Imaging the photophysics of organic semiconductors using polarisation-resolved and near-field optical spectroscopies

Imaging techniques that enable the structure of organic semiconductors to be determined across length scales are essential for optimisation of their luminescence properties. In this study, we prepare well-ordered monolayer films of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI) both on the surface of hexagonal boron nitride (hBN) and confined within few-layer thick hBN vertical heterostructures, and apply polarisation-resolved and tip-enhanced optical spectroscopies to image the effects of molecular orientation and dielectric environment on the photoluminescence (PL) exhibited by this prototype organic semiconductor translated at the micro and nano length scales, respectively. Using this combined approach, we show that PTCDI self-assembles into two discrete types of few-micron-sized grains at sub-monolayer coverage, each exhibiting characteristic shifts in PL emission energy related to their registry on hBN surfaces. Through examination of the near-field PL spectra extracted from images of individual grains, we further reveal the existence of nanoscale inhomogeneities within the molecular layer which influence both the energy of PL emission and ratio of vibronic sidebands and provide compelling evidence that variations in the degree of resonant coupling are present on length scales comparable to the resolution of the near-field measurement. Together these imaging tools enable a more comprehensive understanding of the molecular-level photophysics of organic semiconductor aggregates to be established, accessed under ambient conditions and not requiring low temperatures or ultra-high vacuum typical of complementary analytical approaches. Such information will be critical for example for the optimisation of individual single photon emitters in quantum communication devices and understanding how defects impact the efficiency and spectral sharpness of LEDs.