Stable Organic Radical for Enhancing Metal-Monolayer-Semiconductor Junctions Performance

Among the large amount of families of molecules investigated in molecular junctions (MJs), stable free organic radicals have gained an increasing attention over the last years. 1 Thanks to their open-shell electronic configuration, these molecules are paramagnetic, redox and optically active, which make them appealing species for a variety of applications. 2 Chlorinated trityl radicals, and in particular the perchlorotriphenylmethyl radicals have shown to be highly stable as active molecular units in MJs. 3 Such functional molecules have been recently covalently bound to photoactive, hydrogen-terminated silicon surfaces and the so modified surfaces have been demonstrated to function as light-triggered capacitance switches with good stability. 4 Herein, the charge transport of these systems, employing the open- and closed-shell molecules ( Rad-PTM and α H-PTM , Figure 1a), is investigated as solid-state Metal/monolayer/Semiconductor (MmS) junctions using an eutectic Gallium-Indium liquid metal as the top electrode. A characteristic diode behavior is observed which is tuned by the electronic characteristics of the organic molecule. Our results clearly indicate that the presence of the SOMO-SUMO molecular orbitals impacts on the device performance. The junction incorporating the radical shows an almost two orders of magnitude higher rectification ratio ( R = 10 4.04 ) in comparison with the non-radical one ( R = 10 2.30 ) at ± 1 V bias. Interestingly, the high stability of the fabricated MmS permits to interrogate the system under irradiation, evidencing that at the wavelength where the photon energy is close to the band gap of the radical, there is a clear enhancement of the photoresponse. 5 (1) Ratera, I.; Vidal-Gancedo, J.; Maspoch, D.; Bromley, S. T.; Crivillers, N.; Mas-Torrent, M. J. Mater. Chem. C 2021 , 9 , 10610–10623. (2) Mas-Torrent, M.; Crivillers, N.; Mugnaini, V.; Ratera, I.; Rovira, C.; Veciana, J. J. Mater. Chem. 2009 , 19 , 1691–1695. (3) Bejarano, F.; Olavarria-Contreras, I. J.; Droghetti, A.; Rungger, I.; Rudnev, A.; Gutiérrez, D.; Mas-Torrent, M.; Veciana, J.; Van Der Zant, H. S. J.; Rovira, C.; et al. J. Am. Chem. Soc. 2018 , 140 , 1691–1696. (4) De Sousa, J. A.; Bejarano, F.; Gutiérrez, D.; Leroux, Y. R.; Nowik-Boltyk, E. M.; Junghoefer, T.; Giangrisostomi, E.; Ovsyannikov, R.; Casu, M. B.; Veciana, J.; et al. Chem. Sci. 2020 , 11 , 516–524. (5) De Sousa, J. A.; Pfattner, R.; Gutiérrez, D.; Bromley, S. T.; Veciana, J.; Rovira, C.; Mas-Torrent, M.; Fabre, B.; Crivillers, N. ACS Appl. Mater. Interf ., submitted. Figure 1