Dynamical mean-field theory for molecules and nanostructures

Dynamical mean-field theory (DMFT) has established itself as a reliable and well-controlled approximation to study correlation effects in bulk solids and also two-dimensional systems. In combination with standard density-functional theory (DFT), it has been successfully applied to study materials in which localized electronic states play an important role. It was recently shown that this approach can also be successfully applied to study correlation effects in nanostructures. Here, we provide some details on our recently proposed DFT+DMFT approach to study the magnetic properties of nanosystems [V. Turkowski, A. Kabir, N. Nayyar, and T. S. Rahman, J. Phys.: Condens. Matter 22, 462202 (2010)] and apply it to examine the magnetic properties of small FePt clusters. We demonstrate that DMFT produces meaningful results even for such small systems. For benchmarking and better comparison with results obtained using DFT+U, we also include the case of small Fe clusters. As in the case of bulk systems, the latter approach tends to overestimate correlation effects in nanostructures. Finally, we discuss possible ways to further improve the nano-DFT+DMFT approximation and to extend its application to molecules and nanoparticles on substrates and to nonequilibrium phenomena.     

Theoretical Modeling of Low‐Energy Electronic Absorption Bands in Reduced Cobaloximes

The reduced Co^I states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen‐evolving catalytic activity of these species. In this work we analyze the low‐energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.     

Quantum Chemistry Close to the Fermi Level: Reducing Clusters to Few Active Hole and/or Electron Systems

Various approximate models to describe the electronic properties of some families of clusters are reviewed. They correspond to specific elementary situations close to the Fermi level where one or few electrons are either removed from (or added to) a closed shell wavefunction. Simple hole-particule excitations are also considered. The models discussed involve diatomics-in-molecules schemes, use of pseudopotential framework extended to replace inert atoms, and finally combinations of both techniques for excited states. Applications to electronic structure of alkaline earth clusters, rare-gas systems or chromophores interacting with rare-gas systems are given also prospects for more complex molecular nanosystems or assemblies.     

Acetylenic Coupling: A Powerful Tool in Molecular Construction

Acetylenic coupling is currently experiencing some of the most intensive study of its long history. Rigid and sterically undemanding di- and oligoacetylene moieties, which are frequently encountered in natural products, are finding increasing application as key structural elements in synthetic receptors for molecular recognition. Interesting electronic and optical properties of extensively pi-conjugated systems have spurred research into new linear oligoalkynes and acetylenic carbon allotropes. The synthetic challenges associated with these efforts have in turn spawned new methods. While classical Glaser conditions are still frequently used for homocoupling, the demand for increasingly selective heterocoupling conditions has provided the focus of research over the past decades. These efforts have undoubtedly been hampered by a relatively poor mechanistic understanding of these processes. More recently, palladium-catalyzed coupling methods have led to improvements in both the selectivity and reliability of acetylenic homo- and heterocouplings and paved the way for their application to ever more complicated systems. The variety of acetylenic coupling protocols, the current mechanistic understanding, and their application in natural product and targeted synthesis are discussed comprehensively for the first time in this review, with an emphasis on the most recently developed methods, and their application to the synthesis of complex macromolecular structures.     

Optical properties of ZnO nanostructures: a hybrid DFT/TDDFT investigation

We report on the first principles computational modeling of the electronic and optical properties of ZnO nanosystems. 1D, 2D and 3D ZnO nanostructures with different characteristic size are examined and their lowest optical transition energies are calculated by hybrid TDDFT to investigate the effect of quantum confinement on the optical properties of the systems. For a realistic 3D nanoparticle model we evaluate the influence of oxygen vacancies, including relaxation of the excited states, on the photoluminescence process. The results are in quantitative agreement with experimental data, indicating that neutral oxygen vacancies are likely at the origin of green emission in the ZnO nanostructure. The calculated emission process corresponds to radiative decay from a long-living triplet state, in agreement with the experimental evidence of ～μs emission lifetime and with the results of optically detected magnetic resonance experiments.     

Electronic and magnetic properties of bimetallic ytterbocene complexes: the impact of bridging ligand geometry

Bimetallic ytterbocene complexes with bridging N-heterocylic ligands have been studied extensively in recent years due to their potential applications ranging from molecular wires to single-molecule magnets. Herein, we review our recent results for a series of ytterbocene polypyridyl bimetallic complexes to highlight the versatility and tunability of these systems based on simple changes in bridging ligand geometry. Our work has involved structural, electrochemical, optical, and magnetic measurements with the goal of better understanding the electronic and magnetic communication between the two ytterbium metal centers in this new class of bimetallics.     

Structure and magnetism of VnBz(n+1) sandwich clusters

Results on structural, energetic, electronic, and magnetic properties of linear sandwich VnBzn+1 clusters obtained using high-accuracy density functional computations are presented and analyzed. Energetically close-lying configurations and states of different spin-multiplicities are identified. The computed characteristics are in good agreement with the available experimental data. The computations predict that the most stable forms of the clusters in the size range n >/= 4 are chiral. This feature, combined with the magnetism of these systems, makes them of potential importance as building blocks of nanosystems with coupled optical and magnetic functionalities.     

Optoelectronic Properties of Carbon Nanorings: Excitonic Effects from Time-Dependent Density Functional Theory

The electronic structure and size-scaling of optoelectronic properties in cycloparaphenylene carbon nanorings are investigated using time-dependent density functional theory (TDDFT). The TDDFT calculations on these molecular nanostructures indicate that the lowest excitation energy surprisingly becomes larger as the carbon nanoring size is increased, in contradiction with typical quantum confinement effects. In order to understand their unusual electronic properties, I performed an extensive investigation of excitonic effects by analyzing electron-hole transition density matrices and exciton binding energies as a function of size in these nanoring systems. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in the nanorings. Based on overall trends in exciton binding energies and their spatial delocalization, I find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these carbon nanoring systems.     

Kinetic study of gas phase olefin polymerization with a TiCl4/MgCl2 catalyst. II. Kinetic parameter estimation and model building

Monomer transport and polymerization kinetics are two key phenomena in olefin polymerization with heterogeneous transition metal catalysts. To have a better understanding of these interrelated kinetics and diffusion phenomena, a quantitative calculation of the monomer diffusion directly from experimental study is essential. In this work, a novel temperature-perturbation technique is developed to systematically study the kinetic and diffusion limitations in catalyzed gas phase olefin polymerization. A physical model of the particle growth mechanism as well as its mathematical representation is presented and the diffusion limitations occurring in the system at high temperature are characterized and quantitatively analyzed. Finally, the practical implications of the results of this study on the operation of industrial scale polyolefin reactors are examined. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2075-2096, 1997     

Hybrid nanosystems based on colloidal quantum dots and organic ligands (Review)

The review discusses modern approaches to the synthesis of semiconductor colloidal quantum dots and hybrid nanosystems represented by conjugates of quantum dots and organic ligands. The mechanisms of photoinduced processes taking place in systems of this kind are considered in detail. Data on chemoand photoconvertible hybrid nanosystems are summarized.     

Applications of electrokinetic phenomena in materials science

The discovery of electrokinetic phenomena by Reuss in 1808 and further investigations that gave rise to the concept of the electrical double layer have played an important role in the understanding of colloidal stability. Electrokinetic phenomena are a family of effects in which a liquid moves tangentially to a charged surface. Well-known phenomena of this kind are electrophoresis, electro-osmosis, streaming potential, and sedimentation potential. A review of parameters involved in the electrochemistry of suspensions is made. The practical applications of these phenomena have become widespread in a broad range of research fields such as biomaterials, biofilms, electrokinetic waste remediation, membranes, nuclear and fossil-fired power plants, adhesive and sealant science, and concrete science. The purpose of this paper is to provide an overview of electrokinetic phenomena and their application to surface modification and characterization in a large number of research fields such as corrosion and protection processes, environmental remediation (soil and sediments, transport processes, inorganic pollutants, solid particle surfaces, filter membranes, and biosorption phenomena), cement-based systems, and biological systems.     

β‐Ag3RuO4, a Ruthenate(V) Featuring Spin Tetramers on a Two‐Dimensional Trigonal Lattice

Open‐shell solids exhibit a plethora of intriguing physical phenomena that arise from a complex interplay of charge, spin, orbital, and spin‐state degrees of freedom. Comprehending these phenomena is an indispensable prerequisite for developing improved functional materials. This type of understanding can be achieved, on the one hand, by experimental and theoretical investigations into known systems, or by synthesizing new solids displaying unprecedented structural and/or electronic features. β‐Ag₃RuO₄ may serve as such a model system because it possesses a remarkable anionic structure, consisting of tetrameric polyoxoanions (Ru₄O₁₆)^12?, and is an embedded fragment of a 2D trigonal MO₂ lattice. The notorious frustration of antiferromagnetic (AF) exchange couplings on such lattices is thus lifted, and instead strong AF occurs within the oligomeric anion, where only one exchange path remains frustrated among the relevant six. The strong magnetic anisotropy of the [Ru₄O₁₆]^12? ion, and the effectively orbital nature of its net magnetic moment, implies that this anion may reveal the properties of a single‐molecule magnet if well‐diluted in a diamagnetic matrix.     

Chemisorption-induced spin symmetry breaking in gold clusters and the onset of paramagnetism in capped gold nanoparticles

We present a simple model to describe the induction of magnetic behavior on gold clusters upon chemisorption of one organic molecule with different chemical linkers. In particular, we address the problem of stability of the lowest lying singlet and show that for some linkers there exists a spin symmetry-breaking that lowers the energy and leads to preferential spin density localization on the gold atoms neighboring the chemisorption site. The model is basically an adaptation of the Stoner model for itinerant electron ferromagnetism to finite clusters and it may have important implications for our understanding of surface magnetism in larger nanosystems and its relevance to electronic transport in electrode-molecule interfaces.     

Fast electron density methods in the life sciences--a routine application in the future?

The understanding of mutual recognition of biologically interacting systems on an atomic scale is of paramount importance in the life sciences. Electron density distributions that can be obtained from a high resolution X-ray diffraction experiment can provide--in addition to steric information--electronic properties of the species involved in these interactions. In recent years experimental ED methods have seen several favourable developments towards successful application in the life sciences. Experimental and methodological advances have made possible on the one hand high-speed X-ray diffraction experiments, and have allowed on the other hand the quantitative derivation of bonding, non-bonding and atomic electronic properties. This has made the investigation of a large number of molecules possible, and moreover, molecules with 200 or more atoms can be subject of experimental ED studies, as has been demonstrated by the example of vitamin B12. Supported by the experimentally verified transferability concept of submolecular electronic properties, a key issue in Bader's The Quantum Theory of Atoms in Molecules, activities have emerged to establish databases for the additive generation of electron densities of macromolecules from submolecular building blocks. It follows that the major aims of any experimental electron density work in the life sciences, namely the generation of electronic information for a series of molecules in a reasonable time and the study of biological macromolecules (proteins, polynucleotides), are within reach in the near future.     

Chemistry of two-dimensional MXene nanosheets in theranostic nanomedicine

Transition metal carbide,carbonitride and nitride MXenes,as the emerging two-dimensional(2D)nanomaterials,have aroused burgeoning research interest in a broad range of applications ranging from energy conversion to biomedicines attributing to their distinctive planar nanostructure,physiochemical properties and biological effects.They are featured with fascinating electronic,optical,magnetic,mechanical and thermal properties,which exert significant roles in biomedical applications of 2D MXenes.In this review,we briefly summarize the recent research progress of 2D MXenes and highlight their intrinsic chemistry in theranostic nanomedicines,focusing on the synthetic chemistry for MXenes construction,surface chemistry for surface engineering,physiochemical property for theranostic application and biological chemistry for biosafety evaluation.Furthermore,based on the current achieve ments on MXenes,their potential research directio n,critical challenges and future development in biomedicine are also discussed.It is highly expected that 2D MXene-based nanosystems would have a broad application prospect in theranostic biomedicine provided the current facing critical issues and challenges are adequately solved.     

Effects of resonance occupation on dynamical electronic properties of adsorbates

A great variety of phenomena encountered in the studies of adsorption systems is, in one way or another, determined by the dynamics and the energetics of electronic transitions in adsorbates. Being intrinsically of a quantum nature, these transitions reflect the properties of the unperturbed species, as well as those of the interactions between the adsorbates and substrates that lead to adsorption. A typical feature of chemisorption systems is the occurrence of adsorbate valence electronic resonances which are degenerate to the substrate valence bands. The presence of a resonance may give rise to changes in the properties of the adsorbate electronic transitions relative to the corresponding gas phase characteristics. These changes should, in turn, manifest themselves in a number of the properties of adsorbates, which can be studied by modern surface sensitive experimental methods. In this article, we first briefly review the characteristics of the adsorbate electronic transitions involving valence resonances. Using this as a prerequisite, we present examples of the physical phenomena and events, such as van der Waals scattering from adsorbates and the measurements of the adsorbate spectra by electronic spectroscopies, which can be interpreted by invoking the effects of fractionally occupied valence resonances on the electronic transitions in chemisorbed species.     

Self-Assembly in Natural and Unnatural Systems

Although there are no fundamental factors hindering the development of nanoscale structures, there is a growing realization that “engineering down” approaches, in other words a reduction in the size of structures generated by lithographic techniques below the present lower limit of roughly 1 μm, may become impractical. It has, therefore, become increasingly clear that only by the development of a fundamental understanding of the self-assembly of large-scale biological structures, which exist and function at and beyond the nanoscale, downwards, and the extension of our knowledge regarding the chemical syntheses of small-scale structures upwards, can the gap between the promise and the reality of nanosystems be closed. This kind of construction of nanoscale structures and nanosystems represents the so-called “bottom up” or “engineering up” approach to device fabrication. Significant progress can be made in the development of nanoscience by transferring concepts found in the biological world into the chemical arena. Central to this mission is the development of simple chemical systems capable of instructing their own organization into large aggregates of molecules through their mutual recognition properties. The precise programming of these recognition events, and hence the correct assembly of the growing superstructure, relies on a fundamental understanding and the practical exploitation of non-covalent bonding interactions between and within molecules. The science of supramolecular chemistry—chemistry beyond the molecule in its very broadest sense—has started to bridge the yawning gap between molecular and macro-molecular structures. By utilizing inter-actions as diverse as aromatic π–π stacking and metal–ligand coordination for the information source for assembly processes, chemists have, in the last decade, begun to use biological concepts such as self-assembly to construct nanoscale structures and superstructures with a variety of forms and functions. Here, we provide a flavor of how self-assembly operates in natural systems and can be harnessed in unnatural ones.     

Structures and electronic transport on silicon surfaces

By utilizing a variety of surface superstructures formed on silicon surfaces and atomic layers grown on them, close correlations between the atomic-scale structures and electrical conduction phenomena at the surfaces have been revealed. State-of-art techniques for analyzing and controlling atomic/electronic structures of surfaces are leading to an understanding of the novel electronic transport properties at surfaces. For example, the electrical conduction through surface-state bands, which are inherent in the surface superstructure, has been confirmed in in-situ measurements. An important phenomenon has also been found, where adatoms donate carriers into the surface-state band, resulting in a remarkable enhancement in electrical conductance. The nucleation of the adatoms diminishes such a doping effect. Furthermore, electrical conduction through atomic layers grown on the surfaces, whose growth structures are sensitive to the substrate surface structures, will be also discussed. In this review, we emphasize that the surface electronic transport properties are closely related to the atomic structures and atomistic dynamics on surfaces. The ultimate two-dimensional electron systems, consisting of the surface-state bands and grown atomic layers, are expected to provide a new stage in surface physics, as well as a precursory stage leading to atomic-scale electronics devices.