Theoretical Chemistry for Advanced Nanomaterials

Nowadays, functional analysis is getting indispensable to develop advanced nanomaterials. Functional analysis at electron and atomic levels can be performed from both computational and experimental approaches, together with developments of high-performance computers and experimental instruments. In this chapter, after an explanation of the definition of nanomaterial, typical computational and experimental approaches are briefly introduced. Nanomaterials stand for not only nanosize materials but also materials with nanoscale functionality. After an introduction of representative nanosize materials such as organic nanomaterial, cluster and nanoparticle, nanoscale functionalities in perovskites are overviewed. Finally, recent challenges related to advanced nanomaterials are discussed. Especially, nanospace chemistry, hydrogen society in the future, lithium-ion battery safety and replacement of lithium are mentioned.

Hydride ion, which implies negatively charged hydrogen, has recently attracted much scientific attention from the viewpoint of nanoscale fast ion transport. Molecular orbital calculations based on density functional theory were performed for perovskite fluoride and hydride, in order to investigate the hydride ion-conducting mechanism. In hydride ion-doped KMgF₃ perovskite, it was found that hydride ion conduction occurs, combined with “competitive fluctuation”, which implies that fluorine anion also migrates around local minimum during hydride ion conduction. The activation energy for hydride ion conduction was estimated to be 0.61–0.85 eV. It was also found that hydride ion conduction occurs in KMgH₃ perovskite. The activation energy for hydride ion conduction was estimated to be 0.40–0.61 eV. From the viewpoint of structural stability at high temperature, it was concluded that hydride ion-doped KMgF₃ perovskite is more favourable than KMgH₃ perovskite. In comparison with proton-conducting perovskites, it was concluded that hydride ion conducting perovskites can be utilized as fast ion conductor. Finally, we discuss hydride ion safety and outlook.

This article reviews progress of recently proposed local dielectric constant and polarizability. Local dielectric constant and polarizability are defined only in the formalism of quantum field theory. These quantities are expected to be good tool to the numerical analysis of nanosize materials. Basis set dependence of these quantities is explained with examples, an atom and a cation. The distribution of local polarizability in molecules is explained for covalent molecules, XH_n (X=C, N, O, F, Si, P, S, Cl, Ge, As, Se, and Br). Particularly, the relation between chemical bond and dielectric property is discussed. Application to practical nanosize materials is introduced for hafnium dioxide, which is the leading candidate for next-generation gate dielectric thin film. Particularly, HfLaO_x model is studied for the purpose of the clarification of the effect of the incorporation of La atoms to HfO₂ on the dielectric properties.

This chapter is aimed at analysing the influence that dimensional scaling exerts on the electronic, optical, transport and mechanical properties of materials using both experiments and computer simulations. In particular, to climb the “dimensional ladder” from 0D to 3D, we analyse a specific set of all-carbon allotropes, making the best use of the versatility of this element to combine in different bonding schemes, such as sp ² and sp ³, resulting in architectures as diverse as fullerenes, nanotubes, graphene, and diamond. Owing to the central role of carbon in future emerging technologies, we will discuss a variety of physical observables to show how novel characteristics emerge by increasing or decreasing the dimensional space in which particles can move, ranging from the charge transport in semiconductor (diamond) and semimetallic (graphite) samples to the stress-strain characteristics of several 2D carbon-based materials, to the gas absorption and selectivity in pillared structures and to the thermal diffusion in foams. In this respect, our analysis uses ab initio, multiscale and Monte Carlo (MC) methods to deal with the complexity of physical phenomena at different scales. In particular, the response of the systems to external electromagnetic fields is described using the effective dielectric model of the plasma losses within a Monte Carlo framework, while pressure fields are dealt with the ab initio simulation of the stress-strain relationships. Moreover, in this chapter we present recent theoretical and experimental investigations aimed at producing graphene and other carbon-based materials using supersonic molecular beam epitaxy on inorganic surfaces, starting from fullerene precursors. We mostly focus on the computational techniques used to model various stages of the process on multiple length and time scales, from the breaking of the fullerene cage upon impact to the rearrangement of atoms on the metal surface used to catalyse graphene formation. The insights obtained by our computational modelling of the impact and of the following chemical-physical processes underlying the materials growth have been successfully used to set up an experimental procedure that ended up in the production of graphene flakes by C₆₀ impact on copper surfaces.

Synthetic approaches, structures, and reactivity of group 13–15 needle-shaped oligomers have been reviewed. Computational studies reveal that needle-shaped oligomers are more stable than fullerene-like isomers for all 13–15 pairs. Formation of such oligomers in the gas phase is energetically favorable and feasible from the thermodynamic point of view. However, many competitive reaction pathways are kinetically possible which leads to cascade of reactions and different reaction products. A systematic study of the various effects of structural variations on the electronic properties of Ga-N-based nanorods has been performed. On the basis of DFT computations, we demonstrate that terminal groups have a crucial impact on the electronic properties of the rod-shaped [RGaNH]_3n (R=H, CH₃) oligomers. Oligomers capped with GaR and NH groups adopt almost periodic structure in which terminal groups affect only the very edges of the oligomer. The band gap energy of the [HGaNH]_3n+1 is defined by states localized at the different ends of the oligomer. The value of the band gap is converging fast with increase of n, and for n = 38 it is about 93% of the value of the band gap of the [HGaNH]_3∞ polymer. In contrast, termination of the [HGaNH]_3n rod-shaped oligomer by saturation of dangling bonds with H or CH₃ groups destroys the periodic pattern and increases the number of states, localized at the ends of the oligomer. This way of termination is characterized by systematic change in structural parameters of the oligomer and near exponential decrease of the band gap energy with the oligomer length. The band gap energy for the rod-shaped oligomer of 10 nm of length (n = 38) amounts to only 27% of the value for the band gap of the [HGaNH]_3∞ polymer. Substitution of Ga atoms by Al and In has also been considered. Absorption spectra undergo a red shift if Ga atoms are replaced by In atoms and a small blue shift if Ga atoms are replaced by Al atoms. The effect of electron-donating and electron-withdrawing terminal groups (H, CH₃, F, CF₃) on a dipole moment and energy gap values is found to be significant. The band gap energy of long tube-shaped Ga-N-based oligomers can be tuned within 2 eV by changing the substituents at the ends of the oligomer. A combined effect of all considered factors, substituent groups variations, rod’s elongation, and the way of ends’ termination, can help to vary energy gap of the [HGaNH]_3n rod-shaped oligomer within the range 1–7 eV. Potential applications and further directions are also discussed.

Density functional theory (DFT) as one of molecular simulation techniques has been widely used to become rapidly a powerful tool for research and technology development for the past three decades. In particular, the DFT-based theoretical and fundamental knowledge have shed light on our understanding of the fundamental surface science, catalysis, sensors, materials science, and biology. Oxygen, nitrogen, boron, phosphorus, and sulfur are the most common heteroatoms introduced on the functional carbon nanomaterials surface with different surface functionalities. This book chapter aims to provide a pedagogical narrative of the DFT and relevant computational methods applied for surface chemistry, homogeneous/heterogeneous catalysis, and the fluorescence-based sensing properties of carbon nanomaterials. We overview several representative case studies associated with energy and chemicals production and discuss relevant principles of computationally driven carbon nanomaterials design.

Organometallic clusters composed of transition metal atoms and benzene molecules have been topics of great interest from both fundamental and technological points of view. In this chapter, we review the progress in the physical chemistry of transition metal-benzene clusters. The intrinsic properties of transition metal-benzene clusters as a function of cluster size are investigated by gas-phase experiments, often in combination with quantum chemical calculations. In particular, vanadium-benzene clusters denoted V_nBz_m, showing magic numbers at m = n + 1, n, and n – 1, are characterized to possess multiple-decker sandwich structures, where vanadium atoms and benzene molecules are alternately piled up. Moreover, the discovery of such multiple-decker formation is a cornerstone in bottom-up approaches of molecular magnetism. The interplay of mass spectrometry, laser spectroscopies, and density functional calculations reveals that multiple-decker V_nBz_m clusters exhibit monotonic increase in magnetic moment with the number of layers. Anion photoelectron spectroscopic studies allow direct observations of the geometric and electronic structures of sandwich clusters and their anions. Major progress in this direction includes the recent characterization of tilted multiple-decker sandwich cluster anions composed of manganese atoms and benzene molecules. The sandwich clusters with high-spin characteristics will hopefully be exploited as building blocks in advanced electronic and magnetic nanomaterials via controlled assembly.

Si nanopowder fabricated from Si swarf using the beads milling method exhibits two kinds of photoluminescence (PL), green-PL and blue-PL. Green-PL arises from band-to-band transition of Si nanopowder with band-gap enlarged by the quantum confinement effect. Blue-PL, on the other hand, is attributable to adsorbed 9,10-dimethylanthracene (DMA) impurity in hexane because the structure of the observed PL spectra is nearly identical to that of DMA solvent. The peaked PL spectra arise from vibronic interaction of DMA, and nearly the identical separation energies between the neighboring peaks correspond to the vibrational energy of DMA in the electronic ground-state. The PL intensity of DMA is enhanced by 60,000 times due to adsorption of DMA on Si nanopowder. For excitation photon energies higher than 4.0 eV, new peaks appear in the energy region higher than the (0, 0) band, attributable to transition from vibrational excited-states.

Si nanopowder reacts with water in the neutral pH region between 7 and 9. The hydrogen generation rate strongly depends on pH, while pH doesn’t change after the reaction. Si nanopowder reacts with OH⁻ ions, generating hydrogen, SiO₂, and electrons in the SiO₂ conduction band. Electrons are accepted by water molecules, generating hydrogen and OH⁻ ions. Since OH⁻ ions act as a catalyst, the hydrogen generation rate greatly increases with pH. The generated hydrogen volume vs. the reaction time follows a logarithmic relationship, indicating that migration of OH⁻ ions through the SiO₂ layer is the rate-determining step. The hydrogen generation reaction stops when the SiO₂ thickness reaches to ∼5 nm.

This review article describes a series of studies on glass-ceramic Na⁺ superionic conductors with the Na₅YSi₄O₁₂ (N5)-type structure and with a Na_3+3x−yR _1−xP_ySi_3−yO₉ composition, where R is a rare earth element. In the crystallization of N5-type glass-ceramics, its relatives (Na₃YSi₃O₉ (N3)- and Na₉YSi₆O₁₈ (N9)-type glass-ceramics) structurally belonging to the family of Na_24−3xY_xSi₁₂O₃₆ were found to crystallize as the precursor phase at low temperatures. In order to produce N5 single-phase glass-ceramics, the concentration of both phosphorus and rare earth was found important. The meaning of the composition was evaluated by kinetic study on the phase transformation of metastable N3 or N9 phases to stable N5 phase with Na⁺ superionic conductivity. The possible combinations of x and y became more limited for the crystallization of the superionic conducting phase as the ionic radius of R increased, while the Na⁺ conduction properties were more enhanced in the glass-ceramics of larger R. These results are discussed in view of the structure and the conduction mechanism. Also discussed were the microstructural effects on the conduction properties, which were dependent upon the heating conditions of crystallization. These effects were understood in relation to the grain boundary conduction properties as well as the transmission electron microstructural morphology of grain boundaries. Recent research into the effects of microstructure on conduction properties and microstructural control of Na⁺ superionic conducting glass-ceramics is also introduced. The optimum conditions for crystallization are discussed with reference to the conduction properties and the preparation of crack-free N5-type glass-ceramics. The effects of substituting Si with other elements exhibiting tetrahedral oxygen coordination and substituting Y with various rare earth elements are also discussed in the context of the ionic conductivity of these N5-type glass-ceramics. In addition, results on the improvement in superconductivity by Na⁺ ion implantation and control of the structure by bias crystallization of glasses in an electric field are presented.

To explain the catalytic properties inherent to a number of coatings formed by the method of plasma-electrolytic oxidation (PEO), a series of Ni- and/or Cu-containing coatings on aluminum were investigated by X-ray photoelectron spectroscopy (XPS). It was determined that the main components of the surface layers of these coatings are a variety of oxide structures of the base and electrolyte elements – oxides of aluminum, nickel, copper, and more complicated composite structures. In this chapter, our experimental results are introduced.

Theoretical computations tend to compute electronic properties of increasingly larger systems. To understand the properties, we should rather need small truncated but concise and comprehensible wave functions. For electronic processes, in particular charge transfer, which occur in excited states, we need both the energy and the wave function in order to draw and predict correct conclusions. But the excited states are saddle points in the Hilbert space, and, as shown here, the standard methods for excited states, based on the Hylleraas-Undheim and MacDonald (HUM) theorem, compute indeed the correct energy but may give misleadingly incorrect truncated wave functions, because they search for an energy minimum, not a saddle point (many functions can have the correct energy). Then, where is the saddle point? We shall see the use of a functional F _n of the wave function that has a local minimum at the excited state saddle point, without using orthogonality to approximants of lower-lying states, provided these approximants are reasonable, even if they are crude. Therefore F _n finds a correct, albeit small and concise, thus comprehensible truncated wave function, approximant of the desired excited state saddle point, allowing correct predictions for the electronic process. This could also lead to computational developments of more appropriate (to excited state) truncated basis sets. It is further shown that, via a correct approximant of the 1st excited state, we can improve the ground state. Finally it is shown that, in iterative computations, in cases of “root flipping” (which would deflect the computation), we can use F _n to identify the flipped root. For all the above, demonstrations are given for excited states of He and Li. The grand apophthegm is that HUM finds an energy minimum which, only if the expansion is increased, can approach the excited state saddle point, whereas F _n has local minimum at the saddle point, so it finds it independently of the size of the expansion.

In this chapter, we describe a mixed quantum-classical approach for simulating the dynamics of quantum mechanical phenomena occurring in nanoscale systems. This approach is based on the quantum-classical Liouville equation (QCLE), which prescribes the dynamics of a quantum subsystem coupled to a classical environment. We explain how the QCLE can be solved using a stochastic surface-hopping algorithm and how expectation values of observables can be computed. Schemes for reducing the number of trajectories required in these computations and for ensuring the continuous evolution of the quantum subsystem states along the trajectories are also outlined. To demonstrate the utility of these techniques, we describe two recent applications: vibrational energy transfer in an alpha-helical polypeptide and the field-driven dynamics of a plasmonic metamolecule.