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Interface properties of organic para-hexaphenyl/α-sexithiophene heterostructures deposited on highly oriented pyrolytic graphite.

Schwabegger G, Oehzelt M, Salzmann I, Quochi F, Saba M, Mura A, Bongiovanni G, Vollmer A, Koch N, Sitter H, Simbrunner C - Langmuir (2013)

Bottom Line: As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate.PES measurements show that the interface between p-6P and 6T crystals is sharp on a molecular level without any sign of interface dipole formation or chemical interaction between the molecules.We therefore conclude that the different emission colors of the two 6T phases are caused by different types of molecular aggregation.

View Article: PubMed Central - PubMed

Affiliation: Institute of Semiconductor and Solid State Physics, Johannes Kepler University , Altenbergerstrasse 69, A-4040 Linz, Austria.

ABSTRACT
It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite mica exhibit the intriguing possibility to prepare lasing nanofibers of tunable emission wavelength. For p-6P/6T heterostructures, two different types of 6T emission have been observed, namely, the well-known red emission of bulk 6T crystals and additionally a green emission connected to the interface between p-6P and 6T. In this study, the origin of the green fluorescence is investigated by photoelectron spectroscopy (PES). As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate. The similarity between p-6P/6T heterostructures on muscovite mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning force microscopy (SFM), and optical spectroscopy. PES measurements show that the interface between p-6P and 6T crystals is sharp on a molecular level without any sign of interface dipole formation or chemical interaction between the molecules. We therefore conclude that the different emission colors of the two 6T phases are caused by different types of molecular aggregation.

No MeSH data available.


Related in: MedlinePlus

XPS and UPS spectra ofthe 6T/p-6P/HOPG heterostructure. p-6P (5nm, green curve) is deposited on the bare HOPG substrate (blue). Layersof increasing 6T thickness are deposited onto the p-6P buffer layerwith nominal thicknesses of 0.2 (red), 0.8 (cyan), 1.6 (magenta),3.2 (yellow), and 6.4 nm (black). The left part of the figure showsthe C 1s and S 2p core levels, the middle part shows the valence bandregion (takeoff angle of 45°) together with the secondary electroncutoff (SECO), and the right part shows a schematic energy-level diagramwith energy values deduced from UPS.
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fig5: XPS and UPS spectra ofthe 6T/p-6P/HOPG heterostructure. p-6P (5nm, green curve) is deposited on the bare HOPG substrate (blue). Layersof increasing 6T thickness are deposited onto the p-6P buffer layerwith nominal thicknesses of 0.2 (red), 0.8 (cyan), 1.6 (magenta),3.2 (yellow), and 6.4 nm (black). The left part of the figure showsthe C 1s and S 2p core levels, the middle part shows the valence bandregion (takeoff angle of 45°) together with the secondary electroncutoff (SECO), and the right part shows a schematic energy-level diagramwith energy values deduced from UPS.

Mentions: The measurements were carried out in the form of a thicknessseries, starting with a 5 -nm-thick p-6P layer and 6T layers depositedsubsequently onto this template. The UPS valence band (VB), the secondaryelectron cutoff (SECO) spectra, and the XPS core-level spectra ofcarbon 1s (C 1s) and sulfur 2p (S 2p) were recorded for each thicknesslevel (Figure 5). In the VB region, a peakaround 3.3 eV binding energy (BE) is observed, which is assigned tothe HOPG substrate. Upon p-6P deposition, two distinct peaks ariseat 2.1 and 2.7 eV BE. These peaks are assigned to the p-6P HOMO andHOMO-1, respectively. (See the schematic energy-level diagram in Figure 5.) Upon 6T deposition, a new peak emerges at 1.4eV that originates from the 6T HOMO and is clearly visible at a nominalthickness of only 0.2 nm 6T. Note that the BE of 6T HOMO-1 almostcoincides with the HOMO of p-6P. With increasing 6T thickness, thepeak at 1.4 eV increases in intensity, whereas the p-6P HOMO-1 peakbecomes significantly attenuated. This points toward a sharp interfacebetween p-6P and 6T because UPS is a highly surface-sensitive techniquewith an escape depth for electrons on the order of 1 nm at the givenphoton energy.48 At a nominal 6T thicknessof 1.6 nm, HOMO-1 of p-6P has almost vanished. Clearly, 6T grows onp-6P and completely covers the template layer.


Interface properties of organic para-hexaphenyl/α-sexithiophene heterostructures deposited on highly oriented pyrolytic graphite.

Schwabegger G, Oehzelt M, Salzmann I, Quochi F, Saba M, Mura A, Bongiovanni G, Vollmer A, Koch N, Sitter H, Simbrunner C - Langmuir (2013)

XPS and UPS spectra ofthe 6T/p-6P/HOPG heterostructure. p-6P (5nm, green curve) is deposited on the bare HOPG substrate (blue). Layersof increasing 6T thickness are deposited onto the p-6P buffer layerwith nominal thicknesses of 0.2 (red), 0.8 (cyan), 1.6 (magenta),3.2 (yellow), and 6.4 nm (black). The left part of the figure showsthe C 1s and S 2p core levels, the middle part shows the valence bandregion (takeoff angle of 45°) together with the secondary electroncutoff (SECO), and the right part shows a schematic energy-level diagramwith energy values deduced from UPS.
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC3842851&req=5

fig5: XPS and UPS spectra ofthe 6T/p-6P/HOPG heterostructure. p-6P (5nm, green curve) is deposited on the bare HOPG substrate (blue). Layersof increasing 6T thickness are deposited onto the p-6P buffer layerwith nominal thicknesses of 0.2 (red), 0.8 (cyan), 1.6 (magenta),3.2 (yellow), and 6.4 nm (black). The left part of the figure showsthe C 1s and S 2p core levels, the middle part shows the valence bandregion (takeoff angle of 45°) together with the secondary electroncutoff (SECO), and the right part shows a schematic energy-level diagramwith energy values deduced from UPS.
Mentions: The measurements were carried out in the form of a thicknessseries, starting with a 5 -nm-thick p-6P layer and 6T layers depositedsubsequently onto this template. The UPS valence band (VB), the secondaryelectron cutoff (SECO) spectra, and the XPS core-level spectra ofcarbon 1s (C 1s) and sulfur 2p (S 2p) were recorded for each thicknesslevel (Figure 5). In the VB region, a peakaround 3.3 eV binding energy (BE) is observed, which is assigned tothe HOPG substrate. Upon p-6P deposition, two distinct peaks ariseat 2.1 and 2.7 eV BE. These peaks are assigned to the p-6P HOMO andHOMO-1, respectively. (See the schematic energy-level diagram in Figure 5.) Upon 6T deposition, a new peak emerges at 1.4eV that originates from the 6T HOMO and is clearly visible at a nominalthickness of only 0.2 nm 6T. Note that the BE of 6T HOMO-1 almostcoincides with the HOMO of p-6P. With increasing 6T thickness, thepeak at 1.4 eV increases in intensity, whereas the p-6P HOMO-1 peakbecomes significantly attenuated. This points toward a sharp interfacebetween p-6P and 6T because UPS is a highly surface-sensitive techniquewith an escape depth for electrons on the order of 1 nm at the givenphoton energy.48 At a nominal 6T thicknessof 1.6 nm, HOMO-1 of p-6P has almost vanished. Clearly, 6T grows onp-6P and completely covers the template layer.

Bottom Line: As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate.PES measurements show that the interface between p-6P and 6T crystals is sharp on a molecular level without any sign of interface dipole formation or chemical interaction between the molecules.We therefore conclude that the different emission colors of the two 6T phases are caused by different types of molecular aggregation.

View Article: PubMed Central - PubMed

Affiliation: Institute of Semiconductor and Solid State Physics, Johannes Kepler University , Altenbergerstrasse 69, A-4040 Linz, Austria.

ABSTRACT
It was recently reported, that heterostructures of para-hexaphenyl (p-6P) and α-sexithiophene (6T) deposited on muscovite mica exhibit the intriguing possibility to prepare lasing nanofibers of tunable emission wavelength. For p-6P/6T heterostructures, two different types of 6T emission have been observed, namely, the well-known red emission of bulk 6T crystals and additionally a green emission connected to the interface between p-6P and 6T. In this study, the origin of the green fluorescence is investigated by photoelectron spectroscopy (PES). As a prerequisite, it is necessary to prepare structurally similar organic crystals on a conductive surface, which leads to the choice of highly oriented pyrolytic graphite (HOPG) as a substrate. The similarity between p-6P/6T heterostructures on muscovite mica and on HOPG is evidenced by X-ray diffraction (XRD), scanning force microscopy (SFM), and optical spectroscopy. PES measurements show that the interface between p-6P and 6T crystals is sharp on a molecular level without any sign of interface dipole formation or chemical interaction between the molecules. We therefore conclude that the different emission colors of the two 6T phases are caused by different types of molecular aggregation.

No MeSH data available.


Related in: MedlinePlus