Journal of Physics and Chemistry of Solids 74 (2013) 259–264
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Study on structures and electron field emission of nitrogenated carbon nanotips grown by plasma-enhanced hot filament chemical vapor deposition B.B. Wang a,n, Y.Q. Wang b, C.S. Gong c, R.Z. Wang b, E.Q. Xie c, X.J. Quan a a
College of Chemistry and Chemical Engineering, Chongqing University of Technology, 69 Hongquang Road, Lijiatuo, Banan District, Chongqing 400054, PR China College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR China c School of Physical Science and Technology, Lanzhou University, Lanzhou 73000, PR China b
a r t i c l e i n f o
abstract
Article history: Received 4 July 2012 Received in revised form 30 August 2012 Accepted 26 September 2012 Available online 5 October 2012
Nitrogenated carbon nanotips (NCNTPs) with different structures were synthesized by plasmaenhanced hot filament chemical vapor deposition under different time using methane, nitrogen and hydrogen as the reaction gases. The results of field emission scanning electron microscopy, microRaman spectroscopy and X-ray photoelectron spectroscopy indicate that the NCNTPs are amorphous structure and they have significant changes in the morphologies and components with the growth time. The electron field emission (EFE) from NCNTPs was measured and the results show that the current density increases from about 200 to 2800 mA/cm2 at the field of 12 V/mm depending on their structures and components. According to the growth conditions, the characterization results and the electronic structure of amorphous carbon, the changes in the structures, components and work function of NCNTPs were analyzed. Combined the work function with the characterization results, the EFE properties of NCNTPs were studied. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Vapour deposition C. Raman spectroscopy D. Electrical properties
1. Introduction Recently, carbon nanotips have attracted much attention due to their novel tip structure, unique electronic and mechanic properties and the potential applications in the fields of field emitters, scanning probe microscope tips, nanoindenters, etc. [1–3]. The carbon naotips have good conductivity and can produce a high local field near their tips so that they can become the promising materials for field emitters [4], thus the growth, structures and the electron field emission (EFE) properties of carbon nanotips have been extensively studied in last years [5–12]. In previous work, we studied the EFE properties of nitrogenated carbon nanotips (NCNTPs) [11,12], but the change in the work function of different NCNTPs is not well understood. It is known that the work function plays an important role in the EFE process of materials, thus it is important to understand the work function of NCNTPs for their applications in the field of microelectronic devices. In this work, the change in the work function of different NCNTPs is emphatically analyzed to study the EFE properties of NCNTPs. In the process of fabrication of materials, Plasma is widely employed because it plays important roles in the gaseous ionization and the interaction of ions with materials [13,14], thus the
n
Corresponding author. Tel./fax: þ 86 23 62563221. E-mail address:
[email protected] (B.B. Wang).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.09.016
plasma is usually applied in hot filament chemical vapor deposition system to synthesize carbon materials. In this work, the NCNTPs were synthesized by plasma-enhanced hot filament chemical vapor deposition (PEHFCVD) under different time using methane, nitrogen and hydrogen as the reaction gases. The structural and compositional properties were studied by advanced characterization tools including field emission scanning electron microscopy (FESEM), micro-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The EFE properties were measured in high vacuum system of 10 6 Pa. Beginning with the ion bombardment, the structures and components of NCNTPs were analyzed. According to the electronic structure of amorphous carbon, the work function of NCNTPs was studied. Furthermore, the EFE characteristics of NCNTPs were discussed.
2. Experimental The NCNTPs were synthesized in PEHFCVD system described in Ref. [2]. Briefly, there are three coiled hot tungsten filaments (heated to about 1800 1C) in the reaction chamber to heat and pre-ionize the reaction gases. The substrate was silicon wafer deposited with a thin carbon film, which was heated by the hot filaments to about 870 1C. The distance between the substrate and filament was about 8 mm. A negative bias relative to the filaments was applied to the substrate through a molybdenum holder to produce plasma. The reaction gas was a mixture of methane,
260
B.B. Wang et al. / Journal of Physics and Chemistry of Solids 74 (2013) 259–264
nitrogen and hydrogen, which their flow rates were 20, 15, and 65 sccm (sccm denotes cubic centimeters per minute at standard temperature and pressure), respectively. The working pressure was 2 103 Pa. Before the preparation of NCNTPs, a thin carbon film was deposited on the (1 0 0) single crystal silicon substrate using magnetron sputtering to induce the formation of NCNTPs [15]. During the deposition of carbon film, the vacuum chamber was pumped to about 10 4 Pa using a combination of rotary and turbo-molecular pumps. Then, a high-purity (99.999%) Ar gas was inlet into the chamber, which its flux was 30 sccm. When the work pressure increased to 0.5 Pa, it was maintained at the value. Later, the radio-frequency power was turned on to produce plasma. The power was set at 100 W to deposit the carbon film for 30 min by sputtering carbon target. The thickness of carbon film was about 60 nm. During the growth of NCNTPs, the PEHFCVD chamber was evacuated to a pressure lower than 2 Pa after the substrate was placed into the chamber. Then, methane, nitrogen and hydrogen gases were introduced into the chamber. After the work pressure reached to 2 103 Pa, the filaments were fast heated and the substrate was heated to about 870 1C by the hot filaments. Later, a bias power was turned on to produce plasma and grow the NCNTPs. In this work, we set the bias current to 160 mA and changed the growth time to prepare three specimens A–C, which the growth time was 15, 20 and 25 min, respectively. The morphologies and components of NCNTPs were studied by FESEM, micro-Raman spectroscopy and XPS, respectively. FESEM measurement was performed using a Hitachi S-4800 field emission scanning electron microscope (operated at 15 kV). The XPS measurements were carried out by ESCALAB 250 X-ray photoelectron spectrometer using an Al Ka X-ray source after the specimens were etched about 2 nm by Ar þ . The Raman spectra were recorded in a Renishaw micro-Raman spectroscopy, in which an excitation sources was an Ar þ laser with a wavelength of 514 nm. The EFE characteristics of NCNTPs were measured using a diode configuration in a vacuum system of 10 6 Pa. In the diode configuration, the NCNTP film was used as the cathode and a mirror-polished silicon wafer was used as the anode, and they were spaced by glass fibers with a diameter of 65 mm. During the process of measurement, the voltage was changed from 1 to 800 V.
3. Results Fig. 1(a)–(c) are the FESEM images of the specimens A–C, respectively. From Fig. 1, one can see that the NCNTPs have the similar morphology, but there are obvious differences in their height, top diameter and base width. According to Fig. 1, we obtain the height, base width and top diameter of the typical NCNTPs and they are summarized in Table 1. Fig. 2 shows the Raman spectra of the specimens A–C. As shown in Fig. 2, every spectrum shows two peaks centered about 1354 and 1611 cm 1, which are the D and G peaks of amorphous carbon materials [16–18]. Compared with the G peak of graphite at 1580 cm 1, the G peaks of the specimens have a shift of about 31 cm 1, which originates from the incorporation of nitrogen in the carbon nanotips [19]. In addition, Fig. 2 also shows different ratios of G to D peak for the specimens A–C, which is related to the incorporation of nitrogen in the carbon nanotips. The different growth time results in the difference of nitrogen in the carbon nanotips and it is confirmed by following XPS results. Because nitrogen can drive transformation of sp3 to sp2 carbon in carbon
Fig. 1. FESEM images of different specimens (a): specimen A; (b): specimen B; (c): specimen C.
nitride materials [20], Fig. 2 shows different ratios of G to D peak depending on the growth time. Fig. 3 is the XPS spectra of the specimens A–C, which every spectrum shows a series of peaks. Among these peaks, the peaks located at binding energy (BE) of about 284.8, 398.7 and 532.9 eV are attributed to C 1 s, N 1 s and O 1 s, while other peaks located at about 103, 154.4 and 979.4 eV are attributed to Si 2p, Si 2 s and
B.B. Wang et al. / Journal of Physics and Chemistry of Solids 74 (2013) 259–264
Table 1 The height, base width (BW) and top diameter (D) of NCNTPs of the specimens A–C. Specimen
Height (nm)
BW (nm)
D (nm)
A B C
453–507 293–573 267–467
80–107 107–133 80–107
20–40 13 13–26
1611
2500 1354 Intensity (arb.u)
2000 (A) 1500 1000
(B)
500
(C)
0 1000
1200
1400 Raman shift
1600
1800
2000
(cm-1)
Fig. 2. Raman spectra of different specimens (a): specimen A; (b): specimen B; (c): specimen C.
9
Intensity (arb.u) (X105)
Table 2 The BE of C 1 s, N 1 s and O 1 s peaks and their content Xi in the specimens A–C. Specimen
BEC (eV)
BEN (eV)
BEO (eV)
XC (at%)
XN (at%)
XO (at%)
A B C
284.82 284.81 284.8
398.73 398.6 398.75
532.95 532.91 532.8
38.1 39.2 64.1
4.7 6.4 7.3
37.6 34.9 14.9
attributed to sp2 C–C bonds, aromatic sp2 C–N bonds and nonaromatic sp3 C–N bonds, respectively [24–26]. From Fig. 4(d)–(f), the two deconvoluted peaks are located at BE of 398.6 and 400 eV, which originated from the non-aromatic sp3 C–N bonds and aromatic sp2 C–N bonds, respectively [24–26]. For Fig. 4(g)– (i), the deconvoluted peaks located at BE of about 532.1, 532.9 and 533.1 eV are related to the C ¼O, OH and C–O–C groups [27–29]. Fig. 5 shows the curves of current density J versus the applied electric field E of the specimens A–C, in which the insets are the corresponding Fowler–Nordhelm (F–N) curves. According to Ref. [30], the turn-on field is defined as the electric field when the F–N curve deviates from the straight line. From the F–N curves, the turn-on field is about 4.76, 4.55, and 3.57 V/mm for the specimens A–C, respectively. As shown in Fig. 5, the specimen C can emit a current density of 2.8 mA/cm2 at the field of 12 V/mm, while the current densities are about 0.3 and 0.2 mA/cm2 at the field of 12 V/mm for the specimens A and B, respectively. These data indicate that the specimen C has a better EFE characteristic than the specimens A and B. In addition, the insets in Fig. 5 show that the lnðJ=E2 Þ 1=E curves deviate from F–N curves in the high field region.
C 1s
8 7
Si 2p Si 2s
N 1s
O 1s
C KLL
4. Discussion
(C) 4.1. Analysis of structures and components of NCNTPs
6 5 (b)
4 3 2
(a)
1 0
261
0
100 200 300 400 500 600 700 800 900 1000 1100 Binding energy (eV)
Fig. 3. XPS spectra of the specimens (a): specimen A; (b): specimen B; (c): specimen C.
C KLL, respectively [21–23]. The appearance of the peaks related to Si is related to the etching of hydrogen ions. Because hydrogen has a strong etching effect to amorphous carbon, some places on the substrate are strongly etched so that Si is detected by XPS. Here, we are interested with the peaks related to C, N and O, the detailed BE of C 1 s, N 1 s and O 1 s peaks and their content in the specimens A–C are given in Table 2. To analyze the bond states of C, N and O elements in the NCNTPs, we measured the narrow XPS spectra of the specimens and they are shown in Fig. 4. The wide and asymmetry spectra indicate that they are composed of multipeaks, thus they are fitted using XPS peak 41 software after Shirley background subtraction. As shown in Fig. 4(a)–(c), the three deconvoluted peaks are located at BE of about 284.8, 285.6, 286.6 eV, which are
As shown in Fig. 1, a lot of NCNTPs of the specimen B become sharper than that of the specimens A and C, while the homogeneity of NCNTPs of the specimen A is the best in the three specimens. The change in the morphologies of NCNTPs is related to the ion bombardment under different time. The formation of NCNTPs was carefully studied in Ref. [6], which they can be formed when the sputtering and deposition rates are dynamically balanced. After the formation of NCNTPs, the electric field near the tips of NCNTPs formed by plasma is increased to result in emission of electrons from the NCNTPs [31]. As a result, the ion density near tips of NCNTPs is increased so that the growth rate is enhanced, thus the tips become very sharp at definite time. If the growth time is lasted, some sharp tips of NCNTPs can be removed by sputtering [31,32]. As a result, the new growth surfaces are formed and a growth rate difference occurs on the original and new growth surfaces. The difference in the growth rates will result in the poor homogeneity of NCNTPs, thus Fig. 1 shows that the NCNTPs become sharp and poor homogeneity with the increase in the growth time. The results in Table 2 suggest that the nitrogen in the specimens A–C gradually increases while the oxygen content in the same samples decreases. These changes are associated to the formation of the new growth surfaces and the effect of etching. During the growth of NCNTPs, the new growth surfaces formed by the removing of the tips are irregular [31]. As a result, the nitrogenous ions are easily bonded to the irregular growth surfaces owing to the low nucleation energy on the microdefects [33]. Because the number of new growth surfaces is increased with the growth time and there is much nitrogen to
262
B.B. Wang et al. / Journal of Physics and Chemistry of Solids 74 (2013) 259–264
18 16 (a) C 1s
Specimen A
40
Specimen A
(d) N 1s
12
25
4.5
20
6
4.0
15
4
3.5
10
8
2 280
282
284
286
18 (b) C 1s 16
288
290
Specimen B
14
3.0
7.0
5 394
396
398
400
(e) N 1s
402
404
528 40
Specimen B
6.5
35
6.0
30
5.5
25
5.0
20
4.5
15
4
4.0
10
2
3.5
12 10 8 6
Specimen C
25
8.0
398
400
(f) N 1s
402
404
406
18
Specimen C
7.5
16
7.0
14
6.5
12
15
6.0
10
5.5
284
286
288
290
536
538
Specimen B
530
532
534
(i) O 1s
536
538
Specimen C
6
4.5 282
534
8
5.0
5
532
(h) O 1s
528
20
10
530
5 396
281 282 283 284 285 286 287 288 289 290
30 (c) C 1s
Specimen A
30
5.0
10
(g) O 1s
35
5.5
14
Intensity (arb.u) (x103)
6.0
394
396
398
400
402
404
406
4 528
530
532
534
536
538
Binding energy (eV) Fig. 4. Narrow XPS spectra of C 1 s (a)–(c), N 1 s (d)–(f) and O 1 s (g)–(i).
diffuse in the NCNTPs in a long time, the nitrogen content in the specimen A–C is gradually increased from the specimen A to C. Due to the etching effect of nitrogenous ions, some oxygen atoms absorbed on the surfaces of NCNTPs are etched. Thus, oxygen absorbed on the surfaces is reduced in turn from the specimen A to C. 4.2. Analysis of work function of NCNTPs Fig. 2 indicates that the NCNTPs are amorphous carbon structure. For the amorphous carbon materials, the electronic properties are determined by the aromatic carbon clusters in them [34]. As shown in Fig. 4, the NCNTPs have aromatic sp2 C–N bonds, which imply that nitrogen atoms replace the carbon atoms of sp2 carbon clusters in rings (aromatic carbon clusters). The substitution nitrogen atoms for carbon atoms in aromatic carbon clusters results in the formation of pyridine-like carbon structure [35]. In this carbon structure, three electrons of the nitrogen atom form 3 s bonds and a fourth electron forms a p bond, and the fifth electron is unpaired to enter an antibonding pn state [35]. According to the molecular orbital diagram of pyridine shown in Ref. [36],the N pn state is lower in energy compared to the C pn
state. Thus, there is a low effective pn band edge due to sp2 bonded C–N [36], i.e., the band gap of amorphous carbon becomes narrow owing to the doping of nitrogen and it has been confirmed by experiment [35]. The shrinkage of band gap implies that the Fermi level is close to the conduction band. According to the definition of work function, the work function of amorphous carbon materials will be reduced due to the doping of nitrogen. The nitrogen doping in the amorphous carbon materials is a shallow doping and nitrogen is a n-type dopant in the amorphous carbon materials [37], thus Fermi level fast moves toward the conduction band with the nitrogen content in the amorphous carbon materials [38]. Namely, the work function of the amorphous carbon materials is reduced with the increase in the nitrogen concentration in the amorphous carbon materials. For the specimens A–C, Table 2 indicates that the concentration is gradually enhanced, thus the work function of NCNTPs will be reduced in turn for the specimens A–C. 4.3. Analysis of EFE properties of NCNTPs with diffrent structure Fig. 5 shows that the specimens A–C have different EFE properties due to different structure. For the carbon nanotips,
B.B. Wang et al. / Journal of Physics and Chemistry of Solids 74 (2013) 259–264
400
0
300
-2
ln (J/E2)
Current density (µA/cm2)
are mA/cm2 and V/mm, respectively [39]. From Eq. (1), we can obtain the relation of the slope of F–N curve with the work function and the field enhancement factor and it is, dln J=E2 BF3=2 ¼ : ð2Þ b d 1=E
2
350
250 200
-4
Depending on Eq. (2), the work function can be obtained after the slope and the field enhancement are known. As shown in Fig. 1, there are very short nanowires grown on the tips of NCNTPs, thus the field enhancement factor b should be the product of the field enhancement factor of nanowire bnw and the field enhancement factor of tip btip [40],
-6
150
-8
100
-10
0.1
50
0.2 0.3 1/E (V/µm)
0.4
b ¼ bnw btip
0 0
10
12
2 0
200 ln (J/E2)
Current density (µA/cm2)
8 4 6 Electric field (V/µm)
2
250
150
-2 -4 -6
100
-8 0.1
50
0.2 0.3 1/E (µm/V)
0.4
0 0
8 4 6 Electric field (V/µm)
2
10
12
4
3000 2500 ln (J/E2)
Current density (µA/cm2)
2
2000 1500
0 -2 -4 -6 -8
1000
0.1
500
0.2 0.3 1/E (µm/V)
0.4
0 0
2
4 6 8 Electric field (V/µm)
10
12
Fig. 5. J–E curves of the specimens A–C. The insets are the corresponding F–N curves.
their EFE properties are studied by F–N Eqs. (10–12,30), J¼
AðbEÞ2
F
263
exp
! BF3=2 , bE
ð1Þ
where F is the work function, b is the field enhancement factor, A ¼ 1:54 1010 (AV 2eV), B ¼ 6:83 109 (Vm 1eV 3/2). For the constant A and B, they have different values for different units. For example, B is 6.83 103 (Vmm 1eV 3/2) when the units of J and E
ð3Þ
For the field enhancement factors of nanowire and tip, they can be expressed by
bnw 2 þ l=r,
ð4Þ
btip 10:6þ h=r0 ,
ð5Þ
where l and r are the length and radius of nanowire, and h and r 0 are height and top radius of NCNTP, respectively [41,42]. For a specimen, the electron easily emits from the spot which the field enhancement factor is the largest [43], here we calculate b values of the NCNTPs marked with (A), (B) and (C) in Fig. 1. The structural sizes obtained by Fig. 1 and the calculated b values of NCNTPs (A)–(C) are summarized in Table 3. From Fig. 5, the slopes of F–N curves can be obtained and they are about 92, 75 and 83. According to the b values in Table 3, we use Eq. (2) to estimate the work function of specimens and they are about 3.3, 3.2 and 2.8 eV for the specimens A–C, which are consistence with the analysis in Section 4.2. In Ref. [44], we found that the work function of amorphous carbon cones with impurities is 3.8 eV, which approaches the work function of NCNTPs. This indicates that our analysis in Section 4.2 is reasonable. For the specimen C, it has the lowest work function. In addition, a nitrogen atom can provide a free electron after it replaces a carbon atom in aromatic carbon clusters [35]. Thus, the specimen C has the largest electron concentration due to the highest nitrogen concentration in it. So, Fig. 5 shows that the specimen C has the better EFE characteristic than the specimens A and B. In comparison with the specimen A, the specimen B has a low work function and large field enhancement factor. It seems that the EFE property of the specimen B should be better than that of the specimen A, but Fig. 5 shows that it produces a lower current density than the specimen A in high field region. It is related to the tip damage of NCNTPs during the emission of electrons. From Fig. 1, the tips of the specimen B are such sharp that there is a strong field near the tip and a number of electrons are emitted from it. As a result, it is possible that a high temperature produces at the tip [45] to damage the tip structure, which the phenomenon was observed in the process of electron emission from carbon nanotubes [46,47]. The tip damage results in the reduction of field enhancement factor so that the specimen B produces a lower current density than the specimen A in high field region. Table 3 The l, r, h, r 0 an b values of NCNTPs marked with A, B and C in Fig. 1.
A B C
l (nm)
2r (nm)
h (nm)
2r 0 (nm)
b
429 438 400
23 29 14
86 104 33
15 13 10
444 522 387
264
B.B. Wang et al. / Journal of Physics and Chemistry of Solids 74 (2013) 259–264
The insets in Fig. 5 show that the lnðJ=E2 Þ 1=E curves deviate from F–N curves in the high field region. From the insets, one can see that the slope of the lnðJ=E2 Þ 1=E curves in the high field region is lower than that in the low field region, which indicates that the deviation results from the space-charge effect [48].
5. Conclusion In summary, NCNTPs with different structures were synthesized in PEHFCVD system under different time, which methane, nitrogen and hydrogen were used for the reaction gases. The structures, components and EFE properties of NCNTPs were studied by FESEM, micro-Raman spectroscopy, XPS and high vacuum system, respectively. The EFE results show that the turn-on field of NCNCTPs with different structures is lowered from 4.76 to 3.57 V/mm and the current density increases from about 200 to 2800 mA/cm2 at the field of 12 V/mm depending on their structures and components. Combined with the growth conditions, the change in the structures of NCNTPs was analyzed, which indicates that the change originates from the ion bombardment. According to the electronic structure of amorphous carbon, the change in the work function of NCNTPs was analyzed. The analysis result indicates that the work function of NCNTPs is lowered with the increase of nitrogen content in them. In addition, the nitrogen can increase the density of free electrons. The better EFE properties of NCNTPs originate from the reduction of work function and the increase of free electron density due to the nitrogen doping.
Acknowledgements This work is supported by Chongqing Natural Science Foundation of China (CSTC, 2009BA4027). References [1] K.S. Hazra, P. Rai, D.R. Mohapatra, N. Kulshrestha, R. Bajpai, S. Roy, D.S. Misra, ACS Nano 3 (2009) 2617–2622. [2] B.B. Wang, Q.J. Cheng, Y.A. Chen, K. Ostrikov, R J. Appl. Phys. 110 (2011) 054323–054326. [3] W.J. Lee, C. Li, N. Burke, D. Trimm, J. Patel, Carbon 49 (2011) 2735–2741. [4] C.H. Wei, C.H. Chen, C.-M. Yeh, M.Y. Chen, J. Hwang, A.P. Lee, C.S. Kou, J. Electrochem. Soc. 152 (2005) C366–C369. [5] K. Ostrikov, Rev. Mod. Phys. 77 (2005) 489–511. [6] B.B. Wang, K. Ostrikov, J. Appl. Phys. 105 (2009) 083303–083309. [7] N.G. Shang, S.R.P. Silva, X. Jiang, P. Papakonstantinou, Carbon 49 (2011) 3048–3054. [8] Y. Zhang, L. Zhang, P. Hou, J. Hua, C. Liu, H. Cheng, New Carbon Mater. 2011 (26) (2011) 52–56.
[9] Z.L. Tsakadze, K. Ostrikov, J.D. Long, S. Xu, Diamond Relat. Mater. 13 (2004) 1923–1929. [10] L. Xu, S. Li, Z. Wu, H. Li, D. Yan, C. Zhang, P. Zhang, P. Yan, X. Li, Appl. Phys. A 103 (2011) 59–65. [11] B.B. Wang, Q.J. Cheng, X.X. Zhong, Y.Q. Wang, Y.A. Chen, K. Ostrikov, J. Appl. Phys. 111 (2012) 044317–044319. [12] B.B. Wang, Y.Q. Wang, R.Z. Wang, X.J. Quan, X. Chen, Vacuum 86 (2012) 1815–1821. [13] K.N. Ostrikov, M.Y. Yu, N.A. Azarenkov, J. Appl. Phys. 84 (1998) 4176–4179. [14] N.A. Azarenkov, I.B. Denisenko, K.N. Ostrikov, J. Phys. D: Appl. Phys. 28 (1995) 2465–2469. [15] B.B. Wang, B. Zhang, Diamond Relat. Mater. 16 (2007) 1982–1987. [16] S. Adhikari, H.R. Aryal, D.C. Ghimire, G. Kalita, M. Umeno, Diamond Relat. Mater. 17 (2008) 1666–1668. [17] Gy.J. Kova´cs, M. Veres, M. Koo´s, G. Radno´czi, Thin Solid Films 516 (2008) 7910–7915. [18] J. Sui, J.-J. Lu, Mater. Chem. Phys. 123 (2010) 264–268. [19] J.H. Kaufman, S. Metin, D.D. Saperstein, Phys. Rev. B 39 (1989) 13053–13060. [20] J. Hu, P. Yang, C.M. Lieber, Phys. Rev. B 57 (1998) R3185–R3188. [21] Q. Cheng, S. Xu, K. Ostrikov, Acta Mater. 58 (2010) 560–569. [22] Q. Cheng, S. Xu, K. Ostrikov, J. Mater. Chem. 20 (2010) 5853–5859. [23] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979. [24] S. Song, Q. Gao, J. Jiang, L. Gao, L. Yu, Mater. Lett. 62 (2008) 2520–2523. [25] Z. Zhou, L. Xia, M. Sun, Diamond Relat. Mater. 2004 (13) (2004) 14–21. [26] P. Hammer, R.G. Lacerda, R. Droppa Jr., F. Alvarez, Diamond Relat. Mater. 9 (2000) 577–581. [27] C. Moreno-Castilla, A.F. Pe´rez-Cadenas, F.J. Maldonado-Ho´dar, F. CarrascoMarı´n, J.L.G. Fierro, Carbon 41 (2003) 1157–1167. [28] S.D. Gardner, G. Hem, C. U. P. Jr, Carbon 34 (1996) 1221–1228. [29] S. Gardner, C.S.K. Singamstty, G. Booth, G.-R. He, Carbon 33 (1995) 587–595. [30] Q.Z. Zhao, F. Ciobanu, L.J. Wang, J. Appl. Phys. 105 (2009) 083103–083104. [31] B.B. Wang, G.B. Dong, X.Z. Xu, Appl. Surf. Sci. 258 (2011) 1677–1681. [32] V.I. Merkulov, A.V. Melechko, M.A. Guillorn, D.H. Lowndes, M.L. Simpson, Chem. Phys. Lett. 350 (2001) 381–385. [33] B.B. Wang, W.L. Wang, K.J. Liao, J.L. Xiao, Phys. Rev. B63 (2001) 085412085412. [34] A. Ilie, A.C. Ferrari, T. Yagi, J. Robertson, Appl. Phys. Lett. 76 (2000) 2627–2629. [35] J. Robertson, C.A. Davis, Diamond Relat. Mater. 4 (1995) 441–444. [36] S.R.P. Silva, G.A.J. Amaratunga, Thin Solid Films 270 (1995) 194–199. [37] G.A.J. Amaratunga, S.R.P. Silva, Appl. Phys. Lett. 68 (1996) 2529–2531. [38] Z.Q. Xue, Q.D. Wu, Electron Emission and Electron Spectra, Beijng University Press, Beijing, 1993, p34 (in Chinese). [39] S. Ravipati, C.-J. Kuo, J. Shieh, C.-T. Chou, F.-H. Ko, Microelectron. Reliab. 50 (2010) 1973–1976. [40] R. Seelaboyina, J. Huang, J. Park, D.H. Kang, W.B. Choi, Nanotechnology 17 (2006) 4840–4844. [41] R.G. Forbes, C.J. Edgcombe, U. Valdre , Ultramicroscopy 95 (2003) 57–65. [42] L.D. Filip, J.D. Carey, S.R.P. Silva, J. Appl. Phys. 109 (2011) 5–084527. [43] K.H. Park, S. Lee, K.H. Koh, J. Vac. Sci. Technol., B 24 (2006) 898–902. [44] C.J. Edgcombe, U. Valdre , Philos. Mag. B 82 (2002) 987–1007. [45] S.T. Purcell, P. Vincent, C. Journet, V.T. Binh, Phys. Rev. Lett. 88 (2002) 105502–105504. [46] Z.L. Wang, R.P. Gao, W.A. de Heer, P. Poncharal, Appl. Phys. Lett. 80 (2002) 856–858. [47] T. Kuzumaki, Y. Takamura, H. Ichinose, Y. Horiike, Appl. Phys. Lett. 78 (2001) 3699–3701. [48] Y.C. Choi, Y.M. Shin, D.J. Bae, S.C. Lim, Y.H. Lee, B.S. Lee, Diamond Relat. Mater. 10 (2001) 1457–1464.
