Ultra-Thin GaAs Single-Junction Solar Cells for Self-Powered Skin-Compatible Electrocardiogram Sensors.

GaAs thin-film solar cells have high efficiency, reliability, and operational stability, making them a promising solution for self-powered skin-conformal biosensors. However, inherent device thickness limits suitability for such applications, making them uncomfortable and unreliable in flexural environments. Therefore, reducing the flexural rigidity becomes crucial for integration with skin-compatible electronic devices. Herein, this study demonstrated a novel one-step surface modification bonding methodology, allowing a streamlined transfer process of ultra-thin (2.3 µm thick) GaAs solar cells on flexible polymer substrates. This reproducible technique enables strong bonding between dissimilar materials (GaAs-polydimethylsiloxane, PDMS) without high external pressures and temperatures. The fabricated solar cell showed exceptional performance with an open-circuit voltage of 1.018 V, short-circuit current density of 20.641 mA cm-2, fill factor of 79.83%, and power conversion efficiency of 16.77%. To prove the concept, the solar cell is integrated with a skin-compatible organic electrochemical transistor (OECT). Competitive electrical outputs of GaAs solar cells enabled high current levels of OECT under subtle light intensities lower than 50 mW cm-2, which demonstrates a self-powered electrocardiogram sensor with low noise (signal-to-noise ratio of 32.68 dB). Overall, this study presents a promising solution for the development of free-form and comfortable device structures that can continuously power wearable devices and biosensors.

Precisely Tunable Synthesis of Binder-Free Cobalt Oxide-Based Li-Ion Battery Anode Using Scalable Electrothermal Waves.

Binder-free transition metal oxide-based anodes for lithium-ion batteries (LIBs), having high capacity and abundance, have received considerable attention. However, their low conductivity and unstable charge-discharge cycles must be addressed, and scalable fabrication routes for binder-free design with optimal phase tuning are necessary. Herein, we report a precisely tunable synthesis of binder-free cobalt oxide-based LIB anodes using scalable electrothermal waves. Needle-like nanoarrays of cobalt hydroxide on nickel foams are prepared as precursors, and Joule-heating-driven electrothermal waves passing through the metal foams cause transition to cobalt oxides with preserved structures and adjustable phase tuning of grains and oxygen vacancies. The rapid heating-cooling environment using electrothermal waves causes extreme input thermal energy over the activation energy of phase transitions and metastable phase trapping. This programmable route completes the selective grain characteristics and vacancy concentrations. The electrothermally tuned binder-free LIB anodes employing the CoO/Co3O4@Ni foam-based electrodes exhibit a high-rate capacity (3.7 mAh cm-2) at 2.4 mA cm-2 for 70 charge-discharge cycles. Accumulated electrothermal waves from multiple cycles broaden the tunable ranges of the morphological and chemical transitions causing rapid screening of the optimal phases for LIB anodes. This phase-tuning strategy will inspire precise yet efficient synthesis routes for diverse binder-free electrodes and catalysts.

Electrothermally Driven Nucleation Energy Control of Defective Carbon and Nickel-Cobalt Oxide-Based Electrodes.

Multielement metal/metal oxides/carbon-based support hybrids are promising candidates for high-performance electrodes. However, conventional solid-state synthesis utilizing slow heating-cooling rates is limited by discrepancies in their phase transition temperatures. Herein, we report a rational strategy to control the nucleation energy of defective carbon fibers (DCFs) and Ni-Co-oxide-based electrodes capable of electrochemical activation using electrothermal waves (ETWs). The ETWs, triggered by Joule heating passing through CFs and Ni-Co precursors, induce programmable high-temperature processes via adjustable input powers and durations. The first ETW (∼1500 °C) fabricates the presculpted DCFs, while the second ETW (∼600 °C) directly synthesizes NiCo2O4 spinel nanoparticles on the DCFs. Predesigning DCFs through the Gibbs free energy theory enables tunable control of nucleation energy and solution compatibility with Ni-Co precursors, allowing the morphological and compositional design of the optimal NiCo2O4@DCFs hybrids. Furthermore, they are electrochemically activated to change the morphologies and oxidation states of Ni-Co to more stable wrinkled structures strongly anchored to carbon supports and Ni-Co cations with low oxidation numbers. The activated NiCo2O4@DCFs electrodes exhibit outstanding specific capacitance and long-term cyclic stability (∼1925 F g-1 and ∼115-123% for 20 000 cycles). The ETWs offer a facile yet precise method to predesign carbon supports and subsequently synthesize hybrid electrodes.

Analysis of Blind Reconstruction of BCH Codes.

In this paper, the theoretical lower-bound on the success probability of blind reconstruction of Bose-Chaudhuri-Hocquenghem (BCH) codes is derived. In particular, the blind reconstruction method of BCH codes based on the consecutive roots of generator polynomials is mainly analyzed because this method shows the best blind reconstruction performance. In order to derive a performance lower-bound, the theoretical analysis of BCH codes on the aspects of blind reconstruction is performed. Furthermore, the analysis results can be applied not only to the binary BCH codes but also to the non-binary BCH codes including Reed-Solomon (RS) codes. By comparing the derived lower-bound with the simulation results, it is confirmed that the success probability of the blind reconstruction of BCH codes based on the consecutive roots of generator polynomials is well bounded by the proposed lower-bound.

Scalable Synthesis of Triple-Core-Shell Nanostructures of TiO2 @MnO2 @C for High Performance Supercapacitors Using Structure-Guided Combustion Waves.

Core-shell nanostructures of metal oxides and carbon-based materials have emerged as outstanding electrode materials for supercapacitors and batteries. However, their synthesis requires complex procedures that incur high costs and long processing times. Herein, a new route is proposed for synthesizing triple-core-shell nanoparticles of TiO2 @MnO2 @C using structure-guided combustion waves (SGCWs), which originate from incomplete combustion inside chemical-fuel-wrapped nanostructures, and their application in supercapacitor electrodes. SGCWs transform TiO2 to TiO2 @C and TiO2 @MnO2 to TiO2 @MnO2 @C via the incompletely combusted carbonaceous fuels under an open-air atmosphere, in seconds. The synthesized carbon layers act as templates for MnO2 shells in TiO2 @C and organic shells of TiO2 @MnO2 @C. The TiO2 @MnO2 @C-based electrodes exhibit a greater specific capacitance (488 F g-1 at 5 mV s-1 ) and capacitance retention (97.4% after 10 000 cycles at 1.0 V s-1 ), while the absence of MnO2 and carbon shells reveals a severe degradation in the specific capacitance and capacitance retention. Because the core-TiO2 nanoparticles and carbon shell prevent the deformation of the inner and outer sides of the MnO2 shell, the nanostructures of the TiO2 @MnO2 @C are preserved despite the long-term cycling, giving the superior performance. This SGCW-driven fabrication enables the scalable synthesis of multiple-core-shell structures applicable to diverse electrochemical applications.

Thermoelectric-pyroelectric hybrid energy generation from thermopower waves in core-shell structured carbon nanotube-PZT nanocomposites.

There is an urgent need to develop a suitable energy source owing to the rapid development of various innovative devices using micro-nanotechnology. The thermopower wave (TW), which produces a high specific power during the combustion of solid fuel inside micro-nanostructure materials, is a unique energy source for unusual platforms that cannot use conventional energy sources. Here, we report on the significant enhancement of hybrid energy generation of pyroelectrics and thermoelectrics from TWs in carbon nanotube (CNT)-PZT (lead zirconate titanate, P(Z0.5-T0.5)) composites for the first time. Conventional TWs use only charge carrier transport driven by the temperature gradient along the core materials to produce voltage. In this study, a core-shell structure of CNTs-PZTs was prepared to utilize both the temperature gradient along the core material (thermoelectrics) and the dynamic change in the temperature of the shell structure (pyroelectrics) induced by TWs. The dual mechanism of energy generation in CNT-PZT composites amplified the average peak and duration of the voltage up to 403 mV and 612 ms, respectively, by a factor of 2 and 60 times those for the composites without a PZT layer. Furthermore, dynamic voltage measurements and structural analysis in repetitive TWs confirmed that CNT-PZT composites maintain the original performance in multiple TWs, which improves the reusability of materials. The advanced TWs obtained by the application of a PZT layer as a pyroelectric material contributes to the extension of the usable energy portion as well as the development of TW-based operating devices.

Thermopower Wave-Driven Hybrid Supercapacitor Charging System.

The development of new energy sources and harvesting methods has increased with the rapid development of multiscale wireless and portable systems. A thermopower wave (TW) is a potential portable energy source that exhibits a high power density. TWs generate electrical energy via the transport of charges inside micro- or nanostructured materials. This transport is induced by self-propagating combustion. Despite the high specific power of TWs, the generation of energy by TWs is transient, making a TW device a one-time use source, which is a critical limitation on the further advancement of this technology. Herein, we first report the development of a hybrid supercapacitor charging system driven by consecutive TWs to accumulate multiple amounts of energy generated by the repetitive combustion of the chemical fuel. In this study, hybrid layers composed of a supercapacitor (poly(vinyl alcohol)/MnO2/nickel) and solid fuel layer (nitrocellulose film) were fabricated as one integrated platform. Combustion was initiated by the ignition of the fuel layer, resulting in the production of electrical energy, attributed to the potential difference between two electrodes, and the transport of charges inside one of the electrodes. Electrical energy could simultaneously and directly charge the supercapacitor, and the discharged voltage could be significantly increased in comparison with the voltage level before the application of a TW. Furthermore, the application of multiple TWs in succession in the hybrid supercapacitor charging system successfully allowed for stack voltage amplification, which was synchronized to each TW. The results of this study could be used to understand the underlying phenomena for charging supercapacitors with the variation of thermal energy and to advance the application of TWs as more efficient, practical energy sources.

Manipulation of combustion waves in carbon-nanotube/fuel composites by highly reactive Mg nanoparticles.

Manipulating the interface of micro/nanostructured materials and chemical fuels can change the fundamental characteristics of combustion waves that are generated during a reaction. In this study, we report that Mg/MgO nanoparticles actively amplify the propagation of combustion waves at the interface of multi-walled carbon nanotubes (MWCNTs) and chemical fuels. Fuel/MWCNT and fuel/MWCNT-Mg/MgO composite films were prepared by a facile synthetic method. We present complete physiochemical characterization of these composite films and evaluate the propagating velocities and real-time surface temperatures of combustion waves. Mg/MgO nanoparticles at the interface enhanced the reaction front velocity by 41%. The resulting explosive reactions supplied additional thermal energy to the chemical fuel, accelerating flame propagation. Furthermore, the surface temperatures of the composites with Mg/MgO nanoparticles were much lower, indicating how the transient heat from the reaction would ignite the unreacted fuels at lower surface temperatures despite not reaching the necessary activation energy for a chain reaction. This mechanism contributed to thermopower waves that amplified the output voltage. Furthermore, large temperature gradients due to the presence of nanoparticles increased charge transport inside the nanostructured material, due to the increased thermoelectric effects. This manipulation could contribute to the active control of interfacially driven combustion waves along nanostructured materials, yielding many potential applications.

Voltage amplification of thermopower waves via current crowding at high resistances in self-propagating combustion waves.

Combustion wave propagation in micro/nanostructured materials generates a chemical-thermal-electrical energy conversion, which enables the creation of an unusual source of electrical energy, called a thermopower wave. In this paper, we report that high electrical resistance regimes would significantly amplify the output voltage of thermopower waves, because the current crowding creates a narrow path for charge carrier transport. We show that the structurally defective regions in the hybrid composites of chemical fuels and carbon nanotube (CNT) arrays determine both the resistance levels of the hybrid composites and the corresponding output voltage of thermopower waves. A sudden acceleration of the crowded charges would be induced by the moving reaction front of the combustion wave when the supplied driving force overcomes the potential barrier to cause charge carrier transport over the defective region. This property is investigated experimentally for the locally manipulated defective areas using diverse methods. In this study, thermopower waves in CNT-based hybrid composites are able to control the peak voltages in the range of 10-1000 mV by manipulating the resistance from 10 Ω to 100 kΩ. This controllable voltage generation from thermopower waves may enable applications using the combustion waves in micro/nanostructured materials and better understanding of the underlying physics.

Preparation and Evaluation of Hybrid Composites of Chemical Fuel and Multi-walled Carbon Nanotubes in the Study of Thermopower Waves.

When a chemical fuel at a certain position in a hybrid composite of the fuel and a micro/nanostructured material is ignited, chemical combustion occurs along the interface between the fuel and core materials. Simultaneously, dynamic changes in thermal and chemical potentials across the micro/nanostructured materials result in concomitant electrical energy generation induced by charge transfer in the form of a high-output voltage pulse. We demonstrate the entire procedure of a thermopower wave experiment, from synthesis to evaluation. Thermal chemical vapor deposition and the wet impregnation process are respectively employed for the synthesis of a multi-walled carbon nanotube array and a hybrid composite of picric acid/sodium azide/multi-walled carbon nanotubes. The prepared hybrid composites are used to fabricate a thermopower wave generator with connecting electrodes. The combustion of the hybrid composite is initiated by laser heating or Joule-heating, and the corresponding combustion propagation, direct electrical energy generation, and real-time temperature changes are measured using a high-speed microscopy system, an oscilloscope, and an optical pyrometer, respectively. Furthermore, the crucial strategies to be adopted in the synthesis of hybrid composite and initiation of their combustion that enhance the overall thermopower wave energy transfer are proposed.

An extension and test of Sutherland's concept of differential social organization: the geographic clustering of Japanese suicide and homicide rates.

In an effort to explain the spatial patterning of violence, we expanded Sutherland's (1947) concept of differential social organization to include the level of deviance exhibited by neighboring areas. To test the value of this extension, the geographic clustering of Japanese suicide and homicide rates is assessed using 1985 and 1995 data for prefectures. Univariate results show that both types of violence cluster significantly in space. Inconsistent with the proposed expansion of differential social organization, multivariate results show that proximity to homicide does not increase the risk of homicide in Japan. Results for suicide are supportive and suggest that proximity to higher rates of suicide exposes residents to cultural values that support suicide and to role models who have committed suicide.