Surface Raman spectroscopy, electrochemistry, and X-ray
photoelectron spectroscopy have been
used to study the effects of air exposure on the stability of
self-assembled monolayers (SAMs) formed from
alkanethiols on mechanically polished, smooth Ag and Au surfaces.
Raman spectra exhibit oxidized sulfur
modes after only hours of air exposure. X-ray photoelectron
spectroscopy of the S 2p region provides additional
evidence for sulfur oxidation. Cyclic voltammetry of
Ru(NH3)6
3+ indicates that
oxidized alkanethiol SAMs
retain blocking characteristics toward electron transfer, even after
exposure of the oxidized SAM-surface to a
solubilizing solvent. Control experiments suggest ozone as the
primary oxidant in ambient laboratory air
which causes rapid oxidation of the thiolate moiety. These results
have important ramifications for the general
use of SAMs in many proposed applications.
Indium−tin oxide (ITO) electrodes have been modified with both fluorinated alkyl and aryl phosphonic acids [n-hexylphosphonic acid (HPA) and n-octadecylphosphonic acid (ODPA); 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl phosphonic acid (FHOPA), pentafluorobenzyl phosphonic acid (PFBPA), and tetrafluorobenzyl-1,4-diphosphonic acid (TFBdiPA)]. These are modifiers designed to control both wetting properties toward nonpolar molecular solids and to provide a wide range of tunability in effective surface work function. The molecular nature of surface attachment and changes in electronic and wetting properties were characterized by X-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy (UPS), photoelastic modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and contact angle measurements using both water and hexadecane. Interface dipoles from the PA modifiers contribute to shifts in the low kinetic energy regions of UPS spectra (local vacuum level shifts, which translate into changes in effective surface work function). We show that for ITO surfaces modified with FHOPA, and to a lesser extent with PFBPA, the high work function obtained by oxygen plasma cleaning can be maintained after modification, while decreasing the polar component of surface energy. This approach to oxide surface modification is a strategy that may be beneficial for the modification of transparent conducting oxide surfaces in both organic light emitting diodes and in organic solar cells, where oxide/organic compatibility can affect device performance.
The correlation of Raman spectral indicators for the determination of alkyl chain interactions and conformational order is presented. These investigations probe the conformational order of bulk octadecane and low molecular weight polyethylene as they undergo solid/liquid phase transitions. Spectral indicators are quantitatively correlated to the I[ν a (CH 2 )]/I[ν s (CH 2 )], as this is the primary indicator of rotational and conformational order obtained empirically from Raman spectra. These indicators are interpreted in terms of alkane intramolecular motion, intermolecular interactions between alkyl chains, crystal structure of these solid materials, and the presence of methylene conformers. Results demonstrate that Raman spectroscopy is sensitive to very subtle changes in alkane chain structure and conformation. These results can be used to understand molecular interactions and structure-function relationships in alkane-based materials.
Transparent conducting oxides (TCOs), such as indium tin oxide and zinc oxide, play an important role as electrode materials in organic-semiconductor devices. The properties of the inorganic-organic interface-the offset between the TCO Fermi level and the relevant transport level, the extent to which the organic semiconductor can wet the oxide surface, and the influence of the surface on semiconductor morphology-significantly affect device performance. This review surveys the literature on TCO modification with phosphonic acids (PAs), which has increasingly been used to engineer these interfacial properties. The first part outlines the relevance of TCO surface modification to organic electronics, surveys methods for the synthesis of PAs, discusses the modes by which they can bind to TCO surfaces, and compares PAs to alternative organic surface modifiers. The next section discusses methods of PA monolayer deposition, the kinetics of monolayer formation, and structural evidence regarding molecular orientation on TCOs. The next sections discuss TCO work-function modification using PAs, tuning of TCO surface energy using PAs, and initiation of polymerizations from TCO-tethered PAs. Finally, studies that examine the use of PA-modified TCOs in organic light-emitting diodes and organic photovoltaics are compared.
Thiol-based self-assembled monolayers (SAMs) have been used to tune the effective work function of gold over a range of ca. 1.8 eV via two strategies: (i) the use of ω-functionalized alkanethiols where the tail groups have widely varying electronegativity or (ii) by the creation of two-component SAMs from selected mixtures of methyl-terminated alkanethiols (C16) and alkanethiols fluorinated at the two terminal carbon atoms (C16F2). UV-photoelectron spectroscopy (UPS) was used to monitor changes in effective work function, using shifts in the low kinetic energy edge of these photoemission spectra to quantify the shift in local vacuum level resulting from the interface dipole effect created by the surface modifier. Tail groups on alkanethiol chains varied from −CH3, to −phenyl, −Cl, −Br, and −CF3 or −CF2CF3, which provided a shift in local vacuum level that varied linearly with the calculated molecular dipole moment of the individual modifiers, as observed previously for a more limited range of alkanethiols (J. Phys. Chem. B
2003, 107, 11690). The studies presented here confirm that the intrinsic dipole in the gold−thiolate bond is small (less than 100 meV), whereas the silver−thiolate bond appears to have a strongly polar character, in the direction Ag+−S− (ca. 900 meV). The use of a simple point dipole model to rationalize these apparent shifts in vacuum level was further explored using SAMs derived from various mixtures of C16 and C16F2. The low kinetic energy edge in the UV-photoemission spectra and the effective work function are observed to increase monotonically in energy with increasing C16F2 coverage, confirming that little surface segregation occurs in these self-assembled monolayers over a wide concentration range.
Understanding
the interaction between organic semiconductors (OSCs)
and dopants in thin films is critical for device optimization. The
proclivity of a doped OSC to form free charges is predicated on the
chemical and electronic interactions that occur between dopant and
host. To date, doping has been assumed to occur via one of two mechanistic
pathways: an integer charge transfer (ICT) between the OSC and dopant
or hybridization of the frontier orbitals of both molecules to form
a partial charge transfer complex (CPX). Using a combination of spectroscopies,
we demonstrate that CPX and ICT states are present simultaneously
in F4TCNQ-doped P3HT films and that the nature of the charge
transfer interaction is strongly dependent on the local energetic
environment. Our results suggest a multiphase model, where the local
charge transfer mechanism is defined by the electronic driving force,
governed by local microstructure in regioregular and regiorandom P3HT.
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