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Functional Nanostructured Materials (including low-D carbon)

Unusual Thermal Conductivity of Carbon Nanosheets with Self-Emerged Graphitic Carbon Dots Su-Young Son, Han-Na Jo, Min Park, Gun Young Jung, Dong Su Lee, Sungho Lee, and Han-Ik Joh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01959 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Applied Materials & Interfaces

Unusual Thermal Conductivity of Carbon Nanosheets with Self-Emerged Graphitic Carbon Dots

Su-Young Son†§, Hae-Na Jo†, Min Park‡, Gun Young Jung§, Dong Su Lee*‡, Sungho Lee*†, and Han-Ik Joh*ǁ

†Carbon

Composite Materials Research Center, Korea Institute of Science and

Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeallabuk-do 55324 Republic of Korea ‡Applied

Quantum Composites Research Center, Korea Institute of Science and

Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeallabuk-do 55324 Republic of Korea §School

of Materials Science and Engineering, Gwangju Institute of Science and

Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005 Republic of Korea

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ǁDepartment

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of Energy Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-

gu, Seoul 05023 Republic of Korea

KEYWORDS: thermal conductivities, hierarchical structures, carbon nanosheets, graphitic carbon dots, heat sink materials

ABSTRACT

Thermal conductivity (κ) of 2-dimensional conducting and transparent carbon nanosheets (CNSs) prepared by a catalyst- and transfer-free process is calculated for the first time by the optothermal Raman technique. A systematic structural analysis of CNSs reveals that the thickness of polymer films affects the interaction between molecules and a Si wafer significantly, thus helping determine the ratio of sp2 and sp3 bonding configurations of carbon (C) atoms in the CNS. Notably, holding time of carbonization can realize a hierarchical structure with graphitic carbon dots emerging from the CNS through the


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rearrangement of carbon atoms, leading to the excellent κ value of ~ 540 W/m·K at 310 K. It is demonstrated that an appropriate increase of carbonization time can be an effective approach for improving the ratio of sp2 to sp3-bonded C atoms in the CNS. The thermal conductivity of the CNS with the highest ratio of sp2 to sp3-bonded C atoms exhibits superior behavior than and is comparable to that of reduced graphene oxide and supported graphene, respectively. Finally, when the CNS with the highest κ value of ~ 540 W/m·K was applied to a heater as the heat-dissipating material, the heater showed the temperature decrease by ~ 14 °C compared to the case without the CNS. The catalyst- and transfer-free approach for synthesis of CNSs is highly desirable for use as heat sink materials or substrates with heat dissipation functions for extensively integrated electronic devices.


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Introduction With electronic devices getting more highly integrated, heat management has become a critical issue for durability of internal components. To efficiently remove the heat generated in these devices, heat sink materials with high thermal conductivity such as metals and carbon allotropes have been extensively studied in recent decades.1–5 Among these materials, carbon allotropes exhibit an extraordinary thermal conductivity that ranges from ~ 0.01 (amorphous carbon) to above 2,000 W/m·K (graphene) at room temperature based on their dimension, size, and crystallinity.6–9 In particular, graphene, which is a first truly 2-dimensional (2D) carbon material, has been considered as a promising material for next-generation heat sinks owing to its extremely high and widely tunable thermal conductivity. However, the use of graphene flakes prepared by micromechanical and chemical exfoliation methods in practical applications is hindered by poor mass production and low thermal conductivity, respectively.10 The graphene prepared by chemical vapor deposition method exhibited superior thermal conductivity compared to other heat sink materials. It is well known that the excellent heat conduction


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in graphene originates from the atomically well-aligned structure due to the catalyst effects during its synthesis.7, 11, 12 Polymer-derived carbon nanosheets (CNSs) with similar properties to graphene have received much attention due to their simple processability relative to the chemical vapor deposition method. The process based on carbon fiber (CF) manufacturing is performed under catalyst-free and atmospheric conditions using solid carbon sources such as polyacrylonitrile (PAN), pitch, and polymers of intrinsic porosity.13–17 To synthesize CNSs, a spin-coated polymer film on a silicon (Si) wafer was stabilized at 270 °C in air to arrange PAN molecules into a ladder structure. The stabilized polymer film was carbonized at approximately 1200 °C under H2/Ar atmosphere, resulting in atomically thin CNSs with six-membered carbon rings even though they had a small amount of defects due to the catalyst-free reaction.13 Their electrical, optical, and structural properties could be easily adjusted by controlling experimental variables such as polymer types, polymer concentration, and heating temperature, leading to their wide applications in transparent, conducting, and flexible devices.13,

15, 17

To apply CNSs as heat sink materials, it is

essential to evaluate their thermal conductivity and understand which structural properties


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dominantly affect heat transfer. In addition, CNSs could be used as substrates for electronic devices with heat dissipation functions because one of the main advantages of CNSs is their facile synthesis with their direct formation onto a Si wafer without a transfer process. Typical measurement techniques, such as laser flash, 3-ω, and thermal bridge methods, have been used to accurately evaluate thermal conductivities of heat sink materials. However, these techniques are not suitable for investigating the intrinsic thermal conductivity of 2D materials such as graphene and CNSs due to their atomically thin planar structures. Firstly, Balandin et al., in 2008, had introduced an optothermal Raman measurement technique to measure the thermal conductivity of graphene.5 Since then, the technique has been modified by the Rouff group in 2010,11 and the thermal conductivity of various 2D materials such as transition metal dichalcogenides18–20 and black phosphorus,21 measured by similar approaches have been reported. The key idea of this technique originates from the shifts of phonon frequencies depending on temperature and laser power when the Raman laser is focused at the center of the suspended 2D materials. Two factors enable the use of this technique for calculating


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thermal conductivity of 2D materials.5, 11, 12, 18– 22 First, 2D materials have unique and clear Raman fingerprints. Second, the Raman-active phonon frequency reacts sensitively to temperature changes. Single-layer graphene (SLG) is a good representative example of these effects. The graphitic (G) peak for the SLG at approximately 1,580 cm-1 that arises from the in-plane Raman-active phonon mode in graphite is red-shifted by increase in temperature and laser power. As reported, the extracted thermal conductivity of SLG from the G peak shift is ~ 2,500 W/m·K near 350 K.11 Hence, it is believed that the optothermal Raman measurement has been fairly useful for the investigation of intrinsic thermal conductivities of atomically thin 2D materials and is comparable to the other techniques used for bulk materials.5, 11, 12, 18–22 In this study, we first report the thermal conductivities of PAN-derived CNSs with thicknesses from approximately 1 nm to tens of nm by the optothermal Raman technique, similar to that used for the studies of graphene. The relationship between thermal conductivity and structure of PAN-derived CNSs was also revealed by analyzing their structural properties. On the basis of their relationship, we describe the advanced structure and thermal conductivity of CNSs by fabricating hierarchical structure decorated


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with graphitic carbon dots (GCDs) via control of carbonization time. Lastly, we demonstrate the potential of CNS as a heat sink material via a heater device.

Results and discussion Thermal conductivity of CNSs by thickness control For the systematic study on thermal conductivity of CNSs with various thicknesses, we prepared five kinds of CNSs on a Si wafer by controlling PAN concentration in solvent from 0.5 to 3 wt%; these are designated by CNS 0.5, CNS 0.75, CNS 1.0, CNS 2.0, and CNS 3.0. CNSs were transferred and suspended on gold (Au)-deposited silicon nitride (Si3N4) membranes with holes of diameter 5 μm to prevent thermal scattering from substrates, as shown in Figure 1a. Then, Raman spectra for suspended CNSs were obtained using a 514 nm laser beam with a radius of 0.39 μm for a 50x objective. A detailed information is provided in the supporting information (SI). Based on previous reports, thermal conductivity of 2D materials can be expressed by the following equation, which is induced from a heat diffusion equation in cylindrical coordinates.11


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𝛼 ∙ 𝑙𝑛

𝜅=

[

(𝑅 𝑟0)

2𝜋𝑡 (𝑇𝑚 ― 𝑇𝑎) 𝑄

]

(1)

where κ, 𝑅, 𝑟0, 𝑡, 𝑇𝑚, 𝑇𝑎, and 𝑄 are thermal conductivity, radius of holes, radius of laser beam, thickness of CNSs, measured temperature at the center of CNSs, ambient temperature, and total absorbed power of intrinsic CNSs, respectively. Figures 1b and c show the shifts of G peak position as a function of temperature and laser power, respectively. Since broad D and G peaks were observed in the Raman spectrum of PAN-derived CNS, all spectra were deconvoluted into five peaks using Lorentz and Gaussian functions (Figure S1). Peak positions were obtained by averaging five measurements, which were performed at different holes. It is interesting that the G peaks of CNSs are red-shifted with increase in temperature and laser power. In particular, a linear regression equation, obtained from relationship between temperature and G peak position shown in Figure 1b, is used as a thermometer to predict rising temperature of CNSs due to absorbed laser (Figure 1d). Values of parameters in Equation 1 used to calculate the thermal conductivity of CNSs are presented in experimental section (SI) and Table S1.


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Figure 1. (a) Experimental setup for measuring thermal conductivity of CNSs using the optothermal Raman technique. Optical images of suspended CNS 3.0 on Au-deposited Si3N4 membranes with holes of diameter 5 μm. Variation of G peak position of CNSs with (b) temperature and (c) laser power change. (d) Measured temperature at the center of suspendedCNSs for absorbed power. From this measurement, thermal conductivities of CNSs prepared using PAN polymer with lower concentration exhibited higher values compared to those with higher concentration all over the temperature (Figure 2), indicating that the thickness-dependent properties are similar to those of graphene. Interestingly, thermal conductivities of CNSs with a thickness less than 3 nm decreased sensitively with an increase in temperature,


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while the relatively thick CNSs exhibited a constant value with negligible variation. The 1.4 nm-thick CNS 0.5 exhibited the highest value of ~ 100 W/m·K at 370 K.

Figure 2. Thermal conductivities of CNSs of varying thicknesses as function of temperature.

Correlation between structural characteristics and thermal conductivity of CNSs Heat transfer in carbon materials is predominantly influenced by lattice phonons rather than free electrons except for heteroatom-doped ones.6, 23 Therefore, it is speculated that the thermal conductivity of carbon materials is closely related to their structural properties such as elementary composition, degree of crystallinity, and crystallite size.6, 8, 9 For a systematic study of the relationship between the structural characteristics and thermal


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conductivity of CNSs, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM) measurements were conducted. We conducted XPS analysis to investigate the dependence of the chemical composition of CNSs on their thickness. Figure S2b and Table S2 show the atomic ratio of CNSs calculated by wide-scan survey spectra (Figure S2a). Carbon (C), nitrogen (N), and oxygen (O) are detected in all CNSs. However, O and Si atoms associated with a silicon oxide layer used as the substrate are also detected on CNS 0.5 to 1.0 with the thicknesses of 2.8 nm or smaller due to penetration of the X-ray beam. Hence, we estimated the atomic fraction of O in CNS 0.5 to 1.0 by averaging the O ratios (approximately 1.24 %) from CNS 2.0 to 7.0, for which Si was not detected (Table S2).13 Thus, we believe that PAN-derived CNSs are uniformly composed of approximately C, N, and O of 96.6, 2.2, and 1.2 %, respectively, regardless of their thickness. Figure 3a shows the dependence of the high-resolution C 1s spectra of CNSs on polymer concentrations in the 0.5 ~ 3.0 wt% range indicating that for all samples, six peaks at 284.5, 285.6, 286.4, 287.4, 289.0, and 291.0 eV were observed, corresponding to sp2 (C=C), sp3 (C-C), CO/C-N, C=O/C=N, O-C=O, and π-π* shake up satellite peaks, respectively.24 The relative


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areas based on intensities of deconvoluted C 1s peaks are presented in Figure 3b. Total amounts of sp2 and sp3-bonded C atoms in CNSs are similarly 78.5 ± 0.5 % regardless of their thickness, resulting in the uniform composition of CNSs. However, the ratio of sp2 to sp3-bonded C in CNSs decreased with their increasing thickness. In particular, the ratio of sp2 to sp3-bonded C in CNS 3.0 is found to be 5.5 ± 0.3, which is 16.7 % lower than that in CNS 0.5 (6.6 ± 0.3). This indicated that the content of sp3-bonded C in the thick CNS 3.0 is higher than that in the other CNSs even though PAN-derived CNSs are composed of C, N, and O with similar atomic ratios regardless of their thickness.


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Figure 3. (a) High-resolution C 1s XPS spectra and (b) relative amounts of C 1s composition in CNSs as a function of PAN concentration. To precisely measure the fraction of sp3-bonded C in the CNSs, we perform ultraviolet (UV)-Raman measurements at 244 nm. Visible (VIS)-Raman spectroscopy with the excitation energy of 2.4 eV can only excite phonons in the π-state associated with sp2 sites.25–27 On the other hand, UV-Raman spectroscopy at 244 nm, which has the higher excitation energy of 5.1 eV, is sufficient for exciting phonons both in σ (sp3, diamond-like) and π (sp2, graphite-like) states.25–27 Figure S3 presents Raman spectra of CNS 3.0 measured at 244, 514, and 633 nm laser wavelengths. Only two peaks corresponding to D and G peaks are observed in VIS-Raman spectra (514 and 633 nm laser wavelengths), whereas in the UV-Raman spectrum (244 nm laser wavelength), we can also observe a T-peak near 1,075 cm-1 due to the stretching of sp3 sites.25–27 It is evident that UV-Raman spectroscopy is an informative tool for detecting the presence and quantifying contents of sp3 sites of carbon materials. Figure 4a shows deconvoluted UV-Raman spectra of all CNSs as a function of their thickness. Three unique peaks are observed in deconvoluted UV-Raman spectra at


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approximately 1,075, 1,530, and 1,600 cm-1, corresponding to T, D, and G-peaks, respectively. In addition, similar to XPS results presented in Figure 3, the relative area of the T-peak in CNS 3.0 is larger than that in CNS 0.5 and 1.0 (Figure 4). Meanwhile, Figure S4b shows the thickness dependence of the in-plane crystallite length (La) of CNSs. The La of CNSs had a similar value of approximately 2 nm regardless of their thickness, and their TEM images also showed no significant differences for all CNSs (Figure S5). Therefore, it is believed that the thickness and the ratio of sp2- and sp3-bonded C in CNSs significantly affected heat transfer by lattice vibrations.6


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Figure 4. (a) Deconvoluted ultraviolet (UV)-Raman spectra and (b) relative areas of T, D, and G-peaks in CNSs as a function of PAN concentration.

The fact that the bonding configuration of C atoms is closely related to thermal conductivity, which depends highly on the thickness of the CNSs, implies that formation and orientation of polymer chains are associated with the interactions with the substrate surface. First, PAN molecules are aligned along the spin direction on the substrate by centrifugal force during spin-coating process. Molecules with linear chains are converted to the hexagonal structure of carbon rings with nitrogen atoms via intramolecular reaction


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during the stabilization.28 In a further carbonization, the hybridized structure in CNSs may be formed through the intermolecular interaction between PAN molecules, and surface of the substrate has a strong influence on the formation of sp2-bonded C layers. In the case of atomically thin polymer film, chains directly on the substrate are too difficult to move or orient during formation of the hybridization structure. On the other hand, polymer chains with a relatively weak interaction in a thick film can be freely moved to form intermolecular bonds, such as sp2 and sp3, regardless of bonding configuration. Therefore, the interaction between chain and substrate significantly affects formation of sp2-bonded C layers such as small graphitic units, even though it may show a complementary relationship to the bonding configuration.

Thermal conductivity of hierarchical CNSs with self-emerged graphitic carbon dots It is evident from the above results that the thermal conductivity of CNSs could be enhanced by adjusting thickness and bonding configuration of C atoms. Therefore, we fabricated hierarchical structure of CNSs decorated with GCDs by controlling carbonization time. The CNS 0.5 with the smallest thickness of 1.4 nm was additionally


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heat treated for different holding times from 1 h to 4 h to understand the changes in their structural and thermal properties. These are designated by CNS 1h, CNS 2h, CNS 3h, and CNS 4h, respectively. First, we performed XPS analysis to investigate the dependence of the ratio of sp2 to sp3-bonded C in CNSs on holding time of carbonization. Figure 5a indicates highresolution C 1s spectra of CNSs, and all spectra were deconvoluted in the same manner as described above. Based on deconvoluted C 1s peaks, the relative area of chemical configuration consisting of C atoms were depicted in Figure 5b. Total amounts of sp2- and sp3-bonded C atoms in CNSs were 78.2 ± 0.6 % irrespective of the carbonization time. On the other hand, the ratio of sp2 to sp3-bonded C increased with a longer holding of carbonization. To be specific, while the conventional CNS 0.5 without holding showed the ratio of sp2 to sp3-bonded C of 6.6 ± 0.3, the holding for 3 h at 1150 °C resulted in a significant increase of the value up to 7.3 ± 0.3. It is interesting to note that the ratio of sp2 to sp3-bonded C in CNS 4h decreased sharply down to 6.3 ± 0.3.


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Figure 5. (a) High-resolution C 1s XPS spectra and (b) relative amounts of C 1s compositions in CNSs for different holding times of carbonization. Figure 6 shows TEM images of CNSs fabricated by various holding times of carbonization. It is noteworthy that circular GCDs were formed within CNSs. Few GCDs were observed in the conventional CNS 0.5, however, larger amounts of GCDs were distributed with increasing carbonization holding time up to ~ 3 h. The average diameters of GCDs between 1 and 6 nm are 3.2 ± 0.6 nm, 3.2 ± 0.6 nm, and 5.4 ± 0.6 nm for CNS 1h, 2h, and 3h, respectively. Figure 6f displays a high-resolution TEM (HRTEM) image of the representative GCD with a lattice constant of 3.36 Å, corresponding to a (002) plane


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of graphite.29 Also, the inset in Figure 6f shows the corresponding fast Fourier transform (FFT) pattern, which indicates that a hexagonal lattice originates from a highly crystalline GCD. However, even though GCDs partially existed in CNS 4h, irregular shaped holes with sizes ranging from several nm to tens of nm were formed all over the sheet due to an excessive heat treatment during the carbonization process (Figure 6e). From these results, it is suggested that an appropriate holding time is a very helpful approach for enhancing the ratio of sp2- and sp3-bonded C atoms in CNSs via the hierarchical structure of CNSs decorated with highly crystalline GCDs.


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Figure 6. (a-e) TEM images of CNSs for different holding times of carbonization. (f) A HRTEM image of the representative GCD. The inset is the corresponding FFT pattern.

Thermal conductivities of CNSs with holding time of carbonization were calculated in the same manner as described in SI. Figures 7a and b present the shifts of G peak positions of CNSs depending on temperature and power, respectively. Their thicknesses (Figure S6) measured by AFM are 1.4 nm, 1.3 nm, 1.3 nm, and 1.2 nm in CNS for holding times of 1 h, 2 h, 3 h and 4 h, respectively. Overall, Figure 7c shows the thermal conductivities of CNSs with holding time throughout the temperature range shown. Similar to the tendency of bonding configuration of C atoms, the CNS 3h indicating the highest ratio of sp2- and sp3-bonded C atoms exhibited the highest thermal conductivity of ~ 540 W/m·K at 310 K. We compared the thermal conductivity of CNSs to that of other 2D-carbon materials, such as mechanically exfoliated graphene,5,

22

CVD-grown graphene,11 supported

graphene,30 graphene oxide (GO),9 and reduced-graphene oxide (rGO)9, 31 at near room temperature as a function of their thicknesses, as shown in Figure 7d. The details are


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summarized in Table S3. The CNSs regardless of their thicknesses exhibited superior thermal conductivity to rGO (below 40 W/m·K), even though SLG with the ideal hybridized structure exhibited thermal conductivity above 1,000 W/m·K. In addition, it should be noted that thermal conductivity of the CNS is 50 times higher than that of CF prepared with identical material and under identical conditions such as PAN precursor, stabilization, and carbonization. Considering thermal conductivities of carbon materials, we believe that CNSs prepared directly by catalyst- and transfer-free process would be a promising strategy to resolve thermal dissipation in the super integrated circuit.32 An Au-based heater was prepared to validate the CNSs for heat-dissipating. To this end, we chose the CNS 3h because it had the highest thermal conductivity of ~ 540 W/m·K. The degree of heat dissipation of CNS was measured from a temperature difference of the heater without and with CNS. Detailed experimental methods are described in the SI. Figure 8a indicates the calibration curve of resistance to temperature of the heater. As a result, when the CNS was applied to the heater as the heat-dissipating material, the significant temperature decrease by ~ 14 °C (3 W of heat, 300 W/cm2 of heat flux) was observed (Figures 8b and c). From the results, it is expected that CNS, which


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has the high thermal conductivity, will be utilized as the heat sink material for electronic devices.

Figure 7. Variation of G peak position of CNSs with (a) temperature and (b) laser power changes. (c) Thermal conductivities of CNSs fabricated by holding time of carbonization. (d) Comparison of thermal conductivity between CNSs and other 2D carbon materials depending on their thicknesses. ([5, 22] mechanically exfoliated graphene, [11] CVD-grown graphene, [30] supported graphene, [9] graphene oxide (GO), [9, 31] reduced-graphene oxide (rGO), [33] PAN-based carbon fiber)


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Figure 8. (a) Calibration curve on temperature vs. resistance of the heater. Temperature changes depending on (b) applied power and (c) heat flux in the heater with and without CNS.

Conclusion We demonstrate the potential of PAN-derived CNSs as a heat sink material through the precise measurement of thermal conductivity using the optothermal Raman technique. CNSs exhibited thermal conductivity values ranging from 40 to 100 W/m·K as a function of their thickness, which could be easily controlled by varying the polymer concentration. In particular, the interaction between polymer chain and Si wafer may significantly affect the formation of sp2-bonded C layers, leading to CNSs with different thermal conductivities. Furthermore, the hierarchical CNSs with self-emerged crystalline GCDs were fabricated by controlling the holding time of carbonization. The CNS with the


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highest ratio of sp2 to sp3-bonded C atoms exhibited an excellent κ value of ~ 540 W/m·K at 310 K. Therefore, it is believed that CNSs directly prepared on the Si wafer by a catalyst-free process could be realized as an electronic device substrate with high heat spread functions, even though CNSs have more structural defects and lower thermal conductivity than the SLG prepared by impractical methods.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental section, Raman, XPS, TEM, and depth profile (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D.S.L). *E-mail: [email protected] (S.L.). *E-mail: [email protected] (H.-I.J.).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by a grant from the Korea Institute of Science and Technology (KIST) Institutional program (No. 2Z05400), the Industrial Core Technology Development Program (10052760), the Carbon Cluster Development Program (10083586), funded by the Ministry of Trade, Industry and Energy, Republic of Korea, and Basic Science Research Program through the National Research Foundation of Korea (NRF) (NRF-2018R1D1A1B07045368 and NRF-2018M1A2A2061989).

ABBREVIATIONS CNS, carbon nanosheet; CF, carbon fiber; PAN, polyacrylonitrile; Si, silicon; SLG, single-layer graphene; G, graphitic; GCD, graphitic carbon dot; Au, gold; Si3N4, silicon nitride; XPS, X-ray photoelectron spectroscopy; TEM, transmission electron microscopy; C, carbon; N, nitrogen; O, oxygen; UV, ultraviolet; VIS, visible; La, in-plane crystallite length; FFT, fast Fourier transform; AFM, atomic force microscopy; CVD, chemical vapor deposition; GO, graphene oxide; rGO, reduced-graphene oxide.


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