Thermal Charging of Colloidal Quantum Dots in Apolar Solvents: A Current Transient Analysis

Abstract

We analyze thermal charging in additive-free colloidal CdSe quantum dot (QD) dispersions by means of the transient electric current resulting from a voltage step applied across the QD dispersion. On the basis of the initial current and the total charge separated, we find that the CdSe dispersion behaves as a 1:1 electrolyte where equal fractions of the QDs carry a single positive or a single negative charge. This conclusion is confirmed by a more detailed fitting of the current transient using the Nernst-Planck-Poisson equations. Using equilibrium thermodynamics, we relate the fraction of charged QDs to the QD charging energy. The magnitude of the charging energy corresponds to values found using known models for the charging energy of either a spherical surface in a dielectric or a charge within a dielectric sphere. However, the experimental dependence of the charging energy on the dielectric constant of the solvent is far less pronounced than predicted by these models. A better correspondence is found based on the charging energy of a spherical surface embedded in a compound medium consisting of the ligand shell and the solvent.

Direct determination of absorption anisotropy in colloidal quantum rods

“…As shown before, even in dodecane, a fraction of the QRs carries an electric charge, implying that an applied electric field induces drift of the QRs toward the electrodes. 29 To prevent accumulation of the QRs at the electrodes, the period of oscillation should be shorter than the cell transit time, τ tr = d/μE, i.e., the time it takes for a single rod with mobility μ to cross the cell. For the S5 rods in a 16 μm cell, the cell transit time is about 10 ms for fields of 50 kV/cm.…”

Electrophoretic Deposition of Quantum Dots and Characterisation of Composites

“…Due to the higher dielectric constant of the solvent, there exists a stronger ability of the electrode to attract CQDs out of the solution. The increase in dielectric constant also has the effect of screening charge more effectively between CQDs which has been postulated to have the effect of increasing the population of charged CQDs due to thermal charging [16]. Originally, when looking at literature values, other solvents were also considered for testing, specifically, 1,2-dichlorobenzene and 1,2-dichloroethane, which show dielectric constants of 9.93 and 10.36, respectively, and still relatively low dipole moments (C 2 H 4 Cl 2 , 1.83 D and C 6 H 4 Cl 2 2.14 D), meaning the nonpolar ligand capped CQDs should show some solubility [36].…”

“…Secondly, the presence or absence of charged ligands or counter ions in general can also have a marked effect upon the overall charge of CQD. Finally, the thermodynamic approach termed thermal charging, a process by which an equilibrium exists between the majority non-charged CQD population and a minority of charged CQDs has been used to explain the consistently equal concentrations of negative and positively charged CQDs in solutions in some nonpolar solvents [16].…”

A differential dielectric spectroscopy setup to measure the electric dipole moment and net charge of colloidal quantum dots

“…Dipole moments have been measured using optical spectroscopy 8 or with transient electric birefringence (for rod-like particles). 9 Net charges have been determined by transient current analysis 10 and laser Doppler electrophoresis. 11 Additionally, it has been shown by electrostatic force microscopy that quantum dots can acquire a net charge upon illumination.…”