1

Structure- and composition-dependent electron

field emission from nitrogenated

carbon nanotips

B. B. Wang, C. S. Gong, E. Q. Xie, R. Z. Wang, and K. Ostrikov

The electron field emission (EFE) properties of nitrogenated carbon nanotips

(NCNTPs) were studied under high-vacuum conditions. The NCNTPs were prepared

in a plasma-assisted hot filament chemical vapor deposition system using CH4 and N2

as the carbon and nitrogen sources, respectively. The work functions of NCNTPs were

measured using X-ray photoelectron spectroscopy. The morphological and structural

properties of NCNTPs were studied by field emission scanning electron microscopy,

micro-Raman spectroscopy, and X-ray photoelectron spectroscopy. The field

enhancement factors of NCNTPs were calculated using relevant EFE models based on

the Fowler-Nordheim approximation. Analytical characterization and modeling results

were used to establish the relations between the EFE properties of NCNTPs and their

morphology, structure, and composition. It is shown that the EFE properties of

NCNTPs can be enhanced by the reduction of oxygen termination on the surface as

well as by increasing the ratio of the NCNTP height to the radius of curvature at its

top. These results also suggest that a significant amount of electrons is emitted from

other surface areas besides the NCNTP tops, contrary to the common belief. The

outcomes of this study advance our knowledge on the electron emission properties of

carbon nanomaterials and contribute to the development of the next-generation of

advanced applications in the fields of micro- and opto-electronics.

2

I. INTRODUCTION

Carbon nanomaterials can form sp3-, sp2-, and sp1-hybridized carbon networks and

show diverse electronic, optical, and mechanical properties.1 In particular,

nano-carbons such as carbon nanotubes, graphenes, nanotips, nanocones, and many

other nanostructures have attracted a continuously increasing interest in last years due

to their unique structures and potential applications in the fields of microelectronic

and optoelectronic devices.2-9 Carbon nanotubes are considered to be one of the best

field emitting materials due to the high local field formed near their tops and their

outstanding conductivity which makes it possible to effectively conduct very high

current densities.10

Compared with carbon nanotube forests, carbon nanotips exhibit a high mechanical

strength due to their non-hollow structure as well as a weak emission screening effect

due to the typically large distance between the nanotips within the arrays. Importantly,

the electron field emission (EFE) properties of the carbon nanotips are often superior

to multiwalled carbon nanotubes because their emitting tips are typically in the

low-nanometer size range.10 This is why the carbon nanotips are highly-promising for

the development of the next-generation field-emitting devices and the study of their

EFE properties remains a highly-topical issue in recent years.6, 10-13

However, the relations between the EFE properties of the carbon nanotips and their

3

structural and compositional properties still remain poorly understood. For example, it

was reported13 that the work function and the field enhancement factor of the carbon

nanotips are 5 eV and 3365, respectively. Other authors11 reported totally different

values for the same quantities, namely 1.51 eV for the work function and 1603 for the

field enhancement factor. Why do the carbon nanotips show such large differences in

the values of these two basic EFE parameters?

Physically, the work function is mostly determined by the composition and structure

of the nanotips, while the field enhancement factor is related to their morphological

characteristics. This is why here we study the effects of composition, as well as

structural and morphological properties of the nanotips on their work function and

field enhancement factor. We also note that the changes in the structure and

composition of carbon nanomaterials can cause significant variations of their

electronic properties,14 and hence, the EFE characteristics. Better understanding of

these effects is needed for better control of electron emission from carbon nanotips,

and hence, for their applications in microelectronic and optoelectronic devices. This

paper clarifies these issues by identifying the most important factors that determine

the electron field emission from plasma-produced nitrogenated carbon nanotips.

Reactive plasmas have recently been shown as a versatile nanofabrication tool owing

to its many unique properties such as effective excitation, dissociation, and ionization

of working gases under thermally non-equilibrium conditions.15-17 In this work, we

4

employed a plasma-assisted hot filament chemical vapor deposition (HFCVD) system

to synthesize the nitrogenated carbon nanotips (NCNTPs). Furthermore, catalyst-free

nanomaterials synthesis is a tremendous challenge nowadays,18 so the catalyst was not

used in this work and the NCNTPs were formed directly on a thin carbon film

deposited on a silicon substrate.

The structure and composition of the NCNTPs were studied using advanced analytical

tools including field emission scanning electron microscopy (FESEM), micro-Raman

spectroscopy, and X-ray photoelectron spectroscopy (XPS). The EFE properties of the

NCNTPs were measured in a high-vacuum system. In addition, the work functions of

NCNTPs with different structure were obtained by XPS. We have also used numerical

modeling to calculate the field enhancement factors of different NCNTPs. By

comparing the calculated field enhancement factors with the experimental results, we

have found that electrons can also be emitted from some other areas on NCNTP

surfaces besides their tops. The relation of the EFE properties with the structural and

compositional properties was studied depending on the field enhancement factors and

the work functions of the NCNTPs.

This paper is organized as follows. In Sec. II, the synthesis process of NCNTPs and

their analytical characterization are described in detail. Sec. III presents the

experimental results of the characterization and EFE property measurements, which

include the structural, morphological, and compositional analysis, emission

5

characteristics, and work functions of NCNTPs. In Sec. IV, the work functions and

the field enhancement factors of different NCNTPs are analyzed. Simultaneously, the

relation of the EFE properties of NCNTPs with their structural and compositional

properties is discussed. In the final section, we briefly summarize the results and their

importance for the advanced applications in micro- and optoelectronic devices.

II. EXPERIMENTAL DETAILS

Prior to the growth of NCNTPs, a thin carbon film was deposited on a Si(100) wafer

by RF sputtering. In the sputtering system, a graphite target was used. The working

gas and pressure were Ar (99.999%) and 0.5 Pa, respectively. Before sputtering, the

chamber was firstly pumped till the background pressure of about 10-4 Pa was reached.

Then, 30 sccm of Ar was let into the chamber. By adjusting the gas control valve, the

pressure in the chamber was kept at 0.5 Pa. The RF power supply of 200 W was

turned on for 10 min to deposit the carbon film of ~30 nm thickness.

The growth of NCNTPs was performed in a plasma-assisted HFCVD system

described in detail elsewhere.8 Here, the silicon wafer coated with an amorphous

carbon film was used as the substrate and NH3, N2, and H2 were used as the reaction

gases, whereas the work pressure was about 2×103 Pa. The substrate and reaction

gases were homogeneously heated and fast decomposed by a heating system in the

CVD chamber. This system included 3 tungsten filaments heated to a temperature of

6

about 1800 ºC. The gap between the substrate and the filaments was about 8 mm. This

exposure led to the substrate heating to about 800 ºC. The plasma in the CVD system

was produced using a DC power supply, with the cathode and anode connected with

the substrate and filaments, respectively.

During the growth of NCNTPs, NH3, N2, and H2 were introduced into the chamber

under the background pressure of lower than 2 Pa. When the work pressure was

maintained at about 2×103 Pa, the filaments were fast heated to about 1800 °C by an

AC power supply. Simultaneously, the substrate was heated by the hot filaments till

the temperature reached 800 °C. The DC power supply was then turned on and the

bias current was gradually increased till the plasma glow was generated. Subsequently,

the bias current was set at 160 mA to synthesize the NCNTPs under different sets of

process conditions specified in Table I.

The morphologies of NCNTPs were characterized by a Hitachi S-4800 field emission

scanning electron microscope, which was operated at 15 kV. The Raman spectra of

the NCNTPs were recorded using a Renishaw micro-Raman spectrometer, in which a

514.5 nm line of Ar+ laser was used as the excitation source. The chemical

composition, atomic bonding states of elements, and work functions of NCNTPs were

obtained by an ESCALAB 250 X-ray photoelectron emission spectrometer using an

Al Kα X-ray source. Before the XPS measurements, the specimens were bombarded

by a 2 keV beam of Ar+ ions and the etching depth was about 2 nm. During the

7

analysis of the work function, a negative bias of -5 V was applied to the specimens to

discriminate the signals from the analyzer and the second electron cut-offs.

The measurements of EFE characteristics of NCNTPs were performed using a diode

configuration in a high-vacuum system, where the pressure was ~10-6 Pa. In the diode

configuration, the NCNTP film and a mirror-polished silicon wafer were used as the

cathode and anode, respectively. The two electrodes were separated by glass fibers

with a diameter of 80 µm. In the process of measurement, the voltage was varied from

1 to 800 V.

III. RESULTS

Figure 1 shows the FESEM images of specimens A-C grown under different process

conditions. As shown in Fig.1, the basic shapes of NCNTPs are quite similar.

However, the specific morphological characteristics show a clear difference

depending on the growth conditions. Table II summarizes the heights, bottom widths,

and diameters at the top for NCNTPs depicted in Fig.1.

Figure 2 shows the Raman spectra of specimens A-C. From Fig. 2, one can see that

every spectrum contains two main Raman peaks at about 1353 and 1611 cm-1. These

two peaks are confirmed to be the D and G peaks of amorphous carbon,

respectively.19 Compared with the G peak of graphite at 1580 cm-1, a shift of about 31

8

cm-1 of the G peak of NCNTPs toward the high-frequency side indicates that nitrogen

is incorporated into the carbon nanotips.20

Figure 3 displays the XPS spectra of specimens A-C. As shown in Fig. 3, the spectra

feature Si 2p, C 1s, N 1s, and O 1s peaks located at about 102.8, 284.8, 398.8, and

533.0 eV, respectively. As for the other peaks, the peak located at about 152.5 eV is

related to Si 2s state while the peak located at about 979.2 eV is the Auger KLL peak

of carbon.21 The appearance of Si peak in Fig. 3 is related to the etching of the thin

amorphous carbon by hydrogen ions and radicals during the NCNTP growth. Here,

we are mostly interested in the peaks related to C, N, and O elements. According to

the areas of peaks, the atomic concentrations of elements in the NCNTPs can be

obtained by

( ) ( )( )

/.% 100%

/i i

ii i

AC at

A

χχ

= ×∑

o

, (1)

where iCo

is the atomic concentration of element i, iA represents the area under the

peak of element i in the spectrum, iχ is the sensitivity factor and Σ denotes

summation.21 Table III shows the peak areas obtained by the XPS measurement. The

sensitivity factors for carbon, nitrogen, and oxygen elements are 0.205, 0.38, and 0.63,

respectively.8,21 Using Eq. (1) and the data in Table III, one obtains the atomic

concentrations of carbon, nitrogen, and oxygen elements which are also summarized

in Table III.

The C 1s, N 1s, and O 1s core level XPS spectra of the specimens are shown in Fig. 4

9

and are used to analyze the chemical bonding states of C, N, and O elements in the

NCNTPs. From Fig. 4, it is obvious that the peaks are wide and asymmetric, which

indicate that they contain multiple peaks and can thus be de-convoluted. To deduce

the chemical bonding states, the peaks are fitted using a standard peak fit procedure

after a standard Shirley background subtraction. In Figs. 4(a)-(c), the C 1s core level

XPS spectra of the specimens are fitted by two peaks located at about 284.8 and 286.2

eV. These two peaks are assigned to sp2 carbon bonded to another carbon atom (284.8

eV) or a nitrogen atom (286.2 eV), respectively.12, 22 Similarly, the N 1s core level

XPS spectra of the specimens are fitted by two peaks at about 398.6 and 400.1 eV

shown in Figs. 4 (d)-(f). The two peaks located at about 398.6 and 400.1 eV are

confirmed to originate from the non-aromatic sp3 C-N bonds and aromatic sp2 C-N

bonds, respectively22. For the O 1s core level XPS spectra of the specimens in Fig.4

(g)-(i), spectra (g) and (i) are fitted by two peaks while spectrum (h) is fitted by a

single peak. These peaks are located at about 532.5 and 533.2 eV and they are related

to the -OH and C-O-C groups, respectively.23

The current density-electric field (J-E) characteristic curves of specimens A-C are

shown in Fig. 5(a)-(c), where the corresponding Fowler-Nordhelm (F-N) curves are

plotted in the insets. The turn-on field is defined as the electric field where the F-N

curve departs from a straight line.12 From the F-N curves, the turn-on field is about

2.6, 2.1, and 2.8 V/µm for specimens A-C, respectively. As shown in Fig. 5, when the

applied field is 8.5 V/µm, the current density reaches 910 µA/cm2 for specimen B

10

while it is only about 175 µA/cm2 for specimens A and C. Given the measured

turn-on field and the current density, the EFE property of specimen B is clearly better

compared to specimens A and C.

Figure 6 exhibits the low kinetic energy cut-offs of XPS spectra measured on

specimens A-C with a negative bias of -5 V. According to these spectra, the work

function values can be determined by fitting straight lines into their low kinetic

energy cut-offs and the intersect with the base lines of the spectra.24 From Fig. 6, we

obtain that the work function (the difference of the kinetic energy value of the cut-off

edge with the bias) is 4.66, 4.75, and 4.78 eV for specimens A-C, respectively.

IV. DISCUSSION

The insets in Fig. 5 indicate that the EFE of NCNTPs obeys the F-N approximation

( )2 3 3/21.56 6.83 10exp

EJ

E

ββ

× Φ= − Φ , (2)

where Φ is the work function, β is the field enhancement factor, J is the current

density (µA/cm2), and E is the applied electric field (V/µm).25 Eq. (2) indicates that

the EFE properties of NCNTPs to a large extent are determined by the work function

and the field enhancement factor. In this section, the work function and the field

enhancement factor are firstly analyzed, and then the EFE properties are discussed.

A. Work functions of NCNTPs

11

In Sec. III, the work functions of specimens A-C were obtained to be 4.66, 4.75, and

4.78 eV, which are all smaller compared to bulk graphite (4.8 eV).26 The reduction in

the work function is related to the sp3-like clusters and the incorporation of nitrogen

into the carbon nanotips. As shown in Fig. 4, non-aromatic sp3 C-N and aromatic sp2

C-N bonds have formed in the NCNTPs. In other words, nitrogen effectively

incorporated in the sp3- and sp2-like clusters. The sp3-like clusters in graphite-like

materials provide a reduction of the potential barrier width for electrons escaping into

vacuum,27 while the substitution of a nitrogen atom for a carbon atom in aromatic sp2

carbon materials causes the rise of the Fermi level towards the conduction band,

which effectively reduces the work function.28 This explains the observed reductions

of the work functions of NCNTPs in comparison with bulk graphite.

Figure 2 suggests that the NCNTPs are amorphous structures. For amorphous carbon

materials, the electron field emission mainly originates from sp2 carbon regions,12 i.e.,

the EFE properties of NCNTPs are determined by the sp2 phase of NCNTPs.

According to Fig. 2, the ratio of intensities of G and D peaks is 1.02, 1.03, and 1.02

for specimens A-C. In other words, all the specimens feature the same degree of

graphitization. Therefore, the measured reduction of the work functions of the

specimens is due to the incorporation of nitrogen. From Fig. 4(a)-(c) and Table III, we

obtain the ratios of sp2 C-N phase in sp2 phases and they are 20, 22, and 18% for

specimens A-C, respectively. Thus, the work function of specimen B should be the

12

smallest of all the samples due to the incorporation of nitrogen.28

However, the measurement results indicate that the work function of specimen A is

the lowest, which implies that the work function is also affected by other factors.

Indeed, the surface-dipole effect has an influence on the work function.29 The surface

dipoles originate from the charge transfer between different elements, e.g.,

chemisorbed hydrogen on silicon or oxygen on nickel surfaces.30,31 As shown in Fig.

4(g)-(i), the binding energies (BEs) of oxygen are about 532.5 and 533.2 eV, which

indicate that oxygen terminates the surfaces of NCNTPs because the BE of oxygen

should be lower than 532 eV if oxygen is located in the interior of carbon materials.32

Due to the presence of hydrogen in the reactive chamber, hydrogen also terminates

the surfaces of NCNTPs, hence Fig. 4 (g)-(i) shows the formation of -OH and C-O-C

groups. The formation of these polar bonds indicates that a dipole layer is formed on

the NCNTP surfaces. This in turn affects the electron emission from NCNTPs, namely,

their work functions are altered by the surface-dipole effect. The relation of the work

function with the surface dipole parameters is

( )3/2

0

*

1S

eN

k N

θµε ε α θ

∆Φ = +

, (3)

where N is the number of substrate atoms per area, θ is the fractional coverage of

the adsorbate, *µ is the dipole moment of an isolated surface adsorbate, α is the

polarizability of the dipole, 0ε and Sε are the permittivity of vacuum and the

surface dielectric constant, k is a constant depending on the arrangement of dipoles,

13

and ∆Φ is the resulting variation of the work function.30,31 Equation (3) was

successfully used in the explanation of the experimental results of chemisorptions of

hydrogen on silicon and oxygen on gallium arsenide.30 Here it is noted that *µ is

positive if the negative end of the dipole faces vacuum and the other way around.31

Because the order of electro-negativity among hydrogen, carbon, and oxygen

elements is H < C < O,29 some charges transfer from hydrogen and carbon atoms to

the oxygen atoms to form a dipole layer on the NCNTP surfaces. As a result, the

negative end of the dipole faces vacuum. According to Eq. (3), the work functions of

NCNTPs increase due to the termination of NCNTP surfaces with oxygen. It was

found that ∆Φ increases monotonically with θ when 0∆Φ > .31

The values in Table III indicate that the oxygen content gradually increases from

specimen A to specimen C. Hence, the work functions of specimens A-C should also

follow this trend. According to the above analyses, the work function of specimen C

should be the highest because of its low content of sp2 C-N phase and its high oxygen

content, which is consistent with the measurement results. From Table III, one can see

that the oxygen content of specimen B is about two times higher than in specimen A,

while specimens B and A have a small difference in the content of sp2 C-N phase.

Therefore, the high work function of specimen B is attributed to its high oxygen

content, i.e., a larger extent of surface termination by oxygen.

14

B. Field enhancement characteristics of NCNTPs

From Eq. (2), one can obtain the slope of the F-N curve,

( )( )( )

2 3 3/2ln / 6.83 10

1/

d J E

d E β× Φ= − . (4)

According to Eq. (4) and the insets in Fig. 5, the field enhancement factors are about

2085, 2071, and 1633 for specimens A-C, respectively. These values are obtained

from Eq. (4) after the measured work functions of the relevant samples are used.

From Fig. 1, one can notice short nanowires located on the NCNTP tips. Hence, the

specimens contain hierarchical structures made of the nanotips decorated with thin

nanowires. Consequently, the field enhancement factor should be analyzed using the

multistage approximation33

1

N

s ii

β β β=

= ∏ , (5)

where

( )1 exp 2.3172 /s ts Hβ = − − , (6)

is the factor that accounts for the screening effect of the adjacent tips and iβ is the

field enhancement factor of the i-th-stage emitter.25, 33, 34 Here, s is the average

distance between two tips and tH is the total height including the height of a

NCNTP and the length of the nanowire on the top.25 During the measurement of EFE

properties of NCNTPs, the distance between the cathode and the anode is 80 µm

which is larger than 100ρ , where ρ is the curvature radius of a CNTP top. Under such

15

conditions, the field enhancement factor is calculated as35

2.5tip

hβρ

= + , (7)

where h is the height of the NCNTP. For the nanowire, one has

3N

l

rβ = + , (8)

where l and r are the length of the nanowire and the curvature radius of the

nanowire top, respectively.36

The field enhancement factors of the specimens have been calculated by assuming

that the specimens are two-stage emitters. According to Eqs. (5)-(8), the field

enhancement factors are 302, 279, and 234 for the NCNTPs labeled (A)-(C) in Fig.1,

respectively. This is quite different from the values obtained from the F-N curves. The

significant differences imply that electrons are also emitted from other areas on the

NCNTP surface besides the nanowires located on the tops (see Fig. 1).37 The

nanowires are likely to form from the new growth surface after the tops are etched

through ion bombardment during the growth of NCNTPs.38 Thus, it is possible that

there is an edge at the point where the nanowire joins the nanotip. Near the edge, a

strong electric field is formed.39 Consequently, electrons can be easily emitted from

the edge and the field enhancement factor can be calculated as37

( )1/21 /ir hβ ρ= + . (9)

Following Ref. (34), after this factor is incorporated into Eq. (5), we obtain the field

enhancement factors of about 1932, 2064, and 1264 for specimens A-C, respectively.

In addition, Fig. 1 also shows that the surfaces of NCNTPs are quite rough, which

16

implies that there are some small features or micro-protrusions on the nanotips. If the

contributions of these features and micro-protrusions are included, the field

enhancement factor is further increased and approaches the values obtained from the

F-N plots. From the above analyses, one can thus conclude that electrons are also

emitted from other surface areas of NCNTPs besides their tops. According to Fig. 1,

these areas include the joint edges of the nanowires with the tips and the features or

micro-protrusions on the NCNTP surfaces. In these localized areas, the field

enhancement factor can be further increased due to the edges and much smaller

dimensions of the features or micro-protrusions; as a result, the electron emission can

be very effective. 37,39,40

C. Analysis of EFE properties of NCNTPs

Figure 5 reveals that specimen B shows better EFE properties than specimens A and

C. This difference is related to the structural and thermal factors. From Secs. III and

IV B, one can see that specimen A has a low work function and a high field

enhancement factor, i.e., it can easily emit electrons at the low applied field. Due to

the thermal effect produced by Joule heat during electron emission,41 it is possible that

the nanowires on the NCNTP tops are damaged so that the EFE property is degraded.

A similar phenomenon was observed in the process of electron emission from carbon

nanotubes.42, 43 For specimen C, the high work function and low field enhancement

factor result in a relatively worse EFE performance compared with specimen B; this

17

follows from Eq. (2). According to Eq. (7), the low field enhancement factor of

specimen C results from the low ratio of the height to the curvature radius at the top.

Based on these analyses, the regular NCNTPs with the high ratio of the height to the

curvature radius at the top can be expected to become one of the leading performers in

electron field emission device applications.

V. CONCLUSION

The nitrogenated carbon nanotips were synthesized by plasma-assisted HFCVD under

different conditions and their structures and compositions were investigated by the

advanced characterization tools. In addition, their work functions were obtained by

XPS and are in the 4.66-4.78 eV range depending on the oxygen termination of their

surfaces. The electron field emission properties of the nitrogenated carbon nanotips

were studied under high-vacuum conditions and the results indicate that the turn-on

fields can be as low as 2.1 V/µm, whereas the achievable current densities reach 910

µA/cm2. Modeling of electron field emission was used to calculate the field

enhancement factors. By comparing the field enhancement factors obtained by this

modeling and directly from the experimental data, it was concluded that electrons can

be emitted from some other surface areas of nitrogenated carbon nanotips besides

their tops. The electron field emission properties were systematically analyzed based

on the work functions and the field enhancement factors. The analysis suggests that

the best EFE performance is expected from the nanotips with the high ratio of the

18

height to the curvature radius at the top, which are also decorated by thin nanowires

that emerge from the tops. Therefore, such nanotips hold an outstanding promise as

next-generation electron emitting materials for a diverse range of applications in

microelectronic and optoelectronic devices.

Acknowledgments

This work is supported by Chongqing Natural Science Foundation of China (CSTC,

2009BA4027) and is partially supported by CSIRO’s OCE Science Leadership

Scheme and the Australian Research Council.

19

References

1F. Alibart, M. Lejeune, O. D. Drouhin, K. Zellama, and M. Benlahsen, J. Appl. Phys.

108, 053504 (2010).

2X. Cai, H. Cong, and C. Liu, Carbon 50, 2726 (2012).

3G. Lota, K. Fic, and E. Frackowiak, Energy Environ. Sci. 4, 1592 (2011).

4M. Keidar, I. Levchenko, T. Arbel, M. Alexander, A. M. Waas, and K. Ostrikov,

Appl. Phys. Lett. 92, 043129 (2008).

5G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, F. Pelayo, G.

de Arquer, F. Gatti, and F. H. L. Koppens, Nature Nanotechnol. 7, 363 (2012).

6K. S. Yeong, C. B. Boothroyd, and J. T. L. Thong, Nanotechnology, 17, 3655 (2006).

7N. Shang, W. I. Milne, and X. Jiang, J. Am. Chem. Soc. 129, 8907 (2007).

8B. B. Wang, Q. J. Cheng, Y. A. Chen, and K. Ostrikov, J. Appl. Phys. 110, 054323

(2011).

9G. Y. Zhang, X. D. Bai, E. G. Wang, Y. Guo and W. Guo, Phys. Rev. B 71, 113411

(2005).

10C. H. Wei, C. H. Chen, C.-M. Yeh, M. Y. Chen, J. Hwang, A. P. Lee, and C. S. Kou,

J. Electrochem. Soc. 152, C366 (2005).

11L. Xu, S. Li, Z. Wu, H. Li, D. Yan, C. Zhang, P. Zhang, P. Yan, X. Li, Appl. Phys. A

103, 59 (2011).

12 B. B. Wang, Q. J. Cheng, X. X. Zhong, Y. Q. Wang, Y. A. Chen, and K. Ostrikov, J.

Appl. Phys. 111, 044317 (2012).

13Q. Z. Zhao, F. Ciobanu, and L. J. Wang, J. Appl. Phys. 105, 083103 (2009).

14F. Xiong, Y. Y. Wang, and R. P. H. Chang, Phys. Rev. B 48, 8016 (1993).

15K. Ostrikov, Rev. Mod. Phys. 77, 489 (2005); K. N. Ostrikov, M. Y. Yu, and N. A.

Azarenkov, J. Appl. Phys. 84, 4176 (1998).

16U. Cvelbar, Z. Chen, M. K. Sunkara, and M. Mozetič, Small 4, 1610 (2008); K.

Ostrikov, I. Levchenko, U. Cvelbar, M. Sunkara, and M. Mozetič, Nanoscale 2, 2012

(2010).

20

17 A. Anders, Appl. Phys. Lett. 100, 224104 (2012). 18S. Xu, I. Levchenko, S. Y. Huang, and K. Ostrikov, Appl. Phys. Lett. 95, 111505

(2009); N. A. Azarenkov, I. B. Denisenko, and K. N. Ostrikov, J. Phys. D: Appl.

Phys. 28, 2465 (1995).

19B. B. Wang, Q. J. Cheng, L. H. Wang, K. Zheng, and K. Ostrikov, Carbon 50, 3561

(2012).

20J. H. Kaufman, S. Metin, and D. D. Saperstein, Phys. Rev. B 39, 13053 (1989).

21Q. Cheng, S. Xu, and K. Ostrikov, J. Mater. Chem. 20, 5853 (2010).

22P. Hammer, R. G. Lacerda, R. D. Jr., and F. Alvarez, Diam. Relat. Mater. 9, 577

(2000).

23C. Moreno-Castilla, A. F. Pérez-Cadenas, F. J. Maldonado-Hódar, F.

Carrasco-Marín, J. L. G. Fierro, Carbon 41, 1157 (2003).

24 R. Schlaf, H. Murata, and Z. H. Kafafi, Journal of Electron Spectroscopy and

Related Phenomena 120, 149 (2001).

25J. Xiao, X. Zhang, and G. Zhang, Nanotechnology 19, 295706 (2008).

26M. Shiraishi and M. Ata, Carbon 39, 1913 (2001).

27A. N. Obraztsov, A. P. Volkov, A. I. Boronin, and S. V. Kosheev, Diam. Relat.

Mater. 11, 813 (2002).

28S. H. Lim, H. I. Elim, X. Y. Gao, A. T. S. Wee, W. Ji, J. Y. Lee, and J. Lin, Phys.

Rev. B 73, 045402 (2006).

29J. Shirafuji, and T. Sugino, Diam. Relat. Mater. 5, 706 (1996).

30W. Mönch, P. Koke, and S. Krueger, J. Vac. Sci. Technol. 19, 313 (1981).

31G. N. Luo, K. Yamaguchi, T. Terai, and M. Yamawaki, J. Nucl. Mater. 290–293,

116 (2001).

32C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg,

Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer Corp., 1979, p. 42.

33J. Y. Huang, K. Kempa, S. H. Jo, S. Chen, and Z. F. Ren, Appl. Phys. Let. 87,

053110 (2005).

34F. Solá, A. Biaggi-Labiosa , L. F. Fonseca, O. Resto, M. Lebrón-Colón, and M. A.

Meador, Nanoscale Res. Lett. 4, 431 (2009).

21

35A. J. le Fèbre, L. Abelmann, and J. C. Lodder, J. Vac. Sci. Technol. B 26, 724

(2008).

36R. G. Forbes, C. J. Edgcombe, and U. Valdrè, Ultramicroscopy 95, 57 (2003).

37O. B. Isayeva, M. V. Eliseev, A. G. Rozhnev, and N. M. Ryskin, Sol.-State Electron.

45, 871 (2001).

38B. B. Wang, G. B. Dong, and X. Z. Xu, Appl. Surf. Sci. 258, 1677 (2011).

39J. V. Bladel, Proc. IEEE 71, 901 (1983).

40L. D. Filip, J. D. Carey, and S. R. P. Silva, J. Appl. Phys. 109, 084527 (2011).

41S. T. Purcell, P. Vincent, C. Journet, and V. T. Binh, Phys. Rev. Lett. 88, 105502

(2002)

42Z. L. Wang, R. P. Gao, W. A. de Heer, and P. Poncharal, Appl. Phys. Lett. 80, 856

(2002).

43T. Kuzumaki, Y. Takamura, H. Ichinose, and Y. Horiike, Appl. Phys. Lett. 78, 3699

(2001).

22

Table and figure captions

Table I. Growth conditions: gas flow rates, bias currentbI , applied bias voltage CU ,

and growth time t .

Table II. Morphological characteristics of NCNTPs: heights (H), bottom widths

(BW), and diameters at the top (D).

Table III. Area Ai peaks in Fig. 3 and atomic concentration iCo

of element i, where i

represents carbon, nitrogen, and oxygen.

FIG. 1. FESEM images of specimens A (a), B (b), and C (c).

FIG. 2. (Color online) Raman spectra of specimens A (1), B (2), and C (3).

FIG. 3. (Color online) XPS spectra of specimens A (1), B (2), and C (3).

FIG. 4. (Color online) C 1s, N 1s and O 1s core level XPS spectra of specimens A-C.

FIG. 5. J-E characteristic curves of specimens A-C: (a) A; (b) B; and (c) C. The insets

are the corresponding F-N curves.

FIG. 6. (Color online) Low kinetic energy cut-offs of XPS spectra measured on

specimens A (1), B (2), and C (3).

23

Table I. Growth conditions: gas flow rates, bias currentbI , applied bias voltage CU ,

and growth time t .

Specimen CH4(sccm) N2(sccm) H2(sccm) Ib (mA) UC(V) t(min)

A 20 15 65 160 910-980 20

B 15 15 70 160 920-1000 20

C 10 15 75 160 920-970 20

Table II. Morphological characteristics of NCNTPs: heights (H), bottom widths

(BW), and diameters at the top (D).

Specimen H(nm) BW(nm) D(nm)

A 447-541 53-71 12-29

B 306-624 59-129 12-24

C 200-294 59-94 12

Table III. Area Ai peaks in Fig. 3 and atomic concentration iCo

of element i, where i

represents carbon, nitrogen, and oxygen.

Specimen AC AN AO CC

o

(at.%) NCo

(at.%) OCo

(at.%)

A 42361 9157 39766 70.3 8.2 21.5

B 28338 7410 71362 51 7.2 41.8

C 19422 8274 76252 39.9 9.2 51

24

FIG. 1. FESEM images of specimens A (a), B (b), and C (c).

25

FIG. 2. (Color online) Raman spectra of specimens A (1), B (2), and C (3).

26

FIG. 3. (Color online) XPS spectra of specimens A (1), B (2), and C (3).

27

FIG. 4. (Color online) C 1s, N 1s and O 1s core level XPS spectra of specimens A-C.

28

FIG. 5. J-E characteristic curves of specimens A-C: (a) A; (b) B; and (c) C. The insets

are the corresponding F-N curves.

29

FIG. 6. (Color online) Low kinetic energy cut-offs of XPS spectra measured on

specimens A (1), B (2), and C (3).