Equilibrium Structures of PbSe and CdSe Colloidal Quantum Dots Detected by Dielectric Spectroscopy

Abstract

The permanent electrical dipole moment of colloidal quantum dots is important for their optoelectronic properties and can be determined by dielectric spectroscopy. Until now, however, colloidal interactions have not been taken into account in the interpretation of the spectra. Here, dielectric spectra of PbSe and CdSe colloidal quantum dots dispersed in an apolar liquid are measured from 1 Hz to 10 MHz. At frequencies of 10 kHz−1 MHz, Brownian rotation of nanoparticles with a permanent electric dipole moment is detected. At the lowest concentrations (∼0.1 vol %), the nanoparticles rotate independently of each other, and their dipole moment, for both PbSe and CdSe, is on the order of 40−50 D. At higher concentrations (≥0.3 vol %), the dipolar relaxation becomes slower, indicating the presence of nanoparticle structures. A simple model is used to estimate the interaction strength, which appears to be stronger than expected from the weak dipole moment, and has possibly also contributions from electrical moments of higher order. Our results indicate that nanoparticle interactions in liquid media lead to small equilibrium structures that affect dielectric measurements of the dipole moment already at concentrations of a few tenths of a volume percent.

Size-Dependent Second Virial Coefficients of Quantum Dots from Quantitative Cryogenic Electron Microscopy

“…To account for the experimental second virial coefficients on the order of −100 nm 3 , the required dipole moment is about 300 D, an order of magnitude larger than reported. 17 Clearly, a dipolar interaction alone cannot account for the smaller and negative values of the experimental B 2 values in this work.…”

“…13, 16 Recently, the magnitude of this dipole moment was determined to be 37 D for PbSe QDs with a diameter of 5.7 nm. 17 If we take a dipolar hard-sphere model, 5,14 this dipole moment would still correspond to a positive second virial coefficient on the order of 1000 nm 3 . To account for the experimental second virial coefficients on the order of −100 nm 3 , the required dipole moment is about 300 D, an order of magnitude larger than reported.…”

Improving Indistinguishability of Single Photons from Colloidal Quantum Dots Using Nanocavities

“…where is the relative strength of electric field at colloidal QD location ( / ) and is = 0.35 in our case 10 as the QD sits on the surface of the cavity, denotes the dipole moment of the colloidal QD and ≈ 50D for the QD used 25 . The nanobeam cavity has a decay rate of 1 = / 1 and is coupled to the ring resonator with a coupling rate = 2.1 .…”

“…where 𝜔 𝑜 is the emission frequency equivalent to 𝜆 = 630𝑛𝑚. The QD is coupled to the nanobeam cavity with a coupling rate of 𝑔 which depends on the mode volume of the first cavity and the dipole moment of the QD and is given by 23,24 𝑔 = 𝜂√ 𝜇 2 𝜔 𝑜 2ℏ𝜖 𝑆𝑖𝑁 𝜖 𝑜 𝑉 𝑒𝑓𝑓 where 𝜂 is the relative strength of electric field at colloidal QD location (𝐸 𝐶𝑄𝐷 /𝐸 𝑚𝑎𝑥 ) and is = 0.35 in our case 10 as the QD sits on the surface of the cavity, 𝜇 denotes the dipole moment of the colloidal QD and ≈ 50D for the QD used 25 . The nanobeam cavity has a decay rate of 𝜅 1 = 𝜔 𝑜 /𝑄 1 and is coupled to the ring resonator with a coupling rate 𝐽 = 2.1𝛾.…”

Probing permanent dipoles in CdSe nanoplatelets with transient electric birefringence

“…[23] The values of dipoles for CdSe nanocrystals range from 41 to 98 D for particles with diameters between 2.7 and 5.6 nm and increase with size. [21,23,24] For wurtzite, the measured dipole ranged between 25 and 50 D for spheres and can be as large as 210 D for nanorods. [22] All these values have been measured using dielectric spectroscopy or transient electric birefringence in the case of nanorods.…”