Phosphine oxide monolayers on SiO2 surfaces.

Molecular thin films utilizing phosphonates are widely used in a variety of chemical applications, such as surfactants, stabilization of nanoparticle suspensions, layer by layer film formation, and more. Furthermore, surface chemistry is a valuable tool in the context of semiconducting materials. To date, the studies of monolayer formation of phosphonates have mainly focused on phosphonic acids. However, for certain applications, the less polar phosphine oxides are more desirable. Phosphine oxides are readily soluble in nonpolar organic solvents, such as toluene and mesitylene, and are compatible with many types of semiconducting materials. Importantly, these phosphorous derivatives do not have acidic functionality which can lead to uncontrolled etching or degradation of the substrate surfaces. Recently, we demonstrated a novel application using phosphine oxides for controlled nanoscale doping of materials by utilization of their surface chemical properties. Herein, we report the details of monolayer formation of phosphine oxides on SiO2 substrates, shedding light on the critical role of their chemical substituents in the assembly process. We find the monolayer formation process to be self-limiting, without applying specialized techniques as required for phosphonic acid thin films. Furthermore, the precise nature of the binding interactions between the phosphine oxides and the SiO2 substrate, and the uniformity and surface morphology of the self-assembled monolayers, are found to strongly depend on the chemical, electronic, and steric properties of the substituents. The chemical binding interactions between the phosphine oxides and SiO2 surfaces may involve packing interactions, hydrogen bonding, and/or covalent bonding, depending on the chemical substituents (Figure 1b). Such interactions can lead to monolayer formation involving the P=O molecular sites. Monolayer formation for precursors 1 and 2 was confirmed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), whereas no surface reaction was observed for 3 (Figure 2). H-bonds and covalent bonds are expected to be energetically less favorable for 3 than for 1 and 2 because of the more electron-withdrawing substituents of the P=O group as compared to 1 and 2. This Figure 1. a) The molecular precursors used in this study, and b) proposed H-bond and covalent-bond monolayer formation on the SiO2 surfaces.