Sejeong Won

Tensile characteristics of metal nanoparticle films on flexible polymer substrates for printed electronics applications

Metal nanoparticle solutions are widely used for the fabrication of printed electronic devices. The mechanical properties of the solution-processed metal nanoparticle thin films are very important for the robust and reliable operation of printed electronic devices. In this paper, we report the tensile characteristics of silver nanoparticle (Ag NP) thin films on flexible polymer substrates by observing the microstructures and measuring the electrical resistance under tensile strain. The effects of the annealing temperatures and periods of Ag NP thin films on their failure strains are explained with a microstructural investigation. The maximum failure strain for Ag NP thin film was 6.6% after initial sintering at 150 °C for 30 min. Thermal annealing at higher temperatures for longer periods resulted in a reduction of the maximum failure strain, presumably due to higher porosity and larger pore size. We also found that solution-processed Ag NP thin films have lower failure strains than those of electron beam evaporated Ag thin films due to their highly porous film morphologies.

Double-layer CVD graphene as stretchable transparent electrodes.

The stretchability of CVD graphene with a large area is much lower than that of mechanically exfoliated pristine graphene owing to the intrinsic and extrinsic defects induced during its synthesis, etch-out of the catalytic metal, and the transfer processes. This low stretchability is the main obstacle for commercial application of CVD graphene in the field of flexible and stretchable electronics. In this study, artificially layered CVD graphene is suggested as a promising candidate for a stretchable transparent electrode. In contrast to single-layer graphene (SLG), multi-layer graphene has excellent electromechanical stretchability owing to the strain relaxation facilitated by sliding among the graphene layers. Macroscopic and microscopic electromechanical tensile tests were performed to understand the key mechanism for the improved stretchability, and crack generation and evolution were systematically investigated for their dependence on the number of CVD graphene layers during tensile deformation using lateral force microscopy. The stretchability of double-layer graphene (DLG) is much larger than that of SLG and is similar to that of triple-layer graphene (TLG). Considering the transmittance and the cost of transfer, DLG can be regarded as a suitable candidate for stretchable transparent electrodes.

Improving the stretchability of as-deposited Ag coatings on poly-ethylene-terephthalate substrates through use of an acrylic primer

In this paper, we report that silver films evaporated on poly-ethylene-terephthalate (PET) substrates coated with an acrylic primer can be stretched beyond 70% without fracture. As-deposited films show a larger failure strain than annealed coatings. These observations are rationalized in light of a ductile fracture mechanism where debonding from the substrate coevolves with strain localization. The results of this study indicate that PET substrates coated with an acrylic primer layer may be suitable for stretchable electronics.

Enhancement of titanium nitride barrier metal properties by nitrogen radical assisted metalorganic chemical vapor deposition

Using metalorganic chemical vapor deposition assisted by a nitrogen radical irradiation generated by rf plasma, we have enhanced the quality and the step coverage of titanium nitride barrier metals for the contact holes with a high aspect ratio and a submicron radius. Electrical resistivity measurements show that the film resistivity improves by a factor of five as the proper nitrogen irradiation has been applied. The step coverage in a contact hole with 0.4 μm diam and 3:1 aspect ratio has been improved from 50% to 80% by applying nitrogen plasma, clearly demonstrating the effectiveness of this technique in the conformal deposition of barrier metals for the ultra‐large scale integration. The incident nitrogen radical is believed to play several roles, such as the enhancement of surface migration rate of molecules and the reduction of the amount of hydrocarbon incorporating into the film during the deposition.

Tensile and fatigue behaviors of printed Ag thin films on flexible substrates

Flexible electronics using nanoparticle (NP) printing has been highlighted as a key technology enabling eco-friendly, low-cost, and large-area fabrication. For NP-based printing to be used as a successive alternative to photolithography and vacuum deposition, stretchability and long term reliability must be considered. This paper reports the stretchability and fatigue behavior of 100 nm thick NP-based silver thin films printed on polyethylene-terephthalate substrate and compares it to films deposited by electron-beam evaporation. NP-based films show stretchability and fatigue life comparable to evaporated films with intergranular fracture as the dominant failure mechanism.

Calligraphic ink enabling washable conductive textile electrodes for supercapacitors

The appeal of wearable devices for future electronics has stimulated scientists to unearth novel materials to meet the technological demands of modern society. However, the washability issue still remains a significant challenge. We showed that calligraphic ink, used as a writing tool in East Asian areas for thousands of years, could present a route to translate washable and wearable electrodes into a reality. We prepared washable electrodes by simply coating textiles with the ink. It was observed that the electrical and mechanical performance of the fabricated electrodes remained nearly unchanged even after 10 vigorous laundering cycles using a regular washing machine. In addition, supercapacitors made with those electrodes exhibited excellent cycling stability and high energy/power density. These results establish that everyday calligraphic ink is a simple yet powerful resource for fashioning normal textiles into washable and wearable electrodes for supercapacitors.

Graphene-based stretchable and transparent moisture barrier

We propose an alumina-deposited double-layer graphene (2LG) as a transparent, scalable, and stretchable barrier against moisture; this barrier is indispensable for foldable or stretchable organic displays and electronics. Both the barrier property and stretchability were significantly enhanced through the introduction of 2LG between alumina and a polymeric substrate. 2LG with negligible polymeric residues was coated on the polymeric substrate via a scalable dry transfer method in a roll-to-roll manner; an alumina layer was deposited on the graphene via atomic layer deposition. The effect of the graphene layer on crack generation in the alumina layer was systematically studied under external strain using an in situ micro-tensile tester, and correlations between the deformation-induced defects and water vapor transmission rate were quantitatively analyzed. The enhanced stretchability of alumina-deposited 2LG originated from the interlayer sliding between the graphene layers, which resulted in the crack density of the alumina layer being reduced under external strain.

Thermo-compressive transfer printing for facile alignment and robust device integration of nanowires

Controlled alignment and mechanically robust bonding between nanowires (NWs) and electrodes are essential requirements for reliable operation of functional NW-based electronic devices. In this work, we developed a novel process for the alignment and bonding between NWs and metal electrodes by using thermo-compressive transfer printing. In this process, bottom-up synthesized NWs were aligned in parallel by shear loading onto the intermediate substrate and then finally transferred onto the target substrate with low melting temperature metal electrodes. In particular, multi-layer (e.g. Cr/Au/In/Au and Cr/Cu/In/Au) metal electrodes are softened at low temperatures (below 100 °C) and facilitate submergence of aligned NWs into the surface of electrodes at a moderate pressure (∼5 bar). By using this thermo-compressive transfer printing process, robust electrical and mechanical contact between NWs and metal electrodes can be realized. This method is believed to be very useful for the large-area fabrication of NW-based electrical devices with improved mechanical robustness, electrical contact resistance, and reliability.