microplasma enhancement via the formation of a graphite

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

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 VC 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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II. EXPERIMENT

MCD films were grown on p type silicon substrates using

microwave plasma enhanced chemical vapor deposition

(IPLAS-Cyrannus). The substrates were first thoroughly

cleaned by rinsing the Si wafer sequentially in aqueous

hydrogen peroxide/ammonium hydroxide and hydrogen per-

oxide/hydrochloric acidic solutions. Then the substrates

were ultrasonicated in a solution containing nanodiamond

and titanium powders for 45 min to facilitate the nucleation

of diamond. The substrates were then ultrasonicated in meth-

anol to remove diamond and Ti nanoparticles. The MCD

films were grown in CH4/H2¼ 2/98 sccm plasma excited by

a 1400 W (2.45 GHz) microwave at 65 Torr total pressure for

1 h. The MCD films were coated with a 10-nm-thick layer of

iron by DC sputtering for 1 min. In the first series of studies,

the Fe-coated films were thermally postannealed at 900 �C in

a H2 atmosphere (flow rate of 100 sccm) for 5 min, with

heating and cooling rates of 15 �C/min. The Fe-coated and

postannealed MCD films are henceforth named Fe/pa-MCD

films. In a second series of studies, C2H2 gas (1 sccm) was

introduced into the postannealing chamber and the samples

were maintained at 900 �C for 1, 5, and 10 min, after 900 �C(5 min) postannealing in a H2 atmosphere. The obtained

samples are henceforth referred to as Fe/pa-MCD1, Fe/pa-

MCD2, and Fe/pa-MCD3, respectively.

The morphology and structure of the films were investi-

gated using field emission scanning electron microscopy

(FESEM, Carl Zeiss, SUPRA 55). The Raman spectra were

recorded on a Renishaw micro-Raman spectrometer (Model-

INVIA) in back scattering geometry using the 514.5 nm line of

an Ar-ion laser. The detailed microstructure was examined

using high resolution transmission electron microscopy

(HRTEM, Joel 2100). Field emission measurements were

carried out using a home-built tunable parallel plate capacitor.

The separation of the anode (Mo) tip from the sample

was measured using a digital micrometer and an optical micro-

scope. The EFE properties were analyzed by the Fowler–Nord-

heim (F–N) model.18 The turn-on field was obtained from the

interception of the lines extrapolated from the high-field and

low-field segments of the F–N plots. The plasma illumination

characteristics of the Fe/pa-MCD films were evaluated using a

microplasma device with parallel plate configuration, in which

the indium-tin oxide (ITO) coated glass plates (the anode)

were separated from the cathode (Fe/pa-MCD films) by a fixed

spacer (1.0-mm-thick Teflon). A 8-mm-diameter circular

hole was cut out from the Teflon spacer to form a cylindrical

cavity (Fig. 1). The Ar plasma was excited in between the ITO

and Fe/pa-MCD films by applying a pulsed positive voltage

(0–400 V) to the anode in a vacuum (pressure �100 Torr). The

current density versus applied field was acquired using a

Keitheley 2410 current source electrometer.

III. RESULTS AND DISCUSSION

A. Fe/pa-MCD cathode materials

From the SEM image shown in Fig. 2(a), the deposited

MCD films have faceted grains about 300 nm in size. The

MCD films contain grain boundaries of negligible thickness

and are relatively insulating, resulting in poor EFE properties.

Turn-on voltages of �40 V/lm are needed for the EFE pro-

cess. To ensure that EFE properties were measured,

we wrapped the Cu-foil (attached to the bottom of the Si-sub-

strate) over the film, so that it was in direct contact with the

diamond film. In this configuration, the electrons are trans-

ported along the surface of the films, rather than through the

films. The J–E curves thus measure the surface EFE behavior

of the MCD films. The surface EFE properties of the as-

grown MCD films (curve I) shown in Fig. 3(a) reveal that

these films need only 4.7 V/lm to turn on the surface EFE

process and a surface EFE current density of 1.2 lA/cm2 at a

applied field of 10.8 V/lm is attained. The turn-on field (E0)

and current density (Je) values were extracted from the J–E

curves and are listed in Table I. The Raman spectrum of the

FIG. 1. (Color online) Schematic diagram of the cylindrical microplasma

cavities.

FIG. 2. SEM images of (a) MCDpristine and (b) Fe/pa-processed (900 �C,

5 min in H2) MCD films.

02B108-2 Chen et al.: Microplasma enhancement via the formation of a graphite-like phase 02B108-2

J. Vac. Sci. Technol. B, Vol. 31, No. 2, Mar/Apr 2013


MCD film (curve I) in Fig. 3(b) has a sharp D-band resonance

peak at 1332 cm�1, which corresponds to the C2g-band reso-

nance of the diamond lattice. The �1 -, G*-, �3 -, and D*-band

resonance peaks at 1140, 1350, 1480, and 1580 cm�1, respec-

tively, correspond to sp2-bonded carbon. As Raman spectros-

copy in the visible region (514.5 nm) is several times more

sensitive to sp2-bonded carbons than to sp3-bonded ones, the

presence of sp2-bonded resonance peaks does not imply that

these materials contain a large proportion of sp2-bonded car-

bon species.

The SEM morphology of the Fe-coated MCD films is sim-

ilar to the MCD films (not shown) and the Raman spectrum

[curve II, Fig. 3(b)] is also similar. However, curve II in

Fig. 3(a) shows that the EFE properties are significantly

degraded in the Fe-coated MCD films. The morphology of

the films did not changed for the Fe/pa films at when the

annealing temperatures were lower than 800 �C. Only small

Fe-clusters about a few nanometers in size were formed (not

shown). The Fe-clusters interact with diamond films when

the postannealing temperature increases to 900 �C. Figure

2(b) shows the surface morphology of the Fe/pa MCD films.

In these Fe/pa MCD films (annealed in the presence of hydro-

gen), a network structure with some nanoparticles (white

color) sparsely distributed on the surface is observed, indicat-

ing that Fe-diamond interactions have occurred. In Fig. 3(a)

(curve III), a low E0 and high Je are obtained for the Fe/pa

MCD films. Turn-on voltages of 2.8 V/lm are needed for the

surface EFE process, and a current density of (Je)MCD(NH3)

¼ 128.0 lA/cm2 (at 10.8 V/lm applied field) is achieved. We

deduce that the Fe/pa-processes induce a surface layer, which

is more conducting than the surface of the MCDpristine films.

Sharp D-band resonance peaks (Fig. 3(b), curve III) in the

Raman spectrum indicate that the diamond structure is still

predominant in the Fe/pa-MCD films. However, these peaks

are slightly blue-shifted compared with the MCDpristine films.

Raman spectroscopy reveals the bonding structure in the

whole diamond films and cannot provide clear information

about how the Fe/pa-processes modify the surfaces of the

diamond films.

B. Plasma illumination behavior

From the results, we infer that the formation of a graphite

phase enhances the EFE properties of Fe/pa MCD films.

Therefore, to further enhance the EFE in MCD films is to

increase the proportion of nanographite clusters by modify-

ing the post-treatment process. We introduce C2H2 gas

after the Fe/pa MCD films have been annealed at 900 �C(in H2) and extend the heat-treatment for 1, 5, and 10 min to

give films Fe/pa-MCD1, Fe/pa-MCD2, and Fe/pa-MCD3,

respectively. The SEM morphologies and the Raman charac-

teristics (not shown) were not modified in this additional

post-treatment process. However, the EFE properties of

these MCD films are significantly enhanced. Figure 4 shows

that heating the Fe/pa MCD films to 900 �C in C2H2 further

improves the EFE properties (Table II). For Fe/pa-MCD2,

E0 is 2.2 V/lm and the EFE current density at 8.8 V/lm is

5327.1 lA/cm2.

The utilization of high EFE MCD films as cathode mate-

rials enhances the plasma illumination behavior of micro-

plasma devices. Figure 5 shows that all the microplasma

devices show good plasma illumination characteristics. The

plasma can be ignited at low applied voltages (210–240 V),

which correspond to an igniting field of 0.21–0.24 V/lm.

The plasma current density increases with the applied volt-

age, and plasma illumination current densities of 6.5–8.5

mA/cm2 at applied voltages of 350 V (0.35 V/lm applied

field) are achieved. Plasma illumination is triggered when

the electrons emitted from the cathode (e.g., Fe/pa-MCD

films) gain sufficient kinetic energy for ionizing the gas mol-

ecules (15.7 eV for Ar-species). The Ar-species ionization

FIG. 3. (Color online) (a) EFE properties and (b) Raman spectra of (I)

MCDpristine and (II) as-Fe-coated and (III) Fe/pa-processed (900 �C, 5 min in

H2) MCD films.

TABLE I. EFE properties of the pristine, as Fe-coated and Fe-coated/postan-

nealed MCD films.

Materials

Postannealing condition

(temperature and atmosphere)

E0a

(V/lm)

Jeb

(lA/cm2)

MCDpristine — 4.7 1.2

MCDFe — 5.2 0.9

MCDFe/pa(H2) 900 �C (H2) 2.8 128.0

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.


JVST B - Microelectronics and Nanometer Structures


cross-section increases with the electron kinetic energy and

reaches a maximum value when the electron kinetic energy

is 100 eV. Although the EFE process for Fe/pa-MCD films

can be turned on at low field [(E0)Fe/pa¼ 2.2–2.9 V/lm

compared with (E0)pristine¼ 4.5 V/lm], the onset field for

triggering the illumination process in the cathode does not

significantly change. Moreover, once the plasma was ignited,

the electric field in the cathode is only 0.35 V/lm, which is

less than the turn-on field voltage for triggering the EFE pro-

cess. Therefore, MCD cathode materials can efficiently

produce secondary electrons (under the bombardment of the

Ar-ions) that seem to be the prime factor enhancing the

plasma in microplasma devices, whereas the low turn-on

field for EFE is not. However, the high EFE generated from

Fe/pa-MCD cathodes significantly increases the plasma illu-

mination current density.

C. Microstructure analysis

To understand how the Fe/pa-processes improves the

EFE properties, the microstructures in the MCD films were

examined using TEM. The TEM samples were ion-milled

from the Si side such that the thin foil contained mainly the

materials near the surface region of the MCD films. Figure

6(a) shows the typical bright field image acquired under

scanning transmission electron microscopic mode (STEM)

mode for Fe/pa-MCD films annealed in H2. The inset in

Fig. 6(a) shows the selected area diffraction (SAED) images

acquired in TEM mode. The diffraction spots are arranged in

a ring, implying that this region mainly contains diamond

grains, which were randomly oriented. Structure images of

the region in the center of the large diamond grain near a

zone axis [region A, Fig. 6(a)], shown in Figs. 6(b) and 6(c),

indicate the presence of a large proportion of planar defects.

There are parallel regions of irregular spacing, which are

stacking faults [S.F. in Fig. 6(b)], as implied by streaks along

the (111) direction in the Fourier-transform diffractogram

[FT0b, Fig. 6(b)].19 There also exist parallel regions with reg-

ular spacings (FT0b), which are assigned to hexagonal dia-

mond19 [H.D. in Fig. 6(b)]. Moreover, the FT image

corresponding to Fig. 6(c), FT0c, indicates that these materi-

als consist of twins. Clusters of Fe3C are also observed

[Fe3C in Fig. 6(c)]. The region adjacent to “A” is also a dia-

mond grain but oriented in a nondiffracting direction. The

central diffuse ring is clearly observed in FT0c, implying that

these materials contain nanographitic (or amorphous carbon)

phases. The nanographitic phase is clearly observable when

the grains are oriented away from a zone axis and is less

obvious when the grains are oriented near a zone axis.

The images [Figs. 6(d) and 6(e)] of the regions near the pe-

riphery of the large diamond grains, which are away from the

zone axis, show weak diffraction patterns [region “B,” Fig.

6(a)]. The structure of the minor phases can thus be better

resolved. The FT image in Fig. 6(d), FT0d, shows that Fe-

clusters are present in this region [Fe in Fig. 6(d)], besides the

diamond, whereas the FT0e image indicates the presence of

Fe3C clusters [Fe3C in Fig. 6(e)]. Moreover, the diffuse dif-

fraction ring is clearly observable in FT0d and FT0e, indicating

the presence of a large proportion of nanographite (or amor-

phous carbon) clusters (marked “g”). Presumably, the nano-

graphite (or amorphous carbon) clusters are located on top (or

bottom) of the diamond grains in region A but cannot be

clearly resolved by TEM micrographs. 3D-tomography com-

posed of high angle annular dark field (HAADF) images is

the better technique to resolve the distribution of these phases.

In transmission electron microscopy, the coherently and

elastically scattered electrons form a diffraction contrast

FIG. 4. (Color online) EFE properties of post-treated diamond films: (I)

MCDpristine, (II) Fe/pa-MCD, (III) Fe/pa-MCD1, (IV) Fe/pa-MCD2, (V) Fe/

pa-MCD3.

TABLE II. Surface EFE and plasma illumination properties of the two-step post-treated MCD films.

Materials

Postannealing condition

(temperature and atmosphere) E0a (V/lm) Je

b (lA/cm2) Eic (V/lm) Jplasma

d (lA/cm2)

MCDpristine — 4.5 36.5 — —

Fe/pa-MCD0 900 �C (H2) 3.7 111.5 0.24 6.5

Fe/pa-MCD1 900 �C (H2)þC2H2 (1 min) 2.7 3390.2 0.23 7.0



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.




image that provides structural information. Incoherently

elastically scattered electrons are also present. The scattering

angle is related to the atomic number of the species; the

heavier the species, the larger the scattering angle. When

incoherently scattered electrons are collected by a high

angle annular detector, a HAADF image is observed. The

HAADF-image provides the chemical elemental distribution

in the sample. By changing the camera length (CL) in the

STEM, the electrons that are incoherently (but elastically)

scattered by different species can be resolved. The HAADF-

image represents the spatial distribution of the species,

provided that the correlation between the scattering angle and

the atomic number of the species, inducing the incoherently

scattering process, is known. For the Fe/pa-MCD films shown

in Fig. 6(a), only iron and carbon species are involved. The

contribution of the two species can be clearly resolved by

systematically changing the CL. Figure 7(a) illustrates a typi-

cal HAADF image, which consists of the superposition of the

HAADF images acquired with CL1¼ 400 mm (region A, yel-

low color), CL2¼ 127.3 mm (region B, blue color), and

CL3¼ 93 mm (region C, red color). The energy dispersive x-

ray analysis (EDAX) patterns (in STEM mode) correspond-

ing to the regions marked as A, B, and C in Fig. 7(a) and are

plotted as profiles 1, 2, and 3 in Fig. 7(b), respectively. These

EDAX patterns clearly indicate that profile I contains mostly

carbon species (diamond, graphite, or amorphous carbon).

FIG. 5. (Color online) Plasma illumination properties of MCD films

(a) Fe/pa-MCD0, (b) Fe/pa-MCD1, (c) Fe/pa-MCD2, (d) Fe/pa-MCD3.

FIG. 6. (Color online) (a) TEM bright field and (b) and (c) structure images

of the regions near the center of the diamond grains (region A in “a”) and

(d) and (e) that of the regions in the periphery of the diamond grains (region

B in “a”), showing the presence of planar defects inside the diamond grains

and the existence of Fe-, Fe3C, and nanographitic (or amorphous carbon)

clusters in the surrounding regions.




The EDAX profile 3 contains large Fe-signals from Fe-

clusters. EDAX profile 2 contains both Fe and C signals orig-

inating from Fe3C-clusters. The other small signals are Cu,

Au, and Si, which are presumably contaminants from the

underlying substrates and the Cu-mesh induced in the ion-

milling process.

To further investigate the distribution of species in Fe/

pa-MCD films, TEM 3D-tomography was performed. 3D-

tomography images were constructed from multiple HAADF

images of the samples tilted in incremental (2�) steps and the

images were replayed using the Digital-Micrograph (Joel)software.20 In the construction of the 3D tomograph for the

Fe/pa MCD films, only the HAADF images corresponding to

Fe3C (CL2¼ 127.3 mm) and Fe (CL3¼ 93.0 mm) were super-

imposed. The HAADF images corresponding to diamond

(CL1¼ 400 mm) cannot be used for constructing 3D tomo-

graphs, as the signal from the crystalline diamond signifi-

cantly fluctuates with the tilting angle due to the strong

diffraction of electrons from regions near a zone axis. There-

fore, only the Fe and Fe3C clusters near the periphery of the

diamond grains are observed.

Figures 8(a) and 8(b) show the stereographic projections

of the 3D-tomographs of the Fe3C and the Fe-clusters, respec-

tively. These stereographic projections indicate that both

Fe3C- and Fe-cluster-networks are distributed on top of the

diamond films. The Fe3C-clustes appear exactly at the same

locations as the Fe-clusters. Figure 8(c) shows the stereo-

graphic projection of the 3D-tomograph, after the superposi-

tion of the Fe3C- and Fe-cluster-networks, revealing that the

Fe3C-clusters are always located beneath the Fe-clusters;

the Fe3C-clusters are sandwiched between the Fe-clusters and

the diamond surface. Such observations support the argument

that Fe3C-clusters are formed by Fe–diamond interactions

during the postannealing process. The Fe-clusters catalyti-

cally dissociate the diamond lattice and transport carbon

atoms to the other side of Fe-clusters; the carbon atoms then

reprecipitate to form nanographitic clusters. In the cooling

process, some carbon species are frozen in the Fe-clusters and

FIG. 7. (Color online) (a) HAADF images and (b) EDAX (STEM) profiles of

the MCD films, which were Fe/pa in H2-atmosphere at 900 �C for 5 min.

Spectra I, II, and III correspond to the locations A, B, and C designated in

“a.”

FIG. 8. (Color online) TEM 3D-tomographic projections of the Fe/pa MCD

films: (a) Fe3C-clusters (white color), (b) Fe-clusters (red color), and (c) the

superposition of the Fe3C- and Fe-3D-tomographs.




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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FIG. 9. (Color online) X–Y projection of the TEM 3D-tomographs of the Fe/

pa MCD films, (a) Fe3C-clusters (white color), (b) Fe-clusters (red color),

and (c) the superposition of the Fe3C- and Fe-projections.




http://dx.doi.org/10.1103/PhysRevB.20.624

http://dx.doi.org/10.1103/PhysRevB.20.624

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