Electronic Properties of Tetraazaperopyrene Derivatives on Au(111): Energy-Level Alignment and Interfacial Band Formation

Abstract

N-heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field-effect transistors. Here, we investigated the electronic properties of 1,3,8,10-tetraazaperopyrene derivatives adsorbed on Au(111) using a complementary experimental approach, namely, scanning tunneling spectroscopy and two-photon photoemission combined with state-of-the-art density functional theory. We find signatures of weak physisorption of the molecular layers, such as the absence of charge transfer, a nearly unperturbed surface state, and an intact herringbone reconstruction underneath the molecular layer. Interestingly, molecular states in the energy region of the sp- and d-bands of the Au(111) substrate exhibit hole-like dispersive character. We ascribe this band character to hybridization with the delocalized states of the substrate. We suggest that such bands, which leave the molecular frontier orbitals largely unperturbed, are a promising lead for the design of organic–metal interfaces with a low charge injection barrier.

Influence of Core Substitution on the Electronic Structure of Benzobisthiadiazoles

“…Benzo[1,2- c :4,5-c’]bis[1,2,5]thiadiazoles (BBTs) possess a strong electron-accepting characteristic and inherent small optical band gaps, − which is very beneficial with respect to applications of these compounds as materials in (opto)electronic devices. − In addition, they show auspicious charge carrier mobilities, often ambipolar. , The orientation of the molecules at interfaces to conducting solids and within the films (e.g., in an organic field effect transistor) is important since it influences the electronic structure at the interface, i.e., the energetic positions of occupied and unoccupied states with respect to the Fermi level of the (semi)metal and the charge transport characteristics within the film. − Utilizing high-resolution electron energy loss spectroscopy (HREELS) opens up the unique opportunity to investigate both the molecular orientation at interfaces to (semi)metallic substrates and within thin films − as well as to study the energies of intramolecular electronic transitions (e.g., S 0 → S 1 ), ,,− ,,, including the energetic position of triplet states, ,− which are not accessible with linear optical methods. In contrast to N -heteropolycycles adsorbed on metal surfaces for which so far only a few studies are known in the literature, ,,,,− S-containing organic aromatic compounds, especially thiophenes, have considerably been analyzed regarding their adsorption and electronic structure (see e.g., refs – …”

Influence of N-introduction on the electronic structure and properties of polyacenes: experiment and quantum chemistry in concert

“…This includes electronattachment and electron-detachment levels of the interfaces (transport levels) as well as optical gaps and exciton binding energies. 21,[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] The energies of singlet electronic transitions (e.g. S 0 -S 1 , optical gaps) have been investigated with HREELS, [41][42][43][44][45][46][47][48] Furthermore, HREELS opens up the opportunity to gain insight into the energetic position of triplet states, which are not accessible with conventional (linear) optical methods.…”

Determination of energetic positions of electronic states and the exciton dynamics in a π-expanded N-heterotriangulene derivative adsorbed on Au(111)

“…Vibronic features in 2PPE data have recently been also proposed for another N-heterocyclic compound adsorbed on Au(111) supported by scanning tunneling spectroscopy results. 33 For lower N-HTA-557-P coverages, i.e., around 1 ML conributions from the Au(111) surface, the dbands, the shifted surface state and the first image potential state are visible in the 2PPE data (see ESI †, Fig. S1).…”

“…To gain insight into the energy level alignment, i.e., the energetic positions of adsorbate-derived occupied as well as unoccupied electronic states we conducted photon-energydependent 2PPE measurements. [23][24][25][26][30][31][32][33]35 Fig. 2a shows an exemplarily 2PPE spectrum of 5 monolayer (ML) N-HTA-557-P adsorbed on Au(111) recorded with hv = 4.62 eV (for additional 2PPE data see ESI †).…”

“…2PPE has been proven to be a powerful tool, which provides the unique opportunity for a quantitative determination of the energetic position of occupied as well as unoccupied molecular electronic states (transport levels) but also excitonic states (optical gaps) and therefore exciton binding energies. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] In addition, femtosecond (fs) time-resolved 2PPE allows to gain insights into the electronically excited states dynamics after optical excitation such as exciton formation and decay dynamics.…”