Structure and pressure inside Xe nanoparticles embedded in Al

Crystalline and amorphous xenon nanoparticles were produced in aluminum by ${\text{Xe}}^{+}$ ion implantation and were characterized with high-resolution electron microscopy combined with electron energy loss (EEL) spectroscopy. Unusual distributions of the aspect ratio and of the diameter of the crystalline particles were observed and explained, respectively, by minimization of the surface energy and of the strain energy due to the specific lattice mismatch between Al and Xe. Matrix oxidation was revealed as an important phenomenon accompanying the amorphization of Xe particles. Significant variation of relative EEL peak intensities with Xe particle size was observed and associated with unequal pressure inside different particles. The thus revealed variation was utilized to map the pressure distribution inside individual particles with a nanometer spatial resolution.

Figure 1

HAADF STEM image of Xe precipitates in polycrystalline Al (scale bar 10 nm, $⟨110⟩$ axis normal to the picture). Note the coexistence of crystalline $\left(c_{1,2}\right)$, semicrystalline $\left(s_{1,2}\right)$, and amorphous $\left(a_{1,2}\right)$ particles of similar sizes. Crystalline particles exhibit cubo-octahedral or octahedral $\left(o_{1,2}\right)$ shapes, and the $⟨110⟩$ projection of the particle $o_1$ contains only nine atoms. The particles in the bottom ($c_2$ and $s_2$) and top areas ($c_1$, $o_{1,2}$, and $s_1$) belong to different Al grains; they share a $⟨110⟩$ vertical axis but have different in-plane orientations. The arrow marks one of the numerous small white shapeless features, which do not contain Xe. Drawing over particle $c_1$ outlines the definition of its normalized aspect ratio $A=N_{100}/2N_{111}$ in terms of the number of (111) and (100) Xe planes.

Figure 2

(Color online) Representative line scans of the integrated oxygen $K$-edge EEL signal acquired across a crystalline $\left(c_1\right)$, semicrystalline $\left(s_1\right)$, and amorphous $\left(a_1\right)$ particles of Fig. 1. Oxygen accumulation at the Xe/Al interface is revealed for the amorphous particles.

Figure 3

(Color online) Histograms of Xe nanoparticles in Al as a function (a) of the particle size and (b) of the number of Xe (111) or (100) crystalline planes. Note a coexistence of crystalline, semicrystalline, and amorphous particles in the diameter range 7–10 nm (a). Details of fitting the (111) distribution [dashed line in (b)] are outlined in the text.

Figure 4

(Color online) Top panel: the aspect ratio $A$, defined in Fig. 1, as a function of the volume of the crystalline Xe particles in Al. Bottom panel: surface energy calculated for three particles; minimum energy is set at zero. The energy fluctuations (horizontal dashed line) define the maximal range of $A$ values $\left(A\le 1\right)$, which is presented by lines in the top panel.

Figure 5

(Color online) Electron energy-loss spectrum from an amorphous 40-nm Xe particle in Al and a Xe gas reference spectrum (Ref. 14), both measured with $\sim 0.8\text{eV}$ resolution. The spectra are plotted in a double-logarithmic scale for presentation purposes. Symbols ${\text{O}}_{23}$, ${\text{N}}_{45}$, and ${\text{M}}_{45}$ conventionally identify the associated atomic transitions, while $A$ and $B$ label the peaks used in the pressure measurements discussed below.

Figure 6

(Color online) Bottom part: lines and symbols show background subtracted EEL spectra from Xe particles in Al with the sizes 3, 6, and 40 nm. Second-order polynomials were used as the baselines and the spectra are normalized to the particle diameter. Top part: lines show a reproduction of the high-resolution EEL spectra from Xe gas (Ref. 16) measured in $0°$ and $90°$ scattering geometries. The $90°$ spectrum reveals an extra strong forbidden peak $B_2$, which explains the variation of the $B$-peak intensity with Xe particle size.

Figure 7

(Color online) Solid squares present the ratio of $B$ to $A$ EEL peak intensity (see Figs. 5, 6) as a function of particle size. The latter has been converted into pressure inside the particles using the previously reported data (Ref. 17). Dashed line is a second-order polynomial fit.

Figure 8

Top panel: high angle annular dark field image of Xe nanoparticles (white features) in Al. Bottom panel: map of pressure inside the particles (in GPa) deduced from the $B/A$ EEL peak intensity ratio (see Figs. 6, 7). Note that the top left particle exploded during the scan and that the pressure inside the smallest particles could not be deduced because of the large thickness in this sample area.