Metal complexes-based molecular materials as thin films on silicon substrates

Abstract

A tremendous effort has been devoted in the past two decades to the preparation and study of molecule-based materials derived from organometallic or metalorganic species. The work described in this paper is directed to the study of vapor deposited and electrodeposited thin films of these systems. Amorphous thin films of M(TCNE) x (M = V, Cr, Nb, and Mo; tetracyanoethylene (TCNE): (NC) 2 C=C(CN) 2 ) are grown by chemical vapor deposition (CVD) from organometallic complexes and TCNE on (001)-oriented silicon substrates. The films are characterized by infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and magnetic measurements. Infrared spectra are dominated by CN stretching vibrations and show two main ν CN bands, the positions and relative intensities of which are very similar to those obtained for solution grown M(TCNE) x , y solvent phases (M: first-row transition metal). X-ray photoelectron spectra also confirm the presence of reduced TCNE and low-oxidation state metal atoms in the deposited material. X-ray absorption spectroscopy measurements on Cr(TCNE) x films reveal an irregular octahedral environment for the chromium atoms (Cr–N distance of 2.03 Å and Cr–N–C angle of about 160°). Magnetic studies show that M(TCNE) x as thin films are magnetically ordered at low temperatures. Furthermore, after a several hours exposure to air, CVD-grown V(TCNE) x films exhibit a spontaneous magnetization at room temperature. Ni(dithiolene) 2 -based molecular conductors, namely [( n -C 4 H 9 ) 4 N] 2 [Ni(dcbdt) 2 ] 5 , and TTF[Ni(dmit) 2 ] 2 are electrodeposited on intrinsic silicon wafers used as anodes (dcbdt 2– : 4,5-dicyanobenzene-1,2-dithiolato; TTF: tetrathiafulvalene; dmit 2– : 1,3-dithiole-2-thione-4,5-dithiolato). The films are studied by vibrational spectroscopies (Raman and infrared), X-ray photoelectron spectroscopy, X-ray powder diffraction, scanning electron microscopy, and transport measurements. In the former case, the films exhibit a semiconducting behavior with a room temperature conductivity of about 1.2 × 10 –2 S cm –1 . In the latter case, a metal-like behavior is evidenced in the 14–300 K temperature range. The room temperature conductivity is found to be of ca. 12 S cm –1 , which is a non-negligible value, taking into account the presence of numerous inter-grain contacts within the film.