When the thickness of a metal film is reduced to the nanometer size, its electronic structure will predominantly be governed by the quantum size effect, causing the work function to vary with the film thickness. This issue is of importance in nanoscience because one can realize the electronic structure of the thin film by measuring the work function. In general, one can measure the work function by using photoemission spectroscopy. However, owing to that it is a broad beam technique, it demands the metal film being grown in a layer-by-layer mode; otherwise what has been measured is just an average result over the films of various thicknesses. In order to overcome this limitation, a local probe technique such as scanning tunneling microscopy and spectroscopy (STM and STS) is an option without the need of a uniform film. Using STM, one can obtain the work function of the metal surface by measuring the apparent barrier height. Nevertheless, the general measured error with this method can be as high as 0.3 eV, much inferior to the precision achievable by the broad beam technique.
We have found that the work function of the thin film can be precisely measured by using the Gundlach oscillation in STS. The precision of the measurement can be better than 0.02 eV, which is much better than that of detecting apparent barrier height with STM and is comparable to the photoemission results. This result has been published in Phys. Rev. Lett. 99, 216103, (2007). Since this technique has a high precision, one can use it to detect the subtle variation of the work function for some nanostructures. Therefore, Gundlach oscillation in STS can be a new way for the research of the nanoscience.