Fatigue of metals at nanoscale: Metal thin films and conductive interconnects for flexible device application

Dislocation Evolution in Cyclic-Loaded Cu Nanopillars with Different Configurations

Small-sized metals generally exhibit unusual deformation responses subjected to cyclic loading, since their limited volume cannot effectively accommodate micro-sized dislocation patterns typically found in their bulk counterparts. Here, the cyclic behaviors in Cu nanopillars with different configurations are investigated using in situ transmission electron microscopy fatigue test. Dislocation tangles formed in single- and twinned-crystal nanopillars as a result of cycling-induced operations of multiple slip systems and further unpinning and absorption of pinned dislocations. While, nanopillars configured with low-angle grain boundary (LAGB) underwent the degradation and eventual decomposition of the LAGB due to the cycling-induced emission of grain boundary dislocations, which resulted in high-density mobile dislocations to withstand the cyclic loading. These findings contribute to a systematic and comprehensive understanding of the micro-mechanics of dislocation-related phenomena in the cyclic response of nanoscale metals.

Mechanically Robust, Inkjet-Printable Polymer Nanocomposites with Hybrid Gold Nanoparticles and Metal-like Conductivity

Hybrid core-shell nanoparticles with metal cores and conductive polymer shells yield materials that are sinter-free and highly conductive but mechanically weak. Conventional composites of such nanoparticles are electrically insulating. Here, we introduce microscale phase-separated nanocomposites of hybrid gold-PEDOT:PPS particles in insulating poly(vinyl alcohol) (PVA). They combine electrical conductivities of up to 2.1 × 105 S/m at 10 vol % PVA with increased mechanical adhesion on polyethylene terephthalate and glass substrates. We studied the effects of the PVA molecular weight, hydrolyzation degree, and volume fraction. Composites with 10 vol % highly hydrolyzed PVA at a MW of 89-98 kDa had the highest conductivities and stabilities; highly hydrolyzed PVA even increased the conductivity of the hybrid particle layers. We propose the formation of hydrogen bonds between PVA and PEDOT:PSS that lead to demixing and the formation of stable, structured composites. Finally, we demonstrated the inkjet-printability of inks containing PVA in water with viscosities of 1.6-2.0 Pa s at 50.1 s-1 and prepared bending-resistant electrical leads.

A New Solution to the Grain Boundary Grooving Problem in Polycrystalline Thin Films When Evaporation and Diffusion Meet in Power Electronic Devices

This paper constituted an extension of two previous studies concerning the mathematical development of the grain boundary grooving in polycrystalline thin films in the cases of evaporation/condensation and diffusion taken separately. The thermal grooving processes are deeply controlled by the various mass transfer mechanisms of evaporation-condensation, surface diffusion, lattice diffusion, and grain boundary diffusion. This study proposed a new original analytical solution to the mathematical problem governing the grain groove profile in the case of simultaneous effects of evaporation-condensation and diffusion in polycrystalline thin films by resolving the corresponding fourth-order partial differential equation ∂y∂t=C∂2y∂x2-B∂4y∂x4 obtained from the approximation ∂y∂x2≪1. The comparison of the new solution to that of diffusion alone proved an important effect of the coupling of evaporation and diffusion on the geometric characteristics of the groove profile. A second analytical solution based on the series development was also proposed. It was proved that changes in the boundary conditions of the grain grooving profile largely affected the different geometric characteristics of the groove profile.

An Anti-Fatigue Design Strategy for 3D Ribbon-Shaped Flexible Electronics

Three-dimensional (3D) flexible electronics represent an emerging area of intensive attention in recent years, owing to their broad-ranging applications in wearable electronics, flexible robots, tissue/cell scaffolds, among others. The widely adopted 3D conductive mesostructures in the functional device systems would inevitably undergo repetitive out-of-plane compressions during practical operations, and thus, anti-fatigue design strategies are of great significance to improve the reliability of 3D flexible electronics. Previous studies mainly focused on the fatigue failure behavior of planar ribbon-shaped geometries, while anti-fatigue design strategies and predictive failure criteria addressing 3D ribbon-shaped mesostructures are still lacking. This work demonstrates an anti-fatigue strategy to significantly prolong the fatigue life of 3D ribbon-shaped flexible electronics by switching the metal-dominated failure to desired polymer-dominated failure. Combined in situ measurements and computational studies allow the establishment of a failure criterion capable of accurately predicting fatigue lives under out-of-plane compressions, thereby providing useful guidelines for the design of anti-fatigue mesostructures with diverse 3D geometries. Two mechanically reliable 3D devices, including a resistance-type vibration sensor and a janus sensor capable of decoupled temperature measurements, serve as two demonstrative examples to highlight potential applications in long-term health monitoring and human-like robotic perception, respectively.

Bending Setups for Reliability Investigation of Flexible Electronics

Flexible electronics is a rapidly growing technology for a multitude of applications. Wearables and flexible displays are some application examples. Various technologies and processes are used to produce flexible electronics. An important aspect to be considered when developing these systems is their reliability, especially with regard to repeated bending. In this paper, the frequently used methods for investigating the bending reliability of flexible electronics are presented. This is done to provide an overview of the types of tests that can be performed to investigate the bending reliability. Furthermore, it is shown which devices are developed and optimized to gain more knowledge about the behavior of flexible systems under bending. Both static and dynamic bending test methods are presented.

High Yield Transfer of Clean Large-Area Epitaxial Oxide Thin Films

In this work, we have developed a new method for manipulating and transferring up to 5 mm × 10 mm epitaxial oxide thin films. The method involves fixing a PET frame onto a PMMA attachment film, enabling transfer of epitaxial films lifted-off by wet chemical etching of a Sr₃Al₂O₆ sacrificial layer. The crystallinity, surface morphology, continuity, and purity of the films are all preserved in the transfer process. We demonstrate the applicability of our method for three different film compositions and structures of thickness ~ 100 nm. Furthermore, we show that by using epitaxial nanocomposite films, lift-off yield is improved by ~ 50% compared to plain epitaxial films and we ascribe this effect to the higher fracture toughness of the composites. This work shows important steps towards large-scale perovskite thin-film-based electronic device applications.