Investigations of Graphene on SrTiO3 Single-Crystal using Confocal Raman Spectroscopy

Graphene layers placed on SrTiO3 single-crystal substrates were investigated using temperature-dependent confocal Raman spectroscopy. This approach successfully resolved distinct Raman modes of graphene that are often untraceable in conventional measurements due to the strong Raman scattering background of SrTiO3. Information on defects and strain states was obtained for a few graphene/SrTiO3 samples that were synthesized by different techniques. This confocal Raman spectroscopic approach can shed light on the investigation of not only this graphene/SrTiO3 system but also various two-dimensional layered materials whose Raman modes interfere with their substrates.

Understanding the properties of graphene deposited on various substrates or templates is essential for the advancement of graphene-based device applications. The physical properties of the two-dimensional (2D) carbon atoms in a honeycomb lattice are strongly influenced by the substrate material [1][2][3][4][5] . For example, graphene layers on hexagonal boron nitrides can exhibit a few orders of magnitude higher mobilities than those on silicon substrates 3 . Graphene interfacing with high-k dielectric materials is also promising because high electric fields can be applied directly, achieving ultra-high carrier densities or improved field-effect transistor effects in the graphene layer 5,6 . Strontium titanate (SrTiO3) crystals are considered one of the candidate materials for this perspective due to their high dielectric permittivity at low temperatures 7 . Recently, graphene layers on SrTiO3 single crystals (graphene/SrTiO3) are used as a platform for remote epitaxy in creating free-standing oxide nanomembranes, opening an unprecedented way for functional oxide device applications 8,9 . However, characterizing graphene layers placed on SrTiO3 substrates has been a formidable task. Raman spectroscopy is one of the standard non-destructive tools for inspecting the properties of graphene layers since Raman spectral features can tell us about the sample's defect densities and types 10 , thermal properties 11 , doping levels 12 , and lattice strain 13,14 . For example, there are three well-known Raman scattering processes of graphene, the so-called G mode (i.e., the E2g zone-center scattering), D mode (i.e., the in-plane A1g zone-edge intervalley scattering), and 2D mode (i.e., an overtone of the D mode). Note that the G, D, and 2D modes are relevant to free-carrier doping, defects, and strain, respectively 15 . Observation of these peaks in Raman spectra provides a comprehensive understanding of lab-prepared graphene samples.
Nevertheless, this approach has had limitations for graphene/SrTiO3 samples because a strong multiphonon scattering of SrTiO3 makes both the D mode and G mode peaks untraceable in conventional micro-Raman spectroscopic measurements, as shown in Fig. S1.
In this letter, we report that confocal Raman spectroscopy combined with simple spectral subtraction can be used effectively for characterizing graphene layers placed on SrTiO3 single crystals. By aligning the confocal plane of the laser beam, we can obtain the Raman spectra of graphene layers and SrTiO3 substrates, respectively, with a substantially reduced spectral intensity overlap. This approach of confocal Raman spectroscopy overcomes the drawback of conventional micro-Raman spectroscopic measurements and reveals inelastic light scattering peaks of graphene layers, enabling quantitative spectral analysis. For example, Raman spectra of two different graphene samples that are synthesized using silicon carbide (SiC) and germanium (Ge), respectively, and transferred to SrTiO3 substrates indicate that they have distinct defect densities and types. Temperature-dependent Raman spectra reveal that graphene layers on SrTiO3 experience strain from the substrate that is different from either silicon (SiO2/Si) substrates or copper (Cu) foils. The outcome of this confocal Raman spectroscopic approach on graphene/SrTiO3 provides indispensable information not only in understanding the material but also in the device application for remote epitaxy of functional oxides.  Fig. 1. Note that both measurements are carried out on a single graphene/SrTiO3 sample without a need of additional SrTiO3 reference measurement. We can distinguish the spectral features of graphene (red curve) and SrTiO3 (blue curve) even though there are some common peaks near 1200 -1700 cm -1 between the two. The graphene-focused spectrum (red curve) also exhibits the multiphonon peaks of SrTiO3 due to the confocal plane being thicker than the graphene layer, i.e., 0.5 μm > 0.35 nm. Nevertheless, by subtracting the SrTiO3-focused spectrum (blue curve) from the graphene-focused spectrum (red curve), we can obtain the graphene-only spectrum (black curve), which is consistent with the previously reported Raman spectrum of graphene [18][19][20] . Note that the spectrum of Fig. 1 clearly shows weak features such as the D and Dꞌ modes, which are invisible in conventional micro-Raman spectroscopic measurements, as shown in Fig. S1. We measured a few different graphene/SrTiO3 samples to see if this approach is effective in examining their qualities since the Raman spectral features are correlated with the properties of defects in graphene. We used graphene layers that were respectively grown on SiC and Ge via silicon sublimation of the Si-terminated face of SiC and chemical vapor deposition on hydrogenterminated Ge and transferred to the surface of SrTiO3 substrates using a dry transfer method as reported in Refs. 8, 9, and 21. Figure 2 shows the graphene-only spectra of two different samples, i.e., graphene(SiC)/SrTiO3 and graphene(Ge)/SrTiO3. Note that both the D mode and Dꞌ mode (i.e., the in-plane A1g zone-edge intravalley scattering) peaks of the graphene(Ge)/SrTiO3 sample are significantly higher than those of graphene(SiC)/SrTiO3, implying that the former has larger defect densities than the latter. The intensity ratios between the D and Dꞌ modes (ID/IDꞌ) are approximately 4.5 (for graphene(Ge)/SrTiO3) and 2.7 (for graphene(SiC)/SrTiO3), being smaller than 7 (See Table 1.), implying that both samples possess predominantly vacancy-type defects rather than sp 3type defects, as discussed in Refs. 10 and 22. Nevertheless, the intensity of the 2D mode normalized by the G mode, i.e., I2D/IG, from the graphene(SiC)/SrTiO3 sample is approximately eight times larger than that of the graphene(Ge)/SrTiO3 sample, indicating that the former is higher quality graphene than the latter 10,22 .
Temperature-dependent confocal Raman spectra show that the graphene layer on SrTiO3 substrates exhibits a distinct temperature coefficient compared to other graphene layers transferred to SiO2/Si substrates and grown on Cu foils. Figure S2 shows temperature-dependent Raman spectra of a graphene(SiC)/SrTiO3 sample from room temperature down to 10 K using a custombuilt optical cryostat. All Raman modes of the graphene are shifted to higher energies at low temperatures, which is qualitatively consistent with the previous reports of Refs. 11, 23, and 24.  Fig. 3(a). This observation makes sense because these substrate materials have different thermal expansion coefficients, i.e., 2.6×10 -6 K -1 (SiO2/Si) < 9.0×10 -6 K -1 (SrTiO3) < 16.5×10 -6 K -1 (Cu) 25 . Figure 3(b) shows the temperature dependence of the D and 2D mode energies. Note that both the 6 G mode (in Fig. 3(a)) and D mode (in Fig. 3(b)) are blue-shifted by 15 cm -1 from room temperature to 10 K, whereas the 2D mode is shifted by 31 cm -1 . This unequal temperature-dependent peak shift is understood as the dominant biaxial tensile-strain on the graphene layer by SrTiO3 substrates, which is estimated to be approximately 0.23% (Refs. 26 and 27).
In conclusion, the inelastic light scattering spectrum using confocal Raman spectroscopy provides useful information in understanding the graphene layers under various conditions. We obtained the information on defects and strain states for graphene/SrTiO3 samples, which had been difficult to characterize due to the strong spectral overlap between the graphene and SrTiO3. We suggest that this approach will shed light on the investigation of not only this graphene/SrTiO3 system but also various 2D materials where their substrate's Raman modes interfere with those of the 2D materials.        Figure S1. Conventional Raman spectra of graphene/SrTiO 3 (red curve) and bare SrTiO 3 (blue curve). The black curve is obtained by subtracting the bare SrTiO 3 spectrum from the graphene(SiC)/SrTiO 3 spectrum.