Scanning Tunnelling Microscopy (STM)

Scanning tunnelling microscopy (STM) is an atomic imaging technique invented in 1982 by IBM scientists Gert Binnig and Heinrich Rohrer [52, 145, 146]. The scanning tunnelling microscope has evolved to serve other purposes, for example to probe local electronic structure, manipulate atoms and molecules, and monitor chemical reactions occurring at singular particles or molecules, as well as many others. A schematic of the STM apparatus is shown below.

Figure 44. Basic schematic for a scanning tunnelling microscope.

http://commons.wikimedia.org/wiki/File:ScanningTunnelingMicroscope_schematic.png

A single sharp tip is brought towards the surface of a sample under investigation whilst a potential difference, controlled by a bipotentiostat, is applied between the sample and tip. The sharpness of the tip determines the ultimate resolution of the STM image and atomic resolution is only possible with an atomically sharp tip. (Forming such a tip is not an experimentally difficult process. For example simply cutting a platinum wire with wire cutters may suffice. Another method involves electrochemically etching a tungsten wire in sodium hydroxide solution.)

Initially, coarse control of the tip brings it within a few nanometres of the sample, where quantum tunnelling may occur. At this point the tip is said to have “engaged” the sample. The

process of quantum tunnelling is depicted in figure 45. Here the barrier for traditional electron transfer is given by eVb and is the difference between the higher of the two Fermi levels and

the vacuum level electron energy, i.e the work function. Quantum tunnelling is where electron transfer can occur across the “free space”, db, between the sample and tip, rather than

surmounting this activation energy barrier for electron transfer. This occurs due to overlapping of the sample and tip’s atomic orbitals at short distances and the electron is said to “tunnel” the barrier [52].

The direction of current flow in STM depends on the potential bias applied, if the tip is held at a positive potential relative to the sample, then current flows from the sample to the tip. Current flows the opposite way (tip to sample) if the tip is held negative relative to the sample. Quantum tunnelling is a very short range effect and decays exponentially with distance. Therefore tunnelling current can give very accurate information on the tip-sample distance.

Figure 45. Schematic representation of an electron (e, right) tunnelling through a barrier of "height" (vertical dimension is energy) eVb and thickness (horizontal dimension is spatial) db. e is the elementary charge, -1.6x10-

19 _{C, V is the bias voltage leading to a current flowing between the right and the left "electrode" (side of the} barrier). http://commons.wikimedia.org/wiki/File:TyTunnelling.png

The tunnelling current (I) is described by the following equation.

𝐼 = 𝐶𝑒

Here, C is a constant, W the sample to tip distance and ϕ the work function of the sample. This equation shows that different metals will have different tunnelling currents at the same sample- tip distance due to their unique work function values. This is important in STM as an increase

in tunnelling current may be caused by either encountering a metal in the sample with a lower work function or by a protrusion of atoms on the surface, shortening the tip-sample distance. In STM, the quantum tunnelling effect is exploited in order to gain topographical information on the sample surface by scanning the tip parallel across the sample.

One method of gaining topographical information is by carrying it out in constant current mode. In this mode, as the tip is scanned parallel to the sample surface, its height is free to move perpendicular to the surface (in the z axis) so that a chosen tunnelling current is maintained (see figure 46, left). In this mode, the tip will withdraw from the surface when approaching higher topography and extend towards the surface upon approaching lower topography. An image obtained from STM in constant current mode is that of height vs the x and y axis position.

Figure 46. Comparative schematic of constant current, left, vs. constant height, right, STM. The risk of crashing the STM tip in constant height mode is shown in red. Adapted from images.

http://commons.wikimedia.org/wiki/File:Constant_current.jpg

The shape of the tip formed via the methods described previously has an effect on the topography perceived whilst scanning. Figure 47, shows how the sample may be perceived to have three different topographies when T-shaped (red), V-shaped (green) and U-shaped (blue) tips are scanned across the surface in constant current mode.

Figure 47. Illustrative description of the effect of tip shape on the perceived topography of a sample surface according to STM. http://commons.wikimedia.org/wiki/File:Tipsample.png

The second main mode of STM operation is known as constant height mode. In this mode the position of the tip is locked in the z axis and scanned across the surface (see right, figure 46, right). The increase or decrease in tunnelling current caused by the different sample-tip distance is measured and the readout is of tunnelling current vs the x and y axis position. As the height of the tip is locked, there is no feedback between the tunnelling current and tip position and therefore fast rastering of the tip across the surface is possible. In this mode there is a risk of the tip crashing into the sample upon approaching higher topography, causing damage to the tip and/or the sample.

To gain accurate information on the topography of the sample, the absolute position of the tip must be controlled finely and, for constant current mode, there must be feedback between the tip position and the tunnelling current. This is done by mounting the tip in a piezoelectric scanner; a device which utilizes a material which expands or contracts when a voltage is applied across it, enabling manipulation of the tip’s position on the angstrom scale in the x, y and z axis. In combination with the piezoelectric scanner, a current amplifier is used so that the minute quantum tunnelling current is magnified to a usable level. The amplified current is measured and interpreted by a computer, which then changes the voltages for the x, y and z components of the piezoelectric scanner. This is the basis of the current feedback loop in STM. The above description covers the five basic components of a STM; the metal tip, bipotentiostat, piezoelectric scanner, current amplifier and the hardware/software controlled current feedback loop.