Transmission electron microscopy study of carbon nanophases produced by ion beam implantation

The formation of carbon nanocrystals, produced by ion implantation of carbon ions into fused SiO₂ substrates, followed by 1 h thermal annealing at 1000 °C, in an Ar + 5% H atmosphere has been studied. Combined high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) have been employed for structural characterization of carbon nanophases embedded in the quartz substrate. The dependence of grain size and sample morphology of the carbon nanophases on implantation dose was studied. The carbon nanocrystals formed by the implantation for a dose of 1 × 10¹⁶ C/cm² at 320 keV have been identified as a mixture of c-diamond nanophase and a modified diamond nanophase known as n-diamond. For a higher implantation dose, 5 × 10¹⁶ C/cm², besides n-diamond, another solid carbon nanophase was observed, with a structure known as i-carbon. Following the highest implantation dose 1 × 10¹⁷ C/cm² the sample contained the i-carbon nanophase only. A least-square refinement of SAED patterns was employed for the calculation of unit-cell parameters of identified carbon nanophases.     

Formation and spectral probing of transparent oxyfluoride glass-ceramics containing (Eu2+, Eu3+:BaGdF5) nano-crystals

In the present study, we report the formation of transparent glass-ceramics containing BaGdF₅ nanocrystals under optimum ceramization of SiO₂–BaF₂–K₂O–Sb₂O₃–GdF₃–Eu₂O₃ based oxyfluoride glass and the energy transfer mechanisms in Eu²⁺ → Eu³⁺ and Gd³⁺ → Eu³⁺ has been interpreted through luminescence study. The modification of local environment surrounding dopant ion in glass and glass ceramics has been studied using Eu³⁺ ion as spectral probe. The optimum ceramization temperature was determined from the differential scanning calorimetry (DSC) thermogram which revealed that the glass transition temperature (T_g), the crystallization onset temperature (T_x), and crystallization peak temperature (T_p) are 563 °C, 607 °C and 641 °C, respectively. X-ray diffraction pattern of the glass-ceramics sample displayed the presence of cubic BaGdF₅ phase (JCPDS code: 24-0098). Transmission electron microscopy image of the glass-ceramics samples revealed homogeneous distribution of spherical fluoride nanocrystals ranging 5–15 nm in size. The emission transitions from the higher excited sates (⁵D_J, J = 1, 2, and 3) as well as lowered asymmetry ratio of the ⁵D₀ → ⁷F₂ transition (forced electric dipole transition) to that of the ⁵D₀ → ⁷F₁ transition (magnetic dipole) of Eu³⁺ in the glass-ceramics when compared to glass sample demonstrated the incorporation of dopant Eu³⁺ ions into the cubic BaGdF₅ nanocrystals with higher local symmetry with enhanced ionic nature. The presence of absorption bands of Eu²⁺ ions and Gd³⁺ ions present in the glass matrix or fluoride nanocrystals in the excitation spectra of Eu³⁺ by monitoring emission at 614 nm indicated energy transfer from (Eu²⁺ → Eu³⁺) and (Gd³⁺ → Eu³⁺) in both glass and glass-ceramics samples.     

Size and shape influence of luminescent orthovanadate nanoparticles on their accumulation in nuclear compartments of rat hepatocytes

In this paper the process of nonfunctionalized negatively charged orthovanadate nanoparticle accumulation and redistribution in cells dependent on their shape and size was investigated. Aqueous colloidal solutions of nReVO₄:Eu^3 + (Re = Gd, Y, La) luminescent nanocrystals of different sizes and shapes have been synthesized. The average sizes of spherical particles were 2, 20, and 300 nm, of spindle-like particles – 22 × 6.3 nm, and of rod-like particles – 57 × 4.4 nm. Luminescence of nReVO₄:Eu^3 + nanocrystals was effectively excited by UV and visible irradiation. By means of luminescence microscopy and luminescence microspectroscopy, it has been revealed that spherical nanocrystals with an average diameter of 2 nm tend to accumulate mainly in the rat hepatocyte nuclei in situ and also in the isolated nuclei of these cells. An additional experiment has shown that nanoparticles reveal tropism to nuclear structural components. The penetration into nuclei does not require any modifications of the surface of nanoparticle and is governed by the shape and size of nanoparticle and also is determined by the cellular type.     

Thermal nitridation synthesis of MN (M=Ti,V and Cr) nanocrystals from metals and NH4Cl

Nanocrystalline TiN, VN and CrN have been synthesized by a thermal nitridation reaction between the corresponding metal powder and NH₄Cl at 600 °C in an autoclave. X-ray powders diffraction (XRD) indicated that these nanocystallites had a NaCl-type structure with lattice parameters: a=4.236 Å for TiN, 4.133 Å for VN, 4.145 Å for CrN. Transmission electron microscopy (TEM) showed these products consisted of cubic nanocrystals. X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) techniques revealed that all the products were almost pure nitrides.     

Characterization of H2Ti4O9 with high specific surface area prepared by a delamination/reassembling process

H₂Ti₄O₉ nanocrystals with high specific surface areas were prepared by delamination and reassembly process through a way of ball milling combined with ion exchange reaction. The samples were characterized by X-ray powder diffraction, transmission electron microscopy, chemical analysis, thermal analysis, N₂ adsorption-desorption isotherm, and absorption spectrum. The crystallites of Ti₄O₉²⁻ in the form of titania nanosheets has lateral size less than 50 nm. The specific surface area of H₂Ti₄O₉ nanocrystals depends on pH value of reassembling solution and ball milling time. H₂Ti₄O₉ nanocrystals prepared by ball-milling of K₂Ti₄O₉ for 2 h and suspending in 1 M HCl followed by precipitation at pH 8 had the specific surface area of 328.4 m² g⁻¹ which was about 16 times larger than the fibrous H₂Ti₄O₉ prepared by treating original K₂Ti₄O₉ without ball-milling in 1 M HCl.     

Defect-impurity complexes with high thermal stability in epi-Si n+-p diodes irradiated with MeV electrons

Defect-impurity complexes with high thermal stability which were generated after high temperature annealing of silicon n⁺-p diodes irradiated with 4 MeV electrons at 300 K have been studied by means of deep level transient spectroscopy (DLTS). Such defects are of interest because of their possible application in controlling the carrier lifetime in silicon power devices. The parameters of four deep level traps have been determined and compared with the results of photoluminescence studies on thermal stability of electron-irradiation-induced defects. A donor like trap with an energy level at E_v + 0.39 eV was assigned to a complex incorporating an interstitial carbon atom and two oxygen atoms (C_iO_2i), which gives rise to the P-line (hν = 0.767 eV) in photoluminescence spectra.     

Structural,optical and morphological properties of La,Cu co-doped SnO2 nanocrystals by co-precipitation method

Sn_0.96−xLa_0.04Cu_xO₂ (0 ⩽ x ⩽ 0.03) nanocrystals have been successfully synthesized by employing a simple co-precipitation method. The crystal structure of the synthesized nanocrystals was found to be tetragonal rutile of tin oxide by using X-ray diffraction technique and was not affected by doping. The change in lattice parameters was discussed based on the secondary phase formation and presence of Cu²⁺/Cu³⁺ in LaSnO₂ lattice. The variation in size and shape of the nanocrystals by Cu-doping was discussed using scanning electron microscope. The chemical stoichiometry of Sn, Cu, La and O was confirmed by energy dispersive X-ray spectra. The best optical transparency and lower absorption observed at Sn_0.97La_0.02Cu_0.01O₂ nanocrystals seems to be optimal for industrial applications especially as transparent electrode. The initial blue shift of energy gap from 3.65 eV (Cu = 0%) to 3.78 eV (Cu = 1%) (ΔE_g ≈ 0.13 eV) is due to the distortion in the crystal structure of the host compound and generation of defects. The red shift of energy gap after Cu = 1% is due to the charge-transfer transitions between the metal ions d-electrons and the SnO₂ conduction or valence band. Lattice mode of SnO₂ at 686 cm⁻¹ in Sn_0.98La_0.02O₂ nanocrystals and anti-symmetric SnOSn stretching mode of the surface bridging oxide around 634–642 cm⁻¹ in Cu doped Sn_0.98La_0.02O₂ nanocrystals was confirmed by Fourier transform infrared spectra.     

Thermal stability and annealing behavior of photoluminescence from Eu doped YAG

Eu₂O₃ doped Y₃Al₅O₁₂ (YAG) crystals have been grown using a floating zone technique and evaluated thermal stability and annealing behavior of PL for a fluorescence thermo-sensor application. Color of the crystals grown varies from deep purple to colorless with O₂ concentration of the growth atmosphere and annealing in air. Photoluminescence (PL) peaking at λ = 590, 607, 624, 647 and 709 nm due to Eu³⁺ ions are observed from the crystals under UV excitation. Anomalous temperature dependence of PL intensity, which is observed in as-grown crystals, is improved greatly by annealing through the heat cycle. From annealing behavior of optical absorption spectra, residual Eu²⁺ ions are suggested to be responsible for the de-coloration and the improvement of anomalous temperature dependence of Eu doped YAG crystals.     

Mechanical and transport properties of low-temperature negative thermal expansion material Mn3CuN co-doped with Ge and Si

Anti-perovskite manganese nitrides Mn₃CuN co-doped with Ge and Si show good negative thermal expansion properties at cryogenic temperatures and thus have great potential for cryogenic applications. In this work, Mn₃(Cu_0.6Si_xGe_0.4?x)N (x = 0.05, 0.1, 0.15) were prepared by reactive sintering under pressure. Their structures, densities, electrical resistivities, thermal conductivities and mechanical properties were studied at room and cryogenic temperatures. The results show that the values of electrical resistivities and thermal conductivities of Mn₃(Cu_0.6Si_xGe_0.4?x)N (x = 0.05, 0.1, 0.15) are in the range of 2.5–4.3 × 10^?6 Ω m and 1.9–3.6 W(m K)^?1, respectively. Compression tests indicate the compressive strength and Young’s modulus are about 700 MPa and 110 GPa, respectively.     

Study of physical properties of cobalt oxide (Co3O4) nanocrystals

Cobalt oxide nanocrystals of size 10–15 nm have been prepared by a simple co-precipitation method. The structural investigations have been performed with X-ray diffraction and Transmission Electron Microscopy. Specific surface area of the nanocrystals is 77.5 × 10⁴ cm²/g which have been calculated by X-ray diffraction data. Optical properties are discussed with UV/visible spectroscopy which shows the multiple band gap energies 2.28 eV (O^? II → Co^II) and 1.57 eV (O^? II → Co^III) which suggest the possibility of degeneracy of the valence band. The magnetic behavior is investigated using Vibrating Sample Magnetometer. The Co₃O₄ nanocrystals possess paramagnetic character at room temperature.     

Linear and non-linear optical properties of capped CdTe nanocrystals prepared by mechanical alloying

CdTe nanocrystals were prepared by mechanical alloying the elemental Cd and Te powders. The formation of CdTe with a single cubic phase after 20 h of ball milling was confirmed by X-ray diffraction (XRD). The surface of as-milled CdTe nanoparticles was then capped with polarization TOP/TOPO or (Na₃PO₄)_n organic ligand, which resulted in colorful dispersion solution with optical absorption peaks located at 573 nm and 525 nm, respectively. The third-order non-linearity, namely, the non-linear refraction and two-photon absorption (TPA) coefficient, of the capped CdTe dispersion samples were evaluated using Z-scan technique. The fitting of Z-scan experimental data with a special equation demonstrated that the capped CdTe nanocrystals possess large third-order susceptibilities at resonant wavelength. The non-linear figure of merit (γ/β) for 20 h as-milled CdTe nanocrystals after capping with TOP/TOPO was determined to be ∼ −2 × 10⁻⁵ m, which is nearly 215 times larger than the value reported for bulk CdTe crystals.     

Morphology and phase selective synthesis of CuxO (x = 1, 2) nanostructures and their catalytic degradation activity

Cu_xO (x = 1, 2) nanocrystals have been synthesized by the composite-hydroxide-mediated approach. The obtained nanocrystals were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy, and UV–vis spectrum. The morphology of the nanocrystals changed from sphere-shaped nanostructures to flower-shaped nanostructures, and finally to nanowires associated with phase transformation from CuO to Cu₂O by increasing the temperature. The possible phase transformation mechanism was discussed. The catalytic degradation activity of the Cu_xO (x = 1, 2) nanocrystals to methyl orange was also investigated. The photocatalytic ability of the sphere-shaped nanostructures is much higher than that of the nanowires, owing to its absorption of wider range of light energy. This work provides a new facile synthesis route of Cu_xO (x = 1, 2) nanocrystals and suggests their possible application in organic pollutants removal.     

The synthesis of nanocrystalline YIG in an ammonium nitrate melt

Yttrium iron garnet particles were synthesized in two different ways: first, in an ammonium nitrate melt (ANM) and second, via a solid-state reaction (SSR) route. The structural and magnetic properties of the samples were compared using XRD, SEM and dc magnetization measurements. It was observed for the ANM technique that the phase formation of YIG starts at 1000 °C and then develops with increasing temperature and sintering times. The saturation magnetization, M_s, increases sharply with increasing annealing temperature and then saturates at around 23 emu g^?1 above 1100 °C, while the coercivity decreases due to the increasing particle size. An almost single-phase sample was obtained through ANM route by annealing for 2 h at 1300 °C, after which the YIG fraction in the SSR sample was only 0.34, with M_s = 7.08 emu g^?1. The average particle sizes of the ANM samples were calculated using experimentally determined M_s values. It appeared that they vary from the sub-micron to the micron range, depending on the sintering temperature, and this coincides with the values determined from the SEM micrographs. These samples have homogeneous structures, small grains, good magnetic properties, and do not contain massive agglomerates. Therefore, the synthesis of YIG via the ANM technique represents another alternative to the SSR route.     

Microstructural evolution during annealing and rolling Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass

The microstructural evolution of the Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ bulk metallic glass during annealing and rolling deformation was studied. After annealing at 680 K for 0.5 h, phase separation is observed, and nanocrystallization is further induced by the subsequent rolling deformation. Increasing annealing time to 1.5 h leads to the formation of both nanocrystals and large-size particles of the Zr–Cu fcc phase. After rolling, the volume fraction of nanocrystals increases slightly while the Zr–Cu particles disappear. The presence of phase separation and nanocrystals during annealing reduce the thermal stability of the glass and accelerate the subsequent crystallization driven by rolling. During rolling the two annealed specimens exhibit the good ductility.     

Soft magnetic properties and microstructure of Fe84−xNb2B14Cux nanocrystalline alloys

Magnetic properties and microstructure of new Fe_84−xNb₂B₁₄Cu_x nanocrystalline alloys were investigated. We found that the microstructure was refined and soft magnetic properties of this alloy system were enhanced with proper Cu addition and annealing conditions. It was also discovered that the mean grain size firstly increases, then decreases to a minimum, and finally increases again with increasing annealing temperature for Fe₈₃Nb₂B₁₄Cu₁ nanocrystalline alloy, and this phenomenon was interpreted by the grain growth mechanism. Moreover, after annealing at 813 K for 180 s, Fe₈₃Nb₂B₁₄Cu₁ nanocrystalline alloy shows a fine microstructure with mean grain size of 16 nm, and exhibits excellent soft magnetic properties, such as high saturation magnetic flux density (1.7 T), low coercivity (7 A/m) and high permeability (2.8 × 10⁴). The result indicates that this alloy should have a promising application in the soft magnetic industry.     

Effect of Ni on reactive diffusion between Au and Sn at solid-state temperatures

Influence of Ni on the kinetics of the reactive diffusion between Au and Sn was experimentally studied at solid-state temperatures. Binary Sn–Ni alloys with Ni concentrations of 1, 3 and 5 mass% were used to prepare sandwich (Sn–Ni)/Au/(Sn–Ni) diffusion couples by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures of T = 433, 453 and 473 K for various times in an oil bath with silicone oil. After annealing, AuNiSn₈, AuSn₄, AuSn₂ and AuSn compound layers were observed to form at the (Sn–Ni)/Au interface in the diffusion couple. The total thickness l of the compound layers monotonically increases with increasing annealing time t according to the equation l = k(t/t₀)ⁿ, where t₀ is unit time, 1 s. The exponent takes values between n = 0.29 and 0.37 under the present annealing conditions. Such values of n < 0.5 indicate that the grain boundary diffusion contributes to the rate-controlling process and the grain growth occurs at certain rates. The higher the Ni concentration of the Sn–Ni alloy is, the faster the overall growth of the compound layers occurs. This means that Ni is an accelerator for the reactive diffusion between Au and Sn at solid-state temperatures. The acceleration effect of Ni becomes more remarkable at higher annealing temperatures. Such influence of Ni on the kinetics is mainly attributed to the dependencies of the growth rate of the AuNiSn₈ layer on the composition of the Sn–Ni alloy and the annealing temperature.     

Thermodynamic and kinetic studies for the adsorption of Hg(II) by nano-TiO2 from aqueous solution

Titanium dioxide nanocrystals were employed, for the first time, for the sorption of Hg(II) ions from aqueous solutions. The effects of varying parameters such as pH, temperature, initial metal concentration, and contact time on the adsorption process were examined. Adsorption equilibrium was established in 420 min and the maximum adsorption of Hg(II) on the TiO₂ was observed to occur at pH 8.0. The adsorption data correlated with Freundlich, Langmuir, Dubinin–Radushkevich (D–R), and Temkin isotherms. The Freundlich isotherm showed the best fit to the equilibrium data. The Pseudo-first order and pseudo-second-order kinetic models were studied to analyze the kinetic data. A second-order kinetic model fit the data with the (k₂ = 2.8126 × 10^?3 g mg^?1min^?1, 303 K). The intraparticle diffusion models were applied to ascertain the rate-controlling step. The thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were calculated which showed an endothermic adsorption process. The equilibrium parameter (R_L) indicated that TiO₂ nanocrystals are useful for Hg(II) removal from aqueous solutions.     

Study of optical properties of GeO2 nanocrystals as synthesized by hydrothermal technique

The GeO₂ nanocrystals (α-quartz type structure) with β-phase are synthesized at relatively lower temperature by hydrothermal route using autoclave. All samples are characterized by XRD, FESEM, EDS, TEM, photoluminescence (PL) and UV–vis absorption spectroscopy techniques. Synthesized nanocrystals have uniform shape and uniform size distribution for a particular synthesis condition, which is about 30–300 nm depending on synthesis conditions. The XRD results indicate that grown GeO₂ crystals only shows peak related to α-quartz structure with lattice parameters a = 4.985 Å and c = 5.648 Å. UV–vis absorption spectroscopy measurements reveal the bandgap energies corresponding to the GeO₂ α-quartz structure. Synthesized nanocrystals are capable to emit strong blue light around 425–435 nm under excitation of 300 nm and 325 nm and consequently the as synthesized material can be used in integrated optical devices.     

Optical nonlinearities of BaTiO3 matrix-embedded Au nanoparticles

Composite thin films Au/BaTiO₃ comprising nanometer-sized gold particles embedded in BaTiO₃ matrices were synthesized on MgO(1 0 0) substrates by co-depositing Au and BaTiO₃ targets using pulsed laser deposition technique. The nanostructure of the films and the size distributions of the Au particles were analyzed by high-resolution transmission electron microscopy. Crystal lattice fringes from the Au nanocrystals and BaTiO₃ matrices were observed. The nonlinear optical properties of the Au/BaTiO₃ films were measured using z-scan method at the wavelength of 532 nm with a laser duration of 10 ns. The nonlinear refractive index n₂ and the nonlinear absorption coefficient β were determined to be 2.72 × 10⁻⁶ esu and −1.1 × l0⁻⁶ m/W, respectively.     

Structural and optical properties of In doped Se–Te phase-change thin films: A material for optical data storage

Se_75−xTe₂₅In_x (x = 0, 3, 6, & 9) bulk glasses were obtained by melt quench technique. Thin films of thickness 400 nm were prepared by thermal evaporation technique at a base pressure of 10⁻⁶ Torr onto well cleaned glass substrate. a-Se_75−xTe₂₅In_x thin films were annealed at different temperatures for 2 h. As prepared and annealed films were characterized by X-ray diffraction and UV–Vis spectroscopy. The X-ray diffraction results show that the as-prepared films are of amorphous nature while it shows some poly-crystalline structure in amorphous phases after annealing. The optical absorption spectra of these films were measured in the wavelength range 400–1100 nm in order to derive the extinction and absorption coefficient of these films. It was found that the mechanism of optical absorption follows the rule of allowed non-direct transition. The optical band gap of as prepared and annealed films as a function of photon energy has been studied. The optical band gap is found to decrease with increase in annealing temperature in the present glassy system. It happens due to crystallization of amorphous films. The decrease in optical band gap due to annealing is an interesting behavior for a material to be used in optical storage. The optical band gap has been observed to decrease with the increase of In content in Se–Te glassy system.