Nanostructured Metal Films Research Articles

Programmable Analog Circuits with Neuromorphic Nanostructured Platinum Films

AbstractSelf‐assembled nanoparticle or nanowire networks have recently come under the spotlight as systems able to emulate brain‐like data processing performances by exploiting the memristive character and the wiring of the junctions connecting the nanostructured network building blocks. Recently it is demonstrated that nanostructured Au films, fabricated by the assembling of gold clusters produced in the gas phase, have non‐linear and non‐local electric conduction properties caused by the extremely high density of grain boundaries and the resulting complex arrangement of nanojunctions. These observations suggest that nanostructured metallic films can be explored and exploited as a novel class of neuromorphic systems for unconventional electronic devices. Here it is reported that nanostructured cluster‐assembled Pt films show resistive switching activity, negative differential resistance, and reversible memory effects. These properties allow the use of nanostructured Pt films in programmable analog circuits, such as gain amplifiers and relaxation oscillators with fast response.

Stray Light in 3D Porous Nanostructures of Single‐Crystalline Copper Film

In the design of optical devices and components, geometric structures and optical properties of materials, such as absorption, refraction, reflection, diffraction, scattering, and trapping, have been utilized. Finding the ideal material with certain optical and geometric characteristics is essential for a customized application. Herein, unoxidizable achromatic copper films (ACFs) are fabricated on Al2O3 substrates utilizing an atomic sputtering epitaxy apparatus. ACFs are made up of two regions vertically: a comparatively flat layer region and a 3D porous nanostructured region on top of the flat region. The measured specular reflectance displays low‐pass filter behavior with a sharp cutoff frequency in the infrared spectrum. Furthermore, the measured diffusive reflectance spectra show light‐trapping behavior in the spectral region above the cutoff frequency, where there are no known absorption mechanisms, such as phonons and interband transitions. A focused ion beam scanning electron microscope is utilized to study the thin film's nanostructured region through 3D tomographic analysis in order to comprehend the phenomena that are observed. This work will shed fresh light on the design and optimization of optical filters and light‐trapping employing porous nanostructured metallic thin films.

Photon-Induced Near-Field Electron Microscopy of Nanostructured Metallic Films and Membranes

Photon-Induced Near-Field Electron Microscopy of Nanostructured Metallic Films and Membranes

High-performance plasmonic mid-infrared bandpass filters by inverse design.

Plasmonic spectral filters composed of periodic nanostructured metal films offer novel opportunities for the development of multispectral imaging technologies in the mid-infrared region. However, traditional plasmonic filters, which typically feature simplistic structures such as nanoholes or nanorings, are constrained by a narrow bandpass and significant crosstalk, leading to limited practical performance. Filters designed using inverse techniques allow a substantial degree of freedom in creating intricate structures that align with desired spectral characteristics, including a quasi-square spectral profile, high transmission, wide full width at half maximum, and reduced crosstalk. In this study, we have utilized an inverse design algorithm to engineer high-performance bandpass filters for the mid-infrared range, achieving an average transmittance exceeding 80% within the bandpass window and below 10% in the stop band, which is comparable to that of commercial multilayer Bragg filters. Nanofabrication processes were employed to transfer the designed pattern into the gold film on ZnS substrate that is transparent in the mid-infrared range. The resulting filters exhibit spectral performance analogous to that of the inversely designed models, making them suitable for direct integration with mid-infrared photodetector arrays in multispectral imaging systems.

Nanostructured Black Silicon as a Stable and Surface-Sensitive Platform for Time-Resolved In Situ Electrochemical Infrared Absorption Spectroscopy.

Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a powerful method for probing interfacial chemical processes. However, SEIRAS-active nanostructured metallic thin films for the in situ analysis of electrochemical phenomena are often unstable under biased aqueous conditions. In this work, we present a surface-enhancing structure based on etched black Si internal reflection elements with Au-coatings for in situ electrochemical ATR-SEIRAS. Using electrochemical potential-dependent adsorption and desorption of 4-methoxypyridine on Au, we demonstrate that black Si-based substrates offer advantages over commonly used structures, such as electroless-deposited Au on Si and electrodeposited Au on ITO-coated Si, due to the combination of high stability, sensitivity, and conductivity. These characteristics are especially valuable for time-resolved measurements where stable substrates are required over extended times. Furthermore, the low sheet resistance of Au layers on black Si reduces the RC time constant of the electrochemical cell, enabling a significantly higher time resolution compared to that of traditional substrates. Thus, we employ black Si-based substrates in conjunction with rapid- and step-scan Fourier transform infrared (FTIR) spectroscopy to investigate the adsorption and desorption kinetics of 4-methoxypyridine during in situ electrochemical potential steps. Adsorption is shown to be diffusion-limited, which allows for the determination of the mean molecular area in a fully established monolayer. Moreover, no significant changes in the peak ratios of vibrational modes with different orientations relative to the molecular axis are observed, suggesting a single adsorption mode and no alteration of the average molecular orientation during the adsorption process. Overall, this study highlights the enhanced performance of black Si-based substrates for both steady-state and time-resolved in situ electrochemical ATR-SEIRAS, providing a powerful platform for kinetic and mechanistic investigations of electrochemical interfaces.

Silver Vapor Supersonic Jets: Expansion Dynamics, Cluster Formation, and Film Deposition.

Supersonic jets of metal vapors with carrier gas are promising for producing nanostructured metal films at relatively low source temperatures and high deposition rates. However, the effects of the carrier gas on the jet composition and expansion dynamics, as well as on film properties, remain virtually unexplored. In this work, the free-jet expansion of a mixture of silver vapor with helium in a rarefied regime at an initial temperature of 1373 K is investigated through mass spectrometry and direct-simulation Monte Carlo methods. Introducing the carrier gas into the source is found to result in a transition from a collisionless to a collision-dominated expansion regime and dramatic changes in the Ag jet, which becomes denser, faster, and more forward-directed. The changes are shown to be favorable for the formation of small Ag clusters and film deposition. At a fairly high helium flow, silver Ag2 dimers are observed in the jet, both in the experiment and the simulations, with a mole fraction reaching 0.1%. The terminal velocities of silver atoms and dimers are nearly identical, indicating that the clusters are likely formed due to the condensation of silver vapor in the expanding jet. A high potential of supersonic Ag-He jets for the deposition of nanostructured silver films is demonstrated. The deposited jet Ag2 dimers appear to serve as nucleation centers and, thus, allow for controlling the size of the produced surface nanostructures.

Spin Selectivity-Based Enantiomeric Discrimination by Chiral Nanostructured Au Films

Photomagnetic-chiral anisotropy (PM-ChAC–E) and surface-enhanced Raman scattering-chiral anisotropy (SERS-ChAC–E) are novel enantiomeric discrimination techniques generated by chiroptical response due to the interactions between chiral nanostructured metal films (C) and enantiomers (E). The normal Gauss meter and the Raman scattering spectrometer can be used in these versatile techniques. Here, we report the discrimination mechanism of chiral nanostructured Au films (CNAFs) based on the spin polarization coupling (SPC) between the enantiomers and CNAFs. The CNAFs represent Boerdijk-Coxeter-Bernal helical Au nanofibers with different lengths and widths perpendicularly grown on substrates. The optical activities, magnetic circular dichroism, and magnetic tip conducting atomic force microscopy indicated that the spin selectivity of CNAFs depends on the length of CNAFs. The spin selectivity-dependent selective enhancement of photomagnetic field and Raman scattering suggested that the chirality-induced SPC between the enantiomers and CNAFs gives rise to the phenomena of PM-ChAC–E and consequently SERS-ChAC–E. These findings provide possible mechanism based on spin selectivity in chiral structures and make it a universal theory for enantiomeric discrimination.

Antibacterial Structures Based on Electrically Charged Nanostructured Metal Films on Semiconductors

Antibacterial Structures Based on Electrically Charged Nanostructured Metal Films on Semiconductors

Electroactive nanostructured antibacterial materials

Thin nanostructured metal (Au, Ag) films, magnetron-sputtered on semiconductor (n-type Si) substrate under 6 V voltage exposure for 15 min, exhibit high antibacterial effect against the food pathogens S. aureus and P. aeruginosa. Nanostructures were formed by femtosecond laser ablation, resulting in an array of microspots. The observed effect is caused by the emergence of submicron, laterally periodical static electric and magnetic fields, adjacent to the metal film, causing the abrupt voltage drops, which induce the hyperpolarization of the cell membrane and increase its permeability, resulting in the formation of pores (electroporation) in the membrane and the subsequent apoptosis of the bacterial cell. Additional factors, which enhance the antibacterial effect of the studied materials, are the volume convection in the liquid drop with bacterial culture, caused by the moderate heating of the substrate to 45 °C–50 °C during the electric current flow and electro-taxis of bacteria to the charged nanostructured metal film.

Influence of the nanostructure on the electric transport properties of resistive switching cluster-assembled gold films

The use of Au clusters produced in the gas phase and deposited at low kinetic energy on a substrate allows the bottom-up fabrication of nanostructured metallic thin films with an extremely large number of interfaces, grain boundary junctions and crystal lattice defects. Cluster-assembled gold films exhibit non-ohmic electrical transport properties with a complex resistive switching behavior, exploring discrete resistance states which depend on their structural features (average thickness, resistance value reached on the percolation curve). Their electric conduction properties can be modelled in terms of complex networks of nanojunctions and used to perform binary classification of Boolean functions. The fabrication of devices based on cluster-assembled Au films and exploiting emergent complexity and collective phenomena requires a deep understanding of the influence of the nanoscale structure on the fundamental mechanisms of electrical conduction. Here we present a detailed study of the correlation between the nanostructure and the electrical properties of cluster-assembled gold films by a systematic characterization of the film growth from sub-monolayer to continuous layer beyond the electrical percolation threshold. The influence of different cluster size distributions on the onset of the electrical conduction and the role of defects is investigated by combining in situ and ex situ electrical and structural characterizations.

Novel class of nanostructured metallic glass films with superior and tunable mechanical properties

A novel class of nanostructured Zr50Cu50 (%at.) metallic glass films with superior and tunable mechanical properties is produced by pulsed laser deposition. The process can be controlled to synthetize a wide range of film microstructures including dense fully amorphous, amorphous embedded with nanocrystals and amorphous nano-granular. A unique dense self-assembled nano-laminated atomic arrangement characterized by alternating Cu-rich and Zr/O-rich nanolayers with different local chemical enrichment and amorphous or amorphous-crystalline composite nanostructure has been discovered, while significant in-plane clustering is reported for films synthetized at high deposition pressures. This unique nanoarchitecture is at the basis of superior mechanical properties including large hardness and elastic modulus up to 10 and 140 GPa, respectively and outstanding total elongation to failure (>9%), leading to excellent strength/ductility balance, which can be tuned by playing with the film architecture. These results pave the way to the synthesis of novel class of engineered nanostructured metallic glass films with high structural performances attractive for a number of applications in microelectronics and coating industry.

Role of pH on Nanostructured SERS Active Substrates for Detection of Organic Dyes.

Surface Enhanced Raman Spectroscopy is commonly used as analytical improvement to conventional Raman spectroscopy, able to respond to qualitative diagnostic enquiries, which involve low-concentrated molecular species in complex matrix. In this paper, we described fabrication, characterization and testing of a type of SERS-active substrates realized specifically to detect pigments in work of art. In particular, we detailed the SERS activity of nanostructured noble metal films deposited by pulsed laser ablation onto glass and polishing sheets substrates. The SERS response of the substrates was tested against the presence of some organic dyes in aqueous solutions. Measurements were performed at different pH values, in acidic or basic range, in order to investigate its role in the adsorption mechanism, thus fostering the SERS amplification. In addition, we checked the possible deterioration of the structural properties of the substrates that could occur in presence of alkaline or acidic environment. SERS activity of the substrates was tested against a commonly dye used as a SERS standard (Blue Methylene). Thereafter, substrates have been tested on two organic dyes (Alizarine red-S and Brazilwood), which had proven to be Raman active but present also either a weak Raman scattering cross section and/or a high fluorescence emission. The substrates have proven effective in amplifying Raman scattering of all dyes, quenching troubling fluorescence effects. Furthermore, they have proven to be stable in the pH range between 3 and 11. Furthermore, we carry out of vibrational DFT-calculation of dyes that provide a complete description of the observed SERS spectra.

The Synergy of Topographical Micropatterning and Ta|TaCu Bilayered Thin Film on Titanium Implants Enables Dual-Functions of Enhanced Osteogenesis and Anti-Infection.

Poor osteogenesis and implant-associated infection are the two leading causes of failure for dental and orthopedic implants. Surface design with enhanced osteogenesis often fails in antibacterial activity, or vice versa. Herein, a surface design strategy, which overcomes this trade-off via the synergistic effects of topographical micropatterning and a bilayered nanostructured metallic thin film is presented. A specific microgrooved pattern is fabricated on the titanium surface, followed by sequential deposition of a nanostructured copper (Cu)-containing tantalum (Ta) (TaCu) layer and a pure Ta cap layer. The microgrooved patterns coupled with the nanorough Ta cap layer shows strong contact guidance to preosteoblasts and significantly enhances the osteogenic differentiation in vitro, while the controlled local sustained release of Cu ions is responsible for high antibacterial activity. Importantly, rat calvarial defect models in vivo further confirm that the synergy of microgrooved patterns and the Ta|TaCu bilayered thin film on titanium surface could effectively promote bone regeneration. The present effective and versatile surface design strategy provides significant insight into intelligent surface engineering that can control biological response at the site of healing in dental and orthopedic implants.

Fermi resonance in a molecule adsorbed on plasmonic metal film

AbstractIn this work, the effect of electric field induced by localized plasmon excitation in nanostructured metal film on the vibrational spectrum of adsorbed organic molecule is studied theoretically. By using the Green function formalism, we show that electric field can significantly influence both the frequencies of the molecular vibrations and the anharmonicity constant. Consequently, if Fermi resonance can take place in the adsorbed molecule, it is spectral manifestation (in Raman spectra) will also be modified by the electric field of plasmon. We illustrate the effect with a CO2 molecule adsorbed on nanostructured gold surface, a situation relevant to the application of Surface‐Enhanced Raman Scattering (SERS) or other plasmon‐mediated phenomena. The obtained in these work analytical expressions are shown to describe well spectral features in earlier experimental SERS studies on CO2.

Improved two-temperature modeling of ultrafast thermal and optical phenomena in continuous and nanostructured metal films

In this work, a pump-probe experiment is used to study the ultrafast dynamics of heat transfer in thin gold films and gold nanostructures on glass substrates, following local heating by ultrashort laser pulses. Full spectrotemporal differential reflectivity and transmission maps were obtained for different film thicknesses (30, 50, 80, 150, and 200 nm) and different laser fluences $(0.38 \mathrm{to} 9.5\phantom{\rule{0.16em}{0ex}}{\mathrm{Jm}}^{\ensuremath{-}2})$. For arrays of gold nanorods, the two orthogonal probe polarizations were also acquired. We propose an improved model for these phenomena based on a modified two-temperature model that integrates thermal conduction and the three-dimensional finite element method model to link the spatiotemporal temperature maps to the spectrotemporal optical response maps. The impact of an underlying titanium adhesion layer is reported. Excellent agreement between numerical and experimental data for both the gold films and the nanostructures is shown.

Synthesis of Uniformly Sized Mesoporous Silver Films and Their SERS Application

Nanostructured metal films and particles have been recently exploited as substrates for surface-enhanced Raman spectroscopy (SERS) studies because their intrinsic properties lead to substantial Ram...

Development of reflective co-sputtered nanostructured metallic films

ABSTRACT DC magnetron co-sputtering technique is utilised to develop reflective Ag- and Al-based metal films co-sputtered with Ni deposited on quartz glass substrates. The sputtered films are characterised by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques to investigate microstructure, topology and phase analysis, respectively. Further, average thermo-optical properties such as solar reflectance, absorptance and IR emittance and electrical property such as sheet resistance of the deposited films are evaluated. The reflectance property as a function of wavelength is also investigated. Sputtered Ag and Al films show high (>93%) reflectance, however, co-sputtered Al + Ni and Ag + Ni offer comparatively lesser value, e.g. 45% and 73%, respectively. The significant lower reflectance of Al + Ni is possibly due to the presence of higher amount of Ni in the film and the formation of intermetallic compounds.

Tailoring nanostructured Ni-Nb metallic glassy thin films by substrate temperature

Improving the thermostability and mechanical properties of metastable amorphous materials are critical for their potential applications. Here we report the achievement of a wide variety of nanostructures and properties in Ni-Nb metallic glass thin films (MGTFs) by tuning the substrate temperature (Tsub) during magnetron sputtering. By gradually increasing Tsub close to glass transition temperature (Tg) from room temperature, we evidence the transition from the hierarchical nanostructure with microcracks to the denser nanostructure with a smoother surface and smaller column/particle size. This transition contributes to the enhanced plastic deformation stability and nanomechanical properties, thermal stability, and optical reflectivity in the visible light wavelength. We explain the nanostructure evolution with Tsub based on the competition between the shadowing effect and surface diffusion. The shadowing effect is dominant in low-Tsub films and promotes the voided boundaries, while surface diffusion is remarkably enhanced at high Tsub (0.76–0.87 Tg) to form dense intergrain boundaries. This finding provides a promising avenue towards producing novel functional MGTFs with controllable nanostructures and properties without any post-treatment.

Plasmonic-Active Nanostructured Thin Films

Plasmonic-active nanomaterials are of high interest to scientists because of their expanding applications in the field for medicine and energy. Chemical and biological sensors based on plasmonic nanomaterials are well-established and commercially available, but the role of plasmonic nanomaterials on photothermal therapeutics, solar cells, super-resolution imaging, organic synthesis, etc. is still emerging. The effectiveness of the plasmonic materials on these technologies depends on their stability and sensitivity. Preparing plasmonics-active nanostructured thin films (PANTFs) on a solid substrate improves their physical stability. More importantly, the surface plasmons of thin film and that of nanostructures can couple in PANTFs enhancing the sensitivity. A PANTF can be used as a transducer for any of the three plasmonic-based sensing techniques, namely, the propagating surface plasmon, localized surface plasmon resonance, and surface-enhanced Raman spectroscopy-based sensing techniques. Additionally, continuous nanostructured metal films have an advantage for implementing electrical controls such as simultaneous sensing using both plasmonic and electrochemical techniques. Although research and development on PANTFs have been rapidly advancing, very few reviews on synthetic methods have been published. In this review, we provide some fundamental and practical aspects of plasmonics along with the recent advances in PANTFs synthesis, focusing on the advantages and shortcomings of the fabrication techniques. We also provide an overview of different types of PANTFs and their sensitivity for biosensing.

Plasmonic Pixel Biosensor Based on Grazing Angle Illumination and Computational Imaging

Surface plasmon resonance (SPR) has been adapted for analyzing biomolecular interactions in real time as a label-free technology. However, SPR instruments, which are usually bulky and expensive, require specialized optical components for exciting surface plasmons and detecting spectral signals, and the technique has yet to evolve into a more affordable and accessible analytic tool. In this paper, we demonstrate a plasmonic sensing device integrated with plasmonic pixels for exciting surface plasmons and displaying spectral information. The plasmonic pixels based on continuous nanostructured metallic thin films establish a direct relationship between nanostructure patterns and plasmon resonances in the visible and near-infrared spectrum range. Steep dispersion of Fano resonances induced by incident light from a grazing angle transduces plasmonic spectral responses into a unique two-dimensional image. Spectral information extracted from the plasmonic pixels exhibits an asymmetric Fano resonance behavior with sharp intensity peaks and the full width half maxima (FWHM) down to 10 nm. The relative brightness of the plasmonic pixels allows naked human eyes to detect the formation of a biomolecule monolayer. Most importantly, subnanometer spectral information can be extracted from the plasmonic pixel images and computed to analyze dynamics of chemical and biomolecular binding events in real time. We envision that these nanostructure plasmonic pixel biosensors can provide a compact and low-cost analytic platform to meet the emerging demands, particularly in portable, mobile, and wearable devices for label-free detection and point-of-care diagnostics.