While there has been extensive theoretical work in this
area, establishing a strong case for thermionic cooling, experimental
evidences remain scarce, especially for vacuum
thermionic cooling.9,15,25,26 The most notable recent experimental
cooling result is from Shakouri et al. who reported
about 1 degree of cooling with their semiconductor heterostructure
microcooler at 100 C.9 In this letter, we present a
practical and relatively inexpensive scheme to reduce the
potential barrier for thermionic electron emission using functionalized
carbon nanotube (CNTs) thin films, and report a
large surface cooling effect of as much as 81 C obtained
from such a surface. Even that this 81 C of cooling was
achieved at relatively high cathode temperature of 995 C, it
is a compelling experimental result that demonstrates the
potential of vacuum thermionic cooling.