Effect of diffusion length on the nucleation of chemical vapour deposition diamond

Atomic steps have been suggested as preferential sites for nucleation. However, evidence from recent experiments on diamond growth using faceted sapphire as well as reconstructed silicon substrates shows that atomic steps alone do not always enhance nucleation in the chemical vapour deposition environment. The comparison of the diffusion length of the nucleation precursors and the width of the terraces between the surface steps provides further insights into this nucleation mechanism.     

New developments on size-dependent growth applied to the crystallization of sucrose

The effect of crystal size on the growth rate of sucrose (C₁₂H₂₂O₁₁) at 40 °C is investigated from a theoretical and an experimental point of view. Based on new perspectives resulting from the recently introduced spiral nucleation model [P.M. Martins, F. Rocha, Surf. Sci. 601 (2007) 3400], crystal growth rates are expressed in terms of mass deposition per time and crystal volume units. This alternative definition is demonstrated to be size-independent over the considered supersaturation range. The conventional overall growth rate expressed per surface area units is found to be linearly dependent on crystal size. The advantages of the “volumetric” growth rate concept are discussed. Sucrose dissolution rates were measured under reciprocal conditions of the growth experiments in order to investigate the two-way effect of crystal size on mass transfer rates and on the integration kinetics. Both effects are adequately described by combining a well-established diffusion-integration model and the spiral nucleation mechanism.     

Quasistatic propagation of steps along a phase boundary

We study quasistatic propagation of steps along a phase boundary in a two-dimensional lattice model of martensitic phase transitions. For analytical simplicity, the formulation is restricted to antiplane shear deformation of a cubic lattice with bi-stable interactions along one component of shear strain and harmonic interactions along the other. Energy landscapes connecting equilibrium configurations with periodic and non-periodic arrangements of steps are constructed, and the energy barriers separating metastable states are calculated. We show that a sequential one-by-one step propagation along a phase boundary requires smaller energy barriers than simultaneous motion of several steps.      

Au-induced nanofaceting and the stoichiometry of the Si(7 7 5)-Au surface

The Au-induced changes in the surface morphology of a Si(1 1 1) sample miscut 8° towards have been measured using room temperature scanning tunneling microscopy and low energy electron diffraction. Au coverages of less than 0.06 ML up to 0.43 ML have been investigated. In all cases Au adsorption produces dramatic changes in surface morphology. The Au-induced surface exhibits nanofacets with orientations that depend critically on the amount of Au deposited. Below 0.32 ML, the restructured surface always includes (7 7 5)-Au nanofacets suggesting that the (7 7 5)-Au facet is energetically preferred on this surface. The (7 7 5)-Au facet is oriented 8.5° from [1 1 1] towards and is characterized by 1-d chains spaced 21.3 Å apart that run along the direction. By maximizing the surface area of the (7 7 5) facets and optimizing the associated diffraction pattern we determine that the (7 7 5)-Au reconstruction is optimized at 0.24 ML and corresponds to a stoichiometry of 1.5 Au atoms per 1 × 1 unit cell. We believe that the local Au coverage on the (7 7 5) facet is 0.24 ML, and that the deficit/extra of Au at any particular coverage is accommodated by non-(7 7 5) facets. For example at 0.06 ML the regions of step bunching observed on the clean surface are eliminated and Au-induced (7 7 5) and Au-free (1 1 1)7 × 7 facets are already visible. Up to 0.18 ML the non-(7 7 5) facet is Au free. Beyond 0.32 ML, the (7 7 5)-Au reconstruction is no longer stable and the extra Au is accommodated by the formation of higher angle facets with smaller chain spacings.     

多步斜坡升温——恒温平台石墨炉法直接测定葡萄酒中微量铅

研究了一种直接测定葡萄酒中微量铅的方法，以硝酸－磷酸二氢铵体系为基体改进剂，采用多步斜坡并温，恒温平台石墨炉技术，测定了四个国家五种类别葡萄酒中微量铅，回收率９７％～１０４％，精密度优于３．９％。     

Adsorption states of NO on the Pt(1 1 1) step surface

Using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM), we investigated the adsorption states of NO on the Pt(9 9 7) step surface. At 90 K, we observe three N-O stretching modes at 1490 cm⁻¹, 1631 cm⁻¹ and 1700 cm⁻¹ at 0.2 ML. The 1490 cm⁻¹ and 1700 cm⁻¹ peaks are assigned to NO molecules at fcc-hollow and on-top sites of the terrace, respectively. The 1631 cm⁻¹ peak is assigned to the step NO species. In the present STM results, we observed that NO molecules were adsorbed at the bridge sites of the step as well as fcc-hollow and on-top sites of the terrace. To help with our assignments, density functional theory calculations were also performed. The calculated results indicate that a bridge site of the step is the most stable adsorption site for NO, and its stretching frequency is 1607 cm⁻¹. The interactions between NO species at different sites on Pt(9 9 7) are also discussed.     

Ethylene dissociation on flat and stepped Ni(1 1 1): A combined STM and DFT study

The dissociative adsorption of ethylene (C₂H₄) on Ni(1 1 1) was studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. The STM studies reveal that ethylene decomposes exclusively at the step edges at room temperature. However, the step edge sites are poisoned by the reaction products and thus only a small brim of decomposed ethylene is formed. At 500 K decomposition on the (1 1 1) facets leads to a continuous growth of carbidic islands, which nucleate along the step edges.DFT calculations were performed for several intermediate steps in the decomposition of ethylene on both Ni(1 1 1) and the stepped Ni(2 1 1) surface. In general the Ni(2 1 1) surface is found to have a higher reactivity than the Ni(1 1 1) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the different reaction pathways. In particular the barrier for dissociation is lowered significantly more than the barrier for dehydrogenation, and this is of great importance for the bond-breaking selectivity of Ni surfaces.The influence of step edges was also probed by evaporating Ag onto the Ni(1 1 1) surface. STM shows that the room temperature evaporation leads to a step flow growth of Ag islands, and a subsequent annealing at 800 K causes the Ag atoms to completely wet the step edges of Ni(1 1 1). The blocking of the step edges is shown to prevent all decomposition of ethylene at room temperature, whereas the terrace site decomposition at 500 K is confirmed to be unaffected by the Ag atoms.Finally a high surface area NiAg alloy catalyst supported on MgAl₂O₄ was synthesized and tested in flow reactor measurements. The NiAg catalyst has a much lower activity for ethane hydrogenolysis than a similar Ni catalyst, which can be rationalized by the STM and DFT results.     

Surface morphology and step fluctuations on Ag nanowires

Silver nanowires fabricated using a wet-chemical synthesis have been characterized in UHV using scanning tunneling microscopy. The nanowires have a faceted structure with a high density of steps at the facet boundaries. Monolayer height steps are easily imaged, and have measurable temporal fluctuations at room temperature. Measurement of the step time correlation functions shows that the steps fluctuate via periphery diffusion, with a hopping time constant of 0.10-0.24 s. The symmetry of islands on the facets and the quantitative step characteristics are both consistent with (1 0 0) oriented facets, in agreement with previous TEM characterization.