Semicontinuous Gold Films Research Articles

SERS-Active Substrates Nanoengineering Based on e-Beam Evaporated Self-Assembled Silver Films

Surface-enhanced Raman spectroscopy (SERS) has been intensely studied as a possible solution in the fields of analytical chemistry and biosensorics for decades. Substantial research has been devoted to engineering signal enhanced SERS-active substrates based on semi-continuous nanostructured silver and gold films, or agglomerates of micro- and nanoparticles in solution. Herein, we demonstrate the high-amplitude spectra of myoglobin precipitated out of ultra-low concentration solutions (below 10 μg/mL) using e-beam evaporated continuous self-assembled silver films. We observe up to 105 times Raman signal amplification with purposefully designed SERS-active substrates in comparison with the control samples. SERS-active substrates are obtained by electron beam evaporation of silver thin films with well controlled nanostructured surface morphology. The characteristic dimensions of the morphology elements vary in the range from several to tens of nanometers. Using optical confocal microscopy we demonstrate that proteins form a conformation on the surface of the self-assembled silver film, which results in an effective enhancement of giant Raman scattering signal. We investigate the various SERS substrates surface morphologies by means of atomic force microscopy (AFM) in combination with deep data analysis with Gwyddion software and a number of machine learning techniques. Based on these results, we identify the most significant film surface morphology patterns and evaporation recipe parameters to obtain the highest amplitude SERS spectra. Moreover, we demonstrate the possibility of automated selection of suitable morphological parameters to obtain the high-amplitude spectra. The developed AFM data auto-analysis procedures are used for smart optimization of SERS-active substrates nanoengineering processes.

Conductive scanning probe microscopy of the semicontinuous gold film and its SERS enhancement toward two-step photo-induced charge transfer and effect of the supportive layer

The semicontinuous gold film, enabling various electronic applications including development of surface-enhanced Raman scattering (SERS) substrate, is investigated using conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM) to reveal and investigate local electronic characteristics potentially associated with SERS generation of the film material. Although the gold film fully covers the underlying silicon surface, CAFM results reveal that local conductivity of the film is not continuous with insulating nanoislands appearing throughout the surface due to incomplete film percolation. Our analysis also suggests the two-step photo-induced charge transfer (CT) play the dominant role in the enhancement of SERS intensity with strong contribution from free electrons of the silicon support. Silicon-to-gold charge transport is illustrated by KPFM results showing that Fermi level of the gold film is slightly inhomogeneous and far below the silicon conduction band. We propose that inhomogeneity of the film workfunction affecting chemical charge transfer between gold and Raman probe molecule is associated with the SERS intensity varying across the surface. These findings provide deeper understanding of charge transfer mechanism for SERS which can help in design and development of the semicontinuous gold film-based SERS substrate and other electronic applications.

Probing the hot spot properties of semicontinuous gold films through the fluorescence polarization of CdSe/CdS colloidal nanocrystals

The coupling between thick-shell CdSe/CdS colloidal nanocrystals with the hot spots of a semicontinuous gold film is characterized by measuring simultaneously the photoluminescence decay rate and the linear polarization ratio. The absence of correlations between the two quantities is demonstrated. In contrast with the results obtained with continuous gold films, polarization ratios higher than 80% are achieved for the smallest nanocrystals. This ratio decreases quickly when the nanocrystals size is increased.

Optical reconfiguration and polarization control in semi-continuous gold films close to the percolation threshold.

Controlling and confining light by exciting plasmons in resonant metallic nanostructures is an essential aspect of many new emerging optical technologies. Here we explore the possibility of controllably reconfiguring the intrinsic optical properties of semi-continuous gold films, by inducing permanent morphological changes with a femtosecond (fs)-pulsed laser above a critical power. Optical transmission spectroscopy measurements show a correlation between the spectra of the morphologically modified films and the wavelength, polarization, and the intensity of the laser used for alteration. In order to understand the modifications induced by the laser writing, we explore the near-field properties of these films with electron energy-loss spectroscopy (EELS). A comparison between our experimental data and full-wave simulations on the exact film morphologies hints toward a restructuring of the intrinsic plasmonic eigenmodes of the metallic film by photothermal effects. We explain these optical changes with a simple model and demonstrate experimentally that laser writing can be used to controllably modify the optical properties of these semi-continuous films. These metal films offer an easy-to-fabricate and scalable platform for technological applications such as molecular sensing and ultra-dense data storage.

Surface conductance and microwave scattering in semicontinuous gold films

<p>Semicontinuous gold films 4 nm to 12 nm thick were characterized using patterned coplanar waveguides over a frequency range of 100 MHz to 20 GHz. Such films can form two dimensional fractal aggregates mixed with dielectric voids with unusually large electromagnetic absorption. Surface conductance and microwave absorption were obtained from the measured scattering parameters using a microwave transmission‐reflection model. Within the percolation coverage of gold nanoparticles, when the surface conductance increases from 10<sup>-5</sup> S to 10<sup>-3</sup> S, the film properties transition from dielectric to metallic, and the corresponding microwave transmittance falls far more rapidly than the classical skin depth model would suggest. The resulting microwave absorption attains a peak value in this range, which results from an inhomogeneous localization of an electromagnetic field in fractal structures. The dielectric to metallic transition can be easily identified experimentally from an abrupt change in the phase of the reflection scattering parameter S<sub>11</sub>. The results demonstrate a convenient easurement technique to study electromagnetic properties of surface‐enhanced semicontinuous metallic films for thin broadband absorbers with minimized reflection, and for other microwave applications.</p>

Application of Turkevich Method for Gold Nanoparticles Synthesis to Fabrication of SiO2@Au and TiO2@Au Core-Shell Nanostructures

The Turkevich synthesis method of Au nanoparticles (AuNPs) was adopted for direct fabrication of SiO2@Au and TiO2@Au core-shell nanostructures. In this method, chloroauric acid was reduced with trisodium citrate in the presence of amine-functionalized silica or titania submicroparticles. Core-shells obtained in this way were compared to structures fabricated by mixing of Turkevich AuNPs with amine-functionalized silica or titania submicroparticles. It was found that by modification of reaction conditions of the first method, such as temperature and concentration of reagents, control over gold coverage on silicon dioxide particles has been achieved. Described method under certain conditions allows fabrication of semicontinuous gold films on the surface of silicon dioxide particles. To the best of our knowledge, this is the first report describing use of Turkevich method to direct fabrication of TiO2@Au core-shell nanostructures.

Optical control of fluorescence through plasmonic eigenmode extinction.

We introduce the concept of optical control of the fluorescence yield of CdSe quantum dots through plasmon-induced structural changes in random semicontinuous nanostructured gold films. We demonstrate that the wavelength- and polarization dependent coupling between quantum dots and the semicontinuous films, and thus the fluorescent emission spectrum, can be controlled and significantly increased through the optical extinction of a selective band of eigenmodes in the films. This optical method of effecting controlled changes in the metal nanostructure allows for versatile functionality in a single sample and opens a pathway to in situ control over the fluorescence spectrum.

Percolating plasmonic networks for light emission control.

Optical nanoantennas have revolutionised the way we manipulate single photons emitted by individual light sources in a nanostructured photonic environment. Complex plasmonic architectures allow for multiscale light control by shortening or stretching the light wavelength for a fixed operating frequency, meeting the size of the emitter and that of propagating modes. Here, we study self-assembled semi-continuous gold films and lithographic gold networks characterised by large local density of optical state (LDOS) fluctuations around the electrical percolation threshold, a regime where the surface is characterised by large metal clusters with fractal topology. We study the formation of plasmonic networks and their effect on light emission from embedded fluorescent probes in these systems. Through fluorescence dynamics experiments we discuss the role of global long-range interactions linked to the degree of percolation and to the network fractality, as well as the local near-field contributions coming from the local electro-magnetic fields and the topology. Our experiments indicate that local properties dominate the fluorescence modification.

Strongly modified spontaneous emission decay rate of silicon nanocrystals near semicontinuous gold films

We investigate the spontaneous emission decay rate of Si nanocrystals modified by thin semicontinuous gold films. It has been shown that the mean and standard deviation values of decay rate distribution obtained from the photo-emission decay curve analysis increase due to the deposition of semicontinuous gold films. These values are dependent on gold film thickness and emission wavelength. The observed results are well explained using a point-dipole decay rate model considering the effective dielectric functions of the gold films that exhibit peculiar structures in the localized surface plasmon resonance region.

Distance dependence of the local density of states in the near field of a disordered plasmonic film

We measure the statistical distribution of the photonic local density of states in the near field of a semicontinuous gold film. By varying the distance between the measurement plane and the film, we show that near-field confined modes play a major role in the width of the distribution. Numerical simulations in good agreement with experiments allow us to point out the influence of nonradiative decay channels at short distance.

Enhancing the fluorescence of individual thick shell CdSe/CdS nanocrystals by coupling to gold structures

The fluorescence properties of individual CdSe/CdS nanocrystals (NCs) with a very thick shell and deposited on metallic structures are analyzed in detail. The results obtained for two metallic structures consisting of a continuous or a semi-continuous gold film are compared. Under low pulsed excitation, a strong acceleration of radiative processes is observed. The probability of electron–hole pair recombinations through Auger processes dramatically decreases, resulting in a suppression of blinking and the appearance of biexcitonic cascades. An original method of photons postselection also enables us to determine the decay rate corresponding to biexcitonic recombinations. Finally, a detailed analysis of the excitation process and the photon collection efficiency enables us to discriminate the effect of the gold structure in terms of excitation and fluorescence acceleration. It is found that high collection percentages can be achieved through the modification of NC emission with plasmonic structures.

Plasmon assisted single photon emission of CdSe/CdS nanocrystals deposited on random gold film

We have investigated the modifications of the fluorescence properties of single CdSe/CdS nanocrystals deposited close to a semicontinuous gold film. We report a reduction by a factor of ten of the monoexcitonic state decay rate. Due to the structure of the plasmon resonances, the decay depends on the nanocrystal position. Through the detailed study of the collection efficiency, we show that a large fraction of the plasmons are converted in far field single photons. These results demonstrate the great interest of plasmonic devices to control the single photon emission of colloidal nanocrystals.

Enhanced radiative emission rate of CdSe/ZnS quantum dots on semi-continuous gold films

We have studied the enhancement of radiative emission rate of CdSe/ZnS core-shell quantum dots on semi-continuous gold films which have been prepared by using sputtering technique. The photoluminescence intensity and the decay rate of CdSe/ZnS quantum dots are affected by the semi-continuous gold films. Both the photoluminescence intensity and the decay rate of the quantum dots are greatly enhanced by the semi-continuous gold films near the conductive percolation threshold and that are caused by the enhanced local electric field due to the surface plasmon resonance on the semi-continuous gold films.

Effects of dimension on optical transmittance of semicontinuous gold films

The thickness dependence of dielectric response of semicontinuous gold films evaporated onto glass substrates under various temperatures has been studied. A wide range of mass thicknesses (and filling factors) was considered, up to those corresponding to continuous films. The work includes a brief introduction, description of sample preparation and transmittance measurements, as well as determination of the dielectric response function using simple phenomenological model, namely, the generalized Bruggeman approximation. The approximation considers system's dimension as a varied noninteger quantity near the percolation threshold where the correlation length is large. The results obtained show that our model is valid in the actual range and dielectric properties of films are related to their morphological peculiarities. So, we refine and supplement some of our results reported in an earlier work (Proc. SPIE 3133 (1997) 300).

Negative magnetoresistance in the nearest-neighbour hopping conduction in granular gold film

The low-temperature (0.5–55 K) conduction of semicontinuous gold film vacuum deposited at T≈50 K is studied. The film is near the percolation threshold (thickness 3.25 nm). Its resistance is extremely sensitive to the applied voltage U. At low enough U the film behaves as an insulator (two-dimensional granular metal). In this state the dependences R( T)∝exp(1/ T) (for T⩽20 K) and R( U)∝exp(1/ U)) (for T⩽1 K and U>0.1 V) are observed. Magnetoresistance (MR) is negative and can be described by Δ R( H)/ R(0)∝− H 2/ T. This negative MR which manifests itself for nearest-neighbour hopping is rather uncommon and, up to now, has not been clarified. The possible mechanisms of such case of negative MR are discussed.

Transition from strong to weak electron localization in a percolating gold film under the influence of an electric field

The low-temperature (0.5–55 K) conductivity of a semicontinuous gold film near the percolation threshold is studied. It is found that the film resistance is very sensitive to the applied voltage U. By varying U, the film can be reversibly transformed from the insulating to the metallic type conductivity. This makes it possible to study the metal–insulator transition (MIT) by tuning the electric field. For low U⩽0.05 V, the film behaves as an insulator with the sheet resistance R□ up to 10 MΩ. In this state, the dependences R(T)∝exp(1/T) (for T⩽20 K) and R(U)∝exp(1/U) (for T⩽1 K and U>0.1 V) are observed. At high voltages (U≃10 V), the film has the resistance R□≃5 kΩ and behaves like a “dirty” metal. The magnetoresistance (MR) in the metallic state is positive and corresponds to the weak localization effect. In the insulating state, the MR is negative and is described by the formula ΔR(H)/R(0)∝−H2/T. The negative MR manifests itself for nearest-neighbor hopping. Such behavior is unusual, and its nature is unclear. The dependences of resistance on temperature, voltage, and magnetic field as well as the general nature of the observed MIT are considered on the basis of the obtained results.

Electrical and optical properties of very thin gold films †

The electrical and optical properties of discontinuous and semi-continuous gold films evaporated on to dielectric substrates in a high vacuum have been measured. The electrical properties of the films were characterized by DC conductivity and by the temperature coefficient of resistance: the reflectance and transmittance at near-infrared wavelengths were the optical properties considered. Special attention was given to the films in the vicinity of the percolation threshold.

Optical properties of thin semicontinuous gold films over a wavelength range of 2.5 to 500 microm.

The optical reflectance and transmittance of percolating gold films close to the metal-insulator transition were measured over an extended wavelength range: from 2.5 to 500 \ensuremath{\mu}m. It is shown that the inhomogeneous nature of such films controls the optical properties even at such long wavelengths as 500 \ensuremath{\mu}m, where the typical grain size is 10 nm. Effective-medium theory is shown to be invalid close to the percolation threshold even at 500 \ensuremath{\mu}m. Comparison of the measured data of all the samples with the scaling model of Y. Yagil et al. [Phys. Rev. B 43, 11 342 (1991)] yields excellent agreement over the entire wavelength range. In particular, the short length scale determined by the anomalous diffusion relation is shown to be the relevant length scale for the optical measurements. This scaling model for the optical properties of such films is thus experimentally justified, both qualitatively and quantitatively.

Electron localization in two-dimensional percolating gold films.

We have investigated the temperature dependence and magnetoresistance of thin semicontinuous gold films. We have observed a decrease of the quantum corrections to the conductivity due to two-dimensional localization effects for samples close to the percolation threshold. For these samples, quasi-one-dimensional effects appear to become important, presumably developing around the singly connected links of the percolation network. The experimental data are shown to be consistent with a model based on scaling laws of percolation and localization.