Titanosilicate ETS-10 crystals contain monatomic (diameter 0.67 nm) semiconducting …Ti–O–Ti–O–Ti… chains, which show quantum confinement effects and are regarded as quantum wires [1]. “Quality” of these crystals/quantum wires is expected to affect the performance of electronic devices. It has been hypothesized that larger ETS-10 crystals may have fewer discontinuities in the …Ti–O–Ti–O–Ti… chains, which was indicated by the red shift of the UV absorption edge. Here, Raman and UV-vis spectroscopy were used to probe the “quality” and examine the optical properties of large ETS-10 crystals.

ETS-10 crystals with the average size (L_ave) of 35, 14, 6, and 2 μm were synthesized hydrothermally at 473 K [2, 3]. Raman spectra of these crystals showed all the bands characteristic for ETS-10 [4, 5]. However, the full width at half maximum (FWHM) and the position of the most intense Raman band that was assigned to the symmetric Ti – O stretching vibrations in the …Ti–O–Ti–O–Ti… chains varied from sample to sample. The FWHM of this band increased from 19-21 cm^-1 for the larger (L_ave≥14 µm) crystals to 38-39 cm^-1 for the smaller (L_ave≤6 µm) crystals. In addition, the band position shifted to higher frequency, i.e., from 725 cm^-1 for the crystals with L_ave≥14 µm to 733 cm^-1 for the crystals with L_ave≤6 µm. The band broadening and shift to higher frequency observed for the smaller crystals suggest more stacking defects in these crystals (i.e., wider length distribution of the …Ti–O–Ti–O–Ti… chains). Diffuse reflectance UV-vis spectroscopic analysis showed that the absorption edge of the spectra for the larger ETS-10 crystals appeared progressively more red-shifted from the spectra of the smallest ETS-10 crystals. This red shift may be attributed to the fewer defects present along the …Ti–O–Ti–O–Ti… chains in the larger ETS-10 crystals. Consistent with Raman spectroscopic analysis, this result suggests increased average length of the …Ti–O–Ti–O–Ti… chains embedded in increasingly larger ETS-10 crystals.

[1] C. Lamberti, Micropor. Mesopor. Mater., 30 (1999) 155.

[2] Z. Ji, J. Warzywoda, A. Sacco Jr., Micropor. Mesopor. Mater., 81 (2005) 1.

[3] Z. Ji, J. Warzywoda, A. Sacco Jr., Micropor. Mesopor. Mater., 109 (2008) 1.

[4] N. C. Jeong, M. H. Lee and K. B. Yoon, Angew. Chem., Int. Ed., 46 (2007) 5868.

[5] P.D. Southon and R.F. Howe, Chem. Mater., 14 (2002) 4209.