Jovan Matovic

Negative Refractive Index Metasurfaces for Enhanced Biosensing

In this paper we review some metasurfaces with negative values of effective refractive index, as scaffolds for a new generation of surface plasmon polariton-based biological or chemical sensors. The electromagnetic properties of a metasurface may be tuned by its full immersion into analyte, or by the adsorption of a thin layer on it, both of which change its properties as a plasmonic guide. We consider various simple forms of plasmonic crystals suitable for this purpose. We start with the basic case of a freestanding, electromagnetically symmetrical plasmonic slab and analyze different ultrathin, multilayer structures, to finally consider some two-dimensional “wallpaper” geometries like split ring resonator arrays and fishnet structures. A part of the text is dedicated to the possibility of multifunctionalization where a metasurface structure is simultaneously utilized both for sensing and for selectivity enhancement. Finally we give an overview of surface-bound intrinsic electromagnetic noise phenomena that limits the ultimate performance of a metasurfaces sensor.

Performance limits to the operation of nanoplasmonic chemical sensors: noise-equivalent refractive index and detectivity

We considered figures of merit for chemical and biological sensors based on plasmonic structures and utilizing adsorption/desorption mechanism. The operation of these devices in general is limited by noise determining the minimum detectable refractive-index change. We dedicated our work to the intrinsic noise mechanisms connected with the plasmonic process itself. In contrast, most of the available literature is almost exclusively dedicated to the external noise sources (illumination source and photodetector). Reviewing the refractive-index fluctuations caused by thermal, adsorption-desorption and 1/f noise, we observed a striking analogy between the qualitative behavior of noise in (nano)plasmonic devices and that in semiconductor infrared detectors. The power spectral densities for noise in both of these have an almost identical shape; the adsorption-desorption noise corresponds to generation-recombination processes in detectors, while the other two mechanisms exist in the both types of the devices. Thus the large and mature existing apparatus for infrared detector noise analysis may be applied to the plasmonic sensors. Based on the observed analogy, we formulated the noise-equivalent refractive-index and the specific detectivity as the figures of merit to analyze the ultimate performance of plasmon sensors. The approach is valid for conventional surface plasmon resonance sensors, but also for nanoplasmonic and metamaterial-based devices.

Plasmon modes on laminated nanomembrane-based waveguides

We studied the propagation of plasmonic modes along planar multilayer metal- dielectric structures with finite number of bi-layer unit cells. The dispersion relations for various investigated waveguide structures with the multilayer core and symmetric or asymmetric cladding have been analyzed. In the case of symmetric metallic cladding we have found both TE and TM modes within the light cone, while TM modes only exist outside the light cone. Both symmetric and asymmetric dielectric claddings support modes outside the light cone and of TM-polarization only. Formation of photonic bands and gaps, the structure of their edge lines, and the behavior of modes that cross the edge lines has been investigated. In the subwavelength regime, we have found ordinary surface plasmon polariton dispersion in the forbidden gap that is created via coalescence of the two modes that cross the neighboring band-gap edges. One of those modes can exhibit negative group velocity.

Nanomembrane-based plasmonics

This paper reviews the main properties and applications of nanomembrane-based plasmonic structures, including some results presented here for the first time. Artificial nanomembranes are a novel building block in micro- and nanosystems technologies. They represent quasi-two-dimensional (2D) freestanding structures thinner than 100 nm and with giant aspect ratios that often exceed 1,000,000. They may be fabricated as various quasi-2D metal-dielectric nanocomposites with tailorable properties; they are fully symmetric in an electromagnetic sense and support long-range surface plasmon polaritons. This makes nanomembranes a convenient platform for different plasmonic structures such as subwavelength plasmonic crystals and metamaterials and applications such as plasmon waveguides and ultrasensitive bio/chemical sensors. Among other advantages of nanomembrane plasmonics is the feasibility to fabricate flexible, transferable plasmonic guides applicable to different substrates and dynamically tunable through stretching. There are various approaches to multifunctionalization of nanomembranes for plasmonics, including the use of transparent conductive oxide nanoparticles, but also the incorporation of switchable ion channels. Since the natural counterpart of the artificial nanomembranes are cell membranes, the multifunctionalization of synthetic nanomembranes ensures the introduction of bionic principles into plasmonics, at the same time extending the toolbox of the available nanostructures, materials and functions.

Novel cross-linkers for asymmetric poly-AMPS-based proton exchange membranes for fuel cells

Polymer electrolyte fuel cells (PEFCs) are an ideal solution leading to clean energy by directly converting the fuel’s chemical energy to electricity in order to achieve high degree of efficiency. One of the main components of PEFCs is the proton exchange membrane which should conduct protons but no electrons and should also separate the electrodes and limit fuel crossover. In addition to Nafion®, polymers of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) have been used as a proton-conducting ionomer since sulfonic groups are known for their good proton conductivity. Since poly-AMPS excessively swells or even dissolve in water, we investigated several commercial cross-linkers and new multifunctional monomers to decrease swelling by cross-linking. Formulations with different concentrations of these cross-linkers have been tested constrained in porous polypropylene membranes. Although formulations with commercial cross-linkers (polyethylene glycol diacrylates) already exceeded the conductivity of Nafion®, with some of the synthesized cross-linkers we achieved more than 2.5 times the conductivity of Nafion®. Moreover, the novel amide-based cross-linkers show good hydrolytical stability in contrast to the commercial ones. Finally, we used one of the new cross-linkers to prepare asymmetric membranes and could achieve about 8 times the conductivity of Nafion.

Synthesis of metal nanoparticles using one atmosphere pressure glow plasma

In this paper we propose a method for the synthesis of silver and metal nanoparticles in general. It is based on the reduction of metallic salts dissolved in water by means of non-energetic ions delivered from atmospheric pressure glow discharge plasma. A high-voltage generator with the ability to provide AC and DC voltage components separately is built for this purpose. Using the dielectric barrier discharge plasma reactor it is possible to get uniform rain of ions with tunable current density to penetrate into the salt solution. A porous dielectric plate is employed in the dielectric barrier discharge configuration. The method is tested for synthesis of silver nanoparticles. Obtained nanoparticles were analyzed on SEM and they exhibit spherical shape and sizes in the range of 25 nm to 100 nm.

Thin catalyst layers based on carbon nanotubes for PEM-fuel cell applications

In this study, two approaches are compared to develop thin, multifunctional films of carbon nanotubes (CNT) which are targeted to serve as a catalyst layer in fuel cells. The first is based on the direct deposition of mixed multi- and single-wall CNTs on metalized silicon wafers, using the metallization as a sacrificial layer to subsequently detach the CNT film from the substrate. It is a less time consuming and a straight forward method compared to the alternative under investigation, the layer-by-layer technique (LbL). The LbL uses bilayers of charged nanotubes to slowly build up a film with an exactly defined thickness. The process is well controlled, but the time constants for deposition of each bilayer are rather high (i.e. about 1 h). With additional annealing steps implemented during film generation this method, however, is regarded advantageous as membranes results with improved mechanical stability and a good homogeneity.

Bionic (Nano) Membranes

The goal of this chapter is to offer a concise and clear picture of the most important artificial nanomembrane-related procedures and technologies, including those for fabrication and functionalization, and to present the main properties and potential applications, stressing recent results in the field contributed by the authors. Nanomembranes are probably the most ubiquitous building block in biology and at the same time one of the most primordial ones. Every living cell, from bacteria to the cells in human bodies, has nanomembranes acting as interfaces between the cytoplasm and its surroundings. All metabolic processes proceed through nanomembranes and involve their active participation. Functionally, the man-made nanomembrane strives to mimic this most basic biological unit. The existence of the life itself is a proof that such a fundamental task can be performed. When designing artificial nanomembranes, the whole wealth of structures and processes already enabling and supporting life is at our disposal to recreate, tailor, fine-tune, and utilize them. In some cases, the obstacles are formidable, but then the potential rewards are stunning.

Transfer of nanomembranes from solution to a solid frame via reflow of low surface tension liquids

Nanomembranes represent a novel building block for nanosystems, characterized by a thickness below 100 nm and a giant aspect ratios. A typical procedure for nanomembrane fabrication starts from an ultrathin layer deposited on some kind of a sacrificial substrate. The nanomembrane is released by etching away the substrate, leaving the nanomembrane to freely float in the solvent. This solvent may be water or some other liquid. Many different types of nanomembranes can be made by this procedure, but only a limited number can survive the capillary forces during the extraction from solvent. In this paper we propose a novel, generally applicable method for freeing nanomembranes from the solvent via direct in-situ substitution of the solvent in meniscus with a low surface tension liquid. The proposed method ensures high production yields, is not very dependent on experimental skills, and avoids introduction of contaminants into the nanomembrane structure.

Asymmetric Sol-Gel Proton-Conducting Membrane

Proton-conducting membranes with interpenetrating polymer network morphology have gained attention in recent years for potential replacement of standard Nafion membranes in direct methanol fuel cells. These membranes generally consist of fine interpenetrating domains of proton-conducting and mechanically-supporting polymer phases, which often leads to improvements in mechanical strength and methanol barrier properties. Asymmetric sol-gel membranes comprising proton-conducting channels of cross-linked sulfonic acid functionalized ionomers embedded within a matrix of thermally-resistant, glassy polymer were prepared by photopolymerization starting from a polymer solution and evaluated in our laboratories. These membranes have an integral top skin layer with fine biomimetic proton-conducting channels, which provides a barrier against methanol crossover, on top of a coarser proton-conducting support. Conductivity of asymmetric membranes over a range of initial polymer concentrations and ion-exchange capacities (IEC) was just slightly lower than for the corresponding symmetric membranes. Methanol barrier properties of asymmetric sol-gel membranes were better than that of Nafion 115 membrane. The crosslinking agent functionality had a major effect on membrane conductivity. Use of trifunctional crosslinking agents resulted in significantly higher conductivities than those obtained with bifunctional agents, even surpassing the conductivity of Nafion membranes.