- Nano Express
- Open Access
Formation of PbSe/CdSe Core/Shell Nanocrystals for Stable Near-Infrared High Photoluminescence Emission
© The Author(s) 2010
- Received: 6 April 2010
- Accepted: 5 May 2010
- Published: 1 June 2010
PbSe/CdSe core/shell nanocrystals with quantum yield of 70% were obtained by the “successive ion layer adsorption and reaction” technology in solution. The thickness of the CdSe shell was exactly controlled. A series of spectral red shifts with the CdSe shell growth were observed, which was attributed to the combined effect of the surface polarization and the expansion of carriers’ wavefunctions. The stability of PbSe nanocrystals was tremendously improved with CdSe shells.
- Near infrared
Colloidal IV–VI semiconductor nanocrystals (also known as quantum dots, QDs) are of increasing potential applications in telecommunication, photoelectronic device, and biomedical labeling [1, 2], etc. PbSe QDs are important materials because of the strong confinement effect due to their large Bohr radius and the small band gap in near infrared region. Several approaches have been developed to prepare PbSe QDs with uniform size and high quantum yields [3–5]. However, it has been found that PbSe QDs are not stable [6, 7]. PbSe/PbS  and PbSe/SiO2 core/shell structures have been synthesized to stabilize PbSe QDs. But CdSe should be a better shell material due to the higher stability under air condition, the lower lattice mismatch of ~1%, and the little change of the surface chemistry and physics. It is difficult to grow CdSe shells upon PbSe cores using typical cadmium oleate anion precursor because of high reaction temperatures needed. Hollingsworth’s group recently developed a method of ion exchange to form PbSe/CdSe core/shell structures in which Cd atoms replaced Pb atoms in the outlayers of large PbSe QDs . However, it may not be easy to control the thickness of the CdSe layers. In this work, we employed the “successive ion layer adsorption and reaction (SILAR)” technology  to form air-stable PbSe/CdSe QDs with strong photoluminescence. The quantum yield of PbSe/CdSe QDs was 70%.
where μ and α are the resolved exciton dipole and polarizability, respectively. According to Muller et al.’s work , the spectrum shift of CdSe nanorods depends on the direction of the external electric field. The positive electric field induces red shift, and the negative one leads to blue shift. Since the QDs in this work are spherical (zero dimensional), it is reasonable that their peak shifts are independent of the direction of the electric field. Both positive and negative electric field can cause the emission peak to red shift, and the red shift increases when the electric field is stronger .
It has been known that unpassivated PbSe QDs surface is a Pb atom-rich shell [6, 7]. Therefore, there may be polarization charges on the surface of PbSe QDs which generate surface-polarization energy. However, the polarization charges are neutralized, because Pb atoms on the surface of PbSe QDs connect to oleic acid (the organic ligand used in the synthesis). The quantum yield of fresh PbSe QDs was 85% using IR-26 as a reference. When the PbSe/CdSe core/shell was synthesized, CdSe contacted with Pb atoms instead of oleic acid; this induced the increase of surface polarization charges. The spectra shift to red because of the enhancement of the Stark effect (Fig. 2).
Different crystal lattices and thermal expansivities for PbSe and CdSe will more or less induce surface defects at the interface of the two materials . The carriers will be trapped and result in the enhancement of the Stark effect. Such local fields cause the first exciton peak to shift to red and suppress the emission strength due to a reduced electron–hole wavefunction overlap. Unbalanced charges may also decrease the photoluminescence efficiency (quantum yield) via nonradiative Auger recombination.
The new traps were induced by surface defects depend on the shell growth. Compared with the photoluminescence of one monolayer core/shell QDs, the photoluminescence of two monolayers core/shell QDs increased as shown in Fig. 2b. However, it was found that more shell layers resulted in a decrease in photoluminescence strength (Fig. 2b). That is also because the tensile change at the interface is nonlinear with the shell thickness. When PbSe QDs were covered with two layers of CdSe, the good lattice tensility at the interface reduced the lattice mismatch and therefore increased the photoluminescence strength. When PbSe QDs were covered by three layers of CdSe, the lattice tensility was stronger and hence the photoluminescence strength decreased. Even so the quantum yield was still as high as 70% for our PbSe/CdSe core/shell QDs (IR-26 as the reference).
In conclusion, PbSe/CdSe core/shell QDs with a quantum yield of 70% were synthesized. The surface polarization and the expansion of carriers’ wavefunctions contributed to the spectral red shift. The spectra red shifts during the formation of CdSe shells were calculated, and they exhibited a good fit to the experimental data. The stability of PbSe QDs was dramatically improved by the formation of CdSe shells.
The authors Yu Zhang and Quanqin Dai contributed equally to this work.
The funding supports from the State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, the Worcester Polytechnic Institute, and the National 863 Projects of China (2007AA03Z112, 2007AA06Z112) are acknowledged.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
- Cui D, Xu J, Zhu T, Paradee G, Ashok S, Gerhold M: Harvest of near infrared light in PbSe nanocrystal-polymer hybrid photovoltaic cells. Appl. Phys. Lett. 2006, 88: 183111. Bibcode number [2006ApPhL..88r3111C] Bibcode number [2006ApPhL..88r3111C] 10.1063/1.2201047View ArticleGoogle Scholar
- Levina L, Sukhovatkin V, Musikhin S, Cauchi S, Nisman R, Bazett-Jones DP, Sargent EH: Efficient infrared-emitting PbS quantum dots grown DNA and stable in aqueous solution and blood plasma. Adv. Mater. 2005, 17: 1854. COI number [1:CAS:528:DC%2BD2MXoslGnt7g%3D] COI number [1:CAS:528:DC%2BD2MXoslGnt7g%3D] 10.1002/adma.200401197View ArticleGoogle Scholar
- Murray CB, Sun S, Gaschler W, Doyle H, Betley TA, Kagan CR: Colloidal synthesis of nanocrytals and nanocrystal superlattices. IBM J. Res. Dev. 2001, 45: 47. COI number [1:CAS:528:DC%2BD3MXivVGmurY%3D] COI number [1:CAS:528:DC%2BD3MXivVGmurY%3D] 10.1147/rd.451.0047View ArticleGoogle Scholar
- Yu WW, Falkner JC, Shih BS, Colvin VL: Preparation and characterization of monodisperse PbSe semiconductor nanocrystals in a noncoordinating solvent. Chem. Mater. 2004, 16: 3318. COI number [1:CAS:528:DC%2BD2cXlslersLw%3D] COI number [1:CAS:528:DC%2BD2cXlslersLw%3D] 10.1021/cm049476yView ArticleGoogle Scholar
- Pietryga JM, Schaller RD, Werder D, Stewart MH, Klimov VI, Hollingsworth JA: Pushing the band gap envelope: mid-infrared emitting colloidal PbSe quantum dots. J. Am. Chem. Soc. 2004, 126: 11752. COI number [1:CAS:528:DC%2BD2cXnt1Gmsb4%3D] COI number [1:CAS:528:DC%2BD2cXnt1Gmsb4%3D] 10.1021/ja047659fView ArticleGoogle Scholar
- Dai Q, Wang Y, Zhang Y, Li X, Li R, Zou B, Seo J, Wang Y, Liu M, Yu WW: Stability study of PbSe semiconductor nanocrystals over concentration, size, atmosphere, and light exposure. Langmuir 2009, 25: 12320. COI number [1:CAS:528:DC%2BD1MXnt1Wktro%3D] COI number [1:CAS:528:DC%2BD1MXnt1Wktro%3D] 10.1021/la9015614View ArticleGoogle Scholar
- Moreels I, Fritzinger B, Martins JC, Hens Z: Surface chemistry of colloidal PbSe nanocrystals. J. Am. Chem. Soc. 2008, 130: 15081. COI number [1:CAS:528:DC%2BD1cXht1GnurnJ] COI number [1:CAS:528:DC%2BD1cXht1GnurnJ] 10.1021/ja803994mView ArticleGoogle Scholar
- Xu J, Cui D, Zhu T, Paradee G, Liang Z, Wang Q, Xu S, Wang AY: Synthesis and surface modification of PbSe/PbS core-shell nanocrystals for potential device application. Nanotechnology 2006, 17: 5428. COI number [1:CAS:528:DC%2BD2sXisF2ltw%3D%3D]; Bibcode number [2006Nanot..17.5428X] COI number [1:CAS:528:DC%2BD2sXisF2ltw%3D%3D]; Bibcode number [2006Nanot..17.5428X] 10.1088/0957-4484/17/21/024View ArticleGoogle Scholar
- Tan TT, Selvan ST, Zhao L, Gao S, Ying JY: Size control, shape evolution, and silica coating of near-infrared-emitting PbSe quantum dots. Chem. Mater. 2007, 29: 3112. 10.1021/cm061974eView ArticleGoogle Scholar
- Pietryga JM, Werder DJ, Williams DJ, Casson JL, Schaller RD, Klimov VI, Hollingsworth JA: Utilizing the lability of lead selenide to produce heterostructured nanocrystals with bright, stable infrared emission. J. Am. Chem. Soc. 2008, 130: 4879. COI number [1:CAS:528:DC%2BD1cXjtlShtL4%3D] COI number [1:CAS:528:DC%2BD1cXjtlShtL4%3D] 10.1021/ja710437rView ArticleGoogle Scholar
- Li JJ, Wang AY, Guo W, Keay JC, Mishima TD, Johnson MB, Peng X: Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction. J. Am. Chem. Soc. 2003, 125: 12567. COI number [1:CAS:528:DC%2BD3sXnsVOisLs%3D] COI number [1:CAS:528:DC%2BD3sXnsVOisLs%3D] 10.1021/ja0363563View ArticleGoogle Scholar
- Dai Q, Wang Y, Li X, Zhang Y, Pellegrino DJ, Zhao M, Zou B, Seo J, Wang Y, Yu WW: Size-dependent composition and molar extinction coefficient of PbSe semiconductor nanocrystals. ACS Nano. 2009, 3: 1518. COI number [1:CAS:528:DC%2BD1MXlslSmur0%3D] COI number [1:CAS:528:DC%2BD1MXlslSmur0%3D] 10.1021/nn9001616View ArticleGoogle Scholar
- An JM, Franceschetti A, Zunger A: Electron and hole addition energies in PbSe quantum dots. Phys. Rev. B. 2007, 76: 045401. Bibcode number [2007PhRvB..76d5401A] Bibcode number [2007PhRvB..76d5401A] 10.1103/PhysRevB.76.045401View ArticleGoogle Scholar
- Olkhovets A, Hsu R-C, Lipovskii A, Wise FW: Size-dependent temperature variation of the energy gap in lead-salt quantum dots. Phys. Rev. Lett. 1998, 81: 3539. COI number [1:CAS:528:DyaK1cXmvVCisbk%3D]; Bibcode number [1998PhRvL..81.3539O] COI number [1:CAS:528:DyaK1cXmvVCisbk%3D]; Bibcode number [1998PhRvL..81.3539O] 10.1103/PhysRevLett.81.3539View ArticleGoogle Scholar
- Phillips AC: Introduction to Quantum Mechanics. John Wiley & Sons Ltd., West Sussex, England; 2003:94–99.Google Scholar
- Hyun B, Zhong Y, Bartnik AC, Sun L, Abruña HD, Wise FW, Goodreau JD, Matthews JR, Leslie TM, Borrelli NF: Electron injection from colloidal PbS quantum dots into titanium dioxide nanoparticles. ACS Nano. 2008, 2: 2206. COI number [1:CAS:528:DC%2BD1cXhtlWhu7nF] COI number [1:CAS:528:DC%2BD1cXhtlWhu7nF] 10.1021/nn800336bView ArticleGoogle Scholar
- Querner C, Reiss P, Bleuse J, Pron A: Chelating ligands for nanocrystals’ surface fuctionalization. J. Am. Chem. Soc. 2004, 126: 11574. COI number [1:CAS:528:DC%2BD2cXmvVyqurg%3D] COI number [1:CAS:528:DC%2BD2cXmvVyqurg%3D] 10.1021/ja047882cView ArticleGoogle Scholar
- Yu WW, Qu L, Guo W, Peng X: Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals. Chem. Mater. 2003, 15: 2854. COI number [1:CAS:528:DC%2BD3sXks12msrw%3D] COI number [1:CAS:528:DC%2BD3sXks12msrw%3D] 10.1021/cm034081kView ArticleGoogle Scholar
- Empedocles SA, Bawendi MG: Quantum-confined stark effect in single CdSe nanocrystallite quantum dots. Science 1997, 278: 2114. COI number [1:CAS:528:DyaK1cXhvFCh]; Bibcode number [1997Sci...278.2114E] COI number [1:CAS:528:DyaK1cXhvFCh]; Bibcode number [1997Sci...278.2114E] 10.1126/science.278.5346.2114View ArticleGoogle Scholar
- Muller J, Lupton JM, Lagoudakis PG, Schindler F, Koeppe R, Rogach AL, Feldmann J, Talapin DV, Weller H: Wave function engineering in elongated semiconductor nanocrystals with heterogeneous carrier confinement. Nano Lett. 2005, 5: 2044. COI number [1:STN:280:DC%2BD2Mrktl2mtw%3D%3D]; Bibcode number [2005NanoL...5.2044M] COI number [1:STN:280:DC%2BD2Mrktl2mtw%3D%3D]; Bibcode number [2005NanoL...5.2044M] 10.1021/nl051596xView ArticleGoogle Scholar
- Qian L, Bera D, Tseng T, Holloway PH: High efficiency photoluminescence from silica-coated CdSe quantum dots. Appl. Phys. Lett. 2009, 94: 073112. Bibcode number [2009ApPhL..94g3112Q] Bibcode number [2009ApPhL..94g3112Q] 10.1063/1.3085968View ArticleGoogle Scholar
- Koo BH, Hanada T, Makino H, Yao T: Effect of lattice mismatch on surface morphology of in as quantum dots on (100) In1-xAlxAs/InP. Appl. Phys. Lett. 2001, 79: 4331. COI number [1:CAS:528:DC%2BD3MXpt1yiu7c%3D]; Bibcode number [2001ApPhL..79.4331K] COI number [1:CAS:528:DC%2BD3MXpt1yiu7c%3D]; Bibcode number [2001ApPhL..79.4331K] 10.1063/1.1428763View ArticleGoogle Scholar
- Stouwdam JW, Shan J, van VeggelAndras FCJM, Pattantyus-Abraham AG, Young JF: Photostability of colloidal PbSe and PbSe/PbS core/shell nanocrystals in solution and in the solid state. J. Phys. Chem. C. 2007, 111: 1086. COI number [1:CAS:528:DC%2BD28XhtlektLrJ] COI number [1:CAS:528:DC%2BD28XhtlektLrJ] 10.1021/jp0648083View ArticleGoogle Scholar