Tailoring the thermal and electrical transport properties of graphene films by grain size engineering

Abstract

Understanding the influence of grain boundaries (GBs) on the electrical and thermal transport properties of graphene films is essentially important for electronic, optoelectronic and thermoelectric applications. Here we report a segregation-adsorption chemical vapour deposition method to grow well-stitched high-quality monolayer graphene films with a tunable uniform grain size from ∼200 nm to ∼1 μm, by using a Pt substrate with medium carbon solubility, which enables the determination of the scaling laws of thermal and electrical conductivities as a function of grain size. We found that the thermal conductivity of graphene films dramatically decreases with decreasing grain size by a small thermal boundary conductance of ∼3.8 × 10⁹ W m^-2 K^-1, while the electrical conductivity slowly decreases with an extraordinarily small GB transport gap of ∼0.01 eV and resistivity of ∼0.3 kΩ μm. Moreover, the changes in both the thermal and electrical conductivities with grain size change are greater than those of typical semiconducting thermoelectric materials.

Figure 1. SACVD growth of polycrystalline graphene films with well-controlled grain sizes.

(a) Schematic for the fabrication process of a polycrystalline graphene film. (b) Scanning electron microscope (SEM) image of a graphene film, mostly monolayer, grown on Pt with a mixture of hydrogen (700 standard-state cubic centimetre per minute, sccm) and methane (7 sccm) for 10 min. (c) SEM image of the Pt substrate in b after treating with pure argon (700 sccm) for 20 min, showing that the graphene film has disappeared. (d) SEM image of the Pt substrate in c after treating with a trace of hydrogen (5 sccm) for 20 min, showing that many small graphene domains have appeared. (e) SEM image of a monolayer polycrystalline graphene film formed from d by introducing a low flow rate of methane (0.1 sccm) for 1 h. The reaction temperature was all 900 °C in above cases.

Figure 2. Structural characterization of graphene domains and films.

(a–d) SEM images of graphene domains obtained with a segregation temperature of 900, 950, 1,000 and 1,040 °C, showing that the domain density decreases with segregation temperature. (e–h) False-colour, dark-field image overlays of the graphene films formed by growth and stitching of the graphene domains in a–d. Scale bars, 500 nm. (i,j) High-magnification HRTEM images of graphene films with grain size of ∼200 and ∼700 nm, respectively. The pentagons (blue), heptagons (red) and hexagons (yellow) in the GBs are outlined. All images were processed with an improved Wiener-filtering to remove the noises. Scale bars, 1 nm. (k–n) Histograms of grain sizes of the graphene films in e–h, showing that the grain size is very uniform for each sample.

Figure 3. Thermal transport of graphene films with ∼200 nm-sized grains.

(a) SEM image of a polycrystalline graphene film on a holey SiO₂/Si substrate. Scale bar, 10 μm. (b) Optical image of a polycrystalline graphene film transferred onto a holey SiO₂/Si substrate. Scale bar, 10 μm. (c) Raman map of the polycrystalline graphene film shown in b, and the typical Raman spectra are shown in Supplementary Fig. 12. (d) Raman spectra of the polycrystalline graphene film excited with different power lasers. (e,f) Intensity (e) and position (f) of the 2D peak as a function of laser power.

Figure 4. Thermal and electrical transport of the graphene films with different grain sizes.

(a) Thermal conductivity as a function of grain size with a fit (red curve). The error bars (standard error of the mean, s.e.m.) represent the thermal conductivity variation measured for the same sample. (b) The inverse of thermal conductivity as a function of the inverse of grain size with a fit (red curve), showing a linear relationship. (c) Sheet resistance as a function of grain size with a fit (red curve). (d) Electrical conductivity as a function of grain size with a fit (red curve), showing an exponential relationship. The error bars (s.e.m.) in c and d represent the electrical conductivity variation measured for the same sample and the samples prepared with the same conditions.

Figure 5. Thermal/electrical conductivity change rate of graphene with grain size change rate.

(a) Thermal conductivity change rate of graphene as a function of grain size change rate with a fit (blue curve), showing a linear relationship. (b) Electrical conductivity change rate of graphene as a function of grain size change rate with a fit (blue curve), showing an exponential relationship. The thermal/electrical conductivity change rates of some typical metals (Au⁴⁷, Al⁴⁷, Ag⁴⁸ and Cu⁴⁹) and semiconducting thermoelectric materials (BiSbTe³³, SrTe³⁴ and BiTeSe³⁵) are also shown in different colours for comparison.