43 research outputs found
Disorder induced Coulomb gaps in graphene constrictions with different aspect ratios
We present electron transport measurements on lithographically defined and
etched graphene nanoconstrictions with different aspect ratios including
different lengths (L) and widths (W). A roughly length-independent disorder
induced effective energy gap can be observed around the charge neutrality
point. This energy gap scales inversely with the width even in regimes where
the length of the constriction is smaller than its width (L<W). In very short
constrictions, we observe both resonances due to localized states or charged
islands and an elevated overall conductance level (0.1-1e2/h), which is
strongly length-dependent in the gap region. This makes very short graphene
constrictions interesting for highly transparent graphene tunneling barriers.Comment: 4 pages, 4 figure
Raman spectroscopy on mechanically exfoliated pristine graphene ribbons
We present Raman spectroscopy measurements of non-etched graphene
nanoribbons, with widths ranging from 15 to 160 nm, where the D-line intensity
is strongly dependent on the polarization direction of the incident light. The
extracted edge disorder correlation length is approximately one order of
magnitude larger than on previously reported graphene ribbons fabricated by
reactive ion etching techniques. This suggests a more regular crystallographic
orientation of the non-etched graphene ribbons here presented. We further
report on the ribbons width dependence of the line-width and frequency of the
long-wavelength optical phonon mode (G-line) and the 2D-line of the studied
graphene ribbons
Limitations to Carrier Mobility and Phase-Coherent Transport in Bilayer Graphene
We present transport measurements on high-mobility bilayer graphene fully
encapsulated in hexagonal boron nitride. We show two terminal quantum Hall
effect measurements which exhibit full symmetry broken Landau levels at low
magnetic fields. From weak localization measurements, we extract gate-tunable
phase coherence times as well as the inter- and intra-valley
scattering times and . While is in qualitative
agreement with an electron-electron interaction mediated dephasing mechanism,
electron spin-flip scattering processes are limiting at low
temperatures. The analysis of and points to local strain
fluctuation as the most probable mechanism for limiting the mobility in
high-quality bilayer graphene
Negative quantum capacitance in graphene nanoribbons with lateral gates
We present numerical simulations of the capacitive coupling between graphene
nanoribbons of various widths and gate electrodes in different configurations.
We compare the influence of lateral metallic or graphene side gate structures
on the overall back gate capacitive coupling. Most interestingly, we find a
complex interplay between quantum capacitance effects in the graphene
nanoribbon and the lateral graphene side gates, giving rise to an
unconventional negative quantum capacitance. The emerging non-linear capacitive
couplings are investigated in detail. The experimentally relevant relative
lever arm, the ratio between the coupling of the different gate structures, is
discussed.Comment: 8 pages, 6 figure
Transport in coupled graphene-nanotube quantum devices
We report on the fabrication and characterization of all-carbon hybrid
quantum devices based on graphene and single-walled carbon nanotubes. We
discuss both, carbon nanotube quantum dot devices with graphene charge
detectors and nanotube quantum dots with graphene leads. The devices are
fabricated by chemical vapor deposition growth of carbon nanotubes and
subsequent structuring of mechanically exfoliated graphene. We study the
detection of individual charging events in the carbon nanotube quantum dot by a
nearby graphene nanoribbon and show that they lead to changes of up to 20% of
the conductance maxima in the graphene nanoribbon acting as a good performing
charge detector. Moreover, we discuss an electrically coupled graphene-nanotube
junction, which exhibits a tunneling barrier with tunneling rates in the low
GHz regime. This allows to observe Coulomb blockade on a carbon nanotube
quantum dot with graphene source and drain leads
Simultaneous determination of iron and copper in children's sera by FAAS
Predložena je nova jednostavna metoda plamene atomsko-apsorpcijske spektrometrije (FAAS), za simultano određivanje željeza i bakra u serumu djece. Ona se temelji na predobradbi uzorka u jednom koraku (deproteinizacija s 3 mol L–3 HCl u odnosu 1:1) i kalibraciji u jednom koraku sa standardom pripravljenim u 1.5 mol L–3 HCl.
Tijekom optimizacije metode primijenjen je multifaktorski dizajnirani eksperiment. Preporučena metoda osigurava ispravnost, osjetljivost i preciznost usporedljivu onima referentnih metoda. Novi je pristup jednostavan i brz; on štedi i vrijeme i reagense i uzorke, pri čemu je potonje posebno važno u dječjoj dijagnostici.A new and simple flame atomic-absorption spectrometric (FAAS) method is proposed for simultaneous determination of iron and copper in children's sera. It is based on single-step sample pretreatment (deproteinization with 3 mol L–1 HCl, ratio 1:1) and single-step calibration using 1.5 mol L–1 HCl standard. During method’s optimization a short multifactorial design experiment was used. The proposed method assures accuracy, sensitivity and precision comparable to that of the reference methods. The new approach is simple and time-, labour- and serum-saving, the latter being especially important in pediatric diagnostics
Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature
We present a new fabrication method of graphene spin-valve devices which
yields enhanced spin and charge transport properties by improving both the
electrode-to-graphene and graphene-to-substrate interface. First, we prepare
Co/MgO spin injection electrodes onto Si/SiO. Thereafter, we
mechanically transfer a graphene-hBN heterostructure onto the prepatterned
electrodes. We show that room temperature spin transport in single-, bi- and
trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion
lengths reaching 10m combined with carrier mobilities exceeding 20,000
cm/Vs.Comment: 15 pages, 5 figure
Hot-Carrier Cooling in High-Quality Graphene is Intrinsically Limited by Optical Phonons
Many promising optoelectronic devices, such as broadband photodetectors,
nonlinear frequency converters, and building blocks for data communication
systems, exploit photoexcited charge carriers in graphene. For these systems,
it is essential to understand, and eventually control, the cooling dynamics of
the photoinduced hot-carrier distribution. There is, however, still an active
debate on the different mechanisms that contribute to hot-carrier cooling. In
particular, the intrinsic cooling mechanism that ultimately limits the cooling
dynamics remains an open question. Here, we address this question by studying
two technologically relevant systems, consisting of high-quality graphene with
a mobility >10,000 cmVs and environments that do not
efficiently take up electronic heat from graphene: WSe-encapsulated
graphene and suspended graphene. We study the cooling dynamics of these two
high-quality graphene systems using ultrafast pump-probe spectroscopy at room
temperature. Cooling via disorder-assisted acoustic phonon scattering and
out-of-plane heat transfer to the environment is relatively inefficient in
these systems, predicting a cooling time of tens of picoseconds. However, we
observe much faster cooling, on a timescale of a few picoseconds. We attribute
this to an intrinsic cooling mechanism, where carriers in the hot-carrier
distribution with enough kinetic energy emit optical phonons. During phonon
emission, the electronic system continuously re-thermalizes, re-creating
carriers with enough energy to emit optical phonons. We develop an analytical
model that explains the observed dynamics, where cooling is eventually limited
by optical-to-acoustic phonon coupling. These fundamental insights into the
intrinsic cooling mechanism of hot carriers in graphene will play a key role in
guiding the development of graphene-based optoelectronic devices
Gene Expression Profiling in Gastric Mucosa from Helicobacter pylori-Infected and Uninfected Patients Undergoing Chronic Superficial Gastritis
Helicobacter pylori infection reprograms host gene expression and influences various cellular processes, which have been investigated by cDNA microarray using in vitro culture cells and in vivo gastric biopsies from patients of the Chronic Abdominal Complaint. To further explore the effects of H. pylori infection on host gene expression, we have collected the gastric antral mucosa samples from 6 untreated patients with gastroscopic and pathologic confirmation of chronic superficial gastritis. Among them three patients were infected by H. pylori and the other three patients were not. These samples were analyzed by a microarray chip which contains 14,112 cloned cDNAs, and microarray data were analyzed via BRB ArrayTools software and Ingenuity Pathways Analysis (IPA) website. The results showed 34 genes of 38 differentially expressed genes regulated by H. pylori infection had been annotated. The annotated genes were involved in protein metabolism, inflammatory and immunological reaction, signal transduction, gene transcription, trace element metabolism, and so on. The 82% of these genes (28/34) were categorized in three molecular interaction networks involved in gene expression, cancer progress, antigen presentation and inflammatory response. The expression data of the array hybridization was confirmed by quantitative real-time PCR assays. Taken together, these data indicated that H. pylori infection could alter cellular gene expression processes, escape host defense mechanism, increase inflammatory and immune responses, activate NF-κB and Wnt/β-catenin signaling pathway, disturb metal ion homeostasis, and induce carcinogenesis. All of these might help to explain H. pylori pathogenic mechanism and the gastroduodenal pathogenesis induced by H. pylori infection