Guided Growth of Conductive MoO2@MoS2 Core–Shell Nanowires with Broad Photoluminescence for Improved Device Contacts

Mixed-dimensional heterostructures (MDHs) of layered materials, such as transition-metal dichalcogenides (TMDs), have gained extensive interest, owing to their remarkable optical and electronic properties. Achieving controlled growth of such MDHs is critical for advancing their integration into efficient electronic and optoelectronic devices as well as catalytic systems. However, precise control over the growth direction and orientation of bottom-up TMD-based MDHs remains a challenge. Here, we report on the synthesis of horizontally oriented MoO₂@MoS₂ core–shell nanowires on atomically flat R-, A-, M-plane sapphire and on faceted annealed miscut C-plane 1 ° toward-A sapphire surfaces. The process was generalized and expanded to MoO₂@MoSe₂ MDHs grown epitaxially on C-plane sapphire. Optical microscopy, scanning electron microscopy (SEM), and atomic-force microscopy (AFM) reveal that the nanowires achieve heights of hundreds of nanometers and align along a different set of preferred crystallographic orientations on each surface, indicating epitaxial and graphoepitaxial-guided growth. The optical properties of these core–shell nanowires are primarily determined by the MoS₂ shell, showing varied intensity around the MoS₂ band-edge emission at 1.8 eV and its characteristic A_1g and E_2g Raman modes. Thanks to the highly symmetrical growth and their high crystallinity, the MoO₂@MoS₂ nanowires grown on M-plane sapphire were analyzed using a scanning transmission electron microscope (STEM), confirming a continuous shell of MoS₂ with varying thickness, wrapping a crystalline MoO₂ core. The variation in shell thickness contributes to changes in the band structure along the wire, resulting in a broad-range photoluminescence. While optical behavior is dominated by the MoS₂ shell, the electrical properties are predominantly governed by the MoO₂ core, which demonstrates high conductivity reaching approximately 5 × 10⁴ S·cm^–1. The high conductivity of the horizontally oriented nanowires, coupled with their strong luminescence, makes each component (core and shell) contribute distinct functionalities and opens many opportunities for efficient electronic and optoelectronic devices.