Island Films Research Articles

Identifiable analysis of dispersed precipitates and porosity in ion-irradiated materials

Analytical formulas for the analysis of structural states in ion-irradiated materials, namely dispersed precipitates, gas-filled pores, and island films on a solid substrate are proposed. A method for revealing similarities and differences between the characteristics of the theoretical and experimental size distributions of particles is developed. This method for the identification of distributions allows a correlation to be determined between the features of the transformations of experimental distributions (histograms) and processes in a dispersed system.

Nanosecond infrared laser-induced precipitation of silver nanoparticles in glass

We describe a novel plasmonic hybrid nanostructure based on a silver island film covered with a dielectric silica layer. The thickness of the silica layer is varied from 0 to approximately 46 nm on a single sample, thus allowing for continuous variation of the interaction strength between plasmon excitations in the metallic film and the excited states of pigments comprising photosynthetic complexes used to probe this interaction. While the largest separation between the silver film and photosynthetic complexes provides fluorescence featuring mono-exponential decay, thinner silica spacer distances show bi-exponential decay. The intensity of the fast component, which is attributed to the emission of photosynthetic complexes coupled to plasmon excitations, strongly decreases as a function of the spacer thickness. The interaction is stronger for excitation wavelengths resonant with plasmon absorption in the metallic layer. Full Text: PDF References P. Anger, P. Bharadwaj, L. Novotny, "Enhancement and Quenching of Single-Molecule Fluorescence", Phys. Rev. Lett. 96, 113002 (2006). CrossRef M.A. Noginov, G. Zhu, M. Bahoura, C.E. Small, C. Davison, J. Adegoke, V.P. Drachev, P. Nyga, V.M. Shalaev, "Enhancement of spontaneous and stimulated emission of a rhodamine 6G dye by an Ag aggregate", Phys. Rev. B 74, 184203 (2006). CrossRef K. Ray, R. Badugu, J. R. Lakowicz, "Metal-Enhanced Fluorescence from CdTe Nanocrystals:  A Single-Molecule Fluorescence Study", J. Am. Chem. Soc. 128, 8998 (2006). CrossRef W. Zhang, Y. Chen, C. Hu, Y. Zhang, X. Chen, M. Qiu Zhang, "Effective excitation and control of guided surface plasmon polaritons in a conjugated polymer–silver nanowire composite system", J. Mater. Chem. C 1, 1265 (2013). CrossRef Ł. Bujak, N. Czechowski, D. Piatkowski, R. Litvin, S. Mackowski, T.H.P. Brotosudarmo, R.J. Cogdell, S. Pichler, W. Heiss, "Fluorescence enhancement of light-harvesting complex 2 from purple bacteria coupled to spherical gold nanoparticles", Appl. Phys. Lett. 99, 173701 (2011). CrossRef J.B. Nieder, R. Bittl, M. Brecht, "Fluorescence Studies into the Effect of Plasmonic Interactions on Protein Function", Angew. Chem. Int. Ed. 49, 10217 (2010). CrossRef P. Nyga, V. P. Drachev, M.D. Thoreson, V. M. Shalaev, "Mid-IR plasmonics and photomodification with Ag films", Appl. Phys. B 93, 59 (2008). CrossRef U.K. Chettiar, P. Nyga, M.D. Thoreson, A.V. Kildishev, V.P. Drachev, V.M. Shalaev, "FDTD modeling of realistic semicontinuous metal films", Appl. Phys. B 100, 159 (2010). CrossRef V.P. Drachev, M.D. Thoreson, V. Nashine, E.N. Khaliullin, D. Ben-Amotz, V.J. Davisson, V.M. Shalaev., "Adaptive silver films for surface-enhanced Raman spectroscopy of biomolecules", J. Raman Spectroscopy 36, 648 (2005). CrossRef B. Krajnik, T. Schulte, D. Piątkowski, N. Czechowski, E. Hofmann, S. Mackowski, "SIL-based confocal fluorescence microscope for investigating individual nanostructures", Cent. Eur. J. Phys. 9, 293 (2011). CrossRef

Surface‐enhanced Raman scattering sensing on black silicon

Reactive ion etching was used to fabricate black‐Si over the entire surface area of 4‐inch Si wafers. After 20 min of the plasma treatment, surface reflection well below 2% was achieved over the 300–1000 nm spectral range. The spikes of the black‐Si substrates were coated by gold, resulting in an island film for surface‐enhanced Raman scattering (SERS) sensing. A detection limit of 1 × 10−6 M (at count rate > 102 s−1 . mW−1) was achieved for rhodamine 6G in aqueous solution when drop cast onto a ∼ 100‐nm‐thick Au coating. The sensitivity increases for thicker coatings. A mixed mobile‐on‐immobile platform for SERS sensing is introduced by using dog‐bone Au nanoparticles on the Au/black‐Si substrate. The SERS intensity shows a non‐linear dependence on the solid angle (numerical aperture of excitation/collection optics) for a thick gold coating that exhibits a 10 times higher enhancement. This shows promise for augmented sensitivity in SERS applications.

Tunable Electric Field Enhancement and Redox Chemistry on TiO2 Island Films via Covalent Attachment to Ag or Au Nanostructures

Ag–TiO2 and Au–TiO2 hybrid electrodes were designed by covalent attachment of TiO2 nanoparticles to Ag or Au electrodes via an organic linker. The optical and electronic properties of these systems were investigated using the cytochrome b5 (Cyt b5) domain of sulfite oxidase, exclusively attached to the TiO2 surface, as a Raman marker and model redox enzyme. Very strong SERR signals of Cyt b5 were obtained for Ag-supported systems due to plasmonic field enhancement of Ag. Time-resolved surface-enhanced resonance Raman spectroscopic measurements yielded a remarkably fast electron transfer kinetic (k = 60 s–1) of Cyt b5 to Ag. A much lower Raman intensity was observed for Au-supported systems with undefined and slow redox behavior. We explain this phenomenon on the basis of the different potential of zero charge of the two metals that largely influence the electronic properties of the TiO2 island film.

Surface plasmon enhanced photoluminescence in amorphous silicon carbide films by adjusting Ag island film sizes

Ag island films with different sizes are deposited on hydrogenated amorphous silicon carbide (α-SiC:H) films, and the influences of Ag island films on the optical properties of the α-SiC:H films are investigated. Atomic force microscope images show that Ag nanoislands are formed after Ag coating, and the size of the Ag islands increases with increasing Ag deposition time. The extinction spectra indicate that two resonance absorption peaks which correspond to out-of-plane and in-plane surface plasmon modes of the Ag island films are obtained, and the resonance peak shifts toward longer wavelength with increasing Ag island size. The photoluminescence (PL) enhancement or quenching depends on the size of Ag islands, and PL enhancement by 1.6 times on the main PL band is obtained when the sputtering time is 10 min. Analyses show that the influence of surface plasmons on the PL of α-SiC:H is determined by the competition between the scattering and absorption of Ag islands, and PL enhancement is obtained when scattering is the main interaction between the Ag islands and incident light.

Catalyst and processing effects on metal-assisted chemical etching for the production of highly porous GaN

Metal-assisted chemical etching is a facile method to produce micro-/nanostructures in the near-surface region of gallium nitride (GaN) and other semiconductors. Detailed studies of the production of porous GaN (PGaN) using different metal catalysts and GaN doping conditions have been performed in order to understand the mechanism by which metal-assisted chemical etching is accomplished in GaN. Patterned catalysts show increasing metal-assisted chemical etching activity to n-GaN in the order Ag < Au < Ir < Pt. In addition, the catalytic behavior of continuous films is compared to discontinuous island films. Continuous metal films strongly shield the surface, hindering metal-assisted chemical etching, an effect which can be overcome by using discontinuous films or increasing the irradiance of the light source. With increasing etch time or irradiance, PGaN morphologies change from uniform porous structures to ridge and valley structures. The doping type plays an important role, with metal-assisted chemical etching activity increasing in the order p-GaN < intrinsic GaN < n-GaN. Both the catalyst identity and the doping type effects are explained by the work functions and the related band offsets that affect the metal-assisted chemical etching process through a combination of different barriers to hole injection and the formation of hole accumulation/depletion layers at the metal–semiconductor interface.

Surface enhanced infrared absorption spectra on pulsed laser deposited silver island films

Infrared absorption spectra for self-assembled monolayer of p-nitrothiophenol on Pulsed Laser Deposited (PLD) silver films have been measured at normal incidence of radiation. For a PLD film grown in non-equilibrium conditions, it was confirmed that the particle diameter and the interparticle distance were small compared with those of a vacuum-evaporated film. A film with such morphology is advantageous for infrared absorption enhancement. Results show that the best film for the infrared absorption enhancement is probably that generated with PLD.

The effect of atmospheric tarnishing on the optical and structural properties of silver nanoparticles

We investigate the optical and morphological effects of silver island films exposed to atmosphere. Silver nanoparticles with an average radius below 5 nm were highly tarnished after one week, showing significant morphological changes. Using x-ray photoelectron spectroscopy we demonstrate that the tarnishing is predominantly caused by oxidation and not sulfidation for exposed silver nanostructures under normal conditions. Post-deposition annealing of tarnished nanoparticle films results in further morphological changes which are highly dependent on the exposure time and nominal film thickness. Our results suggest that the size and density of the nanoparticles can be pre-designed by controlling the deposition thickness, tarnishing and annealing. The processing causes a red-shift in the localized surface plasmon resonance due to reduced particle interactions.

Femtosecond optical nonlinearity of Au nanoparticles under their excitation in nonresonant relative to surface plasmon conditions

Nonlinear optical response of Au island films to femtosecond laser pulses is studied in the vicinity but not exactly at the surface plasmon absorption peak (λ spr = 560 nm). The third-order nonlinear optical susceptibility is shown to be Reχ (3) = +1.7 × 10−7 esu and Reχ (3) = +1 × 10−7 esu at the wavelengths 800 and 460 nm, respectively. Kinetics of the optical nonlinear response has been studied for wavelengths 400 and 800 nm. It is believed that the origin of nonlinearity at the wavelengths is related to the free-electron heating in the conduction band and their further thermalization via electron–electron scattering, but at 400 nm the contribution to the nonlinear susceptibility because of interband d → s, p transitions is also possible.

Growth and study of antimony selenide island films

Nanosized Sb2Se3 island films are prepared by the incongruent evaporation of Sb1 − xSex (x = 0.9 at fractions) thin films. The surface morphology of the structures is investigated by optical and atomic force microscopy (AFM). It is shown that an increase in the evaporation temperature leads to a decrease in the surface density of islands.

Surface-Enhanced Raman Scattering and DFT Study of 4,4'-Biphenyldithiol on Silver Surface

Surfaced-enhanced Raman scattering (SERS) of 4,4'-biphenyldithiol (BPDT) has been investigated at a silver island film. Ordinary Raman (OR) spectra of neat sample in solid state and in basic solution have also been taken for comparison. The spectral feature in the SERS spectrum was similar to that for the OR spectrum in basic solution, except for the broadening of ring stretching bands indicative of the presence of surface-phenyl ring π interaction. In contrast, only absence of the C-H stretching band with very small Raman scattering crosssection seemed not pertinent in judging the definitive orientation of molecule. The observed vibrational bands in the SERS spectrum have been assigned by referring to the normal modes and wavenumbers from density functional theory (DFT) calculations of the simple model as 4,4'-biphenyldithiolates bound to two Ag atoms at the both ends. Excellent agreement between the experimental and the calculated results was achieved, which is remarkable considering the level of theory applied.

Largely enhanced single-molecule fluorescence in plasmonic nanogaps formed by hybrid silver nanostructures.

It has been suggested that narrow gaps between metallic nanostructures can be practical for producing large field enhancement. We design a hybrid silver nanostructure geometry in which fluorescent emitters are sandwiched between silver nanoparticles and silver island film (SIF). A desired number of polyelectrolyte layers are deposited on the SIF surface before the self-assembly of a second silver nanoparticle layer. Layer-by-layer configuration provides a well-defined dye position. It allows us to study the photophyical behaviors of fluorophores in the resulting gap at the single molecule level. The enhancement factor of a fluorophore located in the gap is much higher than those on silver surfaces alone and on glass. These effects may be used for increased detectability of single molecules bound to surfaces which contain metallic structures for either biophysical studies or high sensitivity assays.

Simulation and experimental investigation of optical transparency in gold island films

Localized surface plasmons-polaritons represent collective behavior of free electrons confined to metal particles. This effect may be used for enhancing efficiency of solar cells and for other opto-electronic applications. Plasmon resonance strongly affects optical properties of ultra-thin, island-like, metal films. In the present work, the Finite Difference Time Domain (FDTD) method is used to model transmittance spectra of thin gold island films grown on a glass substrate. The FDTD calculations were performed for island structure, corresponding to the Volmer-Weber model of thin film growth. The proposed simulation model is based on fitting of experimental data on nanostructure of ultra-thin gold films, reported in several independent studies, to the FDTD simulation setup. The results of FDTD modeling are then compared to the experimentally measured transmittance spectra of prepared thin gold films and found to be in a good agreement with experimental data.

Effective Temperature Sensing by Irreversible Morphology Evolution of Ultrathin Gold Island Films

An ultrathin gold island film is developed showing efficient temperature sensing when maintaining at certain duration and may be a potential candidate as a temperature marker. The developed gold thin film is based on the energy minimization principle, in which unstable ultrathin films experience morphological instability and self-organization upon thermal dewetting, providing the “finger print” for recording the temperature and duration of the thermal event based on their variation of characteristic optical properties. As compared with other temperature sensing mechanisms and nanostructures, the ultrathin gold film displays an irreversible variation that may be employed ex-situ for extreme conditions in which in situ measurements of the thermal history may not be feasible. A high sensitivity is possible for temperature sensing even at temperatures as low as 100 °C when the time is fixed due to an efficient dewetting process at the nanoscale. This Au-based nanostructure allows fast readout of temperature by simply measuring the surface plasmon absorption. The thermal model was developed based on the correlation among the optical properties, morphological evolution, and the dewetting dynamics and validated with experimental data with accurate determination of temperature within an uncertainty of 4%. The thickness-dependent dewetting behavior further opens up the possibility for designing various nanostructures with controllable sensitivities by simple manipulation of the film thickness and thus dewetting dynamics.

Direct Observation of Aminoglycoside–RNA Binding by Localized Surface Plasmon Resonance Spectroscopy

RNA is involved in fundamental biological functions when bacterial pathogens replicate. Identifying and studying small molecules that can interact with bacterial RNA and interrupt cellular activities is a promising path for drug design. Aminoglycoside (AMG) antibiotics, prominent natural products that recognize RNA specifically, exert their biological functions by binding to prokaryotic ribosomal RNA and interfering with protein translation, ultimately resulting in bacterial cell death. The decoding site, a small internal loop within the 16S rRNA, is the molecular target for the AMG antibiotics. The specificity of neomycin B, a highly potent AMG antibiotic, to the ribosomal decoding RNA site, was previously studied by observing AMG-RNA complexes in solution. Here, we study this interaction using localized surface plasmon resonance (LSPR) transducers comprising gold island films prepared by evaporation on glass and annealing. Small molecule AMG receptors were immobilized on the Au islands via polyethylene glycol (PEG)-thiol linkers, and the interaction with target RNA in solution was studied by monitoring the change in the LSPR optical response upon binding. The results show high-affinity binding of neomycin to 27-nucleotide model A-site RNA sequence in the nanomolar range, while no specific binding is observed for synthetic RNA oligomers (e.g., poly-U). The impact of specific base substitutions in the A-site RNA constructs on binding affinity and selectivity is determined quantitatively. It is concluded that LSPR is a powerful tool for providing molecular insight into small molecule-RNA interactions and for the design and screening of selective antimicrobial drugs.

A microscopic study of strongly plasmonic Au and Ag island thin films

Thin Au and Ag evaporated films (∼5 nm) are known to form island-like growth, which exhibit a strong plasmonic response under visible illumination. In this work, evaporated thin films are imaged with high resolution transmission electron microscopy, to reveal the structure of the semicontinuous metal island film with sub-nm resolution. The electric field distributions and the absorption spectra of these semicontinuous island film geometries are then simulated numerically using the finite difference time domain method and compared with the experimentally measured absorption spectra. We find surface enhanced Raman scattering (SERS) enhancement factors as high as 108 in the regions of small gaps (≤2 nm), which dominate the electromagnetic response of these films. The small gap enhancement is further substantiated by a statistical analysis of the electric field intensity as a function of the nanogap size. Areal SERS enhancement factors of 4.2 × 104 are obtained for these films. These plasmonic films can also enhance the performance of photocatalytic and photovoltaic phenomena, through near-field coupling. For TiO2 photocatalysis, we calculate enhancement factors of 16 and 19 for Au and Ag, respectively. We study the effect of annealing on these films, which results in a large reduction in electric field strength due to increased nanoparticle spacing.

Influence of growth conditions on the lateral grain size of AlN film grown by metal-organic chemical vapor deposition

In this paper, we investigate the effect of growth conditions on the quality of AlN film grown by metal-organic chemical vapor deposition. We test and analyze the influence of different growth conditions, such as nitridation time, growth time of AlN buffer layer and the flow rate of carrier gas, on the lateral grain size of AlN film. It is found that the redution of nitridation time, the increase of growth time of AlN buffer layer, and the reduction of the flow rate of carrier gas can enhance the lateral growth of AlN film and coalescence of islands and increase the lateral grain size of AlN film. So the quality of AlN film is improved.

Mechanism of morphology transformation during annealing of nanostructured gold films on glass

Nanostructured, just-percolated gold films were prepared by evaporation on bare glass. Annealing of the films at temperatures close to or higher than the softening temperature of the glass substrate induces morphological transformation to discrete Au islands and gradual embedding of the formed islands in the glass. The mechanism and kinetics of these processes are studied here using a combination of in situ high-temperature optical spectroscopy; ex situ characterization of the island shape by high-resolution scanning electron microscopy (HRSEM), atomic force microcopy (AFM) and cross-sectional transmission electron microscopy (TEM); and numerical simulations of transmission spectra using the Multiple Multipole Program (MMP) approach. It is shown that the morphological transformation of just-percolated, 10 nm (nominal thickness) Au films evaporated on glass and annealed at 600 °C, i.e., in the vicinity of the substrate glass transition temperature (Tg = 557 °C), proceeds via three processes exhibiting different time scales: (i) fast recrystallization and dewetting, leading to formation of single-crystalline islands (minutes); the initial spectrum characteristic of a continuous Au film is transformed to that of an island film, displaying a surface plasmon (SP) absorption band. (ii) Reshaping and faceting of the single-crystalline islands accompanied by formation of circumferential glass rims around them (first few hours); the overall optical response shows a blue shift of the SP band. (iii) Gradual island embedding in the glass substrate (tens of hours), seen as a characteristic red shift of the SP band. The influence of the annealing atmosphere (air, vacuum) on the embedding process is found to be minor. Numerical modeling of the extinction cross-section corresponding to the morphological transformations during island recrystallization and embedding is in qualitative agreement with the experimental data.

Excitation Volumetric Effect of Plasmon-Enhanced Singlet Oxygen and Super Oxide Generation

In a previous report by us [1], it was demonstrated that the metal enhanced fluorescence (MEF) signal from fluorophores close to metal nanoparticles increased non-linearly with the far-field excitation power (irradiance). The effect was interpreted as an increase in the near-field excitation volume surrounding the metallic nanoparticles and was subsequently referred to as an excitation volumetric effect (EVE), where the technique would potentially allow for the near-field tuning of fluorescence enhancement factors, when observed from the far-field.In this contribution we show that the EVE phenomenon also has a profound effect on the formation of Reactive Oxygen Species (ROS), such as singlet oxygen (1O2) and superoxide anion radical (O2-∗), when sensitizers are placed in close proximity to plasmon supporting substrates. In particular, when the singlet oxygen sensitizer Rose Bengal is placed on a SiFs surface, i.e. on a silver island film [2], the 1O2 response to power is non-linear, and at 100 mW excitation power (535 nm) it is about 5 times higher, as compared to glass reference samples, measured with the commercially available 1O2 probe Sensor Green (SG). We also report a similar power dependence of superoxide generation for Acridine on SiFs surfaces, but using the dihydroethidium O2-∗probe (DHE). We additionally report the influence of various solvents where the singlet oxygen lifetime varies, i.e., water and deuterium oxide.[1] A. I. Dragan and C. D. Geddes, “Excitation volumetric effects (EVE) in metal-enhanced fluorescence,” Phys. Chem. Chem. Phys., vol. 13, no. 9, pp. 3831-3838, 2011.[2] SiFs paper reference here With rod.

Metal-enhanced fluorescence based excitation volumetric effect of plasmon-enhanced singlet oxygen and super oxide generation

In this contribution we show that the Metal-Enhanced Fluorescence (MEF) Excitation Volumetric Effect (EVE), has a profound effect on the formation of Reactive Oxygen Species (ROS), such as singlet oxygen ((1)O2) and superoxide anion radical (O2(-)*), when sensitizers are placed in close proximity to plasmon supporting nanoparticulate substrates. In particular, when the singlet oxygen sensitizer rose bengal is placed on a SiFs surface, i.e. on a silver island film, the (1)O2 response to power is non-linear, and at 100 mW excitation power (535 nm) it is about 5 times higher, as compared to glass control samples, measured with the commercially available (1)O2 probe Sensor Green™. We also report a similar power dependence of superoxide generation for acridine on SiFs surfaces, but using the dihydroethidium O2(-)* probe (DHE). Our findings are consistent with our previously postulated Metal-Enhanced Fluorescence (MEF) and EVE models.