Surfactant-semiconductor interfaces: Perturbation of the photoluminescence of bulk cadmium selenide by adsorption of tri-n-octylphosphine oxide as a probe of solution aggregation with relevance to nanocrystal stabilization

The band edge photoluminescence (PL) of bulk single-crystal n-CdSe is perturbed by adsorption of tri-n-octylphosphine oxide (TOPO) from toluene solution onto the crystal's 0001, 000 (1) over bar, and 11 (2) over bar 0 faces. These are three of the crystal faces observed in studies of CdSe nanocrystals, which are commonly capped with TOPO surfactant molecules to control their properties. At low concentration, where monomeric TOPO dominates the toluene solution composition, reversible PL enhancements are observed, indicating that the adsorbate is acting as a labile Lewis base toward the surface. However, above similar to 10 mM concentration, there is an abrupt reversal in the PL signature, such that net quenching of PL is observed relative to the PL intensity in the toluene reference ambient. The PL changes at concentrations above 10 mM are not reversible with toluene rinsing, and are associated with a strongly bound species of Lewis acidic character that requires exposure to a strong base such as pyridine for desorption. XPS data are consistent with PL measurements in identifying experimental conditions associated with strong and weak binding of TOPO to CdSe. The PL changes can be fit to a dead-layer model in both the low and high concentration regimes, permitting an estimate for TOPO-induced contractions and expansions of the dead-layer thickness of about 100-300 Angstrom, for the 0001 face, which generally yielded the largest PL changes. Equilibrium binding constants were estimated from the Langmuir adsorption isotherm model as being similar to 10(4) M-1 and at least 10(2) M-1 in the low and high concentration regimes, respectively. The concentration at which the PL signature reversal occurs corresponds to incipient aggregate formation in solution based on both P-31 NMR and IR spectral changes. MacroModel calculations indicate that TOPO dimer formation is energetically favorable and that a surface adduct formed from the dimer could be stabilized by multiple surface interactions.