Scanning Tunnelling Microscope
The STM was invented by Binnig, Rohrer, and their co-workers at IBM Research Laboratory, Zurich, Switzerland, in the early 1980s. The 1986 Nobel Prize in Physics was awarded to Binnig and Rohrer for their design of the STM. An STM provides 3-D atomic-scale images of the surface of a sample. It has a stylus with an extremely sharp tip. The stylus scans the surface of the sample from a fixed distance. It is a powerful tool for viewing surfaces at the atomic level. An STM works on the principle of quantum tunnelling. When an atomically sharpened tip under a small voltage is brought close to the surface of a sample, so that the separation is of the order of a nanometre, there is a small change in current in the circuit. This effect is called the quantum tunnelling effect. The induced current is referred to as the tunnelling current.
This current increases as the gap between the tip and the sample decreases. The change in the tunnelling current can be calibrated with respect to the change in gap. In other words, if we scan the tip over the sample surface while keeping the tunnelling current constant, the tip movement depicts the surface topography, because the separation between the tip apex and the sample surface is always constant. Figure illustrates the working principle of an STM. The resolution obtained in an STM is so high that individual atoms can be resolved when the tip apex is atomically sharp.
|Principle of an STM
An STM requires a very sharp stylus tip and an extremely clean sample surface. It uses a sharp metal wire, usually made of tungsten or Pt–Ir alloy as the probe. The tip is prepared either by a mechanical cutter in case of Pt–Ir tip or by electromechanical etching for tungsten tip. The recent advances have made it possible to have an in situ tip growth with application of high voltage while the tip is being faced towards the sample. Thermal field treatments, a nanopillar growth on a tip by pulling it from a heated sample with a special purpose machine (SPM), and so on, have been proposed. In addition, attaching a carbon nanotube to the tip apex has also attracted much interest.
Shows the components of an STM system. A tip is attached at a corner of a scanner, consisting of three rectangular rods of piezo ceramics [Pb (Zr, Ti) O3 (PZT)] that are crossing perpendicularly. The PZT rod can be elongated by increasing the voltage applied between two electrodes on its opposite longitudinal faces. For example, the rod elongates 1–2 nm per 1 V. To scan the tip faster, either a compact and tube-type piezo scanner or a shear piezo scanner is used.
A tunnelling current less than the order of a nanoampere in magnitude is detected by a current amplifier with a conversion ratio of 107−9VA−1. The output of the current amplifier is fed into an absolute-logarithmic amplifier to linearize the relation between the tunnelling current and the separation between the tip and the sample. Afterwards, a reference value Ire is subtracted from the linearized signal, which is a target value for the STM feedback operation to keep the current constant. Then, the signal is input to the feedback control.
|Components of an STM
A suitable set of gain and time constants is selected to maintain a constant current. Finally, the output from the feedback control is amplified with a high-voltage amplifier having an output range higher than ∼100 V, which is applied to the z-piezo. When the tunnelling current exceeds the target value, the feedback control retracts the tip, and conversely, when the tunnelling current decreases from that value, the control brings the tip closer to the sample. To observe an STM image, X–Y piezos are scanned by changing voltages applied to them in saw-like waveforms that are generated by a computer with digital-to-analog converters (DACs). The signal output from the feedback control is fed into an analog-to-digital converter (ADC) installed in the computer. The STM image, processed from the 3-D data of X–Y–Z voltages applied to the scanner, is displayed on a computer monitor and stored in the computer memory.
An STM requires a vibration-free environment. The instrument is provided with airlegs or a mechanical or gas spring system, and a big steel platform. The tip is suspended from a piezotripod.
The three piezo legs control the tip motion within a fraction of an angstrom. This setup permits the STM to be used under high vacuum and also at low temperatures. The entire set-up needs an environmental control system, including an ultra-high vacuum chamber and pumps to keep the tip and the sample clean, a clean gas purging system, a liquid cell with an electrochemical control, and temperature controls for high- and low-temperature observations.
Applications of STM
An STM is a novel surface imaging microscope with atomic resolution. At present, the applications of an STM are numerous. It is used as a powerful high-resolution surface microscope in materials science for samples with electrical conductivity, and its applications will continue to expand in various other fields as well. An STM is a powerful tool for viewing surfaces at the atomic level. It is versatile as it can be used in ultra-high vacuum, in air and various other liquid or gas ambients, and at temperatures ranging from near 0 K to 1000 °C.
Surface Topography Measurement
A microscope called the topographiner had been developed by a team of scientists, led by R. Young, in the USA in the early 1970s. Young applied a high voltage to a sharpened metal tip and scanned it over the sample surface. Although they succeeded in obtaining surface topography on a nanometre scale, they could not achieve atomic resolution due to the shortcomings of the vibration isolation part of the instrument. On the other hand, Binnig and Rohrer successfully developed a stable vibration isolation stage, which made it possible to use the tunnelling mechanism to achieve the desired results.
Today, an STM is the best choice for plotting surface topography of nanomaterials. As long as the structure of the specimen remains stable during scanning and the specimen is a conductor of electricity, the STM provides a high-resolution image of surface topography. When beginning the STM scanning, it is required to obtain the tunnelling current. This is achieved by bringing the tip closer to the sample (the tip and the sample being separated by a few millimetres) using a coarse positioning system. Several types of coarse positioning systems, consisting of piezo ceramics as a main drive, have been developed. To observe an STM image, X–Y piezos are scanned by changing voltages applied to them in saw-like waveforms that are generated by a computer with DACs. As already pointed out, the signal output from the feedback control is fed into an ADC installed in the computer. The STM image, processed from the 3-D data of x–y–z voltages applied to the scanner, is displayed on a computer monitor.