Prediction of a monolayer spin-spiral semiconductor:CoO with a honeycomb lattice

The recent successful fabrication of two-dimensional(2D)CoO with nanometer-thickness motivates us to investigate monolayer CoO due to possible magnetic properties induced by Co atoms.Here,we employ first-principles calculations to show that monolayer CoO is a 2D spin-spiral semiconductor with a honeycomb lattice.The calculated phonon dispersion reveals the monolayer's dynamical stability.Monolayer CoO exhibits a type-I spin-spiral magnetic ground state.The spinspiral state and the direct bandgap character are both robust under biaxial compressive strain(-5%)to tensile strain(5%).The bandgap varies only slightly under either compressive or tensile strain up to 5%.These results suggest a potential for applications in spintronic devices and offer a new platform to explore magnetism in the 2D limit.

Coexistence of Unidirectional Charge Density Waves in LaTe_(3)

The classic rare-earth tritelluride provides an ideal platform to study the strong correlation state owing to its stable structures and abundance of orders.Here we report the observation of an undiscovered charge density wave(CDW)in LaTe_(3)under 4.2 K,the transition temperature of the CDW states is fitted to be 35 K,and confirmed by the evanishment of this CDW at 77 K via using temperature-dependent scanning tunneling microscope/spectroscopy.The coexistence of these CDWs is confirmed by the atomic resolution and beating pattern simulation.It is the first time to observe the coexistence of unidirectional charge density waves system,providing a new platform to understand the competition and evolution between strong correlation states,and get a deeper sight into the phase lag between different order parameters.

Fabrication of large-scale graphene/2D-germanium heterostructure by intercalation

We report a large-scale, high-quality heterostructure composed of vertically-stacked graphene and two-dimensional(2D) germanium.The heterostructure is constructed by the intercalation-assisted technique.We first synthesize large-scale,single-crystalline graphene on Ir(111) surface and then intercalate germanium at the interface of graphene and Ir(111).The intercalated germanium forms a well-defined 2D layer with a 2 × 2 superstructure with respect to Ir(111).Theoretical calculations demonstrate that the 2D germanium has a double-layer structure.Raman characterizations show that the 2D germanium effectively weakens the interaction between graphene and Ir substrate, making graphene more like the intrinsic one.Further experiments of low-energy electron diffraction, scanning tunneling microscopy, and x-ray photoelectron spectroscopy(XPS) confirm the formation of large-scale and high-quality graphene/2D-germanium vertical heterostructure.The integration of graphene with a traditional 2D semiconductor provides a platform to explore new physical phenomena in the future.

Prediction of structured void-containing 1T-PtTe2 monolayer with potential catalytic activity for hydrogen evolution reaction

Two-dimensional(2D)transition metal dichalcogenides(TMDs)have attracted considerable attention because of their unique properties and great potential in nano-technology applications.Great efforts have been devoted to fabrication of novel structured TMD monolayers by modifying their pristine structures at the atomic level.Here we propose an intriguing structured 1T-PtTe2 monolayer as hydrogen evolution reaction(HER)catalyst,namely,Pt4Te7,using first-principles calculations.It is found that Pt4Te7 is a stable monolayer material verified by the calculation of formation energy,phonon dispersion,and ab initio molecular dynamics simulations.Remarkably,the novel structured void-containing monolayer exhibits superior catalytic activity toward HER compared with the pristine one,with a Gibbs free energy very close to zero(less than 0.07 eV).These features indicate that Pt4Te7 monolayer is a high-performance HER catalyst with a high platinum utilization.These findings open new perspectives for the functionalization of 2D TMD materials at an atomic level and its application in HER catalysis.

High quality PdTe_2 thin films grown by molecular beam epitaxy

PdTe2, a member of layered transition metal dichalcogenides （TMDs）, has aroused significant research interest due to the coexistence of superconductivity and type-II Dirac fermions. It provides a promising platform to explore the inter- play between superconducting quasiparticles and Dirac fermions. Moreover, PdTe2 has also been used as a substrate for monolayer antimonene growth. Here in this paper, we report the epitaxial growth of high quality PdTe2 films on bilayer graphene/SiC（0001） by molecular beam epitaxy （MBE）. Atomically thin films are characterized by scanning tunneling microscopy （STM）, X-ray photoemission spectroscopy （XPS）, low-energy electron diffraction （LEED）, and Raman spec- troscopy. The band structure of 6-layer PdTe2 film is measured by angle-resolved photoemission spectroscopy （ARPES）. Moreover, our air exposure experiments show excellent chemical stability of epitaxial PdTe2 film. High-quality PdTe2 films provide opportunities to build antimonene/PdTe2 heterostructure in ultrahigh vacuum for future applications in electronic and optoelectronic nanodevices.

Band engineering of B_2H_2 nanoribbons

Freestanding honeycomb borophene is unstable due to the electron-deficiency of boron atoms. B_2H_2 monolayer, a typical borophene hydride, has been predicted to be structurally stable and attracts great attention. Here, we investigate the electronic structures of B_2H_2 nanoribbons. Based on first-principles calculations, we have found that all narrow armchair nanoribbons with and without mirror symmetry(ANR-s and ANR-as, respectively) are semiconducting. The energy gap has a relation with the width of the ribbon. When the ribbon is getting wider, the gap disappears. The zigzag ribbons without mirror symmetry(ZNR-as) have the same trend. But the zigzag ribbons with mirror symmetry(ZNR-s) are always metallic. We have also found that the metallic ANR-as and ZNR-s can be switched to semiconducting by applying a tensile strain along the nanoribbon. A gap of 1.10 eV is opened under 16% strain for the 11.0-■ ANR-as. Structural stability under such a large strain has also been confirmed. The flexible band tunability of B_2H_2 nanoribbon increases its possibility of potential applications in nanodevices.

Low-temperature growth of large-scale,single-crystalline graphene on Ir(111)

Iridium is a promising substrate for self-limiting growth of graphene. However, single-crystalline graphene can only be fabricated over 1120 K. The weak interaction between graphene and Ir makes it challenging to grow graphene with a single orientation at a relatively low temperature. Here, we report the growth of large-scale, single-crystalline graphene on Ir(111) substrate at a temperature as low as 800 K using an oxygen-etching assisted epitaxial growth method. We firstly grow polycrystalline graphene on Ir. The subsequent exposure of oxygen leads to etching of the misaligned domains.Additional growth cycle, in which the leftover aligned domain serves as a nucleation center, results in a large-scale and single-crystalline graphene layer on Ir(111). Low-energy electron diffraction, scanning tunneling microscopy, and Raman spectroscopy experiments confirm the successful growth of large-scale and single-crystalline graphene. In addition, the fabricated single-crystalline graphene is transferred onto a SiO_2/Si substrate. Transport measurements on the transferred graphene show a carrier mobility of about 3300 cm^2·V^(-1)·s^(-1). This work provides a way for the synthesis of large-scale,high-quality graphene on weak-coupled metal substrates.

Structural and thermal stabilities of Au@Ag core-shell nanoparticles and their arrays:A molecular dynamics simulation

Thermal stability of core-shell nanoparticles(CSNPs)is crucial to their fabrication processes,chemical and physical properties,and applications.Here we systematically investigate the structural and thermal stabilities of single Au@Ag CSNPs with different sizes and their arrays by means of all-atom molecular dynamics simulations.The formation energies of all Au@Ag CSNPs we reported are all negative,indicating that Au@Ag CSNPs are energetically favorable to be formed.For Au@Ag CSNPs with the same core size,their melting points increase with increasing shell thickness.If we keep the shell thickness unchanged,the melting points increase as the core sizes increase except for the CSNP with the smallest core size and a bilayer Ag shell.The melting points of Au@Ag CSNPs show a feature of non-monotonicity with increasing core size at a fixed NP size.Further simulations on the Au@Ag CSNP arrays with 923 atoms reveal that their melting points decrease dramatically compared with single Au@Ag CSNPs.We find that the premelting processes start from the surface region for both the single NPs and their arrays.

Electronic structures of vacancies in Co_(3)Sn_(2)S_(2)

Co_(3)Sn_(2)S_(2)has attracted a lot of attention for its multiple novel physical properties,including topological nontrivial surface states,anomalous Hall effect,and anomalous Nernst effect.Vacancies,which play important roles in functional materials,have attracted increasing research attention.In this paper,by using density functional theory calculations,we first obtain band structures and magnetic moments of Co_(3)Sn_(2)S_(2)with exchange–correlation functionals at different levels.It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co_(3)Sn_(2)S_(2).We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments.The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level.For di-and tri-sulfur vacancies,the localized defect states hybridize with neighboring ones,forming bonding states as well as anti-bonding states.The Sn vacancy on the Sn-terminated surface also introduces localized bond states,which are merged with the valence bands.These results provide a reference for future experimental investigations of vacancies in Co_(3)Sn_(2)S_(2).

Epitaxial synthesis and electronic properties of monolayer Pd2Se3

Two-dimensional(2D)materials received large amount of studies because of the enormous potential in basic science and industrial applications.Monolayer Pd2Se3 is a fascinating 2D material that was predicted to possess excellent thermoelectric,electronic,transport,and optical properties.However,the fabrication of large-scale and high-quality monolayer Pd2Se3 is still challenging.Here,we report the synthesis of large-scale and high-quality monolayer Pd2Se3 on graphene-SiC(0001)by a two-step epitaxial growth.The atomic structure of Pd2Se3 was investigated by scanning tunneling microscope(STM)and confirmed by non-contact atomic force microscope(nc-AFM).Two subgroups of Se atoms have been identified by nc-AFM image in agreement with the theoretically predicted atomic structure.Scanning tunneling spectroscopy(STS)reveals a bandgap of 1.2 eV,suggesting that monolayer Pd2Se3 can be a candidate for photoelectronic applications.The atomic structure and defect levels of a single Se vacancy were also investigated.The spatial distribution of STS near the Se vacancy reveals a highly anisotropic electronic behavior.The two-step epitaxial synthesis and characterization of Pd2Se3 provide a promising platform for future investigations and applications.

Electronic properties of silicene in BN/silicene van der Waals heterostructures

Silicene is a promising 2D Dirac material as a building block for van der Waals heterostructures （vdWHs）. Here we investigate the electronic properties of hexagonal boron nitride/silicene （BN/Si） vdWHs using first-principles calculations. We calculate the energy band structures of BN/Si/BN heterostructures with different rotation angles and find that the electronic properties of silicene are retained and protected robustly by the BN layers. In BN/Si/BN/Si/BN heterostructure, we find that the band structure near the Fermi energy is sensitive to the stacking configurations of the silicene layers due to in- terlayer coupling. The coupling is reduced by increasing the number of BN layers between the silicene layers and becomes negligible in BN/Si/（BN）3/Si/BN. In （BN）n/Si superlattices, the band structure undergoes a conversion from Dirac lines to Dirac points by increasing the number of BN layers between the silicene layers. Calculations of silicene sandwiched by other 2D materials reveal that silicene sandwiched by low-carbon-doped boron nitride or HfO2 is semiconducting.

Band engineering of double-wall Mo-based hybrid nanotubes

Hybrid transition-metal dichalcogenides （TMDs） with different chalcogens on each side （X-TM-Y） have attracted attention because of their unique properties. Nanotubes based on hybrid TMD materials have advantages in flexibility over conventional TMD nanotubes. Here we predict the wide band gap tunability of hybrid TMD double-wall nanotubes （DWNTs） from metal to semiconductor. Using density-function theory （DFT） with HSE06 hybrid functional, we find that the electronic property of X-Mo-Y DWNTs （X = O and S, inside a tube; Y = S and Se, outside a tube） depends both on electronegativity difference and diameter difference. If there is no difference in electron negativity between inner atoms （X） of outer tube and outer atoms （Y） of inner tube, the band gap of DWNTs is the same as that of the inner one. If there is a significant electronegativity difference, the electronic property of the DWNTs ranges from metallic to semiconducting, depending on the diameter differences. Our results provide alternative ways for the band gap engineering of TMD nanotubes.

Local Density of States Modulated by Strain in Marginally Twisted Bilayer Graphene

In marginally twisted bilayer graphene,the Moirépattern consists of the maximized AB(BA)stacking regions,minimized AA stacking regions and triangular networks of domain walls.Here we realize the strain-modulated electronic structures of marginally twisted bilayer graphene by scanning tunneling microscopy/spectroscopy and density functional theory(DFT)calculations.The experimental data show four peaks near the Fermi energy at the AA regions.DFT calculations indicate that the two new peaks closer to the Fermi level may originate from the intrinsic heterostrain and the electric field implemented by back gate is likely to account for the observed shift of the four peaks.Furthermore,the d I/d V map across Moirépatterns with different strain strengths exhibits a distinct appearance of the helical edge states.

Database Construction for Two-Dimensional Material-Substrate Interfaces

Interfacial structures and interactions of two-dimensional(2D)materials on solid substrates are of fundamental importance for fabrications and applications of 2D materials.However,selection of a suitable solid substrate to grow a 2D material,determination and control of 2D material-substrate interface remain a big challenge due to the large diversity of possible configurations.Here,we propose a computational framework to select an appropriate substrate for epitaxial growth of 2D material and to predict possible 2D material-substrate interface structures and orientations using density functional theory calculations performed for all non-equivalent atomic structures satisfying the symmetry constraints.The approach is validated by the correct prediction of three experimentally reported 2D material-substrate interface systems with only the given information of two parent materials.Several possible interface configurations are also proposed based on this approach.We therefore construct a database that contains these interface systems and has been continuously expanding.This database serves as preliminary guidance for epitaxial growth and stabilization of new materials in experiments.

Experimental Synthesis of Strained Monolayer Silver Arsenide on Ag(111)Substrates

Two-dimensional(2 D)materials are playing more and more important roles in both basic sciences and industrial applications.For 2 D materials,strain could tune the properties and enlarge applications.Since the growth of 2 D materials on substrates is often accompanied by strain,the interaction between 2 D materials and substrates is worthy of careful attention.Here we demonstrate the fabrication of strained monolayer silver arsenide(AgAs)on Ag(111)by molecular beam epitaxy,which shows one-dimensional stripe structures arising from uniaxial strain.The atomic geometric structure and electronic band structure are investigated by low energy electron diffraction,scanning tunneling microscopy,x-ray photoelectron spectroscopy,angle-resolved photoemission spectroscopy and first-principle calculations.Monolayer AgAs synthesized on Ag(111)provides a platform to study the physical properties of strained 2 D materials.

Thickness-dependent magnetic order and phase transition in V5S8

V5S8 is an ideal candidate to explore the magnetism at the two-dimensional(2D)limit.A recent experiment has shown that the V5S8 thin films exhibit an antiferromagnetic(AFM)to ferromagnetic(FM)phase transition with reducing thickness.Here,for the first time,using density functional theory calculations,we report the antiferromagnetic order of bulk V5S8,which is consistent with the previous experiments.The specific antiferromagnetic order is reproduced when Ueff=2 eV is applied on the intercalated vanadium atoms within LDA.We find that the origin of the magnetic ordering is from superexchange interaction.We also investigate the thickness-dependent magnetic order in V5S8 thin films.It is found that there is an antiferromagnetic to ferromagnetic phase transition when V5S8 is thinned down to 2.2 nm.The main magnetic moments of the antiferromagnetic and ferromagnetic states of the thin films are located on the interlayered vanadium atoms,which is the same as that in the bulk.Meanwhile,the strain in the thin films also influences the AFM-FM phase transition.Our results not only reveal the magnetic order and origin in bulk V5S8 and thin films,but also provide a set of parameters which can be used in future calculations.

Y_(3)-/X_(3)Y_(4)-THB金属有机网络中本征二维拓扑绝缘体的预测

本工作利用基于密度泛函理论的高通量计算方法,研究了由金属团簇和四羟基苯(THB)分子形成的232个具有Kagome晶格的有机金属网格.在由THB分子和过渡金属原子三聚体(称为Y_(3)-THB)组成的有机金属网格中,有7种网格结构是本征有机拓扑绝缘体.进一步引入过渡金属原子(Tl,Pb,Bi)后得到的X_(3)Y_(4)-THB结构中,发现了9个具有较大自旋轨道耦合能隙的本征有机拓扑绝缘体.在这些本征拓扑绝缘体中,α相Pb_(3)Zn_(4)-THB结构是具有拓扑平带的拓扑绝缘体,其非平庸自旋轨道耦合能隙(97.5 meV)几乎是其平带带宽(22.5 meV)的4倍,是实现分数量子霍尔效应的理想平台.该工作为设计具有大能隙的二维有机拓扑绝缘体提供了新的途径.

ecent progress in the theoretical design of two-dimensional ferroelectric

Two-dimensional(2D)ferroelectrics(FEs),which maintain stable electric polarization in ultrathin films,are a promising class of materials for the development of various miniature functional devices.In recent years,several 2D FEs with unique properties have been successfully fabricated through experiments.They have been found to exhibit some unique properties either by themselves or when they are coupled with other functional materials(e.g.,ferromagnetic materials,materials with 5 d electrons,etc.).As a result,several new types of 2D FE functional devices have been developed,exhibiting excellent performance.As a type of newly discovered 2D functional material,the number of 2D FEs and the exploration of their properties are still limited and this calls for further theoretical predictions.This review summarizes recent progress in the theoretical predictions of 2D FE materials and provides strategies for the rational design of 2D FE materials.The aim of this review is to provide guidelines for the design of 2D FE materials and related functional devices.

“小分子机器”量子结构的构造及其物性

低维量子结构的构筑及调控是物理学向小尺度研究方向延伸的核心问题之一.本文重点围绕高鸿钧及合作者在"小分子机器"量子结构的构造及其物性方面开展的研究,系统地介绍了他们在单原子层次上设计、构造和调控的若干小分子机器量子结构.代表性工作有:(1)构筑了四叔丁基酞菁锌((t-Bu)4-Zn Pc)分子在Au(111)表面上"抛锚"的、带有固定偏心轴的单分子转子及其有序阵列,证实了转动行为的可调控性;(2)实现了单个小分子极限尺度(0.6 nm)的可逆电导转变,进而实现了稳定、重复、可逆的超高密度信息存储中的原理性应用;(3)首次直接证实代表性的"分子机器"轮烷(Rotaxane)分子的构型及其电导的可逆转变,澄清了当时化学界的一个争论热点,该热点也是2016年诺贝尔化学奖的主要成果;(4)通过单个H原子的吸脱附实现了酞菁锰(Mn Pc)分子在Au(111)表面Kondo效应的调控,这种单个自旋量子态的可逆控制,实现了极高密度信息存储(>40 TB/cm^2)的原理性应用;(5)在通过"原子手术"Mn Pc分子所创制的一种新"功能体系"上发现了朗德g因子在原子尺度上具有的空间分布不均匀性,这是长期以来一直未解决的问题.这些系统工作为单分子发电机/无线电辐射器及纳米电子器件等的构造组装与应用奠定了基础.

Emergy assessment of three home courtyard agriculture production systems in Tibet Autonomous Region, China

Home courtyard agriculture is an important model of agricultural production on the Tibetan plateau. Be- cause of the sensitive and fragile plateau environment, it needs to have optimal performance characteristics, including high sustainability, low environmental pressure, and high economic benefit. Emergy analysis is a promising tool for evaluation of the environmental-economic performance of these production systems. In this study, emergy analysis was used to evaluate three courtyard agricultural production models： Raising Geese in Corn Fields （RGICF）, Con- ventional Corn Planting （CCP）, and Pea-Wheat Rotation （PWR）. The results showed that the RGICF model produced greater economic benefits, and had higher sustainability, lower environmental pressure, and higher product safety than the CCP and PWR models. The emergy yield ratio （EYR） and emergy self-support ratio （ESR） of RGICF were 0.66 and 0.11, respectively, lower than those of the CCP production model, and 0.99 and 0.08, respectively, lower than those of the PWR production model. The impact of RGICF （1.45） on the environment was lower than that of CCP （2.26） and PWR （2.46）. The emergy sustainable indices （ESIs） of RGICF were 1.07 and 1.02 times higher than those of CCP and PWR, respectively. With regard to the emergy index of product safety （EIPS）, RGICF had a higher safety index than those of CCP and PWR. Overall, our results suggest that the RGICF model is advantageous and provides higher environmental benefits than the CCP and PWR systems.