Assembly and electronic structure of octithiophene on Au(111)

Charge transport in polymer- and oligomer-based semiconductor materials depends strongly on the structural ordering of the constituent molecules. Variations in molecular conformations influence the electronic structures of polymers and oligomers, and thus impact their charge-transport properties. In this study, we used Scanning Tunneling Microscopy and Spectroscopy (STM/STS) to investigate the electronic structures of different alkyl-substituted oligothiophenes displaying varied torsional conformations on the Au(111) surface. STM imaging showed that on Au(111), oligothiophenes self-assemble into chain-like structures, binding to each other via interdigitated alkyl ligands. The molecules adopted distinct planar conformations with alkyl ligands forming cis- or trans- mutual orientations. For each molecule, by using STS mapping, we identify a progression of particle-in-a-box-like states corresponding to the LUMO, LUMO+1 and LUMO+2 orbitals. Analysis of STS data revealed very similar unoccupied molecular orbital energies for different possible molecular conformations. By using density functional theory calculations, we show that the lack of variation in molecular orbital energies among the different oligothiophene conformers implies that the effect of the Au-oligothiophene interaction on molecular orbital energies is nearly identical for all studied torsional conformations. Our results suggest that cis-trans torsional disorder may not be a significant source of electronic disorder and charge carrier trapping in organic semiconductor devices based on oligothiophenes.

Section: Resultssupporting

confidence: 52%

Oligothiophene wires: impact of torsional conformation on the electronic structure

Kislitsyn

Taber

Gervasi

et al. 2016

“…Note that both states show a dispersion. The peak at E Final − E F = 5.78 eV is an occupied state located at -0.78 eV, which we attribute to photoemission from the HOMO of 8T 27 . When turning the sample toward negative angles (see Fig.…”

Section: Resultsmentioning

confidence: 75%

“…The sub-molecular resolution STM image of a single molecule, as shown in Fig. 1(a), exhibits 8 regular maxima which are related to the 8 thiophene rings within the molecule 27 . In the monolayer regime 8T possesses a well-ordered and densely packed structure in which the molecules are aligned in rows parallel to each others backbone as shown in Fig.1(b).…”

Section: Resultsmentioning

confidence: 99%

“…Since the quality and the properties of these interfaces are substantially dominated by the adsorption of the first molecular layer, it is important to gain detailed information on its geometry. Although the adsorption behavior of 6T on different metal surfaces has been intensively studied [16][17][18][19][20][21][22][23][24][25] , so far only two recent scanning tunneling microscopy (STM) studies investigated the adsorption of 8T on Cu(111) 26 and Au(111) 27 . In the latter we analyzed the adsorption behavior ranging from single molecules to the first two layers of 8T on Au(111).…”

Section: Introductionmentioning

confidence: 99%

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Coverage-dependent adsorption geometry of octithiophene on Au(111)

Varene

Bogner

Meyer

et al. 2012

Self Cite

The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a function of 8T coverage has been studied with low-temperature scanning tunneling microscopy, high resolution electron energy loss spectroscopy as well as with angle-resolved two-photon photoemission and ultraviolet photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying adsorption geometry. Upon reaching the monolayer coverage the orientation of 8T molecules changes towards a tilted configuration, with the long molecular axis parallel to the surface plane, facilitating attractive intermolecular π-π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO - 1) possesses a strong dependence on the adsorption geometry due to the direction of the involved transition dipole moment for the respective photoemission process. The change in molecular orientation as a function of coverage in the first molecular layer mirrors the delicate balance between intermolecular and molecule/substrate interactions. Fine tuning of these interactions opens up the possibility to control the molecular structure and accordingly the desirable functionality.

“…In contrast, for the octithiophene (8T)/Au(111) no energetic shift of the LUMO and LUMO+1 in the first and second layer was found. 24 In that case the molecules were arranged in a tilted geometry with respect to the surface plane, which allowed for intermolecular p-p interaction. With such adsorption geometry, already the adsorbate/ substrate interaction in the first layer is not strong and accordingly, the electronic coupling.…”

Section: Electronic States As a Function Of Coveragementioning

confidence: 99%

Electronic structure and excited state dynamics in a dicyanovinyl-substituted oligothiophene on Au(111)

Bogner

Yang

Corso

et al. 2015

Self Cite

Dicyanovinyl (DCV)-substituted oligothiophenes are promising donor materials in vacuum-processed small-molecule organic solar cells. Here, we studied the structural and the electronic properties of DCV-dimethyl-pentathiophene (DCV5T-Me2) adsorbed on Au(111) from submonolayer to multilayer coverages. Using a multi-technique experimental approach (low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and two-photon photoemission (2PPE) spectroscopy), we determined the energetic position of several affinity levels as well as ionization potentials originating from the lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO), evidencing a transport gap of 1.4 eV. Proof of an excitonic state was found to be a spectroscopic feature located at 0.6 eV below the LUMO affinity level. With increasing coverage photoemission from excitonic states gains importance. We were able to track the dynamics of several electronically excited states of multilayers by means of femtosecond time-resolved 2PPE. We resolved an intriguing relaxation dynamics involving four processes, ranging from sub-picosecond (ps) to several hundred ps time spans. These show a tendency to increase with increasing coverage. The present study provides important parameters such as energetic positions of transport levels as well as lifetimes of electronically excited states, which are essential for designing organic-molecule-based optoelectronic devices.