Atomistic kinetic Monte Carlo study of atomic layer deposition derived from density functional theory

To describe the atomic layer deposition (ALD) reactions of HfO₂ from Hf(N(CH₃)₂)₄ and H₂O, a three‐dimensional on‐lattice kinetic Monte‐Carlo model is developed. In this model, all atomistic reaction pathways in density functional theory (DFT) are implemented as reaction events on the lattice. This contains all steps, from the early stage of adsorption of each ALD precursor, kinetics of the surface protons, interaction between the remaining precursors (steric effect), influence of remaining fragments on adsorption sites (blocking), densification of each ALD precursor, migration of each ALD precursors, and cooperation between the remaining precursors to adsorb H₂O (cooperative effect). The essential chemistry of the ALD reactions depends on the local environment at the surface. The coordination number and a neighbor list are used to implement the dependencies. The validity and necessity of the proposed reaction pathways are statistically established at the mesoscale. The formation of one monolayer of precursor fragments is shown at the end of the metal pulse. Adsorption and dissociation of the H₂O precursor onto that layer is described, leading to the delivery of oxygen and protons to the surface during the H₂O pulse. Through these processes, the remaining precursor fragments desorb from the surface, leaving the surface with bulk‐like and OH‐terminated HfO₂, ready for the next cycle. The migration of the low coordinated remaining precursor fragments is also proposed. This process introduces a slow reordering motion (crawling) at the mesoscale, leading to the smooth and conformal thin film that is characteristic of ALD. © 2013 Wiley Periodicals, Inc.     

Grazing incidence-X-ray fluorescence spectrometry for the compositional analysis of nanometer-thin high-kappa dielectric HfO2 layers.

In future microelectronic devices, SiO2 as a gate dielectric material will be replaced by materials with a higher dielectric constant. One such candidate material is HfO2. Thin layers are typically deposited from ligand-containing precursors in chemical vapor deposition (CVD) processes. In the atomic layer deposition (ALD) of HfO2, these precursors are often HfCl4 and H2O. Obviously, the material properties of the deposited films will be affected by residual ligands from the precursors. In this paper, we evaluate the use of grazing incidence--and total reflection-X-ray fluorescence spectrometry (GI-XRF and TXRF) for Cl trace analysis in nanometer-thin HfO2 films deposited using ALD. First, the results from different X-ray analysis approaches for the determination of Hf coverage are compared with the results from Rutherford backscattering spectrometry (RBS). Next, we discuss the selection of an appropriate X-ray excitation source for the analysis of traces within the high-kappa: layers. Finally, we combine both in a study on the accuracy of Cl determinations in HfO2 layers.     

Chemisorption of HCl to the MgO(001) surface: a DFT study

We use plane wave and embedded cluster ab initio density functional calculations to study adsorption, dissociation and diffusion of the HCl molecule on the MgO(001) surface. The two methods yield comparable results for adsorption of an isolated HCl molecule and complement each other when considering charged species and coverage effects. We find dissociative chemisorption at a coverage smaller than 0.5 monolayer with a Cl(-) ion electrostatically coupled to the OH(-) ion at the surface oxygen site. The adsorption energy of the Cl(-)[dot dot dot](OH)(-) complex is 1.5 eV and the activation energy of Cl(-) diffusion away from OH(-) is 0.6 eV. There is no significant activation energy for rotation of Cl(-) around the adsorption site. At rising coverage, an increase in dipole-dipole repulsion between HCl molecules leads to a lowering of the adsorption energy per HCl and a change of binding towards hydrogen-bridge type as well as a lowering of the activation energy for Cl(-) diffusion. OH(-) formed in the surface due to HCl adsorption has a stretch frequency of 3,083 cm(-1) with Cl(-) associated and 3,648 cm(-1) with Cl(-) removed.     

Multilayer adsorption of water at a rutile TiO2)(110) surface: towards a realistic modeling by molecular dynamics

We present a model combining ab initio concepts and molecular dynamics simulations for a more realistic treatment of complex adsorption processes. The energy, distance, and orientation of water molecules adsorbed on stoichiometric and reduced rutile TiO(2)(110) surfaces at 140 K are studied via constant temperature molecular dynamics simulations. From ab initio calculations relaxed atomic geometries for the surface and the most probable adsorption sites were derived. The study comprises (i) large two-dimensional surface supercells, providing a realistically low concentration of surface oxygen defects, and (ii) a water coverage sufficiently large to model the onset of the growth of a bulk phase of water on the surface. By our combined approach the influence of both, the metal oxide surface, below, and the bulk water phase, above, on the water molecules forming the interface between the TiO(2) surface and the water bulk layer is taken into account. The good agreement of calculated adsorption energies with experimental temperature programmed desorption spectra demonstrates the validity of our model.     

MSINDO study of water adsorption on NiO surfaces

The dissociation of water adsorbed on the surface of NiO was investigated by using the semi-empirical SCF MO method MSINDO. Simulations were based on embedded cluster models representing the (100) surface, with and without a monatomic step. The angle formed between the metal adsorption site and the O–H bond associated with water has been found to be critical to the energetics of the dissociation process. Based on this criterion, it was shown that water dissociation is favorable on the stepped surface, but highly unlikely on the planar surface. In addition, the activation energy required for water dissociation in a monatomic NiO step was considerably lower than for dissociation at the planar surface. The high activation energy associated with water dissociation on the planar surface is attributed to the rigidity of the NiO lattice. Dedicated to Prof. K. Jug in honor of his 65th birthday     

The Surface Chemistry of Water on Fe(100): A Density Functional Theory Study

The formation of water by hydrogenation of atomic oxygen is studied using density functional theory. Atomic oxygen preferentially adsorbs at the four‐fold hollow site, the hydroxyl group prefers the bridge site in a tilted configuration, and water is most stable when adsorbed at the top site with the two O? H bonds parallel to the Fe surface. Water formation by the hydrogenation of oxygen is a highly activated process on the Fe(100) surface, with similar activation energies, in the order of 1.1 eV, for the first and second hydrogen additions. A more favourable route for the addition of the second hydrogen atom involves the disproportionation of hydroxyl groups to form water and adsorbed oxygen. Dissociation of the OH is also likely since the activation energy is similar to that for disproportionation of 0.65 eV. Furthermore, the results show that the dissociation of water on Fe(100) is a non‐activated process: 0.16 eV for the zero‐coverage limit and 0.03 eV when surface oxygen is present. Herein, adsorption energies, structures and vibrational frequencies are presented for several adsorption states at 0.25 ML coverage, as well as the potential energy surface for water formation on Fe(100).     

Insights into the H_2O/V_2O_5 Interface Structure for Optimizing Water-splitting

The interaction of water(H2O) with metal oxide surfaces is of fundamental importance to various fields of science, ranging from batteries to catalysis. In particular, vanadium pentoxide(V2O5) has been widely used as electrode materials for aqueous-battery and catalysts. Herein, theoretical(density functional theory) study gives atomic-scale insights into water monolayers in V2O5 and single-molecule adsorption and dissociation at three low-index surfaces and oxygen-vacancy V2O5(001) surface. The H2O/V2O5 interface structure was identified. The results show that H2O is adsorbed on the stoichiometric V2O5(001) surface with physisorption mechanism, and the dissociation hardly occurs. Water adsorbs as an intact monomer with a computed binding energy of 0.75 eV. The formation of ordered water overlayers has been observed on V2O5(001) surface, suggesting a locally ordered superstructure of molecular water. The molecular H2O adsorption on oxygen-vacancy V2O5(001) surface is stronger than that on the stoichiometric V2O5(001) surface, and H2O can undergo dissociative chemisorption to form a surface hydroxyl group and a H adatom. V2O5 can take the oxygen from H2O, which is consistent with the experimental results.     

Theoretical analysis of initial adsorption of high-κ metal oxides on In(x)Ga(1-x)As(0 0 1)-(4×2) surfaces

Ordered, low coverage to monolayer, high-κ oxide adsorption on group III rich InAs(0 0 1)-(4×2) and In(0.53)Ga(0.47)As(0 0 1)-(4×2) was modeled via density functional theory (DFT). Initial adsorption of HfO(2) and ZrO(2) was found to remove dangling bonds on the clean surface. At full monolayer coverage, the oxide-semiconductor bonds restore the substrate surface atoms to a more bulklike bonding structure via covalent bonding, with the potential for an unpinned interface. DFT models of ordered HfO(2)/In(0.53)Ga(0.47)As(0 0 1)-(4×2) show it fully unpins the Fermi level.     

Interaction of water with clean and oxygen precovered nickel surfaces

The adsorption of water on a Ni(111) single crystal surface, clean as well as precovered with oxygen, has been investigated with thermal desorption spectroscopy (TDS) and measurements of the adsorption-desorption equilibrium combined with XPS (X-ray photoelectron spectroscopy). The measurements have been carried out with water pressures up to 10^–5 mbar on surfaces, which have been either clean or precovered with oxygen. On the clean Ni(111) surface the first adsorbate layer with a maximum coverage of 0.42 ML (monolayers) has a desorption energy of 52 kJ/mol and a preexponential factor of desorption of 10¹⁶s^–1. A second water layer adsorbs with the desorption energy of the ice multilayer but with first order kinetics. On Ni(111) precovered with chemisorbed oxygen an additional state of molecular, more strongly bound water is found, but no dissociation. For higher oxygen precoverages where NiO islands are formed on the surface, also the water dissociation product OH is found adsorbed. On a sample covered with a closed NiO layer, adsorbed OH and molecular water in an energetically not well-defined state are found. High doses of water on oxygen-precovered Ni(111) induce a slow surface modification leading to water dissociation.     

Guanidinate-stabilized monomeric hafnium amide complexes as promising precursors for MOCVD of HfO2

Novel guanidinato complexes of hafnium [Hf{eta2-(iPrN)2CNR2}2(NR2)2] (R2 = Et2, 1; Et, Me, 2; Me2, 3), synthesized by insertion reactions of N,N'-diisopropylcarbodiimide into the M-N bonds of homologous hafnium amide complexes 1-3 and {[mu2-NC(NMe2)2][NC(NMe2)2]2HfCl}2 (4) using a salt metathesis reaction, are reported. Single-crystal X-ray diffraction analysis revealed that compounds 1-3 were monomers, while compound 4 was found to be a dimer. The observed fluxional behavior of compounds 1-3 was studied in detail using variable-temperature and two-dimensional NMR techniques. The thermal characteristics of compounds 1-3 seem promising for HfO2 thin films by vapor deposition techniques. Metal-organic chemical vapor deposition experiments with compound 2 as the precursor resulted in smooth, uniform, and stoichiometric HfO2 thin films at relatively low deposition temperatures. The basic properties of HfO2 thin films were characterized in some detail.     

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.     

Are the reduction and oxidation properties of nitrocompounds dissolved in water different from those produced when adsorbed on a silica surface? A DFT M05‐2X computational study

The reduction and oxidation properties of four nitrocompounds (trinitrotoluene [TNT], 2,4‐dinitrotoluene, 2,4‐dinitroanisole, and 5‐nitro‐2,4‐dihydro‐3H‐1,2,4‐triazol‐3‐one [NTO]) dissolved in water as compared with the same properties for compounds adsorbed on a silica surface were studied. To consider the influence of adsorption, cluster models were developed at the M05/tzvp level. A hydroxylated silica (001) surface was chosen to represent a key component of soil. The PCM(Pauling) and SMD solvation models were used to model water bulk influence. The following properties were analyzed: electron affinity, ionization potential, reduction Gibbs free energy, oxidation Gibbs free energy, and reduction and oxidation potentials. It was found that adsorption and solvation decrease gas phase electron affinity, ionization potential, and Gibbs free energy of reduction and oxidation, and thus, promote redox transformation of nitrocompounds. However, in case of solvation, the changes are more significant than for adsorption. This means that nitrocompounds dissolved in water are easier to transform by reduction or oxidation than adsorbed ones. Among the considered compounds, TNT was found to be the most reactive in an electron attachment process and the least reactive for an electron detachment transformation. During ionization, a deprotonation of adsorbed NTO was found to occur. © 2015 Wiley Periodicals, Inc.     

Water on extended and point defects at MgO surfaces

The interaction of water with extended defects such as mono- and diatomic steps at the MgO(100) surface is investigated through first-principles simulations, as a function of water coverage. At variance with flat MgO(100) terraces, water adsorption is always dissociative on mono- and diatomic steps, as well as on MgO(110) surfaces. In most of the equilibrium configurations, the oxygen of the hydroxyl groups is two- or fourfold coordinated, but single-coordinated OH groups can be stabilized at diatomic step edges. The structural properties of the hydroxyl groups are discussed as a function of their coordination numbers and mutual interactions, as well as the surface defect morphology. It is shown that characteristics of water adsorption are primarily driven by the coordination number of the surface acid-base pair where the dissociation occurs. However, the OH groups resulting from water dissociation are also considerably stabilized by the electrostatic interaction with coadsorbed protons. At low coverage such an interaction, considerably stronger than hydrogen bonding, practically hinders any proton diffusion away from its neighboring hydroxyl. The computed adsorption energies allow us to discuss the onset of water desorption from flat MgO(100) terraces, diatomic and monoatomic steps, and from Mg-O divacancy.     

DFT Study of Metal Atoms Adsorbed at Low-coordinated Sites of MgO （001） Surface

1 INTRODUCTION The interfaces between metals and oxide play a vital role in many industrial applications: hetero- geneous catalysis, microelectronics, thermal barriers, corrosion protection, metal processing and so on[1]. In catalysis, the choice of metal and oxide support is critical in order to obtain a desired reactivity and selectivity[2]. This is due in part to the inherent reac- tivity of the two components. Also the size and shape of the metal particle, which depend on the choice…     

Mechanism for NO2 charging on metal supported MgO

NO2 adsorbed on MgO(100) supported by Ag or Pt is explored by density functional theory calculations. NO2 is weakly adsorbed on MgO(100), with a bond involving minor oxide to adsorbate charge transfer. However, if MgO is supported, then the adsorption energy is considerably enhanced and NO2 is adsorbed as a nitrite (N). Analysis reveals that the NO2 excess charge originates from the oxide side of the oxide/metal interface and that the electron abstraction increases the oxide/metal adhesion. The proposed mechanism is general and should apply for oxidizing surface species.     

Diffuse Reflectance IR Spectra of Molecular Hydrogen and Deuterium Adsorbed on Zinc Oxide

Diffuse reflectance IR spectroscopy is used to study hydrogen and deuterium adsorption on zinc oxide at room temperature and 77 K. At room temperature, H₂ and D₂ molecules are dissociatively adsorbed with the formation of hydrides and hydroxy groups of three types. At 77 K, diffuse reflectance spectra reveal the bands from molecular hydrogen and deuterium in addition to the dissociatively adsorbed forms. The presence of several bands of stretching H–H and D–D vibrations points to the nonuniformity of adsorption sites. This nonuniformity is also confirmed by the fact that, after heating zinc oxide from 77 K to room temperature in an atmosphere of hydrogen, only an insignificant portion of adsorbed molecular hydrogen dissociates. Most of dissociatively adsorbed hydrogen is formed without a molecular precursor. The dissociation of H₂ and D₂ most likely occurs on very active adsorption species so rapidly that the molecular precursor is not observed. The bond energy in molecular deuterium precursors of dissociation estimated from the fundamental vibration frequency and the overtone of D–D vibrations suggests moderate excitation of the bond. This agrees well with the conclusion that the dissociative adsorption of hydrogen and deuterium occurs without a molecular precursor.     

Interaction between water molecules and zinc sulfide nanoparticles studied by temperature-programmed desorption and molecular dynamics simulations

We have investigated the bonding of water molecules to the surfaces of ZnS nanoparticles (approximately 2-3 nm sphalerite) using temperature-programmed desorption (TPD). The activation energy for water desorption was derived as a function of the surface coverage through kinetic modeling of the experimental TPD curves. The binding energy of water equals the activation energy of desorption if it is assumed that the activation energy for adsorption is nearly zero. Molecular dynamics (MD) simulations of water adsorption on 3 and 5 nm sphalerite nanoparticles provided insights into the adsorption process and water binding at the atomic level. Water binds with the ZnS nanoparticle surface mainly via formation of Zn-O bonds. As compared with bulk ZnS crystals, ZnS nanoparticles can adsorb more water molecules per unit surface area due to the greatly increased curvature, which increases the distance between adjacent adsorbed molecules. Results from both TPD and MD show that the water binding energy increases with decreasing the water surface coverage. We attribute the increase in binding energy with decreasing surface water coverage to the increasing degree of surface under-coordination as removal of water molecules proceeds. MD also suggests that the water binding energy increases with decreasing particle size due to the further distance and hence lower interaction between adsorbed water molecules on highly curved smaller particle surfaces. Results also show that the binding energy, and thus the strength of interaction of water, is highest in isolated nanoparticles, lower in nanoparticle aggregates, and lowest in bulk crystals. Given that water binding is driven by surface energy reduction, we attribute the decreased binding energy for aggregated as compared to isolated particles to the decrease in surface energy that occurs as the result of inter-particle interactions.     

Dissociative Adsorption of Water Molecules on Uncharged Surfaces of Indium(111) and Gallium

A possible mechanism of dissociative adsorption (DA) of water on the (111) surface of indium and liquid gallium is investigated within a cluster model for metal using a density functional method (B3LYP). The adsorption interaction of H and O atoms and OH group with these metals is studied. The free energy of DA of H₂ and O₂ molecules is calculated. An analysis of DA is performed both for the case of the metal/vacuum interface and with allowance made for solvation effects within a continuum approach. According to quantum-chemical calculations, DA of water on the In(111) surface is more thermodynamically probable than on gallium. In the case of indium, DA with the participation of a water dimer may have a smaller activation energy compared with DA of monomer H₂O_ads. The data obtained are used to interpret the experimentally observed nonmonotonous dependence of the work function for indium and gallium on the partial pressure of water vapor. The hypothesis about the origin of the absorption band in electroreflectance spectra for the gallium/aqueous solution interface as a result of the electron transfer from an adsorbed water molecule into the metal's conduction band is confirmed.     

甲醇在氧化锌表面吸附的研究

摘要用原位红外和脉冲实验研究了甲醇在氧化锌表面的吸附行为. FTIR结果表明, 甲醇吸附于氧化锌上易生成甲氧基, 且其生成量随着吸附温度的提高而增加. 进一步的研究结果表明, 甲氧基是由甲醇同氧化锌表面的羟基反应生成的, 将其暴露于水蒸汽中后很快消失. 脉冲实验发现, 氧化锌上脉冲甲醇时产生水, 再脉冲水则产生甲醇. 因此甲醇在氧化锌表面吸附生成甲氧基和水的反应是可逆的.     

Adsorption of HCl on the water ice surface studied by X-ray absorption spectroscopy

The adsorption state of HCl at 20 and 90 K on crystalline water ice films deposited under ultrahigh vacuum at 150 K has been studied by X-ray absorption spectroscopy at the O1s K-edge and Cl2p L-edge. We show that HCl dissociates at temperatures as low as 20 K, in agreement with the prediction of a spontaneous ionization of HCl on ice. Comparison between the rate of saturation of the "dangling" hydrogen bonds and the chlorine uptake indicates that hydrogen bonding of HCl with the surface native water "dangling" groups only accounts for a small part of the ionization events (20% at 90 K). A further mechanism drives the rest of the dissociation/solvation process. We suggest that the weakening of the ice surface hydrogen-bond network after the initial HCl adsorption phase facilitates the generation of new dissociation/solvation sites, which increases the uptake capacity of ice. These results also emphasize the necessity to take into account not only a single dissociation event but its catalyzing effect on the subsequent events when modeling the uptake of hydrogen-bonding molecules on the ice surface.