Ligand Controlled Growth of Aqueous II-VI Semiconductor Nanoparticles and Their Self-Assembly

Abstract

Colloidal semiconductor nanoparticles (NPs) contain hundreds to thousands of atoms in a roughly spherical shape with diameters in the range of 1-10 nm. The extremely small particle size confines electron transitions and creates size tunable bandgaps, giving rise to the name quantum dots (QDs). The unique optoelectronic properties of QDs enable a broad range of applications in optical and biological sensors, solar cells, and light emitting diodes. The most common compound semiconductor combination is chalcogenide II-VI materials, such as ZnSe, CdSe, and CdTe. But III-V and group IV as well as more complicated ternary materials have been demonstrated. Coordinating organic ligands are used to cap the NP surface during the synthesis, as a mean of protecting, confining, and separating individual particles. This study investigated the impact of the ligand on particle growth and self-assembly into hierarchical structures. ZnSe QDs were synthesized using an aqueous route with four different thiol ligands, including 3-mercaptopropionic acid (MPA), thioglycolic acid (TGA), methyl thioglycolate (MTG), and thiolactic acid (TLA). The particle growth was monitored as a function of reaction time by converting the band gaps measured using UV-vis spectroscopy into particle sizes. A kinetic model based on a diffusion-reaction mechanism was developed to simulate the growth process. The growth data were fit to this model, yielding the binding strength in the order TLA < MTG ≈ TGA < MPA. This result showed the relationship between the QD growth rates and the chemical structures of the ligands. Ligands containing electron-withdrawing groups closer to the anchoring S atom and branching promoted growth, whereas longer, possibly bidendate, ligands retarded it. Removing TGA ligands from the surface of CdTe QDs in a controlled manner yielded new superstructures that were composed of either intact or fused particles. Purifying as-synthesized QDs by precipitating them using an anti-solvent removed most of the free ligand in solution. Aging this purified QD suspension for a week caused self-assembly of QDs into nanoribbons. The long time needed for self-assembly was due to the slow equilibrium between the ligands on QD surface and in solution. Accelerating the approach to equilibrium by diluting purifed CdTe QDs with organic solvents triggered rapid self-assembly of superstructures within a day, forming various nanostructures from nanoribbons to nanoflowers. The type of nanostructures that formed was determined by the solvation of TGA in the trigger solvent. Extracting the smallest portion of TGA in methanol promoted vectorial growth into ribbons consistent with dipole-dipole attractive and charge-charge repulsive interactions. Removing more of the TGA layer in IPA caused the dots to fuse into webs containing clustered ribbons and branches, and the directional nature of the superstructure was lost. Completely deprotecting the surface in acetone promoted photochemical etching and dissolved the QDs, yielding ower-like structures composed of CdS. Nanocrystal (NC) growth mediated by a ligand was also studied in the organic synthesis of FeS₂ nanocubes. Oleylamine was used not only as the ligand but also the solvent and reductant during the reaction. A one hour reaction between iron (II) chloride and elemental sulfur in oleylamine at 200 ℃ and a S to Fe ratio of 6 yielded phase pure pyrite cubes with dimensions of 87.9±14.1 nm. X-ray diffraction (XRD) spectra and Raman peaks for pyrite at 340, 375, and 426 cm⁻¹ confirmed phase purity. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results showed that the oleylamine remained on the FeS₂ surface as a ligand. The reaction mechanism includes the production of pyrrhotite Fe₁₋ᵪS (0≤x<0.5) via reduction of S⁰ to S²⁻ by oleylamine and the oxidation of pyrrhotite to pyrite with remaining S⁰.