Porous silicon
At the time, Leigh Canham – while working at the Defence Research Agency in England – reasoned that the porous silicon may display quantum confinement effects.
In the published experiment, it was revealed that silicon wafers can emit light if subjected to electrochemical and chemical dissolution.
The growing interest was evidenced in the number of published work concerning the properties and potential applications of porous silicon.
[3] In 2001, a team of scientists at the Technical University of Munich inadvertently discovered that hydrogenated porous silicon reacts explosively with oxygen at cryogenic temperatures, releasing several times as much energy as an equivalent amount of TNT, at a much greater speed.
Explosion occurs because the oxygen, which is in a liquid state at the necessary temperatures, is able to oxidize through the porous molecular structure of the silicon extremely rapidly, causing a very quick and efficient detonation.
Although hydrogenated porous silicon would probably not be effective as a weapon, due to its functioning only at low temperatures, other uses are being explored for its explosive properties, such as providing thrust for satellites.
It was found that the introduction of ethanol eliminates hydrogen and ensures complete infiltration of HF solution within the pores.
Porous silicon formation by stain-etching is particularly attractive because of its simplicity and the presence of readily available corrosive reagents; namely nitric acid (HNO3) and hydrogen fluoride (HF).
Bellet concluded that it was impossible to avoid cracking in thick porous silicon layers under normal evaporating conditions.
Porosity is defined as volume fraction of voids within the PS layer and can be determined easily by weight measurement.
A study by Canham in 1995 found that "a 1 μm thick layer of high porosity silicon completely dissolved within a day of in-vitro exposure to a simulated body fluid".
PS demonstrates optical properties based on porosity and complex refractive indices of Si and the medium inside the pores.
Often, freshly etched porous silicon may be unstable due to the rate of its oxidation by the atmosphere or unsuitable for cell attachment purposes.
The research concluded that "amino silanisation and coating the pSi surface with collagen enhanced cell attachment and spreading".
It was found that high porosity mesoporous layers were completely removed by the simulated body fluids within a day.
In contrast, low to medium porosity microporous layers displayed more stable configurations and induced hydroxyapatite growth.
The human blood plasma contains monomeric silicic acid at levels of less than 1 mg Si/L, corresponding to the average dietary intake of 20–50 mg/day.
It was found that concentration of the acid rose only briefly above the normal 1 mg Si/L level and was efficiently expelled by urine excretion.
[21] The simple adjustment of pore morphology and geometry of porous silicon also offers a convenient way to control its wetting behavior.
Stable ultra- and superhydrophobic states on porous silicon can be fabricated and used in lab-on-a-chip, microfluidic devices for the improved surface-based bioanalysis.
