Dopant induced room temperature phase transition in vanadium dioxide thin films

An ever-increasing energy demand due to rapid urbanization has led to a continuous need for innovative, sustainable, and energy-efficient smart building designs. Windows constitute about ∼ 50% of the total heat loss from a typical household. In this regard, regular window designs could be improved to not only have adequate visible transparency but also modulate IR radiation based on the requirement. The improved smart window design could incorporate either electrochromic or thermochromic coatings, which change their optical behavior in response to external stimuli. Whereas electrochromic coatings require electrical power to function and thus are not suitable for energyefficient smart windows, thermochromic coatings can alter their optical properties solely based on external/internal temperature. One of the promising materials that display thermochromism is vanadium dioxide (VO2), which undergoes a structural phase transition (SPT) from monoclinic to rutile crystal structure at 341 K (68◦C). This structural transition is coupled with a semiconductor-to-metal transition (SMT), which switches the optical behavior of this material from an IR-transparent state to an IR-reflecting state. However, the phase transition temperature (Tc) for pure VO2 is slightly on the higher side and needs to be brought down near room temperature for its commercial applicability. One of the strategies to reduce Tc utilizes the lattice-straining method, wherein the material is strained, causing its transition to occur earlier or later depending on the type of strain. Whereas mechanical straining requires epitaxial thin-film growth and has thickness dependence, chemical straining uses dopant atoms to strain the host lattice and is thickness independent. In this work, unstrained and chemically strained VO2 thin films are deposited on soda–lime glass (SLG) substrates using compound ceramic targets in an RF magnetron sputtering system. Four different dopants, each with varying ionic sizes and concentrations (W6+, Mo6+, Nb5+, and Ta5+), were incorporated to modify the switching behavior and transition temperature (Tc) of the VO2 thin films. The chemically strained thin films were found to have their Tc in the range of 175 K to 334 K, depending on dopant type and concentration. Moreover, chemically strained VO2 shows switching characteristics with Tc = 310 ± 7 K, while also maintaining adequate luminous transmittance of 34.50% and a high infrared switching efficiency of 40.93% at λ = 2500 nm. Furthermore, the role of microstructure (crystallite size, stoichiometry, etc.) in altering the phase transition behavior has been studied. The obtained experimental results could help establish VO2 as a potential next-generation material for various energy-efficient device applications.