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  1. Printed electronics are gaining significant interest due to their design flexibility, low fabrication cost, and rapid design-to-manufacturing turnaround. Conventional substrates for printed electronics are often based on nonbiodegradable polymers such as polyimide that pose high environmental challenges by creating massive e-waste and pollution. As the demand for printed electronics and sensors increases, the ability to print such devices on biodegradable substrates can provide a solution to such environmental problems. However, current printing technologies are based on liquids and inks that are incompatible with biodegradable substrates, such as paper. Here, we present a dry-printing process, namely, a dry additive nanomanufacturing (Dry-ANM) technique, for printing conductive silver lines and patterns on biodegradable papers for flexible hybrid papertronics. Pure and dry nanoparticles are generated by pulsed laser ablation of a silver target that is then transported through a nozzle and directed onto paper substrates, where they are deposited and laser-sintered in real time to form the desired pattern without damaging the paper. The effects of different printing parameters on the paper-burning threshold are investigated, and the electrical properties of the lines are characterized by using different line thicknesses and sintering laser power densities. In addition, the mechanical and electrical properties of the printed lines and patterns are evaluated by bending and twisting tests. Furthermore, the feasibility of printing silver on different paper types is demonstrated. This research can potentially lead to biodegradable and environmentally friendly printed electronics and sensors. 
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    Free, publicly-accessible full text available November 3, 2024
  2. Transition metal spinel oxides comprised of earth-abundant Mn and Co have long been explored for their use in catalytic reactions and energy storage. However, understanding functional properties can be challenging due to differences in sample preparation and the ultimate structural properties of the materials. Epitaxial thin film synthesis provides a novel means of producing precisely controlled materials to explore the variations reported in the literature. In this work, MnxCo3−xO4 samples from x = 0 to x = 1.28 were synthesized through molecular beam epitaxy and characterized to develop a material properties map as a function of stoichiometry. Films were characterized via in situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, and polarized K-edge x-ray absorption spectroscopy. Mn cations within this range were found to be octahedrally coordinated, in line with an inverse spinel structure. Samples largely show mixed Mn3+ and Mn4+ character with evidence of phase segregation tendencies with the increasing Mn content and increasing Mn3+ formal charge. Phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn4+ and Jahn–Teller active Mn3+ octahedra, respectively. Our results help in explaining the reported differences across samples in these promising materials for renewable energy technologies. 
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    Free, publicly-accessible full text available September 1, 2024
  3. Additively manufactured electronics (AMEs), also known as printed electronics, are becoming increasingly important for the anticipated Internet of Things (IoT). This requires manufacturing technologies that allow the integration of various pure functional materials and devices onto different flexible and rigid surfaces. However, the current ink-based technologies suffer from complex and expensive ink formulation, ink-associated contaminations (additives/solvents), and limited sources of printing materials. Thus, printing contamination-free and multimaterial structures and devices is challenging. Here, a multimaterial additive nanomanufacturing (M-ANM) technique utilizing directed laser deposition at the nano and microscale is demonstrated, allowing the printing of lateral and vertical hybrid structures and devices. This M-ANM technique involves pulsed laser ablation of solid targets placed on a target carousel inside the printer head for in-situ generation of contamination-free nanoparticles, which are then guided via a carrier gas toward the nozzle and onto the surface of the substrate, where they are sintered and printed in real-time by a second laser. The target carousel brings a particular target in engagement with the ablation laser beam in predetermined sequences to print multiple materials, including metals, semiconductors, and insulators, in a single process. Using this M-ANM technique, various multimaterial devices such as silver/zinc oxide (Ag/ZnO) photodetector and hybrid silver/aluminum oxide (Ag/Al2O3) circuits are printed and characterized. The quality and versatility of our M-ANM technique offer a potential manufacturing option for emerging IoT. 
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    Free, publicly-accessible full text available August 1, 2024
  4. This work demonstrates the advantage of carrying out silicon ion (Si+) implantation at high temperatures for forming controlled heavily doped regions in gallium oxide. Room temperature (RT, 25 °C) and high temperature (HT, 600 °C) Si implants were carried out into MBE grown (010) β-Ga2O3 films to form ∼350 nm deep Si-doped layers with average concentrations up to ∼1.2 × 1020 cm−3. For such high concentrations, the RT sample was too resistive for measurement, but the HT samples had 82.1% Si dopant activation efficiency with a high sheet electron concentration of 3.3 × 1015 cm−2 and an excellent mobility of 92.8 cm2/V·s at room temperature. X-ray diffraction measurements indicate that HT implantation prevents the formation of other Ga2O3 phases and results in reduced structural defects and lattice damage. These results are highly encouraging for achieving ultra-low resistance heavily doped Ga2O3 layers using ion implantation.

     
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  5. 4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO3 is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d1 Nb4+ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayer structures that may be challenging by PLD. To that end, SrNbO3 thin films were grown using hybrid MBE for the first time using a tris(diethylamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO3 substrates. Varying thicknesses of insulating SrHfO3 capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO3 films to preserve the metastable surface. Grown films were transferred in vacuo for x-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by x-ray absorption near edge spectroscopy and scanning transmission electron microscopy illustrate that the SrHfO3 capping plays an important role in preserving the crystalline quality of the material and the Nb 4d1 metastable charge state under atmospheric conditions.

     
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  6. Nanocrystalline MnFe2O4 has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been sparingly studied as highly ordered thin-film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe2O4 and Fe3O4 spinel oxide films were grown on (001)- and (111)-oriented Nb:SrTiO3 perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). High-resolution X-ray diffraction (HRXRD) and X-ray photoelectron spectroscopy (XPS) show the synthesis of pure phase materials, while scanning transmission electron microscopy (STEM) and reflection high-energy electron diffraction (RHEED) analysis demonstrate island-like growth of (111) surface-terminated pyramids on both (001)- and (111)-oriented substrates, consistent with the literature and attributed to the lattice mismatch between the spinel films and the perovskite substrate. Cyclic voltammograms under a N2 atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe2O4 and Fe3O4. Under an O2 atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, a diffusion-limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe2O4 reported in the literature where the diffusion-limited current is achieved with Mn-based catalysis. This difference is attributed to a low density of Mn surface termination, as determined by the integration of current from CVs collected under N2, in addition to low conductivity through the MnFe2O4 film due to the degree of inversion. Such low densities are attributed to the synthetic method and island-like growth pattern and highlight challenges in studying ORR catalysis with single-crystal spinel materials. 
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  7. Transition metal oxides have long been an area of interest for water electrocatalysis through the oxygen evolution and oxygen reduction reactions. Iron oxides, such as LaFeO 3 , are particularly promising due to the favorable energy alignment of the valence and conduction bands comprised of Fe 3+ cations and the visible light band gap of such materials. In this work, we examine the role of band alignment on the electrocatalytic oxygen evolution reaction (OER) in the intrinsic semiconductor LaFeO 3 by growing epitaxial films of varying thicknesses on Nb-doped SrTiO 3 . Using cyclic voltammetry, we find that there is a strong thickness dependence on the efficiency of electrocatalysis for OER. These measurements are understood based on interfacial band alignment in the system as well as catalytically active surface defect states as confirmed by layer-resolved electron energy loss spectroscopy, electrochemical impedance spectroscopy, and Mott–Schottky measurements. Our results demonstrate the importance of band engineering for the rational design of thin film electrocatalysts for renewable energy sources. 
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