Research

Selective laser sintering

Flexible electronics opened a new class of future electronics. The foldable, light and durable nature of flexible electronics allows vast flexibility in applications such as display, energy devices and mobile electronics. Even though conventional electronics fabrication methods are well developed for rigid substrates, direct application or slight modification of conventional processes for flexible electronics fabrication cannot work. The future flexible electronics fabrication requires totally new low-temperature process development optimized for flexible substrate and it should be based on new material too. Here we present a simple approach to developing a flexible electronics fabrication without using conventional vacuum deposition and photolithography. We found that direct metal patterning based on laser-induced local melting of metal nanoparticle ink is a promising low-temperature alternative to vacuum deposition– and photolithography-based conventional metal patterning processes. The “digital” nature of the proposed direct metal patterning process removes the need for expensive photomask and allows easy design modification and short turnaround time. This new process can be extremely useful for current small-volume, large-variety manufacturing paradigms. Besides, simple, scalable, fast and low-temperature processes can lead to cost-effective fabrication methods on a large-area polymer substrate. The developed process was successfully applied to demonstrate high-quality Ag patterning (2.1 µΩ·cm) and high-performance flexible organic field effect transistor arrays.

Selective laser ablation

We introduce a facile method to enhance the functionality of a patterned metallic transparent conductor through selective laser ablation of metal nanowire percolation network. By scanning focused nanosecond pulsed laser on silver nanowire percolation network, silver nanowires are selectively ablated and patterned without using any conventional chemical etching or photolithography steps. Various arbitrary patterns of silver nanowire transparent conductors are readily created on the percolation network by changing various laser parameters such as repetition rate and power. The macroscopic optical and electrical properties of the percolation network transparent conductor can be easily tuned by changing the conductor pattern design via digital selective laser ablation. Further investigation on the silver nanowire based electrode line prepared by the ablation process substantiates that the general relation for a conducting thin film fails at a narrow width, which should be considered for the applications that requires a high resolution patterns.

Selective laser nanowelding

In the current transparent-conductor industry, indium tin oxide (ITO) film is most frequently used for optoelectronic devices, such as thin-film solar cells, flat-panel displays, and touch-screen panels. However, due to the unpredictable supply of indium, the large material waste involved in production, and the slow production speed, the price of ITO film is increasing rapidly. Most of all, although ITO can be made to coat flexible substrates, its brittle ceramic nature limits its application in flexible and stretchable devices. As an alternative to ITO, Cu NWs have received considerable attention as an alternative to Ag NWs for future transparent conductors because the price of Cu is almost a hundred times cheaper than that of other noble metals, while the electrical conductivity is comparable to the metals with the highest conductivities. Despite those advantages, the actual use of Cu NWs for transparent conductors has been limited mainly due to the fast oxidation problem upon the thermal annealing process in air. The oxidation rate of the Cu nanostructure is drastically higher than bulk Cu because the surface area is significantly increased compared with that in the bulk state. This inherent oxidation problem hinders the development of Cu-NW device fabrication processes carried out in ambient conditions. In this work, the flexible transparent conductor and stretchable electrode based on a Cu-NW percolation network is reported. Its fabrication involves ultrafast plasmonic nanoscale welding using a circularly polarized laser under ambient conditions and room temperatures are used to minimize the Cu oxidation problem, as anticipated from previous studies involving rapid thermal annealing (RTA) techniques.

Selective laser reduction

Nanostructured materials, including metal and metal-oxide nanomaterials, have been widely applied to a variety of electronic devices such as smart human skins, energy device, sensor, and catalyst according to their functionalities. The most obvious role of common metallic nanomaterial is to conduct electrons efficiently and act as an electrode, however, they are easily oxidized by air environments since surface area in nanostructure is maximized compared to the bulk states. As a result, prevention of oxidation through encapsulation or by controlling the ambient has been a significant issue in handling metallic nanomaterials. On the other hand, metal-oxide nanomaterials are mostly free from such problem. Since they are already oxidized naturally or compulsively by neighboring environments or external stimulations, these nanomaterials are much more chemically inert, and thus possess numerous advantages over its metallic states as they can be easily stored without any controlled environment and directly applicable to versatile processes in ambient condition.Laser induced selective photothermochemical reduction is demonstrated to locally and reversibly control the oxidation state of Cu and Cu oxide nanowires in ambient conditions without any inert gas environment. This new concept of “nanorecycling” can monolithically integrate Cu and Cu oxide nanowires by restoring oxidized Cu, considered unusable for the electrode, back to a metallic state for repetitive reuse.

Selective laser growth

For functional nanowire based electronics fabrication, conventionally, combination of complex multiple steps, such as (1) chemical vapor deposition (CVD) growth of nanowire, (2) harvesting of nanowire, (3) manipulation and placement of individual nanowires, and (4) integration of nanowire to circuit are necessary. Each step is very time consuming, expensive, and environmentally unfriendly, and only a very low yield is achieved through the multiple steps. As an alternative to conventional complex multistep approach, original findings are presented on the first demonstration of rapid, one step, digital selective growth of nanowires directly on 3D micro/nanostructures by developing a novel approach; laser induced hydrothermal growth (LIHG) without any complex integration of series of multiple process steps such as using any conventional photolithography process or CVD. The LIHG process can grow nanowires by scanning a focused laser beam as a local heat source in a fully digital manner to grow nanowires on arbitrary patterns and even on the non-flat, 3D micro/nano structures in a safer liquid environment, as opposed to a gas environment. The LIHG process can greatly reduce the processing lead time and simplify the nanowire-based nanofabrication process by removing multiple steps for growth, harvest, manipulation/placement, and integration of the nanowires. LIHG process can grow nanowire directly on 3D micro/nano structures, which will be extremely challenging even for the conventional nanowire integration processes. LIHG does not need a vacuum environment to grow nanowires but can be performed in a solution environment which is safer and cheaper. LIHG can also be used for flexible substrates such as temperature-sensitive polymers due to the low processing temperature. Most of all, the LIHG process is a digital process that does not require conventional vacuum deposition or a photolithography mask.