Defesa de Tese de Doutorado #440 – Henrique Felipe de Melo – 29/05/2025

Nowadays, experimental research advances in several areas of knowledge are deep-rooted in the development and manipulation of nanomaterials, making it essential to explore the fundamental properties of those materials that are candidates for nanotechnology applications. In this thesis, we explore the effects of the temperature, thickness, and substrate on the molybdenum disulfide (MoS2) optical response. In particular, we are interested in the study of the dependence of the excitonic and phononic dynamics of this material on the interfacial constraints and under optical heat generation. We report the dependence of the MoS2 Raman modes on the sample temperature and on the excitation laser power. From the correlation between these two sets of measurements, in a so-called Raman thermometry, we determine the planar thermal conductivity of MoS2 monolayers, bilayers, trilayers, four layers, seven layers and eight layers. We find a nonmonotonic behavior, with the thermal conductivity decreasing from 38 Wm^(-1) K^(-1) to 24 Wm^(-1) K^(-1), as one goes from monolayer to trilayers, and then increasing from 24 Wm^(-1) K^(-1) to 50 Wm^(-1) K^(-1) when the thickness increases from three to eight layers. We associate this behavior with a convolution of two different phonon scattering processes: boundary scattering and interlayer scattering. We also report a monotonic thickness dependence of the interfacial thermal conductance of n-layers of MoS2 on silica/silicon (SiO2/Si), which ranges from 0.9 MWm^(-2) K^(-1) for a monolayer to 3.2 MWm^(-2) K^(-1) for eight layers films. Furthermore, we present a methodology optimized for the analytical extraction of thermal conductivity from Raman thermometry data.
To evaluate the thickness dependence of phonon-interaction processes in MoS_2 samples, we tracked both the A_1g and the E_2g Raman active modes in a temperature range between 80 K and 450 K. The observed peak shifts exhibited an anharmonic character, linked to third and fourth-order phonon interaction processes. The selection rules indicate the decay of Γ-excited optical phonons into two acoustic phonons in the vicinity of the Κ-point of the Brillouin zone, by a three-phonon process, and into three acoustic phonons near the Q-point, by a four-phonon process. Our experimental data demonstrated a correlation between the relative intensity of four-phonon and three-phonon decay channels and the number of MoS2 layers. To explain this correlation, we introduced the concept of “anharmonic availability”, emphasizing the critical role of electron-phonon scattering interactions for the observed phonon decay dynamics.
Additionally, we present temperature-dependent photoluminescence (PL) measurements on monolayers, bilayers, trilayers, and four-layers of MoS2 supported on SiO2/Si substrate. We fit the experimental measurements using Voigt functions and track the photoluminescence peaks corresponding to the B-exciton, A-exciton, and trion over a temperature range of 80 K to 370 K. The O’Donnell and Chen model is employed to predict the optical band gap behavior with increasing temperature, allowing us to investigate the fitting parameters on the number of MoS2 layers. Our results show that the average phonon energy remains independent of the number of layers. However, a strong coupling with the nature of the excitonic process was observed: A-exciton and trion interactions are correlated with zone boundary acoustic phonons, while the B-exciton primarily interacts with optical phonons. Additionally, we observe a layer-dependent exciton-phonon coupling, which is attributed to dielectric screening effects from reduced thickness.
Finally, to understand the intricate interplay between the substrate’s properties and the physical manifestations of the adjacent layers, we study the dependence of the MoS2 photoluminescence on the excitation laser power. We analyze exciton and trion intensities under variations of the excitation laser intensity. By fitting the PL spectra with Voigt functions, we study the dependence of the spectral weight of excitons and trions with laser power. Thereby, we use the mass-action model to describe the interplay between MoS2 free charge density and substrate dielectric features.

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