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

Abstract

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, 0001̄, and 112̄0 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 ∼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 Å, for the 0001 face, which generally yielded the largest PL changes. Equilibrium binding constants were estimated from the Langmuir adsorption isotherm model as being ∼104 M-1 and at least 102 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 31P 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.

Optical properties of functionalized GaN nanowires

“…Excess MPTMS during the surface-modification process could result in the formation of agglomeration leading to a reversed effect as surface modifier. 18 Photoluminescence excitation (PLE) spectra of MPTMS-GaN NWs shown in Fig. 3 are consistent with the strong absorption band suggested at 4.6-4.8 eV, 19 in which the intensity is corrected with the C MPTMS absorption band.…”

Ab initio analysis of monomers and dimers of trialkylphosphine oxides: Structural and thermodynamic stability

“…These results for type II dimers are Ϫ1.39 and Ϫ2.15 kcal/mol. Previous estimative using a macromodel molecular mechanics gives to type II TOPO dimer a value of Ϫ12.7 kcal/mol [19]. Because the entropic change of dimerization is always negative all enthalpy gain at the formation of dimer is cancelled.…”

“…At higher concentrations, stronger binding with Lewis acidic character was observed, represented by quenching of photoluminescence. This abrupt change in photoluminescence signature can be correlated with solution aggregate formation, identified through 31 P NMR and IR spectral measurements [19]. In this case, the characterization of the structural properties of the surfactants can be usefully employed to explain this phenomenon.…”

“…In these cases, it stabilizes the nanoparticles in organic solvents preventing an irreversible aggregation of the nanocrystals [19,30]. An interesting study on perturbation of the photolumines-cence of bulk CdSe demonstrate that at low concentrations TOPO acts like a labile base, enhancing the photoluminescence intensity upon adsorption [19]. At higher concentrations, stronger binding with Lewis acidic character was observed, represented by quenching of photoluminescence.…”

Core–Shell Semiconductor Nanocrystals for Biological Labeling

“…While TOPO is an ideal capping molecule for maintaining QD solubility and optical properties in organic solvents [36], the extremely hydrophobic nature of this surface requires that they be replaced with molecules containing polar groups that allow the QDs to be transferred into aqueous media. With this purpose in mind, several methods have been designed to prepare water-soluble QDs by displacing the TOPO molecules with a more appropriate layer (Fig.…”