Regioselective Growth Mechanism of Single Semiconductor Tips on CdS Nanorods

Abstract

Complex, anisotropic nanocrystals made from two or more components are extremely interesting functional materials that can drive directional, light-activated processes like charge separation and photocatalysis. However, while some synthetic protocols exist, little is known about the reaction mechanism for regioselective, heterogeneous nucleation of a second semiconductor material onto nanocrystal seeds. This paper presents the mechanism that leads to growth of a single tip at one end of CdS nanorods with yields between 50 and 80%. It is shown that the growth of only one tip is a result of tight control of the available, nucleating monomer in the reaction solution by working at a large chalcogenide excess. Conditions that facilitate this reaction pathway are characterized by a kinetic barrier to homogeneous growth. These match those for the formation of metastable magic-size clusters. Through this boundary condition, it can be understood why the formation of telluride tips is favored in comparison to selenides and sulfides, for which the regimes for cluster formation and nucleation on surfaces do not overlap.

Chemoselective Surface Trap-Mediated Metal Growth on Semiconductor Nanocrystals

“…CdTe and ZnTe tips on CdSe/CdS dot-in-rod nanocrystals, denoted CdTe@CdSe/CdS (see Figure 1A) and ZnTe@ CdSe/CdS, were synthesized following a protocol that was recently developed in our group, 22 based on previous work by Deutsch et al 23 Metal tips (Pt, Pd, Au) were grown onto these seed particles according to a modified method originally developed by Mokari et al 4 The reaction temperature was set to 200 °C for Pt and Pd, and to room temperature (27 °C) for Au. The particle dispersion turned optically black upon reaction with the added metal precursors.…”

“…CdTe and ZnTe-Tipped Nanorods. Growth of CdTe and ZnTe tips on CdSe/CdS nanorods was performed according to a previously published protocol by Enders et al, 21,22 which was based on work by Deutsch et al 23 Briefly, a 25 mL three-neck flask equipped with a condenser, thermocouple and septum was loaded with 1.44 g of TOPO, 1.6 mL of ODE, and 1.6 mL of oleylamine, and the mixture was degassed under reduced pressure at 80 °C for 1 h. The flask was then flooded with nitrogen and 0.8 mL (16 nmol) of a dispersion of CdSe/CdS seeded nanorods in TOP were added under inert gas conditions. The reaction mixture was heated to 200 °C and 0.48 mL of Te in TOP (0.1 M) were added with a syringe pump over 1 h. This was followed by the addition of 0.24 mL of Cd oleate or ZnEt 2 in ODE (0.1 M) over 30 min, depending on the targeted tip material.…”

Localized Co2+ Doping of CdSe/CdS Seeded Nanorods

“…Locally transition-metal-doped nanoparticles have the potential to be a valuable tool to gain more insight into exciton dynamics, for example, in transient absorption studies, by using the dopants as a probe for exciton localization. This work adds to the growing toolbox of regioselective, colloidal syntheses that provides symmetry breaking and the asymmetric manipulation of nanoparticles beyond previously published studies. − …”

Design Principles of Colloidal Nanorod Heterostructures

“…For example, these two strategies have been combined to form dot-in-rods with a different composition at one of the tips, such as the single CdTe-tipped CdSe/CdS dot-in-rods. 30,64 A similar asymmetric structure, the Cd(Se,Te)/CdS dot-in-rod with a single CdSe tip shown in Figure 2e, exhibits two-photon upconversion photoluminescence. 65,66 Radial heterostructures can also be produced by heteroepitaxy on nanorod side facets.…”

“…From the original development of these structures, a profusion of particle shapes and compositions are beginning to emerge. For example, these two strategies have been combined to form dot-in-rods with a different composition at one of the tips, such as the single CdTe-tipped CdSe/CdS dot-in-rods. , A similar asymmetric structure, the Cd­(Se,Te)/CdS dot-in-rod with a single CdSe tip shown in Figure e, exhibits two-photon upconversion photoluminescence. , …”