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While some exceptional properties are unique to graphene only (itssignature Dirac-cone gapless dispersion, carrier mobility, recordstrength), other features are common to other two-dimensional materials.The broader family “beyond graphene” offers greaterchoices to be explored and tailored for various applications. Transitionmetal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), and 2D layers of pure elements, like phosphorus or boron, cancomplement or even surpass graphene in many ways and uses, rangingfrom electronics and optoelectronics to catalysis and energy storage.Their availability greatly relies on chemical vapor deposition growthof large samples, which are highly polycrystalline and include interfacessuch as edges, heterostructures, and grain boundaries, as well asdislocations and point defects. These imperfections do not alwaysdegrade the material properties, but they often bring new physicsand even useful functionality. It turns particularly interesting incombination with the sheer openness of all 2D sheets, fully exposedto the environment, which, as we show herein, can change and tunethe defect structures and consequently all their qualities, from electroniclevels, conductivity, magnetism, and optics to structural mobilityof dislocations and catalytic activities. [ABSTRACT FROM AUTHOR]

Wedemonstrate how substrate interfacial chemistry can be utilizedto tailor the physical properties of single-crystalline molybdenumdisulfide (MoS2) atomic-layers. Semiconducting, two-dimensionalMoS2possesses unique properties that are promising forfuture optical and electrical applications for which the ability totune its physical properties is essential. We use self-assembled monolayerswith a variety of end termination chemistries to functionalize substratesand systematically study their influence on the physical propertiesof MoS2. Using electrical transport measurements, temperature-dependentphotoluminescence spectroscopy, and empirical and first-principlescalculations, we explore the possible mechanisms involved. Our datashows that combined interface-related effects of charge transfer,built-in molecular polarities, varied densities of defects, and remoteinterfacial phonons strongly modify the electrical and optical propertiesof MoS2. These findings can be used to effectively enhanceor modulate the conductivity, field-effect mobility, and photoluminescencein MoS2monolayers, illustrating an approach for localand universal property modulations in two-dimensional atomic-layers. [ABSTRACT FROM AUTHOR]

Monolayer molybdenum disulfide (MoS2) is a two-dimensionaldirect band gap semiconductor with unique mechanical, electronic,optical, and chemical properties that can be utilized for novel nanoelectronicsand optoelectronics devices. The performance of these devices stronglydepends on the quality and defect morphology of the MoS2layers. Here we provide a systematic study of intrinsic structuraldefects in chemical vapor phase grown monolayer MoS2, includingpoint defects, dislocations, grain boundaries, and edges, via directatomic resolution imaging, and explore their energy landscape andelectronic properties using first-principles calculations. A richvariety of point defects and dislocation cores, distinct from thosepresent in graphene, were observed in MoS2. We discoverthat one-dimensional metallic wires can be created via two differenttypes of 60° grain boundaries consisting of distinct 4-fold ringchains. A new type of edge reconstruction, representing a transitionstate during growth, was also identified, providing insights intothe material growth mechanism. The atomic scale study of structuraldefects presented here brings new opportunities to tailor the propertiesof MoS2via controlled synthesis and defect engineering. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenide monolayers exhibit ultrahigh surface sensitivity since they expose all atoms to the surface and thereby influence their optoelectronic properties. Here, we report an intriguing lightening of the photoluminescence (PL) from the edge to the interior over time in the WS 2 monolayers grown by physical vapor deposition method, with the whole monolayer brightened eventually. Comprehensive optical studies reveal that the PL enhancement arises from the p doping induced by oxygen adsorption. First-principles calculations unveil that the dissociation of chemisorbed oxygen molecule plays a significant role; i.e., the dissociation at one site can largely promote the dissociation at a nearby site, facilitating the photoluminescence lightening. In addition, we further manipulate such PL brightening rate by controlling the oxygen concentration and the temperature. The presented results uncover the extraordinary surface chemistry and related mechanism in WS 2 monolayers, which deepens our insight into their unique PL evolution behavior.

Two-dimensional transition metal dichalcogenides (TMDCs) show great potential as efficient catalysts for Li-CO 2 batteries. However, the basal plane engineering on TMDCs toward bifunctional catalysts for Li-CO 2 batteries is still poorly understood. In this work, density functional theory calculations reveal that nucleophilic N dopants and electrophilic S vacancies in the ReS 2 plane tailor the interactions with Li atoms and C/O atoms in intermediates, respectively. The electrophilic and nucleophilic dual centers show suitable adsorption with all intermediates during discharge and charge, resulting in a small energy barrier for the rate-determining step. Thus, an efficient bifunctional catalyst is produced toward Li-CO 2 batteries. As a result, the optimal catalyst achieves an ultrasmall voltage gap of 0.66 V and an ultrahigh energy efficiency of 81.1% at 20 μA cm -2 , which is superior to those of previous catalysts under similar conditions. The introduction of electrophilic and nucleophilic dual centers provides new avenues for designing excellent bifunctional catalysts for Li-CO 2 batteries.

Two-dimensional (2D) MXene-loaded single-atom (SA) catalysts have drawn increasing attention. SAs immobilized on oxygen vacancies (O V ) of MXene are predicted to have excellent catalytic performance; however, they have not yet been realized experimentally. Here Pt SAs immobilized on the O V of monolayer Ti 3 C 2 T x flakes are constructed by a rapid thermal shock technique under a H 2 atmosphere. The resultant Ti 3 C 2 T x -Pt SA catalyst exhibits excellent hydrogen evolution reaction (HER) performance, including a small overpotential of 38 mV at 10 mA cm -2 , a high mass activity of 23.21 A mg Pt -1 , and a large turnover frequency of 23.45 s -1 at an overpotential of 100 mV. Furthermore, density functional theory calculations demonstrate that anchoring the Pt SA on the O V of Ti 3 C 2 T x helps to decrease the binding energy and the hybridization strength between H atoms and the supports, contributing to rapid hydrogen adsorption-desorption kinetics and high activity for the HER.

Flexible lithium-sulfur (Li-S) batteries with high mechanical compliance and energy density are highly desired. This manuscript reported that large-area freestanding MXene (Ti 3 C 2 T x ) film has been obtained through a scalable drop-casting method, significantly improving adhesion to the sulfur layer under the continuously bent. Titanium oxide anchored on holey Ti 3 C 2 T x (TiO 2 /H-Ti 3 C 2 T x ) was also produced by the well-controlled oxidation of few-layer Ti 3 C 2 T x , which greatly facilitates lithium ion transport as well as prevents the shuttling of lithium polysulfides. Therefore, the obtained sandwich electrode has demonstrated a high capacity of 740 mAh g -1 at 2 C and a high capacity retention of 81% at 1 C after 500 cycles. Flexible Li-S batteries based on this sandwich electrode have a capacity retention as high as 95% after bending 500 times. This work provides effective design strategies of MXene for flexible batteries and wearable electronics.

The lack of low-cost catalysts with high activity leads to the unsatisfactory electrochemical performance of Li-CO 2 batteries. Single-atom catalysts (SACs) with metal-N x moieties have great potential to improve battery reaction kinetics and cycling ability. However, how to rationally select and develop highly efficient electrocatalysts remains unclear. Herein, we used density functional theory (DFT) calculations to screen SACs on N-doped graphene (SAMe@NG, Me = Cr, Mn, Fe, Co, Ni, Cu) for CO 2 reduction and evolution reaction. Among them, SACr@NG shows the promising potential as an effective electrocatalyst for the reversible Li-CO 2 batteries. To verify the validity of the DFT calculations, a two-step method has been developed to fabricate SAMe@NG on a porous carbon foam (SAMe@NG/PCF) with similar loading of ∼8 wt %. Consistent with the theoretical calculations, batteries with the SACr@NG/PCF cathodes exhibit a superior rate performance and cycling ability, with a long cycle life and a narrow voltage gap of 1.39 V over 350 cycles at a rate of 100 μA cm -2 . This work not only demonstrates a principle for catalysts selection for the reversible Li-CO 2 batteries but also a controllable synthesis method for single atom catalysts.

The development of strategies for tuning the electronic structure of the metal sites in single-atom catalysts (SACs) is the key to optimizing their activity. Herein, we report that iodine doping within the carbon matrix of a cobalt-nitrogen-carbon (Co-N-C) catalyst can effectively modulate its electronic structure and catalytic activity toward the hydrogen evolution reaction (HER). The iodine-doped Co-N-C catalyst shows exceptional HER activity in acid with an overpotential of merely 52 mV at 10 mA cm -2 , a small Tafel slope of 56.1 mV dec -1 , making it among the best SACs based on both precious and nonprecious metals. Moreover, this catalyst possesses a high turnover frequency (TOF) value of 1.88 s -1 (η = 100 mV), which is about 1 order of magnitude larger than that (0.2 s -1 ) of the iodine-free counterpart. Experimental and theoretical studies demonstrate that the introduction of iodine dopants lowers the chemical oxidation state of the Co sites, resulting in the optimized hydrogen adsorption and facilitated HER kinetics. This work provides an alternative strategy to regulate the electronic structure of SACs for improved performance.

Despite recent advances in layered ferromagnets, ferromagnetic interactions in these materials are rather weak. Here, we report pressure-enhanced ferromagnetism in layered CrSiTe 3 flakes revealed by high-pressure magnetic circular dichroism measurements. Below ∼3 GPa, CrSiTe 3 undergoes a paramagnetic-to-ferromagnetic phase transition at ∼32 K, and the field-induced spin-flip in the ferromagnetic phase produces nearly zero hysteresis loops, demonstrating soft ferromagnetism. Above ∼4 GPa, a soft-to-hard ferromagnetic transition occurs, signaled by rectangular-shaped hysteresis loops with finite coercivity and remanent magnetization. Interestingly, as pressure increases, the Curie temperature and coercivity dramatically increase up to ∼138 K and 0.17 T at 7.8 GPa, respectively, in contrast to ∼36 K and 0.02 T at 4.6 GPa. It indicates a remarkable influence of pressure on exchange interactions, which is consistent with DFT calculations. The effective interaction between magnetic couplings and external pressure offers new opportunities in pursuit of high-temperature layered ferromagnets.

As one of the CO 2 capture and utilization technologies, Li-CO 2 batteries have attracted special interest in the application of carbon neutral. However, the design and fabrication of a low-cost high-efficiency cathode catalyst for reversible Li 2 CO 3 formation and decomposition remains challenging. Here, guided by theoretical calculations, CO 2 was utilized to activate the catalytic activity of conventional nitrogen-doped graphene, in which pyridinic-N and pyrrolic-N have a high total content (72.65%) and have a high catalytic activity in both CO 2 reduction and evolution reactions, thus activating the reversible conversion of Li 2 CO 3 formation and decomposition. As a result, the designed cathode has a low voltage gap of 2.13 V at 1200 mA g -1 and long-life cycling stability with a small increase in the voltage gap of 0.12 V after 170 cycles at 500 mA g -1 . Our work suggests a way to design metal-free catalysts with high activity that can be used to activate the performance of Li-CO 2 batteries.

The realization of high-temperature excitonic Bose-Einstein condensation (BEC) in practical materials poses great challenges, because of strict constraints in symmetry, exciton binding, lifetime, and interaction. Here, using first-principles methods and symmetry analysis, we propose a new route to realize high-temperature excitonic BEC in centrosymmetric 2D materials, exploiting the parity symmetry of band edges and reduced Coulomb screening. We demonstrate it by taking monolayer TiS 3 as an example, whose lowest-energy exciton shows small exciton mass, small Bohr radius, large binding, and long lifetime simultaneously. The phase diagram of electron-hole systems is further constructed, showing that both BEC and superfluidity can be realized at high temperature and in a broad range of exciton density. Importantly, we reveal that the high-temperature character of excitonic BEC is robust against thickness, beneficial for its experimental observation. By application of this general strategy to 2D materials in the database, monolayer AuBr and BiS 2 are identified as promising candidates for high-temperature excitonic BEC.

We investigated the evolution of ferromagnetism in layered Fe 3 GeTe 2 flakes under different pressures and temperatures using in situ magnetic circular dichroism (MCD) spectroscopy. We found that the rectangular shape of the hysteresis loop under an out-of-plane magnetic field sweep can be sustained below 7 GPa. Above that pressure, an intermediate state appears in the low-temperature region signaled by an 8-shaped skewed hysteresis loop. Meanwhile, the coercive field and Curie temperature decrease with increasing pressures, implying the decrease of the exchange interaction and the magneto-crystalline anisotropy under pressures. The intermediate phase has a labyrinthine domain structure, which is attributed to the increase of the ratio of exchange interaction to magneto-crystalline anisotropy based on Jagla's theory. Moreover, our calculations reveal a weak structural transition around 6 GPa that corresponds to a significant change in the FeI-FeI bond length, which has strong influences on magnetic interaction. Detailed analysis on exchange interaction and magneto-crystalline anisotropy with pressure shows a consistent trend with experiments.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have emerged as attractive platforms in next-generation nanoelectronics and optoelectronics for reducing device sizes down to a 10 nm scale. To achieve this, the controlled synthesis of wafer-scale single-crystal TMDs with high crystallinity has been a continuous pursuit. However, previous efforts to epitaxially grow TMD films on insulating substrates ( e.g. , mica and sapphire) failed to eliminate the evolution of antiparallel domains and twin boundaries, leading to the formation of polycrystalline films. Herein, we report the epitaxial growth of wafer-scale single-crystal MoS 2 monolayers on vicinal Au(111) thin films, as obtained by melting and resolidifying commercial Au foils. The unidirectional alignment and seamless stitching of the MoS 2 domains were comprehensively demonstrated using atomic- to centimeter-scale characterization techniques. By utilizing onsite scanning tunneling microscope characterizations combined with first-principles calculations, it was revealed that the nucleation of MoS 2 monolayer is dominantly guided by the steps on Au(111), which leads to highly oriented growth of MoS 2 along the ⟨110⟩ step edges. This work, thereby, makes a significant step toward the practical applications of MoS 2 monolayers and the large-scale integration of 2D electronics.

Two-dimensional van der Waals (vdW) magnetic materials are well-recognized milestones toward nanostructured spintronics. An interesting example is CrI 3 ; its magnetic states can be modulated electrically, allowing spintronics applications that are highly compatible with electronics technologies. Here, we report the electric field alone induces the interlayer antiferromagnetic-to-ferromagnetic (AFM-to-FM) phase transition in CrI 3 bilayers with critical field as low as 0.12 V/Å. The AFM-FM energy difference ΔE increases with electric field and is closely related to the field-induced on-site energy difference defined as the splitting between the electronic states of the two vdW layers. Our tight-binding model fits closely with ΔE as a function of electric field and gives a consistent estimation for orbital hopping, exchange splitting, and crystal field splitting. Furthermore, a CrI 3 -based spin field-effect device is suggested with the spin current switched on and off solely by the electric field. These findings not only reveal the physics underlying the transition but also provide guidelines for future discovery and design.

The transformation of 2H-MoS 2 from semiconducting 2H to metallic 1T phases is critical for its electrochemical and device applications, where the formation and dynamics of electronic heterostructures play a key role. Using first-principles calculations, we explore detailed atomic structures and migration processes of such interfaces. While armchair interfacial bonding is severely weakened by the distortion in 1T phase, stable structures form for either Mo- or S-orientated zigzag interfaces with low contact resistance. Different zigzag interfaces have distinct local bonding, which renders interface migration behaviors strongly anisotropic. For Mo-oriented interfaces, both a low formation energy and the migration barrier of the kinks make them prone to fast migration. In contrast, the S-oriented interfaces are more immobile due to the high formation energies of kinks and thus dominate the physical properties of the whole heterostructures. Our findings not only explain various experimental observations but also provide insights into phase transition behaviors in 2D MoS 2 .

Large-scale implementation of electrochemical water splitting for hydrogen evolution requires cheap and efficient catalysts to replace expensive platinum. However, catalysts that work well at high current densities with ultrafast intrinsic activities is still the central challenge for hydrogen evolution. An ideal case is to use single atoms on monolayer two-dimensional (2D) materials, which simplifies the system and in turn benefits the mechanism study, but is a grand challenge to synthesize. Here, we report a universal cold hydrogen plasma reduction method for synthesizing different single atoms sitting on 2D monolayers. In the case of molybdenum disulfide, we design and identify a type of active site, i . e ., unsaturated Mo single atoms on cogenetic monolayer molybdenum disulfide. The catalyst shows exceptional intrinsic activity with a Tafel slope of 36.4 mV dec -1 in 0.5 M H 2 SO 4 and superior performance at a high current density of 400 mA cm -2 with an overpotential of ∼260 mV, based on single flake microcell measurements. Theoretical studies indicate that coordinately unsaturated Mo single atoms sitting on molybdenum disulfide increase the bond strength between adsorbed hydrogen atoms and the substrates through hybridization, leading to fast hydrogen adsorption/desorption kinetics and superior hydrogen evolution activity. This work shines fresh light on preparing highly efficient electrocatalysts for water splitting and other electrochemical processes, as well as provides a general method to synthesize single atoms on two-dimensional monolayers.

Cobalt-nitrogen-functionalized materials have been recognized as promising catalysts for the CO 2 reduction reaction because of their superior activity. In order to further improve their activity, we proposed an optimization method through coordination engineering in cobalt-nitrogen-functionalized porphyrin and graphene. By considering a series of derived structures with coordinating nitrogen atoms substituted by carbon or oxygen atoms, a clear activity trend is obtained by constructing a volcano-type plot for activity against adsorption energies of *CO. Detailed electronic structure analysis shows that the enhanced catalytic activity is due to the lacking of π bonding in Co-O bonds compared to Co-C or Co-N bonds in cobalt-centered motifs. This difference allows us to predict the catalytic activity by using the vacancy formation energy of the cobalt atom. Our work provides a general guideline for a rational design of efficient catalysts, which may stimulate further study of coordination engineering for other key energy conversion processes.

Among two-dimensional (2D) transition-metal dichalcogenides (TMDCs), platinum diselenide (PtSe 2 ) stands in a distinct place due to its fancy transition from type-II Dirac semimetal to semiconductor with a thickness variation from bulk to monolayer (1 ML) and the related versatile applications especially in mid-infrared detectors. However, achieving atomically thin PtSe 2 is still a challenging issue. Herein, we have designed a facile chemical vapor deposition (CVD) method to achieve the synthesis of atomically thin 1T-PtSe 2 on an electrode material of Au foil. Thanks to the high crystalline quality, we have confirmed the complete transition from semimetal to semiconductor from trilayer (3 ML) to 1 ML 1T-PtSe 2 . More importantly, we have found that such atomically thin 1T-PtSe 2 can serve as perfect electrocatalysts, featured with a record high hydrogen evolution reaction (HER) efficiency (comparable to traditional Pt catalyst). Our work is helpful toward the large-scale synthesis, exotic physical property exploration, and intriguing application development of atomically thin TMDCs.

Although dislocations and grain boundaries (GBs) are ubiquitous in large-scale MoS 2 samples, their interaction with phonons, which plays an important role in determining the lattice thermal conductivity of polycrystalline MoS 2 , remains elusive. Here, we perform a systematic study of the heat transport in two-dimensional polycrystalline MoS 2 by both molecular dynamics simulation and atomic Green's function method. Our results indicate that the thermal boundary conductance of GBs of MoS 2 is in the range from 6.4 × 10 8 to 35.3 × 10 8 W m -2 K -1 , which is closely correlated with the overlap between the vibrational density of states of GBs and those of the pristine lattice, as well as the GB energy. It is found that the GBs strongly scatter the phonons with frequency larger than 2 THz, accompanied by a pronounced phonon localization effect and significantly reduced phonon group velocities. Furthermore, by comparing the results from realistic polycrystalline MoS 2 to those from different theoretical models, we observe that the Casimir model is broken down and detailed phonon dynamics at a specific GB should be taken into account to accurately describe the phonon transport in polycrystalline materials. Our findings will provide useful guidelines for designing efficient thermoelectric and thermal management materials based on phonon-GB interaction.

Two-dimensional (2D) metal-semiconductor transition-metal dichalcogenide (TMDC) vertical heterostructures play a crucial role in device engineering and contact tuning fields, while their direct integration still challenging. Herein, a robust epitaxial growth method is designed to construct multiple lattice-matched 2D metal-semiconductor TMDC vertical stacks (VSe 2 /MX 2 , M: Mo, W; X: S, Se) by a two-step chemical vapor deposition method. Intriguingly, the metallic VSe 2 preferred to nucleate and extend from the energy-favorable edge site of the semiconducting MX 2 underlayer to form VSe 2 /MX 2 vertical heterostructures. This growth behavior was also confirmed by density functional theory calculations of the initial adsorption of VSe 2 adatoms. In particular, the formation of Schottky-diode or Ohmic contact-type band alignments was detected for the stacks between VSe 2 and p-type WSe 2 or n-type MoSe 2 , respectively. This work hereby provides insights into the direct integration, band-alignment engineering, and potential applications of such 2D metal-semiconductor stacks in next-generation electronics, optoelectronic devices, and energy-related fields.

The practical application of lithium-sulfur (Li-S) batteries is hindered by their poor cycling stabilities that primarily stem from the "shuttle" of dissolved lithium polysulfides. Here, we develop a nepenthes-like N-doped hierarchical graphene (NHG)-based separator to realize an efficient polysulfide scavenger for Li-S batteries. The 3D textural porous NHG architectures are realized by our designed biotemplating chemical vapor deposition (CVD) approach via the employment of naturally abundant diatomite as the growth substrate. Benefiting from the high surface area, devious inner-channel structure, and abundant nitrogen doping of CVD-grown NHG frameworks, the derived separator favorably synergizes bifunctionality of physical confinement and chemical immobilization toward polysulfides, accompanied by smooth lithium ion diffusions. Accordingly, the batteries with the NHG-based separator delivers an initial capacity of 868 mAh g -1 with an average capacity decay of only 0.067% per cycle at 2 C for 800 cycles. A capacity of 805 mAh g -1 can further be achieved at a high sulfur loading of ∼7.2 mg cm -2 . The present study demonstrates the potential in constructing high-energy and long-life Li-S batteries upon separator modification.

The development of abundant and cheap electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing attention over recent years. However, to achieve low-cost HER electrocatalysis, especially in alkaline media, is still a big challenge due to the sluggish water dissociation kinetics as well as the poor long-term stability of catalysts. In this paper we report the design and synthesis of a two-dimensional (2D) MoS 2 confined Co(OH) 2 nanoparticle electrocatalyst, which accelerates water dissociation and exhibits good durability in alkaline solutions, leading to significant improvement in HER performance. A two-step method was used to synthesize the electrocatalyst, starting with the lithium intercalation of exfoliated MoS 2 nanosheets followed by Co 2+ exchange in alkaline media to form MoS 2 intercalated with Co(OH) 2 nanoparticles (denoted Co-Ex-MoS 2 ), which was fully characterized by spectroscopic studies. Electrochemical tests indicated that the electrocatalyst exhibits superior HER activity and excellent stability, with an onset overpotential and Tafel slope as low as 15 mV and 53 mV dec -1 , respectively, which are among the best values reported so far for the Pt-free HER in alkaline media. Furthermore, density functional theory calculations show that the cojoint roles of Co(OH) 2 nanoparticles and MoS 2 nanosheets result in the excellent activity of the Co-Ex-MoS 2 electrocatalyst, and the good stability is attributed to the confinement of the Co(OH) 2 nanoparticles. This work provides an imporant strategy for designing HER electrocatalysts in alkaline solutions, and can, in principle, be expanded to other materials besides the Co(OH) 2 and MoS 2 used here.

The recent development of two-dimensional transition-metal dichalcogenides in electronics and optoelelectronics has triggered the exploration in spintronics, with high demand in search for half-metallicity in these systems. Here, through density functional theory (DFT) calculations, we predict robust half-metallic behaviors in Co-edge-doped WSe 2 nanoribbons (NRs). With electrons partially occupying the antibonding state consisting of Co 3d yz and Se 4p z orbitals, the system becomes spin-polarized due to the defect-state-induced Stoner effect and the strong exchange splitting eventually gives rise to the half-metallicity. The half-metal gap reaches 0.15 eV on the DFT generalized gradient approximation level and increases significantly to 0.67 eV using hybrid functional. Furthermore, we find that the half-metallicity sustains even under large external strain and relatively low edge doping concentration, which promises the potential of such Co-edge-doped WSe 2 NRs in spintronics applications.

The cathodic oxygen reduction reaction (ORR) is essential in the electrochemical energy conversion of fuel cells. Here, through the NH 3 atmosphere annealing of a graphene oxide (GO) precursor containing trace amounts of Ru, we have synthesized atomically dispersed Ru on nitrogen-doped graphene that performs as an electrocatalyst for the ORR in acidic medium. The Ru/nitrogen-doped GO catalyst exhibits excellent four-electron ORR activity, offering onset and half-wave potentials of 0.89 and 0.75 V, respectively, vs a reversible hydrogen electrode (RHE) in 0.1 M HClO 4 , together with better durability and tolerance toward methanol and carbon monoxide poisoning than seen in commercial Pt/C catalysts. X-ray adsorption fine structure analysis and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy are performed and indicate that the chemical structure of Ru is predominantly composed of isolated Ru atoms coordinated with nitrogen atoms on the graphene substrate. Furthermore, a density function theory study of the ORR mechanism suggests that a Ru-oxo-N 4 structure appears to be responsible for the ORR catalytic activity in the acidic medium. These findings provide a route for the design of efficient ORR single-atom catalysts.

Vertically stacked van der Waals heterostructures composed of compositionally different two-dimensional atomic layers give rise to interesting properties due to substantial interactions between the layers. However, these interactions can be easily obscured by the twisting of atomic layers or cross-contamination introduced by transfer processes, rendering their experimental demonstration challenging. Here, we explore the electronic structure and its strain dependence of stacked MoSe2/WSe2 heterostructures directly synthesized by chemical vapor deposition, which unambiguously reveal strong electronic coupling between the atomic layers. The direct and indirect band gaps (1.48 and 1.28 eV) of the heterostructures are measured to be lower than the band gaps of individual MoSe2 (1.50 eV) and WSe2 (1.60 eV) layers. Photoluminescence measurements further show that both the direct and indirect band gaps undergo redshifts with applied tensile strain to the heterostructures, with the change of the indirect gap being particularly more sensitive to strain. This demonstration of strain engineering in van der Waals heterostructures opens a new route toward fabricating flexible electronics.

The practical recycling of carbon dioxide (CO2) by the electrochemical reduction route requires an active, stable, and affordable catalyst system. Although noble metals such as gold and silver have been demonstrated to reduce CO2 into carbon monoxide (CO) efficiently, they suffer from poor durability and scarcity. Here we report three-dimensional (3D) graphene foam incorporated with nitrogen defects as a metal-free catalyst for CO2 reduction. The nitrogen-doped 3D graphene foam requires negligible onset overpotential (-0.19 V) for CO formation, and it exhibits superior activity over Au and Ag, achieving similar maximum Faradaic efficiency for CO production (∼85%) at a lower overpotential (-0.47 V) and better stability for at least 5 h. The dependence of catalytic activity on N-defect structures is unraveled by systematic experimental investigations. Indeed, the density functional theory calculations confirm pyridinic N as the most active site for CO2 reduction, consistent with experimental results.

Grain boundaries (GBs) of hexagonal boron nitride (h-BN) grown on Cu(111) were investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The first experimental evidence of the GBs composed of square-octagon pairs (4|8 GBs) was given, together with those containing pentagon-heptagon pairs (5|7 GBs). Two types of GBs were found to exhibit significantly different electronic properties, where the band gap of the 5|7 GB was dramatically decreased as compared with that of the 4|8 GB, consistent with our obtained result from density functional theory (DFT) calculations. Moreover, the present work may provide a possibility of tuning the inert electronic property of h-BN via grain boundary engineering.

The challenge in the electrosynthesis of fuels from CO2 is to achieve durable and active performance with cost-effective catalysts. Here, we report that carbon nanotubes (CNTs), doped with nitrogen to form resident electron-rich defects, can act as highly efficient and, more importantly, stable catalysts for the conversion of CO2 to CO. The unprecedented overpotential (-0.18 V) and selectivity (80%) observed on nitrogen-doped CNTs (NCNTs) are attributed to their unique features to facilitate the reaction, including (i) high electrical conductivity, (ii) preferable catalytic sites (pyridinic N defects), and (iii) low free energy for CO2 activation and high barrier for hydrogen evolution. Indeed, DFT calculations show a low free energy barrier for the potential-limiting step to form key intermediate COOH as well as strong binding energy of adsorbed COOH and weak binding energy for the adsorbed CO. The highest selective site toward CO production is pyridinic N, and the NCNT-based electrodes exhibit no degradation over 10 h of continuous operation, suggesting the structural stability of the electrode.

The two-dimensional (2D) transition metal dichalcogenides (TMDC, of generic formula MX2) monolayer displays the "triple-decker" structure with the chemical bond organization much more complex than in well-studied monatomic layers of graphene or boron nitride. Accordingly, the makeup of the dislocations in TMDC permits chemical variability, depending sensitively on the equilibrium with the environment. In particular, first-principles calculations show that dislocations state can be switched to highly mobile, profoundly changing the lattice relaxation and leading to superplastic behavior. With 2D MoS2 as an example, we construct full map for dislocation dynamics, at different chemical potentials, for both the M- and X-oriented dislocations. Depending on the structure of the migrating dislocation, two different dynamic mechanisms are revealed: either the direct rebonding (RB) mechanism where only a single metal atom shifts slightly, or generalized Stone-Wales (SW(g)) rotation in which several atoms undergo significant displacements. The migration barriers for RB mechanism can be 2-4 times lower than for the SW(g). Our analyses show that within a range of chemical potentials, highly mobile dislocations could at the same time be thermodynamically favored, that is statistically dominating the overall material property. This demonstrates remarkable possibility of changing material basic property such as plasticity by changing elemental chemical potentials of the environment.

Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 (M = Mo, W; X = S, Se) can actually improve the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all display a substantial magnetic moment of ∼1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically nonmagnetic. GBs composed of pentagon-heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47°, the structural energetics favor square-octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX2 into a promising 2D magnetic semiconductor.

Guided by the principles of dislocation theory, we use the first-principles calculations to determine the structure and properties of dislocations and grain boundaries (GB) in single-layer transition metal disulfides MS(2) (M = Mo or W). In sharp contrast to other two-dimensional materials (truly planar graphene and h-BN), here the edge dislocations extend in third dimension, forming concave dreidel-shaped polyhedra. They include different number of homoelemental bonds and, by reacting with vacancies, interstitials, and atom substitutions, yield families of the derivative cores for each Burgers vector. The overall structures of GB are controlled by both local-chemical and far-field mechanical energies and display different combinations of dislocation cores. Further, we find two distinct electronic behaviors of GB. Typically, their localized deep-level states act as sinks for carriers but at large 60°-tilt the GB become metallic. The analysis shows how the versatile GB in MS(2) (if carefully engineered) should enable new developments for electronic and opto-electronic applications.

A new dislocation structure-square-octagon pair (4|8) is discovered in two-dimensional boron nitride (h-BN), via first-principles calculations. It has lower energy than corresponding pentagon-heptagon pairs (5|7), which contain unfavorable homoelemental bonds. On the basis of the structures of dislocations, grain boundaries (GB) in BN are investigated. Depending on the tilt angle of grains, GB can be either polar (B-rich or N-rich), constituted by 5|7s, or unpolar, composed of 4|8s. The polar GBs carry net charges, positive at B-rich and negative at N-rich ones. In contrast to GBs in graphene which generally impede the electronic transport, polar GBs have a smaller bandgap compared to perfect BN, which may suggest interesting electronic and optical applications.