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
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- 作者：Julie K. Lorenz and Arthur B. Ellis
- 刊名：Journal of the American Chemical Society
- 出版年：1998
- 出版时间：October 28, 1998
- 年：1998
- 卷：120
- 期：42
- 页码：10970 - 10975
- 全文大小：124K
- 年卷期：v.120,no.42(October 28, 1998)
- ISSN：1520-5126
文摘
The band edge photoluminescence (PL) of bulk single-crystal n-CdSe is perturbed by adsorption oftri-n-octylphosphine oxide (TOPO) from toluene solution onto the crystal's 0001, 000
, and 11
0 faces. Theseare three of the crystal faces observed in studies of CdSe nanocrystals, which are commonly capped withTOPO surfactant molecules to control their properties. At low concentration, where monomeric TOPOdominates the toluene solution composition, reversible PL enhancements are observed, indicating that theadsorbate is acting as a labile Lewis base toward the surface. However, above ~10 mM concentration, thereis an abrupt reversal in the PL signature, such that net quenching of PL is observed relative to the PL intensityin the toluene reference ambient. The PL changes at concentrations above 10 mM are not reversible withtoluene rinsing, and are associated with a strongly bound species of Lewis acidic character that requires exposureto a strong base such as pyridine for desorption. XPS data are consistent with PL measurements in identifyingexperimental conditions associated with strong and weak binding of TOPO to CdSe. The PL changes can befit 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 theLangmuir adsorption isotherm model as being ~104 M-1 and at least 102 M-1 in the low and high concentrationregimes, respectively. The concentration at which the PL signature reversal occurs corresponds to incipientaggregate formation in solution based on both 31P NMR and IR spectral changes. MacroModel calculationsindicate that TOPO dimer formation is energetically favorable and that a surface adduct formed from thedimer could be stabilized by multiple surface interactions.
, and 110 faces. Theseare three of the crystal faces observed in studies of CdSe nanocrystals, which are commonly capped withTOPO surfactant molecules to control their properties. At low concentration, where monomeric TOPOdominates the toluene solution composition, reversible PL enhancements are observed, indicating that theadsorbate is acting as a labile Lewis base toward the surface. However, above ~10 mM concentration, thereis an abrupt reversal in the PL signature, such that net quenching of PL is observed relative to the PL intensityin the toluene reference ambient. The PL changes at concentrations above 10 mM are not reversible withtoluene rinsing, and are associated with a strongly bound species of Lewis acidic character that requires exposureto a strong base such as pyridine for desorption. XPS data are consistent with PL measurements in identifyingexperimental conditions associated with strong and weak binding of TOPO to CdSe. The PL changes can befit 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 theLangmuir adsorption isotherm model as being ~104 M-1 and at least 102 M-1 in the low and high concentrationregimes, respectively. The concentration at which the PL signature reversal occurs corresponds to incipientaggregate formation in solution based on both 31P NMR and IR spectral changes. MacroModel calculationsindicate that TOPO dimer formation is energetically favorable and that a surface adduct formed from thedimer could be stabilized by multiple surface interactions.