Guido Petretto

FireWorks: a dynamic workflow system designed for high-throughput applications

This paper introduces FireWorks, a workflow software for running high‐throughput calculation workflows at supercomputing centers. FireWorks has been used to complete over 50 million CPU‐hours worth of computational chemistry and materials science calculations at the National Energy Research Supercomputing Center. It has been designed to serve the demanding high‐throughput computing needs of these applications, with extensive support for (i) concurrent execution through job packing, (ii) failure detection and correction, (iii) provenance and reporting for long‐running projects, (iv) automated duplicate detection, and (v) dynamic workflows (i.e., modifying the workflow graph during runtime). We have found that these features are highly relevant to enabling modern data‐driven and high‐throughput science applications, and we discuss our implementation strategy that rests on Python and NoSQL databases (MongoDB). Finally, we present performance data and limitations of our approach along with planned future work. Copyright

Adiabatic charge control in a single donor atom transistor

We charge an individual donor quantum dot with an electron originally stored in another quantum dot in its proximity. The single arsenic donor quantum dot and the electrostatic quantum dot in parallel are contained in a silicon nanometric field effect transistor. Their different coupling capacitances with the control and back gates determine a honeycomb pattern at high control gate voltage. It is therefore possible to control the exchange coupling of an electron of the quantum dot with the electrons bound to the donor quantum dot toward the realization of a physical qubit for quantum information processing applications.

Oxygen evolution reaction electrocatalysis on SrIrO3 grown using molecular beam epitaxy

Electrochemical generation of oxygen via the oxygen evolution reaction (OER) is a key enabling step for many air-breathing electrochemical energy storage devices. IrO2 (Ir4+: 5d5) ranks among the most active known OER catalysts. However, it is unclear how the environment of the Ir4+ oxygen-coordination octahedra affects the OER electrocatalysis. Herein, we present the OER kinetics on a single-crystal, epitaxial SrIrO3(100)p perovskite oxide synthesized using molecular-beam epitaxy on a DyScO3(110) substrate. We find that by switching the host structure of the Ir4+ oxygen-coordination octahedra from corner- and edge-sharing rutile (IrO2) to purely corner-sharing perovskite (SrIrO3), the OER activity increases by more than an order of magnitude. We explain our finding with the correlated, semimetal electronic structure of SrIrO3; our density functional theory calculations reveal that the adsorption energetics on SrIrO3 depends sensitively on the electron–electron interaction, whereas for IrO2, it depends rather weakly. This finding suggests the importance of correlations on the OER and the design of future transition metal oxide electrocatalysts.

A comprehensive ab initio study of doping in bulk ZnO with group V elements

Despite the lack of reproducible experimental confirmation, group-V elements have been considered as possible sources of p-type doping in ZnO in the form of simple and complex defects. Using ab initio calculations, based on state-of-the-art hybrid exchange-correlation functional, we study a wide range of defects and defect complexes related with N, P, As, and Sb impurities. We show that none of the candidates for p-type doping can be considered a good source of holes in the valence band due to deep acceptor levels and low formation energies of compensating donor defects. In addition, we discuss the stability of complexes in different regimes.

Switching quantum transport in a three donors silicon fin-field effect transistor

We switch the transport along different paths in a system constituted by a phosphorus donor in a silicon quantum dot in complementary metal-oxide-semiconductor technology, coupled with two donors at the source side. The standard Coulomb blockade pattern created by the transport through the D0 neutral state of an individual donor located in the channel of the device is modified by two additional randomly diffused nearby donors. By varying the control voltages, the fin-field effect transistor acts as a quantum device which may be used to control alternative current paths through different donors.

Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides

We investigate the chemical bonding characteristics of the transition metal dichalcogenides based on their static and dynamical atomic charges within Density Functional Theory. The dynamical charges of the trigonal transition metal dichalcogenides are anomalously large, while in their hexagonal counterparts, their sign is even counterintuitive i.e. the transition metal takes the negative charge. This phenomenon cannot be understood simply in terms of a change in the static atomic charge as it results from a local change of polarization. We present our theoretical understanding of these phenomena based on the perturbative response of the system to a static electric field and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, we establish a link between the sign of the Born effective charge and the

Confinement effects and hyperfine structure in se doped silicon nanowires.

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Metal phosphides as potential thermoelectric materials

-backbonding in organic chemistry and propose an experimental procedure to verify the calculated sign of the dynamical charge in the transition metal dichalcogenides.

Donor wave functions delocalization in silicon nanowires: the peculiar [011] orientation.

We report a density functional study of the electronic properties and hyperfine structure of substitutional selenium in silicon nanowires using plane-wave pseudopotential techniques. We simulated hydrogen passivated [001] oriented nanowires with a diameter up to 2 nm, analyzing the effect of quantum confinement on the defect formation energy and on the hyperfine parameters as a function of the diameter and of the defect position. We show that substitutional Se in silicon has favorable configurations for positions near the surface with possible formation of chalcogen-hydrogen complexes. We also show that hyperfine interactions increase at small diameters, as long as the nanowire is large enough to prevent surface distortion which modifies the symmetry of the donor wave function. Moreover, surface effects lead to strong differences in the hyperfine parameters depending on the Se location inside the nanowire, allowing the identification of an impurity site on the basis of electron paramagnetic resonance spectra.

High-throughput density-functional perturbation theory phonons for inorganic materials

There still exists a crucial need for new thermoelectric materials to efficiently recover waste heat as electrical energy. Although metal phosphides are stable and can exhibit excellent electronic properties, they have traditionally been overlooked as thermoelectrics due to expectations of displaying high thermal conductivity. Based on high-throughput computational screening of the electronic properties of over 48 000 inorganic compounds, we find that several metal phosphides offer considerable promise as thermoelectric materials, with excellent potential electronic properties (e.g. due to multiple valley degeneracy). In addition to the electronic band structure, the phonon dispersion curves of various metal phosphides were computed indicating low-frequency acoustic modes that could lead to low thermal conductivity. Several metal phosphides exhibit promising thermoelectric properties. The computed electronic and thermal properties were compared to experiments to test the reliability of the calculations indicating that the predicted thermoelectric properties are semi-quantitative. As a complete experimental study of the thermoelectric properties in MPs, cubic-NiP2 was synthesized and the low predicted lattice thermal conductivity (∼1.2 W m−1 K−1 at 700 K) was confirmed. The computed Seebeck coefficient is in agreement with experiments over a range of temperatures and the phononic dispersion curve of c-NiP2 is consistent with the experimental heat capacity. The predicted high thermoelectric performance in several metal phosphides and the low thermal conductivity measured in NiP2 encourage further investigations of thermoelectric properties of metal phosphides.