Correlating dynamics and selectivity in adsorption of semiconductor nanocrystals onto a self-organized pattern

Selective adsorption of semiconductor nanocrystals onto an organic self-organized pattern shows a time-dependent behavior. By studying the wetting behavior of delivered solvent (1-phenyloctane) on a lipid self-organized pattern and determining the adhesion energy between semiconductor nanocrystals and substrate, we obtain a correlation between dynamics and selectivity in adsorption of semiconductor nanocrystals onto the pattern by constructing a potential energy landscape. Two consecutive steps for selective adsorption of nanocrystals onto the self-organized pattern have been established: the first one is the molecule exchange of 1-phenyloctane and lipid molecules to form the adsorption sites for nanocrystals, and the second one is the adsorption of nanocrystals onto the adsorption sites due to the strong interaction between nanocrystals and substrate.     

Dynamical properties of the relaxor ferroelectric 0.71Pb(Ni1/3Nb2/3))O3-O.29PbTiO3 probed by Brillouin and Raman scattering

Abstract-Structural phase transitions in relaxor ferroelectric materials remain to be one of the most puzzling issues in materials science. In the present work, we have investigated relaxational and vibrational properties of a relaxor ferroelectric 0.71Pb(Ni(1/3)Nb(2/3))O(3)-O.29PbTiO(3) single crystal by means of inelastic light scattering measurements. Analysis based on the spectra has shown anomalous behaviors at Burns' temperature T(d), freezing temperature T(f) , those are related to a development of polar nanoregions (PNRs). The PNR must play a key role in the relaxor nature, and obtained results provide new insights into the relaxational dynamics in the PNRs and into how they develop below T(f).     

Self-assembly of charged microclusters of CdSe/ZnS core/shell nanodots and nanorods into hierarchically ordered colloidal arrays

A thermodynamically driven self-organization of microclusters of semiconductor nanocrystals with a narrow size distribution into periodic two-dimensional (2D) arrays is an attractive low-cost technique for the fabrication of 2D photonic crystals. We have found that CdSe/ZnS core/shell quantum dots or quantum rods, transferred in aqueous phase after capping with the bifunctional surface-active agent DL-cysteine, form on a poly-L-lysine coated surface homogeneously sized micro-particles, droplet-like spheroid clusters and hexagon-like colloidal crystals self-organized into millimetre-sized 2D?hexagonal assemblies. The presence of an organic molecular layer around the micro-particles prevents immediate contact between them, forming an interstitial space which may be varied in thickness by changing the origin of the molecular layer capping nanocrystals. Due to the high refractive index of CdSe and the low refractive index of the interstitial spaces, these structures are expected to have deep gaps in their photonic band, forming hierarchically ordered 2D arrays of potentially photonic materials.     

Misfit accomodation in epitaxial monolayers on (111) f.c.c. and (110) b.c.c. substrates II: Analytical approach to monolayers in the Nishiyama-Wassermann orientation

In this paper we analyse the stability of (111) f.c.c. ultrathin — of the order of one monolayer-overlayers in the Nishiyama-Wassermann orientation on (110) b.c.c. substrates as a function of lattice parameters and bonding, using a model which has been improved in two respects compared with Part I: the generalization of isotropy in the monolayer to anisotropy and an increase in the number of Fourier terms representing the periodic adsorbate-substrate interaction potential. Analytical tractability required the introduction of constraints which prohibit the determination of the exact conditions for stability. The essential outcome of the theory is the phase boundaries separating the regions of different misfit accomodation modes between overlayer and substrate. Although the model is not an exact representation of the physical system and the analysis is not quantitatively exact either, the predictions should act as excellent guidelines in the search for the properties of such systems.     

R.F. sputtered β-tantalum and b.c.c. tantalum films

Tantalum films have been deposited on both glass and Ta₂O₅-coated substrates by r.f. sputtering. The structure, resistivity ρ and temperature coefficient of resistance TCR of films sputtered in pure argon or reactively in Ar-O₂ or Ar-N₂ have been determined. In pure argon on sufficiently preheated substrates a very pure b.c.c. Ta phase (α^∗-Ta), with ρ∼25 μΩ cm and TCR∼1500 ppm/°C, is formed. Sputtering on substrates which are only slightly preheated or are heated solely by the sputtering process yields a tetragonal Ta phase (β-Ta) with ρ∼165 μΩ cm and TCR∼-160 ppm/°C. With added oxygen the β−Ta phase is produced over the whole partial pressure range investigated. Low partial pressures of nitrogen lead to β-Ta. At higher nitrogen partial pressures b.c.c. Ta (α-Ta) with ρ∼50 μΩ cm and TCR∼650 ppm/°C is formed and finally the plateau region is observed.     

Ion bombardment effects in plasma deposition of hydrogenated amorphous silicon carbide films: A comparative study of d.c. and r.f. discharges

The structure and the properties of hydrogenated amorphous silicon carbide films produced at room temperature by d.c. and r.f. glow discharge decomposition of silane and ethylene were studied with a systematic control of the ion flux at the surface of the growing film. The composition and structure of the films were monitored by measuring their IR absorption, their refractive index and their optical gap. The ion fluxes were determined from the saturation current of a small grid probe located in the substrate holder.It was found that d.c. cathodic and r.f. films show an inorganic structure with a dispersed carbon phase while d.c. anodic films exhibit mainly hydrogenated carbon clusters. These structural changes are thought to result from differences in the energies of the bombarding ions.The versatility of the r.f. and d.c. proximity discharges in comparison with d.c. discharges (anodic and cathodic films) is also emphasized.     

Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

The effects of laser irradiation on γ-Fe₂O₃ 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe₂O₃ phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe₂O₃ nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe₂O₃ is an intermediate phase between γ-Fe₂O₃ and α-Fe₂O₃ and a series of distinct Raman vibrational bands were identified with the ε-Fe₂O₃ phase. The structural transformation of γ-Fe₂O₃ into α-Fe₂O₃ occurs either directly or via ε-Fe₂O₃, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.     

Correlation effects of chaotic and phase-diffusion fields on polarization beats in a V-type three-level system

Abstract

The correlation effects of fourth-order on polarization beats in a V-type three-level system (PBVTS) are investigated using chaotic and phase-diffusion models. It has been found that the temporal behaviour of the beat signal depends on the stochastic properties of the lasers and transverse relaxation rate of the transitions, and the modulation terms of the beat signal depend on the second-order coherence function, which is determined by the laser line shape. Since different stochastic models of the laser field affect only the fourth-order coherence function, they have little influence on the general temporal modulation behaviour of the beat signal. The different roles of the phase fluctuation and amplitude fluctuation have been pointed out in the time domain. The fourth-order coherence function theory of chaotic field is of vital importance in PBVTS. The cases where the pump beams have either narrow band or broadband linewidth are considered and it has been found that for both cases a Doppler-free precision in the measurement of the energy-level splitting between two states can be achieved. Finally, the difference between the PBVTS and DeBeer's ultrafast modulation spectroscopy is discussed as well from a physical viewpoint.     

Fatigue behaviour from application of a.c.

High intensity a.c. is increasingly used in many applications; its use is coming under discussion owing to the thermal fatigue effects that are caused. This work is an investigation of the effect of a.c. on the static strength and fatigue properties of mild steel specimens. Pre-application of a.c. to the specimens did not cause any appreciable change in the static properties such as yield point, ultimate strength and amount of elastic elongation. Slight reduction in the total strain has been measured. Pre-application of a.c. to fatigue tested specimens considerably reduced the endurance limit, an effect similar to that found in cumulative fatigue for prestressed materials. The amount of reduction increases as the intensity of applied a.c. increases.     

Variation of cutting forces in machining of f.c.c. single crystals

In this study, micro-machining of f.c.c. single-crystal materials was investigated based on a hybrid modelling approach combining smoothed particle hydrodynamics and continuum finite element analysis. The numerical modelling was implemented in the commercial software ABAQUS/Explicit by employing a user-defined subroutine VUMAT for a crystal plasticity formulation to gain insight into the underlying mechanisms that drive a plastic response of materials in high deformation processes. The numerical studies demonstrate that cutting force variations in different cutting directions are similar for different f.c.c. crystals even though the magnitudes of the cutting forces are different.     

Exploring for 3D photonic bandgap structures in the 11 f.c.c. space groups

The promise of photonic crystals and their potential applications has attracted considerable attention towards the establishment of periodic dielectric structures that in addition to possessing robust complete bandgaps, can be easily fabricated with current techniques. A number of theoretical structures have been proposed. To date, the best complete photonic bandgap structure is that of diamond networks having Fd3m symmetry (2-3 gap). The only other known complete bandgap in a face-centred-cubic (f.c.c.) lattice structure is that of air spheres in a dielectric matrix (8-9 gap; the so called 'inverse-opal' structure). Importantly, there is no systematic approach to discovering champion photonic crystal structures. Here we propose a level-set approach based on crystallography to systematically examine for photonic bandgap structures and illustrate this approach by applying it to the 11 f.c.c. groups. This approach gives us an insight into the effects of symmetry and connectivity. We classify the F-space groups into four fundamental geometries on the basis of the connectivity of high-symmetry Wyckoff sites. Three of the fundamental geometries studied display complete bandgaps--including two: the F-RD structure with Fm3m symmetry and a group 216 structure with F43m symmetry that have not been reported previously. By using this systematic approach we were able to open gaps between the 2-3, 5-6 and 8-9 bands in the f.c.c. systems.     

Synthesis of Al-Cr-O-N thin films in corundum and f.c.c. structure by reactive r.f. magnetron sputtering

Advanced PVD coatings for metal cutting applications must exhibit a multifunctional property profile including high hardness, chemical inertness and high temperature stability. Recently, ternary Al-Cr-O thin films with mechanical properties similar or superior to conventional aluminium oxide thin films have been suggested as potential materials meeting such demands. These coatings can be deposited at moderate temperatures in PVD processes. In this work, new quaternary Al-Cr-O-N coatings are suggested as alternative for offering thin film materials of high strength, hardness and even toughness. A combinatorial approach to the synthesis of Al-Cr-O-N thin films by means of reactive r.f. magnetron sputtering is presented. A thorough phase analysis of deposited coatings covering a wide range of elemental compositions revealed a well-defined phase transition from a corundum-type α-(Al_1 − x,Cr_x)_2 + δ(O_1 − y,N_y)₃ structure to a CrN-type f.c.c.-(Al_1 − x,Cr_x)_1 + θ(O_1 − y,N_y) structure as a function of the Al/Cr ratio and the nitrogen gas flow ratio. Detailed results on the coatings composition, constitution and microstructure are discussed compared to ternary Al-Cr-O thin films deposited by reactive r.f. magnetron sputtering under nearly identical conditions.     

A Shape‐Induced Orientation Phase within 3D Nanocrystal Solids

When nanocrystals self assemble into ordered superstructures they form functional solids that may inherit the electronical properties of the single nanocrystals. To what extent these properties are enhanced depends on the positional and orientational order of the nanocrystals within the superstructure. Here, the formation of micrometer‐sized free‐standing supercrystals of faceted 20 nm Bi nanocrystals is investigated. The self‐assembly process, induced by nonsolvent into solvent diffusion, is probed in situ by synchrotron X‐ray scattering. The diffusion‐gradient is identified as the critical parameter for controlling the supercrystal‐structure as well as the alignment of the supercrystals with respect to the substrate. Monte Carlo simulations confirm the positional order of the nanocrystals within these superstructures and reveal a unique orientation phase: the nanocrystal shape, determined by the atomic Bi crystal structure, induces a total of 6 global orientations based on facet‐to‐facet alignment. This parallel alignment of facets is a prerequisite for optimized electronic and optical properties within designed nanocrystal solids.     

Magnetic properties of iron-base b.c.c. alloys produced by mechanical alloying

The structure and magnetic properties of Fe-M (M = Zr, Hf, Co or Si) alloy powders produced by mechanical alloying (MA) of elemental powders or a mixture of elemental powders and alloy powders have been examined. The MA Fe-Zr and Fe-Hf powders have a non-equilibrium b.c.c. phase in the composition range above 90 at% Fe. The coercivity of the MA Fe-Zr and Fe-Hf powders exhibits a minimum value (1300 A m⁻¹) combined with a high magnetization of 2.2×10⁻⁴ Wb m kg⁻¹ at 95 at % Fe, which corresponds to the concentration of zero magnetostriction. Although the compacts made from the MA b.c.c. Fe-Zr powders do not exhibit good soft magnetic properties, the permeability of the compacts made from the MA Fe-Co powders annealed at 1173 K for 18 ks in H₂ is nearly the same as that of the alloy ingot produced by conventional casting followed by the same annealing treatment. On leave from ALPS Electric Co. Ltd, Nagaoka 940, Japan.     

Evaporation and deposition of alkyl-capped silicon nanocrystals in ultrahigh vacuum

Nanocrystals are under active investigation because of their interesting size-dependent properties and potential applications. Silicon nanocrystals have been studied for possible uses in optoelectronics, and may be relevant to the understanding of natural processes such as lightning strikes. Gas-phase methods can be used to prepare nanocrystals, and mass spectrometric techniques have been used to analyse Au and CdSe clusters. However, it is difficult to study nanocrystals by such methods unless they are synthesized in the gas phase. In particular, pre-prepared nanocrystals are generally difficult to sublime without decomposition. Here we report the observation that films of alkyl-capped silicon nanocrystals evaporate upon heating in ultrahigh vacuum at 200 degrees C, and the vapour of intact nanocrystals can be collected on a variety of solid substrates. This effect may be useful for the controlled preparation of new quantum-confined silicon structures and could facilitate their mass spectroscopic study and size-selection.     

Crystalline structures and misfit strain inside Er silicide nanocrystals self-assembled on Si(001) substrates

The morphology and crystalline structure of Er silicide nanocrystals self-assembled on the Si(001) substrate were investigated using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). It was found that the nanowires and nanorods formed at 630?°C has dominant hexagonal AlB(2)-type structure, while inside the nanoislands self-organized at 800?°C the tetragonal ThSi(2)-type structure is prevalent. The lattice analysis via cross-sectional high-resolution TEM demonstrated that internal misfit strain plays an important role in controlling the growth of nanocrystals. With the relaxation of strain, the nanoislands could evolve from a pyramid-like shape into a truncated-hut-like shape.     

Measurements of the Degree of Comprehensive Cooling in Stochastically Quenched Microstructures

We clarify the atomistic behavior of a micromechanical structure whose thermally driven stochastic motion has been quenched, using force-feedback techniques. The quenching is observed, via both qualitative and quantitative measurements, to optically clamp one of the vibrational modes of the lever such that the overall body temperature is only reduced slightly. The degree of comprehensive cooling is gauged by examining the reduction in the stochastic vibration of the third vibrational mode of a doubly clamped lever, while the first is quenched, to 143 K. The observation of only slight temperature reductions is confirmed by noting the absence of a phase change in condensing water vapor on a cantilever, although the deflection-magnitude of the fundamental vibrational mode is reduced to an effective temperature of 11 K. Finally, the measured stochastic variation rate is consistent with the lever's mechanical properties, not its thermal properties, demonstrating near-room temperature operation. The results thus imply that each vibrational mode can be reduced to deep sub-Kelvin temperatures independent of the overall thermal state of the lever, thus enabling sub-Brownian sensing in applications such as chemical and radiation detection, and quantum superposition experiments     

Effects of phosphorus doping on structural and optical properties of silicon nanocrystals in a SiO2 matrix

Promise of Si nanocrystals highly depends on tailoring their behaviour through doping. Phosphorus-doped silicon nanocrystals embedded in a silicon dioxide matrix have been realized by a co-sputtering process. The effects of phosphorus-doping on the properties of Si nanocrystals are investigated. Phosphorus diffuses from P-P and/or P-Si to P-O upon high temperature annealing. The dominant X-ray photoelectron spectroscopy P 2p signal attributable to Si-P and/or P-P (130 eV) at 1100 °C indicates that the phosphorus may exist inside Si nanocrystals. It is found that existence of phosphorus enhances phase separation of silicon rich oxide and thereby Si crystallization. In addition, phosphorus has a considerable effect on the optical absorption and photoluminescence properties as a function of annealing temperature.     

Sharp cubic and octahedral morphologies of poly(vinylpyrrolidone)-stabilised platinum nanoparticles by polyol method in ethylene glycol: their nucleation,growth and formation mechanisms

Pt nanoparticles (NPs) were synthesised by a modified polyol method with the addition of silver nitrate. The results showed that the specific shapes of Pt NPs were influenced by the relevant factors, which are the contents of silver nitrate, synthetic time and temperature. A small content of silver nitrate has played an important role in determining their final shapes of platinum NPs. We observed that Pt NPs in the forms of very sharp shapes such as Pt cubes, octahedrons, cuboctahedrons and tetrahedrons have been obtained. In addition, the shape growth mechanisms and formation of Pt NPs have been studied. They exist in both cubic and octahedral shapes. Importantly, Pt nanocrystals can grow into main cubic and octahedral shapes for a short time less than 15?min. Moreover, Pt nanocrystals can also grow into different shapes from cubic and octahedral into spherical ones for several hours. Especially, they exhibited interesting shapes of multiple-branched Pt nanostructures because of their overgrowth and aggregations. Clearly, large cubic and octahedral Pt NPs of 160?nm diameter were observed. The growth and formation of large cubic and octahedral Pt NPs were due to the aggregation of Pt clusters or initial Pt seeds, even small Pt nanocrystals.