Carbon nanotubes (CNTs) were functionalized with a surface coating using magnetron sputter deposition. The CNT samples used were prepared by plasma enhanced chemical vapor deposition and were vertically aligned to the surface of the tungsten substrate. A thin layer of barium strontium oxide approximately 100 run in thickness was deposited on their surface using magnetron sputtering. The oxide coating was uniform, covering the whole surface of the CNTs and significantly lowered the work function while preserving the geometry. The resulting oxide coated CNTs had a work function of 1.9 eV and a field enhancement factor of 467, which led to a significant improvement in both field and thermionic emission. Compared to uncoated CNTs, the field emission was increased by a factor of two, while the thermionic emission increased by more than four orders of magnitude. At 4.4 V/mum, a field emission current of 23.6 muA was obtained from an emitting surface of 0.012 cm2. Similarly, at 1.1 V/mum and 1221 K, a thermionic emission current of 14.6 mA was obtained.