Production of Natural Quantum Wires

Zhaoxia (Ivy) Ji, Julo Warzywoda, Kate Ziemer, and Al Sacco, Jr.

The ability to engineer advanced materials with novel electronic, optical, and magnetic properties is important in the search for smaller, faster, and more efficient products and processes. The electronic properties of semiconductor materials are much different than those of both molecules and bulk materials when at least one dimension becomes smaller than a few tens of nanometers. These are called the quantum confinement effects and can be 0-dimensional (quantum dots), 2-dimensional (quantum wells), or 1-dimensional (quantum wires). The energy quantization effects of quantum wires are prominent; however, quantum wire fabrication has considerable technical complexity and cost. All conventional manufacturing techniques have severe size and geometry limitations since the dimensions processed are on the atomic-scale.

ETS-4 (Figure 1) and ETS-10 (Figure 2) are crystalline titanosilicates with mixed octahedral - tetrahedral framework structures. One of the significant characteristics of ETS type materials is the presence of titanium chains in their framework. These geometrically well-defined chains are formed by the corner sharing TiO6 octahedra, which are isolated from each other by the corner sharing SiO4 tetrahedra. This enables them to behave as one-dimensional monatomic nanowires that exhibit peculiar optical properties determined by quantum confinement. The quantum size effects of titanium chains offer distinctive features for applications in the field of optoelectronics, chemical sensors, and photochemistry. The effective length of the titanium chains in ETS-4 and ETS-10 is limited by the small dimensions of the crystallites, and the presence of lattice defects. Recently, we have disclosed a patent application on a methodology that enables the preparation of large single ETS-4 crystals with controlled characteristics. We have also developed methodologies to control the size, morphology, and defect concentration in ETS-10 crystals, as well as grow large crystals of ETS-10. With these novel methodologies it is possible to tailor both ETS-4 and ETS-10 crystals with desired properties for utilization as quantum wire arrays.

Figure 1: FE-SEM images of the ETS-4 products grown from synthesis mixtures with molar composition 3.6SiO2 : 1TiO2 : 5.5Na2O : xH2SO4 : 230.2H2O: (a) x = 4.4, (b) x = 3.6, (c) x = 3.4, (d) x=3.3 (Yilmaz et al., Microporous and Mesoporous Materials, 2004, 71:167-175).