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HOPG
Highly Oriented Pyrolytic Graphite is a material that consists of many atomic layers of carbon highly oriented among each other. This property makes HOPG the excellent tool for the STM calibration.
HOPG is manufactured at the temperature of 3273K. It was tested that HOPG does not outgas at the temperatures up to 600°C and remains stable at the temperatures up to 2000°C in the inert environment. The parallelism of atomic layers is characterized by "mosaic spread angle". The less this angle the higher the quality of HOPG is.
From the viewpoint of crystallography, graphite belongs to lamellar materials and consists of identical staked planes (Fig. 1.). Carbon atoms within a single plane interact much stronger than with those from adjacent planes. That explains lamellar behavior of graphite. Each atom within a single plane has three nearest neighbours. Network of carbon atoms connected by shortest bonds looks like honeycomb. This two-dimensional and single-atom thick plane is called "graphene".
Fig. 1. Positional relationship between two identical graphene planes A and B.
Graphite structure can be described as an alternite succession of these planes ...ABABAB...
"Usual" graphite, especially natural one, exhibits quite imperfect structure due to plenty of defects and inclusions. A number of technologies are developed for preparation of perfect graphite samples to take advantage of its unique structure. Of these, pyrolysis of organic compounds is the most common and effective.
Some physical properties of graphite are listed below:
Density: 2,266 g/cm3
Thermal conductivities:
along C axis (0001): 8±2 Watt/mK
along surface plane: 1800±200 Watt/mK
HOPG as a perfect substrate for Scanning Probe Microscopy
HOPG terminated with graphene layer can serve as an ideal atomically flat surface to be used as a substrate or standard for SPM investigations (Fig. 2).
Fig. 2. Typical STM image of HOPG surface.
Corresponding fragment of graphene structure is superimposed.
HOPG grades
To characterize perfectness of HOPG, "mosaic spread" term is used. The term originates from X-ray crystallography. Actually, HOPG specimens are polycrystals. Each bulk polycrystal looks like mosaic of microscopic monocrystal grains slightly disoriented with respect to each other. This disordering is responsible for diffracted peak width: the more disordering, the wider the peak. Therefore, perfectness of HOPG can be easily related to a full width of the Cu-Ka radiation peak at half maximum (FWHM) measured in degrees.
There are several grades of HOPG. We use notations developed by Advanced Ceramics Corporation, the leading company in HOPG production.
ZYA Grades: 0.4°±0.1° Mosaic Spread. This is the most perfect HOPG, lateral grain size is typically up to about 10 µm.
Thus, it is the best "cleavable" material exhibiting the smoothest surface that is needed for crucial SPM measurements.ZYB Grades: 0.8°±0.2° Mosaic Spread. Slightly less highly ordered. The lateral grain size is up to 1 µm.
ZYH Grades: 3.5°±1.5° Mosaic Spread. It has a grain size in the range of 30-40 nm.
