Development and application of a 3D image analysis strategy for focused ion beam - Scanning electron microscopy tomography of porous soft materials.

In recent years, the potential of porous soft materials in various device technologies has increased in importance due to applications in fields, such as wearable electronics, medicine, and transient devices. However, understanding the 3-dimensional architecture of porous soft materials at the microscale remains a challenge. Herein, we present a method to structurally analyze soft materials using Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) tomography. Two materials, polymethyl methacrylate (PMMA) membrane and pine wood veneer were chosen as test-cases. FIB-SEM was successfully used to reconstruct the true topography of these materials in 3D. Structural and physical properties were subsequently deduced from the rendered 3D models. The methodology used segmentation, coupled with optimized thresholding, image processing, and reconstruction protocols. The 3D models generated pore size distribution, pore inter-connectivity, tortuosity, thickness, and curvature data. It was shown that FIB-SEM tomography provides both an informative and visual depiction of structure. To evaluate and validate the FIB-SEM reconstructions, porous properties were generated from the physical property analysis techniques, gas adsorption analysis using Brunauer-Emmett-Teller (BET) surface area analysis and mercury intrusion porosimetry (MIP) analysis. In general, the data obtained from the FIB-SEM reconstructions was well-matched with the physical data. RESEARCH HIGHLIGHTS: Porous specimens of both synthetic and biological nature, a poly(methyl methacrylate) membrane and a pine veneer respectively, are reconstructed via FIB-SEM tomography without resin-embedding. Different thresholding and reconstruction methods are explored whereby shadowing artifacts are present with the aid of free open-source software. Reconstruction data is compared to physical data: MIP, gas adsorption isotherms which are analyzed via BET and Barrett-Joyner-Halenda (BJH) analysis to yield a full picture of the materials.

Comparison of Metal Adhesion Layers for Au Films in Thermoplasmonic Applications.

If thermoplasmonic applications such as heat-assisted magnetic recording are to be commercially viable, it is necessary to optimize both thermal stability and plasmonic performance of the devices involved. In this work, a variety of different adhesion layers were investigated for their ability to reduce dewetting of sputtered 50 nm Au films on SiO2 substrates. Traditional adhesion layer metals Ti and Cr were compared with alternative materials of Al, Ta, and W. Film dewetting was shown to increase when the adhesion material diffuses through the Au layer. An adhesion layer thickness of 0.5 nm resulted in superior thermomechanical stability for all adhesion metals, with an enhancement factor of up to 200× over 5 nm thick analogues. The metals were ranked by their effectiveness in inhibiting dewetting, starting with the most effective, in the order Ta > Ti > W > Cr > Al. Finally, the Au surface-plasmon polariton response was compared for each adhesion layer, and it was found that 0.5 nm adhesion layers produced the best response, with W being the optimal adhesion layer material for plasmonic performance.

Less is More: Improved Thermal Stability and Plasmonic Response in Au Films via the Use of SubNanometer Ti Adhesion Layers.

The use of a metallic adhesion layer is known to increase the thermo-mechanical stability of Au thin films against solid-state dewetting, but in turn results in damping of the plasmonic response, reducing their utility in applications such as heat-assisted magnetic recording (HAMR). In this work, 50 nm Au films with Ti adhesion layers ranging in thickness from 0 to 5 nm were fabricated, and their thermal stability, electrical resistivity, and plasmonic response were measured. Subnanometer adhesion layers are demonstrated to significantly increase the stability of the thin films against dewetting at elevated temperatures (>200 °C), compared to more commonly used adhesion layer thicknesses that are in the range of 2-5 nm. For adhesion layers thicker than 1 nm, the diffusion of excess Ti through Au grain boundaries and subsequent oxidation was determined to result in degradation of the film. This mechanism was confirmed using transmission electron microscopy and X-ray photoelectron spectroscopy on annealed 0.5 and 5 nm adhesion layer samples. The superiority of subnanometer adhesion layers was further demonstrated through measurements of the surface-plasmon polariton resonance; those with thinner adhesion layers possessed both a stronger and spectrally sharper resonance. These results have relevance beyond HAMR to all Ti/Au systems operating at elevated temperatures.

Novel in-situ lamella fabrication technique for in-situ TEM.

In-situ transmission electron microscopy is rapidly emerging as the premier technique for characterising materials in a dynamic state on the atomic scale. The most important aspect of in-situ studies is specimen preparation. Specimens must be electron transparent and representative of the material in its operational state, amongst others. Here, a novel fabrication technique for the facile preparation of lamellae for in-situ transmission electron microscopy experimentation using focused ion beam milling is developed. This method involves the use of rotating microgrippers during the lift-out procedure, as opposed to the traditional micromanipulator needle and platinum weld. Using rotating grippers, and a unique adhesive substance, lamellae are mounted onto a MEMS device for in-situ TEM annealing experiments. We demonstrate how this technique can be used to avoid platinum deposition as well as minimising damage to the MEMS device during the thinning process. Our technique is both a cost effective and readily implementable alternative to the current generation of preparation methods for in-situ liquid, electrical, mechanical and thermal experimentation within the TEM as well as traditional cross-sectional lamella preparation.

A Commercial Conducting Polymer as Both Binder and Conductive Additive for Silicon Nanoparticle-Based Lithium-Ion Battery Negative Electrodes.

This work describes silicon nanoparticle-based lithium-ion battery negative electrodes where multiple nonactive electrode additives (usually carbon black and an inert polymer binder) are replaced with a single conductive binder, in this case, the conducting polymer PEDOT: PSS. While enabling the production of well-mixed slurry-cast electrodes with high silicon content (up to 95 wt %), this combination eliminates the well-known occurrence of capacity losses due to physical separation of the silicon and traditional inorganic conductive additives during repeated lithiation/delithiation processes. Using an in situ secondary doping treatment of the PEDOT: PSS with small quantities of formic acid, electrodes containing 80 wt % SiNPs can be prepared with electrical conductivity as high as 4.2 S/cm. Even at the relatively high areal loading of 1 mg/cm(2), this system demonstrated a first cycle lithiation capacity of 3685 mA·h/g (based on the SiNP mass) and a first cycle efficiency of ∼78%. After 100 repeated cycles at 1 A/g this electrode was still able to store an impressive 1950 mA·h/g normalized to Si mass (∼75% capacity retention), corresponding to 1542 mA·h/g when the capacity is normalized by the total electrode mass. At the maximum electrode thickness studied (∼1.5 mg/cm(2)), a high areal capacity of 3 mA·h/cm(2) was achieved. Importantly, these electrodes are based on commercially available components and are produced by the standard slurry coating methods required for large-scale electrode production. Hence, the results presented here are highly relevant for the realization of commercial LiB negative electrodes that surpass the performance of current graphite-based negative electrode systems.