Diamond and Related Materials 12 (2003) 900–904 0925-9635/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. PII:S0925-9635 Ž 03 . 00030-X Effect of isotopic substitution on IR and ESR properties of mass selected ion beam deposited ta-C films G. Fanchini , S.C. Ray , A. Tagliaferro *, V.-M. Tiainen , T. Sajavaara a a a, b b Dip. Fisica and Unita INFM, Politecnico di Torino, I-10129 Torino, Italy a ` Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland b Abstract We have investigated the infrared transparency and electron spin resonance (ESR) signal of natural and C tetrahedral 13 amorphous carbon (ta-C) films deposited by mass selected ion beam. In order to assess the significance of the results, which are averaged over the film, we have monitored the film through micro-Raman, showing that lateral fluctuation in the sp content are 3 limited. We show that ta-C films are transparent to IR radiation and that no hyperfine splitting can be observed in ESR spectra of C samples. The room-temperature line-width is the same (DH s5.5"0.1 G) in all samples. This is attributed to the strong 13 pp signal narrowing caused by spin delocalization and short-range interaction at room temperature.  2003 Elsevier Science B.V. All rights reserved. Keywords: Tetrahedral amorphous carbon; Mass-selected ion beam deposition; Vibrational properties characterization; Defect characterization 1. Introduction Amorphous carbon thin films, without (a-C) or with hydrogen (a-C:H), exhibit great versatility in their phys- ical properties, related with the ability to tune the microstructure by tuning the ratio of sp (graphite-like) 2 to sp (diamond-like) carbon sites w1,2x. However, at a 3 given sp ysp ratio, further degrees of freedom are given 2 3 by the forms of clustering of the sp phase w3x. The 2 sp carbon allotopes can form either planar or distorted 2 aromatic configurations or, even, olefinic chains of different length. The type of clustering of the sp phase 2 is determined also by the ability of the sp phase to 3 accommodate sp clusters of different characteristics 2 w1–3x. Tetrahedralamorphouscarbons (ta-C) representawell known example of the mutual relationship amongst the two phases. They can be grown by a number of deposition techniques involving energetic ions or plasma ion beams, such as filtered cathodic vacuum arc, pulsed- laser-deposition and mass-selected ion beam (MSIB) w1x. During the deposition process, carbon ions ‘sub- plant’ in the growing film and form dense sp bonding 3 w4x. The ‘subplantation’ model suggests that an optimal *Corresponding author. Tel.: q39-11-5647381; fax: q39-11- 5647399. E-mail address: alberto.tagliaferro@polito.it (A. Tagliaferro). energy and a monochromatic ion beam (typically 80– 120 eV) is required to maximise the sp content in ta- 3 C films. The E optical gap of ta-C can reach 3.45 eV 04 w5x. In summary, ta-C’s can be defined as the family of amorphous carbon films exhibiting a high fraction of sp carbon–carbon bonds. Though the perspectives for 3 electronic applications are doubtful, due to the high density of neutral defects w1x, the outstanding optical properties (high gap, IR transparency) and mechanical performances (hardness, low friction) of ta-C make it suitable for several applications w1x. In the present work we aim to check some properties of ta-C films, by monitoring the absence of a shift in the absorption features with isotopic substitution. We focus our attention on the effect of isotopic substitution on the IR absorption and the electron spin resonance (ESR) signal. The absence of hyperfine splitting in the C rich sample and the similarity of the signals in 13 natural (ta- C) and ta- C samples suggest that the 12 13 strong signal narrowing is due to spin delocalization and short-range interaction. 2. Experimental Three ta-C films were prepared by a MSIB deposition system described elsewhere w7,8x,byusinganionenergy known to provide high sp contents ()80%).A C 3 13