ZrO2在Si(100)-2×1表面原子层淀积反应机理的密度泛函理论研究

用密度泛函方法研究了ZrO₂在羟基预处理的Si(100)-2×1表面原子层淀积(ALD)初始反应过程的反应机理, ZrO₂的ALD过程包括两个前体反应物ZrCl₄和H₂O交替的半反应. 两个半反应都经历一个相似的吸附中间体反应路径. 比较单羟基Si表面反应的反应焓变, 可以发现双羟基Si表面反应, 由于相邻羟基的存在, 对ZrCl₄的半反应影响较大, 尤其是化学吸附能增加明显. 而对于H₂O的半反应, 单、双羟基Si表面反应的能量变化不是很明显. 使用内禀反应坐标(IRC)方法, 验证了两个半反应存在相似的过渡态结构和反应机理. 另外, 发现随着温度的升高, 吸附络合物的稳定性降低, 其向反应物方向的解吸附变得容易, 而向产物方向的解离难度增加.     

Inherently Area-Selective Atomic Layer Deposition of Manganese Oxide through Electronegativity-Induced Adsorption

Manganese oxide (MnO_x) shows great potential in the areas of nano-electronics, magnetic devices and so on. Since the characteristics of precise thickness control at the atomic level and self-align lateral patterning, area-selective deposition (ASD) of the MnO_x films can be used in some key steps of nanomanufacturing. In this work, MnO_x films are deposited on Pt, Cu and SiO₂ substrates using Mn(EtCp)₂ and H₂O over a temperature range of 80–215 °C. Inherently area-selective atomic layer deposition (ALD) of MnO_x is successfully achieved on metal/SiO₂ patterns. The selectivity improves with increasing deposition temperature within the ALD window. Moreover, it is demonstrated that with the decrease of electronegativity differences between M (M = Si, Cu and Pt) and O, the chemisorption energy barrier decreases, which affects the initial nucleation rate. The inherent ASD aroused by the electronegativity differences shows a possible method for further development and prediction of ASD processes.     

In situ auger electron spectroscopy study of atomic layer deposition: growth initiation and interface formation reactions during ruthenium ALD on Si-H, SiO2, and HfO2 surfaces

Growth initiation and film nucleation in atomic layer deposition (ALD) is important for controlling interface composition and achieving atomic-scale films with well-defined composition. Ruthenium ALD is studied here using ruthenocene and oxygen as reactants, and growth initiation and nucleation are characterized on several different growth surfaces, including SiO2, HfO2, and hydrogen terminated silicon, using on-line Auger electron spectroscopy and ex-situ X-ray photoelectron spectroscopy. The time needed to reach the full growth rate (typically approximately 1 A per deposition cycle) is found to increase as the surface energy of the starting surface (determined from contact angle measurements) decreased. Growth starts more readily on HfO2 than on SiO2 or Si-H surfaces, and Auger analysis indicates distinct differences in surface reactions on the various surfaces during film nucleation. Specifically, surface oxygen is consumed during ruthenocene exposure, so the nucleation rate will depend on the availability of oxygen and the energetics of surface oxygen bonding on the starting substrate surface.     

Measuring the heat evolved from individual reaction steps in atomic layer deposition

A thermopile has been constructed for detecting heat of reaction during the individual steps taking place in the growth cycles of atomic layer deposition (ALD). The thermopile sensor consists of 64 junctions of thermocouple type K. It has successfully been applied to characterize ALD growth of Al₂O₃ from Al(CH₃)₃ (TMA) and H₂O, and has furthermore been applied to explore energetics of ALD growth for the following combinations of precursors: TMA + O₃, TMA + O₂, TMA + hydroquinone, TiCl₄ + H₂O, and Zn(CH₃CH₂)₂ + H₂O. The thermopile clearly identifies exo/endothermal reaction steps, the effect of surface temperature on exposure to precursors from cold sources as well as variation in the flow of gases, and allows setting up of experiments where variations in precursors and pulsing parameters may provide mechanistic insight into the ALD growth. The sensor represents a new and complementary tool for in situ characterization of thin film growth by ALD.     

Atomic Layer Deposition of Iron Sulfide and Its Application as a Catalyst in the Hydrogenation of Azobenzenes

The atomic layer deposition (ALD) of iron sulfide (FeS_x ) is reported for the first time. The deposition process employs bis(N ,N′ ‐di‐tert‐butylacetamidinato)iron(II) and H₂S as the reactants and produces fairly pure, smooth, and well‐crystallized FeS_x thin films following an ideal self‐limiting ALD growth behavior. The FeS_x films can be uniformly and conformally deposited into deep narrow trenches with aspect ratios as high as 10:1, which highlights the broad applicability of this ALD process for engineering the surface of complex 3D nanostructures in general. Highly uniform nanoscale FeS_x coatings on porous γ‐Al₂O₃ powder were also prepared. This compound shows excellent catalytic activity and selectivity in the hydrogenation of azo compounds under mild reaction conditions, demonstrating the promise of ALD FeS_x as a catalyst for organic reactions.     

Nanopatterning of Si(1 1 1) surfaces by atomic force microscope scratching of an organic monolayer

In this work, we demonstrate selective electroless deposition of Cu into nanoscratches produced on n-type Si(1 1 1) surfaces covered with an organic monolayer. The organic layer (undecylenic acid) was covalently attached to a hydrogen-terminated Si surface. The nanosize scratches were produced with an atomic force microscope (AFM) in contact mode using a diamond-coated tip. Copper was deposited in the scratched regions with an electroless (immersion plating) approach using a 0.05 M CuSO₄ + 1% HF electrolyte. The results show clearly that the organic layer can be used as a mask for the deposition of Cu. Optimization of the electrochemical parameters, leads to a very high selectivity and uniform and well-defined nanostructures. This process represents a novel approach for a direct patterning of Si surfaces using an immersion plating reaction.     

Effect of atomic-layer-deposited TiO2 on carbon-supported Ni catalysts for electrocatalytic glycerol oxidation in alkaline media

Thin TiO₂ layers were deposited onto a carbon-supported Ni catalyst (Ni/C) through atomic layer deposition (ALD) and the resulting TiO₂-coated Ni/C (ALD(TiO₂)-Ni/C) was utilized for electrochemical glycerol oxidation in alkaline media. X-ray photoelectron spectroscopy analysis demonstrated that the Ni surface phase of ALD(TiO₂)-Ni/C mainly consisted of Ni(OH)₂ while that of uncoated Ni/C was a mixed phase of NiO and Ni(OH)₂. The ALD(TiO₂)-Ni/C exhibited electrocatalytic activity at least 2.4 times higher than that of Ni/C. Density functional theory calculations were used to investigate how the modified Ni surface with the TiO₂ coating affects the adsorption/desorption of glycerol.     

Adsorption of TiCl4 and electron donor on defective MgCl2 surfaces and propylene polymerization over Ziegler-Natta catalyst: A DFT study

The formations of defective MgCl₂ surfaces, and subsequent adsorption of Ti species and electron donor, as well as propylene polymerization over the Ziegler-Natta catalyst have been investigated using density functional theory (DFT) method. Twelve possible support models of regular and defective MgCl₂ (110) and (100) surfaces were built. The individual adsorptions of titanium chlorides as mononuclear or dinuclear, and ethyl benzoate (EB) as electron donor, on these models were evaluated. The analysis of energies presented the cases of EB adsorption were generally more stable than titanium chlorides on both surfaces. Thus, EB as internal electron donor mainly prevented TiCl₄ from coordinating on the MgCl₂ surfaces where mostly non-stereospecific active sites could be formed. Exceptionally, A5 the site model with terminal Cl-vacancy on the MgCl₂ support, presented stronger adsorption of TiCl₄ than that of EB on (110) surface. Since the TiCl₄ and ethyl benzoate (EB) would compete to adsorb on the support surface, it seems reasonable to assume that TiCl₄ might predominately occupy this site, which can act as the most plausible active site for propylene polymerization. The first insertion of propylene monomer into the A5 active site model showed that it exhibited good regioselectivity but poor stereospecificity in the absence of electron donor.     

Atomic Layer Deposition (ALD) of Alumina over Activated Carbon Electrodes Enabling a Stable 4 V Supercapacitor Operation

Designing high voltage (>3 V) and stable electrochemical supercapacitors with low self-discharge is desirable for the applications in modern electronic devices. This work demonstrates a 4 V symmetric supercapacitor with stabilized cycling performance through atomic layer deposition (ALD) of alumina (Al₂O₃) on the surface of activated carbon (AC). The 20-cycle ALD Al₂O₃ coated AC delivers 84 % capacitance retention after 1000 charge/discharge cycles under 4 V, contrary to the bare AC cells having only 48 % retention. The extended cycling life is associated with the thickened Stern layer and suppressed oxygen functional group. The self-discharge data also show that the Al₂O₃ coating enables AC cells to maintain 53 % of charge retention after 12 h, which is more than twice higher than that of bare AC cells under the same test protocol of 4 V charging. The curve fitting analysis reveals that ALD coating induced slow self-discharge dominated by ion diffusion mechanism, thus enhancing the AC surface energy.     

Single-step synthesis of TiC mesocrystals on the MWCNTs surface by the pyrolysis of Cp2TiCl2

Titanium carbide mesocrystals have been manufactured by the one-stage metalorganic chemical vapor deposition (MOCVD) technology using the pyrolysis of titanocene dichloride, Cp₂TiCl₂ (Cp = η⁵-C₅H₅) on the surfaces of multi-walled carbon nanotubes (MWCNTs). The initial stages of the Cp₂TiCl₂ gas-phase dissociation were modeled with density functional theory (DFT) calculations. A series of syntheses were carried out with a step-by-step increase of the Cp₂TiCl₂ amount. These experiments allows to observe intermediate stages of self-assembly of TiC mesocrystals. Thus, it was possible to identify that the decomposition of Cp₂TiCl₂ at 900 °C leads to the formation of TiC crystals nanoparticles with their accompanying self-assembly. As a final stage of this process, perfect mesocrystals of a cubic habit with a size of approximately 50 μm were found. The usage of powdered MWCNTs as a substrate in the chemical vapor deposition (CVD) processes provides heterogeneous conditions, which can be promising for the search for new forms of nanostructured materials.     

Conformal Surface Coatings to Enable High Volume Expansion Li‐Ion Anode Materials

An alumina surface coating is demonstrated to improve electrochemical performance of MoO₃ nanoparticles as high capacity/high‐volume expansion anodes for Li‐ion batteries. Thin, conformal surface coatings were grown using atomic layer deposition (ALD) that relies on self‐limiting surface reactions. ALD coatings were tested on both individual nanoparticles and prefabricated electrodes containing conductive additive and binder. The coated and non‐coated materials were characterized using transmission electron microscopy, energy‐dispersive X‐ray spectroscopy, electrochemical impedance spectroscopy, and galvanostatic charge/discharge cycling. Importantly, increased stability and capacity retention was only observed when the fully fabricated electrode was coated. The alumina layer both improves the adhesion of the entire electrode, during volume expansion/contraction and protects the nanoparticle surfaces. Coating the entire electrode also allows for an important carbothermal reduction process that occurs during electrode pre‐heat treatment. ALD is thus demonstrated as a novel and necessary method that may be employed to coat the tortuous network of a battery electrode.     

Atomic layer deposition of TiO2−xNx thin films for photocatalytic applications

Titanium dioxide (TiO₂) is recognized as the most efficient photocatalytic material, but due to its large band gap energy it can only be excited by UV irradiation. Doping TiO₂ with nitrogen is a promising modification method for the utilization of visible light in photocatalysis. In this work, nitrogen-doped TiO₂ films were grown by atomic layer deposition (ALD) using TiCl₄, NH₃ and water as precursors. All growth experiments were done at 500 °C. The films were characterized by XRD, XPS, SEM and UV–vis spectrometry. The influence of nitrogen doping on the photocatalytic activity of the films in the UV and visible light was evaluated by the degradation of a thin layer of stearic acid and by linear sweep voltammetry. Light-induced superhydrophilicity of the films was also studied. It was found that the films could be excited by visible light, but they also suffered from increased recombination.     

Copper nanowire arrays surface wettability control using atomic layer deposition of TiO<Subscript>2</Subscript>

Template two step electrodeposition method and atomic layer deposition were used to synthesize copper nanowires of varied length (1.2 to 26.2 μm) and copper nanowires coated with titanium dioxide. As a result of the atomic layer deposition of TiO₂, coated nanowires demonstrated an up to 10-fold decrease in the wetting angle, compared with uncoated nanowires. It was found the dissipation rate is substantially higher for nanowires coated by the atomic layer deposition method (100 s) as compared with the uncoated copper nanowires (400 s), which assumes the positive properties of water propagation along the surface, necessary for improving the heat transfer. It was also found that the water contact angle for uncoated nanowires and those coated with TiO₂ by the atomic layer deposition (ALD) gradually increases as the samples are kept in air. A gradual increase in wettability was also observed for smooth silicon wafers coated by ALD of TiO₂, which were exposed to air. On the coated silicon substrates, the wetting angle gradually increased from 10° to approximately 56° in the course of four days. In addition, it was shown that copper nanowires coated with TiO₂ by the atomic layer deposition method have an excellent corrosion resistance, compared with uncoated nanowires, when brought in contact with air and water.     

Computational simulations of pore nucleation in silicon(1 1 1)

Float zone n-Si(1 1 1) was electrochemically etched in diluted NH₄F to form porous nuclei. The experimental results were compared with computational simulations of pore nucleation and growth. Electrochemical etching of silicon(1 1 1) results in pore nucleation preferentially localized on the edges of atomic terraces. The initial pore nuclei have diameter and depth of 17 nm and 0.3 nm, respectively. We find a correlation between H-terminated Si(1 1 1) atomic surface morphology and electric field distribution on pore nucleation and growth mechanism. The H-terminated surface is composed from wide (100–200 nm) atomic terraces with steps of 0.3 nm height. Electric field enhancement occurs at the terrace edges leading to focusing the holes trajectories. This leads to weakening of the Si–Si backbonds resulting in easy atom removing. The maximum electric field was observed at terrace edges and at the semispherical pore bottom.     

CsPbBr3 QD/AlOx Inorganic Nanocomposites with Exceptional Stability in Water,Light, and Heat

Herein, the assembly of CsPbBr₃ QD/AlO_x inorganic nanocomposites, by using atomic layer deposition (ALD) for the growth of the amorphous alumina matrix (AlO_x ), is described as a novel protection scheme for such QDs. The nucleation and growth of AlO_x on the QD surface was thoroughly investigated by miscellaneous techniques, which highlighted the importance of the interaction between the ALD precursors and the QD surface to uniformly coat the QDs while preserving the optoelectronic properties. These nanocomposites show exceptional stability towards exposure to air (for at least 45 days), irradiation under simulated solar spectrum conditions (for at least 8 h), and heat (up to 200 °C in air), and finally upon immersion in water. This method was extended to the assembly of CsPbBr_x I_3−x QD/AlO_x and CsPbI₃ QD/AlO_x nanocomposites, which were more stable than the pristine QD films.     

CsPbBr3 QD/AlOx Inorganic Nanocomposites with Exceptional Stability in Water,Light, and Heat

Herein, the assembly of CsPbBr₃ QD/AlO_x inorganic nanocomposites, by using atomic layer deposition (ALD) for the growth of the amorphous alumina matrix (AlO_x ), is described as a novel protection scheme for such QDs. The nucleation and growth of AlO_x on the QD surface was thoroughly investigated by miscellaneous techniques, which highlighted the importance of the interaction between the ALD precursors and the QD surface to uniformly coat the QDs while preserving the optoelectronic properties. These nanocomposites show exceptional stability towards exposure to air (for at least 45 days), irradiation under simulated solar spectrum conditions (for at least 8 h), and heat (up to 200 °C in air), and finally upon immersion in water. This method was extended to the assembly of CsPbBr_x I_3−x QD/AlO_x and CsPbI₃ QD/AlO_x nanocomposites, which were more stable than the pristine QD films.     

Pentacoordinated Al3+‐Stabilized Active Pd Structures on Al2O3‐Coated Palladium Catalysts for Methane Combustion

Supported Pd catalysts are active in catalyzing the highly exothermic methane combustion reaction but tend to be deactivated owing to local hyperthermal environments. Herein we report an effective approach to stabilize Pd/SiO₂ catalysts with porous Al₂O₃ overlayers coated by atomic layer deposition (ALD). ²⁷Al magic angle spinning NMR analysis showed that Al₂O₃ overlayers on Pd particles coated by the ALD method are rich in pentacoordinated Al³⁺ sites capable of strongly interacting with adjacent surface PdO_x phases on supported Pd particles. Consequently, Al₂O₃‐decorated Pd/SiO₂ catalysts exhibit active and stable PdO_x and Pd–PdO_x structures to efficiently catalyze methane combustion between 200 and 850 °C. These results reveal the unique structural characteristics of Al₂O₃ overlayers on metal surfaces coated by the ALD method and provide a practical strategy to explore stable and efficient supported Pd catalysts for methane combustion.     

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.     

Surface modifications of Li-ion battery electrodes with various ultrathin amphoteric oxide coatings for enhanced cycleability

Ultrathin ZnO, ZrO₂, and Al₂O₃ surface coatings are deposited via atomic layer deposition (ALD) with high conformality and atomic scale thickness control to enhance the electrochemical performance of LiMn₂O₄ for applications in lithium ion batteries. Two types of ALD-modified LiMn₂O₄ electrodes are fabricated: one is ALD-coated LiMn₂O₄ composite electrode and the other is electrode composed of ALD-coated LiMn₂O₄ particles and uncoated carbon/polyvinylidenefluoride network. Cycling performance and cyclic voltammetric patterns reveal that ZnO ALD coating is the most effective protective film for improving the electrochemical performance of LiMn₂O₄ at either 25 or 55 °C, followed by ZrO₂ and Al₂O₃. After 100 electrochemical cycles in 1 C at 55 °C, the electrode consisting of LiMn₂O₄ particles coated with six ZnO ALD layers (as thin as ~1 nm) delivers the highest final capacity, more than twice that of the bare electrode. It is also found that amphoteric oxide coating on LiMn₂O₄ particles can enhance the cycleability of LiMn₂O₄ more effectively than coating on the composite electrode. Furthermore, for ALD coating either on the composite electrode or on LiMn₂O₄ particles, the effect of oxide ALD modification for improving capacity retention and increasing specific capacity of LiMn₂O₄ is more phenomenal at elevated temperature than at room temperature.