Determination of the valence band offset at cubic CdSe/ZnTe type II heterojunctions: A combined experimental and theoretical approach

We present a combined experimental and theoretical approach for the determination of the low-temperature valence band offset (VBO) at CdSe/ZnTe heterojunctions with underlying zincblende crystal structure. On the experimental side, the optical transition of the type II interface allows for a precise measurement of the type II band gap. We show how the excitation-power dependent shift of this photoluminescence (PL) signal can be used for any type II system for a precise determination of the VBO. On the theoretical side, we use a refined empirical tight-binding parametrization in order to accurately reproduce the band structure and density of states around the band gap region of cubic CdSe and ZnTe and then calculate the branch point energy (also known as charge neutrality level) for both materials. Because of the cubic crystal structure and the small lattice mismatch across the interface, the VBO for the material system under consideration can then be obtained from a charge neutrality condition, in good agreement with the PL measurements.Comment: 11 pages, 5 figure

Microcurrent-Mediated Modulation of Myofibroblasts for Cardiac Repair and Regeneration

Cardiovascular diseases are a significant cause of illness and death worldwide, often resulting in myofibroblast differentiation, pathological remodeling, and fibrosis, characterized by excessive extracellular matrix protein deposition. Treatment options for cardiac fibrosis that can effectively target myofibroblast activation and ECM deposition are limited, necessitating an unmet need for new therapeutic approaches. In recent years, microcurrent therapy has demonstrated promising therapeutic effects, showcasing its translational potential in cardiac care. This study therefore sought to investigate the effects of microcurrent therapy on cardiac myofibroblasts, aiming to unravel its potential as a treatment for cardiac fibrosis and heart failure. The experimental design involved the differentiation of primary rat cardiac fibroblasts into myofibroblasts. Subsequently, these cells were subjected to microcurrent (MC) treatment at 1 and 2 µA/cm2 DC with and without polarity reversal. We then investigated the impact of microcurrent treatment on myofibroblast cell behavior, including protein and gene expression, by performing various assays and analyses comparing them to untreated myofibroblasts and cardiac fibroblasts. The application of microcurrents resulted in distinct transcriptional signatures and improved cellular processes. Gene expression analysis showed alterations in myofibroblast markers, extracellular matrix components, and pro-inflammatory cytokines. These observations show signs of microcurrent-mediated reversal of myofibroblast phenotype, possibly reducing cardiac fibrosis, and providing insights for cardiac tissue repair

Optical properties of ZnO nanowires: influence of surface treatments and high excitation densities

ZnO nanowires are promising building blocks for optoelectronic applications. Lasers andlight-emitting diodes fabricated from ZnO nanowires will emit in the blue and near-UVspectral region due to the wide band gap of ZnO. Their large refractive index makes themuseful as active waveguides in optoelectronic devices. In addition to the near UV emission,ZnO can show several defect related emission bands in the visible spectral region. By carefullydoping the material, these defect bands can be tailored to generate a white-light spectrumwhich will be beneficial for applications as light emitting diodes. Much research interestfocused on the various defect bands in the past years. Previous work of the author on ZnOnanowires embedded in polymer matrices shows a strong decrease of the defect luminescencefor the embedded samples, combined with a stronger surface exciton emission band.In this work, the dependence of the defect and near-band-edge luminescence of ZnO nanowireson surrounding materials necessary to fabricate nanowire-LED devices is investigated. First,coating of the nanowires with a dielectric shell of amorphous Al2O3 is found to stronglysuppress the defect luminescence while enhancing the surface excitonic emission. This effectis similar to the previously investigated polymer samples and is explained by a model basedon the dielectric properties of the material. A dielectric coating shields the surface states andreduces the band bending typically observed for the ZnO nanowire surfaces. The band bendinginfluences the activation mechanism of deep centers which cause the defect luminescence andredistributes the excitons near the surface in favour of the surface excitonic recombinationmechanism. As a next step, the influence of metallic coatings on the optical properties ofZnO nanowires is investigated and found to result in an opposite behaviour compared to thedielectric coating. An increased defect luminescence and a reduced surface emission bandare observed, completely independent of the work function of the metal. These findings areexplained by the formation of metal-induced gap states at the nanowire surface, which trapexcitons close to the surface.The interaction between plasmonic resonances of metal nanoparticles and excited states ofsemiconductors has attracted much attention in the research community. The use of a plasmasputter-coater is one method to deposit such metal nanoparticles. In this work, experimentalevidence is presented which shows that the plasma of such a system itself changes theoptical properties of ZnO nanowires by implantation of hydrogen into the crystal. The defectluminescence is weakened while the near-UV emission is enhanced, but does not show astronger surface excitonic emission.The lifetime of the charge carriers excited by a fs laser is investigated in pump-and-probeexperiments on ZnO bulk surfaces. Even at excitation densities below the lasing threshold, anelectron-hole-plasma forms with lifetimes of several tens of picoseconds. In order to investigatethe lasing properties of single ZnO nanowires, a setup based on the variable-stripe-lengthmethod is developed and used to determine the modal gain of single nanowires. When excitingwith a fs laser, modal gain values of up to 4900 cm−1 are found which are in good agreementwith theoretical predictions. The gain is found to depend on the nanowire diameter

Optische Eigenschaften von ZnO-Nanodrähten:Einfluss von Oberflächenbehandlungen und hohen Anregungsdichten

ZnO nanowires are promising building blocks for optoelectronic applications. Lasers andlight-emitting diodes fabricated from ZnO nanowires will emit in the blue and near-UVspectral region due to the wide band gap of ZnO. Their large refractive index makes themuseful as active waveguides in optoelectronic devices. In addition to the near UV emission,ZnO can show several defect related emission bands in the visible spectral region. By carefullydoping the material, these defect bands can be tailored to generate a white-light spectrumwhich will be beneficial for applications as light emitting diodes. Much research interestfocused on the various defect bands in the past years. Previous work of the author on ZnOnanowires embedded in polymer matrices shows a strong decrease of the defect luminescencefor the embedded samples, combined with a stronger surface exciton emission band.In this work, the dependence of the defect and near-band-edge luminescence of ZnO nanowireson surrounding materials necessary to fabricate nanowire-LED devices is investigated. First,coating of the nanowires with a dielectric shell of amorphous Al2O3 is found to stronglysuppress the defect luminescence while enhancing the surface excitonic emission. This effectis similar to the previously investigated polymer samples and is explained by a model basedon the dielectric properties of the material. A dielectric coating shields the surface states andreduces the band bending typically observed for the ZnO nanowire surfaces. The band bendinginfluences the activation mechanism of deep centers which cause the defect luminescence andredistributes the excitons near the surface in favour of the surface excitonic recombinationmechanism. As a next step, the influence of metallic coatings on the optical properties ofZnO nanowires is investigated and found to result in an opposite behaviour compared to thedielectric coating. An increased defect luminescence and a reduced surface emission bandare observed, completely independent of the work function of the metal. These findings areexplained by the formation of metal-induced gap states at the nanowire surface, which trapexcitons close to the surface.The interaction between plasmonic resonances of metal nanoparticles and excited states ofsemiconductors has attracted much attention in the research community. The use of a plasmasputter-coater is one method to deposit such metal nanoparticles. In this work, experimentalevidence is presented which shows that the plasma of such a system itself changes theoptical properties of ZnO nanowires by implantation of hydrogen into the crystal. The defectluminescence is weakened while the near-UV emission is enhanced, but does not show astronger surface excitonic emission.The lifetime of the charge carriers excited by a fs laser is investigated in pump-and-probeexperiments on ZnO bulk surfaces. Even at excitation densities below the lasing threshold, anelectron-hole-plasma forms with lifetimes of several tens of picoseconds. In order to investigatethe lasing properties of single ZnO nanowires, a setup based on the variable-stripe-lengthmethod is developed and used to determine the modal gain of single nanowires. When excitingwith a fs laser, modal gain values of up to 4900 cm−1 are found which are in good agreementwith theoretical predictions. The gain is found to depend on the nanowire diameter

Investigation on Nanorod TCO Light-trapping for a-Si:H Solar Cells in Superstrate Configuration

ABSTRACTLight trapping due to rough transparent conductive oxide (TCO) surfaces is a common and industrially applied technique in thin film silicon solar cells. In this study, we demonstrate a novel light trapping solution using electrochemically deposited, highly doped zinc oxide (ZnO) nanorod arrays which goes beyond standard light management concepts. The n-doped ZnO rods enable the application as front electrode in superstrate configuration. We explain our experimental results by multidimensional solar cell simulations and show how the nanorod array geometry influences the cell performance. The requirement is demonstrated to choose an appropriate average nanorod distance which strongly influences the electrical cell characteristics. The results clearly outline the potential of TCO nanorod technology for enhanced light trapping.</jats:p

CdSe/ZnTe superlattices optimized as absorbers for photovoltaics

International audienceWe have grown, by MBE, CdSe/ZnTe superlattices with different periods, to evaluate a new concept of solar cells

CdSe/ZnTe superlattices optimized as absorbers for photovoltaics

International audienceWe have grown, by MBE, CdSe/ZnTe superlattices with different periods, to evaluate a new concept of solar cells