Theoretical Studies on the Deformation Potential, Electron-Phonon Coupling, and Carrier Transports of Layered Systems

The electronic structures, deformation potential, electron-phonon couplings (EPCs), and intrinsic charge transport of layered systems - the sp + sp(2) hybridized carbon allotropes, graphynes (GYs) and graphdiynes (GDYs), as well as sp(2) + sp(3) hybridized structure with buckling, such as stanine - have been investigated theoretically. Computational studies showed that, similar to graphene, some GYs can possess Dirac cones (such as alpha-, beta-, and 6,6,12-GYs), and that the electronic properties of GYs and GDYs can be tuned by cutting into nanoribbons with different widths and edge morphologies. Focusing on the features of Dirac cones, band structure engineering can provide a clue for tuning electronic transport in 2D carbon-based materials. Based on the Boltzmann transport equation and the deformation potential approximation (DPA), the charge carrier mobilities in GYs and GDYs were predicted to be as high as 10(4)-10(5) cm(2).V-1.s(-1) at room temperature. Interestingly, due to lower EPC strength and longer relaxation time, the charge carrier mobility in 6,6,12-GY with double Dirac cones structure was found to be even larger than that of graphene at room temperature. The unique electronic properties and high mobilities of GYs and GDYs make them highly promising candidates for applications in next generation nanoelectronics. Additionally, through the full evaluation of the EPC by density functional perturbation theory (DFPT) and Wannier interpolation, the EPCs with different phonon branches and wave-vectors as well as charge carrier mobilities for graphene, GYs and stanene have been discussed. This showed that the longitudinal acoustic (LA) phonon scattering in the long wavelength limit is the main scattering mechanism for GYs and graphene, and thus the DPA is applicable. Due to stronger LA phonon scattering, the electron mobilities (similar to 10(4) cm(2).V-1.s(-1)) of alpha-GYs and gamma-GYs were predicted to be one order of magnitude smaller than that of graphene at room temperature by full evaluation of the EPC. However, the DPA would fail if there was buckling in the honeycomb structure and the planar symmetry was broken (absence of sigma(h)), such as in stanene, where the inter-valley scatterings from the out-of-plane acoustic (ZA) and transverse acoustic (TA) phonons dominate the carrier transport process and limit the electron mobilities to be (2-3) x 10(3) cm(2).V-1.s(-1) at room temperature. In addition to our calculations, others have also found that the main scattering mechanisms in layered systems with buckling, such as silicene and germanene, are ZA and TA phonons. Thus, these results give us new insights into the role of EPCs and the limitation of the DPA for carrier transport in layered systems. They also indicate that the carrier mobilities of systems without sigma(h)-symmetry can be improved by suppressing the out-of-plane vibrations, for example by clamping by a substrate.