Surface-Guided Core–Shell ZnSe@ZnTe Nanowires as Radial p–n Heterojunctions with Photovoltaic Behavior

The organization of nanowires on surfaces remains a major obstacle toward their large-scale integration into functional devices. Surface–material interactions have been used, with different materials and substrates, to guide horizontal nanowires during their growth into well-organized assemblies, but the only guided nanowire heterostructures reported so far are axial and not radial. Here, we demonstrate the guided growth of horizontal core–shell nanowires, specifically of ZnSe@ZnTe, with control over their crystal phase and crystallographic orientations. We exploit the directional control of the guided growth for the parallel production of multiple radial p–n heterojunctions and probe their optoelectronic properties. The devices exhibit a rectifying behavior with photovoltaic characteristics upon illumination. Guided nanowire heterostructures enable the bottom-up assembly of complex semiconductor structures with controlled electronic and optoelectronic properties

High Thermoelectric Performance in Crystallographically Textured n‑Type Bi₂Te_3–<i>x</i>Se_<i>x</i> Produced from Asymmetric Colloidal Nanocrystals

In the present work, we demonstrate crystallographically textured n-type Bi₂Te_3–<i>x</i>Se_<i>x</i> nanomaterials with exceptional thermoelectric figures of merit produced by consolidating disk-shaped Bi₂Te_3–<i>x</i>Se_<i>x</i> colloidal nanocrystals (NCs). Crystallographic texture was achieved by hot pressing the asymmetric NCs in the presence of an excess of tellurium. During the hot press, tellurium acted both as lubricant to facilitate the rotation of NCs lying close to normal to the pressure axis and as solvent to dissolve the NCs approximately aligned with the pressing direction, which afterward recrystallize with a preferential orientation. NC-based Bi₂Te_3–<i>x</i>Se_<i>x</i> nanomaterials showed very high electrical conductivities associated with large charge carrier concentrations, <i>n</i>. We hypothesize that such large <i>n</i> resulted from the presence of an excess of tellurium during processing, which introduced a high density of donor Te_Bi antisites. Additionally, the presence in between grains of traces of elemental Te, a narrow band gap semiconductor with a work function well below Bi₂Te_3–<i>x</i>Se_<i>x</i>, might further contribute to increase <i>n</i> through spillover of electrons, while at the same time blocking phonon propagation and hole transport through the nanomaterial. NC-based Bi₂Te_3–<i>x</i>Se_<i>x</i> nanomaterials were characterized by very low thermal conductivities in the pressing direction, which resulted in <i>ZT</i> values up to 1.31 at 438 K in this direction. This corresponds to a <i>ca</i>. 40% <i>ZT</i> enhancement from commercial ingots. Additionally, high <i>ZT</i> values were extended over wider temperature ranges due to reduced bipolar contribution to the Seebeck coefficient and the thermal conductivity. Average <i>ZT</i> values up to 1.15 over a wide temperature range, 320 to 500 K, were measured, which corresponds to a <i>ca</i>. 50% increase over commercial materials in the same temperature range. Contrary to most previous works, highest <i>ZT</i> values were obtained in the pressing direction, corresponding to the <i>c</i> crystallographic axis, due to the predominance of the thermal conductivity reduction over the electrical conductivity difference when comparing the two crystal directions