Wan-Ho Chung

In?situ monitoring of flash-light sintering of copper nanoparticle ink for printed electronics

In this work, a flash-light sintering process for Cu nanoinks was studied. In order to precisely monitor the milliseconds flash-light sintering process, a real-time Wheatstone bridge electrical circuit and a high-rate data acquisition system were used. The effects of several flash-light irradiation conditions (irradiation energy, pulse number, on-time, and off-time) and the effects of the amount of poly(N-vinylpyrrolidone) in the Cu nanoink on the flash-light sintering process were investigated. The microstructures of the sintered Cu films were analyzed by scanning electron microscopy. To investigate the oxidation or reduction of the oxide-covered copper nanoparticles, a crystal phase analysis using x-ray diffraction was performed. In addition, the sheet resistance of Cu film was measured using a four-point probe method. From this study, it was found that the flash-light sintered Cu nanoink films have a conductivity of 72 Ωm/sq without any damage to the polyimide substrate. Similar nanoinks are expected to be widely used in printed and flexible electronics products in the near future.

In situ monitoring of a flash light sintering process using silver nano-ink for producing flexible electronics

In this work, a flash light sintering process using silver nano-inks is investigated. A silver nano-ink pattern was printed on a flexible PET (polyethylene terephthalate) substrate using a gravure-offset printing system. The printed silver nano-ink was sintered at room temperature and under ambient conditions using a flash of light from a xenon lamp using an in-house flash light sintering system. In order to monitor the light sintering process, a Wheatstone bridge electrical circuit was devised and changes in the voltage difference of the silver nano-ink were recorded during the sintering process using an oscilloscope. The sheet resistance changes during the sintering process were monitored using the in situ monitoring system devised, under various light conditions (e.g. light energy, on-time and off-time duration, and pulse numbers). The microstructure of the sintered silver film and the interface between the silver film and the PET substrate were observed using a scanning electron microscope, a focused ion beam and an optical microscope. The electrical sheet resistances of the sintered silver films were measured using a four-point probe method. Using the in situ monitoring system devised, the flash light sintering mechanism was studied for each type of light pulse (e.g. evaporation of organic binder followed by the forming of a neck-like junction and its growth, etc).The optimal flash light sintering condition is suggested on the basis of the in situ monitoring results. The optimized flash light sintering process produces a silver film with a lower sheet resistance (0.95 Ω/sq) compared with that of the thermally sintered silver film (2.03 Ω/sq) without damaging the PET substrate or allowing interfacial delamination between the silver film and the PET substrate.

Photonic welding of ultra-long copper nanowire network for flexible transparent electrodes using white flash light sintering

Copper nanowire (Cu NW)-based flexible transparent electronics represent an enormous breakthrough for the development of efficient, scalable and facile processing techniques. From the standpoint of commercialization, a cost-effective and eco-friendly procedure for welding nanowires is imperative to fabricate Cu NW network-based transparent electrodes. In this study, a photonic welding method using a white flash light (WFL) technique was developed in-house for welding Cu NWs in order to produce highly conductive and transparent electrodes under ambient conditions. Flexible Cu NW films with sheet resistances of 128 Ω □−1 and transmittances of >95% at 560 nm on PET substrates were obtained by using light intensity of 1.0 J cm−2 with single pulse and an irradiation time of 5 ms. The WFL technique facilitates localized surface heating and subsequent welding at the junction of the Cu NWs with excellent stability, low sheet resistance and no damage to the polymer substrates. It is expected that the WFL technique will be widely applied to flexible printed and optoelectronic devices such as touch panel displays and solar cells.

Intensive Plasmonic Flash Light Sintering of Copper Nanoinks Using a Band-Pass Light Filter for Highly Electrically Conductive Electrodes in Printed Electronics

In this work, an intensive plasmonic flash light sintering technique was developed by using a band-pass light filter matching the plasmonic wavelength of the copper nanoparticles. The sintering characteristics, such as resistivity and microstructure, of the copper nanoink films were studied as a function of the range of the wavelength employed in the flash white light sintering. The flash white light irradiation conditions (e.g., wavelength range, irradiation energy, pulse number, on-time, and off-time) were optimized to obtain a high conductivity of the copper nanoink films without causing damage to the polyimide substrate. The wavelength range corresponding to the plasmonic wavelength of the copper nanoparticles could efficiently sinter the copper nanoink and enhance its conductivity. Ultimately, the sintered copper nanoink films under optimal light sintering conditions showed the lowest resistivity (6.97 μΩ·cm), which was only 4.1 times higher than that of bulk copper films (1.68 μΩ·cm).

Electrical wire explosion process of copper/silver hybrid nano-particle ink and its sintering via flash white light to achieve high electrical conductivity

In this work, combined silver/copper nanoparticles were fabricated by the electrical explosion of a metal wire. In this method, a high electrical current passes through the metal wire with a high voltage. Consequently, the metal wire evaporates and metal nanoparticles are formed. The diameters of the silver and copper nanoparticles were controlled by changing the voltage conditions. The fabricated silver and copper nano-inks were printed on a flexible polyimide (PI) substrate and sintered at room temperature via a flash light process, using a xenon lamp and varying the light energy. The microstructures of the sintered silver and copper films were observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). To investigate the crystal phases of the flash-light-sintered silver and copper films, x-ray diffraction (XRD) was performed. The absorption wavelengths of the silver and copper nano-inks were measured using ultraviolet-visible spectroscopy (UV-vis). Furthermore, the resistivity of the sintered silver and copper films was measured using the four-point probe method and an alpha step. As a result, the fabricated Cu/Ag film shows a high electrical conductivity (4.06 μΩcm), which is comparable to the resistivity of bulk copper (1.68 μΩcm). In addition, the fabricated Cu/Ag nanoparticle film shows superior oxidation stability compared to the Cu nanoparticle film.

Welding of silver nanowire networks via flash white light and UV-C irradiation for highly conductive and reliable transparent electrodes

In this work, silver nanowire inks with hydroxypropyl methylcellulose (HPMC) binders were coated on polyethylene terephthalate (PET) substrates and welded via flash white light and ultraviolet C (UV-C) irradiation to produce highly conductive transparent electrodes. The coated silver nanowire films were firmly welded and embedded into PET substrate successfully at room temperature and under ambient conditions using an in-house flash white light welding system and UV-C irradiation. The effects of light irradiation conditions (light energy, irradiation time, pulse duration, and pulse number) on the silver nanowire networks were studied and optimized. Bending fatigue tests were also conducted to characterize the reliability of the welded transparent conductive silver nanowire films. The surfaces of the welded silver nanowire films were analyzed via scanning electron microscopy (SEM), while the transmittance of the structures was measured using a spectrophotometer. From the results, a highly conductive and transparent silver nanowire film with excellent reliability could be achieved at room temperature under ambient conditions via the combined flash white light and UV-C irradiation welding process.

UV-assisted flash light welding process to fabricate silver nanowire/graphene on a PET substrate for transparent electrodes

Graphene oxide and silver nanowires were bar coated onto PET substrates and then welded using an ultraviolet (UV)-assisted flash light irradiation process to achieve both high electrical conductivity and low haze. The irradiation process connected adjacent silver nanowires by welding, while simultaneously reducing the graphene oxide to graphene. This process was performed using a custom UV-assisted flash light welding system at room temperature under ambient conditions and was extremely rapid, with processing time of several milliseconds. The effects of varying the weight fractions of the silver nanowires and graphene oxide and of varying the UV-assisted flash light welding conditions (light energy and pulse duration) were investigated. The surface morphologies of the welded silver nanowire/graphene films were analyzed using scanning electron microscopy. Optical characterizations, including transmittance and haze measurements, were also conducted using a spectrophotometer. To test their resistance to oxidation, the welded silver nanowire/graphene films were subjected to high temperature in a furnace (100 °C), and their sheet resistances were measured every hour. The flash light welding process was found to yield silver nanowire/graphene films with high oxidation resistance, high conductivity (14.35 Ω·sq–1), high transmittance (93.46%), and low haze (0.9%). This material showed uniform temperature distribution when applied as a resistive heating film.

Selective wavelength plasmonic flash light welding of silver nanowires for transparent electrodes with high conductivity

In this work, silver nanowires (AgNWs) printed on a polyethylene terephthalate substrate using a bar coater were welded via selective wavelength plasmonic flash light irradiation. To achieve high electrical conductivity and transparent characteristics, the wavelength of the flash white light was selectively chosen and irradiated by using high-pass, low-pass, and band-pass filters. The flash white light irradiation conditions such as on-time, off-time, and number of pulses were also optimized. The wavelength range (400-500 nm) corresponding to the plasmonic wavelength of the AgNW could efficiently weld the AgNW films and enhance its conductivity. To carry out in-depth study of the welding phenomena with respect to wavelength, a multiphysics COMSOL simulation was conducted. The welded AgNW films under selective plasmonic flash light welding conditions showed the lowest sheet resistance (51.275 Ω/sq) and noteworthy transmittance (95.3%). Finally, the AgNW film, which was welded by selective wavelength plasmonic flash light with optical filters, was successfully used to make a large area transparent heat film and dye-sensitized solar cells showing superior performances.

Rapid, cool sintering of wet processed yttria-stabilized zirconia ceramic electrolyte thin films

Here we report a photonic annealing process for yttria-stabilized zirconia films, which are one of the most well-known solid-state electrolytes for solid oxide fuel cells (SOFCs). Precursor films were coated using a wet-chemical method with a simple metal-organic precursor solution and directly annealed at standard pressure and temperature by two cycles of xenon flash lamp irradiation. The residual organics were almost completely decomposed in the first pre-annealing step, and the fluorite crystalline phases and good ionic conductivity were developed during the second annealing step. These films showed properties comparable to those of thermally annealed films. This process is much faster than conventional annealing processes (e.g. halogen furnaces); a few seconds compared to tens of hours, respectively. The significance of this work includes the treatment of solid-state electrolyte oxides for SOFCs and the demonstration of the feasibility of other oxide components for solid-state energy devices.