Observation of electron two-dimensional standing-wave patterns at the surface of an insulating layer by scanning tunneling microscopy

Electron standing wave patterns around single hexa-peri-hexabenzocoronene (HBC) molecules adsorbed on the surface of thin KBr(001) films grown on Ag(111) are observed by scanning tunneling microscopy. We suggest that they result from the scattering, by the molecules, of the electrons populating the surface resonance associated with the lowest image potential state of the insulating film on a metal system. These patterns are nearly independent on the thickness of the film from two to five KBr monolayers, implying that they are localized near the surface of the insulating film. The electrons exhibit a parabolic dispersion relation which yields an effective mass $m^{*}=0.62\pm 0.2m_e$.

Figure 1

(a) Constant current STM image of HBC/KBr(2 ML)/Ag(111). $I=1\mathrm{pA}$, $V=1.5\mathrm{V}$. Inset: HBC structure. (b) Z(V) and (c) dZ/dV spectra recorded on KBr (black) and HBC (red). $I=1\mathrm{pA}$.

Figure 2

Constant current topographic STM images on HBC/KBr(2 ML)/Ag(111) with $I=1\mathrm{pA}$ and $V=3.05\mathrm{V}$ [(a) and (e)], 3.2 V [(b) and (f)], 3.35 V [(c) and (g)], 3.5 V [(d) and (h)]. Images (e)–(h) are images (a)–(d) with an enhanced contrast ($\mathrm{\Delta }Z=30\mathrm{pm}$). Images (i)–(l) are image differences: (i) 3.05–3 V, (j) 3.2–3.15 V, (k) 3.35–3.3 V, and (l) 3.5–3.45 V. The size of all the images is 10 × 10 nm.

Figure 3

Constant current dZ/dV spectra on Ag(111) (black) and on KBr films on Ag(111). Red: 2 ML, green: 3 ML, blue: 4 ML, and magenta: 5 ML. $I=1\mathrm{pA}$. The vertical dotted line is drawn to help visualizing the shift of the FER-1 peak.

Figure 4

Constant current topographic STM images of HBC on different thickness of KBr with $I=1\mathrm{pA}$ and (a)–(d) $V=3.1\mathrm{V}$, and (e) $V=3.05\mathrm{V}$. Images size: 9.6 × 9. 6nm. The Z scale spans 30 pm for all the images.

Figure 5

Dispersion relation extracted from a series of images in the [2.95–3.5 V] bias voltage range, by steps of 50 mV. The red curve is the parabolic fit corresponding to the displayed equation.