Recent Publications : Case Western Reserve University & Université Pierre et Marie Curie
Chiral oily streaks in a smectic A liquid crystal
I. R. Nemitz, A. J. Ferris, E. Lacaze, C. Rosenblatt, Soft Matter, 12 (2016).
The liquid crystal octylcyanobiphenyl (8CB) was doped with the chiral agent CB15 and spin-coated onto a substrate treated for planar alignment of the director, resulting in a film of thickness several hundred nm in the smectic-A phase. In both doped and undoped samples, the competing boundary conditions — planar alignment at the substrate and vertical alignment at the free surface — cause the liquid crystal to break into a series of flattened hemicylinders to satisfy the boundary conditions. When viewed under an optical microscope with crossed polarizers, this structure results in a series of dark and light stripes (“oily streaks”) of period ~ 1 mm. In the absence of chiral dopant the stripes run perpendicular to the substrate’s easy axis. However, when doped with chiral CB15 at concentrations up to c = 4 wt-%, the stripe orientation rotates by a temperature-dependent angle j with respect to the c = 0 stripe orientation, where j increases monotonically with c. j is largest just below the nematic – smectic-A transition temperature TNA and decreases with decreasing temperature. As the temperature is lowered, j relaxes to a steady-state orientation close to zero within ~1 ºC of TNA. We suggest that the rotation phenomenon is a manifestation of the surface electroclinic effect: The rotation is due to the weak smectic order parameter and resulting large director tilt susceptibility with respect to the smectic layer normal near TNA, in conjuction with an effective surface electric field due to polar interactions between the liquid crystal and substrate.
Electroclinic effect in a chiral paranematic liquid-crystal layer above the bulk nematic-to-isotropic transition temperature
I. R. Nemitz, E. Lacaze and C. Rosenblatt, Physical Review E, 93, (2016).
Electroclinic measurements are reported for two chiral liquid crystals above their bulk chiral isotropic – nematic phase transition temperatures. It is found that an applied electric field E induces a rotation θ [∝ Ε] of the director in the very thin paranematic layers that are induced by the cell’s two planar-aligning substrates. The magnitude of the electroclinic coefficient dθ/dE close to the transition temperature is comparable to that of a bulk chiral nematic, as well as to that of a parasmectic region above a bulk isotropic to chiral smectic-A phase. However, dθ/dE in the paranematic layer varies much more slowly with temperature than in the parasmectic phase, and its relaxation time is slower by more than four orders of magnitude than that of the bulk chiral nematic electroclinic effect.
Chiral periodic mesoporous organosilica in a smectic- A liquid crystal: source of the electrooptic response
I. R. Nemitz, K. McEleney, C. Crudden, R. Lemieux, R. Petschek and C. Rosenblatt, Liquid Crystals, 43, (2016).
Chiral periodic mesoporous organosilica (PMO) materials have been shown to deracemize a configurationally achiral, but conformationally racemic liquid crystal in which the PMO is embedded. In particular, application of an electric field E in the liquid crystal’s smectic-A phase results in a rotation of the liquid crystal director by an angle proportional to E, which is detected optically — this is the so-called “electroclinic” effect. Here we present results from electroclinic measurements as a function of frequency and temperature, which allow us to distinguish the component of optical signal that arises from liquid crystal chirality induced within the PMO’s chiral pores from that induced just outside the silica colloids. Our central result is that the overwhelming source of our electrooptic signal emanates from outside the PMO, and that the contribution from the liquid crystal embedded in the chiral pores is much smaller and below the noise level.
Studies of nanocomposites of carbon nanotubes and a negative dielectric anisotropy liquid crystal
P. Kalakonda, R. Basu, I. R. Nemitz, C. Rosenblatt and G. Iannacchione, The Journal Of Chemical Physics, 140, (2014).
The complex specific heat is presented over a wide temperature range for a negative dielectric anisotropy alkoxyphenylbenzoate liquid crystal (9OO4) and carbon nanotube (CNT) composites as a function of CNT concentration. The calorimetric scans were performed under near-equilibrium conditions between 25 and 95 C, first cooling followed by heating for CNT weight percent ranging from w = 0 to 0.2. All 9OO4/CNT mesophases have transition temperatures 1 K higher and a crystallization temperature 4 K higher than that in the pure 9OO4. The crystal phase super heats until a strongly first-order specific heat feature is observed, 0.5 K higher than in the pure 9OO4. The transition enthalpy for the nanocomposite mesophases are 10% lower than that observed in the bulk. The strongly first-order crystallization and melting transition enthalpies are essentially constant over this range of w. Complementary electroclinic measurement on a 0.05 wt% sample, approaching the smectic-C phase from the smectic-A, indicate that the SmA- SmC transition remains mean-field-like in the presence of the CNTs. Given the homogeneous and random distribution of CNT in these nanocomposites, we interpret that these results as arising from the LC-CNT surface interaction pinning orientational order uniformly along the CNT, without pinning the position of the 9OO4 molecule, leading to a net ordering effect for all phases.
Nematic molecular core flexibility and chiral induction
T. Lin, I. R. Nemitz, C. McGrath, C. Schubert, H. Yokoyama, R. Lemieux and C. Rosenblatt, Physical Review E, 88, (2013).
Electroclinic measurements, in which an applied electric field E induces a rotation theta [~E] of the liquid crystal director about the electric field axis in a chiral environment, were performed on several configurationally achiral liquid crystals in the presence of an imposed helical director profile. This imposed twist establishes a chiral symmetry environment for the liquid crystal. It was observed that a conformationally racemic mesogen possessing a flexible phenyl benzoate core exhibits a measurable electroclinic response in the nematic phase. On the other hand, when the phenyl benzoate mesogen is mixed with a mesogen containing a rigid, conformationally achiral core (fluorenone), or with a racemic dopant with an axially chiral core that mimics a mesogen having rigid right- and left-handed conformations (2,2’-spirobiindan-1,1’-dione), the magnitudes of the electroclinic responses were found to decrease sharply, apparently going to zero when extrapolated to the pure 2,2’-spirobiindan-1,1’-dione or fluorenone limit. Note that neither of these additives possesses a nematic phase. The results suggest that the flexibility of the core and its ability to deracemize conformationally in order to compensate the elastic energy cost of the imposed twist is the primary mechanism behind the observed electroclinic response.
Nematic twist cell: Strong chirality induced at the surfaces
T. Lin, I. R. Nemitz, J. Pendery, C. Schubert, R. Lemieux and C. Rosenblatt, Appl. Phys. Lett., 102, (2013).
A nematic twist cell, with easy axes forming an angle θ0 = 20° and thickness d varying continuously across the cell, was filled with a mixture containing a configurationally achiral liquid crystal and a chiral dopant. A linear electrooptic effect, which requires a chiral environment, was observed on application of an ac electric field. This “electroclinic effect” varied monotonically with d , changing sign at 0 d = d where the chiral dopant exactly compensated the imposed pitch. The results indicate that a significant chiral electrooptic effect always exists near the surfaces of a nematic twist cell containing molecules that can be conformationally deracemized. Additionally, this approach can be used to measure the helical twisting power (HTP) of a chiral dopant in a liquid crystal.
Surface topography and rotational symmetry breaking
R. Basu, I. R. Nemitz, Q. Song, R. Lemieux and C. Rosenblatt, Physical Review E, 86, (2012).
A nematic twist cell, in which the two polymer-coated substrates are rubbed and then rotated by an angle theta_0, results in a strong chiral environment at the surfaces. In the presence of an applied electric field, a combination of the chiral environment and the rub-induced breaking of the Cinifnity rotation axis at the surfaces results in a rotation of the molecular director in the substrate plane, viz., a surface electroclinic effect. Using this twist cell geometry, we separate out and quantify the strength of the rub-induced two-fold rotational symmetry from that of the chiral symmetry. Our primary result is that the strength of the mechanically-induced C2 rotational symmetry, which is proportional to the electroclinic response, scales linearly with the rub-induced topographical rms roughness and increases with increasing rubbing strength of the polymer. Our results also suggest that the azimuthal anchoring strength coefficient is relatively insensitive to the strength of the rubbing.
Prior Publications: Bowling Green State University
Heteroepitaxial Growth of Colloidal Nanocrystals onto Substrate Films via Hot-Injection Routes
K. Acharya, E. Khon, T. O’Conner, I. R. Nemitz, A. Klinkova, R. Khnayzer, P. Anzenbacher and M. Zamkov, ACS Nano, 5, (2011).
Suppression of the Plasmon Resonance in Au/CdS Colloidal Nanocomposites
E. Khon, A. Mereshchenko, A. Tarnovsky, K. Acharya, A. Klinkova, N. Hewa-Kasakarage, I. R. Nemitz and M. Zamkov, Nano Letters, 11, (2011).
Tuning the Morphology of Au/CdS Nanocomposites through Temperature-Controlled Reduction of Gold-Oleate Complexes
E. Khon, N. Hewa-Kasakarage, I. R. Nemitz, K. Acharya and M. Zamkov, Chemistry Of Materials, 22, (2010).
Synthesis of PbS/TiO 2 Colloidal Heterostructures for Photovoltaic Applications
K. Acharya, N. Hewa-Kasakarage, T. Alabi, I. R. Nemitz, E. Khon, B. Ullrich, P. Anzenbacher and M. Zamkov, J. Phys. Chem. C, 114, (2010).
Ultrafast Carrier Dynamics in Type II ZnSe/CdS/ZnSe Nanobarbells
N. Hewa-Kasakarage, P. El-Khoury, A. Tarnovsky, M. Kirsanova, I. R. Nemitz, A. Nemchinov and M. Zamkov, ACS Nano, 4, (2010).