Graphene traps for cesium atoms

A significant increase in the surface concentration of cesium atoms intercalated under graphene islands on rhodium has been revealed when annealing the adsorbed layer in a ultrahigh vacuum. Heating leads to a decrease in the area of graphene islands due to the solution of carbon atoms in the metal bulk. At the same time, the edge carbon atoms in the islands, which are coupled with the surface by chemisorptive forces, prevent the leakage of the alkali metal from under the islands. This leads to the significant compression of cesium and to an increase in its surface concentration under the islands by a factor of almost 10. The desorption of cesium is observed only after the complete thermal destruction of graphene islands.     

Thin graphite overlayers: Graphene and alkali metal intercalation

Using LEED and angle resolved photoemission for characterisation we have prepared graphite overlayers with down to monolayer thickness by heating SiC crystals and monitored alkali metal intercalation for the multilayer films. The valence band structure of the monolayer is similar to that calculated for graphene though downshifted by around 0.8 eV and with a small gap at the zone corner. The shift suggests that the transport properties, which are of much present interest, are similar to that of a biased graphene sample. Upon alkali metal deposition the 3D character of the π states is lost and the resulting band structure becomes graphene like. A comparison with data obtained for ex situ prepared intercalation compounds indicates that the graphite film has converted to the stage 1 compounds C₈K or C₈Rb. Advantages with the present preparation method is that the graphite film can be recovered by desorbing small amounts of alkali metal and that the progress of compound formation can be monitored. The energy shifts measured after different deposits indicate that saturation is reached in three steps. Our interpretation is that in the first the alkali atoms are dispersed while the final steps are characterized by the formation of first one and then a second (2 × 2) ordered alkali metal layer adjacent to the uppermost carbon layer.     

Delay times in the sputtering of atoms from alkali-halide crystals during low-energy electron bombardment

Delay times in the sputtering of atoms from RbI, RbBr and NaCl by 540 eV electrons were investigated. This was done by simultaneously using the correlation technique and a slotted disc velocity selector. We observed delays in thermally ejected alkali and halogen atoms. These times were found to be independent of target temperature and are ascribed to lifetimes of excitons. Non-thermal halogen atoms are always sputtered without any delay.     

Graphene growth during benzene pyrolysis on an iridium crystal

The dependence of graphene formation mechanisms during pyrolysis of carbon-containing molecules on iridium on the crystal structure and surface curvature has been studied using field desorption and field electron microscopy. The structure and shape of grown carbon formations, diffusion, desorption, and intercalation of alkali metal atoms have been investigated.     

"Textbook" adsorption at "nontextbook" adsorption sites: halogen atoms on alkali halide surfaces

Density-functional theory and second order M?ller-Plesset perturbation theory calculations indicate that halogen atoms bond preferentially to halide substrate atoms on a series of alkali halide surfaces, rather than to the alkali atoms as might be anticipated. An analysis of the electronic structures in each system reveals that this novel adsorption mode is stabilized by the formation of textbook two-center three-electron covalent bonds. The implications of these findings to, for example, nanostructure crystal growth, are briefly discussed.     

BeXY、MgXY（X、Y=F、Cl、Br）与ClF3和ClOF3形成复合物的理论研究

与卤素原子相比,超卤素表现出电负性更大、结构更丰富、性质更新颖等更加丰富的特征。本文以碱土金属Be、Mg的卤化物BeXY、MgXY（X、Y=F、Cl、Br）为母体,分别与卤素氟化物ClF3、ClOF3形成的复合物为研究对象,设计了12种配合物。采用密度泛函理论研究了这些化合物的结构、垂直电离能(VDE)、绝热电离能(ADE)等性质。研究发现,配合物中存在超卤素阴离子,所表现出来的性质远远优于常见的超卤素阴离子,因此,此类化合物具有很好的应用前景。     

Density functional theory study on the adsorption of alkali metal ions with pristine and defected graphene sheet

The graphene-based materials along with the adsorption of alkali metal ions are suitable for energy conversion and storage applications. Hence in the present work, we have investigated the structural and electronic properties of pristine and defected graphene sheet upon the adsorption of alkali metal ions (Li⁺, Na⁺, and K⁺) using density functional theory (DFT) calculations. The presence of vacancies or vacancy defects enhances the adsorption of alkali ions than the pristine sheet. From the obtained results, it is found that the adsorption energy of Li⁺ on the vacancies defected graphene sheet is higher (3.05?eV) than the pristine (2.41?eV) and Stone–Wales (2.50?eV) defected sheets. Moreover, the pore radius of the pristine and defected graphene sheets are less affected by metal ions adsorption. The increase in energy gap upon the adsorption of metal ions is found to be high in the vacancy defected graphene than that of other sheets. The metal ions adsorption in the defective vacancy sheets has high charge transfer from metal ions to the graphene sheet. The bonding characteristic between the metal ions and graphene sheet are analysed using QTAIM analysis. The influence of alkali ions on the electronic properties of the graphene sheet is examined from the Total Density of States (TDOS) and Partial Density of States (PDOS).     

Tuning the optical properties of phosphorene by adsorption of alkali metals and halogens

Optical properties of phosphorene are tuned by adsorption of alkali metals (Li and Na) and halogens (Br and Cl). It has been found that on increasing the size of alkali metals and halogen adsorbed phosphorene layer the absorption coefficient reduces and shifts towards visible region. The refractive index in alkali metal adsorbed phosphorene increases with size of phosphorene layer. For halogen adsorbed structure it decreases with increase in size of phosphorene layer. Optical absorption is observed to depend on both dielectric constant and refractive index. Since adsorption of alkali and halogen materials modifies the refractive index of phosphorene, absorption is seen to reduce in all cases where refractive index increases due to adsorption even when the dielectric constant was high.     

Migration and localization of metal atoms on strained graphene

Reconstructed point defects in graphene are created by electron irradiation and annealing. By applying electron microscopy and density functional theory, it is shown that the strain field around these defects reaches far into the unperturbed hexagonal network and that metal atoms have a high affinity to the nonperfect and strained regions of graphene. Metal atoms are attracted by reconstructed defects and bonded with energies of about 2?eV. The increased reactivity of the distorted π-electron system in strained graphene allows us to attach metal atoms and to tailor the properties of graphene.     

Observation of negative alkali ions from alkali halides during 248-nm laser irradiation

Below laser fluences where a plasma is formed (the so-called plasma or plume formation threshold) a number of fundamental phenomena can occur where particles such as atomic and molecular ions, atoms and molecular neutrals, and electrons can be emitted. An understanding of such processes is necessary to develop predictive models for material removal from laser irradiated surfaces—at the foundation of laser etching, machining, and pulsed laser deposition. We have reported on a number of the mechanisms for such emission processes. Here, due to space limitations, we present a summary of our studies on the formation of negative alkali ions from single crystal KCl during exposure to pulsed 248-nm radiation at fluences well below the threshold for plasma formation. Despite the high electron affinities of the corresponding halogen atoms, negative halogen ions were not detected. Significantly, the positive and negative alkali ion distributions overlap strongly in time and space, consistent with K formation by the sequential attachment of two electrons to K⁺. Negative alkali ions are also observed under comparable conditions from LiF, NaCl, and KBr. In each material, the strong overlap between the positive and negative alkali ion distributions, and the lack of detected negative halogen ions, suggest that negative ion formation involves a similar mechanism.     

Electronic structure of the Au-intercalated graphene/Ni(111) surface

We report the electronic structure of the Au-intercalated graphene/Ni(111) surface using angle-resolved photoemission spectroscopy and low energy electron diffraction. The graphene/Ni(111) shows no Dirac cone near the Fermi level and a relatively broad C 1s core level spectrum probably due to the broken sublattice symmetry in the graphene on the Ni(111) substrate. When Au atoms are intercalated between them, the characteristic Dirac cone is completely recovered near the Fermi level and the C 1s spectrum becomes sharper with the appearance of a 10?×?10 superstructure. The fully Au-intercalated graphene/Ni(111) surface shows a p-type character with a hole pocket of ～0.034?Å^?1 diameter at the Fermi level. When the surface is doped with Na and K, a clear energy gap of ～0.4?eV is visible irrespective of alkali metal.     

Graphene-nanotube structures: Constitution and formation energy

A new class of carbon nanostructures is considered: covalently (or molecularly) bound graphene and carbon nanotube fragments. As found using semiempirical computations and molecular dynamics methods, the structures formed by bonding nanotube edge atoms to a planar segment of a graphene nanofragment or its edge atoms to nanotube atoms arranged at the element of cylinder and graphene nanoribbons attached to a nanotube by van der Waals forces along the nanotube are the most energetically stable systems.     

Sputtering of molecules during low-energy electron bombardment of alkali halides

The emission of molecules during bombardment of several alkali halides with a 540 eV electron beam has been investigated. Using a time of flight method the energy distributions of the halogen molecules have been measured at various temperatures of the target. The relative halogen molecule to atom ratio has also been examined as a function of the target temperature. It has been found that the molecules were formed from the atoms at the alkali halide surface. This process and the subsequent desorption of the molecules account for the experimental results.     

碱金属原子多极极化率的解析计算及其应用

通过考虑碱金属原子中的价电子在模型势中的运动, 给出了碱金属原子激发态(包括分立谱和连续谱)的波函数, 导出了碱金属原子的多极动态极化率的计算式中所涉及的矩阵元的解析表达式, 实现了碱金属原子多极动态极化率的解析计算. 作为应用, 利用范德瓦尔斯相互作用系数与动态极化率之间的积分关系, 计算了异核基态碱金属原子间的三体相互作用系数, 将计算结果与此前用稳定变分方法得到的结果进行了比较, 结果显示两者是一致的. 此工作为后续研究激发态碱金属原子间的相互作用奠定了基础.     

二维硼碳基纳米结构上的吸附及其性质

综述金属原子与非金属原子和分子在石墨烯、BC₃平面等二维硼碳基纳米结构上的吸附所表现出的各种物理性质及可能的应用.纯净的石墨烯为零带隙的半金属、无磁且自旋轨道耦合效应非常弱,BC₃平面为间接带隙半导体,但金属原子与非金属原子和分子的吸附可能使石墨烯体系在Dirac点处打开带隙、具有强自旋轨道耦合效应,可能使石墨烯体系与二维BC₃体系具有磁有序、超导电性及应用在氢存储上.另外石墨烯表现出非常好的分子探测性能.     

碱金属修饰的多孔石墨烯的储氢性能

基于第一性原理深入研究了碱金属原子(Li,Na,K)修饰的多孔石墨烯(PG)体系的储氢性能,并且通过从头算分子动力学模拟了温度对Li-PG吸附的H2分子稳定性的影响.研究结果表明,PG结构的碳环中心是碱金属原子最稳定的吸附位置,PG单胞最多可以吸附4个碱金属原子,Li原子被束缚最强,金属原子间无团聚的倾向;H2分子通过极化机制吸附在碱金属修饰的PG结构上,每个金属原子周围最多可以稳定地吸附3个H2分子;Li-PG对H2分子的吸附最强(平均吸附能为-0.246 eV/H2),Na-PG对H2分子的吸附较弱(平均吸附能为-0.129 eV/H2),K-PG对H2分子的吸附最弱(平均吸附能为-0.056 eV/H2),不适合用做储氢材料;在不考虑外界压强且温度为300 K的情况下,Li-PG结构可稳定地吸附9个H2分子,储氢量为9.25 wt.%;在400 K时,有7个吸附H2分子脱离Li-PG的束缚,在600-700 K的范围内,吸附H2分子全部脱离了Li-PG体系的束缚.     

Molecular adsorption of NO on free-standing and on graphene-supported Mo<Subscript>3</Subscript>W<Subscript>5</Subscript> cluster: a density functional theory investigation

The adsorption of molecular NO on the free-standing and graphene-supported Mo₃W₅ cluster is studied using methods from the gradient-corrected density functional theory. Before, the effect of the graphene support on the properties of the metal cluster was investigated. The interaction between the metal cluster and the graphene sheet takes place mainly through W atoms, which form up to three bonds with the support. Interaction energies are in the range from 0.6 to 1.5 eV. An amount of charge of about 0.4–0.5 e\(^-\) is transferred from the cluster to the support. Geometric distortions in the metal aggregate are negligible. An important decrease in the magnetic moment of Mo₃W₅ with respect to its free-standing value is observed after the interaction with the support. Molecular NO adsorbs on sites involving W atoms only, both for the free-standing and the supported metal cluster. Adsorption energies are in a range from 2 to 4 eV. A parallel mode is the preferred mode from an energetic point of view. Moreover, for that parallel adsorption mode, the N–O bond is more effectively activated. Magnetic moments change largely after adsorption indicating important rearrangement in the electronic configuration of the metal cluster. An important amount of electronic charge is transferred both from the free-standing and from the supported metal cluster to NO. The amount of charge transferred seems to be closely related to the activation of the N–O bond. The effect of the graphene sheet on the catalytic properties of Mo₃W₅ seems to be negligible, with the exception of some changes in the electronic configuration of the cluster.     

双层h-BN/Graphene结构稳定性及其掺杂特性的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法研究了双层h-BN/Graphene的稳定性及其掺杂特性.研究发现,双层h-BN/Graphene能带结构在K点处有一个小的带隙,在费米能处有类Graphene的线性色散关系.通过施加应变和掺杂来调节带隙,发现掺杂后费米能级附近引入的新能级,主要是N原子的贡献,掺杂后的Na原子和N,C之间存在电荷转移,材料转变为金属性.电荷的转移、载流子密度的增加,在电子元器件中有重要的应用前景.     

Comparisons between adsorption and diffusion of alkali,alkaline earth metal atoms on silicene and those on silicane:Insight from first-principles calculations

The adsorption and diffusion behaviors of alkali and alkaline-earth metal atoms on silicane and silicene are both investigated by using a first-principles method within the frame of density functional theory.Silicane is staler against the metal adatoms than silicene.Hydrogenation makes the adsorption energies of various metal atoms considered in our calculations on silicane significantly lower than those on silicene.Similar diffusion energy barriers of alkali metal atoms on silicane and silicene could be observed.However,the diffusion energy barriers of alkali-earth metal atoms on silicane are essentially lower than those on silicene due to the small structural distortion and weak interaction between metal atoms and silicane substrate.Combining the adsorption energy with the diffusion energy barriers,it is found that the clustering would occur when depositing metal atoms on perfect hydrogenated silicene with relative high coverage.In order to avoid forming a metal cluster,we need to remove the hydrogen atoms from the silicane substrate to achieve the defective silicane.Our results are helpful for understanding the interaction between metal atoms and silicene-based two-dimensional materials.     

Structural,electronic,and magnetic behaviors of 5d transition metal atom substituted divacancy graphene:A first-principles study

Structural, electronic, and magnetic behaviors of 5d transition metal(TM) atom substituted divacancy(DV) graphene are investigated using first-principles calculations. Different 5d TM atoms(Hf, Ta, W, Re, Os, Ir, and Pt) are embedded in graphene, these impurity atoms replace 2 carbon atoms in the graphene sheet. It is revealed that the charge transfer occurs from 5d TM atoms to the graphene layer. Hf, Ta, and W substituted graphene structures exhibit a finite band gap at high symmetric K-point in their spin up and spin down channels with 0.783 μB, 1.65 μB, and 1.78 μB magnetic moments,respectively. Ir and Pt substituted graphene structures display indirect band gap semiconductor behavior. Interestingly, Os substituted graphene shows direct band gap semiconductor behavior having a band gap of approximately 0.4 e V in their spin up channel with 1.5 μB magnetic moment. Through density of states(DOS) analysis, we can predict that d orbitals of 5d TM atoms could be responsible for introducing ferromagnetism in the graphene layer. We believe that our obtained results provide a new route for potential applications of dilute magnetic semiconductors and half-metals in spintronic devices by employing 5d transition metal atom-doped graphene complexes.