Universal spin diffusion length in polycrystalline graphene
Graphene grown by chemical vapor deposition (CVD) is the most promising material for industrial-scale applications based on graphene monolayers. It also holds promise for spintronics; despite being polycrystalline, spin transport in CVD graphene has been measured over lengths up to 30 μm, which is on par with the best measurements made in single-crystal graphene. These results suggest that grain boundaries (GBs) in CVD graphene, while impeding charge transport, may have little effect on spin transport. However, to date very little is known about the true impact of disordered networks of GBs on spin relaxation. Here, by using first-principles simulations, we derive an effective tight-binding model of graphene GBs in the presence of spin–orbit coupling (SOC), which we then use to evaluate spin transport in realistic morphologies of polycrystalline graphene. The spin diffusion length is found to be independent of the grain size, and it is determined only by the strength of the substrate-induced SOC. This result is consistent with the D'yakonov–Perel' mechanism of spin relaxation in the diffusive regime, but we find that it also holds in the presence of quantum interference. These results clarify the role played by GBs and demonstrate that the average grain size does not dictate the upper limit for spin transport in CVD-grown graphene, a result of fundamental importance for optimizing large-scale graphene-based spintronic devices. ; All authors acknowledge support from the European Union Horizon 2020 Programme under Grant Agreement Nos. 696656 and 785219 (Graphene Flagship Core 1 and Core 2). ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and funded by the CERCA Programme/Generalitat de Catalunya. S.M.-M.D and J.-C.C. acknowledge the National Fund for Scientific Research of Belgium [F.R.S.- FNRS] and the Fédération Wallonie-Bruxelles through the "3D nanoarchitecturing of 2D crystals" project (ARC - 16/21-077) for financial support. ; Peer reviewed