The Principles of Operation of Scanning Tunneling Microscope (STM)

Heinrich Rohrer and Gerd K. Binnig, scientists at IBM's Zurich Research Laboratory in Switzerland, are awarded the 1986 Nobel Prize in physics for their work in scanning tunneling microscopy. Binnig and Rohrer were recognized for developing the powerful Scanning Tunneling Microscopy technique. They shared the award with German scientist Ernst Ruska, designer of the first Electron Microscope. STM can form an image of individual atoms on a metal, semiconductor or other conductive sample surface by scanning the tip of a needle over the surface at a height of only a few atomic diameters, so that tunnel current occurs between the tip and the sample.

Main STM techniques are Constant Current and Constant Height modes for "topographic" data acquisition, accomplished by Spectroscopic techniques for "work-function"(barrier height) and "local density of states"(LDOS) profiles acquisition, I(z) and I(V) curves representing chemical and electronic characteristics of surface.

Constant Current and Constant Height Mode

In STM bias voltage is applied between a sharp conductive tip and a conductive sample, so when the sample is approached to a few angstroms from the tip, tunneling current occurs, that indicates proximity of the tip to the sample with very high accuracy. In Constant Current mode (CCM) of operation when scanning sample surface the scanner keeps the current constant by feedback circuit. So vertical displacement of the scanner (feedback signal) reflects surface topography. STM gives true atomic resolution on 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 speed of scanning in CCM is restricted by usage of feedback system. Larger scanning speeds can be obtained by usage of Constant Height mode (CHM), but CCM allows to investigate the samples with developed relief.

Constant Current Mode

Figure 1. Constant Current Mode

Constant Height Mode

Figure 2. Constant Height Mode

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 the 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.

Barrier Height Imaging

The Local Barrier Height (LBH) spectroscopy provides an information about the spatial distribution of the microscopic work function of the surface, as described below.

The tunneling current IT in STM exponentially decays with the tip-sample separation z as

IT ~ exp(-2kz),

where the decay constant is given by

2k = 2(2mU/h2)1/2.

In the LBH imaging, we measure the sensitivity of tunnel current to the tip-sample separation at each pixel of an STM image. The LBH obtained in this method is the so-called apparent barrier height U defined by

U= 0,95(1/IT )2 (dIT /dz)2

This U is customarily compared to an average work function Uav = (Us + Ut )/2, where Ut and Us are the tip and sample work functions, respectively. In many cases, experimental U does not precisely agree with Uav but tends to be slightly smaller. Nevertheless, it is known that U is closely related to the local surface potential (local work function) and is a good measure of it.

The LBH image is obtained by measuring point by point the logarithmic change in the tunneling current with respect to the change in the gap separation, that is, the slope of log I vs. z. In the LBH measurement, the tip-sample distance is modulated sinusoidally by an additional AC voltage applied to the feedback signal for the z-axis piezodevice attached to the tip. The modulation period is chosen to be much shorter than the time constant of the feedback loop in the STM.

Barrier Height Imaging

Figure 3. Barrier Height Imaging

Density of States Imaging

As long as measured in STM current is determined by the tunneling processes through tip-sample surface gap its value depends not only on the barrier height but on the electron density of states also. Accordingly obtained in STM images are not simply images of sample surface relief (topography), these images can be hardly affected by the density of electronic states distribution over the sample surface. Good example of Local Density of States (LDOS) influence on the STM image is well-known image of highly oriented pyrolitic graphite (HOPG) atomic lattice. Only half atoms are visible in STM. Similar case is image of GaAs atomic lattice.

LDOS determining can also help to distinguish chemical nature of the surface atoms. LDOS acquisition is provided simultaneously with the STM images obtaining. During scanning the Bias Voltage is modulated on the value dU, the modulation period is chosen to be much shorter than the time constant of the feedback loop in the STM.

Suitable modulation of tunnel current dI is measured, divided by dU and presented as LDOS image. On Example the topography and LDOS image of HOPG sample are presented.

Density of States Imaging

Figure 4. Density of States Imaging

I(z) Spectroscopy

The I(z) Spectroscopy is related to LBH spectroscopy and can be used for providing an information about the z-dependence of the microscopic work function of the surface. Next important use of the I(z) Spectroscopy is concerned with for testing of the STM tip quality.

I(V) Spectroscopy

In I(V) Spectroscopy (or Current Imaging Tunneling Spectroscopy, CITS) a normal topographic image is acquired at fixed Io and Vo. At each point in the image feedback loop is interrupted and the bias voltage is set to a series of voltages Vi and the tunneling current Ii is recorded. The voltage is then returned to Vo and the feedback loop is turned back on. Each I-V spectra can be acquired in a few milliseconds so there is no appreciable drift in the tip position. This procedure generates a complete current image Ii(x,y) at each voltage Vi in addition to the topographic image z(x,y)|VoIo.

CITS data can be used to calculate a current difference image

DIVi,Vj(x,y) where Vi and Vj bracket a particular surface state, producing an atomic resolved, real space image of a surface state. This technique, for example can be used in UHV to image filled ad-atom states or the dangling bond states for silicon reconstructions.

This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.

For more information on this source, please visit NT-MDT Spectrum Instruments.

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