Two Dimensional Materials has been the focus of much research in the past decade. We review 145 stable two dimensional materials in both bulk and monolayer. We compare their final electronic properties and discuss the results. Specifically, we discuss notable materials that have transitions between bulk and monolayer. Additionally, we use both the bulk and monolayer data to search for structural trends that may be corralated with the electronic properties using machine learning techniques. We find that our machine was able to produce results that predict the basic electronic properties with approximately 65% accuracy. / A Thesis submitted to the Program in Materials Science and Engineering in partial fulfillment of the requirements for the degree of Master of Science. / Summer Semester 2018. / July 17, 2018. / Monolayer, Two Dimensional Materials / Includes bibliographical references. / Jose Mendoza-Cortes, Professor Directing Thesis; Stratos Manousakis, Committee Member; Chen Huang, Committee Member.

Diffraction stress analysis is a commonly used technique to evaluate the properties and performance of different classes of materials from engineering materials, such as steels and alloys, to electronic materials like Silicon chips. Often to better understand the performance of these materials at operating conditions they are also commonly subjected to elevated temperatures and different loading conditions. The validity of any measurement under these conditions is only as good as the control of the conditions and the accuracy and precision of the instrument being used to measure the properties. What is the accuracy and precision of a typical diffraction system and what is the best way to evaluate these quantities? Is there a way to remove systematic and random errors in the data that are due to problems with the control system used? With the advent of device engineering employing internal stress as a method for increasing performance the measurement of stress from microelectronic structures has become of enhanced importance. X-ray diffraction provides an ideal method for measuring these small areas without the need for modifying the sample and possibly changing the strain state. Micro and nano diffraction experiments on Silicon-on-Insulator samples revealed changes to the material under investigation and raised significant concerns about the usefulness of these techniques. This damage process and the application of micro and nano diffraction is discussed.

The three-dimensional integration of microelectronics is a standard that has been actively pursued by numerous researchers in a variety of technical ways over the years. The primary aim of three-dimensional integration is to alleviate the well-known issues associated with device shrinking in conjunction with Moore's Law. In this thesis, we utilize laser-based and other melt-mediated crystallization techniques to create Si thin films that may be of sufficient microstructural quality for use in monolithic thin-film-based three-dimensional integrated circuits (3D-ICs). Beam-induced solidification of initially amorphous or polycrystalline Si films has been actively investigated over the years as an unconventional, yet often-effective, technical means to generate Si films with suitable microstructures for fabricating high-performance electronic devices. Two specific melt-mediated methods that are aimed at crystallizing Si thin films for 3D-ICs are presented. One is referred to as "advanced sequential lateral solidification (SLS)" while the other is referred to as "advanced mixed-phase solidification (MPS)" and we show that these approaches can provide a more 3D-IC-optimal microstructure than can be generated using previous deposition and/or crystallization-based techniques. Advanced SLS, as presented in this thesis, is a novel implementation of the previously-developed directional-SLS method, and is specifically aimed at addressing the microstructural non-uniformity issue that can be encountered in the directional solidification processing of continuous Si films. Films crystallized via the directional-SLS method, for instance, can contain physically distinct regions with varying densities of planar defects and/or crystallographic orientations. As a result, transistors fabricated within such films can potentially exhibit relatively poor device uniformity. To address this issue, we employ advanced SLS whereby Si films are prepatterned into closely-spaced, long, narrow stripes that are then crystallized via directional-SLS in the long-axis-direction of the stripe length. By doing so, one can create microstructurally distinct regions within each stripe, which are then placed within the active channel region of a device. It is shown that when the stripes are sufficiently narrow (less than 2 µm), a bi-crystal microstructure is observed. This is explained based on the change in the interface morphology as a consequence of enhanced heat flow at the edges of the stripe. It is suggested that this bi-crystal formation is beneficial to the approach, as it increases the effective number of stripes within the active channel region. One issue of fundamental and technological significance that is nearly always encountered in laser crystallization is the formation of structural defects, in general, and in particular, twins. Due to the importance of reducing the density of these defects in order to increase the performance of transistors, this thesis investigates the formation mechanism of twins in rapidly laterally solidified Si thin films. These defects have been characterized and examined in the past, but a physically consistent explanation has not yet been provided. To address this situation, we have carried out experiments using a particular version of SLS, namely dot-SLS. This specific technique is chosen because we identify that it is endowed with a fortuitous combination of experimental factors that enable the systematic examination of twinning in laterally grown Si thin films. Based on extensive microstructural analysis of dot-SLS-crystallized regions, we propose that it is the energetics associated with forming a new atomic layer (during growth) in either a twinned or non-twinned configuration heterogeneously at the oxide/film interface that dictate the formation (or absence) of twins. The second method presented in this thesis is that of advanced MPS. The basic MPS approach was originally conceived as a way to generate Si films for solar cells as it is capable of producing large, intragrain-defect-free regions that are predominantly (100) surface-textured. However, the location of the grain boundaries of these equiaxed grains is essentially random, and hence, transistors placed within the interior of the grains would exhibit differing performance compared to those that are place across the grain boundaries. To address this, advanced MPS is introduced and demonstrated as a means to manipulate solidification by seeding from {100} surface-oriented regions and to induce limited directional growth. This is accomplished using a continuous-wave laser with a Gaussian-shaped beam profile wherein a central, completely molten region is surrounded by a ``mixed-phase-region'' undergoing MPS. The technique creates quasi-directional material that consists of large, elongated, parallel, {100} surface-oriented grains. This material is an improvement over previously generated directionally solidified materials, and can allow one to build devices without high angle grain boundaries that are within, and oriented perpendicular to, the active channel. The resulting microstructure is explained in terms of the non-uniform energy density distribution generated by the Gaussian-shaped laser beam, and the corresponding shape and growth of the solid/liquid interface. Based on the observations and considerations from these results, we propose and demonstrate a related scheme whereby a flash-lamp annealing system is utilized in order to induce the advanced MPS condition. This method can potentially time-efficiently crystallize, and create in the process, well-defined regions that are microstructurally suitable for the fabrication of 3D-ICs.

This thesis focused on investigating excimer-laser induced melting and solidification of thin metallic films on SiO2. Two distinct topics were pursued: (1) microstructural manipulation and optimization of Cu films via SLS of as-deposited Cu films on SiO2, and (2) investigation of oriented heterogeneous nucleation via complete melting and subsequent nucleation-initiated solidification of Ni films on SiO2. The work on SLS of Cu films is motivated in large part by the need to improve the properties of Cu films which, among other applications, constitute an essential element in the continued evolution of microelectronic products. The experiments we have conducted show clearly that the film can be, without much difficulty, melted and solidified using pulsed-laser irradiation. Based on the findings from a series of systematic single-shot experiments, we show that SLS can be properly implemented to obtain large-grained Cu films with controlled microstructures and restricted textures. The lateral growth distance was found to increase as a function of increasing incident energy density. This observation is consistent with the findings that were made previously using other materials, and basically indicates that lateral solidification continues until the interface is halted by the interfaces growing from nucleated solids, which are triggered within the liquid matrix ahead of the growing interface. Close examination of the laterally grown grains, which quickly develop 100 rolling direction crystallographic orientation texture due to occlusion of differently oriented grains, reveal, furthermore, that low-angle grain boundaries as well as twins can be generated during the growth. These intra-grain defects are found to appear in a systematic manner (as they are located at specific regions and observed under specific incident energy densities). Significantly longer lateral growth distances observed in Cu films (compared to that of Si films) was attributed to the fact that substantially higher growth rates are expected with simple metallic films at a given interfacial undercooling. The implementation of SLS using Cu films was accomplished quite effectively, as can be expected from the above lateral-growth-related results involving single-shot experiments. We were able to achieve a variety of large-grained, grain-boundary location and grain-orientation controlled Cu films via various SLS techniques. When performed optimally in accordance with the findings made in chapter 2, the resulting microstructure exhibits large grains, which are primarily devoid of intra-grain defects. For example, 2-shot SLS processed Cu films led to strong 100 rolling direction orientation, while avoiding the formation of low-angle grain boundaries and twin-boundaries. The highlight of SLS on Cu films correspond to a version of SLS (referred to as "2-Shot plus 2-Shot" SLS) in which the second 2-shot SLS is performed in the direction perpendicular to the first one. Through this approach, we were able to achieve grain-boundary-location controlled microstructure with a strong 100 orientation texture in all three dimensions (forming, effectively, an ultra-large quasi-single crystal material). Nucleation of solids in laser-quenched Ni films was investigated using EBSD analysis. The surface orientation analysis of nucleated grains obtained within the complete melting regime reveal a clear sign of texture. From these and additional findings from previous work involving Al films, we were able to conclude that systematic heterogeneous nucleation has taken place, and, furthermore, that oriented nucleation of the solids must have taken place. Although always suspected to be the case, it is typically extremely challenging to prove with certainty, in conventional nucleation experiments, that the mechanism of nucleation corresponds to that of a heterogeneous one. Furthermore, although it has been suspected theoretically for over 50 years, experimental results that clearly show that oriented nucleation actually transpires have not been obtained until our work involving Al films; the present findings involving Ni films further strengthen this conclusion as the Ni system removes some of the experimental uncertainties that are associated with Al films, and, furthermore, suggests that the process of oriented nucleation is a general and rather pervasive phenomenon. Additionally, it was observed that the selected orientation changed as a function of incident energy density; in the low energy density regime (above the completed melting threshold) {110}-surface texture was observed, while {111}-surface texture became more dominent at higher densities. Motivated by our experimental work involving Al and Ni that clearly indicates that oriented heterogeneous nucleation is a major path through which heterogeneous nucleation of solids occurs, we have also carried out a 2-dimensional Winterbottom-type thermodynamic analysis that can be used to obtain a better understanding of the phenomenon. In contrast to the previous work on the subject, we consider in our modelling the anisotropic nature of both the solid-liquid and solid-substrate interfacial energy; we advocate that this is the only physically consistent combination. The results show that oriented nucleation can be systematically accounted for as stemming from the expected anisotropic nature of the involved interfacial energies. Furthermore, the analysis also suggests possible reasons for observing a transition in surface texture from one orientation to another.

In this thesis, we present a new beam-induced melt-mediated crystallization process called mixed-phase solidification (MPS) that can produce defect-free, large-grain polycrystalline-Si films with strong (100)-surface texture (>99%) on SiO2. Such a combination of microstructural attributes makes the resulting MPS material well-suited for high-performance electronic and photovoltaic applications. The MPS method was conceived based on a well-known phenomenon of coexisting solid-liquid regions in radiatively-melted Si films on SiO2. Through our investigations, we have discovered that multiple exposures (of an initially amorphous precursor) in air within the solid-liquid coexistence regime can lead to the generation of such a material. In the course of this thesis, we have also identified the optimal processing conditions for obtaining such a microstructure, as well as the physical factors that control the process. A systematic parametric study of the single- and multi-scan MPS process is performed using thin Si films on SiO2 irradiated via a continuous-wave (CW) laser system. We employ an in situ microscopic viewing system to directly observe and understand melting and solidification during the MPS process. Additionally, in order to investigate the grain boundary melting phenomenon, we have conducted "rapid-quench" demarcation experiments and established a one-to-one correspondence between the in situ data and the single-/multi-scan MPS processed microstructure. The experimental results show an incremental increase in grain size and (100)-surface texture with an increase in scan number. The grain size is found to reach an apparent soft saturation value as the number of scans increases. For a given number of scans, a decrease in power or an increase in velocity is found to decrease the grain size and (100)-surface texture. Increases in film thickness lead to an increase in grain size, but a reduction in (100)-surface texturing. Based on what we have experimentally observed, as well as what has been previously established regarding the radiative-melting of Si, we propose a thermodynamic model to account for the microstructural evolution observed in the MPS process (i.e., partial-melting and solidification of polycrystalline-Si films). The model is built on two fundamental considerations: (1) the near-equilibrium environment within which thermodynamic factors dominate the transitions, and (2) the dynamically balanced, yet continuously changing, thermal surroundings. According to our model, the physical melting-solidification sequence for an MPS cycle of polycrystalline-Si films can be described in terms of the melt being initiated first at grain boundaries, and melting and solidification subsequently proceeding primarily laterally at interface-location specific rates as determined by the local curvature and local temperature at a point in the solid/liquid interface. By analyzing the cross-sectional profile of solid/liquid interface, we correlate the local curvature of the interface to the solid-Si/SiO2 interfacial energy and the resulting local equilibrium melting temperature using the Gibbs-Thomson relationship. This local-interface-curvature analysis reveals how the anisotropic nature of the Si/SiO2 interfacial energy and the film thickness affect the surface texture evolution observed during the MPS processing of Si films on SiO2. Our model of MPS is noteworthy in that it is in contrast to the "variable-grain-melting-temperature" argument which has been previously invoked in order to explain similar observations of texture selection; we suggest that such an argument is not thermodynamically consistent, and furthermore, cannot account for the evolution of the microstructure observed in the multi-scan MPS of polycrystalline films. Based on (1) our understanding of the MPS process as interpreted by the above model, and (2) the experimental results, we have also deduced fundamental thermodynamic details of the system. Specifically, by scrutinizing the evolution of the distribution of solid surface orientations in the MPS processed samples, we have extracted the hierarchical order of the Si/SiO2 interfacial energies as a function of grain orientation. We propose, in accordance with our model, that the experimentally observed soft saturation grain size values can be determined from the considerations related to the stable coexistence of solid-liquid mixtures. We substantiate this argument by performing a Mullins- Sekerka-type interface-instability analysis. Specifically, the maximum allowed solid-liquid coexistence distances, which were calculated with an explicit consideration given to the accompanying emissivity changes, were found to correlate well to the experimentally measured grain size value obtained after multi-scan MPS processing. Also, we discuss certain factors encountered during the MPS process (such as slow solidification rates, stable interfaces, and the highly unusual inverted thermal profile) as being responsible for the formation of defect-free grains. Finally, recognizing that the MPS process requires neither the laser nor the scanning of a highly localized beam, we also demonstrate how the MPS process can potentially be implemented in a practical manner using an incoherent light source. Using a xenon-arc flash-lamp system we show how the MPS method can be liberated from the challenges and constraints that are associated with laser-based systems, and thus represents a potentially cost-effective and scalable option.

Measured powder diffraction patterns contain contributions from the sample and the instrument. Most available data analysis software operates on the measured data to extract sample parameters, however, few programs can take sample parameters and rigorously simulate the expected diffraction profile for a given instrument. In this work Monte Carlo methods, within the framework of McStas software, are used for the simulation of neutron diffraction at the SMARTS (Spectrometer for Materials Research at Temperature and Stress) diffractometer in the Los Alamos Neutron Science Center. The simulations include all the instrumental components, such as the moderator, guide system, collimator, detector banks and sample. The results of the simulations are in excellent agreement with the experimental data for different ideal powder samples. The simulations also yield information on the line broadening introduced into the diffraction profile as a function of energy and are used to predict the size and strain limit above which line broadening studies cannot be performed on this instrument. Theoretical derivations of line profile analysis are presented to provide an accurate explanation of the formation of diffraction peaks from the powder sample. This thesis demonstrates how rigorous scattering theory can be used to design optimal diffraction instruments.

This thesis investigates the synthesis and properties of nanocomposite materials comprised of inorganic nanocrystals (NCs) combined with a complementary organic compound utilizing sequential two-step synthesis methods. We demonstrate an enhancement in the mechanical and optical properties of electrophoreticially deposited (EPD) cadmium selendide (CdSe) nanocrystal (NC) films through post-deposition addition of organic ligand molecules and polymeric precursor molecules (monomers). Specifically we show that when these organic compounds are added (i.e. back-filled) into the as-deposited, wet EPD NC film, that fracture in the dried film is suppressed and photoluminscent (PL) efficiency of the inorganic NC phase is greatly increased. We go on to study the synthesis and properties of a novel nanocomposite comprised of inorganic NCs back-filled into a mat of semiconducting poly(3-hexylthiophene) [P3HT] nanowires. P3HT nanowire films are synthesized using a novel method developed as part of this thesis; where P3HT is blended with a sacrificial polymer (polystyrene, PS), leading to spontaneous demixing of the two polymers upon casting, and upon selective removal of the PS phase exposes a dense mat of P3HT nanowires. When back filled with CdSe NCs the composite material exhibits photovoltaic (PV) performance and provides a flexible platform for low-cost, hybrid organic/inorganic NC PV device fabrication. We conclude by showing how the above methods, in conjunction with novel ligand chemistry and lithographic techniques, can be utilized to create a photo-active nanocomposite consisting of lithographically defined, micron-scale, electrodes that are selectively decorated with electron-accepting NCs using EPD, and subsequently back-filled with a complementary electron-donating NC phase. The device architecture and resulting nanocomposite material is capable of lateral exciton separation on a potentially low-cost substrate.

Metallic ferromagnetic (FM) thin film heterostructures play an important role in emerging magnetoelectronic devices, which introduce the spin degree of freedom of electrons into conventional charge-based electronic devices. As the majority of magnetoelectronic devices operate in the GHz frequency range, it is critical to understand the high-frequency magnetization dynamics in these structures. In this thesis, we start with the static magnetic properties of FM thin films and their optimization via the field-sputtering process incorporating a specially designed in-situ electromagnet. We focus on the origins of anisotropy and hysteresis/coercivity in soft magnetic thin films, which are most relevant to magentic susceptibility and power dissipation in applications in the sub-GHz frequency regime, such as magnetic-core integrated inductors. Next we explore GHz magnetization dynamics in thin-film heterostructures, both in semi-infinite samples and confined geometries. All investigations are rooted in the Landau-Lifshitz-Gilbert (LLG) equation, the equation of motion for magnetization. The phenomenological Gilbert damping parameter in the LLG equation has been interpreted, since the 1970's, in terms of the electrical resistivity. We present the first interpretation of the size effect in Gilbert damping in single metallic FM films based on this electron theory of damping. The LLG equation is intrinsically nonlinear, which provides possibilities for rf signal processing. We analyze the frequency doubling effect at small-angle magnetization precession from the first-order expansion of the LLG equation, and demonstrate second harmonic generation from Ni81 Fe19 (Permalloy) thin film under ferromagnetic resonance (FMR), three orders of magnitude more efficient than in ferrites traditionally used in rf devices. Though the efficiency is less than in semiconductor devices, we provide field- and frequency-selectivity in the second harmonic generation. To address further the relationship between the rf excitation and the magnetization dynamics in systems with higher complexity, such as multilayered thin films consisting of nonmagnetic (NM) and FM layers, we employ the powerful time-resolved x-ray magnetic circular dichroism (TR-XMCD) spectroscopy. Soft x-rays have element-specific absorption, leading to layer-specific magnetization detection provided the FM layers have distinctive compositions. We discovered that in contrast to what has been routinely assumed, for layer thicknesses well below the skin depth of the EM wave, a significant phase difference exists between the rf magnetic fields Hrf in different FM layers separated by a Cu spacer layer. We propose an analysis based on the distribution of the EM waves in the film stack and substrate to interpret this striking observation. For confined geometries with lateral dimensions in the sub-micron regime, there has been a critical absence of experimental techniques which can image small-amplitude dynamics of these structures. We extend the TR-XMCD technique to scanning transmission x-ray microscopy (STXM), to observe directly the local magnetization dynamics in nanoscale FM thin-film elements, demonstrated at picosecond temporal, 40 nm spatial and less than 6° angular resolution. The experimental data are compared with our micromagnetic simulations based on the finite element analysis of the time-dependent LLG equation. We resolve standing spin wave modes in nanoscale Ni81 Fe19 thin film ellipses (1000 nm × 500 nm × 20 nm) with clear phase information to distinguish between degenerate eigenmodes with different symmetries for the first time. With the element-specific imaging capability of soft x-rays, spatial resolution up to 15 nm with improved optics, we see great potential for this technique to investigate functional devices with multiple FM layers, and provide insight into the studies of spin injection, manipulation and detection.

The lack of an energetic model for the seemingly spontaneous room temperature recrystallization of electroplated copper thin films has proven to be a technological bottleneck in the optimization of copper interconnect microstructure for the microelectronics industry. The inability to either achieve large grained interconnect microstructures by simple annealing or explain them by a posteriori analyses necessitates a new approach. Synchrotron x-ray diffraction was utilized to obtain real-time grain size, crystallographic texture, and strain data about the recrystallization in the geometrically simpler case of blanket electroplated Cu films. The observation of a bimodal size distribution between as- deposited and recrystallizing grains during led to the development of a theoretical framework for combining x-ray data and the canonical Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetics model. Under this framework, analysis of variations in plated Cu film and vapor deposited underlayer structures established that film recrystallization speed is a function of initial 111 film texture, and that this dependency is modulated by underlayer deposition conditions and plated film thickness. Verification of the new x-ray analysis was performed by combined use of complementary destructive and non-destructive characterization techniques which are more commonly accessible in the industrial setting. These included cross-sectional focused ion beam milling and scanning electron microscopy (x-FIB/SEM), electron back scatter diffraction (EBSD), and four-point probe electrical resistivity measurements. Comparative real-time in situ x-ray and resistivity studies revealed the formation of electron percolation paths which prematurely short-circuited the latter analysis. An effective resistivity model is proposedto extend the current canonical one-dimensional analysis to be compatible with multi-dimensional recrystallization. X-ray analysis of plated films whose initial stress state had been modified by delamination or the photoresist masking of substrate stresses revealed a significant change to the recrystallization kinetics. Complementary real-time EBSD analysis localized the initiation of recrystallization to the free surface of the film. The combination of this with quantitative activation energy measurements was then the basis for a comprehensive theoretical energetics model.

The new complex materials have wide applications in next generation technologies in industrial fields such as electronics, energy production, environment engineering, etc. Understanding their structure is the key in keeping developing new materials and improving their performance. As they are more and more complex in different length scale, new methods that utilize information from different sources and be able to provide complex structural information are on the horizon of this new era. In this thesis, we developed new methods that process the mixed data and provide the extra information that people are interested in. First one is extending the computed tomography technique with other analysis method including texture analysis and Pair Distribution Function (PDF) method. The new methods enable us to study the coupling of desired structural properties, such as texture and local local structure of nano-particles, at meso-scale. For example, by applying the texture-CT analysis on the LiCoO₂ coin cell, we found the texture of LiCoO₂ particles was quite inhomogeneous. By combining PDF and CT method, we successfully studied the catalyst reaction and the participle size distribution in industrial catalyst. Second one is a new method of obtaining reliable anomalous differential Pair Distribution Function (adPDF) by using diffraction data sets in wide energy range and an ad-hoc algorithm that perform the data correction automatically. The new method was demonstrated using both simulated data and real experimental data.