Half of the sun's radiant energy falls outside of the visible spectrum. On a cold day, this extra infrared light provides additional warmth to residential and commercial buildings. On a warm day, it leads to unwanted heating that must be dealt with through energy-intensive climate control methods such as air-conditioning.
Visibly transparent "smart windows" that can modulate the transmission of near infrared light offer one potential cost- and energy-saving measure for modern infrastructure.
To work towards solving this technological challenge, a multidisciplinary team of researchers at Lawrence Livermore National Laboratory (LLNL) developed a new type of electrically controlled, near-infrared smart window that can cut near-infrared light transmission by almost 50%. Their secret ingredient? Vertically aligned carbon nanotubes-tiny, tube-shaped structures made from carbon atoms that are thousands of times thinner than a human hair. The research was published in Nano Letters.
In these smart windows, the carbon nanotubes are grown so they stand upright on the glass, like a microscopic forest. Because these current devices are just millimeters in size, the team noted that scaling-up the technology will be a necessary next step.
Depending on the voltage applied, the nanotubes can either absorb infrared light and block heat from the sun or let the infrared light through. One critical feature of this technology is that once they are put into either a blocking or transparent state, the carbon nanotubes retain charge well, like a battery, and so a continuous voltage is not needed to maintain that state. This offers extremely low power operation, a necessity to drive energy savings for the end user.
To come up with this design, the researchers coupled experimental fabrication and measurements from modeling efforts to better understand the microscopic physics that drive the tunable infrared response, yielding new insights into the optical and electronic physics of vertically aligned carbon nanotubes.
Their work, funded by LLNL's Laboratory Directed Research and Development program, builds on nearly a decade of LLNL research into the synthesis and study of vertically aligned carbon nanotubes.
