We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of ...such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the
D
band intensity, and we show that individual boundaries between coalesced grains impede electrical transport in graphene and induce prominent weak localization, indicative of intervalley scattering in graphene.
► Hexagonally shaped single layer graphene grains are grown by ambient pressure CVD. ► Graphene grain boundaries are formed when grains merge. ► Graphene grains and grain boundaries are visualized by Raman mapping of the intensities of its characteristic peaks. ► Electrical transport is impeded when carriers cross graphene grain boundaries. ► Weak localization is found for transport across an individual graphene grain boundary.
Graphene with a large area was synthesized on Cu foils by chemical vapor deposition under ambient pressure. A 4″
×
4″ graphene film was transferred onto a 6″ Si wafer with a thermally grown oxide ...film. Raman mapping indicates monolayer graphene dominates the transferred graphene film. Gas sensors were fabricated on a 4
mm
×
3
mm size graphene film with a 1
nm palladium film deposited for hydrogen detection. Hydrogen in air with concentrations in 0.0025–1% (25–10,000
ppm) was used to test graphene-based gas sensors. The gas sensors based on palladium-decorated graphene films show high sensitivity, fast response and recovery, and can be used with multiple cycles. The mechanism of hydrogen detection is also discussed.
We report an atomically resolved scanning tunneling microscopy investigation of the edges of graphene grains synthesized on Cu foils by chemical vapor deposition. Most of the edges are ...macroscopically parallel to the zigzag directions of graphene lattice. These edges have microscopic roughness that is found to also follow zigzag directions at atomic scale, displaying many ∼120° turns. A prominent standing wave pattern with periodicity ∼3a/4 (a being the graphene lattice constant) is observed near a rare-occurring armchair-oriented edge. Observed features of this wave pattern are consistent with the electronic intervalley backscattering predicted to occur at armchair edges but not at zigzag edges.
Spin-momentum helical locking is one of the most important properties of the nontrivial topological surface states (TSS) in 3D topological insulators (TIs). It underlies the iconic topological ...protection (suppressing elastic backscattering) of TSS and is foundational to many exotic physics (e.g., majorana fermions) and device applications (e.g., spintronics) predicted for TIs. Based on this spin-momentum locking, a current flowing on the surface of a TI would be spin-polarized in a characteristic in-plane direction perpendicular to the current, and the spin-polarization would reverse when the current direction reverses. Observing such a spin-helical current in transport measurements is a major goal in TI research and applications. We report spin-dependent transport measurements in spin valve devices fabricated from exfoliated thin flakes of Bi2Se3 (a prototype 3D TI) with ferromagnetic (FM) Ni contacts. Applying an in-plane magnetic (B) field to polarize the Ni contacts along their easy axis, we observe an asymmetry in the hysteretic magnetoresistance (MR) between opposite B field directions. The “polarity” of the asymmetry in MR can be reversed by reversing the direction of the DC current. The observed asymmetric MR can be understood as a spin-valve effect between the current-induced spin polarization on the TI surface (due to spin-momentum-locking of TSS) and the spin-polarized ferromagnetic contacts. Our results provide a direct transport evidence for the spin helical current in TSS.
•The first 2-terminal spin valve devices on 3D topological insulators are reported.•The spin-polarized surface current in topological insulators is demonstrated.•A direct transport evidence for the spin-momentum locking of TSS is reported.•Current induced spin polarization in is observed in spin valve measurements.•The results promise applications in spintronics for electrical spin injection.
An atomic-scale study utilizing scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV) is performed on large single crystalline graphene grains synthesized on Cu foil by a chemical vapor ...deposition (CVD) method. After thermal annealing, we observe the presence of periodic surface depressions (stripe patterns) that exhibit long-range order formed in the area of Cu covered by graphene. We suggest that the observed stripe pattern is a Cu surface reconstruction formed by partial dislocations (which appeared to be stair-rod-like) resulting from the strain induced by the graphene overlayer. In addition, these graphene grains are shown to be more decoupled from the Cu substrate compared to previously studied grains that exhibited Moiré patterns.
We show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators using a four-probe transport spectroscopy in a multiprobe ...scanning tunneling microscopy system. We derive a scaling relation of measured resistance with respect to varying interprobe spacing for two interconnected conduction channels to allow quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 against a pure 2D conductance of graphene on SiC substrate. We also quantitatively show the effect of surface doping carriers on the 2D conductance enhancement in topological insulators. The method offers a means to understanding not just the topological insulators but also the 2D to 3D crossover of conductance in other complex systems.
We have investigated the effects of thermal annealing on ex-situ chemically vapor deposited submonolayer graphene islands on polycrystalline Cu foil at the atomic-scale using ultrahigh vacuum ...scanning tunneling microscopy. Low-temperature annealed graphene islands on Cu foil (at ∼430 °C) exhibit predominantly striped Moiré patterns, indicating a relatively weak interaction between graphene and the underlying polycrystalline Cu foil. Rapid high-temperature annealing of the sample (at 700–800 °C) gives rise to the removal of Cu oxide and the recovery of crystallographic features of the copper that surrounds the intact graphene. These experimental observations of continuous crystalline features between the underlying copper (beneath the graphene islands) and the surrounding exposed copper areas revealed by high-temperature annealing demonstrates the impenetrable nature of graphene and its potential application as a protective layer against corrosion.
PDF
