Material-based Devices Research Articles

Recent Advances in In-Memory Computing: Exploring Memristor and Memtransistor Arrays with 2D Materials.

The conventional computing architecture faces substantial challenges, including high latency and energy consumption between memory and processing units. In response, in-memory computing has emerged as a promising alternative architecture, enabling computing operations within memory arrays to overcome these limitations. Memristive devices have gained significant attention as key components for in-memory computing due to their high-density arrays, rapid response times, and ability to emulate biological synapses. Among these devices, two-dimensional (2D) material-based memristor and memtransistor arrays have emerged as particularly promising candidates for next-generation in-memory computing, thanks to their exceptional performance driven by the unique properties of 2D materials, such as layered structures, mechanical flexibility, and the capability to form heterojunctions. This review delves into the state-of-the-art research on 2D material-based memristive arrays, encompassing critical aspects such as material selection, device performance metrics, array structures, and potential applications. Furthermore, it provides a comprehensive overview of the current challenges and limitations associated with these arrays, along with potential solutions. The primary objective of this review is to serve as a significant milestone in realizing next-generation in-memory computing utilizing 2D materials and bridge the gap from single-device characterization to array-level and system-level implementations of neuromorphic computing, leveraging the potential of 2D material-based memristive devices.

Controlled Preparation of High Quality Bubble-Free and Uniform Conducting Interfaces of Vertical van der Waals Heterostructures of Arrays.

Sharp and clean interfaces of van der Waals (vdW) heterostructures are highly demanded in two-dimensional (2D) materials-based devices. However, current assembly methods usually cause interfacial bubbles and wrinkles, hindering carrier interlayer transport. The preparation of a large-scale vdW heterostructure with a bubble-free interface is still a challenge. Although many efforts have been made to eliminate bubbles, the evolution processes of the interfacial bubbles are rarely studied. Here, the interface bubble formation and evolution of the transferred 2D materials and their vdW heterostructure are systemically studied by the atomic force microscopy (AFM) technique and high-resolution surface current mapping. A thermal annealing procedure is developed to reduce the number of bubbles and to improve the quality of interfaces. In addition, influences of the interface residues and nanosteps on bubble evolution are also discussed. Further, we develop the polystyrene (PS)-mediated polydimethylsiloxane (PDMS) transfer technique to realize the high-quality transfer of heterostructure arrays. Finally, high-resolution surface current mapping results confirm that we can now produce highly uniform electrical conduction interfaces of heterojunctions. This study provides guidance for assembling high quality interfaces and paves the way for production of bubble-free heterostructure-based electronic devices with high performance and good uniformity.

Nanomaterials Based Micro/Nanoelectromechanical System (MEMS and NEMS) Devices.

The micro- and nanoelectromechanical system (MEMS and NEMS) devices based on two-dimensional (2D) materials reveal novel functionalities and higher sensitivity compared to their silicon-base counterparts. Unique properties of 2D materials boost the demand for 2D material-based nanoelectromechanical devices and sensing. During the last decades, using suspended 2D membranes integrated with MEMS and NEMS emerged high-performance sensitivities in mass and gas sensors, accelerometers, pressure sensors, and microphones. Actively sensing minute changes in the surrounding environment is provided by means of MEMS/NEMS sensors, such as sensing in passive modes of small changes in momentum, temperature, and strain. In this review, we discuss the materials preparation methods, electronic, optical, and mechanical properties of 2D materials used in NEMS and MEMS devices, fabrication routes besides device operation principles.

Electrical synaptic devices with a high recognition rate based on eco-friendly nanocomposites of a poly(methyl methacrylate) matrix embedded with graphene quantum dots for neuromorphic computing

Artificial synapse devices are currently the subjects of great attention as next-generation hardware for data processing to overcome the problem of data explosion due to the rapid advances in artificial intelligence and cloud computing technology. Nanocomposite-based devices enable unique applications and have several advantages that cannot be achieved in single material-based devices. This study presents binary electrical synapses with digital data storing and analog data processing through a nanocomposite-based active layer composed of poly(methyl methacrylate) (PMMA) with embedded chlorine-functionalized graphene quantum dots (fGQDs) on an indium tin oxide (ITO) substrate. The Al/PMMA-fGQD/ITO devices with an fGQD concentrations of 5 wt% exhibited excellent memory performance with RON/ROFF ratio of 103. Moreover, we demonstrated that our device can successfully emulate biological synaptic functions such as potentiation/depression, short-term/long-term memory, paired-pulse facilitation, learning experience, and spike-timing-dependent plasticity. Furthermore, on the basis of the synaptic behaviors of the devices, they achieved about a 90 % recognition capability when a learning algorithm was used in a single-layer neural network.

Analysis of p-Type Doping in Graphene Induced by Monolayer-Oxidized TMDs.

Doping is one of the most difficult technological challenges for realizing reliable two-dimensional (2D) material-based semiconductor devices, arising from their ultrathinness. Here, we systematically investigate the impact of different types of nonstoichiometric solid MOx (M are W or Mo) dopants obtained by oxidizing transition metal dichalcogenides (TMDs: WSe2 or MoS2) formed on graphene FETs, which results in p-type doping along with disorders. From the results obtained in this study, we were able to suggest an analytical technique to optimize the optimal UV-ozone (UVO) treatment to achieve high p-type doping concentration in graphene FETs (∼2.5 × 1013 cm-2 in this study) without generating defects, mainly by analyzing the time dependency of D and D' peaks measured by Raman spectroscopy. Furthermore, an analysis of the structure of graphene sheets using TEM indicates that WOx plays a better protective role in graphene, compared to MoOx, suggesting that WOx is more effective for preventing the degradation of graphene during UVO treatment. To enhance the practical application aspect of our work, we have fabricated a graphene photodetector by selectively doping the graphene through oxidized TMDs, creating a p-n junction, which resulted in improved photoresponsivity compared to the intrinsic graphene device. Our results offer a practical guideline for the utilization of surface charge transfer doping of graphene toward CMOS applications.

Direct visualization and 3D reconstruction of conductive filaments in aSiO2 material-based memristive device.

Observation of conductive filaments has greatly aided the development of theoretical models of memristive devices. In this work, we visualized and reconstructed the conductive filaments in a Cu/Cu-doped SiO2/W device employing a focused ion beam (FIB) as a milling technique. The SEM images taken from the device after 150 DC sweep cycles showed that Joule heat played a vital role in determining the morphology of a conductive filament, where the vaporization of the conductive filament resulted in the creation of defects, including particles, voids, and cavities. The competition between the formation and vaporization of conductive filaments generally induces a remarkable current fluctuation. Since Cu-doped SiO2 was utilized as the electrolyte, the vapors exfoliated adjacent single layers. FIB milling proceeded in top-down and front-back modes; thus, a 3D model of conductive filaments and defects was constructed according to a series of FIB-SEM images. This methodology is promising for a future failure analysis of memristive devices.

Potassium hydroxide treatment of layered WSe2 with enhanced electronic performances.

2D WSe2-based electronic devices have received much research interest. However, it is still a challenge to achieve high electronic performance in WSe2-based devices. In this work, we report greatly enhanced performances of different thickness WSe2 ambipolar transistors and demonstrate homogeneous WSe2 inverter devices, which are obtained by using a semiconductor processing-compatible layer removal technique via chemical removal of the surface top WOx layer formed by O2 plasma treatment. Importantly, monolayer WSe2 was realised after several consecutive removal processes, demonstrating that the single layer removal is accurate and reliable. After subsequent removal of the top layer WOx by KOH, the fabricated WSe2 field-effect transistors exhibit greatly enhanced electronic performance along with the high electron and hole mobilities of 40 and 85 cm2 V-1 s-1, respectively. Our work demonstrates that the layer removal technique is an efficient route to fabricate high performance 2D material-based electronic devices.

Oxidation-induced modulation of photoresponsivity in monolayer MoS2 with sulfur vacancies.

Two-dimensional transition metal dichalcogenides (TMDs), such as MoS2, hold great promise for next-generation electronics and optoelectronics due to their unique properties. However, the ultrathin nature of these materials renders them vulnerable to structural defects and environmental factors, which significantly impact their performance. Sulfur vacancies (VS) are the most common intrinsic defects in MoS2, and their impact on device performance in oxidising environments remains understudied. This study investigates the impact of VS defects on the photoresponsivity of CVD-grown monolayer MoS2 devices, when exposed to oxidising environments at high temperatures. Our findings reveal a dynamic process of defect generation and healing through oxygen passivation, leading to a significant difference in photocurrent between environments. Temperature-dependent analysis shows defect healing and a notable reduction in defect density upon cooling. This study provides crucial insights into the stability and performance of 2D materials-based devices under varying environmental conditions, essential for designing and controlling the performance of TMD-based devices. Our results pave the way for the development of robust and reliable 2D materials-based electronics and optoelectronics.

Temperature dependent charge carrier dynamics in 2D ternary Cu2MoS4 nanoflakes: An effect of electron-phonon coupling.

Two-dimensional transition metal chalcogenides (2D TMCs) like MoS2, WS2 etc., have established significant dominance in the field of nanoscience and nanotechnology, owing to their unique properties like strong light-matter interaction, high carrier mobility, large photo-responsivity etc. Despite the widespread utilization of these binary TMCs, their potential in the advancement of the optoelectronic research is limited due to the constraints in band tuning and charge carrier lifetime. To overcome these limitations, ternary transition metal chalcogenides have emerged as promising alternatives. Although, the optical properties of these materials have never been explored properly. Herein, we have investigated one such promising member of this group, Cu2MoS4 (CMS) using both steady state and time-resolved spectroscopic techniques. The material exhibits a broad range of visible light absorption, peaking at 576nm. Photoluminescence spectroscopy confirmed the presence of both band gap emission and trap state-mediated emissions. Transient absorption spectroscopy unraveled the excited state charge carrier dynamics of CMS in sub-ps timescale, upon irradiation of visible light. We found significant influence of the trap mediated recombination, while Auger process being dominant at high charge density. We extended our study in a wide temperature range (5-300K), which reveals the impact of electron-phonon coupling strength on the band gap and charge carrier dynamics of this material. This detailed study would draw more attention toward the unexplored optical properties of ternary 2D chalcogenides and will open new avenues for the construction of 2D material-based optical devices.

Light blocking and phase modulation based on thermo-plasmonic hotspots in WS2 monolayer

Here we demonstrate light blocking and phase modulation in nano-plasmonic donate shape rings based on the thermo-plasmonic effect in these structures. For this purpose, we use a laser writing system to fabricate nanohole arrays and cover them with plasmonic gold thin film via a sputtering machine. The chemical vapor deposition method is also used to produce a WS2 layer, which is suitable for light blocking and phase modulation due to the nonlinearity of this two-dimensional material. After theoretically and experimentally evaluating the plasmonic donate-shaped substrate, we use the ellipsometric method to characterize the optical modes of the samples and record the switching manner and light-blocking phenomena under the probe laser excitation set to 980 nm. Our results show phase modulation based on the thermo-plasmonic effect of nano gap in donated double rings and light blocking by thermal expansion of the WS2 layer, which can open new insight into plasmon and two-dimensional material-based devices.

The photodetection characteristics of a Brilliant Blue-FCF implemented device for organic-based optoelectronic applications

The optical properties of sol-gel-based brilliant blue-FCF (BB-FCF) thin film are analyzed for photodetection applications. The effect of the presence of the BB-FCF interface on the microelectronic characteristics of the designated Cu/BB-FCF/n-Si device is elucidated by comparing to its reference Cu/n-Si device. The systematic investigation of photodetection properties of the integrated device is performed between the 20–100 mW/cm2 illumination intensities. First, the optical features of spin-coated BB-FCF thin film are investigated by UV–Vis measurements, and the absorbance of the film is elucidated by the efficient optical absorption in the wavelength of ∼350–700 nm with the calculated optical indirect band gap of 1.72 eV. Next, the frequency-dependent capacitive behavior, charge transport mechanism, and the electronic parameters of both the rectifying Cu/n-Si and the implemented Cu/BB-FCF/n-Si devices like the ideality factor, the barrier height, and series resistance are estimated using the Thermionic emission and Norde's function methods. Subsequently, the photodetection properties of the engineered device with a BB-FCF functional dye interface are studied under a solar simulator with different power intensities. The effect of the illumination intensity and applied reverse bias voltage on the figures of merit, including photoresponsivity, photodetectivity, response speed, and linear dynamic property, are analyzed under an illumination of 20–100 mW/cm2. The designated device with a BB-FCF interface has achieved significant and fast, stable on/off switching sensitivities, with 524 ms and 629 ms rising and falling times, respectively. Therefore, the prepared BB-FCF-based device has good and stable photoresponse performance, and the Cu/BB-FCF/n-Si architecture device may be a strong candidate for photonic and optoelectronic device applications, particularly in rapidly developing organic material-based device technology.

Stamped production of single-crystal hexagonal boron nitride monolayers on various insulating substrates

Controllable growth of two-dimensional (2D) single crystals on insulating substrates is the ultimate pursuit for realizing high-end applications in electronics and optoelectronics. However, for the most typical 2D insulator, hexagonal boron nitride (hBN), the production of a single-crystal monolayer on insulating substrates remains challenging. Here, we propose a methodology to realize the facile production of inch-sized single-crystal hBN monolayers on various insulating substrates by an atomic-scale stamp-like technique. The single-crystal Cu foils grown with hBN films can stick tightly (within 0.35 nm) to the insulating substrate at sub-melting temperature of Cu and extrude the hBN grown on the metallic surface onto the insulating substrate. Single-crystal hBN films can then be obtained by removing the Cu foil similar to the stamp process, regardless of the type or crystallinity of the insulating substrates. Our work will likely promote the manufacturing process of fully single-crystal 2D material-based devices and their applications.

Two-dimensional tellurium-based diodes for RF applications

The research of two-dimensional (2D) Tellurium (Te) or tellurene is thriving to address current challenges in emerging thin-film electronic and optoelectronic devices. However, the study of 2D-Te-based devices for high-frequency applications is still lacking in the literature. This work presents a comprehensive study of two types of radio frequency (RF) diodes based on 2D-Te flakes and exploits their distinct properties in two RF applications. First, a metal-insulator-semiconductor (MIS) structure is employed as a nonlinear device in a passive RF mixer, where the achieved conversion loss at 2.5 GHz and 5 GHz is as low as 24 dB and 29 dB, respectively. Then, a metal-semiconductor (MS) diode is tested as a zero-bias millimeter-wave power detector and reaches an outstanding linear-in-dB dynamic range over 40 dB, while having voltage responsivities as high as 257 V ⋅ W−1 at 1 GHz (up to 1 V detected output voltage) and 47 V ⋅ W−1 at 2.5 GHz (up to 0.26 V detected output voltage). These results show superior performance compared to other 2D material-based devices in a much more mature technological phase. Thus, the authors believe that this work demonstrates the potential of 2D-Te as a promising material for devices in emerging high-frequency electronics.

Thermal strain relaxation of GaAs overgrown on nanovoid based Ge/Si substrate

The integration of high quality III-V materials-based devices on the Si platform is considered as an enabling path towards achieving the long-standing goal of creating low-cost/high-performance devices. However, the heteroepitaxy of III-V compounds on a Si substrate generates high stress due to the difference in thermal expansion coefficient leading notably to crack generation. Here, we report on the reduction of the thermal strain of GaAs and Ge deposited on a Si substrate. The method is based on the formation of buried high density nanovoids by electrochemical etching of Ge/Si virtual substrate followed by direct epitaxial growth of Ge and GaAs. The thermal strain has been investigated for GaAs epilayer by High-Resolution X-Ray Diffraction and Raman spectroscopy showing a 30% decrease in strain. The strain reduction relies on the nanovoids-mediated thermal strain accommodation. Furthermore, photoluminescence measurements showed a 10-fold increase of the intensity, resulting in an improved optical and crystalline quality, making this virtual substrate promising for III-V based devices on Silicon.

Ultra-low power neuromorphic obstacle detection using a two-dimensional materials-based subthreshold transistor

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.

Thickness-Controlled Synthesis of Uniform Hexagonal Boron Nitride Films on Metal Alloys by Chemical Vapor Deposition

Two-dimensional (2D) multilayer hexagonal boron nitride (hBN) is a superior substrate for other 2D materials due to its insulating property, atomic-scale smoothness, and absence of dangling bonds, enhancing electrical or optical properties in high-performance 2D material-based devices. In most fabrication processes for 2D heterostructure devices, thick hBN flakes exfoliated from bulk have been used because of their high quality and simplicity in the process. However, exfoliated hBN flakes are limited in size and thickness control, impeding scale-up production for industrialization. Therefore, new synthesis strategies are required to overcome the drawback of the exfoliation method. In this work, we investigated the synthesis of large-area, multilayer hBN via atmospheric pressure chemical vapor deposition using single crystalline metal and single crystal alloy substrates. The synthesized results are characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, atomic force microscopy, etc. These studies provide a more comprehensive understanding of the growth mechanism of hexagonal boron nitride.

Designing high-efficiency electrode contacts to two-dimensional semiconductor Cu2Se

High-efficiency electrode contact plays a pivotal role in achieving 2D material-based high performance nano devices. Recently, monolayer semiconducting Cu2Se with high stability in air and high carrier mobility of about 103 cm2 V−1 s−1 have been experimentally fabricated. Herein, to design high-efficiency electrode contacts for Cu2Se monolayer, the contact properties between Cu2Se monolayer and a series of experimentally fabricated 2D metals (MX2 in T-phase or H-phase, M = Ni, Nb, Ti, V, Co, Zr, and Ta, X = S, Se and Te) are systematically investigated based on first-principles calculations. Considering both Schottky barriers and tunneling barriers of these metal–semiconductor junctions (MSJs), H-NbSe2 is screened out as the most adaptable electrode for monolayer Cu2Se. Moreover, the fermi level pinning effects are discovered in these MSJs due to the potential steps contributed by the interface dipoles. Our work not only provides useful instruction for the design of high-performance 2D nano devices based on Cu2Se monolayer but also offers insights for prospection into the fundamental electronic properties of MSJs.

Ultracompact MXene V2C-Improved Temperature Sensor by a Runway-Type Microfiber Knot Resonator.

We demonstrate an all-fiber, compact-structure, high-sensing-efficiency temperature sensor using a resonator structure sensor device of a runway type and MXene V2C. The high-quality functional material MXene V2C, synthesized by a simple two-step method, has excellent photothermal conversion performance. As-prepared MXene V2C is integrated into the runway section of a runway-type microfiber knot resonator based on the coupling mechanism between the surface near the field of the fiber and materials. When the temperature variation range is ~25-70 °C, the corresponding transmission light intensity variation is linear, and the maximum normalized sensing efficiency is 2.21 dB/°C/mm. Our work demonstrates that the runway-type structure ensures the compactness of the sensor device and enhances the interaction distance between the material and the microfiber, which provides additional integration strategies for functional material-based sensor devices.

Imaging Fermi-level hysteresis in nanoscale bubbles of few-layer MoS2

The electrical stability and reliability of two-dimensional (2D) crystal-based devices are mainly determined by charge traps in the device defects. Although nanobubble structures as defect sources in 2D materials strongly affect the device performance, the local charge-trapping behaviors in nanobubbles are poorly understood. Here, we report a Fermi-level hysteresis imaging strategy using Kelvin probe force microscopy to study the origins of charge trapping in nanobubbles of MoS2 on SiO2. We observe that the Fermi-level hysteresis is larger in nanobubbles than in flat regions and increases with the height in a nanobubble, in agreement with our oxide trap band model. We also perform the local transfer curve measurements on the nanobubble structures of MoS2 on SiO2, which exhibit enhanced current-hysteresis windows and reliable programming/erasing operations. Our results provide fundamental knowledge on the local charge-trapping mechanism in nanobubbles, and the capability to directly image hysteresis can be powerful tool for the development of 2D material-based memory devices.

Annealing-induced one order of magnitude enhancement in heat dissipation at graphene/SiO2 interface

The potential applications of two-dimensional (2D) materials-based devices in various fields have garnered significant interest. However, the limited heat transfer efficiency across the interface between 2D materials and the substrate has impeded their widespread application. This study presents a novel approach to enhancing the interface thermal transport of a widely used graphene/substrate structure through repeated annealing. Using Raman thermometry, we measured the interface thermal resistance (R) at 9.51 × 10−5 m2 K/W, which was subsequently reduced by an order of magnitude to 5.00 × 10−6 m2 K/W after repeated annealing. Our experimental results, supported by atomic modeling, revealed the significant influence of wrinkles on the interface thermal transport. Atomic force microscopy (AFM) images vividly illustrate the modulation of the surface structure due to repeated annealing, with the initial loose structure becoming tighter and the wrinkle flattened. Further phonon transport analysis revealed a significantly reduced phonon coupling rate between graphene and SiO2 due to the presence of wrinkles in the graphene. Our findings highlight the feasibility of annealing as a means of manipulating interface thermal transport and provide insights into the underlying mechanism, which may inspire further studies on enhancing thermal interface properties.