Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS2

Deterministic grayscale nanotopography to engineer mobilities in strained MoS2 FETs

Field-effect transistors (FETs) based on two-dimensional materials (2DMs) with atomically thin channels have emerged as a promising platform for beyond-silicon electronics. However, low carrier mobility in 2DM transistors driven by phonon scattering remains a critical challenge. To address this issue, we propose the controlled introduction of localized tensile strain as an effective means to inhibit electron-phonon scattering in 2DM. Strain is achieved by conformally adhering the 2DM via van der Waals forces to a dielectric layer previously nanoengineered with a gray-tone topography. Our results show that monolayer MoS2 FETs under tensile strain achieve an 8-fold increase in on-state current, reaching mobilities of 185 cm²/Vs at room temperature, in good agreement with theoretical calculations. The present work on nanotopographic grayscale surface engineering and the use of high-quality dielectric materials has the potential to find application in the nanofabrication of photonic and nanoelectronic devices.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Quantifying the effect of nanosheet dimensions on the piezoresistive response of printed graphene nanosheet networks

Printed networks of 2D nanosheets have found a range of applications in areas including electronic devices, energy storage systems and sensors. For example, the ability to print graphene networks onto flexible substrates enables the production of high-performance strain sensors. The network resistivity is known to be sensitive to the nanosheet dimensions which implies the piezoresistance might also be size-dependent. In this study, the effect of nanosheet thickness on the piezoresistive response of nanosheet networks has been investigated. To achieve this, we liquid-exfoliated graphene nanosheets which were then subjected to centrifugation-based size selection followed by spray deposition onto flexible substrates. The resultant devices show increasing resistivity and gauge factor with increasing nanosheet thickness. We analyse the resistivity versus thickness data using a recently reported model and develop a new model to fit the gauge factor versus thickness data. This analysis allowed us to differentiate between the effect of strain on inter-nanosheet junctions and the straining of the individual nanosheets within the network. Surprisingly, our data implies the nanosheets themselves to display a negative piezo response.

Electrically Controlled High Sensitivity Strain Modulation in MoS2 Field-Effect Transistors via a Piezoelectric Thin Film on Silicon Substrates

Strain can modulate bandgap and carrier mobilities in two-dimensional (2D) materials. Conventional strain-application methodologies relying on flexible/patterned/nanoindented substrates are limited by low thermal tolerance, poor tunability, and/or scalability. Here, we leverage the converse piezoelectric effect to electrically generate and control strain transfer from a piezoelectric thin film to electromechanically coupled 2D MoS2. Electrical bias polarity change across the piezo film tunes the nature of strain transferred to MoS2 from compressive (∼0.23%) to tensile (∼0.14%) as verified through Raman and photoluminescence spectroscopies and substantiated by density functional theory calculations. The device architecture, on silicon substrate, integrates an MoS2 field-effect transistor on a metal-piezoelectric-metal stack enabling strain modulation of transistor drain current (130×), on/off ratio (150×), and mobility (1.19×) with high precision, reversibility, and resolution. Large, tunable tensile (1056) and compressive (-1498) strain gauge factors, electrical strain modulation, and high thermal tolerance promise facile integration with silicon-based CMOS and micro-electromechanical systems.

Substrate-induced strain in molybdenum disulfide grown by aerosol-assisted chemical vapor deposition

Transition metal dichalcogenides have been extensively studied in recent years because of their fascinating optical, electrical, and catalytic properties. However, low-cost, scalable production remains a challenge. Aerosol-assisted chemical vapor deposition (AACVD) provides a new method for scalable thin film growth. In this study, we demonstrate the growth of molybdenum disulfide (MoS2) thin films using AACVD method. This method proves its suitability for low-temperature growth of MoS2thin films on various substrates, such as glass, silicon dioxide, quartz, silicon, hexagonal boron nitride, and highly ordered pyrolytic graphite. The as-grown MoS2shows evidence of substrate-induced strain. The type of strain and the morphology of the as-grown MoS2highly depend on the growth substrate's surface roughness, crystallinity, and chemical reactivity. Moreover, the as-grown MoS2shows the presence of both direct and indirect band gaps, suitable for exploitation in future electronics and optoelectronics.

Manipulating 2D Materials through Strain Engineering

This review explores the growing interest in 2D layered materials, such as graphene, h-BN, transition metal dichalcogenides (TMDs), and black phosphorus (BP), with a specific focus on recent advances in strain engineering. Both experimental and theoretical results are delved into, highlighting the potential of strain to modulate physical properties, thereby enhancing device performance. Various strain engineering methods are summarized, and the impact of strain on the electrical, optical, magnetic, thermal, and valleytronic properties of 2D materials is thoroughly examined. Finally, the review concludes by addressing potential applications and challenges in utilizing strain engineering for functional devices, offering valuable insights for further research and applications in optoelectronics, thermionics, and spintronics.

Engineered two-dimensional nanomaterials based diagnostics integrated with internet of medical things (IoMT) for COVID-19

More than four years have passed since an inimitable coronavirus disease (COVID-19) pandemic hit the globe in 2019 after an uncontrolled transmission of the severe acute respiratory syndrome (SARS-CoV-2) infection. The occurrence of this highly contagious respiratory infectious disease led to chaos and mortality all over the world. The peak paradigm shift of the researchers was inclined towards the accurate and rapid detection of diseases. Since 2019, there has been a boost in the diagnostics of COVID-19 via numerous conventional diagnostic tools like RT-PCR, ELISA, etc., and advanced biosensing kits like LFIA, etc. For the same reason, the use of nanotechnology and two-dimensional nanomaterials (2DNMs) has aided in the fabrication of efficient diagnostic tools to combat COVID-19. This article discusses the engineering techniques utilized for fabricating chemically active E2DNMs that are exceptionally thin and irregular. The techniques encompass the introduction of heteroatoms, intercalation of ions, and the design of strain and defects. E2DNMs possess unique characteristics, including a substantial surface area and controllable electrical, optical, and bioactive properties. These characteristics enable the development of sophisticated diagnostic platforms for real-time biosensors with exceptional sensitivity in detecting SARS-CoV-2. Integrating the Internet of Medical Things (IoMT) with these E2DNMs-based advanced diagnostics has led to the development of portable, real-time, scalable, more accurate, and cost-effective SARS-CoV-2 diagnostic platforms. These diagnostic platforms have the potential to revolutionize SARS-CoV-2 diagnosis by making it faster, easier, and more accessible to people worldwide, thus making them ideal for resource-limited settings. These advanced IoMT diagnostic platforms may help with combating SARS-CoV-2 as well as tracking and predicting the spread of future pandemics, ultimately saving lives and mitigating their impact on global health systems.

Strain-Enhanced Large-Area Monolayer MoS2 Photodetectors

In this study, we show a direct correlation between the applied mechanical strain and an increase in monolayer MoS2 photoresponsivity. This shows that tensile strain can improve the efficiency of monolayer MoS2 photodetectors. The observed high photocurrent and extended response time in our devices are indicative that devices are predominantly governed by photogating mechanisms, which become more prominent with applied tensile strain. Furthermore, we have demonstrated that a nonencapsulated MoS2 monolayer can be used in strain-based devices for many cycles and extensive periods of time, showing endurance under ambient conditions without loss of functionality. Such robustness emphasizes the potential of MoS2 for further functionalization and utilization of different flexible sensors.

Covalently-Bonded Laminar Assembly of Van der Waals Semiconductors with Polymers: Toward High-Performance Flexible Devices

Van der Waals semiconductors (vdWS) offer superior mechanical and electrical properties and are promising for flexible microelectronics when combined with polymer substrates. However, the self-passivated vdWS surfaces and their weak adhesion to polymers tend to cause interfacial sliding and wrinkling, and thus, are still challenging the reliability of vdWS-based flexible devices. Here, an effective covalent vdWS-polymer lamination method with high stretch tolerance and excellent electronic performance is reported. Using molybdenum disulfide (MoS2 )and polydimethylsiloxane (PDMS) as a case study, gold-chalcogen bonding and mercapto silane bridges are leveraged. The resulting composite structures exhibit more uniform and stronger interfacial adhesion. This enhanced coupling also enables the observation of a theoretically predicted tension-induced band structure transition in MoS2 . Moreover, no obvious degradation in the devices' structural and electrical properties is identified after numerous mechanical cycle tests. This high-quality lamination enhances the reliability of vdWS-based flexible microelectronics, accelerating their practical applications in biomedical research and consumer electronics.

Locally Strained 2D Materials: Preparation, Properties, and Applications

2D materials are promising for strain engineering due to their atomic thickness and exceptional mechanical properties. In particular, non-uniform and localized strain can be induced in 2D materials by generating out-of-plane deformations, resulting in novel phenomena and properties, as witnessed in recent years. Therefore, the locally strained 2D materials are of great value for both fundamental studies and practical applications. This review discusses techniques for introducing local strains to 2D materials, and their feasibility, advantages, and challenges. Then, the unique effects and properties that arise from local strain are explored. The representative applications based on locally strained 2D materials are illustrated, including memristor, single photon emitter, and photodetector. Finally, concluding remarks on the challenges and opportunities in the emerging field of locally strained 2D materials are provided.

Lateral junctions of transition metal dichalcogenides as ballistic channels for straintronic applications

In the context of advanced nanoelectronics, two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are gaining considerable interest due to their ultimate thinness, clean surface and high carrier mobility. The engineering prospects offered by those materials are further enlarged by the recent realization of atomically sharp TMD-based lateral junctions, whose electronic properties are governed by strain effects arising from the constituents lattice mismatch. Although most theoretical studies considered only misfit strain, first-principles simulations are employed here to investigate the transport properties under external deformation of a three-terminal device constructed from a MoS2/WSe2/MoS2junction. Large modulation of the current is reported owing to the change in band offset, illustrating the importance of strain on the p-n junction characteristics. The device operation is demonstrated for both local and global deformations, even for ultra-short channels, suggesting potential applications for ultra-thin body straintronics.

Stretchable Thin-Film Transistors Based on Wrinkled Graphene and MoS2 Heterostructures

Two-dimensional (2D) materials are outstanding candidates for stretchable electronics, but a significant challenge is their heterogeneous integration into stretchable geometries on soft substrates. Here, we demonstrate a strategy for stretchable thin film transistors (2D S-TFT) based on wrinkled heterostructures on elastomer substrates where 2D materials formed the gate, source, drain, and channel and characterized them with Raman spectroscopy and transport measurements. The 2D S-TFTs had initial mobility of 4.9 ± 0.7 cm2/(V s). The wrinkling reduced the strain transferred into the 2D materials by a factor of 50, allowing a substrate stretch of up to 23% that could be cycled thousands of times without electrical degradation. The stretch did not alter the mobility but did lead to strain-induced threshold voltage shifts by ΔVT = -1.9 V. These 2D S-TFTs form the foundation for stretchable integrated circuits and enable investigations of the impact of heterogeneous strain on electron transport.

Dynamical control of nanoscale light-matter interactions in low-dimensional quantum materials

Tip-enhanced nano-spectroscopy and -imaging have significantly advanced our understanding of low-dimensional quantum materials and their interactions with light, providing a rich insight into the underlying physics at their natural length scale. Recently, various functionalities of the plasmonic tip expand the capabilities of the nanoscopy, enabling dynamic manipulation of light-matter interactions at the nanoscale. In this review, we focus on a new paradigm of the nanoscopy, shifting from the conventional role of imaging and spectroscopy to the dynamical control approach of the tip-induced light-matter interactions. We present three different approaches of tip-induced control of light-matter interactions, such as cavity-gap control, pressure control, and near-field polarization control. Specifically, we discuss the nanoscale modifications of radiative emissions for various emitters from weak to strong coupling regime, achieved by the precise engineering of the cavity-gap. Furthermore, we introduce recent works on light-matter interactions controlled by tip-pressure and near-field polarization, especially tunability of the bandgap, crystal structure, photoluminescence quantum yield, exciton density, and energy transfer in a wide range of quantum materials. We envision that this comprehensive review not only contributes to a deeper understanding of the physics of nanoscale light-matter interactions but also offers a valuable resource to nanophotonics, plasmonics, and materials science for future technological advancements.

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T' Transition Metal Dichalcogenide Nanosheets

The past few decades have witnessed a notable increase in transition metal dichalcogenide (TMD) related research not only because of the large family of TMD candidates but also because of the various polytypes that arise from the monolayer configuration and layer stacking order. The peculiar physicochemical properties of TMD nanosheets enable an enormous range of applications from fundamental science to industrial technologies based on the preparation of high-quality TMDs. For polymorphic TMDs, the 1T/1T' phase is particularly intriguing because of the enriched density of states, and thus facilitates fruitful chemistry. Herein, we comprehensively discuss the most recent strategies for direct synthesis of phase-pure 1T/1T' TMD nanosheets such as mechanical exfoliation, chemical vapor deposition, wet chemical synthesis, atomic layer deposition, and more. We also review frequently adopted methods for phase engineering in TMD nanosheets ranging from chemical doping and alloying, to charge injection, and irradiation with optical or charged particle beams. Prior to the synthesis methods, we discuss the configuration of TMDs as well as the characterization tools mostly used in experiments. Finally, we discuss the current challenges and opportunities as well as emphasize the promising fields for the future development.

Phase transformations in single-layer MoTe2stimulated by electron irradiation and annealing

Among two-dimensional (2D) transition metal dichalcogenides (TMDs), MoTe2is predestined for phase-engineering applications due to the small difference in free energy between the semiconducting H-phase and metallic 1T'-phase. At the same time, the complete picture of the phase evolution originating from point defects in single-layer of semiconducting H-MoTe2via Mo6Te6nanowires to cubic molybdenum has not yet been reported so far, and it is the topic of the present study. The occurring phase transformations in single-layer H-MoTe2were initiated by 40-80 kV electrons in the spherical and chromatic aberration-corrected high-resolution transmission electron microscope and/or when subjected to high temperatures. We analyse the damage cross-section at voltages between 40 kV and 80 kV and relate the results to previously published values for other TMDs. Then we demonstrate that electron beam irradiation offers a route to locally transform freestanding single-layer H-MoTe2into one-dimensional (1D) Mo6Te6nanowires. Combining the experimental data with the results of first-principles calculations, we explain the transformations in MoTe2single-layers and Mo6Te6nanowires by an interplay of electron-beam-induced energy transfer, atom ejection, and oxygen absorption. Further, the effects emerging from electron irradiation are compared with those produced byin situannealing in a vacuum until pure molybdenum crystals are obtained at temperatures of about 1000 °C. A detailed understanding of high-temperature solid-to-solid phase transformation in the 2D limit can provide insights into the applicability of this material for future device fabrication.

Dielectric Constant in Nanoscale Bubbles on MoS2

Nanoscale bubbles form inevitably during the transfer of two-dimensional (2D) materials on a target substrate due to their van der Waals interaction. Despite a large number of studies based on nanobubble structures with localized strain, the dielectric constant (κ) in nanobubbles of MoS2 is poorly understood. Here, we report κ measurements for nanobubbles on MoS2 by probing the polarization forces based on electrostatic force microscopy. Remarkably, higher κ values of 6-8 independent of the nanobubble size are observed for the nanobubbles as compared to flat regions with a κ of ≈3. We find that the charge carrier increase owing to the strain-induced bandgap reduction is responsible for the enhanced κ of the nanobubbles, where the measured κ is in good agreement with the calculations based on the Clausius-Mossotti relation. Our results provide fundamental information about the strain-induced local dielectric properties of 2D materials and a guide for the design and fabrication of high-performance optoelectrical devices based on 2D materials.

Strain-induced electronic structures and band-gap of few-layer AgInP2S6

The band gap and mechanical control ability of two-dimensional materials largely determine the application value of two-dimensional devices in optical and electronic properties, so the bandgap controllability of two-dimensional materials broadens the application range of multi-functional devices. In the layered van der Waals (vdW) material AgInP2S6, the band gap can be adjusted by the number of layers and flexible strain, and the few layers AgInP2S6have discrete band gap values, which are also relevant for optoelectronic applications. In the strain range of up to 2.7% applied, the band gap can be adjusted, and the film is relatively stable under strain. We further analyzed the physical mechanism of flexible strain band gap regulation and found that strain-regulation reduced the band gap and increased the chemical bond length. These studies open up new opportunities for the future development of vdW material photoelectric resonators represented by AgInP2S6, and have important reference value.

Latest Innovations in 2D Flexible Nanoelectronics

2D materials with dangling-bond-free surfaces and atomically thin layers have been shown to be capable of being incorporated into flexible electronic devices. The electronic and optical properties of 2D materials can be tuned or controlled in other ways by using the intriguing strain engineering method. The latest and encouraging techniques in regard to creating flexible 2D nanoelectronics are condensed in this review. These techniques have the potential to be used in a wider range of applications in the near and long term. It is possible to use ultrathin 2D materials (graphene, BP, WTe2 , VSe2 etc.) and 2D transition metal dichalcogenides (2D TMDs) in order to enable the electrical behavior of the devices to be studied. A category of materials is produced on smaller scales by exfoliating bulk materials, whereas chemical vapor deposition (CVD) and epitaxial growth are employed on larger scales. This overview highlights two distinct requirements, which include from a single semiconductor or with van der Waals heterostructures of various nanomaterials. They include where strain must be avoided and where it is required, such as solutions to produce strain-insensitive devices, and such as pressure-sensitive outcomes, respectively. Finally, points-of-view about the current difficulties and possibilities in regard to using 2D materials in flexible electronics are provided.

Built-in tensile strain dependence on the lateral size of monolayer MoS2 synthesized by liquid precursor chemical vapor deposition

Strain engineering is an efficient tool to tune and tailor the electrical and optical properties of 2D materials. The built-in strain can be tuned during the synthesis process of a two-dimensional semiconductor, such as molybdenum disulfide, by employing different growth substrates with peculiar thermal properties. In this work, we demonstrate that the built-in strain of MoS2 monolayers, grown on a SiO2/Si substrate by liquid precursor chemical vapor deposition, is mainly dependent on the size of the monolayer. In fact, we identify a critical size equal to 20 μm, from which the built-in strain increases drastically. The built-in strain is the maximum for a 60 μm sized monolayer, leading to 1.2% tensile strain with a partial release of strain close to the monolayer triangular vertexes due to the formation of nanocracks. These findings also imply that the standard method for evaluation of the number of layers based on the Raman mode separation can become unreliable for highly strained monolayers with a lateral size above 20 μm.

Highly sensitive, wide-pressure and low-frequency characterized pressure sensor based on piezoresistive-piezoelectric coupling effects in porous wood

Lightweight and highly compressible materials have received considerable attention in flexible pressure sensing devices. In this study, a series of porous woods (PWs) are produced by chemical removal of lignin and hemicellulose from natural wood by tuning treatment time from 0 to 15 h and extra oxidation through H₂O₂. The prepared PWs with apparent densities varying from 95.9 to 46.16 mg/cm³ tend to form a wave-shaped interwoven structure with improved compressibility (up to 91.89 % strain under 100 kPa). The sensor assembled from PW with treatment time of 12 h (PW-12) exhibits the optimal piezoresistive-piezoelectric coupling sensing properties. For the piezoresistive properties, it has high stress sensitivity of 15.14 kPa^-1, covering a wide linear working pressure range of 0.06-100 kPa. For its piezoelectric potential, PW-12 shows a sensitivity of 0.443 V·kPa^-1 with ultralow frequency detection as low as 0.0028 Hz, and good cyclability over 60,000 cycles under 0.41 Hz. The nature-derived all-wood pressure sensor shows obvious superiority in the flexibility for power supply requirement. More importantly, it presents fully decoupled signals without cross-talks in the dual-sensing functionality. Sensor like this is capable of monitoring various dynamic human motions, making it an extremely promising candidate for the next generation artificial intelligence products.

Large band gap quantum spin Hall insulators in plumbene monolayer decorated with amidogen, hydroxyl and thiol functional groups

Two-dimensional Quantum Spin Hall (QSH) insulators featuring edge states that are topologically protected against back-scattering are arising as a novel state of quantum matter. One of the major obstacles to finding QSH insulators operable at room temperature is the insufficiency of suitable materials demonstrating the QSH effect with a large bulk band gap. Plumbene, the latest group-IV graphene analogous material, shows a large SOC-induced band gap opening but the coupling between topological states at different momentum points makes it a topologically trivial insulator. Pristine plumbene can be chemically functionalized to transform it from a conventional insulator to a topologically non-trivial insulator with a considerable bulk band gap. In this work, three new QSH phases in plumbene have been theoretically predicted through functionalization with amidogen (-NH₂), hydroxyl (-OH) and thiol (-SH) groups. The derived electronic properties show non-trivial topological states in plumbene with very high bulk band gaps ranging from 1.0911 eV to as high as 1.1515 eV. External strain can be used to further enhance and tune these bulk gaps, as demonstrated in this work. We also propose a H-terminated SiC (0001) surface as a suitable substrate for the practical implementation of these monolayers to minimize lattice mismatch and maintain their topological order. The robustness of these QSH insulators against strain and substrate effects and the large bulk gaps provide a promising platform for potential applications of future low dissipation nanoelectronic devices and spintronic devices at room temperature.

In Situ Device-Level TEM Characterization Based on Ultra-Flexible Multilayer MoS2 Micro-Cantilever

Current state-of-the-art in situ transmission electron microscopy (TEM) characterization technology has been capable of statically or dynamically nanorobotic manipulating specimens, affording abundant atom-level material attributes. However, an insurmountable barrier between material attributes investigations and device-level application explorations exists due to immature in situ TEM manufacturing technology and sufficient external coupled stimulus. These limitations seriously prevent the development of in situ device-level TEM characterization. Herein, a representative in situ opto-electromechanical TEM characterization platform is put forward by integrating an ultra-flexible micro-cantilever chip with optical, mechanical, and electrical coupling fields for the first time. On this platform, static and dynamic in situ device-level TEM characterizations are implemented by utilizing molybdenum disulfide (MoS₂ ) nanoflake as channel material. E-beam modulation behavior in MoS₂  transistors is demonstrated at ultra-high e-beam acceleration voltage (300 kV), stemming from inelastic scattering electron doping into MoS₂  nanoflakes. Moreover, in situ dynamic bending MoS₂  nanodevices without/with laser irradiation reveals asymmetric piezoresistive properties based on electromechanical effects and secondary enhanced photocurrent based on opto-electromechanical coupling effects, accompanied by real-time monitoring atom-level characterization. This approach provides a step toward advanced in situ device-level TEM characterization technology with excellent perception ability and inspires in situ TEM characterization with ultra-sensitive force feedback and light sensing.

Significant Enhanced Mechanical Properties of Suspended Graphene Film by Stacking Multilayer CVD Graphene Films

Suspended graphene film is of great significance for building high-performance electrical devices. However, fabricating large-area suspended graphene film with good mechanical properties is still a challenge, especially for the chemical vapor deposition (CVD)-grown graphene films. In this work, the mechanical properties of suspended CVD-grown graphene film are investigated systematically for the first time. It is found that monolayer graphene film is hard to maintain on circular holes with a diameter of tens of micrometers, which can be improved greatly by increasing the layer of graphene films. The mechanical properties of CVD-grown multilayer graphene films suspended on a circular hole with a diameter of 70 µm can be increased by 20%, and multilayer graphene films prepared by layer-layer stacking process can be increased by up to 400% for the same size. The corresponding mechanism was also discussed in detail, which might pave the way for building high-performance electrical devices based on high-strength suspended graphene film.

Scaled-Up Synthesis of Freestanding Molybdenum Disulfide Membranes for Nanopore Sensing

2D materials are ideal for nanopores with optimal detection sensitivity and resolution. Among these, molybdenum disulfide (MoS₂ ) has gained traction as a less hydrophobic material than graphene. However, experiments using 2D nanopores remain challenging due to the lack of scalable methods for high-quality freestanding membranes. Herein, a site-directed, scaled-up synthesis of MoS₂ membranes on predrilled nanoapertures on 4-inch wafer substrates with 75% yields is reported. Chemical vapor deposition (CVD), which introduces sulfur and molybdenum dioxide vapors across the sub-100 nm nanoapertures results in exclusive formation of freestanding membranes that seal the apertures. Nucleation and growth near the nanoaperture edges is followed by nanoaperture decoration with MoS₂ , which proceeds until a critical flake curvature is achieved, after which fully spanning freestanding membranes form. Intentional blocking of reagent flow through the apertures inhibits MoS₂ nucleation around the nanoapertures, promoting the formation of large-crystal monolayer MoS₂ membranes. The in situ grown membranes along with facile membrane wetting and nanopore formation using dielectric breakdown enables the recording of dsDNA translocation events at an unprecedentedly high 1 MHz bandwidth. The methods presented here are important steps toward the development of scalable single-layer membrane manufacture for 2D nanofluidics and nanopore applications.

Mechano/acousto-electric coupling between ReS2and surface acoustic wave

Two-dimensional (2D) materials are promising candidates for developing next generation electronic/optoelectronic devices with programmable multi functions, due to their widely tunable properties by various physical stimuli. Mechanical strain is one of the most promising means to effectively modulate the physical properties of 2D materials. Nevertheless, few studies reported micro/nano scale controllable strain application platforms, limiting the development of novel mechano-electrical/optoelectrical devices based on 2D materials. This work proposes surface acoustic wave (SAW) device as a controllable strain modulation platform for 2D materials with sub-micro scale resolution. The platform uses the piezoelectric material (LiNbO₃) as the substrate, which is deposited with interdigitated transducers (IDT) to generate SAW on the surface. The propagation of SAW causes surface deformation, which is then transferred to the 2D materials on the substrate. The period of the surface deformation/strain is related with that of SAW, which is determined by the period of IDT with nano meter scale. It is demonstrated that the photo luminescence spectrum of a 2D ReS₂flake on this platform gradually shifts with the SAW excitation power, which reaches a shift of 3 nm as the SAW excitation power achieves 26 dBm, corresponding to a band gap increase of 5 meV. Meanwhile, the platform is also capable to provide acousto-electric coupling between SAW and 2D materials, which is demonstrated by the shift of the SAW resonant frequency due to the re-distribution of photo-generated carriers in ReS₂upon light illumination.

Strain-Enhanced Mobility of Monolayer MoS2

Strain engineering is an important method for tuning the properties of semiconductors and has been used to improve the mobility of silicon transistors for several decades. Recently, theoretical studies have predicted that strain can also improve the mobility of two-dimensional (2D) semiconductors, e.g., by reducing intervalley scattering or lowering effective masses. Here, we experimentally show strain-enhanced electron mobility in monolayer MoS₂ transistors with uniaxial tensile strain, on flexible substrates. The on-state current and mobility are nearly doubled with tensile strain up to 0.7%, and devices return to their initial state after release of the strain. We also show a gate-voltage-dependent gauge factor up to 200 for monolayer MoS₂, which is higher than previous values reported for sub-1 nm thin piezoresistive films. These results demonstrate the importance of strain engineering 2D semiconductors for performance enhancements in integrated circuits, or for applications such as flexible strain sensors.

Strain Releasing of Flexible 2D Electronics through van der Waals Sliding Contact

Two-dimensional (2D) materials have demonstrated promising potential for flexible electronics, owning to their atomic thin body thickness and dangling-bond-free surface. Here, we report a sliding contact device structure for efficient strain releasing. By fabricating a weakly coupled metal-2D junction with a van der Waals (vdW) gap in between, the applied strain could be effectively released through their interface sliding; hence minimized strain is transferred to the 2D lattice. Therefore, we observed stable device behavior with electrodes stretching over 110%, much higher than 2D devices using evaporated metal contacts. Furthermore, through multicycle straining-releasing measurements, we found the electrodes still form intimate contact with nearly constant contact resistance during sliding, confirming the optimization of device flexibility and electrical properties at the same time. Finally, we demonstrate this vdW sliding contact is a general device geometry and could be well-extended to various 2D or 3D bulk materials, leading to devices with much higher strain tolerance.

Raman Spectroscopic Probe for Nonlinear MoS2 Nanoelectromechanical Resonators

Resonant nanoelectromechanical systems (NEMS) enabled by two-dimensional (2D) semiconductors have been actively explored and engineered for making ultrascaled transducers toward applications in ultralow-power signal processing, communication, and sensing. Although the transduction of miniscule resonant motions has been achieved by low-noise optical or electronic techniques, direct probing of strain in vibrating 2D semiconductor membranes and the interplay between the spectroscopic and mechanical properties are still largely unexplored. Here, we experimentally demonstrate dynamical phonon softening in atomically thin molybdenum disulfide (MoS₂) NEMS resonators by directly coupling Raman spectroscopy with optical interferometry resonance motion detection. In single-layer, bilayer, and trilayer (1L to 3L) MoS₂ circular membrane NEMS resonators, we show that high-amplitude nonlinear resonances can enhance the Raman signal amplitude, as well as introduce Raman modes softening up to 0.8 cm^-1. These results shall pave the way for engineering the coupling and control between collective mechanical vibrations and Raman modes of the constituent crystals in 2D transducers.

Piezoresistive Memories Based on Two-Dimensional Nano-Scale Electromechanical Systems

In this work we present piezoresistive memory-bits based on two-dimensional nano-scale electro-mechanical systems. We demonstrate it is possible to achieve different electrical responses by fine control of micro-structural asymmetries and that information can be encoded in the geometrical configuration of the device and read as in classical ReRAM memories by measuring the current flowing across it. Based on the potential energy landscape of the device, we estimate the energy cost to operate the proposed memories. The estimated energy requirements for a single bit compete with existing technologies.

The era of nano-bionic: 2D materials for wearable and implantable body sensors

Nano-bionics have the potential of revolutionizing modern medicine. Among nano-bionic devices, body sensors allow to monitor in real-time the health of patients, to achieve personalized medicine, and even to restore or enhance human functions. The advent of two-dimensional (2D) materials is facilitating the manufacturing of miniaturized and ultrathin bioelectronics, that can be easily integrated in the human body. Their unique electronic properties allow to efficiently transduce physical and chemical stimuli into electric current. Their flexibility and nanometric thickness facilitate the adaption and adhesion to human body. The low opacity permits to obtain transparent devices. The good cellular adhesion and reduced cytotoxicity are advantageous for the integration of the devices in vivo. Herein we review the latest and more significant examples of 2D material-based sensors for health monitoring, describing their architectures, sensing mechanisms, advantages and, as well, the challenges and drawbacks that hampers their translation into commercial clinical devices.

Strain-Modulated Magnetism in MoS2

Since the experiments found that two-dimensional (2D) materials such as single-layer MoS2 can withstand up to 20% strain, strain-modulated magnetism has gradually become an emerging research field. However, applying strain alone is difficult to modulate the magnetism of single-layer pristine MoS2, but applying strain combined with other tuning techniques such as introducing defects makes it easier to produce and alter the magnetism in MoS2. Here, we summarize the recent progress of strain-dependent magnetism in MoS2. First, we review the progress in theoretical study. Then, we compare the experimental methods of applying strain and their effects on magnetism. Specifically, we emphasize the roles played by web buckles, which induce biaxial tensile strain conveniently. Despite some progress, the study of strain-dependent MoS2 magnetism is still in its infancy, and a few potential directions for future research are discussed at the end. Overall, a broad and in-depth understanding of strain-tunable magnetism is very necessary, which will further drive the development of spintronics, straintronics, and flexible electronics.

Quantitative Strain and Topography Mapping of 2D Materials Using Nanobeam Electron Diffraction

It is known that 2D materials can exhibit a nonflat topography, which gives rise to an inherent strain. Since local curvature and strain influence mechanical, optical, and electrical properties, but are often difficult to distinguish from each other, a robust measurement technique is needed. In this study, a novel method is introduced, which is capable of obtaining quantitative strain and topography information of 2D materials with nanometer resolution. Relying on scanning nanobeam electron diffraction (NBED), it is possible to disentangle strain from the local sample slope. Using the positions of the diffraction spots of a specimen at two different tilts to reconstruct the locations and orientations of the reciprocal lattice rods, the local strain and slope can be simultaneously retrieved. We demonstrate the differences to strain measurements from a single NBED map in theory, simulation, and experiment. MoS2 monolayers with different shapes are used as simulation test structures. The slope and height information are recovered, as well as tensile and angular strain which have an absolute difference of less than 0.2% and 0.2° from the theoretical values. An experimental proof of concept using a freely suspended WSe2 monolayer together with a discussion of the accuracy of the method is provided.

Tunable Electronic Properties of Few-Layer Tellurene under In-Plane and Out-of-Plane Uniaxial Strain

Strain engineering is a promising and fascinating approach to tailoring the electrical and optical properties of 2D materials, which is of great importance for fabricating excellent nano-devices. Although previous theoretical works have proved that the monolayer tellurene has desirable mechanical properties with the capability of withstanding large deformation and the tunable band gap and mobility conductance induced by in-plane strain, the effects of in-plane and out-of-plane strains on the properties of few-layer tellurene in different phases should be explored deeply. In this paper, calculations based on first-principles density functional theory were performed to predict the variation in crystal structures and electronic properties of few-layer tellurene, including the α and β phases. The analyses of mechanical properties show that few-layer α-Te can be more easily deformed in the armchair direction than β-Te owing to its lower Young’s modulus and Poisson’s ratio. The α-Te can be converted to β-Te by in-plane compressive strain. The variations in band structures indicate that the uniaxial strain can tune the band structures and even induce the semiconductor-to-metal transition in both few-layer α-Te and β-Te. Moreover, the compressive strain in the zigzag direction is the most feasible scheme due to the lower transition strain. In addition, few-layer β-Te is more easily converted to metal especially for the thicker flakes considering its smaller band gap. Hence, the strain-induced tunable electronic properties and semiconductor-to-metal transition of tellurene provide a theoretical foundation for fabricating metal–semiconductor junctions and corresponding nano-devices.

Tip-Enhanced Photoluminescence of Freestanding Lateral Heterobubbles

Two-dimensional (2D) semiconducting materials have promising applications in flexible optoelectronics, nanophotonics, and sensing based on the broad tunability of their optical and electronic properties. 2D nanobubbles form exciton funnels due to localized strain that can be used as local emitters for information processing. Their nanoscale optical characterization requires the use of near-field scanning probe microscopy (SPM). However, previous near-field studies of 2D materials were performed on SiO₂/Si and metallic substrates using the plasmonic gap mode to increase the signal-to-noise ratio. Another challenge is the deterministic control of bubble size and location. We addressed these challenges by investigating the photoluminescence (PL) signals of freestanding monolayer lateral WSe₂-MoSe₂ heterostructures under the influence of strain exerted by a plasmonic SPM tip. For first time, we performed tip-enhanced PL imaging of freestanding 2D materials and studied the competition between the PL enhancement mechanisms by nanoindentation as a function of the tip-sample distance. We observed the tunability of PL as a function of bubble size, which opens new possibilities to design optoelectronic nanodevices.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

Revealing the Anisotropic Structural and Electrical Stabilities of 2D SnSe under Harsh Environments: Alkaline Environment and Mechanical Strain

As a promising thermoelectric and semiconducting material, the stability of two-dimensional tin selenide (SnSe) under harsh environments is significant for its practical applications. Here, focusing on the key procedures in the device fabrication process, we report the anisotropic structural and electrical stabilities of SnSe under an alkaline environment and mechanical strain. Due to the anisotropic mechanical properties, the SnSe flakes can naturally form long-straight {011} edge planes during the mechanical exfoliation process. Such a cleavage tendency provides an effective crystal orientation identification method to uncover the orientation-dependent properties. We find that the single-crystalline SnSe flakes experience an anisotropic degradation process with the preferable {011} dissolution planes in the alkaline environment and can be gradually transformed to be polycrystalline consisting of SnSe₂, Sn, and Se nanocrystals. SnSe flakes present an anisotropic electromechanical response with a gauge factor value that reaches ∼-460 under the uniaxial strain along the ⟨011⟩ directions. Our revealed structural and electrical stability of SnSe under harsh environments can provide guidance for the device design, fabrication, and performance evaluation.

Strain-Modulated Dissipation in Two-Dimensional Molybdenum Disulfide Nanoelectromechanical Resonators

Resonant nanoelectromechanical systems (NEMS) based on two-dimensional (2D) materials such as molybdenum disulfide (MoS₂) are interesting for highly sensitive mass, force, photon, or inertial transducers, as well as for fundamental research approaching the quantum limit, by leveraging the mechanical degree of freedom in these atomically thin materials. For these mechanical resonators, the quality factor (Q) is essential, yet the mechanism and tuning methods for energy dissipation in 2D NEMS resonators have not been fully explored. Here, we demonstrate that by tuning static strain and vibration-induced strain in suspended MoS₂ using gate voltages, we can effectively tune the Q in 2D MoS₂ NEMS resonators. We further show that for doubly clamped resonators, the Q increases with larger DC gate voltage, while fully clamped drumhead resonators show the opposite trend. Using DC gate voltages, we can tune the Q by ΔQ/Q = 448% for fully clamped resonators, and by ΔQ/Q = 369% for doubly clamped resonators. We develop the strain-modulated dissipation model for these 2D NEMS resonators, which is verified against our measurement data for 8 fully clamped resonators and 7 doubly clamped resonators. We find that static tensile strain decreases dissipation while vibration-induced strain increases dissipation, and the actual dependence of Q on DC gate voltage depends on the competition between these two effects, which is related to the device boundary condition. Such strain dependence of Q is useful for optimizing the resonance linewidth in 2D NEMS resonators toward low-power, ultrasensitive, and frequency-selective devices for sensing and signal processing.

Mechanical Stress Modulation of Resistance in MoS2 Junctions

Strain engineering is a powerful strategy to control the physical properties of material-enabling devices with enhanced functionality and improved performance. Here, we investigate a modulation of the transport behavior of the two-dimensional MoS₂ junctions under the mechanical stress induced by a tip of an atomic force microscope (AFM). We show that the junction resistance can be reversibly tuned by up to 4 orders of magnitude by altering a tip-induced force. Analysis of the stress-induced evolution of the I-V characteristics indicates a combined effect of the tip-induced strain and strain gradient on the energy barrier height and profile. In addition, we show that the tip-generated flexoelectric effect leads to significant enhancement of the photovoltaic effect in the MoS₂ junctions. A combination of the optical and mechanical stimuli facilitates reversible photomechanical tuning of resistance of the narrow-band 2D semiconductors and development of devices with an enhanced photovoltaic response.

Mechanical sensors based on two-dimensional materials: Sensing mechanisms, structural designs and wearable applications

Compared with bulk materials, atomically thin two-dimensional (2D) crystals possess a range of unique mechanical properties, including relatively high in-plane stiffness and large bending flexibility. The atomic 2D building blocks can be reassembled into precisely designed heterogeneous composite structures of various geometries with customized mechanical sensing behaviors. Due to their small specific density, high flexibility, and environmental adaptability, mechanical sensors based on 2D materials can conform to soft and curved surfaces, thus providing suitable solutions for functional applications in future wearable devices. In this review, we summarize the latest developments in mechanical sensors based on 2D materials from the perspective of function-oriented applications. First, typical mechanical sensing mechanisms are introduced. Second, we attempt to establish a correspondence between typical structure designs and the performance/multi-functions of the devices. Afterward, several particularly promising areas for potential applications are discussed, following which we present perspectives on current challenges and future opportunities

Holey Substrate-Directed Strain Patterning in Bilayer MoS2

Key properties of two-dimensional (2D) layered materials are highly strain tunable, arising from bond modulation and associated reconfiguration of the energy bands around the Fermi level. Approaches to locally controlling and patterning strain have included both active and passive elastic deformation via sustained loading and templating with nanostructures. Here, by float-capturing ultrathin flakes of single-crystal 2H-MoS₂ on amorphous holey silicon nitride substrates, we find that highly symmetric, high-fidelity strain patterns are formed. The hexagonally arranged holes and surface topography combine to generate highly conformal flake-substrate coverage creating patterns that match optimal centroidal Voronoi tessellation in 2D Euclidean space. Using TEM imaging and diffraction, as well as AFM topographic mapping, we determine that the substrate-driven 3D geometry of the flakes over the holes consists of symmetric, out-of-plane bowl-like deformation of up to 35 nm, with in-plane, isotropic tensile strains of up to 1.8% (measured with both selected-area diffraction and AFM). Atomistic and image simulations accurately predict spontaneous formation of the strain patterns, with van der Waals forces and substrate topography as the input parameters. These results show that predictable patterns and 3D topography can be spontaneously induced in 2D materials captured on bare, holey substrates. The method also enables electron scattering studies of precisely aligned, substrate-free strained regions in transmission mode.

Strain Engineering of Low-Dimensional Materials for Emerging Quantum Phenomena and Functionalities

Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron-electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.

Ten Years Progress of Electrical Detection of Heavy Metal Ions (HMIs) Using Various Field-Effect Transistor (FET) Nanosensors: A Review

Heavy metal pollution remains a major concern for the public today, in line with the growing population and global industrialization. Heavy metal ion (HMI) is a threat to human and environmental safety, even at low concentrations, thus rapid and continuous HMI monitoring is essential. Among the sensors available for HMI detection, the field-effect transistor (FET) sensor demonstrates promising potential for fast and real-time detection. The aim of this review is to provide a condensed overview of the contribution of certain semiconductor substrates in the development of chemical and biosensor FETs for HMI detection in the past decade. A brief introduction of the FET sensor along with its construction and configuration is presented in the first part of this review. Subsequently, the FET sensor deployment issue and FET intrinsic limitation screening effect are also discussed, and the solutions to overcome these shortcomings are summarized. Later, we summarize the strategies for HMIs' electrical detection, mechanisms, and sensing performance on nanomaterial semiconductor FET transducers, including silicon, carbon nanotubes, graphene, AlGaN/GaN, transition metal dichalcogenides (TMD), black phosphorus, organic and inorganic semiconductor. Finally, concerns and suggestions regarding detection in the real samples using FET sensors are highlighted in the conclusion.

Programmable patterned MoS2 film by direct laser writing for health-related signals monitoring

The two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising flexible electronic materials for strategic flexible information devices. Large-area and high-quality patterned materials were usually required by flexible electronics due to the limitation from the process of manufacturing and integration. However, the synthesis of large-area patterned 2D TMDs with high quality is difficult. Here, an efficient and powerful pulsed laser has been developed to synthesize wafer-scale MoS₂. The flexible strain sensor was fabricated using MoS₂ and showed high performance of low detection limit (0.09%), high gauge factor (1,118), and high stability (1,000 cycles). Besides, we demonstrated its applications in real-time monitoring of health-related physiological signals such as radial artery pressure, respiratory rate, and vocal cord vibration. Our findings suggest that the laser-assisted method is effective and capable of synthesizing wafer-scale 2D TMDs, which opens new opportunities for the next flexible electronic devices and wearable health monitoring.

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Wafer-scale patterned MoS₂ film has been synthesized by pulsed laser

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The MoS₂ film strain sensor shows low limit detection, high GF, and stability

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The healthy-related singles have been monitored by the MoS₂ film strain sensor

2D Materials for Skin-Mountable Electronic Devices

Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

Spin-crossover nanoparticles anchored on MoS2 layers for heterostructures with tunable strain driven by thermal or light-induced spin switching

In the past few years, the effect of strain on the optical and electronic properties of MoS₂ layers has attracted particular attention as it can improve the performance of optoelectronic and spintronic devices. Although several approaches have been explored, strain is typically externally applied on the two-dimensional material. In this work, we describe the preparation of a reversible 'self-strainable' system in which the strain is generated at the molecular level by one component of a MoS₂-based composite material. Spin-crossover nanoparticles were covalently grafted onto functionalized layers of semiconducting MoS₂ to form a hybrid heterostructure. Their ability to switch between two spin states on applying an external stimulus (light irradiation or temperature change) serves to generate strain over the MoS₂ layer. A volume change accompanies this spin crossover, and the created strain induces a substantial and reversible change of the electrical and optical properties of the heterostructure.

Strain analysis and engineering in halide perovskite photovoltaics

Halide perovskites are a compelling candidate for the next generation of clean-energy-harvesting technologies owing to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskites is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

Two-Dimensional Mechano-thermoelectric Heterojunctions for Self-Powered Strain Sensors

We here demonstrate the multifunctional properties of atomically thin heterojunctions that are enabled by their strong interfacial interactions and their application toward self-powered sensors with unprecedented performance. Bonding between tin diselenide and graphene produces thermoelectric and mechanoelectric properties beyond the ability of either component. A record-breaking ZT of 2.43 originated from the synergistic combination of graphene's high carrier conductivity and SnSe₂-mediated thermal conductivity lowering. Moreover, spatially varying interaction at the SnSe₂/graphene interface produces stress localization that results in a novel 2D-crack-assisted strain sensing mechanism whose sensitivity (GF = 450) is superior to all other 2D materials. Finally, a graphene-assisted growth process permits the formation of high-quality heterojunctions directly on polymeric substrates for flexible and transparent sensors that achieve self-powered strain sensing from a small temperature gradient. Our work enhances the fundamental understanding of multifunctionality at the atomic scale and provides a route toward structural health monitoring through ubiquitous and smart devices.

MoS2/Graphene Photodetector Array with Strain-Modulated Photoresponse up to the Near-Infrared Regime

MoS₂, an emerging material in the field of optoelectronics, has attracted the attention of researchers owing to its high light absorption efficiency, even as an atomically thin layer. However, the covered spectra of the reported MoS₂-based photodetectors are restricted to the visible range owing to their electronic bandgap (∼1.9 eV). Strain engineering, which modulates the bandgap of a semiconductor, can extend the application coverage of MoS₂ to the infrared spectral range. The shrinkage of the bandgap because of the tensile strain on MoS₂ enhances the photoresponsivity in the visible range and extends its sensing capability beyond its fundamental absorption limit. Herein, we report a graphene/MoS₂/graphene metal-semiconductor-metal photodetector (PD) array with a strain-modulated photoresponse up to the spectral range of the near-infrared (NIR). The MoS₂ PD array on a flexible substrate was stretched in the biaxial direction to a tensile strain level of 1.19% using a pneumatic bulging process. The MoS₂-based line-scanning system was implemented by digitizing the output photocurrent of the strained MoS₂ linear array with a low-noise complementary metal-oxide-semiconductor (CMOS) readout integrated circuit (IC) and successfully captured vis-NIR images in foggy conditions. Therefore, we extended the application of the MoS₂ PD array to the NIR regime and demonstrated its use in real-life imaging systems.

Recent developments in 2D transition metal dichalcogenides: phase transition and applications of the (quasi-)metallic phases

The advent of two-dimensional transition metal dichalcogenides (2D-TMDs) has led to an extensive amount of interest amongst scientists and engineers alike and an intensive amount of research has brought about major breakthroughs in the electronic and optical properties of 2D materials. This in turn has generated considerable interest in novel device applications. With the polymorphic structural features of 2D-TMDs, this class of materials can exhibit both semiconducting and metallic (quasi-metallic) properties in their respective phases. This polymorphic property further increases the interest in 2D-TMDs both in fundamental research and for their potential utilization in novel high-performance device applications. In this review, we highlight the unique structural properties of few-layer and monolayer TMDs in the metallic 1T- and quasi-metallic 1T'-phases, and how these phases dictate their electronic and optical properties. An overview of the semiconducting-to-(quasi)-metallic phase transition of 2D-TMD systems will be covered along with a discussion on the phase transition mechanisms. The current development in the applications of (quasi)-metallic 2D-TMDs will be presented ranging from high-performance electronic and optoelectronic devices to energy storage, catalysis, piezoelectric and thermoelectric devices, and topological insulator and neuromorphic computing applications. We conclude our review by highlighting the challenges confronting the utilization of TMD-based systems and projecting the future developmental trends with an outlook of the progress needed to propel this exciting field forward.

Advances in ultrasensitive piezoresistive sensors: from conventional to flexible and stretchable applications

The piezoresistive effect has been a dominant mechanical sensing principle that has been widely employed in a range of sensing applications. This transducing concept still receives great attention because of the huge demand for developing small, low-cost, and high-performance sensing devices. Many researchers have extensively explored new methods to enhance the piezoresistive effect and to make sensors more and more sensitive. Many interesting phenomena and mechanisms to enhance the sensitivity have been discovered. Numerous review papers on the piezoresistive effect have been published; however, there is no comprehensive review article that thoroughly analyses methods and approaches to enhance the piezoresistive effect. This paper comprehensively reviews and presents all the advanced enhancement methods ranging from the quantum physical effect and new materials to nanoscopic and macroscopic structures, and from conventional rigid to flexible, stretchable and wearable applications. In addition, the paper summarises results recently achieved on applying the above-mentioned innovative sensing enhancement techniques in making extremely sensitive piezoresistive transducers.