Bulk terminated NaCl(111) on aluminum: a polar surface of an ionic crystal?

Atomically resolved scanning tunneling microscopy reveals the existence of triangular (111) bulk terminated NaCl islands. The islands can be grown by subsequent adsorption of Na and Cl2 on Al(111) and Al(100) or by conversion of stoichiometric NaCl(100) islands to NaCl(111) via additional Na adsorption. The NaCl(111) islands have Na-Cl-Na sandwich structure. Ab initio calculations of the electronic structure of these islands show that each of the Na atoms carries half a positive elementary charge, leaving the islands neutral and explaining the existence of an otherwise unstable surface.     

Monolayered adatom aggregation induced by metallofullerene molecules on Cu(100)

The adsorption and self-assembly of Gd@C₈₂ molecules on Cu(100) surface have been investigated using scanning tunneling microscopy (STM). The metallofullerene molecules in the assemblies showed two characteristic apparent heights in the STM images. STM manipulation and spectroscopy was performed and revealed the formation of Cu adatom islands underneath the Gd@C₈₂ molecules. The monolayered Cu aggregates were resulted from the adatom–molecule complexation, which is supported by density functional theory (DFT) calculations that show charge transfer and electrostatic interactions between Gd@C₈₂ and adatoms. In addition, sub-molecularly resolved STM images demonstrated the structural and orientational ordering of Gd@C₈₂ assemblies upon thermal annealing. DFT calculations demonstrated that Gd atom located at the lower part of the carbon cage is a favored adsorption configuration for Gd@C₈₂ molecules adsorbed on Cu(100).     

Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-(7 x 7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si₆H₁₂ with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si₆H₁₂ cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10^-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands. Received 30 November 2000     

TEM,XRD and AFM study of poly(<Emphasis Type="Italic">o</Emphasis>-ethoxyaniline) films: new evidence for the formation of conducting islands

The existence of conducting islands in polyaniline films has long been proposed in the literature, which would be consistent with conducting mechanisms based on hopping. Obtaining direct evidence of conducting islands, however, is not straightforward. In this paper, conducting islands were visualized in poly(o-ethoxyaniline) (POEA) films prepared at low pH, using Transmission Electron Microscopy (TEM) and atomic force spectroscopy (AFS). The size of the islands varied between 67 and 470 Å for a pH=3.0, with a larger average being obtained with AFS, probably due to the finite size effect of the atomic force microscopy tip. In AFS, the conducting islands were denoted by regions with repulsive forces due to the double-layer forces. On the basis of X-ray diffraction (XRD) patterns for POEA in the powder form, we infer that the conducting islands are crystalline, and therefore a POEA film is believed to consist of conducting islands dispersed in an insulating, amorphous matrix. From conductivity measurements we inferred the charge transport to be governed by a typical quasi-one dimensional variable range hopping (VRH) mechanism.     

Degeneracy pressure of electrons: a new effect for Pb islands grown on Si(1 1 1) substrate

We have found that the degeneracy pressure of electrons (DPE) inside Pb islands grown on a silicon substrate plays a crucial role in stabilizing the islands. In most cases, at a metal-semiconductor interface charge spilling takes place due to the difference of Fermi energies between the two materials, which makes DPE decrease along with the energy of the system. Based on this new effect, calculations of energy as a function of height are carried out for Pb islands grown on Si(1 1 1)-() and -(7 × 7) phases, which have most stable heights of 5 and 7 monolayers (ML), respectively. Our results explain why these most stable heights are observed. Using this new effect supplemented with experimental data, all the preferred heights of the Pb islands on Si(1 1 1)-(7 × 7) can be explained too.     

Vertical friedel oscillations in interface-induced surface charge modulations of ultrathin quantum islands

Two-dimensional Pb islands of a few atomic layers are grown on the incommensurate Si(111)-Pb surface at low temperatures. Among them, two types of islands having different stacking with the substrate are observed. These islands, respectively, display an alternating image contrast with their thickness. Besides, the contrasts of the islands of different types are complementary to each other layer by layer. These intriguing behaviors do not show significant bias dependence throughout the range from -3 to +3 V and can be explained by the vertical charge oscillation with the growth of a new layer. The charge oscillation in the out-of-plane direction originates from electron scattering by the in-plane potential variation at the Pb/Si interface.     

Search for superheavy elements in galactic cosmic rays

The charge distribution of approximately 6000 nuclei with charge numbers above 55 in galactic cosmic rays has been obtained in the OLIMPIYA project. Three superheavy nuclei with the charge numbers in the range 105 < Z < 130 have been detected. The regression analysis has provided a more accurate estimate of the charge number of one of these nuclei (119 _?6 ⁺¹⁰ with a probability of 95%). Such nuclei should form stability islands. Their detection in nature confirms theoretical predictions and justifies efforts for their synthesis under terrestrial conditions. The model calculations performed in this work possibly can explain the results of some experiments on the investigation of the charge composition of cosmic rays in which particles with charge numbers in the range 94 < Z < 100 were detected (they cannot enter into the composition of primary cosmic radiation because their lifetime is very short). The calculations indicate that events with Z > 92 are due to the fragmentation of heavier nuclei from the stability island, rather than to methodical inaccuracies or fault of instruments. Several such events have been revealed. Thus, the track method makes it possible to obtain the results very important for understanding of the physical picture of the world. The results obtained within the OLIMPIYA project show that the study of tracks of galactic cosmic rays in olivine crystals from meteorites opens new capabilities for the investigation of fluxes and spectra in cosmic rays in the region of heavy and superheavy nuclei.     

Formation of quasi-one-dimensional Cu2O structures by in situ oxidation of Cu(100)

Epitaxial Cu2O islands on a Cu(100) surface formed through oxidizing Cu(100) films at 600 degrees C in an ultrahigh vacuum transmission electron microscope were observed to undergo a shape transition from initially square shaped islands to elongated islands at a critical size of approximately 110 nm. Our experimental data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interface energies and the elastic stress relaxation in the three-dimensional islands. We developed a kinetic model based on oxygen surface diffusion that fits well with the observed volume evolution of the Cu2O islands.     

Non-empirical versus empirical choices for overlapping-sphere radii ratios in SCF-Xα-SW calculations on ClO4 - and SO2

Two non-empirical schemes for establishing ratios of atomic sphere radii for Xα-SW calculations are proposed and compared with known methods of partitioning molecules into atomic fragments along paths of maximum change in electrostatic potential or charge density. In the initial Xα-SW molecular charge distribution, the radius of a sphere around each atom containing the atomic number of electrons is shown to be constant within a few per cent no matter what atomic charges are assumed in constructing the charge distribution. This contrasts with the radius of a sphere containing the number of electrons put in for the atom in calculating its atomic charge density, which shows a significant inverse variation with the assumed charge. Ionization energies for ClO₄ ^- and SO₂ are calculated using ratios of sphere radii established both from the atomic number sphere sizes and from Slater's empirical atomic radii. The results are compared with experiment and with HF-LCAO calculations. The Xα-SW calculations using atomic number radii ratios are in the best overall agreement with experiment.     

Ab-initio calculations of electronic structure and optical properties of TiAl alloy

The electronic structures and optical properties of TiAl intermetallic alloy system are studied by the first-principle orthogonalized linear combination of atomic orbitals method. Results on the band structure, total and partial density of states, localization index, effective atomic charges, and optical conductivity are presented and discussed in detail. Total density of states spectra reveal that (near the Fermi level) the majority of the contribution is from Ti-3d states. The effective charge calculations show an average charge transfer of 0.52 electrons from Ti to Al in primitive cell calculations of TiAl alloy. On the other hand, calculations using supercell approach reveal an average charge transfer of 0.48 electrons from Ti to Al. The localization index calculations, of primitive cell as well as of supercell, show the presence of relatively localized states even above the Fermi level for this alloy. The calculated optical conductivity spectra of TiAl alloy are rich in structures, showing the highest peak at 5.73 eV for supercell calculations. Calculations of the imaginary part of the linear dielectric function show a prominent peak at 5.71 eV and a plateau in the range 1.1-3.5 eV.     

First-principles study of lattice dynamics of LiFePO4

Lattice dynamics of lithium iron orthophosphate (LiFePO₄) isostructural with olivine have been investigated using the first-principles calculations taking into account the on-site Coulomb interaction within the GGA + U scheme. Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves are calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives a convincing evidence for the one-dimensional Li migration tunnel along the [010] direction in LiFePO₄, which has been proposed by other theoretical calculations and experimental observation. The calculated phonon frequencies at the Γ point of the Brillouin zone show good agreement with the available experimental observations.     

Nanowedge island formation on Mo(1 1 0)

The deposition of ultrathin Fe films on the Mo(1 1 0) surface at elevated temperatures results in the formation of distinctive nanowedge islands. The model of island formation presented in this work is based on both experiment and DFT calculations of Fe adatom hopping barriers. Also, a number of classical molecular dynamics simulations were carried out to illustrate fragments of the model. The islands are formed during a transition from a nanostripe morphology at around 2 ML coverage through a Bales-Zangwill type instability. Islands nucleate when the meandering step fronts are sufficiently roughened to produce a substantial overlap between adjacent steps. The islands propagate along the substrate [0 0 1] direction due to anisotropic diffusion/capture processes along the island edges. It was found that the substrate steps limit adatom diffusion and provide heterogeneous nucleation sites, resulting in a higher density of islands on a vicinal surface. As the islands can be several layers thick at their thinnest end, we propose that adatoms entering the islands undertake a so-called “vertical climb” along the sides of the island. This is facilitated by the presence of mismatch-induced dislocations that thread to the sides of the islands and produce local maxima of compressive strain. Dislocation lines also trigger initial nucleation on the surface with 2-3 ML Fe coverage. The sides of the nanowedge islands typically form along low-index crystallographic directions but can also form along dislocation lines or the substrate miscut direction.     

Scanning tunneling spectroscopy of Ni/W(110): bcc and fcc properties in the second atomic layer

Nickel islands are grown on W(110) at elevated temperatures. Islands with a thickness of two layers are investigated with scanning tunneling microscopy. Spectroscopic measurements reveal that nanometer sized areas of the islands exhibit distinctly different apparent heights and dI/dVspectra. Spin polarized and paramagnetic band structure calculations indicate that the spectral features are due to fcc(111) and bcc(110) orientations of the Ni film, respectively.     

Adiabatic theory of particle trapping due to Coulomb crossing of one-dimensional betatron resonance

In the presence of space charge forces, synchrotron oscillations result in periodic modulation of the space charge tune shift, periodic crossing of betatron resonances, and particle trapping in resonance islands. The trapping effect for one-dimensional resonance is considered using classical perturbation theory and the “frozen core” approach to calculation of space charge forces. The beam losses and emittance growth are analyzed for arbitrary order resonance; the numerical results are given for the third-order resonance. The text was submitted by the authors in English.     

Photo-induced charge transfer emission in (E)-3-(4-dimethylamino-naphthalen-1-yl)-acrylic acid and its use as metal ions fluorosensor

In this study, spectroscopic properties of (E)-3-(4-dimethylamino-naphthalen-1-yl)-acrylic acid (DMANAA) have been explored using absorption and emission spectroscopy in combination with quantum chemical calculations. In addition to the local emission, the molecule shows a red-shifted emission from the charge transfer state. The charge transfer emission band is found to be dependent on polarity and hydrogen-bonding ability of the solvents. This photo-induced charge transfer process has been explored theoretically at Density Functional Theory level using twisted intramolecular charge transfer model. Structural calculations and potential energy surfaces along the donor twist coordinate done by Density Functional Theory correlate well with the spectroscopic observations. Addition of metal ions to DMANAA solutions in acetonitrile solvent shows progressive quenching of the charge transfer band and growth of a blue-shifted band, making way for its use as a good fluorosensor for various metal ions.     

Ab Initio Calculations of Differential Cross Sections for Single Charge Transfer in ^3He^2＋ ＋ ^4He Collisions

The single charge transfer process in ^3He^2＋ ＋ ^4He collisions is investigated using the quantum-mechanical molecular- orbital close-coupling method, in which the adiabatic potentials and radial couplings are calculated by using the ab initio multireference single- and double-excitation configuration interaction methods. The differential cross sections for the single charge transfer are presented at the laboratorial energies E = 6 keV and lOkeV for the projectile ^3He^2＋. Comparison with the existing data shows that the present results are better in agreement with the experimental measurements than other calculations in the dominant small angle scattering, which is attributed to the accurate calculations of the adiabatic potentials and the radial couplings.     

Formation mechanism of one-dimensional Si islands on a H/Si(001) surface

The formation mechanism of one-dimensional Si islands on a H/Si(001)-(2x1) surface is studied using scanning tunneling microscopy/spectroscopy and first-principles calculations. We observed that one-dimensional islands that are made from dimer chains are formed at the initial growth stages similar to the bare Si(001) surface. It is found that the number of odd-numbered dimer chains is larger than that of even-numbered dimer chains. We propose the growth processes of the two types of growth edges to explain the observation.     

Raman spectroscopy of the PTCDA–inorganic semiconductor interface: evidence for charge transfer

In the present work, we investigate the vibrational properties of a PTCDA molecule with an additional positive or negative charge using density functional theory. With respect to the calculated vibrational frequencies of the neutral molecule, some modes in particular in the region 1200–1800 cm⁻¹ show large shifts. These calculations are compared with resonant Raman spectra of sub-monolayer PTCDA films on passivated semiconductor surfaces, both before and after annealing the deposited films at elevated temperatures (350 °C). Independent of the sample treatment, the sub-monolayer Raman spectra correspond quite well to reference spectra obtained for thicker films, and we find no evidence for the strong shifts predicted in the calculations for the charged species. From the small changes in the mode frequencies it can be concluded that any charge transfer present involves significantly less than one elementary charge.