Abstract
The colloidal photoluminescent quantum dots (QDs) of CdSe (core) and CdSe/ZnS (core/shell) were synthesized at different temperatures with different growth periods. Optical properties (i.e., UV/Vis spectra and photoluminescent emission spectra) of the resulting QDs were investigated. The shell-protected CdSe/ZnS QDs exhibited higher photoluminescent (PL) efficiency 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-modified QDs were studied. The 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. After the thiol functionalization, the CdSe/ZnS QDs exhibited significantly enhanced PL efficiency and storage stability. Besides surface passivation effect, 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, effects of ligand concentration, type of ligand, and heating on the thiol stabilization of QDs were also discussed.
Highlights
Semiconductor nanocrystals or quantum dots (QDs) have received widespread and growing attention in materials application and have led to many efforts to assess nanoparticles in size-controlled manner
Thiol molecules acted as coligands to passivate the QD surface and possibly as molecular linkers between QDs to improve the optical properties of the QD system
The optical properties of core-shell structure CdSe/ZnS quantum dots were enhanced by cross-linked thiol molecules such as 1,4-benzenedimethanethiol, biphenyl-4,4-dithiol, 1,16hexadecanedithiol, 1,11-undecanedithiol, 1,8-octanedithiol, and 11-mercapto-1-undecanol
Summary
Semiconductor nanocrystals or quantum dots (QDs) have received widespread and growing attention in materials application and have led to many efforts to assess nanoparticles in size-controlled manner. As part of our investigation to overcome these problems, we found that ligand exchange with mercaptoundecanoic acid plus further cross-linking with the amino acid lysine in the presence of dicyclohexylcarbodiimide could form a stable hydrophilic shell [22] Such an organic functionalization reaction required only mild reaction conditions and led to QDs that were individually dispersed in water with good colloidal stability. We report the general features and scope of the ligand-exchange reaction of CdSe/ZnS QDs with a variety of functionalized thiols and dithiols as surface ligands This approach is convenient and the products tolerate organic environments (with neutral alkyl chain tails) or aqueous solutions (with charged COO− tails). The possible formation of dimers, trimers, or other self-assembly clusters by controlled molecular coupling between CeSe/ZnS QDs may offer a powerful mechanism: the ability to control the cluster size of a QDassembly system could improve PL emission and storage stability properties
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