Fig. 1. Sketch of ultrahigh conductivity in NbAs nanobelts. The surface state of NbAs nanobelts is found to host the largest sheet conductivity. The electrons are mostly transported through surface states without encountering large-angle scattering.Two-dimensional materials usually boast high carrier mobility when the carrier density is low. But now researchers have fabricated nanobelts of the transition metal NbAs that show high mobility even when the carrier density is also high [Zhang et al., Nature Materials (2019), https://doi.org/10.1038/s41563- 019-0320-9].
If conventional bulk metals are reduced to the nanoscale, conductivity decreases because surface roughness and defects scatter the charge carriers, reducing their overall mobility. Recently, however, a new type of topological materials called Weyl semimetals have been discovered. In single crystals of these exotic materials, the conduction and valence bands touch at specific points leading to unusual electronic properties and phenomena.
Now a team from Fudan University, the High Magnetic Field Laboratory of the Chinese Academy of Sciences in Heifei, Nanjing University of Science and Technology, Beijing University of Technology, the University of Queensland, Brisbane, ETH Zurich, Trinity College Dublin, and University of California, Davis has designed a new way of synthesizing the Weyl semimetal NbAs. Their approach is based on chemical vapor deposition, taking advantage of the reaction between the metal chloride NbCl5 with hydrogen at high temperatures and low pressures. When carried out in an As atmosphere with a thin (15 nm) Au layer acting as a catalyst, nanobelts of NbAs are produced.
The NbAs nanobelts are highly crystalline, with a large proportion of (001) surfaces, and can be regarded as a three-dimensional version of graphene with specific chirality, explains Faxian Xiu of Fudan University, who led the research.
“We found that the surface states of NbAs nanobelts present the highest sheet conductivity among all two-dimensional systems,” he says.
The team’s exploration of the electrical properties of the NbAs nanobelts reveals that they are metallic, with resistivity more than an order of magnitude lower than the bulk material. Moreover, the room temperature conductivity of NbAs nanobelts is comparable to conventional metallic conductors like Cu, Au, and Ag. These unusual properties can be put down to the unique band structure of these nanobelts, where surface states form an arc-like structure that allows the movement of charge carriers with greatly reduced scattering rates, resulting in high conductivity (Fig. 1).
“Both the mobility and carrier density values in the surface of NbAs nanobelts can achieve high values, unlike other systems in which high carrier density limits the mobility,” points out Xiu. “This unique property comes from the low-scattering-rate nature of Fermi arcs, which form the surface electronic structure in NbAs.”
The ability of Weyl semimetals such as NbAs to apparently overcome the traditional tradeoff between carrier density and mobility could open up the way to highly conductive two-dimensional materials. “[This approach] could be utilized to design proper interconnect materials that link together millions of transistors inside chips,” points out Xiu. “NbAs nanobelts may also have potential prospects in thermoelectric conversion and supercapacitors, where high conductivity is in demand.”
The team now plans to study the thermal and thermoelectric transport properties of NbAs nanobelts to unravel the carrier dynamics of this unusual system further.
This article was originally published in Nano Today 26 (2019), 6-7.