Localized Surface Plasmon Resonance Peak Position Research Articles

A colorimetric assay for H2O2 and glucose based on the morphology transformation of Au/Ag nanocages to nanoboxes

Herein, we introduced a sensitive colorimetric platform for hydrogen peroxide (H2O2) assay based on gold/silver (Au/Ag) nanocages with porous structure. In the presence of H2O2, the morphology of hollow Au/Ag nanocages was converted to closed nanoboxes, altering their localized surface plasmon resonance (LSPR) peak position and the solution color from light blue to deep blue. The morphology transformation and LSPR peak position of Au/Ag nanocages were proportional to H2O2 concentration at the range of 0.1 to 50 µM. The limit of detection (LOD) was obtained to be 0.02 µM, and the relative standard deviation (RSD, for 0.2, 2.0, and 20 µM) was 2.7, 2.3, and 2.9%, respectively. Moreover, a smartphone-based colorimetric sensor was developed for H2O2 assay at the concentration range of 0.25–4.0 µM, with LOD of 0.2 µM and RSD of 3.2, 2.5, and 2.9% (for 0.5, 1.0, and 3.0 µM, respectively). We exploited the established sensor for glucose assay by measuring the generated H2O2 from the enzymatic reaction between glucose and glucose oxidase. There was a linear relationship between LSPR peak wavelength variations and the amount of glucose from 1.0 to 50 µM, with LOD of 0.4 µM and RSD of 3.2, 3.1, and 3.8% (for 2.0, 10, and 30 µM, respectively). The sensor was successfully applied to determine H2O2 and glucose in food and human serum samples, respectively.

Refractive index sensitivity of Au nanostructures in solution and on the substrate

In this work, we present the synthesis and surface immobilization of Au nanostars, Au nanocubes and Au nanorods for localized surface plasmon resonance (LSPR)-based refractometric sensing applications. Au nanostructures exhibiting LSPR peak positions in 500–900 nm spectral range were prepared by seed-mediated synthesis method. The refractive index (RI) sensitivity of all these nanostructures in the colloidal solution were measured and the sample exhibiting highest sensitivity in each category were immobilized on the glass substrate. The surface immobilized nanostructures were investigated for RI sensing. Au nanostars having LSPR peak position at 767 nm exhibited highest RI sensitivity of 484 nm/RIU in solution and 318 nm/RIU on the substrate. This study gives an outline for selecting the Au nanostructures for developing plasmonic sensing platforms.

Robust LSPR Sensing Using Thermally Embedded Au Nanoparticles in Glass Substrates.

The poor adhesion and chemical and thermal stability of plasmonic nanostructures deposited on solid surfaces are a hindrance to the longevity and long-term development of robust localized surface plasmon resonance (LSPR)-based systems. In this paper, we have deposited gold (Au) nanolayers with thicknesses above the percolation limit over glass substrates and have used a thermal annealing treatment at a temperature above the substrate’s glass transition temperature to promote the dewetting, recrystallization, and thermal embedding of Au nanoparticles (NPs). Due to the partial embedding in glass, the NPs were strongly adherent to the surface of the substrate and were able to resist to the commonly used cleaning procedures and mechanical adhesion tests alike. The reflectivity of the embedded nanostructures was studied and shown to be strongly dependent on the NP size/shape distributions and on the degree of NP embedding. Strong optical scattering bands with increasing width and redshifted LSPR peak position were observed with the Au content. Refractive index sensitivity (RIS) values between 150 and 360 nm/RIU (concerning LSPR band edge shift) or between 32 and 72 nm/RIU (concerning LSPR peak position shift) were obtained for the samples having narrower LSPR extinction bands. These robust LSPR sensors can be used following a simple excitation/detection scheme consisting of a reflectance measurement at a fixed angle and wavelength.

Optical absorption analysis of core-shell type Ni@Ag/Au & NiFe@Ag/Au magneto-plasmonic nanostructures

The core-shell nanoparticles are greatly advocated for their usefulness in biomedical applications tending to targeted drug delivery, sensing, and bio-imaging. In this paper, the optical properties of simple; Ni@Ag/Au and complex; NiFe@Ag/Au magnetoplasmonic nanoparticles for their core-shell structures with spherical and prolate geometries are comprehensively investigated using the discrete dipole approximation (DDA) method. It is found that the Localized Surface Plasmon Resonance (LSPR) for absorption (λmax) as well as efficiency (intensity) strongly depend on the geometry of the nanoparticle, aspect ratio, core sizes, and shell thickness. The absorption spectra which are generally found in the 335–717 nm wavelength ranges can be tuned in the ultra violet-near-infrared (UV-NIR) region of the electromagnetic (EM) spectrum for desired biomedical applications. It is discovered that there is a higher tunability of LSPR peak position as well as larger enhancement in the absorption efficiency when the spherical nanoparticle stretches to prolate core-shell configuration. Based on these findings, therefore, it is suggested that the proposed core-shell nanostructures with the tunable optical response in the visible region have potentials in cancer therapy, drug delivery, and optical imaging.

Fine-tuning of localized surface plasmon resonance of metal nanostructures from near-Infrared to blue prepared by nanosphere lithography

Plasmonics using metal nanoparticles (MNPs) has emerged as an important research subject in the field of photonics, electronics, and nanotechnology. Despite spectacular progress in recent years, accurate tuning and modeling of plasmon resonances over a wide spectral range using state-of-the-art fabrication methods are still challenging tasks. Here, we report on the fine-tuning of the localized surface plasmon resonance (LSPR) of metal nanoparticles over a wide spectral range from near-infrared to blue using nanosphere lithography (NSL). In this systematic study, we use NSL to fabricate triangular shaped metal nanostructures using gold, silver, copper, and aluminum. All structures were annealed up to 500 °C under a nitrogen atmosphere in order to study the effect of annealing on the LSPR. Structural changes were investigated using scanning electron microscopy and atomic force microscopy. UV-VIS spectroscopy was used to determine the LSPR spectral position for these structures. The LSPR peak position is ordered as copper, gold, silver, and aluminum (from low to high photon energy—ranging from near-infrared to blue). The rate at which the LSPR changes with respect to the increasing annealing temperature is determined to be (2.3 ± 0.3) nm/°C and (1.3 ± 0.1) nm/°C for Ag and Au, respectively, while Cu MNPs show a two-step relation with a steeper slope of (1.4 ± 0.3) nm/°C initially up to 275 °C followed by a shallower slope of (0.5 ± 0.1) nm/°C. The full width at half maximum of the LSPR increases from gold over silver and copper to aluminum. We also performed finite element method simulations to validate the experimental findings. Our results can have a significant impact in plasmonic applications where fine-tuning and accurate designing of the LSPR are important.

Localized Surface Plasmon Resonance Studies on Pd/C Nano-Composite System: Effect of Metal Concentration and Annealing Temperature.

The effects of metal concentration and annealing temperature on the localized surface plasmon resonance (LSPR) properties of the Pd nanoparticles (NP) dispersed in carbon were investigated. The Pd/C nano-composite thin films with 7 to 39 atomic % concentration of metal content were deposited using the atom beam co-sputtering techniques and subjected to annealing at temperature varying from 300 °C to 600 °C. The UV-vis spectroscopy studies on as-prepared films displayed a Mie scattering profile, but not well-defined LSPR bands were observed for all the values of Pd concentration. This is attributed to the smaller size (3-4 nm) of Pd NPs and rough Pd/C interface, as confirmed from TEM studies. When samples were annealed at a temperature of 300 °C, three broad LSPR absorption bands in the visible region, along with a sharp peak at 210 nm, were observed and the effect of Pd concentration variation was insignificant on their position. The multiple LSPR bands were observed due to agglomeration NPs, which is consistent with earlier reports and is also observed in the TEM images. When annealing temperature was subsequently increased to 500 °C, a blue shift in the LSPR peak position with an increase in the Pd concentration was observed, which phenomena is attributed to the formation of bigger NPs with the formation of sharp NPs-interface at high temperature upon annealing. A monotonic increase in the magnitude and decrease in the FWHM with an increase in concentration suggested change in the dielectric function of sample due to the growth of NPs. This is further confirmed from XRD studies, where strain relaxation and grain growth were observed. The intensity of the SPR peak decreased with an increase in the annealing temperature. The LSPR peak disappeared on annealing at a temperature of 600 °C, suggesting the formation of continuous polycrystalline thin films of Pd. In summary, NPs size, metalmatrix interface, and concentration of metal play key roles in the tailoring the LSPR properties of the Pd.

Dynamics of instability in plasmonic response of nanostructured gold thin films on ambient ageing

Instability in the plasmonic response of nanostructured gold (Au) thin films on ambient ageing has been investigated. Different types of Au nanostructures (island, percolated, and continuous thin film) are obtained at the early stage of thin film growth by variation in film thickness using sputtering technique. Absorbance spectra of as-deposited island Au film shows a systematic blue-shift in the localized surface plasmon resonance (LSPR) peak position on ageing. The rate of blue-shift in the LSPR peak position is fitted with single-exponential decay function to analyse the dynamics of the plasmonic response. The as-deposited percolated Au film displays a transformation of broadened plasmonic response into a wavelength independent absorbance profile on ageing. The immersion of a new plasmonic band in the absorbance spectra is noted for continuous Au thin film on ageing. The change in plasmonic responses of nanostructured Au thin films on ambient ageing is found to be directly correlated with the re-organization of the surface morphology. Finally, ageing-induced solid-state dewetting and crystallization are found to be the main responsible processes of the underlying mechanism for morphological re-organizations, leading to change in the plasmonic response of the nanostructured Au thin films.

Enhancing the Sensitivity of Nanoplasmonic Thin Films for Ethanol Vapor Detection.

Nanoplasmonic thin films, composed of noble metal nanoparticles (gold) embedded in an oxide matrix, have been a subject of considerable interest for Localized Surface Plasmon Resonance (LSPR) sensing. Ethanol is one of the promising materials for fuel cells, and there is an urgent need of a new generation of safe optical sensors for its detection. In this work, we propose the development of sensitive plasmonic platforms to detect molecular analytes (ethanol) through changes of the LSPR band. The thin films were deposited by sputtering followed by a heat treatment to promote the growth of the gold nanoparticles. To enhance the sensitivity of the thin films and the signal-to-noise ratio (SNR) of the transmittance–LSPR sensing system, physical plasma etching was used, resulting in a six-fold increase of the exposed gold nanoparticle area. The transmittance signal at the LSPR peak position increased nine-fold after plasma treatment, and the quality of the signal increased six times (SNR up to 16.5). The optimized thin films seem to be promising candidates to be used for ethanol vapor detection. This conclusion is based not only on the current sensitivity response but also on its enhancement resulting from the optimization routines of thin films’ architectures, which are still under investigation.

Predicting the Size of Silver Nanoparticles from Their Optical Properties

In this study, localized surface plasmon resonance (LSPR) of the spherical silver nanoparticles (AgNPs) was evaluated based on experimental and theoretical viewpoints. In the experimental phase, a facile seed-mediated method was employed to synthesize spherical AgNPs with different sizes. As expected, an increase in the size of AgNPs resulted in a red-shift in LSPR peak position from 390 to 460 nm. Besides, the theoretical LSPR peak position of AgNPs was derived from Mie theory using different refractive indices databases. A comparison between the experimentally obtained data and the theoretical predictions indicated that “Palik” database can lead to a more reliable data regarding the size of AgNPs. Finally, a relation was established for the prediction and estimation of AgNPs’ size through a simple measurement of LSPR position with a UV-Vis spectrometer.

Design of Aluminum Bowtie Nanoantenna Array with Geometrical Control to Tune LSPR from UV to Near-IR for Optical Sensing

Plasmonic nanoantennas have earned strong recognition for their unique capability to confine light from free space into sub-wavelength dimensions with strong electric field (E-field) enhancement factor due to localized surface plasmon resonance (LSPR). Broad spectral tuning of LSPR from ultraviolet (UV) to near-infrared (NIR) is required for incident light wavelength and material sensitive plasmonic applications in different spectral regions. In this article, we introduced and designed a novel aluminum plasmonic platform consisting of a bowtie nanoantenna (BNA) array with metal-insulator-metal (MIM) configuration where LSPR peak position was broadband tunable from UV to NIR through geometric control of antenna parameters. Furthermore, we designed and numerically analyzed a plasmonic biosensor platform that detected concentration of glycerol in de-ionized (DI) water with a concentration in the range of 0 to 40 wt% (refractive index = 1.333 to 1.368) with a sensitivity of 497 nm/RIU (refractive index units). The designed plasmonic platform can also be used as a surface-enhanced Raman scatting (SERS) substrate with enhancement factor as high as 4.82 × 109 for 1042 nm excitation wavelength. The reported hybrid dielectric-metallic plasmonic nanostructured system is a universal plasmonic platform for a wide range of applications including single-molecule SERS, biosensing, fluorescence microscopy, plasmonic nanocavity, nanolasers, and solid-state lighting.

Localized surface plasmon resonance (LSPR) detection of hydrogen gas by Pd2+/Au core/shell like colloidal nanoparticles

A hydrogen detector colloidal solution based on the plasmonic properties of gold nanoparticles (GNPs) is presented. GNPs were prepared by pulsed laser ablation (PLA) of gold target in DI water. PdCl2 solution with different concentration was added to the obtained GNPs colloidal solutions. X-ray diffraction (XRD) confirmed the formation of metallic gold and presence of PdCl2 phases. Transmission electron microscope (TEM) images along with X-ray photoelectron spectroscopy (XPS) revealed formation of the core-shell-like structures of Pd2+/Au NPs. After hydrogenation, TEM revealed that the core-shell morphology changes into free NPs and XPS revealed formation of the metallic Pd phase. Voltammetry analysis showed a well absorption and desorption capability of hydrogen in this gold-PdCl2 plasmonic system. After adding PdCl2 aqueous solution, a red-shift from 522 to 526 nm was observed which was attributed to Pd2+ ion attraction by the negative surface charge of bare GNPs and formation a core-shell like morphology. The optical absorption peaks of PdCl2 (the range 207–236 nm) as well as the LSPR peak of GNPs were traced during diluted hydrogen (0.3–10%) injection in colloidal samples with different Au:Pd molar ratio. It was found that the PdCl2 peaks drop due to Pd2+→Pd0 conversion and more importantly, the gold peak undergoes blue shift due to change in chemical properties of GNPs surrounding. A good correlation between PdCl2 absorption intensity and gold LSPR peak position was found when hydrogen concentration was varied. In this correlation, a desirable detection capability for low concentrations of hydrogen (<4% range, the limit of hydrogen explosion) with a possible large number of points was observed. Finally, a model for the hydrogen sensing mechanism based on the LSPR effect was presented.

Quantitative Analysis of Extinction Coefficients of Tin-Doped Indium Oxide Nanocrystal Ensembles.

The optical extinction coefficients of localized surface plasmon resonance (LSPR) in doped semiconductor nanocrystals (NCs) have intensities determined by the density and damping mechanisms of free charge carriers. We investigate the dependence of the extinction coefficient of tin-doped indium oxide (ITO) NCs on size and dopant concentration and find extinction coefficients as high as 56.6 μm-1 in the near-infrared for 20 nm diameter ITO NCs with 7.5 atomic% Sn. We find ITO NCs to be more efficient infrared light absorbers than metal nanoparticles or molecular dyes. We also find the intensive, volume-normalized extinction coefficient increases significantly with NC doping and NC diameter, but only up to the point of saturation in both cases. We qualitatively analyze trends in LSPR peak position and width to explain the effect of doping and size on extinction.

Flexible tuning of hole-based localized surface plasmon resonance in roxbyite Cu1.8S nanodisks via particle size, carrier density and plasmon coupling

Semiconductor nanocrystals (NCs) heavily doped with cation/anion vacancies or foreign metal ions can support localized surface plasmon resonance (LSPR) in the near-infrared (NIR) and mid-infrared (MIR) spectral wavelengths. Typically, nonstoichiometric copper sulfide Cu2−xS NCs with different x values (0 < x ≤ 1) have attracted numerous attention because of hole-based, particle size, morphology, hole density and crystal phase-dependent LSPR. In spite of excited development of methodology for LSPR manipulation, systematic LSPR tuning of Cu2−xS NCs with a special crystal phase has been limited. Herein, roxbyite Cu1.8S nanodisks (NDs) were selected as a model and their LSPR was readily tuned by particle size, hole density via chemical oxidation and reduction, self-assembly and disassembly in solution and plasmon coupling in multilayer films. Particle size, hole density and plasmon coupling severely affect their LSPR peak position and absorption intensity. Therefore, the ability of flexible LSPR tuning gifts roxbyite Cu1.8S NDs great potential in plasmonic applications, including photocatalysis, photothermal agent, two-photon photochemistry and many others in NIR and MIR regions.

Theoretical investigation of size, shape, and aspect ratio effect on the LSPR sensitivity of hollow-gold nanoshells.

The change in refractive index around plasmonic nanoparticles upon binding to biomolecules is routinely used in localized surface plasmon resonance (LSPR)-based biosensors and in biosensing platforms. In this study, the plasmon sensitivity of hollow gold (Au) nanoshells is studied using theoretical modeling where the influence of shape, size, shell thickness, and aspect ratio is addressed. Different shapes of hollow Au nanoshells are studied that include sphere, disk, triangular prism, rod, ellipsoid, and rectangular block. Multilayered Mie theory and discrete dipole approximation were used to determine the LSPR peak position and LSPR sensitivity as a function of size, shell thickness, shape, and aspect ratio. The change in LSPR peak wavelength per unit refractive index is defined as the sensitivity, and interesting results were obtained from the analysis. The rectangular block and rod-shaped Au nanoshells have shown maximum LSPR sensitivity when compared to other shaped Au nanoshells. In addition, increased sensitivity was observed for higher aspect ratio as well as for smaller shell thicknesses. The results are rationalized based on the inner and outer surface plasmonic coupling.

Hybrid nanostructures of plasmonic gold nanoparticles with a-C:H thin films

Localized surface plasmon resonance (LSPR) of gold nanoparticles (Au NPs) in a granulated film with thickness 2 nm was investigated at the interface of two amorphous hydrogenated carbon (a-C:H) films with optical gap 0.67 eV and 2.7 eV. In these hybrid structures, a blue shift of the LSPR wavelength was observed in contrast with Au NPs on quartz surface at a room temperature. The annealing of such hybrid structures at 200 °C resulted in forming spheroid Au NPs with the dimensions about 11 nm which promotes increasing blue shift of the LSPR spectra. The intensity of the LSPR peak increased significantly after annealing at 300 °C, but Au NPs film morphology and LSPR peak position have not changed. After annealing at 400 °C the LSPR peak intensity in hybrid structure with a narrow gap a-C:H film decreased dramatically but did not change in the sample with a wide gap a-C:H. The observed changes in the LSPR spectra as a result of hybrid structures annealing are associated with the difference in the electronic and atomic structure of a-C:H thin films, as shown by their studies using Raman spectroscopy.

Surface plasmon resonance and polarization change properties in centrosymmetric nanoright-triangle dimer arrays

Dimer nanoparticles in a sandwich structure exhibit a large electric-field intensity enhancement. The dispersion relation between the surface plasmon resonance (SPR) and particle size has not been reported yet, owing to the effects of the particle size, shape, materials, etc. A sandwich structure, which contains a nano-right-triangle dimer array, SiO2 spacer, and Au film, is proposed, with a significant electric-field intensity enhancement and polarization-changing properties. The dependence of the peak positions of the two localized surface plasmon resonance (LSPR) modes as a function of the triangle thicknesses is discussed; different trends are observed for the different LSPR modes. We introduce a concept on the rule for LSPR peak position change, which can contribute to a better understanding of the LSPR modes. In addition, centrosymmetric but not axisymmetric structures, which like in our study exhibit surface plasmon polaritons typically show different responses to a different polarization of the incident light. Here, we showed that our centrosymmetric but not axisymmetric structure can change the linearly polarized light into a circularly or elliptically polarized wave, by surface plasmon-induced polarization properties. Far-field distribution maps are used to study the properties of the surface plasmons-induced circular or elliptic polarization wave. These findings could be employed to better understand the surface plasmon-induced polarization properties showed in previous reports and near-field of surface plasmons. These findings could be employed to better understand the near-field of surface plasmons and polarization properties.

Superiority of localized surface plasmon resonance technique in characterization of ultra-thin metallic films

The comparison and correlation of morphological, optical and crystallographic properties of ultra-thin Au films obtained using field-emission scanning electron microscopy (FESEM), x-ray reflectivity (XRR), UV-visible transmission, and grazing incidence x-ray diffraction (GIXRD) are presented. The Au thin films of different thickness are grown on the glass substrate using the sputtering technique. The particle size, number density and the covered area fraction of Au thin film are obtained from FESEM images. The XRR technique is used to determine the film thickness and surface roughness. The localized surface plasmon resonance (LSPR) response of Au thin films is obtained using UV-Vis transmission spectroscopy. The LSPR peak position and its strength are correlated with film morphology and thickness. Finally, it is shown that LSPR based spectroscopy techniques can provide much better information about morphology and thickness of the Au films up to a resolution of ~1 nm.

Characterizing physical properties and in vivo OCT imaging study of Cu-Sn-S nanocrystals

Highly yield Cu2SnS3 nanocrystals (CTS NCs) have been attracted more attentions in these years, the CTS NCs with strong absorption in near-infrared (NIR) region which can serve as the contrast agent of Optical Coherence Tomography (OCT) imaging. These NCs can be synthesized by facile method, and exhibit a Localized Surface Plasmon Resonance (LSPR) peak in NIR region. The LSPR peak position of the CTS NCs depends on the ratio of copper to tin in the synthesis process. The highest intensity of LSPR at 1380nm when Cu:Sn ratio reach to 9:1. The TEM analysis and X-ray diffraction pattern reveals the formation of CTS NCs with an average size of 6nm and the structure is kesterite crystal phase. In order to apply the NCs in vivo and in vitro study, we used PEGylated phospholipid (DSPE-PEG) to modified NCs, and the colloidal stability and cell viability of DSPE-PEG CTS NCs are very suitable for the in vivo OCT imaging study. To quantitatively analyze the contrast effect of DSPE-PEG CTS NCs, the contrast agent was injected from the tail vein of ICR mice, then applied the SD-OCT system monitor the vein of the mouse pinna for 30 minutes. The results indicated that the DSPE-PEG CTS NCs created an obvious signal in the OCT imaging process, which provide the basis for the application of CTS NCs as the contrast agent for the bioimaging study.

Silver Nanodisk Monolayers with Surface Coverage Gradients for Use as Optical Rulers and Protractors

Colloidal silver nanodisks (AgNDs) are assembled into a monolayer with a coverage density gradient (CDG) on the surface of flat and cylindrical substrates using the Langmuir-Blodgett (LB) technique. Compressing the LB monolayers during transfer to the substrates causes the CDG assembly of the AgNDs. By functionalizing the AgNDs with poly(ethylene glycol), it is possible to control their order inside the LB monolayer assembly by changing the deposition surface pressure. Well-separated AgNDs, 2D aggregates with different numbers of particles, and highly packed 2D arrays are formed as the deposition surface pressure is increased. Localized surface plasmon resonance (LSPR) spectra collected at different separation distances from the highest coverage spot (HCS) of the CDG AgND arrays on a flat substrate are blue-shifted, and the shift increases systematically upon increasing the distance. The relationship among the LSPR peak position, the peak intensity at a fixed wavelength, and the corresponding separation distance from the HCS is fitted exponentially. A similar systematic blue shift in the LSPR spectrum of the CDG AgND monolayer on a cylindrical substrate is obtained when the substrate is rotated at different angles relative to the HCS. The fabricated CDG AgND monolayers can potentially be used for optically measuring distances and angles.

Optical and water repellant properties of Ag/SnO2 bilayer thin films

The optical and water repellant properties of single layer and bilayer films of Ag and SnO2 deposited on glass substrates by thermal evaporation have been reported. Ag/SnO2 bilayers were deposited in two sequences wherein the deposition of SnO2 layer was followed by Ag deposition and vice versa. X-ray diffraction studies show that the Ag films crystallize in the FCC structure and SnO2 is amorphous, while atomic force microscopy images indicate the formation of large clusters of the order of 12 nm. The single layer Ag films exhibit localized surface plasmon resonance (LSPR) that shifts from visible region to the infrared with increase in thickness from 5 to 12 nm. It is observed that, only the Ag films of thickness ≤ 8 nm exhibits LSPR peak whereas the critical thickness is 5 nm for Ag/SnO2 films. A blue shift is observed in the LSPR peak position when the SnO2 layer caps the Ag film. Whereas, the LSPR of Ag is suppressed significantly when the SnO2 layer is introduced between the glass and the Ag film and also when Ag and SnO2 were co-evaporated. Water repellant properties indicate that the pure Ag film has an average contact angle of 104o which decreases to 100o when SnO2 caps the Ag layer and 97o when Ag is deposited on top of the SnO2 buffer layer. Co-evaporated Ag-SnO2 films show a contact angle of 93o.