Comparative study of methods for measuring. the apparent barrier height on an atomic scale

Abstract

Atomic-scale lateral variations in the apparent barrier height, which are a measure of the decay of the conductance with tip-sample distance in the scanning tunneling microscope, have been measured with three different methods. In general, a lateral variation of 10-25% is obtained within the unit cell of the reconstructed Au(110) surface. Furthermore, it is revealed that the absolute values, deduced from measurements of the tip-surface distance as a function of tunnel voltage, are approximately 30% lower than values obtained with other methods. However, in contrast to a previous study, none of the methods result in anomalously low values.In a simple one-dimensional model [1], the tunneling conductance in a scanning tunneling microscope (STM) can be written aswhere I t and V t are the tunneling current and voltage, respectively, ρ local surf is the surface local density of states evaluated at the Fermi energy, φ is the average barrier height in eV, and z is the tip-surface distance in Å. Within this model, an STM image acquired in the constant-current mode can be interpreted as contours of constant ρ local surf . Ever since the invention of the STM [2] such images have provided a wealth of information about surface structures and processes ranging from a micrometer scale down to atomic dimensions.Additional valuable information can be obtained from the decay rate of ρ local surf in the direction normal to the surface. Since this quantity depends on the local chemical nature of the surface, it may offer a way to distinguish between different surface elements/species that cannot be distinguished in a constant-current image. From (1) it is clear that ρ local surf decays with a decay constant given by the average barrier height φ. As a corresponding experimental measure, it has been generally accepted to define an apparent barrier height as φ ap = 1 1.025 d ln(I t /V t ) dz 2

A Scanning Tunneling Microscopy Study of Electrostatic and Proximity Effects in Tip-Assisted Migration and Desorption of a DNA Base Molecule on SrTiO3

“…When the tip approaches the surface from a reference feedback position, the logarithm of tunneling current, log I (Δ z ), increases linearly following a one-dimensional tunneling model. This is a typical result for metal surfaces studied in previous research. − When the tip reaches Δ z = −7 Å, the curves show a steep “current jump”, indicating a change from vacuum tunneling to conductive current. This can be explained by an electrical contact forming between the tip atoms and the surface atoms.…”

“…As the first step, the authors measured tip-displacement Δ z from the reference probe position z ref to the contact position against the surface. The contact position was defined as a “current jump”, indicating the change from vacuum electron tunneling to conductive current. − In the actual measurement of I (Δ z ), the dynamic range of the current amplifier was limited when the tip was scanned along the z -direction over the full tip-displacement range. For this reason, the values of I (Δ z ) were measured as a partial data series obtained at different feedback positions with tunneling currents I of 7, 70, 700, and 7000 pA and a displacement Δ z of ±3 Å.…”

“…(e) Imaging of Tunneling Barrier Height. On the basis of a simple one-dimensional tunneling model, the tunneling barrier-height Φ has been generally defined as where I is the tunneling current and z is the tip−surface separation. − As a corresponding experiment, a lock-in technique was used to measure Δln( I ) with a modulating tip−surface displacement Δ z . The modulation amplitude, frequency, and time constant were Δ z = 1 Å, f = 4 kHz, and τ = 3 ms, respectively.…”

New Insights for Self-Assembled Monolayers of Organothiols on Au(111) Revealed by Scanning Tunneling Microscopy

“…Prior to the tip−SAM surface contact, the ln( I )− Z relationship increases linearly as the tip approaches the surface. The tunneling barrier height, Φ, can be calculated using where I is the tunneling current and Z is the tip−surface separation. − After contact, the tunneling current continues to increase linearly, with a larger slope, as the tip penetrates into the SAM. The first turning point (indicated in Figure A) defines the tip−methyl group contact position, at which I − V curves are acquired, as shown in Figure B.…”

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

“…[10][11][12][13][14][15][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.…”