AW−1/3.326 × 1014 cmHz1/2 W−1 at low operating voltages (gate-source voltage of −0.3 V and drain-source voltage of 0.4 V). Meanwhile, the response time of (NH3 plasma + RTA) phototransistor is significantly reduced to 8–12 ms compared to 33∼31 ms for the RTA phototransistor. Microscopic analyses indicate that the involved mechanisms lie in an effective incorporation of N element into the HfLaON thin film to passivate oxygen vacancies and improve interfacial properties. This work provides an effective way to rectify interface issues and optimize the ferroelectric properties of HfLaON thin films, demonstrating a good application prospects of HfO2-based ferroelectric thin films in 2D material-based photodetection field.

High photoresponsivity MoS2 phototransistor through enhanced hole trapping HfO2 gate dielectric

Phototransistor using 2D semiconductor as the channel material has shown promising potential for high sensitivity photo detection. The high photoresponsivity is often attributed to the photogating effect, where photo excited holes are trapped at the gate dielectric interface that provides additional gate electric field to enhance channel charge carrier density. Gate dielectric material and its deposition processing conditions can have great effect on the interface states. Here, we use HfO2 gate dielectric with proper thermal annealing to demonstrate a high photoresponsivity MoS2 phototransistor. When HfO2 is annealed in H2 atmosphere, the photoresponsivity is enhanced by an order of magnitude as compared with that of a phototransistor using HfO2 without annealing or annealed in Ar atmosphere. The enhancement is attributed to the hole trapping states introduced at HfO2 interface through H2 annealing process, which greatly enhances photogating effect. The phototransistor exhibits a very large photoresponsivity of 1.1 × 107 A W−1 and photogain of 3.3 × 107 under low light illumination intensity. This study provides a processing technique to fabricate highly sensitive phototransistor for low optical power detection.

Correlative Spatial Mapping of Optoelectronic Properties in Large Area 2D MoS2 Phototransistors

Abstract2D materials‐based device performance is significantly affected by film non‐uniformity, especially for large area devices. Here, it investigates the dependence of large area 2D MoS2 phototransistor performance on film morphology through correlative mapping. Monolayer MoS2 films are quazi‐epitaxially synthesized on C‐plane sapphire (Al2O3 ) substrates by chemical vapor deposition, and the growth time and molybdenum trioxide MoO3 precursor volume are varied to obtain variations in film morphology. Raman, photoluminescence, transmittance, and photocurrent maps are generated and compared with each other to obtain a holistic understanding of large area 2D optoelectronic device performance. For example, it shows that the photoluminescence peak shift and intensity can be used to investigate strain and other defects across multiple film morphologies, giving insight into their effects on the photogenerated current in these devices. It also combines photocurrent and absorption maps to generate large area high‐resolution external quantum efficiency and internal quantum efficiency maps for the devices. This study demonstrates the benefit of correlative mapping in the understanding and advancement of large area 2D material‐based electronic and optoelectronic devices.

Nanoscale patterning on layered MoS2 with stacking-dependent morphologies and optical tunning for phototransistor applications

Nanoscale patterning has attracted considerable interest as a novel engineering technology for two-dimensional (2D) nanomaterials for various applications such as electronics, sensors, and catalysts. However, research on nanopatterning technologies that reflect the unique structural and electrical characteristics of 2D nanomaterials remains insufficient. In this study, we present a nanoscale patterning of two different polygons on layered molybdenum disulfide (MoS2), where the nanostructures are controlled by the stacking structure of MoS2. We investigate the dependence of nanopore edge stability on the stacking structure of MoS2 using density functional theory calculations and find the different geometric effects of the two nanopatterns on the band-structure modulation of semiconducting MoS2. Consistent with the theoretical calculations, we fabricate nanoporous MoS2 with hexagonal nanopores for AA’ and triangular nanopores for AB stacked MoS2 films. The AA’ MoS2 has narrow nanoribbons between the two hexagonal nanopores, resulting in a lateral quantum confinement effect, whereas the edge exposure effect is more dominant for the bandgap increase in the triangular nanoporous structure. Nanopatterns contribute to generating in-gap state of MoS2, significantly enhancing the photoelectrical characteristics of MoS2 phototransistors. Especially, the photogating effect is higher for AA’ MoS2 phototransistors with hexagonal nanoholes than AB MoS2 with triangular nanoholes. This study provides new insights into nanoscale patterning and band structure engineering of layered 2D materials for various optoelectrical applications.

Laser shock introduces strained wrinkling of CVD monolayer MoS2 and atomic gapping of plasmonic phototransistors

Monolayer MoS2, an atomic direct bandgap 2D material with excellent mechanical properties, is considered one of the top candidates for next-generation electronics. However, the phototransistor based on MoS2 synthesized through chemical vapor deposition (CVD) has relatively weak optical emissivity and absorption, low field-effect mobility, and a narrow response spectral range These limitations hinder the widespread application of MoS2 in optoelectronic devices. Here, to improve the performance of CVD 2D electronic devices in large-scale applications, we designed a hybrid nano-system with laser shock-enhanced plasmon and strain coupling. Wrinkles of CVD monolayers of MoS2 sandwiched between two layers of Ag nanoparticles (NPs) are tensile strained to form gap-mode localized surface plasmon resonance (LSPR). The strain of the laser shock (LS) leads to the co-deformation of MoS2 and Ag NPs, achieving an extremely narrow gap LSPR (<1 nm). The residual inhomogeneous elastic strain, remaining after the shock, not only enhances the LSPR but also modulates the band structure of MoS2 favorably. In comparison to a phototransistor employing CVD-monolayer MoS2 on a flat substrate (1L-MoS2), the laser-shock phototransistor (LS_Ag-MoS2) is improved by nearly 50 times, the response range is extended to near-infrared, and the field effect mobility is increased by 30 times. In addition, the response time is reduced from 1.28 s to 20 ms, which is comparable to the current high-K dielectric multilayer MoS2 phototransistor. Our work presents an advanced and scalable fabrication method for future applications of ultra-thin and high-performance phototransistors utilizing CVD two-dimensional materials.

Strain Tune Suspended MoS2 for Polarization Photodetection

In situ modulation of molybdenum disulfide (MoS2) structure and breaking its isotopic optical property are crucial for fabricating high‐performance multidimensional photodetectors. However, achieving this remains a challenge. Herein, by introducing a cavity in the channel, the strain tunable suspended MoS2 phototransistor is fabricated. By pumping the environment, a pressure difference between the environment and cavity is generated, inducing strain in the MoS2. Simulation reveals the strain can be tuned up to 0.598% by changing environment pressure. The electrical and photoelectrical properties exhibit significant tuning in response to pressure. During this experiment, the conductance of MoS2 under different pressures shows considerable differentiation between 50 and 100 kPa. As light wavelength decreases, the trend of photocurrent increasing under different pressure becomes more apparent. The differential conductance–voltage curve of different wavelengths at 10 Pa is more dispersed than that at 101 kPa. Those results demonstrate the coupling effect between light and strain. Interestingly, the symmetric photoelectrical property of MoS2 can be broken, resulting in polarization detection. At a pressure of 60 kPa, the dichroic ratio reaches 1.38, similar to intrinsic anisotropic two‐dimensional materials. This can be attributed to the breaking of symmetric structure caused by strain.

Optoelectronic graded neurons for bioinspired in-sensor motion perception.

Motion processing has proven to be a computational challenge and demands considerable computational resources. Contrast this with the fact that flying insects can agilely perceive real-world motion with their tiny vision system. Here we show that phototransistor arrays can directly perceive different types of motion at sensory terminals, emulating the non-spiking graded neurons of insect vision systems. The charge dynamics of the shallow trapping centres in MoS2 phototransistors mimic the characteristics of graded neurons, showing an information transmission rate of 1,200 bit s-1 and effectively encoding temporal light information. We used a 20 × 20 photosensor array to detect trajectories in the visual field, allowing the efficient perception of the direction and vision saliency of moving objects and achieving 99.2% recognition accuracy with a four-layer neural network. By modulating the charge dynamics of the shallow trapping centres of MoS2, the sensor array can recognize motion with a temporal resolution ranging from 101 to 106 ms.

Water‐Stable and Photo‐Patternable Siloxane‐Encapsulated Upconversion Nanoparticles toward Flexible Near‐Infrared Phototransistors

AbstractUpconversion nanoparticles (UCNPs), as near‐infrared (NIR) absorbers, are promising materials for use in flexible NIR photodetectors, which can be applied for wearable healthcare applications due to their advantages in a broad spectral range, high photostability, and biocompatibility. However, to apply UCNPs in wearable and large‐area integrated devices, water stability and micro‐patterning methods are required. In this work, the UCNPs are encapsulated with a siloxane polymer (UCNP@SiOx) via a sol–gel process to enable photo‐patternability and photo‐stabililty in water conditions. The UCNP@SiOx can be photo‐patterned down to micron‐scale feature sizes and exhibit no significant decrease in upconversion photoluminescence (PL) intensities and PL decay time after immersion in water for 2 h. Moreover, UCNP@SiOx is evaluated by an in vitro biocompatibility test and found to be non‐toxic. By integrating the UCNP@SiOx with MoS2 phototransistors (MoS2 + UCNP@SiOx), the devices exhibit enhanced responsivity (0.79 A W−1) and specific detectivity (2.22 × 107 Jones), which are 2.8 times higher than in the bare MoS2 phototransistors, and excellent mechanical durability over 1000 cycles of 20% compression and re‐stretch test. This work opens the way for the facile synthesis of water‐stable and photo‐patternable siloxane‐encapsulated UCNPs and a strategy for fabricating high‐performance flexible NIR phototransistors through wavelength conversion.

Multilayer SnS2/few-layer MoS2 heterojunctions with in-situ floating photogate toward high-performance photodetectors and optical imaging application

Since the successful preparation of the monolayer MoS2 phototransistor, two-dimensional (2D) layered materials (2DLMs) have been regarded as one of the most compelling candidates toward the implementation of the next generation of novel optoelectronic devices and systems. However, most reported 2DLM photodetectors suffer from specific shortcomings, such as low responsivity, large dark current, low specific detectivity, low on/off ratio, and sluggish response rate. Herein, multilayer SnS2/few-layer MoS2 van der Waals heterostructures have been constructed by stacking the MoS2 and SnS2 nanosheets grown by a single atmospheric pressure chemical vapor deposition method. The SnS2/MoS2 heterojunction photodetector demonstrates competitive overall performance with a large on/off ratio of 171, a high responsivity of 28.3 A W−1, and an excellent detectivity of 1.2 × 1013 Jones. In addition, an ultrafast response rate with the response/recovery time down to 1.38 ms/600 is achieved. The excellent properties are associated with the synergy of type-II band alignment of SnS2/MoS2 and the in-situ formed seamless floating photogate, which contribute to separating the photoexcited electron-hole pairs and extending the carrier lifetime. Taking advantage of the excellent photosensitivity, the SnS2/MoS2 device demonstrates proof-of-concept optical imaging application. On the whole, this study provides a distinctive perspective to implement advanced photodetectors with competitive overall performance.

Improved detectivity and response speed of MoS2 phototransistors based on the negative-capacitance effect and defect engineering.

Due to the unique crystal structure, outstanding optoelectronic properties and a tunable band gap from 1.2-1.8 eV, two-dimensional molybdenum disulfide (MoS2) has attracted extensive attention as a promising candidate for future photodetectors. In this work, a negative-capacitance (NC) MoS2 phototransistor is fabricated by using H f 0.5 Z r 0.5 O 2 (HZO) as ferroelectric layer and Al2O3 as matching layer, and a low subthreshold swing (SS) of 39 mV/dec and an ultrahigh detectivity of 3.736×1014 cmHz1/2W-1 are achieved at room temperature due to the NC effect of the ferroelectric HZO. Moreover, after sulfur (S) treatment on MoS2, the transistor obtained a lower SS of 33 mV/dec, a detectivity of 1.329×1014 cmHz1/2W-1 and specially a faster response time of 3-4 ms at room temperature, attributed to the modulation of photogating effect induced by S-vacancy passivation in MoS2 by the S treatment. Therefore, the combination of the defect engineering on MoS2 and the NC effect from ferroelectric thin film could provide an effective solution for high-sensitivity phototransistors based on two-dimensional materials in the future.

Large-Scale MoS2 Pixel Array for Imaging Sensor.

Two-dimensional molybdenum disulfide (MoS2) has been extensively investigated in the field of optoelectronic devices. However, most reported MoS2 phototransistors are fabricated using the mechanical exfoliation method to obtain micro-scale MoS2 flakes, which is laboratory- feasible but not practical for the future industrial fabrication of large-scale pixel arrays. Recently, wafer-scale MoS2 growth has been rapidly developed, but few results of uniform large-scale photoelectric devices were reported. Here, we designed a 12 × 12 pixels pixel array image sensor fabricated on a 2 cm × 2 cm monolayer MoS2 film grown by chemical vapor deposition (CVD). The photogating effect induced by the formation of trap states ensures a high photoresponsivity of 364 AW-1, which is considerably superior to traditional CMOS sensors (≈0.1 AW-1). Experimental results also show highly uniform photoelectric properties in this array. Finally, the concatenated image obtained by laser lighting stencil and photolithography mask demonstrates the promising potential of 2D MoS2 for future optoelectrical applications.

Active pixel sensor matrix based on monolayer MoS2 phototransistor array.

In-sensor processing, which can reduce the energy and hardware burden for many machine vision applications, is currently lacking in state-of-the-art active pixel sensor (APS) technology. Photosensitive and semiconducting two-dimensional (2D) materials can bridge this technology gap by integrating image capture (sense) and image processing (compute) capabilities in a single device. Here, we introduce a 2D APS technology based on a monolayer MoS2 phototransistor array, where each pixel uses a single programmable phototransistor, leading to a substantial reduction in footprint (900 pixels in ∼0.09 cm2) and energy consumption (100s of fJ per pixel). By exploiting gate-tunable persistent photoconductivity, we achieve a responsivity of ∼3.6 × 107 A W-1, specific detectivity of ∼5.6 × 1013 Jones, spectral uniformity, a high dynamic range of ∼80 dB and in-sensor de-noising capabilities. Further, we demonstrate near-ideal yield and uniformity in photoresponse across the 2D APS array.

Spontaneous heteroassembly of 2D semiconducting van der Waals materials in random solution phase

van der Waals heterostructures (vdWHs), consisting of more than one type of atomically thin 2D crystal layers are emerging platforms for interesting electrical, optical, and catalytic applications. High yield production of vdWHs with atomic scale precision is crucial prerequisite for practical utilization. Here we present a generalized approach of random solution phase, high yield heteroassembly of semiconducting vdWHs by exploiting inherent surface charge states of 2D materials as well as chemical affinity of specific ligand end-functionalities. Facile removal of noncovalent functionalized ligands via simple pH reversal enables clean interfaces within vdWHs, yielding outstanding optoelectrical and electrochemical properties driven by fluent interfacial charge transfer among the layered 2D structures. The generality of this procedure is demonstrated by the formation of a series of different vdWHs such as WSe2-MoS2, graphene–MoS2 - and phospherene–WSe2 heterostructures. Atomically thin WSe2–MoS2 phototransistor displayed an exceptionally fast response time with high sensitivity. Graphene–MoS2 overcomes the inherent charge transfer issue of MoS2 for electrochemical catalyst. Phospherene–WSe2 successfully addresses poor ambient stability of phospherene together with enhanced surface activity towards chemical sensing.