Chemical vapor deposition (CVD) is one of the most versatile techniques for the controlled synthesis of functional nanomaterials. When multiple precursors are induced, the CVD process often gives rise to the growth of doped or alloy compounds. In this work, we demonstrate the self-assembly of a variety of 'phase-separated’ functional nanostructures from a single CVD in the presence of various precursors. In specific, with silicon substrate and powder of Mn and SnTe as precursors, we achieved self-organized nanostructures including Si/SiO_x core-shell nanowire heterostructures both with and without embedded manganese silicide particles, Mn₁₁Si₁₉ nanowires, and SnTe nanoplates. The Si/SiO_x core-shell nanowires embedded with manganese silicide particles were grown along the <111> direction of the crystalline Si via an Au-catalyzed vapor-liquid-solid process, in which the Si and Mn vapors were supplied from the heated silicon substrates and Mn powder, respectively. In contrast, direct vapor-solid deposition led to particle-free <110>-oriented Si/SiO_x core-shell nanowires and <100>-oriented Mn₁₁Si₁₉ nanowires, a promising thermoelectric material. No Sn or Te impurities were detected in these nanostructures down to the experimental limit. Topological crystalline insulator SnTe nanoplates with dominant {100} and {111} facets were found to be free of Mn (and Si) impurities, although nanoparticles and nanowires containing Mn were found in the vicinity of the nanoplates. While multiple-channel transport was observed in the SnTe nanoplates, it may not be related to the topological surface states due to surface oxidation. Finally, we carried out thermodynamic analysis and density functional theory calculations to understand the 'phase-separation’ phenomenon and further discuss general approaches to grow phase-pure samples when the precursors contain residual impurities.

Atomically thin layers of group VB transition metal dichalcogenides (TMDs) provide a unique platform for studying two-dimensional (2D) superconductivity and charge density waves. Thus far, the bottom-up synthesis of these 2D TMDs has often involved precursors that are corrosive or toxic, and their lateral sizes are typically only a few micrometers. In this paper, we report the growth of NbS₂ nanoflakes with a thickness down to bilayers and a lateral dimension up to tens of micrometers without using harsh chemical species. NbS₂ nanoflakes either standing or lying with respect to the sapphire substrate were obtained by sulfurization of niobium oxide films that were prepared via pulsed laser deposition. Standing nanoflakes are considered to grow epitaxially on the sapphire substrate according to their ordered orientation, while lying nanoflakes with random orientations were grown directly on top of the niobium oxide films. The Raman spectra of the 3R-phase exhibit strong dependence on the layer thickness, where the A₁ mode softens as the layer number decreases. In contrast to the stable bulk NbS₂, the ultra-thin nanoflakes were oxidized on their top surfaces after prolonged exposure to air, as revealed by X-ray photoelectron spectroscopy. Our work explores an important route to synthesize large-size NbS₂ nanoflakes and studies the oxidation process, which is a critical factor to consider if practical applications should be realized in the future.

Bulk PbTe and alloy compounds thereof are well-known thermoelectric materials for electric power generation. Among these alloys, PbSnTe hosts unique topological surface states that may have improved thermoelectric properties. Here we report on the vapor-transport growth and thermoelectric study of high-quality single-crystalline PbTe and PbSnTe nanowires. The nanowires were grown along the < 001 > direction with dominant {100} facets; the chemical compositions of the wires depend strongly on the substrate position in the growth reactor. We measured the thermopower and electrical and thermal conductivities of individual nanowires to determine the thermoelectric figure of merit ZT. Compared to bulk samples, the PbSnTe nanowires showed both improved thermopower and suppressed thermal conductivity, enhancing the ZTs to ~0.018 and ~0.035 at room temperature. The enhanced thermopower may result from the unique topological surface states; the suppression of thermal conductivity may relate to increased phonon-surface scattering. Compared to PbTe nanowires, the PbSnTe wires have lower thermopower but significantly higher electrical conductivities. This study highlights nanostructuring in combination with alloying as an important approach to enhancing the figure of merit ZT of thermoelectric materials.