Facet Growth Research Articles

Stereo imaging camera model for 3D shape reconstruction of complex crystals and estimation of facet growth kinetics

The principle that the 3D shape of crystals that grow from a solution can be characterised in real-time using stereo imaging has been demonstrated previously. It uses the 2D images of a crystal that are obtained from two or more cameras arranged in defined angles as well as a mathematical reconstruction algorithm. Here attention is given to the development of a new and more robust 3D shape reconstruction method for complicated crystal structures. The proposed stereo imaging camera model for 3D crystal shape reconstruction firstly rotates a digitised crystal in the three-dimensional space and varies the size dimensions in all face directions. At each size and orientation, 2D projections of the crystal, according to the angles between the 2D cameras, are recorded. The contour information of the 2D images is processed to calculate Fourier descriptors and radius-based signature that are stored in a database. When the stereo imaging instrument mounted on a crystalliser captures 2D images, the images are segmented to obtain the contour information and processed to obtain Fourier descriptors and radius-based information. The calculated Fourier descriptors and radius-based signature are used to find the best matching in the database. The corresponding 3D crystal shape is thus found. Potash alum crystals that each has 26 habit faces were used as a case study. The result shows that the new approach for 3D shape reconstruction is more accurate and significantly robust than previous methods. In addition, the growth rates of {111}, {110} and {100} faces were correlated with relative supersaturation to derive models of facet growth kinetics.

In situ TEM observation of dissolution and regrowth dynamics of MoO2 nanowires under oxygen

Direct observation of the dissolution behavior of nanomaterials could provide fundamental insight to understanding their anisotropic properties and stability. The dissolution mechanism in solution and vacuum has been well documented. However, the gas-involved dissolution and regrowth have seldom been explored and the mechanisms remain elusive. We report herein, an in situ TEM study of the dissolution and regrowth dynamics of MoO2 nanowires under oxygen using environmental transmission electron microscopy (ETEM). For the first time, oscillatory dissolution on the nanowire tip is revealed, and, intriguingly, simultaneous layer-by-layer regrowth on the sidewall facets is observed, leading to a shorter and wider nanowire. Combined with first-principles calculations, we found that electron beam irradiation caused oxygen loss in the tip facets, which resulted in changing the preferential growth facets and drove the morphology reshaping.

Formation mechanism and properties of twinned structures in (111) seeded directionally solidified solar grade silicon

The growth structure of photovoltaic multicrystalline silicon formed by directional solidification presents a high fraction of Σ3 and higher order twins. Previous studies proposed that these complex structures are formed by a succession of 2D nucleation events of Σ3 twins on {111} growth facets at the triple line formed by their intersection with the crucible wall, another crystal, or the surface. In this work, we report the reproducible formation of multiple twinned domains inside solar grade Si single crystals grown by directional solidification above a (111) seed. These domains start on a Σ3 twin nucleated inside the crystal bulk, and systematically develop into similar twinned structures characterized by a ternary arrangement of grains in Σ3, Σ9, and Σ27 relationship. The mechanism of formation of the initial twin nucleus is discussed, and a scenario is proposed for the processes of subsequent multiple twinning. The growth competition between twin grains is shown to promote the appearance of incoherent twin boundaries, and dislocations near grain boundaries and in the twin grains themselves. The electrical activity of Σ-boundaries is measured, and the correlation between the structure of the defects and the resulting detrimental electrical activity is then discussed.

Heterogeneous Integration of InGaAs Nanowires on Si(111) for Si Photonics

1. Introduction In recent years, as the advancement of miniaturization of Si LSI in CMOS technology, the electrical connection inside and outside of the chip is facing technical limitations in terms of speed and miniaturization. To solve this problem, optical interconnection and Si photonics, in which various photonic devices are integrated on a Si platform, has been attracting much attention. One of their challenges is light sources, because Si is an indirect-gap semiconductor, and direct integration of highly efficient light emitters based, for instance, on InGaAs has a various problems, such as generation of misfit dislocations due to lattice mismatch between InGaAs and Si, and growth of polar materials on non-polar substrates. A small footprint of nanowires (NWs) grown by selective-area Metal-Organic Vapor-Phase Epitaxy (MOVPE) enables us to overcome these issues because it accommodates lattice mismatch. In this study, we investigated growth behavior of InGaAs NWs, which have compatible bandgap for optical communication band, on Si(111) substrates by selective-area growth. 2. Experimental Si(111) substrates partially masked with 20nm-thick SiO2 for NW growth were prepared by thermal oxidation, electron-beam lithography, and wet chemical etching. InGaAs NWs were grown by selective-area MOVPE, using trimethylgallium (TMGa), trimethylindium (TMIn), and arsine (AsH3) as source materials. To form (111)B polarity on non-polar Si(111) surfaces, special care was taken following Ref. [1]. After the removal of native oxide on Si surfaces by thermal cleaning in a hydrogen atmosphere at above 900°C, AsH3 was supplied for the replacement of outermost Si with As. Then, In order to form a high-quality interface of InGaAs and Si, InGaAs layers were grown firstly by flow-rate modulated epitaxy (FME), in which TMIn/TMGa and AsH3 was supplied alternately with hydrogen intervals. The growth temperature was 670°C. The intended composition of In in InGaAs was 47%, which had a bandgap corresponding to 1.55 μm, and had a lattice mismatch with Si by about 7.7%. 3. Results and discussion We found that the formation of InGaAs NWs critically depended on the diameter d of the NWs (or mask opening). When d was less than 200 nm, hexagonal structures surrounded by vertical {-110} facets appeared and vertical InGaAs NWs were formed in the most of the mask opening. This indicates that (111)B polarity was formed successfully on Si(111) surface and the growth direction of InGaAs was controlled into the vertical <111> direction. However, we also found no InGaAs growth took place in some openings. This is presumably due to a failure of initial nucleation process in some openings. It is noted that number of openings without growth increased as d decreased. On the other hand, when d was larger than 200nm, InGaAs in the hexagonal NW form decreased and anomalous lateral growth over the mask were observed. These results are understood as follows. When d is small, possibility of nucleation of InGaAs on Si is also small. Thus, single, or sometimes, no nuclei were formed in the opening for small d. As d becomes larger, possibility of nucleation increases, resulting in the increase of yield of NW growth. However, if d becomes too large, it is likely that multiple nuclei are formed [2]. Moreover, as the growth proceeds, those nuclei will coalesce but, because of the lattice mismatch between InGaAs and Si, the coalesced InGaAs are thought to contain threading dislocations. Finally, they are ended in laterally overgrown structures, as reported in Ref [2]. Thus, it is important to make sure that a single nucleus is formed for each opening for the growth of NWs. As also reported in Ref. [2], formation of nuclei on Si depends on In composition of InGaAs. In addition, the optimal growth procedure for the NWs on Si is different between InAs [1] and GaAs [3]. These findings suggest that optimal growth conditions, particularly at the initial stage of the growth, is composition-dependent for InGaAs, Thus, optimization of AsH3 thermal treatment and FME process is necessary to increase the yield of NW growth for optical communication band. 4. Summary We have achieved to grow InGaAs NWs vertically on Si(111) substrates with In composition around 47 %, and it was found that the nanowire facet growth changes due to influence of the nucleation and the lattice mismatch. Detailed investigation of InGaAs NWs by m-photoluminescence measurement and electrical characterization will be presented. 5.

Facile synthesis of hierarchically structured BiVO4 oriented along (010) facets with different morphologies and their photocatalytic properties

Hierarchically structured BiVO4 oriented along (010) facets with different morphologies are synthesized by a template-free solvothermal process in glycerol (Gly)-water mixed solvent. The effects of the volume fraction of Gly aqueous solution on the crystal structure, growth degree of (010) facets, morphology and photocatalytic activity were investigated. It is found that the volume fraction of Gly aqueous solution has strong influence on the concentration of the reaction species in the self-assembly process, and consequently induces the different morphology, as well as the crystal growth of the as-prepared BiVO4 oriented along (010) facets. A possible formation mechanism is proposed on the basis of the characterization results of X-ray diffraction and scanning electron microscopy. The four-leaved primrose-like BiVO4 sample exhibits the best photocatalytic activity for the photodegradation of methylene blue (MB) solution under simulated sunlight irradiation. The enhanced photocatalytic activity can be attributed to the improved separation of photogenerated electrons and holes caused by both special morphology and preferential growth of (010) facets.

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes.

This study describes a novel approach for the in situ synthesis of metal oxide-polyelectrolyte nanocomposites formed via impregnation of hydrated polyelectrolyte films with binary water/alcohol solutions of metal salts and consecutive reactions that convert metal cations into oxide nanoparticles embedded within the polymer matrix. The method is demonstrated drawing on the example of Nafion membranes and a variety of metal oxides with an emphasis placed on zinc oxide. The in situ formation of nanoparticles is controlled by changing the solvent composition and conditions of synthesis that for the first time allows one to tailor not only the size, but also the nanoparticle shape, giving a preference to growth of a particular crystal facet. The high-resolution TEM, SEM/EDX, UV-vis and XRD studies confirmed the homogeneous distribution of crystalline nanoparticles of circa 4 nm and their aggregates of 10-20 nm. The produced nanocomposite films are flexible, mechanically robust and have a potential to be employed in sensing, optoelectronics and catalysis.

In Situ Growth and Characterization of Metal Oxide Nanoparticles within Polyelectrolyte Membranes

AbstractThis study describes a novel approach for the in situ synthesis of metal oxide–polyelectrolyte nanocomposites formed via impregnation of hydrated polyelectrolyte films with binary water/alcohol solutions of metal salts and consecutive reactions that convert metal cations into oxide nanoparticles embedded within the polymer matrix. The method is demonstrated drawing on the example of Nafion membranes and a variety of metal oxides with an emphasis placed on zinc oxide. The in situ formation of nanoparticles is controlled by changing the solvent composition and conditions of synthesis that for the first time allows one to tailor not only the size, but also the nanoparticle shape, giving a preference to growth of a particular crystal facet. The high‐resolution TEM, SEM/EDX, UV‐vis and XRD studies confirmed the homogeneous distribution of crystalline nanoparticles of circa 4 nm and their aggregates of 10–20 nm. The produced nanocomposite films are flexible, mechanically robust and have a potential to be employed in sensing, optoelectronics and catalysis.

Preferential Facet Growth of Methylammonium Lead Halide Single Crystals Promoted by Halide Coordination

Perovskite solar cells utilize solution-processed CH3NH3PbI3 thin films as the active layer material, whose morphology and exposed facets directly contribute to the device efficiency and stability. We investigated various conditions for growing CH3NH3PbX3 (X = Br, I) crystals from solutions, and found that bromide, nonaqueous solvents, and higher X−/PbX2 ratios favor the growth of cuboids over dodecahedra. We propose that these factors promote the coordination of X− to Pb2+ on the growing surfaces, thus making the {110}c faces outgrow the {100}c faces.

Floating Silicon Method single crystal ribbon – observations and proposed limit cycle theory

In the Floating Silicon Method (FSM), a single-crystal Si ribbon is grown while floating on the surface of a Si melt. In this paper, we describe the phenomenology of FSM, including the observation of approximately regularly spaced “facet lines” on the ribbon surface whose orientation aligns with (111) crystal planes. Sb demarcation experiments sectioned through the thickness of the ribbon reveal that the solid/melt interface consists of dual (111) planes and that the leading edge facet growth is saccadic in nature, rather than steady-state.To explain this behavior, we propose a heuristic solidification limit cycle theory, using a continuum level of description with anisotropic kinetics as developed by others, and generalizing the interface kinetics to include a roughening transition as well as a re-faceting mechanism that involves curvature and the Gibbs–Thomson effect.

Pt/C nanocatalysts for methanol electrooxidation prepared by water-in-oil microemulsion method

Pt nanoparticles supported on Vulcan XC-72R were synthesized by water-in-oil microemulsion method. By incorporating different amounts of HCl as a capping agent in the precursor-containing water phase, nanoparticle shape was varied. Influencing the growth of certain facets leads to the changes of the particle shape depending on the preferential facets. As a result, nanoparticles exhibit some of the electrochemical features typical for single crystals. Commonly employed synthesis procedure for water-in-oil microemulsion method was altered with the addition of catalyst support in the system and changing the catalyst cleaning steps. Prepared catalysts were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM) and electrochemical methods. Activity and stability for methanol oxidation reaction (MOR), a structure-sensitive reaction, were tested. Electrochemical results reveal the influence of particle size, shape and exposed facets on the electrochemical processes. TEM investigations confirm electrochemical findings, while TGA verifies Pt loading in catalyst powder. Based on the results, optimal HCl concentration for cubic particle formation is determined, and structural effect on MOR activity and stability was tested. Cuboidal NPs show very good reaction activity and fair stability under applied experimental conditions.

Twin-mediated crystal growth: an enigma resolved.

During crystal growth, faceted interfaces may be perturbed by defects, leading to a rich variety of polycrystalline growth forms. One such defect is the coherent Σ3 {111} twin boundary, which is widely known to catalyze crystal growth. These defects have a profound effect on the properties of many materials: for example, electron-hole recombination rates strongly depend on the character of the twin boundaries in polycrystalline Si photovoltaic cells. However, the morphology of the twinned interface during growth has long been a mystery due to the lack of four-dimensional (i.e., space and time resolved) experiments. Many controversial mechanisms have been proposed for this process, most of which lack experimental verification. Here, we probe the real-time interfacial dynamics of polycrystalline Si particles growing from an Al-Si-Cu liquid via synchrotron-based X-ray tomography. Our novel analysis of the time evolution of the interfacial normals allows us to quantify unambiguously the habit plane and grain boundary orientations during growth. This, when combined with direct measurements of the interfacial morphology provide the first confirmation of twin-mediated growth, proposed over 50 years ago. Using the insights provided by these experiments, we have developed a unified picture of the phenomena responsible for the dynamics of faceted Si growth.

Phase-field modeling of twin-related faceted dendrite growth of silicon

We investigated the growth of a twin-related silicon dendrite through a novel phase-field model. The correctness of the model for an equilibrium twined crystal was examined first before we modeled the faceted dendrite growth. The simulated morphologies of 〈112〉 and 〈110〉 faceted dendrites were consistent with experimental observations. The growth orientation of the simulated dendrite depended on the growth rates at the ridges and the re-entrant corners. We further extended the Twin-Plane-Reentrant-Edge (TPRE) mechanism and then proposed a new growth model for faceted dendrites based on our simulations. The undercooling and twin spacing could affect the growth rates at the ridges and the re-entrant corners, which were also explained by the proposed model.

New orientations in the stereographic triangle for self-assembled faceting

Energetically unstable crystalline surfaces, among their uses, can be templates for the growth of periodic arrays of one-dimensional (1D) nanoscale structures. However, few studies have explored self-assembled faceting on high-index (HI) planes inside the stereographic triangle, and extant studies have not produced any criteria for encouraging the formation of one-dimensional periodic arrays. In this Letter, by analyzing the MBE growth of homoepitaxial facets on (631)A GaAs, a HI plane inside the triangle, we present a criteria to produce highly uniform 1D periodic arrays on unexplored surfaces. These families of planes are those belonging to the lines connecting the energetically stable HI GaAs (11 5 2) plane with any of the (100), (110), and (111) planes at the corners of the stereographic triangle. This novel strategy can lead to new possibilities in self-assembling 1D structures and manipulating physical properties, which in turn may result in new HI- and 1D-based experiments and devices.

Influence of Lateral Growth on the Surface Pit Formation of GaN Heteroepitaxial Film Grown by MOCVD

This study made an attempt to understand and control the heteroepitaxial growth of GaN from the view of the essential behaviors of crystal growth. Through a comparison of the nonpolar, polar and semipolar GaN epitaxial film, the influence of lateral growth on the surface pit formation mechanism has been investigated. For a-plane GaN, the lateral growth velocities of the less inclined {20-21} and {11-22} facets are approaching to the velocity of {10-11} facet under high temperature, so that the surface pit was transformed from a triangular shape to a pentagonal one. For c-plane GaN, the size of the surface pit produced by screw dislocation is determined by the lateral growth of the pit facets. A slow lateral growth rate of the inclined facets {10-11} compared with the vertical growth rate of (0001) facet under low V/III ratio would enlarge the pit size. For (11-22) semipolar GaN, surface pit is rarely observed, because the vertical growth velocity of {11-22} plane is slow compared with the lateral growth rate of the inclined facets, such as {11-20} and (0001).DOI: http://dx.doi.org/10.5755/j01.ms.22.2.12931

Shape evolution of carbon supported Pt nanoparticles: From synthesis to application

In this research, a water-in-oil microemulsion method with HCl as a capping agent was applied to synthesize carbon supported Pt catalysts. Varying the concentration of HCl caused changes in the shape of obtained nanoparticles, i.e. preferential growth of certain facets. Addition of catalyst support in the synthesis process facilitated the cleaning procedures necessary to remove the surfactant residues. Prepared catalyst powders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis indicated the influence of HCl addition on the crystallite size and crystal habit. TEM revealed that addition of higher amounts of the capping agent led to the formation of a noticable amount of particles with concave cubic or branched-like structures. Influence of the catalyst particles shape on its electrochemical properties was tested in the oxidations of COads, ammonia and formic acid. The latter one was examined in terms of both activity and stability of as prepared and oxide-annealed (electrochemically treated) catalysts. The results clearly demonstrate that even small changes in the nanoparticle surface structure give rise to distinct modifications in their properties. Concave cubic particles, in comparison to other catalysts, show improved catalytic properties and the contribution of their preferentially oriented {100} facets is electrochemically detectable.

Rapid synthesis of single-phase bismuth ferrite by microwave-assisted hydrothermal method

This paper describes on the fast synthesis of bismuth ferrite by the simple microwave-assisted hydrothermal method. The phase transformation and the preferred growth facets during the synthetic process have been investigated by X-ray diffraction. Bismuth ferrite can be quickly prepared by microwave hydrothermal method by simply controlling the reaction time, which is further confirmed by Fourier Transform infrared spectroscopy and magnetic measurement.

Interfacial Crystallization‐Driven Assembly of Conjugated Polymers/Quantum Dots into Coaxial Hybrid Nanowires: Elucidation of Conjugated Polymer Arrangements by Electron Tomography

A simple and practical “solution‐biphase method” allows the preparation of efficient charge‐transporting 1D nanocrystals with coaxial p–n junctions. It involves gradual diffusion of a top layer of poor solvent (acetonitrile) into a bottom layer of poly(3‐hexyl thiophene)‐b‐poly(2‐vinyl pyridine) (P3HT‐b‐P2VP) conjugated polymers (CPs) and CdSe quantum dots (QDs) dissolved in chloroform. Initial interfacial crystallization‐driven assembly of CPs results in the formation of seeds consisting of dimeric QDs transversely bridged by CPs. Coaxial CPs/QDs hybrid NWs are generated by 1D growth of QDs‐dimeric seeds, enabling tracing of the CPs‐crystallization process via the QDs. Thus, well‐arranged QDs along the longitudinal axis of the NWs infer highly crystalline CPs with edge‐on orientation, as confirmed by electron tomography, UV–vis spectroscopy, and grazing‐incidence wide‐angle X‐ray scattering. This high cristallinity as well as the increased length of the resulting hybrid NWs in solution and the corresponding crystallite size in as‐cast film represent a significant improvement compared to the conventional “one‐pot addition method”. Moreover, although randomly QDs‐attached hybrids of P3HT homopolymer are produced by the solution‐biphase method, branched aggregates with micrometer‐long NW arms are generated from the crystal seeds containing multiple growth facets without precipitate, despite acetonitrile being a nonsolvent.

Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor-Liquid-Solid Nanowire Growth.

Kinking, a common anomaly in nanowire (NW) vapor-liquid-solid (VLS) growth, represents a sudden change of the wire's axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liquid droplets of defined size. Statistical analysis of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diameter and the growth temperature. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to ⟨110⟩, was observed in Ge NWs with a nominal diameter of ∼ 20 nm. This size coincides with a critical diameter at which a spontaneous transition from ⟨111⟩ to ⟨110⟩ growth occurs in the phase field simulations. Larger diameter NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined ⟨111⟩ axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liquid droplet at the tip of the NW.

Tunable morphology with selective faceted growth of visible light active TiO2 thin films by facile hydrothermal method: structural, optical and photocatalytic properties

Selective faceted growth of highly ordered TiO2 (002) nanorods were coated over fluorine doped tin oxide (FTO) substrate prepared by facile and cost effective hydrothermal method for different reaction temperatures (100, 120, 140 and 160 °C). The rutile phase TiO2 and highly preferential oriented along (002) plane were confirmed from the XRD pattern. The TiO2 thin film, prepared with different reaction temperature consisting of anisotropic structures was analysed by SEM and the elemental composition of the films was identified using EDAX. The optical band gap of the TiO2 thin films was found to be 3.5–3.1 eV using UV–visible spectroscopy. The presence of rutile TiO2 phase was confirmed from raman analysis. As a result of photoluminescence studies, improvement of charge separation with low recombination rate was observed in TiO2 thin films. The anionic (Congo Red, Direct Red) and cationic (Rhodamine B and Malachite Green) dye decolorisation of the TiO2 thin films was investigated under visible light from photocatalytic activity.

Triblock copolymer tunes 1-dimensional AgTCNQ nanostructures in aqueous medium by a one-pot reaction

Abstract Metal-organic charge transfer complexes, MTCNQ (M = Ag or Cu; while, TCNQ = 7,7,8, 8-tetracyanoquinodimethane), have received great attentions for their electrical and optical properties. To prepare AgTCNQ, poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol) (P123) has been applied as a soft template for the growth of AgTCNQ. This triblock copolymer is the structure directing agent and necessary to the formation of the AgTCNQ nanostructure. By tuning the concentration of P123, we achieved two novel AgTCNQ nanocrystals (nanospindle and nanocones) for the first time via a one-pot reaction in aqueous solution. In addition, sodium citrate acts as a reductant and ensures the proceeding of the reaction between Ag + and TCNQ particles. A ligand-limited facet growth mechanism has been proposed to explain the shape regulation of AgTCNQ through the facet block effect by P123. This soft template method also enlightens us in the synthesis of other organic crystals with different shapes.