Orientador: Oscar Ferreira de Lima / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-24T18:27:35Z (GMT). No. of bitstreams: 1 Siqueira_EzequielCosta_M.pdf: 11787391 bytes, checksum: 424aa203ebfa572c02c0b2875dde771f (MD5) Previous issue date: 2006 / Resumo: O interesse em sistemas de multicamadas supercondutoras/ferromagnéticas deve-se a algumas questões importantes relacionadas com a supressão da temperatura crítica, à coexistência da supercondutividade e ferromagnetismo e às transições dimensionais da matéria de vórtices. Neste trabalho foram estudados filmes multicamadas preparados por magnetron sputtering, com fórmula geral Ni(x)[Nb(y)/Ni(x)]n, onde x e y são espessuras fixas (4 º A até 20 º A e 87 º A até 250 º A) e n é o número de bicamadas de Ni/Nb. Os filmes foram caracterizados por difração de raios X em baixo e alto ângulo e por medidas de momento magnético como função da temperatura (T) e campo (H), para orientações perpendicular e paralela às camadas. Os resultados das medidas de raios X em baixo ângulo indicaram uma boa estrutura de multicamadas tendo a espessura das bicamadas ficado dentro de 5% dos valores nominais. As medidas em alto ângulo revelaram a texturização das camadas de Ni e Nb nas direções (111) e (110), respectivamente. Este estudo foi concentrado nas transições dimensionais do acoplamento supercondutor e suas implicações sobre a matéria de vórtices. Medidas de magnetização feitas na amostra Ni(20 º A)[Nb(250 º A)/Ni(20 º A)]9 indicaram uma transição 3D-2D do sistema de vórtices, como revelado pelo comportamento do campo crítico H c2 ?? (T) paralelo às camadas. Consistentemente, foi identificada uma possível linha de desacoplamento dos vórtices associada com a transição das linhas de vórtices de Abrikosov em vórtices panquecas, para a orientação de H perpendicular às camadas. Um linha de irreversibilidade foi determinada e sugere diferentes regimes da matéria de vórtices, onde a função exercida pelas camadas ferromagnéticas de Ni pode ser relevante. Todas as amostras estudadas mostraram instabilidades no momento magnético medido, devido à forte interação entre as contribuições supercondutora, das camadas de Nb, e ferromagnética, das camadas de Ni. As amostras com espessuras de Ni menores mostraram um comportamento paramagnético evidenciandoeos efeitos de tamanho finito sobre o magnetismo das camadas de Ni. A temperatura crítica supercondutora nestas amostras (4 K < Tc< 6 K) foi reduzida em relação ao valor apresentado por uma amostra volumétrica de Nb ( Tc= 9,2 K). Em conclusão, todos os efeitos observados neste trabalho mostram uma relação muito próxima com os efeitos de tamanho finito das camadas de Ni e Nb, e com a competição entre supercondutividade e ferromagnetismo / Abstract: The interest in multilayered superconductor/ferromagnetic structures originates from some important questions related to the critical temperature suppression, the coexistence of superconductivity and magnetism, and dimensional transitions of the vortex matter. In this work we have studied multilayered films prepared by magnetron sputtering, with general formula Ni(x)[Nb(y)/Ni(x)]n, where x and y are fixed thicknesses (4 º A to 20 º A and 87 º A to 250 º A), and n is the number of bi-layers of Ni/Nb. The films were characterized by high and low angle X-ray diffraction and by measurements of the magnetic moment as a function of temperature (T) and magnetic field (H), for perpendicular and parallel orientations with respect to the layers. The results of low angle diffraction indicate a good multilayer structure having bi-layer thicknesses within 5% of the nominal values. The high angle results indicate that Ni and Nb layers are textured in (111) and (110) directions. In this study we have concentrated on the dimensional transitions of the superconducting phase and their effects on the vortex matter. Magnetization measurements made in the Ni(20 º A)[Nb(250 º A)/Ni(20 º A) ]9 sample indicated a 3D-2D crossover of the vortex system, as revealed by the parallel upper critical field behavior. Consistently, it was identified a possible decoupling line associated with transition of Abrikosov vortex lines into vortex pancakes, for H perpendicular to the layers. An irreversibility line was determined and it suggests different regimes of the vortex matter, where the role played by the Ni ferromagnetic layers might be relevant. All the samples studied show magnetic moment instabilities, due to the strong interplay between the superconducting Nb layers and the magnetic Ni layers. The samples with more thinner Ni layers showed a paramagnetic behavior in the normal state, revealing a finite size effect on the magnetic properties of the Ni layers. The superconducting critical temperature in those samples (4 K < Tc < 6 K) was reduced in relation to its bulk value (Tc= 9,2 K). In conclusion, all the effects observed in this work have shown a close relation with finite size effects of the Ni and Nb layers, and with the interplay between superconductivity and magnetism / Mestrado / Supercondutividade / Mestre em Física

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京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第23586号 / エネ博第432号 / 新制||エネ||82(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー応用科学専攻 / (主査)教授 松田 一成, 教授 宮内 雄平 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

This thesis describes the fabrication and characterization of two-dimensional transition dichalcogenides (2D TMDs) nanolayers for various applications in electronic and opto-electronic devices applications. In Chapter 1, crystal and optical structure of TMDs materials are introduced. Many TMDs materials reveal three structure polytypes (1T, 2H, and 3R). The important electronic properties are determined by the crystal structure of TMDs; thus, the information of crystal structure is explained. In addition, the detailed information of photon vibration and optical band gap structure from single-layer to bulk TMDs materials are introduced in this chapter. In Chapter 2, detailed information of physical properties and synthesis techniques for molybdenum disulfide (MoS2), tungsten disulfide (WS2), and molybdenum ditelluride (MoTe2) nanolayers are explained. The three representative crystal structures are trigonal prismatic (hexagonal, H), octahedral (tetragonal, T), and distorted structure (Tʹ). At room temperature, the stable structure of MoS2 and WS2 is semiconducting 2H phase, and MoTe2 can reveal both 2H (semiconducting phase) and 1Tʹ (semi-metallic phase) phases determined by the existence of strains. In addition, the pros and cons of the synthesis techniques for nanolayers are discussed. In Chapter 3, the topic of synthesized large-scale MoS2, WS2, and MoTe2 films is considered. For MoS2 and WS2 films, the layer thickness is modulated from single-layer to multi-layers. The few-layer MoTe2 film is synthesized with two different phases (2H or 1Tʹ). The all TMDs films are fabricated using two-step chemical vapor deposition (CVD) method. The analyses of atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy confirm that the synthesis of high crystalline MoS2, WS2, and MoTe2 films are successful. The electronic properties of both MoS2 and WS2 exhibit a p-type conduction with relatively high field effect mobility and current on/off ratio. In Chapter 4, vertically-stacked few-layer MoS2/WS2 heterostructures on SiO2/Si and flexible polyethylene terephthalate (PET) substrates is presented. Detailed structural characterizations by Raman spectroscopy and high-resolution/scanning transmission electron microscopy (HRTEM/STEM) show the structural integrity of two distinct 2D TMD layers with atomically sharp van der Waals (vdW) heterointerfaces. Electrical transport measurements of the MoS2/WS2 heterostructure reveal diode-like behavior with current on/off ratio of ~ 104. In Chapter 5, optically uniform and scalable single-layer Mo1-xWxS2 alloys are synthesized by a two-step CVD method followed by a laser thinning. Post laser treatment is presented for etching of few-layer Mo1-xWxS2 alloys down to single-layer alloys. The optical band gap is controlled from 1.871 to 1.971 eV with the variation in the tungsten (W) content, x = 0 to 1. PL and Raman mapping analyses confirm that the laser-thinning of the Mo1-xWxS2 alloys is a self-limiting process caused via heat dissipation to SiO2/Si substrate, resulting in fabrication of spatially uniform single-layer Mo1-xWxS2 alloy films.

Zweidimensionale (2D) Übergangsmetalldichalcogenide (TMDCs) gehören zu den attraktivsten neuen Materialien für optoelektronische Bauelemente der nächsten Generation. Um die überlegene Funktionalität der mit TMDCs verbundenen Bauelemente zu realisieren, ist ein umfassendes Verständnis ihrer elektronischen Struktur, einschließlich, aber nicht beschränkt auf die Auswirkungen von Defekten auf die elektronischen Eigenschaften und die Ausrichtung der Energieniveaus (ELA) an den TMDCs-Grenzflächen, unerlässlich, aber derzeit nicht ausreichend. Um einen tieferen Einblick in die elektronischen Eigenschaften von TMDCs und den damit verbundenen Grenzflächen in Kombination mit molekularen Halbleitern (MSCs) zu erhalten, untersuchen wir i) die fundamentale Bandstruktur von Monolagen (ML) TMDCs und die durch Schwefelfehlstellen (SVs) induzierte Renormierung der Bandstruktur, um eine solide Grundlage für ein besseres Verständnis der elektronischen Eigenschaften von polykristallinen dünnen Filmen zu schaffen, und ii) die optoelektronischen Eigenschaften ausgewählter MSC/ML-TMDCs-Grenzflächen. Darüber hinaus wird iii) der Einfluss des Substrats auf die elektronischen Eigenschaften einer MSC/ML-TMDC-Grenzfläche untersucht, um das Bauelementedesign zu steuern. Die Charakterisierung erfolgt hauptsächlich durch winkelaufgelöste Photoelektronenspektroskopie (ARPES), ergänzt durch Photolumineszenz (PL), Raman-Spektroskopie, UV-Vis-Absorption, Rastertransmissionselektronenmikroskopie (TEM) und Rasterkraftmikroskopie (AFM). Unsere Ergebnisse tragen zu einem besseren Verständnis der Auswirkungen von Defekten auf ML-TMDC und verwandte Grenzflächen mit MSCs bei, wobei auch die Auswirkungen der Substrate berücksichtigt werden, und sollten dazu beitragen, unser Verständnis des elektronischen Verhaltens in TMDC-verwandten Geräten zu verbessern. / Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are amongst the most attractive emerging materials for next-generation optoelectronic devices. To realize the superior functionality of the TMDCs related devices, a comprehensive understanding of their electronic structure, including but not limited to the impact of defects on the electronic properties and energy level alignment (ELA) at TMDCs interfaces, is essential but presently not sufficient. In an attempt to get a deep insight into the electronic properties of TMDCs and the related interfaces combined with molecular semiconductors (MSCs), we investigate i) the fundamental band structure of monolayer (ML) TMDCs and band structure renormalization induced by sulfur vacancies (SVs), in order to provide a solid foundation for a better understanding the electronic properties of polycrystalline thin films and ii) the optoelectronic properties of selected MSC/ML-TMDC interface. In addition, iii) the impact of the substrate on the electronic properties of the MSC/ML-TMDC interface is investigated for guiding device design. The characterization is mainly performed by using angle-resolved photoelectron spectroscopy (ARPES), with complementary techniques including photoluminescence (PL), Raman spectroscopies, UV-vis absorption, scanning transmission electron microscopy (TEM), and atomic force microscopy (AFM) measurements. Our findings contribute to achieving a better understanding of the impact of defects on ML-TMDC and related interfaces with MSCs considering the substrates’ effect and should help refine our understanding of the electronic behavior in TMDC-related devices.