Emission Quenching in PbSe Quantum Dot Arrays by Short-Term Air

LETTER pubs.acs.org/JPCL

Emission Quenching in PbSe Quantum Dot Arrays by Short-Term Air Exposure Helen E. Chappell,†,‡ Barbara K. Hughes,†,§ Matthew C. Beard,† Arthur J. Nozik,†,§ and Justin C. Johnson*,† †

National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, United States Department of Physics and §Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States

‡

bS Supporting Information ABSTRACT: Clear evidence for two emitting states in PbSe nanocrystals (NCs) has been observed. The flow of population between these two states as temperature increases is interrupted by the presence of nonradiative trap states correlated with the exposure of the NC film to air. Quenching of the higher-energy emission begins after only seconds of exposure, with the effect saturating after several days. Unlike short-term oxygen-related effects in solution, the emission quenching appears to be irreversible, signaling a distinction between surface reactivity in NCs in films and that in solution. The origin of the two emissive centers and the impact of trapping on other NC film properties (e.g., electron/hole mobilities) remain important issues to be resolved. SECTION: Nanoparticles and Nanostructures

L

attice termination at the surface of semiconductor nanocrystals (NCs) leads to dangling bonds, which, if imperfectly passivated, can result in the trapping of charge carriers at the surface. Inorganic or organic passivants have been shown to significantly reduce fast nonradiative decay of excited states, yet even in systems that exhibit a high luminescence quantum yield (such as CdSe/ZnS), defects play a considerable role in relaxation dynamics.1 In lead chalcogenide NCs, surface trapping has not been measured systematically but has been invoked for explaining varying photophysical and electrical behavior of PbSe and PbS quantum dots.2 Particularly important are recent investigations that endeavor to quantitatively measure multiple exciton generation (MEG) through spectroscopy.3 In many cases, NCs can be excited and quickly undergo a “charging” process by which one charge carrier is trapped at the surface for a long period of time. This results in the formation of a trion after absorption of the subsequent laser pulse, leading to complicated transient absorption kinetics from which MEG yields must be unraveled.4,5 Although most recent investigations have been performed on NCs in solution, trapping also occurs, possibly more prominently, in close-packed arrays of NCs, which are the architectures most useful for photovoltaic devices.6 An accurate measure of MEG yields in films treated under various conditions must take into account the role that trapping at the NC surface may play. In addition, several transport-related effects in films are attributed to the presence of trap states: conduction mechanisms7 (i.e., bulk transport versus surface hopping), photoconductive gain,8 and doping.9 r 2011 American Chemical Society

To set the stage for an analysis of the effect of trapping on excited-state relaxation and transport in nanocrystalline PbSe, we have measured temperature-dependent photoluminescence (PL) spectra in ostensibly pristine and air-exposed quantum dot arrays. The temperature-dependent behavior shows clear evidence for the dominance of trap states induced by air exposure that have distinct effects on both low and room temperature luminescence. A myriad of nanocrystalline systems exhibit behavior associated with trapping, and it is not surprising that the lead chalcogenides should also be prone to this phenomenon. Prior measurements of near-unity room-temperature PL quantum yields have suggested the absence of traps in high-quality PbSe NCs; however, those measurements may have overestimated the true emission yield.10 Trap emission, often detectable by its slow decay in transient PL experiments, may also have been obscured by the already very slow radiative decay time of lead chalcogenides11 and the combination of narrow band gap and inhomogeneous and homogeneous line broadening,12 making spectrally resolved detection of traps difficult. Recent investigations of PbSe and PbS have either intentionally or unintentionally exposed the effect that oxidation may have on PL. There are many studies describing photophysical phenomena in NCs for which the level of air exposure (either in the dark or under excitation) was not specifically reported. Received: February 10, 2011 Accepted: March 28, 2011 Published: April 01, 2011 889

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Because even momentary air exposure affects photophysical behavior, we cannot evaluate results for which the exposure is unknown. Instead, we review here a few recent investigations in which air exposure was explicitly controlled. Sykora et al.13 observed an initial, reversible decrease in room-temperature PL intensity upon allowing PbSe NC solution samples to equilibrate in air over hours, correlated with a small blue shift in the first exciton absorption. Long-term (days) oxidation produced a large decrease in effective NC radius and (usually) an increase in PL quantum yield, consistent with the universal band gap dependence of PL. A considerable sensitivity of the PL behavior to NC synthesis conditions was also noted. Moreels et al.14 showed that PbSe nanocrystals synthesized by conventional methods are inherently nonstoichiometric and that exposing the NCs to air in solution causes Pb-oleate to be removed from the surface, although it remains unclear whether an oxidation product remains behind or simply an unpassivated surface. Other investigations have observed significant oxidation from long-term air exposure and have assigned oxidation products to particular species, including oxides, selenates, and selenites.15 Electrical measurements have also noted the extreme sensitivity of transport properties to air exposure, even on time scales for which it is unlikely that true oxidation products could accumulate.16 Finally, Raman measurements of PbSe have demonstrated the lack of any intrinsic PbSe bands except after photo-oxidation or long-term (weeks) dark oxidation.17 Again, an extreme sensitivity to sample synthesis was noted. From a practical standpoint, the degree of oxidation in a NC film is likely to affect both the trap density and its energy with respect to the conduction or valence band. The determination of these parameters is necessary for a true kinetic picture of the fundamental relaxation pathways following photoexcitation, which can be applied toward controlling behavior in particular applications such as photovoltaics or photodetectors. PbSe NCs were synthesized according to conventional procedures,15,18 avoiding air exposure during all preparation steps. PL spectra were recorded for films drop-cast from a hexane/octane solution and subsequently exposed to ambient atmosphere in the dark for durations ranging from 1 min to 63 h. After air exposure, films were flushed with He and maintained in the absence of oxygen thereafter, including during illumination for PL. For each exposure time, the PL spectrum was measured at a series of temperatures between 13 and 325 K. PL experiments were conducted under vacuum (150 K), emission is strongly quenched after prolonged O2 exposure. This effect begins to appear in less than 1 min of exposure to the atmosphere. The shape of IPL(T) changes from having a minimum near 150 K to exhibiting a continuous decrease with increasing temperature. We note the similarity between the PL versus T trend in Figure 2B and those recently reported of PbSe NCs of similar size cast into films.20,21 Further quenching occurs as exposure time is increased, but the changes are most pronounced on a short time scale; the difference in intensity behavior between an airfree film and one exposed for 1 min is far more evident than the difference between a film exposed for 1 min and one exposed for several hours. Additional changes, including blue shifting and quenching of low-temperature emission, can be observed over the course of days. The absorption spectra were measured at room temperature for each exposure time Δt in order to correlate PL observations with the changes in NC size corresponding to the growth of an oxide layer (Figure S1, Supporting Information). Despite the dramatic PL changes, the difference in NC radius between an air-free film and a film exposed for up to 1 h is less than 0.4 Å, as 891

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A general phenomenological model was formulated to describe normalized I(T) curves IPL ðTÞ ¼

suggesting that the formation of a thick oxide layer alters the likelihood of trapping through Y. The fact that the films showed irreversible PL quenching following air exposure suggests that the initial reaction forms an oxidation product at a submonolayer level. Even hundredths of a monolayer may represent several oxide/selenate defects per NC, which, given the appropriate energetics, could be expected to influence excited-state relaxation. There is also a clear difference between the behavior observed in films compared with that in solution. The kinetics of oxygen dissolution and transport in liquid media will slow the overall oxidation process when compared with the direct contact with ambient air afforded by a film. Moreover, in solution, the Pb-oleate capping process is dynamic, and an equilibrium forms between free and bound ligands.14 Introduction of oxygen into solution can temporarily cause the equilibrium to shift toward more free Pb-oleate and a less passivated NC surface, without significant oxidation products forming. Thus, as long as free Pb-oleate is available to bind again to the surface, the equilibrium can shift back when the solution is degassed. In films, no such equilibrium can form due to low ligand mobility, and attack of the NC surface by oxygen is likely to proceed irreversibly. It is interesting to note that device results using both PbSe and PbS NCs have shown great variance in air sensitivity; in some cases, devices improve upon air exposure,27 but in other instances, they degrade.6 Device performance after air exposure also depends on NC size, composition, and surface treatment. A deeper connection between PL emitted from the NCs and their performance in device architectures is desired and is the subject of ongoing investigations.

jA ð1 þ aeΔG=kT Þð1 þ f e
Ea1 =kT þ bðeEph =kT
1Þ
m Þ   jB 1 þ  1
ð1Þ 1 þ aeΔG=kT 1 þ de
Ea2 =kT

The first term in eq 1 is the contribution to the emission from state A, while the second term is from state B. It is assumed that the primary relaxation modes for state A are radiative recombination, reversible transfer to state B, and nonradiative decay through phonon interactions or through trapping pathway Y. The primary relaxation channels for B are radiative recombination, reversible decay to A, and thermally activated nonradiative decay through pathway X. ΔG is the energy separation between states A and B, Ea2 (Ea1) is the activation energy for the nonradiative quenching pathway X (Y), while ΦA and ΦB are proportional to the radiative rates of states A and B, respectively. It should be noted that the parameters Ea1 and Ea2 are activation energies and not absolute energies of trap levels. Thus, states A and B may both access just one trap state but with different activation energies, depending on the spatial localization of A and B wave functions and the chemical nature of the trap state. a represents a statistical weighting of A and B states, b is a phonon coupling parameter, while d and f are frequency factors for transfers B f X and A fY, respectively. The exponent m is related to the average number of phonons of energy Eph necessary to assist nonradiative decay. Portions of the formulation are similar to those considered for CdTe nanocrystals.26 A more complete derivation of eq 1 is described in the Supporting Information. IPL(T) traces were initially fitted individually to eq 1. For all traces, the variables Eph (4 ( 2 meV), ΦB/ΦA (4.0 ( 2), ΔG (
23 ( 6 meV), a (1.3 ( 0.6), Ea1 (8 ( 2 meV), Ea2 (100 ( 30 meV), b (0.14 ( 0.04), and m (4 ( 2) were found to vary by less than a factor of 3 for fits at different air exposure times, without a clear trend with air exposure. It is likely that these parameters are fixed by the NC material and size and thus should not be strongly dependent on the level of oxidation. All variables except f and d were then shared in a simultaneous fit of all IPL(T) data. d varied significantly with increasing air exposure, rising abruptly from less than 10 at Δt = 0 to greater than 102 for Δt = 10 min and continuing to increase to a value above 103 by Δt = 39 h (Figure S2, Supporting Information). The increase in d and can be visualized by the quenching of emission intensity for T > 150 K for all traces except Δt = 0 in Figure 3. The parameter d is expected to be directly proportional to the concentration of the oxidation product that opens pathway X, and thus, its significant increase with minimal air exposure suggests that highly labile surface sites are present for oxygen to react or coordinate with and produce dark trap levels populated with roughly 100 meV activation energy. The less pronounced, but still noticeable, increase in d with prolonged air exposure suggests that a secondary reaction process, possibly involving oxygen attacking partially or fully passivated surfaces, occurs much more slowly. At T < 100 K, additional quenching can be observed as Δt increases, which is reflected in an increasing value of f. The prominence of pathway Y, with Ea1 at roughly 8 meV, is also correlated with increased air exposure, although at the longest exposures, the frequency factor f decreases (Figure S2, Supporting Information),

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional experimental details, absorption spectra, derivation of eq 1, and fitting parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Joey Luther for assistance with sample preparation. H.E.C acknowledges support from the NSF Graduate Research Fellows Program. This work was supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES) under Contract No. DE-AC36-08GO28308 to NREL. ’ REFERENCES (1) Jones, M.; Lo, S. S.; Scholes, G. D. Signatures of Exciton Dynamics and Carrier Trapping in the Time-Resolved Photoluminescence of Colloidal CdSe Nanocrystals. J. Phys. Chem. C 2009, 113, 18632–18642. (2) Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M. Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids. Nano Lett. 2010, 10, 1960–1969. (3) Beard, M. C.; Midgett, A. G.; Law, M.; Semonin, O. E.; Ellingson, R. J.; Nozik, A. J. Variations in the Quantum Efficiency of Multiple 892

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