Browsing by Author "Yakobson, Boris I."
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Item Accelerating multielectron reduction at CuxO nanograins interfaces with controlled local electric field(Springer Nature, 2023) Guo, Weihua; Zhang, Siwei; Zhang, Junjie; Wu, Haoran; Ma, Yangbo; Song, Yun; Cheng, Le; Chang, Liang; Li, Geng; Liu, Yong; Wei, Guodan; Gan, Lin; Zhu, Minghui; Xi, Shibo; Wang, Xue; Yakobson, Boris I.; Tang, Ben Zhong; Ye, RuquanRegulating electron transport rate and ion concentrations in the local microenvironment of active site can overcome the slow kinetics and unfavorable thermodynamics of CO2 electroreduction. However, simultaneous optimization of both kinetics and thermodynamics is hindered by synthetic constraints and poor mechanistic understanding. Here we leverage laser-assisted manufacturing for synthesizing CuxO bipyramids with controlled tip angles and abundant nanograins, and elucidate the mechanism of the relationship between electron transport/ion concentrations and electrocatalytic performance. Potassium/OH− adsorption tests and finite element simulations corroborate the contributions from strong electric field at the sharp tip. In situ Fourier transform infrared spectrometry and differential electrochemical mass spectrometry unveil the dynamic evolution of critical *CO/*OCCOH intermediates and product profiles, complemented with theoretical calculations that elucidate the thermodynamic contributions from improved coupling at the Cu+/Cu2+ interfaces. Through modulating the electron transport and ion concentrations, we achieve high Faradaic efficiency of 81% at ~900 mA cm−2 for C2+ products via CO2RR. Similar enhancement is also observed for nitrate reduction reaction (NITRR), achieving 81.83 mg h−1 ammonia yield rate per milligram catalyst. Coupling the CO2RR and NITRR systems demonstrates the potential for valorizing flue gases and nitrate wastes, which suggests a practical approach for carbon-nitrogen cycling.Item Assessing Carbon-Based Anodes for Lithium-Ion Batteries: A Universal Description of Charge-Transfer Binding(American Physical Society, 2014) Liu, Yuanyue; Wang, Y. Morris; Yakobson, Boris I.; Wood, Brandon C.; Smalley Institute for Nanoscale Science and TechnologyMany key performance characteristics of carbon-based lithium-ion battery anodes are largely determined by the strength of binding between lithium (Li) and sp2 carbon (C), which can vary significantly with subtle changes in substrate structure, chemistry, and morphology. Here, we use density functional theory calculations to investigate the interactions of Li with a wide variety of sp2 C substrates, including pristine, defective, and strained graphene, planar C clusters, nanotubes, C edges, and multilayer stacks. In almost all cases, we find a universal linear relation between the Li-C binding energy and the work required to fill previously unoccupied electronic states within the substrate. This suggests that Li capacity is predominantly determined by two key factors?namely, intrinsic quantum capacitance limitations and the absolute placement of the Fermi level. This simple descriptor allows for straightforward prediction of the Li-C binding energy and related battery characteristics in candidate C materials based solely on the substrate electronic structure. It further suggests specific guidelines for designing more effective C-based anodes. The method should be broadly applicable to charge-transfer adsorption on planar substrates, and provides a phenomenological connection to established principles in supercapacitor and catalyst design.Item Atomic H-Induced Mo2C Hybrid as an Active and Stable Bifunctional Electrocatalyst(American Chemical Society, 2017) Fan, Xiujun; Liu, Yuanyue; Peng, Zhiwei; Zhang, Zhenhua; Zhou, Haiqing; Zhang, Xianming; Yakobson, Boris I.; Goddard, William A. III; Guo, Xia; Hauge, Robert H.; Tour, James M.; NanoCarbon CenterMo2C nanocrystals (NCs) anchored on vertically aligned graphene nanoribbons (VA-GNR) as hybrid nanoelectrocatalysts (Mo2C–GNR) are synthesized through the direct carbonization of metallic Mo with atomic H treatment. The growth mechanism of Mo2C NCs with atomic H treatment is discussed. The Mo2C–GNR hybrid exhibits highly active and durable electrocatalytic performance for the hydrogen-evolution reaction (HER) and oxygen-reduction reaction (ORR). For HER, in an acidic solution the Mo2C–GNR has an onset potential of 39 mV and a Tafel slope of 65 mV dec–1, and in a basic solution Mo2C–GNR has an onset potential of 53 mV, and Tafel slope of 54 mV dec–1. It is stable in both acidic and basic media. Mo2C–GNR is a high-activity ORR catalyst with a high peak current density of 2.01 mA cm–2, an onset potential of 0.93 V that is more positive vs reversible hydrogen electrode (RHE), a high electron transfer number n (∼3.90), and long-term stability.Item Atomistic modeling of nano-materials: From classical to ab initio simulations in different timescales(2008) Lin, Yu; Yakobson, Boris I.Quickly developing computer techniques empower numerical simulations in materials science, which connect abstract theories and empirical experiments. Both deterministic molecular dynamics simulation and stochastic Monte Carlo simulation can employ various levels of theoretical models, from classical potential to the state-of-the-art ab initio method, for different simulation accuracies and needs. After the overview of a variety of methods used in this thesis, namely, classical potential, tight-binding (TB), semi-empirical, and density functional theory (DFT) methods, three following examples demonstrate how the computer-assisted simulations enable us to investigate and predict physical and chemical properties of the nano-materials. Mass diffusion through the graphene layer is the first example, where the DFT saddle point calculations are performed to identify the transition states of carbon absorption, addimer flipping over the graphene layer, and C2 molecule dissociation. In the second example on the cross-linked carbon nanotube bundles, tight-binding method is used for cross-link modeling and energetic stability analysis. Based on the semi-empirical molecular dynamics simulations of the tensile strength testing, a phenomenological model is proposed. After all the parameters are extracted from the quantum chemistry calculations, a series of canonical Monte-Carlo simulations are conducted to statistically analyze the mechanical properties of a nanotube bundle with thousands of cross-links. The last example on silicon nanowire demonstrates how various methods in different levels can be bridged by the energy decomposition in the energetic analysis. A novel electro-mechanical property of the pentagonal silicon nanowire is predicted by the electronic band structure calculations.Item Battery metal recycling by flash Joule heating(AAAS, 2023) Chen, Weiyin; Chen, Jinhang; Bets, Ksenia V.; Salvatierra, Rodrigo V.; Wyss, Kevin M.; Gao, Guanhui; Choi, Chi Hun; Deng, Bing; Wang, Xin; Li, John Tianci; Kittrell, Carter; La, Nghi; Eddy, Lucas; Scotland, Phelecia; Cheng, Yi; Xu, Shichen; Li, Bowen; Tomson, Mason B.; Han, Yimo; Yakobson, Boris I.; Tour, James M.; Welch Institute for Advanced Materials; NanoCarbon Center; Applied Physics Program; Smalley-Curl InstituteThe staggering accumulation of end-of-life lithium-ion batteries (LIBs) and the growing scarcity of battery metal sources have triggered an urgent call for an effective recycling strategy. However, it is challenging to reclaim these metals with both high efficiency and low environmental footprint. We use here a pulsed dc flash Joule heating (FJH) strategy that heats the black mass, the combined anode and cathode, to >2100 kelvin within seconds, leading to ~1000-fold increase in subsequent leaching kinetics. There are high recovery yields of all the battery metals, regardless of their chemistries, using even diluted acids like 0.01 M HCl, thereby lessening the secondary waste stream. The ultrafast high temperature achieves thermal decomposition of the passivated solid electrolyte interphase and valence state reduction of the hard-to-dissolve metal compounds while mitigating diffusional loss of volatile metals. Life cycle analysis versus present recycling methods shows that FJH significantly reduces the environmental footprint of spent LIB processing while turning it into an economically attractive process.Item Breaking of Symmetry in Graphene Growth on Metal Substrates(American Physical Society, 2015) Artyukhov, Vasilii I.; Hao, Yufeng; Ruoff, Rodney S.; Yakobson, Boris I.In graphene growth, island symmetry can become lower than the intrinsic symmetries of both graphene and the substrate. First-principles calculations and Monte Carlo modeling explain the shapes observed in our experiments and earlier studies for various metal surface symmetries. For equilibrium shape, edge energy variations δE manifest in distorted hexagons with different ground-state edge structures. In growth or nucleation, energy variation enters exponentially as ∼eδE/kBT, strongly amplifying the symmetry breaking, up to completely changing the shapes to triangular, ribbonlike, or rhombic.Item Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications(American Chemical Society, 2018) Rao, Rahul; Pint, Cary L.; Islam, Ahmad E.; Weatherup, Robert S.; Hofmann, Stephan; Meshot, Eric R.; Wu, Fanqi; Zhou, Chongwu; Dee, Nicholas; Amama, Placidus B.; Carpena-Nuñez, Jennifer; Shi, Wenbo; Plata, Desiree L.; Penev, Evgeni S.; Yakobson, Boris I.; Balbuena, Perla B.; Bichara, Christophe; Futaba, Don N.; Noda, Suguru; Shin, Homin; Kim, Keun Su; Simard, Benoit; Mirri, Francesca; Pasquali, Matteo; Fornasiero, Francesco; Kauppinen, Esko I.; Arnold, Michael; Cola, Baratunde A.; Nikolaev, Pavel; Arepalli, Sivaram; Cheng, Hui-Ming; Zakharov, Dmitri N.; Stach, Eric A.; Zhang, Jin; Wei, Fei; Terrones, Mauricio; Geohegan, David B.; Maruyama, Benji; Maruyama, Shigeo; Li, Yan; Adams, W. Wade; Hart, A. JohnAdvances in the synthesis and scalable manufacturing of single-walled carbon nanotubes (SWCNTs) remain critical to realizing many important commercial applications. Here we review recent breakthroughs in the synthesis of SWCNTs and highlight key ongoing research areas and challenges. A few key applications that capitalize on the properties of SWCNTs are also reviewed with respect to the recent synthesis breakthroughs and ways in which synthesis science can enable advances in these applications. While the primary focus of this review is on the science framework of SWCNT growth, we draw connections to mechanisms underlying the synthesis of other 1D and 2D materials such as boron nitride nanotubes and graphene.Item Catalytic processes simulated at nano-scale: Growth of graphitic structures and functionalized graphene explained(2011) Ribas, Morgana de Avila; Yakobson, Boris I.Key dynamic processes at nano-scale, such as graphene hydrogenation and fluorination, and carbon nanotube (CNT) growth, cannot be observed in situ in real time. Nevertheless, such processes can be studied through complementary computational methods. This work simulates three important catalytic processes representing the growth of graphitic structures and functionalized graphene. The spillover phenomenon, which has been considered promising for efficient hydrogen storage, includes transfer of H from a metal catalyst to a graphitic receptor, to finally form a graphane island. Although, the spillover is energetically unfavorable to occur on pristine graphene, catalyst saturation provides a way for hydrogen adsorption on the receptor. Ab initio calculations show that the H chemical potential can be increased to a spillover favorable range. Unlike in graphane, upon graphene fluorination different stoichiometric phases form without a nucleation barrier, with the complete CF phase being thermodynamically most stable. After fluorination, graphene electronic properties are transformed from metallic to semiconducting. First-principles and tight-binding methods are used to investigate the patterning of nanoroads and quantum dots on these phases, combining metallic and semiconducting properties on the same sheet. In catalyzed CNT growth the metallic catalyst plays a fundamental role in cap nucleation. Such a mechanism cannot be seen in experiment, nor can it be simulated by first-principles due to its time-scale, yet it can be simulated through molecular dynamics. Tuning the metal-C interaction controls the condition for growth or encapsulation: Surface carbon-diffusion limits the growth below 600 K, and at higher temperatures they depend on cap lift-off. Such tuning can be done through catalyst alloying, as shown through ab initio simulations for Ni-Fe and Cu-Fe bimetallic catalysts. Catalyst shape also plays an important role in CNT growth. The minimization of the Ni surface energy defines the equilibrium crystal shape. Catalyst reshaping is analyzed through C adsorption by first-principles and reactive force fields. The Wulff-construction suggests a significant reduction of the surface energy anisotropy upon C adsorption, based on which a continuum phenomenological model that considers catalyst reshaping in CNT nucleation is formulated. This thesis explains the growth of graphitic structures and functionalized graphene at nano-scale through computational simulations.Item Computational study of defects dynamics in carbon nanotubes and fullerenes(2008) Jiao, Kun; Yakobson, Boris I.In this dissertation, statics and dynamics of defects in fullerenes and carbon nanotubes are studied by various computational model, such as ab initio (DFT, semi-empirical, empirical potential) and several computational methods, such as structure relaxation, molecular dynamics and Monte Carlo simulation. Our investigation shows ozone molecule could etch carbon nanotube by forming a precursor, ester-like structure. Molecular dynamics simulation result presents carbon atoms on tube wall was etched away as CO(gas), which is in good agreement with experimental observation. A specific topological defect, pentagon-heptagon pair, an edge dislocation core in hexagonal lattice, is also studied by dislocation theory with atomistic computer simulations. It is shown how the glide of pentagon-heptagon defects and a particular pseudoclimb, with the atoms directly breaking out of the lattice, work concurrently to maintain the tube perfection. Another type of movement of pentagon-heptagon pair, glide involving 90° bond flipping is also studied. Derived force diagram quantifies the balance between these mechanisms, while simulations show both helical and longitudinal movement of the kinks, in agreement with the forces and with experimental observations. Our theoretical modeling also indicates that pentagons play a critical role in giant fullerene sublimation. Carbon atoms are removed predominantly from the weakest binding energy sites, i.e., the pentagons, leading to the constant evaporation rate. The fullerene cage integrity is attributed to the collective behavior of interacting defects. We also examine the formation and dynamics of edge dislocation in carbon nanotubes theoretically. Our theoretical analyses demonstrate that large edge dislocations, which are prohibited by the Frank energy criterion in 3D-materials, are stable in two-dimensional carbon nanotubes. Recent experimental high resolution transmission electronic microscopy(HRTEM) pictures also support our theoretical model, and show the existence of such large dislocations in nanotubes and their specific pseudo-climb.Item Computational Study of Electronic and Transport Properties of Novel Boron and Carbon Nano-Structures(2013-07-24) Sadrzadeh, Arta; Yakobson, Boris I.; Ajayan, Pulickel M.; Kono, JunichiroIn the first part of this dissertation, we study mainly novel boron structures and their electronic and mechanical properties, using ab initio calculations. The electronic structure and construction of the boron buckyball B80, and boron nanotubes as the α-sheet wrapped around a cylinder are studied. The α-sheet is considered so far to be the most stable structure energetically out of the two dimensional boron assemblies. We will argue however that there are other sheets close in energy, using cluster expansion method. The boron buckyball is shown to have different possible isomers. Characterization of these isomers according to their geometry and electronic structure is studied in detail. Since the B80 structure is made of interwoven double-ring clusters, we also investigate double-rings with various diameters. We investigate the properties of nanotubes obtained from α-sheet. Computations confirm their high stability and identify mechanical stiffness parameters. Careful relaxation reveals the curvature-induced buckling of certain atoms off the original plane. This distortion opens up the gap in narrow tubes, rendering them semi-conducting. Wider tubes with the diameter d 1.7 nm retain original metallic character of the α-sheet. We conclude this part by investigation into hydrogen storage capacity of boron-rich compounds, namely the metallacarboranes. In the second part of dissertation, we switch our focus to electronic and transport properties of carbon nano-structures. We study the application of carbon nanotubes as electro-chemical gas sensors. The effect of physisorption of NO2 gas molecules on electron transport properties of semi-conducting carbon nanotubes is studied using ab initio calculations and Green’s function formalism. It is shown that upon exposure of nanotube to different concentrations of gas, the common feature is the shift in conductance towards lower energies. This suggests that physisorption of NO2 will result in a decrease (increase) in conductance of p-type (n-type) nanotubes with Fermi energies close to the edge of valence and conduction band. Finally we study the effect of torsion on electronic properties of carbon nano-ribbons, using helical symmetry of the structures.Item Computational Study of Growth, Thermodynamics, and Electronic Properties of Nanomaterials(2023-04-21) Ruan, Qiyuan; Yakobson, Boris I.Nanomaterials have gained tremendous research interest over the years due to their unique physical and chemical properties. Computational methods, such as Density Functional Theory (DFT) calculations, are excitingly helpful with the study of nanomaterials, since they provide information under atomic scale. In this thesis, computational study of growth, thermodynamics, and electronics properties of nanomaterials are discussed. Firstly, DFT calculations are performed to study the growth, synthesis, thermodynamics, and stability of borophene, a newly emerged two-dimensional (2D) boron material, in order to give guidance and explanations to experiments. Secondly, the nano-thermodynamics in the phase transition process from graphene to 2D diamond is introduced, to reveal the energy competition during phase transformation that experiments are hard to catch. Next, a diamond-cubic boron nitride heterostructure system is proposed, its enhanced doping abilities over pure diamond are thoroughly investigated by a simple model. Finally, the stability and superconductivities of 2D gallium are studied to provide a new computational method to search materials under confinement.Item Computational Study of Nanomaterials: Properties, Syntheses, and Applications(2022-01-06) Lei, Jincheng; Yakobson, Boris I.Nanomaterials have emerged as an exciting class of materials and one of the most active subjects of research in materials science due to their unique physical and chemical properties with enhanced performance over the bulk counterparts. In this thesis, computational methods, such as density functional theory (DFT) calculations and molecular dynamics (MD) simulations, are employed to study the properties, syntheses and applications of several nanomaterials, including carbon nanotubes (CNTs), transition metal dichalcogenides, and MXenes. Firstly, DFT calculations have been used to predict the stable phase and superconductivity of monolayer Mo2C, a MXene material. Secondly, MD simulations integrated with DFT calculations have been performed to interrogate the synthesis mechanisms of MoS2 and CNTs in chemical vapor deposition growth. Lastly, DFT calculations combined with experimental measurements have been carried out to investigate electrochemical catalysis regarding oxygen and carbon dioxide reduction reactions.Item Computational study of structural and mechanical properties of two-dimensional nanomaterials and their derivatives(2016-11-10) Yang, Yang; Yakobson, Boris I.Two-dimensional Nanomaterials have been demonstrated to show superior properties and promising potential for applications. In this work, we investigate the structures and mechanical properties of several two-dimensional nanomaterials and their derivatives using various computational simulation methods, including boron, carbon nanotube, graphene, and boron nitride. The first part of the thesis focuses on the boron nanostructures. We report a comprehensive first-principles study of the structural and chemical properties of the recently discovered B40 cage. We also discover here a preferred structure of two-dimensional boron using the cluster expansion method and find it to be most table on reactive Cu and Ni. In the second part, an extensive analysis of the graphene grain boundaries is conducted and it is revealed that the sinuous grain boundaries based on dislocation theory and first-principles calculations can be energetically optimal once the global grain boundary line cannot bisect the tilt angle. In addition, we demonstrate here a contrasting behavior for grain boundaries in hybrid two-dimensional materials, which tend to be non-bisector and obey a universal law to optimally match the heterogeneous grains. In the last part, we propose an approach for determining the Gaussian bending modulus of graphene by utilizing carbon torus, whose topology enables its bending energy to be extracted from the coupled in-plane strain energy. Furthermore, we report a unique method to locally determine the mechanical response of individual covalent junctions between carbon nanotubes. Targeted synthesis of desired junction geometries can therefore provide a “structural alphabet” for construction of macroscopic carbon nanotube networks with tunable mechanical response.Item Computational study of synthesis, structure, property and application of low-dimensional materials(2014-07-29) Liu, Yuanyue; Yakobson, Boris I.; Lou, Jun; Tour, James M.Low-dimensional materials have attracted intense interest due to their unconventional properties and promising potential for applications. In this thesis, state-of-art computational methods are employed to study the syntheses, structures, properties, and applications of low-dimensional materials, including carbon nanotube, graphene, boron nitride and transition metal dichalcogenides. Special focus is given to the atomistic mechanism of chemical vapor deposition growth, defect structure and properties, Li-ion battery, and catalytic hydrogen production. Design of novel materials based on Materials Genome approach will also be presented.Item Computer simulations on mechanical and electrical properties of nanoscale materials(2013-12-06) Hua, Ming; Yakobson, Boris I.; Lou, Jun; Kono, JunichiroNanoscale materials have highly regular atomistic structures with very few defects due to their small sizes. The small size and near-perfect structure give such materials unique properties compared with materials at a larger scale. This work investigates the structures and properties of several nanoscale materials using various computer simulation methods. The great strength of carbon nanotubes comes from the strong covalent bonding between carbon atoms, and has been of great interest in research, however both the theoretical and experimental results obtained are in a wide range. In this work, different atomic mechanisms about the nucleation of structural failure are proposed and analyzed, revealing the competition of two routes of forming defects--brittle bond breaking and plastic yield. The relevance of these two routes are shown to be dependent on nanotube symmetry, test time, and temperature. The nanotube strength is decided by the dominant route chosen under these parameters. Helical symmetry exists in many nanoscale structures, but it's far less utilized in computer simulations compared with translational and rotational symmetry. In this work a model for helical symmetry in tight-binding computational method is developed, then the implemented code are used to calculate the structure of thin silicon nanowires, as well as the properties of twisted armchair graphene nanoribbons, such as their deformation energy, band gap, and electrical conductance. Inspired by carbon nanotube, this work also investigates very thin silicon nanotubes. They are shown to have stable structures when filled with various metal atoms along the axis. They can also go through significant structural changes from one stable atomistic configuration to another. Such thin metal-endohedral silicon nanotubes can then combine to form thicker silicide wires that are morphologically identical to experimental disilicide wires synthesized from epitaxial growth.Item Defining shapes of two-dimensional crystals with undefinable edge energies(Springer Nature, 2022) Wang, Luqing; Shirodkar, Sharmila N.; Zhang, Zhuhua; Yakobson, Boris I.The equilibrium shape of crystals is a fundamental property of both aesthetic appeal and practical importance: the shape and its facets control the catalytic, light-emitting, sensing, magnetic and plasmonic behaviors. It is also a visible macro-manifestation of the underlying atomic-scale forces and chemical makeup, most conspicuous in two-dimensional (2D) materials of keen current interest. If the crystal surface/edge energy is known for different directions, its shape can be obtained by the geometric Wulff construction, a tenet of crystal physics; however, if symmetry is lacking, the crystal edge energy cannot be defined or calculated and thus its shape becomes elusive, presenting an insurmountable problem for theory. Here we show how one can proceed with auxiliary edge energies towards a constructive prediction, through well-planned computations, of a unique crystal shape. We demonstrate it for challenging materials such as SnSe, which is of C2v symmetry, and even AgNO2 of C1, which has no symmetry at all.Item Detecting the Biopolymer Behavior of Graphene Nanoribbons in Aqueous Solution(Springer Nature, 2016) Wijeratne, Sithara S.; Penev, Evgeni S.; Lu, Wei; Li, Jingqiang; Duque, Amanda L.; Yakobson, Boris I.; Tour, James M.; Kiang, Ching-Hwa; Richard E. Smalley Institute for Nanoscale Science & TechnologyGraphene nanoribbons (GNR), can be prepared in bulk quantities for large-area applications by reducing the product from the lengthwise oxidative unzipping of multiwalled carbon nanotubes (MWNT). Recently, the biomaterials application of GNR has been explored, for example, in the pore to be used for DNA sequencing. Therefore, understanding the polymer behavior of GNR in solution is essential in predicting GNR interaction with biomaterials. Here, we report experimental studies of the solution-based mechanical properties of GNR and their parent products, graphene oxide nanoribbons (GONR). We used atomic force microscopy (AFM) to study their mechanical properties in solution and showed that GNR and GONR have similar force-extension behavior as in biopolymers such as proteins and DNA. The rigidity increases with reducing chemical functionalities. The similarities in rigidity and tunability between nanoribbons and biomolecules might enable the design and fabrication of GNR-biomimetic interfaces.Item DFT methods applications in nanoscale materials modeling(2021-04-30) Su, Tonghui; Yakobson, Boris I.With the development of simulation methods and explorations in the first-principle calculation, the density functional theory methods have more applications that could predict the properties and explain the results from the nanoscale scope for the experiments. Here, in this thesis, we utilized the DFT methods for the simulation of the material to explore their performance in molecular, 1D, and 2D systems. All these examples showed the precise of single-molecule configurations and also the periodic system properties. In the first molecular system, we combined the geometric configurations with the energy conversion, which converts the photoexcitation energy to mechanical energy output in the type of geometric transform. The DFT methods help to find the pathway in this process and got the numeric efficiency for the system. And we could also summarize a principle to connect the spring constant of a molecule with the mechanical efficiency. In the second example, which is about 1D Se nanowire, we utilized the SWCNT to provide the environment for Se nanowire synthesis and explore the mechanical stability after the encapsulation effects. Also, more stable configurations in SWCNT provides more important properties like charge carrier mobility for nanoscale devices application. With electronic properties and band structure calculation, we discovered Rashba effects in 3H configuration and used strain to control the parameter of Rashba splitting in the band structure, which set the basis and another option for 1D light element spintronics devices. In the third example, monolayer MoS2 with Cu atom doping, we use first-principle calculation collaborating with experiment to provide the microscopic structure exploration, like the doping type and substitutional sites for fixed stoichiometry and also the phase diagram with different potential energy for a different source. With these calculations, we could have a thorough picture of Cu doped MoS2 from both macroscopic and microscopic structures and properties. Our work here shows the important use for DFT calculation for many kinds of systems and properties exploration, which could help with more and more materials synthesis and potential applications.Item Electronic Properties of Functionalized Diamanes for Field-Emission Displays(Amerian Chemical Society, 2023) Tantardini, Christian; Kvashnin, Alexander G.; Azizi, Maryam; Gonze, Xavier; Gatti, Carlo; Altalhi, Tariq; Yakobson, Boris I.Ultrathin diamond films, or diamanes, are promising quasi-2D materials that are characterized by high stiffness, extreme wear resistance, high thermal conductivity, and chemical stability. Surface functionalization of multilayer graphene with different stackings of layers could be an interesting opportunity to induce proper electronic properties into diamanes. Combination of these electronic properties together with extraordinary mechanical ones will lead to their applications as field-emission displays substituting original devices with light-emitting diodes or organic light-emitting diodes. In the present study, we focus on the electronic properties of fluorinated and hydrogenated diamanes with (111), (110), (0001), (101̅0), and (2̅110) crystallographic orientations of surfaces of various thicknesses by using first-principles calculations and Bader analysis of electron density. We see that fluorine induces an occupied surface electronic state, while hydrogen modifies the occupied bulk state and also induces unoccupied surface states. Furthermore, a lower number of layers is necessary for hydrogenated diamanes to achieve the convergence of the work function in comparison with fluorinated diamanes, with the exception of fluorinated (110) and (2̅110) films that achieve rapid convergence and have the same behavior as other hydrogenated surfaces. This induces a modification of the work function with an increase of the number of layers that makes hydrogenated (2̅110) diamanes the most suitable surface for field-emission displays, better than the fluorinated counterparts. In addition, a quasi-quantitative descriptor of surface dipole moment based on the Tantardini–Oganov electronegativity scale is introduced as the average of bond dipole moments between the surface atoms. This new fundamental descriptor is capable of predicting a priori the bond dipole moment and may be considered as a new useful feature for crystal structure prediction based on artificial intelligence.Item Electronics in 2D: heterojunction, property modulation, and growth(2021-02-25) Yu, Henry; Yakobson, Boris I.The crowning achievements of modern electronics stems from decades of research on the fundamental physical, chemical and growth properties of silicon. Ever advancing efficiency in silicon wafer synthesis and our ability to further miniaturize transistors have given us exponential growth in computing speed over the past several decades, a trend famously termed as the Moore’s Law. With the transistor size approaching physical limits of silicon in the near future, the pressure of continuing Moore’s law have made 2D materials a promising candidate for next generation electronics, due to their amazing properties. One prominent feature of 2D materials is their low dimensionality, which causes weak electronic screening compared to 3D systems. This can render traditional device fabrication methods, such as forming contacts or carrier doping, useless, posing a potential challenge for future 2D electronics. Hence, in this thesis I will combine atomistic calculations and continuum models to reveal the new physics of, and propose new ways to build 2D electronic devices, utilizing their low dimensionality. First I will present an analysis on 2D coplanar metal-semiconductor (MS) contacts, which are ubiquitous in electronic devices. Weak electronic screening in low dimensions immediately implies that interface states should have weak effects on 2D MS contacts, in contrast to their 3D counterparts. Using a multi-scale model, I will use Gr|MoS2 as an example to show that the notorious Fermi level pinning is greatly suppressed in 2D MS contacts, due to low dimensionality. Apart from the new physics in 2D heterojunctions, weak screening also makes carrier doping challenging, one of the major means to modulate properties in traditional devices. Fortunately, the layered geometry of 2D materials make strain or defect engineering of properties very easy, through substrate engineering. In this regard, I will present an ab initio based framework to predict the elastic state of 2D crystals on general curved surfaces. Using this framework, I will demonstrate how to transform MoS2 and phosphorene into photonic devices by a nano-pillar surface. Also, as a more specific and exotic example, I will demonstrate how to pattern graphene to make “graphene straintronics”, creating valley current filters or surface tunable superconductors. Finally, for more general surface shapes, the interplay of surface curvature and material growth dynamics causes more drastic changes to the crystal, i.e. formation of defects. Exploiting this phenomenon to achieve defect engineering, I will present a new curved-space phase-field model for 2D material growth on curved surfaces. This model predicts material defect formation either due to the surface topological requirements or the release of elastic energy.
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