Selective Optical Response of Hydrolytically Stable Stratified Si Rugate Mirrors to Liquid Infiltration

Stratified optical filters with distinct spectral features and layered surface chemistry were prepared on silicon substrates with stepwise anodic porosification and thermal carbonization. The use of differing parameters for successive carbonization treatments enabled the production of hydrolytically stable porous silicon-based layered optical structures where the adsorption of water to the lower layer is inhibited. This enables selective shifting of reflectance bands by means of liquid infiltration. The merit of using thermal carbonization for creating layered functionality was demonstrated by comparing the hydrolytic stability resulting from this approach to other surface chemistries available for Si. The functionality of the stratified optical structures was demonstrated under water and ethanol infiltration, and changes in the adsorption properties after 9 months of storage were evaluated. The changes observed in the structure were explained using simulations based on the transfer matrix method and the Bruggeman effective medium approximation. Scanning electron microscopy was used for imaging the morphology of the porous structure. Finally, the adaptability of the method for preparing complex structures was demonstrated by stacking superimposed rugate structures with several reflective bands

Controlling the Epitaxial Growth of Bi₂Te₃, BiTe, and Bi₄Te₃ Pure Phases by Physical Vapor Transport

Bi₂Te₃ is a well-studied material because of its thermoelectric properties and, recently, has also been studied as a topological insulator. However, it is only one of several compounds in the Bi–Te system. This work presents a study of the physical vapor transport growth of Bi–Te material focused on determining the growth conditions required to selectively obtain a desired phase of the Bi–Te system, i.e., Bi₂Te₃, BiTe, and Bi₄Te₃. Epitaxial films of these compounds were prepared on sapphire and silicon substrates. The results were verified by X-ray diffraction, Raman spectroscopy, and Rutherford backscattering spectrometry

Adhesion and Proliferation of Human Mesenchymal Stem Cells from Dental Pulp on Porous Silicon Scaffolds

In regenerative medicine, stem-cell-based therapy often requires a scaffold to deliver cells and/or growth factors to the injured site. Porous silicon (pSi) is a promising biomaterial for tissue engineering as it is both nontoxic and bioresorbable. Moreover, surface modification can offer control over the degradation rate of pSi and can also promote cell adhesion. Dental pulp stem cells (DPSC) are pluripotent mesenchymal stem cells found within the teeth and constitute a readily source of stem cells. Thus, coupling the good proliferation and differentiation capacities of DPSC with the textural and chemical properties of the pSi substrates provides an interesting approach for therapeutic use. In this study, the behavior of human DPSC is analyzed on pSi substrates presenting pores of various sizes, 10 ± 2 nm, 36 ± 4 nm, and 1.0 ± 0.1 μm, and undergoing different chemical treatments, thermal oxidation, silanization with aminopropyltriethoxysilane (APTES), and hydrosilylation with undecenoic acid or semicarbazide. DPSC adhesion and proliferation were followed for up to 72 h by fluorescence microscopy, scanning electron microscopy (SEM), enzymatic activity assay, and BrdU assay for mitotic activity. Porous silicon with 36 nm pore size was found to offer the best adhesion and the fastest growth rate for DPSC compared to pSi comporting smaller pore size (10 nm) or larger pore size (1 μm), especially after silanization with APTES. Hydrosilylation with semicarbazide favored cell adhesion and proliferation, especially mitosis after cell adhesion, but such chemical modification has been found to led to a scaffold that is stable for only 24–48 h in culture medium. Thus, semicarbazide-treated pSi appeared to be an appropriate scaffold for stem cell adhesion and immediate in vivo transplantation, whereas APTES-treated pSi was found to be more suitable for long-term in vitro culture, for stem cell proliferation and differentiation