Abstract
This thesis deals with the properties of semiconductor nanocrystals (ZnS or ZnSe) in the size range (diameter) of 2 nm to 10 nm. The nanocrystals under investigation are doped with the transition metal ions manganese or copper. The goal is to study photoluminescence and electroluminescence from doped ZnS and ZnSe
... read more
nanocrystals, and to describe simulations and theoretical work on the distribution of dopant ions in nanocrystals.
The influence of the synthesis conditions on the properties of ZnS:Mn2+ nanocrystals is described. Different precursors and different ratios of the precursor concentrations were used and interesting effects were observed that could be explaned sucesfully by the proposed luminescence mechanism. This mechanism also explains the observed temperature dependence of the emission intensities and emission profiles. The photoelectrochemical properties of the nanocrystals are presented and discussed. Both anodic and cathodic photocurrent is observed, giving direct evidence for the nanocrystalline nature of the system. Due to the unfavorable kinetics of electron and hole transfer, the photocurrent is small and most of the charge carriers generated by illumination recombine.
Because of the unfavorable kinetics, the synthesis and luminescence properties of nanocrystalline ZnSe:Mn2+ and ZnSe:Cu (prepared via a high-temperature synthesis in a dry-nitrogen atmosphere) was investigated next. The dopant emissions can both be excited via the ZnSe host lattice, indicting incorporation of the dopants into the lattice. Evidence for the preferential formation of Mn2+ dopant pair-states is presented through the luminescence and lifetime measurements. Temperature-dependent photoluminescence spectra and photoluminescence lifetime measurements are also presented and the temperature dependence of the Mn2+ emission energy and bandwidth is explained by electron-phonon coupling to an optical phonon of the ZnSe host lattice. During the growth of the nanocrystals, samples are taken and the luminescence is studied as a function of the particle size. The transition between two growth-mechanisms is observed. The growth-rate of the nanocrystals is strongly dependent on the synthesis temperature. The size of the crystals influences both the ZnSe and the Cu2+ luminescence energies due to the quantum confinement, but not the Mn2+ emission due to the local nature of this transition. Temperature-dependent measurements of the luminescence intensity, lifetime and peak positions are described. The quenching of the Cu2+ related luminescence and concomitant decrease of the lifetime is related to detrapping of an electron from a Coulomb-bound electron-hole pair. Finally, electrodes of ZnSe:Cu nanocrystals on a transparent conducting substrate (indium tin oxide) were fabricated and the potential dependence of the photoluminescence of these electrodes in an aqueous electrolyte was studied. Electroluminescence of the ZnSe:Cu nanocrystalline electrodes was observed but the efficiency was likely to be low.
At the end of the thesis, simulations and a mathematical theory are presented in order to calculate the probability for dopant pair-state formation in a nanocrystal. Knowing this probability is interesting because for certain dopants luminescence properties of single-ions and pair-states of dopant ions differ markedly. As it turns out, the probability for pair-state formation is size dependent due to the changing surface to volume ratio as a function of the crystal size. In the simulations, the probability of finding at least one pair-state in the nanocrystal and the concentration of ions in pair-states were calculated based on many simulations for the same crystal size and dopant concentration. Then a technical part discusses how these results can be approximated (through Stein-Chen Poisson approximation for the probability of finding at least one pair-state) and predicted (by means of an exact analytical expression for the concentration of ions in pair-states) without simulations. These results are very powerful, as they were derived for a general crystal structure and are dependent on the connectivity structure, dopant concentration and crystal size. Therefore, these results are expected to be valid for any nanocrystal and no further simulations will be required.
show less
