ZnO Films Incorporation Study on Macroporous Silicon Structure

In the present work, we developed hybrid nanostructures based on ZnO films deposited on macroporous silicon substrates using the sol–gel spin coating and ultrasonic spray pyrolysis (USP) techniques. The changes in the growth of ZnO films on macroporous silicon were studied using a UV-visible spectrometer, an X-ray diffractometer (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). XRD analysis revealed the beneficial influence of macroporous silicon on the structural properties of ZnO films. SEM micrographs showed the growth and coverage of ZnO granular and flake-like crystals inside the pores of the substrate. The root mean square roughness (RMS) measured by AFM in the ZnO grown on the macroporous silicon substrate was up to one order of magnitude higher than reference samples. These results prove that the methods used in this work are effective to cover porous and obtain nano-morphologies of ZnO. These morphologies could be useful for making highly sensitive gas sensors.

Creation of bilateral structures of macroporous silicon with nanocoatings for solar cells

We have proposed a new technological solution for the creation of solar energy elements using bilateral structures of macroporous silicon to increase the overall efficiency of converting light energy into electricity. Recently, the research on R&D in solar cell technology has focused mainly on crystalline silicon technologies and photovoltaic systems, including organic ones. The main physical phenomenon that determines the prospects of two-dimensional structures of macroporous silicon with nanocoatings as solar cells is the increase in absorption of electromagnetic radiation and photoconductivity as a result of interaction of optical modes with the developed surface of cylindrical macropores with a barrier on the nanocoating-surface boundary. We fabricated two-sided macroporous silicon structures with nanocoatings for solar cells, including silicon technology, organic nanoformations, and photovoltaic system formation. Silicon is a promising material for the manufacture of structures with a cylindrical geometry of air macropores due to the anisotropy of the cheap process of photoelectrochemical etching. The presence of periodically located cylindrical pores separated by silicon columns provides a large effective surface of the samples and enhanced optical and photophysical characteristics of silicon structures. Polymer composites with nanocoatings with CdS nanocrystals and multilayer carbon nanotubes in polyethyleneimine generate charges of opposite sign on both surfaces of the structures under illumination. The formation of bilateral structures of macroporous silicon with nanocoatings increases the overall energy conversion efficiency in solar cells by up to 60 %. In addition, one can use our proposed solar cells in the upper atmosphere.

Low-Frequency Dielectric Relaxation in Structures Based on Macroporous Silicon with Meso-Macroporous Skin-Layer

The spectra of dielectric relaxation of macroporous silicon with a mesoporous skin layer in the frequency range 1–106 Hz during cooling (up to 293–173 K) and heating (293–333 K) are presented. Macroporous silicon (pore diameter ≈ 2.2–2.7 μm) with a meso-macroporous skin layer was obtained by the method of electrochemical anodic dissolution of monocrystalline silicon in a Unno-Imai cell. A mesoporous skin layer with a thickness of about 100–200 nm in the form of cone-shaped nanostructures with pore diameters near 13–25 nm and sizes of skeletal part about 35–40 nm by ion-electron microscopy was observed. The temperature dependence of the relaxation of the most probable relaxation time is characterized by two linear sections with different slope values; the change in the slope character is observed at T ≈ 250 K. The features of the distribution of relaxation times in meso-macroporous silicon at temperatures of 223, 273, and 293 K are revealed. The Havriliak-Negami approach was used for approximation of the relaxation curves ε″ = f(ν). The existence of a symmetric distribution of relaxers for all temperatures was found (Cole-Cole model). A discussion of results is provided, taking into account the structure of the studied object.

1D and 2D polaritons in macroporous silicon structures with nano-coatings

In this paper, we used high-resolution IR absorption spectra to investigate 1D and 2D polaritons in periodical 2D macroporous silicon structures with nano-coatings of SiO2 and CdS, ZnO nanoparticles. The application of high-resolution IR absorption spectroscopy resulted in detection of dipole-active TO vibrations, photon splitting and giant two-polar absorption oscillations with amplitudes of ±107arb.un. As a result, the dispersion law in yz surfaces of macropores change to z direction along macropores. It means additional degree of freedom as vertically polarized light in z direction and horizontally polarized light in x direction resulted in beams splitting and two-photon interference - Hong-Ou-Mandel effect. In our case, 2D resonances of Wannier-Stark electro-optical effect in yz plane correspond to constructive interference of the two-photon states (bosonic behavior), and two-polar resonances in ±z direction are determined by destructive interference of the two-photon states (fermionic behavior). Two-polar oscillations of 1D -polaritons have the ultra-small half-width 0.4–0.6 сm–1 and minimal Rabi frequency of samples 1.0 сm–1 equaled to the resolution of spectral measurements. Furthermore, two-photon interference and 1D polaritons are perspective for high-coherent optical quantum computers on macroporous silicon with nano-coatings and, in addition, for lasers and new metamaterials.

Features of the two-stage formation of macroporous and mesoporous silicon structuresя

The aim of this work was the formation of multilayer structures of macroporous silicon and the study of their structural, morphological, and optical properties in comparison with the properties of multilayer structures of mesoporous silicon. The paper presents the results of the development of techniques for the formation of multilayer structures of porous silicon por-Si by stepwise change in the current with two-stage modes of electrochemical etching.The data on the morphology, composition, and porosity of macroporous and mesoporous silicon samples were obtained using scanning electron microscopy, IR spectroscopy, and X-ray reflectivity. It was shown that with the two-stage growth of porous silicon layers, the depth of the boundary between the layers of the structure was determined by the primary mode of electrochemical etching, while the total layer thickness increased with an increase in the current density of electrochemical etching.A comparative analysis of the relative intensity and fine structure of vibrational modes of IR spectra indicated a significantly more developed specific pore surface and greater sorption capacity of mesoporous silicon as compared to macroporous silicon. REFERENCES 1. Pacholski C. Photonic crystal sensors based on porous silicon. Sensors. 2013;13(4): 4694–4713. https://doi.org/10.3390/s130404694 2. Harraz F. A. Porous silicon chemical sensors and biosensors: A review. Sensors and Actuators B: Chemical. 2014;202: 897–912. https://doi.org/10.1016/j.snb.2014.06.0483. Qian M., Bao X. Q., Wang L. W., Lu X., Shao J., Chen X. S. Structural tailoring of multilayer porous silicon for photonic crystal application. Journal of Crystal Growth. 2006;292(2): 347–350. https://doi.org/10.1016/j.jcrysgro.2006.04.0334. Len’shin A. S., Kashkarov V. M., Turishchev S. Yu., Smirnov M. S., Domashevskaya E. P. Effect of natural aging on photoluminescence of porous silicon. Technical Physics Letters. 2011;37(9): 789–792. https://doi.org/10.1134/S10637850110901245. Kheifets L. I., Neimark A. B. Multiphase processes in porous media. Moscow: Khimiya Publ.; 1982. 320 p. (In Russ.)6. Canham L. Handbook of porous silicon. Switzerland: Springer International Publishing; 2014. 733 p.7. Zimin S. P. Porous silicon – material with new properties. Soros Educational Journal. 2004;8(1): 101–107. Available at: http://window.edu.ru/resource/217/21217/files/0401_101.pdf (In Russ., abstract in Eng.) 8. Seredin P. V., Lenshin A. S., Goloshchapov D. L., Lukin A. N., Arsentyev I. N., Bondarev A. D., Tarasov I. S. Investigations of nanodimensional Al2O3films deposited by ion-plasma sputtering onto porous silicon. Semiconductors. 2015;49(7): 915–920. https://doi.org/10.1134/S10637826150702109. Seredin P. V., Lenshin A. S., Mizerov A. M., Leiste H., Rinke M. Structural, optical and morphological properties of hybrid heterostructures on the basis of GaN grown on compliant substrate por-Si(111). Applied Surface Science. 2019;476: 1049–1060. https://doi.org/10.1016/j.apsusc.2019.01.23910. Seredin P. V., Leiste H., Lenshin A. S., Mizerov A. M. Effect of the transition porous silicon layer on the properties of hybrid GaN/SiC/por-Si/Si(111) heterostructures. Applied Surface Science. 2020;508(145267): 1–14. https://doi.org/10.1016/j.apsusc.2020.14526711. Lenshin A. S., Barkov K. A., Skopintseva N. G., Agapov B. L., Domashevskaya E. P. Influence of electrochemical etching modes under one stage and two Stage formation of porous silicon on the degree of oxidation of its surface layer under natural conditions. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2019;21(4): 534–543. https://doi.org/10.17308/kcmf.2019.21/2364 (In Russ., abstract in Eng.) 12. Buttard D., Dolino G., Bellet D., Baumbach T., Rieutord F. X-ray reflectivity investigation of thin p-type porous silicon layers. Solid State Communications. 1998;109(1): 1–5. https://doi.org/10.1016/S0038-1098(98)00531-613. Lenshin A. S., Seredin P. V., Agapov B. L., Minakov D. A., Kashkarov V. M. Preparation and degradation of the optical properties of nano-, meso‑,and macroporous silicon. Materials Science in Semiconductor Processing. 2015;30: 25–30. https://doi.org/10.1016/j.mssp.2014.09.04014. Ksenofontova O. I., Vasin A. V., Egorov V. V., Bobyl’ A. V., Soldatenkov F. Yu., Terukov E. I., Ulin V. P., Ulin N. V., Kiselev O. I. Porous silicon and its applications in biology and medicine. Technical Physics. 2014;59(1): 66–77. https://doi.org/10.1134/S1063784214010083

Hydrophobic/Oleophilic Structures Based on MacroPorous Silicon: Effect of Topography and Fluoroalkyl Silane Functionalization on Wettability

The effect of the morphology and chemical composition of a surface on the wettability of porous silicon structures is analyzed in the present work. Hydrophobic and superhydrophobic macroporous substrates are attractive for different potential applications. Herein, different hydrophobic macroporous silicon structures were fabricated by the chemical etching of p-type silicon wafers in a solution based on hydrofluoric acid and coated with a fluoro silane self-assembled monolayer. The surface morphology of the final substrate was characterized using a scanning electron microscope. The wettability was assessed from contact angle measurements using water and organic solvents that present low surface energy. The experimental data were compared with the classical wetting states theoretical models described in the literature. Perfluoro-silane functionalized macroporous silicon surfaces presented systematically higher contact angles than untreated silicon substrates. The influence of porosity on the surface wettability of macoporous silicon structures has been established. These results suggest that the combination of etching conditions with a surface chemistry modification could lead to hydrophobic/oleophilic or superhydrophobic/oleophobic structures.