Supplementary Information Experimental Details The multi-walled carbon nanotubes (MWNTs) synthesized by a catalytic chemical vapor deposition method were purchased from Hanwha Company (Korea). The MWNT suspension was prepared by dispersing MWNTs (ca. 10 mg) in dichloroethane (100 ml) using a bath-type sonicator for 500 min and the following centrifugation (6,800 rpm) for 10 min. We have used the following procedure to prepare MWNT emitter. Indium metal film with thickness of 200 nm was deposited on indium tin oxide glass using thermal evaporation. Then, the MWNT suspension was spray-coated on electrode (1 1 cm2) and subsequently thermally treated at 350oC for 20 min in argon in order to assure a strong adhesion between MWNTs and electrode through a melting of the indium layer. After covering a cathode with a 3M-made tape, the tape is pulled out in order to align MWNTs vertically to the substrate as well as to remove the loosely bound MWNTs. The oxygen high voltage annealing was carried out in high vacuum chamber (110-5 Torr) in order to achieve the stable emission condition from uniform MWNT morphology and use the cathode with similar turn-on voltage values.1 The distance between electrodes was kept fixed at 270 m, while the field emission current was measured using a Keithley 248 source measurement unit. All samples were annealed at current density of a 0.1 mA/cm2 for 2 hrs. We fabricated three cathodes with similar turn-on voltage values of nearly ~3 V/m. After the oxygen high voltage annealing process, all samples are measured current-voltage (I-V) test in base pressure. The hafnium oxide coating on MWNT emitter was carried out atomic layer deposition method using tetrakis (ethylmethylamino) hafnium (TEMAH, Hf[N(CH3)C2H5]4) as an hafnium precursor and O2 as an oxidizer. For the atomic layer deposition process, each cycle was composed of four steps as follows. First, a mixture of TEMAH and argon carrier gas (300 sccm) was introduced into the vacuum chamber for 10 s. The chamber was purged with argon (150 sccm) flowing for 5 sec. For the formation of the hafnium oxide (HfOx) layer, oxygen gas (300 sccm) has passed for 5 s, and the resulting by-products or remnant gases were completely removed by the subsequent purging step using an argon (150 sccm) gas for 5 s.2 The MWNT emitters were coated for several cycles (3, 6, and 9 cycles) at 300 °C in order to tune film thickness in the range of 1-12 nm. Transmission electron microscopy (TEM) images were obtained with a JEOL 2010F high-resolution TEM at 200 keV. The chemical state of hafnium oxide film was studies by X-ray photoelectron spectroscopy (XPS, PHI Quantum-2000 spectrometer with Al K). Raman spectra were obtained using a 532-nm laser (JASCO NRS-3100). Field emission characteristics were measured in high vacuum chamber with parallel diode-type configuration, at a base pressure of ~ 4 10-7 Torr. The distance between the hafnium oxide coated MWNT cathode and phosphorus anode was fixed at 270 m and the field emission current was measured using a Keithley 248 source measurement unit. Lastly, we measured field emission characteristics, such as a field enhancement factor, turn-on field, current density, and current stability test according to thickness of protective layer. References 1. Y. I. Song, G. Y. Kim, H. K. Choi, H. J. Jeong, K. K. Kim, C. M. Yang, S. C. Lim, K. H. An, K. T. Jung, Y. H. Lee. Chem. Vap. Depo. 12, 375 (2006). 2. S. Y. Sohn, K. P. Park, D. G. Jung, H. S. Kim, H. Y. Chae, H. M. Kim, J. S. Yi, M. -H. Cho, J. -H. Boo. Jpn. J. Appl. Phys. 46, L461 (2007). FIG. S1 The elemental distribution profile of oxygen and hafnium by energy dispersive spectroscopy of transmission electron microscopy. FIG. S2 (a) The field enhancement factors () and (b) turn-on field of the pristine and their corresponding hafnium oxide-coated carbon nanotubes. FIG. S3 (a, b) Illustration of the field emission and (c) simple band diagram for the field emission mechanism from a wide band gap material (WBGM)-coated nanotube cathode.