Sticking Coefficient Research Articles

The importance of frontier orbital symmetry in the adsorption of diiodobenzene on MoS2(0001)

Evidence of a role of frontier orbital symmetry, in the adsorption process of diiodobenzene on MoS2(0001), appears in the huge differences in the rate of adsorption between 1,3-diiodobenzene, 1,2-diiodobenzene and 1,4-diiodobenzene isomers on MoS2. Experiments indicate that the rate of adsorption of 1,3-diiodobenzene on MoS2(0001) is much greater than that of the 1,2-diodobenzene and 1,4-diiodbenzene isomers. As the differences in calculated diiodobenzene isomer-MoS2 system adsorption energies and electron affinities are negligible, frontier orbital symmetry appears to play a significant role in diiodobenzene adsorption on MoS2(0001). The experimental and theory results, in combination, suggest that a rehybridization requirement, forced by frontier orbital symmetry, comes at a cost of a reduced sticking coefficient. With the introduction of defects, which lower the adsorption site symmetry at the MoS2(0001) surface, the rate of 1,3-diiodobenzene adsorption on MoS2(0001) decreases, while the rates of 1,2-diodobenzene adsorption significantly increases.

The role of faceting in biaxially textured thin films: Columnar morphology and abnormal tilting

The ground-breaking properties of biaxially textured thin films have attracted increasing attention to the characterization and growth theory of their crystal morphologies. In particular, multi-faceted columnar structures developed during oblique angle deposition (OAD) show abnormal tilt angles that have not been previously captured by existing models. Current theories for the formation of biaxially aligned columnar structures overlook the fact that the surface diffusion on individual facets can be finite. In this work, a continuum model incorporating finite adatom mobility, flux-dependent sticking coefficient, and material-specific surface energies is employed to study the growth of a well-known MgO-OAD system. Experimentally observed morphologies are reproduced by simulating the shadowing growth of an array of preferentially oriented single crystals. We show that the abnormal tilting is elusive considering only the effects of faceting and shadowing. A proposed sticking coefficient in our model, determined by the component of adatom momentum parallel to the surface, is responsible for the development of abnormal tilting. The role of faceting is demonstrated by its effect on the resulting columnar morphologies. Using the proposed model, the characteristic morphology and tilting behavior of a CaF2-OAD system are also obtained, which agree with experiments.

Quantitative evaluation of wall chemical effect in hydrogen flame using two-photon absorption LIF

A premixed H2/air flame with N2 dilution (Uin = 533 cm/s, ϕ = 1) was formed in a quartz micro flow reactor with/without a 100-nm thick Inconel coating for the investigation of wall chemical effect of the metal surface. Two-dimensional distributions of OH radical, O atom and H atom in the hydrogen flame were measured via the planar laser induced fluorescence (PLIF) and two-photon absorption laser induced fluorescence (TALIF), respectively. It is found that the distributions of all these three main species in the hydrogen flame are significantly affected by the wall chemical effect. OH, O and H shift downstream in the Inconel-coated channel, and also their concentration becomes lower than those in the less-reactive quartz channel. Based on the measured distributions of OH, O and H over the Inconel surface, the initial sticking coefficients (S0) of the radical quenching model are optimized. It is found that S0 for Inconel are 0.4–0.5, 0.1–0.2 and < 0.05 for OH, O and H, respectively, showing different sticking coefficients for different species for the first time.

The role of dust destruction and dust growth in the evolution of the interstellar medium

ABSTRACT We use Milky Way-like chemodynamical simulations with a new treatment for dust destruction and growth to investigate how these two processes affect the properties of the interstellar medium in galaxies. We focus on the role of two specific parameters, namely fdes (a new parameter that determines the fraction of dust destroyed in a single gas particle vicinity of a supernova) and Cs (the probability that a metal atom or ion sticks to the dust grain after colliding, i.e. the sticking coefficient), in regulating the amount and distribution of dust, cold gas and metals in galaxies. We find that simulated galaxies with low fdes and/or high Cs values not only produce more dust, but they also have a shallower correlation between the dust surface density and the total gas surface density, and a steeper correlation between the dust-to-gas ratio and the metallicity. Only for values of fdes between 0.01 and 0.02, and of Cs between 0.5 and 1 do our simulations produce an average slope of the dust-to-gas ratio versus metallicity relationship that is consistent with observations. fdes values correspond to a total fraction of dust destroyed by a single supernova ranging between 0.42 and 0.44. Finally, we compare predictions of several simulations (with different star formation recipes, gas fractions, central metallicities, and metallicity gradients) with the spatially resolved M101 galaxy, and conclude that metallicity is the primary driver of the spatial distribution of dust, while the dust-to-gas ratio controls the cold gas distribution, as it regulates the atomc-to-molecular hydrogen conversion rate.

Numerical analysis of the effect of surface recombination on N-atom in discharge and post-discharge region

In this paper, the effect of surface recombination on N-atom production is discussed through a one-dimensional simulation of Townsend dielectric barrier discharge in pure N2 based on a fluid model. By comparison of the experimental results, the recommended value of the sticking coefficient of N–N surface recombination is 0.5–1. The spatial-temporal distribution of N-atom of simulation results in discharge and post-discharge agree with experimental results. When the sticking coefficient is 0.5, the primary active species include N, N2(A), and N2(a′). N4+ is the densest positive ion, which can reach 4.77 × 109 cm−3. N-atom can reach the saturation level within about 30 ms. The highest number density is 3.14 × 1014 cm−3 at the position 0.25 mm away from the surface. The numerical simulation results are very consistent with the experimental results. The contribution of surface recombination and three-body recombination for the decay of N-atom are roughly equal in the post-discharge region.

Numerical modeling of SiC by low-pressure chemical vapor deposition from methyltrichlorosilane

Abstract The development of functional relationships between the observed deposition rate and the experimental conditions is an important step toward understanding and optimizing low-pressure chemical vapor deposition (LPCVD) or low-pressure chemical vapor infiltration (LPCVI). In the field of ceramic matrix composites (CMCs), methyltrichlorosilane (CH3SiCl3, MTS) is the most widely used source gas system for SiC, because stoichiometric SiC deposit can be facilitated at 900 °C–1300 °C. However, the reliability and accuracy of existing numerical models for these processing conditions are rarely reported. In this study, a comprehensive transport model was coupled with gas-phase and surface kinetics. The resulting gas-phase kinetics was confirmed via the measured concentration of gaseous species. The relationship between deposition rate and 24 gaseous species has been effectively evaluated by combining the special superiority of the novel extreme machine learning method and the conventional sticking coefficient method. Surface kinetics were then proposed and shown to reproduce the experimental results. The proposed simulation strategy can be used for different material systems.

A Method for Obtaining Liquid-Solid Adsorption Rates from Molecular Dynamics Simulations: Applied to Methanol on Pt(111) in H2O.

Adsorption is an important step in heterogeneous catalysis as it predetermines how many reactant molecules can participate in a surface reaction per unit time. While the rate of adsorption processes is well studied in gas-solid adsorption in both theory and experiment, such rates are still not well studied for liquid-solid adsorption. This is partly because the ever-changing configurations of liquid-phase solvent molecules impede the ability to study a molecule approaching a surface from a liquid phase by either experiment or theory. In this work, we develop a method using molecular dynamics (MD) simulations to study the rate of adsorption in liquid-solid adsorption processes. Specifically, we use MD to model the diffusion of a methanol molecule in aqueous solvent and its adsorption to a Pt(111) surface. We find that by approximating the solute motion as following the same displacement rates as a random walk model, the adsorbed and non-adsorbed states of the methanol molecule near the Pt(111) surface can be discerned and quantified. In particular, this methodology enables extracting a sticking coefficient and a macroscopically relatable adsorption rate. This method can be applied to arbitrary types of reactants and surfaces, as well as different liquid environments, thus providing a general tool for predicting quantitative adsorption rates of liquid-solid adsorption systems.

Biopolyelectrolyte Surface Diffusion Within a Planar Slit Geometry

The effective surface diffusion of biopolyelectrolytes within nanoscale confinement is integral to biosensing, biocatalysis, bioseparations (such as chromatography columns utilizing nanoporous media), as well as biophysical processes. Confinement influences the surface-mediated diffusion of biopolyelectrolyte chains due to the presence of electric double layers and the increased frequency of transient polymer/surface interactions. Here, the diffusion of poly-L-lysine (PLL) in a nanoslit was studied using Convex Lens-induced Confinement (CLiC) single-molecule tracking microscopy. Three surface chemistries were employed to understand and compare the effects of electrostatic and short-range interactions, respectively. In all cases, the effective surface diffusion coefficient increased rapidly with slit height until it saturated at its value for a semi-infinite interface for slit heights <30 nm. The evolution of the diffusion coefficient with slit height was faster for amine-functionalized surfaces with which PLL exhibited stronger short-range interactions. These hydrogen-bonding interactions influenced the intermittent random walk behavior by increasing the waiting times between flights and the re-adsorption probability (a.k.a. sticking coefficient) during flights. Intermittent random walks were simulated within a planar slit geometry, using the appropriate adsorption probabilities and waiting time distributions (obtained from diffusion at a single interface) to account for the characteristic short-range interactions between PLL and each type of surface chemistry. The simulated step-size distributions were in good agreement with experimental measurements, indicating that the fine details of diffusion in nanoscale confinement can be quantitatively described by a model that combines confined Brownian motion with transient surface adsorption and desorption, using parameters obtained only from the behavior at a single interface in a semi-infinite geometry. These results highlight the importance of short-range biopolyelectrolyte/surface interactions on diffusion within confined environments and moreover, demonstrate the complex relationship between electrostatics, confinement, and local polymer and surface chemistry on biopolyelectrolyte surface diffusion within a nanoslit.

Numerical simulation of experimental tests performed on ZAO® non-evaporable-getter pump designed for neutral beam injector applications.

Vacuum systems of neutral beam injectors have very demanding requirements in terms of pumping speed, throughput, and capacity. Due to their high affinity to hydrogenic species, porous sintered Non-Evaporable Getters (NEG) are a possible candidate for deployment in giant hydrogen ion sources and neutral beam injectors for fusion. This paper presents the numerical interpretation of experimental tests on a recently developed NEG cartridge, that is part of a modular pump under development for neutral beam injectors. The cartridge is composed of six stacks of ZAO® porous sintered NEG disks and a heater. It was tested under hydrogen loads relevant for neutral beam injectors, namely, at constant pressure or constant flow, such that the hydrogen pressure was in the range of 20 mPa-40 mPa. The result of the sorption test was reproduced by a three dimensional flow simulation in molecular regime to determine the actual pumping speed, the effective sticking coefficient, and the uniformity of the gas load on the various NEG disks. The procedure developed and the results obtained provide the basic understanding for interpreting the large-scale tests on the modular pump, consisting of 34 of these cartridges.

Interpreting the dynamic equilibrium during evaporation in a cesium environment.

The cesium ovens for the prototype source of the ITER neutral beam injectors are currently tested in the CAesium Test Stand (CATS) facility, with a background pressure of 10-6 mbar. Different diagnostics are here installed: two Langmuir-Taylor detectors allow us to determine the Cs vapour evaporation rate from the oven and the Cs density at different positions in the vacuum chamber; and laser absorption spectroscopy is used to measure the density integrated over a line of sight and a quartz crystal microbalance to detect the cesium mass deposited in time over a surface. In this paper, we present a model to describe the dynamic equilibrium in the evaporation chamber of CATS with the first oven tested in order to gain information about the Cs sticking coefficient at the walls. The model hence includes sticking and energy accommodation of the Cs atoms to the walls, calculates the flux density at the surfaces, and provides the Cs atom density at any location in the volume. By this model, we simulate the Cs evaporation and the equilibrium density, comparing the modeled results with the experimental data. As a result, a sticking coefficient of 2% is obtained.

Including substrate temperature in Berg model for reactive sputtering

Reactive magnetron sputtering deposition can be qualitatively simulated by the well known Berg model. However, such model does not take into account explicitly the dependence of the substrate temperature on the sticking coefficient. The substrate temperature plays a major role in film properties and also affects the hysteresis curves. In order to analyze the effect of substrate heating on the hysteresis curves, the Berg model equations were rewritten by using both the Langmuir and the Kisliuk adsorption models. The theoretical results show that a raise on substrate temperature causes variations in the first critical point, but the adsorptions models exhibit different temperature dependences. In the experimental hysteresis curves obtained for Ti and Al targets using N2 and O2 as reactive gases, the effects of substrate temperature are very small, therefore, negligible.

Optical constants of evaporated amorphous zinc arsenide (Zn3As2) via spectroscopic ellipsometry

We have used spectroscopic ellipsometry to measure the optical constants of evaporated amorphous zinc arsenide (Zn3As2). A five parameter model using a Tauc-Lorentz oscillator was found to fit well each of six amorphous samples deposited on Si3N4/silicon, allowing the layer thicknesses and optical constants to be deduced. Layer thicknesses varied from 20 to 70 nm. The fitted value of the optical gap (Tauc gap) is 0.95 eV, close to the 1.0 eV band gap for crystalline bulk zinc arsenide. A single set of parameters from an ensemble Tauc-Lorentz model can be used to determine the thicknesses of amorphous Zn3As2 layers as long as the layers are ≳ 25 nm thick. Measured film thicknesses do not correlate with targeted thicknesses, likely due to low sticking coefficients of evaporated zinc arsenide.

Role of sulfur in catalyzing fluorine atom fast etching of silicon with smooth surface morphology

Chemical reaction probabilities, defined as the number of silicon atoms removed per incident fluorine atom, have been investigated in mixtures of NF3 and SF6 plasmas in an inductively-coupled plasma reactor. Fluorine atom densities were measured by optical emission actinometry, and isotropic etching rates were measured by the degree of undercutting of SiO2-masked silicon by cross-sectional scanning electron microscopy. In addition, atomic force microscopy was used to examine surface morphology of etched Si surfaces. The F atom reaction probabilities derived from isotropic etching rates indicate an ∼30-fold higher reaction probability in SF6 plasmas compared with values in NF3 plasmas. Surfaces etched in SF6 plasmas were much smoother than those etched in NF3 plasmas. The addition of only 10% SF6 to an NF3 plasma produced a much higher reaction probability (∼5-fold) than in a pure NF3 plasma. This surprising enhancement of reaction probabilities for F with Si in SF6 plasmas is further investigated, based on the mechanism of adsorbed sulfur acting as a catalyst to greatly enhance the etching rate of Si. Dilute sulfur solutions in isopropyl alcohol were allowed to evaporate on the masked Si samples, depositing sulfur in relatively high concentrations near mask edges in ∼2 μm diameter periodic “strings of beads.” The sulfur-dosed sample etched several times faster at the center of each bead than a sample not exposed to sulfur that was placed side by side. The catalytic effect of sulfur is ascribed to an enhanced F sticking coefficient and/or decreased desorption rate on a surface covered with sulfur.

Epitaxial growth of perovskite SrBiO3 film on SrTiO3 by oxide molecular beam epitaxy

Hole-doped perovskite bismuthates such as Ba$_{1-x}$K$_x$BiO$_3$ and Sr$_{1-x}$K$_x$BiO$_3$ are well-known bismuth-based oxide high-transition-temperature superconductors. Reported thin bismuthate films show relatively low quality, likely due to their large lattice mismatch with the substrate and a low sticking coefficient of Bi at high temperatures. Here, we report the successful epitaxial thin film growth of the parent compound strontium bismuthate SrBiO$_3$ on SrO-terminated SrTiO$_3$ (001) substrates by molecular beam epitaxy. Two different growth methods, high-temperature co-deposition or recrystallization cycles of low-temperature deposition plus high-temperature annealing, are developed to improve the epitaxial growth. SrBiO$_3$ has a pseudocubic lattice constant $\sim$4.25 \AA, an $\sim$8.8\% lattice mismatch on SrTiO$_3$ substrate, leading to a large strain in the first few unit cells. Films thicker than 6 unit cells prepared by both methods are fully relaxed to bulk lattice constant and have similar quality. Compared to high-temperature co-deposition, the recrystallization method can produce higher quality 1-6 unit cell films that are coherently or partially strained. Photoemission experiments reveal the bonding and antibonding states close to the Fermi level due to Bi and O hybridization, in good agreement with density functional theory calculations. This work provides general guidance to the synthesis of high-quality perovskite bismuthate films.

Initial Sticking Coefficient Attenuation of Gases in Carbon Monoxide Sensing on Pt80Au14Ti6

It has been proved that Pt80Au14Ti6 can give proper signals as a CO sensitive layer in air at room temperature. Unfortunately, there is no signal anymore from this sample, if it is kept in air more than 24 hours. To overcome this problem, we carried out dry etching technic. The atmosphere testing also has been done in technic etcher chamber by keeping the samples separately in Ar, N2, CO, and O2 with pressure 1 atm for 24 hours without taking out the samples after etching process from the chamber. These experiments found that oxygen causes this loss signal problem. Oxygen can diffuse up to 170 Angstrom under the Pt surface to be subsurface oxygen. This diffusion changes the surface structure of Pt(100)(1x1) to hex-Pt(100) which impacts the attenuating of the initial sticking coefficient of gases. Simulations which have been conducted in this research produced the attenuating factor, i.e., 1 − 0.00585 exp (5.76664Osub), where Osub is the concentration of subsurface oxygen.

Improving the rheology, lubricity, and differential sticking properties of water-based drilling muds at high temperatures using hydrophilic Gilsonite nanoparticles

Recent developments in nanofluids indicate that the addition of nanoparticles to water-based drilling muds can alleviate many drilling problems, including wellbore instability, lost circulation, torque and drag force, differential pipe sticking, and low drilling rates. In this study, the addition of hydrophilic Gilsonite nanoparticles (HGNs) to water-based drilling mud was investigated, and its effects upon the well-cleaning process, rheological properties, filtration loss reduction, lubricity characteristics, and, more importantly, on the differential sticking of the base fluid were evaluated. Hence, initially, the HGNs were characterized by means of Fourier transform infrared spectroscopy, the dynamic light scattering technique, as well as thermogravimetric and differential thermogravimetric analysis. The results showed that after the addition of HGNs, the rheological behavior of the base mud before the hot roll process was improved, and, interestingly, after the hot roll at 149 °C and 4 h, the characteristics of the sample mud were significantly retained, compared to those of the base mud. Also, after the hot roll, the plastic viscosity (PV) improved, the yield point (YP) was remarkably maintained, and the ratio of YP/PV was enhanced. On the other hand, the American Petroleum Institute (API) filtration loss was reduced by 36%, and, additionally, after the hot roll process, the filtration loss reduction and the mud cake thickness in high-pressure high-temperature conditions diminished by 79.4% and 50%, respectively. After the hot rolling process, the lubricity coefficient and torque percentage dropped by 15% and 13.63%, respectively. Also, the HGNs reduced the differential sticking coefficient and increased the likelihood of differential sticking release of the base fluid by 61.5%.

Quantitative Modelling of Deposition of Airborne Molecular Contamination

During the fabrication, micro-chips circuits are very sensitive to all kinds of contamination present in the environments fluid in contact with the wafers [1]. Therefore the fabrication is done in clean rooms and makes use of clean mini-environments (e.g. FOUPs). Historically such clean environments contain an extremely low concentration of particulate contamination, obtained by intensive filtering through different types of particle filters including HEPA filters. In contrast, the air is not purified in terms of molecular, volatile contaminants, except locally inside some specific process equipment. Measurement in state-of-art clean rooms reveal the presence of many molecular contaminants coming from different sources (environmental pollution in the intake air, but also from components and materials in the clean room construction or equipment and from process steps that make use of volatile chemicals). The dominant species are organic compounds comprising a benzene ring with various ligands. The relative molecular concentration is on the order of 1 ppb, corresponding to a molecule volume concentration, C, = 1x1010 molec./cm3. A question that typically arises in this context is that of how clean the air or gas ambient needs to be. This study addresses this issue by presenting a simple but quantitative model, built from first principles, to establish the relation between the amount of contamination in the air and the surface concentration of the contaminants that deposits on the wafer as a function of the exposure time and other relevant parameters. The transfer of contaminants from the air ambient to the surface occurs in 2 steps. Surface reaction: at the surface adhesion of molecules takes placeBulk transport in the air: the contaminants that have deposited are replenished from the volume of the air towards the surface At the surface, the initial adsorption flux, ja, is given by the the thermal bombardment flux (= arrival flux) multiplied with the sticking probability (hi) of species, I, as shown in equation (1) (blue dash-dot line). Where: v th= average thermal velocity (air: 470 m/s) Ci = volumetric density of species i in air ambient adjacent to the surface. The hi = sticking coefficient of species i on the surface, ranges typically from 10-6 to 1 [2,3]. When the resulting surface concentration of species i, si , increases, the reverse process, desorption, will occur simultaneously. A simple mass balance shows that for a case with a sticking coefficient approaching 1 and with a molecule volume concentration of 1x1010 molec./cm3 , (i.e. 1 ppb) a surface concentration of 1x1012 molec./cm2 on an infinitely large flat surface, would require all the contaminants from a volume contained in a layer with a thickness of 1 m adjacent to the surface. This simple assessment illustrates that the supply and transport from the ambient is an important limiting phenomenon. Below the focus is on estimating the potential maximum contamination achieved, assuming a sticking probability = 1. This provides an estimation for the upperlimit for the case with a lower sticking probability. The transport of contaminants in the air occurs by convection and by diffusion. In clean room ambient, laminar flow patterns are quite typical. One can approximate the transport by defining a static boundary layer adjacent to the wafer surface (with a thickness L BL) in which the (lateral) velocity is approximated as 0. Outside the boundary layer the flow is assumed to be intense enough to maintain a constant contamination concentration C i, ∞. During an initial transient phase contaminants diffuse from within the static boundary layer to the surface from a layer proportional to the diffusion length SQRT(Dt) (black dashed line). As soon as the diffusion length reaches the edge of the static boundary layer the supply flux of contaminants reaches a constant value and is determined by diffusion over the static boundary layer thickness (red solid lines). Combining the initial transient and the steady state supply yields equation (2). The values thus obtained for C = 1x1010 molec/cm3 are shown in figure 1 for 3 different values of the boundary layer thickness (LBL). Fig 1 Black and red lines represent the surface contamination as a function of time for a sticking coefficient of 1, C= 1x1010 mol/cm3 and for a static boundary layer thickness of 0.1, 1 or 10 cm. References Saga and T. Hattori, J. Electrochem. Soc 144, p. L253 (1997).Hendriks, Microelectronic Engineering, 25, p. 185 (1994).Den et al., J. Electrochem. Soc 153, G149 (2006). Figure 1

Shape, Size Evolution, and Nucleation Mechanisms of GaAs Nanoislands Grown on (111)Si by Low-Temperature Metal–Organic Vapor-Phase Epitaxy

The shape, size evolution, and nucleation mechanisms of GaAs nanoislands grown at 400 °C on As-stabilized (111)Si by metal–organic vapor phase epitaxy are reported for the first time. GaAs crystallizes in the zincblende phase in the very early nucleation stages until a continuous epilayer is formed. GaAs nanoislands grow (111)-oriented on Si as truncated hexagonal pyramids, bound by six equivalent {120} side facets and a (111) facet at the top. Their diameter and height appear to increase linearly with the deposition time, yielding a constant aspect ratio of ∼1/4. The nanoisland density (before coalescence) stays constant with time at ∼2 × 1010 cm–2, suggesting that the nucleation occurs at specific Si surface sites (defects) during the very early growth stages, rather than being due to the continuous formation of new nuclei. To understand the molecular-level mechanisms driving the low-temperature MOVPE growth of GaAs on Si, we applied a deposition–diffusion–aggregation (DDA) nucleation model, which predicts a linear evolution of the overall GaAs growth rate with surface coverage, in good agreement with experimental observations, under the assumption that direct impingement of trimethylgallium (Me3Ga) molecules onto the nanoisland surface dominates the material nucleation and growth rate; the contribution of Me3Ga adsorbed onto the As-stabilized (111)Si is negligible, pointing out the reduced reactivity of the Si surface (As passivation). Our DDA model allows estimation of the effective reactive sticking coefficient of Me3Ga onto GaAs, which is equal to 2.82 × 10–5: the small value is compatible with the Me3Ga large steric hindrance and the competitive role of methyl radicals in surface adsorption at low temperature.

CHD3 Dissociation on the Kinked Pt(210) Surface: A Comparison of Experiment and Theory

To be able to simulate activated heterogeneously catalyzed reactions on the edge and corner sites of nanoparticles, a method for calculating accurate activation barriers for the reactions is required. We have recently demonstrated that a semiempirical specific reaction parameter (SRP) density functional developed to describe CHD3 dissociation on a flat Ni(111) surface is transferable to describing the same reaction on a stepped Pt(211) surface. In the current work, we compare initial sticking coefficients measured using the King and Wells beam reflectivity technique and calculated from ab initio molecular dynamics trajectories using the same SRP functional for CHD3 dissociation on a kinked Pt(210) surface at a temperature of 650 K. The calculated sticking coefficients overestimate those determined experimentally, with an average energy shift between the two curves of 13.6 kJ/mol, which is over a factor of 3 times higher than the 4.2 kJ/mol limit that defines chemical accuracy. This suggests the SRP functional predicts an activation barrier that is too low for the dissociation on the least coordinated kink atom, which is the site of the lowest energy transition state and where most of the dissociation occurs in the calculations.

Penetration depth variation in atomic layer deposition on multiwalled carbon nanotube forests

Atomic layer deposition (ALD) of Al2O3 on tall multiwalled carbon nanotube forests shows concentration variation with depth in discrete steps. While ALD is capable of extremely conformal deposition in high aspect ratio structures, decreasing penetration depth has been observed over multiple thermal ALD cycles on 1.3 mm tall multiwalled carbon nanotube forests. Scanning electron microscopy imaging with energy dispersive x-ray spectroscopy elemental analysis shows steps of decreasing intensity corresponding to decreasing concentrations of Al2O3. A study of these steps suggests that they are produced by a combination of diffusion limited precursor delivery and the increase in precursor adsorption site density due to nuclei growing during the ALD process. This conceptual model has been applied to modify literature models for ALD penetration on high aspect ratio structures, allowing two parameters to be extracted from the experimental data. The Knudsen diffusion constant for trimethylaluminum (TMA) in these carbon nanotube forests has been found to be 0.3 cm2 s−1. From the profile of the Al2O3 concentration, the sticking coefficient of TMA in the TMA/water thermal ALD process was found to be 0.003.