Limitations
While STS can provide spectroscopic information with amazing spatial resolution, there are some limitations. The STM and STS lack chemical sensitivity. Since the tip-sample bias range in tunneling experiments is limited to, where is the apparent barrier height, STM and STS are 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.
At finite temperatures, the thermal broadening of the electron energy distribution due to the Fermi-distribution limits spectroscopic resolution. At, and the sample and tip energy distribution spread are both . Hence, the total energy deviation is . Assuming the dispersion relation for simple metals, it follows from the uncertainty relation that
where is the Fermi energy, is the bottom of the valence band, is the Fermi wave vector, and is the lateral resolution. Since spatial resolution depends on the tip-sample spacing, smaller tip-sample spacings and higher topographic resolution blur the features in tunneling spectra.
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. Additionally, topographic and spectroscopic data can be recorded simultaneously.
Read more about this topic: Scanning Tunneling Spectroscopy
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