Stefano Cabrini

Gold Nanoparticle Self-Similar Chain Structure Organized by DNA Origami

Here we demonstrate Au nanoparticle self-similar chain structure organized by triangle DNA origami with well-controlled orientation and <10 nm spacing. We show for the first time that a large DNA complex (origami) and multiple AuNP conjugates can be well-assembled and purified with reliable yields. The assembled structure could be used to generate high local-field enhancement. The same method can be used to precisely localize multiple components on a DNA template for potential applications in nanophotonic, nanomagnetic, and nanoelectronic devices.

Formation of Bandgap and Subbands in Graphene Nanomeshes with Sub-10 nm Ribbon Width Fabricated via Nanoimprint Lithography

We fabricated hexagonal graphene nanomeshes (GNMs) with sub-10 nm ribbon width. The fabrication combines nanoimprint lithography, block-copolymer self-assembly for high-resolution nanoimprint template patterning, and electrostatic printing of graphene. Graphene field-effect transistors (GFETs) made from GNMs exhibit very different electronic characteristics in comparison with unpatterned GFETs even at room temperature. We observed multiplateaus in the drain current-gate voltage dependence as well as an enhancement of ON/OFF current ratio with reduction of the average ribbon width of GNMs. These effects are attributed to the formation of electronic subbands and a bandgap in GNMs. Such mesoscopic graphene structures and the nanofabrication methods could be employed to construct future electronic devices based on graphene superlattices.

High-efficiency multilevel zone plates for keV X-rays

The development of high brilliance X-ray sources coupled with advances in manufacturing technologies has led to significant improvements in submicrometre probes for spectroscopy, diffraction and imaging applications. The generation of a small beam spot size is commonly based on three principles: total reflection (as used in optical elements involving mirrors or capillaries), refraction (such as in refractive lenses) and diffraction. The latter effect is employed in Bragg–Fresnel or Soret lenses, commonly known as Fresnel zone plate lenses. These lenses currently give the best spatial resolution, but are traditionally limited to rather soft X-rays—at high energies, their use is still limited by their efficiency. Here we report the fabrication of high-efficiency, high-contrast gold and nickel multistep (quaternary) Fresnel zone plates using electron beam lithography. We achieve a maximum efficiency of 55% for the nickel plate at 7 keV. In addition to their high efficiency, the lenses offer the advantages of low background signal and effective reduction of unwanted diffraction orders. We anticipate that these lenses should have a significant impact on techniques such as microscopy, micro-fluorescence and micro-diffraction, which require medium resolution (500–100 nm) and high flux at fixed energies.

Mapping Local Charge Recombination Heterogeneity by Multidimensional Nanospectroscopic Imaging

Mind the Gap Near-field microscopy has benefited from subwavelength near-field plasmonic probes that make use of the field-concentrating properties of gaps. These probes achieve maximum enhancement only in the tip-substrate gap mode, which can yield large near-field signals, but only for a metallic substrate and for very small tip-substrate gap distances. Bao et al. (p. 1317) designed a probe that unites broadband field enhancement and confinement with bidirectional coupling between far-field and near-field electromagnetic energy. Their tips primarily rely on the internal gap modes of the tip itself, thereby enabling it to image nonmetallic samples. A near-field optical probe designed to maximize its own signal enhancement can be used to image nonmetallic samples. As materials functionality becomes more dependent on local physical and electronic properties, the importance of optically probing matter with true nanoscale spatial resolution has increased. In this work, we mapped the influence of local trap states within individual nanowires on carrier recombination with deeply subwavelength resolution. This is achieved using multidimensional nanospectroscopic imaging based on a nano-optical device. Placed at the end of a scan probe, the device delivers optimal near-field properties, including highly efficient far-field to near-field coupling, ultralarge field enhancement, nearly background-free imaging, independence from sample requirements, and broadband operation. We performed ~40-nanometer–resolution hyperspectral imaging of indium phosphide nanowires via excitation and collection through the probes, revealing optoelectronic structure along individual nanowires that is not accessible with other methods.

Metallic Adhesion Layer Induced Plasmon Damping and Molecular Linker as a Nondamping Alternative

Drastic chemical interface plasmon damping is induced by the ultrathin (∼2 nm) titanium (Ti) adhesion layer; alternatively, molecular adhesion is implemented for lithographic fabrication of plasmonic nanostructures without significant distortion of the plasmonic characteristics. As determined from the homogeneous linewidth of the resonance scattering spectrum of individual gold nanorods, an ultrathin Ti layer reduces the plasmon dephasing time significantly, and it reduces the plasmon scattering amplitude drastically. The increased damping rate and decreased plasmon amplitude are due to the dissipative dielectric function of Ti and the chemical interface plasmon damping where the conduction electrons are transferred across the metal-metal interface. In addition, a pronounced red shift due to the Ti adhesion layer, more than predicted using electromagnetic simulation, suggests the prevalence of interfacial reactions. By extending the experiment to conductively coupled ring-rod nanostructures, it is shown that a sharp Fano-like resonance feature is smeared out due to the Ti layer. Alternatively, vapor deposition of (3-mercaptopropyl)trimethoxysilane on gently cleaned and activated lithographic patterns functionalizes the glass surface sufficiently to link the gold nanostructures to the surface by sulfur-gold chemical bonds without observable plasmon damping effects.

Hyperspectral nanoscale imaging on dielectric substrates with coaxial optical antenna scan probes

An important goal in nano science is to unlock previously inaccessible physics and dynamics within nanoscale systems by combining the efficient nanoscale field confinement/optical resolution (∼10 nm) of optical antennae and the ultrafast temporal resolution (fs) inherent in optical studies with the capabilities of modern scanning probe techniques. Here we report on a significant step toward this goal using a novel nanofabricated coaxial antenna tip capable of recording useful Raman spectra in ∼50 ms to acquire 256 by 256 pixel images on dielectric substrates with a full spectrum at each pixel.

Manipulating nanoscale light fields with the asymmetric bowtie nano-colorsorter.

We present a class of devices called Asymmetric Bowtie nano-Colorsorters. These devices have specifically engineered symmetries enabling them to capture, confine, spectrally filter and steer optical fields while maintaining nanoscale field distributions.

Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces

Using first-principles theory and experiments, chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces are explained and quantified. Density functional theory calculations of the static Raman tensor demonstrate a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential. A straightforward and general analysis is introduced to extract chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.

Electrostatic Force Assisted Exfoliation of Prepatterned Few-Layer Graphenes into Device Sites

We present a novel fabrication method for incorporating nanometer to micrometer scale few-layer graphene (FLG) features onto substrates with electrostatic exfoliation. We pattern highly oriented pyrolytic graphite using standard lithographic techniques and subsequently, in a single step, exfoliate and transfer-print the prepatterned FLG features onto a silicon wafer using electrostatic force. We have successfully demonstrated the exfoliation/printing of 18 nm wide FLG nanolines and periodic arrays of 1.4 mum diameter pillars. Furthermore, we have fabricated graphene nanoribbon transistors using the patterned graphene nanoline. Our electrostatic force assisted exfoliation/print process does not need additional adhesion layers and could be stepped and repeated to deliver the prepatterned graphitic material over wafer-sized areas and allows the construction of graphene-based integrated circuits.

Theta-Shaped Plasmonic Nanostructures: Bringing “Dark” Multipole Plasmon Resonances into Action via Conductive Coupling

Quadrupole plasmon and (octupolar) Fano resonances are induced in lithographically fabricated theta-shaped ring-rod gold nanostructures. The optical response is characterized by measuring the light scattered by individual nanostructures. When the nanorod is brought within 3 nm of the ring wall, a weak quadrupolar resonance is observed due to capacitive coupling, and when a necklike conductive bridge links the nanorod to the nanoring the optical response changes dramatically bringing the quadrupolar resonance into prominence and creating an octupolar Fano resonance. The Fano resonance is observed due to the destructive interference of the octupolar resonance with the overlapping and broadened dipolar resonance. The quadrupolar and Fano resonances are further enhanced by capacitive coupling (near-field interaction) that is favored by the theta-shaped arrangement. The interpretation of the data is supported by FDTD simulation.