in-situ-transmission-electron-microscopy-with-biasing-fabrication

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different ...

RMIT University

Gd-based nanomaterials offer interesting magnetic properties and have been heavily investigated for magnetic resonance imaging. The applicability of these materials beyond biomedical imaging remains limited. The current study explores the applicability of these rare-earth nanomaterials as nanozyme-mediated catalysts for colorimetric sensing of l-cysteine, an amino acid of high biomedical relevance. We show a facile solution-based strategy to synthesize two Gd-based nanomaterials viz. Gd(OH) and GdO nanorods. We further establish the catalytic peroxidase-mimic nanozyme activity of these Gd(OH) and GdO nanorods. This catalytic activity was suppressed specifically in the presence of l-cysteine that allowed us to develop a colorimetric sensor to detect this biologically relevant molecule among various other contaminants. This suppression, which could either be caused due to catalyst poisoning or enzyme inhibition, prompted extensive investigation of the kinetics of this catalytic inhibition in the presence of cysteine. This revealed a competitive inhibition process, a mechanism akin to those observed in natural enzymes, bringing nanozymes a step closer to the biological systems.

Competitive Inhibition of the Enzyme-Mimic Activity of Gd-Based Nanorods toward Highly Specific Colorimetric Sensing of l-Cysteine.

Transparent non-volatile memory devices are desirable for realizing visually-clear integrated systems for information storage. Optical transparency provides advantages in applications such as smart glass electronic devices and wearable electronics. However, achieving high transparency limits the choice of active layers as well as the electrodes; thereby, constraining device processing and performance. Here, we demonstrate bilayer transparent memory cells using room temperature deposited amorphous strontium titanate as the functional material and indium tin oxide electrodes. The entire device is fabricated on glass, making the system highly transparent (>85%) in the visible spectrum. The devices exhibit switching ratios of over two orders of magnitude with measured retention of 10 s and endurance 10 cycles. Through the cross-sectional microstructural analyses it is shown that the asymmetric interfaces and distribution of oxygen vacancies in the bilayer oxide stack are responsible for defining the bipolar resistive switching behaviors. A photoluminescence mapping technique is employed to map the evolution of oxygen vacancies and pinpoint the location of the conductive filament. A transient response to optical excitation (using UV and blue light) is demonstrated in the high resistance state which indicates their potential as multifunctional memories for future transparent electronics.

Transparent amorphous strontium titanate resistive memories with transient photo-response.

Two-dimensional (2D) oxides have a wide variety of applications in electronics and other technologies. However, many oxides are not easy to synthesize as 2D materials through conventional methods. We used nontoxic eutectic gallium-based alloys as a reaction solvent and co-alloyed desired metals into the melt. On the basis of thermodynamic considerations, we predicted the composition of the self-limiting interfacial oxide. We isolated the surface oxide as a 2D layer, either on substrates or in suspension. This enabled us to produce extremely thin subnanometer layers of HfO, AlO, and GdO The liquid metal-based reaction route can be used to create 2D materials that were previously inaccessible with preexisting methods. The work introduces room-temperature liquid metals as a reaction environment for the synthesis of oxide nanomaterials with low dimensionality.

A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides.

Additive manufacturing using selective laser melted titanium (SLM-Ti) is used to create bespoke items across many diverse fields such as medicine, defense, and aerospace. Despite great progress in orthopedic implant applications, such as for "just in time" implants, significant challenges remain with regards to material osseointegration and the susceptibility to bacterial colonization on the implant. Here, we show that polycrystalline diamond coatings on these titanium samples can enhance biological scaffold interaction improving medical implant applicability. The highly conformable coating exhibited excellent bonding to the substrate. Relative to uncoated SLM-Ti, the diamond coated samples showed enhanced mammalian cell growth, enriched apatite deposition, and reduced microbial S. aureus activity. These results open new opportunities for novel coatings on SLM-Ti devices in general and especially show promise for improved biomedical implants.

Polycrystalline Diamond Coating of Additively Manufactured Titanium for Biomedical Applications.

Oxygen vacancies in inorganic semiconductors play an important role in reducing electron-hole recombination, which may have important implications in photocatalysis. Cuprous oxide (Cu2O), a visible light active p-type semiconductor, is a promising photocatalyst. However, the synthesis of photostable Cu2O enriched with oxygen defects remains a challenge. We report a simple method for the gram-scale synthesis of highly photostable Cu2O nanoparticles by the hydrolysis of a Cu(i)-triethylamine [Cu(i)-TEA] complex at low temperature. The oxygen vacancies in these Cu2O nanoparticles led to a significant increase in the lifetimes of photogenerated charge carriers upon excitation with visible light. This, in combination with a suitable energy band structure, allowed Cu2O nanoparticles to exhibit outstanding photoactivity in visible light through the generation of electron-mediated hydroxyl (OH˙) radicals. This study highlights the significance of oxygen defects in enhancing the photocatalytic performance of promising semiconductor photocatalysts.

Oxygen-deficient photostable CuO for enhanced visible light photocatalytic activity.

The surface of an orthopaedic implant plays a crucial role in determining the adsorption of proteins and cell functions. A detailed comparative study has been made of the in vitro osteoblast responses to coarse-grained (grain size: 500 μm), ultrafine-grained (grain size: 100 nm), coarse-porous (pore size: 350 nm), and fine-porous (pore size: 155 nm) surfaces of Ti-20Mo alloy. The purpose was to provide essential experimental data for future design of orthopaedic titanium implants for rapid osseointegration. Systematic original experimental data was produced for each type of surfaces in terms of surface wettability, cell morphology, adhesion, growth, and differentiation. Microscopic evidence was collected to reveal the detailed interplay between each characteristic surface with proteins or cells. Various new observations were discussed and compared with literature data. It was concluded that the coarse-porous surfaces offered the optimum topographical environment for osteoblasts and that the combination of ultrafine grains and considerable grain boundary areas is not an effective way to enhance cell growth and osteogenic capacity. Moreover, pore features (size and depth) have a greater effect than smooth surfaces on cell growth and osteogenic capacity. It proves that cells can discern the difference in pore size in the range of 100-350 nm. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2020-2033, 2018.

A comparative study of the effect of submicron porous and smooth ultrafine-grained Ti-20Mo surfaces on osteoblast responses.

Non-volatile resistive memory devices are theorized to be the most promising pathway towards analog memory and neuromorphic computing. Two-dimensional MoO3 is a versatile planar transition metal oxide, whose properties can be readily tuned, making it anywhere from a wide bandgap semiconductor to a semi-metal. Successful integration of such a planar metal oxide into resistive memory can enable adaptive and low power memory applications. Here, we investigate the non-volatile and reversible resistive switching behaviour of oxygen deficient MoOx in a cross-point metal/insulator/metal (MIM) architecture. Layered MoOx films are synthesised using chemical vapour deposition (CVD) and reveal excellent resistive switching performance with relatively low electroforming and operating voltages. Switching ratios of ∼103 and stable data retention of >104 s are achieved. As such, this work demonstrates the viability of MoOx as a resistive memory element and paves the way for future two-dimensional resistive memory technologies.

Reversible resistive switching behaviour in CVD grown, large area MoO.

The data included in this article provides additional supplementary information on our recent publication describing "Inducing tunable switching behavior in a single memristor" [1]. Analyses of micro/nano-structural and compositional changes induced in a resistive oxide memory during resistive switching are carried out. Chromium doped strontium titanate based resistance change memories are fabricated in a capacitor-like metal-insulator-metal structure and subjected to different biasing conditions to set memory states. Transmission electron microscope based cross-sectional analyses of the memory devices in different memory states are collected and presented.

Data related to the nanoscale structural and compositional evolution in resistance change memories.

Doping semiconductor nanocrystals is a powerful tool to impart new and beneficial optical and electrical properties to the host nanocrystals. Doping has been used to improve the performances of nanocrystal-based devices in applications as diverse as optics, magnetism, electronics, catalysis and sensing. In this work we present a low temperature colloidal synthesis of zinc sulfide (ZnS) nanocrystals doped with indium. Through optimization of the reaction parameters and the doping level, quantum confined (∼2 nm in size) crystalline colloids with highly tunable optical properties are achieved. Using a suite of characterization techniques including X-ray diffraction, high-resolution transmission electron microscopy, optical spectroscopies (absorption, emission, and Raman), compositional analyses and first principles simulations, we investigate the structural, morphological and optical properties of the synthesized nanocrystals. Indium dopants are found to heavily influence the band gap of ZnS. This strategy in addition to traditional methods of size control enables the synthesis of nanocrystals with finely tunable band gaps between ∼3.8 eV-4.3 eV. These doped ZnS nanocrystals are fabricated into selective UV thin-film absorbers and discriminatory proof-of-concept UVA-UVB/C photodetectors.

Augmented band gap tunability in indium-doped zinc sulfide nanocrystals.

The translation of biological synapses onto a hardware platform is an important step toward the realization of brain-inspired electronics. However, to mimic biological synapses, devices till-date continue to rely on the need for simultaneously altering the polarity of an applied electric field or the output of these devices is photonic instead of an electrical synapse. As the next big step toward practical realization of optogenetics inspired circuits that exhibit fidelity and flexibility of biological synapses, optically-stimulated synaptic devices without a need to apply polarity-altering electric field are needed. Utilizing a unique photoresponse in black phosphorus (BP), here reported is an all-optical pathway to emulate excitatory and inhibitory action potentials by exploiting oxidation-related defects. These optical synapses are capable of imitating key neural functions such as psychological learning and forgetting, spatiotemporally correlated dynamic logic and Hebbian spike-time dependent plasticity. These functionalities are also demonstrated on a flexible platform suitable for wearable electronics. Such low-power consuming devices are highly attractive for deployment in neuromorphic architectures. The manifestation of cognition and spatiotemporal processing solely through optical stimuli provides an incredibly simple and powerful platform to emulate sophisticated neural functionalities such as associative sensory data processing and decision making.

Optically Stimulated Artificial Synapse Based on Layered Black Phosphorus.

The high surface area and porosity, and limitless compound and network combinations between the metal ions and organic ligands making up metal-organic frameworks (MOFs) offer tremendous opportunities for their use in many applications. While numerous methods have been proposed for the synthesis of MOF powders, it is often difficult to obtain oriented crystals with these techniques. Further, the need for additional post-synthesis steps to activate the crystals and release them from the substrate presents a considerable production challenge. Here, we report an acoustically-driven microcentrifugation platform that facilitates fast convective solutal transport, allowing the synthesis of MOF crystals in as short as five minutes. The crystals are not only oriented due to long-range out-of-plane superlattice ordering aided by molecular dipole polarization under the acoustoelectric coupling, but also simultaneously activated during the synthesis process.

Acoustomicrofluidic assembly of oriented and simultaneously activated metal-organic frameworks.

With no requirements for lattice matching, van der Waals (vdW) ferromagnetic materials are rapidly establishing themselves as effective building blocks for next-generation spintronic devices. We report a hitherto rarely seen antisymmetric magnetoresistance (MR) effect in vdW heterostructured FeGeTe (FGT)/graphite/FGT devices. Unlike conventional giant MR (GMR), which is characterized by two resistance states, the MR in these vdW heterostructures features distinct high-, intermediate-, and low-resistance states. This unique characteristic is suggestive of underlying physical mechanisms that differ from those observed before. After theoretical calculations, the three-resistance behavior was attributed to a spin momentum locking induced spin-polarized current at the graphite/FGT interface. Our work reveals that ferromagnetic heterostructures assembled from vdW materials can exhibit substantially different properties to those exhibited by similar heterostructures grown in vacuum. Hence, it highlights the potential for new physics and new spintronic applications to be discovered using vdW heterostructures.

Antisymmetric magnetoresistance in van der Waals FeGeTe/graphite/FeGeTe trilayer heterostructures.

Flexible crystals are an emerging class of material with unique properties and a range of potential applications. Their relatively recent development means that mechanical characterisation protocols have not yet been widely established. There is a lack of quantitative flexibility measurements, such as the elastic modulus (Young's modulus), reported in the literature. In this work, we investigate amplitude modulated-frequency modulated atomic force microscopy (AM-FM AFM) as a fast, versatile method for measuring the elastic modulus of single flexible crystals. Specifically, the elastic modulus of single crystals of copper(ii) acetylacetonate (Cu(acac)2) was measured. The elastic modulus for Cu(acac)2 was found to be 4.79 ± 0.16 GPa. Importantly, this technique was able to map the variation in mechanical properties over the surface of the material with nanoscale resolution, showing some degree of correlation between surface morphology and elastic modulus. Additionally, the distribution of elastic modulus values can be measured at different locations on the crystal, giving a statistically robust distribution, which cannot be achieved using other methods.

Measuring the mechanical properties of flexible crystals using bi-modal atomic force microscopy.

Memristors have demonstrated immense potential as building blocks in future adaptive neuromorphic architectures. Recently, there has been focus on emulating specific synaptic functions of the mammalian nervous system by either tailoring the functional oxides or engineering the external programming hardware. However, high device-to-device variability in memristors induced by the electroforming process and complicated programming hardware are among the key challenges that hinder achieving biomimetic neuromorphic networks. Here, a simple hybrid complementary metal oxide semiconductor (CMOS)-memristor approach is reported to implement different synaptic learning rules by utilizing a CMOS-compatible memristor based on oxygen-deficient SrTiO (STO). The potential of such hybrid CMOS-memristor approach is demonstrated by successfully imitating time-dependent (pair and triplet spike-time-dependent-plasticity) and rate-dependent (Bienenstosk-Cooper-Munro) synaptic learning rules. Experimental results are benchmarked against in-vitro measurements from hippocampal and visual cortices with good agreement. The scalability of synaptic devices and their programming through a CMOS drive circuitry elaborates the potential of such an approach in realizing adaptive neuromorphic computation and networks.

Time and rate dependent synaptic learning in neuro-mimicking resistive memories.

Chromism-based optical filters is a niche field of research, due to there being only a handful of electrochromic materials. Typically, electrochromic transition metal oxides such as MoO and WO are utilized in applications such as smart windows and electrochromic devices (ECD). Herein, we report MoO-based electrically activated ultraviolet (UV) filters. The MoO grown on indium tin oxide (ITO) substrate is mechanically assembled onto an electrically activated proton exchange membrane. Reversible H injection/extraction in MoO is employed to switch the optical transmittance, enabling an electrically activated optical filter. The devices exhibit broadband transmission modulation (325-800 nm), with a peak of ∼60% in the UV-A range (350-392 nm). Comparable switching times of 8 s and a coloration efficiency of up to 116 cm C are achieved.

Electrically Activated UV-A Filters Based on Electrochromic MoO.

Enormous efforts have been initiated in the production of biobased fuels and value-added chemicals  biorefinery owing to the scarcity of fossil resources and huge environmental synchronization. Herein, non-noble metal-based metal/mixed metal oxide supported on carbon employing a metal-organic framework as a sacrificial template is demonstrated for the first time in the selective hydrodeoxygenation (HDO) of biomass-derived furfural (FFR) to 2-methyl furan (MF). The aforementioned catalyst (referred to as Cu/CuFeO@C-) exhibited extraordinary catalytic proficiency (100% selectivity toward MF) compared with the conventional Cu/CuFeO@C- catalyst which was prepared by the wet impregnation method. High-resolution transmission electron microscopy and synchrotron X-ray diffraction studies evidenced the existence of both metal (Cu) and mixed metal oxide (CuFeO) phases, in which the metal could help in hydrogenation to alcohol and metal oxide could assist in the hydroxyl group removal step during HDO reaction. The stabilization of encapsulated metal/metal oxide nanoparticles in the carbon matrix, modulation of the electronic structure, and regulation of geometric effects in the Cu/CuFeO@C- are thought to play an important role in its excellent catalytic performance, confirmed by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigations. Furthermore, the structure and activity interconnection was confirmed by  attenuated total reflection-IR studies, which manifested the strong interfacial interaction between FFR and the Cu/CuFeO@C- catalyst. This finding was further supported by NH temperature-programmed desorption analysis, which suggested that the presence of more Lewis/weak acidic sites in this catalyst was beneficial for the hydrogenolysis step in HDO reaction. Additionally, H temperature-programmed reduction studies revealed that the adsorption of H was stronger on the Cu/CuFeO@C- than that over the conventional Cu/CuFeO@C- catalyst; thus, the former catalyst promoted activation of H. A detailed kinetic analysis which demonstrated the lower activation energy barrier along with dual active sites attributed for the activation of the two separate reactions in the HDO process on the Cu/CuFeO@C- catalyst. This work has great implication in developing a highly stable catalyst for the selective upgradation of biomass without deactivation of metal sites in extended catalytic cycles and opens the door of opportunity for developing a sustainably viable catalyst in biomass refinery industries.

Article	Year
Competitive Inhibition of the Enzyme-Mimic Activity of Gd-Based Nanorods toward Highly Specific Colorimetric Sensing of l-Cysteine. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 28838237	2017
Transparent amorphous strontium titanate resistive memories with transient photo-response. Nanoscale\| PubMed ID: 28944813	2017
A liquid metal reaction environment for the room-temperature synthesis of atomically thin metal oxides. Science (New York, N.Y.)\| PubMed ID: 29051372	2017
Polycrystalline Diamond Coating of Additively Manufactured Titanium for Biomedical Applications. ACS applied materials & interfaces\| PubMed ID: 29470044	2018
Oxygen-deficient photostable CuO for enhanced visible light photocatalytic activity. Nanoscale\| PubMed ID: 29543296	2018
A comparative study of the effect of submicron porous and smooth ultrafine-grained Ti-20Mo surfaces on osteoblast responses. Journal of biomedical materials research. Part A\| PubMed ID: 29569836	2018
Reversible resistive switching behaviour in CVD grown, large area MoO. Nanoscale\| PubMed ID: 30141809	2018
Data related to the nanoscale structural and compositional evolution in resistance change memories. Data in brief\| PubMed ID: 30310835	2018
Augmented band gap tunability in indium-doped zinc sulfide nanocrystals. Nanoscale\| PubMed ID: 30488064	2019
Optically Stimulated Artificial Synapse Based on Layered Black Phosphorus. Small (Weinheim an der Bergstrasse, Germany)\| PubMed ID: 31018039	2019
Acoustomicrofluidic assembly of oriented and simultaneously activated metal-organic frameworks. Nature communications\| PubMed ID: 31123252	2019
Antisymmetric magnetoresistance in van der Waals FeGeTe/graphite/FeGeTe trilayer heterostructures. Science advances\| PubMed ID: 31281884	2019
Measuring the mechanical properties of flexible crystals using bi-modal atomic force microscopy. Physical chemistry chemical physics : PCCP\| PubMed ID: 31486450	2019
Time and rate dependent synaptic learning in neuro-mimicking resistive memories. Scientific reports\| PubMed ID: 31659247	2019
Electrically Activated UV-A Filters Based on Electrochromic MoO. ACS applied materials & interfaces\| PubMed ID: 32203662	2020
Leveraging Cu/CuFeO-Catalyzed Biomass-Derived Furfural Hydrodeoxygenation: A Nanoscale Metal-Organic-Framework Template Is the Prime Key. ACS applied materials & interfaces\| PubMed ID: 32314915	2020

Edwin Mayes

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