J. Phys. Chem. C 2008, 112, 6695-6699
6695
Origin of Size-Dependent Energy Transfer from Photoexcited CdSe Quantum Dots to Gold Nanoparticles Mariana Kondon, Junhyung Kim, Nayane Udawatte, and Dongil Lee* Department of Chemistry, Western Michigan UniVersity, Kalamazoo, Michigan 49008 ReceiVed: October 26, 2007; In Final Form: February 14, 2008
The photoluminescence quenching of a CdSe quantum dot by hexanethiolate-monolayer-protected gold clusters (MPCs) with core diameters of 1.1-4.9 nm is described. Experimental evidence suggests that the quenching occurs via an energy transfer mechanism. The energy transfer quenching efficiencies of MPCs were compared using Stern-Volmer plots. The quenching constant (KQ) obtained from the slope of the plot shows an enormous increase from 2.5 × 105 to 2.3 × 108 M-1 by nearly 1000-fold as the core size increases from 1.1 to 4.9 nm. The origin of the size dependence is considered. There is a remarkable linear correlation found between the quenching constant and the MPC core volume. This correlation suggests that the energy transfer quenching efficiency is governed by the absorption cross-section of the MPC quencher that scales as its core volume.
Introduction In recent years, semiconductor-metal nanocomposites have drawn a lot of attention because of their potential applications in photocatalysis and solar energy conversion.1-6 Semiconductor quantum dots (QDs) with tunable band gaps and excellent photostability in particular offer new opportunities for such applications in the visible and infrared regions of solar light.7-10 Earlier investigations have revealed that deposition of noble metal nanostructures on a semiconductor greatly enhances the photoinduced charge carrier separation in light-harvesting systems.11-13 Metal surfaces and nanoparticles are also quenchers of photoexcited states along a directed energy or charge transfer mechanism.14-19 While there is growing interest in the utilization of such nanoparticle composites in many applications,1,4,6,20-22 experimental evidence that reveals factors controlling the excited-state interactions is still limited. We report here quantitative results delineating the origin of the size dependence in photoluminescence (PL) quenching of a CdSe QD by small gold nanoparticles that encompass the transition between bulk and molecular regimes. Monolayer-protected metal clusters (MPCs) are stable, structurally well-defined nanoparticles even in the dried state, which has allowed detailed characterizations of their structures, compositions, and properties.23 The size-dependent optical and electrochemical charging properties of MPCs have been described.24-31 In a previous study,17 Cheng et al. described the fluorescence quenching of small dye molecules by Au MPCs in which they showed the essentiality of the donor-acceptor spectral overlap and the associated nanoparticle core density of electronic states in energy transfer (ET) quenching. More recently, we have demonstrated that the quantized electronic charging properties of Au MPCs can be used to control the transfer of photogenerated electrons from the conduction band of a large band gap semiconductor (TiO2) nanoparticle to Au MPCs.32 The excited-state quenching occurred via charge transfer (CT), and the quenching efficiency was remarkably dependent on the core size of Au MPCs. The origin of the size * To whom correspondence should be addressed. E-mail: dongil.lee@ wmich.edu.
dependence was interpreted by the size-dependent capacitance of Au MPCs. In this paper, we describe the PL quenching of a CdSe QD by Au MPCs. The QD is an octadecylamine-stabilized CdSe nanoparticle. Au MPCs include a series of hexanethiolate (SC6)coated gold nanoparticles,32-35 Au25(SC6)18,34 Au140(SC6)53, Au309(SC6)92, Au807(SC6)163, and Au4033(SC6)453. These neutral, organic-soluble MPCs are abbreviated as Au25, Au140, Au309, Au807, and Au4033, respectively. The PL quenching efficiencies of Au MPCs of different sizes are compared by Stern-Volmer plots.36 The results show a great increase in quenching efficiency with MPC core size. A remarkable correlation is observed between quenching efficiency and MPC core volume, which provides insights into the origin of the size effect in ET quenching in these nanoparticle assemblies. Experimental Section Nanoparticles. Octadecylamine-stabilized CdSe QDs with core diameters determined by transmission electron microscopy (TEM) of 2.6 ( 0.2, 3.4 ( 0.2, and 6.9 ( 0.2 nm were purchased from NN-Labs (Fayetteville, AR). Hexanethiolatecoated Au MPCs with core diameters of 1.1-4.9 nm were prepared as reported previously.32,35 Briefly, MPCs were synthesized by using a modified Brust method33 in which the core size was controlled by employing a varied thiol:gold molar ratio (1:1, 3:1, or 5:1) and solvent fractionated to reduce size dispersity. The isolated MPC core diameters determined by TEM were 1.1 ( 0.2, 1.7 ( 0.2, 2.2 ( 0.2, 2.9 ( 0.2, and 4.9 ( 0.3 nm, corresponding to, respectively, Au25, Au140, Au309, Au807, and Au4033.32-35 TEM images of Au MPC and CdSe QD samples prepared on a Formvar/carbon-coated copper grid (01814F, Ted Pella) were obtained with a JEOL transmission electron microscope (JEM-1230). PL Quenching Experiment. PL quenching of a 3.4 nm diameter CdSe QD by Au MPCs was studied in toluene solutions using a fluorescence spectrometer (LS-50B, PerkinElmer). In a typical experiment, a QD solution was purged with high-purity Ar gas for 30 min to remove oxygen from the solution. A QD concentration of 0.60 µM was selected to obtain
10.1021/jp800766r CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008
6696 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Kondon et al.
Figure 2. (a) PL quenching of a photoexcited CdSe QD by a Au MPC. (b) PL spectral change of a 0.6 µM 3.4 nm CdSe QD colloidal solution upon addition of a Au140 MPC solution. (c) Decrease in the PL intensity upon addition of Au25, Au140, Au309, Au807, and Au4033 MPC solutions. (d) Corresponding Stern-Volmer plots and best fit lines (black lines). The slopes (KQ) are 2.5 × 105, 1.8 × 106, 4.6 × 106, 1.3 × 107, and 2.3 × 108 M-1 for Au25, Au140, Au309, Au807, and Au4033, respectively. Figure 1. (a) Absorption spectra of hexanethiolate-coated Au25, Au140, Au309, Au807, and Au4033 MPCs in toluene. The absorbance values have been normalized to unity at 300 nm for comparison. (b) Absorption (red) and PL (black) spectra of the 3.4 nm diameter CdSe QD in toluene.
a measurable PL intensity and absorbance of
