Characterization of Shell Material on Colloidal CdSe/ZnS Quantum Dots
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Characterization of Shell Material on Colloidal CdSe/ZnS Quantum Dots Zhiheng Yu*, Li Guo**, Hui Du**, Todd Krauss** and John Silcox*** * Physics Department, Cornell University, Ithaca, NY 14853 ** Chemistry Department, University of Rochester, Rochester, NY 14627 *** School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853 ABSTRACT Scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) was used to determine the distribution of ZnS shell material on colloidal core-shell CdSe/ZnS quantum dots (QDs). A sub-nm electron probe was placed at various locations on core-shell QDs to ascertain the chemical distribution of the shell material. While a definite shell of ZnS was detected surrounding the CdSe core, the integrated EELS signals from positions around the QD suggest the distribution of the shell material may not be uniform. A non-uniform shell implies a reduced quality of the QD surface passivation. INTRODUCTION Colloidal II-VI semiconductor quantum dots (QDs) have attained a strong research focus due to their interesting electronic and optical properties and corresponding potential for a wide range of applications, such as improved fluorescent biological labels [1-4], light emitting devices (LEDs) [5], white-light laser sources [6], and near-infrared emitters [7]. Due to greatly improved surface passivation core-shell CdSe/ZnS QDs have shown increased photoluminescence quantum efficiencies (QY) by an order of magnitude [8] over QDs without the shell. Thus, core-shell QDs are even more promising than core QDs for potential colorimetric applications. However, the surface passivation provided by the semiconductor shell is not perfect; photoluminescence QYs are typically only ~ 50% [8], and single CdSe QD photoluminescence exhibits surface-related fluctuations in intensity and spectra [9]. On the other hand, self-assembled InP QDs with Ga0.5In0.5P surface capping do not suffer nearly as much from such surface induced imperfections [10], suggesting that the shell for colloidal semiconductor QDs is incomplete or illformed. Thus, verification of the distribution of the shell material around the core for CdSe/ZnS QDs is essential to better understand their optical properties. Electron energy loss spectroscopy (EELS) is an important tool for the study of the electronic structure and chemical distribution of materials [11]. When fast electrons in an electron microscope (EM) penetrate a thin specimen, they may lose energy to inner-shell bonded electrons. The amount of energy lost is dependent on the specific atom being imaged and therefore EELS can be used to study the chemical composition of the specimen. In this study, we have used EELS to study the shell properties of core-shell CdSe/ZnS QDs by collecting localized core-edge EELS spectra.
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EXPERIMENT Colloidal core-shell CdSe/ZnS QDs were synthesized according to modified versions of the literature methods [12]. EM samples were prepared by depositing ~10 ml of theQD suspension in hexane o
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