Imaging conditions, tunneling

Fig. 2. Schematic diagram of the tunnel gap between sample and tip, with the extension of the electric double layers indicated by the outer Helmholtz plane(OHP). (a) No tip interaction at large tip-sample separation, (b) Overlap of the electric double layers at a distance s = 0.6 nm, which can be achieved by conventional imaging conditions (e.g., Uj = 50 mV It = 2 nA Rt = 2.5 x 107 Q). Inset Dependence of the tunnel gap s on the tunnel resistance Rt for a tunnel barrier of 1.5 eV.

Fig. 11.12 High-resolution STM images of the (2>/3 x 2>/3)R30° -sulfite structure by varying bias voltages at the same imaging area, tunneling conditions (a) +0.3 V, 1 nA (b) -0.3 V, 1 nA. A structural model (c) is proposed according to the STM images and the calculated numbers of Ag adatoms and sulfite moieties in the unit cell. The (1x1) and (2- /3 X 2-j3)R30 unit cells are marked in the model. Reprinted with permission from [22]. Copyright 2007 American Institute of Physics...

Well-defined in situ STM experiments require the use of a bipotentiostat to independently control the electrochemical potential of the tip and substrate relative to some reference electrode. This configuration is distinct from an ultrahigh vacuum (UHV) experiment in which only the bias between the electrodes needs to be specified. In the electrochemical environment, the tip electrode is simultaneously a tunneling probe and an ultramicroelectrode. Consequently, suitable attention must be given to possible faradaic reactions proceeding at the tip as su ested in Fig. 4. These reactions may include redox events as well as deposition and dissolution processes. Under constant current imaging conditions, the set point current is maintained by a combination... 

Figure 8.5 STM image of Ni (110) exposed to CO at 1 x 10-6 mbar (a) raw data (60 x 60 A) (b) and (c) unit cell averaged (30 x 30 A) at two different tunnelling conditions the unit cell is indicated. (Reproduced from Ref. 19).

Figure 8.6 STM image of Ni (11 l)-c(4 x 2) CO structure with (a) (4 x 2) (white) and c(4 x 2) (black) unit cells shown with corresponding corrugation line scan (0.2 A full scale) (b) similar to (a) under different tunnelling conditions and corresponding line scan (0.3 A full scale). (Reproduced from Ref. 20).

Figure 4.7b shows a close-up of Figure 4.7a, a 300 pA constant It contour, which has a corrugation of approximately 100 pm and is located approximately 300 pm from the 02c surface atoms. These values disagree quantitatively with experimental STM results at the same tunneling conditions on two accounts. First, a set point of It= 300 pA is not a particularly large value for constant current STM imaging, and... 

If the tunneling current is from the surface to the tip, the STM images the density of occupied states. If the potential is reversed, the current flows in the other direction, and one images the unoccupied density of states, as the reader can easily understand from Fig. 7.19. This figure also illustrates a necessary condition for STM there must be levels within an energy e-V from the Fermi level on both sides of the tunneling gap, from and to which electrons can tunnel In metals, such levels are practically always available, but when dealing with semiconductors or with adsorbed molecules, this condition may be a limitation. A second condition is that the sample possesses conductivity perfect electrical insulators cannot be measured with STM. 

In any imaging and spectroscopic mode of the STM, a bias is required between the sample and the tip. In an electrochemical solvent, faradaic current between the tip and sample can interfere with, and sometimes completely obscure, the tunneling current. This undesirable situation makes it very difficult to control the feedback and to maintain a constant tunneling gap between the tip and the sample. For example, in our laboratory, we have found that feedback control is lost on our present microscope if the faradaic current, ip, assumes a value greater than one-half that of the tunneling current, it. Use of partially insulated tips alleviates this condition, but unfortunately, does not completely eliminate the problem (57). 

---