Membrane-assisted revelation of the spatial nanoarchitecture of dislocation networks

We propose a method of direct visualization of the spatial nanoarchitecture of dislocation networks which is based on etching away the regions with low density of structural defects from the bulk of GaN epilayers, keeping intact only the threading dislocations and a thin surface film pre-treated with low-energy Ar⁺ ions. The formation of nanometer-thick suspended membrane to which the dislocations are genetically attached provides conditions for the revelation of the spatial nanoarchitecture of dislocation networks using conventional scanning electron microscopy. Complementary monochromatic and panchromatic micro-cathodoluminescence images are presented.     

Electronic structure of the pentacene–gold interface: A density-functional theory study

The structural and electronic properties of a pentacene monolayer adsorbed on the Au(1 1 1) surface have been studied with a density-functional theory (DFT) approach. A thermally stable adsorption geometry of the pentacene monolayer on the gold surface is found, from which the adsorption energy per pentacene molecule can be evaluated. Our results illustrate how the electron charge distribution initially present over the clean gold surface is pushed back upon adsorption of the pentacene monolayer; this push-back (pillow effect) leads to a significant work-function decrease for the modified gold surface. The electronic couplings between the highest occupied molecular orbital of pentacene and the Au(1 1 1) surface and between adjacent pentacene molecules within the monolayer, were extracted from the calculated band structures; the pentacene–gold surface electronic coupling is found to be about five times smaller than the electronic coupling between pentacene molecules.     

Direct Optical Patterning of Quantum Dot Light-Emitting Diodes via In Situ Ligand Exchange

Precise patterning of quantum dot (QD) layers is an important prerequisite for fabricating QD light-emitting diode (QLED) displays and other optoelectronic devices. However, conventional patterning methods cannot simultaneously meet the stringent requirements of resolution, throughput, and uniformity of the pattern profile while maintaining a high photoluminescence quantum yield (PLQY) of the patterned QD layers. Here, a specially designed nanocrystal ink is introduced, “photopatternable emissive nanocrystals” (PENs), which satisfies these requirements. Photoacid generators in the PEN inks allow photoresist-free, high-resolution optical patterning of QDs through photochemical reactions and in situ ligand exchange in QD films. Various fluorescence and electroluminescence patterns with a feature size down to ≈1.5 µm are demonstrated using red, green, and blue PEN inks. The patterned QD films maintain ≈75% of original PLQY and the electroluminescence characteristics of the patterned QLEDs are comparable to thopse of non-patterned control devices. The patterning mechanism is elucidated by in-depth investigation of the photochemical transformations of the photoacid generators and changes in the optical properties of the QDs at each patterning step. This advanced patterning method provides a new way for additive manufacturing of integrated optoelectronic devices using colloidal QDs.     

Hole-vibronic coupling in oligothiophenes: impact of backbone torsional flexibility on relaxation energies

Density functional theory calculations together with highly resolved gas-phase ultraviolet photoelectron spectroscopy have been applied to oligothiophene chains with up to eight thiophene rings. One of the important parameters governing the charge transport properties in the condensed phase is the amount of energy relaxation upon ionization. Here, we investigate the impact on this parameter of the backbone flexibility present in oligothiophenes as a result of inter-ring torsional motions. With respect to oligoacenes that are characterized by a coplanar and rigid backbone, the torsional flexibility in oligothiophenes adds to the relaxation energy and leads to the broadening of the first ionization peak, making its analysis more complex.     

Constant Rate Expressing of Juice from Biological Tissue Enhanced by Pulsed Electric Field

Pulsed electric field (PEF) is an innovative technology, which can be successfully combined with solid/liquid expressing of juice from biological materials. The fresh juicy plants contain the juice in closed cells. The application of PEF ensures the electropermeabilization of cellular membranes, which facilitates the expulsion of liquid from the interior of cells. This article describes the influence of PEF on the kinetics of juice extraction from the layer of sugar beet particles expressed at a constant rate. Experimental study was carried out using a laboratory filter-press cell connected to a PEF treatment system. The PEF was applied by two different modes: as a pretreatment operation before pressing and as a treatment during pressing. The application of PEF to nonpressurized cake leads to increase of energy consumption and higher applied voltage. The PEF treatment of excessively pressurized cakes enhances the juice yield; however, the expressing of juice is significantly delayed. The best result is obtained when the sugar beet tissue is treated with PEF at 1.5-5 bars. The influence of PEF parameters on the juice yield was also studied. The intensity of PEF was varied from 0 to 1000 V/cm and the number of pulses was varied between 0 and 1000. It was established that the optimal parameters of PEF after the cake pressurization at 5 bar were in the next range: intensity of PEF E = 500 V/cm and duration of PEF application 0.03-0.05 s.     

Fabrication of GaN nanowalls and nanowires using surface charge lithography

We demonstrate the possibility for controlled nanostructuring of GaN by focused-ion-beam treatment with subsequent photoelectrochemical (PEC) etching. The proposed maskless approach based on direct writing of surface negative charge that shields the material against PEC etching allows fabrication of GaN nanowalls and nanowires with lateral dimensions as small as 100 nm. The results obtained show that the occurrence of undercut etching inherent to gallium nitride PEC etching depends on the depletion length in doped GaN material, it being nearly fully suppressed in the structures below a critical size of about 200 nm for the investigated GaN layer of doping concentration of 1.7 × 10¹⁷ cm^− 3.     

The Impact of Molecular Orientation on the Photovoltaic Properties of a Phthalocyanine/Fullerene Heterojunction

The anisotropy inherent to many planar organic molecules leads to a high sensitivity of various fundamental processes to the orientation of molecules within films and at heterojunctions. Such processes include absorption, charge and exciton transport, energy levels, and charge transfer, all of which are critical to organic solar cell operation. Here,an in‐depth study of bilayer cells consisting of a donor/acceptor interface between zinc phthalocyanine (ZnPc) and fullerene (C₆₀) is conducted and devices with the typically deposited standing up (edge‐on) orientation are compared to those with ZnPc lying flat (face‐on). The face‐on ZnPc‐based device allows for an increase in all solar cell parameters, substantially increasing power conversion efficiency from 1.5% to 2.8%. Spectrally resolved photocurrent measurements reveal a >50% increase in ZnPc signal, from which only 12% is accounted for by the increase in absorption associated with the face‐on orientation. The increase in internal quantum efficiency is accounted for via an improved charge transfer. The results of this study indicate that proper consideration of the orientation between donor and acceptor needs to be taken in order to fully optimize the numerous processes required for photovoltaic energy conversion.