Microscopic characterizations for 2D material-based advanced electronics.

Two-dimensional (2D) materials have gained significant attention as potential candidates for next-generation electronics, owing to their unique properties such as ultrathin layer thickness, mechanical flexibility, and tunable bandgaps. The distinctive characteristics of 2D materials necessitate the development of nanoscale advanced characterization methods. In this review, we explore the role of microscopy techniques in developing 2D materials-based electronics, from material synthesis and characterization to device performance and reliability. We address the applications of microscopies by delving into the perspectives of channel materials, metal contacts, dielectric materials, and device architectures. Additionally, we provide an outlook on the future directions and potential utilization of microscopy techniques in future 2D semiconductor industry.

MXene-Fiber Composite Membranes for Permeable and Biocompatible Skin-Interfaced Iontronic Mechanosensing.

Artificial ionic sensory systems, bridging the divide between biological systems and electronics, mimic human skin functions but face critical challenges with biocompatibility, comfort, signal stability, and simplifying packaging. Here, we present a simple and permeable skin-interfaced iontronic mechanosensing (SIIM) architecture that integrates human skin as natural ionic material and hierarchically porous MXene-fiber composite membranes as sensing electrodes. The SIIM system eliminates complex ionic material design and multilayer matrix, exhibiting ultrahigh pressure sensitivities (5.4 kPa-1, <75 Pa), a low detection limit (6 Pa), excellent output stability along with high permeability to minimize the impact of sweating on sensing. The noncytotoxic nature of SIIM electrodes ensures excellent biocompatibility (>97% cell coincubational viability), facilitating long-term wearability and high biosafety. Furthermore, the scalable SIIM configuration integrated with matrix smart gloves, effectively monitors human physical movements. This SIIM-based sensor with marked sensing capabilities, structural simplicity, and scalability, holds promising potential in diverse wearable applications.

Revisiting the Epitaxial Growth Mechanism of 2D TMDC Single Crystals.

Epitaxial growth of 2D transition metal dichalcogenides (TMDCs) on sapphire substrates has been recognized as a pivotal method for producing wafer-scale single-crystal films. Both step-edges and symmetry of substrate surfaces have been proposed as controlling factors. However, the underlying fundamental still remains elusive. In this work, through the molybdenum disulfide (MoS2) growth on C/M sapphire, it is demonstrated that controlling the sulfur evaporation rate is crucial for dictating the switch between atomic-edge guided epitaxy and van der Waals epitaxy. Low-concentration sulfur condition preserves O/Al-terminated step edges, fostering atomic-edge epitaxy, while high-concentration sulfur leads to S-terminated edges, preferring van der Waals epitaxy. These experiments reveal that on a 2 in. wafer, the van der Waals epitaxy mechanism achieves better control in MoS2 alignment (≈99%) compared to the step edge mechanism (<85%). These findings shed light on the nuanced role of atomic-level thermodynamics in controlling nucleation modes of TMDCs, thereby providing a pathway for the precise fabrication of single-crystal 2D materials on a wafer scale.

Photoprogrammed Multifunctional Optoelectronic Synaptic Transistor Arrays Based on Photosensitive Polymer-Sorted Semiconducting Single-Walled Carbon Nanotubes for Image Recognition.

The development of neuromorphic optoelectronic systems opens up the possibility of the next generation of artificial vision. In this work, the novel broadband (from 365 to 940 nm) and multilevel storage optoelectronic synaptic thin-film transistor (TFT) arrays are reported using the photosensitive conjugated polymer (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)], F8T2) sorted semiconducting single-walled carbon nanotubes (sc-SWCNTs) as channel materials. The broadband synaptic responses are inherited to absorption from both photosensitive F8T2 and sorted sc-SWCNTs, and the excellent optoelectronic synaptic behaviors with 200 linearly increasing conductance states and long retention time > 103 s are attributed to the superior charge trapping at the AlOx dielectric layer grown by atomic layer deposition. Furthermore, the synaptic TFTs can achieve IOn/IOff ratios up to 106 and optoelectronic synaptic plasticity with the low power consumption (59 aJ per single pulse), which can simulate not only basic biological synaptic functions but also optical write and electrical erase, multilevel storage, and image recognition. Further, a novel Spiking Neural Network algorithm based on hardware characteristics is designed for the recognition task of Caltech 101 dataset and multiple features of the images are successfully extracted with higher accuracy (97.92%) of the recognition task from the multi-frequency curves of the optoelectronic synaptic devices.

Projected performance of Si- and 2D-material-based SRAM circuits ranging from 16 nm to 1 nm technology nodes.

Researchers have been developing 2D materials (2DM) for electronics, which are widely considered a possible replacement for silicon in future technology. Two-dimensional transition metal dichalcogenides are the most promising among the different materials due to their electronic performance and relatively advanced development. Although field-effect transistors (FETs) based on 2D transition metal dichalcogenides have been found to outperform Si in ultrascaled devices, the comparison of 2DM-based and Si-based technologies at the circuit level is still missing. Here we compare 2DM- and Si FET-based static random-access memory (SRAM) circuits across various technology nodes from 16 nm to 1 nm and reveal that the 2DM-based SRAM exhibits superior performance in terms of stability, operating speed and energy efficiency when compared with Si SRAM. This study utilized technology computer-aided design to conduct device and circuit simulations, employing calibrated MoS2 nFETs and WSe2 pFETs. It incorporated layout design rules across various technology nodes to comprehensively analyse their SRAM functionality. The results show that, compared with three-dimensional structure Si transistors at 1 nm node, the planar 2DMFETs exhibited lower capacitance, leading to reduced cell read access time (-16%), reduced time to write (-72%) and lowered dynamic power (-60%). The study highlights the provisional benefits of using planar 2DM transistors to mitigate the performance degradation caused by reduced metal pitch and increased wire resistance in advanced nodes, potentially opening up exciting possibilities for high-performance and low-power circuit applications.

Self-Trapped Exciton Emission in Highly Polar 0D Hybrid Ammonium/Hydronium-Based Perovskites Triggered by Antimony Doping.

Various monovalent cations are employed to construct metal halide perovskites with various structures and functionalities. However, perovskites based on highly polar A-site cations have seldom been reported. Here, a novel hybrid 0D (NH4)x(OH3)3-xInCl6 perovskite with highly polar hydronium OH3+ cations is introduced in this study. Upon doping with Sb3+, hybrid 0D (NH4)x(OH3)3-xInCl6 single crystals exhibited highly efficient broadband yellowish-green (550 nm) and red (630 nm) dual emissions with a PLQY of 86%. The dual emission arises due to Sb3+ occupying two sites within the crystal lattice that possess different polarization environments, leading to distinct Stokes shift energies. The study revealed that lattice polarity plays a significant role in the self-trapped exciton emission of Sb3+-doped perovskites, contributing up to 25% of the Stokes shift energy for hybrid 0D (NH4)x(OH3)3-xInCl6:Sb3+ as a secondary source, in addition to the Jahn-Teller deformation. These findings highlight the potential of Sb3+-doped perovskites for achieving tunable broadband emission and underscore the importance of lattice polarity in determining the emission properties of perovskite materials.

Identifying atomically thin isolated-band channels for intrinsic steep-slope transistors by high-throughput study.

Developing low-power FETs holds significant importance in advancing logic circuits, especially as the feature size of MOSFETs approaches sub-10 nanometers. However, this has been restricted by the thermionic limitation of SS, which is limited to 60 mV per decade at room temperature. Herein, we proposed a strategy that utilizes 2D semiconductors with an isolated-band feature as channels to realize sub-thermionic SS in MOSFETs. Through high-throughput calculations, we established a guiding principle that combines the atomic structure and orbital interaction to identify their sub-thermionic transport potential. This guides us to screen 192 candidates from the 2D material database comprising 1608 systems. Additionally, the physical relationship between the sub-thermionic transport performances and electronic structures is further revealed, which enables us to predict 15 systems with promising device performances for low-power applications with supply voltage below 0.5 V. This work opens a new way for the low-power electronics based on 2D materials and would inspire extensive interests in the experimental exploration of intrinsic steep-slope MOSFETs.

Cascaded compression of size distribution of nanopores in monolayer graphene.

Monolayer graphene with nanometre-scale pores, atomically thin thickness and remarkable mechanical properties provides wide-ranging opportunities for applications in ion and molecular separations1, energy storage2 and electronics3. Because the performance of these applications relies heavily on the size of the nanopores, it is desirable to design and engineer with precision a suitable nanopore size with narrow size distributions. However, conventional top-down processes often yield log-normal distributions with long tails, particularly at the sub-nanometre scale4. Moreover, the size distribution and density of the nanopores are often intrinsically intercorrelated, leading to a trade-off between the two that substantially limits their applications5-9. Here we report a cascaded compression approach to narrowing the size distribution of nanopores with left skewness and ultrasmall tail deviation, while keeping the density of nanopores increasing at each compression cycle. The formation of nanopores is split into many small steps, in each of which the size distribution of all the existing nanopores is compressed by a combination of shrinkage and expansion and, at the same time as expansion, a new batch of nanopores is created, leading to increased nanopore density by each cycle. As a result, high-density nanopores in monolayer graphene with a left-skewed, short-tail size distribution are obtained that show ultrafast and ångström-size-tunable selective transport of ions and molecules, breaking the limitation of the conventional log-normal size distribution9,10. This method allows for independent control of several metrics of the generated nanopores, including the density, mean diameter, standard deviation and skewness of the size distribution, which will lead to the next leap in nanotechnology.

Bi2O2Se-Based Bimode Noise Generator for the Application of Generative Adversarial Networks.

In the emerging technology, the generative aversive networks (GANs), randomness, and unpredictability of inputting noises are the keys to the uniqueness, diversity, robustness, and security of the generated images. Compared with deterministic software-based noise generation, hardware-based noise generation introduces physical entropy sources, such as electronic and photonic noises, to add unpredictability. In this study, bimode Bi2O2Se-based noise generators have been demonstrated for the application of GANs. Harnessing its ultrahigh carrier mobility, excellent air stability, marvelous optoelectronic performance, as well as the unique surface resistive switching effect and defect locations in the energy diagram, Bi2O2Se provides a good material platform to easily integrate with multiple device architectures for generating noises in different physical sources. The noise of the black current mode in a photodetector architecture and the random telegraph noise in a memristor mode were measured, characterized, compared, and analyzed. A method of Markov chain equipped with K-means clustering was carried out to calculate the discrete noise states and the transition probability matrix between them. To evaluate the generated properties of the GANs based on the hardware noise source, the inception score and Fréchet inception distance were evaluated.

Oriented lateral growth of two-dimensional materials on c-plane sapphire.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) represent the ultimate thickness for scaling down channel materials. They provide a tantalizing solution to push the limit of semiconductor technology nodes in the sub-1 nm range. One key challenge with 2D semiconducting TMD channel materials is to achieve large-scale batch growth on insulating substrates of single crystals with spatial homogeneity and compelling electrical properties. Recent studies have claimed the epitaxy growth of wafer-scale, single-crystal 2D TMDs on a c-plane sapphire substrate with deliberately engineered off-cut angles. It has been postulated that exposed step edges break the energy degeneracy of nucleation and thus drive the seamless stitching of mono-oriented flakes. Here we show that a more dominant factor should be considered: in particular, the interaction of 2D TMD grains with the exposed oxygen-aluminium atomic plane establishes an energy-minimized 2D TMD-sapphire configuration. Reconstructing the surfaces of c-plane sapphire substrates to only a single type of atomic plane (plane symmetry) already guarantees the single-crystal epitaxy of monolayer TMDs without the aid of step edges. Electrical results evidence the structural uniformity of the monolayers. Our findings elucidate a long-standing question that curbs the wafer-scale batch epitaxy of 2D TMD single crystals-an important step towards using 2D materials for future electronics. Experiments extended to perovskite materials also support the argument that the interaction with sapphire atomic surfaces is more dominant than step-edge docking.

Interfacial Reconstructed Layer Controls the Orientation of Monolayer Transition-Metal Dichalcogenides.

Growing continuous monolayer films of transition-metal dichalcogenides (TMDs) without the disruption of grain boundaries is essential to realize the full potential of these materials for future electronics and optoelectronics, but it remains a formidable challenge. It is generally believed that controlling the TMDs orientations on epitaxial substrates stems from matching the atomic registry, symmetry, and penetrable van der Waals forces. Interfacial reconstruction within the exceedingly narrow substrate-epilayer gap has been anticipated. However, its role in the growth mechanism has not been intensively investigated. Here, we report the experimental conformation of an interfacial reconstructed (IR) layer within the substrate-epilayer gap. Such an IR layer profoundly impacts the orientations of nucleating TMDs domains and, thus, affects the materials' properties. These findings provide deeper insights into the buried interface that could have profound implications for the development of TMD-based electronics and optoelectronics.

Epitaxial substitution of metal iodides for low-temperature growth of two-dimensional metal chalcogenides.

The integration of various two-dimensional (2D) materials on wafers enables a more-than-Moore approach for enriching the functionalities of devices1-3. On the other hand, the additive growth of 2D materials to form heterostructures allows construction of materials with unconventional properties. Both may be achieved by materials transfer, but often suffer from mechanical damage or chemical contamination during the transfer. The direct growth of high-quality 2D materials generally requires high temperatures, hampering the additive growth or monolithic incorporation of different 2D materials. Here we report a general approach of growing crystalline 2D layers and their heterostructures at a temperature below 400 °C. Metal iodide (MI, where M = In, Cd, Cu, Co, Fe, Pb, Sn and Bi) layers are epitaxially grown on mica, MoS2 or WS2 at a low temperature, and the subsequent low-barrier-energy substitution of iodine with chalcogens enables the conversion to at least 17 different 2D crystalline metal chalcogenides. As an example, the 2D In2S3 grown on MoS2 at 280 °C exhibits high photoresponsivity comparable with that of the materials grown by conventional high-temperature vapour deposition (~700-1,000 °C). Multiple 2D materials have also been sequentially grown on the same wafer, showing a promising solution for the monolithic integration of different high-quality 2D materials.

Statistical Assessment of High-Performance Scaled Double-Gate Transistors from Monolayer WS2.

Scaling of monolayer transition metal dichalcogenide (TMD) field-effect transistors (FETs) is an important step toward evaluating the application space of TMD materials. Although some work on ultrashort channel monolayer (ML) TMD FETs has been published, there exist no comprehensive studies that assess their performance in a statistically relevant manner, providing critical insights into the impact of the device geometry. Part of the reason for the absence of such a study is the substantial variability of TMD devices when processes are not carefully controlled. In this work, we show a statistical study of ultrashort channel double-gated ML WS2 FETs exhibiting excellent device performance and limited device-to-device variations. From a detailed analysis of cross-sectional scanning transmission electron microscopy (STEM) images and careful technology computer aided design (TCAD) simulations, we evaluated, in particular, an unexpected deterioration of the subthreshold characteristics for our shortest devices. Two potential candidates for the observed behavior were identified, i.e., buckling of the TMD on the substrate and loss of gate control due to the source geometry and the high-k dielectric between the metal gate and the metal source electrode.

Bi2O2Se-based integrated multifunctional optoelectronics.

The prominent light-matter interaction in 2D materials has become a pivotal research area that involves either an archetypal study of inherent mechanisms to explore such interactions or specific applications to assess the efficacy of such novel phenomena. With scientifically controlled light-matter interactions, various applications have been developed. Here, we report four diverse applications on a single structure utilizing the efficient photoresponse of Bi2O2Se with precisely tuned multiple optical wavelengths. First, the Bi2O2Se-based device performs the function of optoelectronic memory using UV (λ = 365 nm, 1.1 mW cm-2) for the write-in process with SiO2 as the charge trapping medium followed by a +1 V bias for read-out. Second, associative learning is mimicked with wavelengths of 525 nm and 635 nm. Third, using similar optical inputs, functions of logic gates "AND", "OR", "NAND", and "NOR" are realized with response current and resistance as outputs. Fourth is the demonstration of a 4 bit binary to the decimal converter using wavelengths of 740 nm (LSB), 595 nm, 490 nm, and 385 nm (MSB) as binary inputs and output response current regarded as equivalent decimal output. Our demonstration is a paradigm for Bi2O2Se-based devices to be an integral part of future advanced multifunctional electronic systems.

Graphdiyne-Based Nanofilms for Compliant On-Skin Sensing.

Thin-film electronics pliably laminated onto the epidermis for noninvasive, specific, and multifunctional sensing are ideal wearable systems for health monitoring and information technologies. However, it remains a critical challenge to fabricate ultrathin and compliant skin-like sensors with high imperceptibility and sensitivities. Here we report a design of conductive hydrogen-substituted graphdiyne (HsGDY) nanofilms with conjugated porous structure and inherent softness for on-skin sensors that allow minimization of stress and discomfort with wear. Dominated by the subtle deformation-induced changes in the interdomain tunneling conductance, the engineered HsGDY sensors show continuous and accurate results. Real-time noninvasive spatial mapping of dynamic/static strains in both tensile/compressive directions monitors various body motions with high sensitivity (GF ∼22.6, under 2% strain), fast response (∼60 ms), and long-term durability (∼5000 cycles). Moreover, such devices can dynamically distinguish between the temperature difference and frequency of air inhaled and exhaled through the nostril, revealing a quantitative assessment of the movement/health of the human body. The proof-of-concept strategy provides an alternative route for the design of next-generation wearable organic bioelectronics with multiple electronic functionalities.

Fast water transport and molecular sieving through ultrathin ordered conjugated-polymer-framework membranes.

The development of membranes that block solutes while allowing rapid water transport is of great importance. The microstructure of the membrane needs to be rationally designed at the molecular level to achieve precise molecular sieving and high water flux simultaneously. We report the design and fabrication of ultrathin, ordered conjugated-polymer-framework (CPF) films with thicknesses down to 1 nm via chemical vapour deposition and their performance as separation membranes. Our CPF membranes inherently have regular rhombic sub-nanometre (10.3 × 3.7 Å) channels, unlike membranes made of carbon nanotubes or graphene, whose separation performance depends on the alignment or stacking of materials. The optimized membrane exhibited a high water/NaCl selectivity of ∼6,900 and water permeance of ∼112 mol m-2 h-1 bar-1, and salt rejection >99.5% in high-salinity mixed-ion separations driven by osmotic pressure. Molecular dynamics simulations revealed that water molecules quickly and collectively pass through the membrane by forming a continuous three-dimensional network within the hydrophobic channels. The advent of ordered CPF provides a route towards developing carbon-based membranes for precise molecular separation.

Low-defect-density WS2 by hydroxide vapor phase deposition.

Two-dimensional (2D) semiconducting monolayers such as transition metal dichalcogenides (TMDs) are promising channel materials to extend Moore's Law in advanced electronics. Synthetic TMD layers from chemical vapor deposition (CVD) are scalable for fabrication but notorious for their high defect densities. Therefore, innovative endeavors on growth reaction to enhance their quality are urgently needed. Here, we report that the hydroxide W species, an extremely pure vapor phase metal precursor form, is very efficient for sulfurization, leading to about one order of magnitude lower defect density compared to those from conventional CVD methods. The field-effect transistor (FET) devices based on the proposed growth reach a peak electron mobility ~200 cm2/Vs (~800 cm2/Vs) at room temperature (15 K), comparable to those from exfoliated flakes. The FET device with a channel length of 100 nm displays a high on-state current of ~400 µA/µm, encouraging the industrialization of 2D materials.

Directly Visualizing Photoinduced Renormalized Momentum-Forbidden Electronic Quantum States in an Atomically Thin Semiconductor.

Resolving the momentum degree of freedom of photoexcited charge carriers and exploring the excited-state physics in the hexagonal Brillouin zone of atomically thin semiconductors have recently attracted great interest for optoelectronic technologies. We demonstrate a combination of light-modulated scanning tunneling microscopy and the quasiparticle interference (QPI) technique to offer a directly accessible approach to reveal and quantify the unexplored momentum-forbidden electronic quantum states in transition metal dichalcogenide (TMD) monolayers. Our QPI results affirm the large spin-splitting energy at the spin-valley-coupled Q valleys in the conduction band (CB) of a tungsten disulfide monolayer. Furthermore, we also quantify the photoexcited carrier density-dependent band renormalization at the Q valleys. Our findings directly highlight the importance of the excited-state distribution at the Q valley in the band renormalization in TMDs and support the critical role of the CB Q valley in engineering the quantum electronic valley degree of freedom in TMD devices.

High-κ perovskite membranes as insulators for two-dimensional transistors.

The scaling of silicon metal-oxide-semiconductor field-effect transistors has followed Moore's law for decades, but the physical thinning of silicon at sub-ten-nanometre technology nodes introduces issues such as leakage currents1. Two-dimensional (2D) layered semiconductors, with an atomic thickness that allows superior gate-field penetration, are of interest as channel materials for future transistors2,3. However, the integration of high-dielectric-constant (κ) materials with 2D materials, while scaling their capacitance equivalent thickness (CET), has proved challenging. Here we explore transferrable ultrahigh-κ single-crystalline perovskite strontium-titanium-oxide membranes as a gate dielectric for 2D field-effect transistors. Our perovskite membranes exhibit a desirable sub-one-nanometre CET with a low leakage current (less than 10-2 amperes per square centimetre at 2.5 megavolts per centimetre). We find that the van der Waals gap between strontium-titanium-oxide dielectrics and 2D semiconductors mitigates the unfavourable fringing-induced barrier-lowering effect resulting from the use of ultrahigh-κ dielectrics4. Typical short-channel transistors made of scalable molybdenum-disulfide films by chemical vapour deposition and strontium-titanium-oxide dielectrics exhibit steep subthreshold swings down to about 70 millivolts per decade and on/off current ratios up to 107, which matches the low-power specifications suggested by the latest International Roadmap for Devices and Systems5.

Nanoscale Electronic Transparency of Wafer-Scale Hexagonal Boron Nitride.

Monolayer hexagonal boron nitride (hBN) has attracted interest as an ultrathin tunnel barrier or environmental protection layer. Recently, wafer-scale hBN growth on Cu(111) was developed for semiconductor chip applications. For basic research and technology, understanding how hBN perturbs underlying electronically active layers is critical. Encouragingly, hBN/Cu(111) has been shown to preserve the Cu(111) surface state (SS), but it was unknown how tunneling into this SS through hBN varies spatially. Here, we demonstrate that the Cu(111) SS under wafer-scale hBN is homogeneous in energy and spectral weight over nanometer length scales and across atomic terraces. In contrast, a new spectral feature─not seen on bare Cu(111)─varies with atomic registry and shares the spatial periodicity of the hBN/Cu(111) moiré. This work demonstrates that, for some 2D electron systems, an hBN overlayer can act as a protective yet remarkably transparent window on fragile low-energy electronic structure below.