Large Anisotropy of the Electrical Conductivity of Graphite

Terahertz generation from graphite

“…As mentioned earlier, a possible underlying mechanism for the generation of THz radiation is a transient charge movement along the c-axis of the crystal. The conductivity of graphite along the c-axis is known to be at least three orders of magnitude smaller than the conductivity along the graphene planes [2][3][4][5]. A preferred charge movement along the c-axis, following an ultrafast excitation of graphite, has not been reported in literature to our knowledge.…”

“…At higher powers, the THz generation increases sub-linearly, indicating the onset of saturation. Graphite, in principle, is a material which possesses a centre of inversion symmetry, which prohibits χ (2) processes from the bulk of the crystal [8]. Based on the fact that graphite is more or less opaque to the pump beam, the generation of THz radiation near the surface must be localized to a thin layer consisting of multiple graphene planes within the penetration depth of the pump beam.…”

“…Different graphene planes in graphite are attached to each other by weak van der Waals forces. Thermal, electrical and optical properties of graphite parallel and perpendicular to these graphene layers are known to be different [2,3]. This peculiar structure gives graphite unique qualities which are widely made use of in its different applications [4,5].…”

Giant enhancement in vertical conductivity of stacked CVD graphene sheets by self-assembled molecular layers

“…4 and Supplementary Note 3). The enhancement along in-plane direction is much smaller than that out-of-plane direction, due to different transport mechanism 11 .…”

“…In conventional LBL wet transfer process, the vertical conductivity of stacked CVD graphene films is significantly lower than that of graphite or dry-stacked mechanically exfoliated graphene films 11 . The reasons for low conductivity in LBLstacked graphene films include the following: first, stacked graphene films trap solvent and moisture, hence the measured thickness of monolayer graphene using atomic force microscopy (AFM) is B0.7-1.0 nm compared with the 0.34 nm of exfoliated graphene 4,12,13 ; second, wrinkles on the films arising from surface tension during wet transfer trap air pockets and prevent good electronic coupling between the graphene layers 3,4,14 ; and third, polymer residues from the transfer process [14][15][16][17] .…”