Anisotropic Growth Mechanism Research Articles

Enhanced cross-interfacial growth by thickening transition layers and tailoring grain size of columnar nanotwinned Cu films

Electrodeposited <111>-oriented nanotwinned Cu (NT-Cu) films consist of micron-scale columnar grains and they are thermally stable up to 400 °C. By adding nanoscale equiaxed Cu grains at the bottom of the NT-Cu films, anisotropic large grain growth could be triggered below 200 °C. The grains grew over 50 μm in bonded Cu–Cu films with 5.7 μm thick. Two main factors are identified for the low thermal stability and large grain growth behavior. The grain refining and transition layer thickening are effective to lower the thermal stability of the NT-Cu and reduce the required temperature for recrystallization and grain growth in the NT-Cu film. By increasing the fine-grained layer and refining the grain size of the columnar twinned grains, anisotropic grain growth can occur at 150 °C. The low thermal stability NT-Cu can be employed to completely eliminate the bonding interface at low temperatures in Cu–Cu joints. However, when the fine-grained layer is thicker than a threshold value, normal grain takes place and the bonding interface remains. The mechanism of anisotropic large grain growth was also proposed.

Scalable nano-integration strategy: Controllable three-dimensional monocrystalline GaN nanostructures from nanobelts to nanonetwork

The effective and scalable assembly of 1D nano-units into ordered 3D monocrystalline networks remains a challenge. Based on the crystal structure and growth characteristics of GaN, we have designed a specific epitaxy strategy and successfully achieved large area self-networking of 3D monocrystalline nanostructure on Au-coated β-LiGaO2 (100) substrate formed by the interconnection of GaN nanobelts grown along three equivalent directions of the m-axis [011̅0], [101̅0] and [11̅00]. This well-aligned 3D GaN nanostructure grown by simple CVD method has good reproducibility and the yield of cross-linked junctions are quite high. The mono-crystallinity of this structure were proved by X-ray diffraction (XRD) and transmission electron microscope (TEM) results, especially at the crosslinked junctions. The overall epitaxial relationships between GaN and LiGaO2 was analyzed and the defect-mediated anisotropic VS growth mechanism was deduced from thorough characterization of individual nanobelts. Besides, the morphology of 3D GaN nanostructure including length, width and height was found to be controllable. The E2 (high) phonon peak at 567.4 cm−1 in the Raman spectrum and the strong ultraviolet emission peak centered at 363.3 nm (3.41 eV) in the photoluminescence (PL) spectrum indicate that the 3D GaN nanostructures are nearly strain-less and have high crystalline quality. This large-scale 3D monocrystalline network not only retains the advantage of 1D nanomaterials, but also become easier to operate compared with individual nanowire. The epitaxy assembly strategy is expected to ensure the property homogeneity of chip-scale devices for their real applications and provide a reference for further growth of tunable 3D structures of other materials.

Photo‐Mediated Cascade Growth of Ag Nanocrystals in Flow Reactors for High‐Performance Flexible Transparent Electrodes

AbstractPhoto‐mediated synthesis wielding localized surface plasmon resonance (LSPR) of nanoscale metal is widely applied to sculpt silver (Ag) nanocrystals with controllable dimensions and morphologies. However, this photo‐mediated strategy remains underutilized owing to low efficiency and inferior quality in conventional batch reactors. Here, the effective synthesis of Ag nanocrystals is demonstrated via flow reactors and elucidates the photo‐mediated anisotropic growth mechanism. The investigation reveals that the intersection between excitation wavelengths and LSPR of Ag nanocrystals is a prerequisite for their growth. The final morphology is highly correlated with excitation wavelengths, which modulate the redox potential (reaction barrier) of Ag nanocrystals featuring distinct defective structures. Thus, by utilizing tandem‐connected flow reactors, size‐controllable Ag nanoplates can be yielded in a highly efficient cascade growth manner. In addition, composite conductive ink (submicron/nano‐Ag particles) is employed to create high‐performance flexible transparent electrodes. A sixfold conductivity enhancement especially under low sintering temperature (&lt;80 °C), along with superior transmittance (over 90%), and distinguished cyclic durability (negligible resistance change over 1000 cycles) are achieved simultaneously. The study not only establishes comprehensive insights into the acquisition of well‐defined Ag nanocrystals in flow reactors but also ushers in enormous feasibility toward high‐performance and cost‐effective flexible electronics.

Synthesis and conversion of multi-morphology silver nanoparticles: Anisotropic growth induced by redox cycle

A well-designed strategy utilizing H2O2 as both oxidizing and reducing agent to synthesize multi-morphology silver nanoparticles (AgNPs) was developed to investigate the mechanism of anisotropic growth. At the initial stage, isotropic silver nanospheres were synthesized by pure reduction of H2O2. Then a redox cycle was established, which consisted of mutual conversion between Ag+ and Ag0 caused by reduction and oxidization of H2O2, to induce anisotropic growth of the original nanospheres into popcorn-shaped AgNPs or silver nanoplates with the presence of different concentrations of poly(vinyl pyrrolidone) (PVP). The results indicated that the redox cycle provided a driving force of anisotropic growth by inducing the formation of defects, while PVP played a key role in controlling growth direction. This work provided a high-yield method to synthesize anisotropic AgNPs by combining reduction and oxidization and suggested a hypothesis of the mechanism of anisotropic growth.

Anisotropic Growth of Centimeter-Size CsCu2 I3 Single Crystals with Ultra-Low Trap Density for Aspect-Ratio-Dependent Photodetectors.

Low-dimensional ternary copper iodide metal halide with strong quantum confinement effects has made great progress in optoelectronic fields. However, efficient regulation of anisotropic growth of metal halides single crystal still remains a great challenge. Herein, 2cmsize CsCu2 I3 single crystals with tunable aspect ratio and the trap states (ntrap ) as low as 5.38×109 cm-3 are fabricated by optimized anti-solvent vapor-assisted method, in which the growth cycle is shortened by half. Evidenced by real-time observation and the LaMer growth model, the rapid and anisotropic growth mechanism is ascribed to preferential 1D growth, promoted by high concentration and fast vapor rate. Furthermore, the aspect-ratio-dependent optoelectronic performance is observed, the on-off ratio for 2cm CsCu2 I3 single crystal are enhanced 350 times compared with those of short and thick single crystal, which shows ultrahigh on-off ratio of 1570, D* of 1.34×1012 Jones, Rλ of 276.94mAW-1 , t rise /t decay of 0.37 and 1.08ms, and EQE of 95.53%, which are clearly at very high level among lead-free perovskite-based photodetectors. This study not only provides a new strategy for overcoming anisotropic growth limitations of low-dimensional metal halides, but also paves a way for high-performance optoelectronic applications.

Tailored growth of high‐quality CsPbI3 nanobelts

AbstractNanobelts with a representative rectangular cross section have attracted much interest, owing to their unique applications for exploring novel and efficient optoelectronic nanodevices. In this work, we report the mass production of high‐quality single‐crystalline CsPbI3 nanobelts by a one‐step template‐free process using a solvothermal method. The as‐prepared nanobelts are typically sized in 170–305 nm in width, 50–170 nm in height, and several microns in length, respectively. Accordingly, their width‐to‐height ratio of the cross section could be tailored from 1 to 6.35 by adjusting the amount of Cs2CO3 within the raw materials, representing the formation of CsPbI3 nanobelts in a tailored manner. The nanobelt growth was attributed mainly to a surfactant‐assisted, seed‐mediated anisotropic growth mechanism.

On the origin of controlled anisotropic growth of monodisperse gold nanobipyramids.

We elucidate the crucial role of the cetyl trimethylammonium bromide (CTAB) surfactant in the anisotropic growth mechanism of gold nano-bipyramids, nano-objects with remarkable optical properties and high tunability. Atomistic molecular dynamics simulations predict different surface coverages of the CTAB (positively charged) heads and their (bromide) counterions as function of the gold exposed surfaces. High concentration of CTAB surfactant promotes formation of gold nanograins in solution that work as precursors for the smooth anisotropic growth of more elongated nano-bipyramidal objects. Nanobipyramids feature higher index facets with respect to nanorods, allowing higher CTAB coverages that stabilize their formation and leading to narrower inter-micelles channels that smooth down their anisotropic growth. Absorption spectroscopy and scanning electron microscopy confirmed the formation of nanograins and demonstrated the importance of surfactant concentration on driving the growth towards nano-bipyramids rather than nanorods. The outcome explains the formation of the monodisperse bipyramidal nano-objects, the origin of their controlled shapes and sizes along with their remarkable stability.

Molecular Dynamics Simulations of the Structural Arrangement and Density of Alkylamine Surfactants on Copper Surfaces: Implications for Anisotropic Growth of Copper Nanowires

The understanding of the role of alkylamine surfactants in the anisotropic growth mechanism of copper nanowires (CuNWs) in solution-phase synthesis has implications for the tuning of properties tha...

Growth Mechanism of Gold Nanorods: the Effect of Tip-Surface Curvature As Revealed by Molecular Dynamics Simulations.

An understanding of the anisotropic growth mechanism of gold nanorods (AuNRs) during colloidal synthesis is critical for controlling the nanocrystal size and shape and thus has implications in tuning the properties for applications in a wide range of research and technology fields. In order to investigate the role of the cetyltrimethylammonium bromide (CTAB) coating in the anisotropic growth mechanism of AuNRs, we used molecular dynamics (MD) simulations and built a computational model that considered explicitly the effect of the curvature of the gold surface on CTAB adsorption and therefore differentiated between the CTAB arrangements on flat and curved surfaces, representing the lateral and tip facets of growing AuNRs, respectively. We verified that on a curved surface, a lower CTAB coverage density and larger intermicellar channels are generated compared to those on a flat surface. Using umbrella sampling simulations, we measured the free energy profile and verified that the environment around a curved surface corresponds to an easier migration from the solution to the gold surface for the [AuBr2]- species than does a flat surface. Long unbiased molecular dynamics simulations also corroborated the umbrella sampling results. Therefore, the [AuBr2]- diffusion through the environment of the tips is much more favorable than that in the case of lateral facets. This shows that the surface curvature is an essential component of the anisotropic growth mechanism.

Understanding Anisotropic Growth of Au Penta-Twinned Nanorods by Liquid Cell Transmission Electron Microscopy.

Morphology control of anisotropic nanocrystals is important for tuning shape-dependent physicochemical properties, but the anisotropic growth mechanism remains unclear. Here we investigate the formation of the Au penta-twinned nanorod by liquid cell transmission electron microscopy. It is found that a truncated decahedron forms in the absence of cetyltrimethylammonium bromide (CTAB), whereas in the presence of CTAB, a penta-twinned nanorod forms by producing {100} facets via the reentrant groove and selectively inhibiting atom addition to {100} facets. Density functional theory simulations show that the partial relief of strain energy of the decahedron is caused by the adsorption of Br- ions. Moreover, the selective adsorption of CTAB lowers the surface energy of {100} facets, thus enhancing the nanorod growth. Our work points out the importance of the synergy of strain and surface energy, which provide an in-depth insight into the anisotropic growth of nanorods and lay foundations for the controlled synthesis of nanomaterials.

Mechanism of substrate-induced anisotropic growth of monolayer WS2 by kinetic Monte Carlo simulations

Controlled anisotropic growth of two-dimensional materials provides an approach for the synthesis of large single crystals and nanoribbons, which are promising for applications as low-dimensional semiconductors and in next-generation optoelectronic devices. In particular, the anisotropic growth of transition metal dichalcogenides induced by the substrate is of great interest due to its operability. To date, however, their substrate-induced anisotropic growth is typically driven by the optimization of experimental parameters without uncovering the fundamental mechanism. Here, the anisotropic growth of monolayer tungsten disulfide on an ST-X quartz substrate is achieved by chemical vapor deposition, and the mechanism of substrate-induced anisotropic growth is examined by kinetic Monte Carlo simulations. Results show that, besides the variation of substrate adsorption, the chalcogen to metal (C/M) ratio is a major contributor to the large growth anisotropy and the polarization of undergrowth and overgrowth; either perfect isotropy or high anisotropy can be expected when the C/M ratio equals 2.0 by properly controlling the linear relationship between gas flux and temperature.

Interdigitated back contact silicon heterojunction solar cells featuring an interband tunnel junction enabling simplified processing

This paper reports on the development of an innovative back-contacted crystalline silicon solar cell with passivating contacts featuring an interband tunnel junction at its electron-collecting contacts. In this novel architecture, named “tunnel-IBC”, both the hole collector patterning and its alignment to the electron collector are eliminated, thus drastically simplifying the process flow. However, two prerequisites have to be fulfilled for such devices to work efficiently, namely (i) lossless carrier transport through the tunnel junction and (ii) low lateral conductance within the hole collector in order to avoid shunts with the neighboring electron-collecting regions. We meet these two contrasting requirements by exploiting the anisotropic and substrate-dependent growth mechanism of n- and p-type hydrogenated nano-crystalline silicon layers. We investigate the influence of the deposition temperature and the doping gas concentration on the structural and the selectivity properties of these layers. Eventually, tunnel-IBC devices integrating hydrogenated nano-crystalline silicon layers demonstrate a conversion efficiency up to 23.9%.

Snowflake Like ZnCo2O4‐xF2x/Polypyrrole Composites as High Performance Anode for Lithium Ion Batteries

AbstractIn this work, we develop a facile solvothermal method, sintering process as well as an in situ chemical polymerization route to efficiently fabricate hierarchical snowflake like ZnCo2O4‐xF2x/polypyrrole composite. According to the electron microscope observations, the formation mechanism is proposed as an aggregation process and anisotropic growth mechanism. In favor of the element doping and the structural advantages, the ZnCo2O4‐xF2x/polypyrrole composites display excellent electrochemical performance including a high reversible capacity, excellent cyclic ability, and good rate behavior. The excellent electrochemical performance is attributed to the synergistic effect of the mesoporous structure, F anions doping and coating of PPy layer, which shortens the diffusion path for the Li+ ions, improves the kinetic process, promotes the electron transfer on the electrode/electrolyte interface and buffers the volume change during Li+ ion insertion/extraction process.

Synthesis, characterization and electrochemical properties of different morphological ZnO anchored on graphene oxide sheets

In this study, we reported an optimized facile method to synthesize ZnO particles of three kinds of morphologies, flower-like, rod-like and sphere. Although ZnO grew along the a- and c-axes, however, the anisotropic crystal growth mechanism of ZnO particles was investigated by the ratio of precursor and temperature of the reaction. The characterizations of three ZnO particles were measured by Field-emission scanning electron microscopy, Fourier-transform infrared spectroscopy and X-ray diffraction. In addition, ZnO with three shapes was fixed on gold-printed circuit board to detect their electrochemistry property in a 3% H2O2/phosphate buffer, while ZnO/GO (graphene oxide) composite was obtained by organic linking which showed the different electrochemical results compare to that of ZnO. electron-transfer distance and speed have been changed between the products and support due to the combination of ZnO and GO. The exploration of ZnO of different shape and ZnO/GO composites on their features and properties would provide significant evidence for their application as sensors in extensive fields.

Direct Observation of the Layer-by-Layer Growth of ZnO Nanopillar by In situ High Resolution Transmission Electron Microscopy

Catalyst-free methods are important for the fabrication of pure nanowires (NWs). However, the growth mechanism remains elusive due to the lack of crucial information on the growth dynamics at atomic level. Here, the noncatalytic growth process of ZnO NWs is studied through in situ high resolution transmission electron microscopy. We observe the layer-by-layer growth of ZnO nanopillars along the polar [0001] direction under electron beam irradiation, while no growth is observed along the radial directions, indicating an anisotropic growth mechanism. The source atoms are mainly from the electron beam induced damage of the sample and the growth is assisted by subsequent absorption and then diffusion of atoms along the side surface to the top (0002) surface. The different binding energy on different ZnO surface is the main origin for the anisotropic growth. Additionally, the coalescence of ZnO nanocrystals related to the nucleation stage is uncovered to realize through the rotational motions and recrystallization. Our in situ results provide atomic-level detailed information about the dynamic growth and coalescence processes in the noncatalytic synthesis of ZnO NW and are helpful for understanding the vapor-solid mechanism of catalyst-free NW growth.

Improving Surface Adsorption via Shape Control of Hematite α-Fe2O3 Nanoparticles for Sensitive Dopamine Sensors.

α-Fe2O3 nanoparticles (NPs) with morphologies varying from shuttle to drum were synthesized through an anion-assisted and surfactant-free hydrothermal method by simply varying the ratios of ethanol and water in solvent. Control experiments show that the structural evolution can be attributed to a small-molecular-induced anisotropic growth mechanism in which the growth rate of α-Fe2O3 NPs along the a-, b-, or c-axis was well-controlled. The detailed structural analysis through the high-resolution transmission electron microscope (HRTEM) indicated that shuttle-like Fe2O3 NP surface was covered by high-density atomic steps, which endowed them with the enhanced adsorption and sensor ability toward dopamine (DA). The XPS characterizations indicated that the percentages of the OC component follow the order of shuttle-like Fe2O3 (S-Fe2O3 for short) > pseudoshuttle-like Fe2O3 (Ps-Fe2O3 for short) > polyhedron-like Fe2O3 (Ph-Fe2O3 for short) > drum-like Fe2O3 (D-Fe2O3 for short). Benefits from these structural advantages, the S-Fe2O3 NPs-Nafion composite electrode exhibited remarkable electrochemical detection ability with a wide liner range from 0.2 μM to 0.107 mM and a low detection limit of 31.25 nM toward DA in the presence of interferents.

Formation of Lithium Titanate Hydrate Nanosheets: Insight into a Two-Dimension Growth Mechanism by in Situ Raman

Orthorhombic lepidocrocite-type lithium titanate hydrates (LTH), as one important member among two-dimensional (2D) materials, have drawn much attention due to their potential applications in nanoelectronics, energy conversion, and energy storage devices. However, only a few efforts have focused on an anisotropic growth mechanism that determines the formation kinetics of LTH and/or even their derivatives. In this context, in situ Raman observation was employed to the isothermal crystallization of LTH nanosheet crystals from amorphous precursor precipitates. During isothermal crystallization, the formation of monolayered LTH nuclei is controlled by oxygen diffusion with an activation energy of 0.72 eV; anisotropic growth along (010) planes induces an interblocking effect among LTH nanosheet crystals, which limits their size up to 50 nm. The nucleation–growth crystallization kinetics of LTH nanosheets were accurately interpreted by the modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, which considered their anisotropic 2D growth. Beyond LTH, the present study that combines in situ spectroscopic monitoring with kinetic modeling paves the way toward deeper comprehension and ultimate control of the growth kinetics of 2D titanate materials.

Anisotropic and temperature-dependent growth mechanism of S-phase precipitates in Al–Cu–Mg alloy in relation with GPB zones

By employing atomic-resolution imaging and first principles energy calculations, the growth behavior of S-phase precipitates in a high strength Al–Cu–Mg alloy was investigated. It is demonstrated that the nucleation and growth of the S-phase precipitate are rather anisotropic and temperature-dependent companying with low dimensional phase transformation. There are actually two types of Guinier–Preston (GP) zones that determine the formation mechanism of S-phase at high aging temperatures higher than 180 °C. One is the precursors of the S-phase itself, the other is the structural units or the precursors of the well-known Guinier–Preston–Bagaryatsky (GPB) zones. At high temperatures the later GPB zone units may form around S-phase precipitate and cease its growth in the width direction, leading to the formation of rod-like S-phase crystals; whereas at low temperatures the S-phase precipitates develop without the interference with GPB zones, resulting in S-phase precipitates with lath-like morphology.

Screw Dislocation Assisted Spontaneous Growth of HfB2 Tubes and Rods

The mechanism of anisotropic growth of HfB2 rods has been discussed in this study. HfB2 powder has been synthesized via a sol–gel‐based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron respectively, though a small number of comparative experiments involved amorphous boron as the boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of anisotropic growth of HfB2 has been investigated. It is hypothesized that imperfect oriented attachment of finer HfB2 particles results in screw dislocations in the coarser particles. The screw dislocation facilitates dislocation‐driven growth of particles into anisotropic HfB2 rods.

Anisotropic Grain Growth Mechanism of Mullite Derived from Cobalt Oxide Doped Diphasic-Gels

Abstract The anisotropic grain growth of mullite derived from cobalt oxide doped diphasic-gels was investigated. Results show that cobalt oxide can decrease the crystallization temperature and promote the anisotropic grain growth of the mullite. The aspect ratio of elongated mullite grains mainly depends on the sintering conditions. The crystallization temperature of the mullite derived from cobalt oxide-doped diphasic-gel is lowered to 1200°C. The anisotropic grain growth of the mullite follows the empirical equation Gn-G0n=kt with the growth exponents 3 and 3∼4 for the length and diametric directions, respectively, as 3wt% of cobalt oxide is added. The activation energies of grain growth are 978 kJ/mol for the length and 1125 kJ/mol for the diametric directions, and the growth rate constants at 1600°C are 3.4 and 1.0 for the length and the diametric directions, respectively. The growth mechanism of mullite grains obey the two-dimensional nucleation step growth mechanism.