10,672 research outputs found
In Situ Thermal Decomposition of Exfoliated Two-Dimensional Black Phosphorus
With a semiconducting band gap and high charge carrier mobility,
two-dimensional (2D) black phosphorus (BP), often referred to as phosphorene,
holds significant promise for next generation electronics and optoelectronics.
However, as a 2D material, it possesses a higher surface area to volume ratio
than bulk BP, suggesting that its chemical and thermal stability will be
modified. Herein, an atomic-scale microscopic and spectroscopic study is
performed to characterize the thermal degradation of mechanically exfoliated 2D
BP. From in situ scanning/transmission electron microscopy, decomposition of 2D
BP is observed to occur at ~400 {\deg}C in vacuum, in contrast to the 550
{\deg}C bulk BP sublimation temperature. This decomposition initiates via
eye-shaped cracks along the [001] direction and then continues until only a
thin, amorphous red phosphorous like skeleton remains. In situ electron energy
loss spectroscopy, energy-dispersive X-ray spectroscopy, and energy-loss
near-edge structure changes provide quantitative insight into this chemical
transformation process.Comment: In press: 4 figures in main manuscript, 27 pages with supporting
informatio
Terahertz electron-hole recollisions in GaAs/AlGaAs quantum wells: robustness to scattering by optical phonons and thermal fluctuations
Electron-hole recollisions are induced by resonantly injecting excitons with
a near-IR laser at frequency into quantum wells driven by a
~10 kV/cm field oscillating at THz. At K, up to
18 sidebands are observed at frequencies , with . Electrons and holes recollide with
total kinetic energies up to 57 meV, well above the meV
threshold for longitudinal optical (LO) phonon emission. Sidebands with order
up to persist up to room temperature. A simple model shows that LO
phonon scattering suppresses but does not eliminate sidebands associated with
kinetic energies above .Comment: 5 pages, 4 figure
Dynamical birefringence: Electron-hole recollisions as probes of Berry curvature
The direct measurement of Berry phases is still a great challenge in
condensed matter systems. The bottleneck has been the ability to adiabatically
drive an electron coherently across a large portion of the Brillouin zone in a
solid where the scattering is strong and complicated. We break through this
bottleneck and show that high-order sideband generation (HSG) in semiconductors
is intimately affected by Berry phases. Electron-hole recollisions and HSG
occur when a near-band gap laser beam excites a semiconductor that is driven by
sufficiently strong terahertz (THz)-frequency electric fields. We carried out
experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum
wells. The observed HSG spectra contain sidebands up to the 90th order, to our
knowledge the highest-order optical nonlinearity observed in solids. The
highest-order sidebands are associated with electron-hole pairs driven
coherently across roughly 10% of the Brillouin zone around the \Gamma point.
The principal experimental claim is a dynamical birefringence: the sidebands,
when the order is high enough (> 20), are usually stronger when the exciting
near-infrared (NIR) and the THz electric fields are polarized perpendicular
than parallel; the sideband intensities depend on the angles between the THz
field and the crystal axes in samples with sufficiently weak quenched disorder;
and the sidebands exhibit significant ellipticity that increases with
increasing sideband order, despite nearly linear excitation and driving fields.
We explain dynamical birefringence by generalizing the three-step model for
high order harmonic generation. The hole accumulates Berry phases due to
variation of its internal state as the quasi-momentum changes under the THz
field. Dynamical birefringence arises from quantum interference between
time-reversed pairs of electron-hole recollision pathways
Solvent Exfoliation of Electronic-Grade, Two-Dimensional Black Phosphorus
Solution dispersions of two-dimensional (2D) black phosphorus (BP), often
referred to as phosphorene, are achieved by solvent exfoliation. These
pristine, electronic-grade BP dispersions are produced with anhydrous, organic
solvents in a sealed tip ultrasonication system, which circumvents BP
degradation that would otherwise occur via solvated oxygen or water. Among
conventional solvents, n-methyl-pyrrolidone (NMP) is found to provide stable,
highly concentrated (~0.4 mg/mL) BP dispersions. Atomic force microscopy,
scanning electron microscopy, transmission electron microscopy, Raman
spectroscopy, and X-ray photoelectron spectroscopy show that the structure and
chemistry of solvent-exfoliated BP nanosheets are comparable to mechanically
exfoliated BP flakes. Additionally, residual NMP from the liquid-phase
processing suppresses the rate of BP oxidation in ambient conditions.
Solvent-exfoliated BP nanosheet field-effect transistors (FETs) exhibit
ambipolar behavior with current on/off ratios and mobilities up to ~10000 and
~50 cm^2/(V*s), respectively. Overall, this study shows that stable, highly
concentrated, electronic-grade 2D BP dispersions can be realized by scalable
solvent exfoliation, thereby presenting opportunities for large-area,
high-performance BP device applications.Comment: 6 figures, 31 pages, including supporting informatio
Unified Description of Aging and Rate Effects in Yield of Glassy Solids
The competing effects of slow structural relaxations (aging) and deformation
at constant strain rate on the shear yield stress of simple model
glasses are examined using molecular simulations. At long times, aging leads to
a logarithmic increase in density and . The yield stress also rises
logarithmically with rate, but shows a sharp transition in slope at a rate that
decreases with increasing age. We present a simple phenomenological model that
includes both intrinsic rate dependence and the change in properties with the
total age of the system at yield. As predicted by the model, all data for each
temperature collapse onto a universal curve.Comment: 4 pages, 3 figure
Suppression of electron spin decoherence of the diamond NV center by a transverse magnetic field
We demonstrate that the spin decoherence of nitrogen vacancy (NV) centers in
diamond can be suppressed by a transverse magnetic field if the electron spin
bath is the primary decoherence source. The NV spin coherence, created in "a
decoherence-free subspace" is protected by the transverse component of the
zero-field splitting, increasing the spin-coherence time about twofold. The
decoherence due to the electron spin bath is also suppressed at magnetic fields
stronger than ~25 gauss when applied parallel to the NV symmetry axis. Our
method can be used to extend the spin-coherence time of similar spin systems
for applications in quantum computing, field sensing, and other metrologies.Comment: 20 pages, 4 figure
Jamming under tension in polymer crazes
Molecular dynamics simulations are used to study a unique expanded jammed
state. Tension transforms many glassy polymers from a dense glass to a network
of fibrils and voids called a craze. Entanglements between polymers and
interchain friction jam the system after a fixed increase in volume. As in
dense jammed systems, the distribution of forces is exponential, but they are
tensile rather than compressive. The broad distribution of forces has important
implications for fibril breakdown and the ultimate strength of crazes.Comment: 4 pages, 4 figure
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