Mark Lundie

Electronic and optical properties of reduced graphene oxide

Controlled reduction of graphene oxide is an alternative and promising method to tune the electronic and optically active energy gap of this two-dimensional material in the energy range of the visible light spectrum. By means of ab initio calculations, based on hybrid density functional theory, that combine the Hartree–Fock method with the generalized gradient approximation (GGA), we investigated the electronic, optical, and radiative recombination properties of partially reduced graphene oxide, modelled as small islands of pristine graphene formed in an infinite sheet of graphene oxide. We predict that tuning of optically active gaps, in the wide range from ∼6.5 eV to ∼0.25 eV, followed by the electron radiative transition times in the range from ns to μs, can be effected by controlling the level of oxidization.

Ab initio study of structural and electronic properties of partially reduced graphene oxide

Controlled reduction of graphene oxide (GO) is a promising method to tune the electronic band gap of this two-dimensional material in the energy range of the visible light spectrum. By means of ab initio calculations, based on density functional theory at the generalized gradient approximation level, we investigated electronic properties of partially reduced graphene oxide, modelled as periodic array of small islands of pristine graphene embedded in an infinite sheet of GO. The calculations demonstrated that, by varying the size of the graphene islands from two to eight carbon atoms, it was possible to tune the electronic band gap in a range from 4.38 to 1.31 eV, which is of great importance to the utilization of graphene-based materials in photonic devices.

Bistable Helmholtz dark spatial optical solitons in materials with self-defocusing saturable nonlinearity

We present, to the best of our knowledge, the first exact dark spatial solitons of a nonlinear Helmholtz equation with a self-defocusing saturable refractive-index model. These solutions capture oblique (arbitrary-angle) propagation in both the forward and backward directions, and they can also exhibit a bistability characteristic. A detailed derivation is presented, obtained by combining coordinate transformations and direct-integration methods, and the corresponding solutions of paraxial theory are recovered asymptotically as a subset. Simulations examine the robustness of the new Helmholtz solitons, with stationary states emerging from a range of perturbed input beams.

Analysis of energy gap opening in graphene oxide

The utilisation of graphene structures as photonics materials mandates that an optically active electronic energy gap be formed. Opening of a gap in graphene has been demonstrated by functionalisation with H, F, or O atoms, while experimental observations of graphene oxide have hinted at interesting optical properties, with the potential for absorption of visible light. As such, our analysis is focused on O functionalisation of graphene. We present results from extensive ab initio and hybrid DFT calculations, demonstrating the creation of an optically active gap.

Absorption characteristics of reduced graphene oxide: Application to TCO and solar cells active region

The controlled and patterned reduction of graphene oxide offers a promising method to tune the electronic and optical properties of the material through a wide range. Using ab initio calculations in which the exact exchange energy from Hartree-Fock theory is combined with the exchange-correlation energy obtained from density functional theory (DFT), we studied the electronic, optical, and radiative recombination properties of reduced graphene oxide (rGO). Our model of rGO is based on epoxy functionalised graphene, within which small regions of pristine graphene are formed by reduction. We predict that the gap can be tuned from ~6.85 eV to ~0.25 eV in this manner and that the polarization selective absorption properties can be controlled by manipulating the symmetry of these graphene quantum dots. The optically active can therefore be tuned to ranges suitable for use either as the active medium or a transparent conducting oxide (TCO) in photovoltaic solar cells (PVSCs).

Ab initio parameterisation of the 14 band k·p Hamiltonian: Zincblende study

Despite continued and rapid progress in high performance computing, atomistic level device modelling is still largely out of reach, necessitating the use of quantum mechanical continuum methods, including kcenterdotp perturbation theory. The effective use of such methods requires reliable parameterisation, often obtained from experiment and ab initio calculations. A major limitation of this, the systematic tendency of ab initio density functional theory to underestimate semiconducting material energy band gaps and related properties, can be greatly improved upon by the inclusion of exact exchange, calculated within the Hartree-Fock formalism. We demonstrate that the 14 band kcenterdotp Hamiltonian can be effectively parameterised using this method, at greatly reduced cost in comparison to GW

Universality in relativistic aspirations of Helmholtz soliton pulses

Since the heady days of pioneering temporal optical soliton research [1–3], pulse propagation equations have been almost exclusively of the nonlinear Schrödinger form. These parabolic models are derived from Maxwells equations through a careful handling of the linear dispersion operator, and invoking the near-universal slowly-varying envelope approximation (SVEA). A more general approach to pulse evolution, and hence a potentially more accurate one, may clearly be adopted by seeing what progress can be made when the SVEA is relaxed. When one does so, the governing equation is of the Helmholtz (elliptic or hyperbolic) type. While the precedent of using these more sophisticated models was set in the late 1970s [4], they appear to have received relatively little subsequent attention in the literature. A notable exception is the recent paper by Biancalana and Creatore [5], giving Helmholtz-type pulse models a new physical context in the guise of spatial dispersion.