Removal Of Atoms Research Articles

Coordinative Self‐assembly of π‐Electron Magnetic Porphyrins

π‐Electron magnetic compounds on surfaces have emerged as a powerful platform to interrogate spin interactions at the atomic scale, with great potential in spintronics and quantum technologies. A key challenge is organizing these compounds over large length scales, while elucidating their resulting magnetic properties. Herein, we offer a relevant contribution toward this objective, which consists of using on‐surface synthesis coupled with coordination chemistry to promote the self‐assembly of π‐electron magnetic porphyrin species. A porphyrin precursor equipped with carbonitrile moieties in a trans arrangement was prepared by solution synthesis and deposited on a Au(111)/mica. Depending on the specific growth protocol, surface‐promoted reactions led to the transformation of the precursor into non‐magnetic Au‐CN coordinated porphyrin monomers, covalent porphyrin dimers, and one‐dimensional porphyrin polymers (based on porphyrin monomers or covalent porphyrin dimers), as revealed by scanning probe microscopy studies combined with theoretical calculations. Interestingly, the scanning tunneling microscopy tip could convert such closed‐shell porphyrin units into open‐shell species by the removal of some peripheral hydrogen atoms. The magnetic features (i.e., singlet or triplet ground state) of the porphyrin units comprising the polymers were investigated for polymers of different lengths. No magnetic exchange coupling between adjacent units was observed.

Investigation of the mechanism of [M-H]+ ion formation in photoionized N-alkyl-substituted thieno[3,4-c]-pyrrole-4,6-dione derivatives during higher order MSn high-resolution mass spectrometry.

The mechanism underlying dopant-assisted atmospheric pressure photoionization's (APPI) formation of ions is unclear and still under debate for many chemical classes. In this study, we reexamined the gas-phase reaction mechanisms responsible for the generation of [M-H]+ precursor ions, resulting from the loss of a single hydrogen atom, in a series of N-alkyl-substituted thieno[3,4-c]-pyrrole-4,6-dione (TPD) derivatives. Atmospheric pressure photoionization combined with higher order MS/MSn using high-resolution mass spectrometry (APPI-HR-CID-MSn) and electronic structure calculations using density functional theory were used to determine the chemical structure of observed [M-H]+ ions. As a result, the higher order MSn (n = 3) experiments revealed a reversed Diels-Alder fragmentation mechanism, leading to a common fragment ion at m/z 322 from the studied [M1-5-H]+ ion species. In addition, the calculation for two chemical structure models (N-alkyl-TPD1 and N-alkyl-TPD5) showed that the fragment structure, resulting from the removal of the hydrogen atom connected to the third carbon atom of the N-alkyl side chain, has a more stable cyclic form compared with the linear one. The proposed chemical structure of the N-alkyl TPD ion species, following the loss of a single hydrogen atom, was revealed during APPI-HR-CID-MSn (n= 3) experiments on the [M-H]+ species. Hydrogen radical (H•) abstraction from the alkyl side chain (e.g., hexyl, heptyl, octyl, 2-ethylhexyl, and nonyl) triggered a rearrangement in the radical cation structure of the N-alkyl-TPD derivatives, initiating cyclization and forming a six-membered ring that connects the oxygen atom to the third carbon atom in the alkyl chain. In addition, theoretical calculations supported the APPI-HR-CID-MSn (n = 3) experiments by demonstrating that the proposed chemical structure, resulting from the intramolecular cyclization of the N-alkyl-TPD ion species, was stable in the presence of chlorobenzene. These findings will aid the structural determination and elucidation of molecules with similar core structures.

Catalytic Fe2+ Cation Pair Site for Base-free N-Alkylation of Aromatic Amines with Alcohols.

The development of heterogeneous Fe catalysts is very attractive due to the ubiquitous, abundant, and inexpensive nature of Fe as a resource. However, Fe oxides are commonly inert as catalysts and hence, the design and fabrication of active Fe sites are essential. Herein, the fabrication of an active Fe cation pair site by simple reduction treatment of SiO2-supported FeOx (FeOx/SiO2) is presented. The active Fe cation pair site was formed by the removal of the oxygen atom between Fe cations of the FeOx on SiO2, namely oxygen vacancy formation, which is induced by temperature-controlled reduction treatment. 773 K reduction maximized the Fe cation pair sites without the decomposition of FeOx species, which was an effective catalytic one for the N-alkylation of amines mainly proceeded through Meerwein-Ponndorf-Verley (MPV) type reduction, which is achieved by the stabilization of six-membered ring transition state derived from imines and alcohols over the open active site of the Fe cation pair site.

Denitrogenative Ring-Contraction of Pyridines to a Cyclopentadienyl Skeleton at a Dititanium Hydride Framework.

Selective removal of the nitrogen atom from an aromatic N-heterocycle, such as pyridine, is of significant interest and importance, yet it remains highly challenging. Here, we report an unprecedented denitrogenative ring-contraction reaction of pyridines at a dititanium hydride framework, yielding cyclopentadienyl and nitride species under mild conditions. The reaction of pyridine with a dititanium tetrahydride complex (1) bearing rigid acridane-based PNP-pincer ligands at room temperature produced a cyclopentadienyl/nitride complex (2), in which the two Ti atoms are bridged by a nitride atom and one Ti atom is bonded to a cyclopentadienyl group formed by pyridine denitrogenation and ring-contraction. The reactions of 2-, 3-, and 4-methylpyridines with 1 under similar conditions yielded the same product (3), a methylcyclopentadienyl-ligated analog of 2. When 2,4- or 3,5-dimethylpyridine reacted with 1 at 60 °C, the 1,3-dimethylcyclopentadienyl-ligated analog (5) formed almost quantitatively. The mechanistic details have been elucidated by isolation of key intermediates and density functional theory calculations. It was revealed that the reaction proceeded via coordination of the N atom of a pyridine unit to a Ti atom in 1 followed by H2 release, C═N reduction, two C-N bond cleavage (ring-opening and denitrogenation), and C-C coupling (ring closing). The involvement of C-H activation in an isopropyl group of a PNP ligand at the later stages of the reaction significantly contributed to the stabilization of the denitrogenative ring-contraction product. This work not only provides unprecedented mechanistic insights into denitrogenation of aromatic N-heterocycles but also represents a novel example of skeletal editing of aromatic N-heterocycles.

Electrochemically Promoted Skeleton Transformation of 2,1‐Benzisoxazoles to Alkyl Sulfones

AbstractThis study developed an electrochemically promoted skeleton transformation reaction of 2,1‐benzisoxazoles to synthesize alkyl sulfones. Alkyl sulfones were synthesized in 68–93% yield by using 2,1‐benzisoxazole and sulfonate as substrates, and acetonitrile and water as solvents. Mechanism studies showed that this reaction involves ring opening, removal of the oxygen atom of the 2,1‐benzisoxazoles, and C(sp3)‐sulfonylation under electrochemical conditions.

Investigation on mechanism of mechanical activation and chemical reactions in CMP of diamond assisted by hydroxyl free radicals

Chemical mechanical polishing (CMP) is a crucial manufacturing process for diamond substrate applications. However, due to the extremely high hardness and chemical stability of diamond, its CMP efficiency is extremely low. This study aims to achieve microscopic simulation of atomic oxidation removal at the three-phase interface between diamond abrasive particle, diamond substrate, and hydroxyl free radicals (∙OH) aqueous solution. The mechanical activation behavior, microscopic mechanism, and feasibility of elementary reactions in the ∙OH aqueous solution environment was studied using various perspectives of radial distribution function, central atomic coordination state, atomic bonding and removal forms, energy, electron orbitals, and density functional theory (DFT). The simulation results indicate that during the process of diamond atomic oxidation removal, the atomic density at the first adjacent position centered on one atom decreases, and the stable covalent bond structure is disrupted, resulting in an amorphous phase transition. The chemical adsorption reaction generates C-O and C–H bonds. The C atoms mainly separate from the diamond substrate by the forming products such as carbon monoxide (CO), carbon dioxide (CO2), and carbon chains. The mechanical activation behavior indirectly reduces the activation energy of chemical reactions, while increasing the absolute value of adsorption energy and improving the ability of chemical adsorption solution atoms H and O, as well as ∙OH groups. Based on DFT, it has been verified that the intrusion process of ∙OH groups in the elementary reactions involving the typical product C*O2 is an exothermic reaction that can occur spontaneously, and the energy barrier needs to be overcome by the mechanical action of abrasive particles. At the same time, the energetics of the oxidation of diamond C atoms to generate C*O2 have successfully confirmed that the combined action of ∙OH and diamond abrasive particles is expected to achieve efficient oxidation removal and finishing polishing of diamond atoms.

Functionalized construction of multiple active centers for durability controlling during catalytic combustion of chlorinated aromatic hydrocarbons

The catalytic combustion of chlorine-containing VOCs primarily encounters the bottleneck of catalyst poisoning by chlorine, thereby impeding the further industrial application of this technology. The development of multi-active catalytic materials for discerning chlorine fixation, chlorine transfer, deep oxidation during elimination of chlorine-containing VOCs can mitigate catalyst chlorination and enhance stability. Herein, we utilizing LDHs as catalyst precursors and successfully synthesized flower-like Co2FexCr1-xOy catalyst with different functionalized reaction centers. The addition of Cr improved the surface lattice oxygen, oxygen vacancies, and acid sites of catalyst. The removal efficiency of CB can be achieved >90 % at 247 °C (WHSV = 33000 mL · g−1·h−1). The fixation of dissociated chlorine contributed by Cr not only enhances oxidation performance and directional adsorption of chlorine, but also improves carbon deposition resistance and catalyst sintering resistance significantly. Compared with Co2Fe1Oy, Co2Fe0.67Cr0.33Oy catalyst achieves a remarkable threefold reduction in surface carbon deposition. The DFT calculation revealed that water dissociation energy over the Co site is −0.72 eV, which is significantly lower than Fe (−0.59 eV) and Cr (−0.17 eV), facilitating water dissociation and subsequently removes a substantial amount of accumulated chlorine on Cr to form HCl. With the help of In situ DRIFT and GC/MS analysis, we found the efficient removal of chlorine atoms enhances the exposure of active sites on the catalyst surface with hydrolyzed hydroxyl group, which facilitating the conversion of aromatic hydrocarbons into a greater quantity of maleate and carboxylate species. This phenomenon is beneficial for the subsequent mineralization process of chlorobenzene. This work provide valuable insights for multi-active site catalysis mechanism in practical applications aimed at eliminating chlorinated organic waste gases from industrial emissions.

Dual‐Catalytic Structural Isomerisation as a Route to α‐Arylated Ketones

AbstractIsomerisation reactions provide streamlined routes to organic compounds which are otherwise hard to directly synthesise. The most common forms are positional, geometrical or stereochemical isomerisations which involve the relocation of a double bond or a change in relative location of groups in space. In contrast, far fewer examples of structural (or constitutional) isomerisation exist where the connectivity between atoms is altered. The development of platforms capable of such rearrangement poses a unique set of challenges because chemical bonds must be selectively cleaved, and new ones formed without overall addition or removal of atoms. Here, we show that a dual catalytic system can enable the structural isomerisation of readily available allylic alcohols into more challenging‐to‐synthesise α‐arylated ketones via a H‐atom transfer initiated semi‐pinacol rearrangement. Key to our strategy is the combination of a cobalt catalyst and photocatalyst under reductive, protic conditions which allows intermediates to propagate catalytic turnover. By providing an unusual disconnection to structural motifs which are difficult to access through direct arylation, we anticipate inspiring other advanced catalytic isomerisation strategies that will further retrosynthetic logic for complex molecule synthesis.

Dual-Catalytic Structural Isomerisation as a Route to α-Arylated Ketones.

Isomerisation reactions provide streamlined routes to organic compounds which are otherwise hard to directly synthesise. The most common forms are positional, geometrical or stereochemical isomerisations which involve the relocation of a double bond or a change in relative location of groups in space. In contrast, far fewer examples of structural (or constitutional) isomerisation exist where the connectivity between atoms is altered. The development of platforms capable of such rearrangement poses a unique set of challenges because chemical bonds must be selectively cleaved, and new ones formed without overall addition or removal of atoms. Here, we show that a dual catalytic system can enable the structural isomerisation of readily available allylic alcohols into more challenging-to-synthesise α-arylated ketones via a H-atom transfer initiated semi-pinacol rearrangement. Key to our strategy is the combination of a cobalt catalyst and photocatalyst under reductive, protic conditions which allows intermediates to propagate catalytic turnover. By providing an unusual disconnection to structural motifs which are difficult to access through direct arylation, we anticipate inspiring other advanced catalytic isomerisation strategies that will further retrosynthetic logic for complex molecule synthesis.

In-situ investigation into the atomic-scale removal behavior of gallium nitride

The process of elimination at the atomic level stands as the paramount step in crafting ultra-smooth gallium nitride (GaN) substrates. However, this delicate removal mechanism presents a notable challenge. In this study, we employed a scraping technique utilizing atomic force microscope (AFM) probe manipulation to conduct an in-situ examination of atomic-scale removal behaviors. Through in-situ regular imaging, we uncovered the principles underlying atomic-scale removal. We systematically explored how scraping and imaging techniques impacted surface topography and the behavior of atomic step-terraces removal. Our findings indicated that abrasives knocking facilitates the emergence of atomic step-terraces, while sliding removal techniques hinder the structure. During the scraping process, both the upper and lower sections of the atomic step-terraces were simultaneously eliminated, with material removal rate (MRR) being slower at the bottom. To validate our in-situ observations, we conducted polishing experiments. Furthermore, the coefficient of friction (COF) recorded during chemical mechanical polishing offered insights into the frictional disparities of abrasives on the GaN surface, thereby shedding light on the atomic-scale removal mechanism.

Influences of ultrasonic vibration directions, amplitudes, and frequencies on sapphire polishing studied by molecular dynamics

The sapphire surface morphology, atom removal rate, temperature, polishing force, subsurface damage, dislocation, and stress were explored under different ultrasonic directions, frequencies and amplitudes through molecular dynamics (MD). For both vertical and horizontal vibration, the rising ultrasonic frequency and amplitude will reduce the tangential and normal force, and increase the subsurface temperature and the material removal rate (MRR). Higher frequencies promote the basal dislocation, thus reducing the subsurface damage. Higher amplitudes cause thinner subsurface damage layer under horizontal vibration. However, it is opposite at vertical vibration. The horizontal vibration can obtain a flatter polished surface and a thinner subsurface damage layer due to the longer trajectory and less impact on sapphire surface. This study can provide reference for sapphire high-quality polishing.

Thermoelectric properties of pristine and defective α-graphyne nanoribbons with vacancies present in different positions

In the present work, first-principle thermoelectric property investigations of pristine and defective α-graphyne nanoribbons (α-GYNRs) are conducted. Investigations have also been extended toward variation in temperature affecting the thermoelectric properties. Other objectives of the work presented are to investigate the thermoelectric properties at three temperatures, 300K, 500K, and 700K, of a single vacancy defect in a carbon atom removed from α-GYNRs. Pristine α-GYNRs is an intrinsic band gap semiconductor since defects occur naturally during synthesis, and the band gap significantly increases due to single vacancies and removal of carbon atoms at different positions. It results in a change of electrical conductivity Seebeck coefficient, and thermal conductivity. For some defective structures, with increasing temperature, the Seebeck coefficient increases compared with that of the pure structure and decreases. Also, the total thermal conductivity increases with increasing temperature. The obtained results show that the figure-of-merit value of defective α-GYNRs is much better than pristine α-GYNRs. In addition, ZT decreases in different structures by increasing temperature. Among all the considered structures, the Seebeck coefficient values and the ZT of some defective structures reach about 1.18 and 2.5 times that of the pristine structures at room temperature, respectively.

Development of a ReaxFF Reactive Force Field for Pt/Cl Systems with Application to Platinum Metal Etching with Chlorine and Hydrogen Chloride Gases.

In this study, we present the development of a ReaxFF Pt/Cl/H reactive force field designed to elucidate the etching process by Cl for Pt surfaces. The ReaxFF force field parameters were optimized based on a quantum mechanical training set, which included adsorption energies of Cl and dissociation of HCl on Pt(100) and Pt(111) surfaces, energy/volume relations of PtCl2 crystals, and Cl diffusion on Pt(100) and Pt(111) surfaces. The predictive capability of the force field was further established through molecular dynamics simulations, which investigated the interactions of Cl2 and HCl molecules with the (100) and (111) surfaces of c-Pt crystalline solid slabs. A comparative analysis revealed that the Pt (100) surface exhibited higher susceptibility to chlorination and etching, leading to a more dominant removal of surface Pt atoms, whereas the Pt (111) surface showed greater resistance to these processes. This resistance impeded the access of Cl atoms to the Pt surface, resulting in a slower formation of PtxCly molecules. The etching ratios between HCl and Cl2 were compared with experimental results, yielding satisfactory agreement. This indicates that the developed ReaxFF protocol serves as a valuable tool for studying atomistic-scale details of the etching process in platinum metal systems.

Investigation into atomistic reaction between abrasive and Co in H2O through ReaxFF MD and XPS

Co is expected to replace Cu as the next generation of interconnect metal material and plays an integral role in the field of integrated circuits (IC). However, the reaction mechanism behind Co chemical mechanical polishing is not yet clear. To fill these gaps, the following explorations have been conducted. Reactive force field molecular dynamics (ReaxFF MD) and X-ray photoelectron spectroscopy (XPS) were employed to investigate the atomistic reaction and removal underlying the cobalt (Co) chemical mechanical polishing (CMP) in an aqueous (H2O) environment. It demonstrates the adsorption of H2O onto the Co surface, resulting in the formation of Co-OH, Co-H2O, and other reaction products·H2O molecules also undergo reactions with the abrasive. Furthermore, the OH-terminated surface of the diamond abrasive can chemically interact with the Co substrate, resulting in the formation of C-O-Co bridge bonds. Additionally, the polishing process generates C-Co bonds. Both bonds contribute to the removal of Co atoms, although the former has a relatively smaller effect. A smaller gap between C and Co corresponds to a greater number of C-Co bonds and a faster attainment of dynamic equilibrium. Moreover, as the gap decreases, the normal and frictional force increase within a certain range. However, beyond a certain limit, the friction force decreases. This study aids in uncovering the atomic behavior at the interface during Co CMP and understanding the polishing mechanism.

Catalytic performance of Cs-V-based non-noble soot oxidation catalyst used for DPF and its enhancement by cerium addition

Non-noble metal catalysts have great potential for controlling soot emissions due to their effective oxidation properties and low cost. This study investigated the characteristics of Cs-V-based non-noble metal soot catalyst and Ce doping enhancement effect through comprehensive characterization of physicochemical properties, catalytic activity evaluation, and density functional theory (DFT) theoretical calculations. The results demonstrate that Ce doping improves the degree of surface particle aggregation and the spatial distribution morphology of Cs-V-based catalyst, with a 36.7 % increase in the specific surface area. Additionally, Ce doping enhances the probability of oxygen adsorption and dissociation activation on the Cs-V-based catalyst, accelerating the soot-oxidation process, improving the soot-oxidation activity, with a decrease in the characteristic temperature T10 (temperature at 10 % conversion rate of soot) from 349.7 ℃ to 281.1 ℃, and decreases the activation energy for soot-oxidation to 5.8 kJ/mol. DFT calculations reveal that Ce doping strengthens the pulling effect of interatomic chemical bonds, facilitating the removal of oxygen atoms and dissociated reactive oxygen (Oasd), resulting in an increase in the adsorption energy of C4 molecules and a shorter bond length between C4 molecules and the catalytic surface, facilitating the desorption of post-oxidation soot products.

Exploring Molecular Heteroencoders with Latent Space Arithmetic: Atomic Descriptors and Molecular Operators.

A variational heteroencoder based on recurrent neural networks, trained with SMILES linear notations of molecular structures, was used to derive the following atomic descriptors: delta latent space vectors (DLSVs) obtained from the original SMILES of the whole molecule and the SMILES of the same molecule with the target atom replaced. Different replacements were explored, namely, changing the atomic element, replacement with a character of the model vocabulary not used in the training set, or the removal of the target atom from the SMILES. Unsupervised mapping of the DLSV descriptors with t-distributed stochastic neighbor embedding (t-SNE) revealed a remarkable clustering according to the atomic element, hybridization, atomic type, and aromaticity. Atomic DLSV descriptors were used to train machine learning (ML) models to predict 19F NMR chemical shifts. An R2 of up to 0.89 and mean absolute errors of up to 5.5 ppm were obtained for an independent test set of 1046 molecules with random forests or a gradient-boosting regressor. Intermediate representations from a Transformer model yielded comparable results. Furthermore, DLSVs were applied as molecular operators in the latent space: the DLSV of a halogenation (H→F substitution) was summed to the LSVs of 4135 new molecules with no fluorine atom and decoded into SMILES, yielding 99% of valid SMILES, with 75% of the SMILES incorporating fluorine and 56% of the structures incorporating fluorine with no other structural change.

Towards a better understanding of the surface smoothing effect in gas cluster ion beam processing with molecular dynamics simulation and experiment.

In recent years, the application of gas cluster ion beam (GCIB) technology has made great progress. Due to the similar essence of a monoatomic ion beam, the GCIB also shows flashes of brilliance in material processing. It has been reported that smoothness can be greatly improved after the rough surface is bombarded by the GCIB. This indicates that the GCIB processing has great potential in optical fabrication. Although the surface smoothing effect has been investigated, there is still a lack of dynamic micro-analysis for GCIB processing, which is limited for better understanding the mechanism of smoothing effect. In this paper, the surface smoothing effect in GCIB processing is explicitly investigated with molecular dynamics (MD) simulation and experiment. The principle of GCIB processing is compared with the traditional monoatomic ion-beam based processing, and details of the MD simulation procedure are introduced. Based on this, the dynamic micro-analysis of GCIB processing is conducted under different processing conditions. The simulations reveal the phenomena of atomic removal and migration in GCIB processing, which plays an important role in explaining the mechanism of surface smoothing effect. The experiment was performed on the silicon substrate with the in-house GCIB processing machine. The results indicate that the initial rough surface with dense protrusions can be greatly smoothed, and the root mean square (RMS) value is reduced from 0.586 nm to 0.191 nm. Both simulation and experiment can provide a better understanding of smoothing effect mechanism in GCIB processing.

Electrochemical Weakening Material Strength for Accelerated Thinning of Silicon Carbide Substrates

This paper presents a novel approach to expedite the thinning of silicon carbide (SiC) substrates, a crucial step in enhancing material performance and extending chip longevity by reducing volume resistivity. The traditional methods of SiC substrate thinning, such as diamond wheel grinding and plasma etching, have inherent limitations, including the risk of wafer breakage and high processing costs. In response to these challenges, we propose an innovative technique involving the electrochemical etching attenuation of SiC strength, offering a cost-effective and efficient solution.In this study, we employed an electrochemical method to initially weaken the SiC substrate from the surface to a defined depth, followed by fine-tuning its thickness using a conventional diamond grinding wheel. Remarkably, our results demonstrate that SiC substrates subjected to electrochemical treatment exhibit well-structured lattices free from residual stresses. This novel approach not only reduces the risk of wafer breakage but also enhances the material's integrity (Figure 1).To gain deeper insights into the effects of electrochemistry on SiC material strength, advanced characterization techniques, including scanning electron microscopy, were employed. Our findings reveal that the electrochemically treated SiC surface exhibits a sponge-like morphology, with a cross-sectional profile showcasing prominent columnar structures (Figure 2). This sponge-like structure is attributed to the removal of silicon atoms during anodization, affirming the effectiveness of electrochemical methods in rapidly attenuating the material strength of SiC substrates, thereby facilitating their efficient thinning through grinding without compromising substrate integrity.In summary, this research highlights the potential of electrochemical methodologies for the initial weakening of SiC substrates, leading to accelerated and cost-effective thinning processes. This breakthrough not only expedites SiC substrate thinning but also contributes significantly to the advancement of SiC substrate thinning and grinding techniques, promising substantial benefits for future semiconductor device fabrication. Figure 1

Influencing the surface quality of free-standing wurtzite gallium nitride in ultra-high vacuum: Stoichiometry control by ammonia and bromine adsorption

Gallium nitride (GaN), a wide bandgap semiconductor with absorption and emission in the ultraviolet/visible range, is proposed as an alternative to metallic surfaces for assembling organic molecular structures aiming at optoelectronic applications. However, the formation of a persistent surface oxide layer in air considerably limits the use of GaN for well-defined interfaces. In this work, we have investigated, characterized and processed n-type free-standing c-plane hexagonal wurtzite GaN crystals grown by hydride vapor phase epitaxy and ammonothermal growth methods. Surface cleaning and full removal of the oxide layer on GaN surfaces could be reproducibly achieved via sputtering and annealing cycles, as evidenced by X-ray photoelectron spectroscopy and low-energy electron diffraction. Scanning tunneling microscopy, however, indicated substantial roughening of the GaN surface and the formation of unwanted Ga-rich islands and clusters. Although ammonia (NH3) and bromine (Br) treatments compensated the N/Ga atoms ratio reduced by sputtering, the surface morphology remained rough, exhibiting randomly shaped and distributed hillocks. In addition, we studied the effect of electron bombardment on the surface quality of GaN during NH3 annealing, on-surface debromination and polymerization of 1,3,5-tris(4-bromophenyl) benzene on GaN, and the removal of Ga atoms by Br atoms during the desorption.

Unveiling microstructure effect on nanoscratch behavior of gold-platinum alloys

The microstructural effect on the mechanical behaviors of gold-platinum alloys during nanoscratch is revealed. By adjusting the microstructures (crystallographic orientation, interface type, grain size, twinning thickness), the hardness, plasticity, removal rate and surface roughness of the alloys are significantly improved. The nanoscratch is performed by molecular dynamics (MD). For the single-crystal (SC) alloys, the [11¯0](111)-oriented SC alloy shows the high hardness, wear resistance and low roughness. The high dislocation density induces the strong work hardening while the horizontal twinning boundaries (HTBs) effectively improve the surface quality by hindering the elastic recovery. For the bi-crystal (BC) alloys, the BC alloy with TB exhibits the excellent plasticity and low roughness. TB can uniformly limit the dislocation slip and promote the dislocation nucleation to strengthen the plasticity and surface quality. For the polycrystal (PC) and nanotwinned-polycrystal (NTPC) alloys, the alloys with small grain size or small twinning thickness show the high removal rate and low roughness. The reduction in grain size or twinning thickness inhibits the dislocation motion and promotes the GB or TB migration to improve the surface quality and atomic removal. These results provide an important theoretical basis for the microstructural design and nanofabrication of gold-platinum alloys with smooth surface and remarkable mechanical property, expanding the application for the bimetallic materials.