Xe Nanocrystals Research Articles

Preparation and Analysis of Atom Probe Tips by Xenon Focused Ion Beam Milling.

The damage and ion distribution induced in Si by an inductively coupled plasma Xe focused ion beam was investigated by atom probe tomography. By using predefined patterns it was possible to prepare the atom probe tips with a sub 50 nm end radius in the ion beam microscope. The atom probe reconstruction shows good agreement with simulated implantation profiles and interplanar distances extracted from spatial distribution maps. The elemental profiles of O and C indicate co-implantation during the milling process. The presence of small disc-shaped Xe clusters are also found in the three-dimensional reconstruction. These are attributed to the presence of Xe nanocrystals or bubbles that open during the evaporation process. The expected accumulated dose points to a loss of >95% of the Xe during analysis, which escapes undetected.

Melting and thermodynamic properties of rare gas nanocrystals

A self-consistent statistical method [Phys. Rev. B 66, 054302 (2002)] is used to describe the thermodynamic properties of free rare gas nanocrystals, using thin plates as examples. It is shown that the influence of size on the thermodynamic properties of nanocrystals is due to size-dependent quantization of the vibration spectrum affecting the parameters of a statistical distribution function of atomic displacements and, thus, governing the size dependence of the average values of energetic contributions to the Gibbs free energy of the system. For Xe nanocrystals, we present calculated size dependences of the Debye temperature, heat capacity, interatomic distance, melting temperature, etc.

Xe nanocrystals in Si studied by x-ray absorption fine structure spectroscopy

The structural configuration of Xe clusters, obtained by ion implantation in a Si matrix, has been investigated as a function of the temperature by x-ray absorption fine structure spectroscopy. In contrast with previous results, we demonstrate that an accurate analysis of the data, using high order cumulants, gives evidence of Xe fcc nanocrystals at low temperature, even in the as-implanted Si; expansion of the Xe lattice is always found as a function of the temperature, with no appreciable overpressure. We point out that a dramatic modification of these conclusions can be induced by an incorrect analysis using standard symmetrical pair distribution function $G(r)$; for this reason, all the results were checked by x-ray diffraction measurements.

Expansion and melting of Xe nanocrystals in Si

Xe agglomerates confined in a Si matrix by ion implantation were synthesized with different size depending on the implantation process and/or the thermal treatment. At low temperature Xe nanocrystals are formed, whose expansion and melting were studied in the range 15--$300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Previous high resolution x-ray diffraction spectra were corroborated with complementary techniques such as two-dimensional imaging plate patterns and transmission electron microscopy. We detected fcc Xe nanocrystals whose properties were size dependent. The experiments showed that in annealed samples epitaxial condensation of small Xe clusters, on the cavities of the Si matrix, gave in fact expanded and oriented Xe, suggesting a possible preferential growth of $\mathrm{Xe}\phantom{\rule{0.3em}{0ex}}(311)$ planes oriented orthogonally to the $\mathrm{Si}[02\text{\ensuremath{-}}2]$ direction. On the contrary, small Xe clusters in an amorphous Si matrix have a fcc lattice contracted as a consequence of surface tension. Furthermore, a solid-to-liquid phase transition size dependent was found. Expansion of fcc Xe lattice was accurately determined as a function of the temperature. Overpressurized nanocrystals and/or binary size distributions were disproved.

Fine structure effects and phase transition of Xe nanocrystals in Si

We report on an X-ray diffraction study performed on Xe agglomerates obtained by ion implantation in a Si matrix. At low temperature, Xe nano-crystals were formed in Si with different average sizes according to the preparation procedure. High resolution diffraction spectra were detected as a function of the temperature, in the range 15–300 K, showing evidence of fine structure effects in the growth mode of the Xe nanocrystals. We report the first experimental observation of fcc crystalline agglomerates with a lattice parameter expanded by the epitaxial condensation on the Si cavities, whereas for small agglomerates randomly oriented evidence of a contracted lattice was found. For these nanocrystals, a solid-to-liquid transition temperature, size dependent, was detected; above the transition temperature, a fluid phase was observed. Neither overpressurized clusters were detected at any temperature, nor preferential binary size distribution as reported for a metal matrix.

Melting and crystallization of Xe nanoprecipitates in Al under 1 MeV electron irradiation

Xe nanoprecipitates have been produced by ion implantation into high purity Al at 300 K. With an off‐zone axis high resolution imaging technique, Xe nanocrystals may be clearly structure imaged against a weak structure image of the Al matrix. Under the 1 MeV electron irradiation employed for the HREM observation, the nanoprecipitates exhibit a number of readily observed phenomena including migration within the matrix, changes in shape, faulting, melting, crystallization and coalescence, associated with changes in interface structure. This paper examines the possible mechanisms for melting and crystallization of the nanoprecipitates: beam heating and changes in precipitate volume associated with fluctuations in arrival of Al vacancies and interstitials, or with transport of Xe to and from the Al matrix due to electron irradiation damage in the Al and/or the Xe. Biased fluctuations in the point defect fluxes will produce volume changes in the Al cavity confining the Xe with corresponding deviations from the equilibrium pressure, 2γ/R0, associated with the interfacial tension γ.

What Xe Nanocrystals in Al Can Teach us in Materials Science

ABSTRACTNoble gases are generally very insoluble in solids. For example, Xe implanted into Al at 300 K forms a fine dispersion of crystalline precipitates and, at large enough fluence, fluid precipitates, both of which are stabilized, relative to the gas phase, by the Laplace pressure due to precipitate/matrix interface tensions. High resolution electron microscopy has been performed to determine the largest Xe nanocrystalline precipitate in local equilibrium with fluid Xe precipitates within the Al matrix. From the shape and size of the largest crystal and the Laplace pressure associated with its interface, we show that the interface tensions can be derived by setting the Laplace pressure equal to the pressure for solid/fluid Xe equilibrium derived from bulk Xe compression isotherms at the temperature of equilibration and observation. The Xe/Al interface tensions thus derived are in the range of accepted values of surface tensions for the Al matrix. Furthermore, it is suggested that this same technique may be employed to estimate unknown surface tensions of a solid matrix from the size and shape of maximal nanocrystals of a noble gas element, which have been equilibrated in that matrix at the temperature of observation.

In-situ observation of shape and atomic structure of Xe nanocrystals embedded in aluminium.

An imaging technique to determine in situ the shape and atomic structure of nanosized Xe crystals embedded in Al is described using high-resolution transmission electron microscopy (HRTEM). The Xe nanocrystals, with sizes less than 5 nm were prepared by the implantation of 30 keV Xe+ into Al at room temperature. The fcc Xe nanocrystals are mesotactic with the Al lattice and have a lattice parameter approximately 50% larger than that of Al. HRTEM images of the Xe were not clear in [110] zone axis illumination because of the small number of Xe atoms relative to Al atoms in any atom column. An off-axial imaging technique that consists of tilting the specimen several degrees from a zone axis and defocusing to suppress the Al lattice fringes is employed for the 110 projection of the Xe/Al system and the structure of the Xe nanocrystals is successfully imaged. The Xe images clearly represent projections of cuboctahedra with faces parallel to eight Al {111} planes truncated by six {100} planes. The results of multislice image simulations using a three-dimensional atomic model agreed well with the results obtained by the off-axial imaging technique. The usefulness of the technique is demonstrated with observations of crystal defects introduced into the Xe under intense 1000 keV electron irradiation.

Observation of atomic processes in Xe nanocrystals embedded in Al under 1 MeV electron irradiation

Defect introduction processes in two differently sized Xe nanocrystals embedded in Al were observed with in situ high-resolution electron microscopy. The imaging conditions made it possible to follow the apparent movement of Xe atom columns within a nanocrystal. The positions of individual atom columns were measured on captured video images, before and after defect introduction, from intensity traces along the rows of atom columns. For each of these precipitates, a planar defect appears on {1 1 1}. The observed displacement of Xe atom columns approximately corresponds to the projected displacement produced by a Shockley partial dislocation, bounding an intrinsic stacking fault. Comparison of the displacements of atoms in the two nanocrystals shows that the displacements are reduced at the precipitate/matrix interfaces. This implies that there will be a displacement at interfaces that represents some compromise between the strain field of the partial dislocation and the interface compatibility requirement.

In-situ observation of atomic processes in Xe nanocrystals embedded in al

ABSTRACTSelf-organization processes in Xe nanocrystals embedded in Al are observed with in-situ high-resolution electron microscopy. Under electron irradiation, stacking fault type defects are produced in Xe nanocrystals. The defects recover in a layer by layer manner. Detailed analysis of the video reveals that the displacement of Xe atoms in the stacking fault was rather small for the Xe atoms at boundary between Xe and Al, suggesting the possibility of the stacking fault in Xe precipitate originating inside of precipitate, not at the Al/Xe interface.