Synthesis and bandgap oscillation of uncapped, ZnO clusters by electroporation ofvesicles

Abstract

A convenient preparation is reported for subnanometre size uncapped ZnO quantum dots,which permits the previously inaccessible live observation of the growth of the clusters inthe molecular size regime. The preparation method utilizes electric field-induced transientpore formation (electroporation) in synthetic unilamellar vesicles. This conditionallows for facile monitoring of the time-dependent UV spectra associated withthe growth of the clusters which are found to initially exhibit novel, oscillatingred and blue shifts of the characteristic absorption band, ultimately followedby a monotonic red shift—the latter reflecting cluster growth beyond a size of∼10 Å.Through a comparison of the observed oscillating transition energies with the corresponding trendsfound theoretically by others, the wavelengths of the sequential spectral peaks can be assigned to the(ZnO)1 monomer(5.66 eV), dimer (ZnO)2 (5.19 eV), (ZnO)5 (6.21 eV), (ZnO)12 (5.76 eV), and (ZnO)15 (6.01 eV).Growth beyond (ZnO)15 is associated with the customary monotonic red shift of the absorptionband (5.59 and 4.95 eV). The reason for oscillation of red and blue shiftof the HOMO–LUMO gaps was explained by the structural differences ofZniOi (i = 1–15). Under the experimental conditions used, a stable systemis reached after 12 days. This solution is estimated to contain1.4 × 1017 (ZnO)15 particles, each with agreatest dimension of ∼10 Å.