Nanoscale Compositional Fluctuations Research Articles

Large Nonhysteretic Volume Magnetostriction in a Strong and Ductile High-Entropy Alloy.

Rapid development of smart technologies poses a big challenge for magnetostrictive materials, which should not only permit isotropic and hysteresis-free actuation (i.e., nonhysteretic volume change) in magnetic fields, but also have high strength and high ductility. Unfortunately, the magnetostriction from self-assembly of ferromagnetic domains is volume-conserving; the volume magnetostriction from field-induced first-order phase transition has large intrinsic hysteresis; and most prototype magnetostrictive materials are intrinsically brittle. Here, a magnetic high-entropy alloy (HEA) Fe35Co35Al10Cr10Ni10 is reported that can rectify these challenges, exhibiting an unprecedented combination of large nonhysteretic volume magnetostriction, high tensile strength and large elongation strain, over a wide working temperature range from room temperature down to 100 K. Its exceptional properties stem from a dual-phase microstructure, where the face-centered cubic (FCC) matrix phase with nanoscale compositional and structural fluctuations can enable a magnetic-field-induced transition from low-spin small-volume state to high-spin large-volume state, and the ordered body-centered cubic (BCC) B2 phase contributes to mechanical strengthening. The present findings may provide insights into designing unconventional and technologically important magnetostrictive materials.

Microscopic Structural Evolution during Ultrastable Metallic Glass Formation.

By decreasing the rate of physical vapor deposition, ZrCuAl metallic glasses with improved stability and mechanical performances can be formed, while the microscopic structural mechanisms remain unclear. Here, with scanning transmission electron microscopy and high-energy synchrotron X-ray diffraction, we found that the metallic glass deposited at a higher rate exhibits a heterogeneous structure with compositional fluctuations at a distance of a few nanometers, which gradually disappear on decreasing the deposition rate; eventually, a homogeneous structure is developed approaching ultrastability. This microscopic structural evolution suggests the existence of the following two dynamical processes during ultrastable metallic glass formation: a faster diffusion process driven by the kinetic energy of the depositing atoms, which results in nanoscale compositional fluctuations, and a slower collective relaxation process that eliminates the compositional and structural heterogeneity, equilibrates the deposited atoms, and strengthens the local atomic connectivity.

Unlocking the origin of compositional fluctuations in InGaN light emitting diodes

The accurate determination of the compositional fluctuations is pivotal in understanding their role in the reduction of efficiency in high indium content $In_{x}Ga_{1-x}N$ light-emitting diodes, the origin of which is still poorly understood. Here we have combined electron energy loss spectroscopy (EELS) imaging at sub-nanometer resolution with multiscale computational models to obtain a statistical distribution of the compositional fluctuations in $In_{x}Ga_{1-x}N$ quantum wells (QWs). Employing a multiscale computational model, we show the tendency of intrinsic compositional fluctuation in $In_{x}Ga_{1-x}N$ QWs at different Indium concentration and in the presence of strain. We have developed a systematic formalism based on the autonomous detection of compositional fluctuation in observed and simulated EELS maps. We have shown a direct comparison between the computationally predicted and experimentally observed compositional fluctuations. We have found that although a random alloy model captures the distribution of compositional fluctuations in relatively low In ($\sim$ 18%) content $In_{x}Ga_{1-x}N$ QWs, there exists a striking deviation from the model in higher In content ($\geq$ 24%) QWs. Our results highlight a distinct behavior in carrier localization driven by compositional fluctuations in the low and high In-content InGaN QWs, which would ultimately affect the performance of LEDs. Furthermore, our robust computational and atomic characterization method can be widely applied to study materials in which nanoscale compositional fluctuations play a significant role on the material performance.

Nanoscale compositional fluctuations enabled by dynamic strain-induced austenite reversion in a Mn-rich duplex steel

The solute partitioning under dynamic strain-induced austenite reversion in a Fe-11Mn-1Al-0.07C (in wt.%) steel was studied by atom probe tomography. It was found that the concentrations of Mn and Al fluctuate in austenite at the nanoscale during warm deformation at 650 °C, leading to less stable chemical zones. It is proposed that stored energy in ferrite can change the thermodynamic equilibrium, leading to the formation of austenite with lower Mn and higher Al contents. This paper reveals evidence that stored strain can deviate the thermodynamic equilibrium, and hence could be a new path for engineering the nanostructures in austenite.

Tailoring Mixed Ionic/Electronic Conductivity with Grain Boundaries: (La,Sr)(Ga,Mg)O3-X Case Study

Modified point defect chemistry at grain boundaries can be leveraged to turn a predominantly ionic conductor or a predominantly electronic conductor into a mixed conductor, by changing the grain boundary density [1,2]. There are reports indicating that introducing some electronic conductivity into the ionically conducting electrolyte of a solid oxide electrolysis cell could help to alleviate the problem of delamination at or near the electrolyte-oxygen electrode interface [3]. For this reason, we revisited the possible role of grain boundaries in tailoring mixed conduction in the fast oxide-ion conductor La0.9Sr0.1Ga0.9Mg0.1O3-x (LSGM). LSGM is a candidate electrolyte as it exhibits good thermo-chemo-mechanical compatibility with adjacent perovskite-structured electrodes. Previous work has indicated enhanced p-type electronic transference numbers at the grain boundaries of LSGM compared to the bulk, attributed to space charge effects on the basis of the observed non-linear current-voltage behavior of the grain boundaries at sufficiently high voltages [4]. However, the interplay of processing route, grain size, and dopant concentration in tailoring the extent of this effect had not been investigated. In this work [5] we fabricated dense LSGM pellets with non-dilute doping levels typical of electrolytes, with a range of average grain sizes from ~100 nm to ~6 μm, using field- and pressure- assisted sintering with various post-annealing steps as well as conventional sintering. The electrical transport behavior of grain cores and grain boundaries (GBs) was subsequently studied by impedance spectroscopy as a function of temperature, oxygen partial pressure, and dc bias. The brick layer model and the nano-grain composite model were applied to determine specific GB conductivities and electrical widths from the impedance data. Structure and composition were evaluated by X-ray diffraction, thermogravimetric analysis, transmission electron microscopy, scanning transmission microscopy, and energy dispersive X-ray spectroscopy. With increasing sintering temperature (and grain size), the following trends were observed: increasing pO2-dependence of the GB conductivity indicating higher local electronic transference numbers, higher apparent GB potentials, lower specific GB conductivity, lower pre-exponential factor for specific GB conductivity, increasing electrical GB widths, and greater dc bias dependence of GB conductivity. On the other hand, as expected, grain core electrical behavior was independent of processing route, grain size, and pO2. The results are suggestive of an increasing grain boundary space charge effect with increasing sintering temperature (and grain size), as has been observed for other ionic conductors where grain size and sintering temperature/time were coupled. Additionally, in all cases, the GB electronic transference numbers in these highly doped samples appeared to be smaller than those reported for lightly doped LSGM. Microscopy and EDS revealed that while grain boundaries appeared to be well-crystalline and “clean” in all samples, there was increasing microstructural and compositional homogeneity in samples that were sintered at progressively higher temperatures. The significance of the nanoscale compositional and microstructural fluctuations in the small-grained samples is not yet clear, as the apparent phase purity and density of all the samples was very similar macroscopically. Nonetheless, it seems possible that such inhomogeneity (pores and Sr- rich and Mg-rich areas not limited to GB regions) might limit the space charge contribution at GBs, and that some of the pores could have arisen from residual volatile impurities trapped during the pressure-assisted sintering. Overall, the results confirm that 1) grain boundaries can be sites of mixed conduction in an otherwise predominantly ionically conducting electrolyte and 2) processing route (thermal, pressure, and electrical history) is an important variable for tailoring the magnitude of the local ionic and electronic conductivities and transference numbers. [1] Y.M. Chiang et al., Applied Physics Letters, 69(2), 185-187 (1996).[2] A.M. Saranya et al., Advanced Energy Materials, 5(11), 1500377 (2015). [3] A.V. Virkar et al., International Journal of Hydrogen Energy, 40, 5561-5577 (2015). [4] C.T. Chen et al., Physical chemistry chemical physics, 14, 9047-9049 (2012). [5] T. Chen et al., “Modifying grain boundary ionic/electronic transport in nano-Sr- and Mg- doped LaGaO3-δ by sintering variations,” (under review)

Modulation of plastic flow in metallic glasses via nanoscale networks of chemical heterogeneities

We systematically investigate the microstructures of metallic glasses with nanoscale networks of chemical heterogeneities introduced by the presence of a metastable miscibility gap, and their effects on modulating plastic flow of the alloys. Microstructural analysis of as-quenched alloys and the associated thermodynamic assessment in Cu-Zr-Al-Y metallic glass-forming system suggest that the existence of a metastable miscibility gap can induce not only phase-separated microstructures with sharp phase interfaces but also compositional fluctuations without a clear interface ranging from atomic scale to a few-nanometer scale in the fully amorphous alloys. The statistical analysis of shear avalanches in such compositionally heterogeneous metallic glasses reveals that chemical heterogeneities extending over a few nanometers promote a relatively large population of shear deformation units jammed before the nucleation of mature shear bands. This leads to the multiple nucleation of shear bands and sluggish deformation behavior along them. However, phase interfaces formed by phase separation inside the miscibility gap promote rapid propagation of shear bands at low flow stress, while compositional fluctuations creating non-sharp interfaces emerging at the outside of miscibility gap have relatively high resistance against shear band propagation. We hence suggest that the optimization of nanoscale compositional fluctuations in metallic glasses in terms of topology, percolation and magnitude can be an effective route for improving the materials' damage tolerance upon plastic flow.

Effect of cooling rate after solution treatment on subsequent phase separation during aging of Fe-Cr alloys: A small-angle neutron scattering study

The effect of cooling rate after solution treatment on the initial structure of concentrated binary Fe-Cr alloys and the effect of the initial structure on phase separation during subsequent aging has been investigated. The nano-scale compositional fluctuations in the bulk of the alloys are studied using small-angle neutron scattering and the results are compared with simulations using the Cahn-Hilliard-Cook (CHC) model. The alloys investigated represent different mechanisms of phase separation and at higher Cr content, when spinodal decomposition (SD) is favored, the initial Cr compositional fluctuations due to slow cooling after solution treatment reduce the kinetics of phase decomposition, whereas, at lower Cr composition when nucleation and growth is favored, the kinetics of phase decomposition is more rapid. Regardless of the nominal Cr composition of the alloy, the phase decomposition after extended aging up to 300 h at 748 K is always larger for the more non-random initial structure. The CHC modeling of the cooling process and subsequent initial aging (below 10 h) is in reasonable qualitative agreement with the experimental results for the Fe-40 wt.% Cr alloy decomposing via SD. However, the modeling approach must be refined for accurate quantitative modeling of the full SD process, including coarsening.

Glycolipid Crosslinking is Required for Cholera Toxin to Partition into and Stabilize Ordered Domains

Current models of lipid rafts propose that lipid domains normally exist as nanoscale compositional fluctuations at steady state in cells, but can be stabilized to form functional entities. However, the mechanism behind how stabilized rafts assemble and function remains unclear. Here, we test the role of glycolipid crosslinking as a raft targeting and ordering mechanism using the well-studied raft marker cholera toxin B pentamer (CTxB) as a model. We show that when bound to cell-derived Giant Plasma Membrane Vesicles, a variant of CTxB containing only a single functional GM1 binding site exhibits significantly reduced partitioning in the ordered phase compared to wild type CTxB with five binding sites. Moreover, monovalent CTxB does not increase membrane heterogeneity, unlike wild type CTxB. These results support the long-held hypothesis that CTxB stabilizes raft domains via a crosslinking mechanism and uncover an unexpected role for crosslinking in the targeting of CTxB to ordered domains.

Glycolipid Crosslinking Is Required for Cholera Toxin to Partition Into and Stabilize Ordered Domains

Current models of lipid rafts propose that lipid domains exist as nanoscale compositional fluctuations and these fluctuations can potentially be stabilized into larger domains, consequently better compartmentalizing cellular functions. However, the mechanisms governing stabilized raft assembly and function remain unclear. Here, we test the role of glycolipid crosslinking as a raft targeting and ordering mechanism using the well-studied raft marker cholera toxin B pentamer (CTxB) that binds up to five GM1 glycosphingolipids to enter host cells. We show that when applied to cell-derived giant plasma membrane vesicles, a variant of CTxB containing only a single functional GM1 binding site exhibits significantly reduced partitioning to the ordered phase compared to wild-type CTxB with five binding sites. Moreover, monovalent CTxB does not stabilize membrane domains, unlike wild-type CTxB. These results support the long-held hypothesis that CTxB stabilizes raft domains via a lipid crosslinking mechanism and establish a role for crosslinking in the partitioning of CTxB to ordered domains.

Atom probe tomography of compositional fluctuation in GaInN layers

Thin Ga1−xInxN layers with various indium compositions x were examined by high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Although nanoscale compositional fluctuation was observed in all Ga1−xInxN layers, no appreciable phase separation was observed. Since Ga1−xInxN layers were coherently grown on underlying GaN layers, it is considered that the elastic strain constrains phase separation in the Ga1−xInxN layers and that compositional fluctuations develop in order to minimize the local strain energy. The measured period of compositional fluctuations ranged from 3.4 to 4.9 nm and decreased with growth temperature (Tgrowth). This tendency is suggested to be dominated by the surface migration length of the adatom during the crystal growth.

Compositional Inhomogeneity of Multinary Semiconductor Nanoparticles: A Case Study of Cu2ZnSnS4

Probe-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy were used to characterize the inter- and intraparticle compositional inhomogeneity of multinary Cu2ZnSnS4 (CZTS) nanoparticles. CZTS nanoparticles were prepared following three distinct synthesis protocols described in the literature. Strong fluctuations in composition were observed for Cu and Zn in individual nanoparticles, independent of the synthesis method. Certain particles have regions that have compositions close to that of Cu2SnS3, as well as, in the extreme case, the presence of nearly pure ZnS species. This is an observation that has not been reported in prior studies of these systems and underscores the need to both more carefully study the polydispersity of multinary semiconductor nanoparticles (MSNs) and to improve synthetic protocols and characterization of MSNs. Notably—despite the observation of compositional fluctuations in individual nanoparticles—reactive sintering in Se vapor was shown to reduce the nanoscale compositional fluctuations in the resulting sintered grains, facilitating the use of these heterogeneous particles in optoelectronic devices.

Nano-Scale Pinning Centers in ${\rm Y}_{2}{\rm Ba}_{1}{\rm Cu}_{1}{\rm O}_{5}$ Doped Sm-Ba-Cu-O Superconductor

It has been suggested that the addition of nano-scale RE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> BaCuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (nmRE211, RE = Y, Sm and Nd) in melt-textured growth (MTG) Sm-Ba-Cu-O (SmBCO) materials enhances the critical current density (Jc), especially in high magnetic fields. According to the analysis of pinning properties, the field-induced pinning was attributed to a weakly superconducting phase. This phase was correlated to nano-scale compositional fluctuation regions originated from the peritectic reaction between nano-scale additives and matrix during the melt-growth process. To further elucidate the characteristics of the compositional fluctuation regions, in this study, nano-structure of nmY211 SmBCO samples were investigated using transmission electron microscopy (TEM) and electron energy loss spectra (EELS). Periodic nano-structures with width ranging from few to several tens of nanometers were observed throughout the matrix. Variations in both chemical composition and thickness of these periodic nano-structures were compared with the observations reported in the NEG ((Nd, Eu, Gd)-Ba-Cu-O) samples.

Investigation of composition fluctuations in GaN:Mg using optical transmission spectroscopy, near-field scanning optical microscopy, and scanning Kelvin probe microscopy

Composition fluctuations in GaN:Mg have been investigated by optical transmission spectroscopy, near-field scanning optical microscopy (NSOM), and scanning Kelvin probe microscopy (SKPM). An Urbach tail is observed in the optical transmission spectrum of heavily Mg-doped GaN, with the magnitude of Urbach energy increasing with increasing compositional/potential fluctuations. From the relationship between the Urbach energy and the potential fluctuation magnitude, the spatial size of the compositional fluctuations in GaN:Mg (with the average Mg concentration of ∼4×1019cm−3) is calculated to be 220nm. The nanoscale compositional fluctuations are determined by NSOM and SKPM. A spatial variation of the transmission mode NSOM intensity is observed with a characteristic wavelength of ∼300nm, which is attributed to the inhomogeneous distribution of Mg. The normalized root-mean-square (rms) fluctuation of the transmission mode NSOM intensity for heavily Mg-doped GaN is a factor of 5 greater than that for undoped GaN. Similar microstructure patterns are observed in the NSOM and surface potential images of GaN:Mg as probed by SKPM. The variation of local surface potential (on the order of 100mV) is attributed to the same compositional fluctuations of Mg, which changes the local surface work function. The local Mg concentrations are calculated from the SKPM image. For GaN:Mg with the Mg concentration of 4×1019cm−3, a rms value of 5.4×1019cm−3 of the compositional fluctuations has been observed.

Effect of different nanoscale RE2BaCuO5 additions on the formation of compositional fluctuation in Sm–Ba–Cu–O superconducting bulk materials

This study presents the effect of different nanoscale RE211 additions—Y2BaCuO5 (nmY211), Sm2BaCuO5 (nmSm211), and Nd4Ba2Cu2O10 (nmNd422)—on the nano-scale compositional fluctuation and associated pinning mechanism of the melt-textured growth (MTG) Sm–Ba–Cu–O [SmBCO, of which the composition is Sm123 (Sm1Ba2Cu3Oy) + 25 wt% Sm211 (Sm2BaCuO5)] superconducting bulk materials. The superconductivity and microstructure results indicated that in samples with the addition of these nano-sized particles, nanoscale compositional fluctuations form during the peritectic transformation of melt-growth process, which cause Tc variation on a nanoscale and result in the formation of δTc pinning centers at high magnetic field. The compositional fluctuation regions (δTc pinning centers) are altered by the difference in peritectic temperature, the solubility in the liquid phase, and the ion radius. The direct current transport R-T properties elucidate the change of flux pinning behavior. In addition, the different influence on microstructure and superconductivity between the two methods: mixing rare-earth elements in nanoscale RE211 or in the homogeneous mixed precursor powders [e.g., (Nd, Eu, Gd)–Ba–Cu–O (NEG)] is also discussed.

Direct observation of magnetically induced phase separation in Co-W sputtered thin films

Phase separation of Co-W sputtered thin films having a large magnetocrystalline anisotropy energy have been investigated. A nanoscale compositional fluctuation caused by magnetically induced phase separation was directly confirmed in the films deposited on a heated substrate in analogy with Co-Cr-based alloys. The difference between the phase separation features in Co-W and Co-Cr is attributed to the difference in their elastic energy. It is expected that the phase separation is enhanced by selecting optimum sputtering conditions. The Co-W system, therefore, is considered to be a promising candidate as a base alloy system for high-density recording media.

Magnetically induced phase separation and magnetic properties of Co–Mo hexagonal-close-packed structure thin films

Magnetically induced phase separation along the Curie temperature in the hexagonal-close-packed phase of a Co–Mo binary system has been predicted by thermodynamic calculations. Furthermore, the phase separation and magnetic properties of Co–Mo sputtered thin films have been investigated. Nanoscale compositional fluctuation caused by the magnetically induced phase separation has been confirmed in the films deposited on a heated substrate in the same manner as Co–Cr-based alloys. The magnetic anisotropy constant of the Co–Mo films is larger than that of Co–Cr films. From these results, it is concluded that the Co–Mo system is promising for use as a base alloy system for high density recording media.

Nanoscale Compositional Fluctuations in Single InGaAs/GaAs Quantum Dots

We report the first investigation of compositional disorder in quantum dots with subnanometer precision. We have investigated the compositional profile of a single InGaAs/GaAs quantum dot, of nominal In content x = 0.5, by using energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and atomic force and scanning tunneling microscope (AFM-STM) topography. The compositional analysis at nanoscale reveals a number of unexpected features: (i) In enrichment of the centre of the dot (about 65% In content); (ii) In diffusion outside the dot contour; (iii) In depletion of the wetting layer in the surrounding of the dot.

Nanoscale Compositional Fluctuations in Single InGaAs/GaAs Quantum Dots

We report the first investigation of compositional disorder in quantum dots with subnanometer precision. We have investigated the compositional profile of a single InGaAs/GaAs quantum dot, of nominal In content x = 0.5, by using energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and atomic force and scanning tunneling microscope (AFM-STM) topography. The compositional analysis at nanoscale reveals a number of unexpected features: (i) In enrichment of the centre of the dot (about 65% In content); (ii) In diffusion outside the dot contour; (iii) In depletion of the wetting layer in the surrounding of the dot.

Nanoscale compositional fluctuations in multiple InGaAs/GaAs quantum wires

An accurate analysis of nanoscale compositional fluctuations in InGaAs/GaAs quantum wires grown by metalorganic chemical vapor deposition on V-grooved substrates was performed by means of high-spatial-resolution transmission electron microscopy techniques. Small In fluctuations (2%–3% excess indium), spatially localized over approximately 5 nm, were detected and related to changes in the photoluminescence and photoluminescence excitation spectra.

APFIM studies of some aluminum alloys

We present the latest results of our continuing research on atom probe field ion microscopy on aluminum alloys. The motivation behind the research is to understand the effect of trace elements and nanoscale compositional fluctuations on the development of microstructures in aluminum alloys produced by different manufacturing processes. The nanoscale compositional analyses of Al-Cu-Mg-Ag, Al-Li and AlCeNiFe nanocrystalline alloys are presented.