The modulation of optoelectronic properties, such as the bandgap of a pure-component semiconductor material, is a useful ability that can be achieved by few techniques. Atomic layer deposition (ALD) was used here to experimentally demonstrate the ability to deposit films that exhibit quantum confinement on three-dimensional surfaces. Polycrystalline ZnO films ranging from ~1.5 to 15 nm in thickness were deposited via ALD using diethylzinc and hydrogen peroxide at 100 °C. Conformal, pinhole-free films were deposited on Si wafers and on nanosized spherical SiO₂ particles using an augmented central composite design strategy. Powder x-ray diffraction was used to measure the crystallite size of the films and monitor size evolution on the basis of the number of ALD cycles and thermal annealing post-treatments. The absorbance of the ZnO films on Si wafers and SiO₂ particles was measured using spectroscopic ellipsometry and diffuse transmittance techniques, respectively. Post-deposition annealing steps increased the crystallite size of the films, independently of the coating thickness. The ZnO bandgap was increasingly blue-shifted for films of decreasing crystallite size, approaching +0.3 eV at dimensions of 2–3 nm. The nonlinear bandgap response correlated well with the Brus model. This work represents an experimental demonstration of quantum confinement using ALD on two- and three-dimensional substrates.