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Axial Heterostructures with Phase-Controlled Metastable Segments via Post-Growth Reactions of Ge Nanowires
摘要： Metastable crystal phases promise new functionalities by providing access to materials properties beyond those of thermodynamically stable crystalline solids. While realizing metastable phases in the bulk requires non-equilibrium processes such as rapid quenching or high pressures, size effects at the nanometer scale can promote their formation and stabilization during mild processing. Here we demonstrate the controlled formation of axial heterostructures of alternating stable semiconductor (Ge) and metastable metallic (AuGe) segments via post-growth processing, by decorating Ge nanowires with Au, encapsulating them in graphitic carbon shells, annealing at moderate temperatures to induce alloying of Au and Ge in the melt, followed by slow cooling and crystallization. The process is extended to form heterostructures of binary AgGe and ternary AuAgGe alloy segments in Ge nanowires. The metallic alloys adopt different metastable crystal phases that are selected via the nanowire diameter. This phase selection, explained by a size-dependent solubility of Ge in liquid AuGe (and AgGe) and distinct structural motifs in Ge-rich AuGe melts, allows establishing a previously unknown metastable γ-AgGe metastable in thin axially segmented Ge-AgGe nanowires. The findings demonstrate an avenue for synthesizing axial heterostructures of stable and metastable solid phases in nanowires.
关键词： heterostructures,size effects,AuGe,AgGe,metastable phases,nanowires,phase selection,AuAgGe
更新于2025-09-11 14:15:04
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crystals
摘要： Transition-metal dichalcogenides open novel opportunities for the exploration of exciting new physics and devices. As a representative system, 2H -MoS2 has been extensively investigated owing to its unique band structure with a large band gap, degenerate valleys, and nonzero Berry curvature. However, experimental studies of metastable 1T polytypes have been a challenge for a long time, and electronic properties are obscure due to the inaccessibility of single phase without the coexistence of 1T (cid:2), 1T (cid:2)(cid:2), and 1T (cid:2)(cid:2)(cid:2) lattice structures, which hinder the broad applications of MoS2 in future nanodevices and optoelectronic devices. Using Kx (H2O)yMoS2 as the precursor, we have successfully obtained high-quality layered crystals of the metastable 1T (cid:2)(cid:2)(cid:2)-MoS2 with √3a superstructure and metastable 1T (cid:2)-MoS2 with a × 2a superstructure, as evidenced by structural characterizations through scanning tunneling microscopy, Raman spectroscopy, and x-ray diffraction. It is found that the metastable 1T (cid:2)-MoS2 is a superconductor with onset transition temperature (Tc) of 4.2 K, while the metastable 1T (cid:2)(cid:2)(cid:2)-MoS2 shows either superconductivity with Tc of 5.3 K or insulating behavior, which strongly depends on the synthesis procedure. Both of the metastable polytypes of MoS2 crystals can be transformed to the stable 2H phase with mild annealing at about 70 °C in He atmosphere. These ?ndings provide pivotal information on the atomic con?gurations and physical properties of 1T polytypes of MoS2.
关键词： Transition-metal dichalcogenides,MoS2,metastable phases,superconductivity,structural characterization
更新于2025-09-10 09:29:36