The effect of O 2 plasma treatment on surface energy, topography and surface chemistry of the negative photoresist epoxy novolak SU-8 was investigated by contact angle goniometry, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Directly after plasma treatment, the surfaces were completely wetted by water with a contact angle between water and the SU-8 surface below five degrees. The surface free energy can be increased significantly depending on the plasma dose. The surfaces remained hydrophilic for several months showing a moderate hydrophobic recovery. The surface topography of the plasma treated SU-8 showed a formation of nanoscale aggregates. The rms-roughness of the topography was correlated with the plasma dose. An increased plasma dose induced aggregates of up to 200 nm in size. XPS measurements revealed changes in surface chemistry due to the oxygen plasma process and an increased antimony concentration on the surface.
Wetting properties and surface roughness of SU-8 can be modified by wet chemical and plasma processes. These processes result in an enhanced wettability that can be attributed to an increase of C O and COO groups at the surface. Wet chemical etching with ceric ammonium nitrate (CAN) rendered the SU-8 surface hydrophilic. However, it also led to the accumulation of cerium species on the surface, which may interfere with biochemical reactions in the device. Surface activation could also be achieved with a low-temperature atmospheric argon plasma, which left a smooth surface. Treatment of SU-8 with oxygen plasma led to a stable hydrophilization and an increased surface roughness. Owing to oxygen plasma processing, antimony species were accumulated which could be removed with a cleaning step. In order to maintain the hydrophilization after oxygen plasma treatment for storage, or over several wetting cycles, the surface can be coated with a protein-resistant graft copolymer (PLL-g-PEG).
Resistively switching memory cells (ReRAM) are strong contenders for next-generation non-volatile random access memories. In this paper, we present ReRAM cells on flexible substrates consisting of Ag/spin-on-glass/PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate). The complete cell is fabricated using a standard inkjet printer without additional process steps. Investigations on the spin-on-glass insulating layer showed that low sintering temperatures are sufficient for good switching behavior, providing compatibility with various foils. The cells feature low switching voltages, low write currents, and a high ratio between high and low resistance state of 104. Combined with excellent switching characteristics under bending conditions, these results pave the way for low-power and low-cost memory devices for future applications in flexible electronics.
Printed and flexible sensors are in the focus of recent efforts to establish the advantages of low-cost manufacturing techniques such as screen printing or inkjet printing for printed electronical applications. Devices based on conductive carbon nanotube (CNT) networks within polymeric matrices such as polydimethylsiloxane (PDMS) are already exceeding mere technological demonstrations. Therefore, we investigate the application-oriented behaviour of fully inkjet-printed CNT/PDMS strain sensors under different conditions such as short-and long-term performance. The sensors exhibit a quasi-linear piezoresistive behaviour with vanishing hysteresis to tensile strain. Significant differences in the resistive response between compressive and tensile strain suggest complex re-orientation mechanisms of CNTs inside the matrix. No clear indication for this phenomenon could be observed in the evolution of the CNT network resistance during fatigue measurements within an uncured or cured PDMS matrix, where both scenarios exhibit no visual degradation. However, these measurements over thousands of cycles show different permanent changes in the overall device resistance exhibiting damages but also recovery in the network. Considering these findings facilitates the development of printed sensor devices.
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