NANO LETTERS
Ultrabright PbSe Magic-sized Clusters Christopher M. Evans,† Li Guo,† Jeffrey J. Peterson,† Sara Maccagnano-Zacher,‡ and Todd D. Krauss*,†
2008 Vol. 8, No. 9 2896-2899
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627, School of Applied and Engineering Physics, Cornell UniVersity, Ithaca, New York 14853 Received June 12, 2008; Revised Manuscript Received July 9, 2008
ABSTRACT We present the synthesis and characterization of novel, ultrasmall PbSe magic-sized nanoclusters (MSCs). Unlike the syntheses of highquality semiconductor nanoparticles, the MSC synthesis is straightforward, occurring at room temperature in air over several hours. MSCs have core diameters 50%). With core diameters estimated to be less than 2 nm, these clusters concomitantly address the optimal requirements for in vivo biological labeling: small hydrodynamic radii, high quantum yields (QY), NIR emission, water solubility, and long-term photostability. The PbSe MSC synthesis is based upon the syntheses of larger diameter PbSe QDs15,16 and further details are available in Supporting Information. Briefly, PbO and oleic acid are heated at 150 °C in octadecene, and the solution is then cooled to 20 °C whereby a solution of trioctylphosphineselenide (TOPSe) is subsequently injected. Over the course of a few hours in ambient conditions, the solution darkens * Corresponding author. † University of Rochester. ‡ Cornell University. 10.1021/nl801685a CCC: $40.75 Published on Web 08/01/2008
2008 American Chemical Society
considerably, indicating the formation of PbSe MSCs. Reactions can also be performed under a N2 atmosphere, resulting in MSCs with identical properties to those produced in air. For MSC characterization, absorption spectra were acquired on a Perkin-Elmer UV/vis spectrometer (Lambda 35) and fluorescence spectra were collected with a modular Acton Research fluorometer equipped with a Ge detector. Fluorescence QYs were measured relative to the dye IR125. Dynamic light scattering was performed on a Malvern ZetaSizer Nano ZS using an ultralow volume cuvette with dilute (∼10-7 M) aqueous solutions. Typical semiconductor nanocrystal QD syntheses, such as PbSe QDs grown at higher temperatures (e.g., 150 °C), involve an initial nucleation event followed by growth that results in a continuous red shift of optical features over time.17 In contrast, MSCs are remarkably size stable12 and are easily identified by persistent and isolated peaks in the absorption spectra.13 As seen in Figure 1, the absorption maxima of PbSe MSCs are not strongly dependent on growth time. As the reaction proceeds, absorption peaks grow in at specific wavelengths (625, 690, 750, 820, and 880 nm) favoring lower energy components at long reaction times with only minute variations in peak positions between batches ((5 nm). After background subtraction, each absorption spectrum reveals a complex feature that was modeled by fitting several Gaussian functions, each centered at the specific wavelength corresponding to the individual absorbing component (Supporting Information Figure 1). Figure 2 shows the temporal evolution of each specific MSCs’ contribution to the overall absorption spectrum over the entire reaction period. It is observed that each component grows in at specific times and is eventually consumed for the production of larger species. Therefore, the absorption spectra suggest that instead of growing PbSe QDs with heteroge-
Figure 2. Integrated intensities of the individual Gaussian absorption peaks at 625 (orange circle), 690 (green square), 750 (blue diamond), 820 (red up-triangle) and 880 (gold down-triangle) nm during the PbSe MSC synthesis. The solid line is a fit to a simple function that models the growth and decay of individual cluster species (Supporting Information).
Figure 1. Optical absorption spectra at 10 min intervals (from bottom to top) for the growth solution at 30 °C. During the growth period, the spectral features do not shift, but instead change their relative magnitude. The dashed lines indicate the lowest energy absorption feature for a particular cluster. Absorption peaks at 625, 690, 750, 820, and 880 nm correspond to diameters of 1.0, 1.2, 1.4, 1.6, and 1.8 nm, respectively, based on an extrapolation of the relation between nanoparticle size and the energy of the first exciton absorption assuming a spherical cluster.16
neous diameter distributions, several stable and discretely sized PbSe clusters were formed. Similar conclusions regarding the formation of PbSe MSCs can be obtained from fluorescence spectra taken at specific time intervals during the course of the reaction. Similar to the series of absorption spectra, emission profiles taken over several hours show a persistence in individual peak positions (Figure 3a). However, due to the large (∼100 meV) homogeneous fluorescence line widths of lead-salt QDs,18 and the large stokes shift,16 unambiguously deconvolving individual cluster components of the fluorescence spectrum is challenging. Nonetheless, the magnitude of the Stokes shift is consistent with very small sized nanoparticles.19 In addition to the persistent peaks in the optical spectra, other evidence exists that MSCs of PbSe are being formed. During the synthesis of larger-sized PbSe QDs (at 150 °C), higher energy peaks in the absorption and fluorescence spectra appear, which are similar to the optical spectra for MSCs synthesized at 20 °C (See Supporting Information Nano Lett., Vol. 8, No. 9, 2008
Figure 2). The observation of persistent, high-energy features in absorption and fluorescence spectra, not attributable to higher excitonic states, has also been made during the synthesis of CdSe QDs and assigned to incredibly size stable MSCs present during the QD synthesis.8 Powder X-ray diffraction data (Supporting Information Figure 3) suggest that the PbSe MSCs likely retain their rock-salt crystal structure, although the small size of the MSCs results in weak and broad diffraction peaks. Additionally, using Bragg’s law (nλ ) 2d sin(θ)), we infer a MSC spacing d corresponding to a diameter ∼1.8 nm from the position of a small angle reflection in the XRD (Supporting Information Figure 3). Finally, several simple cubic lead-salt MSC species have been theoretically shown to exist as well.20 It is remarkable that, despite the low temperatures and the presence of oxygen during the synthesis, PbSe MSCs have exceptional fluorescence properties (Figure 3b). Having QYs typically
