506 research outputs found
Structure and electronic transport in graphene wrinkles
Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In
particular, in the large-scale growth of graphene on metallic substrates, high
densities of wrinkles are commonly observed. Despite their prevalence and
potential impact on large-scale graphene electronics, relatively little is
known about their structural morphology and electronic properties. Surveying
the graphene landscape using atomic force microscopy, we found that wrinkles
reach a certain maximum height before folding over. Calculations of the
energetics explain the morphological transition, and indicate that the tall
ripples are collapsed into narrow standing wrinkles by van der Waals forces,
analogous to large-diameter nanotubes. Quantum transport calculations show that
conductance through these collapsed wrinkle structures is limited mainly by a
density-of-states bottleneck and by interlayer tunneling across the collapsed
bilayer region. Also through systematic measurements across large numbers of
devices with wide folded wrinkles, we find a distinct anisotropy in their
electrical resistivity, consistent with our transport simulations. These
results highlight the coupling between morphology and electronic properties,
which has important practical implications for large-scale high-speed graphene
electronics.Comment: 5 figures supplemental information in separated fil
2D Ambipolar Vertical Transistors as Control-free Reconfigurable Logic Devices
As transistor footprint scales down to sub-10 nm regime, the process
development for advancing to further technology nodes has encountered
slowdowns. Achieving greater functionality within a single chip requires
concurrent development at the device, circuit, and system levels.
Reconfigurable transistors possess the capability to transform into both n-type
and p-type transistors dynamically during operation. This transistor-level
reconfigurability enables field-programmable logic circuits with fewer
components compared to conventional circuits. However, the reconfigurability
requires additional polarity control gates in the transistor and potentially
impairs the gain from a smaller footprint. In this paper, vertical transistors
with ambipolar MoTe2 channels are fabricated using the transfer-metal method.
The efficient asymmetric electrostatic gating in source and drain contacts
gives rise to different Schottky barriers at the two contacts. Consequently,
the ambipolar conduction is reduced to unipolar conduction due to different
Schottky barrier widths for electrons and holes. The current flow direction
determines the preferred carrier type. Temperature-dependent measurements
reveal the Schottky barrier-controlled conduction in the vertical transistors
and confirm different Schottky barrier widths with and without electrostatic
gating. Without the complexity overhead from polarity control gates,
control-free vertical reconfigurable transistors promise higher logic density
and lower cost in future integrated circuits
Layer Number Determination and Thickness-dependent Properties of Graphene Grown on SiC
The electronic properties of few-layer graphene grown on the carbon-face of
silicon carbide (SiC) are found to be strongly dependent on the number of
layers. The carrier mobility is larger in thicker graphene because
substrate-related scattering is reduced in the higher layers. The carrier
density dependence of the mobility is qualitatively different in thin and thick
graphene, with the transition occurring at about 2 layers. The mobility
increases with carrier density in thick graphene, similar to multi-layer
graphene exfoliated from natural graphite, suggesting that the individual
layers are still electrically coupled in spite of reports recording non-Bernal
stacking order in C-face grown graphene. The Hall coefficient peak value is
reduced in thick graphene due to the increased density of states. A reliable
and rapid characterization tool for the layer number is therefore highly
desirable. To date, AFM height determination and Raman scattering are typically
used since the optical contrast of graphene on SiC is weak. However, both
methods suffer from low throughput. We show that the scanning electron
microscopy (SEM) contrast can give similar results with much higher throughput
Combining a Fuzzy Matter-Element Model with a Geographic Information System in Eco-Environmental Sensitivity and Distribution of Land Use Planning
Sustainable ecological and environmental development is the basis of regional development. The sensitivity classification of the ecological environment is the premise of its spatial distribution for land use planning. In this paper, a fuzzy matter-element model and factor-overlay method were employed to analyze the ecological sensitivity in Yicheng City. Four ecological indicators, including soil condition,, water condition,, atmospheric conditions and biodiversity were used to classify the ecological sensitivity. The results were categorized into five ranks: insensitive, slightly sensitive, moderately sensitive, highly sensitive and extremely sensitive zones. The spatial distribution map of environmental sensitivity for land use planning was obtained using GIS (Geographical Information System) techniques. The results illustrated that the extremely sensitive and highly sensitive areas accounted for 14.40% and 30.12% of the total area, respectively, while the moderately sensitive and slightly sensitive areas are 25.99% and 29.49%, respectively. The results provide the theoretical foundation for land use planning by categorizing all kinds of land types in Yicheng City
Carrier scattering, mobilities and electrostatic potential in mono-, bi- and tri-layer graphenes
The carrier density and temperature dependence of the Hall mobility in mono-,
bi- and tri-layer graphene has been systematically studied. We found that as
the carrier density increases, the mobility decreases for mono-layer graphene,
while it increases for bi-layer/tri-layer graphene. This can be explained by
the different density of states in mono-layer and bi-layer/tri-layer graphenes.
In mono-layer, the mobility also decreases with increasing temperature
primarily due to surface polar substrate phonon scattering. In
bi-layer/tri-layer graphene, on the other hand, the mobility increases with
temperature because the field of the substrate surface phonons is effectively
screened by the additional graphene layer(s) and the mobility is dominated by
Coulomb scattering.
We also find that the temperature dependence of the Hall coefficient in
mono-, bi- and tri-layer graphene can be explained by the formation of electron
and hole puddles in graphene. This model also explains the temperature
dependence of the minimum conductance of mono-, bi- and tri-layer graphene. The
electrostatic potential variations across the different graphene samples are
extracted.Comment: 18 pages, 7 figure
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