Phase Growth Mechanism Research Articles

Critical Analysis of Phase Evolution, Morphological Control, Growth Mechanism and Photophysical Applications of ZnS Nanostructures (Zero-Dimensional to Three-Dimensional): A Review

ZnS nanostructures are prominent and promising candidates of class II–IV semiconductor materials that can be prepared by sophisticated techniques. Transition of the material from bulk to nanosize brings forth drastic changes in various properties particularly the photophysical properties. In recent years research has been focused on modifying and manipulating the morphologies of ZnS nanostructures for fabricating photocatalysts, photonic devices, biolabeling agent, optical sensors, detectors, and other novel applications. This review article addresses phase evolution (theoretical modeling approach and experimental validation), morphological control, growth mechanisms based on thermodynamic considerations, surface energy driven models, kinematics, template directed growth, etc., and understanding of the photophysical properties of ZnS based on the dimension of nanostructures (zero-dimensional to three-dimensional). A broad overview is presented for various synthesis techniques from the aspect of different ...

Phase constituents and growth mechanism of laser in situ synthesized WC reinforced composite coating with W–C–Ni system

Abstract

Monte Carlo Simulation of Laser-Ablated Particle Splitting Dynamic in a Low Pressure Inert Gas**supported by the Natural Science Foundation of Hebei Province, China (No. A2015201166) and the Natural Science Foundation of Hebei University, China (No. 2013-252)

A Monte Carlo simulation method with an instantaneous density dependent mean-free-path of the ablated particles and the Ar gas is developed for investigating the transport dynamics of the laser-ablated particles in a low pressure inert gas. The ablated-particle density and velocity distributions are analyzed. The force distributions acting on the ablated particles are investigated. The influence of the substrate on the ablated-particle velocity distribution and the force distribution acting on the ablated particles are discussed. The Monte Carlo simulation results approximately agree with the experimental data at the pressure of 8 Pa to 17 Pa. This is helpful to investigate the gas phase nucleation and growth mechanism of nanoparticles.

Volumetric and timescale analysis of phase transformation in single-crystal silicon during nanoindentation

Clarifying the phase transformation process and mechanism of single-crystal silicon induced by high pressure is essential for preparation of new silicon phases. Although many previous researches have focused in this area, the volume of high-pressure phases and the duration of phase transformation are still unclear. In this paper, the volume change and the duration of phase transformation from Si-II phase into Si-XII/Si-III phases were investigated quantitatively by introducing a holding process in the unloading stage of a nanoindentation test. Experimental results indicate that the high-pressure phase volume is dependent strongly on the maximum indentation load and independent of the loading/unloading rate and the holding time at the maximum indentation load, while phase transformation duration is independent of the aforementioned experimental parameters. By analyzing the results, a critical volume of Si-XII/Si-III phases was identified which determines the occurrence of sudden phase transformation, and a modified nucleation and growth mechanism of high-pressure phases was proposed. These results provide new insights into high-pressure phase transformations in single-crystal silicon.

Preparation of planar CH3NH3PbI3 thin films with controlled size using 1-ethyl-2-pyrrolidone as solvent

Recently, planar perovskite solar cells based on CH3NH3PbI3 have attracted many researcher's interest due to their unique advantages such as simple cell architecture, easy fabrication and potential multijunction construction comparing to the initial mesoporous structure. However, the preparation of planar perovskite films with high quality is still in challenge. In this paper, we developed a vapor-assisted solution process using a novel and green solvent of 1-Ethyl-2-pyrrolidone (NEP) instead of the traditional N, N-dimethylformamide (DMF) to construct a high-quality perovskite CH3NH3PbI3 thin film with pure phase, high compactness, small surface roughness and controlled size. The phase evolution and growth mechanism of the perovskite films are also discussed. Utilizing the NEP of low volatility and moderate boiling point as solvent, we dried the PbI2-NEP precursor films at different temperature under vacuum and then obtained PbI2 thin films with different crystalline degree from amorphous to highly crystalline. The perovskite films with crystal size ranged from hundreds of nanometers to several micrometers can be prepared by reacting the PbI2 films of different crystalline degree with CH3NH3I vapor. Moreover, planar-structured solar cells combining the perovskite film with TiO2 and spiro-OMeTAD as the electron and holes transporting layer achieves a power conversion efficiency of 10.2%.

Controllable synthesis of W18O49 nanowire arrays and their application in electrochromic devices

Tungsten oxide nanowire arrays as effective electrochromic working electrodes were fabricated on seed-free FTO glass through a facile solvothermal process. Polyethylene glycol was used as the structure-directing agent to control the crystal growth because of selective absorption on the facet of tungsten oxide crystals. Phase identification and growth mechanism of monoclinic W18O49 nanowire arrays are also extensively discussed. Uniform transparent W18O49 nanowire arrays film exhibits remarkable performance of electrochromic properties, which include high contrast (49.64 % at 632.8 nm), high electrochromic stability (>3000 CV cycles), and fast coloration/bleaching response (tc ~ 7.9 s, tb ~ 1.4 s). These properties of the W18O49 nanowire arrays film endow its promising practical applications in large-area smart windows.

Chemical Vapor Deposition Growth of Monolayer WSe2 with Tunable Device Characteristics and Growth Mechanism Study.

Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot of attention recently, because of their interesting electronic, optical, and mechanical properties. Among large numbers of TMDCs, monolayer of tungsten diselenides (WSe2) is of particular interest since it possesses a direct band gap and tunable charge transport behaviors, which make it suitable for a variety of electronic and optoelectronic applications. Direct synthesis of large domains of monolayer WSe2 and their growth mechanism studies are important steps toward applications of WSe2. Here, we report systematical studies on ambient pressure chemical vapor deposition (CVD) growth of monolayer and few layer WSe2 flakes directly on silica substrates. The WSe2 flakes were characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, and photoluminescence spectroscopy. We investigated how growth parameters, with emphases on growth temperatures and durations, affect the sizes, layer numbers, and shapes of as-grown WSe2 flakes. We also demonstrated that transport properties of CVD-grown monolayer WSe2, similar to mechanically exfoliated samples, can be tuned into either p-type or ambipolar electrical behavior, depending on the types of metal contacts. These results deepen our understandings on the vapor phase growth mechanism of WSe2, and may benefit the uses of these CVD-grown monolayer materials in electronic and optoelectronics.

In Situ TEM Study on Au Mediated Growth of NiSi2 in Si Nanowire: A Vapor-Liquid-Solid Analogy

The vapor-liquid-solid (VLS) mechanism is the prominent growth process for semiconductor nanowires (NWs), for example in Si NW growth. By forming Au-Si eutectic alloy, Si atoms can be sufficiently mobile in the liquid while finding their crystal sites before precipitating out into a solid phase. We report here an in-situ TEM study of new solid-liquid-solid (SLS) Ni disilicide phase growth mechanism in Si NWs that is analogous to the VLS mechanism in being a liquid-mediated growth, but is fundamentally different in terms of nucleation and mass transport. The new mechanism can be potentially used in synthesizing metallic nanowires without templates.

In Situ TEM Study on Au Mediated Growth of NiSi2 in Si Nanowire: A Vapor-Liquid-Solid Analogy

The vapor-liquid-solid (VLS) mechanism is the prominent growth process for semiconductor nanowires (NWs), for example in Si NW growth. Supply atoms of Si are provided by vapor precursors to a supersaturated Au-Si liquid eutectic alloy that mediates the solid nanowire growth. We report here a new solid-liquid-solid (SLS) silicide phase growth mechanism in Si NWs that is analogous to the VLS mechanism in being a liquid-mediated growth, but is fundamentally different in terms of nucleation and mass transport.To observe the growth, we dispersed VLS grown <111> Si NWs on 30 nm TEM silicon nitride membranes with pre-evaporated Ni dots. In-situ TEM heating observations, including real-time video recording, of the interaction between the Au-terminated Si NW and the Ni nanodots were conducted at 700 ºC as a function of time. Figure (a) is a TEM image showing a segment of NiSi2 grown by the distinct gold mediation mechanism, and Figure (b) shows the HR image at the NiSi2/Au and Au/Si interfaces. At the very beginning of the process, the Au-Si tip forms a liquid alloy at the elevated temperature (Figure c). Then, Ni atoms dissolve from the Ni nanodots into the Si NWs and diffuse interstitially to and dissolve in the liquid alloy (Figure d). Upon super-saturation of Ni and Si inside the Au-Ni-Si alloy, a single {111} terminated octahedral NiSi2 nucleus forms in the middle of the liquid alloy (Figure e) and continues to grow until it is large enough to be pinned at the inner surface of the oxidized Si NW. Meanwhile, the Au-Ni-Si liquid alloy starts to move from the tip towards the other end of the Si NW, and catalytically transforms Si to NiSi2 along the moving path (Figure f). Here, two concurrent processes mediated by the moving Au-Ni-Si liquid layer occur: (1) progressive dissolution of the Si NW at the liquid alloy/Si (111) interface, and (2) continuing supply of Ni so that the precipitation of NiSi2 phase occurs at the liquid alloy/NiSi2 interface, which is closer to the tip of the NW. In a similar fashion to the VLS growth of Si, the NiSi2 nucleates at {110} type facets and propagates on its (111) planes. This constitutes a SLS growth process for the NiSi2 phase. With SLS growth, the NiSi2 phase occurs at lower temperatures when mediated by the liquid alloy, compared with the temperatures used in thin film reactions. In contrast NiSi2 is found to nucleate homogeneously in Si NWs without Au mediation only when the temperature is above 800 ºC.Figure captions: (a) TEM bright field image showing NiSi2 forms from the tip of the nanowire with a Au alloy layer moving towards the opposite end. Scale bar is 100 nm (b) HRTEM image of the area around Au. The reaction front is NiSi2 (111) plane that maintains a (111) interface with the solidified Au as demonstrated in the inset FFT patterns. Scale bar is 5 nm. SLS growth process: (c) Ni interstitial diffusion through a Si medium (NW) and accumulation in the Au-Si eutectic alloy. (d) Formation of a liquid Au-Ni-Si ternary alloy at the Si NW tip. (e) Nucleation and growth of a single octahedral NiSi2 crystallite inside the liquid ternary alloy. (f) Continuous dissolution of the Si NW by the eutectic ternary that moves down through the Si NW and concurrent growth of NiSi2 on the other side of the eutectic in the steady state.

In-situ nanoscale imaging of charge transfer of BiNiO3 under high pressure

Over last decades, both synchrotron radiation techniques and high pressure research have made great progress. Advanced synchrotron capabilities with high spatial resolution, high flux, and high energy resolution provides us many new avenues to conduct advanced high pressure researches. In this talk, we will focus on the new developments of the nanoscale imaging techniques on the pressure induced phase separation in three dimensions. BiNiO3 under goes a charge transfer induced phase transition under high pressure or temperature, which shows excellent colossal negative thermal expansion effect [1]. Co-exist of both high density and low density phases over a wide range pressure or temperature plays the key roles on the negative thermal expansion behavior. We utilized a newly developed X-ray absorption near edge spectroscopy tomography method, and successfully resolved the mixture of high/low pressure phases as a function of pressure at tens of nanometer resolution. By choosing incident x-ray energy near Ni absorption edge, the pressure induced valence transition can be mapped at tens of nanometer scale in 3d, which provides crucial information on the HP-LP phase boundary [2]. As temperature driven grain growth upon heating, we can draw fundamental information on the pressure-induced phase growth mechanism.

Interplay between the ionic and electronic transport and its effects on the reaction pattern during the electrochemical conversion in an FeF2 nanoparticle.

Using a charge dependent embedded atom method potential in conjunction with a dynamically adaptive multibody force field, the conversion reaction in an iron difluoride nanoparticle exposed to lithium ions is investigated. The reactions take advantage of the multiple valence states of the cations. A subtle interplay between the ionic and electronic transport, which is not accessible in conventional fixed-charge simulations, has been revealed. The simulated reaction pattern is in close agreement with that observed experimentally at the nanoscale, while providing detailed atomistic mechanisms. Due to difference in the ionic and electronic transport, different stages of reaction are observed and the corresponding phase growth mechanisms have been identified. Initially local Li concentration plays a key role in driving the reaction through amorphous reaction products to the crystalline phases that inhibit Li transport. However, electronic transport and interfacial ion diffusion are shown to be important in creating further transport pathways that allow continued conversion reactions, providing the mechanism that enables the use of these materials in advanced high capacity lithium ion batteries. Such interplay between the ionic and electronic transport will also be important in other materials and devices for energy conversion and storage.

Fluorite Phase Transformations under Vacuum

Complex crystal aggregates from fluorspar vapor phase were grown at specific low-pressure/high-temperature conditions. The quasi-equilibrium of initiated crystal-chemical reactions at the proceeding vapour-crystal phase transformation was strongly dependent on the mass-transport inside an originally designed multicameral crucible, loaded by several portions of natural fluorite. By changing the temperature pressure over the already molten fluorspar portions as well as the gas-permeability of the channels connecting different sections in crucible interior to vacuum ambient, one may control the rate of gaseous-vapour diffusion and the degree of supper-saturation inside the peripheral crucible compartment wherein nucleation and crystal growing occurred. In this way, grown aggregates revealed a complicated habit formed during three-stage growing process provided by relevant thermodynamic and phase. Residual stresses were not observed in the aggregates whereas those in simultaneously grown boules from the non-vaporized melts in crucible cameras were clearly distinguished. The optical transmittance spectra of the boules were obtained considerably better, especially in the UV, comparing to those for crystal aggregates, both showing several peaks of specific light-absorption due to enhanced presence of rare-earth (RE) impurities. The aggregates manifest nearly full reflectivity from Vis to near IR region. The vapor phase growth mechanisms, when natural fluorite with some RE contents has been used, were explained on thermodynamic grounds that shown the manner of reliable control on the phenomenon. The results were anticipated to help for developing new perspective techniques for growth from vapor of several fluoride compounds with complex structure and composition and wide application. It was speculated that similar growth mechanisms of CaF2 crystals were possible on the moon in its very early period of formation.

Controllable synthesis of Ln3+ (Ln = Tb, Eu) doped zinc phosphate nano-/micro-structured materials: phase, morphology and luminescence properties

Ln(3+) (Ln = Tb, Eu) doped zinc phosphate tetrahydrate (ZPT:Ln(3+)) and ammonium zinc phosphate (AZP:Ln(3+)) nano-/micro-structured materials were synthesized in aqueous solution without the addition of any structure-directing agent. The phase structures, morphologies and luminescence properties of the as-synthesized samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy and lifetime. These investigations indicate that different phosphate sources MnH(3-n)PO4 (M = NH4(+) or Na(+), n = 1, 2, 3) can lead to the altering of morphology from nanosheet to microflower, but have no significant effect on the phase structure of the samples. The microlump, nanosheet, and microflower (constructed by the primary microlumps or nanosheets) of orthorhombic ZPT:Ln(3+) could be selectively prepared by adjusting the pH value from 3.5 to 7.0. A mixture of orthorhombic ZPT:Ln(3+) and monoclinic AZP:Ln(3+) with a microflower morphology was obtained when the pH value was adjusted to 8.0. Monoclinic AZP:Ln(3+) microplate, microcube and nanoparticle morphologies were obtained at pH values of 8.5, 9.0 and 11.0 respectively. The phase transformation and growth mechanism of the diverse morphologies were proposed, and ZPT:Ln(3+) (Ln(3+) = Eu or Tb) samples exhibit red or green emission under the excitation of UV light.

Deposition of ZnO nanostructured film at room temperature on glass substrates by activated reactive evaporation

ZnO nanostructured films were deposited on glass substrates at room temperature by activated reactive evaporation technique. Thermal evaporation of zinc at high deposition rate in presence of oxygen plasma resulted in deposition of ZnO nanostructured film with different flower-like morphologies on glass substrates. The structural and morphological properties of these nanostructured films were studied by XRD, SEM and TEM. A gas phase growth mechanism has been proposed for the formation of nanostructures. The room temperature photoluminescence spectra of these films exhibited weak ultraviolet (UV) and strong broad visible emission peaks. Further, the position of visible emission peak is found to vary with morphology of the grown nanostructured films. Photocurrent measurements indicated that these ZnO nanostructured films show high sensitivity to UV light, and hence can be used as efficient UV photodetectors.

Formation mechanism of Cu2ZnSnSe4 absorber layers during selenization of solution deposited metal precursors

Phase-pure Cu2ZnSnSe4 (CZTSe) layers are necessary for achieving efficient thin film solar cells. This requires the knowledge of intermediate phases and their existence regions during the evolution of the CZTSe phase within its homogeneity range. Here we investigate the growth mechanism of different phases when solution deposited metal salt precursors are selenized into CZTSe layers. A combination of in situ and ex situ X-ray diffraction, Raman spectroscopy, energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy at successively increasing substrate temperatures is used to track evolving crystal phases. The growth starts with the fast formation of binary Cu–Se phases that are present between 190°C and 320°C. Overlapping diffraction patterns of CZTSe/Cu2SnSe3/ZnSe phases evolve from 280°C onwards and remain until a final temperature of 550°C. The ternary Cu2SnSe3 phase co-existing with CZTSe between 340°C and 370°C is confirmed by Raman spectroscopy and point EDX measurements. No individual zinc or tin binary phases can be detected. We propose the kinetically driven formation mechanism, which starts with the selenization of Cu requiring the lowest activation energy for reaction, and then proceeds via the gradual incorporation of Sn and Zn to yield the final CZTSe phase.

Diffusion and growth mechanism of phases in the Pd-Sn system

Growth mechanism of phases and atomic mechanism of diffusion are discussed in the Pd-Sn system. The Kirkendall marker plane location indicates that the PdSn4 phase grows because of diffusion of Sn. Atomic arrangement in the crystal indicates that Sn can diffuse through its own sublattice but Pd cannot diffuse unless antisites are present. The negligible diffusion of Pd indicates the absence of Pd antisites. The activation energy value indicates that the contribution from grain boundary diffusion cannot be neglected although experiments were conducted in the homologous temperature range of 0.7–0.79.

Phase transformation process and step growth mechanism of hydroxyapatite whiskers under constant impulsion system

Hydroxyapatite (HAP) whiskers were synthesized using urea as the precipitator by a phase transformation method, and their phase transformation process and growth mechanism were investigated. The results showed that with the decomposition of urea and the corresponding increase of pH value of the reaction system, dicalcium phosphate anhydrous (DCPA) and octacalcium phosphate (OCP) were precipitated at pH of 3.3–4.3; then Ca 2+ and HPO 4 2 − ions began to be released from DCPA at pH values greater than 4.5. Finally HAP whiskers heterogeneously nucleated and grew up into short column crystals along the surface of the OCP flakes. In the absence of the ionic resources, DCPA gradually dissolved and the OCP flakes transformed into HAP continuously and the short columnar HAP whiskers grew up. The aspect ratio of the HAP whiskers with length of 20–100 μm and diameter of 1–2 μm was about 25. The HRTEM and AFM images showed that HAP whiskers grew along the c-axis direction, the (1 0 0) steps were clearly observed at their heads and the straight step lines instead of helical Frank ones were present on the side face of the (1 0 0) steps. The calculation on the basis of the surface energy of the HAP crystal showed that the growth rate of the (0 0 1) plane was the fastest, the growth rate at the homogeneous twist sites was the second and that at heterogeneous twist sites could be the slowest, which were the main factors finally leading to the preferential growth of HAP whiskers along the c-axis direction as well as the formation of the growth steps.

Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum–silicon system

Tracer diffusion coefficients are calculated in different phases in the Mo–Si system from diffusion couple experiments using the data available on thermodynamic parameters. Following, possible atomic diffusion mechanism of the species is discussed based on the crystal structure. Unusual diffusion behaviour is found in the Mo 5Si 3 and Mo 3Si phases, which indicate the nature of defects present on different sublattices. Further the growth mechanism of the phases is discussed and morphological evolution during interdiffusion is explained.

Growth mechanism of phases by interdiffusion and diffusion of species in the niobium–silicon system

The integrated diffusion coefficient of the phases and the tracer diffusion coefficients of the species are determined in the Nb–Si system by the diffusion couple technique. The diffusion rate of Si is found to be faster than that of Nb in both the NbSi2 and Nb5Si3 phases. The possible atomic mechanism of diffusion is discussed based on the crystal structure and on available details of the defect concentration data. The faster diffusion rate of Si in the Nb5Si3 phase is found to be unusual. The growth mechanism of the phases is also discussed on the basis of the data calculated in this study.

Growth mechanism of phases, Kirkendall voids, marker plane position, and indication of the relative mobilities of the species in the interdiffusion zone

Often, wrong conclusions about the mobilities of species are drawn from the position of the Kirkendall marker plane or voids in the interdiffusion zone. To clarify, I have discussed the growth mechanism of the phases and the position of the marker plane depending on the relative mobilities of the species. The formation of different kinds of voids in the interdiffusion zone is discussed. Further, the microstructure that could be found because of the Kirkendall effect is also explained.