We demonstrate that metal carboxylate complexes (L–M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N′,N′-tetramethylbutylene-1,4-diamine, pyridine, N,N,N′,N′-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by 1H NMR spectroscopy and depends most strongly on geometric factors such as sterics and chelation, although also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L–M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface-bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd/Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1Se–2S3/2h absorption, and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) an...

/pdf/ligand-exchange-and-the-stoichiometry-of-metal-chalcogenide-mvgek5d5ry.pdf

Ligand exchange and the stoichiometry of metal chalcogenide nanocrystals: spectroscopic observation of facile metal-carboxylate displacement and binding.

Luminescent semiconductor quantum dots (QDs) are one of the more popular nanomaterials currently utilized within biological applications. However, what is not widely appreciated is their growing role as versatile energy transfer (ET) donors and acceptors within a similar biological context. The progress made on integrating QDs and ET in biological configurations and applications is reviewed in detail here. The goal is to provide the reader with (1) an appreciation for what QDs are capable of in this context, (2) how this field has grown over a relatively short time span, and, in particular, (3) how QDs are steadily revolutionizing the development of new biosensors along with a myriad of other photonically active nanomaterial-based bioconjugates. An initial discussion of QD materials along with key concepts surrounding their preparation and bioconjugation is provided given the defining role these aspects play in the QDs ability to succeed in subsequent ET applications. The discussion is then divided around...

Energy Transfer with Semiconductor Quantum Dot Bioconjugates: A Versatile Platform for Biosensing, Energy Harvesting, and Other Developing Applications.

This review summarizes traditional and recent nonconventional, bioinspired, methods for the aqueous synthesis of colloidal semiconductor quantum dots (QDs). The basic chemistry concepts are critically emphasized at the very beginning as these are strongly correlated with the selection of ligands and the optimal formation of aqueous QDs and their more sophisticated structures. The synergies of biomimetic and biosynthetic methods that can combine biospecific reactivity with the robust and strong optical responses of QDs have also resulted in new approaches to the synthesis of the nanoparticles themselves. A related new avenue is the recent extension of QD synthesis to form nanoparticles endowed with chiral optical properties. The optical characteristics of QD materials and their advanced forms such as core/shell heterostructures, alloys, and doped QDs are discussed: from the design considerations of optical band gap tuning, the control and reduction of the impact of surface traps, the consideration of charg...

Aqueous Based Semiconductor Nanocrystals

We use solution NMR techniques to analyze the organic/inorganic interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. It is shown that the resulting Q-CdSe are stabilized by tightly bound oleic acid species that only exchange upon addition of free oleic acid. The NMR analysis points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a second step. Importantly, we find that every ligand is involved in this exchange process. By addition of oleic acid with a deuterated carboxyl headgroup, we demonstrate that the bound ligands are oleate ions and not oleic acid molecules. This explains why a dynamic adsorption/desorption equilibrium only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the number of oleate ligands to the excess cadmium per CdSe quantum dot, we find a ratio of 2:1. This completes the picture of Q-CdSe as organic/inorgani...

Utilizing self-exchange to address the binding of carboxylic acid ligands to CdSe quantum dots.

Nearly monodisperse zinc-blende CdSe/CdS core/shell nanocrystals were synthesized by epitaxial growth of 1–6 monolayers of CdS shell onto presynthesized zinc-blende CdSe core nanocrystals in one pot To retain the zinc-blende structure, the reaction temperature was lowered to the 100–140 °C range by using cadmium diethyldithiocarbamate as a single-source precursor and primary amine as activation reagents for the precursor Although the wurtzite counterparts grown under the same conditions showed optical properties similar to those reported in the literature, zinc-blende CdSe/CdS core/shell nanocrystals demonstrated surprisingly different optical properties, with ensemble single-exponential photoluminescence decay, significant decrease of photoluminescence peak width by the shell growth, and comparatively high photoluminescence quantum yields The lifetime for the single-exponential ensemble photoluminescence decay of zinc-blende CdSe/CdS core/shell nanocrystals with 3–4 monolayers of CdS shell was reprodu

Crystal structure control of zinc-blende CdSe/CdS core/shell nanocrystals: synthesis and structure-dependent optical properties.

Although wurtzite cadmium selenide quantum dots (wz-CdSe QDs) are one of the best explored colloidal nanomaterials, no detailed investigation of the optical properties of zincblende cadmium selenide quantum dots (zb-CdSe QDs) has been performed until now. Typically, it is assumed that this material shows the same behavior as the wurtzite modification. To investigate this, we present a study on the optical properties of zb-CdSe QDs, yielding the electronic band gap to size relation (sizing curve), the extinction coefficient at short wavelengths, and the oscillator strength of the band gap transition. Comparing these results with literature data on wz-CdSe QDs we observe, despite a deviation of the sizing curve for diameters above 4 nm, a similar extinction coefficient at short wavelengths and a similar oscillator strength.

Optical Properties of Zincblende Cadmium Selenide Quantum Dots

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Optical Properties of Zincblende Cadmium Selenide Quantum Dots