pubs.acs.org/JPCL
Hot Electron Extraction From Colloidal Quantum Dots Anshu Pandey and Philippe Guyot-Sionnest* The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637
ABSTRACT Hot electrons are created in core/shell CdSe/ZnSe colloidal quantum dots by mid-infrared intraband (4 μm) excitation and are probed by timeresolved visible spectroscopy. The hot electron, in the first excited conduction state 1Pe of the CdSe core, is efficiently extracted by tunneling through the ZnSe shell. Electron extraction times are temperature-independent. They range from ∼100 ps for thick, ∼3 nm, uniform ZnSe shells to 1 ns) 1Se electron to become trapped or to recombine with Received Date: September 22, 2009 Accepted Date: October 26, 2009 Published on Web Date: November 05, 2009
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DOI: 10.1021/jz900022z |J. Phys. Chem. Lett. 2010, 1, 45–47
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Figure 1. (a) Schematic showing visible and IR excitation and subsequent intraband relaxation or extraction of the electron. (b) Transient relaxation of the bleach for the two probe wavelengths at B1 (filled circles) and A1 (open circles) shown in the inset. The sample is CdSe/ZnSe with a ∼0.3 nm (approximately one ZnSe monolayer) shell. (c) Same but for a CdSe/ZnS/ZnSe with a total shell thickness of 3.2 ( 0.4 nm. The lines are fits as described in the text.
the hole within a few ps. As previously reported, this effect disappears when the CdSe/ZnSe is capped by a monolayer of CdSe. Therefore, the effect is due to the outer surface of ZnSe, and the electron must have tunneled across the ZnSe shell. This should lead to a verifiable exponential dependence of the dynamics with shell thickness. As shown in Figure 1c, the kinetics are indeed slower with a thicker shell. To quantitatively analyze the effect of the shell thickness, we model the response with two rates shown in Figure 1a. There is the tunneling rate from 1Pe to a trap/recombination center on the shell surface, kT, and the competing intraband relaxation rate from 1Pe to 1Se, kIB. The population P(t) in 1Pe excited by the IR pump decays from its initial value P0 as PðtÞ ¼ P0 expð -ðkIB þ kT ÞtÞ
Figure 2. (a) kT as a function of shell thickness with fits described in the text. (b) kT-1 as a function of temperature for a ∼0.3 nm thick shell (approximately one monolayer ZnSe) on a Wurtzite core.
ð1Þ
while the change in the 1Se population ΔS(t) induced by the IR pump and monitored at B1 is given by kIB ΔSðtÞ ¼ -P0 1 þ ðexpð -ðkT þ kIB ÞtÞ -1Þ ð2Þ kIB þ kT
offsets. The coefficients are in fair quantitative agreement with a simple WKB approximation. Using k = (2m*Eb/p2)1/2, where m* is the ZnSe electron effective mass (∼0.17 me)7 and EB (∼0.15 eV) is the ZnSe barrier height above 1Pe calculated for the zinc blende core/shell, would give k ∼ 0.8 nm-1. The tunneling rates are observed to be temperature-independent from 15 to 300 K, as shown in Figure 2b. This in contrast to typical trapping processes which are activated and slow down at low temperature. However, it is expected for a two-step process if trapping/recombination on the outer surface is much faster than tunneling. In the Supporting Information, a single rate process with a temperature-independent trapping rate is also investigated using a standard electrontransfer model. In summary, the IR pulse creates a hot electron in 1Pe that leads to the rapid removal of the electron from the core states. The sensitivity to the chemical passivation of the outer ZnSe shell surface, the temperature independence, and the exponential dependence on the shell thickness all point to hot electron extraction. Efficient extraction of the 1Pe electron will be useful for mid-IR photodetection. One consequence of the extraction is a noticeable quench of the photoluminescence (PL) when the IR pulse comes soon after the 532 nm pump. The PL quench is readily observed with the CdSe/ZnSe quantum dot with a thin
The ratio ΔS(¥)/ΔS(0) = kT/(kIB þ kT) is the fraction of dots for which the 1Pe electron tunneled out normalized to the dots that had the electron initially excited to 1Pe. For Figure 1b, the data are fitted with an intraband relaxation rate of kIB-1 = 32 ps and a tunneling time constant of kT-1 = 16 ps, for an extraction efficiency of ΔS(¥)/ΔS(0) ∼ 65%. Figure 1c shows a similar response with a thicker ZnSe shell sample with an extraction efficiency of ∼75%, fitted with slower rates of kIB-1 = 90 ps, and kT-1 = 29 ps. Figure 2a shows kT as a function of shell thickness for the samples that were measured. We measured samples grown in a zinc blende as well as a Wurtzite structure, and they show slight but systematic differences. The inverse rates are fitted with the function R-2 exp(2k(R - R0)), where R and R0 are the shell and core radii, respectively. The R-2 terms accounts for an increase of the acceptor state density with shell area, while the exponential increase with shell thickness accounts for tunneling. For core/shell starting with Wurtzite and zinc blende cores, the fitted coefficients k are 0.75 and 0.53 nm-1, respectively. The difference is possibly due to different conduction band
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DOI: 10.1021/jz900022z |J. Phys. Chem. Lett. 2010, 1, 45–47
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Figure 3. (a) Transmission electron microscopy image of branched CdSe/ZnSe (scale bars: 20 nm; inset: 10 nm). (b) PL with (dotted line) and without (solid line) intraband excitation. (c) Bleach recovery at B1. (d) Sample absorption spectrum (solid line) and PLE (dotted line). Above the onset of ZnSe absorption at 420 nm, the deviation indicates only a partial transfer of the excitons generated in ZnSe to the CdSe core.
shell, so that they still fluoresce. A larger IR-induced PL quench is obtained for high ZnSe surface area samples, shown in Figure 3a. These samples have a PL quantum yield of ∼15%. The PL quenches by ∼22% with a single-pass IR pulse and ∼35% in a double-pass, as shown in Figure 3b. For this sample, the transient bleach at B1 is a step-like function with a rise time limited by the ∼8 ps pulse width (Figure 3c). This indicates that 1Pe relaxes within the laser pulse width. kIB should be fast because a luminescent dot has the hole in a delocalized state or a shallow trap, and this should allow electron-hole Auger coupling.8 Nevertheless, Figure 3c also shows that 50% of the dots lose their 1Se electron after IR excitation, and therefore, kT is as fast or faster than kIB. The IR-induced PL quench is visible to the eye, showing the dark mid-IR beam shape and position on a wider fluorescent field. We know of no previous molecular or colloidal systems for which a mid-IR photon can as strongly influence the photophysics in the visible. Finally, we note that if 1Pe electron extraction competed efficiently with the subps excitonic cooling,4 the photoluminescence excitation (PLE) would show a strong deviation with the absorption spectrum around the P exciton at ∼500 nm. This is not the case, as shown in Figure 3d. Therefore, although this work showed efficient 1Pe electron extraction with a cold hole, extraction is inefficient with a “hot” hole. This distinction is relevant to the challenge of extracting hot electrons in photovoltaic devices.9 To conclude, we studied the time evolution of the electronic occupation of the 1Pe and 1Se states of core/shell CdSe/ZnSe colloidal quantum dots that are excited by a resonant, ∼4 μm, intraband pulse subsequent to an interband excitation pulse. We showed that a large fraction (>50%) of the electrons that are excited from 1Se to 1Pe leave the CdSe core faster than relaxing to 1Se. The sensitivity to the outer shell chemical composition, the temperature independence, and the exponential shell thickness dependence all support a hot electron extraction via tunneling across the ZnSe shell to traps or recombination centers on the outer surface. One unusual consequence is that the mid-IR pulse quenches the visible photoluminescence. This work shows the possibility of efficient mid-IR excitation and extraction of electrons in colloidal quantum dots.
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ACKNOWLEDGMENT We thank the U.S. National Science Foundation NSF under Grant No. DMR-0706268 for funding. The authors made use of shared facilities supported by the NSF MRSEC Program under DMR-0820054.
SUPPORTING INFORMATION AVAILABLE Sample preparation, details of measurements, PL quench, and temperature independence. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author:
To whom correspondence should be addressed.
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DOI: 10.1021/jz900022z |J. Phys. Chem. Lett. 2010, 1, 45–47
