Synthesis of lead chalcogenide alloy and core-shell nanowires.

Control over the dimensions and shape of nanostructures represents one of the main challenges in modern materials science. Morphology control of a variety of materials can be achieved using vapor–liquid–solid or solution–liquid–solid techniques to obtain one-dimensional (1D) systems. The unique optical and electrical properties of 1D nanostructures make them one of most important building blocks for nanoscience and nanotechnology applications, and provide the opportunity for their integration in electronic, photonic, thermoelectric, and sensor-based devices. Size control has been traditionally important and necessary to tune the optical and electrical properties of nanomaterials by changing the band gap. This is particularly important in the strong confinement region, where one of the dimensions is smaller than the corresponding excitonic Bohr diameter. Semiconductor alloy and core–shell nanowire systems represent another interesting direction towards functional nanostructures with enhanced structural and property tunability. Herein, we focus on preparing novel 1D heterostructures of IV–VI semiconductor nanomaterials. Lead chalcogenides are known to be good materials for thermoelectrics due to their low thermoconductivity. Pseudobinary (e.g. PbSeTe) and pseudoternary alloys (e.g. PbSnSeTe) have even lower lattice thermal conductivities than the binary compounds due to disorder-induced phonon scattering processes. Lead chalcogenide materials are also good candidates for multiexciton-generation (MEG) solar cells. For example, previous reports showed quantum efficiencies as high as 300% and 700% for PbSe nanoparticles. Heterostructured alloy and core–shell nanomaterials have previously been shown for various materials, mainly II–VI semiconductor nanocrystals. For example, a quasi 1D system of CdSe–ZnS has been reported, other systems include PbSe–PbS core–shell and alloy spherical nanoparticles developed by Lifshitz and co-workers. In addition, Talapin et al. have demonstrated the growth of PbS and Au onto PbSe nanowires. The physical properties of these heterostructured nanosystems are of interest for various applications as shown by the electronic structure calculations carried out by different groups. Here we demonstrate the formation of lead chalcogenide heterostructure nanowires by a solution-phase synthesis at moderate temperatures (see the Experimental Section). Two types of heterostructures (alloy and core–shell) were prepared by changing the concentration and temperature of the reaction. We were able to control the composition of the alloy and the thickness of the shell by changing the growth parameters. Three different systems, PbSexS1 x alloys, and PbSe–PbS and PbSe–PbTe core–shell nanowires were prepared. Achieving these three targeted structures is nontrivial due to various competitive processes such as ripening and formation of pure PbS (PbTe) nanoparticles. The synthesis of PbSe nanowires is based on a previous report by Murray and co-workers. The same procedure was used to prepare the PbSe nanowires used here as templates for further growth to give the alloy and core–shell nanostructures. The diameter of the core nanowires could be controlled and varied from 4 nm up to 100 nm, with a length of a few tens of micrometers. The PbSe nanowires (Figure 1A) were used as templates to form PbSexS1 x alloy wires. Figure 1B shows PbSe0.4S0.6 alloy nanowires that were prepared by the slow addition of Pb and S precursors to a hot solution containing PbSe nanowires. (a detailed description of the synthesis can be found in the Experimental Section). The diameter of the alloy nanowires increased from 6 nm (pure PbSe nanowires) to ca. 10 nm, indicating the incorporation of additional material into the nanowires. Structural characterization of the alloy system was carried out using various methods as shown in Figure 1. Figure 1D shows a high-resolution transmission electron microscopy (HRTEM) image of the PbSe0.4S0.6 nanowires. The latticeresolved image indicates that the nanowires are growing along the h100i direction. X-ray diffraction (XRD) measurements of the alloy nanowires are shown in Figure 1C. The pattern can be indexed to a structure intermediate between the cubic PbSe and cubic PbS bulk phases, which strongly supports the formation of an alloyed structure. An energydispersive X-ray (EDX) spectrum (Figure 1E) taken on a small area of the alloy nanowire, shown in Figure 1D, indicates the presence of Se from the original PbSe nanowires, Pb from the original and added materials, and Cu from the TEM grid. However, due to overlap between the Pb and S peaks, electron energy loss spectroscopy (EELS) was necessary to detect the incorporation of S. The energy loss peak for S was observed at 165 eV (Figure 1F), providing clear evidence for the existence of S in the alloy nanowires. The EDX and EELS spectra were taken from the same area of the nanowire shown in Figure 1D. Tuning the alloy composition can be achieved by simply controlling the reaction conditions. For example, altering the S concentration will act to tune the alloy composition. The actual composition was determined by [*] Dr. T. Mokari, S. E. Habas, Prof. P. Yang Department of Chemistry, University of California Berkeley, CA 94720 (USA) Fax: (+1)510-642-7301 E-mail: [email protected]