Effect of Surface Defects on Auger Recombination in Colloidal CdS

Apr 15, 2011 - Cooney , R. R.; Sewall , S. L.; Dias , E. A.; Sagar , D. M.; Anderson , K. E. H.; ...... M. Lanzi , E. Serra , S. Allard , U. Scherf , ...
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LETTER pubs.acs.org/JPCL

Effect of Surface Defects on Auger Recombination in Colloidal CdS Quantum Dots Yoichi Kobayashi, Takatoshi Nishimura, Hiroshi Yamaguchi, and Naoto Tamai* Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen,Sanda, Hyogo 669-1337, Japan

bS Supporting Information ABSTRACT: The effect of surface states originating from surface defects and capping reagents on Auger recombination in CdS quantum dots (QDs) are investigated by femtosecond transient absorption spectroscopy. Because of the strong size-dependent nature of Auger recombination and surface defects, the size dependence of Auger recombination is also conducted to reveal the effect of surface states. The lifetime of Auger recombination is very similar irrespective of surface states in all size regions and was proportional to D6 (D: QD diameter). This result clearly shows that Auger recombination in CdS QDs does not depend on interfacial electronic structures originating from the surface defects and capping reagents of one monolayer level. SECTION: Nanoparticles and Nanostructures

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emiconductor quantum dots (QDs) are promising materials for solar cells and laser amplifications because of strong Coulomb interactions and efficient carrier multiplication.1 5 In colloidal QDs, by conjugating biomolecules and inorganic materials to the QD surface, various potential applications are obtained such as biological tagging and resonant energy-transfer detection.6 8 On the other hand, the single exciton relaxation processes strongly depend on the capping reagents, surface defects, and environmental conditions.9 11 Defects on the QD surface such as cation or anion dangling bonds make broad electronic states near the band edge, and they serve as hole or electron trappings.12,13 The hot electron relaxation to the band edge is also affected by capping reagents of QDs due to the efficient energy transfer process to the vibrational states of capping reagents.14 A recent state-resolved femtosecond spectroscopic approach quantitatively revealed that a hot hole relaxes to the band edge via nonadiabatic channel mediated by surface ligand vibrations without phonon bottleneck, while a hot electron mainly relaxes via Auger-type energy transfer.15,16 Similarly to single exciton dynamics, QD surface states are expected to affect the multiexcitonic interaction such as Auger recombination. Auger recombination in semiconductor QDs have been extensively investigated from the viewpoints of the effect of QD size, shape, inorganic shell, and so on.1,15 24 Recently, alloyed QDs and giant multishell QDs achieved the significant suppression of Auger recombination by changing their potential shapes.25,26 While these complex structures have opened up a new methodology to suppress Auger recombination, the effect of intrinsic surface properties such as surface defects on Auger recombination has not been comprehensively revealed.15,16,27 29 Klimov et al. reported that the surface passivation has no significant influence on biexciton recombination r 2011 American Chemical Society

dynamics of CdSe QDs,17 and Pandey and Guyot Sionnest reported the insensitivity of hole wave functions on multicarrier recombination rates of CdSe and CdS QDs.28 On the other hand, Auger recombination of CdSe nanorods was strongly influenced by the surface ligands, in which the recombination rate of octadecylphosphonic acid-ligated particles was much slower than that of hexadecanethiol-ligated particles.29 Therefore, the systematic study by using a series of QDs is indispensable. A numerical calculation by empirical pseudopotential theory predicted that Auger recombination mainly occurs at the QD surface, which is determined by the dielectric function at the interior and the surface of QDs.30 A calculation by Fermi’s golden rule also predicted that Auger recombination depends on the abrupt surface inducing a large uncertainty of the electron momentum.31,32 The experimental investigation of the intrinsic surface effect on Auger recombination plays a important role in understanding the theoretically predicted surface effect. Recently, Tyagi and Kambhampati examined the effect of surface charge trapping on multiple exciton recombination and generation of CdSe QDs, and concluded that multicarrier processes often become obscure owing to the surface-induced charge trapping.33 In addition, because of the strong size-dependent nature of Auger recombination and surface defects, the size-dependent analysis is important to reveal the effect of surface states on Auger recombination comprehensively. In this letter, we examined the effect of surface states originating from surface defects and capping reagents on Auger Received: February 25, 2011 Accepted: April 12, 2011 Published: April 15, 2011 1051

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Figure 2. Transient absorption spectra of OA- (a) and GSH-capped (b) CdS QDs with diameters of 2.7 nm. The excitation intensity is 60 μW, which corresponds to ÆN0æ ∼ 3. Figure 1. Absorption (solid lines) and emission spectra (dashed lines) of GSH- (a), OA- (b), and MA-capped (c) CdS QDs.

recombination in colloidal CdS QDs as a function of QD size by femtosecond transient absorption spectroscopy. Surface defects of CdS QDs were confirmed with luminescence spectroscopy. The size dependence of the lifetime of Auger recombination (Auger lifetime) was found to be very similar irrespective of surface states. This result clearly shows that Auger recombination in CdS QDs does not depend on interfacial electronic structures originating from surface defects and capping reagents. The size dependence of Auger recombination in CdS QDs was different from that of CdSe QDs (∼D3), and the possible reason of this deviation is also discussed. CdS QDs capped with L-glutathione (GSH) and myristic acid (MA) were prepared according to the procedure described elsewhere.34,35 CdS QDs capped with oleic acid (OA) were purchased from Aldrich. The QDs diameters (D) were estimated from the first excitonic absorption peak36 and confirmed by transmission electron microscopy (TEM, Tecnai G2 F20 200 keV, FEI). A TEM image of typical GSH-capped CdS QDs are shown in Figure S1a (Supporting Information). The QDs diameters were 3.5 4.9 nm, 2.7 5.9 nm, and 2.5 5.2 nm for MA-, OA-, and GSH-capped CdS QDs, respectively. X-ray scattering images were recorded on a RIGAKU R-AXIS IV imaging plate diffractometer. The diffraction patterns are in the Supporting Information (Figure S1b). The emission quantum yield (Φ) of CdS QDs was determined at room temperature by comparing the integrated emission to that of coumarin 343 in ethanol (Φ = 63%). The emission quantum yield was estimated to be Φ = 2 11% for MA-capped CdS QDs, Φ = 9 55% for OA-capped CdS QDs, and Φ = 5 23% for GSH-capped CdS QDs. UV visible absorption and emission spectra were recorded using U-4100 (Hitachi) and FluoroMax-2 (JobinyvonSpex) spectrophotometers, respectively. Transient absorption spectra were measured by femtosecond pump probe experiments described previously.20 CdS QDs were excited at 266 and 400 nm by an amplified mode-locked Ti:Sapphire laser with the excitation intensity varying from 1 to 200 μW. Absorption

transients were probed by delayed pulses of a femtosecond white-light continuum generated by focusing a fundamental (800 nm) or second harmonic (400 nm) laser pulse into a D2O cell. The temporal resolution was 100 fs and the temporal dispersion of the white-light continuum was collected for the transient absorption spectra. All measurements were performed at room temperature. Figure 1 shows absorption and emission spectra of CdS QDs synthesized by different procedures. In the absorption spectra, the peak associated with the optically allowed 1S3/2(h) 1S(e) transition is observed at the absorption edge in all samples. A sharp 1S3/2(h) 1S(e) peak and several peaks at higher energy are especially observed in MA- and OA-capped CdS QDs (Figure 1b,c), which are probably assigned to 1S1/2(h) 1S(e) and 1P3/2(h) 1P(e) transitions.37,38 The absorption spectra of GSH-capped CdS QDs are broad, and the 1S1/2(h) 1S(e) and 1P3/2(h) 1P(e) peaks cannot be observed, possibly because of a larger size dispersion. In the emission spectra, a sharp excitonic emission peak is observed in MA- and OA-capped CdS QDs. On the other hand, only the broad emission at longer wavelength is observed in GSH-capped CdS QDs, which is associated with the emission from surface defects. This result indicates that the dangling bonds at the QDs surface are not fully passivated with GSH. We measured transient absorption spectra of GSH-, OA-, and MA-capped CdS QDs as a function of the excitation intensity. Figure 2 illustrates the transient absorption spectra of OA- and GSH-capped CdS QDs with average diameters of 2.7 nm. The transient absorption spectra of MA-capped CdS QDs are shown in the Supporting Information (Figure S2a) because their features are very similar to those of OA-capped CdS QDs. The excitation intensity of 60 μW (3.0  1013 photon cm 2) in Figure 2 corresponds to about three excitons per CdS QD on average. The initial average number of excitons per QD, ÆN0æ, was calculated by the equation ÆN0æ = jpσ0, where jp is the pump photon fluence, and σ0 is the QD absorption cross section.39 In both spectra, a negative bleaching peak corresponding to the ground-state 1S3/2(h) 1S(e) absorption is observed. The growth kinetics at the 1S3/2(h) 1S(e) bleaching of GSH- and 1052

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Figure 3. Population dynamics of OA-capped CdS QDs with diameters of 3.4 nm (a) and 4.7 nm (b), and those of GSH-capped CdS QDs with diameters of 3.1 nm (c) and 4.9 nm (d). The solid lines are the singleand two-exponential analyses of the population dynamics.

OA-capped CdS QDs are shortly discussed in the Supporting Information (Figure S3). In Figure S4, we show wide-range transient absorption spectra of GSH-, OA-, and MA-capped CdS QDs. In GSH-capped CdS QDs, a broad transient absorption ranged from 500 to 700 nm was observed, which is probably due to the denser surface states of GSH-capped CdS QDs. On the other hand, a narrow positive peak is observed at a longer wavelength of the 1S3/2(h) 1S(e) bleaching in OA- and MAcapped CdS QDs. This signal has a longer decay component than the experimental window (500 ps) at low excitation intensity (5 μW, ÆN0æ = 0.19), and an additional rise component is observed with increasing excitation intensity (Figure S5a). Klimov and McBranch reported that the positive transient signal of glass-doped CdS QDs is assigned to direct current (dc) Stark effect of the ground state absorption caused by the Augerprocess-induced charge separation.40 However, a positive peak in our experiments exists even at low excitation intensity, and the signal amplitude is linearly proportional to the excitation intensity, which is similar to the trend of the 1S3/2(h) 1S(e) bleaching (Figure S5b). This linearity indicates that the positive signal is due to one photon process. In addition, the rise time (∼6 ps) is slower than the Auger lifetime (2.3 ps for D = 2.7 nm). These results suggest that the positive peak is not due to the Auger process. This positive absorption is probably due to the transition from the band-edge single exciton state to a biexciton state (narrow absorption) or to a higher continuum state (broad absorption).33 In addition, the hole relaxation and trapping processes may be involved at this wavelength region.42 While the transient bleaching is slightly shifted owing to the biexciton state24,41,42 and surface trapping,11 spectral features around the absorption bleaching are almost the same from a few picoseconds to up to hundreds of picoseconds in both samples. At very high excitation intensity (ÆN0æ > 10), slight shift of the bleaching

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spectrum was observed for GSH-capped CdS QDs (∼50 meV). This result suggests that the effect of multiple excitons on the spectral feature of the 1S3/2(h) 1S(e) absorption is negligible. ΔOD at the 1S3/2(h) 1S(e) peak is converted to the instantaneous average number of excitons per QD, ÆN(t)æ, because absorption changes are not linear at higher excitation intensity, i.e., for larger ÆN0æ. The population dynamics were analyzed by the stochastic approach of multiple charge carrier dynamics in semiconductor nanosystems proposed by Barzykin and Tachiya.43 Figure 3 shows population dynamics of OA- and GSHcapped CdS QDs excited at different excitation intensities. The population dynamics of MA-capped CdS are shown in Figure S2b,c. In OA-capped CdS QDs of D = 3.4 nm, the dynamics at low excitation intensity of 20 μW (ÆN0æ = 0.23) are analyzed by a single exponential decay with a lifetime of approximately nanosecond scale, which corresponds to the single exciton decay. With increasing the excitation intensity, an additional fast decay component associated with Auger recombination appears (120, 180 μW; ÆN0æ = 1.3, 1.9). These dynamics are well fitted with a biexponential decay function and the lifetime of Auger recombination (Auger lifetime) is 6.3 ps for D = 3.4 nm (Figure 3a). As the size of CdS QDs increases, the Auger lifetime becomes longer and is 42 ps for D = 4.7 nm (Figure 3b). This behavior is consistent with those of CdSe and CdTe QDs.16,19 In the case of GSH-capped CdS QDs, different trends are expected because of surface defects. However, the intensity-dependent population dynamics of GSH-capped CdS QDs are very similar to that of OA- and MA-capped CdS QDs. These dynamics are well fitted with a biexponential decay function and the Auger lifetime corresponding to D = 3.1 and 4.9 nm is 3.5 (Figure 3c) and 57 ps (Figure 3d), respectively. The amplitudes of the component related to Auger recombination are small as compared with the theoretical predictions, which may be due to the fact that not all the photons are converted to band-edge excitons. Very recently, Tyagi and Kambhampati examined the multicarrier processes by changing surface treatments and concluded that the multicarrier processes often become obscure owing to the surface trapping.33 CdS QDs in our experiments does not have any fast decay components at low excitation intensities in spite of the large amount of surface defects of GSH-capped CdS. Thus the fast lifetime only detected at increased excitation fluences is most probably due to biexciton Auger recombination. We plotted the Auger lifetime logarithmically against QD diameter in Figure 4. The size dependence of the Auger lifetime of CdS QDs capped with different reagents does not show significant discrepancies and scales with ∼D6. This result clearly indicates that Auger recombination in CdS QDs does not depend on surface defects and capping reagents in any size regions. We have shown previously that the size dependence of biexciton Auger recombination varied from D4.6 for thioglycolic acid (TGA)-capped CdTe QDs to D7.0 for OA- and trioctylphosphine-capped CdTe QDs.20 In that study, two possible reasons were considered for the different size dependences of the Auger lifetime. One was the capping reagents, and the other was the formation of a thin CdS gradient at the surface for TGA-capped CdTe QDs. By considering the current result, the different size dependence of Auger recombination in CdTe QDs is most probably due to the formation of the thin CdS gradient. The Auger lifetime of CdS QDs clearly shows a D6 dependence, although Robel et al. reported that the Auger lifetime is 1053

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank to Professor M. Tachiya for fruitful discussions. This work was partially supported by a Grant-inAid for Scientific Research (B, No. 22350012) and a Grant-inAid for Scientific Research on Priority Areas of Molecular Science for Supra Functional Systems (No. 477) from MEXT, Japan. Figure 4. Size dependence of the Auger lifetime of CdS QDs capped with GSH (red circles), OA (blue squares), and MA (green triangles). The gray band indicates a power law dependence of D6, where D is the QD diameter.

proportional to D3 in various QDs (CdSe, PbSe, InAs, and Ge).44 Theoretically, the lifetime of Auger ionization was proportional to Dm (m = 5 7) depending on the band offset.31 In Auger recombination, the Auger lifetime was proportional to Dm (m = 2 4) by considering a density of final states proportional to the volume where the Auger electron can be transferred.45 From the above discussion, the D6 dependence may indicate that CdS QDs are ionized through Auger recombination: Auger ionization may occur in these systems. However, whether Auger electron is ionized could not be clarified in the bleaching analysis of transient absorption, and other experiments such as transient absorption measurements in the near IR region should be performed to analyze the ejected electron. Additionally, detailed numerical calculations suggested that the size dependence of Auger recombination has a strong oscillatory character.31,32 The scaling index of CdSe QDs examined by Pandey and GuyotSionnest was steeper than that examined by Klimov et al. (m > 4) if smaller QDs were included.28 It may become complex to compare the detailed size dependence of Auger recombination in different compounds because of various factors to modify the wave functions of QDs. In conclusion, we examined the effect of surface states originating from surface defects and capping reagents on Auger recombination in various sized CdS QDs by femtosecond transient absorption spectroscopy. The size-dependent analysis clearly shows that Auger recombination in CdS QDs does not depend on these factors. The lifetime of Auger recombination is proportional to D6, and this result is different from those of CdSe QDs. This deviation may indicate the presence of Auger ionization in CdS QDs. The analysis of the ejected electron by transient absorption in the near IR region may give useful information on which process, Auger recombination or Auger ionization, plays an important role.

’ ASSOCIATED CONTENT

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Supporting Information. TEM image, XRD patterns, transient absorption spectra of MA-capped CdS QDs, the growth kinetics at the bleaching peak of OA- and GSH-capped CdS QDs, population dynamics of MA-capped CdS QDs, and the analysis of positive transient absorption of OA-capped CdS QDs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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