Cellulose is our most abundant biopolymer, and hence an important renewable raw material for many materials. Her, we present a small and wide angle X-ray scattering (SAXS/WAXS) study of regenerated cellulose textile fibers, air-gap spun from an ionic liquid solution.[1] Figure 1 shows SAXS and WAXS patterns from two fibers produced with two different draw ratios, DR=2 and 15, respectively. Drawing the fibers result in an increased degree of orientation of the crystalline domains (Figure 1c and d). By analyzing the azimuthal angular dependence of the WAXS pattern, both the crystal degree of orientation and the degree of orientation of amorphous cellulose chains can be obtained, as well as their relative contributions to the total scattering. Thus, offering an accurate determination of the degree of crystallinity. The anisotropic cross-like 2D SAXS pattern, having scattering predominantly perpendicular and parallel to the fiber axis, suggests an internal colloidal structure with oriented crystalline lamellae of ca. 10 nm thickness, embedded within a continuous matrix of amorphous cellulose. The lamellae are oriented with their normal parallel with the fiber axis.

[1] Gubitosi, M., Asaadi, S., Sixta, H., Olsson, U. (2021). Cellulose. 70, 3554

Structure determination of nano-size crystals is always a challenging problem. For quite lots of materials, it is very difficult to synthesize large/good enough crystals for single crystal X-ray diffraction studies. Powder X-ray diffraction (PXRD) is the major method for their atomic structure determination, PXRD is a quite mature technique and lots of powder structures were solved. However, for complicated structures with huge unit cell dimensions or those with crystal sizes smaller than 100nm, it is quite often to have severe peak overlapping problems, which makes it extremely difficult to solve the structure from PXRD alone. Electrons which interact with matter much stronger than X-ray can produce single-crystal-like diffraction from nano-crystalline materials, which makes it possible to collect single-crystal-like diffraction data.

The 3D electron diffraction technique can be used for collecting 3D electron diffraction data. Compared with traditional electron diffraction methods, this technique gives lower dynamical effects and much higher data completeness. Using the intensities abstracted from the data, complicated structures can be directly solved using the similar methods as single-crystal X-ray diffraction. Combining it with other techniques, such as PXRD or even SXRD, more complicated structures can be solved.

A strategy to enhance surface properties of nanocrystals is tailoring their bulk crystalline structure. As an example, the performance of metal nanocatalysts is correlated to lattice distortion induced by the mechanical anisotropy of the crystal structure [1]. We demonstrated that the stress caused by interior interfaces in core@shell nanocrystals results in larger lattice deformations than the elemental lattice mismatch [2]. Plasmonic applications push further the interest for a thorough characterization of the influence of the structural anisotropy on the thermal dynamic disorder.

Here we access the thermal disorder in Pd nanocrystals with molecular dynamics simulation. We focus on cubic nanocrystals, which have a particularly pronounced influence of mechanical anisotropy. We find a marked dependence of dynamic disorder on the crystallographic direction that enhances as crystal size decreases (see Fig. 1). 10 nm nanocrystals show a clear separation of the directional-dynamic disorder profiles. Contrary to theoretical models that ignore mechanical anisotropy, the directional profiles deviate from one another starting with the shortest pair distances.

We extracted an analytical model suitable for include the anisotropic thermal disorder we report here within existing analysis methods of both Bragg and PDF powder scattering profiles. Based on experimentally validated atomistic simulations, the model is calibrated with well-known characteristic material properties such as the bulk MSD and structural mechanical anisotropy (i.e., contrast factor). Finally, we used the whole pair distribution function modelling method [3] to test the model against the analysis of powder X-ray diffraction patterns simulated via Debye scattering equation.

Cadmium selenide nanocrystals (CdSe NCs) are frequently used in optoelectronic devices, as they possess unique optoelectronic properties that are highly sensitive to their size, shape and microstructure. Due to the high sensitivity of the properties to the microstructure features, the structure and microstructure of CdSe NCs must be controlled precisely during the synthesis. The CdSe NCs crystallize in thermodynamically stable wurtzitic structure (space group ), in metastable zinc blende structure (space group ) and in a mixture of both structures [1]. As a result, another critical issue of the CdSe NCs synthesis is the control of phase composition and formation of microstructural defects, as both issues affect the optoelectronic properties additionally [2,3].

The aim of this study was to correlate the size and morphology of CdSe NCs with their phase composition and with the formation of microstructure defects, and to explain the effect of the microstructure defects on the optoelectronic properties of the CdSe NCs. The CdSe NCs under study were produced using hot injection at temperatures between 225°C and 250°C. X-ray diffraction and transmission electron microscopy with high resolution revealed that the CdSe NCs have a size between 3 and 10 nm, and crystallize predominantly in the metastable zinc blende crystal structure. While NCs having a size smaller than 4 nm were practically defect-free, larger particles contained planar defects (stacking faults), which number increased with increasing NC size. When the planar defects appeared randomly in the interior of the NCs, then they led to an anisotropic broadening of the X-ray diffraction lines as typical for isolated stacking faults [4]. When the planar defects appeared on every second cubic lattice plane {111}, then they accomplished the transition of the zinc blende structure of CdSe to the thermodynamically stable wurtzitic modification [5]. A combination of XRD measurements and simulations using DIFFaX revealed that the interplanar spacing along the stacking direction apparently depends on the density and ordering of the planar defects. Our approach is discussed together with the approach of Moscheni et al. [6].

In general, the planar defects located in the interior of the CdSe NCs deteriorate their photoluminescence quantum yield. Additional planar defects originate from the oriented attachment of CdSe NCs along the {111} crystallographic planes. These defects disturb the crystallographic coherence of attached NCs. Consequently, agglomerated NCs are not recognized as large NCs but as separated NCs both by XRD and by photoluminescence.

[1] Bawendi, M. G., Kortan, A. R., Steigerwald, M. L. & Brus, L. E. (1989). J. Chem. Phys. 91, 7282.

[2] Viswanatha, R. & Sarma, D. D. (2009). Chem.: Asian J. 4, 904.

[3] Orfield, N. J., McBride, J. R., Keene, J. D., Davis, L. M. & Rosenthal, S. J. (2015). ACS Nano 9, 831.

[4] Warren, B. E. (1990). X-ray Diffraction. New York: Dover Publication.

[5] Martin, S., Ullrich, C., Šimek, D., Martin, U. & Rafaja, D. (2011). J. Appl. Cryst. 44, 779.

[6] Moscheni, D., Bertolotti, F., Piveteau, L., Protesescu, L., Dirin, D. N., Kovalenko, M. V., Cervellino, A., Pedersen, S., Masciocchi, N. A. & Guagliardi, A. (2018). ACS Nano 12, 12558.

Understanding and governing the complex behavior in magnetic materials at the nanoscale is the key and the challenge not only for fundamental research but also to exploit them in applications ranging from catalysis,[1] to data storage,[2] sorption,[3-4] biomedicine,[5-6] and environmental remediation.[7] In this context, spinel ferrites (M²⁺Fe²O⁴, where M²⁺ = Fe, Co, Mn, etc.) represent ideal magnetic materials for tuning the magnetic properties through chemical manipulations, due to their strong dependence on the cation distribution, spin-canting, interface, size, shape, and interactions. Furthermore, when coupled with other phases (heterostructures), they can display rich and novel physical properties different from the original counterparts (exchange coupling, exchange bias, giant magneto-resistance), allowing them to multiply their potential use.[8] For example, the possibility to tune magnetic anisotropy and saturation magnetization by coupling magnetically hard and soft materials have found usage recently in applications based on magnetic heat induction, such as catalysis or magnetic fluid hyperthermia (MFH).[9] Therefore, it is crucial to engineer core-shell nanoparticles with homogeneous coating and low size dispersity for uniform magnetic response and to maximize the coupling between the hard and soft phases (i.e. the interface).[10] Even though some studies have reported interesting results in the field of magnetic heat induction, a systematic study on an appropriate number of samples for a better comprehension of the phenomena to optimize the performance is needed.

In this contribution, the capability of coupled hard-soft bi-ferrimagnetic nanoparticles to improve the heating ability is exploited to understand the influence of the different features on the performances. This systematic study is then based on the correlation between the heating abilities of three magnetically hard cobalt ferrite cores, covered with magnetically soft spinel iron oxide and manganese ferrite having different thickness, with their composition, structure, morphology and magnetic properties. Direct proof of the core-shell structure formation was provided by nanoscale chemical mapping, with identical results obtained through STEM-EELS, STEM-EDX, and STEM-EDX tomography. ⁵⁷Fe Mössbauer spectroscopy and DC/AC magnetometry proved the magnetic coupling between the hard and the soft phases, thanks also to the comparison among core-shell NPs, ad-hoc prepared mixed cobalt-manganese ferrites NPs, and cobalt ferrite NPs mechanically mixed with manganese ferrite NPs. The heating abilities of the aqueous colloidal dispersions of the three sets of core-shell samples revealed that, in all cases, core-shell nanoparticles showed better performances in comparison with the respective cores, with particular emphasis on the spinel iron oxide coated systems and the samples featuring thicker shells. This scenario entirely agrees with the hypothesis made based on magnetic parameters (saturation magnetization, Néel relaxation times, effective anisotropy) of the powdered samples, and demonstrated the importance of a sophisticated approach based on the synergy of chemical, structural, and magnetic probes down to a single-particle level.

[1] Polshettiwar, V. et al. Chem. Rev., 2011, 111, 3036–3075

[2] Wu, L. et al. Nano Lett., 2014, 14, 3395–3399

[3] Cara, C. et al. J. Mater. Chem. A, 2017, 5, 21688–21698

[4] Cara, C. et al. J. Phys. Chem. C, 2018, 122, 12231–12242

[5] Lim, E. et al. Chem. Rev., 2015. 115, 327-394

[6] Mameli, V. et al. Nanoscale, 2016, 8, 10124–10137

[7] Westerhoff, P. et al. Environ. Sci. Nano, 2016, 3, 1241–1253

[8] Gawande, M.B. et al. Chem. Soc. Rev. 2015, 44, 7540–7590

[9] Lee, J.-H. et al. Nat. Nanotechnol.,2011, 6, 418–422

[10]Sanna Angotzi, M. et al. J. Nanosci. Nanotechnol., 2019, 19, 4954–4963

Lead bromide perovskite nanocrystals (NCs) APbBr₃ in which A: Cs, FA: CH(NH₂)₂, MA: CH₃NH₃ are very promising high-color purity light emitters due to their pure green emission and excellent optical properties. In this present work, the lead bromide perovskite APbBr₃ (A: Cs, FA, MA)nanocrystals have been synthesized by the hot injection method according to. Imran et al ¹ synthesis approach in which the benzoyl bromide was used as halide precursor which can be easily injected into a solution of metal cations to provoke the nucleation and the growth of Lead bromide perovskite NCs. By precisely tuning the relative amount of cation precursors (cesium carbonate and lead acetate for Cs perovskite, formamidine acetate and lead acetate for FA perovskite, and methylamine and lead oxide for MA perovskite), ligands (oleylamine and oleic acid), solvents (octadecene), benzoyl bromide, and the injection temperature (170°C Cs perovskite, 75°C for FA perovskite, 65 °C for MA perovskite), we have been able to synthesize inorganic and organic-inorganic lead bromide perovskite APbBr₃ colloidal nanocubes with excellent control over the size distribution, very high phase purity, and excellent optical properties such as a high green photoluminescence emission efficiency and narrow full width at half-maximum. The resultant lead bromide perovskite nanocrystals (NCs) APbBr₃ present important optical properties, which are among the best-promoting characteristics for a pure green light-emitting device according to the updated recommendation 2020 (Rec. 2020) standard ².

References:

1: Imran, Muhammad, et al. "Benzoyl halides as alternative precursors for the colloidal synthesis of lead-based halide perovskite nanocrystals." Journal of the American Chemical Society 140.7 (2018): 2656-2664.

2: Zhu, Ruidong, et al. "Realizing Rec. 2020 color gamut with quantum dot displays." Optics express 23.18 (2015): 23680-23693.