Correlation between Photooxidation and the Appearance of Raman

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Correlation between Photooxidation and the Appearance of Raman Scattering Bands in Lead Chalcogenide Quantum Dots Jeffrey L. Blackburn,*,† Helen Chappell,†,‡ Joseph M. Luther,† Arthur J. Nozik,†,§ and Justin C. Johnson† †

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

ABSTRACT: In this report, we carefully study the effects of photooxidation on the Raman spectra of lead chalcogenide (PbX) quantum dots (QDs). Photoexcitation of PbS, PbSe, and PbTe QD films at 488 nm with power densities as low as 30 W/cm2 gives rise to several peaks related to both lead(II) oxide and the group VI chalcogenates (PbXO4). The amplitudes of these peaks are shown to increase with continuous laser illumination in air, but are completely absent for samples illuminated under rigorously air-free conditions. These results suggest that the ∼135 cm-1 Raman peak often assigned to an intrinsic PbX LO phonon is more likely an artifact arising from photooxidation. The myriad of potential photooxidation products formed quickly in laser-illuminated, air-exposed PbX QDs suggest that caution should be used in the assignment and interpretation of phonon spectra and phonon-mediated exciton relaxation pathways of these materials, unless the processing and experiments are conducted under airfree conditions. SECTION: Nanoparticles and Nanostructures

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ead chalcogenide (PbX) quantum dots (QDs) are promising candidates for high efficiency, low cost photovoltaics (PV) due to their size-tunable band gap and the recent observation of efficient multiple exciton generation (MEG).1,2 The successful integration of lead chalcogenide QDs into PV requires a detailed understanding of the energy relaxation pathways that control charge carrier populations following photoexcitation. Electron-phonon coupling can be extremely efficient in such highly quantum-confined systems, and electron-phonon interactions likely affect many dynamic processes such as intraband relaxation of excited states and radiative recombination.3-6 It is therefore crucial to understand the detailed phonon spectra for QDs to appreciate the relevant energy scales that influence these electron-phonon interactions. In addition, QD films to be utilized in solar cells may be processed in the presence or absence of oxygen. Recent studies have suggested remarkably different electrical and optical properties of QD films depending on the degree of air exposure, but in most cases the detailed mechanisms underlying these observations are not known.7-10 Thus, there is currently not a full understanding of the primary oxidation and photooxidation products, and how these species affect fundamental photophysical events such as photoluminescence. Lead chalcogenides possess the centro-symmetric rock salt crystal structure, which belongs to the m3m space group or Oh point group. First-order Raman scattering from an ideal rocksalt lattice is forbidden due to the center of inversion symmetry, although some theoretical predictions suggest that higher order scattering should be weakly allowed.11 Despite this restriction, several Raman studies of lead chalcogenide QDs have yielded strong peaks that are often attributed to first-order Raman modes.12,13 However, little attention is typically given to handling the sample in the absence of oxygen, an important r 2011 American Chemical Society

consideration for a material set that is severely prone to oxidation.14-18 Thus, as some recent reports have pointed out, many oxide-related peaks may be mistakenly attributed to intrinsic PbX phonons.17,19 A systematic study of the effects of oxidation on a series of lead chalcogenide QDs is warranted in order to better understand the intrinsic QD vibrational spectra and how these spectra are used to interpret processes such as radiative recombination.12 In this report, we carefully study films of three lead chalcogenide QD samples (PbS, PbSe, and PbTe) in both ambient conditions and in rigorously air-free conditions, which allows us to unambiguously identify oxide-related Raman peaks. We find a number of Raman peaks in air-exposed PbX QD samples that can be attributed to lead oxide (PbO) and chalcogenate peaks (PbXO4 or PbO 3 PbXO4), some of which have been assigned to intrinsic PbX phonon modes in previous reports.12,13 In air-free PbX QD samples, these oxide-related peaks are completely absent, and the Raman spectra contain no peaks that could be assigned to first-order PbX modes. These results are consistent with the a priori expectation of forbidden first-order Raman scattering from the rock salt crystal structure of lead chalcogenides. These results underscore the importance of careful control of the environment for spectroscopic analysis of lead chalcogenide QDs and should guide the analysis of phonon spectra and dynamic processes involving coupling to phonons. PbX QDs capped with oleate were prepared in the absence of oxygen on a Schlenk line, as described previously,20-22 and precipitated several times with ethanol to remove unreacted precursors. The samples Received: January 7, 2011 Accepted: February 9, 2011 Published: February 28, 2011 599

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Figure 1. (a) Absorbance spectra of the three PbX QD samples used in this study. (b) Raman spectra of PbX QD films measured in air. Excitaton was at 488 nm, 20 mW power (75 W/cm2), 90 s integration time.

Figure 2. Time-dependent Raman spectra for (a) PbSe QD film and (b) PbS film, both measured in air. Integration time for each spectrum was three seconds, and the inset labels the total time exposed to 20 mW (75 W/cm2) of 488 nm excitation for each spectrum. Asterisks label peaks that grow with increased exposure time, due to photooxidation of the PbX QDs.

were then brought into a helium atmosphere glovebox (PO2 ∼ 1 ppm). Figure 1a displays the absorbance spectra of the three samples used in this study. Nanocrystal diameter was estimated from first exciton absorbance peaks, and was found to be 6, 8, and 3.5 nm for PbSe, PbS, and PbTe, respectively.23-25 PbX QD films were prepared for Raman measurements by drop-casting a concentrated solution26 of PbX QDs in a 4:1 hexane/octane mixture onto glass microscope slides. Films were prepared to be essentially opaque (optical density > 3 at 488 nm) to maximize the signal-to-noise ratio. After the preparation of a particular PbX QD film, the glass slide covered with the film was split into two pieces. One piece was brought out of the glovebox into ambient conditions with no protection against ambient exposure. The other piece was sealed inside an airtight optical cell within the glovebox before removal into ambient conditions.27 In this report, we will refer to these films as “air-exposed” and “air-free”. The films were stored in the dark before Raman measurements. Raman spectra were recorded in a backscattering configuration, utilizing excitation at 488 nm from an argon ion laser with a spot size of approximately 200 μm at the focal point. Figure 1b shows the Raman spectra of the three air-exposed PbX QD films. The spectrum for each film contains a large number of peaks in the range of ∼100 to 1000 cm-1, some of which are labeled in Figure 1 (vide infra). Importantly, the spectra are each dominated by a strong peak at ∼135 cm-1. In two recent studies of PbSe QDs, the peak at ∼135 cm-1 was attributed to the gamma point longitudinal optical phonon, LO(Γ), of PbSe.12,13 However, several reports on the oxidation of bulk lead chalcogenide surfaces suggest that the peak at ∼135 cm-1 can be assigned to the Pb-O-Pb

stretch in lead (II) oxide (PbO).14-16 Other reports have drawn attention to the incorrect assignment of the strong ∼135 cm-1 band to an intrinsic phonon for PbSe QDs, noting that this band could likely be attributed to photooxidation of the lead chalcogenide surface, although controlled experiments were not performed to confirm this assignment.19 These factors led us to suspect oxidation and/or photooxidation as a potential source for the numerous peaks in the spectra for each of the air-exposed PbX QD films. To determine the contribution of photooxidation to the peaks observed for the air-exposed PbX samples, we performed the following experiment. For a particular air-exposed film, we aligned the focal plane of the Raman excitation laser using a particular spot on the film. Once aligned, we closed the laser shutter so that the laser no longer illuminated the sample, and then translated the film a few millimeters away from the original spot. This allowed us to probe a previously unilluminated spot on the QD film. We then took a series of spectra, integrating 3 s for each spectrum. Thus, the total illumination time for experiment x (x = 1, 2, 3, etc.) was 3x seconds. The results of this experiment for the air-exposed PbSe and PbS QD films are shown in Figure 2. Note that the signal-to-noise ratios for these spectra are considerably lower than those for the spectra in Figure 1, since the total integration time is much lower (3 s for Figure 2 versus 180 s for Figure 1). However, even with the short integration time, we observe a number of peaks for both the PbSe and PbS QD films that grow as the total illumination time increases.28 It follows that these peaks, marked with asterisks in each figure, are 600

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associated with photooxidation of the PbX QD surface during laser illumination, and are not due to phonons that are intrinsic to the unoxidized QDs. The strong proclivity for photooxidation of the PbX QDs, demonstrated in Figure 2a,b, suggests that if intrinsic first-order Raman modes for the PbX QDs were to be observed, they would

likely be obscured by oxide-related peaks in ambient conditions. To fully eliminate oxide-related peaks from the spectra, we measured Raman spectra for films loaded into airtight windowed cells within the helium glovebox. This ensures an extremely low pressure of oxygen in the cell (∼1 ppm) during Raman excitation, minimizing the effects of photooxidation. Figure 3 compares the Raman spectra for air-exposed and air-free PbSe QD films. Note that the air-free sample is background-corrected by subtracting the spectrum obtained from a diffuse scattering silicon substrate in an air-free cell that is identical to the cell used for the PbSe film. This correction is done for several reasons.29 It is immediately apparent that the air-free PbSe film is completely devoid of the numerous spectral features observed for the airexposed film. Within our sensitivity limit, we can find no peaks that could be attributed to first-order Raman modes of the PbSe QDs. This observation is also consistent for PbS and PbTe films. Importantly, these results imply that essentially every peak in the range of ∼100 to 1000 cm-1 for the PbX QD films is associated with photooxidation of the PbX QD surfaces. With the knowledge that the rich spectra observed in Figure 1 arise primarily from photooxidation of the PbX QDs, we attempt to identify the numerous peaks based on previous literature reports. Table 1 summarizes the peaks observed for each sample and their probable assignments. We first assign the peaks in the 600-1000 cm-1 range to XO4 species arising from anion oxidation in the PbX QDs. We first

Figure 3. Comparison of Raman spectra for a PbSe QD film measured in air (air-exposed) and measured in an airtight cell (air-free), both excited at 75 W/cm2 (488 nm). The asterisk marks the position of the TO phonon of the silicon film used to background-correct the PbSe film spectrum.

Table 1. Summary of Observed Raman Peaks in Spectra for Air-Exposed (ox) and Air-Free (non-ox) Dropcast PbX QD Filmsa PbX PbS (ox)

PbS (nonox) PbSe (ox)

PbSe(nonox) PbTe (ox)

peak position (cm-1)

strength

photoresponse

assignment

theoretical/literature (cm-1)

961 cm-1

med

yes

PbSO4

96018

602

weak

yes

PbO 3 PbSO4

60114

429

weak

yes

PbO 3 PbSO4

320

weak

?

PbO 3 PbSO4

270

weak

?

Pb-O-Pb overtone

264

176

med

yes

PbO

16815

132

strong

yes

Pb-O-Pb stretch

13519 14415

NA

NA

NA

NA

NA

43014 33414

782

med

yes

SeO4/HSeO4

84030/77931

368

med

yes

PbSeO4

35830

274

strong

yes

Pb-O-Pb overtone

272

176

shoulder

yes

PbO

16815

136

strong

yes

Pb-O-Pb stretch

13519 14415

NA

NA

NA

NA

NA

743

strong

yes

TeO3/TeO3þ1

730-79032

677

strong

yes

TeO4 trigonal bipyramids

68032

420

weak

?

Te-O-Te bending vibrations

420-45032

270 176

weak shoulder

? yes

Pb-O-Pb overtone PbO

268 16815

134

strong

yes

Pb-O-Pb stretch

13519 14415

PbTe (nonox)

NA

NA

NA

NA

NA

The “photoreponse” column indicates whether or not the peak is observed to grow as a function of time with laser exposure in air, as demonstrated in Figure 2. A question mark (?) in the photoresponse column indicates a peak that is below our sensitivity in this experiment due to the short integration time. The “assignment” column indicates potential assignments, as deduced from previous reports (reference value and citation listed in last column). a

601

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The Journal of Physical Chemistry Letters note that the peak at 971 cm-1 for the PbS QD film is a well-known peak for lead sulfate, PbSO4.18 The PbSe sample has a peak at 782 cm-1 that we assign to the PbSeO4, although this peak falls slightly lower than the typical peak position for the strong Ag mode of bulk PbSeO4.30 Interestingly, the position we observe coincides better with that of HSeO4 salts,31 a discrepancy that we do not currently understand. Duverger et al. previously assigned the ∼680 cm-1 peak in oxidized tellurium films to TeO4 trigonal bipyramids.32 In that report, they also observed a peak between 730 and 790 cm-1 that they assigned to TeO3/TeO3þ1 species and a broad peak between 420 450 cm-1 that was assigned to Te-O-Te bending vibrations.32 In the oxidized PbTe QD film shown in Figure 1, we observe these peaks at 743 cm-1 and 420 cm-1, respectively. There is also a vibrational mode for R-PbO at ∼420 cm-1 that explains the appearance of a peak in this area for each of the PbX QD films. As discussed above, the range of 100 to 400 cm-1 is dominated by a very strong peak at ∼135 cm-1 and a shoulder of varying intensity at ∼172 cm-1. Studies on lead chalcogenide oxidation have revealed peaks at ∼135 cm-1 and ∼170 cm-1 upon formation of lead(II) oxide, PbO. The first of these peaks is typically assigned to the Pb-O-Pb symmetrical stretch. Although a recent study of PbSe QDs assigned the peak at ∼170 cm-1 to the overtone of the X point LO phonon, 2LO(X),13 our experiments suggest this peak is actually oxide-related. Each chalcogenide film contains a peak near 270 cm-1 that can be assigned to the second overtone of the Pb-O-Pb peak at ∼135 cm-1. It is unclear why this peak varies significantly in intensity and is particularly strong for PbSe. The peaks at 320, 429, and 602 cm-1 for the PbS QD sample are in the correct range for peaks that have been observed for PbO 3 PbSO4,14 in accordance with the sulfate peak observed at 961 cm-1. The prominent peak observed at 368 cm-1 for the PbSe sample coincides with the strongest peak previously observed for powdered PbSeO4, the ν4 peak typically observed near 358 cm-1,30 supporting our assignment of the 782 cm-1 peak to a selenate mode. The assignments made above suggest that essentially all of the peaks observed for the air-exposed PbX QD films can be assigned to vibrations associated with lead(II) oxide or corresponding chalcogenate species (PbXO4 or PbO 3 PbXO4). Recent XPS studies on air-exposed PbSe QD films have found a variety of oxidation products, primarily dominated by lead oxides and hydroxides.9,10 The most likely selenium oxidation products were attributed to Se4þ species such as SeO2 and SeO32-, with a notable absence of the more stable SeO42-.9,10 Sykora et al. noted that the absence of SeO42- suggested that the formation of this phase was kinetically inhibited at room temperature due to an activation barrier. Our results demonstrate that, in contrast to room temperature oxidation, photooxidation arising from laser excitation of PbX QDs at modest power densities (vide infra) generates primarily X6þ oxides in lieu of X4þ oxides. We note that dark oxidation for a relatively short period of time (hours), which can only produce a submonolayer coverage of oxidation, did not cause any oxide-related peaks to appear in our Raman spectra. This may be an indication that more aggressive oxidation (such as photooxidation or many days of dark oxidation) would be required for the Pb-O-Pb stretch near 135 cm-1 to appear. It is important to consider the power density at which photooxidation occurs for the PbX QD films studied here, so that valid comparisons can be made among reports in the literature. We first observe the oxide-related peaks at power levels above ∼8 mW for 488 nm excitation. Given our spot size of ∼200 μm, this lower limit of power density is ∼30 W/cm2. Several recent PL studies have noted photooxidation effects at power densities in the 1000-2000 W/cm2

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range.7,8 However, it is difficult to directly compare our results to other Raman reports, which in some cases do not delineate focused spot sizes or power levels, or whether the sample was excited in ambient or air-free conditions. We can compare to one recent report that excited PbSe QD films at 632.8 nm at power levels of e10 mW with a spot size of 1 μm,13 which translates to a power density of e1.3 MW/cm2. In this report, the dominant peak observed at ∼134 cm-1 was attributed to the intrinsic LO(Γ) phonon of the PbSe QDs. Even with the less energetic photons of the 632.8 nm excitation source relative to the 488 nm photons employed in our study, the 5 orders of magnitude larger power density employed in that study strongly implies that photooxidation of the PbSe QD films should significantly influence their results even in the presence of trace amounts of oxygen. In conclusion, we show strong evidence that first-order Raman scattering from intrinsic PbX modes is not observed for QD films studied in the rigorous absence of air. When excited in ambient conditions at power densities as low as 30 W/cm2, photooxidation of the PbX QD surface generates a large number of peaks that can be readily assigned to lead(II) oxide and X6þ (e.g., PbSeO4) oxidation products. The results suggest that the ∼135 cm-1 peak often assigned to an intrinsic PbX vibrational mode is more likely an artifact arising from photooxidation. The lack of a Raman LOphonon band in air-free PbX films and the myriad of potential photooxidation products formed quickly in laser-illuminated, airexposed PbX lead to questions about traditional interpretation of exciton and charge carrier relaxation in these materials, which are often processed and studied in air. We are currently working toward correlating the degree of oxidation observed by Raman scattering with other photophysical processes, such as photoluminescence, as well as solar cell performance.

’ ACKNOWLEDGMENT We thank Danielle Smith and Barbara Hughes for assistance with sample preparation. Nanocrystal synthesis, film preparation, and characterization were 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). Raman spectroscopy was supported by the Solar Photochemistry program of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract No. DE-AC36-08GO28308 to NREL. Helen Chappell acknowledges support from the NSF Graduate Research Fellows Program. ’ REFERENCES (1) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; A., S.; Efros, A. L. Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots. Nano Lett. 2005, 5, 865–871. (2) Schaller, R. D.; Klimov, V. I. High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Phys. Rev. Lett. 2004, 92, 186601. (3) Guyot-Sionnest, P.; Wehrenberg, B.; Yu, D. Intraband Relaxation in CdSe Nanocrystals and the Strong Influence of the Surface Ligands. J. Chem. Phys. 2005, 123, 074709–7. (4) Wehrenberg, B. L.; Wang, C.; Guyot-Sionnest, P. Interband and Intraband Optical Studies of PbSe Colloidal Quantum Dots. J. Phys. Chem. B 2002, 106, 10634–10640. (5) Harbold, J. M.; Du, H.; Krauss, T. D.; Cho, K.-S.; Murray, C. B.; Wise, F. W. Time-resolved Intraband Relaxation of Strongly Confined 602

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(26) Solution concentration estimated at ∼5 mg/mL concentration, with an optical density over 1 at the first exciton, if measured in a 1 cm cuvette. (27) The airtight optical cell consists of two windowed UHV con-flat pieces that are sealed together with a copper gasket, making an airtight metal-to-metal seal. (28) We note that the PbTe QD film showed similar behavior, but this data is not shown for brevity. (29) First, the diffuse scattering from the silicon substrate is quite similar to the scattering obtained for the PbX films, allowing us to subtract diffuse-scattered Rayleigh light that leaks through the 488 nm notch filter. Second, no obvious PbX-related peaks were observed for the air-free PbX samples, necessitating a well-characterized sample of identical form factor on which to align the focal plane of the Raman excitation. The strong transverse optical (TO) mode of silicon at 515 cm-1 allows us to ensure proper alignment of the air-free cell, after which alignment, the cell containing the PbX film is dropped into place. (30) Scheuermann, W.; Schutte, C. J. H. Raman and Infrared Spectra of SrSeO4 and PbSeO4. J. Raman Spectrosc. 1973, 1, 619–627. (31) Pawlowski, A.; Polomska, M.; Hilczer, B.; Szczesniak, L.; Pietraszko, A. Superionic Phase Transition in Rb3D(SeO4)2 Single Crystals. J. Power Sources 2007, 173, 781–787. (32) Duverger, X.; Bouazaoui, M.; Turrell, S. Raman Spectroscopic Investigations of the Effect of the Doping Metal on the Structure of Binary Tellurium-Oxide Glasses. J. Non-Cryst. Solids 1997, 220, 169– 177.

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