Self-assembling of nanocavities in ${\mathrm{TiO}}_2$ dispersed with Au nanoclusters

There has been considerable research effort on tailoring the nonlinear optical properties of dielectric materials by dispersing nanometer-sized metallic clusters in them. It has been proposed that the optical response of this type of material is related to the quantum antidots (vacancy clusters), which is spatially located at the interface between the metal cluster and the dielectric matrix. In order to clarify the vacancy clustering behavior as well as its correlation with Au clustering, single crystal ${\mathrm{TiO}}_2$ has been implanted with Au ions at $975\mathrm{K}$ and subsequently annealed at $1275\mathrm{K}$ for $10\mathrm{h}$. A characteristic self-assembling of nanocavities along the boundary between the region of Au clusters and the free surface has been observed in the present system. These cavities are faceted along ${\mathrm{TiO}}_2\left(110\right)$ and have a size of $\sim 10\mathrm{nm}$. High angle annular dark-field (HAADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) revealed that vacancy clusters of $\sim 2\mathrm{nm}$ in size also exist in the Au populated regions. Formation of cavities in Au-irradiated ${\mathrm{TiO}}_2$ strongly indicates that vacancy clustering processes prevail over Frenkel-pair recombination. Furthermore, the Au atoms substitution for Ti in ${\mathrm{TiO}}_2$ is also directly observed by STEM-HAADF imaging and by channeling Rutherford backscattering spectrometry.

Figure 1

(Color online) (a) Cross-sectional TEM images showing the general microstructural features of the Au implanted ${\mathrm{TiO}}_2$. The dark spots and the lightly contrasted regions represent Au clusters and cavities, respectively. Note the alignment of the cavities along the boundary layer separating the Au cluster populated region and the Au cluster free region. The inset shows the orientation of the (110) plane faceted cavity with respect to the crystal orientation of ${\mathrm{TiO}}_2$. (b) A representative HRTEM image showing the region includes a cavity. The cavity is faceted along the ${\mathrm{TiO}}_2$ (110) lattice planes. The inset shows the crystal structure with the (110) plane marked.

Figure 2

(Color online) (a) High resolution TEM image showing an Au cluster with an orientation relationship of $\mathrm{Au}⟨110⟩∕∕{\mathrm{TiO}}_2\left[001\right]$ and $\mathrm{Au}\left\{111\right\}∕∕{\mathrm{TiO}}_2\left(110\right)$. The cluster is facetted along the $\mathrm{Au}\left\{111\right\}$ and $\mathrm{Au}\left\{112\right\}$ planes. (b) Interface structural model for Au facetted along $\mathrm{Au}\left(112\right)∕∕{\mathrm{TiO}}_2\left(110\right)$ (inset of the right-hand corner) and the simulated HRTEM image (inset at the middle).

Figure 3

(Color online) (a) STEM-HAADF image showing Au cluster, cavities, and Ti columns in which Au substitutes for Ti as indicated by the arrows. The simulated STEM image was overlaid for which half of the Ti atoms in the center column were substituted by Au atoms as shown in (b). (b) Atomic structural model for which half of the Ti atoms in the center column are substituted by Au atoms and the correspondingly simulated STEM image. Note the high brightness in the Au substituted atomic column.