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This thesis describes research into the interaction between electrons and various (pseudo) two-dimensional materials. This research is using two approaches: in Chapters 3 and 4 a low-energy electron microscope is used, and in Chapters 5 and 6 transport properties are studied. Chapter 1 introduces the concept of a two-dimensional material. First, the various kinds of such materials are illustrated. Secondly, the specific materials used in this thesis will be treated. We will see that two-dimensionality can be achieved in different ways: first of all top-down in a method where layers are peeled off a crystal until a single atomic layer remains. Secondly: bottom-up, in a method where a single layer is created from smaller components. Chapter 2 introduces the setup which was used for the measurements in Chapters 3 and 4. In these chapters, we will look at materials using electrons, in a low-energy electron microscope (LEEM). A regular microscope works by illuminating a sample with light. In a microscope, we observe bright and dark patches (corresponding to reflection and absorption of the light, respectively), as well as colors (corresponding to reflection and absorption of different wavelengths or energies of the light). We can also magnify objects using lenses. The LEEM works in a very comparable way, with the major difference that we do not use light (i.e. photons) but electrons to image the sample. An image is formed by electrons after interaction with the sample has taken place. This image can also be magnified, and contains bright and dark patches, from which the interaction of the material with the electrons can be established. Besides this, it is possible to change the electron energy in the setup, which makes it possible to measure the interaction at different energies. In the third Chapter we use the LEEM’s ability to measure the atomic orientation of thin layers of crystal. We look at graphene, a two-dimensional lattice of carbon atoms. This graphene was grown on a wafer. Contrary to peeling a crystal to atomically thin layers, this growth method is compatible with industrial processes, which require large slabs of graphene in predictable shapes. In developing these growing methods, it turns out to be difficult to grow large pieces of single-crystal material. With LEEM we look at differences in angular orientation in a layer of graphene. The motivation for this is that boundaries between such domains have a negative influence on the conductive properties of the material. In the fourth Chapter a method is extended to measure and visualize band structures in two-dimensional materials. We look specifically at molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN). The method (scanning ARRES) rapidly scans the electron bream across the first Brillouin zone. This gives a complete image of the band structure of these materials at energies above the Fermi level plus work function. The fifth and sixth Chapters concern single layer superstructures built out of nanocrystals. The building blocks are lead selenide (PbSe) single crystals in the form of a truncated cube, with a diameter of about 5 nm. By allowing these crystals to organize on a fluid surface, a single layer of crystals emerges. These crystals bond covalently in the direction of the atomic lattice. The material which emerges from this process can have multiple shapes, in this thesis we study the square structure. In Chapter 5 we study the conductance properties of such a structure at room temperature, under the influence of an ionic-fluid gate. This gate makes is possible to achieve high charge densities in these structures. We measure high mobilities for these systems, in the order of 1 cm2/Vs. In the sixth Chapter these samples are cooled to approximately 4 K. Despite the high mobilities measured in Chapter 5, the dependence of the conductance with temperature shows that transport is dominated by a hopping process and not by band transport, at the length scale of these samples.
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This thesis describes research into the interaction between electrons and various (pseudo) two-dimensional materials. This research is using two approaches: in Chapters 3 and 4 a low-energy electron microscope is used, and in Chapters 5 and 6 transport properties are studied. Chapter 1 introduces the concept of a two-dimensional material. First, the various kinds of such materials are illustrated. Secondly, the specific materials used in this thesis will be treated. We will see that two-dimensionality can be achieved in different ways: first of all top-down in a method where layers are peeled off a crystal until a single atomic layer remains. Secondly: bottom-up, in a method where a single layer is created from smaller components. Chapter 2 introduces the setup which was used for the measurements in Chapters 3 and 4. In these chapters, we will look at materials using electrons, in a low-energy electron microscope (LEEM). A regular microscope works by illuminating a sample with light. In a microscope, we observe bright and dark patches (corresponding to reflection and absorption of the light, respectively), as well as colors (corresponding to reflection and absorption of different wavelengths or energies of the light). We can also magnify objects using lenses. The LEEM works in a very comparable way, with the major difference that we do not use light (i.e. photons) but electrons to image the sample. An image is formed by electrons after interaction with the sample has taken place. This image can also be magnified, and contains bright and dark patches, from which the interaction of the material with the electrons can be established. Besides this, it is possible to change the electron energy in the setup, which makes it possible to measure the interaction at different energies. In the third Chapter we use the LEEM’s ability to measure the atomic orientation of thin layers of crystal. We look at graphene, a two-dimensional lattice of carbon atoms. This graphene was grown on a wafer. Contrary to peeling a crystal to atomically thin layers, this growth method is compatible with industrial processes, which require large slabs of graphene in predictable shapes. In developing these growing methods, it turns out to be difficult to grow large pieces of single-crystal material. With LEEM we look at differences in angular orientation in a layer of graphene. The motivation for this is that boundaries between such domains have a negative influence on the conductive properties of the material. In the fourth Chapter a method is extended to measure and visualize band structures in two-dimensional materials. We look specifically at molybdenum disulfide (MoS2) and hexagonal boron nitride (hBN). The method (scanning ARRES) rapidly scans the electron bream across the first Brillouin zone. This gives a complete image of the band structure of these materials at energies above the Fermi level plus work function. The fifth and sixth Chapters concern single layer superstructures built out of nanocrystals. The building blocks are lead selenide (PbSe) single crystals in the form of a truncated cube, with a diameter of about 5 nm. By allowing these crystals to organize on a fluid surface, a single layer of crystals emerges. These crystals bond covalently in the direction of the atomic lattice. The material which emerges from this process can have multiple shapes, in this thesis we study the square structure. In Chapter 5 we study the conductance properties of such a structure at room temperature, under the influence of an ionic-fluid gate. This gate makes is possible to achieve high charge densities in these structures. We measure high mobilities for these systems, in the order of 1 cm2/Vs. In the sixth Chapter these samples are cooled to approximately 4 K. Despite the high mobilities measured in Chapter 5, the dependence of the conductance with temperature shows that transport is dominated by a hopping process and not by band transport, at the length scale of these samples.
Graphene's maximized surface-to-volume ratio, high conductance, mechanical strength, and flexibility make it a promising nanomaterial. However, large-scale graphene production is typically cost-intensive. This manuscript describes a microbial reduction approach for producing graphene that utilizes the bacterium Shewanella oneidensis in combination with modern nanotechnology to enable a low-cost, large-scale production method. The bacterial reduction approach presented in this paper increases the conductance of single graphene oxide flakes as well as bulk graphene oxide sheets by 2.1 to 2.7 orders of magnitude respectively while simultaneously retaining a high surface-area-to-thickness ratio. Shewanella-mediated reduction was employed in conjunction with electron-beam lithography to reduce one surface of individual graphene oxide flakes. This methodology yielded conducting flakes with differing functionalization on the top and bottom faces. Therefore, microbial reduction of graphene oxide enables the development and up-scaling of new types of graphene-based materials and devices with a variety of applications including nano-composites, conductive inks, and biosensors, while avoiding usage of hazardous, environmentally-unfriendly chemicals.
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Graphene's maximized surface-to-volume ratio, high conductance, mechanical strength, and flexibility make it a promising nanomaterial. However, large-scale graphene production is typically cost-intensive. This manuscript describes a microbial reduction approach for producing graphene that utilizes the bacterium Shewanella oneidensis in combination with modern nanotechnology to enable a low-cost, large-scale production method. The bacterial reduction approach presented in this paper increases the conductance of single graphene oxide flakes as well as bulk graphene oxide sheets by 2.1 to 2.7 orders of magnitude respectively while simultaneously retaining a high surface-area-to-thickness ratio. Shewanella-mediated reduction was employed in conjunction with electron-beam lithography to reduce one surface of individual graphene oxide flakes. This methodology yielded conducting flakes with differing functionalization on the top and bottom faces. Therefore, microbial reduction of graphene oxide enables the development and up-scaling of new types of graphene-based materials and devices with a variety of applications including nano-composites, conductive inks, and biosensors, while avoiding usage of hazardous, environmentally-unfriendly chemicals.
Self-assembled nanocrystal solids show promise as a versatile platform for novel optoelectronic materials. Superlattices composed of a single layer of lead-chalcogenide and cadmium-chalcogenide nanocrystals with epitaxial connections between the nanocrystals, present outstanding questions to the community regarding their predicted band structure and electronic transport properties. However, the as-prepared materials are intrinsic semiconductors; to occupy the bands in a controlled way, chemical doping or external gating is required. Here, we show that square superlattices of PbSe nanocrystals can be incorporated as a nanocrystal monolayer in a transistor setup with an electrolyte gate. The electron (and hole) density can be controlled by the gate potential, up to 8 electrons per nanocrystal site. The electron mobility at room temperature is 18 cm2/(V s). Our work forms a first step in the investigation of the band structure and electronic transport properties of two-dimensional nanocrystal superlattices with controlled geometry, chemical composition, and carrier density.
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Self-assembled nanocrystal solids show promise as a versatile platform for novel optoelectronic materials. Superlattices composed of a single layer of lead-chalcogenide and cadmium-chalcogenide nanocrystals with epitaxial connections between the nanocrystals, present outstanding questions to the community regarding their predicted band structure and electronic transport properties. However, the as-prepared materials are intrinsic semiconductors; to occupy the bands in a controlled way, chemical doping or external gating is required. Here, we show that square superlattices of PbSe nanocrystals can be incorporated as a nanocrystal monolayer in a transistor setup with an electrolyte gate. The electron (and hole) density can be controlled by the gate potential, up to 8 electrons per nanocrystal site. The electron mobility at room temperature is 18 cm2/(V s). Our work forms a first step in the investigation of the band structure and electronic transport properties of two-dimensional nanocrystal superlattices with controlled geometry, chemical composition, and carrier density.
Recent observations of destructive quantum interference in single-molecule junctions confirm the role of quantum effects in the electronic conductance properties of molecular systems. These effects are central to a broad range of chemical and biological processes and may be beneficial for the design of single-molecule electronic components to exploit the intrinsic quantum effects that occur at the molecular scale. Here we show that destructive interference can be turned on or off within the same molecular system by mechanically controlling its conformation. Using a combination of ab initio calculations and single-molecule conductance measurements, we demonstrate the existence of a quasiperiodic destructive quantum-interference pattern along the breaking traces of π-stacked molecular dimers. The results demonstrate that it is possible to control the molecular conductance over more than one order of magnitude and with a sub-ångström resolution by exploiting the subtle structure-property relationship of π-stacked dimers.
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Recent observations of destructive quantum interference in single-molecule junctions confirm the role of quantum effects in the electronic conductance properties of molecular systems. These effects are central to a broad range of chemical and biological processes and may be beneficial for the design of single-molecule electronic components to exploit the intrinsic quantum effects that occur at the molecular scale. Here we show that destructive interference can be turned on or off within the same molecular system by mechanically controlling its conformation. Using a combination of ab initio calculations and single-molecule conductance measurements, we demonstrate the existence of a quasiperiodic destructive quantum-interference pattern along the breaking traces of π-stacked molecular dimers. The results demonstrate that it is possible to control the molecular conductance over more than one order of magnitude and with a sub-ångström resolution by exploiting the subtle structure-property relationship of π-stacked dimers.
