Free-standing Gold Films Research Articles

Enhanced Photothermal Steam Generation and Gold Using the Efficiency of Ultralight Gold Foam with Hierarchical Porosity

Controlling the optical properties of metal plasma nanomaterials through structure manipulation has attracted great attention for solar steam generation. However, realizing broadband solar absorption for high-efficiency vapor generation is still challenging. In this work, a free-standing ultralight gold film/foam with a hierarchical porous microstructure and high porosity is obtained through controllably etching a designed cold-rolled (NiCoFeCr)99Au1 high-entropy precursor alloy with a unique grain texture. During chemical dealloying, the high-entropy precursor went through anisotropic contraction, resulting in a larger surface area compared with that from the Cu99Au1 precursor although the volume shrinkage is similar (over 85%), which is beneficial for the photothermal conversion. The low Au content also results in a special hierarchical lamellar microstructure with both micropores and nanopores within each lamella, which significantly broadens the optical absorption range and makes the optical absorption of the porous film reach 71.1-94.6% between 250 and 2500 nm. In addition, the free-standing nanoporous gold film has excellent hydrophilicity, with the contact angle reaching zero within 2.2 s. Thus, the 28 h dealloyed nanoporous gold film (NPG-28) exhibits a rapid evaporation rate of seawater under 1 kW m-2 light intensity, reaching 1.53 kg m-2 h-1, and the photothermal conversion efficiency reaches 96.28%. This work demonstrates the enhanced noble metal gold using efficiency and solar thermal conversion efficiency by controlled anisotropic shrinkage and forming a hierarchical porous foam.

Fast attenuation of high-frequency acoustic waves in bicontinuous nanoporous gold

We studied the formation and attenuation of GHz elastic waves in free-standing nanoporous gold films by MeV ultrafast electron diffraction and finite element simulations. By tracing the evolution of the high frequency acoustic waves in time domain, we found that the bicontinuous nanoporous structure in nanoporous gold films results in three-dimensionally acoustic response with low coherence, leading to fast attenuation of the elastic waves in comparison with solid gold films. The morphologically dominated dynamics indicates the nanoporosity plays an important role in the high-frequency acoustic energy relaxation, which shines a light on the applications of dealloyed nanoporous materials in nanodevices and sensors as GHz and THz acoustic filters and dampers.

Electron energy loss spectroscopy on freestanding perforated gold films

We report on a combined far- and near-field study of surface plasmon polaritons on freestanding perforated gold films. The samples are fabricated by focused ion beam milling of {a periodic hole array into} a carbon membrane followed by thermal evaporation of gold and plasma ashing of the carbon film. Optical transmission spectra show a series of characteristic features, which can be attributed to the excitation of surface plasmon modes via the periodic nanohole array. The corresponding near-field distributions are mapped by electron energy-loss spectroscopy. Besides the optically bright surface plasmon modes, we observe in the near-field an additional dark plasmon mode, which is absent in the normal incidence far-field spectra. Our experimental results are in good agreement with numerical computations based on a discontinuous Galerkin time-domain method.

Thickness dependency of creep crack propagation mechanisms in submicrometer-thick gold films investigated using in situ FESEM and EBSD analysis

To clarify the thickness dependency of creep crack propagation mechanisms, creep crack propagation experiments were carried out on submicrometer-thick metallic films; freestanding gold films with thicknesses of ~120 nm (thinner) and ~340 nm (thicker) were subjected to in situ field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) analysis. In both the thicker and thinner films, creep crack stably propagated at room temperature by repeating the processes of (i) stretch and local thinning ahead of the creep crack, (ii) void formation at grain boundaries in the creep-damaged region, and (iii) void growth and coalescence of voids or that between the main crack and voids. The creep-damaged region ahead of the creep crack tip was narrower, shorter and shallower in the thinner films compared to that of the thicker films. In the thicker films, the creep cracks propagated through transgranular creep fracture, whereas in the thinner films, the creep cracks propagated by transgranular and intergranular creep fracture, indicating that creep crack propagation mechanisms depend on film thickness.

Mechanical properties of nanoporous gold subjected to tensile stresses in real-time, sub-microscopic scale

Ductile metals infiltrated with interconnected nanopores through selective dealloying gain useful properties but become macroscopically brittle due to flow localization. The mechanical behavior of nanoporous metals is dependent on a complex relationship between the deformation of the nanoporous foam structure and the deformation of the individual ligaments. Recent simulations and in situ experiments have revealed many insights into the deformation behavior of nanoporous metals, but it remains unclear how to engineer the structure to reduce flow localization. We perform transmission electron microscopy in situ tensile experiments on freestanding nanoporous gold films and observe the morphology evolution of both the interconnected structure and individual ligaments during deformation. Most ligaments fractured through plastic instability after large plastic elongation. We also observed several unexpected results such as instances of strain hardening and nanoscale brittle fracture in individual ligaments. Our observations suggest that highly curved ligaments and a wider distribution of ligament diameters could each contribute to ductility and fracture toughness.

Optimizing the Nonlinear Optical Response of Plasmonic Metasurfaces.

Controlling the nonlinear optical response of nanoscale metamaterials opens new exciting applications such as frequency conversion or flat metal optical elements. To utilize the already well-developed fabrication methods, a systematic design methodology for obtaining high nonlinearities is required. In this paper we consider an optimization-based approach, combining a multiparameter genetic algorithm with three-dimensional finite-difference time domain (FDTD) simulations. We investigate two choices of the optimization function: one which looks for plasmonic resonance enhancements at the frequencies of the process using linear FDTD, and another one, based on nonlinear FDTD, which directly computes the predicted nonlinear response. We optimize a four-wave-mixing process with specific predefined input frequencies in an array of rectangular nanocavities milled in a thin free-standing gold film. Both approaches yield a significant enhancement of the nonlinear signal. Although the direct calculation gives rise to the maximum possible signal, the linear optimization provides the expected triply resonant configuration with almost the same enhancement, while being much easier to implement in practice.

Self-referenced single-shot THz detection.

We demonstrate a self-referencing method to reduce noise in a single-shot terahertz detection scheme. By splitting a single terahertz pulse and using a reflective echelon, both the signal and reference terahertz time-domain waveforms were measured using one laser pulse. Simultaneous acquisition of these waveforms significantly reduces noise originating from shot-to-shot fluctuations. We show that correlation function based referencing, which is not limited to polarization dependent measurements, can achieve a noise floor that is comparable to state-of-the-art polarization-gated balanced detection. Lastly, we extract the DC conductivity of a 30 nm free-standing gold film using a single THz pulse. The measured value of σ0 = 1.3 ± 0.4 × 107 S m-1 is in good agreement with the value measured by four-point probe, indicating the viability of this method for measuring dynamical changes and small signals.

Understanding the extremely low fracture toughness of freestanding gold thin films by in-situ bulge testing in an AFM

The fracture toughness of freestanding gold films with thicknesses between 60nm and 320nm was determined by bulge testing to be around 2MPam1/2. This surprisingly low value confirms the trend also observed for other metals that thin films exhibit only a fraction of the bulk fracture toughness. In order to understand this behavior, the fracture process of freestanding gold films with a crack introduced by focused ion beam (FIB) milling was observed in-situ in an atomic force microscope (AFM). AFM scans of the crack tip region show stable crack growth mainly along grain boundaries. Plastic deformation is localized in a narrow corridor in front of the crack tip. A large plastic zone, as one would typically expect under plane stress, is not observed. Instead, strong local necking is evidenced. We conclude that the spatial confinement of the plastic deformation is the primary reason for the low fracture toughness of metallic thin films.

Growing gold fractal nano-structures and studying changes in their morphology as a function of film growth rate

We investigate the formation of fractal like nano-structures on free standing gold films grown via surfactant mediated thin film growth process. We determine these structures to be confined within the first few monolayers of the thin film. Their chemical composition is identical to that of the Au film, although their density is different from the surrounding film. We observe changes in the morphology of these fractal structures by controlling the film growth rate, which spans across three orders of magnitude. From our study, we quantify the morphological changes in the fractal structure via a roundness parameter and we suggest an empirical relation between the roundness parameter and the growth rate. The study shows an inverse relationship between the roundness parameter and the growth rate and also that the fractal to compact morphological transition is continuous.

Uncertainty Analysis of Melting and Resolidification of Gold Film Irradiated by Nano- to Femtosecond Lasers Using Stochastic Method

A sample-based stochastic model is presented to investigate the effects of uncertainties of various input parameters, including laser fluence, laser pulse duration, thermal conductivity constants for electron, and electron–lattice coupling factor, on solid–liquid phase change of gold film under nano- to femtosecond laser irradiation. Rapid melting and resolidification of a free-standing gold film subject to nano- to femtosecond laser are simulated using a two-temperature model incorporated with the interfacial tracking method. The interfacial velocity and temperature are obtained by solving the energy equation in terms of volumetric enthalpy for control volume (CV). The convergence of variance (COV) is used to characterize the variability of the input parameters, and the interquartile range (IQR) is used to calculate the uncertainty of the output parameters. The IQR analysis shows that the laser fluence and the electron–lattice coupling factor have the strongest influences on the interfacial location, velocity, and temperatures.

自立金属ナノ薄膜のその場電界放射走査型電子顕微鏡観察疲労/クリープ強度試験法の開発

Experimental technique of in situ FESEM fatigue/creep experiments has been developed to investigate the mechanisms of fatigue/creep crack propagation in freestanding metallic nano-films. We developed an in situ FESEM fatigue/creep dual-mode testing machine which was able to conduct the fatigue or creep experiments inside the FESEM chamber. The testing machine has the capability to apply high-frequency cyclic load or constant load under load-control conditions to the freestanding metallic nano-film specimens. In situ FESEM observations of the fatigue crack propagation in 523-nm-thick freestanding copper (Cu) films confirmed that intrusions/extrusions were formed ahead of the fatigue crack tip in the lower stress intensity factor range (ΔK), and the size of the intrusions/extrusions increased as the number of stress cycles increased. The fatigue crack then propagated preferentially through these intrusions/extrusions. In the higher ΔK, the fatigue crack propagated in tensile fracture mode. In addition, in situ FESEM observations of the creep crack propagation in 391-nm-thick freestanding gold (Au) films confirmed that voids were formed ahead of the creep crack tip, and the crack then propagated by coalescence of the voids and the crack. These results indicate that this experimental technique is effective to clarify the mechanisms and mechanics of fatigue/creep crack propagation in freestanding nano-films.

Large-scale growth of sharp gold nano-cones for single-molecule SERS detection

Quasi-periodic Au nano-cone arrays uniformly sprout on centimeter-sized free-standing nanoporous gold (NPG) films via epitaxial plating, and the nano-cones@NPG serve as a high-performance SERS substrate for single molecule detection.

Design and fabrication of a 3D-structured gold film with nanopores for local electric field enhancement in the pore

Three-dimensionally structured gold membrane films with nanopores of defined, periodic geometries are designed and fabricated to provide the spatially localised enhancement of electric fields by manipulation of the plasmons inside nanopores. Square nanopores of different size and orientation relative to the pyramid are considered for films in aqueous and air environments, which allow for control of the position of electric fields within the structure. Designs suitable for use with 780 nm light were created. Here, periodic pyramidal cavities produced by potassium hydroxide etching to the {111} planes of (100) silicon substrates are used as templates for creating a periodic, pyramidal structured, free-standing thin gold film. Consistent with the findings from the theoretical studies, a nano-sized hole of 50 nm square was milled through the gold film at a specific location in the cavity to provide electric field control which can subsequently used for enhancement of fluorescence or Raman scattering of molecules in the nanopore.

Ultrafast electron crystallography of heterogeneous structures: Gold-graphene bilayer and ligand-encapsulated nanogold on graphene

Here, we report studies of structures comprising a nanoscale gold-graphene bilayer and ligand-encapsulated nanogold on graphene multilayers. The observed time scale for the heating dynamics of the gold layer is significantly slower, when compared to previous results on free-standing gold films, and is independent of the level of carrier excitation. A model is proposed which incorporates the local carrier excitation in the gold layer, carrier relaxation in the graphene layer and heating of the gold layer by thermal conduction. When gold is isolated from graphene by ligand encapsulation, the carriers become again localized, consistent with the two phase description.

Localized surface plasmon resonance of nanoporous gold

We report the plasmonic properties of free-standing nanoporous gold (NPG) films with an intricate bicontinuous nanostructure. Two characteristic plasmon bands of NPG have been detected in absorption spectra. One at ∼490 nm, resulting from the resonant absorption of gold films, is independent of nanopore sizes and dielectric surroundings. The other at ∼550–650 nm, arising from the excitation of localized surface plasmon resonance, shows obvious band shift with the nanopore sizes and dielectric indices of surrounding media, suggesting that NPG is a promising candidate as plasmonic sensors for organic and biologic molecule detection. This study also shines light on the underlying mechanisms of surface enhanced spectroscopy of NPG.

Modeling of Ultrafast Phase Change Processes in a Thin Metal Film Irradiated by Femtosecond Laser Pulse Trains

Ultrashort laser pulses can be generated in the form of a pulse train. In this paper, the ultrafast phase change processes of a 1 μm free-standing gold film irradiated by femtosecond laser pulse trains are simulated numerically. A two-temperature model coupled with interface tracking method is developed to describe the ultrafast melting, vaporization, and resolidification processes. To deal with the large span in time scale, variable time steps are adopted. A laser pulse train consists of several pulse bursts with a repetition rate of 0.5–1 MHz. Each pulse burst contains 3–10 pulses with an interval of 50 ps–10 ns. The simulation results show that with such configuration, to achieve the same melting depth, the maximum temperature in the film decreases significantly in comparison to that of a single pulse. Although the total energy depositing on the film will be lifted, more energy will be transferred into the deeper part, instead of accumulating in the subsurface layer. This leads to lower temperature and temperature gradient, which is favorable in laser sintering and laser machining.

Membrane protein biosensing with plasmonic nanopore arrays and pore-spanning lipid membranes.

Integration of solid-state biosensors and lipid bilayer membranes is important for membrane protein research and drug discovery. In these sensors, it is critical that the solid-state sensing material does not have adverse effects on the conformation or functionality of membrane-bound molecules. In this work, pore-spanning lipid membranes are formed over an array of periodic nanopores in free-standing gold films for surface plasmon resonance (SPR) kinetic binding assays. The ability to perform kinetic assays with a transmembrane protein is demonstrated with α-hemolysin (α-HL). The incorporation of α-HL into the membrane followed by specific antibody binding (anti-α-HL) red-shifts the plasmon resonance of the gold nanopore array, which is optically monitored in real time. Subsequent fluorescence imaging reveals that the antibodies primarily bind in nanopore regions, indicating that α-HL incorporation preferentially occurs into areas of pore-spanning lipid membranes.

Electromechanical Characterization of Au Thin Films using Micro-tensile Testing

This paper presents the test technique about measurement of electrical resistance changes of thin films during tensile testing. In this work, we used a real-time digital image correlation strain measurement system coupled with micro-tensile testing unit and voltage/current sourcemeter. This system has the advantage of real time displacement monitoring with a resolution of 50 nm during the micro-tensile testing, with the ability to measure the variation in electrical resistance of the specimen at the same time. We obtained the complete testing data for the stress–strain curve and associated electrical resistance-strain curve for 1 and 2 μm-thick freestanding gold films. Young’s modulus was about 61~69 GPa and 0.2% offset yield strength was about 361~402 MPa. In case of the electrical resistance, rapid change was observed under the elastic regime, while less obvious under the plastic regime. We also conducted finite element analysis, and this result implied that the electrical resistivity would not be constant during micro-tensile testing.

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 E y 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.

Melting and resolidification of gold film irradiated by nano- to femtosecond lasers

Rapid melting and resolidification of a free-standing gold film subject to nano- to femtosecond laser pulses are investigated using the two-temperature model in conjunction with an interfacial tracking method. The interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, in the ultra-fast phase-change process are obtained by considering the interfacial energy balance and nucleation dynamics. A nonlinear electron heat capacity and a temperature-dependent electron–lattice coupling factor for the rapid phase change are taken into account. Effects of laser pulse width and fluence on melting and resolidification are also studied.