Optical and X-ray scattering studies of the electric field-induced orientational order in colloidal suspensions of pigment nanorods

Abstract

Under pulsed or a.c. electric fields, colloidal suspensions of nanorods can show strong electro-optic effects, such as the Kerr effect, with fast response times (a few ms), which makes them good candidates for some commercial applications. For this purpose, suspensions of Pigment red 176 nanorods in dodecane were recently developed and their physical properties have been studied. We report here on the investigation of the orientational order induced by electric fields in isotropic suspensions of pigment nanorods by three different techniques: transient electric birefringence, transient electric dichroism, and in-situ small-angle X-ray scattering under electric field ("Electro-SAXS"). We show that, in spite of the apolar character of the solvent, the Maxwell-Wagner-O'Konski mechanism (i.e. the polarization of the counter-ion cloud around each particle) is responsible for the field-induced alignment of the nanorods. Although the particles are only weakly charged and the dielectric constant of dodecane is low, the pigment nanorods effectively behave as metallic particles in an insulating matrix and reach strong values (S~0.5) of the induced nematic order parameter at moderate field amplitudes (~1 V/µm). This study confirms the feasibility of using suspensions of Pigment red 176 nanorods in dodecane for electro-optic applications.

Dynamic Plasmonic Pixels

“…If the NRs were dispersed in polar, high-dielectric-constant solvents such as water and methanol, then NR alignment also did not occur, whereas nonpolar, low-dielectric-constant solvents have been shown to be effective hosts for alignment of metal NRs. The influence of the type of solvent on the alignment behavior is complex and includes considerations such as the contrast between the solvent and NR dielectric constants, − the effect of the electric double layer at the NR–solvent interface, − charge accumulation at the electrodes, , and ionic impurity mobility, many of which are frequency-dependent factors. Applying a uniform electric field across the suspension is also important to minimize the potential translational redistribution of NRs that would arise due to dielectrophoresis in a non-uniform field. , …”

Probing permanent dipoles in CdSe nanoplatelets with transient electric birefringence

“…The field-induced birefringence was measured in real time, under polarizing microscope (Leitz Ortholux II), with the apparatus described in detail in references [49,83]. It consists of a stabilized light source, an optical compensator introducing an additional constant phase shift, a photo-multiplier tube (PMT), a load resistor R L transforming the PMT anode current in a voltage difference, a differential amplifier with band-pass filters (AM 502, Tektronix), and a digital oscilloscope (DSO-X 2004A, Agilent Technologies) that accumulates the signal up to 64000 counts.…”

How antagonistic salts cause nematic ordering and behave like diblock copolymers

“…Nematic order parameters commonly used in the analysis of liquid crystal structures measure the overall homogenity of nematic orientation. They are suitable when one is interested in deviations from some prefered direction, for example given by an applied electric field, surface patterning or initially ordered state [31][32][33] , or to determine the nematic homogenity of some spontaneously ordered steady-state 34,35 . In our case, ordering progresses without any inherently preferred direction and we are interested in the time evolution of the size of nematically ordered domains.…”