We report on the observation of a fine structure in ion tracks in amorphous SiO2 using small angle x-ray scattering measurements. Tracks were generated by high energy ion irradiation with Au and Xe between 27 MeV and 1.43 GeV. In agreement with molecular dynamics simulations, the tracks consist of a core characterized by a significant density deficit compared to unirradiated material, surrounded by a high density shell. The structure is consistent with a frozen-in pressure wave originating from the center of the ion track as a result of a thermal spike.

Aging in super glassy polymers such as poly(trimethylsilylpropyne) (PTMSP), poly(4‐methyl‐2‐pentyne) (PMP), and polymers with intrinsic microporosity (PIM‐1) reduces gas permeabilities and limits their application as gas‐separation membranes. While super glassy polymers are initially very porous, and ultra‐permeable, they quickly pack into a denser phase becoming less porous and permeable. This age‐old problem has been solved by adding an ultraporous additive that maintains the low density, porous, initial stage of super glassy polymers through absorbing a portion of the polymer chains within its pores thereby holding the chains in their open position. This result is the first time that aging in super glassy polymers is inhibited whilst maintaining enhanced CO2 permeability for one year and improving CO2/N2 selectivity. This approach could allow super glassy polymers to be revisited for commercial application in gas separations.

A combination of conventional and synchrotron-based techniques has been used to characterize the sizedependent structural and thermal properties of Ge nanocrystals ͑NCs͒ embedded in a silica ͑a-SiO 2 ͒ matrix. Ge NC size distributions with four different diameters ranging from 4.0 to 9.0 nm were produced by ion implantation and thermal annealing as characterized with small-angle x-ray scattering and transmission electron microscopy. The NCs were well represented by the superposition of bulklike crystalline and amorphous environments, suggesting the formation of an amorphous layer separating the crystalline NC core and the a-SiO 2 matrix. The amorphous fraction was quantified with x-ray-absorption near-edge spectroscopy and increased as the NC diameter decreased, consistent with the increase in surface-to-volume ratio. The structural parameters of the first three nearest-neighbor shells were determined with extended x-ray-absorption finestructure ͑EXAFS͒ spectroscopy and evolved linearly with inverse NC diameter. Specifically, increases in total disorder, interatomic distance, and the asymmetry in the distribution of distances were observed as the NC size decreased, demonstrating that finite-size effects govern the structural properties of embedded Ge NCs. Temperature-dependent EXAFS measurements in the range of 15-300 K were employed to probe the mean vibrational frequency and the variation of the interatomic distance distribution ͑mean value, variance, and asymmetry͒ with temperature for all NC distributions. A clear trend of increased stiffness ͑higher vibrational frequency͒ and decreased thermal expansion with decreasing NC size was evident, confirming the close relationship between the variation of structural and thermal/vibrational properties with size for embedded Ge NCs. The increase in surface-to-volume ratio and the presence of an amorphous Ge layer separating the matrix and crystalline NC core are identified as the main factors responsible for the observed behavior, with the surrounding a-SiO 2 matrix also contributing to a lesser extent. Such results are compared to previous reports and discussed in terms of the influence of the surface-to-volume ratio in objects of nanometer dimensions.

Swift heavy-ion irradiation of elemental metal nanoparticles (NPs) embedded in amorphous SiO 2 induces a spherical to rodlike shape transformation with the direction of NP elongation aligned to that of the incident ion. Large, once-spherical NPs become progressively more rodlike while small NPs below a critical diameter do not elongate but dissolve in the matrix. We examine this shape transformation for ten metals under a common irradiation condition to achieve mechanistic insight into the transformation process. Subtle differences are apparent including the saturation of the elongated NP width at a minimum sustainable, metal-specific value. Elongated NPs of lesser width are unstable and subject to vaporization. Furthermore, we demonstrate the elongation process is governed by the formation of a molten ion-track in amorphous SiO 2 such that upon saturation the elongated NP width never exceeds the molten ion-track diameter. Ion-solid interactions during swift heavy-ion irradiation (SHII) are dominated by inelastic processes in the form of electron excitation and ionization while, in contrast, the influence of elastic processes such as ballistic displacements is negligible. Macroscopically, amorphous SiO 2 (a-SiO 2 ) undergoes a volume-conserving anisotropic deformation when subjected to SHII such that thin freestanding layers contract and expand, respectively, in directions parallel and perpendicular to that of the incident ion [1]. The viscoelastic model [2,3], based on a transient thermal effect, successfully explains this so-called ion hammering. Microscopically, energy is deposited along the ion path, from incident ion to matrix electrons, and is then dissipated within a narrow cylinder of material surrounding the ion path. The heat flow in both the electron and lattice subsystems is well described as functions of time and radial distance by the inelastic thermal spike (i-TS) model [4,5]. When the temperature of the lattice exceeds that required for melting, the material along the ion path is molten and upon quenching an ion track is formed. Recently, we measured the molten ion-track diameter in a-SiO 2 as a function of electronic stopping power [6]. The ion-track radial density distribution consisted of an under-dense core and over-dense shell (relative to unirradiated material), the formation of which was attributed to a quenched-in pressure wave emanating from the ion-track center [6].Elemental metal nanoparticles (NPs) embedded in a-SiO 2 and subjected to SHII can undergo an intriguing shape transformation where once-spherical NPs become progressively more rodlike with the direction of elongation aligned along that of the incident ion. This phenomenon has been reported for several metals under a wide range of SHII conditions, with Refs. [7][8][9][10][11][12][13][14][15][16][17] citing selected examples. Freestanding metallic NPs irradiated under comparable conditions do not change shape, demonstrating the embedding a-SiO 2 matrix must have a role in the shape transformation process [8,17]. An unambiguous ...

Pt nanoparticles ͑NPs͒ formed by ion-beam synthesis in amorphous SiO 2 were irradiated with Au ions in the energy range of 27-185 MeV. Small-angle x-ray scattering ͑SAXS͒ and transmission electron microscopy were used to characterize an irradiation-induced shape transformation within the NPs. A simple yet effective way of analyzing the SAXS data to determine both NP dimensions is presented. A transformation from spherical to rodlike shape with increasing irradiation fluence was observed for NPs larger than an energy-dependent threshold diameter, which varied from 4.0 to 6.5 nm over 27-185 MeV. NPs smaller than this threshold diameter remained spherical upon irradiation but decreased in size as a result of dissolution. The latter was more pronounced for the smallest particles. The minor dimension of the transformed NPs saturated at an energydependent value comparable to the threshold diameter for elongation. The saturated minor dimension was less than the diameter of the irradiation-induced molten track within the matrix. We demonstrate that Pt NPs of diameter 13 nm reach saturation of the minor dimension beyond a total-energy deposition into the matrix of 20 keV/ nm 3 .

The temperature-dependent evolution of atomic vibrations in crystalline and amorphous InP has been studied using extended x-ray absorption fine-structure ͑EXAFS͒ spectroscopy. Measurements were performed at the In K edge for temperatures in the range of 20-295 K. In crystalline InP, the first nearest-neighbor ͑NN͒ EXAFS Debye-Waller factor, representative of the correlated mean-square relative displacement ͑MSRD͒ parallel to the bond direction, is considerably smaller than the uncorrelated mean-square displacement ͑MSD͒ determined from x-ray diffraction measurements. In contrast, the MSRD perpendicular to the bond direction agrees well with the MSD. This clearly demonstrates that vibrations of two neighboring atoms relative to each other are strongly reduced along the bond direction but are unhindered perpendicular to it, consistent with the wellknown behavior of III-V semiconductors where bond bending is energetically favored over bond stretching. With increasing interatomic distance, the correlation of atomic motion quickly vanishes as manifested by increased EXAFS Debye-Waller factors. For the third NN shell the value closely approaches the MSD demonstrating the nearly uncorrelated motion of atoms only three shells apart. In the amorphous phase, only information about the first NN shell is accessible although the latter is now comprised of both P and In atoms. The EXAFS Debye-Waller factors are significantly higher than in the crystalline phase but exhibit a very similar temperature dependence. This results from strongly increased structural disorder in the amorphous phase whereas the thermally induced disorder is very similar to that in crystalline InP. A correlated Einstein model was fitted to the Debye-Waller factors yielding Einstein temperatures that vary as functions of the vibrational phase difference and reduced mass of the atomic pair, and represent a measure of the strength and thermal evolution of the corresponding relative vibrations.

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