NANO LETTERS
Fluorescence Spectroscopy of Single Lead Sulfide Quantum Dots
2006 Vol. 6, No. 3 510-514
Jeffrey J. Peterson and Todd D. Krauss* Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627 Received December 30, 2005; Revised Manuscript Received February 1, 2006
ABSTRACT We report the observation of fluorescence from single PbS quantum dots (QDs) using confocal microscopy in an inert atmosphere. Single PbS QDs exhibit a narrowing of the fluorescence line width (relative to the ensemble) and fluorescence intermittency (“blinking”), consistent with the properties of other single QDs. However, single-particle line widths averaged 100 meV, indicating a significant homogeneous component to the ensemble line width. In addition, the duration of an on/off blinking event has a probability density that decays according to an inverse power law, indicating a broad distribution of kinetic rates to multiple on/off states.
Semiconductor quantum dots (QDs) composed of lead chalcogenides (PbS, PbSe, PbTe) are unique materials that are ideal for fundamental studies of strongly quantumconfined systems and may also enable important future technological advances. From a technological perspective, lead chalcogenide QDs are of interest because they are among the few materials that can provide tunable electronic transitions at important near-infrared (NIR) wavelengths. Indeed, electroluminescent devices operating at wavelengths important for telecommunications (1.3-1.55 µm) that incorporate PbSe QDs,1,2 and water-soluble PbS QDs for in vivo microscopic imaging that absorb and emit in the NIR “biological transparency window,” have been reported.3 In addition, the observation of optical gain and amplified spontaneous emission from PbSe QDs opens the door to possible tunable NIR lasers,4 and multiexciton generation in PbSe QDs may increase the efficiencies of future solar cells.5,6 Fundamental studies of PbS and PbSe QDs are frequently motivated by the fact that the large exciton Bohr radii, aB, of PbSe (46 nm) and PbS (20 nm) permit levels of quantum confinement that are not possible for other II-VI or III-V materials.7 Furthermore, the lead chalcogenides’ exciton Bohr radii are composed of nearly equal contributions from the electron and hole.7 In contrast to other II-VI and III-V materials, in which the hole aB is ∼1 nm (such as CdSe), the large hole Bohr radius of the lead chalcogenides allows both carriers to be extremely quantum-confined. Such extreme confinement of both carriers is predicted to enhance nearly all of the size-dependent properties that are frequently of interest in nanostructured materials.8-10 PbSe and PbS can be synthesized as colloidal QDs with diameters ranging between 2 and 20 nm and with narrow * Corresponding author. E-mail:
[email protected]. 10.1021/nl0525756 CCC: $33.50 Published on Web 02/21/2006
© 2006 American Chemical Society
size distributions.11-17 These QDs exhibit well-defined exciton peaks in their absorption spectra and band-edge luminescence that can be tuned throughout the NIR spectral region (Figure 1A). Studies of the optical properties of PbSe QDs at room temperature have reported photoluminescence (PL) lifetimes of ∼300 ns and fluorescence quantum yields (QYs) as high as 80%;12,13 similar results have been reported for PbS QDs.17,18 Although the long PL lifetimes can be accounted for by a quantitative consideration of dielectric screening,12 it has remained a challenge to rationalize the high QYs of PbSe and PbS QDs. One important technique for the elucidation of QD optical properties is single QD fluorescence spectroscopy, which eliminates averaging over all members of the ensemble and can reveal fundamental properties otherwise masked in ensemble measurements. Single QD fluorescence spectroscopy has been accomplished successfully for several other colloidal semiconductor materials, including CdSe,19-21 CdS,22 CdTe,23 Si,24,25 and InP.26 Understanding key elements of lead chalcogenide QD photophysics, such as the homogeneous line width and the origin of the high QY, clearly would benefit from studies on the single-particle level. However, because of difficulties in synthesis and poor sample photostability, such studies have been extremely difficult and have not been reported previously. Here we report fluorescence measurements from single PbS QDs using confocal microscopy at 300 K. In air, PbS QDs spun-coated on quartz coverslips photobleach quickly, necessitating a N2 purge of the local atmosphere. Single PbS QDs exhibit non-Lorentzian line shapes with line widths on the order of 100 meV, indicating a significant homogeneous component to the ensemble line width. Measurements of fluorescence intensity time traces from a single QD reveal the presence of fluorescence intermittency, with an inverse
Figure 1. (A) Absorption spectra for a series of different-sized colloidal PbS QDs. (B) Typical absorption (left curve) and fluorescence (right curve) spectra of ∼3-nm-diameter PbS QDs. All spectra were obtained in air at 300 K in tetrachloroethylene.
power law describing the decay of probability density for on and off times. PbS QDs were synthesized according to the general method of Hines and Scholes.17 (For further details, see the Supporting Information.) The absorption spectra of colloidal QD samples were collected with a Perkin-Elmer UV/vis/ NIR spectrometer (Lambda 19), and fluorescence spectra were collected with a modular Acton Research fluorometer equipped with a Ge detector. Fluorescence QYs were measured relative to the dye IR 125. Confocal microscope samples were prepared by spin-casting solutions of QDs (∼1 × 10-8 M) in 0.1% poly(methyl methacrylate)/toluene (g/ mL) onto a quartz coverslip, embedding the single PbS QDs in a thin polymer film. Single PbS QD fluorescence measurements were obtained using epi-fluorescence confocal microscopy with excitation at 632.8 nm. The excitation laser was reflected by a dichroic mirror to a high numerical aperture (NA) objective (100×, NA 1.3), and focused to a point spot (diameter ∼400 nm) on the sample surface. QD fluorescence was collected through the same objective, passed through the dichroic mirror and a holographic notch filter, and directed to an imaging spectrometer mounted with a thermoelectrically cooled, Si charge-coupled detector (CCD, Princeton Instruments, Spec10). Images were created by scanning a piezoelectric stage (Mad City Labs, NanoH70) in 0.5-µm steps through a 10 × 10 µm2 area. Typical capture times for images and fluorescence spectra were 370 ms and 300 s, respectively, with typical excitation intensities of ∼10 kW/ cm2. PbS QDs with diameters >4 nm emit at wavelengths >1000 nm, requiring IR-sensitive array detectors, which have typical noise levels an order of magnitude greater than cooled Si CCDs. To utilize Si CCDs for these measurements, we synthesized the smallest possible PbS QDs (diameter ∼3 nm), which fluoresce at wavelengths
