Alfonso Martín

Asymmetric Pentagonal Metal Meshes for Flexible Transparent Electrodes and Heaters

Metal meshes have emerged as an important class of flexible transparent electrodes. We report on the characteristics of a new class of asymmetric meshes, tiled using a recently discovered family of pentagons. Micron-scale meshes were fabricated on flexible polyethylene terephthalate substrates via optical lithography, metal evaporation (Ti 10 nm, Pt 50 nm), and lift-off. Three different designs were assessed, each with the same tessellation pattern and line width (5 μm), but with different sizes of the fundamental pentagonal unit. Good mechanical stability was observed for both tensile strain and compressive strain. After 1000 bending cycles, devices subjected to tensile strain showed fractional resistance increases in the range of 8-17%, while devices subjected to compressive strain showed fractional resistance increases in the range of 0-7%. The performance of the pentagonal metal mesh devices as visible transparent heaters via Joule heating was also assessed. Rapid response times (∼15 s) at low bias voltage (≤5 V) and good thermal resistance characteristics (213-258 °C cm2/W) were found using measured thermal imaging data. Deicing of an ice-bearing glass coupon on top of the transparent heater was also successfully demonstrated.

Facile Formation of Ordered Vertical Arrays by Droplet Evaporation of Au Nanorod Organic Solutions

Droplet evaporation is a simple method to induce organization of Au nanorods into ordered superstructures. In general, the self-assembly process occurs by evaporation of aqueous suspensions under strictly controlled experimental conditions. Here we present formation of large area ordered vertical arrays by droplet evaporation of Au nanorod organic suspensions. The uncontrolled (free air) evaporation of such suspensions yielded to formation of ordered nanorod domains covering the entire area of a 5 mm diameter droplet. Detailed investigation of the process revealed that nanorods organized into highly ordered vertical domains at the interface between solvent and air on a fast time scale (minutes). The self-assembly process mainly depended on the initial concentration of nanorod solution and required minimal control of other experimental parameters. Nanorod arrays displayed distinct optical properties which were analyzed by optical imaging and spectroscopy and compared to results obtained from theoretical calculations. The potential use of synthesized arrays as surface-enhanced Raman scattering probes was demonstrated with the model molecule 4-aminobenzenthiol.

Flexible SERS active substrates from ordered vertical Au nanorod arrays

SERS active flexible substrates were fabricated by droplet deposition of Au nanorod solutions on rigid surfaces and subsequent stamping of assembled nanorod arrays on photographic paper and plastic surfaces. With this method plasmonic SERS substrates whose active area was constituted by Au nanorods assembled in closely spaced vertical arrays were fabricated. The SERS performances of the flexible active substrates were investigated with model molecule 4-aminobenzenethiol (4-ABT), where detection in the μM concentration range and Enhancement Factors (EF) of 105 were obtained. To demonstrate field-based applications, detection of food contaminant crystal violet (CV) below the recommended 5 nM limit of detection was achieved. Also, detection of drug-marker benzocaine traces by a swabbing technique was demonstrated.

Synthesis, optical properties and self-assembly of gold nanorods

Noble metal nanostructures of different aspect ratios were synthesised and optically characterised at individual nanorod level. Rayleigh scattering spectroscopy/scanning electron microscopy measurements were performed to uniquely correlate optical signatures with nanorod size and shape. Scattering spectra of nanorods were dominated by the intense longitudinal surface plasmon resonance (SPR) band in the near-infrared part of the spectrum. This band was found to be highly shape and size dependent. Droplet evaporation techniques and application of dielectrophoretic forces have been used to organise nanorod dispersions into ordered arrays. Depending on the technique and nanoparticle size used, nanorods were found to form one, two or three dimensional (1D, 2D and 3D) superstructures. Within these superstructures nanorods organised themselves into end-to-end lines (1D), side-to-side fashion (2D) or hexagonal arrangements (3D).

Au nanorod plasmonic superstructures obtained by a combined droplet evaporation and stamping method

A combined droplet evaporation and stamping method is presented for the fabrication of Au nanorod superstructures. Specifically, domains of nanorods parallel to the substrate in a close-packed side-to-side fashion are obtained by evaporation of Au chlorobenzene solutions, followed by stamping of the dried droplet on transparent substrates. To understand and optimize the assembly mechanism, synthetic parameters affecting the droplet evaporation process are carefully investigated. The optical characterization of individual domains shows markedly anisotropic extinction, confirming the high degree of internal order generated by aligned nanorods. In addition, the unique orientation of domains produces a unique distribution of color intensities, which is used for the initial demonstration of a novel plasmonic encoding/decoding system.

Plasmonic detection of mercury via amalgam formation on surface-immobilized single Au nanorods

Abstract Au nanorods were used as plasmonic transducers for investigation of mercury detection through a mechanism of amalgam formation at the nanorod surfaces. Marked scattering color transitions and associated blue shifts of the surface plasmon resonance peak wavelengths (λmax) were measured in individual nanorods by darkfield microscopy upon chemical reduction of Hg(II). Such changes were related to compositional changes occurring as a result of Hg–Au amalgam formation as well as morphological changes in the nanorods’ aspect ratios. The plot of λmax shifts vs. Hg(II) concentration showed a linear response in the 10–100 nM concentration range. The sensitivity of the system was ascribed to the narrow width of single nanorod scattering spectra, which allowed accurate determination of peak shifts. The system displayed good selectivity as the optical response obtained for mercury was one order of magnitude higher than the response obtained with competitor ions. Analysis of mercury content in river and tap water were also performed and highlighted both the potential and limitation of the developed method for real sensing applications.

Non-resonant Raman spectroscopy of individual ZnO nanowires via Au nanorod surface plasmons

We present a non-resonant Raman spectroscopy study of individual ZnO nanowires mediated by Au nanorod surface plasmons. In this approach, selective excitation of the plasmonic oscillations with radiation energy below the semiconductor bandgap was used to probe surface optical modes of individual ZnO nanowires without simultaneous excitation of bulk phonons modes or band-edge photoluminescence. The development of a reproducible method for decoration of nanowires with colloidal Au nanorods allowed performing an extensive statistical analysis addressing the variability and reproducibility of the Raman features found in the hybrid nanostructures. An estimated field enhancement factor of 103 was calculated, which greatly exceeded previously reported values, and resulted in the detection of a surface optical mode not observable in bare ZnO nanowires under comparable experimental conditions. The role played by Au nanorods in the observed enhancement was investigated both theoretically and experimentally. Specifically, evidence of the superior capabilities in enhancing Raman signals of nanorod longitudinal surface plasmons compared to nanorod transversal surface plasmons is provided. Finite-difference-time domain (FDTD) simulations were used to support the experimental findings and corroborate the use of plasmonic resonances for spectroscopic investigation of individual semiconductor nanostructures.