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Superlattices
Kinds of Superlattices Terms modified by Superlattices Selected AbstractsObservation of a Ternary Nanocrystal Superlattice and Its Structural Characterization by Electron Tomography,ANGEWANDTE CHEMIE, Issue 51 2009Drei die zusammenpassen: PbSe-Nanokristalle mit zweierlei Durchmessern (blau und grün) und CdSe-Nanokristalle (rot) bauen einen ternären Kolloidkristall auf (siehe Bild). Einer elektronentomographischen Untersuchung zufolge ist das Übergitter isostrukturell mit dem Atomgitter von AlMgB4. [source] Interdiffusion phenomena in InGaAs/GaAs superlattice structuresCRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2010B. Sar, kavak Abstract We have studied structural properties of InGaAs/GaAs superlattice sample prepared by Molecular Beam Epitaxy (MBE) using high resolution X-ray diffractometer (HRXRD). Increasing strain relaxation and defect generations are observed with the increasing Rapid Thermal Annealing (RTA) temperature up to 775 °C. The higher temperatures bring out relaxation mechanisms; interdiffusion and favored migration. The defect structure and the defects which are observed with the increasing annealing temperature were analyzed. Firstly, the in-plane and out-of-plane strains after the annealing of sample were found. Secondly, the structural defect properties such as the parallel X-ray strain, perpendicular X-ray strain, misfit, degree of relaxation, x composition, tilt angles and dislocation that are obtained from X-ray diffraction (XRD) analysis were carried out at every temperature. As a result, we observed that the asymmetric peaks especially in asymmetric (224) plane was affected more than symmetric and asymmetric planes with lower polar or inclination angles due to c-direction at low temperature. These structural properties exhibit different unfavorable behaviors for every reflection direction at the increasing temperatures. The reason is the relaxation which is caused by spatially inhomogeneous strain distribution with the increasing annealing temperature. In the InGaAs superlattice samples, this process enhances preferential migration of In atoms along the growth direction. Further increase in the annealing temperature leads to the deterioration of the abrupt interfaces in the superlattice and degradation in its structural properties. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Heat-Transport Mechanisms in SuperlatticesADVANCED FUNCTIONAL MATERIALS, Issue 4 2009Yee Kan Koh Abstract The heat transport mechanisms in superlattices are identified from the cross-plane thermal conductivity , of (AlN)x,(GaN)y superlattices measured by time-domain thermoreflectance. For (AlN)4.1 nm,(GaN)55,nm superlattices grown under different conditions, , varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest ,, , of (AlN)4 nm,(GaN)y superlattices decreases monotonically as y decreases, ,,=,6.35,W m,1 K,1 at y,=,2.2,nm, 35 times smaller than , of bulk GaN. For long-period superlattices (y,>,40,nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G,,,620 MW m,2 K,1. For y,<,40,nm, the apparent value of G increases with decreasing y, reaching G,,,2 GW m,2 K,1 at y,<,3,nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)4.1 nm,(GaN)4.9,nm superlattice irradiated by 2.3 MeV Ar ions to a dose of 2,×,1014 ions cm,2 is reduced by <35%, suggesting that heat transport in these short-period superlattices is dominated by long-wavelength acoustic phonons. Calculations using a Debye-Callaway model and the assumption of a boundary scattering rate that varies with phonon-wavelength successfully capture the temperature, period, and ion-dose dependence of ,. [source] Combinatorial Hierarchically Ordered 2D Architectures Self-assembled from Nanocrystal Building Blocks,ADVANCED MATERIALS, Issue 19 2008Xiangxing Xu A one-step, low-cost, and general nanocrystal self-assembly method that covers both the nanometer-scale superlattice and macroscale ordered patterns,that is, hierarchical architectures,is developed. This approach can generate various functional, compositional, and dimensional combinatorial architectures. It may give new opportunities in applications of catalysis, electronics, energy, magnetic devices, and bio-techniques. [source] Self-Assembly and Self-Orientation of Truncated Octahedral Magnetite Nanocrystals,ADVANCED MATERIALS, Issue 18 2006K. Zheng Monodisperse magnetite nanoparticles (12,nm) in the shape of Wigner,Seitz crystals self-assemble into a body-centered cubic superlattice (see figure) in which the nanoparticles possess the same crystallographic orientations. Shape plays a critical role in controlling the orientation of the nanoparticles in self-assembly. Both self-assembly and self-orientation of nanoparticles are important for technical applications such as high-density magnetic storage. [source] Structural anisotropy of InGaAs/GaAs(001) quantum dot chains structuresPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2007V. P. Kladko Abstract We have studied the structural properties of ordered InGaAs/GaAs(001) quantum dot chains multilayer by high-resolution X-ray diffraction. Two systems of lateral satellites, one of which being inclined with respect to the sample surface normal, i.e. the growth direction [001], were observed. The measured inclination of 30.0° ± 2.5° does not affect the diffraction profile from planar superlattice (SL), i.e. SL peaks are not inclined with respect to the GaAs substrate peak. We identify the splitting of coherent SL satellites for all orders as well as for two perpendicular directions. This splitting most likely indicates that two discrete periods exist in SL structure. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Highly reliable blue-violet inner stripe laser diodes using planar regrowth of AlGaN/GaN superlattice cladding layerPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2007K. Fukuda Abstract We describe highly reliable operation of a novel planar inner stripe blue-violet laser diode (BV-LD). A planar regrowth technique makes it possible to simultaneously fabricate a low-resistive superlattice (SL) structure both on the current-injecting narrow stripe area and on the AlN current-blocking layer. This allows for low operating voltage and over 1000 h stable operation at a single-mode output power of 200 mW at 80 °C. The results obtained here clearly indicate that this inner stripe laser structure is a desirable candidate for a reliable high-power light source for the next generation of optical disc systems. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Fields of deformation anisotropy exploration in multilayered (In,Ga)As/GaAs structures by high-resolution X-ray scatteringPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2006O. Yefanov Abstract The results of investigation of In0.3Ga0.7As/GaAs superlattice by high-resolution X-ray scattering are presented. The influence of lattice distortion on diffraction curves (DC) were analyzed with dynamical diffraction theory. It allowed to explain azimuth dependence of experimental diffraction curves. Anisotropic changes in the shape of InGaAs lattice unit cell were shown and measured. The influence of smooth borders between hetero-layers were analyzed. Comparative analysis of different gradient functions on the hetero-border influence on diffraction curves was done. Parameters of heterojunction in investigated samples were determined with the help of DC modelling. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Magnon energy gap and the magnetically structural symmetry in a three-layer ferrimagnetic superlatticePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2006Rong-ke Qiu Abstract The magnon energy band in a ferrimagnetic superlattice with three layers in a unit cell is studied by employing retarded Green's functions and the spin-wave method. Two modulated energy gaps ,,13 and ,,23 are evaluated systematically, which exist in the magnon energy band along the Kx -direction perpendicular to the plane of the superlattice. It is revealed that the energy gap ,,13 has a direct relation with the symmetry among the spin quantum numbers and the interlayer exchange couplings, while the energy gap ,,23 relates to the symmetry among these spin quantum numbers only. These symmetries differ from the symmetry of crystallographic point groups. We define the magnetically structural symmetry that is dominated mainly by the magnetic parameters. The absence of the energy gap at a certain condition means that the system has a high magnetically structural symmetry. The magnetically structural symmetry of the superlattice, which is an intrinsic property, strongly affects the magnon energy band structure and thus the magnetic behaviors of the system. Furthermore, two complete bandgaps are observed to extend through the Brillouin zone (referred to as "magnonic crystal") in this superlattice system. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Interlayer coupling in magnetic superlattices with electron density inhomogeneitiesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2004W. Gruhn Abstract We discuss the influence of spatial inhomogeneities of the free electron density on the magnetic interaction between magnetic layers of the superlattice, mediated across nonmagnetic, metallic spacer. Using the modified total energy approach, we prove that the TM or RE superlattices the additional scattering of free electrons on magnetic ion multipole moment increases the ferroquadrupolar biquadratic coupling between magnetic layers. It is shown also that the nonuniform free electron density generates contribution to the interlayer coupling being of the Dzialoshinsky-Moriya type. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Optical and magnetic properties of the DyN/GaN superlatticePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003Y. K. Zhou Abstract Ferromagnet/semiconductor DyN/GaN superlattice structures were grown on SiC substrate by radio frequency molecular beam epitaxy. Formation of DyN was confirmed by X-ray diffraction measurements. The temperature dependence of the photoluminescence (PL) spectra for the DyN/GaN superlattice and GaDyN samples was studied. Sharp PL peaks from two samples showed no shift with temperature, indicating the intra-atomic f,f transition at Dy ions. Comparison of the PL spectra for the DyN/GaN superlattice and GaDyN suggests that GaDyN is formed at the GaN/DyN interface. The DyN/GaN superlattice exhibited two ferromagnetic components; one is major at low temperatures (<50 K) and the other is major at high temperatures (>350 K). The former shows large coercivity Hc of about 1800 Oe at 7 K, which was originated from DyN layers. The latter may come from GaDyN, which was formed at the GaN/DyN interface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Vibrational modes in a square cross-section InAs/InP nanowire superlatticePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Seiji Mizuno Abstract We study theoretically the phonon modes in a square cross-section nanowire superlattice (NWSL) consisting of InAs and InP. The phonon modes in the NWSL composed of cubic materials are classified into five types, i.e., A1, A2, B1, B2, and E modes. We calculate the dispersion relations of each phonon mode and corresponding displacement fields for this NWSL. The A1, A2, B1, B2, and E modes have features of dilatational, torsional, stretching, shear, and flexural modes, respectively. We examine the effects of both the superlattice modulation along the wire axis and lateral confinement of phonons. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Deterministic self-organization: Ordered positioning of InAs quantum dots by self-organized anisotropic strain engineering on patterned GaAs (311)BPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2009E. Selçuk Abstract Laterally ordered InGaAs quantum dot (QD) arrays, InAs QD molecules, and single InAs QDs in a spot-like periodic arrangement are created by self-organized anisotropic strain engineering of InGaAs/GaAs superlattice (SL) templates on planar GaAs (311)B substrates in molecular beam epitaxy. On shallow- and deep-patterned substrates the respectively generated steps and facets guide the self-organization process during SL template formation to create more complex ordering such as periodic stripes, depending on pattern design. Here we demonstrate for patterns such as shallow- and deepetched round holes and deep-etched zigzag mesas that the self-organized periodic arrangement of QD molecules and single QDs is spatially locked to the pattern sidewalls and corners. This extends the concept of guided self-organization to deterministic self-organization. Absolute position control of the QDs is achieved without one-to-one pattern definition. This guarantees the excellent arrangement control of the ordered QD molecules and single QDs with strong photoluminescence emission up to room temperature, which is required for future quantum functional devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Monolithically integrated UV/IR-photodetectors based on an AlN/GaN-based superlattice grown on an AlGaN buffer layerPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009Daniel Hofstetter Abstract In this article, we demonstrate closely spaced, monolithically integrated photodetectors in two largely different wavelength ranges. The epitaxial structure of the devices was grown by plasma-assisted molecular-beam epitaxy on an AlN-on-sapphire template; it consists of a Si-doped AlGaN thin film, and a nearly strain compensated 40 period AlN/GaN superlattice with 1.0 nm thick GaN quantum wells and 2.0 nm thick AlN barriers. The entire structure is covered with an AlGaN cap layer. The superlattice acts as active region for the infrared detector, while the AlGaN buffer layer serves as active area for the ultraviolet detector. While the photovoltaic near-infrared detector has a center wavelength of 1.37 ,m, the photoconductive ultraviolet detector has a long wavelength cutoff at 250 nm. The two detectors could be operated up to room temperature with reasonable sensitivities. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Observation of the second-nearest-neighbor Bloch oscillation in a GaAs/AlAs superlatticePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2008T. Hasegawa Abstract We have systematically investigated the quantum beat of the miniband excitons and the Bloch oscillation in the Wannier-Stark (WS) localization as a function of applied bias voltage in a GaAs (6.8 nm)/AlAs (0.9 nm) superlattice embedded in a p - i - n diode structure. The coherent dynamics behaviors were detected with a reflection-type pump-probe technique. We clearly observed the transformation process from the miniband-exciton quantum beat to the Bloch oscillation with an increase in applied bias voltage producing an internal electric field. The noteworthy finding is the fact that the Bloch oscillation with the frequency of vBO = 2eFD /h appears in a weak localization regime in addition to the usual Bloch oscillation with vBO = eFD /h in a strong localization regime, where F is the electric field, and D is the superlattice period. The frequency of 2eFD /h indicates that the newly observed Bloch oscillation is due to the wave-packet motion in the second-nearest-neighbor space range. The results described above are explained by the electric-field-strength dependence of the envelope-function localization, which is estimated from the envelope-function profile calculated by a transfer-matrix method and the excitonic transitions observed by electroreflectance spectroscopy. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photoluminescence, Raman scattering and vertical transport of CdSe/CdMgSe superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2006I. I. Reshina Abstract Photoluminescence and photoluminescence excitation spectra, vertical transport of photoexcited charge carriers and excitons and Raman scattering by optical phonons have been studied for the first time in low-strained superlattices CdSe/CdMgSe grown by molecular-beam epitaxy on InAs substrates. The vertical transport was studied by purely optical means involving an enlarged quantum well built into the superlattice. The enlarged quantum well served as a sink for the photoexcited carriers and excitons that have tunnelled through the superlattice. The measurements conducted in temperature range 2,150 K and also under in-plane strong magnetic fields show that vertical transport occurs mainly by free heavy-hole excitons but in superlattices with 5.9 nm and 7.3 nm periods it is not of the Bloch type. A comparison of the calculated energies of the interband transitions with the experimental data provides the valence-band ofsett in the range 0.4,0.5. The Raman spectra indicate a two-mode behavior of the optical phonons in CdMgSe barriers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The evolution of the electric field in an optically excited semiconductor superlatticePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 8 2005Alvydas Lisauskas Abstract We report on time-resolved photocurrent spectroscopy of an intrinsic GaAs/Al0.3Ga0.7As superlattice subsequent to femtosecond optical excitation. Information on the spatio-temporal evolution of the densities of electrons and holes and on the internal electric field is obtained by tracing Wannier-Stark photocurrent spectra as a function of delay time for various bias fields and pump excitation intensities. The experimental results are supplemented by simulations. We employ the combined information to define the conditions to be met for succesful pump-probe Bloch gain experiments. In particular, we find that field screening sets on upper limit for the carrier density of 1016 cm,3, and that the time window during which gain should be found is defined by the duration of the sweep-out of the optically injected electrons from the superlattice which occurs within about 10 ps after excitation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Well-width dependence of coupled Bloch-phonon oscillations in biased InGaAs/InAlAs superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004Michael Först Abstract The coupling between Bloch oscillations and longitudinal optical (LO) phonons is investigated in ternary In0.53Ga0.47As/In0.52Al0.48As superlattices of different well widths. In femtosecond time-resolved studies, a strong increase of the coherent LO phonon amplitudes is observed when the Bloch oscillations are subsequently tuned into resonance with the different optical phonon modes of the ternary semiconductor materials. In a narrow-well superlattice where electronic minibands are energetically shifted close to the confining barrier potential, the phonon amplitudes are asymmetrically enhanced on the high-frequency edge of the resonance. Here, at high electric fields, field induced tunneling into above-barrier continuum states leads to a rapid dephasing of Bloch oscillations. The associated polarization change provides an additional excitation process for coherent LO phonons. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Quasiperiodic impurity energy spectra of GaAs/GaxAl1,xAs superlatticesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2004M. S. Vasconcelos Abstract In this work we consider a generalized Fibonacci quasiperiodic superlattice (GFQPSL), within a tight-binding model, in which its nearest-neighbor-hopping matrix elements are distributed according to the generalized Fibonacci sequence. The electronic density of states (DOS) is then determined by using a Green function method based on Dyson's equation together with a transfer-matrix treatment. The resulting energy spectrum is then determined, considering initial physical parameters according to the scheme used in the experimental realization of a GFQPSL. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Towards a classification of icosahedral viruses in terms of indexed polyhedraACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2006A. Janner The standard Caspar & Klug classification of icosahedral viruses by means of triangulation numbers and the more recent novel characterization of Twarock leading to a Penrose-like tessellation of the capsid of viruses not obeying the Caspar,Klug rules can be obtained as a special case in a new approach to the morphology of icosahedral viruses. Considered are polyhedra with icosahedral symmetry and rational indices. The law of rational indices, fundamental for crystals, implies vertices at points of a lattice (here icosahedral). In the present approach, in addition to the rotations of the icosahedral group 235, crystallographic scalings play an important rôle. Crystallographic means that the scalings leave the icosahedral lattice invariant or transform it to a sublattice (or to a superlattice). The combination of the rotations with these scalings (linear, planar and radial) permits edge, face and vertex decoration of the polyhedra. In the last case, satellite polyhedra are attached to the vertices of a central polyhedron, the whole being generated by the icosahedral group from a finite set of points with integer indices. Three viruses with a polyhedral enclosing form given by an icosahedron, a dodecahedron and a triacontahedron, respectively, are presented as illustration. Their cores share the same polyhedron as the capsid, both being in a crystallographic scaling relation. [source] The fine structure of X-ray diffuse scattering in the vicinity of high-angle superlattice Bragg reflectionsACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2003Fine structure of X-ray diffuse scattering Triple-axis X-ray diffractometry was used to study diffuse scattering from an AlAs/GaAs superlattice grown on an [001]-oriented GaAs substrate by molecular beam epitaxy. Reciprocal-space maps were obtained around the 002 reflection from the superlattice and its low-angle first-order satellite. The data obtained reveal quasi-Bragg diffuse-scattering sheets caused by conformal behavior of interfacial roughness as well as amplification of diffuse scattering when the incoming or outgoing angle is nearly equal to the Bragg angle of the superlattice or substrate. The observed features of diffuse-scattering fine structure are explained within the framework of the distorted-wave Born approximation. Nevertheless, this approximation is shown to be incorrect for quantitative analysis of diffuse scattering. In particular, the observed domination in intensity of the incoming Bragg features over the outgoing ones is shown to reflect the decay rate of the coherent X-ray field through the diffuse-scattering channel, which is not negligible relative to the coherent diffraction. [source] Two-dimensionally modulated structure of the rare-earth polysulfide GdS2,x (x = 0.18 , 13/72)ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2003Rafael Tamazyan The crystal structure of GdS2,x is determined by single-crystal X-ray diffraction as a 144-fold superstructure of the ZrSSi structure type. The superstructure is described as a two-dimensional, commensurately modulated structure with the superspace group P4/n(,,½)(00)(ss) and with , = 1/4 and , = 1/3. Structure refinements within the classical approach, employing the 144-fold supercell, fail because most of the superlattice reflections have zero intensities within the experimental resolution. Within the superspace approach the absent superlattice reflections are systematically classified as higher-order satellite reflections. Accordingly, the superspace approach has been used to refine the structure model comprising the basic structure positions and the amplitudes of the modulation functions of the three crystallographically independent atoms. The quality of fit to the diffraction data and the values of the refined parameters are independent of the assumption on the true symmetry (incommensurate or a 12,×,12,×,2, I -centred superlattice with different symmetries). Arguments of structural plausibility then suggest that the true structure is a superstructure with space group I, corresponding to sections of superspace given by (t1, t2) equal to [(4n, 1)/48, (4m, 3)/48] or [(4n, 3)/48, (4m, 1)/48] (n and m are integers). Analysis of the structure, employing both superspace techniques (t plots) and the supercell structure model all show that the superstructure corresponds to an ordering of vacancies and an orientational ordering of S dimers within the square layers of the S2 atoms. [source] Fabrication of Ordered Nanostructured Arrays Using Poly(dimethylsiloxane) Replica Molds Based on Three-Dimensional Colloidal CrystalsADVANCED FUNCTIONAL MATERIALS, Issue 10 2009Hong Kyoon Choi Abstract Hexagonally arrayed structures of colloidal crystals with uniform surface are a good candidate for master molds to be used in soft lithography. Here, the fabrication of periodically arrayed nanostructures using poly(dimethylsiloxane) (PDMS) molds based on three-dimensionally (3D) ordered colloidal crystals is reported. A robust, high-quality 3D colloidal-crystal master molds is prepared using the colloidal suspension containing a water-soluble polymer. The surface patterns of the 3D colloidal crystals can then be transferred onto a polymer film via soft lithography, by means of the replication of the surface pattern with PDMS. Various hexagonally arrayed nanostructure patterns can be fabricated, including close-packed and non-close-packed 2D arrays and honeycomb structures by the structural modification of the 3D colloidal-crystal templates. The replicated hexagonally arrayed structures can also be used as templates for producing colloidal crystals with 2D superlattices. [source] Heat-Transport Mechanisms in SuperlatticesADVANCED FUNCTIONAL MATERIALS, Issue 4 2009Yee Kan Koh Abstract The heat transport mechanisms in superlattices are identified from the cross-plane thermal conductivity , of (AlN)x,(GaN)y superlattices measured by time-domain thermoreflectance. For (AlN)4.1 nm,(GaN)55,nm superlattices grown under different conditions, , varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest ,, , of (AlN)4 nm,(GaN)y superlattices decreases monotonically as y decreases, ,,=,6.35,W m,1 K,1 at y,=,2.2,nm, 35 times smaller than , of bulk GaN. For long-period superlattices (y,>,40,nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G,,,620 MW m,2 K,1. For y,<,40,nm, the apparent value of G increases with decreasing y, reaching G,,,2 GW m,2 K,1 at y,<,3,nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)4.1 nm,(GaN)4.9,nm superlattice irradiated by 2.3 MeV Ar ions to a dose of 2,×,1014 ions cm,2 is reduced by <35%, suggesting that heat transport in these short-period superlattices is dominated by long-wavelength acoustic phonons. Calculations using a Debye-Callaway model and the assumption of a boundary scattering rate that varies with phonon-wavelength successfully capture the temperature, period, and ion-dose dependence of ,. [source] Functional Perovskites , From Epitaxial Films to Nanostructured Arrays,ADVANCED FUNCTIONAL MATERIALS, Issue 24 2008Ionela Vrejoiu Abstract Functional perovskite materials gain increasing significance due to their wide spectrum of attractive properties, including ferroelectric, ferromagnetic, conducting and multiferroic properties. Due to the developments of recent years, materials of this type can conveniently be grown, mainly by pulsed laser deposition, in the form of epitaxial films, multilayers, superlattices, and well-ordered arrays of nanoislands. These structures allow for investigations of preparation,microstructure,property relations. A wide variation of the properties is possible, determined by strain, composition, defect contents, dimensional effects, and crystallographic orientation. An overview of our corresponding work of recent years is given, particularly focusing on epitaxial films, superlattices and nanoisland arrays of (anti)ferroelectric and multiferroic functional perovskites. [source] "Charge Leakage" at LaMnO3/SrTiO3 InterfacesADVANCED MATERIALS, Issue 5 2010Javier Garcia-Barriocanal Direct evidence for charge leakage at the interface of epitaxial SrTiO3/LaMnO3 superlattices with atomically sharp interfaces is provided. The direction of charge leakage can be reversed by changing the LMO/STO thickness ratio. This result will be important for the understanding of some of the reported limitations of oxide devices involving manganite/titanate interfaces. [source] Magnetically Tunable Metal,Insulator SuperlatticesADVANCED MATERIALS, Issue 4 2010Masao Nakamura Design and control of nanometer-scale electronic phase separation are demonstrated in high-quality manganite superlattices composed of a ferromagnetic metal and an antiferromagnetic insulator (see figure). Bicritical competition of these phases sensitively controls the magnetic and electronic properties of the superlattices as a whole. A magnetic field can effectively tune the volume fraction of each phase and consequently the position of the phase domain boundaries. [source] Ordered Gold Nanoarrays: 3D Ordered Gold Strings by Coating Nanoparticles with Mesogens (Adv. Mater.ADVANCED MATERIALS, Issue 17 200917/2009) Liquid-crystal-coated gold nanoparticles exhibit 3D long-range ordering in strings jacketed by rod-like mesogens (shown in green) with controllable interparticle spacing. The work reported on p. 1746 by Goran Ungar and co-workers demonstrates a new method of assembling ordered metal nanoparticle superlattices, in a step towards developing self-assembled metamaterials. The cover shows red, yellow, and blue nanoparticles lying at different heights and forming a rhombohedral lattice. [source] 3D Ordered Gold Strings by Coating Nanoparticles with MesogensADVANCED MATERIALS, Issue 17 2009Xiangbing Zeng Gold nanoparticles covered with a nematic liquid-crystal ligand laterally attached via a thioalkyl spacer and a thioalkane "diluent" exhibit 3D ordering in strings jacketed by the mesogens with controllable interparticle spacing. The particles form rhombohedral, hexagonal, and rectangular columnar superlattices, not the usual packing modes of spheres. [source] Monodisperse Nanoparticles of Ni and NiO: Synthesis, Characterization, Self-Assembled Superlattices, and Catalytic Applications in the Suzuki Coupling Reaction,ADVANCED MATERIALS, Issue 4 2005J. Park Monodisperse spherical Ni nanoparticles with diameters of 2 nm, 5 nm, and 7 nm were synthesized from the thermal decomposition of a Ni,oleylamine complex. Ni nanocrystal superlattices were generated via the controlled evaporation of solvent (see Figure). The nanoparticles were successfully used as catalysts for the Suzuki coupling reaction, and were readily oxidized to produce NiO nanoparticles. [source] | ||||||||||||||||||||||||||||