Au Film Thickness Research Articles

Effect of indirect irradiation on surface morphology of Au film by nanosecond laser

Indirect nanosecond-pulsed laser ablation of Au film of 10 nm–600 nm has been performed to investigate the ablated surface morphology of Au film as compared to direct pulsed laser ablation. This work shows that both the direct and indirect ablations give similar trend of ablation threshold fluence for film thickness equal or less than 170 nm. In this region, the ablation mechanism is driven by melting and vaporization. However, for film thickness greater than 170 nm, indirect ablation exhibits superior film quality with 3.0–3.6 factors less applied laser fluence compared to direct ablation and thus has no heat-affected zone and no recast material. As a result, the ablation mechanism for indirect ablation is driven by photomechanical process. This work highlighted the significant role of irradiation orientation (direct and indirect) as well as the correlation of heat penetration depth with different Au film thickness in influencing the ablation mechanism of Au film on glass substrate.

Controlling Growth High Uniformity Indium Selenide (In2Se3) Nanowires via the Rapid Thermal Annealing Process at Low Temperature

High uniformity Au-catalyzed indium selenide (In2Se3) nanowires are grown with the rapid thermal annealing (RTA) treatment via the vapor-liquid-solid (VLS) mechanism. The diameters of Au-catalyzed In2Se3 nanowires could be controlled with varied thicknesses of Au films, and the uniformity of nanowires is improved via a fast pre-annealing rate, 100 °C/s. Comparing with the slower heating rate, 0.1 °C/s, the average diameters and distributions (standard deviation, SD) of In2Se3 nanowires with and without the RTA process are 97.14 ± 22.95 nm (23.63%) and 119.06 ± 48.75 nm (40.95%), respectively. The in situ annealing TEM is used to study the effect of heating rate on the formation of Au nanoparticles from the as-deposited Au film. The results demonstrate that the average diameters and distributions of Au nanoparticles with and without the RTA process are 19.84 ± 5.96 nm (30.00%) and about 22.06 ± 9.00 nm (40.80%), respectively. It proves that the diameter size, distribution, and uniformity of Au-catalyzed In2Se3 nanowires are reduced and improved via the RTA pre-treated. The systemic study could help to control the size distribution of other nanomaterials through tuning the annealing rate, temperatures of precursor, and growth substrate to control the size distribution of other nanomaterials.Graphical Rapid thermal annealing (RTA) process proved that it can uniform the size distribution of Au nanoparticles, and then it can be used to grow the high uniformity Au-catalyzed In2Se3 nanowires via the vapor-liquid-solid (VLS) mechanism. Comparing with the general growth condition, the heating rate is slow, 0.1 °C/s, and the growth temperature is a relatively high growth temperature, > 650 °C. RTA pre-treated growth substrate can form smaller and uniform Au nanoparticles to react with the In2Se3 vapor and produce the high uniformity In2Se3 nanowires. The in situ annealing TEM is used to realize the effect of heating rate on Au nanoparticle formation from the as-deposited Au film. The byproduct of self-catalyzed In2Se3 nanoplates can be inhibited by lowering the precursors and growth temperatures.

(Invited) Hot-Electron Driven Photocatalysis and Photoexcited Charge Carrier Dynamics in TiO2-Passivated Photocatalysts

We report enhanced photocatalysis for H2 evolution and CO2 reduction using TiO2-passivated InP and GaAs photocathodes.1-3 The TiO2 layer makes the InP semiconductor photochemically stable. This represents a major step forward in photocatalysis, which has typically been limited to metal oxide materials. In addition to making these surfaces stable, the TiO2 film, deposited by atomic layer deposition (ALD), also provides a substantial enhancement in the efficiency of H2 evolution. We find that passivating GaAs with just a few nm of TiO2 produces a shift in the onset potential of H2 evolution by +0.35 V at 1 mA/cm2 and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE). Here, thinner TiO2 films produce a larger enhancement than thicker films, which correlates with the higher density of O-vacancies (i.e., Ti3+ surface states) observed in these thinner films using X-ray photoemission spectroscopy (XPS). While TiO2 films 1-5nm thick produce large enhancements, no enhancement is observed for TiO2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO2films. We also report photocatalytic CO2 reduction with water to produce methanol using TiO2-passivated InP nanopillar photocathodes under visible wavelength illumination.2 Again, the TiO2 passivation layer provides a stable photocatalytic surface and substantial enhancement in the photoconversion efficiency and selectivity through the introduction of O-vacancies associated with the nonstoichiometric growth of TiO2 by ALD. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO2 surface, which serve as catalytically active sites in the CO2 reduction process. PW-DFT shows that CO2 binds stably to these oxygen vacancies and CO2 gains an electron (−0.897e) spontaneously from the TiO2 support. The TiO2 film increases the Faraday efficiency of methanol production by a factor 5.7X under an applied potential of −0.6 V vs NHE, which is 1.3 V below the E o (CO2/CO2 −) = −1.9 eV standard redox potential. In order to further understand the strong dependence of these photocatalysts on TiO2 thickness over the range of 0−15 nm, we performed cross-sectional high resolution transmission electron microscopy (HRTEM) of GaAs/TiO2 heterojunctions.3 Thinner films (1−10 nm) are amorphous and show enhanced catalytic performance with respect to bare GaAs. HRTEM images and electron energy loss spectroscopy (EELS) maps show that the native oxide of GaAs is removed by the TiCl4 ALD precursor, which is corrosive. Thicker TiO2 films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO2films are conducting. We also report measurements of hot electron-driven photocatalytic water splitting using Au films with and without TiO2 coatings.4 In these structures, a thin (3-10nm) film of TiO2is deposited using atomic layer deposition (ALD) on top of a 100nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (~µA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1 µA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film.

Localized surface plasmon sensor based on gold island films using a hetero-core structured optical fiber.

This paper reports the effect of applying an Au island film to the hetero-core structured optical fiber, which is fabricated by annealing thin Au films with thicknesses of 3 and 5 nm and its sensing performance for the refractive index changes. We experimentally observe that novel localized surface plasmon resonance spectra for 1.333 RIU appear in the visible-to-near-infrared region, depending on the shape of Au islands. The absorbance and spectral sensitivity for a given refractive index region of the tested solvents were obtained to be 4.81 AU/RIU and 517 nm/RIU, respectively, in the range of 1.333-1.384 RIU in the case of 5 nm thick Au film annealed at 900°C.

Au cluster formation on a pore containing membrane under the various surface treatments

In this report, the authors will investigate the formation of Au clusters on the nanoscale membrane formed during various surface treatments such as electron beam irradiations, Ga ion focused ion beam (FIB) technique, and thermal treatment. Nanoapertures on the freestanding Au film were fabricated by using FIB technique, and a nanometer scale membrane created in the aperture by various surface treatments. Transmission electron microscopy reveals that Au clusters has formed on the membrane after the sample storage at room temperature for several months. In addition, Au clusters on the carbon-containing membrane were also observed after surface treatments of Ga ion beam etching, and thermal heating of freestanding 40 nm thick Au film at temperatures ranging from 400 to 800 °C. Spinodal decomposition, spinodal dewetting, and coalescence of the Au particles on the carbon-containing membrane were also observed.

Self-Organized Freestanding One-Dimensional Au Nanoparticle Arrays.

One-dimensional Au nanoparticle arrays encapsulated within freestanding SiO2 nanowires are fabricated by thermal oxidation of Au-coated Si nanowires with controlled diameter and surface modulation. The nanoparticle diameter is determined by the Si nanowire diameter and Au film thickness, while the interparticle spacing is independently controlled by the Si nanowire modulation. The optical absorption of randomly oriented Au nanoparticle arrays exhibits a strong plasmonic response at 550 nm. Scanning transmission electron microscopy (STEM)-electron energy loss spectrum (EELS) of nanoparticle arrays confirmed the same plasmonic response and demonstrated uniform optical properties of the Au nanoparticles. The plasmonic response in the STEM-EELS maps is primarily confined around the vicinity of the nanoparticles. On the other hand, examination of the same nanowires by energy-filtered transmission electron microscopy also revealed significant enhancement in the plasmonic excitation in the regions in between the nanoparticles. This versatile route to synthesize one-dimensional Au nanoparticle arrays with independently tailorable nanoparticle diameter and interparticle spacing opens up opportunities to exploit enhanced design flexibility and cost-effectiveness for future plasmonic devices.

Process Optimization of Pressure-Assisted Rapid Ag Sintering Die Attach for 300 °C Applications

Ag sintering is a promising die-attach technique to fabricate digital and analog thick-film modules for 300 °C applications. In this paper, a low-temperature, pressure-assisted rapid sintering process was developed. Process optimization decreased the sintering time to 1 min, which allowed the use of an FC150 thermocompression flip-chip/die bonder for automated die attach instead of a hydraulic press, improving the manufacturability. The porosity of the sintered Ag layer was decreased from 30% (pressureless assembly) to 15% with the application of a low pressure (7.6 MPa) during the 1-min sintering process for large die. The average shear strength for the 3 $\times $ 3-mm2 die was 70.7 MPa, and the 8 $\times8$ -mm2 die could not be sheared off due to the 100-kg force limit of the shear test module. Both Ag and Au metallization (die and substrate) were studied. A new substrate metallization combination was found that was compatible with thick-film Au metallized substrates.

Assembly of gold nanoparticles into aluminum nanobowl array

We mimic unique honeycomb structure as well as its functions of storing honey and pollen to assemble Au nanoparticle pattern on honeycomb-like Al nanobowl array by utilizing solid state dewetting process. Patterned Au nanoarrays of ‘one particle per bowl’ with tunable plasmonic bands ranging from the visible to the near-infrared region are fabricated by finely selecting the initial thickness of Au film, the geometry of Al nanobowl array and the thermal treatment parameters. This work presents a powerful approach to assemble Au nanoparticles into high density nanoarrays with superior spatial resolution, offering highly concentrated electromagnetic fields for plasmonic sensor applications.

Polycrystalline Au Nanomembrane as a Tool for Two-Tone Micro/Nanolithography

A Au nanomembrane is utilized as a hydroxyl ion channel to develop negative and positive-tone micro/nanolithography. Less than 50 nm thick Au films, that is, Au nanomembranes, have nanopores through which ions can pass. Micro- and nanostructures can be fabricated by using an Au nanomembrane-deposited Al layer with KOH solution loading on it. Hydroxyl ions diffuse over Au nanomembranes to react with an underneath Al layer. Depending on the pH of the fabrication process, which is governed by the volume of KOH solution loaded onto an Au nanomembrane-deposited Al layer, protruded aluminum hydroxide structures (positive tone lithography) or micro/nanotunnels (negative tone lithography) are able to be obtained. A flexible THz photonic structure and microfluidic channels have been demonstrated as possible applications of these structures.

(Invited) A Versatile Plasmonic Film for Biosensing and Photocatalytic Applications

Novel Au@TiO2 plasmonic films were fabricated by individually placing Au nanoparticles into TiO2 nanocavity arrays through a sputtering and dewetting process. These discrete Au nanoparticles in TiO2 nanocavities showed strong visible-light absorption due to the plasmonic resonance. Surface-enhanced Raman spectra (SERS) of this plasmonic film showed a high sensityvity to organic molecule at low concentration (down to 0.1 micro molar). Photoelectrochemical studies demonstrated that the developed Au@TiO2 plasmonic films exhibited significantly enhanced catalytic activities towards oxygen reduction reactions with an onset potential of 0.92 V (vs reversible hydrogen electrode), electron transfer number of 3.94, and limiting current density of 5.2 mA cm-2. The deposited Au film thickness remarkably affected the catalytic activities of Au@TiO2, and TiO2 coated with 5 nm Au (Au5@TiO2) exhibited the best catalytic function to reduce oxygen. The plasmon-enhanced reductive activity is attributed to the surface plasmonic resonance of embedded Au nanoparticles in TiO2 nanocavities and suppressed electron recombination.

Investigation on the plasmon Talbot effect of finite-sized periodic arrays of metallic nanoapertures

We present an in-depth and systematical investigation on the plasmon Talbot effect of finite-sized two-dimensional (2D) periodic metallic nanoaperture arrays. The nanoaperture shapes, fill factor, lattice distribution, array size, film thickness, material property and polarization state of the incident light are considered, and the inherent influencing rules are summarized via the three-dimensional (3D) finite-difference time-domain (FDTD) numerical simulations. The nanoaperture shapes, fill factor or array size seems to express a tiny influence on Talbot effect, which shows a good agreement with our previously reported experimental results. Besides, square lattice brings out a much more uniform Talbot pattern than the triangular distribution, and the smaller array period should be taken to estimate the Talbot distance when it comes to a rectangular distribution. Furthermore, the thickness of Au film is suggested to within the range of 50~100 nm, which gives a broadest Talbot contour. It is also found out that the elliptical shape of hotspots is closely related to the linearly polarization state of the light source, showing an asymmetric electromagnetic field. The research contributes to a better understanding of the optical transmission features through periodic metallic nanoaperture arrays, which provides opportunities for the potential applications such as nanofabrication, optoelectronics, and imaging.

Low temperature rapid formation of Au-induced crystalline Ge films using sputtering deposition

We report low temperature (100–170°C) and rapid (10min) formation of crystalline Ge films between Au catalyst film and quartz glass substrate using a radio frequency magnetron sputtering deposition. The formation rate of crystalline Ge films between Au catalyst film and quartz glass substrate is proportional to the deposition rate of Ge film, namely the flux of Ge atoms. To obtain insights on the formation mechanism of crystalline Ge films, we studied dependence of grain size of Au films on annealing temperature and Au film thickness. Crystalline Ge films formed below Au films have random crystalline orientation with in-plane grain size from below 1μm. Small crystalline grain size of Au films is needed to form rapidly Au induced crystalline Ge films.

Evidence of Plasmonic Induced Photocatalytic Hydrogen Production on Pd/TiO2 Upon Deposition on Thin Films of Gold

H2-production from renewables using sunlight is probably the holy grail of modern science and technology. Among the many approaches for increasing reaction rates, by increasing light absorption, plasmonic materials are often invoked. Yet, most plasmonic metals on semiconductors are also good for Schottky barrier formation. In this work, we are presenting evidences of de-coupling the plasmonic from Schottky effects on photoreaction. To conduct this we have systematically changed the under-layer gold film thickness and associated particle size. On top of the thin film layer, we have deposited the exact amount of a prototypical Schottky-based photo-catalyst (Pd/TiO2). We found up to 4 times increase in the H2-production rate at a critical Au film thickness (8 nm-thick). Below this thickness, the plasmonic response is not too strong while above it, the PR decays in favor of the Drude absorption mode. The reaction requires the presence of both UV (to excite the semiconductor) and visible light (to excite Au particles) in order to obtain high hydrogen production, 800 µmol/gCatal.min (probably the highest direct hydrogen (not current) production rate reported on a performing catalyst). The enhancement origin is quantitatively traced to its computed electric field strength (EFS). Adding a dielectric (SiO2) in between the Au thin layer and the catalyst exponentially decreased the reaction rate and EFS, with increasing its thickness. This work indicates the possibility of making an active and stable photo-catalyst from fundamental concepts yet further progress on the structural (technological) front is needed to make a practical catalyst.Graphical abstract

Long-range surface plasmon resonance and surface-enhanced Raman scattering on X-shaped gold plasmonic nanohole arrays.

A multilayered architecture including a thin Au film supporting an X-shaped nanohole array and a thick continuous Au film separated by a Cytop dielectric layer is reported in this work. Long-range surface plasmon resonance (LR-SPR) was generated at the top Au/water interface, which also resulted in a long-range surface-enhanced Raman scattering (LR-SERS) effect. LR-SPR originates from the coupling of surface plasmons (SPs) propagating along the opposite sides of the thin Au film embedded in a symmetric refractive index environment with Cytop (n = 1.34) and water (n = 1.33). The finite-difference time-domain (FDTD) simulation method was used to investigate the optimal dimensions of the substrate by studying the reflectance spectra and electric field profiles. The calculated optimal structure was then fabricated via electron beam lithography, and its LR-SERS performance was demonstrated by detecting rhodamine 6G and 4-mercaptobenzoic acid in the refractive index-matched environment. We believe that this structure as a LR-SPR or LR-SERS substrate can have broad applications in biosensing.

Multiple surface plasmon resonances of square lattice nanohole arrays in Au-SiO2-Au multilayer films

The optical properties and the local electromagnetic field enhancement of a multilayer structure with square lattice nanohole arrays in Au-SiO2-Au multilayer films are numerically studied using finite-difference time domain method. Simulation results demonstrate that the multiple surface plasmons (SP) resonances consist of SP on the air/Au interface, SiO2/Au interfaces (the middle layers), Au/SiO2 interface (the lower layer) and coupling modes on the Au film and Au film. We investigated some of structure parameters that influence the SP resonances of the multilayer nanostructure. Adjustment of the thickness of SiO2 film (H2), the diameter (R) of circular nanoholes, the periods (C) of square lattice and the thickness of Au film (H1) could change the absorption intensity and the SP resonances. The simulation of the electromagnetic field distributions shows that the location of the local electromagnetic field enhancement can specify the different SP resonances patterns. Dipole, quadrupole, and twelve-pole SP resonances modes can be found in the multilayer nanostructure. These studies are important for applications using multiple SP resonances.

Temperature coefficient of resistance and thermal expansion coefficient of 10-nm thick gold films

A methodology to simultaneously estimate the temperature coefficient of resistance (αR) and the thermal expansion coefficient (αT) of metallic films with thickness in the nanometric range in a film/substrate system is discussed. An analytical model which takes into account the thermo-resistivity and the piezo-resistivity effects to estimate αR and αT of metallic films from experimental data obtained at room conditions is proposed. The methodology is first validated by using 100-nm thick Au films which yields values close to the bulk, providing confidence on the reported values. The proposed methodology was used to obtain αR and αT of 10-nm thick Au films deposited by thermal evaporation with three deposition rates onto two substrates. The results show that for 10-nm thick Au films αR presents similar values than previous reports, meanwhile αT is between 5 and 6 times higher than the corresponding bulk value; the arrangement of the atoms during the films deposition yields only minor variation in such a thermal parameter.

Size-Dependent Surface Migration of Au Alloys on Si Nanowires at Different Cooling Rates.

Understanding metal alloy migration in metal-catalyzed nanowires growth is a prerequisite for improving its potential applications in the field of nanodevices. Here, we explored the surface migration of Au alloys in vertically aligned Si nanowires with different cooling rates. We varied the diameter of Au alloys by controlling the thickness of Au film as a catalyst for SiNW growth, and found that the behavior of Au alloys migration is dependent on size of Au alloys. In addition, the size-dependent migration mechanism of Au alloys was investigated at different cooling rates, which is related to different Au-Si eutectics.

Component Attachment with Pressureless Ag Sintering for 300°C Applications

With an increased demand for high-power and high-temperature electronics, Ag sintering paste has been considered a promising Pb-free die-attach material candidate for these applications. Extensive research has been carried out investigating pressure and pressureless Ag sintering for die attach. In this work, passive component (chip resistor) attachment with Ag sintering was explored. Due to termination geometry differences between resistors and dies, different processing procedures and parameters were developed. For PtAu terminated resistors, the mean shear force of as-built samples on thick-film Ag metallized substrates was 90 N, but dropped to 18.6 N after 1,500 h at 300°C. Formation of a dense Ag layer near the PtAu resistor termination and a void region near the thick-film metallization was observed in cross sections after 1,000 h at 300°C. For PdAg terminated resistors with a plated Ni/Au finish, the initial shear force results were low due to Ag diffusion along Au metallization surface. For PdAg terminated resistors with Ag thick-film substrates, the initial shear force was ~60 N and remained in the range of 50–70 N during aging at 300°C for 1,500 h. A new thick-film metallization (Au + Ag) was developed to enable the use of thick-film Au interconnect metallization.

Au thin‐film electrodes based potentiometric CO 2 sensor using Li 3 PO 4 as both the reference material and the solid electrolyte

Thin-film Au electrodes prepared by magnetron sputtering method were used to develop a potentiometric CO2 gas sensor. The thermal evaporated Li3PO4 played the role of electrolyte and sensing material. This design can simplify the fabrication process compared with the conventional solid electrolyte potentiometric CO2 sensor (c-sensor) and improve the performance compared with the thick-film Au electrodes sensor (t-sensor). The designed CO2 sensor (d-sensor) presented good response characteristics at the CO2 concentration range of 250–2500 ppm and the electromotive force (EMF), ΔEMF/dec, response and recovery time were investigated. The EMF values of the sensor were linearly dependent on logarithm of CO2 partial pressure at the working temperatures between 420°C and 500°C. The d-sensor showed a sensitivity of 79.1–93.7 mV/dec at working temperatures range of 420–500°C which was much higher than the sensitivities of c-sensor and the t-sensor. The response and recovery time of the fabricated sensors were 10 and 14 s at working temperature of 500°C, respectively. The surface morphology of Au thin films was considered to increase the three-phase interface area and be good for the reaction gases to diffuse from Au film to lithium phosphate, allowing rapid chemical reaction equilibrium, getting a more stable EMF output.

Growth and morphology control of β-Ga2O3 nanostructures by atmospheric-pressure CVD

Growth experiments of monoclinic gallium oxide (β-Ga2O3) nanostructures were performed by the atmospheric-pressure CVD using metal gallium (Ga) beads and water (H2O) as source materials and a Au film as a catalyst in terms of substrate materials, Au film thickness, growth temperature and growth time. Local structure analysis based on transmission electron microscope (TEM) observations and selective area electron diffraction (SAED) measurements revealed the successful growth of single crystalline β-Ga2O3 nanowires (NWs) through vapor-liquid-solid (VLS) growth mechanism. The average diameter of the NWs was changed in the range from ~60 to ~1500nm under the growth conditions picked up in this paper. The critical temperature for the VLS growth using the 30nm-thick Au catalytic film was experimentally estimated to be ~789°C. The diversity of shapes of nanostructures was probably due to the temperature evolution of Au catalytic particle size, the film growth on the NW sides by vapor-solid (VS) growth mechanism, and the epitaxial relation between the substrate and the individual NW. The photoluminescence intensity of the blue band emission at ~490nm relative to that of the ultraviolet band emission at ~360nm became larger with increasing average diameter, suggesting the increase in the native defects formed near the surface region.