The intrinsic anisotropy of NbSe2 provides a favorable prerequisite of second harmonic generation (SHG) and rich possibilities for tailoring its nonlinear optical (NLO) properties. Here we report the highly efficient SHG of mechanically exfoliated NbSe2 flakes. The nonlinear optical response changes with excitation wavelengths, layer thicknesses, and polarizations of the excitation laser. The anisotropic SHG response further exhibits the intrinsic non-centrosymmetric crystal structure and could effectively assign the crystalline orientation of NbSe2 flakes. Interestingly, although NbSe2 flakes with tens of nanometers thickness experience attenuations in SHG performance, more efficient SHG anisotropy ratios were obtained, which are around 4 times higher than that of the 5-layer counterpart. The determined second-order nonlinearities of NbSe2 flakes (monolayer: â¼1.0 × 103 pm/V; 3-layer: â¼73 pm/V) are comparable to those of the commonly reported two-dimensional materials (e.g., MoS2, WSe2, graphene) with the same number of layers and much higher than those of commercial nonlinear optical crystals.
Developing efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) bifunctional electrocatalysts is attractive for rechargeable metal-air batteries. Meanwhile, single metal atoms embedded in 2D layered transition metal chalcogenides (TMDs) have become a very promising catalyst. Recently, many attentions have been paid to the 2D ReS2 electrocatalyst due to its unique distorted octahedral 1T' crystal structure and thickness-independent electronic properties. Here, the catalytic activity of different transition metal (TM) atoms embedded in ReS2 using the density functional theory is investigated. The results indicate that TM@ReS2 exhibits outstanding thermal stability, good electrical conductivity, and electron transfer for electrochemical reactions. And the Ir@ReS2 and Pd@ReS2 can be used as OER/ORR bifunctional electrocatalysts with a lower overpotential for OER (ηOER ) of 0.44 V and overpotentials for ORR (ηORR ) of 0.26 V and 0.27 V, respectively. The excellent catalytic activity is attributed to the optimal adsorption strength for oxygen intermediates coming from the effective modulation of the electronic structure of ReS2 after Ir/Pd doping. The results can help to deeply understand the catalytic activity of TM@ReS2 and develop novel and highly efficient OER/ORR electrocatalysts.
The search for suitable electrode materials is crucial for the development of high-performance Na-ion batteries (NIBs). In recent years, significant attention has been drawn to two-dimensional (2D) oxides as potential NIB electrode materials. In this study, employing the first-principles density functional theory method, we investigate the thermodynamic and kinetic properties of Na adsorption and diffusion behavior on the 2D TiO2(010) monolayer. Our findings demonstrate that the 2D anatase TiO2(010) monolayer exhibits enhanced thermodynamic stability. Furthermore, the Na atoms preferentially adsorb on the top of oxygen atoms within the TiO2(010) monolayer, and their diffusion along the [100] direction is characterized by a low energy barrier of 0.054 eV. This comprehensive analysis sheds light on the structural stability, preferred adsorption sites, and diffusion paths of Na atoms on the 2D anatase TiO2(010) monolayer, providing valuable insights into the nature of the material's structure and Na ion transport. Moreover, the 2D structure of the TiO2matrix facilitates short Na diffusion lengths and a large electrode/electrolyte interface, thereby demonstrating the potential of this material as an NIB electrode material.
The electrochemical hydrogen evolution reaction (HER) effectively produces clean, renewable, and sustainable hydrogen; however, the development of efficient electrocatalysts is required to reduce the high energy barrier of the HER. Herein, we report two excellent single-atom (SA)/metal-organic framework (MOF) composite electrocatalysts (PtSA-MIL100(Fe) and PtSA-MIL101(Cr)) for HER. The obtained PtSA-MIL100(Fe) and PtSA-MIL101(Cr) electrocatalysts exhibit overpotentials of 60 and 61 mV at 10 mA cm-2, respectively, which are close to that of commercial Pt/C (38 mV); they exhibit overpotentials of 310 and 288 mV at 200 mA cm-2, respectively, which are comparable to that of commercial Pt/C (270 mV). Theoretical simulations reveal that Pt SAs modulate the electronic structures of the MOFs, leading to the optimization of the binding strength for H* and significant enhancement of the HER activity. This study describes a novel strategy for preparing desirable HER electrocatalysts based on the synergy between SAs and MIL-series MOFs. Using MIL-series MOFs to support SAs could be valuable for future catalyst design.
The remote excitation and remote-controlling of the localized surface plasmon resonance (LSPR) in a heterotype and hollow gold nanosheet (HGNS) is studied using FDTD simulations. The heterotype HGNS contains an equilateral and hollow triangle in the center of a special hexagon, which forms a so-called hexagon-triangle (H-T) heterotype HGNS. If we focus the incident-exciting laser on one of the vertexes of the center triangle, the LSPR could be achieved among other remote vertexes of the outer hexagon. The LSPR wavelength and peak intensity depend sensitively on factors such as the polarization of the incident light, the size and symmetry of the H-T heterotype structure, etc. Several groups of the optimized parameters were screened out from numerous FDTD calculations, which help to further obtain some significant polar plots of the polarization-dependent LSPR peak intensity with two-petal, four-petal or six-petal patterns. Remarkably, based on these polar plots, the on-off switching of the LSPR coupled among four HGNS hotspots could be remote-controlled simply via only one polarized light, which shows promise for its potential application in remote-controllable surface-enhanced Raman scattering (SERS), optical interconnects and multi-channel waveguide switches.
In this study, composites CoFe2O4/Fe were successfully synthesized by in situ oxidation, and their composition, structure, and magnetic properties have been investigated. According to the analysis of X-ray photoelectron spectrometry measure results, the cobalt ferrite insulating layer was completely coated on the surface of Fe powder particles. The evolution of the insulating layer during the annealing process has been discussed, which is correlated to effects on the magnetic properties of the composites CoFe2O4/Fe. The amplitude permeability of the composites reached a maximum of 110, and their frequency stability reached 170 kHz with a relatively low core loss of 253.6 W/kg. Therefore, the composites CoFe2O4/Fe has potential application in the field of integrated inductance and high-frequency motor, which is conducive to energy conservation and carbon reduction.
Janus materials, as a family of multifunctional materials with broken mirror symmetry, have played a great role in piezoelectric, valley-related, and Rashba spin-orbit coupling (SOC) applications. Using first-principles calculations, it is predicted that monolayer 2H-GdXY (X, Y = Cl, Br, I) will combine giant piezoelectricity, intrinsic valley splitting and a strong Dzyaloshinskii-Moriya interaction (DMI), resulting from the intrinsic electric polarization, spontaneous spin polarization and strong spin-orbit coupling. Opposite Berry curvatures and unequal Hall conductivities at the K- and K'-valleys of monolayer GdXY are promising for storing information through the anomalous valley Hall effect (AVHE). Through construction of the spin Hamiltonian and micromagnetic model, we obtained the primary magnetic parameters of monolayer GdXY as a function of the biaxial strain. Due to the dimensionless parameter κ having strong tunability, monolayer GdClBr is promising to host isolated skyrmions. The present results are expected to enable the application of Janus materials in piezoelectricity, spin- and valley-tronics and the formation of chiral magnetic structures.
A glucose sensor with high sensitivity and low detection limit is vital for human beings' health. Herein, a CoO nanoneedle array with an unique electronic structure was successfully constructed by a hydrothermal and subsequent high-temperature calcination process. The optimized CoO-400 nanoneedles exhibit a larger electrochemical active surface area, beneficial electronic structure, favorable lattice distortion, and abundant active sites, which effectively promote electrochemical properties toward glucose sensing. The glucose sensor constructed by CoO-400 nanoneedles shows a high sensitivity of 84.23 mA cm-2 mM-1 and low detection limit of 4.4 × 10-7 M, superior to the results from most previous reports. Moreover, outstanding anti-interference ability, superior long-term stability, good repeatability, and satisfactory reproducibility in glucose detection for CoO-400 nanoneedles are also demonstrated.
Cobalt-based sulfides with variable valence states and unique physical and chemical properties have shown great potential as oxygen evolution reaction (OER) catalysts for electrochemical water-splitting reactions. However, poor morphological characteristics and a small specific surface area limit its further application. Here, hexagonal single-crystal two-dimensional (2D) CoS nanosheets with different thicknesses are successfully prepared by an atmospheric-pressure chemical vapor deposition method. Because of the advantages of the 2D structure, more exposed catalytic active sites, better reactant adsorption ability, accelerated electron transfer, and enhanced electrical conductivities can be achieved from the thinnest 5 nm CoS nanosheets (CoS-5), significantly improving OER performance. The electrochemical tests manifest that CoS-5 show an overpotential of 290 mV at 10 mA cm-2 and a Tafel slope of 65.6 mV dec-1 in the OER in an alkaline solution, superior to those for other thicknesses of CoS, bulk CoS, and RuO2. For the mechanistic investigation, the lowest charge transfer resistance (Rct) and the highest double-layer capacitance (Cdl) were obtained for CoS-5, demonstrating the faster OER kinetics and the larger active area. Density functional theory calculations further reveal the enhanced density of states around the Fermi level and higher H2O molecule adsorption energy for thinner CoS nanosheets, promoting its intrinsic catalytic activity. Moreover, the two-electrode system with CoS-5 as the anode and Pt/C as the cathode requires only 1.56 V to attain 10 mA cm-2 in the overall water-splitting reaction. We believe that this study will provide a fresh view for thickness-dependent catalytic performance and offers a new material for the study of electronic and energy devices.
Recently prepared layered MoSi2N4 exhibits excellent stability and semiconductor properties, adding building blocks for two-dimensional families. In this research, we present the spin-orbit coupling and valley-related properties of monolayer WSi2N4 family. Better than transition metal dichalcogenides, the structural symmetry of WSi2N4 monolayer can be different by changing the stacking of three parts in the monolayers, resulting in a Rashba spin-orbit field. The vertical and horizontal polarization will lift the degeneration of the in-plane and out-of-plane polarized spin, respectively. The gradient of potential energy and the proportion of d orbitals play dominant roles. The in-plane orbitals contribute to the out-of-plane spin polarization, while the out-of-plane orbitals contribute to the in-plane spin polarization. The characteristics of a Rashba semiconductor can be utilized in spin/valley Hall effects, as well as the regulation of the spin direction of the valley electrons, promoting the manipulation of multiple degrees of freedom of electrons in monolayer materials.
Group-VA two-dimensional layered materials in a puckered honeycomb structure exhibit strong in-plane anisotropy and have emerged as new platforms for novel devices. Here, we report on systematic Raman investigations on exfoliated b-As flakes on the Ag1 and Ag2 polarization dependence on their symmetry, excitation wavelength, and flake thickness. The intensity maximums of both phonons are corrected in the b-As armchair direction under 633 nm excitation regardless of the flake thickness upon considering optical birefringence effects and interference effects. The intensity ratio of Ag1 to Ag2 modes under 532 nm excitation is useful for b-As crystalline orientation identification. Temperature-dependent Raman investigations reveal the linearly anharmonic behaviors of both phonons in the range from 173 to 293 K and a slightly greater first-order temperature coefficient in the zigzag direction. Our findings give deep insight into the in-plane phonon anisotropy and anharmonicity of b-As and provide a step toward future device applications.
Strain engineering has become one of the effective methods to tune the electronic structures of materials, which can be introduced into the molecular junction to induce some unique physical effects. The various γ-graphyne nanoribbons (γ-GYNRs) embedded between gold (Au) electrodes with strain controlling have been designed, involving the calculation of the spin-dependent transport properties by employing the density functional theory. Our calculated results exhibit that the presence of strain has a great effect on transport properties of molecular junctions, which can obviously enhance the coupling between the γ-GYNR and Au electrodes. We find that the current flowing through the strained nanojunction is larger than that of the unstrained one. What is more, the length and strained shape of the γ-GYNR serves as the important factors which affect the transport properties of molecular junctions. Simultaneously, the phenomenon of spin-splitting occurs after introducing strain into nanojunction, implying that strain engineering may be a new means to regulate the electron spin. Our work can provide theoretical basis for designing of high performance graphyne-based devices in the future.
The quasimetallic 1T' phase 2D transition-metal dichalcogenides (TMDs) consist of 1D zigzag metal chains stacked periodically along a single axis. This gives rise to its prominent physical properties which promises the onset of novel physical phenomena and applications. Here, the in-plane electronic correlations are explored, and new mid-infrared plasmon excitations in 1T' phase monolayer WSe2 and MoS2 are observed using optical spectroscopies. Based on an extensive first-principles study which analyzes the charge dynamics across multiple axes of the atomic-layered systems, the collective charge excitations are found to disperse only along the direction perpendicular to the chains. Further analysis reveals that the interchain long-range coupling is responsible for the coherent 1D charge dynamics and the spin-orbit coupling affects the plasmon frequency. Detailed investigation of these charge collective modes in 2D-chained systems offers opportunities for novel device applications and has implications for the underlying mechanism that governs superconductivity in 2D TMD systems.
The intricate features of many-body interactions and spin-orbit coupling play a significant role in numerous physical phenomena. Particularly in two-dimensional transition metal dichalcogenides (2D-TMDs), excitonic dynamics are a key phenomenon that promises opportunities for diverse range of device applications. Here, we report the direct observation of a visible-range three-dimensional resonant exciton and its associated charged exciton in monolayer tungsten diselenide, as compared to monolayer molybdenum disulfide. A comprehensive experimental study that includes high-resolution TEM, Raman, high-resolution spectroscopic ellipsometry over a wide temperature range down to 4 K, high-energy temperature, and excitation power-dependent photoluminescence spectroscopy has been conducted. It is supported by first-principles calculations to unravel the influence of spin-orbit coupling in the formation of the resonant exciton and to identify its in-plane and out-of-plane features. Furthermore, we study the impact of temperature and thickness on the spin-orbit coupling strength in 2D-TMDs. This work is crucial in creating a platform in the fundamental understanding of high-energy resonant exciton in layered two-dimensional systems and that such high-energy optoelectronic features make them an increasingly attractive candidate for novel electronic and optoelectronic applications particularly in the aspects of solar cells and light-emitting diodes via the manipulation of excitonic states.
Artificial van der Waals (vdW) heterojunctions assembled by atomically-thin two-dimensional (2D) materials have demonstrated new physical phenomena and unusual properties, thus triggering new electronic, optoelectronic, valleytronic and photocatalytic application. Herein, the electronic band structures of different vdW heterojunctions based on ternary Mo1-x W x Y2 (Y = S, Se; x = 0-1) monolayer with five stacking orders (AA, AA[Formula: see text], A[Formula: see text]B, AB, AB[Formula: see text]) have been investigated using first principle calculations. The direct/indirect band gap has been obtained in the AA[Formula: see text] stacking type-II heterojunctions, ranging from 0.538 eV to 1.260 eV, that are determined by the interlayer distances and stoichiometries. The estimated power conversion efficiency of the AA[Formula: see text] stacking type-II heterojunction varied from 9.1% to 23.4%. The type-I heterojunctions have also been predicted when semiconducting 2H-MoTe2 monolayer stacks with the specific Mo1-x W x Se2 monolayer, which are MoTe2/Mo0.25W0.75Se2 and MoTe2/Mo0.5W0.5Se2. The reported theoretical results can provide broader 2D materials design possibility for the functional devices.
Using first-principles calculations based on density functional theory, we studied the electronic and magnetic properties of phosphorene co-doped with Cl and a metal atom (including Sc, Ti, V, Cr, Mn, Fe, Co, and Ni). It is found that Cl atom doping makes it much easier for metallic atoms to dope into phosphorene. Phosphorene co-doped with Cl and V, Cr, Mn, or Fe is magnetic, which is determined by the number of valence electrons. Taking V-Cl and Co-Cl co-doped phosphorene as an example, analyses are carried out on the reasonable selection of the doping sites, which distinctly affect the stability, band gap and magnetic moment. The stability is closely relevant to the electronegativity of impurity atoms. With the biaxial strain ranging from -4% to 4%, the magnetic moment of V-Cl co-doped phosphorene and the band gap of Co-Cl co-doped phosphorene are greatly tunable between 1.757-0.951 µB and 0.687-0.496 eV, which come from the electron transfer from V to the surrounding P atoms and the weakened bond between Co and Cl, respectively. These investigations provide a reference for regulating the electronic structure and magnetic properties of diluted magnetic semiconductors and promote the applications of phosphorene in spintronics and nanodevices.
2D transition metal dichalcogenides (2D-TMDs) and their unique polymorphic features such as the semiconducting 1H and quasi-metallic 1T' phases exhibit intriguing optical and electronic properties, which can be used in novel electronic and photonic device applications. With the favorable quasi-metallic nature of 1T'-phase 2D-TMDs, the 1H-to-1T' phase engineering processes are an immensely vital discipline exploited for novel device applications. Here, a high-yield 1H-to-1T' phase transition of monolayer-MoS2 on Cu and monolayer-WSe2 on Au via an annealing-based process is reported. A comprehensive experimental and first-principles study is performed to unravel the underlying mechanism and derive the general trends for the high-yield phase transition process of 2D-TMDs on metallic substrates. While each 2D-TMD possesses different intrinsic 1H-1T' energy barriers, the option of metallic substrates with higher chemical reactivity plays a significantly pivotal role in enhancing the 1H-1T' phase transition yield. The yield increase is achieved via the enhancement of the interfacial hybridizations by the means of increased interfacial binding energy, larger charge transfer, shorter interfacial spacing, and weaker bond strength. Fundamentally, this study opens up the field of 2D-TMD/metal-like systems to further scientific investigation and research, thereby creating new possibilities for 2D-TMDs-based device applications.
Using first-principles calculations, we find Li-intercalated bilayer arsenene with AB stacking is dynamically stable, which is different from pristine bilayer with AA stacking. Electron-phonon coupling of the stable Li-intercalated bilayer arsenene are dominated by the low frequency vibrational modes (Eâ³(1), [Formula: see text](1), E'(1) and acoustic modes) and lead to an superconductivity with T c = 8.68 K with isotropical Eliashberg function. Small biaxial tensile strain (2%) can improve T c to 11.22 K due to the increase of DOS and phonon softening. By considering the fully anisotropic Migdal-Eliashberg theory, T c are found to be enhanced by 50% and exhibits a single anisotropic gap nature. In addition, considering its nearly flat top valence band which is favorable for high temperature superconductivity, we also explore the superconducting properties of hole-doped monolayer arsenene under different strains. the unstrained monolayer arsenene superconducts at T c = 0.22 K with 0.1 hole/cell doping. By applying 3% biaxial strain, T c can be lifted up strikingly to 6.69 K due to a strong Fermi nesting of the nearly flat band. Then T c decreases slowly with strain. Our findings provide another insight to realize 2D superconductivity and suggest that the strain is crucial to further enhance the transition temperature.
The piezoelectric and elastic properties of a molecular piezoelectric meta-nitroaniline (mNA) in its single-crystal form were investigated in the framework of first-principles density functional perturbation theory (DFPT). Results support the recent experimental findings those despite being soft and flexible, mNA's piezoelectric coefficients are an order of magnitude greater than that of ZnO and LiNbO3. A molecular-level insight into the piezoelectric properties of mNA is provided. These results are helpful not only for better understanding mNA, but also for developing new piezoelectric materials.
