Surface fluxes of Si and C adatoms at initial growth stages of SiC quantum dots

Self-assembly of highly stoichiometric SiC quantum dots still remains a major challenge for the gas/plasma-based nanodot synthesis. By means of a multiscale hybrid numerical simulation of the initial stage (0.1-2.5 s into the process) of deposition of SiCSi (100) quantum dot nuclei, it is shown that equal Si and kst atom deposition fluxes result in strong nonstoichiometric nanodot composition due to very different surface fluxes of Si and C adatoms to the quantum dots. At this stage, the surface fluxes of Si and C adatoms to SiC nanodots can be effectively controlled by manipulating the SiC atom influx ratio and the Si (100) surface temperature. It is demonstrated that at a surface temperature of 800 K the surface fluxes can be equalized after only 0.05 s into the process; however, it takes more then 1 s at a surface temperature of 600 K. Based on the results of this study, effective strategies to maintain a stoichiometric ([Si] [C] =1:1) elemental ratio during the initial stages of deposition of SiCSi (100) quantum dot nuclei in a neutral/ionized gas-based process are proposed.

Effective control of ion fluxes over large areas by magnetic fields : from narrow beams to highly uniform fluxes

An effective control of the ion current distribution over large-area (up to 103 cm2) substrates with the magnetic fields of a complex structure by using two additional magnetic coils installed under the substrate exposed to vacuum arc plasmas is demonstrated. When the magnetic field generated by the additional coils is aligned with the direction of the magnetic field generated by the guiding and focusing coils of the vacuum arc source, a narrow ion density distribution with the maximum current density 117 A m-2 is achieved. When one of the additional coils is set to generate the magnetic field of the opposite direction, an area almost uniform over the substrate of 103 cm2 ion current distribution with the mean value of 45 A m-2 is achieved. Our findings suggest that the system with the vacuum arc source and two additional magnetic coils can be effectively used for the effective, high throughput, and highly controllable plasma processing.

Nanostructures of various dimensionalities from plasma and neutral fluxes

A complex multi-scale model and numerical simulations are used to demonstrate, by simulating the development of patterns of nanotips, nanowalls, nanoislands and nanovoids of a characteristic size of 5-100 nm, a greater degree of determinism in the formation of various nanostructures by using the low-density, low-temperature plasma-based processes. It is shown that in the plasma, in contrast to the neutral gas-based processes, one can synthesize nanostructures of various dimensionalities and shapes with a larger surface density, desired geometrical parameters and narrower size distribution functions. This effect is mainly attributed to strong ion focusing by irregular electric fields in the nanopatterns, which effectively redistributes the influxes of plasma-generated building units and thus provides a selective control of their delivery to the growing nanostructures.

Effect of ambipolar fluxes on nanoparticle charging in low-pressure glow discharges

The effect of ambipolar fluxes on nanoparticle charging in a typical low-pressure parallel-plate glow discharge is considered. It is shown that the equilibrium values of the nanoparticle charge in the plasma bulk and near-electrode areas are strongly affected by the ratio S ath i of the ambipolar flux and the ion thermal velocities. Under typical experimental conditions the above ratio is neither S ath i≪ 1 nor S ath i≫1, which often renders the commonly used approximations of the purely thermal or "ion wind" ion charging currents inaccurate. By using the general approximation for the ambipolar drift-affected ion flux on the nanoparticle surface, it appears possible to obtain more accurate values of the nanoparticle charge that usually deviate within 10-25 % from the values obtained without a proper accounting for the ambipolar ion fluxes. The implications of the results obtained for glow discharge modeling and nanoparticle manipulation in low-pressure plasmas are discussed.

Ge/Si quantum dot formation from non-uniform cluster fluxes

The controlled growth of ultra-small Ge/Si quantum dot (QD) nuclei (≈1 nm) suitable for the synthesis of uniform nanopatterns with high surface coverage, is simulated using atom-only and size non-uniform cluster fluxes. It is found that seed nuclei of more uniform sizes are formed when clusters of non-uniform size are deposited. This counter-intuitive result is explained via adatom-nanocluster interactions on Si(100) surfaces. Our results are supported by experimental data on the geometric characteristics of QD patterns synthesized by nanocluster deposition. This is followed by a description of the role of plasmas as non-uniform cluster sources and the impact on surface dynamics. The technique challenges conventional growth modes and is promising for deterministic synthesis of nanodot arrays.

The relationship between N2O, NO, and N2 fluxes from fertilized and irrigated dryland soils of the Aral Sea Basin, Uzbekistan

Microbial respiratory reduction of nitrous oxide (N2O) to dinitrogen (N2) via denitrification plays a key role within the global N-cycle since it is the most important process for converting reactive nitrogen back into inert molecular N2. However, due to methodological constraints, we still lack a comprehensive, quantitative understanding of denitrification rates and controlling factors across various ecosystems. We investigated N2, N2O and NO emissions from irrigated cotton fields within the Aral Sera Basin using the He/O2 atmosphere gas flow soil core technique and an incubation assay. NH4NO3 fertilizer, equivalent to 75 kg ha−1 and irrigation water, adjusting the water holding capacity to 70, 100 and 130% were applied to the incubation vessels to assess its influence on gaseous N emissions. Under soil conditions as they are naturally found after concomitant irrigation and fertilization, denitrification was the dominant process and N2 the main end product of denitrification. The mean ratios of N2/N2O emissions increased with increasing soil moisture content. N2 emissions exceeded N2O emissions by a factor of 5 ± 2 at 70% soil water holding capacity (WHC) and a factor of 55 ± 27 at 130% WHC. The mean ratios of N2O/NO emissions varied between 1.5 ± 0.4 (70% WHC) and 644 ± 108 (130% WHC). The magnitude of N2 emissions for irrigated cotton was estimated to be in the range of 24 ± 9 to 175 ± 65 kg-N ha−1season−1, while emissions of NO were only of minor importance (between 0.1 to 0.7 kg-N ha−1 season−1). The findings demonstrate that for irrigated dryland soils in the Aral Sera Basin, denitrification is a major pathway of N-loss and that substantial amounts of N-fertilizer are lost as N2 to the atmosphere for irrigated dryland soils.

Methane and nitrous oxide fluxes in annual and perennial land-use systems of the irrigated areas in the Aral Sea Basin

Land use and agricultural practices can result in important contributions to the global source strength of atmospheric nitrous oxide (N2O) and methane (CH4). However, knowledge of gas flux from irrigated agriculture is very limited. From April 2005 to October 2006, a study was conducted in the Aral Sea Basin, Uzbekistan, to quantify and compare emissions of N2O and CH4 in various annual and perennial land-use systems: irrigated cotton, winter wheat and rice crops, a poplar plantation and a natural Tugai (floodplain) forest. In the annual systems, average N2O emissions ranged from 10 to 150 μg N2O-N m−2 h−1 with highest N2O emissions in the cotton fields, covering a similar range of previous studies from irrigated cropping systems. Emission factors (uncorrected for background emission), used to determine the fertilizer-induced N2O emission as a percentage of N fertilizer applied, ranged from 0.2% to 2.6%. Seasonal variations in N2O emissions were principally controlled by fertilization and irrigation management. Pulses of N2O emissions occurred after concomitant N-fertilizer application and irrigation. The unfertilized poplar plantation showed high N2O emissions over the entire study period (30 μg N2O-N m−2 h−1), whereas only negligible fluxes of N2O (<2 μg N2O-N m−2 h−1) occurred in the Tugai. Significant CH4 fluxes only were determined from the flooded rice field: Fluxes were low with mean flux rates of 32 mg CH4 m−2 day−1 and a low seasonal total of 35.2 kg CH4 ha−1. The global warming potential (GWP) of the N2O and CH4 fluxes was highest under rice and cotton, with seasonal changes between 500 and 3000 kg CO2 eq. ha−1. The biennial cotton–wheat–rice crop rotation commonly practiced in the region would average a GWP of 2500 kg CO2 eq. ha−1 yr−1. The analyses point out opportunities for reducing the GWP of these irrigated agricultural systems by (i) optimization of fertilization and irrigation practices and (ii) conversion of annual cropping systems into perennial forest plantations, especially on less profitable, marginal lands.

On the relationship between finger width, velocity, and fluxes in thermohaline convection

Double-diffusive finger convection occurs in many natural processes.The theories for double-diffusive phenomena that exist at present consider systems with linear stratification in temperature and salinity. The double-diffusive systems with step change in salinity and temperature are, however, not amenable to simple stability analysis. Hence factors that control the width of the finger, velocity, and fluxes in systems that have step change in temperature and salinity have not been understood so far. In this paper we provide new physical insight regarding factors that influence finger convection in two-layer double-diffusive system through two-dimensional numerical simulations. Simulations have been carried out for density stability ratios (R-rho) from 1.5 to 10. For each density stability ratio, the thermal Rayleigh number (Ra-T) has been systematically varied from 7x10(3) to 7x10(8). Results from these simulations show how finger width, velocity, and flux ratios in finger convection are interrelated and the influence of governing parameters such as density stability ratio and the thermal Rayleigh number. The width of the incipient fingers at the time of onset of instability has been shown to vary as Ra-T-1/3. Velocity in the finger varies as Ra(T)1/3/R-rho. Results from simulation agree with the scale analysis presented in the paper. Our results demonstrate that wide fingers have lower velocities and flux ratios compared to those in narrow fingers. This result contradicts present notions about the relation between finger width and flux ratio. A counterflow heat-exchanger analogy is used in understanding the dependence of flux ratio on finger width and velocity.

Sensible heat fluxes during the active and break phases of the southwest monsoon over the Indian region

Based on the theory given by Saltzman and Ashe (1976), sensible heat fluxes are calculated for the active and break phases of the southwest monsoon over the Indian region. The conclusion drawn is that the sensible heat flux is generally larger during the break monsoon situation when compared with that for the active monsoon situation. The synoptic heat flux is negligible when compared with mean and diurnal heat fluxes over the Indian region even during the monsoon season.

Urban energy fluxes in Helsinki and their high frequency spectral corrections

Inadvertent climate modification has led to an increase in urban temperatures compared to the surrounding rural area. The main reason for the temperature rise is the altered energy portioning of input net radiation to heat storage and sensible and latent heat fluxes in addition to the anthropogenic heat flux. The heat storage flux and anthropogenic heat flux have not yet been determined for Helsinki and they are not directly measurable. To the contrary, turbulent fluxes of sensible and latent heat in addition to net radiation can be measured, and the anthropogenic heat flux together with the heat storage flux can be solved as a residual. As a result, all inaccuracies in the determination of the energy balance components propagate to the residual term and special attention must be paid to the accurate determination of the components. One cause of error in the turbulent fluxes is the fluctuation attenuation at high frequencies which can be accounted for by high frequency spectral corrections. The aim of this study is twofold: to assess the relevance of high frequency corrections to water vapor fluxes and to assess the temporal variation of the energy fluxes. Turbulent fluxes of sensible and latent heat have been measured at SMEAR III station, Helsinki, since December 2005 using the eddy covariance technique. In addition, net radiation measurements have been ongoing since July 2007. The used calculation methods in this study consist of widely accepted eddy covariance data post processing methods in addition to Fourier and wavelet analysis. The high frequency spectral correction using the traditional transfer function method is highly dependent on relative humidity and has an 11% effect on the latent heat flux. This method is based on an assumption of spectral similarity which is shown not to be valid. A new correction method using wavelet analysis is thus initialized and it seems to account for the high frequency variation deficit. Anyhow, the resulting wavelet correction remains minimal in contrast to the traditional transfer function correction. The energy fluxes exhibit a behavior characteristic for urban environments: the energy input is channeled to sensible heat as latent heat flux is restricted by water availability. The monthly mean residual of the energy balance ranges from 30 Wm-2 in summer to -35 Wm-2 in winter meaning a heat storage to the ground during summer. Furthermore, the anthropogenic heat flux is approximated to be 50 Wm-2 during winter when residential heating is important.

Below-ground processes in meadow soil under elevated ozone and carbon dioxide : - Greenhouse gas fluxes, N cycling and microbial communities

This thesis focuses on how elevated CO2 and/or O3 affect the below-ground processes in semi-natural vegetation, with an emphasis on greenhouse gases, N cycling and microbial communities. Meadow mesocosms mimicking lowland hay meadows in Jokioinen, SW Finland, were enclosed in open-top chambers and exposed to ambient and elevated levels of O3 (40-50 ppb) and/or CO2 (+100 ppm) for three consecutive growing season, while chamberless plots were used as chamber controls. Chemical and microbiological analyses as well as laboratory incubations of the mesocosm soils under different treatments were used to study the effects of O3 and/or CO2. Artificially constructed mesocosms were also compared with natural meadows with regards to GHG fluxes and soil characteristics. In addition to research conducted at the ecosystem level (i.e. the mesocosm study), soil microbial communities were also examined in a pot experiment with monocultures of individual species. By comparing mesocosms with similar natural plant assemblage, it was possible to demonstrate that artificial mesocosms simulated natural habitats, even though some differences were found in the CH4 oxidation rate, soil mineral N, and total C and N concentrations in the soil. After three growing seasons of fumigations, the fluxes of N2O, CH4, and CO2 were decreased in the NF+O3 treatment, and the soil NH4+-N and mineral N concentrations were lower in the NF+O3 treatment than in the NF control treatment. The mesocosm soil microbial communities were affected negatively by the NF+O3 treatment, as the total, bacterial, actinobacterial, and fungal PLFA biomasses as well as the fungal:bacterial biomass ratio decreased under elevated O3. In the pot survey, O3 decreased the total, bacterial, actinobacterial, and mycorrhizal PLFA biomasses in the bulk soil and affected the microbial community structure in the rhizosphere of L. pratensis, whereas the bulk soil and rhizosphere of the other monoculture, A. capillaris, remained unaffected by O3. Elevated CO2 caused only minor and insignificant changes in the GHG fluxes, N cycling, and the microbial community structure. In the present study, the below-ground processes were modified after three years of moderate O3 enhancement. A tentative conclusion is that a decrease in N availability may have feedback effects on plant growth and competition and affect the N cycling of the whole meadow ecosystem. Ecosystem level changes occur slowly, and multiplication of the responses might be expected in the long run.

Sampling frequency affects estimates of annual nitrous oxide fluxes

Quantifying nitrous oxide (N(2)O) fluxes, a potent greenhouse gas, from soils is necessary to improve our knowledge of terrestrial N(2)O losses. Developing universal sampling frequencies for calculating annual N(2)O fluxes is difficult, as fluxes are renowned for their high temporal variability. We demonstrate daily sampling was largely required to achieve annual N(2)O fluxes within 10% of the best estimate for 28 annual datasets collected from three continents, Australia, Europe and Asia. Decreasing the regularity of measurements either under- or overestimated annual N(2)O fluxes, with a maximum overestimation of 935%. Measurement frequency was lowered using a sampling strategy based on environmental factors known to affect temporal variability, but still required sampling more than once a week. Consequently, uncertainty in current global terrestrial N(2)O budgets associated with the upscaling of field-based datasets can be decreased significantly using adequate sampling frequencies.

Carbon balance and component CO2 fluxes in boreal Scots pine stands

The forest vegetation takes up atmospheric carbon dioxide (CO2) in photosynthesis. Part of the fixed carbon is released back into the atmosphere during plant respiration but a substantial part is stored as plant biomass, especially in the stems of trees. Carbon also accumulates in the soil as litter and via the roots. CO2 is released into the atmosphere from these carbon stocks in the decomposition of dead biomass. Carbon balance of a forest stand is the difference between the CO2 uptake and CO2 efflux. This study quantifies and analyses the dynamics of carbon balance and component CO2 fluxes in four Southern Finnish Scots pine stands that covered the typical economic rotation time of 80 years. The study was based on direct flux measurements with chambers and eddy covariance (EC), and modelling of component CO2 fluxes. The net CO2 exchange of the stand was partitioned into component fluxes: photosynthesis of trees and ground vegetation, respiration of tree foliage and stems, and CO2 efflux from the soil. The relationships between the component fluxes and the environmental factors (light, temperature, atmospheric CO2, air humidity and soil moisture) were studied with mathematical modelling. The annual CO2 balance varied from a source of about 400 g C/m2 at a recently clearcut site to net CO2 uptake of 200 300 g C/m2 in a middle-aged (40-year-old) and a mature (75-year-old) stand. A 12-year-old sapling site was at the turning point from source to a sink of CO2. In the middle-aged stand, photosynthetic production was dominated by trees. Under closed pine canopies, ground vegetation accounted for 10 20% of stand photosynthesis whereas at the open sites the proportion and also the absolute photosynthesis of ground vegetation was much higher. The aboveground respiration was dominated by tree foliage which accounted for one third of the ecosystem respiration. Rate of wood respiration was in the order of 10% of total ecosystem respiration. CO2 efflux from the soil dominated the ecosystem respiratory fluxes in all phases of stand development. Instantaneous and delayed responses to the environmental driving factors could predict well within-year variability in photosynthetic production: In the short term and during the growing season photosynthesis follows primarily light while the seasonal variation is more strongly connected to temperature. The temperature relationship of the annual cycle of photosynthesis was found to be almost equal in the southern boreal zone and at the timberline in the northern boreal zone. The respiratory fluxes showed instantaneous and seasonal temperature relationships but they could also be connected to photosynthesis at an annual timescale.