145 research outputs found
Non-invasive Scanning Raman Spectroscopy and Tomography for Graphene Membrane Characterization
Graphene has extraordinary mechanical and electronic properties, making it a
promising material for membrane based nanoelectromechanical systems (NEMS).
Here, chemical-vapor-deposited graphene is transferred onto target substrates
to suspend it over cavities and trenches for pressure-sensor applications. The
development of such devices requires suitable metrology methods, i.e.,
large-scale characterization techniques, to confirm and analyze successful
graphene transfer with intact suspended graphene membranes. We propose fast and
noninvasive Raman spectroscopy mapping to distinguish between freestanding and
substrate-supported graphene, utilizing the different strain and doping levels.
The technique is expanded to combine two-dimensional area scans with
cross-sectional Raman spectroscopy, resulting in three-dimensional Raman
tomography of membrane-based graphene NEMS. The potential of Raman tomography
for in-line monitoring is further demonstrated with a methodology for automated
data analysis to spatially resolve the material composition in micrometer-scale
integrated devices, including free-standing and substrate-supported graphene.
Raman tomography may be applied to devices composed of other two-dimensional
materials as well as silicon micro- and nanoelectromechanical systems.Comment: 23 pages, 5 figure
Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes
Monolayer graphene exhibits exceptional electronic and mechanical properties,
making it a very promising material for nanoelectromechanical (NEMS) devices.
Here, we conclusively demonstrate the piezoresistive effect in graphene in a
nano-electromechanical membrane configuration that provides direct electrical
readout of pressure to strain transduction. This makes it highly relevant for
an important class of nano-electromechanical system (NEMS) transducers. This
demonstration is consistent with our simulations and previously reported gauge
factors and simulation values. The membrane in our experiment acts as a strain
gauge independent of crystallographic orientation and allows for aggressive
size scalability. When compared with conventional pressure sensors, the sensors
have orders of magnitude higher sensitivity per unit area.Comment: 20 pages, 3 figure
Large Scale Integration of Graphene Transistors for Potential Applications in the Back End of the Line
A chip to wafer scale, CMOS compatible method of graphene device fabrication
has been established, which can be integrated into the back end of the line
(BEOL) of conventional semiconductor process flows. In this paper, we present
experimental results of graphene field effect transistors (GFETs) which were
fabricated using this wafer scalable method. The carrier mobilities in these
transistors reach up to several hundred cmVs. Further, these
devices exhibit current saturation regions similar to graphene devices
fabricated using mechanical exfoliation. The overall performance of the GFETs
can not yet compete with record values reported for devices based on
mechanically exfoliated material. Nevertheless, this large scale approach is an
important step towards reliability and variability studies as well as
optimization of device aspects such as electrical contacts and dielectric
interfaces with statistically relevant numbers of devices. It is also an
important milestone towards introducing graphene into wafer scale process
lines
Interplay of the volume and surface plasmons in the electron energy loss spectra of C
The results of a joint experimental and theoretical investigation of the C60
collective excitations in the process of inelastic scattering of electrons are
presented. The shape of the electron energy loss spectrum is observed to vary
when the scattering angle increases. This variation arising due to the electron
diffraction of the fullerene shell is described by a new theoretical model
which treats the fullerene as a spherical shell of a finite width and accounts
for the two modes of the surface plasmon and for the volume plasmon as well. It
is shown that at small angles, the inelastic scattering cross section is
determined mostly by the symmetric mode of the surface plasmon, while at larger
angles, the contributions of the antisymmetric surface plasmon and the volume
plasmon become prominent.Comment: 11 pages, 3 figure
Hybridization-related correction to the jellium model for fullerenes
We introduce a new type of correction for a more accurate description of
fullerenes within the spherically symmetric jellium model. This correction
represents a pseudopotential which originates from the comparison between an
accurate ab initio calculation and the jellium model calculation. It is shown
that such a correction to the jellium model allows one to account, at least
partly, for the sp2-hybridization of carbon atomic orbitals. Therefore, it may
be considered as a more physically meaningful correction as compared with a
structureless square-well pseudopotential which has been widely used earlier.Comment: 16 pages, 10 figure
Vacancy decay in endohedral atoms: the role of non-central position of the atom
We demonstrate that the Auger decay rate in an endohedral atom is very
sensitive to the atom's location in the fullerene cage. Two additional decay
channels appear in an endohedral system: (a) the channel due to the change in
the electric field at the atom caused by dynamic polarization of the fullerene
electron shell by the Coulomb field of the vacancy, (b) the channel within
which the released energy is transferred to the fullerene electron via the
Coulomb interaction. % The relative magnitudes of the correction terms are
dependent not only on the position of the doped atom but also on the transition
energy \om. Additional enhancement of the decay rate appears for transitions
whose energies are in the vicinity of the fullerene surface plasmons energies
of high multipolarity. % It is demonstrated that in many cases the additional
channels can dominate over the direct Auger decay resulting in pronounced
broadening of the atomic emission lines. % The case study, carried out for
Sc@C, shows that narrow autoionizing resonances in an
isolated Sc within the range \om = 30... 45 eV are dramatically
broadened if the ion is located strongly off-the-center. % Using the developed
model we carry out quantitative analysis of the photoionization spectrum for
the endohedral complex ScN@C and demonstrate that the additional
channels are partly responsible for the strong modification of the
photoionization spectrum profile detected experimentally by
M\"{u}ller et al. (J. Phys.: Conf. Ser. 88, 012038 (2008)).Comment: 32 pages, 11 figure
Formalism of collective electron excitations in fullerenes
We present a detailed formalism for the description of collective electron
excitations in fullerenes in the process of the electron inelastic scattering.
Considering the system as a spherical shell of a finite width, we show that the
differential cross section is defined by three plasmon excitations, namely two
coupled modes of the surface plasmon and the volume plasmon. The interplay of
the three plasmons appears due to the electron diffraction of the fullerene
shell. Plasmon modes of different angular momenta provide dominating
contributions to the differential cross section depending on the transferred
momentum.Comment: 11 pages, 2 figures; submitted to the special issue "Atomic Cluster
Collisions: Structure and Dynamics from the Nuclear to the Biological Scale"
of Eur. Phys. J.
Angular Dependences of Third Harmonic Generation from Microdroplets
We present experimental and theoretical results for the angular dependence of
third harmonic generation (THG) of water droplets in the micrometer range (size
parameter ). The THG signal in - and -polarization obtained
with ultrashort laser pulses is compared with a recently developed nonlinear
extension of classical Mie theory including multipoles of order .
Both theory and experiment yield over a wide range of size parameters
remarkably stable intensity maxima close to the forward and backward direction
at ``magic angles''. In contrast to linear Mie scattering, both are of
comparable intensity.Comment: 4 pages, RevTeX, 3 figures available on request from
[email protected], submitted to PR
Surface plasmons in metallic structures
Since the concept of a surface collective excitation was first introduced by
Ritchie, surface plasmons have played a significant role in a variety of areas
of fundamental and applied research, from surface dynamics to surface-plasmon
microscopy, surface-plasmon resonance technology, and a wide range of photonic
applications. Here we review the basic concepts underlying the existence of
surface plasmons in metallic structures, and introduce a new low-energy surface
collective excitation that has been recently predicted to exist.Comment: 14 pages, 14 figures, to appear in J. Opt. A: Pure Appl. Op
A diabetes-predictive amino acid score and future cardiovascular disease.
AimsWe recently identified a metabolic signature of three amino acids (tyrosine, phenylalanine, and isoleucine) that strongly predicts diabetes development. As novel modifiable targets for intervention are needed to meet the expected increase of cardiovascular disease (CVD) caused by the diabetes epidemic, we investigated whether this diabetes-predictive amino acid score (DM-AA score) predicts development of CVD and its functional consequences.Methods and resultsWe performed a matched case-control study derived from the population-based Malmö Diet and Cancer Cardiovascular Cohort (MDC-CC), all free of CVD. During 12 years of follow-up, 253 individuals developed CVD and were matched for age, sex, and Framingham risk score with 253 controls. Amino acids were profiled in baseline plasma samples, using liquid chromatography-tandem mass spectrometry, and relationship to incident CVD was assessed using conditional logistic regression. We further examined whether the amino acid score also correlated with anatomical [intima-media thickness (IMT) and plaque formation] and functional (exercise-induced myocardial ischaemia) abnormalities. Compared with the lowest quartile of the DM-AA score, the odds ratio (95% confidence interval) for incident CVD in subjects belonging to quartiles 2, 3, and 4 was 1.27 (0.72-2.22), 1.96 (1.07-3.60), and 2.20 (1.12-4.31) (P(trend) = 0.010), respectively, after multivariate adjustment. Increasing quartile of the DM-AA score was cross-sectionally related to carotid IMT (P(trend) = 0.037) and with the presence of at least one plaque larger than 10 mm(2) (P(trend) = 0.001). Compared with the lowest quartile of the DM-AA score, the odds ratio (95% confidence interval) for inducible ischaemia in subjects belonging to quartiles 2, 3, and 4 was 3.31 (1.05-10.4), 4.24 (1.36-13.3), and 4.86 (1.47-16.1) (P(trend) = 0.011), respectively.ConclusionThis study identifies branched-chain and aromatic amino acids as novel markers of CVD development and as an early link between diabetes and CVD susceptibility
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