Plasmonic Thin Films Research Articles

SERS and SE-FTIR platforms from plasmonic bismuth thin films for label-free multiplexed biomolecules detection

SERS and SE-FTIR platforms from plasmonic bismuth thin films for label-free multiplexed biomolecules detection

Deposition of alternative plasmonic ZrHfN thin films via closed-field dual-cathode DC unbalanced magnetron sputtering for enhanced SEF substrate applications

We present the preparation of zirconium hafnium nitride thin films as an alternative plasmonic sensing material. The ZrHfN thin films were deposited by closed-field dual-cathode DC unbalanced magnetron sputtering without an external substrate heating/biasing. A comprehensive investigation into the effect of zirconium sputtering current on the physical structural, and chemical compositions was systematically characterized by grazing-incidence X-ray diffraction, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Our results indicate that the optimal ZrHfN thin films deposited at 500 mA-IZr exhibit good crystallinity and high surface roughness, which presents excellent surface-enhanced fluorescent substrate performance for detecting rhodamine 6G at a limit of detection of 5.99 × 10−8 M, an enhancement factor of 15.62 ± 0.79-fold and displaying reusability through 25 cycles. These findings suggest promising prospects for developing ZrHfN-based SEF sensor chips for diverse applications in the future.

Thermo-plasmon-assisted all-optical switches based on coupled micro-cavities

Given the major applications of optical switches in the next generation electronics, plasmon assisted all-optical switch is proposed based on plasmonic micro ring arrays covered with the graphene layer. For this purpose, we use a laser writing system to fabricate micro hole arrays and cover them by plasmonic gold thin film via sputtering machine, with chemical vapor deposition method being used to produce graphene layer. After theoretically and experimentally evaluating the plasmonic substrate, we use ellipsometric method to characterize the optical modes of the samples. Furthermore, the all-optical and gated switches response is recorded for the fabricated sample. Our results show phase delay plasmon induced switching as well as thermo-plasmonic affected benefit for the proposed switching structure in the order of 0.29 s (duty cycle). The suggested switch with its low power consumption, small size, and easier and low-cost fabrication process can open new insight into plasmon-assisted all-optical switches.

On the Measurements of the Surface-Enhanced Raman Scattering Spectrum: Effective Enhancement Factor, Optical Configuration, Spectral Distortion, and Baseline Variation

In this paper, a comprehensive theoretical framework for understanding surface-enhanced Raman scattering (SERS) measurements in both solution and thin-film setups, focusing on electromagnetic enhancement principles, was presented. Two prevalent types of SERS substrates found in the literature were investigated: plasmonic colloidal particles, including spherical and spheroid nanoparticles, nanoparticle diameters, and thin-film-based SERS substrates, like ultra-thin substrates, bundled nanorods, plasmonic thin films, and porous thin films. The investigation explored the impact of analyte adsorption, orientation, and the polarization of the excitation laser on effective SERS enhancement factors. Notably, it considered the impact of analyte size on the SERS spectrum by examining scenarios where the analyte was significantly smaller or larger than the hot spot dimensions. The analysis also incorporated optical attenuations arising from the optical properties of the analyte and the SERS substrates. The findings provide possible explanations for many observations made in SERS measurements, such as variations in relative peak intensities during SERS assessments, reductions in SERS intensity at high analyte concentrations, and the occurrence of significant baseline fluctuations. This study offers valuable guidance for optimizing SERS substrate design, enhancing SERS measurements, and improving the quantification of SERS detection.

Plasmonic Titanium Nitride Nanohole Arrays for Refractometric Sensing.

Group IVB metal nitrides have attracted great interest as alternative plasmonic materials. Among them, titanium nitride (TiN) stands out due to the ease of deposition and relative abundance of Ti compared to those of Zr and Hf metals. Even though they do not have Au or Ag-like plasmonic characteristics, they offer many advantages, from high mechanical stability to refractory behavior and complementary metal oxide semiconductor-compatible fabrication to tunable electrical/optical properties. In this study, we utilized reactive RF magnetron sputtering to deposit plasmonic TiN thin films. The flow rate and ratio of Ar/N2 and oxygen scavenging methods were optimized to improve the plasmonic performance of TiN thin films. The stoichiometry and structure of the TiN thin films were thoroughly investigated to assess the viability of the optimized operation procedures. To assess the plasmonic performance of TiN thin films, periodic nanohole arrays were perforated on TiN thin films by using electron beam lithography and reactive ion etching methods. The resulting TiN periodic nanohole array with varying periods was investigated by using a custom microspectroscopy setup for both reflection and transmission characteristics in various media to underline the efficacy of TiN for refractometric sensing.

Tuneable Plasmonic Resonances Of A Dynamic Thin Film Of Ultrasmall Nanocrystals Modified In the Anti-Galvanic Reduction Process.

Ultrasmall gold nanoparticles (NPs) have revolutionized nanotechnology as they are an excellent starting substrate for the synthesis of organic-inorganic hybrid materials with photonic or energy conversion applications, often with a responsive nature. However, ultrasmall NPs do not sustain plasmonic resonances, preventing their use in plasmon-related applications. In the presented work, we show a method of chemical modification of ultrasmall gold nanoparticles in order to fabricate dynamically controlled plasmonic thin films. For this purpose, we used the Anti-Galvanic Reduction process (AGR) to modify the surface of small gold nanoparticles, inducing plasmonic properties without notable size increases. Au@Ag NPs are then modified with liquid crystal-like organic ligands. The obtained NPs can assemble into densely packed films with long-range order and temperature-dependent structural properties. Namely, we detect two, fully reversible phase transitions between the hexagonal and cubic symmetries. The combination of AGR and organic surface modifications enabled us to demonstrate the possibility of managing plasmonic properties in the thin film of ~2 nm diameter metallic NPs.

Light blocking and phase modulation based on thermo-plasmonic hotspots in WS2 monolayer

Here we demonstrate light blocking and phase modulation in nano-plasmonic donate shape rings based on the thermo-plasmonic effect in these structures. For this purpose, we use a laser writing system to fabricate nanohole arrays and cover them with plasmonic gold thin film via a sputtering machine. The chemical vapor deposition method is also used to produce a WS2 layer, which is suitable for light blocking and phase modulation due to the nonlinearity of this two-dimensional material. After theoretically and experimentally evaluating the plasmonic donate-shaped substrate, we use the ellipsometric method to characterize the optical modes of the samples and record the switching manner and light-blocking phenomena under the probe laser excitation set to 980 nm. Our results show phase modulation based on the thermo-plasmonic effect of nano gap in donated double rings and light blocking by thermal expansion of the WS2 layer, which can open new insight into plasmon and two-dimensional material-based devices.

Synthesis and optical properties of Ag/Au-TiO2 plasmonic composite thin films

In this work, composite thin films based on titanium dioxide and noble metal nanoparticles (Ag, Au and bimetallic Ag/Au alloys) (Me-TiO2) were synthesized using the RF magnetron sputtering technique. The obtained thin films were characterized by TEM, SEM, XRD analysis, and Raman spectroscopy. It was observed that annealing in an argon atmosphere led to the crystallization of the initially amorphous as-deposited TiO2 matrix. The analysis of transmission spectra revealed that the composite thin films exhibited two light absorption regions: the first is local minima in the visible range associated with localized surface plasmon resonance (LSPR) phenomena; the second is light absorption due to the energy band gap. The study demonstrates the possibility of tuning these parameters in the composite films by changing the composition of the metal NPs. The LSPR minima for Ag-TiO2 and Au-TiO2 films were located at about 485 nm and 606 nm, respectively. In the composite thin films with bimetallic ∼Ag0.54/Au0.46 alloy nanoparticles, the position of the absorption peak was found to be at 555 nm. The energy band gap of these films also varies almost linearly, decreasing with an increase in the Au content, so that the largest value among the annealed Me-TiO2 composites was observed for the Ag-TiO2.

Optical properties of plasmonic titanium nitride thin films from ultraviolet to mid-infrared wavelengths deposited by pulsed-DC sputtering, thermal and plasma-enhanced atomic layer deposition

We present a comparative study of the optical properties of 50 nm-thick titanium nitride (TiN) films deposited on a silicon substrate by pulsed-DC sputtering, thermal, and plasma-enhanced atomic layer deposition. Silicon was chosen as the optimal material for the complementary metal–oxide–semiconductor (CMOS) integration. Using ellipsometry and Fourier-transform infrared (FTIR) spectroscopy, broadband permittivity of the films was obtained from a series of free-space reflection measurements in the wavelength range of 210 nm to 20.0μm (∼47619–500 cm−1). Our particular focus is on the influence of the deposition method and its process temperature on the TiN plasmonic properties. We used grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, X-ray reflectometry, atomic force microscopy, and electrical resistivity measurements to analyze the composition of thin films and to elucidate the effect of different deposition parameters on their optical properties. All measurements were carried out at room temperature and the thickness of all film samples was fixed to 50 nm to exclude the influence of these factors on the optical properties. We demonstrate that PEALD-deposited films have smoother surfaces than those deposited by the sputtering technique, while sputtered films have higher densities. In addition, by changing the temperature of both processes, it is possible to influence the stoichiometry and crystal orientation of the layers. Based on this characterization, we found out that TiN thin film deposited by pulsed-DC at 600 °C gives the best plasmonic response (high negative permittivity ϵ1 and relatively low losses associated with ϵ2, where ϵ1 and ϵ2 are the real and imaginary parts of permittivity, respectively) among all samples for the entire wavelength range under study. The same film also exhibits the excellent resistivity of 29 μΩcm. These results suggest that there is a direct relationship between stoichiometry, structural quality, and oxygen incorporation in TiN films and their plasmonic response.

Tunable electron scattering mechanism in plasmonic SrMoO3 thin films

4d transition metal perovskite oxides serve as suitable testbeds for the study of strongly correlated metallic properties. Among these, SrMoO3 (SMO) exhibits remarkable electrical conductivity at room temperature. The temperature-dependent resistivity (ρ(T)) exhibits a Fermi-liquid behavior below the transition temperature T∗, reflecting the dominant electron-electron interaction. Above T∗, electron-phonon interaction becomes more appreciable. In this study, we employed the power-law scaling of ρ(T) to rigorously determine the T∗. We further demonstrate that the T∗ can be modified substantially by ∼40 K in epitaxial thin films. It turns out that the structural quality determines T∗. Whereas the plasma frequency could be tuned by the change in the electron-electron interaction via the effective mass enhancement, we show that the plasmonic properties are more directly governed by the electron-impurity scattering. The facile control of the electron scattering mechanism through structural quality modulation can be useful for plasmonic sensing applications in the visible region.

Sensitivity enhanced tunable plasmonic biosensor using two-dimensional twisted bilayer graphene superlattice

Abstract This study theoretically demonstrated an insight for designing a novel tunable plasmonic biosensor, which was created by simply stacking a twisted bilayer graphene (TBG) superlattice onto a plasmonic gold thin film. To achieve ultrasensitive biosensing, the plasmonic biosensor was modulated by Goos–Hänchen (GH) shift. Interestingly, our proposed biosensor exhibited tunable biosensing ability, largely depending on the twisted angle. When the relative twisted angle was optimized to be 55.3°, such a configuration: 44 nm Au film/1-TBG superlattice could produce an ultralow reflectivity of 2.2038 × 10−9 and ultra-large GH shift of 4.4785 × 104 µm. For a small refractive index (RI) increment of 0.0012 RIU (refractive index unit) in sensing interface, the optimal configuration could offer an ultra-high GH shift detection sensitivity of 3.9570 × 107 µm/RIU. More importantly, the optimal plasmonic configuration demonstrated a theoretical possibility of quantitatively monitoring severe acute respiratory syndrome coronavirus (SARS-CoV-2) and human hemoglobin. Considering an extremely small RI change as little as 3 × 10−7 RIU, a good linear response between detection concentration of SARS-CoV-2 and changes in differential GH shift was studied. For SARS-CoV-2, a linear detection interval was obtained from 0 to 2 nM. For human hemoglobin, a linear detection range was achieved from 0 to 0.002 g/L. Our work will be important to develop novel TBG-enhanced biosensors for quantitatively detecting microorganisms and biomolecules in biomedical application.

Fluorophore-induced plasmonic current for solar energy collection.

Biological chromophores and certain model organisms present a unique opportunity in solar energy harvesting. Currently, solar energy conversion efficiency ranges from mid to upper teens, necessitating multiple panels to generate electricity. This is a problem since many agricultural farms in the sunbelt are transitioning into solar farms, supporting the growing demand for electricity, and further deepening existing food crisis. Therefore, the system's efficiency, stability, and longevity are crucial. Advances in plasmonic materials offer promising results due to their compatibility with fluorescent biosensors and various fluorophores, suggesting potential applications in solar energy conversion. In this presentation, we discuss a technique called fluorophore-induced plasmonic current (FIPC), where plasmonic thin films and a fluorophore solution are used to generate current as a function of fluorophore concentration. In addition, we consider a few fabrication methods to increase the current response and extend the lifetime of the system. Key takeaway suggests a working range of concentration where the magnitude of current change is proportional to the increase in concentration. Our preliminary data on thin film fabrication demonstrates an increase in current response within the working range as well as lower signal to noise, leading to a stable baseline current. Future work in this project will combine our present system with molecules such as chlorophyll a and b, along with photoautotrophs like algae.

Anodization parameters influence on anodic aluminum oxide formed above the silver island film

Optical properties of hybrid plasmonic thin film structure in the form of anodic alumina porous matrix above silver island film on the quartz substrate have been investigated. Silver nanoparticle film in the bottom of the structure has been obtained by physical vapor deposition in a vacuum chamber. The silver island film with the island of average diameter of 100 nm has been formed after annealing in the air atmosphere. Above the silver nanoparticle film an aluminum film has been deposited by the E-beam evaporation. As a result of one-step straight anodization, a nanoporous alumina thin film has been formed. The obtained structures were investigated, using spectroscopy and electron microscopy methods. The structure reflectance and optical density spectra have been obtained and analyzed for different anodization times and currents. To compare the results, the reflectance and optical density spectra have been obtained for silver nanoparticles and anodic alumina. When anodization times are increased, structure reflection coefficient spectra become more like the same characteristic for anodic aluminum oxide, which can be explained by film oxidation. At the same time a red shift of reflectance spectrums is observed in the structures with bigger maximum anodization currents. This effect has been observed in other works and can be explained by the increasing distance between the pores. A numerical modeling of optical properties with the help of Mie calculator for the structure with the nanoparticle size of 100 nm has shown that the results of the modeling can be compared to the experimentally obtained optical density spectra. The modeling was performed using spherical approximation. To obtain more precise results for alumina film thickness and nanoparticle optical properties, a silver nanoparticle form factor has to be considered. The results of this work can be used in sensors, optical coatings and photon sources fabrication methods. These can be used in screens, optical schemes and many other plasmonic devices.

Temperature-dependent optical properties of low-loss plasmonic SrMoO3 thin films

SrMoO3 (SMO) thin films are deposited on LaAlO3 substrates by magnetron sputtering. The effects of ambient temperature on the structural, electrical, and optical properties of the films are investigated. As the temperature increases from 23°C to 800°C, the SMO film exhibits high crystallinity and low electrical resistivity, and the real part of dielectric functions becomes less negative in the visible and near-IR wavelength range, and the epsilon near zero (ENZ) wavelength increases from 460 nm to 890 nm. The optical loss of the SMO film is significantly lower than that of Au, and its plasmonic performance is comparable to or even higher than TiN in the temperature range of 23°C to 600°C. These studies are critical for the design of high-temperature SMO-based plasmonic devices.

Capillary-force-assisted self-assembly of gold nanoparticles into highly ordered plasmonic thin films for ultrasensitive SERS.

In this study, a capillary device based on the surface plasmon-enhanced Raman scattering effect was prepared by a simple and easy method. First, the capillary was treated with APTES solution. Due to the electrostatic effect, gold nanoparticles could be easily and tightly assembled in the capillary inner wall. On this basis, the effects of changing the concentration of APTES, the concentration of colloids and the soaking time of the capillary in the colloids on the assembly of gold nanoparticles on the inner wall of the capillary were studied, and the SERS enhancement effect under different conditions was analyzed, and the optimal solution was successfully found. At the same time, the reason why the capillary substrate shows better SERS performance than the traditional planar substrate is deeply discussed. Since the nanoparticles can be attached to the upper and lower surfaces of the inner wall of the capillary, the utilization rate of nanoparticles and laser is improved, thereby achieving higher enhancement. For the detection of the probe molecule rhodamine 6G, it was proved that the substrate has good uniformity and the lowest detection limit can reach 10-10 M. Finally, the real-life pesticide thiram and the food additive aspartame were tested, and the detection limits could reach 10-6 M and 0.25 g L-1. It is confirmed that the prepared capillary shows excellent SERS performance and can be used for rapid detection in various fields.

Strongly coupled plasmonic systems on optical fiber sensors: A study on nanomaterial properties

New paths to increase the sensing performance of plasmonic sensors have been reported in recent years. There are several methodologies to achieve such purpose, namely by optimizing the nanostructure, nanomaterial and even the sensing platform. Recently the use nanoparticles over plasmonic thin films have been reported and shown sensitivity enhancement, when compared to a bare thin film. Nevertheless, a nanomaterial combination between NP and thin film has not been studied. In this work it was studied such plasmonic materials in order to optimize not only refractometric sensitivity but also decrease the resultant plasmonic band width. It was found that for Au, Ag and Cu thin films, the deposition of plasmonic nanoparticles resulted in an overall refractometric sensitivity and figure of merit (FOM) increase. The larger FOM increase was obtained for the Ag thin film, from 42 to 162 when coupled to Si nanoparticles. The greater sensitivity increase was achieved for a Cu thin film coupled to a Si nanoparticle, with an increase from 1745 to 3230 nm/RIU.

Plasmon resonance of gold and silver nanoparticle arrays in the Kretschmann (attenuated total reflectance) vs. direct incidence configuration

While the behaviour of plasmonic solid thin films in the Kretschmann (also known as Attenuated Total Reflection, ATR) configuration is well-understood, the use of discrete nanoparticle arrays in this optical configuration is not thoroughly explored. It is important to do so, since close packed plasmonic nanoparticle arrays exhibit exceptionally strong light-matter interactions by plasmonic coupling. The present work elucidates the optical properties of plasmonic Au and Ag nanoparticle arrays in both the direct normal incidence and Kretschmann configuration by numerical models, that are validated experimentally. First, hexagonal close packed Au and Ag nanoparticle films/arrays are obtained by air–liquid interfacial assembly. The numerical models for the rigorous solution of the Maxwell’s equations are validated using experimental optical spectra of these films before systematically investigating various parameters. The individual far-field/near-field optical properties, as well as the plasmon relaxation mechanism of the nanoparticles, vary strongly as the packing density of the array increases. In the Kretschmann configuration, the evanescent fields arising from p- and s-polarized (or TM and TE polarized) incidence have different directional components. The local evanescent field intensity and direction depends on the polarization, angle of incidence and the wavelength of incidence. These factors in the Kretschmann configuration give rise to interesting far-field as well as near-field optical properties. Overall, it is shown that plasmonic nanoparticle arrays in the Kretschmann configuration facilitate strong broadband absorptance without transmission losses, and strong near-field enhancement. The results reported herein elucidate the optical properties of self-assembled nanoparticle films, pinpointing the ideal conditions under which the normal and the Kretschmann configuration can be exploited in multiple light-driven applications.

Optimization of Au:CuO Thin Films by Plasma Surface Modification for High-Resolution LSPR Gas Sensing at Room Temperature.

In this study, thin films composed of gold nanoparticles embedded in a copper oxide matrix (Au:CuO), manifesting Localized Surface Plasmon Resonance (LSPR) behavior, were produced by reactive DC magnetron sputtering and post-deposition in-air annealing. The effect of low-power Ar plasma etching on the surface properties of the plasmonic thin films was studied, envisaging its optimization as gas sensors. Thus, this work pretends to attain the maximum sensing response of the thin film system and to demonstrate its potential as a gas sensor. The results show that as Ar plasma treatment time increases, the host CuO matrix is etched while Au nanoparticles are uncovered, which leads to an enhancement of the sensitivity until a certain limit. Above such a time limit for plasma treatment, the CuO bonds are broken, and oxygen is removed from the film’s surface, resulting in a decrease in the gas sensing capabilities. Hence, the importance of the host matrix for the design of the LSPR sensor is also demonstrated. CuO not only provides stability and protection to the Au NPs but also promotes interactions between the thin film’s surface and the tested gases, thereby improving the nanocomposite film’s sensitivity. The optimized sensor sensitivity was estimated at 849 nm/RIU, which demonstrates that the Au-CuO thin films have the potential to be used as an LSPR platform for gas sensors.

Multifunctional logic gates based on resonant transmission at atomic-plasmonic structure

Regarding the confinement of light at nanoscale dimensions in plasmonic structures, we try to show the impact of hot atomic vapor spectroscopy on a miniaturized scale. In such a combined structure, resonant coupling of the atom to plasmonic mode provides diverse ways to control the optical response of the system. We fabricate an atomic plasmonic cell based on Rubidium atomic vapor and gold plasmonic thin film onto the Kretschmann setup to introduce resonant coupling (EIT-like) of atom-plasmons as a tunable all-optical bandpass filter, switch, or logic gates. These all-optical devices such as NOR and XNOR logic gates are well done based on the filter by incidence angle of light, temperature as well as the external magnetic field. We believe the possibility of easy modulation of atomic susceptibility, not only through direct alteration on atoms but also through common methods available for modulation of plasmonic mode, has the potential to design and fabricate modern all-optical devices.

Immobilization of Streptavidin on a Plasmonic Au-TiO2 Thin Film towards an LSPR Biosensing Platform.

Optical biosensors based on localized surface plasmon resonance (LSPR) are the future of label-free detection methods. This work reports the development of plasmonic thin films, containing Au nanoparticles dispersed in a TiO2 matrix, as platforms for LSPR biosensors. Post-deposition treatments were employed, namely annealing at 400 °C, to develop an LSPR band, and Ar plasma, to improve the sensitivity of the Au-TiO2 thin film. Streptavidin and biotin conjugated with horseradish peroxidase (HRP) were chosen as the model receptor–analyte, to prove the efficiency of the immobilization method and to demonstrate the potential of the LSPR-based biosensor. The Au-TiO2 thin films were activated with O2 plasma, to promote the streptavidin immobilization as a biorecognition element, by increasing the surface hydrophilicity (contact angle drop to 7°). The interaction between biotin and the immobilized streptavidin was confirmed by the detection of HRP activity (average absorbance 1.9 ± 0.6), following a protocol based on enzyme-linked immunosorbent assay (ELISA). Furthermore, an LSPR wavelength shift was detectable (0.8 ± 0.1 nm), resulting from a plasmonic thin-film platform with a refractive index sensitivity estimated to be 33 nm/RIU. The detection of the analyte using these two different methods proves that the functionalization protocol was successful and the Au-TiO2 thin films have the potential to be used as an LSPR platform for label-free biosensors.