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.
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.
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