Formation Of Nanocrystallites Research Articles

Morphology, Particle Size Distribution, Aggregation, and Crystal Phase of Nanocrystallites in the Urine of Healthy Persons and Lithogenic Patients

The first step in urinary stone formation is the nucleation of urinary mineral from supersaturated urine. The formed nuclei then grow or/and aggregate to a pathological size. Thus, the nanocrystallites in urine may be related to the formation of urinary stones. Nanocrystallites with a size of less than 1000 nm in the urine samples of 85 healthy persons and 65 lithogenic patients were comparatively investigated using laser scattering spectroscopy, TEM, and X-ray diffraction. Most of the nanocrystallites in healthy urine samples were spheroidal, less aggregated, well-dispersed, and with a narrow particle size distribution from about 20 to 350 nm. In contrast, most of the particles in lithogenic urines had sharply angled edges and tips, were aggregated, and had a broad particle size distribution from 1.1 to 1000 nm. More calcium oxalate dihydrate (COD) nanocrystallites were found in healthy urine; however, more calcium oxalate monohydrate (COM) nanocrystallites were found in lithogenic urine. The morphology, particle size, aggregation, and crystal phase of nanocrystallites in the urine of lithogenic patients are pronouncedly different from those of healthy persons. The results suggest, in ascending order of importance, that making nanocrystallites rounded, diminishing their size differentiation, and decreasing their aggregation in urine by physical and chemical methods maybe the means to prevent urinary stone formation. The most crucial among the four differences is the crystal phase differential of calcium oxalate (CaOxa). That is, the formation of COD nanocrystallites in urine can be considered as being relatively more favorable in preventing stone formation than the formation of COM nanocrystallites, which are in accord with those found for larger crystallites.

Fine particles in amorphous silicon germanium thin films

AbstractWe have regulated the number density and the size of the fine particles/clusters called nanoparticles into the a‐SiGe:H films deposited from silane and germane diluted in hydrogen by applying a square wave pulse modulations (SWPM) of the rf (13.56 MHz) amplitude in plasma enhanced chemical vapor deposition (PECVD). The plasma “on” time Ton and “off” time Toff were controlled over a wide range from 300 μsec to 30 msec. Growth rate (γr) of the number density (Nr) of the particles of size r shows a lognormal dependence on r. Tendency towards formation of nanocrystallites was observed on reducing the Ton. Ge content within nanocrystallites was found to increase with decrease in nanocrystallites size. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanocrystallites formation in a‐SiC by low power plasma enhanced chemical vapour deposition

AbstractFormation of nanocrystallites in the amorphous silicon carbide thin films deposited from silane methane mixture diluted in argon by plasma enhanced CVD has been studied under different levels of rf power density and Ar dilution. Systematic transformations between different polymorphic phases of SiC have been observed with changes in rf power and Ar dilution. At low Ar dilution no H‐SiC (α‐SiC) nanocrystallites was observed. Only traces of β‐SiC of Si were observed. With increase in Ar dilution formation of α‐SiC nanocrystallites was observed. Increase of rf power density at this dilution enhanced α‐SiC nanocrystallites but reduced β‐SiC slightly. Variation of SiC peak in the infra red absorption corresponds closely with the changes in β‐SiC nanocrystallites in the layers. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of Si Nanocrystallites in Al-Added Amorphous Si Films by Electron Beam Irradiation

Si nanocrystallites were formed in Al-added amorphous Si films (Al/a-Si, a-AlxSi1-x) by the irradiation of a focused electron beam. In-situ heating of the a-Al0.025Si0.975 films was performed at temperatures up to 400 °C. The size, shape, and concentration of the Si crystallites varied significantly with the film temperature and electron beam irradiation time. Si nanocrystallites with a mean size of ∼10 nm were formed when the films were kept at 200 °C, and irradiated using an electron beam with a current density of 15.7 pA/cm2. The total crystallite volume fraction in the films increased from ∼9.2 to ∼94.8% with increasing temperature from 100 to 400 °C. The estimated activation energy for the crystallization in the a-Al0.025Si0.975 film under the electron beam irradiation was 0.78±0.05 eV.

10.1007/s11451-008-2023-y

Growth and the optical properties of epitaxial heterostructures Si(111)/(CrSi2 nanocrystallites)/Si(111) based on nanosized islands of chromium disilicide (CrSi2) on Si(111) were studied using low-energy electron diffraction, atomic-force microscopy, and optical reflection and transmission spectroscopy. The heterostructures with thicknesses of 0.1, 0.3, 0.6, 1.0, and 1.5 nm were formed by reactive epitaxy at a temperature of 500°C followed by the epitaxial growth of silicon at 750°C. The specific features of changes in the density and sizes of CrSi2 islands on the silicon surface were determined at T = 750°C as the chromium layer thickness was increased. It was established that, in the heterostructures with chromium layer thicknesses exceeding 0.6 nm, a small part of faceted Cr2Si2 nanocrystallites (NCs) emerge into near-surface region of the silicon, which is confirmed by the data from optical reflectance spectroscopy and an analysis of the spectral dependence of the absorption coefficient. A critical size of NCs is shown to exist above which their shift to the silicon surface is hampered. The decreased density of emerging NCs at chromium layer thicknesses of 1.0–1.5 nm is associated with the formation of coarser NCs within a silicon layer, which is confirmed by the data from differential reflection spectroscopy.

Microstructure and mechanical properties of magnetron co-sputtered Ni–Al coatings

Binary Ni–Al alloy coatings were fabricated by magnetron co-sputtering with dual targets of pure Ni and Al metals. The chemical composition variation, microstructure evolution and related properties of the coatings caused by sputtering power modulation were investigated. The Ni–Al coatings with Al contents ranging from 2.7 to 24.9 at.% were manipulated through dual-gun sputtering. The as-deposited Ni–Al films revealed crystalline and nano-crystalline microstructures with respect to sputtering powers. Microstructure consisting of a significant crystallization feature of a metastable (Ni,Al) matrix and Ni 3Al precipitations was observed for the coatings under high sputtering powers, while coatings deposited at lower sputtering powers exhibited a (Ni,Al) solid solution polycrystalline structure with (111) preferred orientation. The coatings with (Ni,Al) matrix and Ni 3Al precipitations exhibited a higher hardness than those with a single (Ni,Al) phase. The formation of (Ni,Al) nano-crystallites and Ni 3Al phases was the strengthening mechanism for Ni–Al coatings deposited under high energy input during sputtering. With the increase in Al sputtering power, a slight decrease in grain size due to a higher Al concentration in the Ni–Al coatings was observed. The variation in hardness of the magnetron co-sputtered Ni–Al coatings could be attributed to the grain size and microstructure evolution.

Fine sructure of Na5Lu9F32 nanocrystallites formed at the initial stages of crystallization

X-ray diffraction investigations have demonstrated that the initial stages of the formation and growth of Na5Lu9F32 nanocrystallites from a powder mixture of lutetium fluoride, ammonium fluoride, and sodium fluoride during the solid-phase synthesis are accompanied by complex structural transformations, among which a cyclic transformation of the type single-phase → two-phase → single-phase state can be distinguished. It is shown that the two-phase state consists of two isomorphous phases with a small misfit of the lattice parameters. The isomorphous phases have common extended interphase boundaries, along which a continuous change of the lattice parameters occurs as one phase transforms into the other. A long-term annealing of the two-phase state leads to the final single-phase state, which is characterized by the growth of nanocrystallites to the scale of microcrystallites.

Magnetic and sensitive magnetoelastic properties of Finemet nanostructured ribbon

Soft-magnetic Fe73.5Cu1Nb3Si13.5B9 (Finemet) ribbon has been fabricated by using melt-spinning techniques. After annealing at suitable temperature the ribbon changes from an amorphous to crystalline state which related to the formation of Fe nanocrystallites. Study on the magnetic and magnetoelastic properties of the ribbon is presented. Furthermore, based on the fabricated ribbon stress sensors are simply constructed. The sensors showed high sensitivity of 3.8 mV/MPa as well as a wide working range up to 17 MPa. These sensors are potential for practical applications such as detecting small stress and movement in civil structures.

A novel femtosecond-laser formation of CdS nanocrystallites in zirconia matrices

A novel method for direct laser writing of two-dimensional cadmium sulfide (CdS) semiconductor nanoparticle microstructures is reported. A two photon or a higher-order multiphoton absorption process, originating from femtosecond laser pulses, was used to decompose CdS precursors dispersed in a zirconia thin film previously dip-coated on a glass substrate. The kinetics of nanoparticle formation as a function of laser power were monitored in situ by photoluminescence spectroscopy. Raman spectroscopy was also performed to characterize the structural changes of the zirconia matrix under irradiation and to verify the formation of CdS nanoparticles. Results show that CdS nanoparticles were formed by two-photon absorption (TPA) with or without the help of an additional carbazole photoinitiator.

Study of the atomic structure and dynamics of Cu-Fe nanocomposites by X-ray diffraction and inelastic neutron scattering

The atomic structure and the dynamics of Cu1 − x Fe x samples (x = 0.15, 0.5, and 0.85) obtained by melt spinning from the elements that are immiscible under standard conditions has been investigated. The change in the physical properties of the system obtained in this way is due to the formation of nanocrystallites.

Ion beam induced modification in GeO x thin films: A phase separation study

Formation of Ge crystallites in GeO x matrix as a result of heavy ion irradiation is reported. Micro-Raman spectra of the films show the evolution of Ge crystallite regions in GeO x thin films upon ion irradiation with 100 MeV Au. Transmission electron microscopic studies of the irradiated films revealed the presence of Ge crystallites. Crystallinity of the Ge nanoparticles was confirmed by high-resolution electron microscope images. Atomic force microscopy was employed to study the modifications in surface morphology of GeO x films before and after irradiation. Formation of Ge nanocrystallites has been explained on the basis of phenomenon of ion beam induced phase separation.

Photoluminescence of Silicon Nanocrystals Embedded in Silicon Oxide

The bonding structure and the optical properties of silicon-rich silicon oxide films which were prepared using plasma-enhanced chemical vapor deposition (PECVD) with thermal oxidation were studied in detail. The composition and bonding structures were explored using X-ray photoelectron spectroscopy (XPS). The luminescent properties were studied using Raman spectroscopy and photoluminescence (PL) measurements. Results show that high-temperature (approximately 1000 degrees C) annealing of as-deposited Si-rich silicon oxide gives rise to phase separation and formation of crystalline Si phases in the oxide films. Upon high temperature annealing, the PL (550-900 nm) intensity was significantly reduced in stoichiometric oxide or in low Si-content (prepared with [N2O]/[SiH4] gas flow ratio > 2.5) films whereas an opposite trend was found in higher Si-rich samples. Particularly, for the red band (650 nm) and the infrared band (750 nm), the PL intensity was significantly enhanced by annealing slightly Si-rich samples ([N2O] / [SiH4] = 2.5) at 1100 degrees C. This observation was explained by the formation of Si nanocrystallites via phase separation reaction and the removal of defect-related absorption centers during annealing.

Structural study of device quality silicon germanium thin films deposited by pulsed RF plasma CVD

Microstructures in silicon germanium thin films deposited by pulsed rf plasma CVD have been studied with the help of small-angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM). With lowering of the pulse duty cycle the size of the particles incorporated in the films from the plasma decreases. However, the particles become more symmetric in shape and crystalline in nature. At 75% duty cycle the films have the highest photosensitivity. The increase in SAXS scattering at 75% has been explained by the formation of uniform-size nanocrystallites of SiGe. Urbach energy variation with the duty cycle also suggests the formation of nanocrystallites.

Photoluminescence from high-pressure-annealed silicon dioxide

Effect of treatment at high temperature (HT) (up to 1200°C) under high hydrostatic pressure (HP) (up to 1.5GPa) on the photoluminescence (PL) of silicon dioxide films was investigated. The authors found that the PL intensity of the oxide films grown in pure oxygen can be significantly enhanced by the HT-HP treatment. Four PL peaks, at wavelengths of about 570, 620, 720, and 950nm, were found. The 570 and 950nm peaks are attributed to the defect centers and amorphous Si, respectively. The luminescence at 620 and 720nm is attributed to the quantum confinement effect involving the silicon nanocrystallites embedded in the oxide film. The PL intensities are strongly governed by the preparation conditions of the as-grown oxide layer as well as by the annealing conditions. Raman study indicates that both amorphous and crystalline Si phases coexist in the HT-HP processed samples. The formation of Si nanocrystallites is attributed to the phase separation effect involving Si suboxide.

Room temperature synthesis of 1-hexanethiolate capped [formula omitted] quantum dots, in Triton X-100 water-in-oil microemulsions

1-Hexanethiolate capped copper(I) selenide quantum dots were synthesized at room temperature by the method of arrested precipitation in Triton X-100, water-in-oil microemulsions. A shoulder at 450 nm observed in the UV–vis spectrum was associated to the formation of size-quantized particles. X-ray and electron diffraction analysis revealed the formation of nanocrystallites of cubic Cu 2 − x Se (Berzelianite) phase having a lattice constant of 5.70 ± 0.07 Å. This phase has been otherwise known to form at room temperature, only with the assistance of ultrasound/microwave/ γ -radiation or in presence of excess selenium. Using Debye–Scherrer equation, the average particle size was estimated to be ca. 7.3 nm. Two major peaks at 1114 cm −1 and 619 cm −1 in the FT-IR spectra, were fitted to Cu–S stretching and bending, respectively. It suggests the formation of stable Cu–S bond between surface copper ions and the adsorbed 1-hexanethiolate.

Growth mechanism of nanocrystalline silicon at the phase transition and its application in thin film solar cells

Nanocrystalline (or microcrystalline) silicon (nc-Si:H or μc-Si:H) is an absorber material that is crucial for obtaining thin film silicon tandem solar cells with high efficiency. This material is conventionally produced by direct plasma-enhanced chemical vapor deposition (PECVD), which is based on the dissociation of silane (SiH 4) and hydrogen (H 2) in a radiofrequency (rf) plasma. During the last few decades, the plasma deposition parameter regime has been explored intensively, in particular to improve the quality of nc-Si:H and to increase its deposition rate. The desired formation of nanocrystallites occurs under growth conditions close to the transition regime. A technique that is not plasma based is hot-wire CVD (HWCVD), which is based on catalytic decomposition of SiH 4 and dissociation of H 2 gases on a hot filament. Because source gases are catalytically decomposed, the method is often referred to as catalytic CVD (Cat-CVD). The HWCVD technique has been shown to be a viable method for the deposition of silicon-based thin films and solar cells. We discuss the progress at our laboratory. Specifically, recent single junction and multijunction n–i–p-type solar cells are highlighted. Improvements have been achieved by addressing, among others, the morphology of the light-scattering rough surfaces, to avoid cavities and shunting paths, and by studying the structural evolution of the nanocrystalline phase during growth. The obtained solar cells are highly stable against light soaking.

Deposition of nano‐crystalline lead chalcogenide thin films using a simple electrochemical technique

AbstractNanocrystalline thin films of lead chalcogenides (PbS, PbSe and PbTe) were deposited on transparent conducting oxide (TCO) coated glass substrates at room temperature using a simple electrochemical (galvanic) technique, from suitable electrolytic solutions and without the application of any external bias. X‐ray characterization revealed that PbS, PbSe and PbTe thin films were all of cubic phase. The formation of nanocrystallites with dense and compact surface morphology was detected from FESEM and AFM measurements. The nanocrystalline films showed ‘blue shifted’ IR absorption. Hall measurements showed the materials to be p‐type in nature with carrier concentration in the range 1019 – 1020/cm3. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface nanostructure formation mechanism of 45 steel induced by supersonic fine particles pombarding

By means of supersonic fine particles bombarding (SFPB), a nanostructured surface layer up to 15 μm was fabricated on a 45 steel plate with ferrite and pearlite phases. To reveal the grain refinement mechanism of SFPB-treated 45 steel, microstructure features of various sections in the treated surface were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Grain size increases with an increase of depth from the treated surface. Plastic deformation and grain refinement processes are accompanied by an increase in strain. Plastic deformation in the proeutectoid ferrite phases has precedence over the pearlite phases. Grain refinement in the ferrite phases involves: the onset of dislocation lines (Dls), dislocation tangles (DTs) and dense dislocation walls (DDWs) in the original grains; the formation of fine lamellar and roughly equiaxed cells separated by DDWs; by dislocation annihilation and rearrangement, the transformation of DDWS into subboundaries and boundaries and the formation of submicron grains or subgrains; the successive subdivision of grains to finer and finer scale, resulting in the formation of highly misoriented nano-grains. By contrast, eutectoid cementite phase accommodated strain in a sequence as follows: onset of elongated, bended and shear deformation under deformation stress of ferrites, short and thin cementites with a width of about 20-50 nm and discontinuous length were formed. Shorter and thinner cementites were developed into ultra-fine pieces under the action of high density dislocation and strains. At the top surface, some cementites were decomposed under severe plastic deformation. Experimental evidences and analysis indicate that surface nanocrystallization of 45 steel results from dislocation activities, high strains and high strain rate are necessary for the formation of nanocrystallites.

A magnetically recyclable heterogeneous catalyst: Cobalt nano-oxide supported on hydroxyapatite-encapsulated γ-Fe2O3 nanocrystallites for highly efficient olefin oxidation with H2O2

High catalytic activity of cobalt nano-oxide supported on hydroxyapatite-encapsulated magnetic γ-Fe2O3 nanocrystallites had been prepared. HR-TEM characterization demonstrated the formation of γ-Fe2O3 nanocrystallites within the HAP matrix with a mean diameter of 1–3 nm. The mixed Co3O4 and CoFe2O4 oxides were existed on the surface of CoHAP-γ-Fe2O3. The CoHAP-γ-Fe2O3 exhibited good catalytic activity for the oxidation of various alkenes based on a cooperation effect between these two oxides. The magnetic properties of the CoHAP-γ-Fe2O3 provided a convenient route for separation of the catalyst from the reaction mixture by application of an external permanent magnet. The spent catalyst could be recycled without appreciable loss of catalytic activity.

Synthesis of thin diamond films from faceted nanosized crystallites

Diamond films consist of crystallites having nanometer grains were deposited using low methane concentration by hot filament chemical vapor deposition (HFCVD). The results show that films consist of nanodiamond grains with grain sizes ranging from 20 nm to 200 nm having thickness dependent size. Increasing the deposition time, the grain size increases and hence the thickness of the film increases. The diamond nucleation (nucleation density 10 10 cm −2) is comparable to that obtained by biasing the substrate. The use of low methane concentration for the formation of nano crystallites improves the quality of the film as indicated by Raman spectroscopy. The distance between the filament and substrate is increased while maintaining the substrate temperature. The effects of this large separation on the gas phase chemistry are discussed which helps to understand the reduced size of the crystallites under input gas ratios when microcrystallines are obtained.