Detailed experimental investigation of the barrier-height lowering and the tip-sample force gradient during STM operation in air

Abstract

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 ...

Electron tunneling across an interfacial water layer inside an STM junction: tunneling distance, barrier height and water polarization effect

“…It is obvious that the STM junction is filled with a layer of non-insulating materials, most likely water molecules condensed from atmospheric moisture, as has been suggested in previous studies [11][12][13][14]. The tunneling distance measured in this work agrees with the result for aqueous phase tunneling, a value of s ≤ 10 Å at R t = 10 7 Ω has been deduced from I − z characteristics for an Au tip and an Au sample in an electrochemical environment [8].…”

“…For example, * To whom correspondence should be addressed water inside an STM junction greatly increases the tunneling distance compared with that in ultrahigh vacuum (UHV) [8]. The local barrier height for electron tunneling (φ) is unusually low in water [8][9][10][11][12]15] compared with UHV [16][17][18][19]. These phenomena should somehow be related to the physics of electron transfer across interfacial water, but this remains uncertain at present.…”

“…An electrochemical environment [8][9][10] or humid atmosphere [11][12][13][14] provides an STM junction filled with water, in the latter case condensed from atmospheric moisture [13,14]. In the aqueous STM studies, however, many aspects are still not well resolved or perhaps have not been given enough detailed consideration.…”

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

“…Anomalously low values (below 1 eV) have often been reported [3,5,7], which have been explained as being consequences of an elastic contact between tip and surface [8]. However, this explanation seems to fail for experiments carried out under ambient conditions on a Au sample [9]. Furthermore, discrepancies between results obtained by different ways of measuring have caused some debate in the literature.…”

Charge Transfer between STM Tip and Au(100) in Dry, H2O, and D2O Atmospheres

“…The different values of barrier height for gold tip compared to Pt-Ir tip may be caused by materials difference but also differences in tip shape, 15) surface impurities, 15) local density of states, 16) and conductance. [16][17][18] The apparent barrier height is also well known to depend on the local work function, 13,19) and the work function of Pt is larger than that of Au.…”