  The regulation of temperature at the macro and microscale is a major
energy-consuming process of humankind. Modern cooling systems account for 15%
of the global energy consumption and are responsible for 10% of greenhouse gas
emissions. Due to global warming, a ten-fold growth in the demand of cooling
technologies is expected in the next 30 years, thus linking global warming and
cooling needs through a worrying negative feedback loop. Here, we propose an
inexpensive solution to this global challenge based on a single-layer of silica
microspheres self-assembled on a soda-lime glass substrate. This
two-dimensional (2D) crystal acts as a visibly translucent thermal blackbody
for above-ambient daytime radiative cooling and can be used to passively
improve the thermal performance of devices that undergo critical heating during
operation. The temperature of a crystalline silicon wafer was found to be 14K
lower during daytime when covered with our thermal emitter, reaching an average
temperature difference of 19K when the structure was backed with a silver
layer. In comparison, the soda-lime glass used as a reference in our
measurements lowered the temperature of the silicon wafer by just 5 K. The
cooling power density of this rather simple radiative cooler under direct
sunlight was found to be up to 350 W/m2 when applied to hot surfaces with
relative temperatures of 50 K above the ambient. This is crucial to radiatively
cool down electronic devices, such as solar cells, where an increase in
temperature has drastic effects on performance. Our 2D thermo-functional
material includes a single layer of spheres emitting long-wave radiation
through the infrared atmospheric window and over a broad wavelength ange, thus
providing effective radiative cooling using the outer space as a heat sink at 3
K.
