Abstract
Various types of 2% Cu-incorporated CdS (Cu:CdS) quantum dots (QDs) with very similar sizes have been prepared via a water soluble colloidal method. The locations of Cu impurities in CdS host nanocrystals have been controlled by adopting three different synthetic ways of doping, exchange, and adsorption to understand the impurity location-dependent relaxation dynamics of charge carriers. The oxidation state of incorporated Cu impurities has been found to be +1 and the band-gap energy of Cu:CdS QDs decreases as Cu2S forms at the surfaces of CdS QDs. Broad and red-shifted emission with a large Stokes shift has been observed for Cu:CdS QDs as newly produced Cu-related defects become luminescent centers. The energetically favored hole trapping of thiol molecules, as well as the local environment, inhibits the radiative recombination processes of Cu:CdS QDs, thus resulting in low photoluminescence. Upon excitation, an electron is promoted to the conduction band, leaving a hole on the valence band. The hole is transferred to the Cu+ d-state, changing Cu+ into Cu2+, which then participates in radiative recombination with an electron. Electrons in the conduction band are ensnared into shallow-trap sites within 52 ns. The electrons can be further captured on the time scale of 260 ns into deep-trap sites, where electrons recombine with holes in 820 ns. Our in-depth analysis of carrier relaxation has shown that the possibilities of both nonradiative recombination and energy transfer to Cu impurities become high when Cu ions are located at the surface of CdS QDs.
Highlights
With the great growth of interest in the field of nanochemistry, the study of nanocrystalline semiconductors has become increasingly important over a few decades due to their unique optical and electrical properties compared with the respective ones of bulk semiconductors [1, 2]
The crystallinity of CdS host nanocrystals has been affected by incorporated Cu ions due to lattice mismatches, the oxidation state of incorporated Cu impurities has been found to be +1, and the bandgap energy of Cu:CdS Quantum dots (QD) gradually decreases as Cu2S forms at the surfaces of CdS QDs
The broader emission band with a large Stokes shift has been observed for Cu:CdS QDs as newly produced Cu-related defects play an important role in the emission process
Summary
With the great growth of interest in the field of nanochemistry, the study of nanocrystalline semiconductors has become increasingly important over a few decades due to their unique optical and electrical properties compared with the respective ones of bulk semiconductors [1, 2]. In the nucleation-doping strategy, the formation of the host nanocrystals has been reported to occur under established synthetic conditions and to be quenched by lowering the reaction temperature [4]. Active dopant precursors were introduced and doping took place without the growth of the host. Growth doping was realized by mixing dopant and host precursors during nucleation. The reaction conditions were tuned to be sufficiently mild to make the dopant precursor inactive, and the growth of the host became the only process, over-coating the dopant. The doping strategy has affected the dopant locations and the host optical properties, little research has been achieved to reveal the influence of dopant locations on the luminescence properties of host QDs systematically
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