Negative Doping in semiconducting 2H-MoS 2 and surface functionalisation Aleksandra Krajewska 1,2 and Aidan McDonald 1,2 1 School of Chemistry, Trinity College Dublin, Ireland, 2 CRANN/AMBER, Trinity College Dublin, Ireland Molybdenum disulfide (MoS 2 ) is a graphene-like, layered material with semiconducting properties that is naturally present in the Earth’s crust. Upon exfoliation from the bulk to the two-dimensional (2D) scale a transition from the indirect to direct bandgap with the emission in the visible range occurs. However, the incorporation of the thermodynamically stable form of 2D-MoS 2 (hexagonal symmetry, 2H) into multi-component devices had been challenging due to the hydrophobicity and intentness of the surface. 1,2 On the other hand, the metastable form of 2D-MoS 2 (tetragonal, 1T) with metallic properties has shown to be easier to functionalise and integrate into complex structures due to activation of the surface with negative doping. 3,4 The aim of our work was to activate the surface of 2H-MoS 2 to achieve similar reactivity as has been previously observed with 1T-MoS 2 . Liquid phase exfoliated 2H-MoS 2 was reacted with NaBH 4 , which resulted in a mild reduction of the surface and introduction of the negative doping within the material (n-2H-MoS 2 ) as identified by the zetapotential and emission spectroscopy measurements (Figure 1). At the same time, emission and absorption spectroscopies confirmed the preservation of the semiconducting phase of 2H-MoS 2 . A thorough characterisation of the material with TEM, XPS, pXRD, TGA, DRIFT and Raman spectroscopies did not reveal any signs of functionalisation, deterioration, or phase transition in the material. n-2H-MoS 2 displayed a very different dispersibility in common solvents compared to pristine 2H-MoS 2 , favouring mediums with a high dielectric constant capable of stabilisation of the charge at the electrical double layer. Similar behaviour was observed for the metallic 1T-MoS 2 . Moreover, it was demonstrated that upon drop-casting, the dispersion of n-2H-MoS 2 formed a uniform coat, free from any agglomerates. The potential of integration of 2H-MoS 2 into more complex devices has also been demonstrated by the functionalisation of n-2H-MoS 2 with various organohalides (Figure 1). This has allowed for the incorporation of functionalities of varying chemistry like methyl, hydroxyl, amide, methoxysilane and pyridine. Covalent tethering of new functionalities was identified by XPS, pXRD, TGA, DRIFT and Raman spectroscopies. The functionalisation allows for the incorporation of 2H-MoS 2 with various other materials including polar and non-polar surfaces, silicones, inorganic complexes and proteins.
Figure 1. Scheme of the reaction of 2H-MoS 2 with NaB 4 and subsequent functionalisation. References 1. P. K. Chow, E. Singh, B. C. Viana, J. Gao, J. Luo, J. Li, Z. Lin, A. L. Elías, Y. Shi, Z. Wang, M. Terrones and N. Koratkar, ACS Nano , 2015, 9 , 3023–3031. 2. Y. Wang and M. Chhowalla, Nat. Rev. Phys. , 2022, 4 , 101–112. 3. D. Voiry, A. Goswami, R. Kappera, C. de C. C. e Silva, D. Kaplan, T. Fujita, M. Chen, T. Asefa and M. Chhowalla, Nat. Chem. , 2015, 7 , 45–49. 4. X. Chen, M. Assebban, M. Kohring, L. Bao, H. B. Weber, K. C. Knirsch and A. Hirsch, J. Am. Chem. Soc. , 2022, 144 , 9645–9650.
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