Crystallographic Facets Research Articles

Effect of additives on the morphology of single-crystal Au nanosheet synthesized using the polyol process

Triangular and hexagonal-shaped single-crystal Au nanosheets were prepared by the reduction of HAuCl4 with the polyol method, after addition of a polyvinylpyrolidone (PVP) capping agent and salts. Both capping agent and salts played a crucial role during nanosheet growth. The high PVP concentration was found to result in a mixed Au morphology of sheets and spherical particles, which could be tailored with the addition of salts to provide solely nanosheets. The main product is always nanosheets after the salts have been added. The selective adsorption of PVP and salt ions onto certain crystallographic facets is thought to be important in the proposed growth mechanism. This synthetic route is simple and fast, and can be extended to other morphology metal particles with careful salt and capping agent selection.

ChemInform Abstract: Shape Control of Platinum and Palladium Nanoparticles for Catalysis

AbstractReview: 94 refs.

Fatigue crack growth of AA2219 under different aging conditions

The fatigue crack growth of commercial AA2219 has been examined under different aging treatments, namely, naturally aged (NA), under aged (UA), peak aged (PA) and over aged (OA) conditions. From the near threshold stress intensity range (Δ K NTH), the alloy in the NA condition is found to have the highest resistance to fatigue crack initiation. The crack growth rate increases and the near threshold stress intensity range decreases with advancing aging. This observation is found to be consistent with lower levels of crack closure and decreasing levels of tortuosity in crack path for PA and OA tempers. The inhomogeneous transcrystalline slip in the UA condition results in the slower crack growth at low stress intensity range (Δ K). The fracture morphology changes from crystallographic facets near the threshold region to clearly developed ductile striations in the Paris power-law regime to microvoid coalescence in the high Δ K regions.

High index, reactive facet-controlled synthesis of one-dimensional single crystalline rare earth hydroxide nanobelts

Nanomaterials that are enclosed by high index, reactive facets exhibit a significantly enhanced chemical reactivity, leading to superior performances in electronics, photonics, energy conversion and storage, as well as interfacing with living cells for various biological purposes. This article investigates the formation of single crystalline rare earth hydroxide nanobelts with selective control of high energy surfaces. Combining experimental and theoretical efforts, a carboxylic group and carbon–carbon double bond of oleic acid molecules are found to reduce the (0001) surface energy by 60%, leading to the growth of nanobelts and the stabilization of their high energy surfaces. These findings shed light on the growth of one-dimensional nanostructures that exhibit a simultaneous control over the crystallographic facets, shapes, and sizes. Our results engender a versatile synthesis method for nanomaterials that exhibit enhanced physical and chemical properties by overcoming their energy barrier to grow with high energy surfaces.

GaN-based surface-emitting laser with two-dimensional photonic crystal acting as distributed-feedback grating and optical cladding

We demonstrate GaN-based distributed-feedback surface-emitting lasers grown on sapphire substrates with a two-dimensional (2D) square-lattice photonic crystal (PhC) that forms a high-aspect-ratio void array (void diameter: 65 nm and depth: 220 nm). The 2D PhC layer acts as both distributed-feedback grating and p-type optical cladding of a separate confinement heterostructure. To form the 2D PhC in the nitride semiconductor layers, we developed an embedding process that uses mass-transport phenomena. Crystallographic facets appeared on the inner walls of the embedded voids. Room-temperature lasing action was observed at 406.0 nm for a PhC lattice constant of 162.5 nm. The threshold current density was 9.7 kA/cm2 for a 120-μm-square p-contact electrode.

Aqueous synthesis of single-crystalline ZnO prisms on graphite substrates

Morphological change from ZnO films to ZnO prisms is achieved by an electrochemical deposition method on a graphite substrate at the low temperature of 70 °C. The ZnO prisms, which are prism-shaped ZnO rods with a wide diameter, have hexagonal well-defined crystallographic facets. The ZnO prism is 1.4 μm in diameter and 1.3 μm in length. Transmission electron microscopy and electron diffraction patterns indicate that the ZnO prisms have a single-crystalline wurtzite structure with c-axis orientation. Additionally, cathodoluminescence shows that the annealed ZnO prisms in nitrogen gas emit a significant level of near-band-edge ultraviolet light.

Fatigue resistance of the grain size transition zone in a dual microstructure superalloy disk

Mechanical property requirements vary with location in nickel-based superalloy disks. In order to maximize the associated mechanical properties, heat treatment methods have been developed for producing tailored microstructures. In this study, a specialized heat treatment method was applied to produce varying grain microstructures from the bore to the rim portions of a powder metallurgy processed nickel-based superalloy disk. The bore of the contoured disk consisted of fine grains to maximize strength and fatigue resistance at lower temperatures. The rim microstructure of the disk consisted of coarse grains for maximum resistance to creep and dwell crack growth at high temperatures up to 704 °C. However, the fatigue resistance of the grain size transition zone was unclear, and needed to be evaluated. This zone was located as a band in the disk web between the bore and rim. Specimens were extracted parallel and transverse to the transition zone, and multiple fatigue tests were performed at 427 and 704 °C. Mean fatigue lives were lower at 427 °C than 704 °C. Specimen failures often initiated at relatively large grains, which failed on crystallographic facets. Grain size distributions were characterized in the specimens, and related to the grains initiating failures as well as location within the transition zone. Fatigue life decreased with increasing maximum grain size. Correspondingly, mean fatigue resistance of the transition zone was slightly higher than that of the rim, but lower than that of the bore. The scatter in limited tests of replicates was comparable for all transition zone locations examined.

Photoelectrochemical Stability of Electrodeposited Cu2O Films

We report on the photoelectrochemical stability of electrodeposited films of cuprous oxide (Cu2O) in aqueous solutions. p-type Cu2O was electrodeposited from an alkaline lactate solution onto Au substrates with a (111) preferred orientation. The as-deposited films exhibit a strong (111) orientation and a columnar morphology, terminating at the surface with pyramidal features. Stability of the photoelectrochemical response was studied via repeated cycling polarization and by monitoring the photocurrent within a voltage window where only the CuO/Cu2O transformation would occur. The Cu2O film morphology changes from a dense structure to a more stable network of elongated leaf-like crystals. We attributed this behavior to the energy differences among the various crystallographic facets of Cu2O, with the {111} facets being the most stable. Photocurrent evolution over time was explained in terms of an initial increase in surface area at the start of the process, followed by a decrease due to increased recombina...

Femtosecond laser ablation of sapphire on different crystallographic facet planes by single and multiple laser pulses irradiation

Ablation of sapphire on different crystallographic facet planes by single and multiple laser pulses irradiation was carried out with a femtosecond pulsed laser operating at a wavelength of 780 nm and a pulse width of 164 fs. The quality and morphology of the laser ablated sapphire surface were evaluated by scanning electron microscopy and atomic force microscopy. For single laser pulse irradiation, two ablation phases were observed, which have a strong dependency on the pulse energy. The volume of the ablated craters kept an approximately linear relationship with the pulse energy. The threshold fluences of the two ablation phases on different crystallographic facet planes were calculated from the relationship between the squared diameter of the craters and pulse energy. With multiple laser pulses irradiation, craters free of cracks were obtained in the ‘gentle’ ablation phase. The threshold fluence for N laser pulses was calculated and found to decrease inversely to the number of laser pulses irradiating on the substrate surface due to incubation effect. The depth of the craters increased with the number of laser pulses until reaching a saturation value. The mechanism of femtosecond laser ablation of sapphire in two ablation phases was discussed and identified as either phase explosion, Coulomb explosion or particle vaporization. The choice of crystallographic facet plane has little effect on the process of femtosecond laser ablation of sapphire when compared with the parameters of the femtosecond laser pulses, such as pulse energy and number of laser pulses. In the ‘gentle’ ablation phase, laser-induced periodic surface structures (LIPSS) with a spatial period of 340 nm were obtained and the mechanism of the LIPSS formation is discussed. There is a potential application of the femtosecond laser ablation to the fabrication of sapphire-based devices.

Fatigue life prediction of small notched Ti–6Al–4V specimens using critical distance

This study investigated the method of estimating the fatigue strength of small notched Ti–6Al–4V specimen using the theory of critical distance that employs the stress distribution in the vicinity of the notch root. Circumferential-notched round-bar fatigue tests were conducted to quantify the effects of notch radius and notch depth on fatigue strength. The fatigue tests show that the larger notch radius increases the fatigue strength and the greater notch depth decreases the fatigue strength. The theory of critical distance assumes that fatigue damage can be correctly estimated only if the entire stress field damaging the fatigue fracture process zone is taken into account. Critical distance stress is defined as the average stress within the critical distance from notch root. The region from the notch root to the critical distance corresponds to the fatigue fracture process zone for crack initiation. It has been found that a good correlation exists between the critical distance stress and crack initiation life of small notched specimens if the critical distance is calibrated by the two notched fatigue failure curves of different notch root radii. The calibrated critical distances did not vary clearly over a wide range of fatigue failure cycles from medium-cycle low-cycle fatigue regime to high-cycle fatigue regime and have an almost constant value. This critical distance corresponds to the size of crystallographic facet at the fatigue crack initiation site for the wide range of fatigue cycles.

Growth of AlN single crystalline boules

We have obtained high-quality, crack-free AlN wafers using a convex thermal field inside the growth chamber. Free-standing AlN boules of 15 mm in height and 15 mm in diameter were grown. The carbon concentration was found to be similar in all parts of the boule (∼8×10 18 cm −3) while the initial O concentration was higher (∼1×10 19 cm −3) and slightly decreased during growth. It was found that O incorporated differently on different crystallographic faces. High resolution XRD showed a continuous improvement in crystal quality as a function of boule length. The full width at half maximum (FWHM) of the double crystal rocking curves decreased from 78 in at the beginning of growth to 13 in at the growth end. To the best of our knowledge, this is the first report on impurity incorporation on different crystallographic facets obtained from the same boule.

Self‐Assembly, Disassembly, and Reassembly of Gold Nanorods Mediated by Bis(terpyridine)–Metal Connectivity

Since the advent of nanotechnology, a primary challenge has been to assemble materials from predesigned building blocks for the fabrication of nanometer-sized materials. Metal nanocrystals (NCs) have attracted significant attention due to their unique shape and size, as well as their tuneable electrical, optical and catalytic properties. A bottomup approach has generally been used for constructing the desired multi-unit composites from individual components. Although extensive studies have been performed on spherically shaped NCs, research on anisotropic nanoparticles, for example, nanorods (NRs) with a distinct 1D or linear structure, is limited. Since the properties of NCs are mostly sizeand shape-dependent, NRs have the potential to show novel as well as improved properties, such as in surface-enhanced Raman scattering (SERS) effects or optical and fluorescent properties, when compared to isotropic nanospheres (NSs). c,3] Geometrically, the aspect ratio (the length of major axis divided by the width of minor axis) of NRs is >1 and their cofacial assembly produces more angularly complex structures than that of the NSs. Side-to-side assemblies of NRs have been achieved by introducing active sites on the sides of the rods and subsequently connecting via DNA, electrostatic interaction, and more recently with click chemistry; however, arrangement of NRs in an endto-end orientation, for example, forming chains or branched structures, requires a regiospecific functionalization. Toward this goal, the tips of NRs have been modified with different metals, which were subsequently termed nanodumbbells, for participation in direct end-to-end assembly. It is well-known that Au NRs have different crystallographic facets, which comprise the ends and side surfaces. Recently, it was found that these side facets have a higher surface energy, and in turn, adsorb more bilayer surfactant, for example, cetyltrimethylammonium bromide (CTAB); notably, the yield of these surface-modified nanorods has been shown to be supplier (CTAB) dependent. Current progress on direct end-to-end Au NR assembly has focused on using biocompatible connectors, a,w-dithiols, and synthetic polymers. As well, organometallic connections, such as bis(terpyridine)-transition metal complexes, are also of interest due to their potential to impart unique electrical, optical, and photovoltaic properties. For example, hybrid Au NCs and bis(terpyridine)–Ru complexes have been fabricated and shown to exhibit enhanced electrochemical properties. Herein, we report the first example of a bottom-up approach for the end-to-end linear and branched assembly of Au NRs into multicomponent structures using [(disulfide-modified terpyridine)2–M ] (M=Fe or Cd) interconnectors and their facile disassembly and reassembly. The synthesis of disulfide-modified monoterpyridine (SStpy) 2, the [(SS-tpy)2–M ] complexes 3 (M=Fe), and 4 (M= Cd) is shown in Scheme 1. In a typical Gabriel synthesis, 4’(4-hydroxyphenyl)-2,2’:6’2’’-terpyridine was treated with N-(3-bromopropyl)phthalimide to afford the intermediate protected amine, which was subsequently deprotected (H2NNH2·H2O) to give the amine-modified monoterpyridine 1. The structure was supported (H NMR) by signals for the new OCH2 (4.17 ppm) and CH2NH2 (3.20 ppm) moieties; a dominate peak (ESI-MS) at m/z 383.3 [M+H] further confirms the structure. Disulfide-modified monoterpyridine 2 [a] Y.-T. Chan, Prof. Dr. G. R. Newkome Departments of Polymer Science and Chemistry The University of Akron, Akron, OH 44325-4717 (USA) Fax: (+1)330-972-2413 E-mail : newkome@uakron.edu [b] Dr. S. Li Excel Polymers, 14330 Kinsman Rd. Burton, OH 44325 (USA) [c] Dr. C. N. Moorefield Maurice Morton Institute for Polymer Science The University of Akron, Akron, OH 44325 (USA) [d] Dr. P. Wang ChemNano Materials, 2220 High St. Suite 605 Cuyahoga Falls, OH 44221 (USA) [e] Prof. Dr. C. D. Shreiner Chemistry Department, Hiram College, P.O. Box 67 Hiram, OH 44234 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000040.

Effect of aging treatments on the fatigue crack growth and threshold behavior of AA 2219 aluminium alloy

The fatigue crack growth and threshold behavior of AA2219 Al-alloy has been examined for naturally aged (NA), under aged (UA), peak aged (PA) and over aged (OA) conditions. Significant differences were observed in the fatigue crack growth at low and high stress intensity ranges. The alloy in the NA condition possesses the highest resistance to fatigue crack initiation which can be viewed from the values of achieved threshold stress intensity range, ΔKTH. In the intermediate ΔK (Paris law) regime, all the four materials fall into a single band. In the high ΔK regime (> 20 MPa√m) PA condition exhibits better crack growth resistance than under aged conditions. The inhomogeneous transcrystalline slip in the UA condition results in the slower crack growth at low ΔK. The fracture morphology changes from crystallographic facets in the threshold region to clearly developed ductile striations in the Paris law regime to microvoid coalescence in the high ΔK region.

Gigacycle fatigue behavior of high strength aluminum alloys

Fatigue in the very long life regime has assumed great significance in recent years particularly after it has been established that a fatigue limit doesn’t exist in most cases. The paper deals with investigations of gigacycle fatigue and near threshold fatigue crack growth behaviors of AA7075-T6, AA6061-T6 and AA2024-T3 Al alloys using piezoelectric accelerated fatigue at 20KHz. Experiments were conducted in air at room temperature on hourglass-shaped specimens under completely reversed loading conditions (R=−1). It was noticed that failure continues to take place right up to the gigacycle (109 cycles) range, at nominal cyclic stresses. The high cycle fatigue fractures are generally rough in appearance and seen to initiate predominantly from the surface. Furthermore, they contain a significant number of voids, exhibit crystallographic faceting, and discernible fatigue striations. Gigacycle fatigue fracture of AA2024-T3, however, shows features of ductile tearing instead of striations. The studies also show that fatigue crack growth rates of small cracks are greater than those of large cracks for almost the same stress intensity factor range ΔK, and that some small cracks may grow at ΔK values below the large crack threshold in the near-threshold crack growth regime.

In situ observations of the nucleation and growth of atomically sharp graphene bilayer edges

Using in situ transmission electron microscopy, we observed the nucleation and growth of graphene bilayer edges (BLE) with “fractional nanotube”-like structure from the reaction of graphene monolayer edges (MLEs). Most BLEs showed atomically sharp zigzag or armchair crystallographic facets in contrast to the atomically rough MLEs with irregular shapes, suggesting that the BLEs are much more stable and crystallographically anisotropic. Our direct observations and theoretical studies (geometric models and ab initio calculations) provide important clues for tailoring the edge structure and transport properties of multi-layer graphene.

Body-centered cubic phase in 3-arm star mesogens: a torsional tapping AFM and GISAXS study

The mode of liquid crystal (LC) self-assembly of asymmetric three-arm star oligobenzoate mesogens has been investigated by torsional tapping mode AFM imaging, and by bulk and grazing-incidence X-ray diffraction. It was confirmed that the cubic liquid crystal phase has Imm symmetry and established that it consists of spherical aggregates arranged on a body-centered lattice, rather than having a bicontinuous structure. Molecular simulation suggests that, in spite of their perceived rigidity, the oligobenzoate mesogens are folded, often sharply, within the supramolecular spheres, and that they act effectively as conical objects. The recently introduced torsional tapping AFM technique has allowed high resolution and contrast to be obtained from the soft mobile surface of the samples, showing the (110) plane of highly ordered supramolecular spheres. Notably the fine structure of the observed steps reveals that nearly isolated micelles still preserve their integrity. Finally, the equilibrium habits of the cubic LC droplets were shown by optical microscopy to be polyhedral with crystallographic facets, rather than spherical. This suggests that the air–LC interface is below the roughening transition temperature, which is attributed to the whole micelles rather than the isolated molecules acting as the interacting units.

Growth of pentatwinned gold nanorods into truncated decahedra

The growth mechanism from pentatwinned (PTW) gold nanorods into truncated quasi-decahedral particles when a gold salt (HAuCl₄) is reduced by N, N-dimethylformamide (DMF) in the presence of poly(vinylpyrrolidone), was elucidated through a combination of different techniques, including transmission and scanning electron microscopy, high resolution TEM and selected area electron diffraction. Particles with intermediate shapes between the original pentatwinned Au nanorods, used as seeds, and the final quasi-decahedral particles were obtained by simply tuning the [HAuCl₄] to [seeds] ratio. From the thorough structural analysis of all the intermediate morphologies obtained, it was concluded that gradual morphology changes are related to the preferential growth of higher energy crystallographic facets. As a result of the particle growth and concomitant decreased anisotropy, a progressive blue-shift of the surface plasmon resonance bands of the nanoparticles was registered by vis-NIR extinction spectroscopy.

Shape control of platinum and palladium nanoparticles for catalysis

Platinum and palladium are important catalysts for a wide variety of industrial processes. With the increasing demands of these materials, the development of high-performance catalysts is an important area of research, and as a result, shape control synthesis has become one of the leading research focuses. This minireview surveys the different approaches in solution-phase synthesis that have been successfully adopted for achieving shaped platinum and palladium nanoparticles that are enclosed with specific crystallographic facets. In addition, catalytic studies of the shaped nanoparticles are highlighted, in which promising results have been reported in terms of enhanced activity and selectivity. The future outlook discusses the aspects in synthesis and catalysis to be considered for the development of highly efficient and effective catalysts.

Fatigue crack nucleation and growth behaviors of laser melting deposited Ti–6Al–2Zr–Mo–V

The fatigue crack nucleation and growth behaviors of laser melting deposited (LMD) Ti–6Al–2Zr–Mo–V alloy are studied. A fine α/β lamellar microstructure is obtained. High-cycle fatigue test was conducted at room temperature. Fatigue fracture surfaces are examined by SEM. Fatigue crack initiates by cleavage fracture along crystallographic facets in α and propagates by cleavage facets and tearing of α/β lamellar colonies in fatigue crack initiation region. At applied stress less than 750 MPa, fatigue crack nucleation is characterized by the crystallographic cleavage facets of α lamellae and tearing of β lamellae. Striations are observed parallel or perpendicular to the lamellar orientation. Two types of steps are observed in the crack propagation. The mechanisms of fatigue nucleation and growth in laser deposited Ti–6Al–2Zr–Mo–V alloy are related to the slip behaviors and the interactions between crack and colony boundaries.

Evolution of the defect structure of reverted austenite upon pearlite transformation in a steel 120G4

Evolution of the microstructure of a steel 120G4 after pearlite transformation and subsequent heating to temperatures of 680–1200°C has been investigated by transmission electron microscopy. It has been established that with a rise in the heating temperature the fraction of the carbide phase decreases and the austenite defect structure changes from microtwins and stacking faults to dislocations. Upon heating to T = 700–720°C, in the restored (reverted) austenite, wide twins with a regular crystallographic faceting, whose appearance is seemingly connected with the specific features of the pearlite transformation rather than with relaxation of internal stresses, have been discovered experimentally for the first time.