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Ultraviolet and visible Raman spectroscopies of the graphitic BCx phases
Diamond & Related Materials 18 (2009) 1123–1128
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Ultraviolet and visible Raman spectroscopies of the graphitic BCx phases P.V. Zinin a,⁎, X.R. Liu b, L.C. Ming a, S.K. Sharma a, Y. Liu c, S.M. Hong b a b c
School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA High Pressure Laboratory, Southwest Jiaotong University, Chengdu, Sichuan, China National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an Shanxi, China
a r t i c l e
i n f o
Article history: Received 7 May 2008 Received in revised form 21 November 2008 Accepted 12 February 2009 Available online 27 February 2009 Keywords: B-C BC3 BC2N Superhard Raman scattering
a b s t r a c t Ultraviolet (UV) Raman and visible Raman spectroscopies were applied to study the graphitic BCx (g-BCx) phases. The Raman spectra of the g-BCx phases excited with UV laser at 244 nm have one main peak: a G peak (approximately at 1590 cm− 1), and do not have the D peak (around 1350 cm− 1) characteristic for Raman spectra of disordered graphitic phases. The D peak can be detected in all g-BCx phases when green (534 nm) or near-infrared (785 nm) lasers are used for Raman scattering excitation. The positions of the G and D peaks were found to be independent (within the experimental errors) of the B/C ratio. The pattern of the peaks in UV Raman spectra of g-BC2.1 phase indicates that the additional peaks centered at 1089 cm− 1 should be assigned to the Eg mode of B4C vibration rather than to the T mode characteristic to amorphous graphite. The high signal-to-noise (S/N) ratio and lack of fluorescence of the UV Raman spectra allow an accurate measure of bandwidth and frequency of the G peaks. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Boron-rich carbon graphitic BCx phases are of interest for several reasons. First, the graphitic BC3 phase is predicted to be a superconductor at a relatively high temperature (Tc 22 K) [1]. Second, graphitic BCx phases are used as precursors in high-pressure hightemperature synthesis of diamond-like BCx phases [2,3]. These materials should possess a unique combination of excellent physical and chemical properties such as great hardness and high chemical inertness [4,5], and they are predicted to be superconductors [6]. Superconductivity was found in boron-doped diamond synthesized at high pressure (~9 GPa) and temperature (2500–2800 K) [7]. Recently, two new diamond-like BCx phases have been synthesized from g-BCx under high pressure and high temperature (HPHT) in a diamond anvil cell: a cubic BC1.6 phase and a diamond-like BC3 phase [2,3]. The structure of the diamond-like BCx phases obtained at HPHT depends on the structure and chemical bonding of the starting g-BCx phases, and therefore full characterization of the g-BC x phases is of importance for successful synthesis of the new diamond-like BCx phases. Raman spectroscopy is commonly used for characterization in chemistry, because vibrational information is very specific for the chemical bonds in molecules. It has been already applied for characterization of graphitic and diamond-like BCx phases, and for observation of the phase transition under high pressure [2,3,8]. Unfortunately, the strong fluorescence and photoluminescence that
⁎ Corresponding author. E-mail address: [email protected] (P.V. Zinin). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.02.025
accompany visible Raman excitation of g-BCx phases limit the sensitivity that these Raman excitation wavelengths are able to achieve [9]. An effective way to diminish fluorescence and photoluminescence background on Raman spectra of graphitic and diamond-like phases is to use UV laser excitation [9–11]. We report here the use of a deep UV Raman excitation and compare the spectral information content of UV spectra with that of visible Raman excitation for the graphitic BC1.8, BC2.1, BC5.4 and BC8.2 phases. The UV Raman spectroscopy was found to be an effective tool in characterization of the graphite [12] and various graphitic phases [10,13,14] including hexagonal BN (h-BN) [15]. It is demonstrated here that in the case of diamond-like materials [9,16], UV Raman measurements of g-BCx phases yield significantly greater information content than do visible Raman measurements. 2. Experiment The BCx phases were obtained by two different deposition techniques: (a) two specimens, BC2.1 and BC5.4, were prepared by high-pressure thermal chemical vapor deposition (HPCVD) method using C2H2, BCl3 (as carbon and boron sources) and H2 (30 mbar, 1225 °C) [17]; (b) BCx film consisting of two layers with different concentrations of boron, BC1.8 and BC8.2 (Fig. 1). The film was deposited at 950 °C on carbon substrate (for 60 h) by low-pressure chemical vapor deposition (LPCVD) from BCl3 and CH4 as boron and carbon sources, respectively. The detailed description of the LPCVD technique can be found in the PhD thesis [18] and is similar to that presented with some modification in the Ref. [19]. The CVD reaction under high temperature (~1000 °C) is one of the main routes for synthesis of g-BCx phases [20–24] since 1986 [25].
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7DW, United Kingdom). Sample damage is an issue in Raman measurements, particularly from UV excitation [10]. Raman spectra were collected by varying the acquisition times, both with and without sample rotation. We found that for measuring resonant UV spectra of the graphitic BCx and h-BN specimens, rotation is not required. Consecutive replicate measurements were performed to ensure no photoalteration of the samples was induced during 244 nm excitation. The calibration of the UV spectrograph was conducted with the sharp 1332 cm− 1 Raman peak of diamond. The visible and infrared Raman measurements were performed using confocal Raman microscopes operating at different excitation wavelengths: inVia Raman (Renishaw), confocal Raman system (WiTec alpha300), and RXN system (Kaiser Optical Systems, Inc., Arbor, MI, USA). In the inVia Raman system, the Raman spectra are excited by an Invictus 514 nm green laser (Ion Laser Technology, UT, USA). In the micro-Raman RXN system, the Raman spectra are excited by an Invictus 785-nm NIR laser (Kaiser Optical Systems, Inc., Ann Arbor, MI, USA). With the WiTec alpha300 confocal microscope, the Raman spectra are excited by a Nd-YAG green (532-nm) laser (Coherent Compass, Dieburg, Germany).
Fig. 1. SEM images of the g-BCx film-deposited graphite substrate by the LPCVD method: (a) cross-section of the specimen showing two layers with different boron concentration: g-BC1.8 and g-BC8.2; (b) top surface of the specimen showing the marked area.
The elemental compositions of the graphitic phases were studied with an SEM system (JEOL JSM-5900) equipped with an energydispersive detector for quantitative chemical analysis of the BCx phase [2,3]. For the HPCVD BCx film the composition was found to be x = 5.38 ± 0.91 for BC5.4 and x = 2.09 ± 0.14 for the BC2.1 phase. Nominally, the LPCVD film contains an outer 5-µm layer of BC8.2 and an inner 5-µm layer of BC1.8 as shown in Fig. 1a. Because a small amount of oxygen was detected in both layers, the composition for the LPCVD film should be correctly represented as BCxOy where x = 8.15 ± 2.17 and y = 0.94 ± 0.32 for the outer; and x = 1.75 ± 0.13 and y = 0.03 for the inner layers. We also measured elemental composition of the marked area of the top surface BC10.2 phase where the C/B ratio was found to be x = 10.19 ± 1.69 (Fig. 1b) with a very small amount of oxygen (less than 1%). It is likely that most of the oxygen was introduced during cutting and polishing of the surface shown in Fig.1. Indeed, measurements of the composition of the intact top surface of BCx film show that concentration of the oxygen is ten times lower than that of the top surface after polishing. As concentration of the boron may be different along the film surface (8b x b 10), the Raman scattering measurements were conducted at the same area where the composition of the sample was measured. Following the procedure developed in Ref. [26], we marked the area of interest (for instance, the cross-section shown in Fig. 1b) before SEM-EDX measurements. Then, this area was found with SEM and with an optical microscope attached to the confocal Raman system; corresponding EDX and Raman scattering measurements were made. The UV Raman spectra acquired at 244 nm were excited using a frequency-doubled Ar ion laser (85-SHG, Lexel Laser Inc., Fremont, CA, USA). The spectra were collected with a 40× objective using Renishaw confocal Raman microscope “inVia” (Renishaw, Gloucestershire GL12
Fig. 2. UV (244 nm) and visible (514 nm) Raman spectra of the g-BC2.1 specimen measured with Renishaw system: (a) UV spectrum taken with UV ×40 objective; integration time was 10 min; laser power on sample was 0.05 mW; (b) visible Raman spectrum (averaged over five spectra) taken with ×20 objective; integration time was 1 min; laser power on sample was 2 mW.
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where layers in the cross-section of the g-BCx coating were better seen than in the Renishaw system. The UV Raman spectra of the g-BC2.1 and gBC5.4 phases each have one strong peak (G peak): at 1586 cm− 1 for gBC2.1 and another at 1585 cm− 1 for g-BC5.4. The G peak of the g-BC5.4 phase has an asymmetrical shape and can be considered as a convolution of two peaks, the G peak and a broad band at 1539 cm− 1. The second strongest peak for both graphitic phases is located around 1100 cm− 1: it is 1089 cm− 1 for g-BC2.1 (Fig. 2a) and 1083 for g-BC5.4 (Fig. 3a). This peak is more pronounced for the g-BC2.1 phase than that for the g-BC5.4 phase: the ratio of area of this peak to G peak is I1089/IG =0.35 for the g-BC2.1 and I1083/IG = 0.007 for the g-BC5.4 phase. It is difficult to recognize additional peaks in the UV Raman spectrum of the g-BC5.4 phase, but an additional sharp weak peak at 717 cm− 1, a broad peak centered at 782 cm− 1 and a weak peak at 1004 cm− 1 sitting on the shoulder of the 1089 cm− 1 peak can be distinguished in the Raman spectrum of the g-BC2.1 phase. Visible (514 nm) Raman spectra of the g-BC2.1, g-BC5.4 phases have also D peaks (Figs. 2b and 3b): centered at 1364 and 1354 cm− 1 respectively. The peak around 1100 cm− 1 cannot be distinguished in the visible Raman spectra of either of the g-BC2.1 or g-BC5.4 phases. It might be that they are hidden by a strong and broad D peak.
Fig. 3. The UV (244 nm) and visible (514 nm) Raman spectra of the g-BC5.4 specimen measured with the Renishaw system: (a) the complete UV spectrum (500–2000 cm− 1) taken with UV ×40 objective; integration time was 10 min; laser power on sample was 0.05 mW; (b) visible Raman spectrum (averaged over five spectra) taken with ×20 objective; integration time was 1 min; laser power on sample was 2 mW.
We found that several additional peaks always appear on the UV Raman spectrum when the surface of the specimen is a good reflector and can be detected only on polished highly reflecting samples. For instance, the UV Raman spectrum collected from shiny aluminum plate contains sharp peaks at 1436, 1534, 1555, 1688 and 2328 cm− 1, indicating that these peaks are resulted from Raman scattering inside the UV laser system. This set of peaks was subtracted from the UV Raman spectra of BCx and B4C phases when it was detected. 3. Results The UV and visible spectra of the g-BC2.1 and g-BC5.4 are shown in Figs. 2 and 3 respectively; UV and near-infrared Raman spectra of g-BC1.8 phase are shown in Fig. 4. The appearance of the Raman spectra of the gBC8.2 phase is similar to those of g-BC1.8. Positions of the peaks are shown in Table 1. Peak-fitting analysis (dashed lines) was performed using “Grams Software” (version 7.02, Thermo Fisher Scientific, Inc). Background subtraction procedure was applied for green and near-infrared Raman spectra. UV Raman spectra need only offset correction. We used infrared laser excitation (785 nm) for measuring spectra of the crosssection of the BCx film since this laser was attached to the Kaiser system
Fig. 4. UV (244 nm) and near-infrared (785 nm) Raman spectra of the g-BC8.15 specimen: (a) the UV spectrum is an average over five spectra taken with Renishaw system and ×40 objective; integration time was 10 min; laser power on sample was 0.2 mW; (b) the near-infrared spectrum is an average over five spectra taken with Kaiser system and ×50 objective: integration time was 2 min; laser power on sample was 2 mW.
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Table 1 Wavenumbers of the D and G Raman peaks (cm− 1) of g-BCx phases and graphite measured in this work and taken from literature. Graphitic specimens (excitation used) g-BC1.8a (244 nm) g-BC1.8a (532 nm) g-BC1.8a (785 nm) g-BC2.1a (244 nm) g-BC2.1a (514 nm) g-BC5.4a (244 nm) g-BC5.4a (514 nm) g-BC1.6b (488 nm) gp-BC3c (633 nm) g-BC4d (514 nm) g-BC5e (514 nm) g-BCa5 (514 nm) g-BC6f (514 nm) g-BCa8.2 (244 nm) g-BCa8.2 (532 nm) g-BCa8.2 (785 nm) g-BCa10.2 (244 nm) g-BCa10.2 (785 nm) Graphiteg
D peak 1354 (250) 1319 (158) 1364 (201) 1354 (158) 1351 1365 1344 1366 1360 1349 (242) 1319 (150) 1324 (130)
G peak 1583 (29) 1573(110) 1586 (73) 1586 (53) 1584 (102) 1585 (44) 1589 (80) 1587 1582 1561 1556 1585 1590 1590 (75) 1570 (100) 1584 (77) 1585 (82) 1565 (77) 1581
Numbers in parenthesis are full width at half-maximum (FWHM) of the correspondent peaks. a This study. The average over five measurements. b From Ref. [8]. c From Ref. [3]. d From Ref. [30]. e From Ref. [37]. f From Ref. [21]. g From Ref. [35].
to graphite, and therefore to the splitting of the G peak in the Raman spectra of the g-BCx phases. The high S/N ratio and lack of fluorescence of the UV Raman spectra allow accurate measure of bandwidth and frequency of a firstorder phonon band with accuracy better than that measured with green and near-infrared (Figs. 2–4). Comparison of the full width at half-maximum (FWHM) of the G peak measured by deep UV Raman and by visible Raman shows that the FWHM of the G peak in UV Raman spectra (40–50 cm− 1, see Table 1) is 1.5–2 times lower than those measured using conventional Raman: 60 cm− 1 for BC4 [30], 80 cm− 1 for g-BC3 [3], 80 cm− 1 for g-BC1.6 [8] and 120 cm− 1 for gBC5. A comparative analysis of the positions of the G peaks measured with UV, green and near-infrared laser excitation for different BCx phases shows that difference in the G peak position excited by different laser can be as high as 20 wavenumbers (Table 1). The lowest FWHM of the G peak on UV Raman spectra implies that the most reliable position of the G peak can be measured with UV excitation. Recently, hexagonal sheets of BC, BC3 and BC7 were examined theoretically and their vibration energies compared to that of graphene by Lowther et al. [31]. Calculations on various g-BCx structures indicate that the origin of the peaks centered at 1580– 1590 cm− 1 for g-BCx phases is similar to that of the E2g Raman active mode of graphite [32], also called a G peak [33]. The G peak in graphite
The UV Raman spectra of the g-BC1.8 and g-BC8.2 phases are similar to those of g-BC2.1, g-BC5.4 having one strong peak at 1583 cm− 1 for g-BC1.8 (Fig. 4a) and one at 1590 cm− 1 for g-BC8.2. Peaks around 1100 cm− 1 are not seen on the UV Raman spectra of the g-BCx films obtained by the LPCVD technique. We found that the g-BCx specimen obtained by the LPCVD technique look like a metallic solid after polishing. It is in agreement with a suggestion made by Kouvetakis et al. [25] pointing out that electrical conductivity of BCx phases should be greater than that of graphite because the electron deficiency of each boron atom creates a “hole-carrier” in the valence band. The Raman spectra excited with near-infrared laser (785 nm) have a D peak centered at 1319 cm− 1 for both g-BC1.8 and g-BC8.2 phases, and G peaks at 1586 cm− 1 for g-BC1.8 phase and 1584 cm− 1 for g-BC8.2 phase (Table 1). Measurements of the UV Raman spectrum of the h-BN phase (Fig. 5a) show that the position of the main peak (1365 cm− 1) is in good agreement with that known from literature (1364 cm− 1) [15]. The phonon eigenvector of this mode is a doubly degenerate in-plane optical mode with E2g symmetry [15,27]. The Raman spectrum of the h-BN phase has a weak peak at 922 cm− 1 (Fig. 5b). Within the accuracy of the UV spectrometer (1–2 cm− 1), it is in coincidence with the peak centered at 920 cm− 1 detected in Ref. [15]. Analysis of the UV Raman measurements demonstrates that positions of the Raman peaks of h-BN can be measured with high absolute accuracy and are well reproduced. 4. Discussion The G peak of the g-BCx phases has a more complex spectral structure (Figs. 2a, 3a and 4a) than that of h-BN (Fig. 5); it is similar in shape to the G peak of single-wall carbon nanotubes (SWNTs) [28,29]. Due to the folding of the graphite and to the symmetry-breaking effects associated with the nanotube curvature, the E2g peak in the Raman spectra of graphite splits into several modes with different symmetries in the Raman spectra of SWNTs. Boron atoms in the g-BCx phases could lead to the breaking hexagonal symmetry characteristic
Fig. 5. Portions of the Raman spectra of the g-BN specimen taken with UV ×40 lens: (a) frequency range from 600 to 2000 cm− 1; (b) frequency range from 400 to 1100 cm− 1. Integration time was 10 min and the laser power on sample was 0.05 mW.
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Fig. 6. Positions of the G and D peaks as a function of the C/B ratio.
is due to the bond stretching of all pairs of sp2 atoms in both rings and chains [34]. Theoretical simulations also demonstrate that the next highest energy band of g-BCx phases (around 1350 cm− 1) is a Raman active mode and is associated with E vibrations [31]. A strong peak around 1350 cm− 1 is usually detected in disordered graphite or in diamond-like materials: the D mode [32,35]. The D peak is due to the breathing modes of sp2 atoms in rings [10] and its intensity is associated with the size of the sp2 carbon clusters [32,36]. The D band intensity of diamond-like materials decreases as the excitation frequency increases and it is not detected on the UV Raman spectra of disordered graphite [9]. The D peak is absent from the UV Raman spectra of the g-BCx phases, indicating that it might have the same origin as in the case of graphite. The effect of the B/C ratio on the behavior of the G peak with visible excitation (488 nm and 514.5 nm) has already been investigated for graphite-like ByC1 − y films for boron concentrations in the range 0 b y b 0.17 [37]. Thin films containing ByC1 − y phases were prepared using a CVD method by varying the relative proportions of benzene C6H6 and boron trichloride BCl3 in a Vactronic CVD-300-M reactor at 900 °C and 5 Torr. The G and D visible Raman peaks are observed to soften as a result of boron substitution: G mode—from 1590 cm− 1 for B/C = 0 to 1535 cm− 1 for B/C = 0.17; D mode—from 1370 to 1345 cm− 1 as B/C ratio increases from 0 to 0.17. This behavior is in agreement with ab initio calculations of the vibration of the planar g-BCx predicting that G and D modes should be softening with an increased boron [31]. Positions of the D and G peaks on nearinfrared (785 nm), visible (514 nm) and UV Raman spectra collected from g-BCx phases in this study are shown in Fig. 6. It shows that the position of the G peak measured with resonant Raman or with visible scattering for g-BCx graphite-like specimens does not follow the rule found in Refs. [31,37]: the position of the G peak is independent of the B/C ratio; when concentration of the boron increases from 0.1 (BC10) to 0.55 BC1.8: the inclination of the least-squares line of the G peak position as a function of B/C ratio is not statistically significant different from the horizontal line for peaks measured with UV, visible and infrared lasers excitations. Similarly, the position of the D peak measured by near-infrared and green laser excitation is found to be independent of the C/B ratio (Fig. 6). The discrepancy between our results and the theoretical predictions [31] certainly is not related to the accuracy of the theoretical simulations. Experimental and theoretical studies of the phonon-dispersion curves of the stable and metastable BC3 honeycomb sheets with different lattice constants demonstrated that the observed curves are in good agreement with the theoretical curves calculated on the basis of the ab initio theory [38]. One of the plausible explanations of the discrepancy between our
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results and theoretical predictions is related to the structure of the BCx phases obtained by CVD techniques. The phonon-dispersion curves of the g-BC3 phase [38] and theoretical simulations of the G peak position as a function of boron concentration [31] were performed on flat hexagonal sheets containing no defects for plane g-BCx structures. Experimental measurements of the phonon-dispersion curves were made on the g-BC3 layer on an NbB2 surface assuming that g-BC3 is flat. It is possible that the atomic structure of g-BCx phases obtained by CVD deposition cannot be modeled as a perfect graphene-like plane sheet. Recently, periodic density functional theory calculations of the structure and electronic properties of single-layered carbon nitride graphene C3N4 phases revealed that graphene-like phases can spontaneously adopt a stable buckled geometry [39]. It is likely that buckling and defects have effects on the vibrational properties of the BCx phases. Similarly to the G peak, the position of the D peak was found to be independent of the B/C ratio (Fig. 6) when concentration of the boron increases from 0.1 (BC10) to 0.55 (BC1.8). The absence of the softening of the D mode with the increase of the boron concentration might be related to the fact that nature of the mode detected with green and infrared excitation is similar to that in graphite. Being attributed to structure in the vibrational density of states, the D band in graphite is a breathing mode of A1g symmetry and arises from the wave-vector selection-rule relaxation from finite-crystal-size effects [35]. The characteristic feature of the D band in graphite is that its intensity decreases with an increase of the frequency of excitation [9]. Similar behavior is exhibited by the D peak of the BCx phases measured in this paper: for BC1.8 the ratio of the peak areas between D and G peaks is zero at 244 nm, 3.0 at 532 nm, and 3.6 at 785 nm. Therefore, the absence of the D peak on the UV Raman spectra of the g-BCx phases and similarity in the behavior of the D peak with wavelength of the Raman scattering excitation indicate that it might have the same origin as in the case of graphite. The Raman active band predicted around 1350 cm− 1 in Ref. [31] is likely to be hidden by a broad D band in visible Raman spectra of g-BCx phases due to disordering in the g-BCx phase, and it is not excited by UV irradiation. The UV Raman spectra of g-BC2.1 and g-BC5.4 phases show an additional peak at approximately 1100 cm− 1 (Figs. 2a and 3a). A peak centered at approximately 1100 cm− 1, the so-called T peak, has been detected in UV Raman spectra of tetrahedral amorphous carbon (ta-C) [16,40], disordered, amorphous, and diamond-like carbon, and it was attributed to C–C sp3 vibrations [10]. We postulate that the peaks centered at 1089 cm− 1 for g-BC2.1 and at 1083 cm− 1 for g-BC5.4 in the UV Raman spectra are related to the presence of B4C in the g-BCx phases. Raman scattering of B4C has been measured conventionally
Fig. 7. The UV (244 nm) Raman spectrum of the B4C powder: the UV spectrum is an average over three spectra taken with the Renishaw system and 40× objective; integration time was 5 min; laser power on sample was 0.2 mW.
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[41] and using FT-Raman system [42], but it has not been studied with UV laser excitation. Fig. 7 shows the UV Raman spectrum of the B4C obtained from “Sigma-Aldrich, St. Louis, USA”. It has a strong but broad band centered at 1084 cm− 1 assigned to the breathing mode of icosahedrons in B4C [43]. A weak but relatively sharp peak at 723 cm− 1 can be assigned to the Eg mode of B4C vibration [43] and a broad band around 807 cm− 1 detected in the UV Raman spectrum of B4C (Fig. 7) can be assigned to the A1g mode that was detected in the infrared Raman spectrum of B4C at 797 cm− 1. There is also a weak and broad peak at 990 cm− 1 sitting on the shoulder of the 1084 cm− 1. It is related to the B4C chain rotating [43]. The peak at 1590 cm− 1 (Fig. 7) is due to some amount of amorphous carbon in the B4C powder [44]. Comparison between UV Raman spectra of B4C and BC2.1 demonstrates that main peaks (723, 803, 990 and 1084 cm− 1) of the UV Raman spectrum of B4C in the region of 600–1150 cm− 1 can be found in the UV spectrum of g-BC2.1 (717, 782, 1004 and 1092 cm− 1) indicating that that the g-BC2.1 phases contain some amount of B4C. It is also likely that a weak and broad peak centered at 1069 cm− 1 and detected in the visible (514 nm) Raman spectrum of the g-BC4 phase [30] was due to the presence of B4C in the graphitic phase. It was assigned to T mode characteristic to graphite in Ref. [30], however the T mode should appear only in UV excitation [10]. The area ratio I1098/IG for g-BC2.1 is approximately 50 times (area ratio 0.35) higher than that for the g-BC5.4 phase (0.007). It indicates that g-BC2.1 phases contain sufficient amounts of B4C and that the C/B ratio of the graphitic phase should be lower than 2.1. We do not know the exact concentration of the B4C in the g-BC2.1 phase, therefore we do not remove it from Fig. 6 since it does not change the trend showing therein. Detection of the B4C phase by UV Raman scattering and determination of its concentration is of importance for synthesis of the new diamond-like phases from g-BCx phases [2,3,45] as it requires the knowledge of the B/C ratio in the starting graphitic and synthesized diamond-like phases. With UV excitation, it is possible to obtain Raman spectra with a very low fluorescent background that does not substantially interfere with the collection of the vibrational spectrum of interest [46]. Therefore, the ratio I1089/IG of the Raman peaks could be used as a measure of the amount of the B4C in the g-BCx phase after an appropriate calibration. Calibration of intensities of g-BCx and B4C phases is beyond the scope of this work; however, our preliminary data indicate that the cross sections of the UV Raman scattering by g-BCx phases and B4C are of the same order. 5. Conclusions The Raman spectrum of the g-BCx phases excited with UV laser at 244 nm has one main peak (a G peak at approximately 1590 cm− 1), and does not have the D peak characteristic of disordered graphitic phases. The position of the G peak measured with resonant Raman scattering was found to be independent (within the experimental errors) of the B/C ratio when concentration of the boron increased from 0.1 (BC10) to 0.55 BC1.8. A peak around 1350 cm− 1 (D peak) can be detected in all g-BCx phases when green or infrared lasers are used for Raman scattering excitation. The D mode was predicted to be Raman active in g-BCx phases and should exhibit softening with increased boron concentration [31]. Similarly to the G peak, the position of the D peak in this study on several g-BCx phases is also independent of the B/C ratio. The absence of the softening of the D mode with the increase of the B/C ratio might be related to the fact that the mode detected with green and infrared excitation is similar in nature to that in graphite. The pattern of the peaks in the UV Raman spectra of g-BC2.1 phase indicates that the additional peaks centered at 1089 cm− 1 should be assigned to the Eg mode of B4C vibration rather than to the T mode characteristic to amorphous graphite. The possibility of detecting the B4C in BCx phases may provide a method for quantitative determina-
tion of the B4C concentration in the BCx phases. Such a method is of importance for high-pressure synthesis of novel cubic BCx phases. Acknowledgments This work was supported by U.S. Department of Energy Grant no. DE-FG02-07ER46408. We also are grateful to J. E. Lowther for useful discussion and V. Solozhenko for providing the graphitic phases used in this experiment and Tayro Acosta for his help in conducting UV Raman measurements. References [1] F.J. Ribeiro, M.L. Cohen, Phys. Rev. B 69 (2004) 212507 (4 pages). [2] P.V. Zinin, L.C. Ming, I. Kudryashov, N. Konishi, M.H. Manghnani, S.K. Sharma, J. Appl. Phys. 100 (2006) 013516. [3] P.V. Zinin, L.C. Ming, I. Kudryashov, N. Konishi, S.K. Sharma, J. Raman Spectrosc. 38 (2007) 1362. [4] E.A. Ekimov, V. Sidorov, R.A. Sadykov, N.N. Mel'nik, S. Gierlotka, A. Presz, Diamond Relat. Mater. 14 (2005) 437. [5] P.V. Zinin, V.L. Solozhenko, A.J. Malkin, L.C. Ming, J. Mater. Sci. 40 (2005) 3009. [6] J.E. Moussa, M.L. Cohen, Phys. Rev. B 77 (2008) 8. [7] E.A. Ekimov, V.A. Sidorov, E.D. Bauer, N.N. Mel'nik, N.J. Curro, D.O. Thompson, S.M. Stishov, Nature 428 (2004) 542. [8] P.V. Zinin, I. Kudryashov, N. Konishi, L.C. Ming, V.L. Solozhenko, S.K. Sharma, Spectrochim. Acta, Part A 61 (2005) 2386. [9] R.W. Bormett, S.A. Asher, R.E. Witowski, W.D. Partlow, R. Lizewski, F. Pettit, J. Appl. Phys. 77 (1995) 5916. [10] A.C. Ferrari, J. Robertson, Phys. Rev. B 64 (2001) 075414. [11] S.A. Asher, Ann. Rev. Phys. Chem. 39 (1988) 537. [12] J. Maultzsch, S. Reich, C. Thomsen, Phys. Rev. B 70 (2004) 155403. [13] T.R. Ravindran, J.V. Badding, Solid State Commun. 121 (2002) 391. [14] T.R. Ravindran, J.V. Badding, J. Mater. Sci. 41 (2006) 7145. [15] S. Reich, A.C. Ferrari, R. Arenal, A. Loiseau, B.I.J. Robertson, Phys. Rev. B 71 (2005) 205201. [16] V.I. Merkulov, J.S. Lannin, C.H. Munro, S.A. Asher, V.S. Veerasamy, W.I. Milne, Phys. Rev. Lett. 78 (1997) 4869. [17] T. Shirasaki, A. Derre, M. Menetrier, A. Tressaud, S. Flandrois, Carbon 38 (2000) 1461. [18] Y.S. Liu, qFundamental manufacturing technique by CVD/CVI route for B-C ceramic (in chinese),q Northwestern Polytechnical University, Xi'an, China, PhD thesis 2008. [19] Y.S. Liu, L.F. Cheng, L.T. Zhang, W.B. Yang, Y.D. Xu, Int. J. Appl. Ceram. Tech. 5 (2008) 305. [20] K.M. Krishnan, Appl. Phys. Lett. 58 (1991) 1857. [21] C.T. Hach, L.E. Jones, C. Crossland, P.A. Thrower, Carbon 37 (1999) 221. [22] B.M. Way, J.R. Dahn, T. Tiedje, K. Myrtle, M. Kasrai, Phys. Rev. B 46 (1992) 1697. [23] D.L. Fecko, L.E. Jones, P.A. Thrower, Carbon 31 (1993) 637. [24] S. Flandrois, B. Ottaviani, A. Derre, A. Tressaud, J. Phys. Chem. Solids 57 (1996) 741. [25] J. Kouvetakis, R.B. Kaner, M.L. Sattler, N. Bartlett, J. Chem. Soc. Chem. Commun. (1986) 1758. [26] M. Prasad, M.H. Manghnani, Y. Wang, P. Zinin, R.A. Livingston, J. Mater. Sci. 35 (2000) 3607. [27] R. Geick, C.H. Perry, G. Rupprecht, Phys. Rev. 146 (1966) 543. [28] M.A. Pimenta, A. Marucci, S.A. Empedocles, M.G. Bawendi, E.B. Hanlon, A.M. Rao, P.C. Eklund, R.E. Smalley, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B 58 (1998) R16016. [29] A. Jorio, A.G. Souza Filho, G. Dresselhaus, M.S. Dresselhaus, A.K. Swan, M.S. Ünlü, B.B. Goldberg, M.A. Pimenta, J.H. Hafner, C.M. Lieber, R. Saito, Phys. Rev. B 65 (2002) 155412. [30] V.L. Solozhenko, O. Kurakevich, A.U. Kuznetsov, J. Appl. Phys. 102 (2007) 1691. [31] J.E. Lowther, P.V. Zinin, L.C. Ming, Phys. Rev. B 79 (2009) 033401. [32] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [33] S. Reich, C. Thomsen, Phil. Trans. R. Soc. Lond. A 362 (2004) 2271. [34] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [35] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [36] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. [37] J.G. Naeini, B.M. Way, J.R. Dahn, J.C. Irwin, Phys. Rev. B 54 (1996) 144. [38] H. Yanagisawa, T. Tanaka, Y. Ishida, E. Rokuta, S. Otani, C. Oshima, Phys. Rev. B 73 (2006) 045412. [39] M. Deifallah, P.F. McMillan, F. Cora, J. Phy. Chem. C 112 (2008) 5447. [40] K.W.R. Gilkes, H.S. Sands, D.N. Batchelder, J. Robertson, W.I. Milne, Appl. Phys. Lett. 70 (1997) 1980. [41] D.R. Tallant, T.L. Aselage, A.N. Campbell, D. Emin, Phys. Rev. B 40 (1989) 5649. [42] H. Werheit, R. Schmechel, U. Kuhlmann, T.U. Kampen, W. Monch, A. Rau, J. Alloy. Comp. 291 (1999) 28. [43] H. Werheit, H.W. Rotter, F.D. Meyer, H. Hillebrecht, S.O. Shalamberidze, T.G. Abzianidze, G.G. Esadze, J. Solid State Chem. 177 (2004) 569. [44] J. McCauley, Private communication (2008). [45] J.E. Lowther, J. Phys. Condens. Mat. 17 (2005) 3221. [46] J.W. Ager III, R.K. Nalla, K.L. Breeden, R.O. Ritchie, J. Biomed. Opt. 10 (2005) 034012.
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Ultraviolet and visible Raman spectroscopies of the graphitic BCx phases P.V. Zinin a,⁎, X.R. Liu b, L.C. Ming a, S.K. Sharma a, Y. Liu c, S.M. Hong b a b c
School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA High Pressure Laboratory, Southwest Jiaotong University, Chengdu, Sichuan, China National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an Shanxi, China
a r t i c l e
i n f o
Article history: Received 7 May 2008 Received in revised form 21 November 2008 Accepted 12 February 2009 Available online 27 February 2009 Keywords: B-C BC3 BC2N Superhard Raman scattering
a b s t r a c t Ultraviolet (UV) Raman and visible Raman spectroscopies were applied to study the graphitic BCx (g-BCx) phases. The Raman spectra of the g-BCx phases excited with UV laser at 244 nm have one main peak: a G peak (approximately at 1590 cm− 1), and do not have the D peak (around 1350 cm− 1) characteristic for Raman spectra of disordered graphitic phases. The D peak can be detected in all g-BCx phases when green (534 nm) or near-infrared (785 nm) lasers are used for Raman scattering excitation. The positions of the G and D peaks were found to be independent (within the experimental errors) of the B/C ratio. The pattern of the peaks in UV Raman spectra of g-BC2.1 phase indicates that the additional peaks centered at 1089 cm− 1 should be assigned to the Eg mode of B4C vibration rather than to the T mode characteristic to amorphous graphite. The high signal-to-noise (S/N) ratio and lack of fluorescence of the UV Raman spectra allow an accurate measure of bandwidth and frequency of the G peaks. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Boron-rich carbon graphitic BCx phases are of interest for several reasons. First, the graphitic BC3 phase is predicted to be a superconductor at a relatively high temperature (Tc 22 K) [1]. Second, graphitic BCx phases are used as precursors in high-pressure hightemperature synthesis of diamond-like BCx phases [2,3]. These materials should possess a unique combination of excellent physical and chemical properties such as great hardness and high chemical inertness [4,5], and they are predicted to be superconductors [6]. Superconductivity was found in boron-doped diamond synthesized at high pressure (~9 GPa) and temperature (2500–2800 K) [7]. Recently, two new diamond-like BCx phases have been synthesized from g-BCx under high pressure and high temperature (HPHT) in a diamond anvil cell: a cubic BC1.6 phase and a diamond-like BC3 phase [2,3]. The structure of the diamond-like BCx phases obtained at HPHT depends on the structure and chemical bonding of the starting g-BCx phases, and therefore full characterization of the g-BC x phases is of importance for successful synthesis of the new diamond-like BCx phases. Raman spectroscopy is commonly used for characterization in chemistry, because vibrational information is very specific for the chemical bonds in molecules. It has been already applied for characterization of graphitic and diamond-like BCx phases, and for observation of the phase transition under high pressure [2,3,8]. Unfortunately, the strong fluorescence and photoluminescence that
⁎ Corresponding author. E-mail address: [email protected] (P.V. Zinin). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.02.025
accompany visible Raman excitation of g-BCx phases limit the sensitivity that these Raman excitation wavelengths are able to achieve [9]. An effective way to diminish fluorescence and photoluminescence background on Raman spectra of graphitic and diamond-like phases is to use UV laser excitation [9–11]. We report here the use of a deep UV Raman excitation and compare the spectral information content of UV spectra with that of visible Raman excitation for the graphitic BC1.8, BC2.1, BC5.4 and BC8.2 phases. The UV Raman spectroscopy was found to be an effective tool in characterization of the graphite [12] and various graphitic phases [10,13,14] including hexagonal BN (h-BN) [15]. It is demonstrated here that in the case of diamond-like materials [9,16], UV Raman measurements of g-BCx phases yield significantly greater information content than do visible Raman measurements. 2. Experiment The BCx phases were obtained by two different deposition techniques: (a) two specimens, BC2.1 and BC5.4, were prepared by high-pressure thermal chemical vapor deposition (HPCVD) method using C2H2, BCl3 (as carbon and boron sources) and H2 (30 mbar, 1225 °C) [17]; (b) BCx film consisting of two layers with different concentrations of boron, BC1.8 and BC8.2 (Fig. 1). The film was deposited at 950 °C on carbon substrate (for 60 h) by low-pressure chemical vapor deposition (LPCVD) from BCl3 and CH4 as boron and carbon sources, respectively. The detailed description of the LPCVD technique can be found in the PhD thesis [18] and is similar to that presented with some modification in the Ref. [19]. The CVD reaction under high temperature (~1000 °C) is one of the main routes for synthesis of g-BCx phases [20–24] since 1986 [25].
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7DW, United Kingdom). Sample damage is an issue in Raman measurements, particularly from UV excitation [10]. Raman spectra were collected by varying the acquisition times, both with and without sample rotation. We found that for measuring resonant UV spectra of the graphitic BCx and h-BN specimens, rotation is not required. Consecutive replicate measurements were performed to ensure no photoalteration of the samples was induced during 244 nm excitation. The calibration of the UV spectrograph was conducted with the sharp 1332 cm− 1 Raman peak of diamond. The visible and infrared Raman measurements were performed using confocal Raman microscopes operating at different excitation wavelengths: inVia Raman (Renishaw), confocal Raman system (WiTec alpha300), and RXN system (Kaiser Optical Systems, Inc., Arbor, MI, USA). In the inVia Raman system, the Raman spectra are excited by an Invictus 514 nm green laser (Ion Laser Technology, UT, USA). In the micro-Raman RXN system, the Raman spectra are excited by an Invictus 785-nm NIR laser (Kaiser Optical Systems, Inc., Ann Arbor, MI, USA). With the WiTec alpha300 confocal microscope, the Raman spectra are excited by a Nd-YAG green (532-nm) laser (Coherent Compass, Dieburg, Germany).
Fig. 1. SEM images of the g-BCx film-deposited graphite substrate by the LPCVD method: (a) cross-section of the specimen showing two layers with different boron concentration: g-BC1.8 and g-BC8.2; (b) top surface of the specimen showing the marked area.
The elemental compositions of the graphitic phases were studied with an SEM system (JEOL JSM-5900) equipped with an energydispersive detector for quantitative chemical analysis of the BCx phase [2,3]. For the HPCVD BCx film the composition was found to be x = 5.38 ± 0.91 for BC5.4 and x = 2.09 ± 0.14 for the BC2.1 phase. Nominally, the LPCVD film contains an outer 5-µm layer of BC8.2 and an inner 5-µm layer of BC1.8 as shown in Fig. 1a. Because a small amount of oxygen was detected in both layers, the composition for the LPCVD film should be correctly represented as BCxOy where x = 8.15 ± 2.17 and y = 0.94 ± 0.32 for the outer; and x = 1.75 ± 0.13 and y = 0.03 for the inner layers. We also measured elemental composition of the marked area of the top surface BC10.2 phase where the C/B ratio was found to be x = 10.19 ± 1.69 (Fig. 1b) with a very small amount of oxygen (less than 1%). It is likely that most of the oxygen was introduced during cutting and polishing of the surface shown in Fig.1. Indeed, measurements of the composition of the intact top surface of BCx film show that concentration of the oxygen is ten times lower than that of the top surface after polishing. As concentration of the boron may be different along the film surface (8b x b 10), the Raman scattering measurements were conducted at the same area where the composition of the sample was measured. Following the procedure developed in Ref. [26], we marked the area of interest (for instance, the cross-section shown in Fig. 1b) before SEM-EDX measurements. Then, this area was found with SEM and with an optical microscope attached to the confocal Raman system; corresponding EDX and Raman scattering measurements were made. The UV Raman spectra acquired at 244 nm were excited using a frequency-doubled Ar ion laser (85-SHG, Lexel Laser Inc., Fremont, CA, USA). The spectra were collected with a 40× objective using Renishaw confocal Raman microscope “inVia” (Renishaw, Gloucestershire GL12
Fig. 2. UV (244 nm) and visible (514 nm) Raman spectra of the g-BC2.1 specimen measured with Renishaw system: (a) UV spectrum taken with UV ×40 objective; integration time was 10 min; laser power on sample was 0.05 mW; (b) visible Raman spectrum (averaged over five spectra) taken with ×20 objective; integration time was 1 min; laser power on sample was 2 mW.
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where layers in the cross-section of the g-BCx coating were better seen than in the Renishaw system. The UV Raman spectra of the g-BC2.1 and gBC5.4 phases each have one strong peak (G peak): at 1586 cm− 1 for gBC2.1 and another at 1585 cm− 1 for g-BC5.4. The G peak of the g-BC5.4 phase has an asymmetrical shape and can be considered as a convolution of two peaks, the G peak and a broad band at 1539 cm− 1. The second strongest peak for both graphitic phases is located around 1100 cm− 1: it is 1089 cm− 1 for g-BC2.1 (Fig. 2a) and 1083 for g-BC5.4 (Fig. 3a). This peak is more pronounced for the g-BC2.1 phase than that for the g-BC5.4 phase: the ratio of area of this peak to G peak is I1089/IG =0.35 for the g-BC2.1 and I1083/IG = 0.007 for the g-BC5.4 phase. It is difficult to recognize additional peaks in the UV Raman spectrum of the g-BC5.4 phase, but an additional sharp weak peak at 717 cm− 1, a broad peak centered at 782 cm− 1 and a weak peak at 1004 cm− 1 sitting on the shoulder of the 1089 cm− 1 peak can be distinguished in the Raman spectrum of the g-BC2.1 phase. Visible (514 nm) Raman spectra of the g-BC2.1, g-BC5.4 phases have also D peaks (Figs. 2b and 3b): centered at 1364 and 1354 cm− 1 respectively. The peak around 1100 cm− 1 cannot be distinguished in the visible Raman spectra of either of the g-BC2.1 or g-BC5.4 phases. It might be that they are hidden by a strong and broad D peak.
Fig. 3. The UV (244 nm) and visible (514 nm) Raman spectra of the g-BC5.4 specimen measured with the Renishaw system: (a) the complete UV spectrum (500–2000 cm− 1) taken with UV ×40 objective; integration time was 10 min; laser power on sample was 0.05 mW; (b) visible Raman spectrum (averaged over five spectra) taken with ×20 objective; integration time was 1 min; laser power on sample was 2 mW.
We found that several additional peaks always appear on the UV Raman spectrum when the surface of the specimen is a good reflector and can be detected only on polished highly reflecting samples. For instance, the UV Raman spectrum collected from shiny aluminum plate contains sharp peaks at 1436, 1534, 1555, 1688 and 2328 cm− 1, indicating that these peaks are resulted from Raman scattering inside the UV laser system. This set of peaks was subtracted from the UV Raman spectra of BCx and B4C phases when it was detected. 3. Results The UV and visible spectra of the g-BC2.1 and g-BC5.4 are shown in Figs. 2 and 3 respectively; UV and near-infrared Raman spectra of g-BC1.8 phase are shown in Fig. 4. The appearance of the Raman spectra of the gBC8.2 phase is similar to those of g-BC1.8. Positions of the peaks are shown in Table 1. Peak-fitting analysis (dashed lines) was performed using “Grams Software” (version 7.02, Thermo Fisher Scientific, Inc). Background subtraction procedure was applied for green and near-infrared Raman spectra. UV Raman spectra need only offset correction. We used infrared laser excitation (785 nm) for measuring spectra of the crosssection of the BCx film since this laser was attached to the Kaiser system
Fig. 4. UV (244 nm) and near-infrared (785 nm) Raman spectra of the g-BC8.15 specimen: (a) the UV spectrum is an average over five spectra taken with Renishaw system and ×40 objective; integration time was 10 min; laser power on sample was 0.2 mW; (b) the near-infrared spectrum is an average over five spectra taken with Kaiser system and ×50 objective: integration time was 2 min; laser power on sample was 2 mW.
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Table 1 Wavenumbers of the D and G Raman peaks (cm− 1) of g-BCx phases and graphite measured in this work and taken from literature. Graphitic specimens (excitation used) g-BC1.8a (244 nm) g-BC1.8a (532 nm) g-BC1.8a (785 nm) g-BC2.1a (244 nm) g-BC2.1a (514 nm) g-BC5.4a (244 nm) g-BC5.4a (514 nm) g-BC1.6b (488 nm) gp-BC3c (633 nm) g-BC4d (514 nm) g-BC5e (514 nm) g-BCa5 (514 nm) g-BC6f (514 nm) g-BCa8.2 (244 nm) g-BCa8.2 (532 nm) g-BCa8.2 (785 nm) g-BCa10.2 (244 nm) g-BCa10.2 (785 nm) Graphiteg
D peak 1354 (250) 1319 (158) 1364 (201) 1354 (158) 1351 1365 1344 1366 1360 1349 (242) 1319 (150) 1324 (130)
G peak 1583 (29) 1573(110) 1586 (73) 1586 (53) 1584 (102) 1585 (44) 1589 (80) 1587 1582 1561 1556 1585 1590 1590 (75) 1570 (100) 1584 (77) 1585 (82) 1565 (77) 1581
Numbers in parenthesis are full width at half-maximum (FWHM) of the correspondent peaks. a This study. The average over five measurements. b From Ref. [8]. c From Ref. [3]. d From Ref. [30]. e From Ref. [37]. f From Ref. [21]. g From Ref. [35].
to graphite, and therefore to the splitting of the G peak in the Raman spectra of the g-BCx phases. The high S/N ratio and lack of fluorescence of the UV Raman spectra allow accurate measure of bandwidth and frequency of a firstorder phonon band with accuracy better than that measured with green and near-infrared (Figs. 2–4). Comparison of the full width at half-maximum (FWHM) of the G peak measured by deep UV Raman and by visible Raman shows that the FWHM of the G peak in UV Raman spectra (40–50 cm− 1, see Table 1) is 1.5–2 times lower than those measured using conventional Raman: 60 cm− 1 for BC4 [30], 80 cm− 1 for g-BC3 [3], 80 cm− 1 for g-BC1.6 [8] and 120 cm− 1 for gBC5. A comparative analysis of the positions of the G peaks measured with UV, green and near-infrared laser excitation for different BCx phases shows that difference in the G peak position excited by different laser can be as high as 20 wavenumbers (Table 1). The lowest FWHM of the G peak on UV Raman spectra implies that the most reliable position of the G peak can be measured with UV excitation. Recently, hexagonal sheets of BC, BC3 and BC7 were examined theoretically and their vibration energies compared to that of graphene by Lowther et al. [31]. Calculations on various g-BCx structures indicate that the origin of the peaks centered at 1580– 1590 cm− 1 for g-BCx phases is similar to that of the E2g Raman active mode of graphite [32], also called a G peak [33]. The G peak in graphite
The UV Raman spectra of the g-BC1.8 and g-BC8.2 phases are similar to those of g-BC2.1, g-BC5.4 having one strong peak at 1583 cm− 1 for g-BC1.8 (Fig. 4a) and one at 1590 cm− 1 for g-BC8.2. Peaks around 1100 cm− 1 are not seen on the UV Raman spectra of the g-BCx films obtained by the LPCVD technique. We found that the g-BCx specimen obtained by the LPCVD technique look like a metallic solid after polishing. It is in agreement with a suggestion made by Kouvetakis et al. [25] pointing out that electrical conductivity of BCx phases should be greater than that of graphite because the electron deficiency of each boron atom creates a “hole-carrier” in the valence band. The Raman spectra excited with near-infrared laser (785 nm) have a D peak centered at 1319 cm− 1 for both g-BC1.8 and g-BC8.2 phases, and G peaks at 1586 cm− 1 for g-BC1.8 phase and 1584 cm− 1 for g-BC8.2 phase (Table 1). Measurements of the UV Raman spectrum of the h-BN phase (Fig. 5a) show that the position of the main peak (1365 cm− 1) is in good agreement with that known from literature (1364 cm− 1) [15]. The phonon eigenvector of this mode is a doubly degenerate in-plane optical mode with E2g symmetry [15,27]. The Raman spectrum of the h-BN phase has a weak peak at 922 cm− 1 (Fig. 5b). Within the accuracy of the UV spectrometer (1–2 cm− 1), it is in coincidence with the peak centered at 920 cm− 1 detected in Ref. [15]. Analysis of the UV Raman measurements demonstrates that positions of the Raman peaks of h-BN can be measured with high absolute accuracy and are well reproduced. 4. Discussion The G peak of the g-BCx phases has a more complex spectral structure (Figs. 2a, 3a and 4a) than that of h-BN (Fig. 5); it is similar in shape to the G peak of single-wall carbon nanotubes (SWNTs) [28,29]. Due to the folding of the graphite and to the symmetry-breaking effects associated with the nanotube curvature, the E2g peak in the Raman spectra of graphite splits into several modes with different symmetries in the Raman spectra of SWNTs. Boron atoms in the g-BCx phases could lead to the breaking hexagonal symmetry characteristic
Fig. 5. Portions of the Raman spectra of the g-BN specimen taken with UV ×40 lens: (a) frequency range from 600 to 2000 cm− 1; (b) frequency range from 400 to 1100 cm− 1. Integration time was 10 min and the laser power on sample was 0.05 mW.
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Fig. 6. Positions of the G and D peaks as a function of the C/B ratio.
is due to the bond stretching of all pairs of sp2 atoms in both rings and chains [34]. Theoretical simulations also demonstrate that the next highest energy band of g-BCx phases (around 1350 cm− 1) is a Raman active mode and is associated with E vibrations [31]. A strong peak around 1350 cm− 1 is usually detected in disordered graphite or in diamond-like materials: the D mode [32,35]. The D peak is due to the breathing modes of sp2 atoms in rings [10] and its intensity is associated with the size of the sp2 carbon clusters [32,36]. The D band intensity of diamond-like materials decreases as the excitation frequency increases and it is not detected on the UV Raman spectra of disordered graphite [9]. The D peak is absent from the UV Raman spectra of the g-BCx phases, indicating that it might have the same origin as in the case of graphite. The effect of the B/C ratio on the behavior of the G peak with visible excitation (488 nm and 514.5 nm) has already been investigated for graphite-like ByC1 − y films for boron concentrations in the range 0 b y b 0.17 [37]. Thin films containing ByC1 − y phases were prepared using a CVD method by varying the relative proportions of benzene C6H6 and boron trichloride BCl3 in a Vactronic CVD-300-M reactor at 900 °C and 5 Torr. The G and D visible Raman peaks are observed to soften as a result of boron substitution: G mode—from 1590 cm− 1 for B/C = 0 to 1535 cm− 1 for B/C = 0.17; D mode—from 1370 to 1345 cm− 1 as B/C ratio increases from 0 to 0.17. This behavior is in agreement with ab initio calculations of the vibration of the planar g-BCx predicting that G and D modes should be softening with an increased boron [31]. Positions of the D and G peaks on nearinfrared (785 nm), visible (514 nm) and UV Raman spectra collected from g-BCx phases in this study are shown in Fig. 6. It shows that the position of the G peak measured with resonant Raman or with visible scattering for g-BCx graphite-like specimens does not follow the rule found in Refs. [31,37]: the position of the G peak is independent of the B/C ratio; when concentration of the boron increases from 0.1 (BC10) to 0.55 BC1.8: the inclination of the least-squares line of the G peak position as a function of B/C ratio is not statistically significant different from the horizontal line for peaks measured with UV, visible and infrared lasers excitations. Similarly, the position of the D peak measured by near-infrared and green laser excitation is found to be independent of the C/B ratio (Fig. 6). The discrepancy between our results and the theoretical predictions [31] certainly is not related to the accuracy of the theoretical simulations. Experimental and theoretical studies of the phonon-dispersion curves of the stable and metastable BC3 honeycomb sheets with different lattice constants demonstrated that the observed curves are in good agreement with the theoretical curves calculated on the basis of the ab initio theory [38]. One of the plausible explanations of the discrepancy between our
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results and theoretical predictions is related to the structure of the BCx phases obtained by CVD techniques. The phonon-dispersion curves of the g-BC3 phase [38] and theoretical simulations of the G peak position as a function of boron concentration [31] were performed on flat hexagonal sheets containing no defects for plane g-BCx structures. Experimental measurements of the phonon-dispersion curves were made on the g-BC3 layer on an NbB2 surface assuming that g-BC3 is flat. It is possible that the atomic structure of g-BCx phases obtained by CVD deposition cannot be modeled as a perfect graphene-like plane sheet. Recently, periodic density functional theory calculations of the structure and electronic properties of single-layered carbon nitride graphene C3N4 phases revealed that graphene-like phases can spontaneously adopt a stable buckled geometry [39]. It is likely that buckling and defects have effects on the vibrational properties of the BCx phases. Similarly to the G peak, the position of the D peak was found to be independent of the B/C ratio (Fig. 6) when concentration of the boron increases from 0.1 (BC10) to 0.55 (BC1.8). The absence of the softening of the D mode with the increase of the boron concentration might be related to the fact that nature of the mode detected with green and infrared excitation is similar to that in graphite. Being attributed to structure in the vibrational density of states, the D band in graphite is a breathing mode of A1g symmetry and arises from the wave-vector selection-rule relaxation from finite-crystal-size effects [35]. The characteristic feature of the D band in graphite is that its intensity decreases with an increase of the frequency of excitation [9]. Similar behavior is exhibited by the D peak of the BCx phases measured in this paper: for BC1.8 the ratio of the peak areas between D and G peaks is zero at 244 nm, 3.0 at 532 nm, and 3.6 at 785 nm. Therefore, the absence of the D peak on the UV Raman spectra of the g-BCx phases and similarity in the behavior of the D peak with wavelength of the Raman scattering excitation indicate that it might have the same origin as in the case of graphite. The Raman active band predicted around 1350 cm− 1 in Ref. [31] is likely to be hidden by a broad D band in visible Raman spectra of g-BCx phases due to disordering in the g-BCx phase, and it is not excited by UV irradiation. The UV Raman spectra of g-BC2.1 and g-BC5.4 phases show an additional peak at approximately 1100 cm− 1 (Figs. 2a and 3a). A peak centered at approximately 1100 cm− 1, the so-called T peak, has been detected in UV Raman spectra of tetrahedral amorphous carbon (ta-C) [16,40], disordered, amorphous, and diamond-like carbon, and it was attributed to C–C sp3 vibrations [10]. We postulate that the peaks centered at 1089 cm− 1 for g-BC2.1 and at 1083 cm− 1 for g-BC5.4 in the UV Raman spectra are related to the presence of B4C in the g-BCx phases. Raman scattering of B4C has been measured conventionally
Fig. 7. The UV (244 nm) Raman spectrum of the B4C powder: the UV spectrum is an average over three spectra taken with the Renishaw system and 40× objective; integration time was 5 min; laser power on sample was 0.2 mW.
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[41] and using FT-Raman system [42], but it has not been studied with UV laser excitation. Fig. 7 shows the UV Raman spectrum of the B4C obtained from “Sigma-Aldrich, St. Louis, USA”. It has a strong but broad band centered at 1084 cm− 1 assigned to the breathing mode of icosahedrons in B4C [43]. A weak but relatively sharp peak at 723 cm− 1 can be assigned to the Eg mode of B4C vibration [43] and a broad band around 807 cm− 1 detected in the UV Raman spectrum of B4C (Fig. 7) can be assigned to the A1g mode that was detected in the infrared Raman spectrum of B4C at 797 cm− 1. There is also a weak and broad peak at 990 cm− 1 sitting on the shoulder of the 1084 cm− 1. It is related to the B4C chain rotating [43]. The peak at 1590 cm− 1 (Fig. 7) is due to some amount of amorphous carbon in the B4C powder [44]. Comparison between UV Raman spectra of B4C and BC2.1 demonstrates that main peaks (723, 803, 990 and 1084 cm− 1) of the UV Raman spectrum of B4C in the region of 600–1150 cm− 1 can be found in the UV spectrum of g-BC2.1 (717, 782, 1004 and 1092 cm− 1) indicating that that the g-BC2.1 phases contain some amount of B4C. It is also likely that a weak and broad peak centered at 1069 cm− 1 and detected in the visible (514 nm) Raman spectrum of the g-BC4 phase [30] was due to the presence of B4C in the graphitic phase. It was assigned to T mode characteristic to graphite in Ref. [30], however the T mode should appear only in UV excitation [10]. The area ratio I1098/IG for g-BC2.1 is approximately 50 times (area ratio 0.35) higher than that for the g-BC5.4 phase (0.007). It indicates that g-BC2.1 phases contain sufficient amounts of B4C and that the C/B ratio of the graphitic phase should be lower than 2.1. We do not know the exact concentration of the B4C in the g-BC2.1 phase, therefore we do not remove it from Fig. 6 since it does not change the trend showing therein. Detection of the B4C phase by UV Raman scattering and determination of its concentration is of importance for synthesis of the new diamond-like phases from g-BCx phases [2,3,45] as it requires the knowledge of the B/C ratio in the starting graphitic and synthesized diamond-like phases. With UV excitation, it is possible to obtain Raman spectra with a very low fluorescent background that does not substantially interfere with the collection of the vibrational spectrum of interest [46]. Therefore, the ratio I1089/IG of the Raman peaks could be used as a measure of the amount of the B4C in the g-BCx phase after an appropriate calibration. Calibration of intensities of g-BCx and B4C phases is beyond the scope of this work; however, our preliminary data indicate that the cross sections of the UV Raman scattering by g-BCx phases and B4C are of the same order. 5. Conclusions The Raman spectrum of the g-BCx phases excited with UV laser at 244 nm has one main peak (a G peak at approximately 1590 cm− 1), and does not have the D peak characteristic of disordered graphitic phases. The position of the G peak measured with resonant Raman scattering was found to be independent (within the experimental errors) of the B/C ratio when concentration of the boron increased from 0.1 (BC10) to 0.55 BC1.8. A peak around 1350 cm− 1 (D peak) can be detected in all g-BCx phases when green or infrared lasers are used for Raman scattering excitation. The D mode was predicted to be Raman active in g-BCx phases and should exhibit softening with increased boron concentration [31]. Similarly to the G peak, the position of the D peak in this study on several g-BCx phases is also independent of the B/C ratio. The absence of the softening of the D mode with the increase of the B/C ratio might be related to the fact that the mode detected with green and infrared excitation is similar in nature to that in graphite. The pattern of the peaks in the UV Raman spectra of g-BC2.1 phase indicates that the additional peaks centered at 1089 cm− 1 should be assigned to the Eg mode of B4C vibration rather than to the T mode characteristic to amorphous graphite. The possibility of detecting the B4C in BCx phases may provide a method for quantitative determina-
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