Nanostructured Metal Films Research Articles

Polarization-dependent transmission through subwavelength anisotropic aperture arrays

The use of polarized light as an approach to further control the extraordinary transmission (EOT) through nanostructured metallic films has recently gained attention. In this work, it is shown that aperture shape and orientation not only determine the intensity of the polarized light emitted, corroborating the previous work of others, but also can be used to spectrally tune the relative peak intensity of surface plasmon polaritons modes. The high extinction ratio of high aspect ratio apertures lends itself to the creation of micron sized structures that emit at different wavelengths depending upon the orientation of linearly polarized incident light. This has many potential applications including the prospect of color shifting pixels for high definition television (HDTV) and thin film electroluminescent (TFEL) devices as well as novel polarization mode dispersion control components.

Fabrication of Nanostructured Thin Films Using Porous Alumina Templates

ABSTRACTWe demonstrate a method for fabricating a variety of nanostructured metallic thin films by using one simple approach based on the use of anodic aluminum oxide (AAO) membrane as masking template. Highly-ordered large-area metallic nanopore- or nanotip-arrays (e.g., Ni, Ag, Au), with various sizes and shapes, were deposited onto AAO membranes through e-beam evaporation. Free standing nano-patterned metallic films were obtained after chemically removing the template and characterized using scanning electron microscopy and electrochemical methods. The nanostructured films will find use in a number of applications, such as plasmonics, electrochemical and biomedical analysis, and catalysis.

Propagation of Femtosecond Surface Plasmon Polariton Pulses on the Surface of a Nanostructured Metallic Film: Space-Time Complex Amplitude Characterization

Ultrashort surface plasmon polariton (SPP) pulses, propagating on the surface of a nanostructured metallic film, are characterized in space and time using time-resolved spatial-heterodyne imaging. Optical pulses are coupled from free space into various surface modes using a 2D array of circular nanoholes, and spatial amplitude and phase characteristics of the scattered surface field are measured with femtosecond-scale time resolution. Demonstrated in-plane focusing of SPP pulse provides additional electromagnetic field localization with possible applications in SPP nanophotonics, nonlinear surface dynamics, biochemical sensing, and ultrafast surface studies.

Nanostructured metal films: Fabrication and catalytic properties

Thin nanostructured films consisting of monodispersive and amorphous Cu, Ni, or Pd nanoparticles were prepared by means of laser electrodispersion technique. These films were deposited on dielectric (thermally oxidized silicon) or semiconductor (naturally oxidized Si) supports, and the resulting particle surface density was closely controlled by the time of deposition. The nanoparticle size was 5 nm for Cu, 2.5 nm for Ni, and 2 nm for Pd. In all cases, the relative particle size dispersion was less than 10%. The most important common feature of the fabricated catalysts is their unusually high activity, which has been observed in several test reactions including chlorohydrocarbon conversions (Cu and Ni) and hydrogenation (Ni and Pd). The observed strong dependencies of the catalytic activity on the particle surface density and on the nature of the support clearly indicate that alongside with small particle size and their amorphous state, inter-particle interactions or particle interaction with the support play decisive role in determining the catalytic activity.

Suppression of radiative losses of surface polaritons on nanostructured thin metal films

The strong electromagnetic coupling between surface plasmon polariton modes on opposite interfaces of a finite thickness periodically nanostructured metal film has been studied. Surface polariton dispersion and associated electromagnetic field distributions have been analyzed. It was shown that at a frequency that corresponds to the crossing of film Bloch modes of different symmetries, the radiative losses of surface polaritons that are related to the polaritons' coupling to light during propagation on the structured surface are suppressed.

Dislocation-based modeling of the mechanical behavior of epitaxial metallic multilayer thin films

A 3D dislocation cellular automaton model is employed to simulate yield and hardening in nanostructured metallic multilayer thin films during in-plane, biaxial tensile loading. The films consist of two types of alternating, single-crystalline FCC layers with (0 0 1) epitaxy, a mismatch in stress-free lattice parameter, but no elastic modulus mismatch. The simulations monitor the operation of interfacial and threading sources with lengths greater than the individual layer thickness. At larger layer thickness, strength increases with decreasing layer thickness, due to slip confinement to individual layers. At sufficiently small layer thickness, slip confinement is not possible, even during initial stages of plastic deformation. Consequently, strength is not controlled by layer thickness but rather by source length, as well as coherency stress and interfacial barrier strength. Here, strength may increase, decrease, or reach a plateau depending on how source length and barrier strength vary with layer thickness.

Optical response in nanostructured thin metal films with dielectric over-layers

Thin metal films which contain nano-size pores yield higher than expected transmittance and larger than expected times for transmission of near infra red radiation. An optically equivalent layer with complex refractive index (n*,k*) can model measured specular transmittance and reflectance, when scattering is weak. The way surface plasmon effects impact on these measured indices is considered. A strongly elevated n* is linked to trad the time for a surface plasmon to re-radiate and hence delay transmission times, and reduced imaginary part k*, to resonant channelling via voids. Measurement of n* thus allows an estimate of trad. The sensitivity of (n*,k*) to surface effects is illustrated using an insulating overlayer to modify the surface states. Resultant measured changes in n*,k* are substantial. Results are for two nanostructured metal systems characterised with 400,000× scanning electron microscopy.

Polarization control of optical transmission of a periodic array of elliptical nanoholes in a metal film.

The spectral dependences of polarized optical transmission of a metal film with a periodic array of elliptical nanoholes have been studied. Such nanostructured metal films exhibit enhanced broadband optical transmission that one can control by selecting the polarization of incident and (or) transmitted light.

Fabrication of nanostructured Al 2CuMg thin film by femtosecond pulsed laser ablation

A nanostructured Al 2CuMg thin film was formed by ablating reactive arc melted targets with femtosecond (10 −15 s range) laser pulses in a high vacuum chamber (10 −6 Torr). The films were characterized by secondary electron microscopy, energy dispersive analysis of X-rays, X-ray diffraction and transmission electron microscopy. Discussion is focused on the morphology of the film produced by this technique. It was observed that the femtosecond ablation technique could be used to form nanostructured metallic thin films.

Elasticity Study of Nanostructured Al and Al-Si(Cu) Films

In order to get an insight into the elastic property of nanostructured fcc metal films, the Young's modulus, E f , and the internal friction, Q - 1 f , in Al-Si(Cu) s p and Al s p films prepared by rf-sputtering and those in Al-Si(Cu) v e and Al v e films by vacuum evaporation were studied for the thickness, d, range of 4 to 300 nm, where the mean grain size was below 40 nm. A decrease in E f and an increase in Q - 1 f with decreasing d associated with the grain boundary anelastic process (GBAP) activated above 200 K are commonly observed. GBAP in the nanostructured Al is hardly modified by alloying with Si and Cu, and is very similar to GBAP reported in nanostructured Ag and Au. It is indicated that the elastic property of Al-Si(Cu) s p , Al s p , Al-Si(Cu) v e and Al v e nanocrystalline films is governed by GBAP, the internal stress and the surface oxide layer.

Nanostructured Metal Surfaces Fabricated by a Nonlithographic Template Method

We have developed a simple and completely nonlithographic preparation technique for free-standing nanostructured metallic films with a close-packed hexagonal array of metallic nanoembossments, by utilizing electrochemically prepared textured aluminum sheets as a replication master in conjunction with electrochemical deposition of metals. Microscopic analyses using field emission scanning electron microscopy and atomic force microscopy revealed that the present nanofabrication technique offers a highly facile and economical way for the production of periodic metallic nanostructures in a large area with high fidelity in pattern transfer as well as with a good degree of flexibility in materials.

Optical properties of nanostructured metal films.

Nanostructured metal films of platinum, gold and silver up to 675 nm thick we prepared by electrochemical deposition through templates of 700 nm diameter polystyrene spheres assembled as hexagonal close packed monolayer on an evaporated gold surface followed by removal of the template by dissolution in tetrahydrofuran. The reflection spectra of the films at normal incidence were recorded as a function of film thickness and the spectra correlated with the local visual appearance of the film and the surface structure from SEM. For thin films, below one quarter sphere height, the spectra show a single reflectivity dip at a wavelength just below the sphere diameter consistent with surface-plasmon grating-like behaviour. For the thicker films several reflectivity dips are observed which move towards longer wavelength with increasing film thickness. This behaviour is shown to be consistent with a model in which light reflected from the top of the structure interferes with light reflected from within the spherical segment cavities in the film.

Near-field photonics: surface plasmon polaritons and localized surface plasmons

Surface plasmon polaritons and localized surface plasmons are discussed in the context of photonic applications. Near-field imaging of scattering, reflection, interference and localization of surface polaritons is reviewed, and approaches for the implementation of elements of surface polariton optics are presented. Surface plasmon polaritonic crystals and their role in the determination of optical properties of periodically nanostructured metal films are described. Non-linear effects related to surface polaritons and localized surface plasmons allowing control of optical properties of nanostructured metal films with light are discussed. Surface plasmon optics opens up numerous possibilities for application of these intrinsically two-dimensional excitations in passive and active devices of all-optical integrated circuits.

How light gets through periodically nanostructured metal films: a role of surface polaritonic crystals.

The physical origin of the enhanced optical transmission of periodically structured films related to surface plasmon polaritons is discussed from first principles. The enhancement of transmission through smooth, randomly rough and periodically nanostructured films is considered. Analysis shows that any metal (or dielectric) nanostructured film can exhibit enhanced transmission in certain spectral ranges corresponding to surface plasmon (or phonon) polariton Bloch mode states on a periodic structure. Resonant tunnelling via these states is responsible for the transmission enhancement. The properties of surface polaritonic crystals are analogous to those of photonic crystals and can find numerous applications for scaling down optical devices to nanometric dimensions as well as for designing novel nanostructured materials whose optical properties are determined by surface polariton interaction in a periodic structure.

Characterization of magnetization processes in nanostructured rare earth-transition metal films

We synthesize rare earth-transition metal (RE-TM) amorphous films using the electrodeposition method (RE=Nd, Gd and TM=Co). Nanocrystructured RE-TM films are prepared by thermal treatment of as-synthesized films below the glass–crystal transition temperature. Based on the magnetoelastic effect, the magnetization processes in nanostructured samples are characterized by acoustic internal friction measurements using the vibrating-reed technique. Since internal friction and the Young’s modulus are sensitive to grain boundary and magnetic domains movement, this technique seems to characterize the effects of nanostructures on the magnetization processes in RE-TM films well. We find that the magnetoelastic effect in nanostructured RE-TM film increases with an increase in grain size.

The secret of early nanomaterials is revealed, thanks to transmission electron microscopy

Lustre ceramic decoration was revealed by analytical electron microscopy to have been the first nanostructured metallic thin film made by man. Considering this type of decoration in the context of cultural heritage, this is a remarkable discovery in the history of technology, because nanocrystal films have been produced empirically since medieval times. This meant that a high level of technological knowledge of materials science was required to obtain and to reproduce lustre layers. An analytical approach by means of transmission electron microscopy (TEM), with energy-dispersive X-ray spectroscopy (EDS) and electron diffraction (ED) facilities, allowed us to resolve the microstructure and chemical composition of the lustre.

Enhanced nonlinear optical conversion from a periodically nanostructured metal film.

We demonstrate an approximately10(4) increase in conversion efficiency for optical second-harmonic generation (SHG) from a periodically nanostructured metal structure consisting of a single subwavelength aperture in a thin silver film surrounded by a set of concentric surface grooves. The forward-transmitted second-harmonic radiation from this structure is measured relative to that from an identical aperture with no surrounding surface periodicity. We explain the observed SHG enhancement quantitatively in terms of a measured 120x increase in the strength of the fundamental radiation in the vicinity of the aperture resulting from the periodic nanostructuring.

Light tunneling via resonant surface plasmon polariton states and the enhanced transmission of periodically nanostructured metal films: An analytical study

An analytical treatment of optical transmission through periodically nanosructured metal films capable of supporting surface-plasmon polaritons is presented. The optical properties of such metal films are governed by surface polariton behavior in a periodic surface structure forming a surface polaritonic crystal. Due to different configurations of the electromagnetic field of surface polariton modes, only states of even Brillouin zones are responsible for the optical transmission enhancement at normal incidence. The transmission enhancement is related to photon tunneling via resonant states of surface polariton Bloch modes in which the energy buildup takes place. Surface polariton states of at least one of the film interfaces contribute to the transmission resonance which occurs due to tunnel coupling between photons and surface polaritons on the opposite interfaces. Under double-resonance conditions, resonant tunneling between surface polariton states of both interfaces is achieved, which leads to further enhancement of the transmission efficiency. The double-resonance conditions occur not only in the case of a film in symmetric environment but can also be engineered for a film on a substrate. Light tunneling via surface polariton states can take place directly through a structured metal film and does not necessarily require holes in a film.

A micromachined calorimetric gas sensor: an application of electrodeposited nanostructured palladium for the detection of combustible gases.

Palladium films with regular nanoarchitectures were electrochemically deposited from the hexagonal (H1) lyotropic liquid crystalline phase of the nonionic surfactant octaethyleneglycol monohexadecyl ether (C16EO8) onto micromachined silicon hotplate structures. The H1-e Pd films were shown to have high surface areas (approximately 28 m2 g(-1)) and to act as effective and stable catalysts for the detection of methane in air on heating to 500 degrees C. The response of the H1-e Pd-coated planar pellistors was found to be linearly proportional to the concentration of methane between 0 and 2.5% in air with a detection limit below 0.125%. Our results show that the electrochemical deposition of nanostructured metal films offers a promising approach to the fabrication of micromachined calorimetric gas sensors for combustible gases.

Electroless Deposition and Patterning of Morphologically Complex Precious Metal Films on Semiconductor Surfaces

ABSTRACTPrecious metals are choice materials for a myriad of applications due their high electrical conductivity, resistance to corrosion, and ligand binding specificity. Indispensable in modern electronics fabrication, precious metals also enjoy widespread use as catalysts, support substrates, and sensor elements. Recent progress towards metallization on diminishing size regimes has imposed increasingly stringent demands upon thin film preparation methodologies. Metallization techniques employed in ultra large scale integration (ULSI) device fabrication, nanoelectromechanical systems (NEMS), and arrayed nanosensors will require unparalleled control of surface morphology, deposition rate, and substrate adhesion without sacrificing throughput or cost effectiveness. Furthermore, precious metal films of this type are essential for fundamental investigations aimed at elucidating the intricate nature of interfacial topics ranging from self-assembled monolayers (SAMs) to heterogeneous catalysis. In contrast to complex and expensive vacuum methods of metallization, research in our laboratory has focused on the preparation of precious metal thin films on semiconductor substrates via electroless deposition. Thin and thick films of gold, platinum, and palladium nanoparticles have been prepared as a result of the immersion of germanium and gallium arsenide substrates into dilute, aqueous solutions of tetrachloraurate (III), tetrachloroplatinate (II), and tetrachloropalladate (II), respectively. This methodology yields nanostructured precious metal films with control over surface morphology and deposition rate. Moreover, metal films prepared in this manner exhibit excellent adhesion to the underlying semiconductor substrate. The resultant films were characterized utilizing scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning probe microscopy (SPM). This method provides for the facile interfacing of metal nanostructures with group (IV) and III-IV compound semiconductor surfaces.