Academic literature on the topic 'Twistronics'

Graphene twistronics using multilayer graphene is presented in such a way that it provides a metamaterial effect. This manuscript also analyzes the prediction of behavior using machine learning. The metamaterial effect is achieved by twisting the graphene layers. Graphene twistronics is a new concept for changing the electrical and optical properties of bilayer graphene by applying a small angle twist between the layers. The angle twists of 5°, 10°, and 15° are analyzed for the proposed graphene twistronics design. Tuning in the absorption spectrum is achieved by applying small twists to the angles of the bilayer graphene. Results in the form of absorption, conductivity, permeability, permittivity, and impedance are presented for different twist angles. The twisted graphene layers also demonstrate negative permittivity and negative permeability, similar to metamaterials. These negative refraction properties of graphene twistronics provide flexibility and transparency, which can be applied in photovoltaic applications. Machine-learning-based regression models are used to reduce the simulation time and resources. The results show that a regression model can reliably estimate intermediate wavelength absorption values with an R2 of 0.9999.

The twistronics, which is arising from the moiré superlattice of the small angle between twisted bilayers of 2D materials like graphene, has attracted much attention in the field of 2D materials and condensed matter physics. The novel physical properties in such systems, like unconventional superconductivity, come from the dispersionless flat band that appears when the twist reaches some magic angles. By tuning the filling of the fourfold degeneracy flat bands, the desired effects are induced due to the strong correlation of the degenerated Bloch electrons. In this article, we review the twistronics in twisted bi- and multi-layer graphene (TBG and TMG), which is formed both by transfer assembly of exfoliated monolayer graphene and epitaxial growth of multilayer graphene on SiC substrates. Starting from a brief history, we then introduce the theory of flat band in TBG. In the following, we focus on the major achievements in this field: (a) van Hove singularities and charge order; (b) superconductivity and Mott insulator in TBG and (c) transport properties in TBG. In the end, we give the perspective of the rising materials system of twistronics, epitaxial multilayer graphene on the SiC.

Abstract In Twistronics we study the effect of relative twist between the layers of a material on the electronic properties of that layered materials. There are expected to be hundreds of layered materials which can give rise to thousands or even more layered materials with combination of layers and relative twist between the layers. There is a great possibility to encounter many exotic electronic properties in these rather less explored layered materials with relative twist between the layers. There is a lot to explore and understand, to unlock full potential of twistronics. A lot of theoretical and experimental studies have been done and are being done to explore the electronic properties of twisted bilayer graphene, making it a good material to start with to explore the hidden potential of twistronics. Here we present a simple theoretical model study for commensurate twisted bilayer graphene. Starting from understanding of moire pattern in twisted bilayer graphene our study goes through writing of tight binding Hamiltonian, computational codes to determine various model parameters, solution of Hamiltonian to obtain quasi particle energies and density of states near Fermi energy in twisted bilayer graphene. Our theoretical calculations have produced flat band near Dirac point and Van-Hove singularities near Fermi energy, which agree qualitatively with recent experimental studies.

The revolution of electronic devices has shown substantial enhancement in terms of functionality, structure and higher efficiency over the last few decades. One recent approach is the replacement of the most fundamental component of the electronic device with carbon nanostructures. The unique properties of carbon-based nanostructures make them widely used in many fields ranging from material science, energy and environment, biology and medicine. In this PhD thesis, we theoretically investigate and identify, through accurate quantum mechanical first-principles calculations, promising carbon-based structures for applications in the nano-technology industry such as molecular-devices, semiconductors, and superconductors. The Density Functional Theory (DFT) approach will be used as it is the most widely employed and successful method to solve the Schrödinger equation for the quantum mechanical description of the atomic structures. We apply DFT with self-consistent non-equilibrium Green’s function (NEGF) to model the conductance of azulene quinone derivatives which are placed between semi-infinite gold electrodes which allows experimentalists to synthesis and design unprecedented molecular devices constituted of such non-alternant hydrocarbons for moletronics applications. From molectronics to twistronics, we explored the electronic and transport characteristics of twisted hetero bilayer graphene/phosphorene systems with such features could have an impact on enhancing the applications of two-dimensional twisted structures in nano electronics. Subsequently, we have shown how graphene and/or boron-nitride can maintain a crucial role in protecting a well-known two-dimensional superconductor, magnesium diboride, while maintaining its electronic properties. Finally, we investigate how both monolayers phosphorene and molybdenum disulfide have high potential gas-sensing capabilities for Hydrogen Peroxide because detecting such a cancerous molecule with real-time and cost-effective two-dimensional electrochemical sensors, would highly benefit in early diagnosis required for efficient treatment. All studies undertaken will yield not only a better understanding of the properties and applications of carbon-based nano-electronic devices, but also the prediction and identification of new promising device materials and structures.

Two-dimensional (2D) materials are one or few atoms thick layered materials. The interaction between the layers in their parent three-dimensional material is weak. Therefore, one can stack different 2D materials on top of each other like “Lego”, or one can rotate one of the layers on top of another layer of 2D materials. The ability to controllably “stack” and “twist” is unique to these materials and provides a great platform to manipulate the electronic, vibrational, and optical properties. Experimental evidence of correlated insulating states, superconductivity, ferromagnetism in the case of twisted bilayer graphene at a certain rotation angle has led to a flurry of research activity in understanding the behavior of electrons in these materials. However, two important facets attracted very little attention: effects of twisting on the collective vibration of atoms (i.e. phonons), and structural reconstruction of rigidly twisted moiré lattice. In this thesis, we explore the layer and twist angle dependence of the phonon modes in several 2D materials. We combine membrane theory and molecular dynamics simulations to show that layer breathing modes can be mapped consistently to vibrations of a simple linear chain model. Our study provides a simple and efficient way to probe the interlayer interaction in few layers of 2D materials. The introduction of twist between two layers gives rise to a large scale moiré lattice. We find that the Raman active phonon modes, especially low-frequency shear and layer breathing modes, are quite sensitive to the twist angle. We discover the existence of phason modes (with frequency 1 cm −1 , comparable to acoustic modes) for any nonzero twist, corresponding to an effective translation of the moiré lattice by relative displacement of the constituent layers in a nontrivial way. Our calculations shed new insights into the origin of friction at the nanoscale. An important step in understanding the exotic electronic and optical properties of the moiré lattices is the inclusion of the effects of structural relaxation of the un-relaxed moiré lattices. All the studies conducted on moiré materials to date presume that the moiré lattice constant of the un-relaxed twisted structure remains intact even after relaxation. We explore if novel lattice reconstructions of the moiré lattices are possible and the consequences of such lattice reconstructions on the electronic properties. In the last part of the thesis, in collaboration with experimentalists, we investigate the softening and broadening of the high-frequency phonon modes due to temperature, doping, and twist angle in MoS 2 , a prototypical transition metal dichalcogenide. This thesis has been organized as follows: • In Chapter 1, we describe the motivations behind studying properties of 2D materi- als, focusing on moiré materials. We point out some key experimental and theoretical 1challenges in the field of moiré materials. In the end, we also highlight the issues addressed in this thesis and summarize the key results. • In Chapter 2, we describe the methods adopted in this thesis. We use multiscale simulations to efficiently compute the structural, vibrational, and electronic proper- ties presented in this thesis. All the electronic structure calculations are performed with first-principles density-functional theory (DFT) based calculations. We briefly summarize key concepts behind DFT. We also outline some of the technical aspects of our DFT calculations. All the structure predictions and vibrational properties calculations are performed using molecular dynamics (MD) simulations. We briefly summarize some key concepts of MD simulations. • In Chapter 3, we develop an efficient strategy to compute breathing modes of 2D ma- terials, including the finite temperature anharmonic effects. Relative out-of-plane dis- placements of the constituent layers of 2D materials give rise to unique low-frequency breathing modes. The breathing modes can be used as a direct probe to determine layer thickness using Raman spectroscopy. We compare our calculations with exper- iments and first-principles calculations and find that they are in excellent agreement with each other. • In Chapter 4, we computationally explore the engineering of phonons with the twist angle in TMD bilayers. We establish that the phonons and related properties can be controlled by twisting, and we refer to this engineering as “twistnonics”. The sensitiveness of low-frequency phonon modes with twist angle can be used to monitor structural reconstruction. Moreover, we show that the velocities of the phason modes are quite sensitive to the twist angle, unlike the acoustic modes. Our study reveals the possibility of an intriguing θ-dependent superlubric to pinning behavior and the existence of phason modes in all two-dimensional materials. • In Chapter 5, we demonstrate a dramatic reconstruction of moiré lattices in twisted transition metal dichalcogenides for θ > 58.4 ◦ . Our calculations suggest that the presumption that the moiré lattice constant of the rigidly twisted structure continues to characterize the relaxed structure is not always valid. We also find multiple flat bands both near the valence and conduction band edges in the reconstructed lattice, which can lead to the realization of exotic correlated electronic states. • In Chapter 6, we use several techniques to investigate the temperature, doping, and twist angle dependence of the high-frequency Raman modes in MoS 2 and compare our results directly to the experiments. We compute the temperature dependence of the phonon modes using DFT based calculations incorporating three-phonon processes, and the doping dependence of the modes by explicitly computing electron-phonon coupling matrix elements with DFT. On the other hand, the twist angle dependence of the modes is computed with classical simulations. • In Chapter 7, we summarize and provide some future directions based on the work presented in this thesis.