Surface potential of a polar oxide film: FeO on Pt(111)

We have studied a thin FeO film on Pt(111) using scanning tunneling microscopy. The corrugation of the Moiré pattern, that arises due to a lattice mismatch between the oxide film and the substrate, is found to increase dramatically when the microscope is operated in the field-emission regime. This contrast enhancement can be attributed to variations in the energy at which field-emission resonances are observed. We assign this effect to variations of the surface potential within the Moiré unit cell. Using a simple electrostatic model we show that in this oxide film with polar termination, such variations can be induced by very subtle differences in geometry.

Figure 1

(Color online) $75\times 50{\mathrm{\AA }}^2$ STM images at sample bias voltages of (a) $20\mathrm{mV}$ and (b) $4500\mathrm{mV}$. (c) Line profiles from the STM images across the Moiré unit cell. $I_t=0.3\mathrm{nA}$.

Figure 2

(Color online) $dI∕dV$ (top) and topographic images. Image size $88\times 88{\mathrm{\AA }}^2$, $I_t=0.3\mathrm{nA}$.

Figure 3

(Color online) (a) $dI∕dV$ spectra obtained with closed feedback loop, taken at the three positions on the FeO film indicated in the inset (left). (b) Energies at which field-emission resonances are observed. The lines represent fits to the points with Eq. (1); first-order field-emission resonance have not been included in the fit. The inset shows the resulting values for the barrier $\Phi$.

Figure 4

Hard sphere models of the (a) top, (b) fcc, and (c) hcp regions in the FeO over-layer. Side view is sketched in (d) along with the electric field due to the polar bilayer for two different $\mathrm{Fe}-\mathrm{O}$ separations, $d_1$ and $d_2$.

Figure 5

(Color online) (a) Tunneling current vs tip-sample separation recorded at the two most distinct locations in the FeO unit cell. The solid lines represent fits to Eq. (2). (b) Resulting values for the barrier $\Phi$, and (c) $d\ln I∕dz$ image. Image size $57\times 57{\mathrm{\AA }}^2$, $V_s=4500\mathrm{mV}$, $I_t=0.3\mathrm{nA}$, $\Delta z_{\mathrm{rms}}=1\mathrm{\AA }$.