Scanning tunneling microscopy and spectroscopy of NaCl overlayers on the stepped Cu(311) surface: Experimental and theoretical study

The physical properties of ultrathin NaCl overlayers on the stepped Cu(311) surface have been characterized using scanning tunneling microscopy (STM) and spectroscopy, and density-functional calculations. Simulations of STM images and differential conductance spectra were based on the Tersoff-Hamann approximation for tunneling with corrections for the modified tunneling barrier at larger voltages and calculated Kohn-Sham states. Characteristic features observed in the STM images can be directly related to calculated electronic and geometric properties of the overlayers. The measured apparent barrier heights for the mono-, bi-, and trilayers of NaCl and the corresponding adsorption-induced changes in the work function, as obtained from the distance dependence of the tunneling current, are well reproduced and explained by the calculated results. The measurements revealed a large reduction of the tunneling conductance in a large voltage range, resembling a band gap. However, the simulated spectrum showed that only the onset at positive sample voltages may be viewed as a valence-band edge, whereas the onset at negative voltages is caused by the drastic effect of the electric field from the tip on the tunneling barrier.

Figure 1

Experimental STM images and calculated LDOS images of a NaCl monolayer in the $p(3\times 2)$-II structure. (a) Calculated LDOS image at an average distance of 7.9 Å from the Cu(311) surface. The surface unit cell used in the calculation, see Fig. 4 (right), is indicated. (b) Experimental STM image with tunneling parameters 210 pA and −24 meV. In (a) and (b) the area shown is $15\times 15{\mathrm{\AA }}^2$.

Figure 2

Experimental STM images and calculated LDOS images of a NaCl monolayer in the $p(3\times 2)$-I structure. (a) Calculated LDOS image at an average distance from the Cu(311) surface of 7.9 Å. The surface unit cell used in the calculation, see Fig. 4 (left), is indicated. (b) Experimental STM image with tunneling parameters 210 pA and 24 meV. In (a) and (b) the area shown is $15\times 15{\mathrm{\AA }}^2$.

Figure 3

Experimental STM image and calculated LDOS images of NaCl bilayer structures on Cu(311). (a) and (c) Calculated LDOS image for the $p(3\times 2)$-I and $p(3\times 2)$-II structures, respectively, at an average distance of 9.0 Å from the Cu(311) surface. The surface unit cells used in the calculations, see Fig. 4, are indicated. (b) Experimental STM image of the $p(3\times 2)$-I bilayer structure with tunneling parameters 360 pA and 100 meV. The STM images of the $p(3\times 2)$-I and $p(3\times 2)$-II structures could not be discriminated. In (a), (b), and (c) the area shown is $15\times 15{\mathrm{\AA }}^2$.

Figure 4

Ordered structures of a NaCl monolayer on Cu(311): (left) $p(3\times 2)$-I and (right) $p(3\times 2)$-II. The dark and light filled circles represent Cu atoms of the first and second layer, respectively, and the large white and small black open circles represent ${\mathrm{Cl}}^{-}$ and ${\mathrm{Na}}^{+}$ ions, respectively. The unit cells used in the calculations are indicated. In the $p(3\times 2)$-I structure, all ${\mathrm{Cl}}^{-}$ ions sites are equivalent, whereas there are two inequivalent ${\mathrm{Na}}^{+}$ ion sites. In the $p(3\times 2)$-II structure, all ${\mathrm{Na}}^{+}$ ion sites are equivalent, whereas there are two inequivalent ${\mathrm{Cl}}^{-}$ ions sites; see also Fig. 1 in Ref. 6.

Figure 5

Experimental $dI∕dV$ spectrum for a monolayer NaCl on Cu(311). The tip-sample distance corresponds to a current set point of $I=0.3\mathrm{nA}$ at $V=3.8\mathrm{V}$. This distance is about 6 Å larger than the distance for the imaging parameter set used in Figs. 1, 2.

Figure 6

(Color) Calculated interface energies and geometrical parameters of a NaCl (a) $p(3\times 2)$-II mono-, (b) $p(3\times 2)$-I mono-, (c) $p(3\times 2)$-I bi-, and (d) $p(3\times 2)$-I trilayer on the (311) surface of Cu(311). The small, medium, and large spheres represent Na, Cu, and Cl, respectively. The interface layer for the bi- and trilayer correspond to the $p(3\times 2)$-I structure. The geometrical parameters are given in Å. The adsorption energies $E_a$ are given in eV per NaCl pair and is defined as $E_a=(E_{n\mathrm{Na}\mathrm{Cl}}+E_{(l-n)\mathrm{Na}\mathrm{Cl}∕\mathrm{Cu}}-E_{l\mathrm{Na}\mathrm{Cl}∕\mathrm{Cu}})∕m$, where $l$ is the number of NaCl layers, $m$ is the number of NaCl in the interface, and $n=1$, $n=1$, 2, and $n=1$, 2, 3, for mono-, bi-, and trilayers, respectively. $n$ is shown as superscipts of $E_a$ for the bi- and trilayers.

Figure 7

Partial $s$, $p$, and $d$ density of states (PDOS) around the topmost ${\mathrm{Cl}}^{-}$ ions in a NaCl (a) tri-, (b) bi-, and (c) monoayer on Cu(311). Note that the scales of the partial density of states for the different layers differ by several orders of magnitude.

Figure 8

Calculated $dI∕dV$ spectra for a monolayer NaCl in the $p(3\times 2)$-I structure on Cu(311). The solid and dotted lines are calculated from Eq. (1) using the full energy dependence of ${\rho }_S\left(ϵ\right)$ and a constant ${\rho }_S\left(ϵ\right)$, ${\rho }_S\left(ϵ\right)={\rho }_S\left({ϵ}_F\right)$, respectively. The dashed-dotted line is the result obtained using only the LDOS contribution to $dI∕dV$ [first term in Eq. (1)]. In the calculations we used $z_s=2\mathrm{\AA }$, $s=10\mathrm{\AA }$, and ${\mathrm{\Phi }}_{\mathrm{av}}=3.5\mathrm{eV}$.

Figure 9

(a) Electron-density difference, $\mathrm{\Delta }\rho (\text{electrons}∕{\mathrm{\AA }}^3)$ and (b) $\ln \mathbf{\left(}\rho \left({ϵ}_F\right)\mathbf{\right)}$ (arbitrary units) along $\left[01\overline{1}\right]$ for the $p(3\times 2)$-I monolayer structure. The position of the ${\mathrm{Cl}}^{-}$, at $d\left[01\overline{1}\right]=0$, 3.94 and 7.71 Å, are indicated. The solid and dashed lines indicate the positions of the ${\mathrm{Cl}}^{-}$ ions and the Cu surface reference plane, respectively. Both $\mathrm{\Delta }\rho$ and $\ln \mathbf{\left(}\rho \left({ϵ}_F\right)\mathbf{\right)}$ have been truncated.