Electron transport mechanism of titania chains in the framework of titanosilicates ETS-4 and ETS-10

Current techniques for preparation of low-dimensional devices present technical difficulties and concomitant costs. This is especially true in the fabrication of aligned quantum wire arrays. Microporous crystalline titanosilicate materials, ETS-4 and ETS-10, are hypothesized to have naturally occurring quantum wires in their framework. They contain monatomic titania chains (...Ti-O-Ti-O-Ti...) isolated from each other by a highly siliceous matrix. These titania chains (∼ 6.7 A in diameter) in ETS-4 run in only the b direction, while they run in both a and b directions in ETS-10. Electron transport properties of titania chains --as hypothesized quantum wires- in the framework of individual ETS-4 and ETS-10 crystals were studied. Current-voltage performance (I-V curves) of both ETS-4 and ETS-10 were investigated at room temperature by placing 2 microprobes on the surface of the individual crystal. It was verified that conduction occurs through the defects in titania chains. Furthermore, the main feature of the current-voltage curve was identified as a current peak due to resonant electron tunneling between titania chains through the insulating layer. Peak to Valley Ratio (PVCR) for 15mu interval was determined as 3.4 and 5.4 for ETS-4 and ETS-10, respectively. It was observed that the PVCR's are decreasing with increasing intervals in the direction of titania chains and for intervals larger than 150mu (ETS-4 only) these current peaks disappeared. Current-voltage curves were also obtained in the dimension/direction where there are no titania chains. The observed current jumps with a higher PVCR supports the hypothesis of resonance electron tunneling through insulating layer of SiO4. Low temperature current-voltage behavior of ETS-4 was investigated by performing the device integration of large individual crystal. Non linear I-V behavior at low voltages was observed. These nonlinear curves were attributed to electron transport through discrete titania chains via tunneling. Current values were observed to be decreasing with decreasing temperatures as expected for bulk semiconductors. However, at higher voltages, similar to the results obtained by probe station at room temperature, current jumps followed by negative differential resistance was observed. PVCR of the peaks were observed to be increasing and peaks shifted to higher resonance voltage values with decreasing temperature. Increase in PVCR upon decreasing temperature also supports the hypothesis of resonance tunneling through insulating siliceous matrix. These results taken in total provide a basis for electron transport mechanism of the monatomic titania chains in ETS crystals which can be utilized in future's optoelectronic devices.