Efficient thermoelectric power conversion and cooling requires materials with high electrical conductivity s, high Seebeck coefficient a, and low thermal conductivity k.[1–3] Theory predicts that nanostructuring can increase the figure of merit ZT (¼ sa2T/k) beyond the bulk value, owing to k decrease from enhanced phonon scattering at nanostructure boundaries,[4] and increase in s and a from quantum confinement.[1, 5] The promise of higher ZT in nanowires and nanoparticles owing to a greater degree of confinement than in 2D quantum wells [1] has stimulated the exploration of new approaches to synthesize nanostructures of bismuth telluride (Bi2Te3-based thermoelectric materials have the highest reported ZT in the bulk form [1, 6]) and its alloys.[7–9] Nanorods are of particular interest because they are suitable for building heat-pumping circuits for device cooling, and allow the study of thermal and electrical properties of individual nanostructures through contact formation. Aligned Bi2Te3 nanorods can be obtained by electrochemical deposition in porous alumina templates,[10, 11] but are typically polycrystalline and exhibit low charge-carrier mobility.[1] Soft-templating approaches utilizing molecular agents are attractive for synthesizing single-crystal nanorods,[12] which are more conducive for high ZT. Moreover, soft-templating can facilitate template removal during processing and harvest template–nanostructure interactions for passivation or doping.[12]

Here, we demonstrate a new approach to obtain core/shell bismuth telluride/bismuth sulfide nanorods with shell branching by using a biomolecular surfactant, L-glutathionic acid (LGTA). We show that crystallographic twinning of Bi2S3 driven by Bi-LGTA ligand desorption is the primary mechanism of shell branching, which can be controlled by adjusting the LGTA concentration, reaction temperature, and time. Such branched nanostructures and their formation mechanism are different from the nanotetrapod heterostructures of CdSe and CdTe [13–15] obtained by exploiting lattice mismatch between allotropic polytypes. In a typical synthesis, we added orthotelluric acid to aqueous BiCl3 in concentrated HNO3 mixed with LGTA. The solutions were reffuxed in a mixture of ethylene glycol and polyethylene glycol (PEG) at 140 or 195 8C for up to 24hours. Scanning electron microscopy (SEM) images from samples obtained by quenching the reactions at different times reveal nanorods (see Fig. 1) that, upon closer examination, are rectangular prisms with slightly convex facets and lengths ranging between 100–4500nm. The lateral dimensions span 35–290nm, and the width of the largest area face of the prism is a factor of 2–3 greater than that of the smaller-face dimension. The nanorod morphology strongly depends on the reaction temperature and time, and LGTA concentration. Reactions with low LGTA/Bi3þ ratios at 1408C for 7hours treaction 24hours yield branched nanostructures (Fig. 1a–c). High LGTA/Bi3þ ratios result in unbranched nanorods that are longer and wider (Fig. 1d and e). Increasing the reaction temperature to 195 8C results in longer nanorods, with the reappearance of branches after reffuxing for 5 hours (Fig. 1f). Carrying out the reaction at 195 8C for 24hours yields macroscopic agglomerates, likely resulting from thermally induced degradation of the soft templates. For both LGTA/Bi3þ concentration levels, the nanorod length and width increase with increasing reaction time (see Supporting Information Fig. S1), with the latter being smaller.