Luminescence enhancement of colloidal quantum dots by strain compensation
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Luminescence enhancement of colloidal quantum dots by strain compensation Y. Lu, Y.Q. Zhang, and X. A. Cao Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, U.S.A ABSTRACT We have investigated the effects of two different strain-relief bilayer shell structures on the luminescent properties of colloidal CdSe quantum dots (QDs). CdSe QDs with a straincompensated ZnS/ZnCdS bilayer shell were synthesized using the successive ion layer adsorption and reaction technique and their crystallinity of was examined by X-ray diffraction. The QDs enjoyed the benefits of excellent exciton confinement by the ZnS intermediate shell and strain compensation by the ZnCdS outer shell. The resulting CdSe/ZnS/ZnCdS QDs exhibited 40% stronger photoluminescence and a smaller peak redshift upon shell growth compared to conventional CdSe/ZnCdS/ZnS core/shell/shell QDs with an intermediate lattice adaptor. CdSe/ZnS/ZnCdS QD light-emitting diodes (LEDs) had a luminance of 558 cd/m2 at 20 mA/cm2, 28% higher than that of CdSe/ZnCdS/ZnS QD-LEDs. The former also had better spectral purity. These results suggest that nanocrystal shells may be strain-engineered in a different way to achieve QDs of high crystalline and optical quality well suited for full-color display applications. INTRODUCTION Colloidal nanocrystal quantum dots (QDs) synthesized by low-cost solution processes have many attributes which make them suitable for optoelectronic applications [1-3]. However, the performance of QD-based optoelectronic devices, including light-emitting diodes (LEDs), and solar cells, is lagging significantly behind that of devices based on conventional bulk materials [1]. Due to the high surface-to-volume ratio inherent to nanocrystals, the device performance is largely determined by the surface properties of QDs. Surface defects trap carriers and enhance nonradiative recombination [3,4]. Organic ligands introduced for dispersion during colloidal synthesis may protect the surfaces of QDs in solutions, but they cannot provide full passivation of the QDs in device structures as many ligands may detach from the QD surfaces during device processing [4]. A common strategy is to better passivate the QD surface with a thin shell of a wider band gap semiconductor, forming core/shell (CS) nanoparticles [3]. The shell reduces the number of surface dangling bonds and physically separates the optically active core from its surrounding medium. Consequently, core/shell QDs have much improved photoluminescence (PL) quantum yield (QY) and stability against photo-oxidation as compared to QD cores. Among many CS QDs, CdSe/ZnS CS QDs have been a subject of extensive research [3, 5-10]. ZnS possesses a much larger band gap than CdSe and forms a Type-I heterojunction with CdSe [3]. Even though ZnS provides effective electronic passivation of CdSe core QDs, it is not an ideal shell material because its lattice constant is ~12% smaller than that of CdSe. The large lattice mismatch prevents the epitaxial growth of a thick ZnS
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