106 research outputs found
Mixed ab initio quantum mechanical and Monte Carlo calculations of secondary emission from SiO2 nanoclusters
A mixed quantum mechanical and Monte Carlo method for calculating Auger
spectra from nanoclusters is presented. The approach, based on a cluster
method, consists of two steps. Ab initio quantum mechanical calculations are
first performed to obtain accurate energy and probability distributions of the
generated Auger electrons. In a second step, using the calculated line shape as
electron source, the Monte Carlo method is used to simulate the effect of
inelastic losses on the original Auger line shape. The resulting spectrum can
be directly compared to 'as-acquired' experimental spectra, thus avoiding
background subtraction or deconvolution procedures. As a case study, the O K-LL
spectrum from solid SiO2 is considered. Spectra computed before or after the
electron has traveled through the solid, i.e., unaffected or affected by
extrinsic energy losses, are compared to the pertinent experimental spectra
measured within our group. Both transition energies and relative intensities
are well reproduced.Comment: 9 pageg, 5 figure
Lorentz Symmetry in QFT on Quantum Bianchi I Space-Time
We develop the quantum theory of a scalar field on LQC Bianchi I geometry. In
particular, we focus on single modes of the field: the evolution equation is
derived from the quantum scalar constraint, and it is shown that the same
equation can be obtained from QFT on an "classical" effective geometry. We
investigate the dependence of this effective space-time on the wavevector of
the mode (which could in principle generate a deformation in local
Lorentz-symmetry), focusing our attention on the dispersion relation. We prove
that when we disregard backreaction no Lorentz-violation is present, despite
the effective metric being different than the classical Bianchi I one. A
preliminary analysis of the correction due to inclusion of backreaction is
briefly discussed in the context of Born-Oppenheimer approximation.Comment: 14 pages, v3. Corrected a reference in the bibliograph
Angle selective backscattered electron contrast in the low-voltage scanning electron microscope: simulation & experiment for polymers
Recently developed detectors can deliver high resolution and high contrast images of nanostructured carbon based materials in low voltage scanning electron microscopes (LVSEM) with beam deceleration. Monte Carlo Simulations are also used to predict under which exact imaging conditions purely compositional contrast can be obtained and optimised. This allows the prediction of the electron signal intensity in angle selective conditions for back-scattered electron (BSE) imaging in LVSEM and compares it to experimental signals. Angle selective detection with a concentric back scattered (CBS) detector is considered in the model in the absence and presence of a deceleration field, respectively. The validity of the model prediction for both cases was tested experimentally for amorphous C and Cu and applied to complex nanostructured carbon based materials, namely a Poly(N-isopropylacrylamide)/Poly(ethylene glycol) Diacrylate (PNIPAM/PEGDA) semi-interpenetration network (IPN) and a Poly(3-hexylthiophene-2,5-diyl) (P3HT) film, to map nano-scale composition and crystallinity distribution by avoiding experimental imaging conditions that lead to a mixed topographical and compositional contrast
Energy Deposition around Swift Carbon-Ion Tracks in Liquid Water
Energetic carbon ions are promising projectiles used for cancer radiotherapy. A thorough knowledge of how the energy of these ions is deposited in biological media (mainly composed of liquid water) is required. This can be attained by means of detailed computer simulations, both macroscopically (relevant for appropriately delivering the dose) and at the nanoscale (important for determining the inflicted radiobiological damage). The energy lost per unit path length (i.e., the so-called stopping power) of carbon ions is here theoretically calculated within the dielectric formalism from the excitation spectrum of liquid water obtained from two complementary approaches (one relying on an optical-data model and the other exclusively on ab initio calculations). In addition, the energy carried at the nanometre scale by the generated secondary electrons around the ion's path is simulated by means of a detailed Monte Carlo code. For this purpose, we use the ion and electron cross sections calculated by means of state-of-the art approaches suited to take into account the condensed-phase nature of the liquid water target. As a result of these simulations, the radial dose around the ion's path is obtained, as well as the distributions of clustered events in nanometric volumes similar to the dimensions of DNA convolutions, contributing to the biological damage for carbon ions in a wide energy range, covering from the plateau to the maximum of the Bragg peak
Angle selective backscattered electron contrast in the low-voltage scanning electron microscope: Simulation and experiment for polymers
AbstractRecently developed detectors can deliver high resolution and high contrast images of nanostructured carbon based materials in low voltage scanning electron microscopes (LVSEM) with beam deceleration. Monte Carlo Simulations are also used to predict under which exact imaging conditions purely compositional contrast can be obtained and optimised. This allows the prediction of the electron signal intensity in angle selective conditions for back-scattered electron (BSE) imaging in LVSEM and compares it to experimental signals. Angle selective detection with a concentric back scattered (CBS) detector is considered in the model in the absence and presence of a deceleration field, respectively. The validity of the model prediction for both cases was tested experimentally for amorphous C and Cu and applied to complex nanostructured carbon based materials, namely a Poly(N-isopropylacrylamide)/Poly(ethylene glycol) Diacrylate (PNIPAM/PEGDA) semi-interpenetration network (IPN) and a Poly(3-hexylthiophene-2,5-diyl) (P3HT) film, to map nano-scale composition and crystallinity distribution by avoiding experimental imaging conditions that lead to a mixed topographical and compositional contras
Electronic excitation spectra of cerium oxides: from ab initio dielectric response functions to Monte Carlo electron transport simulations
Nanomaterials made of the cerium oxides CeO and CeO have a broad
range of applications, from catalysts in automotive, industrial or energy
operations to promising materials to enhance hadrontherapy effectiveness in
oncological treatments. To elucidate the physico-chemical mechanisms involved
in these processes, it is of paramount importance to know the electronic
excitation spectra of these oxides, which are obtained here through
high-accuracy linear-response time-dependent density functional theory
calculations. In particular, the macroscopic dielectric response functions
of both bulk CeO and CeO are derived, which compare
remarkably well with the available experimental data. These results stress the
importance of appropriately accounting for local field effects to model the
dielectric function of metal oxides. Furthermore, we reckon the materials
energy loss functions \mbox{Im} (-1/\bar{\epsilon}), including the accurate
evaluation of the momentum transfer dispersion from first-principles. In this
respect, by using a Mermin-type parametrization we are able to model the
contribution of different electronic excitations to the dielectric loss
function. Finally, from the knowledge of the electron inelastic mean free path,
together with the elastic mean free path provided by the relativistic Mott
theory, we carry out statistical Monte Carlo (MC) charge transport simulations
to reproduce the major features of the reported experimental reflection
electron energy loss (REEL) spectra of cerium oxides. The good agreement with
REEL experimental data strongly supports our approach based on MC modelling
informed by ab initio calculated electronic excitation spectra in a broad range
of momentum and energy transfers.Comment: 21 pages, 19 figure
Novel organic photovoltaic polymer blends: A rapid, 3-dimensional morphology analysis using backscattered electron imaging in the scanning electron microscope
Finding the optimal morphology of novel organic photovoltaic (OPV) polymer blends is a major obstacle slowing the development of more efficient OPV devices. With a focus on accelerating the systematic morphology optimisation process, we demonstrate a technique offering rapid high-resolution, 3-dimensional blend morphology analysis in the scanning electron microscope. This backscattered electron imaging technique is used to investigate the morphological features and lengthscales defining the promising PffBT4T-2OD:PC70BM blend system and show how its photovoltaic performance is related to the nature of its phase separation. Low-voltage backscattered electron imaging can be used to probe for structure and domain stacking through the thickness of the film, as well as imaging surface morphology with highly competitive spatial resolution. For reference, we compare our results with equivalent images of the widely studied P3HT:PC60BM blend system. Our results also demonstrate that backscattered electron imaging offers significant advantages over conventional cross-sectional imaging techniques, and show that it enables a fast, systematic approach to control 3-dimensional active layer morphology in polymer:fullerene blends
Quantitative secondary electron imaging for work function extraction at atomic level and layer identification of graphene
Two-dimensional (2D) materials usually have a layer-dependent work function, which require fast
and accurate detection for the evaluation of their device performance. A detection technique with
high throughput and high spatial resolution has not yet been explored. Using a scanning electron
microscope, we have developed and implemented a quantitative analytical technique which allows
effective extraction of the work function of graphene. This technique uses the secondary electron
contrast and has nanometre-resolved layer information. The measurement of few-layer graphene flakes
shows the variation of work function between graphene layers with a precision of less than 10meV. It is
expected that this technique will prove extremely useful for researchers in a broad range of fields due to
its revolutionary throughput and accuracy
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