ULTIMATE COMPUTING - Quantum Consciousness Studies

194 NanoTechnology
to a tunneling current of electrons (Figures 10.2, 10.3 and 10.4). The tunneling
current depends exponentially on the tip-to-substrate separation (about an order of
magnitude per angstrom or tenth nanometer). Depending on the substrate, typical
tunneling currents and voltages are on the order of nano-amperes and millivolts,
respectively. A servo system uses a feedback control that keeps the tip to
substrate separation constant by modulating the voltage across a piezoelectric
positioning system. As the tip is scanned across the surface, variations in this
voltage, when plotted, correspond to surface topography. Detailed surface
topography maps of various materials have been obtained which demonstrate
individual atoms like cobblestones. By varying and measuring different
combinations of tunneling current, voltage, and distance, different types of
information may be obtained about the surface being probed.
The basic modes of operation of an STM, described by Hansma and Tersoff
(1987), are summarized in Table 10.1. Here i, v, and h are the tunneling current,
the voltage across the gap, and the gap size respectively. Mode I is used to
measure the topography of the surface of a metal or semiconductor and is the
slowest mode since the electro-mechanical servo system must follow the shape of
the surface during the scanning operation.