Structural investigations on an amorphous Se90Te10 alloy produced by mechanical alloying using EXAFS, cumulant expansion and RMC simulations

We investigated an amorphous Se(90)Te(10) alloy produced by mechanical alloying using two different approaches. First, we used extended x-ray absorption fine structure (EXAFS) spectroscopy and the cumulant expansion method using the Einstein model for the temperature dependence of the cumulants to obtain the cumulants C(*)(1), C(*)(2), and C(*)(3). From these, we found information about the structure of the alloy as well as the thermal and structural disorder, anharmonicity of the effective interatomic pair potentials, thermal expansion of the Se-Se and Se-Te bonds and asymmetry of the partial distribution functions g(Se-Se)(r) and g(Se-Te)(r). The cumulants C(*)(1), C(*)(2), and C(*)(3) also allowed us to reconstruct the g(EXAFS)(ij)(r,T) functions from EXAFS. Then, we made reverse Monte Carlo (RMC) simulations using the total structure factor S(K) obtained from synchrotron x-ray diffraction and the EXAFS oscillations χ(k) on the Se K edge as input data to obtain the g(RMC)(ij)(r) functions. Both methods furnished very similar g(ij)(r) functions, and the structural data obtained from them were also very similar. The results obtained from both methods showed the presence of Se-Te pairs indicating that there is alloying at the atomic level. In addition, we could not find any evidence of the presence of Te clusters in the alloy.     

Thermal and optical studies of an amorphous InSe9 alloy produced by mechanical alloying

In this paper we investigated thermal and optical properties of an amorphous alloy of the In–Se system. The amorphous InSe₉ alloy was produced by mechanical alloying and it was studied using Differential Scanning Calorimetry and microPhotoluminescence spectroscopy techniques, and from them several properties, such as glass transition and crystallization temperatures and energies and the optical gap energy were determined and compared to the values found in other alloys of the In–Se system. This comparison revealed some differences among our alloy produced by mechanical alloying and alloys produced by other techniques, which is a clear indication of the influence of the fabrication technique in their physical properties. The main differences occur in the activation energies associated with the glass transition and crystallization processes and also in the optical gap energy.     

EXAFS and cumulant expansion studies of an amorphous Se90P10 alloy produced by mechanical alloying

We investigated the local atomic order of an amorphous Se₉₀P₁₀ alloy produced by Mechanical Alloying through EXAFS measurements on Se K edge at five temperatures followed by a cumulant expansion analysis. We obtained a lot of structural information such as average coordination numbers and interatomic distances, structural and thermal disorder, asymmetry of the pair distribution functions g_ij(r), anharmonicity of the interatomic potential, thermal expansion and Einstein and Debye temperatures for the Se-Se and Se-P pairs. We also reconstructed the g_ij(r) functions for the Se-Se and Se-P pairs at the temperatures investigated.     

EXAFS and XRD studies of an amorphous Co57Ti43 alloy produced by mechanical alloying

We have investigated the local atomic order of an amorphous Co₅₇Ti₄₃ alloy produced by mechanical alloying by means of x-ray diffraction and EXAFS analyses on Co and Ti K-edges. Average coordination numbers and average interatomic distances between first neighbors where found from EXAFS and compared with those determined using an additive hard sphere (AHS) model associated with an deconvolution, and also with data from bcc- Co₂Ti compound. EXAFS results obtained indicated a shortening in the Co-Ti and Ti-Ti average interatomic distances when compared to those found by using the AHS-RDF method and an increase in the Co-Co and Ti-Ti average interatomic distances and a shortening in the Co-Ti one when compared to the interatomic distances found in the bcc- Co₂Ti compound. In spite of these differences, average coordination numbers obtained from EXAFS and AHS-RDF are similar to each other and also to those found in bcc- Co₂Ti.     

Reverse Monte Carlo simulations and Raman scattering of an amorphous GeSe4 alloy produced by mechanical alloying

The short and intermediate range order of an amorphous GeSe₄ alloy produced by Mechanical Alloying were studied by Reverse Monte Carlo simulations of its X-ray total structure factor and Raman scattering. The simulations were used to compute the , and partial distribution functions and the , and partial structure factors. We calculated the coordination numbers and interatomic distances for the first and second neighbors. The data obtained indicate that the structure of the alloy has important differences when compared to alloys prepared by other techniques. There are a high number of Se-Se pairs in the first shell, and some of the tetrahedral units formed seemed to be connected by Se-Se bridges.     

Structural and optical properties of amorphous hydrogenated silicon carbonitride films produced by PECVD

Amorphous hydrogenated silicon carbonitride thin films (a-Si:C:N:H), deposited by plasma enhanced chemical vapour deposition (PECVD) using hexamethyldisilazane (HMDSN) as monomer and Ar as feed gas, have been investigated for their structural and optical properties as a function of the deposition RF plasma power, in the range of 100-300 W. The films have been analysed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV-vis-NIR spectrophotometry and atomic force microscopy (AFM). From the analysis of the FT-IR spectra it results that the films become more amorphous and inorganic as RF plasma power increases. The incorporation of oxygen in the deposited layers, mainly due to the atmospheric attack, has been evaluated by XPS and FT-IR spectroscopy. Reflectance/transmittance spectra, acquired in the range of 200-2500 nm, allow to descrive the film absorption edge for interband transitions. A relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition RF plasma power has been investigated. The reduction of the optical energy gap from 3.85 to 3.69 eV and the broadening of the optical absorption tail with RF plasma power increasing from 100 to 300 W are ascribed to the growth of structural disorder, while the increase of the refractive index, evaluated at 630 nm, is attributed to a slight densification of the film. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become rougher when RF plasma power increases. The wettability of the film has been studied and related to the chemical composition and to the morphology of the deposited layers.     

Study of nanocrystalline and amorphous powders prepared by mechanical alloying

The process of mechanical alloying consists of intimate mixing and mechanical working of elemental powders in a high-energy ball mill. It has been well established that this process is able to produce nanocrystalline and amorphous material. In this study, the structural effects of mechanical alloying of pure Fe, Fe₅₀W₅₀ and Fe₅₀Mo₅₀ powders were investigated by X-ray diffraction and Mössbauer spectroscopy. For all cases, nanocrystalline and/or amorphous fractions were found after milling. The resulting particle size was determined by X-ray diffraction. Pure Fe does not amorphize even after prolonged milling times. For the nanocrystalline powder, significant changes in the linewidth and the hyperfine field are found. Powder mixtures of Fe₅₀Mo₅₀ and Fe₅₀W₅₀ are completely amorphous after milling times of 10 h, as seen by Mössbauer spectroscopy, but nanocrystalline fractions of the non-iron part are still found in X-ray diffraction. Also in the amorphous state, further changes in the hyperfine parameters are found with increasing milling time.     

Behaviour of ultrasonic velocities in amorphous Se90Ge10 and Se85Ge15 alloys near their glass transition

Precise measurements of 10 MHz frequency longitudinal and shear wave velocities are reported in amorphous SeGe alloys near their glass transition temperature T _g . There is a sharp decrease of the velocities near T _g , but the reduction in velocities appears smaller than expected.     

Aging of a nanostructured Zn50Se50 alloy produced by mechanical alloying

The aging at room temperature of a nanocrystalline equiatomic ZnSe alloy produced by mechanical alloying was investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques. The measured XRD patterns showed the presence of the peaks corresponding to the crystalline trigonal selenium (c-Se) phase. It is observed that the ZnSe phase is stable with aging. The appearance of the c-Se phase is attributed to the migration of Se atoms located at the interfacial component of the as-milled ZnSe nanostructure with aging.     

Comparison among the local atomic order of amorphous TM-Ti alloys (TM = Co,Ni, Cu) produced by mechanical alloying studied by EXAFS

We have investigated the local atomic structure of amorphous TM-Ti alloys (TM = Co, Ni, Cu) produced by Mechanical Alloying by means of EXAFS analyses on TM and Ti K-edges. Coordination numbers and interatomic distances for the three alloys where found and compared. EXAFS results obtained indicated a shortening in the unlike pairs TM-Ti as the difference between d electrons of TM and Ti atoms increases, suggesting an increase in the chemical short range order (CSRO) from TM = Co to Cu.Received: 12 October 2003, Published online: 9 April 2004PACS: 61.43.Dq Amorphous semiconductors, metals, and alloys - 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. - 81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation     

A metastable phase forming during crystallization of an amorphous Ni70Mo10P20 alloy

The evolution of the structure of an amorphous Ni₇₀Mo₁₀P₂₀ alloy during annealing in the temperature range from 673 to 873 K is studied using x-ray diffraction and transmission electron microscopy. It is found that a previously unknown metastable phase forms during crystallization of the amorphous Ni₇₀Mo₁₀P₂₀ alloy. The morphology of the newly formed crystals is studied, the space group and the lattice constant are determined, and the atom positions for the phase that forms are proposed. It is found that the metastable phase is a phosphide whose composition is close to (Ni(Mo))₃P. We established that, as the temperature or the duration of annealing is increased further, the metastable phase turns into the equilibrium Ni₃P phase. The orientation relations between the metastable and equilibrium phases are determined.     

Mössbauer study on surface crystallization behavior of amorphous Fe90Zr10 alloy ribbon

The precipitous drop of crystallization temperature at the surface of amorphous Fe₉₀Zr₁₀ ribbon is confirmed by TMS and CEMS. The deficiency in Zr at the surface amorphous phase, caused by the absorption and the diffusion of oxygen, is found to be responsible for it. The final crystalline products are assigned to α-Fe, Fe₃Zr and Fe₂Zr, whereas only α-Fe precipitates at the surfaces. Moreover, it is indicated that the crystallization behavior of the bulk is influenced by the ambient gases during annealing and quite different between in vacuum and in air.     

Structural,electronic, and optical properties of bulk Cu2Se

By using first-principles calculations within the density functional theory and the many-body perturbation theory, we investigate the structural, electronic, and optical properties of bulk Cu₂Se with a recently discovered low-temperature layered configuration. We demonstrate that the effects of the van der Waals forces significantly modify the interlayer binding and distance in the layered Cu₂Se, while the band gap is invariant. Our density functional theory and post-processing GW calculations reveal that for the layered structure, GW correction remedies the serious band-gap underestimation of the density functional theory from 0.12 eV to 0.99 eV. By solving the Bethe–Salpeter equation, we find that the optical gap of the layered Cu₂Se is 0.86 eV, which is in close agreement with previous experimental observations. In addition, we show that the high-temperature fluorite structure has no band gap, even after GW correction, explaining that the band gap controversy among the theories stems from different structural models. This work may serve as an important guide in designing and evaluating photovoltaic devices using Cu₂Se-based materials.     

Structural,vibrational, optical,photoluminescence, thermal,dielectric, and mechanical studies on zinc(tris) thiourea sulfate single crystal: A noticeable effect of organic dye

In this work, uranine-dyed zinc(tris) thiourea sulfate(ZTS) monocrystals, 26 mm×15 mm×10 mm in size, were synthesized by the solution method at ambient temperature. Their purity, crystallinity, lattice parameters, and functional modes were studied by x-ray diffraction, Fourier transform-infrared spectroscopy(FT-IR), and FT-Raman spectroscopy analyses. The sodium ion content of the crystals from the dye was confirmed by elemental analysis. The diffused reflectance spectral analysis of the dyed crystal revealed a characteristic absorption band at 490 nm attributed to the presence of the dye. The calculated band gaps of the non-dyed and dyed crystals were 4.53 and 4.57 e V, respectively. A green emission peak at ～(512 ± 4) nm was observed in the photoluminescence spectrum of the uranine-dyed crystals. A differential scanning calorimetry study confirmed that the thermal stability improved owing to the addition of the dye. Dielectric and microhardness studies were conducted to examine the significant improvements in the corresponding properties of dyed crystals. The results demonstrated the competency of the dyed ZTS crystals for applications in optoelectronic devices.     

Phase segregation and crystallization in the amorphous alloy Ni70Mo10P20

Structural evolution of the amorphous alloy Ni₇₀Mo₁₀P₂₀ has been studied by x-ray diffraction, and by following transmission and high-resolution electron microscopy annealing both above and below the glass-transition temperature. When annealed above this temperature, the amorphous phase undergoes segregation into regions about 100 nm in size having different chemical composition. Diffraction from such samples produces diffuse rings, and the scattering vector corresponding to the maximum intensity varies from point to point within the interval of 4.88 to 4.78 nm⁻¹. When occurring between the glass-transition and crystallization temperatures, crystallization produces groups of nanocrystals, 20–30 nm in size, which are in direct contact with one another and form a polymorphic mechanism. The crystallization mechanism changes when the annealing temperature is brought below the glass-transition point. At these temperatures the amorphous matrix crystallizes entectically with formation of eutectic colonies. Fiz. Tverd. Tela (St. Petersburg) 40, 1577–1580 (September 1998)     

Structural,optical and electrical properties of CuIn5Se8 and CuGa5Se8

Ingots of CuIn₅Se₈ and CuGa₅Se₈ were prepared by direct fusion of the stoichiometric mixture of the elements. The analysis of X-ray powder diffraction data showed the presence of one single phase with tetragonal structure for CuGa₅Se₈ and two phases with hexagonal and tetragonal structure for CuIn₅Se₈. The lattice parameters a and c were refined by means of the program NBS*AIDS83. The phase transition temperatures were obtained by Differential Thermal Analysis measurements performed on samples sealed in evacuated quartz ampoules. Transmittance measurements were used to determine the absorption coefficient α. The values of E_g were obtained from a plot of (αhν)² vs. hν. The electrical resistivity was measured from 10 to 400 K using a four-wire configuration. The activation energies were estimated from the Arrhenius plot of the resistivities.     

Structural,thermal and optical study of nanocrystalline silicon produced by ball milling

The ball milling process was used to obtain nanocrystalline cubic silicon. Between 5 and 10 h of milling, amorphous silicon was also formed. The differential scanning calorimetry (DSC) spectrum of a powder milled for 10 h showed that the amorphous–crystalline phase transition occurs at about 450 °C. According to Raman spectroscopy and X‐ray diffraction results, volume fractions of the crystalline, interfacial and amorphous phases were about 30, 39 and 31%, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Structural,optical and electrical properties of sulfur-incorporated amorphous carbon films

Amorphous carbon–sulfur (a-C:S) composite films were prepared by vapor phase pyrolysis technique. The structural changes in the a-C:S films were investigated by electron microscopy. A powder X-ray diffraction (XRD) study depicts the two-phase nature of a sulfur-incorporated a-C system. The optical bandgap energy shows a decreasing trend with an increase in the sulfur content and preparation temperature. This infers a sulfur incorporation and pyrolysis temperature induced reduction in structural disorder or increase in sp ² or π-sites. The presence of sulfur (S 2p) in the a-C:S sample is analyzed by the X-ray photoelectron spectroscopy (XPS). The sp ³/sp ² hybridization ratio is determined by using the XPS C 1s peak fitting, and the results confirm an increase in sp ² hybrids with sulfur addition to a-C. The electrical resistivity variation in the films depends on both the sulfur concentration and the pyrolysis temperature.