Heat is a familiar form of energy transported from a hot side to a colder side of an object, but not a notion associated with microscopic measurements of electronic properties. A temperature difference within a material causes charge carriers, electrons or holes, to diffuse along the temperature gradient inducing a thermoelectric voltage. Recently, we have demonstrated that local thermoelectric measurements can yield high sensitivity imaging of structural disorder on the atomic and nanometer scales. The thermoelectric measurement acts to amplify the variations in the local density of states at the Fermi-level, giving high differential contrast in thermoelectric signals.

Using this imaging technique, we uncovered the dimensional evolution of disorder in epitaxial graphene, which is the evolution of strain-response patterns with increasing thickness: spot, line, and domain patterns. Compressive strain energy in the graphene layer and the interaction energy between graphene layers are considered responsible for the pattern evolution. Disorder and strain can have large and spatially localized electronic signatures, even for subtle changes in atomic positions. We detect these signatures with an enhanced sensitivity by exploiting thermopower that reflects distortions in the electronic structure, which enabled us to image the structural change due to strain.
Structural disorder appears with point defects in the first layer of epitaxial graphene, which generate dislocations and give rise to soliton-like domain-wall line patterns separating regions of the different interlayer stacking of the second graphene layer. We show that the domain wall is mediated by a topological point defect that is comprised of a pair of carbon pentagon and heptagon, a dislocation core.