59 research outputs found
Effect of oxygen plasma etching on graphene studied with Raman spectroscopy and electronic transport
We report a study of graphene and graphene field effect devices after
exposure to a series of short pulses of oxygen plasma. We present data from
Raman spectroscopy, back-gated field-effect and magneto-transport measurements.
The intensity ratio between Raman "D" and "G" peaks, I(D)/I(G) (commonly used
to characterize disorder in graphene) is observed to increase approximately
linearly with the number (N(e)) of plasma etching pulses initially, but then
decreases at higher Ne. We also discuss implications of our data for extracting
graphene crystalline domain sizes from I(D)/I(G). At the highest Ne measured,
the "2D" peak is found to be nearly suppressed while the "D" peak is still
prominent. Electronic transport measurements in plasma-etched graphene show an
up-shifting of the Dirac point, indicating hole doping. We also characterize
mobility, quantum Hall states, weak localization and various scattering lengths
in a moderately etched sample. Our findings are valuable for understanding the
effects of plasma etching on graphene and the physics of disordered graphene
through artificially generated defects.Comment: 10 pages, 5 figure
Electronic Transport in Chemical Vapor Deposited Graphene Synthesized on Cu: Quantum Hall Effect and Weak Localization
We report on electronic properties of graphene synthesized by chemical vapor
deposition (CVD) on copper then transferred to SiO2/Si. Wafer-scale (up to 4
inches) graphene films have been synthesized, consisting dominantly of
monolayer graphene as indicated by spectroscopic Raman mapping. Low temperature
transport measurements are performed on micro devices fabricated from such CVD
graphene, displaying ambipolar field effect (with on/off ratio ~5 and carrier
mobilities up to ~3000 cm^2/Vs) and "half-integer" quantum Hall effect, a
hall-mark of intrinsic electronic properties of monolayer graphene. We also
observe weak localization and extract information about phase coherence and
scattering of carriers.Comment: shortened version, published on APL. See version 1 for more Raman
data
Towards Exascale Computation for Turbomachinery Flows
A state-of-the-art large eddy simulation code has been developed to solve
compressible flows in turbomachinery. The code has been engineered with a high
degree of scalability, enabling it to effectively leverage the many-core
architecture of the new Sunway system. A consistent performance of 115.8
DP-PFLOPs has been achieved on a high-pressure turbine cascade consisting of
over 1.69 billion mesh elements and 865 billion Degree of Freedoms (DOFs). By
leveraging a high-order unstructured solver and its portability to large
heterogeneous parallel systems, we have progressed towards solving the grand
challenge problem outlined by NASA, which involves a time-dependent simulation
of a complete engine, incorporating all the aerodynamic and heat transfer
components.Comment: SC23, November, 2023, Denver, CO., US
Control and Characterization of Individual Grains and Grain Boundaries in Graphene Grown by Chemical Vapor Deposition
The strong interest in graphene has motivated the scalable production of high
quality graphene and graphene devices. Since large-scale graphene films
synthesized to date are typically polycrystalline, it is important to
characterize and control grain boundaries, generally believed to degrade
graphene quality. Here we study single-crystal graphene grains synthesized by
ambient CVD on polycrystalline Cu, and show how individual boundaries between
coalescing grains affect graphene's electronic properties. The graphene grains
show no definite epitaxial relationship with the Cu substrate, and can cross Cu
grain boundaries. The edges of these grains are found to be predominantly
parallel to zigzag directions. We show that grain boundaries give a significant
Raman "D" peak, impede electrical transport, and induce prominent weak
localization indicative of intervalley scattering in graphene. Finally, we
demonstrate an approach using pre-patterned growth seeds to control graphene
nucleation, opening a route towards scalable fabrication of single-crystal
graphene devices without grain boundaries.Comment: New version with additional data. Accepted by Nature Material
The Human Activity Radar Challenge: benchmarking based on the ‘Radar signatures of human activities’ dataset from Glasgow University
Radar is an extremely valuable sensing technology for detecting moving targets and measuring their range, velocity, and angular positions. When people are monitored at home, radar is more likely to be accepted by end-users, as they already use WiFi, is perceived as privacy-preserving compared to cameras, and does not require user compliance as wearable sensors do. Furthermore, it is not affected by lighting condi-tions nor requires artificial lights that could cause discomfort in the home environment. So, radar-based human activities classification in the context of assisted living can empower an aging society to live at home independently longer. However, challenges remain as to the formulation of the most effective algorithms for radar-based human activities classification and their validation. To promote the exploration and cross-evaluation of different algorithms, our dataset released in 2019 was used to benchmark various classification approaches. The challenge was open from February 2020 to December 2020. A total of 23 organizations worldwide, forming 12 teams from academia and industry, participated in the inaugural Radar Challenge, and submitted 188 valid entries to the challenge. This paper presents an overview and evaluation of the approaches used for all primary contributions in this inaugural challenge. The proposed algorithms are summarized, and the main parameters affecting their performances are analyzed
Effects of Crystalline Anisotropy and Indenter Size on Nanoindentation by Multiscale Simulation
Nanoindentation processes in single crystal Ag thin film under different crystallographic orientations and various indenter widths are simulated by the quasicontinuum method. The nanoindentation deformation processes under influences of crystalline anisotropy and indenter size are investigated about hardness, load distribution, critical load for first dislocation emission and strain energy under the indenter. The simulation results are compared with previous experimental results and Rice-Thomson (R-T) dislocation model solution. It is shown that entirely different dislocation activities are presented under the effect of crystalline anisotropy during nanoindentation. The sharp load drops in the load–displacement curves are caused by the different dislocation activities. Both crystalline anisotropy and indenter size are found to have distinct effect on hardness, contact stress distribution, critical load for first dislocation emission and strain energy under the indenter. The above quantities are decreased at the indenter into Ag thin film along the crystal orientation with more favorable slip directions that easy trigger slip systems; whereas those will increase at the indenter into Ag thin film along the crystal orientation with less or without favorable slip directions that hard trigger slip systems. The results are shown to be in good agreement with experimental results and R-T dislocation model solution
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