Optical transition energies of isolated molecular monomers and weakly interacting two-dimensional aggregates

Abstract

The optical excitation energies of organic dye molecules are often said to depend sensitively on the polarizability of the utilized substrate. To this end, we employ differential reflectance spectroscopy (DRS) to analyze the S 0 → S 1 fundamental transition energies observed for 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as a function of coverage on various surfaces, such as sp 2-bonded insulating layers [graphene and hexagonal boron nitride (h-BN)], and noble metals pre-covered by a molecular wetting layer which prevents hybridization of the second-layer molecules with the metal states. We elucidate the optical absorbance behavior of PTCDA layers grown on h-BN/Rh(111) and on h-BN/Pt(111) and characterize their structures by means of scanning tunneling microscopy. Surprisingly, although the dielectric properties of the employed substrates differ substantially, only two main transition energies are observed: (i) PTCDA HE essentially mimics the behavior of isolated monomers on surfaces (particularly at submonolayer coverage), while (ii) PTCDA LE , red-shifted by ≈ 70 meV (≈ 560 cm −1), is attributed to two-dimensional densely packed aggregates. This red-shift is in remarkable accordance with previous investigations for PTCDA on NaCl(100) and, therefore, likely arises from the same physical effects, namely the formation of two-dimensional excitonic bands and the polarizability of neighboring molecules within the monolayer. In distinction from earlier studies, we conclude that the polarizabilities of the employed substrates do not constitute the dominant contribution to the molecular S 0 → S 1 transition energies observed here.

Noncontact AFM and differential reflectance spectroscopy joint analyses of bis-pyrenyl thin films on bulk insulators: Relationship between structural and optical properties

“…to name a few. The adsorption of molecules on substrates was also intensively studied, among which is the exemplary case of perylene on mica(0001) [32,50,51], on Au(111) [32], on KCl(001) [51], on graphene, and hexagonal boron nitride [52], but as well, ethylene on Si(001) [53], phthalocyanine derivatives on Au(111) [54] and Si(001)-2 × 1 [55], perfluoropentacene on Ag(110) [42] and on SiO 2 [56], pentacene on quartz (0001) [57] and on SiO 2 [56], α-sexithiophene on Cu(110)-(2 × 1)O [58], etc. Other results are reviewed in Refs.…”

Self-assembly and spectroscopic fingerprints of photoactive pyrenyl tectons on hBN/Cu(111)

“…Indeed, DFT calculations have predicted the adsorption of the pyrene with the aromatic rings centered above the N-positions of a free-standing hBN sheet [81]. Preferred alignments have also been reported for low coverages of PTCDA and MnPc on the strongly corrugated hBN/Rh(111) support [21,22,80], with computational modeling showing PTCDA rings positioned above the N sites of a hBN flake [14]. In contrast, no preferred orientations have been identified for a hydrocarbon lander molecule (i.e., DBP) on hBN/Pt(111) [19].…”

“…However, studies comparing orbital-resolved STM/STS data to optical gaps are scarce [86]. In addition to this effect, the adsorption and supramolecular organization on hBN could sensitively affect the optical transitions [14,16,21]. For example, the fluorescence peak energy of perylene derivatives was reduced considerably (0.3-0.4 eV) upon the adsorption on bulklike hBN [14].…”

“…As a promising alternative to bulk insulators, ultrathin dielectric films can act as decoupling layers but maintain the possibility to perform STM and STS measurements [17]. Atomically-thin hBN sheets attracted considerable interest as such spacer layers [18] and can promote site-dependent decoupling and adsorption [19,20], yielding access to optical transitions [21] as well as allowing for orbital-resolved STM imaging [19,[21][22][23]. For instance, hBN/Cu(111) [24][25][26][27] features a work function template with a moiré superstructure: Depending on the registry of the layer and substrate atoms, the surface is divided in areas of low and high local work function, denoted as "pores" and "wires", respectively [28][29][30][31].…”

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

“…To investigate the decoupling of an organic molecule from a metal substrate by a monolayer of hBN we chose PTCDA and the Cu(111) surface. PTCDA serves as a planar model molecule for investigations of organic layers on surfaces [20][21][22][23]. Monolayers of hBN on Cu(111) have been investigated by several groups and are, hence, rather well understood [24][25][26][27][28][29].…”