Enhancement of Out-of-Plane Charge Transport in a Vertically Stacked Two-Dimensional Heterostructure Using Point Defects

Defect Engineering in Bi-Based Photo/Electrocatalysts for Nitrogen Reduction to Ammonia

Main group Bi-based materials have gained popularity as N2 reduction reaction (NRR) photo/electrocatalysts due to their ability to inhibit competitive H2 evolution reactions (HER) and the unique N2 adsorption activities. The introduction of defects in Bi-based catalysts represents a highly effective strategy for enhancing light absorption, promoting efficient separation of photogenerated carriers, optimizing the activity of free radicals, regulating electronic structure, and improving catalytic performance. In this review, we outline the various applications of state of the defect engineering in Bi-based catalysts and elucidate the impact of vacancies on NRR performance. In particular, the types of defects, methods of defects tailoring, advanced characterization techniques, as well as the Bi-based catalysts with abundant defects and their corresponding catalytic behavior in NRR were elucidated in detail. Finally, the main challenges and opportunities for future development of defective Bi-based NRR catalysts are discussed, which provides a comprehensive theoretical guidance for this field.

Ultrafast Laser Processing of 2D Materials: Novel Routes to Advanced Devices

Ultrafast laser processing has emerged as a versatile technique for modifying materials and introducing novel functionalities. Over the past decade, this method has demonstrated remarkable advantages in the manipulation of 2D layered materials, including synthesis, structuring, functionalization, and local patterning. Unlike continuous-wave and long-pulsed optical methods, ultrafast lasers offer a solution for thermal heating issues. Nonlinear interactions between ultrafast laser pulses and the atomic lattice of 2D materials substantially influence their chemical and physical properties. This paper highlights the transformative role of ultrafast laser pulses in maskless green technology, enabling subtractive, and additive processes that unveil ways for advanced devices. Utilizing the synergetic effect between the energy states within the atomic layers and ultrafast laser irradiation, it is feasible to achieve unprecedented resolutions down to several nanometers. Recent advancements are discussed in functionalization, doping, atomic reconstruction, phase transformation, and 2D and 3D micro- and nanopatterning. A forward-looking perspective on a wide array of applications of 2D materials, along with device fabrication featuring novel physical and chemical properties through direct ultrafast laser writing, is also provided.

The 3D Controllable Fabrication of Nanomaterials with FIB-SEM Synchronization Technology

Nanomaterials with unique structures and functions have been widely used in the fields of microelectronics, biology, medicine, and aerospace, etc. With advantages of high resolution and multi functions (e.g., milling, deposition, and implantation), focused ion beam (FIB) technology has been widely developed due to urgent demands for the 3D fabrication of nanomaterials in recent years. In this paper, FIB technology is illustrated in detail, including ion optical systems, operating modes, and combining equipment with other systems. Together with the in situ and real-time monitoring of scanning electron microscopy (SEM) imaging, a FIB-SEM synchronization system achieved 3D controllable fabrication from conductive to semiconductive and insulative nanomaterials. The controllable FIB-SEM processing of conductive nanomaterials with a high precision is studied, especially for the FIB-induced deposition (FIBID) 3D nano-patterning and nano-origami. As for semiconductive nanomaterials, the realization of high resolution and controllability is focused on nano-origami and 3D milling with a high aspect ratio. The parameters of FIB-SEM and its working modes are analyzed and optimized to achieve the high aspect ratio fabrication and 3D reconstruction of insulative nanomaterials. Furthermore, the current challenges and future outlooks are prospected for the 3D controllable processing of flexible insulative materials with high resolution.

Recent progresses on ion beam irradiation induced structure and performance modulation of two-dimensional materials

Two-dimensional (2D) materials are receiving significant attention for both fundamental research and industrial applications due to their unparalleled properties and wide application potential. In this case, the controllable modulation of their structures and properties is essential for the realization and further expansion of their applications. Accordingly, ion beam irradiation techniques, with large scope to adjust parameters, high manufacturing resolution, and a series of advanced equipment being developed, have been demonstrated to have obvious advantages in manipulating the structure and performance of 2D materials. In recent years, many research efforts have been devoted to uncovering the underlying mechanism and control rules regarding ion irradiation induced phenomena in 2D materials, aiming at fulfilling their application potential as soon as possible. Herein, we review the research progress in the interaction between energetic ions and 2D materials based on the energy transfer model, type of ion source, structural modulation, performance modification of 2D materials, and then their application status, aiming to provide useful information for researchers in this field and stimulating more research advances.

Ion irradiation of monolayer graphene-Nd:YAG hybrid waveguides: fabrication and laser

Hybrid waveguides consisting of two-dimensional layered materials pad on the surface of optical waveguides suffer from a nonuniform and loose contact between the two-dimensional material and the waveguide, which can reduce the efficiency of the pulsed laser. Here, we present high-performance passively Q-switched pulsed lasers in three distinct structures of monolayer graphene-Nd:YAG hybrid waveguides irradiated by energetic ions. The ion irradiation enables the monolayer graphene a tight contact and strong coupling with the waveguide. As a result, Q-switched pulsed lasers with narrow pulse width and high repetition rate are obtained in three designed hybrid waveguides. The narrowest pulse width is 43.6 ns, provided by the ion-irradiated Y-branch hybrid waveguide. This study paves the way toward developing on-chip laser sources based on hybrid waveguides by using ion irradiation.

Regulating the Electrical and Mechanical Properties of TaS2 Films via van der Waals and Electrostatic Interaction for High Performance Electromagnetic Interference Shielding

Low-dimensional transition metal dichalcogenides (TMDs) have unique electronic structure, vibration modes, and physicochemical properties, making them suitable for fundamental studies and cutting-edge applications such as silicon electronics, optoelectronics, and bioelectronics. However, the brittleness, low toughness, and poor mechanical and electrical stabilities of TMD-based films limit their application. Herein, a TaS2 freestanding film with ultralow void ratio of 6.01% is restacked under the effect of bond-free van der Waals (vdW) interactions within the staggered 2H-TaS2 nanosheets. The restacked films demonstrated an exceptionally high electrical conductivity of 2,666 S cm-1, electromagnetic interference shielding effectiveness (EMI SE) of 41.8 dB, and absolute EMI SE (SSE/t) of 27,859 dB cm2 g-1, which is the highest value reported for TMD-based materials. The bond-free vdW interactions between the adjacent 2H-TaS2 nanosheets provide a natural interfacial strain relaxation, achieving excellent flexibility without rupture after 1,000 bends. In addition, the TaS2 nanosheets are further combined with the polymer fibers of bacterial cellulose and aramid nanofibers via electrostatic interactions to significantly enhance the tensile strength and flexibility of the films while maintaining their high electrical conductivity and EMI SE.This work provides promising alternatives for conventional materials used in EMI shielding and nanodevices.

Ultrafast photoresponse of vertically oriented TMD films probed in a vertical electrode configuration on Si chips

Integrated photodetectors based on transition metal dichalcogenides (TMDs) face the challenge of growing their high-quality crystals directly on chips or transferring them to the desired locations of components by applying multi-step processes. Herein, we show that vertically oriented polycrystalline thin films of MoS₂ and WS₂ grown by sulfurization of Mo and W sputtered on highly doped Si are robust solutions to achieve on-chip photodetectors with a sensitivity of up to 1 mA W^-1 and an ultrafast response time in the sub-μs regime by simply probing the device in a vertical arrangement, i.e., parallel to the basal planes of TMDs. These results are two orders of magnitude better than those measured earlier in lateral probing setups having both electrodes on top of vertically aligned polycrystalline TMD films. Accordingly, our study suggests that easy-to-grow vertically oriented polycrystalline thin film structures may be viable components in fast photodetectors as well as in imaging, sensing and telecommunication devices.

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their heterostructures have attracted significant interest in both academia and industry because of their unusual physical and chemical properties. They offer numerous applications, such as electronic, optoelectronic, and spintronic devices, in addition to energy storage and conversion. Atomic and structural modifications of van der Waals layered materials are required to achieve unique and versatile properties for advanced applications. This review presents a discussion on the atomic-scale and structural modifications of 2D TMDs and their heterostructures via post-treatment. Atomic-scale modifications such as vacancy generation, substitutional doping, functionalization and repair of 2D TMDs and structural modifications including phase transitions and construction of heterostructures are discussed. Such modifications on the physical and chemical properties of 2D TMDs enable the development of various advanced applications including electronic and optoelectronic devices, sensing, catalysis, nanogenerators, and memory and neuromorphic devices. Finally, the challenges and prospects of various post-treatment techniques and related future advanced applications are addressed.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Manipulation of Band Alignment in Two-Dimensional Vertical WSe2/BA2PbI4 Ruddlesden-Popper Perovskite Heterojunctions via Defect Engineering

Transition metal dichalcogenides (TMDs), two-dimensional (2D) layered Ruddlesden-Popper perovskite material, and their heterojunctions have attracted a great deal of interest in optoelectronic applications. Although various approaches for modulating their properties and applications have been demonstrated, knowledge of the interface band alignment and defect engineering on the TMD/2D perovskite heterojunction is still lacking. Herein, the optoelectronic properties and defect engineering of the WSe₂/BA₂PbI₄ heterojunction have been investigated with density functional theory simulations. We find that the WSe₂/BA₂PbI₄ van der Waals heterojunction maintains an indirect bandgap and S-scheme alignment, facilitating the efficient splitting of light excited carriers across the interface. Importantly, we find that defect engineering could manipulate the band alignment. The introduction of the BA vacancies could switch the interface from the S-scheme to the typical type II interface, whereas Se vacancies would facilitate recombination at the S-scheme interface. Our work proves that the interfacial properties of heterojunctions can be regulated by defect modulation to address different optoelectronic applications.

Recent Advances on Tuning the Interlayer Coupling and Properties in van der Waals Heterostructures

2D van der Waals (vdW) heterostructures are receiving increasing research attention due to the theoretically amazing properties and unprecedented application potential. However, the as-synthesized heterostructures are generally underperforming due to the weak interlayer coupling, which inspires the researchers to find ways to modulate the interlayer coupling and properties, realizing the tailored performance for actual applications. There have been a lot of publications regarding the controllable regulation of the structures and properties of 2D vdW heterostructures in the past few years, while a review work summarizing the current advances is not yet available, though it is significant. This paper conducts a state-of-the-art review regarding the current research progress of performance modulation of vdW heterostructures by different techniques. First, the general synthesis methods of vdW heterostructures are summarized. Then, different performance modulation techniques, that is, mechanical-based, external fields-assisted, and particle beam irradiation-based methods, are discussed and compared in detail. Some of the newly proposed concepts are described. Thereafter, applications of vdW heterostructures with tailored properties are reviewed for the application prospects of the topic around this area. Moreover, the future research challenges and prospects are discussed, aiming at triggering more research interest and device applications around this topic.

Atomic insights into the influence of Bi doping on the optical properties of two-dimensional van der Waals layered InSe

As a narrow-gap semiconductor, III-VI two-dimensional (2D) van der Waals layered indium selenide (InSe) has attracted a lot of attention due to excellent physical properties. For potential optoelectronic applications, the tunability of the optical property is challenging, e.g., the modulation of optical bandgap commonly by element doping. However, the deep understanding of the influence of element doping on the microstructure and the optical properties lacks of systematic investigation. In this work, by using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we investigate the influence of Bi doping on controlling of the microstructure and optical properties of InSe single crystal in detail. The results show that Bi doping can introduce additional stacking faults in InSe single crystal, and more importantly, the atomic spacing and lattice constant of Bi-doped InSe are changed a lot as compared to that of the undoped one. Further optical characterizations including photoluminescence and transmission spectra reveal that Bi-doping can broaden the transmission wavelength range of InSe and make its optical bandgap blue-shift, which can also be physically interpreted from the doping-induced structure change. Our work expands new ideas for the optical property modulation of 2D thin-layer materials and brings new possibilities for the development of thin-layer InSe optical devices.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Molecular dynamics simulations of ion beam irradiation on graphene/MoS2 heterostructure

The interaction between ion irradiation and two-dimensional (2D) heterostructures is important for the performance modulation and application realization, while few studies have been reported. This paper investigates the influence of Ar ion irradiation on graphene/MoS2 heterostructure by using molecular dynamics (MD) simulations. The generation of defects is studied at first by considering the influence factors (i.e., irradiation energy, dose, stacking order, and substrate). Then uniaxial tensile test simulations are conducted to uncover the evolution of the mechanical performance of graphene/MoS2 heterostructure after being irradiated by ions. At last, the control rule of interlayer distance in graphene/MoS2 heterostructure by ion irradiation is illustrated for the actual applications. This study could provide important guidance for future application in tuning the performance of graphene/MoS2 heterostructure-based devices by ion beam irradiation.

Defect Engineering of Out-of-Plane Charge Transport in van der Waals Heterostructures for Bi-Direction Photoresponse

Defects are ubiquitous in two-dimensional (2D) transition-metal dichalcogenides (TMDs), generated by the initial growth- or the postprocessing. However, the defects may play negative roles in the photoelectronic properties of TMDs due to the reduction of in-plane transport of carriers. In this work, we demonstrate that the Se-vacancy defects in MoSe₂ side of the van der Waal heterostructure is able to switch direction of out-of-plane charge transport. Photoresponse spectra showed defect density enable modified surface potential of MoSe_2-x, leading to the barrier reverse between graphene and MoSe_2-x and switches of the photoresponse from the negative to the positive. This unexpected property stemmed from appearance of midgap states by defects at heterostructure, as demonstrated by the density functional theory calculation and scanning tunneling microscope results. MoSe_2-0.2/graphene heterostructure has a broadband response ranging from 450 to 1064 nm and exhibits comparable or higher positive responsivity (5.4 × 10³ A/W to -15.3 × 10³ A/W at 632.8 and 5.7 × 10³ A/W to -1.2 × 10³ A/W at 1064 nm) to the negative one of the pristine MoSe₂/graphene. Based on defect-engineered heterostructures, we construct optoelectronic OR and AND logic devices with a broadband operation. Our work elucidates an alternative avenue to tailor the out-of-plane charge transport in TMD-based heterostructure through defects, and potentially invokes applicable utilization for 2D photodetectors and optoelectronic logic gates.

Interface chemistry of two-dimensional heterostructures - fundamentals to applications

Two-dimensional heterostructures (2D HSs) have emerged as a new class of materials where dissimilar 2D materials are combined to synergise their advantages and alleviate shortcomings. Such a combination of dissimilar components into 2D HSs offers fascinating properties and intriguing functionalities attributed to the newly formed heterointerface of constituent components. Understanding the nature of the surface and the complex heterointerface of HSs at the atomic level is crucial for realising the desired properties, designing innovative 2D HSs, and ultimately unlocking their full potential for practical applications. Therefore, this review provides the recent progress in the field of 2D HSs with a focus on the discussion of the fundamentals and the chemistry of heterointerfaces based on van der Waals (vdW) and covalent interactions. It also explains the challenges associated with the scalable synthesis and introduces possible methodologies to produce large quantities with good control over the heterointerface. Subsequently, it highlights the specialised characterisation techniques to reveal the heterointerface formation, chemistry and nature. Afterwards, we give an overview of the role of 2D HSs in various emerging applications, particularly in high-power batteries, bifunctional catalysts, electronics, and sensors. In the end, we present conclusions with the possible solutions to the associated challenges with the heterointerfaces and potential opportunities that can be adopted for innovative applications.

Defect-Induced in Situ Atomic Doping in Transition Metal Dichalcogenides via Liquid-Phase Synthesis toward Efficient Electrochemical Activity

Transition metal dichalcogenides (TMDs), due to their fascinating properties, have emerged as potential next-generation semiconducting nanomaterials across diverse fields of applications. When combined with other material systems, precise control of the intrinsic properties of the TMDs plays a vital role in maximizing their performance. Defect-induced atomic doping through introduction of a chalcogen vacancy into the TMDs lattices is known to be a promising strategy for modulating their characteristic properties. As a result, there is a need to develop tunable and scalable synthesis routes to achieve vacancy-modulated TMDs. Herein, we propose a facile liquid-phase ligand exchange approach for scalable, uniform, and vacancy-tunable synthesis of TMDs films. Varying the relative molar ratio of the chalcogen to transition metal precursors enabled the in situ modulation of the chalcogen vacancy concentrations without necessitating additional post-treatments. When employed as the electrocatalyst in the hydrogen evolution reaction (HER), the vacancy-modulated TMDs, exhibiting a synergetic effect on the energy level matching to the reduction potential of water and optimized free energy differences in the HER pathways, showed a significant enhancement in the hydrogen production via the improved charge transfer kinetics and increased active sites. The proposed approach for synthesizing tunable vacancy-modulated TMDs with wafer-scale synthesis capability is, therefore, promising for better practical applications of TMDs.

Modulation of photocarrier relaxation dynamics in two-dimensional semiconductors

Due to strong Coulomb interactions, two-dimensional (2D) semiconductors can support excitons with large binding energies and complex many-particle states. Their strong light-matter coupling and emerging excitonic phenomena make them potential candidates for next-generation optoelectronic and valleytronic devices. The relaxation dynamics of optically excited states are a key ingredient of excitonic physics and directly impact the quantum efficiency and operating bandwidth of most photonic devices. Here, we summarize recent efforts in probing and modulating the photocarrier relaxation dynamics in 2D semiconductors. We classify these results according to the relaxation pathways or mechanisms they are associated with. The approaches discussed include both tailoring sample properties, such as the defect distribution and band structure, and applying external stimuli such as electric fields and mechanical strain. Particular emphasis is placed on discussing how the unique features of 2D semiconductors, including enhanced Coulomb interactions, sensitivity to the surrounding environment, flexible van der Waals (vdW) heterostructure construction, and non-degenerate valley/spin index of 2D transition metal dichalcogenides (TMDs), manifest themselves during photocarrier relaxation and how they can be manipulated. The extensive physical mechanisms that can be used to modulate photocarrier relaxation dynamics are instrumental for understanding and utilizing excitonic states in 2D semiconductors.

Are two-dimensional materials radiation tolerant?

Two-dimensional (2D) materials have many unique properties, which can be exploited in various applications. In particular, electronic devices based on 2D materials should ideally be suited for the operation in outer cosmic space due to their low weight, small size and low power consumption. This brings about the issue of their radiation hardness, or tolerance, which has recently been addressed in a number of studies. The results of these investigations are somewhat counterintuitive: although one can naively expect that atomically thin structures should easily be destroyed by the beams of energetic particles, the devices made from 2D materials were reported to exhibit extraordinary radiation hardness. In this Focus article, an overview of the recent studies on the subject is given, followed by the discussion of the origin of the reported high tolerance, which is inherently related to the response of 2D materials, the systems with the reduced dimensionality, to irradiation. The analysis of the experimental and theoretical data on the behavior of 2D systems under irradiation indicates that although free-standing 2D materials can indeed be referred to as radiation resilient systems under irradiation conditions corresponding to the outer space, this is generally not the case, as the environment, e.g., the substrate, can strongly influence the radiation tolerance of 2D materials and devices based on these systems.

Vacancy-Induced Antibacterial Activity of XS2-y Quantum Dots against Drug-Resistant Bacteria for Treatment of Bacterial Keratitis

Due to the widespread antibiotic-resistant microbes and the slow development in antibiotics, innovative new antibacterial agents are eagerly desired to control infection in the resistance era. Here, it is demonstrated that the antibacterial ability against drug-resistant bacteria can be endowed to transition metal dichalcogenides (XS₂ , X = Mo/W) quantum dots by sulfur vacancies, and their application in bacterial keratitis. The sulfur vacancies are generated by the ion irradiation with the controlled influences, which ensures the one-way electron transport from the external environment to XS₂ leading to a strong reactive oxygen speciesindependent oxidative stress. With the concentration of 140 µg mL^-1 of XS_2-0.1 quantum dots, the sterilization efficiency of Gram-positive Staphylococcus aureus and methicillin-resistant Staphylococcus aureus more than 99.9% within 20 min at room temperature in the dark is realized. For biomedical application against bacterial keratitis, it is observed that the occurrence of severe clinical manifestation like ocular perforation can be prevented. This work demonstrates the vacancy as a novel, simple, and effective strategy to tune XS₂ as the antibacterial agent with a fast response and no reliance on light that has significant potential therapeutic effects on clinical drug resistant bacterial keratitis.

Effect of localized helium ion irradiation on the performance of synthetic monolayer MoS2 field-effect transistors

Helium ion irradiation is a known method of tuning the electrical conductivity and charge carrier mobility of novel two-dimensional semiconductors. Here, we report a systematic study of the electrical performance of chemically synthesized monolayer molybdenum disulfide (MoS₂) field-effect transistors irradiated with a focused helium ion beam as a function of increasing areal irradiation coverage. We determine an optimal coverage range of approx. 10%, which allows for the improvement of both the carrier mobility in the transistor channel and the electrical conductance of the MoS₂, due to doping with ion beam-created sulfur vacancies. Larger areal irradiations introduce a higher concentration of scattering centers, hampering the electrical performance of the device. In addition, we find that irradiating the electrode-channel interface has a deleterious impact on charge transport when contrasted with irradiations confined only to the transistor channel.

Interface engineering of two-dimensional transition metal dichalcogenides towards next-generation electronic devices: recent advances and challenges

Over the past decade, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous research interest for future electronics owing to their atomically thin thickness, compelling properties and various potential applications. However, interface engineering including contact optimization and channel modulations for 2D TMDCs represents fundamental challenges in ultimate performance of ultrathin electronics. This article provides a comprehensive overview of the basic understanding of contacts and channel engineering of 2D TMDCs and emerging electronics benefiting from these varying approaches. In particular, we elucidate multifarious contact engineering approaches such as edge contact, phase engineering and metal transfer to suppress the Fermi level pinning effect at the metal/TMDC interface, various channel treatment avenues such as van der Waals heterostructures, surface charge transfer doping to modulate the device properties, and as well the novel electronics constructed by interface engineering such as diodes, circuits and memories. Finally, we conclude this review by addressing the current challenges facing 2D TMDCs towards next-generation electronics and offering our insights into future directions of this field.

Extrapolated Defect Transition Level in Two-Dimensional Materials: The Case of Charged Native Point Defects in Monolayer Hexagonal Boron Nitride

Defect formation energy as well as the charge transition level (CTL) plays a vital role in understanding the underlying mechanism of the effect of defects on material properties. However, the accurate calculation of charged defects, especially for two-dimensional materials, is still a challenging topic. In this paper, we proposed a simplified scheme to rescale the CTLs from the semilocal to the hybrid functional level, which is time-saving during the charged defect calculations. Based on this method, we systematically calculated the formation energy of four kinds of intrinsic point defects in two-dimensional hexagonal boron nitride (2D h-BN) by uniformly scaling the supercells by which we found a time-saving method to obtain the "special vacuum size" (Komsa, H.-P.; Berseneva, N.; Krasheninnikov, A. V.; Nieminen, R. M. Phys. Rev. X, 2014, 4, 031044). Native defects including nitrogen vacancy (V_N), boron vacancy (V_B), nitrogen atom anti-sited on boron position (N_B), and boron atom anti-sited on nitrogen position (B_N) were calculated. The reliability of our scheme was verified by taking V_N as a probe to conduct the hybrid functional calculation, and the rescaled CTL is within the acceptable error range with the pure HSE results. Based on the results of CTLs, all the native point defects in the h-BN monolayer act as hole or electron trap centers under certain conditions and would suppress the p- or n-type electrical conduction of h-BN-based devices. Our rescale method is also suitable for other materials for defect charge transition level calculations.

2D materials towards ultrafast photonic applications

Two-dimensional materials are now excelling in yet another arena of ultrafast photonics, including optical modulation through optical limiting/mode-locking, photodetectors, optical communications, integrated miniaturized all-optical devices, etc.

A Review of Recent Applications of Ion Beam Techniques on Nanomaterial Surface Modification: Design of Nanostructures and Energy Harvesting

Nanomaterials have gained plenty of research interest because of their excellent performance, which is derived from their small size and special structure. In practical applications, to acquire nanomaterials with high performance, many methods have been used to modulate the structure and components of materials. To date, ion beam techniques have extensively been applied for modulating the performance of various nanomaterials. Energetic ion beams can modulate the surface morphology and chemical components of nanomaterials. In addition, ion beam techniques have also been used to fabricate nanomaterials, including 2D materials, nanoparticles, and nanowires. Compared with conventional methods, ion beam techniques, including ion implantation, ion irradiation, and focused ion beam, are all pure physical processes; these processes do not introduce any impurities into the target materials. In addition, ion beam techniques exhibit high controllability and repeatability. Here, recent progress in ion beam techniques for nanomaterial surface modification is systematically summarized and existing challenges and potential solutions are presented.