PSi Samples Research Articles

Monitoring of degradation of porous silicon photonic crystals using digital photography.

We report the monitoring of porous silicon (pSi) degradation in aqueous solutions using a consumer-grade digital camera. To facilitate optical monitoring, the pSi samples were prepared as one-dimensional photonic crystals (rugate filters) by electrochemical etching of highly doped p-type Si wafers using a periodic etch waveform. Two pSi formulations, representing chemistries relevant for self-reporting drug delivery applications, were tested: freshly etched pSi (fpSi) and fpSi coated with the biodegradable polymer chitosan (pSi-ch). Accelerated degradation of the samples in an ethanol-containing pH 10 aqueous basic buffer was monitored in situ by digital imaging with a consumer-grade digital camera with simultaneous optical reflectance spectrophotometric point measurements. As the nanostructured porous silicon matrix dissolved, a hypsochromic shift in the wavelength of the rugate reflectance peak resulted in visible color changes from red to green. While the H coordinate in the hue, saturation, and value (HSV) color space calculated using the as-acquired photographs was a good monitor of degradation at short times (t < 100 min), it was not a useful monitor of sample degradation at longer times since it was influenced by reflections of the broad spectral output of the lamp as well as from the narrow rugate reflectance band. A monotonic relationship was observed between the wavelength of the rugate reflectance peak and an H parameter value calculated from the average red-green-blue (RGB) values of each image by first independently normalizing each channel (R, G, and B) using their maximum and minimum value over the time course of the degradation process. Spectrophotometric measurements and digital image analysis using this H parameter gave consistent relative stabilities of the samples as fpSi > pSi-ch.

Photoluminescence spectra of nitrogen-rich InN thin films grown on Si(110) and photoelectrochemical etched Si(110)

Indium nitride (InN) thin films were deposited on anisotropic silicon [Si(110)] and photoelectrochemical etched silicon [Psi(110)] substrates by reactive radio-frequency sputtering. All deposited films showed wurtzite nanocrystalline InN films with a (101) preferred growth orientation. The optical properties of the nitrogen-rich InN thin films were investigated under various deposition gas concentrations. Strong photoluminescence was observed in the 1.8 eV–1.98 eV energy range for nanocrystalline InN grown on Si(110) at room temperature. The lower energy emission peak was achieved for InN grown on PSi(110) samples.

Study of the optical properties of a thermoresponsive polymer grafted onto porous silicon scaffolds

In this report, a polymer-filled porous silicon (pSi) structure is described that is able to detect changes in temperature around a critical value en route to developing a temperature sensor deployed in wounds dressings that signals inflammation or infection of the wound bed. Using surface-initiated atom transfer radical polymerization (SI-ATRP), thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains are grafted onto pSi layers with different porosity and pore size and the optical changes (effective optical thickness below and above the lower critical solution temperature (LCST)) are monitored via interferometric reflectance spectroscopy. Six etching conditions and three different surface functionalization conditions are explored in order to optimise the optical response to temperature change. Thermally oxidised pSi samples with the highest investigated porosity (80%) show the largest optical response and will be the target for developing optical sensors of wound temperature.

Detection of nerve agent stimulants based on photoluminescent porous silicon interferometer

Porous silicon (PSi) exhibiting dual optical properties, both Fabry-Pérot fringe and photolumincence, was developed and used as chemical sensors. PSi samples were prepared by an electrochemical etch of p-type silicon under the illumination of 300-W tungsten lamp during the etch process. The surface of PSi was characterized by cold field-emission scanning electron microscope. PSi samples exhibited a strong visible orange photoluminescence at 610 nm with an excitation wavelength of 460 nm as well as Fabry-Pérot fringe with a tungsten light source. Both reflectivity and photoluminescence were simultaneously measured under the exposure of organophosphate vapors. An increase of optical thickness and quenching photoluminescences under the exposure of various organophosphate vapors were observed.

Photo and electroluminescence of porous silicon layers

Porous silicon layers were prepared by both chemical and electrochemical methods on n- and ptype Si substrates. In the former technique, light emission was obtained from p-type and n-type samples. It was found that intense light illumination during the preparation process was essential for PSi formation on n-type substrates.An efficient electrochemical cell with some useful features was designed for electrochemical etching of silicon. Various preparation parameters were studied and photoluminescence emissions ranging from dark red to light blue were obtained from PSi samples prepared on p-type substrates. N-type samples produced emissions ranging from dark red to orange-yellow. Electroluminescence of porous silicon samples showed that the color of the emission was the same as the photoluminescence color of the sample, and its intensity and duration depended on the current density passed through the sample. The effects of exposure of samples to air, storage in vacuum, and heat-treatment in air on luminescence intensity of the samples and preparation of patterned porous layers were also studied.Keywords: Porous silicon layers; photoluminescence; electroluminescenceDOI: http://dx.doi.org/10.3126/bibechana.v8i0.4897 BIBECHANA 8 (2012) 46-52

Use of electrochemical methods to examine different surface preparation methods for organic coatings on steel

For preparation of steel surfaces before applying an organic coating, waterjetting is a modern, effective and environmentally friendly method. However there is a lack of data on its performance in terms of the subsequent protection afforded by the paint compared with more conventional methods. Hence previously exposed steel panels had their surfaces prepared in a variety of ways (abrasion, blasting with garnet entrained in water (10 K psi) and waterjetting without garnet at two velocities (20 K psi and 40 K psi)) prior to coating with two typical maintenance coatings. These were then exposed under immersion condition in the laboratory and monitored using the DC resistance method. Another set of samples were exposed in a salt spray test in accordance with ASTM B 117 for 500 h. At the end of both tests DC measurements were carried out and the samples were inspected visually in terms of blistering and rust. Results showed that both the immersion test and the salt spray test differentiated the surface preparations methods in the same order. Thus control panels performed the best. Nearly as good though were the two (20 K psi and 40 K psi) sets of waterjetted samples. The abraded samples were variable but overall provided less protection. The panels blasted with garnet entrained in water (10 K psi) samples proved to be the worst. Characterisation of the surfaces prior to coatings using the scanning electron microscope was also conducted. A tentative explanation for the differing protection afforded involving the integrity of the oxide film was suggested. Further work is needed to substantiate this. Overall, with samples in the immersion test, results using electrochemical methods proved themselves to be very effective in assessing and even predicting performance in a relatively short period of time. The results also correlated with the salt spray test. Also the work showed that waterjetting is a very effective way of preparing the surface prior to maintenance painting.

Photoluminescence Properties of Anodic Si Layers Formed in HF/Oxidant Electrolytes

Anodic layers are formed on p-Si substrates by etching in a or a mixed solution. The scanning electron microscopy (SEM) image on the -formed layer reveals that the sample has a double-layered [highly resistive top/porous Si (PSi)-like bottom] structure, while the -formed surface provides an SEM image popularly observed on PSi layers formed in a HF/alcohol solution. The -formed sample emits light in the green region but frequently shows a gradual change in the photoluminescence (PL) spectrum from green to red upon exposure to air. The -formed sample exhibits a stable red emission, which is typically observed in conventional PSi samples. The temperature-dependent PL properties of such samples are quite different. The -formed sample can be explained in terms of the usual Arrhenius expression; however, the -formed sample exhibits a multiple-peak structure at below , but it disappears at high temperatures. The X-ray photoelectron and Fourier transform IR spectroscopies suggest that the highly resistive top layer formed in the solution is of any compound of the hydroxyl group or others. The final sample treatment may be a key point for obtaining air-stable, green-emitting layers in the solution.

Hydrogen and oxygen concentration analysis of porous silicon

Elastic recoil detection and Rutherford backscattering spectrometry combined with the nuclear reaction analysis method have been applied for the determination of oxygen, hydrogen and carbon concentration depth profiles of aged p+-type porous Si layers of different low and medium porosities. The plan view and cross-section scanning electron microscopy images have provided with information about both the diameter and silicon skeleton structure of the pores. The concentration depth profiles reveal the existence of a non-homogeneous subsurface porous film several hundred nanometers thick for all the studied samples. Differences in the atomic composition among low and medium porosity layers and the possible origin of various impurities are discussed. The maximum H content in PSi has been observed at the depth of 200–600nm, while the highest oxygen concentration is typical of 200nm thick subsurface layers. The highest obtained ratio of H/Si atomic concentrations reaches the value of 2 for the PSi samples with porosity P of 66%, comparing to NH/NSi=0.27 in the case of P=25% PSi.

Recycling of harbor sediment as lightweight aggregate

Sediment sampled from Taichung Harbor was mixed with local reservoir sediment at different weight ratios to prepare lightweight aggregate at 1050, 1100, and 1150 °C. A pressure of 3000 or 5000 psi was used to shape the powder mixtures into pellets before the heating processes. The results indicate that the leaching levels of trace metals from the lightweight aggregate samples are considerably reduced to levels less than Taiwan Environmental Protection Administration regulatory limits. Increasing final process temperature tends to reduce the bulk density and crushing intensity of lightweight aggregate with a concomitant increase in water sorption capability. Lightweight aggregate with the lowest bulk density, 0.49 g cm −3 for the 5000 psi sample, was obtained with the heating process to 1150 °C. Based on the X-ray absorption near edge structure results, FeSO 4 decomposition with a concomitant release of SO x ( x = 2, 3) is suggested to play an important role for the bloating process in present study.

Detection of Avidin Based on Rugate-structured Porous Silicon Interferometer

Biosensor based on rugate PSi interferometer for the detection of avidin has been described. Rugate PSi fabricated by applying a computer-generated pseudo-sinusoidal current waveform has been prepared for the application as a label-free biosensor based on porous silicon interferometer. The fabrication, optical characterization, and surface derivatization of a rugate PSi has been described. The method to fabricate biotin-derivatized rugate PSi has been investigated. The surface and cross sectional morphology of rugate PSi are obtained with SEM. FT-IR spectroscopy is used to characterize the oxidation and functionalization reaction of rugate PSi sample. Binding of the avidin into the biotin-derivatized rugate PSi induces a change in refractive index. A red-shift of reflectivity by 18 nm in the reflectivity spectrum is observed, when the biotin-modified rugate PSi was exposed to a flow of avidin.

Detection of organophosphates based on surface-modified DBR porous silicon using LED light

Organophosphonate nerve agents have been detected with DBR PSi samples containing a copper (II) sulfate. The detection efficiency is increased when a LED is used as an incident light source. DBR porous silicon containing copper (II) sulfate shows a TEP detection limit of about 150 ppb for 1 min, when a LED (λem=590 nm) is used as an incident light source.

Differential elastic scattering and backscattering cross sections of H, Si and O in the low energy (2–50 eV) range for electron spectroscopy on porous Si

Hydrogen is found to influence the porous Si/PSi system. H atoms cannot be detected by AES or XPS, but profound effects have been observed on PSi using the elastic peak electron spectroscopy (EPES) in the 50–100 eV range after HF treatment of PSi samples. Limited numbers of low energy cross sections are available in the literature. The data of Joy et al are confined to 20 and 50 eV. The differential cross sections of electrons have been calculated in the present work in the 2–50 eV range for H, Si and O, i.e. elements composing the surface of PSi. The relativistic and nonrelativistic PWEM method was used in calculations. The electron-atom interaction was described by the TFD potential, using a recent approximation, in view of considerable errors of earlier approximations. Some experimental data are available for gaseous hydrogen. They turn out to be in good agreement with our calculated values. The total elastic, scattering, the total backscattering and the backscattering cross sections integrated within the angular range (124–176 °) of RFA, σ RFA, have been calculated for monolayer of H, Si and O. They strongly increase with decreasing energy. In the case of Si, a broad maximum is observed and strong decrease below 20 eV, with a deep minimum between 2 and 10 eV. Here σ RFA,H ⪢ σ RFA,Si, EPES is promising in this very low energy range for the system H/Si, analogous to H/W.