Shape, Size, and Assembly Control of PbTe Nanocrystals

Abstract

The small band gap and the large Bohr radius 1 of the Pbchalcogenide enable unique optical, electrical, and chemical properties, presenting this family of materials as a good candidate for potential application in solar cells, 2-4 thermoelectric (TE) 5 devices, telecommunication, 6 field effect transistors (FET), Tremendous efforts have been put toward controlling the properties of the nanocrystals by controlling their size due to quantum confinement phenomena. Several groups have reported controlled synthesis of PbTe nanocrystals, 18 all obtaining cube-shape structures with sizes ranging from 15 to 20 nm. Herein, we demonstrate the synthesis of PbTe nanocrystals with controllable shape and size. We establish a new method for controlling the shape of Pb-chalcogenide materials by controlling the reaction kinetic, using proper surfactant, the right temperature, and changing the molar ratio between Pb and Te. The reaction was carried out using trioctylphosphine (TOP) or diphenyl ether (DPE) as the growth solvent and phosphonic acid or amines (hexadecylamine, HDA, and dodecylamine, DDA) as the stabilizer surfactant. Two separate precursor solutions were prepared for Pb and Te. The Pb solution contained Pb-acetate trihydrate and oleic acid in TOP or DPE, and the Te solution was prepared by dissolving Te powder in TOP (see Supporting Information). The precursor was first injected at 250°C followed by a constant growth temperature of 170-180°C for 3-4 min. The PbTe nanoparticle structures were studied by high-resolution transmission electron microscopy (HRTEM) and are shown to be single-crystalline as presented in Achieving shape control using surfactant becomes one of the most studied effects in the nanoparticle shape control field. To study the effect of the surfactant, we have also tested tetradecylphosphonic acid (TDPA) or one of its derivatives (octadecyl-and hexylphosphonic acids), instead of the amines. The phosphonic acid leads to a slow growth rate; it takes ∼10 s for the nucleation to take place after the precursors' injection (change in the color of growth solution from colorless to black) compared to an immediate nucleation when the amine is used. This is due to the strong binding of the phosphonic acid to the nanoparticles' surface. The ratio between the precursors (Pb:Te) was also examined for this surfactant, and it was changed from 5:1 to 1:5. Cube particles were obtained when the ratio between the Pb:Te precursors is 1:5. However, the cuboctahedra were produced when the ratio for Pb:Te is 5:1 to 1:3. Octahedral particles were not achieved by using the phosphonic acid surfactant, and again this could be related to the fact that growing octahedra nanocrystals required a fast growth rate of the (100) direction compared to the (111) facet, and this is not achievable using phosphonic acid surfactant since the growth rate is slower using phosphonic acid. Size uniformity of the cubic, cuboctahedral, and octahedral nanoparticles prepared using TOP and phosphonic acid can be controlled by the proper choice of surfactants and the reaction time (cuboctahedra shown in The cubic, octahedral, and cuboctahedral shape PbTe particles could have a special interest for thermoelectric and photovoltaic application once they are processed into thin film form. We have so far already established an approach for assembling the PbTe nanocrystals and succeeded in preparing a large area close-packed film using the Langmuir-Blodgett technique, as shown i