Induced Charge Redistribution Research Articles

Interfacial defects induced electronic property transformation at perovskite SrVO3/SrTiO3 and LaCrO3/SrTiO3 heterointerfaces.

Unravelling the atomic structure and chemical species of interfacial defects is critical to understanding the origin of interfacial properties in many heterojunctions. Here, by combining advanced transmission electron microscopy, spectroscopy and first-principles calculations, we demonstrate interfacial Ti diffusion in SrVO3/SrTiO3 and LaCrO3/SrTiO3 heterointerfaces and uncover that the interfacial defects induce a significant change in electronic properties by showing an electronic transformation from the insulating state to metallic state at SrVO3/SrTiO3 heterointerfaces due to the hybridization of interfacial Ti d, O p and V d, and a metallic to insulating state transformation at LaCrO3/SrTiO3 because of Ti-Cr mixing induced charge redistribution in the interfacial layer.

Mechanical and electronic coupling in few-layer graphene and hBN wrinkles: a first-principles study

Wrinkle engineering is an important pathway to develop novel functional devices of two-dimensional materials. By combining first-principles calculations and continuum mechanics modelling, we have investigated the wrinkling of few-layer graphene and hexagonal boron nitride (hBN) and provide a way to estimate their bending stiffness. For few-layer wrinkles under the same strain, the magnitude of structural deformation of each constituent layer gradually decreases from bottom to top layers, while interlayer interaction increases with increasing layer number. Comparing with monolayer wrinkles, the electronic properties of few-layer wrinkles are more sensitive to bending deformation as mechanical and electronic coupling induce charge redistribution at the wrinkles, making few-layer graphene and hBN wrinkles suitable for electromechanical system application.

Van der Waals interactions and the limits of isolated atom models at interfaces.

Van der Waals forces are among the weakest, yet most decisive interactions governing condensation and aggregation processes and the phase behaviour of atomic and molecular matter. Understanding the resulting structural motifs and patterns has become increasingly important in studies of the nanoscale regime. Here we measure the paradigmatic van der Waals interactions represented by the noble gas atom pairs Ar–Xe, Kr–Xe and Xe–Xe with a Xe-functionalized tip of an atomic force microscope at low temperature. Individual rare gas atoms were fixed at node sites of a surface-confined two-dimensional metal–organic framework. We found that the magnitude of the measured force increased with the atomic radius, yet detailed simulation by density functional theory revealed that the adsorption induced charge redistribution strengthened the van der Waals forces by a factor of up to two, thus demonstrating the limits of a purely atomic description of the interaction in these representative systems.

A DFT study of atomic geometry and electronic structures for oxidized Al0.25Ga0.75N (0 0 1) (2 × 2) reconstruction surfaces

In order to study formation energies, atomic geometry, electronic structures, and surface states of oxidized Al0.25Ga0.75N (001) (2×2) reconstruction surfaces, models of Al0.25Ga0.75N (001) (2×2) reconstruction surfaces with different coverages of oxygen atoms were built, and first-principles calculations were performed based on density functional theory (DFT). The results of these calculations show that formation energy of the oxidized Al0.25Ga0.75N (001) (2×2) reconstruction surfaces decreases as the increasing of O coverage under O-rich conditions. O atom prefers to locate on the top of GaGa and GaAl dimers. After relaxation, the length values of GaO and AlO bonds are 1.856Å and 1.853Å, respectively. With smaller electronegativity values, Ga and Al atoms donate electrons to O atom, and the dangling bonds of Ga and Al atoms on the surface were occupied. At Fermi level, the partial density of states (PDOS) of Ga and Al atoms on the topmost surface decrease significantly after Ga and Al atoms bond with O atoms, showing that surface states disappear. O atoms adsorption or replacement on Al0.25Ga0.75N (001) (2×2) reconstruction surface induces charge redistribution, which in turn forms Ga(Al)O dipoles oriented to O atoms. With Ga(Al)O dipoles, work functions of oxidized surfaces are larger than that of clean surface, and the work function increases as O coverage increases.

Surface-dependence of interfacial binding strength between zinc oxide and graphene

Density functional theory calculations have revealed strong surface-dependence of the binding strength between ZnO and graphene, attributed to induced charge redistribution owing to the in-plane and out-of-plane dipole moment of the oxide surface.

Chemical reactivity and band-gap opening of graphene doped with gallium, germanium, arsenic, and selenium atoms.

Herein, the effects of substitutional doping of graphene with Ga, Ge, As, and Se are shown. Ge exhibits the lowest formation energy, whereas Ga has the largest one. Ga- and As-doped graphene display a reactivity that is larger than that corresponding to a double vacancy. They can decompose H2 and O2 easily. Variation of the type and concentration of dopant makes the adjustment of the interlayer interaction possible. In general, doping of monolayer graphene opens a band gap. At some concentrations, Ga doping induces a half metallic behavior. As is the element that offers the widest range of gap tuning. Heyd-Scuseria-Ernzerhof calculations indicate that it can be varied from 1.3 to 0.3 eV. For bilayer graphene, the doped sheet induces charge redistribution in the perfect underneath sheet, which opens a gap in the range of 0.05-0.4 eV. This value is useful for developing graphene-based electronics, as the carrier mobility of the undoped sheet is not expected to alter.

Simultaneous deposition of cerium oxide and gold nanostructures-characterization and analytical properties toward glucose electro-oxidation and sensing

Here, we present for the first time an Au–CeO2 nanocomposite synthesized in the presence of a mild pressure of oxygen (1.33 Pa) by the cross-beam laser deposition technique. The deposit was grown on a conductive (current collector) substrate composed of a porous network of carbon microfibers. The structure and morphology of the deposit were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XPS analysis demonstrated that the interaction of Au with CeO2 during deposition induced charge redistribution in the ceria nanoparticles (Ce3+,4+), resulting in positively charged Au species (Au3+). TEM analysis revealed that the basal planes of the carbon fibers are coated with a monolayer of interconnected nanoparticles, whereas their outer walls are covered with an ultra-thin film (∼40 nm thickness) formed of nanoparticles gathering into columns. The direct electrocatalytic oxidation of glucose in a neutral medium, such as phosphate buffered saline (pH = 7.2) solution, has been investigated with cyclic voltammetry, chronoamperometry and square-wave voltammetry. Sensitivity as high as 57.5 μA cm−2 mM−1 up to 10 mM of glucose and a low detection limit of 10 μM were obtained with square-wave voltammetry. Such electrocatalytic activity is because of the presence of cationic gold (electronic effect) and the size reduction of gold particles (size effect). This interesting analytical performance in addition to the low loading of gold (100 μg cm−2) makes the laser fabricated Au–CeO2 nanocomposite electrode potentially promising for glucose fuel cells, assessment of glucose in biological fluids, as well as in the analysis of food and beverages.

Tuning the Properties of Pd Nanoclusters by Ligand Coatings: Electronic Structure Computations on Phosphine, Thiol, and Mixed Phosphine–Thiol Ligand Shells

Tuning the properties of palladium nanoparticles using different protecting ligand shells is an important step toward the application-orientated design of nanoparticles for nanoelectronics and catalysis. We present a density functional theoretical characterization of Pd13 and Pd55 metal cores protected by only thiol, only phosphine, and mixed phosphine–thiol ligand shells. We analyze the ligand contributions to the frontier orbitals and the charge redistribution between the ligand shell and the metal core and show that these properties control the values of the charging energy and the catalytic activity. The charge transfer character of the metal–ligand interaction is influenced by the presence of other ligands in the capping system indicating a cooperative effect in the ligand induced charge redistribution. Because of the interplay between the stabilization of the frontier orbital due to the contribution of the sulfur and the charge donation by the phosphine, the charging energy of the mixed phosphine–thiol protected cluster is larger than that of the only phosphine and the only thiol systems. The complementary point of view is adopted for rationalizing the catalytic properties of the clusters by analyzing the effect of the interaction with the metallic core on the properties of the ligand. The impact of solvation on the electronic structure of the ligand capped Pd13 cluster is investigated by including explicitly a layer of water molecules in the model system.

Improved Interaction of Hydrogen on Transition-Metal-Doped Al(100) Stepped Surface

Using the density functional theory with generalized gradient approximation, we have studied in detail the enhanced hydrogen interaction with a transition-metal-doped Al(100) stepped surface. Judging from the calculated total energies, the early transition metals prefer to dope at the lower edge sites of a surface step. The Sc, Ti, and V donate electrons, whereas the Cr and Mn gain electrons. The low energy costs for activating both the H2 splitting and the H atomic diffusion show improved catalytic performances. In the transition states, hydrogen would bond to both transition metal and Al atoms for H2 splitting on Sc- and Ti-doped surfaces, whereas it would only develop a rather weak interaction with the metals in the other studied materials. The charge transfer results in a 0.8 e charge gain and 0.4 A increase in bond length of H2, facilitating H2 dissociation on Sc- and Ti-doped surfaces. However, in the other studied materials, the presence of hydrogen only induces charge redistribution, resulting in ...

Feasibility of occurrence of different types of protonated base pairs in RNA: a quantum chemical study.

Protonated nucleobases have significant roles in facilitating catalytic functions of RNA, and in stabilizing different structural motifs. Reported pKa values of nucleobase protonation suggest that the population of neutral nucleobases is 10(3)-10(4) times higher than that of protonated nucleobases under physiological conditions (pH ∼ 7.4). Therefore, a molecular level understanding of various putative roles of protonated nucleobases cannot be achieved without addressing the question of how their occurrence propensities and stabilities are related to the free energy costs associated with the process of protonation under physiological conditions. With water as the proton donor, we use advanced QM methods to evaluate the site specific protonation propensities of nucleobases in terms of their associated free energy changes (ΔGprot). Quantitative follow up on the energetics of base pair formation and database search for evaluating their occurrence frequencies, reveal a lack of correlation between base pair stability and occurrence propensities on the one hand, and ease of protonation on the other. For example, although N7 protonated adenine (ΔGprot = 40.0 kcal mol(-1)) is found to participate in stable base pairing, base pairs involving N7 protonated guanine (ΔGprot = 36.8 kcal mol(-1)), on geometry optimization, converge to a minima where guanine transfers its extra proton to its partner base. Such observations, along with examples of weak base pairs involving N3 protonation of cytosine (ΔGprot = 37.0 kcal mol(-1)) are rationalized by analysing the protonation induced charge redistributions which are found to significantly influence, both positively and negatively, the hydrogen bonding potentials of different functional sites of individual nucleobases. Protonation induced charge redistribution is also found to strongly influence (i) the aromatic character of the rings of the participating bases and (ii) hydrogen bonding potential of the free edges of the protonated base pair. Comprehensive analysis of a non-redundant RNA crystal structure dataset further reveals that, while availability of stabilization possibilities determine the feasibility of occurrence of protonated bases, their occurrence context and specific functional roles are important factors determining their occurrence propensities.

Adsorption of β-d-glucose and cellobiose on kaolinite surfaces: Density functional theory (DFT) approach

In this study, we investigate the kaolinite surfaces and their interaction with β-d-glucose and cellobiose using density functional theory calculations. We found that their molecular adsorption energy on kaolinite depends on i) the characteristics of the kaolinite surfaces such as the hydroxylated (001) surface or the siloxane 001¯ surface and ii) a molecular orientation of the monomer on the surface. The adsorption energy of the β-d-glucose and the cellobiose on the hydroxylated (001) surface are significantly greater (almost 200%) than that on the siloxane 001¯ surface since the hydroxyl group can form hydrogen bond more efficiently than the oxygen in siloxane group. Through Mulliken population analysis, we found that the hydrogen bond formation induces charge redistribution of the kaolinite surfaces. Therefore, the hydroxylated (001) surface undergoes more significant charge redistribution due to more hydrogen bond formation with adsorbate molecules in comparison to the siloxane 001¯ surface.

Selective sensing of volatile organic compounds using novel conducting polymer–metalnanoparticle hybrids

Conducting polymer–metal nanoparticle hybrids, fabricated by assembling metalnanoparticles on top of functionalized conducting polymer film surfaces using conjugatedlinker molecules, enable the selective sensing of volatile organic compounds (VOCs). Inthese conducting polymer–metal nanoparticle hybrids, selectivity is achieved by assemblingdifferent metals on the same conducting polymer film. This eliminates the need to developeither different polymers chemistries or device configurations for each specific analyte. Inthe hybrids, chemisorption of the analyte vapor induces charge redistribution inthe metal nanoparticles and changes their work function. The conjugated linkermolecule causes this change in the work function of the tethered nanoparticlesto affect the electronic states in the underlying conducting polymer film. Theresult is an easily measurable change in the resistance of the hybrid structure.The fabrication of these sensing elements involved the covalent assembly of nickel (Ni) and palladium(Pd) metal nanoparticles on top of poly(3,4-ethylenedioxythiophene-co-thiophene-3-aceticacid), poly(EDOT-co-TAA), films using 4-aminothiophenol linker molecules. The change inresistance of hybrid Pd/poly(EDOT-co-TAA) and Ni/poly(EDOT-co-TAA) hybrid films toacetone and toluene, respectively, is observed to be in proportion to their concentrations.The projected detection limits are 2 and 10 ppm for toluene and acetone, respectively. Anegligible response (resistance change) of the Pd/poly(EDOT-co-TAA) films to tolueneexposure confirmed its selectivity for detecting acetone. Similarly, lack of response toacetone confirmed the selectivity of the Ni/poly(EDOT-co-TAA) stacks for detectingtoluene. It is anticipated that the assembly of other metals such as Ag, Au and Cu on topof poly(EDOT-co-TAA) would provide selectivity for detecting and discriminating otherVOCs.

Active role of buried ultrathin oxide layers in adsorption of O 2 on Au films

Ultrathin oxide layers can exhibit special behavior by enabling the coupling of structural distortions and charge transfer beyond that allowed in the bulk. In this work, we show from first-principles calculations that ultrathin layers of titania, a prototypical oxide, are active in stabilizing adsorption of O 2 on Au overlayers. The adsorbed O 2 molecules induce charge redistribution in Au that penetrates to the Au–titania interface, which responds through structural distortions that lower the total energy of the system. We suggest that this effect may be of more general nature and useful in catalysis.

Thermal expansion and compressibility in superconductingNaxCoO2∙4xD2O(x≈1∕3): Evidence for pressure-induced charge redistribution

We have performed thermal expansion and compressibility measurements on the recently discovered superconducting material NaxCoO2*4xD2O (x=1/3) using neutron powder diffraction over the temperature range 10-295 K and the pressure range 0-0.6 GPa. Pressure measurements were done in a helium-gas pressure cell. Both the thermal expansion and compressibility are very anisotropic, with the largest effects along the c axis, as would be expected for a layered material with weak hydrogen bonding nominally along the c axis. Near room temperature, the anisotropies of the thermal expansion and compressibility of the hexagonal crystal structure are nearly the same [(Dc/c)/(Da/a)=3-4], with a 100 C change in temperature being roughly equivalent to 0.2 GPa pressure. This would imply that changes in atom position parameters are also the same, but this is not the case. While the effects of temperature on the atom positions are essentially what one might expect, the effects of pressure are surprising. With increasing pressure, the thickness of the CoO2 layer increases, due to the combined effects of an increasing Co-O bond length and changes in the O-Co-O angles of the CoO6 octahedra. We conclude that this unusual effect results from pressure-induced strengthening of the hydrogen bonding between the Nax(D2O)4x layers and the CoO2 layers. The strengthening of these hydrogen bonds requires that charge be moved from the somewhere else in the structure; hence, there is a pressure induced charge redistribution that weakens (lengthens) the Co-O bonds and changes the electronic structure of the superconducting CoO2 layers.

Effect of Li doping and Li, La co-doping on the structure and physical properties of LaBa2Cu3Oy

Polycrystalline samples of LaBa 2 Cu 3− x Li x O y (0⩽ x ⩽0.7) and La 1+ x Ba 2− x Cu 3− x Li x O y (0⩽ x ⩽0.7) were synthesized by the solid-state reaction method. The structure and physical properties were investigated by means of X-ray diffraction (XRD), resistivity, infrared (IR) transmission spectra and electronic spin resonance (ESR). With Li doping, an orthorhombic–tetragonal transition takes place as 0.3< x ⩽0.5. While for La 1+ x Ba 2− x Cu 3− x Li x O y , the system remains orthorhombic structure within the entire doping range. Li doping and Li, La co-doping induce charge redistribution in the two systems and the single Li doping causes Cu 2+ spins to localize, especially for the x ⩾0.5 samples. The changes of the IR and ESR spectra are analysed and discussed in detail.

Effect of Li doping and Li, La co-doping on the structure and physical properties of LaBa 2Cu 3O y

Polycrystalline samples of LaBa 2Cu 3− x Li x O y (0⩽ x⩽0.7) and La 1+ x Ba 2− x Cu 3− x Li x O y (0⩽ x⩽0.7) were synthesized by the solid-state reaction method. The structure and physical properties were investigated by means of X-ray diffraction (XRD), resistivity, infrared (IR) transmission spectra and electronic spin resonance (ESR). With Li doping, an orthorhombic–tetragonal transition takes place as 0.3< x⩽0.5. While for La 1+ x Ba 2− x Cu 3− x Li x O y , the system remains orthorhombic structure within the entire doping range. Li doping and Li, La co-doping induce charge redistribution in the two systems and the single Li doping causes Cu 2+ spins to localize, especially for the x⩾0.5 samples. The changes of the IR and ESR spectra are analysed and discussed in detail.

Field penetration induced charge redistribution effects on the field emission properties of carbon nanotubes—a first-principle study

The effect of field penetration induced charge redistribution on the field emission properties of carbon nanotubes (CNTs) have been studied by the first-principle calculations. It is found that the carbon nanotube becomes polarized under external electric field leading to a charge redistribution. The resulting band bending induced by field penetration into the nanotube tip surface can further reduce the effective workfunction of the carbon nanotubes. The magnitude of the redistributed charge Δ Q is found to be nearly linear to the applied external field strength. In addition, we found that the capped (9, 0) zigzag nanotube demonstrates better field emission properties than the capped (5, 5) armchair nanotube due to the fact that the charge redistribution of π electrons along the zigzag-like tube axis is easier than for the armchair-like tube. The density of states (DOS) of the capped region of the nanotube is found to be enhanced with a value 30% higher than that of the sidewall part for the capped (5, 5) nanotube and 40% for the capped (9, 0) nanotube under an electric field of 0.33 V/Å. Such enhancements of the DOS at the carbon nanotube tip show that electrons near the Fermi level will emit more easily due to the change of the surface band structure resulting from the field penetration in a high field.

Mutation induced modulation of hydrogen bonding to P700 studied using FTIR difference spectroscopy.

Site-directed mutagenesis in combination with Fourier transform infrared difference spectroscopy has been used to study how hydrogen bonding modulates the electronic and physical organization of P700, the primary electron donor in photosystem I. Wild-type PS I particles from Chlamydomonas reinhardtii and a mutant in which ThrA739 is changed to alanine [TA(A739) mutant] were studied. ThrA739 is thought to provide a hydrogen bond to the chlorophyll-a' molecule of P700 (the two chlorophylls of P700 (P700(+)) will be called P(A) and P(B) (P(A)(+) and P(B)(+))). The mutation considerably alters the (P700(+)-P700) FTIR difference spectra. However, we were able to describe all of the mutation induced changes in the difference spectra in terms of difference band assignments that were proposed recently (Hastings, G., Ramesh, V. M., Wang, R., Sivakumar, V. and Webber, A. (2001) Biochemistry 40, 12943-12949). Upon comparison of mutant and wild type (P700(+)-P700) FTIR difference spectra, it is shown that (1) the 13(3) ester carbonyl modes of P(A) and P(B) are unaltered upon mutation of ThrA739 to alanine. (2) The 13(3) ester carbonyl modes of P(A)(+)/P(B)(+) upshift/downshift upon mutation. These oppositely directed shifts indicate that the mutation modifies the charge distribution over the pigments in the P700(+) state, with charge on P(B) being relocated onto P(A). We also show that the 13(1) keto carbonyl mode of P(B)/P(B)(+) is unaltered/downshifted upon mutation, as is expected for the above-described mutation induced charge redistribution in P700(+). Although the 13(3) ester carbonyl modes of the chlorophylls of P700 in the ground state are unaltered upon mutation, the 13(1) keto carbonyl mode of P(A) upshifts upon mutation, as does the 13(1) keto carbonyl mode of P(A)(+). For P700 in the ground state, bands that we associate with HisA676/HisB656 upshift/downshift upon mutation. For the P700(+) state, bands that we associate with HisA676/HisB656 also upshift/downshift upon mutation. These observations are also consistent with the notion that the mutation leads to the charge on P(B)(+) being relocated onto P(A)(+). In addition, we suggest that a hydrogen bond to the 13(1) keto carbonyl of P(A) is still present in the TA(A739) mutant, probably mediated through an introduced water molecule.

Carbon nanotube self-doping: Calculation of the hole carrier concentration

The hole carrier concentration of semiconducting carbon nanotubes is calculated as a function of tube diameter based on a self-doping mechanism caused by curvature induced charge redistribution of the bonding orbitals. The results show that nanotube hole carrier concentration is inversely proportional to the tube diameter and agree well with field-effect measurements and Raman spectroscopy on pristine carbon nanotubes.

Phase transitions in two-dimensional anisotropic chain systems: submonolayers of Sr adsorbed on Mo(112)

Ordered phases of Sr on Mo(112) and their phase transitions have been studied up to one physical monolayer as a function of both coverage and temperature using optical LEED. Starting at a coverage of 0.07, islands of a p(8×1) structure are formed at 100 K, which coexist with a disordered lattice gas. The formation of incommensurate structures with properties of floating solids starts already at coverages slightly above the completed p(8×1) commensurate structure ( θ=0.125). The latter itself behaves like a floating solid and undergoes a depinning transition at T≈125 K, similar to the next commensurate structure, p(5×1), which is formed at θ=0.20. Floating solids are found in the whole coverage range between 0.12 and 0.23. At higher coverage coexistence between p(5×1) and c(2×2) structures is found, which melt by forming intermediate two-dimensional eutectics, i.e. coexistence regions with their melts, with an eutectic point at θ=0.37, T eu=310 K. Close to a coverage of 0.5 a homogenous phase is formed, which disorders by a continuous phase transition, as explicitly tested by determination of the critical exponents β of the order parameter and ν of the correlation length. It is shown that the system belongs to the universality class of the Ising model. An incommensurate phase is again formed at higher coverage due to uniaxial compression of the layers. The behaviour at low coverages can be qualitatively understood assuming lateral interactions along the furrows mainly caused by dipole–dipole interactions and electrostatic screening of the adsorbate induced charge redistribution.