76 research outputs found
Transport properties of annealed CdSe nanocrystal solids
Transport properties of artificial solids composed of colloidal CdSe
nanocrystals (NCs) are studied from 6 K to 250 K, before and after annealing.
Annealing results in greatly enhanced dark and photocurrent in NC solids, while
transmission electron microscopy (TEM) micrographs show that the inter-dot
separation decreases. The increased current can be attributed to the
enhancement of inter-dot tunneling caused by the decreased separation between
NCs and by chemical changes in their organic cap. In addition, the absorption
spectra of annealed solids are slightly red-shifted and broadened. These
red-shifts may result from the change of the dielectric environment around the
NCs. Our measurements also indicate that Coulomb interactions between charges
on neighboring NCs play an important role in the tunneling current.Comment: 24 pages,4 figures, 1 tabl
Measuring Charge Transport in an Amorphous Semiconductor Using Charge Sensing
We measure charge transport in hydrogenated amorphous silicon (a-Si:H) using
a nanometer scale silicon MOSFET as a charge sensor. This charge detection
technique makes possible the measurement of extremely large resistances. At
high temperatures, where the a-Si:H resistance is not too large, the charge
detection measurement agrees with a direct measurement of current. The device
geometry allows us to probe both the field effect and dispersive transport in
the a-Si:H using charge sensing and to extract the density of states near the
Fermi energy.Comment: 4 pages, 4 figure
Sub-optical resolution of single spins using magnetic resonance imaging at room temperature in diamond
There has been much recent interest in extending the technique of magnetic
resonance imaging (MRI) down to the level of single spins with sub-optical
wavelength resolution. However, the signal to noise ratio for images of
individual spins is usually low and this necessitates long acquisition times
and low temperatures to achieve high resolution. An exception to this is the
nitrogen-vacancy (NV) color center in diamond whose spin state can be detected
optically at room temperature. Here we apply MRI to magnetically equivalent NV
spins in order to resolve them with resolution well below the optical
wavelength of the readout light. In addition, using a microwave version of MRI
we achieved a resolution that is 1/270 size of the coplanar striplines, which
define the effective wavelength of the microwaves that were used to excite the
transition. This technique can eventually be extended to imaging of large
numbers of NVs in a confocal spot and possibly to image nearby dark spins via
their mutual magnetic interaction with the NV spin.Comment: 10 pages, 8 figures, Journal of Luminescence (Article in Press
Design of photonic crystal microcavities for cavity QED
We discuss the optimization of optical microcavity designs based on 2D
photonic crystals for the purpose of strong coupling between the cavity field
and a single neutral atom trapped within a hole. We present numerical
predictions for the quality factors and mode volumes of localized defect modes
as a function of geometric parameters, and discuss some experimental challenges
related to the coupling of a defect cavity to gas-phase atoms.Comment: 12 pages, 16 figure
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Large-area epitaxial growth of curvature-stabilized ABC trilayer graphene.
The properties of van der Waals (vdW) materials often vary dramatically with the atomic stacking order between layers, but this order can be difficult to control. Trilayer graphene (TLG) stacks in either a semimetallic ABA or a semiconducting ABC configuration with a gate-tunable band gap, but the latter has only been produced by exfoliation. Here we present a chemical vapor deposition approach to TLG growth that yields greatly enhanced fraction and size of ABC domains. The key insight is that substrate curvature can stabilize ABC domains. Controllable ABC yields ~59% were achieved by tailoring substrate curvature levels. ABC fractions remained high after transfer to device substrates, as confirmed by transport measurements revealing the expected tunable ABC band gap. Substrate topography engineering provides a path to large-scale synthesis of epitaxial ABC-TLG and other vdW materials
An adaptive inelastic magnetic mirror for Bose-Einstein condensates
We report the reflection and focussing of a Bose-Einstein condensate by a new
pulsed magnetic mirror. The mirror is adaptive, inelastic, and of extremely
high optical quality. The deviations from specularity are less than 0.5 mrad
rms, making this the best atomic mirror demonstrated to date. We have also used
the mirror to realize the analog of a beam-expander, producing an ultra-cold
collimated fountain of matter wavesComment: 4 pages, 4 figure
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
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