Phase Change Materials (VO2)

Vanadium dioxide is an interesting material that reversibly switches between a semiconductor and a metal at a temperature of 68 C.  In the semiconducting state it is a cubic crystal, and in the metallic state it is a hexagonal crystal. The switching produces large changes in its resistivity, optical constants and stress.

Our interest in VO2 is for its applications in optical limiters and switchable polarizers. Optical limiters are front-end components that protect sensitive electronics and cameras from high energy laser threats. The rise in temperature in the VO2 film can be designed to make it switch from a highly transparent state to a highly reflective state. Wiregrid polarizers can also be made by periodically patterning the VO2. By raising the temperature, it can switch between a polarizing and a birefringent optical element.

Unfortunately, VO2 is not easy to grow. The process window is narrow because vanadium has many different stable oxidation states besides VO2. growth requires a specific substrate temperature (typically 500-600 C) and a specific oxygen partial pressure. Our VO2 films are produced using Ion-Assisted Deposition (IAD) by thermal evaporation. We use vanadium as the source metal along with oxygen ions under highly controlled conditions to produce VO2 thin films.

Most of the commercially available uncooled thermal cameras contain a variant of VO2. The sensing element is VOx, which is a mixture of several different vanadium oxides. Instead of rapidly switching at 68 C, it exhibits a slow switching, which is better suited as a temperature sensing element.

 

 

Publications

  1. Pengfei Guo, David Lombardo, and Andrew M. Sarangan. “Vanadium dioxide switchable components based on wiregrids for mid-infrared applications”. Nanoengineering: Fabrication, Properties, Optics, and Devices XIV, page 43. SPIE, 2017. doi:10.1117/12.2272758
  2. Mengyang Zou, Chuan Ni, and Andrew Sarangan. “Ion-assisted evaporation of vanadium dioxide thin films”. Proc. SPIE Nanoengineering: Fabrication, Properties, Optics, and Devices XIII, page 99271Q, 2016. doi:10.1117/12.2238491