The reverse bias leakage current in macroscopic GaN Schottky diodes is found to be insensitive to barrier height. Using a scanning current–voltage microscope, we show that the reverse bias current occurs at small isolated regions, while most of the sample is insulating. By comparing the current maps to topographic images and transmission electron microscopy results, we conclude that reverse bias leakage occurs primarily at dislocations with a screw component. Furthermore, for a fixed dislocation density, the V/III ratio during the molecular beam epitaxial growth strongly affects reverse leakage, indicating complex dislocation electrical behavior that is sensitive to the local structural and/or chemical changes.

The photosystem I (PS I) protein is one of nature's most efficient light harvesting complexes and exhibits outstanding optoelectronic properties. Here we demonstrate how metal nanoparticles which act as artificial antennas can enhance the light absorption of the protein. This hybrid system shows an increase in light absorption and of circular dichroism over the entire absorption band of the protein rather than at the specific plasmon resonance wavelength of spherical metal nanoparticles (NPs). This is explained by broad-resonant and nonresonant field enhancements caused by metal NP aggregates, by the high dielectric constant of the metal, and by NP-PS I-NP antenna junctions which effectively enhance light absorption in the PS I.

The large multiprotein complex, photosystem I (PSI), which is at the heart of light-dependent reactions in photosynthesis, is integrated as the active component in a solid-state organic photovoltaic cell. These experiments demonstrate that photoactive megadalton protein complexes are compatible with solution processing of organic-semiconductor materials and operate in a dry non-natural environment that is very different from the biological membrane.

The photoelectronic nature of a dried photosystem I protein attached to a metal surface is studied using various spectroscopic techniques. The proteins are found to be optically active after the chemical adsorption. In addition, energy-resolved photoelectronic measurements indicate that the interaction of photosystem I with the metal surface leads to new molecule/substrate states, yielding energy states different from those of the individual components. Such interactions increase the spectral-response range beyond the absorption spectrum of photosystem I and are expected to improve the energy-conversion efficiency of devices based on this system.

The photosystem I (PS I) reaction center is a chlorophyll protein complex located in thylakoid membranes of chloroplasts and cyanobacteria. PS I mediates a light-induced electron transfer through a serial of redox reactions.[1] It is intriguing to incorporate the PS I into optoelectronic circuits, since the PS I exhibits outstanding optoelectronic properties found only in the photosynthetic systems. The quantum yield for absorbing a photon within the whole complex is determined to be close to 100 %, while the energy yield for the process is approximately 58 %.[1] The nanoscale dimension and the generation of 1 V photovoltage further makes the PS I reaction center a promising unit for applications in molecular optoelectronics. [2][3][4][5][6] Utilizing a unique cysteine (Cys) mutation at the end of PS I, we demonstrate a four-step chemical procedure based on carbodiimide chemistry for covalent binding of PS I proteins to carbon nanotubes (CNTs). [7] The method allows studying hybrid nanosystems for the construction of optoelectronic devices based on PSI-CNTs heterostructures. Three variations in the design of PSI-CNT hybrid structures are presented which allow exploiting the potential of PS I as an integrated part of CNT nanodevice for optoelectronic applications. Recently, we have demonstrated the possibility to covalently bind PS I directly to gold surfaces [5] and indirectly via a small linker molecule to GaAs surfaces. [6] To this end, amino acids in the extra membrane loops of the PS I facing the cytoplasmic side of the bacterial membrane (oxidizing side) were mutated to cysteines (Cys) enabling the formation of covalent bonds with a metal surface or a chemically functionalized GaAs surface. The Cys located at extra membranal loops of the protein do not have steric hindrance, when placed on a solid surface e.g. of a gold electrode or CNTs as shown here.The mutations D235C/Y634C were selected near the special chlorophyll pair P700 to allow close proximity between the reaction center and the CNTs. [5] As depicted by white arrows in Figure 1, here we utilize a PS I with two mutants on the oxidizing side of the PS I. This single sided mutant ensures a high outcome of our chemical self-assembly procedure. For a variation of our chemical scheme we also use bipolar (BM) mutants, where the mutations are located at both the oxidizing (white arrows) and the reducing side of the PS I (gray arrow). Our self-assembly approach facilitates efficient electronic junctions and avoids disturbance in the function of the reaction center. The covalent attachment of the PS I through the Cys further ensures the structural stability of the self-assembled, oriented PS I. As demonstrated recently, [5,6] a dry oriented monolayer of PS I assembled on gold electrodes and GaAs surfaces exhibits charge transfer between PS I and the solid state surface. In this work, we extend the above chemical scheme in order to covalently attach PS I proteins to CNTs. The hybrid systems are characterized by atomic force microscopy and COMMUNICATION