Microplasma enhancement via the formation of a graphite-like phase on diamond cathodes Huang-Chin Chen and I-Nan Lin a) Department of Physics, Tamkang University, Tamsui, New Taipei 251, Taiwan Shiu-Cheng Lou and Chulung Chen Department of Photonics Engineering, Yuan-Ze University, Chung-Li 32003, Taiwan Ray-Her Tang and Wen-Ching Shih Graduate Institute in Electro-Optical Engineering, Tatung University, Taipei 104, Taiwan Shen-Chuan Lo and Li-Jen Lin Materials and Chemical Research Labs, ITRI, Hsinchu, Taiwan 310, Taiwan Chi-Young Lee a) Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan (Received 3 August 2012; accepted 14 November 2012; published 10 December 2012) Enhanced electron field emission (EFE) properties in microcrystalline diamond (MCD) films that have been Fe-coated and postannealed are observed. Additionally, improved microplasma characteristics are also observed when these materials are used as cathodes. The turn-on field for inducing the EFE process decreases from 4.7 V/lm for pristine MCD films to 2.2 V/lm for the Fe-coated/postannealed ones, whereas the EFE current density at an applied field of 8.8 V/lm increases from 36.5 to 5327.1 lA/cm 2 . Transmission electron microscopy, in conjunction with high-angle annular dark field and 3D-tomography studies, reveals that enhanced EFE in the Fe-coated/postannealed MCD films is due to the graphite-like phase on the surface of diamond films. The authors infer that the Fe-coating interacts with the diamond in the postannealing process to dissolve carbons and reprecipitate them in nanographite networks. This process is similar to the formation of carbon nanotubes by the dissolution and reprecipitation of carbon species at the presence of nanosized Fe catalysts. The utilization of high EFE diamond films as cathode materials enhances the microplasma, as the ignition field for initiating the plasma is lowered and a high plasma current density is attainable. V C 2013 American Vacuum Society. [http://dx.doi.org/10.1116/1.4769373] I. INTRODUCTION Diamond films have negative electron affinity properties 1 and many desirable physical/chemical properties. 2–5 They have been the focus of intensive research and are especially used as electron field emitters. However, the large electronic band gap (5.5 eV) in diamond films significantly hinders the electron field emission (EFE) behavior due to the lack of free electrons required for field emission. Good electron field emitters require both a sufficient supply of electrons from back contact materials and effective transport and efficient emission from the emitting sites. Doping the diamond films with boron 6,7 or nitrogen 8,9 ions introduces new interband states within the band gap, which facilitates the transport of electrons from the valence band to the conduction band and thereby improves the EFE in these materials. However, the EFE in these materials is not satisfactory, as the conductivity of diamond materials is still not as good as nanographite materials. 10,11 Efforts to improve the conductivity of dia- mond films have to date been unsuccessful. Even recently developed nanocrystalline or ultrananocrystalline diamond films, which contain grain boundaries of considerable conductivity cannot achieve the same level of EFE proper- ties as the nanocarbon materials. 12–14 However, it has been reported that EFE in diamond films can be improved by coat- ing a thin layer of metallic Fe on the diamond films followed by postannealing of the samples in a reducing atmosphere. 15 On the other hand, diamond usually exhibits a large ion- induced secondary electron emission coefficient (c-coeffi- cient) owing to the wide energy band gap (5.5 eV) pf the materials. 16,17 Such properties, together with a large resist- ance to ion bombardment damage, make diamond a suitable candidate for cathode materials of a microplasma devices. Moreover, the charging effect of the conventional cathode materials, MgO, can be circumvented due to high conductiv- ity of the diamond films. It is expected that the utilization of diamond films as cathode materials for microplasma devices not only facilitates the ignition of the plasma but also is beneficial for sustaining the plasma. In this paper, we further enhanced the electron field emis- sion properties of the two-step processed diamond films by modifying the annealing process and explored the reasons for enhanced EFE properties in Fe-coated/annealed micro- crystalline diamond (MCD) films using transmission elec- tron microscopy, especially the 3D tomographic technique. Moreover, we used these EFE materials as cathodes and observed the enhanced behavior of the plasma. a) Authors to whom correspondence should be addressed; electronic addresses: [email protected]; [email protected]02B108-1 J. Vac. Sci. Technol. B 31(2), Mar/Apr 2013 2166-2746/2013/31(2)/02B108/8/$30.00 V C 2013 American Vacuum Society 02B108-1 Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jvb.aip.org/jvb/copyright.jsp
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Microplasma enhancement via the formation of a graphite-like phaseon diamond cathodes
Huang-Chin Chen and I-Nan Lina)
Department of Physics, Tamkang University, Tamsui, New Taipei 251, Taiwan
Shiu-Cheng Lou and Chulung ChenDepartment of Photonics Engineering, Yuan-Ze University, Chung-Li 32003, Taiwan
Ray-Her Tang and Wen-Ching ShihGraduate Institute in Electro-Optical Engineering, Tatung University, Taipei 104, Taiwan
Shen-Chuan Lo and Li-Jen LinMaterials and Chemical Research Labs, ITRI, Hsinchu, Taiwan 310, Taiwan
Chi-Young Leea)
Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan
(Received 3 August 2012; accepted 14 November 2012; published 10 December 2012)
Enhanced electron field emission (EFE) properties in microcrystalline diamond (MCD) films that
have been Fe-coated and postannealed are observed. Additionally, improved microplasma
characteristics are also observed when these materials are used as cathodes. The turn-on field for
inducing the EFE process decreases from 4.7 V/lm for pristine MCD films to 2.2 V/lm for the
Fe-coated/postannealed ones, whereas the EFE current density at an applied field of 8.8 V/lm
increases from 36.5 to 5327.1 lA/cm2. Transmission electron microscopy, in conjunction with
high-angle annular dark field and 3D-tomography studies, reveals that enhanced EFE in the
Fe-coated/postannealed MCD films is due to the graphite-like phase on the surface of diamond
films. The authors infer that the Fe-coating interacts with the diamond in the postannealing
process to dissolve carbons and reprecipitate them in nanographite networks. This process is
similar to the formation of carbon nanotubes by the dissolution and reprecipitation of carbon
species at the presence of nanosized Fe catalysts. The utilization of high EFE diamond films as
cathode materials enhances the microplasma, as the ignition field for initiating the plasma is
lowered and a high plasma current density is attainable. VC 2013 American Vacuum Society.
[http://dx.doi.org/10.1116/1.4769373]
I. INTRODUCTION
Diamond films have negative electron affinity properties1
and many desirable physical/chemical properties.2–5 They
have been the focus of intensive research and are especially
used as electron field emitters. However, the large electronic
band gap (5.5 eV) in diamond films significantly hinders the
electron field emission (EFE) behavior due to the lack of
free electrons required for field emission. Good electron field
emitters require both a sufficient supply of electrons from
back contact materials and effective transport and efficient
emission from the emitting sites. Doping the diamond films
with boron6,7 or nitrogen8,9 ions introduces new interband
states within the band gap, which facilitates the transport of
electrons from the valence band to the conduction band and
thereby improves the EFE in these materials. However, the
EFE in these materials is not satisfactory, as the conductivity
of diamond materials is still not as good as nanographite
materials.10,11 Efforts to improve the conductivity of dia-
mond films have to date been unsuccessful. Even recently
developed nanocrystalline or ultrananocrystalline diamond
films, which contain grain boundaries of considerable
conductivity cannot achieve the same level of EFE proper-
ties as the nanocarbon materials.12–14 However, it has been
reported that EFE in diamond films can be improved by coat-
ing a thin layer of metallic Fe on the diamond films followed
by postannealing of the samples in a reducing atmosphere.15
On the other hand, diamond usually exhibits a large ion-
induced secondary electron emission coefficient (c-coeffi-
cient) owing to the wide energy band gap (5.5 eV) pf the
materials.16,17 Such properties, together with a large resist-
ance to ion bombardment damage, make diamond a suitable
candidate for cathode materials of a microplasma devices.
Moreover, the charging effect of the conventional cathode
materials, MgO, can be circumvented due to high conductiv-
ity of the diamond films. It is expected that the utilization of
diamond films as cathode materials for microplasma devices
not only facilitates the ignition of the plasma but also is
beneficial for sustaining the plasma.
In this paper, we further enhanced the electron field emis-
sion properties of the two-step processed diamond films by
modifying the annealing process and explored the reasons
for enhanced EFE properties in Fe-coated/annealed micro-
crystalline diamond (MCD) films using transmission elec-
tron microscopy, especially the 3D tomographic technique.
Moreover, we used these EFE materials as cathodes and
observed the enhanced behavior of the plasma.
a)Authors to whom correspondence should be addressed; electronic
aE0: The turn-on applied field designated as the interception of the straight lines extrapolated from the high field and low field segments of the F–N plot.bJe: The EFE current density at an applied field of Ea¼ 10.8 V/lm.cEi: The ignition field designated to initiate the plasma in the microplasma devices that was estimated from the images of the microplasma devices under
increasing applied voltage.dJplasma: The plasma current density at an applied field of Ea¼ 0.35 V/lm.
02B108-4 Chen et al.: Microplasma enhancement via the formation of a graphite-like phase 02B108-4
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jvb.aip.org/jvb/copyright.jsp
form Fe3C-clusters. When the Fe-clusters are large (or the
postannealing period is short), the carbon species cannot
diffuse all the way through the Fe-cluster. The surface of the
Fe-clusters do not react with the diamond. Only the bottom
part of the Fe-clusters transform into Fe3C-clusters. As the
nanographitic phase is associated more closely with the Fe3C-
clusters (than the Fe-clusters), the distribution of the nanogra-
phite clusters is the same as that of the Fe3C-cluster networks.
Figures 9(a) and 9(b) shows the X–Y projections of the
3D-tomographs of the Fe3C and the Fe-clusters, respectively,
and Fig. 9(c) shows the superposition of the two. These
results infer, again, that the Fe-clusters are distributed in
the same locations as the Fe3C-clusters and the Fe-cluster net-
works are located on top of the Fe3C-clusters. While the X–Y
projection of the 3D-tomograph shown in Fig. 9(c) is similar
to the HAADF image shown in Fig. 4(a), there are differences
between the two images. The HAADF image in Fig. 7(a) is
the superposition of the diamond (CL1, in yellow), Fe3C
(CL2, in blue), and Fe (CL1, in red) HAADF images. The
sequence of superposition can be arbitrarily arranged. Figure
7(a) is obtained by assuming that the Fe-HAADF image is
located on top of the Fe3C-HAADF image. In contrast, when
constructing the 3D-tomograph, no prior knowledge on how
Fe interacts with diamond is necessary to correctly superim-
pose the images. The series of the images taken with different
tilting angles locate the HAADF images in the correct
position. Therefore, Figs. 8(c) and 9(c) unambiguously show
that the Fe-cluster network is located on top of the Fe3C-clus-
ter network.
Our results indicate that the formation of the graphite
phase is closely related to the presence of Fe-clusters. We
infer that the Fe-clusters catalytically dissociate the diamond
at postannealing temperatures, then the dissolved carbon
atoms are transported to the other side of the Fe-clusters
where they reprecipitate to form nanographite clusters. This
process is similar to the formation of carbon nanotubes by
the dissolution and reprecipitation of carbon species in the
presence of nanosized Fe catalysts.14,15
IV. CONCLUSIONS
The surface EFE properties of diamond films were
improved by incorporating a Fe-layer and a postannealing
process. TEM observations indicate that the dominant factor
contributing to enhance EFE in Fe/pa-MCD films is the for-
mation of a nanographitic phase. We infer that the mecha-
nism for the formation of the nanographite phase involves the
dissociation of carbon atoms from diamond in the presence
of Fe-clusters under high temperature postannealing condi-
tions. The carbon species were dissolved in the Fe-clusters,
transported through the clusters, and then reprecipitated on
the other side of the Fe-clusters, forming a nanographite
layer. The utilization of these high EFE diamond films as
cathode materials lowers the ignition field for initiating the
plasma and increases the plasma current density.
ACKNOWLEDGMENTS
The authors would like to thank the National Science
Council, Republic of China for supporting this research
through Project Nos. NSC 99-2119-M-032-003-MY2 and
NSC100-2113-M-007-006.
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