Research Article Synthesis and Optical Properties of Thiol Functionalized CdSe/ZnS (Core/Shell) Quantum Dots by Ligand Exchange Huaping Zhu, 1,2 Michael Z. Hu, 1 Lei Shao, 3 Kui Yu, 4 Reza Dabestani, 1 Md. Badruz Zaman, 4 and Shijun Liao 2 1 Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, USA 2 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China 3 Key Lab for Nanomaterials, Ministry of Education, Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, China 4 Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, ON, Canada K1A 0R6 Correspondence should be addressed to Michael Z. Hu; hum1@ornl.gov Received 24 January 2014; Accepted 10 February 2014; Published 20 March 2014 Academic Editor: Binbin Weng Copyright © 2014 Huaping Zhu et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Te colloidal photoluminescent quantum dots (QDs) of CdSe (core) and CdSe/ZnS (core/shell) were synthesized at diferent temperatures with diferent growth periods. Optical properties (i.e., UV/Vis spectra and photoluminescent emission spectra) of the resulting QDs were investigated. Te shell-protected CdSe/ZnS QDs exhibited higher photoluminescent (PL) efciency and stability than their corresponding CdSe core QDs. Ligand exchange with various thiol molecules was performed to replace the initial surface passivation ligands, that is, trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP), and the optical properties of the surface- modifed QDs were studied. Te thiol ligand molecules in this study included 1,4-benzenedimethanethiol, 1,16-hexadecanedithiol, 1,11-undecanedithiol, biphenyl-4,4  -dithiol, 11-mercapto-1-undecanol, and 1,8-octanedithiol. Afer the thiol functionalization, the CdSe/ZnS QDs exhibited signifcantly enhanced PL efciency and storage stability. Besides surface passivation efect, such enhanced performance of thiol-functionalized QDs could be due to cross-linked assembly formation of dimer/trimer clusters, in which QDs are linked by dithiol molecules. Furthermore, efects of ligand concentration, type of ligand, and heating on the thiol stabilization of QDs were also discussed. 1. Introduction Semiconductor nanocrystals or quantum dots (QDs) have received widespread and growing attention in materials appli- cation and have led to many eforts to assess nanoparticles in size-controlled manner. When the sizes of such QDs become close to or smaller than the bulk exciton Bohr radius, the electronic and optical properties of these nanomaterials show a signifcant change from their corresponding bulk proper- ties, which are called quantum size efects. One such efect is the quantization of the bulk valence band and conduction band, resulting in discrete atomic-like transitions that shif to higher energies as the size of nanocrystals decreases. Because of their unique size-dependent optical and electronic properties, semiconductor nanocrystals are likely to play a key role in the emerging new feld of nanotechnology, the application of which ranges from optoelectronic devices to biological fuorescence marking [1, 2]. In order to study the fundamental properties of these QDs and explore their use in specifc applications, convenient access to ligand-stabilized QDs possessing active functional groups is essential. Te lack of general synthetic strategies to introduce specifc functionalities into the terminal positions of the ligand has limited detailed experimental investigations of the QD properties and reactivities [3]. QDs are not very stable in liquid solution and the underlying cause of this instability is still not totally clear. To stabilize QDs, the most common method used is chemically attaching a monolayer of organic molecules to the atoms on the surface of the nanocrystals. Tese organic molecules attached to Hindawi Publishing Corporation Journal of Nanomaterials Volume 2014, Article ID 324972, 14 pages http://dx.doi.org/10.1155/2014/324972