Optical spectroscopic studies in the time and frequency domain of graphene and fabricated patterned graphene structures

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Κατσιαούνης, Σταύρος

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Phonon lifetimes are crucial physical parameters that play a vital role in phonon transport and the thermal conductivity of two-dimensional materials. Despite extensive research on the ultrafast dynamics of phonons in photoexcited graphene since 2008, there are still open questions to be answered. Notably, a thorough investigation of the phonon dynamics on polycrystalline chemical vapor deposited (CVD) graphene along with a direct comparison with single crystal exfoliated samples has not been reported in the literature prior to this study. Furthermore, to date, the influence of doping on phonon lifetimes in graphene has received limited attention. In this study, we have developed a unique Time-Resolved Incoherent Anti-Stokes Raman Scattering (TRIARS) setup to directly investigate the ultrafast dynamics of G phonons in graphene crystals and examine how various parameters, such as the production method, number of layers, underlying substrate, and doping level, influence phonon lifetimes. Our investigation revealed that the presence of an underlying substrate significantly reduces the G phonon lifetime of single layer graphene. For two and three exfoliated graphene layers, we observed that the lifetime gradually approaches that of graphite (≈2.2 ps corresponding to ≈2.4 cm-1). The G phonon lifetime in CVD graphene samples consistently appeared lower than that of exfoliated ones, mainly due to the presence of various structural defects like wrinkles and grain boundaries. We have also determined the electron-phonon coupling strength of graphite at 10.6 cm-1 which aligned excellently with theoretical predictions. Furthermore, we conducted a thorough investigation of the phonon dynamics in pristine and p-type doped CVD graphene, observing an approximately 14% decrease in the G phonon lifetime with doping levels up to 𝐸𝐹=270 meV below the Dirac point. We also noted a 30% decrease in the electron-phonon coupling constant, 𝜆 𝛤, of CVD graphene samples compared to exfoliated ones. Moreover, it was shown that phonon-defect scattering constitutes a significant contribution in the linewidth of the G Raman band in CVD graphene samples. Additionally, we explored the formation of nanopores in graphene under ambient conditions by irradiating its lattice with below ablation threshold ultrashort laser pulses. This phenomenon has not been thoroughly investigated prior to the present study. Existing methods for nano-perforation of graphene sheets have encountered challenges related to scalability and cost efficiency. In this study, we propose an experimental protocol that addresses these issues by introducing a novel technique to fabricate nanoscale pores (10 – 40 nm) in CVD graphene membranes. The nanopore network was visualized and quantified using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), while Raman spectroscopy was employed to establish a correlation between the nano-perforated area and nano-topographic imaging. We have shown that Raman imaging is a powerful tool for the identification of the nanoporous area and its combination with AFM revealed that nanopores of certain size and distribution are formed under certain experimental conditions. Finally, based on graphene's interaction with ultrashort laser pulses we submitted and were awarded a patent titled "Graphene-based 3D optical memory" by the Hellenic Industrial Property Organization.



Graphene, Time resolved Raman, Raman spectroscopy, Phonon lifetimes, Phonon dynamics, Phonon linewidths, Electron - phonon coupling, Nitric acid doping, Pump - Probe, Kerr lens modelocking, Nanopores, Femtosecond laser irradiation