We consider new possibilities for cooling by vacuum tunneling. We examine a nanogeometry and find that large cooling currents can be obtained by a combination of energy selective tunneling of electrons and thermionic emission. The energy selective tunneling is a result of the special form of a potential barrier which has wider gap for low energy electrons, which results in electrons above the Fermi level being the principal tunneling component. Numerical calculations show that available material with work functions about 1.0 eV are useful for cooling. For gaps of 5-15 nm, which are well within the present state of the art, only a small external voltage ͑1-3 V͒ is required to create large currents and a useful Peltier coefficient of about 0.3, and cooling power of 100 W/cm 2 .
We have completed an investigation of cooling at room temperature by thermionic emission. The use of a small nm-sized gap lowered the vacuum barrier between the electrodes, enabling emission from surfaces with work functions of ∼1 eV at room temperature. We utilized a microfabricated cantilever with a cesiated metal coating on the tip, and an integrated thermometer to initiate and control an emission current of 1–10 nA, and to detect the resulting temperature changes. Using a lock-in technique, temperature changes of 0.1–1.0 mK were observed, corresponding to cooling power of 1–10 nW. The amplitude of this signal and its dependence on emission current and bias voltage are in good agreement with our model. Possible applications for cooling and energy conversion are discussed.
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