Ab initio approaches to Resonant Raman Spectroscopy of Transition Metal Dichalcogenides

Doctoral thesis (2017)

This thesis deals with the theory and simulation of resonant Raman spectroscopy in two-dimensional transition metal dichalcogenides. We present two different ab initio approaches. In the first, we ... [more ▼]

This thesis deals with the theory and simulation of resonant Raman spectroscopy in two-dimensional transition metal dichalcogenides. We present two different ab initio approaches. In the first, we calculate the Raman susceptibility tensor as a function of laser energy from the change of the dielectric susceptibility with atomic displacements. In the second, we formulate the Raman tensor in terms of time-dependent perturbation theory and calculate it using electron-light and electron-phonon coupling matrix elements obtained from density functional theory. We investigate the role of resonance, quantum interference and excitonic effects in the Raman spectra of single and triple-layer MoTe2. We compare our simulations with experimental results, explaining the dependence of the Raman intensities on the excitation energy. We demonstrate that the two approaches are formally and numerically equivalent in the adiabatic limit. In addition, the second approach allows to include the dynamical effects and captures a shift of the intensities with respect to the adiabatic case. This method is also more computationally efficient and is extended to include temperature effects using many-body perturbation theory. We have implemented both of these methods in a software package with interfaces to open source ab initio codes. Furthermore, we have developed web based tools to visualize excitonic states and phonon modes. [less ▲]

Excitons in boron nitride single layer

in Physical Review. B, Condensed Matter (2016), 94(125303),

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed ... [more ▼]

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence (π) and conduction (π∗) bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme. [less ▲]

Phonon-limited carrier mobility and resistivity from carbon nanotubes to graphene

in Physical Review. B, Condensed Matter and Materials Physics (2015)

Under which conditions do the electrical transport properties of one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent? We have performed atomistic calculations of the phonon ... [more ▼]

Under which conditions do the electrical transport properties of one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent? We have performed atomistic calculations of the phonon-limited electrical mobility in graphene and in a wide range of CNTs of different types to address this issue. The theoretical study is based on a tight-binding method and a force-constant model from which all possible electron-phonon couplings are computed. The electrical resistivity of graphene is found in very good agreement with experiments performed at high carrier density. A common methodology is applied to study the transition from one to two dimensions by considering CNTs with diameter up to 16 nm. It is found that the mobility in CNTs of increasing diameter converges to the same value, i.e., the mobility in graphene. This convergence is much faster at high temperature and high carrier density. For small-diameter CNTs, the mobility depends strongly on chirality, diameter, and the existence of a band gap. [less ▲]

Moiré-induced replica of graphene phonons on Ir(111)

in Annalen der Physik (2014), 526(9-10), 372-380

The phonon dispersion of singly oriented graphene on Ir(111) has been determined by angle-resolved inelastic electron scattering. Replica of graphene phonon bands are induced by the moire superstructure ... [more ▼]

The phonon dispersion of singly oriented graphene on Ir(111) has been determined by angle-resolved inelastic electron scattering. Replica of graphene phonon bands are induced by the moire superstructure. Calculations for a linear chain of C atoms attached to an infinitely heavy substrate reveal that imposing a superstructure by periodically varying the C-C interaction and the C-substrate coupling induces replicated phonons at wave vectors reflecting the supercell periodicity. Deviations between the phonon dispersion of graphene on Ir(111) and of pristine graphene are analyzed and rationalized in terms of the weak graphene-Ir(111) interaction. [less ▲]