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

Abstract

Carborane (C2B10H12) molecules are unique precursors for self-assembled monolayer (SAM) applications. These 3D, icosahedral (12-vertex) molecules allow for the formation of well-ordered layers, potentially with fewer defects than traditional linear alkyl SAMs, and are amenable to cross-linking via labile H atoms with a variety of mechanisms including heat, plasma, and radiation (e.g., UV, e-beam). We have investigated the deposition of SAMs of 1,2-dithiol-ortho-carborane (O) and 9-thiol-meta-carborane (M) on copper from the vapor phase in ultrahigh-vacuum conditions, along with the effect of thermal (150–400 °C) and plasma (N2, H2, Ar, O2) postgrowth treatment. Both films show rapid deposition in the vapor phase with approximate monolayer or few layer coverage, as well as a mixture of physisorption (thiol adsorption) and chemisorption (thiolate binding). Both films additionally show notable stability to thermal treatment up to 400 °C, with only gradual (not abrupt) decrease in boron coverage and minor changes in chemical composition/makeup; however, with the monothiol derivative M showing greater loss of boron and greater oxidation. Nitrogen (N2) plasma treatment leads to partial nitrogenation of boron, while hydrogen (H2) and argon (Ar) plasma treatments lead to partial oxidation of boron, with some parasitic growth (increase in B coverage) especially for the dithiol derivative O. Oxygen (O2) plasma treatment shows an aggressive oxidation (of boron, carbon, sulfur, and copper), but appears to passivate the layers (i.e., the boron oxide based layer formed remains stable and does not change with further plasma exposure). Both heat and plasma treatments reduce copper oxide at the interface and increase thiolate binding (thus conceivably stabilizing the films). Indeed, exposure to N2 plasma appears to further stabilize the films toward heat treatment. These findings highlight that these carboranethiol SAMs represent a robust option for various functional and protective layer applications, and that heat and plasma may be used to further stabilize these films, to modify their properties, or as part of a more complex fabrication scheme.