Two-Dimensional Crystals and van der Waals Heterostructures based on Inorganic and Molecular Strongly Correlated Layered Materials

“En resumen, toda obra grande es el fruto de la paciencia y de la perseverancia, combinadas con una atención orientada tenazmente durante meses y aun años hacia un objeto particular”. “In summary, all great work is the fruit of patience and perseverance, combined with tenacious concentration on a subject over a period of months or years”. Santiago Ramón y Cajal (Reglas y consejos sobre investigación científica. Madrid: Academia de Ciencias Exactas, Físicas y Naturales, 1897). The present Thesis constitutes a journey –a romance of many dimensions, quoting Edwin Abbot Abbot– from 3D to 2D and, hopefully (we let the curious Reader judge), “Through Flatland to Thoughtland” (citing, again, Abbot). Thus, it can be understood as the addition of different strongly correlated lives: the life as a crystal grower, the life as an electrical contact maker, the life as an exfoliator, the life as a lab plumber, the life as…, but, overall, the life as an ever-learning process. Towards a better understanding of the present Thesis, a brief summary including the principal aspects of the work developed in each Chapter is provided below. The order of the Chapters reflects the chronological learning process that has taken place: from solid-state crystal growth (Chapter I) to the fabrication of different set-ups for the assembly and electrical characterization of atomically-thin layers (Chapter VII), from the study of 3D inorganic compounds (Chapter I and Chapter II) to the isolation of atomically-thin layers of molecular systems (Chapter III) followed by the fabrication of inorganic (Chapter IV and Chapter V) and hybrid molecular/inorganic (Chapter VI) van der Waals heterostructures based on 2D materials. Every Chapter consists on an initial brief overview, followed by an introduction (both historical and conceptual). Next, it is discussed the different topics to be covered (thus, results and discussion). Finally, different conclusions are sketched followed by detailed appendices and references. As a general concept, the Chapters are conceived to be short and easy-reading (or, at least, that has been ourintention). Thus, specific details as data analysis and reproducibility over devices are shown as appendixes. Chapter I constitutes the cornerstone of the present manuscript since it implies the initial crystal growth process for the ulterior study of their physical properties. In particular, it describes succinctly the different solid-state techniques employed in the Thesis as well as the main family of compounds grown: transition metal dichalcogenides, transition metal phosphochalcogenides and metal halides. Chapter II discusses the properties of bulk 1T-TaS2 as a quantum spin liquid based on muon spin relaxation and heat capacity. Our results evidence the existence of different competing quantum phases in this compound. Chapter III introduces the interest for two-dimensional materials and deals with the mechanical exfoliation of layered molecular compounds with magnetic properties. It constitutes the first report on the isolation of a molecular monolayer by mechanical exfoliation. Chapter IV describes the fabrication of vertical van der Waals heterostructures based on few-layers graphene and atomically thin-layers of 1T-TaS2 for exploring the interplay between dimensionality and out-of-plane correlations. Chapter V focuses on superconducting van der Waals heterostructures based on few-layers of NbSe2 and TaS2, highlighting the formation of electronic barriers at the interfaces. Chapter VI reports on the sensing of the spin state of molecular materials with two-dimensional materials based on the fabrication of hybrid molecular/inorganic van der Waals heterostructures (formed by spin-crossover systems and few-layers graphene or WSe2 monolayers). Chapter VII compiles briefly some of the plumber actions behind the present Thesis as the development of new equipment and techniques that were not present in the laboratory before. General Conclusions summarizes some of the most relevant aspects developed in this Thesis, although specific conclusions are included at the end of each chapter. Resumen de la Tesis Doctoral is a resume of the Thesis in Spanish.

“En resumen, toda obra grande es el fruto de la paciencia y de la perseverancia, combinadas con una atención orientada tenazmente durante meses y aun años hacia un objeto particular”. “In summary, all great work is the fruit of patience and perseverance, combined with tenacious concentration on a subject over a period of months or years”. Santiago Ramón y Cajal (Reglas y consejos sobre investigación científica. Madrid: Academia de Ciencias Exactas, Físicas y Naturales, 1897). The present Thesis constitutes a journey –a romance of many dimensions, quoting Edwin Abbot Abbot– from 3D to 2D and, hopefully (we let the curious Reader judge), “Through Flatland to Thoughtland” (citing, again, Abbot). Thus, it can be understood as the addition of different strongly correlated lives: the life as a crystal grower, the life as an electrical contact maker, the life as an exfoliator, the life as a lab plumber, the life as…, but, overall, the life as an ever-learning process. Towards a better understanding of the present Thesis, a brief summary including the principal aspects of the work developed in each Chapter is provided below. The order of the Chapters reflects the chronological learning process that has taken place: from solid-state crystal growth (Chapter I) to the fabrication of different set-ups for the assembly and electrical characterization of atomically-thin layers (Chapter VII), from the study of 3D inorganic compounds (Chapter I and Chapter II) to the isolation of atomically-thin layers of molecular systems (Chapter III) followed by the fabrication of inorganic (Chapter IV and Chapter V) and hybrid molecular/inorganic (Chapter VI) van der Waals heterostructures based on 2D materials. Every Chapter consists on an initial brief overview, followed by an introduction (both historical and conceptual). Next, it is discussed the different topics to be covered (thus, results and discussion). Finally, different conclusions are sketched followed by detailed appendices and references. As a general concept, the Chapters are conceived to be short and easy-reading (or, at least, that has been ourintention). Thus, specific details as data analysis and reproducibility over devices are shown as appendixes. Chapter I constitutes the cornerstone of the present manuscript since it implies the initial crystal growth process for the ulterior study of their physical properties. In particular, it describes succinctly the different solid-state techniques employed in the Thesis as well as the main family of compounds grown: transition metal dichalcogenides, transition metal phosphochalcogenides and metal halides. Chapter II discusses the properties of bulk 1T-TaS2 as a quantum spin liquid based on muon spin relaxation and heat capacity. Our results evidence the existence of different competing quantum phases in this compound. Chapter III introduces the interest for two-dimensional materials and deals with the mechanical exfoliation of layered molecular compounds with magnetic properties. It constitutes the first report on the isolation of a molecular monolayer by mechanical exfoliation. Chapter IV describes the fabrication of vertical van der Waals heterostructures based on few-layers graphene and atomically thin-layers of 1T-TaS2 for exploring the interplay between dimensionality and out-of-plane correlations. Chapter V focuses on superconducting van der Waals heterostructures based on few-layers of NbSe2 and TaS2, highlighting the formation of electronic barriers at the interfaces. Chapter VI reports on the sensing of the spin state of molecular materials with two-dimensional materials based on the fabrication of hybrid molecular/inorganic van der Waals heterostructures (formed by spin-crossover systems and few-layers graphene or WSe2 monolayers). Chapter VII compiles briefly some of the plumber actions behind the present Thesis as the development of new equipment and techniques that were not present in the laboratory before. General Conclusions summarizes some of the most relevant aspects developed in this Thesis, although specific conclusions are included at the end of each chapter. Resumen de la Tesis Doctoral is a resume of the Thesis in Spanish.