Nanocrystal Grains Research Articles

Surface crystallization of amorphous fused silica during electron cyclotron resonance plasma etching

After low pressure fluorine plasma ecthing and oxygen ion passivation, a crystallized layer composed of SiO2 nano-crystal grains is observed in an amorphous fused silica surface. The depth of crystallized layer is at least several hundreds nanometers. Fluorine and carbon ion are generated from Ar/CF4 by the method of electron cyclotron resonance (ECR). F ion breaks Si-O band of initial silica surface layer and releases O ion. Carbon ion combines with oxygen ion, and turns into CO2, and SiF4 is generated from fluorine and silicon. After initial surface layer is removed, unsaturated Si atom remains. Si dangling bond recombines with new O ion and then creates crystallized -cristobalite nano-crystal grains under a high temperature.

Study on crystalline to amorphous structure transition of Cr coatings by magnetron sputtering

The influence of deposition rate on crystalline to amorphous microstructure transition of Cr coatings was studied through preparation of Cr coatings deposited onto silicon wafers using magnetron sputtering technique. The microstructure and morphology of Cr coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results show that Cr coating prepared at 400W exhibits dense columnar crystalline structure and the crystallite size and crystallization rate are increased expressly in the initial 5min. When the deposition rate achieved to the maximum, Cr coating shows a case of infinite periodic renucleation where new crystals are assumed to be nucleated periodically on the surfaces of growing crystals and strong persistence of the columnar growth morphology is apparent. However, Cr coating exhibits overall microstructure of amorphous phase mixed with a few nano-crystal grains as the deposition rate decreases to the minimum.

Pressure threshold and dynamics of nucleation for Si nano-crystal grains prepared by pulsed laser ablation

Si nano-crystal grains are prepared by pulsed laser ablation in low pressure Ar at room temperature through changing the gas pressure and the distance between target and substrate. The morphologies and compositions of samples are characterized by scanning electron microscopy images, Raman scattering spectra and X-ray diffraction spectra.The pressure threshold for Si grain formation is obtained to be 0.6 Pa at a laser fluence of 4 J/cm2, distance between target and substarate of 3 cm, and room temperature.Combining the fluid mechanics model and the nucleation division model, the dynamics process of nucleation is analyzed.The Monte Carlo simulation shows that the nucleation of nano-crystal grains is determined jointly by temperature and supersaturated density.

外加氦气流下脉冲激光沉积制备纳米硅晶粒成核区宽度的计算

外加氦气流下脉冲激光沉积制备纳米硅晶粒成核区宽度的计算

Structural evolution and optical characterization in argon diluted Si:H thin films obtained by plasma enhanced chemical vapor deposition

The structural evolution and optical characterization of hydrogenated silicon (Si:H) thin films obtained by conventional radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) through decomposition of silane diluted with argon were studied by X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, transmission electron microscopy (TEM), and ultraviolet and visible (UV-vis) spectroscopy, respectively. The influence of argon dilution on the optical properties of the thin films was also studied. It is found that argon as dilution gas plays a significant role in the growth of nano-crystal grains and amorphous network in Si:H thin films. The structural evolution of the thin films with different argon dilution ratios is observed and it is suggested that argon plasma leads to the nanocrystallization in the thin films during the deposition process. The nanocrystallization initiating at a relatively low dilution ratio is also observed. With the increase of argon portion in the mixed precursor gases, nano-crystal grains in the thin films evolve regularly. The structural evolution is explained by a proposed model based on the energy exchange between the argon plasma constituted with Ar* and Ar+ radicals and the growth regions of the thin films. It is observed that both the absorption of UV-vis light and the optical gap decrease with the increase of dilution ratio.

Preparation of size controllable copper nanocrystals for nonvolatile memory applications

A method of fabricating Cu nanocrystals embedded in SiO2 dielectric film for nonvolatile memory applications by magnetron sputtering is introduced in this paper. The average size and distribution density of Cu nanocrystal grains are controlled by adjusting experimental parameters. The relationship between nanocrystal floating gate micro-structure and its charge storage capability is also discussed theoretically.

Synthesis of nanoporous silicon carbide ceramics by thermal evaporation process

New nanoporous β-SiC ceramics were synthesized by a simple thermal evaporation method with commercial silicon powder and activated carbon fragments. The results of scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and X-ray diffraction indicated that the microstructure of the β-SiC nanoporous ceramics was uniform and consistent with the pore size of 50–100 nm. The β-SiC nanocrystal grains of 50–200 nm were accumulated together to form a nanopore network. The formation mechanism was attributed to a template synthesis process, in which activated carbon fragments were employed as the template and they reacted with vaporized silicon through a vapor–solid way.

Characterization and magnetic properties of electrospun Co1−x Zn x Fe2O4 nanofibers

By the electrospinning and calcination techniques, we have prepared uniform nanofibers of Co1−xZnxFe2O4 (0.0≤x≤0.5) ferrites with diameters of 110–130 nm. The Co1−xZnxFe2O4 nanofibers are single-phase spinels and the lattice constant with Zn content deviates from the Vegard’s law for these Co1−xZnxFe2O4 nanofibers. The Co1−xZnxFe2O4 nanocrystal grains by which are built nanofibers increase with calcination temperature. Variations of coercivity and saturation magnetization with calcination temperature can be explained in terms of the grain-size (D) effect. The coercivity (Hc) of Co0.5Zn0.5Fe2O4 nanofibers varies as D0.65 and basically follows the predicted D2/3 dependence based on the random anisotropy model in a D range below the single-domain size around 40 nm. The saturation magnetization of Co1−xZnxFe2O4 nanofibers initially increases with increasing Zn content, reaches a maximum value at x=0.3 and then decreases with further increase of Zn content, while the coercivity exhibits a continuous reduction with the increase of Zn content.

The effect of cathodic current density on the microstructure of electrochemically produced nanostructured powders of Ni xMo 1– x alloys

Nanostructured Ni x Mo 1– x alloy powders were produced by electrochemical deposition from nickel–sulphate and sodium–molybdate ammonia solutions in the current density range of 50–500 mA cm −2. The chemical composition of the powders has been found to depend on the cathodic current density for the simultaneous reactions of induced molybdenum and nickel codeposition and hydrogen evolution. As the cathodic current density increases, over the current density range of 50–500 mA cm −2, the content of molybdenum in the alloy decreases from 7 mol.% to 0.4 mol.%. The molybdenum content decreases rapidly with an increase in current density from 50 mA cm −2 to 200 mA cm −2. A further increase in current density produces a slight decrease in the molybdenum content. X-ray diffraction (XRD) analysis determined that the obtained powders contained an amorphous phase and nanocrystals arranged in a face-centred cubic lattice (FCC). The amorphous phase exists between nanocrystal grains, its percentage in the produced powders being relatively small. The size of nanocrystals is dependent on the cathodic current density for powder production and on the molybdenum content in the alloy. The increase in current density from 50 mA cm −2 to 200 mA cm −2 induces an increase in mean nanocrystal value from 13 nm to 21 nm. Further current density increases result in a decrease in the mean nanocrystal size to 17.5 nm at j = 500 mA cm −2. The size of the crystals produced in the current density range of 50–200 mA cm −2 is generally dependent on the molybdenum content in the alloy. The crystal dimensions of the powders produced at j > 200 mA cm −2 were affected by the cathodic current density. The increases in both the cathodic current density for powder production and the molybdenum content in Ni x Mo 1– x alloys resulted in a greater amorphous phase percentage and smaller crystal grains of the FCC phase with a higher density of chaotically distributed dislocations and a higher level of microstrain.

Shock-Assisted Superficial Hexagonal-to-Cubic Phase Transition in GaN/Sapphire Interface Induced by Using Ultra-violet Laser Lift-Of Techniques

Ultra-violet (KrF excimer laser, λ = 248 nm) laser lift-of (LLO) techniques have been operated to the GaN/sapphire structure to separate GaN from the sapphire substrate. Hexagonal to cubic phase transformation induced by the ultra-violet laser lift-of (UV-LLO) has been characterized by micro-Raman spectroscopy, micro-photoluminescence, along with high-resolution transmission electron microscopy (HRTEM). HRTEM indicates that UV-LLO induced phase transition takes place above the LLO interface, without phase transition under the LLO interface. The formed cubic GaN often exists as nanocrystal grains attaching on the bulk hexagonal GaN. The half-loop-cluster-like UV-LLO interface indicates that the LLO-induced shock waves has generated and played an assistant role in the decomposition of the hexagonal GaN and in the formation of cubic GaN grains at the LLO surface.

Mechanism of surface nanocrystallization in pure nickel induced by high-current pulsed electron beam

High-current pulsed electron beam （HCPEB） technique was applied to irradiate the samples of polycrystalline pure nickel. Microstructures of the irradiated surface and sub-surface were investigated by using scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The experimental results show that the surface layer melted, and 2 μm depth remelted layer was formed on the top surface. Superfast solidification of melted layer results in the formation of nano-structure of 80 nm grain size in the remelted layer. In the sub-surface regions, the structures of dislocation walls and sub- dislocation walls in it with band shape were induced by severe plastic deformation, which become the dominant structures 5—15 μm below the irradiated surface. Those defect structures crossing with each other results in grain refinement, and nanocrystal grains about 10 nm in size were produced in the sub-surface layer.

Growth and characteristics of laser deposited anatase and rutile TiO 2 films on Si substrates

Titanium dioxide thin films have been synthesized on Si (100) substrates using pulsed laser deposition method (KrF: 248 nm, 20 ns, 5 Hz). The emission spectra of the plasma induced by ablating TiO 2 target in the oxygen or argon ambient gas were analyzed. The influences of substrate temperature and ambient gas pressure on the structural properties of TiO 2 films were discussed. The X-ray diffraction results show that the films deposited at 750 °C are (004)-oriented anatase phase and (110)-oriented rutile phase under the oxygen and argon pressure of 5 Pa, respectively. The scanning electron microscopy images indicate that the TiO 2 films have a uniform and smooth surface and are composed of nanocrystal grains. The pure anatase and rutile phase TiO 2 films are further proved by Raman spectromicroscopy. In addition, the optical transmission spectra and Fourier Transform infrared spectroscopy of the films were also studied.

Exciton Lifetime in Sio2 Layers with Embedded Silicon Nanocrystals as a Function of the "Dark" Fraction Of Nanocrystals in the System

Previously reported data on photoluminescence quenching in ion-beam irradiated silicon dioxide layers with embedded silicon nanocrystals were used to analyze the exciton lifetime in such systems versus the quenched fraction of nanocrystals. Estimates of the characteristic time of exciton hopping between adjacent nanocrystal grains, the length of excitonic wavefunction decay into the dielectric, and the cross-section of exciton recombination at deep-level centers introduced by the irradiation into the SiO2 matrix are given.

Structural state and phase composition of nickel surface layers modified by high-intensity implantation of titanium ions

This paper reports on the results of experimental investigations into the microstructure and the elemental and phase compositions of ion-alloyed nickel surface layers modified by high-intensity implantation of titanium ions. It is established that the implantation of titanium ions into nickel surface layers up to 1600 nm thick leads to the formation of intermetallic phases (namely, NiTi, Ni3Ti, and NiTi2), a solid solution of titanium in nickel, titanium oxides of different stoichiometries, and titanium carbide TiC. It is demonstrated that the phases formed in the ion-alloyed nickel surface layers have a nanocrystalline structure. The average size of the nanocrystal grains is equal to 40 nm.

Molecular dynamics simulation of deformation-induced crystallization mechanism in amorphous Ti3Al alloy

The crystallization of Ti3Al amorphous alloy during tensile deformation was studied with molecular dynamics. The simulation results show that the crystallization of amorphous alloy is induced by the local plastic deformation. The microstructure evolution was analyzed in the tension process. It is found that the nucleation and growth are induced by the shear localization and the final crystal phase has fcc structure. It is also found that the crystallization process is accompanied by strengthening of the material. The nanocrystal grains embedded in the amorphous phase can enhance the rigidity of the sample.

Improving mechanical properties of amorphous carbon nitride films by titanium doping

Titanium doped amorphous carbon nitride (a-CNx) films with a nitrogen content of ∼24at.% were synthesized by radio frequency magnetron sputtering method. The effects of incorporating Ti on the mechanical properties of a-CNx films were investigated by nanoindentation, scanning electron microscope, x-ray diffraction spectra, and x-ray photoelectron spectroscopy, respectively. It was found that nanometer sized TiN crystallites were formed and embedded in the a-CNx matrix, causing an enhancement of hardness from ∼28to∼40GPa in the a-CNx films. The improved mechanical properties with addition of Ti are attributed to the densified microstructure due to the development of fine grain size of TiN and sp3 C–N bonds. These TiN nanocrystal grains are separated by an amorphous phase, preventing dislocation movement and hence enhancing the hardness of the film. The increased sp3 C–N bond fraction induced by incorporation of Ti also plays an important role in the enhancement of hardness.

Influence of Si content and growth condition on the microstructure and mechanical properties of Ti–Si–N nanocomposite films

Thin films of Ti–Si–N have been prepared by ion beam assisted deposition (IBAD) from two Ti and Si targets. The silicon concentration in the deposited coatings is varied between 0 and 23.7 at.%. The influence of Si content and growth conditions on the microstructure and mechanical properties were investigated using XPS, AFM, XRD and nanoindenter. These nanocomposite coatings exhibit improved mechanical properties in comparison with TiN deposited under the same condition. The hardness measured by nanoindentation reached 42 GPa in Ti–Si–N films containing 11.32 at.% of Si, whereas TiN films only had a value of about 18 GPa. AFM showed that the finest grain size of Ti–Si–N appeared to be 5 nm when Si content was 11.32 at.%. From XPS and XRD results, the microstructures of the high hardness samples were found to consist of nanocrystal TiN grains and amorphous Si 3N 4.

Theoretical study on the influence of the microstructure of Fe73.5Cu1Nb3Si13.5B9 on its giant magneto-impedance effect

Based on the experimental results of atomic force microscope observation of the microstructure and the giant magneto-impedance (GMI) effect of Fe73.5Cu1Nb3Si13.5B9 nanocrystalline alloy, we propose a model for exploring the influence of the microstructure on the GMI effect in this kind of Fe-based nanocrystalline soft magntic alloy which explains successfully the GMI behavior at low frequency. The model possesses the main features of the sandwiched model while eliminating its deficiencies. This model is called the mesoscopic model. We also point out the influence of the electrical conductivity σ and the permeability μ of nanocrystal grains on the GMI effect of the Fe-based nanocrystalline alloys.

Low-resistance gas sensors fabricated from multiwalled carbon nanotubes coated with a thin tin oxide layer

Gas sensors have been fabricated from multiwalled carbon nanotubes (MWNTs) coated with a thin tin oxide layer, and have been used to detect oxidizing and reducing gases down to a ppm level. The barriers between the tin oxide nanocrystal grains on the MWNTs dominate the sensor resistance in different gases, and the conducting carriers in the MWNTs have a low resistance, which make the resistance of the sensors much lower than that of SnO2 nanobelt sensors. The resistance is 130kΩ in air, 230kΩ in 2ppm NO2, and 2.8MΩ in 50ppm NO2, so that impedance matching with amplifying circuits can be easily achieved.

Preparation and characterization of nanocrystal grain TiO2 porous microspheres

Using the monomer droplets as templates, nanocrystal grain TiO2 porous microspheres were initially prepared by reversed suspension polymerization and sol–gel method. The properties of TiO2 microspheres were studied by DSC, TG, XRD, SEM and BET. The results showed: porous microspheres were mainly composed of anatase TiO2 nanocrystal grains, and the size of crystal grains was between 20 and 40nm. The diameter of TiO2 porous microspheres was 200–500μm. BET analysis showed that the surface area of TiO2 porous microspheres was 87.40m2/g; the pore distribution possessed monodispersity, the mean pore size was 8.65nm. And the degradation effect of TiO2 microspheres on KN-R dye was studied. The results showed that the microspheres prepared by this method have high photocatalytic activity.