| Two-dimensional(2D)materials approach our lives with outstanding mechanical,thermal,electrical,and other properties.They provide new ideas for the application of micro-nano materials in the fields of optoelectronics,bionics,catalysis,and energy storage.However,in practical engineering applications,2D materials inevitably interact with other materials.The thermal strain between the 2D material and the substrate resulting from the device's self-heating or the ambient temperature change could cause a serious thermal expansion mismatch at the contact interface.It will affect the realization of the device's original function,and even destroy the structure or shorten the service life of the device.A fundamental physical property of the material-the thermal expansion coefficient(TEC)is involved.Its accurate measurement would compensate for the thermal strain caused by the thermal expansion mismatch,protecting the device.The TEC study helps us to understand the thermal properties of 2D materials.It will provide an important reference for the structural design and application of related functional devices of 2D materials,which has practical guiding significance.Graphene and molybdenum disulfide(Mo S2)are representatives of 2D materials.They have different application fields.The former is semi-metallic with zero bandgaps,while the latter is the semiconductor with a certain bandgap.Not only does their research on the thermal expansion property provide a reference for the application of materials,but the methods and ideas can be extended to other 2D materials as well.This study focuses on the single-layer graphene(SLG),single-layer Mo S2,using Raman spectroscopy to extract their TECs.The main research contents are as follows.Highly oriented pyrolytic graphite and Mo S2 crystals were used in our experiments.SLG was mechanically exfoliated on the Si/Si O2 substrate.By combining mechanical peeling and dry transfer,and improving the transfer process,the substrate supported and suspended single-layer Mo S2were prepared on the PDMS,Si/Si O2,and gold-plated Si/Si O2 substrate with array holes,respectively.The samples were characterized.Based on the comprehensive analysis of Raman spectra,the TECs of SLG were extracted by excluding defects,native strain and doping effects.The temperature range was controlled to avoid the effect of defects created during the high-temperature treatment on the Raman frequency shifts.The frequency shifts during two thermal cycles were measured in the range of303-503 K.The frequency shifts in the first heating process were inconsistent with the subsequent processes.It was ascribed to the native strain generated during the sample preparation process and the doping effect between graphene and the substrate.Therefore,by excluding the first thermal cycle,the negative TECs of graphene were calculated,which were much closer to the previous theoretical predictions.The temperature-dependent Raman spectra of substrate-supporting and suspended Mo S2during thermal cycles were measured and the TECs of of monolayer samples were obtained.It was found that when the laser power was less than 1.1 m W,the Raman characteristic peak frequency shifts of Mo S2 were not affected.,A comparative analysis of the Raman frequency shifts during thermal cycles showed that the substrate-supporting Mo S2 had released the native strain due to the dry transfer process.The Raman frequency shifts in the first heating process were consistent with the subsequent processes.The suspended part of the single-layer Mo S2was still affected by the supporting part of the substrate.After fully considering this factor,the TECs of Mo S2 were calculated.It was comprehensively analyzed the temperature-dependent Raman spectra of the characteristic peaks of SLG and Mo S2 during the two thermal cycles and the TECs were obtained by reasonably extracting the temperature-dependent Raman frequency shifts.Our work will provide the design basis for the fabrication of micro/nanoelectromechanical devices and lay the foundation for the engineering application in the next-generation nano-electromechanical devices. |
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