ARTICLE pubs.acs.org/Langmuir
PbSe/CdSe and PbSe/CdSe/ZnSe Hierarchical Nanocrystals and Their Photoluminescence Yu Zhang,†,|| Quanqin Dai,†,|| Xinbi Li,‡ Jianyu Liang,‡ Vicki. L. Colvin,§ Yiding Wang,†,* and William W. Yu†,§,* †
State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun 130012, People's Republic of China ‡ Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States § Department of Chemistry and Center for Biological and Environmental Nanotechnology, Rice University, Houston, Texas 77005, United States
bS Supporting Information ABSTRACT: Multiple CdSe and ZnSe semiconductor shells were grown on PbSe semiconductor spherical cores with monolayer control. For CdSe shell coating, we found that there was little room to further increase the quantum yields of freshlymade high-quality PbSe nanocrystals that already owned very high initial values because of their good surface status; but there was great improvement for the PbSe nanocrystals with low initial quantum yields because of the poor surface status. Nonetheless, the quantum yield for the latter case could not reach the former’s value. Additional ZnSe shells on PbSe/CdSe could further increase the quantum yield and protect the nanocrystals from air oxidation. The observed phenomena in the synthesis of the PbSe/CdSe and PbSe/CdSe/ZnSe core/shell structures were explained through the carrier wave function expansion and the surface polarization.
1. INTRODUCTION The recognition of the importance for the near-infraredemitting nanocrystals (NIR NCs) has been growing rapidly in recent years. One significant application of NIR NCs is their deployment as optical amplifying media for telecommunications based on silica fiber technology, which has an optimal transmission window of 1.3 to 1.55 μm.1,2 Another evolving field in biotechnology is the use of NIR NCs as fluorescent contrast agents for biomedical imaging in living tissues.3,4 Other potential applications include electroluminescent devices,5,6 photodetectors,7 and photovoltaics for solar energy conversion.810 The IVVI group of semiconducting materials comprises some of the most promising materials for NIR applications. Colloidal lead chalcogenide NCs, PbX (X = S, Se, and Te), have recently been actively studied for their applications. These PbX semiconducting materials possess many unique and unusual properties, such as narrow bandgaps (0.41, 0.28, and 0.31 eV for PbS, PbSe, and PbTe, respectively, at room temperature),11 large static dielectric constants,12 small and approximately equal masses of the electron and hole,13 and large Bohr radii.14,15 The large exciton Bohr radius of PbSe (46 nm) results in a much stronger quantum confinement with respect to IIVI and IIIV semiconductor materials when they are in a similar nanoscale regime. Murray et al. first reported the hot-injection synthesis of PbSe NCs, which was to inject trioctylphopshine-selenium into a diphenylether solution of lead oleate at elevated temperatures.16 r 2011 American Chemical Society
The particle size can be tuned from 3.5 to 15 nm,1619 and these NCs possess extraordinary optical properties, such as high photoluminescence efficiency or quantum yield (QY),15 possible multiple exciton generation,2026 and a high nonlinear refractive index.27 For the materials aspect of PbSe NCs, some particular interests are as follows: (1) to improve the QY; (2) to improve the stability of photoluminescence (PL) intensity and peak position; (3) to reduce its toxicity for practical applications. PbSe NCs are often passivated by oleic acid (OA) and a small amount (05%) of trioctylphopshine (TOP).28 The OA ligands bind to surface Pb atoms. The oxidization occurs on the PbSe surface under air, which eventually induces the departure of the surface Pb atoms from the particle surface together with OA ligands which may exist as Pb(OA)2; the effective PbSe core size is thus reduced. Generally, forming a shell is a good way to improve the QY and the chemical and optical stability, as well as to reduce the toxicity of semiconductor NCs (minimizing the leaking of toxic components in the core, such as Pb2+ in PbSe NCs).29 PbS30 and SiO231 have been employed to form the shells over PbSe core NCs. But the different lattice constants of PbS and PbSe can Received: April 23, 2011 Revised: June 18, 2011 Published: June 24, 2011 9583
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Langmuir induce the excessive loss of the QY, and the surface chemistry and physics may be changed by the growth of SiO2. Recently, an etching-ion exchange method was developed by Hollingsworth’s group to form PbSe/CdSe core/shell structures and improve the QY of PbSe NCs that had very low initial values.29 In this work, the successive ion layer adsorption and reaction (SILAR) technology was used to prepare PbSe/CdSe and PbSe/ CdSe/ZnSe core/shell NCs. Through the comparison of the core/shell synthesis for fresh and oxidized PbSe core NCs, the surface state was proven to be a critical factor for PL change in the formation of PbSe/CdSe NCs. Compared with the most commonly used shell material ZnS, ZnSe has a lower lattice parameter mismatch relative to CdSe. Therefore, PbSe/CdSe/ ZnSe core/multishell structure was employed to further improve the stability of PbSe NCs and reduce the core material’s toxicity.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Lead oxide (PbO, Aldrich, 99.99%), selenium powder (Se, Aldrich, 99.99%), oleic acid (OA, Aldrich, 90%), 1-octadecene (ODE, Aldrich, 90%), tributyphosphine (TBP, Aldrich, 97%), cadmium cyclohexanebutyrate (Alfa), zinc 2-ethylhexanoate (Alfa), and oleylamine (OLA, City Chemical, 98%) were used in the synthesis. Acetone, chloroform, methanol, hexanes, and toluene were purchased from VWR. All chemicals were used directly without further treatment. 2.2. Synthesis of PbSe Nanocrystals. PbSe NCs were synthesized based on Colvin’s work to get uniform sizes and high QYs.17 In detail, 0.892 g PbO (4.000 mmol), 2.600 g OA (8.000 mmol) and 12.848 g ODE were loaded into a three-neck flask. The mixture was heated to 170 °C under N2 flow until a colorless solution was obtained. A 6.400 g 10% Se-TBP solution (prepared in a glovebox) was quickly injected into the above vigorously stirred solution. The temperature dropped immediately and was then kept at 148 °C for the growth of PbSe NCs. After four minutes, the reaction was stopped by injecting excessive room-temperature toluene. The resulting NCs were purified via the chloroformmethanol extraction and acetone precipitation method.32,33 The purified products were redispersed in hexanes for the evaluation of their optical properties and follow-up reactions. 2.3. Oxidization of PbSe Nanocrystals. The above fresh-made PbSe NC solution was kept overnight without light under air. The next day, the oxidized PbSe NCs were purified again and kept in hexanes for core/shell synthesis. The absorption and PL spectra were recorded for the calculation of particle size and concentration. 2.4. Injection Solutions for Shell Growth. There were three injection solutions. One was cadmium injection solution (0.04 M) prepared by dissolving 0.1804 g cadmium cyclohexanebutyrate in 8.1300 g OLA at 60 °C under N2 flow; it was a clear colorless solution. The second was selenium injection solution (0.04 M) prepared by dissolving 0.0316 g selenium in 7.8800 g ODE at 220 °C under N2 flow; it was a clear yellow solution. These two injection solutions were cooled down to room temperature for shell synthesis. The third was zinc injection solution (0.04 M) made by mixing 0.7036 g zinc 2-ethylhexanoate with 39.4000 g ODE at room temperature; it was a clear colorless solution. For each injection, a calculated volume of a given injection solution was loaded into a syringe following a standard air-free procedure.34 2.5. Synthesis of PbSe/CdSe Core/Shells Nanocrystals. The synthesis of PbSe/CdSe core/shell NCs was reported in a previous study.35 Here we followed the same procedure to prepare the core/shell structure using two different core NCs: fresh-made PbSe NCs and oxidized PbSe NCs. The average thickness of one monolayer of CdSe was taken as 0.35 nm, so one additional layer of growth would increase the diameter of a nanocrystal by 0.7 nm. In a typical experiment with 1.000 104 mmol of 4.8 nm PbSe core, 3.315 102 mmol of Cd and
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Se precursors were needed for the first layer of the shell growth, and 4.212 102 mmol of Cd and Se precursors completed the formation of the second layer. In actuality, 1.000 104 mmol PbSe NCs (4.8 nm, concentration determined by Dai’s work36) was dispersed in 5 mL of hexanes, then mixed with 1.500 g of ODA and 5.000 g of ODE and loaded into a 25-mL three-neck flask. The hexanes were first removed from the system using a mechanical pump at room temperature for 30 min. Subsequently, the reaction mixture was heated to 120 °C under N2 flow. Then, the predetermined amounts of the cadmium and selenium solutions were alternatively injected into the three-neck flask with syringes following a standard air-free procedure. The reaction was finally quenched by the injection of toluene. Methanol extraction plus acetone precipitation were employed to purify the products. The PbSe/CdSe NCs were redispersed into hexanes for PL and absorption measurements. CdSe shell growth on oxidized PbSe NCs was also achieved using the same experimental method.
2.6. Synthesis of PbSe/CdSe/ZnSe Core/Shell/Shell Nanocrystals. The average thickness of one monolayer of ZnSe was taken as 0.32 nm, so one layer growth would increase the nanocrystal diameter by 0.64 nm. Therefore, for 1.000 104 mmol of PbSe/CdSe NCs (4.8 nm PbSe core with 1.4 nm CdSe shell), the injection dosages of Zn and Se precursors for the first and second layers were 5.127 102 and 6.131 102 mmol, respectively. After the growth of two layers CdSe shell on PbSe core NCs, the predetermined amounts of the zinc and selenium solutions were alternatively injected into the three-neck flask syringes using a standard air-free procedure. The reaction was stopped by the injection of toluene, and methanol extraction and acetone precipitation were applied to purify the products. The PbSe/CdSe/ZnSe NCs were redispersed into hexanes for PL and absorption measurements. 2.7. Characterization. The absorption and photoluminescence spectra were recorded by a Perkin-Elmer Lambda 9 UVvis-NIR spectrometer and a CM110 Spectralproducts NIR spectrometer with a 904 nm-diode laser module as the excitation source, respectively. For transmission electron microscope (TEM) specimen preparation, nanocrystal particles dispersed in hexanes were dropped on carbon-coated copper grids and then dried naturally. The images of the particles were taken using a JEOL FasTEM-2010 TEM. A JEOL JSM-840 scan electron microscope (SEM) was employed to collect the energy dispersive X-ray (EDX) analysis data.
3. RESULTS 3.1. CdSe Shell Growth on Different PbSe Core NCs. For fresh-made high-quality PbSe core NCs (4.8 nm, Figure 1a), the CdSe shell formation was found to have consistent red shifts for both absorption and PL spectral peaks upon the increase of shell layers (11 nm for the first layer and 10 nm for the second layer35). The PL intensity showed an initial little drop for the first layer and the subsequent increment for the second layer. CdSe shells effectively prevented PbSe cores from oxidation: the PL spectrum and the QY of the resulting NCs did not change for at least a month in air. Further characterization of these core/shell NCs by EDX is presented in Figure S1a in Supporting Information. We hypothesize that shell formation with another semiconductor material may not increase the QY if the core NCs already have very high values. This is verified by the above CdSe shell formation on fresh-made PbSe NC with the QY of 85%. After two monolayer CdSe shell growth, the QY of PbSe/CdSe NCs is 70%. We also hypothesize that the same shell formation may greatly increase the QY for low-quality core NCs that have very low values. Since the synthesis method we used17 cannot make low-quality PbSe NCs (low QY), we instead tried to oxidize these PbSe NCs with air.37 The oxidized PbSe NCs had a very low QY 9584
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Figure 1. Transmission electron microscopic images of 4.8 nm PbSe (a), 6.2 nm PbSe/CdSe (b), and 7.5 nm PbSe/CdSe/ZnSe (c) NCs.
Figure 2. The evolution of the absorption (a) and photoluminescence (b) spectra of PbSe/CdSe core/shell NCs upon the growth of two monolayer CdSe shells over 4.8 nm oxidized PbSe cores.
(∼6%), which were used as the core particles to grow CdSe shell on them. Experimental results showed that the QY was dramatically increased after two layers of CdSe shell formation—nearly 10 times increase was obtained (Figure 2). After two monolayer CdSe shell growth upon the oxidized PbSe NCs, the absorption peak shifts from 1476 to 1502 nm and the PL peak shift from 1493 to 1520 nm. 3.2. PbSe/CdSe/ZnSe NCs and Their Stability. Figure 3 shows the evolution of the absorption and PL spectra for ZnSe shell formation on the PbSe/CdSe core/shell NCs based on the fresh-made PbSe core NC (4.8 nm fresh PbSe core with 1.4 nm CdSe shell). Similar to the CdSe shell growth,35 red shift of the peaks were also observed from both the absorption and PL spectra upon the growth of ZnSe shell. After two monolayer ZnSe shell growth, the absorption peak shifts from 1510 to 1539 nm and the PL peak shift from 1528 to 1558 nm. The average thickness of the monolayer-ZnSe shell is 0.32 nm. The
Figure 3. The evolution of the absorption (a) and photoluminescence (b) spectra of PbSe/CdSe/ZnSe core/multishell NCs upon the growth of two monolayers CdSe shells and two monolayers ZnSe shells over 4.8 nm PbSe cores. After the growth of two monolayers ZnSe shells, the PbSe/ CdSe/ZnSe NCs also show superior stability to PbSe core NCs (c).
growth of two layers of ZnSe over a PbSe/CdSe NCs therefore increased the particle diameter by about 1.3 nm as observed in Figure 1. EDX also showed the existence of ZnSe shell (Figure S1b in Supporting Information). The PL intensities of PbSe/CdSe/ZnSe NCs could be kept for at least three weeks without decrease (Figure 3c).
4. DISCUSSION 4.1. Carriers Expansion in Core/Shell Structures. An increase of shell layers will result in the decline of the quantum confinement on the electronic transitions in PbSe NCs and render the expansion of the carriers’ wave functions out of the core regions.30 Figure 4 shows the energy level of core/shell structure. The related parameters of PbSe, CdSe and hexane are listed in Table 1.30,32,38,39 When PbSe NCs and solvent contact directly, the confined potential of electron and hole are V2e = (1.95)(eV) (4.87) 9585
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Table 1. LUMO, HOMO, me and mh of PbSe, CdSe and Hexane material
Figure 4. Schematic diagram of energy levels in PbSe/CdSe core/shell structure.
(eV) = 2.92 eV and V2h = (5.15)(eV) (7.45) (eV) = 2.3 eV in PbSe NCs, respectively; when PbSe core and CdSe shell contacts, the confined potentials of electron and hole in PbSe NCs are V1e = (4.45)(eV) (4.87) (eV) = 0.42eV and V1h = (5.15)(eV) (6.19)(eV) = 1.04 eV, respectively. In the PbSe/CdSe core/shell structure, the carriers’ confined potential in PbSe core will decrease and it will be easier for the carriers to go through the interface barrier. Therefore, the wave functions of electron and hole in PbSe core will spread into CdSe shell and their energies will decrease. However, the confined potential of electron (V1e = 0.42 eV) is lower than the confined potential of hole (V1h = 1.04 eV). In this case, the electrons in semiconductor’s conduction band with low work function will transfer into the one with high work function but the holes spread in the contrary way. Although the effective masses of electron and hole for PbSe are almost the same (me = 0.070, mh = 0.068), the smaller diffusion barrier of electron will result in an easier diffusion than hole. Therefore, the hole wave function will be relatively confined in PbSe core while the electron wave function will relatively expand to CdSe shell. Coulomb attraction becomes weaker due to the decrease of the wave functions’ overlap. However, the quantum confinement energy decreases more than Coulomb attraction at the same time. Thus, the absorption and PL spectra will eventually shift to red because of the combined changes in quantum confinement energy and Coulomb attraction. 4.2. Stark Effect in Core/Shell Structures. The surfacepolarization energy also affects gap energy, this is the Stark effect. There are always some defect states on the surface of unpassivated PbSe NCs, and the trapped charge carriers will induce localized electric fields. The surface polarization energy ΔEpol versus local electric field ξ can be expressed as follows:40 1 ΔEpol ¼ μξ þ Rξ2 þ 3 3 3 2 where μ and R are the resolved exciton dipole and polarizability, respectively. Because the present nanocrystals are spherical, the direction of the electric field cannot affect the peak shifts. Both positive and negative electric fields make the emission peak shift to red, and this red shift will increase when the electric field is stronger.41 High-quality PbSe NCs are well capped with oleic acid and they possess very high QYs.28,36 This means the carries are neutralized very well and there are few traps on the particle surface. In this case, some more surface traps may be induced by
LUMO (eV)
HOMO (eV)
me (m0)
mh (m0)
PbSe
4.87
5.15
0.07
0.068
CdSe hexane
4.45 1.95
6.19 7.45
0.13 1.0
0.45 1.0
lattice mismatch from the shell material. The different crystal lattices and thermal expansivities for PbSe and CdSe more or less induce lattice mismatch at the interface of the two materials for the fresh PbSe NCs.42 The Stark effect will be enhanced by the trapped carriers, and the emission strength will be suppressed by these local fields due to a reduced electronhole wave function overlap (see Figure 2 in ref 31). Unbalanced charges may also result in the decrease of QY via nonirradiative Auger recombination. Therefore, it is hard to improve or keep the very high QY of fresh-made high-quality PbSe NCs through extra shell structures.34 In general, PbSe NCs has a higher QYs compared with CdSe NCs due to the better passivation induced by OA-capped Pb shell structure.28 The lattice parameter mismatch between CdSe and ZnSe (6.3%) is significantly lower than that of the most commonly used shell material ZnS (10.6%). Therefore, ZnSe is a better shell candidate for CdSe. ZnSe shell with two monolayers was considered to be an optimal way to increase the passivation of CdSe surface and therefore improved the QYs. A thicker deposited ZnSe shell was reported to induce the QY decrease, which was probably due to the higher concentration of structural defects created within the thicker ZnSe shell.43 The oxidation results in a loss of surface Pb atoms and oleic acid ligands.28 Therefore, for the oxidized PbSe NCs, the destructive oxidation induces a lot more surface traps and they trap many carriers. The CdSe shells effectively eliminate most of these traps through good passivation. The QY hence can be increased almost 10 times with two CdSe layers (Figure 2b). In comparison to the CdSe shell growth over fresh-made and oxidized PbSe NCs, we found that it was not easy to improve the QY of high-quality PbSe NCs (good surface status and high initial QY) after the shell formation; but it was very easy to increase the QY for low-quality PbSe NCs (poor surface status and low initial QY) multiple times after shell growth.
5. CONCLUSIONS High-quality air-stable PbSe/CdSe and PbSe/CdSe/ZnSe NCs have been synthesized through layer-by-layer control. The formation and QYs of core/shell structures with fresh-made and oxidized PbSe NCs have been investigated for the surface effect. It is hard to further improve the QY of high-quality PbSe NCs, but it is easy to increase the QY of low-quality PbSe NCs—the QY can be enhanced for about 10 times. However, the QY for the latter case cannot reach the former’s value. Additional ZnSe shells on PbSe/CdSe can further increase the QY and protect the PbSe core NCs from air oxidation. These observed phenomena can be understood through the carrier wave function expansion and the surface polarization. ’ ASSOCIATED CONTENT
bS
Supporting Information. EDS spectra for the PbSe/ CdSe and PbSe/CdSe/ZnSe NCs showing the existence of Pb,
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Langmuir Cd, Se (a) and Pb, Cd, Zn, Se (b), respectively. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (W.W.Y.); wangyiding47@ gmail.com (Y.W.). )
Author Contributions
These authors contributed equally to this work.
’ ACKNOWLEDGMENT This work is supported by the National 863 Projects of China (2007AA03Z112, 2007AA06Z112), State Key Laboratory on Integrated Optoelectronics, and College of Electronic Science and Engineering in Jilin University. ’ REFERENCES (1) Harrison, M. T.; Kershaw, S. V.; Burt, M. G.; Rogach, A. L.; Kor-nowski, A.; Eychm€uller, A.; Weller, H. Pure. Appl. Chem. 2000, 72, 295–307. (2) Kershaw, S. V.; Harrison, M.; Rogach, A. L.; Kornowski, A. IEEE J. Sel. Top. Quantum Electron. 2000, 6, 534–543. (3) Lim, Y. T.; Kim, S.; Nakayama, A.; Stott, N. E.; Bawendi, M. G.; Frangioni, J. V. Mol. Imaging 2003, 2, 50–64. (4) Rogach, A. L.; Eychm€uller, A.; Hickey, S. G.; Kershaw, S. V. Small 2007, 3, 536–557. (5) Bakueva, L.; Musikhin, S.; Hines, M. A.; Chang, T.-W. F.; Tzolov, M.; Scholes, G. D.; Sargent, E. H. Appl. Phys. Lett. 2003, 82, 2895–2897. (6) Steckel, J. S.; Coe-Sullivan, S.; Bulovic, V.; Bawendi, M. G. Adv. Mater. 2003, 15, 1862–1866. (7) McDonald, S. A.; Konstantatos, G.; Zhang, S. G.; Zhang, S. G.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138–142. (8) Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2004, 92, 186601/ 1–4. (9) Choi, J. J.; Lim, Y.; Santiago-Berrios, M. B.; Oh, M.; Hyun, B.; Sun, L.; Bartnik, A. C.; Goedhart, A.; Malliaras, G. G.; Abru~ na, H. D.; Wise, F. W.; Hanrath, T. Nano Lett. 2009, 9, 3749–3755. (10) Law, M.; Beard, M. C.; Choi, S.; Luther, J. M.; Hanna, M. C.; Nozik, A. J. Nano Lett. 2008, 8, 3904–3910. (11) Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P.; Micic, O. I.; Ellingson, R. J.; Nozik, A. J. J. Am. Chem. Soc. 2006, 128, 3241–3247. (12) Bailey, P. T.; O’Brien, M. W.; Rabii, S. Phys. Rev. 1969, 179, 735–739. (13) Baskoutas, S.; Terzis, A. F. Mater. Sci. Eng. B-Solid 2008, 147, 280–283. (14) Wise, F. W. Acc. Chem. Res. 2000, 33, 773–780. (15) 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. (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) Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L. Chem. Mater. 2004, 16, 3318–3322. (18) Sashchiuk, A.; Amirav, L.; Bashouti, M.; Krueger, M.; Sivan, U.; Lifshitz, E. Nano Lett. 2004, 4, 159–165. (19) Sashchiuk, A.; Langof, L.; Chaim, R.; Lifshitz, E. J. Cryst. Growth 2002, 240, 431–438. (20) Schaller, R. D.; Klimov, V. I. Phys. Rev. Lett. 2004, 92, 186601/ 1–4. (21) Schaller, R. D.; Agranovich, V. M.; Klimov, V. I. Nat. Phys. 2005, 1, 189–194.
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