Synthesis of Superconducting Nanocables of FeSe Encapsulated in Carbonaceous Shell

The recent discovery of superconductivity in iron selenide has attracted considerable attention due to the simplicity of composition, unconventional nature of superconductivity, and ease of synthesis. We have synthesized superconducting FeSe nanowires with a simple catalyst-aided vapor transport reaction at 800 °C in an inert atmosphere. The precursors were chosen to be elemental Se and iron acetylacetonate [Fe^III(C₅H₈O₂)₃]. These vaporized very easily, thereby facilitating transport, and also contributed to the formation of a carbonaceous shell encapsulating the FeSe nanowires. The superconductivity of these nanocables was confirmed through magnetic measurements and a <i>T</i>_c of ≈8 K was obtained for an ensemble of nanocables. The length of FeSe filling inside the carbon nanofibers could be varied by controlling the reaction conditions while the diameter of nanowires was dependent on the thickness of Au–Pd coating used as a catalyst. Extensive analysis through high-resolution microscopy revealed that there was considerable lattice contraction of FeSe in the nanocable up to about 3.6% along the <i>c</i>-direction leading to a reduced spacing between the (001) lattice planes. Interestingly, this compression was more pronounced near the catalyst-FeSe interface and was reduced further along the length of the nanocable. The presence of carbon nanofibers as a shell around the FeSe protected the FeSe nanowires from both atmospheric O₂ and moisture attack, as was evident from the very long ambient condition shelf life of these nanocables, and also makes them more stable under e-beam irradiation

Enhancement of Superconducting <i>T</i>_c (33 K) by Entrapment of FeSe in Carbon Coated Au–Pd₁₇Se₁₅ Nanoparticles

FeSe has been an interesting member of the Fe-based superconductor family ever since the discovery of superconductivity in this simple binary chalcogenide. Simplicity of composition and ease of synthesis has made FeSe, in particular, very lucrative as a test system to understand the unconventional nature of superconductivity, especially in low-dimensional models. In this article we report the synthesis of composite nanoparticles containing FeSe nanoislands entrapped within an <i>ent</i>-FeSe-Pd₁₆Se₁₅–Au nanoparticle and sharing an interface with Pd₁₇Se₁₅. This assembly exhibits a significant enhancement in the superconducting <i>T</i>_<i>c</i> (onset at 33 K) accompanied by a noticeable lattice compression of FeSe along the ⟨001⟩ and ⟨101⟩ directions. The <i>T</i>_<i>c</i> in FeSe is very sensitive to application of pressure and it has been shown that with increasing external pressure <i>T</i>_<i>c</i> can be increased almost 4-fold. In these composite nanoparticles reported here, immobilization of FeSe on the Pd₁₇Se₁₅ surface contributes to increasing the effect of interfacial pressure, thereby enhancing the <i>T</i>_<i>c</i>. The effect of interfacial pressure is also manifested in the contraction of the FeSe lattice (up to 3.8% in ⟨001⟩ direction) as observed through extensive high-resolution TEM imaging. The confined FeSe in these nanoparticles occupied a region of approximately 15–25 nm, where lattice compression was uniform over the entire FeSe region, thereby maximizing its effect in enhancing the <i>T</i>_<i>c</i>. The nanoparticles have been synthesized by a simple catalyst-aided vapor transport reaction at 800 °C where iron acetylacetonate and Se were used as precursors. Morphology and composition of these nanoparticles have been studied in details through extensive electron microscopy