We review our work on combinatorial search and investigation of morphotropic phase boundaries (MPBs) in chemically substituted BiFeO3 (BFO). Utilizing the thin-film composition spread technique, we discovered that rare-earth (RE = Sm, Gd, and Dy) substitution into the A-site of the BFO lattice results in a structural phase transition from the rhombohedral to the orthorhombic phase. At the structural boundary, both the piezoelectric coefficient and the dielectric constant are substantially enhanced. It is also found that the observed MPB behavior can be universally described by the average A-site ionic radius as a critical parameter, indicating that chemical pressure effect due to substitution is the primary cause for the MPB behavior in RE-substituted BFO. Our combinatorial investigations were further extended to the A- and B-site cosubstituted BFO in the pseudoternary composition spread of (Bi1−xSmx)(Fe1−yScy)O3. Clustering analysis of structural and ferroelectric property data of the fabricated pseudoternary composition spread reveals close correlations between the structural and ferroelectric properties. We show that the evolution in structural and ferroelectric properties is controlled solely by the A-site Sm substitution and not the B-site Sc substitution.

Multichromophoric dendrimers are increasingly being considered for solar energy systems. To design materials with suitably efficient photon collection demands a thorough understanding of crucial photophysical conditions and electrodynamic mechanisms, many of which prove to emulate photosynthetic systems. Key parameters include the chromophore absorption properties, the generation, branching and folding of the dendrimer, and the presence of a spectroscopic gradient. Driving excitation towards a trap, resonance energy transfer favors migration between nearest neighbor chromophores. In modeling the progress of excitation from antenna chromophores towards the trap, a propensity matrix method has broad applicability, giving physical insights of generic validity. Calculations on specific dendrimers are best served by quantum chemistry models; again, links with photobiological systems can be discerned. Two important optically nonlinear features are cooperative energy pooling, and two-photon energy transfer. Branch multiplicity and the polar or polarizable nature of the chromophores also play important roles in determining energy harvesting characteristics.

In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the chemical and electronic structure of these components. Because of the high complexity of the Cu2ZnSn(S,Se)4 compound semiconductor absorber material alone, this is particularly true for kesterite-based devices. Hence, this paper reviews our recent progress in the characterization of Cu2ZnSnS4 (CZTS) thin films. It is demonstrated that a combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the article will be on secondary impurity phases, electronic structure, native oxidation, and the CZTS surface composition.

We have studied dynamic thermo-mechano-chemical responses of reactive metallic systems, both in clouds of small oxygen-free particles (∼1–10 μm in diameter) produced by fracturing Zr-rich bulk metallic glass and in pure Zr metal foils (∼25 μm thin), under thermal (laser ablation or pulse electrical heating) and mechanical loadings. The mechanical fracture/fragmentation and fragments reactions were time resolved using an integrated set of fast six-channel optical pyrometer, high-speed microphotographic camera, and time- and angle-resolved synchrotron x-ray diffraction. These small-scale tabletop real-time experiments performed on or near surfaces of reactive metals provide fundamental data, in atomistic scales or of particle clouds, regarding fragmentation mechanics, combustion mechanisms and kinetics, and dynamics of energy release under thermal and mechanical loadings. We present the results of pure Zr and Zr-rich amorphous metals, not only signifying diversified combustion mechanisms depending on microstructures, particle sizes, oxygen pressure, and ignition conditions but also providing fundamental data that can be used to develop and validate thermochemical and mechanochemical models for reactive materials.

We use the first-principles GW + Bethe–Salpeter equation approach to study the electronic structure and optical absorption spectra of uniaxial strained graphene. Applied strain induces an anisotropic Fermi velocity and tilts the axis of the Dirac cone. As a result, the optical response of strained graphene is dramatically changed; the optical absorption is anisotropic; the characteristic single optical absorption peak of pristine graphene is split into two peaks with enhanced excitonic effects. Within the infrared regime, the optical absorbance of uniaxial strained graphene is no longer a constant because of the broken symmetry and anisotropic excitonic effects. Within the visible-light regime, we observe a prominent optical absorption peak due to an enhanced red shift by electron–hole interactions, enabling us to change the visible color and transparency of stretched graphene. Finally, we also reveal enhanced excitonic effects within the ultraviolet regime, where a few nearly bound excitons are identified.

Polycrystalline randomly oriented CaMnO3 films were successfully deposited on sapphire substrates by soft chemistry methods. The precursor solutions were obtained from a mixture of metal acetates dissolved in acids. The Seebeck coefficient and the electrical resistivity were measured in the temperature range of 300 K < T < 1000 K. Modifications of thermal annealing procedures during the deposition of precursor layers resulted in different power factor values. Thermal annealing of CaMnO3 films at 900 °C for 48 h after four-layer depositions (route A) resulted in a pure perovskite phase with higher power factor and electrical resistivity than four-layer depositions of films annealed layer by layer at 900 °C for 48 h (route B). The studied films have negative Seebeck coefficients indicative of n-type conduction and electrical resistivities showing semiconducting behavior.

The effect of crystallographic orientation and sample volume on incipient plasticity of commercially pure polycrystalline nickel 200 was investigated using electron back scatter diffraction (EBSD) and nanoindentation. A nickel sample was annealed, and grain orientations were determined using EBSD. Specifically oriented grains were indented using tips of nominal radii 100, 1000, and 1300 nm. The onset of plasticity in relatively defect-free small volumes is characterized by a sharp “pop-in” event during load-controlled nanoindentation. Grains in the (001) orientation yield at higher pressures; larger radii tips are more sensitive to orientation effects but yield at lower stresses. Subsurface defects may result in the dominance of tip radius over orientation, indicated by statistical analysis of yield points. An activation volume analysis, in conjunction with the yield pressure as a function of tip size, suggests that dislocation loops may play a critical role in causing these effects.

(Zr, Hf)NiSn-based half-Heusler alloys with refined grains were prepared by melt spinning and spark plasma sintering. The grain size of the melt-spun (MS) thin ribbons varied from ∼500 nm to ∼3 μm. X-ray diffraction analysis showed that single phased alloys were obtained. Nanoscale precipitates dispersed in the matrix could be observed in both the MS ribbons and sintered bulk samples, which increased the carrier concentration and electrical conductivity. The lattice thermal conductivity decreased by more than 20% below 100 K and 5–20% from 200 to 1000 K, compared with the levitation melted counterparts, due to the refined grain sizes. The maximum dimensionless figure of merit ZT value reached ∼0.9 for the MS Hf0.6Zr0.4NiSn0.98Sb0.02sample.

Recently, tremendous progress has been made toward the application of organic light-emitting diodes (OLEDs) in full color flat panel displays and other devices. This article reviews and discusses our recent progress in extended development of emissive semi-interpenetrating polymer networks (E-semi-IPNs) and hybrid quantum dots (QDs)–polymer nanocomposites for use in multicolor and multilayer OLED pixels through low-cost solution processing. Our semi-IPNs with high solvent resistance, containing an inert polymer network and conjugated polymers, served in different layers of OLED devices. These semi-IPNs do not require complicated chemical modification to OLED materials; therefore, many state-of-the-arts conjugated polymers can be utilized to achieve red–green–blue and white OLEDs by tuning formulations. Our research findings on hybrid QD–oligomer nanocomposites lead to the successful design and synthesis of QD–polymer hybrid nanocomposites, which were used to build proof-of-the-concept devices showing good promise in providing excellent color purity and stability from QDs and solution processability from hybrid nanocomposites.

Recent extensive nanomechanical experiments have revealed that the instantaneous strength and plasticity of a material can be significantly affected by the size (of sample, microstructure, or stressed zone). One more important property to be added into the list of size-dependent properties is time-dependent plastic deformation referred to as creep; it has been reported that the creep becomes more active at the small scale. Analyzing the creep in the small scale can be valuable not only for solving scientific curiosity but also for obtaining practical engineering information about the lifetime or durability of advanced small-scale structures. For the purpose, nanoindentation creep experiments have been widely performed by far. Here we critically review the existing nanoindentation creep methods and the related issues and finally suggest possible novel ways to better estimate the small-scale creep properties.

Systematic studies have been carried out for investigating the mechanical properties of carbon nanotube (CNT)-reinforced phenolic resin. In this work, phenolic resin/CNT nanocomposite were processed using two different techniques: (i) three-roll calender device (TRC) and (ii) aqueous solution processes. In both techniques, up to 2 wt% CNT was used. In this study, it was observed that the values of tensile strength, Young’s modulus, shear stress, impact resistance and also that of the tribological properties increased directly proportional to carbon nanotube volume content. Halpin-Tsai, Voigt-Reuss and Cox equations were adopted to fit the experimental data of the tensile strength and Young’s modulus of the multiwalled carbon nanotube/phenolic resin composite. Morphological analysis was done utilizing scanning electron microscopy (SEM) and good dispersion of nanotubes within the phenolic resin matrix was revealed. Also according to the results presented in this work, SEM showed that TRC processing gave better dispersion than aqueous solution mixing. The observation that the values for mechanical properties increased more for TRC than aqueous mixed samples is consistent with this.

Raman scattering spectra and ferroelectric properties of epitaxial tetragonal Pb(Zr, Ti)O3 were investigated for polar axis-oriented thin films with various Zr/(Zr + Ti) ratios and by changing the ratios from 0 to 0.50 at different measurement temperatures. The chosen films in the thickness range of 1–2 μm present the advantage of showing small residual strain. The E (TO) modes were successfully isolated using cross-polarization configurations, while A1 (TO) and B1 modes were activated using parallel polarization configurations. Systematic changes in Raman peak positions were observed with changes in the Zr/(Zr + Ti) ratios at different measurement temperatures. It was found in both cases that the tetragonal distortion (c/a-1) and the value of square of spontaneous polarization (Ps2) linearly increased with increasing ω2[A1(1TO)], where a and c are the lattice parameters of a and c-axes. This indicates that monitoring A1(1TO) mode is efficient as a characterization method of ferroelectricity. It can also be used as a novel nondestructive process check or reliability assessment technique during fabrication of microelectromechanical systems (MEMS) using piezoelectric materials.

A metal-supported solid oxide fuel cell (SOFC) using Ce0.8Sm0.2O2 (Sm-doped ceria, SDC) buffer layer and La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte films showed a small degradation in the cell performance after a long-term operation because of La migration from the electrolyte to the buffer layer, resulted in a formation of a less conductive phase. Thus, various ceramic materials such as doped ceria and perovskite-related oxides were investigated for an effective buffer layer with respect to fabricating reliable metal-supported SOFCs using a LSGM electrolyte film. In particular, La-doped CeO2 (LDC) and Pr-doped LaCrO3 (LPCr) were investigated as buffer layer material since the materials showed chemical compatibility with the LSGM and anode materials. The cell using a LDC buffer layer showed a prior stability during the operation for 100 h at 973 K, while the power density of the cell was slightly low owing to the low electrical conductivity of LDC compared with that of SDC or LPCr. In contrast, the cell using a LPCr buffer layer revealed significantly low open circuit voltage (OCV) and power density, which were attributed to Pr decomposition in the LPCr caused by the reactivity with water vapor. However, the metal-supported cell with a multilayer electrolyte film including LSGM/LPCr/SDC layers showed an almost theoretical OCV and reasonably high power density with no degradation after a long-term operation for 100 h at 973 K, suggesting that the LPCr layer effectively prevented La migration and the SDC layer led to avoid the Pr decomposition. Thus, a LPCr is an effective buffer layer material for reliable metal-supported SOFCs using a LSGM electrolyte thin film.

In-situ tensile tests have been performed in a dual beam focused ion beam and scanning electron microscope on as-grown and prestrained single-crystal molybdenum-alloy (Mo-alloy) fibers. The fibers had approximately square cross sections with submicron edge lengths and gauge lengths in the range of 9–41 μm. In contrast to previously observed yield strengths near the theoretical strength of 10 GPa in compression tests of ∼1–3-μm long pillars made from similar Mo-alloy single crystals, a wide scatter of yield strengths between 1 and 10 GPa was observed in the as-grown fibers tested in tension. Deformation was dominated by inhomogeneous plastic events, sometimes including the formation of Lüders bands. In contrast, highly prestrained fibers exhibited stable plastic flow, significantly lower yield strengths of ∼1 GPa, and stress–strain behavior very similar to that in compression. A simple, statistical model incorporating the measured dislocation densities is developed to explain why the tension and compression results for the as-grown fibers are different.

Laser interference patterning (LIP) and the hereby induced microstructure modifications have been investigated in gold/yttria-stabilized zirconia nanocomposite films. Transmission electron microscopy was used to study the influence of the laser treatment on the structure and microstructure of the samples. The impact of LIP on the friction coefficient has been evidenced. The initial microstructure consisted of gold nanograins homogeneously distributed in the yttria-stabilized zirconia matrix. A noticeable growth and coalescence of gold nanograins occurred near the surface in specific regions. Simultaneously, a foamy morphology, mostly consisting of gold crystals, was formed at the surface and is responsible for a drastic diminution of the friction coefficient after patterning. Furthermore, the influence of the film topography on the friction behavior is analyzed using Abbott–Firestone curves. In contrast to thermal annealing, the laser treatment proposed here is a fast procedure to partially relocate gold at the film surface and provide a local solid lubrication.

Phase structures and electrical properties of lead-free piezoelectric (1−x)(Bi0.5Na0.5)TiO3–x(K0.5Na0.5)NbO3 (BNT–xKNN) ceramics with 0.08 ≤ x ≤ 0.19 were systematically investigated. Results showed that a phase transition from a tetragonal to a pseudocubic phase occurred in this system, as KNN content increases. The addition of KNN shifted both the depolarization temperature Td and rhombohedral–tetragonal phase transition temperature TR-T to lower temperatures and tended to enhance the relaxor behavior of the ceramics, which was well explained by the microdomain–macrodomain transition theory with calculating criterion K. At x = 0.10–0.11, Td reached room temperature (RT), which accordingly induced an enhancement of the unipolar strain that peaks at a value of 0.22% was obtained. Furthermore, as the compositions (x = 0.12–0.15) have Td below RT, samples exhibited high electrostrictive response with large electrostrictive coefficient Q33 (0.017–0.019 m4/C2) and good thermostability comparable with that of traditional Pb-based electrostrictors.

In this paper, ink-jet printing was used to deposit poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester blend as active layer and a comparison study of three printing methods [multiarray (MA), single layer array, and multilayer array] was performed. For organic photovoltaics (OPVs) fabricated using MA or multilayer arrays, the efficiency was less than 1% independent of printing parameters. When single layer print pattern was used, the device performance improved significantly and an efficiency of 1.29% was obtained, indicating that the thin films fabricated using a single layer are more suitable for OPVs than films obtained by overlapping of multiple layers. The influence of annealing parameters on electrical and optical thin film properties was also investigated. The study found that the optimum annealing condition for the printed OPVs is solvent annealing at 60 °C, yielding an efficiency of 1.99%.

Nanofibers (NFs) of Ga-doped ZnO (GZO) were prepared by electrospinning of polymer–salts solution. Sintering profiles reported in the literature led to loss of the fibrous structure. Hence, the morphology, thermal stability, and phase composition of green and sintered fibers were investigated as function of sintering conditions to elucidate this degradation process. Optimal results were obtained at 400 °C for 30 min. This low temperature sintering of GZO fibers has not been previously reported. The fibers were porous with a significant surface area, making it possible to test their sensitivity to environmental changes. In particular, the response of the GZO NFs to changes in humidity was demonstrated for the first time. The electrical and sensing properties of single NFs prepared at these conditions were studied using a field-effect transistor mode.

In this article, the sizes of the volumes sampled by nanoindentation tests for hardness and modulus measurements are studied using finite element simulations. The zones of influence for hardness and modulus in single-phase systems are determined by modeling a hemispherical particle in a matrix, with properties close to those of each other, and monitoring the deviation of the measured values from those of the particle. It is found that, for hardness testing of elastic-perfectly plastic materials, the intrinsic hardness of the particle is measured as long as the plastic region is still within the particle, i.e., the contact radius is one half or less of the particle radius. Thus, in a hardness test of a single-phase material, all of the plastically deforming material, and only the plastically deforming material, contributes to the hardness measured. In contrast, the zone influencing the modulus is not restricted to a specific volume near the indenter. The modulus measured from the elastic response at the indentation point is dependent upon the entire specimen. A relationship is developed to describe the observed behavior of the measured modulus, that holds true for both sink-in and pile-up material behavior and for different indenter cone angles.