Cross-sectional Scanning Electron Microscopy Research Articles

Ion assisted near-complete filling of high aspect ratio trenches for 3-D neutron detectors

A single-step process has been developed for the conformal deposition of boron carbide in high aspect ratio trenches fabricated in an n-type (111) silicon substrate. Plasma enhanced chemical vapor deposition was used to deposit boron carbide from ortho-carborane (OC2B10H12) precursor mixed with helium and argon gas. The depositions were carried out under substrate self-bias conditions to achieve a high fill factor using forward-directed ions. Nearly complete filling of 25 µm deep trenches with high-aspect ratio (> 6:1) via conformal coating of side walls has been confirmed from cross-sectional scanning electron microscopy studies. The results presented in this article establish that the process technology developed for achieving conformal deposition of the neutron stopping layer is suitable for the fabrication of Silicon p-i-n based three-dimensional thermal neutron detectors with high neutron detection efficiency.

DESIGNING AND FABRICATION OF A LOW-COST DIP-COATER FOR RAPID PRODUCTION OF UNIFORM THIN FILMS

In this work, a self-made dip-coater equipment was developed for the fabrication of thin films. The assembly of the apparatus was carried out using simple mechanical and electronic pieces, recycle parts, and spending an inexpensive budget suitably. The production, software design, and features of the device were focused on the sol-gel dip-coating method, which involves gravitational draining and drying processes, as well as continued condensation reactions. The dip-coater was based on the Arduino microcontroller and a step motor. The immersion speed in the solution, the waiting time, and the withdrawal process were typed by a digital control panel, where the optimal range found for speed was 0.1 – 6.0 mm s-1 without vibration interferences. The total fabrication cost of the fabricated dip coater was less than 100 USD and the assembly process was not complicated. Finally, the performance of the dip-coater was evaluated through the deposit of copper oxide and iron oxide films on fluorine-doped tin oxide glass substrates layer by layer. The field emission scanning electron microscopy cross-section images confirm the formation of thin films with thickness in the nanoscale range, with good stability and sameness achieved through the control thickness during the dip-coating method under ambient conditions.

Low-Temperature Self-Aligned-Silicide-Capable Transistor Process Using Solid-Phase-Epitaxy and Lift-Off for Hybrid Substrates

Details of a large-area-compatible sub-600 °C process for the fabrication of high mobility field-effect transistors on single crystalline silicon are presented. It is shown that lift-off and solid-phase-epitaxy (SPE) can be used in conjunction with plasma enhanced chemical vapor deposition (PECVD) to create MOSFET transistors with mobilities comparable to other high-performance techniques. Furthermore, use of an amorphous silicon/silicon oxynitride (SiOxNy) sacrificial bilayer is shown to make the process capable of self-aligned silicidation. The topography of the deposition is studied using cross-sectional scanning electron microscopy (SEM). It is shown that a successful lift-off can be achieved through the proper design of the sacrificial bilayer despite the high degree of step coverage generally exhibited by PECVD films. Cross-sectional transmission electron microscopy (TEM) is used to reveal the microstructure of the epitaxy and silicidation. Current-voltage characteristics of the fabricated transistors are presented and the field effect mobility of electrons on the fabricated devices is reported. The developed process is particularly useful in applications where high mobility transistors need to be built on silicon/nonsilicon hybrid platforms that exclude high processing temperatures.

Enhanced thermo-mechanical, thermal and EMI shielding properties of MWNT/MAgPP/PP nanocomposites prepared by extrusion

In this work, multiwalled nanotube/maleic anhydride–grafted polypropylene/polypropylene (MWNT/MAgPP/PP) nanocomposites are produced using an industrially relevant twin-screw extrusion process. The composites with varying nanotube content in the range of 0.5 wt% to 2 wt% were synthesized and evaluated for their thermo-mechanical (i.e., Vicat softening temperature (VST), heat deflection temperature (HDT)), thermal (i.e., Melting (Tm) temperature, crystallization (Tc) temperature and thermal decomposition (Td) temperature), and electromagnetic interference (EMI) shielding properties. The scanning electron microscopy (SEM) of composite cross-sections reveal uniform dispersion of MWNTs in the polymer matrix. Overall, the MWNT content in the composites was found to influence the composite properties with higher nanotube loadings resulting in a higher increase in the measured properties over the MAgPP/PP sample. The composite containing 2wt% MWNTs exhibited the highest increase of 33.42%, 15.52%, 15.42%, 10.23%, over the pure MAgPP/PP sample in its Tdi, Tdf, VST and HDT values, respectively. The electromagnetic interference (EMI) shielding effectiveness of 2 mm thick sample containing 2wt% MWNTs content was measured to be 5.9 dB in the X-Band frequency range of 8–12.4 GHz, which corresponds to 74.29% attenuation of incident wave power. The results obtained suggest the viability of the masterbatch based extrusion method as a potential strategy for the large-scale production of nanocomposites.

Washcoating of catalytic particulate filters studied by time-resolved X–ray tomography

In this paper we introduce a new application of time-resolved X-ray tomography (XRT) to imaging of the washcoating process in catalytic monolith filters with a honeycomb structure (DPF, GPF). The method is applied to several alumina washcoats with different particle size distributions deposited on a cordierite substrate. The results bring new insights into the drying dynamics of the porous filter wall as well as the on-wall catalyst layer, including the process of layer shrinking and cracking. The XRT results are supplemented by cross-section scanning electron microscopy images and mercury intrusion porosimetry of the final structures. In addition to the statistical analysis of the washcoat distribution, the impact of the washcoat structure on filter pressure loss is determined by experiments.

Lithium plating on the anode for lithium-ion batteries during long-term low temperature cycling

Occurrence of lithium plating on the anode is a severe side reaction in the lithium-ion batteries, which brings cell capacity degradation and reduces the cell safety. This paper focuses on 37Ah commercial lithium-ion batteries and clarifies the evolution of lithium plating during long-term low temperature (−10 °C) cycling. The tested cells are analyzed at different degradation stages, named “fixed-point analysis”, to evaluate the evolution process of lithium plating. It is found that the capacity fade and the internal resistance increase exhibit a decelerated trend with a turning point around 450 cycles. The loss of cyclable lithium caused by lithium plating is deemed to be the main reason behind the battery degradation. Post-mortem analysis including scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) is conduced to reveal the mechanism. The thickness and morphology change of plated lithium, and the plated lithium covered by solid electrolyte interface film, are disclosed by the surface and cross-section SEM images from the fixed-point analysis. XPS analysis further reveals the composition of solid electrolyte interface film, and the plated lithium existing. In a nutshell, lithium plating is inhomogeneous and has high spatial dependence.

(Invited) Interfaces in Solid-State Batteries

The use of solid electrolyte separators has the potential to revolutionise the safety of batteries by removing the need for flammable liquid electrolytes, as well as improve their energy densities by enabling the use of lithium metal anodes.1 However, significant challenges need to be overcome before solid-state batteries (SSBs) are able to compete with the performance of conventional Li-ion batteries, with many of these challenges occurring at the interfaces of the solid electrolyte. To enable the use of a Li metal anode, the challenges of the anode/solid electrolyte interface must be surmounted, the most significant of these being contact loss on battery discharge and dendrite infiltration into the solid electrolyte on charge.2–4 In addition, new hybrid approaches utilising both liquid and solid electrolyte must be explored.A promising approach to integrating a cathode into a SSB is a hybrid cell in which the solid electrolyte and cathode are separated by a conventional liquid electrolyte. This overcomes the problem of creating and maintaining intimate contact between two ceramic phases during cycling. However, in these hybrid cells a new interface, the solid electrolyte/liquid electrolyte interface, is introduced.5,6 In our recent work, we have assessed the stability of this interface and tracked the growth of solid electrolyte interphase (SEI) with time by a variety of techniques including electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and galvanostatic cycling (Figure 1).7 These results lead to conclusions as to the feasibility of such an interface in a SSB. Figure 1: Schematic of a hybrid solid electrolyte/liquid electrolyte cell set over i) an SEM showing SEI growth at the interface and ii) a graph showing increase in resistance with time due to the development of this SEI, obtained by EIS To enable the use of an energy dense Li metal anode, solid electrolyte separators must be impervious to penetration by dendrites. However, dendrite penetration of ceramic electrolytes is observed during charge (i.e. plating the Li metal anode) above a critical rate of current, termed the critical current density on plating (CCP). Better understanding how dendrites grow and the factors that affect critical current densities is of vital importance to enable rational design of materials and to make informed choices for operating conditions of a SSB. Our current research seeks to approach this challenge in two ways; to achieve a better mechanistic understanding of Li ingress into solid electrolytes by studying the process of plating Li metal, and to understand and prevent Li/solid electrolyte contact loss during the stripping process. Results from studies on both dendrite growth and contact loss will be presented. The stripping process has been investigated in Li/Li6PS5Cl/Li and Na/Na-β’’-alumina/Na cells, revealing the formation of voids above a critical current density on stripping (CCS). Studies utilising 3-electrode cells, cross-sectional SEM and operando X-ray CT showed that that above CCS voids form and grow, leading to higher local current densities at the remaining areas of contact, leading to dendrite formation on plating.3,8 For both solid electrolytes, CCS was observed to be < CCP. This means that CCS rather than to CCP limits the rate at which it is possible to cycle a SSB. CCS is found to be dependent on the rate of creep of the metal anode. In this context, realistic operating conditions which can increase this rate of creep during battery operation will be discussed. J. Janek and W. G. Zeier, Nat. Energy, 1, 16141 (2016).M. J. Wang, R. Choudhury, and J. Sakamoto, Joule, 1–14 (2019)J. Kasemchainan et al., Nat. Mater., 18, 1105–1111 (2019)K. Kerman, A. Luntz, V. Viswanathan, Y.-M. Chiang, and Z. Chen, J. Electrochem. Soc., 164, A1731–A1744 (2017)T. Abe, F. Sagane, M. Ohtsuka, Y. Iriyama, and Z. Ogumi, J. Electrochem. Soc., 152, A2151 (2005).M. Weiss et al., Electrochem. Energy Rev., 3, 221–238 (2020)J. Liu et al., Joule, 4, 101–108 (2020).D. Spencer Jolly et al., ACS Appl. Mater. Interfaces, 12, 678-685 (2020) Figure 1

Impact of Electrode Thick Spot Irregularities on Polymer Electrolyte Membrane Fuel Cell Performance

Polymer electrolyte membrane fuel cells (PEMFC) have great potential for power generation in many applications, but to implement them on a larger scale, understanding of quality requirements, tolerances, and controls during manufacturing must improve. As part of this broad challenge, irregularities resulting from the membrane electrode assembly (MEA) production process must be studied to determine if they impact the performance of the cell.1,2 One potential irregularity is a locally increased thickness of the coated or deposited electrode. In order to understand and provide insight into the impact of such irregularities, we intentionally created thick spots in the center of cathode electrodes, either by liquid casting or ultrasonic spraying of the additional catalyst ink. The thick spots fabricated by these two different approaches exhibited different sizes, morphologies, and Pt loading distributions, which were carefully characterized by optical microscopy, X-ray fluorescence spectroscopy (XRF), and scanning electron microscopy (SEM). Edge-protected MEAs were fabricated using both pristine electrodes and electrodes with thick spot irregularities. The effect of these irregularities on MEA structure were investigated by cross-sectional SEM. Electrochemical performance, including electrochemically active surface area (ECSA), H2/O2 polarization curves, mass activity and H2/air polarization curves, was measured to study the impact of the electrode irregularities on catalyst utilization and PEMFC initial performance. NREL’s segmented cell was utilized to investigate the resulting spatial performance of a representative thick spot MEA.The results indicate that thick spot irregularities that are present in the center of the cathode electrode impact initial cell performance although the impact of the thick spots on ECSA and mass activity was minimal. The latter suggests that the number of active Pt sites and the kinetics were not significantly altered. However, the H2/air polarization curves indicated that liquid-cast thick spots and large sprayed thick spots decrease cell performance dramatically at current densities above 0.8 A/cm2. Based on cross-sectional SEM analysis, the likely causes of this performance drop may originate from (i) delamination between the GDL and CL, (ii) the presence of thin spots in the membrane due to deformation associated with the thick spot, (iii) additional compression of the GDL and/or (iv) thick catalyst layer sections inhibiting local mass transport. Spatial performance diagnostics confirmed that the performance loss of MEAs containing liquid-cast thick spots is more pronounced at high current density than at low current density. Moreover, the performance of the area surrounding the liquid-cast thick spot is impacted as well, likely due to the various morphological changes resulting from the irregularity. We expect that the morphological changes resulting from these irregularities could serve as seed points for premature cell failure in long-term PEMFC operation. Future work will focus on the impact of electrode thick spot irregularities on PEMFC performance-over-time and lifetime, for both reinforced and non-reinforced membranes.

Design of Non-Ammoniacal Solution for Electrodeposition of CuO on Cu2o Substrate

Semiconductor materials are attracting increasing attention as photoelectrodes and photovoltaic devices. Cuprous oxide (Cu2O), which has been intensively studied due to its high absorption coefficient, low cost, and low toxicity, is a p-type semiconductor with a bandgap energy of approximately 2.1 eV. However, Cu2O has a narrow solar absorption region due to its high bandgap energy. CuO (bandgap energy of 1.4 eV) has been combined with Cu2O to increase the absorption wavelength range [1]. Thermal oxidation of Cu2O surfaces for forming CuO/Cu2O bi-layer films has been reported [2]. However, this thermal oxidation creates nanopores and voids in the films and degrades the film performance. In this study, an electrochemical formation of CuO on a Cu2O surface is investigated. Previously reported methods of electrochemical CuO formation include an anodic electrodeposition using an ammoniacal aqueous solution [3]. In ammoniacal solutions, however, the Cu2O surface is unstable and can be dissolved as Cu(I)-ammine complexes. In this study, suitable non-ammoniacal ligands, which can be used to stabilize Cu2O during CuO electrodeposition, are examined through thermodynamic calculations, and the electrodeposition behavior of CuO films on Cu2O is experimentally investigated.An appropriate ligand, 2-aminoethanol, i.e., mono-ethanolamine (MEA), was obtained by drawing the pH speciation diagram using thermodynamic data [4, 5]. It is difficult to use MEAs to form complexes with Cu(I) and easy to form with Cu(II), and MEAs are relatively inexpensive and versatile. Figure 1 shows that CuO is stable at a pH region lower than the region where the Cu(II)-MEA complex is dominant, suggesting that it is feasible to achieve CuO electrodeposition using a local pH drop, through, for example, anodic oxygen evolution reaction.In a 0.05 M Cu(II) + 0.125 M MEA aqueous solution without NaOH addition (pH = 8.5), precipitates − probably hydroxide − were formed (Figure 2(b)). In contrast, when the pH was increased to 12.5, a deep blue aqueous solution was obtained without any precipitates (Figure 2(c)). While a set of existing stability constants [5] indicated the presence of CuO or Cu(OH)2 precipitates in the 0.05 M Cu(II) + 0.125 M MEA aqueous solution at a pH value of 12.5, a clear solution was obtained at [Cu(II)] = 0.05 M, [MEA] = 0.125 M, and pH = 12.5. A Cu2O substrate was immersed in this solution for 5 h to confirm the stability of Cu2O in the solution. From the results, almost no weight change was observed, unlike the case of ammoniacal solutions, when nitrogen degassing was carried out for 30 min before performing the immersion test to prevent the oxidation of Cu2O by dissolved oxygen. Potentiostatic electrodeposition of CuO was conducted on +0.45 V vs. Ag/AgCl reference electrode at 313 K. Cu2O/FTO —Cu2O was electrodeposited on fluorine-doped tin oxide (FTO) in advance [2]— and Pt plates were used as both the working electrode (WE) and the counter electrode. Figure 3 shows the X-ray diffraction (XRD) profiles of the Cu2O/FTO WE before and after the electrodeposition. Along with cubic Cu2O, monoclinic CuO was detected. Additionally, the cross-sectional field-emission scanning electron microscopy (FE-SEM) images depicted in Fig. 4 indicate a bi-layer structure. Therefore, we conclude that CuO was stacked, keeping the Cu2O layer intact. Figure 5 shows the visible spectroscopy near-infrared region (vis-NIR) absorbance spectra of the Cu2O and CuO/Cu2O films. The absorption edge of the Cu2O films was 630 nm, while that of the CuO/Cu2O films shifted to 900 nm, demonstrating that the CuO/Cu2O films had a broader absorption wavelength range than pure Cu2O or pure CuO films. In summary, a non-ammoniacal bath is surveyed from the thermodynamics database, and CuO/Cu2O bi-layer films are electrochemically fabricated.

Examination of Conductive Properties of ZrO2 Film Formed By Self-Oxidation Method

By forming an oxide film on the surface of metallic zirconium, ZrO2 works as a corrosion resistant film and prevents the permeation of hydrogen. This material is expected to be utilized as a plasma facing material for fusion reactors, because zirconium and ZrO2 have excellent compatibility at high temperature under the redaction atmosphere. Vapor deposition methods such as electron beam vapor deposition and pulse laser vapor deposition are the main methods of forming oxide films on metallic zirconium surfaces. In this study, the self-oxidation method using water vapor was employed as a film forming method, because a dense film of zirconium oxide can be obtained on the surface of metallic zirconium by the heat treatment at 500 °C in water vapor. Although the self-oxidation method using water vapor is promising, the previous reports have been limited yet. An oxide film is formed on the surface of metal Zr by a self-oxidation method in a water vapor atmosphere, and its conductive properties due to the difference in the thickness of each layer of the film are examined.The fine structure of the preparing film was observed with a field emission Scanning Electron Microscope (SEM). The film thickness was determined as the average film thickness from the cross-sectional SEM image using image processing software Image Processing and Analysis in JAVA (Image J). The O/Zr ratio in the film was analyzed by line analysis of an Energy Dispersive X-ray analyzer (EDX). The Crystallinity and formed phase of the film were identified by Electron Back Scatter Diffraction (EBSD) analysis and "grazing incidence X-ray diffraction". impedance measurement was performed using SP-150 electrochemical measurement system of Bio-Logic Sciences Instruments. It was decided to separate the conductivity of each layer by measuring the oxide film prepared by adjusting the film thickness.In the previous studies, it was revealed from the SEM observation of the cross section of the oxide film that the oxide film was not a single layer but a laminated structure whose O/Zr ratio gradually decreases with increasing of the film depth. Therefore, by changing the X-ray incident angle (θ = 1 ~ 12 °) by XRD analysis, we analyzed the change of the formed phase in the thickness direction and found that each phase is not an amorphous phase but a crystalline phase. The sample formed by steam oxidation was subjected to EDS line analysis in the direction perpendicular to the oxide film cross section, and the O/Zr ratio of each layer was measured. It was observed that the atomic ratios of the three kinds of layer, bottom layer, second layer and top layer were observed to be 0.3-0.6, 0.6, and 1.5 or more, respectively. The samples with the various thickness of oxide film were prepared and the specific electrical resistances of each film were measured with the impedance measurements. The electrical conductive properties of each layer were investigated with the fitting of equation whose parameters are the thickness and the specific electrical resistance of each layer.

(Invited) Advanced Microscopy Techniques for Studying the Durability of Fuel Cells

Durability is still one of the main aspects for which significant progress is expected to allow a larger deployment of fuel cell vehicles. It is therefore crucial to further study the degradation mechanisms of the MEA components and to quantify them in relation to the fuel cell operation conditions. The different electron microscopy techniques (Fig. 1), including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), have shown their essential contribution to offer deep insight into several degradation mechanisms and will likely continue to have an impact on future efforts to improve durability.TEM has been widely used to characterize Pt based PEMFC electro-catalysts and provides information on the size distribution of nanoparticles (NPs), their morphology and crystallographic structure. In addition, microscopes equipped with a Cs probe corrector and associated with electron energy loss spectroscopy (EELS) or X-ray energy dispersion spectroscopy (EDS) allow analyzing the chemical composition of each atomic column. Therefore, TEM characterization has been widely used to develop new high-performance catalysts with well-controlled chemistry/morphology and to study their stability.The main Pt NP degradation mechanism that occurs during fuel cell operation is the electrochemical Ostwald ripening leading to a decrease in the number of NPs smaller than 4 nm associated with an increase in the average NP size (usually from 2-3 nm to 4-5 nm). If the cathode potential becomes higher than 0.9 - 1V, the greater dissolution of the small NPs produces a large amount of Pt ions that migrate toward the membrane where they are reduced by the H2 crossing over to form membrane Pt precipitates. Similar results are observed for Pt alloy NPs but in this case, the electrochemical Ostwald ripening mechanism leads to an increase in the thickness of the Pt shell surrounding the alloy core (typically from 0.6 to 1 nm)1 that affects their activity. Indeed, the negative standard potential of the non-noble elements (Co, Ni...) prevents the reduction of their ions within the MEA and only Pt is redeposited on the larger NPs. Ions of the non-noble elements migrate into the electrolyte and lower its proton conductivity. The membrane contamination can be detected by SEM/EDS analyses performed on MEA cross-section but taking care during sample preparation to prevent altering the ions distribution. This contamination can be limited by avoiding using Pt alloy NPs smaller than 4 nm. .More recently, electron tomography analyses have revealed that when high surface area carbon are used, many Pt NPs are located inside the carbon and that during ageing tests, these interior NPs are less affected by the coarsening mechanism2,3. It is assumed that the confinement of the NPs inside small pores increases their interspacing and thus hampering the electrochemical Ostwald ripening mechanism. The development of carbon support with a well-controlled porosity allowing an optimal NP confinement seems to be a very promising research direction for which electron tomographic analyses will be essential.On the other hand, the carbon support must have a high oxidation resistance to prevent structural collapse of the porous cathode catalyst layer if high potential values are accidentally reached as can locally happen during the start-up/shut-down phases. This severe cathode degradation by carbon corrosion is highlighted on MEA cross-section SEM images. Today FIB-SEM analyses providing 3D images of the porous structure offer the possibility to go further by measuring the evolution of the porosity.Ionomer is also an important MEA component whose degradation dramatically limits PEMFC durability. MEA cross-section SEM images can reveal the physical degradation of the membrane such as thinning. In the electrode, however, the study of ionomer degradation remains challenging because high resolution 3D images are required. For this purpose, electron tomography experiments are still under development.Many degradation studies used to concern MEA aged during accelerated stress test specific for each component. In our laboratory, we are more focused on identifying degradation phenomena that occur in stacks operating in or near real-world conditions. For these studies, we couple electron microscopy analyses with in-situ local measurements using segmented devices to select the regions of interest and to relate local degradation to local performance and conditions1. Acknowledgement : this project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation program under grant agreement No. 779565 (ID-Fast) and previously No. 621216 (SecondAct).

The effect of nitrogen concentration on N-doped diamond-like carbon films prepared by plasma-electrolytic method

Nitrogen-doped diamond-like carbon films were deposited on tool steel substrate througha plasma electrolytic technique. These coatings were fabricated using the hydrocarbons containing nitrogen vapor as a nitrogen and source. The formation of films was proved by Raman spectroscopy, X-ray diffraction patterns, and cross-section scanning electron microscopy. According to the results, by increasing the urea content, the highest amount of Raman intensity ratio for graphite/disordered graphite (ID/IG) was obtained 3.57. On the other hand, N-doped DLC, graphite and nitride phases in X-ray diffraction patterns were observed. The N-doped films had better tribological properties because of significant nitride phases and less surface roughness with an optimum amount of 1.229 μm for the sample with 4 wt. % urea. The friction coefficient of films was obtained as about 0.1–0.25. Subsequently, the sample with 4 wt. % urea indicated the maximum wear resistance by 70 m compared with 10 m resistance for the sample with 0 wt. % urea.

Fabrication and characterization of copper (II) oxide/iron (III) oxide thin film heterostructures for trace arsenic (III) removal in water

In this study, nano-heterostructures based on copper (II) oxide/iron (III) oxide (CuO/α-Fe2O3) thin films were fabricated by a dip-coating technique using aqueous solutions. The heterostructures were deposited on fluorine-doped tin oxide glass substrates varying the CuO film thickness. From a detailed characterization using Fourier transform-infrared and X-ray diffraction the formation of CuO (tenorite)/α-Fe2O3 (hematite) was demonstrated. Atomic force microscopy provided valuable information on the growth of α-Fe2O3 crystals in the heterostructure with a conical-shaped surface. Meanwhile, the field emission scanning electron microscopy cross-section images confirm the formation of well-defined CuO layers under the α-Fe2O3 layers. The optical band gap energies for the heterostructures obtained were estimated from the diffuse reflectance spectra and ranged from 1.41 to 1.51 eV. Photoluminescence analysis revealed an improved separation and faster transfer of photogenerated electrons and holes for the heterostructures. The removal arsenic from an aqueous solution was achieved through the direct adsorption for As(III) and visible light oxidation to As(V). An enhancement of removal efficiency of As(III) for the heterostructures fabricated compared to pristine oxides was obtained.

Electrodeposition of NiSn-rGO Composite Coatings from Deep Eutectic Solvents and Their Physicochemical Characterization

The present work describes, for the first time, the electrodeposition of NiSn alloy/reduced graphene oxide composite coatings (NiSn-rGO) obtained under pulse current electrodeposition conditions from deep eutectic solvents (choline chloride: ethylene glycol eutectic mixtures) containing well-dispersed GO nanosheets. The successful incorporation of the carbon-based material into the metallic matrix has been confirmed by Raman spectroscopy and cross-section scanning electron microscopy (SEM). A decrease in the crystallite size of the coating was evidenced when graphene oxide was added to the electrolyte. Additionally, the topography and the electrical properties of the materials were investigated by atomic force microscopy (AFM). The corrosion behavior in 0.5 M NaCl solution was analyzed through potentiodynamic polarization and electrochemical impedance spectroscopy methods for different immersion periods, up to 336 h, showing a slightly better corrosion performance as compared to pure NiSn alloy.

Tailoring material properties of electrochemically deposited Al film from chloroaluminate ionic liquid for microsystem technology using pulsed deposition

To practically utilize an electrochemically deposited Al film in microsystems, its material properties have to be elucidated, along with the influence of different deposition conditions. In this study, microstructure, electrical, mechanical and thermal properties of electrochemically deposited Al film from a chloroaluminate ionic liquid electrolyte are comprehensively investigated. The nanoindentation hardness and elastic modulus of the film varied between 486–908 MPa and 41–102 GPa, respectively, depending on the deposition condition. The electrical resistivity and the coefficient of thermal expansion varied between 51–160 nΩ m and 2–20 ppm/K, respectively. The results are compared to those of Al thin films deposited using other techniques. The characterized properties were found to be associated to the microstuctures of the deposits, which can be tailored by the deposition conditions. The microstructures of the deposits were comprehensively characterized by cross-sectional scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) spectroscopy. These results are useful as a design guide for the future applications of the film.

Toward Brain-on-a-Chip: Human Induced Pluripotent Stem Cell-Derived Guided Neuronal Networks in Tailor-Made 3D Nanoprinted Microscaffolds.

Brain-on-a-chip (BoC) concepts should consider three-dimensional (3D) scaffolds to mimic the 3D nature of the human brain not accessible by conventional planar cell culturing. Furthermore, the essential key to adequately address drug development for human pathophysiological diseases of the nervous system, such as Parkinson's or Alzheimer's, is to employ human induced pluripotent stem cell (iPSC)-derived neurons instead of neurons from animal models. To address both issues, we present electrophysiologically mature human iPSC-derived neurons cultured in BoC applicable microscaffolds prepared by direct laser writing. 3D nanoprinted tailor-made elevated cavities interconnected by freestanding microchannels were used to create defined neuronal networks-as a proof of concept-with two-dimensional topology. The neuronal outgrowth in these nonplanar structures was investigated, among others, in terms of neurite length, size of continuous networks, and branching behavior using z-stacks prepared by confocal microscopy and cross-sectional scanning electron microscopy images prepared by focused ion beam milling. Functionality of the human iPSC-derived neurons was demonstrated with patch clamp measurements in both current- and voltage-clamp mode. Action potentials and spontaneous excitatory postsynaptic currents-fundamental prerequisites for proper network signaling-prove full integrity of these artificial neuronal networks. Considering the network formation occurring within only a few days and the versatile nature of direct laser writing to create even more complex scaffolds for 3D network topologies, we believe that our study offers additional approaches in human disease research to mimic the complex interconnectivity of the human brain in BoC studies.

On the Structural, Morphological, and Electrical Properties of Carbon Nanowalls Obtained by Plasma-Enhanced Chemical Vapor Deposition

In this study, we investigated the morphological, structural, and electrical properties of carbon nanowall (CNW) structures obtained by plasma-enhanced chemical vapour deposition (PECVD) and underlined the induced effects of argon/nitrogen (Ar/N2) postsynthesis plasma treatment on the electrical behaviour. The top view and cross-section scanning electron microscopy micrographs revealed that the fabricated samples are about 18 μm height, and the edges are less than 10 nm. The Raman analysis showed the presence of the specific peaks of graphene-based materials, i.e., D-band, G-band, D′-band, 2D-band, and D+G-band. The average values of the electrical resistance of fabricated samples were evaluated by current-voltage characteristics acquired at room temperature, in the ranges of 0 V–0.2 V, and an increase was noticed with about 50% after the Ar/N2 postsynthesis plasma treatment compared to pristine samples. Moreover, the Hall measurements proved that the obtained CNW structures had p-type conductivity (Hall coefficient was 0.206 m3/C), and the concentration of charge carriers was 7.8×1019 cm-3, at room temperature.

Layer Configurations for Al-Ge Eutectic Wafer Bonding

Introduction Heterogeneous integration between CMOS and MEMS devices can be realized by using Al-Ge which are compatible materials in both fabrication environments. Al is widely used in the CMOS-industry and does not involve any contamination risk in contrast to other eutectic systems like Au-Si[1]. Si-Ge is also widely used to augment transistor switching speed [2]. The eutectic reaction for Al-Ge takes place around 420oC, which is CMOS-compatible[3], [4].The limitation of the Al-Ge system is the need for removal of the surface oxides. It was previously reported that Ge can be deposited on top of Al using evaporation or sputtering without vacuum-break [4]. This prevents oxide formation on the Al layer. By contrast, GeO2 is easy to remove with a dilute-HF dip [5]. The current work investigates using a single layer or dual layers of Al and Ge for bonding. Depositing single layers is advantageous since it enables patterning by dry etch and avoids the need for a lift-off process. The layers are deposited in separate tools, so oxide formation is expected. The effect of the presence of the oxide layers will be investigated. Method Al and Ge layers are deposited on 200 mm wafers with 500 nm thermal oxide. Thicknesses are matched to eutectic composition. (300 nm Ge and 500 nm Al) The wafers are bonded and then analyzed using scanning acoustic microscopy (SAM) and cross-sectional scanning electron microscopy (X-SEM). Results and discussion The first group of wafers are prepared with a single layer of Al and Ge on either substrate while the second group has a double layer of either Al on top of Ge or Ge on top of Al. All wafers are joined by using forming gas pre-treatment at 390oC, 15 min of solid state inter-diffusion at 410oC followed by bonding for 30 min at 450oC with 25 kN of force. The bonding recipe is illustrated in figure 1. The wafers are analyzed using SAM and X-SEM. The SAM results are shown in figure 2.In figure 2.a the result of the single layer configuration bond is visible. Micro-voids are visible in the center. The outer region has larger voids. The X-SEM image of the bonding interface confirms the presence of small voids, as shown in figure 3. The formation of the eutectic in the single layer configuration is only possible where the two wafers are in contact. Non-uniformity and roughness could be preventing full contact and result in voids.The SAM result of the Al on top of Ge double layer configuration bond is shown in Figure 2.b, the outer region is mostly unbonded. The bonding interface is clearly visible on the X-SEM image, as shown in figure 4. There is no interface between the Al and the Ge layer visible. The native oxide on top of the Al layer could be preventing the two sides from mixing but the Ge oxide layer does not seem to hinder the eutectic formation.The SAM image of the Ge on top of Al configuration bond is shown in figure 2.c. The wafer looks well bonded. X-SEM reveals a triple layer structure. (see figure 5) The bond interface is clearly visible and additionally two thinner interfaces are visible. These could be the remainder of the native Al-oxide. Conclusion Three configurations of Al-Ge eutectic bonds were investigated using SAM and X-SEM. The highest bond uniformity is obtained with Ge being the top layer prior to bonding. The presence of the oxide layers could be preventing the mixing of the eutectic across the bonding interface. In this regard especially the Al-oxide seems problematic.[1] B. Pathangey, L. D. McCarthy, and D. C. Skilbred, “Effect of metal contaminants in pre-gate oxide cleans for sub-100-nm devices,” IEEE Trans. Device Mater. Reliab., vol. 5, no. 4, pp. 631–638, Dec. 2005, doi: 10.1109/TDMR.2005.861161.[2] M. L. Lee, E. A. Fitzgerald, M. T. Bulsara, M. T. Currie, and A. Lochtefeld, “Si, SiGe, and Ge channels for high-mobility metal-oxide-semiconductor field-effect transistors,” J. Appl. Phys., vol. 97, no. 1, p. 011101, Jan. 2005, doi: 10.1063/1.1819976.[3] H. Takeuchi, A. Wung, X. Sun, R. T. Howe, and T.-J. King, “Thermal Budget Limits of Quarter-Micrometer Foundry” IEEE Trans. Electron Devices, vol. 52, no. 9, pp. 2081–2086, Sep. 2005, doi: 10.1109/TED.2005.854287.[4] A. Hilton and D. Temple, “Wafer-Level Vacuum Packaging of Smart Sensors,” Sensors, vol. 16, no. 11, p. 1819, Oct. 2016, doi: 10.3390/s16111819.[5] B. Onsia et al., “The Influence of Wet Treatments on the Germanium Wafer Surface,” Solid State Phenom., vol. 103–104, pp. 27–30, Apr. 2005, doi: 10.4028/www.scientific.net/SSP.103-104.27. Figure 1

Failure Analysis and Defects Characterization of Polymer Electrolyte Membrane Fuel Cells after Relative Humidity Cycling

Water content in the proton exchange membrane fuel cell (PEMFC) changes due to the spatiotemporal interplay of temperature, humidity, and electrochemical reactions. The humidity fluctuations result in membrane swelling and shrinking, and consequently, in-plane tensile stress development that causes mechanical failures [1,2]. In this study, commercial catalyst coated membranes (CCMs) were tested, under a relative humidity (RH) cycling accelerated stress test (AST), between oversaturated and fully-dry states. After the failure of the membrane, a conventional bubble jig test was performed on the membrane electrode assemblies (MEAs) to identify the defective regions. For further understanding of the morphology around the defects and the associated failure mechanism, the identified failure regions were examined using scanning electron microscopy (SEM) cross-section imaging and X-ray computed tomography (XCT) to be referenced with the beginning of life (BOL) sample [3]. The failed regions showed a wide range of defects, from small pinholes to fully-ruptured CCMs, in various forms and patches. There was good conformity between the size of the bubble obtained from the bubble jig test and the morphology of the defects in the images. Further, possible microstructural changes, i.e. porosity and ionomer distribution of the RH cycled catalyst layers were evaluated for the first time by scanning-transmission electron microscopy- energy dispersive spectroscopy (STEM-EDS) analysis along with an in-house developed quantitative method [3,4]. The STEM-EDS mapping revealed a changed ionomer distribution as well as a decrease in the catalyst layer porosity with respect to the BOL sample. The novel analysis approach revealed additional mechanisms of MEA degradation in the RH cycled MEAs, beside the membrane failure modes. To further explain the failure mechanisms, the nuclear magnetic resonance (NMR) was used to investigate the perfluorosulfonic acid (PFSA) chain and structure changes due to the RH cycling and differentiate any mechanical defect from chemical alteration of the chains in the membrane [5]. The NMR spectrum showed irregular broadening of the 19F peaks, which can be correlated to the mechanical changes in the fluorine chains of the membrane. Complementary characterization techniques such as Fourier transform infra-red spectroscopy (FTIR) and Raman spectroscopy analyses results will be shown for the catalyst layer to further explain the failure modes. Consequently, a comprehensive characterization methodology was developed for better understanding of the failure mode from the RH cycling AST.Keywords: RH cycling, Failure Analysis, Polymer Electrolyte Membrane, Fuel Cell Durability.

Temperature Dependence on Tensile Mechanical Properties of Sintered Silver Film.

This paper investigates the influence of temperature on tensile mechanical properties of sintered silver (s-Ag) films with 8–10 μm in thickness for fundamental reliability design of semiconductor systems. The s-Ag film sintered under a pressure of 60 MPa possesses the porosity (p) around 5% determined from cross-sectional scanning electron microscope (SEM) images. The stress–strain (S-S) curves of s-Ag and pure silver (p-Ag) films are obtained using originally designed uniaxial tensile tester at temperatures from 25 °C to 150 °C. The S-S curves of p-Ag indicate ductile behavior irrespective of temperature, whereas those of s-Ag indicate brittle-ductile transition at 120 °C. Compared with p-Ag, s-Ag possesses low Young’s modulus (E) and high ultimate tensile strength (UTS) below 80 °C. The mechanism of brittle-ductile transition is discussed based on fracture surface observation results.