Aarhus University researchers have created a self-regulating smart window that blocks heat—without electricity, sensors, or moving parts. Powered by sunlight, the window uses silver nanorings and solvent evaporation to adaptively reduce infrared transmission while staying crystal clear.
Study: Thermoplasmonic Nanorings for Passive Solar-Responsive Smart Windows in Energy-Efficient Building Applications. Image Credit: Melinda Nagy/Shutterstock.com
In a recent breakthrough, researchers from Aarhus University introduced a smart window technology that operates without wiring, sensors, or electronics. Their study, published in the journal Advanced Functional Materials, demonstrated an innovative solar-responsive design using a light-sensitive hybrid material composed of thermoplasmonic silver nanorings.
These transparent nanostructures aim to dynamically regulate heat transmission based on solar intensity while maintaining visible light levels, enabling windows to adapt naturally to changing sunlight.
The findings represent a key step toward energy-efficient and sustainable architecture by reducing reliance on mechanical cooling and lowering greenhouse gas emissions.
Addressing Energy Challenges in Modern Buildings
Energy-efficient building technologies are essential for addressing global energy consumption challenges, especially in regions where cooling demands exceed heating needs. Conventional windows often allow excessive solar heat gain, resulting in higher indoor temperatures, increased reliance on air conditioning, and higher energy costs.
To mitigate these issues, engineers are exploring intelligent materials that can adapt to environmental conditions.
One promising approach involves thermoplasmonic nanostructures, which leverage localized surface plasmon resonance (LSPR) to alter thermal and optical properties in response to sunlight. Unlike traditional electrochromic or mechanically adjustable smart windows, which require external power, this passive technology enables the dynamic regulation of solar heat transmission while maintaining natural light levels.
About the Study
For the study, the researchers designed metallic nanorings for integration into double-glazed window systems, enabling passive control of solar heat transmission. Using a combination of hole-mask colloidal lithography (HCL) and physical vapor deposition (PVD), they produced uniform arrays of silver (Ag) nanorings with diameters of approximately 200 nm. This fabrication method ensured precise control over the size, spacing, and uniformity of the components.
The nanorings, embedded within a transparent coating, act as microscopic antennas that selectively absorb near-infrared (NIR) light, responsible for most solar heat gain, while allowing visible light to pass through. This maintains interior brightness while reducing unwanted heat transmission.
When exposed to sunlight, the nanorings generate localized heating, which triggers the evaporation of a thin overlayer of solvent, such as ethanol or butanol, inside the double-glazed cavity. This phase change from liquid to vapor decreases the local refractive index, shifting the localized surface plasmon resonance (LSPR) peak and enabling real-time modulation of the optical response without the need for external power sources or sensors.
The experimental setup incorporated these coatings into a double-glazed configuration consisting of a plain glass layer and a nanoring-coated layer. To validate the mechanism, ultraviolet-visible (UV-Vis) and near-infrared (NIR) spectrophotometry, as well as finite-difference time-domain (FDTD) simulations, were employed, which confirmed that the nanorings’ resonance and heat regulation behavior closely matched theoretical predictions.
The researchers also compared nanorings to solid nanodiscs and found that the rings provided greater spectral tunability and sensitivity to environmental refractive index changes.
Study Findings
The study provided crucial insights into the performance of thermoplasmonic silver nanorings and their potential for passive solar regulation. The nanorings responded strongly to changes in the surrounding refractive index, with the LSPR peak shifting from approximately 995 nm in air to approximately 1115 nm in ethanol (EtOH). This redshift confirmed their ability to adapt to varying environmental conditions.
Temperature-dependent experiments demonstrated a reversible blueshift in the LSPR peak as the surrounding medium warmed, caused by evaporation of the solvent layer from the nanoring surfaces under heat, which lowers the refractive index around the structures. This behavior supports the use of the nanorings for real-time, passive control of solar heat gain.
When integrated into a prototype double-glazed window system, the nanoring coating reduced NIR transmission under natural sunlight, resulting in a measured temperature drop of more than 4 °C on objects placed behind the coated window while still allowing natural light to pass through.
Another important observation was the reversible nature of the thermal response. The windows allowed more heat to enter during cooler conditions and blocked it during intense sunlight, ensuring comfort the whole day without relying on power or mechanical systems. This dynamic regulation was demonstrated under both LED-based NIR illumination and full-day solar testing in Denmark, confirming the system’s viability under real-world environmental conditions.
The nanoring coatings also demonstrated strong thermal cycling durability, with consistent optical response over repeated heating and cooling cycles.
Practical Applications for Sustainable Building Design
This research has significant implications for modern building design, particularly in the development of smart windows that dynamically regulate solar heat.
Thermoplasmonic nanorings integrated into window coatings can control solar gain, reducing reliance on mechanical cooling and lowering energy consumption. These nanorings block harmful radiation while allowing visible light to pass through, enhancing occupant comfort by minimizing glare and heat buildup. They are suitable for both new constructions and retrofits, improving indoor comfort and building sustainability without the need for renovations.
The system’s response can also be tuned based on the choice of solvent. For example, ethanol and butanol provide different phase-change temperatures, enabling custom window performance depending on the regional climate. This tunability, combined with compatibility with scalable fabrication methods, makes the technology adaptable for global use.
Additionally, the nanoring layers could potentially be integrated with existing low-emissivity (Low-E) coatings to improve thermal control while maintaining visible transparency.
Conclusion
The development of a passive, light-responsive window coating using silver nanorings marks an important advancement in energy-efficient building design. This technology allows windows to regulate heat naturally, reducing cooling loads and improving indoor comfort, all without the need for external power sources or sensors.
These findings underscore the potential of thermoplasmonic nanorings to enhance smart window technology through passive solar control. Looking ahead, key areas for future research include refining fabrication techniques for large-scale manufacturing, boosting the long-term stability of the nanostructures, and evaluating alternative solvents with different vapor pressures and refractive indices.
The research team also recommended exploring complementary materials to further improve thermal performance and integrating nanoring layers with Low-E coatings to reduce both solar heat gain and internal heat re-radiation.
With continued progress, silver nanoring-based coatings could play a central role in the evolution of smart window systems, helping buildings manage energy more efficiently, lowering carbon emissions, and supporting a more sustainable built environment.
Journal Reference
González, X, B., & et al. (2025, September). Thermoplasmonic Nanorings for Passive Solar-Responsive Smart Windows in Energy-Efficient Building Applications. Advanced Functional Materials, e18295. DOI: 10.1002/adfm.202518295, https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202518295
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.