Matteo Altissimo

The Dark Side of Plasmonics

Plasmonic dark modes are pure near-field modes that can arise from the plasmon hybridization in a set of interacting nanoparticles. When compared to bright modes, dark modes have longer lifetimes due to their lack of a net dipole moment, making them attractive for a number of applications. We demonstrate the excitation and optical detection of a collective dark plasmonic mode from individual plasmonic trimers. The trimers consist of triangular arrangements of gold nanorods, and due to this symmetry, the lowest-energy dark plasmonic mode can interact with radially polarized light. The experimental data presented confirm the excitation of this mode, and its assignment is supported with an electrostatic approximation wherein these dark modes are described in terms of plasmon hybridization. The strong confinement of energy in these modes and their associated near fields hold great promise for achieving strong coupling to single photon emitters.

E-beam lithography for micro-nanofabrication.

Electron beam lithography (EBL) is one of the tools of choice for writing micro- and nanostructures on a wide variety of materials. This is largely due to the fact that modern EBL machines are capable of writing nanometer-sized structures on areas up to mm(2). The aim of this contribution is to give technical and practical backgrounds in this extremely flexible nanofabrication technique.

Self‐Assembly of Vertically Aligned Gold Nanorod Arrays on Patterned Substrates

The main objective of this thesis was to develop a fabrication method combining bottom-up and top-down approaches to self-assemble anisotropic building blocks into advanced nanostructures on patterned substrates. Building blocks (gold nanorods) were synthesised following a seed-mediated protocol and subsequently purified by a fractionated precipitation strategy to remove nanoparticulate byproducts formed during the chemical synthesis. Gold nanorods (GNRs) were self-assembled into discrete vertically aligned arrays based on capillary and convective forces into templates fabricated by means of lithography processes. Patterned substrates were fabricated by a series of cleanroom processes to provide different templates for the self-assembly of GNRs. The role of the patterned surface was to guide and confine the fabrication of vertically aligned GNR arrays by providing a chemical and geometrical template. Recessed gold features were produced on silica-coated silicon wafers with a variety of shapes depending on the application envisioned for the GNR arrays. Square templates were fabricated by photolithography while rectangular patterns were produced by electron beam lithography. Surface treatments were carried out to endow the patterned substrates with a wettability contrast, required for the GNR self-assembly. The hydrophilicity of the gold surface was increased by a UV-ozone treatment and the silica surface was passivated with a PEG-silane functionalisation making it hydrophobic. The seed-mediated synthesis is a well-known method to produce GNRs but it also inevitably yields nanoparticulate byproducts. In a typical synthesis of GNRs with an aspect ratio of 3, three types of impurities can be identified: (1) large spherical nanoparticles, (2) nanoplates and (3) high aspect ratio nanorods. A size- and shape-selective purification strategy was developed to remove these three nanoparticulate byproducts from GNR solutions. The purification method exploits the sharp size-dependent colloidal stability threshold exhibited by gold nanoparticles functionalised with thiol-PEG-carboxyl. The ligand provides gold nanoparticles an excellent colloidal stability due to electrostatic interparticle repulsion. These repulsions forces can be attenuated to induce nanoparticle precipitation beyond specific thresholds of ionic strength or ethanol concentrations. Based on this concept, a two-step protocol enabled the separation of GNR from nanoparticulate byproducts increasing the purity of the as-synthesised GNR solution from 88.6% to 98.1%. The fabrication of vertically aligned GNR arrays was achieved by capillary and convective assembly on patterned substrates. The wettability contrast directed the self-assembly of GNR arrays onto predefined areas with an unprecedented accuracy. Two main factors have shown to play crucial roles in the self-assembly process. The temperature controlled the confinement of GNR arrays inside the template whereas the GNR concentration influenced the quality of the hexagonal close-packed (hcp) ordering of standing GNRs. When the self-assembly was performed at 45˚C with a GNR concentration of 9 nM, the nanostructures comprised three layers of vertically aligned nanorods with an interparticle distance of 5 nm and an excellent hcp ordering over a long range (substrate scale). The surface-enhanced Raman scattering (SERS) activity of these nanostructures has exhibited a sensitivity up to 36 times, when compared to a commercial SERS substrate.

Quantification of ZnO Nanoparticle Uptake, Distribution, and Dissolution within Individual Human Macrophages

The usefulness of zinc oxide (ZnO) nanoparticles has led to their wide distribution in consumer products, despite only a limited understanding of how this nanomaterial behaves within biological systems. From a nanotoxicological viewpoint the interaction(s) of ZnO nanoparticles with cells of the immune system is of specific interest, as these nanostructures are readily phagocytosed. In this study, rapid scanning X-ray fluorescence microscopy was used to assay the number ZnO nanoparticles associated with ∼1000 individual THP-1 monocyte-derived human macrophages. These data showed that nanoparticle-treated cells endured a 400% elevation in total Zn levels, 13-fold greater than the increase observed when incubated in the presence of an equitoxic concentration of ZnCl2. Even after excluding the contribution of internalized nanoparticles, Zn levels in nanoparticle treated cells were raised ∼200% above basal levels. As dissolution of ZnO nanoparticles is critical to their cytotoxic response, we utilized a strategy combining ion beam milling, X-ray fluorescence and scanning electron microscopy to directly probe the distribution and composition of ZnO nanoparticles throughout the cellular interior. This study demonstrated that correlative photon and ion beam imaging techniques can provide both high-resolution and statistically powerful information on the biology of metal oxide nanoparticles at the single-cell level. Our approach promises ready application to broader studies of phenomena at the interface of nanotechnology and biology.

Quantitative comparison of preparation methodologies for x-ray fluorescence microscopy of brain tissue

X-ray fluorescence microscopy (XFM) facilitates high-sensitivity quantitative imaging of trace metals at high spatial resolution over large sample areas and can be applied to a diverse range of biological samples. Accurate determination of elemental content from recorded spectra requires proper calibration of the XFM instrument under the relevant operating conditions. Here, we describe the manufacture, characterization, and utilization of multi-element thin-film reference foils for use in calibration of XFM measurements of biological and other specimens. We have used these internal standards to assess the two-dimensional distribution of trace metals in a thin tissue section of a rat hippocampus. The data used in this study was acquired at the XFM beamline of the Australian Synchrotron using a new 384-element array detector (Maia) and at beamline 2-ID-E at the Advanced Photon Source. Post-processing of samples by different fixation techniques was investigated, with the conclusion that differences in solvent type and sample handling can significantly alter elemental content. The present study highlights the quantitative capability, high statistical power, and versatility of the XFM technique for mapping trace metals in biological samples, e.g., brain tissue samples in order to help understand neurological processes, especially when implemented in conjunction with a high-performance detector such as Maia.

Differential interference contrast x-ray microscopy with twin zone plates

X-ray imaging in differential interference contrast (DIC) with submicrometer optical resolution was performed by using a twin zone plate (TZP) setup generating focal spots closely spaced within the TZP spatial resolution of 160 nm. Optical path differences introduced by the sample are recorded by a CCD camera in a standard full-field imaging and by an aperture photodiode in a standard scanning transmission x-ray microscope. Applying this x-ray DIC technique, we demonstrate for both the full-field imaging and scanning x-ray microscope methods a drastic increase in image contrast (approximately 20×) for a low-absorbing specimen, similar to the Nomarski DIC method for visible-light microscopy.

X-ray lithography for micro- and nano-fabrication at ELETTRA for interdisciplinary applications

ELETTRA (http://www.elettra.trieste.it/index.html) is a third generation synchrotron radiation source facility operating at Trieste, Italy, and hosts a wide range of research activities in advanced materials analysis and processing, biology and nano-science at several various beam lines. The energy spectrum of ELETTRA allows x-ray nano-lithography using soft (1.5 keV) and hard x-ray (10 keV) wavelengths. The Laboratory for Interdisciplinary Lithography (LIILIT) was established in 1998 as part of an Italian national initiative on micro- and nano-technology project of INFM and is funded and supported by the Italian National Research Council (CNR), INFM and ELETTRA. LILIT had developed two dedicated lithographic beam lines for soft (1.5 keV) and hard x-ray (10 keV) for micro- and nano-fabrication activities for their applications in engineering, science and bio-medical applications. In this paper, we present a summary of our research activities in micro- and nano-fabrication involving x-ray nanolithography at LILITs soft and hard x-ray beam lines.

Current status of the TwinMic beamline at Elettra: a soft X-ray transmission and emission microscopy station

The current status of the TwinMic beamline at Elettra synchrotron light source, that hosts the European twin X-ray microscopy station, is reported. The X-ray source, provided by a short hybrid undulator with source size and divergence intermediate between bending magnets and conventional undulators, is energy-tailored using a collimated plane-grating monochromator. The TwinMic spectromicroscopy experimental station combines scanning and full-field imaging in a single instrument, with contrast modes such as absorption, differential phase, interference and darkfield. The implementation of coherent diffractive imaging modalities and ptychography is ongoing. Typically, scanning transmission X-ray microscopy images are simultaneously collected in transmission and differential phase contrast and can be complemented by chemical and elemental analysis using across-absorption-edge imaging, X-ray absorption near-edge structure or low-energy X-ray fluorescence. The lateral resolutions depend on the particular imaging and contrast mode chosen. The TwinMic range of applications covers diverse research fields such as biology, biochemistry, medicine, pharmacology, environment, geochemistry, food, agriculture and materials science. They will be illustrated in the paper with representative results.

Three-dimensional micro- and nanostructuring by combination of nanoimprint and x-ray lithography

We present results on a lithographic approach that combines nanoimprint (NIL) and x-ray lithography (XRL) for fabricating unconventional three-dimensional (3D) polymer structures. The use of XRL for structuring a prepatterned resist by NIL gives rise to high-resolution high-aspect-ratio structures whose overall profile is enveloped by the original 3D imprinted profile. The technological potential of this method has been demonstrated by patterning several different types of structures with XRL on an hexagonal array of hemispheres previously obtained by nanoimprinting.

Fabrication of three-dimensional stamps for embossing techniques by lithographically controlled isotropic wet etching

The excellent characteristics that nanoimprint lithography (NIL) shows as a two-dimensional patterning technique, such as resolution and throughput, are shared with its three-dimensional (3D) structuring version. Nevertheless, the potentialities of NIL for 3D structuring remain largely unexplored and unexploited, hindered by the difficulties in the fabrication of high quality masters with throughput suitable for practical purposes. We present a technique that allows production of stamps with tightly controlled three-dimensional profiles in the 10nm–100μm scale range. This technique consists of a sequence of lithographic steps alternated with isotropic wet etching processes performed on a quartz substrate. Scanning electron microscopy and atomic force microscopy characterization shows that 3D structures with very accurate shape control and nanometer scale surface roughness can be formed with this method. Quartz stamps have been employed in nanoimprint, hot embossing, or casting processes to shape complex p...