Temperature dependence of the Raman spectra of multilayer graphene nanoribbons fabricated by unzipping method

https://doi.org/10.1016/j.diamond.2020.108047Get rights and content

Highlights

  • Thermal property of multilayer graphene nanoribbons was studied by Raman spectroscopy.

  • The G band frequency was monotonically downshifted with increasing temperature.

  • The temperature-dependent of the G band is explained by anharmonic phonon interaction.

  • The thermal property of MLGNRs is compared with graphite and other nanocarbons.

Abstract

The temperature dependence of Raman spectra of multilayer-graphene nanoribbons (MLGNRs) fabricated by unzipping method was investigated in the temperature range from 300 K to 700 K. MLGNRs with the width of ~200 nm are isolated and individually measured. The frequency of G band is monotonically downshifted with increasing temperature. The change in the G band frequency with temperature is reversible in thermal cycles with heating and cooling. By linear fitting, the temperature coefficient is estimated to be about −0.021 cm−1/K. This value is smaller than −0.028 cm−1/K of carbon nanotubes and larger than −0.011 and −0.016 cm−1/K of graphite and graphene, respectively, as reported previously. This means that MLGNRs are thermally stable compared with carbon nanotubes with curvatures, whereas the thermal stability of MLGNRs is lower than those of graphene and graphite. The better fitting to the G band frequency shift with temperature is obtained with nonlinear quadratic curve. From the theoretical analysis of the fitted quadratic curve, it is clarified that the downshift of G band frequency with increasing temperature is attributed to the anharmonic phonon interaction, especially 4-phonon process rather than 3-phonon process. Comparing with other nanocarbon materials reported so far, it is suggested that the strength of the anharmonic phonon interaction depends on the layer number and size of graphene.

Introduction

Graphene, a two-dimensional honeycomb lattice of carbon atoms, has remarkable electric, mechanical, thermal, and optical properties [1]. In particular, the carrier mobility of graphene is predicted theoretically more than 250,000 cm2 V−1 s−1 at room temperature, therefore, graphene is expected to be applied as a next-generation electronic device. However, applications in semiconductor technologies such as a transistor are restricted because graphene has a zero-band gap [2]. Graphene nanoribbons (GNRs), defined quasi-one-dimensional graphene with nanometer-width, have been attracted great attention as one of attempt to open a band gap in graphene [3]. Numerous studies about their fabrications and properties have been reported, but especially their basic thermal properties are still not clear due to few experimental studies. The understanding of the thermal properties of GNRs is important for their device applications such as field effect transistors [4] and sensors [5], since the devices composed of heterogeneous composites with the interfaces are fabricated and operated in various temperature environment.

Generally, Raman spectroscopy is an indispensable method to obtain important information such as structural and electronic properties of carbon materials [6]. For the evaluation of thermal property, the temperature dependence of Raman spectra has been reported for graphite [[7], [8], [9]], graphene [[10], [11], [12], [13], [14], [15]], carbon nanotubes [[16], [17], [18], [19], [20], [21]]. The comparison and discussion of the temperature dependence are needed for practical applications. However, the temperature dependence of GNRs has not been reported yet to the best of our knowledge.

In this study, we measured the temperature dependence of the Raman spectra of multilayer GNRs (MLGNRs) fabricated by unzipping method [22] which is one of the most industrial synthesis method [23,24] to understand the thermal characteristics of MLGNRs. Especially, we aimed to observe a completely isolated GNR without bundles and segregations to understand the intrinsic thermal property of MLGNRs. As a result, we succeeded in observing the intrinsic temperature dependence of a MLGNR for the first time. The temperature-dependent behavior with non-linear quadratic curve will be explained by the theoretical model including anharmonic phonon interaction with 3- and 4-phonon processes, and compared with typical nanocarbon materials has been reported so far.

Section snippets

Materials and methods

MLGNRs fabricated by a method of unzipping MWCNTs were purchased from SIGMA-ALDRICH (Prod. No. 797774) [25]. 10 mg of MLGNRs were dispersed in 10 ml of ethanol with a homogenizer (Dr. HIELSCHER UP400S) and dropped onto SiO2 (285 nm)/Si substrate. In order to avoid aggregations of MLGNRs, the SiO2/Si substrate was placed on a hot plate heated to about 80 °C before dropping. The prepared sample was observed by using an optical microscope (OLYMPUS BX51) and a field emission scanning electron

Results and discussion

Fig. 1(a) shows the typical SEM image of a MLGNR dispersed on SiO2/Si substrate at 300 K. A magnified figure of the square area in (a) is shown in Fig. 1(b). Note that such isolated samples were selected in Raman measurements. It is found that the length and width of the MLGNR are about 5–10 μm and 200 nm respectively. These features are in good agreement with those in one of typical of GNRs reported so far [22]. However, it was hard to successfully measure the thickness of MLGNRs by using an

Conclusion

We have succeeded in observing the intrinsic temperature dependence of a completely isolated MLGNR with a width of ~200 nm. The downshift of the G band frequency is clearly observed with increasing temperature. The change in the G band is reversible in the thermal cycles between 300 K and 700 K. This reversible behavior reflects intrinsic thermal properties without damages. The temperature-dependent behavior with non-linear quadratic curve is explained by theoretical model with anharmonic

Author's contributions

All authors contributed equally to this work.

Author statement

Marina Tsujimoto: Conceptualization, Methodology, Writing-Original Draft. Makoto Tanimura: Formal analysis, Investigation, Resources. Masaru Tachibana: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was in part supported by Iketani Science and Technology Foundation (0291078-A) and JSPS KAKENHI (17K06797) in Japan.

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