Band-inverted gaps in InAs/GaSb and GaSb/InAs core-shell nanowires

Exploring Strategies toward Synthetic Precision Control within Core-Shell Nanowires

ConspectusAchieving precision and reproducibility in terms of physical structure and chemical composition within arbitrary nanoscale systems remains a "holy grail" challenge for nanochemistry. Because nanomaterials possess fundamentally distinctive size-dependent electronic, optical, and magnetic properties with wide-ranging applicability, the ability to produce homogeneous and monodisperse nanostructures with precise size and shape control, while maintaining a high degree of sample quality, purity, and crystallinity, remains a key synthetic objective. Moreover, it is anticipated that the methodologies developed to address this challenge ought to be reasonably simple, scalable, mild, nontoxic, high-yield, and cost-effective, while minimizing reagent use, reaction steps, byproduct generation, and energy consumption.The focus of this Account revolves around the study of various types of nanoscale one-dimensional core-shell motifs, prepared by our group. These offer a compact structural design, characterized by atom economy, to bring together two chemically distinctive (and potentially sharply contrasting) material systems into contact within the structural context of an extended, anisotropic configuration. Herein, we describe complementary strategies aimed at resolving the aforementioned concerns about precise structure and compositional control through the infusion of careful "quantification" and systematicity into customized, reasonably sustainable nanoscale synthetic protocols, developed by our group. Our multipronged approach involved the application of (a) electrodeposition, (b) electrospinning, (c) a combination of underpotential deposition and galvanic displacement reactions, and (d) microwave-assisted chemistry to diverse core-shell model systems, such as (i) carbon nanotube-SiO₂ composites, (ii) SnO₂/TiO₂ motifs, (iii) ultrathin Pt-monolayer shell-coated alloyed metal core nanowires, and (iv) Cu@TiO₂ nanowires, for applications spanning optoelectronics, photocatalysis, electrocatalysis, and thermal CO₂ hydrogenation, respectively.In so doing, over the years, we have reported on a number of different characterization tools involving spectroscopy (e.g., extended X-ray absorption fine structure (EXAFS) spectroscopy) and microscopy (e.g., high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM)) for gaining valuable insights into the qualitative and quantitative nature of not only the inner core and outer shell themselves but also their intervening interface. While probing the functional catalytic behavior of a few of these core-shell structures under realistic operando conditions, using dynamic, in situ characterization techniques, we found that local and subtle changes in chemical composition and physical structure often occur during the reaction process itself. As such, nuanced differences in atomic packing, facet exposure, degree of derivatization, defect content, and/or extent of crystallinity can impact upon observed properties with tangible consequences for performance, mechanism, and durability.

Radial heterojunction based on single ZnO-CuxO core-shell nanowire for photodetector applications

ZnO-Cu_xO core-shell radial heterojunction nanowire arrays were fabricated by a straightforward approach which combine two simple, cost effective and large-scale preparation methods: (i) thermal oxidation in air of a zinc foil for obtaining ZnO nanowire arrays and (ii) radio frequency magnetron sputtering for covering the surface of the ZnO nanowires with a Cu_xO thin film. The structural, compositional, morphological and optical properties of the high aspect ratio ZnO-Cu_xO core-shell nanowire arrays were investigated. Individual ZnO-Cu_xO core-shell nanowires were contacted with Pt electrodes by means of electron beam lithography technique, diode behaviour being demonstrated. Further it was found that these n-p radial heterojunction diodes based on single ZnO-Cu_xO nanowires exhibit a change in the current under UV light illumination and therefore behaving as photodetectors.