Noble Metal Films Research Articles

Fabrication of 250-nm-hole arrays in glass and fused silica by UV laser ablation

Parallel nanohole drilling in glass using an ArF excimer laser (193nm) is demonstrated. For the first time, hole arrays with 500nm pitch and individual holes with 250nm diameter and more than 100nm depth are fabricated by phase mask imaging using a Schwarzschild objective. Holes in soda lime glass are drilled by direct ablation; fused silica is processed by depositing a SiOx-film on SiO2, patterning the SiOx by ablation, and finally oxidizing the remaining SiOx to SiO2. Thermally induced ordered dewetting of noble metal films deposited on such templates may be used for the fabrication of plasmonic devices.

Enhanced Bifunctional Oxygen Catalysis in Strained LaNiO3 Perovskites.

Strain is known to greatly influence low-temperature oxygen electrocatalysis on noble metal films, leading to significant enhancements in bifunctional activity essential for fuel cells and metal-air batteries. However, its catalytic impact on transition-metal oxide thin films, such as perovskites, is not widely understood. Here, we epitaxially strain the conducting perovskite LaNiO3 to systematically determine its influence on both the oxygen reduction and oxygen evolution reaction. Uniquely, we found that compressive strain could significantly enhance both reactions, yielding a bifunctional catalyst that surpasses the performance of noble metals such as Pt. We attribute the improved bifunctionality to strain-induced splitting of the eg orbitals, which can customize orbital asymmetry at the surface. Analogous to strain-induced shifts in the d-band center of noble metals relative to the Fermi level, such splitting can dramatically affect catalytic activity in this perovskite and other potentially more active oxides.

Ion-beam assisted laser fabrication of sensing plasmonic nanostructures.

Simple high-performance, two-stage hybrid technique was developed for fabrication of different plasmonic nanostructures, including nanorods, nanorings, as well as more complex structures on glass substrates. In this technique, a thin noble-metal film on a dielectric substrate is irradiated by a single tightly focused nanosecond laser pulse and then the modified region is slowly polished by an accelerated argon ion (Ar+) beam. As a result, each nanosecond laser pulse locally modifies the initial metal film through initiation of fast melting and subsequent hydrodynamic processes, while the following Ar+-ion polishing removes the rest of the film, revealing the hidden topography features and fabricating separate plasmonic structures on the glass substrate. We demonstrate that the shape and lateral size of the resulting functional plasmonic nanostructures depend on the laser pulse energy and metal film thickness, while subsequent Ar+-ion polishing enables to vary height of the resulting nanostructures. Plasmonic properties of the fabricated nanostructures were characterized by dark-field micro-spectroscopy, Raman and photoluminescence measurements performed on single nanofeatures, as well as by supporting numerical calculations of the related electromagnetic near-fields and Purcell factors. The developed simple two-stage technique represents a new step towards direct large-scale laser-induced fabrication of highly ordered arrays of complex plasmonic nanostructures.

Plasmonic thin films for application in improved chromogenic windows

Nanocomposites consisting of noble metal nanoparticles in a transparent matrix exhibit plasmonic absorption in the visible wavelength range, and conducting oxide nanoparticles display a localized plasma absorption in the near infrared. The optical properties of nanocomposites are commonly modelled by effective medium theories, which describe the effective dielectric function of the composite using as input the dielectric functions of the constituents and their respective volume fractions. Plasmonic effects can be exploited in the design of energy-efficient windows in order to obtain improved performance. Electrochromic coatings that switch in the near infrared make use of the modulation of the plasma absorption of oxide nanoparticles due to charge density modulation induced by an external voltage. Plasmonic thermochromic switching in the near infrared has the potential to be significantly larger than in the case of a thin film. Very thin noble metal films are an interesting alternative to conducting oxides as transparent contacts to electrochromic devices. However, in this latter case plasmonic effects are to be avoided rather than exploited.

Implantation of plasmonic nanoparticles in SiO2 by pulsed laser irradiation of gold films on SiOx-coated fused silica and subsequent thermal annealing

The pulsed UV-laser irradiation of thin noble metal films deposited on glass substrates leads to the incorporation of metal particles in the glass, if a sufficiently high laser fluence is applied. This process is called laser implantation. For the implantation of gold into pure fused silica, high laser fluences (∼1J/cm2 at 193nm laser wavelength) are required. Using a SiOx (x≈1) coated SiO2-substrate, the implantation of gold into this coating can be accomplished at significantly lower fluences starting from 0.2J/cm2 (comparable to those used for standard glass). Particles with diameters in the range of 10–60nm are implanted to a depth of about 40nm as identified by transmission electron microscopy. An additional high temperature annealing step in air leads to the oxidation of SiOx to SiO2, without influencing the depth distribution of particles significantly. Only superficial, weakly bound particles are released and can be wiped away. Absorption spectra show a characteristic plasmon resonance peak at 540nm. Thus, pure silica glass (SiO2) with near surface incorporated plasmonic particles can be fabricated with this method. Such material systems may be useful for example as robust substrates for surface enhanced Raman spectroscopy (SERS).

Reliable fabrication of plasmonic nanostructures without an adhesion layer using dry lift-off

Lift-off is the most commonly used pattern-transfer method to define lithographic plasmonic metal nanostructures. A typical lift-off process is realized by dissolving patterned resists in solutions, which has the limits of low yield when not using adhesion layers and incompatibility with the fabrication of some specific structures and devices. In this work, we report an alternative ‘dry’ lift-off process to obtain metallic nanostructures via mechanical stripping by using the advantage of poor adhesion between resists and noble metal films. We show that this dry stripping lift-off method is effective for both positive- and negative-tone resists to fabricate sparse and densely-packed plasmonic nanostructures, respectively. In particular, this method is achieved without using an adhesion layer, which enables the mitigation of plasmon damping to obtain larger field enhancement. Dark-field scattering, one-photon luminescence and surface-enhanced Raman scattering measurements were performed to demonstrate the improved quality factor of the plasmonic nanostructures fabricated by this dry lift-off process.

Hot-Electron Intraband Luminescence from Single Hot Spots in Noble-Metal Nanoparticle Films.

Disordered noble-metal nanoparticle films exhibit highly localized and stable nonlinear light emission from subdiffraction regions upon illumination by near-infrared femtosecond pulses. Such hot spot emission spans a continuum in the visible and near-infrared spectral range. Strong plasmonic enhancement of light-matter interaction and the resulting complexity of experimental observations have prevented the development of a universal understanding of the origin of light emission. Here, we study the dependence of emission spectra on excitation irradiance and provide the most direct evidence yet that the continuum emission observed from both silver and gold nanoparticle aggregate surfaces is caused by recombination of hot electrons within the conduction band. The electron gas in the emitting particles, which is effectively decoupled from the lattice temperature for the duration of emission, reaches temperatures of several thousand Kelvin and acts as a subdiffraction incandescent light source on subpicosecond time scales.

Spectroscopic Ellipsometry Characterization of Pd Thin Film Grown by Atomic Layer Deposition

Ultrathin noble metal films have attracted lots of attentions in fuel cell catalysts, hydrogen storage, emission control, etc. Among many synthesis methods, atomic layer deposition (ALD) is a powerful one to fabricate ultrathin films with great precision [1]. The measurement of such noble metal films is quite important to both understand the initial growth behavior and guide the film fabrication process. Spectroscopic ellipsometry (SE) is a fast and non-destructive thin film characterization technique with atomic sensitivity [2]. The initial growth of noble metals, however, is in island growth mode and bulk metal model could not correlate with such structures. Similar structures have been reported by the Maxwell-Garnett Effective medium approximation (MG-EMA) model to study the optical properties of Au, Ag thin films [3-4]. In this paper, we choose the MG-EMA model with General oscillator (GO) to study the size and volume fraction. In this study, MG-EMA model has been chosen to study Pd ultrathin film grown on Si substrate, as well as its growth on self-assembled monolayers (SAMs) modified substrate respectively. The GO model with one Drude and two Lorentz oscillators is firstly applied to fix the optical constants of Pd. Compared with Pd bulk model, MG-EMA model with GO is more reliable to predict the film thickness verified by X-ray reflection test. The stable growth rate on Si substrate reveals our methods are feasible and is in accordance with the quartz crystal microbalance measurement. For Pd coverage, MG-EMA fitting result is similar with the statistical computation from scanning electron microscope when cycles of Pd ALD cycles are over 400, while large bias exists for cycles under 400, might be due to that air is not the proper filling medium between nanoparticles. Then we change the filling medium into SAMs as a comparison, better fitting performance is obtained. It is demonstrated that the filling medium between nanoparticles is important for the application of MG-EMA model.

Enhanced Optical Chirality through Locally Excited Surface Plasmon Polaritons

The possibility to enhance chiral light-matter interactions through plasmonic nanostructures provides entirely new opportunities for greatly improving the detection limits of chiroptical spectroscopies down to the single molecule level. The most pronounced of these chiral interactions occur in the ultraviolet (UV) range of the electromagnetic spectrum, which is difficult to access with conventional localized plasmon resonance based sensors. Although Surface Plasmon Polaritons (SPPs) on noble metal films can sustain resonances in the desired spectral range, their transverse magnetic nature has been an obstacle for enhancing chiroptical effects. Here we demonstrate, both analytically and numerically, that SPPs excited by near-field sources can exhibit rich and non-trivial chiral characteristics. In particular, we show that the excitation of SPPs by a chiral source not only results in a locally enhanced optical chirality but also achieves manifold enhancement of net optical chirality. Our finding that SPPs facilitate a plasmonic enhancement of optical chirality in the UV part of the spectrum is of great interest in chiral bio-sensing.

Growth and photoemission spectroscopic studies of ultrathin noble metal films on graphite

Growth of Cu, Ag and Au thin films on graphite(0 0 0 1) surface and possible formation of quantum well (QW) states originating due to the confinement of thin film sp electrons within the band gap of graphite along ΓM symmetry direction are investigated using low-energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES). Higher surface diffusivity and surface energy of Cu on graphite surface led to cluster growth and does not reveal any quantum size effect, while Ag and Au films grow epitaxially in spite of large lattice mismatch. However, better surface ordering has been achieved by growing Ag and Au at low temperature (LT), followed by room-temperature (RT) annealing which are evident from LEED and the presence of sharp Shockley-type surface state (SS) at Fermi level (EF). ARPES study of Ag films on graphite does not show any QW states, whereas Au films demonstrate a very sharp SS, Au bulk bands and well-resolved QW states or resonances. The observed low in-plane dispersions of these Au QW states or resonances are compared with the dispersions obtained in the previous Au QW state studies as well as for free-standing Au films.

Theoretical study of visible light refraction phenomena occurring at noble metal–air interfaces

Based on the generalized reflection and refraction theory, the visible light wavelength-dependent refraction behaviors occurring at both Ag–air and Au–air interfaces are qualitatively attributed to the non-synchronized variations of electric and magnetic parameters of the light. Numerical results show that distribution of time-averaged Poynting vector of the transmitted light traveling through a wedge shaped noble metal film may be affected by width and divergence of the incident light beam, energy losses, and so on. Careful consideration of effects of multi-factors on the refraction phenomena are necessary to establish reasonable relationship among measured results by using different experimental methods. Our work may provide a new possible angle of view to understand the refraction phenomena occurring at noble metal–air interface.

(Invited) Recent Progress in the Study of Single Molecule Electrochemistry Using Surface-Enhanced and Tip-Enhanced Raman Spectroscopy

During the last few years, there has been an explosion of interest and activity in the field of plasmonics. The goal of plasmonics is to control and manipulate light on the nanometer length scale using the properties of the collective electronic excitations in noble metal films or nanoparticles, known as surface plasmons. An improved understanding of the interactions between adsorbed molecules and plasmonic nanostructures (i.e., molecular plasmonics) is having a significant impact in a number of research areas including electrochemistry, surface science, catalysis for energy conversion and storage, the materials science of nanoparticles, biomedical diagnostics, art conservation science, and nanolithography. In the first part of this lecture, I will provide some background material on the basic physical concepts underlying molecular plasmonics with an emphasis on surface-enhanced Raman spectroscopy (SERS), localized surface plasmon resonance (LSPR) spectroscopy, and tip-enhanced Raman spectroscopy (TERS). In the second part of this lecture, I will focus in on our most recent work in studying single molecule electrochemistry using surface-enhanced and tip-enhanced Raman spectroscopy. I will first describe the development of an instrument that has the capability of combining tip-enhanced Raman spectroscopy at the solid/liquid interface with scanning electrochemical microscopy (SECM). Next the single molecule Raman spectroelectrochemistry of Rhodamine 6G (R6G) at acetonitrile/Ag interface will be discussed. Finally, I will discuss the path forward toward the goal of understanding heterogeneous electron transfer at the single molecule level.

Surface roughness-correlated SERS effect on Au island-deposited substrate

Evaporation and sputtering-deposition of noble metal films on substrates are two most popular and easiest methods for creating stable surface-enhanced Raman scattering (SERS)-active metal nanoparticles (NPs). However, intrinsically low SERS effect and unclearly inherent relationship between SERS effect and surface roughness of created Au NPs make SERS-related applications are limited. Here, we present the enhanced SERS effect observed on Au nanoislands (AuNIs)-deposited substrates prepared by thermal evaporation and followed by post-annealing treatment (PAT). Experimental results indicate that the SERS intensity of Rhodamine 6G (R6G) adsorbed on AuNIs, corresponding to average particle diameter of ca. 220nm and to mean roughness (Ra) of 50nm, can be increased by one order of magnitude after PAT at 700°C. Meanwhile, the AuNIs (annealed at 700°C)-deposited silicon substrate demonstrates a large enhancement factor (EF) of 2.2×107. The relative standard deviation is ca. 7% in SERS measurements. In addition, detailed relationships of Ra derived from analysis of atomic force microscopy (AFM) and SERS effect are correlated for metal particles smaller and larger than 300nm.

Colossal spin Hall effect in ultrathin metallic films

We predict spin Hall angles up to 80% for ultrathin noble metal films with substitutional Bi impurities. The colossal spin Hall effect is caused by enhancement of the spin Hall conductivity in reduced sample dimension and a strong reduction of the charge conductivity by resonant impurity scattering. These findings can be exploited to create materials with high efficiency of charge to spin current conversion by strain engineering.

Bare and protected sputtered-noble-metal films for surface-enhanced Raman spectroscopy

Sputtered silver and gold films with different surface morphologies have been prepared and coated with a benzenethiol self-assembled monolayer. Rough noble metal films showed strong Raman features assigned to adsorbed benzenethiol molecules upon irradiation over a wide energy range in the visible spectrum, which disclosed the occurrence of a significant surface-enhanced Raman scattering with maximal enhancement factors as high as 6×106. In addition, the adsorption of ethanethiol onto silver surfaces hinders their corrosion over days while preserving mostly intact enhancement properties of naked silver. This study may be applied to develop stable and efficient metalized probes for tip-enhanced Raman spectroscopy.

Phenomenological understanding of dewetting and embedding of noble metal nanoparticles in thin films induced by ion irradiation

The present experimental work provides the phenomenological approach to understand the dewetting in thin noble metal films with subsequent formation of nanoparticles (NPs) and embedding of NPs induced by ion irradiation. Au/polyethyleneterepthlate (PET) bilayers were irradiated with 150 keV Ar ions at varying fluences and were studied using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (X-TEM). Thin Au film begins to dewet from the substrate after irradiation and subsequent irradiation results in spherical nanoparticles on the surface that at a fluence of 5 × 1016 ions/cm2 become embedded into the substrate. In addition to dewetting in thin films, synthesis and embedding of metal NPs by ion irradiation, the present article explores fundamental thermodynamic principles that govern these events systematically under the effect of irradiation. The results are explained on the basis of ion induced sputtering, thermal spike inducing local melting and of thermodynamic driving forces by minimization of the system free energy where contributions of surface and interfacial energies are considered with subsequent ion induced viscous flow in substrate.

Porous gold cluster film prepared from Au@BSA microspheres for electrochemical nonenzymatic glucose sensor

To obtain hierarchically porous gold-cluster film as the direct electrochemical sensing interface for nonenzymatic glucose detection, a facile, cost-effective, environmentally friendly and bottom-up approach was developed by calcination of Au@BSA microspheres. The fabricated gold-cluster film was composed of the network structure interconnected with Au particles and the disordered 3D hierarchical pores. Due to large surface area, high electroconductivity and excellent electrocatalysis towards glucose oxidation, the as-prepared sensor showed high current response so that the glucose determination could be carried out at a less positive potential where common interferents (ascorbic acid, acetamidophenol and uric acid) were not oxidized. Importantly, 0.15M chloride ions (Cl−) had no poisoning effect on the gold-cluster electrode. Meanwhile, the sensor also exhibited a rapid response (5 s), high sensitivity (10.76μAmM−1cm−2), a low limit of detection (1μM), a wide and useful linear range (0.01−10mM), good operational stability and storage stability. This work not only demonstrated an excellent direct electrochemical sensing interface for nonenzymatic detection of glucose, but also would extend to other noble metal films and metal oxide films as various electrodes and electrochemical interface.

Transparent Metal Films for Detection of Single-Molecule Optical Absorption by Scanning Tunneling Microscopy

Atomically flat, conductive, and transparent noble metal films are produced to extend the wavelength range of room-temperature single-molecule optical absorption detected by scanning tunneling microscopy (SMA-STM). Gold films grown on a platinum underlayer to 15 nm total thickness, deposited by electron beam evaporation onto c-plane sapphire substrates, show sufficient light transmission for backside illumination for laser-assisted STM experiments. Low resistance, transparency, and the atomically flat island surfaces make these good substrates for SMA-STM studies. Monte Carlo lattice kinetics were simulated to allow for a better understanding of the growth modes of the Pt–Au films and of the achieved morphologies. SMA-STM is detected for a quantum dot deposited by aerosol spraying onto Pt–Au films, demonstrating the suitability of such films for single-molecule absorption spectroscopy studies.

Electroless Deposition By Surface Limited Redox Replacement in One Cell Configuration

The deposition of thin noble metal films is an area of wide interest for many different industrial applications. These films allow for an application to take advantage of the beneficial properties of said metal of interest while minimizing cost. Thusly, thin films are currently being applied as low-cost, high-activity fuel cell catalysts, photovoltaic semiconductors, and protective coatings in circuitization of electronic devices.Surface limited redox replacement (SLRR) is a technique that has been applied to depositing ultra thin films of metals and alloys and has also shown the ability to grow epitaxial metal layers utilizing the monolayer limited nature of underpotential deposition (UPD). In SLRR an UPD monolayer of a sacrificial element is formed via a potential pulse. This layer is sequentially replaced by a more noble metal at free corrosion conditions. This sequence of events ultimately yields the deposition of one layer of the more noble metal 1. This technique can be then applied to subsequent events resulting in perfectly controlled layer by layer metal growth 2-6.During SLRR the potential pulse that is applied to form the UPD layer has conventionally been performed by an external power device (potentiostat). In this work instead of the application of external power, a corrosion pair scenario is realized in a three-electrode configuration with a Pb wire in solution serving as the “executive” electrode, a Hg/Hg2SO4 reference electrode and a Au polycrystalline disc used as working electrode. The difference in equilibrium potentials of the executive and working electrode coupled with specific aspects of the kinetics of the Pb-Au corrosion pair allow for efficient control of the potential applied to the Au substrate. More specifically, electrical contact between executive and working electrodes results in the application of a potential to the Au electrode, nearly identical to the Pb equilibrium potential. This ultimately leads to the formation of a sacrificial Pb UPD layer on the Au surface. Subsequent disconnection of the electrical contact allows for the electroless, in nature, SLRR to occur. This technique can consequentially be used to coat a substrate metal surface with another metal or alloy of interest as illustrated in Figure 1. In the present report, the proposed approach has been applied to an ideal system in the plating of Ag on Au in order to ascertain the protocol’s applicability for metal deposition. The work has also been extended to the industrially more relevant system of Pt on Au. Adopting the herein presented routine, SLRR can be simplified and prove even more cost effective with no need for external power source and little to no energy cost, ultimately improving industrial viability.

A Study of Back Electrode Stacked With Low Cost Reflective Layers For High-Efficiency Thin-Film Silicon Solar Cell

In this study, commercially available white paint is used as a pigmented dielectric reflector (PDR) in the fabrication of a low-cost back electrode stack with an Al-doped ZnO (AZO) layer for thin-film silicon solar cell applications. An initial AZO film was deposited by the radio-frequency magnetron sputtering method. In order to obtain the highest transmittance and lowest resistivity of AZO film, process parameters such as sputtering power and substrate temperature were investigated. The optimal 100-nm-thick AZO film with low resistivity and high transmittance in the visible region are 6.4 × 10−3 Ω·cm and above 80%, respectively. Using glue-like white paint doped withTiO2 nanoparticles as the PDR enhances the external quantum efficiency (EQE) of a microcrystalline silicon absorptive layer owing to the doped white particles improving Fabry–Pérot interference (FPI), which raises reflectance and scattering ability. To realize the cost down requirement, decreasing the noble metal film thickness such as a 30-nm-thick silver reflector film, and a small doping particle diameter (D50 = 135 nm) and a high solid content (20%) lead to FPI improvement and a great EQE, which is attributed to improved scattering and reflectivity because of optimum diameter (Dopt) and thicker PDR film. The results indicate that white paint can be used as a reflector coating in low-cost back-electrode structures in high-performance electronics.