189 research outputs found
First-principles calculations and bias-dependent STM measurements at the alpha-Sn/Ge(111) surface: a clear indication for the 1U2D configuration
The nature of the alpha-Sn/Ge(111) surface is still a matter of debate. In
particular, two possible configurations have been proposed for the 3x3 ground
state of this surface: one with two Sn adatoms in a lower position with respect
to the third one (1U2D) and the other with opposite configuration (2U1D). By
means of first-principles quasiparticle calculations we could simulate STM
images as a function of bias voltage and compare them with STM experimental
results at 78K, obtaining an unambiguous indication that the stable
configuration for the alpha-Sn/Ge(111) surface is the 1U2D. The possible
inequivalence of the two down Sn adatoms is also discussed.Comment: Submitted to PR
Detection of topological phase transitions through entropy measurements: the case of germanene
We propose a characterization tool for studies of the band structure of new
materials promising for the observation of topological phase transitions. We
show that a specific resonant feature in the entropy per electron dependence on
the chemical potential may be considered as a fingerprint of the transition
between topological and trivial insulator phases. The entropy per electron in a
honeycomb two-dimensional crystal of germanene subjected to the external
electric field is obtained from the first principle calculation of the density
of electronic states and the Maxwell relation. We demonstrate that, in
agreement to the recent prediction of the analytical model, strong spikes in
the entropy per particle dependence on the chemical potential appear at low
temperatures. They are observed at the values of the applied bias both below
and above the critical value that corresponds to the transition between the
topological insulator and trivial insulator phases, while the giant resonant
feature in the vicinity of zero chemical potential is strongly suppressed at
the topological transition point, in the low temperature limit. In a wide
energy range, the van Hove singularities in the electronic density of states
manifest themselves as zeros in the entropy per particle dependence on the
chemical potential.Comment: 8 pages, 5 figures; final version published in PR
Detection of heavy metals in water using graphene oxide quantum dots: an experimental and theoretical study
In 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
Work function, deformation potential, and collapse of Landau levels in strained graphene and silicene
We perform a systematic {\it ab initio} study of the work function and its
uniform strain dependence for graphene and silicene for both tensile and
compressive strains. The Poisson ratios associated with armchair and zigzag
strains are also computed. Based on these results, we obtain the deformation
potential, crucial for straintronics, as a function of the applied strain.
Further, we propose a particular experimental setup with a special strain
configuration that generates only the electric field, while the pseudomagnetic
field is absent. Then, applying a real magnetic field, one should be able to
realize experimentally the spectacular phenomenon of the collapse of Landau
levels in graphene or related two-dimensional materials.Comment: 9 pages, 7 figures; final version published in PR
Engineering Silicon Nanocrystals: Theoretical study of the effect of Codoping with Boron and Phosphorus
We show that the optical and electronic properties of nanocrystalline silicon
can be efficiently tuned using impurity doping. In particular, we give
evidence, by means of ab-initio calculations, that by properly controlling the
doping with either one or two atomic species, a significant modification of
both the absorption and the emission of light can be achieved. We have
considered impurities, either boron or phosphorous (doping) or both (codoping),
located at different substitutional sites of silicon nanocrystals with size
ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped
nanocrystals have the lowest impurity formation energies when the two
impurities occupy nearest neighbor sites near the surface. In addition, such
systems present band-edge states localized on the impurities giving rise to a
red-shift of the absorption thresholds with respect to that of undoped
nanocrystals. Our detailed theoretical analysis shows that the creation of an
electron-hole pair due to light absorption determines a geometry distortion
that in turn results in a Stokes shift between adsorption and emission spectra.
In order to give a deeper insight in this effect, in one case we have
calculated the absorption and emission spectra going beyond the single-particle
approach showing the important role played by many-body effects. The entire set
of results we have collected in this work give a strong indication that with
the doping it is possible to tune the optical properties of silicon
nanocrystals.Comment: 14 pages 19 figure
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