130 research outputs found
Calibration of piezoelectric positioning actuators using a reference voltage-to-displacement transducer based on quartz tuning forks
We use a piezoelectric quartz tuning fork to calibrate the displacement of
ceramic piezoelectric scanners which are widely employed in scanning probe
microscopy. We measure the static piezoelectric response of a quartz tuning
fork and find it to be highly linear, non-hysteretic and with negligible creep.
These performance characteristics, close to those of an ideal transducer, make
quartz transducers superior to ceramic piezoelectric actuators. Furthermore,
quartz actuators in the form of a tuning fork have the advantage of yielding
static displacements comparable to those of local probe microscope scanners. We
use the static displacement of a quartz tuning fork as a reference to calibrate
the three axis displacement of a ceramic piezoelectric scanner. Although this
calibration technique is a non-traceable method, it can be more versatile than
using calibration grids because it enables to characterize the linear and
non-linear response of a piezoelectric scanner in a broad range of
displacements, spanning from a fraction of a nanometer to hundreds of
nanometers. In addition, the creep and the speed dependent piezoelectric
response of ceramic scanners can be studied in detail.Comment: 9 pages, 3 figure
Atomically thin mica flakes and their application as ultrathin insulating substrates for graphene
We show that it is possible to deposit, by mechanical exfoliation on SiO2/Si
wafers, atomically thin mica flakes down to a single monolayer thickness. The
optical contrast of these mica flakes on top of a SiO2/Si substrate, which
depends on their thickness, the illumination wavelength and the SiO2 substrate
thickness, can be quantitatively accounted for by a Fresnel law based model.
The preparation of atomically thin insulating crystalline sheets will enable
the fabrication of ultrathin defect-free insulating substrates, dielectric
barriers or planar electron tunneling junctions. Additionally, we show that
few-layer graphene flakes can be deposited on top of a previously transferred
mica flake. Our transfer method relies on viscoelastic stamps, as those used
for soft lithography. A Raman spectroscopy study shows that such an all-dry
deposition technique yields cleaner and higher quality flakes than conventional
wet-transfer procedures based on lithographic resists.Comment: 11 pages, 5 figures, 1 graphical abstrac
Periodic spatial variation of the electron-phonon interaction in epitaxial graphene on Ru(0001
We have performed low temperature scanning tunnelling spectroscopy (STS)
measurements on graphene epitaxially grown on Ru(0001). An inelastic feature,
related to the excitation of a vibrational breathing mode of the graphene
lattice, was found at 360 meV. The change in the differential electrical
conductance produced by this inelastic feature, which is associated with the
electron-phonon interaction strength, varies spatially from one position to
other of the graphene supercell. This inhomogeneity in the electronic
properties of graphene on Ru(0001) results from local variations of the
carbon-ruthenium interaction due to the lattice mismatch between the graphene
and the Ru(0001) lattices.Comment: 6 Pages, 3 figure
Fabrication and characterization of metallic nanowires
The shape of metallic constrictions of nanoscopic dimensions (necks) formed using a scanning tunneling microscope is shown to depend on the fabrication procedure. Submitting the neck to repeated plastic deformation cycles makes it possible to obtain long necks or nanowires. Point-contact spectroscopy results show that these long necks are quite crystalline, indicating that the repeated cycles of plastic deformation act as a “mechanical annealing” of the neck.This work was supported by the DGICYT under Contract Nos. MAT95-1542 and PB94-0382
Strong modulation of optical properties in black phosphorus through strain-engineered rippling
Controlling the bandgap through local-strain engineering is an exciting
avenue for tailoring optoelectronic materials. Two-dimensional crystals are
particularly suited for this purpose because they can withstand unprecedented
non-homogeneous deformations before rupture: one can literally bend them and
fold them up almost like a piece of paper. Here, we study multi-layer black
phosphorus sheets subjected to periodic stress to modulate their optoelectronic
properties. We find a remarkable shift of the optical absorption band-edge of
up to ~0.7 eV between the regions under tensile and compressive stress, greatly
exceeding the strain tunability reported for transition metal dichalcogenides.
This observation is supported by theoretical models which also predict that
this periodic stress modulation can yield to quantum confinement of carriers at
low temperatures. The possibility of generating large strain-induced variations
in the local density of charge carriers opens the door for a variety of
applications including photovoltaics, quantum optics and two-dimensional
optoelectronic devices.Comment: 16 pages main text + 13 pages S
Microheater actuators as a versatile platform for strain engineering in 2D materials
We present microfabricated thermal actuators to engineer the biaxial strain
in two-dimensional (2D) materials. These actuators are based on microheater
circuits patterned onto the surface of a polymer with a high thermal expansion
coefficient. By running current through the microheater one can vary the
temperature of the polymer and induce a controlled biaxial expansion of its
surface. This controlled biaxial expansion can be transduced to biaxial strain
to 2D materials, placed onto the polymer surface, which in turn induces a shift
of the optical spectrum. Our thermal strain actuators can reach a maximum
biaxial strain of 0.64 % and they can be modulated at frequencies up to 8 Hz.
The compact geometry of these actuators results in a negligible spatial drift
of 0.03 um/deg, which facilitates their integration in optical spectroscopy
measurements. We illustrate the potential of this strain engineering platform
to fabricate a strain-actuated optical modulator with single-layer MoS2.Comment: 5 main text figures + 5 supp. info. figure
- …