Adsorbate-induced stiffening, quantum thermopower and photoresponse in two-dimensional materials

• Autores: Simon Aurel Svatek
• Directores de la Tesis: Nicolás Agraït de la Puente (dir. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2017
• Idioma: español
• Tribunal Calificador de la Tesis: Julio Gómez Herrero (presid.), Cristina Gómez-Navarro (secret.), Herre van der Zant (voc.), Peter Belton (voc.), Andres Castellanos Gomez (voc.)
• Programa de doctorado: Programa de Doctorado en Física de la Materia Condensada, Nanociencia y Biofísica por la Universidad Autónoma de Madrid; la Universidad de Murcia y la Universidad de Oviedo
• Materias:
  • Física
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • The feature size in semiconductor electronics has reduced over the last decades, thereby decreasing production costs and power consumption. If one dimension of a material is reduced the foreseeable limit is represented by a two-dimensional (2D) material, that is a crystal made of a single layer of atoms. The research field of 2D materials has emerged since the isolation of a graphene, a single layer of carbon atoms, from bulk graphite in 2004. Meanwhile, single layers of a variety of materials have been studied, thereby expanding the catalogue of 2D materials, which now includes the quasi-metal graphene, semiconductors (e.g. MoS2, InSe), and insulators (e.g. BN). The possibility to incorporate these materials into functional devices offer a new platform for the study of condensed matter phenomena and for the development of novel technologies. The content of this thesis is concerned with the characterisation of these materials in conditions relevant for applications, i. e. in the presence of adsorbates and solvents, a temperature gradient, and under illumination.

    In chapter 2 an overview of 2D materials is given. This includes a summary of their properties and a discussion of the possibility to combine these materials into stacked structures. This chapter also includes a description of the physical phenomena which are relevant to the experiments, namely molecular self-assembly on surfaces, the theoretical model for photovoltaic semiconductor devices, and the thermoelectric effect.

    Chapter 3 provides an overview of the experimental techniques. This includes a description of the basic principles of scanning probe microscopy and the experimental setups. In particular, a conductive atomic force microscope has been developed which allows the application of a thermal gradient between the sample and the tip. It allows high-accuracy measurements of voltages with errors in the µV range. Further, a short summary of the microfabrication techniques to form 2D material devices is given.

    In chapter 4, a morphological study of graphene in the presence of adsorbates and solvents is presented. Self-assembled monolayers of alkanes, tetratetracontane, were formed on graphene and imaged by scanning tunnelling microscopy in a liquid, tetradecane, whereby the graphene was supported by either a stiff or a soft substrate. The soft support was prepared by transferring graphene onto mica which involves the immersion of the sample into water. The hydrophilicity of mica causes water to be trapped at the graphene/mica interface causing the graphene to be responsive to molecular adsorption. We observe that the adsorption of alkanes induces curvature and anisotropic stiffness in the graphene with a symmetry axis along the alkanes. These effects are not observed when the graphene is supported by a stiff support, i.e. BN, in which case the molecular adsorption is reminiscent to the adsorption on graphite. The findings show that molecular adsorption can influence the mechanical properties of graphene and, thus, is relevant to applications in electronic materials, membrane technologies, and micromechanical systems.

    In chapter 5 the thermoelectrical properties of MoS2 in the cross-plane direction are explored. The thickness of thin layers of MoS2 is between ~1 nm for a monolayer and ~4 nm for six-layer MoS2. Hence, the observed thermoelectric effect is the quantum-equivalent of the classical property of a material to generate a voltage from a thermal gradient. It is quantified by the quantum thermopower which has, before this work, only been directly measured in single molecule junctions, which have values up to 40 µV/K and atomic contacts with values up to 5 µV/K. In this study quantum thermopower values of up to 330 µV/K in few-layer crystals have been found. The findings suggest that 2D materials are promising for thermoelectrical applications and probing the cross-plane direction could provide a novel platform for the study of quantum thermoelectric effects.

    2D materials can be assembled into vertical stacks in a predefined, chosen sequence. The properties of such heterostructures differ from the properties of their isolated constituents. Each new sequence represents therefore a novel artificial material. The small effort to form such materials and the unusual properties and new phenomena which arise in such structures have sparked a great amount of scientific interest. In this research field, there are many open questions, one of which has arisen from the finding that the Shockley diode model, which describes the charge carrier dynamics of a classical p-n junction, does not hold for p-n junctions made of monolayers. Few-layer material is important for applications and the question arises whether the Shockley diode model holds in an intermediate thickness-regime between monolayer and bulk. In chapter 6 the carrier dynamics in few-layer p-n junctions made of few-layer MoS2 with a total thickness of ~10 nm is investigated, showing good agreement with the Shockley diode model. Furthermore, the low thickness enables to influence the charge carrier density in the material by an electrostatic field which can be generated by applying a voltage to a gate-electrode. It is found that the characteristic diode parameters show a strong gate-dependence, which is discussed comprehensively. The findings demonstrate potential for MoS2 based solar cells and lays groundwork for gate-tunability of efficiency and photovoltaic response in ultrathin p-n junctions.

    Another type of 2D material heterostructure, in which a 2D material is sandwiched between graphene electrodes, is discussed in chapter 7. The motivation to form such sandwich-devices is to increase the charge carrier extraction efficiency by exploiting the low resistance of graphene in comparison with 2D semiconductors. The development of a fabrication strategy to form graphene/InSe/graphene vertical stacks from centimetre-scale graphene and exfoliated InSe flakes, providing a route to large area scalability, is discussed in chapter 6. The optoelectronic properties of these stacks have been measured and we find that, due to a large extraction efficiency of photogenerated charge carriers, we have fabricated one of the most photosensitive 2D material photodetectors reported to date.