High-resolution Electron Microscopy Studies Research Articles

A high-resolution analytical scanning transmission electron microscopy study of the early stages of spinodal decomposition in binary Fe–Cr

Spinodal decomposition (SD) is an important phenomenon in materials science and engineering. For example, it is considered to be responsible for the 475°C embrittlement of stainless steels comprising the bcc (ferrite) or bct (martensite) phases. Structural characterization of the evolving minute nano-scale concentration fluctuations during SD in the Fe–Cr system is, however, a notable challenge, and has mainly been considered accessible via atom probe tomography (APT) and small-angle neutron scattering. The standard tool for nanostructure characterization, viz. transmission electron microscopy (TEM), has only been successfully applied to late stages of SD when embrittlement is already severe. However, we here demonstrate that the structural evolution in the early stages of SD in binary Fe–Cr, and alloys based on the binary, are accessible via analytical scanning TEM. An Fe–36wt% Cr alloy aged at 500°C for 1, 10 and 100h is investigated using an aberration-corrected microscope and it is found that highly coherent and interconnected Cr-rich regions develop. The wavelength of decomposition is rather insensitive to the sample thickness and it is quantified to 2, 3 and 6nm after ageing for 1, 10 and 100h, which is in reasonable agreement with prior APT analysis. The concentration amplitude is more sensitive to the sample thickness and acquisition parameters but the TEM analysis is in good agreement with APT analysis for the longest ageing time. These findings open up for combinatorial TEM studies where both local crystallography and chemistry is required.

Exchange bias in ferrite hollow nanoparticles originated by complex internal magnetic structure

Iron-oxide hollow nanospheres (HNS) may present unusual magnetic behavior as a consequence of their unique morphology. Here, we report the unusual magnetic behavior of HNS that are 9 nm in diameter. The magnetic properties of HNS originate in their complex magnetic structure, as evidenced by Mössbauer spectroscopy and magnetization measurements. We observe a bias in the hysteresis when measured at very low temperature in the field cooling protocol (10 kOe). In addition, dc (static) and ac (dynamic) magnetization measurements against temperature and applied field reveal a frustrated order of the system below 10 K. High-resolution transmission electron microscopy (HRTEM) studies reveal that the HNS are composed of small crystalline clusters of about 2 nm in diameter, which behave as individual magnetic entities. Micromagnetic simulations (using conjugate gradient in order to minimize the total energy of the system) reproduce the experimentally observed magnetic behavior. The model considers the hollow particles as constituted by small ordered clusters embedded in an antiferromagnetic environment (spins localized outside the clusters). In addition, the surface spins (in both inner and outer surfaces of the HNS) are affected by a local surface anisotropy. The strong effective magnetic anisotropy field of the clusters induces the bias observed when the system is cooled in the presence of a magnetic external field. This effect propagates through the exchange interaction into the entire particle.

Structural anomaly in the high-entropy alloy ZrNbTiTaHf

We present a high-resolution scanning transmission electron microscopy study on the microstructure of a non-equiatomic high-entropy alloy with the composition of Zr12.7 Nb30.8 Ti17.7 Ta30.8 Hf8.0 (at.%). We identify a novel inter-grain phase (IGP) that compositionally and structurally differs from the surrounding body-centred cubic host. In particular, we find that the IGP has a composition of Zr52.8 Nb6.9 Ti4.6 Ta20.6 Hf15.1 (at.%) and that it solidifies in a face-centred cubic crystal lattice structure. The occurrence of the latter is unexpected and remarkable since all possible binary phase diagrams of the involved elements only show body-centred cubic and hexagonal close-packed crystal structures. Therefore, to validate our experimental findings we have conducted parameter-free ab-initio calculations based on density-functional theory and the coherent-potential approximation. The simulations support our experimental findings showing that for the composition of the IGP, the face-centred cubic crystal structure is indeed the most stable one.

Impact of non-ionic surfactant chemical structure on morphology and stability of polystyrene nanocomposite latex

Polystyrene (PS) colloid particles in presence of non-ionic surfactant-modified clay particles were prepared by the free-radical polymerization of styrene monomers in emulsion. Three different types of non-ionic surfactants, sorbitan monopalmitate (Span®40), polyethylene glycol octadecyl ether (Brij®S10), and polyoxyethylene (9) nonylphenylether (Igepal®Co-630) were used for the preparation of surfactant-modified clay. High-resolution transmission electron microscopy studies showed that few colloid PS particles with clay mineral layers at the surface were obtained; the particle sizes were observed to be in the micrometer size range, and stable dispersions were obtained when Span®40 and Igepal®Co-630 modified clay minerals were used as stabilizers. The clay mineral particles were observed to be mostly encapsulated by PS latex particles, and a typical morphology was observed when Brij®S10-modified clay was used as a stabilizer. This strategy can be applied to develop stable polymer latex particles via emulsion polymerization.

High pressure synthesis of amorphous TiO2 nanotubes

Amorphous TiO2 nanotubes with diameters of 8-10 nm and length of several nanometers were synthesized by high pressure treatment of anatase TiO2 nanotubes. The structural phase transitions of anatase TiO2 nanotubes were investigated by using in-situ high-pressure synchrotron X-ray diffraction (XRD) method. The starting anatase structure is stable up to ∼20GPa, and transforms into a high-density amorphous (HDA) form at higher pressure. Pressure-modified high- to low-density transition was observed in the amorphous form upon decompression. The pressure-induced amorphization and polyamorphism are in good agreement with the previous results in ultrafine TiO2 nanoparticles and nanoribbons. The relationship between the LDA form and α-PbO2 phase was revealed by high-resolution transmission electron microscopy (HRTEM) study. In addition, the bulk modulus (B0 = 158 GPa) of the anatase TiO2 nanotubes is smaller than those of the corresponding bulks and nanoparticles (180-240 GPa). We suggest that the unique open-ended nanotube morphology and nanosize play important roles in the high pressure phase transition of TiO2 nanotubes.

Ultrasound-assisted synthesis and characterization of ultrathin copper nanowhiskers

Nanoscale copper is of great importance in material science due to its excellent catalytic properties. This work proposed a facile one-step route to synthesize ultrathin copper nanowhisker by using ultrasound-assisted polyol method. High-resolution transmission electron microscope (HRTEM) studies showed that the copper nanowhiskers with diameters of 1.5–5nm and lengths of 20–60nm were obtained. The traces of the copper microplates were formed as the byproduct. The effect of ultrasound was also investigated. The results indicated that the ultrasonic power played an important role in the formation of copper nanowhiskers. The formation mechanism for copper nanowhiskers had also been proposed.

Thermoelectric properties of spark plasma sintered lead telluride nanocubes

Abstract

Large-Scale Molecular Dynamics and High-Resolution Transmission Electron Microscopy Study of Graphene Grain Boundaries

Large-Scale Molecular Dynamics and High-Resolution Transmission Electron Microscopy Study of Graphene Grain Boundaries

Stir casting process for manufacture of Al–SiC composites

Stir casting is an economical process for the fabrication of aluminum matrix composites. There are many parameters in this process, which affect the final microstructure and mechanical properties of the composites. In this study, micron-sized SiC particles were used as reinforcement to fabricate Al-3 wt% SiC composites at two casting temperatures (680 and 850 °C) and stirring periods (2 and 6 min). Factors of reaction at matrix/ceramic interface, porosity, ceramic incorporation, and agglomeration of the particles were evaluated by scanning electron microscope (SEM) and high-resolution transition electron microscope (HRTEM) studies. From microstructural characterizations, it is concluded that the shorter stirring period is required for ceramic incorporation to achieve metal/ceramic bonding at the interface. The higher stirring temperature (850 °C) also leads to improved ceramic incorporation. In some cases, shrinkage porosity and intensive formation of Al4C3 at the metal/ceramic interface are also observed. Finally, the mechanical properties of the composites were evaluated, and their relation with the corresponding microstructure and processing parameters of the composites was discussed.

Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing.

Growth of freestanding nano- and microstructures with complex morphologies is a highly desired aspect for real applications of nanoscale materials in various technologies. Zinc oxide tetrapods (ZnO-T), which exhibit three-dimensional (3D) shapes, are of major importance from a technological applications point of view, and thus efficient techniques for growth of different varieties of tetrapod-based networks are demanded. Here, we demonstrate the versatile and single-step synthesis of ZnO-T with different arm morphologies by a simple flame transport synthesis (FTS) approach, forming a network. Morphological evolutions and structural intactness of these tetrapods have been investigated in detail by scanning electron microscopy, X-ray diffraction, and micro-Raman measurements. For a deeper understanding of the crystallinity, detailed high-resolution transmission electron microscopic studies on a typical ZnO tetrapod structure are presented. The involved growth mechanism for ZnO tetrapods with various arm morphologies is discussed with respect to variations in experimental conditions. These ZnO-T have been utilized for photocatalytic degradation and nanosensing applications. The photocatalytic activities of these ZnO-T with different arm morphologies forming networks have been investigated through the photocatalytic decolorization of a methylene blue (MB) solution under UV light illumination at ambient temperature. The results show that these ZnO-T exhibit strong photocatalytic activities against MB and its complete degradation can be achieved in very short time. In another application, a prototype of nanoelectronic sensing device has been built from these ZnO-T interconnected networks and accordingly utilized for UV detection and H2 gas sensing. The fabricated device structures showed excellent sensing behaviors for promising practical applications. The involved sensing mechanisms with respect to UV photons and H2 gas are discussed in detail. We consider that such multifunctional nanodevices based on ZnO tetrapod interconnected networks will be of interest for various advanced applications.

Characterization of InSb Nanoparticles Synthesized Using Inert Gas Condensation

Nanoparticles (NPs) of indium antimonide (InSb) were synthesized using a vapor phase synthesis technique known as inert gas condensation (IGC). NPs were directly deposited, at room temperature and under high vacuum, on glass cover slides, TEM grids and (111) p-type silicon wafers. TEM studies showed a bimodal distribution in the size of the NPs with average particle size of 13.70 nm and 33.20 nm. The Raman spectra of InSb NPs exhibited a peak centered at 184.27 cm−1, which corresponds to the longitudinal optical (LO) modes of phonon vibration in InSb. A 1:1 In-to-Sb composition ratio was confirmed by energy dispersive X-ray (EDX). X-ray diffractometer (XRD) and high-resolution transmission electron microscopy (HRTEM) studies revealed polycrystalline behavior of these NPs with lattice spacing around 0.37 and 0.23 nm corresponding to the growth directions of (111) and (220), respectively. The average crystallite size of the NPs obtained using XRD peak broadening results and the Debye-Scherrer formula was 25.62 nm, and the value of strain in NPs was found to be 0.0015. NP’s band gap obtained using spectroscopy and Fourier transform infrared (FTIR) spectroscopy was around 0.43–0.52 eV at 300 K, which is a blue shift of 0.26–0.35 eV. The effects of increased particle density resulting into aggregation of NPs are also discussed in this paper.

Influence of doping on crystal growth, structure and optical properties of nanocrystalline CaTiO3: a case study using small-angle neutron scattering

The effect of dopant size (ionic radius) on the crystal growth, structure and optical properties of nanocrystalline calcium titanate, CaTiO3 (CTO), have been studied using small-angle neutron scattering. X-ray diffraction, along with high-resolution transmission electron microscopy, confirms the growth of pure nanocrystalline CTO. Rietveld analysis reveals that the difference of ionic radii between dopant and host ions induces strain within the lattice, which significantly affects the lattice parameters. The induced strain, due to the difference of ionic radii, causes the shrinkage of the optical band gap, which is manifested by the redshift of the absorbance band. Mesoscopic structural analysis using scattering techniques demonstrates that the ionic radius of the dopant influences the agglomeration behaviour and particle size. A high-resolution transmission electron microscopy study reconfirms the formation of pure highly crystalline CTO nanoparticles.

High-resolution transmission electron microscopy study on reversion of Al2CuMg precipitates in Al–Cu–Mg alloys under irradiation

Image deconvolution analyses showed that reversion of S-Al2CuMg precipitates occurred in an Al–Cu–Mg alloy during high-resolution transmission electron microscopy observations. A fraction of Mg and Cu atoms in the precipitates diffused into Al matrix due to electron beam irradiation at 300kV, resulting in structural/chemical reversion of the precipitates. The structural reversion of the S-Al2CuMg precipitates is closely related with irradiation-induced displacement of atoms. The strong attraction between Cu and Mg atoms might assist the sub-threshold displacement of Cu atoms. One transitional structure is determined to be S′′-Al10Cu3Mg3, a precursor of S-Al2CuMg. Two other transitional structures, Al3CuMg and Al18Cu5Mg5 which have the same lattice parameters of a=c=0.405nm as that of S′′-Al10Cu3Mg3, but different b values, are suggested.

A Study into the Significant Improvement in Performance of Silicon Nanowire Electrodes through the Use of Electrolyte Additives

Si nanowires (NWs) are a promising anode material for next generation lithium-ion batteries due to their large maximum theoretical capacity (3579 mAh/g) when compared with conventional graphitic based materials (372 mAh/g). NWs also provide good electrical conductivity along their length, have a high interfacial area in contact with the electrolyte, have an optimal short diffusion distance for Li-ion transport and can be grown directly from current collectors, eliminating the need for binders and conductive additives. However before Si NWs can be realized as a viable alternative to graphite it is essential that suitable electrolytes are developed that result in a stable SEI layer formation over the initial cycles. Here we present a systematic study of the effect of common LIB electrolyte additives such as FEC and VC on the capacity retention, rate capability and SEI layer composition of binder-free Si NW electrodes. The efficacy of 5 different electrolyte additives in various combinations and used in conjunction with standard carbonate based electrolytes was investigated. We show that small quantities of additives can improve the cycle life and rate capability of the Si NWs significantly when compared with a standard 1M LiPF6 in EC/DEC electrolyte. Using XPS and EIS techniques we elucidate the effect of each additive combination on the composition and electrical properties of the SEI layer and correlate these results with the improved performance. Furthermore, through an ex-situ high-resolution transmission electron microscopy and high-resolution scanning electron microscopy study we show that the additives can also affect a change in morphology of the active material with cycling, with the better performing additives inducing a change from a NW morphology to a stable porous network like structure over initial cycles.

Silicon-Germanium Heterostructure Nanowires As High Capacity Lithium-Ion Battery Anodes - an Electrochemical and Structural Investigation

Li-alloying nanowires (NWs) are promising anode materials for next generation lithium-ion batteries due to their large maximum theoretical capacity when compared with conventional graphitic based materials (372 mAh/g). Ge NWs, in particular, have shown great promise, exhibiting stable capacities of ̴ 900 mAh/g over 1000s of cycles and excellent rate capability due to the high rate of diffusivity of Li in Ge and its inherent high conductivity.1, 2 The superior capacity retention of Ge is attributed to the complete restructuring of the NWs that occurs within the first 100 cycles to form a continuous porous network that is mechanically robust. The possibility of incorporating Si into this stable network of Ge is very appealing, resulting in electrodes that combine the very high capacity of Si with the excellent rate characteristics and capacity retention of Ge. Here we report the formation of high-performance and high-capacity lithium-ion battery anodes from high-density Si-Ge heterostructure nanowire arrays grown directly from the current collector. The unique morphology of the active material consists of Si NW branches grown from a Ge NW backbone. By tuning the ratio of Si to Ge in the material it is possible to design electrodes for either high-capacity applications (high Si : Ge mass ratio) or high rate capability applications (high Ge : Si mass ratio). We show by ex-situ high-resolution transmission electron microscopy (HRTEM) and high-resolution scanning electron microscopy (HRSEM) studies that the NW array transforms into a mechanically robust, porous network of Si and Ge ligaments. Once this network is formed it is highly stable, maintaining capacities of 1700 mAh/g over 200 cycles. The electrode material described here also has the advantage of being formed in a low energy, rapid synthetic protocol. Using a very simple reaction protocol, NW growth occurs by thermolytic decomposition of an organometallic precursor onto a preheated substrate in an inert atmosphere. 1. T. Kennedy, E. Mullane, H. Geaney, M. Osiak, C. O’Dwyer, K. M. Ryan, Nano Letters 14, 716-723(2014), 2. E. Mullane, T. Kennedy, H. Geaney, K. M. Ryan, ACS Applied Materials & Interfaces , 6, 18800-18807 (2014).

Effect of starting materials on the wear performance of NiTi-based composites

NiTi alloys have exhibited significant potential as a matrix of wear resistant composites. In this study, in order to examine the effect of starting materials on the wear performance of NiTi-based composites, both elemental Ni/Ti and prealloyed NiTi powders were used to fabricate NiTi–6wt% nano-Al2O3 composites using hot isostatic pressing (HIP). Nanoindentation and microhardness test results indicate that the composite samples produced from the elemental Ni/Ti powders exhibited higher hardness and lower pseudoelasticity properties than those of the samples fabricated from the prealloyed NiTi powders; this is attributed to the higher amount of Ni3Ti/NiTi2 phases in the samples from the elemental powders. The wear test results also confirm the superior performance of the composite samples produced from the elemental powders. The high-resolution transmission electron microscopy studies of the wear debris demonstrate that the heavy local plastic deformation induced during the wear test appeared to inhibit the pseudoelasticity during the wear process. Hence, it can be deduced that the role of the hardness enhancement is more pronounced than that of the pseudoelasticity reduction in the wear performance of NiTi-based composite samples.

Nanocomposite of carbon nanotubes/silica nanoparticles and their use for adsorption of Pb(II): from surface properties to sorption mechanism

This paper demonstrates the synthesis of multi-wall carbon nanotubes and silica nanocomposite (CNT/SiO2). Successful realization of MWCNT/SiO2 nanostructure was observed by scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and high-resolution transmission electron microscopy studies. The as-prepared nanocomposite was evaluated as an adsorbent to remove lead, Pb(II), from aqueous solutions. The resulting MWCNT/SiO2 manifests propitious adsorption performance (~95%) over silica nanoparticles (~50%) and CNTs (~45%). Lagergren’s pseudo-first order, pseudo-second order and intraparticle diffusion models were used to analyse the kinetic data obtained at different initial Pb(II) concentrations. The adsorption kinetic data were described well by the pseudo-second order model with R2 of 0.99. The activation energy (Ea) of the adsorption process was calculated as 15.8 kJ mol−1. Adsorption data were described well by the Langmuir and Temkin models. The positive values of both ∆H° (29.4 kJ/mol) and ∆S° (116.8 J/mol K) obtained suggest an endothermic reaction and in increase in randomness at the solid–liquid interface during the adsorption of Pb(II) on the nanocomposite. The negative ∆G° values indicate a spontaneous adsorption process. Elemental dispersive X-ray analysis and mapping confirmed the adsorption of Pb(II) on the nanocomposite surface. The work also highlights the recyclability of the nanocomposite with high efficiencies and supports its potential for environmental applications. It is anticipated that the results bear broad potential in the sorption domain for the design of efficient and reusable adsorbent.

HRTEM investigation of phase stability in alumina–zirconia multilayer thin films

Phase stability of nanostructured thin films can be significantly different from the stability of the same materials in bulk form because of the increased contribution from surface and interface effects. Zirconia (ZrO2), stabilized in tetragonal and cubic phases, is a technologically important material and is used for most high temperature applications. In literature, zirconia can be found to be stabilized in its high temperature phases down to room temperature via two routes, doping with divalent or trivalent cations and crystallite size controls. Apart from these, in the alumina/zirconia thin-film multilayer system, a constraining effect on the zirconia layers provides another route to stabilization of the tetragonal zirconia phase at room temperature. However, in such nanostructured geometries, at high temperatures, the small diffusion lengths involved can influence the phase stability. The present work deals with the high-resolution transmission electron microscope (HRTEM) studies of pulsed laser ablated alumina–zirconia thin-film multilayers in the as deposited state and annealed up to 1473 K at 2 × 10−5 mbar. Conventional techniques such as X-ray diffraction lack the ability to detect localized phase changes at nanometre length scales and also for the low volume fraction of newly formed phases. Cross-sectional HRTEM techniques have been successful in detecting and characterizing these interactions.

Platinum on carbonaceous supports for glycerol hydrogenolysis: Support effect

Metal vapor synthesis (MVS) technique was applied to generate Pt-nanoparticles of different size (<1.3nm and 2.5nm) deposited onto carbonaceous supports, mainly characterized by a different surface area. The supported catalysts were employed in the glycerol hydrogenolysis reaction carried out under basic reaction conditions at 433 and 453K to obtain 1,2-propanediol as the main liquid product. Comparison of the composition of the liquid- and gas-phase products obtained by the different catalysts showed a clear dependence of aqueous-phase reforming, water–gas shift reaction activity as well as 1,2-propanediol chemoselectivity on the degree of Pt-sintering occurring on different carbon supports. High-resolution transmission electron microscopic and X-ray powder diffraction studies carried out on as-synthesized and recovered heterogeneous catalysts provided clear evidences that a high surface area carbon support, such as Ketjen Black EC-600JD, notably retards nanoparticle aggregation.

Nanoformulation of poly(ethylene glycol) polymerized organic insect repellent by PIT emulsification method and its application for Japanese encephalitis vector control

The utilization of increased dosage of insect repellents to overcome mosquito resistance has raised environmental concerns globally. In accord to this, we have formulated an efficacious, water-dispersive, nanometric formulation of a poor water-soluble insect repellent, diethylphenylacetamide (DEPA) by poly(ethylene glycol) (PEG) polymerization followed by PIT emulsification method. The critical micelle concentration of PEG in the spontaneously emulsified conventional DEPA droplets was determined, based on the droplets physical stability. Subjecting them to PIT emulsification yielded monodispersed polymeric nanomicelles of DEPA (Nano DEPA) with hydrodynamic mean diameter of 153.74nm. The high-resolution scanning and transmission electron microscopic studies revealed the characteristic core–shell structure of micelle. The comparative efficacy of Bulk DEPA and Nano DEPA was evaluated by larvicidal and WHO cone bioassay against the Japanese encephalitis vector Culex tritaeniorhynchus. The median lethal concentrations (48h) for 3rd instars C. tritaeniorhynchus larvae were found to be 0.416mg/L for Bulk DEPA and 0.052mg/L for Nano DEPA, respectively. The median knockdown concentrations (60min) for the two to three-day-old, sucrose-fed, female adult mosquitoes were 5.372% (v/v) and 3.471% (v/v) for Bulk and Nano DEPA, respectively. Further investigation by histopathological and biochemical studies propound that Nano DEPA exerted better bioefficacy as comparative to its bulk form even at minimal exposure concentrations. Hence, Nano DEPA will serve as an effective alternate in controlling the vector expansion with reduced dosage.