Hydrocarbon decomposition kinetics on the Ir(111) surface.

The kinetics of the thermal decomposition of hydrocarbons on the Ir(111) surface is determined using kinetic Monte Carlo (kMC) and rate equations simulations, both based on the density functional theory (DFT) calculated energy barriers of the involved reaction processes. This decomposition process is important for understanding the early stages of epitaxial graphene growth where the deposited hydrocarbon acts as a carbon feedstock for graphene formation. The methodology of the kMC simulations and the rate equation approaches is discussed and a comparison between the results obtained from both approaches is made in the case of the temperature programmed decomposition of ethylene for different initial coverages. The theoretical results are verified against experimental data from in situ X-ray photoelectron spectroscopy (XPS) experiments. Both theoretical approaches give reasonable results; however we find that, as expected, rate equations are less reliable at high coverages. We find that the agreement between experiment and theory can be improved in all cases if slight adjustments are made to the energy barriers in order to account for the intrinsic errors in DFT. Finally we extend our approach to the case where hydrocarbon species are dosed onto the substrate continuously, as in the chemical vapour deposition (CVD) graphene growth method. For ethylene and methane the thermal decomposition mechanism is determined, and it is found that in both cases the formation of C monomers is to be expected, which is limited by the presence of hydrogen atoms.

A free energy study of carbon clusters on Ir(111): Precursors to graphene growth.

It is widely accepted that the nucleation of graphene on transition metals is related to the formation of carbon clusters of various sizes and shapes on the surface. Assuming a low concentration of carbon atoms on a crystal surface, we derive a thermodynamic expression for the grand potential of the cluster of N carbon atoms, relative to a single carbon atom on the surface (the cluster work of formation). This is derived taking into account both the energetic and entropic contributions, including structural and rotational components, and is explicitly dependent on the temperature. Then, using ab initio density functional theory, we calculate the work of formation of carbon clusters CN on the Ir(111) surface as a function of temperature considering clusters with up to N = 16 C atoms. We consider five types of clusters (chains, rings, arches, top-hollow, and domes), and find, in agreement with previous zero temperature studies, that at elevated temperatures the structure most favoured depends on N, with chains and arches being the most likely at N<10 and the hexagonal domes becoming the most favourable at all temperatures for N>10. Our calculations reveal the work of formation to have a much more complex character as a function of the cluster size than one would expect from classical nucleation theory: for typical conditions, the work of formation displays not one but two nucleation barriers, at around N = 4-5 and N = 9-11. This suggests, in agreement with existing LEEM data, that five atom carbon clusters, along with C monomers, must play a pivotal role in the nucleation and growth of graphene sheets, whereby the formation of large clusters is achieved from the coalescence of smaller clusters (Smoluchowski ripening). Although the main emphasis of our study is on thermodynamic aspects of nucleation, the pivotal role of kinetics of transitions between different cluster types during the nucleation process is also discussed for a few cases as illustrative examples.

Changing renin substrates in human pregnancy.

Renin substrate of high molecular weight was identified in plasma from pregnant women, non-pregnant subjects and in samples of umbilical arterial and venous plasma, using gel filtration chromatography. The proportion existing in the high molecular weight form in pregnant women was significantly increased above that of non-pregnant subjects by 11 weeks after the last menstrual period and continued to rise to term. In contrast, the proportion in fetal plasma was significantly lower than that of the non-pregnant group. High molecular weight renin substrate was itself heterogeneous and appeared to consist of four sub-groups. In non-pregnant adults the first peak, eluting near the void volume of the column, was the largest. In the fetus this profile was reversed. The elution profile in maternal plasma had changed markedly by as early as 8 weeks of gestation. Thereafter, it was possible to see increasing dominance of the minor 'intermediate' peaks visible in the non-pregnant adult until, in the third trimester, the high molecular weight form was eluted as a large broad band with little evidence of the individual peaks. Oestrogens are the most likely stimulus for the increased production of the high molecular weight renin substrate in pregnant women. In the fetus, although the same form of renin substrate is stimulated, the actual amount in the high molecular weight range is very small. This may be because oestrogens are sulphated in the fetus or because the fetal liver is not very responsive to the stimulation. The similarity between the profiles of mother and fetus may indicate a common source of production of renin substrate in the feto-placental unit.

The effect of changes in blood gas tension upon the renin-angiotensin system of the newborn infant.

Renin and renin-substrate concentrations were measured in the paired umbilical artery and vein blood of 20 babies born by elective caesarean section, and 36 babies born vaginally. Blood pH, carbon dioxide tension and oxygen tension were measured in all samples. Babies born vaginally were significantly more acidaemic than those delivered by caesarean section. The cord artery blood also had a significantly greater carbon dioxide tension at vaginal delivery. A decrease in pH of either cord artery or vein blood, irrespective of mode of delivery, was associated with an increase in renin concentration. Renin-substrate concentration tended to be lower in the cord vein blood of acidaemic babies born vaginally, such that the ratio renin: renin-substrate concentration was negatively correlated with pH. At caesarean section the renin concentration of cord vein blood was positively correlated with carbon dioxide tension and increased dramatically at oxygen tensions less than 3kPa. This was not observed in babies born vaginally, but the cord artery substrate concentration in these infants was positively correlated with carbon dioxide tension. Thus changes in the renin-angiotensin system at birth may partly be affected by blood gas tension.

Studies on the effect of mode of delivery on the renin-angiotensin system in mother and fetus at term.

Plasma renin substrate (PRS) and renin (PRC) concentrations were measured in the paired umbilical artery and vein blood of 54 babies and in the venous blood of 23 of their mothers at vaginal delivery. Similar samples were collected from 31 babies and their mothers at elective caesarean section. Fetal renin concentrations in both umbilical artery and vein blood were markedly increased at vaginal delivery compared with values at operative delivery. Renin substrate concentration was correlated with the length of second stage of labour. Umbilical vein PRC and PRS were greater than corresponding umbilical artery values at vaginal, but not at operative delivery, suggesting that some factor associated with delivery might stimulate the placenta to produce renin and renin substrate during vaginal delivery. Maternal and umbilical concentrations of both renin and renin substrate were unrelated, supporting the view that the maternal and fetal compartments are independent. It is suggested that hyperactivity of the renin-angiotensin system may be an integral part of the marked changes in cardiovascular homeostasis which occur at birth.