Two-Dimensional Materials for Highly Efficient and Stable Perovskite Solar Cells

Perovskite solar cells(PSCs)offer low costs and high power conversion efficiency.However,the lack of long-term stability,primarily stemming from the interfacial defects and the sus-ceptible metal electrodes,hinders their practical application.In the past few years,two-dimensional(2D)materials(e.g.,graphene and its derivatives,transitional metal dichalcogenides,MXenes,and black phosphorus)have been identified as a promising solution to solving these problems because of their dangling bond-free surfaces,layer-dependent electronic band structures,tunable functional groups,and inherent compactness.Here,recent progress of 2D material toward efficient and stable PSCs is summarized,including its role as both interface materials and electrodes.We discuss their beneficial effects on perovskite growth,energy level alignment,defect passivation,as well as blocking external stimulus.In particular,the unique properties of 2D materials to form van der Waals heterojunction at the bottom interface are emphasized.Finally,perspectives on the further development of PSCs using 2D materials are provided,such as designing high-quality van der Waals heterojunction,enhancing the uniformity and coverage of 2D nanosheets,and developing new 2D materials-based electrodes.

Image segmentation of exfoliated two-dimensional materials by generative adversarial network-based data augmentation

Mechanically cleaved two-dimensional materials are random in size and thickness.Recognizing atomically thin flakes by human experts is inefficient and unsuitable for scalable production.Deep learning algorithms have been adopted as an alternative,nevertheless a major challenge is a lack of sufficient actual training images.Here we report the generation of synthetic two-dimensional materials images using StyleGAN3 to complement the dataset.DeepLabv3Plus network is trained with the synthetic images which reduces overfitting and improves recognition accuracy to over 90%.A semi-supervisory technique for labeling images is introduced to reduce manual efforts.The sharper edges recognized by this method facilitate material stacking with precise edge alignment,which benefits exploring novel properties of layered-material devices that crucially depend on the interlayer twist-angle.This feasible and efficient method allows for the rapid and high-quality manufacturing of atomically thin materials and devices.

Control of light-matter interactions in two-dimensional materials with nanoparticle-on-mirror structures

Light–matter interactions in two-dimensional(2D)materials have been the focus of research since the discovery of graphene.The light–matter interaction length in 2D materials is,however,much shorter than that in bulk materials owing to the atomic nature of 2D materials.Plasmonic nanostructures are usually integrated with 2D materials to enhance the light–matter interactions,offering great opportunities for both fundamental research and technological applications.Nanoparticle-on-mirror(NPo M)structures with extremely confined optical fields are highly desired in this aspect.In addition,2D materials provide a good platform for the study of plasmonic fields with subnanometer resolution and quantum plasmonics down to the characteristic length scale of a single atom.A focused and up-to-date review article is highly desired for a timely summary of the progress in this rapidly growing field and to encourage more research efforts in this direction.In this review,we will first introduce the basic concepts of plasmonic modes in NPo M structures.Interactions between plasmons and quasi-particles in 2D materials,e.g.,excitons and phonons,from weak to strong coupling and potential applications will then be described in detail.Related phenomena in subnanometer metallic gaps separated by 2D materials,such as quantum tunneling,will also be touched.We will finally discuss phenomena and physical processes that have not been understood clearly and provide an outlook for future research.We believe that the hybrid systems of2D materials and NPo M structures will be a promising research field in the future.

Recent progress on valley polarization and valley-polarized topological states in two-dimensional materials

Valleytronics, using valley degree of freedom to encode, process, and store information, may find practical applications in low-power-consumption devices. Recent theoretical and experimental studies have demonstrated that twodimensional(2D) honeycomb lattice systems with inversion symmetry breaking, such as transition-metal dichalcogenides(TMDs), are ideal candidates for realizing valley polarization. In addition to the optical field, lifting the valley degeneracy of TMDs by introducing magnetism is an efficient way to manipulate the valley degree of freedom. In this paper, we first review the recent progress on valley polarization in various TMD-based systems, including magnetically doped TMDs,intrinsic TMDs with both inversion and time-reversal symmetry broken, and magnetic TMD heterostructures. When topologically nontrivial bands are empowered into valley-polarized systems, valley-polarized topological states, namely valleypolarized quantum anomalous Hall effect can be realized. Therefore, we have also reviewed the theoretical proposals for realizing valley-polarized topological states in 2D honeycomb lattices. Our paper can help readers quickly grasp the latest research developments in this field.

Electrocatalytic stability of two-dimensional materials

In electrocatalysis,two-dimensional(2D)materials have attracted extensive interests due to their unique electronic structure and physical properties.In recent years,many efforts have been devoted to improving the catalytic activity of 2D materials.However,the stability of 2D materials under catalytic conditions,as a critical issue,requires better understanding for any practical applications.This review summarizes recent progress in electrocatalytic stability of 2D materials,including four intrinsic factors that affect the stability of 2D materials:1.Weak interactions between 2D catalyst and substrate;2,delamination of 2D catalyst layers;3.metastable phase of 2D materials;4.chemistry and environmental instability of 2D materials.Meanwhile,some corresponding solutions are summarized for each factor.In addition,this review proposes potential routes for developing 2D catalytic materials with both high activity and stability.

Dynamic behavior of tunneling triboelectric charges in two-dimensional materials

Although the research history of triboelectrification has been more than 2000 years, there are still many problems to be solved so far.The use of scanning probe microscopy provides an important way to quantitatively study the transfer, accumulation, and dissipation of triboelectric charges in the process of triboelectrification. Two-dimensional materials are considered to be key materials for new electronic devices in the post-Moore era due to their atomic-scale size advantages. If the electrostatic field generated by triboelectrification can be used to replace the traditional gate electrostatic field, it is expected to simplify the structure of two-dimensional electronic devices and reconfigure them at any time according to actual needs. Here, we investigate the triboelectrification process of various two-dimensional materials such as MoS_(2), WSe_(2),and ZnO. Different from traditional bulk materials, after two-dimensional materials are rubbed, the triboelectric charges generated may tunnel through the two-dimensional materials to the underlying substrate surface. Because the tunneling triboelectric charge is protected by the twodimensional material, its stable residence time on the substrate surface can reach more than 7 days, which is more than tens of minutes for the traditional triboelectric charge. In addition, the electrostatic field generated by the tunneling triboelectric charge can effectively regulate the carrier transport performance of two-dimensional materials, and the source–drain current of the field effect device regulated by the triboelectric floating gate is increased by nearly 60 times. The triboelectric charge tunneling phenomenon in two-dimensional materials is expected to be applied in the fields of new two-dimensional electronic devices and reconfigurable functional circuits.

Radiofrequency sensing systems based on emerging two-dimensional materials and devices

As one of the most promising platforms for wireless communication,radiofrequency(RF)electronics have been widely advocated for the development of sensing systems.In particular,monolayer and few-layer two-dimensional(2D)materials exhibiting extraordinary electrical properties not only can be integrated to improve the performance of RF circuits,but also to display exceptional sensing capabilities.This review provides an in-depth perspective of current trends and challenges in the application of 2D materials for RF biochemical sensing,including:(i)theoretical bases to achieve different sensing schemes;(ii)unique properties of 2D materials for reasoning their applications in RF sensing;(iii)developments in 2D RF sensors to facilitate the practice of biochemical sensors with ever-demanding sensitivities,as well as their potential uses in meeting the requirements and challenges of biochemical sensors in the Internet-of-Things era.

Waveguide-integrated optical modulators with two-dimensional materials

Waveguide-integrated optical modulators are indispensable for on-chip optical interconnects and optical computing.To cope with the ever-increasing amount of data being generated and consumed,ultrafast waveguide-integrated optical modulators with low energy consumption are highly demanded.In recent years,two-dimensional(2D)materials have attracted a lot of attention and have provided tremendous opportunities for the development of high-performance waveguide-integrated optical modulators because of their extraordinary optoelectronic properties and versatile compatibility.This paper reviews the state-of-the-art waveguide-integrated optical modulators with 2D materials,providing researchers with the developing trends in the field and allowing them to identify existing challenges and promising potential solutions.First,the concept and fundamental mechanisms of optical modulation with 2D materials are summarized.Second,a review of waveguide-integrated optical modulators employing electro-optic,all-optic,and thermo-optic effects is provided.Finally,the challenges and perspectives of waveguide-integrated modulators with 2D materials are discussed.

Unfolding the structure-property relationships of Li_(2)S anchoring on two-dimensional materials with high-throughput calculations and machine learning

Lithium-sulfur(Li-S)batteries are notable for their high theoretical energy density,but the‘shuttle effect’and the limited conversion kinetics of Li-S species can downgrade their actual performance.An essential strategy is to design anchoring materials(AMs)to appropriately adsorb Li-S species.Herein,we propose a new three-procedure protocol,named InfoAd(Informative Adsorption)to evaluate the anchoring of Li_(2)S on two-dimensional(2D)materials and disclose the underlying importance of material features by combining high-throughput calculation workflow and machine learning(ML).In this paradigm,we calculate the anchoring of Li_(2)S on 12552D A_(x)B_(y)(B in the VIA/VIIA group)materials and pick out 44(un)reported nontoxic 2D binary A_(x)B_(y)AMs,in which the importance of the geometric features on the anchoring effect is revealed by ML for the first time.We develop a new Infograph model for crystals to accurately predict whether a material has a moderate binding with Li_(2)S and extend it to all 2D materials.Our InfoAd protocol elucidates the underlying structure-property relationship of Li_(2)S adsorption on 2D materials and provides a general research framework of adsorption-related materials for catalysis and energy/substance storage.

Different angle-resolved polarization configurations of Raman spectroscopy: A case on the basal and edge plane of two-dimensional materials

Angle-resolved polarized Raman（ARPR） spectroscopy can be utilized to assign the Raman modes based on crystal symmetry and Raman selection rules and also to characterize the crystallographic orientation of anisotropic materials.However, polarized Raman measurements can be implemented by several different configurations and thus lead to different results. In this work, we systematically analyze three typical polarization configurations： 1） to change the polarization of the incident laser, 2） to rotate the sample, and 3） to set a half-wave plate in the common optical path of incident laser and scattered Raman signal to simultaneously vary their polarization directions. We provide a general approach of polarization analysis on the Raman intensity under the three polarization configurations and demonstrate that the latter two cases are equivalent to each other. Because the basal plane of highly ordered pyrolytic graphite（HOPG） exhibits isotropic feature and its edge plane is highly anisotropic, HOPG can be treated as a modelling system to study ARPR spectroscopy of twodimensional materials on their basal and edge planes. Therefore, we verify the ARPR behaviors of HOPG on its basal and edge planes at three different polarization configurations. The orientation direction of HOPG edge plane can be accurately determined by the angle-resolved polarization-dependent G mode intensity without rotating sample, which shows potential application for orientation determination of other anisotropic and vertically standing two-dimensional materials and other materials.

Photodetecting and light-emitting devices based on two-dimensional materials

Two-dimensional （2D） materials, e.g., graphene, transition metal dichalcogenides （TMDs）, and black phosphorus （BP）, have demonstrated fascinating electrical and optical characteristics and exhibited great potential in optoelectronic applications. High-performance and multifunctional devices were achieved by employing diverse designs, such as hybrid systems with nanostructured materials, bulk semiconductors and organics, forming 2D heterostructures. In this review, we mainly discuss the recent progress of 2D materials in high-responsive photodetectors, light-emitting devices and single photon emitters. Hybrid systems and van der Waals heterostructure-based devices are emphasized, which exhibit great potential in state-of-the-art applications.

Two-dimensional materials-decorated microfiber devices for pulse generation and shaping in fiber lasers

Two-dimensional（2D） materials have been regarded as a promising nonlinear optical medium for fabricating versatile optical and optoelectronic devices. Among the various photonic applications, the employment of 2D materials as nonlinear optical devices such as saturable absorbers for ultrashort pulse generation and shaping in ultrafast lasers is one of the most striking aspects in recent years. In this paper, we review the recent progress of 2D materials based pulse generation and soliton shaping in ultrafast fiber lasers, and particularly in the context of 2D materials-decorated microfiber photonic devices. The fabrication of 2D materials-decorated microfiber photonic devices, high performance mode-locked pulse generation, and the nonlinear soliton dynamics based on pulse shaping method are discussed. Finally, the challenges and the perspective of the 2D materials-based photonic devices as well as their applications are also discussed.

Recent progress on integrating two-dimensional materials with ferroelectrics for memory devices and photodetectors

Two-dimensional （2D） materials, such as graphene and MoS2 related transition metal dichalcogenides （TMDC）, have attracted much attention for their potential applications. Ferroelectrics, one of the special and traditional dielectric materials, possess a spontaneous electric polarization that can be reversed by the application of an external electric field. In recent years, a new type of device, combining 2D materials with ferroelectrics, has been fabricated. Many novel devices have been fabricated, such as low power consumption memory devices, highly sensitive photo-transistors, etc. using this technique of hybrid systems incorporating ferroelectrics and 2D materials. This paper reviews two types of devices based on field effect transistor （FET） structures with ferroelectric gate dielectric construction （termed FeFET）. One type of device is for logic applications, such as a graphene and TMDC FeFET for fabricating memory units. Another device is for optoelectric applications, such as high performance phototransistors using a graphene p-n junction. Finally, we discuss the prospects for future applications of 2D material FeFET.

Thermal properties of two-dimensional materials

Two-dimensional （2D） materials, such as graphene, phosphorene, and transition metal dichalcogenides （e.g., MoS2 and WS2）, have attracted a great deal of attention recently due to their extraordinary structural, mechanical, and physical properties. In particular, 2D materials have shown great potential for thermal management and thermoelectric energy generation. In this article, we review the recent advances in the study of thermal properties of 2D materials. We first review some important aspects in thermal conductivity of graphene and discuss the possibility to enhance the ultra-high thermal conductivity of graphene. Next, we discuss thermal conductivity of MoS2 and the new strategy for thermal management of MoS2 device. Subsequently, we discuss the anisotropic thermal properties of phosphorene. Finally, we review the application of 2D materials in thermal devices, including thermal rectifier and thermal modulator.

A review on spatial self-phase modulation of two-dimensional materials

Self-diffraction appears when the strong laser goes through two-dimensional material suspension,and this spatial self-phase modulation(SPPM)phenomenon can be used to measure nonlinear optical parameters and achieve optical switch.At present,the mechanism of SPPM is still ambiguous.The debate mainly focuses on whether the phenomenon is caused by the nonlinear refractive index of the two-dimensional material or the thermal effect of the laser.The lack of theory limits the dimension of the phase modulation to the radius of the diffraction ring and the vertical imbalance.Therefore,it is urgent to establish a unified and universal SSPM theoretical system of two-dimensional material.

Epitaxy of Ⅲ-nitrides on two-dimensional materials and its applications

Ⅲ-nitride semiconductor materials have excellent optoelectronic properties,mechanical properties,and chemical stability,which have important applications in the field of optoelectronics and microelectronics.Two-dimensional(2D)materials have been widely focused in recent years due to their peculiar properties.With the property of weak bonding between layers of 2D materials,the growth ofⅢ-nitrides on 2D materials has been proposed to solve the mismatch problem caused by heterogeneous epitaxy and to develop substrate stripping techniques to obtain high-quality,low-cost nitride materials for high-quality nitride devices and their extension in the field of flexible devices.In this progress report,the main methods for the preparation of 2D materials,and the recent progress and applications of different techniques for the growth ofⅢ-nitrides based on 2D materials are reviewed.

Surface-enhanced Raman spectroscopy chips based on two-dimensional materials beyond graphene

Surface-enhanced Raman spectroscopy(SERS) based on two-dimensional(2 D) materials has attracted great attention over the past decade. Compared with metallic materials, which enhance Raman signals via the surface plasmon effect, 2 D materials integrated on silicon substrates are ideal for use in the fabrication of plasmon-free SERS chips, with the advantages of outstanding fluorescence quenching capability, excellent biomolecular compatibility, tunable Fermi levels, and potentially lowcost material preparation. Moreover, recent studies have shown that the limits of detection of 2 D-material-based SERS may be comparable with those of metallic substrates, which has aroused significant research interest. In this review, we comprehensively summarize the advances in SERS chips based on 2 D materials. As several excellent reviews of graphene-enhanced Raman spectroscopy have been published in the past decade, here, we focus only on 2 D materials beyond graphene, i.e., transition metal dichalcogenides, black phosphorus, hexagonal boron nitride, 2 D titanium carbide or nitride, and their heterostructures. We hope that this paper can serve as a useful reference for researchers specializing in 2 D materials, spectroscopy, and diverse applications related to chemical and biological sensing.

Magnus Hall Effect in Two-Dimensional Materials

The Magnus Hall effect(MHE) is a new type of linear-response Hall effect, recently proposed to appear in two-dimensional(2D) nonmagnetic systems at zero magnetic field in the ballistic limit. The MHE arises from a self-rotating Bloch electron moving under a gradient-electrostatic potential, analogous to the Magnus effect in the macrocosm. Unfortunately, the MHE is usually accompanied by a trivial transverse signal, which hinders its experimental observation. We systematically investigate the material realization and experimental measurement of the MHE, based on symmetry analysis and first-principles calculations. It is found that both the out-ofplane mirror and in-plane two-fold symmetries can neutralize the trivial transverse signal to generate clean MHE signals. We choose two representative 2D materials, monolayer MoS_(2), and bilayer WTe_(2), to study the quantitative dependency of MHE signals on the direction of the electric field. The results are qualitatively consistent with the symmetry analysis, and suggest that an observable MHE signal requires giant Berry curvatures. Our results provide detailed guidance for the future experimental exploration of MHE.

Strategies to improve electrocatalytic and photocatalytic performance of two-dimensional materials for hydrogen evolution reaction

Two-dimensional materials(2D)with unique physicochemical properties have been widely studied for their use in many applications,including as hydrogen evolution catalysts to improve the efficiency of water splitting.Recently,typical 2D materials MoS2,graphene,MXenes,and black phosphorus have been widely investigated for their application in the hydrogen evolution reaction(HER).In this review,we summarize three efficient strategies—defect engineering,heterostructure formation,and heteroatom doping—for improving the HER performance of 2D catalysts.The d-band theory,density of states,and Fermi energy level are discussed to provide guidance for the design and construction of novel 2D materials.The challenges and prospects of 2D materials in the HER are also considered.

Field-effect transistors based on two-dimensional materials for logic applications

Field-effect transistors （FETs） for logic applications, graphene and MoS2, are discussed. These materials have based on two representative two-dimensional （2D） materials, drastically different properties and require different consider- ations. The unique band structure of graphene necessitates engineering of the Dirac point, including the opening of the bandgap, the doping and the interface, before the graphene can be used in logic applications. On the other hand, MoS2 is a semiconductor, and its electron transport depends heavily on the surface properties, the number of layers, and the carrier density. Finally, we discuss the prospects for the future developments in 2D material transistors.