Porous silicon (PS) is one of the most studied porous materials since it has a wide range of applications, such as the development of electroluminescent devices [1], combustible batteries [2], biosensors [3], and pharmaceutical administration devices [4]. The main method to obtain PS is electrochemical etching, due to the capacity to control porosity, the diameter of the pores, and the thickness of the porous layers. The potential to apply PS in diverse areas results from its structural, mechanical, optical, electrical, thermal, physicochemical, and biochemical properties, which are related to morphology and pore size. As the porous layer and the pore size can be controlled during the obtaining process, its properties can be adjusted according to the desired application. The control of the PS properties can be achieved by manipulating its structural parameters, modifying the chemistry of its surface, and introducing other materials. This opens the possibility of creating PS-based devices, since they have advantages such as: its compatible with silicon technology, it is a cheap material, it is easy to obtain, with high reproducibility, capable of operating at room temperature, and it has a big specific surface.
An important optical characteristic of PS that awoke worldwide scientific interest, and it is not in crystalline silicon, it is its photoluminescence efficiency at room temperature. In the case of PS, photoluminescence can vary from red to blue (with adequate manufacturing conditions), depending on the thickness of the silicon filaments and the surface characteristics; this property opens the posibility of manufacturing a light-emitting devices or a laser [1]. In reference to its chemical properties, there are reports that the specific surface can be up to 500 m2/gm, which is equivalent to a surface bigger than a soccer court, in a small volume [5]. This specific big surface is precisely the characteristic that gives PS its ability to chemically react on its surface [6]. For example, it has been observed that when putting PS in contact with other materials, such as steam, organic gases, or even the environment, the chemistry of its surface is modified [7]. Due to this, PS can be applied to the development of biosensors.
Despite the positive characteristics of PS, there are problems when it is applied for the development of devices. For example, in the development of electroluminescent devices, there are problems with stability due to the material having a large internal surface and therefore a tendency to suffer from a chemical change when exposed to air, which can generate changes in the luminescent emission [8]. Another issue with a large internal surface is developing an efficient electrical contact [5]. Moreover, the porous films have very low conductivity, which causes bad efficiency due to the high operating current needed [9]. As was demonstrated by S. Ménard et al [10], which presented results that show an increase in the resistivity with the increment in the porosity. Even the idea of using PS as an electric insulator is considered. Also in the development of biosensors, the PS shows stability problems, due to the large specific surface. The porous layer is susceptible to changes in the pH, when the pH reaches alkaline levels, these changes can even dissolve the porous layer [7]. A possible solution for this kind of problem is the incorporation of other materials that reduce the effective area and improve electrical properties without the loss of the optic and chemical properties. The material or element to introduce has to be incorporated into the porous silicon by methods that do not affect the chemical, optic, and morphological characteristics. Another solution can be the compression of the porous layer, searching for the reduction of the effective surface preserving the main characteristics of the PS. In this work the compression of PS powder compressed into a tablet was chosen.
This study presents the results of the characterization of a PS tablet. The analysis of photoluminescence showed that the luminescent emission from the PS tablet has a slight shift of the maximum of emission, due to the compression process, therefore keeping its potential to be applied in the development of optic devices. The x-ray diffraction (XRD) characterization showed that the PS tablet has similar characteristics to amorphous silicon after the compression process. The morphological characterization was carried out through scanning electron microscopy (SEM), the images showed a compact surface with some irregularities. The morphology also was studied by profilometry and a rough surface was observed. The chemical properties of the surface were characterized with Fourier transform infrared spectroscopy (FTIR) and showed a hydrogenated surface, which can be used as a platform for the development of medical and biological devices [11]. The semi-quantitative chemical analysis was carried out by energy dispersive X-ray spectroscopy (EDS) and showed an oxidized surface.