Flat Au Films Research Articles

Direct fabrication and characterization of gold nanohole arrays.

We demonstrate a one-step fabrication method to realize desired gold (Au) nanoholes arrays by using a one-photon absorption based direct laser writing technique. Thanks to the optically induced thermal effect of Au material at 532 nm excitation wavelength, the local temperature at the laser focus area can reach as high as 600°C, which induces an evaporation of the Au thin film resulting in a metallic nanohole. By controlling the laser spot movement and exposure time, different two-dimensional Au nanoholes structures with periodicity as small as 500 nm have been demonstrated. This allows obtaining plasmonic nanostructures in a single step without needing the preparation of polymeric template and lift-off process. By this direct fabrication technique, the nanoholes do not have circular shape as the laser focusing spot, due to the non-uniform heat transfer in a no-perfect flat Au film. However, the FDTD simulation results and the experimental measurement of the transmission spectra show that the properties of fabricated plasmonic nanoholes arrays are very close to those of ideal plasmonic nanostructures. Actually, the plasmonic resonance depends strongly on the periodicity of the metallic structures while the heterogeneous form of the holes simply enlarges the resonant peak. Furthermore, it is theoretically demonstrated that the non-perfect circular shape of the Au hole allows amplifying the electromagnetic field of the resonant peak by several times as compared to the case of perfect circular shape. This could be an advantage for application of this fabricated structure in laser and nonlinear optics domains.

Bio-inspired plasmonic leaf for enhanced light-matter interactions

Abstract The mathematical concept of fractals is widely applied to photonics as planar structures ranging from terahertz resonators, optical antennas, to photodetectors. Here, instead of a direct mathematical abstract, we design a plasmonic leaf with fractal geometry from the outline of a leaf from Wargrave Pink. The enhanced light-matter interactions are observed numerically from the improvement in both absorption and near-field intensification. To demonstrate the effect experimentally, a three-dimensional fractal structure is realised through direct laser writing, which significantly improves the photothermal conversion. By virtue of the self-similarity in geometry, the artificial leaf improves the absorption of a 10-nm-thick gold film with 14 × temperature increment compared to flat Au film. Not limited to the proof-of-concept photothermal experiment demonstrated here, the fractal structure with improved light-matter interactions can be utilised in a variety of applications ranging from non-linear harmonic generation, plasmonic-enhanced fluorescence, to hot electron generation for photocatalysis.

Theoretical Investigation of a Simple Design of Triple-Band Terahertz Metamaterial Absorber for High-Q Sensing

This paper presents a simple metamaterial design to achieve the triple-band near-perfect absorption response that can be used in the area of sensor application. The introduced absorber consists of an array of Au strip and a bulk flat Au film separated by an insulator dielectric layer. Three narrow-band resonance absorption peaks are obtained by superposing three different modes (a fundamental mode resonance and two high-order responses) of the Au strip. These resonance modes (in particular of the last two modes) have large sensitivity to the changes of the surrounding index, overlayer thickness and the refractive index of the overlayer.

Functionalizing gold with single strand DNA: novel insight into optical properties via combined spectroscopic ellipsometry and nanolithography measurements.

We have studied the self-assembly of 22-base oligonucleotides bound by a short alkyl thiol linker (C6-ssDNA) on flat Au films. The self-assembled monolayer (SAM) was modified by addition of a spacer (mercaptohexanol, MCH). Molecular depositions were monitored in situ by spectroscopic ellipsometry (SE). SAMs were characterized in a liquid environment by coupling SE (difference spectra method) with Atomic Force Microscope (AFM) measurements. We exploited the biofilm thickness obtained by AFM nanolithography and imaging to solve the refractive index/thickness correlation in optical measurements on ultrathin molecular layers. The combined SE/AFM analysis provided reliable estimates of the thickness and the refractive index of the biofilm in the NIR region (650-1300 nm) and revealed new aspects of DNA molecular organization: exposure to MCH leads to an increase of both film thickness and refractive index, which points to a reorganization of C6-ssDNA film. We show that the contribution of the thiol/Au interface has to be included in the optical model to obtain a more reliable determination of the refractive index of the biofilm in a liquid. The careful, correlative characterization of the mixed C6-ssDNA/MCH SAM represents a key step towards the optimization of a robust detection scheme based on helix-helix hybridization.

Electrical and magnetic properties of hemozoin nanocrystals

Hemozoin crystals, also known as malaria pigment, are the by-products of hemoglobin degradation by the Plasmodium parasite during its intra-erythrocytic development. Although it is well known that they are responsible for the peculiar paramagnetic behavior of infected red blood cells, their physical properties are not well known. Here, we show that synthetic hemozoin nanocrystals display an insulating and paramagnetic behavior. Conductive atomic force microscopy measurements on crystals dispensed over flat Au films revealed that hemozoin crystals are insulating, with a breakdown field larger than 5 × 107 V m−1. The magnetic susceptibility, measured with a vibrating sample magnetometer, is 4.1 ± 0.6 × 10−4, compatible with a paramagnetic behavior. These results are confirmed by impedimetric and magnetophoretic measurements on aqueous suspensions of hemozoin crystals.

Tuning adhesion forces between functionalized gold colloidal nanoparticles and silicon AFM tips: role of ligands and capillary forces.

Adhesion forces between functionalized gold colloidal nanoparticles (Au NPs) and scanning probe microscope silicon tips were experimentally investigated by atomic force microscopy (AFM) equipped with PeakForce QNM (Quantitative Nanoscale Mechanics) module. Au NPs were synthesized by a seed-mediated process and then functionalized with thiols containing different functional groups: amino, hydroxy, methoxy, carboxy, methyl, and thiol. Adhesion measurements showed strong differences between NPs and silicon tip depending on the nature of the tail functional group. The dependence of the adhesion on ligand density for different thiols with identical functional tail-group was also demonstrated. The calculated contribution of the van der Waals (vdW) forces between particles was in good agreement with experimentally measured adhesive values. In addition, the adhesion forces were evaluated between flat Au films functionalized with the same molecular components and silicon tips to exclude the effect of particle shape on the adhesion values. Although adhesion values on flat substrates were higher than on their nanoparticle counterparts, the dependance on functional groups remained the same.

Transferrable Plasmonic Au Thin Film Containing Sub-20 nm Nanohole Array Constructed via High-Resolution Polymer Self-Assembly and Nanotransfer Printing.

The fabrication and characterization of nanoscale hole arrays (NHA) have been extensively performed for a variety of unique characteristics including extraordinary optical transmission phenomenon observed for plasmonic NHAs. Although the size miniaturization and hole densification are strongly required for enhancement of high-frequency optical responses, from a practical point-of-view, it is still not straightforward to manufacture NHA using conventional lithography techniques. Herein, a facile, cost-effective, and transferrable fabrication route for high-resolution and high-density NHA with sub-50 nm periodicity is demonstrated. Solvent-assisted nanotransfer printing with ultrahigh-resolution combined with block copolymer self-assembly is used to fabricate well-defined Si nanomesh master template with 4-fold symmetry. An Au NHA film on quartz substrate is then obtained by thermal-evaporation on the Si master and subsequent transfer of the sample, resulting in NHA structure having a hole with a diameter of 18 nm and a density over 400 holes/μm2. A resonance peak at the wavelength of 650 nm, which is not present in the transmittance spectrum of a flat Au film, is observed for the Au NHA film. Finite-difference time-domain (FDTD) simulation results propose that the unexpected peak appears because of plasmonic surface guiding mode. The position of the resonance peak shows the sensitivity toward the change of the refractive index of surrounding medium, suggesting it as a promising label-free sensor application. In addition, other types of Au nanostructure arrays such as geometry-controlled NHA and nanoparticle arrays (NPAs) shows the outstanding versatility of our approach.

Gold-rich ligament nanostructure by dealloying Au-based metallic glass ribbon for surface-enhanced Raman scattering

A new method to fabricate an Au-rich interconnected ligament substrate by dealloying the Au-based metallic glass ribbon for surface-enhanced Raman scattering (SERS) applications was investigated in this study. Specifically, three substrates, Au film, Au-based metallic glass ribbon, and dealloyed Au-based metallic glass ribbon, were studied. The dealloyed surface showed ligament nanostructure with protruding micro-islands. Based on the field emission scanning electron microscopy, reflection and scattering measurements, the dealloyed Au-based metallic glass provided a large surface area, multiple reflections, and numerous fine interstices to produce hot spots for SERS enhancements. The SERS signal of analyte, p-aminothiophenol, in the micro-island region of dealloyed Au-based metallic glass was about 2 orders of magnitude larger than the flat Au film. Our work provides a new method to fabricate the inexpensive and high SERS enhancements substrates.

Enhanced Efficient Light Emission of Er(Yb/Y) Silicates at the Wavelength of 1.53μm with Au Plasmonic Arrays

In this paper, we present a novel method to increase photoluminescence (PL) efficiency at the wavelength of 1.53 $\mu$ m ( $\lambda =$ 1.53 $\mu$ m) by depositing Au plasmonic arrays on $\text{SiO}_\text{2}\text{/Er}\text{(Yb/Y)}$ silicates. Theoretical analysis reveals that the PL enhancement is caused by the resonant coupling of surface plasmon polaritons from photoemission in $\text{Er(Yb/Y)}$ and the electric field of incident light at the $\text{Au/SiO}_{2}$ interface. Numerical investigations show that by using the finite-difference-time-domain (FDTD) method, the change of the peak shape and bandwidth of transmission spectra at $\lambda =$ 1.53 $\mu$ m depends strongly on the arrays period ( ${a_0}$ ) and $\text{Au}$ film thickness ( ${t_{\text{Au}}}$ ). It is demonstrated that, by experiments, the plasmonic arrays of ${a_0} =$ 1082 nm and ${t_{\text{Au}}} =$ 50 nm can enhance the PL intensity two times, compared with a flat Au film, which can serve as an excellent guideline for designing and optimizing a highly efficient solid-state light source.

Carbon Nanodots with Sub-Nanosecond Spontaneous Emission Lifetime.

Compared with most mature cadmium-containing quantum dots (QDs), carbon nanodots (CNDs) are a new class of colloidal nanomaterials that exhibit unique photoluminescence (PL) properties while being nontoxic and easily manufactured using low-cost precursor materials. However, solid-state CNDs exhibit poor PL quantum yields (PL-QYs) and inefficient radiative transition, which significantly hinders their practical use in optoelectronic devices. To address this issue, plasmonic nanoantennas consisting of Au nanorods (Au-NRs) deposited on a flat Au film with inserted dielectric layers were used to enhance the spontaneous emission of solid-state CNDs with broad spectral linewidth. Using steady-state, time-resolved, and spatial-resolved PL measurements, we found that after coupling to plasmonic nanogaps (PNGs), the PL emission was significantly enhanced, accompanied by a PL lifetime shortening to the sub-nanosecond range (≈140 ps). According to the experimental data, the radiative transition is strongly accelerated and can thus overcome the metal loss, leading to a large PL enhancement. Our demonstration can pave the way to the design of eco-friendly nanoemitters with sub-nanosecond PL lifetime for promising applications in light-emitting devices.

Extrinsic surface-enhanced Raman scattering detection of influenza A virus enhanced by two-dimensional gold@silver core–shell nanoparticle arrays

A surface enhanced Raman scattering (SERS) based biosensor using a direct immunoassay platform was demonstrated for influenza A detection. The sensitivity was improved ~4 times by using a well-tuned Au@Ag 2D array instead of a flat Au film.

Large‐Area Resonance‐Tuned Metasurfaces for On‐Demand Enhanced Spectroscopy

We show an effective procedure for lateral structure tuning in nanoimprint lithography (NIL) that has been developed as a vertical top‐down method fabricating large‐area nanopatterns. The procedure was applied to optical resonance tuning in stacked complementary (SC) metasurfaces based on silicon‐on‐insulator (SOI) substrates and was found to realize structure tuning at nm precision using only one mold in the NIL process. The structure tuning enabled us to obtain fine tuning of the optical resonances, offering cost‐effective, high‐throughput, and high‐precision nanofabrication. We also demonstrate that the tuned optical resonances selectively and significantly enhance fluorescence (FL) of dye molecules in a near‐infrared range. FL intensity on a SC metasurface was found to be more than 450‐fold larger than the FL intensity on flat Au film on base SOI substrate.

Ultrasensitive hybrid SERS substrate for rapid detection of trace chemicals

We demonstrate here a simple and effective approach for fabricating surface-enhanced Raman spectroscopy substrates by hybridizing Au films with silver particle clusters. A trace amount of aqueous chemical is dropped on the Au film covered substrate, followed by the addition of colloidal Ag nanoparticles (Ag NPs). The combination of Ag clusters and Au film promotes the enhancement ability of the substrate, as confirmed by a finite difference time domain simulation. The electromagnetic field tends to concentrate at the junctions between the Ag NPs and the flat Au film, and yields additional ‘hot spots’. The detect limitation of the hybrid substrate is 10 pM, with an enhancement factor about 1.5 × 107 when rhodamine 6G is employed as the probe molecule. The hybrid substrate showed good homogeneity, with a relative standard deviation about 10.7%. Moreover, the hybrid substrate is utilized for melamine detection with a detection limit of 10 nM (1.2 ppb). The simple procedure, high sensitivity, and good homogeneity make the hybrid substrate a promising tool for trace chemical detection.

Extraordinary terahertz absorption bands observed in micro/nanostructured Au/polystyrene sphere arrays

Terahertz (THz) time-domain spectroscopy is carried out for micro/nanostructured periodic Au/dielectric sphere arrays on Si substrate. We find that the metal-insulator transition can be achieved in THz bandwidth via varying sample parameters such as the thickness of the Au shell and the diameter of the Au/dielectric sphere. The Au/polystyrene sphere arrays do not show metallic THz response when the Au shell thickness is larger than 10 nm and the sphere diameter is smaller than 500 nm. This effect is in sharp contrast to the observations in flat Au films on Si substrate. Interestingly, the Au/polystyrene sphere arrays with a 5-nm-thick Au shell show extraordinary THz absorption bands or metallic optical conductance when the diameter of the sphere is larger than 200 nm. This effect is related to the quantum confinement effect in which the electrons in the structure are trapped in the sphere potential well of the gold shell.

Broadband Optical Absorption Enhancement in Au-Coated ZnO Nanotips

The enhancement of broadband optical absorption in zinc oxide (ZnO) nanotip (NT) arrays coated with evaporated gold (Au) on fluorine-doped SnO2 (FTO)/glass by a simple hydrothermal growth and subsequent Au evaporation is reported. As the core of ZnO NT arrays is gradually coated with evaporated Au, the reflectance is slightly increased at lambda > approximately 800 nm while the transmittance is decreased at lambda approximately 400-1800 nm. For both FTO/glass and ZnO NT arrays on FTO/glass, the coating of Au improves the light absorption due to the antireflective geometry compared to the flat Au films and the absorptance is also enhanced by increasing the nominal thickness of Au with evaporation time. For the ZnO NT arrays with an Au evaporated for 600 s at 0.5 A/s, a high absorptance of >72% is achieved over the wavelength range of 250-2000 nm, indicating a significant increase due to the enhanced antireflection property as well as the increased surface area compared to the Au-coated FTO/glass without ZnO NT arrays.

Atomic force microscopy investigation of the kinetic growth mechanisms of sputtered nanostructured Au film on mica: towards a nanoscale morphology control

The study of surface morphology of Au deposited on mica is crucial for the fabrication of flat Au films for applications in biological, electronic, and optical devices. The understanding of the growth mechanisms of Au on mica allows to tune the process parameters to obtain ultra-flat film as suitable platform for anchoring self-assembling monolayers, molecules, nanotubes, and nanoparticles. Furthermore, atomically flat Au substrates are ideal for imaging adsorbate layers using scanning probe microscopy techniques. The control of these mechanisms is a prerequisite for control of the film nano- and micro-structure to obtain materials with desired morphological properties. We report on an atomic force microscopy (AFM) study of the morphology evolution of Au film deposited on mica by room-temperature sputtering as a function of subsequent annealing processes. Starting from an Au continuous film on the mica substrate, the AFM technique allowed us to observe nucleation and growth of Au clusters when annealing process is performed in the 573-773 K temperature range and 900-3600 s time range. The evolution of the clusters size was quantified allowing us to evaluate the growth exponent 〈z〉 = 1.88 ± 0.06. Furthermore, we observed that the late stage of cluster growth is accompanied by the formation of circular depletion zones around the largest clusters. From the quantification of the evolution of the size of these zones, the Au surface diffusion coefficient was evaluated in . These quantitative data and their correlation with existing theoretical models elucidate the kinetic growth mechanisms of the sputtered Au on mica. As a consequence we acquired a methodology to control the morphological characteristics of the Au film simply controlling the annealing temperature and time.

Determination of Surface Area and Porosity of Small, Nanometer-Thick Films by Quartz Crystal Microbalance Measurement of Gas Adsorption

The Brunauer-Emmett-Teller (BET) surface area of 15 nm-thick films made of TiO2/polyelectrolyte bilayer was determined by quartz crystal microbalance (QCM) measurement of N2 and Ar adsorption isotherms at 77 K. The measurements were carried out using a home-built vacuum chamber that includes built-in 9 MHz QCM and cryostat units. As little as 1 ng of the adsorbed gas could be detected, and the BET surface area of a flat Au film (ca. 0.5 cm2) on an oscillator was determined within an experimental error of +/-5%. The titania/polymer composite film gives N2 and Ar adsorption isotherms consisting of a less-pronounced type-I curve and a break at around p/p0 = 0.7. This behavior is ascribed to the presence of irregular micropores and 6 nm phi-mesopores in the composite film. An analysis of the isotherms shows that the porosity of the composite film is about 12%, which is much smaller than that of bulk titania gel powder. The greater density appears to be related to the reported superior properties (robustness and resistance to electrical breakdown) of the organic/inorganic multilayer film. We conclude that the QCM-based, high-precision measurement of gas adsorption is a powerful tool for investigation of the detailed morphology of nanometer-thick films.

Growth of thin, flat, epitaxial ( [formula omitted]) oriented gold films on c-cut sapphire

A new growth procedure has been developed to prepare thin (<50 nm), flat (roughness ∼0.3–1 nm rms), epitaxial Au films on top of an insulating substrate without using a magnetic seedlayer. This could be obtained by depositing first a Nb seedlayer (⩽1 nm) at room temperature on top of c-cut sapphire followed by evaporating Au at a rate of 0.05–0.1 nm/s at 300 °C. This procedure resulted in (1 1 1) oriented flat Au films even for thickness well below 50 nm. The flatness of these films was consistently confirmed by atomic force and scanning tunneling microscopy, their excellent epitaxial quality (rocking width 0.1°–0.3°) by X-ray diffractometry and reflected high energy electron diffraction.

X-ray characteristics of atomically flat gold films deposited by ICB

Atomically flat Au films on SiO 2 have been deposited by an ionized cluster beam (ICB). The roughness of the film surface was reduced with increasing thickness at high acceleration voltage. Average roughness of the film deposited at I e = 100 mA and V a = 3 kV was 0.25 nm by analysis of X-ray reflectivity. The films have shown high reflectivity of X-rays (AI K α) at a wavelength of 0.834 nm. The reflectivity was 95% at the incident angle of 0.75° and was 10% larger than the value calculated by using Fresnel's equation, in which the surface roughness was assumed to be zero.

Preparation of atomically flat gold films by ionized cluster beam

We have prepared atomically flat Au films by using an ionized cluster beam. The films were deposited on polyimide and SiO 2 at room temperature in an ultrahigh vacuum of 4 × 10 −9 Torr using different electron currents for ionization and different acceleration voltages. Using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), the film surface was found to be improved with increasing acceleration voltage. The root-mean-square roughness measured was 0.25 nm for the films deposited on SiO 2 at I e = 100 mA and V a = 3 kV. The films showed high reflectivity for X-rays at a wavelength of 0.834 nm.