The thermodynamic stability of Ge1−xSnx alloys is investigated across the full composition range by employing density functional theory (DFT) in conjunction with the cluster expansion formalism (CE). Configurational, vibrational, and electronic entropy contributions are estimated to allow computation of alloy free energy at finite temperatures. Germanium and tin are found to be immiscible in the bulk up to temperatures approaching germanium's melting point, and much higher than that of tin. Since the main contribution to alloy destabilization is found to be related to the large difference in atomic radii between atomic constituents, the possibility of stabilizing the alloy by reducing segregation through epitaxial constraints in thin films is explored. For germanium-tin alloys, the (001) substrate orientation is preferred for epitaxial growth as it allows for the largest degree of out-of-plane relaxation. Epitaxial films have been simulated by biaxially straining bulk alloy cells as to constrain their lattice spacing to that of substrates lattice-matched to x = 0, and approximately x = 0.5, and x = 1. We conclude that due to the large difference in elastic constants between the components, epitaxial films with high tin content grown on lattice-matched substrates exhibit the greatest stability.

Type of publicationArticle (peer-reviewed) Density functional theory and density functional tight binding are applied to model electron transport in copper nanowires of approximately 1-and 3-nm diameters with varying crystal orientation and surface termination. The copper nanowires studied are found to be metallic irrespective of diameter, crystal orientation, and/or surface termination. Electron transmission is highly dependent on crystal orientation and surface termination. Nanowires oriented along the [110] crystallographic axis consistently exhibit the highest electron transmission while surface oxidized nanowires show significantly reduced electron transmission compared to unterminated nanowires. Transmission per unit area is calculated in each case; for a given crystal orientation we find that this value decreases with diameter for unterminated nanowires but is largely unaffected by diameter in surface oxidized nanowires for the size regime considered. Transmission pathway plots show that transmission is larger at the surface of unterminated nanowires than inside the nanowire and that transmission at the nanowire surface is significantly reduced by surface oxidation. Finally, we present a simple model which explains the transport per unit area dependence on diameter based on transmission pathways results.

Original citationSanchez-Soares, A., O'Donnell, C. and Greer, J. C. (2016) Electronic structure properties of nanowires (NWs) with diameters of 1.5 and 3 nm based on semimetallic α − Sn are investigated by employing density functional theory and perturbative GW methods. We explore the dependence of electron affinity, band structure, and band-gap values with crystallographic orientation, NW crosssectional size, and surface passivants of varying electronegativity. We consider four chemical terminations in our study: methyl (CH 3 ), hydrogen (H), hydroxyl (OH), and fluorine (F). Results suggest a high degree of elasticity of Sn-Sn bonds within the Sn NWs' cores with no significant structural variations for nanowires with different surface passivants. Direct band gaps at Brillouin-zone centers are found for most studied structures with quasiparticle corrected band-gap magnitudes ranging from 0.25 to 3.54 eV in 1.5-nm-diameter structures, indicating an exceptional range of properties for semimetal NWs below the semimetal-to-semiconductor transition. Band-gap variations induced by changes in surface passivants indicate the possibility of realizing semimetal-semiconductor interfaces in NWs with constant cross-section and crystallographic orientation, allowing the design of novel dopant-free NW-based electronic devices.

For semimetal nanowires with diameters on the order of 10 nm, a semimetal-to-semiconductor transition is observed due to quantum confinement effects. Quantum confinement in a semimetal lifts the degeneracy of the conduction and valence bands in a "zero" gap semimetal or shifts energy levels with a "negative" overlap to form conduction and valence bands. For semimetal nanowires with diameters less than 10 nm, the band gap energy can be significantly larger than the thermal energy at room temperature resulting in a new class of semiconductors suitable for nanoelectronics. As a nanowire's diameter is reduced, its surface-to-volume ratio increases rapidly leading to an increased impact of surface chemistry on its electronic structure. Energy level shifts to states in the vicinity of the Fermi energy with varying surface electronegativity are shown to be comparable in magnitude to quantum confinement effects arising in nanowires with diameters of a few nanometer; these two effects can counteract one another leading to semimetallic behavior at nanowire cross sections at which confinement effects would otherwise dominate. Abruptly changing the surface terminating species along the length of a nanowire can lead to an abrupt change in the surface electronegativity. This can result in the formation of a semimetal-semiconductor junction within a monomaterial nanowire without impurity doping nor requiring the formation of a heterojunction. Using density functional theory in tandem with a Green's function approach to determine electronic structure and charge transport, respectively, current rectification is calculated for such a junction. Current rectification ratios of the order of 10-10 are predicted at applied biases as low as 300 mV. It is concluded that rectification can be achieved at essentially molecular length scales with conventional biasing, while rivaling the performance of macroscopic semiconductor diodes.

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