(5cc) Quantum Wires in Titanosilicate Ets-4

Yilmaz, B., CAMMP/Northeastern University
Warzywoda, J., Northeastern University
Sacco, Jr., A., Northeastern University

One-dimensional nanostructures (quantum wires) represent a link between molecular and solid state physics. They exhibit electrical, optical, magnetic and thermodynamic properties that are essentially related to their extremely small dimensions. These ?quantum size effects? are a consequence of the quantum confinement of the carriers. Quantum wires offer great promise for novel applications, especially in the fields of microelectronics, optoelectronics and non-linear optics. However, quantum wire preparation presents a problem of considerable technical complexity and cost. Techniques such as ?conventional microfabrication? have severe size and geometry limitations, due to their inherent atomic scale dimensions. ETS-4 is a nano-porous crystalline titanosilicate. Its framework contains linear monatomic ?Ti?O?Ti?O?Ti? chains, which are effectively isolated by an insulating siliceous matrix. These chains can exhibit quantum confinement effects. However, randomly located defects, frequently observed in the ETS-4 crystal structure, produce unwanted discontinuities. Additionally, since the ?Ti?O?Ti?O?Ti? chains run along the b-axis, there is a need to control the morphology and size of ETS-4 crystals (in the b direction), as well as the formation of lattice defects. Methods for controlling morphology, crystal quality, relative amount of defects and degree of intergrowth of ETS-4 were developed. A progressive change of ETS-4 morphology from polycrystalline spherulites to monolithic crystals with rectangular prism morphology was achieved by adjusting the synthesis mixture composition 3.6 SiO2 : 1 TiO2 : 5.5 Na2O : x H2SO4 : 230.2 H2O, where x was varied between 3.3 and 4.4. The X-ray powder diffraction and energy dispersive X-ray spectroscopy indicate a smaller number of Ti vacancies (defects) in monolithic crystals compared to spherulites. The largest dimension of these monolithic crystals was determined to coincide with the ?Ti?O?Ti?O?Ti? chains running in the b-direction. Variation of cation type and concentration allowed the effective control of the size of monolithic ETS-4 crystals. This novel strategy resulted in large ETS-4 crystals with an average length of ~200 μm. Single crystal X-ray diffraction analysis demonstrated that these are single crystals with no detected intergrowth. Detailed investigation of the optical band gap transition in ETS-4 was performed for the first time. A significant shift of the optical band gap more than 60 nm to the deeper UV region was observed for ETS-4 when compared to the bulk titania polymorphs. Such a blue shift is the verification of the quantum confinement of the titania chains in ETS-4, and indicates that these chains are indeed quantum wires. The diffuse reflectance UV-vis (DR-UV-vis) and Raman spectra acquired for diverse ETS-4 samples exhibited varied characteristics. This was attributed to variations in the quantum wire quality. Integration of the ETS-4 quantum wire arrays into micro devices was explored. A photolithography based fabrication methodology was developed to individually address an ETS-4 crystal and incorporate it into a micro device. Utilizing this methodology a novel micro device with an ETS-4 crystal as the quantum wire array component was prepared. Highly oriented (b-out-of-plane) ETS-4 films were also fabricated in situ on various substrates. Since the quantum wires in ETS-4 crystals run in the b-direction only, in principle these films are aligned quantum wire arrays. These films can be utilized to test the potential of ETS-4 crystals as quantum wire components of thermoelectric, optoelectronic and photovoltaic devices.