Contrasting Structural Reconstructions, Electronic Properties, and Magnetic Orderings along Different Edges of Zigzag Transition Metal Dichalcogenide Nanoribbons

Abstract

Two-dimensional transition metal dichalcogenides represent an emerging class of layered materials exhibiting various intriguing properties, and integration of such materials for potential device applications will necessarily invoke further reduction of their dimensionality. Using first-principles approaches, here we investigate the structural, electronic, and magnetic properties along the two different edges of zigzag MX (M = Mo, W; X = S, Se) nanoribbons. Along the M edges, we reveal a previously unrecognized but energetically strongly preferred (2 × 1) reconstruction pattern, which is universally operative for all the four systems (and possibly more), characterized by an elegant self-passivation mechanism through place exchanges of the outmost X and M edge atoms. In contrast, the X edges undergo a much milder (2 × 1) or (3 × 1) reconstruction for MoX or WX, respectively. These contrasting structural preferences of the edges can be exploited for controlled fabrication of properly tailored transition metal dichalcogenide nanoribbons under nonequilibrium growth conditions. We further use the zigzag MoX nanoribbons to demonstrate that the Mo and X edges possess distinctly different electronic and magnetic properties, which are significant for catalytic and spintronic applications.

Fabrication of MoSe2 nanoribbons via an unusual morphological phase transition

“…This explains why in Fig. 3c, the electronic structures of the edges expand over 3 nm (spectra #4–#11 and #22–#31), much wider than the calculated ones33. We have used different tips and found that the width of this extended edge region varies, confirming that this extended edge width is due to the probe shape.…”

“…At this point it is worthwhile to note that, almost all experimental results, including the STS data in the present study, show a finite bandgap at the edge46; in contrast, most theoretical calculations would predict a metallic3233 edge. Such an inconsistency might be due to the lacking of accurate atomic models of the zigzag edges.…”

“…3. As mentioned earlier, ideally bulk truncated TMD nanoribbons within the stable 1H structure have two different types of zigzag edges, one Mo-terminated and the other Se-terminated, but only one type of armchair edge3233. Therefore, the asymmetry of the edge morphology strongly suggests that the long and distinctly different edges of the nanoribbons are zigzag edges.…”

“…To date, only limited successes have been demonstrated in the fabrication of TMD nanoribbons using top-down approaches2930; a viable bottom-up approach for controlled mass production of high-quality TMD nanoribbons remains to be discovered. This standing obstacle severely hinders potential advances in exploring their exotic electronic31, magnetic3233, optical3435 and catalytic36 properties for device applications.…”

“…By properly controlling the substrate temperature and Se over-pressure, the MoSe 2 atomic layers undergo a three-stage shape transformation: from fractal to compact 2D nanoislands, and eventually to nanoribbons, in contrast to the traditional two-stage growth behaviour involving only the transformation from the fractal to compact regime. Measurements using in situ scanning tunnelling microscopy/spectroscopy (STM/STS) and ex situ transmission electron microscopy (TEM) reveal that the nanoribbons extend in the zigzag direction, with distinctly different edges whose ideally bulk truncated forms are commonly called Mo- and Se-terminated33. Locally, the Se-terminated edge contains more kinks than the Mo-terminated edge.…”