Flat-surface-assisted and self-regulated oxidation resistance of Cu(111)

Refining metallic nano-copper by electron-rich black carbon for superior Fenton-like catalysis in water purification: The capacitive regulation of corrosive electron transfer

Transition metals are promising catalysts for environmental remediation. However, their low reactivity, poor stability and weak reusability largely limit practical applications. Herein, we report that the electron-rich dissolved black carbon (DBC) incorporated into the nanoscale zero-valent copper (nZVCu) can boost intrinsic reactivity, structural stability and cyclic reusability for superior peroxymonosulfate (PMS) activation and pollutant degradation. A series of refractory pollutants can be effectively removed on the DBC/nZVCu, in comparison with the nZVCu reference. Hydroxyl radical (‧OH) is identified as the dominant reactive oxygen species by electron spin resonance (ESR) and chemical quenching tests, mediated by the metastable Cu(III) as the key reactive intermediate. The electron-rich DBC protects nanoscale Cu from oxidative corrosion to slow down the surface formation of inert CuO layer, rendered by the thermodynamically and dynamically capacitive regulation of corrosive electron transfer from metallic core. By this refining way, the conducive DBC improves the neighboring utilization of reactive electron during metal corrosion, oxidant activation, radical generation and pollutant degradation in Fenton-like catalysis. Our findings suggest that the ubiquitous DBC can be an efficient chelating agent to refine transition metals by serving as the surface deactivator and electron mediator, and take new insights into their environmental and agricultural geochemistry.

Selective atomic sieving across metal/oxide interface for super-oxidation resistance

Surface passivation, a desirable natural consequence during initial oxidation of alloys, is the foundation for functioning of corrosion and oxidation resistant alloys ranging from industrial stainless steel to kitchen utensils. This initial oxidation has been long perceived to vary with crystal facet, however, the underlying mechanism remains elusive. Here, using in situ environmental transmission electron microscopy, we gain atomic details on crystal facet dependent initial oxidation behavior in a model Ni-5Cr alloy. We find the (001) surface shows higher initial oxidation resistance as compared to the (111) surface. We reveal the crystal facet dependent oxidation is related to an interfacial atomic sieving effect, wherein the oxide/metal interface selectively promotes diffusion of certain atomic species. Density functional theory calculations rationalize the oxygen diffusion across Ni(111)/NiO(111) interface, as contrasted with Ni(001)/NiO(111), is enhanced. We unveil that crystal facet with initial fast oxidation rate could conversely switch to a slow steady state oxidation.

Hole-Carrier-Dominant Transport in 2D Single-Crystal Copper

In 2D noble metals like copper, the carrier scattering at grain boundaries has obscured the intrinsic nature of electronic transport. However, it is demonstrated that the intrinsic nature of transport by hole carriers in 2D copper can be revealed by growing thin films without grain boundaries. As even a slight deviation from the twin boundary is perceived as grain boundaries by electrons, it is only through the thorough elimination of grain boundaries that the hidden hole-like attribute of 2D single-crystal copper can be unmasked. Two types of Fermi surfaces, a large hexagonal Fermi surface centered at the zone center and the triangular Fermi surface around the zone corner, tightly matching to the calculated Fermi surface topology, confirmed by angle-resolved photoemission spectroscopy (ARPES) measurements and vivid nonlinear Hall effects of the 2D single-crystal copper account for the presence of hole carriers experimentally. This breakthrough suggests the potential to manipulate the majority carrier polarity in metals by means of grain boundary engineering in a 2D geometry.

Vacancy Ordering in Ultrathin Copper Oxide Films on Cu(111)

Oxide thin films grown on metal surfaces have wide applications in catalysis and beyond owing to their unique surface structures compared to their bulk counterparts. Despite extensive studies, the atomic structures of copper surface oxides on Cu(111), commonly referred to as "44" and "29", have remained elusive. In this work, we demonstrated an approach for the structural determination of oxide surfaces using element-specific scanning tunneling microscopy (STM) imaging enhanced by functionalized tips. This approach enabled us to resolve the atomic structures of "44" and "29" surface oxides, which were further corroborated by noncontact atomic force microscopy (nc-AFM) measurements and Monte Carlo (MC) simulations. The stoichiometry of the "44" and "29" frameworks was identified as Cu23O16 and Cu16O11, respectively. Contrary to the conventional hypothesis, we observed ordered Cu vacancies within the "44" structure manifesting as peanut-shaped cavities in the hexagonal lattice. Similarly, a combination of Cu and O vacancies within the "29" structure leads to bean-shaped cavities within the pentagonal lattice. Our study has thus resolved the decades-long controversy on the atomic structures of "44" and "29" surface oxides, advancing our understanding of copper oxidation processes and introducing a robust framework for the analysis of complex oxide surfaces.

Orientation-dependent oxidation behavior of Cu under In-situ E-Beam irradiation

Herein, we present an atomic in-situ investigation of Cu oxidation along different orientations stimulated by high-energy electron beams (E-Beam) in transmission electron microscopy (TEM). By following the microstructural evolution of the Cu substrate in real time, high-resolution TEM (HRTEM) images reveal an orientation-dependent oxidation mechanism, whereby Cu along [110] zone axis migrates onto the surface and be oxidized while Cu along [100] zone axis is oxidized completely both in bulk and at the surface. The different oxidation mechanisms can be attributed to the differing diffusion rates of oxygen in Cu structures along directions. Moreover, the growth of Cu oxides is found to follow a layer-by-layer mechanism, where Cu mostly migrates onto nanocrystal {110} planes. This behavior would lead to the oxides wider in geometric shape and therefore promote the aggregation of adjacent oxides. These findings have important implications for the practical use of copper-based materials in oxidizing environments.

A Method Probing High-Temperature Oxidation Behavior of Crystalline Materials

To date, the oxidation behavior of crystal materials is not fully understood; additional research is needed to understand the oxidation of materials. Herein, density functional theory (DFT) calculations and a 3D kinetic Monte Carlo (KMC) model are used to investigate the infiltration and diffusion behaviors of oxygen atoms within the crystal. Oxygen molecules readily adsorbes on crystal surfaces of the material and rapidly dissociates, verified by both first-principles calculations and energy-dispersive spectrometer (EDS) results. The infiltration ability of oxygen atoms into the inner crystal layers is affected by the surrounding oxygen atom, lattice compactness, and other factors. Energy-barrier calculations show that crystal thin/dense layers have significant effects on the crystal oxidation process, so high-pressure technology is used to investigate this correlation experimentally. KMC calculations and thermogravimetric analyses (TGA) show the infiltration behavior of oxygen atoms in the main crystal plane (211) toward the inner layers has the highest proportion to the actual high-temperature oxidation behavior of the title material. The results of both the KMC calculations and thermal experiments show the material peeled off upon further oxidation, which accelerates oxidation. At the same time, high-pressure treatment increases the oxidation resistance of materials at lower temperatures (<600 °C).

Oxygen Impurity-Tuned Structure and Adhesion Properties of the Cu/SiO2 Interface

The properties of the Cu/SiO2 interface usually deteriorate in the complex atmospheric environment, which may limit its performance and application in the engineering. Using the reactive molecular dynamics method, we investigate how the mechanical behaviors of the Cu/SiO2 interface change as it interacts with oxygen impurities. The interfacial oxidation degree could be enhanced as O2 penetrates into the interface area. This makes the interfacial structure disordered and is not conducive to the survival of Cu-O-Si bondings, which reduces the tensile and shear strengths of the interface. To improve the abrupt bonding property change at the interface and modify the interfacial adhesion properties, O impurities are introduced at the Cu interstitial sites near the interface. By doing so, the interface strength can be significantly enhanced due to the production of typical O-Cu-O bondings while the regular interfacial structure is retained. Meanwhile, the interfacial oxidation also changes the tensile failure site and shearing sliding mode of the interface, i.e., from inside the oxide to between oxide and Cu. The findings of this work may not only advance the understanding of interaction mechanism between oxygen impurities and the Cu/SiO2 interface but also provide new insights into optimizing the bonding properties of the metal/oxide interface.

Adsorption Structures, Vibrational Raman Spectra and Chemical Binding Properties of Thioglycolic Acid on Cu(111) Surfaces: A DFT Study

Copper is widely used in everyday life and industrial production because of its good electrical and thermal conductivity. To overcome copper oxidation and maintain its good physical properties, small organic molecules adsorbed on the surface of copper make a passivated layer to further avoid copper corrosion. In this work, we have investigated thioglycolic acid (TGA, another name is mercaptoacetic acid) adsorbed on copper surfaces by using density functional theory (DFT) calculations and a periodical slab model. We first get five stable adsorption structures, and the binding interaction between TGA and Cu(111) surfaces by using density of states (DOS), indicating that the most stable configuration adopts a triple-end binding model. Then, we analyze the vibrational Raman spectra of TGA adsorbed on the Cu(111) surface and make vibrational assignments according to the vibrational vectors. Finally, we explore the temperature effect of the thermodynamically Gibbs free energy of TGA on the Cu(111) surface and the antioxidant ability of the small organic molecular layer of copper oxidation on the copper surface. Our calculated results further provide evidences to interpret the stability of adsorption structures and antioxidant properties of copper.

The damage mechanism in copper studied using in situ TEM nanoindentation

Copper (Cu) has a soft-plastic nature, which makes it susceptible to damages from scratching or abrasive machining, such as lapping and polishing. It is a challenge to control these damages as the damage mechanism is elusive. Nonetheless, controlling damages is essential, especially on the atomic surfaces of Cu. To interpret the damage mechanism, in situ transmission electron microscopy (TEM) nanoindentation was performed using a cube-corner indenter with a radius of 57 nm at a loading speed of 5 nm s-1. Experimental results showed that damages originate from dislocations, evolve to stack faults, and then form broken crystallites. When the indentation depth was 45 nm at a load of 20 μN, damages comprised dislocations and stacking faults. After increasing the depth to 67 nm and load to 30 μN, the formation of broken crystallites initiated; and the critical depth was 67 nm. To validate the damage mechanism, fixed-abrasive lapping, mechanical polishing, and chemical mechanical polishing (CMP) were conducted. Firstly, a novel green CMP slurry containing silica, hydrogen peroxide, and aspartic acid was developed. After CMP, a surface roughness Ra of 0.2 nm was achieved with a scanning area of 50 μm × 50 μm; and the thickness of the damaged layer was 3.1 nm, which included a few micro-stacking faults. Lapping and mechanical polishing were carried out using a silicon carbide plate and cerium slurry, with surface roughness Ra values of 16.42 and 1.74 nm, respectively. The damaged layer of the former with a thickness of 300 nm comprised broken crystallites, dislocations, and stacking faults and that of the latter with a thickness of 33 nm involved several stacking faults. This verifies that the damage mechanism derived from in situ TEM nanoindentation is in agreement with lapping and polishing. These outcomes propose new insights into understanding the origin of damages and controlling them, as well as obtaining atomic surfaces using a novel green CMP technique for soft-plastic metals.

Effect of Copper Surface Roughness on the High-Temperature Structural Stability of Single-Layer-Graphene

Graphene (Gr) has shown great potential in the field of oxidation protection for metals. However, numerous studies have shown that Gr will suffer structural degradation on metal surface during high-temperature oxidation, which significantly limited the effectiveness of their oxidation protection. Therefore, understanding the degradation mechanism of Gr is of great interest to enhance their structural stability. Here, the effect of copper (Cu) surface roughness on the high-temperature structural stability of single-layer graphene (SLG) was examined using Cu covered with SLG as a model material. SLG/Cu with different roughness values was obtained via high-temperature annealing of the model material. After high-temperature oxidation at 500 °C, Raman spectra analysis showed that the defect density of the oxidized SLG increased from 41% to 81% when the surface roughness varied from 37 nm to 81 nm. Combined with density functional theory calculations, it was found that the lower formation energy of the C-O bond on rough Cu surfaces (0.19 eV) promoted the formation of defects in SLG. This study may provide guidance for improving the effectiveness of SLG for the oxidation protection of metallic materials.

Controlled Growth of Single-Crystal Graphene Wafers on Twin-Boundary-Free Cu(111) Substrates

Single-crystal graphene (SCG) wafers are needed to enable mass-electronics and optoelectronics owing to their excellent properties and compatibility with silicon-based technology. Controlled synthesis of high-quality SCG wafers can be done exploiting single-crystal Cu(111) substrates as epitaxial growth substrates recently. However, current Cu(111) films prepared by magnetron sputtering on single-crystal sapphire wafers still suffer from in-plane twin boundaries, which degrade the SCG chemical vapor deposition. Here, it is shown how to eliminate twin boundaries on Cu and achieve 4 in. Cu(111) wafers with ≈95% crystallinity. The introduction of a temperature gradient on Cu films with designed texture during annealing drives abnormal grain growth across the whole Cu wafer. In-plane twin boundaries are eliminated via migration of out-of-plane grain boundaries. SCG wafers grown on the resulting single-crystal Cu(111) substrates exhibit improved crystallinity with >97% aligned graphene domains. As-synthesized SCG wafers exhibit an average carrier mobility up to 7284 cm2 V-1 s-1 at room temperature from 103 devices and a uniform sheet resistance with only 5% deviation in 4 in. region.

Efficient Catalytic Production of Hydrogen Peroxide Using Tin-containing Zeolite Fixed Palladium Nanoparticles with Oxidation Resistance

The metal surfaces tend to be oxidized in air through dissociation of the O-O bond of oxygen to reduce the performances in various fields. Although several ligand modification routes have alleviated the oxidation of bulky metal surfaces, it is still a challenge for the oxidation resistance of small-size metal nanoparticles. Herein, we fixed the small-size Pd nanoparticles in tin-contained MFI zeolite crystals, where the tin acts as an electron donor to efficiently hinder the oxidation of Pd by weakening the adsorption of molecular oxygen and suppressing the O-O cleavage. This oxidation-resistant Pd catalyst exhibited superior performance in directly synthesizing hydrogen peroxide from hydrogen and oxygen, with the productivity of hydrogen peroxide at ≈10,170 mmol gPd -1 h-1 , steadily outperforming the catalysts tested previously. This work leads to the hypothesis that tin is an electron donor to realize oxidation-resistant Pd within zeolite crystals for efficient catalysis to overcome the limitation of generally supported Pd catalysts and further motivates the use of oxidation-resistant metal nanoparticles in various fields.

Enhanced copper anticorrosion from Janus-doped bilayer graphene

The atomic-thick anticorrosion coating for copper (Cu) electrodes is essential for the miniaturisation in the semiconductor industry. Graphene has long been expected to be the ultimate anticorrosion material, however, its real anticorrosion performance is still under great controversy. Specifically, strong electronic couplings can limit the interfacial diffusion of corrosive molecules, whereas they can also promote the surficial galvanic corrosion. Here, we report the enhanced anticorrosion for Cu simply via a bilayer graphene coating, which provides protection for more than 5 years at room temperature and 1000 h at 200 °C. Such excellent anticorrosion is attributed to a nontrivial Janus-doping effect in bilayer graphene, where the heavily doped bottom layer forms a strong interaction with Cu to limit the interfacial diffusion, while the nearly charge neutral top layer behaves inertly to alleviate the galvanic corrosion. Our study will likely expand the application scenarios of Cu under various extreme operating conditions.

The Role of Grain Boundary Sites for the Oxidation of Copper Catalysts during the CO Oxidation Reaction

The oxidation of transition metal surfaces is a process that takes place readily at ambient conditions and that, depending on the specific catalytic reaction at hand, can either boost or hamper activity and selectivity. Cu catalysts are no exception in this respect since they exhibit different oxidation states for which contradicting activities have been reported, as, for example, in the catalytic oxidation of CO. Here, we investigate the impact of low-coordination sites on nanofabricated Cu nanoparticles with engineered grain boundaries on the oxidation of the Cu surface under CO oxidation reaction conditions. Combining multiplexed in situ single particle plasmonic nanoimaging, ex situ transmission electron microscopy imaging, and density functional theory calculations reveals a distinct dependence of particle oxidation rate on grain boundary density. Additionally, we found that the oxide predominantly nucleates at grain boundary-surface intersections, which leads to nonuniform oxide growth that suppresses Kirkendall-void formation. The oxide nucleation rate on Cu metal catalysts was revealed to be an interplay of surface coordination and CO oxidation behavior, with low coordination favoring Cu oxidation and high coordination favoring CO oxidation. These findings explain the observed single particle-specific onset of Cu oxidation as being the consequence of the individual particle grain structure and provide an explanation for widely distributed activity states of particles in catalyst bed ensembles.

Self-Oxidation Resistance of the Curved Surface of Achromatic Copper

Copper surfaces that exhibit a wide range of achromatic colors while still metallic have not been studied, despite advancements in antireflection coatings. A series of achromatic copper films grown with [111] preferred orientation by depositing 3D porous nanostructures is introduced via coherent/incoherent atomic sputtering epitaxy. The porous copper nanostructures self-regulate the giant oxidation resistance by constructing a curved surface that generates a series of monoatomic steps, followed by shrinkage of the lattice spacing of one or two surface layers. First-principles calculations confirm that these structural components cooperatively increase the energy barrier against oxygen penetration. The achromaticity of the single-crystalline porous copper films is systematically tuned by geometrical parameters such as pore size distribution and 3D linkage. The optimized achromatic copper films with high oxidation resistance show an unusual switching effect between superhydrophilicity and superhydrophobicity. The tailored 3D porous nanostructures can be a candidate material for numerous applications, such as antireflection coatings, microfluidic devices, droplet tweezers, and reversible wettability switches.

Polarization Dependence of Laser-Induced Dynamics on Non-Flat Metal Surfaces: A Time-Dependent Density Functional Theory Approach

Real-time time-dependent density functional theory was used to study the laser-pulse-induced ion dynamics on metal surfaces featuring rows of atomic ridges. In contrast to atomically flat surfaces, the rows of atomic ridges induce anisotropy on the surface even in surface-parallel directions. This anisotropy causes the laser-induced ion dynamics to depend on the orientation of the laser polarization vector in the surface-parallel directions. This polarization dependence occurs for both copper (111) and aluminum (111) surfaces, indicating that the existence of localized d orbitals in the electronic system does not play a crucial role. The difference in kinetic energies between ions on the ridges and those on the planar surface reached a maximum when the laser polarization vector was perpendicular to the rows of ridges but parallel to the surface. A simple mechanism for the polarization dependence and some potential applications in laser processing are discussed.

Electrosynthesis of polymer-grade ethylene via acetylene semihydrogenation over undercoordinated Cu nanodots

The removal of acetylene impurities remains important yet challenging to the ethylene downstream industry. Current thermocatalytic semihydrogenation processes require high temperature and excess hydrogen to guarantee complete acetylene conversion. For this reason, renewable electricity-based electrocatalytic semihydrogenation of acetylene over Cu-based catalysts is an attractive route compared to the energy-intensive thermocatalytic processes. However, active Cu electrocatalysts still face competition from side reactions and often require high overpotentials. Here, we present an undercoordinated Cu nanodots catalyst with an onset potential of -0.15 V versus reversible hydrogen electrode that can exclusively convert C₂H₂ to C₂H₄ with a maximum Faradaic efficiency of ~95.9% and high intrinsic activity in excess of -450 mA cm^-2 under pure C₂H₂ flow. Subsequently, we successfully demonstrate simulated crude ethylene purification, continuously producing polymer-grade C₂H₄ with <1 ppm C₂H₂ for 130 h at a space velocity of 1.35 × 10⁵ml g_cat^-1 h^-1. Theoretical calculations and in situ spectroscopies reveal a lower energy barrier for acetylene semihydrogenation over undercoordinated Cu sites than nondefective Cu surface, resulting in the excellent C₂H₂-to-C₂H₄ catalytic activity of Cu nanodots.

Atomistic Origins of Reversible Noncatalytic Gas-Solid Interfacial Reactions

Noncatalytic gas-solid reactions are a large group of heterogeneous reactions that are usually assumed to occur irreversibly because of the strong driving force to favor the forward direction toward the product formation. Using the example of Ni oxidation into NiO with CO2, herein, we demonstrate the existence of the reverse element that results in the NiO reduction from the countering effect of the gaseous product of CO. Using in situ electron microscopy observations and atomistic modeling, we show that the oxidation process occurs via preferential CO2 adsorption along step edges that results in step-flow growth of NiO layers, and the presence of Ni atoms on the flat NiO surface promotes the nucleation of NiO layers. Simultaneously, the NiO reduction happens via preferential step-edge adsorption of CO that leads to the receding motion of atomic steps, and the presence of Ni vacancies in the NiO surface facilitates the CO-adsorption-induced surface pitting. Temperature and CO2 pressure effect maps are constructed to illustrate the spatiotemporal dynamics of the competing NiO redox reactions. These results demonstrate the rich gas-solid surface reaction dynamics induced by the coexisting forward and reverse reaction elements and have practical applicability in manipulating gas-solid reactions via controlling the gas environment or atomic structure of the solid surface to steer the reaction toward the desired direction.

Coherent consolidation of trillions of nucleations for mono-atom step-level flat surfaces

Constructing a mono-atom step-level ultra-flat material surface is challenging, especially for thin films, because it is prohibitively difficult for trillions of clusters to coherently merge. Even though a rough metal surface, as well as the scattering of carriers at grain boundaries, limits electron transport and obscures their intrinsic properties, the importance of the flat surface has not been emphasised sufficiently. In this study, we describe in detail the initial growth of copper thin films required for mono-atom step-level flat surfaces (MSFSs). Deposition using atomic sputtering epitaxy leads to the coherent merging of trillions of islands into a coplanar layer, eventually forming an MSFS, for which the key factor is suggested to be the individual deposition of single atoms. Theoretical calculations support that single sputtered atoms ensure the formation of highly aligned nanodroplets and help them to merge into a coplanar layer. The realisation of the ultra-flat surfaces is expected to greatly assist efforts to improve quantum behaviour by increasing the coherency of electrons.

Robust Helical Dichroism on Microadditively Manufactured Copper Helices via Photonic Orbital Angular Momentum

Three-dimensional chiral metallic metamaterials have already attracted extensive attention in the wide research fields of chiroptical responses. These artificial chiral micronanostructures, possessing strong chiroptical signals, show huge significance in next-generation photonic devices and chiroptical spectroscopy techniques. However, most of the existing chiral metallic metamaterials are designed for generating chiroptical signals dependent on photonic spin angular momentum (SAM). The chiral metallic metamaterials for generating strong chiroptical responses by photonic orbital angular momentum (OAM) remain unseen. In this work, we fabricate copper microhelices with opposite handedness by additively manufacturing and further examine their OAM-dominated chiroptical response: helical dichroism (HD). The chiral copper microhelices exhibit differential reflection to the opposite OAM states, resulting in a significant HD signal (∼50%). The origin of the HD can be theoretically explained by the difference in photocurrent distribution inside copper microhelices under opposite OAM states. Moreover, the additively manufactured copper microhelices possess an excellent microstructural stability under varying annealing temperatures for robust HD responses. Lower material cost and noble-metal-similar optical properties, accompanied with well thermal stability, render the copper microhelices promising metamaterials in advanced chiroptical spectroscopy and photonic OAM engineering.

Effect of Water Vapor on Oxidation Processes of the Cu(111) Surface and Sublayer

Copper-based catalysts have different catalytic properties depending on the oxidation states of Cu. We report operando observations of the Cu(111) oxidation processes using near-ambient pressure scanning tunneling microscopy (NAP-STM) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The Cu(111) surface was chemically inactive to water vapor, but only physisorption of water molecules was observed by NAP-STM. Under O₂ environments, dry oxidation started at the step edges and proceeded to the terraces as a Cu₂O phase. Humid oxidation of the H₂O/O₂ gas mixture was also promoted at the step edges to the terraces. After the Cu₂O covered the surface under humid conditions, hydroxides and adsorbed water layers formed. NAP-STM observations showed that Cu₂O was generated at lower steps in dry oxidation with independent terrace oxidations, whereas Cu₂O was generated at upper steps in humid oxidation. The difference in the oxidation mechanisms was caused by water molecules. When the surface was entirely oxidized, the diffusion of Cu and O atoms with a reconstruction of the Cu₂O structures induced additional subsurface oxidation. NAP-XPS measurements showed that the Cu₂O thickness in dry oxidation was greater than that in humid oxidation under all pressure conditions.

Stabilization of Copper-Based Biochips with Alumina for Biosensing Application

Surface plasmon resonance devices typically rely on the use of gold-coated surfaces, but the use of more abundant metals is desirable for the long-term development of plasmonic biochips. As a substitute for gold, thin copper films have been deposited on glass coverslips by thermal evaporation. As expected, these films immersed in a water solution initially exhibit an intense plasmonic resonance comparable to gold. However, without protection, an angle-resolved optical analysis shows a rapid degradation of the copper, characterized by a continuous angular shift of the plasmonic resonance curve. We show that copper films protected with a thin layer of aluminum oxide of a few nanometers can limit the oxidation rate for a sufficient time to perform some standard measurements. As the process is simple and compatible with the current biochip production technique, such an approach could pave the way for the production of alternative and more sustainable biochips.

Atomic-level flatness on oxygen-free copper surface in lapping and chemical mechanical polishing

Oxygen-free copper (OFC) serves as a core component of high-end manufacturing, and requires high surface quality. It is always a significant challenge to manufacture high-quality atomic-level surfaces. In this study, SiO₂ nanospheres with good dispersibility were prepared and a late-model environmentally friendly chemical mechanical polishing (CMP) slurry was developed. The CMP slurry consists of SiO₂ nanospheres, CeO₂ nanospheres, H₂O₂, NaHCO₃, polyaspartic acid and deionized water. After CMP, the average roughness (Sa) of the OFC wafer reached 0.092 nm with an area of 50 × 50 μm². Atomic-level flatness on the oxygen-free copper surface was acquired, which has never been reported before. Moreover, the mechanical removal mechanism of abrasive particles and the chemical reactions during lapping and CMP are proposed in detail. The thickness and composition of the damaged layer after lapping and polishing were analyzed. The lapping-damaged layer consists of a lattice distortion region, moiré fringes, grain boundary, superlattice and edge dislocations, and the polishing-damaged layer contains a handful of stacking faults with single-layer or multi-layer atoms. The chemical action involves three reactions: oxidation, corrosion and chelation. The processing method and its mechanistic explanation pave the way for the fabrication of high-performance OFC surfaces for use in vacuum, aerospace, military and electronic industries.

Enhancement of fatigue resistance by recrystallization and grain growth to eliminate bonding interfaces in Cu-Cu joints

Cu–Cu joints have been adopted for ultra-high density of packaging for high-end devices. However, cracks may form and propagate along the bonding interfaces during fatigue tests. In this study, Cu–Cu joints were fabricated at 300 °C by bonding 〈111〉-oriented nanotwinned Cu microbumps with 30 μm in diameter. After temperature cycling tests (TCTs) for 1000 cycles, cracks were observed to propagate along the original bonding interface. However, with additional 300 °C-1 h annealing, recrystallization and grain growth took place in the joints and thus the bonding interfaces were eliminated. The fatigue resistance of the Cu–Cu joints is enhanced significantly. Failure analysis shows that cracks propagation was retarded in the Cu joints without the original bonding interface, and the electrical resistance of the joints did not increase even after 1000 cycles of TCT. Finite element analysis was carried to simulate the stress distribution during the TCTs. The results can be correlated to the failure mechanism observed by experimental failure analysis.