Group IV photonics is on the way to be integrated with electronic circuits, making information transfer and processing faster and more energy efficient. Light sources, a critical component of photonic integrated circuits, are still in development. Here, we compare Multi-Quantum-Well (MQW) light emitting diodes (LEDs) with Ge0.915Sn0.085 wells and Si0.1Ge0.8Sn0.1 to a reference Ge0.915Sn0.085 homojunction LED. Material properties as well as band structure calculations are discussed, followed by optical investigations. Electroluminescence spectra acquired at various temperatures indicate an effective carrier confinement for electrons and holes in the GeSn quantum wells and confirm the excellent performance of GeSn/SiGeSn MQW light emitters.
Electrospinning technology has been widely recognized because of its ability to synthesize nanoscale fibers that are structurally similar to fibrillar structure of the natural extracellular matrix (ECM). Rendering the nanofiber surface to be biofunctional is critical for the successful application of the electrospinning technology in biomedical applications. Limitations in typical conjugation chemistry and physical adsorption procedures might be overcome by using polydopamine (pDA) coating inspired by adhesive proteins secreted by marine mussels. This perspective paper attempts to highlight an emerging area of the unique combination of electrospinning with pDA surface functionalization. The scientific progress and understandings of pDA coating chemistry mechanisms, coating processes and characterization with aids of nanoscale analytical techniques are reviewed and discussed. The intrinsic biomimetic morphological characteristics of the electrospun nanofibers united with the unique advantages of the pDA associated bio-functionalization have endowed a range of successful applications, especially in the interesting and important field of bioengineering.
A combination of the unique hosting properties of cyclodextrins (CDs) and the peculiar UV-responsive trans-cis isomerization of the guest molecule azobenzene has endowed light-responsibility of the inclusion complex (IC). The IC of 4-aminoazobenzene (AAB) and hydroxypropyl-b-cyclodextrin (HPbCD), with its inherent viscosity from hydrogen bondings between CDs and p-p stacking between AABs, was electrospun into nanofibers from water without using any carrier polymer matrix. The integrity of electrospun ICs was proven by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), together with Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The homogeneous distribution of HPbCD-AAB-IC was confirmed by surface chemistry mapping using timeof-flight secondary ion mass spectrometry (ToF-SIMS). The UV response of ICs prior to, during and post electrospinning was investigated. UV irradiation prior to electrospinning caused precipitation of AAB from the aqueous IC solution. UV irradiation during electrospinning flight demonstrated the interruption of ICs and consequently broader diameter distributions were obtained. Post-spinning UV irradiation induced topography and adhesion force changes on the electrospun nanofiber surfaces, demonstrated by in situ atomic force microspectroscopy (AFM) quantitative nanomechanical mapping. The present study is the first case where the supramolecule with stimuli response was electrospun into nanofibers with retained activity.
Upconversion of sub-band-gap photons constitutes a promising way for improving the efficiency of silicon-based solar cells beyond the Shockley-Queisser limit. 1500 nm to 980 nm upconversion by trivalent erbium ions is well-suited for this purpose, but the small absorption cross section hinders real-world applications. We employ tailored gold nanostructures to vastly improve the upconversion efficiency in erbium-doped TiO 2 thin films. The nanostructures are found using topology optimization and parameter optimization and fabricated by electron beam lithography. In qualitative agreement with a theoretical model, the samples show substantial electric-field enhancements inside the upconverting films for excitation at 1500 nm for both s-and p-polarization under a wide range of incidence angles and excitation intensities. An unprecedented upconversion enhancement of 913 ± 51 is observed at an excitation intensity of 1.7 W cm −2 . We derive a semi-empirical expression for the photonically enhanced upconversion efficiency, valid for all excitation intensities. This allows us to determine the upconversion properties needed to achieve significant improvements in real-world solar-cell devices through photonic-enhanced upconversion.
Upconversion of sunlight with energy below the band gap of a solar cell is a promising technique for enhancing the cell efficiency, simply by utilizing a larger part of the solar spectrum. The present topical review addresses this concept and discusses the material properties needed for an efficient upconversion process with focus on both silicon and organic solar cells. To design efficient upconverters, insight into topics such as quantum-optics, nano-optics, numerical modeling, optimization, material fabrication, and material characterization is paramount, and the necessary concepts are introduced throughout the review. Upconversion modeling is done using rate equations, while optical modeling is done by solving Maxwell's equations using the finite element method. Topology optimization is introduced and used to generate geometries of gold nanoparticles capable of greatly enhancing the upconversion yield. Fabrication and experimental characterization methods are discussed. Some recent results are presented and finally the possibility of designing upconverting materials capable of increasing the short-circuit current in a solar cell is discussed.
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