Jing Kong

A vast amount of scientific knowledge is embedded in journal articles as unstructured text, creating challenges for efficiently extracting detailed insights. Traditionally, expert-authored reviews summarize research progress, but they often struggle to capture the intricate synthesis protocols in individual papers and provide limited quantitative comparisons of experimental techniques. Recent advancements in machine learning, particularly natural language processing (NLP), have enabled automated text mining and information extraction. However, in materials science, most approaches have focused on refining model architectures rather than addressing domain-specific challenges such as data annotation and the extraction of complex synthesis details. We present a machine learning framework for extracting synthesis protocols of 2D materials, including graphene and TMDs, from publications spanning 1980-2022. By combining named entity recognition (NER) and extractive question answering (EQA), we retrieve both categorical and numerical synthesis parameters. Generative models are further used to summarize and generate experimental recipes, enabling knowledge transfer across material systems. Our domain-specific, fine-tuned models offer improved precision and interpretability compared to general-purpose approaches. This scalable framework helps unlock hidden insights from literature, supporting data-driven synthesis optimization and accelerating materials discovery.

Single-orientation stitching of graphene has emerged as the predominant method for growth of large-area, high-quality graphene films. Particularly noteworthy is graphene grown on single-crystalline Cu(111)/sapphire substrates, which exhibits exceptionally planar oriented stitching due to the atomically smooth substrate, facilitating the formation of continuous, high-quality graphene monolayer. These single-orientation stitches have conventionally been regarded as seamless with negligible defect concentrations. In this article, we present experimental observations regarding graphene grown on single-crystalline Cu(111)/sapphire substrates. Among the graphene flakes with single-orientation, our findings reveal two major merging behaviors: one producing the expected seamless stitching, and another unexpectedly generating structural defects that create nanoscale pathways permitting water permeation. Notably, we identify a unique merging structure-overlapped junction, in which the edge of one graphene flake overlaps and lies atop the edge of another flake, rather than forming a continuous atomic stitch. This discovery challenges the conventional anticipation of single-orientation stitched graphene films as seamless single crystalline film, while offers unique perspective for graphene applications in molecular sieving, selective filtration membranes, and protective coatings.

Chemical dopants-induced phase boundary engineering has boosted electrical properties of (K, Na)NbO3-based piezoceramics, yet the underlying mechanisms governing these improvements remain unclear. Here, we elucidate these mechanisms through comprehensive multi-scale structural analysis (atomic-to-nanoscale-to-mesoscale) on two representative solid-solutions, namely (K, Na, Li)NbO3 and (K, Na)NbO3-(Bi0.5Na0.5)ZrO3. By utilizing neutron pair distribution function analysis, scanning transmission electron microscope, first-principle calculations, and phase-field simulations, our results reveal distinct atomic-scale mechanism underlying phase boundary engineering. In (K, Na, Li)NbO3, convergent off-center displacements of Li atoms induce an interplay between displacive and order-disorder phase transition; while in (K, Na)NbO3-(Bi0.5Na0.5)ZrO3, divergent off-center displacements of Bi atoms trigger a predominant order-disorder type phase transition. These atomic-scale structural characteristics directly correlate with mesoscopic ferroelectric domains and ultimately determine macroscopic electrical properties. This work elucidates the role of chemical dopants in phase boundary engineering from a multi-scale perspective, establishing a framework for designing lead-free piezoceramics with enhanced electrical properties and advancing the development of eco-friendly piezoceramics. The authors reveal distinct atomic-scale mechanisms underlying phase boundary engineering formation and explain the different electrical properties of two (K, Na)NbO3 based materials with the similar phase boundary, providing a new mechanistic insight.

Two-dimensional (2D) semiconductors have emerged as promising candidates for enabling complementary metal-oxide-semiconductor (CMOS) technology in post-silicon electronics. However, a significant performance gap between 2D p-type and n-type transistors hampers their immediate industrial application. In this Comment, we discuss recent advances in high-performance 2D p-type transistors, outline a roadmap for their potential development, and propose benchmark performance metrics to guide future progress. The performance of p-type transistors based on 2D semiconductors has not yet reached the level required for the realization of competitive complementary metal-oxide-semiconductor (CMOS) circuits. In this Comment, the authors discuss the recent developments, current challenges, and future outlook of 2D p-type transistors.

Symmetry breaking in van der Waals materials enables the realization of quantum states and advanced device functionalities. Janus transition-metal dichalcogenides (TMDs) exhibit distinctive nonlinear optical properties due to their broken out-of-plane mirror symmetry. However, the dynamic control of second harmonic generation (SHG) anisotropy and resonance behavior via optical excitation remains elusive. In this work, we investigate the SHG response of Janus MoSSe/MoS 2 heterostructures with 2H and 3R stackings. We can tune the SHG response by varying the incident photon wavelength from 800 to 1000 nm, which shows a resonance-dependent enhancement in intensity and a deviation from 6-fold symmetry, indicating wavelength-dependent anisotropy. The ratio between maximum and minimum intensity in the armchair directions, associated with the SHG anisotropy, reaches a value of 1.73 at the excitation wavelength of 1000 nm. Group theory analysis and first-principles calculations reveal that the observed anisotropy arises from optically induced strain. Our findings highlight the role of symmetry breaking and optical resonance contributing to the optomechanical tuning of SHG anisotropy, offering opportunities for developing Janus TMD-based photonic devices for frequency conversion, light generation, and optical switching.

van der Waals (vdW) heterojunctions have emerged as highly promising candidates for next-generation spintronic devices, owing to their exceptional interface quality, scalability, and tunable electronic properties. Here, we report a vdW superconducting heterojunction based on the Fe3GaTe2/CsV3Sb5/Fe3GaTe2, which combines the strong perpendicular magnetic anisotropy of Fe3GaTe2 with the superconductivity and charge density wave order of the Kagome metal CsV3Sb5. Notably, this superconducting heterojunction exhibits a magnetoresistance of 0.25% at 2 K, approximately two times higher than that observed at elevated temperatures. The magnetic proximity effect in CsV3Sb5 modulates superconductivity by suppressing spin-singlet Cooper pairing and enabling spin-triplet states. The interplay between magnetism and superconductivity not only elucidates the coexistence of competing electronic orders in CsV3Sb5 but also highlights the potential of such heterojunctions for energy-efficient, high-performance spintronic devices. Our work establishes Fe3GaTe2/CsV3Sb5/Fe3GaTe2 as a promising platform for engineering spin-polarized superconducting states and advancing quantum computing technologies.

Two-dimensional (2D) van der Waals (vdW) materials hold the potential for ultrascaled ferroelectric (FE) devices due to their silicon compatibility and robust polarization down to atomic scale. However, the inherently weak vdW interactions enable facile sliding between layers, introducing complexities beyond those encountered in conventional ferroelectric materials and presenting substantial challenges in uncovering intricate switching pathways. Here, we combine atomic-resolution imaging under in situ electrical biasing conditions with first-principles calculations to unravel the atomic-scale switching mechanisms in SnSe, a vdW group IV monochalcogenide. Our results uncover the coexistence of a consecutive 90° switching pathway and a direct 180° switching pathway from antiferroelectric (AFE) to FE order in this vdW system. Atomic-scale investigations and strain analysis reveal that the switching processes simultaneously induce interlayer sliding and compressive strain, while the lattice remains coherent despite the presence of multidomain structures. These findings elucidate vdW ferroelectric switching dynamics at atomic scale and lay the foundation for the rational design of 2D ferroelectric nanodevices.

Characterization of atomic-scale materials traditionally requires human experts with months to years of specialized training. Even for trained human operators, accurate and reliable characterization remains challenging when examining newly discovered materials such as two-dimensional (2D) structures. This bottleneck drives demand for fully autonomous experimentation systems capable of comprehending research objectives without requiring large training data sets. In this work, we present ATOMIC (Autonomous Technology for Optical Microscopy & Intelligent Characterization), an end-to-end framework that integrates foundation models to enable fully autonomous, zero-shot characterization of 2D materials. Our system integrates the vision foundation model (i.e., Segment Anything Model), large language models (i.e., ChatGPT), unsupervised clustering, and topological analysis to automate microscope control, sample scanning, image segmentation, and intelligent analysis through prompt engineering, eliminating the need for additional training. When analyzing typical MoS 2 samples, our approach achieves 99.7% segmentation accuracy for single layer identification, which is equivalent to that of human experts. In addition, the integrated model is able to detect grain boundary slits that are challenging to identify with human eyes. Furthermore, the system retains robust accuracy despite variable conditions, including defocus, color-temperature fluctuations, and exposure variations. It is applicable to a broad spectrum of common 2D materials─including graphene, MoS 2, WSe 2, SnSe─regardless of whether they were fabricated via top-down or bottom-up methods. This work represents the implementation of foundation models to achieve autonomous analysis, providing a scalable and data-efficient characterization paradigm that transforms the approach to nanoscale materials research.

Second-Harmonic Generation In their Research Article (10.1002/adpr.202500033), Kung-Hsuan Lin and co-workers show that few-layer SnSe with ferroelectric stacking exhibits second harmonic generation (SHG) efficiencies up to three orders of magnitude higher than conventional nonlinear crystals. The SHG response is highly anisotropic and strongly dependent on the excitation wavelength.

Abstract 2D Janus transition metal dichalcogenides (TMDs) are promising candidates for various applications including non‐linear optics, energy harvesting, and catalysis. These materials are usually synthesized via chemical conversion of pristine TMDs. Nanometer‐scale characterization of the obtained Janus materials’ morphology and local composition is crucial for both the synthesis optimization and the future device applications. In this work, we present the results of cross‐correlated atomic force microscopy (AFM) and tip‐enhanced Raman spectroscopy (TERS) study of Janus monolayers synthesized by the hydrogen plasma‐assisted chemical conversion of MoSe 2 and MoS 2 . We demonstrate that the choice of both the growth substrate and the starting TMD influences the residual strain, thereby shaping the nanoscale morphology of the resulting Janus material. Furthermore, by employing TERS imaging, we show the presence of nanoscale islands (≈20 nm across) of MoSe 2 ‐ (MoS 2 ‐) vertical heterostructures originating from the bilayer nanoislands in the precursor monolayer crystals. The understanding of the origins of nanoscale defects in Janus TMDs revealed in this study can help with further optimization of the Janus conversion process towards uniform and wrinkle‐/crack‐free Janus materials. Moreover, this work shows that cross‐correlated AFM and TERS imaging is a powerful and accessible method for studying nanoscale composition and defects in Janus TMD monolayers.

Electrocatalysis is key to sustainable energy conversion and storage, but its efficiency is limited by scaling laws between reactant adsorption and desorption. Multisite catalysts promises to overcome these limits, but challenges in fabrication and characterization hinder its validation. We present a platform to study and optimize multisite electrocatalysis. Leveraging van der Waals stacked 2D materials, we create catalytic edge assemblies with precise activity variations, enabling atomically engineered site separation and interaction. This approach enables the identification of multisite catalysts that enhance the hydrogen evolution reaction (HER) beyond single-site Sabatier scaling. Altering atomic-scale site separations reverts the system to single-site mechanisms, highlighting the importance of intermediate transport. Direct evidence of intermediate exchange is provided by electrostatic control of the sites, supported by ab initio simulations. We further engineer bifunctional catalysts for the oxygen evolution reaction (OER) and HER, achieving superior neutral water splitting. These findings enable the catalytic cascade design and complex electrochemical synthesis.

The second‐harmonic generation (SHG) susceptibilities of few‐layer SnSe with ferroelectric stacking are investigated using both experimental and theoretical approaches. Theoretical calculations predict a maximum bulk SHG susceptibility of 2444 pm V −1 at 1.2 eV, which is three orders of magnitude larger than that of typical nonlinear crystals. Experimentally, a maximum value of 1424 pm V −1 at 1.19 eV in close agreement with the theoretical prediction is measured. The anisotropic SHG patterns observed experimentally align with theoretical predictions based on the material's point group symmetry. The photon‐energy dependence of SHG patterns is also measured within the range of 1.19 to 1.55 eV to explore the relative strengths of various SHG susceptibilities. Notably, the measured is significantly larger than the theoretical value of bulk AC‐SnSe, likely due to the strain effects and the mixing of ferroelectric and antiferroelectric stacking configurations in the practical SnSe few‐layer samples.

Janus transition metal dichalcogenides (TMDCs) with intrinsic broken mirror symmetry and vertical dipole moment provide an additional degree of freedom to manipulate material symmetry down to atomic-layer thickness. However, despite advances in synthesis strategies, fundamental understanding of this atomic substitution process remains limited, which has impeded their implementation in advanced devices. Here, by using a room-temperature atomic-layer substitution (RT-ALS) strategy, we systematically investigate the critical factors facilitating the high-yield conversion of Janus TMDCs on diverse substrates. Combining Raman spectroscopy probes, X-ray photoelectron spectroscopy (XPS) measurements, and density functional theory (DFT) calculations, we demonstrate that substrates with enhanced electron doping or larger surface polarity substantially benefit the conversion of Janus TMDCs reaching a near-unity yield. Intriguingly, the strong affinity between Janus TMDCs and substrates (e.g., Au) brings about abnormal Raman spectroscopic phenomena. These findings highlight the significance of substrates in achieving the reliable synthesis of Janus two-dimensional materials with improved homogeneity on various substrates. In addition, this takes us one step closer to utilizing Janus TMDCs as a versatile platform in next-generation optoelectronic devices, sensors, and quantum technologies.

Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are promising candidates for various applications in non-linear optics, energy harvesting, and catalysis. These materials are usually synthesized via chemical conversion of pristine TMDs. Nanometer-scale characterization of the obtained Janus materials' morphology and local composition is crucial for both the synthesis optimization and the future device applications. In this work, we present a cross-correlated atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) study of Janus $\mathrm{Mo}_{\mathrm{Se}}^{\mathrm{S}}$ and Janus $\mathrm{Mo}_{\mathrm{S}}^{\mathrm{Se}}$ monolayers synthesized by the hydrogen plasma-assisted chemical conversion of $\mathrm{MoSe}_2$ and $\mathrm{MoS}_2$, respectively. We demonstrate how the choice of the growth substrate and the starting TMD affects the morphology of the resulting Janus material. Furthermore, by employing TERS imaging, we demonstrate the presence of nanoscale islands (~20 nm across) of $\mathrm{MoSe}_2$-$\mathrm{Mo}_{\mathrm{Se}}^{\mathrm{S}}$ ($\mathrm{MoS}_2$-$\mathrm{Mo}_{\mathrm{S}}^{\mathrm{Se}}$) vertical heterostructures originating from the bilayer nanoislands in the precursor monolayer crystals. The understanding of the origins of nanoscale defects in Janus TMDs revealed in our study can help with further optimization of the Janus conversion process towards uniform and wrinkle-/crack-free Janus materials. Moreover, our work shows that cross-correlated AFM and TERS imaging is a powerful and accessible method for studying nanoscale composition and defects in Janus TMD monolayers.

This paper studies highly scaled, multichannel MoS<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> transistors (MCTs) through both experiments and simulations. A low-temperature MoS<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> synthesis enables a novel integration scheme for MCTs on 8″ wafers with self-aligned source/drain contacts, multiple gate-dielectric-MoS<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> channel-dielectric-gate layers stacked on 8″ Si wafers, reduced lithography steps and no sacrificial layers. A 1-channel, double-gate MoS<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> transistor features <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$386\ \mu \mathrm{A}/\mu \mathrm{m}$</tex> on current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{I}_{\text{ON}})$</tex> at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{V}_{\text{DS}}=1\ \mathrm{V}$</tex>, subthreshold swing SS = 85 mV/dec, and record performance among 2D gate-all-around (GAA) transistors and MCTs. A low drain-induced-barrier-lowering DIBL = 28 mV/V is also demonstrated. We also report the first functional high performance 2-channel MoS<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> transistor with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$673\ \mu \mathrm{A}/\mu \mathrm{m}\ \mathrm{I}_{\text{ON}}$</tex> and 88 mV/dec SS at 400 nm channel length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{L}_{\text{Ch}})$</tex>. In parallel, local-back gate transistors with 1.5 nm physical oxide thickness and 4 nm physical channel length were fabricated to calibrate the device model at extreme scales for design-technology co-optimization (DTCO). Thanks to the DTCO analysis, a projection of multi-channel transistor's scaling, including power, and performance for “1 nm” technology node and beyond is presented.

In this work, we report the graphene-promoted formation of an interfacial oxide layer when certain metals are deposited on graphene. We probe interfacial oxide formation through the observation that several metals, when 10–12 nm in thickness and deposited on graphene on a transparent substrate, show a change in optical contrast compared to that in areas where the metal directly contacts the substrate. Aluminum shows this effect, while platinum and nickel do not exhibit such a pronounced optical contrast change. To understand this phenomenon, we characterize the Al-graphene, Ti-graphene, and Ni-graphene interfaces using techniques including X-ray photoelectron spectroscopy depth profiling, X-ray reflectivity, and Raman spectroscopy. These techniques show the presence of oxide at the buried metal–graphene interface for the cases of aluminum and titanium deposition, and we discuss how this explains the change in optical contrast. We show that this process is sensitive to the background vacuum level during deposition. In the case of nickel, we did not observe the presence of an oxide. Building upon these findings, we propose structures for Al-graphene, Ti-graphene, and Ni-graphene interfaces. We propose a model based on the metal work function and interaction with graphene that can guide the metals for which interfacial oxidation is to be expected, and we discuss the role of the deposition conditions in controlling the extent of oxide formation. These observations provide important implications for various devices using graphene as either the channel or the contact. Depending on whether a metal–graphene interfacial oxide is desirable and its functionality, these findings will afford guidance for their fabrications in the future.

Ternary content-addressable memory (TCAM) is promising for data-intensive artificial intelligence applications due to its large-scale parallel in-memory computing capabilities. However, it is still challenging to build a reliable TCAM cell from a single circuit component. Here, we demonstrate a single transistor TCAM based on a floating-gate two-dimensional (2D) ambipolar MoTe 2 field-effect transistor with graphene contacts. Our bottom graphene contacts scheme enables gate modulation of the contact Schottky barrier heights, facilitating carrier injection for both electrons and holes. The 2D nature of our channel and contact materials provides device scaling potentials beyond silicon. By integration with a floating-gate stack, a highly reliable nonvolatile memory is achieved. Our TCAM cell exhibits a resistance ratio larger than 1000 and symmetrical complementary states, allowing the implementation of large-scale TCAM arrays. Finally, we show through circuit simulations that in-memory Hamming distance computation is readily achievable based on our TCAM with array sizes up to 128 cells.

The controlled vapor-phase synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is essential for functional applications. While chemical vapor deposition (CVD) techniques have been successful for transition metal sulfides, extending these methods to selenides and tellurides often faces challenges due to uncertain roles of hydrogen (H 2 ) in their synthesis. Using CVD growth of MoSe 2 as an example, this study illustrates the role of a H 2 -free environment during temperature ramping in suppressing the reduction of MoO 3, which promotes effective vaporization and selenization of the Mo precursor to form MoSe 2 monolayers with excellent crystal quality. As-synthesized MoSe 2 monolayer-based field-effect transistors show excellent carrier mobility of up to 20.9 cm 2 /(V·s) with an on–off ratio of 7 × 10 7 . This approach can be extended to other TMDs, such as WSe 2, MoTe 2, and MoSe 2 /WSe 2 in-plane heterostructures. Our work provides a rational and facile approach to reproducibly synthesize high-quality TMD monolayers, facilitating their translation from laboratory to manufacturing.

The optimization of nanoscale optical devices and structures will enable the exquisite control of planar optical fields. Polariton manipulation is the primary strategy in play. In two-dimensional heterostructures, the ability to excite mixed optical modes offers an additional control in device design. Phonon polaritons in hexagonal boron nitride have been a common system explored for the control of near-infrared radiation. Their hybridization with graphene plasmons makes these mixed phonon polariton modes in hexagonal boron nitride more appealing in terms of enabling active control of electrodynamic properties with a reduction of propagation losses. Optical resonators can be added to confine these hybridized plasmon–phonon polaritons deeply into the subwavelength regime, with these structures featuring high quality factors. Here, we show a scalable approach for the design and fabrication of heterostructure nanodisc resonators patterned in chemical vapor deposition-grown monolayer graphene and h -BN sheets. Real-space mid-infrared nanoimaging reveals the nature of hybridized polaritons in the heterostructures. We simulate and experimentally demonstrate localized hybridized polariton modes in heterostructure nanodisc resonators and demonstrate that those nanodiscs can collectively couple to the waveguide. High quality factors for the nanodiscs are measured with nanoscale Fourier transform infrared spectroscopy. Our results offer practical strategies to realize scalable nanophotonic devices utilizing low-loss hybridized polaritons for applications such as on-chip optical components.

One of the exotic expectations in the 2D curved spacetime is the geometric potential from the curvature of the 2D space, still possessing unsolved fundamental questions through Dirac quantization. The atomically thin 2D materials are promising for the realization of the geometric potential, but the geometric potential in 2D materials is not identified experimentally. Here, the curvature-induced ring-patterned bound states are observed in structurally deformed 2D semiconductors and formulated the modified geometric potential for the curvature effect, which demonstrates the ring-shape bound states with angular momentum. The formulated modified geometric potential is analogous to the effective potential of a rotating charged black hole. Density functional theory and tight-binding calculations are performed, which quantitatively agree well with the results of the modified geometric potential. The modified geometric potential is described by modified Gaussian and mean curvatures, corresponding to the curvature-induced changes in spin-orbit interaction and band gap, respectively. Even for complex structural deformation, the geometric potential solves the complexity, which aligns well with experimental results. The understanding of the modified geometric potential provides us with an intuitive clue for quantum transport and a key factor for new quantum applications such as valleytronics, spintronics, and straintronics in 2D semiconductors.

Cardiac microtissues provide a promising platform for disease modeling and developmental studies, which require the close monitoring of the multimodal excitation-contraction dynamics. However, no existing assessing tool can track these multimodal dynamics across the live tissue. We develop a tissue-like mesh bioelectronic system to track these multimodal dynamics. The mesh system has tissue-level softness and cell-level dimensions to enable stable embedment in the tissue. It is integrated with an array of graphene sensors, which uniquely converges both bioelectrical and biomechanical sensing functionalities in one device. The system achieves stable tracking of the excitation-contraction dynamics across the tissue and throughout the developmental process, offering comprehensive assessments for tissue maturation, drug effects, and disease modeling. It holds the promise to provide more accurate quantification of the functional, developmental, and pathophysiological states in cardiac tissues, creating an instrumental tool for improving tissue engineering and studies.

2D metal chalcogenides (MCs) have garnered significant attention from both scientific and industrial communities due to their potential in developing next-generation functional devices. Vapor-phase deposition methods have proven highly effective in fabricating high-quality 2D MCs. Nevertheless, the conventionally high thermal budgets required for synthesizing 2D MCs pose limitations, particularly in the integration of multiple components and in specialized applications (such as flexible electronics). To overcome these challenges, it is desirable to reduce the thermal energy requirements, thus facilitating the growth of various 2D MCs at lower temperatures. Numerous endeavors have been undertaken to develop low-temperature vapor-phase growth techniques for 2D MCs, and this review aims to provide an overview of the latest advances in low-temperature vapor-phase growth of 2D MCs. Initially, the review highlights the latest progress in achieving high-quality 2D MCs through various low-temperature vapor-phase techniques, including chemical vapor deposition (CVD), metal-organic CVD, plasma-enhanced CVD, atomic layer deposition (ALD), etc. The strengths and current limitations of these methods are also evaluated. Subsequently, the review consolidates the diverse applications of 2D MCs grown at low temperatures, covering fields such as electronics, optoelectronics, flexible devices, and catalysis. Finally, current challenges and future research directions are briefly discussed, considering the most recent progress in the field.

Van der Waals (vdW) ferroelectrics have attracted significant attention for their potential in next-generation nano-electronics. Two-dimensional (2D) group-IV monochalcogenides have emerged as a promising candidate due to their strong room temperature in-plane polarization down to a monolayer limit. However, their polarization is strongly coupled with the lattice strain and stacking orders, which impact their electronic properties. Here, we utilize four-dimensional scanning transmission electron microscopy (4D-STEM) to simultaneously probe the in-plane strain and out-of-plane stacking in vdW SnSe. Specifically, we observe large lattice strain up to 4% with a gradient across ~50 nm to compensate lattice mismatch at domain walls, mitigating defects initiation. Additionally, we discover the unusual ferroelectric-to-antiferroelectric domain walls stabilized by vdW force and may lead to anisotropic nonlinear optical responses. Our findings provide a comprehensive understanding of in-plane and out-of-plane structures affecting domain properties in vdW SnSe, laying the foundation for domain wall engineering in vdW ferroelectrics.

Our previous study observed the localization-delocalization transition and critical quantum fluctuations of the local density of states (LDOS) on the structurally disordered two-dimensional (2D) semiconductor $\mathrm{Mo}{\mathrm{S}}_{2}$. This transition corresponds to the metal-insulator transition (MIT) reported in transport measurements. The structural disorder in $\mathrm{Mo}{\mathrm{S}}_{2}$ caused curvature-induced band gap fluctuations, leading to charge localization and unusual band edge flattening through doping. The critical behavior for the MIT was analyzed using autocorrelation and multifractality of LDOS mapping results. However, the effect of structural disorder on critical points has not been fully explored. Here, we systematically investigated the impact of structural disorder on band tail formation and critical doping concentration by examining the radial-averaged autocorrelation and multifractality of LDOS in 2D semiconductors. Our finding indicates that the radial-averaged autocorrelation and multifractality of LDOS characterize the band tail ranges and band edge flattening in disordered 2D semiconductors. Decaying regions in the radial-averaged autocorrelation profile and first-order derivative of singularity peak positions determine band tail ranges. Increased structural disorder led to larger band tail widths near valence and conduction band edges, while the doping-induced band edge flattening altered band tail widths for each valence and conduction band. As the band edge is flattened due to doping, the LDOS map near the critical energy becomes uniform, exhibiting a divergence in the localization length. The average value of conduction and valence band tail widths remained almost constant regardless of doping control, serving as a representative value for the degree of structural disorder. For the MIT, we found that the critical doping concentration depends on the degree of structural disorder in 2D semiconductors. Our findings provide valuable insights into the fundamental physics of structurally disordered 2D semiconductors in relevance to quantum phase transitions, which could have important implications for designing and optimizing electronic/optoelectronic devices based on 2D materials.

Nonlinear optical responses in second harmonic generation (SHG) of van der Waals heterobilayers, Janus MoSSe/MoS 2, are theoretically optimized as a function of strain and stacking order by adopting an exchange-correlation hybrid functional and a real-time approach in first-principles calculation. We find that the calculated nonlinear susceptibility, χ (2), in AA stacking (550 pm/V) becomes three times as large as AB stacking (170 pm/V) due to the broken inversion symmetry in the AA stacking. The present theoretical prediction is compared with the observed SHG spectra of Janus MoSSe/MoS 2 heterobilayers, in which the peak SHG intensity of AA stacking becomes four times as large as AB stacking. Furthermore, a relatively large, two-dimensional strain (4%) that breaks the C 3 v point group symmetry of the MoSSe/MoS 2, enhances calculated χ (2) values for both AA (900 pm/V) and AB (300 pm/V) stackings 1.6 times as large as that without strain.

Nanoporous graphene has shown great promise for membrane separations. Its atomic thickness, remarkable mechanical, chemical, and thermal robustness could enable ultrahigh-flux membrane processes addressing persistent challenges in a wide range of separation needs. On the other hand, molecular separations across nanoporous graphene rely dominantly on size-based mechanisms (e.g, size exclusion) that is highly sensitive to the nanopore size and size distribution of nanopore ensemble created on the graphene. However, among existing nanopore creation methods, the process of nanopore nucleation is often coupled with nanopore expansion, which results in a lognormal nanopore size distribution with a long tail. It remains a challenge to obtain both high density and narrow size distribution of nanopores in graphene. Here, we report a cascaded compression approach to engineering nanopores in monolayer graphene for molecular separations with the assistance of electrical control of chemical vapor deposition of graphene[1]. 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 of expansion, a new batch of nanopores is created, leading to increased nanopore density by each cycle. As a result, high-density nanopores with a short-tail size distribution are obtained by the cascaded compression that show high rejection and ultrafast organic solvent nanofiltration exceeding the state-of-the-art. Reference [1] Wang J, Park J H, Lu A Y, et al. Electrical Control of Chemical Vapor Deposition of Graphene. Journal of the American Chemical Society, 20

Chemical vapor deposition (CVD) is widely used for the efficient growth of low dimensional materials. Typically, the synthesis parameters such as gas flow rate, temperature, pressure etc. are being optimized in order to achieve desirable results such as high-quality materials. Up to now, gas phase electro-chemical reactions have not been widely considered in CVD. In this talk, our explorations of using applied voltages to control the CVD synthesis results will be presented. We have found that applying an electric field between the copper substrate and a counter electrode has significant impacts on the growth of graphene. Electrochemical effect and ionic collision effect are observed in different conditions. Switching electric field can also be used to twist the chirality of carbon nanotubes. It is anticipated that electrical control during CVD synthesis could open up new ways to assist the synthesis of low dimensional materials in the future.

Abstract Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and certain topological properties, such as the Berry curvature and the quantum metric tensor, have attracted considerable interest. Here, we report giant room-temperature nonlinearities in non-centrosymmetric two-dimensional topological materials—the Janus transition metal dichalcogenides in the 1 T’ phase, synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently show orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities in 1 T’ MoSSe (e.g., &gt; 50 times higher than 2 H MoS 2 for 18 th order harmonic generation; &gt; 20 times higher than 2 H MoS 2 for terahertz emission). We link this giant nonlinear optical response to topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and heralds the applications of topological materials in optoelectronics down to the monolayer limit.