Nucleation and Growth of CdSe on ZnS Quantum Crystallite Seeds, and Vice Versa, in Inverse Micelle Media

Abstract

Composite semiconductor crystallites involving CdSe grown on an ZnS seed,and vice versa, have been synthesized and "capped" with organic ligands in inverse micelle solutions. These composite particles, as well as capped seed crystallites of CdSe and ZnS, are isolated, purified, and characterized for relative atomic composition, structure, and electronic properties. The Debye X-ray scattering equation, when solved for these layered particles, shows that powder X-ray scattering is insensitive to a small foreign inclusion. A simple theoretical model for the LUMO and HOMO of layered crystallites shows that a small (<15-A diameter) interior foreign seed causes only small shifts of the lowest excited state, to either higher or lower energies. The capped CdSe seed and the capped CdSe portion of the layered particle grown on a ZnSe seed undergo low-temperature (169 "C) annealing to give near-single-crystal X-ray scattering. However, CdSe annealing is blocked by a surface ZnS layer which is ca. 4 A thick. While growth to make composite particles does occur, neither particle shows evidence for epitaxial growth.Semiconductor quantum crystallites (20-60-A diameter) have incomplete band structure development, even though the bulk unit cell is present. Their optical and electronic properties are size dependent, and within the past 5 years synthetic procedures have been developed to prepare and stabilize uniform crystallites.I~* Inverse micelle solutions provide a medium for synthesis of stable, size-selected semiconductor colloid^.^ CdSe crystallites in inverse micelles do not fuse with one another because of the surfactant, yet they can grow larger if either ionic or organometallic sources of Cd and Se atoms are added.4 This surface reactivity was exploited in an organic "capping" reaction:(1) Steigerwald, M. L.; Brus, L. E.

Quantum Optics: Colloidal Fluorescent Semiconductor Nanocrystals (Quantum Dots) in Single-Molecule Detection and Imaging

“…The alternation of trapping and untrapping events results in intermittent fluorescence, or blinking typical of QDs, visible at the single-molecule level (Nirmal et al 1996) and reduces the overall quantum yield (QY), defined as the ratio of emitted to absorbed photons. One way to eliminate these "trap" states and protect surface atoms from photooxidation and other chemical reactions is to grow a few atomic layers thick protec- (Dabbousi et al 1997;Eychmüller et al 1993;Kortan et al 1990;Mews et al 1994;Peng et al 1997). This "shell" layer can increase the QY to values up to ∼90 % (Reiss et al 2002) by eliminating surface traps and better confining the exciton into the core.…”

Rational synthesis and the structure-property relationships of nanoheterostructures: a combinative study of experiments and theory

“…The earliest mention of NHSs accompanied the nascent stage of nanoscience, as exemplified by the core-shell noble metal or metal chalcogenide (ME) nanoparticles (NPs), in the 1990s. 8 Then, the high-quality core-shell ME NPs 9 were further developed with the advent of the synthesis of high-temperature organometallics that created monodisperse NPs. The synthesis of island-grown NHSs originated from noble-metal loaded metal-oxides (the first example was Ag-ZnO) in 2004.…”

Influence of solvent on the morphology and photocatalytic properties of ZnS decorated CeO2 nanoparticles

“…An example of such a system is CdSe-ZnS core-shell particles prepared from zinc diethyldithiocarbamate Zn(S 2 CNEt 2 ) in an organic medium, such as tri-n-octylphosphine, 22 in the presence of oleylamine as a surfactant additive. 23 Earlier synthesis methods involved two different (and hazardous) precursors: diethyl zinc and bis(trimethylsilyl)sulfide, 24,25 zinc inorganic salts (chlorate and acetate) precipitated by sodium and ammonium sulfide, 26,27 have also been employed. Simpler inorganic precursors (metal oxide and elemental sulfur) have been proposed, however, these required high temperatures (230 C) for dissolution.…”