627 research outputs found
Compressibility of graphene
We develop a theory for the compressibility and quantum capacitance of
disordered monolayer and bilayer graphene including the full hyperbolic band
structure and band gap in the latter case. We include the effects of disorder
in our theory, which are of particular importance at the carrier densities near
the Dirac point. We account for this disorder statistically using two different
averaging procedures: first via averaging over the density of carriers
directly, and then via averaging in the density of states to produce an
effective density of carriers. We also compare the results of these two models
with experimental data, and to do this we introduce a model for inter-layer
screening which predicts the size of the band gap between the low-energy
conduction and valence bands for arbitary gate potentials applied to both
layers of bilayer graphene. We find that both models for disorder give
qualitatively correct results for gapless systems, but when there is a band gap
at charge neutrality, the density of states averaging is incorrect and
disagrees with the experimental data.Comment: 10 pages, 7 figures, RevTe
Optical and transport gaps in gated bilayer graphene
We discuss the effect of disorder on the band gap measured in bilayer
graphene in optical and transport experiments. By calculating the optical
conductivity and density of states using a microscopic model in the presence of
disorder, we demonstrate that the gap associated with transport experiments is
smaller than that associated with optical experiments. Intrinsic bilayer
graphene has an optical conductivity in which the energy of the peaks
associated with the interband transition are very robust against disorder and
thus provide an estimate of the band gap. In contrast, extraction of the band
gap from the optical conductivity of extrinsic bilayer graphene is almost
impossible for significant levels of disorder due to the ambiguity of the
transition peaks. The density of states contains an upper bound on the gap
measured in transport experiments, and disorder has the effect of reducing this
gap which explains why these experiments have so far been unable to replicate
the large band gaps seen in optical measurements.Comment: 5 pages, 5 figures, RevTeX. Published versio
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Selective Lanthanide Sensing with Gold Nanoparticles and Hydroxypyridinone Chelators.
The octadentate hydroxypyridinone chelator 3,4,3-LI(1,2-HOPO) is a promising therapeutic agent because of its high affinity for f-block elements and noncytotoxicity at medical dosages. The interaction between 3,4,3-LI(1,2-HOPO) and other biomedically relevant metals such as gold, however, has not been explored. Gold nanoparticles functionalized with chelators have demonstrated great potential in theranostics, yet thus far, no protocol that combines 3,4,3-LI(1,2-HOPO) and colloidal gold has been developed. Here, we characterize the solution thermodynamic properties of the complexes formed between 3,4,3-LI(1,2-HOPO) and Au3+ ions and demonstrate how under specific pH conditions the chelator promotes the growth of gold nanoparticles, acting as both reducing and stabilizing agent. 3,4,3-LI(1,2-HOPO) ligands on the nanoparticle surface remain active and selective toward f-block elements, as evidenced by gold nanoparticle selective aggregation. Finally, a new colorimetric assay capable of reaching the detection levels necessary for the quantification of lanthanides in waste from industrial processes is developed based on the inhibition of particle growth by lanthanides
The influence of interlayer asymmetry on the magnetospectroscopy of bilayer graphene.
We present a self-consistent calculation of the interlayer asymmetry in bilayer graphene caused by an applied electric field in magnetic fields. We show how this asymmetry influences the Landau level spectrum in bilayer graphene and the observable inter-Landau level transitions when they are studied as a function of high magnetic field at fixed filling factor as measured experimentally in Ref. [1]. We also analyze the magneto-optical spectra of bilayer flakes in the photon-energy range corresponding to transitions between degenerate and split bands of bilayers
Aromatic emission from the ionised mane of the Horsehead nebula
We study the evolution of the Aromatic Infrared Bands (AIBs) emitters across
the illuminated edge of the Horsehead nebula and especially their survival and
properties in the HII region. We present spectral mapping observations taken
with the Infrared Spectrograph (IRS) at wavelengths 5.2-38 microns. A strong
AIB at 11.3 microns is detected in the HII region, relative to the other AIBs
at 6.2, 7.7 and 8.6 microns. The intensity of this band appears to be
correlated with the intensity of the [NeII] at 12.8 microns and of Halpha,
which shows that the emitters of the 11.3 microns band are located in the
ionised gas. The survival of PAHs in the HII region could be due to the
moderate intensity of the radiation field (G0 about 100) and the lack of
photons with energy above about 25eV. The enhancement of the intensity of the
11.3 microns band in the HII region, relative to the other AIBs can be
explained by the presence of neutral PAHs. Our observations highlight a
transition region between ionised and neutral PAHs observed with ideal
conditions in our Galaxy. A scenario where PAHs can survive in HII regions and
be significantly neutral could explain the detection of a prominent 11.3
microns band in other Spitzer observations.Comment: 9 pages, 9 figures, accepted for publication in A&
SPIRE-FTS observations of RCW 120
The expansion of Galactic HII regions can trigger the formation of a new
generation of stars. However, little is know about the physical conditions that
prevail in these regions. We study the physical conditions that prevail in
specific zones towards expanding HII regions that trace representative media
such as the photodissociation region, the ionized region, and condensations
with and without ongoing star formation. We use the SPIRE Fourier Transform
Spectrometer (FTS) on board to observe the HII region RCW 120.
Continuum and lines are observed in the m range. Line intensities
and line ratios are obtained and used as physical diagnostics of the gas. We
used the Meudon PDR code and the RADEX code to derive the gas density and the
radiation field at nine distinct positions including the PDR surface and
regions with and without star-formation activity. For the different regions we
detect the atomic lines [NII] at m and [CI] at and m,
the ladder between the and levels and the
ladder between the and levels, as well as CH in absorption. We find gas temperatures in the range K for
densities of , and a high column density on the order
of that is in agreement with dust
analysis. The ubiquitousness of the atomic and CH emission suggests the
presence of a low-density PDR throughout RCW 120. High-excitation lines of CO
indicate the presence of irradiated dense structures or small dense clumps
containing young stellar objects, while we also find a less dense medium
() with high temperatures (K).Comment: 11 pages, 11 figures, accepted by A&
Dust processing in photodissociation regions - Mid-IR emission modelling
Mid-infrared spectroscopy of dense illuminated ridges (or photodissociation
regions, PDRs) suggests dust evolution. Such evolution must be reflected in the
gas physical properties through processes like photo-electric heating or H_2
formation. With Spitzer Infrared Spectrograph (IRS) and ISOCAM data, we study
the mid-IR emission of closeby, well known PDRs. Focusing on the band and
continuum dust emissions, we follow their relative contributions and analyze
their variations in terms of abundance of dust populations. In order to
disentangle dust evolution and excitation effects, we use a dust emission model
that we couple to radiative transfer. Our dust model reproduces extinction and
emission of the standard interstellar medium that we represent with diffuse
high galactic latitude clouds called Cirrus. We take the properties of dust in
Cirrus as a reference to which we compare the dust emission from more excited
regions, namely the Horsehead and the reflection nebula NGC 2023 North. We show
that in both regions, radiative transfer effects cannot account for the
observed spectral variations. We interpret these variations in term of changes
of the relative abundance between polycyclic aromatic hydrocarbons (PAHs,
mid-IR band carriers) and very small grains (VSGs, mid-IR continuum carriers).
We conclude that the PAH/VSG abundance ratio is 2.4 times smaller at the peak
emission of the Horsehead nebula than in the Cirrus case. For NGC2023 North
where spectral evolution is observed across the northern PDR, we conclude that
this ratio is ~5 times lower in the dense, cold zones of the PDR than in its
diffuse illuminated part where dust properties seem to be the same as in
Cirrus. We conclude that dust in PDRs seems to evolve from "dense" to "diffuse"
properties at the small spatial scale of the dense illuminated ridge.Comment: 11 pages, 11 figures, accepted for publication in A&
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