Two-dimensional Atomic Crystals Research Articles

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.

Graphene-based spintronics

Graphene, the first isolated two-dimensional atomic crystal, is about to pass its 20th year. The last decade has been a critical period for graphene to gradually move from the laboratory to practical applications, and the research on the spin-related physical properties and various spintronic applications of graphene is still enduring. In this review, we systematically retrospect the important and state-of-art progresses about graphene-based spintronics. First, spin–orbit coupling and various tuning means in graphene have been introduced, such as adatoms, electrical control, and the proximity effect. Second, several methods for inducing magnetism in graphene are summarized, including defect, atom doping, proximity effect, and the recently attractive twisted magic-angle. Third, graphene-based lateral and vertical spin valves are discussed, along with some emergent spin transport properties, including spin injection, scattering, and relaxation. Fourth, graphene-based spin logic circuits for spin communications and multifunctional spin logic devices are exhibited. Finally, some significant opportunities and challenges of graphene-based spintronics for the fundamental physics and practical applications in the future are briefly discussed.

Exciton states in molecular superlattices

The structure of molecular superlattices consists of van der Waals heterostructures of two-dimensional atomic crystals (ultrathin films) connected to each other with regular and periodic patterns. Inspired by the increasing importance of excitons for optical spectroscopy and the engineering of photonic materials in recent years, we have calculated the energy spectra of excitons in superlattices in this theoretical study. The number of film layers and the ratio between the exciton energy transfer within each motif (film) and the exciton energy transfer between neighboring motifs (between films) are important parameters that determine the behavior in an external electromagnetic field.

Measurement of the surface susceptibility of single-layer atomic crystal by the photonic spin Hall effect in momentum space

Fast and robust measurement of the surface susceptibility is still in urgent need for investigation and application of atomically thin crystals. In this work, we propose an effective method to measure the surface susceptibility of single-layer graphene by detecting the photonic spin Hall effect in momentum space. At a graphene interface, the beam separations with different spin states contain an angular component due to the existence of surface susceptibility. By implementing a postselection with real weak value in our scheme, the contribution of the spatial spin-Hall shift is excluded and the angular one is amplified for detection. It is demonstrated that treating the angular shift as a pointer allows for a reliable determination of the surface susceptibility. Our method may pave a way for the optical parameter characterization of two-dimensional atomic crystals via angular metrology.

Electron Transport Properties of Graphene/WS2 Van Der Waals Heterojunctions.

Van der Waals heterojunctions of two-dimensional atomic crystals are widely used to build functional devices due to their excellent optoelectronic properties, which are attracting more and more attention, and various methods have been developed to study their structure and properties. Here, density functional theory combined with the nonequilibrium Green's function technique has been used to calculate the transport properties of graphene/WS2 heterojunctions. It is observed that the formation of heterojunctions does not lead to the opening of the Dirac point of graphene. Instead, the respective band structures of both graphene and WS2 are preserved. Therefore, the heterojunction follows a unique Ohm's law at low bias voltages, despite the presence of a certain rotation angle between the two surfaces within the heterojunction. The transmission spectra, the density of states, and the transmission eigenstate are used to investigate the origin and mechanism of unique linear I-V characteristics. This study provides a theoretical framework for designing mixed-dimensional heterojunction nanoelectronic 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.

Effects of adsorbed molecular ordering to the superconductivity of a two-dimensional atomic layer crystal

The effect of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) adsorption on the physical properties of the two-dimensional (2D) atomic layer superconductor (ALSC) In/Si(111)-($\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$) has been studied by angle-resolved photoelectron spectroscopy, transport measurements, and scanning tunneling microscopy. Hole doping from the adsorbed molecules has been reported to increase the superconducting transition temperature ${T}_{\mathrm{c}}$ of this ALSC, and the molecular spin tends to decrease it. Owing to its large electron affinity and its nonexistent spin state, the adsorption of PTCDA was expected to increase ${T}_{\mathrm{c}}$. However, the PTCDA adsorption dopes only a small number of holes in the In layers and causes a suppression of ${T}_{\mathrm{c}}$ with a sharp increase in the normal-state sheet resistance followed by an insulating transition. Taking the disordering of the arrangement of PTCDA into account, we conclude that the increase in resistance is due to the localization effect originating from the random potential that is induced by the disordered PTCDA molecules. The present result also indicates the importance of the crystallinity of a 2D molecular film adsorbed on ALSCs.

Polarons in two-dimensional atomic crystals

The polaron is the archetypal example of a quasiparticle emerging from the interaction between fermionic and bosonic fields in quantum field theory. In crystalline solids, polarons are formed when electrons and holes become dressed by the quanta of lattice vibrations. While experimental signatures of polarons in bulk three-dimensional materials abound, only rarely have polarons been observed in two-dimensional atomic crystals. Here, we shed light on this asymmetry by developing a quantitative ab initio theory of polarons in atomically-thin crystals. Using this conceptual framework, we unravel the real-space structure of the recently-observed hole polaron in hexagonal boron nitride, we discover an unexpected critical condition for the existence of polarons in two-dimensional crystals, and we establish the key materials descriptors and the universal laws that underpin polaron physics in two dimensions.

Broadband enhancement of absorption by two-dimensional atomic crystals modeled as non-Hermitian photonic scattering

We report the design and fabrication of a vertical structure using a distributed Bragg reflector and dielectric material layer to achieve optimized optical absorption enhancement for a stack of monolayer WS2 and MoS2, namely, a tenfold increase in absorption over a 100 nm spectral range. Our research indicates that we can approach over 50% absorption by finely tuning the thickness of the spacer layer. Our theoretical model shows that the dependence of the absorption coefficient on the spacer thickness can be understood as a solution of a non-Hermitian Schrödinger equation. These results advance the development of broadband optical devices, including solar energy conversion and sensitive optical sensors, by using two-dimensional excitonic materials.

Advanced electrochemical energy storage and conversion on graphdiyne interface

The research in graphdiyne (GDY) has experienced a rapid growth period in the first decade after its birth. As a new two-dimensional (2D) atomic crystal, GDY has unique structures consisting of both sp and sp² hybridized carbon atoms, and exhibits many unprecedented intrinsic properties to scientists. Due to the inherent characteristics of GDY, some new phenomena and properties have been discovered in a wide range of research fields. GDY has made substantial breakthroughs in fundamental and applied science, formed innovative scientific concepts, and made great achievements. In these fields, the electrochemical energy storage and conversion are two important and impressive fields for the fundamental applicative investigations. This review focuses on the utilization of GDY as advanced electrochemical interface for the electrochemical energy storage and conversion. It begins with an introduction of the superiorities and inherent compatibilities of GDY as the electrochemical interface. Then, GDY's recent achievements in electrochemical storage and conversion are commented, from which we can appreciate the inherent advantages of GDY as a crucial material for alternating and innovating the electrochemical interface. Finally, new insights into the challenges and further perspectives on the GDY interface for the electrochemical energy storage and conversion are discussed, aiming at prompting in-depth investigation and their performance in practical applications.

Positive and negative Goos–Hänchen shifts in anisotropic two-dimensional atomic crystals

In this paper, the positive and negative Goos–Hänchen (GH) shifts in anisotropic two-dimensional atomic crystals are systematically investigated. As an example, the GH shifts including the spatial GH shift and the angular GH shift in the black phosphorus-substrate are studied. It is found that the spatial GH shift and the angular GH shift are sensitive to the incident angle, photon energy, optical axis angle and doping concentration. Further studies have shown that the signs variations of the spatial GH shift stem from a phase mutation of the reflection coefficients, and the angular GH shift is proportional to the angular derivative of the modulus of the reflection coefficients. Based on the above results, a systematic method of positive and negative regulations of GH shifts is explored, and it is found that the positive and negative of spatial GH shift can be flexibly controlled by altering the photon energy or rotating the optical axis angle, and the signs of angular GH shift can be easily regulated by changing the incident angle. Finally, combined with weak measurement techniques, an application scheme for accurately sensing the optical axis angle of black phosphorus based on the amplified GH shifts is proposed. These results can be extensively extended to other anisotropic two-dimensional atomic crystals.

A Capillary-Force-Assisted Transfer for Monolayer Transition-Metal-Dichalcogenide Crystals with High Utilization.

The capillary-force-assisted transfer has shown application potential for constructing two-dimensional (2D) electronic and optoelectronic devices for the advantage of free of spin coating the organic compound and etching the substrate. Currently, the transfer mechanism remains obscure. The capillary adhesion mechanism and capillary invasion separation mechanism were proposed independently and rarely discussed in a comprehensive manner. What is more, the integrity and utilization remain to be improved. Here, we developed the capillary-force-assisted transfer method with high utilization and integrity. Uniformity of water transport was improved by introducing water from the sidewall of the small polydimethylsiloxane (PDMS) stamp driven by capillary force. The transfer integrity rate increased, and the location of the complete samples became predictable. The transfer utilization increased as the limited water transportation minimized the impact on the surrounding WS2. The monolayer triangle WS2 crystals from adjacent areas on the sapphire substrate were transferred one after another. Besides, local mechanical exfoliation of the continuous WS2 thin films was demonstrated, implying that the capillary adhesion is strong enough to break the strong in-plane covalent bond and overcome the van der Waals force between WS2 and sapphire substrate. Finally, the water transport model between two surfaces with different hydrophobicity combinations was derived on the basis of the Young-Laplace equation. The analysis of water transport between different interfaces reveals how capillary adhesion and capillary invasion work together to achieve capillary force transfer. This study highlights the potential of the capillary-force-assisted transfer as an efficient technique for fabricating van der Waals structures based on two-dimensional atomic crystals, especially periodic structures.

Chiral molecular intercalation superlattices

The discovery of chiral-induced spin selectivity (CISS) opens up the possibility to manipulate spin orientation without external magnetic fields and enables new spintronic device designs1-4. Although many approaches have been explored for introducing CISS into solid-state materials and devices, the resulting systems so far are often plagued by high inhomogeneity, low spin selectivity or limited stability, and have difficulties in forming robust spintronic devices5-8. Here we report a new class of chiral molecular intercalation superlattices (CMIS) as a robust solid-state chiral material platform for exploring CISS. The CMIS were prepared by intercalating layered two-dimensional atomic crystals (2DACs) (such as TaS2 and TiS2) with selected chiral molecules (such as R-α-methylbenzylamine and S-α-methylbenzylamine). The X-ray diffraction and transmission electron microscopy studies demonstrate highly ordered superlattice structures with alternating crystalline atomic layers and self-assembled chiral molecular layers. Circular dichroism studies show clear chirality-dependent signals between right-handed (R-) and left-handed (S-) CMIS. Furthermore, by using the resulting CMIS as the spin-filtering layer, we create spin-selective tunnelling junctions with a distinct chirality-dependent tunnelling current, achieving a tunnelling magnetoresistance ratio of more than300per cent and a spin polarization ratio of more than60per cent. With a large family of 2DACs of widely tunable electronic properties and a vast selection of chiral molecules of designable structural motifs, the CMIS define a rich family of artificial chiral materials for investigating the CISS effect and capturing its potential for new spintronic devices.

Insight into the Heterogeneity of Longitudinal Plasmonic Field in a Nanocavity Using an Intercalated Two-Dimensional Atomic Crystal Probe with a ∼7 Å Resolution.

Quantitative measurement of the plasmonic field distribution is of great significance for optimizing highly efficient optical nanodevices. However, the quantitative and precise measurement of the plasmonic field distribution is still an enormous challenge. In this work, we design a unique nanoruler with a ∼7 Å spatial resolution, which is based on a two-dimensional atomic crystal where the intercalated monolayer WS2 is a surface-enhanced Raman scattering (SERS) probe and four layers of MoS2 are a reference layer in a nanoparticle-on-mirror (NPoM) structure to quantitatively and directionally probe the longitudinal plasmonic field distribution at high permittivity by the quantitative SERS intensity of WS2 located in different layers. A subnanometer two-dimensional atomic crystal was used as a spacer layer to overcome the randomness of the molecular adsorption and Raman vibration direction. Combined with comprehensive theoretical derivation, numerical calculations, and spectroscopic measurements, it is shown that the longitudinal plasmonic field in an individual nanocavity is heterogeneously distributed with an unexpectedly large intensity gradient. We analyze the SERS enhancement factor on the horizontal component, which shows a great attenuation trend in the nanocavity and further provides precise insight into the horizontal component distribution of the longitudinal plasmonic field. We also provide a direct experimental verification that the longitudinal plasmonic field decays more slowly in high dielectric constant materials. These precise experimental insights into the plasmonic field using a two-dimensional atomic crystal itself as a Raman probe may propel understanding of the nanostructure optical response and applications based on the plasmonic field distribution.

Boosting the performance of single-atom catalysts via external electric field polarization

Single-atom catalysts represent a unique catalytic system with high atomic utilization and tunable reaction pathway. Despite current successes in their optimization and tailoring through structural and synthetic innovations, there is a lack of dynamic modulation approach for the single-atom catalysis. Inspired by the electrostatic interaction within specific natural enzymes, here we show the performance of model single-atom catalysts anchored on two-dimensional atomic crystals can be systematically and efficiently tuned by oriented external electric fields. Superior electrocatalytic performance have been achieved in single-atom catalysts under electrostatic modulations. Theoretical investigations suggest a universal “onsite electrostatic polarization” mechanism, in which electrostatic fields significantly polarize charge distributions at the single-atom sites and alter the kinetics of the rate determining steps, leading to boosted reaction performances. Such field-induced on-site polarization offers a unique strategy for simulating the catalytic processes in natural enzyme systems with quantitative, precise and dynamic external electric fields.

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.

Photonic spin Hall effect in twisted few-layer anisotropic two-dimensional atomic crystals

Motivated by the emergence of stacked two-dimensional materials with a twist angle, we examine the photonic spin Hall effect in twisted few-layer anisotropic two-dimensional atomic crystals. By developing a generalized model for arbitrary layers of anisotropic two-dimensional atomic crystals, the photonic spin Hall effect of twisted few-layer black phosphorus without interlayer interaction is demonstrated and discussed. We find that the twist between different layers results in the arise of the in-plane spin Hall shift, and the shift is very sensitive to the twist angle. While the in-plane spin Hall shifts are symmetric about the origin as a function versus the twist angle, the transverse spin Hall shifts are symmetric about the axis. And the photonic spin Hall shifts exhibit periodic changes versus the thickness of the interlayer due to the optical Fabry-P\'erot resonance. The results may provide different aspects for analysis and control of the optical properties of twisted multilayer two-dimensional materials.

Soft flexural waves and self-localization of wrinkling modes of single- and few-layer graphene under compression in a compliant matrix

This Research Letter studies the evolution under external compression of the wrinkling modes of single- and few-layer graphene or another two-dimensional atomic crystal embedded in or placed on a compliant matrix, or on a dense fluid in a gravitational field. An analytical model based on the nonlinear bending elasticity of the graphene is developed, which shows that the compressive surface stress causes spatial localization of the extended sinusoidal wrinkling mode with a solitonlike envelope, with the localization length decreasing with overcritical external strain. The parameters of the extended sinusoidal wrinkling mode are determined from the conditions of the anomalous softening of the flexural surface acoustic wave with predominant out-of-plane and suppressed in-plane surface displacements, propagating along the graphene in a compliant anisotropic matrix. Self-localization of the wrinkling modes finally results in the formation of strongly localized modes with approximately one-period sinusoidal profiles and external-strain-independent wavelengths. Self-localization of the wrinkling modes is governed by the derived Ginzburg-Landau-type nonlinear envelope-function equation with a negative dispersive term, which we relate with the graphene-nonlinearity-induced repulsion between soft flexural surface acoustic waves with negative effective mass. One- and two-dimensional wrinkling patterns and two types of strongly localized graphene wrinkling modes with different symmetry are described.

Strong coupling between excitons in a two-dimensional atomic crystal and quasibound states in the continuum in a two-dimensional all-dielectric asymmetric metasurface

We investigate the interaction between excitons in a two-dimensional atomic crystal and quasibound states in the continuum (q-BIC) in a two-dimensional all-dielectric asymmetric metasurface. By introducing coherent and incoherent coupling terms in a coupled oscillator model, we demonstrate the coexistence of coherent and incoherent coupling processes in the strongly coupled system and the resultant sub/superradiant polariton states. Based on the multipole decomposition method and near-field analysis of full wave simulations, we study the microscopic excitation of multipole components in q-BIC and their coupling to excitons. We reveal that not only the magnetic dipole but also the interference between electric dipole and its toroidal counterpart dominate in the exciton-BIC strong coupling regime. The fractions of the magnetic and electric dipole ingredients are modulated by the topology of the two-dimensional all-dielectric metasurface, exhibiting distinctly different features in the high- and low-energy hybrid modes in the strong coupling system. Our findings are expected to be of importance for both fundamental research in TMD-based light-matter interactions and practical applications in the design of novel, tunable exciton-polariton devices with high compactibility.