Abstract: The demand for innovative thermal management materials with superior thermal conductivity and electrical insulating properties has significantly increased with the development of portable and flexible electronic gadgets. Fluorinated graphene (FG) has recently attracted the attention of the scientific community because of its exceptional thermal conductivity and electrical insulating qualities. This work aims to provide a detailed analysis of the structure‐property relationships inherent in FG, including both chemical and physical properties, and to explain the FG manufacturing process. Special attention should be paid to a thorough analysis of the thermodynamic conduction mechanism exhibited by FG, including the effects of corrugation size, fluorine coverage, and fluorine atom distribution on its thermal conductivity. The essay also examines in‐depth the most current and cutting‐edge developments addressing the utilisation of FG as a functional filler in composite‐modified polyimide (PI) materials. Furthermore, it has been noted as a crucial component in answering the needs for possible applications by maximising thermal conductivity and mechanical qualities in FG/PI composites through particular FG structural engineering and increased FG‐PI interaction. As a result, these elements serve as the main focus of ongoing research projects, highlighting important directions for development and investigation. read less

Abstract: Molecular motion and bond dissociation are two of the most fundamental phenomena underpinning properties of molecular materials. We have entrapped HF and H2O molecules within the fullerene C60 cage, encapsulated within a single-walled carbon nanotube (X@C60)@SWNT where X = HF or H2O. (X@C60)@SWNT represents a class of molecular nanomaterial composed of a guest within a molecular host within a nanoscale host, enabling investigations of the interactions of isolated single di- or tri-atomic molecules with the electron beam. The use of the electron beam simultaneously as a stimulus of chemical reactions in molecules and as a sub-Å resolution imaging probe allows investigations of the molecular dynamics and reactivity in real time and at the atomic scale, which are probed directly by chromatic and spherical aberration corrected high resolution transmission electron microscopy (Cc/Cs-corrected HRTEM) imaging, or indirectly by vibrational electron energy loss spectroscopy (EELS) in situ during scanning transmission electron microscopy (STEM) experiments. Experimental measurements indicate that the electron beam triggers homolytic dissociation of the H-F or H-O bonds, respectively, causing the expulsion of the hydrogen atoms from the fullerene cage, leaving fluorine or oxygen behind. Due to a difference in the mechanisms of penetration through the carbon lattice available for F or O atoms, atomic fluorine inside the fullerene cage appears to be more stable than the atomic oxygen under the same conditions. The use of (X@C60)@SWNT, where each molecule X is 'packaged' separately from each other, in combination with the electron microscopy methods and density functional theory (DFT) modelling in this work, enable bond dynamics and reactivity of individual atoms to be probed directly at the single-molecule level. read less

Abstract: Carbon nanothread (C-NTH) is a new ultrathin one-dimensional sp3 carbon nanostructure, which exhibits promising applications in novel carbon nanofibers and nanocomposites. Recently, researchers have successfully developed a new alternative structure - ultrathin carbon nitride nanothread (CN-NTH). In this work, we investigate the mechanical properties of CN-NTHs through large-scale molecular dynamics simulations. Comparing with their C-NTH counterparts, CN-NTHs are found to exhibit a higher tensile and bending stiffness. In particular, because of the bond redistribution, the CN-NTHs in the polymer I group and tube (3,0) group are found to possess a higher failure strain than their C-NTH counterparts. However, the CN-NTH in the polytwistane group has a smaller failure strain compared with the pristine C-NTH. According to the atomic configurations, the presence of nitrogen atoms always leads to stress/strain concentrations for the nanothreads under tensile deformation. This study provides a comprehensive understanding of the mechanical properties of CN-NTHs, which should shed light on their potential applications such as fibers or reinforcements for nanocomposites. read less

Abstract: This paper presents a parametric study to investigate the effect of relevant design variables on mechanochemical reaction and mechanical properties of a mechanophore-embedded thermoset polymer matrix. Mechanophores emit fluorescence when a specific covalent bond breaks due to external stress, and thus have attracted immense research interest as a damage sensor. Recently, a mechanophore named dimeric 9-anthracene carboxylic acid (Di-AC) was synthesized successfully and incorporated into epoxy-based thermoset polymer matrix to detect damage precursor. However, there is significant potential in modeling the complex mechanochemistry associated with the Di-AC to obtain a better understanding of this mechanophore and its interaction with the host thermoset material. In this study, a hybrid MD simulation methodology is employed to explore this complex mechanochemistry along with the investigation of the effect of design parameters on the mechanophore performance. The hybrid MD simulation method enables the simulation of Di-AC synthesis, epoxy curing, and mechanical loading test; therefore, the experimental process performed can be emulated accurately. Previously, the hybrid MD method showed the capability of capturing experimentally observed phenomena such as early signal detection and yield strength variation between neat epoxy system and epoxy with 5 wt% Di-AC thermoset polymer. In this paper, the effect of curing temperature on mechanophore activation and mechanical properties is investigated. A series of temperatures are used in the curing simulation, which are experimentally achievable. Results show that curing temperature below glass transition temperature maintains early signal detection and yield strength decreases when the curing temperature increases above the glass transition temperature. Good correlation is observed with experimental results. read less

Abstract: A one-dimensional nanostructure with sp3 -hybridized carbon atoms, namely, poly[5]asterane (PA), is predicted by means of electronic structure calculations and reactive molecular dynamics simulations. Thermochemical analysis based on homodesmotic reactions showed that the formation of poly[5]asterane is more favorable than that of polytriangulane and comparable to that of polytwistane. A plane-wave DFT approach gave a computed Young's modulus of about 0.84 TPa, which is quite promising and comparable to those of other sp3 -hybridized nanothreads. Simulations of the desorption of hydrogen atoms from PA showed a high activation energy (Ea ≈52 kcal mol-1 ), which again indicates substantial chemical stability. Interestingly, PA was shown to exhibit auxetic behavior (negative Poisson's ratio). Thus, PA is advocated as a new mechanically and chemically stable nanothread with exotic auxetic behavior. read less

Abstract: Mechanochemistry, in particular in the form of single-molecule atomic force microscopy experiments, is difficult to model theoretically, for two reasons: Covalent bond breaking is not captured accurately by single-determinant, single-reference quantum chemistry methods, and experimental times of milliseconds or longer are hard to simulate with any approach. Reactive force fields have the potential to alleviate both problems, as demonstrated in this work: Using nondeterministic global parameter optimization by evolutionary algorithms, we have fitted a reaxFF force field to high-level multireference ab initio data for disulfides. The resulting force field can be used to reliably model large, multifunctional mechanochemistry units with disulfide bonds as designed breaking points. Explorative calculations show that a significant part of the time scale gap between AFM experiments and dynamical simulations can be bridged with this approach. read less

Abstract: We use objective boundary conditions and self-consistent charge density-functional-based tight-binding to simulate at the atomistic scale the formation of helices in narrow graphene nanoribbons with armchair edges terminated with fluorine and hydrogen. We interpret the microscopic data using an inextensible, unshearable elastic rod model, which considers both bending and torsional strains. When fitted to the atomistic data, the simple rod model uses closed-form solutions for a cubic equation to predict the strain energy and morphology at a given twist angle and the crossover point between pure torsion and a helix. Our modeling and simulation bring key insights into the origin of the helical graphene morphologies stored inside of carbon nanotubes. They can be useful for designing chiral nanoribbons with tailored properties. read less

Abstract: We propose systems that allow a tuning of the phonon transmission function T(ω) in graphene nanoribbons by using C13 isotope barriers, antidot structures, and distinct boundary conditions. Phonon modes are obtained by an interatomic fifth-nearest neighbor force-constant model (5NNFCM) and T(ω) is calculated using the non-equilibrium Green’s function formalism. We show that by imposing partial fixed boundary conditions it is possible to restrict contributions of the in-plane phonon modes to T(ω) at low energy. On the contrary, the transmission functions of out-of-plane phonon modes can be diminished by proper antidot or isotope arrangements. In particular, we show that a periodic array of them leads to sharp dips in the transmission function at certain frequencies &ohgr; &ngr; ?> which can be pre-defined as desired by controlling their relative distance and size. With this, we demonstrated that by adequate engineering it is possible to govern the magnitude of the ballistic transmission functions T ( &ohgr; ) ?> in graphene nanoribbons. We discuss the implications of these results in the design of controlled thermal transport at the nanoscale as well as in the enhancement of thermo-electric features of graphene-based materials. read less

Abstract: In mechanochemistry, forces are used to initiate chemical reactions. Although mechanochemical reactions have been conducted for millennia, a fundamental understanding of the relevant processes at the molecular level is still unavailable. Nevertheless, mechanochemistry is a lively field with an extraordinarily wide range of applications. Several approaches to apply forces to single molecules are commonly used today. One such approach is Single-Molecule Force Spectroscopy, where forces are transmitted from the cantilever of an Atomic Force Microscope to a macromolecule that is anchored to a glass surface. Moving the cantilever away from the surface exerts a pulling force on the molecule. In another class of mechanochemical experiments, ultrasound baths are used to rupture polymers mechanically or to transduce forces to a mechanically susceptible moiety. This capability of ultrasound baths is due to the collapse of cavitational bubbles in the liquids, which generates tensile forces. Furthermore, ball milling or grinding techniques can be used to crush solids, thereby applying forces to molecules. This procedure can be applied to make and break covalent bonds. Due to the lack of solvent, mechanochemical synthesis in a ball mill shows enormous potential as a sustainable and environment-friendly alternative to thermochemistry. Despite this rich body and long history of experimental mechanochemical procedures, the underlying processes are not well understood at the molecular level. However, such a comprehension is desperately needed for the optimization of mechanochemical syntheses. The use of quantum chemical methods to describe mechanochemical processes, which is called quantum mechanochemistry, has proven to be of tremendous value in understanding mechanochemistry at its most fundamental level. Quantum chemical methods for the description of molecules under external forces afford predictions on force-induced changes in molecular geometry, reactivity and spectroscopic properties. Moreover, force analysis tools are available that can be used to identify the mechanically relevant degrees of freedom in a molecule or its force-bearing scaffold, thereby rationalizing mechanochemical reactivity. During my PhD work, I have developed the JEDI (Judgement of Energy DIstribution) analysis, which is a quantum chemical force analysis tool for the distribution of stress energy in a mechanically deformed molecule. Based on the harmonic approximation, an energy is calculated for each bond, bending and torsion in a molecule, thus allowing the identification of the mechanically most strained regions in a molecule as well as the rationalization of mechanochemical processes. When a molecule is stretched, some internal modes store more energy than others. This leads to particularly large displacements of certain modes and to the preconditioning of selected bonds for rupture. Using the JEDI analysis I investigated the mechanochemical properties of polymer strands that are tangled into knots. In analogy to ropes, polymer strands are weakened by the ubiquitous overhand knot by approximately 50% and the point of bond rupture is located at the “entry” or “exit” of the knot. The JEDI analysis revealed the reason for this behavior. Upon stretching, most stress energy is stored in the torsions of the curved part of the knot and only a remarkably small amount of energy is used to stretch the bonds that ultimately break. This observation leads to the physical picture that the knot “chokes off” the chain in its immediate vicinity. In this process, the torsions act as work funnels that effectively localize the mechanical energy in the knot, thus preconditioning the covalent bonds at its entry and exit for bond rupture. Besides the description of mechanical deformation in the ground state, the JEDI analysis can be used in the electronically excited state to quantify the energy gained by relaxation on the excited state potential energy surface (PES). For this, the harmonic approximation needs to be applicable on the excited state PES of interest. The physical process that is described by the excited state JEDI analysis is fundamentally different from the ground state variant. While in the ground state JEDI analysis the distribution of stress energy in a mechanically deformed molecule is analyzed, i.e. energy is expended for deformation, the excited state JEDI analysis quantifies the energy gained by the relaxation of each internal mode upon relaxation on the excited state PES, i.e. energy becomes available. With the excited state JEDI analysis, the mechanical efficiency of molecular photoswitches can be calculated. The spatial extension of a photoswitch that undergoes, e.g., cis-trans-photoisomerization, changes significantly during this process and forces are exerted on the environment. However, other internal modes of the photoswitch that do not contribute to the change in spatial extension change as a side effect of the relaxation on the excited state PES and a certain amount of energy is wasted on them. This effect limits the mechanical efficiency of photoswitches. Using the excited state JEDI analysis, I investigated the mechanical efficiency of the stiff-stilbene photoswitch, which had been used in an experiment in literature to accelerate the electrocyclic ring opening of cyclobutene by photoisomerization. I found that the mechanical efficiency of stiff-stilbene is much too low to account for the observed enhancement of the reaction. A much more reasonable physical explanation is that excess energy from absorption of a photon is dissipated as heat, which accelerates the rupture of the thermally labile bond in cyclobutene. Furthermore, I used the JEDI analysis to investigate methods for the stabilization of strained hydrocarbons. Angle-strained cycloalkynes with a ring size smaller than eight carbon atoms are highly unstable under normal laboratory conditions, since the C≡C−C bond angles deviate substantially from linearity. Applying an external force in an appropriate direction partially linearizes these bond angles, thus leading to a stabilization of the cycloalkyne. Incorporating cycloheptyne into a macrocycle with stiff-stilbene, however, does not lead to a significant stabilization, since the mechanical efficiency of stiff-stilbene is low. Coupling cycloheptyne to another strained hydrocarbon, on the other hand, stabilizes the molecule tremendously. In particular, cycloheptyne was coupled to a strained cyclophane in a condensed macrocycle and I found that appropriate linking leads to a loss of strain in both hydrocarbons. In addition to the development of the JEDI force analysis tool, I used existing quantum mechanochemical methods to develop molecular force probes. This class of molecules can be incorporated into larger systems like polymers and proteins and can be used to monitor forces acting in different regions of the macromolecules in real-time via force-induced changes in the spectroscopic signals of the force probes. I found that the reduction of point group symmetry upon mechanical deformation of the molecular force probe is a profitable feature, since electronically excited states that are degenerate in the unperturbed state can split up upon application of an external force. This effect can lead to the generation of new peaks in the spectrum, thus allowing the precise identification of the force probe signal. Additionally, I incorporated molecular force probes into the backbone of proteins without disturbing their natural fold. The formation of hydrogen bonds between the force probe and neighboring strands in a β-sheet preserves the secondary and tertiary structure of the protein and allows the identification of the pulling direction. The application of forces along and perpendicular to the backbone yields pronounced and clearly distinguishable signals of the force probes in the infrared and Raman spectra. Advantageously, the intensities of these signals are proportional to the external force at selected points of the spectrum, which makes the molecules “force rulers”. The signals of the force probes can be intensified and shifted to a transparent window in the protein spectrum by isotopic substitution. read less

Abstract: : The abundant and renewable lignin resource has attracted increasingly attention. Supercritical carbon dioxide has excellent properties in catalytic gasification. In this paper, a molecular model of softwood-lignin was established to as a reactant. ReaxFF molecular dynamics simulation method was used to simulate the gasification of softwood in supercritical carbon dioxide environment with nano-platinum particles as catalysts. The method of slow temperature rise (80K/ps) was adopted, and the system was studied at a constant temperature 1700K. The cleavage of different bond at the initial stage of reaction were studied, and the different gasification production under the action of catalysts in supercritical carbon dioxide were analyzed and compared with the non-catalyst condition. The result shows with the addition of catalyst, the amount of CO 2 cracking increases, the number of H 2 and CO molecules produced was increased, and the reaction rate increases. read less

Abstract: Erythritol has been proposed as an inert surrogate for developing theoretical and computational models to study aging in energetic materials. In this work, we present a comparison of mechanical and shock properties of erythritol computed using the ReaxFF reactive force field and from ab initio calculations employing density functional theory (DFT). We screened eight different ReaxFF parameterizations, of which the CHO parameters developed for hydrocarbon oxidation provide the most accurate predictions of mechanical properties and the crystal structure of erythritol. Further validation of the applicability of this ReaxFF parameterization for modeling erythritol is demonstrated by comparing predictions of the elastic constants, crystal structure, vibrational density of states, and Hugoniot curves against DFT calculations. The ReaxFF predictions are in close agreement with the DFT simulations for the elastic constants and shock Hugoniot when the crystal is loaded along its c axis but show as much as 30% disagreement in the elastic constants in the a b plane and 12% difference in shock pressures when shocked along the a or b crystal axes. Last, we compare thermomechanical properties predicted from classical molecular dynamics with those calculated using the quasi-harmonic approximation and show that quantum mechanical effects produce large discrepancies in the computed values of heat capacity and thermal expansion coefficients compared with classical assumptions. Combining classical molecular dynamics predictions of mechanical behavior with phonon-based calculations of thermal behaviors, we show that predicted shock-induced temperatures for pressures up to 6.5 GPa do not exceed the pressure-dependent melting point of erythritol. read less

Abstract: This study provides an enhanced corrosion resistance of epoxy resin (EP) by embedding fluorinated graphene (FG) into the epoxy matrix. FG with different fluorine contents was obtained by reacting nitrogen trifluoride (NF3) gas with GO and then incorporated into the EP matrix to fabricate the different composites. Through a series of characterization methods, the chemical composition and microstructures of FG were systematically analyzed, and its corrosion resistance was also studied. Results revealed that F atoms were bonded to the GO surface to form C–F covalent bonds, and an FG lamellar thickness less than 2 nm. The contact angle of the coatings increased with the incorporation of FG, and the coating resistance of FG2/EP coating was 3 orders of magnitude more than that of the EP coating after immersion for 4080 h. Thus, the incorporation of FG into epoxy matrix significantly enhanced its hydrophobic properties and barrier performance, which was beneficial to improving the long-term corrosion resistance of the coating. read less

Abstract: Using density functional theory calculations, we investigate and report the electronic structure basis of symmetry-breaking in atomic structures and variations in extreme mechanical properties of a number of widely used polymers, namely, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, and three structural variants of PVDF. All of these polymers consist of C–C bonds in the backbone and a combination of C–H and C–F bonds attached to the backbone. Our results show that the relative proportion and arrangement of the C–H and C–F bonds, hereafter called the chemical heterogeneity, lead to two distinct sets of structural symmetries and mechanical properties. The set with the lower chemical heterogeneity possesses higher structural symmetry and exhibits higher strength and toughness. Electronic population analysis shows that chemical heterogeneity breaks the structural symmetry and alters deformation induced “electron redistribution patterns” in the C–C backbone. Strong variations in electron redistribution serve as the basis for distinctive mechanical behavior among the polymers: the higher the structural variation and electron redistribution, the lower the bond rupture force and toughness of the polymer. The bond rupture process is initiated by a decrease in the electronic population in the backbone, while the electronic interactions at the C–H and C–F bonds dictate where, how, and which bonds rupture. Furthermore, in the linear regime of mechanical deformation, the force–strain behavior is insensitive to the chemical heterogeneity and controlled by the strong covalent interactions of the C–C bonds. In the nonlinear regime, the chemical heterogeneity dramatically reduces the strength and toughness of the polymer chain. read less

Abstract: Hydrogenation and fluorination have been presented as two possible methods to open a bandgap in graphene, required for field-effect transistor applications. In this work, we present a detailed study of the phonon-limited mobility of electrons and holes in hydrogenated graphene (graphane) and fluorinated graphene (graphene fluoride). We pay special attention to the out-of-plane acoustic (ZA) phonons, responsible for the highest scattering rates in graphane and graphene fluoride. Considering the most adverse cut-off for long-wavelength ZA phonons, we have obtained electron (hole) mobilities of 28 (41) cm2 V−1 s−1 for graphane and 96 (30) cm2 V−1 s−1 for graphene fluoride. Nonetheless, for a more favorable cut-off wavelength of ∼2.6 nm, significantly higher electron (hole) mobilities of 233 (389) cm2 V−1 s−1 for graphane and 460 (105) cm2 V−1 s−1 for graphene fluoride are achieved. Moreover, while complete suppression of ZA phonons can increase the electron (hole) mobility in graphane up to 278 (391) cm2 V−1 s−1, it does not affect the carrier mobilities in graphene fluoride. Velocity–field characteristics reveal that the electron velocity in graphane saturates at an electric field of ∼4 × 105 V cm−1. Comparing the mobilities with other two-dimensional (2D) semiconductors, we find that hydrogenation and fluorination are two promising avenues to realize a 2D semiconductor while providing good carrier mobilities. read less

Abstract: In this paper we present a successful approach for the generation of partially fluorinated graphene structures. A computationally simple model optimized on a large density functional theory dataset quickly and precisely predicts experimentally observed structures. From the analysis of the structural diversity of fluorinated graphene in a wide range of synthesis temperatures, the general structural patterns are identified and the conditions for their achievement are determined. In addition, to facilitate further studies of fluorinated graphene, we present a ready-to-use GenCF code that implements the described structure generator. read less

Abstract: Within the framework of stochastic reactive molecular dynamics simulations a statistical method for generating fluorinated graphene structures with desirable fluorine distribution is developed. Electronic transport properties of fluorinated graphene in a wide range of functionalization degrees and system ordering are investigated. A strong correlation between irregularities in fluorine distribution and electronic properties is found. In particular, proposed consideration allows for the reproduction of both the experimentally observed electron–hole asymmetry in transport properties of fluorinated graphene and a recently revealed conductivity peak at 10% fluoride content. read less

Abstract: We developed ReaxFF parameters for phosphorus and hydrogen to give a good description of the chemical and mechanical properties of pristine and defected black phosphorene. ReaxFF for P/H is transferable to a wide range of phosphorus- and hydrogen-containing systems including bulk black phosphorus, blue phosphorene, edge-hydrogenated phosphorene, phosphorus clusters, and phosphorus hydride molecules. The potential parameters were obtained by conducting global optimization with respect to a set of reference data generated by extensive ab initio calculations. We extended ReaxFF by adding a 60° correction term, which significantly improved the description of phosphorus clusters. Emphasis was placed on the mechanical response of black phosphorene with different types of defects. Compared to the nonreactive SW potential ( Jiang , J.-W. Nanotechnology 2015 , 26 , 315706 ), ReaxFF for P/H systems provides a significant improvement in describing the mechanical properties of the pristine and defected black phosphorene, as well as the thermal stability of phosphorene nanotubes. A counterintuitive phenomenon is observed that single vacancies weaken the black phosphorene more than double vacancies with higher formation energy. Our results also showed that the mechanical response of black phosphorene is more sensitive to defects in the zigzag direction than that in the armchair direction. In addition, we developed a preliminary set of ReaxFF parameters for P/H/O/C to demonstrate that the ReaxFF parameters developed in this work could be generalized to oxidized phosphorene and P-containing 2D van der Waals heterostructures. That is, the proposed ReaxFF parameters for P/H systems establish a solid foundation for modeling of a wide range of P-containing materials. read less

Abstract: In quantum mechanochemistry, quantum chemical methods are used to describe molecules under the influence of an external force. The calculation of geometries, energies, transition states, reaction rates, and spectroscopic properties of molecules on the force-modified potential energy surfaces is the key to gain an in-depth understanding of mechanochemical processes at the molecular level. In this review, we present recent advances in the field of quantum mechanochemistry and introduce the quantum chemical methods used to calculate the properties of molecules under an external force. We place special emphasis on quantum chemical force analysis tools, which can be used to identify the mechanochemically relevant degrees of freedom in a deformed molecule, and spotlight selected applications of quantum mechanochemical methods to point out their synergistic relationship with experiments. read less

Abstract: The association and dispersion of C60 fullerene and C60F48 fluorinated fullerene in molecular and ionic solvents are studied using molecular dynamics simulations and dissolution experiments, with the aim of improving our understanding of nanocarbon materials interacting with liquid media. Fullerenes have uniform sizes and are devoid of edges and were chosen as models of wider classes of carbon nanomaterials. We parametrized a new atomistic force field for fluorinated nanocarbon materials and then used molecular dynamics to calculate structural quantities and potential of mean force. The aggregation in different solvents was assessed by UV/vis spectroscopy. The solvents considered include water, organic molecules, and ionic liquids, and we studied the effects of functional groups on the cation and of changing anions. Ionic liquids with a long alkyl chain on the cation are better solvents for C60F48 than for C60, revealing the stronger hydrophobic nature of C60F48. Surprisingly, an ionic liquid with a benzy... read less

Abstract: Recent years have witnessed a large volume of works on the modification of graphene; however, an understanding of the associated morphology or mechanical properties changes is still lacking, which is vital for its engineering implementation. By taking the C4F fluorination as an example, we find that the morphology of both graphene sheet (GS) and graphene nanoribbon (GNR) can be effectively tailored by fluorination patterning via molecular dynamics simulations. The fluorine atom produces out-of-plane forces which trigger several intriguing morphology changes to monolayer graphene, including zigzag, folded, ruffle, nanoscroll, and chain structures. Notably, for multilayer GNR, the delamination and climbing phenomena of the surface layer are observed. Further studies show that the fluorination pattern can also be utilized to modulate the mechanical properties of graphene, e.g., about 40% increase of the effective yield strain is observed for the examined GNR with fluorination patterns. This study not only de... read less

Abstract: Based upon the first principles calculations, a new (2 × 1) reconstructed edge structure (edge-4) with a triangle-pentagon pair topological defect at its edges is found for the zigzag Si nanoribbon, which is different from the previously found ones (edge-2 and edge-3) and more stable in energy than them. More interestingly, it is found that the edge-2 and edge-3 can transform into the new edge-4 under a little bit compression force along the ribbon edge, and the edge-4 could also be transformed into the edge-3 by a tensile strain larger than 9%. The calculated vibrational modes of the edge-4 show that two new characteristic vibrational edge defect modes appear at 434 cm(-1) and 515 cm(-1), which could be used in experiment to distinguish easily the new edge-4 from the edge-2 and edge-3. Finally, a sharp peak near the Fermi level is found to exist in the projected density of states from the edge's pz orbital of edge-4, making its energy bands spin-split and the antiferromagnetic state to be its ground state. read less

Abstract: Graphene derivatives containing covalently bound halogens (graphene halides) represent promising two-dimensional systems having interesting physical and chemical properties. The attachment of halogen atoms to sp(2) carbons changes the hybridization state to sp(3), which has a principal impact on electronic properties and local structure of the material. The fully fluorinated graphene derivative, fluorographene (graphene fluoride, C1F1), is the thinnest insulator and the only stable stoichiometric graphene halide (C1X1). In this review, we discuss structural properties, syntheses, chemistry, stabilities, and electronic properties of fluorographene and other partially fluorinated, chlorinated, and brominated graphenes. Remarkable optical, mechanical, vibrational, thermodynamic, and conductivity properties of graphene halides are also explored as well as the properties of rare structures including multilayered fluorinated graphenes, iodine-doped graphene, and mixed graphene halides. Finally, patterned halogenation is presented as an interesting approach for generating materials with applications in the field of graphene-based electronic devices. read less

Abstract: Low-dimensional Material: Structure-property Relationship and Applications in Energy and Environmental Engineering Hang Xiao In the past several decades, low-dimensional materials (0D materials, 1D materials and 2D materials) have attracted much interest from both the experimental and theoretical points of view. Because of the quantum confinement effect, low-dimensional materials have exhibited a kaleidoscope of fascinating phenomena and unusual physical and chemical properties, shedding light on many novel applications. Despite the enormous success has been achieved in the research of low-dimensional materials, there are three fundamental challenges of research in lowdimensional materials: 1) Develop new computational tools to accurately describe the properties of lowdimensional materials with low computational cost. 2) Predict and synthesize new low-dimensional materials with novel properties. 3) Reveal new phenomenon induced by the interaction between low-dimensional materials and the surrounding environment. In this thesis, atomistic modelling tools have been applied to address these challenges. We first developed ReaxFF parameters for phosphorus and hydrogen to give an accurate description of the chemical and mechanical properties of pristine and defected black phosphorene. ReaxFF for P/H is transferable to a wide range of phosphorus and hydrogen containing systems including bulk black phosphorus, blue phosphorene, edge-hydrogenated phosphorene, phosphorus clusters and phosphorus hydride molecules. The potential parameters were obtained by conducting global optimization with respect to a set of reference data generated by extensive ab initio calculations. We extended ReaxFF by adding a 60° correction term which significantly improved the description of phosphorus clusters. Emphasis was placed on the mechanical response of black phosphorene with different types of defects. Compared to the nonreactive SW potential of phosphorene, ReaxFF for P/H systems provides a significant improvement in describing the mechanical properties of the pristine and defected black phosphorene, as well as the thermal stability of phosphorene nanotubes. A counterintuitive phenomenon was observed that single vacancies weaken the black phosphorene more than double vacancies with higher formation energy. Our results also showed that the mechanical response of black phosphorene is more sensitive to defects in the zigzag direction than that in the armchair direction. Since ReaxFF allows straightforward extensions to the heterogeneous systems, such as oxides, nitrides, the proposed ReaxFF parameters for P/H systems also underpinned the reactive force field description of heterogeneous P systems, including Pcontaining 2D van der Waals heterostructures, oxides, etc. Based on the evolutionary algorithm driven structural search, we proposed a new stable trisulfur dinitride (S3N2) 2D crystal that is a covalent network composed solely of S-N σ bonds. S3N2 crystal is dynamically, thermally and chemically stable as confirmed by the computed phonon spectrum and ab initio molecular dynamics simulations. GW calculations showed that the 2D S3N2 crystal is a wide, direct band-gap (3.92 eV) semiconductor with a small hole effective mass. The anisotropic optical response of 2D S3N2 crystal was revealed by GW-BSE calculations. Our result not only marked the prediction of the first 2D crystal composed of nitrogen and sulfur, but also underpinned potential innovations in 2D electronics, optoelectronics, etc. Inspired by the discovery of S3N2 2D crystal, we proposed a new 2D crystal, diphosphorus trisulfide (P2S3), based on the extensive evolutionary algorithm driven structural search. The 2D P2S3 crystal was confirmed to be dynamically, thermally and chemically stable by the computed phonon spectrum and ab initio molecular dynamics simulations. This 2D crystalline phase of P2S3 corresponds to the global minimum in the Born-Oppenheimer surface of the phosphorus sulfide monolayers with 2:3 stoichiometry. It is a wide band gap (4.55 eV) semiconductor with P-S σ bonds. The electronic properties of P2S3 structure can be tuned by stacking into multilayer P2S3 structures, forming P2S3 nanoribbons or rolling into P2S3 nanotubes, expanding its potential applications for the emerging field of 2D electronics. Then we showed that the hydrolysis reaction is strongly affected by relative humidity. The hydrolysis of CO3 with n = 1-8 water molecules was investigated by ab initio method. For n = 15 water molecules, all the reactants follow a stepwise pathway to the transition state. For n = 6-8 water molecules, all the reactants undergo a direct proton transfer to the transition state with overall lower activation free energy. The activation free energy of the reaction is dramatically reduced from 10.4 to 2.4 kcal/mol as the number of water molecules increases from 1 to 6. Meanwhile, the degree of the hydrolysis of CO3 is significantly increased compared to the bulk water solution scenario. The incomplete hydration shells facilitate the hydrolysis of CO3 with few water molecules to be not only thermodynamically favorable but also kinetically favorable. We showed that the chemical kinetics is not likely to constrain the speed of CO2 air capture driven by the humidity-swing. Instead, the pore-diffusion of ions is expected to be the time-limiting step in the humidity driven CO2 air capture. The effect of humidity on the speed of CO2 air capture was studied by conducting CO2 absorption experiment using IER with a high ratio of CO3 to H2O molecules. Our result is able to provide valuable insights to designing efficient CO2 air-capture sorbents. Lastly, the self-assembly mechanism of one-end-open carbon nanotubes (CNTs) suspended in an aqueous solution was studied by molecular dynamics simulations. It was shown that two one-end-open CNTs with different diameters can coaxially self-assemble into a nanocapsule. The nanocapsules formed were stable in aqueous solution under ambient conditions, and the pressure inside the nanocapsule was much higher than the ambient pressure due to the van der Waals interactions between two parts of the nanocapsule. The effects of the normalized radius difference, normalized inter-tube distance and aspect ratio of the CNT pairs were systematically explored. The electric field response of nanocapsules was studied with ab initio molecular dynamics simulations, which showed that nanocapsules can be opened by applying an external electric field, due to the polarization of carbon atoms. This discovery not only shed light on a simple yet robust nanocapsule self-assembly mechanism, but also underpinned potential innovations in drug delivery, nano-reactors, etc. read less

Abstract: Graphene, as a typical two-dimensional nanometer material, has shown its unique application potential in electrical characteristics, thermal properties, and thermoelectric properties by virtue of its novel electronic structure. The field of traditional material modification mainly changes or enhances certain properties of materials by mixing a variety of materials (to form a heterostructure) and doping. For graphene as well, this paper specifically discusses the use of traditional modification methods to improve graphene’s electrical and thermoelectrical properties. More deeply, since graphene is an atomic-level thin film material, its shape and edge conformation (zigzag boundary and armchair boundary) have a great impact on performance. Therefore, this paper reviews the graphene modification field in recent years. Through the change in the shape of graphene, the change in the boundary structure configuration, the doping of other atoms, and the formation of a heterostructure, the electrical, thermal, and thermoelectric properties of graphene change, resulting in broader applications in more fields. Through studies of graphene’s electrical, thermal, and thermoelectric properties in recent years, progress has been made not only in experimental testing, but also in theoretical calculation. These aspects of graphene are reviewed in this paper. read less