Fast Electrochromic Switching for Optical Hardening Applications

摘要

Infrared optical switching devices based on closed-tube-vapor-transport grownsingle crystals of group IV metal dichalcogenides were investigated. Reversible infrared reflectance modulation was identified in alkali metal intercalated HfS2 and ZrS2. A reflectance driven transmittance modulation of 0.0 1 % to 50% from 2-16 micrometers was observed with LixHfS2 single crystals chemically lithiated and delithiated. Similar, although less dramatic, modulation was obtained with AgxHfS2 crystals. Efforts to develop practical optical switching devices based on this IR modulation focused on the incorporation of electroactive and photoactive electron donor-acceptor complexes in the van der Waals' gaps of the crystals. Electron donor (e.g., Li or Ag) intercalation is readily achieved because of coulombic stabilization of the intercalated crystal through intercalation of the chalcogens (e.g., S) and the donor cation. Intercalation of electron acceptors (e.g., I and Br) into reduced (donor intercalated) dichalcogenides was found energetically unfavorable and resulted in deintercalation of the donor. Mixed-metal (group IV and V) dichalcogenide crystals, that are intrinsically n-type, were also used as hosts for acceptor intercalation. The as-grown mixed-metal crystals had higher electrical conductivities and were more IR reflective than pure HfS2 or ZrS2. Attempts to intercalate I and Br into the mixed-metal crystals resulted in extensive exfoliation. The use of metallocenes, principally chromocene, to create a 5A c-axis expansion was successful and led to the preparation Of I2 'intercalated' chromocene-HfS2. The nature of the bonding between the intercalated I2 and both the chromocene and HfS2 remains unknown.