Tuning the Properties of 2D Materials Using Organic Molecules

Surface functionalization using organic molecules is an effective method to dope two-dimensional (2D) materials due to their high surface areas. Furthermore, by tuning the properties of the molecule on ordered 2D crystals, surface functionalization can be used to precisely control the electronic properties of 2D materials.¹ In previous studies, we investigated the doping power of novel organic electron donors (OEDs) based on 4,4’-bipyridine on monolayer MoS₂ using field effect transistor (FET) measurements before and after surface functionalization, as well as various spectroscopic characterizations. For example, using the reductant DMAP-OED, we demonstrated that both the surface coverage and reduction potential of the molecule are critically important for controlling the carrier concentration in MoS₂. We experimentally estimated the doping power of DMAP-OED to be between 0.6 – 1.3 electrons per molecule, with good agreement with DFT results.¹ Our more recent study showed that a more compact reductant, Me-OED, is an even greater dopant, achieving record levels of electron donation on MoS₂ FETs.²<br/><br/>Here, I discuss the organic reductant tetrakis(dimethylamino)ethyelene (TDAE), a molecular dopant which has never been studied before for 2D functionalization but could greatly impact the electrical properties of 2D FET devices based on criteria developed in our previous research. The relative position of the Fermi level of many 2D transition medal dichalcogenides (TMDCs), like MoS₂, falls below the reduction potential of TDAE, resulting in facile n-type doping to the 2D surface. More importantly, the small size and planar structure of TDAE molecules should allow for greater molecular surface coverage, and thus more electronic charge doping to surfaces. Our research is aided by spectroscopic and atomic force microscopic characterization. This project is conducted on MoS₂, but it can be expanded to other 2D TMCDs and materials. Our results highlight how the physical characteristics of a molecule are at least as important as their chemical affinity for charge donation, allowing us to realize unprecedented levels of doping to MoS₂.<br/><br/><br/>[1] Yarali, M.; Zhong, Y.; Reed, S. N.; Wang, J.; Ulman, K. A.; Charboneau, D. J.; Curley, J. B.; Hynek, D. J.; Pondick, J. V.; Yazdani, S.; Hazari, N.; Quek, S. Y.; Wang, H.; Cha, J. J., Near-Unity Molecular Doping Efficiency in Monolayer MoS₂.<br/>Advanced Electronic Materials 2021, 7 (2), 2000873.<br/><br/>[2] Reed-Lingenfelter, S. N.; Chen, Y.; Yarali, M.; Charboneau, D. J.; Curley, J. B.; Hynek, D. J.; Wang, M.; Williams, N. L.; Hazari, N.; Quek, S. Y.; Cha, J. J., Compact Super Electron-Donor to Monolayer MoS2. Nano Lett. 2022, 22 (11), 4501-4508.