Li adatoms on graphene: doping and intervalley scattering

Charge doping by alkali atoms (Li) deposited under cryogenic ultrahigh-vacuum conditions is arguably the simplest of all adatom effects on graphene. We present an experimental investigation of Li adatoms on epitaxial and CVD graphene, focusing on the crucial role of high temperature annealing prior to cryogenic deposition for efficient doping and a measurement of how the adatoms affect the intervalley scattering rate. While doping saturated at 2$\times$ 10$^{\mathrm{13}}$ e/cm$^{\mathrm{-2}}$ on unannealed graphene, independent of previous processing, a 700 K/900 K anneal prior to cryogenic deposition caused the saturated doping level to rise one order of magnitude to above 10$^{\mathrm{14}}$ e/cm$^{\mathrm{-2\thinspace }}$[1]. For the annealed samples, long range Coulomb interaction due to Li adatoms caused a dramatic enhancement of the intervalley scattering, a result that contradicts the naive expectation that short range scattering is necessary for intervalley scattering but is qualitatively consistent with theoretical predictions in Ref. [2]. [1] A. Khademi, et al.,~arXiv:1610.00301. [2] P. Boross, et al., Phys. Rev. B. 92, 035420 (2015).