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Thermal conductivity reduction of multilayer graphene with fine grain sizes

  • Woomin Lee
  • Kenneth David Kihm
  • Seung Hwan KoEmail author
Letter
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Abstract

The thermal conductivities of monolayer graphene with 4.1 μm grain size and multilayer graphene with 0.3 μm grain sizes are measured by optothermal Raman technique. The number of layers and average grain sizes of graphene are controlled by copper thickness and synthesis conditions in chemical vapor deposition system. In addition, the graphene samples are suspended on a thorough 8-μm hole substrate via PMMA transfer method to avoid substrate effects. As a result, the thermal conductivity graphene is significantly reduced from 3000 to 660 W/m K at 320 K with increasing graphene layers and decreasing grain sizes due to the enhanced phonon scattering. The multilayer graphene with fine grain sizes that has the suppressed thermal conductivity can be useful for application in various field, such as thermoelectric material and thermal rectification.

Keywords

Thermal conductivity reduction CVD graphene Multilayer Grain size 

List of symbols

A

Area

α

An integral function of r0 and R

d

Average grain size

n

The number of grains

Q

Absorbed heat

R

Hole-suspended graphene of radius

r0

Laser beam radius

T

Temperature

Ta

Ambient temperature

Tm

Measured graphene temperature

t

Thickness of graphene

k

Thermal conductivity

1 Introduction

Graphene [1], that is a two-dimensional material of carbon atoms in the form of hexagonal lattice, has many novel properties, such as high electrical mobility [2], optical transparency [3], and superior mechanical strength [4]. Especially, graphene has a extraordinarily high thermal conductivity [5] due to long phonon mean free path affiliated with the strong carbon bondings in two-dimensional lattice [6, 7]. The high thermal conductivity is significantly reduced with increasing the number of layers [8] and decreasing grain sizes [9], respectively. However, the reduced thermal conductivity of graphene can be useful for the application in various fields, such as thermoelectric material [10] and thermal rectification [11]. Therefore, we tried to intentionally reduce the thermal conductivity of graphene by increasing the number of graphene layer and decreasing grain sizes for the application.

In this study, the thermal conductivity of multilayer graphene with sub-μm grain size is measured and compared to monolayer graphene. In CVD system, monolayer graphene with μm grain size and multilayer graphene with sub-μm grain size are synthesized by controlling synthesis conditions and the thickness of copper catalyst. Then, graphene samples are transferred on a through 8-μm hole via PMMA transfer method [12] to avoid substrate effects. Finally, the thermal conductivities of graphene samples are measured and compared by optothermal Raman technique [5].

2 Sample preparation and experiment

2.1 Sample preparation

Polycrystalline graphene was synthesized by manipulating synthesis conditions in CVD (Chemical Vapor Deposition) system. In general, the initial nucleation density increases when synthesis temperature is low and methane flow rates are high [13, 14, 15, 16]. In addition, the initial grain density increases using the copper catalyst with lower purity [17]. The initial nucleation density is directly related to the average grain sizes of graphene, so the proper synthesis recipe and copper purity allow the CVD graphene to have various grain sizes. Furthermore, the purity of copper catalyst plays a critical role in determining the number of graphene layers that can be attributed to the enhanced catalytic capability due to surface impurities of copper catalyst [18], thus multilayer graphene can be grown on the lower purity of copper catalyst. As a result, graphene samples with different layers and grain sizes were synthesized by controlling synthesis conditions and copper purity as summarized in Table 1.
Table 1

The purity of copper and synthesis conditions to obtain graphene samples with different grain sizes and number of layers

Grain size (μm)

4.1

0.3

Number of layers

1

3

The purity of copper (%)

99.9999

99.9

Synthesis temperature (°C)

1000

900

CH4:H2 pressure ratio

30:5

80:5

Based on the initial nucleation density of SEM images in Fig. 1a, the average grain sizes of graphene were defined as the effective diameter of each polygonal grain, i.e., \(\left( {\pi d^{2} /4} \right) \times n = A\) where d is the average grain size, n is the number of grains, and A is area. The average grain sizes of the two graphene are 4.1 and 0.3 μm, respectively.
Fig. 1

a SEM images of different nucleation density in the initial growth state. b Optical image of the suspended graphene with 4.1 μm grain size on the 8 μm hole substrate. c Raman spectra of graphene samples. d Schematic illustration of measuring thermal conductivity by optothermal Raman method

The number of graphene layers was estimated by the measured absorbance because the optical absorbance linearly increases with increasing the number of graphene layers [3]. The optical absorbance of graphene was determined from the transmission measurements with and without graphene laid on a thorough hole pattern. The measured absorbance of graphene samples were \(3.32 \pm 0.33\) and \(8.86 \pm 1.02\). The graphene with grain size of 4.1 μm seems to monolayer because the measured absorbance falls within the known absorbance range of monolayer CVD graphene [19, 20, 21, 22]. On the other hand, the measured absorbance of graphene with grain size of 0.3 μm is much higher than monolayer about 3 times, so the effective number of graphene layers is estimated to 3 layers.

As shown in Fig. 1b, all graphene samples were suspended on the 8-μm hole pattern substrate via poly(methyl methacrylate) transfer method [12] to avoid substrate effect on thermal transport in graphene sheets. And the hole diameter of 8 μm is sufficiently large so that the absorbed heat at the center region of the suspended graphene can be assumed to transport to the hole edge by diffusion [9].

In Fig. 1c, the Raman spectra of monolayer graphene with 4.1 μm grain size show high G peak to 2D peak intensity ratio that is the typical footprint of single-layer graphene [23].

Also, there is no D peak around 1350 cm−1, showing the sheet quality of the graphene is very excellent [24, 25]. However, D peak is more pronounced and G peak to 2D peak intensity ratio decreases due to multilayers [26] and small grain sizes [9] for the multilayer graphene with 0.3 μm grain size.

2.2 Experiment

In this research, the thermal conductivity of graphene samples was measured by optothermal Raman method [5, 21] with measurements of temperature-dependent 2D peak shifts in the Raman spectra as depicted in Fig. 1d. When the absorbed heat from incident laser is diffusively conducted along graphene and the convection loss from graphene surface to air is ignored, the thermal conductivity of the suspended graphene k is given as
$$k = \frac{{\ln \left( {{R \mathord{\left/ {\vphantom {R {r_{0} }}} \right. \kern-0pt} {r_{0} }}} \right)}}{{2\pi t\left( {{{T_{\text{m}} - T_{\text{a}} } \mathord{\left/ {\vphantom {{T_{\text{m}} - T_{\text{a}} } Q}} \right. \kern-0pt} Q}} \right)}}\alpha$$
(1)
where R is the hole-suspended graphene of radius, r0 is the laser beam of nominal radius, t is the thickness of graphene, Ta is ambient temperature, and Q is the absorbed heat. The measured graphene temperature Tm is determined by 2D Raman shifts and \(\alpha\) represents an integral function of r0 and R [9].
As shown in Eq. (1), the absorbed power and temperature rise of graphene by laser irradiation should be measured to determine the thermal conductivity of graphene where other parameters are known values except for Tm and Q. At first, the temperature coefficients of Raman 2D peak shift were measured as shown Fig. 2. Raman 2D peak has a linear trend in the experiment temperature ranges (300–500 K) [19, 21, 27], so the graphene temperature can be estimated using the temperature coefficients. The measured temperature coefficients are dω/dT = − 0.0.328 cm−1/K and − 0.0511 cm−1/K for the multilayer graphene with 0.3 μm grain sizes and the monolayer graphene with 4.1 μm grain size, respectively.
Fig. 2

The temperature coefficients of Raman 2D peak shifts with increasing temperature for two graphene samples

3 Results and discussions

Figure 3 shows the measured thermal conductivities of monolayer graphene with grain size of 4.1 μm and multilayer graphene with grain size of 0.3 μm. In addition, a previous result of exfoliation graphene that shows the highest thermal conductivity of the measured thermal conductivity of graphene [5] is plotted in Fig. 3 to compare with our results. The 25% error bars are added because the average errors of the optothermal Raman measurement was 25% in the previous study [9]. The thermal conductivities of multilayer graphene with 0.3 μm grain size are further lower than those of monolayer graphene with 4.1 μm grain size. The results can be attributed to two reasons; one is that the enhanced grain boundary scattering because smaller grain sizes limit the long-wavelength phonon contributions, resulting in lowered thermal conductivities [9]. The other is that the out-of-plane acoustic modes, which dominantly carry the absorbed heat in graphene sheets, are restricted due to the increase of graphene layers [28].
Fig. 3

The measured thermal conductivities of monolayer graphene with grain sizes of 4.1 μm and multilayer graphene with grain sizes of 0.3 μm

The thermal conductivities of two graphene samples decrease with increasing temperature due to the enhanced Umklapp scattering at higher temperature [7, 29]. Grain boundary scattering largely depends on grain boundary density or grain sizes unlike Umklapp scattering that has strong temperature dependency [30]. As a result, the negative temperature dependence of thermal conductivities is weakened with decreasing grain sizes from ~ T−1.91 to ~ T−0.86 in Fig. 3, because the heat is transported by largely grain boundary scattering rather than Umklapp scattering for graphene with smaller grain sizes.

4 Conclusions

In summary, the measured thermal conductivities of graphene with different layers and grain sizes were compared to effectively reduce thermal transport in graphene. By manipulation of synthesis conditions and copper thickness, monolayer graphene with 4.1 μm grain size and multilayer graphene with 0.3 μm grain size were synthesized in CVD system. The measured thermal conductivities of multilayer graphene with 0.3 μm grain size (660–330 W/m K for 320 K < T < 550 K) is significantly lower than those of monolayer graphene with 4.1 μm grain size (3000–1280 W/m K for 320 K < T < 550 K).

Notes

Acknowledgements

This research was primarily supported by the Nano-Material Technology Development Program (R2011-003-2009) and Basic Science Research Program (2017R1A2B3005706), and Global Frontier R&D Program on Center for Multiscale Energy System (Grant No. 2012-054172) through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning, and was also partially supported by the Magnavox Professorship fund from the University of Tennessee (R0-1137-3164) and Institute of Engineering Research at Seoul National University.

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Copyright information

© The Korean Society of Mechanical Engineers 2019

Authors and Affiliations

  • Woomin Lee
    • 1
  • Kenneth David Kihm
    • 2
  • Seung Hwan Ko
    • 1
    • 3
    • 4
    Email author
  1. 1.School of Mechanical and Aerospace EngineeringSeoul National UniversitySeoulRepublic of Korea
  2. 2.Mechanical, Aerospace, and Biomedical EngineeringThe University of TennesseeKnoxvilleUSA
  3. 3.Institute of Advanced Machinery and Design (SNU-IAMD)Seoul National UniversitySeoulRepublic of Korea
  4. 4.Institute of Engineering ResearchSeoul National UniversitySeoulRepublic of Korea

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