Design study of a micro illumination platform based on GaN microLED arrays.

The design study of a micro illumination tool based on GaN microLED arrays is presented. The high spatio-temporal resolution and the capability of generating fully customized optical patterns that characterize the proposed platform would enable the manipulation of biological systems, e.g., for optogenetics applications. Based on ray tracing simulations, the design aspects that mainly affect the device performance have been identified, and the related structural parameters have been optimized to improve the extraction efficiency and the spatial resolution of the resulting light patterns. Assuming that the device is a bottom emitter, and the light is extracted from the n-side, the presence of mesa-structures on the p-side of the GaN layer can affect both the efficiency and the resolution, being optimized for different values of the mesa-side inclination angle. The full width at half maximum (FWHM) of the extracted spots is mainly determined by the substrate thickness, and the relation between the FWHM and the array pitch represents a criterion to define the resolution. Namely, when F W H M

Ablation threshold of GaN films for ultrashort laser pulses and the role of threading dislocations as damage precursors.

The laser-induced ablation threshold of c-plane GaN films upon exposure to ultrashort laser pulses was investigated for different wavelengths from the IR to the UV range and pulse widths between 0.34 and 10 ps. The one-pulse ablation threshold ranges between 0.15 and 3 J/cm2 and shows an increase with the wavelength and the pulse width, except for deep UV pulses. Based on a rate equation model, we attribute this behavior to the efficiency of seed carrier generation by interband absorption. In addition, the multi-pulse ablation threshold was analyzed. Accumulation effects are more prominent in case of IR than with UV pulses and are closely linked to damage precursors. By a thorough structural investigation, we demonstrate that threading dislocations, especially those with a screw component, significantly contribute to laser damage, since they provide a variety of dispersed states within the band gap.

A Novel Approach for a Chip-Sized Scanning Optical Microscope.

The recent advances in chip-size microscopy based on optical scanning with spatially resolved nano-illumination light sources are presented. This new straightforward technique takes advantage of the currently achieved miniaturization of LEDs in fully addressable arrays. These nano-LEDs are used to scan the sample with a resolution comparable to the LED sizes, giving rise to chip-sized scanning optical microscopes without mechanical parts or optical accessories. The operation principle and the potential of this new kind of microscope are analyzed through three different implementations of decreasing LED dimensions from 20 µm down to 200 nm.

Toward three-dimensional hybrid inorganic/organic optoelectronics based on GaN/oCVD-PEDOT structures.

The combination of inorganic semiconductors with organic thin films promises new strategies for the realization of complex hybrid optoelectronic devices. Oxidative chemical vapor deposition (oCVD) of conductive polymers offers a flexible and scalable path towards high-quality three-dimensional inorganic/organic optoelectronic structures. Here, hole-conductive poly(3,4-ethylenedioxythiophene) (PEDOT) grown by oxidative chemical vapor deposition is used to fabricate transparent and conformal wrap-around p-type contacts on three-dimensional microLEDs with large aspect ratios, a yet unsolved challenge in three-dimensional gallium nitride technology. The electrical characteristics of two-dimensional reference structures confirm the quasi-metallic state of the polymer, show high rectification ratios, and exhibit excellent thermal and temporal stability. We analyze the electroluminescence from a three-dimensional hybrid microrod/polymer LED array and demonstrate its improved optical properties compared with a purely inorganic microrod LED. The findings highlight a way towards the fabrication of hybrid three-dimensional optoelectronics on the sub-micron scale.

An Imidazolium-Based Lipid Analogue as a Gene Transfer Agent.

A dicationic imidazolium salt is described and investigated towards its application for gene transfer. The polar head group and the long alkyl chains in the backbone contribute to a lipid-like behavior, while an alkyl ammonium group provides the ability for crucial electrostatic interaction for the transfection process. Detailed biophysical studies regarding its impact on biological membrane models and the propensity of vesicle fusion are presented. Fluorescence spectroscopy, atomic force microscopy and confocal fluorescence microscopy show that the imidazolium salt leads to negligible changes in lipid packing, while displaying distinct vesicle fusion properties. Cell culture experiments reveal that mixed liposomes containing the novel imidazolium salt can serve as plasmid DNA delivery vehicles. In contrast, a structurally similar imidazolium salt without a second positive charge showed no ability to support DNA transfection into cultured cells. Thus, we introduce a novel and variable structural motif for cationic lipids, expanding the field of lipofection agents.

Interaction of imidazolium-based lipids with phospholipid bilayer membranes of different complexity.

In recent years, alkylated imidazolium salts have been shown to affect lipid membranes and exhibit general cytotoxicity as well as significant anti-tumor activity. Here, we examined the interactions of a sterically demanding, biophysically unexplored imidazolium salt, 1,3-bis(2,6-diisopropylphenyl)-4,5-diundecylimidazolium bromide (C11IPr), on the physico-chemical properties of various model biomembrane systems. The results are compared with those for the smaller headgroup variant 1,3-dimethyl-4,5-diundecylimidazolium iodide (C11IMe). We studied the influence of these two lipid-based imidazolium salts at concentrations from 1 to about 10 mol% on model biomembrane systems of different complexity, including anionic heterogeneous raft membranes which are closer to natural membranes. Fluorescence spectroscopic, DSC, surface potential and FTIR measurements were carried out to reveal changes in membrane thermotropic phase behavior, lipid conformational order, fluidity and headgroup charge. Complementary AFM and confocal fluorescence microscopy measurements allowed us to detect changes in the lateral organization and membrane morphology. Both lipidated imidazolium salts increase the membrane fluidity and lead to a deterioration of the lateral domain structure of the membrane, in particular for C11IPr owing to its bulkier headgroup. Moreover, partitioning of the lipidated imidazolium salts into the lipid vesicles leads to marked changes in lateral organization, curvature and morphology of the lipid vesicles at high concentrations, with C11IPr having a more pronounced effect than C11IMe. Hence, these compounds seem to be vastly suitable for biochemical and biotechnological engineering, with high potentials for antimicrobial activity, drug delivery and gene transfer.

Directly addressable GaN-based nano-LED arrays: fabrication and electro-optical characterization.

The rapid development of display technologies has raised interest in arrays of self-emitting, individually controlled light sources atthe microscale. Gallium nitride (GaN) micro-light-emitting diode (LED) technology meets this demand. However, the current technology is not suitable for the fabrication of arrays of submicron light sources that can be controlled individually. Our approach is based on nanoLED arrays that can directly address each array element and a self-pitch with dimensions below the wavelength of light. The design and fabrication processes are explained in detail and possess two geometries: a 6 × 6 array with 400 nm LEDs and a 2 × 32 line array with 200 nm LEDs. These nanoLEDs are developed as core elements of a novel on-chip super-resolution microscope. GaN technology, based on its physical properties, is an ideal platform for such nanoLEDs.

Effects of the deep-sea osmolyte TMAO on the temperature and pressure dependent structure and phase behavior of lipid membranes.

We studied the interaction of lipid membranes with the deep-sea osmolyte trimethalamine-N-oxide (TMAO), which is known to stabilize proteins most efficiently against various environmental stress factors, including high hydrostatic pressure (HHP). Small-angle X-ray-scattering was applied in combination with fluorescence and infrared spectroscopy, calorimetric and AFM measurements to yield insights into the influence of TMAO on the supramolecular structure, hydration level, lipid order as well as the phase behavior of one- and three-component model biomembranes, covering a large region of the temperature-pressure phase space. Our results show that TMAO has not only a marked effect on the conformational dynamics and stability of proteins and nucleic acids, but also on lipid bilayer systems. The gel-to-fluid phase transition is shifted to higher temperatures with increasing TMAO concentration, and the lipid order parameter increases in the fluid lipid phase. Strong H-bonding with bulk water and preferential exclusion of TMAO from the lipid headgroup region leads to a drastic loss of water in the interlamellar space of fully hydrated multivesicular lipid assemblies. HHP leads to an increase of the lipid order parameter of fluid membranes as well, resulting in an increase of the lipid length. Such effect is rather small, however, and the marked effect TMAO imposes on the interlamellar spacing of the lipid bilayers is not significantly affected by temperature and high pressure. Furthermore, the lateral organization of heterogeneous model membranes changes upon addition of the cosolvent. TMAO leads to a coalescence of lipid domains, probably due to an increase of the line tension between liquid ordered and disordered domains in such raft-like lipid bilayer structures.

Impact of Y3+-ions on the structure and phase behavior of phospholipid model membranes.

Trivalent yttrium cations are able to mimic the behavior of Ca2+ in many important biochemical processes, and their application in medicinal chemistry has increased in recent years. While the effect of mono- and divalent salts on lipid membranes has been studied extensively, the effect of trivalent cations, such as Y3+, on the structure and phase behavior of lipid bilayers is largely unknown. Here, we studied the effect of YCl3 on the structure, phase behavior and thermodynamic parameters of zwitterionic DPPC, 20% anionic DPPC/DPPG (80/20) and 10% anionic DOPC/DOPG/DPPC/DPPG/cholesterol (20/5/45/5/25) model biomembrane systems using Fourier-transform infrared spectroscopy, differential scanning calorimetry, Laurdan fluorescence spectroscopy, confocal fluorescence microscopy, zeta potential measurements and atomic force microscopy, covering a wide range of salt concentrations, temperature and pressure. Y3+ ions penetrate deep into the lipid headgroup region and are coordinated to the phosphate groups, resulting in a stronger lipid packing and partial dehydration of the headgroup region. Increasing Y3+ concentration leads to a pronounced increase of the gel-to-fluid phase transition temperature of the phospholipid bilayers, owing to an increased lateral compression pressure, particularly for anionic lipid membranes. Increased lipid chain order and phase segregation of anionic membranes is fostered at high salt concentrations owing to lipid sorting. The fluid-to-gel phase transition pressure decreases significantly with the concentration of the trivalent ion, most pronounced for the negatively charged lipid vesicles. Remarkably, the Y3+-induced ordering effect is much stronger than a hydrostatic pressure-induced ordering of the lipid chains.