Highly oriented pyrolytic graphite (HOPG) is a highly pure and ordered form of synthetic
graphite
Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
. It is characterised by a low
mosaic
A mosaic () is a pattern or image made of small regular or irregular pieces of colored stone, glass or ceramic, held in place by plaster/Mortar (masonry), mortar, and covering a surface. Mosaics are often used as floor and wall decoration, and ...
spread angle, meaning that the individual graphite crystallites are well aligned with each other. The best HOPG samples have mosaic spreads of less than 1 degree.
Note that the term "highly ''ordered'' pyrolytic graphite" is sometimes used for this material, but IUPAC favors "highly ''oriented''".
Synthesis
The method used to produce HOPG is based on the process used to make
pyrolytic graphite, but with additional tensile stress in the basal-plane direction. This produces improved alignment of the graphite crystallites and an interplanar spacing close to that observed in natural graphite. The "stress recrystallization" of graphite was first described by L. C. F. Blackman and
Alfred Ubbelohde in 1962.
The diameters of the individual crystallites in HOPG are typically in the range 1–10 μm.
Application
HOPG is used in
x-ray optics as a monochromator and in
scanning probe microscopy
Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. SPM was founded in 1981, with the invention of the scanning tunneling microscope, an instrument for imaging ...
as a substrate and for magnification calibration.
References
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Allotropes of carbon