Lithography-free 2D nanoribbons line up

Two-dimensional materials demonstrate unusual properties from superconductivity to ferroelectricity, attractive for nanoscale electronic devices. Large, regular arrays of 2D materials hold potential for high-density devices and integrated circuits but synthesis is challenging. Now researchers from Nanyang Technological University in Singapore and Southeast University, and Suzhou Institute of NanoTech and Nano-Bionics in China have developed a one-step chemical vapor deposition (CVD) synthesis that produces regular arrays of single-crystal MoTe₂ nanoribbons directly on SiO₂/Si substrates without the need for post-processing [Deng et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2022.06.002].

“Patterned 2D materials have sparked a wide range of applications like logic integrated circuits, photoelectric devices, nonlinear optics, infrared detectors, and biosensors,” points out first author of the study, Ya Deng of Nanyang Technological University. “Preparing patterned materials is always the cornerstone for nano-device applications.”

Generating such materials usually requires special templates or electron-beam lithography to produce patterns after synthesis. But Kun Zhou and Zheng Liu, who led the effort, and their colleagues produced regular arrays of highly dense, well-aligned MoTe₂ nanoribbons on normal substrates without any step features or post processing. The molten salt assisted CVD approach produces large arrays (exceeding 100 microns) of single-crystalline MoTe₂ nanoribbons approximately 53-58 nm wide separated by gaps of 56 nm, with around 10 nanoribbons per micron.

“The nanoribbon arrays [we produced] are based on a joint synthesis mechanism composed of a crystal-structure boosted self-etching process associated with a solid-liquid-vapor (SLV) mode,” explains Deng.

The researchers believe that the self-etching process that occurs in SLV mode arises from the interaction H₂ gas flow during CVD growth and the presence of NaCl in the precursor. Although the etching effect of H₂ on 2D materials like graphene and MoS₂ is well known, it does not typically produce such ordered structures. The combination of NaCl assisted by H₂ produces the regular etching effect observed.

The approach offers a concise and efficient strategy for synthesizing patterned materials in a controllable way. The high-density, highly aligned 2D arrays could prove useful as ultrathin channel material for large-scale integrated circuits. The nanoribbons are also rich in edge states, which could be exploited in catalysis or sensing. Moreover, since MoTe₂ possesses edge supercurrents, synthesized arrays could hold great potential for exploring topological superconductors.

“We are now trying to synthesize larger MoTe₂ single crystal area arrays,” says Deng, “and using the synthesized arrays as templates to obtain other molybdenum chalcogenide (MoS₂, MoSe₂) arrays and heterostructures.”