Hirotaka Koga

Hirotaka Koga
Osaka University | Handai · The Institute of Scientific and Industrial Research (ISIR)

PhD

About

101
Publications
12,883
Reads
How we measure 'reads'
A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more
3,720
Citations
Additional affiliations
April 2010 - March 2012
The University of Tokyo
Position
  • PostDoc Position
April 2004 - March 2010
Kyushu University
Position
  • PhD Student

Publications

Publications (101)
Preprint
Biomimicry and the development of sophisticated materials can derive inspiration from the nanoscale component orientation and hierarchical structures found in living organisms. Herein, a simple and flexible strategy to construct a sophisticated hierarchical structure based on electrophoretic and electrochemical deposition is reported. Cellulose nan...
Article
Full-text available
Semiconducting nanomaterials with 3D network structures exhibit various fascinating properties such as electrical conduction, high permeability, and large surface areas, which are beneficial for adsorption, separation, and sensing applications. However, research on these materials is substantially restricted by the limited trans-scalability of thei...
Article
Full-text available
Humidity sensing plays an important role in diverse applications in the fields of agriculture, healthcare, skincare, and environmental monitoring. With the advent of the “trillion sensor” society, there is a growing demand for humidity sensors that can be made from abundant and renewable resources. Cellulose, the most abundant and renewable bioreso...
Article
Full-text available
Heat dissipation has become increasingly important in electronics. Conventional convection cooling systems have significant material and dimensional constraints, and they have difficulty meeting the heat dissipation, miniaturization, and flexibility requirements of next-generation smart electronics. Here, we used kirigami (the traditional art of pa...
Article
Full-text available
Biomass-derived three-dimensional (3D) porous nanocarbons have attracted much attention due to their high surface area, permeability, electrical conductivity, and renewability, which are beneficial for various electronic applications, including energy storage. Cellulose, the most abundant and renewable carbohydrate polymer on earth, is a promising...
Article
Full-text available
High hygroscopicity is an unavoidable feature of cellulose materials, and it is important for clarifying the humidity dependence of the target function toward precise utilization. Recently, cellulose nanofiber films have been found to have higher thermal conductivity than plastics and glass, and they are expected to be used as flexible thermal cond...
Article
Full-text available
Water is detrimental to electronic devices because it easily causes short circuits. The use of sealing to prevent water permeation is considered as the current conventional solution; however, the trend for developing flexible and stretchable electronic circuits has placed severe demands on waterproof technologies. This report describes a coating th...
Article
Full-text available
Chitin, a natural polysaccharide polymer, forms highly crystalline nanofibers and is expected to have sophisticated engineering applications. In particular, for development of next-generation heat-transfer and heat-insulating materials, analysis of the thermal conductivity is important, but the thermal conductivity properties of chitin nanofiber ma...
Article
We propose a new methodology for direct evaluation of the degree of fibrillation of fibrillating pulp suspensions through the pixel-resolved retardation distribution. Through simple normalization by just injecting a pulp suspension with a certain concentration into a quartz flow channel with a constant cross-sectional shape, the degree of fibrillat...
Article
Full-text available
Three-dimensional porous and defect-rich nitrogen-doped nanocarbon is successfully prepared by facile pyrolysis of crab shell-derived chitin nanofiber paper, delivering promising performance in both photosensing and energy storage.
Article
Full-text available
As a renewable nanomaterial, transparent nanopaper is one of the promising materials for electronic devices. Although conventional evaporation drying method endows nanopaper with superior optical properties, the long fabrication time limits its widely use. In this work, we propose a multi-stage drying method to achieve high-speed fabrication of cle...
Article
Full-text available
It is essential to build multiaxis oriented nanocellulose films in the plane for developing thermal or optical management films. However, using conventional orientation techniques, it is difficult to align nanocelluloses in multiple directions within the plane of single films rather than in the thickness direction like the chiral nematic structure....
Article
Plasmonic nanoparticles, such as gold nanoparticles (AuNPs), have been actively applied in solar vapor generation for seawater desalination and water purification, owing to their photothermal heating performances. Such nanoparticles have been frequently anchored within porous supporting materials to ensure easy handling and water absorption. Howeve...
Article
Thermal transport modulating materials show great potential to address the heat problems in a wide range of engineering fields. However, tuning the thermal conductivity of solid-state materials is practically difficult because it requires specific or extreme stimulation, such as chemical composition change, a phase transition, or large applied fluc...
Chapter
Flexible and wearable electronics have recently attracted increasing attention, because they can open up new possibilities for next-generation portable devices. Cellulose paper serves various purposes in daily life, including in writing, printing, wrapping, and packaging applications. Since paper materials are sustainable, mass produced, inexpensiv...
Article
A nanopaper sensor device that combines humidity sensing, wireless information transmission, and degradability has been fabricated using wood-derived nanopaper as the substrate and dielectric layers. The nanopaper shows excellent suitability for capacitor dielectric layers because of its high dielectric constant, insulating properties suitable for...
Article
Full-text available
In this paper, transparent electrodes with dense Cu@Ag alloy nanowires embedded in stretchable substrates are successfully fabricated by a high-intensity pulsed light (HIPL) technique within one step. The intense light energy not only induces rapid mutual dissolution between the Cu core and the Ag shell to form dense Cu@Ag alloy nanowires but also...
Article
Progress toward the concept of “a trillion sensor universe” requires sensor devices to become more abundant, ubiquitous, and be potentially disposable. Here, we report a paper-based disposable molecular sensor device constructed from a nanowire sensor based on common zinc oxide (ZnO), a wood-derived biodegradable cellulose nanofiber paper substrate...
Article
The intrinsic birefringence of cellulose is one of the most fundamental optical parameters for analyzing and developing various cellulosic materials. However, the previously reported values greatly vary depending on the problems occurred due to the measured cellulose sample or method, and it is still a challenge to evaluate the intrinsic birefringe...
Article
Full-text available
Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and how the colloidal CNFs align i...
Article
Thin-film wearable electronics are required to be directly laminated on to human skin for reliable, sensitive bio-sensing but with minimal irritation to the user after long-time use. Excellent heat management films with strongly anisotropic thermal conductivity (K) and adequate breathability are increasingly desirable for shielding the skin from he...
Article
Electrochemical techniques were employed to study the electrochemical corrosion behavior of Sn-9Zn-xTi (x = 0, 0.05, 0.1, 0.2 wt.%) lead-free solders in neutral 0.5M NaCl solution, aiming to figure out the effect of Ti content on the corrosion properties of Sn-9Zn, providing information for the composition design of Sn-Zn-based lead-free solders fr...
Article
Full-text available
Nanopaper prepared from holocellulose pulp is one of the best substrates for flexible electronics because of its high thermal resistance and high clear transparency. However, the clearness of nanopaper decreases with increasing concentration of the starting cellulose nanofiber dispersion—with the use of a 2.2 wt % dispersion, for example—resulting...
Article
Functional molecules play a significant role in the development of high-performance composite materials. Functional molecules should be well dispersed (ideally dissolved) and supported within an easy-to-handle substrate, to take full advantage of their functionality and ensure easy handling. However, simultaneously achieving the dissolution and sup...
Article
The Back Cover picture shows a renewable wood pulp paper reactor for highly efficient, green, and sustainable chemical manufacturing. This “paper reactor” contains hierarchical micro/nanoscale pores tailored from wood pulp fibers, which can act as both convective-flow and rapid-diffusion channels for efficient access of reactants to metal nanoparti...
Article
Full-text available
Continuous-flow nanocatalysis by metal nanoparticle (NP) catalyst-anchored flow reactors has recently provided an excellent platform for effective chemical manufacturing. However, there has been limited progress in porous structure design and recycling systems for metal NP-anchored flow reactors to create more efficient and sustainable catalytic pr...
Article
Electrochemical behavior of Zn-xSn (x = 40 wt%,30 wt%,20 wt%) high-temperature lead-free alloys was investigated in aerated 0.5 M NaCl solution by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. OCP results show that the corrosion state of the alloys working surface reached a r...
Article
Full-text available
The aim of this study is to clarify light scattering mechanism of hazy transparent cellulose nanopaper. Clear optical transparent nanopaper consists of 3–15 nm wide cellulose nanofibers, which are obtained by the full nanofibrillation of pulp fibers. At the clear transparent nanopaper with 40 μm thickness, their total transmittance are 89.3–91.5% a...
Article
A biaxially wave-shaped polydimethylsiloxane (PDMS) surface was developed simply by using a taro leaf as the template. The resulting leaf-templated PDMS (L-PDMS) possesses a micro-sized curved interface structure, which is greatly beneficial for the exact embedding of a silver nanowire (AgNW) network conductive film covering the L-PDMS surface. The...
Article
Submicron Cu particle ink was developed to successfully achieve high reliable and high conductive Cu patterns on low-cost, transparent, flexible substrates by an optimized two-step sintering process involving low temperature heat-welding and subsequent flash light sinter-reinforcement. The Cu ink contains a special additive of Cu-ammino complex mad...
Article
Silver nanowires (AgNWs) are excellent candidate electrode materials in next-generation wearable devices due to their high flexibility and high conductivity. In particular, patterning techniques for AgNWs electrode manufacture are very important in the roll-to-roll printing process to achieve high throughput and special performance production. It i...
Article
Full-text available
Single-use disposable nonvolatile memory devices hold promise for novel applications in internet of everything (IoE) technology by storing the health status of individual humans in daily life. However, conventional memory devices are not disposable because they are mostly composed of non-renewable, non-biodegradable and sometimes toxic materials, c...
Article
The recent trend toward the Internet of Things (IoT) has increased the importance of technologies for the mass production of high-sensitivity flexible antennas for efficient wireless communication. Here, we describe the rapid low-temperature fabrication of high-sensitivity flexible antennas based on hot-water sintering of silver (Ag) precursor-base...
Conference Paper
Mass production techniques to provide highly sensitive and low-cost antennas are demanded to achieve the Internet of Things. Printing is a suitable technique for mass production of low-cost antennas. In this study, three types of printed silver nanowire (AgNW) antennas were fabricated by preparing inks containing AgNWs of different width and length...
Conference Paper
Flexible electronics is expected technology in the future, the bonding material may also be required flexibility. STPE is a promising material that has both flexibility and low-temperature curability, it is possible to satisfy the characteristics required in the near future bonding material.
Article
Full-text available
Copper nanowire (CuNW) conductors have been considered to have a promising perspective in the area of stretchable electronics due to the low price and high conductivity. However, the fabrication of CuNW conductors suffers from harsh conditions, such as high temperature, reducing atmosphere, and time-consuming transfer step. Here, a simple and rapid...
Article
Featuring outstanding electrical and optical properties, silver nanowires (AgNWs) have been regarded as one of the most promising candidates for ITO to manufacture transparent conductive electrodes. However, the poor long-term stability of bare AgNWs due to sulfidation/oxidation corrosion is an unavoidable and urgent problem in practical applicatio...
Article
Recent progress in portable and wearable electronics has promoted a growing demand for high-performance and flexible energy-storage devices that are abundant and affordable. Because reduced graphene oxide (rGO), originating from inexpensive graphite, serves as a higher-performance energy-storage electrode than conventional activated carbons and car...
Article
The transparent electrode based on silver nanowire (AgNW) networks is one promising alternative of indium tin oxide film in particular for advanced flexible and printable electronics. However, the widespread application of AgNW electrode is hindered by its poor long-term reliability. Although the reliability can be improved by applying traditional...
Article
Optically transparent cellulose nanopaper is one of the best candidate substrates for flexible electronics. Some types of cellulose nanopaper are made of mechanically or chemically modified cellulose nanofibers. Among these, nanopapers produced from chemically modified cellulose nanofibers are the most promising substrate because of their lower pow...
Conference Paper
Full-text available
Silver nanowires (AgNWs) transparent electrodes have attracted considerable attention owing to their high electrical and thermal conductivity, optical transparency, flexibility on many polymer substrates, and simple solution process. Many reports only focused on improving the performance of AgNWs electrodes while ignored the application lifetime co...
Conference Paper
The transparent electrode based on silver nanowire (AgNW) networks has emerged as an outstanding alternative of the electrode based on indium tin oxide (ITO) film in particular for flexible electronics. The polymeric films are popularly applied as flexible substrates and thus the electrode employing AgNWs and polymeric film is potential to be widel...
Article
Printed electronics (PE) has been emerged as one of the key manufacturing processes for a wide variety of electronics products not only for home appliances but also for vehicles and for business-scale. The advances of nanomaterials, especially metallic and organic conductive materials, for PE brought printed electronics into the reality. Sensing co...
Article
Full-text available
Transparent electrodes based on silver nanowires (AgNWs) films have attracted considerable attention owing to their high electrical and thermal conductivity, optical transparency, and flexibility. However, the long-term reliability of AgNWs electrodes, has seldom been studied. In the work, the effects of storing environment, such as, natural light,...
Article
Full-text available
Multi-layer graphene oxide (MGO) has attracted considerable interest in conductive polymer composites. However, in most cases the optimal MGO content is determined by using complex procedure. Without the most complicated works in processing polymerized samples followed by testing various properties, here, a facile strategy is proposed to directly i...
Article
Portable electronic devices such as smartphones and tablets have become widespread in recent years, and this has promoted demand for reducing their size and weight. Making such portable devices flexible is also an aim of next-generation wearable electronics. Antenna components allow the transmission and reception of radio waves as information signa...
Article
Full-text available
We discuss the successful use of chemically-modified cellulose paper as a microstructured catalytic reactor for the production of useful chemicals. The chemical modification of cellulose paper was achieved using a silane-coupling technique. Amine-modified paper was directly used as a base catalyst for the Knoevenagel condensation reaction. Methacry...
Article
The next-generation application of pressure sensors is gradually being extended to include electronic artificial skin (e-skin), wearable devices, humanoid robotics and smart prosthetics. In these advanced applications, high sensing capability is an essential feature for high performance. Although surface patterning treatments and some special elast...
Article
Silver nanoparticle lines are fabricated on flexible and highly heat-resistant nanopaper substrates made from densely packed 15 to 60 nm cellulose nanofibers. The conductivity of the lines is enhanced to the level of bulk silver.
Article
Full-text available
Transparent electrode based on silver nanowires (AgNWs) emerges as an outstanding alternative of indium tin oxide film especially for flexible electronics. However, the conductivity of AgNWs transparent electrode is still dramatically limited by the contact resistance between nanowires at high transmittance. Polyvinylpyrrolidone (PVP) layer adsorbe...
Chapter
In this chapter, we describe a new type of paper substrate based on cellulose nanofibers for flexible electronic applications. Cellulose nanofiber paper, referred to as nanopaper in this study, has high optical transparency like that of polyethylene terephthalate (PET) films and a low coefficient of thermal expansion comparable to that of glass. Th...
Chapter
Printing techniques, such as ink-jet printing, screen printing, and flexography, are promising alternatives to conventional photolithography for the production of electronic devices. The advantages of these techniques include low manufacturing costs, environmental sustainability, manufacturing simplicity, and high material usage. Among these techni...
Article
We demonstrate the fabrication of chitin nanofibril aerogels and their successful application as base catalysts for the production of useful chemicals. Squid-pen chitin nanofibrils (ChNF) with primary C2-amine groups on their crystalline surfaces were fabricated into highly porous aerogels with high specific surface areas up to 289 m2 g-1 using fre...
Conference Paper
Silver nanowires (AgNWs) are synthesized with various lengths and diameters depending on the reaction parameters and the stirring speed in polyol process. AgNWs > 80 μm in length are prepared at 110°C without stirring, while shorter wires with only 20∼30 μm even below 5 μm become the main products at reaction temperature from 160 to 200°C. These Ag...
Conference Paper
High-dielectric materials have played essential roles in various electronic applications such as capacitor, transistor, antenna and pressure sensor. In recent years, paper has attracted much attention as a potential substrate for low-cost flexible electronics. However, the dielectric constant (k) of conventional paper is low due to the high porosit...
Article
Full-text available
On the development of flexible electronics, a highly flexible nonvolatile memory, which is an important circuit component for the portability, is necessary. However, the flexibility of existing nonvolatile memory has been limited, e.g. the smallest radius into which can be bent has been millimeters range, due to the difficulty in maintaining memory...
Article
Single- and multi-layer transparent conductive oxide (TCO) thin films exhibiting high performance, good packing density and low surface/interface roughness are deposited on silica glass substrates by the sol–gel method. The crystal and microstructural properties of the TCO thin films are evaluated as an alternate to films prepared by ultra-high vac...
Article
Silver nanowires >60 μm and even 100 μm in length have been synthesized using a polyol process by adjusting the stirring speed at 130 °C. The length is over three times longer than that of normal AgNWs. These wires have a uniform 60 nm diameter, independent of the stirring speed. At 91% transmittance at 550 nm, AgNW films fabricated at room tempera...
Article
Full-text available
We demonstrate the fabrication of highly transparent conductive networks on a cellulose nanofiber paper, called cellulose nanopaper. Uniform coating of the conductive nanomaterials, such as silver nanowires (AgNWs) and carbon nanotubes, is achieved by simple filtration of their aqueous dispersions through the cellulose nanopaper, which acts as both...
Article
Eco-friendly and low-cost cellulose nanofiber paper (nanopaper) is a promising candidate as a novel substrate for flexible electron device applications. Here, a thin transparent nanopaper-based high-mobility organic thin-film transistor (OTFT) array is demonstrated for the first time. Nanopaper made from only native wood cellulose nanofibers has ex...