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Biomolecules such as α-helices are exclusively right-handed without contamination by their left-handed counterparts. Within the abiotic world, chiral or external perturbations have to be applied to the abiotic helical foldamers or polymers to control and bias their helical screw sense. Otherwise, it is not possible to separate racemic helical foldamers composed of achiral building blocks. By designing two complementary "sticky" groups, incorporating them into the ends of helical pentamers and taking advantage of the energetically more favored full overlap, which involves helical backbones, we succeeded in demonstrating, for the first time, a spontaneous resolution of racemic helices into their enantiopure single-handed helical forms via chiral crystallization without the use of chiral auxiliary or external stimuli. © 2012 The Royal Society of Chemistry.

We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched between two N-doped graphene nanoribbons (GNRs) based on the density functional theory and nonequilibrium Greens function methods. Our calculated results clearly reveal that the FePc molecular junction has high spin-filter efficiency as well as negative differential resistance (NDR). The zero-bias conductance through FePc molecule is dominated by the spin-down electrons, and the observed NDR originates from the bias-dependent effective coupling between the FePc molecular orbitals and the narrow density of states of electrodes. The remarkable high spin-filter efficiency and NDR are robust regardless of the edge shape and the width of GNRs, and the N-doping site in GNRs. These predictions indicate that FePc junction holds great promise in molecular electronics and spintronics applications. © 2012 American Institute of Physics.

Here we report the synthesis and characterization of a series of annelated benzotriazole based polymers with variable bandgap. Benzobistriazole monomers reported by us previously were desymmetrized using partial reduction ring opening followed by ring closure to produce a wide range of annelated benzotriazole based monomers. These monomers were co-polymerized with a fluorene moiety to give polymers with bandgaps ranging from 1.16 to 2.41 eV. © 2012 American Chemical Society.

Let's twist again: A "clean reaction" strategy based on thermally eliminating lactam bridges from a soluble acene precursor through a retro-Diels-Alder reaction gives a new, stable, green heptatwistacene (see structure). The molecule has a twist angle of 23.49°, is very stable even in air, and has an optical band gap of 1.82 eV. © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.

Quantum tunneling of magnetization (QTMs), stemming from their importance for understanding materials with unconventional properties, has continued to attract widespread theoretical and experimental attention. However, the observation of QTMs in the most promising candidates of molecular magnets and few iron-based compounds is limited to very low temperature. Herein, we first highlight a simple system, ultrasmall half-metallic V 3O 4 quantum dots, as a promising candidate for the investigation of QTMs at high temperature. The quantum superparamagnetic state (QSP) as a high temperature signature of QTMs is observed at 16 K, which is beyond absolute zero temperature and much higher than that of conventional iron-based compounds due to the stronger spin-orbital coupling of V 3+ ions bringing high anisotropy energy. It is undoubtedly that this ultrasmall quantum dots, V 3O 4, offers not only a promising candidate for theoretical understanding of QTMs but also a very exciting possibility for computers using mesoscopic magnets.

Detailed studies on water-binding aquafoldamers are presented that illustrate the potential use of the elongated larger aquafoldamers for recognizing larger water clusters of diverse topologies. A novel self-trapping dimerization mode involving two tetramer molecules is proposed, which is consistent with the obtained varying experimental evidences. © The Royal Society of Chemistry 2012.

First-principles calculations are performed to investigate the unzipping mechanism of carbon nanotubes (CNTs) into narrow graphene nanoribbons (GNRs) upon oxidation. By treating possible adsorptive structures, we found that, upon further oxidation, epoxy pairs tear the CNT up with an initial energy barrier of 0.59 eV (armchair) and 0.60 eV (zigzag), and the following steps of unzipping CNT become much easier because of the stress induced by the carbonyl pair. However, for zigzag CNTS, the unzipped edge structures of nanoribbons cannot be controlled, because of the similar stability of different oxidation process, which means zigzag CNTs should be avoided in producing high-quality GNRs. © 2012 The Royal Society of Chemistry.

Vladimir Korepin completed his undergraduate study at Saint Petersburg State University. He was subsequently employed by the Mathematical Institute of Academy of Sciences of Russia. He completed his graduate and postdoctoral studies at this Institute in 1977 and he continued to work there till 1989. His scientific adviser was Ludwig Faddeev. In 1985, he received a doctor of sciences degree in mathematical physics from the Council of Ministers of the Russian Federation.

Here we report the syntheses of low bandgap polymers based on benzobisthiadiazole (BBT), thiadiazoloquinoxaline (TQ) and pyrazinoquinoxaline (PQ) core structures with different aromatic substituents. The effects of changing core structures from BBT to PQ and also substituents from biphenyl and bithienyl on the photophysical, electrochemical and morphology of the polymers were studied. These polymers were incorporated into solar cell devices as donors, with PC[71]BM as acceptors, and their device performances were correlated with their properties. It was found that the effect of these structural changes has significant consequences on the overall device performances. © 2012 The Royal Society of Chemistry.

The unprecedented applications of two-dimensional (2D) atomic sheets in spintronics are formidably hindered by the lack of ordered spin structures. Here we present first-principles calculations demonstrating that the recently synthesized dimethylmethylene-bridged triphenylamine (DTPA) porous sheet is a ferromagnetic half-metal and that the size of the band gap in the semiconducting channel is roughly 1 eV, which makes the DTPA sheet an ideal candidate for a spin-selective conductor. In addition, the robust half-metallicity of the 2D DTPA sheet under external strain increases the possibility of applications in nanoelectric devices. In view of the most recent experimental progress on controlled synthesis, organic porous sheets pave a practical way to achieve new spintronics. © 2012 American Chemical Society.

Although cation-vacancies can induce localized magnetic moments in semiconductors, the collective magnetism is impeded by low vacancy concentration. To improve the vacancy concentration, we study the effect of external hydrostatic strain on the vacancy formation energy. Our first-principles calculations discover that vacancy formation energy is significantly reduced in ionic semiconductors with the monotonic volume contraction, while only slightly decreased in covalent semiconductors. Especially for ZnO, the equilibrium concentration of cation-vacancies has been improved by 10 9 times. We predicted that strain can be used to produce d 0 magnetism in ionic semiconductors much easier in experiments. © 2012 American Institute of Physics.

Boron Carbide is one the hardest and lightest material that is also relatively easier to synthesis as compared to other superhard ceramics like cubic boron nitride and diamond. However, the brittle nature of monolithic advanced ceramics material hinders its use in various engineering applications. Thus, strategies that can toughen the material are of fundamental and technological importance. One approach is to use nanostructure materials as building blocks, and organize them into a complex hierarchical structure, which could potentially enhance its mechanical properties to exceed that of the monolithic form. In this paper, we demonstrated a simple approach to synthesize oneand two-dimension nanostructure boron carbide by simply changing the mixing ratio of the initial compound to influence the saturation condition of the process at a relatively low temperature of 1500°C with no catalyst involved in the growing process. Characterization of the resulting nanostructures shows B 13C 2, which is a superhard phase of boron carbide as its hardness is almost twice as hard as the commonly known B 4C. Using ab-initio density functional theory study on the elastic properties of both B 12C 3 and B 13C 2, the high hardness of B 13C 2 is consistent to our calculation results, where bulk modulus of B 13C 2 is higher than that of B 4C. High resolution transmission electron microscopy of the nanoflakes also reveals high density of twinning defects which could potentially inhibit the crack propagation, leading to toughening of the materials. Copyright © 2012 American Scientific Publishers.

Microelectronic circuits/arrays produced via high-speed printing instead of traditional photolithographic processes offer an appealing approach to creating the long-sought after, low-cost, large-Area flexible electronics. Foremost among critical enablers to propel this paradigm shift in manufacturing is a stable, solution-processable, high-performance semiconductor for printing functionally capable thin-film transistors-fundamental building blocks of microelectronics. We report herein the processing and optimisation of solution-processable polymer semiconductors for thin-film transistors, demonstrating very high field-effect mobility, high on/off ratio, and excellent shelf-life and operating stabilities under ambient conditions. Exceptionally high-gain inverters and functional ring oscillator devices on flexible substrates have been demonstrated. This optimised polymer semiconductor represents a significant progress in semiconductor development, dispelling prevalent skepticism surrounding practical usability of organic semiconductors for high-performance microelectronic devices, opening up application opportunities hitherto functionally or economically inaccessible with silicon technologies, and providing an excellent structural framework for fundamental studies of charge transport in organic systems.

We investigate charge transport across metalmoleculemetal junctions, i.e.hexagonal and triangular nanographene molecular layers sandwiched between Pt and Pd thin films, as measured by ballistic-electron-emission spectroscopy (BEEM). The measured shape of currentvoltage curves cannot be explained in the framework of existing BEEM theories of bulk inorganic semiconductors. We develop a tight-binding model for the BEEM process and propose that the energetic dispersion of molecular layers and the dephasing effect due to the interface states account for the anomalous BEEM currentvoltage behavior and play an important role in determining the shape of the curve. The electronphonon scattering can also affect the shape of currentvoltage curves. © 2012 IOP Publishing Ltd.

Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3 eV in epitaxial graphene. Surprisingly, the resonant exciton occurs at room temperature and for a very large number of graphene layers N≈75, thus suggesting a poor screening in graphene. The optical conductivity (σ1) of a resonant exciton scales linearly with the number of graphene layers (up to at least 8 layers), implying the quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4 eV, which is a mixture of interband transitions from π to π* at the M point and a π plasmonic excitation, is observed. In contrast, for graphite the resonant exciton is not observable but strong interband transitions are seen instead. Supported by theoretical calculations, for N≤ 28 the σ1 is dominated by the resonant exciton, while for N> 28 it is a mixture between exitonic and interband transitions. The latter is characteristic for graphite, indicating a crossover in the electronic structure. Our study shows that important elementary excitations in graphene occur at high binding energies and elucidate the differences in the way electrons interact in graphene and graphite. © 2011 American Physical Society.

Herein we report a facile synthesis method using surfactant cetyltrimethylammonium bromide (CTAB) as a capping agent for controlling the crystal size and morphology of zeolitic imidazolate framework-8 (ZIF-8) crystals in aqueous systems. The particle sizes can be precisely adjusted from ca. 100 nm to 4 μm, and the morphology can be changed from truncated cubic to rhombic dodecahedron. This journal is © The Royal Society of Chemistry.

Disruption of the cellular membrane by the amyloidogenic peptide, islet amyloid polypeptide (IAPP), has been considered as one of the mechanisms of β-cell death during type 2 diabetes. The N-terminal region (residues 1-19) of the human version of IAPP is suggested to be primarily responsible for the membrane-disrupting effect of the full-length hIAPP peptide. However, the detailed assembly mode of hIAPP1-19 with membrane remains unclear. To gain insight into the interactions of hIAPP1-19 oligomer with the model membrane, we have employed coarse-grained molecular dynamics self-assembly simulations to study the aggregation of hIAPP1-19 fragments in the binary lipid made of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and anionic dipalmitoylphosphatidylserine (DPPS) in the presence and absence of different levels of cholesterol content. The membrane-destabilizing effect of hIAPP1-19 is found to be modulated by the presence of cholesterol. In the absence of cholesterol, hIAPP1-19 aggregates prefer to locate inside the bilayer, forming pore-like assemblies. While in the presence of cholesterol molecules, the lipid bilayer becomes more ordered and stiff, and the hIAPP1-19 aggregates are dominantly positioned at the bilayer-water interface. The action of cholesterol may suggest a possible way to maintain the membrane integrity by small molecule interference. © 2011 American Physical Society.

Logic operation is the key of digital electronics and spintronics. Based on spin-dependent transport property of zigzag graphene nanoribbons studied using nonequilibrium Green's function method and density functional theory, we propose a complete set of all-carbon spin logic gates, in which the spin-polarized current can be manipulated by the source-drain voltage and magnetic configuration of the electrodes. These logic gates allow further designs of complex spin logic operations and pave the way for full implementation of spintronics computing devices. © 2011 American Institute of Physics.

The effect of plasma parameters on the nanostructures formed during physical vapor deposition growth of carbon films has been studied. It was shown that the formation and nature of nanostructures strongly depend on plasma density and ion energy during the deposition. High plasma density results in formation of nanocrystals with preferred orientation even at low negative substrate bias (300 V) while at low plasma density higher substrate bias (500 V) is required for the nanocrystals. Moreover, at the same plasma density the nature of the nanostructures strongly depends on the ion energy. At higher ion energies, carbon nanotubes are formed in the microstructure while at lower ion energies, graphitic nanostructures are more stable. It was also found that prior to the formation of preferred orientation, an amorphous layer is formed at the silicon/carbon interface. Through electron energy loss spectroscopy, it is shown that the structure of this layer strongly depends on ion energy during the deposition. © 2010 Elsevier Ltd. All rights reserved.

One-pot, multi-molecular macrocyclization allows the highly selective preparation of pentagon-shaped circular aromatic pentamers mediated by an inward-pointing continuous hydrogen-bonding network. © 2011 The Royal Society of Chemistry.

We report on the electric-field-induced reversible metal-insulator transition (MIT) of the insulating LaAlO3 thin films observed in metal/LaAlO3/Nb-SrTiO3 heterostructures. The switching voltage depends strongly on the thickness of the LaAlO3 thin film which indicates that a minimum thickness is required for the MIT. A constant opposing voltage is required to deplete the charges from the defect states. Our experimental results exclude the possibility of diffusion of the metal electrodes or oxygen vacancies into the LaAlO3 layer. Instead, the phenomenon is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO3 that forms a continuum state with the conduction band of the Nb-SrTiO3. Once this continuum (metallic) state is formed, the state remains stable even when the voltage bias is turned off. The thickness dependent reverse switch-on voltage and the constant forward switch-off voltage are consistent with our model. The viewpoint proposed here can provide an alternative mechanism for resistive switching in complex oxides. © 2011 American Physical Society.

We have done the detailed theoretical investigation by performing calculations for numerous LaAlO3/SrTiO3 heterostructures using the generalized gradient approximation. We find that external electric field can cooperate or compete with spontaneous polarization and effectively modulate band dispersion around the Fermi level. Positive field can induce a reversible insulator-metal transition in insulating heterostructures. Rather than the Ti-dxy interface state that emerges in insulator-metal transition without external field, the pd-anti-bonding state composed of the O-pz and the Ti-d3z2-1 at SrTiO3 surface exchanges its hole with the electron of the O-px,p y orbitals (bonding states) at LaAlO3 surface with the presence of positive field. Associated with insulator-metal transition, there is a remarkable change of local work functions in heterostructure, which suggests that LaAlO3/SrTiO3 heterostructures are the promising candidates for novel nanoscale oxide electronic devices and technology, for instance, the Schottky diodes with adjustable barrier height. © 2011 American Chemical Society.

We demonstrate the information on molecular vibrational modes via the second derivative (d2IB/dV2) of the ballistic electron emission spectroscopy (BEES) current. The proposed method does not create huge fields as in the case of conventional derivative spectroscopy and maintains a zero bias across the device. BEES studies carried out on three different types of large polycyclic aromatic hydrocarbon (PAH) molecular layers show that the d2IB/dV2 spectra consist of uniformly spaced peaks corresponding to vibronic excitations. The peak spacing is found to be identical for molecules within the same PAH family though the BEES onset voltage varies for different molecules. In addition, injection into a particular orbital appears to correspond to a specific vibrational mode as the manifestation of the symmetry principle. © 2011 IOP Publishing Ltd.

Described in this study is a conceptually new class of five-fold-symmetric cavity-containing planar pentameric macrocycles with their interior decorated by five convergently aligned, properly spaced carbonyl oxygen atoms. These cation-binding oxygens enclose a hydrophilic lumen of 2.85 Å in radius and thus display high-affinity binding toward alkali metal cations, and possibly many other cations, too. Arising from their high-affinity recognition of metal ions, these planar macrocycles form cation- or ion-pair-induced one-dimensional columnar aggregates, and subsequently fascinating fibrillation results. © 2011 American Chemical Society.

The dynamics of structure evolution of nanodiamonds ranging from 22 to 318 atoms of various shapes is studied by density functional tight-binding molecular dynamics. The spherical and cubic nanodiamonds can be transformed into fullerene-like structures upon heating. A number of the transformed fullerenes consist of pentagons and hexagons only. Others contain squares, heptagons, and octagons. One simulated fullerene is an isomer of C60. The temperature of the transformation depends on the size, shape, and orientation of initial cluster. To be transformed into onion-like fullerenes, the spherical nanodiamonds should have 200 atoms or more, while the cubic ones require 302 atoms or more. The time-resolved energy profiles of all the transformations clearly reveal three-stage transformation character. During the first stage, the energy reduces quickly due to converting sp3 carbon with dangling bond at the surface into sp2 one, and the formation of partial sp2 envelope wrapping the cluster. For the second stage, energy decreases slowly. The remaining interior carbon atoms come to the surface through the hole in the sp2 envelope, and similar amount of sp 3 and sp2 atoms coexist. The third stage involves the closure of holes, accompanied by the detachment of C2 molecules and carbon chains from the edges. The energy decreases relatively fast in this stage. The proposed three-stage transformation pathway holds for all the simulations performed in this work, including those with the instant heating. © 2011 American Chemical Society.

Recent experimental study revealed the optical conductivity of La 1-xCaxMnO3 over a wide range of energy and the occurrence of spectral weight transfer as the system transforms from a paramagnetic insulating to a ferromagnetic metallic phase. We propose a model and calculation within the dynamical mean-field theory to explain this phenomenon. We find the role of oxygens in mediating the hopping of electrons between manganeses as the key that determines the structures of the optical conductivity. In addition, by parametrizing the hopping integrals through magnetization, our result suggests a possible scenario that explains the occurrence of spectral weight transfer, in which the ferromagnetic ordering increases the rate of electron transfer from O2p orbitals to upper Mneg orbitals while simultaneously decreasing the rate of electron transfer from O2p orbitals to lower Mneg orbitals, as temperature is varied across the ferromagnetic transition. With this scenario, our optical conductivity calculation shows very good quantitative agreement with the experimental data. © 2011 American Physical Society.

Water-binding foldamers have been rarely studied. By orienting both H-bond donors and acceptors toward their interior, two pyridine-derived crescent-shaped folding oligoamides were found to be capable of trapping both conventional and unconventional water dimer clusters in their cavity (∼2.5 Å radius). In the unconventional water dimer cluster, the two water molecules stay in contact via an unusual H-H interaction (2.25 Å) rather than the typical H-bond. © 2011 American Chemical Society.

Two anomalous bias dependent resistive peaks induced by the SrTiO 3 structural phase transitions at 55 and 110 K were observed in a LaAlO3/SrTiO3 and Nb:SrTiO3 rectifying junction when the LaAlO3/SrTiO3 was depleted under reverse bias. At these transition temperatures, the barrier between LaAlO3/ SrTiO3 and Nb:SrTiO3 showed abrupt changes in the tunneling energy under forward bias. The peak at 110 K was an insulator-metal phase transition while the peak at 55 K was a metal-insulator one. We propose that the phase transitions of the SrTiO3 substrate influence the charge transfer to the LaAlO3/SrTiO3 layer, giving rise to these anomalous resistive peaks. © 2011 American Institute of Physics.

A series of forcible curved circular DNAs (cDNAs) were prepared to investigate the recognition features of human Topoisomerase I (hTopo I). The IC 50 can be modulated by the curvature degrees of cDNA. In addition, preferential bindings of hTopo I to cDNA with high curvature have been observed by AFM and EMSA. © 2011 The Royal Society of Chemistry.

Regular pentagonal adhesive tiles were obtained by designing a cyclic fluoropentamer with an internal CF⋯HN H-bonded network and "sticky" edges that can glue the molecules together through intermolecular CArH⋯O=C H bonds. In the crystal they form the mathematically predicted densest all-pentagon 2D lattice with a packing density of 0.921 (see picture). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

It is demonstrated that the right and left handedness of DNA supercoils can be engineered precisely and readily at the molecular level in vitro through utilization of the invading property of peptide nucleic acid. © 2011 The Royal Society of Chemistry.

New insights into understanding and controlling the intriguing phenomena of spontaneous merging (kissing) and the self-assembly of monolithic Y- and T-junctions is demonstrated in the metal-organic chemical vapor deposition growth of GaAs nanowires. High-resolution transmission electron microscopy for determining polar facets was coupled to electrostatic-mechanical modeling and position-controlled synthesis to identify nanowire diameter, length, and pitch, leading to junction formation. When nanowire patterns are designed so that the electrostatic energy resulting from the interaction of polar surfaces exceeds the mechanical energy required to bend the nanowires to the point of contact, their fusion can lead to the self-assembly of monolithic junctions. Understanding and controlling this phenomenon is a great asset for the realization of dense arrays of vertical nanowire devices and opens up new ways toward the large scale integration of nanowire quantum junctions or nanowire intracellular probes. © 2011 American Chemical Society.

Electrochemical impedance spectroscopy measurements of pulsed laser deposited single crystal anatase TiO2 thin films with various concentrations of Ta substituting for Ti were carried out. The qualities of the films were characterized by X-ray diffraction and Rutherford back scattering-channeling measurements. UV-visible measurements show a systematic increase of the bandgap with Ta incorporation. Corresponding Mott-Schottky plot was applied to obtain a continuous shift of the flat band potential with increasing free charge carrier (provided by Ta) concentration. This was verified theoretically by ab initio calculation which shows that extra Ta d-electrons occupy Ti t2g orbital with increasing Ta concentration, thereby pushing up the Fermi level. The Mott-Schottky results were consistent when compared with Hall effect and temperature dependent resistivity measurements. From the measured deviation of carrier densities from Hall and Mott-Schottky measurements we have estimated the static dielectric constant of the TiO 2 as a function of Ta incorporation, not possible from capacitive measurements. © 2011 Author(s).

We report the synthesis and characterization of pyrazino[2,3-g]quinoxaline derivatives with a systematic change in the substituents at the 2, 3, 7 and 8 positions and with or without 2-thienyl at the 5 and 10 positions to study the substituent effect and quinoid character of such a system. We performed density functional theory calculations using the B3LYP functional and 6-31G* basis set under the geometry optimization condition, together with the resonance effect of a valence bond theory to understand the electronic structures of these molecules. It was found that a combination of conjugation and cross-conjugation effects is responsible for the observed trends in their electronic properties, thus giving insights into designing molecules utilizing such effects. © 2011 The Royal Society of Chemistry.

Molecular dynamics simulations of aquaporin Z homotetramer which is a membrane protein facilitating rapid water movement through the plasma membrane of Escherichia coli were performed. Initial configurations were taken from the open and closed states of crystal structures separately. The resulting water osmotic permeability (pf) and diffusive permeability (pd) displayed distinct features. Consistent with previous studies, the side chain conformation of arginine189 was found to mediate the water permeability. A potential of mean force (PMF) as a function of the distance between NH1 of R189 and carbonyl oxygen of A117 was constructed based on the umbrella sampling technique. There are multiple local minima and transition states on the PMF. The assignment of the open or closed state was supported by the permeability pf, calculated within trajectories in umbrella sampling simulations. Our study disclosed a detailed mechanism of the gated water transport.

We apply density functional theory to study the optical properties of armchair graphene nanoribbons (AGNRs) functionalized from both edge and surface employing quasiparticle corrections and many body, that is, electron-hole, interactions. The variation in band gaps is scrutinized in terms of bonding character and the localization of wave functions. We have determined the family and functionalization dependence of quasiparticle correction, absorption spectrum, exciton binding strength, and its wave function's spatial characteristics. In particular, all the excitons exhibit non-Frenkel character that results from the interplay among the extended π electron states and weakly screened columbic interactions. The functionalized AGNRs hold promising potential in optoelectronic applications. © 2010 American Chemical Society.

Impact ionization and Auger recombination in nanoscale spherical quantum dots (QDs) have been Studied theoretically. It is shown that due to the strong quantum confinement of both electrons in the conduction band and holes ill the valence band. impact ionization and Auger recombination energies in these QDs call be oil the order of a few millielectronvolts when various selection rules are fulfilled, which are Much higher than spontaneous radiative emission energies. This explains the experimentally reported high Occurrence rates of the multiple exciton generation (MEG) effect in QDs. However, due to quantum confinement, the energy States are discrete in QDs, especially for low-energy states where the densities of states are low. This implies that only a limited number of high-energy electron states call interact with (i.e., impact ionize) low-energy hole states in QDs having certain values of radii due to the energy conservation requirement. This explains the vastly scattered experimental data and difficulties ill Utilizing the MEG effect in practice., QC 20100525

Using density functional theory calculations combined with nonequilibrium Green's function method, we report the electronic, magnetic, and transport properties of iron-cyclooctatetraene (Fe-COT) sandwich clusters. Four Fe n COTn+1 (n = 1-4) clusters with linear sandwich structure are highly stable because of the strong Fe-COT coupling. The ground state of Fe-COT clusters is ferromagnetic, Fe atoms couple ferromagnetically to the neighboring COT rings, and the large total magnetic moments increase with the number of Fe atoms. The spin-polarized transport calculations indicate that Fe-COT clusters coupled to gold electrodes act as nearly perfect spin-filters. The revealed properties indicate that the Fe-COT clusters would be ideal materials for promising molecular spintronics. © 2010 American Chemical Society.

Nano-size carbon particles such as fullerenes, carbon onions and nanodiamond represent the 0-D analoges of three-dimensional sp2 -bonded graphite and sp3 -bonded diamond. Recently it has been found that upon heating 0-D nanodiamonds transform into carbon onions 1′2. Such finding promt questions: whether similar changes of structure and hybridization observed in 0-D nanodiamond are to be expected in 1-D. Will diamond nanowires transform into single-walled nanotubes or multi-walled nanotubes? © 2010 IEEE.

In this work, we illustrated a novel design strategy that allows systematically tunable interior properties (effective cavity size, steric crowdedness, and hydrophobicity) contained within a novel class of shape-persistent aromatic pentamers to take place on a scale below 3 Å. Such finely tunable structural features are complimented by experimentally observable functional variations in ion-binding potential. Results of the selective, differential binding affinities of three circular pentamers for Li+, Na+, K+, Rb+, and Cs +, substantiated by metal-containing crystal structures and computational modeling, are detailed. © 2010 American Chemical Society.

The symbolic method (including normal ordering. antinormal ordering and Weyl ordering symbol) is usually utilized to tackle miscellaneous operators which have different commutative relations. Considering the Weyl ordering symbol's remarkable properties, we have efficiently and conveniently derived the Wigner distribution function for field damping in a squeezed bath and a vacuum bath respectively, and then examined the decoherence processes from the plots of Wigner function and its contour in quantum phase space. Alternatively, we can employ a general Wigner operator under phase space transform to calculate distribution function and discuss the damping process. © 2010 Springer Science+Business Media, LLC.

Small nanodiamonds (22-172 atoms) of various shapes were studied by tight-binding molecular dynamics. It was found that spherical and cubic nanodiamonds with number of atoms more than 40 formed closed-shaped fullerene-like structures under graduate heating. The smallest fullerene-like structure obtained in the simulations contained 35 atoms. One of the observed fullerenes was an isomer of the C60 fullerene and had a shape which is close to spherical one. The temperature and the time of the transformation depended on the size of the nanodiamod. The heating and cooling rate also influenced the final structure and a number of distortions in the final fullerene. The mechanism of the transformation was studied. Stability of the fullerenes was confirmed by DFT optimization. Diamond nanowires of 5-8 Å in diameter also can transform into damaged CNT under heating. ©2010 IEEE.

Biasing the conformational preferences of aromatic oligoamides by internally placing intramolecular hydrogen bonds has led to a series of stably folded molecular strands. This article presents the results from extensive solid-state, solution, and computational studies on these folding oligomers. Depending on its backbone length, an oligoamide adopts a crescent or helical conformation. Surprisingly, despite the highly repetitive nature of the backbone, the internally placed, otherwise very similar intramolecular hydrogen bonds showed significantly different stabilities as demonstrated by hydrogen-deuterium exchange data. It was also observed that the hydrogen-bonding strength can be tuned by adjusting the substituents attached to the exterior of the aromatic backbones. Examining the amide hydrogen-deuterium exchange rates of trimers revealed that a six-membered hydrogen bond nearing the ester end is the weakest among all the four intramolecular hydrogen bonds of a molecule. This observation was verified by ab initio quantum mechanical calculations at the level of B3LYP/6-31G. Such a "weak point" creates the "battle of the bulge" where backbone twisting is centered, which is consistently observed in the solid-state structures of the four trimer molecules studied. In the solid state, the oligomers assemble into interesting one-dimensional structures. A pronounced columnar packing of short oligomers (i.e., dimers, trimers, and tetramer) and channel-like, potentially ion-conducting stacks of longer oligomers (i.e., tetramer, pentamer, and hexamer) were observed. © 2010 American Chemical Society.

Impact ionization and Auger recombination in nanoscale spherical quantum dots (QDs) have been studied theoretically. It is shown that due to the strong quantum confinement of both electrons in the conduction band and holes in the valence band, impact ionization and Auger recombination energies in these QDs can be on the order of a few millielectronvolts when various selection rules are fulfilled, which are much higher than spontaneous radiative emission energies. This explains the experimentally reported high occurrence rates of the multiple exciton generation (MEG) effect in QDs. However, due to quantum confinement, the energy states are discrete in QDs, especially for low-energy states where the densities of states are low. This implies that only a limited number of high-energy electron states can interact with (i.e., impact ionize) low-energy hole states in QDs having certain values of radii due to the energy conservation requirement. This explains the vastly scattered experimental data and difficulties in utilizing the MEG effect in practice. © 2010 American Chemical Society.

Electronic structures, magnetic, and transport properties of organometallic sandwich molecular wire (SMW) constructed with altering europium (Eu) atoms and cyclooctatetraene (COT) rings have been studied by spin-polarized first-principles calculations and nonequilibrium Green's function method. The one-dimensional Eu-COT SMW is predicted to be a ferromagnetic semiconductor. The calculated average magnetic moment per Eu atom is about 7 μB. The total magnetic moment (TMM) of Eun (COT)n+1 (n=1-4) sandwich clusters increases linearly with the number of Eu atoms and the linear relation TMM=7n-2 holds, which agrees nicely with experimental observations. The spin transportation calculations show that Eu-COT clusters coupled to gold electrodes are nearly perfect spin filters. In addition, we find that this large transmission spin polarization is very robust regardless of the contact configuration of the junctions, which is very important for the future molecular spintronics. © 2009 American Institute of Physics.

We study a triangular frustrated antiferromagnetic Heisenberg model with nearest-neighbor interactions J1 and third-nearest-neighbor interactions J3 by means of Schwinger-boson mean-field theory. By setting an antiferromagnetic J3 and varying J1 from positive to negative values, we disclose the low-temperature features of its interesting incommensurate phase. The gapless dispersion of quasiparticles leads to the intrinsic T2 law of specific heat. The magnetic susceptibility is linear in temperature. The local magnetization is significantly reduced by quantum fluctuations. We address possible relevance of these results to the low-temperature properties of NiGa2S 4. From a careful analysis of the incommensurate spin wavevector, the interaction parameters are estimated as J1≈-3.8755K and J 3≈14.0628K, in order to account for the experimental data. © 2009 IOP Publishing Ltd.

The robust magnetic state of recently synthesized hybrid carbon nanostructures, i.e., nanobuds, is predicted through comprehensive spin-polarized density-functional calculations. The effects of chirality, curvature, and topology on the magnetism of nanobuds are scrutinized by detailed electronic structure analysis. The substantial emergent amounts of unpaired spins originate in the presence of carbon radicals introduced by the geometry-induced electronic frustration. The location of radicals is mainly on the nanotube surface within the connecting region with fullerene, rather than surfaces with negative Gaussian curvature. The magnetic nanobuds hold great promise in the field of spintronics owing to their ready accessibility by experimental synthesis and fabrication. © 2009 The American Physical Society.

We report first-principles calculations of the electronic and transport properties of diarylethene molecules sandwiched between metallic armchair (5, 5) carbon nanotubes with open and closed configurations. The calculated transmission spectra of two configurations are distinctively different near the Fermi level, and their profiles can be tuned by the gate bias voltage. The on-off ratio of currents between the closed and open configurations under a gate bias voltage is predicted to be more than two orders of magnitude, which reproduces the essential features of the experimental results. Moreover, we find that the switching property of diarylethene molecular junctions is very robust to the anchoring configuration, substituent of cyclopentene, and choice of electrodes. © 2009 Elsevier B.V. All rights reserved.

A hydroxyl Schiff base molecule was synthesized from tetraethylenetetramine and 4-nitrosalicylaldehyde and explored as an ionophore with ionic liquid, cation excluder in PVC membranes showing remarkable sulfate selectivity; the optimal recognition site was predicted by density function theory (DFT) simulation. © The Royal Society of Chemistry.