170 research outputs found
Designing novel Sn-Bi, Si-C and Ge-C nanostructures, using simple theoretical chemical similarities
A framework of simple, transparent and powerful concepts is presented which is based on isoelectronic (or isovalent) principles, analogies, regularities and similarities. These analogies could be considered as conceptual extensions of the periodical table of the elements, assuming that two atoms or molecules having the same number of valence electrons would be expected to have similar or homologous properties. In addition, such similar moieties should be able, in principle, to replace each other in more complex structures and nanocomposites. This is only partly true and only occurs under certain conditions which are investigated and reviewed here. When successful, these concepts are very powerful and transparent, leading to a large variety of nanomaterials based on Si and other group 14 elements, similar to well known and well studied analogous materials based on boron and carbon. Such nanomaterias designed in silico include, among many others, Si-C, Sn-Bi, Si-C and Ge-C clusters, rings, nanowheels, nanorodes, nanocages and multidecker sandwiches, as well as silicon planar rings and fullerenes similar to the analogous sp2 bonding carbon structures. It is shown that this pedagogically simple and transparent framework can lead to an endless variety of novel and functional nanomaterials with important potential applications in nanotechnology, nanomedicine and nanobiology. Some of the so called predicted structures have been already synthesized, not necessarily with the same rational and motivation. Finally, it is anticipated that such powerful and transparent rules and analogies, in addition to their predictive power, could also lead to far-reaching interpretations and a deeper understanding of already known results and information
Comprehensive Ab Initio Study of Electronic, Optical, and Cohesive Properties of Silicon Quantum Dots of Various Morphologies and Sizes up to Infinity
We present a comprehensive and integrated model-independent ab initio study of the structural, cohesive, electronic, and optical properties of silicon quantum dots of various morphologies and sizes in the framework of all-electron "static" and time-dependent density functional theory (DFT, TDFT), using the well-tested B3LYP and other properly chosen functional(s). Our raw ab initio results for all these properties for hydrogen-passivated nanocrystals of various growth models and sizes from 1 to 32 Å are subsequently fitted, using power-law dependence with judicially selected exponents, based on dimensional and other plausibility arguments. As a result, we can not only reproduce with excellent accuracy known experimental and well-tested theoretical results in the regions of overlap but also extrapolate successfully all the way to infinity, reproducing the band gap of crystalline silicon with almost chemical accuracy as well as the cohesive energy of the infinite crystal with very good accuracy. Thus, our results could be safely used, among others, as interpolation and extrapolation formulas not only for cohesive energy and band gap but also for interrelated properties, such as dielectric constant and index of refraction of silicon nanocrystals of various sizes all the way up to infinity. © 2016 American Chemical Society
Photo-absorption spectra of small hydrogenated silicon clusters using the time-dependent density functional theory
We present a systematic study of the photo-absorption spectra of various
SiH clusters (n=1-10, m=1-14) using the time-dependent density
functional theory (TDDFT). The method uses a real-time, real-space
implementation of TDDFT involving full propagation of the time dependent
Kohn-Sham equations. Our results for SiH and SiH show good
agreement with the earlier calculations and experimental data. We find that for
small clusters (n<7) the photo-absorption spectrum is atomic-like while for the
larger clusters it shows bulk-like behaviour. We study the photo-absorption
spectra of silicon clusters as a function of hydrogenation. For single
hydrogenation, we find that in general, the absorption optical gap decreases
and as the number of silicon atoms increase the effect of a single hydrogen
atom on the optical gap diminishes. For further hydrogenation the optical gap
increases and for the fully hydrogenated clusters the optical gap is larger
compared to corresponding pure silicon clusters.Comment: 6 pages, 5 figure
Phonon Localization in One-Dimensional Quasiperiodic Chains
Quasiperiodic long range order is intermediate between spatial periodicity
and disorder, and the excitations in 1D quasiperiodic systems are believed to
be transitional between extended and localized. These ideas are tested with a
numerical analysis of two incommensurate 1D elastic chains: Frenkel-Kontorova
(FK) and Lennard-Jones (LJ). The ground state configurations and the
eigenfrequencies and eigenfunctions for harmonic excitations are determined.
Aubry's "transition by breaking the analyticity" is observed in the ground
state of each model, but the behavior of the excitations is qualitatively
different. Phonon localization is observed for some modes in the LJ chain on
both sides of the transition. The localization phenomenon apparently is
decoupled from the distribution of eigenfrequencies since the spectrum changes
from continuous to Cantor-set-like when the interaction parameters are varied
to cross the analyticity--breaking transition. The eigenfunctions of the FK
chain satisfy the "quasi-Bloch" theorem below the transition, but not above it,
while only a subset of the eigenfunctions of the LJ chain satisfy the theorem.Comment: This is a revised version to appear in Physical Review B; includes
additional and necessary clarifications and comments. 7 pages; requires
revtex.sty v3.0, epsf.sty; includes 6 EPS figures. Postscript version also
available at
http://lifshitz.physics.wisc.edu/www/koltenbah/koltenbah_homepage.htm
Atomistic simulations of self-trapped exciton formation in silicon nanostructures: The transition from quantum dots to nanowires
Using an approximate time-dependent density functional theory method, we
calculate the absorption and luminescence spectra for hydrogen passivated
silicon nanoscale structures with large aspect ratio. The effect of electron
confinement in axial and radial directions is systematically investigated.
Excited state relaxation leads to significant Stokes shifts for short nanorods
with lengths less than 2 nm, but has little effect on the luminescence
intensity. The formation of self-trapped excitons is likewise observed for
short nanostructures only; longer wires exhibit fully delocalized excitons with
neglible geometrical distortion at the excited state minimum.Comment: 10 pages, 4 figure
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