Scanning Transmission Electron Microscopy Measurements Research Articles

Challenges to magnetic doping of thin films of the Dirac semimetal Cd3As2

Magnetic doping of topological quantum materials provides an attractive route for studying the effects of time-reversal symmetry breaking. Thus motivated, we explore the introduction of the transition metal Mn into thin films of the Dirac semimetal Cd3As2 during growth by molecular beam epitaxy. Scanning transmission electron microscopy measurements show the formation of a Mn-rich phase at the top surface of Mn-doped Cd3As2 thin films grown using both uniform doping and delta doping. This suggests that Mn acts as a surfactant during epitaxial growth of Cd3As2, resulting in phase separation. Magnetometry measurements of such samples indicate a ferromagnetic phase with out-of-plane magnetic anisotropy. Electrical magneto-transport measurements of these films as a function of temperature, magnetic field, and chemical potential reveal a lower carrier density and higher electron mobility compared to pristine Cd3As2 films grown under similar conditions. This suggests that the surfactant effect might also serve to getter impurities. We observe robust quantum transport (Shubnikov-de Haas oscillations and an incipient integer quantum Hall effect) in very thin (7 nm) Cd3As2 films despite being in direct contact with a structurally disordered surface ferromagnetic overlayer.

Effects of polishing pressure and sliding speed on the material removal mechanism of single crystal diamond in plasma-assisted polishing

Plasma-assisted polishing (PAP) was confirmed to be high-efficiency and high-quality when applied to single crystal diamond (SCD) substrates. The effects of polishing pressure applied to the SCD substrate and sliding speed between the polishing plate and SCD substrate in PAP were systematically investigated in this study. Higher polishing pressures or sliding speeds resulted in a higher material removal rate (MRR) of SCD substrate, and the highest MRR achieved 5.3 μm/h. In the case of PAP conducted at low polishing pressures such as 62.5, 81.3 kPa, scanning white light interferometer (SWLI) (84-μm square) and atomic force microscope (AFM) (5-μm square) measurements revealed an atomic-scale smooth surface without the surface texture depending on the crystal direction and the lowest Sq roughness of 0.3 nm (84-μm square) was achieved. Correspondingly, at low polishing pressure of 62.5 kPa, MRRs in PAP along the <100> and <110> directions were nearly identical, suggesting that isotropic removals occurred. In the case of PAP performed at high polishing pressures such as 143.8, 246.9, and 350.0 kPa, SWLI and AFM measurements revealed a rough surface with bands of grooves along <100> direction. Correspondingly, at high polishing pressures of 246.9 and 350.0 kPa, MRRs in PAP along <100> direction were ~11 and ~12 times faster than that in <110> direction, respectively, suggesting that anisotropic removals occurred. Moreover, scanning transmission electron microscopy and angle-resolved X-ray photoelectron spectroscopy measurements confirmed that no damage or non-diamond layer was present on the PAP-processed SCD substrate. However, as the sliding speed varied, both SWLI and AFM measurements demonstrated that Sq roughness remained nearly constant with the same polishing pressure applied to the SCD substrate.

Superconducting 2D NbS2 Grown Epitaxially by Chemical Vapor Deposition.

Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting great attention because of their interesting low-temperature properties such as superconductivity, magnetism, and charge density waves (CDW). However, further studies and practical applications are being slowed down by difficulties in synthesizing high-quality materials with a large grain size and well-determined thickness. In this work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D NbS2 crystals on a sapphire substrate, with a thickness-dependent structural phase transition. NbS2 crystals are epitaxially aligned by the underlying c-plane sapphire resulting in high-quality growth. The thickness of NbS2 is well controlled by growth parameters to be between 1.5 and 10 nm with a large grain size of up to 500 μm. As the thickness increases, we observe in our NbS2 a transition from a metallic 3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electrical transport measurements. A Berezinskii-Kosterlitz-Thouless (BKT) superconducting transition occurs in the CVD-grown 2H-phase NbS2 below the transition temperature (Tc) of 3 K. Our work demonstrates thickness and phase-controllable synthesis of high-quality superconducting 2D NbS2, which is imperative for its practical applications in next-generation TMDC-based electrical devices.

Grain refinement effect on the Ti-45Nb alloy electrochemical behavior in simulated physiological solution

In this study, the influence of microstructural refinement induced by the high-pressure torsion (HPT) on the corrosion resistance of the Ti-45Nb (mass%) alloy was investigated. The alloy characteristics before and after the HPT deformation were analyzed by electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM), x-ray diffraction (XRD), and Vickers microhardness measurements, while the alloy corrosion behavior in simulated physiological conditions was examined by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) analysis. Detailed microstructural analyses revealed that the HPT deformation led to significant grain refinement of the Ti-45Nb alloy exhibiting an ultra-fine grained (UFG) microstructure along with a substantial increase of hardness. Results also indicated that the grain refinement did not affect the alloy phase composition since β-Ti and Ti4Nb phases were present in the microstructure before and after the HPT deformation. Even though the Ti-45Nb alloy in both, coarse-grained (CG) and UFG, conditions shows high corrosion resistance in Ringer's solution at 37 °C, it was observed that the HPT treatment additionally improved the alloy corrosion properties. Namely, more rapid formation of the passivating layer with better barrier properties on the UFG alloy surface was recorded and resulted in better corrosion resistance of the alloy after HPT deformation. An increase of the grain contact area in the refined microstructure caused an increase of the diffusive transfer along the grain boundaries, accelerated the formation of a less defective protective barrier surface layer, and promoted the alloy surface passivation in the simulated physiological conditions.

Zn-Induced Defect Complexity for the High Thermoelectric Performance of n-Type PbTe Compounds.

Although defect engineering is the core strategy to improve thermoelectric properties, there are limited methods to effectively modulate the designed defects. Herein, we demonstrate that a high ZT value of 1.36 at 775 K and a high average ZT value of 0.99 in the temperature range from 300 to 825 K are realized in Zn-containing PbTe by designing complex defects. By combining first-principles calculations and experiments, we show that Zn atoms occupy both Pb sites and interstitial sites in PbTe and couple with each other. The contraction stress induced via substitutional Zn on Pb sites alleviates the swelling stress by Zn atoms occupying the interstitial sites and promotes the solubility of interstitial Zn atoms in the structure of PbTe. The stabilization of Zn impurity as a complex defect extends the region of PbTe phase stability toward Pb0.995Zn0.02Te, while the solid solution region in the other direction of the ternary phase diagram is much smaller. The evolution of defects in PbTe was further explicitly corroborated by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) and positron annihilation measurement. The Zn atoms compensate the Pb vacancies (VPb) and Zn interstitials (Zni) significantly improve the electron concentration, producing a high carrier mobility of 1467.7 cm2 V-1 s-1 for the Pb0.995Zn0.02Te sample. A high power factor of 4.11 mW m-1 K-2 is achieved for the Pb0.995Zn0.02Te sample at 306 K. This work provides new insights into understanding the nature and evolution of the defects in n-type PbTe as well as improving the electronic and thermal transport properties toward higher thermoelectric performance.

Scattering Matrix Determination in Crystalline Materials from 4D Scanning Transmission Electron Microscopy at a Single Defocus Value

Recent work has revived interest in the scattering matrix formulation of electron scattering in transmission electron microscopy as a stepping stone toward atomic-resolution structure determination in the presence of multiple scattering. We discuss ways of visualizing the scattering matrix that make its properties clear. Through a simulation-based case study incorporating shot noise, we shown how regularizing on this continuity enables the scattering matrix to be reconstructed from 4D scanning transmission electron microscopy (STEM) measurements from a single defocus value. Intriguingly, for crystalline samples, this process also yields the sample thickness to nanometer accuracy with no a priori knowledge about the sample structure. The reconstruction quality is gauged by using the reconstructed scattering matrix to simulate STEM images at defocus values different from that of the data from which it was reconstructed.

Searching novel complex solid solution electrocatalysts in unconventional element combinations

Despite outstanding accomplishments in catalyst discovery, finding new, more efficient, environmentally neutral, and noble metal-free catalysts remains challenging and unsolved. Recently, complex solid solutions consisting of at least five different elements and often named as high-entropy alloys have emerged as a new class of electrocatalysts for a variety of reactions. The multicomponent combinations of elements facilitate tuning of active sites and catalytic properties. Predicting optimal catalyst composition remains difficult, making testing of a very high number of them indispensable. We present the high-throughput screening of the electrochemical activity of thin film material libraries prepared by combinatorial co-sputtering of metals which are commonly used in catalysis (Pd, Cu, Ni) combined with metals which are not commonly used in catalysis (Ti, Hf, Zr). Introducing unusual elements in the search space allows discovery of catalytic activity for hitherto unknown compositions. Material libraries with very similar composition spreads can show different activities vs. composition trends for different reactions. In order to address the inherent challenge of the huge combinatorial material space and the inability to predict active electrocatalyst compositions, we developed a high-throughput process based on co-sputtered material libraries, and performed high-throughput characterization using energy dispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (SEM), X-ray diffraction (XRD) and conductivity measurements followed by electrochemical screening by means of a scanning droplet cell. The results show surprising material compositions with increased activity for the oxygen reduction reaction and the hydrogen evolution reaction. Such data are important input data for future data-driven materials prediction.

Demonstration of epitaxial growth of strain-relaxed GaN films on graphene/SiC substrates for long wavelength light-emitting diodes

Strain modulation is crucial for heteroepitaxy such as GaN on foreign substrates. Here, the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by metal-organic chemical vapor deposition is demonstrated. Graphene was directly prepared on SiC substrates by thermal decomposition. Its pre-treatment with nitrogen-plasma can introduce C–N dangling bonds, which provides nucleation sites for subsequent epitaxial growth. The scanning transmission electron microscopy measurements confirm that part of graphene surface was etched by nitrogen-plasma. We study the growth behavior on different areas of graphene surface after pre-treatment, and propose a growth model to explain the epitaxial growth mechanism of GaN films on graphene. Significantly, graphene is found to be effective to reduce the biaxial stress in GaN films and the strain relaxation improves indium-atom incorporation in InGaN/GaN multiple quantum wells (MQWs) active region, which results in the obvious red-shift of light-emitting wavelength of InGaN/GaN MQWs. This work opens up a new way for the fabrication of GaN-based long wavelength light-emitting diodes.

Absence of ferromagnetism in MnBi2Te4/Bi2Te3 down to 6 K

We successfully fabricated a ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}/{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ heterostructure by incorporating Mn and Te inside the topmost quintuple layer of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, as unambiguously confirmed by low-energy electron diffraction $I\text{\ensuremath{-}}V$ and scanning transmission electron microscopy measurements. The surface-state Dirac cone of the heterostructure showed little change compared to that of pristine ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, and x-ray magnetic circular dichroism measurements showed that the system was paramagnetic down to 5.6 K. These results are in contrast to previous works on related materials that showed magnetic order around 10 K as well as theoretical predictions and suggest the intricacy of the magnetic properties of two-dimensional van der Waals magnets.

Pt3Co@Pt Core@shell Nanoparticles as Efficient Oxygen Reduction Electrocatalysts in Direct Methanol Fuel Cell

AbstractCore‐shell structured bimetallic platinum‐metal (Pt−M) nanoparticles, as a new class of active and stable nanocatalysts, have shown many advantages in increasing the utilization of precious Pt and improving electrocatalytic performances. Here, a core‐shell Pt3Co@Pt supported on porous graphitic carbon (denoted as Pt3Co@Pt/C) is synthesized via a simple thermal method, and further used as an efficient electrocatalyst for oxygen reduction reaction (ORR) in the direct methanol fuel cell. An atomic‐resolution high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) measurement combining with line‐profile analysis reveals that the average thickness of Pt‐shell is around 0.4–0.6 nm, forming an ultrathin catalytic layer. Given its unique geometric and electronic structure, the as‐prepared Pt3Co@Pt/C displays highly enhanced electrocatalytic ORR activity and stability, boosted anti‐methanol poisoning ability with a high onset potential and an exceptional half‐wave potential in 0.1 M HClO4 solution. Impressively, its mass activity (0.71 mA mgPt−1) and specific activity (2.75 mA cmPt−2) for ORR are 3.7‐ and 8.1‐fold higher than those of commercial Pt/C catalyst, respectively. The Pt3Co@Pt/C nanocatalysts show remarkable tolerance against methanol poisoning, evidenced by the in situ Fourier‐transform infrared (FTIR) spectroscopy. This work points out a path for the design of high‐performance nanocatalysts for accelerating the development of clean energy technologies involving ORR.

Formation Mechanism of Rounded SiGe-Etch Front in Isotropic SiGe Plasma Etching for Gate-All-Around FETs

We investigated the formation mechanism of a rounded silicon-germanium (SiGe)-etch front (rounding) in gate-all-around field-effect transistor (GAA-FET) manufacturing. This rounding is created by the isotropic etching of the SiGe layer after anisotropic etching of the SiGe/Si stack, which degrades device characteristics. The etch-time dependence of the rounding amount during isotropic SiGe etching with nitrogen trifluoride plasma indicates that rounding is mainly formed in an initial stage of SiGe etching, namely, etch time less than 15 s. Cross-sectional scanning transmission electron microscopy and energy dispersive x-ray spectroscopy (STEM EDX) measurement indicated that a Ge-containing layer formed on the sidewall of SiGe/Si patterns before isotropic SiGe etching. From these results, we propose a formation model of SiGe rounding below. The Ge composition in the Ge-containing layer has a gradient due to ion-assisted Ge diffusion during anisotropic etching of the SiGe/Si stack. This gradient induces rounding during isotropic SiGe etching because the etch rate of the SiGe layer decreases as the Ge composition decreases. To validate our model, the Ge-containing layer after anisotropic etching was removed by post-etch treatment and the Ge spectra on the sidewall was reduced to the detection limit of STEM EDX. As a result, the rounding amount after isotropic SiGe etching improved from 2.7 to 1.8 nm. This reduction indicates that the formation of the Ge-containing layer during anisotropic etching of the SiGe/Si stack is one of the main causes of rounding after isotropic SiGe etching.

Reverse Water–Gas Shift Chemical Looping Using a Core–Shell Structured Perovskite Oxygen Carrier

Reverse water–gas shift chemical looping (RWGS-CL) offers a promising means of converting the greenhouse gas of CO2 to CO because of its relatively low operating temperatures and high CO selectivity without any side product. This paper introduces a core–shell structured oxygen carrier for RWGS-CL. The prepared oxygen carrier consists of a metal oxide core and perovskite shell, which was confirmed by inductively coupled plasma mass spectroscopy (ICP-MS), XPS, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements. The perovskite-structured shell of the prepared oxygen carrier facilitates the formation and consumption of oxygen defects in the metal oxide core during H2-CO2 redox looping cycles. As a result, amounts of CO produced per unit weight of the core–shell structured oxygen carriers were higher than that of a simple perovskite oxygen carrier. Of the metal oxide cores tested, CeO2, NiO, Co3O4, and Co3O4-NiO, La0.75Sr0.25FeO3-encapsulated Co3O4-NiO was found to be the most promising oxygen carrier for RWGS-CL, because it was most productive in terms of CO production and exhibited long-term stability.

Improved Efficiency of Ultraviolet B Light‐Emitting Diodes with Optimized p‐Side

The effects of design and thicknesses of different optically transparent p‐current spreading layers [short‐period superlattice, superlattice (SL), and bulk p‐] as well as the type and thickness of the p‐GaN cap layer on the electrical and optical characteristics of 310 nm ultraviolet light‐emitting diodes (LEDs) are investigated. Scanning transmission electron microscopy measurements display self‐organized composition variations in the nonpseudomorphically grown SLs, reducing the effect of increased hole injection efficiency of a SL. In addition, the effect leads to an increased operation voltage. In contrast, the bulk p‐AlGaN layer has a uniform composition and the corresponding LEDs show only a slightly lower output power along with a lower operating voltage. If the thickness of the p‐AlGaN bulk layer in the LED is reduced from 150 nm to 50 nm, the output power increases and the operating voltage decreases. Finally, LEDs with a nonuniform ‐GaN cap layer from a 3D island‐like growth mode feature the highest output power and operating voltage. In contrast, the output power and operating voltage of LEDs with a smooth and closed cap depend on the thickness of ‐GaN. The highest output power and lowest operating voltage are achieved for LEDs with the thinnest ‐GaN cap.

Combined κ-carbide precipitation and recovery enables ultra-high strength and ductility in light-weight steels

Light-weight high-manganese steels with high aluminum and carbon contents are known to possess high age-hardening potential due to the precipitation of nano-sized κ-carbides. Although in recrystallized material the precipitation of the κ-phase has been widely researched, the influence of pre-deformation/deformation microstructure on the formation of κ-carbides is still unknown. The present study aims at gaining an increased understanding of the underlying mechanisms (and their interplay) active during annealing after pre-deformation, i.e. concurrent strengthening by precipitation of κ-carbides and softening by recovery/recrystallization. Therefore, a Fe-29.8Mn-7.65Al-1.11C steel was age-hardened in deformed/cold-rolled as well as in recrystallized condition. The microstructure and tensile behavior of the material in both states after annealing treatments at 600–800 °C were investigated by means of light optical microscopy, (high-resolution) scanning transmission electron microscopy, x-ray diffraction and hardness measurements as well as tensile tests. The focus was put on understanding the interplay between microstructural defects (e.g. dislocations), precipitation of κ-carbides, and recovery/recrystallization. The fundamental dependence of strengthening and softening mechanisms on the precondition, annealing parameters and correlation with the mechanical properties are discussed.

Stacking fault-associated polarized surface-emitted photoluminescence from zincblende InGaN/GaN quantum wells

Zincblende InGaN/GaN quantum wells offer a potential improvement to the efficiency of green light emission by removing the strong electric fields present in similar structures. However, a high density of stacking faults may have an impact on the recombination in these systems. In this work, scanning transmission electron microscopy and energy-dispersive x-ray measurements demonstrate that one-dimensional nanostructures form due to indium segregation adjacent to stacking faults. In photoluminescence experiments, these structures emit visible light, which is optically polarized up to 86% at 10 K and up to 75% at room temperature. The emission redshifts and broadens as the well width increases from 2 nm to 8 nm. Photoluminescence excitation measurements indicate that carriers are captured by these structures from the rest of the quantum wells and recombine to emit light polarized along the length of these nanostructures.

Synthesis of Three-Layer Perovskite Oxynitride K2Ca2Ta3O9N·2H2O and Photocatalytic Activity for H2 Evolution under Visible Light.

Substitution of oxide anions (O2-) in a metal oxide for nitrogen (N3-) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden-Popper phase three-layer oxynitride perovskite of K2Ca2Ta3O9N·2H2O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K2LaTa2O6N·1.6H2O. Topochemical ammonolysis reaction of a Dion-Jacobson phase oxide KCa2Ta3O10 at 1173 K in the presence of K2CO3 resulted in a single-phase layered perovskite, K2Ca2Ta3O9N·2H2O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K2Ca2Ta3O9N·2H2O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K2Ca2Ta3O9N·2H2O modified with a Pt cocatalyst generated H2 from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H2 evolution activity of K2Ca2Ta3O9N·2H2O was an order of magnitude lower than that of K2LaTa2O6N·1.6H2O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K2Ca2Ta3O9N·2H2O was shorter than that in K2LaTa2O6N·1.6H2O, which could explain the low photocatalytic activity of K2Ca2Ta3O9N·2H2O.

On-chip selective growth of SnO2 nanowires for DNA sensor development

Tin oxides nanowires were selectively grown on the working electrode of a sensor by using modified chemical vapor deposition technique with supporting shadow mask. Time and distance evolution of Sn mass transport in correlation with carrier gas pressure and flow rates in the chemical vapor deposition system was described by Comsol Multiphysics. The simulated density of SnO2 nanowires was in good agreement with that obtained by scanning electron microscope image. X-ray diffraction, high-resolution transmission electron microscopy, scanning transmission electron microscopy measurements showed that nanowire structure was tetragonal with lattice constants a = 0.474 nm and c = 0.318 nm. The nanowires were around 25 nm in diameter and 3.5 μm in length. A simple immobilization of a DNA sensor was conducted by a surface modification of tin oxide nanowire using 3-amino-propyl-tri-ethoxysilance hydroxylation in vapor phase; creating the amino functional groups to which the phosphate groups of probe ss-DNA (Epstein-Barr virus) bound. Differential voltage measurements were conducted to detect DNA hybridization. The response time was 2.5 s and the limit of detection was 3.2 pM.

Structural and photoelectric properties of tensile strainedBiFeO3

An in-depth structural study of a 23-nm-thick ${\mathrm{BiFeO}}_{3}$ film grown on orthorhombic ${\mathrm{NdScO}}_{3}\phantom{\rule{4pt}{0ex}}{\left(110\right)}_{O}$ substrates demonstrates the presence of a mixed phases. Atomic resolution scanning transmission electron microscopy measurements reveal an out-of-plane stripe domain structure typical of rhombohedral ${\mathrm{BiFeO}}_{3}$ films but with a polarization component along pseudocubic ${\ensuremath{\langle}100\ensuremath{\rangle}}_{PC}$ or canted from the ${\ensuremath{\langle}111\ensuremath{\rangle}}_{PC}$ towards the in-plane direction. Photovoltaic measurements display an anomalous modulation of the open circuit voltage as temperature is decreased that is attributed to a structural change associated with a transition to a single structural phase.

Transformation and Evaporation of Surface Adsorbents on a Graphene "Hot Plate".

Dynamic surface modification of suspended graphene at high temperatures was directly observed with in situ scanning transmission electron microscopy (STEM) measurements. The suspended graphene devices were prepared on a SiN membrane substrate with a hole so that STEM observations could be conducted during Joule heating. Current–voltage characteristics of suspended graphene devices inside the STEM chamber were measured while monitoring and controlling the temperature of graphene by estimating the electrical power of the devices. During the in situ STEM observation at high temperatures, residual hydrocarbon adsorbents that had remained on graphene effectively evaporated creating large, atomically clean graphene areas. At other places, dynamic changes in the shape, position, and orientation of adsorbents could be directly observed. The temperature of the suspended graphene sample was estimated to reach up to 2000 K during the experiment, making graphene an efficient high-temperature micrometer-sized electron-transparent hot plate for future experiments in microscopes.

Adsorption Site Regulation to Guide Atomic Design of Ni-Ga Catalysts for Acetylene Semi-Hydrogenation.

Atomic regulation of metal catalysts has emerged as an intriguing yet challenging strategy to boost product selectivity. Here, we report a density functional theory-guided atomic design strategy for the fabrication of a NiGa intermetallic catalyst with completely isolated Ni sites to optimize acetylene semi-hydrogenation processes. Such Ni sites show not only preferential acetylene π-adsorption, but also enhanced ethylene desorption. The characteristics of the Ni sites are confirmed by multiple characterization techniques, including aberration-corrected high-resolution scanning transmission electron microscopy and X-ray absorption spectrometry measurements. The superior performance is also confirmed experimentally against a Ni5 Ga3 intermetallic catalyst with partially isolated Ni sites and against a Ni catalyst with multi-atomic ensemble Ni sites. Accordingly, the NiGa intermetallic catalyst with the completely isolated Ni sites shows significantly enhanced selectivity to ethylene and suppressed coke formation.