Using a combined scanning-tunnel-microscopy (STM)/atomic-force microscopy (AFM) setup, we have measured simultaneously the apparent barrier height and the tip-sample force gradient for various tip-sample distances of an STM configuration in air. Our results reconfirm the existence of repulsive tip-sample forces during the STM operation with inert metal systems in air. Also, the gradient of such forces is of the correct sign that is necessary to cause the lowering of the apparent barrier heights through elastic deformations. However, our results indicate that the model that is based on the elastic deformation due to the tip-sample forces does not provide a satisfactory explanation of the lowering of the barrier heights for the case of gold in air.In scanning tunneling microscopy, the tunnel barrier height P, in eV, can be extracted (up to a good approximation) from the expression: $ =0 976d (ln. I)/ds, where I is the tunnel current and s is the tip-sample distance in angstroms. ' The barrier height is generally taken as the average of the work functions of the tip and of the sample. However, it is well known that when the barrier heights are measured with scanning tunneling microscopy (STM), unusually low values are obtained. As an example, Binnig et a/. , in their pioneering work, reported a value of /=0. 6 eV for the W-Pt system, under what they called a moderate vacuum. ' The expected barrier height for this system is about 5 eV. The situation for some other materials such as graphite is even more drastic. These extremely low values of barrier heights are not due to an intrinsic deficiency that is associated with the STM, since under clean UHV conditions, reasonable values for barrier heights have been observed. ' Thus, the contamination that is inevitably present on surfaces under non-UHV conditions must be responsible. It is known that surface impurities can change work-function values. However, these changes are small compared to the drastic lowering of barrierheight values that are observed commonly in STM measurements. Improving over the model by Soler et al. , Coombs and Pethica, and independently Mamin et a/. , put forward a model to explain the lowering of barrier-height values in STM measurements.In this model, which is originally suggested for the case of graphite, there exists a repulsive interaction between the tip and the sample. This repulsive interaction is mediated by the surface contaminants that are present between the tip and the sample. Under this situation, when the tip moves a distance LL away from the sample, the actual increase in the tipsample distance is M =hz+EF /k, where hI' is the increase in the force between the tip and the sample, and k is the spring constant that characterizes the elastic deformation of the tip-sample system in the tunnel direction.What the STM measures is the tip motion da, not the actual change in tunnel distance hs. Thus the STM measures an apparent barrier height P"which is related to the barrier height P in Eq. (1) byIf it is taken that F increases as the ...

We report an experimental setup of the ballistic-electron-emission microscope (BEEM) that is applicable to the study of thin free-standing metal films and metal-vacuum interfaces. We have used the technique to obtain spatially averaged BEEM spectra of thin free-standing gold films. Using such a spectrum, we obtained a rather coarse value of the work function for gold, which is compatible with some other reported values.