[OPEN ACCESS] Rod Packing in Chiral Nematic Cellulose Nanocrystal Dispersions Studied by Small-Angle X-ray Scattering and Laser Diffraction

Christina Schütz, Michael Agthe, Andreas B. Fall, Korneliya Gordeyeva, Valentina Guccini, Michaela Salajková, Tomás S. Plivelic, Jan P. F. Lagerwall, German Salazar-Alvarez, and Lennart Bergström
Langmuir 31 (2015) 6507–6513.
DOI: 10.1021/acs.langmuir.5b00924

Abstract

The packing of cellulose nanocrystals (CNC) in the anisotropic chiral nematic phase has been investigated over a wide concentration range by small-angle X-ray scattering (SAXS) and laser diffraction. The average separation distance between the CNCs and the average pitch of the chiral nematic phase have been determined over the entire isotropic–anisotropic biphasic region. The average separation distances range from 51 nm, at the onset of the anisotropic phase formation, to 25 nm above 6 vol % (fully liquid crystalline phase) whereas the average pitch varies from ≈15 μm down to ≈2 μm as ϕ increases from 2.5 up to 6.5 vol %. Using the cholesteric order, we determine that the twist angle between neighboring CNCs increases from about 1° up to 4° as ϕ increases from 2.5 up to 6.5 vol %. The dependence of the twisting on the volume fraction was related to the increase in the magnitude of the repulsive interactions between the charged rods as the average separation distance decreases.

[OPEN ACCESS] 2D to 3D crossover of the magnetic properties in ordered arrays of iron oxide nanocrystals - Nanoscale (RSC Publishing)

Nanoscale 5 (2013) 953-960.

Bertrand Faure , Erik Wetterskog , Klas Gunnarsson , Elisabeth Josten , Raphaël P. Hermann , Thomas Brückel , Jens Wenzel Andreasen , Florian Meneau , Mathias Meyer , Alexander P Lyubartsev , Lennart Bergström , German Salazar-Alvarez and Peter Svedlindh

DOI: 10.1039/C2NR33013JD

http://pubs.rsc.org/en/Content/ArticleLanding/2013/NR/c2nr33013j

Abstract:

The magnetic 2D to 3D crossover behavior of well-ordered arrays of monodomain γ-Fe2O3 spherical nanoparticles with different thicknesses has been investigated by magnetometry and Monte Carlo (MC) simulations. Using structural information of the arrays obtained from grazing incidence small-angle X-ray scattering and scanning electron microscopy together with experimentally determined values for the saturation magnetization and magnetic anisotropy of the nanoparticles, we show that MC simulations can reproduce the thickness-dependent magnetic behavior. The magnetic dipolar particle interactions induce a ferromagnetic coupling that increases in strength with decreasing thickness of the array. The 2D to 3D transition in the magnetic properties is mainly driven by a change in the orientation of the magnetic vortex states with increasing thickness, becoming more isotropic as the thickness of the array increases. Magnetic anisotropy prevents long-range ferromagnetic order from being established at low temperature and the nanoparticle magnetic moments instead freeze along directions defined by the distribution of easy magnetization directions.

Hamaker Constants of Iron Oxide Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/la201387d

Langmuir, 2011, 27 (14), pp 8659–8664
Bertrand Faure, German Salazar-Alvarez, and Lennart Bergström

DOI: 10.1021/la201387d

Abstract:

The Hamaker constants for iron oxide nanoparticles in various media have been calculated using Lifshitz theory. Expressions for the dielectric responses of three iron oxide phases (magnetite, maghemite, and hematite) were derived from recently published optical data. The nonretarded Hamaker constants for the iron oxide nanoparticles interacting across water, A_1w1 = 33 – 39 zJ, correlate relatively well with previous reports, whereas the calculated values in nonpolar solvents (hexane and toluene), A₁₃₁ = 9 – 29 zJ, are much lower than the previous estimates, particularly for magnetite. The magnitude of van der Waals interactions varies significantly between the studied phases (magnetite < maghemite < hematite), which highlights the importance of a thorough characterization of the particles. The contribution of magnetic dispersion interactions for particle sizes in the superparamagnetic regime was found to be negligible. Previous conjectures related to colloidal stability and self-assembly have been revisited on the basis of the new Lifshitz values of the Hamaker constants.

[OPEN ACCESS] Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes

http://pubs.acs.org/doi/abs/10.1021/nl200126v [OPEN ACCESS]

Nano Lett., 2011, 11 (4), pp 1651–1656
Sabrina Disch, Erik Wetterskog, Raphaël P. Hermann, German Salazar-Alvarez, Peter Busch, Thomas Brückel, Lennart Bergström, and Saeed Kamali

DOI: 10.1021/nl200126v

Abstract:

Grazing incidence small-angle scattering and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays.