270 research outputs found
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Probing Atomic-Scale Properties of Organic and Organometallic Molecules by Scanning Tunneling Spectroscopy
No full textThe study of molecular physics has become increasingly important from both a scientific and technological viewpoint. The physical behavior of materials at nanometer length scales holds many surprises and the potential technological applications of molecular science arevast. This dissertation focuses on the fundamental physics of molecules adsorbed to metallic and semiconducting surfaces. Using a scanning tunneling microscope, four different molecular systems, C60, Gd@C82, tetramantane, and tetracyanoethylene (TCNE), were studied. The main effects investigated were (1) how can the properties of these molecules be atomically controlled, (2) how do metal surfaces affect molecular properties, (3) how do electron-electron and electron-vibration coupling influence molecular behavior, and (4) how do spins behave in molecule-scalestructures. For C60 we demonstrate a fine control of molecular properties such as energy levels, electron-electron interactions, and electron-vibration interactions via potassiumdoping. We also find that metal surfaces strongly influence the electronic screening and ordering of adsorbed molecules. In Gd@C82 and tetramantane molecules, the spatialdistribution of the electron-vibration coupling is found to be very inhomogeneous at sub-nanometer (< 10-9 m) length scales. In titanocene, we find that Au(111) induces molecular dissociation, with titanocene fragments displaying a spin-induced Kondo effect. The final molecule, TCNE, displays variable surface coupling and also enables tunable magnetic exchange coupling between covalently bonded spin centers in Vx(TCNE)y complexes
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Scanning Tunneling Microscopy of Graphene Quantum Dots
No full textQuantum confinement of Dirac fermions is an important frontier in graphene research. In this dissertation, we report on our use of a scanning tunneling microscope (STM) to investigate electrostatically confined Dirac fermions in graphene quantum dots. We first describe a technique for patterning embedded gates in backgated graphene/hexagonal boron nitride (hBN) heterostructures by STM manipulation of defect charges within the hBN substrate. In conjunction with a tunable backgate, this allows us to engineer p-n junctions in monolayer and bilayer graphene whose geometries can be flexibly designed with nanoscale precision. Using scanning tunneling spectroscopy (STS), we image and spatially characterize the behavior of Dirac fermions in the vicinity of p-n junctions and show that circular p-n junctions in monolayer and bilayer graphene act as gate-tunable quantum dots with unique energy spectra. For monolayer graphene quantum dots, comparison with theoretical simulations of the massless Dirac equation enables us to identify each experimentally observed spectroscopic peak as a quantum dot eigenstate with a unique set of quantum numbers. In bilayer graphene, we demonstrate a gate-tunable evolution of locally gated graphene from classical dots to quantum dots and achieve control over the number of massive Dirac fermions contained in a quantum dot by using the STM tip as a top gate. Furthermore, we explore the electronic properties of quantum double dots and non-circular monolayer graphene p-n junctions using spatially resolved STS. Our work yields insight into the spatial behavior of Dirac fermions under the influence of local electrostatic potentials and provides a platform for further experimental investigation of physics related to p-n junctions in graphene
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Investigating Diffusion Using Video Scanning Tunneling Microscopy and Deep Learning
No full textDiffusion is a ubiquitous phenomenon encompassing a wide range of physical systems from atomic scale motion to cellular locomotion. Diffusion plays a central role in practical applications such as electrochemistry, batteries, and thin film growth. The external control of diffusion properties presents new possibilities for understanding fundamental physical phenomena as well as a basis for next generation technological advances. The contributions of this thesis include the fabrication, modeling, and characterization of diffusion on gate-tunable nanodevices. We first derive the fundamental results of diffusion which form the basis of our experimental analysis. Afterwards, we describe the procedures for fabrication and algorithms for characterization of diffusion in video scanning tunneling microscopy (STM). The physical platform upon which most of these results are based on is a single-layer graphene on hexagonal boron nitride (h-BN) field-effect transistor (FET) with fluorinated tetracyanoquinodimethane (F4TCNQ) adsorbates, which were chosen due to favorable gate-tunable electronic properties. This device architecture enables control of diffusion through external temperature control, application of lateral and vertical electric fields, and substrate engineering. The analysis presented in this work provides rich sandbox for locally probing diffusion behavior in different system regimes. In particular, we find that device level control allows us to tune the density of adsorbates on the device due to the proximity of molecular orbitals to the device Fermi level and the device classical capacitance. Charged molecules form uniform arrays in response to the gate voltage to screen the applied field. In addition, we see that gate voltage can directly control diffusion through the modification of transition state energies as a result of molecule charging. We show that substrate level control in the form of moiré superlattice engineering can suppress diffusion by inducing anomalous subdiffusion in short time scales. Such behavior is explained using thermal equilibrium statistics for diffusion which show that the introduction of the moiré superlattice increases the number of unique energy states available to the diffusing particle. As a result, the size of the state space increases which in turn increases the time required to visit all available states. For short times, the entirety of the state space is not visited with high likelihood, a violation of the thermodynamic hypothesis that all accessible states are equiprobable, a phenomenon known as ergodicity breaking. Despite the challenges inherent in uncovering this behavior under experimental constraints of data sparsity using canonical statistical methods, we demonstrate that it is possible to observe this phenomenon by using deep learning
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Scanning Tunneling Microscopy Study of Graphene Electronic Nanostructures
No full textThis dissertation focuses on the study of the electronic properties of graphene using scanning tunneling microscopy (STM). A particular focus is the behavior of charge carriers in monolayer graphene around a single Coulomb impurity at the microscopic scale, and the interactions between two vertically stacked graphene layers. In order to probe the intrinsic electronic properties of graphene, hexagonal boron nitride (BN) was employed as a new substrate for supporting graphene due to its ultra-flatness and high purity. The surface roughness and charge inhomogeneity of graphene on BN were measured to be significantly smaller than graphene on SiO using STM. The high quality of graphene on BN enables us to investigate the fundamental physics question of how Dirac fermions in graphene respond to a single Coulomb impurity. This problem is divided into two separate regimes (subcritical and supercritical) depending on the strength of the Coulomb potential. The subcritical regime was investigated by using charge-tunable cobalt trimers on graphene, and the graphene LDOS around these charged impurity showed electron-hole asymmetry but no bound states. The supercritical regime was achieved by manipulating calcium dimers into clusters in order to surpass the supercritical charge threshold, and atomic-collapse quasi-bound states were demonstrated. The screening properties of graphene under various doping conditions were also measured using charge-stable calcium monomer adatoms, and the characteristic screening length of graphene was observed to scale inversely with its Fermi wavevector. Besides depositing charged impurities, the electronic structure of graphene can also be modified via vertical layer stacking. The lattice orientation difference between two contacting layers of graphene induces a series of Moir\'{e} bands which manifest themselves as Van Hove singularities and other higher-order spectroscopic features that can be measured in twisted bilayer graphene. The results presented in this dissertation contribute to our understanding of the microscopic electronic structure of graphene under the influence of charged impurities and layer-layer interactions, and shed new insight on how we can tailor graphene properties for use in real world applications
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On-Surface Synthesis and Local Electronic Structure Characterization of Low-Dimensional Nano-Materials
No full textUnderstanding of the electronic properties of new materials is of central importance for science and technology. In particular, low-dimensional nano-materials exhibit exotic properties that are determined by quantum mechanics. Rapid development of reticular chemistry has enabled the syntheses of a variety of low-dimensional condensed matter systems that are formed by linking up molecular building blocks. This dissertation focuses on the on-surface synthesis of such materials and consequent characterization of their local electronic structure with scanning tunneling microscopy (STM).The structure － property relationship of two low-dimensional nano-materials will be described: quasi-one-dimensional (1D) graphene nanoribbons (GNRs) and two-dimensional (2D) covalent organic frameworks (COFs). In the case of GNRs, we first show that two compatible molecular precursors with different sizes lead to GNR heterojunctions with atomically precise interfaces. In order to test predictions for how doped GNRs should behave, we synthesized B-doped GNRs with different doping concentrations on Au(111). Characterization of their electronic properties revealed a symmetry-dependent hybridization between the dopant states and the underlying Au substrate. We also describe our efforts to synthesize N=11 armchair GNRs through sp3-to-sp2 conversion of carbon atoms. In the case of 2D COFs, we first show that formation of imine linkages within COF366-OMe causes a downshift in the frontier orbital energy of porphyrin cores due to the electron-withdrawing characteristics of imine bonds. We then demonstrate the ability to achieve a 2D lattice of type II heterojunctions through judicious choice of molecular precursors that result in an asymmetrical bonding scheme within a 2D COF.Our studies show that on-surface synthesis can lead to the realization of target nanostructures that are unreachable via solution-based methods, as demonstrated by intra-molecular reactions that lead to new “nano-graphene” species
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Tuning Electrostatic Potentials for Imaging the Quantum Properties of Massless Dirac Fermions in Graphene
No full textGraphene, a two-dimensional (2D) honeycomb lattice of sp2-bonded carbon atoms, is renowned for its many extraordinary properties. Not only does it have an extremely high carrier mobility, exceptional mechanical strength, and fascinating optical behavior, graphene additionally has an interesting energy-momentum relationship that is emergent from its space group symmetry. Graphene's low-energy electronic excitations consist of quasiparticles whose energies disperse linearly with wavevector and obey a 2D massless Dirac equation with a modified speed of light. This fortuitous circumstance allows for the exploration of ultra-relativistic phenomena using conventional tabletop techniques common to solid state physics and material science. Here I discuss experiments that probe these ultra-relativistic effects via application of scanning tunneling microscopy (STM) and spectroscopy (STS) to graphene field-effect transistors (FETs) in proximity with charged impurities.The first part of this dissertation focuses on the ultra-relativistic Coulomb problem. Depending on the strength of the potential, the Coulomb problem for massless Dirac particles is divided into two regimes: the subcritical and the supercritical. The subcritical regime is characterized by an electron-hole asymmetry in the local density of states (LDOS) and, unlike in nonrelativistic quantum mechanics, does not support bound states. In contrast, the supercritical regime hosts quasi-bound states that are analogous to ``atomic collapse" orbits predicted to occur in atoms with nuclear charge Z > 170. By using an STM tip to directly position calcium (Ca) impurities on a graphene surface, we assembled "artificial nuclei" and observed a transition between the subcritical and supercritical regimes with increasing nuclear charge. We also investigated the screening of these charged impurities by massless Dirac fermions while varying the graphene carrier concentration with an electrostatic gate.The second part of this dissertation focuses on the ultra-relativistic harmonic oscillator. We developed a method for manipulating charged defects inside the boron nitride (BN) substrate underneath graphene to construct circular graphene p-n junctions. These p-n junctions were effectively quantum dots that electrostatically trapped graphene's relativistic charge carriers, and we imaged the interference patterns corresponding to this quantum confinement. The observed energy-level spectra in our p-n junctions closely matched a theoretical spectrum obtained by solving the 2D massless Dirac equation with a quadratic potential, allowing us to identify each observed state with principal and angular momentum quantum numbers.The results discussed here provide insight into fundamental aspects of relativistic quantum mechanics and into graphene properties pertinent to technological applications. In particular, graphene's response to electrostatic potentials determines the scope in which its charge carriers can be directed and harnessed for useful purposes. Furthermore, many of the results contained in this dissertation are expected to generalize to other Dirac materials
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Exploring Graphene Nanoribbons Using Scanning Probe Microscopy and Spectroscopy
No full textGraphene nanoribbons (GNRs) are strips of graphene, featuring narrow widths at the nanometer scale. A GNR may be considered as a structure cut out of graphene, which is a two dimensional honeycomb lattice of sp2 carbon atoms. Cutting graphene in different ways may be understood as imposing different boundary conditions on graphene, and therefore the electronic structures of GNRs are dependent on their geometries. Fascinating properties of graphene nanoribbons ranging from width-dependent semiconducting energy gaps to localized edge magnetization are predicted in theory. These properties, together with their ultra-thin nature, give GNRs great potential in future electronic applications. This dissertation focuses on the fundamental relations between the geometry and the electronic structure of GNRs, and explores bottom-up strategies to synthesize GNRs via molecular self-assembly. Using scanning tunneling microscopy (STM) and spectrocopy (STS), chiral and ultra-narrow armchair GNRs and width-modulated GNR heterojunctions were studied. The localized edge states in chiral GNRs derived from unzipping carbon nanotubes were explored and evidence is shown that these states are spin-polarized. We further modified the chiral GNR edges with hydrogen plasma, and determined both the terminal hydrogen-bonding structure and the edge electronic structure by combining STM and ab initio simulation. Bandgap tuning of bottom-up synthesized armchair GNRs was demonstrated via development of a new molecular building block. We find that the energy gap of wider N = 13 armchair GNRs is 1.4 ± 0.1 eV, 1.2 eV smaller than the bandgap of a narrower N = 7 armchair GNR. In addition, width-modulated GNR heterojunctions were obtained by fusing segments of two different molecular building blocks, and were characterized to possess electronic structure similar to type I semiconductor junctions. As an effort to develop an alternative route toward synthesis of GNRs, we imaged and studied single-molecule enediyne chemical reactions on metallic surfaces with non-contact atomic force microscopy (nc-AFM). This bond-resolved imaging technique allows us to extract an unparalleled insight into the chemistry involved in complex enediyne cyclization cascades on surfaces
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Photomechanical Response of Molecular Nanostructures
No full textThis dissertation focuses on the optomechanical behavior of molecular adsorbates on semiconducting and metal surfaces. Specifically, light-induced conformational changes of an azobenzene derivative on a semiconducting surface and infrared active vibrational modes of diamondoid molecules on a metal surface were studied in a scanning tunneling microscopy setup.Using conventional scanning tunneling microscopy, the self-assembly and light-induced mechanical switching of 3,3',5,5'-tetra-tert-butylazobenzene deposited on GaAs(110) were explored at the single-molecule level. After exposure to ultraviolet light, tetra-tert-butylazobenzene molecules exhibited conformational changes attributed to trans to cis photoisomerization. Photoisomerization of the molecules was observed to occur preferentially in one-dimensional stripes, a behavior that is significantly different from optically induced switching behavior observed when these molecules were placed on a gold surface.To characterize infrared absorption of submonolayers of molecules on electrically conducting crystals, a new scanning-tunneling-microscopy-based spectroscopy technique was developed. The technique employs a scanning tunneling microscope as a precise detector to measure the expansion of a molecule-decorated crystal that is irradiated by infrared light from a tunable laser source. Using this technique, the infrared absorption spectra of adamantane, [121]tetramantane, and [123]tetramantane on a Au(111) surface were obtained. Significant differences between the infrared spectra for the two tetramantane isomers show the power of the new technique to differentiate chemical structures even when single-molecule-resolved scanning tunneling microscopy images look quite similar. For adamantane on Au(111), the simplest studied system, ab initio calculations allowed for the interpretation of the microscopic vibrational dynamics revealed by the measurements. The infrared spectrum of an adamantane monolayer on Au(111) was found to be substantially modified with respect to the gas-phase infrared spectrum due to both the adamantane-adamantane interaction and adamantane-gold interactions. The results presented in this dissertation improve our understanding of photoisomerizational and optically induced vibrational properties of molecules placed in a condensed matter environment, and highlight the important role played by the molecule-surface and molecule-molecule interactions. These results are highly relevant to the field of molecular electronics and optically actuated molecular nanomachines
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Probing Atomic-Scale Properties of Magnetic and Optoelectronic Nanostructures
No full textThis dissertation presents scanning tunneling microscopy and spectroscopy studies of individual molecules and graphene nanoribbons (GNRs) bound to a substrate. Understanding the local electronic properties of these systems is importance from a fundamental physics viewpoint and for advancing potential technological applications in nanoelectronics. Two molecular systems, tetracyanoethylene (TCNE), and bithiophene naphthalene diimide (BND), were investigated. The basic questions addressed are (1) how do molecules respond to a condensed matter environment (i.e. a metal or semiconducting surface), (2) how do spins behave in molecule-scale structures, and (3) how do the intrinsic electronic properties of molecules affect their self-assembly behavior. We find that TCNE molecules display variable surface coupling and enable tunable magnetic exchange coupling between covalently bonded spin centers in Vx(TCNE)y complexes. We also were able to determine the TCNE adsorption site within a molecular monolayer on Ag(100) through a combination of inelastic electron tunneling spectroscopy and density functional theory calculations. We find that BND molecules exhibit type-II heterojunction energy level alignment. The interplay between the bipolar electronic nature of the molecule and the substrate results in different self-assembly patterns on a Au(111) surface. In GNRs we have demonstrated the presence of magnetic edge states for chiral nanoribbons with atomically smooth edges. We have further controlled GNR edges via hydrogen plasma etching, and have determined their exact edge termination
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Exploring Self-Assembly and Photomechanical Switching Properties of Molecules at Surfaces
No full textThe possible reduction of functional devices to molecular length scales provides many exciting possibilities for enhanced speed, device density, and new functionality. Optical actuation of nanomechanical systems through the conversion of light to mechanical motion is particularly desirable because it promises reversible, ultra-fast, remote operation. Past studies in this area have mainly focused on solution-based molecular machine ensembles, but surface-bound photomechanical molecules are expected to be important for future applications in molecular machines, molecular electronics, and functional surfaces. Cryogenic ultra-high-vacuum scanning tunneling microscopy has been employed to study the surface-based photomechanical switching properties of a promising class of photomechanical molecule called azobenzene.In the case of tetra-tert-butyl-functionalized azobenzene (TTB-AB) molecules adsorbed on Au(111) reversible switching by means of ultraviolet and visible excitation is experimentally observed at the single-molecule level. The presence of the metallic surface leads to a significant change of the molecular photoswitching properties: (i) photoswitching cross section is significantly reduced compared to the molecules in solution environment, (ii) photoswitching directionality is strongly affected. (iii) correlation between molecular ordering, electronic structure, and photoswitching capability is observed. (iv) new photoswitching dynamical mechanisms become operative.The results presented in this thesis provide insight into our understanding of photoswitching and adsorption properties of surface-bound molecules and elucidate the important role of molecule-surface interactions and molecule-molecule interactions
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