Research on flexible thin film transistors (TFTs) has been steadily under way, with the next generation devices being noted. Molybdenum disulfide (MoS2), one of the two-dimensional materials, has excellent optical, electrical and chemical properties and is used in field effect transistors (FETs). However, most of the synthesis studies are concentrated on chemical vapor deposition, and it is necessary to make a method to manufacture MoS2 while maintaining crystallinity through an inexpensive solution process.



In this study, MoS2 FET was implemented through a solution process by selectively modifying the edge of MoS2 only with organic compounds, without using metal ions at all. The paper was published in ACS Nano (2017, 11, 12832-12839) by the researchers led by Hyoyoung LEE (CINAP-IBS, Professor of Chemistry) with Mun Seok JEONG (Professor of Energy Science). To exfoliate the 2D MoS2 bulk materials into single-layer edge-1T basal-2H MoS2 sheets, they used 4-carboxy-benzenediazonium (+N2-benzene-COOH) tetrafluoroborate. Edge-exposed nucleophilic sulfides of the bulk MoS2 can act as substitutes for the electrophiles of +benzene-COOH diazonium salts. The chemically attached polar COOH functional group facilitates the exfoliation by creating an increased electrostatic repulsion force between the MoS2 nanosheets. As a result, the configuration of the resulting exfoliated HOOC-benzene-MoS2 sheets (e-MoS2) is changed to the conducting 1T or 1T′ phase through functionalization at the edge part, while remaining in the semiconducting 2H phase at the basal plane.



Compared with n-BuLi exfoliated MoS2 (n-MoS2) and basal plane functionalized MoS2 (b-MoS2), which mainly have metallic 1T or 1T′ phase during the chemical treatment showing the on/off ratio of <100, e-MoS2 has a relatively high mobility (μ= 1.2 cm2/(V s) and on/off ratio (= 106) due to the low number of defect sites in the e-MoS2 nanosheets. PDDA (polydiallyldimethylammonium chloride) is used as a cationic layer, while e-MoS2 is used as an anionic layer for the channel film formation. Cross-linking via hydrogen bonding of the negatively charged COOH of the e-MoS2 sheets with the help of a cationic polymer provided good film formation for solution processing TFT channels. The e-MoS2 TFT exhibited great electrical performance (average mobility of 170.8 cm2/(V s) at 1 V and on/off ratio of 106) on SiO2/Si substrates and also a high mobility of 36.34 cm2/(V s) (on/off ratio of 103) on PDMS/PET substrates for flexible TFTs.