Atom Probe Tomographic Imaging of PbS Quantum Dot Formation on Neodymium Clusters in Silicate Glasses

The first 3-D direct observation of clusters of Nd oxide inside silicate glasses was achieved using atom probe tomography. Three-dimensional elemental maps of major chemical elements in glasses such as Si, Al, Zn and O showed no evidence of regions that had concentrations higher than the average values, whereas the Nd aggregated into regions of high concentration. Elemental maps of Nd and Pb recorded from the glasses containing PbS QDs showed highly-concentrated areas of both elements at the same locations; this result indicates that PbS QDs formation started in association with the Nd clusters.

Nd (Fig. 2g) were distributed homogeneously. Nd concentration ([Nd]) was high in many regions. This is the first visual three-dimensional (3D) mapping of RE clusters in a glass. Clustering of RE ions in solid matrices has been considered as a major source of the energy transfers that degrade the emission efficiencies of REs. For example, fluorescence intensity of the Tb 3+ : 5 D 3 → 7 F 5 transition decreases as [Tb 3+ ] in the glass increases 37 . Indirect evidence such as decrease in fluorescent lifetimes and fluorescent intensities of RE ions at certain energy levels have been used to propose the presence of clustering, but this is the first 3-D direct observation of regions with RE ion concentration higher than those expected from the homogeneous distribution.
We further performed computational analysis using the iso-surface imaging method to visualize areas of high [Nd] clearly. The 1.40% Nd-iso surface analysis shows clear images of Nd-rich regions inside the matrix (Fig. 3a). A similar analysis for Si did not reveal any images of clustered areas, because Si was distributed homogeneously in the specimen so the analysis did not form a closed surface (Fig. 3b). The elemental concentration profile (Fig. 3c)  www.nature.com/scientificreports www.nature.com/scientificreports/    (Fig. 4g).
We also conducted iso-surface compositional analyses for areas of clustered Nd and Pb. Results of the analyses with 1.40% Nd iso-surface concentration contour showed Nd-rich regions (Fig. 5a) similar to those in Fig. 3a. In addition, areas with high [Pb] (i.e., PbS QDs) were detected in glasses (Fig. 5b). In the same analysis, Si and Al atoms did not show clustering. Combination of Fig. 5a,b proved that Nd clusters almost coincide with the location of PbS QDs (Fig. 5c); this result suggests that the growth of PbS QDs is closely related to the presence of Nd clusters (Fig. 5d) and it is consistent with the results of electron energy loss spectroscopy (EELS) (Fig. S1) 28 . The elemental concentration profile along one QD showed that [Pb] increased to ~10 at.%, and [Nd] increased to 12 at.% (Fig. 5e).  www.nature.com/scientificreports www.nature.com/scientificreports/ Nd cluster analysis using the maximum separation method. We visualized Nd clusters in 3D elemental mapping (Fig. 2) and iso-surface images (Fig. 3). We also used the maximum-separation method. The value of χ 2 is usually determined from the distribution of Nd inside the matrix: χ 2 = 0 suggests a random distribution; χ 2 > 0 indicate that clusters exist [38][39][40] . The glass specimen that contained 5.0 mol.% of Nd 2 O 3 had χ 2 ~0.93 before heat treatment. This result is another evidence of a non-uniform and heterogeneous distribution of Nd in the glass matrix.
Afterwards, the Nd-Nd nearest-neighbor analysis between cluster and matrix was obtained from Fig. 6 to determine the nearest neighbor distance D max between Nd atoms. Evaluation considered 14471 blocks of 100 atoms. D max was ~1.0 nm in the clusters ~1.5 nm in the glass matrix. Other parameters considered are Order, which is the number of nearest Nd atoms around a specific Nd atom; N min , which is the minimum size of cluster that can be considered as a cluster; and E, which is the erosion distance at which an atom can be considered to have moved from the cluster to the matrix 41  We have demonstrated the presence of Nd clusters in silicate glasses by using atom probe tomography and 3D elemental mapping. Distributions of the major chemical elements that constitute the (e.g., Si, Al, O) showed no evidence clustering. When the glasses were heat-treated at 500 °C for 30 h to precipitate the PbS QDs, areas of high [Nd] and [Pb] formed in the glasses, and closely coincide with each other; this result indicates that formation of PbS QDs most probably started at the Nd clusters.

Method
Glass preparation. A base glass with a nominal composition of 50SiO 2 -5Al 2 O 3 -25NaO-10BaO-8ZnO-2ZnS-0.8PbO (in mol%) and a Nd-glass with an additional 5 mol% of Nd 2 O 3 were prepared using a conventional melt-quenching process. Starting powders were mixed and melted in an alumina crucible at 1340 °C for 30 min. Each melt was poured onto a brass mold that had been preheated at 380 °C, then was quenched by pressing with another brass plate. Resulting glasses were annealed at 380 °C for 2 h to eliminate residual thermal stress. Specimens were polished to ~150-μm thickness, then used as a source for fabrication by FIB of specimens for APT.
Atom probe tomography (Apt). APT analysis (CAMECA, LEAP4000X HR) was performed using a pulsed UV laser (λ = 355 nm) and each atom was detached from the surface by irradiating the laser beam with  Inset table: compositional difference between clusters and glass matrix.