HOPG (Highly Oriented Pyrolytic Graphite)

Highly Oriented Pyrolytic Graphite (HOPG) is a type of pure, highly laminar graphite used as an atomic-scale calibration standard for atomic force microscopy (AFM) and scanning tunnelling microscopy and STM). It has a very smooth, flat surface and good electrical conductivity which make it particularly suitable for STM measurements.

HOPG substrates are particularly favoured due to their easy preparation as their layered structure allows a completely new, clean, smooth and conductive surface to be prepared by removing the topmost layers using sticky tape. HOPG itself is an interesting object for STM investigations. One can measure the surface roughness, microscopic surface features, arrangement of the carbon atoms on the HOPG surface, etc. HOPG images at the atomic level can also be used for calibrating STM for high-resolution imaging. HOPG is also a useful substrate for investigation of other materials which the user wishes to investigate by AFM or STM. The surface consists of many flat areas as well as randomly located steps. Single steps have a well-defined height of 0.34 nm and can be used for calibration.
z direction.

For use in STM the main HOPG parameters are the size of crystallites and the number of interlayer defects, which, in turn, defines the number of layers splitting of the sample. The mosaic distribution of crystallites doesn’t matter, so there is no sense to make measurement of FWHM that entails additional costs for selection and certification of the samples. HOPG can also be used in X-Ray or neutron monochromator applications, in which case the mosaic spread is of primary importance, so each crystal must be certified by FWHM measured on a diffractometer.