arXiv subjects

A. Lombardo

Publications and source records attributed to A. Lombardo.

27 records · Page 2Linked to original sources

arXiv2011

Strong Plasmonic Enhancement of Photovoltage in Graphene

Amongst the wide spectrum of potential applications of graphene, ranging from transistors and chemical-sensors to nanoelectromechanical devices and composites, the field of photonics and optoelectronics is believed to be one of the most promising. Indeed, graphene's suitability for high-speed photodetection was demonstrated in an optical communication link operating at 10 Gbit/s\cite. However, the low responsivity of graphene-based photodetectors compared to traditional III-V based ones is a potential drawback. Here we show that, by combining graphene with plasmonic nanostructures, the efficiency of graphene-based photodectors can be increased by up to 20 times, due to field concentration in the area of a p-n junction. Additionally, wavelength and polarization selectivity can be achieved employing nanostructures of different geometries.

cond-mat.mtrl-sci↗

arXiv2011

The Shear Mode of Multi-Layer Graphene

We uncover the interlayer shear mode of multi-layer graphene samples, ranging from bilayer-graphene (BLG) to bulk graphite, and show that the corresponding Raman peak measures the interlayer coupling. This peak scales from~43cm-1 in bulk graphite to~31cm-1 in BLG. Its low energy makes it a probe of near-Dirac point quasi-particles, with a Breit-Wigner-Fano lineshape due to resonance with electronic transitions. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions

cond-mat.mes-hall↗

arXiv2011

Quantifying defects in graphene via Raman spectroscopy at different excitation energies

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities for a given defect density strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an inter-defect distance ~3nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.

cond-mat.mtrl-sci↗

arXiv2011

Atomic force microscope nanolithography of graphene: cuts, pseudo-cuts and tip current measurements

We investigate atomic force microscope nanolithography of single and bilayer graphene. In situ tip current measurements show that cutting of graphene is not current driven. Using a combination of transport measurements and scanning electron microscopy we show that, while indentations accompanied by tip current appear in the graphene lattice for a range of tip voltages, real cuts are characterized by a strong reduction of the tip current above a threshold voltage. The reliability and flexibility of the technique is demonstrated by the fabrication, measurement, modification and re-measurement of graphene nanodevices with resolution down to 15 nm.

cond-mat.mes-hall↗

arXiv2010

Tilted potential induced coupling of localized states in a graphene nanoconstriction

We use the charged tip of a low temperature scanning probe microscope to perturb the transport through a graphene nanoconstriction. Maps of the conductance as a function of tip position display concentric halos, and by following the expansion of the halos with back-gate voltage we are able to identify an elongated domain over the nanoconstriction where they originate. Amplitude modulations of the transmission resonances are correlated with the gradient of the tip-induced potential and we analyze this in terms of modified coupling between localized states.

cond-mat.mes-hall↗

arXiv2010

Surface Enhanced Raman Spectroscopy of Graphene

Surface enhanced Raman scattering (SERS) exploits surface plasmons induced by the incident field in metallic nanostructures to significantly increase the Raman intensity. Graphene provides the ideal prototype two dimensional (2d) test material to investigate SERS. Its Raman spectrum is well known, graphene samples are entirely reproducible, height controllable down to the atomic scale, and can be made virtually defect-free. We report SERS from graphene, by depositing arrays of Au particles of well defined dimensions on graphene/SiO$_2$(300nm)/Si. We detect significant enhancements at 633nm. To elucidate the physics of SERS, we develop a quantitative analytical and numerical theory. The 2d nature of graphene allows for a closed-form description of the Raman enhancement. This scales with the nanoparticle cross section, the fourth power of the Mie enhancement, and is inversely proportional to the tenth power of the separation between graphene and the nanoparticle. One consequence is that metallic nanodisks are an ideal embodiment for SERS in 2d.

cond-mat.mes-hall↗

arXiv2009

Scanning gate microscopy of current-annealed single layer graphene

We have used scanning gate microscopy to explore the local conductivity of a current-annealed graphene flake. A map of the local neutrality point (NP) after annealing at low current density exhibits micron-sized inhomogeneities. Broadening of the local e-h transition is also correlated with the inhomogeneity of the NP. Annealing at higher current density reduces the NP inhomogeneity, but we still observe some asymmetry in the e-h conduction. We attribute this to a hole doped domain close to one of the metal contacts combined with underlying striations in the local NP.

cond-mat.mes-hall↗

arXiv2009

Making Graphene Luminescent

We show that strong photoluminescence (PL) can be induced in single-layer graphene on using an oxygen plasma treatment. PL characteristics are spatially uniform across the flakes and connected to elastic scattering spectra distinctly different from those of gapless pristine graphene. Oxygen plasma can be used to selectively convert the topmost layer when multi-layer samples are treated

cond-mat.other↗

arXiv2008

Uniaxial Strain in Graphene by Raman Spectroscopy: G peak splitting, Gruneisen Parameters and Sample Orientation

Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. Since its experimental discovery, graphene continues to attract enormous interest, in particular as a new kind of matter, in which electron transport is governed by a Dirac-like wave equation, and as a model system for studying electronic and phonon properties of other, more complex, graphitic materials[1-4]. Here, we uncover the constitutive relation of graphene and probe new physics of its optical phonons, by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E2g optical mode splits in two components, one polarized along the strain and the other perpendicular to it. This leads to the splitting of the G peak into two bands, which we call G+ and G-, by analogy with the effect of curvature on the nanotube G peak[5-7]. Both peaks red shift with increasing strain, and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The singly degenerate 2D and 2D' bands also red shift, but do not split for small strains. We study the Gruneisen parameters for the phonons responsible for the G, D and D' peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance[8, 9]

cond-mat.mtrl-sci↗