pubs.acs.org/NanoLett
Origin of High Photoluminescence Efficiencies in CdSe Quantum Belts Yi-Hsin Liu,†,§ Virginia L. Wayman,†,§ Patrick C. Gibbons,‡,§ Richard A. Loomis,*,†,§ and William E. Buhro*,†,§ †
Department of Chemistry, ‡ Department of Physics, and § the Center for Materials Innovation, Washington University, St. Louis, Missouri 63130-4899 ABSTRACT CdSe quantum belts (QBs) having lengths of 0.5-1.5 µm and thicknesses of 1.5-2.0 nm exhibit high photoluminescence (PL) efficiencies of ∼30%. Epifluorescence studies establish the PL spectra to be uniform along single QBs, and nearly the same from QB to QB. Photogenerated excitons are shown to be effectively delocalized over the entire QBs by position-selective excitation. Decoration of the QBs with gold nanoparticles indicates a low density of surface-trap sites, located primarily on the thin belt edges. The high PL efficiencies and effective exciton delocalization are attributed to the minimization of defective {11¯00} edge surface area or edge-top/bottom (face) line junctions in QBs relative to quantum wires having roughly isotropic cross sections, for which very low PL quantum efficiencies have been reported. The results suggest that trap sites can be minimized in pseudo-one-dimensional nanocrystals, such that the facile transport of energy and charge along their long axes becomes possible. KEYWORDS Exciton diffusion, decoration, surface traps, quantum wires
W
e report that thin CdSe quantum belts (QBs)1,2 having lengths of 0.5-1.5 µm exhibit uniform photoluminescence (PL) with efficiencies of ∼30%.3 Absorption spectra are consistent with one-dimensional (1D) quantum confinement in the thickness dimension,3 indicating that exciton-confinement properties are essentially identical to those in the previously reported CdSe nanoribbons,1 nanoplatelets,3 and nanosheets.4 Epifluorescence experiments establish that the PL is polarized along the long dimension of the QBs, and that photogenerated excitons are effectively delocalized along their entire lengths at room temperature.5,6 Images of Au-decorated QBs suggest that the high PL efficiencies and long-range exciton delocalization are due to a low density of surface trap sites, which are distributed primarily at the edges of the QBs. The results show that pseudo-1D semiconductor nanocrystals can have excellent passivation and optical properties, and effectively transport excitons and presumably charge carriers with minimal losses to nonradiative-recombination pathways. Pseudo-1D semiconductor nanocrystals such as quantum wires7-9 (QWs) and quantum rods (QRs)7,10 are under active consideration for solar-cell applications because they can in principle transport photogenerated carriers over longer distances and with fewer hopping events than corresponding devices constructed with nanocrystal (colloidal) quantum dots (QDs).7 However, efficient performance of QR and QW devices requires that carriers not be disproportionately lost to nonradiative recombination. PL efficiencies provide a measure of the quality of surface passivation and the scarcity
of trap sites that induce nonradiative recombination. Although the ensemble (as opposed to single-nanocrystal) PL efficiencies in CdSe QDs are now routinely >60%,11-13 those in QRs are lower (20-30%),14,15 and those previously reported in QWs are disappointingly low (0.1-0.3%).16-21 The increased axial dimension in QRs and QWs allows excitons, electrons, and/or holes to sample a much larger nanocrystal-surface area and thus experience a higher probability for encountering a trap site. Thus, in equally passivated QDs, QRs, and QWs losses to nonradiative recombination will be highest in the QWs and lowest in the QDs.19 Even so, we now demonstrate experimentally that nonradiative traps can be effectively minimized in CdSe QBs, establishing that pseudo-1D semiconductor nanocrystals having an extended axial dimension can exhibit photoluminescence efficiencies competitive with those of QRs and about half of those of QDs. The CdSe QBs were synthesized from cadmium acetate and selenourea in n-octylamine by a slight modification of the procedure of Peng and co-workers (see Supporting Information),2 and were structurally and morphologically similar to the CdSe nanoribbons reported by Hyeon and coworkers.1 The QBs were initially obtained in well-aligned bundles (Figure 1a),1 which were unbundled by mild sonication in the presence of oleylamine (10% w/w in toluene, Figure 1b). High-resolution transmission electron microscopy (HRTEM) images of the intact bundles established that the QBs had thicknesses of 1.5-2.0 nm (Figure 1c). TEM and HRTEM (Figure 1d,e) images of the unbundled QBs established their widths to be 7-15 nm, and their lengths to be 0.5-1.5 µm. X-ray diffraction (XRD) patterns of the QBs indexed to a wurtzite crystal structure (Supporting Information Figure S1) and Fourier transforms of and lattice spacings
* To whom correspondence should be addressed. E-mail: (W.E.B.) buhro@ wustl.edu; (R.A.L)
[email protected]. Received for review: 11/9/2009 Published on Web: 12/16/2009 © 2010 American Chemical Society
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DOI: 10.1021/nl903740p | Nano Lett. 2010, 10, 352-357
FIGURE 2. Extinction (black) and emission (red) spectra from a dispersion of unbundled CdSe QBs. The absorption features are assigned according to the text.
(0.12m0),24 m* h is the hole effective mass (m* B ) 0.90m0, m* A ) 0.45m0),22 Ebbulk is the bulk exciton binding energy (14.8 meV),19 and m0 is the free electron mass. This model suffers from the standard EMA assumptions of isotropic, nondegenerate bands, and parabolic band shapes.22 As written in eq 1 it assumes infinite-potential barriers at the quantumwell surfaces,23 it neglects the crystal-field splitting of the A and B bands (bulk value24 ∆cf ) 39 meV), and as applied here it assumes bulk values for the carrier effective masses. As such, the model is quite approximate. Even so, good fits to the energies of the first two transitions in Figure 2 were obtained from eq 1 using a well thickness of t ) 1.80 nm, central to the range experimentally determined by TEM (see above). The 1B-1e transition was calculated at 2.79 eV, and the 1A-1e, transition was calculated at 2.95 eV, which is very close to the energies experimentally observed (Figure 2). However, the energy calculated by eq 1 for the 2B-2e transition (6.08 eV) was far from the feature assigned to the 2B-2e transition in Figure 2, as expected due to the crudeness of the theoretical model.22,23 Therefore, the extinction data, and their similarity to those previously reported for CdSe nanoribbons,1 nanoplatelets,3 and nanosheets,4 established that the CdSe QBs exhibited the quantum-confinement effects expected of quantum wells3 and were thus 1Dconfined systems.
FIGURE 1. TEM (a,b) and HRTEM (c-e) images of bundled and unbundled CdSe QBs. (a) Bundled QBs; (b) unbundled QBs; (c) bundled QBs with their thin edges in the image plane; (d,e) unbundled QBs with their large top (bottom) surfaces in the image plane.
in HRTEM images indicated the crystallographic orientation given in Figure 1d (Supporting Information Figure S2).1 The wurtzite lattice parameters were determined to be a ) 4.12 ( 0.04 and 4.20 ( 0.06 Å, and c ) 6.89 ( 0.02 and 6.8 ( 0.1 Å by XRD and HRTEM lattice spacings, respectively. These values were smaller than the bulk CdSe lattice parameters (a ) 4.30 Å, c ) 7.02 Å) by similar amounts as in the previous report.1 An extinction spectrum from a dispersion of unbundled CdSe QBs is given in Figure 2. This and other spectra collected from the QBs are very similar to those previously reported for CdSe nanoribbons,1 nanoplatelets,3 and nanosheets,4 all of which have thicknesses comparable to the QBs studied here and thus have only one strongly confined dimension (smaller than the exciton Bohr radius of 5.4 nm22 for CdSe). Therefore, analogously to Ithurria and Dubertret,3 we assigned the three absorption features (from lower to higher energy) to 1B-1e, 1A-1e, and 2B-2e quantumwell transitions, respectively (A ≡ light hole, B ≡ heavy hole, and e ≡ electron).23 (The light-hole and heavy-hole bands in hexagonal semiconductors are typically labeled A and B, respectively.24) The energies (Ens) of wurtzite CdSe quantum-well transitions may be calculated to first order using an overly simple effective-mass-approximation, particle-in-a-box (EMA-PIB) model (eq 1),3,22,23,25 where Eg is the bulk band gap (1.751 eV),24 h is Planck’s constant, n is a quantum number, t is the well thickness, m* e is the electron effective mass © 2010 American Chemical Society
En ) Eg +
[
]
h2n2 1 1 - 4Ebbulk + 2 m* * m 8t e h
(1)
The proposed beltlike morphology of the nanostructures was therefore confirmed by several forms of evidence. The HRTEM images closely resembled those of the analogous CdSe nanoribbons previously reported by Hyeon.1 In the images, the nanostructures exhibited two distinct spatial dimensions corresponding to the thicknesses (Figure 1c) and widths (Figure 1d,e) of the belts. The broad top/bottom and thin edge surfaces indexed to {112¯0} and {11¯00} faces, respectively (Supporting Information Figure S2), which are geometrically orthogonal. Finally, the quantum well-like 353
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FIGURE 3. (a) A wide-field PL microscopy image of unbundled CdSe QBs spin-coated onto a coverslip; (b) a PL microscopy image of a CdSe QB; (c) PL spectra recorded from individual CdSe QBs (various colors) plotted with the ensemble CdSe QD PL spectrum (black) obtained from a solvent dispersion of QBs. FIGURE 4. PL microscopy images of a CdSe QB illuminated at the (a) center of the QB, and (b) edge of the QB. The white circles represent the positions of the illumination spots. The PL intensity (black) along the length of each QB is shown below each image with Lorentzian fits to the data (red). The full-widths at half maxima of the Lorentzian fits are 1.0(2) and 1.3(5) µm for (a) and (b), respectively. In (b), the dotted curve (blue) represents a convolution of a 1.2 µm length QB with the resolution of a 550 nm Lorentzian (see text).
extinction spectra confirmed 1D confinement and therefore that the nanostructures possessed only a single thin dimension. Consequently, the nanostructures were QBs. A PL spectrum from a dispersion of unbundled CdSe QBs is also shown in Figure 2. The emission features from the ensembles of QBs were narrow (fwhm ) 50 meV), and Stokes shifted by 25 meV (451 nm - 447 nm ) 4 nm) from the 1B-1e absorption. The PL efficiencies (quantum yields, QYs) of the as-prepared QB bundles were determined to be 7 ( 3% (see the Supporting Information). Upon addition of n-oleylamine, the QYs increased to 15 ( 5% and after sonication increased further to 30 ( 10%.3 However, the optimized PL efficiency was unstable and decreased upon prolonged exposure to n-oleylamine (with a time scale of days), exposure to air (with a time scale of hours), and exposure to light and air (with a time scale of minutes). The optimized QB PL QYs were considerably larger than those previously reported for the analogous CdSe nanoribbons (1-2%).1 Dispersions of unbundled QBs were deposited onto glass coverslips for epifluorescence microscopy and spectroscopy. Although unbundled, we cannot assert that the resulting images and spectra were obtained entirely from single QBs. Evidence for residual bundles of a few to several QBs was observed in places. As most of the QBs were only slightly longer (0.5-1.5 µm) than the resolution spot size (0.4-0.6 µm), most appeared in the epifluorescence images as slightly elongated features (Figure 3a,b). However, the QBs exhibited polarized emission parallel to the elongated dimension of the features (Supporting Information Figure S3), as expected © 2010 American Chemical Society
for 1D nanostructures.26-28 The emission of the QBs did not change appreciably as a function of excitation power density (2.0-320 W/cm2); however, the QBs began to photobleach rapidly at higher power densities (>150 W/cm2). The emission spectra collected from various QBs were nearly identical to one another (Figure 3c), and comparably narrow (mean Lorentzian fwhm ) 48 ( 15 meV) to the ensemble PL described above (mean Lorentzian fwhm ) 50 meV). The similarity of the epifluorescence and ensemble PL spectra attests to the thickness uniformity among the QBs in the ensemble specimens. The 2D delocalization of excitons within the QBs was demonstrated by position-selective excitation and epifluorescence microscopy. A long QB (l ≈ 1.2 µm) was irradiated with a ∼425 nm diameter illumination spot overlapping the center region of the QB (Figure 4a). The resulting PL image revealed intense emission with a PL-intensity profile that fit well to a Lorentzian line shape with a fwhm of 1.0 µm centered at the point of irradiation, independent of excitation power density. The increase in the breadth of this profile over the experimental spatial resolution, 500-550 nm, is evidence for exciton delocalization or diffusion. 354
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The emission intensity in the PL image obtained for the same QB decreased as the center position of the irradiation spot was moved to a point just off the end of the QB (Figure 4b). However, the maximum PL intensity did not shift toward the end of the QB closest to the irradiation region; rather, it was still observed near the center of the QB. A Lorentzian fit to the intensity profile revealed a fwhm of ∼1.3 µm, which is more than two times the experimental limit. The PL-intensity profile for an equal probability of emission along the entire length of a narrow QB should appear as a convolution of the image resolution with a line representing the length of the QB. An emission simulation obtained using a Lorentzian with a fwhm ) 550 nm for the image resolution and a continuous, uniform distribution of emission sites along the QB length of 1.2 µm is included in Figure 4b as a dotted line. The agreement of the simulation and data suggests a nearly uniform probability for radiative exciton recombination along the entire length of the QB. Low-temperature studies of ZnSe quantum wells have demonstrated quasi-ballistic transport29 of hot excitons with coherence lengths of 300-400 nm30 and total propagation lengths of >1 µm.31 Accordingly, we interpret our results as indicating the effective delocalization (transport) of 2D excitons over the full span of the individual QBs. To account for the high PL QYs and whole-belt delocalization of photogenerated excitons, the density and distribution of surface defects in the QBs were characterized by gold decoration. The method (see the Supporting Information) was adapted from the procedures of Banin,32 Mokari,33 and co-workers, who previously demonstrated selective Aunanoparticle deposition on the high-energy tips and surfacedefect sites of CdSe QRs32 and QWs.33 As an historical aside, we note that analogous decoration strategies were first employed to identify the surface defects associated with Griffith cracks in glass34,35 and dislocations in crystals.36-38 TEM images of the gold-decorated QBs are shown in Figure 5. Remarkably, the Au nanoparticles were deposited almost exclusively on the edges of the CdSe QBs, and at considerably lower densities than the corresponding decoration densities previously observed for CdSe QWs.33 The results suggested comparatively low surface-defect populations in the QBs with these defects, presumably in proportion to carrier-trapping sites, predominantly located within the thin belt edges. The identity of the defective edge surfaces was established by the crystallographic orientation of the QBs. We confirmed here (see the Supporting Information) the same crystallographic orientation previously determined for the closely related CdSe nanoribbons.1 The large-area, top and bottom QB surfaces are parallel to the nonpolar {112¯0} wurtzite crystal faces (see Figure 6a). Previous theoretical studies of the amine passivation and reconstruction of {112¯0} wurtzite CdSe facets has established them to be stable and well-passivated with an amine ligand bonded to © 2010 American Chemical Society
FIGURE 5. TEM images of gold-decorated CdSe QBs. Note that the Au nanoparticles are deposited primarily upon the edges of the QBs.
each surface Cd site.39-41 The long, thin, edge surfaces are parallel to the nonpolar {11¯00} wurtzite crystal faces. (This family of symmetrically equivalent crystal faces is more commonly identified as the {101¯0} family,39-41 but the (11¯00) member of the family is perpendicular to the (112¯0) face, as is required by the geometry of the QBs.) The {11¯00} facets have only a slightly different surface structure than the {112¯0} facets, and the theoretical studies suggest that they are also stable and well-passivated.39,41 Even so, our decoration results suggest that the QB {11¯00} edges are more prone to defects than are the {112¯0} top and bottom surfaces. Although they have been theoretically modeled,4,39-41 we note that the precise structures of these {112¯0} and {11¯00} surfaces are unknown. Roughness is evident along the edges of the QBs (Figure 1d, e), just as thickness variations are evident in individual QBs (Figure 1c). The accumulation of defects on the {11¯00} edges may be intrinsic to the edgesurface structure, or result from the growth mechanism. The important point is that the QBs are so thin relative to their width that the defective {11¯00} surfaces occupy only about 14% of the total QB surface area, effectively minimizing the population of trapping sites. Therefore, the high PL QYs and extensive exciton delocalization in the CdSe QBs appear to 355
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angular QB, leading to a larger relative population of surfacetrapping sites in QWs than in QBs. In summary, CdSe QBs exhibit high PL QYs and long exciton diffusion distances, consistent with the facile transport of energy and charge along their long axes. Their excellent properties are due to the highly anisotropic QB cross sections and specifically to the minimization of surfaces or edges prone to trapping sites. The challenge to extending these excellent properties to QWs more generally is identifying means to diminish defective side facets or edge junctions, or devising specific passivation schemes for such surfaces or edges where trapping sites tend to accumulate. Acknowledgment. We are grateful to the National Science Foundation for funding this work under Grants CHE-0518427 (W.E.B.) and DMR-0906966 (R.A.L.). We thank Dr. Fudong Wang for assistance with HRTEM. We are grateful to Dr. Jeffrey B. Neaton (Molecular Foundry, Lawrence Berkeley National Laboratory) for suggesting the possible location of the trap-site defects on the edge line junctions.
FIGURE 6. Crystallographic assignment of the surfaces (a) expressed in CdSe QBs,1 and (b) proposed for CdSe QWs.39 The {11¯00}family of crystallographically equivalent faces, which is more commonly identified as the {101¯0} family, includes the (101¯0), (11¯00), (01¯10), (1¯010), (1¯100), and (011¯0) faces. The {112¯0} family includes the (112¯0), (21¯1¯0), (12¯10), (1¯1¯20), (2¯110), and (1¯21¯0) faces.
Supporting Information Available. Preparative procedures, spectroscopic methods and calculations, TEM and XRD methods, an XRD pattern of CdSe QBs, HRTEM Fouriertransform and lattice-spacing images, and polarized-emission data (11 pages). This material is available free of charge via the Internet at http://pubs.acs.org.
be accidents of the growth morphology and possibly the growth mechanism. We are now able to conjecture about the differing PL behaviors of CdSe QWs and QBs. Colloidal CdSe QWs having roughly isotropic cross sections exhibit PL QYs of only 0.1-0.3%.16-21 Furthermore, the fluorescence intermittency in CdSe QWs is predominantly localized blinking or twinkling over small length scales,16,20,21 which demonstrates that excitons diffuse over only short distances prior to encountering trap sites. These characteristics differ significantly from the 30% QYs and whole-belt exciton delocalization observed here for CdSe QBs. The long axis for both wurtzite CdSe QWs42,43 and QBs lies along the [0001] direction,1 and so their side facets should be from the same crystallographic families. Manna and co-workers argue convincingly that the nearly isotropic cross section of QRs and QWs is best accommodated by a dodecahedral-prism geometry having an alternating sequence of {112¯0} and {11¯00} side facets of equal surface area (Figure 6b).39 Therefore, the {11¯00} surface area, which may be inherently more defective, rises from about 14% in the QBs to about 50% in the QWs. Consequently the natural population of surface-trapping sites may be several times greater in the QWs. Alternatively, the trap sites might be preferentially located in the edge line junctions between {112¯0} and {11¯00} surfaces rather than on the {11¯00} edge surfaces. Because the decorating Au nanoparticles have dimensions of 3-5 nm, and the thickness of the CdSe QBs is e2 nm, we cannot distinguish edge surface decoration from edge line-junction decoration. However, the number of such line junctions in a dodecahedral-prismatic QW is three times that in a rect© 2010 American Chemical Society
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