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The Raman Spectroscopy and XPS investigation of CVD diamond after fast-neutron irradiation
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The Raman Spectroscopy and XPS investigation of CVD diamond after fastneutron irradiation You-hang Liua, Jia-liang Zhanga, Fang-liang Wua, Ming Qia,*, Li-fu Heib, Fan-xiu Lvb a b
National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing, 210093, China University of Science and Technology Beijing, Beijing, 100083, China
ARTICLE INFO
ABSTRACT
Keywords: Fast-neutron irradiation Radiation-induced defects Raman spectroscopy X-ray photoelectron spectroscopy Artificial diamond
Artificial diamonds grown by the method of chemical vapor deposition (CVD) were investigated using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) after they were irradiated with fast neutrons up to a fluence of 3.3 × 1017 neutrons/cm2. Compared with the unirradiated one, the Raman spectra of irradiated diamonds showed a broadening and asymmetric Raman peak shape. In addition, there is a new peak at 1200 cm−1 in the irradiated-surface Raman spectra, which can be attributed to the satellite peak due to Fano interference. The XPS results showed that more ether bonds will be formed on the surface of irradiated diamonds after acid cleaning, and the introduced oxygen atoms also provide carriers for the Fano effect.
1. Introduction
2. Experiment
In recent years, diamond has been arousing great interest of researchers for its application in the harsh radiation environment due to its excellent thermal conduction, large resistivity, and especially strong radiation hardness. For example, the large hadron collider (LHC) planned to increase its luminosity to five times higher than before in the upcoming HL-LHC phase. At such high luminosity, present Forward Calorimeter (FCal) which is located near the collision point of ATLAS Detector may malfunction. The mini-Forward Calorimeter that uses artificial single crystal diamond as its sensitive material is potentially projected to mount between the FCal and collision point to alleviate this problem [1]. In order to investigate their performance in harsh radiation environment, numerous irradiation tests for various diamond detectors have been done yet [2–4]. However, despite the irradiation test of diamond detectors has a long history, there are still more ambiguities about the damage of energetic-particle irradiation on the diamond materials. In this work, we present the characterization results (by means of Raman spectroscopy and XPS) of fast-neutron irradiated and unirradiated diamond sensors. By comparing and analyzing the spectra of different samples, it could help us for better understanding the mechanism of damage caused by neutron irradiation. Moreover, the XPS spectra of neutron-irradiated diamond were reported for the first time to the best of our knowledge.
2.1. Sensor preparing
⁎
The single crystal diamond samples we used were grown on a commercially available high pressure high temperature (HPHT) synthetic type-Ib (100) single-crystal diamond plate via a 30-kW DC arc plasma jet chemical vapor deposition (CVD). The polycrystalline diamonds were grown by the same method but on a polycrystalline diamond substrate. After growth, the epitaxial diamond layers were separated by laser cutting and then mechanically polished. The freestanding polished diamonds were boiled in the acid mixture (20 % nitric acid; 80 % sulfuric acid) in order to remove any metal contaminants, and the surfaces of diamonds were terminated by oxygens after acid treatment. The purpose of fast-neutron irradiation was to investigate the radiation hardness and performance of diamond sensors under the analogous irradiation condition of HL-LHC, and the diamond samples must be metallized for the dynamic response signal readout. Meanwhile, a D.C. high voltage was loaded during the whole irradiation period. Prior to irradiation, composite metal films composed of Ti-W-Au were sputtered on the upper and lower surfaces of the free-standing diamond wafers after careful surface cleaning with alcohol and acetone followed by the annealing (Fig. 1). After metallization, the good ohmic contact was formed between the metal electrodes and diamond samples.
Corresponding author. E-mail address: [email protected] (M. Qi).
https://doi.org/10.1016/j.mtcomm.2019.100699 Received 25 May 2019; Accepted 13 October 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: You-hang Liu, et al., Materials Today Communications, https://doi.org/10.1016/j.mtcomm.2019.100699
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Fig. 1. (a) Schematic diagram of the synthetic diamond sensor and (b) the top view photograph of the test sensor.
Fig. 2. Schematic diagram of the irradiation facility at IBR-2: 1-Active core of IBR-2 reactor, 2-Water moderator, 3-Samples, 4-Metallic container to hold the samples, 5-The first biological shield, 6-Al section, 7-The second biological shield, 8-Cylinder with water as biological shield, 9-Elbowed pipe, 10-The dumper.
Fig. 4. The “survey” XPS of ird-PCD, ird-SCD, PCD, and SCD. (yellow line: irdPCD, blue line: ird-SCD, purple line: PCD, green line: SCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 3. The surface Raman spectra of ird-SCD and ird-PCD, which have been shifted down for clarity, with the raw cross-section Raman spectra of ird-SCD; the inset corresponding to the Raman spectra of ird-SCD and ird-PCD before irradiation. (yellow line: ird-PCD, blue line: ird-SCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 1 The elements and their concentration found in XPS on four sample surfaces. (N/ D = Not Detected).
2.2. Irradiation experiment The fast-neutron irradiation was carried out using IBR-2 reactor at Dubna, Russia. A schematic diagram of the irradiation facility of IBR-2 reactor is shown in Fig. 2. The diamond sensors were irradiated for more than 11 days and the total fluence reached 3.3 × 1017 neutrons/cm2, which is equivalent to the dose of ten-years operation of HL-LHC. The response of the diamond sensors was continuously measured during the whole irradiation. As there was still strong induced radioactivity on these sensors after irradiation experiment, the characterization of these samples was not taken until the radioactivity reduced to a harmless level more than two years later. 2
Element
Carbon
Oxygen
Nitrogen
Fluorine
SCD ird-SCD PCD ird-PCD
89.69 86.63 88.13 87.23
9.50 % 12.28 % 7.24 % 10.98 %
0.81 1.09 4.63 1.79
N/D N/D N/D Trace
% % % %
% % % %
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Fig. 5. The high-resolution C 1s (a), O 1s (b), and N 1s (c) XPS of the four samples after calibration. (green line: SCD, blue line: ird-SCD, yellow line: ird-PCD, purple line: PCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
wavenumber from 1000 to 1800 cm−1. The Full Wave at Half Maximum (FWHM) of the single-phonon peak is around 2 cm−1 for single crystal diamond while it is always a little larger for polycrystalline crystals. As shown in the inset of Fig. 3, the single-phonon peak of the artificial single crystal diamond is at 1334.05 cm−1 and the FWHM is 1.95 cm−1 approximately while the FWHM of the polycrystalline diamond is around 4.12 cm−1 before irradiation. The blue shift of the single-phonon peak can be attributed to the local stress in the lattice. The Raman spectra of irradiated single crystal diamond (ird-SCD, NTSCD-2-3#) and irradiated polycrystalline diamond (ird-PCD, NTPCD-2-1#), also shown in Fig. 3, exhibited a broadening and asymmetric single-phonon peak. There are two possible reasons for the asymmetry of Raman peaks except for the ambient parameters such as temperature, stress, etc. One possibility is the phonon confinement due to the low dimensionality of crystals [12]. The asymmetry of Raman peaks caused by phonon confinement always shows a wider half-width and a higher background at the lower wavenumber side of the peak. The cross-section Raman spectra of ird-SCD displayed a higher background below 1300 cm−1 as shown in Fig. 3, suggesting that the asymmetry of crosssection Raman peaks is mainly caused by phonon confinement. The other possible reason for the asymmetry of Raman peaks is Fano interference, which is mainly caused by the interference of continuum states and discrete levels lying in between the continuum [13,14] for doped semiconductors. Unlike the phonon confinement that causes nearly identical asymmetric peak shape, the p-type doping results in a wider half-width and a higher background at the larger wavenumber side of Raman peaks while the n-type doping results in a similar asymmetry with the phonon confinement. Moreover, the biggest difference between the asymmetry of Raman peaks caused by these two effects is that the Fano effect will lead to anti-resonance on the opposite side of the wider half-width side, and the satellite peak due to the antiresonance is always regarded as the signature of Fano interference [15–17]. Despite the similar asymmetric peak shape for surface and crosssection Raman spectra, the surface Raman spectra have their unique broad peaks at around 1200 and 1450 cm−1 wavenumbers. The shoulder peak at around 1400–1500 cm−1 was ascribed to microcrystalline graphite carbons on the irradiated surface [18–21], and we considered that the peak at 1200 cm−1 was due to the satellite peak caused by Fano effect. In other words, we think that the asymmetry of the cross-section single-phonon peak is mainly due to the phonon confinement and the asymmetry of the irradiated-surface peak is the result of the interaction between the phonon confinement and Fano interference. In addition to the asymmetric single-phonon Raman peak, an obvious sharp Raman peak can be observed at wavenumber of 1634 cm−1 for Raman spectra of irradiated diamonds. The structure causing the Raman peak near 1630 cm−1 was so-called “glassy carbons” for neutron-irradiated diamonds [7,10], and this structure can be attributed to the localized three-fold bonding [5,10,22] or the split interstitial defect
2.3. Demetallization The metal films must be removed for the characterization experiments after the irradiation. Different mixed acids were used to restore uncoated diamonds. In the first step, the diamond sensors were immersed in aqua regia to remove the outermost golden. Subsequently, ultrasound cleaning of diamond sensors with mixed acid (hydrogen fluoride with little nitric acid) was used to remove residual tungsten and titanium. The acids cleaning has little if any influence on the characterization of unirradiated diamonds as the acid-cleaned diamonds have the identical Raman spectra with the virgin one. 2.4. Characterization After removal of metal films, several irradiated diamonds were characterized by means of surface Raman spectra, cross-section Raman spectra and XPS separately. Other samples, a single crystal diamond (SCD) and a polycrystalline diamond (PCD) without any irradiation or metallization treatment, were also characterized by the Raman and XPS as a contrast. Raman spectroscopy is a widely used experimental method to study the damage caused by ion implantation [5,6] or neutron irradiation [7–10] in diamond materials because of its capacity of distinguishing the allotropes of carbon such as diamond and graphite. The surface and cross-section Raman spectra were obtained at approximately 300 K using a Renishaw RM2000 micro-Raman spectrometer fitted with laser wavelengths of 514 nm. As the thickness of the irradiated samples ranges from 220 to 250 μm, the cross-section Raman spectra of the irradiated samples were measured at a depth from 10 μm to 210 μm below the irradiated surface with the step of 40 μm. The cross section was obtained just before the characterization experiment by mechanical breaking instead of laser cutting to avoid local heating. XPS, as a surface characterization method, is widely used for the chemical state identification of one or more elements in materials. The usage of XPS in the analysis of diamond surfaces began at the 70 s of last century [11]. However, there are few reports on the analysis of irradiated diamond surfaces using XPS method. XPS analysis was performed on the sample surfaces with a PHI Quantera SXM using a monochromatic Al Kα X-ray radiation (hν=1486.6 eV). The software “Multipak” and no charge compensation method was used for obtaining the XPS spectra. The diameter of the X-ray beam was 200 μm and the incident angle is 45° with respect to the surface plane. “Survey” spectra with the scanning step of 1.0 eV were used for the element identification while the high-resolution spectra of C 1s, O 1s and N 1s with the step of 0.1 eV were used for the calculation of the detailed composition. 3. Results and discussion 3.1. Raman spectra Raman spectra of virgin diamonds are relatively simple and only show one strong single-phonon peak at 1332 cm−1 in the range of 3
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Fig. 6. The deconvolution of high-resolution (a) C 1s and (b) O 1s XPS of the four samples. (green line: SCD, blue line: ird-SCD, purple line: PCD, yellow line: irdPCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Table 2 The content and exact binding energy of deconvolution components for the C 1s and O 1s XPS of the four samples. The binding energy for the C4 component comes from the data of SCD and PCD, the binding energy of C4 component in ird-SCD is given in brackets. spectra
C 1s
C4/C3
components Binding energy (eV)
C1 287.3
C2 286.1
C3 285.2
SCD ird-SCD PCD ird-PCD
4.12 % 10.14% 5.4 % 10.68 %
6.34 % 18.98% 6.80 % 10.14 %
74.63 43.21 65.03 37.56
C4 284.2 (284.6) % % % %
14.91 27.68 22.73 41.62
% % % %
O 1s O1 533.2
0.20 0.64 0.35 1.11
64.06 44.27 56.46 20.97
O2/O1 O2 532.2
% % % %
35.94 55.73 43.54 79.03
% % % %
0.56 1.26 0.77 3.77
Fig. 7. The reduced diamond surface model before neutron irradiation, after neutron irradiation and after acid cleaning. (grey sphere: carbon atoms, red sphere: oxygen atoms, blue sphere: hydrogen atoms). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
[6]. The origin of the peak at around 1630 cm−1 has yet to be confirmed.
detailed assignment of these components is discussed later. The binding energies of the C1 and C2 components are 287.3 and 286.1 eV respectively as shown in Fig. 6(a). The C1 component has the largest FWHM in the four components (2.0 eV for SCD and PCD; 3.1 eV for irradiated diamonds). Considering the surfaces of diamonds terminated with oxygens after acid cleaning, we proposed that the C1 component could be regarded as the synthesis peak of some components such as carboxyl (CeOOH, 288.7 eV) [25], ketone radical (C]O, 287.3 eV) [23,25], hydroxyl radical (CeOeH, 285.7 eV) [26], and so on. It should be reminded that the surfaces of SCD and PCD were also terminated with oxygens as they were boiled in the acid mixture after growth. The C2 component may be attributed to ether bonds (CeOeC, 286.0 eV) [23]. The proportion of C1 and C2 components in the irradiated samples both increased as shown in Table 2, which is consistent with the increasing proportion of oxygen as listed in Table 1. The C3 components are dominant in all samples except ird-PCD. Hence, this component is ascribed to the pure diamond structure and was used for calibration with 285.2 eV, which is the standard value of sp3-bonded carbons [24,27–30]. In general, the binding energy difference between sp2 and sp3 components is 1.0 eV, the same with the difference between the C3 and C4 components in SCD and PCD. However, this difference decreases to 0.5 and 0.6 eV respectively in the two irradiated diamonds. The lower difference between sp3 and sp2 components has been reported in the XPS spectra of graphene oxide (0.7 eV) [31], nanodiamonds (0.7 eV) [26], diamond-like-carbons (0.6 eV) [32], and primitive heteroepitaxial synthetic diamonds (0.3 eV) [33]. Therefore, the C4 component was attributed to the sp2-bonded carbons and the binding energy shift of the C4 components may be caused by
3.2. XPS XPS was used for detecting the transformation of the surface chemical state and determining the possible formation of new elements on the diamond surfaces during fast-neutron irradiation. The “survey” spectra of the SCD, PCD, ird-SCD (NTSCD-2-3#), and ird-PCD (NTPCD2-1#) are given in Fig. 4 and the elements found are listed in Table 1. The quantitative analysis of concentration was determined by the respective peak area of high-resolution XPS and the elemental sensitivity factors. The high-resolution spectra of C 1s, O 1s, and N 1s are given in Fig. 5. Nitrogen and oxygen are elements that discovered on the surfaces of all four samples except for carbon. The trace fluorine that was only found in the ird-PCD may be ascribed to the chemical adsorption of fluorine on the surface during metal film removal. The above results showed that nuclear transmutation (but isotopes cannot be excluded) almost impossibly occurred on the diamond surface during fast-neutron irradiation below the fluence of 3.3 × 1017 neutrons/cm2. The deconvolution of the C 1s and O 1s high-resolution spectra is given in Fig. 6(a) and (b) respectively and the background subtraction was Shirley type. The residual noise after deconvolution was also given in Fig. 6 with red lines. The C 1s spectra were decomposed into four components (C1, C2, C3, C4, arrangement from the high binding energy to low) as done previously [23,24]. As taking into account the oxygenterminated surface of the diamonds and the irradiation effect, the 5
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irradiation-induced defects. The irradiated diamonds exhibited a larger proportion of the C4 component than SCD and PCD, and the proportion of C4 component even exceeded C3 in the ird-PCD. This result is consistent with the fact that polycrystalline samples are easier to suffer the irradiation damage. The O 1s spectra were decomposed into two components as shown in Fig. 6(b). The O2 component (532.2 eV) at lower binding energy may be ascribed to the ether bond [34] which is the favoured oxygen-terminated state on the (100) diamond surfaces [35,36]. The O1 component (533.2 eV) can be ascribed to the other oxygen terminal groups such as hydroxyl and carbonyl, which are also observed on the (100) surfaces [37]. For our irradiated diamond samples, the proportion of oxygen and the ratio of O2/O1 both have an obvious increase as shown in Tables 1 and 2 respectively. Hence, we put forward a proposal that after irradiation, the vacancies formed on the diamond surfaces will lead to the formation of more oxygen-terminated groups during the acid cleaning, especially ether bonds (considering the increased proportion of O2 component), as illustrated by the schematic diagram in Fig. 7. Meanwhile, the involvement of oxygen atoms could provide carriers for Fano interference. For polycrystalline diamond that is easier to suffer radiation damage, ird-PCD exhibits the highest O2 concentration and the most intensity Raman peak at 1200 cm−1 that we attribute to the satellite peak due to the Fano effect as discussed in the previous section, which could provide a referential proof for our proposal. Moreover, we realized that the newly formed ether bonds are slightly different from the chemical absorbed one because of the different lattice environment as shown in Fig. 7, which was supported by the broadening FWHM of O2 components in the irradiated samples.
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4. Conclusion The appearance of the asymmetric single-phonon peak and some new peaks in Raman spectra of neutron-irradiated diamonds is due to lattice distortion under the fast neutron irradiation. By comparing the surface and cross-section Raman spectra of ird-SCD, we proposed that the Raman peak at 1200 cm−1 appeared in the irradiated surface Raman spectra may be ascribed to the satellite peak due to Fano effect. The XPS results showed that the vacancies induced by fast-neutron irradiation would provide the “nuclei points” for the formation of ether bonds “CeOeC” on irradiated diamond surfaces. Furthermore, the introduction of oxygen atoms would cause the breakdown of selection rule in the Raman spectroscopy associated with a loss of the long-range periodicity of the diamond lattices and this effect could induce some new features in the Raman spectroscopy. Some other characterization experiments of these fast-neutron irradiated diamond samples, such as SEM, TEM, etc., and the corresponding dynamic response performance degradation are still under study. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the “International Science & Technology Cooperation Program of China” (Contract No. 2015DFG02100), The Ministry of Science and Technology of the People's Republic of China. References [1] J. Turner, Upgrade plans for ATLAS forward calorimetry for the HL-LHC, Phys. Procedia 37 (2012) 301–308, https://doi.org/10.1016/j.phpro.2012.02.376.
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P. Jiricek, I. Bieloshapka, Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods, J. Electron Spectros. Relat. Phenomena 195 (2014) 145–154, https://doi.org/10.1016/j.elspec.2014.07.003. J.F. Morar, F.J. Himpsel, G. Hollinger, J.L. Jordan, G. Hughes, F.R. McFeely, C 1 s excitation studies of diamond (111). I. Surface core levels, Phys. Rev. B 33 (1986) 1340–1345, https://doi.org/10.1103/PhysRevB.33.1340. F. Arezzo, E. Severini, N. Zacchetti, An XPS study of diamond films grown on differently pretreated silicon substrates, Surf. Interface Anal. 22 (1994) 218–223, https://doi.org/10.1002/sia.740220149. C. Fang, Y. Zhang, W. Shen, S. Sun, Z. Zhang, L. Xue, X. Jia, Synthesis and characterization of HPHT large single-crystal diamonds under the simultaneous influence of oxygen and hydrogen, CrystEngComm. 19 (2017) 5727–5734, https://doi. org/10.1039/c7ce01349c. N.W. Makau, T.E. Derry, Study of oxygen on the three low index diamond surfaces by XPS, Surf. Rev. Lett. 10 (2003) 295–301, https://doi.org/10.1142/ S0218625X03005189. M.J. Rutter, J. Robertson, Ab initio calculation of electron affinities of diamond surfaces, Comput. Mater. Sci. 10 (1998) 330–333, https://doi.org/10.1016/S09270256(97)00104-3. P.E. Pehrsson, T.W. Mercer, J.A. Chaney, Thermal oxidation of the hydrogenated diamond (100) surface, Surf. Sci. 497 (2002) 13–28, https://doi.org/10.1016/ S0039-6028(01)01677-6.
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The Raman Spectroscopy and XPS investigation of CVD diamond after fastneutron irradiation You-hang Liua, Jia-liang Zhanga, Fang-liang Wua, Ming Qia,*, Li-fu Heib, Fan-xiu Lvb a b
National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing, 210093, China University of Science and Technology Beijing, Beijing, 100083, China
ARTICLE INFO
ABSTRACT
Keywords: Fast-neutron irradiation Radiation-induced defects Raman spectroscopy X-ray photoelectron spectroscopy Artificial diamond
Artificial diamonds grown by the method of chemical vapor deposition (CVD) were investigated using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) after they were irradiated with fast neutrons up to a fluence of 3.3 × 1017 neutrons/cm2. Compared with the unirradiated one, the Raman spectra of irradiated diamonds showed a broadening and asymmetric Raman peak shape. In addition, there is a new peak at 1200 cm−1 in the irradiated-surface Raman spectra, which can be attributed to the satellite peak due to Fano interference. The XPS results showed that more ether bonds will be formed on the surface of irradiated diamonds after acid cleaning, and the introduced oxygen atoms also provide carriers for the Fano effect.
1. Introduction
2. Experiment
In recent years, diamond has been arousing great interest of researchers for its application in the harsh radiation environment due to its excellent thermal conduction, large resistivity, and especially strong radiation hardness. For example, the large hadron collider (LHC) planned to increase its luminosity to five times higher than before in the upcoming HL-LHC phase. At such high luminosity, present Forward Calorimeter (FCal) which is located near the collision point of ATLAS Detector may malfunction. The mini-Forward Calorimeter that uses artificial single crystal diamond as its sensitive material is potentially projected to mount between the FCal and collision point to alleviate this problem [1]. In order to investigate their performance in harsh radiation environment, numerous irradiation tests for various diamond detectors have been done yet [2–4]. However, despite the irradiation test of diamond detectors has a long history, there are still more ambiguities about the damage of energetic-particle irradiation on the diamond materials. In this work, we present the characterization results (by means of Raman spectroscopy and XPS) of fast-neutron irradiated and unirradiated diamond sensors. By comparing and analyzing the spectra of different samples, it could help us for better understanding the mechanism of damage caused by neutron irradiation. Moreover, the XPS spectra of neutron-irradiated diamond were reported for the first time to the best of our knowledge.
2.1. Sensor preparing
⁎
The single crystal diamond samples we used were grown on a commercially available high pressure high temperature (HPHT) synthetic type-Ib (100) single-crystal diamond plate via a 30-kW DC arc plasma jet chemical vapor deposition (CVD). The polycrystalline diamonds were grown by the same method but on a polycrystalline diamond substrate. After growth, the epitaxial diamond layers were separated by laser cutting and then mechanically polished. The freestanding polished diamonds were boiled in the acid mixture (20 % nitric acid; 80 % sulfuric acid) in order to remove any metal contaminants, and the surfaces of diamonds were terminated by oxygens after acid treatment. The purpose of fast-neutron irradiation was to investigate the radiation hardness and performance of diamond sensors under the analogous irradiation condition of HL-LHC, and the diamond samples must be metallized for the dynamic response signal readout. Meanwhile, a D.C. high voltage was loaded during the whole irradiation period. Prior to irradiation, composite metal films composed of Ti-W-Au were sputtered on the upper and lower surfaces of the free-standing diamond wafers after careful surface cleaning with alcohol and acetone followed by the annealing (Fig. 1). After metallization, the good ohmic contact was formed between the metal electrodes and diamond samples.
Corresponding author. E-mail address: [email protected] (M. Qi).
https://doi.org/10.1016/j.mtcomm.2019.100699 Received 25 May 2019; Accepted 13 October 2019 2352-4928/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: You-hang Liu, et al., Materials Today Communications, https://doi.org/10.1016/j.mtcomm.2019.100699
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Fig. 1. (a) Schematic diagram of the synthetic diamond sensor and (b) the top view photograph of the test sensor.
Fig. 2. Schematic diagram of the irradiation facility at IBR-2: 1-Active core of IBR-2 reactor, 2-Water moderator, 3-Samples, 4-Metallic container to hold the samples, 5-The first biological shield, 6-Al section, 7-The second biological shield, 8-Cylinder with water as biological shield, 9-Elbowed pipe, 10-The dumper.
Fig. 4. The “survey” XPS of ird-PCD, ird-SCD, PCD, and SCD. (yellow line: irdPCD, blue line: ird-SCD, purple line: PCD, green line: SCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 3. The surface Raman spectra of ird-SCD and ird-PCD, which have been shifted down for clarity, with the raw cross-section Raman spectra of ird-SCD; the inset corresponding to the Raman spectra of ird-SCD and ird-PCD before irradiation. (yellow line: ird-PCD, blue line: ird-SCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Table 1 The elements and their concentration found in XPS on four sample surfaces. (N/ D = Not Detected).
2.2. Irradiation experiment The fast-neutron irradiation was carried out using IBR-2 reactor at Dubna, Russia. A schematic diagram of the irradiation facility of IBR-2 reactor is shown in Fig. 2. The diamond sensors were irradiated for more than 11 days and the total fluence reached 3.3 × 1017 neutrons/cm2, which is equivalent to the dose of ten-years operation of HL-LHC. The response of the diamond sensors was continuously measured during the whole irradiation. As there was still strong induced radioactivity on these sensors after irradiation experiment, the characterization of these samples was not taken until the radioactivity reduced to a harmless level more than two years later. 2
Element
Carbon
Oxygen
Nitrogen
Fluorine
SCD ird-SCD PCD ird-PCD
89.69 86.63 88.13 87.23
9.50 % 12.28 % 7.24 % 10.98 %
0.81 1.09 4.63 1.79
N/D N/D N/D Trace
% % % %
% % % %
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Fig. 5. The high-resolution C 1s (a), O 1s (b), and N 1s (c) XPS of the four samples after calibration. (green line: SCD, blue line: ird-SCD, yellow line: ird-PCD, purple line: PCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
wavenumber from 1000 to 1800 cm−1. The Full Wave at Half Maximum (FWHM) of the single-phonon peak is around 2 cm−1 for single crystal diamond while it is always a little larger for polycrystalline crystals. As shown in the inset of Fig. 3, the single-phonon peak of the artificial single crystal diamond is at 1334.05 cm−1 and the FWHM is 1.95 cm−1 approximately while the FWHM of the polycrystalline diamond is around 4.12 cm−1 before irradiation. The blue shift of the single-phonon peak can be attributed to the local stress in the lattice. The Raman spectra of irradiated single crystal diamond (ird-SCD, NTSCD-2-3#) and irradiated polycrystalline diamond (ird-PCD, NTPCD-2-1#), also shown in Fig. 3, exhibited a broadening and asymmetric single-phonon peak. There are two possible reasons for the asymmetry of Raman peaks except for the ambient parameters such as temperature, stress, etc. One possibility is the phonon confinement due to the low dimensionality of crystals [12]. The asymmetry of Raman peaks caused by phonon confinement always shows a wider half-width and a higher background at the lower wavenumber side of the peak. The cross-section Raman spectra of ird-SCD displayed a higher background below 1300 cm−1 as shown in Fig. 3, suggesting that the asymmetry of crosssection Raman peaks is mainly caused by phonon confinement. The other possible reason for the asymmetry of Raman peaks is Fano interference, which is mainly caused by the interference of continuum states and discrete levels lying in between the continuum [13,14] for doped semiconductors. Unlike the phonon confinement that causes nearly identical asymmetric peak shape, the p-type doping results in a wider half-width and a higher background at the larger wavenumber side of Raman peaks while the n-type doping results in a similar asymmetry with the phonon confinement. Moreover, the biggest difference between the asymmetry of Raman peaks caused by these two effects is that the Fano effect will lead to anti-resonance on the opposite side of the wider half-width side, and the satellite peak due to the antiresonance is always regarded as the signature of Fano interference [15–17]. Despite the similar asymmetric peak shape for surface and crosssection Raman spectra, the surface Raman spectra have their unique broad peaks at around 1200 and 1450 cm−1 wavenumbers. The shoulder peak at around 1400–1500 cm−1 was ascribed to microcrystalline graphite carbons on the irradiated surface [18–21], and we considered that the peak at 1200 cm−1 was due to the satellite peak caused by Fano effect. In other words, we think that the asymmetry of the cross-section single-phonon peak is mainly due to the phonon confinement and the asymmetry of the irradiated-surface peak is the result of the interaction between the phonon confinement and Fano interference. In addition to the asymmetric single-phonon Raman peak, an obvious sharp Raman peak can be observed at wavenumber of 1634 cm−1 for Raman spectra of irradiated diamonds. The structure causing the Raman peak near 1630 cm−1 was so-called “glassy carbons” for neutron-irradiated diamonds [7,10], and this structure can be attributed to the localized three-fold bonding [5,10,22] or the split interstitial defect
2.3. Demetallization The metal films must be removed for the characterization experiments after the irradiation. Different mixed acids were used to restore uncoated diamonds. In the first step, the diamond sensors were immersed in aqua regia to remove the outermost golden. Subsequently, ultrasound cleaning of diamond sensors with mixed acid (hydrogen fluoride with little nitric acid) was used to remove residual tungsten and titanium. The acids cleaning has little if any influence on the characterization of unirradiated diamonds as the acid-cleaned diamonds have the identical Raman spectra with the virgin one. 2.4. Characterization After removal of metal films, several irradiated diamonds were characterized by means of surface Raman spectra, cross-section Raman spectra and XPS separately. Other samples, a single crystal diamond (SCD) and a polycrystalline diamond (PCD) without any irradiation or metallization treatment, were also characterized by the Raman and XPS as a contrast. Raman spectroscopy is a widely used experimental method to study the damage caused by ion implantation [5,6] or neutron irradiation [7–10] in diamond materials because of its capacity of distinguishing the allotropes of carbon such as diamond and graphite. The surface and cross-section Raman spectra were obtained at approximately 300 K using a Renishaw RM2000 micro-Raman spectrometer fitted with laser wavelengths of 514 nm. As the thickness of the irradiated samples ranges from 220 to 250 μm, the cross-section Raman spectra of the irradiated samples were measured at a depth from 10 μm to 210 μm below the irradiated surface with the step of 40 μm. The cross section was obtained just before the characterization experiment by mechanical breaking instead of laser cutting to avoid local heating. XPS, as a surface characterization method, is widely used for the chemical state identification of one or more elements in materials. The usage of XPS in the analysis of diamond surfaces began at the 70 s of last century [11]. However, there are few reports on the analysis of irradiated diamond surfaces using XPS method. XPS analysis was performed on the sample surfaces with a PHI Quantera SXM using a monochromatic Al Kα X-ray radiation (hν=1486.6 eV). The software “Multipak” and no charge compensation method was used for obtaining the XPS spectra. The diameter of the X-ray beam was 200 μm and the incident angle is 45° with respect to the surface plane. “Survey” spectra with the scanning step of 1.0 eV were used for the element identification while the high-resolution spectra of C 1s, O 1s and N 1s with the step of 0.1 eV were used for the calculation of the detailed composition. 3. Results and discussion 3.1. Raman spectra Raman spectra of virgin diamonds are relatively simple and only show one strong single-phonon peak at 1332 cm−1 in the range of 3
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Fig. 6. The deconvolution of high-resolution (a) C 1s and (b) O 1s XPS of the four samples. (green line: SCD, blue line: ird-SCD, purple line: PCD, yellow line: irdPCD). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Table 2 The content and exact binding energy of deconvolution components for the C 1s and O 1s XPS of the four samples. The binding energy for the C4 component comes from the data of SCD and PCD, the binding energy of C4 component in ird-SCD is given in brackets. spectra
C 1s
C4/C3
components Binding energy (eV)
C1 287.3
C2 286.1
C3 285.2
SCD ird-SCD PCD ird-PCD
4.12 % 10.14% 5.4 % 10.68 %
6.34 % 18.98% 6.80 % 10.14 %
74.63 43.21 65.03 37.56
C4 284.2 (284.6) % % % %
14.91 27.68 22.73 41.62
% % % %
O 1s O1 533.2
0.20 0.64 0.35 1.11
64.06 44.27 56.46 20.97
O2/O1 O2 532.2
% % % %
35.94 55.73 43.54 79.03
% % % %
0.56 1.26 0.77 3.77
Fig. 7. The reduced diamond surface model before neutron irradiation, after neutron irradiation and after acid cleaning. (grey sphere: carbon atoms, red sphere: oxygen atoms, blue sphere: hydrogen atoms). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
[6]. The origin of the peak at around 1630 cm−1 has yet to be confirmed.
detailed assignment of these components is discussed later. The binding energies of the C1 and C2 components are 287.3 and 286.1 eV respectively as shown in Fig. 6(a). The C1 component has the largest FWHM in the four components (2.0 eV for SCD and PCD; 3.1 eV for irradiated diamonds). Considering the surfaces of diamonds terminated with oxygens after acid cleaning, we proposed that the C1 component could be regarded as the synthesis peak of some components such as carboxyl (CeOOH, 288.7 eV) [25], ketone radical (C]O, 287.3 eV) [23,25], hydroxyl radical (CeOeH, 285.7 eV) [26], and so on. It should be reminded that the surfaces of SCD and PCD were also terminated with oxygens as they were boiled in the acid mixture after growth. The C2 component may be attributed to ether bonds (CeOeC, 286.0 eV) [23]. The proportion of C1 and C2 components in the irradiated samples both increased as shown in Table 2, which is consistent with the increasing proportion of oxygen as listed in Table 1. The C3 components are dominant in all samples except ird-PCD. Hence, this component is ascribed to the pure diamond structure and was used for calibration with 285.2 eV, which is the standard value of sp3-bonded carbons [24,27–30]. In general, the binding energy difference between sp2 and sp3 components is 1.0 eV, the same with the difference between the C3 and C4 components in SCD and PCD. However, this difference decreases to 0.5 and 0.6 eV respectively in the two irradiated diamonds. The lower difference between sp3 and sp2 components has been reported in the XPS spectra of graphene oxide (0.7 eV) [31], nanodiamonds (0.7 eV) [26], diamond-like-carbons (0.6 eV) [32], and primitive heteroepitaxial synthetic diamonds (0.3 eV) [33]. Therefore, the C4 component was attributed to the sp2-bonded carbons and the binding energy shift of the C4 components may be caused by
3.2. XPS XPS was used for detecting the transformation of the surface chemical state and determining the possible formation of new elements on the diamond surfaces during fast-neutron irradiation. The “survey” spectra of the SCD, PCD, ird-SCD (NTSCD-2-3#), and ird-PCD (NTPCD2-1#) are given in Fig. 4 and the elements found are listed in Table 1. The quantitative analysis of concentration was determined by the respective peak area of high-resolution XPS and the elemental sensitivity factors. The high-resolution spectra of C 1s, O 1s, and N 1s are given in Fig. 5. Nitrogen and oxygen are elements that discovered on the surfaces of all four samples except for carbon. The trace fluorine that was only found in the ird-PCD may be ascribed to the chemical adsorption of fluorine on the surface during metal film removal. The above results showed that nuclear transmutation (but isotopes cannot be excluded) almost impossibly occurred on the diamond surface during fast-neutron irradiation below the fluence of 3.3 × 1017 neutrons/cm2. The deconvolution of the C 1s and O 1s high-resolution spectra is given in Fig. 6(a) and (b) respectively and the background subtraction was Shirley type. The residual noise after deconvolution was also given in Fig. 6 with red lines. The C 1s spectra were decomposed into four components (C1, C2, C3, C4, arrangement from the high binding energy to low) as done previously [23,24]. As taking into account the oxygenterminated surface of the diamonds and the irradiation effect, the 5
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irradiation-induced defects. The irradiated diamonds exhibited a larger proportion of the C4 component than SCD and PCD, and the proportion of C4 component even exceeded C3 in the ird-PCD. This result is consistent with the fact that polycrystalline samples are easier to suffer the irradiation damage. The O 1s spectra were decomposed into two components as shown in Fig. 6(b). The O2 component (532.2 eV) at lower binding energy may be ascribed to the ether bond [34] which is the favoured oxygen-terminated state on the (100) diamond surfaces [35,36]. The O1 component (533.2 eV) can be ascribed to the other oxygen terminal groups such as hydroxyl and carbonyl, which are also observed on the (100) surfaces [37]. For our irradiated diamond samples, the proportion of oxygen and the ratio of O2/O1 both have an obvious increase as shown in Tables 1 and 2 respectively. Hence, we put forward a proposal that after irradiation, the vacancies formed on the diamond surfaces will lead to the formation of more oxygen-terminated groups during the acid cleaning, especially ether bonds (considering the increased proportion of O2 component), as illustrated by the schematic diagram in Fig. 7. Meanwhile, the involvement of oxygen atoms could provide carriers for Fano interference. For polycrystalline diamond that is easier to suffer radiation damage, ird-PCD exhibits the highest O2 concentration and the most intensity Raman peak at 1200 cm−1 that we attribute to the satellite peak due to the Fano effect as discussed in the previous section, which could provide a referential proof for our proposal. Moreover, we realized that the newly formed ether bonds are slightly different from the chemical absorbed one because of the different lattice environment as shown in Fig. 7, which was supported by the broadening FWHM of O2 components in the irradiated samples.
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4. Conclusion The appearance of the asymmetric single-phonon peak and some new peaks in Raman spectra of neutron-irradiated diamonds is due to lattice distortion under the fast neutron irradiation. By comparing the surface and cross-section Raman spectra of ird-SCD, we proposed that the Raman peak at 1200 cm−1 appeared in the irradiated surface Raman spectra may be ascribed to the satellite peak due to Fano effect. The XPS results showed that the vacancies induced by fast-neutron irradiation would provide the “nuclei points” for the formation of ether bonds “CeOeC” on irradiated diamond surfaces. Furthermore, the introduction of oxygen atoms would cause the breakdown of selection rule in the Raman spectroscopy associated with a loss of the long-range periodicity of the diamond lattices and this effect could induce some new features in the Raman spectroscopy. Some other characterization experiments of these fast-neutron irradiated diamond samples, such as SEM, TEM, etc., and the corresponding dynamic response performance degradation are still under study. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the “International Science & Technology Cooperation Program of China” (Contract No. 2015DFG02100), The Ministry of Science and Technology of the People's Republic of China. References [1] J. Turner, Upgrade plans for ATLAS forward calorimetry for the HL-LHC, Phys. Procedia 37 (2012) 301–308, https://doi.org/10.1016/j.phpro.2012.02.376.
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[26] [27] [28]
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