Preparation of Ni nano-particles monolayer on a polyimide substrate

Abstract

Nanoparticles were produced on a polyimide (PI) film through reaction of a Ni80Fe20 thin film on a PI precursor layer during its imidization reaction. The PI precursor used in thes experiment was p-phenylene biphenyltetracarboximide (BPDA-PDA) type polyamic acid (Dupont, PI2610D) dissolved in N-Methyl-2-Pyrrolidinone. A 3.5-nm-thick Ni80Fe20 film was deposited using magnetron sputtering on a Si substrate that was spin coated with the PI precursor. The thickness of the spin-coated PI precursor was controlled so that the final cured thickness of the PI layer was 40 nm. Thus-prepared Ni80Fe20/PI samples were cured at 400ºC for 1 hour under vacuum. From the TEM plan view image, nanoparticles were estimated that the average particle size was 4.1 ± 1.4 nm and the volume fraction of the particles was approximately 24% with irregular shape. The cross-sectional image of nanoparticles indicated that they were formed in a mono-layer; there was minimal vertical migration of metal during imidization reaction. It can be also observed, the vertical height of the particles is ~ 5 nm, indicating that the particles are more or less symmetric in shape. Electron diffraction pattern of the nanoparticles was indexed to a fcc structure although the diffraction rings were rather broadened due to the extremely small particle size. The XPS spectra near the Fe and Ni L absorption edges suggest that Fe was preferentially consumed during imidization, leaving behind islands of Ni, which coalesced into spherical particles during curing. Micro-beam EDS line scanning the particles and PI matrix confirmed the fact since Ni signal was confined on the particles while Fe was found throughout the matrix. We used the nanoparticles as a template for growing nano-sized structure. When ~5 nm Co was deposited on top of nanoparticles using atomic layer deposition, the Co film, instead of forming a continuous film, broke apart into discrete islands whose size approximately matches that of the Ni nanoparticles. Thus, we demonstrate that a simple and economic method of synthesized a mono-layer of metallic particles on top of an organic film, which can be used as a basis for building functional nano-structures to attain desired properties.