Structural Bistability in RbI Monolayers on Ag(111)

Abstract

Alkali halides are well-known for their tendency to form rock-salt-like crystal structures. Here we present a scanning tunneling microscopy study of a previously unreported alternative structure of one such alkali halide, RbI. When deposited on Ag(111) at a low submonolayer surface coverage, RbI forms islands with hexagonally coordinated atomic structures, in contrast to the expected rock-salt structures typically observed for such alkali halide films on metal surfaces. At a near-monolayer RbI surface coverage, we observe the coexistence of the hexagonally coordinated phase and a square-coordinated rock-salt-like RbI phase that is analogous to that observed for other alkali halides. Our density functional theory calculations for this system highlight the role of RbI−Ag interfacial charge transfer in defining the RbI structure and the impact of local atomic coordination on the RbI−Ag charge-transfer interaction.

Modulation of Carbon Nanotube Electronic Structure by Grain Boundary Defects in RbI on Au(111)

“…Our results show localized states analogous to the image potential states observed in the experiment, where the energies are slightly lower due to the well-known tendency of DFT to underestimate band gaps. These states are mostly centered on Rb atoms, which is in line with previous results on the atomic contributions for states in this energy range for RbI . Despite being mostly centered on Rb atoms, the effect of the stabilizing potential at iodine atoms is clearly important, with the Rb atoms closest to (furthest from) the most (least) stabilizing potentials showing the lowest (highest) energy peaks.…”

“…The strength of this interaction can be gauged by the net electron count of adatoms, which, for example, for iodine atoms in the pristine RbI structure ranges from +0.20 (for the most Au-coordinated) to +0.61 (for the least Au-coordinated iodine atoms). This suggests that the high Au–I coordination facilitates electron transfer from the adlayer to the substrate, in line with previous results for RbI on Ag(111) . The net electron counts of iodine atoms in the GBD, which fall in the range of +0.20 to +0.14 electron, are lower than for iodine atoms in the pristine RbI structure, on average (+0.45 electron).…”

“…While the GBD atomic structure is not clear from the STM topography, the latter does allow us to constrain the types of possible GBD structures. Specifically, STM topography shows that RbI monolayers have a hexagonal structure rather than the typical square alkali–halide structure observed for NaCl, a consequence of the weaker alkali–halide interaction, analogous to previous findings for RbI on Ag(111) . Furthermore, an analysis of STM topography of the GBD from Figure shows that the GBD arises at the interface of two RbI monolayers displaced with respect to each other by one Au atomic spacing along the ⟨100⟩ direction parallel to the GBD (see Figure a and Supporting Information Figures S4 and S5a).…”

“…Visualizations of all structures were made using the VESTA package . Simulations of STS were performed with the Wentzel–Kramers–Brillouin approximation, using methods described elsewhere …”

“…An interesting alternative to NaCl is RbI, which has been shown to induce less energy broadening in STS measurements, allowing the observation of finer electronic structural details in naphthalocyanine and oligothiophene molecules. More importantly, unlike NaCl, which grows in a rock-salt-like structure with a low density of defects on a variety of metal surfaces, RbI exhibits a higher density of defects and significantly more variability in its structure on Au(111) and Ag(111), a consequence of a stronger RbI–metal interaction enabled by a weaker Rb–I bond strength. , …”