HOPG Crystals

HOPG is obtained by high-temperature annealing under uniaxial pressure of Pyrolytic Carbon. It is a highly anisotropic mosaic crystal with a near ideal structure and a high purity, defined by the manufacturing process.

The angular distribution of the C-axis of the crystallites is called mosaic spread or mosaicity and becomes the only parameter describing the quality (grade) of HOPG. Mosaic spread of commercially available bulk HOPG crystals ranges from 0,4 up to 3,5 degrees.

Grade	Mosaic spread	Nominal thickness	Minimum size	Maximum size
AGraphZ	0.4+/-0.1 degrees	2(-1) mm	10 mm x 10 mm	30 mm x 20 mm
AGraphY	0.8+/-0.2 degrees	2 (-1) mm	10 mm x 10 mm	50 mm x 50 mm
AGraphX	1.7+/-0.4 degrees	2 (-1) mm	10 mm x 10 mm	50 mm x 50 mm
AGraphH	3.5+/-1.5 degrees	2 (-1) mm	10 mm x 10 mm	50 mm x 50 mm
single-bent HOPG R=450, 220, 145 mm	0.6+/-0.2 degrees	2 (-1) mm	10 mm x 10 mm	30 mm x 20 mm

HOPG is a mosaic crystal consisted of crystallites that are very well aligned in the direction perpendicular to the basal plane of graphite crystal and are randomly oriented in the basal plane.

Optigraph GmbH offers HOPG crystals as flat or cylindrically bent plates of different grades. Cylindrically bent bulk HOPG crystals are available only in a few standard radii.

HOPG Monochromatos

HOPG crystals are widely used as X-rays and neutron monochromators.

As mosaic crystals, HOPG exhibits the highest integral reflectivity, which is at least an order of magnitude higher than the reflectivity of all other crystals used for X-rays. Resolution achieved on a crystal can be improved by mosaic focusing.

HOPG monochromators are usually plates of 1-2 mm thick, flat or cylindrically bent.

HOPG monochromators offer moderate resolution (wide energy window), but high flux. It is used for moderate preliminary monochromatization and as broadband filters. Mosaic focusing, which occurs when the crystal is placed equidistant between the source and the detector, narrows the energy bandwidth.

Cylindrical shape of classical (bulk) HOPG monochromator is manufactured, in contrast to Graphite Optics, directly by annealing under pressure on concave/convex dies. Due to high anisotropy of thermal expansion coefficients in HOPG, the final radius of the crystal after cooling differs from the radius of the dies. As the result, only a limited number of proved radii are commercially available. The need for crystals of a wide range of precise shapes led to Graphite Optics based on flexible films.

HOPG monochromators are used not only for X-rays , but also for neutrons. HOPG is a very common filter for removing high-order components from a beam of monochromatic neutrons selected by diffraction on other monochromators. For this application material with mosaic spread of 1.5-3.0 degree are usually used.

Choosing the HOPG of required grade for neutron application, one should take into account that practically all manufacturers of HOPG control mosaic spread using CuKa X-rays, but not neutrons. The crystal volume penetrated by X-rays is much thinner than the one penetrated by neutron, and the mosaic spread measured with neutrons is usually larger than the value obtained by X-ray. The difference strongly depends on the size, thickness and grade of HOPG crystals under consideration, as well as on neutrons energy [Freund A., 2022].

More about Mosaic Focusing

STM Substrates

HOPG crystals are a most widely used substrate for a scanning Probe Microscopy (SPM), namely Scanning Tunnel Microscopy (STM) and Atomic Force Microscopy (AFM).

The atomically flat surface and good electrical conductivity in the basal plane ensure the applicability of HOPG crystal as a STM- substrate. The regular graphite structure serves as a gauge for sizing the objects under study.

HOPG has a layered structure, which simplifies the surface renewal necessary for this application. By pressing a scotch tape to the basal surface of HOPG sample and peeling it off, one easily gets a fresh conductive surface.

STM substrates are usually plates of 10x10mm² or 12x12mm² and 1-2 mm thick.

The quality of HOPG crystals in this application is determined by the size on the grains and crystallites, by the amount of broken C-C planes on the basal surface and the thickness of the layer pilling off during the surface renewal. These properties are defined by defect structure, resulting from the annealing . The mosaic spread of the samples as such does not play a role. The grade of the crystals reflects the annealing conditions (for given technology and manufacturer) and is used to qualify SPM-substrates. Certification of HOPG intended for SPM substrate is carried out by random sampling (one crystal out 10 from a batch prepared at the similar annealing conditions is measured).

AgraphZ substrates

AgraphY substrates

AgraphH substrates

Optigraph mainly offers double-sided samples. The entire sample belongs to the same grade and has a uniform structure. A substrate can be used on both sides.

Historically, there are also one-side samples. Those crystals are cut from the outer part of the annealed plate, which, upon annealing, contacts the dies and undergoes a significant thermal gradient. This leads to a sharp gradient of the crystal properties, and the crystal corresponds to the declared grade only from the side faced to the inner volume. Since the thermal gradient in vicinity of the dies is not very stable, the properties gradient of one-side sample is unpredictable and could not be guarantied. One-side samples are cheaper and may be supplied upon request.

Model objects

HOPG crystals as an approximation of a single graphite crystal .

Designed as an approximation of a single graphite crystal, which is extremely rare in nature in sizes more than one mm, the HOPG crystal remains a model object for more than 50 years in research on carbon materials, for example, the graphite behavior in electrical cells [Bussetti G., 2021].

The real grain structure of HOPG, however, is very complex and strongly depends on the annealing conditions. Due to historical reasons, HOPG is characterized only by a mosaic spread.

Mosaic spread reflects a disorder of the crystallites and grains in the direction perpendicular to the basal plane of HOPG. This characteristic is valid for optical applications and for material produced within the same technology and production line. In the applications sensitive to the defect structure, HOPG crystals with the same mosaicity, especially from different production sources can behave quite different . Sometimes this leads to a mismatch of the results obtained by different researchers.

If various forms of well-aligned PG are chosen as model objects, then a significant difference in production conditions and, consequently, in the defect structure between HOPG, HOPG-flex and HAPG allows a better understanding of the processes under study.