Grafeno e nanofitas de grafeno vêm, cada vez mais, atraindo o interesse da comunidade científica devido as suas notáveis propriedades. Neste trabalho realizou-se um estudo sistemático da estabilidade de defeitos do tipo divacância, vacância e Stone-Wales em grafeno e nanofitas de grafeno com bordas zigzag. Para tal, fizeram-se cálculos de primeiros princípios, baseados em teoria do funcional da densidade (DFT) na aproximação GGA com o uso de pseudopotenciais ultrasoft e uma base de ondas planas. Também foram feitas simulações de imagens de STM para os defeitos nas nanofitas. Além disso, foram encontrados dois novos defeitos, não publicados em nenhum outro lugar (até onde vai o conhecimento do autor), com energia de formação muito baixa. / Graphene and graphene nanoribbons have been attracting a lot of interest from the scientific community because of their novel properties. In this work, a systematic research has been done on the stability and energetics of divacancy, vacancy and Stone-Wales defects in graphene and zigzag graphene nanoribbons. With this goal in mind, ab initio density functional calculations within the GGA approximation, using ultrasoft pseudopotentials and a plane wave basis were done. Also, STM images were simulated for some selected defects. Besides, two new defects, not published elsewhere (to the best knowledge of the author), with very low formation energy are reported.

The acoustoelectric effect in graphene and graphene nanoribbons (GNRs) on lithium niobate surface acoustic wave (SAW) devices was studied experimentally. Monolayer graphene produced by chemical vapour deposition was transferred to the SAW devices. The photoresponse of the acoustoelectric current (Iae) was characterised as a function of SAW frequency and intensity, and illumination wavelength (using 450 nm and 735 nm LEDs) and intensity. Under illumination, the measured Iae increased by more than the measured decrease in conductivity, while retaining a linear dependence on SAW intensity. The latter is consistent with the piezoelectric interaction between the graphene charge carriers and the SAWs being described by a relatively simple classical relaxation model. A larger increase in Iae under an illumination wavelength of 450 nm, compared to 735 nm at the same intensity, is consistent with the generation of a hot carrier distribution. The same classical relaxation model was found to describe Iae generated in arrays of 500 nm-wide GNRs. The measured acoustoelectric current decreases as the nanoribbon width increases, as studied for GNRs with widths in the range 200 – 600 nm. This reflects an increase in charge carrier mobility due to increased doping, arising from damage induced at the nanoribbon edges during fabrication. 2 Lastly, the acoustoelectric photoresponse was studied as a function of graphene nanoribbon width (350 – 600 nm) under an illumination wavelength of 450 nm. Under illumination, the nanoribbon conductivity decreased, with the largest percentage decrease seen in the widest GNRs. Iae also decreased under illumination, in contrast to the acoustoelectric photoresponse of continuous graphene. A possible explanation is that hot carrier effects under illumination lead to a greater decrease in charge carrier mobility than the increase in acoustoelectric attenuation coefficient. This causes the measured decrease in Iae.

The strategic importance of microelectronics is reflected in its ubiquity in the global production network and in our daily lives. Above all, the microelectronics revolution has been enabled and driven by the scalability of the silicon transistor and the computational efficiency of its CMOS architecture. While the semiconductor industry has been incredibly adept at pushing the boundaries of scaling in the last few decades, many factors suggest that silicon technology is running into scientific and practical limitations to further scaling. Thus, the push for a beyond-silicon computing platform is imperative. Akin to the transition from bipolar to MOSFET technology or from NMOS to CMOS architecture, the industry is once again looking for the next disruptive technology to continue the exponential growth of computing power. In 2004, two research groups, one from the University of Manchester and the other from Georgia Tech, reported on the electrical properties of ultrathin graphite. Their findings demonstrated the stability of graphene, an atomic layer of graphite, as well as its superb carrier mobility, spurring the semiconductor industry to invest in the material as a candidate for a beyond-silicon computing platform. Within this framework, this thesis explores the promise of graphene as a material and technological platform for electronic devices. The objectives of the thesis are (i) to elucidate opportunities and challenges in the design and fabrication of graphene field-effect devices, and (ii) to advance a new device platform based on graphene p-n junctions.

Over the past decade there has been a great deal of interest in graphene, a 2-dimensional allotrope of carbon with exceptional mechanical and electrical properties. Its outstanding mobility, minimal size, and mechanical stability make it an appealing material for use in next generation electronic devices. Epitaxial graphene growth on silicon carbide is a reliable, scalable method for the production of high quality graphene films. Recent work has shown that the SiC can be patterned prior to graphitization, in order to selectively grow graphene nanostructures. Graphene nanoribbons grown using this technique do not suffer from the rough edges caused by lithographic patterning, and recent measurements have revealed extraordinary transport properties. In this thesis the magnetic properties of these nanoribbons are investigated through spin polarized current injection. The sensitivity of these nanoribbons to spin polarized current is interesting from a fundamental physics standpoint, and may find applications in future spintronic devices.

Grafeno e nanofitas de grafeno vêm, cada vez mais, atraindo o interesse da comunidade científica devido as suas notáveis propriedades. Neste trabalho realizou-se um estudo sistemático da estabilidade de defeitos do tipo divacância, vacância e Stone-Wales em grafeno e nanofitas de grafeno com bordas zigzag. Para tal, fizeram-se cálculos de primeiros princípios, baseados em teoria do funcional da densidade (DFT) na aproximação GGA com o uso de pseudopotenciais ultrasoft e uma base de ondas planas. Também foram feitas simulações de imagens de STM para os defeitos nas nanofitas. Além disso, foram encontrados dois novos defeitos, não publicados em nenhum outro lugar (até onde vai o conhecimento do autor), com energia de formação muito baixa. / Graphene and graphene nanoribbons have been attracting a lot of interest from the scientific community because of their novel properties. In this work, a systematic research has been done on the stability and energetics of divacancy, vacancy and Stone-Wales defects in graphene and zigzag graphene nanoribbons. With this goal in mind, ab initio density functional calculations within the GGA approximation, using ultrasoft pseudopotentials and a plane wave basis were done. Also, STM images were simulated for some selected defects. Besides, two new defects, not published elsewhere (to the best knowledge of the author), with very low formation energy are reported.

Recent advances in experimental and computational methods have opened up new directions in graphene fundamental studies. In addition to understanding the basic properties of this material and its quasi-one dimensional structures, significant efforts are devoted to describing their long ranged dispersive interactions. Other two-dimensional materials, such as silicene, germanene, and transition metal dichalcogenides, are also being investigated aiming at finding complementary to graphene systems with other "wonder" properties. The focus of this work is to utilize first principles simulations methods to build our basic knowledge of structure-interaction relations in two-dimensional materials and design their properties. In particular, mechanical folding and extended defects in zigzag and armchair graphene nanoribbons can be used to modulate their electronic and spin polarization characteristics and achieve different stacking patterns. Our simulations concerning zigzag silicene nanoribbons show width-dependent antiferromagnetic-ferromagnetic transitions unlike the case of zigzag graphene nanoribbons, which are always antiferromagnetic. Heterostructures, build by stacking graphene, silicene, and MoS$_2$, are also investigated. It is found that hybridization alters the electronic properties of the individual layers and new flexural and breathing phonon modes display unique behaviors in the heterostructure compositions. Anchored to SiC substrate graphene nanoribbons are also proposed as possible systems to be used in graphene electronics. Our findings are of importance not only for fundamental science, but they could also be used for future experimental developments.

Graphene is a single layer of carbon atoms arranged in honeycomb lattice. Over a long period of time it was treated as a hypothetical material to understand the properties of other allotropes of carbon, such as graphite, carbon nanotube etc. Half decade back, a single layer of graphene was finally isolated and since then the field has observed a flurry of activities. Low energy excitations in graphene are massless Dirac Fermions and quantum electrodynamic effects can be observed at room temperature in graphene, which makes it very popular among the condensed matter community. In addition graphene also shows many interesting mesoscopic effects, which is the focus of the present work. We study the electronic, magnetic and structural properties of the graphene nanostructures. The entire thesis based on the results and findings obtained from the present investigation is organized as follows. Chapter 1: provides a general introduction to the properties of graphene and graphene based nanostructures. Chapter2:describes the theoretical tools used in this thesis to investigate the properties of graphene nanoribbons. The first two chapters are meant to give the reader an overview about the field of graphene and a few of the computational techniques commonly used to investigate the properties of graphene. The following chapters are the new findings reported in this thesis. Chapter3:shows how zigzag graphene nanoribbons respond in a non-linear fashion when edges are subjected to some external potential such as magnetic field. Such response originates from the edge states present in zigzag ribbons and thus not observed in armchair nanoribbons. In the limit of ribbon width W→∞, an edge magnetic field produces a moment of ~ 1/3 per edge atom even for an infinitesimally small field, which is clearly a signature of non-linear response. Response of a finite width nanoribbon is size dependent and also depends on ln(V), the applied field. This is akin to Weber-Fechner law of audio visual perceptions. It is interesting to note that nature does provide a “quantum realization” of this in the form of biological sensing organs like the ear and eye. The magnetic response is found to scale inversely with the ribbon width. Chapter4:deals with the magnetic properties of the zigzag graphene nanoribbon. This is also a special property of the geometry of the zigzag edges and not observed in armchair nanoribbons. Our investigation reveals that the electron-electron repulsion (Hubbard U) energy creates a delta function like edge magnetic field in zigzag graphene nanoribbons. Starting from this, magnetic properties of zigzag graphene nanoribbons can be qualitatively and quantitatively explained from the non-linear response of zigzag nanoribbons. Zigzag graphene nanoribbons can exist in two possible ‘magnetic states’: antiferro (AF) where the two opposite edges have antiparallel magnetic moment and ferro (FM) where moment is parallel in the two opposite edges. First we describe the properties of undoped zigzag nanoribbons. They have AF ground state. Continuum theory can explain the size dependent bandgap and magnetic moment of the ground state. We present the first explicit derivation of the gap. Then we show that hole doping can change the ground state to FM, which is metallic. Thus the system has the property of magnetoresistance, which can be exploited by doping zigzag graphene nanoribbons externally with some gate voltage or internally by some electron acceptor element, such as boron. The critical doping for transition depends inversely with the ribbon width. We have found that the ‘phase transition’ on hole doping is a common phenomena for zigzag terminated nanostructures, such as hexagonal nanodots. Chapter5:discusses the effects of random edge shapes and random potential (Anderson disorder) on the magnetic properties of zigzag graphene nanostructures. Defects and disorders in the form of edge shape randomness and random potentials arising from substrate are very common in graphene. Our study reveals that edge state magnetism is very robust to shape randomness of the terminating edges of nanostructures; as long as there are three to four repeat units of a zigzag edge, the edge state magnetism is preserved. We also discover some “high energy” edges (ones where the edge atoms have only one nearest neighbor) can have very large moments compared to even the zigzag edges. Edge magnetism is also found to be robust to relatively small Anderson disorders, because a slowly varying small potential does not scatter the edge states. Chapter6:reveals how edge functionalization by O atom and OHgroup changes the properties of the zigzag graphene nanoribbons. Functionalization by various different molecules is a very popular method of tuning the properties of graphene. We have shown that it is possible to tune the properties of zigzag graphene nanoribbons by edge functionalization. Further, we have found that structures with clustered functionalization leads to “spatially” varying electronic structure, which can lead to interesting possibilities for electronic devices. Chapter7:describes structural stability, electronic and magnetic properties of graphene nanoribbons in presence of topological defects such as Stone-Wales defects. Our study reveals that the sign of stress induced by a SW defect in a graphene nanoribbon depends on the orientation of the SW defect with respect to the ribbon edge and the relaxation of the structure to relieve this stress determines its stability. Local warping or wrinkles arise in graphene nanoribbon when the stress is compressive, while the structure remains planar otherwise. The specific consequences to armchair and zigzag graphene nanoribbon can be understood from the anisotropy of the stress induced by a SW defect embedded in bulk graphene. We also have found localized electronic states near the SW defect sites in a nanoribbon. However, warping results in delocalization of electrons in the defect states. We have observed that, in zigzag graphene nanoribbons magnetic ordering weakens due to the presence of SW defects at the edges and the ground state is driven towards that of a nonmagnetic metal.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention owing to their unique electrical, optical, mechanical, and thermal properties not found in their 3D counterparts. They can be obtained by mechanical, chemical, or electrochemical exfoliation. However, these strategies lack uniformity and produce defect-rich samples, making it impossible for large-scale device fabrication. Chemical vapor deposition (CVD) method emerges as the viable candidate to create atomically thin specimens at the technologically relevant scale. However, the large-scale growth of monolayer TMD films with spatial homogeneity and high electrical performance remains an unsolved challenge. The spatial inhomogeneity and the associated grain boundaries between randomly oriented domains culminate to the deleterious quality of TMDs, breaking of the long-range crystalline periodicity and introduction of insidious strain. Recent research efforts have therefore dedicated to obtaining the single crystallinity of 2D materials by controlling the orientation and dimensionality to obtain a large-scale and grain boundary-free monolayer films for Si-comparable electron mobility and overcoming the scaling limitation of traditional Si-based microelectronics,. In the first part of this thesis, orientation and dimensionality controlling of TMDs are discussed. To this end, we systematically study the growth of stereotypical molybdenum disulfide (MoS2) monolayer on a c-plane sapphire with CVD to elucidate the factors controlling their orientation. We have arrived at the conclusion that the concentration of precursors- that is, the ratio between sulfur and molybdenum oxide, plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS2 monolayers. Later, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer, and single-crystalline MoS2 nanoribbons on β-gallium (iii) oxide (β-Ga2O3) (100) substrates. LDE MoS2 nanoribbons have spatial uniformity over a long-range and transport characteristics on par with those seen in exfoliated benchmarks. In the second part, we theoretically reveal and experimentally determine the origin of resonant modulation of contrast as a result of the residual 3-fold astigmatism in modern scanning transmission electron microscopy (STEM) and its unintended impact on violating the power-law dependence of contrast on coordination modes between the transition metal and chalcogenide atoms.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第20728号 / エネ博第356号 / 新制||エネ||70(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 坂口 浩司, 教授 萩原 理加, 教授 佐川 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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