Exciton Emission and Light‐Induced Charge Separation in Colloidal ZnO Nanocrystals

Abstract

Adsorption of organic molecules at ZnO nanoparticle surfaces enables the transfer of energy or charge across resulting organic–inorganic interfaces and, consequently, determines the optoelectronic performance of ZnO‐based hybrids. We investigated adsorption‐induced changes with photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy on aqueous colloidal ZnO dispersions. Citrate and acetate ion adsorption increases or decreases radiative exciton annihilation at hν=3.3 eV and at room temperature, respectively. Searching for a correspondence between PL emission and the yield of trapped charge carriers originating from exciton separation (using photon energies of hν=4.6 eV and fluxes of ṄPh=1014 cm−2 s−1 for excitation), we found that there is a negligible fraction of paramagnetic products that originates from exciton separation. Upon polychromatic excitation with significantly higher photon fluxes (ṄPh=1016 cm−2 s−1), ZnO‐specific shallow defects trap unpaired electrons in citrate‐ and acetate‐functionalized samples. The adsorption‐dependent PL intensity changes and the excitation parameter dependent yield of separated charges (EPR) in colloidal ZnO nanoparticles underline that the distribution over the different exciton annihilation channels sensitively depends on interface composition and the intensity of the photoexcitation light.

From Anhydrous Zinc Oxide Nanoparticle Powders to Aqueous Colloids: Impact of Water Condensation and Organic Salt Adsorption on Free Exciton Emission

“…With this very first systematic comparison of the free exciton emission of ZnO nanocrystals, which were grown in the gas phase, processed, and measured under water-free conditions, with identical nanoparticles in contact with a condensed water phase, we discuss important optical property changes that can originate from the conversion of the anhydrous powder into an aqueous colloidal dispersion. The present investigation connects to a previously performed study on aqueous ZnO colloids that aimed at the relationship between adsorption-induced photoluminescence property changes and the yield of photogenerated charges that are accessible to electron paramagnetic resonance spectroscopy . Here, we used two widely employed surfactants, citrate ions producing negative ζ-potential values, and acetate ions, the adsorption of which shifts the corresponding ζ-potential values to more positive values.…”

“…In the present case, namely, there is no correspondence between adsorbate-induced intensity changes related to the free exciton emission at λ = 380 nm and the broad emission band centered at λ = 670 nm, the intensity of which remains constant throughout all of the experiments performed for this study and which therefore is also not correlated with the observed ζ-potential changes. Hence, under the experimental conditions, applied citrate and acetate adsorption have no significant influence on potential energy transfer steps that convert a fraction of the exciton energy into a defect-related emission in the visible light range …”

“…Hence, under the experimental conditions, applied citrate and acetate adsorption have no significant influence on potential energy transfer steps that convert a fraction of the exciton energy into a defect-related emission in the visible light range. 33 …”

“…The present investigation connects to a previously performed study on aqueous ZnO colloids that aimed at the relationship between adsorption-induced photoluminescence property changes and the yield of photogenerated charges that are accessible to electron paramagnetic resonance spectroscopy. 33 Here, we used two widely employed surfactants, citrate ions producing negative ζ-potential values, and acetate ions, the adsorption of which shifts the corresponding ζ-potential values to more positive values. We will demonstrate that the corresponding trends in the intensity of the free exciton emission at λ = 380 nm ( hv = 3.3 eV), which is absent on dehydroxylated ZnO nanoparticle powders, show a clear dependence on the ζ-potential in aqueous dispersion.…”

“…During colloidal processing of ZnO nanoparticles, surfactant molecules and polymers are adsorbed from solution onto the particles surfaces. ,,, The adsorption of citrate or acetate ions, for instance, changes the ζ-potential values of the particle dispersions, enables one to increase electric double-layer repulsion for the stabilization of colloidal dispersions, and affects the free exciton emission intensity . Changes in the broad defect-related luminescence band are reported for other inorganic or organic adsorbates. − Apart from a number of studies that focus on the impact of specific molecules, − a general understanding of how adsorbed surfactants and the solvents’ dielectric properties can affect energy and intensity of photoluminescence bands is far from complete.…”