Electronic properties of assemblies of zno quantum dots

Abstract

Electron transport in an assembly of ZnO quantum dots has been studied using an electrochemically gated transistor. The electron mobility shows a step-wise increase as a function of the electron occupation per quantum dot. When the occupation number is below two, transport occurs by tunnelling between S orbitals. Transport becomes three times faster when the occupation number is between two and eight; tunnelling now occurs between the P orbitals. Electron transport is thus critically determined by the quantum properties of the building blocks.

The results for aqueous electrolytes are compared with the results obtained with propylene carbonate solutions. The storage and long-range transport of electrons in a porous assembly of ZnO quantum dots is found to depend strongly on the interpenetrating electrolyte. The screening of the electron charge is less effective in the case of an assembly permeated with a propylene carbonate electrolyte solution. The effect of temperature on the conductance of ZnO assemblies permeated with propylene carbonate and ethanol is reported.

The optical transitions in few-electron artificial atoms strongly confined in ZnO nanocrystals with a diameter between 3 and 6 nm are reported. We use an assembly of weakly coupled ZnO nanocrystals in which electrons are injected electrochemically; the average electron number is obtained from the injected charge and the number of quantum dots in the assembly. The charged ZnO nanocrystals show broad spectra in the near IR, the shape and total absorption intensity being determined by the average electron number only. The spectra can be explained by taking into account the allowed electric dipole transitions between the atom-like orbitals of the ZnO nanocrystals and the size-distribution of the nanocrystals in the sample.

Assemblies of ZnO nanocrystals can be charged with electrons; the average electron number can be varied in a controlled way between zero and ten. The luminescence of such charged assemblies due to UV excitation has been studied and compared with the luminescence of quantum-dot suspensions. The luminescence of ZnO assemblies depends on the electron number and the nature of the interpenetrating electrolyte. Remarkably, with increasing electron number, the green (defect) luminescence quenches, while the exciton luminescence increases. Thus, quenching of the exciton luminescence by an Auger-type process is unimportant in charged ZnO quantum dots. This forms a strong contrast with results obtained with Si and CdSe quantum dots.

Electron transport in macroporous GaP networks permeated with an electrolyte solution has been studied by analysis of the photocurrent response upon a small-amplitude modulation of the light intensity. It is found that under conditions close to a steady-state, electron transport is non-dispersive, characterized by a single transit time that depends on the thickness of the porous layer and the background light intensity. An analysis of the results show that the transit time is determined by multiple trapping and detrapping of electrons in interfacial states close to the electron Fermi level which is determined by the steady-state light intensity. As a result the density-of-states (DOS) function in a considerable region of the band gap can be determined from the transit time, when the energy of the electron Fermi level is changed by means of the background light-intensity.