Following in real-time the two-step assembly of nanoparticles into mesocrystals in levitating drops

Michael Agthe, Tomás S. Plivelic, Ana Labrador, Lennart Bergström, German Salazar-Alvarez

Nano Letters (2016)

DOI: 10.1021/acs.nanolett.6b02586

Abstract:

Mesocrystals composed of crystallographically-aligned nanocrystals are present in biominerals and assembled materials which show strongly directional properties of importance for mechanical protection and functional devices. Mesocrystals are commonly formed by complex biomineralisation processes and can also be generated by assembly of anisotropic nanocrystals. Here, we follow the evaporation-induced assembly of maghemite nanocubes into mesocrystals in real-time in levitating drops. Analysis of time-resolved small angle X-ray scattering data and ex-situ scanning electron microscopy together with interparticle potential calculations show that the substrate-free, particle-mediated crystallization process proceeds in two stages involving the formation and rapid transformation of a dense, structurally disordered phase into ordered mesocrystals. Controlling and tailoring the particle-mediated formation of mesocrystals could be utilized to assemble designed nanoparticles into new materials with unique functions.

[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.

[PhD defence] Christina Schütz – Fabrication of nanocellulose-based materials - Liquid crystalline phase formation and design of inorganic–nanocellulose hybrids

Christina Schütz​ has (literally) nailed her thesis and the dissertation will be on May 29th (See link). The full text of the thesis can be found at DiVA.

Spin excitations in cubic maghemite nanoparticles studied by time-of-flight neutron spectroscopy

Phys. Rev. B 89, 064402 (2014)
DOI: 10.1103/PhysRevB.89.064402

S. Disch, R. P. Hermann, E. Wetterskog, A. A. Podlesnyak, K. An, T. Hyeon, G. Salazar-Alvarez, L. Bergström, and Th. Brückel

Abstract:

We have determined the field dependence of collective magnetic excitations in iron oxide nanoparticles of cubic shape with 8.42(2) nm edge length and a narrow log normal size distribution of 8.2(2)% using time-of-flight neutron spectroscopy. The energy dependence of the uniform precession modes was investigated up to 5 T applied field and yields a Landé factor g=2.05(2) as expected for maghemite (γ-Fe2O3) nanoparticles. A large effective anisotropy field of BA,eff=0.45(16) T was determined, in excellent agreement with macroscopic measurements. This anisotropy is attributed to enhanced shape anisotropy in these monodisperse cubic nanoparticles. The combination of our results with macroscopic magnetization information provides a consistent view of the energy scales of superparamagnetic relaxation and collective magnetic excitations in magnetic nanoparticles.

Correlating material-specific layers and magnetic distributions within onion-like Fe3O4/MnO/γ-Mn2O3 core/shell nanoparticles

J. Appl. Phys. 113, 17B531 (2013)

K. L. Krycka; J. A. Borchers; M. Laver; G. Salazar-Alvarez; A. López-Ortega; M. Estrader; S. Suriñach; M. D. Baró; J. Sort; J. Nogués

DOI: 10.1063/1.4801423

Abstract
The magnetic responses of two nanoparticle systems comprised of Fe₃O₄/γ-Mn₂O₃ (soft ferrimagnetic, FM/hard FM) and Fe₃O₄/MnO/γ-Mn₂O₃ (soft FM/antiferromagnetic, AFM/hard FM) are compared, where the MnO serves to physically decouple the FM layers. Variation in the temperature and applied field allows for Small Angle Neutron Scattering (SANS) measurements of the magnetic moments both parallel and perpendicular to an applied field. Data for the bilayer particle indicate that the graded ferrimagnetic layers are coupled and respond to the field as a single unit. For the trilayer nanoparticles, magnetometry suggests a Curie temperature (T_C) ≈ 40 K for the outer γ-Mn₂O₃component, yet SANS reveals an increase in the magnetization associated with outer layer that is perpendicular to the applied field above T_C during magnetic reversal. This result suggests that the γ-Mn₂O₃ magnetically reorients relative to the applied field as the temperature is increased above 40 K.

[OPEN ACCESS] Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation

Nanoscale, 2013, Just Accepted Manuscript

Sabrina Disch,  Erik Wetterskog,  Raphaël P. Hermann,  Denis Korolkov,  Peter Busch,  Peter Boesecke,  Olivier Lyon,  Ulla Vainio,  German Salazar-Alvarez,  Lennart Bergström and Thomas Brückel

DOI: 10.1039/C3NR33282A

Abstract:

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure - similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

Resolving Material-Specific Structures within Fe3O4|γ-Mn2O3 Core|Shell Nanoparticles Using Anomalous Small-Angle X-ray Scattering

ACS Nano 2013, 7, 921-931.

Kathryn L. Krycka, Julie A. Borchers, German Salazar-Alvarez, Alberto López-Ortega , Marta Estrader, Sonia Estradé, Elin Winkler, Roberto Daniel Zysler, Jordi Sort, Francesca Peiró, Maria Dolors Baró, Chi-Chang Kao and Josep Nogués

DOI: 10.1021/nn303600e

http://pubs.acs.org/doi/abs/10.1021/nn303600e

Abstract:

The material specific structure of monodispersed Fe3O4|γ-Mn2O3 core|shell nanoparticles is determined using multiple energy, anomalous, small-angle x-ray scattering (ASAXS). The contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe3O4|γ-Mn2O3 core|shell nanoparticles with a diameter of 8.2 nm ± 0.2 nm consist of a core with a composition near Fe3O4 surrounded by a (MnXFe1−X)3O4 shell with a graded composition, i.e., ranging from X ≈ 0.40 at the inner shell toward X ≈ 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe3O4cores without a coating shell. Importantly, the present analysis enhances the sensitivity of the method with regard to the chemical boundaries and internal nanoparticle morphology compared with traditional approaches. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multi-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.