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J. Phys. Chem. B 2001, 105, 436-443
Ultrafast Carrier Dynamics in CdSe Nanocrystals Determined by Femtosecond Fluorescence Upconversion Spectroscopy David F. Underwood, Tadd Kippeny, and Sandra J. Rosenthal* Department of Chemistry, Vanderbilt UniVersity, Box 1822-B, NashVille, Tennessee 37235 ReceiVed: August 25, 2000; In Final Form: October 25, 2000
Femtosecond fluorescence upconversion has been utilized to study the band edge and deep trap emission dynamics of cadmium selenide (CdSe) nanocrystals (NC’s) ranging in size from 27 to 72 Å in diameter. Both the band edge rise time and decay show a direct correlation to NC size, and a rise time that depends on excitation energy. Surface-oxidized and non-oxidized NC’s display the same band edge fluorescence decay kinetics, but the relative amplitudes of the short and long components differ. The deep trap emission that appears within 2 ps is attributed to ultrafast relaxation of a surface selenium dangling bond electron to the valence band where it combines radiatively with the initial photogenerated hole. By this process, the large amplitude of the band edge emission that is attributed to direct electron/hole recombination is attenuated within the initial 2-6 ps. The long lifetime of the band edge emission originates from a triplet state, with an energy lying just below the lowest electronic level consistent with the “Dark Exciton”. The extended deep trap emission arises from the relaxation of the excited-state conduction band electron to a surface-localized hole or vice-versa. A new model is presented which describes these mechanisms for exciton relaxation in CdSe quantum dots.
I. Introduction Semiconducting nanocrystals such as CdSe have gained tremendous attention in recent years because of their unique optical and electronic properties that stem from quantum confinement. Their robust nature, tunable optical properties, and large oscillator strengths make them ideal candidates for the building blocks of future optoelectronic devices such as optical switches or photovoltaics. As the kinetics of the photogenerated electron-hole pair (the exciton) necessarily defines the operating parameters of such devices, it is clear that an understanding of the exciton relaxation pathway is an integral part of the design process. Current synthetic methods for the production of highquality CdSe nanocrystals afford an inner crystalline lattice that is pure and homogeneous in structure - i.e., there are no point defects that introduce new electronic states. This means that the fate of the photogenerated electron and hole are largely, if not completely, dictated by the NC size and surface properties. Recently there have been numerous reports dismissing the existence of the phonon bottleneck in quantum dots, a bulk property1 which mediates the “preparation” of excitons and their energy relaxation through electron/phonon coupling in the lattice.2-6 Prabhu et al.3 found a 7-10 ps exciton formation constant for bulk CdSe using an upconversion technique. Efros, Kharchenko, and Rosen7 proposed an Auger-like mechanism for the ultrafast sub-picosecond relaxation found in semiconducting NC’s, which was later substantiated in two papers by Klimov et al.8,9 The rate of exciton formation and relaxation observed in this study validates the absence of a phonon bottleneck in CdSe NC’s. Other ultrafast techniques have been utilized previously to discern the dynamics of excitons in bulk3 CdSe and NC’s, including femtosecond pump-probe,5,8,10-12 femtosecond transient hole burning,13 fluorescence line-narrowing,14 and photon * Author to whom correspondence should be addressed.
echo.15 Most of these ultrafast measurements display a 1-2 ps rise time for the population of the excited state, followed by a 2-6 ps decay, and a nano- or even microsecond lifetime for the full relaxation to the ground state. Unlike femtosecond pump-probe laser spectroscopies, fluorescence upconversion is unique in its ability to preferentially probe the excited state (the exciton), without interference from other processes such as excited-state absorption or ground-state recovery.16 In the present study, a tunable optical parametric amplifier was used as the pump source so that samples could be excited close to, or above the onset of the first excitonic transition (1S3/2-1Se) in the static absorption spectrum. Although the 2S3/2(h)-1Se transition17 is allowed in the region defined by the bandwidth of our pulses (see Figure 1), theoretically their oscillator strengths are such that at most they may contribute ∼10%. The next higher-energy 1P3/2 - 1Pe transition does have the same relative oscillator strength as the lowest energy state,17 but its excitation was avoided by keeping the pump energy lower than the required transition energy. The laser power and sample concentrations were set to ensure that only one exciton was formed per NC; in this configuration, the upconversion experiment yields accurate dynamic measurements free from effects such as bi- or multi-exciton annihilation10 or excitation high into the conduction band states. The surface properties of our NC samples are believed to strongly influence the dynamics of the exciton. Figure 2 shows our Rutherford backscattering spectroscopy (RBS) data18 which indicates that the surfaces of CdSe NC’s are comprised of on average 30% selenium atoms. The remaining 70% of the surface consists of Cd atoms which are mostly passivated with trioctylphosphine oxide (TOPO) ligands. The bare selenium atoms on the surface give rise to dangling bonds which lead to midgap states.19,20 Semiempirical tight-binding models of CdSe NC’s by Leung and Whaley20 report that the surface relaxation in NC’s is similar to that of bulk semiconductor surfaces, and
10.1021/jp003088b CCC: $20.00 © 2001 American Chemical Society Published on Web 12/22/2000
Ultrafast Carrier Dynamics in CdSe Nanocrystals
Figure 1. Typical static UV-visible absorbance and fluorescence spectra of TOPO-capped CdSe NC’s in toluene (34 Å NC’s shown). The OPA laser bandwidth is shown as an average excitation position for all experiments, approximately 15-20 nm below the first exciton transition. Inset: Energy diagram depicting the excitation energy (545 nm (2.27 eV)), band edge emission (569 nm (2.18 eV)), and deep trap emission (885 nm (1.40 eV)).
that the majority of the surface states arise from selenium sp3 hybridized dangling bond orbitals.21,22 The deep trap emission has been the focus of many experimental23,24 and theoretical25-30 investigations in an attempt to elucidate the role of surface states. Lifshitz et al. have performed photoluminescence and optically detected magnetic resonance studies that conclude the existence of a nonexcitonic band attributed to recombination between shallow trapped electrons and deep trapped holes associated with the surface of the NC.23,24 In an effort to characterize the ultrafast electronic properties of CdSe nanocrystals, we have utilized fluorescence upconversion spectroscopy to examine: the excitation and emission wavelength dependence of the band edge emission, the size dependence of the kinetics of band edge and deep trap emission, and surface-oxidized vs non-oxidized samples to determine the role of surface passivants in the relaxation process. II. Experimental Section A. Cadmium Selenide Nanocrystals. TOPO-passivated CdSe NC’s were prepared in the strict absence of oxygen by the pyrolysis of organometallic precursors in TOPO at 360 °C using the method of Murray, Norris, and Bawendi,31 as modified by the Alivisatos group.21 The NC’s were characterized by steady state UV-visible absorption (Cary 50 Bio) and fluorescence (PC1, ISS) spectroscopy (Figure 1), powder X-ray diffraction, and high-resolution transmission electron microscopy (TEM)(CM20, Phillips). The size distribution of our samples were determined to have a deviation of (5% by TEM. All of the optical measurements, including the ultrafast spectroscopy, were conducted at room temperature with a NC concentration such that the optical density of the lowest excitonic transition was ∼0.9 in anhydrous toluene distilled from sodium benzophenone. Preparation of NC’s greater than 45 Å in diameter in a strictly oxygen-free environment resulted in samples that displayed practically no steady state band edge or deep trap emission. We attribute this observation to the fact that only the majority of Cd atoms are passivated by TOPO at this point, leaving the surface Se atoms bare with a sp3 lone pair dangling bond
J. Phys. Chem. B, Vol. 105, No. 2, 2001 437 directed normal to the NC surface (ignoring surface reconstruction). These sites can prohibit electron/hole recombination (thus reducing or quenching emission) by trapping the hole at the surface; the smaller NC’s do not demonstrate this behavior, presumably because the electron and hole wave functions have a greater overlap for smaller sized nanocrystals. Samples
