Self-organized Superlattice Research Articles

Possibility and Optimized Conditions for Spontaneous Superlattice Formation in Stabilized Zirconia Thin Films Grown Under In Situ Magnetic Field

Possibility and Optimized Conditions for Spontaneous Superlattice Formation in Stabilized Zirconia Thin Films Grown Under In Situ Magnetic Field

The effect of elastic anisotropy on the symmetry selection of irradiation-induced void superlattices in cubic metals

Self-organized microstructures and patterns have been widely observed in non-equilibrium physical systems. In particular, irradiation in metals creates far-from-equilibrium environments, in which the competing dynamics of defect production and annihilation can lead to unique self-organized superlattice structures, e.g., void and gas bubble superlattices. From a physical point of view, the superlattice structures are dictated by the intrinsic symmetry breaking in the metals, i.e., anisotropy caused by the breaking of continuous rotational symmetry. In the literature, two distinctive anisotropies, elastic anisotropy and diffusion anisotropy of interstitials, have been proposed to be the origins of superlattice formation. However, it is still unclear which anisotropy dominates the symmetry selection of superlattice structures. In this paper, we study elastic anisotropy and its effect on the symmetry of void superlattices. By using theoretical analyses and phase field simulations, we show that elastic anisotropy in cubic metals can lead to either face-centered cubic or simple cubic superlattices depending on the Zener anisotropy ratio. The superlattices formed under this elastic anisotropy mechanism must form under the influence of spinodal decomposition, as the mechanism requires perturbations in the vacancy concentration field to develop into spatially-static concentration waves. We compare to existing work on symmetry selection in superlattices via diffusion anisotropy and to experimental observations, and we suggest that concentration wave development under the influence of elastic anisotropy is not the mechanism for symmetry selection during the formation of irradiation-induced void superlattices, but that diffusion anisotropy could be the dominant mechanism.

Truth and Myth of Phase Coexistence in Methylammonium Lead Iodide Perovskite Thin Film via Transmission Electron Microscopy.

Hybrid organic-inorganic perovskites have attracted substantial interest as the most favorable prospective material for efficient photovoltaic and optoelectronic devices. However, their extreme sensitivity to electron beam radiation makes it difficult to obtain their intrinsic structure by transmission electron microscopy and can even lead to significant misidentifications. In 2018, the coexistence of methylammonium lead iodide (MAPbI3 ) in the cubic and tetragonal phase using electron microscopy and electron diffraction techniques was reported in article "Self-Organized Superlattice and Phase Coexistence inside Thin Film Organometal Halide Perovskite". Herein, however, that claim is challenged by comparing their experimental data to simulated diffraction patterns and arguing that their perovskite samples may have been damaged due to excessive electron beam irradiation. Consequently, true phase coexistence was not observed in that previously reported work, rather merely the decomposition products of MAPbI3 .

Three-Dimensional Superlattice of PbS Quantum Dots in Flakes.

In the last two decades, many experiments were conducted in self-organization of nanocrystals into two- and three-dimensional (3D) superlattices and the superlattices were synthesized and characterized by different techniques, revealing their unusual properties. Among all characterization techniques, X-ray diffraction (XRD) is the one that has allowed the confirmation of the 3D superlattice formation due to the presence of sharp and intense diffraction peaks. In this work, we study self-organized superlattices of quantum dots of PbS prepared by dropping a monodispersed colloidal solution on a glass substrate at different temperatures. We showed that the intensity of the low-angle XRD peaks depends strongly on the drying time (substrate temperature). We claim that the peaks are originated from the 3D superlattice. Scanning electron microscopy images show that this 3D superlattice (PbS quantum dots) is formed in flake’s shape, parallel to the substrate surface and randomly oriented in the perpendicular planes.

Lead Halide Perovskite Nanocrystals: From Discovery to Self-assembly and Applications.

Lead halide perovskites (LHPs) of the general formula APbX3 (A=Cs+, CH3NH3+, or CH(NH2)2+; X=Cl, Br, or I) have recently emerged as a unique class of low-cost, versatile semiconductors of high optoelectronic quality. These materials offer exceptionally facile solution-based engineerability in the form of bulk single crystals, thin films, or supported and unsupported nanostructures. The lattermost form, especially as colloidal nanocrystals (NCs), holds great promise as a versatile photonic source, operated via bright photoluminescence (PL) in displays or lighting (energy down-conversion of blue light into green and red), or via electroluminescence in light-emitting diodes. In this article we discuss the recent history of the development of highly-luminescent NCs of LHPs, the current state-of-the-art of this class of materials, and the future prospects of this highly active research field. We also report the demonstration of long-range ordered, self-organized superlattice structures obtained from cube-shaped colloidal CsPbBr3NCs using drying-mediated self-assembly.

Collective vibration of discharge current filaments in a self-organized pattern within a dielectric barrier discharge

We report on the collective vibration of discharge current filaments in a self-organized superlattice pattern within an air-argon dielectric barrier discharge system. The period of collective vibration decreases as the air content increases. An antisymmetric normal vibration mode of discharge current filaments is observed. The measurements of the spatiotemporal behavior of the superlattice pattern show that the discharge of the vibrating dot (filament) is asynchronous with that of its eight neighboring dots, including four big dots and four small dots. The forces exerted on the vibrating dot are analyzed, and a motion equation is set up in each half cycle of the applied voltage. It is found that the motion of the small dot is not an ideal vibration but is a vibration with fluctuating amplitude, which is in good agreement with experiments.

Formation of Self‐Organized Superlattice Nanotube Arrays – Embedding Heterojunctions into Nanotube Walls

Formation of Self-Organized Superlattice Nanotube Arrays – Embedding Heterojunctions into Nanotube Walls

Experimental study on self-organized hexagonal superlattice pattern in dielectric barrier discharge in argon/air

A new type of hexagonal superlattice pattern is observed under three different boundary conditions by using a dielectric barrier discharge device with water electrode. The power spectrum of hexagonal superlattice pattern and its angular distributions are shown. The phase diagram of the hexagonal superlattice pattern as a function of air concentration and applied voltage is obtained. The correlation measurements indicate that the hexagonal superlattice pattern is an interllacing of two different transient sublattices. The selection of pattern formation in quadrangular boundary with different area is studied. It is found that the hexagonal superlattice pattern is a typical type of nonlinear self-organized pattern, and the formation of square superlattice pattern is dependent upon the boundary conditions.

Structural characterization and stress-relaxation behavior of superlattice Cu 5Zr

A self-organized superlattice containing modulated layers in Cu5Zr was manufactured by casting a Cu–Zr system. Substitutional Zr atoms caused compressive stress in the superlattice so that periodic stress fields were formed with a long-range order along the 〈1 1 0〉 directions on {1 1 1} for stress relaxation. On the other hand, it was demonstrated theoretically by atomistic simulations that a superlattice could form easily due to the large negative enthalpy of formation in the structure (−12.890 kJ/mol of atoms).

Nanopatterning the electronic properties of gold surfaces with self-organized superlattices of metallic nanostructures

The self-organized growth of nanostructures on surfaces could offer many advantages in the development of new catalysts, electronic devices and magnetic data-storage media. The local density of electronic states on the surface at the relevant energy scale strongly influences chemical reactivity, as does the shape of the nanoparticles. The electronic properties of surfaces also influence the growth and decay of nanostructures such as dimers, chains and superlattices of atoms or noble metal islands. Controlling these properties on length scales shorter than the diffusion lengths of the electrons and spins (some tens of nanometres for metals) is a major goal in electronics and spintronics. However, to date, there have been few studies of the electronic properties of self-organized nanostructures. Here we report the self-organized growth of macroscopic superlattices of Ag or Cu nanostructures on Au vicinal surfaces, and demonstrate that the electronic properties of these systems depend on the balance between the confinement and the perturbation of the surface states caused by the steps and the nanostructures' superlattice. We also show that the local density of states can be modified in a controlled way by adjusting simple parameters such as the type of metal deposited and the degree of coverage.

Self-organized superlattices along the [001] growth direction in In 0.52Al 0.48As layers grown on nominally (001) InP substrates by molecular beam epitaxy

Horizontal self-organized superlattice structures consisting of alternating In-rich and Al-rich layers formed naturally during solid-source molecular beam epitaxy (MBE) growth of In 0.52Al 0.48As on exactly (001) InP substrates, with In and Al fluxes unchanged. The growth temperatures were changed from 490 to 510 ∘C, the most commonly used growth temperature for In 0.52Al 0.48As alloy. No self-organized superlattices (SLs) were observed at the growth temperature 490 ∘C, and self-organized SLs were observed in InAlAs layers at growth temperatures ranging from 498 to 510 ∘C. The results show that the period of the SLs is very highly regular, with the value of ∼6 nm, and the composition of In or Al varies approximately sinusoidally along the [001] growth direction. The theoretical simulation results confirm that the In composition modulation amplitude is less than 0.02 relative the In composition of the In 0.52Al 0.48As lattice matched with the InP substrate. The influence of InAs self-organized quantum wires on the spontaneously formed In x Al 1− x As/In y Al 1− y As SLs was also studied and the formation of self-organized In x Al 1− x As/In y Al 1− y As SLs was attributed to the strain-mediated surface segregation process during MBE growth of In 0.52Al 0.48As alloy.

Self-organized vertical superlattices in epitaxial GaInAsSb

Self-organized superlattices are observed in GaInAsSb epilayers grown nominally lattice matched to vicinal GaSb substrates. The natural superlattice (NSL) is detected at the onset of growth and is inclined by an additional 4° with respect to the (001) terrace of the vicinal GaSb substrate. This tilted NSL intersects the surface of the epilayer, and the NSL period is geometrically correlated with the periodicity of surface undulations. While the underlying driving force for this phase separation arises from solution thermodyamics, the mechanism for the self-organized microstructure is related to local strain associated with surface undulations. By using a template with surface undulations, the tilted NSL can be induced in layers with alloy compositions that normally do not exhibit this self-organized microstructure under typical growth conditions.

Self-organized superlattice formation during crystal growth from continuous beam fluxes.

Alloy superlattice structures consisting of alternating Si-rich and C-rich layers form spontaneously during gas-source molecular beam epitaxy of Si(1-y)C(y) on Si(001) from constant Si2H6 and CH3SiH3 precursor fluxes at T(s)=725-750 degrees C. The self-organized patterning is due to a complex interaction among competing surface reactions. During growth of the initial Si-rich layer, strain-driven C segregation to the subsurface results in charge transfer from surface Si atom dangling bonds to C backbonds. This decreases the Si2H6 sticking probability, and, hence, the instantaneous deposition rate, thereby enhancing C segregation. The Si-rich layer continues until a critical C coverage is reached allowing nucleation of a C-rich layer which grows until the excess subsurface C is depleted. The process then repeats with periods tunable through the choice of T(s) and y(avg).

Observation of self-organized superlattice in AlGaInAsSb pentanary alloys

An unexpected self-organized superlattice structure has been observed in the AlGaInAsSb pentanary alloys grown by metalorganic vapor-phase epitaxy. The samples were studied by transmission electron microscopy, double-crystal x-ray diffraction, and secondary ion mass spectrometry measurements. The modulation strength and period of the self-organized superlattice are correlated to the alloy composition.

Complex ordering in ternary wurtzite nitride alloys

Atomic ordering in AlGaN films produced by plasma assisted molecular beam epitaxy (PA-MBE) was investigated with X-ray diffraction (XRD) and transmission electron microscopy selected area diffraction (TEM-SAD). Under nitrogen-rich growth conditions, films undergo 1×1 monolayer cation ordering along the [0001] growth direction. This type of ordering is stronger in N-polar films than in Ga-polar films and is sensitive to the growth ratio of group-III to group-V flux. These observations are in qualitative agreement with the first principal calculations by Northrup and co-workers. Under group-III rich growth conditions of Ga-polar films, the XRD data is consistent with the quasi-random stacking of spontaneously formed 14-ML and 12-ML superlattice structures. Reciprocal space mapping shows a small tilt (0.25°) between the crystallographic c-axis of the wurtzite structure and the superlattice axis. This result can be explained by the presence of c-axis oriented superlattice domains with a parallel step translation between neighboring domains. This result is in qualitative agreement with a recently proposed model by Venezuela and co-workers of self-organized superlattice formation during step flow growth of strained alloys on vicinal surfaces.

Self-organized superlattice patterns with two slightly differing wave numbers.

We report on the observation of superlattices that occur spontaneously in a nonlinear optical system with O(2) symmetry. A secondary bifurcation from hexagons yields patterns formed by twelve wave vectors. Besides irregular patterns these may either be quasiperiodic patterns or superlattices built from two classes of wave vectors differing slightly in their length. Both classes of wave vectors stem from only one pattern-forming instability. The wave vectors fit on a hexagonal or a square grid. In the former case the set of wave vectors can be decomposed into two hexagonal triads, whereas in the case of the square grid squeezed triads occur.

Self-Organized Superlattices in GaInAsSb Grown on Vicinal Substrates

ABSTRACTSelf-organized superlattices are observed in GaInAsSb epilayers grown lattice matched to vicinal GaSb substrates. The natural superlattice (NSL) is oriented at a slight angle of about 4° with respect to the vicinal (001) GaSb substrate. This vertical composition modulation is detected at the onset of growth. Layers in the NSL are continuous over the lateral extent of the substrate. Furthermore, the NSL persists throughout several microns of deposition. The NSLs have a period ranging from 10 to 30 nm, which is dependent on deposition temperature and GaInAsSb alloy composition. While the principle driving force for this type of phase separation is chemical, the mechanism for the self-organized microstructure is related to local strains associated with surface undulations. By using a substrate with surface undulations, the tilted NSL can be induced in layers with alloy compositions that normally do not exhibit this self-organized microstructure under typical growth conditions. These results underscore the complex interactions between compositional modulation and morphological perturbations.

Epitaxial growth and self-organized superlattice structures in AlGaN films grown by plasma assisted molecular beam epitaxy

Aluminum gallium nitride alloys were grown by molecular beam epitaxy and their film composition, structure and microstructure were investigated by Rutherford backscattering spectroscopy, Atomic Force Microscopy, X-ray diffraction, Transmission Electron Microscopy and High Resolution Electron Microscopy. The kinetics of growth was investigated and the result show that at the growth temperature of 750 °C the sticking coefficient of Ga varies monotonically from practically 0 to 1 as the growth varies from group-III to -V rich regime. Correspondingly, the surface morphology changes from atomically smooth in the group-III regime to relatively rough in the nitrogen-rich regime. The X-ray diffraction and TEM studies revealed the existence of three types of spontaneously formed superlattice structures, along the [0001] direction, with periodicities of 2-, 7- and 12-monolayers. While the two-monolayer ordering is preferred under group-V rich conditions of growth, the 7- and 12-monolayer ordering were observed under group-III rich conditions of growth. Off-axis X-ray diffraction shows the absence of in-plane ordering and that periodic stacking faults are not present in the superlattice structures.

Analysis of composition fluctuations in Al xGa 1 −xN

Composition fluctuations in Al x Ga 1 −x N-layers ( x=0.25 and 0.35) are investigated on an atomic scale by high-resolution transmission electron microscopy (HRTEM). The samples were grown by plasma induced molecular beam epitaxy on Al 2O 3 (0001). A strain state analysis of the cross-sectional HRTEM micrographs is performed with the digital analysis of lattice images (DALI) program package. Composition profiles on an atomic scale are derived by the measurement of the distances between intensity maxima positions in the HRTEM image. The analyses revealed different areas in the Al x Ga 1 −x N-layers with either homogeneous or ‘striped’ contrast. In the striped areas the analyses indicate a strong decomposition that leads to the formation of self-organized superlattice structures.

Electronic-structure calculations of self-organized PbS-Bi2S3−(Ag2S)(113) twinning superlattices

The electronic structure of natural, self-organized twinning superlattices based on PbS-${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ and PbS-${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}{\ensuremath{-}\mathrm{A}\mathrm{g}}_{2}\mathrm{S}$ alloys is calculated and discussed. Superlattices based on PbS-${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}$ are predicted to be semiconductors, having subbands separated by significant minigaps, as in the conventional ``manmade'' superlattices. On the other hand, the two variations of PbS-${\mathrm{Bi}}_{2}{\mathrm{S}}_{3}{\ensuremath{-}\mathrm{A}\mathrm{g}}_{2}\mathrm{S}$ superlattices that we have proposed calculations on, are likely to be semimetals. Since these self-organized superlattices should boast high stability against interdiffusion and interface roughness problems, we believe they merit further theoretical and experimental research, and may be found to be suitable for possible applications.