Black silicon is a semiconductor material, a surface modification of silicon with very low reflectivity and correspondingly high absorption of visible (and infrared) light.

The modification was discovered in the 1980s as an unwanted side effect of reactive ion etching (RIE).^[1][2] Other methods for forming a similar structure include electrochemical etching, stain etching, metal-assisted chemical etching, and laser treatment.

Black silicon has become a major asset to the solar photovoltaic industry as it enables greater light to electricity conversion efficiency^[3] of standard crystalline silicon solar cells, which significantly reduces their costs.^[4]

Black silicon is a needle-shaped surface structure where needles are made of single-crystal silicon and have a height above 10 µm and diameter less than 1 µm.^[2] Its main feature is an increased absorption of incident light—the high reflectivity of the silicon, which is usually 20–30% for quasi-normal incidence, is reduced to about 5%. This is due to the formation of a so-called effective medium^[5] by the needles. Within this medium, there is no sharp interface, but a continuous change of the refractive index that reduces Fresnel reflection. When the depth of the graded layer is roughly equal to the wavelength of light in silicon (about one-quarter the wavelength in vacuum) the reflection is reduced to 5%; deeper grades produce even blacker silicon.^[6] For low reflectivity, the nanoscale features producing the index graded layer must be smaller than the wavelength of the incident light to avoid scattering.^[6]

The unusual optical characteristics, combined with the semiconducting properties of silicon make this material interesting for sensor applications. Potential applications include:^[7]

• Image sensors with increased sensitivity
• Thermal imaging cameras
• Photodetector with high-efficiency through increased absorption.^{[8][9][10][11]}
• Mechanical contacts and interfaces^[2]
• Terahertz applications.^{[12][13][14][15]}
• Solar cells^{[16][17][18][19]}
• Antibacterial surfaces^[20] that work by physically rupturing bacteria's cellular membranes.
• Surface enhanced Raman spectroscopy^[21]
• Semiconductor chemical or gas sensors^{[22][23][24][25][26][27][28][29][30][31][32]}

When the material is biased by a small electric voltage, absorbed photons are able to excite dozens of electrons. The sensitivity of black silicon detectors is 100–500 times higher than that of untreated silicon (conventional silicon), in both the visible and infrared spectra.^[42][43]

A group at the National Renewable Energy Laboratory reported black silicon solar cells with 18.2% efficiency.^[19] This black silicon anti-reflective surface was formed by a metal-assisted etch process using nano particles of silver. In May 2015, researchers from Finland's Aalto University, working with researchers from Universitat Politècnica de Catalunya announced they had created black silicon solar cells with 22.1% efficiency^[44][45] by applying a thin passivating film on the nanostructures by Atomic Layer Deposition, and by integrating all metal contacts on the back side of the cell.

A team led by Elena Ivanova at Swinburne University of Technology in Melbourne discovered in 2012^[46] that cicada wings were potent killers of Pseudomonas aeruginosa, an opportunist germ that also infects humans and is becoming resistant to antibiotics. The effect came from regularly-spaced "nanopillars" on which bacteria were sliced to shreds as they settled on the surface.

Both cicada wings and black silicon were put through their paces in a lab, and both were bactericidal. Smooth to human touch, the surfaces destroyed Gram-negative and Gram-positive bacteria, as well as bacterial spores.

The three targeted bacterial species were P. aeruginosa, Staphylococcus aureus and Bacillus subtilis, a wide-ranging soil germ that is a cousin of anthrax.

The killing rate was 450,000 bacteria per square centimetre per minute over the first three hours of exposure or 810 times the minimum dose needed to infect a person with S. aureus, and 77,400 times that of P. aeruginosa. However, it was later proven that the quantification protocol of Ivanova's team was not suitable for these kind of antibacterial surfaces.

• Quantum efficiency of a solar cell
• Solasys
• University of Wisconsin-Madison. "'Stealth' material hides hot objects from infrared eyes." ScienceDaily. www.sciencedaily.com/releases/2018/06/180622174752.htm (accessed 23 June 2018).