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Anisotropic Nanoparticles

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Participants :

Elodie Chaudan (thèse), Khalid Lahlil, Jacques Peretti, Thierry Gacoin, Jongwook Kim

Publications :

 [1] Kim, J. et al. LaPO4 mineral liquid crystalline suspensions with outstanding colloidal stability for electro-optical applications. Adv. Funct. Mater. (2012) 22, 4949-4956
 [2] Kim, J. et al. Optimized combination of intrinsic and form birefringence in oriented LaPO4 nanorod assemblies. Appl. Phys. Lett. (2014) 105, 061102
 [3] Kim, J. et al. Optically anisotropic thin films by shear-oriented assembly of colloidal nanorods Adv. Mater. 25, (2013) 3295-3300

Synthesis and assembly of anisotropic nano-objects is a key issue for developing functional materials performing anisotropic physical properties such as birefringence, ferromagnetism, and polarized luminescence. Moreover, dynamic control of the organized structure in the soft matter state is challenging and promising for applications to switchable devices. Our strategy is to synthesize functional nanocrystals with highly anisotropic crystallinity and morphology, and exploit their spontaneous- and directed-assemblies to obtain collective anisotropic properties.

Figure 1. Liquid crystalline phase behaviour of colloidal
LaPO4. [1] nanorods1
Figure 2. Thin film retardation plate made by shear-directed assembly of LaPO4 nanorods [3].

For instance, we synthesized permanently stabilized colloidal suspensions of monocrystalline LaPO4 nanorods which exhibit remarkable liquid crystalline (LC) behavior. And we investigated the anisotropic optical properties emerging both from the intrinsic crystalline anisotropy and the organized structures1. In this system, concentrated colloidal nanorods with repulsive surface interaction self-assemble into nematic and columnar LC phases in accordance with the Onsager’s theoretical model of ‘hard rods’. The orientation of LaPO4 nanorods in both isotropic and LC phases can be controlled with the externally applied electric field resulting in switchable optical birefringence1, 2. The high Kerr constant (BKerr) obtained with the athermal LC behavior is beneficial for applications to electro-optical devices (e.g. LC displays) with high power light sources such as lasers. Also, phase-tuned nematic rod gel shows fast sol-gel transition in response to shear (thixotropy). We use this property for directed assembly of thin films with defect-free rod orientation3. Synergistic contributions of form birefringence and intrinsic birefringence of LaPO4 crystal permit Δnfilm = 0.13. This film may replace expensive and size-limited calcite waveplates.