Charge redistribution in the formation of one-dimensional lithium wires on Cu(001)

Abstract

We describe the formation of one-dimensional lithium wires on a Cu͑001͒ substrate, providing an atomicscale description of the onset of metallization in this prototypical adsorption system. A combination of helium atom scattering and density-functional theory reveals pronounced changes in the electronic charge distribution on the formation of the c͑5 ͱ 2 ϫ ͱ 2͒R45°Li/Cu͑001͒ structure, as in-plane bonds are created. Charge donation from Li-substrate bonds is found to facilitate the formation of stable, bonded, and depolarized chains of Li adatoms that coexist with an interleaved phase of independent adatoms. The resultant overlayer has a commensurate charge distribution and lattice modulations but differs fundamentally from structurally similar charge-density wave systems. Self-organized, nanometric wires are ideal prototypes for exploring the electronic changes that occur during the process of bond formation at a surface. Epitaxial deposition onto intrinsically anisotropic substrates-such as ͑110͒ oriented, 1 stepped, 2-6 or reconstructed 7 single crystals-has been particularly useful in this regard. The approach can yield long, robust wires that are easier to study than freestanding structures.8 It is also of interest for the study of lowdimensional phenomena, including charge-density waves ͑CDWs͒, 9,10 which have been identified in a variety of lowdimensional p-block 11 and d-block 3,9 epitaxial systems. In contrast, one-dimensional ͑1D͒ s-block systems have not previously been identified but could offer nonlocal, freeelectronlike properties that would contrast with the local nature of d-band systems. In the case of alkali metals on lowindex surfaces, a difficulty in forming 1D surface structures is the nature of interadsorbate interactions.12,13 Heavy alkali metal adatoms are highly polarized and tend toward isotropic, hex-type overlayers. As we demonstrate in the present work, light alkali metals behave differently, a fact that can be attributed to stronger and more localized adatom-substrate interactions. The resulting atomic chains of Li have a 1D structural character and they offer a unique opportunity for the observation of the charge redistribution as metallic bonds form in the surface plane. We use a combination of helium atom scattering ͑HAS͒ and density-functional theory ͑DFT͒ to probe directly the electronic structure of these anisotropic Li films, which we show to be characterized by inhomogeneous electronic-charge distributions and the coexistence of two distinct Li adatom states.Previous electron 14,15 and atom 16 diffraction studies have shown that a series of well-ordered submonolayer Li adatom structures forms on Cu͑001͒ at low temperature. Upon deposition, Li adatoms condense from a dilute lattice gas into an ordered c͑2 ϫ 2͒ structure at 0.5 monolayers ͑ML͒ with adatoms occupying fourfold hollow sites, 17,18 as sketched in Fig. 1͑a͒. Adatoms in the c͑2 ϫ 2͒ structure have a large separation and Ne scattering indicates that they act as independent Einstein oscillators: 16,19 both factors po...

Energetic Ground State Calculations, Electronic Band Structure at Surfaces

“…For example, the dipole-dipole repulsion of adsorbed Na atom pairs on Cu(001) is modified from a pure R −3 law (R being the Na-Na distance) because of the reduction of the dipole moment as they come closer: neglecting this effect overestimates the repulsion energy by 20 meV when Na atoms are two Cu unit cells apart [153]. Reduction in the adsorption dipole induced by local density increases contribute to the stability of quasi-1D structures observed and computed for Li/Cu(001) [154,155] and Na/Cu(001) [156,157] as saturation coverage is approached. Atom scattering experiments demonstrate a 1D corrugation of the probe-surface interaction that, linked to the surface electron density through the effective medium theory, is actually due to larger extension of the density towards vacuum, as the workfunction is locally reduced because of the increased density of dipoles.…”

The surface phase diagram of Li/Cu(001) explored by helium atom scattering

“…Thus, they can be considered as strained, high density domain boundaries separating narrow stripes of c(2 × 2) surface. The adatoms still all occupy four-fold hollow sites, but their proximity within the dislocation lines can cause a slight in-plane, repulsive displacement [8,27,29].…”

“…Indeed, at a coverage of 0.8 ML the surface Li density, in atoms per unit area, exceeds that of the (110) planes of bulk Li and the Li monolayer would therefore be strained. From a spacefilling perspective, and given that the ladder structures are already accommodated by a slight lateral buckling [8,27,29], it is likely that regime IV spans the onset of incommensurate growth. An incommensurate structure would allow the interadatom distances to equilibriate and therefore remove the type of heterogeneous adatom distribution that gave rise to strong diffraction in the case of the ladder structures.…”

“…A general picture of the adsorption process has been derived by measurement of the work function, the adsorbate core-level shifts and the local structure, etc, through Kelvin probe, x-ray photoemission and low energy electron diffraction (LEED) studies, respectively [2]. Helium atom scattering (HAS) has also examined dynamical aspects [5,6] and the thorny problem of metallization in alkali metal overlayers [7,8]. Together, this work has revealed a surprisingly rich variation of electronic and structural properties as a function of alkali metal coverage and down the alkali metal group.…”

“…Above 0.5 ML, the Li overlayer is observed to undergo further condensation into several more complex, higher-order commensurate structures, including c(7 [4]. We recently outlined DFT and HAS measurements that indicated the formation of in-plane, inter-Li bonding in these phases, arising from the introduction of dislocation lines in the c(2 × 2) starting structure, as coverage increases beyond 0.5 ML [8]. One aim of the present work is to provide full details of that HAS analysis, focusing on the derivation of an accurate Hesurface interaction potential for the ladder structures.…”