Claire Mormiche

The role of steps in the dynamics of hydrogen dissociation on Pt(533)

The dissociative adsorption of H2 and D2 on Pt(533) (Pt{4(111)×(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D2 from (100) step sites is observed at lowest exposures in TPD (assigned β3) at 375 K. Saturation of this peak at ΘH=0.14 corresponds to the filling of half of the available four-fold sites at the (100) step edge. At higher coverages, additional desorption takes place from the (111) terraces in a broad peak below 300 K similar to that observed (assigned β1 and β2) for the Pt(111) surface. The incident kinetic energy (Ei), surface temperature (Ts), coverage (ΘD), and incident angle (Φ) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S0) first decreases with increasing kinetic energy over the range 0<Ei(meV)<150 (low energy component), and subsequently increases (high energy component). Comparison with D2 dissociation on Pt(111), where (S0) incre...

The dynamics of the dissociative adsorption of methane on Pt(533)

The influence of well-defined steps on the dynamics of the dissociative chemisorption of methane on Pt(533) has been investigated using molecular beam techniques. The initial dissociative chemisorption probability S0 has been determined as a function of incident energy Ei, angle of incidence θi, and surface temperature TS. For incident kinetic energies in the range 26<Ei(meV)<1450, the initial dissociation probability of CH4 on the Pt(533) surface is higher than on Pt(111), for all surface temperatures investigated. This enhancement in dissociation is associated with the additional direct sticking mediated by the step sites, with no evidence for any additional indirect dynamical channel to dissociation induced by the step sites in the range of energies studied. The Ei dependence can be separated into the contributions of the (111) terraces and the (100) steps. The latter exhibits an effective activation barrier for dissociation ≈300 meV lower than the (111) terraces. The angular dependence can also be int...

The blocking of the step-mediated indirect channel to hydrogen dissociation by oxygen on Pt(5 3 3)

The initial dissociative sticking probability, S-0, of H-2 on clean and oxygen modified Pt(5 3 3) was measured as a function of incident kinetic energy, E-i, surface temperature, T-s, and coverage, ?(H). At low oxygen coverages, where oxygen adsorption leads to oxygen atoms decorating step sites, the indirect channel to hydrogen dissociation operating in the energy regime E-i<150 meV, is blocked. This blocking is shown to be complete when the (1 0 0) step sites are saturated by oxygen at a coverage of ?(O) = 0.12. For this oxygen decorated surface, a second indirect channel, mediated by an accommodated physisorbed precursor on the clean surface at E-i<30 meV, remains intact, as does the higher energy direct dissociation channel characteristic of the (1 1 1) terraces. A similar weak T-s dependence is exhibited for the very low energy indirect channel on the clean and oxygen modified surface. The low energy (physisorbed precursor) indirect channel, and the direct channel, are blocked only as oxygen is atomically adsorbed on the (1 1 1) terraces. The ?(H) dependence measured at various energies on the clean and oxygen decorated surface at ?(O) = 0.12 also indicate the blocking of the step-mediated indirect channel which is characterised by a weak ?(H) dependence. These results are consistent with the conclusion that the indirect channel to hydrogen dissociation operating in the energy regime E-i<150 meV, suggested to involve an unaccommodated precursor, is mediated by step sites. The importance of the poisoning of step sites in reactions involving hydrogen dissociation from a thermalised isotropic gas at 300 K is also considered.

High throughput methodology for synthesis, screening, and optimization of solid state Lithium ion electrolytes

A study of the lithium ion conductor Li(3x)La(2/3-x)TiO(3) solid solution and the surrounding composition space was carried out using a high throughput physical vapor deposition system. An optimum total ionic conductivity value of 5.45 × 10(-4) S cm(-1) was obtained for the composition Li(0.17)La(0.29)Ti(0.54) (Li(3x)La(2/3-x)TiO(3)x = 0.11). This optimum value was calculated using an artificial neural network model based on the empirical data. Due to the large scale of the data set produced and the complexity of synthesis, informatics tools were required to analyze the data. Partition analysis was carried out to determine the synthetic parameters of importance and their threshold values. Multivariate curve resolution and principal component analysis were applied to the diffraction data set. This analysis enabled the construction of phase distribution diagrams, illustrating both the phases obtained and the compositional zones in which they occur. The synthetic technique presented has significant advantages over other thin film and bulk methodologies, in terms of both the compositional range covered and the nature of the materials produced.

Novel metal gates for high κ applications

The development of gate systems suitable for high κ dielectrics is critical to the advancement of complementary metal-oxide-semiconductor (CMOS) devices. Both the effective work function and material stability are key parameters to these systems. A systematic study of metal gates of the composition HfxSi1-x (0.25 ≤ x ≤ 1) is demonstrated here, including XPS, XRD and four point probe measurements. The effective work function of each material is evaluated and it is shown that it can be tuned from 4.5 to less than 4.0 eV. Suitable work functions for n-channel metal-oxide-semiconductor applications (4.05 ± 0.2 eV) were achieved using hafnium rich compositions; however, XPS and diffraction measurements confirmed that these materials demonstrated a high propensity to oxidise, causing the reduction of the underlying oxides, making them unsuitable for commercial application.

Direct and indirect channels to molecular dissociation at metal and metal alloy surfaces

Publisher Summary This chapter discusses two limiting dynamical routes for the dissociation of a molecule at a surface—direct and indirect channels. The direct channel involves the breaking of the molecular bond during the initial collision of the molecule with the surface. The effective activation barrier presented during the collision is a function of the internal molecular degrees of freedom and the position of impact in the surface unit cell. This channel is the most dynamically accessible, because energy partition within the degrees of freedom of the molecule can be selected, and the dynamical outcome of the encounter provides a map of the potential energy surface. This is perhaps best exemplified by the activated dissociation of hydrogen on copper, which has become a model system in the study of direct dissociation dynamics. The indirect channel encompasses a class of dissociation that takes place through the total or partial accommodation of a molecular “precursor” that subsequently goes on to dissociate. Entrance into the precursor state can be either activated or nonactivated depending on its electronic surface configuration with respect to that of the gas phase molecule. Like the direct channel, this part of the potential is accessible dynamical. The probability that the precursor will go on to dissociate depends on the partition between dissociation and desorption, with the energy provided, for accommodated molecules, by the surface.