Thermal Stability Switches on Potential of Borophene Crystals

Tokyo Institute of Technology researchers have investigated borophene liquid crystals that demonstrate high thermal stability and optical switching behavior, even at low voltages. The findings highlight the potential of borophene oxide-derived liquid crystals for use in a variety of applications that include those that conventional organic liquid crystals or inorganic materials are unable to support.

More specifically, the research team said, its findings demonstrate the feasibility of inorganic liquid devices in harsh environments. Device applications for the research include on/off optical switching based on dynamic scattering.

Traditionally, the development of borophene liquid crystals requires a very narrow temperature range, which hinders their large-scale application. Borophene’s monolayer structure contains a network of boron bonds that grants the material high flexibility, which can be beneficial for the generation of a liquid state at low temperatures. However, borophene’s instability poses challenges in the transition to a liquid state.

Inset: Chemical structure of the prepared borophene oxide layers. White arrows indicate the borophene oxide atomic layers and purple spheres represent potassium cations. A Japanese research team has demonstrated the feasibility of inorganic liquid devices for use in harsh environments. Courtesy of Tetsuya Kambe, Tokyo Institute of Technology.

Borophene oxide can improve the stability of the internal boron network, in turn stabilizing the entire structure. This property of borophene oxide enables the creation of liquid crystals, which are typically required when using other 2D materials.

At first, the researchers looked at previously tested methods to generate borophene oxide layers (BoL) as crystals (BoL-C); they then converted BoL-C to liquid crystals (BoL-LC) by heating them up to temperatures of 105 to 200 ºC. They observed that the resultant dehydration weakened the interactions between the interlayers of the BoL-C, which is desirable for its flexibility.

The researchers then looked at BoL-LC using polarized optical microscopy to look at its structural properties. They found that BoL-LC sheets are stacked parallel to the surface of the liquid drop with a slightly curved form. This spherulite orientation of borophene sheets was later confirmed using scanning electron microscopy.

An analysis of the phase transition features showed that phase transition (P-ii/P-i) occurred around 100 ºC for BoL-LC. In fact, both transition phases exhibited high thermal stability at extreme temperatures. The team also observed a highly ordered orientation of the P-ii phase.

(a) The phase change from P-i to P-ii. (b) Changes from crystal to P-ii phase. The photographs show time-dependent polarized optical microscopic images after rapid-cooling from 200 °C (P-i) to room temperature. Waiting times are 0, 3, and 40 min. The radial straight lines on the sample (0 min) suggested the crystalline phase, and they gradually disappeared. Courtesy of Tetsuya Kambe, Tokyo Institute of Technology.

The team created a dynamic scattering device using BoL-LC to test its optical switching behavior. Unlike other organic liquid crystals, the BoL-based device responded well to voltages as low as 1 V.

“Although a liquid crystal device using graphene oxide has been reported previously, it was a lyotropic liquid crystal, with a strong dependence on the solution concentration. Therefore, the previously reported material is different from the liquid borophene created in this study, without the use of any solvents,” said Tetsuya Kambe, a professor at Tokyo Institute of Technology.

The team also found that even upon direct exposure to fire, BoL-LC was noncombustible, which confirms that BoL-LC in a liquid state with an ordered layer structure can exist over a wider range of temperatures — a property that had not been observed thus far in other organic materials, the researchers said.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-28625-w).