Application of ONETEP to Surfaces, Nanostructures and Crystalline Defects
The combination of the large system sizes achievable by using ONETEP and the high-accuracy obtainable through quantum mechanical simulations have allowed researchers using ONETEP to perform groundbreaking first-principles simulations of surfaces, nanostructures and defects.
Amorphization of Germanium Nanoparticles under Pressure
Finite size effects have significant consequences for the the phase behavior of matter under extreme pressure. This is particularly true for semiconductor nanoparticles such as Germanium, where amorphous phases are readily accessible. In collaboration with experimental researchers at Queen Mary University, Niccolo Corsini performed ONETEP calculations on Ge nanoparticles that have been able to reveal the details of the transformation mechanism into a new high density phase—amorphous metallic Ge.
In a recent publication in Nano Letters we report on the remarkable behavior of small (under ∼5 nm) matrix-free Ge nanoparticles under hydrostatic compression. It is drastically different from both larger nanoparticles and bulk Ge. We show that pressure drives surface-induced amorphization leading to Ge–Ge bond overcompression and eventually to a polyamorphic semiconductor-to-metal transformation.
Acid-base dissociation mechanisms at the silica-water interface
Silanol groups at the silica-water interface determine not only the surface charge, but also have an important role in the binding of ions and biomolecules. As the pH is increased above pH 2, the silica surface develops a net negative charge primarily due to deprotonation of the silanol group. An improved understanding of the energetics and mechanisms of this fundamentally important process should further our understanding of the relevant dynamics. Researchers at the University of Southampton used ab initio molecular dynamics simulations with ONETEP to investigate the mechanisms of surface neutralization and charging in the presence of OH-; and H3O+ respectively. This charging mechanism has received little attention in the literature. The protonation or deprotonation of isolated silanols in the presence of H3O+ or OH-, respectively, was shown to be a highly rapid, exothermic reaction with no significant activation energy. This process occurred via a concerted motion of the protons through ‘water wires’. Geometry optimisations of large water clusters at the silica surface demonstrated proton transfer to the surface occurring via the rarely discussed ’proton holes’ mechanism. This indicates that surface protonation is possible even when the hydronium ion is distant (at least 4 water molecules separation) from the surface
- Deprotonation of silica in water from H3O2- anion - video of ab initio MD simulation
- Protonation of silica in water - video of ab initio MD simulation
- Deprotonation of silica in water from OH- anion - video of ab initio MD simulation
Self-Assembled Quantum Dots in a Nanowire
Quantum Dots in semiconductor nanowires are a promising technology for quantum photonics, with applications in single-photon sources, solid-state lighting, solar cells and nano-sensing. Recent work of David O'Regan and co-workers has involved using ONETEP to model a new class of quantum dot, which self-assembles in core–shell GaAs/AlGaAs nanowires, to help understand the unusual microscopic origins of its excellent optical properties.
This new quantum dot emits in the red, very sharply, is highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unlike conventional quantum dots, however, its emission is blue-shifted relative to the band-gap of the GaAs nanowire core. In this study, ONETEP electronic structure calculations on up to 12168 atoms helped to show that these bright optical transitions originate in quantum confinement due to the self-assembled Al-rich quantum dot barriers.
- Self-assembled quantum dots in a nanowire system for quantum photonics
M. Heiss, Y. Fontana, A. Gustafsson, G. Wüst, C. Magen, D. D. O'Regan, J. W. Luo, B. Ketterer, S. Conesa-Boj, A. V. Kuhlmann, J. Houel, E. Russo-Averchi, J. R. Morante, M. Cantoni, N. Marzari, J. Arbiol, A. Zunger, R. J. Warburton, and A. Fontcuberta i Morral,
Nature Materials doi:10.1038/nmat3557 (2013).
Nanocrystals such as nanorods (nanocrystals with high aspect ratio) are currently of great interest since their optoelectronic properties may be tuned by changing their size and shape. The large system sizes achievable using ONETEP have enabled researchers at Imperial and Southampton to perform simulations on whole nanorods, and have used these to identify the factors that determine how charge is distributed within these systems as well as other electronic properties. We have shown that surface termination plays a key role and have proposed a Fermi level pinning model to explain the variation in dipole moment with size, composition and surface chemistry.
- Linear-Scaling Density Functional Theory Simulations of Polar Semiconductor Nanorods , N. D. M. Hine, P. W. Avraam, P. Tangney, P. D. Haynes, J. Phys. Conf. Ser 367, 012002 (2012).
- Fermi level pinning can determine polarity in semiconductor nanorods, P. W. Avraam, N. D. M. Hine, P. Tangney, P. D. Haynes, Phys. Rev. B 85, 115404 (2012).
- Factors influencing the distribution of charge in polar nanocrystals, P. W. Avraam, N. D. M. Hine, P. Tangney, P. D. Haynes, Phys. Rev. B. 83 241402 (2011).
- Large-scale first principles and tight-binding density functional theory calculations on hydrogen-passivated silicon nanorods, N. Zonias, P. Lagoudakis and C.-K. Skylaris, J. Phys.: Condens. Matter 22, 025303 (2010).
Defects in Al2O3 and Si
The simulation of defects in bulk material presents a challenge for first-principles methods, as the disruption caused by the defect centre on the crystalline lattice can be extremely long-ranged. This requires the use of large system sizes in order to sufficiently approach the dilute limit to accurately calculate fundamental defect properties such as the formation energy.
In their study of the silicon vacancy, Corsetti and Mostofi have shown that ONETEP can be used to simulate point defects in large supercells with a level of accuracy comparable to conventional plane-wave methods. This opens up the possibility of performing routine studies of isolated defects and defect complexes in extremely large crystals and nanostructures.
In earlier work, Hine (with Haynes, Mostofi and Payne) developed new techniques for sparse matrix algebra which allowed fast and efficient treatment of multiplication of matrices which were of very variable sparsity, as are encountered in simulation of a large chunk of a solid with LS-DFT. Their paper in J. Chem. Phys demonstrated convergence of defect formation energies with system size in a strongly-ionic solid (Al2O3).
- The silicon vacancy: insights from large-scale electronic structure calculations and maximally-localized Wannier functions, F. Corsetti, A. A. Mostofi, Phys. Rev. B. 84, 035209 (2011)..
- Linear-scaling density-functional simulations of charged point defects in Al2O3 using hierarchical sparse matrix algebra, N. D. M. Hine, P. D. Haynes, A. A. Mostofi, and M. C. Payne, J. Chem. Phys. 133, 114111 (2010).