Van der Waals (vdW) heterostructures assembled from graphene and hexagonal boron nitride (h-BN) provide a platform for investigating fundamental physics and also novel electronic properties that could be exploited for devices. Graphene/h-BN heterostructures have higher carrier mobility and better device performance when compared with traditional graphene-based devices on SiO2/Si substrate. Vertical interlayer tunneling in graphene/h-BN/graphene structures also exhibit negative differential resistance (NDR). These electrical properties have attracted considerable attention for energy band engineering and device performance optimization.

Due to the intrinsic vdW forces between the layers, graphene stacked on h-BN tends to be misoriented relative to the h-BN layer. Interlayer electron transport through a graphene / rotated h-BN / graphene heterostructure is strongly affected by the misorientation angle θ of the h-BN with respect to the graphene layers with different physical mechanisms governing the transport in different regimes of angle, Fermi level, and bias. The different mechanisms and their resulting signatures in resistance and current are analyzed using two different models, a tight-binding, nonequilibrium Green's function model and an effective continuum model. The qualitative features resulting from the two different models compare well. In the large-angle regime (θ > 4°), the change in the effective h-BN bandgap seen by an electron at the K point of the graphene causes the resistance to monotonically increase with angle by several orders of magnitude reaching a maximum at θ = 30°. It does not affect the peak-to-valley current ratios in devices that exhibit negative differential resistance. In the small-angle regime (θ < 4°), Umklapp processes open up new conductance channels that manifest themselves as non-monotonic features in a plot of resistance versus Fermi level that can serve as experimental signatures of this effect. For small angles and high bias, the Umklapp processes give rise to two new current peaks on either side of the direct tunneling peak.

Electronic properties of a bilayer graphene/h-BN heterostructure are studied using a continuum model. The simulation results show that the resistance at the secondary Dirac cone as function of vertical electric field exhibits strong electron-hole asymmetry. First principles simulations were used to understand the effect of a rotated h-BN substrate on the electronic properties of trilayer graphene. Finally, tetra-layer graphene's transport properties are studied using a tight-binding model and Boltzmann transport theory. The interband and intraband scattering mechanisms give a good explanation of the experimental results.

The discovery of graphene's unique electronic and thermal properties has moti-

vated the search for new two-dimensional materials. Examples of these materials

include the layered two-dimensional transition metal dichalcogenides (TMDC) and

metal mono-chalcogenides. The properties of the TMDCs (eg. MoS2, WS2, TaS2,

TaSe2) and the metal mono-chalcogenides (eg. GaSe, InSe, SnS) are diverse - ranging from semiconducting, semi-metallic and metallic. Many of these materials exhibit strongly correlated phenomena and exotic collective states such as exciton condensates, charge density waves, Lifshitz transitions and superconductivity. These properties change as the film thickness is reduced down to a few monolayers.

We use first-principles simulations to discuss changes in the electronic and the

vibrational properties of these materials as the film thickness evolves from a single atomic monolayer to the bulk limit. In the semiconducting TMDCs (MoS2, MoSe2,WS2 and WSe2) and monochalcogenides (GaS, GaSe, InS and InSe) we show confining these materials to their monolayer limit introduces large band degeneracies or non-parabolic features in the electronic structure. These changes in the electronic structure results in increases in the density of states and the number of conducting modes. Our first-principles simulations combined with a Landauer approach show these changes can lead to large enhancements up to an order of magnitude in the thermoelectric performance of these materials when compared to their bulk structure. Few monolayers of the TMDCs can be misoriented with respect to each other due to the weak van-der-Waals (vdW) force at the interface of two monolayers. Misorientation of the bilayer semiconducting TMDCs increases the interlayer van-der-Waals gap distance, reduces the interlayer coupling and leads to an increase in the magnitude

of the indirect bandgap by up to 100 meV compared to the registered bilayer.

In the semi-metallic and metallic TMDC compounds (TiSe2, TaS2, TaSe2) a phase

transition to a charge density wave (CDW) ground state occurs at a temperature that is unique to each material. Confining these materials to a single monolayer or few-monolayers can increase or decrease their CDW transition temperature and change the magnitude of the CDW energy gap. We show the low energy Raman modes observed in 1T-TaSe2 and 1T-TaS2 in their CDW ground state can emerge from zone folded phonons due to the reconstruction of the lattice in the bulk and monolayer structures. In 1T-TiSe2 the driving mechanism of the CDW is excitonic condensation.We show the excitonic gap of the monolayer and bilayer structures can increase by up to a factor of 3 compared to the excitonic gap of the bulk structure.

The singular density of states and the two Fermi wavevectors resulting from a ring-shaped or "Mexican hat'' valence band give rise to unique trends in the charged impurity scattering rates and charged impurity limited mobilities. Ring shaped valence bands are common features of many monolayer and few-layer two-dimensional materials including the III-VI materials GaS, GaSe, InS, and InSe. The wavevector independence of the screening, calculated within the random phase approximation, is so strong that it is the dominant factor determining the overall trends of the scattering rates and mobilities with respect to temperature and hole density. Charged impurities placed on the substrate and in the 2D channel are considered. The different wavevector dependencies of the bare Coulomb potentials alter the temperature dependence of the mobilities. Moving the charged impurities 5 {\AA} from the center of the channel to the substrate increases the mobility by an order of magnitude by suppressing the large wavevector backscattering within the outer Fermi ring.

Using first-principles calculations, We investigate the cobalt (111) surface as an alternative substrate for the growth of hexagonal boron nitride (h-BN). We find the adsorption energies of B and N to be larger

on the Co(111) surface compared to the commonly used Cu(111) surface. Trace concentrations of carbon within the Co(111) substrate are found to lower the adsorption energies of B and N on Co(111). The most favorable binding sites and the migration barriers between these sites are also elucidated using nudged elastic band calculations.

Despite multiple successful applications of high-throughput computational materials design from first principles, there are a number of factors that inhibit its future adoption. Of particular importance are the limited ability to provide high fidelity in a reliable manner and the limited accessibility to non-expert users. We present example applications of a novel approach, where high-fidelity first-principles simulation techniques, density functional theory with hybrid screened exchange (HSE) and the GW approximation, are standardized and made available online in an accessible and repeatable setting. We apply this approach to extract electronic band gaps and band structures for a diverse set of 847 materials ranging from pure elements to III-V and II-VI compounds, ternary oxides and alloys. We find that for HSE and G0W0, the average relative error fits within 20%, whereas for conventional generalized gradient approximation the error is 55%. For HSE we find the average calculation time on an up-to-date server centrally available from a public cloud provider fits within a 48 hours window. This work provides a cost-effective, accessible and repeatable practical recipe for performing high-fidelity first-principles calculations of electronic materials in a high-throughput manner.

In the first part of the manuscript we have investigated low-frequency 1/f noise in the boron nitride – graphene – boron nitride heterostructure field-effect transistors on Si/SiO2 substrates (f is a frequency). The device channel was implemented with a single layer graphene encased between two layers of hexagonal boron nitride. The transistors had the charge carrier mobility in the range from ~30000 to ~36000 cm2/Vs at room temperature. It was established that the noise spectral density normalized to the channel area in such devices can be suppressed to ~510-9 μm2 Hz-1 , which is a factor of 5 – 10 lower than that in non-encapsulated graphene devices on Si/SiO2. The physical mechanism of noise suppression was attributed to screening of the charge carriers in the channel from traps in SiO2 gate dielectric and surface defects. The obtained results are important for the electronic and optoelectronic applications of graphene.

In the second part of this work we report on the current-carrying capacity of the nanowires made from the quasi-1D van der Waals metal tantalum triselenide capped with quasi-2D boron nitride. The chemical vapor transport method followed by chemical and mechanical exfoliation was used to fabricate the mm-long TaSe3 wires with the lateral dimensions in the 20 to 70 nm range. Electrical measurements establish that the TaSe3/h-BN nanowire heterostructures have a breakdown current density exceeding 10 MA/cm2 — an order-of-magnitude higher than that for copper. Some devices exhibited an intriguing step-like breakdown, which can be explained by the atomic thread bundle structure of the nanowires. The quasi-1D single crystal nature of TaSe3 results in a low surface roughness and in the absence of the grain boundaries. These features can potentially enable the downscaling of the nanowires to the lateral dimensions in a few-nm range. Our results suggest that quasi-1D van der Waals metals have potential for applications in the ultimately downscaled local interconnects.

Interfacial coupling between the top and the bottom surface states in thin-film 3-dimensional (3D) topological insulators (TIs) destroys the momentum-spin (k-s) locking chirality, and thus prohibits the good transport properties of TI surface states. Our theoretical investigations in 3D TI thin films show that this effect only occurs near the band gaps due to the spin mismatch between the opposing surfaces. By tuning the position of the Fermi level, interfacial tunneling and in-plane semi-classical transport present unique signatures due to this mismatch. These results indicate the possibility to restore the bulk transport properties of 3D TIs in the case of thin films.

As a real-space non-trivial topological order in solid states, the skyrmion phase in B20 compounds or heavy metal interfaces is considered as a strong candidate to implement next generation storage units or spintronic devices. Using coherent transport modeling, we demonstrate that a `topological spin Hall effect' (TSHE) can be achieved in some circumstances due to individual magnetic skyrmions. In order to evaluate the device application possibilities, a topological charge analysis is carried out to identify and quantify the topological protection in each skyrmion. Based on this analysis, an on-wafer solution to individually create magnetic skyrmions is proposed. The feasibility and stability of the proposal is numerically evaluated by solving Landau-Lifshitz-Gilbert (LLG) equation.

Besides the physics that governs the operations in electronic or spintronic devices, the requirement of signal integrity gives rise to another fundamental limit to enhance the performance of next-generation devices. System designers have proposed many solutions to overcome this limit, such as differential signaling or multi-port passive networks. Characterization of these designs calls for reliable algorithms to remove the effects of access structures: the de-embedding techniques. Taking impedance mismatch as a perturbation quantity, we establish a simplified model of uniform multi-conductor transmission lines. A novel multi-port de-embedding technique is proposed theoretically and tested experimentally.

Self-assembled nanostructures, composed of inorganic and organic materials, have multiple applications in the fields of engineering and nanotechnology. Experimental research using nanoscaled materials, such as semiconductor/metallic nanocrystals, nanowires (NW), and carbon nanotube (CNT)-molecular systems have potential applications in next generation nano electronic devices. Many of these molecular systems exhibit electronic device functionality. However, experimental analytical techniques to determine how the chemistry and geometry affects electron transport through these devices does not yet exist.

Using theory and modeling, one can approximate the chemistry and geometry at

the atomic level and also determine how the chemistry and geometry governs electron current. Nanoelectronic devices however, contain several thousand atoms which makes quantum modeling difficult. Popular atomistic modeling approaches are capable of handling small molecular systems, which are of scientific interest, but have little engineering value. The lack of large scale modeling tools has left the scientific and engineering community with a limited ability to understand, explore, and design complex systems of engineering interest.

To address these issues, I have developed a high performance general quantum charge transport model based on the non-equilibrium Green function (NEGF) formalism using density functional theory (DFT) as implemented in the FIREBALL software. FIREBALL is a quantum molecular dynamics code which has demonstrated the ability to model large molecular systems. This dissertation project of integrating NEGF into FIREBALL provides researchers with a modeling tool capable of simulating charge current in large inorganic / organic systems.

To provide theoretical support for experimental efforts, this project focused on CNT-molecular systems, which includes the discovery of a CNT-molecular resonant tunneling diode (RTD) for electronic circuit applications. This research also answers basic scientific questions regarding how the geometry and chemistry of CNT-molecular systems affects electron transport.

Due to the self-passivated and dangling bond free surfaces of two-dimensional (2D), a variety of vertical heterostructures are designed with a wide range of bandgap and material properties. We use first-principles simulations to investigate the electric field, strain and magnetic proximity effect in 2D heterostructures.

Both monolayer WSe2/monolayer MoSe2 and bilayer WSe2/monolayer MoSe2 form intrinsic type II heterojunction. As the electric field is ramped from negative to positive, the band structure of both structures shows a transition from indirect to direct bandgap. The bilayer WSe2/monolayer MoSe2 even shows a transition from type I heterojunction to type II heterojunction under negative electric field.

HfSe2/SnS2 is indirect bandgap heterostructure and shows a coherent superposition of the conduction band wavefunctions of the individual layers at conduction band minimum (CBM). The CBM without electric field is weighted towards SnS2 layer, a vertical electric field of 0.2 V/Å, pointing from HfSe2 to SnS2 layer, reverses the weights of the conduction band wavefunction. Placing graphene on HfSe2/SnS2 results in significant charge transfer from graphene to the heterostructure, and the trilayer system forms a negative Schottky barrier contact. The contact resistance of graphene on HfSe2/SnS2 calculated from a tunneling Hamiltonian indicates an excellent low-resistance contact.

PtSe2/SnS2 forms a Mexican hat in the valence bands around Γ. The in-plane biaxial strain can significantly tune the band structures of PtSe2/SnS2. Under tensile strain the height of Mexican hat is more than six times of the value without strain; while under compressive strain, a semiconducting to metallic transition is observed. Graphene in contact with SnS2 layer of PtSe2/SnS2 heterostructure forms negative Schottky barrier.

Recent experiments demonstrating proximity induced ferromagnetism in graphene motivate this study of commensurate EuO/graphene/EuO heterostructures. Using insights from lattice symmetries of EuO/graphene/EuO heterostructures, we developed a model Hamiltonian that includes proximity induced exchange splitting, spin-orbit coupling, and inter-valley interactions with parameters fitted to ab initio calculations. The intervalley interaction opens a trivial gap preventing the system from crossing into a non-trivial state. The model Hamiltonian is analyzed to determine the conditions under which the heterostructures can exhibit topologically non-trivial bands.

We explore the excitonic, electronic, phononic and thermal properties of low-dimensional materials, specifically the two-dimensional and quasi-one-dimensional transition metal chalcogenides. The possibility of observing Bose-Einstein exciton condensation (BEC) in transition metal dichalcogenides (TMDs) has been analyzed at three different levels of theory. We find that, in the strong coupling regime, mean field theory with either an unscreened or screened interlayer interaction predicts a room-temperature condensate. However, intralayer interactions can essentially renormalize the quasiparticle dispersion, which can be captured by many-body GW formalism. In the strong coupling regime, the improved BEC theory predicts that intralayer interactions have a large impact on the condensate order parameter, as well as on its functional dependencies on effective mass and carrier density. We also explore the thermal properties of 2D materials, specifically in the misoriented bilayer graphene (m-BLG) system, using ab initio density functional theory (DFT) and phonon Boltzmann transport equation (BTE). we find that the lattice thermal conductivity of m-BLG reduces to almost half of its unrotated counterpart. To explain the phonon dynamics, we analyze the phonon dispersions, phonon velocity distributions, occupations, density of states and heat capacity, both before and after misorientation. Detailed calculation of the phonon-phonon scattering lifetime reveals that, the increased umklapp scattering in the acoustic and quasi-acoustic phonon branches is the main reason for the reduced thermal conductivity in m-BLG system. We also explore the thermal conductivity of quasi-1D materials, specifically TaSe3 and NbS3, using ab initio DFT and phonon BTE. We find that both materials exhibit highly anisotropic thermal transport. A thermal conductivity of 6.3 W/mK (70.6 W/mK) is observed for metallic TaSe3 (semiconducting NbS3) along the chain direction. In-depth study of velocity and lifetime distribution shows that lower scattering and higher phonon velocity in NbS3 are the reasons behind such higher thermal conductivity. The umklapp scattering process is found to be the dominant phonon scattering mechanism in this family of low-dimensional materials. We also investigate the electronic and vibrational properties of different phases of the quasi-1D material NbS3. We find that the dimerized phase NbS3-IV is a semiconductor, whereas the undimerized phase NbS3-V is a metal. Similarity between the band dispersions of phase-I and phase-IV arises from the similarity in their structures, in spite of some stacking and chiral faults. Both phase-I and phase-IV are dynamically stable, whereas the phonon dispersion in phase-V exhibits instability along the inter-chain and growth direction, indicating a possible charge density wave ground state. Finally, we explore the band alignment properties of different quasi-1D transition metal trichalcogenides (TMTs). From the DFT calculations, we can identify several TMTs as promising candidates for ohmic contacts and tunnel FET devices.

Misorientation of two layers of bilayer graphene leaves distinct signatures in the electronic properties and the phonon modes. The effect on the thermal conductivity has received the least attention and is the least well understood.

In this work, the in-plane thermal conductivity of twisted bilayer graphene (TBG) is investigated as a function of temperature and interlayer misorientation angle using nonequilibrium molecular dynamics (NEMD). The central result is that with rotation angles larger than 13 degrees, the calculated thermal conductivities decrease approximately linearly with the increasing lattice constant of the commensurate TBG unit cell. Comparisons of the phonon dispersions show that misorientation has a negligible effect on the low-energy phonon frequencies and velocities. However, the larger periodicity of TBG reduces the Brillouin zone size to the extent that the zone edge acoustic phonons are thermally populated. This allows Umklapp scattering to reduce the lifetimes of the phonons contributing to the thermal transport, and consequently, to reduce the thermal conductivity. This explanation is supported by direct calculation of reduced phonon lifetimes in TBG based on density functional theory (DFT) for larger rotation angles.

Nothing was previously known about the questions about how small twist angles (< 13 degrees) affect the thermal conductivity of TBG, and how it approaches its aligned value as the twist angle approaches 0 degrees. To provide insight into these questions, we perform large scale NEMD calculations on commensurate TBG structures with angles down to 1.87 degrees. The results show a smooth, non-monotonic behavior of the thermal conductivity with respect to the commensurate lattice constant. As the commensurate lattice constant increases, the thermal conductivity initially decreases by 50%, and then it returns to 90% of its aligned value as the angle is reduced to 1.89 degrees. These same qualitative trends are followed by the trends in the shear elastic constant, the wrinkling intensity, and the out-of-plane ZA2 phonon frequency. The picture that emerges of the physical mechanism governing the thermal conductivity is that misorientation reduces the shear elastic constant; the reduced shear elastic constant enables greater wrinkling, and the greater wrinkling reduces the thermal conductivity. The small-angle behavior of the thermal conductivity raises the question of how do response functions approach their aligned values as the twist angle approaches 0 degrees. Is the approach gradual, discontinuous, or a combination of the two?

Much attention has been given recently to the material data science. A particular emphasis is placed on low dimensional materials exhibiting novel electrical and thermal properties. An improved dimension classifier model has been created to identify the quasi-1D materials that are often classified within the 2D material family. The algorithm is based on the fact that quasi-1D materials contain different bond lengths within the unit cell. The model can identify known quasi-1D material based on the structural data from Material Project Database. Using the optimized distributed gradient boosting model (XGBoost), both the band gap and the magnetization properties can be predicted from structural and elemental features. By fitting the XGBoost model with 15,000 kinds of materials, the accuracy of the predictions on the 5000 testing samples is greater than 91%. The mean absolute error of the band gap prediction is only 0.148 eV. Additionally, 1,025 kinds of magnetic materials have been identified among 5000 kinds of materials. According to the feature importance analysis, the most correlated feature for band gap prediction is the number of the valence electrons. While, for the magnetic material classification, it is the elemental period.

Half-integer conductance, the signature of Majorana edge

modes, was recently observed in a thin-film

magnetic topological insulator/superconductor bilayer.

Chapter 2 of this thesis analyzes a scheme for gate control of Majorana zero modes in such systems. Gating the top surface of the thin-film magnetic topological insulator controls the topological phase in the region underneath the gate. The voltage of the transition depends on the gate width, and narrower gates require larger voltages. Relatively long gates are required, on the order of 2 µm, to prevent hybridization of the end modes and to allow the creation of Majorana zero modes at low gate voltages. Applying voltage to T-shaped and I-shaped gates localizes the Majorana zero modes at their ends. This scheme may provide a facile method for implementing quantum gates for topological quantum computing.

The extremely small energy splitting of Majorana zero modes caused by s-wave pairing makes identifying them experimentally very challenging.

A heterostructure between a magnetic TI and a high Tc superconductor, which has an order of magnitude enhancement in the induced pairing gap, may offer a more feasible approach.

In Chapter 3, we study the effect of top surface electrical gating on a TI / high-Tc-superconductor with dx2-y2 pairing.

We calculated the phase diagram using the Kubo formula to find the required condition for the topological superconductivity Chern number ±1, which is necessary for obtaining Majorana modes.

We show that chiral Majorana modes appear by applying the gating potential to the top surface of the TI. We also show that we are able to change their propagation direction by only changing the top gating potential. In the end, we apply the gating potential locally to create localized Majorana zero modes. Although bound states appear at zero energy, and they are robust against an increasing gating potential,

we find that the wave functions of these zero-energy modes

do not fulfill the wavefunction condition of a Majorana zero mode,

namely that a Majorana mode is its own anti-particle.

In Chapter 4, the effect of an antiferromagnetic Skyrmion on the tunneling magnetoresistance of a ferromagnet/insulator/antiferromagnet/ferromagnet heterostructure is numerically investigated.

The tunneling magnetoresistance in the presence of a Bloch type antiferromagnetic Skyrmion is significantly reduced when the polarization of the ferromagnetic leads is anti-parallel.

The amplitude of the output signal, caused by the transmission of current in the presence of Skyrmion, depends on the resistance change induced by the Skyrmion. The change in the resistance can be engineered by changing the thickness of the insulator and magnetization strength of the ferromagnetic leads. This scheme can be used for electrical detection, and the read-out of antiferromagnetic Skyrmions in the future of Skyrmion based antiferromagnetic memories.