Rai Ayushi, Bienz Siiri, Hansen Vidar F, Zenobi Renato, Kumar Naresh
Department of Mechanical and Structural Engineering and Materials Science, University of Stavanger, N-4036 Stavanger, Norway.
Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1-5/10, 8093 Zurich, Switzerland.
ACS Appl Mater Interfaces. 2025 Jun 4;17(22):32625-32634. doi: 10.1021/acsami.5c04827. Epub 2025 May 22.
Thermochromic vanadium dioxide (VO) thin films, known for their reversible metal-insulator transition (MIT) near 68 °C, are promising candidates for energy-efficient applications such as smart window coatings. However, optimizing their structural and interfacial properties to enhance thermochromic performance remains a significant challenge. Traditional characterization techniques such as X-ray diffraction and transmission electron microscopy face inherent limitations in simultaneously providing detailed chemical information and nanoscale spatial resolution─capabilities that are essential for resolving localized structural heterogeneity and interfacial phenomena. This study employs hyperspectral tip-enhanced Raman spectroscopy (TERS) imaging to address these limitations and investigate the nanoscale structure of pristine VO and VO/TiO thin films. TERS imaging revealed nanoscale regions of lattice deformations and nanocrystallites with different orientations in VO thin films, resulting in a high density of grain boundaries that elevate the MIT temperature. In VO/TiO bilayers, TERS detected coexisting anatase and brookite phases in the TiO layer, with tensile strain in the brookite phase and the VO/TiO interface characterized by localized intermixing and strain. These novel insights underscore the polycrystalline nature of the thin films grown with pulsed layer deposition technique and highlight the critical role of nanoscale structural and interfacial properties in determining thermochromic performance of VO-based thin films. Furthermore, this study demonstrates the effectiveness of TERS as a robust nanoanalytical tool for advancing the design of VO-based smart coatings and functional materials.
热致变色二氧化钒（VO₂）薄膜以其在68°C附近的可逆金属-绝缘体转变（MIT）而闻名，是智能窗涂层等节能应用的有前途的候选材料。然而，优化其结构和界面性能以提高热致变色性能仍然是一项重大挑战。传统的表征技术，如X射线衍射和透射电子显微镜，在同时提供详细的化学信息和纳米级空间分辨率方面存在固有局限性，而这些能力对于解决局部结构异质性和界面现象至关重要。本研究采用高光谱针尖增强拉曼光谱（TERS）成像来解决这些局限性，并研究原始VO₂和VO₂/TiO₂薄膜的纳米级结构。TERS成像揭示了VO₂薄膜中晶格变形的纳米级区域和不同取向的纳米晶体，导致高密度的晶界，从而提高了MIT温度。在VO₂/TiO₂双层膜中，TERS检测到TiO₂层中共存的锐钛矿相和板钛矿相，板钛矿相和VO₂/TiO₂界面存在拉伸应变，其特征是局部混合和应变。这些新的见解强调了采用脉冲层沉积技术生长的薄膜的多晶性质，并突出了纳米级结构和界面性能在决定VO₂基薄膜热致变色性能方面的关键作用。此外，本研究证明了TERS作为一种强大的纳米分析工具在推进VO₂基智能涂层和功能材料设计方面的有效性。
