  The electronic band structure of twisted bilayer graphene develops van Hove
singularities whose energy depends on the twist angle between the two layers.
Using Raman spectroscopy, we monitor the evolution of the electronic band
structure upon doping using the G peak area which is enhanced when the laser
photon energy is resonant with the energy separation of the van Hove
singularities. Upon charge doping, the Raman G peak area initially increases
for twist angles larger than a critical angle and decreases for smaller angles.
To explain this behavior with twist angle, the energy of separation of the van
Hove singularities must decrease with increasing charge density demonstrating
the ability to modify the electronic and optical properties of twisted bilayer
graphene with doping.
