Coulomb engineering of the bandgap and excitons in two-dimensional materials

The ability to control the size of the electronic bandgap is an integral part of solid-state technology. Atomically thin two-dimensional crystals offer a new approach for tuning the energies of the electronic states based on the unusual strength of the Coulomb interaction in these materials and its environmental sensitivity. Here, we show that by engineering the surrounding dielectric environment, one can tune the electronic bandgap and the exciton binding energy in monolayers of WS2 and WSe2 by hundreds of meV. We exploit this behaviour to present an in-plane dielectric heterostructure with a spatially dependent bandgap, as an initial step towards the creation of diverse lateral junctions with nanoscale resolution

New Layered Materials and Functional Nanoelectronic Devices

This thesis introduces functional nanomaterials including superatoms and carbon nanotubes (CNTs) for new layered solids and molecular devices. Chapters 1-3 present how we incorporate superatoms into two-dimensional (2D) materials. Chapter 1 describes a new and simple approach to dope transition metal dichalcogenides (TMDCs) using the superatom Co6Se8(PEt3)6 as the electron dopant. Doping is an effective method to modulate the electrical properties of materials, and we demonstrate an electron-rich cluster can be used as a tunable and controllable surface dopant for semiconducting TMDCs via charge transfer. As a demonstration of the concept, we make a p-n junction by patterning on specific areas of TMDC films. Chapter 2 and Chapter 3 introduce new 2D materials by molecular design of superatoms. Traditional atomic van der Waals materials such as graphene, hexagonal boron-nitride, and TMDCs have received widespread attention due to the wealth of unusual physical and chemical behaviors that arise when charges, spins, and vibrations are confined to a plane. Though not as widespread as their atomic counterparts, molecule-based layered solids offer significant benefits; their structural flexibility will enable the development of materials with tunable properties. Chapter 2 describes a layered van der Waals solid self-assembled from a structure-directing building block and C60 fullerene. The resulting crystalline solid contains a corrugated monolayer of neutral fullerenes and can be mechanically exfoliated. Chapter 3 describes a new method to functionalize electroactive superatoms with groups that can direct their assembly into covalent and non-covalent multi-dimensional frameworks. We synthesized Co6Se8[PEt2(4-C6H4COOH)]6 and found that it forms two types of crystalline assemblies with Zn(NO3)2, one is a three-dimensional solid and the other consists of stacked layers of two-dimensional sheets. The dimensionality is controlled by subtle changes in reaction conditions. CNT-based field-effect transistor (FETs), in which a single molecule spans an oxidatively cut gap in the CNT, provide a versatile, ground-state platform with well-defined electrical contacts. For statistical studies of a variety of small molecule bridges, Chapter 4 presents a novel fabrication method to produce hundreds of FETs on one single carbon nanotube. A large number of devices allows us to study the stability and uniformity of CNT FET properties. Moreover, the new platform also enables a quantitative analysis of molecular devices. In particular, we used CNT FETs for studying DNA-mediated charge transport. DNA conductance was measured by connecting DNA molecules of varying lengths to lithographically cut CNT FETs

Trouble-makers in cytologic interpretation of the uterine cervix

The development and standardization of cytologic screening of the uterine cervix has dramatically decreased the prevalence of squamous cell carcinoma of the uterine cervix. Advances in the understanding of biology of human papillomavirus have contributed to upgrading the histologic diagnosis of the uterine cervix; however, cytologic screening that should triage those that need further management still poses several difficulties in interpretation. Cytologic features of high grade intraepithelial squamous lesion (HSIL) mimics including atrophy, immature metaplasia, and transitional metaplasia, and glandular lesion masquerades including tubal metaplasia and HSIL with glandular involvement are described with accentuation mainly on the differential points. When the cytologic features lie in a gray zone between the differentials, the most important key to the more accurate interpretation is sticking to the very basics of cytology; screening the background and cellular architecture, and then scrutinizing the nuclear and cytoplasmic details

Machine Learning Classification of Buildings for Map Generalization

A critical problem in mapping data is the frequent updating of large data sets. To solve this problem, the updating of small-scale data based on large-scale data is very effective. Various map generalization techniques, such as simplification, displacement, typification, elimination, and aggregation, must therefore be applied. In this study, we focused on the elimination and aggregation of the building layer, for which each building in a large scale was classified as “0-eliminated,” “1-retained,” or “2-aggregated.” Machine-learning classification algorithms were then used for classifying the buildings. The data of 1:1000 scale and 1:25,000 scale digital maps obtained from the National Geographic Information Institute were used. We applied to these data various machine-learning classification algorithms, including naive Bayes (NB), decision tree (DT), k-nearest neighbor (k-NN), and support vector machine (SVM). The overall accuracies of each algorithm were satisfactory: DT, 88.96%; k-NN, 88.27%; SVM, 87.57%; and NB, 79.50%. Although elimination is a direct part of the proposed process, generalization operations, such as simplification and aggregation of polygons, must still be performed for buildings classified as retained and aggregated. Thus, these algorithms can be used for building classification and can serve as preparatory steps for building generalization

Two-Dimensional Nanosheets from Redox-Active Superatoms

We describe a new approach to synthesize two-dimensional (2D) nanosheets from the bottom-up. We functionalize redox-active superatoms with groups that can direct their assembly into multidimensional solids. We synthesized Co₆Se₈­[PEt₂(4-C₆H₄COOH)]₆ and found that it forms a crystalline assembly. The solid-state structure is a three-dimensional (3D) network in which the carboxylic acids form intercluster hydrogen bonds. We modify the self-assembly by replacing the reversible hydrogen bonds that hold the superatoms together with zinc carboxylate bonds via the solvothermal reaction of Co₆Se₈­[PEt₂(4-C₆H₄COOH)]₆ with Zn­(NO₃)₂. We obtain two types of crystalline materials using this approach: one is a 3D solid and the other consists of stacked layers of 2D sheets. The dimensionality is controlled by subtle changes in reaction conditions. These 2D sheets can be chemically exfoliated, and the exfoliated, ultrathin 2D layers are soluble. After they are deposited on a substrate, they can be imaged. We cast them onto an electrode surface and show that they retain the redox activity of the superatom building blocks due to the porosity in the sheets

Patterning Superatom Dopants on Transition Metal Dichalcogenides

This study describes a new and simple approach to dope two-dimensional transition metal dichalcogenides (TMDCs) using the superatom Co₆Se₈(PEt₃)₆ as the electron dopant. Semiconducting TMDCs are wired into field-effect transistor devices and then immersed into a solution of these superatoms. The degree of doping is determined by the concentration of the superatoms in solution and by the length of time the films are immersed in the dopant solution. Using this chemical approach, we are able to turn mono- and few-layer MoS₂ samples from moderately to heavily electron-doped states. The same approach applied on WSe₂ films changes their characteristics from hole transporting to electron transporting. Moreover, we show that the superatom doping can be patterned on specific areas of TMDC films. To illustrate the power of this technique, we demonstrate the fabrication of a lateral p–n junction by selectively doping only a portion of the channel in a WSe₂ device. Finally, encapsulation of the doped films with crystalline hydrocarbon layers stabilizes their properties in an ambient environment

Enhancement of Exciton–Phonon Scattering from Monolayer to Bilayer WS2

Layered transition metal dichalcogenides exhibit the emergence of a direct bandgap at the monolayer limit along with pronounced excitonic effects. In these materials, interaction with phonons is the dominant mechanism that limits the exciton coherence lifetime. Exciton-phonon interaction also facilitates energy and momentum relaxation, and influences exciton diffusion under most experimental conditions. However, the fundamental changes in the exciton-phonon interaction are not well understood as the material undergoes the transition from a direct to an indirect bandgap semiconductor. Here, we address this question through optical spectroscopy and microscopic theory. In the experiment, we study room-temperature statistics of the exciton line width for a large number of mono- and bilayer WS2 samples. We observe a systematic increase in the room-temperature line width of the bilayer compared to the monolayer of 50 meV, corresponding to an additional scattering rate of similar to 0.1 fs(-1). We further address both phonon emission and absorption processes by examining the temperature dependence of the width of the exciton resonances. Using a theoretical approach based on many-body formalism, we are able to explain the experimental results and establish a microscopic framework for exciton-phonon interactions that can be applied to naturally occurring and artificially prepared multilayer structures

Coverage-Dependent Modification of the Surface Electronic Structure of an Organic-Semiconductor-Adsorbate Layer

The electronic structure of a hexa-<i>cata</i>-hexabenzocoronene (HBC)/Cu­(111) interface is investigated by two-photon photoemission over a range of coverage from 0 to 2 ML monolayers. It is found that increasing the HBC coverage shifts the vacuum level of the Cu substrate until this shift saturates at a coverage of ∼2 ML. Over this same range of coverage, the Shockley and the bare-surface Cu(111) image-potential states are shown to be quenched, while new unoccupied states appear and grow in strength with coverage. The use of momentum- and polarization-resolved photoemission spectra reveals that the new states are modified image states