bluuebunny/crossref_metadata_2025_split huggingface dataset:
Novel Electromechanical Phenomena at the Nanoscale: Phenomenological Theory and Atomistic Modeling
Authors: Tagantsev Alexander K. , Meunier Vincent , Sharma Pradeep 
Published: February 2011

AbstractIn the past two decades, the fact that “small is different” has been established for a wide variety of phenomena, including electrical, optical, magnetic, and mechanical behavior of materials. However, one largely untapped but potentially very important area of nanoscience involves the interplay of electricity and mechanics at the nanoscale. In this article, predicated on both phenomenological approaches and atomistic calculations, we summarize the state-of-the-art in understanding electromechanical coupling at the nanoscale. First, we address flexoelectricity—the coupling of strain gradient to polarization. Flexoelectricity exists in both piezoelectric and nonpiezoelectric dielectrics. As a high-order spatial-dispersion effect, the flexoelectricity becomes more and more important with the reduction of the spatial scale of the problem. Exploitation of this phenomenon and the associated nanoscale size effects can lead to tantalizing applications, such as “piezoelectric nanocomposites without using piezoelectric materials.” The second issue concerns electromechanical effects at the dielectric/metal interface. An interface in solids typically exhibits a lower symmetry compared to that of the associated adhering materials. This symmetry reduction can drastically affect the electromechanical and dielectric behavior of the material at the nanoscale.
Probing Diffusion Kinetics with Secondary Ion Mass Spectrometry
Authors: De Souza Roger A. , Martin Manfred 
Published: February 2011

AbstractSecondary ion mass spectrometry (SIMS) is a powerful analytical technique for determining elemental and isotopic distributions in solids. One of its main attractions to researchers in the field of solid-state ionics is its ability to distinguish between isotopes of the same chemical element as a function of position in a solid. With enriched stable isotopes as diffusion sources, this allows self-diffusion kinetics in solids to be studied. In this article, taking oxygen isotope diffusion in oxides as our main example, we present the standard experimental method, and, subsequently, we discuss several promising developments, in particular the opportunities offered by thin-film geometries, and the investigation of inhomogeneous systems, including possible fast diffusion along grain boundaries and making space-charge layers at interfaces “visible.” These examples demonstrate that SIMS is capable of probing mass transport processes over various length scales, ranging from some nanometers to hundreds of micrometers.
Current Status of GaN-Based Solid-State Lighting
Authors: Nakamura Shuji 
Published: February 2011

AbstractThe continuous improvement in luminous efficacy of “white” light-emitting-diode (LED) sources offers the potential of considerable energy savings in general lighting applications. Recent experiments at UCSB have demonstrated 117 lumens per watt (lm/W) in white LEDs, with further improvements expected in the near future. Considerable progress has also been achieved using nonpolar GaN, such as a-plane {1120} and m-plane {1100} GaN, or semipolar GaN substrates. Such devices avoid the deleterious effects of charge separation due to spontaneous and piezoelectric polarization inherent in most c-axis-oriented devices.
Detached Bridgman Growth—A Standard Crystal Growth Method with a New Twist
Authors: Cröll Arne , Volz Martin P. 
Published: February 2011

AbstractBridgman or vertical gradient freeze (VGF) crystal growth processes have several advantages compared to other melt growth methods, especially the possibility to achieve a low level of thermal stress and low dislocation densities in the grown crystals. However, crystals grown in contact with a crucible usually suffer from mechanical stress during cooling, reducing the structural quality. The “detached” or “dewetted” Bridgman growth avoids this problem and has recently been investigated in more detail as a promising tool to improve crystal quality. Detached growth, where the crystal is separated from the crucible wall by a gap of 10–100 μm, was found originally in some microgravity experiments going back to 1975. Considerable improvements of crystal quality were reported for those cases; however, the reasons for the detachment were not fully understood. In the last 10–15 years, theoretical investigations as well as new experiments have shown beyond a doubt that detached growth can, in principle, be achieved in Earth's gravity with the same advantages that were demonstrated in the crystals grown under microgravity. It could be shown that the ability to achieve detachment depends on a complex interplay of the wetting of the melt with the crucible and the crystal as well as the pressure balance in the system, including the hydrostatic pressure, the gas pressure above the melt, and the pressure below the melt. It turns out that for stable detachment, only, specific combinations of meniscus shape, gap size, wetting angle, growth angle, and pressures work. The conditions that lead to detachment are thus highly specific for a given system.
Double Tungstate and Molybdate Crystals for Laser and Nonlinear Optical Applications
Authors: Zharikov E.V. , Zaldo C. , Díaz F. 
Published: February 2011

AbstractThe renaissance of double tungstate (DW) and double molybdate (DMo) crystals is due to the vigorous recent progress in growth and characterization of these materials in the form of both bulk crystals with centimeter dimensions and epitaxial layers. “Double” refers to compounds having two ions with the generic composition MT(XO4)2 (where M = Li+, Na+, or K+; T = Ln3+, Y3+, or Bi3+; and X = W6+ or Mo6+). These compounds crystallize primarily in two phases belonging to the monoclinic (C2/c) and tetragonal (l4) structures. The specific properties of both types of crystals and the major technological challenges associated with the Czochralski and top-seeded solution growth of rare-earth doped crystals are presented. The performance of specific crystals is discussed in terms of their applications as tunable, thin-disk, and ultrashort (<100 fs) pulsed lasers or as nonlinear elements for laser Raman shifting.
High-Performance Piezoelectric Single Crystals of Complex Perovskite Solid Solutions
Authors: Ye Zuo-Guang 
Published: February 2011

AbstractRelaxor-based single crystals of complex perovskite solid solutions, Pb(Mg1/3Nb2/3)O3–PbTiO3[PMN–PT] and Pb(Zn1/3Nb2/3)O3–PbTiO3[PZN–PT], exhibit extraordinary piezoelectric performance, with extremely high piezoelectric coefficients, very large electromechanical coupling factors, and exceptionally high strain levels. These materials outperform the currently used Pb(Zr1–xTix)O3[PZT] ceramics, making them the materials of choice for the next generation of electromechanical transducers for a broad range of advanced applications. In this article, recent major advances in the development of piezocrystals are reviewed in terms of crystal growth, piezoelectric properties, crystal chemistry, domain structure, and device applications.
Nonpolar and Semipolar Group III Nitride-Based Materials
Authors: Speck J. S. , Chichibu S. F. 
Published: February 2011

AbstractGaN and its alloys with InN and AlN are materials systems that have enabled the revolution in solid-state lighting and high-power/high-frequency electronics. GaN-based materials naturally form in a hexagonal wurtzite structure and are naturally grown in a (0001) c-axis orientation. Because the wurtzite structure is polar, GaN-based heterostructures have large internal electric fields due to discontinuities in spontaneous and piezoelectric polarization. For optoelectronic devices, such as light-emitting diodes and laser diodes, the internal electric field is generally deleterious as it causes a spatial separation of electron and hole wave functions in the quantum wells, which, in turn, likely decreases efficiency. Growth of GaN-based heterostructures in alternative orientations, which have reduced (semipolar orientations) or no polarization (nonpolar) in the growth direction, has been a major area of research in recent years. This issue highlights many of the key developments in nonpolar and semipolar nitride materials and devices.
Development of Nonpolar and Semipolar InGaN/GaN Visible Light-Emitting Diodes
Authors: Feezell Daniel F. , Schmidt Mathew C. , DenBaars Steven P. , Nakamura Shuji 
Published: February 2011

AbstractThis article reviews the development of nonpolar and semipolar InGaN/GaN light-emitting diodes (LEDs), emphasizing structures on freestanding bulk GaN. A brief history of LED development on each orientation is provided, followed by a discussion of the most relevant and recent results. The context is related to several current LED issues, such as the realization of high-efficiency white solid-state lighting, potential solutions to the “green gap,” and applications for polarized emitters. The section on nonpolar LEDs highlights high-power violet and blue emitters and considers the effects of indium incorporation and substrate miscut. The section on semipolar GaN reviews the development of LEDs in the violet, blue, green, and yellow regions and highlights the potential of InGaN/GaN LEDs as an alternative technology to AlInGaP for yellow emitters. A brief review of polarization anisotropy also is included for each orientation. Finally, a two source white light system utilizing a nonpolar blue LED and a semipolar yellow LED is presented.
Semipolar III Nitride Semiconductors: Crystal Growth, Device Fabrication, and Optical Anisotropy
Authors: Funato Mitsuru , Kawakami Yoichi 
Published: February 2011

AbstractSemipolar InGaN/GaN quantum wells (QWs) are quite attractive as visible light emitters. One of the reasons is that a better optical transition probability is expected because of weaker internal electric fields, compared to conventional polar QWs. In addition, in-plane optical polarization anisotropy, which is absent in conventional QWs, is another relevant property because it affects device design and also may provide a means for novel applications. We revealed that the in-plane optical anisotropy in semipolar QWs switched from one direction perpendicular to the [0001] crystal axis to the perpendicular direction as the In composition increases. This is a property unique to semipolar QWs and enables, for example, to make cavity mirrors of laser diodes by cleavage. In this article, we describe the concept of semipolar planes and fabrication of high-quality epitaxial films for semipolar QWs. Furthermore, we discuss device fabrication and optical polarization anisotropy.
Nonpolar-Oriented GaN Films for Polarization-Sensitive and Narrow-Band Photodetectors
Authors: Grahn Holger T. 
Published: February 2011

AbstractThis article reviews the optical polarization properties of unstrained and strained GaN films with a nonpolar orientation. In unstrained a -plane GaN films, the A exciton becomes completely linearly polarized perpendicular to the c-axis, whereas the B and C excitons are only partially polarized. In m -plane or a -plane GaN films under anisotropic in-plane compressive strain, all three interband transitions between the three uppermost valence bands and the conduction band can become linearly polarized for sufficiently large strain values. The complete linear polarization can be directly observed in reflection, transmission, or photoreflectance by a polarization-dependent energy gap. This complete linear polarization can be used to realize polarization-sensitive photodetectors in the ultraviolet spectral range, which do not need a polarization filter in front of the photodetector. By combining a polarization filter and photodetector or two photodetectors from the same material with their c-axes oriented perpendicular to each other, a narrowband photodetection configuration can be achieved in the ultraviolet spectral range with a band width below 8 nm. Since both realizations are also polarization sensitive, a configuration with four photodetectors is necessary to achieve narrow-band sensitivity regardless of the polarization state of the incident light. At the same time, the configuration with four photodetectors allows for the determination of the absolute angle of polarization.
Engineering Materials for Regenerative Medicine
Authors: Prasad Shastri V. , Lendlein Andreas 
Published: February 2011

AbstractThe mammalian physiology represents a level of sophistication in materials design, assembly, and function that has yet to be replicated by the modern tools of materials science. Although, the building blocks of our body (pluripotent stem and progenitor cells) are still available within our tissues, the absence of the biological and structural cues that drove the development process early on, in an adult, limits our ability to regenerate after an injury. The goal of regenerative medicine is therefore to recapitulate embryonic events within an artificially defined materials space (i.e., the niche) so that the repair processes can be triggered using our reservoir of stem cells. This engineering of the regenerative niche will require an interdisciplinary exercise involving materials scientists, biologists, and clinicians. The success of this exercise will hinge on our ability to develop materials that incorporate principles of wound healing, lessons from immunology and developmental biology, and knowledge of cellular mechanics and molecular biology such that they can mimic the cellular environment, instruct cells to make fate decisions, and direct the hierarchical organization of tissues. This article presents the current state of this challenge in the implementation of regenerative therapies.
Designing and Engineering Stem Cell Niches
Authors: Teixeira Ana I. , Hermanson Ola , Werner Carsten 
Published: February 2011

AbstractStem cells have received a lot of attention due to great promises in medical treatment, for example, by replacing lost and sick cells and re-constituting cell populations. There are several classes of stem cells, including embryonic, fetal, and adult tissue specific. More recently, the generation of so-called induced pluripotent stem (iPS) cells from differentiated cells has been established. Common criteria for all types of stem cells include their ability to self-renew and to retain their ability to differentiate in response to specific cues. These characteristics, as well as the instructive steering of the cells into differentiation, are largely dependent on the microenvironment surrounding the cells. Such “stem cell friendly” microenvironments, provided by structural and biochemical components, are often referred to as niches. Biomaterials offer attractive solutions to engineer functional stem cell niches and to steer stem cell state and fatein vitroas well asin vivo. Among materials used so far, promising results have been achieved with low-toxicity and biodegradable polymers, such as polyglycolic acid and related materials, as well as other polymers used as structural “scaffolds” for engineering of extracellular matrix components. To improve the efficiency of stem cell control and the design of the biomaterials, interfaces among stem cell research, developmental biology, regenerative medicine, chemical engineering, and materials research are rapidly developing. Here we provide an introduction to stem cell biology and principles of niche engineering and give an overview of recent advancements in stem cell niche engineering from two stem cell systems—blood and brain.
Wound Healing Versus Regeneration: Role of the Tissue Environment in Regenerative Medicine
Authors: Atala Anthony , Irvine Darrell J. , Moses Marsha , Shaunak Sunil 
Published: February 2011

AbstractOne of the major challenges in the field of regenerative medicine is how to optimize tissue regeneration in the body by therapeutically manipulating its natural ability to form scar at the time of injury or disease. It is often the balance between tissue regeneration, a process that is activated at the onset of disease, and scar formation, which develops as a result of the disease process that determines the ability of the tissue or organ to be functional. Using biomaterials as scaffolds often can provide a “bridge” for normal tissue edges to regenerate over small distances, usually up to 1 cm. Larger tissue defect gaps typically require both scaffolds and cells for normal tissue regeneration to occur without scar formation. Various strategies can help to modulate the scar response and can potentially enhance tissue regeneration. Understanding the mechanistic basis of such multivariate interactions as the scar microenvironment, the immune system, extracellular matrix, and inflammatory cytokines may enable the design of tissue engineering and wound healing strategies that directly modulate the healing response in a manner favorable to regeneration.
Progress in Carbon Nanotube Electronics and Photonics
Authors: Avouris Phaedon , Martel Richard 
Published: February 2011

AbstractIn electronics and photonics, intrinsic properties of semiconducting materials play a dominant role in achieving high-performance devices and circuits. In this respect, carbon nanotubes are prime candidates because of their exceptionally high carrier mobility, low capacitance, and strong optical response (direct bandgap). Although these properties compare very favorably with those of crystalline silicon, several issues related to their synthesis, processing, and assembly have challenged efforts for making electronic and photonic devices. Tremendous progress, nevertheless, has been achieved over the years, and much has been learned from novel photonic devices and electronic circuits. We review some of the developments in nanotube transistor performance optimization, ac operation, nanotube circuits, self-assembly, thin-film devices, and nanotube optical devices such as light emitters and detectors. We also examine the issues and opportunities that still exist.
New Nanofabrication Strategies: Inspired by Biomineralization
Authors: Aizenberg Joanna 
Published: February 2011

AbstractNature produces a wide variety of exquisite mineralized tissues, fulfilling diverse functions. Organisms exercise a level of molecular control over the detailed nano- and microstructure of the biomaterials that is unparalleled in today's technology. Our understanding of the underlying design principles of biomaterials provides ample opportunities for developing new approaches to materials fabrication at the nanometer and micrometer scale. It is clear that valuable materials lessons can be taught by any organism. I will exemplify this point by describing new nano- and microfabrication strategies and devices that have been inspired by the studies of biomineralization in echinoderms. The topics will include self-assembly, control of crystallization, synthesis of adaptive optical structures, hybrid materials, and novel actuation systems at the nanoscale level.
MEMS for In Situ Testing—Handling, Actuation, Loading, and Displacement Measurements
Authors: Haque M.A. , Espinosa H.D. , Lee H.J. 
Published: February 2011

AbstractMechanical testing of micro- and nanoscale materials is challenging due to the intricate nature of specimen preparation and handling and the required load and displacement resolution. In addition, in Situ testing requires the entire experimental setup to be drastically miniaturized, because conventional high-resolution microscopes or analytical tools usually have very small chambers. These challenges are increasingly being addressed using microelectromechanical systems (MEMS)-based sensors and actuators. Because of their very small size, MEMS-based experimental setups are the natural choice for materials characterization under virtually all forms of in Situ electron, optical, and probe microscopy. The unique advantage of such in Situ studies is the simultaneous acquisition of qualitative (up to near atomic visualization of microstructures and deformation mechanisms) and quantitative (load, displacement, flaw size) information of fundamental materials behavior. In this article, we provide a state-of-the-art overview of design and fabrication of MEMS-based devices for nanomechanical testing. We also provide a few case studies on thin films, nanowires, and nanotubes, as well as adhesion-friction testing with a focus on in Situ microscopy. We conclude that MEMS devices offer superior choices in handling, actuation, and force and displacement resolutions. Particularly, their tight tolerances and small footprints are difficult to match by off-the-shelf techniques.
Ancient Egyptian Pigments: The Examination of Some Coffins from the San Diego Museum of Man
Authors: Scott David A. 
Published: February 2011

AbstractEgyptian pigments include the most successful synthetic blue pigment made, which has been in use for more than 2500 years. The nature of some Egyptian pigments, such as malachite, azurite, green earth, and calcite are briefly reviewed. The principal artworks examined here are a group of several 26th Dynasty coffins in the collection of the San Diego Museum of Man. This article examines the nature of the wooden substrate, binder, pigments, and alteration products. The conservation needs of the collection are of special importance. The pigments used in the coffins are malachite, orpiment, carbon black, calcite, and red ocher. Gum Arabic was used as a binder. Wood anatomical studies identified fig as the wood. This article also covers the degradation product, oxammite, resulting from the microbiological decay of textile threads on a Hawk Mummy.
Electronic Structure and Dynamics at Organic Donor/Acceptor Interfaces
Authors: Zhu Xiaoyang , Kahn Antoine 
Published: February 2011

AbstractWe present our understanding of the electronic energy landscape and dynamics of charge separation at organic donor/acceptor interfaces. The organic/organic interface serves as a valuable point of reference and plays an important role in emerging electronic and optoelectronic applications, particularly organic photovoltaics (OPVs). The key issue on electronic structure at organic donor/acceptor interfaces is the difference in the lowest unoccupied molecular orbitals or that in the highest occupied molecular orbitals. This difference represents an energy gain needed to overcome the exciton binding energy in a charge-separation process in OPV. A sufficiently large energy gain favors the formation of charge transfer (CT) states that are energetically close to the charge-separation state. At an organic donor/acceptor interface in an OPV device, these high-energy CT states, also called hot CT excitons, are necessary intermediates in a successful charge-separation process.
Time-Resolved X-Ray Scattering from Coherent Excitations in Solids
Authors: Trigo Mariano , Reis David 
Published: February 2011

AbstractRecent advances in pulsed x-ray sources have opened up new opportunities to study the dynamics of matter directly in the time domain with picosecond to femtosecond resolution. In this article, we present recent results from a variety of ultrafast sources on time-resolved x-ray scattering from elementary excitations in periodic solids. A few representative examples are given on folded acoustic phonons, coherent optical phonons, squeezed phonons, and polaritons excited by femtosecond lasers. Next-generation light sources, such as the x-ray-free electron laser, will lead to improvements in coherence, flux, and pulse duration. These experiments demonstrate potential opportunities for studying matter far from equilibrium on the fastest time scales and shortest distances that will be available in the coming years.