We report on an easy-to-use, successful, and reproducible route to synthesize functionalized graphite oxide (GO) and its conversion to graphene-like materials through chemical or thermal reduction of GO. Graphite oxide containing hydroxyl, epoxy, carbonyl, and carboxyl groups loses mainly hydroxyl and epoxy groups during reduction, whereas carboxyl species remain untouched. The interaction of functionalized graphene with fluorescent methylene blue (MB) is investigated and compared to graphite, fully oxidized GO, as well as thermally and chemically reduced GO. Optical absorption and emission spectra of the composites indicate a clear preference for MB interaction with the GO derivatives containing a large number of functional groups (GO and chemically reduced GO), whereas graphite and thermally reduced GO only incorporate a few MB molecules. These findings are consistent with thermogravimetric, X-ray photoelectron spectroscopic, and Raman data recorded at every stage of preparation. The optical data also indicate concentration-dependent aggregation of MB on the GO surface leading to stable MB dimers and trimers. The MB dimers are responsible for fluorescence quenching, which can be controlled by varying the pH value.

Knowledge on surface potential dynamics is crucial for understanding the performance of modern-type nanoscale devices. We describe an electrical pump-probe approach in Kelvin-probe force microscopy that enables a quantitative measurement of dynamic surface potentials at nanosecond-time and nanometer-length scales. Also, we investigate the performance of pump-probe Kelvin-probe force microscopy with respect to the relevant experimental parameters. We exemplify a measurement on an organic field effect transistor that verifies the undisturbed functionality of our pump-probe approach in terms of simultaneous and quantitative mapping of topographic and electronic information at a high lateral and temporal resolution.

color-tunable emission, they are particularly attractive for applications in self-emissive flat-panel displays. [2,3] Due to the compatibility of OLED technology with low-cost production approaches, such as solutionbased printing methods, OLEDs are also considered for large-area, thin, lightweight, and glare-free ambient illumination. [4-7] In the future, commercial devices may also exploit the fact that OLEDs are compatible with a range of different substrates, including mechanically flexible ones. [8-10] There has also been interest in adapting OLED technology for applications beyond the display and lighting sector, e.g., for biomedical use or optical communications. [11-16] Meanwhile, a number of other material platforms, e.g., perovskites and quantum dots, quickly gain prominence for thin-film LED light-sources, and each of these have specific benefits and challenges when compared to OLED technology. [17-20] In order to compare different materials and device architectures, reliable and accurate measurements of device efficiency are crucial. Yet, there are a number of difficulties associated with this, which mainly arise from the extended emissive area of OLEDs and other thin-film LEDs, from the use of transparent and often light-guiding substrates, and from the fact that the angular emission characteristics can vary drastically between devices. Despite decades of research into OLEDs, the community still uses a number of different measurement techniques and there is no universal standard. [21] Up to this date, a significant number of publications report efficiency estimates from oversimplified measurements and a substantial fraction of these appear to overestimate the real device efficiency. Here, we describe the implementation of a goniometer-based measurement setup that can record the electroluminescence spectrum of an OLED or other LED with extended emissive area under different angles to accurately determine their emission characteristics and efficiency. We explain in detail the design, assembly, and alignment of the goniometer and the procedure used to extract the device efficiency from the measured data. We provide examples that illustrate how the angle-resolved measurement leads to a significantly more accurate efficiency value, compared to, e.g., just recording the intensity emitted in the forward direction. Our motivation for using an angleresolved method over other approaches, in particular over a measurement based on an integrating sphere, is that it is more flexible and less prone to calibration artefacts. In addition, the The accurate characterization of thin-film light emitting diodes (LEDs)-including organic light emitting diodes (OLEDs), perovskite LEDs, and quantum dot LEDs-is crucial to the understanding of the factors that influence their efficiency and thus to the fabrication of LEDs with improved performance and stability. In addition, detailed information about the angular characteristics of LED emission is useful to assess the suitability of individual architectures, e.g., for display ap...

One of the great challenges in designing modern organic field-effect transistors is lowering the injection barriers that arise at the interfaces between the metallic electrodes and the semiconducting transport channel. Currently, these barriers are quantified mostly by time-independent and external inspection, techniques lacking temporal insight into the most relevant switching dynamics. We address this problem here by pump-probe Kelvin-probe force microscopy, which combines the high spatial resolution of standard Kelvin-probe force microscopy with a pump-probe, enabling time resolution down to nanoseconds. When investigating a dynamically operated pentacene-based organic field-effect transistor, pump-probe Kelvin-probe force microscopy is capable of in-situ probing the temporal charge evolution at any sample spot within the device. Thus, Schottky-barriers arising at the boundaries between electrodes and transport channel are identified as speed bumps for high-speed organic field-effect transistor operation, manifested by residual charges that are retained within the organic film upon switching the device.

Important dynamic processes in mechanobiology remain elusive due to a lack of tools to image the small cellular forces at play with sufficient speed and throughput. Here, we introduce a fast, interference-based force imaging method that uses the illumination of an elastic deformable microcavity with two rapidly alternating wavelengths to map forces. We show real-time acquisition and processing of data, obtain images of mechanical activity while scanning across a cell culture, and investigate sub-second fluctuations of the piconewton forces exerted by macrophage podosomes. We also demonstrate force imaging of beating neonatal cardiomyocytes at 100 fps which reveals mechanical aspects of spontaneous oscillatory contraction waves in between the main contraction cycles. These examples illustrate the wider potential of our technique for monitoring cellular forces with high throughput and excellent temporal resolution.