Anisotropic Growth of PbSe Nanocrystals on Au-Fe3O4 Hybrid Nanoparticles

Two of the most important research directions in the preparation of nanocrystals are the growth of non-spherical quantum-confined structures and nanocrystal heterostructures. Non-spherical structures provide a means of understanding the effect of dimensionality and structural anisotropy on quantum-confined optoelectronic behavior and also introduce new degrees of freedom in engineering nanocrystal-based devices. Semiconductor nanorods have potential technological advantages over spherical nanocrystals in applications such as polarized light emitters and photovoltaics. Preparation of nanocrystal heterostructures enables new combinations of material properties, such as photonic and magnetic properties, to be achieved. This opens up new possibilities for investigating interactions between nanoscale components of different materials, and new technological applications based on combinations of material properties not attainable in homogeneous nanocrystals. Colloidal chemistry provides the basis of a modular approach for integrating different materials and, therefore, different functionalities in these nanocrystal heterostructures. Anisotropic growth of semiconductors with the wurtzite crystal structure has been achieved using multiple surfactants, exploiting inherent differences in growth rates between different crystal directions, which are modulated by differences in the affinity of surfactants for the different crystal faces. In other cases, a solution-liquid-solid (SLS) method that employs molten metal particles as seeds for heterogeneous nucleation and growth of semiconductor wires has been used. For groups IV, II–VI, and III–V materials, there is a substantial body of knowledge on the growth of anisotropic nanostructures. However, understanding of anisotropic growth of IV– VI materials is less well developed. Hull et al. have prepared long, branched PbSe wires using an SLS approach, and Cho et al. have prepared PbSe nanowires and nanorings via controlled oriented aggregation of PbSe particles. However, these methods do not provide a route to dispersible nanorods with tunable length and aspect ratio that can be chemically processed. Yong et al. recently reported noble metal seeded growth of PbSe nanorods and branched nanostructures using extremely low concentrations of metal nanoparticles. Although the effect of the metal seed particles on PbSe morphology in that study was clear, no direct evidence of PbSe rods growing from the metal seeds was presented, and no free Au seed particles could be found in the reaction products, probably because of the extremely low seed particle concentrations used. Thus, the mechanism of this effect on morphology remained somewhat unclear. For InAs and InP, mixtures of rods and wires of different lengths were synthesized by Kan et al. using Au nanoparticles as seeds, followed by multiple steps of centrifugation to achieve length control. Here, we present a technique to grow PbSe nanocrystals with different morphologies (dotlike, rodlike, and branched) on Au–Fe3O4 binary hybrid nanoparticles. The Au–Fe3O4 binary hybrid nanoparticles are ‘peanut-shaped’, each consisting of a smaller spherical Au nanoparticle attached to, and slightly embedded in, a larger Fe3O4 nanoparticle, as shown in the center of Figure 1 and also in Figure S1 in the Supporting Information. When these particles are used to seed growth of PbSe, the semiconductor nucleates only on the Au portion, initially forming ternary hybrid nanoparticles. By varying the PbSe precursor-to-seed particle ratio, the extent of growth can be controlled to yield a dot-shaped PbSe component (structure 1 in Fig. 1), a rod-shaped PbSe component (structures 2a and 2b), multiple rod-shaped PbSe components (structures 3a and 4a) or a multibranched PbSe component (structures 3b, 4b, and 5). The dominant observed morphology proceeds clockwise from structure 1 to structure 5 as the precursor-to-seed ratio is increased, but multiple morphologies may co-exist in the same sample. Rod-shaped and multibranched PbSe components cleave from the Au–Fe3O4 binary C O M M U N IC A IO N S