Multinuclear NMR and EPR Studies on Si-Doped Diamond-like Carbon

Recently, there is high interest in the development of diamond-like carbon (DLC) for a variety of applications in high strength, low-wear and low-friction film coatings.  For instance, tool surfaces comprised of DLC are useful for machining electronic and optical components. Another example involved durable coatings for magnetic storage devices. Performance enhanced materials can be prepared by incorporating a few percent Si into the DLC. This has been done using plasma immersion ion-implantation and deposition (PIIID) by Professor Robert Carpick and his group at the University of Pennsylvania, who have provided materials for our investigation.  During sample fabrication, the physical/chemical properties can be tailored, and correspondingly a large and variable combination of bulk and surface structures including dangling bonds will be present.  In order to better understand these materials, we have carried out a study of the atomic level structure of Si-doped DLC using solid-state ¹H, ¹³C and ²⁹Si nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR).  NMR is used to quantify the protons and investigate the carbon and silicon local structural environments, while EPR is employed to measure the unpaired electron spin concentration which is attributed to dangling bond defects.  

The ratio of carbon hybridizations (sp²:sp³) and the hydrogen content are two significant factors that gauge the mechanical properties of DLCs.  We have found a sp²:sp³ ratio of 58:42, as measured by ¹³C MAS NMR for a Si-DLC sample, which contrasts with earlier studies of pure DLC materials.^(1,2,3)  Qualitatively, further evidence of the differences between sp² and sp³ carbons is borne out through bonding scenarios with hydrogen, as this can be observed with the enhanced sp² signal intensity via ¹³C-¹H cross-polarization (CP).  The normalized ¹H NMR signal intensity gives an unexpectedly large hydrogen content as much as 5% by weight, though some of this is attributed to adsorbed water.  In addition to these findings, ²⁹Si-¹H CP experiments provide direct evidence of silicon-hydrogen bonds.  Unpaired electron spins in dangling bond defects are quantified from normalized EPR spectra.  We measure roughly 2 × 10²⁰unpaired electron spins/gram for Si-DLC.  

These preliminary measurements provide an interesting glimpse into the structure of PIIID prepared Si-doped DLCs.  Many questions remain concerning the distributions of sp² and sp³ carbons, Si-tetrahedra, etc., as well as how the structure can incorporate so many protons and defects.  Similar NMR/EPR measurements for annealed samples (50^o to 350^oC) are in progress, as these are expected to provide valuable information concerning structural changes resulting from treatment at elevated temperature, which is important for device applications.

Reference:

(1)  P.K. Chu, L.Li, Mater. Chem. Phys.,  96(2006) 253-277

(2)  A.I.Shames, A.M.Panich,et al., J Phys. Chem. Solids, 63(2002) 1993-2001

(3)  N. Zumbulyadis, J.Chem.Phys., 86(1987) 1162-1166

Acknowledgement:

This work was supported by the National Science Foundation through a subcontract to Hunter College from the University of Pennsylvania.