Single fluorescent molecules embedded in the bulk of host crystals have proven to be excellent probes of the dynamics in their nano environment, thanks to their narrow (about 0.1 {\mu}eV) optical linewidth of the 0-0 zero-phonon line (0-0 ZPL) at cryogenic temperatures. However, the optical linewidths of the 0-0 ZPL have been found to increase dramatically as the single molecules are located closer to a surface or interface, while no 0-0 ZPL has been detected for organic molecules on any surface. Here, we study single fluorescent terrylene molecules adsorbed on the surface of hexagonal boron-nitride (hBN) substrates. Our low-temperature results show for the first time the observation of the 0-0 ZPL of fluorescent molecules on a surface. With resonant excitation, we find 0-0 ZPL linewidths down to values that are about 10 times larger than the Fourier limit of 45 +/- 3 MHz, dictated by the fluorescence lifetime. We compare our results for molecules deposited on the surfaces of annealed and non-annealed hBN flakes and we see a marked improvement in the spectral stability of the emitters after annealing. Our high-resolution spectra obtained on terrylene suggest the possibility of employing hBN in combination with a wide variety of single molecule emitters for investigation of physical phenomena at surfaces or for use in nanophotonic devices.
@article{arxiv.2207.03812,
title = {Sharp zero-phonon lines of single organic molecules on a hexagonal boron-nitride surface},
author = {Robert Smit and Arash Tebyani and Jil Hameury and Sense Jan van der Molen and Michel Orrit},
journal= {arXiv preprint arXiv:2207.03812},
year = {2023}
}