Electrical transport through molecules has been much studied since it was proposed that individual molecules might behave like basic electronic devices, and intriguing single-molecule electronic effects have been demonstrated. But because transport properties are sensitive to structural variations on the atomic scale, further progress calls for detailed knowledge of how the functional properties of molecules depend on structural features. The characterization of two-terminal structures has become increasingly robust and reproducible, and for some systems detailed structural characterization of molecules on electrodes or insulators is available. Here we present scanning tunnelling microscopy observations and classical electrostatic and quantum mechanical modelling results that show that the electrostatic field emanating from a fixed point charge regulates the conductivity of nearby substrate-bound molecules. We find that the onset of molecular conduction is shifted by changing the charge state of a silicon surface atom, or by varying the spatial relationship between the molecule and that charged centre. Because the shifting results in conductivity changes of substantial magnitude, these effects are easily observed at room temperature.
In this report we present a straightforward new technique for fabricating nanotips. This approach is based on spatially controlling the reaction of nitrogen gas with the surface atoms of a tungsten tip in a field ion microscope (FIM). Confining this field-assisted etching reaction to the shank has enabled us to produce single-atom tips with an apex radius far sharper than the nominal 10 nm radius of curvature tips we start with. Tip sharpening is evidenced in several ways. The FIM imaging voltage drops dramatically from, typically, 4.4 to 1.6 kV. Nanotip formation is also evident from the increase in the FIM magnification and the decrease in the apex area, which are monitored throughout the experiment. A subsequent field evaporation allows the nanotip to be sequentially deconstructed to further describe the extraordinary sharp tip that was formed. We also demonstrate the utility of these nanotips for the scanning tunneling microscope.
The scientific and technical challenges involved in building the planar electrical connection of an atomic scale circuit to N electrodes (N > 2) are discussed. The practical, laboratory scale approach explored today to assemble a multi-access atomic scale precision interconnection machine is presented. Depending on the surface electronic properties of the targeted substrates, two types of machines are considered: on moderate surface band gap materials, scanning tunneling microscopy can be combined with scanning electron microscopy to provide an efficient navigation system, while on wide surface band gap materials, atomic force microscopy can be used in conjunction with optical microscopy. The size of the planar part of the circuit should be minimized on moderate band gap surfaces to avoid current leakage, while this requirement does not apply to wide band gap surfaces. These constraints impose different methods of connection, which are thoroughly discussed, in particular regarding the recent progress in single atom and molecule manipulations on a surface.
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