Probing the electronic structure of bistable single-layer RbI structures on Ag(111)

Strong interactions present in single-layer RbI grown on metal surfaces have been shown to result in the surprising coexistence of bistable RbI structures. However, it is not well known how these structural differences affect the electronic behavior of the RbI film. Here, we use scanning tunneling microscopy/spectroscopy (STM/STS) to probe the image potential electronic states (IPSs) of the RbI/Ag(111) system, which are known to be highly sensitive to the electronic properties of ultrathin insulating films. By comparing our experimental STS measurements to first-principles numerical simulations of the IPSs, we calculate the work function, dielectric constant, and conduction band energy for the two distinct structure types of RbI on Ag(111). In addition, our results show that local variations in the IPS behavior, evident in both a moiré pattern and grain boundary defects, can be attributed to electrostatic potentials present in the RbI structure.

Figure 1

Representative STS of IPS on RbI/Ag(111). (a) STM topography ($V_{\mathrm{b}}=-50$ mV, $I$ = −200 pA) of a region of RbI showing coexistence of hexagonal (left side) and square (right side) RbI structures. (b) Representative STS spectra ($I$ = 10 pA, $V_{\mathrm{pp}}=40$ mV) recorded on each RbI structure type, where the feedback was left on during STS acquisition, in line with previous studies of IPS [29].

Figure 2

Experimental and simulated IPS energies. Comparison of simulated (red) to experimental (black) IPS energies. The horizontal bars are the standard error of experimental STS data obtained with five different tips summed in quadrature with the measurement's uncertainty resulting from the amplitude of bias oscillation in the STS measurement (40 mV peak to peak).

Figure 3

STS measurements of spatial modulation of IPS energy in square RbI. (a) STM topography ($V_{\mathrm{b}}=2$ V, $I$ = 10 pA) of a square RbI monolayer, indicating the path along which a progression of STS measurements were taken in (b). (b) Progression of STS measurements taken along the spatial path shown in (a). (c) Spatial variation in downshifted first IPS, showing STS data from the horizontal dashed line in (b). (d) Spatial variation in DFT-calculated net electron counts, averaged along the ⟨100⟩ RbI direction orthogonal to the STS path. (e) Individual STS spectra showing distinctions in energy of the lowest-energy IPS for different regions of the moiré supercell. The spectra correspond to the position of the vertical lines in (b). (f) Portion of the STS spectra from (e), zoomed in to highlight difference in the first IPS energy.

Figure 4

STS results for localized IPS at GBD in hexagonal RbI. (a) STM topography ($V_{\mathrm{b}}=50$ mV, $I$ = 10 pA) of GBD formed between two discontinuous domains of hexagonal RbI, where the pristine unit cell is shown in white and the discontinuity is highlighted by the double-sided arrows. (b) Representative STS measurements taken on (dashed curve) and away from (solid curve) the GBD shown in (a). The energy ranges used for two-dimensional STS mapping are overlaid in red (c) and blue (d). (c), (d) Two-dimensional STS mapping of the GBD shown in (a), where the mapped energies are chosen to show the spatial behavior of the downshifted IPS at the GBD (c) and the IPS of the pristine hexagonal monolayer (d). The horizontal dashed line shows the orientation of the GBD as in (a).

Figure 5

DFT-calculated potential for GBD in hexagonal RbI. (a) DFT-optimized structure of the GBD shown in Fig. 4. The solid horizontal arrows show the lattice vector of the pristine hexagonal RbI structure, the dashed arrow shows how the pristine lattice vector propagates across the GBD, and the double-sided arrow shows the discontinuity between the structures on either side of the GBD. The segmented solid arrow shows the path along which the change in potential is sliced in (b). (b) Change in potential energy of electrons due to adsorption of RbI $[\mathrm{\Delta }E\left(r\right)=E{\left(r\right)}_{\mathrm{Ag}/\mathrm{RbI}}-E{\left(r\right)}_{\mathrm{Ag}}-E{\left(r\right)}_{\mathrm{RbI}}]$, is shown in the direction orthogonal to the surface. (c) Vertical slices of the potential from (b) are shown for several iodine atoms with distinct adsorption geometry, as indicated by the vertical lines shown in (b).