Materials Science

A Graded Improvement

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Science  29 Feb 2008:
Vol. 319, Issue 5867, pp. 1163
DOI: 10.1126/science.319.5867.1163a

Refractive index is a key parameter to consider in selecting materials for optics and photonics applications, as it determines the extent of reflection and refraction when light impinges on an interface. Unfortunately, optimal choice of this parameter often necessitates compromising other material properties. Kim et al. show that they can conveniently tailor the refractive index of a single material-the transparent conductor indium tin oxide (ITO)—for device applications. Using oblique angle deposition, they are able to grow porous films consisting of arrays of oriented rods. The porosity can be controlled by changing the angle of the ITO vapor flux, thus tuning the refractive index from a bulk value of 2.19 to below 1.3. The authors exploit this tunability to grow a six-layer gradient coating on a light-emitting diode (LED), in which the ITO acts as both a coating and a conducting layer. By gradually reducing the refractive index, they eliminate almost all Fresnel reflection and thereby improve the output of the LED by 24% compared to a device made with a bulk ITO layer. — MSL

Adv. Mater. 20, 801 (2008).

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