Zinc oxide nanoclusters have been predicted as promising building blocks for cluster-assembled materials with unprecedented properties. Here, for the first time these clusters are probed by time-resolved photoelectron spectroscopy and characterized in detail by density functional theory. Their validity as building blocks for cluster-assembled materials is confirmed via rigid cage-like structures facilitating three-dimensional aggregation in combination with large band gaps that are nevertheless significantly lower than any known ZnO polymorph. In addition, electron-hole pair localization in the excited state of the cluster anions combined with their structural rigidity leads to extraordinary long-lived states above the band gap virtually independent of the cluster size, defying the rule "every atom counts".

Small semiconductor nanoclusters exhibit unique properties often very different from those of their atomic and bulk counterparts. Their better understanding and characterization is expected to be useful in the development of highly functional cluster-assembled materials (CAMs) with tunable properties. In this work, the structural and electronic properties of size-selected (CdO) n – clusters were examined by conventional and time-resolved spectroscopy (TR-PES) combined with density functional theory (DFT) calculations. The observed highly symmetric alternant-cage structures and large band gaps confirm the validity of those species as CAM building blocks. Moreover, our results demonstrate a striking similarity between (CdO) n – and (ZnO) n – clusters of the same size that is not restricted to the ground state but also comprises properties of excited states. We suggest that in general valence isoelectronic small binary (XY) n clusters might exhibit similar structures and comparable properties. The long lifetimes of excited states observed in our experiment are proposed as a general probe helpful in identifying suitable structures for CAMs and hence simplifying their design.

The impact of size-selected Ag clusters on graphite: anSTM study S J Carroll, P Weibel, B von Issendorff et al. - Recent citations

Bare metal clusters have properties that make them interesting for applications in photochemistry and photovoltaics. Long-lived excited states are a prerequisite for such applications, because in them the energy of the photon can be stored. Clusters have a low density of states and long-lived excited states should therefore occur frequently. However, in fact, such states are a rarity, as indicated by time-resolved photoelectron data of mass-selected cluster anions. And there is another puzzling observation: only clusters with narrow peaks in their photoelectron spectra exhibit long-lived excited states. Both findings can be explained if internal conversion, i.e. the conversion of electronic excitation energy into vibrational excitations, is the major relaxation mechanism in clusters. It becomes more likely, if a change of the electronic configuration results in a large geometry change, which is probably the case for most clusters. Only clusters with a weak coupling between geometric and electronic structure may have long-lived excited states and narrow peaks.