The special features of adsorption and the kinetics of decomposition of monosilane molecules on the epitaxial surface of silicon

Analytic equations relating the rate of the incorporation of silicon atoms into a growing crystal to the characteristic frequency of the pyrolysis of silane molecules on the surface of silicon were obtained over the temperature range corresponding to the epitaxial growth of silicon films. As distinct from the earlier works, it was assumed that adsorbed silicon atoms and monosilane molecules formed double bonds with the surface. The data of technological experiments for the most extensively used pyrolysis models obtained thus far were used to determine the region of the characteristic frequencies of the decomposition of hydride molecule radicals adsorbed on the surface of a silicon plate over the temperature range 450–700°C. The temperature dependence of the frequency of monosilane molecule decomposition was shown to be to a great extent determined by the form of the temperature dependence of the $ \tilde v_{SiH_2 }^0 $ \tilde v_{SiH_2 }^0 preexponential factor. It was also found that the characteristic frequency of the decomposition of silane molecules was sensitive to the stage of pyrolysis at which hydrogen atoms released from silane molecules were captured by the surface. Decomposition occurred at the highest rate if hydrogen molecules were adsorbed at the stage of the adsorption of monosilane. The lowest rate of decomposition was observed if hydrogen molecules were adsorbed at the stage of the decomposition of radicals already captured by the surface. The temperature dependence of the coefficient of adsorption of monosilane molecules was characterized by a negative activation energy of the process for almost all the most important system models over the temperature range of growth. At elevated growth temperatures, the adsorption of monosilane molecules by the surface of silicon proceeded via an intermediate state characterized by the difference of desorption and chemisorption energies on the order of 0.28 eV.     

Exact results for the adsorption of a semiflexible copolymer chain in three dimensions

Lattice model of directed self-avoiding walk has been solved analytically to investigate adsorption–desorption phase transition behaviour of a semiflexible sequential copolymer chain on a two-dimensional impenetrable surface perpendicular to the preferred direction of the walk of the copolymer chain in three dimensions. The stiffness of the chain has been accounted by introducing an energy barrier for each bend in the walk of the copolymer chain. Exact value of adsorption–desorption transition points have been determined using the generating function method for the cases in which one type of monomer is having interaction with the surface, namely (i) no interaction (ii) attractive interaction and (iii) repulsive interaction. Results obtained in each of the case show that for stiffer copolymer chain adsorption transition occurs at a smaller value of monomer surface attraction than a flexible copolymer chain. These features are similar to that of a semiflexible homopolymer chain adsorption.     

Adsorption, reaction and desorption of hydrogen on modified Pd(1 1 1) surfaces

The interaction of hydrogen (deuterium) with different modified Pd(1 1 1) surfaces has been investigated. The focus was put on the energy and angel dependence of the desorbing molecules from oxygen covered, potassium covered and vanadium oxide covered surfaces. Conventional adsorption/desorption as well as permeation/desorption experiments were performed. For the oxygen covered surface optimum reaction rates for water production and the energy distribution of the reaction products were determined, both for the reaction of oxygen with molecular hydrogen as well as with atomic hydrogen. Potassium on the surface enhances the activation barrier for hydrogen adsorption resulting in a hyper-thermal desorption flux and a forward focused angular distribution of desorption. Permeation/desorption of deuterium from ultra-thin vanadium oxide films yield mainly thermalized desorbing molecules or slightly hyper-thermal, depending on the oxidation state of the surface oxide.     

水分子在水化蒙脱石表面脱附行为的第一性原理研究

使用密度泛函理论计算研究了水分子在蒙脱石(MMT)表面的脱附作用.水分子在MMT表面的脱附受到配衡离子(Li~+,Na~+,或K~+),MMT表面和吸附水分子的共同作用.通过势能面扫描得到了脱附水分子在MMT表面脱附的能量路径.通过对脱附水分子能量路径的分析发现:水分子在MMT表面的脱附能垒受到配衡离子种类的影响,半径较小的配衡离子具有较大的脱附能垒.此外,吸附水的存在能降低脱附水分子的脱附能垒,使脱附过程容易触发.     

水分子在水化蒙脱石表面脱附行为的第一性原理研究

使用密度泛函理论计算研究了水分子在蒙脱石(MMT)表面的脱附作用．水分子在MMT 表面的脱附受到配衡离子(Li+,Na+,或K+),MMT表面和吸附水分子的共同作用．通过势能面扫描得到了脱附水分子在MMT表面脱附的能量路径．通过对脱附水分子能量路径的分析发现：水分子在MMT 表面的脱附能垒受到配衡离子种类的影响,半径较小的配衡离子具有较大的脱附能垒．此外,吸附水的存在能降低脱附水分子的脱附能垒,使脱附过程容易触发．     

汞在Ti2NO2 MXene表面吸附氧化的第一性原理计算

汞是一种有毒的重金属,在生产生活中以各种形式排放的汞对生态及人类健康都存在一定程度的威胁.因此,寻找高效的汞吸附剂具有十分重要的意义.本文基于密度泛函理论的第一性原理计算方法,研究了汞在Ti₂NO₂(MXene)和具有一个氧空位缺陷的Ti₂NO₂(Ov-Ti₂NO₂)上的吸附和氧化机理.计算结果表明Hg⁰在Ti₂NO₂表面的吸附为物理吸附,在Ov-Ti₂NO₂表面为化学吸附. Ti₂NO₂表面氧空位的存在可以改善HgO与Ov-Ti₂NO₂之间的相互作用,从而使吸附能提高116 kJ/mol. Hg⁰在Ov-Ti₂NO₂表面氧化为HgO的反应能垒为92.55 kJ/mol,小于其在Ti_2<...     

Hydrogen adsorption on graphene: a first principles study

We present a systematic ab initio study of atomic hydrogen adsorption on graphene. The characteristics of the adsorption process are discussed in relation with the hydrogenation coverage. For systems with high coverage, the resultant strain due to substrate relaxation strongly affects H atom chemisorption. This leads to local structural changes that have not been pointed out to date, namely localized surface curvature. We demonstrate that the hydrogen chemisorption energy barrier is independent of the optimization technique and system size, being associated with the relaxation and rehybridization of the sole adsorbent carbon atom. On the other hand, the H desorption barrier is very sensitive to a correct structural relaxation and is also dependent on the degree of system hydrogenation.     

Molecular adsorption and growth of n-butane adlayers on Pt(1 1 1)

The molecular adsorption of n-butane and the growth of n-butane adlayers on Pt(1 1 1) was investigated using molecular beam techniques, temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED). It is found that as the surface coverage of n-butane increases, structural changes occur in the adlayer at surface temperatures near 98 K that are accompanied by changes in the binding energy and mobility of the adsorbed species. The film growth process can be divided into four distinct coverage regimes. At low coverages (θ<0.14 ML, where 1 ML is defined as one butane molecule per Pt atom) a disordered monolayer forms in which the butane molecules prefer to lie parallel to the surface in order to minimize their binding energy. At coverages from 0.14 to 0.20 ML, ordered regions develop within the monolayer in which the butane molecules also lie parallel to the surface. The binding energy in the ordered phase is lower than that in the disordered phase due to repulsive intermolecular interactions. A more densely-packed ordered phase begins to form at 98 K after the low-coverage ordered phase saturates at 0.20 ML. The experimental results suggest that the n-butane molecules tilt away from the surface in the high-coverage ordered phase. Finally, a disordered second layer phase forms after the high coverage ordered phase saturates at 0.35 ML. The molecules in the second layer are very mobile at 98 K and rapidly diffuse to the edges of the beam spot. Interchange of molecules between the second layer and ordered monolayer is found to govern the net rate of second layer diffusion at surface temperatures less than 133 K. The adsorption probability of n-butane on Pt(1 1 1) continuously increases with increasing coverage, with no significant dependencies on the structure of the n-butane adlayer. This finding indicates that the long-range arrangements and molecular orientations of a mobile alkane adlayer have a negligible influence on the intrinsic adsorption dynamics, suggesting that the energy transfer processes that facilitate adsorption are highly localized.     

Molecular beam study of the apparent activation barrier associated with adsorption and desorption of hydrogen on copper

Molecular beam techniques are employed to study the adsorption and desorption of H₂ on the (100), (110), and stepped (310) crystal faces of copper. Each crystal is exposed simultaneously to a supersonic molecular beam of H₂ (energy variable from 1.6 to 10.7 kcal/mole) and a highly dissociated beam of deuterium. The majority of the H₂ molecules are scattered from the surface (i.e., are not adsorbed), while a portion of the remaining molecules adsorb dissociatively and react catalytically with adsorbed deuterium atoms to form HD molecules. These HD molecules desorb, and their angular distribution is measured by a rotatable mass spectrometer. For all three crystal faces, the distributions of desorbed HD deviate significantly from diffuse emission and are in excellent agreement with the results of our previous permeation study. From the dependence of the HD signal on the energy and incident angle of the H₂ beam, it appears that there are substantial energy barriers to adsorption, with these barriers depending on crystallographic orientation and acting essentially perpendicular to the surfaces. Both the energy dependence of the dissociative adsorption probability and the shapes of the HD angular distributions are nearly identical for the stepped (310) and (100) surfaces, thereby suggesting that ledge sites are not the principal regions responsible for adsorption of hydrogen on copper. The estimated adsorption probabilities versus energy are “S” shaped curves which appear to level off at values considerably less than unity. A comparison of our results with a very simple model with a single energy barrier to adsorption is qualitatively but not quantitatively satisfactory. An interpretation which includes a distribution of energy barriers is suggested.     

The dynamics of hydrogen adsorption on polycrystalline uranium

The dynamics of dissociative hydrogen adsorption on clean polycrystalline uranium has been studied using temperature programmed desorption and supersonic molecular beams. The initial sticking probability was measured as a function of incident kinetic energy between 3 and 255 meV. Two adsorption channels were observed; a non-activated direct channel was shown to be active over the entire energy range studied and a low energy indirect channel that was characterised by a decrease in sticking probability with increasing beam energy, and an insensitivity to both surface temperature and a range of hydrogen coverages. Together these results suggest the existence of an unaccommodated molecular precursor that has sufficient lifetime and mobility to locate favourable sites and dissociatively adsorb.     

Theoretical study of the effect of spin-selective adsorption of water molecules on MgO surface

The effect of spin-selective adsorption of water molecules on the surface of MgO crystal is theoretically studied. The study is performed using two different approaches, i.e., quantum-chemical simulation and an analytical calculation in a quasiclassical approximation. The adsorption energy is calculated using the B3LYP density functional and the 6–311G* basis set. The calculated value of the adsorption energy 0.70 eV agrees well with an experimental value of 0.65 eV. It is established that the energy difference of adsorbed ortho- and para-water molecules is negligible and, thus, the difference of the adsorption energies is completely determined by the energy difference of free molecules in ortho- and para-states. It follows from the analytical calculation that this result is essentially general and is related not only to an MgO surface, but to any other surface on which the energy barrier for rotation of the adsorbed molecule is much larger than the corresponding rotational constant. Based on this, the conclusion is reached that the effect of spin-selective adsorption on these surfaces should not be observed under normal conditions.     

Kinetics of protein adsorption/desorption mediated by pH-responsive polymer layer

We propose a new way of regulating protein adsorption by using a p H-responsive polymer. According to the theoretical results obtained from the molecular theory and kinetic approaches, both thermodynamics and kinetics of protein adsorption are verified to be well controlled by the solution p H. The kinetics and the amount of adsorbed proteins at equilibrium are greatly increased when the solution environment changes from acid to neutral. The reason is that the increased p H promotes the dissociation of the weak polyelectrolyte, resulting in more charged monomers and more stretched chains.Thus the steric repulsion within the polymer layer is weakened, which effectively lowers the barrier felt by the protein during the process of adsorption. Interestingly, we also find that the kinetics of protein desorption is almost unchanged with the variation of p H. It is because although the barrier formed by the polymer layer changes along with the change of p H,the potential at contact with the surface varies equally. Our results may provide useful insights into controllable protein adsorption/desorption in practical applications.     

On the adsorption and desorption of H2 at metal surfaces

Recent technological developments have made possible measurements of the distribution of internal levels of molecules desorbing from a hot surface. Such measurements provide new information concerning the desorption process and the potential energy surface (PES) that governs it. Associative, or re-combinative desorption is of particular interest because the distributions of internal levels reflect the manner in which the molecular bond is formed as the desorbing species leaves the surface. As the simplest associative desorption systems, H₂ and D₂ adsorbing on and desorbing from metal surfaces deserve special attention and serve as prototypes for systems with a more complex chemistry. In this note I review briefly from the theoretical point of view some features of the interaction of H₂ with metals and their relevance to associative adsorption and dissociative sticking.     

基于第一性原理的CVD金刚石涂层表面氧原子作用分析

采用第一性原理方法研究了氧原子在CVD金刚石涂层表面吸附形成的两种氧掺杂结构的差异及脱附CO的难易程度.仿真计算结果表明：氧原子在金刚石表面顶位和桥位吸附形成C=O羰结构和C-O-C醚结构,改变与其直接成键的局部金刚石结构;C-O-C结构吸附能比C=O结构大,其结构更加稳定;C=O结构断键脱附形成CO的能垒比C-O-C结构更低,CVD金刚石涂层表面脱附CO主要是以C=O断键形成;氢终止表面能够增强碳原子之间成键,提高C=O脱附的能垒,而氧终止表面作用相反,降低脱附能垒.     

The adsorption of oxygen on a stepped platinum single crystal surface

The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.     

The kinetics of field adsorption

In an applied field of ～ 1 V/Å or higher, gas atoms and molecules near the electrode surface can be attracted to the surface and be field adsorbed there. Field adsorption can occur at a temperature much higher than that in ordinary physical adsorption. For example, field adsorption of He and Ne on metal surfaces can occur at a temperature above 100 K. The kinetic of field adsorption on spherical, cylindrical and field ion emitter surfaces are discussed. Methods for measuring the field adsorption energy, the gas supply constant, and other physical parameters using the pulsed-laser time-of-flight atom-probe are described. These parameters are essential for describing the kinetics of field adsorption. We consider field adsorption and desorption of both atomic and molecular gases. In molecular gases, field dissociation of field desorbed species can occur. A proper account of this effect has to be included in the data analysis. A case with two adsorption states at different adsorption sites is also considered. Field adsorption must play an important role in the early stages of cloud formation, and in the adsorption-desorption and catalytical reactions on the surface of interstellar particles, etc.     

Relation between adsorption and catalysis in the case of NiO and Co3O4

Reversed flow-inverse gas chromatography is a quick, precise and effective methodology to characterize physicochemical properties of adsorbents. This is extended to the experimental measurement of the adsorption energy distribution function as well as of the differential energy of adsorption due to lateral interactions of molecules adsorbed on two catalysts, namely Co₃O₄ and NiO. Thus, the nature and the strength of the adsorbate-adsorbent and adsorbate-adsorbate interactions are extracted in order to give detailed answers to the questions: (a) where are the molecules on the heterogeneous surface and (b) which is the nature of the surface chemical bonds? Thus, adsorption of 1-butene was found to take place immediately and irreversibly. It holds a deep relation between adsorption and catalysis of 1-butene over these catalysts. As a consequence, the adsorption of 1-butene in the presence of hydrogen leads to isobutane and/or n-butane, depending on the temperature. It can be seen from the adsorption/desorption kinetic constants that the adsorption of 1-butene on Co₃O₄ is one order higher than over NiO. This fact in connection with the bigger activation energy and the lower kinetic coefficients concerning hydrogenation reaction over NiO shows that Co₃O₄ is a better catalyst for this kind of catalysis.     

Associative desorption of adsorbed oxygen promoted by translational energy of incident O2 molecules

Effects of the translational energy of incident oxygen molecules on surface processes from dissociative adsorption to subsequent associative desorption on Pt(1 1 1) have been investigated by means of supersonic molecular beam scattering. The reaction rate of associative desorption increases with the incident translational energy. The enhanced associative desorption can be attributed to nonthermal diffusion of dissociated oxygen, promoted by the translational energy of incoming molecules.     

Kinetics of adsorption and desorption using Auger electron spectroscopy: Application to xenon covered (0001) graphite

Three regimes of condensation have been observed between 74 and 80 K in the adsorption and desorption of a submonolayer film of xenon. The first one corresponds to thej condensation or evaporation of a two-dimensional (2D) ‘gas’, the second one to the growth of 2D crystal in the presence of the 2D gas, and the third one to the completion of the 2D crystal on the (0001) graphite face. Zero order kinetics for both adsorption and desorption is found in the large range of coverage (0.3 < θ <0.9) where the two phases coexist on the surface. The activation energy of desorption of the 2D crystal is measured; its value (～6 kcal mole^?1) is in fair agreement with the value of the latent heat of evaporation of this phase (5.5 or 5.7 kcal mole^?1) determined previously. No activation energy of nucleation has been observed during the adsorption process. The growth rate is controlled by the incident flux only.