2D nanomaterials: A solution for plates

Plates can be cut from gallium-oxide nanoribbons using a sulfur solution.

The size and shape of nanomaterials determine their intrinsic properties and behavior. Following the success of graphene — two-dimensional (2D) sheets of carbon exhibiting novel electronic properties —there has been increasing interest in 2D forms of other materials, with shapes also including ribbons and plates. So far, methods of preparing such 2D structures have involved high-temperature (600–1300 °C) chemical vapor deposition. Now, Juan Xu and colleagues in Korea¹ have developed a much less daunting method, instead cutting plates from nanoribbons of gallium oxide in the solution phase.

Fig. 1: Self-assembled gallium-oxide/gallium-sulfide nanoplates.

The method arose serendipitously when the researchers attempted to make gallium sulfides by treating gallium-oxide nanoribbons with sulfur solution at 230 °C. They found that the nanoribbons were ‘cut’ into plates (Fig. 1); and that very little gallium sulfide was formed. The plates were around 20 nm long, with a thickness of close to 1.2 nm, which corresponds to a single unit-cell of gallium oxide (Ga₂O₃). The composition of the plates was determined by a range of X-ray techniques, which all confirmed that Ga₂O₃ was the main product. Energy-dispersive X-ray spectroscopy showed that the plates contained 6 atomic percent of sulfur. The researchers assumed the sulfur was present in gallium sulfide (Ga₂S₃), and calculated the ratio of Ga₂O₃ to Ga₂S₃ to be 95.7:4.3. To produce nanoplates, the concentration of the sulfur solution needed to be in the range of 0.11–0.17 moles. Below this range, a mixture of ribbons and plates was obtained, and above 0.24 M, no nanomaterials were detected.

Xu and his colleagues suggest that the plate formation is a result of the sulfur reacting at defect sites in the gallium-oxide nanoribbons to form gallium sulfide. The lattice mismatch between the oxide and sulfide creates a weak point that allows the ribbon to break up into plates. This theory is supported by photoemission spectroscopy measurements, which showed that emission from defects was much weaker for the plates than for the nanoribbons.

The researchers expect various future applications, including electrodes and photovoltaic devices. Synthesis of colloidal 2D nanomaterials could lead to large-area device fabrication, although the organic surfactants on the surface of the nanomaterials would need to be removed.