Ultrafast spectroscopy of two dimensional semiconductors

This thesis explores the optical properties of two-dimensional crystals by means of time-resolved spectroscopic techniques. Two-dimensional (2D) crystals are atomically thin materials consisting of planes of atoms held together by strong covalent/ionic bonding to form layers, which, conversely, are stacked by much weaker van-der-Waals forces. The strong 2D quantum con nement induces changes in the electronic band structure and optical behavior of isolated single layers (1L), giving rise to exciting and yet unexplored physical e ects. By far the most studied 2D crystal is graphene: 1L of carbon atoms arranged into a honeycomb structure. It is a semimetal, endowed with extraordinary sheet resistance, carrier mobility and conductivity[1]. However, the absence of an electronic band gap has hindered its application in optoelectronics, motivating the search for alternative semiconducting 2D crystals. Among the semiconducting layered materials, the transition metal dichalcogenides (TMDs) are the most promising candidates to substitute graphene, in particular, the compounds of molybdenum and tungsten. These latter are indirect-gap semiconductors in the multilayer form, which eventually have a transition to direct gap once reduced to single layers. The body of the text concentrates on three major published results concerning the charge and spin carriers dynamics in MoS2 and the hot electrons relaxation in encapsulated graphene. Methods and experimental setups developed during the thesis are also discussed. We have performed a study of the non-equilibrium optical properties of 1L-MoS2, and the obtained results are straightforwardly extended to the other semiconducting TMDs, which share a common photo-physics. We measured the transient absorption ( A) over a broad spectral range in order to monitor simultaneously three excitonic resonances in the visible range, commonly named as A, B and C transitions. To this aim, the standard transient absorption setup has been coupled with a confocal microscope, which enables to identify micrometer-size samples. We compared the transient absorption spectra at 100fs time delay after excitation using three di erent pump energies. After the photo-excitation, the excitonic absorptions are bleached ( A < 0) and red-shifted absorptions ( A > 0) appear, independent of the pump energy. The comparison with ab-initio simulations enables to explain this experimental response in terms of energy renormalization of both the electronic band gap (Eg) and the exciton binding energies (Eb). The combination of these two processes, results into a few meV red-shift of the absorption resonances, referred to as bandgap renormalization (BGR). This phenomenon has been observed in bulk systems but at much higher photo-excited carrier densities. In 2D crystals, it can be observed in low excitation regime because of the enhancement of many-body interactions which can be traced back to both the reduced dielectric screening and the 2D quantum con nement. A further evidence of the impact of dimensionality on 1L-MoS2 optical response, is the relaxation dynamics of valley polarization. The electronic gap in 1L-TMDs appears iii iv at two di erent valleys, at K and K' points of the Brillouin zone. Because of the breaking of spatial inversion symmetry, valley and spin degrees of freedom are strongly coupled and, by means of circularly polarized light, the two di erent valleys can be selectively populated, inducing a population di erence between the two valleys called valley polarization. We investigate the relaxation dynamics of the valley polarization by a combination of two non-equilibrium optical techniques: time-resolved Faraday rotation (TRFR) and time-resolved circular dichroism (TRCD). In the TRCD, the sample is excited with a circularly polarized pulse, resonant with the optical gap to induce the valley polarization, and the transient transmittivity ( T=T ) of co- and counter- circularly polarized pulses is measured in order to probe the population dynamics of the same and the opposite valley. In TRFR experiments, instead, the relaxation is monitored measuring the rotation of the linear polarization of a probe pulse whose energy is tuned well below the absorption gap. In these conditions, the signals are only sensitive to intervalley scattering. The observed fast dynamics, suggest that the depolarization is driven by e cient electron-hole Coulomb exchange interaction, a further proof of the enhancement of many body interactions in low dimensional materials. Beyond the eld of 2D crystals, isolated layers can also be reassembled into van der Waals heterostructures, made by stacking atomic planes in a chosen sequence. Among them, graphene encapsulation by hexagonal boron nitride (hBN) has demonstrated successful in decreasing the defects density and doping of graphene, improving its electrical properties. We have studied the optical properties of hBN encapsulated graphene in low excitation regime using a high sensitivity transient absorption setup. When graphene absorbs light, a distribution of hot electron is created, whose cooling dynamics is strongly in uenced by the phononic properties of the encapsulant material. We compare experimental data of transient absorption in the near infrared range with THz pump probe measurements and time-resolved photocurrent data, all indicating that the complete relaxation is achieved in less than 10 picoseconds (ps). We are able to rationalize this relaxation scenario in terms of near- eld coupling between the hot carriers in graphene and out-of plane, high-momentum hyperbolic phonons of hBN, which facilitate e cient energy transfer from graphene, boosting the electron cooling to few ps. Reading guideline The rst chapter of this thesis is dedicated to a brief introduction to the investigated materials, i.e. graphene and single layer MoS2 and their equilibrium optical properties. The second chapter presents the investigation of hot carrier relaxation in hBN-encapsulated graphene, while, in the third chapter, the evidences of bandgap renormalization in single layer MoS2 are discussed. The study of the intervalley scattering in single layer MoS2 is detailed in the fourth chapter. In the fth chapter three published articles focused on the study of the relaxation dynamics of few layers TMDs are attached. In the sixth chapter, a study of the non linear optical properties of graphene-protected copper is reported. Conclusions are nally drawn in the seventh chapter. During the PhD I continued a parallel research activity devoted to the study of vibrational properties of glasses initiated during the Master thesis. The most signi cant results are attached in the appendix B. The transient absorption microscope built during the PhD was used to study the spatial inhomogeneity of the optical properties of a perovskite crystal which is reported in the appendix C.

Questa tesi è dedicata allo studio delle proprietà opto-elettroniche di cristalli bidimensionali. Diverse tecniche di spettroscopia risolta in tempo vengono utilizzate per studiare le proprietà ottiche fuori dall'equilibrio di grafene incapsulato in nitruro di boro e di cristalli bidimensionali di dicalcogenuri di metalli di transizione (TMDs). In un primo capitolo introduttivo vengono discusse le proprietà ottiche all'equilibrio dei sistemi studiati, a cui seguono tre capitoli dedicati alla presentazione di tre dei principali lavori pubblicati durante il dottorato. Nello specifico, viene presentato lo studio del raffreddamento di elettroni fotoeccitati in grafene incapsulato mediante assorbimento transiente ad alta sensibilità. Viene quindi discusso l'effetto di rinormalizzazione fotoindotta delle energie di eccitazione di cristalli bidimensionali di molibdenite osservato tramite assorbimento transiente ad ampia copertura spettrale. Infine viene presentato lo studio della dinamica di depolarizzazione delle valli nella molibdenite bidimensionale tramite misure di dicroismo circolare e di rotazione Faraday risolte in tempo. Il rilassamento all'equilibrio dei cristalli bidimensionali di dicalcogenuri di metalli di transizione composti da pochi strati atomici è stato quindi studiato mediante assorbimento transiente che ha messo in luce uno scenario universale di diseccitazione comune a tutti i TMDs. Conclusioni generali sul ruolo della ridotta dimensionalità nel determinare le proprietà optoelettroniche dei cristalli bidimensionali sono tratte sulla base dei risultati sperimentali raccolti.