Superlattice Orientation Research Articles

First-Principles Study of n*AlN/n*ScN Superlattices with High Dielectric Capacity for Energy Storage.

As a paradigm of exploiting electronic-structure engineering on semiconductor superlattices to develop advanced dielectric film materials with high electrical energy storage, the n*AlN/n*ScN superlattices are systematically investigated by first-principles calculations of structural stability, band structure and dielectric polarizability. Electrical energy storage density is evaluated by dielectric permittivity under a high electric field approaching the uppermost critical value determined by a superlattice band gap, which hinges on the constituent layer thickness and crystallographic orientation of superlattices. It is demonstrated that the constituent layer thickness as indicated by larger n and superlattice orientations as in (111) crystallographic plane can be effectively exploited to modify dielectric permittivity and band gap, respectively, and thus promote energy density of electric capacitors. Simultaneously increasing the thicknesses of individual constituent layers maintains adequate band gaps while slightly reducing dielectric polarizability from electronic localization of valence band-edge in ScN constituent layers. The AlN/ScN superlattices oriented in the wurtzite (111) plane acquire higher dielectric energy density due to the significant improvement in electronic band gaps. The present study renders a framework for modifying the band gap and dielectric properties to acquire high energy storage in semiconductor superlattices.

Orientational Disorder in Epitaxially Connected Quantum Dot Solids.

Periodic arrays of strongly coupled colloidal quantum dots (QDs) may enable unprecedented control of electronic band structure through manipulation of QD size, shape, composition, spacing, and assembly geometry. This includes the possibilities of precisely engineered bandgaps and charge carrier mobilities, as well as remarkable behaviors such as metal-insulator transitions, massless carriers, and topological states. However, experimental realization of these theoretically predicted electronic structures is presently limited by structural disorder. Here, we use aberration-corrected scanning transmission electron microscopy to precisely quantify the orientational disorder of epitaxially connected QD films. In spite of coherent atomic connectivity between nearest neighbor QDs, we find misalignment persists with a standard deviation of 1.9°, resulting in significant bending strain localized to the adjoining necks. We observe and quantify a range of out-of-plane particle orientations over thousands of QDs and correlate the in-plane and out-of-plane misalignments, finding QDs misoriented out-of-plane display a statistically greater misalignment with respect to their in-plane neighbors as well. Using the bond orientational order metric ψ4, we characterize the 4-fold symmetry and introduce a quantification of the local superlattice (SL) orientation. This enables direct comparison between local orientational order in the SL and atomic lattice (AL). We find significantly larger variations in the SL orientation and a statistically robust but locally highly variable correlation between the orientations of the two differently scaled lattices. Distinct AL and SL behaviors are observed about a grain boundary, with a sharp boundary in the AL orientations, but a more smooth transition in the SL, facilitated by lattice deformation between the neighboring grains. Coupling between the AL and SL is a fundamental driver of film growth, and these results suggest nontrivial underlying mechanics, implying that simplified models of epitaxial attachment may be insufficient to understand QD growth and disorder when oriented attachment and superlattice growth occur in concert.

Silicon Nanocrystal Superlattice Nucleation and Growth

Colloidal dodecene-passivated silicon (Si) nanocrystals were dispersed in hexane or chloroform and deposited onto substrates as face-centered cubic superlattices by slowly evaporating the solvent. The uniformity of the nanocrystals enables extended order; however, the solvent and the evaporation protocol significantly influence the self-assembly process, determining the morphology of the films, the extent of order, and the superlattice orientation on the substrate. Chloroform yielded superlattices with step-flow growth morphologies and (111)SL, (100)SL, and (110)SL orientations. Hexane led to mostly island morphologies when evaporated at room temperature with exclusively (111)SL orientations. Higher evaporation temperatures led to more extensive step-flow deposition. A model for the surface diffusion of nanocrystals adsorbed on the superlattice surface is developed.

Theoretical study of interfacial damping in perpendicular anisotropy superlattices along multiple crystal orientations

Damping, representing the loss of magnetic energy from the electrons to the lattice through the spin-orbit interaction, is calculated for Co/Pt and Co/Pd superlattices grown along the (001), (111), and (011) orientations. The damping consists of two contributions: interfacial and, usually, bulk. The interfacial damping shows dependence on the superlattice orientation. The origin of the interfacial damping is due to both the distorted electronic states at the interface and the spin-orbit interaction in the weakly polarized nonmagnetic Pt/Pd layers deposited on Co layers. The density of states around the Fermi level provides the spin-flip channels and closely correlates with the damping value. The damping shows asymmetry in the two transverse directions of the spin for spins at most angles. The damping for out-of-plane magnetization can be as much as 1.7 times larger than that of in-plane magnetization.

Smectic Nanorod Superlattices Assembled on Liquid Subphases: Structure, Orientation, Defects, and Optical Polarization

Directing the orientation of anisotropic nanocrystal assemblies is important for harnessing the shape-dependent properties of nanocrystal solids in devices. We control the orientation of smectic B superlattices of CdSe/CdS dot-in-rod nanocrystals through assembly on different polar interfaces and quantify the superlattice orientation through correlated small- and wide-angle grazing-incidence diffraction. Small-angle scattering is used to determine the phase of the nanorod superlattices and their preferential growth directions from the subphase. Wide-angle diffraction is used to quantify the orientations of nanorods within the superlattices and with respect to the substrate. Not only are the nanorod long axes aligned within the structures, but truncation of the short axes also coaligns the crystal axes of the nanorods with the zone axes in assembled smectic B crystals. Three dimensional orientational alignment of nanocrystals in superlattices is highly desirable in device applications. Depending on the sub...

Studies of Liquid Crystalline Self-Assembly of GdF3 Nanoplates by In-Plane, Out-of-Plane SAXS

Directed self-assembly of colloidal nanocrystals into ordered superlattices enables the preparation of novel metamaterials with diverse functionalities. Structural control and precise characterization of these superlattices allow the interactions between individual nanocrystal building blocks and the origin of their collective properties to be understood. Here, we report the directed liquid interfacial assembly of gadolinium trifluoride (GdF(3)) nanoplates into liquid crystalline assemblies displaying long-range orientational and positional order. The macroscopic orientation of superlattices is controlled by changing the subphases upon which liquid interfacial assembly occurs. The assembled structures are characterized by a combination of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) measurements performed on a laboratory diffractometer. By doping GdF(3) nanoplates with europium (Eu(3+)), luminescent phosphorescent superlattices with controlled structure are produced and enable detailed structural and optical characterization.

Limited local electron-lattice coupling in manganites: An electron diffraction study

$(\mathrm{Pr},\mathrm{Ca})\mathrm{Mn}{\mathrm{O}}_{3}$ is the archetypal charge-ordered manganite, but in ${\mathrm{Pr}}_{0.48}{\mathrm{Ca}}_{0.52}\mathrm{Mn}{\mathrm{O}}_{3}$, we find (using convergent-beam electron diffraction and dark-field images) that the superlattice period is locally incommensurate with respect to the parent lattice and that the superlattice orientation possesses significant local variations. This suggests that local electron-lattice coupling never overwhelmingly dominates the rich physics of manganites, even in the most extreme scenarios that produce the largest colossal magnetoresistance effects.

Helium gas-bubble superlattice formation in molybdenum

The gas-bubble superlattice is a striking phenomenon where small helium bubbles of uniform size are regularly arranged on a space lattice having the same symmetry as the crystal lattice of the host metal but with a lattice spacing some twenty times greater. Typically, bubble diameters are ∼2 nm and bubble concentrations are ∼1 × 10 25 m −3. Superlattices have been observed in all three metal types: bcc, fcc and hcp. The most common superlattice orientation is parallel with the host lattice. However, for the fcc metals, and the bcc metal vanadium the superlattice also contains structural variants – regions where the ordered bubble array has an orientation that is rational with, but different from, that of the crystal lattice of the host metal. Here we report on the structure of the bubble superlattice in the bcc metal molybdenum. Recently the proposal has been made that this superlattice could act as a photonic crystal for soft X-rays. An assessment is made of the quality of the superlattice in molybdenum in relation to this proposal.

Effect of Annealing on Microstructure of Ni80Fe20/Cu Multilayers

[Ni80Fe20/Cu]15 multilayers were fabricated by dc-magnetron sputtering and annealed at 150, 250 and 350℃, respectively. The structures were investigated by low-angle and high-angle X-ray diffraction. It was found that, as the annealing temperature increases, the [111] preferred orientation of superlattices is improved slightly, while the superlattice period, interplane distance, average multilayer coherence length decrease. The interfacial roughness increases with the increase of annealing temperature and/or annealing time, this can be attributed to the Ni80Fe20 and Cu sublayers has been revealed by simulation of high-angle X-ray diffraction, and its thickness increases as the annealing temperature of annealing time increases. The simulation results furthermore showed that the interplane distances of the Ni80Fe20 layer keeps constant, and that the Cu layer decreases slightly as the annealing temperature increases.

Lateral potential modulation in InAs/AlSb quantum wells by wet etching

Lateral superlattices are fabricated on InAs/AlSb quantum wells using wet chemical etching. For magnetic fields where the cyclotron diameter matches an integer multiple of the superlattice period commensurability oscillations in the magnetoresistance are observed. The spin-splitting of the Landau levels can be resolved for high magnetic fields. In this regime the height of the maxima in magnetoresistance depends on the direction of current flow with respect to the superlattice orientation.

Electronic band structure of the (GaAs)1/(InAs)1 (111) superlattice.

We present a first-principles calculation of the electronic band structure of (GaAs${)}_{1}$/(InAs${)}_{1}$ (111) superlattice, performed by using ab initio norm-conserving pseudopotentials. The folding in the smaller Brillouin zone and the new symmetry properties of the electronic states have been thoroughly investigated by a comparison with the band structure of the corresponding disordered alloy treated within the virtual-crystal approximation. The interactions between the folded states giving rise to energy splittings and electronic charge localization on given layers have been analyzed at the high-symmetry points. The effects of the new ordering potential, introduced by differentiating the two cations in the virtual crystal, and of the strain potential---arising when the superlattice is subjected to a lateral strain on the (111) planes---are separately investigated. We find a strong charge localization in the first two conduction states at the \ensuremath{\Gamma} point and a considerable minimum-band-gap reduction in the new ordered structure with respect to the virtual crystal. The strength of this localization and the fundamental band gap depend on the superlattice orientation and on the strain condition.

Helium scattering from the layered compound single crystal surface of 1T-TaS 2 in the temperature region of charge density wave reconstruction

The charge density wave (CDW) phases of the transition metal dichalcogenide 1T-TaS 2 and the transitions between them have been studied with a helium beam scattering apparatus. The known superlattice orientation ( Δφ = -13.9°), its lattice vector ( g s1 = 0.6 A ̊ −1 ) as well as the main and superlattice corrugation amplitudes ( ζ 0 = 0.055, ζ 1 = 0.04 A ̊ ) have been confirmed. From a best fit of the Bragg peak intensities via the Kirchhoff approximation an attractive well of D(HeTaS 2) = 8.7 meV has been determined.

Helium scattering from the layered compound single crystal surface of 1T-TaS 2 in the temperature region of charge density wave reconstruction

The charge density wave (CDW) phases of the transition metal dichalcogenide 1T-TaS2 and the transitions between them have been studied with a helium beam scattering apparatus. The known superlattice orientation (Δφ = -13.9°), its lattice vector (gs1 = 0.6 A−1) as well as the main and superlattice corrugation amplitudes (ζ0 = 0.055, ζ1 = 0.04 A) have been confirmed. From a best fit of the Bragg peak intensities via the Kirchhoff approximation an attractive well of D(HeTaS2) = 8.7 meV has been determined.