NANOFILM - New metallic nanocomposites for micro and nanofabrication

Metal nanoparticles are of great interest because of their unique physical properties such as plasmonic absorption. By incorporating nanoparticles into polymers, nanocomposites with interesting properties can be made. These nanocomposites can be useful within optoelectronics, electrochemistry or catalysts. The possibility to effectively structure the nanocomposites are however a limiting factor. In this project the UV sensitive photoresist SU-8 gold and silver nanocomposites have been fabricated which can be deposited and structured using standard micro and nanofabrication processes. In the beginning, a method named as ex situ has been attempted to develop the nanocomposites. In this method, synthesised nanoparticles were incorporated into the photoresist matrix through effective solvent exchange from water to cyclopentanone, the solvent of the primarily used SU-8. The solvent exchange was achieved by first stabilizing the nanoparticles using the block co-polymers PVP/VA and PVA-COOH. The water was then secondly removed by co-evaporating under reduced pressure in the presence of target solvent. PVA-COOH was discarded after a jellification when in contact with cyclopentanone was discovered. A technique called pre grafting was found to be most effective for the stabilisation process. In pre grafting the nanoparticles were formed in the presence of PVP-VA. For the two metal nanocomposites attempted to be fabricated only the gold SU-8 nanocomposite was successfully and reproducibly produced with the ex situ method. An in situ method was then tried for the development of both gold and silver SU-8 nanocomposites. In this procedure, the metal precursors were incorporated with SU-8 and the respective nanoparticles formed within the photoresist matrix. The nanoparticles should preferably be formed during the heat treatment or UV irradiation after spin coating. It was able possible to dissolve the gold precursor directly into the photoresist, but nanoparticles with large size distribution were formed within a time frame of 20 s. This made further processes such as spinning and formation of a homogeneous thin layer of nanocomposite tedious. In the case of silver, a co-solvent was needed to incorporate silver nitrate into SU-8. In the first attempt DMSO was used as the co-solvent, but as DMSO is a Lewis base, it prohibited complete cross-linking during the UV irradiation step. Acetonitrile was then chosen for as co-solvent which resulted in a successfully structured in situ silver SU-8 nanocomposite. For both the ex situ and in situ nanocomposites structuring was initially found to be troublesome. Complete cross-linking of the SU-8 was only found to be possible after removing the filter from the aligner which blocks wavelengths below 350 nm. The obtained resolution was similar to what could be obtained with unmodified SU-8. It has been found that for the silver nanocomposites a further hard bake step at 300◦C in the end of the process resulted in the additional formation of numerous nanoparticles. The heating does not result in growth or agglomeration of the nanoparticles. It was also found that for high loads of silver in the nanocomposite, the additional heating results in a conductive composite. The conductive silver nanocomposite has been compared to a commercially available product, and although SEM investigations show that the structure of the fabricated composite appears to be preferable compared to the commercially available one, the obtained conductivity needs to be improved before it is competitive. All nanocomposites have been investigated to show that the nanoparticles maintain their plasmonic absorption and that the absorption is proportional to the nanoparticle concentration. The position of the plasmonic absorption peak was found to be red shifted with approximately 10 nm compared to that of pre synthesised nanoparticles in SU-8 or cyclopentanone. It was for the silver nanocomposites found that the additional heat treatment blue shifted the plasmonic absorption peak such that it again resembled that of the synthesised nanoparticles. In conclusion this project has resulted in the successful fabrication of new metallic nanocomposites which can be used for micro- and nanofabrication. The process for the in situ formation of nanoparticles is, however, not completely understood and should be investigated in the future