Silicon nanowire

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As a silicon nanowire ( English silicon nano wire ) are massive silicon structures with a diameter in the nanoscale denotes that for applications in micro-electronics and the battery technology are suitable. The diameter ranges from a few nanometers to a few hundred nanometers , depending on the application . The length of the wire significantly exceeds the diameter and varies greatly depending on the application. Although nanowires by definition have a maximum diameter of 100 nanometers, silicon nanowires with a diameter of more than 100 nanometers are often referred to as nanowires. Wires with diameters in the micrometer range are called whiskers .

Manufacturing

Growth of silicon nanowires

A distinction is made between two basic principles in production: top-down and bottom-up .

In the top-down approach, the wires are etched from a solid piece of silicon . Monocrystalline silicon wafers are particularly suitable as starting materials . The wafer is coated with a transition metal as a catalyst where there shouldn't be any wires at the end . The wafer is then electrolessly etched with hydrofluoric acid. Etching processes are mainly used in microelectronic applications.

In the bottom-up approach, silicon nanowires are grown on a substrate. A special chemical vapor deposition process is used for this purpose; see VLS mechanism . Gold particles serve as the catalyst . Since gold leads to undesirable effects in semiconductor technology , the VLS mechanism is unsuitable for microelectronics.

Delimitations

Silicon nanowire is just a geometric shape of silicon. In terms of its crystal structure and properties, it does not differ from ordinary single-crystal or amorphous silicon. The small dimensions can lead to quantum effects . However, this applies to all solid bodies with very small dimensions. Nanotubes made of silicon, on the other hand, are an allotrope of silicon and have fundamentally different properties. Silicon is semiconducting, while silicon nanotubes can have metallic conductivity.

Both silicon nanowire and nanoporous silicon can be produced by hydrofluoric acid etching of single-crystal silicon.

  • In nanoporous silicon, electrical etching creates elongated pores in the solid.
  • In the case of silicon nanowire, the solid body is etched using catalyst-assisted, non-electrical etching in such a way that only the wires remain.

Applications

microelectronics

In microelectronics, the term nanowire is used for particularly narrow silicon tracks in the range of a few nanometers, where quantum effects occur. These nanowires usually run parallel to the substrate. Examples include:

  • Silicon nanowire as a gate between the source and drain in a field effect transistor .
  • As a (bio) sensor, especially by functionalizing the surface.

Anode in the lithium-ion battery

Silicon is a promising electrode material for anodes in lithium-ion batteries . Theoretically, silicon has the highest specific capacity of all materials at 4200 mAh / g. More lithium ions can be stored in the silicon crystal than in pure lithium metal. When the battery is charged, lithium ions are stored in the crystal of the silicon anode, which increases its size by more than 300%. This change in size severely damages the silicon solid and thus leads to rapid degradation of the anode. Using silicon nanowire can significantly increase the service life of a silicon anode, as the wire can expand without disturbing neighboring wires. A field of silicon nanowire is created perpendicular to the substrate. Silicon nanowire anodes have a capacity that is more than ten times higher than that of conventional graphite electrodes. However, this does not have to mean that the battery capacity increases by a factor of 10, since the cathode and electrolyte also have a great influence on the battery properties. The service life of the anode is increased by using silicon nanowire, but this does not completely solve the problem of degradation. Coated nanowire anodes are a possible approach for longer life cycles. Nanoporous silicon and silicon nanotubes are further alternatives for the anode material.

Other uses

Researchers have developed superhydrophobic surfaces based on silicon nanowires. To do this, the scientists grew nanowires on etched pyramids in the micrometer range. Such a nanostructure-on-microstructure approach can also be found in the lotus effect . The resulting structure was made hydrophobic by a coating. The result is almost complete non-wettability.

Individual evidence

  1. ↑ a b c d Rui Huang, Jing Zhu: Silicon nanowire array films as advanced anode materials for lithium-ion batteries In: Materials Chemistry and Physics Volume 121, Issue 3, 2010, pp. 519-522, doi: 10.1016 / j. matchemphys.2010.02.017 .
  2. ↑ a b Candace K. Chan et al .: High-performance lithium battery anodes using silicon nanowires . In: Nature Nanotechnology . tape 3 , no. 1 , January 2008, p. 31-35 , doi : 10.1038 / nnano.2007.411 .
  3. ^ Brian J. Landi, Matthew J. Ganter, Cory D. Cress, Roberta A. DiLeo, Ryne P. Raffaelle: Carbon nanotubes for lithium ion batteries . In: Energy & Environmental Science . tape 2 , no. 6 , June 2, 2009, p. 638-654 , doi : 10.1039 / B904116H .
  4. ↑ Xiaocheng Li, Beng Kang Tay, Philippe Miele, Arnaud Brioude, David Cornu: Fabrication of silicon pyramid / nanowire binary structure with superhydrophobicity . In: Applied Surface Science . tape 255 , no. 16 , May 30, 2009, pp. 7147-7152 , doi : 10.1016 / j.apsusc.2009.03.047 .