Li Adatoms Research Articles

Hydrogen adsorption on Li metal in boron-substituted graphene: An ab initio approach

The characteristics of hydrogen adsorption on Li metal atoms dispersed on graphene with boron substitution is investigated including Li clustering, hydrogen bonding characteristics, and the open metal states of Li adatom using density functional theory calculations. It is found that Li atoms are well dispersed on boron-substituted graphene and can form the (2 × 2) pattern because clustering of Li atoms is hindered by the repulsive Coulomb interaction between Li atoms. One Li adatom dispersed on the double side of graphene can absorb up to 8 hydrogen molecules corresponding to a 13.2% hydrogen storage capacity. In addition, the adsorption behaviors of non-hydrogen atoms such as C and B are calculated to determine whether Li atoms can remain as the open metal state in boron-substituted graphene.

Noble and alkali adatoms on a –Ag surface: a first-principles study

Using first-principles calculations, we present a comprehensive study on the atomicand electronic structures of metal adatoms (noble metals Ag, Au, Cu and alkalimetals Li, Na, K) adsorbed on a –Ag (hereafter -Ag) surface. We found that adsorption of noble and alkali adatoms can inducesignificant structural changes in the topmost Ag layer. The most striking andinteresting results are the immersion of the noble and Li adatoms into the substrateAg layer and the finding of the most stable configurations with three adatomsincorporating into or being adsorbed on the surface dependent on their atomic radii.We also found that the almost empty two-dimensional free-electron-like bands1 and its band foldings1* of the originalsurface band s1 of the -Ag surface split into a gap at the surface Brillouin zone (SBZ) boundary withadsorption of an adatom. The two surface bands gradually move downwards and thes1 band is gradually filled with an increase of coverage. Thes1 band is fully occupied with the largest band gap ∼ 0.25 eV betweenthe s1 and s1* bands at the critical coverage of 0.14 monolayers (ML) [three adatoms in a –Ag (hereafter -Ag) unit cell], which corresponds to the most stable adsorption phase. Although theadsorption configurations are different, both the noble and alkali adatom adsorptions giverise to similar electronic structures at low coverages, indicating a free-electron-likecharacter of the adsorption surfaces.

Lithium adsorption on zigzag graphene nanoribbons

We have studied the adsorption of Li atoms at the hollow sites of graphene nanoribbons (zigzag and armchair), graphene, and fullerenes by means of density functional theory calculations including local and semilocal functionals. The binding energy of a Li atom on armchair nanoribbons [of about 1.70 eV for local spin density approximation (LSDA) and 1.20 eV for Perdew–Burke–Ernzerhof (PBE)] is comparable to the corresponding value in graphene (1.55 and 1.04 eV for LSDA and PBE, respectively). Notably, the interaction between Li and zigzag nanoribbons is much stronger. The binding energy of Li at the edges of zigzag nanoribbons is about 50% stronger than in graphene for the functionals studied here. While the charge transfer between the Li adatom and the zigzag nanoribbon significantly affects the magnetic properties of the latter providing an additional interaction mechanism that is not present in a two-dimensional graphene or armchair nanoribbons, we find that the morphology of the edges, rather than magnetism, is responsible for the enhanced Li-nanoribbon interaction.

Lithium adsorption on MgO(100) and its defects: Charge transfer, structure, and energetics

The adsorption energetics and growth of lithium vapor on MgO(100) at 300 K was studied using microcalorimetry, in combination with low-energy electron diffraction (LEED), low-energy ion scattering (ISS), Auger electron spectroscopy (AES), and work-function measurements. The MgO(100) samples were films of $\ensuremath{\sim}4\text{ }\text{nm}$ thickness grown on a Mo(100) single crystal. The initial sticking probability of lithium was $\ensuremath{\sim}0.97$, reaching unity by 0.5 monolayer (ML). The AES and ISS signals vary with Li coverage up to 3 ML as expected if the Li atoms stay within the layer where they initially hit (i.e., with no interlayer transport). Initially, lithium adsorbs strongly at the intrinsic surface defects and as two-dimensional (2D) lithium clusters, with a heat of adsorption of 260 kJ/mol. The heat approaches the heat of sublimation of bulk Li (159 kJ/mol) by 0.4 ML, due to the growth of 2D and then three-dimensional (3D) Li islands. Argon-ion sputtering of the surface increases the defect density and the probability for adsorbing Li to find a defect, and thus the heat of adsorption at low coverages. When defects only are being populated, Li exhibits a heat of adsorption of 410 kJ/mol. Comparing heats with recent density functional theory (DFT) calculations suggests that the defect sites are under-coordinated O atoms at steps or kinks, or related structures at dislocations. The work function decreases by $\ensuremath{\sim}1.8\text{ }\text{eV}$ within the first 0.5 ML and then increases to near the value of bulk Li(solid), $\ensuremath{\sim}2.6\text{ }\text{eV}$, by 3 ML. These results support recent DFT calculations predicting stronger electron transfer from Li to the MgO when at steps and kinks than at terraces, and decreasing charge transfer as 2D Li clusters grow. The work function starts to increase when the growth mode becomes dominated by growth of 3D Li(solid). In spite of a large amount of electron transfer from Li to MgO, Li adatoms have attractive interactions that lead to 2D clustering. For 1-nm-thick MgO films, the heat of adsorption was higher by 60--20 kJ/mol than for 4 nm films in the entire range from 0 to 0.7 ML, where adsorption in the first layer dominates.

Defect Sites and Their Distributions on MgO(100) by Li and Ca Adsorption Calorimetry

Chemical bonding to oxide surfaces is often dominated by surface defects, but their nature remains elusive. Calorimetric measurements of Ca and Li adsorption energies on MgO(100) and ion-damaged MgO(100), when combined with density functional theory (DFT) calculations and kinetic modeling, are shown to be a powerful way to assess the nature of the defect sites on oxide surfaces and their lateral distributions. While ion sputtering causes a strong increase in the initial adsorption energy for Li on MgO(100) at 300 K, the initial adsorption energy for Ca is independent of the extent of sputtering. This result and the measured coverage dependence of the adsorption energies of Ca and Li on MgO(100) surfaces with approximately 5, 12, and 25% defects were simulated with a kinetic model based on DFT input regarding site binding energies and adatom migration barriers. Reproducing the experimental results required models with distinct probability distributions of local defect concentrations for the differing extents of ion damage. A key difference between Li and Ca revealed by DFT and necessary to reproduce their differing adsorption energy versus ion damage measurements is the much greater tendency for a diffusing Li adatom to remain locked in place once it reaches a terrace site neighboring an occupied step or kink site, thus nucleating a 2D island on a terrace. In contrast, Ca adatoms thermally diffuse from such sites quickly, to seek out the remaining defect sites. The model also reproduces the measured Li and Ca film morphology seen by ion scattering spectroscopy.

Substrate-mediated interaction and electron-induced diffusion of single lithium atoms on Ag(001)

The adsorption properties and the mechanism of induced diffusion of single Li atoms on Ag(001) have been investigated at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by means of the low-temperature scanning-tunneling microscopy. On the basis of Li-Li pair distances, we have observed that there is no preferential adsorption site on Ag(001). Li adatoms form pairs with one preferred adatom distance, most likely due to long-range interaction between Li adatoms mediated by electrons from the bulk. The manipulation experiments show that diffusion of single Li adatoms can be induced upon electron transfer into an unoccupied electronic state connected to a single Li adatom. The existence of such Li-induced state is also confirmed by spectroscopic measurements.

Lithium-induced dimer reconstructions on Si(001) studied by photoelectron spectroscopy and band-structure calculations

The electronic and atomic structure of Si(001) with 0.5 and 1 ML of lithium have been studied experimentally using angle resolved ultraviolet photoelectron spectroscopy, Si $2p$ core level spectroscopy, and low energy electron diffraction. Experimental surface state dispersions are compared with recent theoretical results in the literature and with results from additional density functional theory calculations. Four adsorption configurations for the 0.5 ML $2\ifmmode\times\else\texttimes\fi{}2$ surface and three configurations for the 1 ML $2\ifmmode\times\else\texttimes\fi{}1$ surface are compared. Fittings of Si $2p$ core level data support the alternation of strongly and weakly buckled Si dimers of the $2\ifmmode\times\else\texttimes\fi{}2$ models and symmetric Si dimers of the $2\ifmmode\times\else\texttimes\fi{}1$ models based on the relative intensities of the surface components. As a tool to differentiate between the different $2\ifmmode\times\else\texttimes\fi{}2$ and $2\ifmmode\times\else\texttimes\fi{}1$ models surface state dispersions are used since they are sensitive to the positions of the Li adatoms.

Adsorption and diffusion of an alkali-metal adatom on transition-metal dichalcogenides

Employing total-energy density-functional theory calculations the adsorption energies, the diffusion pathways, and the barriers for a Li adatom on stepped ${\mathrm{TiSe}}_{2}$ surfaces with full relaxation of the atomic environment were studied. The minimum-energy paths for Li diffusion on a ${\mathrm{TiSe}}_{2}$ (0001) surface and across the $(10\overline{1}0)$ and $(\overline{1}2\overline{1}1)$ steps have been calculated. At the $(\overline{1}2\overline{1}1)$ step edge an Ehrlich-Schwoebel barrier of $0.31\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was obtained, which is additional to the diffusion barrier of $0.25\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ on the flat surface. The energies for subsequent diffusion within the van der Waals gap have been calculated, too.

Atomic and electronic structure of the Si(0 0 1)2 × 1–Li chemisorption system at 1.0 monolayer coverage

Ab initio plane-wave pseudopotential density functional theory (DFT) calculations have been performed to determine the atomic and electronic structure of the Si(0 0 1)2 × 1–Li adsorption system at 1.0 monolayer (ML) coverage. Chemisorption of the lithium atoms is found to result in a minimum energy configuration characterized by symmetric Si dimers in agreement with the results of high-resolution core-level photoelectron spectroscopy. The transition from the asymmetric Si dimers of the clean Si(0 0 1) surface to the symmetric dimers of the Li chemisorbed surface is due to charge transfer from the Li adatoms to the substrate. The dispersion of the occupied electronic surface state bands is found to be in good agreement with the angle-resolved photoemission data. The nature of the lowest energy unoccupied surface state band suggests that silicide formation may occur at coverages greater than 1.0 ML.

Atomic structure and theoretical scanning tunneling microscopy images of Li zigzag chains on the GaAs(110) surface

We have investigated the Li/GaAs(110) surface by combining the low-energy electron diffraction (LEED) and the ab initio pseudopotential calculations. The potential energy surface for the single atomic adsorption shows the existence of two inequivalent adsorption sites. At 0.5 monolayer coverage of Li, a well defined $1\ifmmode\times\else\texttimes\fi{}2$ LEED pattern is observed. The atomic structure at this coverage is characterized by Li zigzag chains running in the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction. This structure is stabilized by electron transfer from the Li adatoms to the dangling bonds of certain surface Ga atoms. The simulated scanning tunneling microscopy images show linear (asymmetric zigzag) chains along the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction for positive (negative) sample biases.

Cluster expansion technique: An efficient tool to search for ground-state configurations of adatoms on plane surfaces

The cluster expansion technique is applied to describe the energetics of adatom configurations on plane substrate surfaces at submonolayer coverage and to find the stable configuration in thermodynamic equilibrium. The power of the method is demonstrated for the case of Li adatoms on Mo(112).

First-principles calculation of Li adatom structures on the Mo(112) surface

The formation and structure of the ordered phases of Li atoms adsorbed on the Mo(112) surface are investigated by performing first-principles density-functional calculations. Large inward relaxation for the first atomic interlayer distance of the clean Mo(112) surface is found. The vertical relaxations of the substrate are only little influenced by the adsorbed Li atoms. The magnitude of lateral shifts of atoms along the atomic rows is found to be small and in line with measured values. The energetics of chain structures of Li adatoms for coverages $0.125<~\ensuremath{\Theta}<~1$ monolayer is determined and the $p(4\ifmmode\times\else\texttimes\fi{}1)$ and $p(2\ifmmode\times\else\texttimes\fi{}1)$ adatom structures are found to be the most favored ones in agreement with experiment. The binding energy of Li atoms decreases with increasing coverage. The calculated work function changes with Li adatom coverage are in good agreement with experiment. The energy barriers for adatom diffusion are determined and the energetics of lateral interactions between adatoms is discussed.

Diffusion and intercalation of alkali atoms in transition metal dichalcogenides

Alkali metal atoms deposited on transition metal dichalcogenides (TMDCs) can either diffuse on the surface or intercalate into the van der Waals gaps of the substrate. In order to understand the interplay between adsorption and intercalation of alkali atoms at these layered crystals, we performed a detailed study of the migration and energetics of Li adatom on TiSe2(0001) surface, employing total-energy density-functional theory calculations. To get insight into stable and metastable states, diffusion paths and diffusion barriers for the Li adatom, we calculated the adatom's potential energy surface, to find e.g. the hcp site energetically slightly preferred over the fcc site. Results are also given for other alkali atoms. To obtain information about the adsorbate–substrate bond, the change in the valence electron density and orbital distribution was studied. The paths of direct intercalation for the alkali atom from the surface through the three layered TiSe2 sandwich into the van der Waals gap were investigated. It was found that the alkali atom has to overcome an energy barrier of at least 3.81 eV to make the direct intercalation possible.

An investigation of adatom bonding at metals by the electron localization function

In this article, within an ab initio density functional framework, we study the charge rearrangement in chemisorption of an isolated atom on a simple metal surface described by jellium. We focus our attention on the electron localization function (ELF) and illustrate how this function can complement the total charge density contour plots for the understanding of chemisorption. The ELF of Li, Cl, Si, and Fe adatoms are discussed. In all such cases the ELF shows itself as a wealthier, more immediate and visually informative pattern of the charge rearrangement. By varying the atom–surface distance of Fe, the formation of the chemisorption bond and the role played by the majority and the minority spin populations are discussed.

Adsorption of Li on Ni(110) surfaces at low and room temperature

We present the investigation of Li adsorption on Ni(110) surfaces at low temperature (LT) and room temperature (RT) by means of Auger electron spectroscopy, low-energy electron diffraction, thermal desorption spectroscopy and work function measurements, in ultrahigh vacuum. At RT, from the early stages of deposition ( Θ≤0.1 ML) Li induces a (1×2) reconstruction of the Ni(110) surface. The (1×2) reconstruction is lifted at 0.5 ML. A further increase of Li coverage leads to a disordered surface, with an intermixing of Li adatoms and the Ni substrate with a tendency toward Li–Ni alloy formation. A gradual heating of the Li-covered Ni(110) surface gives an Li(3×1), an Li(5×1) structure and the (1×2) reconstruction before the complete desorption of Li. Near saturation ( Θ≈2 ML), Li forms a nearly metallic overlayer. At LT (150 K), deposition of Li up to 1 ML gives a sequence of c(2×2), (7×2), (3×2), (4×2), (5×4) and finally (1×1) structures, which have been determined using structure factor calculations. These structures are attributed to a continuous stretching of deposited Li atoms along the [11̄0] direction of the Ni(110) surface. The Li–Ni interaction is a thermally activated process and decreases the stability of the Ni surface, leading to its reconstruction at RT. However, at LT the Li–Li interaction dominates and the effect of Li adatoms on the substrate structure is limited. The change of the (1×2) reconstruction to the ideal (1×1) bulk-like surface occurs at the beginning of the transition of ionic to metallic bonding of the Li overlayer.

Surface diffusion measurements from digitized FEM images: analysis of local brightness fluctuations

An extension of the field-emission current fluctuation method to study surface diffusion on the surface of a field emitter tip is proposed. The present approach is based on an analysis of the local brightness fluctuations extracted from digitized FEM video-images instead of the conventional probe-hole technique. “Virtual probe-holes” which correspond to probed surface regions of nanometer size can be placed in chosen areas of the image and the integrated intensity in these windows can be monitored with the time resolution of video frames (40 ms in our case). For the Li/Pt[1 0 0] tip system, the autocorrelation and crosscorrelation analyses of the local FEM brightness fluctuations have been performed. From a comparison of the experimentally obtained and theoretically calculated autocorrelation functions and from the crosscorrelation functions we determine the surface diffusion parameters for Li adatoms on the apex plane of the [1 0 0]-oriented Pt field emitter tip.

COMPLETION OF THE DETERMINATION OF COMPLEX SURFACE STRUCTURES FORMED ON LOW-INDEX PLANES OF COPPER SINGLE CRYSTAL BY Li DEPOSITION: Cu(110)-(4×1)-3Li

With increasing Li coverage on Cu(110), (n×1)(4≤n≤8) structures follow the (1×2) missing-row structure at room temperature. We determine the (4×1) structure by a dynamical low-energy electron diffraction (LEED) analysis with symmetrized automated tensor LEED. This completes the solution of the complex surface structures formed on low-index planes of copper single crystal by Li deposition. Every fourth Cu row in the [001] direction is substituted by Li atoms and the remaining Cu rows are covered by Li adatoms. Features of this structure, denoted as Cu (110)-(4×1)-3 Li , are very similar to those of the previously determined Cu (001)-(3×3)-5 Li , Cu (001)-(4×4)-10 Li and Cu (111)-(2×2)-3 Li structures.

STM observation of restructured Cu(001) surfaces induced by Li deposition

We have studied the (2 × 1), (3 × 3) and (4 × 4) structures formed on Cu(001) with increasing Li coverage at 300 K using scanning tunneling microscope (STM). The STM images of the three phases support the previously determined structures by low-energy electron diffraction analysis. Details of the STM images of the (4 × 4) structure are discussed on the basis of the determined structure. A cluster of four Li adatoms on a small island of nine Cu atoms is observed as a single protrusion and the protrusions form the (4 × 4) lattice. We found migration of vacancies and domain boundaries. On the whole, however, the (4 × 4) structure is stable at 300 K on terraces.

Atomic and electronic structure of Li-adsorbed Si(100) surfaces

We investigate the atomic and electronic structure of Li-adsorbed Si(100) surfaces through first-principles pseudopotential calculations. We find that Li adatoms interact mainly with the dangling-bond orbitals of Si dimers, however, no hydrogenlike directional bonds exist between the Li and substrate Si atoms, based on calculated scanning tunneling microscopy images. The analysis of charge densities demonstrates a large charge transfer from the Li adatom to a dangling-bond orbital of a Si dimer, which is responsible for a large decrease of work function at submonolayer coverages. The Li atoms form linear chains either by occupying the interdimer bridge sites along the dimer row direction or the dimer bridge and cave sites perpendicular to the dimer row. As Li coverage (\ensuremath{\Theta}) increases, repulsive interactions between the adatoms in the chains are also found to increase. Thus, at 1-ML Li coverage, the stable adsorptions sites are changed to either the cave and pedestal sites or the valley-bridge and pedestal sites, with all the Si dimers symmetric, resulting in a $2\ifmmode\times\else\texttimes\fi{}1$ structure. From the calculated formation energies, we find a $1\ifmmode\times\else\texttimes\fi{}1$ phase to be thermodynamically stable at $\ensuremath{\Theta}=2,$ in contrast to other alkali-metal adsorptions.

Electronic properties of the single-domain Li/Si(001) surface

We report surface electronic band structures of the single-domain Li-adsorbed Si(001) surface investigated by using synchrotron uv radiation. The spectral changes observed for the first two Li-induced ordered structures, a (2 x 2)-Li and a (2 x 1)-Li, suggest that these surfaces consist of asymmetric dimers as the basic building ingredients for the structures and that Li adatoms locate primarily at or near the up-Si atoms of the asymmetric dimers. Results of the line-shape analysis of Si 2p core level shifts as a function of the Li coverage support the asymmetric dimer models for the first two ordered structures and account for the rigid shifts in the surface bands.