Thermal Conductivity and Pressure-Dependent Raman Studies of

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Thermal Conductivity and Pressure Dependent Raman Studies of Vertical Graphene Nanosheets Karuna Kara Mishra, Subrata Ghosh, Ravindran Ramamoorthy Thoguluva, Sankarakumar Amirthapandian, and Mohammed Kamruddin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08754 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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The Journal of Physical Chemistry

Thermal Conductivity and Pressure Dependent Raman Studies of Vertical Graphene Nanosheets K. K. Mishra,* Subrata Ghosh, T. R. Ravindran, S. Amirthapandian, M. Kamruddin Material Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India

E-mail: [email protected] Abstract Thermal and mechanical properties of graphene sheet are of significant importance in the areas of thermal and stress management, respectively. Here we report the thermal conductivity and high pressure behaviors of unsupported vertical graphene nanosheets (VGNs) grown by electron cyclotron resonance-plasma enhanced chemical vapor deposition method. Structural morphology of the as grown VGNs on SiO2/Si substrate suggests a homogeneous, uniformly interconnected network of graphene sheets standing vertically on a basal nanographitic layer. On examination of edges of exfoliated sheets using transmission electron microscopy, seven layers of graphene is estimated. The frequency of the G-band (E2g-in plane mode) is found to vary linearly with temperature. The first order temperature coefficient for G-band is found to be 1.446 × 10-2 cm1

K-1. Using the G-band temperature coefficient and its position dependence on excitation laser

power, the thermal conductivity of the VGNs at room temperature is estimated to be 250 (19) Wm-1K-1. The effect of pressure (P) on the G-mode frequency (ω) of unsupported VGNs is investigated by in-situ Raman spectroscopic studies up to 40 GPa using a diamond Anvil cell. Above 16 GPa, discontinuity in ω vs P curve suggests a disruption of long-range order in the graphene layers resulting in a deviation from two-dimensional layer structure. Persistence of local sp2-hybridization up to 40 GPa is evident from the presence of G-band at this highest pressure. Upon decompression, VGN is found to recover its original ordered structure. Keywords: Graphene, Raman scattering, Phonon, Thermal conductivity, High pressure, Chemical vapor deposition

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Introduction Graphene and its derivatives have attracted significant research interests because of their extraordinary physical, optical, and electronic properties.1-9 A unique linear energy dispersion relation,1 exceptional mobility of charge carriers,3 potential for realizing quantum spin Hall effect10 and ballistic conduction11 make graphene a suitable candidate for high-speed nanoelectronics.6,12 VGNs are known as carbon nanowalls, a close relative to graphene, received strong attention due to its unique geometry. The interesting characteristics of this nanoarchitecture includes large 3-dimensional network structure, huge sharp edges, high aspect ratio with length in the range of several nano meters to micro-meters and capability of easy functionalization.13,14,15 It consists of vertically aligned graphene nanosheets, each sheet composed of a few layers of graphene.13,14,15 The physical properties of these nanosheets can differ from other nanocarbon materials. Similar to graphene, VGNs exhibit strong Raman scattering intensity.16,17 Therefore, it is possible to elucidate structural and transport properties from an analysis of Raman spectral parameters.16-18 Study of vibrational behavior of VGNs is necessary to understand various physical phenomena such as thermal conductivity, thermal expansion, specific heat which essentially arise due to anharmonic phonons.18-19 Anharmonicity in interatomic interaction potential causes temperature dependence of phonon frequency19 and the shift in phonon frequency can arise from a combination of two effects: a purely anharmonic contribution related to phonon-phonon interaction and a quasi-harmonic contribution due to volume change (thermal expansion). Secondly, the implementation of the electronic devices (size of a few nm to micro-meters), depends on the thermal management efficiency and hence largely depends on the nature of their thermal conductivity. Therefore, estimation of thermal conductivity of VGNs to determine their heat dissipation ability is important for applications and 2 ACS Paragon Plus Environment

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also from a fundamental point of view. Since a micro-Raman spectrometer can act as a heater and thermometer (as explained below), Raman scattering is used as an opto-thermal and nondestructive technique for evaluating the thermal conductivity in several 2-D carbon materials including graphene.20-24 Using laser excitation as a source of heat, the local temperature can be increased, resulting in red-shifting of G-band. By monitoring the G-band in Raman spectra with temperature and laser power excitation separately often thermal conductivity in these materials are accurately evaluated.20-24 Recently, separate Raman spectroscopic studies on temperature and laser power excitation dependent studies on G-band were used to extract the thermal conductivity on graphenes and carbon nanotubes.20-25 At ambient conditions ultra high thermal conductivity in the range 4800-5300 Wm-1K-1 was estimated in a single layer graphene using Raman spectroscopy.20 Thermal conductivity of the three dimensional nanofoam, synthesized by CVD, is reported to be ~11 Wm-1K-1 using Raman spectroscopic results.25 Inhomogeneous distribution of nanofoam and small diameter of pores were attributed for this significant reduction in thermal conductivity. Therefore, Raman spectroscopic studies on unsupported VGNs, comprising of few layer graphene, can be useful to estimate its thermal conductivity. Besides its thermal conductivity, studies on mechanical properties of VGNs are also scarce. Recent studies on elastic properties and intrinsic strength of graphene established it as an exceptionally strong material.26 Extremely high breaking strength of value ~42 Nm-1 and Young’s modulus of about ~1 TPa were observed. Quasi- and non-hydrostatic pressure effect up to 50 GPa on graphene reveals it to be the most stable and healable structure under stress.27 At ~16 GPa, a deviation from its two-dimensional layer structure is concluded from the behaviour of strain dependence of G-band shift. Under compression, carbon K-edge spectroscopic studies on graphite using X-ray inelastic scattering revealed a change of half of its bonding character



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from π- to σ-bonds.28 Effect of compression up to ~8 GPa on supported graphene with different pressure transmitting media (PTM) suggests that PTM has no role on their Raman spectral features.29Although graphene based materials are of great practical importance, the study of effect of pressure on graphene materials is still scarce. In this work, we report temperature and pressure dependent Raman studies of VGNs grown by electron cyclotron resonance-plasma enhanced chemical vapor deposition (ECRPECVD) method. The morphology and microstructures of the as grown VGNs sample were examined using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), respectively. The thermal conductivity of the suspended VGNs is evaluated by performing a temperature dependent Raman spectroscopic study and independently measuring laser power excitation dependencies of G-band in Raman spectra. We have carried out in-situ high-pressure Raman spectroscopic studies up to 40 GPa in a diamond-anvil cell to explore the possibility of disorder in graphene at high pressure. The few layer graphene in VGNs is likely to become disordered at ~16 GPa. The Raman spectra were analyzed quantitatively to obtain the E2g mode frequency with pressure. Using the reported strain data,30 Grüneisen parameter of E2g in-plane mode is estimated and compared with the reported graphene materials. To the best of our knowledge, there are no earlier reports of temperature and pressure dependent Raman studies on VGNs or its thermal conductivity.

Experimental details VGN films were deposited on SiO2/Si substrates by ECR-PECVD method.16,17 400 nm thin film of SiO2 was grown by thermal oxidation on n-Si(100) substrate of resistivity 2-8 Ohm4 ACS Paragon Plus Environment

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cm. We have chosen CH4 (5N purity) as hydrocarbon source and Ar (3N purity) as carrier gas. Prior to growth, the substrates were cleaned by acetone, isopropyl alcohol, de-ionized water followed by drying with N2 gas, and loaded into the chamber. The chamber was evacuated to 106

mbar by rotary and turbo molecular pumps. The substrates were heated to 1023 K at a heating

rate of 30 K/min and held at 1023 K for 30 min. Subsequently, they were cleaned by Ar plasma using microwave (2.45 GHz) power of 200 W at 1023 K for 10 min. Thereafter, CH4 of 5 sccm is fed into the chamber through mass flow controller along with Ar gas of 25 sccm for 60 min at 400 W microwave power. The substrate temperature and chamber pressure were maintained at 1023 K and 2.8 × 10-3 mbar, respectively during growth. After the growth, the plasma was turned off and the samples were annealed at the growth temperature for about 30 minutes. The samples were subsequently allowed to cool down naturally to room temperature under Ar gas atmosphere. Morphological analysis of the films was investigated by FE-SEM (Supra 55, Zeiss, Germany) at an accelerating voltage of 5 kV and the micro-structure of the sample was characterized using a high resolution (HR) TEM (Libra 200 FE, Zeiss, Germany) operated at 200 kV. X-rays photo emission spectrometer (M/s SPECS, Germany) was used to examine surface chemical bonding in VGNs. In-situ temperature dependent Raman spectra were recorded from both the substrate supported and suspended VGNs, in 100-500 K range using 514.5 nm line from an

Ar-ion laser as excitation. It can be mentioned here that SiO2/Si substrate on which the VGNs were grown was annealed at 673 K for 4 hrs in air. The VGNs that were peeled in this process of annealing were then gently lifted and detached from the SiO2/Si substrate using a wooden toothpick. A piece of the peeled VGNs was placed on a quartz cover slip to carry out the Raman spectroscopic measurements. Since the contact of the VGNs’ bottom end with quartz is loose, 5 ACS Paragon Plus Environment


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one can treat this as suspended VGNs. Raman spectra were recorded using a micro Raman spectrometer (InVia, Renishaw, UK). A 20 X long distance objective was used to focus the laser beam to ~1 µm spot on the sample. Measurements were carried out using a Linkam heatingcooling stage (THMS 600) ensuring a temperature stability of ± 0.1 K. In-situ high pressure Raman spectroscopic measurements were carried out using a compact, symmetric diamond anvil cell (DAC) with diamonds of culet diameter 500 µm. A few flakes of VGN sample were loaded into a 200-µm hole of a pre-indented stainless-steel gasket in the DAC. 4:1 methanol-ethanol mixture was used as the pressure transmitting medium. Ruby was loaded along with the sample as a pressure calibrant. Measurements were carried out up to ~40 GPa, covering the wave number range 1000-1900 cm-1. Raman spectra were also recorded during the pressure-reducing cycle. The spectra were analyzed using Lorentzian line shapes using PeakFit software (JANDEL). Results and Discussion A. Microstructure The morphology of as-grown VGNs on SiO2 substrate (Figure 1) shows that VGNs are homogeneous, uniform and form an interconnected network of graphene sheets standing vertically on the nanographitic base layer. The nanographitic base layer is the interface layer between the substrates and vertical network. Each vertical sheet is composed of a few layers of graphene; its length varies from few nanometers to micrometers. The vertical height of VGNs is found to be 770 nm (inset Figure 1). The microstructure of graphene sheets was studied using transmission electron microscopy (TEM). Figure 2(a) shows a bright field TEM micrograph recorded from graphene sheets. The number of layers in a graphene sheet was determined by


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examining the edges of the sheets parallel to the electron beam. An example is shown in Figure 2(b) where the graphene sheet consisted of 7 layers and measuring about 2.45 nm thick with the distance between consecutive layers measured to be ~0.35 nm. This is in accord with the (002) lattice spacing confirmed with the JCPDS card No.00-056-0159. Figure 2(c) shows HRTEM image of the graphene sheet and the inset depicts FFT pattern obtained from the marked region. The filtered inverse fast Fourier transform (IFFT) image corresponding to that region is shown in Figure 2(d). The lattice spacing for (100), (010) and (-110) planes were measured to be ~0.22 nm. B. Temperature dependent Raman spectroscopy Figure 3 shows the Raman spectra of the SiO2/Si supported VGNs at room temperature in the frequency range 1000-3500 cm-1. Three characteristic strong Raman bands are seen at 1354, 1585, and 2700 cm-1. The G-band at 1585 cm-1 is a doubly degenerate in-plane Raman active E2g mode and other two are A1g mode and its first overtone.27 The 1585 cm-1 band is a Brillouin zone centre phonon, essentially contributed by a first-order Raman scattering process and representing the optical phonon mode activated by intra-layer sp2 bonded carbon atoms. On the other hand, the D-band at 1354 cm-1 and G′-band at ~2700 cm-1 originate from a second order process; where the former is resulting from a defect-assisted one phonon process and the latter G′ band involves two optical phonons near the zone centre of the Brillouin zone.31 The observed line width of G′ band is broad (~76 cm-1), suggesting that our VGNs are composed of few layer of graphene.31 The other less pronounced defect assisted bands known to be activated by double resonance processes such as D′′, D′, D+D′′, D+D′ and 2D′ are located at 1110, 1622, 2460, 2946 and 3235 cm-1, respectively.31,32 These bands are strongly dispersive with excitation energy31,33



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and are in general absent in case of single layer graphene. In order to examine the surface chemistry in VGNs, we have recorded the C1s XPS spectrum and analyzed using GaussianLorentzian lines shape (Supplementary Figure S1). The individual deconvoluted peak corresponds to sp2 C=C, sp3 C-H, C=O and C-OH bonding.16 Therefore, the observation of high intensity of D-band in our Raman spectra is due to several factors such as ion induced defects during growth, presence of edge states, sp3 bonded C-H species.16 However, our TEM data suggest high crystalline nature of the sample, The behaviour of these defects and their contribution to the defect band intensity is reported in our earlier report.16 Temperature dependent Raman spectroscopic measurements on suspended VGNs were carried out in the range 100-500 K and spectra recorded at few temperatures are shown in Figure 4(a). It can be mentioned here that the thickness of the VGN sheets (i.e., height of the vertical walls) on SiO2/Si substrate is less than a micron as measured by SEM (Figure 1) and other dimensions of the used film in experiment are of the order of 1 mm. Therefore, this geometry ensures that VGN sheets are still vertical after peeling from the SiO2/Si substrate and placing them on quartz cover slip. One can notice that upon increase in temperature, the G-band shows a monotonic red-shift and its width is insensitive to temperature in line with those observed in other graphene materials.34 Figure 4(b) shows the G-band positions as a function of temperature. Since the temperature-induced change in this band position is small, a relatively large scatter of the experimental data is obtained. The variation of this band position can be fitted to a linear equation ω(T)= ω0 + χT, where ω0 and χ are the fitting parameters that represent the mode wave number and first-order temperature coefficient of the G-band, respectively. The fitting yields ω0 = 1587 cm-1 and χ = -1.47(1) × 10-2 cm-1/K. It can be mentioned that higher order anharmonic contribution to ω(T) are omitted here since they are significant at high temperature only. For 8 ACS Paragon Plus Environment

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convincing, a comparison of experimental temperature coefficients of other graphene-based materials is presented in Table 1. In the present case, χ value is low, and close to that obtained for single and bi-layer graphene,34 indicating its few layer graphene structure. As mentioned earlier, the temperature dependence of phonon frequency at constant pressure arises due to anharmonicity involved in the lattice potential energy and contributed by dual effects: (a) contribution due to changes in interatomic interactions caused by a purely volume change and (b) a true anharmonic contribution due to intrinsic phonon-phonon

anharmonic interaction.35 ∂ = − + ∂ , where γ is the mode Grüneisen parameter and α

is the thermal expansion coefficient of material. The first term on the right hand side is the quasiharmonic contribution and the second is the true anharmonic (pure-temperature) effect. Hence, the true anharmonic contribution to G-band in VGNs can be estimated by using the values of γ, α and the slope of G-band frequency vs temperature. In order to examine whether the substrate has any role on the temperature coefficient of G-band, we measured the temperature dependent Raman spectra on substrate (SiO2/Si) supported VGNs and their G-band position variation with temperature is analyzed (Figure 5a and b). One can see that the G-band position exhibits a similar decreasing trend with temperature as noticed in our unsupported VGNs. The temperature coefficient is estimated as χs = -1.34(1) × 10-2 cm1

/K. The fact that the temperature coefficient χs differs by ~10% for suspended VGNs implies

that the substrate effect cannot be neglected. This is consistent with the prediction of Chen et al.18but contrary to the report of Balandin34 et al. As discussed earlier, laser excitation focused on the sample with a spot size of about 1 µm causes local heating leading to a rise in temperature. Variation of laser power leads to a 9 ACS Paragon Plus Environment


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change in local temperature which in turn affects the position of phonon modes. The rate of change of G-band position with laser power can be used to estimate the thermal conductivity of graphene based materials. The expression for heat conduction through a surface of area A is:

= − ∮ ∇. ,20 where the term on the left-hand side denotes rate of heat flow, K and T

represent thermal conductivity and absolute temperature, respectively. In case of smaller laser spot size comparable to the size of the suspended sample, this expression has been further simplified by Balandin et al,20 assuming radial heat flow from the centre of the sample:

K=χ -1, where χ is the first order temperature coefficient of G-band, δω/δP is the variation in G-band position with excitation laser power, and h is thickness of material. Furthermore, the behaviour of G-band with different laser power is useful in only suspended VGNs materials because of the fact that the heat generated by the laser excitation has to conduct through the VGNs and to avoid any extraneous dissipation of local heat energy to the substrate in case of substrate supported VGNs. Therefore, to make heat conduction free from substrate effect, we measured Raman spectra on substrate-free VGNs sit on a copper grid (normally used for TEM). The length of the suspended VGN flakes vary up to 1 µm, and the copper grid acts as a heat sink. A schematic diagram of laser heating of free VGNs on a copper grid is shown in Figure 6a. Raman spectra measured at several incident excitation laser powers is shown in Figure 6b. Upon increasing laser power, increase in the spectral intensity and monotonous red-shift of the G-band position are observed. The red-shift of G-band position indicates a rise in local temperature of the sample surface. Figure 6c shows the variation of Gband position as a function of laser excitation power. The δω/δP of the G-band is obtained by a linear fit to the plot and is found to be -3.91(4) cm-1mW-1. We have also measured Raman


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spectra on large number of free VGNs with respect to laser power and obtained the average value of δω/δP = -3.8(3) cm-1mW-1. Using our experimental values of δω/δP, temperature coefficient of G-band χ = -1.47(1) × 10-2 cm-1/K and thickness of VGNs (h) = 2.45 nm in the above expression, the in-plane thermal conductivity of VGNs is estimated as 250(19) Wm-1K-1. It is illustrative to compare the thermal conductivity of VGNs with those in other graphene based materials such as single and few-layer graphene, defects induced graphene, and nanofoam graphene (Table 1). The present thermal conductivity K value is found to be much lower than that for single and few-layer graphene, which were reported to be between 5000 20 to 1500 Wm1

K-1.36 The reduction in thermal conductivity with increase in graphene layer is suggested to be

due to increase in the phonon Umklapp scattering and coupling of phonons across the cross plane.36 The thermal conductivity of defect induced graphene is found to be decreasing with increasing defect density in the range 1800-400 Wm-1K-1.37 This was explained as due to phonon-point defect scattering. As pointed earlier, in case of three dimensional graphene nanofoam,25 the thermal conductivity was found to reduce substantially to ~11 Wm-1K-1. This reduction was attributed to inhomogeneous distribution of nanofoams and structural parameters like surface roughness. In the present case, the much larger value of K for VGNs than that reported for nanofoam could be ascribed to more uniform, crystalline and homogeneous distribution of VGNs, as observed from our morphological and structural studies (FE-SEM and HRTEM). C. High-pressure Raman spectroscopy The D and G′ bands of the VGN sample are masked by strong scattering from diamond anvils of the symmetric cell, preventing us from obtaining these Raman bands, especially in the low pressure range. Zone boundary phonons like D, D′ and G′ are known to be originated from 11 ACS Paragon Plus Environment


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double resonance (DR) Raman scattering processes. Therefore, in high pressure (strain) experiments, a change in relative position of Dirac cones in its electronic band structure can vary the DR scattering conditions, resulting in a change in these phonons. On the other hand, for Gband, a fundamental vibration, the same zone centre Raman active phonon can be probed with pressure. Therefore, Raman spectroscopic studies of G-band response to pressure are examined to gain insights about graphitic materials.27-29 In order to examine the changes that occur in the G-band of unsupported VGNs at high pressure, we have recorded in-situ Raman spectra using the DAC up to 40 GPa (Figure 7). A prominent peak and a shoulder are located at 1585 and 1626 cm-1 which are the G- and D′-band, respectively in the ambient pressure spectrum. Upon increasing pressure, these bands shift to higher wave numbers. The G-band could be followed throughout the pressure range whereas the D′ band which appears as a shoulder at ambient could not be identified beyond 5.2 GPa, due to insufficient intensity. Hardening of G-band with pressure is as expected since under the influence of pressure the intra-planar C-C bonds in graphene layer get compressed.27,30 Beyond 16 GPa, the G-band is weak and broad, possibly because of the existence of disorder in the graphene layer, consistent with X-ray diffraction results of Mao et al.28 Existence of disorder in graphene layers in VGNs will be discussed later. A broad D-band is seen at ~1544 cm-1 above 30 GPa (Fig. 7). Raman spectra were also recorded during the decompression cycle (Figure 7). In reverse cycle, at 0.3 GPa, the G-band re-appears at ~1585 cm-1, similar to that observed in forward cycle at this pressure. This suggests that the sample recovers fully from a disordered state at a high pressure to its original state when the pressure is released and indicating its high degree of healable structure. The pressure dependence of the Raman bands is shown in Figure 8. The D′ and D-bands exhibit hardening; while the former is not identified beyond 5.2 GPa, the 12 ACS Paragon Plus Environment

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latter is visible only above ~30 GPa. Upon increase in pressure, the G-band exhibits hardening in the complete range of pressure. However, as can be seen (See Figure 8) at ~16 GPa, a clear change in slope from 4.02 cm-1 GPa-1 to 2.36 cm-1GPa-1 is observed. Interestingly, high pressure investigation on crystalline graphite using synchrotron X-ray inelastic scattering inferred that at ~16 GPa, half of the inter-planar C-C π-bonds convert to σ-bonds along the adjacent layers and the other half remain as π-bonds because the layers come closer (at or below interlayer distance ~2.8 Å)28 upon compression. In fact with increasing pressure the bridging C atoms along graphitic layers come closer and participate in σ-bond between alternating layers whereas the non-bridging C atoms remain only in π-bond along the layer. Since these bridging C atoms in a planar layer shift either upward or downward in equal measure along the carbon chains to participate in σ-bond between alternating layers, it results in a buckling of graphitic layers. In the present case, the slope change around the same pressure suggests a similar effect in our few layer VGNs also. The significant broadening of G-band beyond 16 GPa could be due to the positional disorder of C-atoms in planar layer structure and hence resulting in buckling of our few layer graphene. Broadening in spectral features19 and in diffraction peaks28 are known to be caused by presence of disorder in lattice. One can see from Figure 7 that with increase in pressure the Gband broadens significantly by a factor of 2.5 between 16 and 40 GPa. This increase in width is expected due to positional disorder of C-atoms leading to a distribution of the C-C bond lengths and bond angles. Similar disordering of C-atom in graphene layers has been reported in other graphene based systems such as few layer graphene27 and graphite28. In order to verify whether the weakening of the Raman G-band is due to the growth of positional disorder, we have analyzed the integrated intensity of the G-band above the background (Supplementary Figure S2a). One can see indeed a reduction in intensity of the G-band as a function of pressure.



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Analysis of the integrated intensity of the background between 1480-1900 cm-1 (Supplementary Figure S2b) shows a corresponding increase with pressure. Therefore, a redistribution of the G-

band intensities into the background is evident and hence the growth of positional disorder in the high-pressure is understandable. The presence of G-band at the highest pressure suggests the dominance of the C-C sp2 hybridization along the graphene layer over the sp3 hybridization across the graphene layers, consistent with a recent finding on few-layer graphene.27 The band frequency during the decompression cycle (Figure 8) suggests a complete reversible nature of the spectral change, unlike in other graphitic systems such as graphite, carbon nano-tubes which irreversibly exhibit amorphization at high pressure.38 In order to compare the pressure dependence behaviours of G-band in our VGNs with other graphitic compounds, viz., 1 layer, 2 layer, 7 layer, and graphite, we have reproduced the data from Refs. 27 and 29 for these materials and plotted along with our obtained experimental G-band position for VGNs in Figure 9. One can notice that upon increasing pressure a faster rate of shift of G-peak in 1 layer graphene and least shift in graphite, whereas rate of change in other materials are observed to be decreasing with increasing number of graphene layers. The pressure dependence of G-band in the present VGNs is found to be almost matching with the reported few-layer graphene consisting of 7-layers27 (Figure 9) suggesting our VGN sheet consists of 7layers of graphene. This is exactly observed in our HRTEM analysis (discussed earlier). Figure 10 shows the variation of normalized G-band frequency (∆ω/ωo) with respect to change in strain (∆ε). ∆ω/ωo is obtained from our present Raman experimental result and strain values (c/co) is estimated by using the X-ray diffraction data on graphite from Ref. 29. It can be mentioned here that the lattice parameter c of 7 layers graphene reported27 to matches closely with that of graphite. The slope of the ∆ω/ωo vs c/co is obtained by linear fits to the plot; this 14 ACS Paragon Plus Environment

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slope essentially represents the mode Grüneisen parameter for the G-band.39 One can see a discontinuity in the slope at around 16 GPa, suggesting a possible transition to a disordered crystal structure above this pressure. A similar kind of discontinuity has been reported in a fewlayer graphene under compression.27 The Grüneisen parameters for the G-band are estimated as 0.37(1) and 0.67(5) at below and above 16 GPa, respectively; the latter value (γ = 0.67) is found to be two times higher than that obtained for the ordered graphene state (γ = 0.37). This suggests that the in-plane G-band becomes easily compressible in the disorder graphene, possibly due to buckling of layers, and the disordered graphene is sensitive to pressure compared to the ambient ordered state. Studies of few layer graphene under compression by Clark et al.27 indicates the same order of these Grüneisen parameters (0.26 and 0.92 across ~16 GPa) along with a similar increasing trend in the pressure induced disorder phase. On the otherhand, the results of uniaxial strain on single layer graphene39 indicate Grüneisen parameter (γ ~ 1.99) that is an order of magnitude larger than our results pointing towards larger compression along a-axis of graphene layer. Conclusions We have reported the temperature and pressure dependent Raman studies of in-plane E2g mode in unsupported vertical graphene nanosheets prepared by plasma enhanced chemical vapour deposition on SiO2/Si substrate. The E2g mode frequency shows red-shift and varies linearly with temperature. The first order temperature coefficient of E2g mode is found to be 1.446 × 10-2 cm-1K-1. Using our experimentally obtained temperature coefficient of E2g mode and laser excitation power dependencies of E2g mode, the thermal conductivity of the VGNs at ambient temperature is estimated to be 243.35 Wm-1K-1. The pressure dependence of the E2g mode in VGNs suggests a deviation from the long range order in graphene layer structure in 15 ACS Paragon Plus Environment


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graphene sheet around 16 GPa. The Grüneisen parameters of G-band are obtained and compared with other graphitic materials. High stability and healable ability of VGNs under pressure is also illustrated. AUTHOR INFORMATION

* Corresponding author Dr. Karuna K. Mishra (K.K.Mishra) Condensed Matter Physics Division, Material Science Group, Indira Gandhi Centre for Atomic Research Kalpakkam 603102, India [email protected], [email protected] (K. K. Mishra) Tel./Fax: +91 44 27480081 22261

AUTHOR CONTRIBUTIONS K.K.M. planned, performed Raman spectroscopic experiments, analyzed the data and wrote the manuscript. S.G. synthesized the samples and contributed to the growth and morphology section. S.A. performed HR-TEM of the samples. All authors discussed the results, commented on the manuscript and gave approval to the final version of the manuscript. S.G., T.R.R., M.K., and S.A. contributed to the revision of the manuscript. NOTES The authors declare no competing financial interests.

Associated Content Supporting Information Available Figure S1. C1s XPS spectrum of VGNs. Individual deconvoluted peaks corresponding to sp2 C-C, sp3 CH, C=O and C-OH bonding are also shown. Figure S2. (a) Evolution of the integrated intensity of the Gband at high pressure, and (b) The pressure dependence of background Raman intensity arising from positional disorder. This material is available free of charge via the Internet at http://pubs.acs.org/.


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Acknowledgment We acknowledge S. R. Polaki for SEM and N. G. Krishna for XPS facilities. We thank the Director, IGCAR and Director, MSG, Kalpakkam for encouragement and support.

Reference

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(33) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 2006, 97, 187401-187403. (34) Calizo, I.; Balandin, A. A.; Bao, W.; Miao, F.; Lau, C. N. Temperature dependence of the Raman spectra of graphene and graphene multilayers. Nano Lett. 2007, 7, 2645-2649. (35) Weinstein, B.; Zallen, R. Light Scattering in Solids IV; Cardona, M., Güntherodt, G., Eds.; Springer, Berlin, 1984; Vol. 4, pp. 463. (36) Ghosh, S.; Bao, W.; Nika, D. L.; Subrina, S.; Pokatilov, E. P; Lau, C. N.; Balandin, A. A. Dimensional crossover of thermal transport in few-layer graphene. Nat. Mater. 2010, 9, 555-558. (37) Malekpour, H.; Ramnani, P.; Srinivasan, S.; Balasubramanian, G.; Nika, D. L.; Mulchandani, A.; Lake, R. K; Balandin, A. A. Thermal conductivity of graphene with defects induced by electron beam irradiation. Nanoscale 2016, 8, 14608-14616. (38) Chen, J.-Y.; Kim, M.; Yoo, C.-S. High structural stability of single wall carbon nanotube under quasi-hydrostatic high pressures. Chem. Phys. Lett. 2009, 479, 91-94. (39) Mohiuddin, T. M. G; Lombardo, A.; Nair, R. R.; Bonetti, A.; Savini, G.; Jalil, R.; Bonini, N.;. Basko, D. M.; Galiotis, C.; Marzari, N.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C. Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Grüneisen parameters, and sample orientation. Phys. Rev. B 2009, 79, 205433-205440. (40) Klemens, P. G. Theory of the A-plane thermal conductivity of graphite. J. Wide Bandgap Mater. 2000, 7, 332-339.


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Table caption Table 1 Thermal conductivity and temperature coefficient ∂ω/∂T for G-band (E2g mode) of our PE-CVD grown VGNs and other reported graphene based structure.

∂ω/∂T ×10 (cm-1/K)

Thermal conductivity (Wm-1K-1)

Single layer graphene20

-1.62

4800-5300

Bilayer graphene36

-1.5

3000

Highly oriented pyrolytic graphite 34,40 Few layers graphene with defects37 VGNs (This work)

-1.1

2000

-1.33

1800-400

-1.47(1)

250(19)

Nanofoam graphene25

-

11

Material

-2



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Figure captions Figure 1 (color online) SEM micrograph of vertical graphene nanosheets (VGNs) and inset: represents the cross sectional view of VGNs.

Figure 2 (color online) (a) The bright field TEM image recorded from graphene sheet, (b) HRTEM image recorded from a folded edge, shows 7 layers of graphene, (c) HRTEM image of the graphene sheet and FFT is given as inset (taken from marked region), and (d) inverse fast Fourier transform (IFFT) of the inset of (c). Figure 3 (color online) Raman spectra from VGNs measured at room temperature. The G-band at ~1585, D-band at ~1354, and G′-band at ~2700 cm-1 are clearly seen. Solid curves are the Lorentzian least-square fits to the data.

Figure 4 (color online) (a) Raman spectra of suspended VGNs measured at different temperatures. Solid curves are the Lorentzian least-square fits to the ambient data including a suitable background and (b) Temperature dependence of the frequencies of Raman G-band in VGNs. Lines through the data are linear least squares fit.

Figure 5 (color online) (a) Raman spectra of substrate supported VGNs measured at different temperatures. Solid curves are the Lorentzian least-square fits to the ambient data including a suitable background and (b) Temperature dependence of the frequencies of Raman G-band in VGNs. Lines through the data are linear least squares fit.

Figure 6 (color online) Schematic representation of excitation laser heating on suspended VGNs and resulting heat conduction, (b) Raman spectra of suspended VGNs measured at different incident laser powers. Solid curves are the Lorentzian least-square fits to the data including a suitable background. Vertical dashed line indicates the red-shift of G-band position with laser power, (c) Mode frequencies of G-band as a function of excited laser powers in the suspended VGNs. Lines through the data are linear least squares fit. The errors bars are smaller than symbols.


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Figure 7 (color online) Raman spectra of VGNs recorded at different pressures. Solid curves are the Lorentzian least-square fits to the data including a suitable background. The ↓ marks locate the D-band above 30 GPa. The D-band below 30 GPa is obscured by the influence of strong Raman signal from diamond anvils cell. The spectra shown by (r) denoted to pressure release. For the sake of clarity, the spectra are vertically shifted.

Figure 8 (color online) Pressure dependencies of the G, D and D′-band frequencies. The G-band slope in the ω vs P curve of VGNs changes at around 16 GPa, suggesting the deviation from 2Dlayer structure. Solid circles (red color) indicate the G-band behaviour in the decompression cycle. Lines through the data are linear least squares fit. Vertical dashed line indicates the discontinuity in G-band slope. Solid circles (black color) indicate the G-band behaviour with pressure previously obtained in 7-layer graphene from Ref. [27]. The errors bars are smaller than the size of symbols.

Figure 9 (color online) Pressure dependencies of the G-band position of the present VGNs. For its comparisons the same for previously obtained in various graphene (1, 2 and 7 layers) and graphite from Refs. [27, 29] are also included. The pressure dependence of the VGNs is most likely to 7-layer graphene. Lines are least-square fits, for guide to the present experimental data. The errors bars are smaller than the size of symbols.

Figure 10 (color online) The normalized G-band (∆ω/∆ωo) vs strain (-∆ε) of the present VGNs. Vertical dashed line indicates the discontinuity in slope at around 16 GPa, suggesting the loss of long range order in layer structure of graphene. Strain (-∆ε) is reproduced from Ref. [30]. The errors bars are smaller than the size of symbols.



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Figure 1 Mishra et al.


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Table of Contents Graphic


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Figure 1 (color online) SEM micrograph of vertical graphene nanosheets (VGNs) and inset: represents the cross sectional view of VGNs. 352x239mm (72 x 72 DPI)

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Figure 2 (color online) (a) The bright field TEM image recorded from graphene sheet, (b) HRTEM image recorded from a folded edge, shows 7 layers of graphene, (c) HRTEM image of the graphene sheet and FFT is given as inset (taken from marked region), and (d) inverse fast Fourier transform (IFFT) of the inset of (c).

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Figure 3 (color online) Raman spectra from VGNs measured at room temperature. The G-band at ~1585, Dband at ~1354, and G′-band at ~2700 cm-1 are clearly seen. Solid curves are the Lorentzian least-square fits to the data. 204x176mm (300 x 300 DPI)



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Figure 4 (color online) (a) Raman spectra of suspended VGNs measured at different temperatures. Solid curves are the Lorentzian least-square fits to the ambient data including a suitable background and (b) Temperature dependence of the frequencies of Raman G-band in VGNs. Lines through the data are linear least squares fit. 97x172mm (300 x 300 DPI)


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Figure 4 (color online) (a) Raman spectra of suspended VGNs measured at different temperatures. Solid curves are the Lorentzian least-square fits to the ambient data including a suitable background and (b) Temperature dependence of the frequencies of Raman G-band in VGNs. Lines through the data are linear least squares fit. 114x177mm (300 x 300 DPI)



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Figure 5 (color online) (a) Raman spectra of substrate supported VGNs measured at different temperatures. Solid curves are the Lorentzian least-square fits to the ambient data including a suitable background and (b) Temperature dependence of the frequencies of Raman G-band in VGNs. Lines through the data are linear least squares fit. 92x180mm (300 x 300 DPI)


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Figure 5 (color online) (a) Raman spectra of substrate supported VGNs measured at different temperatures. Solid curves are the Lorentzian least-square fits to the ambient data including a suitable background and (b) Temperature dependence of the frequencies of Raman G-band in VGNs. Lines through the data are linear least squares fit. 126x177mm (300 x 300 DPI)



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Figure 6 (color online) Schematic representation of excitation laser heating on suspended VGNs and resulting heat conduction, (b) Raman spectra of suspended VGNs measured at different incident laser powers. Solid curves are the Lorentzian least-square fits to the data including a suitable background. Vertical dashed line indicates the red-shift of G-band position with laser power, (c) Mode frequencies of Gband as a function of excited laser powers in the suspended VGNs. Lines through the data are linear least squares fit. The errors bars are smaller than symbols. 166x74mm (150 x 150 DPI)


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Figure 7 (color online) Raman spectra of VGNs recorded at different pressures. Solid curves are the Lorentzian least-square fits to the data including a suitable background. The ↓ marks locate the D-band above 30 GPa. The D-band below 30 GPa is obscured by the influence of strong Raman signal from diamond anvils cell. The spectra shown by (r) denoted to pressure release. For the sake of clarity, the spectra are vertically shifted. 90x174mm (300 x 300 DPI)



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Figure 8 (color online) Pressure dependencies of the G, D and D′-band frequencies. The G-band slope in the ω vs P curve of VGNs changes at around 16 GPa, suggesting the deviation from 2D-layer structure. Solid circles (red color) indicate the G-band behaviour in the decompression cycle. Lines through the data are linear least squares fit. Vertical dashed line indicates the discontinuity in G-band slope. Solid circles (black color) indicate the G-band behaviour with pressure previously obtained in 7-layer graphene from Ref. [27]. The errors bars are smaller than the size of symbols. 214x170mm (300 x 300 DPI)


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Figure 9 (color online) Pressure dependencies of the G-band position of the present VGNs. For its comparisons the same for previously obtained in various graphene (1, 2 and 7 layers) and graphite from Refs. [27, 29] are also included. The pressure dependence of the VGNs is most likely to 7-layer graphene. Lines are least-square fits, for guide to the present experimental data. The errors bars are smaller than the size of symbols. 223x172mm (300 x 300 DPI)



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Figure 10 (color online) The normalized G-band (∆ω/∆ωo) vs strain (-∆ε) of the present VGNs. Vertical dashed line indicates the discontinuity in slope at around 16 GPa, suggesting the loss of long range order in layer structure of graphene. Strain (-∆ε) is reproduced from Ref. [30]. The errors bars are smaller than the size of symbols. 119x170mm (300 x 300 DPI)


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Thermal and mechanical properties of vertical graphene nanosheet (VGN), a close relative of graphene, are of importance in the areas of thermal and stress management, respectively. VGNs were synthesized by the plasma enhanced chemical vapour deposition method. Using a Raman spectrometer as an opto-thermal technique, the thermal conductivity of 7-layer VGN graphene at room temperature is estimated as 250 Wm1K-1. The effect of pressure up to 40 GPa, using diamond anvil cell suggests a disruption in long-range order in the graphene layers at 16 GPa. Upon decompression, VGNs is found to be healable under large stress. 320x153mm (96 x 96 DPI)


Thermal Conductivity and Pressure-Dependent Raman Studies of

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