Orientation Of Nanocrystals Research Articles

The combined effects of electrolyte addition and mechanical shearing on cellulose nanocrystal alignment

AbstractThis research explored the impact of four different electrolytes on the orientation of cellulose nanocrystals (CNCs) in shear-cast films prepared from aqueous CNC gels. Changes in the aqueous CNC gels’ rheological properties with electrolyte addition were correlated to the orientation and optical properties of dried CNC films. Film alignment was qualitatively assessed using cross-polarized optical microscopy and quantified by order parameters computed by UV–Vis transmission spectroscopy. Electrolyte addition resulted in an increased alignment in dried CNC films. For pure CNCs, the film order parameters remained constant at approximately 0.3 for shear rates from 20 s−1 to 100 s−1. However, higher order parameters were achieved in the presence of electrolytes. Notably, an order parameter of 0.88 was achieved at a shear rate of only 20 s−1. In addition, films produced from dispersions containing electrolytes exhibited improved clarity and haze. The results of this work highlight that electrolyte addition can enable higher order parameters at lower shear rates and facilitate the development of aligned CNC films for applications such as polarizers, clear coatings, and piezoelectric materials. Graphical abstract

Effect of shear rate on orientation of cellulosic nanofibers and nanocrystals in poly(butylene adipate‐co‐terephthalate) based composites

Effect of shear rate on orientation of cellulosic nanofibers and nanocrystals in poly(butylene adipate‐<scp><i>co</i></scp>‐terephthalate) based composites

Effect of the crystallographic orientation of the surface of single-crystal Si wafers on the endotaxial growth of NiSi2 nanoplates

A multi-technique analysis was used to investigate how the orientation of single-crystal Si wafer surfaces affects the size, shape and orientation of NiSi2 nanocrystals grown within the wafers through the thermal diffusion of Ni atoms from a nickel-doped thin film deposited on the surface. Nickel-doped thin films were prepared on silicon wafers with three distinct crystallographic orientations, [001], [110] and [111]. Three sets of samples were then annealed at 500, 600 and 700°C for 2 h. Regardless of crystallographic orientation or annealing temperature, NiSi2 nanoplates with a nearly hexagonal shape grew close to the external surface of the wafers, aligning their larger surfaces parallel to one of the planes of the Si{111} crystallographic form. The crystallographic orientation and annealing temperature in the 500–700°C range did not significantly affect the final values of the average diameter and thickness of the nanoplates. However, significant differences were noted in the number of nanoplates formed in Si wafers with different crystallographic orientations. The results indicate that these observed differences are correlated with the number of pre-existing defects in the wafers that influence the heterogeneous nucleation process. In addition, the average size and size dispersion were determined for pores at the surface of the Si wafers formed due to the etching process used for native oxide removal.

Predictive Modeling of Nanocrystal Orientation in Superlattices: Insights from Ligand Entropy.

The self-assembly of nanocrystals (NCs) into close-packed, ordered superlattices (SLs) is of broad, engineering interest. The coherent orientation of polyhedral nanocrystals within NC SLs enhances electronic, magnetic, and vibrational coupling, leading to a variety of emergent phenomena. Here, we show that coherent orientation of polyhedral NCs in many SLs can be understood simply by considering its effect on the conformational entropy of surface ligands. We report the predicted nanocrystal orientations and entropic driving force to orient for a broad range of nanocrystal shapes and superlattice unit cells, and we show that ligand entropy is sufficient to reproduce a host of reported experimental and computational observations. We additionally use this framework to predict the expected distribution of interstitial species such as solvent or unbound ligands in an oriented NC SL. This work offers intuition for understanding the orientation of NCs in superlattices and a future framework for analyzing multinary structures.

4D-STEM Mapping of Nanocrystal Reaction Dynamics and Heterogeneity in a Graphene Liquid Cell.

Chemical reaction kinetics at the nanoscale are intertwined with heterogeneity in structure and composition. However, mapping such heterogeneity in a liquid environment is extremely challenging. Here we integrate graphene liquid cell (GLC) transmission electron microscopy and four-dimensional scanning transmission electron microscopy to image the etching dynamics of gold nanorods in the reaction media. Critical to our experiment is the small liquid thickness in a GLC that allows the collection of high-quality electron diffraction patterns at low dose conditions. Machine learning-based data-mining of the diffraction patterns maps the three-dimensional nanocrystal orientation, groups spatial domains of various species in the GLC, and identifies newly generated nanocrystallites during reaction, offering a comprehensive understanding on the reaction mechanism inside a nanoenvironment. This work opens opportunities in probing the interplay of structural properties such as phase and strain with solution-phase reaction dynamics, which is important for applications in catalysis, energy storage, and self-assembly.

Imaging of ultrafast photoexcited electron dynamics in pentacene nanocrystals on a graphite substrate.

Understanding molecular film growth on substrates and the ultrafast electron dynamics at their interface is crucial for advancing next-generation organic electronics. We have focused on studying the ultrafast photoexcited electron dynamics in nanoscale organic crystals of an aromatic molecule, pentacene, on a two-dimensional material of graphite substrate. Through the use of time-resolved two-photon photoelectron emission microscopy (2P-PEEM), we have visualized the ultrafast lateral evolution of photoexcited electrons. By resonantly tuning the incident photon to excite pentacene molecules, polarization-dependent 2P-PEEM has revealed that pentacene nanocrystals (sub- to several μm) on the substrate exhibit a preferential orientation, in which a molecular π-orbital contacts the substrate in a "lying flat" orientation, facilitating electron transfer to the substrate. The time-resolved 2P-PEEM captures the motion of excited electrons in a femto- to pico-second timescale, clearly imaging the ultrafast charge transfer and lateral expansion two-dimensionally on the graphite substrate. Moreover, we found that the lying-flat molecular orientation of pentacene nanocrystals is transformable into a "standing-up" one through gentle heating up to 50 °C. These experimental insights using time-resolved 2P-PEEM will be highly valuable in enhancing the photofunctionalities of organic electronic devices by controlled molecular deposition.

Uniaxial orientation of cellulose nanocrystals by zone-casting technique

Cellulose nanocrystals (CNCs) are biomass-based nanoparticles with attractive properties. Using a zone-casting technique, transparent films 2 cm wide and 4 cm long, and 2 µm thick, with uniaxially oriented CNCs were prepared from aqueous suspension of CNCs. The nanocrystals were aligned within the entire film perpendicular to the zone-casting direction. The orientation of the CNCs was confirmed by polarized light microscopy, X-ray diffraction and atomic force microscopy. The intensity of the light transmitted through the films depended on its polarization direction and was the strongest for the light polarized perpendicularly to the crystal orientation direction. The orientation of CNCs in the films resulting in optical anisotropy makes them promising materials for applications in optoelectronics.

Highly strong and tough silk by feeding silkworms with rare earth ion-modified diets

Nature-derived silk fibers possess excellent biocompatibility, sustainability, and mechanical properties, yet producing strong and tough silk fibers in a facile and large-scale manner remains a significant challenge. Herein, we report a simple method for preparing strong and tough silk fibers by feeding silkworms rare earth ion-modified diets. The resulting silk fibers exhibit significantly increased tensile strength and toughness, with average values of 0.85 ± 0.07 GPa and 156 ± 13 MJ m−3, respectively, and maximum values of 0.97 ± 0.04 GPa and 188 ± 19 MJ m−3, approaching those of spider dragline silk. Our findings suggest that the incorporation of rare earth ions (La3+ or Eu3+) into the silk fibers contributes to this enhancement. Structure analysis reveals a reduction in content and an improvement in orientation of β-sheet nanocrystals in silk fibers. X-ray photoelectron spectroscopy analysis confirms the chemical interaction between rare earth ions with β-sheet nanocrystals. The structural evolution and chemical interactions lead to the simultaneous enhancement in both strength and toughness. This work presents a simple, scalable, and effective strategy for producing ultra-strong and tough silk fibers with potential applications in areas requiring super structural materials, such as personal protection and aerospace.

Fabrication of perovskite solar cells with PCE of 21.84% in open air by bottom-up defect passivation and stress releasement

The archetypical n-type semiconductor material, tin oxide (SnO2), is common and well-known electron transport layer (ETL) material in regular perovskite solar cells (PSCs) due to the superior electron mobility and suitable energy level alignment with perovskite. However, plentiful detrimental defects concentrated at perovskite/SnO2 ETLs interface and unfavorable residual stress within perovskite films, compromise and dampen the photovoltaic performance and shelf stability of PSCs. Herein, alkali metal salts (i.e., 2, 2′-biquinoline-4, 4′-dicarboxylic acid alkali metal salts (BCA-2H, BCA-2Na, and BCA-2K)) are employed as pre-buried modifiers in SnO2 films to achieve a holistic amelioration of ETLs, perovskite photoactive layer, and their interface heterojunction. The BCA modification facilitates crystal orientation and crystalline of SnO2 nanocrystals, instrumental in decreasing trap state densities within functional layers, releasing interfacial residual tensile strain, and upgrading perovskite film performance. In comparison to BCA-2H- and BCA-2Na-doped ETLs, the BCA-2K decoration corroborate better defect passivation and stress release effect originating from preferable interface enhancement and desirable surface chemical properties. Consequently, the PSCs based on SnO2-BCA-2K ETL fabricated under open-air conditions deliver the champion efficiency of 21.84% accompanied by negligible hysteresis. Moreover, the unpacked devices maintain 81.2% of its original value after 1200 h storage under an ambient condition of 55 ± 5% (relative humidity, RH) and 25 ± 5 ℃ with open-circuit conditions.

Mapping nanocrystal orientations via scanning Laue diffraction microscopy for multi-peak Bragg coherent diffraction imaging.

The recent commissioning of a movable monochromator at the 34-ID-C endstation of the Advanced Photon Source has vastly simplified the collection of Bragg coherent diffraction imaging (BCDI) data from multiple Bragg peaks of sub-micrometre scale samples. Laue patterns arising from the scattering of a polychromatic beam by arbitrarily oriented nanocrystals permit their crystal orientations to be computed, which are then used for locating and collecting several non-co-linear Bragg reflections. The volumetric six-component strain tensor is then constructed by combining the projected displacement fields that are imaged using each of the measured reflections via iterative phase retrieval algorithms. Complications arise when the sample is heterogeneous in composition and/or when multiple grains of a given lattice structure are simultaneously illuminated by the polychromatic beam. Here, a workflow is established for orienting and mapping nanocrystals on a substrate of a different material using scanning Laue diffraction microscopy. The capabilities of the developed algorithms and procedures with both synthetic and experimental data are demonstrated. The robustness is verified by comparing experimental texture maps obtained with Laue diffraction microscopy at the beamline with maps obtained from electron back-scattering diffraction measurements on the same patch of gold nanocrystals. Such tools provide reliable indexing for both isolated and densely distributed nanocrystals, which are challenging to image in three dimensions with other techniques.

Online detection of orientation of cellulose nanocrystals in a capillary flow with polarization-sensitive optical coherence tomography

Significant importance in the stiffness of materials, such as filaments and films, made of elongated components, has been attributed to orientation. Thus, the control of orientation during the manufacturing of materials has been the target of process optimization for long time. Measuring orientation during the process allows to better grasp the means to control it. In fact, such online tools would enable on-fly process control and optimization improving the flexibility with regards to the raw materials used, and the application requirements. In this article, we will discuss a method based on polarization-sensitive optical coherence tomography utilized as a light-weight online measurement tool of particle (here cellulose nanocrystals) orientation for the purposes of manufacturing next generation products by providing the appropriate interpretation of the retardation images with the help of modelling.

Analysis of the crystal phase and orientation of nanocrystals and nanorods of MoOx thin films

Nanoscale materials, which are also known as nanomaterials, have attracted considerable attention owing to their unique physical properties and discrete band structures. Recently, studies have focused on the enhanced chemical reactions of nanomaterials with large specific surface areas for several applications, such as gas sensing. In this study, we investigated the orientation of MoOx nanorods and plate-like crystals constituting molybdenum oxide nanostructured thin films by transmission electron microscopy and three-dimensional X-ray diffraction. The nanorods are composed of Mo8O23 (non-stoichiometric composition) and are elongated along the b-axis, although they are developed in random directions with respect to the substrate. The plate-like crystals comprise α-MoO3 and they are strongly oriented along the 0k0 plane horizontally to the substrate surface. Thus, the crystal structure and orientation of individual components of MoOx nanostructured thin films are clarified via transmission electron microscopy and three-dimensional X-ray diffraction studies.

Texture development in Cu-Ag-Fe triphase immiscible nanocomposites with superior thermal stability

Nanocrystalline metals attract intensive interests due to their high strength, excellent wear resistance and superior radiation. However, grain coarsening at elevated temperatures can lead to significant property degradation, impairing their potentials for high temperature applications. Here, we report a unique interplay of immiscible Cu, Ag, Fe grains in stabilizing the grain structure and tailoring the texture development. The Cu50Ag50 alloy experiences drastic grain coarsening after annealing at 600 °C and 700 °C. In contrast, the Cu-Ag-Fe nanocomposites exhibit much smaller grain size upon annealing. Interestingly, while the Cu50Ag50 alloy displays random nanocrystal orientation, highly textured Cu-Ag-Fe nanocomposites form. Furthermore, the composition of Fe plays a dominant role in changing the texture of nanocomposites from (111) Ag (Cu) and (110) Fe in Cu45Ag45Fe10, to prominent (110) Ag (Cu) and (100) Fe in Cu33Ag33Fe34. The fundamental mechanisms behind the texture formation and evolution are discussed. This study provides a fresh perspective to the design of stable nanocomposites with tunable texture.

Uniformly aligned flexible magnetic films from bacterial nanocelluloses for fast actuating optical materials

Naturally derived biopolymers have attracted great interest to construct photonic materials with multi-scale ordering, adaptive birefringence, chiral organization, actuation and robustness. Nevertheless, traditional processing commonly results in non-uniform organization across large-scale areas. Here, we report magnetically steerable uniform biophotonic organization of cellulose nanocrystals decorated with superparamagnetic nanoparticles with strong magnetic susceptibility, enabling transformation from helicoidal cholesteric (chiral nematic) to uniaxial nematic phase with near-perfect orientation order parameter of 0.98 across large areas. We demonstrate that magnetically triggered high shearing rate of circular flow exceeds those for conventional evaporation-based assembly by two orders of magnitude. This high rate shearing facilitates unconventional unidirectional orientation of nanocrystals along gradient magnetic field and untwisting helical organization. These translucent magnetic films are flexible, robust, and possess anisotropic birefringence and light scattering combined with relatively high optical transparency reaching 75%. Enhanced mechanical robustness and uniform organization facilitate fast, multimodal, and repeatable actuation in response to magnetic field, humidity variation, and light illumination.

Revealing the Aqueous Sequential Growth Mechanism between Au and Ag Nanocrystals of Segmented Ag-Au-Ag Heterojunction Nanorods via Redox Reaction Kinetics

Although the growth mechanisms (e.g., seed-induced growth and capping agent orientation) of bimetal nanocrystals (e.g., core–shell, alloy, segmented, and branched) from artificial experimental speculation and theoretical calculation have been widely accepted, precisely revealing their growth mechanisms is still tremendously challenging. In this work, we utilized redox reaction kinetics for the first time to successfully reveal the aqueous sequential growth mechanism between Au and Ag nanocrystals of segmented Ag-Au-Ag heterojunction nanorods (HJNRs) in a one-step and high-temperature aqueous system. Herein, electrode potentials of different electrical pairs (e.g., Ag+/Ag and AuCl4–/Au) at 200 °C could be calculated through using the Helgeson–Kirkham–Flowers state and other equations, from which whether Au and Ag nanocrystals grew successively and formed segmented Ag-Au-Ag HJNRs could be correctly assessed. The redox reaction kinetics mechanism can also explain well the aqueous-phase growth mechanisms of other bimetal nanocrystals and paves a promising avenue for the design and synthesis of other one-dimensional segmented metal nanostructures.

Microstructure and Intrinsic Strain of Nanocrystals in Ferroelectric (Na,K)NbO3 Nanofibers.

Densely woven highly crystallized biocompatible sodium–potassium niobate Na0.35K0.65NbO3 fibers with an average diameter of 100–200 nm and several hundreds of microns in length were sintered by the sol–gel calcination-assisted electrospinning technique. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) confirmed preferential cube-on-cube [001] orientation of nanocrystals within the fiber’s body, separated by a low angle grain boundary. The Williamson–Hall method was employed to analyze the broadening of XRD reflections and to accurately determine the size and intrinsic strain of nanocrystal fiber aggregates. The main objective of this article is to test the potential capacity of direct XRD analysis to noninvasively control crystallite size and lattice distortion in core-shell coaxial nanofibers.

Nanotwin assisted reversible formation of low angle grain boundary upon reciprocating shear load

Severe plastic deformation of metals is known to lead to superior properties that cannot be achieved by any traditional metallurgic process. Origin of the superior properties is perceived to be closely associated with grain refinement, a fundamental process during the severe plastic deformation, which is essentially the formation of new grain boundaries. However, the atomistic mechanism of grain boundary formation remains largely obscure. Here, by using in-situ transmission electron microscopy and molecular dynamic simulation, we reveal, for the first time at atomic level, shear-induced low-angle grain boundary (LAGB) formation processes in Au nanocrystal. We discover the LAGB formation is accomplished through inward propagation of nanotwins accompanied by dislocations gliding on twin boundaries, a nanotwin-mediated dislocation slip mechanism, which shows reversible characteristic under reciprocating shear load and is affected by the nanocrystal microstructure and orientation. Our result unveils unprecedented atomistic insights on shear driven grain refinement towards nanostructure of superior properties.

Selenate Se(VI) reduction to elemental selenium on heterojunctioned rutile/brookite nano-photocatalysts with enhanced charge utilization

Newly synthesized nanocrystalline TiO2 mixed-phase photocatalysts are shown to surpass in Selenium (VI) reduction effectiveness the commercial P25 nanoTiO2 photocatalyst Heterojunctioned rutile (63 %)/brookite (37 %) (designated as R/b) nanoTiO2 is found to be the most effective photocatalyst achieving 95 % photo-assisted reductive deposition vs. 25 % only by the benchmark P25 composed of anatase (80 %) and rutile (A/r). This is attained at very high photocharge utilization with at least 5-fold less formic acid as hole scavenger. The superior performance of R/b is attributed to a favorable band alignment at the heterojunction and its 1D-dominated nanocrystal orientation enabling efficient co-adsorption and enhanced charge separation. The deposition of elemental Se further “auto-catalyzes” the photocatalytic process by enabling visible light activity with simultaneous suppression of charge recombination. In-depth probing of the R/b photocatalyst after selenium removal provides evidence of a 2-step mechanism involving (a) reductive adsorption of Se(VI)aq → Se(IV)surf with the participation of co-adsorbed formate and Ti3+ defects, and photoreduction of Se(IV)surf → Se(0)surf.

Rheo-SAXS study of shear-induced orientation and relaxation of cellulose nanocrystal and montmorillonite nanoplatelet dispersions.

The development of robust production processes is essential for the introduction of advanced materials based on renewable and Earth-abundant resources. Cellulose nanomaterials have been combined with other highly available nanoparticles, in particular clays, to generate multifunctional films and foams. Here, the structure of dispersions of rod-like cellulose nanocrystals (CNC) and montmorillonite nanoplatelets (MNT) was probed using small-angle X-ray scattering within a rheological cell (Rheo-SAXS). Shear induced a high degree of particle orientation in both the CNC-only and CNC:MNT composite dispersions. Relaxation of the shear-induced orientation in the CNC-only dispersion decayed exponentially and reached a steady-state within 20 seconds, while the relaxation of the CNC:MNT composite dispersion was found to be strongly retarded and partially inhibited. Viscoelastic measurements and Guinier analysis of dispersions at the shear rate of 0.1 s-1 showed that the addition of MNT promotes gel formation of the CNC:MNT composite dispersions. A better understanding of shear-dependent assembly and orientation of multi-component nanoparticle dispersions can be used to process materials with improved mechanical and functional properties.

Влияние фотонной обработки на повышение термоэлектрической добротности твердого раствора Bi2Te3 − Bi2Se3

In the present work, the phase compositions, morphology, structure, and thermal conductivity of Bi2Te3 – хSeх-based semiconductor wafers before and after the photon treatment (PT) were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and laser flash methods. The semiconductor plates 10 × 10 × 2 mm in size were prepared by electric discharge cutting from the briquettes, synthesized by hot pressing of Bi2Te3 – хSeх powder. Then, the plates were annealed at 570 K in an argon atmosphere for 24 h. The PT was carried out in an Ar atmosphere by irradiation of gas-discharge xenon lamps with pulses of 1.0 and 1.4 s and energy density ranging from 125 to 175 J/cm2. As revealed, the PT initiates the formation of a thin surface layer with an inhomogeneous nanocrystalline structure and an arbitrary nanocrystals orientation. The volume of material manifests a large-block textured crystalline structure with the original elemental and phase compositions formed during the extrusion. Thereby, a gradient nanostructured region composed of nano-sized and large Bi2Te3 – хSeх crystals with tunneling contacts is formed in the process of PT. We revealed that the PT changes the material bandgap slightly, decreases the concentration of charge carriers, increasing their mobility. The scattering of carriers and photons on linear defects dominates in a wide temperature range in the studied samples. Thereby, the figure of merit rise s by 8 % after PT, due to a decrease in the phonon component of thermal conductivity.