Hartwin Peelaers

First-principles investigation of graphene fluoride and graphane

Different stoichiometric configurations of graphane and graphene fluoride are investigated within density functional theory. Their structural and electronic properties are compared, and we indicate the similarities and differences among the various configurations. Large differences between graphane and graphene fluoride are found that are caused by the presence of charges on the fluorine atoms. A new configuration that is more stable than the boat configuration is predicted for graphene fluoride. We also perform GW calculations for the electronic band gap of both graphene derivatives. These band gaps and also the calculated Youngs moduli are at variance with available experimental data. This might indicate that the experimental samples contain a large number of defects or are only partially covered with H or F.

High-voltage field effect transistors with wide-bandgap β-Ga2O3 nanomembranes

Nanoscale semiconductor materials have been extensively investigated as the channel materials of transistors for energy-efficient low-power logic switches to enable scaling to smaller dimensions. On the opposite end of transistor applications is power electronics for which transistors capable of switching very high voltages are necessary. Miniaturization of energy-efficient power switches can enable the integration with various electronic systems and lead to substantial boosts in energy efficiency. Nanotechnology is yet to have an impact in this arena. In this work, it is demonstrated that nanomembranes of the wide-bandgap semiconductor gallium oxide can be used as channels of transistors capable of switching high voltages, and at the same time can be integrated on any platform. The findings mark a step towards using lessons learnt in nanomaterials and nanotechnology to address a challenge that yet remains untouched by the field.

Hydrogenated cation vacancies in semiconducting oxides

Using first-principles calculations we have studied the electronic and structural properties of cation vacancies and their complexes with hydrogen impurities in SnO(2), In(2)O(3) and β-Ga(2)O(3). We find that cation vacancies have high formation energies in SnO(2) and In(2)O(3) even in the most favorable conditions. Their formation energies are significantly lower in β-Ga(2)O(3). Cation vacancies, which are compensating acceptors, strongly interact with H impurities resulting in complexes with low formation energies and large binding energies, stable up to temperatures over 730 °C. Our results indicate that hydrogen has beneficial effects on the conductivity of transparent conducting oxides: it increases the carrier concentration by acting as a donor in the form of isolated interstitials, and by passivating compensating acceptors such as cation vacancies; in addition, it potentially enhances carrier mobility by reducing the charge of negatively charged scattering centers. We have also computed vibrational frequencies associated with the isolated and complexed hydrogen, to aid in the microscopic identification of centers observed by vibrational spectroscopy.

Fundamental limits on optical transparency of transparent conducting oxides: Free-carrier absorption in SnO2

Transparent conducting oxides combine high electrical conductivity with transparency to visible light. However, the large concentration of free electrons introduces a source of absorption that limits the transparency. Here, we evaluate the importance of phonon-assisted free-carrier absorption in SnO2 completely from first principles. Our results show that absorption is modest in the visible and much stronger in the UV and infrared. We also provide insight into the mechanisms that govern absorption in different wavelength regimes.

Phonon Band Structure of Si Nanowires: A Stability Analysis

We present full ab initio calculations of the phonon band structure of thin Si nanowires oriented along the [110] direction. Using these phonon dispersion relations, we investigate the structural stability of these wires. We found that all studied wires were stable also when doped with either B or P, if the unit cell was taken sufficiently large along the wire axis. The evolution of the phonon dispersion relations and of the sound velocities with respect to the wire diameters is discussed. Softening is observed for acoustic modes and hardening for optical phonon modes with increasing wire diameters.

Exciton-dominated dielectric function of atomically thin MoS2 films

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

First-principles study of van der Waals interactions in MoS2 and MoO3.

Van der Waals interactions play an important role in layered materials such as MoS2 and MoO3. Within density functional theory, several methods have been developed to explicitly include van der Waals interactions. We compare the performance of several of these functionals in describing the structural and electronic properties of MoS2 and MoO3. We include functionals based on the local density or generalized gradient approximations, but also based on hybrid functionals. The coupling of the semiempirical Grimme D2 method with the hybrid functional HSE06 is shown to lead to a very good description of both structural and electronic properties.

First-principles study of the mobility ofSrTiO3

We investigate the electronic and vibrational spectra of

Fundamental limits on the electron mobility of β-Ga2O3

{\mathrm{SrTiO}}_{3}

Multilayer transition-metal dichalcogenide channel Thin-Film Transistors

, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of