  Thermal stealth and camouflage have been intensively studied for blending
objects with their surroundings against remote thermal image detection.
Adaptive control of infrared emissivity has been explored extensively as a
promising way of thermal stealth, but it still requires an additional feedback
control. Passive modulation of emissivity, however, has been remained as a
great challenge which requires a precise engineering of emissivity over wide
temperature range. Here, we report a drastic improvement of passive camouflage
thin films capable of concealing thermal objects at near room temperature
without any feedback control, which consists of a vanadium dioxide (VO2) layer
with gradient tungsten (W) concentration. The gradient W-doping widens the
metal-insulator transition width, accomplishing self-adaptive thermal stealth
with a smooth change of emissivity. Our simple approach, applicable to other
similar thermal camouflage materials for improving their passive cloaking, will
find wide applications, such as passive thermal camouflage, urban energy-saving
smart windows, and improved infrared sensors.
