136 research outputs found
Optoelectronic Properties and Excitons in Hybridized Boron Nitride and Graphene Hexagonal Monolayers
We explain the nature of the electronic band gap and optical absorption
spectrum of Carbon - Boron Nitride (CBN) hybridized monolayers using density
functional theory (DFT), GW and Bethe-Salpeter equation calculations. The CBN
optoelectronic properties result from the overall monolayer bandstructure,
whose quasiparticle states are controlled by the C domain size and lie at
separate energy for C and BN without significant mixing at the band edge, as
confirmed by the presence of strongly bound bright exciton states localized
within the C domains. The resulting absorption spectra show two marked peaks
whose energy and relative intensity vary with composition in agreement with the
experiment, with large compensating quasiparticle and excitonic corrections
compared to DFT calculations. The band gap and the optical absorption are not
regulated by the monolayer composition as customary for bulk semiconductor
alloys and cannot be understood as a superposition of the properties of
bulk-like C and BN domains as recent experiments suggested
Exciton Radiative Lifetimes in Two-Dimensional Transition Metal Dichalcogenides
Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX_2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS2 and in two MX_2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS_2. The MoS_2/WS_2 and MoSe_2/WSe_2 heterobilayers exhibit very long-lived (âŒ20â30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion
Duration of untreated illness predicts 3-year outcome in patients with obsessive-compulsive disorder: A real-world, naturalistic, follow-up study
Duration of untreated illness (DUI) is a predictor of outcome in psychotic and affective disorders. The few available data on the effect of DUI in obsessive-compulsive disorder (OCD) suggest an association between longer DUI and poorer response to treatments. This is a real-world, naturalistic, follow-up study evaluating the impact of DUI on long-term clinical outcomes. The sample consists of 83 outpatients with OCD with a mean DUI of 7.3 (5.8) years. Patients with symmetry/ordering cluster symptoms were younger at onset of the disease (20.4 ± 7.9 vs. 27.8 ± 10.6; p<.05, d = 0.79), had a longer duration of the illness (10.1 ± 4.6 vs. 6.8 ± 4.6, p<.05; d = 0.53) and a longer DUI (7.9 ± 6.5 vs. 5.4 ± 3.6, p<.05, d = 0.49) compared to patients not presenting with those symptoms. Fifty-nine patients completed the follow-up, and 33.9% (N = 20) met the criteria for partial remission, scoring <15 at the Y-BOCS for at least eight weeks. Patients in partial remission for more than 40% of the follow-up were defined as âgood outcomeâ and they had a significantly shorter DUI compared to patients with âpoor outcomeâ. Access to adequate treatments is highly delayed in patients with OCD. DUI is strongly associated with poor treatment outcomes. Therefore, strategies to ensure an early diagnosis and treatment are needed
Study of a Nonlocal Density scheme for electronic--structure calculations
An exchange-correlation energy functional beyond the local density
approximation, based on the exchange-correlation kernel of the homogeneous
electron gas and originally introduced by Kohn and Sham, is considered for
electronic structure calculations of semiconductors and atoms. Calculations are
carried out for diamond, silicon, silicon carbide and gallium arsenide. The
lattice constants and gaps show a small improvement with respect to the LDA
results.
However, the corresponding corrections to the total energy of the isolated
atoms are not large enough to yield a substantial improvement for the cohesive
energy of solids, which remains hence overestimated as in the LDA.Comment: 4 postscript figure
Many-body perturbation theory calculations using the yambo code
International audienceyambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as quantum-espresso and abinit. yambo's capabilities include the calculation of linear response quantities (both independent-particle and including electron-hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron-phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools
Semiconducting Monolayer Materials as a Tunable Platform for Excitonic Solar Cells
The recent advent of two-dimensional monolayer materials with tunable
optoelectronic properties and high carrier mobility offers renewed
opportunities for efficient, ultra-thin excitonic solar cells alternative to
those based on conjugated polymer and small molecule donors. Using
first-principles density functional theory and many-body calculations, we
demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with
commonly used acceptors such as PCBM fullerene or semiconducting carbon
nanotubes can provide excitonic solar cells with tunable absorber gap,
donor-acceptor interface band alignment, and power conversion efficiency, as
well as novel device architectures. For the case of CBN-PCBM devices, we
predict the limit of power conversion efficiencies to be in the 10 - 20% range
depending on the CBN monolayer structure. Our results demonstrate the
possibility of using monolayer materials in tunable, efficient, polymer-free
thin-film solar cells in which unexplored exciton and carrier transport regimes
are at play.Comment: 7 pages, 5 figure
Ab initio optical properties of Si(100)
We compute the linear optical properties of different reconstructions of the
clean and hydrogenated Si(100) surface within DFT-LDA, using norm-conserving
pseudopotentials. The equilibrium atomic geometries of the surfaces, determined
from self-consistent total energy calculations within the Car-Parrinello
scheme, strongly influence Reflectance Anisotropy Spectra (RAS), showing
differences between the p(2x2) and c(4x2)reconstructions. The Differential
Reflectivity spectrum for the c(4x2) reconstruction shows a positive peak at
energies < 1 eV, in agreement with experimental results.Comment: fig. 2 correcte
Search for Gamma Ray Bursts with the Argo-YBJ Detector in Scaler Mode
We report on the search for Gamma Ray Bursts (GRBs) in the energy range 1-100
GeV in coincidence with the prompt emission detected by satellites using the
Astrophysical Radiation with Ground-based Observatory at YangBaJing (ARGO-YBJ)
air shower detector. Thanks to its mountain location (Yangbajing, Tibet, P.R.
China, 4300 m a.s.l.), active surface (about 6700 m**2 of Resistive Plate
Chambers), and large field of view (about 2 sr, limited only by the atmospheric
absorption), the ARGO-YBJ air shower detector is particularly suitable for the
detection of unpredictable and short duration events such as GRBs. The search
is carried out using the "single particle technique", i.e. counting all the
particles hitting the detector without measurement of the energy and arrival
direction of the primary gamma rays.
Between 2004 December 17 and 2009 April 7, 81 GRBs detected by satellites
occurred within the field of view of ARGO-YBJ (zenith angle < 45 deg). It was
possible to examine 62 of these for >1 GeV counterpart in the ARGO-YBJ data
finding no statistically significant emission. With a lack of detected spectra
in this energy range fluence upper limits are profitable, especially when the
redshift is known and the correction for the extragalactic absorption can be
considered. The obtained fluence upper limits reach values as low as 10**{-5}
erg cm**{-2} in the 1-100 GeV energy region.
Besides this individual search for a higher energy counterpart, a statistical
study of the stack of all the GRBs both in time and in phase was made, looking
for a common feature in the GRB high energy emission. No significant signal has
been detected.Comment: accepted for publication in Ap
Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results
Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole
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