Magic sized clusters of cadmium selenide have been documented in numerous recent reports and are believed to play crucial roles in nanocrystal synthesis,– including as a solute reservoir for larger nanocrystals and as building blocks for the assembly of sheet and ribbon structures., Many of these studies report a cluster with lowest energy absorption near 420 nm. Based on the energy of the 420 nm transition and a variety of other analytical techniques, including mass spectrometry, a structure with ~1.5 nm dimensions and a molecular formula of (CdSe)33–34 has been proposed., A nonstoichiometric formula, Cd35Se28, with a carboxylate and amine ligand shell has also been proposed. All of these clusters show very similar absorption spectra, though a variety of surfactants and starting cadmium and selenium sources have been used in their preparation.
Magic Size III-V Semiconductor Clusters: Synthesis and Properties
Jiajia Ning, Uri Banin
Institute of Chemistry and the Center for Nanoscience and Nanotechnology,
The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Magic size semiconductor clusters represent a molecular limit for semiconductor nanocrystals. Such clusters, previously mostly studied for II-VI and IV-VI semiconductors, are characterized by well defined size with particular stability often assigned to their closed-shell architecture. is formed only by clusters containing a well-defined number of atoms. Here we synthesized magic size nanoclusters of III-V semiconductors which have been rarely studied, including both magic sized InP nanocrystals (MInP NCs), and InAs clusters. The systems were characterized by absorption, Xray diffraction, and their thermal stability in the synthesis.
Furthermore due to large surface to volume ratio inherent to the clusters, the emission of MInP NCs is very weak. To address this, we also developed and studied the growth of protective high band gap ZnS shell onto the MInP NCs via layer-by-layer method. The properties of such coated MInP NCs were characterized. Transition between different emission properties was observed using both spectral measurements and fluorescence lifetimes. Several mechanisms for the observed emission behavior and the transition will be discussed.
Role of Magic-Sized Clusters in the Synthesis of CdSe Nanorods.
The dynamics of the CdSe nanorod synthesis reaction have been studied, giving attention to the kinetics of magic-sized clusters (MSCs) that form as intermediates in the overall reaction. The MSCs have a distinct absorption peak, and the kinetics of this peak give insight into the overall reaction mechanism. In these studies, the reaction mixture consists primarily of Cd(phosphonate)2 and trioctyl phosphine selenium in a solution of trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO). We find that the rate at which precursors react to form CdSe monomers and the rates at which monomers react to form nanoparticles can be varied by changing the chemistry of the reaction mixture. Decreasing the TOP concentration decreases the extent to which selenium is bound, both in the precursors and on the particles’ surfaces, and thereby increases both the precursor to monomer and monomer to particle reaction rates. Decreasing the phosphonate concentration decreases the extent to which phosphonate binds cadmium in the precursors and on the surface of the nanoparticles, also increasing the rates of both reactions. This is also accomplished by the addition of inorganic acids which protonate the phosphonates. The presence of inorganic acids (impurities) is the primary reason that the overall synthesis reaction is faster in solutions made with technical grade rather than purified TOPO. The TOP and phosphonic acid concentrations are coupled because excess phosphonic acids react with TOP, forming TOPO and less strongly binding species, specifically phosphinic acids, phosphine oxides, and phosphines.