High-resolution Transmission Electron Microscopy Measurements Research Articles

Heteroepitaxial ZnO films on diamond: Optoelectronic properties and the role of interface polarity

We demonstrate the growth of heteroepitaxial ZnO films on (110) diamond substrates by molecular beam epitaxy and report on a major advance in structural quality, as confirmed by XRD and high-resolution TEM measurements. The growth direction is found to be along the polar c-axis with Zn-polarity, deduced from annular bright-field scanning transmission electron microscopy imaging. This is important information, as simulations of the electronic band structure reveal the ZnO polarity to dominate the electronic structure of the interface: the formation of a two-dimensional electron gas on the ZnO side or a two-dimensional hole gas on the diamond side are predicted for Zn- and O-polarity, respectively. In addition, photoluminescence and absorption studies exhibit good optical properties and reveal stimulated emission for optical excitation above a threshold of 30 kW/cm2.

A catalyst-free growth of aluminum-doped ZnO nanorods by thermal evaporation.

The growth of Al:ZnO nanorods on a silicon substrate using a low-temperature thermal evaporation method is reported. The samples were fabricated within a horizontal quartz tube under controlled supply of O2 gas where Zn and Al powders were previously mixed and heated at 700°C. This allows the reactant vapors to deposit onto the substrate placed vertically above the source materials. Both the undoped and doped samples were characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) measurements. It was observed that randomly oriented nanowires were formed with varying nanostructures as the dopant concentrations were increased from 0.6 at.% to 11.3 at.% with the appearance of ‘pencil-like’ shape at 2.4 at.%, measuring between 260 to 350 nm and 720 nm in diameter and length, respectively. The HRTEM images revealed nanorods fringes of 0.46 nm wide, an equivalent to the lattice constant of ZnO and correspond to the (0001) fringes with regard to the growth direction. The as-prepared Al:ZnO samples exhibited a strong UV emission band located at approximately 389 nm (E g = 3.19 eV) with multiple other low intensity peaks appeared at wavelengths greater than 400 nm contributed by oxygen vacancies. The results showed the importance of Al doping that played an important role on the morphology and optical properties of ZnO nanostructures. This may led to potential nanodevices in sensor and biological applications.

Enhanced electrical properties of electrospun nylon66 nanofibers containing carbon nanotube fillers and Ag nanoparticles

In this study, we describe the preparation and characterization of electrospun Nylon66 composite nanofibers incorporated with carbon nanotubes (CNT) fillers and silver nanoparticles. We have incorporated the composites in to Nylon66 nanofibers to enhance the characteristics of the resultant composite nanofibers. The resultant composite nanofibers were characterized by using field-emission scanning electron microscopy, energy dispersive X-ray analysis, high-resolution transmission electron microscopy, X-ray diffraction, and current-voltage (I–V) measurement analysis. The morphology of the composite nanofibers exhibited densely arranged mesh-like ultrafine nanofibers which were strongly bound in between the main fibers. From I–V characteristics, it was observed that the incorporation of CNT fillers and Ag nanoparticles in to electrospun Nylon66 composite nanofibers can be significantly enhanced the electrical properties.

Influence of amorphous and crystalline type precursor intermediates on the morphology and luminescence properties of green emitting β-Ca2SiO4:Eu2+ phosphor synthesized using a liquid phase precursor method

Influence of precursor intermediates (PI) on the synthesis of high efficient green emitting β-Ca2SiO4:Eu2+ (β-C2S:Eu) phosphors were demonstrated by a novel liquid-phase-precursor (LPP) method utilizing (a) CaCl2–EuCl3, (b) Ca(NO3)2–EuCl3, (c) CaCl2–Eu(NO3)3, and (d) Ca(NO3)2–Eu(NO3)3 combinations as precursors. The crystallinity and morphology of the PI analyzed using X-ray diffraction and high-resolution transmission electron microscopy measurements showed both the amorphous and crystalline type precursor intermediates (API and CPI) depending upon the metal-source materials. The final morphology and luminescence properties of β-C2S:Eu phosphors were found to be strongly influenced by the crystallinity (amorphous and crystalline) and morphology of the PIs. The APIs have led to regular-shaped homogeneous phosphor particles with higher luminescence when compared with the CPIs.

Achieve high-quality InGaN/GaN multiple quantum wells on La0.3Sr1.7AlTaO6 substrates

High-quality InGaN/GaN multiple quantum wells (MQWs) have been epitaxially grown on La0.3Sr1.7AlTaO6 (LSAT) (111) substrates by radio-frequency molecular beam epitaxy. The as-grown InGaN/GaN MQWs are characterized by X-ray technique of high-resolution X-ray diffraction (HRXRD), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) for structural and optical properties. The clear and sharp Pendellösung fringes from the typical HRXRD θ–2θ scan patterns reveal the abrupt InGaN/GaN interfaces and well designed MQWs periodicities, which are confirmed by the HRTEM measurement. A sharp and narrow PL peak of InGaN/GaN MQWs grown on LSAT substrates is observed at 445nm with a full width at half-maximum of 22nm conducted at room temperature, which shows the high optoelectronic properties. This work presents an alternative substrate for achieving high-quality InGaN/GaN MQWs for the potential application of optoelectronic devices.

Plasma-assisted synthesis and high-resolution characterization of anisotropic elemental and bimetallic core-shell magnetic nanoparticles.

Magnetically anisotropic as well as magnetic core–shell nanoparticles (CS-NPs) with controllable properties are highly desirable in a broad range of applications. With this background, a setup for the synthesis of heterostructured magnetic core–shell nanoparticles, which relies on (optionally pulsed) DC plasma gas condensation has been developed. We demonstrate the synthesis of elemental nickel nanoparticles with highly tunable sizes and shapes and Ni@Cu CS-NPs with an average shell thickness of 10 nm as determined with scanning electron microscopy, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements. An analytical model that relies on classical kinetic gas theory is used to describe the deposition of Cu shell atoms on top of existing Ni cores. Its predictive power and possible implications for the growth of heterostructured NP in gas condensation processes are discussed.

Composition-controlled ternary Rh–Pd–Pt solid-solution alloy nanoparticles by laser irradiation of mixed solution of metallic ions

Abstract

Optimal growth and thermal stability of crystalline Be0.25Zn0.75O alloy films on Al2O3(0001)

The influence of growth temperature on the synthesis of BexZn1−xO alloy films, grown on highly-mismatched Al2O3(0001) substrates, was studied by synchrotron x-ray scattering, high-resolution transmission electron microscopy and photoluminescence measurements. A single-phase BexZn1−xO alloy with a Be concentration of x = 0.25, was obtained at the growth temperature, Tg = 400 °C, and verified by high-resolution transmission electron microscopy. It was found that high-temperature growth, Tg≥600 °C, caused phase separation, resulting in a random distribution of intermixed alloy phases. The inhomogeneity and structural fluctuations observed in the BexZn1−xO films grown at high temperatures are attributed to a variation in Be composition and mosaic distribution via atomic displacement and strain relaxation.

Ferromagnetism in homogeneous (Al,Co)-codoped 4H-silicon carbides

In view of the recent controversies on above room-temperature (RT) ferromagnetism (FM) in transition-metal (TM) doped silicon carbides (SiC), the present paper aims to shed some light on the natural origin of long-range magnetic order by investigating the (Al, Co)-doped 4H-SiC, both experimentally and theoretically. A combination of characterizations means including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray absorption near-edge structure spectroscopy (XANES) measurement eliminated the presence of any nanoclusters or secondary phases as the source of FM. X-ray absorption near-edge structure spectroscopy analyses provided convincing evidence that no secondary phases such as Co metallic clusters were present when Co and Al are homogeneously inserted in the SiC matrix. RT FM originates from a composite mechanism based on the Ruderman–Kittel–Kasuya–Yosida, and double-exchange interactions. The dopant Al is found to stabilize the crystal structure as well as show the experimental possibility of tuning the magnetization by codoping.

Electrospun polyacrylonitrile nanofibers supported Ag/Pd nanoparticles for hydrogen generation from the hydrolysis of ammonia borane

A high-performance hydrogen generation system based on the electrospun polyacrylonitrile (PAN)/Ag/Pd composite nanofibers, which were prepared by microwave reducing the electrospun PAN/AgNO3 nanofibers and followed by a replacement reaction has been demonstrated. The morphology of the as-prepared PAN/Ag/Pd composite nanofibers and the metal nanoparticles on the fibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. It has been demonstrated that the obtained PAN/Ag/Pd composite nanofibers possess fine morphology and high catalytic activities for H2 generation from aqueous solution of ammonia borane (NH3BH3, AB). The H2 generation test exhibited that the catalyst had excellent catalytic activity (with turnover frequency (TOF) of 377.2 mol H2 h−1 (mol Pd)−1), good recycle stability and easy-separation from the reaction system. This new kind of nanofibers possesses great potential application for the new clean energy development.

Revealing the atomic and electronic structure of a SrTiO3/LaNiO3/SrTiO3 heterostructure interface

The atomic structures of SrTiO3 (STO)/LaNiO3 (LNO)/STO heterostructure interfaces were investigated by spherical aberration-corrected (CS) (scanning) transmission electron microscopy. Atomic displacement and lattice distortion measurements and electron energy loss spectroscopy (EELS) were used to quantitatively analyze the distortion of the interfacial octahedra and the bond length at the interfaces. Combined with high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy analyses, two distinct interfacial atomic terminating layers are unambiguously determined. Ensuing quantitative HRTEM measurements revealed that the Ni-O bond length in the interfacial octahedral is elongated at the bottom interface (–NiO2-SrO–). Atomic displacement shows structural relaxation effects when crossing the interfaces and lattice distortions across the interface is more pronounced in LNO than in STO. The Ti/O atomic ratio, La and Ti relative atomic ratio as derived by EELS quantification indicate non-stoichiometric composition at the interfaces. Distinct fine structures of Ti-L2,3 edge and O-K edge at the bottom and top interfaces are observed. By comparison, we are able to estimate Ti valency at both interfaces. Combining the structural distortions and Ti valency, the polar discontinuity and charge transfer at the interfaces are discussed.

Epitaxial growth of vanadium nitride thin films by laser molecule beam epitaxy

Vanadium nitride thin films were epitaxially grown on SrTiO3 and sapphire substrates by ablating vanadium target in activated N2 atmosphere. The epitaxial orientation relationships of film/substrate are [001]VN//[001]SrTiO3 and [111]VN/[0001]α−Al2O3. Atomic force microscope measurements show the smooth surfaces of the films. High-resolution transmission electron microscope measurement was performed on the cross-section of VN/SrTiO3 sample, which shows a good epitaxial growth. Temperature-dependent resistance measurements demonstrate that the films have typical metallic conductivity.

Measurements of strain in AlGaN/GaN HEMT structures grown by plasma assisted molecular beam epitaxy

The AlGaN/GaN high electron mobility transistor (HEMT) structures were grown on the (0001) HVPE bulk GaN substrates using plasma-assisted molecular-beam epitaxy (PAMBE). The AlGaN layers of 12% or 21% Al were grown to nominal 20nm thickness after which the 3nm thick GaN cap was added. High-resolution X-ray diffraction (HRXRD) and TEM measurements were used to determine crystallographic quality, composition and strain distribution in these samples. It was shown that 12% Al samples are uniform while 21% samples exhibit large compositional nonuniformity. The mismatch strain is localized close to interface on GaN side while on AlGaN side penetrates deeply, in more nonuniform way, especially for higher Al content sample.

Clean Method for the Synthesis of Reduced Graphene Oxide-Supported PtPd Alloys with High Electrocatalytic Activity for Ethanol Oxidation in Alkaline Medium

In this article, a clean method for the synthesis of PtPd/reduced graphene oxide (RGO) catalysts with different Pt/Pd ratios is reported in which no additional components such as external energy (e.g., high temperature or high pressure), surfactants, or stabilizing agents are required. The obtained catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), induced coupled plasma atomic emission spectroscopy (ICP-AES), and electrochemical measurements. The HRTEM measurements showed that all of the metallic nanoparticles (NPs) exhibited well-defined crystalline structures. The composition of these Pt-Pd/RGO catalysts can be easily controlled by adjusting the molar ratio of the Pt and Pd precursors. Both cyclic voltammetry (CV) and chronoamperometry (CA) results demonstrate that bimetallic PtPd catalysts have superior catalytic activity for the ethanol oxidation reaction compared to the monometallic Pt or Pd catalyst, with the best performance found with the PtPd (1:3)/RGO catalyst. The present study may open a new approach for the synthesis of PtPd alloy catalysts, which is expected to have promising applications in fuel cells.

Epitaxial growth of homogeneous single-crystalline AlN films on single-crystalline Cu (1 1 1) substrates

The homogeneous and crack free single-crystalline AlN thin films have been epitaxially grown on single-crystalline Cu (111) substrates with an in-plane alignment of AlN [11–20]//Cu [1–10] by pulsed laser deposition (PLD) technology with an integrated laser rastering program. The as-grown AlN films are studied by spectroscopic ellipsometry, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), polarized light microscopy, high-resolution X-ray diffraction, and high-resolution transmission electron microscopy (HRTEM). The spectroscopic ellipsometry reveals the excellent thickness uniformity of as-grown AlN films on the Cu (111) substrates with a root-mean-square (RMS) thickness inhomogeneity less than 2.6%. AFM and FESEM measurements indicate that very smooth and flat surface AlN films are obtained with a surface RMS roughness of 2.3nm. The X-ray reflectivity image illustrates that there is a maximum of 1.2nm thick interfacial layer existing between the as-grown AlN and Cu (111) substrates and is confirmed by HRTEM measurement, and reciprocal space mapping shows that almost fully relaxed AlN films are achieved only with a compressive strain of 0.48% within ∼321nm thick films. This work demonstrates a possibility to obtain homogeneous and crack free single-crystalline AlN films on metallic substrates by PLD with optimized laser rastering program, and brings up a broad prospect for the application of acoustic filters that require abrupt hetero-interfaces between the AlN films and the metallic electrodes.

Formic Acid-Assisted Synthesis of Palladium Nanocrystals and Their Electrocatalytic Properties

Palladium (Pd) nanocrystals have been synthesized by using formic acid as the reducing agent at room temperature. When the concentration of formic acid was increased continuously, the size of Pd nanocrystals first decreased to a minimum and then increased slightly again. The products have been investigated by a series of techniques, including X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), UV-vis absorption, and electrochemical measurements. The formation of Pd nanocrystals is proposed to be closely related to the dynamical imbalance of the growth and dissolution rate of Pd nanocrystals associated with the adsorption of formate ions onto the surface of the intermediates. It is found that small Pd nanocrystals showed blue-shifted adsorption peaks compared with large ones. Pd nanocrystals with the smallest size display the highest electrocatalytic activity for the electrooxidation of formic acid and ethanol on the basis of cyclic voltammetry and chronoamperometric data. It is suggested that both the electrochemical active surface area and the small size effect are the key roles in determining the electrocatalytic performances of Pd nanocrystals. A "dissolution-deposition-aggregation" process is proposed to explain the variation of the electrocatalytic activity during the electrocatalysis according to the HRTEM characterization.

Au–ZnO hybrid nanoflowers, nanomultipods and nanopyramids: one-pot reaction synthesis and photocatalytic properties

The preparation of noble metal-semiconductor hybrid nanocrystals with controlled morphologies has received intensive interest in recent years. In this study, facile one-pot reactions have been developed for the synthesis of Au-ZnO hybrid nanocrystals with different interesting morphologies, including petal-like and urchin-like nanoflowers, nanomultipods and nanopyramids. In the synthesis strategy, oleylamine-containing solution serves as the reaction medium, and the in situ generated Au seeds play an important role in the subsequently induced growth of ZnO nanocrystals. With the aid of several surfactants, hybrid nanocrystals with different morphologies that have considerable influences on their optical and photocatalytic activities are readily achieved. Through high-resolution transmission electron microscopy measurements, an observed common orientation relationship between ZnO and Au is that ZnO nanocrystals prefer to grow with their polar {001} facets on the {111} facets of Au nanocrystals, and well-defined interfaces are evident. Surface plasmon resonance bands of Au with different positions are observed in the UV-vis spectra, and the UV and visible emissions of ZnO are found to be dramatically reduced. Finally, the as-prepared Au-ZnO nanocrystals exhibit excellent photocatalytic activity for the photodegradation of rhodamine B compared with pure ZnO nanocrystals. The Au-ZnO hybrid nanopyramids show the highest catalytic efficiency, which is correlated with the exposed crystal facets, crystallinity and the formation of hybrid nanostructures. The as-prepared Au-ZnO hybrid nanocrystals are expected to find diverse potential applications in the fields such as photocatalysis, solar energy conversion, sensing and biological detection.

Synthesis of polyol based Ag/Pd nanocomposites for applications in catalysis

The synthesis of polyvinylpyrrolidone seed mediated Ag/Pd allied nanobimetallic particles was successfully carried out by the simultaneous reduction of the metal ions in ethylene glycol, diethylene glycol, glycerol, pentaerythritol and sodium borohydride solution. The optical measurement revealed the existence of peak broadening that causes diffusion processes of the metal sols to decrease making it possible to monitor the changes spectrophotometrically. This, together with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high resolution TEM measurements strongly support the conclusion that intimately alloyed clusters were formed and the particle growth anisotropy is diffusion limited. Finally, the catalytic potential of the nanocomposites was investigated using 4-nitrophenol in the presence of sodium borohydride at 299K; a good linear fitting of ln(A/A0) versus the reaction time was obtained, indicating pseudo-first-order kinetics.

Spectroscopy and Femtosecond Dynamics of Water Soluble Type I CdSe/ZnS Core–Shell Quantum Dot

Thiol-capped type I CdSe/ZnS core–shell quantum dot nanostructures have been synthesized at low temperature in water; and then characterized by steady-state absorption and photoluminescence (PL) studies and high resolution TEM (HRTEM) measurements. On excitation of CdSe quantum dot predominantly surface state emission was detected, however on ZnS shell formation prominent exciton emission of CdSe with much higher overall quantum yield was observed. Femtosecond up-conversion measurements reveal that exciton emission found to decay much faster as compared to that of surface state emission. Lifetime of CdSe exciton emission found to increases with ZnS shell thickness. Femtosecond transient absorption studies have been carried out on these QD and core–shell material at 400 nm laser light and monitored the transients in the visible region to study charge carrier dynamics in ultrafast time scale. On laser excitation electron–hole pairs are generated which are detected by induced absorption signal for the charge carriers in visible region and immediate bleach at excitonic position for both QD and QD core–shell. Carrier quenching studies has been carried out for both CdSe and CdSe/ZnS by using benzoquinone (BQ, electron quencher) and pyridine (Py, hole quencher) suggest that although CdSe/ZnS form type I core–shell, still both electron and hole found to be leaked through ZnS shell from CdSe core.

Biosynthesis of silver nanoparticles from the marine seaweed Sargassum wightii and their antibacterial activity against some human pathogens

In this paper, we have reported on biological synthesis of nano-sized silver and its antibacterial activity against human pathogens. The nanoparticles of silver were formed by the reduction of silver nitrate to aqueous silver metal ions during exposure to the extract of marine seaweed Sargassum wightii. The optical properties of the obtained silver nanoparticles were characterized using UV–visible absorption and room temperature photoluminescence. The X-ray diffraction results reveal that the synthesized silver nanoparticles are in the cubic phase. The existence of functional groups was identified using Fourier transform infrared spectroscopy. The morphology and size of the synthesized particles were studied with atomic force microscope and high-resolution transmission electron microscope measurements. The synthesized nanoparticles have an effective antibacterial activity against S. aureus, K. pneumoniae, and S. typhi.