Controlling the Quantum Wire Quality in Crystalline Titanosilicates Ets-4 and Ets-10

Bilge Yilmaz, Juliusz Warzywoda, and Albert Sacco Jr. Department of Chemical Engineering, CAMMP/Northeastern University, 360 Huntington Ave, 147 Snell Engineering Center, Boston, MA 02115

Nanowires thinner than ten nanometers in diameter exhibit optical, electrical, magnetic and thermodynamic properties related to their extremely small dimensions. These “quantum size effects” are a consequence of the quantum confinement of the carriers. Nanowires with these quantum size effects are called quantum wires. Quantum wires have attracted attention due to the distinct features (e.g., enhanced transport characteristics) they offer for novel applications, especially in the fields of microelectronics, optoelectronics and non-linear optics. However, their fabrication still presents a problem of considerable technical complexity and cost. Conventional nano-manufacturing techniques have severe size and geometry limitations since atomic-scale dimensions are required. ETS-4 and ETS-10 are members of the Engelhard TitanoSilicate (ETS) family of mixed octahedral/tetrahedral microporous framework materials. In their crystalline frameworks [TiO6] octahedra are linked to form linear …Ti – O – Ti – O – Ti… chains, which are isolated from one another by an insulating siliceous matrix made of [SiO4] tetrahedra. These chains exhibit quantum confinement effects and can behave as quantum wires. These monatomic …Ti – O – Ti – O – Ti… chains (quantum wires) are the thinnest wires that can be hypothesized. However, ETS materials are highly prone to intergrowth and defects. As a result these chains are broken at random points in the crystals, resulting in the discontinuation of the quantum wires. A study was undertaken to examine the ways of controlling the quality of quantum wires in ETS materials. Diffuse reflectance UV-vis (DR-UV-vis) spectroscopy and Raman spectroscopy were utilized for characterization of the quantum wires in a variety of ETS-4 and ETS-10 samples. Detailed investigation of the optical band gap transition in ETS materials was performed. A blue shift of the optical band gap (> 60 nm) was observed for the ETS-4 and ETS-10 samples when compared to bulk titania polymorphs (i.e., anatase, rutile). Such a blue shift can be considered as the verification of the quantum confinement of the titania chains in ETS structures. The DR-UV-vis and Raman spectra acquired for diverse ETS-4 and ETS-10 samples that differed in crystal morphology, degree of intergrowth, and surface topography exhibited different characteristics, which were hypothesized to be related to the quantum wire quality. X-ray diffraction and energy dispersive X-ray spectroscopy analyses provided information on the relative amount of Ti vacancies (defects) along the titania chains (quantum wires) in these diverse ETS samples. Models were proposed for quantum wire growth in ETS-4 and ETS-10 frameworks and potential causes of defect formation along the quantum wires were identified. Comparison of the mechanisms for quantum wire growth in the framework and the spectroscopic behaviour for ETS-4 and ETS-10 revealed that the control of quantum wire quality via synthesis is more feasible in ETS-4.