Optical and electronic properties of the two-dimensional materials inse, wse2 and mos2
- Autores: Mauro Brotóns i Gisbert
- Directores de la Tesis: Juan Francisco Sánchez Royo (dir. tes.)
- Lectura: En la Universitat de València ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: María Carmen Asensio Ariño (presid.), Alfredo Segura García del Río (secret.), Brian David Gerardot (voc.)
- Programa de doctorado: Programa Oficial de Doctorado en Física
- Materias:
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- Resumen
The field of two-dimensional materials has recently attracted the attention of the scientific community.The new properties that these materials show when their thickness is reduced down to the nanometer scale allows to envisage their potential application for both fundamental and technological research in a variety of fields including catalysis, energy storage, sensing, optoelectronic devices and quantum information.
This thesis presents the study of the optical and electronic properties of different two-dimensional van der Waals materials: the extensively studied transition-metal dichalcogenides MoS2 and WSe2 and the much less studied two-dimensional InSe. In the case of InSe, this thesis starts by providing an experimental and numerical study of the optical contrast of InSe nanosheets deposited on different substrates in order to determine the conditions that maximize it, which is a fundamental requirement for a fast and reliable localization of nanosheets with a particular thickness. Our results show that the optical contrast of InSe nanosheets deposited onto the extensively used SiO2(300 nm)/Si substrates can be optimized by illumination through standard wide band-pass filters. This approach turns out to be a good strategy because of the easy integration of band-pass filters in regular optical microscopy systems. Moreover, we have also demonstrated both experimentally and numerically that the optical contrast method for InSe can also be generalized to broadband illumination. In this way, our calculations have revealed that among the currently used SiO2/Si substrates, those with a coating layer of SiO2 of around 110 nm will favor optical detection of InSe nanosheets as thin as one single layer under white light illumination. In addition, we have studied the optical contrast of InSe nanosheets deposited on transparent substrates. Our results suggest that the best optical contrast is achieved for available transparent substrates with low real refractive indices.
Following the study of the optical contrast of InSe nanosheets, this PhD dissertation provides an extension of the experimental and theoretical study of the effects that quantum-size confinement has on the electronic, vibrational, and optical properties of InSe nanosheets with a view to investigate their potential integration into optoelectronic applications. Our results demonstrate that the progressive enhancement of quantum confinement effects has deep effects on lattice dynamics, electronic, and optical properties. The most evident effect of quantum-size confinement appears on the electronic properties of extended states. Density functional theory calculations have predicted a huge increase of the electronic band gap by more than 1 eV for single-layer InSe, which has been experimentally demonstrated by means of room temperature micro-photoluminescence in InSe as its thickness decreases from bulk to the single layer. Such a wide band gap tuning has resulted to be one of the largest optical windows observed so far in the bulk to single-layer transition of a given semiconductor.
The study of the optical and electronic properties of InSe nanosheets presented in this thesis is concluded by demonstrating that the morphological modification of InSe nanosheets through nanotexturing is able to enhance the room-temperature light emission intensity of this semiconductor for a wide range of material thicknesses.
With regard to WSe2, this PhD work presents a non-resonant, near-resonant and resonant laser excitation study of three-dimensionally confined excitons in single-layer WSe2. Our results demonstrate resonance fluorescence from a quantum emitter in single-layer WSe2 in spite of significant spectral fluctuations and background laser scattering. Moreover, despite the challenges that the spectral fluctuations present for quantum control and resonance fluorescence, we have shown its utility for high-resolution photoluminescence excitation spectroscopy spectroscopy, which has yield the direct observation of a threedimensionally confined weakly fluorescent exciton state that is energetically blue shifted by ~4.8 meV. We have also proved that resonant excitation of this blue-shifted state provides a single-photon source. The resonance fluorescence and laser spectroscopy techniques demonstrated in this thesis raise the prospect for indistinguishable single-photon generation and investigations of the spin and valley coherence of strongly confined excitons in two-dimensional WSe2.
In this PhD dissertation we also approach the numerical modeling of light emission by different two-dimensional materials embedded in multilayer planar structures. Despite two-dimensional materials have promising applications in optoelectronics, photonics and quantum technologies, their intrinsically low light absorption limits their performance and, consequently, potential light-emitting devices must be accurately engineered for optimal operation. In this sense, we have applied a transfer matrix-based source term method to study and optimize light emission processes in different two-dimensional materials such as MoS2, WSe2, h-BN and graphene. First we demonstrate that the implemented analytical model accurately accounts for experimental results reported for representative two-dimensional materials such as graphene and single-layer MoS2. Then, the model has been used to propose structures to optimize light emission by exciton recombination in single-layer MoS2, light extraction from arbitrarily oriented dipole single layers, and single-photon emission in two-dimensional materials. Also, it has been successfully applied to retrieve exciton-cavity interaction parameters from MoS2 microcavity experiments.
Finally, we have also approached the question of the hidden spin-polarized nature of the bulk electronic bands of 2H- MoS2 by optical absorption measurements and density functional theory calculations performed under high-pressure conditions. By applying pressure, we experimentally and theoretically show that the pressure dependence of the excitonic optical transitions (specifically, the exciton binding energy) can be only understood by considering that the two highest valence bands and the two lowest conduction bands at the K-point of the Brillouin zone are spin-polarized, with a spin sequence that agrees with that expected by calculations reported so far on single-layer MoS2. Moreover, we have also approached experimentally and theoretically the study of the evolution of the indirect band gap of bulk MoS2 under pressure. Our results indicate that a semiconductor-to-metal transition is expected to occur at ~35 GPa.
