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
(110-5 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.