Nanodiamonds containing negatively charged nitrogen vacancy (NV−) centres are highly promising biolabels due to NV− photostability and spectral range. For effective cell tracking we require NV− to be stable in this charge state, but it is known that nanodiamond surfaces may alter the NV charge state; intermittent fluorescing and conversion to NV0 are frequently observed. Different models have been proposed linking surface termination type to the resultant NV charge state, but a full understanding has not yet been reached. For the work presented in this paper, we use density functional theory to examine how the NV electronic structure in nanodiamond clusters changes with different surfaces. We move beyond examining fully terminated surfaces and focus on the role of surface radicals, based on recent research showing pH also affects NV charge. Our work shows that surface radicals can explain the intermittent fluorescence observed for hydrogenated surfaces, and that different absorbates on the surface influence the resultant NV charge for a specified termination type. We have found that both the termination type, and the absorbates on the surface, play important roles in determining NV charge and should be considered together when predicting surface coverage that will stably produce NV−.

The neutral silicon-vacancy complex in diamond is of interest for quantum applications due to its favourable optical properties relative to both its negative charge state and the nitrogen-vacancy centre. To establish an uncharged form, co-doping with electrically active impurities has been suggested, and although complexes with hydrogen or nitrogen have been identified, complexes with boron are largely unstudied. This report presents results from density-functional modelling of SiB, SiVB and SiVBH complexes, some of which are expected to produce highly characteristic magnetic signatures. Critically for the neutral silicon-vacancy complex, we find that boron binds less strongly than nitrogen or hydrogen.

A quantum-chemical study of the positive charge-state of the nitrogen-vacancy center in diamond is presented. Charge control of this promising qubit candidate is a focus of diamond quantum technology research, as currently charge stability relating to surfaces and nearby defects causes some difficulties for quantum applications. To demonstrate full charge control over the nitrogen vacancy, all three charge states should be identified and the processes that lead to charge state changes understood. However, experimental markers for the positive state remain elusive compared to the readily detectable zero-phonon lines of the neutral and negative. In this work we present predicted hyperfine and zero-field splitting tensors as clear signatures of the normally spinless NV+ ((1)A(1) ground state) by probing a long-lived spin-triplet excited-state similar to 0.7 eV above the NV+ ground state. We find a relatively narrow excitation energy range of approximately 0.7-1.1 eV between excitation into an E-3 state and conversion into the neutral charge state. To provide insight into the thermal stability of the positive charge state, we have calculated binding and diffusion energies for both charged and uncharged systems. We predict, given that the activation energies are only weakly charge state dependent, all three charge states would diffuse in the 1600-1900 degrees C range, but the positive state has a significantly lower binding energy, suggesting that it will dissociate at temperatures around 1000 degrees C rather than migrate.

Experimental data indicate that carbon vacancies incorporated in active regions of SiC devices are important electrical defects, responsible for device limiting effects such as carrier lifetime reduction. For field-effect transistors that include a 4H-SiC/SiO2 interface, such as at the gate, the oxidation pro- cess is understood to introduce native defects to the SiC, including injection of carbon self-interstitials and vacancies, that diffuse into the active layer and interact with other defects and impurities. It is therefore important to understand the migration behaviour of primary native defects such as VC in the vicinity of 4H-SiC/SiO2 interfaces. We report here the results of a density-functional theory investi- gation into the diffusion of the carbon vacancy in such a region. We conclude that the migration of VC is significantly hindered in the immediate vicinity of the interface, with the energy of diffusion barrier being approximately 15% greater than the corresponding diffusion in bulk 4H-SiC.

Aiming to better understand the reactivity of graphene-based materials, the present work employs density functional theory that provides detailed information about spin-density distributions for single and contiguous pairs of carbene-like active sites. In order to examine the extent to which different models, methodologies, and approximations affect the outcome, our calculations employ the AIMPRO, QuantumEspresso and Gaussian program packages. Models are in the form of polycyclic aromatic hydrocarbons (PAHs) and graphene nanoribbons (GNRs), both isolated and within supercells with periodic boundary conditions. Benchmarking calculations for the phenyl radical and cation are also presented. General agreement is found among the methods and also with previous studies. A significant electron spin polarization (spin density >1.096 electron spin) on the active sites is seen in both periodic and cluster systems, but it tends to be lower for GNRs than graphene clusters. The effect of the functional seems to be much more important than the position of singularities at the edges of the GNRs. Finally, we show the interactions and effects on spin density when a single site lies at the edge of a bilayer GNR, where bonding between layers may occur under specific circumstances.

In recent years, a plethora of theoretical carbon allotropes have been proposed, none of which has been experimentally isolated. We discuss here criteria that should be met for a new phase to be potentially experimentally viable. We take as examples Haeckelites, 2D networks of sp(2)-carbon-containing pentagons and heptagons, and "penta-graphene," consisting of a layer of pentagons constructed from a mixture of sp(2)- and sp(3)-coordinated carbon atoms. In 2D projection appearing as the "Cairo pattern," penta-graphene is elegant and aesthetically pleasing. However, we dispute the author's claims of its potential stability and experimental relevance.

The formation of extended defects in graphene from the coalescence of individual mobile vacancies can significantly alter its mechanical, electrical and chemical properties. We present the results of ab initio simulations which demonstrate that the strain created by multi-vacancy complexes in graphene determine their overall growth morphology when formed from the coalescence of individual mobile lattice vacancies. Using density functional theory, we map out the potential energy surface for the motion of mono-vacancies in the vicinity of multi-vacancy defects. The inhomogeneous bond strain created by the multi-vacancy complexes strongly biases the activation energy barriers for single vacancy motion over a wide area. Kinetic Monte Carlo simulations based on rates from ab initio derived activation energies are performed to investigate the dynamical evolution of single vacancies in these strain fields. The resultant coalescence processes reveal that the dominant morphology of multi-vacancy complexes will consist of vacancy lines running in the two primary crystallographic directions, and that more thermodynamically stable structures, such as holes, are kinetically inaccessible from mono-vacancy aggregation alone.

The future exploitation of the exceptional properties of nanocrystal (NC) thin films deposited from liquid dispersions of nanoparticles relies upon our ability to produce films with improved electrical properties by simple and inexpensive means. Here, we demonstrate that the electronic conduction of solution-processed NC films can be strongly enhanced without the need of postdeposition treatments, via specific molecules adsorbed at the surfaces of adjacent NCs. This effect is demonstrated for Si NC films doped with the strong molecular oxidizing agent tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Density functional calculations were carried out with molecule-doped superlattice solid models. It is shown that, when populated by electrons, hybrid molecule/NC states edge (and may actually resonate with) the conduction-band states of the NC solid. This provides extra electronic connectivity across the NC network as the molecules effectively flatten the electronic potential barriers for electron transfer across the otherwise vacuum-filled network interstitialcies.

First-principles calculations are used to investigate the structure, electronic and optical properties of silicon nanocystals with chlorine-passivated surface. The nanocrystals considered were approximately spherical, with diameters between 1.5 and 3.0 nm. We show that the nanocrystals with chlorinated surface have a smaller bandgap, lower optical absorption threshold, and greater ionization energy and electron affinity than hydrogenated silicon nanocrystals of the same size.

The modification of the electronic structure of silicon nanocrystals using an organic dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), is investigated using first-principles calculations. It is shown that physisorbed F4-TCNQ molecules have the effect of oxidizing the nanocrystal, attracting the charge density towards the F4-TCNQ-nanocrystal interface, and decreasing the excitation energy of the system. In periodic F4-TCNQ/nanocrystal superlattices, F4-TCNQ is suggested to enhance exciton separation, and in the presence of free holes, to serve as a bridge for electron/hole transfer between adjacent nanocrystals.

Diamond surfaces with suitable adsorbed chemical species can exhibit both negative and positive electron affinities, arising from the complex electrostatic interplay between adsorbates and surface carbon atoms of diamond lattice. We presents the results of density functional calculations into the energetics and the electron affinity of diamond (100) surfaces terminated with the oxides of selected transition metals. We find that for a correct stoichiometry, oxides of transition metals, such as Ti and Zn, exhibit a large negative electron affinity of around 3 eV. The desorption of transition metal oxides is found to be highly endothermic. We therefore propose that transition metal oxides are promising for the surface coating of diamond-based electron emitters, as these exhibit higher thermal stability in comparison to the commonly used CsO terminations, while retaining the advantage of inducing a large negative electron affinity.

The preferred location of boron in oxidized free-standing Si nanoparticles was investigated using a first-principles density functional approach. The nanoparticles were modeled by a silicon core about 1.5nm in diameter surrounded by an outer shell of SiO2 with a thickness of about 0.5nm, and considered negatively charged. The calculated formation energies indicate that B is equally stable in the Si core and in the SiO2 shell, showing preference for interface sites. This indicates that, in contrast with phosphorus, the ratio of the boron concentration in the silicon core to that of the silicon shell will not be improved over one upon thermal annealing. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS2) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown.

Effects of the ethylene carbonate (EC) solvent on Li insertion and diffusion in Si anodes are studied using density functional theory. On both (1 0 0) and (1 1 1) reconstructed surfaces of Si, a semi-dissociated (SD) configuration of EC is stable and most favorable for Li insertion, lowering its barrier by up to 0.2 eV vs a clean surface. The less stable molecular adsorption has little effect on Li insertion and diffusion, while the surface ketone formed by dissociating the SD configuration at a cost of 0.6 eV has a strong detrimental effect on Li insertion, increasing its barrier by up to 0.4 eV.

We analyse the formation energy of interstitial boron (Bi) and the properties of the defect resulting from its association with an oxygen dimer (BiO2i) to evaluate the possibility that it may be the slow-forming centre responsible for the light-induced degradation of B-doped Si solar cells. However, we find that the formation energy of Bi is too high, and therefore its concentration is negligible. Moreover, we find that the lowest energy form of BiO2i is a shallow donor, and the deep donor form is high in energy. Lowest energy structure of the BiO2i defect.

The preferred location of boron and phosphorus in oxidized free-standing Si nanoparticles was investigated using a first-principles density functional approach. The calculated formation energies indicate that P should segregate to the silicon core, whereas B is equally stable in the Si and SiO_2 regions. Our models thus suggest that, in contrast with nanocrystals with H-terminated surfaces, the efficiency of phosphorus incorporation in oxidized Si nanoparticles can be improved by thermal annealing.

Substitutional group III and group V elements, though commonly used as shallow dopants in bulk silicon, have a limited efficiency in silicon nanocrystals. In this work, we use first principles models of 1.5 nm ninocrystals with hydride- and silanol-terminated surfaces to understand how oxidation influences the 7 segregation and deactivation of dopants at the surface and the dopant binding energies. We show that the surface oxygen layer changes drastically the radial dependence of the dopant formation energy both for donors and for acceptors, but that, independently from the oxidation, dopant diffusion does not take place at operating conditions. Additionally, we show that the oxidation increases the electron binding energy of the P, As, and Sb and decreases the hole binding energy of B, Al, Ga, and In.

We have investigated, using density functional simulations, the energetics and the electronic properties of oxides of selected transition metals, TMs, adsorbed onto a dia-mond (001) surface. We find that stoichiometric oxides of TMs, particularly Ti and Zn, influence the electron affinity of diamond strongly. The electron affinities of stoichiomet-ric oxides of Ti and Zn are calculated to be around −3 eV, significantly higher than 1.9 eV of commonly used H–termination. The reactions of TMs with an oxygenated diamond are found to be highly exothermic. Based upon the energetics and the electronic properties, we propose that in the regime of ultra thin films, oxides of TMs are promising options for surface coating of diamond–based electron emitters, as these coatings are compatible with semiconductor device fabrication processes, while having the benefit of inducing a large negative electron affinity.

We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nanotubes. This results in lower formation energies than any other nonfunctionalized edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighboring bulk graphene by a chain of insulating sp(3)-carbon atoms, and introduce van Hove singularities into the graphene density of states.