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Two-dimensional (2D) materials attract enormous attention and show promising applications in many fields of science and technologies (nanodevices, energy storage/harvest and catalytic processes, etc.). Pentagonal compounds emerge as a new family in 2D materials along with classic trigonal transition metal dichalcogenides and MXenes, which have been intensively investigated to date. Encouraged by the successful synthesis of pentagonal PdSe2 using CVD method, we explore nine pentagonal monolayers, MX2 (M = Ni, Pd & Pt, and X = S, Se & Te), based on the first-principles calculations. We find that all MX2 are dynamically and thermodynamically stable, and intrinsic semiconductors. Our results show that PdTe2 exhibits excellent potential application in solar water splitting due to optimal band gap and suitable band-edge positions matching with both the water reduction and oxidation potentials (0 and 1.23 V vs. NHE). We further find that the majority of MX2 monolayers (except NiTe2) are applicable in photocatalytic oxygen production. Our findings are expected to shed light on the possible synthesis of pentagonal MX2 and their application in photocatalytic water splitting.

Two-dimensional (2D) elemental metals have attracted considerable attention because of their fascinating physical properties and potential diverse applications. Information on the structures, mechanical properties, and dynamical stabilities of single atom thick phases of metallic elements in the periodic table has been previously reported. Here, the magnetic properties of 45 2D metals in three lattice structures (hexagonal, square, and honeycomb) were systematically explored using density functional theory calculations. Although it is well known that there are only five magnetic elements in their 3D counterparts, it is interesting to find that 18 of the 45 2D elemental metals are endowed with magnetism due to coordination number decreases and energy band narrowing of the out-of-plane orbitals. In particular, the honeycomb lattice is more likely to be magnetic than the square lattice. Most of the magnetic 2D metals are ferromagnetic and only a few are anti-ferromagnetic. It is further shown that the 2D metals with higher structural symmetry have smaller magnetic moments. These results provide a foundation for the further study of their magnetic/spintronic properties and their potential applications, such as in catalysis and gas sensing.

Due to the unique properties and diverse applications, two-dimensional (2D) nanomaterials have been widely investigated in these years. In this work, a group of new pentagonal 2D transitional-metal sulphide monolayers (MS, M = Fe, Mn, and V) are designed based on density-functional-theory (DFT) calculations. We show that all three monolayer structures are thermally, dynamically and mechanically stable. We find that FeS, MnS and VS are metallic, and FeS and MnS are also magnetic, in which the magnetic moments are contributed by the Fe and Mn atoms, respectively. We further find that these MS monolayers show high activities for hydrogen evolution reactions (HER) as the calculated Gibbs free energies are close to zero electron volt (eV), especially for FeS and VS. Our findings broaden the family of noble-metal-free two-dimensional materials, and also help to develop low cost commercial HER catalysts.

High-performance nanodevices require fast and reversible tunability of electronic and optical properties under external stimuli. In the current work, using first-principles simulations and non-equilibrium Green function transport calculations, we demonstrate that bending can effectively and asymmetrically modulate the optoelectronic properties of Janus transition-metal dichalcogenides (J-TMDCs), due to their out-of-plane flexoelectric gating. The dynamic correlation of the electronic and optical behaviors is revealed by the bending-induced interplay between the quantum confined giant Stark effect and deformation potential. The nonsymmetric directional-information encoded in the concave and convex bending motions and the intrinsic dipole of the atomically thin film renders J-TMDCs promising for wearable motion sensors and chemical sensors.

MoN2 monolayer is a newly discovered two-dimensional material with strong ferromagnetic ordering in its ground state. We found through the first-principle calculations that the ferromagnetic spin ordering within MoN2 varies dramatically due to moderate structural deformations. For example, absorption of a Fe atom on the top of the Mo atom results in structural deformation and the magnetic moment decreases significantly. Further exploration by manually moving the N atom horizontally towards the Mo atom or the hollow site reveals that the ferromagnetic spin ordering vanishes only if the displacement of the N atom from its equilibrium position is larger than 0.5 A. Based on these findings, we further propose and demonstrate a nanomechanical modulation by imposing redial deformation to sensitively recover or quench the spin polarization within a MoN2 nanotube. Our work suggests the great potential of the 2D MoN2 material in the nanoscale electronic and spintronic applications.

One of the major challenging issues in lithium-sulfur (Li-S) batteries is that the lithium polysulfides easily dissolve in the electrolyte and shuttle between anode and cathode. To overcome this problem, an ideal anchoring materials is urgent. In the current work, we explore the potential application of BC2N allotropes (approximated by Type-I and Type-II structures) as anchoring materials for Li-S batteries by using first-principles method with van der Waals interaction. Type-I is a direct-bandgap semiconductor, while type-II is semi-metallic due to inversion symmetry of its structure. They show remarkable but not too strong chemical interaction with S8 and Li2Sx clusters and enhanced electrical conductivity. In addition, the introduction of dopants and defects result in semiconductor-to-metal transition on type-I BC2N, and enhancement of binding energies and still keep intact Li2Sx clusters. Our findings demonstrate that defective and doped BC2N monolayers are promising anchoring materials for Li-S batteries.

The conventional layered-growth modes of atom/small molecule self-assembly on solid surfaces are usually obtained from the “mean field approximation” description of adsorbate–substrate interactions, which neglected the differences in kinetic parameters between different adsorption configurations. Here, through a multiscale computational method, we showed a novel mixed layered self-assembly growth of C60 molecules on an oxygen-passivated Fe(001)-p(1 × 1)O substrate through considering differences in kinetic parameters between adsorption configurations in detail. The simulated mixed growth is consistent with the experimental observation (Picone, et al. Appl. Mater. Interfaces 2016, 8, 26418) in which regions of large C60 islands that have been nucleated coexist with areas of well-separated C60 molecules. With a simple general model, the mixed layered-growth mode can be applied for other large molecules’ self-assembly, provided that the multidegrees of freedom in adsorption configurations give distinct growt...

Owning the merits of both lead-free and air-stable, the double-perovskite Cs2AgBiBr6 has attracted increasing attention, but suffers low visible-light absorption coefficient due to its large indirect bandgap. Moreover, the electronic structure of its synthesized films has not been explored clearly yet. In this work, we developed a general and promising method to fabricate continuous, uniform and highly orientated Cs2AgBiBr6 films in large scale on various substrates through capillary-assisted dip-coating method. Strikingly, those optimized films are single crystalline verified by φ-scan XRD. Its electronic structure was carefully studied independently by multi-photo-physical characterizations. Its bandgap can be tuned from 2.65 to 2.25 eV by changing the substrate temperature in growth from 40 to 160 °C. Essentially, their work-function (WF) was determined at −5.01 eV and WF-VBM is around 2 eV. This novel band structure with typical n-type characteristic, was further confirmed by DFT calculations, which reveals that the Cs interstitials and Br vacancies derived deep defect levels were fixed around its Fermi level, closer to the conduction band. This conclusion is different from its widely accepted p-type feature, but definitely deepens our understanding of this material and inspires us to find more valuable strategies of modulating its band structure and optoelectronic properties.

Two-dimensional (2D) materials with versatile properties are promising for diverse applications. In this work, a new family of 2D pentagonal NbX (penta-NbX, X = S, Se or Te) monolayers are designed to achieve the objectives. We demonstrate that these new materials are stable against mechanical strains, lattice dynamics and thermal fluctuations, because of the co-existence of ionic and covalent bonding between the Nb and X elements in these materials. We find that penta-NbX changes from metal to semiconductor as X changes from S/Se to Te. We show that penta-NbTe is a direct band-gap semiconductor with ultra-high carrier mobility (in the order of ~104 cm2 V−1 s−1), which is higher than or comparable to that of most 2D semiconductors and promising for ultra-fast electronics. We further show that metallic penta-NbS are catalytically active for hydrogen evolution reaction because of its low overpotential over a wide range of hydrogen coverages. We expect that the penta-NbX monolayers may find applications in electronics and electrocatalysis.

It is widely accepted that quantum confinement and strain effect display opposite impacts on the band gap size of spherical or tubular transition metal dichalcogenide (TMDC) nanostructures. However, our extensive ab initio calculations and correlation of the band edge evolution of single-wall (SW) TMDC nanotubes (NTs) to their in-plane or out-of-plane orbital characters reveal that the previous interpretation of the band gap evolution behavior (which focused on strain energy) can be revised to the cooperation of deformation potential and flexoelectricity. Specifically, we scrutinize the band profile of multiwall (MW) TMDC NTs and assign the experimentally observed red/blue shift in excitonic transition energy to the decoupling effect arising from flexoelectric field rather than from the commonly expected quantum confinement effect. More importantly, we further apply these novel insights to nested Janus TMDC NTs, which offer an unprecedented platform to realize both negative electron and negative hole compressibilities without the electron correlation effect. Such compressibility gives rise to negative quantum capacitance. This in turn endows these 1D van der Waals heterostructures with emerging applications in hysteresis-free steep-slope transistors and multivalued logic devices.

Searching photocatalysts for efficient hydrogen production has been a challenging issue for solar-energy harvesting. Using co-catalyst is proven to be an effective approach to improve the efficienc...

Herein, we implement first-principles calculations to design Li7P3S11-xOx at an atomic scale, aiming to obtain stable Li7P3S11-xOx-type solid electrolyte materials with good Li+ conductivity. After searching for chemical potentials, Li2O2 is expected to be the potential raw material, and it can afford the most favorable growth environment for the synthesis of Li7P3S11-xOx (x = 0.25, 0.50, 0.75 and 1). Among these compounds, it is found that Li7P3S10.25O0.75 exhibits the most desirable Li+ conductivity of 109 mS cm-1 at 300 K, which is far higher than that of Li7P3S11 (50 mS cm-1 at 300 K). By structural analysis, it is demonstrated that the Li diffusion pathway in Li7P3S10.25O0.75 is significantly broadened relative to that in Li7P3S11 (71.38 A3vs. 69.48 A3), which breaks the bottleneck during Li diffusion. Moreover, the resistance of Li ion diffusion in Li7P3S10.25O0.75 decreases due to the balance of interactions between Li and its neighbouring atoms at the transition state, which induces a much lesser energy barrier of Li7P3S10.25O0.75 than that of Li7P3S11 (0.20 eV vs. 0.31 eV). Moreover, introducing Li vacancies is unlikely to alter the essence of the inherent superionic conductivity of Li7P3S10.25O0.75. Furthermore, Li7P3S10.25O0.75 can maintain good thermal stability and similar electrochemical stability to Li7P3S11. This study successfully clarifies the role of oxygen in enhancing the Li+ conductivity of Li7P3S11-xOx. Moreover, it affords a new strategy to design other solid-state electrolytes with good Li+ conductivity.

The strong interaction between transition metal (TM) atoms and semiconductor surface atoms may diminish the magnetic moments of the TM atoms and prevent them from being used as single atom spin-based devices. A carbon cage that can encapsulate TM atoms and isolate them from interacting with surface atoms is considered to protect the magnetic moments of the TM atoms. We have studied the magnetic moments of Fe, Co, and Ni atoms adsorbed inside the corner hole of Si(111)-(7 × 7) by using first-principles calculations based on the density functional theory. The results show that when Co and Ni atoms are directly adsorbed inside the corner hole, the magnetic moments are 1.353μB and 0, respectively. However when a C60 cage is used to encapsulate the atoms, the magnetic moments increase to 1.849μB and 0.884μB, respectively. The results show a clear protecting effect of a carbon cage. For Fe with and without C60, the magnetic moments are 2.909μB and 2.825μB, respectively. The presence of a C60 cage can also maintain their magnetic moments. Further analysis shows that the TM atoms possess magnetic moments when the conduction electrons are localized around them. All the results can be well understood in the framework of the Anderson impurity model. Our results demonstrate that a carbon cage may effectively protect the magnetic moments of TM atoms. This provides a new strategy for developing single atom spin-based devices on semiconductors.

Investigations of van der Waals (vdW) heterostructures based on distinct low-dimensional materials have attracted significant attention for higher performance devices. Here we use density functional theory computations to scrutinize the band alignment in one-dimensional (1D) vdW heterostructures. In particular, using nanoribbons (NRs) encapsulated inside nanotubes (NTs) based on ten binary-compounds of group IV–IV and group III–V elements, we identified both momentum-matched and -mismatched type II heterostructures with gaps varying from 0.56 eV to 4.37 eV. In addition, we demonstrate a substantial reduction (up to near 0.95 eV) in the staggered band gap of BN compounds by both transverse electric field and longitudinal tensile strain. These findings are favorable for enhancing light harvesting through a wide spectrum and reducing the carrier recombination; our designed heterostructures are expected to offer opportunities for photocatalytic water splitting with safe storage of H2 products inside the NTs.

Highly efficient, earth-abundant, and low-cost photocatalysts are widely pursued for solar-driven hydrogen generation from water. Herein, we first report vanadium disulfide (VS2) with high hydrogen evolution reaction (HER) activity both in basal and edges to be the co-catalyst of graphitic carbon nitride (g-C3N4) for ultrahigh solar-driven hydrogen production. VS2-decorated g-C3N4 shows an impressing photocatalytic hydrogen evolution with a rate of 87.4 μmol/h, 26 times higher than pristine g-C3N4. Our combined experimental and computational studies reveal that the excellent efficiency of the composite is attributed to: (1) effective electron-hole separation and electron transfer from g-C3N4 to VS2, resulting from the optimal band alignment between VS2 and g-C3N4 and metallic characteristic of VS2; (2) fast hydrogen generation on the surface due to the high surface area and excellent HER activity of VS2. Our findings demonstrate that VS2/g-C3N4 may be applicable in solar-driven water splitting, and the design principle can be applied to search for novel photocatalysts.

Searching for earth-abundant and efficient electrocatalysts for large-scale hydrogen generation from the electrolysis of water is of paramount importance. Incorporating nonmetal or metal elements into host catalysts is a possible way to tune the catalytic capability of an electrocatalyst on the hydrogen evolution reaction (HER). Here, a simple and facile way was presented to synthesize N and V coincorporated Ni nanosheets on self-supported conductive carbon paper (NV–Ni/CP) as noble-metal-free catalysts for the HER. Compared with Ni/CP, the NV–Ni/CP shows better HER property with low overpotential of 95 mV (10 mA cm–2), small Tafel slope (140 mV dec–1), and superior long-term stability. We further show that the outstanding HER performance of NV–Ni/CP is ascribed to the increased active sites and enhanced conductivity. It is expected that the coincorporation of nonmetal and metal elements will provide more chances to enhance the HER catalytic ability and extend the scope of cost-effective electrocatalysts.

Graphene has been widely studied as an electrocatalyst for hydrogen evolution reaction (HER). However, pure flat graphene is catalytically inert in HER. In this work, we investigate the effect of curvature on the improvement in the catalytic activity of pure and doped graphenes. We find that the HER performance can be dramatically improved on waved-graphene due to localized chemical potential and Pt-analogous activity can be achieved at suitable compression. (1) For pure graphene, the calculated HER performance increases more than 50% as tuned by curvature due to reduced calculated Gibbs free energies. (2) For B- and N-doped graphene, their optimal HER catalytic ability occurs at low curvature conditions. (3) For metal-doped graphene, Mo-doped graphene exhibits excellent catalytic ability in HER at certain compressions. (4) For nitrogen-metal co-doped graphene, N–Ni and N–V co-doped graphenes can be tuned by curvature to show outstanding performance in HER with their exothermal formation energies. Our cal...

The fast development of high-performance devices for diverse applications requires nanoscale materials with multifunctional properties, motivating theoretical exploration into novel two-dimensional (2D) materials. In this work, we propose a new family of 2D nanomaterials, Janus transition metal oxides and chalcogenides MXY (M = Ti, Zr, or Hf; X = S or Se; Y = O or S; X ≠ Y) monolayers, for their versatile applications. We find that the Janus MXY monolayers are semiconductors with a wide range of band gaps ranging from 0.739 to 2.884 eV. We show that TiSO, ZrSO, and HfSO monolayers are promising candidates for photocatalysis because of their suitable band gaps and optimal redox potentials for water splitting, and ZrSeS and HfSeS monolayers are suitable candidates for nanoscale electronics because of their high carrier mobility. We further show that TiSO, ZrSO, and ZrSeO monolayers possess large piezoelectric properties because of the broken inversion symmetry stemmed from the different atomic sizes and electronegativities of the X and Y elements, which are better or comparable to other 2D and bulk piezoelectric materials. Our study demonstrates that the 2D Janus MXYs may find versatile applications into photocatalysts, electronics, sensors, and energy harvesting/conversion.

Organic–inorganic halide perovskites have attracted increasing interest for solar-energy harvesting because of the simple fabrication process, high efficiency, and low cost. In this work, we systematically investigate the structural and electronic properties, and stability of two-dimensional (2D) hybrid organic–inorganic perovskites (HOIPs) based on density-functional-theory calculations. We explore a general rule to predict the bandgap of the 2D HOIP: its bandgap decreases as the thickness increases and the size of metal atom decreases as well as that of the halide atom increases. We find that effective mass of hole increases as the thickness of 2D HOIP increases. Importantly, the 2D HOIPs exhibit high stability on the resistance of water and oxygen than bulk HOIPs due to high positive adsorption energy. Our results confirm that the 2D HOIPs may be excellent alternatives to the unstable bulk HOIPs in solar energy harvesting with improved performance due to suitable bandgap, small carrier effective mass, ...

Iron fluoride with high operating voltage and theoretical energy density has been proposed as a high-performance cathode material for Li-ion batteries. However, the inertness of pristine bulk FeF3 results in poor Li kinetics and cycling life. Developing nanosheet-based electrode materials is a feasible strategy to solve these problems. Herein, on the basis of first-principles calculations, first the stability of FeF3 (012) nanosheet with different atomic terminations under different environmental conditions was systematically studied, then the Li-ion adsorption and diffusion kinetics were thoroughly probed, and finally the voltages for different Li concentrations were given. We found that F-terminated nanosheet is energetically favorable in a wide range of chemical potential, which provide a vehicle for lithium ion diffusion. Our Li-ion adsorption and diffusion kinetics study revealed that (1) the formation of Li dimer is the most preferred, (2) the Li diffusion energy barrier of Li dimer is lower than is...

Two dimensional (2D) materials with hexagonal building blocks have received tremendous interest in recent years and show promise as nanoscale devices for versatile applications. Herein, we propose a new family of 2D pentagonal CrX (X = S, Se or Te) monolayers (penta-CrX) for applications in electronics, spintronics and photocatalysis. We find that the 2D penta-CrX monolayers are thermally, structurally and mechanically stable. The penta-CrX monolayers are antiferromagnetic and semiconducting. We show that the magnetism is attributed to the super-exchange induced by the ionic interactions between the Cr and X atoms and can be enhanced upon applying tension. We further show that the penta-CrS and penta-CrSe monolayers show good redox potentials versus a normal hydrogen electrode, and their band gaps are comparable to the energy of a photon in the visible light region, indicating their capability of maximal utilization of solar energy for water splitting. With intrinsic semiconducting and controllable magnetic properties, the proposed penta-CrX monolayers may hold promise as flexible spintronics and photocatalysts.

The development of two-dimensional (2D) materials with high conductivity and catalytic activity is important for the proposed hydrogen economy. Herein, we design a new family of 2D monolayers, pentagonal transition-metal borides/carbides (penta-TMBs and penta-TMCs), as electrocatalysts for the hydrogen evolution reaction (HER) on the basis of density functional theory (DFT). We find that all of the stable 2D penta-TMBs/TMCs are metallic, and 2D WB and HfC are ferromagnetic metals. We demonstrate that penta-TMBs and penta-TMCs show high catalytic performance for the HER. The Gibbs free energy for the adsorption of hydrogen atoms on catalyst surfaces (such as WB and ZrC) is close to the ideal value of zero electron volt (eV). Our findings highlight a new family of promising noble metal-free HER catalysts and provide new insight into the design of advanced 2D materials.

Active catalysts for nitrogen fixation (N2-fixation) have been widely pursued through constant efforts for industrial applications. Here, we report a family of catalysts, MXenes (M2X: M = Mo, Ta, Ti, and W; X = C and N), for application in N2-fixation based on density functional theory calculations. We find that the catalytic performance of MXenes strongly depends on the reaction energy in each reaction step. More exothermic steps lead to higher catalytic performance in the course of N2-fixation. We show that the reaction energy in N2-fixation is strongly affected by the charge transfer: (1) if N atoms gain more electrons in a step, the reaction is exothermic with a larger reaction energy; (2) if N atoms lose electrons in a step, the reaction is endothermic in general. We further show that Mo2C and W2C are highly active for N2-fixation due to their exothermic reactions and strong charge transfer, which may be applicable in the chemical-engineering industry.

Two dimensional (2D) nanomaterials have received increasing interest because of their unique properties for versatile applications. In this work, we present a first-principles study on a new family of 2D nanostructures, Janus transition metal chalcogenide MSX (M = Ti or V; and X = C, N, Si, or P) monolayers, for their multifunctional applications. In this work, we show that the Janus MSXs possess diverse electronic and magnetic properties, and can be semiconducting or metallic, and nonmagnetic or magnetic, depending on their composition. We find that a TiSC monolayer with a 1H phase (TiSC-1H) is suitable as a cathode for Li ion batteries and anode materials for Na and Mg ion batteries due to its high open circuit voltage (OCV) (2.121 eV) for Li, and low OCVs upon Na (0.676 eV) and Mg (1.044 eV) intercalation, respectively. Importantly, TiSC-1H shows fast charge/discharge rates, good cycling stability, and high storage density as electrode materials for rechargeable batteries because of low ion diffusion barriers, small volume expansion and high specific capacity. We further show that TiSP-1H has the best performance in the hydrogen evolution reaction due to both its catalytic activities on the surfaces and relatively low overpotentials upon hydrogenation. Our study demonstrates that the 2D Janus MSXs may find multifunctional applications in nanodevices, spintronics, catalysis, and electrochemical energy storage.

Two-dimensional (2D) nanomaterials with tunable electronic and magnetic properties are promising for their versatile applications in multifunctional devices. In this work, we present a first-principles study on the electronic and magnetic transitions of 2D Ti2NO2 monolayer achieved by applying tension. We show that 2D Ti2NO2 experiences a transition from ferromagnetic half-metal to antiferromagnetic semiconductor as tension increases up to a certain level. We find that charge redistribution occurs with the extension of bond length due to the suppressed crystal field splitting. We further show that the charge transfer between eg and t2g orbitals gives rise to the shift of the energy levels and opens a gap near the Fermi level, resulting in the electronic transition. The semiconducting nature favors the superexchange coupling, leading to the magnetic transition from the ferromagnetic state to the antiferromagnetic state. We expect that the 2D Ti2NO2 monolayer with tunable electronic and magnetic properties ...

The porphyrin macrocyclic core features dynamic conformational transformations in free space because of its structural flexibility. Once attached to a substrate, the molecule–substrate interaction often restricts this flexibility and stabilizes the porphyrin in a specific conformation. Here using molecular dynamic and density-functional theory simulations and scanning tunneling microscopy and spectroscopy, we investigated the conformation relaxation and stabilization processes of two porphyrin derivatives (5,15-dibromophenyl-10,20-diphenylporphyrin, Br2TPP, and 5,15-diphenylporphyrin, DPP) adsorbed on Au(111) and Pb(111) surfaces. We found that Br2TPP adopts either dome or saddle conformations on Au(111) but only the saddle conformation on Pb(111), whereas DPP deforms to a ruffled conformation on Au(111). We also resolved the structural transformation pathway of Br2TPP from the free-space conformations to the surface-anchored conformations. These findings provide unprecedented insights revealing the confo...

2D materials have been interesting for applications into nanodevices due to their intriguing physical properties. In this work, four types of unique structures are designed that are composed of MXenes and C/N-Si layers (CNSi), where MXene is sandwiched by the CNSi layers with different thicknesses, for their practical applications into integrated devices. The systematic calculations on their elastic constants, phonon dispersions, and thermodynamic properties show that these structures are stable, depending on the composition of MXene. It is found: 1) different from MXene or N-functionalized MXene (M2 CN2 ), SiN2 /M2 X/SiN2 possess new electronic properties with free carriers only in the middle, leading to 2D free electron gas; 2) CNSi/MXene/CNSi shows an intrinsic Ohmic semiconductor-metal-semiconductor (S-M-S) contact, which is potential for applications into nanodevices; and 3) O/M2 C/SiN2 and N/M2 C/OSiN are also stable and show different electronic properties, which can be semiconductor or metal as a whole depending on the interface. A method is further proposed to fabricate the 2D structures based on the industrial availability. The findings may provide a novel strategy to design and fabricate the 2D structures for their application into nanodevices and integrated circuits.

Due to Fermi-level pinning in metal-two-dimensional MoS2 junctions, improving the performance of MoS2-based electrical devices is still under extensive study. The device performance of few-layer MoS2 depends strongly on the number of layers. In this work, via density-functional theory calculations, a comprehensive understanding from the atomistic view was reached for the interlayer interaction between metal and few-layer MoS2 with phase-engineering and intercalation doping, which are helpful for improving the contact performance. These two methods are probed to tune the performance of few-layer MoS2-based field-effect transistors, and both of them can tune the Schottky barrier height. Phase-engineering, which means that the MoS2 layer in contact with metal is converted to the T phase, can transform the Schottky barrier from n- to p-type. Intercalation doping, which takes advantage of annealing and results in metal atom interaction in between MoS2 layers, makes the MoS2 layers become quasi-freestanding and converts the indirect bandgap into direct bandgap. Our atomistic insights help improve the performance of few-layer MoS2-based electronic devices.

It is a great challenge to explore cheap, abundant and eco-friendly electrocatalysts for hydrogen evolution reaction (HER). Here, we report the fabrication of VS2 nanoflowers with 1T phase by a simple hydrothermal method and their electrocatalytic performance in the HER. We find that the VS2 nanoflowers show comparable HER performance to Pt in acids, including an ultra-low onset potential of 32 mV, a Tafel slope of 34 mV dec−1 which resembles that of Pt, and a small overpotential of 58 mV (54 mV for Pt) at a current density of 10 mA cm−2. High stability and almost 100% faradaic efficiency indicate the practical application of VS2 nanoflowers in the HER. Our first-principles calculations reveal that the thermoneutral Gibbs free energy of hydrogen adsorption on both the basal and edge sites of the 1T-VS2 monolayer can be achieved under certain hydrogen coverage and the monolayer shows good conductivity, which contribute to the impressive catalytic performance of VS2 nanoflowers. We expect that the VS2 nanostructures may be applicable in electrocatalysis with high efficiency.

Photocatalytic water splitting is an environmentally friendly technique for hydrogen production. In this work, we report a novel photocatalyst consisting of two-dimensional (2D) titanium carbide (Ti2C) and graphitic carbon nitride (g-C3N4). We observe substantially enhanced water splitting activities due to the efficient synergistic interaction between Ti2C and g-C3N4. Optimal properties are achieved in the g-C3N4 with a loading of 0.4 wt% Ti2C with a hydrogen production rate of 47.5 μmol h−1, which is 14.4 times as much as that in the case using pure g-C3N4, and it even outperforms Pt-loaded g-C3N4. We further show that the Ti2C/g-C3N4 has high stability and good reproducibility. We expect that the Ti2C/g-C3N4 can be a photocatalyst for large scale applications because both Ti2C and g-C3N4 are low-cost, abundant, and nontoxic.

Layer-number-dependent performance of metal-semiconductor junctions (MSJs) with multilayered two-dimensional (2D) semiconductors has attracted increasing attention for their potential in ultrathin electronics and optoelectronics. However, the mechanism of the interaction and the resulting charge transfer/redistribution at the two kinds of interfaces in MSJ with multilayered 2D semiconductors, namely, the metal-semiconductor (M-S) and the semiconductor-semiconductor (S-S) interfaces, have not been well understood until now, although that is important for the overall Schottky barrier height and the energy-band-offset between different layers of the 2D semiconductors. Here, based on state-of-the-art density functional theory calculations, the mechanisms of bonding and asymmetric electron redistribution at the M-S and S-S interfaces of metal-bilayer MoS2 junctions are revealed. Multiple mechanisms collectively contribute to the electron redistribution at the two kinds of interfaces, and the dominant mechanism depends on both the dimensionality (2D vs 3D) and the work function of metal electrodes. For the M-S interface, the pushback effect and metal-induced gap states play a dominant role for MSJs with 3D metal, while the covalent-like quasi-bonding feature appears for MSJs with medium-work-function 2D metals, and charge transfer plays a main role for MSJs with 2D metals that have very large or small work functions. For the S-S interface, it inherits the electron-redistribution behavior of the M-S interface for MSJs with 2D metal, while opposite electron-redistribution appears in MSJs with 3D metal. These mechanisms provide general insights and new concepts to better understand and use MSJs with multilayered 2D semiconductors.

Carbon dioxide (CO2) normally exists in a gaseous molecular state under ambient conditions, while two-dimensional (2D) crystals of CO2 have not been reported yet. In this work, based on density functional theory and the particle swarm optimization method, we unveil two CO2 2D crystals with space groups of P4[combining macron]M2 and Amm2. Our results show that these structures have excellent thermal, dynamic, and mechanical stability. The new structures are insulators with an indirect or direct band gap, while the indirect band gap can be tuned to be direct with small uniaxial strains. More importantly, the P4[combining macron]M2 structure has an in-plane negative Poisson's ratio, which is due to the interaction of the lattice symmetry and the local CO4 tetrahedron symmetry. In addition, the Amm2 sheet has a very large electronic band gap (>9 eV), which is the largest in all known 2D materials. Enthalpy curves indicate that these 2D structures may be obtained from the ambient phase of CO2 under high pressure. This work presents new structures of CO2, and because of their excellent performance in terms of stability, mechanical and electronic properties, they potentially have broad applications.

Germanium phosphide (GeP), a new member of the Group IV-Group V compounds, is introduced into the fast growing 2D family with experimental and theoretical demonstration of strong anisotropic physical properties. The indirect band gap of GeP can be drastically tuned from 1.68 eV for monolayer to 0.51 eV for bulk, with highly anisotropic dispersions of band structures. Thin GeP shows strong anisotropy of phonon vibrations. Moreover, photodetectors based on GeP flakes show highly anisotropic behavior with anisotropic factors of 1.52 and 1.83 for conductance and photoresponsivity, respectively. This work lays the foundation and ignites future research interests in Group IV-Group V compound 2D materials.

Two dimensional (2D) nanomaterials have demonstrated huge potential in wide applications from nanodevices to energy harvesting/storage. In this work, we propose a new class of 2D monolayers, nitrogen-functionalized MXenes (Nb2CN2 and Ta2CN2), based on density-functional theory (DFT). We find that these monolayers are direct semiconductors with near linear energy dispersions at the Γ point. M2CN2 monolayers have significant small effective mass and show an ultra-high mobility of up to 106 cm2 V-1 s-1. We show that the electronic structures of the M2CN2 monolayers can be easily controlled by biaxial and uniaxial strains. Importantly, the carrier mobility and direct band gap can be dramatically increased within a certain range of strain. A direct-indirect band gap transition can be triggered and the band gap can be tuned under strain. The tunable electronic properties are attributed to the structural changes and charge redistribution under stain. Our findings demonstrate that N-functionalized MXenes are promising materials for nanodevices with high speed and low power.

An interesting in-plane anisotropic layered dimetal chalcogenide Ta2NiS5 is introduced, and the optical and electrical properties with respect to its in-plane anisotropy are systematically studied. The Raman vibration modes have been identified by Raman spectra measurements combined with calculations of phonon-related properties. Importantly, the Ta2NiS5 flakes exhibit strong anisotropic Raman response under the angle-resolved polarized Raman spectroscopy measurements. We found that Raman intensities of the Ag mode not only depend on rotation angle but are also related to the sample thickness. In contrast, the infrared absorption with light polarized along the a axis direction is always larger than that in the c axis direction regardless of thickness under the polarization-resolved infrared spectroscopy measurements. Remarkably, the first-principles calculations combined with angle-resolved conductance measurements indicate strong anisotropic conductivity of Ta2NiS5. Our results not only prove Ta2NiS5 is ...

So far the Tetra-MoN2 has been reported as the most stable structure of MoN2, which is a nonmagnetic semiconductor with an indirect band gap. With fluorine atoms adsorbed on the surface, the fluorinated Tetra-MoN2 (F-Tetra-MoN2) exhibits rich magnetic behaviors mediated by the polarized p electron of N and induced by small strains and low-concentration charge doping. Coverage or distance of F adsorbates also causes dramatic change of the magnetic properties. F adsorbates trend to aggregation which induces stronger spin polarization in F-Tetra-MoN2. These results suggest an efficient route to tune the magnetic properties of fluorinated Tetra-MoN2.

Molecular doping of inorganic semiconductors is a rising topic in the field of organic/inorganic hybrid electronics. However, it is difficult to find dopant molecules which simultaneously exhibit strong reducibility and stability in ambient atmosphere, which are needed for n-type doping of oxide semiconductors. Herein, successful n-type doping of SnO2 is demonstrated by a simple, air-robust, and cost-effective triphenylphosphine oxide molecule. Strikingly, it is discovered that electrons are transferred from the R3P+ O- σ-bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course is verified by multi-photophysical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO2 , which enlarges the built-in field from 0.01 to 0.07 eV and decreases the energy barrier from 0.55 to 0.39 eV at the SnO2 /perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.

Two-dimensional (2D) semiconductors SnP3 are predicted, from first-principles calculations, to host moderate band gaps (0.72 eV for monolayer and 1.07 eV for bilayer), ultrahigh carrier mobility (~104 cm2 V−1 s−1 for bilayer), strong absorption coefficients (~105 cm−1) and good stability. Moreover, the band gap can be modulated from an indirect character into a direct one via strain engineering. For experimental accessibility, the calculated exfoliation energies of monolayer and bilayer SnP3 are smaller than those of the common arsenic-type honeycomb structures GeP3 and InP3. More importantly, a semiconductor-to-metal transition is discovered with the layer number N > 2. We demonstrate, in remarkable contrast to the previous understandings, that such phase transition is largely driven by the correlation between lone-pair electrons of interlayer Sn and P atoms. This mechanism is universal for analogues phase transitions in arsenic-type honeycomb structures (GeP3, InP3 and SnP3).

Due to the intriguing anisotropic optical and electrical properties, low-symmetry 2D materials are attracting a lot of interest both for fundamental studies and fabricating novel electronic and optoelectronic devices. Identifying new promising low-symmetry 2D materials will be rewarding toward the evolution of nanoelectronics and nano-optoelectronics. In this work, germanium diarsenide (GeAs2 ), a group IV-V semiconductor with novel low-symmetry puckered structure, is introduced as a favorable highly anisotropic 2D material into the rapidly growing 2D family. The structural, vibrational, electrical, and optical in-plane anisotropy of GeAs2 is systematically investigated both theoretically and experimentally, combined with thickness-dependent studies. Polarization-sensitive photodetectors based on few-layer GeAs2 exhibit highly anisotropic photodetection behavior with lineally dichroic ratio up to ≈2. This work on GeAs2 will excite interests in the less exploited regime of group IV-V compounds.

Via the first-principles approach based on the hybrid functional, the stable configurations and electronic and magnetic properties of fluorinated two-dimensional BeN2 (F-BeN2) have been studied. F atoms are adsorbed on Be ion sites and each fluorine adsorbate induces robust 1 μB magnetic moment. As a result, a F-BeN2 sheet becomes a ferromagnetic half-metal from a semiconducting intrinsic BeN2. The semiconducting bandgap of the half-metallic F-BeN2 sheets varies from 2.26 to 2.92 eV with different F coverages. With external strains, F-BeN2 maintains the ferromagnetic and half-metallic features and the semiconducting bandgap changes only a little. It suggests that F-BeN2 can be a good light-element magnet with promising potential for spintronic applications.