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 compell