Thursday, June 30, 2016

The critical material nearly everyone overlooks

Condensed matter physics is tough to popularize, and yet aspects of it are absolutely ubiquitous in modern technologies.  For example:  Nearly every flat panel display, from the one on your phone to your computer monitor to your large television, takes advantage of an underappreciated triumph of materials development, a transparent conducting layer.  Usually, when a material is a good conductor of electricity, it tends to be (when more than tens of nm thick) reflective and opaque.   Remember, light is an electromagnetic wave.  If the electric field from the light can make the mobile charge in the material move, and if that charge can keep up with the rapid oscillations (1014 Hz and faster!) of the electric field, then the light tends to be reflected rather than transmitted.  This is why polished aluminum or silver can be used as a mirror.

The dominant technology for transparent conductors is indium tin oxide (ITO), which manages to thread between two constraints.  It's a highly doped semiconductor.  The undoped indium oxide material has a band gap of 3 eV, meaning that violet light with a shorter wavelength than about 350 nm will have enough energy to be absorbed, by kicking electrons out of the filled valence band and into the conduction band.  Longer wavelength light (most of the visible spectrum) doesn't have enough energy to make those transitions, and thus the material is transparent for those colors.   ITO has had enough tin added to make the resulting material fairly conducting at low frequencies (say those relevant for electronics, but much lower than the frequency of visible light).  However, because of the way charge moves in ITO (see here or here for a nice article), it does not act reflective at visible frequencies.   This material is one huge enabling technology for displays!  I remember being told that the upper limit on LCD display size was, at one point, limited by the electrical conductivity of the ITO, and that we'd never have flat screens bigger than about a meter diagonal.  Clearly that problem was resolved.

Indium isn't cheap.  There are many people interested in making cheaper (yet still reasonably transparent) conducting layers.  Possibilities include graphene (though even at monolayer thickness it does absorb about 2% in the visible) and percolative networks of metal nanowires (or nanotubes).    Unfortunately, because of the physics described above, it would appear that transparent aluminum  (in the sense of having true bulk metal-like properties but optical transparency in the visible) must remain in the realm of science fiction.

1 comment:

DanM said...

Are there any materials that have reasonable conductivity (similar to ITO, say) at DC but that are transparent to terahertz radiation? Hm...