- Email: [email protected]
From a-C to nanographene by chemical nano-engineering
Accepted Manuscript From a-C to Nanographene by Chemical Nano-Engineering
Shilpa Ramakrishna, Rajiv O. Dusane PII:
S0254-0584(18)30280-3
DOI:
10.1016/j.matchemphys.2018.04.018
Reference:
MAC 20513
To appear in:
Materials Chemistry and Physics
Received Date:
26 December 2017
Revised Date:
28 March 2018
Accepted Date:
05 April 2018
Please cite this article as: Shilpa Ramakrishna, Rajiv O. Dusane, From a-C to Nanographene by Chemical Nano-Engineering, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys. 2018.04.018
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ACCEPTED MANUSCRIPT
From a-C to Nanographene by Chemical NanoEngineering Shilpa Ramakrishnaa‡, Rajiv O. Dusanea*‡ aSemiconductor
Thin Films and Plasma Processing Laboratory, Department of Metallurgical
Engineering and Materials Science, Indian Institute of Technology Bombay (IITB), Powai, Mumbai, India 400076
Highlights
Effect of post deposition atomic hydrogen treatment of a-C thin films on copper surface for different time duration has been brought out.
It is observed that the atomic hydrogen treatment at 600°C transforms the a-C film to a significant density of nanographene domains.
These findings open up an interesting pathway to fabricate graphene at low temperature.
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ABSTRACT .
Interaction of atomic hydrogen with amorphous carbon (a-C) grown on Cu substrate has been explored for the first time. Here we report the investigations performed at 600°C substrate temperature. This research finds its significance in understanding the role of atomic hydrogen in establishing the growth of graphene at low substrate temperatures. After exposing the a-C film to atomic hydrogen for various durations, we observe that atomic hydrogen reacts with the a-C film on Cu surface leading to the formation of nanographene. With the increase in exposure time, graphene nuclei grow in number and dimension, forming a polycrystalline network of nanographene domains. High resolution transmission electron microscope images reveal this structural transformation, which has also been substantiated by Raman spectra. The chemical compositional analysis is performed using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Based on these observations, we explain the probable mechanism which elucidates the role of atomic hydrogen in transforming a-C into sp2 hybridized hexagonal structures in the presence of Cu at 600°C.
KEYWORDS: Hot wire chemical vapor deposition, nanographene, atomic hydrogen, low temperature growth, growth mechanism. 1. Introduction Graphene is most commonly grown on Cu by chemical vapor deposition (CVD) using CH4 as the precursor at around 1000°C [1-7] or higher [8, 9]. And also by other CVD techniques like plasma enhanced CVD and hot wire chemical vapor deposition (HWCVD) at around
900°C
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[10] and 1000°C [11, 12] respectively. Several approaches have been employed to bring down the growth temperature of graphene by these techniques.[13-19] Understanding the role of atomic hydrogen during the carbon film deposition plays a crucial role in the chemistry of nucleation and growth of graphene, diamond-like carbon, polymeric films, and diamond films. Such a study also enables us to tailor the properties of carbon films for various applications. The atomic hydrogen interaction during the growth controls the proportion of sp2 and sp3 bonded carbon contents in the film. This in turn decides the various physical and chemical properties of the diamond-like carbon film.[20, 21] However there are reports of post deposition treatment of the a-C film with atomic hydrogen leading to various modifications in the film structure.[22, 23] Also atomic hydrogen induced chemical erosion of a-C film has been reported.[22-25] In this paper, we report the results of our investigations on the atomic hydrogen induced structural evolution of a-C film deposited on Cu at 600°C using hot wire chemical vapor process (HWCVP), which leads to growth of nanographene. Such a study remains unexplored and we expect our approach contributes significantly towards an understanding of the mechanism and feasibility of nucleation and growth of graphene on Cu at reduced substrate temperature. This work could open up the opportunities to further explore the chemistry of hot wire generated atomic hydrogen, in order to bring down the growth temperature of graphene and thus lowering the thermal budget of its production. We have observed the changes in the film microstructure using transmission electron microscope (TEM) and Raman spectroscopy for different time durations of atomic hydrogen treatment. The chemical compositional changes are observed via X-ray photoelectron
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spectroscopy
(XPS),
Fourier
transform
infrared
spectroscopy
(FTIR)
supported
by
Photoluminescence (PL) study. 2. Materials and Methods 2.1 Sample Preparation We have performed all our experiments using an indigenously designed HWCVD system. HWCVD is a cost-effective growth technique which allows the deposition over a range of substrate temperatures.[26, 27] This is also a decoupled CVD system, wherein the gas dissociation takes place at the filament surface while the deposition occurs on the substrates located 5-7 cm away from the filament. The filament is made up of tungsten and is subject to joule heating. We have maintained a filament temperature of around 2000°C in all our experiments and the temperature is measured using a two colour optical pyrometer. In the present case the substrates are placed at a distance of 3 cm from the filament. The chamber is evacuated initially to a base pressure of 3×10-6 mbar and the substrate is heated to 600°C which is measured using a thermocouple. Copper foils (99.98% trace metal basis, Sigma Aldrich) of 0.25 mm thickness are used as the substrate for growth. The foils are mechanically polished and cleaned via ultra-sonication in acetone followed by DI water, isopropanol and finally with acetic acid to reduce any oxide present on the surface. Copper foils are annealed at 600°C in hydrogen atmosphere for 1 h. Then 5 sccm of methane is inlet into the chamber while the tungsten filament is heated to 2000°C. The dissociated reactive carbon species deposit on the annealed copper surface at 600°C. The deposition is performed for a very short duration of 60 s at 3×10-4 mbar pressure. After the film is deposited, 100 sccm of hydrogen gas is introduced into the chamber at a pressure of 5 Torr. The film was then treated with atomic hydrogen generated by the filament
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heated to 2000°C for 30 min, 60 min and 120 min respectively. The time scale of deposition and atomic hydrogen treatment is shown pictorially in Figure 1.
Figure 1. The a-C growth and exposure of as-grown film to atomic hydrogen represented on a time scale. 2.2 Transfer Method In order to examine the as grown film before and after atomic hydrogen exposure under TEM, the samples are transferred onto TEM grid by PMMA transfer method similar to the procedure mentioned in the ref [28]. The as-grown carbon film on Cu foil is spin-coated with PMMA solution and then cured at 120°C for 2 min. The opposite side of the copper foil was polished to remove the carbon film. The 0.5 × 0.5 cm2 × 250 μm thick Cu substrate is then etched away by 1 M FeCl3 solution in 3M HCl for over a period of ∼18 h. The PMMA/carbon film stack is cleaned with deionized water and placed on the required substrate and dried. A small amount of PMMA solution is coated onto the PMMA/graphene to dissolve the precoated PMMA, then the PMMA is slowly cured at room temperature and dissolved by acetone.
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3 Characterization Details The structural evolution of the film subsequent to atomic hydrogen treatment at 600°C was determined using high resolution transmission electron microscope operated at 200 kV ( JEOL JEM 2100F). Raman spectroscopy measurements were performed using 532 nm laser at 1.5 mW power (Witec Alpha300 RS). The chemical compositional analysis was determined by X ray photoelectron spectroscopy using Al Kα radiation (energy of 1486.6 eV; Kratos Analytical, Axis Supra). The peaks were deconvoluted using ESCApe software. The optical absorption measurements were performed using double beam absorption spectrometer at 2 nm resolution (Jasco
V-530).
The
photoluminescence
measurements
were
recorded
on
Jasco
spectrofluorometer (FP-8500). A Bruker Hyperion 3000 FTIR spectrometer equipped with Ge crystal was used for the analysis in attenuated total reflectance (ATR) mode. 4 Results and Discussion Figure 2a shows the HRTEM image of the as-grown carbon film before atomic hydrogen exposure. There are no lattice fringes observed in the image, the film remains to be amorphous in nature. Figure 2b shows the magnified image of the as-grown film after 30 min of annealing in vacuum at a low pressure of 3×10-6 mbar at 600°C. Even after annealing for 30 min, there is no observable change in the film microstructure and the film remains amorphous in nature. Figures 2c-d show the HRTEM image of the as-grown a-C film after 30 min and 60 min of atomic hydrogen exposure respectively. The lattice fringes visible in the image after 30 min of exposure, confirm the crystallization of a-C film. The inset shows the Fast Fourier transform (FFT) of the HRTEM image and the spots have a hexagonal symmetry and their distance from the center measure an interplanar spacing of 0.20 nm. After 60 min of atomic hydrogen exposure, the a-C film shows the lattice fringes. The crystallization of a-C film occurs simultaneously at several
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places on the substrate with increase in the diameter of crystallites. The inset shows the FFT of the region marked within the blue box which shows a hexagonal symmetry. The image obtained after inverse FFT is shown in Figure 2e.[29] The image identifies a sequence of the hexagonal structure with atoms appearing in the brighter contrast (Figure 2e) and the corresponding lattice parameter is measured to be 0.234 nm. This value is close to the lattice parameter of exfoliated graphene, a = 0.246 nm (Figure 2f). The small difference in the measured value is due to the resolution limit of the microscope. This confirms the transformation of a-C to nanographene domains. The number density of nanographene domains increases significantly as compared to 30 min exposure. Figure 2g shows the HRTEM image after 120 min of atomic hydrogen exposure. Formation of a polycrystalline structure with increased domain size as compared to 60 min exposure is observed and the lattice fringes marked in the image denote an inter-planar spacing of 0.21 nm which corresponds to the distance between the {10-10} set of lattice planes of graphene. The FFT of the image shows a ring pattern corresponding to a polycrystalline network. The film morphology consists of patches of thinner regions appearing lighter; this could be due to the removal of material attributed to the chemical erosion[25] of the amorphous component by atomic hydrogen, which becomes noticeable with increased exposure duration. Hence on exposure to hot wire generated atomic hydrogen the a-C undergoes crystallization leading to formation of nanographene which grows in size and number density with increase in time of exposure.
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Figure 2. (a) HRTEM image of HWCVD grown carbon film showing amorphous structure before atomic hydrogen exposure. (b) HRTEM image of HWCVD grown a-C film after 30min of annealing in vacuum. (c-d) HRTEM images showing crystallization of HWCVD grown a-C film after 30 min and 60 min of atomic hydrogen exposure respectively and the insets show the respective FFT. (e) Inverse FFT of the region marked within yellow box. The image shows hexagonal crystal structure with lattice parameter 0.234 nm close to the lattice parameter of exfoliated graphene crystal structure (f), 0.246 nm. (g) HRTEM image after 120 min of atomic hydrogen exposure. Further in order to analyze the thickness of a-C film and to estimate the number of layers of graphene formed after 30 min of atomic hydrogen exposure, we have performed cross-sectional TEM characterization. Figure 3a shows the cross-sectional TEM image of amorphous carbon on copper and figure 3b is a higher resolution image of a portion of the film identified by the square.
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The lamella of sample for the cross-sectional TEM imaging was prepared by focused ion beam (FIB) milling. The images show a variation in thickness from approximately 6.2 - 8.3 nm. The variation in thickness could be due to uneven copper surface. Figure 3c-d show the crosssectional TEM images of the amorphous carbon film after 30 min of exposure to atomic hydrogen. Before FIB milling the nanographene film grown on copper is covered by a layer of aluminum and platinum in order to avoid ion beam damage as can be seen in figure 3c which also shows that the nanographene is multilayered. Figure 3d shows the presence of 8 layers of graphene with an interlayer distance of 0.335nm. The lateral dimensions are around 6 nm and 4 nm. It is to be noted that the film contains many such domains with different dimensions.
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Figure 3. (a) Cross sectional TEM image of amorphous carbon on Copper. (b) Magnified image of amorphous carbon film. (c) Cross sectional TEM image of the nanographene on Copper. (d) Magnified image of the nanographene showing the presence of multilayers The Raman spectra of HWCVD a-C before (black curve) and after atomic hydrogen exposure for 30 min (green curve), 60 min (red curve) and 120 min (blue curve) respectively are shown in Figure 4a. The common characteristic Raman features of graphitic materials are G- band ( 1581-
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1600 cm-1), [30] which arises due to the relative motion of sp2 bonded carbon atoms and the Dband at around ̴1350 cm-1 , arises due to breathing mode vibrations of the aromatic ring [30, 31]. In case of graphene, a vacancy like defect leads to activation of D-mode.[32] Therefore in case of a-C, ID/IG ratio gives information related to graphene-like cluster formation. In this case, we observe that with an increase in the atomic hydrogen treatment time to 60 min, there is an increase in ID/IG (4b). This is due to the formation of nanographene domains which contribute to the breathing mode vibration. However, beyond 60 min of atomic hydrogen treatment, there is a decrease in ID/IG ratio (Figure 4b). This is due to larger crystallite size in the polycrystalline structure. As we move slightly away from the nanocrystalline regime towards larger crystalline domain size ID/IG ratio decreases. This is in agreement with the amorphization trajectory proposed by A. C. Ferrari et al.[33] which shows that it is difficult to quantify the film quality on the basis of the ID/IG ratio alone as the ratio decreases both when the film is in the amorphous regime and crystalline regime. Figure 4b shows the variation of G-band FWHM and ID/IG ratio with the time of exposure to atomic hydrogen. The FWHM of the G-band shows a monotonic decrease with the atomic hydrogen exposure time due to the larger size of the graphene domains. The reports from A. C. Ferrari et al.[33, 34] correlate the increase of grain size with the decrease in FWHM of the G-peak. These results confirm the atomic hydrogen induced transformation of a-C to nanocrystalline film.
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Figure 4. (a) Raman spectra of HWCVD grown a-C on Cu foil (black), HWCVD a-C on Cu foil after 30 min (green), 60 min (red) and 120 min (blue) of atomic hydrogen exposure. (b) Variation of the ratio of ID/IG (blue) and FWHM of G-band (black) with an increase in time of exposure of a-C film to atomic hydrogen. The XPS measurements are performed to examine the chemical composition of HWCVD grown a-C on Cu, before and after hot wire generated atomic hydrogen exposure. Figure 5a shows XPS survey scan peaks corresponding to C1s at 284.4 eV and O1s at 532 eV. The C:O atomic ratios before and after 120 min of atomic hydrogen are 2.54 and 4.16 respectively. The deconvolution of the XPS high resolution scan (Figure 5b-c) for C 1s shows peaks corresponding to C=C at 284.4 eV, C-OH and C=O peaks centered at 285.7 eV and 288.1 eV respectively.[11, 35, 36] Before atomic hydrogen exposure, the a-C film contain 80.34 ± 1.98% C=C, 10.25 ± 1.45% C=O and 9.41 ± 1.64% C-OH. After 120 min of atomic hydrogen exposure, there is a reduction in oxygen content existing in the film. The C=O and C-OH content reduces to 6.58 ±
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0.79% and 1.65 ± 2.1% respectively. Besides C=C content increases from 80.34 ± 1.98% to 91.77 ± 1.74%. This confirms the reduction in sp3 type defects[32] present due to oxygen functional groups and increase in sp2 hybridized carbon atoms due to atomic hydrogen interaction at 600°C. The FTIR spectra of HWCVD deposited a-C film before atomic hydrogen exposure and after 120 min of exposure to atomic hydrogen is shown in Figure 5d. The FTIR measurements are performed in attenuated total reflectance mode (ATR). A strong and broad peak corresponding to O-H stretching vibration band around 3300 – 3400 cm-1 is observed to decrease in intensity after 120 min of atomic hydrogen exposure.[37, 38] And also the bands at around 1070 cm-1 and 1130 cm-1 corresponding to O-C-O stretching and wagging respectively from epoxy groups[37, 38] are also observed to reduce. Hence the atomic hydrogen reduces the oxygen functional groups present. The peaks at around
840 cm-1,
1600 cm-1,
1650 cm-1,
assigned to C-H bending in disubstituted aromatic ring, aromatic and olefinic C=C stretching respectively[37-39] are observed to increase in intensity. However, the peak intensity at 2922 cm-1 corresponding to C-H asymmetric stretching[38] vibration has significantly minimized after atomic hydrogen exposure for 120 min. Both the XPS and FTIR spectra confirm that atomic hydrogen effectively reduces oxygen functional groups present in the film and leads to increment in C=C group.
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Figure 5. (a) XPS survey scan for HWCVD a-C on Cu before and after 120 min of atomic hydrogen exposure respectively. (b) XPS high resolution scan for C 1s core level peak before atomic hydrogen exposure. (c) XPS scan for C 1s core level peak after 120 min of atomic hydrogen exposure. The peaks are deconvoluted into its components. (d) FTIR spectra of HWCVD deposited a-C film before atomic hydrogen exposure and after 120 min of atomic hydrogen exposure.
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The optical absorption and photoluminescence (PL) spectra of the as-grown film and after atomic hydrogen exposure were measured. Figures 6a-b show the UV–vis absorption spectra of the as-grown film and after 120 min of atomic hydrogen exposure respectively. In both the cases, the film exhibits a strong absorbance in the deep UV range around 230 nm corresponding to π– π* transition of aromatic C=C bonds and a broad weak shoulder around 280 nm which extends up to 360 nm. This has been previously attributed to defect states arising from oxygen functional groups present in the film.[40] The PL excitation spectra were acquired at wavelength 342 nm for both the samples as shown in Figure 6a-b. It shows pronounced excitation band at around 230 nm and a weaker band at 280 nm respectively which is consistent with the overlapping absorption spectra. The PL emission spectra at 230 nm excitation wavelength, before and after 120 min of atomic hydrogen exposure are shown in Figure 6a-b. The PL emission spectra are deconvoluted into two Gaussian peaks identified as Ip1 and Ip2 as shown in figure 6c and 6d. The peak Ip2 is centered at 340 nm and Ip1 is centered at around 420 nm. In the case of GO, PL peak appearing at around 354 nm (Ip2) corresponds to the π-π* transitions [41]. In our case, the peak before atomic hydrogen exposure corresponding to π-π* transitions is at 338 nm and after 120 min of atomic hydrogen the peak shifts to 344 nm. This could be due to increase in the size of graphene domain with atomic hydrogen exposure. The broad peak between 350 nm and 500 nm labeled as Ip1 corresponds to the transition states induced by oxygen functional groups. With the reduction of oxygen impurities, the relative intensity of the peak Ip1 decreases, as the optical transitions arising due to oxygen states decrease due to the reduction by atomic hydrogen.[42] The simultaneous increase in the Ip1 strongly correlates with the percentage increase in C=C content as observed in XPS results and also with the increase in graphene domain size as observed in the HRTEM images in Figure 2c, 2d, and 2h respectively.
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Figure 6. (a) UV-visible absorption, PL excitation (measurement wavelength-342 nm) and PL emission spectra (excitation wavelength 230 nm) of HWCVD a-C film before atomic hydrogen exposure and after 120 min of atomic hydrogen exposure (b). (c) PL emission spectrum (excitation wavelength 230 nm) of HWCVD a-C film acquired before atomic hydrogen exposure and after 120 min atomic hydrogen exposure (d) deconvoluted into two peaks labeled as Ip1 and Ip2. Based on FTIR observations and the kinetic analysis of hydrocarbon erosion by atomic hydrogen as reported by A. Horn et al.[25], we discuss the probable mechanism of atomic hydrogen induced transformation of a-C into nanographene domains on Cu surface at 600°C. As seen in Figure 4d, the peak at about 2922 cm-1 corresponds to C‒H asymmetric stretching
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vibration from CH2 and CH, and the peak at about 2852 cm-1 corresponds to C‒H asymmetric stretching vibration from CH2,[37, 38] hydrogen is bonded to sp3 hybridized carbon. These signatures corresponding to C-H group significantly reduce with atomic hydrogen exposure. Consequently, there is an increase in peak intensity corresponding to aromatic and olefinic (C=C) stretching at around ̴1600 cm-1 and ̴1650 cm-1 respectively.[37, 38] The reaction with atomic hydrogen leads to breaking of C-H bond in sp3 bonded carbon atoms leading to dehydrogenation. The subsequent redistribution of electrons leads to sp2 hybridized carbon and they undergo aromatization in the presence of Cu at 600°C leading to the formation of nanographene. This reaction is accompanied by the erosion of hydrocarbon in the form of free radicals of methane or methyl as proposed by A. Horn et al.[25] The other plausible dehydrogenation mechanism could be thermal decomposition via hydrocarbon split off.[25] In our case, we rule out this possibility as, after 30 min of annealing at 600°C, the film remains amorphous in nature as shown in the TEM image (Figure 2b). Only on exposure to atomic hydrogen for 30 min, the formation of graphene crystallites are visible, which are clusters of sp2 bonded aromatic structures. We conclude the graphene nucleation to be a result of formation of sp2 carbon species due to atomic hydrogen induced dehydrogenation reaction. This analysis is strongly supported by FTIR data. With a prolonged time of exposure to atomic hydrogen, a significant amount of filmforming sp2 carbon species (C=C) are available to undergo aromatization and we see an increase in size of graphene crystallites (as observed in HRTEM image Figure 2d). Thus for the first time, we demonstrate the atomic hydrogen induced transformation of the a-C film via defect healing to nanocrystalline graphene at 600°C.
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AUTHOR INFORMATION Corresponding Author * email-id: [email protected]; phone (o): *+912225767633. Present Addresses Semiconductor thin films and Plasma Processing Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, India -400076
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.
ACKNOWLEDGMENT We acknowledge DST-FIST, at ME&MS, IIT Bombay for the FIB/SEM tool enabling the TEM sample preparation. We also thank SAIF/CRNTS for the TEM, FTIR characterization facilities. S.RK acknowledges financial support from UGC, Gov. of India. S.RK would like to thank Dr. V. Pandey for his jelp in commissioning the HWCVD setup and also would like to further thank him and R. Chulliyil for useful discussion. S.RK would also like to thank M. Kumawat, A. Pant, L. Gurnani for TEM characterization and A. Kumar for TEM sample preparation.
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ABBREVIATIONS HWCVD, Hot wire chemical vapor deposition; PL, Photoluminescence; HRTEM, High resolution transmission electron microscope; FTIR, Fourier transform infrared spectroscopy; XPS, X-ray photoelectron spectroscopy; FFT, Fast Fourier Transform.
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For Table of Contents Only/ Graphical Abstract
25
Shilpa Ramakrishna, Rajiv O. Dusane PII:
S0254-0584(18)30280-3
DOI:
10.1016/j.matchemphys.2018.04.018
Reference:
MAC 20513
To appear in:
Materials Chemistry and Physics
Received Date:
26 December 2017
Revised Date:
28 March 2018
Accepted Date:
05 April 2018
Please cite this article as: Shilpa Ramakrishna, Rajiv O. Dusane, From a-C to Nanographene by Chemical Nano-Engineering, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys. 2018.04.018
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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From a-C to Nanographene by Chemical NanoEngineering Shilpa Ramakrishnaa‡, Rajiv O. Dusanea*‡ aSemiconductor
Thin Films and Plasma Processing Laboratory, Department of Metallurgical
Engineering and Materials Science, Indian Institute of Technology Bombay (IITB), Powai, Mumbai, India 400076
Highlights
Effect of post deposition atomic hydrogen treatment of a-C thin films on copper surface for different time duration has been brought out.
It is observed that the atomic hydrogen treatment at 600°C transforms the a-C film to a significant density of nanographene domains.
These findings open up an interesting pathway to fabricate graphene at low temperature.
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ABSTRACT .
Interaction of atomic hydrogen with amorphous carbon (a-C) grown on Cu substrate has been explored for the first time. Here we report the investigations performed at 600°C substrate temperature. This research finds its significance in understanding the role of atomic hydrogen in establishing the growth of graphene at low substrate temperatures. After exposing the a-C film to atomic hydrogen for various durations, we observe that atomic hydrogen reacts with the a-C film on Cu surface leading to the formation of nanographene. With the increase in exposure time, graphene nuclei grow in number and dimension, forming a polycrystalline network of nanographene domains. High resolution transmission electron microscope images reveal this structural transformation, which has also been substantiated by Raman spectra. The chemical compositional analysis is performed using X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Based on these observations, we explain the probable mechanism which elucidates the role of atomic hydrogen in transforming a-C into sp2 hybridized hexagonal structures in the presence of Cu at 600°C.
KEYWORDS: Hot wire chemical vapor deposition, nanographene, atomic hydrogen, low temperature growth, growth mechanism. 1. Introduction Graphene is most commonly grown on Cu by chemical vapor deposition (CVD) using CH4 as the precursor at around 1000°C [1-7] or higher [8, 9]. And also by other CVD techniques like plasma enhanced CVD and hot wire chemical vapor deposition (HWCVD) at around
900°C
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[10] and 1000°C [11, 12] respectively. Several approaches have been employed to bring down the growth temperature of graphene by these techniques.[13-19] Understanding the role of atomic hydrogen during the carbon film deposition plays a crucial role in the chemistry of nucleation and growth of graphene, diamond-like carbon, polymeric films, and diamond films. Such a study also enables us to tailor the properties of carbon films for various applications. The atomic hydrogen interaction during the growth controls the proportion of sp2 and sp3 bonded carbon contents in the film. This in turn decides the various physical and chemical properties of the diamond-like carbon film.[20, 21] However there are reports of post deposition treatment of the a-C film with atomic hydrogen leading to various modifications in the film structure.[22, 23] Also atomic hydrogen induced chemical erosion of a-C film has been reported.[22-25] In this paper, we report the results of our investigations on the atomic hydrogen induced structural evolution of a-C film deposited on Cu at 600°C using hot wire chemical vapor process (HWCVP), which leads to growth of nanographene. Such a study remains unexplored and we expect our approach contributes significantly towards an understanding of the mechanism and feasibility of nucleation and growth of graphene on Cu at reduced substrate temperature. This work could open up the opportunities to further explore the chemistry of hot wire generated atomic hydrogen, in order to bring down the growth temperature of graphene and thus lowering the thermal budget of its production. We have observed the changes in the film microstructure using transmission electron microscope (TEM) and Raman spectroscopy for different time durations of atomic hydrogen treatment. The chemical compositional changes are observed via X-ray photoelectron
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spectroscopy
(XPS),
Fourier
transform
infrared
spectroscopy
(FTIR)
supported
by
Photoluminescence (PL) study. 2. Materials and Methods 2.1 Sample Preparation We have performed all our experiments using an indigenously designed HWCVD system. HWCVD is a cost-effective growth technique which allows the deposition over a range of substrate temperatures.[26, 27] This is also a decoupled CVD system, wherein the gas dissociation takes place at the filament surface while the deposition occurs on the substrates located 5-7 cm away from the filament. The filament is made up of tungsten and is subject to joule heating. We have maintained a filament temperature of around 2000°C in all our experiments and the temperature is measured using a two colour optical pyrometer. In the present case the substrates are placed at a distance of 3 cm from the filament. The chamber is evacuated initially to a base pressure of 3×10-6 mbar and the substrate is heated to 600°C which is measured using a thermocouple. Copper foils (99.98% trace metal basis, Sigma Aldrich) of 0.25 mm thickness are used as the substrate for growth. The foils are mechanically polished and cleaned via ultra-sonication in acetone followed by DI water, isopropanol and finally with acetic acid to reduce any oxide present on the surface. Copper foils are annealed at 600°C in hydrogen atmosphere for 1 h. Then 5 sccm of methane is inlet into the chamber while the tungsten filament is heated to 2000°C. The dissociated reactive carbon species deposit on the annealed copper surface at 600°C. The deposition is performed for a very short duration of 60 s at 3×10-4 mbar pressure. After the film is deposited, 100 sccm of hydrogen gas is introduced into the chamber at a pressure of 5 Torr. The film was then treated with atomic hydrogen generated by the filament
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heated to 2000°C for 30 min, 60 min and 120 min respectively. The time scale of deposition and atomic hydrogen treatment is shown pictorially in Figure 1.
Figure 1. The a-C growth and exposure of as-grown film to atomic hydrogen represented on a time scale. 2.2 Transfer Method In order to examine the as grown film before and after atomic hydrogen exposure under TEM, the samples are transferred onto TEM grid by PMMA transfer method similar to the procedure mentioned in the ref [28]. The as-grown carbon film on Cu foil is spin-coated with PMMA solution and then cured at 120°C for 2 min. The opposite side of the copper foil was polished to remove the carbon film. The 0.5 × 0.5 cm2 × 250 μm thick Cu substrate is then etched away by 1 M FeCl3 solution in 3M HCl for over a period of ∼18 h. The PMMA/carbon film stack is cleaned with deionized water and placed on the required substrate and dried. A small amount of PMMA solution is coated onto the PMMA/graphene to dissolve the precoated PMMA, then the PMMA is slowly cured at room temperature and dissolved by acetone.
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3 Characterization Details The structural evolution of the film subsequent to atomic hydrogen treatment at 600°C was determined using high resolution transmission electron microscope operated at 200 kV ( JEOL JEM 2100F). Raman spectroscopy measurements were performed using 532 nm laser at 1.5 mW power (Witec Alpha300 RS). The chemical compositional analysis was determined by X ray photoelectron spectroscopy using Al Kα radiation (energy of 1486.6 eV; Kratos Analytical, Axis Supra). The peaks were deconvoluted using ESCApe software. The optical absorption measurements were performed using double beam absorption spectrometer at 2 nm resolution (Jasco
V-530).
The
photoluminescence
measurements
were
recorded
on
Jasco
spectrofluorometer (FP-8500). A Bruker Hyperion 3000 FTIR spectrometer equipped with Ge crystal was used for the analysis in attenuated total reflectance (ATR) mode. 4 Results and Discussion Figure 2a shows the HRTEM image of the as-grown carbon film before atomic hydrogen exposure. There are no lattice fringes observed in the image, the film remains to be amorphous in nature. Figure 2b shows the magnified image of the as-grown film after 30 min of annealing in vacuum at a low pressure of 3×10-6 mbar at 600°C. Even after annealing for 30 min, there is no observable change in the film microstructure and the film remains amorphous in nature. Figures 2c-d show the HRTEM image of the as-grown a-C film after 30 min and 60 min of atomic hydrogen exposure respectively. The lattice fringes visible in the image after 30 min of exposure, confirm the crystallization of a-C film. The inset shows the Fast Fourier transform (FFT) of the HRTEM image and the spots have a hexagonal symmetry and their distance from the center measure an interplanar spacing of 0.20 nm. After 60 min of atomic hydrogen exposure, the a-C film shows the lattice fringes. The crystallization of a-C film occurs simultaneously at several
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places on the substrate with increase in the diameter of crystallites. The inset shows the FFT of the region marked within the blue box which shows a hexagonal symmetry. The image obtained after inverse FFT is shown in Figure 2e.[29] The image identifies a sequence of the hexagonal structure with atoms appearing in the brighter contrast (Figure 2e) and the corresponding lattice parameter is measured to be 0.234 nm. This value is close to the lattice parameter of exfoliated graphene, a = 0.246 nm (Figure 2f). The small difference in the measured value is due to the resolution limit of the microscope. This confirms the transformation of a-C to nanographene domains. The number density of nanographene domains increases significantly as compared to 30 min exposure. Figure 2g shows the HRTEM image after 120 min of atomic hydrogen exposure. Formation of a polycrystalline structure with increased domain size as compared to 60 min exposure is observed and the lattice fringes marked in the image denote an inter-planar spacing of 0.21 nm which corresponds to the distance between the {10-10} set of lattice planes of graphene. The FFT of the image shows a ring pattern corresponding to a polycrystalline network. The film morphology consists of patches of thinner regions appearing lighter; this could be due to the removal of material attributed to the chemical erosion[25] of the amorphous component by atomic hydrogen, which becomes noticeable with increased exposure duration. Hence on exposure to hot wire generated atomic hydrogen the a-C undergoes crystallization leading to formation of nanographene which grows in size and number density with increase in time of exposure.
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Figure 2. (a) HRTEM image of HWCVD grown carbon film showing amorphous structure before atomic hydrogen exposure. (b) HRTEM image of HWCVD grown a-C film after 30min of annealing in vacuum. (c-d) HRTEM images showing crystallization of HWCVD grown a-C film after 30 min and 60 min of atomic hydrogen exposure respectively and the insets show the respective FFT. (e) Inverse FFT of the region marked within yellow box. The image shows hexagonal crystal structure with lattice parameter 0.234 nm close to the lattice parameter of exfoliated graphene crystal structure (f), 0.246 nm. (g) HRTEM image after 120 min of atomic hydrogen exposure. Further in order to analyze the thickness of a-C film and to estimate the number of layers of graphene formed after 30 min of atomic hydrogen exposure, we have performed cross-sectional TEM characterization. Figure 3a shows the cross-sectional TEM image of amorphous carbon on copper and figure 3b is a higher resolution image of a portion of the film identified by the square.
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The lamella of sample for the cross-sectional TEM imaging was prepared by focused ion beam (FIB) milling. The images show a variation in thickness from approximately 6.2 - 8.3 nm. The variation in thickness could be due to uneven copper surface. Figure 3c-d show the crosssectional TEM images of the amorphous carbon film after 30 min of exposure to atomic hydrogen. Before FIB milling the nanographene film grown on copper is covered by a layer of aluminum and platinum in order to avoid ion beam damage as can be seen in figure 3c which also shows that the nanographene is multilayered. Figure 3d shows the presence of 8 layers of graphene with an interlayer distance of 0.335nm. The lateral dimensions are around 6 nm and 4 nm. It is to be noted that the film contains many such domains with different dimensions.
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Figure 3. (a) Cross sectional TEM image of amorphous carbon on Copper. (b) Magnified image of amorphous carbon film. (c) Cross sectional TEM image of the nanographene on Copper. (d) Magnified image of the nanographene showing the presence of multilayers The Raman spectra of HWCVD a-C before (black curve) and after atomic hydrogen exposure for 30 min (green curve), 60 min (red curve) and 120 min (blue curve) respectively are shown in Figure 4a. The common characteristic Raman features of graphitic materials are G- band ( 1581-
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1600 cm-1), [30] which arises due to the relative motion of sp2 bonded carbon atoms and the Dband at around ̴1350 cm-1 , arises due to breathing mode vibrations of the aromatic ring [30, 31]. In case of graphene, a vacancy like defect leads to activation of D-mode.[32] Therefore in case of a-C, ID/IG ratio gives information related to graphene-like cluster formation. In this case, we observe that with an increase in the atomic hydrogen treatment time to 60 min, there is an increase in ID/IG (4b). This is due to the formation of nanographene domains which contribute to the breathing mode vibration. However, beyond 60 min of atomic hydrogen treatment, there is a decrease in ID/IG ratio (Figure 4b). This is due to larger crystallite size in the polycrystalline structure. As we move slightly away from the nanocrystalline regime towards larger crystalline domain size ID/IG ratio decreases. This is in agreement with the amorphization trajectory proposed by A. C. Ferrari et al.[33] which shows that it is difficult to quantify the film quality on the basis of the ID/IG ratio alone as the ratio decreases both when the film is in the amorphous regime and crystalline regime. Figure 4b shows the variation of G-band FWHM and ID/IG ratio with the time of exposure to atomic hydrogen. The FWHM of the G-band shows a monotonic decrease with the atomic hydrogen exposure time due to the larger size of the graphene domains. The reports from A. C. Ferrari et al.[33, 34] correlate the increase of grain size with the decrease in FWHM of the G-peak. These results confirm the atomic hydrogen induced transformation of a-C to nanocrystalline film.
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Figure 4. (a) Raman spectra of HWCVD grown a-C on Cu foil (black), HWCVD a-C on Cu foil after 30 min (green), 60 min (red) and 120 min (blue) of atomic hydrogen exposure. (b) Variation of the ratio of ID/IG (blue) and FWHM of G-band (black) with an increase in time of exposure of a-C film to atomic hydrogen. The XPS measurements are performed to examine the chemical composition of HWCVD grown a-C on Cu, before and after hot wire generated atomic hydrogen exposure. Figure 5a shows XPS survey scan peaks corresponding to C1s at 284.4 eV and O1s at 532 eV. The C:O atomic ratios before and after 120 min of atomic hydrogen are 2.54 and 4.16 respectively. The deconvolution of the XPS high resolution scan (Figure 5b-c) for C 1s shows peaks corresponding to C=C at 284.4 eV, C-OH and C=O peaks centered at 285.7 eV and 288.1 eV respectively.[11, 35, 36] Before atomic hydrogen exposure, the a-C film contain 80.34 ± 1.98% C=C, 10.25 ± 1.45% C=O and 9.41 ± 1.64% C-OH. After 120 min of atomic hydrogen exposure, there is a reduction in oxygen content existing in the film. The C=O and C-OH content reduces to 6.58 ±
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0.79% and 1.65 ± 2.1% respectively. Besides C=C content increases from 80.34 ± 1.98% to 91.77 ± 1.74%. This confirms the reduction in sp3 type defects[32] present due to oxygen functional groups and increase in sp2 hybridized carbon atoms due to atomic hydrogen interaction at 600°C. The FTIR spectra of HWCVD deposited a-C film before atomic hydrogen exposure and after 120 min of exposure to atomic hydrogen is shown in Figure 5d. The FTIR measurements are performed in attenuated total reflectance mode (ATR). A strong and broad peak corresponding to O-H stretching vibration band around 3300 – 3400 cm-1 is observed to decrease in intensity after 120 min of atomic hydrogen exposure.[37, 38] And also the bands at around 1070 cm-1 and 1130 cm-1 corresponding to O-C-O stretching and wagging respectively from epoxy groups[37, 38] are also observed to reduce. Hence the atomic hydrogen reduces the oxygen functional groups present. The peaks at around
840 cm-1,
1600 cm-1,
1650 cm-1,
assigned to C-H bending in disubstituted aromatic ring, aromatic and olefinic C=C stretching respectively[37-39] are observed to increase in intensity. However, the peak intensity at 2922 cm-1 corresponding to C-H asymmetric stretching[38] vibration has significantly minimized after atomic hydrogen exposure for 120 min. Both the XPS and FTIR spectra confirm that atomic hydrogen effectively reduces oxygen functional groups present in the film and leads to increment in C=C group.
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Figure 5. (a) XPS survey scan for HWCVD a-C on Cu before and after 120 min of atomic hydrogen exposure respectively. (b) XPS high resolution scan for C 1s core level peak before atomic hydrogen exposure. (c) XPS scan for C 1s core level peak after 120 min of atomic hydrogen exposure. The peaks are deconvoluted into its components. (d) FTIR spectra of HWCVD deposited a-C film before atomic hydrogen exposure and after 120 min of atomic hydrogen exposure.
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The optical absorption and photoluminescence (PL) spectra of the as-grown film and after atomic hydrogen exposure were measured. Figures 6a-b show the UV–vis absorption spectra of the as-grown film and after 120 min of atomic hydrogen exposure respectively. In both the cases, the film exhibits a strong absorbance in the deep UV range around 230 nm corresponding to π– π* transition of aromatic C=C bonds and a broad weak shoulder around 280 nm which extends up to 360 nm. This has been previously attributed to defect states arising from oxygen functional groups present in the film.[40] The PL excitation spectra were acquired at wavelength 342 nm for both the samples as shown in Figure 6a-b. It shows pronounced excitation band at around 230 nm and a weaker band at 280 nm respectively which is consistent with the overlapping absorption spectra. The PL emission spectra at 230 nm excitation wavelength, before and after 120 min of atomic hydrogen exposure are shown in Figure 6a-b. The PL emission spectra are deconvoluted into two Gaussian peaks identified as Ip1 and Ip2 as shown in figure 6c and 6d. The peak Ip2 is centered at 340 nm and Ip1 is centered at around 420 nm. In the case of GO, PL peak appearing at around 354 nm (Ip2) corresponds to the π-π* transitions [41]. In our case, the peak before atomic hydrogen exposure corresponding to π-π* transitions is at 338 nm and after 120 min of atomic hydrogen the peak shifts to 344 nm. This could be due to increase in the size of graphene domain with atomic hydrogen exposure. The broad peak between 350 nm and 500 nm labeled as Ip1 corresponds to the transition states induced by oxygen functional groups. With the reduction of oxygen impurities, the relative intensity of the peak Ip1 decreases, as the optical transitions arising due to oxygen states decrease due to the reduction by atomic hydrogen.[42] The simultaneous increase in the Ip1 strongly correlates with the percentage increase in C=C content as observed in XPS results and also with the increase in graphene domain size as observed in the HRTEM images in Figure 2c, 2d, and 2h respectively.
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Figure 6. (a) UV-visible absorption, PL excitation (measurement wavelength-342 nm) and PL emission spectra (excitation wavelength 230 nm) of HWCVD a-C film before atomic hydrogen exposure and after 120 min of atomic hydrogen exposure (b). (c) PL emission spectrum (excitation wavelength 230 nm) of HWCVD a-C film acquired before atomic hydrogen exposure and after 120 min atomic hydrogen exposure (d) deconvoluted into two peaks labeled as Ip1 and Ip2. Based on FTIR observations and the kinetic analysis of hydrocarbon erosion by atomic hydrogen as reported by A. Horn et al.[25], we discuss the probable mechanism of atomic hydrogen induced transformation of a-C into nanographene domains on Cu surface at 600°C. As seen in Figure 4d, the peak at about 2922 cm-1 corresponds to C‒H asymmetric stretching
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vibration from CH2 and CH, and the peak at about 2852 cm-1 corresponds to C‒H asymmetric stretching vibration from CH2,[37, 38] hydrogen is bonded to sp3 hybridized carbon. These signatures corresponding to C-H group significantly reduce with atomic hydrogen exposure. Consequently, there is an increase in peak intensity corresponding to aromatic and olefinic (C=C) stretching at around ̴1600 cm-1 and ̴1650 cm-1 respectively.[37, 38] The reaction with atomic hydrogen leads to breaking of C-H bond in sp3 bonded carbon atoms leading to dehydrogenation. The subsequent redistribution of electrons leads to sp2 hybridized carbon and they undergo aromatization in the presence of Cu at 600°C leading to the formation of nanographene. This reaction is accompanied by the erosion of hydrocarbon in the form of free radicals of methane or methyl as proposed by A. Horn et al.[25] The other plausible dehydrogenation mechanism could be thermal decomposition via hydrocarbon split off.[25] In our case, we rule out this possibility as, after 30 min of annealing at 600°C, the film remains amorphous in nature as shown in the TEM image (Figure 2b). Only on exposure to atomic hydrogen for 30 min, the formation of graphene crystallites are visible, which are clusters of sp2 bonded aromatic structures. We conclude the graphene nucleation to be a result of formation of sp2 carbon species due to atomic hydrogen induced dehydrogenation reaction. This analysis is strongly supported by FTIR data. With a prolonged time of exposure to atomic hydrogen, a significant amount of filmforming sp2 carbon species (C=C) are available to undergo aromatization and we see an increase in size of graphene crystallites (as observed in HRTEM image Figure 2d). Thus for the first time, we demonstrate the atomic hydrogen induced transformation of the a-C film via defect healing to nanocrystalline graphene at 600°C.
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AUTHOR INFORMATION Corresponding Author * email-id: [email protected]; phone (o): *+912225767633. Present Addresses Semiconductor thin films and Plasma Processing Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, India -400076
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.
ACKNOWLEDGMENT We acknowledge DST-FIST, at ME&MS, IIT Bombay for the FIB/SEM tool enabling the TEM sample preparation. We also thank SAIF/CRNTS for the TEM, FTIR characterization facilities. S.RK acknowledges financial support from UGC, Gov. of India. S.RK would like to thank Dr. V. Pandey for his jelp in commissioning the HWCVD setup and also would like to further thank him and R. Chulliyil for useful discussion. S.RK would also like to thank M. Kumawat, A. Pant, L. Gurnani for TEM characterization and A. Kumar for TEM sample preparation.
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ABBREVIATIONS HWCVD, Hot wire chemical vapor deposition; PL, Photoluminescence; HRTEM, High resolution transmission electron microscope; FTIR, Fourier transform infrared spectroscopy; XPS, X-ray photoelectron spectroscopy; FFT, Fast Fourier Transform.
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