  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.
