Daniele Selli

Optimizing electronic structure and quantum transport at the graphene-Si(111) Interface: An ab initio density-functional study

We use abxa0initio density-functional calculations to determine the interaction of a graphene monolayer with the Si(111) surface. We find that graphene forms strong bonds to the bare substrate and accommodates the 12% lattice mismatch by forming a wavy structure consisting of free-standing conductive ridges that are connected by ribbon-shaped regions of graphene, which bond covalently to the substrate. We perform quantum transport calculations for different geometries to study changes in the transport properties of graphene introduced by the wavy structure and bonding to the Si substrate. Our results suggest that wavy graphene combines high mobility along the ridges with efficient carrier injection into Si in the contact regions.

Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultralow thermal conductivity

Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultra-low thermal conductivity Shiyun Xiong 1,2 , ∗ Daniele Selli 2 , Sanghamitra Neogi 3 , and Davide Donadio 4† arXiv:1705.03143v1 [cond-mat.mtrl-sci] 9 May 2017 1 Functional Nano and Soft Materials Laboratory (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123 , P.R. China 2 Max Planck Institute for Polymer Research, Ackermannweg 10, 55218 Mainz, Germany 3 Department of Aerospace Engineering Sciences, University of Colorado Boulder, Boulder, Colorado 80309, USA 4 Department of Chemistry, University of California Davis, One Shields Ave. Davis, 95616, CA A detailed understanding of the relation between microscopic structure and phonon propagation at the nanoscale is essential to design materials with desired phononic and thermal properties. Here we uncover a new mechanism of phonon interaction in surface oxidized membranes, i.e., native oxide layers interact with phonons in ultra-thin silicon membranes through local resonances. The local resonances reduce the low frequency phonon group velocities and shorten their mean free path. This effect opens up a new strategy for ultralow thermal conductivity design as it complements the scattering mechanism which scatters higher frequency modes effectively. The combination of native oxide layer and alloying with germanium in concentration as small as 5% reduces the thermal conductivity of silicon membranes to 100 time lower than the bulk. In addition, the resonance mechanism produced by native oxide surface layers is particularly effective for thermal condutivity reduction even at very low temperatures, at which only low frequency modes are populated. Controlling terahertz vibrations and heat transport in nanostructures has a broad impact on several ap- plications, such as thermal management in micro- and nano-electronics, renewable energies harvesting, sensing, biomedical imaging and information and communication technologies [1–8]. Significant efforts have been made to understand and engineer heat transport in nanoscale silicon due to its natural abundance and technological relevance [9–12]. In the past decade researchers ex- plored strategies to obtain silicon based materials with low thermal conductivity (TC) and unaltered electronic transport coefficients, so to achieve high thermoelectric figure of merit and enable silicon-based thermoelectric technology[11–18]. From the earlier studies it was recognized that low- dimensional silicon nanostructures, such as nanowires, thin films and nano membranes feature a largely re- duced TC, up to 50 times lower than that of bulk at room temperature. TC reduction becomes more promi- nent with the reduction of the characteristic dimension of the nanostructures [19–22]. Theory and experiments consistently show that surface disorder and the pres- ence of disordered material at surfaces play a major role in determining the TC of nanostructures [12, 23–25]. However, a comprehensive understanding of the physi- cal mechanisms underlying so large TC reduction is lack- ing. The effect of surface roughness and surface disor- der on phonons has been so far interpreted in terms of phonon scattering [26–30], but scattering would not ac- count for mean free path reduction of long-wavelength low-frequency modes. Recent theoretical work demon- strated that surface nanostructures, such as nanopillars at the surface of thin films or nanowires, can efficiently reduce TC through resonances, a mechanism that is in- trinsically different from scattering [31, 32]. Surface res- onances alter directly phonon dispersion relations by hy- bridizing with propagating modes in the nanostructures, thus hampering their group velocity. In this Communication we unravel the effect of native oxide surface layers on thermal transport in ultra-thin silicon membrane models that closely resemble experi- ments, by atomistic molecular dynamics simulations. We show that the observed low TC in these systems, and plausibly in other oxide coated silicon nanostructures, is predominantly due to resonances analogous to those occurring in nanophononic metamaterials. It is worth stressing that surface oxide layers in low dimensional Si materials grow spontaneously at atmospheric conditions, and do not require any specific processing. Our simu- lations ascertain the occurrence of low frequency reso- nant modes that hybridize with the acoustic branches (ω . 4 THz) of the membrane, effectively suppressing their mean free path (MFP). This discovery opens up the possibility to further optimize the TC of ultra-thin silicon membranes by combining resonances with mass scattering, which affects phonons with higher frequency (ω & 4 THz). We show that alloying the crystalline core of ultra-thin membranes with a small percentage of sub- stitutional germanium atoms brings forth ultra-low TC in silicon membranes with technologically viable thick- ness [33]. All TCs are calculated with equilibrium molecular dy- namics (EMD) simulations at 300 K using LAMMPS [34] with interatomic interactions described by the widely used Tersoff potential [35–37]. The equations of mo- tion are integrated with the velocity Verlet algorithm

Theoretical investigation of the electronic structure and quantum transport in the graphene–C(111) diamond surface system

We investigate the interaction of a graphene monolayer with the C(111) diamond surface using ab initio density functional theory. To accommodate the lattice mismatch between graphene and diamond, the overlayer deforms into a wavy structure that binds strongly to the diamond substrate. The detached ridges of the wavy graphene overlayer behave electronically as free-standing polyacetylene chains with delocalized π electrons, separated by regions containing only sp(3) carbon atoms covalently bonded to the (111) diamond surface. We performed quantum transport calculations for different geometries of the system to study how the buckling of the graphene layer and the associated bonding to the diamond substrate affect the transport properties. The system displays high carrier mobility along the ridges and a wide transport gap in the direction normal to the ridges. These intriguing, strongly anisotropic transport properties qualify the hybrid graphene-diamond system as a viable candidate for electronic nanodevices.

Framework reconstruction between hR8 and cI16 germaniums: A molecular dynamics study

Using molecular dynamics simulations and a Density Functional based Tight Binding method, the metastable germanium allotropes hR8 and cI16 are shown to interconvert by means of two sets of quasi-1D chains. The first set hosts sequences of SN2 inversions of Ge tetrahedral centers, and represents the activated step. The second set does not imply any reconstruction, but assists the first one in propagating the reconstruction. The overall process is commenced by bond nucleation, followed by chain formation and reconstruction into either structure. A novel intermediate metastable phase is visited in the process. Elementary steps of chemical reactivity are accessible due to the appropriate time and spatial resolution of the methods used. This paves the way for a chemical understanding of structure reconstruction and metastable phase formation in solid materials.