Colloidal nanostructures as building blocks for macroscopic thermoelectric materials with electron-crystal phonon-glass
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1267-DD08-12
Colloidal nanostructures as building blocks for macroscopic thermoelectric materials with electron-crystal phonon-glass properties Marcus Scheele1, Niels Oeschler2, Katrin Meier2, Andreas Kornowski1, Christian Klinke1, and Horst Weller1 1
University of Hamburg, Institute of Physical Chemistry, Grindelallee 117, 20146 Hamburg, Germany 2 Max Planck Institute of Chemical Physics of Solids, Noethnitzer Strasse 40, 01187 Dresden, Germany ABSTRACT We demonstrate the shape- and size-controlled synthesis of colloidal ~10 nm bismuth telluride nanoparticles stabilized by organic ligands in solution. Post-synthetic ligand exchange with oleic acid allows for a quick and simple ligand removal by consecutive washing with basic ammonia solution. Mild spark plasma sintering yields a macroscopic nanostructured bulk solid with nanograins unaltered in size and shape. We present the full thermoelectric characterization with an emphasis on the thermal properties of this material. It will be shown that thus prepared nanostructured bulk solids possess significantly altered physical properties typical for materials with high surface-to-volume-ratios. These alterations have the potential to lead to improved thermoelectric performances benefiting from their phonon-glass electron-crystal behavior. INTRODUCTION Fabrication methods of nanostructured bulk solids can generally be divided into topdown and bottom-up techniques. As an example for the former, ball-milling of macroscopic ingots to nanograins followed by hot-pressing has led to remarkable improvements in the thermoelectric figure of merit (ZT) of these materials [1]. Bottom-up techniques such as hydrothermal [2] and wet-chemical methods [3] have been exploited as an alternative to nanograin synthesis followed by sintering to macroscopic pellets. To control the rate of crystal grain growth and stabilize the highly energetic surfaces of small nanograins, long-chained coordinating organic molecules such as thiols, carboxylic acids, or amines are added to the reaction mixture. These molecules, referred to in the following as ligands, play a key role in bottom-up synthesis and allow excellent control over shape, size, and size distribution of the nanograins. Since thermoelectric properties are known to depend strongly on these parameters for grains on the nanoscale [4], bottom-up nanograin synthesis offers exciting possibilities to designing high-performance thermoelectric materials. PbSe nanograins obtained by wetchemical synthesis were reported to show a remarkable increase in thermopower attributed to sharp spikes in the density of states [5]. However, a major draw-back in previous works was the often low electric conductivity in the final nanostructured bulk solids. This can immediately be understood when imagining the two component structure of individual nanograins obtained by wet-chemical synthesis: The inorganic, highly conductive nanograin core and the organic, insulating shell made of ligands protecting the core. As necessary as the presence of the ligands was throu
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