Electrochemical fabrication of textured Bi2Te3 nanowire arrays

Initiated by the theoretical works of Hicks and Dresselhaus [1], the interest in nanostructured materials as more efficient thermoelectric materials has increased enormously. Recent experimental results on structures ranging from superlattices [2] to Si nanowires [3,4] emphasize the potential of tailoring materials properties by implementing finiteand quantum-size effects to circumvent classical limitations. In particular, Bi2Te3 nanowires are expected to exhibit important enhancements of the thermoelectric efficiency in comparison to their bulk counterparts. Our research focuses on two important aspects of nanoscale thermoelectrics: (i) fabricating thermoelectric nanowires with very small well-controlled diameters, serving as low-dimensional model systems, and (ii) demonstrating that the preferential crystalline orientation of the nanowires can be tuned by the synthesis parameters. Bi2Te3 nanowire arrays were electrochemically grown in a thermostated 3-electrode setup with a saturated calomel electrode (SCE) using ion-track etched polycarbonate (PC) membranes with controlled geometrical characteristics prepared at GSI. PC foils with thicknesses of 10 μm (Makrofol KG, Bayer) and 30 μm (Makrofol N, Bayer) were irradiated at the UNILAC with heavy ions (fluence: 10 – 5x10 ions/cm) and subsequently etched in 6M NaOH at 50°C. Figure 1 shows STEM-in-SEM images of segments of Bi2Te3 wires grown in 10 μm thick PC. Systematic investigations to attain smaller structures led to fabrication of nanowires with average diameter 19 ± 2 nm.