Effect of Heat and Plasma Treatment on Carboranethiol Self-Assembled Monolayers on Copper

Carborane (C2B10H12)moleculesare unique precursors for self-assembled monolayer (SAM) applications.These 3D, icosahedral (12-vertex) molecules allow for the formationof well-ordered layers, potentially with fewer defects than traditionallinear alkyl SAMs, and are amenable to cross-linking via labile Hatoms with a variety of mechanisms including heat, plasma, and radiation(e.g., UV, e-beam). We have investigated the deposition of SAMs of1,2-dithiol-ortho-carborane (O) and9-thiol-meta-carborane (M) on copperfrom the vapor phase in ultrahigh-vacuum conditions, along with theeffect of thermal (150-400 degrees C) and plasma (N-2, H-2, Ar, O-2) postgrowth treatment. Both filmsshow rapid deposition in the vapor phase with approximate monolayeror few layer coverage, as well as a mixture of physisorption (thioladsorption) and chemisorption (thiolate binding). Both films additionallyshow notable stability to thermal treatment up to 400 degrees C, withonly gradual (not abrupt) decrease in boron coverage and minor changesin chemical composition/makeup; however, with the monothiol derivative M showing greater loss of boron and greater oxidation. Nitrogen(N-2) plasma treatment leads to partial nitrogenation ofboron, while hydrogen (H-2) and argon (Ar) plasma treatmentslead to partial oxidation of boron, with some parasitic growth (increasein B coverage) especially for the dithiol derivative O. Oxygen (O-2) plasma treatment shows an aggressive oxidation(of boron, carbon, sulfur, and copper), but appears to passivate thelayers (i.e., the boron oxide based layer formed remains stable anddoes not change with further plasma exposure). Both heat and plasmatreatments reduce copper oxide at the interface and increase thiolatebinding (thus conceivably stabilizing the films). Indeed, exposureto N-2 plasma appears to further stabilize the films towardheat treatment. These findings highlight that these carboranethiolSAMs represent a robust option for various functional and protectivelayer applications, and that heat and plasma may be used to furtherstabilize these films, to modify their properties, or as part of amore complex fabrication scheme.