36 research outputs found
Evolution of the electronic band structure of twisted bilayer graphene upon doping
The electronic band structure of twisted bilayer graphene develops van Hove
singularities whose energy depends on the twist angle between the two layers.
Using Raman spectroscopy, we monitor the evolution of the electronic band
structure upon doping using the G peak area which is enhanced when the laser
photon energy is resonant with the energy separation of the van Hove
singularities. Upon charge doping, the Raman G peak area initially increases
for twist angles larger than a critical angle and decreases for smaller angles.
To explain this behavior with twist angle, the energy of separation of the van
Hove singularities must decrease with increasing charge density demonstrating
the ability to modify the electronic and optical properties of twisted bilayer
graphene with doping.Comment: 10 pages, 4 figure
Pressure-induced commensurate stacking of graphene on boron nitride
Combining atomically-thin van der Waals materials into heterostructures
provides a powerful path towards the creation of designer electronic devices.
The interaction strength between neighboring layers, most easily controlled
through their interlayer separation, can have significant influence on the
electronic properties of these composite materials. Here, we demonstrate
unprecedented control over interlayer interactions by locally modifying the
interlayer separation between graphene and boron nitride, which we achieve by
applying pressure with a scanning tunneling microscopy tip. For the special
case of aligned or nearly-aligned graphene on boron nitride, the graphene
lattice can stretch and compress locally to compensate for the slight lattice
mismatch between the two materials. We find that modifying the interlayer
separation directly tunes the lattice strain and induces commensurate stacking
underneath the tip. Our results motivate future studies tailoring the
electronic properties of van der Waals heterostructures by controlling the
interlayer separation of the entire device using hydrostatic pressure.Comment: 17 pages, 4 figures and supplementary information. Updated to
published versio
Electric Field Control of Soliton Motion and Stacking in Trilayer Graphene
The crystal structure of a material plays an important role in determining
its electronic properties. Changing from one crystal structure to another
involves a phase transition which is usually controlled by a state variable
such as temperature or pressure. In the case of trilayer graphene, there are
two common stacking configurations (Bernal and rhombohedral) which exhibit very
different electronic properties. In graphene flakes with both stacking
configurations, the region between them consists of a localized strain soliton
where the carbon atoms of one graphene layer shift by the carbon-carbon bond
distance. Here we show the ability to move this strain soliton with a
perpendicular electric field and hence control the stacking configuration of
trilayer graphene with only an external voltage. Moreover, we find that the
free energy difference between the two stacking configurations scales
quadratically with electric field, and thus rhombohedral stacking is favored as
the electric field increases. This ability to control the stacking order in
graphene opens the way to novel devices which combine structural and electrical
properties
Band Structure Mapping of Bilayer Graphene via Quasiparticle Scattering
A perpendicular electric field breaks the layer symmetry of Bernal-stacked
bilayer graphene, resulting in the opening of a band gap and a modification of
the effective mass of the charge carriers. Using scanning tunneling microscopy
and spectroscopy, we examine standing waves in the local density of states of
bilayer graphene formed by scattering from a bilayer/trilayer boundary. The
quasiparticle interference properties are controlled by the bilayer graphene
band structure, allowing a direct local probe of the evolution of the band
structure of bilayer graphene as a function of electric field. We extract the
Slonczewski-Weiss-McClure model tight binding parameters as
eV, eV, and eV.Comment: 12 pages, 4 figure