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

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 in the molecular size regime. The preparation method utilizes electric field-induced transient pore formation (electroporation) in synthetic unilamellar vesicles. This condition allows for facile monitoring of the time-dependent UV spectra associated with the growth of the clusters which are found to initially exhibit novel, oscillating red and blue shifts of the characteristic absorption band, ultimately followed by 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 trends found 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 absorption band (5.59 and 4.95 eV). The reason for oscillation of red and blue shift of the HOMO-LUMO gaps was explained by the structural differences of Zn(i)O(i) (i = 1-15). Under the experimental conditions used, a stable system is reached after 12 days. This solution is estimated to contain 1.4 × 10(17) (ZnO)(15) particles, each with a greatest dimension of ∼10 Å.