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Detection of carbon nitride by resonant Raman spectroscopy
Diamond and Related Materials 13 (2004) 1558–1560
Detection of carbon nitride by resonant Raman spectroscopy J. Robertson* Engineering Department, Cambridge University, Trumpington St, Cambridge CB2 1PZ, UK
Abstract Multi-wavelength is proposed as a means to test if a new phase is truly C3N4 . If it is C3N4, then the Raman modes will not disperse as the excitation wavelength changes. However, if the phase contains any p bonding, the Raman modes will disperse. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Raman mode; Spectroscopy; C3N4
The prediction by Liu and Cohen w1x of the existence a b-C3N4 phase with a bulk modulus and hardness similar to diamond has led to an enormous effort to synthesise this material. However, there has been no agreed success on how it is grown. The main reason C3N4 has not been made is that it is less stable than competing phases w2x, particularly graphite plus molecular nitrogen N2. This paper describes a Raman method for eliminating the sp2 phases from consideration. DeVries w3x analysed over 400 reports of the synthesis of C3N4 up to 1997. He concluded that none were actually C3N4, including that of Yur et al. w4x. Many authors used X-ray diffraction to identify the crystal structure of C3N4. However, part of the problem is the large unit cell of C3N4, which means that the diffraction pattern has many lines. There is also the a-C3N4 phase, which has a double sized unit cell. Matsumoto et al. w5x noted that all diffraction lines must be present and in the correct position to verify C3N4, and this has not happened. The difficulty of preparing C3N4 should have been obvious. Sidgwick w6x many years ago noted that nitrogen and carbon do not usually form single bonds, except in highly hydrogenated cases such as amines. This observation was emphasised by Badzian et al. w7x. This inhibits chemical vapour deposition routes to C3N4. The main reason for the difficulty of making C3N4 is that it is less stable than competing phases of graphite plus N2. As C3N4 is denser than the combination of graphite plus N2, one could try to use ion beam methods and the process of subplantation to grow it as a dense *Tel.: q44-1223-332689; fax: q44-1223-332662. E-mail address: [email protected] (J. Robertson).
phases w8x. However, the ion bombardment also causes disordering and this would result in an amorphous phase. Note that ion beam deposition is possible for cubic BN, because BN is more ionic than C3N4, which has a low ionicity w1x, and ion bombardment of compounds over a critical ionicity creates the crystalline phase not the amorphous phase w9x. Indeed, Muhl w10x notes that most growth processes have created amorphous CNx with nitrogen contents well below the required 57%. Badzian et al. w7x and Muhl w10x also noted a key point is that the presence of Si will stabilise a Si3N4like phase b-SiC2N4. This is crystalline. The Si can diffuse up from the substrate. Many early reports had Si impurity. SiC2N4 is an interesting ceramic, but it is not C3N4. The methods used to characterise possible C3N4 are X-ray diffraction w5x, infra-red (IR) spectroscopy w11– 13x, X-ray photoemission spectroscopy w14x, electron energy loss spectroscopy and its equivalent, X-ray near edge spectroscopy w15x and nuclear magnetic resonance (NMR)w16x. We have already noted the requirements for XRD. For IR, the main point is that C3N4 contains only C–N single bonds, and no C–C, C_ C or C_ N bonds. The bond stretching mode of single C–N bonds is lower than that of C_ N bonds, and lies approximately 1310 to 1380 cmy1 w11,12x. It is in the same range as that of C–C bonds in diamond 1332 cmy1. There should be no modes above 1400 cmy1. XPS has often been used to try to distinguish bonding in carbon nitrides, using the shifts of the C and N core levels. The problem is that the C–N bond ionicity is low, so the shifts are small.
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.085
J. Robertson / Diamond and Related Materials 13 (2004) 1558–1560
1559
Fig. 2. Schematic diagram of the excitation in (a) wide gap semiconductor, (b) narrow gap semiconductor like Si, (c) low dimensional system like graphite.
Fig. 1. Electronic density of states of C3N4.
EELS and XANES are an ideal means to prove the presence of C3N4. The electronic structure w17x is shown in Fig. 1. As there are only s bonds present in C3N4, there are no empty p* states in this material. Therefore, there should be no p* peak at 285 eV in the C 1s spectrum or at 402 eV in the N 1s spectrum. So far, no material has passed this test. However, EELS and XANES are not so widely available. Hence, we propose the use of Raman spectroscopy as a more widely available test of C3N4. Raman is the scattering of photons by phonons, by lattice polarisability. The polarisation occurs by the excitation of an electron-hole pair. The excitation could be to a continuum of states (Fig. 2b), as in the conduction band of Si, or to a virtual state in the band gap, as occurs usually in a wide band gap material like diamond (a). The third situation occurs in low dimensional cases, in which the excitation resonates with a specific valence to conduction transition (c). Case (b) is actually like case (c), but in three dimensions, the density of states shows no singularities. The low dimensionality in case (c) creates a singularity in the electronic density of states, which emphasises the resonance. The importance of the resonance case is that there is then a selection rule w18x, which couples the allowed phonon wavenumber (q) to the electron wavenumber (k). This therefore couples the phonon energy to the electronic excitation energy, via the phonon and electronic band structures. This is very notable in graphite w19x where a double resonance creates a selection rule of qs2k, where both wavevectors are measured from the K point of the Brillouin zone. The key effect of resonance is that the Raman phonon energy must disperse as the photon energy is varied. All sp2 and sp1 p bonded systems will have dispersing Raman modes, as seen in for example amorphous carbon w18,20x or in
polyacetylene. In contrast, sp3 and only s bonded systems show non-dispersing Raman modes (e.g. diamond). This simple rule can be applied to C3N4. As C3N4 is only s bonded, any Raman mode should not disperse as the laser excitation energy is changed. This, crystallinity and the absence of any Si should act as a strong test on whether a phase is really C3N4. An example of Raman dispersion is shown in Fig. 3 for relatively highly sp3 bonded amorphous CNx. The G mode at approximately 1600 cmy1 is due to the residual sp2 bonding, and it is clearly seen to disperse, as the laser energy is varied w21x.
Fig. 3. Dispersion of Raman peaks due to sp2 bonding in amorphous CNx.
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Acknowledgments The author is grateful to A.C. Ferrari for the many Raman spectra. References w1 x w2 x w3 x w4 x w5 x w6 x w7 x w8 x w9 x
A.Y. Liu, M. Cohen, Science 24 (1989) 841. D.M. Teeter, R.J. Hemley, Science 271 (1996) 53. R.C. DeVries, Innovation 1 (1997) 161. K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu, Phys. Rev. B 49 (1994) 5034. S. Matsumoto, E.Q. Xie, F. Izumi, Diamond Relat. Mater. 8 (1999) 1175. N.V. Sidgwick, The Organic Chemistry of Nitrogen, 2nd ed, Oxford, 1937. A. Badzian, T. Badzian, R. Roy, W. Drawl, Thin Solid Films 354 (1999) 148. D. Marton, K.J. Boyd, A.H. Albayati, S.S. Todorov, J.W. Rabalais, Phys. Rev. Lett. 73 (2003) 118. J. Robertson, Diamond Relat. Mater. 5 (1996) 519.
w10x S. Muhl, J.M. Mendez, Diamond Relat. Mater. 8 (1999) 1809. w11x Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Appl. Phys. Lett. 73 (1998) 915. w12x C.T. Kuo, J.Y. Wu, T.R. Lu, Mater. Chem. Phys. 72 (2001) 251. w13x L.W. Yin, M.S. Li, G. Luo, J.L. Sui, J.M. Wang, Chem. Phys. Lett. 369 (2003) 483. w14x C. Ronning, H. Feldermann, R. Merk, H. Hofsass, P. Reinke, J.U. Thiele, Phys. Rev. B 58 (1998) 2207. w15x N. Hellgren, J. Guo, C. Sathe, A. Agui, J. Nordgren, Y. Luo, et al., Appl. Phys. Lett. 79 (2001) 4348. w16x Y.G. Yoon, B.G. Pfrommer, F. Mauri, S.G. Louie, Phys. Rev. Lett. 80 (1998) 3388. w17x J. Robertson, C.A. Davis, Diamond Relat. Mater. 4 (1995) 441. w18x A.C. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 075 414. w19x C. Thomsen, S. Reich, Phys. Rev. Lett. 85 (2000) 5214. w20x J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2002) 129–281. w21x A.C. Ferrari, S.E. Rodil, J. Robertson, Phys. Rev. B 67 (2003) 155 306.
Detection of carbon nitride by resonant Raman spectroscopy J. Robertson* Engineering Department, Cambridge University, Trumpington St, Cambridge CB2 1PZ, UK
Abstract Multi-wavelength is proposed as a means to test if a new phase is truly C3N4 . If it is C3N4, then the Raman modes will not disperse as the excitation wavelength changes. However, if the phase contains any p bonding, the Raman modes will disperse. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Raman mode; Spectroscopy; C3N4
The prediction by Liu and Cohen w1x of the existence a b-C3N4 phase with a bulk modulus and hardness similar to diamond has led to an enormous effort to synthesise this material. However, there has been no agreed success on how it is grown. The main reason C3N4 has not been made is that it is less stable than competing phases w2x, particularly graphite plus molecular nitrogen N2. This paper describes a Raman method for eliminating the sp2 phases from consideration. DeVries w3x analysed over 400 reports of the synthesis of C3N4 up to 1997. He concluded that none were actually C3N4, including that of Yur et al. w4x. Many authors used X-ray diffraction to identify the crystal structure of C3N4. However, part of the problem is the large unit cell of C3N4, which means that the diffraction pattern has many lines. There is also the a-C3N4 phase, which has a double sized unit cell. Matsumoto et al. w5x noted that all diffraction lines must be present and in the correct position to verify C3N4, and this has not happened. The difficulty of preparing C3N4 should have been obvious. Sidgwick w6x many years ago noted that nitrogen and carbon do not usually form single bonds, except in highly hydrogenated cases such as amines. This observation was emphasised by Badzian et al. w7x. This inhibits chemical vapour deposition routes to C3N4. The main reason for the difficulty of making C3N4 is that it is less stable than competing phases of graphite plus N2. As C3N4 is denser than the combination of graphite plus N2, one could try to use ion beam methods and the process of subplantation to grow it as a dense *Tel.: q44-1223-332689; fax: q44-1223-332662. E-mail address: [email protected] (J. Robertson).
phases w8x. However, the ion bombardment also causes disordering and this would result in an amorphous phase. Note that ion beam deposition is possible for cubic BN, because BN is more ionic than C3N4, which has a low ionicity w1x, and ion bombardment of compounds over a critical ionicity creates the crystalline phase not the amorphous phase w9x. Indeed, Muhl w10x notes that most growth processes have created amorphous CNx with nitrogen contents well below the required 57%. Badzian et al. w7x and Muhl w10x also noted a key point is that the presence of Si will stabilise a Si3N4like phase b-SiC2N4. This is crystalline. The Si can diffuse up from the substrate. Many early reports had Si impurity. SiC2N4 is an interesting ceramic, but it is not C3N4. The methods used to characterise possible C3N4 are X-ray diffraction w5x, infra-red (IR) spectroscopy w11– 13x, X-ray photoemission spectroscopy w14x, electron energy loss spectroscopy and its equivalent, X-ray near edge spectroscopy w15x and nuclear magnetic resonance (NMR)w16x. We have already noted the requirements for XRD. For IR, the main point is that C3N4 contains only C–N single bonds, and no C–C, C_ C or C_ N bonds. The bond stretching mode of single C–N bonds is lower than that of C_ N bonds, and lies approximately 1310 to 1380 cmy1 w11,12x. It is in the same range as that of C–C bonds in diamond 1332 cmy1. There should be no modes above 1400 cmy1. XPS has often been used to try to distinguish bonding in carbon nitrides, using the shifts of the C and N core levels. The problem is that the C–N bond ionicity is low, so the shifts are small.
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.085
J. Robertson / Diamond and Related Materials 13 (2004) 1558–1560
1559
Fig. 2. Schematic diagram of the excitation in (a) wide gap semiconductor, (b) narrow gap semiconductor like Si, (c) low dimensional system like graphite.
Fig. 1. Electronic density of states of C3N4.
EELS and XANES are an ideal means to prove the presence of C3N4. The electronic structure w17x is shown in Fig. 1. As there are only s bonds present in C3N4, there are no empty p* states in this material. Therefore, there should be no p* peak at 285 eV in the C 1s spectrum or at 402 eV in the N 1s spectrum. So far, no material has passed this test. However, EELS and XANES are not so widely available. Hence, we propose the use of Raman spectroscopy as a more widely available test of C3N4. Raman is the scattering of photons by phonons, by lattice polarisability. The polarisation occurs by the excitation of an electron-hole pair. The excitation could be to a continuum of states (Fig. 2b), as in the conduction band of Si, or to a virtual state in the band gap, as occurs usually in a wide band gap material like diamond (a). The third situation occurs in low dimensional cases, in which the excitation resonates with a specific valence to conduction transition (c). Case (b) is actually like case (c), but in three dimensions, the density of states shows no singularities. The low dimensionality in case (c) creates a singularity in the electronic density of states, which emphasises the resonance. The importance of the resonance case is that there is then a selection rule w18x, which couples the allowed phonon wavenumber (q) to the electron wavenumber (k). This therefore couples the phonon energy to the electronic excitation energy, via the phonon and electronic band structures. This is very notable in graphite w19x where a double resonance creates a selection rule of qs2k, where both wavevectors are measured from the K point of the Brillouin zone. The key effect of resonance is that the Raman phonon energy must disperse as the photon energy is varied. All sp2 and sp1 p bonded systems will have dispersing Raman modes, as seen in for example amorphous carbon w18,20x or in
polyacetylene. In contrast, sp3 and only s bonded systems show non-dispersing Raman modes (e.g. diamond). This simple rule can be applied to C3N4. As C3N4 is only s bonded, any Raman mode should not disperse as the laser excitation energy is changed. This, crystallinity and the absence of any Si should act as a strong test on whether a phase is really C3N4. An example of Raman dispersion is shown in Fig. 3 for relatively highly sp3 bonded amorphous CNx. The G mode at approximately 1600 cmy1 is due to the residual sp2 bonding, and it is clearly seen to disperse, as the laser energy is varied w21x.
Fig. 3. Dispersion of Raman peaks due to sp2 bonding in amorphous CNx.
J. Robertson / Diamond and Related Materials 13 (2004) 1558–1560
1560
Acknowledgments The author is grateful to A.C. Ferrari for the many Raman spectra. References w1 x w2 x w3 x w4 x w5 x w6 x w7 x w8 x w9 x
A.Y. Liu, M. Cohen, Science 24 (1989) 841. D.M. Teeter, R.J. Hemley, Science 271 (1996) 53. R.C. DeVries, Innovation 1 (1997) 161. K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu, Phys. Rev. B 49 (1994) 5034. S. Matsumoto, E.Q. Xie, F. Izumi, Diamond Relat. Mater. 8 (1999) 1175. N.V. Sidgwick, The Organic Chemistry of Nitrogen, 2nd ed, Oxford, 1937. A. Badzian, T. Badzian, R. Roy, W. Drawl, Thin Solid Films 354 (1999) 148. D. Marton, K.J. Boyd, A.H. Albayati, S.S. Todorov, J.W. Rabalais, Phys. Rev. Lett. 73 (2003) 118. J. Robertson, Diamond Relat. Mater. 5 (1996) 519.
w10x S. Muhl, J.M. Mendez, Diamond Relat. Mater. 8 (1999) 1809. w11x Y.K. Yap, S. Kida, T. Aoyama, Y. Mori, T. Sasaki, Appl. Phys. Lett. 73 (1998) 915. w12x C.T. Kuo, J.Y. Wu, T.R. Lu, Mater. Chem. Phys. 72 (2001) 251. w13x L.W. Yin, M.S. Li, G. Luo, J.L. Sui, J.M. Wang, Chem. Phys. Lett. 369 (2003) 483. w14x C. Ronning, H. Feldermann, R. Merk, H. Hofsass, P. Reinke, J.U. Thiele, Phys. Rev. B 58 (1998) 2207. w15x N. Hellgren, J. Guo, C. Sathe, A. Agui, J. Nordgren, Y. Luo, et al., Appl. Phys. Lett. 79 (2001) 4348. w16x Y.G. Yoon, B.G. Pfrommer, F. Mauri, S.G. Louie, Phys. Rev. Lett. 80 (1998) 3388. w17x J. Robertson, C.A. Davis, Diamond Relat. Mater. 4 (1995) 441. w18x A.C. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 075 414. w19x C. Thomsen, S. Reich, Phys. Rev. Lett. 85 (2000) 5214. w20x J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2002) 129–281. w21x A.C. Ferrari, S.E. Rodil, J. Robertson, Phys. Rev. B 67 (2003) 155 306.