Charge Transfer at Organic/Inorganic Interfaces and the Formation of Space Charge Regions Studied with Infrared Light

We present in situ infrared spectroscopy as a powerful tool for the qualitative and quantitative analysis of the charge transfer through the prototypical interface between the organic semiconductor 4,4′-bis­(<i>N</i>-carbazolyl)-1,1′-biphenyl (CBP) and MoO₃ that in organic electronic devices is often used to improve their performance. Due to the different infrared vibrational spectra, charged and neutral species of CBP molecules can be well distinguished, which allows the measurement of the amount of charged species in the vicinity of the interface. The quantitative analysis of CBP thickness-dependent infrared transmission spectra delivered the extension of the space charge region from the interface into the CBP on a nanometer scale. The clear influence of the deposition sequence on these interface properties was clarified by further studies of the inverted layer structures

Solid Phase Synthesis of Short Peptide-Based Multimetal Tags for Biomolecule Labeling

We describe an unprecedented solid phase peptide synthesis (SPPS) of short peptide-based multimetal tags designated as elemental tags for the quantification of biomolecules via inductively coupled plasma mass spectrometry (ICP-MS). The macrocyclic chelator 1,4,7,10-tetraazacyclododecane <i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>‴-tetra acetic acid (DOTA) was attached to the side chain of <i>N</i>-α-(9-fluorenylmethoxycarbonyl)-l-lysine (Fmoc-Lys-OH) and metalated with a lanthanide to provide a building block for Fmoc-based SPPS. Thereby, in contrast to existing strategies for the synthesis of DOTA–peptide conjugates, an already metalated DOTA-amino acid was used as a building block for SPPS. The DOTA-lanthanide complex was stable throughout the whole SPPS, even during the final cleavage in concentrated trifluoroacetic acid. This indicates that the strategy to first metalate the Fmoc-Lys­(DOTA)-OH and to utilize the metal coordination to protect the carboxyl groups of DOTA offers an alternative to conventional synthetic routes using <i>tert</i>-butyl protected DOTA. Several small peptides containing up to four metal ions were synthesized, among them peptides carrying defined metal sequences consisting of two different lanthanides. The peptides were N-terminally maleimide-functionalized, thus introducing a moiety for conjugation to thiol-bearing biomolecules. The final objective of this work was the signal enhancement in ICP-MS-based DNA quantification assays. To evaluate the performance of the multimetal peptide tags in assay, they were applied to label thiol-modified 15mer DNA oligonucleotide probes. These served as reporter probes in a model sandwich-type hybridization assay. Thereby, we found that the ICP-MS signal increased linearly with the number of lanthanide ions attached to the reporter probe

Spatial Extent of Plasmonic Enhancement of Vibrational Signals in the Infrared

Infrared vibrations of molecular species can be enhanced by several orders of magnitude with plasmonic nanoantennas. Based on the confined electromagnetic near-fields of resonantly excited metal nanoparticles, this antenna-assisted surface-enhanced infrared spectroscopy enables the detection of minute amounts of analytes localized in the nanometer-scale vicinity of the structure. Among other important parameters, the distance of the vibrational oscillator of the analyte to the nanoantenna surface determines the signal enhancement. For sensing applications, this is a particularly important issue since the vibrating dipoles of interest may be located far away from the antenna surface because of functional layers and the large size of biomolecules, proteins, or bacteria. The relation between distance and signal enhancement is thus of paramount importance and measured here with <i>in situ</i> infrared spectroscopy during the growth of a probe layer. Our results indicate a diminishing signal enhancement and the effective saturation of the plasmonic resonance shift beyond 100 nm. The experiments carried out under ultra-high-vacuum conditions are supported by numerical calculations

Comprehensive Molecular Characterization of a Cisplatin-Specific Monoclonal Antibody

Despite their immense and rapidly increasing importance as analytical tools or therapeutic drugs, the detailed structural features of particular monoclonal antibodies are widely unknown. Here, an antibody already in use for diagnostic purposes and for molecular dosimetry studies in cancer therapy with very high affinity and specificity for cisplatin-induced DNA modifications was studied extensively. The molecular structure and modifications as well as the antigen specificity were investigated mainly by mass spectrometry. Using nano electrospray ionization mass spectrometry, it was possible to characterize the antibody in its native state. Tandem-MS experiments not only revealed specific fragments but also gave information on the molecular structure. The detailed primary structure was further elucidated by proteolytic treatment with a selection of enzymes and high resolution tandem-MS. The data were validated by comparison with known antibody sequences. Then, the complex glycan structures bound to the antibody were characterized in all detail. The Fc-bound oligosaccharides were released enzymatically and studied by matrix-assisted laser desorption/ionization mass spectrometry. Overall 16 different major glycan structures were identified. The binding specificity of the antibody was investigated by applying synthetic single and double stranded DNA oligomers harboring distinct Pt adducts. The antibody–antigen complexes were analyzed by mass spectrometry under native conditions. The stability of the complex with double stranded DNA was also investigated

Correlation between Chemical and Electronic Properties of Solution-Processed Nickel Oxide

Solution-processed nickel oxide (sNiO) is known to be an excellent charge-selective interlayer in optoelectronic devices. Its beneficial properties can be further enhanced by an oxygen plasma (OP) treatment. In order to elucidate the mechanism behind this improvement, we use infrared transmission and X-ray photoelectron spectroscopy to probe the bulk and surface properties of the sNiO. We find that increasing the annealing temperature of the sNiO not only increases the structural order of the material but also reduces the concentration of nickel hydroxide species present in the bulk and on the surface of the film. This results in a decrease of the work function, while an additional OP treatment raises the work function to between 5.5 and 5.6 eV. For all annealing temperatures investigated, the consequences of the OP treatment are identified as reactions of both NiO and β-Ni­(OH)₂ to form thin β-NiOOH phases in the first atomic layers. Our results emphasize the importance of understanding the correlation between the preparation and resulting properties of sNiO layers and provides further insight into the interpretation of interface properties of NiO

Dopant Diffusion in Sequentially Doped Poly(3-hexylthiophene) Studied by Infrared and Photoelectron Spectroscopy

The diffusivity of dopants in semiconducting polymers is of high interest as it enables methods of sequential doping but also affects device stability. In this study, we investigate the diffusion of a bulky sequentially deposited p-dopant in poly­(3-hexylthiophene) (P3HT) thin films using nondestructive <i>in situ</i> infrared (IR) spectroscopy and photoelectron spectroscopy (PES). We probe dopant diffusion into the polymer film at varying coverage by differentially evaluating electron transfer in the bulk and at the surface. Thereby it is possible to determine dopant coverages at which both electron transfer and incorporation of dopants are saturated. By use of PES, neutral and charged dopants can be distinguished, revealing that charged dopants are less mobile in the diffusion process than neutral molecules. We further compare the diffusivity in semicrystalline and fully amorphous P3HT. We find that at high coverage semicrystalline P3HT seems to yield a higher capacity for dopants than fully amorphous P3HT. A temperature-dependent measurement of sequential doping shows directly that the incorporation of dopants is thermally activated and requires temperatures close to room temperature

Photo-Cross-Linkable Polymeric Optoelectronics Based on the [2 + 2] Cycloaddition Reaction of Cinnamic Acid

We report the synthesis of cinnamic acid-functionalized conjugated polymers, which are cross-linked via [2 + 2] cycloaddition by UV illumination, reducing their solubility. The cross-linking reaction was investigated by a combination of FTIR and optical spectroscopy, and an optimum condition for the solubility modulation of thin films, a major challenge in the solution-phase fabrication of layered optoelectronic devices, was reached. As proof of concept, OLEDs were fabricated, using these conjugated polymers as emissive layers

Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface

Structure–Property Relationship of Phenylene-Based Self-Assembled Monolayers for Record Low Work Function of Indium Tin Oxide

Studying the structure–property relations of tailored dipolar phenyl and biphenylphosphonic acids, we report self-assembled monolayers with a significant decrease in the work function (WF) of indium–tin oxide (ITO) electrodes. Whereas the strengths of the dipoles are varied through the different molecular lengths and the introduction of electron-withdrawing fluorine atoms, the surface energy is kept constant through the electron-donating <i>N</i>,<i>N-</i>dimethylamine head groups. The self-assembled monolayer formation and its modification of the electrodes are investigated via infrared reflection absorption spectroscopy, contact angle measurements, and photoelectron spectroscopy. The WF decrease in ITO correlates with increasing molecular dipoles. The lowest ever recorded WF of 3.7 eV is achieved with the fluorinated biphenylphosphonic acid

Controlled Molecular Orientation of Inkjet Printed Semiconducting Polymer Fibers by Crystallization Templating

Here we present the controlled deposition of highly aligned poly­(3-hexylthiophene-2,5-diyl) (P3HT) fibers by inkjet printing. The functional ink consists of the crystallization agent 1,3,5-trichlorobenzene (TCB), the carrier solvent chlorobenzene, and the semiconducting polymer P3HT. The inkjet printing process was designed in such a way that the drying zone migrates in the printing direction, effectively growing the TCB out of solution and forcing the P3HT chains to align in the printing direction. The films are deposited in arbitrary shapes on a variety of substrates, thus demonstrating the full freedom of design necessary for the digital fabrication of future integrated circuits. We demonstrate by optical and structural investigations that P3HT arranges in a nontrivial empty-core–shell structure with the long molecular axis in the fiber direction while the short axis extends in a radial fashion. Such arrangement induces a fourfold increase in field-effect mobility along the fiber direction as compared to the isotropic printed reference