Direct determination of absorption anisotropy in colloidal quantum rods

Abstract

We propose a direct method to determine absorption anisotropy of colloidal quantum rods. In this method, the rods are aligned in solution by using an alternating electric field and we measure simultaneously the resulting average change in absorption. We show that a frequency window for the electric field exists in which the change in absorbance as a function of field strength can be analyzed in terms of the quantum-rod dipole moment and the absorption coefficient for light that is polarized parallel or perpendicular to the long axis of the rod. The approach is verified by measuring the absorbance change of CdSe rods at 400 nm as a function of field strength, where we demonstrate excellent agreement between experiment and theory. This enables us to propose improved values for the CdSe quantum-rod extinction coefficient. Next, we analyze CdSe/CdS dot-in-rods and find that the absorption of the first exciton transition is fully anisotropic, with a vanishing absorption coefficient for light that is polarized perpendicularly to the long axis of the rods.

Exciton Fine Structure of CdSe/CdS Nanocrystals Determined by Polarization Microscopy at Room Temperature

“…33 R e = f c f a , hereafter referred to as the dielectric effect parameter, is the ratio of electric field attenuation between the major and minor axis. In our case f c > f a and consequently R e > 1…”

Full alignment of dispersed colloidal nanorods by alternating electric fields

“…Although electrical alignment has mainly been used to characterize nanorod properties such as the dipole moment 30 or the absorption anisotropy, 31 it can be regarded as a technical solution to produce functional solid lms with strong anisotropy in optical absorption, emission or scattering. For example, alignment-in-solution can be achieved in a polymerizable medium, where a triggered polymerization can lock the rod orientation to produce self-supporting lms or sheets of aligned nanorods.…”

“…33 In addition, there are no reports about reaching the regime of full nanorod alignment in the literature. Using the available combination of applied elds and quantum rod dipoles, Kamal et al only obtained partial orientation, as the electrostatic energy gained by orienting the rods was at best about equal to the thermal energy, 31 while an even weaker orientation can be deduced from the gures provided by Li et al…”

Orientational Control of Colloidal 2D-Layered Transition Metal Dichalcogenide Nanodiscs via Unusual Electrokinetic Response

“…20 This is because the absorption (A) of the spheroids depends on the angle (γ) between the light polarization and the major axis of the spheroid as A= A ║ cos 2 γ + A ⊥ sin 2 γ, where A ║ and A ⊥ are the absorption parallel and perpendicular to the major axis, respectively. 21 In order to confirm that the observed dichroism in TiS 2 nanodiscs reflects the orientational order, ∆A/A was measured as a function of the polarization angle under both E x and E z electric field. Figure 4a shows the peak value of the fractional change in the absorbance (∆A/A peak ) from TiS 2 nanodiscs as a function of the polarization angle (θ p ) under square wave E x Page 6 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 (green) and E z (black) electric fields.…”