Goal: Nanoscale systems offer a diverse set of new properties for the improvement of existing technologies.  To fully exploit these materials new analytical techniques must be developed to understand the complex relationship between diverse parameters.  Our goal is to find new ways to determine and describe the important figures of merit for nanoscale systems in a way that these methods are broadly applicable to future technology development.

Studies:

• Describe the effect of variation in grain size, as determined using AFM (Figure 1) and XRD (Figure 2b), to the band gap determined using spectroscopic ellipsometry of mixed Mo_xW_1-xO₃ films.  The inverse relationship between grain size and band gap is attributed to quantum size effects (Figure 2c).
• Characterize the porous network of surfactant templated TiO₂ before and after the infusion of Au nanoparticles.  Spectroscopic ellipsometry is used to show that the Au particles are aligned inside the columnar mesopores (Figure 3).

Figure 1.  1 x 1 mm² AFM phase images for WO₃ films deposited with (a) 5ms (b) 50ms (c) 100ms and (d) 300ms pulse widths.  Estimated band gaps are indicated for each film.

Figure 2. (a) The band gaps of Mo_xW_1-xO₃ films are determined using the imaginary refractive index derived by ellipsometry and an indirect Tauc plot.  (b) The grain size is determined using Scherrer analysis of the XRD pattern.  (c) A clear relationship between grain size and band gap is observed.

Figure 3. Real (a) and imaginary (b) components of the complex refractive index for mesoporous TiO₂ (green dots) as well as the normal (solid) and extreme (red dash) refractive index of mesoporous TiO₂ infused with 20 wt. % Au.  The large k_e in the visible spectrum indicates alignment of the Au particles along the film’s z axis.

Significance: Meeting the diverse challenges of future technology development in applications such as sensors, energy storage devices, catalysts, solar cells, etc. requires both new materials and the capability to characterize them.  Characterization is made difficult by the fact that these materials often have complex compositions and are disordered over long length scales. Additionally, at the nanoscale level material properties arise from a combination of structure and composition.  To address these issues we are developing techniques that can be applied to real world systems and elucidate how the interplay of film composition and structure alters material performance.

Related Publications:

May, R. A.; Kondrachova, L. V.; Hahn, B. P.; Stevenson, K. J. “Optical Constants of Electrodeposited Mixed Molybdenum-Tungsten Oxide Films Determined by Variable Angle Spectroscopic Ellipsometry”. J. Phys. Chem. C.  in press, 2007.

Students currently involved in this project: Alan May