We report on the near-field coupling of individual gold nanoantennas arranged in tip-to-tip dimer configuration, leading to strong electromagnetic field enhancements in the infrared, which is of great interest for sensing applications such as surface-enhanced infrared spectroscopy. We quantitatively evaluated the enhancement of vibrational excitations of a 5 nm thick test layer of 4,4'-bis(N-carbazolyl)-1,1'-biphenyl as a function of different gap sizes. The dimers with the smallest gaps under investigation (∼3 nm) lead to more than 1 order of magnitude higher signal enhancement with respect to gaps of 50 nm width. The comparison of experimental data and finite-difference time-domain simulations reveals a nonperfect filling of the gaps with sizes below 10 nm, which means that morphological information on the nanoscale is obtained additionally to chemical information.

We report on the impact of the differing spectral near- and far-field properties of resonantly excited gold nanoantennas on the vibrational signal enhancement in surface-enhanced infrared absorption (SEIRA). The knowledge on both spectral characteristics is of considerable importance for the optimization of plasmonic nanostructures for surface-enhanced spectroscopy techniques. From infrared micro-spectroscopic measurements, we simultaneously obtain spectral information on the plasmonic far-field response and, via SEIRA spectroscopy of a test molecule, on the near-field enhancement. The molecular test layer of 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) was deposited on the surface of gold nanoantennas with different lengths and thus different far-field resonance energies. We carefully studied the Fano-type vibrational lines in a broad spectral window, in particular, how the various vibrational signals are enhanced in relation to the ratio of the far-field plasmonic resonance and the molecular vibrational frequencies. As a detailed experimental proof of former simulation studies, we show the clearly red-shifted maximum SEIRA enhancement compared to the far-field resonance.

Efficient charge transport in organic semiconductors and at their interfaces with electrodes is crucial for the performance of organic moleculebased electronic devices. Band formation fosters effective transport properties and can be found in organic single crystals of large π-stacking aromatic molecules. However, at molecule/metal interfaces, hybrid band formation and band dispersion is a rarely observed phenomenon. Using angle-resolved twophoton photoemission supported by density functional theory calculations, we demonstrate such band formation for two different molecule/metal systems, namely tetrathiafulvalene/Au(111) and tetrafluoro-tetracyanoquinodimethane/ Au(111), in the energy region of occupied as well as unoccupied electronic states. In both cases, strong adsorbate/substrate interactions result in the formation of interface states because of hybridization between localized molecular states and delocalized metal bands. These interface states exhibit significant dispersions. Our study reveals that hybridization in combination with an extended well-ordered adsorption structure of the π-conjugated organic molecules is a striking concept to receive and experimentally observe band formation at molecule/metal interfaces.

Generating well-defined molecular structures at inorganic/organic interfaces and within molecular films is fundamental for charge carrier transport and thus the performance of organic molecule-based (opto)­electronic devices. Here we show by means of low-energy electron diffraction that tetrafluorotetracyano­quinodimethane (F4TCNQ) grows in an epitaxial fashion on the Au(111) surface, resulting in a unit cell which consists of one molecule. In this well-ordered crystalline films we found the formation of an extended space charge region and a dispersing unoccupied electronic molecular state using energy- and angle-resolved two-photon photoemission. The latter finding is a clear proof for band formation in the crystalline molecular structure. We suggest that the high electron affinity of F4TCNQ and a bandlike electron transport are responsible for the formation of the space charge region. Using F4TCNQ as a hole injection layer may open the opportunity to manipulate the hole injection barrier in a controlled way via variation of the F4TCNQ layer thickness.

Narrow graphene nanoribbons (GNRs) exhibit electronic and optical properties that are not present in extended graphene. Most importantly, they possess band gaps in the order of a few electron volts, which has been subject to numerous studies. Here we report on the experimental observation of exctionic states in the band gap of N = 7 armchair GNRs (7-GNR) on Au(111) and Au(788) using energy-and angle-resolved two-photon photoemission spectroscopy. Thereby, an exciton binding energy in the 7-GNR on Au(111) of 160 ± 60 meV has been determined. On the stepped Au(788) surface, the exciton binding energy is in the same range.

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(N-carbazolyl)-1,1′-biphenyl (CBP) and MoO 3 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. ■ INTRODUCTIONIn organic electronics the energy level alignment at interfaces between electrodes and organic semiconductors is crucial for the effective barrier heights for charge extraction (organic photovoltaics (OPVs)) and/or injection (organic field-effect transistors (OFETs)/organic light-emitting diodes (OLEDs)) and thus for the overall device performance. Inserting suitable interlayer materials is one approach to modify the energetics at such interfaces. For example, for many years, thin transitionmetal oxide (TMO) layers between organic semiconductors and the metal electrodes have been used to improve device performance. 1,2 For these systems the increased efficiency has been explained in terms of a decreased effective hole injection barrier due to a change of the work function of the contact and the energy level alignment at the interface between the TMO and the organic layer. 3−6 The origin of these effects and their experimental evaluation as well as their description by theoretical models based on molecular properties such as the density of states of the involved frontier orbitals are topics of recent publications. 7−12 Different experimental techniques are used to investigate the evolution of energy levels at the interface and the associated interfacial charge transfer (CT) that is also termed contact doping. In addition to the frequently used UV photoemission spectroscopy (UPS) and Kelvin probe measurements, which measure the potential profile as a consequence of the transferred charges, 1,7,13 CT at interfaces also has been investigated with differential-reflection spectroscopy in the UV−vis range. 12 Especially for the latter method, the broad spectral features complicate an easy evaluation of these. Very recently, Shallcross et al. showed that X-ray photoemission spectroscopy (XPS) can be used to investigate the extent of contact doping at interfaces of high work function materials, such as indium tin oxide (ITO) or MoO x -covered ITO and solution-processed thin layers of poly(3-hexylthiophene-2,5-diyl) (P3HT). 11 Since a better understanding of CT effects on the molecular level seems to be crucial for the optimal device design, fur...

Optically induced processes in organic materials are essential for light harvesting, switching, and sensor technologies. Here we studied the electronic properties of the tetracyanoquinodimethane(TCNQ)/Au(111) interface by using two-photon photoemission spectroscopy. For this interface we demonstrated the lack of charge-transfer interactions, but we found a significant increase in the sample work function due to UV-light illumination, while the electronic structure of the TCNQ-derived states remain unaffected. Thereby the work function of the interface can be tuned over a wide range via the photon dose. We assigned this to a photoinduced metal-to-molecule electron transfer creating negative ions. The electrons are bound by a small potential barrier. Thus thermal activation reverses the process resulting in the original work function value. The presented photoinduced charge transfer at the TCNQ/Au(111) interface can be used for continuous work function tuning across the substrate's work function, which can be applied in device-adapted hole-injection layers or organic UV-light sensors.

The use of areal characterization of surface texture with high accuracy in a quality control process requires reliability. Therefore, regular inspection of the measurement systems is needed. Important metrological features of a measurement system in dimensional metrology are the amplification factor and linearity. This paper presents a simple method for characterizing the axial scanning system of areal topography measuring instruments with little expense and effort, well suited for industrial routine calibration in the field. The method is based on employing a single material measure with a range of step heights. It is shown that the amplification factor and linearity deviations can be determined and adjusted for large axial measurement ranges.