857 research outputs found
Electrical and Noise Characteristics of Graphene Field-Effect Transistors: Ambient Effects and Noise Sources
We fabricated a large number of single and bilayer graphene transistors and
carried out a systematic experimental study of their low-frequency noise
characteristics. A special attention was given to determining the dominant
noise sources in these devices and the effect of aging on the current-voltage
and noise characteristics. The analysis of the noise spectral density
dependence on the area of graphene channel showed that the dominant
contributions to the low-frequency electronic noise come from the graphene
layer itself rather than from the contacts. Aging of graphene transistors due
to exposure to ambient for over a month resulted in substantially increased
noise attributed to the decreasing mobility of graphene and increasing contact
resistance. The noise spectral density in both single and bilayer graphene
transistors either increased with deviation from the charge neutrality point or
depended weakly on the gate bias. This observation confirms that the
low-frequency noise characteristics of graphene transistors are qualitatively
different from those of conventional silicon metal-oxide-semiconductor
field-effect transistors.Comment: 26 pages with 8 figure
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
Micro-Raman Spectroscopy of Mechanically Exfoliated Few-Quintuple Layers of Bi(2)Te(3), Bi(2)Se(3) and Sb(2)Te(3) Materials
Bismuth telluride - Bi(2)Te(3)- and related compounds have recently attracted
strong interest owing to the discovery of the topological insulator properties
in many members of this family of materials. The few-quintuple films of these
materials are particularly interesting from the physics point of view. We
report results of the micro-Raman spectroscopy study of the "graphene-like"
exfoliated few-quintuple layers of Bi(2)Te(3), Bi(2)Se(3) and Sb(2)Te(3). It is
found that crystal symmetry breaking in few-quintuple films results in
appearance of A1u-symmetry Raman peaks, which are not active in the bulk
crystals. The scattering spectra measured under the 633-nm wavelength
excitation reveals a number of resonant features, which could be used for
analysis of the electronic and phonon processes in these materials. In order to
elucidate the influence of substrates on the few-quintuple-thick topological
insulators we examined the Raman spectra of these films placed on mica,
sapphire and hafnium-oxide substrates. The obtained results help to understand
the physical mechanisms of Raman scattering in the few-quintuple-thick films
and can be used for nanometrology of topological insulator films on various
substrates.Comment: 19 pages; 7 figure
Photo-Thermoelectric Effect at a Graphene Interface Junction
We investigate the optoelectronic response of a graphene interface junction,
formed with bilayer and single-layer graphene, by photocurrent (PC) microscopy.
We measure the polarity and amplitude of the PC while varying the Fermi level
by tuning a gate voltage. These measurements show that the generation of PC is
by a photo-thermoelectric effect. The PC displays a factor of ~10 increase at
the cryogenic temperature as compared to room temperature. Assuming the
thermoelectric power has a linear dependence on the temperature, the inferred
graphene thermal conductivity from temperature dependent measurements has a
T^{1.5} dependence below ~100 K, which agrees with recent theoretical
predictions
Nanoscale Mechanical Drumming Visualized by 4D Electron Microscopy
With four-dimensional (4D) electron microscopy, we report in situ imaging of the mechanical drumming of a nanoscale material. The single crystal graphite film is found to exhibit global resonance motion that is fully reversible and follows the same evolution after each initiating stress pulse. At early times, the motion appears âchaoticâ showing the different mechanical modes present over the micron scale. At longer time, the motion of the thin film collapses into a well-defined fundamental frequency of 1.08 MHz, a behavior reminiscent of mode locking; the mechanical motion damps out after âŒ200 ÎŒs and the oscillation has a âcavityâ quality factor of 150. The resonance time is determined by the stiffness of the material, and for the 75 nm thick and 40 ÎŒm square specimen used here we determined Youngâs modulus to be 1.0 TPa for the in-plane stressâstrain profile. Because of its real-time dimension, this 4D microscopy should have applications in the study of these and other types of materials structures
Graphene -- Based Nanocomposites as Highly Efficient Thermal Interface Materials
We found that an optimized mixture of graphene and multilayer graphene -
produced by the high-yield inexpensive liquid-phase-exfoliation technique - can
lead to an extremely strong enhancement of the cross-plane thermal conductivity
K of the composite. The "laser flash" measurements revealed a record-high
enhancement of K by 2300 % in the graphene-based polymer at the filler loading
fraction f =10 vol. %. It was determined that a relatively high concentration
of single-layer and bilayer graphene flakes (~10-15%) present simultaneously
with thicker multilayers of large lateral size (~ 1 micrometer) were essential
for the observed unusual K enhancement. The thermal conductivity of a
commercial thermal grease was increased from an initial value of ~5.8 W/mK to
K=14 W/mK at the small loading f=2%, which preserved all mechanical properties
of the hybrid. Our modeling results suggest that graphene - multilayer graphene
nanocomposite used as the thermal interface material outperforms those with
carbon nanotubes or metal nanoparticles owing to graphene's aspect ratio and
lower Kapitza resistance at the graphene - matrix interface.Comment: 4 figure
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