Phonovoltaic. II. Tuning band gap to optical phonon in graphite
Abstract
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 (Δ Ee ,g ), as discussed in Paper I [C. Melnick and M. Kaviany, Phys. Rev. B 93, 094302 (2016), 10.1103/PhysRevB.93.094302]. 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 Δ 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≃Δ Ee ,g≫kBT and a method for tuning and evaluating pV materials.
- Publication:
-
Physical Review B
- Pub Date:
- March 2016
- DOI:
- 10.1103/PhysRevB.93.125203
- Bibcode:
- 2016PhRvB..93l5203M