1,721,082 research outputs found
Tuning the optical absorption and exciton bound states of germanene by chemical functionalization
No full textAbstract We present a comprehensive study of buckled honeycomb germanene functionalized with alternately bonded side groups hydroxyl (–H), methyl (–CH3) and trifluoro methyl (–CF3). By means of most modern theoretical and computational methods we determine the atomic geometries versus the functionalizing groups. The quasiparticle excitation effects on the electronic structure are taken into account by means of exchange-correlation treatment within the GW framework. The Bethe–Salpeter equation is solved ab initio to derive optical spectra including excitonic and quasiparticle effects. Band edge excitons are investigated in detail. The binding properties are compared with those resulting from model studies. The functionalization leads to significantly modified band structures compared with pristine germanene. The Dirac bands near the K point are destroyed and direct gaps appear at the point. Together with the many-body effects, quasiparticle gaps of 2.3, 1.8 or 1.0 eV result for –H, –CH3 and –CF3 functionalization. Totally different absorption spectra are found for in-plane and out-of-plane light polarization. Strongly bound excitons are visible below the quasiparticle band edge with binding energies of about 0.5, 0.4 or 0.3 eV. The nature of these band-gap excitons is investigated via their wave function, the contribution of various interband combinations and the dipole selection rules
Detection of Heavy Metals in Water Using Graphene Oxide Quantum Dots: An Experimental and Theoretical Study
Full text linkIn this work, we investigate by ab initio calculations and optical experiments the sensitivity of graphene quantum dots in their use as devices to measure the presence, and concentration, of heavy metals in water. We demonstrate that the quenching or enhancement in the optical response (absorption, emission) depends on the metallic ion considered. In particular, two cases of opposite behaviour are considered in detail: Cd2+, where we observe an increase in the emission optical response for increasing concentration, and Pb2+ whose emission spectra, vice versa, are quenched along the concentration rise. The experimental trends reported comply nicely with the different hydration patterns suggested by the models that are also capable of reproducing the minor quenching/enhancing effects observed in other ions. We envisage that quantum dots of graphene may be routinely used as cheap detectors to measure the degree of poisoning ions in water
Evolution of the Electronic and Optical Properties of Meta-Stable Allotropic Forms of 2D Tellurium for Increasing Number of Layers
Full text linkIn this work, ab initio Density Functional Theory calculations are performed to investigate the evolution of the electronic and optical properties of 2D Tellurium—called Tellurene—for three different allotropic forms ([Formula: see text]-, [Formula: see text]- and [Formula: see text]-phase), as a function of the number of layers. We estimate the exciton binding energies and radii of the studied systems, using a 2D analytical model. Our results point out that these quantities are strongly dependent on the allotropic form, as well as on the number of layers. Remarkably, we show that the adopted method is suitable for reliably predicting, also in the case of Tellurene, the exciton binding energy, without the need of computationally demanding calculations, possibly suggesting interesting insights into the features of the system. Finally, we inspect the nature of the mechanisms ruling the interaction of neighbouring Tellurium atoms helical chains (characteristic of the bulk and [Formula: see text]-phase crystal structures). We show that the interaction between helical chains is strong and cannot be explained by solely considering the van der Waals interaction
Electronic and optical properties of Si and Ge nanocrystals: an ab-initio study
No full textFirst-principles calculations within density functional theory and
many-body perturbation theory have been carried out in order to
investigate the structural, electronic and optical properties of un-
doped and doped silicon nanostructures. We consider Si nanoclus-
ters co-doped with B and P. We find that the electronic band gap is
reduced with respect to that of the undoped crystals, suggesting the
possibility of impurity based engineering of electronic and optical
properties of Si nanocrystals. Finally, motivated by recent sugges-
tions concerning the chance of exploiting Ge dots for photovoltaic
nanodevices, we present calculations of the electronic and optical
properties of a Ge 35 H 36 nanocrystal, and compare the results with
those for the corresponding Si 35 H 36 nanocrystals and the co-doped
Si 33BPH36
Honeycomb silicon on alumina: Massless Dirac fermions in silicene on substrate
No full textWe predict the stability of a graphenelike silicene sheet on one monolayer of aluminum oxide. We find that the honeycomb buckled structure of silicene is not broken upon interaction with one monolayer of Al2O3 in the kagome geometry. As a consequence, the electronic band structure shows unperturbed cones with massless Dirac fermions embedded into Al2O3-derived bands. The heterostructure conserves the topological character of the freestanding silicene. The optical properties of the silicene/Al2O3 bilayers clearly maintain the characteristic infrared universal limit of pi alpha. A quantized absorbance N center dot pi alpha (with N silicene layers) also occurs for stacking of multilayers
Transitions in Xenes between excitonic, topological and trivial insulator phases: influence of screening, band dispersion and external electric field
Full text linkUsing a variational approach, the binding energies of the lowest bound
excitons in Xenes under varying electric field are investigated. The internal
exciton motion is described both by Dirac electron dispersion and in
effective-mass approximation, while the screened electron-hole attraction is
modeled by a Rytova-Keldysh potential with a 2D electronic polarizability
. The most important parameters as spin-orbit-induced gap
, Fermi velocity and are taken from ab initio
density functional theory calculations. In addition, is
approximated in two different ways. The relation of and is ruled by
the screening. The existence of an excitonic insulator phase with
sensitively depends on the chosen . The values of and
are strongly modified by a vertical external electric bias
, which defines a transition from the topological into a trivial insulator
at , with the exception of plumbene. Within the Dirac approximation,
but also within the effective mass description of the kinetic energy, the
treatment of screening dominates the appearance or non-appearance of an
excitonic insulator phase. Gating does not change the results: the prediction
done at zero electric field is confirmed when a vertical electric field is
applied. Finally, Many-Body perturbation theory approaches based on the Green's
function method, applied to stanene, confirm the absence of an excitonic
insulator phase, thus validating our results obtained by ab initio modeling of
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