743 research outputs found
Thermodynamic calculations on the catalytic growth of multiwall carbon nanotubes
We have developed a thermodynamic model of the catalytic growth of multiwall
carbon nanotubes from hydrocarbon precursors at elevated temperature. Using
this model we have computed the heat distribution, and carbon concentration in
the catalyst. Calculations delivered a analytical formula for the growth time
and growth rate. We find that the growth is mainly driven by a concentration
gradient within the catalyst, rather than a temperature gradient.Comment: 9 pages, 3 figures, 1 tabl
Two-dimensional magnetotransport in Bi2Te2Se nanoplatelets
Single-crystalline Bi2Te2Se nanoplates with thicknesses between 8 and 30 nm
and lateral sizes of several micrometers were synthesized by a vapour-solid
growth method. Angle-dependent magnetoconductance measurements on individual
nanoplates revealed the presence of a two-dimensional weak anti-localization
effect. In conjunction with gate-dependent charge transport studies performed
at different temperatures, evidence was gained that this effect originates from
the topologically protected surface states of the nanoplates
Photon super-bunching from a generic tunnel junction
Generating correlated photon pairs at the nanoscale is a prerequisite to
creating highly integrated optoelectronic circuits that perform quantum
computing tasks based on heralded single-photons. Here we demonstrate
fulfilling this requirement with a generic tip-surface metal junction. When the
junction is luminescing under DC bias, inelastic tunneling events of single
electrons produce a photon stream in the visible spectrum whose super-bunching
index is 17 when measured with a 53 picosecond instrumental resolution limit.
These photon bunches contain true photon pairs of plasmonic origin, distinct
from accidental photon coincidences. The effect is electrically rather than
optically driven - completely absent are pulsed lasers, down-conversions, and
four-wave mixing schemes. This discovery has immediate and profound
implications for quantum optics and cryptography, notwithstanding its
fundamental importance to basic science and its ushering in of heralded photon
experiments on the nanometer scale
Raman spectroscopy and field emission measurements on catalytically grown carbon nanotubes
We used microcontact printing to pattern a silicon surface with an
iron-containing catalytic solution. Multi-wall carbon nanotubes were
subsequently grown on the patterned areas by chemical vapor deposition at
temperatures between 650 and 1000C. We demonstrate that the diameter of the
catalytically grown multi-wall nanotubes increases with the deposition
temperature. Raman spectroscopy has been used to investigate the crystalline
character of the obtained structures and it is found that the fraction of the
nano-crystalline shell increases with the temperatures. The measurement of the
field emission properties shows a correlation between the tube diameter and the
emission field values.Comment: 6 pages, 6 figures, 1 tabl
Illuminating the dark corridor in graphene: polarization dependence of angle-resolved photoemission spectroscopy on graphene
We have used s- and p-polarized synchrotron radiation to image the electronic
structure of epitaxial graphene near the K-point by angular resolved
photoemission spectroscopy (ARPES). Part of the experimental Fermi surface is
suppressed due to the interference of photoelectrons emitted from the two
equivalent carbon atoms per unit cell of graphene's honeycomb lattice. Here we
show that by rotating the polarization vector, we are able to illuminate this
'dark corridor' indicating that the present theoretical understanding is
oversimplified. Our measurements are supported by first-principles
photoemission calculations, which reveal that the observed effect persists in
the low photon energy regime.Comment: 5 pages, 4 figure
Single charge and exciton dynamics probed by molecular-scale-induced electroluminescence
Excitons and their constituent charge carriers play the central role in
electroluminescence mechanisms determining the ultimate performance of organic
optoelectronic devices. The involved processes and their dynamics are often
studied with time-resolved techniques limited by spatial averaging that
obscures the properties of individual electron-hole pairs. Here we overcome
this limit and characterize single charge and exciton dynamics at the nanoscale
by using time-resolved scanning tunnelling microscopy-induced luminescence
(TR-STML) stimulated with nanosecond voltage pulses. We use isolated defects in
C thin films as a model system into which we inject single charges and
investigate the formation dynamics of a single exciton. Tuneable hole and
electron injection rates are obtained from a kinetic model that reproduces the
measured electroluminescent transients. These findings demonstrate that TR-STML
can track dynamics at the quantum limit of single charge injection and can be
extended to other systems and materials important for nanophotonic devices
Quantum Brownian motion at strong dissipation probed by superconducting tunnel junctions
We have studied the temporal evolution of a quantum system subjected to
strong dissipation at ultra-low temperatures where the system-bath interaction
represents the leading energy scale. In this regime, theory predicts the time
evolution of the system to follow a generalization of the classical
Smoluchowski description, the quantum Smoluchowski equation, thus, exhibiting
quantum Brownian motion characteristics. For this purpose, we have investigated
the phase dynamics of a superconducting tunnel junction in the presence of high
damping. We performed current-biased measurements on the small-capacitance
Josephson junction of a scanning tunneling microscope placed in a low impedance
environment at milli-Kelvin temperatures. We can describe our experimental
findings by a quantum diffusion model with high accuracy in agreement with
theoretical predications based on the quantum Smoluchowski equation. In this
way we experimentally demonstrate that quantum systems subjected to strong
dissipation follow quasi-classical dynamics with significant quantum effects as
the leading corrections.Comment: 5 pages, 4 figure
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