Nanoscale Heterogeneity Research Articles

Energetic and structural studies of amorphous Ca1−xMgxCO3·nH2O (0 ⩽ x ⩽ 1)

Early stage amorphous precursors provide a low energy pathway for carbonate mineralization. Many natural deposits of carbonate minerals and biogenic calcium carbonate (both amorphous and crystalline) include significant amounts of Mg. To understand the role of magnesium-containing amorphous precursors in carbonate mineralization, we investigated the energetics and structure of synthetic amorphous Ca–Mg carbonates with composition Ca1−xMgxCO3·nH2O (0⩽x⩽1) using isothermal acid solution calorimetry and synchrotron X-ray scattering experiments with pair distribution function (PDF) analysis. Amorphous magnesium carbonate (AMC with x=1) is energetically more metastable than amorphous calcium carbonate (ACC with x=0), but it is more persistent (crystallizing in months rather than days under ambient conditions), probably due to the slow kinetics of Mg2+ dehydration. The Ca1−xMgxCO3·nH2O (0⩽x⩽1) system forms a continuous X-ray amorphous series upon precipitation and all intermediate compositions are energetically more stable than a mixture of ACC and AMC, but metastable with respect to crystalline carbonates. The amorphous system can be divided into two distinct regions. For x=0.00–0.47, thermal analysis is consistent with a homogeneous amorphous phase. The less metastable compositions of this series, with x=0.0–0.2, are frequently found in biogenic carbonates. If not coincidental, this may suggest that organisms take advantage of this single phase low energy amorphous precursor pathway to crystalline biogenic carbonates. For x⩾0.47, energetic metastability increases and thermal analysis hints at nanoscale heterogeneity, perhaps of a material near x=0.5 coexisting with another phase near pure AMC (x=1). The most hydrated amorphous phases, which occur near x=0.5, are the least metastable, and may be precursors for dolomite formation.

A mean-field model for anelastic deformation in metallic-glasses

In this article, a simple mean-field model is developed for quantitatively understanding the anelastic deformation in metallic-glasses (MGs). By relating the anelastic strain of an MG under a constant stress to the constrained configurational transition of liquid-like phases, we are able to derive the dynamic equation that captures the essential features of anelastic deformation hitherto reported. The important implications of this mean-field model are also discussed with reference to the recent findings of nano-scale structural heterogeneity in MGs.

Multi-scale structuration of glasses: Observations of phase separation and nanoscale heterogeneities in glasses by Z-contrast scanning electron transmission microscopy

Chemical fluctuations are probed in glasses obtained with different thermal histories using scanning transmission electron microscopy in high angle annular dark field mode. Direct imaging of the glass structure is obtained at the sub-nanometer scale with Z-contrast. Macroscopic glass-in-glass separation is probed in a slowly quenched melt where chemical resolution allows the determination of the regions associated with Zr/Zn atoms. In a quickly cooled glass, exempt of macroscopic phase separation, a segregation of Zr/Zn atoms is still evidenced but on a different length scale, suggesting either the beginnings of glass separation or intrinsic features of the glass structure. Glass inhomogeneities must be taken into account to have a quantitative evaluation of nucleation processes. These “nano”-heterogeneities can be associated with the nucleation of zirconia phases, giving important clues to understand the nucleation pathways and the structural role of nucleating agents in aluminosilicate glasses.

Nanomechanical Properties of Polyethylene Glycol Brushes on Gold Substrates

A necessary step in advancing the use of polyethylene glycol (PEG) surface coatings in critical biotechnological applications such as cancer treatments is to provide direct and reliable nanoscale property characterization. Measurements for such characterization are currently provided by scanning probe methods, which are capable of assessing heterogeneity of both surface coverage and properties with nanoscale spatial resolution. In particular, atomic force microscopy (AFM) can be used to detect and quantify the heterogeneity of surface coverage, whereas atomic force spectroscopy can be used to determine mechanical properties, thereby revealing possible heterogeneity of properties within coatings. In this work, AFM and force spectroscopy were used to characterize the morphology and mechanical properties of thiol-functionalized PEG surface coatings on flat gold substrates in aqueous PEG solution. Thiol-functionalized PEG offers a direct and simple method of attachment to gold substrates without intermediate anchoring layers and therefore can be exploited in developing PEG-functionalized gold nanoparticles. AFM was used to investigate the morphology of the PEG coatings as a function of molecular weight; the commonly observed coverage was in the form of sparse, brushlike islands. Similarly, force spectroscopy was utilized to study the mechanical properties of the PEG coatings in compression and tension as a function of molecular weight. A constitutive description of the mechanical properties of PEG brushes was achieved through a combinatorial analysis of the statistical responses acquired in both compression and tension tests. Such a statistical characterization provides a straightforward procedure to assess the nanoscale heterogeneity in the morphology and properties of PEG coverage.

Hydration of humic and fulvic acids studied by DSC

Qualitative and quantitative aspects of hydration of four humic acids (HA) and three fulvic acids (FA) originating from different sources were investigated. DSC experiments at subambient temperatures were carried out in order to monitor differences in ice behavior originating from freezable water surrounding humic molecules. It was found that kinetic effects play a significant role in hydration processes of both HA and FA. In fact, the hydration took part over 21 days which was detected as a progressive decrease in ice melting enthalpy. Simultaneously, the peak shapes and positions changed indicating structural changes in the physical structure of the humic substances. In case of FA, the dependency of melting enthalpy on water concentration showed a linear trend resembling a complete hydration previously observed for water-soluble hydrophilic polymers. In contrast, the melting enthalpy of some HA increased in a step-like way with increasing water content, suggesting preservation of original hydrophobic scaffold during the hydration. The differences between the rather young FA and the rather old HA lead to the conclusion that water can play a significant role in processes of humification. We assume that separation of hydrophobic and hydrophilic domains and thus increase in nanoscale heterogeneity represents an important physical contribution to the overall humification process. It was also demonstrated that the higher content of oxygen in humic molecules is not the only indicator of higher water holding capacity. Instead the porosity of humic matrix seems to contribute as additional parameter into these processes.

Ultrafast optical demagnetization manipulates nanoscale spin structure in domain walls

During ultrafast demagnetization of a magnetically ordered solid, angular momentum has to be transferred between the spins, electrons, and phonons in the system on femto- and picosecond timescales. Although the intrinsic spin-transfer mechanisms are intensely debated, additional extrinsic mechanisms arising due to nanoscale heterogeneity have only recently entered the discussion. Here we use femtosecond X-ray pulses from a free-electron laser to study thin film samples with magnetic domain patterns. We observe an infrared-pump-induced change of the spin structure within the domain walls on the sub-picosecond timescale. This domain-topography-dependent contribution connects the intrinsic demagnetization process in each domain with spin-transport processes across the domain walls, demonstrating the importance of spin-dependent electron transport between differently magnetized regions as an ultrafast demagnetization channel. This pathway exists independent from structural inhomogeneities such as chemical interfaces, and gives rise to an ultrafast spatially varying response to optical pump pulses.

Physical ageing of the contact line on colloidal particles at liquid interfaces

Young's law predicts that a colloidal sphere in equilibrium with a liquid interface will straddle the two fluids, its height above the interface defined by an equilibrium contact angle. This has been used to explain why colloids often bind to liquid interfaces, and has been exploited in emulsification, water purification, mineral recovery, encapsulation and the making of nanostructured materials. However, little is known about the dynamics of binding. Here we show that the adsorption of polystyrene microspheres to a water/oil interface is characterized by a sudden breach and an unexpectedly slow relaxation. The relaxation appears logarithmic in time, indicating that complete equilibration may take months. Surprisingly, viscous dissipation appears to play little role. Instead, the observed dynamics, which bear strong resemblance to ageing in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. These results may provide clues to longstanding questions on colloidal interactions at an interface.

Pressure-induced magnetoresistivity reversal in magnetite

The magnetoresistivity (MR) of nano-powdered magnetite has been measured under pressures up to 19.0 GPa and magnetic fields up to 15 kG. An unexpected quasi-linear positive MR below 6.0 GPa and an MR transition from positive to negative around 6.0 GPa have been found. The abnormal positive MR below 6.0 GPa originates from the confinement effect caused by nano-scale heterogeneity at the contact surface between Mo and Fe3O4 sample and the high negative spin polarization of magnetite. The MR transition at about 6.0 GPa comes from a phase transition from half-metal to metal induced by pressure.

Potential of AFM–nanothermal analysis to study the microscale thermal characteristics in soils and natural organic matter (NOM)

This exploratory study evaluates the potential of nanothermal analysis (nTA) coupled with atomic force microscopy (AFM) of soil samples for understanding physicochemical processes in soil and for linking the nanospatial and microspatial distribution of thermal characteristics with the macroscopic properties of soil samples. Soil and reference samples were investigated by differential scanning calorimetry and AFM-nTA. nTA was conducted on 16 points of each AFM image in two subsequent heating cycles (55–120°C and 55–300°C, respectively). Thermograms were subdivided into characteristic types and their spatial distribution was compared between sample replicates and materials. Thermogram types consisted of partly structured expansion and compression phases, suggesting material-specific thermal profiles. The distribution of thermogram types reflected sample-dependent nanoscale and microscale heterogeneity. Indications for water molecule bridge transitions were found by nTA in peat and soil. Organic materials generally revealed strong expansion and irreversible compression phases, latter probably due to the collapse of pore and aggregate structures. In contrast to charcoal and manure, peat shows strong expansion below 120°C and compression only above 120°C. All investigated samples are heterogeneous on the nanoscale and microscale with respect to thermal behaviour. AFM-nTA allows distinguishing numerous different materials on nanometre and micrometre scales in soil samples. The material-dependent characteristics will help in understanding and learning more about the nanoscale distribution of different materials and properties. Related to the macroscopic thermal behaviour, this will allow studying links between the properties of biogeochemical interfaces and the processes governed by them.

TiCu-Based Bulk Metallic Glasses Exhibiting Large Compressive Plastic Strain <sup></sup>

TiCu-based bulk metallic glasses with large plasticity (up to 10%) were fabricated by copper mold suction casting. The detailed studies showed that the large plasticity resulted from the nanoscale heterogeneities formed upon solidification, which resulted in extensively shear band formation, branching and interaction.

Introducing a strain-hardening capability to improve the ductility of bulk metallic glasses via severe plastic deformation

The great technological potential for bulk metallic glasses (BMGs) arises primarily because of their superior mechanical properties. To realize this potential, it is essential to overcome the severe ductility limitations of BMGs which are generally attributed to shear localization and strain softening. Despite much international effort, progress in improving the ductility of BMGs has been limited to certain alloys with specific compositions. Here, we report that severe plastic deformation of a quasi-constrained volume, which prevents brittle materials from fracture during the plastic deformation, can be used to induce strain hardening and to reduce shear localization in BMGs, thereby giving a significant enhancement in their ductility. Structural characterizations reveal the increased free volume and nanoscale heterogeneity induced by severe plastic deformation are responsible for the improved ductility. This finding opens a new and important pathway towards enhanced ductility of BMGs.

Chemical and phase heterogeneity of Ti-Fe-Mn-silica microspherules in crushed ore samples

A Fe-Ti-Mn-silicate microspherule from the heavy mineral fraction of crushed geological samples from an underground mine (Kellyam gold deposit, Yakutia) was examined by transmission electron microscopy. The 400-μm microspherule has a complex chemical composition and contains the following elements (in order of decreasing content): O, Ti, Si, Mn, Fe, Al, Mg, Ca, and K. The material of the micro-spherule shows a polyphase structure and chemical heterogeneity with separation into amorphous oxide-silicate (SiO2) and crystalline metallic (α-Ti + FeTi) constituents. The spherical shape and the character of internal heterogeneity suggest that the microspherule was formed owing to the rapid solidification of a melt droplet. The nanoscale heterogeneity of the microspherule is a consequence of a two-stage process. (1) Liquid immiscibility in the initial melt droplet before its solidification resulted in the incomplete spatial separation of Si + Al and Ti + Fe + Mn and appearance of SiO2 globules with admixtures of Al, Ca, K, Mg, Ti, Fe, and Mn approximately 100 nm in size. (2) Chemical heterogeneity developed subsequently both within the globules and within the matrix at the stage of melt solidification or after the solidification. The second-stage chemical heterogeneity was manifested in (a) the separation of heavy minor element oxides (Ti, Fe, and Mn) from the light elements Si, Al, K, Ca, and Mg(?) within the globules and (b) the separation of the matrix material into crystalline metallic (α-Ti + FeTi) and amorphous oxide-silicate [SiO2-MgO-Mn3O4(?)-Fe2O3(?)-TiO2(?)] parts. The obtained results do not rule out the possibility of formation of such microspherules during drilling operations.

How Kondo-holes create intense nanoscale heavy-fermion hybridization disorder

Replacing a magnetic atom by a spinless atom in a heavy-fermion compound generates a quantum state often referred to as a "Kondo-hole". No experimental imaging has been achieved of the atomic-scale electronic structure of a Kondo-hole, or of their destructive impact [Lawrence JM, et al. (1996) Phys Rev B 53:12559-12562] [Bauer ED, et al. (2011) Proc Natl Acad Sci. 108:6857-6861] on the hybridization process between conduction and localized electrons which generates the heavy-fermion state. Here we report visualization of the electronic structure at Kondo-holes created by substituting spinless thorium atoms for magnetic uranium atoms in the heavy-fermion system URu(2)Si(2). At each thorium atom, an electronic bound state is observed. Moreover, surrounding each thorium atom we find the unusual modulations of hybridization strength recently predicted to occur at Kondo-holes [Figgins J, Morr DK (2011) Phys Rev Lett 107:066401]. Then, by introducing the "hybridization gapmap" technique to heavy-fermion studies, we discover intense nanoscale heterogeneity of hybridization due to a combination of the randomness of Kondo-hole sites and the long-range nature of the hybridization oscillations. These observations provide direct insight into both the microscopic processes of heavy-fermion forming hybridization and the macroscopic effects of Kondo-hole doping.

Characterization of Original Bioapatite and Nano-Scale Heterogeneities in Fossil Bone with Transmission and Scanning Electron Microscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Identifying the role of nanoscale heterogeneities in pitting behaviour of Al-based metallic glass

To clarify the correlation of nanoscale heterogeneity with corrosion in Al-based metallic glasses, three model alloys with a single nanoscale α-Al, Al 3Ni or Al 11Ce 3 phase embedded in amorphous Al-Ni-Ce alloy matrix were obtained directly by melt quenching. The results indicated that the high pitting corrosion resistance of AM alloys was not deteriorated by nanocrystalline α-Al precipitation; whereas the pitting potential was slightly decreased and considerably reduced relative to their amorphous state due to the precipitation of nanocrystalline Al 3Ni or Al 11Ce 3 respectively. Such a pitting sensitivity of different types of heterogeneities attributes to the nano-scale pit initiation events.

Characterization of Nanoscale Mechanical Heterogeneity in a Metallic Glass by Dynamic Force Microscopy

We report nanoscale mechanical heterogeneity of a metallic glass characterized by dynamic force microscopy. Apparent energy dissipation with a variation of ~12%, originating from nonuniform distribution of local viscoelasticity, was observed. The correlation length of the heterogeneity was measured to be ~2.5 nm, consistent with the dimension of shear transformation zones for plastic flow. This study provides the first experimental evidence on the nanoscale viscoelastic heterogeneity in metallic glasses and may fill the gap between atomic models and macroscopic glass properties.

Nanoscale heterogeneity in alkyl-methylimidazolium bromide ionic liquids

High-energy x-ray diffraction measurements and atomistic molecular dynamics (AMD) numerical simulations have been carried out on 1-alkyl-3-methylimidazolium bromide ionic liquids, C(n)mimBr, with n = 2, 4, and 6. Excellent agreement between experiment and simulation is obtained, including the region of the low-Q peak that has proved problematic in previous work in the literature. In the partial structure analysis of the AMD results, a distinct peak develops at the leading edge of the ring-ring pair distribution function and shifts to lower r with increasing alkyl chain length, indicating that the preferential parallel and antiparallel alignment of neighboring cation rings plays a larger role with increasing chain length. The ring-ring, anion-anion, and ring-anion partial structure factors are dominated by strong charge-ordering peaks around 1.1 Å(-1), corresponding to a distance between neighboring polar entities of D(2) = 5.7 Å. In contrast, the tail-tail S(Q) is dominated by the low-Q peak that rises and moves to lower Q with increasing chain length; the length scale of this structural heterogeneity D(1) increases from about 10 Å in C(2)mimBr to 14.3 Å in C(4)mimBr and 18.8 Å in C(6)mimBr. Both the length scale of the structural heterogeneity and its anomalous temperature dependence in the C(n)mimBr liquids studied here show considerable similarity to results in the literature for C(n)mimPF(6) liquids, indicating a remarkable insensitivity to the form and size of the anion. Our results are consistent with the concept of nanoscale heterogeneity with small, crystal-like moieties.

Towards well-structured fullerene–phthalocyanine composites: Co-aggregation of fullerene C 60 with tetrakis(4-amino-5-phenoxy)phthalocyanine

The paper reports on binary composites prepared from fullerene C 60 and π-rich cobalt tetrakis(4-amino-5-phenoxy)phthalocyanine (CoPc) being a mixture of four geometrical isomers. Co-aggregation has been conducted under aqueous and non-aqueous conditions by the Langmuir–Blodgett (LB) technique and drop-casting from o-xylene, respectively. The prepared LB composites are badly disordered whereas the cast ones are predominantly structured: depending on the C 60/CoPc ratio, the wire-, worm-, or bead-like particles are observed. Optical spectroscopy is used to reveal the aggregation state of fullerene in the composites. The nano-scale heterogeneities resulting from homomolecular aggregation of fullerene are revealed only in the former case. Smooth co-aggregation under non-aqueous conditions is attributed to intermediate formation of the ternary C 60–CoPc–xylene solvates. Formation of the well-shaped composite particles seems to be an optional post-aggregation process being favored by the molecular symmetry of the CoPc isomers.

Effect of hydrophobic nanopatches within an ionic surface on the structure of liquids

The structures of liquid water and isopropanol have been studied as a function of the size of a hydrophobic patch present in a model hydrophilic surface via molecular dynamics simulations. A significant anisotropy extending into the first few solvent layers is found over the patch which suggests implications for many real-world systems in which nanoscale heterogeneity is found.

Local heterogeneity in the distribution of iron ions in CR-spinel from Ural ultramafic massifs: Evidence from Mössbauer spectroscopy data

The crystal chemistry of Fe ions in Cr-spinel from the largest Ural ultramafic massifs has been studied by Mossbauer spectroscopy at room temperature and the boiling temperature of liquid nitrogen. The spectra substantially depend on the mineral composition (stoichiometry) and measurement temperature; Fe2+ and Fe3+ doublets significantly overlap; the Fe2+ doublet lines are markedly broadened. According to the Mossbauer data, the degree of iron oxidation is 7–35% and appreciably differs from that in the stoichiometric approximation. The disturbance of integral stoichiometry by di- and trivalent cations (deviation of the Me3+/Me2+ value from 2.0) may be caused not only by partial inversion of the mineral structure but also by local micro- and nanoscale heterogeneity of the mineral, clustering of Fe2+ and Al (Cr, Fe3+) cations, and the appearance of associates. Possible application of the QS-distribution method for analyzing nonequivalent nuclear iron states and the thermal dynamics of Mossbauer spectra for studying local clustering effects of iron cations is discussed. It is shown that these approaches give new information on local heterogeneity of structural sites occupied by iron ions.