NCAFM2023 Programme Booklet

CHARACTERIZING NONLINEAR OPTICAL PROPERTIES OF 2D CONJUGATED POLYMERS

S. Briesenick, 1 A. Czarnecki 1 , M. Cowie 1 , Z. Schumacher 2 , and P. Grütter 1

1 Department of Physics, McGill University, 3600 Rue University, QC H3A 2T8, Montréal, Canada 2 Department of Physics, ETH Zürich, Otto-Stern-Weg 1, 8093 Zürich, Switzerland Correspondent author email: simon.briesenick@mail.mcgill.ca

Two-dimensional π-conjugated polymer (2DCP) films have recently gained significant attention in optoelectronics due to their unique electronic structure, which exhibits both Dirac cones and flat bands [1]. The two-dimensional extended π-conjugated network in 2DCPs results in delocalized electrons that are responsible for their optoelectronic properties. However, until recently, 2DCPs have suffered from small domain sizes and a large number of defects, severely limiting their application. Fortunately, recent advancements in the synthesis of mesoscale, ordered 2DCPs with domain sizes >100nm and much-reduced defect densities have been achieved, enabling the direct observation of the Dirac cone for the first time in organic 2D materials [1]. Recent progresses in FM-AFM made it possible to characterize the second-order nonlinear susceptibility χ (2) with <10nm spatial resolution on 2D transition metal dichalcogenides [2]. Femtosecond laser pulses induce a nonlinear polarization in the sample, which can be detected due to the variations in the electrostatic forces between the tip and the sample using FM-AFM. Our work aims at applying FM-AFM detected local non-linear optical response to 2DCPs to correlate charge carrier separation and recombination as well as single photon emission sites by correlating locally measured χ (2) with atomic scale structure, including defects.

Fig. (a) Molecular structure of precursor monomer TBTANG and STM images of P 2 TANG on Au(111) (8x8 nm 2 (bottom left) and 90x90 nm 2 (right)) [1] (b) AFM topography image of a MoSe 2 island on a Si substrate and tr-AFM measurement on the substrate and on the MoSe 2 layer at the indicated regions on the left. As the delay time between the fs laser pulses is varied, the frequency shift traces out an interferometric autocorrelation of the electric fields with envelopes (red and orange) proportional to χ (2) .

References [1] G. Galeotti et al., Nature Materials, 2020, 19 , 8. [2] Z. Schumacher et al., Proceedings of the National Academy of Sciences, 2020, 117 , 33.

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