H. Pinto

Mechanisms of doping graphene

We distinguish three mechanisms of doping graphene. Density functional theory is used to show that electronegative molecules like tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and electropositive metals like K dope graphene p- and n- type, respectively. These dopants are expected to lead to a decrease in carrier mobility arising from Coulomb scattering but without any hysteresis effects. Secondly, a novel doping mechanism is exhibited by Au which dopes bilayer graphene but not single layer. Thirdly, electrochemical doping is effected by redox reactions and can result in p-doping by humid atmospheres and n-doping by NH 3 and toluene.

Theory of the birefringence due to dislocations in single crystal CVD diamond

Single crystal diamond grown by chemical vapour deposition (CVD) often exhibits strain induced birefringence arising from bundles of edge dislocations lying almost parallel to the [001] growth axis. The birefringent pattern changes when the crossed-polarizers are rotated with respect to the underlying lattice. For polarizers parallel to ⟨110⟩ directions, the birefringence pattern consists of four bright petals with dark arms along ⟨110⟩. For polarizers parallel to ⟨100⟩, the birefringence pattern consists of eight petals of weaker intensity with dark arms along ⟨110⟩ and ⟨100⟩ directions. We evaluate the birefringence intensity by using isotropic elasticity theory and find that these patterns can be explained by a specific dislocation arrangement which is consistent with x-ray topographic studies.

Point and extended defects in chemical vapour deposited diamond

Homoepitaxially grown single crystal CVD diamond has a dislocation arrangement not seen in natural IIa diamond. Whereas in the latter, dislocations lie on (111) planes along [10] directions, dislocations in CVD diamond lie nearly along the [001] growth axis and are arranged in bundles with almost four fold rotational symmetry. Their large strain is easily seen in birefringence and we model the arrangements in the bundle to account for the singular behaviour of the birefringence when the polarisers are rotated with respect to the crystal. The optical absorption spectrum of brown CVD diamond displays a continuum similar to that found in brown IIa natural diamonds but in addition broad point defect induced bands. We investigate the suggestion that the 2.38 eV (560 nm) peak is related to VNH. We suggest that the breadth of these bands is related to the large structural change when the defect is excited.