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Effects of experimental conditions on the growth of vertically aligned carbon nitride nanocone arrays
Thin Solid Films 529 (2013) 71–75
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effects of experimental conditions on the growth of vertically aligned carbon nitride nanocone arrays X.N. Fu, L. Chen, X.J. Liu, J.S. Lai, J. Sun, Z.F. Ying, J.D. Wu, H.L. Lu, N. Xu ⁎ Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Available online 29 May 2012 Keywords: Carbon nitride nanocone Abnormal glow discharge Tip−substrate distance
a b s t r a c t Vertically aligned carbon nitride nanocone (CNNC) arrays were prepared on Ni-covered (100) silicon wafers by an abnormal glow discharge plasma assisted chemical vapor deposition method. In order to control the growth of the CNNC arrays, the distance of the anode tip to the substrate surface was adjusted for it affected the contents and activities of the species in the plasmas leading to the CNNC growth. Based on the characterization of the as-grown thin films and the analysis of the growth environments, the effects of the experimental conditions on the growth of the CNNC arrays were studied and their growth mechanism was discussed. The tip−substrate distance strongly affects the CNNC growth. Under appropriate experimental conditions, the vertically-aligned and intact CNNC arrays with the β-C3N4 microstructure and the minimum tip curvature diameter of only 3–4 nm could be fabricated. This kind of CNNC arrays have many potential applications, such as tips for microscopes, electron-emitting units in field emission displays, electron-capture electrodes of solar cells etc. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since β-C3N4 is predicted to possess distinguished elastic modulus and hardness even higher than those of diamond [1], much attention has been paid to its theoretical and experimental studies. So far, five different structures of C3N4, i.e. the hexagonal α-C3N4 and β-C3N4, the pseudo-cubic and cubic C3N4, and the graphitic C3N4, have been proposed [2]. Among them, the structure of β-C3N4 is similar to silicon nitride. Carbon-based nanostructures have many applications in electron-emitting panels, sensors, solar cells and nano-electronic devices [3–7]. Following the earlier theoretical study of carbon nitride films, the possibility of partial substitution of nitrogen for carbon in a pure carbon nanostructure to form a binary CN compound has been predicted [8,9]. Doping of nitrogen into graphite or diamond nanostructures, or better direct constructing carbon nitride nanostructures, can enhance their field emissivity, increase their mechanical, thermal and chemical stabilities, and make their electronic and optical properties manipulated by varying the composition. Various synthesis techniques have been devised to synthesize C3N4, including chemical vapor deposition, physical vapor deposition, solvothermal method etc. [10–12]. Most of products synthesized by these techniques are amorphous CNx films due to the difficulties of the dissociation of N2 and the easiness of the formation of graphite structure from carbon. Although extensive studies on the synthesis
⁎ Corresponding author. Tel.: + 86 21 65642150; fax: + 86 21 65641344. E-mail address: [email protected] (N. Xu). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.05.057
and characterization of carbon nitride have been reported, a simple, appropriate and effective synthetic way is still eagerly anticipated. To fabricate C3N4 nanostructures such as carbon nitride nanocones (CNNCs) with high aspect ratio is even a greater challenge with the current methods. The production of active nitrogen atoms and the effective control of gas-phase reactions between nitrogen-related and carbonrelated precursors are very important for the formation of carbon nitride [13,14]. Recently, an abnormal glow discharge plasma assisted chemical vapor deposition (GPCVD) method combined with highly dense plasma and bias enhancement has been utilized to prepare carbon nitride films and CNNC arrays [15–17]. In this article, the effects of the distance between the anode tip and the substrate surface on the growth of the CNNC arrays by the GPCVD method were studied in order to better control the CNNC growth and further study the growth mechanism. The tip-substrate distance, which affected the substrate temperature and the contents and activities of the nitrogen-related and carbon-related precursors in the plasmas, were regulated to control the morphology, composition, and structure of the growing nanostructures in attempts to obtain well vertically aligned CNNC arrays for potential applications. 2. Experimental details By using the GPCVD method, the CNNC arrays were grown on Nicovered Si (100) wafers. The experiment device is made up of a direct-current (DC) plasma source and a vacuum chamber. The DC plasma source with a CH4/N2 mixture inlet provides an intense reactive plasma (as shown in Fig. 1) [18]. Si (100) wafers were used as substrates
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Morphology,microstructure and chemical bonding state of the samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800, accelerating voltage of 1.0 kV), transmission electron microscopy (TEM, Hitachi 800, accelerating voltage of 200 kV) with high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI 5000 C, Al Kα 1486.6 eV). The optical emission spectra (OES) from the abnormal glow discharge plasma were measured by a spectrometer (Acton Reseglowh, Spectra Pro 500i) and detected by a gated intensified charge-coupled device (Andor Technology, iStar DH720). The substrate temperatures were measured by a thermal couple contacting with the backs of the substrates and connected to a digital multimeter (VICTOR VC9808). 3. Results and discussion
Fig. 1. Schematic diagram of the GPCVD set-up. 1−anode holder Cu, 2−tip of the coneshaped anode, 3−hollow cathode Mo, 4−holder, 5−Si substrate.
and put on a graphitic bracket which was fixed on a water-cooled hollow-cylinder cathode in the source. The vacuum chamber with a 10− 4 Pa base vacuum is connected with the DC plasma source. The sample preparation involves two steps. In the first step, Ni catalyst layers were deposited on Si (100) wafers by pulsed laser deposition (PLD). The Si (100) wafers were polished with 0.5 μm diamond powders to generate slight scratches, which is beneficial to increase the distribution density of the as-grown CNNCs. Before putting into the deposition chamber, the scratched wafers were ultrasonically cleaned in ethanol and acetone, etched in HF solution (H2O:HF=4:1) and rinsed in deionized water to remove the absorbed dust, grease and silicon oxide layers, respectively. In PLD, 100 nm thick Ni layers as catalyst were deposited on 1×1 cm2 scratched Si (100) substrates under 2×10− 4 Pa for 5 min without substrate-heating. The wavelength, pulse energy, repetition of the used Nd:YAG laser were 532 nm, 50 mJ and 10 Hz, respectively. The target of PLD was made from pure Ni and the distance between the target and substrate was 3 cm. In the second step, CNNC arrays were prepared by the GPCVD method. The distance between the tip of the cone-shaped anode and the surface of the prepared Ni-covered wafer was regulated from 1 to 10 mm. The plasma source generated the reactive plasma through the abnormal glow discharge with a CH4/N2 mixture inlet under a total pressure of 666 Pa. The gas flow rates of CH4 and N2 were fixed at 0.4 or 0.8 sccm and 20 sccm, respectively, and the discharge current was set to 150 mA. The abnormal glow discharge lasted for 45 min. The tip−substrate distance was regulated to find out the changes of the size, distribution density and morphology of the as-grown CNNCs under the same other experimental conditions.
The typical FESEM images of the morphology of the as-grown CNNC array are given in Fig. 2. Fig. 2a and b shows the CNNCs grown on a Nicovered Si (100) wafer for 45 min (with a CH4/N2 flow rate ratio of 1/ 50, a discharge current of 150 mA, and a tip−substrate distance of 1 mm). In Fig. 2a, the as-grown nanocrystals have a conical shape and are vertically aligned. Fig. 2b shows three integral nanocones with bottom diameters of about 200–280 nm and heights of about 200–450 nm. The crystallinity of the CNNCs was determined by HRTEM and SAED. Fig. 3a–c gives the TEM and HRTEM images and SAED pattern of a CNNC picked off from the sample resulting in Fig. 2. The HRTEM image in the upper-right inset of Fig. 3b shows the lattice spacing of 0.32 nm, which is well consistent with the β-C3N4 (110) plane [2,19]. Fig. 3c shows the corresponding SAED pattern taken from the indicated area on the CNNC in Fig. 3b, which is also in accordance with the single crystalline hexagonal β-C3N4. For further examining the stoichiometry and chemical bonding state of carbon and nitrogen in the as-grown CNNCs, the XPS were measured on a 0.6×0.6 cm2 area of the sample prepared with a CH4/N2 flow rate ratio of 1/50 after Ar± sputtered for 15 min (3 kV, 6 μA/cm2). The C1s at 284.6 eV was taken as the reference. The [N]/[C] atomic ratio in this sample is about 0.64, which is smaller than that of β-C3N4 (about 1.33). It can be explained that a lot of carbon was absorbed on the surface of the sample for exposure to air and the CNx film with low nitrogen content was deposited at the places without grown CNNCs. The C1s1/2 and N1s1/2 core electron peaks of the XPS are shown in Fig. 4a and b, and both of them are further fitted with two and three Gaussian-peak components using the peak fitting software (AugerScan Version 3.2), respectively. As shown in Fig. 4a, the left deconvoluted peak 1 of C1s1/2 at 284.3 eV is derived from graphite or amorphous carbon, and the right deconvoluted peak 2 at 285.7 eV can be assigned to C\N bonding [21]. The ratio of C\N bondings to C\C ones is about 0.61:1. In Fig. 4b, the peak of N 1s is deconvoluted into three Gaussian peaks at 398.4, 399.8 and 401.2 eV, respectively. The most intense deconvoluted peak 1 at 398.4 eV is attributed to sp3-hybridized
Fig. 2. FESEM images of a CNNC array: (a) CNNCs grown on a Ni-covered Si (100) wafer with a Ni catalyst layer of about 100 nm, a CH4/N2 flow rate ratio of 1/50, a discharge current of 150 mA and a tip–substrate distance of 1 mm, and (b) three integral nanocones selected from image of (a).
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Fig. 3. (a) TEM image of a sample prepared with a CH4/N2 flow rate ratio of 1/50. (b) HRTEM micrograph of a CNNC taken from the sample resulting in the image (a). (c) The corresponding SAED pattern from the marked place in the image (b).
N\C bonding [20]. The deconvoluted peak 2 (399.8 eV) and 3 (401.2 eV) are assigned to sp2 hybridized N\C bonding and N_N or N_O one, respectively. From Fig. 4b, it could be estimated that the contents of sp3N\C, sp2-N\C and N_N or N_O bonding are 57.2%, 36.3% and 6.5%,
Fig. 4. XPS spectra of (a) C 1s and (b) N 1s core electron peaks of the CNNC film synthesized on a Ni-covered Si (100) wafer with a CH4/N2 flow rate ratio of 1/50.
respectively. The peak at 398.4 eV takes up about 57.2% room of N1s1/2 peak, implying that the sp3-hybridized N\C bonding is the predominant bonding type for the N atoms in the sample. Considering that C\C, N_N or N_O bondings may come from the exterior atmosphere, it can be inferred that the more C atoms contained in the as-grown CNNCs are bonded with N atoms, while most of the contained N atoms are bonded with C atoms into sp3-hybridized N\C bonding. The gas-phase reactions between nitrogen-related and carbonrelated precursors in the plasmas are very important for the formation of CNNCs. In order to understand the influence of the plasmas locating at the substrate surface on the CNNC growth, only the distance of the anode tip to the substrate surface was changed while the other experimental conditions remained unchanged. FESEM images of the morphology of CNNC arrays grown on scratched Si (100) wafers for 45 min. with a catalyst layer thickness of about 100 nm, a discharge current of 150 mA, a CH4/N2 flow rate ratio of 1/25 and different tip–substrate distances of 1, 4 and 7 mm are given in Fig. 5. These images show that the morphology of the as-grown CNNCs is determined by the tip–substrate distance. In Fig. 5, it could be found that the as-grown CNNCs with a tip– substrate distance of 1 mm are intact, sharp and densely distributed, and the as-grown CNNCs become incomplete, blunt, sparsely distributed and smaller as the tip–substrate distance increases. Comparing the three images, the tip–substrate distance of 1 mm is considered to be suitable for obtaining intact CNNCs. DC plasma can lead to high temperature on the built-in substrate surface. In order to understand the effects of substrate temperature on the growth of CNNCs, the substrate temperatures at different tip–substrate distances were measured by a thermal couple contacting with the backs of the substrates. With the same discharge current of 150 mA and CH4/ N2 flow rate ratio of 1/25, the tip–substrate distance was changed from 1 to 10 mm. The corresponding substrate temperatures at tip–substrate distances of 1, 4, 7, 10 mm were measured to be 765, 770, 782 and 798 °C, respectively. The substrate temperature slightly increased and had no obvious change with increment of the tip–substrate distance. This situation may be due to the increment of the tip–substrate distance and the induced decrement of the discharge resistance, and further led to increment of the input power. In comparison with the FESEM images of Fig. 3, it could be inferred that the substrate temperature of above 765 °C is appropriate for the growth of CNNCs and the changes of substrate temperature within a tip–substrate distance of 1–10 mm have no significant effects on the growth of CNNCs, for higher substrate temperature should be more conducive to the crystallization of growing material.
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Actually, the emission from N2 or N2+ was also promoted due to emissive excitation promotion from the collisions with methane-related species to some extent. At the mean time, the lines of N atoms or atomic ions, which should appear at 500–800 nm (not shown here), were quenched by the promoted collisions and reactions with other species. Fig. 6b shows the OES taken at different tip–substrate distances. Fig. 7a and b gives the emission intensity of CN, N2+ and N2 and the intensity ratios of N2+/CN and N2+/N2 against the tip–substrate distances in Fig. 6b, respectively. In Fig. 7b, it is apparent that the intensity ratios of N2+/CN and N2+/N2 increases as the tip–substrate distance decreases although their absolute intensity decreases (as shown in Fig. 7a), which means that N2+ is more and more active in comparison with CN radicals and N2. It could be found in Fig. 5 that morphology of the as-grown CNNCs also becomes better as the tip–substrate distance decreases. It is inferred that the formation of the β-C3N4 is determined by the balance of the deposition rates of nitrogen-related precursors (N2+,N2,N+ and N) and carbon-related ones (CN and CHx). A high deposition rate of nitrogenrelated precursors should be beneficial to form sp3-hybridized C\N bondings,which is essential to form β-C3N4 structure, and avoid generating C_N, C_C etc. on the substrate. On the other hand,as a carbon nitride nucleus increases in height, the incidence of feedstock ion flux at the top region will be intensified due to locally stronger focusing of the electric field surrounding the top, so the more deposited species will be sputtered off and the diameter of the top will be cut down. In this way, the CNNCs with sharp tips are eventually formed. Increment of
Fig. 5. FESEM images of the CNNCs prepared with different tip–substrate distances of: (a) 1 mm, (b) 4 mm and (c) 7 mm, respectively.
For further understanding the morphological variations of the CNNCs grown at different tip–substrate distances, it is necessary to know the composition of the plasmas and the activities of the feedstock species in the plasmas. Aiming to clarify the action of the plasma in the growth of the CNNCs, OES from the abnormal glow discharge plasma with different tip–substrate distances were measured. Fig. 6a shows a typical OES of the plasma at a CH4/N2 flow rate ratio of 1/50 and a tip–substrate distance of 10 mm (collected from 350 to 440 nm). In Fig. 6a, the predominant thorny lines accompanied by the weak ones centering at 388.2 and 421.6 nm respectively were produced by CN-species (transition B2Σ+ − X2Σ+, Δν = 0 and Δν=−1), the peaks at 357.8, 380.4, 399.7 and 405.8 nm came from N2 and the ones at 375.4 and 391.3 nm resulted from N2+ [21]. The strong CN-related emission suggests the existence of large numbers of CN radicals in the plasma. As the methane is introduced into the pure nitrogen inlet for glow discharge, main lines originating from CN radicals at 388.2 and 421.6 nm, which require lower activation energy, would suppressed those from N2 or N2+.
Fig. 6. (a) OES of the abnormal glow discharge plasma collected from 350 to 440 nm with a CH4/N2 flow rate ratio of 1/50 and a tip–substrate distance of 10 mm. (b) OES of the abnormal glow discharge plasma with a CH4/N2 flow rate ratio of 1/50 and different tip–substrate distances of 1, 4, 7 and 10 mm, respectively.
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and N2 mixture inlet. FESEM, TEM, HRTEM and SAED show that the as-grown CNNCs could have intact cone-shape, the vertical alignment, the β-C3N4 microstructure and the minimum tip curvature diameter of 3–4 nm. The FESEM shows that the size, distribution density and morphology of the as-grown CNNCs are strongly affected by the tip–substrate distance. The tip–substrate distance of 1–2 mm is the optimal condition for the growth of the CNNCs with intact coneshape and good crystallinity. The intact CNNC arrays synthesized by GPCVD method have potential applications in microscopes, fieldemission-displays, solar cells etc. Acknowledgments The authors acknowledge the supports from the National Natural Science Foundation of China (grant no.10875030) and National Basic Research Program of China (973 Program) (grant no.2010CB933703). References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Fig. 7. (a) The emission intensity of CN (388.2 nm), N2+ (391.3 nm) and N2 (380.4 nm) against the tip–substrate distances taken from Fig. 6(b). (b) The intensity ratios of N2+/CN (391.3/388.2 nm) and N2+/N2 (391.3/380.4 nm) against the tip–substrate distances in graph (a), respectively.
[17] [18] [19] [20]
the intensity ratios of N2±/CN and N2±/N2 should be conducive to this process. 4. Conclusions The vertically aligned CNNC arrays have been synthesized on Nicoated Si (100) substrates through the GPCVD method using a CH4
[21]
A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. D.M. Teter, R.J. Hemley, Science 271 (1996) 53. Y.T. Jan, H.C. Hsieh, C.F. Chen, Diam. Relat. Mater. 8 (1999) 772. G. Zhang, X. Jiang, E. Wang, Science 300 (2003) 472. C.J. Huang, Y.K. Chih, J. Hwang, J. Appl. Phys. 94 (2003) 6796. M. Terrones, P. Redlich, N. Grobert, S. Trasobares, W.K. Hsu, H. Terrones, Y.Q. Zhu, J.P. Hare, C.L. Reeves, A.K. Cheetham, M. Rühle, H.W. Kroto, D.R.M. Walton, Adv. Mater. 11 (1999) 655. A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Adv. Mater. 17 (2005) 1648. S. Stafstro¨m, Appl. Phys. Lett. 77 (2000) 3941. G. Zhang, W. Duan, B. Gu, Appl. Phys. Lett. 80 (2002) 2589. C. Niu, Y.Z. Lu, C.M. Lieber, Science 261 (1993) 334. Y. Peng, T. Ishigaki, S. Horiuchi, Appl. Phys. Lett. 73 (1998) 3671. D.R. Miller, J.J. Wang, E.G. Gillan, J. Mater. Chem. 12 (2002) 2463. Y.F. Zhang, Z.H. Zhou, H.L. Li, Appl. Phys. Lett. 68 (1996) 634. T.Y. Yen, C.P. Chou, Appl. Phys. Lett. 67 (1995) 2801. W. Hu, N. Xu, Y.Q. Shen, T.W. Zhang, Mater. Sci. Eng., A 432 (2006) 142. W. Hu, X.F. Xu, Y.Q. Shen, J.S. Lai, X.N. Fu, J.D. Wu, Z.F. Ying, N. Xu, J. Electron. Mater. 39 (2010) 381. W.Hu.N. Xu, X.F. Xu, J.D. Wu, Y.Q. Shen, Z.F. Ying, Chem. Vap. Depos. 15 (2009) 306. N. Xu, Y.C. Du, Z.F. Ying, Z.M. Ren, F.M. Li, Rev. Sci. Instrum. 68 (1997) 2994. K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu, I.C. Wu, Phys. Rev. B: Condens. Matter 49 (1994) 5034. F. Le Normand, J. Hommet, T. Szouml, C. Fuchs, E. Fogarassy, Phys. Rev. B 64 (2001) 5416. W. Hu, J. Tang, J. Wu, J. Sun, Y. Shen, X. Xu, N. Xu, Phys. Plasmas 15 (2008) 073502.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effects of experimental conditions on the growth of vertically aligned carbon nitride nanocone arrays X.N. Fu, L. Chen, X.J. Liu, J.S. Lai, J. Sun, Z.F. Ying, J.D. Wu, H.L. Lu, N. Xu ⁎ Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Available online 29 May 2012 Keywords: Carbon nitride nanocone Abnormal glow discharge Tip−substrate distance
a b s t r a c t Vertically aligned carbon nitride nanocone (CNNC) arrays were prepared on Ni-covered (100) silicon wafers by an abnormal glow discharge plasma assisted chemical vapor deposition method. In order to control the growth of the CNNC arrays, the distance of the anode tip to the substrate surface was adjusted for it affected the contents and activities of the species in the plasmas leading to the CNNC growth. Based on the characterization of the as-grown thin films and the analysis of the growth environments, the effects of the experimental conditions on the growth of the CNNC arrays were studied and their growth mechanism was discussed. The tip−substrate distance strongly affects the CNNC growth. Under appropriate experimental conditions, the vertically-aligned and intact CNNC arrays with the β-C3N4 microstructure and the minimum tip curvature diameter of only 3–4 nm could be fabricated. This kind of CNNC arrays have many potential applications, such as tips for microscopes, electron-emitting units in field emission displays, electron-capture electrodes of solar cells etc. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since β-C3N4 is predicted to possess distinguished elastic modulus and hardness even higher than those of diamond [1], much attention has been paid to its theoretical and experimental studies. So far, five different structures of C3N4, i.e. the hexagonal α-C3N4 and β-C3N4, the pseudo-cubic and cubic C3N4, and the graphitic C3N4, have been proposed [2]. Among them, the structure of β-C3N4 is similar to silicon nitride. Carbon-based nanostructures have many applications in electron-emitting panels, sensors, solar cells and nano-electronic devices [3–7]. Following the earlier theoretical study of carbon nitride films, the possibility of partial substitution of nitrogen for carbon in a pure carbon nanostructure to form a binary CN compound has been predicted [8,9]. Doping of nitrogen into graphite or diamond nanostructures, or better direct constructing carbon nitride nanostructures, can enhance their field emissivity, increase their mechanical, thermal and chemical stabilities, and make their electronic and optical properties manipulated by varying the composition. Various synthesis techniques have been devised to synthesize C3N4, including chemical vapor deposition, physical vapor deposition, solvothermal method etc. [10–12]. Most of products synthesized by these techniques are amorphous CNx films due to the difficulties of the dissociation of N2 and the easiness of the formation of graphite structure from carbon. Although extensive studies on the synthesis
⁎ Corresponding author. Tel.: + 86 21 65642150; fax: + 86 21 65641344. E-mail address: [email protected] (N. Xu). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.05.057
and characterization of carbon nitride have been reported, a simple, appropriate and effective synthetic way is still eagerly anticipated. To fabricate C3N4 nanostructures such as carbon nitride nanocones (CNNCs) with high aspect ratio is even a greater challenge with the current methods. The production of active nitrogen atoms and the effective control of gas-phase reactions between nitrogen-related and carbonrelated precursors are very important for the formation of carbon nitride [13,14]. Recently, an abnormal glow discharge plasma assisted chemical vapor deposition (GPCVD) method combined with highly dense plasma and bias enhancement has been utilized to prepare carbon nitride films and CNNC arrays [15–17]. In this article, the effects of the distance between the anode tip and the substrate surface on the growth of the CNNC arrays by the GPCVD method were studied in order to better control the CNNC growth and further study the growth mechanism. The tip-substrate distance, which affected the substrate temperature and the contents and activities of the nitrogen-related and carbon-related precursors in the plasmas, were regulated to control the morphology, composition, and structure of the growing nanostructures in attempts to obtain well vertically aligned CNNC arrays for potential applications. 2. Experimental details By using the GPCVD method, the CNNC arrays were grown on Nicovered Si (100) wafers. The experiment device is made up of a direct-current (DC) plasma source and a vacuum chamber. The DC plasma source with a CH4/N2 mixture inlet provides an intense reactive plasma (as shown in Fig. 1) [18]. Si (100) wafers were used as substrates
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Morphology,microstructure and chemical bonding state of the samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800, accelerating voltage of 1.0 kV), transmission electron microscopy (TEM, Hitachi 800, accelerating voltage of 200 kV) with high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI 5000 C, Al Kα 1486.6 eV). The optical emission spectra (OES) from the abnormal glow discharge plasma were measured by a spectrometer (Acton Reseglowh, Spectra Pro 500i) and detected by a gated intensified charge-coupled device (Andor Technology, iStar DH720). The substrate temperatures were measured by a thermal couple contacting with the backs of the substrates and connected to a digital multimeter (VICTOR VC9808). 3. Results and discussion
Fig. 1. Schematic diagram of the GPCVD set-up. 1−anode holder Cu, 2−tip of the coneshaped anode, 3−hollow cathode Mo, 4−holder, 5−Si substrate.
and put on a graphitic bracket which was fixed on a water-cooled hollow-cylinder cathode in the source. The vacuum chamber with a 10− 4 Pa base vacuum is connected with the DC plasma source. The sample preparation involves two steps. In the first step, Ni catalyst layers were deposited on Si (100) wafers by pulsed laser deposition (PLD). The Si (100) wafers were polished with 0.5 μm diamond powders to generate slight scratches, which is beneficial to increase the distribution density of the as-grown CNNCs. Before putting into the deposition chamber, the scratched wafers were ultrasonically cleaned in ethanol and acetone, etched in HF solution (H2O:HF=4:1) and rinsed in deionized water to remove the absorbed dust, grease and silicon oxide layers, respectively. In PLD, 100 nm thick Ni layers as catalyst were deposited on 1×1 cm2 scratched Si (100) substrates under 2×10− 4 Pa for 5 min without substrate-heating. The wavelength, pulse energy, repetition of the used Nd:YAG laser were 532 nm, 50 mJ and 10 Hz, respectively. The target of PLD was made from pure Ni and the distance between the target and substrate was 3 cm. In the second step, CNNC arrays were prepared by the GPCVD method. The distance between the tip of the cone-shaped anode and the surface of the prepared Ni-covered wafer was regulated from 1 to 10 mm. The plasma source generated the reactive plasma through the abnormal glow discharge with a CH4/N2 mixture inlet under a total pressure of 666 Pa. The gas flow rates of CH4 and N2 were fixed at 0.4 or 0.8 sccm and 20 sccm, respectively, and the discharge current was set to 150 mA. The abnormal glow discharge lasted for 45 min. The tip−substrate distance was regulated to find out the changes of the size, distribution density and morphology of the as-grown CNNCs under the same other experimental conditions.
The typical FESEM images of the morphology of the as-grown CNNC array are given in Fig. 2. Fig. 2a and b shows the CNNCs grown on a Nicovered Si (100) wafer for 45 min (with a CH4/N2 flow rate ratio of 1/ 50, a discharge current of 150 mA, and a tip−substrate distance of 1 mm). In Fig. 2a, the as-grown nanocrystals have a conical shape and are vertically aligned. Fig. 2b shows three integral nanocones with bottom diameters of about 200–280 nm and heights of about 200–450 nm. The crystallinity of the CNNCs was determined by HRTEM and SAED. Fig. 3a–c gives the TEM and HRTEM images and SAED pattern of a CNNC picked off from the sample resulting in Fig. 2. The HRTEM image in the upper-right inset of Fig. 3b shows the lattice spacing of 0.32 nm, which is well consistent with the β-C3N4 (110) plane [2,19]. Fig. 3c shows the corresponding SAED pattern taken from the indicated area on the CNNC in Fig. 3b, which is also in accordance with the single crystalline hexagonal β-C3N4. For further examining the stoichiometry and chemical bonding state of carbon and nitrogen in the as-grown CNNCs, the XPS were measured on a 0.6×0.6 cm2 area of the sample prepared with a CH4/N2 flow rate ratio of 1/50 after Ar± sputtered for 15 min (3 kV, 6 μA/cm2). The C1s at 284.6 eV was taken as the reference. The [N]/[C] atomic ratio in this sample is about 0.64, which is smaller than that of β-C3N4 (about 1.33). It can be explained that a lot of carbon was absorbed on the surface of the sample for exposure to air and the CNx film with low nitrogen content was deposited at the places without grown CNNCs. The C1s1/2 and N1s1/2 core electron peaks of the XPS are shown in Fig. 4a and b, and both of them are further fitted with two and three Gaussian-peak components using the peak fitting software (AugerScan Version 3.2), respectively. As shown in Fig. 4a, the left deconvoluted peak 1 of C1s1/2 at 284.3 eV is derived from graphite or amorphous carbon, and the right deconvoluted peak 2 at 285.7 eV can be assigned to C\N bonding [21]. The ratio of C\N bondings to C\C ones is about 0.61:1. In Fig. 4b, the peak of N 1s is deconvoluted into three Gaussian peaks at 398.4, 399.8 and 401.2 eV, respectively. The most intense deconvoluted peak 1 at 398.4 eV is attributed to sp3-hybridized
Fig. 2. FESEM images of a CNNC array: (a) CNNCs grown on a Ni-covered Si (100) wafer with a Ni catalyst layer of about 100 nm, a CH4/N2 flow rate ratio of 1/50, a discharge current of 150 mA and a tip–substrate distance of 1 mm, and (b) three integral nanocones selected from image of (a).
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Fig. 3. (a) TEM image of a sample prepared with a CH4/N2 flow rate ratio of 1/50. (b) HRTEM micrograph of a CNNC taken from the sample resulting in the image (a). (c) The corresponding SAED pattern from the marked place in the image (b).
N\C bonding [20]. The deconvoluted peak 2 (399.8 eV) and 3 (401.2 eV) are assigned to sp2 hybridized N\C bonding and N_N or N_O one, respectively. From Fig. 4b, it could be estimated that the contents of sp3N\C, sp2-N\C and N_N or N_O bonding are 57.2%, 36.3% and 6.5%,
Fig. 4. XPS spectra of (a) C 1s and (b) N 1s core electron peaks of the CNNC film synthesized on a Ni-covered Si (100) wafer with a CH4/N2 flow rate ratio of 1/50.
respectively. The peak at 398.4 eV takes up about 57.2% room of N1s1/2 peak, implying that the sp3-hybridized N\C bonding is the predominant bonding type for the N atoms in the sample. Considering that C\C, N_N or N_O bondings may come from the exterior atmosphere, it can be inferred that the more C atoms contained in the as-grown CNNCs are bonded with N atoms, while most of the contained N atoms are bonded with C atoms into sp3-hybridized N\C bonding. The gas-phase reactions between nitrogen-related and carbonrelated precursors in the plasmas are very important for the formation of CNNCs. In order to understand the influence of the plasmas locating at the substrate surface on the CNNC growth, only the distance of the anode tip to the substrate surface was changed while the other experimental conditions remained unchanged. FESEM images of the morphology of CNNC arrays grown on scratched Si (100) wafers for 45 min. with a catalyst layer thickness of about 100 nm, a discharge current of 150 mA, a CH4/N2 flow rate ratio of 1/25 and different tip–substrate distances of 1, 4 and 7 mm are given in Fig. 5. These images show that the morphology of the as-grown CNNCs is determined by the tip–substrate distance. In Fig. 5, it could be found that the as-grown CNNCs with a tip– substrate distance of 1 mm are intact, sharp and densely distributed, and the as-grown CNNCs become incomplete, blunt, sparsely distributed and smaller as the tip–substrate distance increases. Comparing the three images, the tip–substrate distance of 1 mm is considered to be suitable for obtaining intact CNNCs. DC plasma can lead to high temperature on the built-in substrate surface. In order to understand the effects of substrate temperature on the growth of CNNCs, the substrate temperatures at different tip–substrate distances were measured by a thermal couple contacting with the backs of the substrates. With the same discharge current of 150 mA and CH4/ N2 flow rate ratio of 1/25, the tip–substrate distance was changed from 1 to 10 mm. The corresponding substrate temperatures at tip–substrate distances of 1, 4, 7, 10 mm were measured to be 765, 770, 782 and 798 °C, respectively. The substrate temperature slightly increased and had no obvious change with increment of the tip–substrate distance. This situation may be due to the increment of the tip–substrate distance and the induced decrement of the discharge resistance, and further led to increment of the input power. In comparison with the FESEM images of Fig. 3, it could be inferred that the substrate temperature of above 765 °C is appropriate for the growth of CNNCs and the changes of substrate temperature within a tip–substrate distance of 1–10 mm have no significant effects on the growth of CNNCs, for higher substrate temperature should be more conducive to the crystallization of growing material.
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Actually, the emission from N2 or N2+ was also promoted due to emissive excitation promotion from the collisions with methane-related species to some extent. At the mean time, the lines of N atoms or atomic ions, which should appear at 500–800 nm (not shown here), were quenched by the promoted collisions and reactions with other species. Fig. 6b shows the OES taken at different tip–substrate distances. Fig. 7a and b gives the emission intensity of CN, N2+ and N2 and the intensity ratios of N2+/CN and N2+/N2 against the tip–substrate distances in Fig. 6b, respectively. In Fig. 7b, it is apparent that the intensity ratios of N2+/CN and N2+/N2 increases as the tip–substrate distance decreases although their absolute intensity decreases (as shown in Fig. 7a), which means that N2+ is more and more active in comparison with CN radicals and N2. It could be found in Fig. 5 that morphology of the as-grown CNNCs also becomes better as the tip–substrate distance decreases. It is inferred that the formation of the β-C3N4 is determined by the balance of the deposition rates of nitrogen-related precursors (N2+,N2,N+ and N) and carbon-related ones (CN and CHx). A high deposition rate of nitrogenrelated precursors should be beneficial to form sp3-hybridized C\N bondings,which is essential to form β-C3N4 structure, and avoid generating C_N, C_C etc. on the substrate. On the other hand,as a carbon nitride nucleus increases in height, the incidence of feedstock ion flux at the top region will be intensified due to locally stronger focusing of the electric field surrounding the top, so the more deposited species will be sputtered off and the diameter of the top will be cut down. In this way, the CNNCs with sharp tips are eventually formed. Increment of
Fig. 5. FESEM images of the CNNCs prepared with different tip–substrate distances of: (a) 1 mm, (b) 4 mm and (c) 7 mm, respectively.
For further understanding the morphological variations of the CNNCs grown at different tip–substrate distances, it is necessary to know the composition of the plasmas and the activities of the feedstock species in the plasmas. Aiming to clarify the action of the plasma in the growth of the CNNCs, OES from the abnormal glow discharge plasma with different tip–substrate distances were measured. Fig. 6a shows a typical OES of the plasma at a CH4/N2 flow rate ratio of 1/50 and a tip–substrate distance of 10 mm (collected from 350 to 440 nm). In Fig. 6a, the predominant thorny lines accompanied by the weak ones centering at 388.2 and 421.6 nm respectively were produced by CN-species (transition B2Σ+ − X2Σ+, Δν = 0 and Δν=−1), the peaks at 357.8, 380.4, 399.7 and 405.8 nm came from N2 and the ones at 375.4 and 391.3 nm resulted from N2+ [21]. The strong CN-related emission suggests the existence of large numbers of CN radicals in the plasma. As the methane is introduced into the pure nitrogen inlet for glow discharge, main lines originating from CN radicals at 388.2 and 421.6 nm, which require lower activation energy, would suppressed those from N2 or N2+.
Fig. 6. (a) OES of the abnormal glow discharge plasma collected from 350 to 440 nm with a CH4/N2 flow rate ratio of 1/50 and a tip–substrate distance of 10 mm. (b) OES of the abnormal glow discharge plasma with a CH4/N2 flow rate ratio of 1/50 and different tip–substrate distances of 1, 4, 7 and 10 mm, respectively.
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and N2 mixture inlet. FESEM, TEM, HRTEM and SAED show that the as-grown CNNCs could have intact cone-shape, the vertical alignment, the β-C3N4 microstructure and the minimum tip curvature diameter of 3–4 nm. The FESEM shows that the size, distribution density and morphology of the as-grown CNNCs are strongly affected by the tip–substrate distance. The tip–substrate distance of 1–2 mm is the optimal condition for the growth of the CNNCs with intact coneshape and good crystallinity. The intact CNNC arrays synthesized by GPCVD method have potential applications in microscopes, fieldemission-displays, solar cells etc. Acknowledgments The authors acknowledge the supports from the National Natural Science Foundation of China (grant no.10875030) and National Basic Research Program of China (973 Program) (grant no.2010CB933703). References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Fig. 7. (a) The emission intensity of CN (388.2 nm), N2+ (391.3 nm) and N2 (380.4 nm) against the tip–substrate distances taken from Fig. 6(b). (b) The intensity ratios of N2+/CN (391.3/388.2 nm) and N2+/N2 (391.3/380.4 nm) against the tip–substrate distances in graph (a), respectively.
[17] [18] [19] [20]
the intensity ratios of N2±/CN and N2±/N2 should be conducive to this process. 4. Conclusions The vertically aligned CNNC arrays have been synthesized on Nicoated Si (100) substrates through the GPCVD method using a CH4
[21]
A.Y. Liu, M.L. Cohen, Science 245 (1989) 841. D.M. Teter, R.J. Hemley, Science 271 (1996) 53. Y.T. Jan, H.C. Hsieh, C.F. Chen, Diam. Relat. Mater. 8 (1999) 772. G. Zhang, X. Jiang, E. Wang, Science 300 (2003) 472. C.J. Huang, Y.K. Chih, J. Hwang, J. Appl. Phys. 94 (2003) 6796. M. Terrones, P. Redlich, N. Grobert, S. Trasobares, W.K. Hsu, H. Terrones, Y.Q. Zhu, J.P. Hare, C.L. Reeves, A.K. Cheetham, M. Rühle, H.W. Kroto, D.R.M. Walton, Adv. Mater. 11 (1999) 655. A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Adv. Mater. 17 (2005) 1648. S. Stafstro¨m, Appl. Phys. Lett. 77 (2000) 3941. G. Zhang, W. Duan, B. Gu, Appl. Phys. Lett. 80 (2002) 2589. C. Niu, Y.Z. Lu, C.M. Lieber, Science 261 (1993) 334. Y. Peng, T. Ishigaki, S. Horiuchi, Appl. Phys. Lett. 73 (1998) 3671. D.R. Miller, J.J. Wang, E.G. Gillan, J. Mater. Chem. 12 (2002) 2463. Y.F. Zhang, Z.H. Zhou, H.L. Li, Appl. Phys. Lett. 68 (1996) 634. T.Y. Yen, C.P. Chou, Appl. Phys. Lett. 67 (1995) 2801. W. Hu, N. Xu, Y.Q. Shen, T.W. Zhang, Mater. Sci. Eng., A 432 (2006) 142. W. Hu, X.F. Xu, Y.Q. Shen, J.S. Lai, X.N. Fu, J.D. Wu, Z.F. Ying, N. Xu, J. Electron. Mater. 39 (2010) 381. W.Hu.N. Xu, X.F. Xu, J.D. Wu, Y.Q. Shen, Z.F. Ying, Chem. Vap. Depos. 15 (2009) 306. N. Xu, Y.C. Du, Z.F. Ying, Z.M. Ren, F.M. Li, Rev. Sci. Instrum. 68 (1997) 2994. K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu, I.C. Wu, Phys. Rev. B: Condens. Matter 49 (1994) 5034. F. Le Normand, J. Hommet, T. Szouml, C. Fuchs, E. Fogarassy, Phys. Rev. B 64 (2001) 5416. W. Hu, J. Tang, J. Wu, J. Sun, Y. Shen, X. Xu, N. Xu, Phys. Plasmas 15 (2008) 073502.