Field-emission characteristics of self-assembled Si-capped Ge quantum dots on Si (001) with different Si coverages have been investigated. During capping with Si, Ge quantum dots exhibit a dramatic shape transition from multi-faceted domes to truncated pyramids, and eventually to nanorings. With an appropriate amount of Si-capping to form the truncated pyramids, the field-emission behaviors of Si-capped Ge quantum dots were found to be improved significantly. Based on transmission electron microscope examinations, this improvement can be attributed to the sharper apex of the truncated pyramids as compared to the uncapped domes. However, further Si-capping could degrade the field-emission properties owing to the flattening of Ge islands features. This work provides a useful scheme to utilize self-assembled Si-capped Ge quantum dots as field-emitter arrays. (c) 2005 Elsevier B.V. All rights reserved.