Alexandru Bujor

Bio: Alexandru Bujor is an academic researcher from University of Bucharest. The author has contributed to research in topics: Etching (microfabrication) & Total external reflection. The author has co-authored 3 publications.

In-Depth Analysis of Porous Si Electrodes for Supercapacitors

Abstract: Supercapacitors have presented promising results in energy storage until now, making them a possible candidate for becoming a mainstream rechargeable power source, which could be easily integrated ...

From Chip Size to Wafer-Scale Nanoporous Gold Reliable Fabrication Using Low Currents Electrochemical Etching.

Abstract: We report a simple, scalable route to wafer-size processing for fabrication of tunable nanoporous gold (NPG) by the anodization process at low constant current in a solution of hydrofluoric acid and dimethylformamide. Microstructural, optical, and electrochemical investigations were employed for a systematic analysis of the sample porosity evolution while increasing the anodization duration, namely the small angle X-ray scattering (SAXS) technique and electrochemical impedance spectroscopy (EIS). Whereas the SAXS analysis practically completes the scanning electronic microscopy (SEM) investigations and provides data about the impact of the etching time on the nanoporous gold layers in terms of fractal dimension and average pore surface area, the EIS analysis was used to estimate the electroactive area, the associated roughness factor, as well as the heterogeneous electron transfer rate constant. The bridge between the analyses is made by the scanning electrochemical microscopy (SECM) survey, which practically correlates the surface morphology with the electrochemical activity. The results were correlated to endorse the control over the gold film nanostructuration process deposited directly on the substrate that can be further subjected to different technological processes, retaining its properties. The results show that the anodization duration influences the surface area, which subsequently modifies the properties of NPG, thus enabling tuning the samples for specific applications, either optical or chemical.

X-ray scattering profiles: revealing the porosity gradient in porous silicon

Abstract: Porous silicon layers with different porosities were prepared by adjusting the anodization current density of the electrochemical etching process, starting from highly doped p-type crystalline silicon wafers. The microstructural parameters of the porous layers were assessed by high-resolution X-ray diffraction, total external reflection, scanning electron microscopy and nitro­gen adsorption–desorption analysis. Furthermore, both the surface porosity and the mean porosity for the entire volume of the samples were estimated by employing total external reflection measurements and X-ray reciprocal-space mapping, respectively. The results clearly indicate that the surface porosity is different from the mean porosity, and the presence of a depth porosity gradient is suggested. To evaluate the porosity gradient in a nondestructive way, a new laboratory method using the grazing-incidence X-ray diffraction technique is reported. It is based on the analysis of the X-ray scattering profiles of the porous layers to obtain the static Debye–Waller factors. In this way, a description of the porosity gradient in a quantitative framework becomes possible, and, as a result, it was shown that the porosity increases exponentially with the X-ray penetration depth. Moreover, a strong dependence between the porosity gradient and the anodization current was demonstrated. Thus, in the case of the lowest anodization current (e.g. 50 mA cm−2) a variation of only 15% of the porosity from the surface to the interface is found, but when applying a high anodization current of 110 mA cm−2 the porosity close to the bulk interface is almost three times higher than at the surface.

Fabrication of Nitrogen/Boron Highly Co‐Doped Graphene Electrode for Enhanced Electrochemical Performance

Abstract: A facile hydrothermal and freeze-drying approach was employed to prepare high co-doped content (8.16 % N and 6.31 % B) N, B co-doped graphene gel (N/BGR). Positively charged [BMIm][BF4] ionic liquid as N/B source and negatively charged graphene oxide (GO) as graphene precursor are capable of combining and self-assembly by electrostatic force. The N/BGR electrode presented enhanced capacitance performance that could be associated with the 3D porous structures that provide space for ion migration and redox active sites that provide pseudocapacitance.

Investigation of the relationship between porosity and luminescent properties of porous silicon

Abstract: In this work we obtained porous silicon with different porosity by electrochemical etching and studied their photoluminescence. Two well-known photoluminescence mechanisms of porous silicon related to the composition and morphology of the surface have been discovered, and it has been established at what porosity values they prevail. It is shown that an increase in the porosity index leads to an increase in the intensity of photoluminescence.