pubs.acs.org/Langmuir © 2009 American Chemical Society
Stability Study of PbSe Semiconductor Nanocrystals over Concentration, Size, Atmosphere, and Light Exposure )
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Quanqin Dai,† Yingnan Wang,‡ Yu Zhang,†, ,^ Xinbi Li,† Ruowang Li,† Bo Zou,*,‡ JaeTae Seo,§ Yiding Wang, ,^ Manhong Liu,# and William W. Yu*,† †
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Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609, ‡State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China, §Department of Physics, Hampton University, Hampton, Virginia 23668, State Key Laboratory on Integrated Optoelectronics and ^College of Electronic Science and Engineering, Jilin University, Changchun 130012, China, and #College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China Received May 1, 2009. Revised Manuscript Received May 30, 2009
Infrared-emitting PbSe nanocrystals are of increasing interest in both fundamental research and technical application. However, the practical applications are greatly limited by their poor stability. In this work, absorption and photoluminescence spectra of PbSe nanocrystals were utilized to observe the stability of PbSe nanocrystals over several conventional factors, that is, particle concentration, particle size, temperature, light exposure, contacting atmosphere, and storage forms (solution or solid powder). Both absorption and luminescence spectra of PbSe nanocrystals exposed to air showed dependence on particle concentration, size, and light exposure, which caused large and quick blue-shifts in the optical spectra. This air-contacted instability arising from the destructive oxidation and subsequent collision-induced decomposition was kinetically dominated and differed from the traditional thought that smaller particles with lower concentrations shrank fast. The photoluminescence emission intensity of the PbSe nanocrystal solution under ultraviolet (UV) exposure in air increased first and then decreased slowly; without UV irradiation, the emission intensity monotonously decreased over time. However, if stored under nitrogen, no obvious changes in absorption and photoluminescence spectra of the PbSe nanocrystals were observed even under UV exposure or upon being heated up to 100 C.
Introduction Ultraviolet (UV)- and visible-emitting chalcogenide semiconductor nanocrystals have presented numerous potential applications in the past decade, including biomedical labeling/imaging,1-6 light-emitting diodes (LEDs),7,8 lasers,9,10 and solar cells,11-13 because of their advantages in photostability, size-dependent broad-band absorption, and narrow emission.14,15 In the nearand mid-infrared wavelength range, traditional organic dyes only offer broad emission spectra with very low photoluminescence *Corresponding authors. E-mail:
[email protected] (W.W.Y.); zoubo@jlu. edu.cn (B.Z.). (1) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2016. (2) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016–2018. (3) Yu, W. W.; Chang, E.; Falkner, J. C.; Zhang, J.; Al-Somali, A. M.; Sayes, C. M.; Johns, J.; Drezek, R.; Colvin, V. L. J. Am. Chem. Soc. 2007, 129, 2871–2879. (4) Yu, W. W.; Chang, E.; Drezek, R.; Colvin, V. L. J. Biomed. Nanotechnol. 2006, 2, 225–228. (5) Zhou, D.; Ying, L.; Hong, X.; Hall, E. A.; Abell, C.; Klenerman, D. Langmuir 2008, 24, 1659–1664. (6) Sonesson, A. W.; Elofsson, U. M.; Callisen, T. H.; Brismar, H. Langmuir 2007, 23, 8352–8356. (7) Colvin, V. L.; Schlamp, M. C.; Allvisatos, A. P. Nature 1994, 370, 354–357. (8) Sundar, V. C.; Lee, J.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102–1105. (9) Kazes, M.; Lewis, D. Y.; Ebenstein, Y.; Mokari, T.; Bannin, U. Adv. Mater. 2002, 14, 317–321. (10) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314–317. (11) Huynh, W.; Peng, X.; Alivisatos, A. P. Adv. Mater. 1999, 11, 923–927. (12) Pradhan, S.; Chen, S.; Wang, S.; Zou, J.; Kauzlarich, S.; Louie, A. Y. Langmuir 2006, 22, 787–793. (13) Zaban, C. A.; Micic, O. I.; Gregg, B. A.; Nozik, A. J. Langmuir 1998, 14, 3153–3156. (14) Yu, W. W. Expert Opin. Biol. Ther. 2008, 8, 1571–1581. (15) Larson, D. R.; Zipfel, W. R.; Williams, R. M.; Clark, S. W.; Bruchez, M. P.; Wise, F. W.; Webb, W. W. Science 2003, 300, 1434–1436.
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(PL) efficiencies. In comparison, PbSe semiconductor nanocrystals not only exhibit narrow emission peaks in a wide wavelength range of 1100-4000 nm16-21 with extremely high quantum efficiencies (up to nearly 90%),19,21 but also have ultrafast electronic depolarizations, making them excellent materials for optical switches and biological imaging.20,22,23 Recent reports have indicated that PbSe semiconductor nanocrystals can potentially revolutionize solar cell technology,24,25 because they exhibit an efficient “carrier multiplication”, a phenomenon in which the absorption of one single high-energy photon can produce as many as seven electron-hole pairs. However, the full applications of PbSe semiconductor nanocrystals are not realistic yet due to their instability,26,27 which motivated this stability study on them. Absorption and photoluminescence spectra of PbSe semiconductor nanocrystals were (16) Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. Dev. 2001, 45, 47–56. (17) Du, H.; Chen, C.; Krishnan, R.; Krauss, T. D.; Harbold, J. M.; Wise, F. W.; Thomas, M. G.; Silcox, J. Nano Lett. 2002, 2, 1321–1324. (18) Pietryga, J. M.; Schaller, R. D.; Werder, D.; Stewart, M. H.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2004, 126, 11752–11753. (19) Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L. Chem. Mater. 2004, 16, 3318–3322. (20) Wise, F. W. Acc. Chem. Res. 2000, 33, 773–780. (21) Wehrenberg, B. L.; Wang, C.; Guyot-Sionnest, P. J. Phys. Chem. B 2002, 106, 10634–10640. (22) Lim, Y. T.; Kim, S.; Nakayama, A.; Stott, N. E.; Bawendi, M. G.; Frangioni, J. V. Mol. Imaging 2003, 2, 50–64. (23) Weissleder, R. Nat. Biotechnol. 2001, 19, 316–317. (24) Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2004, 92, 186601/1–186601/4. (25) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Nano Lett. 2005, 5, 865–871. (26) Stouwdam, J. W.; Shan, J.; van Veggel, F. C. J. M. J. Phys. Chem. C 2007, 111, 1086–1092. (27) Pietryga, J. M.; Werder, D. J.; Williams, D. J.; Casson, J. L.; Schaller, R. D.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2008, 130, 4879–4885.
Published on Web 06/12/2009
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employed to study the nanocrystal instability affected by the several major factors of particle concentration, particle size, temperature, UV exposure, room light or no light (darkness), inert gas (nitrogen) or air contact, and material states of solution or solid powder. This detailed instability study of PbSe semiconductor nanocrystals rendered us some interesting phenomena that differed from our common thought, and potentially facilitated in finding ways to both stabilize the nanocrystals and realize their great potentials in practical applications.
Experimental Section PbSe nanocrystals were synthesized by following the literature procedure in Yu et al.’s article.19 In detail, 0.892 g (4.00 mmol) of PbO, 2.260 g (8.00 mmol) of oleic acid, and 12.848 g of octadecene were loaded into a three-neck flask and heated to 180 C under nitrogen flow. After the yellow PbO powder completely disappeared, the temperature was set to 170 C. Then 6.400 g of selenium-trioctylphosphine solution (containing 0.640 g of selenium; prepared in a glovebox) was swiftly injected into the above vigorously stirred solution. After the injection, the temperature of the solution was reset to 140 C for the nanocrystal growth. When PbSe nanocrystals grew to the desired size, excessive roomtemperature toluene was carefully added into the flask to quickly decrease the temperature and finally quench the reaction process. The absorption measurements of the as-prepared PbSe nanocrystals could be disturbed by excessive reaction precursors and reaction solvents retained in the solution. They were removed first by extracting with methanol and second by precipitating with acetone.19,28,29 The purified PbSe semiconductor nanocrystals were redispersed in tetrachloroethylene for evaluating their optical properties of both absorption and emission. The PbSe nanocrystal solutions were sealed in optical cuvettes to prevent the evaporation of tetrachloroethylene, which might bring in particle concentration changes during the storage time. For the nanocrystal stability analysis related to particle concentrations, solid powder, 4.32, 0.86, and 0.43 μM PbSe nanocrystals in tetrachloroethylene were evaluated. In the case of solid powder form, enough solid PbSe nanocrystal samples were kept during storage, and only a small fraction was redispersed in tetrachloroethylene in order to measure the spectra. Three particle sizes (5.2, 5.7, and 6.0 nm) of the PbSe nanocrystals were used to study the size effect on stability. UV light (365 nm), room light, and darkness were utilized to study the effect of light exposure on the nanocrystal stability. In case of darkness, PbSe nanocrystal samples were stored in dark environment and were only exposed to the spectral light source for ∼1 min during their optical spectrum recording. All the above studies were in either inert nitrogen gas or atmospheric air at room temperature. For the stability analysis of PbSe nanocrystals at different temperature conditions, the nanocrystal samples were heated and kept at 60 C for 20 min, then 80 C for 20 min, and 100 C for 10 min; the absorption and PL spectra were taken after these samples were quickly cooled to room temperature using cold tetrachloroethylene.
Results and Discussion As a direct bandgap semiconductor material, PbSe nanocrystals exhibit strong size-dependent optical properties. The absorption spectra of PbSe nanocrystals with average diameters of 3-7 nm are shown in Figure 1a. Well-defined absorption peaks of the as-prepared PbSe nanocrystals are apparent. This reflects the narrow size distribution of PbSe nanocrystals, which can be further confirmed by transmission electron microscopy (TEM) observation as shown in Figure 1b. The absorption spectra and TEM image in Figure 1 were obtained right after the (28) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368–2371. (29) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300–4308.
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Figure 1. (a) Near-infrared absorption spectra of the as-prepared (fresh) PbSe semiconductor nanocrystals. (b) TEM image of a fresh PbSe nanocrystal sample right after the synthesis.
PbSe nanocrystal samples were synthesized and purified (“fresh samples”). The instability of the fresh PbSe nanocrystals were observed under different influence factors of particle concentration, particle size, temperature, and exposure light of UV, room illumination, or no light (darkness). The following are several storage conditions of PbSe nanocrystals as solid or solutions in either nitrogen or air before measuring their optical spectra: (i) 5.0 nm nanocrystals with different particle concentrations (solid, 4.32, 0.86, or 0.43 μM) under darkness in air; (ii) 1.0 μM nanocrystal solutions with different particle sizes (5.2, 5.7, and 6.0 nm) under room light in air; (iii) 0.86 μM nanocrystal solutions with particle size of 5.0 nm under UV or room light in air; (iv) 0.86 μM nanocrystal solutions with particle size of 5.0 nm under UV light or darkness in nitrogen; (v) 4.7 nm nanocrystal solutions kept at different temperatures under room light in nitrogen. Figure 2 illustrates the temporal evolution of absorption spectra of PbSe nanocrystals with different storage concentrations (solid powder (Figure 2a), 4.32 μM (Figure 2b), 0.86 μM (Figure 2c), and 0.43 μM (Figure 2d), condition i), which exhibits spectral blue-shifts over storage time. The blue-shifts for different particle concentrations can be clearly seen in Figure 3a. A higher concentration has a faster blue-shift, which indicates a concentration-dependence on the instability of PbSe nanocrystals. Similarly, particle size-dependent instability measurement of PbSe nanocrystals, as shown in Figure 3b, demonstrates faster blueshifts with larger particle sizes (condition ii). Moreover, the blueshift of PbSe nanocrystals is sensitive to light exposure sources (Figure 4, condition iii). The PbSe nanocrystals exposed to UV radiation displayed faster blue-shifts compared with the ones under room light exposure. In contrast to the spectral blue-shifts of PbSe nanocrystals with air contact, there was no obvious spectral change for the same PbSe nanocrystals in nitrogen, with either light exposure or temperature changes (Figure 5, conditions iv and v). Corresponding to the blue-shifts of absorption spectra, PL emission blue-shifts of PbSe nanocrystals under air contact were DOI: 10.1021/la9015614
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Figure 2. Temporal evolution of the absorption spectra of 5.0 nm PbSe nanocrystal samples stored under darkness in air: (a) PbSe nanocrystal powder and (b-d) PbSe nanocrystal solutions with particle concentrations of 4.32, 0.86, and 0.43 μM, respectively.
Figure 3. (a) Temporal blue-shifts of the first excitonic absorption peaks of 5.0 nm PbSe nanocrystal powder and solutions stored under darkness in air. (b) Temporal blue-shifts of the first absorption peaks of different-sized PbSe nanocrystals exposed to room light in air; the concentrations of these PbSe nanocrystals were identical (1.0 μM).
also observed (Figure 6). Additionally, it can be seen that the emission intensity of solution samples increased during the beginning period of UV irradiation (Figure 6a). The maximal increase was about 2.0 times (to 55% quantum yield) for the amplitude of PL emission maximum. It is known that the PL 12322 DOI: 10.1021/la9015614
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emission properties of semiconductor nanocrystals are very sensitive to the nanocrystal surface states. UV or laser irradiation can significantly enhance the emission intensities of surfactantmolecule-capped plain chalcogenide semiconductor nanocrystals, which is called photoenhancement.30-34 This photoenhancement effect is attributed to the relaxation process with an extremely important component of charge-carrier recombination at surface trap states.31 Photogenerated electrons and holes may be trapped at the external adatoms on the nanocrystal surface.32 These surface trap states can be stabilized by the light-induced rearrangement of surfactant molecules, which causes thermalization back to the lowest exciton state and thus enhances the emission intensities.31 We believe PbSe plain nanocrystals follow the same mechanism of photoenhancement. While we are handling the plain nanocrystals, the relative work of Alivisatos’ group focused on the core/shell structures.35 Photoenhancement of core/shell structured nanocrystals is probably due to the chemical bond rearrangement or defect relocation at the core-shell interface through a photochemical process. This process is accompanied by surface reconstructions, resulting in the reduction of surface trap states. Comparatively, the surface reconstruction contributed to photoenhancement of core/shell structured nanocrystals is similar to those of the plain nanocrystals. As the air-exposed irradiation continued (Figure 6a), the emission intensity of PbSe nanocrystals decreased. This is consistent with the observation of CdSe nanocrystals.31 It was found that the emission intensities of colloidal CdSe nanocrystals increased during the first period of constant illumination and subsequently decreased if the illumination continued. This intensity decrease is due to the photooxidation,31,36 which is also the reason to cause the emission intensity of PbSe nanocrystals to decrease monotonously over time when no UV light irradiated (Figure 6b). The oxidation accompanied by dissolution of nanocrystals leads to the particle size shrinking and the damage of the particle surface. Size shrinking results in spectral blue-shifts, and surface damage decreases the PL emission intensity. Dissolution of PbSe nanocrystals by destructive oxidation was previously presented by Stouwdam and co-workers.26 They observed that PbSe nanoparticles exhibited a decrease in their effective diameter of about 0.3 nm after being stored in air and room light for 2 months. This result was recently reobserved by Moreels et al.,37 who reported an effective diameter reduction of about 0.5 nm under ambient conditions. Although they concluded that oxidation might not lead to a complete loss of oleic acid ligands (retaining about 70% of the surface ligands after 1 month), we found that, with the help of UV irradiation, oxidation could completely release the oleic acid ligands and damage PbSe nanoparticles in the solution within a shorter period. As shown in Figure 7, an original black PbSe nanocrystal solution gradually turned into a clear colorless solution with air contact and UV irradiation for 12 days, which was attributed to the destructive oxidation and dissolution. The role of oxidation was also (30) Cordero, S. R.; Carson, P. J.; Estabrook, R. A.; Strouse, G. F.; Buratto, S. K. J. Phys. Chem. B 2000, 104, 12137–12142. (31) Jones, M.; Nedeljkovic, J.; Ellingson, R. J.; Nozik, A. J.; Rumbles, G. J. Phys. Chem. B 2003, 107, 11346–11352. (32) Lifshitz, E.; Glozman, A.; Litvin, I. D.; Porteanu, H. J. Phys. Chem. B 2000, 104, 10449–10461. (33) Aldana, J.; Wang, Y. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 8844–8850. (34) Tsay, J. M.; Doose, S.; Pinaud, F.; Weiss, S. J. Phys. Chem. B 2005, 109, 1669–1674. (35) Manna, L.; Scher, E. C.; Li, L. S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136–7145. (36) Nida, D. L.; Nitin, N.; Yu, W. W.; Colvin, V. L.; Richards-Kortum, R. Nanotechnology 2008, 19, 035701/1–035701/6. (37) Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. J. Am. Chem. Soc. 2008, 130, 15081–15086.
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Figure 4. Temporal evolution of the absorption spectra of 5.0 nm PbSe nanocrystals exposed to UV light (a) and room light (b) in air. Both of the PbSe nanocrystal concentrations shown here were identical (0.86 μM). (c) Temporal blue-shifts of the first excitonic absorption peaks derived from panels (a) and (b).
Figure 5. Temporal evolution of the absorption spectra of PbSe nanocrystal solution stored in nitrogen atmosphere under darkness (a), UV light (b), or different temperatures with exposure to room light (c).
Figure 6. Temporal evolution of the emission spectra of air-contacted 5.0 nm PbSe nanocrystal solution either exposed to UV light (a) or in darkness (b).
evidenced by Sapra et al.’s study on the spectral features of both the nanocrystals in inert gas and the oxidized products.38 They proposed an oxidation process from the nanocrystal surface inward. This interior process for each particle resulted in effective (38) Sapra, S.; Nanda, J.; Pietryga, J. M.; Hollingsworth, J. A.; Sarma, D. D. J. Phys. Chem. B 2006, 110, 15244–15250.
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Figure 7. Decomposition of PbSe nanocrystals. Fresh black PbSe nanocrystal solution stored in air (left, vial) finally turned into clear and colorless solution (right, cuvette) under 12 day UV irradiation.
size decreases and spectral blue-shifts. Hollingsworth and colleagues utilized this instability to controllably synthesize PbSe/CdSe core/shell nanocrystals.27 The resulting core/shell nanocrystals were stable against fading and spectral shifting, and could further undergo additional ZnS shell growth to produce PbSe/CdSe/ZnS DOI: 10.1021/la9015614
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Figure 8. Temporal HWHM evolution of 5.0 nm PbSe nanocrystals (0.86 μM) under dark in air. HWHM is the half-width at halfmaximum to the longer wavelength side of the first absorption peak.
nanostructures. This makes it possible to employ PbSe nanocrystals as stable, bright, and biocompatible near-infrared optical labels. Such instability could also be applied to air-sensitive sensors. Destructive oxidation on the surface of PbSe nanocrystals is also one of the reasons for the concentration- and size-dependent phenomena illustrated in Figure 3. The more important reason for these phenomena is the particle collision in solution. From the kinetic theory of gas, the average gas molecule collision frequency (f) is proportional to πd2nν, where d is the effective gas molecule diameter (πd2 is the cross-sectional area of that gas molecule), n is the number density, and ν is the average moving rate of that gas molecule. It says the gas molecule collision frequency is proportional to the molecule size (d) and the density (n). Applying this theory to the PbSe nanoparticle solution, where the nanoparticles collide with each other, larger particles and higher concentrations lead to higher collision frequency than smaller particles and lower concentrations. We assume the oxidized surface is not a sturdy structure,26,27,37 so the collision can peel it off; it is accompanied by new surfaces exposed to air and oxidized quickly. Apparently, higher collision frequency accelerates this process, leading to the faster effective size shrinking of PbSe nanocrystals. Therefore, we observed the phenomena shown in Figure 3, in which PbSe nanocrystals with higher concentrations and larger particle sizes (corresponding to higher collision frequency) have faster blueshifts in their spectra. These observations are different from the traditional thought that smaller particles shrink fast, because the traditional process is thermodynamically controlled (through
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surface energy), while the spectral blue-shift here is kinetically controlled (through particle collision). In addition to the blue-shifts, the narrowing of the size distributions was observed from the recorded spectra (Figure 8). The spectral narrowing was analyzed with half-width at halfmaximum (HWHM) to the longer wavelength side of the first absorption peak. The HWHM can be used as a convenient index of size distributions; a smaller HWHM implies a narrower size distribution for the same kind of semiconductor nanocrystals.19 A similar size distribution narrowing was reported for PbS semiconductor nanocrystals, where the narrowing was simply attributed to the solvents used in the experiments.39 However, our observation indicates that in addition to the spectral blue-shifts the size distribution narrowing is also attributable to the oxidation in air and the subsequent collision-sponsored decomposition of the surface oxides. Without air exposure (i.e., under nitrogen), there were no blue-shifts observed (Figure 5), while air contact with the help of other influence factors, such as UV irradiation, leads to extremely fast oxidation. On the other hand, any nanocrystal sample is actually a particle ensemble with some larger and smaller sizes in addition to the medium ones. Largersized particles shrink (blue-shifts in the spectra) faster than the smaller ones (Figure 3b), resulting in a narrower size distribution over time. In other words, the observed self-narrowing is due to the destructive oxidation and subsequent collision-induced sizedependent decomposition.
Conclusion The stability of PbSe semiconductor nanocrystals over several conventional physical conditions has been systematically studied. PbSe semiconductor nanocrystals under air exposure exhibited instability dependent on particle concentrations, particle sizes, and light conditions. These air-contacted instability trends were due to the destructive oxidation and the kinetically collisioninduced decomposition, which differed from the traditional thermodynamic mechanism. In contrast, under inert nitrogen gas, the absorption and photoluminescence properties of the PbSe semiconductor nanocrystals were preserved even with UV irradiation or upon being heated at mild temperatures. Acknowledgment. This work was supported by the Worcester Polytechnic Institute, the National Science Foundation of China (20773043), the National Basic Research Program of China (2005CB724400), and the Outstanding Youth Promotion Foundation of Shandong (2008BS09009). The work at Hampton University was supported by the National Science Foundation (HRD-0734635, HRD-0630372, and ESI-0426328/002). (39) Hines, M. A.; Scholes, G. D. Adv. Mater. 2003, 15, 1844–1849.
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