bluuebunny/crossref_metadata_2025_split huggingface dataset:
Development of Bulk GaN Crystals and Nonpolar/Semipolar Substrates by HVPE
Authors: Fujito Kenji , Kubo Shuichi , Fujimura Isao 
Published: February 2011

AbstractThe remarkable progress in nonpolar and semipolar devices based on gallium nitride (GaN) in recent years has been driven by not only advancements in the epitaxial growth technique but also improvements in the quality of bulk nonpolar and semipolar GaN substrates. At present, high-quality nonpolar/semipolar substrates are only made by slicing thick bulk GaN crystals grown by hydride vapor-phase epitaxy (HVPE). Although HVPE is currently the most successful method for obtaining high-quality bulk GaN crystals, it is still difficult to obtain uniform crystals with large diameters and thicknesses. The size of the nonpolar/semipolar substrates has been limited by the growth thickness along the c-axis of bulk GaN crystals. Here we review the growth of bulk GaN crystals by HVPE to achieve high-quality and large-sized nonpolar and semipolar substrates.
GaN-Based Light-Emitting Diodes on Selectively Grown Semipolar Crystal Facets
Authors: Scholz Ferdinand , Wunderer Thomas , Neubert Barbara , Feneberg Martin , Thonke Klaus 
Published: February 2011

AbstractIn this article, we briefly review a particular approach to fabricate light-emitting diode (LED) structures on the semipolar side facets of triangular GaN stripes grown by selective area epitaxy. This approach enables a significant reduction of the internal piezoelectric fields in the LED's active area, while still maintaining the well-established c-direction as the main epitaxial growth direction for GaN-based devices on large area substrates. For the latter, these internal fields are responsible for the lower efficiency of GaN-based LEDs in the longer (green) wavelength range. The reduced internal fields of such semipolar LEDs can be directly determined by photoluminescence (PL) investigations on pre-biased LED structures and further confirmed by time-resolved PL studies. The epitaxial growth behavior is strongly facet-dependent, leading to different surface flatnesses on different semipolar facets formed by this procedure and different – indium incorporation efficiencies. An increased indium uptake on semipolar {1101} facets as compared to conventional c-plane layers can help to shift the LED emission to longer wavelengths near 500 nm, despite the significantly reduced field-dependent Stark shift, which helps to reach the green wavelength range in polar LEDs.
Polytype Replication in Heteroepitaxial Growth of Nonpolar AlN on SiC
Authors: Suda Jun , Horita Masahiro 
Published: February 2011

AbstractZinc-blende and wurtzite are the most common structures for binary compound semiconductors. Aluminum nitrides (AIN), one of the most promising materials for deep ultraviolet light-emitting diodes, have a wurtzite structure as an equilibrium phase due to its strong ionicity. Silicon carbide (SiC) is widely used as a substrate for heteroepitaxial growth of AlN, since SiC has a hexagonal structure whose lattice constant is close to that of AIN. Different from other compound semiconductors, SiC can have many different crystalline structures, called polytypism. Among various polytypes of SiC, large-size high-quality wafers are available for 4H and 6H structures. When AlN is grown on a 4H- or 6H-SiC basal plane (0001), normal, wurtzite-structured AIN is obtained. On the other hand, when AlN is grown on a nonbasal SiC plane, such as nonpolar (1100) or (1120), what is expected? If ideal growth is realized, AIN will follow the crystalline structure of SiC (i.e., the polytype of the SiC substrate will be replicated to the AIN epitaxial layer). Nonpolar nitride growth has attracted much attention to eliminate undesirable internal electric fields due to the polarization in nitride heterostructures. In addition, nonpolar nitride growth on SiC also allows an opportunity to obtain nitrides with new crystalline structures. In this article, the polytype replication growth of AIN on nonpolar SiC substrates is reviewed.
Degradable, Multifunctional Cardiovascular Implants: Challenges and Hurdles
Authors: Jung Friedrich , Wischke Christian , Lendlein Andreas 
Published: February 2011

AbstractPolymer-coated and polymer-based cardiovascular implants are essential constituents of modern medicine and will proceed to gain importance with the demographic changes toward a society of increasing age-related morbidity. Based on the experiences with implants such as coronary or peripheral stents, which are presently widely used in clinical medicine, several properties of the next generation of cardiovascular implants have been envisioned that could be fulfilled by multifunctional polymers. The challenge is to combine tailored mechanical properties and rapid endothelialization with controlled drug release in order to modulate environmental cells and tissue. Additionally, degradability and sensitivity to external stimuli are useful in several applications. A critical function in terms of clinical complications is the hemocompatibility. The design of devices with improved hemocompatibility requires advancedin vitrotest setups as discussed in depth in this article. Finally, degradable, multifunctional shape-memory polymers are introduced as a promising family of functional polymers that fulfill several requirements of modern implants and are of high relevance for cardiovascular application (e.g., stent technology). Such multifunctional polymers are a technology platform for future cardiovascular implants enabling induced autoregeneration in regenerative therapies.
Ultrananocrystalline and Nanocrystalline Diamond Thin Films for MEMS/NEMS Applications
Authors: Sumant Anirudha V. , Auciello Orlando , Carpick Robert W. , Srinivasan Sudarsan , Butler James E. 
Published: February 2011

AbstractThere has been a tireless quest by the designers of micro- and nanoelectro mechanical systems (MEMS/NEMS) to find a suitable material alternative to conventional silicon. This is needed to develop robust, reliable, and long-endurance MEMS/NEMS with capabilities for working under demanding conditions, including harsh environments, high stresses, or with contacting and sliding surfaces. Diamond is one of the most promising candidates for this because of its superior physical, chemical, and tribomechanical properties. Ultrananocrystalline diamond (UNCD) and nanocrystalline diamond (NCD) thin films, the two most studied forms of diamond films in the last decade, have distinct growth processes and nanostructures but complementary properties. This article reviews the fundamental and applied science performed to understand key aspects of UNCD and NCD films, including the nucleation and growth, tribomechanical properties, electronic properties, and applied studies on integration with piezoelectric materials and CMOS technology. Several emerging diamond-based MEMS/NEMS applications, including high-frequency resonators, radio frequency MEMS and photonic switches, and the first commercial diamond MEMS product—monolithic diamond atomic force microscopy probes—are discussed.
Graphene: Materially Better Carbon
Authors: Fuhrer Michael S. , Lau Chun Ning , MacDonald Allan H. 
Published: February 2011

AbstractGraphene, a single atom–thick plane of carbon atoms arranged in a honeycomb lattice, has captivated the attention of physicists, materials scientists, and engineers alike over the five years following its experimental isolation. Graphene is a fundamentally new type of electronic material whose electrons are strictly confined to a two-dimensional plane and exhibit properties akin to those of ultrarelativistic particles. Graphene's two-dimensional form suggests compatibility with conventional wafer processing technology. Extraordinary physical properties, including exceedingly high charge carrier mobility, current-carrying capacity, mechanical strength, and thermal conductivity, make it an enticing candidate for new electronic technologies both within and beyond complementary metal oxide semiconductors (CMOS). Immediate graphene applications include high-speed analog electronics and highly conductive, flexible, transparent thin films for displays and optoelectronics. Currently, much graphene research is focused on generating and tuning a bandgap and on novel device structures that exploit graphene's extraordinary electrical, optical, and mechanical properties.
Epitaxial Graphenes on Silicon Carbide
Authors: First Phillip N. , de Heer Walt A. , Seyller Thomas , Berger Claire , Stroscio Joseph A. , Moon Jeong-Sun 
Published: February 2011

AbstractThis article reviews the materials science of graphene grown epitaxially on the hexagonal basal planes of SiC crystals and progress toward the deterministic manufacture of graphene devices. We show that the growth of epitaxial graphene on Si-terminated SiC(0001) differs from growth on the C-terminated SiC(0001) surface, resulting in, respectively, strong and weak coupling to the substrate and to successive graphene layers. Monolayer epitaxial graphene on either surface displays the expected electronic structure and transport characteristics of graphene, but the non-graphitic stacking of multilayer graphene on SiC(0001) determines an electronic structure much different from that of graphitic multilayers on SiC(0001). This materials system is rich in subtleties, and graphene grown on the two polar faces of SiC differs in important ways, but all of the salient features of ideal graphene are found in these epitaxial graphenes, and wafer-scale fabrication of multi-GHz devices already has been achieved.
Recent Developments in Carbon Nanotube Sorting and Selective Growth
Authors: Liu Jie , Hersam Mark C. 
Published: February 2011

AbstractDue to their high carrier mobilities, electromigration resistance, and tailorable optical properties, carbon nanotubes are promising candidates for high-performance electronic and optoelectronic applications. However, traditional synthetic methods have lacked control over the structure and properties of carbon nanotubes. This polydispersity problem has confounded efforts to take carbon nanotubes from the research laboratory to the marketplace, especially for electronic and optoelectronic applications, where reliable and reproducible performance is paramount. In recent years, the research community has devoted significant effort to this issue, leading to substantial advances in the preparation of monodisperse carbon nanotube materials. This article highlights the most recent and promising developments from two perspectives: post-synthetic sorting and selective growth of carbon nanotubes of predetermined physical and electronic structure. These complementary approaches have yielded improved uniformity in carbon nanotube materials, resulting in impressive advances in carbon nanotube electronic and optoelectronic technology.
Quantitative In Situ Mechanical Testing in Electron Microscopes
Authors: Legros M. , Gianola D.S. , Motz C. 
Published: February 2011

AbstractThis article is devoted to recent progress in the area of in situ electron microscopy (scanning and transmission) and will focus on quantitative aspects of these techniques as applied to the deformation of materials. Selected recent experiments are chosen to illustrate how these techniques have benefited from improvements ranging from sample preparation to digital image acquisition. Known for its ability to capture the underlying phenomena of plastic deformation as they occur, in situ electron microscopy has evolved to a level where fully instrumented micro- and nanomechanical tests can be performed simultaneously.
In Situ Scanning Probe Microscopy Nanomechanical Testing
Authors: Li Xiaodong , Chasiotis Ioannis , Kitamura Takayuki 
Published: February 2011

AbstractScanning probe microscopy (SPM) has undergone rapid advancements since its invention almost three decades ago. Applications have been extended from topographical imaging to the measurement of magnetic fields, frictional forces, electric potentials, capacitance, current flow, piezoelectric response and temperature (to name a few) of inorganic and organic materials, as well as biological entities. Here, we limit our focus to mechanical characterization by taking advantage of the unique imaging and force/displacement sensing capabilities of SPM. This article presents state-of-the-art in situ SPM nanomechanical testing methods spanning (1) probing the mechanical properties of individual one-dimensional nanostructures; (2) mapping local, nanoscale strain fields, fracture, and wear damage of nanostructured heterogeneous materials; and (3) measuring the interfacial strength of nanostructures. The article highlights several novel SPM nanomechanical testing methods, which are expected to lead to further advancements in nanoscale mechanical testing and instrumentation toward the exploration and fundamental understanding of mechanical property size effects in nanomaterials.
In SituElastic Strain Measurements—Diffraction and Spectroscopy
Authors: Spolenak R. , Ludwig W. , Buffiere J.Y. , Michler J. 
Published: February 2011

AbstractUnderstanding the mechanical properties of materials is crucial for their reliable application as bulk materials as well as in a miniaturized form. The deformation of materials is usually non-uniform and, hence, needs to be characterized on a local level. The following article focuses on thein-Situdetermination of mechanical stresses in crystalline materials during deformation. This can be achieved by both diffraction as well as spectroscopical methods, where the elastic strain is the parameter measured, which is subsequently converted into stresses by the application of Hooke's law. Asin Situmeasurements require rapid data acquisition in conjunction with reasonable penetration depths, we will focus on x-rays. However, the different techniques described can be applied to any other diffraction probe as well. The description of diffraction techniques, which span the range from averaging techniques to 2D and 3D strain mapping, is complemented by a section on Raman spectroscopy as an alternative method for stress determination for non-metallic materials. Local stresses also can be correlated to local defect densities.
Organic/Inorganic Hybrids for Solar Energy Generation
Authors: Hsu Julia W.P. , Lloyd Matthew T. 
Published: February 2011

AbstractOrganic and hybrid (organic/inorganic) solar cells are an attractive alternative to traditional silicon-based photovoltaics due to low-temperature, solution-based processing and the potential for rapid, easily scalable manufacturing. Using oxide semiconductors, instead of fullerenes, as the electron acceptor and transporter in hybrid solar cells has the added advantages of better environmental stability, higher electron mobility, and the ability to engineer interfacial band offsets and hence the photovoltage. Further improvements to this structure can be made by using metal oxide nanostructures to increase heterojunction areas, similar to bulk heterojunction organic photovoltaics. However, compared to all-organic solar cells, these hybrid devices produce far lower photocurrent, making improvement of the photocurrent the highest priority. This points to a less than optimized polymer/metal oxide interface for carrier separation. In this article, we summarize recent work on examining the polymer structure, electron transfer, and recombination at the polythiophene-ZnO interface in hybrid solar cells. Additionally, the impact of chemical modification at the donor-acceptor interface on the device characteristics is reviewed.
Cooperative Electronic and Magnetic Properties of Self-Assembled Monolayers
Authors: Naaman Ron , Vager Zeev 
Published: February 2011

AbstractSelf-assembled monolayers (SAMs) of organic dipolar molecules have new electronic and magnetic properties that result from their organization, despite the relatively weak interaction among the molecules themselves. Here we review the origin of this cooperative effect and summarize work performed on spin selective electron transmission through SAMs. The spin selectivity observed, in some cases, is consistent with a model in which a SAM containing chiral dipolar molecules behaves like a magnetic layer. The magnetic properties result in the SAMs behaving as spin filters, even without applying an external magnetic field to the layer.
Advances in Scattering Probes for Materials
Authors: Evans Paul G. , Billinge Simon J.L. 
Published: February 2011

AbstractRecent advances in x-ray and neutron sources, optics, and scattering methods are heralding a new age in the study of the structure and properties of complex materials. By providing unprecedented resolution in real space, reciprocal space, and time, new techniques address materials characterization challenges beyond anything possible before, at length scales ranging from the atomic scale to the mesoscale, and at times as short as femtoseconds. The high degree of coherence of third-generation synchrotron sources permits a new level of precision in the quantitative description and analysis of diffraction and scattering and allows beams with sizes probing individual nanostructures to be produced. As a result, in situx-ray and neutron analysis techniques now provide insight into the structure of nanomaterials and yield a more precise set of metrics describing the nanometer-scale structure of materials. Time resolution and in situ studies allow application of these techniques to materials driven far from equilibrium and to the challenging environment associated with materials processing.
X-Ray Probes for In Situ Studies of Interfaces
Authors: Fong D.D. , Lucas C.A. , Richard M.-I. , Toney M.F. 
Published: February 2011

AbstractSurfaces and buried interfaces play critical roles in many environmental, catalytic, and tribological processes and in a wide variety of applications, including microelectronics and optoelectronics. Interfacial structure and composition are closely coupled to their surroundings, and probes that yield information about materials in situ are essential to obtain a thorough understanding of interface functions and properties. The highly brilliant, hard x-rays available from synchrotron light sources can easily penetrate through gas or liquid environments, or even solid thin-film overlayers, and enable real-time monitoring of the evolving chemistry and structure of the interface with atomic-scale resolution. Here we review the in situ study of interfaces by a variety of synchrotron x-ray scattering techniques and provide several examples of their application in electrochemical processes and thin-film island growth. We also discuss recent advances in analytical techniques and x-ray optics that are facilitating the in situ study of surfaces and buried interfaces with direct imaging.
Advances in Structural Studies of Materials Using Scattering Probes
Authors: Huq Ashfia , Welberry Richard , Bozin Emil 
Published: February 2011

AbstractX-ray and neutron diffraction have been two key techniques for structural characterization of materials since their inception. If single crystals of the materials of interest cannot be synthesized, one has to resort to powder diffraction. This results in the loss of three-dimensional orientation information of the crystal, and one has to contend with the one-dimensional information that is inherent to powder diffraction, making it harder to analyze the data. The structural study of contemporary materials and their remarkable properties is a challenging problem, particularly when properties of interest result from interplay of multiple degrees of freedom. Very often these are associated with structural defects or relate to different length scales in a material. The signature of the defect-related phenomenon is visible as diffuse scattering in the diffraction pattern, and the signals associated with diffuse scattering are orders of magnitude smaller than Bragg scattering. Given these limitations, it is crucial to have high-resolution and high-intensity data along with the ability to carry out theoretical interpretation that goes beyond periodic lattice formalism of crystallography. Great advances have been achieved due to the advent of synchrotron and neutron sources, along with the availability of high-speed computational algorithms allowing materials scientists to work with a very small amount of sample (both single crystal and powder) and analyze vast amounts of data to unravel detailed structural descriptions that were not previously possible. This article presents some of these great advances in using scattering probes for materials characterization.
The Polyvalent Gold Nanoparticle Conjugate—Materials Synthesis, Biodiagnostics, and Intracellular Gene Regulation
Authors: Mirkin Chad A. 
Published: February 2011

AbstractAdvances in nanoscale directed assembly strategies have enabled researchers to analogize atomic assembly via chemical reactions and nanoparticle assembly, creating a new nanoscale “periodic table.” We are just beginning to realize the nanoparticle equivalents of molecules and extended materials and are currently developing the ground rules for creating programmable nanometer-scale coordination environments. The ability to create a diverse set of nanoscale architectures from one class of nanoparticle building blocks would allow for the synthesis of designer materials, wherein the physical properties of a material could be predicted and controlled a priori. Our group has taken the first steps toward this goal and developed a means of creating tailorable assembly environments using DNA-nanoparticle conjugates. These nanobioconjugates combine the discrete plasmon resonances of gold nanoparticles with the synthetically controllable and highly selective recognition properties of DNA. Herein, we elucidate the beneficial properties of these materials in diagnostic, therapeutic, and detection capabilities and project their potential use as nanoscale assembly agents to realize complex three-dimensional nanostructures.
Surface Modification Technologies for Durable Space Polymers
Authors: Kleiman J.I. 
Published: February 2011

AbstractMany polymers, paints, and organic-based materials exposed to the space environment undergo dramatic changes and irreversible degradation of physical and functional characteristics. While many protective approaches, including protective coatings and mechanical metal foil wrapping or cladding—especially for synthesized bulk materials, are used to reduce the effects of the space environment, the protection of such materials in space remains a major challenge, especially for future long-duration exploration missions or permanent space stations. In addition to the traditional approaches, surface modification processes are used increasingly to protect or to impart new properties to materials used in the space environment. This article presents a brief overview of the present situation in the field of surface modification of space materials. A number of surface modification solutions that differ from the traditional protective coating approaches are discussed that change the surface properties of treated materials, thus protecting them from the hazards of low Earth orbit and geostationary orbit environments or imparting new functional properties. Examples of their testing, characterization, and applications are provided.