Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40 nm diameter Bi 2 Te 3 nanowires/alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2ϫ 10 −7 m 2 s −1 at 150 K to 6.9ϫ 10 −7 m 2 s −1 at 300 K and is lower than thermal diffusivity of unfilled

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... alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride. Recent developments in nanostructured thermoelectric materials show that highly efficient thermoelectric energy conversion could be forthcoming. 1 Among emerging nanostructures, thermoelectric nanowires receive considerable attention from researchers. 2-5 Bismuth telluride ͑Bi 2 Te 3 ͒ nanowires are particularly interesting because bulk Bi 2 Te 3 is the most efficient of materials used in thermoelectric applications near room temperature. They are also appealing for device applications because can be fabricated inexpensively through electrochemical deposition in nanochanneled alumina templates. [6] [7] [8] [9] The templates are also perceived to serve as structural support for the fragile nanowires. 10 However, heat leakage through the anodic alumina layer may affect the overall thermoelectric efficiency. Knowledge of the thermal transport in Bi 2 Te 3 nanowires/alumina nanocomposite is essential for device applications. The thermal properties of the nanocomposite can be also used to gain insight into the thermal properties of embedded nanowires. Thermal conductivity measurements using the 3 method 11,12 have been reported for Si nanowires embedded in a polymer matrix. 13 This letter reports experimental thermal properties of a 40 nm Bi 2 Te 3 nanowires/alumina nanocomposite over the temperature range between 150 K and 300 K. The nanocomposite consists of porous anodic alumina of 30% porosity with highly ordered parallel cylindrical pores of 40 nm di-ameter filled with Bi 2 Te 3 in proportion better than 95% through electrodeposition. Temperature-dependent thermal diffusivity characterization was carried out with a photothermoelectric technique, described briefly in the following section. The magnitude of the thermal conductivity reduction in 40 nm Bi 2 Te 3 nanowires in the direction parallel to the wire axis is assessed from the measured thermal diffusivity using an effective medium model. Temperature-dependent thermal diffusivity characterization of a 40 nm Bi 2 Te 3 alumina nanocomposite has been carried out with a photothermoelectric technique. 14,15 The experimental setup is schematically represented in Fig. 1 . In this method ac optical heating was applied to the front side of the sample and the temperature rise was detected at the a)