Abstract:
The quest for materials capable of realizing the next generation of electronic and photonic devices continues to fuel research on the electronic, optical and vibrational properties of graphene. Single-layer graphene (SLG) and multi-layer graphene (MLG) flakes with less than ten layers each show a distinctive band structure. How to understand the electronic excitation of SLG and FLG is a crucial issue in the fundamental physics and device application. Raman spectroscopy is one of the most useful and versatile tools to access phonons in graphene samples. The interaction between phonon and electronic excitation is a direct way to probe the electronic properties of SLG and MLG.
We show that by performing Raman measurements at different excitation energies, the variation of the 2D intensity relative to the G peak of heavily-doped SLG with excitation energy allows one to assess its Fermi energy for a given doping level, and a Fermi level shift of up to ~0.85 eV can be measured for Stage-1 intercalation graphite compounds by FeCl3, where each graphene layer in compounds behaves as a decoupled heavily doped SLG. We also show that detection of Raman modes down to ~10 cm-1 is possible using three BragGrate notch filters in combination with a single monochromator.
We focus on the low-energy shear mode in graphene layers, whose frequency corresponds to ~42 cm-1 of the E2g mode in bulk graphite. We uncover the equivalent mode for MLGs and show that it provides a direct measurement of the interlayer coupling. The corresponding shear modes can be well-fitted with a Breit-Wagner-Fano lineshape, which arises as quantum interference between the shear mode and a continuum of Raman-active electronic transitions. This makes it a probe for the quasiparticles near the Dirac point by quantum interference.
Graphite is not the only layered material. MoS2 is another typical layered material. The unique valley-selective circular dichroism in monolayer MoS2 by circularly polarized photoluminescence will be briefly reported.