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.
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.
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.
KEYWORDS: Sensors, Infrared detectors, Microelectromechanical systems, Infrared sensors, Solids, Thermography, Temperature metrology, Mechanical sensors, Infrared radiation, Signal to noise ratio
The ultimate performance of modern mechanical sensors based on bimaterial cantilevers significantly lags behind the
maximum values as limited by thermal fluctuations. Even more, the signal-to-noise ratios of novel MEMS sensors fall
behind the characteristics of the previous generations of mechanical sensors fabricated by macroscopic production
technologies. In this paper we present for the first time a comparative analysis of the ultimate detection limits of MEMS
sensors based on bimaterial cantilever displacement and detectors based of longitudinal elongation of an equivalent
cantilever. The starting point of our analysis was a definition of the correspondence between the transversal
displacement of a bimaterial cantilever and the longitudinal elongation of the equivalent simple cantilever. These two
structures generally cannot be directly compared, since the bimaterial cantilever displacement depends on 14 variables,
while the longitudinal elongation of the simple cantilever depends on 7 parameters only. However, we show that under
certain conditions a full correspondence can be established between the parameters of these two structures. The
expansion coefficient is used here in its general sense to describe the linear length change as a function of a given
external variable, for instance temperature, analyte concentration, photonic flux, etc.
We analyzed the intrinsic noise of plasmonic sensors caused by the adsorption-desorption of gaseous analytes on the
sensor surface. We analyzed a general situation when there is a larger number of different species in the environment.
We developed our model and applied it to calculate various analyte mixtures, including some environmental pollutants,
toxic and dangerous substances. The spectral density of mean square refractive index fluctuations follows a dependence
similar to that of generation-recombination noise in photodetectors, flat at lower frequencies and sharply decreasing at
higher. Some of the calculated noise levels are well within the detection range of conventional surface plasmon
resonance sensors. One of the obvious conclusions is that AD noise may be an important limiting factor in monitoring
process kinetics by nanoplasmonic sensors. An AD noise peak is observed in temperature dependence of mean square
refractive index fluctuations, thus sensor operating temperature may be optimized to obtain larger signal to noise ratio. A
significant property of AD noise is its increase with the plasmon sensor area decrease, which means that it will be even
more pronounced in modern nanoplasmonic devices. Our consideration is valid both for conventional surface plasmon
resonance devices and for general nanoplasmonic devices. This research could be of importance in diverse areas such as
environmental sensing, homeland security, forensic applications, life sciences, etc.
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.
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