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Two-dimensional (2-D) materials consist of atomically thin crystal layers bound by the van der Waals force. Recently, the popularity of these materials has been on the rise, primarily due to their many potential applications in electronics, optoelectronics and catalysis.

This is especially true for solution-processable 2-D semiconductor nanosheets, such as MoS2, which show particular potential for the development of large-area thin-film electronics. Compared to conventional zero- and one-dimensional nanostructures, which are typically restricted by surface dangling bonds and associated trapping states at grain boundaries, 2-D nanosheets have dangling-bond-free surfaces, resulting in a clean interface within a thin film and thus excellent charge transport.

Despite their potential benefits, preparing high-quality solution-processable 2-D semiconductor nanosheets comes with a number of challenges. For instance, MoS2 nanosheets and thin films created using lithium intercalation and exfoliation are negatively affected by the presence of the metallic 1T phase, and thus show poor electrical performance.

"In the conventional lithium (Li) intercalation process, the insertion of each Li+ ion entails the injection of one electron into the host crystals," Prof. Xiangfeng Duan, one of the researchers who carried out the study, told TechXplore. "The intercalation of a large number of Li+ leads to massive electron injection into the MoS2 crystal (1 e per formula unit in LiMoS2) that induces the undesired semiconducting 2H to metallic 1T phase transition."

Past studies suggest that this unfavourable phase transition only occurs when the electron injection exceeds a certain threshold, that of 0.29 e per MoS2 formula unit. Based on these findings, Duan and his colleagues devised a new approach to prepare semiconductor 2-D nanosheets, in which the electron injections are chemically manipulated to be below this observed threshold.

"We came up with the idea to reduce the electron injection into the hosting 2-D crystals and prevent the undesired phase transition by replacing the small Li+ (d ≈ 2 Å) with larger cations, such as quaternary ammonium (d ≈ 20 Å for THAB)" Prof. Duan explained. "The bulky size of the quaternary ammonium molecules naturally limits the number of molecules that can fit into the hosting crystal and thus the number of electrons injected, which prevents the undesired phase transition to the metallic 1T phase."

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