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In Situ Fabrication of Freestanding Single-Atom-Thick 2D Metal/Metallene and 2D Metal/ Metallene Oxide Membranes: Recent Developments
In recent years, two-dimensional (2D) materials have attracted a lot of research interest as they exhibit several fascinating properties. However, outside of 2D materials derived from van der Waals layered bulk materials only a few other such materials are realized, and it remains difficult to confirm their 2D freestanding structure. Despite that, many metals are predicted to exist as 2D systems. In this review, the authors summarize the recent progress made in the synthesis and characterization of these 2D metals, so called metallenes, and their oxide forms, metallene oxides as free standing 2D structures formed in situ through the use of transmission electron microscopy (TEM) and scanning TEM (STEM) to synthesize these materials. Two primary approaches for forming freestanding monoatomic metallic membranes are identified. In the first, graphene pores as a means to suspend the metallene or metallene oxide and in the second, electron-beam sputtering for the selective etching of metal alloys or thick complex initial materials is employed to obtain freestanding single-atom-thick 2D metal. The data show a growing number of 2D metals/metallenes and 2D metal/ metallene oxides having been confirmed and point to a bright future for further discoveries of these 2D materials.
Erratum: Exploring the 3D structure and defects of a self-assembled gold mesocrystal by coherent X-ray diffraction imaging (Nanoscale (2021) DOI: 10.1039/D1NR01806J)
Correction for ‘Exploring the 3D structure and defects of a self-assembled gold mesocrystal by coherent X-ray diffraction imaging’ by Jerome Carnis et al., Nanoscale, 2021, DOI: 10.1039/D1NR01806J.
Study of TiAl thin films on piezoelectric CTGS substrates as an alternative metallization system for high-temperature SAW devices
Ti/Al multilayer films with a total thickness of 200 nm were deposited on the high-temperature (HT) stable piezoelectric Ca3TaGa3Si2O14 (CTGS) as well as on thermally oxidized Si (SiO2/Si) reference substrates. The Ti–Al films were characterized regarding their suitability as an alternative metallization for electrodes in HT surface acoustic wave devices. These films provide the advantage of significantly lower costs and in addition also a significantly lower density as compared to Pt, which allows a greater flexibility in device design. To realize a thermal stability of the films, AlNO cover as well as barrier layers at the interface to the substrate were applied. The samples were annealed for 10 h at up to 800 °C in high vacuum (HV) and at 600 °C in air and analyzed regarding the γ-TiAl phase formation, film morphology, and possible degradation. The Ti/Al films were prepared either by magnetron sputtering or by e-beam evaporation and the different behavior arising from the different deposition method was analyzed and discussed. For the evaporated Ti/Al films, AlNO barriers with a lower O content were used to evaluate the influence of the composition of the AlNO on the HT stability. The sputter-deposited Ti/Al films showed an improved γ-TiAl phase formation and HT stability (on SiO2/Si up to 800 °C in HV and 600 °C in air, on CTGS with a slight oxidation after annealing at 800 °C in HV) as compared to the evaporated samples, which were only stable up to 600 °C in HV and in air.
Long-term high-temperature behavior of Ti–Al based electrodes for surface acoustic wave devices
The long-term high-temperature behavior of Ti–Al based electrodes for the application in surface acoustic wave (SAW) sensor devices was analyzed. The electrodes were obtained by e-beam evaporation of Ti/Al multilayers on the high-temperature stable piezoelectric Ca3TaGa3Si2O14 (CTGS) substrates and structuring via the lift-off process. AlNO (25 at.% Al; 60 at.% N and 15 at.% O) cover and barrier layers were applied as protection against oxidation from the surrounding atmosphere and to prohibit a chemical reaction with the substrate. The samples were annealed at temperatures up to 600 °C in air for a duration of up to 192 h. Scanning and transmission electron microscopy were used to evaluate the morphology and degradation of the electrodes as well as of the extended contact pads. The results revealed that the Ti–Al based electrodes remained unoxidized after annealing for 192 h at 400 and 500 °C and for 24 h at 600 °C. After the heat treatment for 192 h at 600 °C, a strong oxidation of the structured electrodes occurred, which was less pronounced within the pads. In summary, the investigation showed that Ti–Al based SAW devices are a cost efficient alternative for long-term applications up to at least 500 °C and short- and medium-term applications up to 600 °C.
Beyond Janus Geometry: Characterization of Flow Fields around Nonspherical Photocatalytic Microswimmers
Catalytic microswimmers that move by a phoretic mechanism in response to a self-induced chemical gradient are often obtained by the design of spherical janus microparticles, which suffer from multi-step fabrication and low yields. Approaches that circumvent laborious multi-step fabrication include the exploitation of the possibility of nonuniform catalytic activity along the surface of irregular particle shapes, local excitation or intrinsic asymmetry. Unfortunately, the effects on the generation of motion remain poorly understood. In this work, single crystalline BiVO4 microswimmers are presented that rely on a strict inherent asymmetry of charge-carrier distribution under illumination. The origin of the asymmetrical flow pattern is elucidated because of the high spatial resolution of measured flow fields around pinned BiVO4 colloids. As a result the flow from oxidative to reductive particle sides is confirmed. Distribution of oxidation and reduction reactions suggests a dominant self-electrophoretic motion mechanism with a source quadrupole as the origin of the induced flows. It is shown that the symmetry of the flow fields is broken by self-shadowing of the particles and synthetic surface defects that impact the photocatalytic activity of the microswimmers. The results demonstrate the complexity of symmetry breaking in nonspherical microswimmers and emphasize the role of self-shadowing for photocatalytic microswimmers. The findings are leading the way toward understanding of propulsion mechanisms of phoretic colloids of various shapes.
Surface Modified β-Ti-18Mo-6Nb-5Ta (wt%) Alloy for Bone Implant Applications: Composite Characterization and Cytocompatibility Assessment
Commercially available titanium alloys such as Ti-6Al-4V are established in clinical use as load-bearing bone implant materials. However, concerns about the toxic effects of vanadium and aluminum have prompted the development of Al- and V-free β-Ti alloys. Herein, a new alloy composed of non-toxic elements, namely Ti-18Mo-6Nb-5Ta (wt%), has been fabricated by arc melting. The resulting single β-phase alloy shows improved mechanical properties (Young’s modulus and hardness) and similar corrosion behavior in simulated body fluid when compared with commercial Ti-6Al-4V. To increase the cell proliferation capability of the new biomaterial, the surface of Ti-18Mo-6Nb-5Ta was modified by electrodepositing calcium phosphate (CaP) ceramic layers. Coatings with a Ca/P ratio of 1.47 were obtained at pulse current densities, −jc, of 1.8–8.2 mA/cm2, followed by 48 h of NaOH post-treatment. The thickness of the coatings has been measured by scanning electron microscopy from an ion beam cut, resulting in an average thickness of about 5 μm. Finally, cytocompatibility and cell adhesion have been evaluated using the osteosarcoma cell line Saos-2, demonstrating good biocompatibility and enhanced cell proliferation on the CaP-modified Ti-18Mo-6Nb-5Ta material compared with the bare alloy, even outperforming their CaP-modified Ti-6-Al-4V counterparts.
Aluminum based high temperature thin film electrode system for wireless sensors
Self-sustained, wireless high-temperature stable sensors are developed, which are based on an aluminum alloy as the electrode metallization. Due to its cost-effectiveness accompanied by a high-temperature stability, this alloy substitutes and outperforms the commonly applied expensive Pt- and Ir-based metals. For the first time, a comprehensive structural, electrical and high-frequency characterization of these surface acoustic wave (SAW) sensors is shown. They are based on Catangasite (Ca3TaGa3Si2O14, CTGS) in combination with properly structured cover and barrier layers for the metallization. The frequency characteristics is determined up to 700 °C by ex situ and in situ methods. In addition, the morphology of the AlRu electrodes is analyzed after the thermal loadings and the temperature dependent sheet resistance is measured. The results reveal a reproducible and linear correlation between the applied temperature and the sheet resistance as well as the resonant frequency. In addition, hardly any degradation of the electrodes is detected after the thermal loadings. The observed high-temperature stability of the devices up to at least 700 °C demonstrates the large potential of the AlRu based SAW sensors as a cost-efficient alternative to expensive noble metal based sensors in industrial applications for the support of energy efficient operation.
Stress and Microstructure Evolution in Mo Thin Films without or with Cover Layers during Thermal-Cycling
The intrinsic stress behavior and microstructure evolution of Molybdenum thin films were investigated to evaluate their applicability as a metallization in high temperature microelectronic devices. For this purpose, 100 nm thick Mo films were sputter-deposited without or with an AlN or SiO2 cover layer on thermally oxidized Si substrates. The samples were subjected to thermal cycling up to 900 °C in ultrahigh vacuum; meanwhile, the in-situ stress behavior was monitored by a laser based Multi-beam Optical Sensor (MOS) system. After preannealing at 900 °C for 24 h, the uncovered films showed a high residual stress at room temperature and a plastic behavior at high temperatures, while the covered Mo films showed an almost entirely elastic deformation during the thermal cycling between room temperature and 900 °C with hardly any plastic deformation, and a constant stress value during isothermal annealing without a notable creep. Furthermore, after thermal cycling, the Mo films without as well as with a cover layer showed low electrical resistivity (≤10 μΩ·cm).
Tailoring the stoichiometry of C3N4 nanosheets under electron beam irradiation
Two-dimensional polymeric graphitic carbon nitride (g-C3N4) is a low-cost material with versatile properties that can be enhanced by the introduction of dopant atoms and by changing the degree of polymerization/stoichiometry, which offers significant benefits for numerous applications. Herein, we investigate the stability of g-C3N4 under electron beam irradiation inside a transmission electron microscope operating at different electron acceleration voltages. Our findings indicate that the degradation of g-C3N4 occurs with N species preferentially removed over C species. However, the precise nitrogen group from which N is removed from g-C3N4 (C–N–C, [double bond, length as m-dash]NH or –NH2) is unclear. Moreover, the rate of degradation increases with decreasing electron acceleration voltage, suggesting that inelastic scattering events (radiolysis) dominate over elastic events (knock-on damage). The rate of degradation by removing N atoms is also sensitive to the current density. Hence, we demonstrate that both the electron acceleration voltage and the current density are parameters with which one can use to control the stoichiometry. Moreover, as N species were preferentially removed, the d-spacing of the carbon nitride structure increased. These findings provide a deeper understanding of g-C3N4.
Progress and challenges in using sustainable carbon anodes in rechargeable metal-ion batteries
Rechargeable lithium-ion batteries (LIBs) are one of the most promising alternatives to effectively bypass fossil fuels. However, long-term energy application of LIBs could be restricted in the future due to the increased production cost of LIB arising from the shortage and inaccessibility of Li in the Earth's crust. Na or K have been considered as substitutes for Li but in spite of their natural abundance, they suffer from low gravimetric/volumetric energy density. An alternative to increase the efficiency of sodium-ion battery (SIBs) and potassium-ion battery (KIBs) is to focus on finding the high‐performing negative electrode, the anode. The large volume changes of alloying and conversion type anodes for KIBs and SIBs make hard carbons to a better option on this regard than usual graphitic carbons, but a key obstacle is the reliance on unsustainable sources. Thus, biomass-derived carbon could offer a promising alternative, and it has indeed been in the focus of much recent work. This review highlights the recent advances in using carbon extracted from various biomass sources in rechargeable Li-, Na-, and K-ion batteries. Maximizing the energy and power densities as well as the lifetime of carbon anodes require an exploration of the right balance between carbon structures, pore morphology, chemical composition and alkali metal-ion storage. Thus, in this review, first, we take stock of key challenges and opportunities to extract carbon from various plants structural components and identify the extracted carbon structure compared to graphite-like structure. Then, we provide an overview on morphological and structural modification of the extracted carbons. Finally, we show how the physicochemical properties, structural alignment and morphological variation of the biomass-derived carbon can affect the storage mechanism and electrochemical performance. The extensive overview of this topic provided here is expected to stimulate further work on environmentally friendly battery design and towards the optimization of the battery performance. Electrode materials in alkali-metal-ion batteries that are based on biomass-derived carbon may allow not only a technical breakthrough, but also an ethically and socially acceptable product.