“Heat trap”: Light-induced localized heating and thermionic electron emission from carbon nanotube arrays

Light-induced heating of a typical bulk conductor to thermionic electron emission temperatures usually requires high-power lasers. This is because of the efficient dissipation of heat generated at the illuminated spot to the surroundings, since electrical conductors are normally also good thermal conductors. We show that the situation can be drastically different in a carbon nanotube forest and a spot on the surface of the forest can be heated to above 2000 K using a low-power beam of visible light, leading to localized thermionic electron emission. This unique phenomenon may be explained by a rapid drop in thermal conductivity with increase in temperature, leading to a positive feedback that thermally isolates an island on the forest. Applications include thermoelectrics, photocathodes, optical switches, solar cells and even solar displays.

► We illuminate a spot on the sidewall of a carbon nanotube forest using a low-power visible laser.
► Although nanotubes are thermally conductive, the spot heats to thermionic emission temperatures with an incandescent glow.
► The illuminated spot becomes effectively thermally isolated, while maintaining electrical conductivity.
► Thermionic electron emission is thus achieved with very low laser power, opening the door to various novel applications.