Nanocrystalline Au Films Research Articles

Formation of Au nanostructures on the surfaces of annealed [formula omitted] thin films

One of the promising methods for high-throughput fabrication of nanostructures is the spontaneous dewetting of an as-deposited thin film. The formation process of Au nanostructures (AuNS) on TiO2 thin films consisted of two steps: formation of Au thin films and post-deposition annealing at 500∘C temperature in Ar gas environment for 60 minutes. TiO2 thin films were deposited using reactive magnetron sputtering method on SiO2 and fused quartz substrates and soaked at 500∘C and 900∘C temperatures to understand the influence of crystalline structure and surface morphology of Au and TiO2 thin films on the formation of AuNS. Properties of the deposited thin films were investigated using XRD, SEM, AFM, and RAMAN. The mean diameter (Dmean), mean area of AuNS (Amean), and mean distance between centroids of nearest neighbor AuNS (NNDcentr) increased exponentially when thicker nanocrystalline Au films were annealed. Standard deviation from Dmean and coefficient of variation also increased indicating higher dispersion of AuNS diameter. It was found that the influence of surface roughness increased by annealing thicker nanocrystalline Au thin films. Dmean, Amean, and NNDcentr increased and the density of AuNS (n) decreased (hAu: 10 nm and 15 nm) with decreasing Rq of TiO2 thin films. The peaks of absorbance (500 nm- 750 nm) were broad and not symmetrical indicating multiple localized surface plasmon resonance (LSPR) from various size AuNS. The position of peak shifted to the longer wavelengths when Dmean and NNDcentr was larger.

Achieving very high cycle fatigue performance of Au thin films for flexible electronic applications

The fatigue damage behavior of the nanocrystalline Au films on polyimide substrates was investigated. It was found that the very high-cycle fatigue damage resistance of the Au film was significantly enhanced by at least a factor of ∼2 in supported loading through adding an ultrathin Ti interlayer at the Au film/polyimide interface. Such a better fatigue damage resistance is mainly ascribed to the effective suppression of voiding at the Au film/polyimide interface through modulation of the Au/Ti interface, and thus the propensity of the cyclic strain localization and grain boundary cracking is reduced. The finding may provide a potential strategy for the design of flexible devices with ultra-long fatigue life.

Cumulative shear strain–induced preferential orientation during abnormal grain growth near fatigue crack tips of nanocrystalline Au films

Abstract

Enabling remote quantum emission in 2D semiconductors via porous metallic networks

Here we report how two-dimensional crystal (2DC) overlayers influence the recrystallization of relatively thick metal films and the subsequent synergetic benefits this provides for coupling surface plasmon-polaritons (SPPs) to photon emission in 2D semiconductors. We show that annealing 2DC/Au films on SiO2 results in a reverse epitaxial process where initially nanocrystalline Au films gain texture, crystallographically orient with the 2D crystal overlayer, and form an oriented porous metallic network (OPEN) structure in which the 2DC can suspend above or coat the inside of the metal pores. Both laser excitation and exciton recombination in the 2DC semiconductor launch propagating SPPs in the OPEN film. Energy in-/out- coupling occurs at metal pore sites, alleviating the need for dielectric spacers between the metal and 2DC layer. At low temperatures, single-photon emitters (SPEs) are present across an OPEN-WSe2 film, and we demonstrate remote SPP-mediated excitation of SPEs at a distance of 17 μm.

Deformation-mechanism dependent stretchability of nanocrystalline gold films on flexible substrates

Abstract

Creep behavior of nanocrystalline Au films as a function of temperature

Freestanding nanocrystalline Au films, subjected to nominally elastic loads at 25–110 °C, demonstrated high primary (10−7–10−4 s−1) and steady-state creep rates (10−8–10−5 s−1). The deformation mechanisms for creep were strongly temperature dependent: grain boundary sliding-based creep dominated at room temperature and 50 °C, while the contribution of dislocation-mediated creep increased at 80 and 110 °C. The effect of applied stress on primary and steady-state creep strain at different temperatures was captured well by a non-linear model that was based on the kinetics of thermal activation. Multi-cycle creep experiments showed that at room temperature virtually all the primary strain accumulated during each forward creep cycle was recovered upon complete unloading. As the contribution of dislocation-mediated creep increased with temperature, the ratio of strain recovery to primary strain accumulated during each cycle was reduced due to the accumulation of plastic strain at higher temperatures. Notably, at all temperatures, the steady-state creep rate decreased after the first creep cycle. Moreover, the entire creep response remained virtually unchanged in all subsequent cycles, which implies that the first creep cycle resulted in mechanical annealing. This conclusion was further supported by calculations of the activation entropy: A reduction in its magnitude between the first and all subsequent creep cycles at all temperatures pointed out to mechanical annealing of initial material defects during the first loading cycle. The negative values of the calculated activation entropy indicated that entropy changes due to annihilation of defects-dominated entropy changes associated with the generation of new defects. Finally, the activation entropy for steady-state creep was temperature insensitive, but increased with stress, which is consistent with an increase in defect generation at higher stresses.

Quantifying and observing viscoplasticity at the nanoscale: highly localized deformation mechanisms in ultrathin nanocrystalline gold films.

This study unveils the stress relaxation transient deformation mechanisms in 100 nm-thick, nanocrystalline Au films thanks to a robust quantitative in situ TEM MEMS nanomechanical testing approach to quantify stress relaxation and to perform in situ observations of time-dependent deformation in ultrathin nanocrystalline films. The relaxation is characterized by a decrease in plastic strain rate of more than one order of magnitude over the first ∼30 minutes (from 10(-4) to less than 10(-5) s(-1)). For longer relaxation experiments, the plastic strain rate decreases down to 10(-7) s(-1) after several hours. The power-law exponent n, relating plastic strain rate and stress, continuously decreases from initial large values (n from 6 to 14 at t = 0) down to low values (n ∼ 1-2) after several hours. In situ TEM observations reveal that the relaxation behavior is initially accommodated by highly localized, sustained, intergranular and transgranular dislocation motion. Over time, the dislocation sources become less operative or exhausted, leading to a transition to grain-boundary-diffusion based mechanisms. The results also highlight a promising technique for nanoscale characterization of time-dependent deformation.

Mechanisms of solid-state dewetting of thin Au films in different annealing atmospheres

We studied the initial stages of solid-state dewetting of thin nanocrystalline Au films deposited on sapphire substrates during annealing in air and in the flow of forming gas. The interrupted annealing technique was employed to study the evolution of the size and morphology of individual holes in the film. The holes exhibited comparable expansion rates and similar topographies comprising an elevated rim in both annealing atmospheres. In particular, we observed a terraced topography of the hole rim with sharp steps associated with individual grain boundaries. The holes in the film annealed in air exhibited more branched, tortuous lateral morphology than their counterparts in the film annealed in forming gas. We interpreted these experimental finding in terms of the effect of annealing atmosphere on the surface anisotropy of Au. We developed dewetting models which consider Au self-diffusion along the Au–sapphire interface as a factor controlling the kinetics of the dewetting process. The predictions of our models are in a good agreement with experimental findings.

The effect of solute segregation on strain localization in nanocrystalline thin films: Dislocation glide vs. grain-boundary mediated plasticity

Deformation mechanisms of nanocrystalline Au and AuCu thin films on compliant substrates were investigated by synchrotron-based in situ tensile testing and Automated Crystal Orientation Mapping using transmission electron microscopy. The results demonstrate that intragranular dislocation plasticity, inferred from evolution of deformation texture, is responsible for the formation of periodic and ordered shear bands in AuCu films. In contrast, pure Au films deform homogeneously without shear band formation and without evolution of deformation texture. Cu solutes are deemed to pin grain boundaries thereby enforce dislocation glide, while in pure Au, plasticity is carried by grain boundary shear and grain boundary migration.

A thermally activated model for room temperature creep in nanocrystalline Au films at intermediate stresses

Uniaxial tension creep experiments conducted on free-standing nanocrystalline Au films with 40 nm grain size showed high initial primary creep rates of 3.3 × 10−8–2.7 × 10−7 s−1, reaching steady-state creep rates of 5.5 × 10−9–1.1 × 10−8 s−1 after 5–6 h. The isochronous creep curves pointed to the need for a nonlinear creep model based on the kinetics of thermal activation, which captured well the effect of applied stress amplitude. The implications of this model on the long term stability of nanocrystalline films were explored numerically for periodic stress profiles.

Grain growth inhibition in thin nanocrystalline Au films by grain boundary diffusion and oxidation of Ti

We studied grain growth in thin nanocrystalline Au films deposited on a sapphire substrate with and without an ultrathin Ti underlayer (adhesion promoter). The samples were annealed at 200°C for 2h in air. The reference thin Au film without a Ti underlayer exhibited significant grain growth during annealing, whereas no changes in microstructure of the Au layer were observed in the Au/Ti bilayers. This stabilization of the microstructure of the Au layer was attributed to thermal grain boundary grooves on the Au surface filled with Ti oxide. The grooves exhibited an elongated morphology characterized by a low apparent dihedral angle value, atypical for thermal grain boundary grooves in pure metals. We demonstrated that grooves with this morphology are very efficient in pinning grain boundary motion. We also developed a quantitative model of grain boundary grooving coupled with grain boundary interdiffusion in thin bilayer films. The model predicted the formation of long, narrow grooves at the grain boundaries, which are very efficient in suppressing grain growth in nanocrystalline thin films.

Grain boundary structure dependent fracture in nanocrystalline Au films

Nanocrystalline Au films were in situ strained in a high resolution transmission electron microscope, which demonstrated that the diffusion-assisted intergranular fracture was the dominant failure mode. Grain orientation with respect to grain boundaries (GBs) imposes important effect on the crack propagation and blunting. The low surface energy and high diffusion mobility of {111} planes lead to a notch-like crack. The stress concentration at the tip may help breaking {111} planes layer by layer and thus advance the crack. Cracks can be diverted from the preset path by GBs and grow into the grain interior, which has never been revealed by other experiments and molecular dynamics simulations.

Strain rate sensitivity of nanocrystalline Au films at room temperature

The effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85 and 1.76 μm free-standing microscale tension specimens tested over eight decades of strain rate, between 6 × 10 −6 and 20 s −1. The elastic modulus was independent of the strain rate, 66 ± 4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 575–895 MPa and 675–940 MPa, respectively, with the yield strength reaching the tensile strength at strain rates faster than 10 −1 s −1. The activation volumes for the two film thicknesses were 4.5 and 8.1 b 3, at strain rates smaller than 10 −4 s −1 and 12.5 and 14.6 b 3 at strain rates higher than 10 −4 s −1, while the strain rate sensitivity factor and the ultimate tensile strain increased below 10 −4 s −1. The latter trends indicated that the strain rate regime 10 −5–10 −4 s −1 is pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5–6 h) and significant primary creep with initial strain rate of the order of 10 −7 s −1.

Atomic-Scale-Deformation-Dynamics (ASDS) of Nanowires and Nanofilms

Nanowires and nanofilms are fundamental building blocks of micro and nano-electronics for both of bottom-up and top-down technologies. Monitoring and recording the mechanical property dynamics at atomic scale are important to understand the atomic mechanism of new and surprising nano-phenomena and design new applications. Through years’ endeavors, we developed tensile and/or bending in-situ atomic-lattice resolution electron microscopy methods and equipments for nanowires and successfully conducted atomic-lattice resolution mechanical tests on individual nano-objects. With this, we observed the brittle materials SiC and Si nanowires (NWs) become highly ductile at room temperature. The crystalline structural evolution processes corresponding to the occurrence of unusual large strain plasticity includes the dislocation initiation, dislocation accumulation and amorphorization as well as the necking of the one dimensional nanowires were fully recorded at atomic scale and in real time. We also expand the experimental methods and equipments to two-dimensional nanofilms. An example of tensile experiment on nano-crystalline Au films is presented. The deformation mechanisms of nano-crystalline gold films were observed at the atomic scale and real-time. At the mean time, an atomic scale the crack blunting behavior was captured and the plastic deformation mechanism of the single nano-crystalline was revealed.

Molecular Dynamics Simulation of Laser Melting of Nanocrystalline Au

The mechanisms and kinetics of short pulse laser melting of single crystal and nanocrystalline Au films are investigated on the basis of the results of simulations performed with a model combining the molecular dynamics method with a continuum-level description of the laser excitation and subsequent relaxation of the conduction band electrons. A description of the thermophysical properties of Au that accounts for the contribution of the thermal excitation of d band electrons is incorporated into the model and is found to play a major role in defining the kinetics of the melting process. The effect of nanocrystalline structure on the melting process is investigated for a broad range of laser fluences. At high fluences, the grain boundary melting in nanocrystalline films results in a moderate decrease of the size of the crystalline grains at the initial stage of the laser heating and is followed by a rapid (within several picoseconds) collapse of the crystal structure in the remaining crystalline parts of t...

Surface-enhanced Raman scattering of molecules adsorbed on nanocrystalline Au and Ag films formed at the organic–aqueous interface

Surface-enhanced Raman scattering (SERS) of pyridine adsorbed on ultrathin nanocrystalline Au and Ag films generated at the liquid–liquid interface has been investigated. The shifts and intensification of bands formed with these films comprising metal nanoparticles are comparable to those found with other types of Au and Ag substrates. SERS of rhodamine 6G adsorbed on Ag films has also been studied. The results demonstrate that nanocrystalline metal films prepared by the simple method involving the organic–aqueous interface can be used effectively for SERS investigations.

Strain rate effects on the mechanical behavior of nanocrystalline Au films

The effect of fabrication, film thickness, and strain rate on the mechanical behavior of Au films with 100 nm (evaporated gold) and 200 nm (electroplated gold) average grain sizes was investigated. Uniaxial tension was imposed at 10 − 3 –10 − 6 s − 1 strain rates on evaporated 0.5 μm and 0.65 μm thick Au specimens, and at 10 − 2 –10 − 5 s − 1 on electroplated 2.8 μm thick Au specimens. Strain rates between 10 − 3 and 10 − 5 s − 1 had a marked impact on the ultimate strain of evaporated films and less significant effect on their yield and saturation stress. The ductility increased with decreasing strain rate and it varied between 2–4.5% for 500–650 nm thick films and 3.4–10.6% for 2.8 μm thick films. When compared at the same strain rate, the thick electroplated films were more ductile than the thin evaporated films, but their yield and saturation stresses were lower, possibly due to their larger grain size. Qualitatively, the stress–strain behavior was consistent at all rates except at the slowest that resulted in significantly different trends. A marked decrease of the maximum strength, effective Young's modulus, and yield strength occurred at 10 − 6 s − 1 for thin, and at 10 − 5 s − 1 for thick films, while for 500 nm thin films multiple stress localizations per stress–strain curve were recorded. Because of temperature, applied stress, and grain size considerations this behavior was attributed to dislocation creep taking place at a strain rate comparable to the applied strain rate.

On the development of the <111> fiber texture in nanocrystalline gold during growth and annealing

ABSTRACTThe evolution during growth and subsequent annealing of the <111> fiber texture in magnetron-sputtered nanocrystalline Au films has been studied experimentally using X-ray diffraction with synchrotron radiation. To quantitatively investigate this fiber texture, grain orientation distributions were recorded in situ during growth and during subsequent annealing using Bragg-Brentano geometry. The (111) diffraction intensity was measured as a function of the sample tilt χ, the tilt axis lying at the intersection of the film surface and the scattering plane. As a quantitative measure of the texture, we used the width of the orientation distributions. The grain-orientation distributions narrowed during annealing. The activation energy for the process behind this texture change was found to be 0.64 ± 0.05 eV, close to the activation energy for grain boundary self-diffusion in nanocrystalline Au. This and the narrowing of the grain orientation distributions led us to suggest that the observed changes in texture originated from grain rotations and not from grain growth. Grain growth did not take place at the lower temperatures, where changes in orientation distributions were observed.