12,867 research outputs found
Environmental, Thermal, and Electrical Susceptibility of Black Phosphorus Field Effect Transistors
Atomic layers of black phosphorus (P) isolated from its layered bulk make a
new two-dimensional (2D) semiconducting crystal with sizable direct bandgap,
high carrier mobility, and promises for 2D electronics and optoelectronics.
However, the integrity of black P crystal could be susceptible to a number of
environmental variables and processes, resulting in degradation in device
performance even before the device optical image suggests so. Here, we perform
a systematic study of the environmental effects on black P electronic devices
through continued measurements over a month under a number of controlled
conditions, including ambient light, air, and humidity, and identify evolution
of device performance under each condition. We further examine effects of
thermal and electrical treatments on inducing morphology and, performance
changes and failure modes in black P devices. The results suggest that
procedures well established for nanodevices in other 2D materials may not
directly apply to black P devices, and improved procedures need to be devised
to attain stable device operation.Comment: in Journal of Vacuum Science & Technology B (2015
Radio Sources in the NCP Region Observed with the 21 Centimeter Array
We present a catalog of 624 radio sources detected around the North Celestial
Pole (NCP) with the 21 Centimeter Array (21CMA), a radio interferometer
dedicated to the statistical measurement of the epoch of reionization (EoR).
The data are taken from a 12 h observation made on 2013 April 13, with a
frequency coverage from 75 to 175 MHz and an angular resolution of ~ 4 arcmin.
The catalog includes flux densities at eight sub-bands across the 21CMA
bandwidth and provides the in-band spectral indices for the detected sources.
To reduce the complexity of interferometric imaging from the so-called "w" term
and ionospheric effects, the present analysis are restricted to the east-west
baselines within 1500 m only. The 624 radio sources are found within 5 degrees
around the NCP down to ~ 0.1 Jy. Our source counts are compared, and also
exhibit a good agreement, with deep low-frequency observations made recently
with the GMRT and MWA. In particular, for fainter radio sources below ~ 1 Jy,
we find a flattening trend of source counts towards lower frequencies. While
the thermal noise (~0.4 mJy) is well controlled to below the confusion limit,
the dynamical range (~10^4) and sensitivity of current 21CMA imaging is largely
limited by calibration and deconvolution errors, especially the grating lobes
of very bright sources, such as 3C061.1, in the NCP field which result from the
regular spacings of the 21CMA. We note that particular attention should be paid
to the extended sources, and their modeling and removals may constitute a large
technical challenge for current EoR experiments. Our analysis may serve as a
useful guide to design of next generation low-frequency interferometers like
the Square Kilometre Array.Comment: 16 pages, 21 figures, 7 tables, 1 machine readable table, accepted
for publication in Ap
Fixed trace -Hermite ensembles: Asymptotic eigenvalue density and the edge of the density
In the present paper, fixed trace -Hermite ensembles generalizing the
fixed trace Gaussian Hermite ensemble are considered. For all , we prove
the Wigner semicircle law for these ensembles by using two different methods:
one is the moment equivalence method with the help of the matrix model for
general , the other is to use asymptotic analysis tools. At the edge of
the density, we prove that the edge scaling limit for -HE implies the
same limit for fixed trace -Hermite ensembles. Consequently, explicit
limit can be given for fixed trace GOE, GUE and GSE. Furthermore, for even
, analogous to -Hermite ensembles, a multiple integral of the
Konstevich type can be obtained.Comment: 16 page
Resolving and Tuning Mechanical Anisotropy in Black Phosphorus via Nanomechanical Multimode Resonance Spectromicroscopy
Black phosphorus (P) has emerged as a layered semiconductor with a unique
crystal structure featuring corrugated atomic layers and strong in-plane
anisotropy in its physical properties. Here, we demonstrate that the crystal
orientation and mechanical anisotropy in free-standing black P thin layers can
be precisely determined by spatially resolved multimode nanomechanical
resonances. This offers a new means for resolving important crystal orientation
and anisotropy in black P device platforms in situ beyond conventional optical
and electrical calibration techniques. Furthermore, we show that
electrostatic-gating-induced straining can continuously tune the mechanical
anisotropic effects on multimode resonances in black P electromechanical
devices. Combined with finite element modeling (FEM), we also determine the
Young's moduli of multilayer black P to be 116.1 and 46.5 GPa in the zigzag and
armchair directions, respectively.Comment: Main Text: 13 Pages, 4 Figures; Supplementary Information: 5 Pages, 2
Figures, 2 Table
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