Scanning Tunneling Microscopy (STM)

Scanning Tunneling Microscopy (STM)


HOW DOES THE STM WORK?

The STM doesn’t work the way a conventional microscope does, using optics to magnify a sample. Instead a sharp (1-10 nm) probe that is electrically conductive is scanned just above the surface of an electrically conductive sample. The principle of STM is based on tunneling of electrons between this conductive sharp probe and sample.

Basic principles

Scanning tunneling microscopy (STM) is an experimental technique based on the principles of quantum tunneling of electrons between two electrodes separated by a potential barrier, typically used for imaging surfaces of materials with sub-atomic resolution. It was pioneered by Gerd Binnig and Heinrich Rohrer in 1982, and was awarded a Nobel Prize in Physics in 1986.

Typical STM experimental setup consists of a sharp metallic tip which is brought within several angstroms of a conducting surface using a 3-dimensional piezoelectric scanner (Figure 1(a)). This scanner can position the tip both laterally (in the xy – plane) and vertically (in the z – direction) with sub-angstrom precision. Application of voltage V between the tip and the sample allows electrons to quantum-mechanically tunnel between the two (Figure 1(b)). The resulting tunneling current can be calculated using the time-dependent perturbation theory. If a positive V is applied to the sample, the Fermi level of the sample shifts down with respect to the Fermi level of the tip, and electrons tunnel from the occupied states of the tip into the empty states of the sample (Figure 1(b)).

Figure 1. Basic principles of STM and STS.

  • The first member of SPM family, scanning tunneling microscopy (STM), was developed In 1982, by Gerd Binnig and Heinrich Rohrer at IBM in Zurich created the ideas of STM (Phys. Rev. Lett., 1982, vol 49, p57). Both of the two people won 1986 Nobel prize in physics for their brilliant invention.
  • • STM tip should be conducting (metals, like Pt); • STM plays with the very top (outermost) atom at the tip and the nearest atom on sample; so the whole tip is not necessarily very sharp in shape, different from the case of AFM, where spatial “contact” is necessary and crucial for feedback.

What is Tunneling?


Tunneling is a phenomenon that describes how electrons flow (or tunnel) across two objects of differing electric potentials when they are brought into close proximity to each other. When a voltage is applied between probe and surface electrons will flow across the gap (probe-sample distance) generating a measurable current.











The tunneling phenomena can be explained by quantum mechanics. Tunneling originates from the wavelike properties of electrons. When two conductors are close enough there is an overlapping of the electron wavefunctions. Electrons can then diffuse across the barrier between the probe (tip-terminating ideally in a single atom) and the sample when a small voltage is applied. The resulting diffusion of electrons is called tunneling. A more detailed quantum mechanics explanation can be found.1

Fig01.png
Figure 1. Tunneling of electrons from an etched STM probe (i.e. tungsten wire) to a conductive surface.

An important characteristic of tunneling is that the amplitude of the current exhibits an exponential decay with the distance, d. One way to describe this relationship is by the equation:

I∼Ve–cd

  • I = Tunneling current
  • V = voltage between probe and sample
  • c = constant
  • d = probe-sample separation distance

Key factor for STM: Very small changes in the probe-sample separation induce large changes in the tunneling current! (i.e. at a separation of a few Å the current rapidly decreases)

This dependency on tunneling current and probe-sample distance allows for precise control of probe-sample separation, resulting in high vertical resolution(<1 Å) . Furthermore, tunneling is only carried out by the outermost single atom of the probe. This allows for high lateral resolution (<1 Å).

How are the sharp STM probes made?

Commercial probes are available but often users make their own probes. A common method is to electrochemically etch tungsten, W, wire in NaOH to create a sharp probe. A problem with W probes is that they oxidize over time. Platinum iridium (Pt-Ir) is preferred for use in air because platinum does not easily oxidize. The tiny fraction of Iridium in the alloy makes it much harder. The Pt-Ir tips are usually shaped by cutting Pt-Ir wire with a wire cutter.

It should be noted that a tip does not necessarily have to be one perfect point.

STM Probe

Fig03.png
Figure 3. STM probe magnified to an atomic scale.

How does the probe move across the surface?

A simple analogy to describe SPM is to think of a stylus of a turntable scanning across a record, Figure 4. However unlike the stylus in a turntable, the probe in SPM does not make direct contact with the surface.

In STM a voltage is applied between the metallic probe and the sample, typically (0-3 V). When the probe is close to the surface (2-4 Å ) the voltage will result in a current, due to tunneling between the probe and sample. When the probe is far away from the surface, the current is zero. The tunneling current produced is low (pA-nA) but can be monitored using amplifiers. A 3D scanner with an electronic feedback loop is used to raster the probe across the sample to obtain a topographical image and monitor the tunneling current.

Fig05.png
Figure 5. Schematic of STM with a piezoelectric scanner and feedback loop.

Imaging Methods

There are two methods of imaging in STM:

1) Constant Current

A constant tunneling current is maintained during scanning (typically 1 nA). This is done by vertically (z) moving the probe at each (x,y) data point until a “setpoint” current is reached. The vertical position of the probe at each (x,y) data point is stored by the computer to form the topographic image of the sample surface. This method is most common in STM.

Fig08.png
Figure 8. Constant current imaging with STM.

2) Constant Height

In this approach the probe-sample distance is fixed. A variation in tunneling current forms the image. This approaching allows for faster imaging, but only works for flat samples.

Fig09.png
Figure 9. Constant height imaging with STM. (d= fixed probe-sample distance).

WHAT ARE THE LIMITATIONS OF STM?

Although the STM itself does not need vacuum to operate (it works in air as well as under liquids), ultrahigh vacuum (UHV) is required to avoid contamination of the samples from the surrounding medium.

  • Complex and expensive instrumentation - especially (UHV) version
  • Subject to noise (electrical, vibration)
  • Must fabricate probes - dull probes or multiple tips at the end of probe can create serious artifacts
  • Only works for conductive samples: metals, semiconductors
    • samples can be “altered” to be conductive by coating with Au, but this coating can mask/hide certain features or degrade imaging resolution

References

  1. For more detail on tunneling and quantum mechanics you can go here: http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmdet.htm
  1. Bonnell, D. A., Ed. Scanning Probe Microscopy and Spectroscopy: Theory, Techniques, and Applications; Wiley-VCH: New York, 2001.

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