Fig. 1. (Color online) Synthesis of thin Na₂Ta₄O₁₁ flakes via CVD setup and crystal structure of Na₂Ta₄O₁₁. (a) Schematic diagram of the CVD setup used for synthesis of Na₂Ta₄O₁₁ flakes. (b) Top view of a single layer of TaO₇ pentagonal bipyramids. (c) Polyhedral view of the local coordination environments in Na₂Ta₄O₁₁ for a single TaO₆ octahedron. The O, Ta and Na atoms are represented by blue, silver and red balls, respectively.

Fig. 2. (Color online) Characterizations of CVD-grown thin Na₂Ta₄O₁₁ flakes. (a) Representative optical image. (b) AFM characterization. (c) XRD characterization. (d) Typical Raman spectra of Na₂Ta₄O₁₁ flakes.

Fig. 3. (Color online) Structure characterization of CVD-grown Na₂Ta₄O₁₁ flakes on sapphire. XPS core level spectra of (a) Na 1s, (b) O 1s, and (c) Ta 4f. (d) HRTEM image and (e) FFT pattern image of thin Na₂Ta₄O₁₁ flakes. (f) SEM image for analysis of EDS. (g) The corresponding EDS spectra of the Na₂Ta₄O₁₁ flake in (f).

Fig. 4. (Color online) Deposition time-dependent optical images of Na₂Ta₄O₁₁ flake. (a–d) Optical images of Na₂Ta₄O₁₁ with deposition time at 30, 50, 60, and 90 min. Scale bars: 20 µm. (e–g) Representative AFM images of Na₂Ta₄O₁₁ flake with deposition time from 30 to 60 min. (h) Average size (purple star) and thickness (blue square) as a function of growth time.