HiPIMS deposition of VO2 thermochromic coating for enhanced optical properties

• Main Author: Vu, Tuan Duc
• Other Authors: Long Yi
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Microelectronics and semiconductor materials::Thin films; Engineering::Materials::Composite materials
• Online Access: https://hdl.handle.net/10356/160650
• Institution: Nanyang Technological University

Smart windows are defined by their ability to regulate incoming solar radiation in order to reduce energy consumption of buildings by modulating the heat intake. Vanadium dioxide (VO2) is one of the potential candidates for smart window materials due to its ability to reversibly transit from monoclinic VO2(M) to rutile VO2(R) at a near room temperature. As a result of this transition, the infrared radiation (IR) transparent VO2(M) abruptly becomes IR opaque and effectively regulates the heat intake by solar radiation. Despite the promising potential, VO2 has not been commercialized due to various technical difficulties which hinder its feasibilities outside of laboratories environment: high transition temperature (τc) of 68oC; low luminous transmission (Tlum) of around 40%; and low solar modulation (ΔTsol) less than 25%. Currently, various methods have been used to fabricate VO2 thin films in attempts to improve its intrinsic properties and improve the efficiency of high quality VO2 thin film fabrication process. One of those methods is high-power impulse magnetron sputtering (HiPIMS). HiPIMS is a physical vapor deposition method that utilizes high power density in short periodic pulses to increase the energy of sputtered atoms while protecting the integrity of the target, promising higher film quality than traditional methods of evaporation or conventional sputtering. This thesis detailed the development of HiPIMS deposition techniques for VO2-based devices with enhanced thermochromic performance. In chapter 4, a HiPIMS deposition technique for VO2(M) thin film on a commercial material, such as soda-lime glass substrate, was established with a high deposition rate of 5.7 nm/min. The VO2(M) deposited on soda-lime glass exhibits excellent crystallinity and thermochromic properties (highest luminous transmission Tlum ≈ 30.4%, and solar modulation ΔTsol ≈ 12%) in comparison with similarly prepared VO2 on high temperature glass and quartz substrates. The high crystallinity rendered by short deposition duration and high ionization in HiPIMS process opens opportunities to apply high quality VO2(M) thin film onto a variety of substrate more efficiently. Expanding from the established foundational method in chapter 4, modification to the substrate was made through development of a template-based bottom-up approach to economically produce controlled biomimetic antireflective nanostructured coatings. The Greta-oto butterfly has transparent wings with extraordinary omnidirectional anti-reflection behaviour, owing to the unusual nanostructures with random height and space distribution on its wing surface which are difficult to reproduce en masse. To mimic such structure, chapter 5 of this thesis reported a low-cost bottom-up approach by stacking monolayer-aligned AgNWs meshes, followed by deposition of overcoats. Such AgNWs mesh provides the template to grow nanostructures, imitating that on the Greta-oto’s wings which consists of randomly situated nano-cones with controllable mean pitches and heights. The resulting nanostructure in VO2/AgNWs systems showed enhanced anti-reflections and thermochromism with up to 70% reduced omnidirectional reflectance, and 37% increase in Tlum while retaining ΔTsol in comparison with pure VO2 film of similar thickness. VO2 is severely limited in commercialization potential due to its instability in the operational environment. Protective layering to combat VO2 degradation often requires extra fabrication steps which can be complex and costly. A new strategy to fabricate VO2 nanorod nanocomposite thermochromic smart window with record high 33-year service life was discussed in chapter 6. By introducing seeding, one-step HiPIMS process was achieved which comprises of guided growth of VO2 nanorod embedded within amorphous V2O5 matrix. Furthermore, the unique nanorod structure with lowered transition temperature τc of 56.6 °C has tunable solar modulation depending on the solar incident angle. This new strategy provides a solution to address VO2 Achilles’ heel, its instability, which could facilitate the applications of this near room temperature phase change materials with respect to not only thermochromic smart windows but also other applications. This thesis is a step-by-step investigation of VO2 thermochromic devices fabrication using HiPIMS process. The optimal deposition parameters and the appropriate substrate and process modifications are ascertained with the goal of improving VO2 thermochromism, optical modulation, and service life.