Raman Spectroscopy on 1D Graphene Nanoribbons: From Structure to Electron-Phonon Interactions

Low-dimensional materials (1D and 2D) are promising candidates as building blocks of future electronics and optoelectronics. Controllable bandgap, strong light-matter interaction, and sub-nanometer thickness are among their favorable properties for various applications. In this regard, graphene nanoribbons (GNRs) are especially interesting, due to their remarkable physical properties that are critically determined by their width and edge structure [1]. A comprehensive characterization of these materials is a crucial learning step toward their reliable integration in devices.<br/> <br/>In this work, we showcase the use of various types of Raman spectroscopy along with density functional theory (DFT) calculations, for the investigation of intrinsic and defect-induced structural, vibrational, and optoelectronic properties of GNRs, with special emphasis on probing the interaction in-between GNRs and with various substrates (metal and oxides). The 9-atom wide armchair GNR (9-AGNR) is chosen as an example system.<br/> <br/>We start by using low-temperature, polarization Raman spectroscopy measured with multi-wavelength excitation (488, 532, and 785 nm) to investigate the differences in fundamental and defect-activated phonons of 9-AGNRs grown uniaxially aligned on gold (Au(788)) and transferred to oxide substrates using the process from [2]. We probe the possible interaction between the GNRs by investigating two samples with distinct surface coverages: a full monolayer (ML) and 0.3 ML assuming that the interaction between the GNRs decreases by decreasing the surface coverage. Raman measurements on these two samples show particular differences in the spectral region corresponding to C-H vibrations (1000 to 1400 cm^-1), suggesting possible activation of interaction-induced Raman peaks.<br/> <br/>Next, we use temperature-dependent Raman measurements, in the range from 70 to 500 K, in order to probe phonon-electron and phonon-phonon coupling in these systems. We systematically investigate the dependence of Raman peak positions and widths of G, D and C-H related phonons with temperature. We show that, for the full monolayer GNRs on the Au (788) substrate, the electron-phonon coupling is dominating up to temperatures of 250 K. This is a surprising feature, considering that 9-AGNRs grown on Au substrates, have a semiconducting behavior with a bandgap of about 1.4 eV [3]. This should diminish electron-phonon coupling for all phonons with energies lower than 1.4 eV (11200 cm^-1). We explain this behavior in terms of interactions between adjacent GNRs and/or between GNRs and the substrate. Finally, we discuss how electron-phonon and phonon-phonon coupling can have a profound effect on the electronic properties of these materials, which can be especially crucial for device performances. <br/> <br/>[1] Cai et al. Nature 466 (2010)<br/>[2] Borin Barin et al. ACS Applied NanoMaterials 2 (2019)<br/>[3] Talirz et al. ACS Nano 1, (2017)