Chapter 7 - The phonon fingerprint of 2D solids Flashcards
What is the difference between absorption in an indirect gap semiconductor and a direct gap?
In an indirect band gap semiconductor, there is a slight possibility of absorption across the band-gap due to interaction with a phonon (for momentum conservation). Otherwise the energy required will need to match a direct band gap.
How is the absorbance in TMDC monolayers?
High, about 5-10%. Factor 2-3 larger compared to bulk TMDC.
What is a Stokes-shift?
Stokes-shift is when light is inelastically scattered on a phonon (Raman scattered), and loses the amount of energy it took to excite the phonon. It is therefore red-shifted.
What is an Anti-Stokes-shift?
Anti-Stokes-shift is when the light interacts with an excited phonon-state, and is reemitted with an extra emount of energy equal to the excited state. It is therefore blue-shifted.
How can the inelastic light scattering (ILS) process be described?
It can be described either macroscopically (frequency shift due to change in electric susceptibility caused by excitations) or by using third order time dependent perturbation theory.
How is Inelastic Light Scattering described macroscopically?
The polarization of the medium is given by the classical dipolar moments of individual atoms. Macroscopic polarization modulated by excitation, e.g. phonons (lattice vibrations). This leads to a fluctuation of electric susceptibility. The frequency shift is then due to this change in electric susceptibility. The change in electric susceptibility is proportional to change in polarizability.
Why is Raman spectroscopy sensitive to number of layers?
Because the more layers, the more stiff the out-of-plane vibrations become, thus shifted towards higher energy. For in-plane vibrations the energy is shifted towards less energy. Due to this, we can easily count the number of layers.
What is the requirement for ILS activity?
That the change in polarization is different from 0.
Draw the Feynman diagram for the microscopic description of ILS.
See slides.
Which phonon modes are present in graphene?
The shear mode (G), the breathing mode (D, only active if defects are present), and double phonon mode (2D).
What can be said about the shear phonon mode in graphene?
It is a zone center phonon mode, and has no dispersion. Its intensity changes with number of layers (increases with more layers).
What can be said about the breathing phonon mode in graphene?
It is a mode at the K point. Only allowed if there are defects present, and is therefore a fingerprint for defects. It is active at armchair edges. It carries information about the electronic band structure.
What can be said about the double phonon mode in graphene?
It is a double resonant process. It carries information about the electronic band structure. The lineshape is sensitive to number of layers. Active in pristine graphene.
What does the ratio between the intensity of the D and G peaks in a graphene Rama spectrum tell us?
It indicates the defect density and thus the size of the individual grains.
What is the difference between IR and Raman active modes?
IR active modes need to a change in the electric dipole. Raman active modes need a change in the polarizability, that is, a change in the size, shape or direction of the electron cloud surrounding the molecule.
