The influence of an interfacial hBN layer on the fluorescence of an organic molecule

We investigated the ability of a single layer of hexagonal boron nitride (hBN) to decouple the excited state of the organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) from the supporting Cu(111) surface by Raman and fluorescence (FL) spectroscopy. The Raman fingerprint-type spectrum of PTCDA served as a monitor for the presence of molecules on the surface. Several broad and weak FL lines between 18,150 and 18,450 cm^-1 can be detected, already from the first monolayer onward. In contrast, FL from PTCDA on a bare Cu(111) surface is present only from the second PTCDA layer onward. Hence, a single layer of hBN decouples PTCDA from the metal substrate to an extent that a weak radiative FL decay of the optical excitation can occur. The different FL lines can be ascribed to different environments of the adsorption sites, namely molecules adsorbed at surface defects, in large ordered domains, and located in the second layer.