258 research outputs found
Difference frequency laser spectroscopy of the nu3 fundamental band of NH<sup> + </sup><sub>2</sub>
The nu3 band of NH + 2 in the X-tilde 3B1 ground electronic state was observed in direct absorption with a tunable difference frequency laser spectrometer in the 3 µ region, using velocity modulation detection. NH + 2 and NH + 3 ions were generated in an ac discharge of He and NH3, or of He, N2, and H2. Fifty-three rovibrational transitions were measured and fit to a triplet A-reduced Hamiltonian to determine rotational, centrifugal distortion, and spin–rotation constants. The band origin was found to be nu0=3359.932 cm^−1, in excellent agreement with a recent calculation of Jensen, Bunker, and McLean. Indirect evidence from the spectrum suggested that NH + 2 is quasilinear, but selection rules prevented a determination of the A rotational constant
Statistical mechanics of double-stranded semi-flexible polymers
We study the statistical mechanics of double-stranded semi-flexible polymers
using both analytical techniques and simulation. We find a transition at some
finite temperature, from a type of short range order to a fundamentally
different sort of short range order. In the high temperature regime, the
2-point correlation functions of the object are identical to worm-like chains,
while in the low temperature regime they are different due to a twist
structure. In the low temperature phase, the polymers develop a kink-rod
structure which could clarify some recent puzzling experiments on actin.Comment: 4 pages, 3 figures; final version for publication - slight
modifications to text and figure
Levy statistics and anomalous transport in quantum-dot arrays
A novel model of transport is proposed to explain power law current
transients and memory phenomena observed in partially ordered arrays of
semiconducting nanocrystals. The model describes electron transport by a
stationary Levy process of transmission events and thereby requires no time
dependence of system properties. The waiting time distribution with a
characteristic long tail gives rise to a nonstationary response in the presence
of a voltage pulse. We report on noise measurements that agree well with the
predicted non-Poissonian fluctuations in current, and discuss possible
mechanisms leading to this behavior.Comment: 7 pages, 2 figure
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Incorporation of Iron Oxide Nanoparticles and Quantum Dots into Silica Microspheres
We describe the synthesis of magnetic and fluorescent silica microspheres fabricated by incorporating maghemite (γ-Fe2O3) nanoparticles (MPs) and CdSe/CdZnS core/shell quantum dots (QDs) into a silica shell around preformed silica microspheres. The resultant 500 nm microspheres have a narrow size distribution and show uniform incorporation of QDs and MPs into the shell. We have demonstrated manipulation of these microspheres using an external magnetic field with real-time fluorescence microscopy imaging.Engineering and Applied Science
Cathodoluminescence and photoluminescence of highly luminescent CdSe/ZnS quantum dot composites
We report room-temperature cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSenanocrystals, 33 and 42 Ã… in diameter, embedded in a ZnS matrix. The thin-filmquantum dot composites were synthesized by electrospray organometallic chemical vapor deposition.Cathodoluminescence and photoluminescence are dominated by the sharp band-edge emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad window (470-650 nm) by varying the size of the dots. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and the voltage and current density applied
Metabolic Tumor Profiling with pH, Oxygen, and Glucose Chemosensors on a Quantum Dot Scaffold
Acidity, hypoxia, and glucose levels characterize the tumor microenvironment rendering pH, pO2, and pGlucose, respectively, important indicators of tumor health. To this end, understanding how these parameters change can be a powerful tool for the development of novel and effective therapeutics. We have designed optical chemosensors that feature a quantum dot and an analyte-responsive dye. These noninvasive chemosensors permit pH, oxygen, and glucose to be monitored dynamically within the tumor microenvironment by using multiphoton imaging.National Cancer Institute (U.S.) (Grant R01-CA126642
Electronic transport in films of colloidal CdSe nanocrystals
We present results for electronic transport measurements on large
three-dimensional arrays of CdSe nanocrystals. In response to a step in the
applied voltage, we observe a power-law decay of the current over five orders
of magnitude in time. Furthermore, we observe no steady-state dark current for
fields up to 10^6 V/cm and times as long as 2x10^4 seconds. Although the
power-law form of the decay is quite general, there are quantitative variations
with temperature, applied field, sample history, and the material parameters of
the array. Despite evidence that the charge injected into the film during the
measurement causes the decay of current, we find field-scaling of the current
at all times. The observation of extremely long-lived current transients
suggests the importance of long-range Coulomb interactions between charges on
different nanocrystals.Comment: 11 pages, 10 figure
Synthesis of CdS and CdSe nanocrystallites using a novel single-molecule precursors approach
The synthesis of CdS and CdSe nanocrystallites using the thermolysis of several dithioor
diselenocarbamato complexes of cadmium in trioctylphosphine oxide (TOPO) is reported.
The nanodispersed materials obtained show quantum size effects in their optical spectra
and exhibit near band-edge luminescence. The influence of experimental parameters on
the properties of the nanocrystallites is discussed. HRTEM images of these materials show
well-defined, crystalline nanosized particles. Standard size fractionation procedures can
be performed in order to narrow the size dispersion of the samples. The TOPO-capped CdS
and CdSe nanocrystallites and simple organic bridging ligands, such as 2,2¢-bipyrimidine,
are used as the starting materials for the preparation of novel nanocomposites. The optical
properties shown by these new nanocomposites are compared with those of the starting
nanodispersed materials
Statistical mechanics of semiflexible ribbon polymers
The statistical mechanics of a ribbon polymer made up of two semiflexible
chains is studied using both analytical techniques and simulation. The system
is found to have a crossover transition at some finite temperature, from a type
of short range order to a fundamentally different sort of short range order. In
the high temperature regime, the 2-point correlation functions of the object
are identical to worm-like chains, while in the low temperature regime they are
different due to a twist structure. The crossover happens when the persistence
length of individual strands becomes comparable to the thickness of the ribbon.
In the low temperature regime, the ribbon is observed to have a novel
``kink-rod'' structure with a mutual exclusion of twist and bend in contrast to
smooth worm-like chain behaviour. This is due to its anisotropic rigidity and
corresponds to an {\it infinitely} strong twist-bend coupling. The
double-stranded polymer is also studied in a confined geometry. It is shown
that when the polymer is restricted in a particular direction to a size less
than the bare persistence length of the individual strands, it develops zigzag
conformations which are indicated by an oscillatory tangent-tangent correlation
function in the direction of confinement. Increasing the separation of the
confining plates leads to a crossover to the free behaviour, which takes place
at separations close to the bare persistence length. These results are expected
to be relevant for experiments which involve complexation of two or more stiff
or semiflexible polymers.Comment: 20 pages, 11 figures. PRE (in press
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