Spectroscopic, structural, and electrical characterization of thin films vapor-deposited from the spin-crossover complex Fe(phen)_2 (NCS)_2

Abstract

Thin films (~100 nm) have been prepared of the prototypical spin-crossover complex Fe(phen)2(NCS)2 (phen = 1,10-phenanthroline). Initial attempts to prepare these films by direct vapor deposition yielded films of a different material. Through extensive FT-IR, Raman, UV-Vis, and x-ray photoelectron spectroscopy it is shown that these as-deposited films are the ferroin-based tris complex [Fe(phen)3](SCN)2. Structural characterization by AFM and powder XRD reveals them to be smooth and amorphous. When heated, the [Fe(phen)3](SCN)2 films are converted first to Fe(phen)2(SCN)2 and then to a third species postulated to be Fe(phen)(NCS)2 which is likely a one-dimensional coordination polymer. On the other hand, deposition from Fe(phen)2(NCS)2 onto heated substrates produces a mixture of these three materials. The identity of the Fe(phen)2(NCS)2 films is proved by additional spectroscopic, structural, and magnetic characterization. Magnetometry reveals them to remain spin-crossover active albeit with a more gradual and incomplete spin-transition than the bulk material. The films are found to be granular in nature and deep crevices were observed at the grain boundaries. Within the optical microscope, the coloring of the grains is seen to be dependent upon sample orientation. Powder XRD indicates texturing of crystalline domains where the crystallographic c-axis is parallel to the surface normal. This represents a new process for production of Fe(phen)2(NCS)2 films. With the aim of realizing the potential for spin-crossover materials to modulate electrical conduction and vise versa, electrical characterization has been performed as a function of temperature on vertical junction devices incorporating the prepared Fe(phen)2(NCS)2 films. In order to prevent penetration of the top electrode through the cracks and crevices in the organometallic layer, a multiple sequential deposition and annealing process was developed to produce films with a continuous surface topography. A small change in the weak electrical conductivity of these devices appears at the spin transition temperature, demonstrating for the first time in this important material a coupling of the electrical conductivity and magnetic spin state. Here, the HS state has a higher electrical conductivity. Incorporation of LiF interfacial layers between the Fe(phen)2(NCS)2 and the metal electrodes improves conduction slightly but tunneling still appears to be the current-limiting mechanism. Electrical measurements were also performed on devices made with the related complex [Fe(phen)3](SCN)2. Such films were much more conductive - as good as other typical organic semiconductor materials. All together, this work establishes the potential for this family of complexes to be incorporated into thin-film based electrical devices whose operation is based on the spin-crossover effect or otherwise.