| Constant Height mode |
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| STM | |
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In STM bias voltage is applied between a sharp onductive tip and a
conductive sample, so when the sample is pproached to a few angstroms
from the tip, tunneling current occurs, that indicates proximity of the
tip to the sample with very high ccuracy. In Constant Height mode (CHM)
of operation the scanner of STM oves the tip only in plane, so that
current between the tip and the sample surface visualizes the sample
relief. Because in this mode the djusting of the surface height is not
needed a higher scan speed can e obtained. CHM can only be applied if
the sample surface is very flat, because surface corrugations higher
than 5-10 A will cause the ip to crash. The weak feedback is still
present to maintain a constant verage tip-sample distance. As the
information on the surface structure is obtained via the current, a
direct gauging of height ifferences is no longer possible. STM gives true atomic resolution on the some samples even at ambient conditions. Scanning tunneling microscopy can be applied to study conductive surfaces or thin nonconductive films and small objects deposited on conductive substrates. The tunnel currents registered in the course of the measurement are sufficiently small - up to 0.03 nA (with a special STM head - up to 0.01 nA), so it is possible to investigate also low conductivity surfaces, in particular biological objects. Among the STM disadvantages one can mention the complexity of the results interpretation for some surfaces since the surface image received in the STM investigation mode is determined not only by the surface relief but also by density of states, bias voltage sign and value, current value etc. For example on the highly oriented pyrolitic graphite surface one can see only each second atom. It is concerned with special arrangement of wave functions density of states. References
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