Phonon anharmonicities in supported graphene
Graphical abstract
Introduction
Due to the unique physical properties of graphene, it is a promising material for a variety of applications [1]. In particular, it demonstrates high values of carrier mobility and thermal conductivity [[1], [2], [3], [4]]. Electrical and thermal transport in graphene are known to be strongly affected by anharmonic phonon-related effects such as electron-phonon coupling [[5], [6], [7]] or high-order phonon-phonon interactions [[8], [9], [10], [11]], including coupling of optical and acoustic phonon modes. Consequently, these processes are directly related to such common problems of nanoelectronics as increasing electrical conductivity or removing heat from the device functional elements, as well as applied tasks such as determining graphene thermal conductivity [8,12], which underlines the importance of investigating the anharmonic phonon properties of graphene.
At the same time, various nanoelectronic applications require graphene layers supported by the substrates, which in turn affect physical properties of the two-dimensional material. Graphene phonon dispersion is very sensitive to different interatomic forces such as graphene-substrate interaction [13]. As it was shown in Ref. [14] for graphene on such a common substrate as SiO2/Si, coupling of acoustic phonons to the substrate Rayleigh waves may linearize the dispersion, increasing hybridized mode group velocity. Phonons can leak through the interface [15] or scatter on it [16]. As graphene gets adsorbed on the substrate surface, modification of graphene phonon dispersion can be significant; it will expectedly lead to stronger anharmonic effects in atom oscillations, making an experimental study of supported graphene phonon properties relevant.
Raman spectroscopy is a powerful tool for graphene studies, being a versatile non-destructive method to obtain information about graphene phonon properties [17]. The purpose of the present study is to experimentally determine and analyze anharmonic phonon temperature shifts and broadening for supported graphene by Raman spectroscopy in order to establish the substrate effect on the anharmonic phonon properties of graphene.
Section snippets
Experimental
Graphene was synthesized on copper foil at 1020 °C by chemical vapor deposition (CVD) with CH4 flow of 40 sccm and hydrogen flow of 10 sccm. Copper foil (Alfa Aesar, 99.999%, 10 × 30 cm2, 25 μm thick) was pre-annealed at 1060 °C under hydrogen flow of 300 sccm and argon flow of 2000 sccm at a pressure of <10−4 Torr for 1–2 h inside the chamber. After graphene growth, Cu foil was cooled down to room temperature. The cooling rate was described in detail elsewhere [18].
Graphene was transferred to
Results and discussion
Room-temperature Raman spectra of the experimental graphene are presented in Fig. 1, where G and 2D features are observed, typical for this material [17]. The absence of a distinguishable D peak in the spectra of undoped graphene indicates low defect density [17]; for nitrogen-doped graphene, G peak position is upshifted by ∼ 3 cm−1, with that of 2D being normal, indicating electron doping together with 2D and G peak intensity ratio of 1.4–1.6 [23]. Another disorder-induced band D′ is also
Conclusion
Temperature-dependent Raman measurements were performed for graphene as-grown on copper, transferred to SiO2/Si, copper and Al2O3, and nitrogen-doped graphene on SiO2/Si in the temperature range of 20–294 K. Different G and 2D peak position and linewidth temperature dependencies were obtained and analyzed taking into account contributions of anharmonic phonon effects, graphene thermal expansion, substrate-induced strain and electron-phonon coupling. Determined anharmonic constants for 3- and
Acknowledgments
This work was supported by Belarusian State Program for Research "Photonics, opto- and microelectronics" and Russian Foundation of Basic Researches (individual project 16-33-60229), as well as partially supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (No. 2017R1D1A1B03035102 and 2017R1D1A1B03032759). The authors of this work are grateful to Prof. L.V. Yashina and Ph.D. Student A. I. Belova for XPS studies of
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