Role of the electric field in surface electron dynamics above the vacuum level

Abstract

Scanning tunneling spectroscopy (STS) is used to study the dynamics of hot electrons trapped on a Cu(100) surface in field emission resonances (FER) above the vacuum level. Differential conductance maps show isotropic electron interference wave patterns around defects whenever their energy lies within a surface projected band gap. Their Fourier analysis reveals a broad wave vector distribution, interpreted as due to the lateral acceleration of hot electrons in the inhomogeneous tipinduced potential. A line-shape analysis of the characteristic constant-current conductance spectra permits to establish the relation between apparent width of peaks and intrinsic line-width of FERs, as well as the identification of the different broadening mechanisms.

Localization, splitting, and mixing of field emission resonances induced by alkali metal clusters on Cu(100)

“…7(b)], the FER's appear equally well in the dI/dV curves and in the total transmission at the tip Fermi level 12 for a given Z(V ). In fact, the presence of resonances in the latter is the reason for the appearance of peaks in the corresponding dI/dV spectrum.…”

“…U tip is then calculated within the flat tip approximation on the basis of well-tested models. 11,12 This allows inclusion of the applied electric field in the tunneling junction and an efficient treatment of the varying tip-sample distance, mandatory to compare with the STS data taken at high bias voltages. Finally, U abs is the imaginary potential introduced inside the metal to account for the inelastic electron-electron scattering events.…”

“…To identify them in our model calculations, we use the same strategy as in the experiment: compare the point spectra taken on top of the clean metal surface with the spectra on top of the alkali island. For the clean metal surface, the 1D version of the WPP procedure 12 has been used. The calculated constant current distance-voltage characteristic Z(V ) shows sharper (energies in the projected band gap) or smoother (energies outside the gap) steps at values V i that essentially correspond to the different resonance energies E i = eV i [see Fig.…”

“…This pattern is ascribed to the density modulation created by the scattering of FER electrons at the cluster. 12 Besides the localization of FER's at single Li clusters, we have also studied FER's of closely spaced Li clusters. Some examples of dI/dV maps are presented in Fig.…”

“…Since the energetics of FER's is to a large extent determined by the electric field at the junction, the interpretation of the data is relatively simple. This can be done using one-dimensional models, 11,12 as long as both the tip radius of curvature and the lateral extension of the surface area of interest are large compared to the tip-surface distances (a few nanometers when operating the STM at high bias voltages).…”