Influence of band structure on the apparent barrier height in scanning tunneling microscopy

Abstract

The apparent height of the tunneling barrier in scanning tunneling microscopy measured on Au͑111͒, Ag͑111͒, and Cu͑111͒ surfaces is found to vary significantly with the bias voltage. In particular, the apparent barrier height a is asymmetric with respect to the bias polarity on all three surfaces, in contrast to simple interpretations of a in terms of an average work function of tip and sample. Model calculations of the tunneling current, which take band-structure effects into account, describe the experimental observations. A fundamental physical property of a metal surface is its work function , which is defined as the minimum work required to remove an electron from the metal at T =0 K. Knowledge about at the atomic scale can improve the understanding of chemical surface processes such as heterogeneous catalysis and adsorption. 1,2 Scanning tunneling microscopy ͑STM͒ offers the possibility to study of conductive samples at the atomic scale 3,4 using the exponential variation in the tunneling current I with the tip excursion z. 5,6 From I͑z͒ data an apparent barrier height a may be determined, which-within the WKB approximation of the tunneling current through a rectangular barrier-is related to the sample work function s by 7,8where t is the work function of the tip apex, V is the applied tunneling bias voltage, and −e is the electron charge. Hence, the effect of the finite voltage on a is assumed to be linear. A more realistic barrier shape, which takes into account a classical image potential, leads to minor modifications. 9 Atomistic calculations for tunneling between an Au͑100͒ sample and tip confirm this picture in the zero-bias limit. 6 The number of publications using measurements of a to characterize surfaces is considerable. Often a is evaluated at elevated sample voltages with ͉V͉ ӷ 0.1 V assuming that Eq. ͑1͒ is valid. 10-16 On the other hand, surprisingly few reports on voltage-resolved a measurements are available. [16][17][18][19][20][21] For example, a on the reconstructed Au͑111͒ surface was reported to show a voltage polarity dependence, which was suggested to result from the surface dipole layer originating from the reconstruction. 19 In a subsequent study no voltage polarity dependence was found. 20 Calculations predicted a voltage polarity dependence of a on Al͑100͒ and excluded the possibility that the polarity-induced difference may be due to the formation of an additional surface dipole layer. 22 Overall, more work appears to be required to clarify the origin of the voltage dependency of a on metal surfaces and its interpretation.Here, we report low-temperature STM results for voltageresolved apparent barrier heights a on the ͑111͒ surfaces of Au, Ag, and Cu. Probing the unoccupied states of defect-free surface areas we find similar behavior for all surfaces. a remains rather constant for sample voltages up to V Ϸ 3.5 V. At higher voltages, a decreases within some hundred millivolts by ⌬ a Ϸ 2 eV and then undergoes oscillations due to Gundlach resonances. 17 When probing the occupied sta...

Superatom orbitals of C60on Ag(111): Two-photon photoemission and scanning tunneling spectroscopy

“…These procedures require knowledge of the absolute tip-sample distance and apparent tunneling barrier height, which can depend strongly on the bias and tip conditions. 27 Other factors, such as the tip-sample coupling, tip density of states, etc., also must be considered. Use of the ln derivative corrects for the z and V dependence of the transmission function while avoiding the potential of introducing additional error through incorrect assumptions for these values.…”

Simulation of spin-polarized scanning tunneling microscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)

“…The absolute value ensures that the transmission probability is symmetric in the positive and negative bias range with the minimum at zero bias. 27,28 This way we circumvent the problem of the bias-asymmetric contribution from the sample LDOS to the differential conductance. 29 We use this energy-dependent vacuum decay for an ideal, electronically featureless, and maximally spin-polarized tip model.…”

Local tunneling decay length and Kelvin probe force spectroscopy

“…Consequently, one expects that from the decay length determined from current-distance spectra the barrier height can be extracted, which-in simplest approximationis the mean of the work function of the tip and the sample. It has been early realized that several corrections are needed, because of the tunneling junction being three-dimensional in nature [4][5][6], the applied bias voltage [7][8][9] or image charge effects [10] altering the barrier's shape, for example. In this context a so-called effective barrier height has been introduced, to still make use of the simple description.…”