Browsing by Subject "DFT"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    A Theoretical Study on Porous-Silicon Based Synapse Design for Neural Hardware
    (IEEE, 2021-12) Sikder, Orthi; Schubert, Peter; Electrical and Computer Engineering, School of Engineering and Technology
    Porous silicon (po-Si) is a form of silicon (Si) with nanopores of tunable sizes and shapes distributed over the bulk structure. Although crystalline Si (c-Si) is already established as one of the most advantageous and promising elements for its technological significance, the additional key aspect of po-Si is its large surface area with respect to its small volume which is beneficial for surface chemistry. In this work, we explore the design of a po-Si based synaptic device and investigate its potential for neuromorphic hardware. First, we analyze several electrical properties of po-Si through density functional theory (Ab Initio/ first principle) calculation. We show that the presence of intra-pore dangling states appears within the bandgap region of po-Si. While the bandgap of the po-Si is well known to be higher than c-Si yielding low carrier density and higher resistance, the appearance of these dangling states can significantly participate in electronic transport through hopping mechanism. Then, we analyze the electric-field driven modulation in the dangling bond through controlled intra-pore Si-H bond dissociation. Such modulation of the dangling state density further allows the tenability of the po-Si conductance. Finally, we theoretically evaluate the current-voltage characteristics of our proposed po-Si based synaptic devices and determine the possible range of obtainable conductivity for different porosity. Our analysis signifies that the integration of such devices in the synaptic fabric can enable significantly denser and energy-efficient neuromorphic hardware.
  • Thumbnail Image
    Item
    Computational characterization of enzyme-bound thiamin diphosphate reveals a surprisingly stable tricyclic state: implications for catalysis
    (Beilstein, 2019-01-16) Planas, Ferran; McLeish, Michael J.; Himo, Fahmi; Chemistry and Chemical Biology, School of Science
    Thiamin diphosphate (ThDP)-dependent enzymes constitute a large class of enzymes that catalyze a diverse range of reactions. Many are involved in stereospecific carbon–carbon bond formation and, consequently, have found increasing interest and utility as chiral catalysts in various biocatalytic applications. All ThDP-catalyzed reactions require the reaction of the ThDP ylide (the activated state of the cofactor) with the substrate. Given that the cofactor can adopt up to seven states on an enzyme, identifying the factors affecting the stability of the pre-reactant states is important for the overall understanding of the kinetics and mechanism of the individual reactions. In this paper we use density functional theory calculations to systematically study the different cofactor states in terms of energies and geometries. Benzoylformate decarboxylase (BFDC), which is a well characterized chiral catalyst, serves as the prototypical ThDP-dependent enzyme. A model of the active site was constructed on the basis of available crystal structures, and the cofactor states were characterized in the presence of three different ligands (crystallographic water, benzoylformate as substrate, and (R)-mandelate as inhibitor). Overall, the calculations reveal that the relative stabilities of the cofactor states are greatly affected by the presence and identity of the bound ligands. A surprising finding is that benzoylformate binding, while favoring ylide formation, provided even greater stabilization to a catalytically inactive tricyclic state. Conversely, the inhibitor binding greatly destabilized the ylide formation. Together, these observations have significant implications for the reaction kinetics of the ThDP-dependent enzymes, and, potentially, for the use of unnatural substrates in such reactions.
  • Thumbnail Image
    Item
    Correcting soft errors online in fast fourier transform
    (ACM, 2017) Liang, Xin; Chen, Jieyang; Tao, Dingwen; Li, Sihuan; Wu, Panruo; Li, Hongbo; Ouyang, Kaiming; Liu, Yuanlai; Song, Fengguang; Chen, Zizhong; Computer and Information Science, School of Science
    While many algorithm-based fault tolerance (ABFT) schemes have been proposed to detect soft errors offline in the fast Fourier transform (FFT) after computation finishes, none of the existing ABFT schemes detect soft errors online before the computation finishes. This paper presents an online ABFT scheme for FFT so that soft errors can be detected online and the corrupted computation can be terminated in a much more timely manner. We also extend our scheme to tolerate both arithmetic errors and memory errors, develop strategies to reduce its fault tolerance overhead and improve its numerical stability and fault coverage, and finally incorporate it into the widely used FFTW library - one of the today's fastest FFT software implementations. Experimental results demonstrate that: (1) the proposed online ABFT scheme introduces much lower overhead than the existing offline ABFT schemes; (2) it detects errors in a much more timely manner; and (3) it also has higher numerical stability and better fault coverage.
  • Thumbnail Image
    Item
    Density Functional Theory (DFT) study of hydrogen storage in porous silicon
    (2018) Boaks, Mawla; Schubert, Peter
    Based on plane wave DFT calculation, we carried out micro level investigation of hydrogen storage in nanoporous silicon (npSi). One quarter of a hexagonal pore with Palladium catalyst placed at the surface has been studied for hydrogen dissociation, spillover, bond hopping, and diffusion for both single catalyst atom and small catalyst cluster consisting of multiple catalyst atoms. All the DFT computations were done in one of the biggest research supercomputer facilities of the world, Big Red II. We opted ABINIT, an open source DFT tool for our computations. Our calculation revealed low dissociation, spillover, and bond hoping energy barrier. The energy required to be provided from external sources to fully recharge the storage medium from a gaseous source at a completely empty state has also been evaluated. Hydrogen diffusion along the inner surface of the pore as a means of bond hopping and the possibility of quantum tunneling, a low temperature phenomena used to spontaneously go over an otherwise less likely high energy barrier have been studied as well. Using these micro level parameter values evaluated from the DFT study, the performance of any potential hydrogen storage material can be compared to a set of characteristics sought in an efficient storage media. Thus, the micro scale feasibility of this novel npSi material based hydrogen storage technology was studied as a part of a STTR Phase I project.
  • Thumbnail Image
    Item
    Effect of vacancies and edges in promoting water chemisorption on titanium-based MXenes
    (Springer, 2023-04-01) Marquis, Edoardo; Benini, Francesca; Anasori, Babak; Rosenkranz, Andreas; Righi, Maria Clelia; Mechanical and Energy Engineering, School of Engineering and Technology
    The functionality of two-dimensional (2D) transition metal carbides and nitrides (MXenes) in technological applications greatly depends on their wettability. For instance, MXenes' layer stability against degradative oxidation is notably reduced when stored in aqueous solutions, leading to the transformation into oxides. In this work, we study water adsorption on Ti-based MXenes by ab initio calculations. The energy gains for the molecular adsorption on Tin+1XnT2 is evaluated as a function of the termination (T = F, O, OH, mixture), the carbon/nitrogen ratio (X = C, N), the layer thickness (n) and water coverage. MXenes' hydrophilicity tends to increase due to the presence of defects as vacancies and flake edges. We demonstrate that physical adsorption occurs through hydrogen bonding on both defect-free layers and layers containing C/N or Ti atomic vacancies, with -OH terminations providing the strongest interactions (0.40-0.65 eV). In contrast, strong water chemisorption is observed on surfaces with a single termination vacancy (0.60-1.20 eV), edges (0.75-0.85 eV), and clusters of defects (1.00-1.80 eV). We verified that the presence of undercoordinated Ti atoms on the surface is the key factor in promoting H2O chemisorption, i.e., the degradative oxidation.