Electron confinement effects in the EPR spectra of colloidal n-type ZnO quantum dots

Additional unpaired electrons have been introduced into colloidal ZnO quantum dots (QDs) photochemically and investigated by experimental and theoretical methods to test various possible descriptions of their wave functions. For n-type ZnO QDs with diameters between 3.0 and 7.0 nm, electron paramagnetic resonance (EPR) spectroscopy reveals size-dependent g* values in the range 1.960 < g* < 1.968 that are temperature independent and that rule out highly localized wave function descriptions. The size dependence of g* is described well using a k.p perturbation expression, indicating similarity between these quantum confined electrons and free carriers in bulk ZnO. Model calculations confirm that significant electron density can reside on the QD surfaces only for surface well depths that can be experimentally excluded. Together, these results allow a firm experimental description of the photogenerated electrons as delocalized within the conduction bands of the colloidal ZnO QDs, making them excellent candidates for investigation of spin effects in semiconductor nanostructures.