Scanning Tunneling Microscopy (STM)
STM is the oldest of the scanning probe microscopes and involves bringing an atomically sharp tip within a couple of nanometres of the sample. When a bias is applied between the tip and sample, electrons will tunnel from the one to the other and this current can be used to measure the topography of the sample. The unique aspect of STM is that is can readily be used to acquire images with atomic resolution which is not generally true for the other SPMs. However, because a current is being measured only conducting samples can be examined and hence this limits the range of possible samples.
Scanning Tunnelling Microscopy (STM) relies on “tunnelling current” between the probe and the sample to sense the topography of the sample. The STM probe, a sharp metal tip (in the best case, atomically sharp), is positioned a few atomic diameters above a conducting sample which is electrically biased with respect to the tip. At a distance under 1 nanometre, a tunnelling current will flow from sample to tip. In operation, the bias voltages typically range from 10 to 1000 mV while the tunnelling currents vary from 0.2 to 10 nA. The tunnelling current changes exponentially with the tip-sample separation, typically decreasing by a factor of two as the separation is increased 0.2 nm. The exponential relationship between the tip separation and the tunnelling current makes the tunnelling current an excellent parameter for sensing the tip-to-sample separation. In essence, a reproduction of the sample surface is produced by scanning the tip over the sample surface and sensing the tunnelling current. The first STM operated in ultrahigh vacuum on cryogenically cooled samples. In the years following its invention in 1981, many variations on the STM theme appeared. P.K. Hansma and J. Tersoff wrote a good review article on the subject, containing experiments, theory, and over 100 references for The Journal of Applied Physics, 61 pp. R1-23, 1987.
STM relies on a precise scanning technique to produce very high-resolution, three dimensional images of sample surfaces. The STM scans the sample surface beneath the tip in a raster pattern while sensing and outputting the tunnelling current to the computer control station. The digital signal processor (DSP) in the workstation controls the Z position of the piezo based on the tunnelling current error signal. The STM can operate in both constant height and constant current modes. The DSP always adjusts the height of the tip based on the tunnelling current signal, but if the feedback gains are low the piezo remains at a nearly constant height while tunnelling current data is collected. This is constant height mode. With the gains high, the piezo height changes to keep the tunnelling current nearly constant, and changes in piezo height are used to construct the image. This is constant current mode. The exponential relationship between tip-sample separation and tunnelling current allows the tip height to be controlled very precisely. For example, if the tunnelling current stays within 20 percent of the set-point value (the current to be maintained by the feedback system), the variation in the tip-sample separation is less than 0.02 nm.