Interfaces play an important role in emerging organic electronic applications. In order to optimize and control the performance in organic devices such as organic solar cells, a comprehensive understanding of the contacts is essential. However, despite the vast progress made, a fundamental theory of the physical processes taking place at the contacts is still lacking. In this work, a numerical device model is used to clarify the effect of imperfect contacts in organic bulk heterojunction solar cells. The effect of increased injection barriers, reduced surface recombination, interfacial minority carrier doping, and traps for majority carriers at the electrodes causing reduced efficiencies is simulated. Two distinctly different underlying mechanisms leading to different S-shaped features are found, both leading to an effective shift of the built-in voltage. In the case of an extraction barrier to majority carriers at the contact, such as reduced surface recombination, the S kink is due to an induced diffusion potential. In the case of interfacial doping or traps, the S kink results from band bending caused by the fixed or trapped space charge. We derive analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish the mechanisms. We show how to experimentally differentiate between these effects and provide tools to extract the relevant physical parameters.

The neutral biexciton cascade of single quantum dots is a promising source of entangled photon pairs. The character of the entangled state is determined by the energy difference between the excitonic eigenstates known as fine-structure splitting (FSS). Here we reduce the magnitude of the FSS by simultaneously using two independent tuning mechanisms: in-plane magnetic field and vertical electric field. We observe that there exists a minimum possible FSS in each quantum dot which is independent of these tuning mechanisms. However, with simultaneous application of electric and magnetic fields, we show the FSS can be reduced to its minimum value as the energy of emission is tuned over several meV with a 5-T magnet.

Mechanical resonators shaped like microcantilevers have been demonstrated as a platform for very sensitive detection of chemical and biological analytes. However, their use as an analytical tool requires fundamental understanding of the molecular absorption-induced effects in the static and dynamic sensor response. The effect of absorption-induced surface stress on the microcantilever response is here investigated using palladium hydride formation. It is shown that the dynamic and static responses of the cantilever monitored simultaneously exhibit excellent correlation with the crystalline phases of hydride formation. However, the resonance frequency shifts and quasistatic bending are observed to be independent during the solid solution phase. Importantly, absorption-induced changes in the elastic parameters of the palladium film are found to play a dominant role in the static and dynamic response. The presented results help in discerning between changes in swelling and elastic modulus that control the cantilever response as well as the relationships between these parameters.

A phenomenon that can be exploited for the manipulation of magnetization without the conventional current-generated magnetic fields is magnetoelastic coupling, which might, thus, pave the way for low-power data-storage devices. Here, we report a quantitative analysis of the magnetic uniaxial anisotropy induced by piezoelectric strain in Ni nanostructured squares. By applying strain, the magnetic domains in Ni nanostructured squares can be manipulated by the magnetoelastic effect in the Ni. The strain-induced anisotropy displaces the domain walls in the square leading to changes in the domain sizes. By comparing the experiments with micromagnetic simulations, the resulting uniaxial anisotropy is quantified. We find a good agreement for a magnetostrictive constant of ${\lambda }_s=-26\mathrm{ppm}$, confirming a full strain transfer from the piezoelectric to the Ni nanostructures and the retainment of a bulklike ${\lambda }_s$.

The miniaturization of optical devices is a prerequisite for broadband data-processing technology to compete with cutting-edge nanoelectronic circuits. For these future nano-optical circuits, controlling the spatial and temporal evolution of surface plasmons, i.e., propagating optical near fields at metal-insulator interfaces, is a key feature. Here, we design, optimize, and fabricate a nanoscale directional coupler with one input and two output ports, a device that is an essential element of nano-optical circuits. The directional coupler is based on a two-wire transmission line supporting two plasmonic eigenmodes that can be selectively excited. By manipulating the input polarization of ultrashort pulses and pulse pairs and by characterizing the light emitted from both output ports, we demonstrate open-loop ultrafast spatial and spatiotemporal coherent control of plasmon propagation. Because of the intuitive and optimized design, which exploits a controlled near-field interference mechanism, varying the linear input polarization is enough to switch between both output ports of the nanoscale directional coupler. Since we exploit the interference of a finite spectrum of eigenmodes, our experiments represent a very intuitive classical analogue to quantum control in molecules.

We demonstrate microwave manipulation of the spin states of electrically injected spin-polarized electrons in silicon. Although the silicon channel is bounded by ferromagnetic metal films, we show that moderate microwave power can be applied to the devices without altering the device operation significantly. Resonant microwave irradiation is used to induce spin rotation of spin-polarized electrons as they travel across a silicon channel, and the resultant spin polarization is subsequently detected by a ferromagnetic Schottky barrier spin detector. These results demonstrate the potential for combining advanced electron spin resonance techniques to complement the study of semiconductor spintronic devices beyond standard magnetotransport measurements.

We study the charge transfer in multilayer graphene from first principles. We find that highly oriented (Bernal) and misoriented (turbostratic) multilayers show similar charge distributions despite their different electronic structure. We quantify the charge transfer and doping distribution in turbostratic graphene layers, where the screening is affected by vanishing density of states near the Fermi level. The results are in good agreement with an analytic model accounting for the electrostatic interaction and the band filling and point out the importance of system-specific interactions between the surface and the first layer. We find that graphene is an outstanding material for ultrathin electrodes, as most of the benefits of multilayer graphene can be captured with bilayers.

Inkjet printing deposits droplets with a well-controlled narrow size distribution. This paper aims at improving experimental and numerical methods for the optimization of drop formation. We introduce a method to extract the one-dimensional velocity profile inside a single droplet during drop formation. We use a novel experimental approach to capture two detailed images of the very same droplet with a small time delay. The one-dimensional velocity within the droplet is resolved by accurately determining the volume distribution of the droplet. We compare the obtained velocity profiles to a numerical simulation based on the slender jet approximation of the Navier-Stokes equation and we find very good agreement.

Listening is a complex sound selection process thought to be located in the auditory cortex. A biophysically motivated Hopf model of the mammalian cochlea reveals that pitch, a main characteristic in the perception of sound, is already materialized at the level of the mammalian hearing sensor. Here, we provide evidence that major elements of listening may similarly be implemented at the auditory periphery by means of efferent connections to the cochlea that tune the hearing sensor towards an auditory object of interest. The cochlea model we use in our investigations is advocated by its performance quality, the simplicity by which efferent control can be implemented, and by the closeness of the control results compared to the biological data. We tune the Hopf parameters to target on a sound, using pitch as the guiding feature. How well we achieve our goal is tested on real-world sounds and measured by a specifically developed tuning-error measure. The results provide a first estimate of how much the peripheral hearing system can assist a listener in focusing on an auditory signal and, thus, what is contributed by the auditory cortex.

The refractive index of biological specimens is a source of intrinsic contrast that can be explored without any concerns of photobleaching or harmful effects caused by extra contrast agents. In addition, the refractive index contains rich information related to the metabolism of cells at the cellular and subcellular levels. Here, we report a no-moving-parts approach that provides three-dimensional refractive-index maps of biological samples continuously flowing in a microfluidic channel. Specifically, we use line illumination and off-axis digital holography to record the angular spectra of light scattered from flowing samples at high speed. Applying the scalar diffraction theory, we obtain accurate refractive-index maps of the samples from the measured spectra. Using this method, we demonstrate label-free three-dimensional imaging of live RKO human colon cancer cells and RPMI8226 multiple myeloma cells, and obtain the volume, dry mass, and density of these cells from the measured three-dimensional refractive-index maps. Our results show that the reported method, alone or in combination with the existing flow cytometry techniques, shows promise as a quantitative tool for stain-free characterization of a large number of cells.

We present a method by which first-principles calculations are linked quite naturally to experimental observables, which determine and characterize the state of an electrochemical system. The method is based on the formation energies of ions in solution and utilizes a grand canonical framework. This is particularly well suited to use in conjunction with ab initio calculations, since it allows charged defects to be treated individually, rather than requiring the consideration of full charge-neutral reactions. It allows the identification of a region of water stability, the $p\mathrm{H}$ scale, and the electrode potential. We discuss and demonstrate the possibilities and capabilities of the approach by the extraction of ion-hydration energies from the Nernst equation and the construction of a $\text{potential}/p\mathrm{H}$ diagram for zinc by an alternative route deviating from the standard approach used to obtain Pourbaix diagrams.