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An efficient phonovoltaic (pV) material requires a highly energetic optical phonon (Ep,O kBT ) with linewidth dominated by the electron-phonon (e-p) coupling and resonant with its electronic band gap ( Ee,g), as discussed in Paper I [C. Melnick and M. Kaviany, Phys. Rev. B 93, 094302 (2016)]. No current material combines these properties. While graphite (graphene) has the former two, it lacks a band gap. Opening and tuning the band gap in graphite is challenging due to the stability of the Dirac point, e.g., under a uniaxial strain <0.25. We tune its band gap through partial hydrogenation using extensive ab initio calculations and find a stable graphame structure with Ee,g Ep,O 200 meV, C128H1×24. We calculate the e-p coupling in tuned C128H1×24 and graphene and show that the transition from π -π∗ (graphene) to σ -σ ∗ (graphane) bands suppresses the electron-phonon coupling, such that optical phonons in C128H1×24 primarily downconvert, and it does not achieve a high figure of merit (ZpV < 0.1). Ab initio phonon-phonon couplings are calculated for graphane and graphene to support this result. Overall, we develop a material with Ep,O Ee,g kBT and a method for tuning and evaluating pV materials.

Phonovoltaic. II. Tuning band gap to optical phonon in graphite