Haoran Yang

Rational synthesis of ultrathin n-type Bi2Te3 nanowires with enhanced thermoelectric properties.

A rational yet scalable solution phase method has been established, for the first time, to obtain n-type Bi(2)Te(3) ultrathin nanowires with an average diameter of 8 nm in high yield (up to 93%). Thermoelectric properties of bulk pellets fabricated by compressing the nanowire powder through spark plasma sintering have been investigated. Compared to the current commercial n-type Bi(2)Te(3)-based bulk materials, our nanowire devices exhibit an enhanced ZT of 0.96 peaked at 380 K due to a significant reduction of thermal conductivity derived from phonon scattering at the nanoscale interfaces in the bulk pellets, which corresponds to a 13% enhancement compared to that of the best n-type commercial Bi(2)Te(2.7)Se(0.3) single crystals (~0.85) and is comparable to the best reported result of n-type Bi(2)Te(2.7)Se(0.3) sample (ZT = 1.04) fabricated by the hot pressing of ball-milled powder. The uniformity and high yield of the nanowires provide a promising route to make significant contributions to the manufacture of nanotechnology-based thermoelectric power generation and solid-state cooling devices with superior performance in a reliable and a reproducible way.

Nontoxic and Abundant Copper Zinc Tin Sulfide Nanocrystals for Potential High-Temperature Thermoelectric Energy Harvesting

Improving energy/fuel efficiency by converting waste heat into electricity using thermoelectric materials is of great interest due to its simplicity and reliability. However, many thermoelectric materials are composed of either toxic or scarce elements. Here, we report the experimental realization of using nontoxic and abundant copper zinc tin sulfide (CZTS) nanocrystals for potential thermoelectric applications. The CZTS nanocrystals can be synthesized in large quantities from solution phase reaction and compressed into robust bulk pellets through spark plasma sintering and hot press while still maintaining nanoscale grain size inside. Electrical and thermal measurements have been performed from 300 to 700 K to understand the electron and phonon transports. Extra copper doping during the nanocrystal synthesis introduces a significant improvement in the performance.

Synthesis and Thermoelectric Properties of Compositional-Modulated Lead Telluride–Bismuth Telluride Nanowire Heterostructures

We demonstrate the rational solution-phase synthesis of compositional modulated telluride nanowire heterostructures containing lead telluride (PbTe) and bismuth telluride (Bi2Te3). By tuning the ratio between PbTe and Bi2Te3 through adjusting the amount of critical reactants and precursors during the synthesis, the influence of composition on the thermoelectric properties of the nanowire heterostructures has been investigated in hot pressed nanocomposite pellets. Measurements of the thermoelectric properties show strongly reduced thermal conductivity that leads to an enhanced thermoelectric figure of merit (ZT) of 1.2 at 620 K.

Enhanced Thermoelectric Properties in Bulk Nanowire Heterostructure-Based Nanocomposites through Minority Carrier Blocking

To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride-silver telluride (PbTe-Ag2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe-Ag2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag2Te nanowire-based nanocomposite produced in similar method, the PbTe-Ag2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe-Ag2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting.

Topological insulator Bi2Te3 films synthesized by metal organic chemical vapor deposition

Topological insulator (TI) materials such as Bi2Te3 and Bi2Se3 have attracted strong recent interests. Large scale, high quality TI thin films are important for developing TI-based device applications. In this work, structural and electronic properties of Bi2Te3 thin films deposited by metal organic chemical vapor deposition (MOCVD) on GaAs (001) substrates were characterized via X-ray diffraction (XRD), Raman spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and electronic transport measurements. The characteristic topological surface states (SS) with a single Dirac cone have been clearly revealed in the electronic band structure measured by ARPES, confirming the TI nature of the MOCVD Bi2Te3 films. Resistivity and Hall effect measurements have demonstrated relatively high bulk carrier mobility of ~350 cm^2/Vs at 300K and ~7,400 cm^2/Vs at 15 K. We have also measured the Seebeck coefficient of the films. Our demonstration of high quality topological insulator films grown by a simple and scalable method is of interests for both fundamental research and practical applications of thermoelectric and TI materials.

Composition Modulation of Ag2Te Nanowires for Tunable Electrical and Thermal Properties

In this article, we demonstrated that composition modulation of Ag2Te nanowires can be achieved during the self-templated transformation of Te nanowires into Ag2Te nanowires during solution phase synthesis, which provides a mean to tune the carrier density of the Ag2Te nanowires. Both nearly stoichiometric and Ag-rich nanowires have been synthesized, which give rise to p-type and n-type Ag2Te nanocomposites after hot press, respectively. The electrical and thermal properties of the two kinds of samples have been measured. Theoretical modeling based on the near-equilibrium Boltzmann transport equations has been used to understand the experimental results. We found that ZT of the heavily doped n-type sample reaches 0.55 at 400 K, which is the highest ZT value reported for Ag2Te at the same temperature mainly due to the reduced thermal conductivity by the nanostructures. Theoretical analysis on the carrier transport shows that the power factor is also very well optimized in the doped Ag2Te sample considering the reduced carrier mobility by the nanostructures.

Flexible Nanocrystal-Coated Glass Fibers for High-Performance Thermoelectric Energy Harvesting

Recent efforts on the development of nanostructured thermoelectric materials from nanowires (Boukai, A. I.; et al. Nature 2008, 451, (7175), 168-171; Hochbaum, A. I.; et al. Nature 2008, 451, (7175), 163-167) and nanocrystals (Kim, W.; et al. Phys. Rev. Lett. 2006, 96, (4), 045901; Poudel, B.; et al. Science 2008, 320, (5876), 634-638; Scheele, M.; et al. Adv. Funct. Mater. 2009, 19, (21), 3476-3483; Wang, R. Y.; et al. Nano Lett. 2008, 8, (8), 2283-2288) show the comparable or superior performance to the bulk crystals possessing the same chemical compositions because of the dramatically reduced thermal conductivity due to phonon scattering at nanoscale surface and interface. Up to date, the majority of the thermoelectric devices made from these inorganic nanostructures are fabricated into rigid configuration. The explorations of truly flexible composite-based flexible thermoelectric devices (See, K. C.; et al. Nano Lett. 2010, 10, (11), 4664-4667) have thus far achieved much less progress, which in principle could significantly benefit the conversion of waste heat into electricity or the solid-state cooling by applying the devices to any kind of objects with any kind of shapes. Here we report an example using a scalable solution-phase deposition method to coat thermoelectric nanocrystals onto the surface of flexible glass fibers. Our investigation of the thermoelectric properties yields high performance comparable to the state of the art from the bulk crystals and proof-of-concept demonstration also suggests the potential of wrapping the thermoelectric fibers on the industrial pipes to improve the energy efficiency.

The effects of the size and the doping concentration on the power factor of n-type lead telluride nanocrystals for thermoelectric energy conversion.

For the first time, we demonstrate a successful synthesis of colloidal n-type lead telluride nanocrystals doped with iodine. By tuning the reaction time and iodine concentration in the precursor solution, nanocrystals with different sizes and doping concentrations are synthesized. The Seebeck coefficient and electrical conductivity of the nanocrystals are measured on nanocrystal thin films fabricated by dip-coating glass substrates in the nanocrystals solution. Investigations on the influence of size and doping concentration on the electrical properties have been performed. The results show that the size of the nanocrystals significantly influences the electrical conductivity but not the Seebeck coefficient of nanocrystal films, while higher doping concentration leads to lower Seebeck coefficient but higher electrical conductivity in the nanocrystal films. Proof-of-concept thin-film thermoelectric modules are also fabricated using both p-type and n-type PbTe nanocrystals for the conversion of thermal energy into electrical energy.

Structure and Thermoelectric Properties of Spark Plasma Sintered Ultrathin PbTe Nanowires

Solution-synthesized thermoelectric nanostructured materials have the potential to have lower cost and higher performance than materials synthesized by solid-state methods. Herein we present the synthesis of ultrathin PbTe nanowires, which are compressed by spark plasma sintering at various temperatures in the range of 405-500 °C. The resulting discs possess grains with sizes of 5-30 μm as well as grains with sizes on the order of the original 12 nm diameter PbTe nanowires. This micro- and nanostructure leads to a significantly reduced thermal conductivity compared to bulk PbTe. Careful electron transport analysis shows suppressed electrical conductivity due to increased short-range and ionized defect scatterings, while the Seebeck coefficient remains comparable to the bulk value. The PbTe nanowire samples are found unintentionally p-type doped to hole concentrations of 2.16-2.59 × 10(18) cm(-3). The maximum figure of merit achieved in the unintentionally doped spark plasma sintered PbTe nanowires is 0.33 at 350 K, which is among the highest reported for unintentionally doped PbTe at low temperatures.

Environmentally benign synthesis of ultrathin metal telluride nanowires.

Metal telluride nanowires are attractive materials for many applications, yet most synthesis recipes require hazardous reducing agents such as hydrazine or sodium borohydride. We describe a two-step synthesis of various metal tellurides with nanowire morphology using a nonhazardous reducing agent, ascorbic acid. In the first step, Te grows one-dimensionally to form ultrathin nanowires; in the second step, these nanowires are converted to metal telluride nanowires by adding metal precursors. Analysis of the reaction products versus time provides insights into the growth and conversion mechanisms as well as the reaction rates.