in your element Silicon beyond the valley
Mietek Jaroniec refl ects on how silicon, whether bonded with other elements in a variety of materials, in high purity for electronic devices, or in its newer ‘black silicon’ form, continues to be invaluable in many aspects of our lives.
ilicon is, aft er oxygen, the most synthesized in the presence of surfactant abundant element of the Earth’s crust. templates — a synthesis strategy that has SBut although about 75% of the Earth is created almost unlimited opportunities made from silica — or silicon dioxide (SiO2), for the development of nanomaterials, the main constituent of silicate minerals such as nanoporous silica-based particles, such as sand, quartzite or granite — for catalysis, separations, environmental elemental silicon itself is only occasionally cleanup, drug delivery and found in nature and was unknown until nanotechnology. the nineteenth century. In 1811 Gay-Lussac While discussing silica, it and Th enard probably obtained impure would be remiss to neglect amorphous silicon by heating potassium the large-scale formation of with silicon tetrafl uoride; however, the silica structures with nanoscale discovery of this element is usually credited Silicon valley: Scanning electron precision by various marine to Berzelius, who in 1824 added extra microscopy image of the laser- organisms. Understanding this washings to isolate pure silicon. It is now irradiated silicon surface (inset), ‘biosilifi cation’ process occurring in produced on a large scale by heating silica which resembles a fragment of Bryce Canyon nature off ers tremendous opportunities for with carbon in an electric furnace, at high National Park in Utah, USA. the development of environmentally benign temperatures (1,900–2,350 °C) far exceeding syntheses of novel silicon-based materials, its melting point (1,414 °C). and could eventually lead to advances in According to the US Geological to solar cells, liquid-crystal displays, and biosensors, biocatalysis and the engineering Survey, the world’s reserves of pure silicon semiconductor-based detectors. Remarkable of biomolecules — a fi eld now known as (including that synthesized) exceeded fi ve advances in microelectronics have been ‘silicon biotechnology’. million tons in 2007 — the best indicator made possible by the miniaturization of Another amazing and technologically of its importance in today’s technology. silicon integrated circuits, and this fi eld promising discovery shows the importance Over 90% is consumed for the production is now heading towards nanoelectronics. of unveiling microstructure and of silicon-containing chemicals and alloys. Th e use of porous silicon, for instance, nanostructure. In 1998, Mazur and his Aluminium-rich alloys, for example, are for the development of a range of sensors team at Harvard University showed that commonly used in the automobile industry; arose from its luminescence properties and the irradiation of a silicon wafer with silicones (which feature silicon–oxygen and large surface areas. Th e high-purity silicon femtosecond laser pulses in the presence silicon–carbon bonds) have found extensive required for such microelectronic devices is of a sulfur-containing gas transforms its applications as greases, resins, rubbers and obtained by a complex multi-step process, shiny surface into a forest of microscopic sealants; silica, in the form of sand, is also usually involving the transformation of spikes, which resembles Bryce Canyon a basic ingredient of glass and concrete, a crude metallic silicon to chlorosilanes in Utah, USA (pictured). Normally, the some of the most widely used materials; and (compounds containing silicon–chlorine surface of silicon refl ects a substantial aerogel, an extremely light form of silica bonds). Aft er separation and purifi cation amount of light — but this ‘black silicon’ because 90% of its volume is occupied by these are reduced with hydrogen to the strongly absorbs visible light by trapping pores, is a very effi cient insulating material. polycrystalline silicon used to make silicon it between its spikes, which makes it very Without diminishing the enormous wafers (smooth thin discs). promising for solar cells. It also absorbs importance of these applications, the Th e richness of silicon chemistry is infrared radiations with wavelengths as greatest impact of silicon on today’s amazing; new discoveries arise continuously long as 2,500 nm and, surely, novel optical technology and lifestyle is accounted for by in this fi eld. And although the surface and electronic applications of black silicon only a small fraction of the world’s reserves area of 1 g of sand particles is extremely can be expected in the future. Th is example (about 5%) — the high-purity silicon used in small, the internal surface area of the same shows that silicon, although known for a variety of electronic devices, ranging from amount of silica particles with accessible almost 200 years, can still amaze us. ❐ computer microchips and power transistors nanoscale pores (about 3 nm) can easily exceed 1,000 m2 (approximately the area MIETEK JARONIEC is in the Department of of an Olympic-size swimming pool). Such Chemistry, Kent State University, Kent, Ohio particles with ordered nanopores are 44242, USA. e-mail: [email protected] Na Mg AlSi P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga
166 NATURE CHEMISTRY | VOL 1 | MAY 2009 | www.nature.com/naturechemistry
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