Several kinds of flexible and stretchable devices were fabricated by laser direct writing. A stretchable Ag paste was employed as a conductive material, which showed the suitable resistance stability in the application to a stretchable electronic device. A stretchable Ag paste coating on a polyurethane film was applied to the fabrication of a stretchable in-plane micro-supercapacitor (MSC) consisting of a carbon electrode which was prepared by laser ablation of a carbon nanotube (CN) layer on the stretchable Ag paste coating. The cyclic voltammogram of the device showed the capacitive characteristics even during stretching. The in-plane MSC consisting of CN electrode can be worked as a power source for a digital clock during stretching up to 44% elongation. A stretchable Ag paste coating pattern was also applied to an antenna-type sensor, where a screen printing mask for the stretchable Ag paste coating was prepared by laser ablation. A black-dye containing gel film on a silk cloth was patterned by galvano-scanning of ns pulse laser beam. A dipole-type antenna pattern consisting of a stretchable Ag paste coating was prepared on a polyurethane film. A reversible change of return loss at 5.317 GHz during stretching and shrinking of the antenna pattern was observed. A flexible antenna type sensor was also applied to agri-sensing, where the sensing mechanism was based on the change of the resonant properties of the antenna-type sensor caused by the dielectric condition change around the device. A flexible spiral antenna-type sensor on a silicone elastomer sheet was fabricated by printing of a stretchable Ag paste through a gel film mask, where a spiral antenna pattern was drawn by laser ablation of a black gel film on the silicone elastomer sheet. The sensing of the drying process of a plant leaf was observed by the antenna type sensor. The change of the return loss spectrum can be attributed to the change of the water content in a plant tissue which influenced the dielectric properties of the plant leaf.
Laser direct writing based on a graphene hybrid material was studied to develop the on-demand fabrication of an antenna-type sensor device related to IoT technology. A few-layer graphene oxide (GO) water dispersion formed a gel-like fluid and worked as a binder for CuO nanorods (NRs). A Go/CuO NRs hybrid coated flexible polymer substrate was scanned by a 445 nm semiconductor laser through a Galvano-scanner. Optical microscope images showed the Cu grain growth with increasing the laser power although a laser scanning under excess laser power condition formed an inhomogeneous film with the formation of an isolated large Cu grain. The laser-reduction of CuO NRs was enhanced by the presence of GO, which is an effective reductive agent for metal oxide nano-materials. Raman spectroscopy showed the formation of a reduced graphene oxide (rGO) and the disappearance of CuO signals in the Raman spectrum of a laser-scanned GO/CuO NRs hybrid film. The formation of metallic Cu from CuO NRs was also confirmed by XPS Cu2p spectra, which showed the disappearance of satellite peaks assigned to CuO. A meander line antenna pattern was drawn by laser direct writing on a GO/CuO NRs hybrid film and then unirradiated area was removed by water etching. The antenna-type sensor showed resonance peaks in the region from 1 to 6 GHz. The changes of resonance frequency and return loss were studied by dropping various solvents on the antenna-type chemical sensor. A resonance peak at around 5.50 GHz was sensitive to the dielectric changes depending on the solvents. A remarkable enhancement of the return loss and resonance frequency shift was caused by a solvent with the higher dielectric constant. The return loss change and dielectric constant of solvent showed a clear relationship.
Laser reduced graphene oxide-based interdigitated microelectrodes were functionalized with TiO2 nanoparticles towards sensor applications. Two kinds of interdigitated microelectrodes were prepared by laser direct writing using graphene oxide (GO) and TiO2 nanoparticles. One is a TiO2 nanoparticle-deposited interdigitated microelectrode consisting of GO and laser-induced reduced graphene oxide (rGO), where the rGO/GO/rGO structure was prepared by laser direct writing on a GO-coated PET film and then a TiO2 sol solution was drop-casted on the electrode. Another is a TiO2/rGO hybrid interdigitated microelectrode prepared by laser direct writing on a TiO2 nanoparticle-GO hybrid film. The UV light sensitivity of the TiO2 nanoparticle-deposited rGO/GO/rGO interdigitated microelectrode and the oxygen quenching behavior were applied to oxygen sensing. The output voltage from the TiO2 nanoparticle-deposited rGO/GO/rGO structure in the AC detection mode under 369 nm LED irradiation showed clear relationship with the degree of vacuum. The sensing behavior was based on the photo-generated carrier quenching by oxygen. The irradiation of a 405 nm blue violet laser to a TiO2 nanoparticle-GO hybrid film caused the crystal phase transition from anatase to rutile TiO2 accompanying the melting of anatase nanoparticles. The TiO2/rGO hybrid interdigitated microelectrode consisting of anatase TiO2, rutile TiO2, and rGO was prepared by laser direct writing. The TiO2/rGO hybrid interdigitated microelectrode showed the response to visible light irradiation.
The electronic interconnection based on solution processes using nanomaterials is one of the key technologies in the printed electronics towards flexible and wearable devices in IoT. The laser direct writing of three kinds of conductive micropatterns and the device applications were studied. The first one is a Cu micro-grid structure using a Cu nanoparticle ink and the application to a strain sensor. The second one is a reduced graphene oxide (rGO) interdigitated electrode prepared by laser-induced reduction of graphene oxide (GO) and the application of an rGO/GO/rGO interdigitated microelectrode to a humidity sensor. The third one is a carbon interdigitated electrode prepared by laser carbonization of a polyimide (PI) film and the application to an in-plane micro-supercapacitor (MSC). One of the important features in wearable devices is the stability and reliability under bending conditions. The influences of bending in the electric properties were studied for the Cu micro-grid and an rGO/GO/rGO interdigitated electrode. Remarkable resistance change of the Cu micro-grid structure was observed in bending experiment. An unusual large resistance change was explained by a nanostructure remaining even after sintering. On the other hand, the rGO/GO/rGO structure showed excellent stability of the output signal in humidity sensing experiments against bending. Such a stable electronic properties against bending stress was attributed to the electronic conductivity based on π−π interaction between graphene planes. A crystalline layered structure of graphene planes were clearly observed in TEM images for an rGO film.
Micropatterns consisting of reduced graphene oxide (rGO) and graphene oxide (GO) were fabricated via laser direct writing where an insulating GO was converted to a conductive rGO by laser-induced reduction. The rGO/GO microinterdigitated electrode showed humidity sensing behaviors. The influence of the shape and size of the rGO/GO microinterdigitated electrode was studied to discuss the sensing mechanism and to improve the sensitivity. Three kinds of rGO(w)/GO(g) micropatterns were prepared by laser direct writing on a GO-coated PET (polyethylene terephthalate) substrate using a CW 405 nm semiconductor laser, where w and g are the rGO electrode width and the GO gap width between rGO electrodes in micrometer unit, respectively. The humidity sensing properties of rGO(400)/GO(100), rGO(160)/GO(40), and rGO(80)/GO(20) were studied by monitoring the output voltage waveform from an electronic circuit consisting an rGO/GO interdigitated electrode and resistors in AC mode operation. The sensitivity enhancement induced by charging property of an rGO/GO interdigitated electrodes was observed accompanying transient decay of the output voltage. The shape and size of an rGO/GO interdigitated electrode remarkably influenced the humidity sensing behaviors depending on the contribution ratio of C and R parameters.
Micro-supercapacitors with small size, light weight, flexibility while maintaining high energy and power output are required for portable miniaturized electronics. The fabrication methods and materials should be cost-effective, scalable, and easily integrated to current electronic industry. Carbon materials have required properties for high-performance flexible supercapacitors, including high specific surface areas, electrochemical stability, and high electrical conductivity, as well as the high mechanical tolerance. Laser direct writing method is a non-contact, efficient, single-step fabrication technique without requirements of masks, post-processing, and complex clean room, which is a useful patterning technique, and can be easily integrated with current electronic product lines for commercial use. Previously we have reported micro-supercapacitors fabricated by laser direct writing on polyimide films in air or Ar, which showed highcapacitive performance. However, the conductivity of the carbon materials is still low for fast charge-discharge use. Here, we demonstrated the fabrication of flexible carbon/Au composite high-performance MSCs by first laser direct writing on commercial polyimide films followed by spin-coating Au nanoparticles ink and second in-situ laser direct writing using the low-cost semiconductor laser. As-prepared micro-supercapacitors show an improved conductivity and capacitance of 1.17 mF/cm2 at a high scanning rate of 10,000 mV/s, which is comparable to the reported capacitance of carbon-based micro-supercapacitors. In addition, the micro-supercapacitors have high bend tolerance and long-cycle stability.
The selective metallization on a flexible polymer film via laser direct writing and the following additive metallization process was studied as an alternate to conventional semi-additive process in the fabrication of printed circuit board. A Cu micropattern was fabricated on a polyimide film via CW blue-violet laser direct writing using a Cu nanoparticle ink and applied to a seed layer for the Cu electroplating. The on-demand processing of a Cu micropattern whose line width and thickness are ca. 5 and 2 μm, respectively, was achieved by combination of a laser-written seed micropattern and the following electro-plating. The homogeneity of the Cu micropatterns prepared from Cu nanoparticles was easily improved by combination with the following Cu plating.
The 2D and 3D laser direct writing using Cu nanoparticle ink were studied using a compact blue-violet semiconductor laser. The laser direct writing based on a motion controller and G-code language program on PC enabled to prepare various kinds of shaped Cu grids which can be easily designed on-demand. A Cu grid pattern was prepared on a flexible and transparent polymer substrate and applied to a stress sensor. A star-shaped grid was fabricated a polymer substrate and the performance as a stress sensor for detecting the motion of human hand devises was demonstrated toward wearable electronics. In the preliminary 3D study, we have employed the layer-by-layer formation of a 3D structure, where cycle of the spin-coating of a metal nanoparticle ink and the laser direct writing were repeated. The 3D microstructures prepared by the 3D laser direct writing using Ag and Cu nanoparticle inks suggested the possibility of a 3D interconnection.
Micro-/nano-scale power supply units with high energy and high power densities are critical components for the development of compact miniaturized portable electronic devices. Supercapacitors have attracted many research attentions due to their high power density, robust cycle performance, pollution-free operation, and maintenance-free features. Besides, the properties of small size, light weight, and flexibility are also required. On-chip microsupercapacitors (MSCs) have the potential acting as power supply units in portable devices, due to their simplified packaging processes and compatibility to the integrated circuits. However, the fabrication methods and materials should be cost-effective, scalable, and compatible to current electronic industry. Carbon materials own high specific surface areas, electrochemical stability, and high electrical conductivity, which are critical parameters for high-power supercapacitors. Moreover, the high mechanical tolerance makes them good candidates for flexible wearable devices. Therefore, MSCs based on carbon materials would satisfy the requirements of portable electronics. In this work, we demonstrated the fabrication of carbon MSCs by laser direct writing on commercial polyimide sheets in Ar with lowcost semiconductor cw-laser with a wavelength of 405nm. The obtained structures are macro-nanostructures comprising graphitized and amorphous carbon with relatively smooth surfaces and low resistance, in compared with the structures obtained by laser writing in air. As-prepared micro-supercapacitors show a high capacitance of about 14.9 mF/cm2 at a scanning rate of 10 mV/s, which is comparable to the reported highest capacitance of carbon-based supercapacitors fabricated by pulse-laser writing.