Macroporous Silicon Research Articles

Effect of CBD growth times on the ZnO microrods prepared on macroporous silicon

Effect of CBD growth times on the ZnO microrods prepared on macroporous silicon

Surfactant-Free Aqueous Fabrication of Macroporous Silicone Monoliths for Flexible Thermal Insulation

Abstract Hydrophobic silicone macroporous materials prepared in an aqueous solution by the sol–gel method have been considered for various applications such as separation media, heat insulators, and liquid nitrogen adsorbents. In the conventional preparation process, surfactants are used to suppress phase separation to obtain a uniform bulk material. However, a large amount of solvent and time is required to remove them before drying, which hinders industrial-scale synthesis. By copolymerizing tetra-, tri-, and bifunctional organosilicon alkoxides in an aqueous acetic acid–urea solution, flexible macroporous silicone monoliths were successfully obtained. The marshmallow-like monoliths recovered their original shape even after 80% uniaxial compression and significant bending and water repellency. The thermal conductivity of those materials was ∼0.035 W m−1 K−1 and did not increase even under 60% uniaxial compression. This characteristic property can be used for thermal insulation on surfaces with various shapes and in confined spaces under harsh conditions.

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.

X-ray reflectivity investigation of multilayer macroporous silicon structures

In this work, the X-ray reflectivity was used to study the porosity of multilayer macroporous silicon samples obtained under various conditions. The porosity calculation is based on a change in the position of the critical angle of total external reflection resulting from a decrease in the density of the porous silicon layer. Our findings show that the absence of photoluminescence in the samples is due to a porosity of about 30 % in the surface layer. The morphological features were characterized by scanning electron and atomic force microscopy.

In English

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.

(Invited) Scratch Controled Electrochemical Production of Macropores in Silicon

Electrochemical etching of macropores in Si can be done either randomly (wild pores) or lithographically defined. When the points to be etched are not defined, the etching process starts at defects, or statistically distributed on the surface, just limited by the space-charge-region created in the semiconductor as effect of the electric field applied [1]. Ordered pores are usually accomplished by lithography plus a preliminary etching step for the formation of inverted pyramids [2]. At the pits of the pyramids the electric field is more intense during the electrochemical etching process, causing that the probability to etch pores is higher at those points. Defining the positions where pores are etched is necessary for different applications, where order is important. However, this makes the process to produce pores, which is not necessarily clean, more expensive with the need clean-room appliances (masking layer deposition and lithography).In order to simplify the production of ordered micropores, in this work it is proposed to take advantage of the increased probability of surface defects to act as nucleation points of growing pores. An electronic grade single side polished p-type Si wafer, with resistivity of 20 Ωcm was used as starting material. The electrolyte was a 10 % (v/v) HF solution in DMF. The wafer was scratched with fine sand paper previous to the etching process. Electrochemical etching was performed aplying a current density of 20 mA/cm^2 for 10 min. Macropores grew mainly along the scratches (see Figure). The spacing between pores depends on the distances between scratches. The regions without scratches have a lower probability to be etched. However, if the defects introduced by the sand paper are cured by a short chemical etching process in KOH, the probability to electrochemically etch those sections is further increased, in comparison with the sections with scratches and no KOH treatment. With the observations, it is straight to think that it is possible to do deffect engingineering to tune the electrochemical etching of micropores in Si.[1] E. Quiroga-González, H. Föll, book chapter "Chapter 2: Fundamentals of Silicon Porosification via Electrochemical Etching", in "Porous Silicon: Formation and Properties, Volume 1", editor G. Korotcenkov, CRC Press. (2015). ISBN: 9781482264548.[2] E. Quiroga-González, E. Ossei-Wusu, J. Carstensen, H. Föll, J. Electrochem. Soc. 158(11) E119 (2011). Figure 1

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.

Different Electrode Configurations for NH3 Gas Sensing Based on Macro Porous Silicon Layer

Different Electrode Configurations for NH3 Gas Sensing Based on Macro Porous Silicon Layer

In English

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.
 
 
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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.

Heterostructure of TiO2 and macroporous silicon: The simplest relaxation oscillator

A heterostructure composed of TiO 2 and macroporous silicon in this study may represent the simplest relaxation oscillator ever reported. It consisted of macroporous Si with TiO 2 grown on its pore walls, and metallic contacts. This is also the first relaxation oscillator of its kind. Such oscillators are commonly formed by a capacitor or an inductor and a feedback loop with a switching device operating with a threshold voltage. The dielectric characteristics of TiO 2 (capacitive behavior) in combination with the tunnel diode characteristics of the heterostructure (presenting a negative differential resistance, when activated at a threshold voltage) are shown to give rise to the relaxation oscillations. • The present device represents the simplest relaxation oscillator ever reported, and the first one of TiO 2 and Si. • The key characteristic of the device in order to obtain oscillations is that it behaves as tunnel diode. • Metal Assisted Chemical Etching of Si followed by TiO 2 deposition by hydrothermal synthesis is the simple procedure to prepare the device.

Distribution of Excess Charge Carriers in Bilateral Macroporous Silicon with Different Thicknesses of Porous Layers

Distribution of Excess Charge Carriers in Bilateral Macroporous Silicon with Different Thicknesses of Porous Layers

Active liquid flow control through a polypyrrole-coated macroporous silicon membrane toward chemical stimulation applications

In this paper, we demonstrate the voltage-controlled flow of organic liquid through a macroporous silicon structure. The demonstrated flow control structure consists of a silicon-based macropore membrane and flow control units formed on polypyrrole (PPy)-coated pores, which are actuated by its electrochemical wetting change. Conformal coating of a PPy layer along macropore silicon walls is achieved with the help of spray-deposited tin-oxide (SnO2) layer on the pores. The transport of organic liquid through the developed membrane is successfully controlled by low-voltage external bias on the PPy layer. Cyclic deliveries of organic liquid were demonstrated using the proposed concept. Considering the highly-ordered fluidic channel structure formed in silicon, which can have multiple separated electrodes, the proposed concept has the potential to achieve an actuated fluidic dispenser in an arrayed form.

In English

In this paper, we investigated high-resolution IR absorption and reflection spectra in one-sided periodical 2D macroporous silicon structures with nano-coatings of SiO2 and CdS, ZnO nanoparticles, as well as two-sided structures of macroporous silicon without nano-coatings. After changing the resolution of spectra measurements from 2 to 1 cm–1, the oscillation period of Wannier-Stark electro-optical effect decreases by 3 times, and absorption increases by 60–100 times; and for two-sided structures the oscillation period decreases by 1.5 times and absorption increases by 25–30 %. In addition, giant absorption oscillations with positive and negative amplitudes of 107 arb. un. were evaluated in spectral regions of Si–Si–bonds and Pb centers. Similar oscillations in the reflection spectra have much less amplitudes up to 4·104 arb. un. In the spectral area of the transverse phonon ωTO (Si–Si–bonds) absorption spectra of 2D macroporous silicon structures consistent fully with data for phonon polaritons in microresonators as a result of resonance interaction of dipole-active vibrations with the frequency of ωTO in thin films with the surface modes of microresonator. In addition, microresonators interact both with each other in one-sided macroporous silicon structures and in the system of two-sided macroporous silicon. The giant absorption oscillations testify the strong interaction of surface polaritons with photons. The coherence of oscillations and large-scale spatial correlation are а result of exciton-polariton condensation on macropores as microresonators. In 2D macroporous silicon structures with nano-coatings band bending on the surface of the macropores are significant. Therefore, the generated photoelectrons link with holes, forming electron-hole pairs named as exciton-polaritons according to phenomenon of Bose-Einstein condensation.

Incorporation of Zinc Oxide on Macroporous Silicon Enhanced the Sensitivity of Macroporous Silicon MSM Photodetector

In this study, metal-semiconductor-metal (MSM) photodetectors (PDs) was fabricated on the macroporous silicon substrate, (mPS) followed by growth of zinc oxide (ZnO) on the mPS substrate (ZnO/mPS) with addition of nickel (Ni) contact in finger mask pattern. The mPS was formed by electrochemical etching technique with an estimated etching time of 15 min by a continuous current flow of 25 mA. The ZnO nanostructures were synthesized by cost-effective chemical bath deposition (CBD) process. Field Effect Scanning Electron Microscopy (FESEM) and X-ray Diffractometer (XRD) revealed the randomly distributed of upper-morphologies and crystalline properties of x-shaped mPS as well as the distribution of ZnO/mPS. A good Schottky contact with superior photoconductivity, photo-responsivity, and photo-sensitivity have been demonstrated from these MSM PDs under illumination of 383 nm and 422 nm light sources. An enhanced photoconductivity at 422 nm had been demonstrated from Ni/ZnO/mPS/Ni PD which was believed due to the incorporation of energy properties at ZnO/mPS interface with a greater energy band gap of 3.15 eV. The responsivity of Ni/mPS/Ni PD at 383 nm and 422 nm were 0.088 A W−1 and 0.171 A W−1, respectively. While the Ni/ZnO/mPS/Ni PD shows the double photo-responsivity for both wavelengths, with the values of 0.160 A W−1 and 0.385 A W−1, respectively. In overall, the incorporation of ZnO/mPS structure drawn a great significance in PDs performance due to unique tunable photo-responsivity, higher sensitivity, and faster rise and decay time which attributed to the swift recombination process.

Efficient, fast response, and low cost sensor for NH3 gas molecules based on SnO2: CuO/macroPSi nanocomposites

In the present study, a procedure of the inserting of SnO2:CuO nanoparticles with different CuO nanoparticles contents within a macroporous silicon layer (macroPSi) gas sensor was prepared and successfully investigated. The macroPSi was effectively fabricated by laser assisted etching process, and CuO nanoparticles loaded with SnO2 with a high value of surface area were successfully synthesized by the spray pyrolysis method. Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FE-SEM) manifested a novel morphology for CuO Bucky particles inside the pores and a nano nail like structure for SnO2 with a small average grain size of CuO Bucky particles with 30% content. This morphology of nanocomposites improved the sensing performance for NH3 gas. A higher sensitivity with a very swift response and recovery times of 4 s and 55 s, respectively, was obtained with 150 ppm of NH3 gas at the room temperature. This improvement in gas sensor performance is strongly related to the higher specific surface areas and smaller particle size with a higher surface roughness of SnO2 and CuO nanoparticles within the nanocomposites.

High-coherent oscillations in IR spectra of macroporous silicon with nanocoatings

High-coherent oscillations in IR spectra of macroporous silicon with nanocoatings

Residual Deformations and Mechanical Stresses in Macroporous and Nonporous Silicon Under Normal Etching Conditions

Lattice changes of macroporous silicon were studied under variations in the etching conditions of macropores. It was established that under all the etching conditions, additional mechanical stresses and corresponding deformations arise up to the appearance of dislocations in the macroporous layer at elevated voltage or current.

Controllable formation of plasmonic gold nanoparticles by pulsed laser–induced etching

This work discusses the preparation and formation of macro porous silicon (PSi) with silicon nano pillars; synthesized via laser–induced etching. The role of laser pulse duty cycle (10–100%) was investigated for silicon nano pillars samples creation using laser diode; characterized by its short wavelength (405 nm) and high laser intensity (500 mW/cm2). Morphological and spectroscopic aspects of the Si nanocrystallite sizes and plasmonic Au–NP, surface topography, roughness and thickness of the created layer were investigated by the scanning probe microscopy, photoluminescence measurements, the analysis of (FESEM) images and (XRD) patterns. The results revealed well-regulated Si nano pillars layers after adjusting the laser duty cycle under the same laser intensity and etching time. The histogram of Si nano pillars sizes, surface roughness and altitude of nano pillars within macro PSi created at 20% laser duty cycle showed specific characteristics; due to the minimal heat accumulation within the pillars and limited probability of damaging the pillars’ morphological aspects. The Si nano pillars were employed to synthesize and control the plasmonic features of Au-NPs. The histogram of hotspot regions and plasmonic Au-NPs sizes and their specific surface areas and grain size showed well-controlled structures after adjusting the laser pulse duty cycle.