Direct observation of controlled growth of oriented metal-chalcogenide nanowires

One-dimensional (1D) transition-metal chalcogenide nanowires (TMC-NWs) are flexible metallic structures with a general formula of X₆Y₆ (X = Mo, W; Y = S, Se, and Te)^1-3 and show tuneable metallic characteristics.^{1, 4-6} Benefiting from their unique structures and electrical properties, TMC-NWs show the potential to serve as conductive interconnections between nanodevices⁵.

Although significant progress has been made in the growth of TMC-NWs^6-10, their orientation control is still challenging due to its 1D nature, resulting in an isotropic distribution of NW bundles.

In this work, we successfully applied in-situ electrical bias to 2D molybdenum ditelluride (MoTe₂) and simultaneously observed a real-time structural transformation to oriented nanowires in a scanning transmission electron microscope (STEM) with atomic resolution. Under the applied bias voltage, 2D MoTe₂ gradually converts into highly oriented 1D Mo₆Te₆ NWs along zigzag edges with sharp interfaces. The novelty of the experiment can be summarized as follows:

1)    In-situ observation of structural evolution of MoTe₂ under electrical bias. This is realized by transferring the graphite-sandwiched 2D MoTe₂ heterostructure to the microelectromechanical system (MEMS) chip. With graphite layers connected to electrodes, we can apply vertical electrical bias to the confined 2D MoTe₂ flake.

2)    Atomic resolution of the electrical in-situ experiment. This is realized by the excellent stability of the MEMS-based in-situ system for applying bias and also the rational design of the 3-layer heterostructure. The graphite electrodes are quite thin compared to the MoTe₂ which makes them electron transparent.

3)    First-principles calculations suggest anisotropic migration of Te atoms. Density functional theory (DFT) calculations confirm that the growth of highly oriented NWs originates from both the confinement effect of the graphite electrodes and the local Joule heating generated by the applied bias.

Our work demonstrates alternative strategies to prepare metal-semiconductor heterojunctions in 2D materials. Moreover, we also show the feasibility of studying phase transformations of 2D materials under electrical excitation at the atomic scale by introducing graphite electrodes, which greatly expands in-situ characterization techniques.