Commercial buildings in the United States alone account for nearly 40% of the total energy consumption. Among them, electricity is the largest energy source for buildings. Therefore, the design of new energy efficient materials and technologies is crucial to meet goals such as the Net-Zero Energy Commercial Building Initiative (CBI) put forward by the U.S. Department of Energy (DOE). Approaches to block or reflect intense sunlight when needed, and to capture the sun’s light and heat when desirable, remain complex and expensive to fabricate.

A low cost, durable composite film used as a window “shade” where the transparency can be tailored to adjust heat and light. The film could also be mounted between two panes of glass and motor controlled.

A composite film consisting of a thin layer of quasi-amorphous array of silica nanoparticles (NPs) is embedded in a bulk elastomeric polymer film.

• It is highly transparent (>90% transmittance in the visible wavelength) in the initial state.
• Upon mechanical stretching, the transmittance is dramatically reduced to 30% and displays angle-independent structural color at a strain >30%.
• The displayed reflective color is a so-called structural color, which could be tuned by the silica NP size; the color displayed is invariant with the applied strain.
• The switch between transparency and colored states has been reversibly cycled over 1000 times without losing the film’s structural and optical integrity.

The initial state is transparent vs. technologies such as polymer dispersed liquid crystals and electrochromic displays.

• The switch between transparency and colored states has been reversibly cycled at least 1000 times without losing the film’s structural and optical integrity.
• The “window” can be massproduced using low-cost methods from abundant materials.
• In addition to light blocking smart windows, it can also be used for display or security applications.