Scanning tunneling spectroscopy
In scanning tunneling microscopy, a metal tip is moved over a conducting sample without making physical contact.
The electron energy is set by the electrical potential difference (voltage) between the sample and the tip.
The change of the current with the energy of the electrons is the simplest spectrum that can be obtained, it is often referred to as an I-V curve.
As is shown below, it is the slope of the I-V curve at each voltage (often called the dI/dV-curve) which is more fundamental because dI/dV corresponds to the electron density of states at the local position of the tip, the LDOS.
Scanning tunneling spectroscopy is an experimental technique which uses a scanning tunneling microscope (STM) to probe the local density of electronic states (LDOS) and the band gap of surfaces and materials on surfaces at the atomic scale.
[1] Generally, STS involves observation of changes in constant-current topographs with tip-sample bias, local measurement of the tunneling current versus tip-sample bias (I-V) curve, measurement of the tunneling conductance,
Since the tunneling current in a scanning tunneling microscope only flows in a region with diameter ~5 Å, STS is unusual in comparison with other surface spectroscopy techniques, which average over a larger surface region.
The origins of STS are found in some of the earliest STM work of Gerd Binnig and Heinrich Rohrer, in which they observed changes in the appearance of some atoms in the (7 x 7) unit cell of the Si(111) – (7 x 7) surface with tip-sample bias.
[2] STS provides the possibility for probing the local electronic structure of metals, semiconductors, and thin insulators on a scale unobtainable with other spectroscopic methods.
The tunneling matrix element, describes the energy lowering due to the interaction between the two states.
[3] For higher bias voltages, the predictions of simple planar tunneling models using the Wentzel-Kramers Brillouin (WKB) approximation are useful.
[2] The tip is often regarded to be a single molecule, essentially neglecting further shapes induced effects.
A small, high frequency sinusoidal modulation voltage is superimposed on the D.C. tip-sample bias.
[5] In practice, the modulation frequency is chosen slightly higher than the bandwidth of the STM feedback system.
Such effects arise from the capacitance between the tip and the sample, which grows as the modulation frequency increases.
[2] In order to obtain I-V curves simultaneously with a topograph, a sample-and-hold circuit is used in the feedback loop for the z piezo signal.
In variable-spacing scanning tunneling spectroscopy (VS-STS), the same steps occur as in CS-STS through turning off the feedback.
Lock-in detection and modulation techniques are used to find the conductivity, because the tunneling current is a function also of the varying tip-sample spacing.
[8] Current-imaging-tunneling spectroscopy (CITS) is an STS technique where an I-V curve is recorded at each pixel in the STM topograph.
One approach to improving the experimental design is by applying feature-oriented scanning (FOS) methodology.
By plotting the magnitude of I on a log scale versus the tip-sample bias, the band gap can clearly be determined.
Although determination of the band gap is possible from a linear plot of the I-V curve, the log scale increases the sensitivity.
Usually, the WKB approximation for the tunneling current is used to interpret these measurements at low tip-sample bias relative to the tip and sample work functions.
[2] Although the tunneling transmission probability T is generally unknown, at a fixed location T increases smoothly and monotonically with the tip-sample bias in the WKB approximation.
diverges as V approaches 0, preventing investigation of the local electronic structure near the Fermi level.
Additionally, the voltage dependence of T, which is usually unknown, can vary with position due to local fluctuations in the electronic structure of the surface.
is the apparent barrier height, STM and STS only sample valence electron states.
Element-specific information is generally impossible to extract from STM and STS experiments, since the chemical bond formation greatly perturbs the valence states.
[4] At finite temperatures, the thermal broadening of the electron energy distribution due to the Fermi-distribution limits spectroscopic resolution.
[3] Despite these limitations, STS and STM provide the possibility for probing the local electronic structure of metals, semiconductors, and thin insulators on a scale unobtainable with other spectroscopic methods.
