Structural characterization of nanometer-sized crystalline Pd by x-ray-diffraction techniques

Size dependence of phosphorus doping in silicon nanocrystals

Doping of silicon nanocrystals (Si-NCs) is one of the major challenges for silicon nanoscale devices. In this work, phosphorus (P) doping in Si-NCs which are embedded within an amorphous silicon matrix is realized together with the growth of Si-NCs by plasma-enhanced chemical vapor deposition under a tunable substrate direct current (DC) bias. The variation of phosphorus concentration with substrate bias can be explained by the competition of bonding processes of Si-Si and P-Si bonds. The formation of Si-Si and P-Si bonds is differently influenced by the ion bombardment controlled by the substrate bias, due to their bonding energy difference. We have studied the influences of grain size on P doping in Si-NCs. Free carrier concentration, which is provided by activated P atoms, decreases with decreasing grain size due to increasing formation energy and activation energy of P atoms incorporated in Si-NCs. Furthermore, we have studied the P locations inside Si-NCs and hydrogen passivation of P in the form of P-Si-H complexes using the first-principles method. Hydrogen passivation of P can also contribute to the reduced free carrier concentration in smaller Si-NCs. These results provide valuable understanding of P doping in Si-NCs.

Structure and morphology of shape-controlled Pd nanocrystals

Pd nanocrystals were produced with uniform truncated-cube shape and a narrow size distribution, yielding controlled surface area fractions from low Miller index ({100}, {110}, {111}) crystalline facets. Details on the structure and morphology of the nanocrystals were obtained by combining X-ray powder diffraction line profile analysis, high-resolution transmission electron microscopy and surface electrochemistry based on Cu underpotential deposition.

Correlating sampling and intensity statistics in nanoparticle diffraction experiments

In a previous article [Öztürk, Yan, Hill & Noyan (2014).J. Appl. Cryst.47, 1016–1025] it was shown that the sampling statistics of diffracting particle populations within a polycrystalline ensemble depended on the size of the constituent crystallites: broad X-ray peak breadths enabled some nano-sized particles to contribute more than one diffraction spot to Debye–Scherrer rings. Here it is shown that the equations proposed by Alexander, Klug & Kummer [J. Appl. Phys.(1948),19, 742–753] (AKK) to link diffracting particle and diffracted intensity statistics are not applicable if the constituent crystallites of the powder are below 10 nm. In this size range, (i) the one-to-one correspondence between diffracting particles and Laue spots assumed in the AKK analysis is not satisfied, and (ii) the crystallographic correlation between Laue spots originating from the same grain invalidates the assumption that all diffracting plane normals are randomly oriented and uncorrelated. Such correlation produces unexpected results in the selection of diffracting grains. For example, three or more Laue spots from a given grain for a particular reflection can only be observed at certain wavelengths. In addition, correcting the diffracted intensity values by the traditional Lorentz term, 1/cos θ, to compensate for the variation of particles sampled within a reflection band does not maintain fidelity to the number of poles contributing to the diffracted signal. A new term, cos θB/cos θ, corrects this problem.

Tuning oxygen impurities and microstructure of nanocrystalline silicon photovoltaic materials through hydrogen dilution

As a great promising material for third-generation thin-film photovoltaic cells, hydrogenated nanocrystalline silicon (nc-Si:H) thin films have a complex mixed-phase structure, which determines its defectful nature and easy residing of oxygen impurities. We have performed a detailed investigation on the microstructure properties and oxygen impurities in the nc-Si:H thin films prepared under different hydrogen dilution ratio treatment by the plasma-enhanced chemical vapor deposition (PECVD) process. X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and optical transmission spectroscopy have been utilized to fully characterize the microstructure properties of the nc-Si:H films. The oxygen and hydrogen contents have been obtained from infrared absorption spectroscopy. And the configuration state of oxygen impurities on the surface of the films has been confirmed by X-ray photoelectron spectroscopy, indicating that the films were well oxidized in the form of SiO2. The correlation between the hydrogen content and the volume fraction of grain boundaries derived from the Raman measurements shows that the majority of the incorporated hydrogen is localized inside the grain boundaries. Furthermore, with the detailed information on the bonding configurations acquired from the infrared absorption spectroscopy, a full explanation has been provided for the mechanism of the varying microstructure evolution and oxygen impurities based on the two models of ion bombardment effect and hydrogen-induced annealing effect.

Passivation of nanocrystalline silicon photovoltaic materials employing a negative substrate bias

Hydrogenated nanocrystalline silicon (nc-Si:H) shows great promise in the application of third-generation thin film photovoltaic cells. However, the mixed-phase structure of nc-Si:H leads to many defects existing in this important solar energy material. Here we present a new way to passivate nc-Si:H films by tuning the negative substrate bias in plasma-enhanced chemical vapor deposition. Microstructures of the nc-Si:H films prepared under a negative bias from 0 to -300 V have been characterized using Raman, x-ray diffraction, transmission electron microscope, and optical transmission techniques. A novel passivation effect on nc-Si:H films has been identified by the volume fraction of voids in nc-Si:H, together with the electrical properties obtained by electron spin resonance and effective minority lifetime measurements. The mechanism of the passivation effect has been demonstrated by infrared spectroscopy, which illustrates that the high-energy H atoms and ions accelerated by an appropriate bias of -180 V can form more hydrides along the grain boundaries and effectively prevent oxygen incursions forming further Si-O/Si interface dangling bonds in the nc-Si:H films. The detrimental influence of a bias over -180 V on the film quality due to the strong ion bombardment of species with excessively high energy has also been observed directly from the surface morphology by atomic force microscopy.

TWO-STATE STRUCTURE OF NANOMETER-SIZED Cu PARTICLES

Molecular dynamics simulations have been employed to investigate the static and dynamic properties of unsupported spherical Cu particles with size ranging between 1 and 10 nm. The potential energy, the structural arrangement, and the mobility of atomic species were studied for each nanometer-sized particle within the temperature range between 300 K and the melting point. Two distinct regions, namely an internal domain and a surface layer, can be identified within each nanoparticle on the basis of the radial profiles of such quantities. The atomic species belonging to the interior of the particle display a bulk-like behavior. By contrast, the surface layer is characterized by an excess potential energy associated with a disordered structure. At relatively low temperatures, the surface atoms possess structural and energetic features intermediate between the ones of a superheated bulk solid and of an undercooled bulk liquid. Pre-melting processes at the surface are also evident at temperatures close to the melting point. The nanometer-sized particles can be thus regarded as heterogeneous two-state systems consisting of roughly distinguishable bulk-like and surface regions.

Size effects in the magnetic behaviour of TbAl(2) milled alloys

The study of the magnetic properties depending upon mechanical milling of the ferromagnetic polycrystalline TbAl(2) material is reported. Rietveld analysis of the x-ray diffraction data reveals a decrease in the grain size down to 14 nm and a -0.15% variation in the lattice parameter, after 300 h of milling time. Irreversibility in the zero field cooled-field cooled (ZFC-FC) dc susceptibility and clear peaks in the ac susceptibility between 5 and 300 K show that the long-range ferromagnetic structure is inhibited in favour of a disordered spin arrangement below 45 K. This glassy behaviour is also deduced from the variation of the irreversibility transition with the field (H(2/3)) and frequency. The magnetization process of the bulk TbAl(2) is governed by domain-wall thermal activation processes. In contrast, in the milled samples, cluster-glass properties arise as a result of cooperative interactions due to the substitutional disorder. The interactions are also influenced by the nanograin structure of the milled alloys, showing a variation in coercivity with the grain size, below the crossover between the multi- and single-domain behaviours.

Sonochemical production of a non-crystalline phase of palladium

A non-crystalline phase of palladium was produced by cavitation technique starting from a solution of palladium acetylacetonate and toluene. The microscopic structure of the sample, a very fine powder, was investigated by X-ray diffraction and it showed the characteristic features of a disordered system. Further details about the phase of the sample were gained by studying the crystallization process induced by thermal treatments.