Scanning Transmission Electron Microscopy Characterization Research Articles

Atomic layer deposited Al2O3 as a protective overlayer for focused ion beam preparation of plan-view STEM samples

Preservation of analyte integrity during focused ion beam (FIB) sample preparation is a significant challenge in the scanning transmission electron microscopy (STEM) characterization of plan-view samples with sensitive surface chemistries. This can preclude the characterization of atomic arrangements, nanoscale surface coverages, and distributions and morphologies of functional molecular materials composed of surface-immobilized metal nanoparticles, clusters or coordination complexes. This work demonstrates effective protection of Pt nanoparticle (NP) morphology through a plan-view FIB lift-out and thinning procedure by encapsulating the sample surface in an Al2O3 overlayer grown by atomic layer deposition (ALD). High-angle annular dark field (HAADF)-STEM analysis was used in concert with energy dispersive X-ray spectroscopy (EDS) to identify and image sub-10 nm features attributed to Pt and to evaluate the distribution of implanted Ga+ (derived from the FIB milling beam). ALD is a mild chemical vapor deposition (CVD) technique that has the capability to generate dense, pinhole-free films with tunable compositions and properties, making this ALD-FIB procedure applicable to many sample architectures for plan-view lamella preparation and STEM analysis.

Atomic origin of magnetic coupling of antiphase boundaries in magnetite thin films

Revealing the magnetic coupling nature of boundary defects is crucial for in-depth understanding of the behavior and properties of magnetic materials and devices. Here, magnetite (i.e., Fe3O4) thin films were grown epitaxially on (100) SrTiO3 single-crystal substrates by pulsed laser deposition. Atomic-scale scanning transmission electron microscopy characterizations reveal that three types of antiphase boundaries (APBs) are formed in the Fe3O4 thin film. They are the (100) APB that is formed on the (100) plane with a crystal translation of (1/4)a[011¯], the type I and type II (110) APBs that are both formed on the (110) plane with the same crystal translation of (1/4)a[101] but different terminated atomic planes. The type I (110) APB is terminated at the atomic plane with mixed tetrahedral- and octahedral-sites Fe atoms, the type II (110) APB is terminated at the octahedral-site Fe plane. First-principles calculations reveal that the (100) APB and the type I (110) APB tend to form the ferromagnetic coupling that will not decrease the spin polarization of Fe3O4 films, while the type II (110) APB prefers to form the antiferromagnetic coupling that will degrade the magnetic properties. The magnetic coupling modes of the APBs are closely related to the Fe-O-Fe bond angles across the boundaries.

Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors.

Tuning the charge transport properties of two-dimensional transition metal dichalcogenides (TMDs) is pivotal to their future device integration in post-silicon technologies. To date, co-doping of TMDs during growth still proves to be challenging, and the synthesis of doped WSe2, an otherwise ambipolar material, has been mainly limited to p-doping. Here, we demonstrate the synthesis of high-quality n-type monolayered WSe2 flakes using a solid-state precursor for Se, zinc selenide. n-Type transport has been reported with prime electron mobilities of up to 10 cm2 V-1 s-1. We also demonstrate the tuneability of doping to p-type transport with hole mobilities of 50 cm2 V-1 s-1 after annealing in air. n-Doping has been attributed to the presence of Zn adatoms on the WSe2 flakes as revealed by X-ray photoelectron spectroscopy (XPS), spatially resolved time of flight secondary ion mass spectroscopy (SIMS) and angular dark-field scanning transmission electron microscopy (AD-STEM) characterization of WSe2 flakes. Monolayer WSe2 flakes exhibit a sharp photoluminescence (PL) peak at room temperature and highly uniform emission across the entire flake area, indicating a high degree of crystallinity of the material. This work provides new insight into the synthesis of TMDs with charge carrier control, to pave the way towards post-silicon electronics.

Combined effect of Sn addition and pre-ageing on natural secondary and artificial ageing of Al\u2013Mg\u2013Si alloys

Both Sn addition and pre-ageing are known to be effective in maintaining the artificial ageing potential after natural ageing of Al–Mg–Si alloys. In this study, the combined effects of Sn addition and pre-ageing at 100 °C or 180 °C on natural secondary ageing and subsequent artificial ageing of an alloy AA6014 were investigated using hardness, electrical resistivity, differential scanning calorimetry and transmission electron microscopy characterizations. It is found that pre-ageing can suppress natural secondary ageing and improve the artificial ageing hardening kinetics and response after 1 week of natural secondary ageing in both alloys with and without Sn addition. The effect of pre-ageing at 100 °C is more pronounced in the Sn-free alloy while the combination of pre-ageing at 180 °C and adding Sn shows superiority in suppressing natural secondary ageing and thus avoiding the undesired hardening before artificial ageing. Moreover, when natural ageing steps up to 8 h are applied before pre-ageing at 100 °C, the effect of pre-ageing in Sn-added alloy can be further improved. The influence of Sn on vacancies at different ageing temperatures is discussed to explain the observed phenomena.Graphical abstract

Cu/Ni-NiOx Nanoparticles Distributed on Graphene as Catalysts for the Methanolysis of Ammonia Borane to Produce Hydrogen

Ultrasmall Cu/Ni-NiOx nanoparticles (NPs) had been successfully prepared by attaching and growing Ni-NiOx NPs on the surface of Cu NPs and then uniformly dispersed onto graphene (G) surface. High-angle annular dark-field–scanning transmission electron microscopy (HAADF-STEM) characterization confirmed that Ni-NiOx NPs were uniformly distributed on the surface of Cu NPs. The catalytic methanolysis of ammonia borane (AB) proved that Cu/Ni-NiOx NPs possessed high catalytic hydrogen (H2) evolution activity, and the highest turnover frequency (TOF) was 17.72 mol H2·mol–1 Cu·min–1. Comparative experiments revealed that the high catalytic activity originated from the abundant catalytic active sites of Cu/Ni-NiOx NPs. The attachment of Ni-NiOx on the surface of Cu NPs that dispersed on G (G-Cu/Ni-NiOx) can not only effectively improve the antioxidative performance of Cu NPs under ambient conditions but also promote the methanolysis of AB to produce H2.

Scission of C–O and C–C linkages in lignin over RuRe alloy catalyst

The performance of lignin depolymerization is basically determined by the interunit C–O and C–C bonds. Numerous C–O bond cleavage strategies have been developed, while the cleavage of C–C bond between the primary aromatic units remains a challenging task due to the high dissociation energy of C–C bond. Herein, a multifunctional RuRe alloy catalyst was designed, which exhibited exceptional catalytic activity for the cleavage of both C–O and C–C linkages in a broad range of lignin model compounds (β-1, α-5, 5–5, β-O-4, 4-O-5) and two stubborn lignins (kraft lignin and alkaline lignin), affording 97.5% overall yield of monocyclic compounds from model compounds and up to 129% of the maximum theoretical yield of monocyclic products based on C–O bonds cleavage from realistic lignin. Scanning transmission electron microscopy (STEM) characterization showed that RuRe (1:1) alloy particles with hexagonal close-packed structure were homogeneously dispersed on the support. Quasi-in situ X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) indicate that Ru species were predominantly metallic state, whereas Re species were partially oxidized; meanwhile, there was a strong interaction between Ru and Re, where the electron transfer from Re to Ru was occurred, resulting in great improvement on the capability of C–O and C–C bonds cleavage in lignin conversion.

Morphology Control and Crystalline Quality of p-Type GaN Shells Grown on Coaxial GaInN/GaN Multiple Quantum Shell Nanowires.

The morphology and crystalline quality of p-GaN shells on coaxial GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated using metal-organic chemical vapor deposition. By varying the trimethylgallium (TMG) flow rate, Mg doping, and growth temperature, it was verified that the TMG supply and growth temperature were the dominant parameters in the control of the p-GaN shape on NWs. Specifically, a sufficiently high TMG supply enabled the formation of a pyramid-shaped NW structure with a uniform p-GaN shell. The ratio of the growth rate between the c- and m-planes on the NWs was calculated to be approximately 0.4545. High-angle annular dark-field scanning transmission electron microscopy characterization confirmed that no clear extended defects were present in the n-GaN core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell above the c-plane MQS region, only a few screw dislocations and Frank-type partial dislocations appeared at the interface between the serpentine c-plane MQS and the p-GaN shell near the tips. This suggested that the crystalline quality of the MQS structure can trigger the formation of screw dislocations and Frank-type partial dislocations during the p-GaN growth. The growth mechanism of the p-GaN shell on NWs was also discussed. To inspect the electronic properties, a prototype of a micro light-emitting diode (LED) with a chip size of 50 × 50 μm2 was demonstrated in the NWs with optimal growth. By correlating the light output curve with the electroluminescence spectra, three different emission peaks (450, 470, and 510 nm) were assignable to the emission from the m-, r-, and c-planes, respectively.

Preparation and MRI performances of core-shell structural PEG salicylic acid-gadolinium composite nanoparticles

Gadolinium (III)-based T1 contrast agents have been widely used in clinical MRI. In this study, salicylic acid-gadolinium chelate was prepared via directly coordination reaction of gadolinium ion and salicylic acid. Then, three polyethylene glycols with different molecular weight were modified on the surface of salicylic acid-gadolinium by simple chemical coupling to construct three core-shell structural Gd based composites. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization results show that the composite is a spherical particle with a diameter of about 100–200 nm. The longitudinal relaxation rate r1 of the Gal-PEG-2000 is 11.097 (mmol/L)−1/s, and the ratio of r2/r1 is as low as 2.53. The composite shows good liver and intestines MRI performances after being used in in vivo imaging, showing a good prospect of biological application.

Enhanced high-temperature age-hardening behavior and mechanical properties of Al–Mg–Si alloys via microalloying with Cd

The effects of trace (0.05 at.%) Cd addition on the high-temperature (250 °C) age-hardening behavior, precipitate microstructure and room-temperature tensile properties of an Al-0.8Mg-0.8Si (wt.%) alloy were investigated. The results showed that the Cd addition not only accelerated the precipitation kinetics but also increased the peak hardness of Al–Mg–Si alloys. The quantitative analysis of the peak-aged precipitates based on (high-resolution) transmission electron microscopy images indicated that the Cd addition refined the size of needle-shaped precipitates and increased their number density. High-angle annular dark-field scanning transmission electron microscopy characterizations revealed that the Cd addition stimulated the heterogeneous nucleation of β" on the early formed nanosized Cd-rich precipitates and Cd-containing QP lattices, resulting in a much-refined precipitate microstructure. Furthermore, the Cd addition also enhanced the room-temperature tensile properties of the peak-aged alloy. The Cd-added alloy exhibited higher yield strength, ultimate tensile strength, and total elongation than the Cd-free alloy, which was due to the refined precipitate microstructure and narrowed precipitate-free zone along the grain boundaries.

Crystal Growth and Characterization of n-GaN in a Multiple Quantum Shell Nanowire-Based Light Emitter with a Tunnel Junction.

Here, we systematically investigated the growth conditions of an n-GaN cap layer for nanowire-based light emitters with a tunnel junction. Selective-area growth of multiple quantum shell (MQS)/nanowire core-shell structures on a patterned n-GaN/sapphire substrate was performed by metal-organic vapor phase epitaxy, followed by the growth of a p-GaN, an n++/ p++-GaN tunnel junction, and an n-GaN cap layer. Specifically, two-step growth of the n-GaN cap layer was carried out under various growth conditions to determine the optimal conditions for a flat n-GaN cap layer. Scanning transmission electron microscopy characterization revealed that n++-GaN can be uniformly grown on the m-plane sidewall of MQS nanowires. A clear tunnel junction, involving 10-nm-thick p++-GaN and 3-nm-thick n++-GaN, was confirmed on the nonpolar m-planes of the nanowires. The Mg doping concentration and distribution profile of the p++-GaN shell were inspected using three-dimensional atom probe tomography. Afterward, the reconstructed isoconcentration mapping was applied to identify Mg-rich clusters. The density and average size of the Mg clusters were estimated to be approximately 4.3 × 1017 cm-3 and 5 nm, respectively. Excluding the Mg atoms contained in the clusters, the remaining Mg doping concentration in the p++-GaN region was calculated to be 1.1 × 1020 cm-3. Despite the lack of effective activation, a reasonably low operating voltage and distinct light emissions were preliminarily observed in MQS nanowire-based LEDs under the optimal n-GaN cap growth conditions. In the fabricated MQS-nanowire devices, carriers were injected into both the r-plane and m-plane of the nanowires without a clear quantum confinement Stark effect.

The interfacial structure underpinning the Al-Ga liquid metal embrittlement: disorder vs. order gradients

Embrittlement of polycrystalline aluminum by liquid gallium represents a classic example of liquid metal embrittlement, whereas the interfacial atomic structure of Ga penetration front remains elusive despite decades of investigation. We performed aberration-corrected scanning transmission electron microscopy characterization to uncover the atomistic structure of Ga-enriched complexion within Al general grain boundaries. Multiple-layer adsorption of Ga with ordering gradients is revealed, consisting of disordered Ga layers at the GB core that are sandwiched by two more ordered adsorbate layers connecting to both aluminum grains. The ordering gradients within the interfacial complexion are further verified by the hybrid Monte Carlo and molecular dynamic simulations. This study advances our fundamental understandings of complexion structures within general grain boundaries.

Influence of NOx on the activity of Pd/θ-Al2O3 catalyst for methane oxidation: Alleviation of transient deactivation

Alumina supported Pd catalyst (Pd/Al2O3) is active for complete oxidation of methane, while often suffers transient deactivation during the cold down process. Herein, heating and cooling cycle tests between 200 and 900°C and isothermal experiments at 650°C were conducted to investigate the influence of NOx on transient deactivation of Pd/θ-Al2O3 catalyst during the methane oxidation. It was found that the co-fed of NO alleviated transient deactivation in the cooling ramp from 800 to 500°C, which was resulted from the in situ formation of NO2 during the process of methane oxidation. Over the Pd/θ-Al2O3, thermogravimetric analysis and O2 temperature programmed oxidation measurements confirmed that transient deactivation was due to the decomposition of PdO particles and the hysteresis of Pd reoxidation, while the metal Pd entities were less active for methane oxidation than the PdO ones. CO pulse chemisorption and scanning transmission electron microscopy characterizations rule out the NO2 effect on Pd size change. Powder X-ray diffraction and X-ray photoelectron spectroscopy characterizations were used to obtain palladium status of Pd/θ-Al2O3 before and after reactions, indicating that in lean conditions at 650°C, the presence of NO2 increases the content of active PdO on the catalyst surface, thus benefits methane oxidation. Homogeneous reaction between CH4, O2, and NOx may be partially responsible for the alleviation above 650°C. The interesting research of alleviation in transient deactivation by NOx, the components co-existing in exhausts, are of great significance for the application.

DFT and HAADF-STEM Investigations of the Zn Effects on β″ Phase Structure in a Zn Added Al-Mg-Si-Cu Alloy

First-principles calculations were conducted to investigate the effects of Zn on the structure of β″ phase. The effects of Cu, which was often added in the alloy, were also taken into consideration. Firstly, single Zn or Cu atom was doped on different sites of the β″ phase. Then the formation enthalpies and lattice constants of doped β″ phases were calculated. The results showed that it was more energetically favorable for single Zn or Cu atom to occupy Si3/Al sites than other sites. Furthermore, different quantities of Zn or Cu atoms were doped on Si3/Al sites. With the amounts of doping atoms increasing, the formation enthalpies of β″ phases doped by Zn were lower than which doped by Cu, indicating that it was more preferential for Zn to enter the β″ phase when Zn content was higher than Cu. Additionally, the doping of Zn could reduce the formation enthalpies of the β″ phase, which promoted the formation of the β″ phases. As a result, the aging hardening response of the alloy was improved. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization was also conducted on a peak-aging Zn added Al-Mg-Si-Cu alloy. The HAADF-STEM image of β″ phase showed that the occupancies of Zn atoms were just on the Si3/Al sites and substituted all the Al atoms, which was consistent with the results of first-principles calculations.

Direct Laser Patterning of a 2D WSe2 Logic Circuit

AbstractCarrier doping is the basis of the modern semiconductor industry. Great efforts are put into the control of carrier doping for 2D semiconductors, especially the layered transition metal dichalcogenides. Here, the direct laser patterning of WSe2 devices via light‐induced hole doping is systematically studied. By changing the laser power, scan speed, and the number of irradiation times, different levels of hole doping can be achieved in the pristine electron‐transport‐dominated WSe2, without obvious sample thinning. Scanning transmission electron microscopy characterization reveals that the oxidation of the laser‐radiated WSe2 is the origin of the carrier doping. Photocurrent mapping shows that after the same amount of laser irradiation, with increasing thickness, the laser patterned PN junction changes from the pure lateral to the vertical‐lateral hybrid structure, accompanied by the decrease in the open circuit voltage. The vertical‐lateral hybrid PN junction can be tuned to a pure lateral one by further irradiation, showing possibilities to construct complex junction profiles. Moreover, a NOR gate circuit is demonstrated by direct patterning of p‐doped channels using laser irradiation without introducing passive layers and metal electrodes with different work functions. This method simplifies device fabrication procedures and shows a promising future in large scale logic circuit applications.

Preservation stability of chemically synthesized graphite oxide slurry and reduced graphene oxide powder

Reduced graphene oxide (RGO) powder and graphite oxide slurry were stored under natural conditions for different times, and then they were characterized in the RGO powder state. The influences of preservation time on the physical and chemical performances of RGO were investigated by different analysis methods. The results indicate that they are relatively stable in X-ray diffraction and Raman activity, whereas changes are detectable in their scanning electron microscopy and transmission electron microscopy characterizations. Importantly, with the increase of preservation time, the specific capacitances at a current density of 1 A/g maintain around 150 F/g for those two groups of RGO samples, indicating that the electrochemical performance of RGO powder fabricated by chemical route is fairly stable. In contrast, there is an obvious decrease in electrical conductivity, and after being stored for 180 days, the electrical conductivities remain only 16.4% and 15.1% of their initial values, respectively. These test results can not only arouse some new research ideas about improving the stability of chemically derived RGO powder and graphite oxide slurry, but also provide important references to their practical applications for industrial production.

Ferroelectricity of as-deposited HZO fabricated by plasma-enhanced atomic layer deposition at 300 °C by inserting TiO2 interlayers

We report the observation of ferroelectricity in hafnium-zirconium-oxide thin films in the as-deposited state, namely, after deposition at a low temperature of 300 °C without post-metallization annealing. The Hf0.5Zr0.5O2 (HZO) thin film was interposed between two TiO2 interlayers, and all films were produced by plasma enhanced atomic layer deposition and integrated into a TiN-based metal-insulator-metal capacitor. The ferroelectric nature of the as-deposited HZO film was evaluated by a polarization-voltage hysteresis loop, and a 2Pr value of ∼7.4 μC/cm2 was achieved. Grazing incidence x-ray diffraction measurements and atomic-resolution scanning transmission electron microscopy characterization revealed the co-existence of fully crystallized polar orthorhombic and monoclinic phases of the dielectric in the as-deposited sample. We concluded that the nucleation and growth of the crystalline polar non-centrosymmetric orthorhombic phases in the 10 nm HZO thin film were prompted by the available energy from the plasma and the tensile lattice mismatch strain provided by the TiO2 interlayer.

Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysis

Summary Synthesis of liquid fuel (C5+ hydrocarbons) via CO2 hydrogenation generally involves cascade catalysis of reverse water gas shift (RWGS) and Fischer-Tropsch synthesis (FTS) reactions over heterogeneous catalysts. Because of thermodynamic limitations of the cascade reactions, the reported heterogeneous catalysts usually suffer from high temperature (above 300°C) and low selectivity. Here, we report the liquid fuel production from CO2 hydrogenation by coupling homogeneous RuCl3 and heterogeneous Ru0 catalysts, which proceeded at the lowest temperature reported so far (180°C) and reached the highest C5+ selectivity to date (71.1%). The TOF of the CO2 hydrogenation reached 9.5 h−1, which is comparable with that of the reported Ru0 catalyzed FTS reaction using syngas. Detailed study indicates that synergy of homogeneous and heterogeneous catalysis is the key for the outstanding performance.

Wafer-Scale Growth of Pristine and Doped Monolayer MoS2 Films for Electronic Device Applications

Scalable production and controlled doping of large-area two-dimensional transition-metal dichalcogenide films are fundamental steps toward their applications in electronic devices. Although a variety of methods for preparation of wafer-scale transition-metal dichalcogenide films have been developed, it is still challenging to realize homogeneous doping of the large-area films to modulate their electronic properties. In this paper, we report a new chemical vapor deposition (CVD) method for preparation of wafer-scale pristine and doped monolayer MoS2 films on 2-inch sapphire wafers. The molybdenum precursors are supplied in a "face-to-face" manner from a silica gel plate to the sapphire wafer, which guarantees uniform nucleation and growth of monolayer MoS2. This method can be used to prepare substitutionally doped monolayer MoS2 films. By using ReCl3 as the dopant precursor, we have obtained continuous Re-doped monolayer MoS2 films on sapphire wafers. Elemental analysis confirms successful Re-doping of the MoS2 film. Spherical aberration-corrected scanning transmission electron microscopy characterization reveals that the Re atoms are incorporated at the substitutional Mo sites in the MoS2 lattice. The incorporation of Re atoms leads to n-type doping of MoS2 as evidenced by Kelvin probe force microscope studies. Electrical measurements reveal that the transport properties of the Re-doped monolayer MoS2 is dramatically enhanced as compared with the pristine MoS2. The CVD method developed in this study can be applied to the production of a variety of two-dimensional transition-metal dichalcogenide films suitable for applications in electronic devices.

Anomalous carrier dynamics and localization effects in nonpolar m-plane InGaN/GaN quantum wells at high temperatures

Nonpolar InGaN/GaN quantum wells (QWs) have gained significance for efficient light emitting since it eliminates polarization-related effects and allows for higher radiative capability. Although much progress has been made in analyzing carrier localization effects at cryogenic temperatures, carrier dynamics at above room temperatures are still not well understood. In this work, we have observed and explored anomalous carrier dynamics of two nonpolar m-plane InGaN/GaN QWs at high temperatures by combining scanning transmission electron microscopy (STEM) and photophysical characterization. Both experimental and theoretical results suggest that carrier lifetime in both samples increases with temperature in a certain range of temperature. However, the reduced integrity and uniformity of QWs make carriers more prone to the nonradiative Shockley-Reed-Hall recombination as temperature rises. Moreover, both acoustic and optical phonon scatterings dominate from 300 to 600 K through the analysis on the evolution of photoluminescence spectra. Overall, these detailed studies provide insights into approaches to evaluate carrier dynamics at elevated temperatures and improve emitting performance to further push the practical efficiency limit.

Controllable CO adsorption determines ethylene and methane productions from CO2 electroreduction

Among all CO2 electroreduction products, methane (CH4) and ethylene (C2H4) are two typical and valuable hydrocarbon products which are formed in two different pathways: hydrogenation and dimerization reactions of the same CO intermediate. Theoretical studies show that the adsorption configurations of CO intermediate determine the reaction pathways towards CH4/C2H4. However, it is challenging to experimentally control the CO adsorption configurations at the catalyst surface, and thus the hydrocarbon selectivity is still limited. Herein, we seek to synthesize two well-defined copper nanocatalysts with controllable surface structures. The two model catalysts exhibit a high hydrocarbon selectivity toward either CH4 (83%) or C2H4 (93%) under identical reduction conditions. Scanning transmission electron microscopy and X-ray absorption spectroscopy characterizations reveal the low-coordination Cu0 sites and local Cu0/Cu+ sites of the two catalysts, respectively. CO-temperature programed desorption, in-situ attenuated total reflection Fourier transform infrared spectroscopy and density functional theory studies unveil that the bridge-adsorbed CO (COB) on the low-coordination Cu0 sites is apt to be hydrogenated to CH4, whereas the bridge-adsorbed CO plus linear-adsorbed CO (COB + COL) on the local Cu0/Cu+ sites are apt to be coupled to C2H4. Our findings pave a new way to design catalysts with controllable CO adsorption configurations for high hydrocarbon product selectivity.