Thesis (M.S.)--Ohio University, June, 2005. / Title from PDF t.p. Includes bibliographical references (p. 46-50)

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisor: Dim-Lee Kwong. Vita. Includes bibliographical references.

Thesis (Ph.D.)--Ohio University, March, 2007. / Title from PDF t.p. Includes bibliographical references.

A large family of materials, collectively known as "van der Waals materials," have attracted enormous research attention over the past decade following the realization that they could be isolated into individual crystalline monolayers, with charge carriers behaving effectively two-dimensionally. More recently, an even larger class of composite materials has been realized, made possible by combining the isolated atomic layers of different materials into "van der Waals heterostructures," which can exhibit electronic and optical behaviors not observed in the parent materials alone. This thesis describes efforts to characterize the atomic-scale structural and electronic properties of these van der Waals materials and heterostructures through scanning tunneling microscopy measurements. The majority of this work addresses the properties of monolayer and few-layer graphene, whose charge carriers are described by massless and massive chiral Dirac Hamiltonians, respectively. In heterostructures with hexagonal boron nitride, an insulating isomorph of graphene, we observe electronic interference patterns between the two materials which depend on their relative rotation. As a result, replica Dirac cones are formed in the valence and conduction bands of graphene, with their energy tuned by the rotation. Further, we are able to dynamically drag the graphene lattice in these heterostructures, owing to an interaction between the scanning probe tip and the domain walls formed by the electronic interference pattern. Similar dragging is observed in domain walls of trilayer graphene, whose electronic properties are found to depend on the stacking configuration of the three layers. Scanning tunneling spectroscopy provides a direct method for visualizing the scattering pathways of electrons in these materials. By analyzing the scattering, we can directly infer properties of the band structures and local environments of these heterostructures. In bilayer graphene, we map the electrically field-tunable band gap and extract electronic hopping parameters. In WSe₂, a semiconducting transition metal dichalcogenide, we observe spin and layer polarizations of the charge carriers, representing a coupling of the spin, valley and layer degrees of freedom.

In this thesis, a wide breadth of topics related to the field of surface science are addressed using density functional theory (DFT). Specifically, five studies with relevance to molecular electronics and heterogeneous catalysis are presented, with a particular focus on interadsorbate interactions, reactivity and characterisation of molecules on transition metal surfaces. The first part of this work focuses on giving strong theoretical underpinning to the atomic-scale observations provided by scanning tunnelling microscopy (STM) experiments conducted by my group colleagues. The theoretical calculations presented here provide support to the experimental evidences but also serve to unravel information that is inaccessible from the experiments. On the one hand, the variety of results obtained in this thesis using standard DFT methods serve to highlight the capabilities of the computationally low-demanding methods for modelling processes occurring on metal surfaces. On the other hand, we notice that these workhorse methods in DFT have inherent limitations for providing an accurate description of some properties, in particular binding energies. This, further improvements in the level of theory are necessary for advancing the computational accuracy of standard DFT methods in materials science. The second part of this thesis is devoted to highlight the high level of accuracy obtained by the new theoretical approaches in the field of materials science. Due to the recent implementation of new algorithms combined with the increasing computer power that is available to the scientific community, these sophisticated methods are becoming more accessible for modelling solid-state systems. Here, the recent implementation of the random-phase approximation (RPA) for solids is employed to perform to benchmark study on the adsorption of benzene on different close-packed transition metal surfaces. The development of new theoretical tools is also essential to improve our predictive capabilities in surface science. A novel approach to correct vibrational intensities by including anharmonicities using density functional perturbation theory (DFPT) is proposed. The new method is tested for the adsorption of different organic molecules on various transition metal surfaces. The results obtained by this implementation demonstrate excellent improvements for predicting accurate spectra of molecules on surfaces.

This thesis presents the study for the temperature-driven and current-driven metal-insulator phase transition (MIT) in Ca₂RuO₄ via home-built variable temperature Scanning Tunneling Microscope. Atomically resolved topography images along with temperature dependence of resistivity are taken verifying the quality of the single crystals used in this experiment. Tunneling spectra are measured under various temperatures across the Tmi = 357K, which clearly shows spectra evolution with temperature and the difference between the room- temperature insulator phase and the high-temprature metal phase. Compared with DMFT calculation, the STS indicates lattice structure plays a vital role in the phase transition. Same measurement is conducted on the crystals under a DC current, thanks to a custom designed sample holder. The evolution of the tunneling spectra with source current demon- strates similarity with the one of temperature-driven MIT. The comparison between the spectra taken at high-temperature metalic state and the high-current metalic state high- lights the similarity of these 2 phases, with both showing a DOS transfer from 1eV to lower energy, when compared with the ground state. Combined with a variety of other studies via transport, scattering technique and infrared thermal imaging, it is found that the local temperature dominates both temperature-driven and current-driven MIT. It is very likely that the current-driven is caused by the inevitable Joule heating generated by the current, indicating the high-current metallic phase might be the same with high-current metallic phase. Finally, surface roughness and autocorrelation length analysis suggests an inhomo- geneous surface topography stemmed from the coexistence of the insulating S* phase and conducting L* phase under current.

This dissertation describes the synthesis of novel, small molecule building blocks and the phenomena that are revealed when they are studied in unusual environments. Chapter 1 is divided into two sections, serving as introductions for chapters 2-3 and 4-5 respectively. The first section introduces the scanning tunneling microscope break junction (STM-BJ) as a new environment for reaction chemistry. Chapter 2 describes a truly single molecule reaction in which azobenzenes are formed in the STM-BJ one molecule at a time. Chapter 3 describes an electrostatically driven Ullmann coupling reaction of biphenyl iodides in the STM-BJ. In contrast to the reaction described in chapter 2, this reaction occurs throughout the solution in the presence of an electric field that surrounds the nanoelectrodes. The second half of chapter 1 provides a primer on the photoisomerization of azobenzenes, a class of small molecule dyes whose unique photochromic profile and switchable properties have been exploited across a wide range of fields. Chapter 4 introduces azobenzenes as photoswitchable ligands on superatomic clusters. Chapter 5 details the phenomenon and mechanism of white light driven rolling of Cu(I) isocyanoazobenzene crystals. Finally, chapter 6 is a self-contained research project. It details an organocatalytic O₂-coupled oxidation platform that capitalizes on the capacity of hydrazines to undergo rapid autoxidation to diazenes.

Graphene, a single atom thick hexagonally bonded sheet of carbon atoms, was first isolated in 2004 opening a whole new field in condensed matter research and material engineering. Graphene has hosted a whole array of novel physics phenomena as its carriers move at near the speed of light governed by the Dirac Hamiltonian, it has few scattering sites, it is easily gate-tunable, and hosts exciting 2D physics amongst many other properties. Graphene was only the tip of the iceberg in 2D research as researchers have since identified a whole family of materials with similar layered atomic structures allowing isolation into several atom thick monolayers. Monolayer material properties range from metals to semiconductors, superconductors, magnets and most other properties found in 3D materials. Naturally, this has led to making fully 2D heterostructures to study exciting physics and explore applications such as 2D transistors. It has recently been found that not only can you stack these materials at will but you can also tune their properties with an inter-layer twist between layers which at precise twist angles yields on-demand electronic correlations that can be easily tuned with experimental knobs leading to novel correlated phases. The pioneering techniques towards understanding each 2D material and heterostructures thereof have usually been with transport and optics. These techniques are inherently bulk macroscopic measurements which do not give insights into the nanoscale properties such as atomic-scale features or the nanoscale heterostructure properties that govern the systems. Atomic-scale structural and electronic insights are crucial towards understanding each system and providing proper guidelines for comprehensive theoretical understandings. In this thesis, we study the atomic-scale structural and electronic properties of various 2D systems using ultra-high vacuum (UHV) scanning tunneling microscopy and spectroscopy (STM/STS), a technique which utilizes electron tunneling with an atomically sharp tip to visualize atomic structure and low-energy spectroscopic properties. We focus on three major types of systems: twisted graphene heterostructures (magic angle twisted bilayer graphene and small angle double bilayer graphene), bulk and monolayer semiconducting transition metal dichalcogenides (TMDs), and 2D heterointerfaces (TMD - metal and graphene p-n junctions). We establish a number of state of the art methods to study these 2D systems in their cleanest, transport-experiment-like forms using surface probes like STM/STS including robust, clean, reliable contact methods and procedures towards studying micronscale exfoliated 2D samples atop hexagonal boron nitride (hBN) as well as photo-assisted STM towards studying semiconducting TMDs and other poorly conducting materials at low temperatures (13.3 Kelvin). We begin with one of the most currently mainstream topics of twisted bilayer graphene (tBG) where, near the magic angle of 1.1◦ the first correlated insulating and superconducting states in graphene were observed. A lack of detailed understanding of the electronic spectrum and the atomic-scale influence of the moir´e pattern had precluded a coherent theoretical understanding of the correlated states up til our work. We establish novel, robust methods to measure these micron-scale samples with a surface scanning probe technique. We directly map the atomic-scale structural and electronic properties of tBG near the magic angle using scanning tunneling microscopy and spectroscopy (STM/STS). Contrary to previous understandings (which predicted two flat bands with a several meV separation in the system), we observe two distinct van Hove singularities (vHs) in the local density of states (LDOS) around the magic angle, with a doping-dependent separation of 40-57 meV. We find that the vHs separation decreases through the magic angle with a lowest measured value of 7-13 meV at 0.79◦ . When doped near half moir´e band filling where the correlated insulating state emerges, a correlation-induced gap splits the conduction vHs with a maximum size of 6.5 meV at 1.15◦ , dropping to 4 meV at 0.79◦ . We find that more crucial to the magic angle than the vHs separation is that the ratio of the Coulomb interaction (U) to the bandwidth (t) of each individual vHs is maximized (as opposed to the proximity of the individual vHs’s), indicating that indeed electronic correlations are very important and suggesting a Cooper-like pairing mechanism based on electron-electron interactions. This establishes that magic angle tBG is to be understood in a single vHs picture where the band-width of the vHs is minimized. Spectroscopy maps show that three-fold (C3) rotational symmetry of the LDOS is broken in magic angle tBG, with an anisotropy that is strongest near the Fermi level, and is highly enhanced when the doping is in the vicinity of the correlated gap, indicating the presence of a strong electronic nematic susceptibility or even nematic order in tBG in regions of the phase diagram where superconductivity is observed. We next turn to twisted double bilayer graphene (tDBG), a system that is similar to tBG in phenomenology but turns out to be quite different. Correlated insulating and superconducting states were also found using transport in tDBG at a magic angle of 1.2-1.3◦ and ABC rhombohedral trilayer graphene aligned to hBN (ABC-tLG/hBN) with some stark differences such as displacement field tunable correlated states. We perform the first atomic-scale structural and electronic studies of small-angle tDBG as well as ABCA four layer rhombohedral stacked graphene and compare the findings to tBG. We first find that the moir´e pattern formed by tDBG is fundamentally different from tBG in that instead of hosting AB/BA Bernal stacking regions, it hosts BABA/ABCA (Bernal/rhombohedral) stacking domains. While we find this for small angle tDBG, these structural arguments will apply at all angles including the magic angle indicating that the flat bands and electron densities in tDBG are likely dominated at the ABCA sites. We use small angle tDBG to study large domains of four-layer ABCA graphene, revealing its displacement field dependent low energy spectroscopic structure and the flat band structure that comes with the four layer rhombohedral stacking which hosts the flattest band measured in any system of a 3-5 meV half-width. Furthermore, we measure the emergence of a 9.5 meV correlated gap in ABCA four-layer graphene at neutrality indicating that even without a hBN moir´e, ABCA graphene will likely host correlated states purely due to a flat band. These correlated states could be insulating or even superconducting in nature and the study thereof could provide crucial insight into whether superconductivity is related to Mott insulator physics as is suggested in the cuprates. When coupled to an hBN moir´e, these correlated states may be even stronger than that of magic angle tBG, magic angle tDBG and (most cer- tainly) ABC-tLG/hBN. Finally, we show that at Bernal - four-layer rhombohedral domain boundaries, there exists a topologically protected helical surface edge state. We next turn to the semiconducting TMDs. We find that semiconducting MoTe2 and MoSe2 have long range magnetic ordering as measured by muon spin resonance and SQUID at critical temperatures of 40 K and 100 K respectively. Using atomic-resolution STM/STS, we find that the semiconducting TMDs have a variety of intrinsic defects, one of which (a molybdenum substitution for a chalcogen, Mosub) we postulate using DFT is the cause of the long-range magnetism in the semiconducting TMDs which are not expected to host magnetism in their pristine structures. This finding establishes these semiconducting TMDs as magnetically ordered and adds them to the family of potential dilute magnetic semiconductor materials (the uniform robust fabrication of which has been sought-after for decades) which could have applications in spintronics. We then perform 13.3 Kelvin measurements (for the first time in these materials to our knowledge) on the same crystals using photoassisted STM, a technique that we establish to enable this low temperature measurement. The photo-assisted STM measurements reveal that not only are these defects magnetic but they host localized structural distortions which cover a large areas of the crystal surfaces. We find that these structural distortions are localized charge density waves due to a very high amount of localized doping that comes from the defects, putting the materials into a locally metallic regime and causing a phonon instability (found by phonon DFT). This finding of localized charge density waves in these high-quality semiconducting 2D materials is highly atypical for a semiconductor system and could have implications towards all techniques. The charge density waves could also be related to the measured magnetism as they have a much larger area of coverage in MoSe2 as opposed to MoTe2 which could be related to the critical temperature difference. We finally turn to two types of heterointerfaces, the first being metal-monolayer MoS2 junctions. We present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer MoS2 using STM/STS. Our measurements reveal that the electronic properties of these junctions, at the fundamental limit of a minimized Schottky barrier, are dominated by 2D metal induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (LDOS) of the MoS2 within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of about 0.55 nm near mid gap to as long as 2 nm near the band edges and are nearly identical for Au, Pd and graphite contacts, indicating that this is a universal property of the monolayer 2D semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact. These findings are generally applicable to any 2D semiconductor. We next look at another type of heterointerface, this time purely electronic in nature, graphene p-n junctions. Graphene p-n junctions should host interesting electron-optical properties such as electron collimation and Veselago lensing. While vague signatures of these have been observed, robust, definitive control of these properties are still lacking. We present the first atomic-scale characterization of state-of-the-art graphene p-n junctions using STM/STS revealing their current imperfections including significant electron-hole asymmetry, nonlinearity, roughness and intrinsic doping. We model the implications thereof and show that these imperfections strongly hinder electron-optical applications. Finally we explore the origin of these imperfections and potential avenues towards realizing better graphene p-n junction devices that may host much improved electron-optical properties.

The incorporation of molecules as low-cost and stable structures in electronic circuits is a promising strategy to miniaturize electronic components. Although single-molecule electronics is still at an early phase, the investigation of charge transport through single molecules is fundamentally important to understand the relevant scientific concepts and technological applications. In this dissertation, we measured and modulated the charge transport perpendicular to the plane of small benzene derivatives. In contrast to the conventional strategy to link molecules to electrodes via anchoring groups, we used the electrode potential to control the geometry of molecules and to form the junctions through π-system-metal electrode interactions. Using a combination of electrochemical STM (EC-STM) imaging and STM-BJ methods, the measurement of charge transport through single, flat oriented tetrafluoroterephthalic acid (TFTPA) molecules on an electrified Au (111) electrode showed that, at potentials below the potential of zero-charge (pzc) of Au(111), the molecules lie flat on the electrode and form highly ordered structures. The conductance of TFTPA, along the axis perpendicular to the benzene plane, is 0.24 ± 0.04 G0, consistent with reports for other molecules oriented flat in the junction. The configuration dependent conductivity has been confirmed by first-principles non-equilibrium Green’s function computation performed by Professor John Perdew and Dr. Haowei Peng at Temple University. Hence, the electrochemical surface potential can be employed to control the orientation of molecules to access a new charge transport measurement axis. Building on our previous results (Chapter 3), we studied charge transport through two fundamentally important molecules, tetracyanoquinodimethane (TCNQ) and tetrafluorotetracyanoquinodimethane (F4TCNQ) to determine the effect of molecule-electrode binding while maintaining the same core molecular structure. The findings show that on the negatively charged Au(111), the flat-oriented TCNQ and F4TCNQ molecules exhibit similar but high conductance of ~ 0.22 ± 0.01 G0 and 0.24 ± 0.01 G0, respectively. In addition to the high conductance, two peaks at 0.02 G0, and 0.05 G0 were detected for both molecules, assigned to the bidentate-bidentate and monodentate-bidentate configurations. Density functional theory (DFT) and non-equilibrium Green’s function (NEGF) calculations were performed by Professor Manuel Smeu and Dr. Stuart Shepard at Binghamton University to determine the conductance of four distinct molecular configurations. The results show how the orientation of molecules in the junction and the molecule-electrode denticity influence the molecular orbital offsets relative to the Fermi level and the consequent charge transport. The electronic structure and charge transport through single molecules can be modulated using various functional groups. Interestingly, our previous findings (Chapters 3, and 4) showed that the conductance perpendicular to the plane of TFTPA and TCNQ/F4TCNQ was similar to the parent molecules (TPA and TCNQ). Thus, it appeared that fluorination did not significantly change charge transport properties perpendicular to the molecular plane. Building on our previous studies, we measured the conductance through mesitylene substituted with electron-withdrawing groups (e.g., NO2, Br) or with electron-donating groups (e.g., CH3) to determine if other groups might impact conductance. Our results showed that the conductance perpendicular to the molecular plane increases by introducing electron-withdrawing groups and decreases as electron-donating groups are introduced to the mesitylene molecule. Density functional theory (DFT) and non-equilibrium Green’s function (NEGF) calculations were performed to rationalize our experimental findings (By Professor Smeu and Dr. Stuart Shepard). We demonstrated that the changes in the conductance perpendicular to the molecular plane correlate well with the Hammett constant of the corresponding functional groups, indicating the importance of the nature and strength of chemical substituents on the degree of conductance modulations at least for mesitylene derivatives. Following up on the modulation of charge transport through the intrinsic properties of molecules, we investigated the effect of solvent polarity on conductance of single molecules. Particularly, we focused on charge transport through dimethylaminobenzonitrile (DMABN), a molecule that shows unique behavior, such as noticeable bulk electronic modulations in response to the physical properties of the solvents in which the molecule is immersed, e.g., dual fluorescence in polar environment, due to the stabilized intramolecular charge transfer (TICT) state. Our charge transport results show that the conductance of DMABN in a polar solvent (acidified water) is ~ten times higher than the value observed in toluene (nonpolar solvent). The conductance of a molecule with no TICT properties shows no solvent polarity-dependent conductance, indicating that the intrinsic properties of DMABN (i.e., the TICT effect) play a critical role in the enhanced conductivity in the polar solvent. Molecular dynamics calculations (performed by Professor Manuel Smeu, and Dr. Stuart Shepard) suggest that the DMABN molecule can undergo internal rotation in the junction in polar solvents, result in a higher conductance compare to the planar geometry. Our results demonstrate that molecules exhibiting TICT properties can be promising candidates to design molecular devices with sensing and switching functionalities. The findings of this dissertation, in combination with the calculations (via collaboration with computational experts), show that the intrinsic and extrinsic properties of junctions, e.g., the geometry of molecule within the junction, the charge transport axis, the molecule-electrode binding, the characteristics and electronic structure of the molecules investigated, and the physical properties of the environment, influence charge transport through single molecules. This fundamental understanding and the ability to control charge transport through single molecules may allow the design of practical devices, e.g., large-scale molecular architectures and circuits, molecular switches, and sensors. / Chemistry

This thesis explores electron transport across single-molecule circuits via a combination of theory and experiment. Chapter 1 begins by introducing the diverse motivations for studying single-molecule electronics within engineering, chemistry and physics. Key aspects of the theory of electron transport across single-molecule circuits are summarized, before describing the modified scanning tunneling microscope technique used to measure single-molecule circuits. Chapter 2 presents a new theoretical approach to calculating quantum interference, which allows interference effects to be easily visualized within a matrix. The approach demonstrates that interference is vital to molecular-scale transport and accounts for conductance decay with length across molecular wires. In Chapter 3, a novel chemical design strategy is used to exploit destructive quantum interference in a series of long molecular wires containing a central benzothiadiaole unit. Scanning tunneling microscope-break junction measurements show the wires exhibit extremely nonlinear current-voltage characteristics, and the conductance of a six-nanometer molecule can be modulated by a factor of 10,000. Chapter 4 details how the scanning tunneling microscope setup may be modified to incorporate electrochemical impedance spectroscopy. Impedance measurements are then used to interrogate the solvent environment and measure capacitance. Chapter 5 demonstrates solvent-induced shifts in molecular conductance can be correlated with changes in junction capacitance. Together, the chapters in this thesis provide a framework for using chemical design to develop single-molecule circuits with functional properties.