39 research outputs found
Self-assembly and electron-beam-induced direct etching of suspended graphene nanostructures
We report on suspended single-layer graphene deposition by a
transfer-printing approach based on polydimethylsiloxane stamps. The transfer
printing method allows the exfoliation of graphite flakes from a bulk graphite
sample and their residue-free deposition on a silicon dioxide substrate. This
deposition system creates a blistered graphene surface due to strain induced by
the transfer process itself. Single-layer-graphene deposition and its
"blistering" on the substrate are demonstrated by a combination of Raman
spectroscopy, scanning electron microscopy and atomic-force microscopy
measurements. Finally, we demonstrate that blister-like suspended graphene are
self-supporting single-layer structures and can be flattened by employing a
spatially-resolved direct-lithography technique based on electron-beam induced
etching.Comment: 17 pages, 5 figure
Stretching graphene using polymeric micro-muscles
The control of strain in two-dimensional materials opens exciting
perspectives for the engineering of their electronic properties. While this
expectation has been validated by artificial-lattice studies, it remains
elusive in the case of atomic lattices. Remarkable results were obtained on
nanobubbles and nano-wrinkles, or using scanning probes; microscale strain
devices were implemented exploiting deformable substrates or external loads.
These devices lack, however, the flexibility required to fully control and
investigate arbitrary strain profiles. Here, we demonstrate a novel approach
making it possible to induce strain in graphene using polymeric micrometric
artificial muscles (MAMs) that contract in a controllable and reversible way
under an electronic stimulus. Our method exploits the mechanical response of
poly-methyl-methacrylate (PMMA) to electron-beam irradiation. Inhomogeneous
anisotropic strain and out-of-plane deformation are demonstrated and studied by
Raman, scanning-electron and atomic-force microscopy. These can all be easily
combined with the present device architecture. The flexibility of the present
method opens new opportunities for the investigation of strain and
nanomechanics in two-dimensional materials
Reflectionless tunneling in planar Nb/GaAs hybrid junctions
Reflectionless-tunneling was observed in Nb/GaAs superconductor/semiconductor
junctions fabricated through a two-step procedure. First, periodic
-doped layers were grown by molecular beam epitaxy near the GaAs
surface, followed by an As cap layer to protect the surface during {\it
ex-situ} transfer. Second, Nb was deposited by dc-magnetron sputtering onto the
GaAs(001) 2 4 surface {\it in-situ} after thermal desorption of the
cap layer. The magnetotransport behavior of the resulting hybrid junctions was
successfully analyzed within the random matrix theory of phase-coherent Andreev
transport. The impact of junction morphology on reflectionless tunneling and
the applicability of the fabrication technique to the realization of complex
superconductor/semiconductor mesoscopic systems are discussed.Comment: 10 pages, 3 figures, to be published in Appl. Phys. Let
Electrostatic force microscopy and potentiometry of realistic nanostructured systems
We investigate the dependency of electrostatic interaction forces on applied
potentials in Electrostatic Force Microscopy (EFM) as well as in related local
potentiometry techniques like Kelvin Probe Microscopy (KPM). The approximated
expression of electrostatic interaction between two conductors, usually
employed in EFM and KPM, may loose its validity when probe-sample distance is
not very small, as often realized when realistic nanostructured systems with
complex topography are investigated. In such conditions, electrostatic
interaction does not depend solely on the potential difference between probe
and sample, but instead it may depend on the bias applied to each conductor.
For instance, electrostatic force can change from repulsive to attractive for
certain ranges of applied potentials and probe-sample distances, and this fact
cannot be accounted for by approximated models. We propose a general
capacitance model, even applicable to more than two conductors, considering
values of potentials applied to each of the conductors to determine the
resulting forces and force gradients, being able to account for the above
phenomenon as well as to describe interactions at larger distances. Results
from numerical simulations and experiments on metal stripe electrodes and
semiconductor nanowires supporting such scenario in typical regimes of EFM
investigations are presented, evidencing the importance of a more rigorous
modelling for EFM data interpretation. Furthermore, physical meaning of Kelvin
potential as used in KPM applications can also be clarified by means of the
reported formalism.Comment: 20 pages, 7 figures, 1 tabl
Revealing the atomic structure of the buffer layer between SiC(0001) and epitaxial graphene
On the SiC(0001) surface (the silicon face of SiC), epitaxial graphene is
obtained by sublimation of Si from the substrate. The graphene film is
separated from the bulk by a carbon-rich interface layer (hereafter called the
buffer layer) which in part covalently binds to the substrate. Its structural
and electronic properties are currently under debate. In the present work we
report scanning tunneling microscopy (STM) studies of the buffer layer and of
quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling
the buffer layer from the SiC(0001) substrate by means of hydrogen
intercalation. Atomic resolution STM images of the buffer layer reveal that,
within the periodic structural corrugation of this interfacial layer, the
arrangement of atoms is topologically identical to that of graphene. After
hydrogen intercalation, we show that the resulting QFMLG is relieved from the
periodic corrugation and presents no detectable defect sites
Little-Parks effect in single YBaCuO sub-micron rings
The properties of single submicron high-temperature superconductor (HTS)
rings are investigated. The Little-Parks effect is observed and is accompanied
by an anomalous behavior of the magnetic dependence of the resistance, which we
ascribe to non-uniform vorticity (superfluid angular momentum) within the ring
arms. This effect is linked to the peculiar HTS-relationship between the values
of the coherence length and the London penetration depth.Comment: 14 pages, 3 figure
Fluorescence lifetime microscopy unveils the supramolecular organization of liposomal Doxorubicin
The supramolecular organization of Doxorubicin (DOX) within the standard Doxoves® liposomal formulation (DOX®) is investigated using visible light and phasor approach to fluorescence lifetime imaging (phasor-FLIM). First, the phasor-FLIM signature of DOX® is resolved into the contribution of three co-existing fluorescent species, each with its characteristic mono-exponential lifetime, namely: crystallized DOX (DOXc, 0.2 ns), free DOX (DOXf, 1.0 ns), and DOX bound to the liposomal membrane (DOXb, 4.5 ns). Then, the exact molar fractions of the three species are determined by combining phasor-FLIM with quantitative absorption/fluorescence spectroscopy on DOXc, DOXf, and DOXb pure standards. The final picture on DOX® comprises most of the drug in the crystallized form (∼98%), with the remaining fractions divided between free (∼1.4%) and membrane-bound drug (∼0.7%). Finally, phasor-FLIM in the presence of a DOX dynamic quencher allows us to suggest that DOXf is both encapsulated and non-encapsulated, and that DOXb is present on both liposome-membrane leaflets. We argue that the present experimental protocol can be applied to the investigation of the supramolecular organization of encapsulated luminescent drugs/molecules all the way from the production phase to their state within living matter