HOPG

"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. Pyrolytic graphite is a graphite material with a high degree of preferred crystallographic orientation of the c-axes perpendicular to the surface of the substrate, obtained by graphitization heat treatment of pyrolytic carbon or by chemical vapor deposition at temperatures above 2500°K. Hot working of pyrolytic graphite by annealing under compressive stress at approximately 3300°K results in highly oriented pyrolytic graphite (HOPG). Thus HOPG is a highly-ordered form of high-purity pyrolytic graphite (impurity level is of the order of 10 ppm ash or better).

HOPG is characterized by the highest degree of three-dimensional ordering. The density, parameters of the crystal lattice, preferable orientation in a plane (0001) and anisotropy of the physical properties of the HOPG are close to those for natural graphite mineral. In particular, like mica, HOPG belongs to lamellar materials because its crystal structure is characterized by an arrangement of carbon atoms in stacked parallel layers – the two-dimensional and single-atom thick form of carbon that is called graphene. Graphite structure can be described as an alternate succession of these identical staked planes. Carbon atoms within a single plane interact much stronger than with those from adjacent planes. That explains characteristic cleaving behavior of graphite. Graphene – planar, hexagonal arrangement of carbon atoms. The lattice of graphene consists of two equivalent interpenetrating triangular carbon sublattices A and B, each one contains a half of the carbon atoms. Each atom within a single plane has three nearest neighbors: the sites of one sublattice (A – marked by red) are at the centers of triangles defined by tree nearest neighbors of the other one (B – marked by blue). The lattice of graphene thus has two carbon atoms, designated A and B, per unit cell, and is invariant under 120° rotation around any lattice site. Network of carbon atoms connected by the shortest bonds looks like honeycomb. But in bulk HOPG, even in bilayer graphene, A- and B-sites C atoms become inequivalent (including those on the surface): two coupled hexagonal lattices on the neighbor graphene sheets are arranged according to Bernal ABAB stacking, when every A-type atom in the upper (surface) layer is located directly above an A-type atom in the adjacent lower layer, whereas B-type atoms do not lie directly below or above an atom in the other layer, but sit over a void – a center of a hexagon. Figure illustrates the assumed non-equivalent types of carbon atoms. Thus in each layer the atoms form a grid of correct hexagons with distances between atoms equal 0.1415 nm. The distance between layers is equal 0.3354 nm that gives a theoretical value of density ρ = 2.265 g/cm³.

HOPG terminated with graphene layer is an excellent tool for using in scanning probe microscopy as a substrate or calibration standard at atomic levels of resolution. This is an easily renewable material with an extremely smooth surface. It has an ideal atomically flat surface and provides a background with only carbon in the elemental signature thus making results in a featureless background. This is vital for SPM measurements that require uniform, flat, and clean substrates, for samples where elemental analysis is to be done.

In an atomic resolution SPM image of HOPG, there are two possible images. Under ideal conditions, SPM images of HOPG surface reveal a lattice of dark spots with a lattice parameter of 0.246 nm. The six C atoms composed in hexagonal ring surrounding each spot give a bright signal, which leads to a true honeycomb atomic pattern (symmetric contrast). The center to center atomic distance is 0.1415 nm.

The atomic pattern normally observed in most SPM images of graphite under usual conditions shows an asymmetric positive contrast: bright spots originate from only three C atoms out of each set of six from a graphene hexagon unit cell of the graphite lattice. Each apparent atom is surrounded by six nearest neighbors. The distance between any two of these atoms is 0.246 nm. This contrast is commonly reported for HOPG surfaces. The characteristic features of the SPM images are readily interpreted in terms of the A–B stacking in graphite: the asymmetry in the surface atom environment results in a threefold symmetry (‘‘three-for-six’’) pattern.

Fig. Schematic representation of the structure of the bulk hexagonal graphite crystal. The dashed lines show the axes of bulk unit cell. Side insets: top view of the basal plane of graphite and schematic representation of the surface structure (carbon atoms) of graphite most viewed by SPM, where every other atom is enhanced (right-side inset) and viewed under ideal conditions, where every single atom is seen (left-side inset).

Actually, similar to mica, real HOPG specimens are layered polycrystals. Each bulk polycrystal looks like mosaic of microscopic monocrystal grains of different sizes. The structure is columnar, the columns run vertically within the flat slab of the material, and the grain boundaries can be seen on the lateral surfaces. The grains are slightly disoriented with respect to each other. An angular spread of the c-axes of the crystallites is of the order of 1 degree. The surface of specimen consists of many randomly placed steps – result of the cleaving process: single atomic steps and steps of several or dozens of atomic layers. Although the heights of multilayer hills and valleys are not calibrated, single steps have the well defined height of 0.34 nm and can be used for calibration in z direction. To characterize the angle of deviation of the grain's boundaries from the perpendicular axis of the columnar structure, a measure of the parallelism of grains – perfectness of HOPG samples, a "mosaic spread" term is used. The lower the mosaic spread, the more highly ordered the HOPG. The term originates from X-ray crystallography. The disordering results in broadening of the (002) diffraction peak: the more disordering, the wider the peak. Therefore, perfectness of HOPG can be easily related to a Full Width at Half Maximum (FWHM) of the Cu-K_a rocking curve (radiation peak) measured in degrees – "mosaic spread angle". Thus, the less this angle, the higher the quality of HOPG. The size of grains also varies with the mosaic spread. The lower mosaic spread results in a freshly cleaved surface that exhibits the smaller number of the steps due to the bigger size of grains. The higher the quality, the less the roughness of the surface. The lower level grade material is also more "cleavable" allowing the bigger number of cleavings per sample.

All the other physical characteristics of graphite, including atom-to-atom distance, that is an atomic property of carbon, are independent of its grade and remain the same for all types of HOPG. Due to the anisotropic nature of HOPG such characteristics as thermal conductivity and electrical resistivity are different in different directions: along the basal plane and along the principal axis c (perpendicular to the basal plane).

HOPG is a high-stable material. It remains stable at the temperatures up to 500°C in air and up to two-three thousand Celsius degrees in the vacuum or inert environment. It also exhibits high chemical inertness to just about everything.

Physical Characteristics of HOPG (@ 300°K)