Dense arrays of vertically aligned titania nanotubes (see Figure) are created directly on silicon substrates by combining atomic layer deposition with an alumina template-based fabrication approach. This method enables excellent control over the nanotube dimensions, including their wall thickness, diameter, and height. Nanotubes were fabricated with tube walls thin enough (< 3 nm) to exhibit a wall-thickness-dependent blue-shift in the optical absorption spectra of the arrays.