As one of the most promising two-dimensional materials, graphene’s outstanding electrical, mechanical and optical properties have made many new devices possible. Its ultra-thin thickness makes both the fabrication and characterization of graphene-based device challenging. In my thesis, I will discuss different approaches to fabricate and characterize graphene devices for the use in transport and THz radiation devices, as well as strain engineering. These approaches could be potentially used to produce next generation electrical devices, photonics devices and generate ultra-high pseudomagnetic fields. To analyze graphene’s quality, I use multi-variable Raman spectroscopy for identifying graphene’s defect density, strain and doping. A case study of strain redistribution on a silicon dioxide grating with sub-diffraction limit resolution of the strain variation is presented, where atomic force microscopy and Raman spectroscopy are applied for characterization. The strain redistribution is used to determine the strain dependent friction between graphene and the substrate. This work has also lead to more precise determination of the strain and shear response using the 2D phonon band. Improvements of the electrical property of graphene is achieved by using graphene encapsulated between atomically flat hBN layers as well as tuning surface hydrophobicity via substrate salinization. This also provides an improved method to optically determine charge density in graphene with order of magnitude enhancement in sensitivity.

Investigates the mechanical and electronic properties of two-dimensional nanomaterials under strain, addressing gaps in the existing literature. The primary challenge with these materials is the inconsistent application of high strain rates and the absence of experimental data at low temperatures. To overcome these challenges, we develop Microelectromechanical Systems (MEMS)-based devices for characterizing 2D nanomaterials and semiconductor materials at low temperatures. Four MEMS-based devices are developed to facilitate this characterization. The first device is a unique MEMS testing platform with on-chip actuation, sensing, and feedback control systems, capable of applying controlled displacements to nanoscale specimens while minimizing temperature fluctuations. To achieve this, MEMS thermal actuators with an axial stiffness of 40253.6 N/m are used. Capacitive sensors and V-beam amplification mechanisms are designed for precise measurement. The second device, the cascaded MEMS device, employs horizontal and vertical V-shaped structures to measure stress-strain curves of 2D nanomaterials at low temperatures. The third device is a customized MEMS electrostatic actuator for bending tests on silicon material under low-temperature conditions. Finally, two MEMS rotational structures, including a novel C-shaped structure, are developed to amplify movement. The MEMS devices are fabricated using bulk micromachining and deep reactive-ion etching (DRIE) with silicon-on-insulator (SOI) wafers, incorporating underpass technology for electrical isolation within the MEMS-based testing platforms. To optimize DRIE etching parameters for creating underpass islands in SOI MEMS, a study was conducted considering a total of nine wafers, divided into two batches for fabrication process, and examining their behavior concerning the etching process. The devices are optically characterized at room temperature and tested in a vacuum environment and at low temperatures using scanning tunneling microscope (STM) tool.

<div>Multifunctional characteristics of Bismuth-based oxides offer great opportunities to design a variety of devices exploiting either a single functionality or the synergistic multifunctionalities. In the past decades, strain engineering of thin films arose as a solution for fabrication of novel structures with highly desired properties. In this thesis, strain engineering has been applied to Bismuth-based oxides to explore the strain effect on thin film structures and functionalities.</div><div>BiFeO₃ (BFO) servers as the first study platform, because of its strain-induced phase transition and the corresponding diverse polarization properties. The strain effect of SrRuO₃ (SRO)-buffered substrates on ferroelectric and optical properties of BFO thin films has been investigated. A wide range of strain states have been achieved in BFO films. The ferroelectricity and bandgap have been effectively tuned even with partial strain relaxation. However, pure BFO suffers from high leakage current and large coercive field. To overcome these limitations, Sm-doped BFO (BSFO) systems emerged and has been used in controlling the microstructure and properties of BFO. Our detailed structure analysis proves the Sm doping amount in BSFO thin films can be tuned effectively via deposition temperature. Consequently, the Sm dopant influences phase formation of BSFO and the macroscopic ferroelectric properties. </div><div>Another member in Bismuth-based oxide family, Bi₂WO₆ (BWO), has been selected as the base material for the design of the two-phase nanocomposites, because of its unique layered structure and ferroelectric property. To introduce ferromagnetic component into BWO, two methods have been explored. The first method incorporates Mn cations into the BWO matrix (BWMO), and the second method couples CoFe2O4 (CFO) as secondary phase with BWO to form a vertically aligned nanocomposite (VAN) system. Both systems exhibit robust ferromagnetic and ferroelectric response at room temperature and demonstrate their promise as room temperature multiferroics for future spintronics and memory applications. </div><div>The studies in this dissertation demonstrate the great structure flexibility and tunable functionalities of BFO and BWO systems. It shows the potential structure modification and property control of other Bi-based oxides. In the last chapter, new experimental plans and directions are proposed. The connections between the strain engineering and the tunable material properties are being built for various applications. </div><div><br></div>

No description available.

2D materials like graphene and MoS_2 are atomically thin, extremely strong and flexible, making them attractive for integration into strain engineered devices. Strain on these materials can change physical properties, as well as induce exotic physics, not typically seen in solid-state systems. Here, we probe the novel physics arising from distorted lattices of 2D materials, strained by nanopillars indentation and microelectromechanical systems (MEMS), using Raman and photoluminescence (PL) spectroscopy. From nanopillars strained multilayer MoS_2, we observe exciton and charge carrier funneling due to strain, inducing dissociation of excitons in to free electron-hole pairs in the indirect material. Using MEMS devices, we were able to dynamically strain monolayer and multilayer graphene. Multilayer graphene under MEMS strain showed signatures of loss in Bernal stacking due to shear of the individual layers, indicating that MEMS can be used to tune the layer commensuration with tensile strain. We further explore simulation of pseudo-magnetic fields (PMFs) generated in monolayer graphene strained by MEMS, using machine learning, to accelerate and optimize the strength and uniformity of the PMF in new graphene geometries. Nanopillars provide non-uniform, centrally biaxial strain to multilayer MoS_2 transferred on top. Raman E^1_2g and PL redshift across the pillar confirms 1-2% strain in the material. We also observe a softening in the A_1g Raman mode and an enhancement in the overall PL with an increase in radiative trions, under strain. The changes in these charge-dependent features indicates funneling of charge carriers and neutral excitons to the apex of the pillar, as strain locally deforms the band structure of the conduction and valence bands. DFT calculations of the band structure in bilayer MoS_2 under biaxial strain shows the conduction band is lowered, further increasing the indirectness of multilayer MoS_2. This should cause the PL intensity to decrease, whereas we observe an increase in MoS_2 PL intensity under strain. We theorize that this is due to a dissociation of excitons into free electron-hole pairs. The increase in charge carrier densities due to strain leads to a renormalization of the local band structure and increased dielectric screening, supporting free electron-hole recombination at the K-point without momentum restrictions. In turn, electron-hole recombination occurs around the K-point inducing a high intensity PL, which opens attractive opportunities for utilization in optoelectronic devices. MEMS chevron actuators can dynamically strain 2D materials, which we demonstrate through uniaxial strain in CVD and exfoliated graphene. We use a novel microstructure assisted transfer technique which can deterministically place materials on non-planar surfaces like MEMS devices. Building on previously reported 1.3% in monolayer MoS2 from our group, we report tunable 0.3% strain in CVD monolayer graphene and 1.2% strain in multilayer exfoliated graphene using MEMS chevron actuators, detected by Raman spectroscopy. The asymmetric-to-symmetric strain evolution of the 2D phonon line shape in multilayer graphene is evidence of changes in interlayer interactions, caused by shearing between layers. This demonstrates that MEMS can be used to tune the commensuration in few layer 2D materials, which is a promising avenue towards Moiré engineering. Using machine learning, we also simulate optimal monolayer graphene geometries for generating strong, uniform pseudo-magnetic fields by MEMS strain. The coupled use of finite-element methods, variational auto-encoder, and auxiliary neural network accelerates the search for PMFs in strained graphene, while optimizing the graphene shape for fabrication through electron-beam lithography. Our experimental and simulated work creates a road-map for rapid advancement in zero-field quantum Hall effect devices using graphene-integrated MEMS actuators.

Graphene, a single atomic layer of graphite, has many exciting electronic and mechanical properties. On a fundamental level, the quasi-relativistic behaviour of the charge carriers in graphene arises from the honeycomb-like atomic structure. Deforming the lattice changes the lengths of the carbon-carbon bonds, breaking the hopping symmetry between carbon sites. Mathematically, elastic strain in a graphene membrane can be described by additional terms in the low-energy effective Hamiltonian, analogous to the vector potential of an external magnetic field. Hence, certain non-uniform strain geometries produce so-called `pseudo-magnetic fields', leading to a predicted zero-field quantum Hall effect. These fictitious magnetic fields are distinct from an external magnetic field in that they are only observed by charge carriers within the membrane, and have opposing polarity for electrons in the K and K' valleys, preserving time-reversal symmetry of the lattice as a whole. Deforming graphene in the non-uniform manner required to produce a homogeneous pseudo-magnetic field has proven to be a huge technological challenge, however, restricting experimental evidence to scanning tunnelling spectroscopy measurements on, for example, highly deformed nanobubbles formed by the thermal expansion of an epitaxially grown sheet on a platinum substrate. These results stimulated a large amount of interest in strain-engineering electron transport in graphene, partly due to the extreme magnitude of the observed pseudo-magnetic field, a direct consequence of the strain components strongly varying over the space of a few nanometres, but the formation of nanobubbles is a highly stochastic process which cannot be reliably reproduced. Subsequent research found a way to fabricate nanobubbles with a high degree of consistency, but the measurements were still limited to local-probe techniques due to the nanoscale size of the devices. As such, a method to reliably induce a homogeneous pseudo-magnetic field within a micron-sized membrane would be an attractive proposition, and is the basis for the work presented within this thesis. The non-uniform strain required precludes a simple bending or elongation of the substrate, hence a more local method is required. A novel nanostructure consisting of suspended gold beams surrounding a graphene membrane will deform upon cooling to cryogenic temperatures, and crucially, the actuation mechanism can be designed to produce any configuration of strain, including uniaxial strain, triaxial strain and a fan-shaped deformation, the latter two of which are predicted to create homogeneous pseudo-magnetic fields within a membrane. Strain patterns which are predicted to produce experimentally significant pseudo-magnetic fields (~1 T) may be generated with complex actuation beams that are physically achievable. Furthermore, the actuation mechanisms may be utilised as electrical contacts to the membrane, allowing its conductivity to be measured in the context of a two- or multi-terminal measurement, in conjunction with an external magnetic field. The design of the devices was developed using finite-element analysis, and the behaviour verified by low-temperature imaging of prototypes. While, after careful annealing, some conventional two-terminal suspended devices exhibited quantum Hall features at very low fields, the fabricated strain-inducing devices did not display pseudo-Landau quantisation, nor Landau quantisation, due to the difficulties of using current annealing to clean devices post-fabrication. The presented work, however, could pave the way towards observing signatures of pseudo-magnetic fields in a range of experimental measurements, as well as creating alternative strain geometries.

The strain introduced by lattice mismatch is a built-in characteristic in Si/SiGe heterostructures, which has significant influences on various material properties. Proper design and precise control of strain within Si/SiGe heterostructures, i.e. the so-called “strain engineering”, have become a very important way not only for substantial performance enhancement of conventional microelectronic devices, but also to allow novel device concepts to be integrated with Si chips for new functions, e.g. Si-based optoelectronics. This thesis thus describes studies on two subjects of such strain-engineered Si/SiGe heterostructures grown by molecular beam epitaxy (MBE). The first one focuses on the growth and characterizations of delicately strain-symmetrized Si/SiGe multi-quantum-well/superlattice structures on fully relaxed SiGe virtual substrates for light emission in the THz frequency range. The second one investigates the strain relaxation mechanism of thin SiGe layers during MBE growth and post-growth processes in non-conventional conditions. Two types of THz emitters, based on different quantum cascade (QC) intersubband transition schemes, were studied. The QC emitters using the diagonal transition between two adjacent wells were grown with Si/Si0.7Ge0.3 superlattices up to 100 periods. It was shown that nearly perfect strain symmetry in the superlattice with a high material quality was obtained. The layer parameters were precisely controlled with deviations of ≤ 2 Å in layer thickness and ≤ 1.5 at. % in Ge composition from the designed values. The fabricated emitter devices exhibited a dominating emission peak at ~13 meV (~3 THz), which was consistent with the design. An attempt to produce the first QC THz emitter based on the bound-to-continuum transition was made. The structures with a complicated design of 20 periods of active units were extremely challenging for the growth. Each unit contained 16 Si/Si0.724Ge0.276 superlattice layers, in which the thinnest one was only 8 Å. The growth parameters were carefully studied, and several samples with different boron δ-doping concentrations were grown at optimized conditions. Extensive material characterizations revealed a high crystalline quality of the grown structures with an excellent growth control, while the heavy δ-doping may introduce layer undulations as a result of the non-uniformity in the strain field. Moreover, carrier lifetime dynamics, which is crucial for the THz QC structure design, was also investigated. Strain-symmetrized Si/SiGe multi-quantum-well structures, designed for probing the carrier lifetime of intersubband transitions inside a well between heavy hole 1 (HH1) and light hole 1 (LH1) states with transition energies below the optical phonon energy, were grown on SiGe virtual substrates. The lifetime of the LH1 excited state was determined directly with pump-probe spectroscopy. The measurements indicated an increase of lifetime by a factor of ~2 due to the increasingly unconfined LH1 state, which agreed very well with the theory. It also showed a very long lifetime of several hundred picoseconds for the holes excited out of the well to transit back to the well through a diagonal process. Strained SiGe grown on Si (110) substrates has promising potentials for high-speed microelectronics devices due to the enhanced carrier mobility. Strain relaxation of SiGe/Si(110) subjected to different annealing treatments was studied by X-ray reciprocal space mapping. The in-plane lattice mismatch was found to be asymmetric with the major strain relaxation observed in the lateral [001] direction. It was concluded that this was associated to the formation and propagation of conventional a/2&lt;110&gt; dislocations oriented along [110]. This was different from the relaxation observed during growth, which was mainly along in-plane [110]. A novel MBE growth process to fabricate thin strain-relaxed Si0.6Ge0.4 virtual substrates involving low-temperature (LT) buffer layers was investigated. At a certain LT-buffer growth temperature, a dramatic increase in the strain relaxation accompanied with a decrease of surface roughness was observed in the top SiGe, together with a cross-hatch/cross-hatch-free transition in the surface morphology. It was explained by the association with a certain onset stage of the ordered/disordered transition during the growth of the LT-SiGe buffer. / Kisel(Si)-baserad mikroelektronik har utvecklats under en femtioårsperiod till att bli basen för vår nuvarande informationsteknologi. Förutom att integrera fler och mindre komponenter på varje kisel-chip så utvecklas metoder att modifiera och förbättra materialegenskaperna för att förbättra prestanda ytterligare. Ett sätt att göra detta är att kombinera kisel med germanium (Ge) bl.a. för att skapa kvantstrukturer av nanometer-storlek. Eftersom Ge-atomerna är större än Si-atomerna kan man skapa en töjning i materialet vilket kan förbättra egenskaperna, ex.vis hur snabbt laddningarna (elektronerna) rör sig i materialet. Genom att variera Gekoncentrationen i tunna skikt kan man skapa skikt som är antingen komprimerade eller expanderade och därmed ger möjlighet att göra strukturer för tillverkning av nya typer av komponenter för mikroelektronik eller optoelektronik. I detta avhandlingsarbete har Si/SiGe nanostrukturer tillverkats med molekylstråle-epitaxi-teknik (molecular beam epitaxy, MBE). Med denna teknik byggs materialet upp på ett substrat, atomlager för atomlager, med mycket god kontroll på sammansättningen av varje skikt. Samtidigt kan töjningen av materialet designas så att inga defekter skapas alternativt många defekter genereras på ett kontrollerat sätt. I denna avhandling beskrivs detaljerade studier av hur töjda i/SiGe-strukturer kan tillverkas och ge nya potentiella tillämpningar ex.vis som källa för infraröd strålning. Studierna av de olika töjda skikten har framför allt gjorts med avancerade röntgendiffraktionsmätningar och transmissionselektronmikroskopi.

Strain engineering is used as a means of manufacturing micro- and nano- scale particles with the ability to reversibly alter their geometry from three dimensional tubes to two dimensional flat layers. These particles are formed from a bi-layer of two dissimilar materials, one of which is the elastomeric material polydimethylsiloxane (PDMS), deposited under stress on a sacrificial substrate. Upon the release of the bi-layer structure from the substrate, interfacial residual stress is released resulting in the formation of tubes or coils. These particles possess the ability to dramatically alter their geometry and, consequently, change some properties that are reversible and can be triggered by a stimulus. This work focuses on the material selection and manufacturing of the bi-layer structures used to create the responsive particles and methods for characterizing and controlling the responsive nature of the particles. Furthermore, the potential of using these particles for a capture/release application is explored, and a systematic approach to scale up the manufacturing process for such particles is provided. This includes addressing many of the problems associated with fabricating ultra-thin layers, tuning the size of the particles, understanding how the stress accumulated at the interface of a bi-layer structure can be used as a tool for triggering a response as well as developing methods (i.e. experiments and applications) that allow the demonstration of this response.

Microbial-based industrial production has experienced a revolutionary development in the last decades as chemical industry has shifted its focus towards more sustain- able production of fuels, building blocks for materials, polymers, chemicals, etc. The strain engineering and optimization programs for industrially relevant phenotypes tackle three challenges for increased production: optimization of titer, productivity, and yield. The yield of production is function of the robustness of the microbe, generally associated with complex phenotypes. The poor understanding of complex phenotypes associated with increased production poses a challenge for the rational design of strains of more robust microbial producers. Laboratory adaptive evolution is a strain engineering technique used to provide fundamental biological insight through observation of the evolutionary process, in order to uncover molecular determinants associated with the desired phenotype. In this dissertation, the development of different methodologies to study complex phenotypes in microbial systems using laboratory adaptive evolution is described. Several limitations imposed for the nature of the technique were discussed and tackled. Three different cases were studied. Initially, the n-butanol tolerance in Escherichia coli was studied in order to illustrate the effect of clonal interference in microbial systems propagated under selective pressure of an individual stressor. The methodology called Visualizing Evolution in Real Time (VERT) was developed, to aid in mapping out the adaptive landscape of n-butanol tolerance, allowing the uncovering of divergent mechanisms of tolerance. A second case involves the study of clonal interference of microbial systems propagated under several stressors. Using VERT, Saccharomyces cerevisiae was evolved in presence of hydrolysates of lignocellulosic biomass. Isolated mutants showed differential fitness advantage to individual inhibitors present in the hydrolysates; however, some mutants exhibited increased tolerance to hydrolysates, but not to individ- ual stressors. Finally, dealing with the problem of using adaptive evolution to increase production of secondary metabolites, an evolutionary strategy was successfully designed and applied in S. cerevisiae, to increase the production of carotenoids in a short-term experiment. Molecular mechanisms for increased carotenoids production in isolates were identified.

The focus of this thesis is on using mechanical strain to tailor the electronic properties of graphene. The first half covers the electro-mechanical coupling for graphene in different configurations, namely a hexagonal Y-junction, various shaped bubbles on different substrates, and with kirigami cuts. For all of these cases, a novel combination of tight-binding electronic structure calculations and molecular dynamics is utilized to demonstrate how mechanical loading and deformation impacts the resulting electronic structure and transport. For the Y-junction, a quasi-uniform pseudo magnetic field induced by strain restricts transport to Landau-level and edge-state-assisted resonant tunneling. For the bubbles, the shape and the nature of the substrate emerge as decisive factors determining the effectiveness of the nanoscale pseudo magnetic field tailoring in graphene. Finally, for the kirigami, it is shown that the yield and fracture strains of graphene, a well-known brittle material, can be enhanced by a factor of more than three using the kirigami structure, while also leading to significant enhancements in the localized pseudo magnetic fields. The second part of the thesis focuses on dissipation mechanisms in graphene nanomechanical resonators. Thermalization in nonlinear systems is a central concept in statistical mechanics and has been extensively studied theoretically since the seminal work of Fermi, Pasta, and Ulam (FPU). Using molecular dynamics and continuum modeling of a ring-down setup, it is shown that thermalization due to nonlinear mode coupling intrinsically limits the quality factor of nanomechanical graphene drums and turns them into potential test beds for FPU physics. The relationship between thermalization rate, radius, temperature and prestrain is explored and investigated.