Neutral C60 effusive source for atomic collisions with fullerene

Review of scientific instruments, Vol.72, Nº9

Simulations of parasitic and intrinsic capacitances related to Multiple-Gate-Field-Effect Transistor architectures

Report for the scientific sojourn carried out at the Université Catholique de Louvain, Belgium, from March until June 2007. In the first part, the impact of important geometrical parameters such as source and drain thickness, fin spacing, spacer width, etc. on the parasitic fringing capacitance component of multiple-gate field-effect transistors (MuGFET) is deeply analyzed using finite element simulations. Several architectures such as single gate, FinFETs (double gate), triple-gate represented by Pi-gate MOSFETs are simulated and compared in terms of channel and fringing capacitances for the same occupied die area. Simulations highlight the great impact of diminishing the spacing between fins for MuGFETs and the trade-off between the reduction of parasitic source and drain resistances and the increase of fringing capacitances when Selective Epitaxial Growth (SEG) technology is introduced. The impact of these technological solutions on the transistor cut-off frequencies is also discussed. The second part deals with the study of the effect of the volume inversion (VI) on the capacitances of undoped Double-Gate (DG) MOSFETs. For that purpose, we present simulation results for the capacitances of undoped DG MOSFETs using an explicit and analytical compact model. It monstrates that the transition from volume inversion regime to dual gate behaviour is well simulated. The model shows an accurate dependence on the silicon layer thickness,consistent withtwo dimensional numerical simulations, for both thin and thick silicon films. Whereas the current drive and transconductance are enhanced in volume inversion regime, our results show thatintrinsic capacitances present higher values as well, which may limit the high speed (delay time) behaviour of DG MOSFETs under volume inversion regime.

The sigma factor AlgU (AlgT) controls exopolysaccharide production and tolerance towards desiccation and osmotic stress in the biocontrol agent Pseudomonas fluorescens CHA0.

A variety of stress situations may affect the activity and survival of plant-beneficial pseudomonads added to soil to control root diseases. This study focused on the roles of the sigma factor AlgU (synonyms, AlgT, RpoE, and sigma(22)) and the anti-sigma factor MucA in stress adaptation of the biocontrol agent Pseudomonas fluorescens CHA0. The algU-mucA-mucB gene cluster of strain CHA0 was similar to that of the pathogens Pseudomonas aeruginosa and Pseudomonas syringae. Strain CHA0 is naturally nonmucoid, whereas a mucA deletion mutant or algU-overexpressing strains were highly mucoid due to exopolysaccharide overproduction. Mucoidy strictly depended on the global regulator GacA. An algU deletion mutant was significantly more sensitive to osmotic stress than the wild-type CHA0 strain and the mucA mutant were. Expression of an algU'-'lacZ reporter fusion was induced severalfold in the wild type and in the mucA mutant upon exposure to osmotic stress, whereas a lower, noninducible level of expression was observed in the algU mutant. Overexpression of algU did not enhance tolerance towards osmotic stress. AlgU was found to be essential for tolerance of P. fluorescens towards desiccation stress in a sterile vermiculite-sand mixture and in a natural sandy loam soil. The size of the population of the algU mutant declined much more rapidly than the size of the wild-type population at soil water contents below 5%. In contrast to its role in pathogenic pseudomonads, AlgU did not contribute to tolerance of P. fluorescens towards oxidative and heat stress. In conclusion, AlgU is a crucial determinant in the adaptation of P. fluorescens to dry conditions and hyperosmolarity, two major stress factors that limit bacterial survival in the environment.

Raman scattering of InSb quantum dots grown on InP substrates

In this paper we present the Raman scattering of self-assembled InSb dots grown on (001) oriented InP substrates. The samples were grown by pulsed molecular beam epitaxy mode. Two types of samples have been investigated. In one type the InSb dots were capped with 200 monolayers of InP; in the other type no capping was deposited after the InSb dot formation. We observe two peaks in the Raman spectra of the uncapped dot, while only one peak is observed in the Raman spectra of the capped dots. In the case of the uncapped dots the peaks are attributed to LO-like and TO-like vibration of completely relaxed InSb dots, in agreement with high resolution transmission electron microscopy photographs. The Raman spectra of the capped dot suggest a different strain state in the dot due to the capping layer.

Graphoepitaxy of CeO2 on MgO and its application to the fabrication of 45° grain boundary Josephson junctions of YBa2Cu3O7-x

We communicate a detailed study of the epitaxial growth of CeO2 on MgO. The key feature of the growth is the dependence of the in¿plane orientation of the CeO2 epitaxial layer on the MgO surface morphology. Atomic force microscopic (AFM) measurements, x¿ray analyses, as well as high¿resolution transmission electron microscopy (HRTEM) investigations reveal that on rough substrates a cube¿on¿cube growth of CeO2 on MgO occurs while on smooth substrates the CeO2 unit cell is rotated around the surface normal by 45° with respect to the MgO unit cell when the deposition rate is low (~0.3 Å/s) during the first stages of growth. This growth mechanism can be used for a defined fabrication of 45° grain boundaries in the CeO2 layer by controlling the surface roughness of the MgO substrate. This report demonstrates that these 45° grain boundaries may be used to fabricate YBa2Cu3O7¿x Josephson junctions.

The biogenic origin of needle fibre calcite

Needle fibre calcite is one of the most ubiquitous habits of calcite in vadose environments (caves deposits, soil pores, etc.). Its origin, either through inorganic, indirect or direct biological processes, has long been debated. In this study, investigations at 11 sites in Europe, Africa and Central America support arguments for its biogenic origin. The wide range of needle morphologies is the result of a gradual evolution of the simplest type, a rod. This rod is the elementary brick which, by aggregation and welding, builds more complex needles. The absence of cross-welded needles implies that they are welded in a mould, or under a longitudinal and unidirectional constraint, before being released inside the soil pores. The difference between the lengthening of the needles and the c axis can be explained by the existence of needles observed under a scanning electron microscope in organic sleeves, which can act as a mould during rod growth. Complex morphologies with epitaxial outgrowths on straight rods cannot have grown entirely inside organic microtubes; they must result from soil diagenesis after the release of straight rods in a soil-free medium. Whisker crystals are interpreted as the result of growth and coalescence of euhedral crystals on a rod. Rhomb chains are considered to be the consequence of successive epitaxial growth steps on a needle during variations in growth conditions. Isotopic signatures for needle fibre calcite vary from -16.63[per mille] to +1.10[per mille] and from -8.63[per mille] to -2.25[per mille] for Delta13C and Delta18O, respectively. The absence of high Delta18O values for needle fibre calcite precludes a purely physicochemical origin (evaporative) for this particular habit of calcite. As epitaxial growth cannot precipitate in the same conditions as initial needles, needle fibre calcite stable isotopic signatures should be used with caution as a proxy for palaeoenvironmental reconstructions. In addition, it is suggested that the term needle fibre calcite should be kept for the original biogenic form. The other habit should be referred to as epitaxial forms of needle fibre calcite.

Reduced ceria nanofilms from structure prediction

Experimentally, Ce2O3 films are used to study cerium oxide in its fully or partially reduced state, as present in many applications. We have explored the space of low energy Ce2O3 nanofilms using structure prediction and density functional calculations, yielding more than 30 distinct nanofilm structures. First, our results help to rationalize the roles of thermodynamics and kinetics in the preparation of reduced ceria nanofilms with different bulk crystalline structures (e.g. A-type or bixbyite) depending on the support used. Second, we predict a novel, as yet experimentally unresolved, nanofilm which has a structure that does not correspond to any previously reported bulk A2B3 phase and which has an energetic stability between that of A-type and bixbyite. To assist identification and fabrication of this new Ce2O3 nanofilm we calculate some observable properties and propose supports for its epitaxial growth.

Unusual bismuth-containing surface layers of III-V compound semiconductors

The understanding and engineering of bismuth (Bi) containing semiconductor surfaces are signi cant in the development of novel semiconductor materials for electronic and optoelectronic devices such as high-e ciency solar cells, lasers and light emitting diodes. For example, a Bi surface layer can be used as a surfactant which oats on a III-V compound-semiconductor surface during the epitaxial growth of IIIV lms. This Bi surfactant layer improves the lm-growth conditions if compared to the growth without the Bi layer. Therefore, detailed knowledge of the properties of the Bi/III-V surfaces is needed. In this thesis, well-de ned surface layers containing Bi have been produced on various III-V semiconductor substrates. The properties of these Bi-induced surfaces have been measured by low-energy electron di raction (LEED), scanning-tunneling microscopy and spectroscopy (STM), and synchrotron-radiation photoelectron spectroscopy. The experimental results have been compared with theoretically calculated results to resolve the atomic structures of the studied surfaces. The main ndings of this research concern the determination of the properties of an unusual Bi-containing (2×1) surface structure, the discovery and characterization of a uniform pattern of Bi nanolines, and the optimization of the preparation conditions for this Bi-nanoline pattern.

Femtosecond real-time probing of reactions. 12. Vectorial dynamics of transition states

Femtosecond time-resolved techniques with KETOF (kinetic energy time-of-flight) detection in a molecular beam are developed for studies of the vectorial dynamics of transition states. Application to the dissociation reaction of IHgI is presented. For this system, the complex [I---Hg---I](++)* is unstable and, through the symmetric and asymmetric stretch motions, yields different product fragments: [I---Hg---I](++)* -> HgI(X^2/sigma^+) + I(^2P_3/2) [or I*(^2P_l/2)] (1a); [I---Hg---I](++)* -> Hg(^1S_0) + I(^2P_3/2) + I(^2P_3/2) [or I* (^2P_1/2)] (1 b). These two channels, (1a) and (1b), lead to different kinetic energy distributions in the products. It is shown that the motion of the wave packet in the transition-state region can be observed by MPI mass detection; the transient time ranges from 120 to 300 fs depending on the available energy. With polarized pulses, the vectorial properties (transition moments alignment relative to recoil direction) are studied for fragment separations on the femtosecond time scale. The results indicate the nature of the structure (symmetry properties) and the correlation to final products. For 311-nm excitation, no evidence of crossing between the I and I* potentials is found at the internuclear separations studied. (Results for 287-nm excitation are also presented.) Molecular dynamics simulations and studies by laser-induced fluorescence support these findings.

Femtosecond real-time probing of reactions. 13. Multiphoton dynamics of IHgI

Real-time studies of the dynamics were performed on the reaction of HgI_2 in a molecular beam. Excitation was by either one or multi pump photons (311 nm), leading to two separate sets of dynamics, each of which could be investigated by a time-delayed probe laser (622 nm) that ionized the parent molecule and the fragments by REMPI processes. These dynamics were distinguished by combining the information from transients taken at each mass (HgI_2, HgI, I_2, Hg, and I) with the results of pump (and probe) power dependence studies on each mass. A method of plotting the slope of the intensity dependence against the pump-probe time delay proved essential. In the preceding publication, we detailed the dynamics of the reaction initiated by a one photon excitation to the A-continuum. Here, we present studies of higher-energy states. Multiphoton excitation accesses predissociative states of HgI_2, for which there are crossings into the symmetric and asymmetric stretch coordinates. The dynamics of these channels, which lead to atomic (I or Hg) and diatomic (HgI) fragments, are discussed and related to the nature of the intermediates along the reaction pathway.

Fundamental interactions of molecules (Na_2, Na_3) with intense femtosecond laser pulses

The real-time dynamics of multiphoton ionization and fragmentation of molecules Na_2 and Na_3 has been studied in molecular beam experiments employing ion and electron spectroscopy together with femtosecond pump-probe techniques. Experiments with Na_2 and Na_3 reveal unexpected features of the dynamics of the absorption of several photons as seen in the one- and three-dimensional vibrational wave packet motion in different potential surfaces and in high laser fields: In Na_2 a second major resonance-enhanced multiphoton ionization (REMPI) process is observed, involving the excitation of two electrons and subsequent electronic autoionization. The possibility of controlling a reaction by controlling the duration of propagation of a wave packet on an electronically-excited surface is demonstrated. In high laser fields, the contributions from direct photoionization and from the second REMPI process to the total ion yield change, due to different populations in the electronic states participating in the multiphoton ionization (MPI) processes. In addition, a vibrational wave packet motion in the electronic ground state is induced through stimulated emission pumping by the pump laser. The 4^1 \summe^+_g shelf state of Na_2 is given as an example for performing frequency spectroscopy of highlying electronic states in the time domain. Pure wave packet effects, such as the spreading and the revival of a vibrational wave packet, are investigated. The three-dimensional wave packet motion in the Na_3 reflects the normal modes in the X and B states, and shows in addition the pseudorotational motion in the B state in real time.

Femtosecond pump-probe study of the spreading and recurrence of a vibrational wave packet in Na_2

The motion of a vibrational wave packet in the bound A(^1 \summe^+_u) electronic state of the sodium dimer is detected in a femtosecond pump/probe molecular beam experiment. For short times harmonic motion is seen in the total ion yield of Na^+_2 as a function of delay time between the two laser pulses. The spreading of the wave packet results in the loss of the periodic variation of the ion signal. For longer delay times (47 ps) the wave packet regains its initial form which is reflected in the revival structure of the Na^+_2 signal. Time-dependent quantum calculations reproduce the measured effects.

Femtosecond dynamics of molecules and clusters

The real-time dynamics of multiphoton ionization and fragmentation of molecules - Na_2 , Na_3 - and clusters - Na_n, Hg_n - has been studied in molecular beam experiments employing ion and electron spectroscopy together with femtosecond pump-probe techniques. Experiments with Na_2 and Na_3 reveal unexpected features of the dynamics of the absorption of several photons as seen in the one- and three dimensional vibrational wave packet motion in different potential surfaces and in high laser fields. Cluster size dependent studies of physical properties such as absorption resonances, lifetimes and decay channels have been performed using tunable femtosecond light pulses in resonance enhanced multiphoton ionization (REMPI) of the cluster size under investigation. This method failed in ns-laser experiments due to the ultrafast decay of the studied cluster. For Na_n, cluster we find that for cluster sizes n \le 21 molecular excitations and properties prevail over collective excitations of plasmon-like resonances. In the case of Hg_n cluster prompt formation of singly and doubly charged cluster are observed up to n \approx 60. The transient multiphoton ionization spectra show a 'short' time wave packet dynamics, which is identical for singly and doubly charged mercury clusters while the 'long' time fragmentation dynamics is different.

Femtosecond real-time probing of reactions

Femtosecond reaction dynamics of OClO in a supersonic molecular beam are reported. The system is excited to the A^2A_2 state with a femtosecond pulse, covering a range of excitation in the symmetric stretch between v_1 = 17 to v_1 = 11 (308-352 nm). A time-delayed femtosecond probe pulse ionizes the OClO, and OClO^+ is detected. This ion has not been observed in previous experiments because of its ultrafast fragmentation. Transients are reported for the mass of the parent OClO as well as the mass of the ClO. Apparent biexponential decays are observed and related to the fragmentation dynamics: OClO+hv \rightarrow (OClO)^{(++)*} \rightarrow ClO+O \rightarrow Cl+O_2. Clusters of OClO with water (OClO)_n (H_2 0)_m with n from 1 to 3 and m from 0 to 3 are also observed. The dynamics of the fragmentation reveal the nuclear motions and the electronic coupling between surfaces. The time scale for bond breakage is in the range of 300-500 fs, depending on v_1; surface crossing to form new intermediates is a pathway for the two channels of fragmentation: ClO+O (primary) and Cl+O_2 (minor). Comparisons with results of ab initio calculations are made.

Model catalyst studies of the strong metal-support interaction: Surface structure identified by STM on Pd nanoparticles on TiO2(110)

Model catalysts of Pd nanoparticles and films on TiO2 (I 10) were fabricated by metal vapour deposition (MVD). Molecular beam measurements show that the particles are active for CO adsorption, with a global sticking probability of 0.25, but that they are deactivated by annealing above 600 K, an effect indicative of SMSI. The Pd nanoparticles are single crystals oriented with their (I 11) plane parallel to the surface plane of the titania. Analysis of the surface by atomic resolution STM shows that new structures have formed at the surface of the Pd nanoparticles and films after annealing above 800 K. There are only two structures, a zigzag arrangement and a much more complex "pinwheel" structure. The former has a unit cell containing 7 atoms, and the latter is a bigger unit cell containing 25 atoms. These new structures are due to an overlayer of titania that has appeared on the surface of the Pd nanoparticles after annealing, and it is proposed that the surface layer that causes the SMSI effect is a mixed alloy of Pd and Ti, with only two discrete ratios of atoms: Pd/Ti of 1: 1 (pinwheel) and 1:2 (zigzag). We propose that it is these structures that cause the SMSI effect. (c) 2005 Elsevier Inc. All rights reserved.