Characterization of oligothiophene films by high resolution electron energy loss spectroscopy

Pellegrino, Olivier; Vilar, M. Rei; Horowitz, Gilles; Kouki, Fayçal; Garnier, F.; Silva, J.D. Lopes da; Rego, A.M. Botelho do

doi:10.1016/s0040-6090(98)00639-7

Cited by 15 publications

(8 citation statements)

References 16 publications

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“…The narrow peak a 4 ,observed in the 4T/Au spectrum around 0.20 ± 0.08 eV above G 4 , does not appear in the 4T-SAM spectrum.Similar structures were also observed in the 5T/Au and 6T/Au spectra, as well as in the 6T/Au UV/vis absorption spectra, and were assigned to the upper Davydov component, that is, to 1 a g → b u . The DS value is, anyhow, quite controversial, and theoretical calculations are still in progress .…”

Section: Resultssupporting

confidence: 73%

“…In contrast, spectra recorded with evaporated oligothiophene films (nT, n standing for the number of cycles) do not exhibit this individual character of the aromatic cycles. Instead, they are dominated by wide bands with thresholds corresponding to the gap values [12][13][14][15] revealing, in comparison to the pendant groups above mentioned, strong crystalline effects. These values are in agreement with those obtained by optical methods on single crystals.…”

Section: Introductionmentioning

confidence: 99%

“…Both vibrational and electron transitions are studied and compared to the optical and HREELS spectra of Roligothiophene (nT) evaporated thin films previously studied. [12][13][14] It is shown that the energy thresholds of electronic excitations of these nT-SAMs exhibit a similar dependence versus the conjugated chain length of the oligothienyl moieties. The role of intermolecular interactions is discussed.…”

Section: Introductionmentioning

confidence: 99%

“…The present study deals with the HREELS characterization of the electronic structure of Au(111) surfaces covered with SAMs of α-terthienylhexanethiol (3T-SAMs) and α-quaterthienyldodecanethiol (4T-SAMs) and, more precisely, with the interaction among the thienyl functionalities placed at the film−vacuum interface. Both vibrational and electron transitions are studied and compared to the optical and HREELS spectra of α-oligothiophene ( n T) evaporated thin films previously studied. − It is shown that the energy thresholds of electronic excitations of these n T-SAMs exhibit a similar dependence versus the conjugated chain length of the oligothienyl moieties. The role of intermolecular interactions is discussed.…”

Section: Introductionmentioning

confidence: 99%

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HREELS Study of Self-Assembled Monolayers of Alkylthiols Functionalized with Oligothiophene Moieties: Extreme Surface Analysis and Evidence for Intermolecular Interactions

Vilar¹,

Lang²,

Horowitz³

et al. 2003

Langmuir

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SAMs (self-assembled monolayers) of alkanethiols functionalized by R-terthienyl (3T-SAMs) and R-quaterthienyl (4T-SAMs) were formed on a gold surface presenting (111) terraces and studied by highresolution electron energy loss spectroscopy (HREELS). The angular width of the elastic peak leads to a domain size of ca. 90 Å, comparable to that of the Au(111) terraces of the substrate. Enhancement of the spectral resolution by using a new generation spectrometer enables the improvement of the spectral analysis in the energy range of the vibrational losses. A detailed analysis of the differential cross sections of the different vibrational peaks indicates the nature of their mechanisms: impact mechanisms show that the extreme surface of the monolayers is mainly constituted by thienyl moieties with the thienyl density in 3T-SAMs higher than that in 4T-SAMs; dipolar mechanisms show that molecules are standing up slightly tilted on the substrate; a resonance via a negative ion formation was detected around 4.5 eV. On the other hand, the electronic excitation domain leads to excitonic gap values of 2.62 ( 0.08 and 2.90 ( 0.08 eV for 4T-SAMs and 3T-SAMs, respectively. A sharp feature located at 0.28 ( 0.08 above the electronic threshold was detected in the 3T-SAM spectra. This feature, also appearing in the spectra of the R-oligothiophene evaporated films, is here assigned to a vibronic excitation. Its intensity is always related to the organization degree of the SAMs. An additional band without equivalence in the optical spectra appearing in the 4T-SAM and 3T-SAM HREELS spectra with thresholds at 3.58 ( 0.08 and 4.15 ( 0.08 eV, respectively, is assigned to the electronic gap. The values of the excitonic and electronic gaps are well correlated with those found in the corresponding spectra of evaporated R-quaterthiophene, R-quinquethiophene, and R-sexithiophene films formerly studied. These results lead to the conclusion that thienyl functionalities present strong intermolecular interactions.

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Section: Resultssupporting

confidence: 73%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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HREELS Study of Self-Assembled Monolayers of Alkylthiols Functionalized with Oligothiophene Moieties: Extreme Surface Analysis and Evidence for Intermolecular Interactions

Vilar¹,

Lang²,

Horowitz³

et al. 2003

Langmuir

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“…4) [3,4,17]. The accepted HOMO-LUMO energy gap for PDI-8CN 2 is ~2.4 eV [19], while for T6 is ~2.1 eV [26]. The perylene/Au films are characterized by a LUMO position really close to the metal Fermi level, leading to a very low electron injection barrier from Au.…”

mentioning

confidence: 99%

Surface doping in T6/PDI-8CN2 heterostructures investigated by transport and photoemission measurements

Aversa

Verucchi

Tatti

et al. 2012

Applied Physics Letters

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ABSTRACT.In this paper, we discuss the surface doping in sexithiophene (T6) organic field-effect transistors by N,N'-bis (n-octyl)-dicyanoperylenediimide (PDI-8CN 2 ). We show that an accumulation heterojunction is formed at the interface between the organic semiconductors and that the consequent band bending in T6 caused by PDI-8CN 2 deposition can be addressed as the cause of the surface doping in T6 transistors. Several evidences of this phenomenon have been furnished both by electrical transport and photoemission measurements, namely the increase in the conductivity, the shift of the threshold voltage and the shift of the T6 HOMO peak towards higher binding energies.

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Surface analysis of oligothiophene films using HREELS: molecular orientation effects

Vilar

Horowitz

Lang

et al. 1999

Adv. Mater. Opt. Electron.

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The surface of evaporated films of α‐oligothiophenes—quaterthiophene (4T), quinquethiophene (5T) and sexithiophene (6T)—was studied by high‐resolution electron energy loss spectroscopy (HREELS), extending the common vibrational domain of energy loss to electronic excitation and secondary emission. Vibrational spectra show that the molecular orientation is not the same on graphite and gold substrates. HREELS results corroborate those obtained by reflection–absorption infrared spectroscopy (RAIRS), confirming that graphite substrates induce films where molecules lie flat on the surface, whereas gold substrates generate a molecular orientation close to perpendicularity. Using these two differently oriented samples, we demonstrate the existence of two relaxation channels for the electron surface interaction: electronic excitation and ionisation. HREELS spectra recorded with incident electron energy between 2 and 7 eV reveal that secondary electron emission originating from ionisations is always more efficient for escape directions perpendicular to the plane of the molecule. This is here associated with the π orbital origin of this emission. Copyright © 2000 John Wiley & Sons, Ltd.

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Characterization of oligothiophene films by high resolution electron energy loss spectroscopy

Cited by 15 publications

References 16 publications

HREELS Study of Self-Assembled Monolayers of Alkylthiols Functionalized with Oligothiophene Moieties: Extreme Surface Analysis and Evidence for Intermolecular Interactions

HREELS Study of Self-Assembled Monolayers of Alkylthiols Functionalized with Oligothiophene Moieties: Extreme Surface Analysis and Evidence for Intermolecular Interactions

Surface doping in T6/PDI-8CN2 heterostructures investigated by transport and photoemission measurements

Surface analysis of oligothiophene films using HREELS: molecular orientation effects

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