46815-AC10
Fundamental Investigations of Thermal-to-Electrical Power Conversion in Lithographically Patterned Bismuth Telluride (Bi2Te3) Nanowire Arrays
PbTe and Bi2Te3 are two conventional thermoelectric materials that were discovered in the 1950's. Interest in these materials has been heighted by theoretical predictions that low dimensional versions of these materials, such as quantum wells (2D) and nanowires (1D), will possess much better thermoelectric properties than the bulk 3D materials. These predictions have sparked a lot of work on the synthesis of nanowires composed of these two materials, but there are few measurements of thermoelectric properties for either because of the difficulty associated with making these measurements - a difficulty that is compounded by the fact that the as-synthesized nanowires are often free-standing and relatively short (l < 50 µm).
A technique for patterning nanowires of PbTe on the surfaces of dielectrics would simplify their incorporation into circuits thereby facilitating the measurement of thermoelectric parameters. With this as motivation, we have been developing a method that involves the fabrication of a sacrificial, horizontal template using conventional microfabrication methods. Our method, called lithographically patterned nanowire electrodeposition (LPNE) uses photolithography to prepare a 3-sided "nano-form" into which a nanowire can be electrodeposited using the horizontal nickel edge that defines the back surface of this nano-form. The LPNE method can be used to prepare air-suspended PbTe nanowires as follows: First, a 1-2 µm thick photoresist (PR) layer (Shipley S1800 series) is spin-coated onto a glass surface. The nano-form architecture is then created atop this photoresist-covered surface (see Figure): A nickel layer was thermally deposited, a positive-tone PR was coated onto this nickel layer and photopatterned, then (step 1) the sample was irradiated with a UV-lamp to flood-expose the top-photoresist patterns that were not previously exposed/developed. A crucial point is that after this flood-exposure, no developing is carried out. Next (step 2) the exposed nickel is etched using HNO3 to produce a 500 nm deep undercut at the edges of the exposed regions. The horizontal trench formed around the perimeter of the exposed region is the nano-form into which the PbTe nanowires will be electrodeposited. The height of this nano-form equals the thickness of the nickel layer which defines the vertical back of the trench. This nano-form follows the contour of the photopatterned region. PbTe nanowire synthesis occurs next (step 3). In step 4, the (previously exposed) top-PR layer was removed in a developing solution prior to the routine removal of nickel patterns in step 5 leaving free-standing nanowires on top of an intact and unexposed layer of bottom-PR.
In addition to permitting the patterning of nanowires onto surfaces, the LPNE method provides a natural way to make arrays of thousands of chemically and structurally identical nanowires. These arrays can be used to carry out high quality characterization on oriented samples using powder x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). We have used the feedback provided by these techniques to refine our synthesis conditions, culminating in nanowires that are stoichiometric, crystalline, and highly pure .
In our future work, we will be advancing from this point in two main directions: First, we will be attempting to synthesize suspended nanowires composed of a second material: Bi2Te3. Secondly, we are developing reliable methods for measuring the thermoelectric properties of these nanowire arrays especially their thermal conductivity, Seebeck coefficients, and electrical conductivities. The final objective of this project is to determine whether highly efficient thermoelectric materials can be obtained by forming conventional materials, like PbTe and Bi2Te3, into nanowires and if so, what nanowire parameters (length, width, and composition) permit the optimization of their thermoelectric performance.
