Nanoscale Domain Structures Research Articles

Stochastic Insulator-to-Metal Phase Transition-Based True Random Number Generator

An oscillator-based true random number generator (TRNG) is experimentally demonstrated by exploiting inherently stochastic threshold switching in the insulator-to-metal transition (IMT) in vanadium dioxide. Through experimentation and modeling, we show that the origin of stochasticity arises from small perturbations in the nanoscale domain structure, which are then subsequently amplified through a positive feedback process. Within a 1T1R oscillator, the stochastic cycle-to-cycle variations in the IMT trigger voltage result in random timing jitter, which is harnessed for a TRNG. The randomness of the IMT TRNG output is validated using the NIST SP800-22 statistical test.

High-resolution charge carrier mobility mapping of heterogeneous organic semiconductors

Organic electronic device performance is contingent on charge transport across a heterogeneous landscape of structural features. Methods are therefore needed to unravel the effects of local structure on overall electrical performance. Using conductive atomic force microscopy, we construct high-resolution out-of-plane hole mobility maps from arrays of 5000 to 16 000 current-voltage curves. To demonstrate the efficacy of this non-invasive approach for quantifying and mapping local differences in electrical performance due to structural heterogeneities, we investigate two thin film test systems, one bearing a heterogeneous crystal structure [solvent vapor annealed 5,11-Bis(triethylsilylethynyl)anthradithiophene (TES-ADT)—a small molecule organic semiconductor] and one bearing a heterogeneous chemical composition [p-DTS(FBTTh2)2:PC71BM—a high-performance organic photovoltaic active layer]. TES-ADT shows nearly an order of magnitude difference in hole mobility between semicrystalline and crystalline areas, along with a distinct boundary between the two regions, while p-DTS(FBTTh2)2:PC71BM exhibits subtle local variations in hole mobility and a nanoscale domain structure with features below 10 nm in size. We also demonstrate mapping of the built-in potential, which plays a significant role in organic light emitting diode and organic solar cell operation.

Fabrication of nano-scale domain structures in bulk Mg-doped LiNbO3 crystals

We present a new method to fabricate nano-scale domain structures in bulk Mg-doped LiNbO3 crystals with the assistance of thermal fixing technique. Domain structures with hundred-nanometer width and millimeter length can be fabricated using this method. SEM image reveals a feature of smooth sidewall for the nano-scale domain structures.

Scattering effect of the well-ordered MgB4 impurity phase in two-step sintered polycrystalline MgB2 with glycine addition

Glycine-doped MgB2 bulk was prepared by two-step sintering in this study, first at 750 °C and then 900 °C. The MgB4 particles are induced to precipitate where the dislocations concentrated after C substitution or along the steps of screw dislocation during crystal growth, forming ordered MgB4 arrays throughout the MgB2 grain. By means of atomic force microscope, the detected magnetic domains are arranged in agreement with the ordered MgB4 particles after the measurement of magnetic hysteresis loop, which supported that the nano-scale MgB4 domain structure brought strong scattering effects and indicated that atomic force microscopy could test the role of the impurities. As a result, the extrapolating upper critical field H c2(0 K) is enhanced to 22.8 T for the sample with ordered MgB4, while only 18.1 T for the un-doped sample underwent the same sintering program. Besides, carbon substitution contributed to the enhancement of H c2 as well.

Nanoscale Domain Structure and Defects in a 2-D WO3 Layer on Pd(100)

A stoichiometric two-dimensional (2-D) WO3 layer has been fabricated by vapor-phase deposition of (WO3)3 clusters onto a Pd(100) surface and characterized by a combined experimental/theoretical multitechnique approach. The oxide forms a WO2 + O bilayer with a well-ordered c(2 × 2) structure, displaying at the full monolayer coverage a regular nanoscale pattern of antiphase domain boundaries, as revealed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) and rationalized by DFT analysis as a consequence of elastic strain relief. The stability of the WO2 + O bilayer is provided by polarity compensation via charge rearrangement at the WO3/Pd interface and allows for surface redox chemistry via reversible release and restoration of oxygen atoms of the tungstyl or W═O groups.

Light control of orbital domains: case of the prototypical manganite La0.5Sr1.5MnO4

Control of electronic and structural ordering in correlated materials on the ultrafast timescale with light is a new and emerging approach to disentangle the complex interplay of the charge, spin, orbital and structural degree of freedom. In this paper we present an overview of how orbital order and orbital domains can be controlled by near IR and THz radiation in the layered manganite La0.5Sr1.5MnO4. We show how near-IR pumping can efficiently and rapidly melt orbital ordering. However, the nanoscale domain structure recovers unchanged demonstrating the importance of structural defects for the orbital domain formation. On the contrary, we show that pulsed THz fields can be used to effectively orientate the domains. In this case the alignment depends on the in-plane electric field polarisation and is induced by an energy penalty that arises from THz field induced hopping of the localised charges.

Room temperature ferroelectricity in continuous croconic acid thin films

Ferroelectricity at room temperature has been demonstrated in nanometer-thin quasi 2D croconic acid thin films, by the polarization hysteresis loop measurements in macroscopic capacitor geometry, along with observation and manipulation of the nanoscale domain structure by piezoresponse force microscopy. The fabrication of continuous thin films of the hydrogen-bonded croconic acid was achieved by the suppression of the thermal decomposition using low evaporation temperatures in high vacuum, combined with growth conditions far from thermal equilibrium. For nominal coverages ≥20 nm, quasi 2D and polycrystalline films, with an average grain size of 50–100 nm and 3.5 nm roughness, can be obtained. Spontaneous ferroelectric domain structures of the thin films have been observed and appear to correlate with the grain patterns. The application of this solvent-free growth protocol may be a key to the development of flexible organic ferroelectric thin films for electronic applications.

A glimpse at topological structures in multiferroic materials

A glimpse at topological structures in multiferroic materials

Abnormal magnetic ordering and ferromagnetism in perovskite ScMnO3 film

Bulk multiferroic ScMnO3 is the stable hexagonal phase, and it is very difficult to prepare its perovskite orthorhombic phase even under high pressure. We fabricated the orthorhombic ScMnO3 thin film by pulsed laser deposition through suitable substrate LaAlO3 and found that nano-scale twin-like domains are naturally formed in the thin film. Magnetic properties of the orthorhombic ScMnO3 thin films show that, besides normal antiferromagnetic ordering at 47 K, an anomalous magnetic transition occurs at 27 K for 60 nm film and at 36 K for 150 nm film only along the c-axis, which is absent in the ab-plane. Moreover, the second magnetic transition for both films is suppressed when the applied field increases from 1 kOe to 10 kOe. In addition, the ferromagnetism shows up in both films at 10 K, and saturation magnetization increases dramatically in 60 nm film compared with 150 nm film. We propose that the second magnetic transition might be more of lattice strain effect and also related to magnetism-induced ferroelectric polarization in orthorhombic RMnO3 thin films and low-temperature ferromagnetic properties in our films originate from the nano-scale twin-like domain structure.

Self-assembled domain structures: From micro- to nanoscale

The recent achievements in studying the self-assembled evolution of micro- and nanoscale domain structures in uniaxial single crystalline ferroelectrics lithium niobate and lithium tantalate have been reviewed. The results obtained by visualization of static domain patterns and kinetics of the domain structure by different methods from common optical microscopy to more sophisticated scanning probe microscopy, scanning electron microscopy and confocal Raman microscopy, have been discussed. The kinetic approach based on various nucleation processes similar to the first-order phase transition was used for explanation of the domain structure evolution scenarios. The main mechanisms of self-assembling for nonequilibrium switching conditions caused by screening ineffectiveness including correlated nucleation, domain growth anisotropy, and domain–domain interaction have been considered. The formation of variety of self-assembled domain patterns such as fractal-type, finger and web structures, broad domain boundaries, and dendrites have been revealed at each of all five stages of domain structure evolution during polarization reversal. The possible applications of self-assembling for micro- and nanodomain engineering were reviewed briefly. The review covers mostly the results published by our research group.

Hierarchically structured hematite architectures achieved by growth in a silica hydrogel.

Biomineralization strategies include the use of hydrogels to direct the formation of composite, single-crystal-like structures with unique structure-property profiles. Application of similar synthetic approaches to transition-metal oxides has the promise to yield low-temperature routes to hierarchically structured crystals that are optimized for a range of applications. Here, growth of hematite (α-Fe2O3) within a silica hydrogel resulted in hierarchical, mosaic crystals preferentially expressing catalytically active {110} facets, which are absent in solution-grown controls. Quantitative structural and compositional analysis reveals architectural changes that begin with the incorporation of silicon into the hematite lattice and propagate through to the nanoscale domain structure and assembly, leading to microscale morphologies that show improved photocatalytic performance. This work demonstrates the potential of applying bioinspired crystallization techniques to design functional oxides with multiscale architectural features.

Novel multiferroicity in GdMnO3 thin films with self-assembled nano-twinned domains.

There have been many interests in exploring multiferroic materials with superior ferroelectric and magnetic properties for the purpose of developing multifunctional devices. Fabrication of thin films plays an important role in achieving this purpose, since the multiferroicity can be tuned via strain, dimensionality, and size effect, without varying the chemical composition. Here, we report exotic multiferroic behaviors, including high-TC (~75 K) ferroelectric state, a large spontaneous polarization (~4900 μC/m2) and relatively strong ferromagnetism emerging at ~105 K, in orthorhombic GdMnO3/SrTiO3 (001) thin films with self-assembled nano-scale twin-like domains. We propose a possible ab-plane spiral-spin-order phase to be responsible for the large spontaneous polarization in the films, which can only be stabilized by relatively high magnetic field H > 6 T in the bulk crystals. It is suggested that the nano-scale twin-like domain structure is essential for the high temperature ferroelectricity and ferromagnetism of the thin films.

Live Imaging of Reversible Domain Evolution in BaTiO3 on the Nanometer Scale Using in-situ STEM and TEM

1. Department of Physics and Astronomy, School of Mathematics and Physics, Queen's University Belfast, UK, BT7 1NN 2. FEI Company, Europe NanoPort, Achtseweg Noord 5, 5651 GG Eindhoven, The Netherlands There is an increasing interest in novel ferroic materials, especially in device applications such as transistors, memory devices, tunneling barriers, etc.. The functionality of such materials is enabled by the reversible switching between equivalent states (or domains) that form to minimize the system’s free energy. This switching behavior depends strongly on the domain structure pattern and their mobility under external stimuli (electrical, mechanical, temperature, etc.). There is a strong need to study this switching in detail. Nanoscale domain structures and their specific switching behavior strongly influence the material responses and properties such as dielectric permittivity, piezoelectric coefficients and remnant polarization. Fortunately in-situ (scanning) transmission electron microscopy (S/TEM) represents a powerful technique for studying ferroic materials and their switching behavior with resolutions down to the atomic scale. Here, the domain pattern evolution in BaTiO

Temperature driven nano-domain evolution in lead-free Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 piezoceramics

Hierarchical micro- and nanoscale domain structures in Pb-free Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 piezoceramics were investigated by transmission electron microscopy. In situ heating and cooling studies of domain structure evolution reveal an irreversible domain transformation from a wedge-shaped rhombohedral nanodomain structure to a lamellar tetragonal domain structure, which could be associated with strong piezoelectricity in Ba(Zr0.2Ti0.8)O3-50(Ba0.7Ca0.3)TiO3 piezoceramics.

Phosphorescent light-emitting diodes using triscarbazole/bis(oxadiazole) hosts: comparison of homopolymer blends and random and block copolymers

Examples of blends of carbazole- and bis(oxadiazole)benzene-based side-chain polymers have recently been reported to be efficient host materials for phosphorescent emitters in organic light-emitting diodes. Here, the properties and performance of a physical blend of polynorbornene homopolymers with triscarbazole and bis(oxadiazole)benzene side chains are compared to those of random and block copolymers of the corresponding triscarbazole- and bis(oxadiazole)benzene-functionalized monomers. Green-emitting devices in which the blend is used a host for Ir(ppy)3 are significantly more efficient than those based on copolymers. Differential scanning calorimetry and solid-state NMR data show that there is no macroscale separation between the two polymers in the blend. The NMR data suggest that there are significant differences in the dimensionality and characteristic length of nanoscale domain structures in the block copolymer and the blend. Use of Ir(pppy)3 in place of Ir(ppy)3 leads to even more efficient light-emitting diodes, with external quantum efficiencies of up to ca. 21% (at 100 cd m−2).

Dynamic magnetic imaging by alternating force magnetic force mmicroscopy

Recently, magnetic force microscope (MFM) for dynamic imaging of AC magnetic field has attracted considerable attention due to its potential applications and special requirements in industry. In this paper, we develop an alternating force MFM technique based on the frequency modulation of MFM tip oscillation, which provides a powerful tool for the development of key technologies in magnetic information storage industry. Different from conventional MFM, the main points of the present work are: 1) the investigation of the frequency-modulation phenomenon; 2) optimization of the MFM tip parameter, and introduction of the MFM signal processing apparatus; 3) observation of the AC magnetic field. For dynamic evaluation of AC magnetic field, we need to theoretically analyze the mechanical and magnetic properties of MFM tips, to technically develop the MFM signal processing apparatus, and to experimentally image the dynamic magnetic signals. Finally, we demonstrate the alternating force MFM technique, which can measure and analyze the nano-scale magnetic domain structures in advanced magnetic materials.

Nanoscale domain structures in 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 (91PZN-9PT) single crystals studied by piezoresponse forcemicroscopy

High-resolution domain studies have been performed in 91PZN-9PT single crystals by piezoresponse force microscopy. Nanometer- and micron-sized strip-shaped 180º and parallel 90º domains were observed in the rhombohedral (R) and tetragonal (T) phases. Domain patterns with the typical sizes 5–200 nm were observed on the (001)cub surface of unpoled sample. The existence of nanodomains in the sample is tentatively attributed to the relaxor nature of PZN-PT where small polar clusters may form under zero-field cooling conditions. The domain observation confirms the structure of the morphotropic 91PZN-9PT crystals, which is composed of both the R and T (or monoclinic) orientations states. The outstanding piezoelectric properties may result from the cooperative response of the microscale/nanoscale domain structure.

Study of Nanoscale Domain Structure and Elastic Response of Lead-Free Piezoelectric Ceramics by Scanning Probe Microscopy

The nanoscale domain structure and variation of local elasticity in lead-free piezoelectric ceramics were investigated using home-made piezoresponse force microscope (PFM) and low-frequency scanning probe acoustic microscope (SPAM) based on a commercial atomic force microscope. The static domain structure and domain dynamics during polarization reversal were studied by PFM, while SPAM investigations showed the elastic and subsurface ferroelectrics structures. PFM and SPAM provided powerful tools for measurements of the local electromechanical and elastic properties of lead-free piezoelectric ceramics.

Polymer electrolyte membranes based on poly(arylene ether sulfone) with pendant perfluorosulfonic acid

Poly(arylene ether sulfone)-based ionomers with sulfonate groups of varying acidity (perfluoroalkyl sulfonate, aryl sulfonate and alkyl sulfonate) were synthesized via borylation of aromatic C–H bonds and Suzuki coupling with sulfonated phenyl bromides. Properties of the ionomers, such as thermal stability, water uptake, ion exchange capacity, morphology and proton conductivity, were analyzed with respect to the effect of the sulfonate group. Superacidic fluoroalkyl sulfonated ionomers displayed much higher conductivity at low relative humidity than less acidic aryl and alkyl sulfonated ionomers in spite of their lower ion exchange capacities. The water uptake of the membranes correlated with their IEC, regardless of the acid group identity. The membranes with fluoroalkyl and alkyl sulfonate groups had similar hydration numbers as a function of RH, but the hydration number of the aromatic sulfonate sample was greater than the other polymers. Ionic domain structure analysis by atomic force microscopy, transmission electron microscopy and small-angle X-ray scattering revealed that all of the aromatic ionomers in this study had a small, disorganized phase structure. These results demonstrate that the primary influence on the proton conductivity of these randomly sulfonated copolymers is the acid strength while the nanoscale domain structure plays a secondary role in the low RH proton transport.

Domain Nanotechnology in Ferroelectric Single Crystals: Lithium Niobate and Lithium Tantalate Family

The recent achievements in domain nanotechnology in single crystals of lithium niobate and lithium tantalate family have been reviewed. The formation of nanoscale domain structures has been studied with high spatial resolution using modern experimental methods of domain visualization. Criteria of the investigated ferroelectric material selection and recent experimental achievements have been presented. The physical basis of the nanodomain engineering based on the role of nanoscale domains in formation of the domain patterns in highly non-equilibrium switching conditions has been discussed. The obtained knowledge allows us to manufacture the highly efficient periodically poled crystals for laser light frequency conversion.