Published papers

Metal–organic frameworks (MOFs) have attracted considerable interest as adsorbent materials because of their high molecular adsorption capability. To fully utilize these properties in electrically operable microstructures, integration with conductive structures is essential, as it addresses the inherent electrical limitations of MOFs. Conventional approaches for fabricating MOF-integrated electrically conductive structures typically require separate steps for material synthesis, patterning, and integration with conductive structures. In this study, we demonstrate single-step laser direct writing of arbitrarily patterned microstructures composed of MOF-5 and graphitic carbon for CO2 sensing. The laser process forms MOF-5, which has been reported to exhibit CO2 adsorption capability, together with conductive graphitic carbon within a single written microstructure, eliminating the need for separate fabrication and integration steps. The fabricated microstructures exhibit resistance changes upon CO2 adsorption by MOF-5, while the incorporated graphitic carbon generates Joule heating under applied voltage, thereby promoting desorption of the adsorbed CO2. These results indicate that single-step laser direct writing is a promising strategy for fabricating MOF-integrated microstructures that combine adsorption capability with electrical conductivity, providing a basis for broader integration of MOFs into functional microscale devices.

Efficient removal of toxic and persistent dyes from water is crucial for water purification. A promising approach involves using carbon-based electrodes for electrolysis. In this process, increasing the electrode surface area enhances the interfacial contact between the electrode and the solution, thereby improving dye removal efficiency. Accordingly, electrode materials with high electrical conductivity and a large surface area are desired to further enhance dye removal efficiency. In this study, we report the fabrication of conductive and porous structures directly on textiles via laser-induced graphitization. Porous graphitic carbon structures were fabricated by laser irradiation of textiles immersed in a lignin dispersion, which induced laser-induced graphitization in the irradiated area. The fabricated structures were successfully applied as electrodes for the electrolytic removal of the model dye, methyl orange (MO), from an aqueous solution. Lignin was used as both a carbon precursor and a flame retardant by immersing the textiles in a lignin dispersion prior to laser irradiation. By controlling lignin concentration and laser irradiation conditions, graphitic carbon structures were fabricated that retained the macroscopic fiber bundle morphology and exhibited microscale porosity on bundle surfaces. The fabricated electrodes with high electrical conductivity and a porous structure enabled electrolytic removal of MO. Among the conditions investigated, electrodes fabricated at 10 wt% lignin and a laser scanning speed of 100 mm/s showed the highest removal amount (0.99 mg/g). The proposed method offers a facile, one-step route for fabricating water purification devices capable of efficient dye removal from aqueous solutions using laser irradiation on lignin-immersed textiles.

Thermoelectric power generation, which directly converts temperature differences in the environment into electrical energy, has attracted considerable attention as a next-generation power supply technology because of its maintenance-free operation. Laser-induced graphitization has been investigated as a fabrication method for thermoelectric generators because carbon-based materials are readily available and offer lower environmental impact, and the maskless direct-writing process enables simple fabrication with high design flexibility. However, graphitic carbon structures formed by laser-induced graphitization generally exhibit p-type thermoelectric response, which was associated with absorption of atmospheric species such as O2 and H2O. In this study, graphitic carbon structures exhibiting process-tunable p- and n-type thermoelectric response are fabricated by laser irradiation using polyimide as a precursor and aniline as a nitrogen source. Moreover, as a proof-of-concept, thermoelectric generators composed of these structures are fabricated, and power generation is demonstrated using the fabricated generators. The proposed method provides an approach for further enhancing the output of thermoelectric generators fabricated by laser-induced graphitization.

Zinc–air batteries (ZABs) possess high energy density and utilize earth-abundant zinc, making them attractive candidates for next-generation sustainable energy storage devices. To meet the increasing demand for flexible power sources in wearable devices, flexible ZABs have been studied by integrating the cathode and gel electrolyte into a single structure. However, the conventional fabrication of such integrated structures often requires a complicated and multistep fabrication process, which increases manufacturing costs. In this study, a gel electrolyte-cathode integrated structure was fabricated in two steps. First, we fabricated graphitic carbon (GC) on the gel electrolyte by laser-induced graphitization of lignin-containing agarose hydrogel. Second, we added gold chloride solution dropwise onto GC to decorate GC with Au clusters. The performance of ZABs prepared with the structures decorated with Au clusters was improved, exhibiting higher open-circuit voltages and maximum power densities than those without Au cluster decoration. Furthermore, the Au clusters remained decorated on GC even after the drying–reswelling process, demonstrating that the prepared ZABs can function by reswelling even after drying. The new, simple method proposed in this study for fabricating a gel electrolyte-cathode integrated structure is expected to be an effective strategy for the practical application of flexible ZABs.

Thermoelectric power generation is a renewable energy technology that converts thermal energy into electrical energy by using temperature gradients. The fabrication of thermoelectric generators requires thermoelectric materials with different Seebeck coefficients to be arranged according to the temperature gradient and desired device geometry. In this study, we demonstrated thermoelectric power generation using a conductive graphitic carbon structure fabricated via the laser-induced graphitization of a marine biodegradable polymer. For the first time, laser-induced graphitization of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), a marine biodegradable polymer, was achieved. Furthermore, we successfully fabricated spatially selective structures with different Seebeck coefficients, an essential requirement for thermoelectric generator construction by solely controlling the laser irradiation parameters. The fabricated structures generated thermoelectric power not only from temperature differences in the atmosphere but also from thermal gradients between water and air, demonstrating their potential applicability in both terrestrial and marine environments. The proposed method offers a sustainable approach to the fabrication of thermoelectric generators.

Hydrogels have attracted attention as highly biocompatible supporting materials for wearable devices. An antenna is a key component for wireless transmission of information collected by the devices worn on the human body. Furthermore, integrating antennas into the devices eliminates the need for external wiring, which could obstruct the movement of the human body. In this study, we demonstrated the fabrication of planar conductive structures by laser-induced graphitization of tannic acid-containing alginate hydrogels and the fabrication of microstrip patch antennas (MPAs) using the fabricated structures. In a single laser scan onto the hydrogels, cracks were formed in the fabricated structures. For the fabrication of continuous planar structures, we used a method leveraging the water absorption property of hydrogels. By the method, in which an aqueous solution of tannic acid was added to the structures, followed by rescanning the laser beam to convert tannic acid to graphitic carbon, the number of cracks decreased. Additionally, it is confirmed that the fabricated structures can be applied to the radiation patches of hydrogel-based MPAs. A decrease in the resonant frequency of the fabricated MPAs corresponding to the decrease in the number of cracks is observed.

Light-driven control of the flow velocity inside a hydrogel microchannel is demonstrated by fabricating metal microstructures by multiphoton photoreduction. A technique to evaluate the local strain and compression of the hydrogel will also be described.

In this work the method of laser induced reverse transfer is applied for a stabile implantation of gold and silver nanoparticles into glass substrate. The method is based on laser ablation of gold and silver targets covered by soda-lime glass. The experiments are performed using nanosecond laser pulses at wavelength of 1064 nm. The illumination of the target through the glass side results in ablation and embedding of the ablated material into the glass. At certain conditions the implantation process is highly efficient as multiple washing of the substrate with water and even a wiping do not remove the deposited material. Detailed analyses of the processed area are performed in order to clarify the morphology, composition and the structure of the material. The mechanisms of nanoparticle formation and their embedding is discussed on a basis of heat diffusion model describing the evolution of the temperature during laser interaction. Application of the formed structures in Surface Enhanced Raman Spectroscopy (SERS) is demonstrated, as a high-sensitive detection of Ammonium nitrate and organic dyes is obtained. Multiple use of the obtained materials in SERS is also evidenced.

The patterning of electrically conductive structures directly onto the polydimethylsiloxane (PDMS) surface is crucial for the realization of flexible and elastic devices. Herein, defocused femtosecond laser pulses are irradiated for the fabrication of highly crystalline graphitic carbon structures on PDMS and a high electrical conductivity of ≈51.6 S m−1 is achieved. Utilizing the elasticity of PDMS and the high conductivity of fabricated structures, a fingertip-sized pressure sensor, which can detect pressures as low as ≈0.1 Pa, is realized. Moreover, a novel ring-shaped heart rate (HR) monitor, capable of measuring the HR from the index finger, is demonstrated, indicating the potential of fabricated structures in miniature and wearable healthcare applications.

In this work, the ablation rate and surface modifications are studied of aluminum nitride ceramic treated by a pulsed nanosecond Nd:YAG laser at four wavelengths—266 nm, 355 nm, 532 nm, and 1064 nm. The ablation depth is presented as a function of the laser fluence and the number of pulses applied. A decrease of the ablation efficiency with the increase of both laser fluence and number of pulses is observed for all wavelengths used. It is found that the laser treatment leads to the formation of a variety of micro and nanostructures on the surface of the material that are strongly dependent on the processing conditions. The laser ablation at the fundamental wavelength leads to the appearance of periodic structures on the surface. Such effect is not observed for the other wavelengths used. It is further found that the laser processing also leads to the formation a conductive layer for all wavelengths. Using a model based on the heat diffusion equation that takes into account the process of ceramic decomposition, the ablation dynamics is followed, and the dependence of the surface layers thickness on the processing conditions is estimated. The results obtained are used to explain the experimentally observed dependences.

This work presents results on nanosecond laser-induced surface structuring of AlN ceramic. The process under investigation is performed using the fundamental wavelength of Nd:YAG laser system. It is shown that the change of the ambient air pressure from atmospheric one to 10-4 Torr, leads to modification of the surface morphology – from ripples structure with a periodicity of about the incident irradiation wavelength, to an oriented periodic microstructure with a characteristic period in the range 10–15 µm. In all cases when the periodic structures are formed, a clear orientation perpendicular to the incident irradiation polarization is observed. The properties of the oriented periodic microstructures (OPMSs) formed at the lowest pressure condition are studied in more details, as the influence of the processing conditions on their characteristics is presented. It is found that the formation of OPMS requires a minimal laser pulse number and application of laser fluence above certain threshold. The surface of the fabricated structures is conductive, as the performed analyses indicate that the effect is related to presence aluminium in the processed areas. Possible mechanisms of formation of these structures are discussed. Ablation experiments using irradiation at 532 and 266 nm are also reported and discussed. The presented results may have significant impact when laser processing of AlN ceramics is considered, and for the design of structures with potential applications in microelectronics, tribology, and aluminium-based UV plasmonics.

We coated triangular-shaped silver nanoparticles, a type of anisotropic nanoplate (NPL), with silica (i.e., prepared Ag@SiO2 NPLs). When we irradiated Ag@SiO2 NPLs with nanosecond-pulsed laser light for 10 s, the triangular shape changed to spherical because of the photothermal effect. A high laser power exposed the silver core, and the particles exhibited strong antimicrobial activity. In contrast, at a moderate laser power, the silica layer crystallized, and the particles’ antimicrobial activity decreased. Thus, a combination of Ag@SiO2 NPLs and an appropriately tuned power of pulsed laser irradiation facilitated a decreased or an increased antimicrobial activity.

Utilizing renewable resources for electrodes realizes the sustainable fabrication of a supercapacitor with high environmental friendliness. Laser-based graphitization of biomass has been emerging as a promising technique for patterning the electrodes of a supercapacitor with renewable resources. Herein, simultaneous patterning and microstructuring of laser-induced graphene (LIG) on a renewable biomass resource, bamboo, by a laser-based graphitization technique was demonstrated. By irradiating femtosecond laser pulses onto bamboo, graphitization and microstructuring were both induced simultaneously, forming conductive structures with high surface area. Furthermore, LIG patterned on bamboo by our method was used as the electrodes of supercapacitors. NaCl was selected as the electrolyte for the fabrication of supercapacitors. The proposed method realizes the fabrication of environmentally-friendly supercapacitors comprised of all renewable biomass resources.

With the rapid development of the Internet of Things (IoT), numerous electronic devices are expected to be installed in the natural environment. Triboelectric nanogenerators (TENGs) can be reliable devices providing on-site, clean, and sustainable power generation in nature. The fabrication of metal-free, biodegradable TENGs will reduce the environmental burden and disposal cost after usage. In this study, we demonstrate the direct fabrication of conductive graphitic carbon on a biodegradable polymer composite by laser-induced graphitization and apply the structures for metal-free biodegradable TENGs. Laser-induced graphitization of poly(lactic acid) (PLA) composite sheets was achieved for the first time by mixing alkaline lignin powder as a filler. The fabricated TENGs could generate electricity, and the power density reached ∼1.98 mW/m2 at a load resistance of 200 MΩ when the fabricated TENG was driven at a contact frequency of ∼1 Hz and a contact pressure of 1 N. Power generation by contact with water and plants was also demonstrated, showing its applicability for power generation in natural environments. The proposed method in this study offers the facile fabrication of biodegradable devices and the development of sustainable power generation devices.

In this study, we patterned structures composed of graphitic and silicon carbide nanocrystals by the laser-induced carbonization of polydimethylsiloxane. The average size of the observed silicon carbide nanocrystals varied depending on the energy per laser pulse, or pulse energy, used for patterning, where larger crystals were observed for structures patterned with higher pulse energies. Moreover, the electrical property of the patterned structure shifted from conductive to semiconductive, as the pulse energy for patterning increased. To the best of our knowledge, this is the first demonstration of the patterning of structures exhibiting measurable semiconducting properties by the laser-induced carbonization of polymers.

We demonstrate the direct fabrication of graphitic carbon on a biodegradable polymer composite by laser-induced graphitization and applied the structures for the electrode of metal-free biodegradable triboelectric nanogenerators (TENGs). The laser-induced graphitization of fully biodegradable composite sheets composed of alkaline lignin powder and poly(L-lactic acid) (PLLA) was realized. The fabricated TENGs generated electricity by contact with synthetic polymers and natural materials such as water and plant leaves, indicating their applicability for on-site power generation in the natural environment. The proposed method provides the facile fabrication of biodegradable devices and the development of sustainable power generation.

An electric double-layer capacitor (EDLC), which is one of the pivotal energy storage devices, provides rapid charge–discharge capabilities and an extended cycle life. Areal capacitance, a key indicator of EDLC performance, increases with the specific surface area of its electrodes. This study demonstrates a method for significantly increasing the specific surface area in the laser-induced graphitization of biodegradable polymers by incorporating NaHCO3 into the composite sheet, generating not only microscale pores but also a large number of nanoscale fine pores. Furthermore, it shows that using these structures as EDLC electrodes leads to a substantial increase in areal capacitance. An increase in the number of fine pores formed in the structure and a corresponding rise in the areal capacitance of the fabricated EDLC are observed with the increase in the NaHCO3 weight ratio. Notably, the composite sheets are composed of natural-derived, biodegradable materials, while NaHCO3 is known for its low corrosivity and biotoxicity. The proposed method thus offers a pathway for fabricating energy storage devices with minimal environmental impact, ensuring their eco-friendly disposal post-use.

In laser-induced graphitization, decreasing the linewidth of patterned structures is key to enhancing the potential in microdevice fabrication. In this study, we demonstrated that graphitic carbon structures with narrow linewidth can be directly patterned in laser-induced graphitization of cellulose nanofiber (CNF) film by immersing the film in CaI2 solution. The linewidth of the structures increased with increasing laser power, indicating that the linewidth can be readily controlled. Based on the above findings, we patterned multiple concentric structures with precisely controlled linewidth to fabricate a Fresnel zone plate, which can focus laser light in the visible wavelength range.

The integration of soft electronic devices with biological systems has garnered increasing attention for applications such as bioinspired soft robotics and wearable health monitors. These devices are designed to interface with biological surfaces or tissues, mimicking the transmission of electrical signals through neural tissues while ensuring high biocompatibility and flexibility. Conductive polymer hydrogels, combining conductive polymers and hydrogels, have emerged as promising materials owing to their flexibility, stretchability, and biocompatibility. In the realization of advanced soft electronic devices, it is key to employ materials with high flexibility and biocompatibility and, moreover, develop techniques for the fabrication of precise and localized conductive structures inside hydrogels. In this study, we demonstrated the spatially selective polymerization of conductive polymers inside a thermoresponsive hydrogel. A poly(N-isopropylacrylamide) (PNIPAM) hydrogel was immersed in a precursor solution of polyaniline (PANI), and polymerization was induced by femtosecond laser pulse irradiation. Conductive polymer microstructures were successfully fabricated inside the hydrogel. Furthermore, using the thermal responsiveness of the PNIPAM hydrogel, we demonstrated that the electrical resistance of the PANI structures changed in response to temperature. The present method provides a strategy for the precise and localized fabrication of conductive structures inside hydrogels, offering an approach to the fabrication of soft electronic devices.