Formation of Au nano-patterns on various substrates using simplified nano-transfer printing method

For future device applications, fabrication of the metal nano-patterns on various substrates, such as Si wafer, non-planar glass lens and flexible plastic films become important. Among various nano-patterning technologies, nano-transfer print method is one of the simplest techniques to fabricate metal nano-patterns. In nano-transfer printing process, thin Au layer is deposited on flexible PDMS mold, containing surface protrusion patterns, and the Au layer is transferred from PDMS mold to various substrates due to the difference of bonding strength of Au layer to PDMS mold and to the substrate. For effective transfer of Au layer, self-assembled monolayer, which has strong bonding to Au, is deposited on the substrate as a glue layer.In this study, complicated SAM layer coating process was replaced to simple UV/ozone treatment, which can activates the surface and form the -OH radicals. Using simple UV/ozone treatments on both Au and substrate, Au nano-pattern can be successfully transferred to as large as 6 in. diameter Si wafer, without SAM coating process. High fidelity transfer of Au nano-patterns to non-planar glass lens and flexible PET film was also demonstrated.     

Direct writing of micro/nano-scale patterns by means of particle lens arrays scanned by a focused diode pumped Nd:YVO4 laser

A practical approach to a well-known technique of laser micro/nano-patterning by optical near fields is presented. It is based on surface patterning by scanning a Gaussian laser beam through a self-assembled monolayer of silica micro-spheres on a single-crystalline silicon (Si) substrate. So far, the outcome of this kind of near-field patterning has been related to the simultaneous, parallel surface-structuring of large areas either by top hat or Gaussian laser intensity distributions. We attempt to explore the possibility of using the same technique in order to produce single, direct writing of features. This could be of advantage for applications in which only some areas need to be patterned (i.e. local area selective patterning) or single lines are required (e.g. a particular micro/nano-fluidic channel). A diode pumped Nd:YVO₄ laser system (wavelength of 532 nm, pulse duration of 8 ns, repetition rate of 30 kHz) with a computer-controlled 3 axis galvanometer beam scanner was employed to write user-defined patterns through the particle lens array on the Si substrate. After laser irradiation, the obtained patterns which are in the micro-scale were composed of sub-micro/micro-holes or bumps. The micro-pattern resolution depends on the dimension of both the micro-sphere’s diameter and the beam’s spot size. The developed technique could potentially be employed to fabricate photonic crystal structures mimicking nature’s butterfly wings and anti-reflective “moth eye” arrays for photovoltaic cells.     

The surface plasmon resonance of metal-film nanohole arrays

In this paper we investigated the enhanced transmission and surface plasmon resonance through a thin gold film with a periodic array of subwavelength nanoholes. Both freestanding gold-film nanohole arrays and gold-film nanohole arrays deposited on a gallium arsenide (GaAs) substrate are considered. Periodic arrays of nanoholes exhibit two different surface plasmon resonance features: localized waveguide resonance and the well-recognized photonic crystal resonance. The tangential electric field component $Ey$ is nonzero only in the hole region for a freestanding gold-film nanohole array, but it can exist in the hole region and in the metallic region for a gold-film nanohole array deposited on a GaAs substrate.     

Patterning thin metallic film via laser structured weakly bound template

A weakly bound buffer material is structured on a surface by interfering low power laser beams, as a template for patterning metallic thin films deposited on top. The excess buffer material and metal layer are subsequently removed by a second uniform laser pulse. This laser pre-structured buffer layer assisted patterning procedure is demonstrated for gold layer forming a grating on a single crystal Ru(1 0 0) under UHV conditions, using Xe as the buffer material. Millimeters long, submicron (0.65 μm) wide wires can be obtained using laser wavelength of 1.064 μm with sharp edges of less than 30 nm, as determined by AFM. This method provides an all-in-vacuum metallic film patterning procedure at the submicron range, with the potential to be developed down to the nanometer scale upon decreasing the patterning laser wavelength down to the UV range.     

Catalyst patterning methods for surface-bound chemical vapor deposition of carbon nanotubes

We present three different catalyst preparation and patterning techniques for plasma-enhanced chemical vapor deposition of carbon nanostructures from acetylene and ammonia mixtures. The different merits and potential areas of application are highlighted for each technique as compared to the benchmark of e-beam-lithography patterning. Maskless, focused ion beam written Pt can nucleate aligned carbon nanofibers, thereby allowing a sub-100 nm lateral resolution on non-planar substrate geometries combined with an in-situ monitoring. Ion beam milling additionally enables the pre-shaping and marking of the substrate, which is shown for the growth of individual nanofibers on the apex of commercial scanning probe tips. Pulsed electrochemical deposition was used to form Ni and Fe catalyst islands of controlled size and density. This is also demonstrated on complex substrate geometries such as carbon cloth. Nanocontact printing was employed to deposit a highly purified Co colloid in regular patterns with feature sizes down to 100 nm onto silicon wafers for low cost patterning over large areas. We analyze the catalyst restructuring upon exposure to elevated temperatures for each technique and relate this to the nucleated nanofiber dimensions and array densities. The flexibility in catalyst and substrate material allows a transfer of our achievements to catalyst-assisted growth of nanostructures in general facilitating their hierarchical device integration and future application. PACS 81.16.Rf; 81.16.Hc; 61.46.+w     

Near field localization mediated by a single gold nanoparticle embedded in transparent matrix: Application for surface modification

In this work the near field properties of a single gold nanoparticle embedded in transparent host medium are investigated theoretically. The analysis of the electromagnetic field in the near field zone is obtained by finite difference time domain (FDTD) simulation technique. The nanoscale system consists of a transparent layer in which a gold particle with diameters of D = 200 or 80 nm is embedded, is situated on a substrate surface. Laser pulse at wavelength of 800 nm irradiates normally this system. It is found that the field in the vicinity of the particle is enhanced, and at a certain condition the zone with the highest enhancement is localized on the substrate surface. Furthermore, the near field characteristics are found to be controllable by the dielectric properties of the host material, substrate, parameters of the incident irradiation and particle size. With the increase of the refractive index of the host medium, both the magnitude of the near field on the substrate and the characteristic size of the field enhanced zone decrease. The influence of the particle size and polarization of the incident laser irradiation on the near filed properties of the system are also presented. The proposed configuration can be applied for a multiple nanoprocessing and an integrated near field source with a spatial resolution of D/3.     

Electromagnetic field distribution on a rough gold thin film: An experimental study as a function of the gold thickness

The electromagnetic field distribution on an illuminated rough gold surface has been investigated by apertureless scanning near-field optical microscopy. The sample consists of an alumina substrate with a variable gold coverage ranging from 0 to 30 monolayers (ML). For such small thicknesses, the metal layer is not continuous but exhibits a certain roughness. We have studied the influence of this thickness on the electromagnetic field localization on the surface. For a gold coverage smaller than 10 equivalent monolayers, the electromagnetic field is almost uniform on the surface. For 10 and 14 ML, the field becomes inhomogeneous and isolated, localized peaks start to be visible. The width of the peaks is smaller than 50 nm. Above 14 ML, strong variations are apparent everywhere on the sample. Their amplitude tends to saturate beyond 24 ML. A complete statistical study of the sample (standard deviation, Fourier analysis) is performed.     

Surface modification of silicate glasses by nanoimprint using nanostriped NiO thin film molds

Nanoscale surface modification of silicate glasses was examined by applying nanoimprint technique using a nanostriped NiO thin film mold. The mold had the pattern composed of regularly arranged straight nanogrooves, which was formed by high-temperature annealing of the Li-doped NiO epitaxial thin film deposited on the atomically stepped sapphire (α-Al₂O₃ single crystal) substrate. The glass imprint was proceeded through the simple steps of heating (∼600 °C), pressing (∼1 kPa) and then cooling in air. The nanoimprinted glass surface transferred reversely from the mold exhibited the multi nanowire array having an interval of ∼80 nm, wire width of ∼70 nm, and wire height of ∼20 nm.     

Picosecond acoustics in vegetal cells: Non-invasive in vitro measurements at a sub-cell scale

A 100 fs laser pulse passes through a single transparent cell and is absorbed at the surface of a metallic substrate. Picosecond acoustic waves are generated and propagate through the cell in contact with the metal. Interaction of the high frequency acoustic pulse with a probe laser light gives rise to Brillouin oscillations. The measurements are thus made with lasers for both the opto-acoustic generation and the acousto-optic detection, and acoustic frequencies as high as 11 GHz can be detected, as reported in this paper. The technique offers perspectives for single cell imaging. The in-plane resolution is limited by the pump and probe spot sizes, i.e. ∼1 μm, and the in-depth resolution is provided by the acoustic frequencies, typically in the GHz range. The effect of the technique on cell safety is discussed. Experiments achieved in vegetal cells illustrate the reproducibility and sensitivity of the measurements. The acoustic responses of cell organelles are significantly different. The results support the potentialities of the hypersonic non-invasive technique in the fields of bio-engineering and medicine.     

Probing the diffusion of vacuum ultraviolet (λ = 172 nm) induced oxidants by nanoparticles immobilization

Vacuum ultraviolet (VUV, λ = 172 nm) patterning of alkyl monolayer on silicon surface has been demonstrated with emphasis on the diffusion of VUV induced oxygen-derived active species, which are accountable for the pattern broadening. The VUV photons photo-dissociates the atmospheric oxygen and water molecules into the oxygen-derived active species (oxidants). These oxidants photo-oxidize the hexadecyl (HD) monolayer in VUV irradiated regions (Khatri et al., Langmuir. 24 (2008) 12077), as well as the little concentration of oxidants diffuses towards the masked areas. In this study, we performed VUV patterning at a vacuum pressure of 10 Pa to track the diffusion pathways for the oxidants with help of gold nanoparticles (AuNPs; ? = 10 nm) immobilization. At VUV irradiated sites AuNPs are found as uniformly distributed, but adjacent to the pattern boundary we observed quasi-linear arrays of AuNPs, which are determined by diffusion pathways of the oxidants. The diffusion of oxidants plays vital role in pattern broadening. The site selective anchoring of AuNPs demonstrates the utility of VUV photons for the construction of functional materials with microstructural architecture.     

Impact of laser produced X-rays on the surface of gold

In the paper an attempt has been made to use the laser-induced plasma as an X-ray source for the growth of nanostructures on the surface of gold. For this purpose, an Nd:YAG laser operated at second harmonics (λ = 532 nm, E = 400 mJ) is used to produce plasma from analytical grade 5N pure Al, Cu and W targets. An analytical grade (5N pure) gold substrate was irradiated by X-rays generated from Al, Cu and W plasma under the vacuum ∼10⁻⁴ Torr. The surface was analyzed by two techniques, XRD and AFM. The aberrations in the XRD peaks show that there are significant structural changes in the exposed gold, in terms of decreased reflection intensities, increased dislocation line density, changes in the d-spacing and disturbance in the periodicity of the planes. AFM used to explore the topography of the irradiated gold reveals that regardless of the source, nm sized hillocks have been produced on the gold surface. The roughness of the surface has increased due to the growth of these hillocks.     

Enhanced surface evolution induced by the molecular desorption in dodecanethiol self-assembled monolayer on Au(1 1 1)

The evolution of the surface structure in dodecanethiol self-assembled monolayer on Au(1 1 1) substrate has been studied with ultra high vacuum scanning tunneling microscopy at several temperatures. The structure of substrate Au(1 1 1) surface changed suddenly at a temperature of 110 °C. The enhanced mobility of the substrate gold atoms at this temperature is attributed to the desorption of the dodecanethiol molecules.     

Controlled preparation of well-ordered transition metal oxide layers on a metallic surface

A method to create various well-ordered two dimensional transition metal oxide films on a metallic substrate has been exploited. The formation of an intermediate amorphous layer with controllable metal-oxygen stoichiometry serves as an important precursor condition for the final transformation into a mono-phase, crystalline oxide layer via mild annealing. As a key ingredient serves a Cu₃Au(1 0 0) substrate covered by oxygen. The flat Cu-O topmost layer stops completely intermixing of the substrate material with the subsequently evaporated transition metal film. Likewise the wetting of the surface is considerably enhanced and a homogeneous oxidation of the film is strongly promoted. The proposed technique appears to be widely efficient for preparation of various two dimensional oxide films covering the entire Cu₃Au(1 0 0) substrate. Its usefulness is demonstrated successfully for vanadium, niobium and molybdenum to produce a set of single-phase transition metal oxides of different stoichiometry and geometrical structure. All created oxides are found to be thermally stable at least up to a substrate temperature of 800 K.     

Growth mechanisms for wire-like epitaxial gold silicide islands on Si(1 1 0) surfaces

Epitaxial islands grown on various substrates are usually strained because of differences in lattice constants of the materials of the island and the substrate. Shape transition in the growth of strained islands has been proposed as a mechanism for strain relief and a way to form self-organized quantum wires. Shape transition usually leads to an elongated island growth. However, an elongated island growth may also be due to an anisotropic diffusion of material, the anisotropy being imposed by the symmetry of the substrate surface. In the present example, growth of gold silicide wire-like nanostructures on a Si(1 1 0) surface has been investigated by photoemission electron microscopy (PEEM). Growth of elongated unidirectional gold silicide islands, with an aspect ratio as large as 12:1, has been observed by PEEM following gold deposition on the Si substrate and subsequent annealing at the Au-Si eutectic temperature. Distribution of the width and the length of the gold silicide islands as a function of island area shows a feature similar to that for the shape transition. However, detailed investigations reveal that the elongated growth of gold silicide islands is rather mainly due to anisotropic diffusion of gold due to the twofold symmetry of the (1 1 0) surface of the Si substrate.     

Peptide-induced patterning of gold nanoparticle thin films

In this work, the use of patterned proteins and peptides for the deposition of gold nanoparticles on several substrates with different surface chemistries is presented. The patterned biomolecule on the surface acts as a catalyst to precipitate gold nanoparticles from a precursor solution of HAuCl₄ onto the substrate. The peptide patterning on the surfaces was accomplished by physical adsorption or covalent attachment. It was shown that by using covalent attachment with a linker molecule, the influence of the surface properties from the different substrates on the biomolecule adsorption and subsequent nanoparticle deposition could be avoided. By adjusting the reaction conditions such as pH or HAuCl₄ concentration, the sizes and morphologies of deposited gold nanoparticle agglomerates could be controlled. Two biomolecules were used for this experiment, 3XFLAG peptide and bovine serum albumin (BSA). A micro-transfer molding technique was used to pattern the peptides on the substrates, in which a pre-patterned poly(dimethylsiloxane) (PDMS) mold was used to deposit a lift-off pattern of polypropylmethacrylate (PPMA) on the various substrates. The proteins were either physically adsorbed or covalently attached to the substrates, and an aqueous HAuCl₄ solution was applied on the substrates with the protein micropatterns, causing the precipitation of gold nanoparticles onto the patterns. SEM, AFM, and Electron Beam Induced Current (EBIC) were used for characterization.     

Direct observation of slow morphological transformations and wetting behavior of pulsed laser deposited sub-monolayer gold on (0 0 0 1) sapphire in atmosphere

Using high-resolution atomic force microscope we observed in ambient atmosphere the slow morphological transitions of the incipient adlayer of gold grown on (0 0 0 1) sapphire substrate by pulsed laser deposition. The equivalent average uniform thickness of the gold deposition was about 0.55 Å, which is about one-fourth of its monolayer. A dynamic simulation revealed that about 10% of the gold was implanted into the substrate up to the depth of about 3.3 nm and the top monolayer of the sapphire surface was almost completely depleted of oxygen atoms due to the preferential sputtering by the plume particles. The gold adlayer transformed into a labile phase which enhanced the surface roughness and had a preferred orientation of a wavy structure during 24 h of the deposition. The auto-correlation function of this wavy structure in labile metastable phase revealed two-fold symmetry and provided a preferential size of about 4 nm (peak to peak) with a mean separation of 8 nm. At the end of about 6 days this phase was found to completely transform into an apparently de-wetted phase of beads with average in-plane diameter of ∼20 nm and height of ∼7 nm having large size distribution. Each bead was seen to have coating of a concentric corona layer, which might be that of the condensed moisture or other gaseous species from atmosphere because subjecting these samples to vacuum removed this layer. These observations shed light on the dynamics of the pulsed laser deposited metastable gold adlayer in the incipient stage of its growth on sapphire and their wetting or de-wetting mechanisms in ambient atmosphere.     

Influence of the substrate,metal overlayer and lattice neighbors on the focusing properties of colloidal microspheres

Calculations of the field distribution in colloidal SiO₂ microspheres are presented. Two cases are considered: small particles on a Si substrate irradiated by the 266 nm light, and larger ones, covered with a gold film and irradiated at 800 nm. Substrate, neighboring spheres and sputtered metal overlayer all significantly modify the field pattern and magnitude. Reflected light is focused inside the spheres, which may lead to their damage. The results can be useful in the analysis of microspheres-assisted nano-patterning.     

A new imaging method for gold-surface adsorbates based on anomalous reflection

A new imaging method is proposed for ultrathin films with a thickness of a few nanometers, based on the anomalous reflection (AR) of gold. In the AR effect, the reflectivity fairly decreases for blue or purple light (380 nm < λ < 480 nm) with the existence of a transparent dielectric layer at a gold surface. Thus, a thin gold film can be used as an imaging platform. Clear AR images are obtained for a microarray of protein (avidin) spots of diameter 120 μm with gaps of size 50 μm between the spots (36 spots/mm²). The resolution of the AR imaging is governed solely by the illumination spot size. AR imaging is a promising technique for high throughput analysis of biomolecular detection in a microarray format.     

Adsorption of gold on hydrogen terminated Si(0 0 1): Formation of chain structure

Possible formation of stable Au atomic wire on the hydrogen terminated Si(0 0 1): 3×1 surface is investigated under the density functional formalism. The hydrogen terminated Si(0 0 1): 3×1 surface is patterned in two different ways by removing selective hydrogen atoms from the surface. The adsorption of Au on such surfaces is studied at different sub-monolayer coverages. At 4/9 monolayer (ML) coverage, zigzag continuous Au chains are found to be stable on the patterned hydrogen terminated Si(0 0 1): 3×1 surface. The reason for the stability of the wire structures at 4/9 ML coverage is explained. It is to be noted that beyond 4/9 ML coverage, the additional Au atoms may introduce clusters on the surface. The continuous atomic gold chains on the substrate may be useful for the fabrication of atomic scale devices.     

A study of AFM-based scratch process on polycarbonate surface and grating application

We report on the possibility of applying atomic force microscope (AFM) lithography to draw micro/nano-structures on the surface of a polycarbonate (PC) substrate. We also fabricated a grating structure on the PC surface using the scratch method. An AFM silicon tip coated with a diamond layer was utilized as a cutting tool to scratch the surface of the sample. In order to obtain pattern depth deeper than the control method of interaction force, we used a scanner movement method which the sample scanner moves along the Z-axis. A grating of 100 μm × 150 μm was fabricated by the step and repeat method wherein the sample stage is moved in the direction of the XY-axis. The period and the depth of the grating are 500 and 50 nm, respectively. Light of 632.8 nm wavelength was diffracted on the surface of the PC substrate.