Brad D. Malone

Multiply folded graphene

The folding of paper, hide, and woven fabric has been used for millennia to achieve enhanced articulation, curvature, and visual appeal for intrinsically flat, two-dimensional materials. For graphene, an ideal twodimensional material, folding may transform it to complex shapes with new and distinct properties. Here, we present experimental results that folded structures in graphene, termed grafold, exist, and their formations can be controlled by introducing anisotropic surface curvature during graphene synthesis or transfer processes. Using pseudopotential-density-functional-theory calculations, we also show that double folding modifies the electronic band structure of graphene. Furthermore, we demonstrate the intercalation of C60 into the grafolds. Intercalation or functionalization of the chemically reactive folds further expands grafold’s mechanical, chemical, optical, and electronic diversity.

EPW: A program for calculating the electron-phonon coupling using maximally localized Wannier functions

Abstract EPW ( E lectron– P honon coupling using W annier functions) is a program written in Fortran90 for calculating the electron–phonon coupling in periodic systems using density-functional perturbation theory and maximally localized Wannier functions. EPW can calculate electron–phonon interaction self-energies, electron–phonon spectral functions, and total as well as mode-resolved electron–phonon coupling strengths. The calculation of the electron–phonon coupling requires a very accurate sampling of electron–phonon scattering processes throughout the Brillouin zone, hence reliable calculations can be prohibitively time-consuming. EPW combines the Kohn–Sham electronic eigenstates and the vibrational eigenmodes provided by the Quantum ESPRESSO package (see Giannozzi et al., 2009 [1] ) with the maximally localized Wannier functions provided by the wannier90 package (see Mostofi et al., 2008 [2] ) in order to generate electron–phonon matrix elements on arbitrarily dense Brillouin zone grids using a generalized Fourier interpolation. This feature of EPW leads to fast and accurate calculations of the electron–phonon coupling, and enables the study of the electron–phonon coupling in large and complex systems. Program summary Program title: EPW Catalogue identifier: AEHA_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEHA_v1_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: GNU Public License No. of lines in distributed program, including test data, etc.: 304 443 No. of bytes in distributed program, including test data, etc.: 1 487 466 Distribution format: tar.gz Programming language: Fortran 90 Computer: Any architecture with a Fortran 90 compiler Operating system: Any environment with a Fortran 90 compiler Has the code been vectorized or parallelized?: Yes, optimized for 1 to 64 processors RAM: Heavily system dependent, as small as a few MB Supplementary material: A copy of the “EPW/examples” directory containing the phonon binary files can be downloaded Classification: 7 External routines: MPI, Quantum-ESPRESSO package [1], BLAS, LAPACK, FFTW. (The necessary Blas, Lapack and FFTW routines are included in the Quantum-ESPRESSO package [1].) Nature of problem: The calculation of the electron–phonon coupling from first-principles requires a very accurate sampling of electron–phonon scattering processes throughout the Brillouin zone; hence reliable calculations can be prohibitively timeconsuming. Solution method: EPW makes use of a real-space formulation and combines the Kohn–Sham electronic eigenstates and the vibrational eigenmodes provided by the Quantum-ESPRESSO package with the maximally localized Wannier functions provided by the wannier90 package in order to generate electron–phonon matrix elements on arbitrarily dense Brillouin zone grids using a generalized Fourier interpolation. Running time: Single processor examples typically take 5–10 minutes. References: [1] P. Giannozzi, et al., J. Phys. Condens. Matter 21 (2009), 395502, http://www.quantum-espresso.org/ .

Optoelectronic properties of single-layer, double-layer, and bulk tin sulfide: A theoretical study

SnS is a metal monochalcogenide suitable for use as absorber material in thin film photovoltaic cells. Its structure is an orthorhombic crystal of weakly coupled layers, each layer consisting of strongly bonded Sn-S units. We use first-principles calculations to study model single-layer, double-layer, and bulk structures of SnS in order to elucidate its electronic structure. We find that the optoelectronic properties of the material can vary significantly with respect to the number of layers and the separation between them: the calculated band gap is wider for fewer layers (2.72 eV, 1.57 eV, and 1.07 eV for single-layer, double-layer, and bulk SnS, respectively) and increases with tensile strain along the layer stacking direction (by ∼55 meV/1% strain).

Identifying Structural Flow Defects in Disordered Solids Using Machine-Learning Methods

We use machine-learning methods on local structure to identify flow defects-or particles susceptible to rearrangement-in jammed and glassy systems. We apply this method successfully to two very different systems: a two-dimensional experimental realization of a granular pillar under compression and a Lennard-Jones glass in both two and three dimensions above and below its glass transition temperature. We also identify characteristics of flow defects that differentiate them from the rest of the sample. Our results show it is possible to discern subtle structural features responsible for heterogeneous dynamics observed across a broad range of disordered materials.

Structural stability and electronic properties of low-index surfaces of SnS

Thin film photovoltaic cells are increasingly important for cost-effective solar energy harvesting. Layered SnS is a promising absorber material due to its high optical absorption in the visible and good doping characteristics. We use first-principles calculations based on density functional theory to study structures of low-index surfaces of SnS using stoichiometric and oxygen-containing structural models, in order to elucidate their possible effect on the efficiency of the photovoltaic device. We find that the surface energy is minimized for the surface with orientation parallel to the layer stacking direction. Compared to stoichiometric surfaces, the oxygen-containing surfaces exhibit fewer electronic states near the band gap. This reduction of near-gap surface states by oxygen should reduce recombination losses at grain boundaries and interfaces of the SnS absorber, and should be beneficial to the efficiency of the solar cell.

First principles study of phosphorus and boron substitutional defects in Si-XII

We present a first principles study of boron and phosphorus substitutional defects in Si-XII. Recent results from nanoindentation experiments reveal that the Si-XII phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. Using the hybrid functional of Heyd, Scuseria and Ernzerhof (HSE), we examine the formation energies of the B and P defects at the two distinct atomic sites in Si-XII to find on which site the substitutional defects are more easily accommodated. We also estimate the thermodynamic transition levels of each defect in its relevant charge states. The hybrid calculations also give an independent prediction that Si-XII is semiconducting, in agreement with recent experimental data.