Doctor of Philosophy / Department of Chemical Engineering / Vikas Berry / Placidus Amama / The evolution of unique electrical, optical, thermal, mechanical, and chemical properties in two-dimensional (2D) nanomaterials due to the atomic confinement in the z-direction has ignited tremendous technology promises. With that promise comes a challenge of incorporating 2D nanomaterials into practical applications, enabling their large-area growth and using covalent or van der Waal bonding to extent and control their properties in electronic applications. This PhD thesis establishes the following results: (a) successfully developing of scalable processes for direct growth of large-area graphene, h-BN, and MoS₂-on-h-BN on SiO₂/Si substrate, (b) demonstrating an electronic sensor for the defection of molecular motion by covalently interfacing 2D nanomaterials with photo-mechanical molecules, and (c) establishing the modulation of structural, electrical, thermal properties of 2D nanomaterials by covalently interfacing metal nanoparticles with 2D nanomaterials. A promising scalable route for large-area growth of 2D nanomaterial on a dielectric substrate is to perform chemical vapor deposition (CVD). Via two patented processes, we have synthesized graphene films directly on a SiO₂ substrate via carbon-diffusion through copper grains, and h-BN film on SiO₂ substrate via surface oxide assisted mechanism. The continuous graphene film grown with large coverage on SiO₂ substrate possessed a crystalline sp² domain size of 140 nm with low defect density (as indicated by low Raman I[subscript]D/I[subscript]G~0.1). The sheet resistance of this turbostratic stacking graphene was ~4 kOhm/sq, with a charge carrier mobility of ~250 cm²V⁻¹s⁻¹. Unprecedented, large coverage of directly grown h-BN film on SiO₂ substrate was demonstrated. This h-BN film showed a 6-fold smoothness enhancement compared to that of SiO₂ substrate. Such smoothness and the nature of free dangling bond of h-BN film reduced Coulombic long range scattering, leading to the 5-fold enhancement in the conductivity of the MoS₂, which is directly grown on the underlying h-BN platform. The next-generation molecular electromechanical systems require controlled manipulation and detection of molecular motion to build systems which respond to molecular mechanics. To achieve this, we covalently interfaced photo-mechanical molecules (azobenzene) (density = 2.5 nm⁻²) onto trilayer graphene (37.5% sp² coverage), where high sensitivity of this trilayer graphene due to high quantum capacitance (6.3 microF/cm²) and carrier confinement was leveraged. This enabled graphene to sensitively detect azobenzene isomerization, where one hundred molecules generated one charged carriers in the graphenic platform (2.44 x 10¹² holes/cm²). As mentioned before, surface modification of 2D nanomaterials opens an avenue to incorporate them into rational applications. We demonstrated the ability to interface noble metal nanoparticles (gold, silver) selectively onto a MoS₂ lattice (60° angular displacement) via both diffusion limited aggregation and instantaneous reaction arresting (using microwaves). Such gold nanoparticle interfaces allowed the modulation of electrical, and thermal properties, confirmed by Raman, electrical, and thermal studies. Consequently, a remarkably capacitive interaction between gold and thin MoS₂ sheet showed a 9-fold increase of effective gate capacitance with low Schottky barrier (14.52 meV), and a 1.5-fold increase in thermal conductivity with a low carrier-transport thermal-barrier (44.18 meV). This long-term work has established the following points: 1) Scalable routes for the growth of 2D nanomaterials, which can be extended to synthesize complex hetero/lateral architectures for integrated thin film circuitries. Furthermore, 2) covalent functionalization of 2D nanomaterials with nanoparticles and molecular systems can futuristically develop rational interfaces with other 2D heterostructures, and molecular machines.

Thesis advisor: Michael Graf / Bismuth is an interesting element to study because the low effective mass of its charge carriers makes the material sensitive to quantum confinement effects. When bismuth is reduced to the nanoscale two interesting phenomena may occur: it may transition from a semimetal to a semiconductor, or charge carriers in special surface states may begin to dominate the behavior of the material. Arrays of bismuth nanowires of various diameters were studied to investigate these possibilities. The magnetoresistance of the arrays was measured and the period of Shubnikov-de Haas oscillations suggested an increase in the effective mass and density of the material’s charge carriers for small nanowire diameters. These increases suggested that electrons were present in surface states and strongly influenced the material’s behavior when its dimensions were sufficiently reduced. The magnetization of the nanowire arrays was also measured and the lack of de Haas-van Alphen oscillations for certain diameter nanowires suggested that electrons were not present in surface states and that instead the material was transitioning from a semimetal to a semiconductor. Heat capacity measurements were planned to reconcile the two experiments. My detailed calculations demonstrated that heat capacity measurements were feasible to determine the presence, or absence, of surface charge carriers. Because the electronic contribution to the material’s heat capacity is small a calorimeter platform was constructed with ultra-low heat capacity components. / Thesis (BS) — Boston College, 2009. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: College Honors Program. / Discipline: Physics.

The strong and distinctive excitonic interactions are among one of the most interesting aspects of the newly discovered family of two-dimensional semiconductors, monolayers of transition metal dichalcogenides (TMDC). In this dissertation, we explore two types different types of excitonic states in these materials beyond the isolated exciton in its radiative ground state. In the first part of this thesis, we examine higher-order excitonic states, involving correlations between more than a single electron and hole in the usual configuration of an exciton. In particular, we demonstrate the existence of four-body correlated or biexciton states in monolayer WSe₂. The biexciton is identified as a sharply defined state in photoluminescence spectra at high exciton density. The biexciton binding energy, i.e., the energy required to separate it into to isolated excitons, is found to be 52 meV , which is more than an order of magnitude greater than that in conventional quantum-well structures. Such high binding energy arises not only from the two-dimensional carrier confinement, but also from reduced and non-local dielectric screening. These results open the way for the creation of new correlated excitonic states linking the degenerate valleys in TMDC crystals, as well as more complex many-body states such as exciton condensates or the recently reported dropletons. In the second part of this thesis, two chapters are devoted to the identification and characterization of intrinsic lower-energy dark excitonic states in monolayer WSe₂. These optically forbidden transitions arise from the conduction band spin splitting, which was previously neglected as it only arises from higher-order spin-orbit coupling terms. First, by examining light emission using temperature-dependent photoluminescence and time-resolved photoluminescence, we indirectly probe and identify the existence of dark states that lies ~30 meV below the optically bright states. The presence of the dark state is manifest in pronounced quenching of the bright exciton emission observed at reduced temperature. To extract exact energy levels and actually utilize these dark states, as the second step, we sought direct spectroscopic identification of these states. We achieve this by applying an in-plane magnetic field, which mixes the bright and spin forbidden dark excitons. Both neutral and charged dark excitonic states have been identified in this fashion, and their energy levels are in good agreement with ab-initio calculations using GW-BSE approach. Moreover, due to the protection from their spin structure, much enhanced emission and valley lifetime were observed for these dark states. These studies directly reveal the excitonic spin manifolds in this prototypical two-dimensional semiconductor and provide a new route to control the optical and valley properties of these systems.

Nanophotonics is a fast-growing area of both scientific significance and practical value for applications. Nanophotonics studies the interaction between light and electronic systems in nanomaterials and nanostructures as well as the behavior of light in nanometer scales. It covers many hot topics such as plasmonics, two-dimensional materials, and silicon photonics. Increasing attention is given to the area and nanophotonics is expected to have significant impact on future technology advances. This thesis work focuses on three aspects of nanophotonics. The first aspect is in exploring the nonlocal effect and surface correction for nanometer-length-scale plasmonic structures. Plasmonics is the study of the interaction between electromagnetic fields and free electrons in a metal. It exploits the unique optical properties of metallic nanostructures to enable routing and manipulation of light at the nanoscale, where nonlocal effect becomes important. Here we introduce a new surface hydrodynamic model for plasmon propagation at interfaces, which incorporates both nonlocality and surface contributions. This surface correction is calculated via a discontinuity in the normal component of the electric displacement in conjunction with Feibelman's d-parameters, thus enabling rapid numerical calculation of nanostructures without requiring a full quantum calculation because of its large computational requirement. We examine numerical calculations of surface plasmon polaritons propagation at a single interface structure, and then for a more complex thin-film structures. The second aspect is investigating the third-harmonic generation in thick multilayer graphene. Graphene is the first two-dimensional material to be discovered and has attracted much interest because of its remarkable two-dimensional electronic, optical, mechanical, and thermal properties. Multilayer graphene, can be seen as stacking of monolayer graphene, and it offers an array of properties that are of interest for optical physics and devices. We describe the layer-dependent for third-harmonic generation in thick multilayer graphene on quartz substrate. The third harmonic signal of multilayer graphene exhibits a complex dependence on its layer number showing that the optimal third harmonic signal at 24 layers, in good agreement with two theoretical models. The third aspect is an exploration in silicon photonics of design and demonstration of a differential phase shift keying demodulator based on coherent perfect absorption effect. Silicon photonics is considered a potential future communication system mainly due to its compact footprint, dense integration, and compatibility with mature silicon integrated circuit manufacturing. Differential phase shift keying based system offers advantages, e.g., dispersion tolerance, improved sensitivity, and does not require coherent detection. Coherent perfect absorption uses a ring resonator works for the critical coupling condition at resonance frequency. This work shows a new compact demodulator circuit can be integrated in all optical-system.

This thesis explores the synthesis and characterisation of a range of molecular wheels containing unpaired electron spins. These molecular spin systems are of considerable interest, both for the insight they provide into the physics of such systems and for their potential as quantum bits ("qubits") in a quantum information processing device. In particular, this thesis explores using these wheels to meet criteria 1 and 5 of the DiVincenzo criteria. The synthesis of a novel homometallic and nonametallic ring of CrIII ions is introduced, along with extensive physical characterisation. Inelastic Neutron Scattering measurements suggest that the molecule has an almost degenerate S = 1/2 ground state with only 0.1 meV separation, making this ring a near perfect example of a Type I frustrated spin system. Chemical modification of the heterometallic {Cr7M} family of wheels with both hard and soft Lewis base functionality is also explored. Using a triphenylphosphine derivative, the coordination chemistry of a highly sterically hindered mono-substituted triphenylphosphine derivative with gold is explored, yielding new arrangements of the wheels. Changes in the electronic and steric properties of the system are studied by a combination of 31P NMR spectroscopy and DFT modelling, revealing dramatic changes in the phosphorus donor properties. The effect of this ligand substitution on the anisotropy tensor of CoII contained in a heterometallic {Cr7Co} ring is explored using variable temperature 1H NMR spectroscopy. Using a combination of the experimentally observed 1H NMR dipolar shifts and computational modelling, a significant change in the anisotropy tensor of the cobalt is found. Finally, as part of a g-engineering approach to qubit design the chemistry of the octametallic {Cr7Ni} ring functionalised with triphenylphosphine oxide is introduced. Initial efforts towards developing a hybrid {Cr7Ni}2Ln (Ln = Gd, Eu) qubit system, along with characterisation by EPR and luminescence spectroscopy, suggest that this may be a route to developing a qubit with the capacity for optical control of the communication.

Functionalizing surfaces with amine groups through the hydrolytic condensation of aminotrialkoxysilanes is a typical approach when modifying silica particles for use in bioimaging, enzyme immobilization, and other applications. This processing step can be eliminated if amine-functionalized silica particles are directly prepared without using aminotrialkoxysilanes. Here, a one-pot, ambient-condition, water-phase method to synthesize silica-based nanoparticles (NPs) that present surface amine groups is described. The formation mechanism involves the electrostatic crosslinking of cationic polyallylamine hydrochloride by citrate anions and the infusion of the formed polymer/salt aggregates by silicic acid. The particles were unimodal with average diameters in the range of 40 to 100 nm, as determined by the size of the templating polymer/salt aggregates. Colorimetric analysis using Coomassie brilliant blue and zeta potential measurements confirmed the presence of surface amine groups of the hybrid silica/polymer NPs. Surface charge calculations indicated the hybrid NPs had a lower amine surface density than aminopropyltriethoxysilane-functionalized silica (0.057 #/nm 2 vs. 0.169 #/nm 2 at pH 7).

Using scanning tunneling microscopy (STM) we have explored complex surface adsorbed molecules, nanocars, on Au(111) and the parameters related to the direct translation of these molecules by the STM tip. Specifically, the molecules focused on here were functionalized with C 60 or trans ruthenium complexes. With low tunneling currents the molecules could be imaged at room temperature. Increasing the tunneling current allowed us to bring the tip closer to individual molecules and reposition them on the surface. Below specific current and bias voltage conditions the molecules remained stationary, while in other cases the tip interaction was strong enough to drastically damage or eject the molecule from the field of view. High temperature scans revealed the effect of the wheel activation energy relative to the underlying surface as the different wheeled nanocars began diffusing at different temperatures confirming the manipulation measurements.

Noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR), in which incident light causes a collective oscillation of a nanoparticle's free electrons. This phenomenon results in unique optical properties, including enhanced electric fields near the particle surface and an extinction peak at the resonant wavelength. The LSPR extinction peak's location is sensitive to the refractive index of the surrounding medium, especially in the volume closest to the particle surface. This makes plasmonic nanoparticles ideal for biosensing: their refractive index sensitivity can be used to transduce molecular binding signals. A method has been developed to use the optical extinction of films of gold nanorods to track antibody-antigen interactions in real time, resulting in a label-free kinetic immunoassay based on LSPR. Also, this method has been adapted to scattering spectra of single gold bipyramids. The single-particle approach has allowed the label-free detection of single biomolecules with kinetics information. These methods have future applications to both molecular biology and clinical assays.

Advances in several research areas increase the need for more sophisticated fabrication techniques and better performing materials. Tackling this problem from a bottom-up perspective is currently an active field of research. The bottom-up fabrication procedure offers sub-nanometer accurate manipulation. At this time, candidates to achieve nanomanipulation include chemical (self-assembly), biotechnology methods (DNA-based), or using controllable physical forces (e.g. electrokinetic forces, mechanical forces). In this thesis, new methods and techniques for mechanical nanomanipulation using probe force interaction are developed. The considered probes are commonly used in Atomic Force Microscopes (AFMs) for high resolution imaging. AFM-based mechanical nanomanipulation will enable arranging nanoscale entities such as nanotubes and molecules in a precise and controlled manner to assemble and produce novel devices and systems at the nanoscale. The novelty of this research stems from the development of new modeling of the physics and mechanics of the tip interaction with nanoscale entities, coupled with the development of new smart cantilevers with multiple degrees of freedom. The gained knowledge from the conducted simulations and analysis is expected to enable true precision and repeatability of nanomanipulation tasks which is not feasible with existing methods and technologies.

The control and manipulation of infrared (IR) radiation beyond the capabilities of natural materials using silicon carbide (SiC), metamaterials, or a combination thereof, is presented. Control is first demonstrated using SiC, a polar crystal that exhibits a dielectric permittivity less than zero in the mid-IR range, through the excitation of tightly confined surface phonon-polaritons (SPPs), thus enabling a multitude of applications not possible with conventional dielectrics. Optimal, or critical coupling to SPPs is explored in SiC films through Otto-configuration attenuated total reflection. One practical application based on Otto-coupled SPPs is presented: IR refractive index sensing is shown for three pL-scale fluid analytes. It is then demonstrated that when two SiC films are brought to a few-micron separation, IR radiation can excite surface modes that possess phase velocities near the speed of light, a property required for efficient table-top particle accelerators. Metamaterials are engineered with subwavelength structure and possess optical properties not found in nature. Two such metamaterials will be introduced: metal films perforated with arrays of rectangular holes display the ability to control IR light polarization through spoof surface plasmon excitation, and metal/dielectric multilayers patterned with subwavelength-pitch corrugations display frequency-tunable, wide-angle, perfect IR absorption. Two experiments, which have implications in polarization control and thermal emission, combine the benefits of SiC with those of metamaterials: extraordinary optical transmission and absorption are observed in SiC hole arrays, and the design of individual SiC antennas permits the control of the bulk metamaterial responses of impedance and absorption/emission. Finally, a new optical beamline based on Fourier transform IR spectroscopy was designed, built, characterized, and implemented, serving as the major experimental objective of this dissertation. The novel beamline, which confines radiation to a 200-micron diameter and enables angle-dependent IR spectroscopy, was verified using multiple metamaterial structures. / text