We evaluate and compare the sensitivity of gold nanodisks on silica substrates and nanoholes made in silica-supported gold films, two of the most common sensor structures used in plasmonic biosensing. An alumina overcoat was applied by atomic layer deposition (ALD) to precisely control the interfacial refractive index and determine the evanescent plasmonic field decay length. The results are in good agreement with analytical models and biomolecular binding experiments for the two substrates. We found that nanodisks outperform nanoholes for thin dielectric coatings (<∼20 nm), while the opposite holds true for thicker coatings (>∼20 nm). The optimum nanoplasmonic transducer element for a given biorecognition reaction can be chosen based on experimentally determined bulk sensitivities/noise levels and theoretically estimated evanescent field decay lengths.
In comparison with structures containing nanoparticles, relatively little effort has been spent on developing void-based structures, although many studies have looked at the optical properties of nanoholes in thin metal fi lms since the discovery of "extraordinary" transmission. [ 6 ] The optical properties of individual nanoholes and arrays thereof are now fairly well understood, [ 7 ] especially for the case of thicker (typically hundreds of nm) metal fi lms, where each interface supports its own surface plasmon polariton (SPP). Nanohole arrays in thinner fi lms (typically ∼50 nm or less), where the individual SPPs hybridize into two new modes, [8][9][10] are now also fairly well studied. [11][12][13] Due to their unique geometry, solid state nanopores in general [ 14 ] and plasmonic nanopores in particular [ 15,16 ] have proven valuable in several unique applications. For instance, since a continuous metal fi lm can operate as an electrode, additional sensing techniques can easily be implemented. [ 17,18 ] However, fabrication and optical characterization of more advanced nanopore-based structures such as apertures penetrating several fi lms is scarce. In a few cases holes continue into a dielectric supporting fi lm. [ 16,19,20 ] For the case of metal-insulator-metal (MIM) fi lms, structures with high void fractions and relatively large apertures have been fabricated [ 21,22 ] and evaluated as negative refractive index (RI) metamaterials in the near infrared. [ 21,23,24 ] Overall, the concept of MIM thin fi lms is interesting due to the appearance of new SPP modes and the possibility to utilize such modes, e.g., in waveguides. [25][26][27] However, to date nobody has presented apertures in MIM fi lms that have even a single of the following three properties: Connecting two compartments, small diameters (on the order of ∼100 nm or less) and a low (tens of percent) void fraction. As a consequence, the optical response of such structures has only been theoretically estimated [28][29][30] and there are no experimental studies on SPP excitation by nanopore arrays in MIM fi lms.In this work we show for the fi rst time fabrication of arrays of very small (down to at least 50 nm) pores penetrating suspended MIM fi lms and connecting two reservoirs (without representing a total void fraction of more than ∼10%). The dispersions and fi elds for the SPP modes of the system are solved for by extending a theoretical formalism we recently introduced. [ 31 ] With emphasis on the hybridization of the bonding mode surface plasmons, we demonstrate how the nanopore arrays can be used to excite SPP modes in the MIM structures using visible A novel type of plasmonic nanopore array in a metal-insulator-metal thin fi lm is presented. The optical properties of this structure are described using a generic theoretical framework for surface waves in a coupled multilayer system. The characteristic spacing (short-range order) of the pores enables grating-type coupling to hybridized surface plasmons, with stronger coupling to some modes tha...
Improved understanding of thermal deactivation processes of supported nanoparticles via sintering is needed to increase the lifetime of catalysts. To monitor sintering processes under industrially relevant application conditions, in situ experimental methods compatible with elevated temperatures, high pressures, and harsh chemical environments are required. Here, we experimentally demonstrate the applicability of in situ indirect nanoplasmonic sensing (INPS) to investigate the sintering of Pt model catalysts on flat alumina and silica supports in O 2 and NO 2 atmospheres in real time and under operating conditions. Moreover, by means of finite-difference time-domain (FDTD) electrodynamics simulations, we establish a generic scaling approach to account for different inherent sensitivities of INPS sensor platforms featuring dielectric support layers with different refractive index. On the basis of these findings, we identify a universal, support-, and sintering-environment-independent correlation between INPS centroid shift signal and mean Pt particle size during the sintering process. As a first demonstration of the new possibilities offered by INPS and the obtained universal scaling to study sintering processes under different applied conditions, we investigate the dependence of the sintering rate of a SiO 2 -supported Pt model catalyst on the O 2 concentration in Ar carrier gas. We find a clear dependence of the sintering rate on the O 2 concentration in the 0.05−0.5% O 2 concentration range.
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