Growth of CdSe Quantum Rods and Multipods Seeded by Noble-Metal Nanoparticles

Semiconductor nanocrystals (NCs) have emerged as an important class of materials because of their tunable optoelectronic properties that arise from quantum size effects. They can be used as active components in functional nanocomposites, chemical sensors, biomedicine, optoelectronics, and nanoelectronics. More recently, NCs of different shapes, including rods, bipods, tripods, tetrapods, and cubes have been fabricated. These nonspherical NCs serve as ideal model systems for studying anisotropic optoelectronic effects, including polarized emission, and quantum rod (QR) lasing. They may also serve as building blocks for complex nanostructures in nanoelectronics and nanomedicine. Here, we report the preparation of CdSe QRs and multipods via heterogeneous nucleation on noble-metal nanoparticles. This allows their preparation at much milder conditions (225 °C, 0.066 M CdO) than previously reported (> 260 °C, > 0.4 M CdO). The CdSe NCs initially grow as multipods that subsequently cleave to form freestanding QRs with very high photoluminescence quantum yields (PL QYs). Template-free shape control during the growth of NCs depends on the ability to achieve different growth rates on different crystal faces within the same NC. This occurs in an anisotropic crystal structure, such as the wurtzite structure of CdSe, when a single growth direction is favored over others. In this system, polymorphism is also possible, and a key parameter is the energy difference between different polymorphs. In the case of CdSe and CdTe, NCs may nucleate with the zinc blende structure, followed by growth of the wurtzite structure on these nuclei to produce tetrapods. The energy difference between the two crystal structures is small enough that both are accessible at typical reaction temperatures. This mechanism has been associated with the observation of kinetically promoted tetrapod structures of CdSe and CdTe. Generally, the colloidal growth of nonspherical NCs is achieved by one of two methods. In one approach, the reaction is carried out in the presence of two surfactants with significantly different binding abilities to the NC faces, such as phosphonic acid and a long-chain carboxylic acid or amine. The strongly adsorbed phosphonic acid slows the growth of the NC and results in a preferential growth along the c-axis of the wurtzite structure. In this method, a high precursor concentration is maintained, often via multiple injections of the precursors into the reaction pot during the growth of the NC. A mixture of carboxylic acid and amine, without a phosphonic acid, does not induce anisotropic NC growth, but yields spherical NCs (Fig. S1 in the Supporting Information). Another approach is the solution–liquid–solid (SLS) method, analogous to the vapor–liquid–solid approach for growing nanowires from vapor precursors. This method uses metallic nanoparticles as seeds to promote anisotropic crystal growth. The metallic seed particles melt, precursor atoms dissolve in them, and crystal growth occurs at the metal’s liquefied surface. This provides a lower-energy path to nucleation than homogeneous nucleation in the vapor or solution phase. NC rods or wires of materials including InP, InAs, and Si have been prepared using metallic nanoparticles as seeds. Growth of CdSe wires by the SLS method using bismuth-coated gold nanoparticles has been reported, although those experiments were carried out using technical grade (90 %) trioctylphospine oxide containing phosphonic acids that may also promote anisotropic growth. To the best of our knowledge, the use of pure noble-metal nanoparticles to aid the growth of nonspherical II–VI NCs has not previously been demonstrated. Here, we report a single-pot colloidal synthesis of CdSe multipods and rods using nanoparticles of pure Au, Ag, Pd, or Pt as seeds. The CdSe multipods occur both as simple homogeneous multipods and as heteromultipods with the Au particle at the center of the structure (where Au is the seed), as shown schematically in Figure 1. The CdSe particles were produced using each type of metal nanoparticle (see Supporting C O M M U N IC A TI O N S