STM operation modes (rus. режимы измерений на СТМ) — scanning tunnelling microscopy (STM) has five main variable parameters. These are horizontal coordinates x and y, height z, the bias voltage V and tunnelling current I. Depending on variation of these parameters, three basic modes of STM measurement are distinguished: 1) constant current mode, in which I and V are held constant, x and y change with movement of the scanning tip, and z is measured; 2) constant height mode (also known as current imaging mode), where z and V are held constant, x and y change during the scan, and I is measured; and 3) scanning tunnelling spectroscopy (STS), which is a whole set of modes with variation of V.

Description

Constant Current Mode is the most frequently used mode of obtaining STM images. In this mode, a needle moves across the surface of the sample while voltage and current are kept constant. In order to maintain a constant current at a fixed voltage, the tracking system continuously adjusts the vertical position of the needle varying the voltage Vz in the z-piezoelectric element. In the ideal case of a homogeneous (from the point of view of electronics) surface, constant current means a constant gap between the tip and the surface, i.e., while scanning the needle path follows all the features of the surface topography (Fig. a). The surface element height is determined directly from Vz. Such measurements produce the surface topography as a function of the needle position z(x, y).

In constant height mode, the surface is scanned with the needle at a constant voltage Vz in the z-piezoelectric element while measuring the tunnelling current I as a function of the needle position (Fig. b). The voltage V between the tip and the sample is kept constant, and the servo system feedback is turned off. In this case, the surface bump will be reflected in higher tunnelling current when passed by the needle . This mode provides for higher scanning speed compared to constant current mode, because the servo system does not need to react to all the features of the surface passing under the needle. This feature is particularly valuable when studying dynamic processes in real time, particularly when recording STM video. The drawback of this mode is the difficulty of quantifying the surface topography from the tunnel current changes.

Scanning tunnelling spectroscopy (STS) is a set of methods of scanning tunnelling microscopy in which the voltage between the tip and the sample is varied to obtain the information on the local electronic structure of the surface.

Illustrations

Schematic representation of operation of a scanning tunneling microscope in (a) constant curren
Schematic representation of operation of a scanning tunneling microscope in (a) constant current mode and (b) constant height mode.

Authors

  • Zotov Andrey V.
  • Saranin Alexander A.

Source

  1. Oura K. et al. Surface Science: An Introduction // Springer, 2010 - 452 pp.

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