Kartikey Thakar

Accurate, timely and selective detection of moving obstacles is crucial for reliable collision avoidance in autonomous robots. The area- and energy-inefficiency of CMOS-based spiking neurons for obstacle detection can be addressed through the reconfigurable, tunable and low-power operation capabilities of emerging two-dimensional (2D) materials-based devices. We present an ultra-low power spiking neuron built using an electrostatically tuned dual-gate transistor with an ultra-thin and generic 2D material channel. The 2D subthreshold transistor (2D-ST) is carefully designed to operate under low-current subthreshold regime. Carrier transport has been modeled via over-the-barrier thermionic and Fowler–Nordheim contact barrier tunneling currents over a wide range of gate and drain biases. Simulation of a neuron circuit designed using the 2D-ST with 45 nm CMOS technology components shows high energy efficiency of ~3.5 pJ per spike and biomimetic class-I as well as oscillatory spiking. It also demonstrates complex neuronal behaviors such as spike-frequency adaptation and post-inhibitory rebound that are crucial for dynamic visual systems. Lobula giant movement detector (LGMD) is a collision-detecting biological neuron found in locusts. Our neuron circuit can generate LGMD-like spiking behavior and detect obstacles at an energy cost of <100 pJ. Further, it can be reconfigured to distinguish between looming and receding objects with high selectivity. We also show that the spiking neuron circuit can function reliably with ±40% variation in the 2D-ST current as well as up to 3 dB signal-to-noise ratio with additive white Gaussian noise in the input synaptic current.

Various analog applications, such as phase switching, have been demonstrated using either ambipolar or anti-ambipolar transport in two-dimensional materials. However, the availability of only one transport mode severely limits the application scope and range. This work demonstrates electrostatically reconfigurable and tunable ambipolar and anti-ambipolar transport in the same field-effect transistor using a photoactive ambipolar WSe₂ channel with gate-controlled channel and Schottky barriers. This enables the realization of in-phase, out-of-phase, and double-frequency sinusoidal output signals under dark and illumination conditions. The output waveforms were used to generate phase-, frequency-, and amplitude-modulated analog schemes for 2- and 3-bit data transmission. Evaluation of all possible schemes for their power consumption, error probability, and implementation complexity highlights the importance of switching between ambipolar and anti-ambipolar modes of transport for best transmission performance. A dual-metal contact transistor with improved linearity for harmonic and excess power suppression demonstrates further performance enhancement. Generic device architecture and operation makes this work adaptable to any ambipolar material amenable to electrostatic control.

Layered transition metal dichalcogenides have shown tremendous potential for photodetection due to their non-zero direct bandgaps, high light absorption coefficients and carrier mobilities, and ability to form atomically sharp and defect-free heterointerfaces. A critical and fundamental bottleneck in the realization of high performance detectors is their trap-dependent photoresponse that trades off responsivity with speed. This work demonstrates a facile method of attenuating this trade-off by nearly 2x through integration of a lateral, in-plane, electrostatically tunable p-n homojunction with a conventional WSe₂ phototransistor. The tunable p-n junction allows modulation of the photocarrier population and width of the conducting channel independently from the phototransistor. Increased illumination current with the lateral p-n junction helps achieve responsivity enhancement upto 2.4x at nearly the same switching speed (14–16 µs) over a wide range of laser power (300 pW–33 nW). The added benefit of reduced dark current enhances specific detectivity (D*) by nearly 25x to yield a maximum measured flicker noise-limited D* of 1.1×10¹² Jones. High responsivity of 170 A/W at 300 pW laser power along with the ability to detect sub-1 pW laser switching are demonstrated.

PN heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current-rectifiers, photodetectors, and photovoltaic devices. However, a direct or near-direct heterointerface bandgap for enhanced photogeneration in high light-absorbing few-layer vdW materials remains unexplored. In this work, for the first time, density functional theory calculations show that the heterointerface of few-layer group-6 transition metal dichalcogenide (TMD) WSe₂ with group-7 ReS₂ results in a sizeable (0.7 eV) near-direct type-II bandgap. The interlayer IR bandgap is confirmed through IR photodetection and micro-photoluminescence measurements demonstrate type-II alignment. Few-layer flakes exhibit ultrafast response time (5 μs) at high responsivity (3 A/W) and large photocurrent-generation and responsivity-enhancement at the hetero-overlap region (10-100X). Large open-circuit voltage of 0.64 V and short-circuit current of 2.6 μA enable high output electrical power. Finally, long term air-stability and facile single contact metal fabrication process make the multi-functional few-layer WSe₂/ReS₂ heterostructure diode technologically promising for next-generation optoelectronics.

Transition metal dichalcogenides (TMDCs) are a family of two-dimensional layered materials (2DLMs) with extraordinary optical properties. They present an attractive option for future multi-functional and high-performance optoelectronics. However, much remains to be done to realize a mature technology for commercial applications. In this review article, we describe the progress and scope of TMDC devices in optical and photonic applications. Various photoresponse mechanisms observed in such devices and a brief discussion on measurement and analysis methods are described. Three main types of optoelectronic devices, namely photodetectors, photovoltaics and light-emitting devices are discussed in detail with a focus on device architecture and operation. Examples showing experimental integration of 2DLM-based devices with silicon photonics are also discussed briefly. A wide range of data for key performance metrics is analysed with insights into future directions for device design, processing and characterization that can help overcome present gaps and challenges.

Stabilizing defective domains of sub‐monolayer thickness in 2D materials plays a significant role in tuning the electronic and optical properties. The chemical vapor deposition growth of WS₂ monolayers with control over phase and uniformly nucleated defect domains is reported. Multifold photoluminescence (PL) enhancement (≈20‐fold) is achieved by stabilizing defect‐dominated domains toward design of nanoscale optical devices. While the observed luminescence variation in the form of alternating triangles arises from planar heterostructure (1H–1T′) formation, defect domain present at the center of WS₂ heterophase monolayer causes 20‐fold enhancement in photoluminescence. The observed luminescence enhancement is correlated with sulfur defects and confirmed by site‐specific spatial X‐ray photoemission spectroscopy and in situ PL measurements at cryogenic temperatures. Detailed microstructural and spectroscopic observations indicate the partial stripping of monolayers by removal of sulfur and incorporation of oxygen leading to increased excitonic emissions. The demonstrated atomic‐scale manipulation to stabilize defect domains over large area with enhanced luminescence behavior is relevant for developing next‐generation nanoscale photonic devices.

Rhenium disulfide (ReS₂) is an attractive candidate for photodetection applications owing to its thickness independent direct bandgap. Despite various photodetection studies using two-dimensional semiconductors, the trade-off between responsivity and response-time under varying measurement conditions has not been studied in detail. This report presents a comprehensive study of the architectural, laser power and gate-bias dependence of responsivity and speed in supported and suspended ReS₂ phototransistors. Photocurrent scans show uniform photogeneration across the entire channel due to enhanced optical absorption and direct bandgap in multilayer ReS₂. High responsivity of 4 A/W (at 50 ms response time) and low response time of 20 μs (at 4 mA/W responsivity) make this one of the fastest reported transition-metal-dichalcogenide photodetectors. Occupancy of intrinsic (bulk ReS₂) and extrinsic (ReS₂/SiO₂ interface) traps is modulated using gate-bias to demonstrate tunability of the response time (responsivity) over 4 orders (15×) of magnitude highlighting the versatility of these photodetectors. Differences in the trap distributions of suspended and supported channel architectures, and their occupancy under different gate-biases, enable switching the dominant operating mechanism between either photogating or photoconduction. Further, a new metric that captures intrinsic photodetector performance by including the trade-off between its responsivity and speed, besides normalizing for the applied bias and geometry, is proposed and benchmarked for this work.

Stacking of various van der Waals (vdW) materials to assemble heterostructures has led to extended functionalities in terms of electrical transport and optoelectronic applications surpassing individual material performances [1]-[5]. Few layered (FL) WSe ₂ is chemically stable and shows prominent p-type conduction when contacted with a high work function metal like Pt [6]. ReS ₂ , on the other hand, is an n-type semiconductor which has a direct bandgap invariant of thickness and shows good optical response [7]. Therefore, for the first time, by combining WSe ₂ and ReS ₂ as a p-n heterostructure, we report the electrical and optoelectronic properties of the stack.