Master thesis “Properties of graphenoid layers” deals with materials of graphenoid nature such as graphene, graphene oxide and its reduced state. The paper effectively summarize basic theoretical knowledge in the first half of its range. In the second half the project deals with practical part consisted of experiments with application of graphene oxide solvents, its analysis and especially comparing properties of non-reduced graphene oxide with its chemically reduced form. Material is examined in a form of applied thin layers on different substrates.

This thesis covers the structural, electronic, magnetic, and vibrational properties of graphene and silicene. In Chapter I, we will start with an introduction to graphene and silicene. In Chapter II, we will briefly discuss about the methodology (i. e. density functional theory)In Chapter III, we will introduce band gap opening in graphene either by introducing defects/doping or by creating superlattices with h-BN substrate. In Chapter IV, we will focus on the structural and electronic properties of K and Ge-intercalated graphene on SiC(0001). In addition, the enhancement of the superconducting transition temperature in Li-decorated graphene supported by h-BN substrate will be discussed. In Chapter V, we will discuss the vibrational properties of free-standing silicene. In addition, superlattices of silicene with h-BN as well as the phase transition in silicene by applying an external electric field will be discussed. The electronic and magnetic properties transition metal decorated silicene will be discussed, in particular the realization of the quantum anomalous Hall effect will be addressed. Furthermore, the structural, electronic, and magnetic properties of Mn decorated silicene supported by h-BN substrate will be discussed. The conclusion is included in Chapters VI. Finally, we will end with references and a list of publications for this thesis.

In this thesis, the growth of 2D materials, in particular graphene and FeO2 on crystalline supports, is studied by a multitude of surface-sensitive techniques. The mechanisms of graphene growth in ultra-high vacuum and high Ar pressure are explored, and a simple device for the manufacturing of high-quality, monocrystalline graphene on SiC is described. The electronic and chemical properties of B and N dopants on graphene are characterized by means of STM/AFM with CO-functionalized tips and supported by DFT calculations. The chemical interaction of a probe molecule (FePc) with doped graphene is also investigated. The long-standing controversy of the so-called "biphase" reconstruction of α - Fe2O3(0001) is resolved by the discovery of a complete FeO2 overlayer in this phase. The structure of this overlayer is investigated by means of STM, LEEM and DFT calculations. A thorough description of the routes to obtain single phases over the whole surface of α - Fe2O3(0001) is provided.

In two dimensions, non-trivial topology and enhanced correlation lead to amazing physical phenomena. Graphene offers a high-quality, ultra-tunable and integratable two dimensional electron system in the study of interacting and topological quantum fluids. In this thesis we studied in detail various emergent quantum phenomena of electron fluids due to both strong in-plane and out-of-plane interaction between electrons in single and multi-layer graphene systems. Using magnetoresistance measurement in the corbino disk geometry, we manged to quantitatively measure the viscosity of electrons in monolayer and bilayer graphene as a function of carrier density and temperature. We demonstrated a crossover between degenerate Fermi liquid and non-degenerate electron-hole liquid. In the quantum Hall regime, we applied the corbino geometry as a probe of the incompressible sample bulk, improving significantly the resolution of fragile quantum Hall states compared to Hall bar devices. The improved resolution enables quantitative studies over a much broader parameter space in both singlelayer and multi-layer graphene system. In double-layer graphene where two vertically stacked graphene layers are in close proximity but electrically separated by a thin hBN tunnel barrier, we observed sequence of FQHS which can be perfectly described by two-component composite fermion theory. Using a combination of different measurement configuration, we found evidence for a novel type of two-component non-abelian FQHS. At \nu = 1 in double-layer graphene where ground states of indirect excitons occur, we mappped out the entire phase diagram. We realized BEC-BCS crossover in the exciton condensation phase tunable with both magnetic field and electrostatic gating. At small exciton filling fraction, we discovered Wigner crystal of excitons. Lastly, we realized a strongly correlated triple-layer quantum Hall system with independent control of carrier density in each layer and demonstrated three-layer coherent quantum Hall effect at total integer filling fraction and possibly fractional filling fraction.

In this thesis, I firstly report high field measurements of graphene/NbN junctions, in which NbN makes edge contact to graphene. Transport measurements at zero field demonstrate clear features associated with both retro and specular Andreev reflection. By applying perpendicular magnetic field, field dependence of junction transparency at Quantum Hall (QH) / superconductor (SC) interface is calculated and explained by a picture of superposition of electron and hole edge excitation. Zeeman splitting is induced in graphene by applying in plane magnetic field. We observe changes in the Andreev reflection spectrum that are consisting with spin splitting of the graphene band structure. This edge contact technique provides the opportunity to create hybrid SC/graphene or SC/QH system to illustrate new physics such as non-Abelian zero modes of Majorana physics. Secondly, other potential material candidates for SC/graphene junctions are discussed, high field transport measurement of FeSeTe/graphene junction is discussed, Superconductor/quantum spin Hall (QSH) interface and superconductor-graphene-superconductor weak link are also discussed, respectively. At last, via contact, a new contact method for two-dimensional materials, especially air-sensitive materials is discussed, the via contact method provides a new and reliable fabrication technique for two dimensional materials.

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

The Fourth Industrial Revolution is driven by cyber-physical systems, in which sensors link the real and virtual worlds. A global explosion of physical sensors seamlessly connected to networks is expected to produce trillions of sensors annually. To accommodate sustainable sensor production, it is crucial to minimize the consumption of raw materials, the complexity of fabrication, and waste discharge while improving sensor performance and wearability. Graphene has emerged as an excellent candidate material for its electrical and mechanical characteristics; however, its economic impact has been hindered by complex and energy-intensive processes. Meanwhile, printed electronics offer a compelling range of merits for scalable, high-yield, low-cost manufacturing of graphene. Among them, the one-step laser scribing process has enabled a simultaneous formation and patterning of porous graphene in a solid-state and opened new perspectives for versatile and widely tunable physical sensing platforms. This dissertation introduces flexible, lightweight, and robust Laser-Scribed Graphene sensor solutions for detecting various physical parameters, such as strain, flow, deflection, force, pressure, temperature, conductivity, and magnetic field. Multifunctionality was obtained by exploiting the direct laser scribing process combined with the flexible nature of polyimide and the piezoresistivity of porous graphene. The outstanding properties of LSG, such as low cytotoxicity, biocompatibility, corrosion resistance, and ability to function under extreme pressure and temperature conditions, allowed targeting diverse emerging applications. As a wearable device in healthcare, the LSG sensor was utilized to monitor motions involving joint bandings, such as finger folding, knee-related movements, microsleep detection, heart rate monitoring, and plantar pressure measurements. The marine ecosystem was used as an illustrative sensor application to cope with harsh environments. To this end, the sensor measured the velocity of underwater currents, pressure, salinity, and temperature while monitoring the movement of marine animals. The sensitivity to the magnetic field remained stable up to 400 °C, making the LSG sensor a viable option for high-temperature applications. In robotics, the LSG sensor was developed for velocity profile monitoring of drones and as a soft tactile sensor. The study provides insights into methods of improving sensor performance, opportunities, and challenges facing a tangible realization of LSG physical sensors.

The lithium-ion battery is the most preferred choice for energy storage, for example, in electric vehicle batteries and electronic devices. These commonly utilized transition metal-based cathodes and graphite anodes. However, replacing the active materials with organic, redox-active materials is of great interest since these organic batteries are excelling in charging speed and cycling stability. Therefore, in the present thesis, the synthesis and characterization of potential organic electroactive materials, mainly polymers, are investigated. Concerning the structure of the polymers, linear polymers, three-dimensional / crosslinked polymers, as well as dendrimers, were chosen. The electroactive subunits include viologen, imide, triphenylamine, porphyrin, and ferrocene, either as homopolymer or copolymer, as well as active materials like graphene oxide (GO) or electrolytes. The characterization of the structures was performed by means of NMR, FTIR spectroscopy, and elemental analysis. The electrochemical properties of products were investigated by the cyclic voltammetry (CV) technique. Electrodes were prepared by drop-casting a solution of the polymers onto a current collector, and the (dis)charge performance was investigated. To enhance the conductivity of the layers, composites of the polymers with GO were prepared. Since the performance depends on the electrolyte composition, different types of solvents and salts were used and compared. The capacities in a thin film of pure polymers and dendrimers were much smaller than in the composite film with rGO. These performances are based on the molecular self-assembly of polymers and dendrimers on individual GO sheets yielding colloidal polymer/dendrimer@GO and efficient GO/rGO transformation electrocatalyzed by polymers and dendrimers. However, the stability and capacity of some polymers and dendrimers such as P2, P5, P6 and G2 were not optimal in this type of composite film. Moreover, the peak potential in the positive charge range assigned to the nitrogen centre of triphenylamine and porphyrin was found to decrease after the first scan, which is probably due to a dissolution of the film. Therefore different methods were used to composite polymer or dendrimer with GO such as reducing GO before mixing. As noticed that the redox behaviour of amine and ferrocene are reversible, but the stability of radical cation species is not stable in organic solvent after oxidation. Besides the preparation of electrodes by drop-casting, the layer-by-layer process was used by alternate dipping between cationic polymer solution and anionic GO or Poly(sodium p-styerenesulfonate) (PSS) solution. PSS acts as a counter ion for the polymer, which changes the moving species in the electrolyte from anion to cation. As noted that a large cation (TBA+) shows lower capacity compared to small cations (Li+, K+). Apart from the CV, quartz crystal microbalance (QCM) was used to monitor layer growth.

Reduced graphene oxide (rGO) has attracted considerable interest recently as the low cost and chemical stable derivative of pristine graphene with application in many applications such as energy storage, water purification and electronic devices. This dissertation thoroughly investigated stacked rGO membrane fabrication process by vacuum-driven filtration, discovered asymmetry of the two surfaces of the rGO membrane, explored application perspectives of the asymmetric rGO membrane in fog collection and microstructure patterning, and disclosed membrane compaction issue during water filtration and species rejection. In more details, this dissertation revealed that, with suitable pore size, the filtration membrane substrate would leave its physical imprint on the bottom surface of the rGO membrane in the form of surface microstructures, which result in asymmetric dynamic water wettability properties of the two surfaces of the rGO membrane. The asymmetric wettability of the rGO membrane would lead to contrasting fog harvesting behavior of its two surfaces. The physical imprint mechanism was further extended to engineering pre-designed patterns selectively on the bottom surface of the rGO membrane. This dissertation, for the first time, reported the water flux and rejection kinetics, which was related to the compaction of the rGO membrane under pressure in the process of water filtration.

Despite the weak interaction between electrons and atomic vibrations (phonons) in the one-atom thick crystal of carbon called graphene, the scattering of electrons off phonons limits coherent electron transport in pristine devices over mesoscopic length scales. The future of graphene as a replacement to silicon and other materials in advanced electronic devices will depend on the success of controlling and optimizing electronic transport. In this dissertation, we explore the electron-phonon interaction via Raman scattering, elucidating the effects of filling and emptying charge states on the phonons in both the metallic state and when levels are quantized by an applied perpendicular magnetic field. In zero magnetic field, the phonon energy shifts due to electronic screening by charge carriers. Previously, a logarithmic divergence of the phonon energy was predicted as a function of the charge carrier density. For the first time, we observe signatures of this logarithmic divergence at liquid He temperatures after vacuum annealing on single layers. We also measure the electron-phonon coupling strength, Fermi velocity, and broadening of electronic quantum levels from Raman scattering and correlate these parameters to electronic transport. In a strong perpendicular magnetic field, the energy bands split into discrete Landau levels. Here, we observe kinks and splitting of the optical phonon energy, even when the Landau level transitions are far from resonant with the phonons. We discover that the kinks are attributed to charge filling of Landau levels, as understood from a linearized model based on electron-phonon interactions. Moreover, we show that material parameters determined without magnetic fields also describe phonon behavior in high magnetic fields.