455 research outputs found
A theoretical model for single molecule incoherent scanning tunneling spectroscopy
Single molecule scanning tunneling spectroscopy (STS), with dephasing due to
elastic and inelastic scattering, is of some current interest. Motivated by
this, we report an extended Huckel theory (EHT) based mean-field
Non-equilibrium Green's function (NEGF) transport model with electron-phonon
scattering treated within the self-consistent Born approximation (SCBA).
Furthermore, a procedure based on EHT basis set modification is described. We
use this model to study the effect of the temperature dependent dephasing, due
to low lying modes in far-infrared range for which hw<<kT, on the resonant
conduction through highest occupied molecular orbital (HOMO) level of a phenyl
dithiol molecule sandwiched between two fcc-Au(111) contacts. Furthermore, we
propose to include dephasing in room temperature molecular resonant conduction
calculations.Comment: 12 pages, 5 figure
Strain and field modulation in bilayer graphene band structure
Using an external electric field, one can modulate the bandgap of Bernal
stacked bilayer graphene by breaking A-~B symmetry. We analyze strain effects
on the bilayer graphene using the extended Huckel theory and find that reduced
interlayer distance results in higher bandgap modulation, as expected.
Furthermore, above about 2.5 angstrom interlayer distance, the bandgap is
direct, follows a convex relation to electric field and saturates to a value
determined by the interlayer distance. However, below about 2.5 angstrom, the
bandgap is indirect, the trend becomes concave and a threshold electric field
is observed, which also depends on the stacking distance.Comment: 3 pages, 5 figures - v1 and v2 are the same, uploaded twice - v3,
some typos fixed and a reference adde
Armchair graphene nanoribbons: Electronic structure and electric field modulation
We report electronic structure and electric field modulation calculations in
the width direction for armchair graphene nanoribbons (acGNRs) using a
semi-empirical extended Huckel theory. Important band structure parameters are
computed, e.g. effectives masses, velocities and bandgaps. For the three types
of acGNRs, the pz orbital tight-binding parameters are extracted if feasible.
Furthermore, the effect of electric field in the width direction on acGNRs
dispersion is explored. It is shown that for the two types of semiconducting
acGNRs, an external electric field can reduce the bandgap to a few meV with
different quantitative behavior.Comment: 5 pages, 5 figure
An Extended Huckel Theory based Atomistic Model for Graphene Nanoelectronics
An atomistic model based on the spin-restricted extended Huckel theory (EHT)
is presented for simulating electronic structure and I-V characteristics of
graphene devices. The model is applied to zigzag and armchair graphene
nano-ribbons (GNR) with and without hydrogen passivation, as well as for
bilayer graphene. Further calculations are presented for electric fields in the
nano-ribbon width direction and in the bilayer direction to show electronic
structure modification. Finally, the EHT Hamiltonian and NEGF (Nonequilibrium
Green's function) formalism are used for a paramagnetic zigzag GNR to show
2e2/h quantum conductance.Comment: 5 pages, 8 figure
Accurate Modeling of the Cubic and Antiferrodistortive Phases of SrTiO3 with Screened Hybrid Density Functional Theory
We have calculated the properties of SrTiO3 (STO) using a wide array of
density functionals ranging from standard semi-local functionals to modern
range-separated hybrids, combined with several basis sets of varying
size/quality. We show how these combination's predictive ability varies
significantly, both for STO's cubic and antiferrodistortive (AFD) phases, with
the greatest variation in functional/basis set efficacy seen in modeling the
AFD phase. The screened hybrid functionals we utilized predict the structural
properties of both phases in very good agreement with experiment, especially if
used with large (but still computationally tractable) basis sets. The most
accurate results presented in this study, namely those from
HSE06/modified-def2-TZVP, stand as the most accurate modeling of STO to date
when compared to the literature; these results agree well with experimental
structural and electronic properties as well as providing insight into the band
structure alteration during the phase transition.Comment: 14 pages, 6 figure
Ultrafast Electronic Energy Transfer in an orthogonal molecular dyad
The St Andrews group acknowledges support from the European Research Council (grant number 321305) and the Engineering and Physical Sciences Research Council (grant EP/L017008/1). I.D.W.S. also acknowledges support from a Royal Society Wolfson Research Merit Award.Understanding electronic energy transfer (EET) is an important ingredient in the development of artificial photosynthetic systems and photovoltaic technologies. Although EET is at the heart of these applications and crucially influences their light-harvesting efficiency, the nature of EET over short distances for covalently bound donor and acceptor units is often not well understood. Here we investigate EET in an orthogonal molecular dyad (BODT4) in which simple models fail to explain the very origin of EET. Based on nonadiabatic ab initio molecular dynamics calculations and fluorescence depolarization experiments we gain detailed microscopic insights into the ultrafast electro-vibrational dynamics following photoexcitation. Our analysis offers molecular-level insights into these processes and reveals that it takes place on timescalesĀ ā²Ā 100 fs and occurs through an intermediate charge-transfer state.PostprintPeer reviewe
Molecular structure and the twist-bend nematic phase : the role of terminal chains
Peer reviewedPostprin
An atomistic quantum transport solver with dephasing for field-effect transistors
Extended Huckel theory (EHT) along with NEGF (Non-equilibrium Green's
function formalism) has been used for modeling coherent transport through
molecules. Incorporating dephasing has been proposed to theoretically reproduce
experimental characteristics for such devices. These elastic and inelastic
dephasing effects are expected to be important in quantum devices with the
feature size around 10nm, and hence an efficient and versatile solver is
needed. This model should have flexibility to be applied to a wide range of
nano-scale devices, along with 3D electrostatics, for arbitrary shaped contacts
and surface roughness. We report one such EHT-NEGF solver with dephasing by
self-consistent Born approximation (SCBA). 3D electrostatics is included using
a finite-element scheme. The model is applied to a single wall carbon nanotube
(CNT) cross-bar structure with a C60 molecule as the active channel. Without
dephasing, a negative differential resistance (NDR) peak appears when the C60
lowest unoccupied molecular orbital level crosses a van Hove singularity in the
1D density of states of the metallic CNTs acting as contacts. This NDR
diminishes with increasing dephasing in the channel as expected.Comment: to appear in Journal of Computational Electronic
Knot-isomers of Moebius Cyclacene: How Does the Number of Knots Influence the Structure and First Hyperpolarizability?
Four large ring molecules composed by 15 nitrogen-substituted benzene rings,
named as "knot-isomers of Moebius cyclacene", i.e. non-Moebius cyclacenes
without a knot (0), Moebius cyclacenes with a knot (1), non-Moebius cyclacenes
with two knots (2), and Moebius cyclacenes with three knots (3), are
systematically studied for their structures and nonlinear optical properties.
The first hyperpolarizability (beta_0) values of these four knot-isomers
structures are 4693 (0) < 10484 (2) < 25419 (3) < 60846 au (1). The beta_0
values (60846 for 1, 10484 for 2 and 25419 au for 3) of the knot-isomers with
knot(s) are larger than that (4693 au for 0) of the knot-isomer without a knot.
It shows that the beta_0 value can be dramatically increases (13 times) by
introducing the knot(s) to the cyclacenes structures. It is found that
introducing knots to cyclacenes is a new means to enhance the first
hyperpolarizability.Comment: 12 pages, 4 figure
- ā¦