Distinguishing different stackings in layered materials via luminescence spectroscopy

Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from ``indirect'', finite-momentum, excitons via coupling to finite-momentum phonons. We show that the differences in peak positions, number of peaks and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed.Comment: Main: 6 pages and 4 figures, Supplementary: 6 pages and 7 figure

Distinguishing Different Stackings in Layered Materials via Luminescence Spectroscopy

peer reviewedDespite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from “indirect,” finite-momentum excitons via coupling to finite-momentum phonons.We show that the differences in peak positions, number of peaks, and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed

Effetto della deformazione meccanica e di vibrazioni reticolari sulle proprietà eccitoniche di semiconduttori quasi-2D

Lo sviluppo di dispositivi opto-elettronici innovativi richiede la comprensione dei meccanismi fisici che governano l’interazione fra luce ed elettroni. Recentemente, diversi lavori sperimentali e teorici hanno dimostrato il ruolo rilevante degli effetti eccitonici nella caratterizzazione di sistemi 2D. Vari lavori sperimentali hanno inoltre mostrato come le proprietà eccitoniche di questi materiali vengano modificate dalla dipendenza degli eccitoni dai gradi di libertà reticolari. In questa tesi abbiamo analizzato tale interazione da un punto di vista teorico, sviluppando metodi computazionali utilizzati sia per ottenere una descrizione accurata di osservabili altrimenti non accessibili utilizzando un approccio ab initio a causa della elevata complessità dei sistemi studiati, sia per interpretare esperimenti di catodoluminescenza e scattering inelastico di raggi X. In primo luogo abbiamo calcolato le risonanze eccitoniche nel singolo strato di C3N, risolvendo l’equazione di Bethe Salpeter (BSE) partendo da un modello Tight Binding (TB) delle bande elettroniche e sviluppando una descrizione semplificata dello screening elettronico. In questo modo abbiamo ottenuto una miglior efficienza computazionale rispetto a calcoli completamente ab initio, pur mantenendo lo stesso grado di accuratezza; utilizzando questo approccio, abbiamo analizzato come uno strain uniassiale possa modificare le proprietà ottiche del sistema imperturbato, classificando le risonanze eccitoniche secondo le simmetrie del sistema e giustificando la presenza di anisotropia ottica indotta dallo strain applicato; infine abbiamo discusso la presenza di bande eccitoniche con andamento non analitico e di come tali bande siano influenzate da una deformazione meccanica esterna. L’analisi sul C3N è stata infine completata con una caratterizzazione completamente ab initio delle proprietà ottiche del bistrato di C3N, con due possibili stackings (ovvero AB e AB’) per investigare l’effetto dell’interazione fra i diversi strati sulle proprietà eccitoniche del singolo layer. In secondo luogo abbiamo implementato un metodo per la simulazione di spettri di luminescenza, basato sul calcolo dell’interazione eccitone-fonone. Tale schema è stato utilizzato nell’interpretazione di esperimenti di catodoluminescenza (CL) in bulk-BN, con due differenti stackings, ovvero AA’ (hBN) e ABC (BN romboedrico). Attraverso questi calcoli è stato possibile caratterizzare la struttura fine del segnale CL osservato sperimentalmente, analizzando le differenze fra gli spettri ottenuti per diverse sequenze di stacking dei layers in termini dei diversi modi fononici coinvolti nel processo di emissione di luce. Infine abbiamo sviluppato un codice di post-processing con cui è possibile simulare spettri di scattering inelastico di raggi X (IXS) da vibrazioni reticolari, partendo da dispersioni fononiche ottenute completamente ab initio. Tale schema è stato applicato all’interpretazione di esperimenti IXS su bulk-MoS2 ad elevate pressioni.This thesis centers on the effects of lattice dynamics and structural distortions on the excited-state properties of quasi two-dimensional (2D) semiconductors from a theoretical point of view: a new avenue of research which only recently is becoming amenable to predictive computational approaches. We analyse the coupling between excitonic resonances and vibrations, and show its importance for the accurate spectroscopic characterization of prototype systems. The fundamental understanding of the microscopic physical mechanisms governing light-matter interaction, including the effects of strain, is expected to be of key relevance for the design of innovative optoelectronic devices based on such materials. To gain deep insight in these mechanisms, we combine accurate first-principles calculations --based on Density Functional Theory and Many Body Perturbation Theory-- with judicious quantitative models. We then develop simple computational schemes to obtain accurate predictions, while greatly increasing the efficiency with respect to state-of-the-art ab initio methods. In this way, we are able to study complex systems that would otherwise be out of reach. We focus on the effects of strain, stacking geometries, lattice vibrations and pressure in selected relevant systems, and analyse and understand recent cathodoluminescence (CL) and inelastic X-ray scattering (IXS) experiments. First, we investigate graphene-like 2D polyaniline (also known as C3N), focusing on how uniaxial strain can tune its optical properties. We compute excitonic resonances by solving the Bethe-Salpeter equation in a tight-binding model for the electronic bands including a simplified description of the electronic screening. This model retains the accuracy of our fully ab initio calculations at greatly decreased computational cost. We also classify excitons according to the symmetries of the systems, explain the optical anisotropy of the perturbed monolayer and the non-analytic behaviour of the excitonic bands. Our analysis on C3N progresses with the ab-initio characterisation of bilayers in different stacking motifs (namely AB, AB' and AA'), where we explain the anomalous quenching of the optical absorption spectrum as induced by the interlayer coupling. Second, we implement a scheme to predict luminescence spectra, based on calculated exciton-phonon coupling terms. We analyse CL experiments on bulk Boron Nitride (BN) in two stacking motifs: AA' (hBN) and ABC (rhombohedral BN). Our calculations accurately reproduce the fine structure of the observed CL signal and explain the differences in the spectra by revealing the role of out-of-plane phonon branches involved in the photon emission process. Finally, we develop a tool to simulate the dynamical structure factor of IXS, starting from phonon dispersions DFT Density Functional Perturbation Theory. We employ this tool to provide guidance and a sound interpretation of ultra-high pressure IXS experiments and related structural transitions in MoS2

Dynamical diffraction effects in STEM orbital angular momentum resolved electron energy-loss magnetic chiral dichroism

In this paper, we explore the properties of dynamical diffraction coefficients in orbital angular momentum resolved electron energy-loss magnetic chiral dichroism spectra, in a scanning transmission electron microscopy setup. We demonstrate that for basic zone axis geometries with fourfold or threefold symmetry the coefficients are constrained to have simplified forms. By exploiting these properties, we show how a dichroism spectrum accessible using this technique is only weakly dependent on sample thickness and, more generally, on dynamical diffraction effects. Our results indicate that in such cases it is possible to determine the orbital and spin components of atomic magnetic moments approximately from experimental spectra without the need for additional dynamical diffraction calculations

Orbital Angular Momentum and Energy Loss Characterization of Plasmonic Excitations in Metallic Nanostructures in TEM

Recently, a new device to measure the orbital angular momentum (OAM) electronic spectrum after elastic/inelastic scattering in a transmission electron microscope has been introduced. We modified the theoretical framework needed to describe conventional low-loss electron energy loss spectroscopy (EELS) experiments in transmission electron microscopes (TEM) to study surface plasmons in metallic nanostructures, to allow for an OAM post selection and devise new experiments for the analysis of these excitations in nanostructures. We found that unprecedented information on the symmetries and on the chirality of the plasmonic modes can be retrieved even with limited OAM and energy resolutions

Orbital angular momentum resolved electron magnetic chiral dichroism

We propose to use the recently introduced orbital angular momentum spectrometer in a transmission electron microscope to perform electron magnetic chiral dichroism experiments, dispersing the inelastically scattered electrons from a magnetic material in both energy and angular momentum. The technique offers several advantages over previous formulations of electron magnetic chiral dichroism as it requires much simpler experimental conditions in terms of specimen orientation and thickness. A simulation algorithm, based on the multislice description of the beam propagation, is used to anticipate the advantages of the approach over current electron magnetic chiral dichroism implementations. Numerical calculations confirm an increased magnetic signal to noise ratio with in plane atomic resolution