Home » Chemical and electronic properties of semiconducting oligomer and thiol monolayer interfaces. by Yan Ge
Chemical and electronic properties of semiconducting oligomer and thiol monolayer interfaces. Yan Ge

Chemical and electronic properties of semiconducting oligomer and thiol monolayer interfaces.

Yan Ge

Published
ISBN : 9780549983057
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109 pages
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 About the Book 

This thesis includes research topics related to interfacial studies between metals, alkanethiol monolayers, and organic semiconducting polymers. These are important materials for electronic device applications such as organic light emitting diodesMoreThis thesis includes research topics related to interfacial studies between metals, alkanethiol monolayers, and organic semiconducting polymers. These are important materials for electronic device applications such as organic light emitting diodes (OLED), organic field effect transistors (OFETs) and photovoltaics.-X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) have been used to study the interfacial electronic properties of thiophene and alpha-sexithiophene (6T) adsorbed at 130 K in ultrahigh vacuum on top of 1H,1 H,2H,2H - perfluorodecanethiol (PFDT) self-assembled monolayers (SAMs) on gold. The binding energy of the F 1 s XPS peak decreases following thiophene and 6T adsorption, indicating charge transfer to the fluorinated SAMs. UPS measurements substantiate charge transfer, with the valence features of thiophene and 6T in contact with the monolayer appearing at higher ionization energies compared to thicker layers. The energies of the UPS-measured secondary electron cutoffs of 6T deposited on PFDT/Au illustrate the absence of a common vacuum level between the organic layers at the PFDT-6T interface and the presence of a -0.9 eV interface dipole. Similar measurements performed for 6T deposition on self-assembled octadecanethiol (ODT) give a weaker interface dipole of opposite sign (phi=+0.4 eV). The relatively large value and sign of the 6T/PFDT/Au interface dipole suggest that charge transfer to the PFDT-covered surface results in the formation of dipoles with their negative ends toward the Au surface, in contrast to the 6T-ODT interface. The effects of a SAM layer on X-ray-induced oligomerization, which is known to occur for condensed thiophene, were also investigated. Comparison of the thickness of oligomeric thiophene formed by Mg Kalpha X-ray irradiation on clean and PFDT-covered gold surfaces demonstrates that a thicker oligomer layer forms on the SAM covered surface, suggesting that the spacing provided by the SAM reduces quenching of electronic excitations that lead to X-ray-induced oligomerization.-XPS and UPS have also been used to investigate deposition of buckminsterfullerene (C60) on gold surfaces and the interfacial electronic properties between C60 and alpha-sexithiophene. C60 and 6T films have been prepared by thermal evaporation in ultrahigh vacuum on clean gold substrates at room temperature. XPS measurements of the carbon intensity as a function of C60 dose demonstrate layer-by-layer growth of on the gold surface, with a high coverage work function of 4.5 eV. Independent of the deposition order, the vacuum level offset of 0.6 +/- 0.05 eV matches the work function difference between the C60/Au and 6T/Au surfaces, indicating the vacuum levels of the organic layers remain pinned to the Fermi level. 6T film donates electron density to the C60 layer, as evidenced by movement of the 6T (C60) highest occupied molecular orbitals (HOMO) toward higher (lower) ionization energies. This is consistent with the known donor-acceptor nature of thiophene-fullerene complexes.-Due to electron tunnelling, organic thin films are generally believed to be in electrical contact with a photoelectron spectrometer when adsorbed on metal substrates. The Fermi level of the organic film and the spectrometer are normally aligned because the organic film is in electrical contact with the substrate. However, in this thesis, we show that this generally applicable model is incorrect for systems of potassium deposited on alkanethiol self-assembled monolayers absorbed on gold substrates. The results of the photoemission experiments clearly illustrate that the methyl and methylene groups in alkanethiol monolayers on gold surfaces are not in adequate electronic contact with the metal surface such that their photoelectron spectra may be referenced to the Fermi level without shifting. The shift of the C is core level is not due to chemical bonding effects or electron transfer between the carbon atoms and the deposited potassium. Instead, it is a result of electronic isolation of the methyl and methylene groups from the surface, with incomplete charge compensation causing the observed shifts. In contrast, the S 2p core level is in electronic contact with the surface. This most likely arises from its near proximity to the surface. The implications of these findings are that interpretation of photoelectron spectra of thin organic films must not assume that changes in observed peak positions are due to chemical effects.