Structural and electronic decoupling of a large organic molecule from a metal surface by a single layer of hexagonal boron nitride

Abstract

In the present work, the ability of one single layer of hexagonal boron nitride (hBN) to decouple the organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) from an underlying Cu(111) surface was investigated. The decoupling was probed by fluorescence (FL) spectroscopy of the PTCDA molecule adsorbed on the hBN surface. An analysis of the topographical and the electronic structure of the system PTCDA/hBN/Cu(111) was performed with spotprofile analysis low energy electron diffraction (SPA-LEED), the normal incidence x-ray standing waves technique (NIXSW), x-ray photoelectron spectroscopy (XPS), and ultraviolet photoemission spectroscopy (UPS). To gain insight into the growth process of the hBN layer, temperature programmed desorption (TPD) was used. The investigation comprises three steps.<br /> First, the structure of the substrate hBN/Cu(111) was analyzed and the growth process of the hBN layer was investigated. The structural analysis revealed the very weak interactions at the hBN/Cu(111) interface. Despite a small lattice mismatch of 2.0% the structure is incommensurate. The hBN layer is slightly buckled with an amplitude of (0.42 ± 0.05) Å, however, compared to the large vertical distance between hBN and the Cu(111) surface of 3.24 Å this buckling is very small and the hBN layer can be considered locally flat. A further indication for weak interfacial interactions is the wide range of azimuthal orientations of the hBN domains. The orientations are determined during the growth process which is separated into three steps. First, hBN domains rotated by 30° relative to the Cu(111) surface grow at surface defects, then the hBN domains on terraces grow aligned with the metal substrate and reach the largest sizes of all hBN domains, and finally, randomly oriented hBN domains fill the remaining gaps between larger domains.<br /> Then, the PTCDA molecule on hBN/Cu(111) was investigated with respect to its geometric and electronic structure. For the PTCDA/hBN interface, too, an incommensurate structure, a large vertical distance, and disorder in the azimuthal orientations of the adsorbate domains were found. Furthermore, the investigations revealed a small desorption energy of PTCDA molecules, a strong resemblance of the C1s XPS spectra of PTCDA/hBN/Cu(111) and PTCDA multilayers, and no differential shifts of orbital energies of PTCDA on hBN/Cu(111) compared to the gas phase of PTCDA. All of these observations point to the very weak interactions between PTCDA and the hBN/Cu(111) substrate. Stronger interfacial interactions have only been found for hBN layers with a higher density of structural defects. Here, the underlying Cu(111) surface could influence the azimuthal orientation of the PTCDA domains.<br /> Finally, the optical properties of PTCDA/hBN/Cu(111) were measured and compared to those of PTCDA/Cu(111) where the first PTCDA layer decouples the higher layers of PTCDA from the metal surface. Raman lines were measured for both systems and compared to PTCDA on other substrates. On both, hBN/Cu(111) and Cu(111), fluorescence from molecules at surface defects and from molecules adsorbed in higher layers (the second and third PTCDA layers on hBN/Cu(111) and on Cu(111), respectively, and higher) was observed at 18,450 cm^-1 and at 18,150 cm^-1, respectively. Only on hBN/Cu(111), fluorescence from ordered PTCDA domains in the first layer was found at 18,300 cm^-1. However, the fluorescence intensity of PTCDA on hBN/Cu(111) is low and the life times of the excited states amounts to only (1.6 ± 4.4) · 10^-14 s.<br /> These investigations showed that a single layer of hBN is able to decouple the PTCDA molecule from an underlying Cu(111) surface only to a certain degree. On the one hand, PTCDA on hBN/Cu(111) structurally resembles PTCDA molecules in the gas phase rather than those adsorbed on metal surfaces. On the other hand, the hBN layer does not decouple the PTCDA molecule enough to prevent the quenching of an electronic excitation. Despite the reduction of the charge-transfer at the interface, the fluorescence intensities and quantum yields are small.