Ir Substrates Research Articles

DFT study on ORR catalyzed by bimetallic Pt-skin metals over substrates of Ir, Pd and Au

Bimetallic Pt-skin catalyst is a class of near-surface alloy (NSA) that owns a high degree of control over composition. Herein, density functional theory (DFT) is used to calculate the energetics of oxygen reduction reaction (ORR) on Pt-skin over Ir, Pd and Au substrates. A Brønsted-Evans-Polanyi (BEP) relationship can be determined for the oxygen molecule dissociation. The binding energy of both atomic oxygen and hydroxyl radical is found to correlate well with the d band center of Pt-skin atoms. Their catalytic activities show the volcano relationship as the positions of each substrate in the periodic table. The effect of surface strain, band structure and charge transfer on the d band center is well studied, and it can be found that the surface strain effect plays a dominant role for all Pt-skin catalysts. Ir substrate makes the d band center of Pt-skin go far away from the Fermi level, while Au substrate makes it move towards the Fermi level. Being different from both Ir and Au, Pd substrate makes the d band center of Pt-skin comparable with the monometallic Pt.

Easy Approach to Graphene Growth on Ir(111) and Ru(0001) from Liquid Ethanol

AbstractThe growth of a high‐quality complete graphene layer is successfully achieved for Ir(111) and Ru(0001) substrates using liquid ethanol as a precursor. Metallic substrates, which are cleaned in ultra‐high vacuum conditions, were ex‐situ immersed in liquid ethanol followed by the controlled in situ thermal annealing. The process of graphene formation and its quality are carefully monitored using X‐ray photoelectron spectroscopy, low‐energy electron diffraction, and scanning tunneling microscopy methods. It is found that graphene formation starts at 400 °C via ethanol decomposition and desorption of oxygen from the surface leading to the formation of the high‐quality complete graphene layer at 1000 °C. The results of the systematic angular‐resolved photoelectron spectroscopy experiments confirm the high quality of the obtained graphene layer, and it concludes that such an approach offers an easy, quick, and reproducible method to synthesize large‐scale graphene on different metallic substrates.

Ambipolar Doping of Monolayer FeSe by Interface Engineering

We report on ambipolar modulation doping of monolayer FeSe epitaxial films grown by molecular beam epitaxy and in situ spectroscopic measurements via a cryogenic scanning tunneling microscopy. It is found that hole doping kills superconductivity in monolayer FeSe films on metallic Ir(001) substrates, whereas electron doping from polycrystalline IrO2/SrTiO3 substrate enhances significantly the superconductivity with an energy gap of 10.3 meV. By exploring substrate-dependent superconductivity, we elucidate the essential impact of substrate work functions on the superconductivity of monolayer FeSe films. Our results therefore offer a valuable reference guide for further enhancement of the transition temperature T c in FeSe-based superconductors by interface engineering.

Surface microstructures and oxygen evolution properties of cobalt oxide deposited on Ir(111) and Pt(111) single crystal substrates

AbstractWe investigated the oxygen evolution reaction (OER) activity changes of cobalt oxide (CoOx) thin films on Ir(111) and Pt(111) substrates by repeated OER measurements in 0.1 M KOH. Atomic force microscopy and X‐ray photoelectron spectroscopy analysis of the as‐prepared CoOx/Ir(111) and CoOx/Pt(111) showed similar surface morphologies of the CoOx thin films and almost the same OER overpotentials, which were estimated to be around 430 mV. However, after three OER measurements, the overpotential of CoOx/Ir(111) decreased by 70 mV, whereas that of CoOx/Pt(111) increased slightly. Structural analysis showed that CoOx/Ir(111) revealed the island‐like nanostructures of CoOx dispersed on Ir(111) surface, accompanied by the generation of CoOOH. In contrast, for CoOx/Pt(111), the Pt(111) substrate remains covered by the CoOx thin film. The results suggest that the interaface at CoOx (CoOOH) nano‐islands and Ir(111) substrate are responsible for reducing the OER overpotential.

Growth of high-quality inch-diameter heteroepitaxial diamond layers on sapphire substrates in comparison to MgO substrates

Recently, a 2-inch-diameter high-quality (001) diamond wafer was prepared that was heteroepitaxially grown on a (001) Ir buffer layer/(112¯0) A-plane sapphire substrate. Herein, we investigate residual strain and curvature of diamond in a relation with Ir/sapphire substrate and Ir/(001) MgO substrate. Diamond layers grown on Ir substrates have tensile strain, whereas Ir buffer layers on sapphire or MgO substrates have compressive strain. However, the magnitude of these strains in diamond and Ir layers grown on sapphire is considerably lower than that of the same layers grown on MgO. The lower tensile strain in the diamond on Ir/sapphire leads to the growth of a 2-inch-diameter layer without cracking. The convex curvature radius of the diamond layers grown on sapphire is larger than that observed on the MgO substrate. The difference in the coefficients of thermal expansion between the diamond and sapphire is considerably smaller than that of the MgO substrate. The low tensile strain of diamond is caused by the difference in the lattice constants of the diamond and Ir/sapphire substrate.

Analysis of Surface Microstructures Formed on Ir Substrate under Different Bias Conditions by Microwave Plasma Chemical Vapor Deposition

To understand the process window of bias‐enhanced nucleation of diamond, it is important to study surface modifications of Ir substrates that occur under different conditions. Herein, various microstructures formed on Ir substrates are reported. The first microstructure contains rectangular particles with a size of several hundred nanometers obtained at a relatively high pressure of 12.5 kPa and a high temperature of 880 °C; these particles consist of epitaxially grown Ir3Si species. The second microstructure includes particles oriented along the [110] direction; these particles, however, consist of etched Ir pillars with a height of 400 nm when the substrate temperature increases to 1110 °C. Both microstructures form because of Ir sputtering due to the bombardment of energetic charged ions and the transport of Ir surface atoms. This study demonstrates two types of surface modifications under different bias‐enhanced nucleation conditions and provides a method for preparing an epitaxial Ir–Si compound that can be applied in various silicon‐based electronic devices.

Characterization of Substrates and Surface-Enhancement in Atomic Force Microscopy Infrared Analysis of Amyloid Aggregates.

Atomic force microscopy infrared (AFM-IR) spectroscopy is an emerging analytical technique that can be used to probe the structural organization of specimens with nanometer spatial resolution. A growing body of evidence suggests that nanoscale structural analysis of very small (<10 nm) biological objects, such as viruses and amyloid aggregates, requires substrates that must fit strict criteria of low surface roughness and low IR background, simultaneously. In this study, we examine the suitability of a broad range of substrates commonly used in AFM and IR fields, and we determined that silicon, zinc sulfide, and calcium fluoride are the most ideal substrates for nanoscale imaging of amyloid oligomers, protein aggregates that are directly linked to the onset and progression of neurodegenerative diseases. Our data show that these substrates provide the lowest roughness and the lowest background in the 800-1800 cm-1 spectral window from all examined AFM and IR substrates. We also investigate a contribution of surface enhancement in AFM-IR by the direct comparison of signal intensities from oligomers located on silicon and gold-coated silicon surfaces. We found that metallization of such substrates provides a factor of ~7 enhancements to the IR signal and induces an equivalent enhancement of the sample background in the 950-1250 cm-1 spectral region.

Silicene Growth through Island Migration and Coalescence

Silicene, the Si based counterpart to graphene is predicted to demonstrate similar desirable features to graphene, possessing a Dirac cone in the band-structure and Fermi velocities comparable to graphene. It has also been predicted to demonstrate several exotic properties such as the quantum spin Hall effect, chiral superconductivity and giant magnetoresistance. It has the added benefit of being easy to integrate into current industrial device engineering processes. Silicene monolayers have been grown on Ag (111) and Ir (111) substrates and recently a working field-effect transistor has been demonstrated at room temperature. While it is possible to characterize the final monolayer structure, it is impossible to probe experimentally the evolving atomic structure during growth.We perform massively-parallel molecular dynamics (MD) simulations to study long time monolayer silicene growth on an Ir (111) surface. We observe an intricate multi-stage growth process driven by atomic and cluster migration on the surface. Initial growth involves formation of sub-nanometer clusters via adatom surface diffusion. Subsequently, these clusters rearrange spontaneously with each additional Si atom, forming clusters containing 4–7 member rings. Growth of each cluster through adatom adhesion is accompanied by the formation of larger islands through cluster migration and coalescence. Coalescence of smaller, more mobile islands into larger clusters is aided by the internal rearrangement of rings within each cluster. This flexibility, both of clusters and their constituent atoms, allows the impinging clusters to reorient after first contact and form a more perfect union. We also report on the effect of temperature and flux on the growth process and the final nanostructure. Our study provides atomistic insights into the early stage growth mechanisms of silicene which can be significant for controlled synthesis of its 2D monolayers. Reference: Cherukara, M. J., Narayanan, B., Chan, H., & Sankaranarayanan, S. K. (2017). Silicene growth through island migration and coalescence. Nanoscale, 9(29), 10186-10192. Figure 1

A simple descriptor for binding and charge transfer at blue phosphorene-metal interfaces

We have performed density functional theory calculations to study the binding and charge transfer at interfaces between blue phosphorene and various metal substrates. The substrates considered are Al(111), Ag(111), Au(111), Pt(111), Zr(0001) and Sc(0001). The substrates suitable for use for synthesis and exfoliation of blue phosphorene, or instead for use as electrodes for either making contacts or charge doping blue phosphorene, are identified. We find that both the charge transfer and strength of binding correlate excellently with a simple descriptor D = Δ χ / Δ R , where Δ χ is the difference in Pauling electronegativities between the metal and phosphorus, and Δ R the corresponding difference in covalent radii. The predictions of the descriptor are validated by subsequent calculations on Pd(111), Cu(111) and Ir(111) substrates. This easily computed descriptor should help in the rapid identification of two-dimensional material + metal combinations suitable for specific applications. • We study interfaces between blue phosphorene and various metal substrates using density functional theory caalculations. • We formulate a descriptor = Δχ/Δℛ, where Δχ and Δℛ are the differences in electronegativities and atomic sizes respectively. • This descriptor can be used to predict both the charge transfer and the strength of binding at the interface.

Silicene-Like Domains on IrSi3 Crystallites

Recently, silicene, the graphene equivalent of silicon, has attracted a lot of attention due to its compatibility with Si-based electronics. So far, silicene has been epitaxy grown on various crystalline surfaces such as Ag(110), Ag(111), Ir(111), ZrB2(0001) and Au(110) substrates. Here, we present a new method to grow silicene via high temperature surface reconstruction of hexagonal IrSi3 nanocrystals. The h-IrSi3 nanocrystals are formed by annealing thin Ir layers on Si(111) surface. A detailed analysis of the STM images shows the formation of silicene like domains on the surface of some of the IrSi3 crystallites. We studied both morphology and electronic properties of these domains by using both scanning tunneling microscopy/spectroscopy and first-principles calculation methods.

In Situ stress Measurements during Strained-Layer Electrodeposition

Enhanced catalytic activity has been demonstrated in elastically strained thin films, pseudomorphically grown on substrates with an appropriate lattice mismatch. The enhancement in catalytic activity is typically attributed to both electronic interactions and the epitaxial mismatch strain in the overlayer. The magnitude of the strain is often estimated from lattice-constant measurements using x-ray diffraction. An additional probe gaining popularity in electrochemistry involves the measurement of surface and growth stress using cantilever curvature. Cantilever curvature offers a nice complement to the direct strain measurementand can provide an in situ measurement while the strained layer is being formed. This talk will focus on the biaxial growth stress that develops during the self-terminated electrodeposition of ultrathin Pt films onto a variety of cantilever substrates that introduce different levels of misfit strain into the Pt. The cantilever was a borosilicate glass strip measuring 60 mm × 3 mm × 0.108 mm and was metallized on one side by electron beam evaporation, resulting in substrates with strong (111) crystallographic orientation. The electrolyte was 0.5 mol/L NaCl containing 3 mmol/L K2PtCl4. Deposition consisted of stepping the potential from the open circuit potential (~ -0.1V vs SSE) down to -1.2V for 10s and then back to -0.1V for another 30s, following the procedure established by Liu et al.1Figure 1 shows the stress response for Au, Pt and Ir substrates. Pt grown on Au exhibits considerable tensile stress, which is consistent with the +3.9% lattice mismatch between Au and Pt. In contrast, Pt deposition onto a Pt cantilever deposits homoepitaxially as an unstrained continuation of the substrate. The slight increase in stress with continued deposition is likely microstructural. Pt grown on Ir, with its -2.1% lattice misfit, initially exhibits compressive stress, before leveling off and turning tensile, similar to the growth of Pt on Pt. Although it is tempting to focus on lattice misfit as the primary source of stress, one must consider other contributions, such as step line tension associated with the growth of small islands, possible intermixing, and surface roughness. References Y. Liu, D. Gokcen, U. Bertocci and T. P. Moffat, Science 338, 1327 (2012). Figure 1

Tiered deposition of sub-5 nm ferroelectric Hf1-xZrxO2 films on metal and semiconductor substrates

Using a tiered deposition approach, Hf1-xZrxO2 (HZO) films with varying atomic layer deposition (ALD) cycles from 36 to 52 cycles were grown on Ge, Ir, and TiN substrates in single runs and annealed at 500 °C. 40 ALD cycle films grown on Ir exhibit a switched polarization (Psw) of 13 μC/cm2, while those grown on Ge and TiN did not exhibit measurable Psw values until 44 and 52 ALD cycles, respectively. High-resolution cross-sectional transmission electron microscopy confirmed these results; the ferroelectric films are crystalline with defined lattice fringes, while non-ferroelectric films remain amorphous. 52 ALD cycle 1:1 HZO grown on Ge had the highest Psw of all the films fabricated at 39 μC/cm2, while the 1:1 HZO grown on TiN displayed continuous wake-up and no fatigue up to 1010 cycles with the Psw increasing from &amp;lt;1 μC/cm2 to 21 μC/cm2.

On the Extended Holstein–Hubbard Model for Epitaxial Graphene on Metal

A model that combines the extended Hubbard model involving intra-atomic and interatomic Coulomb repulsion and the Holstein model describing the interaction of a band electron with an Einstein phonon is proposed. Three regions of the phase diagram are considered. The regions correspond to the states of spin- and charge-density waves and the state uniform in spin and charge. Numerical estimations for the Rh, Ir, and Pt substrates show that Coulomb interaction plays a leading part, making possible transitions from the uniform state to the states of spin- and charge-density waves.

Расширенная модель Холстейна-Хаббарда для эпитаксиального графена на металле

AbstractA model that combines the extended Hubbard model involving intra-atomic and interatomic Coulomb repulsion and the Holstein model describing the interaction of a band electron with an Einstein phonon is proposed. Three regions of the phase diagram are considered. The regions correspond to the states of spin- and charge-density waves and the state uniform in spin and charge. Numerical estimations for the Rh, Ir, and Pt substrates show that Coulomb interaction plays a leading part, making possible transitions from the uniform state to the states of spin- and charge-density waves.

Platinum Monolayer on Ir-Pd-Based Nanoparticles for Oxygen Reduction Reaction

Proton exchange membrane fuel cells (PEMFCs) have gained extensive attention as one of the most promising technologies for transportation due to their high energy generation efficiency and low environmental impacts. However, there’re still many technical difficulties and challenges hindering their widespread commercial application, among which the most important ones are high loading (high cost) of Pt [1] and its dissolution [2] during fuel cell operation on the cathode side of fuel cell. The approach of synthesizing electrocatalysts with a monolayer of Pt provides an effective way of reducing Pt contents while maintaining or even improving their electrochemical activities for oxygen reduction reaction (ORR) [3]. However, in addition to have beneficial catalytical influence on Pt shell, the material from which the cores are made, must be stabile in highly oxidative environment of the fuel cell cathode. Pt monolayer (PtML) catalyst deposited on Pd and Pd-based alloys cores showed higher ORR activity and stability compared to a pure Pt catalyst [2, 4]. Due to the incomplete Pt shell, Pd (a noble metal) shows some instability under the prolonged highly oxidizing conditions that can occur during fuel cell operation. Since Ir is one of the most stable transition metals, some research has been conducted on PtML catalysts deposited on Ir and Ir-M (M=Fe, Co, Ni, Cu) alloy nanoparticle substrates [5, 6]. In this work, low-Pt content catalysts consist of a PtML on carbon-supported IrPd (IrPd/C) and IrPd-shell Ni-core (IrPd/Ni/C) nanoparticles were examined for ORR activity and stability. IrPd/C nanoparticles, with molar ratio of Ir:Pd=1:4, were prepared using PdCl2 and (NH4)2IrCl6 as metal precursors, citric acid as a complexing agent, and NaBH4 as a reducing agent. The IrPd/Ni/C nanoparticles were synthesized by first preparing Ni nanoparticles supported on carbon (Ni/C) which were subsequently immersed in solution containing Pd and Ir ions (Ir:Pd=1:4) to form PdIr shell by partially galvanically displacing Ni. After that, IrPd/C and IrPd/Ni/C nanoparticles were filtered and dried in vacuum oven. At the end, they were annealed in pure H2 atmosphere at 550°C for 2 hours. The PtMLIrPd/C and PtMLIrPd/Ni/C catalysts were prepared by depositing a Pt monolayer onto these particles by displacing a Cu monolayer formed at underpotentials in 0.05 M CuSO4 and 0.1M H2SO4 solution. The average diameter of IrPd/C nanoparticles is ~ 4 nm (Fig. 1) while for the IrPd/Ni/C nanoparticles is ~ 10 nm (not shown). Polarization curves showed that both the catalysts of interests in this work have better ORR activities than PtML catalysts deposited on commercial Pd/C (30wt% NE-H) (PtMLPd/C). The half-wave potentials and initial Pt mass activities of PtMLIrPd/C (882 mV, 1.71 A/mgPt) and PtMLIrPd/Ni/C catalysts (877 mV, 1.61 A/mgPt) are much high than those for PtMLPd/C (854 mV, 0.555 A/mgPt) (Fig. 2). Moreover, after 5,000 cycles of accelerated durability test, the PtMLIrPd/C still has a relatively high Pt mass activity of 1.16 A/mgPt, while that of PtMLIrPd/Ni/C drops to 0.775 A/mgPt but still exceeds the DOE 2020 technical targets for electrocatalysts for transportation applications [7]. More detailed discussion on the ORR activity and stability of these catalysts will be presented at the meeting. Acknowledgements This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Unraveling the Influence of Metal Substrates on Graphene Nucleation from First-Principles Study

Using ab initio calculations, we systematically investigate graphene nucleation on 10 representative metal substrates that have been used in graphene growth by chemical vapor deposition. We find that the metal substrates can be divided into three categories with respect to the competition between carbon–carbon (C–C) and carbon–metal (C–M) interactions, which leads to the distinct critical size (Nc) dependence of the smallest graphene precursor on the substrates. The C–M interactions are weak on Ag, Au, Cu, and Co substrates, and the chemical potential of carbon decreases monotonically to approach that of graphene as the size of the carbon clusters increases. We observed an Nc around C13–C14 corresponding to the structural transition from a linear chain to sp2 configuration on these substrates. In contrast, the C–M interactions are strong on Ru, Pt, Rh, and Ir substrates, and the extremely stable carbon monomer thermodynamically determines the larger Nc about C19. The third category is Ni and Pd substrates...

Rapid Synthesis of Nonepitaxial Multilayer Silicene and Germanene Assisted By Plasma/Ion Implantation

Owing to the zero-bandgap of graphene, its application for channel material of switching devices is almost totally limited even if there are some methods for the bandgap opening of graphene, which is unstable or temporary. Thus the synthesis and applications of transition metal dichalcogenides (TMDs) have been widely studied since they possess bandgaps with a transformation from indirect to direct as the monolayer is reached. Besides mechanical exfoliation, the chemical vapor deposition (CVD) is the most mature technique for the growth of TMDs so far. However, the as-grown TMD films on SiO2/Si are non-continuous, and the transfer step seriously deteriorating the quality of TMD films is still inevitable since the SiO2 corroded by S or Se during the CVD process loses the insulating capability. Moreover, almost all of the processes for device fabrication need to be altered in the future, if the TMD materials would be commercialized in the semiconductor industry. Hence there are lots of issues that should be overcome for the real applications of TMDs. In fact, Si and Ge that belong to group IV in periodic table as well have been commercialized in the semiconductor industry for a long while. Hence the 2D Si and Ge that would be compatible with the present process and technology became the newly topics in the material science. According to theoretical calculation, the crystal structure of 2D Si and Ge belongs to one kind of hexagonal lattice named lonsdaleite structure. Thus the 2D Si and Ge, so called silicene and germanene, are actually the single-layered lonsdaleite Si and Ge. It is worth to mention that although silicene and germanene are also with zero-band gap like graphene, it is easier to open the band gap by applying a vertical electric field due to the non-equivalent atomic positions or adopting surface adsorption; meanwhile, the doping effect could be accomplished by introducing the strain. However, the vast challenge in synthesis of them is the naturally lack of sp2 orbital hybridization in Si and Ge owing to the larger ionic radii than that of C, so that they may not be directly obtained from bulk by mechanical exfoliation. Recently, computational studies concerning the physical properties of silicene and germanene were performed by many groups. Silicene and germanene have been grown on Ag, Au, Al, Ir, ZrB2/Si, and Pt substrates using ultra-high vacuum (UHV) epitaxy systems. At present, the first issue in formation of silicene and germanene might be the inevitable transfer process, which would deteriorate the quality of it, for device applications in the future, if the metals are used as the substrates during growth. Up to now, all of the studies only used STM combined with a diffraction analysis to demonstrate the formation of silicene and germanene. Hence multiple analyses should be necessary to acquire more information for further identification of materials. In our first work for synthesis of 2D materials, the multilayer graphene has been successfully synthesized by the plasma-assisted process introducing nitrogen ions into a 4H-SiC substrate to cause the preferential silicon nitridation and simultaneously condense carbon atoms onto the surface during thermal treatment. Then we expanded the process to synthesize multilayer germanene on a SiGe/Si substrate. Furthermore, we modified the plasma-assisted process by replacing the N2 plasma immersion with the B ion implantation for direct synthesis of multilayer silicene on a 4H-SiC substrate.

Molecular Dynamics Simulations of Silicene Monolayer Growth

Silicene, the Si based counterpart to graphene is predicted to demonstrate similar desirable features to graphene, possessing a Dirac cone in the band-structure and Fermi velocities comparable to graphene. It has also been predicted to demonstrate several exotic properties such as the quantum spin Hall effect, chiral superconductivity, giant magnetoresistance, and has the added benefit of being easy to integrate into current industrial device engineering processes. Silicene monolayers have been grown on Ag (111) and Ir (111) substrates and recently a working field-effect transistor has been demonstrated at room temperature. While it is possible to characterize the final monolayer structure, it is impossible to probe experimentally the evolving atomic structure during growth. We use large-scale molecular dynamics (MD) simulations to study the growth of silicene monolayers on Ag (111) and Ir (111) surfaces. Initially, individual Si atoms aggregate to form small clusters containing 3-6 Si atoms with different ring geometries. Over time, these rings merge to form larger clusters, but containing extended line defects and grain structures. Figure 1 A-D shows the growth process on an Ag (111) surface, while Figure 1 E shows the final grown structure on an Ir (111) surface. Interestingly, structures grown on the Ag surface are prone to more defects, in particular extended regions of sub-surface growth, while the structures grown on Ir are relatively defect free. We also explore the effect of different growth conditions such as temperature and deposition flux on the structure of the monolayers. Finally, we study the mechanical and thermal properties of these in-silico grown monolayers and compare their performance to previously reported data for ideal structures. The processing-structure-property correlations we draw will enable experimental processes to be tailored to obtain target device properties. Figure 1

Oxygen Reduction Reaction Activity and Durability for Model Pt Shell Layers on Ir(111) Prepared by Molecular Beam Epitaxy

We fabricated well-defined Pt–Ir bimetallic surfaces through monolayer-thick Pt epitaxial growth on Ir(111) substrates (Pt n ML/Ir(111):n = 1–4) using molecular beam epitaxy in ultrahigh vacuum (UHV). The surface structures of Pt n ML/Ir(111) were verified using reflection high-energy electron diffraction in UHV. Electrochemical characterization tests, including evaluations of oxygen reduction reaction (ORR) activity and durability, were performed using cyclic voltammetry and rotating disk electrode techniques in 0.1 M HClO4. Regarding the initial ORR activity for Pt n ML/Ir(111) surfaces, Pt2ML/Ir(111) surfaces exhibited the highest ORR activity (24 times that of the clean Pt(111) surfaces). Initial ORR activities declined when square-wave potential cycles (PCs) were applied between 0.6 and 1.0 V vs. reversible hydrogen electrode. The Pt4ML/Ir(111) surface exhibited the highest ORR activity even after 5000 PC loadings (6 times that of the clean Pt(111) surfaces), suggesting that Pt-shell/Ir-core core–shell nanoparticles have potential for use as practical Pt-based bimetallic ORR catalysts.

Wafer bowing control of free-standing heteroepitaxial diamond (100) films grown on Ir(100) substrates via patterned nucleation growth

The potential of patterned nucleation growth (PNG) technique to control the wafer bowing of free-standing heteroepitaxial diamond films was investigated. The heteroepitaxial diamond (100) films were grown on an Ir(100) substrate via PNG technique with different patterns of nucleation regions (NRs), which were dot-arrays with 8 or 13μm pitch aligned to <100> or <110> direction of the Ir(100) substrate. The wafer bows and the local stress distributions of the free-standing films were measured using a confocal micro-Raman spectrometer. For each NR pattern, the stress evolutions within the early stage of diamond growth were also studied together with a scanning electron microscopic observation of the coalescing diamond particles. These investigations revealed that the NR pattern, in terms of pitch and direction of dot-array, strongly affects the compressive stress on the nucleation side of the diamond film and dominantly contributes to the elastic deformation of the free-standing film. This indicates that the PNG technique with an appropriate NR pattern is a promising solution to fabricate free-standing heteroepitaxial diamond films with extremely small bows.