15 research outputs found
Thermopower in hBN/graphene/hBN superlattices
Thermoelectric effects are highly sensitive to the asymmetry in the density
of states around the Fermi energy and can be exploited as probes of the
electronic structure. We experimentally study thermopower in high-quality
monolayer graphene, within heterostructures consisting of complete hBN
encapsulation and 1D edge contacts, where the graphene and hBN lattices are
aligned. When graphene is aligned to one of the hBN layers, we demonstrate the
presence of additional sign reversals in the thermopower as a function of
carrier density, directly evidencing the presence of the moir\'e superlattice.
We show that the temperature dependence of the thermopower enables the
assessment of the role of built-in strain variation and van Hove singularities
and hints at the presence of Umklapp electron-electron scattering processes. As
the thermopower peaks around the neutrality point, this allows to probe the
energy spectrum degeneracy. Further, when graphene is double-aligned with the
top and bottom hBN crystals, the thermopower exhibits features evidencing
multiple cloned Dirac points caused by the differential super-moir\'e lattice.
For both cases we evaluate how well the thermopower agrees with Mott's
equation. Finally, we show the same superlattice device can exhibit a
temperature-driven thermopower reversal from positive to negative and vice
versa, by controlling the carrier density. The study of thermopower provides an
alternative approach to study the electronic structure of 2D superlattices,
whilst offering opportunities to engineer the thermoelectric response on these
heterostructures.Comment: 9 pages, 3 figure
Proton and Li-Ion Permeation through Graphene with Eight-Atom-Ring Defects
Defect-free graphene is impermeable to gases and liquids but highly permeable
to thermal protons. Atomic-scale defects such as vacancies, grain boundaries
and Stone-Wales defects are predicted to enhance graphene's proton permeability
and may even allow small ions through, whereas larger species such as gas
molecules should remain blocked. These expectations have so far remained
untested in experiment. Here we show that atomically thin carbon films with a
high density of atomic-scale defects continue blocking all molecular transport,
but their proton permeability becomes ~1,000 times higher than that of
defect-free graphene. Lithium ions can also permeate through such disordered
graphene. The enhanced proton and ion permeability is attributed to a high
density of 8-carbon-atom rings. The latter pose approximately twice lower
energy barriers for incoming protons compared to the 6-atom rings of graphene
and a relatively low barrier of ~0.6 eV for Li ions. Our findings suggest that
disordered graphene could be of interest as membranes and protective barriers
in various Li-ion and hydrogen technologies
Thermopower in hBN/graphene/hBN superlattices
Thermoelectric effects are highly sensitive to the asymmetry in the density of states around the Fermi energy and can be exploited as probes of the electronic structure. We experimentally study thermopower in high-quality monolayer graphene, within heterostructures consisting of complete hBN encapsulation and 1D edge contacts, where the graphene and hBN lattices are aligned. When graphene is aligned to one of the hBN layers, we demonstrate the presence of additional sign reversals in the thermopower as a function of carrier density, directly evidencing the presence of the single-aligned moiré superlattice. We show that the temperature dependence of the thermopower enables the assessment of the role of built-in strain variation and van Hove singularities and hints at the presence of Umklapp electron-electron scattering processes. As the thermopower peaks around the neutrality point, this allows to probe the energy spectrum degeneracy. Further, when graphene is double aligned with the top and bottom hBN crystals, the thermopower exhibits features evidencing multiple cloned Dirac points caused by the differential super-moiré superlattice. For both cases we evaluate how well the thermopower agrees with Mott's equation. Finally, we show the same moiré superlattice device can exhibit a temperature-driven thermopower reversal from positive to negative and vice versa, by controlling the carrier density. The study of thermopower provides an alternative approach to study the electronic structure of 2D superlattices, whilst offering opportunities to engineer the thermoelectric response on these heterostructures
Exploring room temperature spin transport under band gap opening in bilayer graphene
Abstract We study the room-temperature electrical control of charge and spin transport in high-quality bilayer graphene, fully encapsulated with hBN and contacted via 1D spin injectors. We show that spin transport in this device architecture is measurable at room temperature and its spin transport parameters can be modulated by opening of a band gap via a perpendicular displacement field. The modulation of the spin current is dominated by the control of the spin relaxation time with displacement field, demonstrating the basic operation of a spin-based field-effect transistor