Tight-binding Quantum Chemical Molecular Dynamics Research Articles

Atom-by-Atom and Sheet-by-Sheet Chemical Mechanical Polishing of Diamond Assisted by OH Radicals: A Tight-Binding Quantum Chemical Molecular Dynamics Simulation Study.

Ultraflat and damage-free single-crystal diamond is a promising material for use in electronic devices such as field-effect transistors. Diamond surfaces are conventionally prepared by the chemical mechanical polishing (CMP) method, although the CMP efficiency remains a critical issue owing to the extremely high hardness of diamond. Recently, OH radicals have been demonstrated to be potentially useful for improving the CMP efficiency for diamond; however, the underlying mechanisms are still elusive. In this work, we applied our previously developed CMP-specialized tight-binding quantum chemical molecular dynamics simulator to comprehensively elucidate the CMP mechanisms of diamond assisted by OH radicals. Our simulation results indicate that the diamond surface is oxidized by reactions with OH radicals and then a concomitant surface reconstruction takes place due to the distorted and unstable nature of the oxidized diamond surface structure. Furthermore, we interestingly reveal that the reconstruction of the diamond surface ultimately leads to two distinct removal mechanisms: (i) gradual atom-by-atom removal through the desorption of gaseous molecules (e.g., CO2 and H2CO3) and (ii) drastic sheet-by-sheet removal through the exfoliation of graphitic ring structures. Hence, we propose that promoting the oxidation-induced graphitization of the diamond surface may provide a route to further improving the CMP efficiency.

Role of OH Termination in Mitigating Friction of Diamond-like Carbon under High Load: A Joint Simulation and Experimental Study.

Diamond-like carbon (DLC) has recently attracted much attention as a promising solid-state lubricant because it exhibits low friction, low abrasion, and high wear resistance. Although we previously reported the reason why H-terminated DLC exhibits low friction based on a tight-binding quantum chemical molecular dynamics (TB-QCMD) simulation, experimentally, the low-friction state of H-terminated DLC is not stable, limiting its application. In the present work, our TB-QCMD simulations suggest that H/OH-terminated DLC could give low friction even under high loads, whereas H-terminated DLC could not. By using gas-phase friction experiments, we confirm that OH termination can indeed provide much more stable lubricity than H termination, validating the predictions from simulations. We conclude that H/OH-terminated DLC is a new low-friction material with high load capacity and high stable lubricity that may be suitable for practical use in industrial applications.

Cooperative roles of chemical reactions and mechanical friction in chemical mechanical polishing of gallium nitride assisted by OH radicals: tight-binding quantum chemical molecular dynamics simulations.

Chemical mechanical polishing (CMP) is a key manufacturing process for applying gallium nitride (GaN), especially the Ga-face GaN, to semiconductor devices such as laser diodes. However, the CMP efficiency for GaN is very low due to its high hardness and chemical stability. Experimentally, OH radicals appear able to improve the CMP efficiency of GaN polished by a SiO2 abrasive grain, whereas the mechanisms of the OH-radical-assisted CMP process remain unclear because experimental elucidation of the complex chemical reactions occurring among GaN substrate, abrasive grain, and OH radicals is difficult. In this work, we used our previously developed tight-binding quantum chemical molecular dynamics simulator to study the OH-radical-assisted CMP process of the widely employed Ga-face GaN substrate polished by an amorphous SiO2 abrasive grain in an effort to understand how OH radicals assist the CMP process and then aid the development of next-generation CMP techniques. Our simulations revealed that the OH-radical-assisted CMP process of GaN occurs via the following three basic reaction steps: (i) first, all hydrogen terminations on the GaN surface are replaced by OH terminations through continuous reactions with OH radicals; (ii) after the substrate is fully terminated by OH, the hydrogen atoms of these OH terminations are removed by reacting with newly added OH radicals, which forms H2O molecules and leaves energetic oxygen atoms with dangling bonds on the surface; and (iii) finally, these energetic oxygen atoms intrude inside the substrate with concomitant dissociation of Ga-N bonds and the generation of N2 and gallium hydroxide molecules, which accumulatively lead to the removal of N and Ga atoms from the substrate.

Tribochemical reactions and graphitization of diamond-like carbon against alumina give volcano-type temperature dependence of friction coefficients: A tight-binding quantum chemical molecular dynamics simulation

Diamond-like carbon (DLC) is a promising solid lubricant used as a protective coating to reduce friction against alumina. Friction properties of DLC/alumina are strongly affected by temperature. To improve the friction performance of DLC, we investigate the friction behaviors of DLC/alumina at various temperatures and reveal the mechanisms by using our tight-binding quantum chemical molecular dynamics method. We observe an interesting volcano-type temperature dependence of friction coefficients in our friction simulations. Friction coefficients of DLC/alumina are low and show little change at 300–600 K because no tribochemical reactions occur at the interface. However, as the temperature increases, friction coefficients increase at 600–800 K and subsequently decrease at 800–1000 K. At 600–800 K, interfacial C-O and C-Al bonds between two substrates are formed during friction, leading to a high friction coefficient. Interestingly, further increment of temperature to 800–1000 K induces the graphitization of DLC. The graphite-like surface suppresses the interfacial bond formation, reducing the friction coefficient. We reveal that the volcano-type temperature dependence of friction coefficients is due to the tribochemical reactions generating interfacial bonds at 600–800 K and the graphitization of DLC reducing the number of interfacial bonds at 800–1000 K.

Tight-Binding Quantum Chemical Molecular Dynamics Study on the Friction and Wear Processes of Diamond-Like Carbon Coatings: Effect of Tensile Stress.

Diamond-like carbon (DLC) coatings have attracted much attention as an excellent solid lubricant due to their low-friction properties. However, wear is still a problem for the durability of DLC coatings. Tensile stress on the surface of DLC coatings has an important effect on the wear behavior during friction. To improve the tribological properties of DLC coatings, we investigate the friction process and wear mechanism under various tensile stresses by using our tight-binding quantum chemical molecular dynamics method. We observe the formation of C-C bonds between two DLC substrates under high tensile stress during friction, leading to a high friction coefficient. Furthermore, under high tensile stress, C-C bond dissociation in the DLC substrates is observed during friction, indicating the atomic-level wear. These dissociations of C-C bonds are caused by the transfer of surface hydrogen atoms during friction. This work provides atomic-scale insights into the friction process and the wear mechanism of DLC coatings during friction under tensile stress.

Atomistic Mechanisms of Chemical Mechanical Polishing of a Cu Surface in Aqueous H2O2: Tight-Binding Quantum Chemical Molecular Dynamics Simulations.

We applied our original chemical mechanical polishing (CMP) simulator based on the tight-binding quantum chemical molecular dynamics (TB-QCMD) method to clarify the atomistic mechanism of CMP processes on a Cu(111) surface polished with a SiO2 abrasive grain in aqueous H2O2. We reveal that the oxidation of the Cu(111) surface mechanically induced at the friction interface is a key process in CMP. In aqueous H2O2, in the first step, OH groups and O atoms adsorbed on a nascent Cu surface are generated by the chemical reactions of H2O2 molecules. In the second step, at the friction interface between the Cu surface and the abrasive grain, the surface-adsorbed O atom intrudes into the Cu bulk and dissociates the Cu-Cu bonds. The dissociation of the Cu-Cu back-bonds raises a Cu atom from the surface that is mechanically sheared by the abrasive grain. In the third step, the raised Cu atom bound to the surface-adsorbed OH groups is removed from the surface by the generation and desorption of a Cu(OH)2 molecule. In contrast, in pure water, there are no geometrical changes in the Cu surface because the H2O molecules do not react with the Cu surface, and the abrasive grain slides smoothly on the planar Cu surface. The comparison between the CMP simulations in aqueous H2O2 and pure water indicates that the intrusion of a surface-adsorbed O atom into the Cu bulk is the most important process for the efficient polishing of the Cu surface because it induces the dissociation of the Cu-Cu bonds and generates raised Cu atoms that are sheared off by the abrasive grain. Furthermore, density functional theory calculations show that the intrusion of the surface-adsorbed O atoms into the Cu bulk has a high activation energy of 28.2 kcal/mol, which is difficult to overcome at 300 K. Thus, we suggest that the intrusion of surface-adsorbed O atoms into the Cu bulk induced by abrasive grains at the friction interface is a rate-determining step in the Cu CMP process.

Tight-binding quantum chemical molecular dynamics simulations for the elucidation of chemical reaction dynamics in SiC etching with SF6/O2 plasma.

We used our etching simulator [H. Ito et al., J. Phys. Chem. C, 2014, 118, 21580-21588] based on tight-binding quantum chemical molecular dynamics (TB-QCMD) to elucidate SiC etching mechanisms. First, the SiC surface is irradiated with SF5 radicals, which are the dominant etchant species in experiments, with the irradiation energy of 300 eV. After SF5 radicals bombard the SiC surface, Si-C bonds dissociate, generating Si-F, C-F, Si-S, and C-S bonds. Then, etching products, such as SiS, CS, SiFx, and CFx (x = 1-4) molecules, are generated and evaporated. In particular, SiFx is the main generated species, and Si atoms are more likely to vaporize than C atoms. The remaining C atoms on SiC generate C-C bonds that may decrease the etching rate. Interestingly, far fewer Si-Si bonds than C-C bonds are generated. We also simulated SiC etching with SF3 radicals. Although the chemical reaction dynamics are similar to etching with SF5 radicals, the etching rate is lower. Next, to clarify the effect of O atom addition on the etching mechanism, we also simulated SiC etching with SF5 and O radicals/atoms. After bombardment with SF5 radicals, Si-C bonds dissociate in a similar way to the etching without O atoms. In addition, O atoms generate many C-O bonds and COy (y = 1-2) molecules, inhibiting the generation of C-C bonds. This indicates that O atom addition improves the removal of C atoms from SiC. However, for a high O concentration, many C-C and Si-Si bonds are generated. When the O atoms dissociate the Si-C bonds and generate dangling bonds, the O atoms terminate only one or two dangling bonds. Moreover, at high O concentrations there are fewer S and F atoms to terminate the dangling bonds than at low O concentration. Therefore, few dangling bonds of dissociated Si and C atoms are terminated, and they form many Si-Si and C-C bonds. Furthermore, we propose that the optimal O concentration is 50-60% because both Si and C atoms generate many etching products producing fewer C-C and Si-Si bonds are generated. Finally, we conclude that our TB-QCMD etching simulator is effective for designing the optimal conditions for etching processes in which chemical reactions play a significant role.

Prediction of low-energy boron doping profile for ultrashallow junction formation by hybrid molecular dynamics method

Our original hybrid method combining tight-binding quantum chemical and classical molecular dynamics was first applied to the low-energy doping process of boron into a silicon substrate, which has a depth of more than 10 nm that is needed to evaluate an ultrashallow junction position. Tight-binding quantum chemical molecular dynamics calculation was used for an injected boron atom and surrounding silicon atoms within a sphere with a radius of 0.5 nm centered at the boron atom. This method is advantageous in treating the many-body collision effect and electron–electron interaction, which are more important in low-energy doping, compared with the Monte Carlo method with binary collision approximation. A comparison with a plasma doping experiment was also carried out. The junction positions were 6.2 nm for boron doping at an initial kinetic energy of 200 eV in the simulation results and 6.4 nm for 200 eV in the experimental results. Good agreement between simulation and experimental results indicates that our hybrid molecular dynamics method is applicable to doping profile prediction in a silicon structure with a depth of more than 10 nm that is needed to evaluate ultrashallow junction formation.

Tight-Binding Quantum Chemical Molecular Dynamics Simulations of Mechanisms of SiO2 Etching Processes for CF2 and CF3 Radicals

The plasma etching of SiO2 by CF2 and CF3 radicals is investigated by using our etching simulator based on tight-binding quantum chemical molecular dynamics method. During etching simulations, C–F and Si–O bonds dissociate and C–O and Si–F bonds are generated. Moreover, CO, CO2, COF, and COF2 molecules form, which is consistent with previous experimental studies. We also examine the dependence of the etching mechanism of CF2 and CF3 radicals on the kinetic energy of irradiation. At a low kinetic energy of 10 eV, a CF2 radical dissociates more Si–O bonds than a CF3 radical does. This is because the high chemical reactivity of the CF2 diradical accelerates the etching process. At a high kinetic energy of 150 eV, a CF3 radical dissociates more Si–O bonds than a CF2 radical does. This is because a CF3 radical generates a greater number of reactive F atoms than a CF2 radical does and thus forms more Si–F bonds. Thus, we conclude that our etching simulator modeled the different SiO2 etching mechanisms of CF2 an...

Tight-binding quantum chemical molecular dynamics simulations of the low friction mechanism of fluorine-terminated diamond-like carbon films

The super-low friction mechanism of fluorine-terminated diamond-like carbon (F-terminated DLC) is investigated by using our tight-binding quantum molecular dynamics code and compared with that of hydrogen-terminated DLC (H-terminated DLC). Under a contact pressure of 1 GPa, F- and H-terminated DLC show smooth sliding and low friction coefficients of 0.07 and 0.04, respectively. The ion radius of fluorine is larger than that of hydrogen, which leads to the larger asperity of the F-terminated DLC surface. Thus, the friction coefficient of F-terminated DLC is slightly larger than that of H-terminated DLC. We also perform friction simulations under contact pressures of 3 and 7 GPa. Under a contact pressure of 3 GPa, the friction coefficients are 0.09 and 0.13 for F- and H-terminated DLC, respectively. F-terminated DLC shows the same friction behavior as seen under a contact pressure of 1 GPa, whereas the C–C bond formation reaction is observed at the interface of H-terminated DLC under a contact pressure of 3 GPa, leading to a slightly higher friction coefficient than when under a contact pressure of 1 GPa. Thus, under a contact pressure of 3 GPa, F- and H-terminated DLC show different friction behaviors. Furthermore, under a high contact pressure of 7 GPa, bond formation and dissociation are observed at the friction interface in F- and H-terminated DLC. C–C bond formation is observed more frequently in H-terminated DLC than in F-terminated DLC, and the lifetime of C–C bonds in H-terminated DLC is much longer. At this higher pressure, H-terminated DLC shows a high friction coefficient of 0.42 due to strong C–C bonds at the friction surface, whereas F-terminated DLC shows a low friction coefficient of 0.08. The strong repulsive interaction at the interface of F-terminated DLC that arises from the large negative charge and ion size of fluorine maintains the distance between DLC films under a high contact pressure. This prevents strong C–C bond formation at the friction surface, which results in the low friction properties of F-terminated DLC. We suggest that the friction properties of DLC films under a high contact pressure are improved by F termination.

Tight-Binding Quantum Chemical Molecular Dynamics of Oxygen Migration of Rh-supporting CeO2 Surfaces

ABSTRACTThe electronic properties of the interface between Rh clusters and CeO2 (111), (110) and (100) surfaces were studied using an isothermal-isobaric (NPT) ensemble at 773 K and 101.343 kPa using the tight binding-quantum chemical molecular dynamics (TB-QCMD) method. The amount of electronic exchange by interaction at the interface between the supported Rh55 clusters and each CeO2 surface was investigated quantitatively. A comparison of the mean square displacement (MSD) showed that the topmost oxygens on the Rh-supporting CeO2 surface exhibited higher mobility than those of the bare CeO2 surface. Although the mobility of the topmost oxygens on the bare CeO2 surface was in the order (100) > (110) > (111), this sequence was altered by the presence of Rh, so that the oxygen mobility for the more open (110) surface was the largest. The amount of electron exchange that occurred between Rh and the CeO2 (110) surface was also larger than for the (111) or (100) surface. The Ce 4f orbitals on the CeO2 (110) surface exhibited the strongest mixing with Rh 4d orbitals, which simultaneously caused restructuring and instability of the topmost Ce-O bonds. This enhancement of oxygen migration in the presence of Rh was occurred together with an increase in the number of oxygen vacancies on the ceria surface. This was because the topmost oxygens was shifted to have a stronger affinity with Rh and thus formed stronger bonds with Rh than with Ce.

Different Crystal Growth Mechanisms of Si(001)-(2 × 1):H during Plasma-Enhanced Chemical Vapor Deposition of SiH3 and SiH2 Radicals: Tight-Binding Quantum Chemical Molecular Dynamics Simulations

We use tight-binding quantum chemical molecular dynamics to investigate the crystal growth mechanisms of H-terminated Si(001)-(2 × 1) during plasma-enhanced chemical vapor deposition of SiH3 and SiH2 radicals. We find that crystal growth by SiH3 radical deposition consists of two stages: (1) the first SiH3 radical abstracts a surface-terminating H atom and produces a dangling bond, and (2) a second SiH3 radical is adsorbed on the dangling bond. Thus, at least two SiH3 radicals are required for generating a new Si–Si bond. Interestingly, during SiH2 deposition, a SiH2 radical can be directly adsorbed onto a H-terminated site without H abstraction by another SiH2 radical. Thus, one SiH2 radical is sufficient for generating a new Si–Si bond. This SiH2 radical crystal growth mechanism is different from the SiH3 radical mechanism. The direct adsorption process consists of a two-step chemical reaction: (1) the SiH2 radical abstracts a surface-terminating H atom and produces a dangling bond and a SiH3 radical, a...

Early stage oxidation of Ni–Cr binary alloy (1 1 1), (1 1 0) and (1 0 0) surfaces: A combined density functional and quantum chemical molecular dynamics study

First-principles and tight-binding quantum chemical molecular dynamics were used in this study. The chemisorption energies of O and OH on the Ni–Cr (110) surface are lower than those of other surfaces. The oxygen 2p orbitals hybridise with Ni 3d, 4s and small amounts of p orbitals for the (100) surface while Ni p orbitals have no contribution for the (110) surface, which might reduce the adsorption energy. Additionally, oxygen acquires the maximum depth into the Ni–Cr (110) surface. Applied strain increases the oxygen diffusivity. This study reveals that the Ni–Cr (110) surface is easier for oxygen diffusion accordingly oxidation accelerates.

Quantum Chemical Molecular Dynamics Study of the Water–Gas Shift Reaction on a Pd/MgO(100) Catalyst Surface

The water–gas shift (WGS) reaction on a Pd/MgO(100) catalyst surface was studied using the tight binding-quantum chemical molecular dynamics (TB-QCMD) method. Molecular adsorption of CO was observed. In contrast, we observed that H2O adsorption occurs first molecularly but the molecule then dissociates on the surface. The resultant hydroxyl group reacts with preadsorbed CO to form an OCOH intermediate and a single H atom. This process is relevant as the initial hydroxylation step, and it is part of the catalyzed hydrolysis mechanism. During the molecular dynamics simulation the OCOH intermediate inverted into an H–CO2 like molecule and finally HCO2 decomposed to CO2 and H. Later on, the resultant H interacts with the previously dissociated single H atom (H released from the H–OH dissociation) and forms the WGS product H–H molecule. It was observed that the CO2 desorbed from the supported Pd cluster while the H2 molecule remains attached to the Pd cluster during the simulation. The geometries and dissociation energies of water molecules were obtained and the type of adsorption assessed. Chemical changes, changes in electronic and adsorption states, and structural changes were also investigated through TB-QCMD calculations, which indicate that the metal-oxide interface plays an essential role in the catalysis, helping in the dissociation of water and the formation of the OCOH intermediate. The present study indicates that the MgO(100) support has a strong interaction with the Pd catalyst, which may cause an increase in Pd activity as well as enhancement of the metal catalyst dispersion, hence, increasing the rate of the WGS reaction. Furthermore, from the molecular dynamics and electronic structure calculations, we have identified a number of consequences for the interpretation and modeling of the WGS reaction.

Early Stage Oxidation Initiation at Different Grain Boundaries of fcc Fe–Cr Binary Alloy: A Computational Chemistry Study

Tight-binding quantum chemical molecular dynamics method has been applied in order to study the Σ3 (111), Σ5 (100) and random grain boundaries oxidation initiation mechanism of fcc Fe–Cr binary alloy in a boiling water reactor environment. The metal–water interaction at high temperatures causes diffusion of environmental species and segregation of metallic atoms. Water molecules favorably permeate through the random grain boundary (GB) to find the space generated by atomic rearrangement, although it is difficult to diffuse in the Σ3 (111) and Σ5 (100) grain boundaries. Moreover, applied strain creates extra spaces in the lattice that can facilitate the absorption of environmental species. The highly positively charged chromium and the negatively charged oxygen atoms or OH remain along the GB by forming bonds. The GB atoms selectively lose their valence electrons when dissociated atoms adsorb, indicating that the oxidation process is a possible mechanism of intergranular cracking initiation.

Chemical Reaction Dynamics of SiO2 Etching by CF2 Radicals: Tight-Binding Quantum Chemical Molecular Dynamics Simulations

The plasma etching of SiO2 by CF2 radicals was investigated using a newly developed etching process simulator based on tight-binding quantum chemical molecular dynamics (TB-QCMD). CF2 radicals were continuously irradiated on the SiO2(001) surface and then the dissociations of the C–F and Si–O bonds were observed. We also observed the generation of CO and CO2 molecules and Si–F bonds, which is in good agreement with previous experiments. The formation of etching holes was realized after the continuous irradiation of CF2 radicals. Furthermore, the effect of radical velocity on etching efficiency was also examined. The ratio of penetration depth to the width of irradiated atoms was examined for the evaluation of etching efficiency. The ratio increases as the irradiation velocity of CF2 radicals increases. Our TB-QCMD etching process simulator is capable of predicting etching rate and aspect ratio depending on the velocity of irradiated radicals.

J061054 量子分子動力学法に基づくリチウムイオン電池の劣化プロセスシミュレーション(〔J061-05〕燃料電池・二次電池におけるナノ・マイクロ現象とマクロ性能(5):各種電池)

To clarify the degradation mechanism of lithium-ion battery cathode, we studied the degradation process of the cathode caused by a chemical reaction with ethylene carbonate (EC) via our tight-binding quantum chemical molecular dynamics (TB-QCMD) method. We simulated the dynamics of the chemical reaction of LiCoO_2(010) surface and EC. In addition, our calculated atomic bond population revealed that a Co-O bond of the LiCoO_2(010) surface weakened when an O atom bound with a C atom of EC. This weakening of the Co-O bond indicates that the O atom of the surface is easily removed from the surface and the Co atom is reduced. It is known that the structural change of the cathode after the reduction of the Co atom causes the degradation of the lithium-ion battery cathode. Thus, we suggested that the adsorption of EC resulting in the reduction of Co atoms was the initial process of the degradation of the lithium-ion battery cathode.

6AM2-C-6 Tight-Binding量子分子動力学法と第一原理分子動力学法による窒化炭素腹界面の低摩擦機構に関する研究(6AM2-C OS6 マイクロナノトライボロジー(1))

We performed friction simulations of hydrogen terminated carbon nitride (H-CNX) using our first-principles molecular dynamics (FPMD) and tight-binding quantum chemical molecular dynamics (TB-QCMD) simulators. In FPMD simulation, H-CN_X showed low friction coefficient of 0.15 and 0.10 at 5 and 10 GPa, respectively. This is because terminated hydrogen atoms prevented the bond generation at the films interface. In TB-QCMD simulation, under 1 GPa, H-CN_X and H-DLC also showed low friction coefficients of 0.05 because terminated hydrogen atoms prevented the bond generation at the films interface. On the other hand, under 5 GPa, while H-CN_X showed a low friction coefficient of 0.07, a friction coefficient of H-DLC takes a high value of 0.42. Although C-C bonds were generated at the interface of H-DLC films under high pressure, nitrogen atoms of H-CN_X prevented C-C bond generation at the interface. We found that H-CN_X is more stable and shows lower friction than H-DLC under high pressure.

2410 第一原理分子動力学法とTight-Binding量子分子動力学法によるトライボケミカル反応ダイナミクスシミュレーション

2410 第一原理分子動力学法とTight-Binding量子分子動力学法によるトライボケミカル反応ダイナミクスシミュレーション

An atomic study of hydrogen effect on the early stage oxidation of transition metal surfaces

Density functional theory (DFT) and tight-binding quantum chemical molecular dynamics (QCMD) have been applied to analyze the role of interstitial hydrogen in the process of oxygen adsorption to Ni (111) and Cr-doped Ni (111) surfaces and diffusion within these metal surfaces. The DFT calculations demonstrate that the fcc hollow and octahedral sites are the most favorable for hydrogen adsorption on the surface and subsurface, respectively. A clean metal surface has a slight inward relaxation in the topmost layer, whereas the metal atoms show outward relaxation (2%) due to interstitial hydrogen. The adsorption energies of oxygen and OH have decreased to 0.26 and 0.13 eV, respectively, and the metal atomic bond further extended in the range of 1–2% in order to hydrogen remained interstitial site. Hydrogen changes to a negatively charged in the interstitial site by receiving electron. The QCMD results reveal that the oxygen penetration depth increases when hydrogen occupy into interstitial octahedral site. The deeply diffused or interstitial hydrogen receives electrons from the metal. Additionally, interstitial hydrogen initiates the charge transfer and extends the metal atomic bond. The localized process weakens the metal–metal bonds and it makes the surface chemically active for further interaction. This process can help oxygen or other species to diffuse into the structure. As a result, the subsurface hydrogen accelerates the early stage of oxidation initiation.