2D Atomic Crystals Research Articles

Van der Waals epitaxial growth of single-crystal molecular film.

Epitaxy is the cornerstone of semiconductor technology, enabling the fabrication of single-crystal film. Recent advancements in van der Waals (vdW) epitaxy have opened new avenues for producing wafer-scale single-crystal 2D atomic crystals. However, when it comes to molecular crystals, the overall weak vdW force means that it is a significant challenge for small molecules to form a well-ordered structure during epitaxy. Here we demonstrate that the vdW epitaxy of Sb2O3 molecular crystal, where the whole growth process is governed by vdW interactions, can be precisely controlled. The nucleation is deterministically modulated by epilayer-substrate interactions and unidirectional nuclei are realized through designing the lattice and symmetry matching between epilayer and substrate. Moreover, the growth and coalescence of nuclei as well as the layer-by-layer growth mode are kinetically realized via tackling the Schwoebel-Ehrlich barrier. Such precise control of vdW epitaxy enables the growth of single-crystal Sb2O3 molecular film with desirable thickness. Using the ultrathin highly oriented Sb2O3 film as a gate dielectric, we fabricated MoS2-based field-effect transistors that exhibit superior device performance. The results substantiate the viability of precisely managing molecule alignment in vdW epitaxy, paving the way for large-scale synthesis of single-crystal 2D molecular crystals.

2D Atomic-Molecular Heterojunctions toward Brainoid Applications.

Brainoid computing using 2D atomic crystals and their heterostructures, by emulating the human brain's remarkable efficiency and minimal energy consumption in information processing, poses a formidable solution to the energy-efficiency and processing speed constraints inherent in the von Neumann architecture. However, conventional 2D material based heterostructures employed in brainoid devices are beset with limitations, performance uniformity, fabrication intricacies, and weak interfacial adhesion, which restrain their broader application. The introduction of novel 2D atomic-molecular heterojunctions (2DAMH), achieved through covalent functionalization of 2D materials with functional molecules, ushers in a new era for brain-like devices by providing both stability and tunability of functionalities. This review chiefly delves into the electronic attributes of 2DAMH derived from the synergy of polymer materials with 2D materials, emphasizing the most recent advancements in their utilization within memristive devices, particularly their potential in replicating the functionality of biological synapses. Despite ongoing challenges pertaining to precision in modification, scalability in production, and the refinement of underlying theories, the proliferation of innovative research is actively pursuing solutions. These endeavors illuminate the vast potential for incorporating 2DAMH within brain-inspired intelligent systems, highlighting the prospect of achieving a more efficient and energy-conserving computing paradigm.

Excitons at the interface of 2D TMDs and molecular semiconductors.

Van der Waals heterostructures (vdWHs) of vertically stacked two-dimensional (2D) atomic crystals have been used to elicit intriguing phenomena stemming from strong electronic correlations, magnetic textures, and interlayer excitons spawned at the heterointerface. However, vdWHs comprised of heterointerfaces between these 2D atomic crystal lattices and molecular assemblies are emerging as equally intriguing platforms supporting properties to be harnessed for photovoltaic energy conversion, photodetection, spin-selective charge injection, and quantum emission. In this perspective, we summarize recent research examining exciton dynamics in heterostructures between semiconducting 2D transition metal dichalcogenides and molecular organic semiconductors. We discuss methods for assembly of these heterostructures, the nature of interlayer or charge-transfer excitons at transition-metal dichalcogenide (TMD)-molecule interfaces, explicit exciton transfer between organics and TMDs, and other interfacial phenomena driven by the merger of these two material classes. We also suggest key new research directions extending the remit of these 2D atomic-molecular lattice heterointerfaces into the domains of condensed matter physics, quantum sensing, and energy conversion.

Dynamic strain coupling driven by structural phase transition in mixed-dimensional 2H-MoS2/VO2 van der Waals heterointerfaces

Mixed-dimensional van der Waals (vdW) heterostructures, integrated two-dimensional (2D) atomic crystals with three-dimensional (3D) functional materials, offer a powerful means to manipulating physical properties and generating unprecedented functionalities. Understanding interfacial couplings at those hetero (homo)-interfaces is indispensable for exploring new optical and electronic devices. Herein, we investigated dynamically phase-transition-driven strain coupling across a vdW heterointerface through integrating 2D layered 2H-MoS2 nanoflakes onto 3D phase-change VO2 epitaxial thin films. The Raman peak positions of the in-plane and out-of-plane vibration modes E2g1 and A1g from the 2H-MoS2 nanoflakes show a phonon softening and reversible hysteresis loop as a function of temperature in this mixed-dimensional vdW 2H-MoS2/(1¯11)-VO2/(11¯02)-Al2O3 heterostructure, originating from the co-action of temperature-dependent anharmonicity in 2H-MoS2 and reversible structural phase transition (SPT)-induced in-plane tensile strain from the VO2 thin film. Accordingly, the integrated Raman scattering intensity of these two feature peaks of the 2H-MoS2 nanoflakes increased (decreased) as the temperature increased (decreased), exhibiting a hysteresis loop in the SPT and metal–insulator transition region of VO2. Additionally, the peak integrated intensity enhancement ratio of the E2g1 and A1g vibration modes was approximately 2.3 and 2.8, respectively. These results indicate that the dynamically SPT-driven in-plane tensile strain from the bottom VO2 layer interfacially couples with the adjacent 2H-MoS2 nanoflakes and results in a reduction in the electronic transition energy, leading to an enhancement in the Raman scattering intensity of 2H-MoS2. Our work holds promise for dynamic strain control of lattice dynamics and electron–phonon interaction of 2D materials for functional electronic and photoelectronic devices.

Atomic engineering of two-dimensional materials via liquid metals

The usage of liquid metal or alloy enables the fabrication of various 2D atomic crystals. This review fully examines the synthetic methods, condition-behavior relationship, and atomic engineering in this field, emphasizing a promising future.

Experimental Realization of Monolayer α-Tellurene.

2D materials emerge as a versatile platform for developing next-generation devices. The experimental realization of novel artificial 2D atomic crystals, which does not have bulk counterparts in nature, is still challenging and always requires new physical or chemical processes. Monolayer α-tellurene is predicted to be a stable 2D allotrope of tellurium (Te), which has great potential for applications in high-performance field-effect transistors. However, the synthesis of monolayer α-tellurene remains elusive because of its complex lattice configuration, in which the Te atoms are stacked in tri-layers in an octahedral fashion. Here, a self-assemble approach, using three atom-long Te chains derived from the dynamic non-equilibrium growth of an a-Si:Te alloy as building blocks, is reported for the epitaxial growth of monolayer α-tellurene on a Sb2 Te3 substrate. By combining scanning tunneling microscopy/spectroscopy with density functional theory calculations, the surface morphology and electronic structure of monolayer α-tellurene are revealed and the underlying growth mechanism is determined. The successful synthesis of monolayer α-tellurene opens up the possibility for the application of this new single-element 2D material in advanced electronic devices.

Physically informed machine-learning algorithms for the identification of two-dimensional atomic crystals

After graphene was first exfoliated in 2004, research worldwide has focused on discovering and exploiting its distinctive electronic, mechanical, and structural properties. Application of the efficacious methodology used to fabricate graphene, mechanical exfoliation followed by optical microscopy inspection, to other analogous bulk materials has resulted in many more two-dimensional (2D) atomic crystals. Despite their fascinating physical properties, manual identification of 2D atomic crystals has the clear drawback of low-throughput and hence is impractical for any scale-up applications of 2D samples. To combat this, recent integration of high-performance machine-learning techniques, usually deep learning algorithms because of their impressive object recognition abilities, with optical microscopy have been used to accelerate and automate this traditional flake identification process. However, deep learning methods require immense datasets and rely on uninterpretable and complicated algorithms for predictions. Conversely, tree-based machine-learning algorithms represent highly transparent and accessible models. We investigate these tree-based algorithms, with features that mimic color contrast, for automating the manual inspection process of exfoliated 2D materials (e.g., MoSe2). We examine their performance in comparison to ResNet, a famous Convolutional Neural Network (CNN), in terms of accuracy and the physical nature of their decision-making process. We find that the decision trees, gradient boosted decision trees, and random forests utilize physical aspects of the images to successfully identify 2D atomic crystals without suffering from extreme overfitting and high training dataset demands. We also employ a post-hoc study that identifies the sub-regions CNNs rely on for classification and find that they regularly utilize physically insignificant image attributes when correctly identifying thin materials.

Layered Semiconductor Cr0.32Ga0.68Te2.33 with Concurrent Broken Inversion Symmetry and Ferromagnetism: A Bulk Ferrovalley Material Candidate.

The valleytronic state found in group-VI transition-metal dichalcogenides such as MoS2 has attracted immense interest since its valley degree of freedom could be used as an information carrier. However, valleytronic applications require spontaneous valley polarization. Such an electronic state is predicted to be accessible in a new ferroic family of materials, i.e., ferrovalley materials, which features the coexistence of spontaneous spin and valley polarization. Although many atomic monolayer materials with hexagonal lattices have been predicted to be ferrovalley materials, no bulk ferrovalley material candidates have been reported or proposed. Here, we show that a new non-centrosymmetric van der Waals (vdW) semiconductor Cr0.32Ga0.68Te2.33, with intrinsic ferromagnetism, is a possible candidate for bulk ferrovalley material. This material exhibits several remarkable characteristics: (i) it forms a natural heterostructure between vdW gaps, a quasi-two-dimensional (2D) semiconducting Te layer with a honeycomb lattice stacked on the 2D ferromagnetic slab comprised of the (Cr, Ga)-Te layers, and (ii) the 2D Te honeycomb lattice yields a valley-like electronic structure near the Fermi level, which, in combination with inversion symmetry breaking, ferromagnetism, and strong spin-orbit coupling contributed by heavy Te element, creates a possible bulk spin-valley locked electronic state with valley polarization as suggested by our DFT calculations. Further, this material can also be easily exfoliated to 2D atomically thin layers. Therefore, this material offers a unique platform to explore the physics of valleytronic states with spontaneous spin and valley polarization in both bulk and 2D atomic crystals.

Van der Waals integration of artificial heterostructures and high-order superlattices

<p indent="0mm">The recent blossom in 2D atomic crystals (2DACs) has ignited intense interest in a new type of bond-free van der Waals heterostructures (vdWHs), in which distinct material components are physically brought together within a vdW distance and held together by weak vdW interactions. Without direct chemical bonding between the constituent materials, the vdWHs negate the lattice matching requirements in typical epitaxially bonded heterostructures. Here we briefly summarize the key advances in the construction and fundamental investigation of versatile vdWHs from diverse 2DACs and beyond, and highlight a unique class of vdW superlattices (vdWSLs) consisting of alternating 2D atomic layers and/or self-assembled molecular layers, with tailored structural symmetry, electronic band modulation, interlayer coupling, and chirality. Lastly, we conclude with a brief outlook on the opportunities in exploring such artificial materials to unlock previously inaccessible physical limits and enable new device concepts beyond the reach of the existing materials.

Using in-plane anisotropy to engineer Janus monolayers of rhenium dichalcogenides

The new class of Janus two-dimensional (2D) transition-metal dichalcogenides with two different interfaces are currently gaining increasing attention due to their distinct properties different from the typical 2D materials. Here, we show that in-plane anisotropy of a 2D atomic crystal, like ReS$_{2}$ or ReSe$_{2}$, allows formation of a large number of inequivalent Janus monolayers. We use first-principles calculations to investigate the structural stability of 29 distinct ReX$_{2-x}$Y$_{x}$ ($\mathrm{X,Y \in \{S,Se\}}$) structures, which can be obtained by selective exchange of exposed chalcogens in a ReX$_{2}$ monolayer. We also examine the electronic properties and work function of the most stable Janus monolayers and show that the large number of inequivalent structures provides a way to engineer spin-orbit splitting of the electronic bands. We find that the breaking of inversion symmetry leads to sizable spin splittings and spontaneous diople moments than are larger than those in other Janus dichalcogenides. Moreover, our caluclations suggest that the work function of the Janus monolayers can be tuned by varying the content of the substituting chalcogen. Our work demonstrates that in-plane anisotropy provides additional flexibility in sub-layer engineering of 2D atomic crystals.

Single-Atom Tailoring of Two-Dimensional Atomic Crystals Enables Highly Efficient Detection and Pattern Recognition of Chemical Vapors

Low-dimensional semiconductor materials, such as single-walled carbon nanotubes, two-dimensional (2D) atomic crystals, and organic frameworks, have been widely adapted as ideal platforms to construct various chemo/biosensors with satisfying sensitivity. However, the general drawbacks in chemiresistive devices, including high operation temperatures, low response to low-polarity molecules, and poor selectivity, have limited their real-world applications. In this study, 2D materials (graphene, MoS2, and WSe2) were systematically functionalized with series of monodispersed single atomic sites (Pt, Co, and Ru) through a facile approach to construct single-atom sensors (SASs) for the detection of VOCs at room temperature. The structural and catalytic characteristics of SAs successfully translated into enhanced gas-sensing performance, with a 1-2 orders of magnitude increase in relative response to ethanol (@5 ppm) and acetone (@20 ppm) vapors (in all M-2D SASs as compared to pristine substrates), high selectivity to VOCs against relative humidity (M-WSe2 SASs), and fast response/recovery time (11/58 s for Pt-Graphene and 22/48 s for Pt-MoS2 to 50 ppm ethanol, 9/57 s for Pt-Graphene and 15/75 s for Pt-MoS2 to 200 ppm acetone) that are several times faster than the pristine 2D materials. Density functional theory (DFT) calculations revealed the signaling mechanism in SASs, and the data were further trained to build machine learning (ML) models for predicting the adsorption energies and sensing performance using the features of adsorption heights, metal charge, and charge transfer between the adsorbed VOCs and SASs sites. Finally, the rich combination of the metal single atoms and 2D atomic crystal supports were converted to cross-sensitive SA sensor array that allows for detection and identification of different VOCs.

Local phase transition at crack edges of Mo1−xWxTe2 polymorphs

The phase engineering of two-dimensional (2D) materials has attracted considerable attention in recent years, but nanometer-scale samples and experimental techniques have limited the rigorous study of the local phase engineering of 2D atomic crystals. Here, we report on a nanometer-scale probe of phase evolution in the Mo1−xWxTe2 alloys, which exhibit a structural transition between the semiconducting 2H phase and the metallic 1T′ phase. We found that many crack edges are created by the accumulated tungsten atoms, resulting in poor crystallinity in the 2H-Mo1−xWxTe2 flakes at the vicinity of phase transition with a tungsten content, x = 0.08. With mild laser irradiation of the 2H-Mo1−xWxTe2 flakes, the 2H phase region near a crack edge was easily converted to the 1T′ phase, while other areas apart from the crack edge region retained their 2H phase under the same laser treatment. Our study clarifies the role of locally accumulated atoms and crystallinity for the phase engineering of 2D materials, which opens the way to various applications based on heterophase structures, ranging from energy conversion and storage to optoelectronic devices.

Endoepitaxial growth of monolayer mosaic heterostructures.

The controllable growth of two-dimensional (2D) heterostructure arrays is critical for exploring exotic physics and developing novel devices, yet it remains a substantial synthetic challenge. Here we report a rational synthetic strategy to fabricate mosaic heterostructure arrays in monolayer 2D atomic crystals. By using a laser-patterning and an anisotropic thermal etching process, we create periodic triangular hole arrays in 2D crystals with precisely controlled size and atomically clean edges, which function as robust templates for endoepitaxial growth of another 2D crystal, to obtain monolayer mosaic heterostructures with atomically sharp heterojunction interfaces. Systematic microstructure and spectroscopic characterizations reveal periodic modulation of chemical compositions, lattice strains and electronic band gaps throughout the mosaic heterostructures. The robust growth of the monolayer mosaic heterostructures with a high level of synthetic control opens a pathway for band structure engineering and spatially modulating the potential landscapes in the atomically thin 2D crystals, establishing a designable material platform for fundamental studies and development of complex devices and integrated circuits from 2D heterostructures.

Plasmon-induced thermal tuning of few-exciton strong coupling in 2D atomic crystals

Strong light–matter interaction in 2D materials at the few-exciton level is important for both fundamental studies and quantum optical applications. Characterized by a fast coherent energy exchange between photons and excitons, strongly coupled plasmon–exciton systems in 2D materials have been reported with large Rabi splitting. However, large Rabi splitting at the few-exciton level generally requires large optical fields in a highly confined mode volume, which are difficult to achieve for in-plane excitons in 2D materials. In this work, we present a study of a strongly coupled gold dimer antenna with a sub-10 nm gap on a monolayer tungsten disulphide ( W S 2 ), with an estimated number of excitons of 4.67 ± 0.99 . We demonstrate that varying the spatial mode overlap between the plasmonic field and the 2D material can result in up to a ∼ t e n f o l d increase in the number of excitons, a value that can be further actively tuned via plasmon-induced heating effects. The demonstrated results would represent a key step toward quantum optical applications operating at room temperatures.

Sensing by Surface Work Function Modulation: High Performance Gas Sensing using van der Waals Stacked Bipolar Junction Transistor

We propose and demonstrate a new principle for sensing. Impact of light, radiation, charge particle and gas molecular etc. can induce the change of the surface work function of a solid. This will result in a profound effect to a device’s output, such as current and voltage. Such an effect is in particular outstanding in a nanodevice using van der Waals stacked two-dimensional (2D) atomic crystals. In this paper, a study of a gas sensing device relying on such a principle is presented. Van der Waals bipolar junction transistors (V2D-BJT) based on vertically stacked MoS2/WSe2/MoS2 and WSe2/MoS2/WSe2 heterostructures are separately fabricated to detect NH3 and NO2 gas. Kelvin probe force microscope measurement shows that the work function change at the overlapped region of MoS2/WSe2/MoS2 heterostructure upon exposure to 50 ppm NH3 gas is 80 meV. In addition, the MoS2/WSe2/MoS2 V2D-BJT manifest superior sensing performances in detecting NH3, while the WSe2/MoS2/WSe2 V2D-BJT exhibited superior NO2 sensing properties in terms of high sensitivity (18), low power dissipation (12 nW), fast response (26 s) and recovery (14 s) time. We demonstrate that the NH3 and NO2 gas can be selectively detected by the V2D-BJT through changing the van der Waals heterostructure stacking sequences.

Strain-Engineered Rippling and Manipulation of Single-Layer WS2 by Atomic Force Microscopy

Surface ripple, as an important factor of corrugations in two-dimensional (2D) atomic crystals, plays important roles in determining their mechanical and physical properties. Here, we systematicall...

Electrical Switch of Poisson's Ratio in IV-VI Monolayers via Pseudophase Transitions.

The recent discovery of negative Poisson's ratio (NPR) in two-dimensional (2D) atomic crystals has stimulated extensive research interest in their intriguing physical properties. Here, via density functional theory (DFT) calculations, we reveal in the family of 2D IV-VI semiconductors that an iso-symmetry structure variation concerning the switch of the cation (IV) versus anion (VI) in the outermost layers leads to the change of sign of Poisson's ratio. Such iso-symmetry structural pseudo-phase transition can be induced by external strains, as well as electric fields, realizing the possibility of an electrically switchable Poisson effect. The phase transition process could involve a dynamic intermediate state with an alternative cation/anion switch in the frequencies of 2-3 THz according to the real-time time-dependent DFT (rt-TDDFT) calculations. The results open the way for studying pseudophases in 2D materials associated with sharply different physical properties, such as Poisson's ratio, for electromechanical and optoelectronic applications.

Molecular Dynamics Simulations Reveal Orientation-Dependent Nanotoxicity of Black Phosphorene toward Dimeric Proteins

Phosphorene, a monolayer of elemental phosphorus, has emerged as one of the most significant two-dimensional (2D) atomic crystals in the post-graphene era, as a potential candidate for semiconducto...

Two-dimensional aluminium, gallium, and indium metallic crystals by first-principles design

Rapidly emerging two-dimensional (2D) atomic layer crystals exhibit diverse, tunable electronic properties. They appear to be more flexible than 3D crystals with greater versatility and improved functionality in a wide range of potential applications. Among these 2D materials, metallic crystals are relatively unexplored although two allotropes of gallenene (2D gallium) have been synthesized on a range of substrates. Based on these experimental findings, we investigate systematically the group 13 metals using first-principles density functional theory calculations and an unbiased structural search. In this study, the electronic structure, bonding characteristics, and phonon properties of predicted 2D allotropes of group 13 metals are calculated, including the expected effects of strain induced by substrates on the dynamical stability. Theoretical results predict that most group 13 elements have one or more stable 2D allotropes with the preferred allotrope depending on the cell shape relaxation and strain, indicating that the substrate will determine the overall allotrope preferred. This demonstrates a new avenue for the discovery of thermodynamically stable 2D metallic layers, with properties potentially suitable for electronic and optoelectronic applications.

Recent Development of Gas Sensing Platforms Based on 2D Atomic Crystals.

Sensors, capable of detecting trace amounts of gas molecules or volatile organic compounds (VOCs), are in great demand for environmental monitoring, food safety, health diagnostics, and national defense. In the era of the Internet of Things (IoT) and big data, the requirements on gas sensors, in addition to sensitivity and selectivity, have been increasingly placed on sensor simplicity, room temperature operation, ease for integration, and flexibility. The key to meet these requirements is the development of high-performance gas sensing materials. Two-dimensional (2D) atomic crystals, emerged after graphene, have demonstrated a number of attractive properties that are beneficial to gas sensing, such as the versatile and tunable electronic/optoelectronic properties of metal chalcogenides (MCs), the rich surface chemistry and good conductivity of MXenes, and the anisotropic structural and electronic properties of black phosphorus (BP). While most gas sensors based on 2D atomic crystals have been incorporated in the setup of a chemiresistor, field-effect transistor (FET), quartz crystal microbalance (QCM), or optical fiber, their working principles that involve gas adsorption, charge transfer, surface reaction, mass loading, and/or change of the refractive index vary from material to material. Understanding the gas-solid interaction and the subsequent signal transduction pathways is essential not only for improving the performance of existing sensing materials but also for searching new and advanced ones. In this review, we aim to provide an overview of the recent development of gas sensors based on various 2D atomic crystals from both the experimental and theoretical investigations. We will particularly focus on the sensing mechanisms and working principles of the related sensors, as well as approaches to enhance their sensing performances. Finally, we summarize the whole article and provide future perspectives for the development of gas sensors with 2D materials.