{\theta}-Tunable Photoluminescence from Interlayer Excitons in Twisted Bilayer Graphene

TL;DR

This study demonstrates angle-dependent photoluminescence in twisted bilayer graphene, revealing interlayer excitons with tunable binding energies and suggesting potential for angle-controlled optoelectronic applications.

Contribution

It provides experimental evidence of interlayer excitons in tBLG with angle-dependent properties, supported by spectroscopic data and theoretical models.

Findings

Photoluminescence intensity varies with stacking angle.

Interlayer excitons exhibit binding energies of 0.5 to 0.7 eV.

Resonant PL suggests formation of bound excitons rather than simple van Hove singularity effects.

Abstract

Using resonant 2-photon excitation of interlayer electrons in twisted bilayer graphene (tBLG), we resolve photoluminescence (PL) that tunes spectrally with stacking angle, {\theta}. This weak signal is 4- 5$\times$ larger than the non-resonant background and is emitted from the interlayer band anti-crossing regions traditionally associated with van Hove singularity resonances. However, our observation of resonant PL emission with delayed ~1 ps electronic thermalization suggests interlayer carriers may instead form bound-excitons. Using both the 2-photon PL and intraband transient absorption spectra, we observe bright and dark state peak-splitting associated with an interlayer exciton binding energy ranging from 0.5 to 0.7 eV for {\theta} = 8$^o$ to 17$^o$. These results support theoretical models showing interlayer excitons in tBLG are stabilized by a vanishing exciton-coupling strength to the metallic continuum states. This unexpected dual metal-exciton optical property of tBLG suggests possible {\theta}-tuneable control over carrier thermalization, extraction and emission in optical graphene-based devices.