Self-Assembled Monolayers of Phosphorus–Nitrogen Cages

Abstract

This study investigates the deposition of the phosphorus–nitrogen (PN) cage P2N6 onto Au and Cu surfaces using both vapor-phase and solution-phase methods. The P2N6 cage sublimes cleanly and reaches an onset of volatility near 83 °C, suitable for vapor-phase deposition processes. Quartz crystal microbalance experiments on Au show self-limiting uptake of 61.1 ng·cm–2 that corresponds to 1.56 ± 0.16 molecules·nm–2 and closely aligned with computational close-packed monolayer coverage. Scanning tunneling microscopy reveals the formation of long-range ordered zigzag ribbon domains on Au(111), with a measured structural density of 1.55 molecules·nm–2, in excellent agreement with both the QCM and modeling. X-ray photoelectron spectra were collected of P2N6 on Au and Cu surfaces and displayed P 2p and N 1s signals that match intact PN cage environments. Work functions decreased upon adsorption of 4.3 to 3.1 eV for P2N6 on Au after vapor deposition, and from 4.2 to 3.5 eV for Cu, which indicates monolayer formation. Notably, the solution-phase deposition process of P2N6 on Cu causes substrate etching, identifying vapor deposition as the reliable pathway for Cu functionalization. Time-of-flight SIMS detected 237 m/z of the molecular ion that confirm cage retention on the surface and the characteristic mass fragments. Selective adsorption studies of patterned Au/SiO2 and Cu/SiO2 substrates revealed a metal-selective adsorption with high selectivity coefficients (Au, S = 0.96; Cu, S = 0.93). These results establish PN cages for their potential to be a thermally stable and modular platform for selective surface functionalization, with vapor-phase compatibility.