Over the last two decades, two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDCs) have invoked tremendous interest of the scientific community due to their unique electronic and optical properties. While TMDCs hold great promise as a potential replacement for silicon for scaling transistors beyond sub-3 nm technology node, graphene holds great potential as transparent electrodes and optical phase-modulators for next-generation photonic devices. In addition to the aforementioned applications, these 2D devices also provide a great platform for studying novel physical phenomena associated with 2D materials such as Moiré interactions, valley-dependent spintronics, and correlated electron physics. In order to realize high-performance 2D material based devices, advancement of three key aspects are imperative - (1) analytical modeling to gauge insights into the electrostatics and current transport in 2D devices, (2) development of efficient techniques for fabricating 2D devices, and (3) understanding the fundamental limitations of the existing characterization techniques and developing better methods. We started by modeling the unique electrostatics of the 2D lateral p-n junctions, wherein we developed analytical expressions for the electric field, electrostatic potential, and depletion width across 2D lateral p-n junctions. We extend these expressions for use in lateral 3D metal-2D semiconductor junctions and lateral 2D heterojunctions. The results show a significantly larger depletion width (~ 2 to 20x) for 2D junctions compared to conventional 3D junctions. Further, we show that the depletion widths at metal-2D semiconductor junctions can be significantly modulated by the surrounding dielectric environment and semiconductor doping density. Finally, we derived a minimal dielectric thickness for a symmetrically-doped 2D lateral p-n junction, above which the out-of-plane simulation region boundaries minimally affect the simulation results. After electrostatics, we attempted to understand the current transport in 2D material-based devices. Typically used back-gated field-effect transistors (BGFETs) are often modeled as Schottky barrier (SB)-MOSFETs assuming that the current flow is limited by the source-contact in the OFF state, while the channel limits the current in the ON state. Here, using an analytical model and drift-diffusion simulations, we show that the channel limits the overall current in the OFF state and vice versa, contrary to past studies. For top-contacted BGFETs, we modeled different current paths at a top-contacted metal-2D semiconductor junction and illustrated the unique “corner effect”—where the potential change and current transport are dominated by the metal-2D semiconductor edge and the associated lateral region. We determined that the edge transport supersedes the vertical current injection in monolayer TMDCs and hence, to reduce contact resistance in 2D devices degenerate doping of channel region next to contact regions is of paramount importance. After developing models to theoretically analyze these devices, we focused on understanding the shortcomings in the existing characterization techniques affecting the extraction of important device parameters such as contact resistance, SBH, and channel mobility. We prove that the transfer length estimated using the standard TLM measurement techniquecan severely overestimate the true transfer length. We also discuss the large discrepancy in SBH values extracted using the Arrhenius method compared to their theoretical values. Using our analytical modeling, we attribute this to the presence of long channel regions in experimental devices. Furthermore, we highlight that the presence of large contact resistance results in underestimation of channel mobilities which renders Kelvin measurements such as four-probe and Hall-bar measurements imperative for 2D devices. Finally, we introduced a unique etch and doping method using self-limiting oxidation which allows us to design and fabricate various high-performance 2D devices. We first used the method to demonstrate a selective, damage-free atomic layer etch (ALE) that enables layer-by-layer removal of monolayer WSe₂ without altering the physical, optical, and electronic properties of the underlying layers. Using a comprehensive set of characterization techniques, we show that the quality of our ALE processed layers is comparable to that of pristine layers of similar thickness. Further, using graphene as a testbed, we demonstrate the use of a sacrificial monolayer WSe₂ layer to protect the channel, which is etched in the final process step in a technique we call Sacrificial WSe₂ with ALE Processing (SWAP). Furthermore, the top oxidized layer acts like an atomically thin degenerate p-type dopant for a large variety of semiconductors such as graphene, carbon nanotubes, and WSe2. We show that the TOS-doped graphene yields a low sheet resistance due to high mobility at a very high hole density that remains active even at 1.5 K. We apply this principle to improve the transmittance of graphene (>99%) at telecommunication bandwidth (1.5 to 1.6 𝜇m), that makes it a suitable replacement for Indium tin oxide (ITO) as a transparent electrode.

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The addition of fillers to a polymer matrix to endow soft materials with desirable properties has been a focused area of study over many decades and composite materials based on this idea are being increasingly incorporated into several end use products. Yet, almost always, the focus is on maximizing a particular property set for a unique polymer/filler combination for a specific application, which might not necessarily be translatable into another application. To exploit possible synergies, there is a need to develop materials that have the potential to perform multiple functions at the same time rather than a singular function. In this vein of thought, materials constructed using only polymer grafted nanoparticles (GNPs) have the potential to be one such class of materials as they have been shown to display a whole host of unique property sets – ranging from improved mechanical strength, enhancements in the gas and condensable penetrant transport properties, improvements in thermal conductivity, tunability of impact mitigation to more exotic behavior related to development of phononic bandgaps and quasi-crystalline materials. This thesis explores some of the structure-dynamics-property relations of some of the unique property sets described above and aims to provide insights into the nanoscale properties that lead to the improvements observed in macroscopic properties. In the first 2 chapters, we study the effect of tethering polymer chains to a spherical surface on the segmental and local vibrational dynamics of grafted polymer chains in an ensemble of GNPs. In the field of gas transport, the hopping motion of gas molecules inside a non-porous polymer matrix is facilitated by the motion of polymer segments, yet the understanding between the coupling of the two is very poor. By utilizing GNPs in which the diffusivity of gases is controlled by varying graft chain molecular weights, we can show that segmental dynamics of the polymer chains operating on a length scale of ~ 1 nm are positively correlated with the observed enhancements in diffusivities observed previously. We also propose that the inefficient packing of polymer chains leads to a decrease in the barriers of motion of the polymer segments, which is ultimately responsible for allowing penetrant molecules to move through the polymer phase much faster than a corresponding homopolymer melt. By utilizing a similar time and length scale approach, we can also explain the observed increases in thermal conductivities through the vibrational motion of polymer chains. This reaffirms the important role nanoscale polymer dynamics plays in both mass and thermal transport. In the next few chapters, we switch gears and focus on the microscopic structure and dynamics of the nanoparticles and how they impact the mechanical properties in suspensions. By studying the translational and vibrational motion of the GNPs, we find that the vibrational amplitude of a singular GNP decreases with increasing chain length all while the motion of the NP becomes faster, a phenomenon that we can associate with unjamming of the GNPs. This transition from jamming to unjamming is also visible in the local and long wavelength structure of the GNPs as well as the sound velocity through the material. Through these observations we can show that there is an intricate link between the structure and the relevant mechanical properties. Lastly, by building on the understanding laid out in the first few chapters, we propose that static features measurable through scattering are indicators of the enhanced transport properties of GNP based membranes. This also provides structural insights into the correlation between the structure of the polymer phase and the transport of penetrants. Each of the chapters touch upon a unique aspect of the structure and dynamics of different components of a GNP at different time and length scales, and how they are possibly linked to the several different property sets or dynamic features exhibited by the constructs, while also providing possible microscopic explanations for the same.

Dip-pen nanolithography (DPN) is a scanning probe microscopy-based nanofabrication method that relies on a fluid-coated atomic force microscope probe for the deposition of material on a substrate with nanometer-scale resolution. The ability to tailor the structure and chemical composition of materials at the nanometer length scale is enabling in elds ranging from medical diagnostics to nano-electronics. While DPN is among the highest resolution additive manufacturing techniques to date, the conguration of ink on the probe and the process of ink transport are poorly understood. Specically, the inking and patterning procedures are susceptible to variations in the ambient environmental conditions and currently not all aspects of the processes are reliably controlled. Thus, a key challenge barring the widespread adoption of DPN beyond a research tool is reproducibility. We hypothesize that closed-loop control over the inking and patterning process could address this irreproducibility, however techniques to monitor the quantity and concentration of ink on the tip of the probe have not been yet developed. Here, we study the mechanics of atomic force microscope (AFM) probes throughout the inking and patterning process to understand if the behavior of the ink can be studied in situ. In particular, we develop an approach for conning ink to the tip of an AFM probe, which is critical for reliable patterning and modeling the mechanics of the probe. Then, we nd that the quantity of ink on an AFM probe can be determined in situ by observing the shift in the natural frequency of the probe. Finally, we show that this method allows for the observation and quantication of the ink deposited on a substrate, in real time. Collectively, these approaches lay the groundwork for a closed-loop implementation of DPN in which the inking and patterning processes are performed with drastically improved reliability. Given that these techniques are easily implemented on any commercial AFM, we expect that they could lead to new applications in the study of nanoscale soft materials. / 2017-11-04T00:00:00Z

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Carbon nanomaterials, such as carbon nanotubes and graphene, have attracted significant interest over the past several years due to their outstanding and unusual combination of physical properties. These properties can be modified in a controllable way by chemical functionalization in order to enable specific technological applications. One example is hydrogenation, achieved by the exposure of these materials to a source of atomic hydrogen. This process has been considered for hydrogen storage purposes and for the control of the band gap of these materials for applications in carbon-based electronics. Hydrogen atoms are chemisorbed onto the surface of these materials, introducing sp3-hybridized C-C bonds in a structure originally formed by delocalized sp2 C-C bonding. This bonding transition causes severe structural and morphological changes to the graphene layers/walls. Also, it has been demonstrated that the exposure of multi-walled carbon nanotubes (MWCNTs) to a H2 plasma leads to the formation of diamond nanocrystals embedded within the nanotube walls. This thesis presents a computational analysis of the effects of hydrogen chemisorption on the structure and morphology of graphene and single-walled carbon nanotubes (SWCNTs), as well as of the different nanostructures that can be generated upon formation of inter-shell and inter-layer sp 3 C-C bonds in MWCNTs and few-layer graphene (FLG), respectively. The analysis is based on a synergistic combination of atomic-scale modeling tools, including first-principles density functional theory (DFT) calculations and classical molecular-dynamics (MD) and Monte Carlo (MC) simulations. The results demonstrate that SWCNTs and graphene swell upon hydrogenation and provide interpretations to experiments reported in the literature; this swelling depends strongly on the hydrogen surface coverage. A MC/MD-based compositional relaxation procedure generates configurations whose arrangements of H atoms are in excellent agreement with experimental observations. Detailed structural analysis of the hydrogenated surfaces is carried out, providing information which cannot be extracted easily from conventional experimental techniques. The findings of the analysis are used to explain the limitations on the maximum H storage capacity of SWCNT bundles upon their exposure to an atomic H flux. Furthermore, it is demonstrated that the structures resulting from formation of inter-shell or inter-layer C-C bonds are stable and provide seeds for the nucleation of crystalline carbon phases embedded into the shells and layers of the MWCNT and FLG structures, respectively. The key parameter that determines the type and size of the generated nanocrystals is the chiral-angle difference between adjacent layers/walls in the original structure. A novel type of carbon structure, consisting of fullerene-like caged configurations embedded within adjacent graphene layers, has been discovered for the case where the graphene layers are rotated with respect to each other; interestingly, one class of these structures retains the unique and desired electronic properties of single-layer graphene.

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