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Displaying Results 1 - 6 of 6 on page 1 of 1

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A first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps (2014)

Legesse, Merid; Nolan, Michael; Fagas, Gíorgos

A first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps (2014)

Legesse, Merid; Nolan, Michael; Fagas, Gíorgos

Abstract:

In this paper, we use a model of hydrogenated amorphous silicon generated from molecular dynamics with density functional theory calculations to examine how the atomic geometry and the optical and mobility gaps are influenced by mild hydrogen oversaturation. The optical and mobility gaps show a volcano curve as the hydrogen content varies from undersaturation to mild oversaturation, with largest gaps obtained at the saturation hydrogen concentration. At the same time, mid-gap states associated with dangling bonds and strained Si-Si bonds disappear at saturation but reappear at mild oversaturation, which is consistent with the evolution of optical gap. The distribution of Si-Si bond distances provides the key to the change in electronic properties. In the undersaturation regime, the new electronic states in the gap arise from the presence of dangling bonds and strained Si-Si bonds, which are longer than the equilibrium Si-Si distance. Increasing hydrogen concentration up to saturatio...

http://hdl.handle.net/10468/2917

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Doped colloidal InAs nanocrystals in the single ionized dopant limit (2019)

Biaye, Moussa; Amit, Yorai; Gradkowski, Kamil; Turek, Natalia; Godey, Sylvie; Makoudi, ...

Doped colloidal InAs nanocrystals in the single ionized dopant limit (2019)

Biaye, Moussa; Amit, Yorai; Gradkowski, Kamil; Turek, Natalia; Godey, Sylvie; Makoudi, Younes; Deresmes, Dominique; Tadjine, Athmane; Delerue, Christophe; Banin, Uri; Mélin, Thierry

Abstract:

We investigate the electronic properties of individual n-type (Cu) doped and p-type (Ag) doped InAs colloidal nanocrystals (NCs) in the 2–8 nm size range from their charge transfers toward a highly oriented pyrolytic graphite (HOPG) substrate, using ultrahigh vacuum Kelvin probe force microscopy (KPFM) with elementary charge sensitivity at 300 K. The NC active dopant concentration is measured as ND = 8 × 1020 cm–3 and NA > 5 × 1020 cm–3 for n- and p-type doping, respectively. The electrostatic equilibrium between the NC and the HOPG reference substrate is investigated and reveals an enhancement of the Fermi-level mismatch between the NCs and the HOPG substrate at reduced NC sizes, both for n- and p-type doping. It also shows, for n-type doped NCs with smallest sizes (∼2 nm), the existence of a full depletion regime, in which smallest NCs contain single ionized dopants. Results are compared with self-consistent tight-binding calculations of the electronic structure of InAs NCs, in...

http://hdl.handle.net/10468/8402

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Effect of N interstitial complexes on the electronic properties of GaAs1−xNx alloys from first principles (2019)

Querales-Flores, José D.; Ventura, Cecilia I.; Fuhr, Javier D.

Effect of N interstitial complexes on the electronic properties of GaAs1−xNx alloys from first principles (2019)

Querales-Flores, José D.; Ventura, Cecilia I.; Fuhr, Javier D.

Abstract:

Although several approaches have been used in the past to investigate the impact of nitrogen (N) on the electronic structure of GaAs1−xNx alloys, there is no agreement between theory and experiments about the importance of the different N interstitial defects in these alloys, and their nature is still unknown. Here we analyse the impact of five different N defects on the electronic structure of GaAs1−xNx alloys, using density-functional methods: we calculate electronic states, formation energies, and charge transition levels. The studied defects include NAs, AsGa, AsGa-NAs substitutional defects and (N-N)As, (N-As)As split-interstitial complex defects. Our calculated defect formation energies agree with those reported by Zhang et al. [Phys. Rev. Lett. 86, 1789 (2001)], who predicted these defects. Among the interstitial defects, we found that (N-As)As emerges as the lowest energy configuration in comparison with (N-N)As, in agreement with recent experiments [Jen et al., Appl. Phys. ...

http://hdl.handle.net/10468/9047

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First principles static and dynamic calculations for the transition metal hydride series MH4L3 (M = Fe, Ru and Os; L = NH3, PH3 and PF3) (2015)

Sieffert, Nicolas; Kendrick, Thomas; Tiana, Davide; Morrison, Carole A.

First principles static and dynamic calculations for the transition metal hydride series MH4L3 (M = Fe, Ru and Os; L = NH3, PH3 and PF3) (2015)

Sieffert, Nicolas; Kendrick, Thomas; Tiana, Davide; Morrison, Carole A.

Abstract:

We present a first principles static and dynamical study of the transition metal hydride series MH4L3 (M = Fe, Ru and Os; L = NH3, PH3 and PF3), with a view to arriving at an understanding of how the variation in the electronic properties of the metal sites and ligands can influence the dynamics of the resulting complexes. A broad range of behaviour was observed, encompassing stable classical minima (M = Os, L = NH3 and M = Ru, L = PH3) to stable eta(2)-H-2 non-classical minima (M = Fe, L = PF3 and M = Ru, L = PH3 or PF3), with the other structures exhibiting dynamical behaviour that spontaneously converted between the classical and non-classical states during the molecular dynamics simulations. The importance of a small L-axial-M-L-axial angle in stabilising the non-classical state is highlighted, as is a short eta(2)-H-2 center dot center dot center dot H-cis distance in non-classical complexes that spontaneously convert to the classical form. We also investigated the changes in t...

http://hdl.handle.net/10468/6413

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Linear Electronic and Optical Processes in Fullerene Thin Films (2005)

Moghal, Jonathan; Brant, Naomi; Phelan, Siobhan; Byrne, Hugh J.; Chambers, Gordon

Linear Electronic and Optical Processes in Fullerene Thin Films (2005)

Moghal, Jonathan; Brant, Naomi; Phelan, Siobhan; Byrne, Hugh J.; Chambers, Gordon

Abstract:

The electrical properties of C60 have been extensively studied in both the solid and solution phases. The vibrational spectroscopy of C60 is predominantly molecular in character. However electronic spectroscopy reveals features, which are specific to the solid. These features have been attributed to intermolecular charge transfer states. The relative importance of these inter – and intramolecular processes in terms of their contribution to the electronic transport is discussed. Cyclic voltammetry is employed to generate charged molecular species, which also contribute to the conduction process and comparisons to optical excited states species are drawn. The cyclic voltammetry was monitored in situ with vibrational spectroscopy so as to observe any shifts in the C60 spectrum due to charging. The current voltage characteristics of thin film sandwich structures fabricated by vacuum are then presented and discussed. A strongly non-linear behaviour is observed, a sharp increase in the de...

https://arrow.dit.ie/radcon/7

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Reduction mechanisms of the CuO(111) surface through surface oxygen vacancy formation and hydrogen adsorption (2013)

Maimaiti, Yasheng; Nolan, Michael; Elliott, Simon D.

Reduction mechanisms of the CuO(111) surface through surface oxygen vacancy formation and hydrogen adsorption (2013)

Maimaiti, Yasheng; Nolan, Michael; Elliott, Simon D.

Abstract:

We studied the reduction of CuO(111) surface using density functional theory (DFT) with the generalized gradient approximation corrected for on-site Coulomb interactions (GGA + U) and screened hybrid DFT (HSE06 functional). The surface reduction process by oxygen vacancy formation and H2 adsorption on the CuO(111) surface is investigated as two different reduction mechanisms. It is found that both GGA + U and HSE06 predict the same trend in the relative stability of oxygen vacancies. We found that loss of the subsurface oxygen is initially thermodynamically favourable. As the oxygen vacancy concentration increases, mixture of subsurface and surface vacancies is energetically preferred over full reduction of the surface or subsurface monolayer. The reduction of Cu2+ to Cu+ is found to be more favourable than that of Cu+ to Cu0 in the most stable vacancy structures at all concentrations. Consistent with the oxygen vacancy calculations, H2 adsorption occurs initially on under-coordinat...

http://hdl.handle.net/10468/2420

Displaying Results 1 - 6 of 6 on page 1 of 1