Effect of strain on geometric and electronic structures of graphene on Ru(0001) surface

The atomic and electronic structures of a graphene monolayer on a Ru(0001) surface under compressive strain are investigated by using first-principles calculations. Three models of graphene monolayers with different carbon periodicities due to the lattice mismatch are proposed in the presence and the absence of the Ru(0001) substrate separately. Considering the strain induced by the lattice mismatch, we optimize the atomic structures and investigate the electronic properties of the graphene. Our calculation results show that the graphene layers turn into periodic corrugations and there exist strong chemical bonds in the interface between the graphene N×N superlattice and the substrate. The strain does not induce significant changes in electronic structure. Furthermore, the results calculated in the local density approximation (LDA) are compared with those obtained in the generalized gradient approximation (GGA), showing that the LDA results are more reasonable than the GGA results when only two substrate layers are used in calculation.     

Adsorption behavior of triphenylene on Ru(0001) investigated by scanning tunneling microscopy

As a representative of small aromatic molecules, triphenylene(TP) has markedly high carrier mobility and is an ideal precursor for building graphene nanostructures. We mainly investigated the adsorption behavior of TP molecules on Ru(0001) by using scanning tunneling microscopy(STM). In submonolayer regime, TP molecules are randomly dispersed on Ru(0001) and the TP overlayer can be thoroughly dehydrogenated and converted into graphene islands at 700 K. Due to weak interaction between TP molecules and graphene, the grooves formed among graphene islands have confinement effect on TP molecules. TP adopts a flat-lying adsorption mode and has two adsorption configurations with the 3-fold molecular axis aligned almost parallel or antiparallel to the ■ direction of the substrate. At TP coverages of 0.6 monolayer(ML)and 0.8 ML, the orientational distributions of the two adsorption configurations are equal. At about 1.0 ML, we find the coexistence of locally ordered and disordered phases. The ordered phase includes two sets of different superstructures with the symmetries of ■R23.41° and p(4 × 4), respectively. The adsorption behavior of TP on Ru(0001) can be attributed to the delicate balance between molecule–substrate and molecule–molecule interactions.     

Tuning of the periodicity of stable self-organized metallic templates

The atomic and electronic structures of Pb bilayer/Pt(111) are investigated with two theoretical calculations.We find that the stable (2 × 2)/(3 × 3) Pb/Pt(111) structure is a promising candidate for being used as a template with self-organized ordered Pb semi-cluster array on the first Pb monolayer.This stable structure can realize the ordered Au single-atom array around the Pb semi-clusters that can cause selective adsorption of noble atoms.The size of Pb magic number semi-cluster plays a more important role in determining the periodicity of the template than the lattice constant misfit between the substrate and the overlayer.This leads to quite a different periodicity between the two stable templates,which are (2 × 2)/(3 × 3) Pb/Pt(111) and Pb/Cu(111).Therefore,by considering the size of the stable semi-clusters and carefully selecting different substrate materials,we can tune the density of Pb semi-clusters as the nucleation points and then tune the periodicity of the stable template.     

Structures of Pt clusters on graphene doped with nitrogen, boron, and silicon: a theoretical study

The structures of Pt clusters on nitrogen-,boron-,silicon-doped graphenes are theoretically studied using densityfunctional theory.These dopants(nitrogen,boron and silicon) each do not induce a local curvature in the graphene and the doped graphenes all retain their planar form.The formation energy of the silicon-graphene system is lower than those of the nitrogen-,boron-doped graphenes,indicating that the silicon atom is easier to incorporate into the graphene.All the substitutional impurities enhance the interaction between the Pt atom and the graphene.The adsorption energy of a Pt adsorbed on the silicon-doped graphene is much higher than those on the nitrogen-and boron-doped graphenes.The doped silicon atom can provide more charges to enhance the Pt-graphene interaction and the formation of Pt clusters each with a large size.The stable structures of Pt clusters on the doped-graphenes are dimeric,triangle and tetrahedron with the increase of the Pt coverage.Of all the studied structures,the tetrahedron is the most stable cluster which has the least influence on the planar surface of doped-graphene.     

Formation of graphene on Ru（0001） surface

We report on the formation of a graphene monolayer on a Ru(0001) surface by annealing the Ru(0001) crystal. The samples are characterized by scanning tunnelling microscopy (STM) and Auger electron spectroscopy (AES). STM images show that the Moir\'{e} pattern is caused by the graphene layer mismatched with the underlying Ru(0001) surface and has an $N\times N$ superlattice. It is further found that the graphene monolayer on a Ru(0001) surface is very stable at high temperatures. Our results provide a simple and convenient method to produce a graphene monolayer on the Ru(0001) surface, which is used as a template for fabricating functional nanostructures needed in future nano devices and catalysis.     

Ab initio calculations on oxygen vacancy defects in strained amorphous silica

The effects of uniaxial tensile strain on the structural and electronic properties of positively charged oxygen vacancy defects in amorphous silica(a-SiO2)are systematically investigated using ab-initio calculation based on density functional theory.Four types of positively charged oxygen vacancy defects,namely the dimer,unpuckered,and puckered four-fold(4×),and puckered five-fold(5×)configurations have been investigated.It is shown by the calculations that applying uniaxial tensile strain can lead to irreversible transitions of defect structures,which can be identified from the fluctuations of the curves of relative total energy versus strain.Driven by strain,a positively charged dimer configuration may relax into a puckered 5×configuration,and an unpuckered configuration may relax into either a puckered 4×configuration or a forward-oriented configuration.Accordingly,the Fermi contacts of the defects remarkably increase and the defect levels shift under strain.The Fermi contacts of the puckered configurations also increase under strain to the values close to that of Eα′center in a-SiO2.In addition,it is shown by the calculations that the relaxation channels of the puckered configurations after electron recombination are sensitive to strain,that is,those configurations are more likely to relax into a two-fold coordinated Si structure or to hold a puckered structure under strain,both of which may raise up the thermodynamic charge-state transition levels of the defects into Si band gap.As strain induces more puckered configurations with the transition levels in Si band gap,it may facilitate directly the development of oxide charge accumulation and indirectly that of interface charge accumulation by promoting proton generation under ionization radiation.This work sheds a light on understanding the strain effect on ionization damage at an atomic scale.     

Screening of Local Magnetic Moment by Electrons of Disordered Graphene

Based on the Anderson impurity model and self-consistent approach, we investigate the condition for the screening of a local magnetic moment by electrons in graphene and the influence of the moment on electronic properties of the system. The results of numerical calculations carried out on a finite sheet of graphene show that when the Fermi energy is above the single occupancy energy and below the double occupancy energy of the local impurity, a magnetic state is possible. A phase diagram in a parameter space spanned by the Coulomb energy U and the Fermi energy is obtained to distinguish the parameter regions for the magnetic and nonmagnetic states of the impurity. We find that the combined effect of the impurity and finite size effect results in a large charge density near the edges of the finite graphene sheet. The density of states exhibits a peak at the Dirac point which is caused by the appearance of the edge states localized at the zigzag edges of the sheet.     

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

In marginally twisted bilayer graphene, the Moiré pattern 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 ...     

Dissociations of O2 molecules on ultrathin Pb（111） films： first-principles plane wave calculations

Using first-principles calculations,we systematically study the dissociations of O₂ molecules on different ultrathin Pb（111） films.According to our previous work revealing the molecular adsorption precursor states for O₂,we further explore why there are two nearly degenerate adsorption states on Pb（111） ultrathin films,but no precursor adsorption states existing at all on Mg（0001） and Al（111） surfaces.The reason is concluded to be the different surface electronic structures.For the O₂ dissociation,we consider both the reaction channels from gas-like and molecularly adsorbed O₂ molecules.We find that the energy barrier for O₂ dissociation from the molecular adsorption precursor states is always smaller than that from O₂ gas.The most energetically favorable dissociation process is found to be the same on different Pb（111） films,and the energy barriers are found to be influenced by the quantum size effects of Pb（111） films.     

High quality sub-monolayer,monolayer, and bilayer graphene on Ru（0001）

High quality sub-monolayer, monolayer, and bilayer graphene were grown on Ru(0001). For the sub-monolayer graphene, the size of graphene islands with zigzag edges can be controlled by the dose of ethylene exposure. By increasing the dose of ethylene to 100 Langmuir at a high substrate temperature(800℃), high quality single-crystalline monolayer graphene was synthesized on Ru(0001). High quality bilayer graphene was formed by further increasing the dose of ethylene while reducing the cooling rate to 5℃/min. Raman spectroscopy revealed the vibrational states of graphene, G and 2D peaks appeared only in the bilayer graphene, which demonstrates that it behaves as the intrinsic graphene. Our present work affords methods to produce high quality sub-monolayer, monolayer, and bilayer graphene, both for basic research and applications.     

Stability and optoelectronic property of low-dimensional organic tin bromide perovskites

The toxicity and degradation of hybrid lead-halide perovskites hinder their extensive applications.It is thus of great importance to explore non-toxic alternative materials with excellent stability and optoelectronic property.We investigate the atomic structures and optoelectronic properties of non-toxic organic tin bromide perovskites(OTBP)with one/zerodimensional(1D/0D)structures by first-principles calculations.The calculated atomic structures show that the 1D/0D OTBPs are stable and the structure of inorganic octahedra in 0D is higher order than that in 1D.Moreover,the origination of exceptional purity emitting light in experiments is explained based on the calculated electronic structure.     

Influence of alkali metal superoxides on structure,electronic,and optical properties of Be_(12)O_(12) nanocage:Density functional theory study

The effect of alkali metal superoxides M_3O(M = Li,Na,K) on the electronic and optical properties of a Be_(12)O_(12) nanocage was studied by density functional theory(DFT) and time-dependent density functional theory(TD-DFT).The energy gaps(Eg) of all configurations were calculated.Generally,the adsorption of alkali metal superoxides on the Be_(12)O_(12) nanocage causes a decrease of Eg.Electric dipole moment μ,polarizability α,and static first hyperpolarizability β were calculated and it was shown that the adsorption of alkali metal superoxides on Be_(12)O_(12) increases its polarizability.It was found that the absorption of M_3 O on Be_(12)O_(12) nanocluster improves its nonlinear optical properties.The highest first hyperpolarizability(β≈ 214000 a.u.) is obtained in the K_3O–Be_(12)O_(12)nanocluster.The TD-DFT calculations were performed to investigate the origin of the first hyperpolarizabilities and it was shown that a higher first hyperpolarizability belongs to the structure that has a lower transition energy.     

Comparison of electronic structure between monolayer silicenes on Ag(111)

√√The electronic structures of monolayer silicenes(4 × 4 and■×■R13.9o) grown on Ag(111) surface are studied by scanning tunneling spectroscopy(STS) and density functional theory(DFT) calculations. While both phases have similar electronic structures around the Fermi level, significant differences are observed in the higher energy unoccupied states.The DFT calculations show that the contributions of Si 3pz orbitals to the unoccupied states are different because of their different buckled configurations.     

Boundary graphene layer effect on surface plasmon oscillations in a quantum plasma half-space

The effect of graphene on unique features of surface plasmon-polariton excitations near the interface of vacuum and quantum plasma half-space is explored using a quantum hydrodynamic model including the Fermi electron temperature and the quantum Bohm potential together with the full set of Maxwell equations.It is found that graphene as a conductive layer significantly modifies the propagation properties of surface waves by making a change on the corresponding wave dispersion relation.It is shown that the presence of graphene layer on the interface of vacuum and plasma leads to a blue-shift in the surface Plasmon frequency.The results of present study must be contributed to the modern electronic investigations.     

First-principles study of the optical properties of defect electronic structure and chalcopyrite CdGa2Te4

<正>The electronic and optical properties of the defect chalcopyrite CdGa₂Te₄ compound are studied based on the first-principles calculations.The band structure and density of states are calculated to discuss the electronic properties and orbital hybridized properties of the compound.The optical properties,including complex dielectric function,absorption coefficient,refractive index,reflectivity,and loss function,and the origin of spectral peaks are analysed based on the electronic structures.The presented results exhibit isotropic behaviours in a low and a high energy range and an anisotropic behaviour in an intermediate energy range.     

Real-space observation on standing configurations of phenylacetylene on Cu(111) by scanning probe microscopy

The adsorption configurations of molecules adsorbed on substrates can significantly affect their physical and chemical properties. A standing configuration can be difficult to determine by traditional techniques, such as scanning tunneling microscopy(STM) due to the superposition of electronic states. In this paper, we report the real-space observation of the standing adsorption configuration of phenylacetylene on Cu(111) by non-contact atomic force microscopy(nc-AFM).Deposition of phenylacetylene at 25 K shows featureless bright spots in STM images. Using nc-AFM, the line features representing the C–H and C–C bonds in benzene rings are evident, which implies a standing adsorption configuration. Further density functional theory(DFT) calculations reveal multiple optimized adsorption configurations with phenylacetylene breaking its acetylenic bond and forming C–Cu bond(s) with the underlying copper atoms, and hence stand on the substrate.By comparing the nc-AFM simulations with the experimental observation, we identify the standing adsorption configuration of phenylacetylene on Cu(111). Our work demonstrates an application of combining nc-AFM measurements and DFT calculations to the study of standing molecules on substrates, which enriches our knowledge of the adsorption behaviors of small molecules on solid surfaces at low temperatures.     

Stability, defect and electronic properties of graphane-like carbon-halogen compounds

We perform first-principles total energy calculations to investigate the stabilities and the electronic structures of graphane-like structures of carbon-halogen compounds, where the hydrogen atoms in the graphane are substituted by halogen atoms. Three halogen elements, fluorine (F), chlorine (Cl) and bromine (Br), are considered, and the graphane-like structures are named as CF, CCl and CBr, respectively. It is found that for the single-atom adsorption, only the F adatom can be chemically adsorbed on the graphene. However, the stable graphane-like structures of CF, CCl and CBr can form due to the interaction between the halogen atoms. The carbon atoms in the stable CF, CCl and CBr compounds are in the sp³ hybridization, forming a hexagonal network similar to the graphane. The electronic band calculations show that CF and CCl are semiconductors with band gaps of 3.28 eV and 1.66 eV, respectively, while CBr is a metal. Moreover, the molecular dynamics simulation is employed to clarify the stabilities of CF and CCl. Those two compounds are stable at room temperature. A high temperature (≥1200 K) is needed to damage CF, while CCl is destroyed at 700 K. Furthermore, the effects of a vacancy on the structure and the electronic property of CF are discussed.     

First-principles study of the electronic and optical properties of the （Y,N）-codoped anatase TiO2 photocatalyst

First-principles plane-wave pseudopotential calculations are performed to study the geometrical structures,formation energies,and electronic and optical properties of Y-doped,N-doped,and(Y,N)-codoped TiO 2.The calculated results show that Y and N codoping leads to lattice distortion,easier separation of photogenerated electron-hole pairs and band gap narrowing.The optical absorption spectra indicate that an obvious red-shift occurs upon Y and N codoping,which enhances visible-light photocatalytic activity.     

Super-strong interactions between multivalent anions and graphene

Based on the density functional theory(DFT) calculations,we showed that the interactions between different valence anions(PO_4~(3-),CH_3 PO_4~(2-),(CH_3)_2 PO_4) and graphene significantly increased as the valence of anion increased from negative monovalence to negative trivalence.The adsorption energy of(CH_3)_2 PO_4~-on the electron-rich graphene flake(C_(84)H_(24))is-8.3 kcal/mol.The adsorption energy of CH_3 PO_4~(2-) on the electron-rich graphene flake(C84 H24) is-48.0 kcal/mol,which is about six times that of(CH_3)_2 PO_4 adsorption on electron-rich graphene flake(C84 H24) and is even much larger than that of CO_3~(2-) adsorption on electron-deficient aromatic ring C_6 F_6(-28.4 kcal/mol).The adsorption energy of PO_4~(3-)on the electron-rich graphene flake(C84 H24) is-159.2 kcal/mol,which is about 20 times that of(CH_3)_2 PO_4 adsorption on the graphene flake(C84 H24).The super-strong adsorption energy is mainly attributed to the orbital interactions between multivalent anions and graphene.This work provides new insights for understanding the interaction between multivalent anions and π-electron-rich carbon-based nanomaterials and is helpful for the design of graphene-based DNA biosensor.     

Electronic and transport properties of V-shaped defect zigzag MoS_2 nanoribbons

Based on the nonequilibrium Green＇s function （NEGF） in combination with density functional theory （DFT） calcu- lations, we study the electronic structures and transport properties of zigzag MoS2 nanoribbons （ZMNRs） with V-shaped vacancy defects on the edge. The vacancy formation energy results show that the zigzag vacancy is easier to create on the edge of ZMNR than the armchair vacancy. Both of the defects can make the electronic band structures of ZMNRs change from metal to semiconductor. The calculations of electronic transport properties depict that the currents drop off clearly and rectification ratios increase in the defected systems. These effects would open up possibilities for their applications in novel nanoelectronic devices.