Introduction
Highly oriented pyrolytic graphite (HOPG) is a highly pure and ordered form of synthetic graphite. It is characterized by a low mosaic 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. The layers are stacked together via van der Waals interactions and can be exfoliated into thin 2D layers. It is used in x-ray optics as a monochromator and in scanning probe microscopy as a substrate for magnification calibration.
Fig.1. Highly oriented pyrolytic graphite
Properties
The diameters of the individual crystallites in HOPG are typically in the range 1 – 10 μm. Thin films of HOPG afford the opportunity to realize crystal optics with arbitrary geometry by mounting it on a mould of any shape. A specific feature of a HOPG is its mosaicity accompanied by mosaic focusing and high integral reflectivity. Another interesting feature is, due to the mosaic spread of the HOPG crystals, that it is possible also with a beam of low divergence to record a spectrum in a broad energy range even within one laser shot. That means that the HOPG spectrometer can act as a polychromator. The flatness and scarcity of microscopic defects on the HOPG surface as compared to the PG surface is obvious from the STM scans (Fig 2).
Fig.2. STM images of the graphite platforms: (a) Image of HOPG (b) image of PG.
(Microchemical Journal 1999, 61, 247–261)
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.
Applications
X-ray diagnostics and spectroscopy: Due to the characteristic of the mosaicity accompanied by mosaic focusing and high integral reflectivity, HOPG can be used as powerful optics for x-ray diagnostics as well as for x-ray absorption and emission spectroscopy.
Atomic Absorption Spectrometry: HOPG is characterized by a high degree of c-axis alignment, has been employed to study electrothermal atomization. The strong alignment would imply that relatively few active sites and imperfections are present on the surface compared to PCPG and PG, reducing interactions between analytes and the substrate.
Biosensor: HOPG sensors offer the potential for biosensing applications which is related to its electronic properties. HOPG is chemically similar to graphene but is used as a low cost development model for surface functionalization research.
