(263b) Characterization of Quantum Wires in Engelhard Titanosilicates ETS-4 and ETS-10

Today's computers and electrical devices are limited by the size and conductivity of the wires that compose them. Quantum wires have diameters less than 10 nm, which is the de Broglie wavelength of an electron, and high conductivity. Consequently, they could lead to much faster and smaller computers. Single quantum wires are difficult to fabricate and arrange into circuits or arrays, and easy to break. Titanosilicate ETS-4 and ETS-10 crystals may provide a solution to this problem since they are considered to have naturally occurring quantum wire arrays that are formed by the semiconducting monatomic ?Ti?O?Ti?O? chains in their structures.¹ The monatomic chains are 6.7Å in diameter and are insulated from each other by silica. ETS-4 has chains that run in the crystal b direction while ETS-10 has non-intersecting chains that run in the crystal a and b directions. TEM analysis has shown chains are 250 Å long on average.² The advantage of quantum wire arrays such as these in ETS-4 and ETS-10 over single wires is that electrons can tunnel between wires when there are defects in these wires. Also, although silica in the ETS materials provides a barrier to electrons, when electrons reach the right energy they are able to tunnel through the insulator to other wires. This phenomenon is called resonance tunneling.³

Large monolithic ETS-4 crystals with rectangular prism morphology and length (i.e., crystal b dimension) up to ~200 μm, and large plate-like ETS-10 crystals up to ~80 μm in a and b directions were synthesized and used in optical (UV-vis) and current-voltage (I-V) characterization. Such crystals are more desirable for analysis and applications due to the convenient morphology/size, and because larger crystals have been shown to have fewer defects. DR-UV-vis analysis of large ETS-4 and ETS-10 crystals showed a blue shift of 50 to 60 nm when compared to bulk titania. This is expected since quantum confinement increases bandgap energy.⁴ I-V curves were measured using electrical microprobes at room temperature and crystals integrated into a device with gold electrodes for low temperatures. For both types of crystals at room temperature, current was observed to increase non-linearly with voltage, peaked at 5 V for ETS-4 (8 V for ETS-10), then experienced negative resistance and decreases dramatically.⁵ These initial results are consistent with quantum tunneling theory, although other phenomena are also possible. Further work is needed to verify these results and fully characterize I-V behavior for both ETS-4 and ETS-10.

1. B. Yilmaz, J. Warzywoda, A. Sacco Jr., J. Crystal Growth, 271 (2004) 325-331.

2. S. M. Kuznicki, et al, Nature, 412 (2001) 720-724.

3. H. Sakaki, Journal of Crystal Growth, 251 (2003) 9-16.

4. B. Yilmaz, PhD Thesis, Northeastern University, Boston, 2006.

5. Ö. Özkanat, MS Thesis, Northeastern University, Boston, 2009.