Two-dimensional Crystal Growth Research Articles

In Situ Imaging of Two-Dimensional Crystal Growth Using a Heat-Resistant Optical Microscope.

Revealing low-dimensional material growth dynamics is critical for crystal growth engineering. However, in a practical high-temperature growth system, the crystal growth process is a black box because of the lack of heat-resistant imaging tools. Here, we develop a heat-resistant optical microscope and embed it in a chemical vapor deposition (CVD) system to investigate two-dimensional (2D) crystal growth dynamics. This in situ optical imaging CVD system can tolerate temperatures of ≤900 °C with a spatial resolution of ∼1 μm. The growth of monolayer MoS2 crystals was studied as a model for 2D crystal growth. The nucleation and growth process have been imaged. Model analysis and simulation have revealed the growth rate, diffusion coefficient, and spatial distribution of the precursor. More importantly, a new vertex-kink-ledge model has been suggested for monolayer crystal growth. This work provides a new technique for in situ microscopic imaging at high temperatures and fundamental insight into 2D crystal growth.

Non-isothermal crystallization kinetics for CaO–SrO–B2O3–Al2O3–Bi2O3–SiO2 glass system with Ta2O5 addition

The effect of Ta2O5 at the rates of 1, 2, 3 and 4 % mol on the CaO–SrO–B2O3–Al2O3–Bi2O3–SiO2 glass system was examined by the melt-quenching at 1400 °C. Non-isothermal crystallization kinetics of the compounds were calculated using Differential Scanning Calorimetry (DSC). The kinetics study revealed activation energy values of 109.4 kJ/mol for the T1 composition, 302.14 kJ/mol for the T2 composition, 471.5 kJ/mol for the T3 composition, and 344.7 kJ/mol for the T4 composition, as determined by applying the Kissinger method. According to the average of Avrami parameters, two-dimensional crystal growth was observed in T1 glass and surface crystallization was observed in all other glass. The glass forming ability (GFA) of the glasses was investigated with the values obtained as a result of HSM analysis. In the XRD analysis performed after heat treatment at 850 °C, crystal phase development was observed in the glasses. As a result of the crystallization process, it was observed that wollastonite, strontium aluminum silicate, strontium aluminum oxide and bismuth oxide phases were formed in T1, T2 and T3 glasses.

Non-Isothermal Analysis of the Crystallization Kinetics of Amorphous Mg72Zn27Pt1 and Mg72Zn27Ag1 Alloys.

In this study, thin ribbons of amorphous Mg72Zn27Pt1 and Mg72Zn27Ag1 alloys with potential use in biomedicine were analyzed in terms of the crystallization mechanism. Non-isothermal annealing in differential scanning calorimetry (DSC) with five heating rates and X-ray diffraction (XRD) during heating were performed. Characteristic temperatures were determined, and the relative crystalline volume fraction was estimated. The activation energies were calculated using the Kissinger method and the Avrami exponent using the Jeziorny-Avrami model. The addition of platinum and silver shifts the onset of crystallization towards higher temperatures, but Pt has a greater impact. In each case, Eg > Ex > Ep (activation energy of the glass transition, the onset of crystallization, and the peak, respectively), which indicates a greater energy barrier during glass transition than crystallization. The highest activation energy was observed for Mg72Zn27Pt1 due to the difference in the size of the atoms of all alloy components. The crystallization in Mg72Zn27Ag1 occurs faster than in Mg72Zn27Pt1, and the alloy with Pt has higher (temporary) thermal stability. The Avrami exponent (n) values oscillate in the range of 1.7-2.6, which can be interpreted as one- and two-dimensional crystal growth with a constant/decreasing nucleation rate during the process. Moreover, the lower the heating rate, the higher the nucleation rate. The values of n for Mg72Zn27Pt1 indicate a greater number of nuclei and grains than for Mg72Zn27Ag1. The XRD tests indicate the presence of α-Mg and Mg12Zn13 for both Mg72Zn27Pt1 and Mg72Zn27Ag1, but the contribution of the Mg12Zn13 phase is greater for Mg72Zn27Ag1.

Growth of two-dimensional crystals enabled by unusual mass transport

Growth of two-dimensional crystals enabled by unusual mass transport

Polytypic Two-Dimensional FeAs with High Anisotropy.

In the realm of two-dimensional (2D) crystal growth, the chemical composition often determines the thermodynamically favored crystallographic structures. This relationship poses a challenge in synthesizing novel 2D crystals without altering their chemical elements, resulting in the rarity of achieving specific crystallographic symmetries or lattice parameters. We present 2D polymorphic FeAs crystals that completely differ from bulk orthorhombic FeAs (Pnma), differing in the stacking sequence, i.e., polytypes. Preparing polytypic FeAs outlines a strategy for independently controlling each symmetry operator, which includes the mirror plane for 2Q-FeAs (I4/mmm) and the glide plane for 1Q-FeAs (P4/nmm). As such, compared to bulk FeAs, polytypic 2D FeAs shows highly anisotropic properties such as electrical conductivity, Young's modulus, and friction coefficient. This work represents a concept of expanding 2D crystal libraries with a given chemical composition but various crystal symmetries.

Surface-confined two-dimensional mass transport and crystal growth on monolayer materials

Surface-confined two-dimensional mass transport and crystal growth on monolayer materials

First-principles calculations of specular reflection of high-energy electrons during the two-dimensional crystal growth

RHEED_DIFF_2D is an open-source software capable of performing qualitative numerical simulations of changes to the intensity of RHEED oscillations in the function of the layer deposited, employing various models of scattering crystal potential for heteroepitaxial structures of bcc (001) films nucleated on a fcc (001) surface, including the possible existence of various diffuse scattering models through the layer parallel to the surface. This model can be used for interpreting data weakly dependent on the azimuth of the incident electron beam direction and for 2D monolayer-by-monolayer growth modes. The program does not use any external data sources. New version program summaryProgram title: RHEED_DIFF_2DCPC Library link to program files:https://doi.org/10.17632/83ht2kc8xt.1Licensing provisions: GNU General Public License 3Programming language used: C++Journal reference of previous version: Computer Physics Communications 185 (2014) 3001–3009Does the new version supersede the previous version?: No. It is a supplement to the previous version.Reasons for new versionResponding to users' feedback we present a practical procedure of construction of simulation program, which facilitates the calculation of changes to the intensity of RHEED oscillations in the function of the layer deposited for 2D monolayer-by-monolayer growth modes, employing various models of scattering crystal potential for heteroepitaxial structures of bcc (001) Fe thin layers nucleated on the fcc (001) Ag substrate. The forms of these potentials can be freely defined by the user. The proposed approach may be generalized and applied to other metallic bcc/fcc or fcc/bcc interfaces.Nature of problemBulk iron has a bcc structure at room temperature and ferromagnetic below 920 K. It has been noticed that Fe monolayers grown on nonmagnetic substrates show a variety of interesting properties depending on the size of the lattice misfit between the substrate and growing layers, and on the thickness of the deposited material [1,2]. The fundamental scientific problem in such research is to specify both interface type and growth mechanism for subsequent layers. Researchers and technologists manufacturing two-dimensional nanoscale systems frequently use RHEED measurements in the function of the deposited layers to control growth of samples at the atomistic level of accuracy.Experimental data indicate that the iron grows on the Ag(001) surface as a bcc α-Fe film rotated by 450 relative to the underlying lattice [3]. For this reason, the net point spacing in plane (001) of Ag surface is 2.889 Å while that of bulk Fe(001) is 2.8665 Å. This leads to a compressive in-plane misfit (aFe – aAg/√2)/(aAg/√2) ≈-0.78%. This slight mismatch causes that during nucleation and in the early stage of growth, Fe should grow in 2D monolayer-by-monolayer mode on the Ag(001) substrate.Solution methodRHEED intensities are calculated within the general framework described in Ref. [4] with the model of the scattering potential for heterostructures:(1)UCombined(z)=∑nUsubstrate(z)+∑n(Ulayer(z)+Uaddlayer(z)),where the Uaddlaer represents the diffuse scattering on the topmost layer.Summary of revisionsThe presented version RHEED_DIFF_2D of the computer program implements an iterative algorithm for calculating the intensity of the specularly reflected electron beam as a function of growing layers. The pseudocode listed below illustrates the main assumptions of this algorithm.for all growing layers for all coverages of the overlayer for all elements of the scattering matrix Calculating the elements of the scattering matrix for the selected model of the scattering potential of the overlayer end for Calculating the amplitude of the specularly reflected electron beam end forend for

Stamped production of single-crystal hexagonal boron nitride monolayers on various insulating substrates

Controllable growth of two-dimensional (2D) single crystals on insulating substrates is the ultimate pursuit for realizing high-end applications in electronics and optoelectronics. However, for the most typical 2D insulator, hexagonal boron nitride (hBN), the production of a single-crystal monolayer on insulating substrates remains challenging. Here, we propose a methodology to realize the facile production of inch-sized single-crystal hBN monolayers on various insulating substrates by an atomic-scale stamp-like technique. The single-crystal Cu foils grown with hBN films can stick tightly (within 0.35 nm) to the insulating substrate at sub-melting temperature of Cu and extrude the hBN grown on the metallic surface onto the insulating substrate. Single-crystal hBN films can then be obtained by removing the Cu foil similar to the stamp process, regardless of the type or crystallinity of the insulating substrates. Our work will likely promote the manufacturing process of fully single-crystal 2D material-based devices and their applications.

A kinetic approach for valorisation of hydrodesulphurisation catalyst residue into mullite

Mullite (3Al2O3·2SiO2) is a silicoaluminate widely used for the production of ceramics, and it is usually synthesised from different raw materials with solid-state reactions. In this work, petrochemical industry catalyst waste (Ni–Mo waste) composed principally of alumina and silica has been envisaged as a suitable precursor to prepare mullite via thermal treatment. The kinetics of mullite crystallisation from the catalyst waste were studied with differential thermal analysis (DTA). The starting Ni–Mo waste was an amorphous material, and the crystallisation process takes place at temperatures between 1200 and 1300 °C. The activation energies calculated with isothermal and nonisothermal methods were 1059 and 743 kJ mol-1, respectively. The Avrami exponent (n) obtained with the Ligero method and the parameter m calculated with the Matusita method indicated two-dimensional crystal growth, probably through a diffusion-controlled bulk crystallisation mechanism from a constant number of nuclei. The frequency factor calculated by the isothermal treatment was 8 x 1033 s-1.

A simple strategy for synthesis of b-axis-oriented MFI zeolite macro-nanosheets

Zeolite nanosheets with shortened diffusion paths and high external surface areas have drawn significant attention. This unique morphology of zeolites can endow them with reduced diffusion resistance and high accessibility of active sites, and thereby good catalytic performance. Exploring simple synthesis strategies for zeolite nanosheets is mostly desired in zeolite chemistry. Herein, MFI zeolite nanosheets with a thickness of 50-100 nm along the b -axis and a length of 1 μm along the c -axis, namely zeolite macro-nanosheets, have been successfully synthesized via the strategy of combining pre-aging and pH regulation without the use of complex templates/additives and fluorine media. Pre-aging at a relatively high temperature of 90 °C and low alkalinity for crystallization (pH = 7.0~8.5) are found to play key roles in controlling macro-nanosheet synthesis, which can promote the formation of MFI zeolite precursors and then dissociate them into smaller ones for the following oriented aggregation during hydrothermal treatment. Crystallization kinetic studies suggest that the two-dimensional crystal growth is the intrinsic behavior of zeolite crystallization under employed conditions. This synthesis strategy can be easily extended to the synthesis of heteroatom-containing MFI macro-nanosheets, such as ZSM-5 and TS-1.

A General Analytical Solution for Two-Dimensional Columnar Crystal Growth during Laser Beam Welding of Thin Steel Sheets

A technique for calculating the main solidification parameters for a two-dimensional columnar crystal growth during complete penetration laser beam welding of thin steel sheets was developed. Given that the weld pool interface is described by Lamé curves (superellipses) within the horizontal plane of growth, general analytical solutions were derived for the geometry of the crystal axis and the corresponding growth rate and cross-sectional area of the crystal. A dimensionless analysis was performed to provide insights on the dependence of the solidification parameters on the shape and dimensions of the rear part of the weld pool boundary. The derived solutions were applied for the case of complete penetration laser beam keyhole welding of 2 mm thick 316L austenitic chromium-nickel steel sheets. It was shown that the reconstruction of the weld pool boundary with Lamé curves provides higher accuracy and flexibility compared to results obtained with elliptical functions. The validity of the proposed technique and the derived analytical solutions was backed up by a comparison of the obtained solutions to known analytical solutions and experimentally determined shapes and sizes of the crystals on the top surface of the sheet. The dimensions of the calculated crystal axis correlated well with the experimentally obtained results.

Scaling laws for two-dimensional dendritic crystal growth in a narrow channel.

We investigate analytically and computationally the dynamics of two-dimensional needle crystal growth from the melt in a narrow channel. Our analytical theory predicts that, in the low supersaturation limit, the growth velocity V decreases in time t as a power law V∼t^{-2/3}, which we validate by phase-field and dendritic-needle-network simulations. Simulations further reveal that, above a critical channel width Λ≈5l_{D}, where l_{D} is the diffusion length, needle crystals grow with a constant V<V_{s}, where V_{s} is the free-growth needle crystal velocity, and approaches V_{s} in the limit Λ≫l_{D}.

Utilization of gasification slag and petrochemical incineration fly ash for glass ceramic production.

This study investigated glass ceramics produced using coal gasification slag (CGS) and petrochemical incineration fly ash (PIFA) to immobilize hazardous heavy metals such as Cr and As. However, the crystallization kinetics and stabilization behavior mechanism of different heavy metals in the petrochemical incineration fly ash-derived glass-ceramics remains unclear. And X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and inductively coupled plasma mass spectrometry were used to characterize glass and crystalline products. In this paper, we reported the crystallization kinetics and chemical leaching characteristics of the glass ceramic. A low crystallization activation energy of 121.49kJ/mol was achieved from crystallization peak of several different heating rates around 850°C, implying that it is easier to produce the glass ceramics at that temperature. The Avrami parameter of the former crystallization was determined to be 1.23 ± .12, which indicated two-dimensional crystal growth with heterogeneous nucleation. The toxicity characteristic leaching procedure results indicated that the heavy metals were well solidified, and that the leaching concentration was significantly lower than the limit specified by governmental agencies. The potentially toxic element index of the parent glass and the two glass ceramics were 11.7, 5.8, and 3.6, respectively. Therefore, the conversion of hazardous petrochemical incineration fly ash and other solid waste into environmentally friendly glass ceramics shows considerable potential and reliability.

Atomic scale insights into the epitaxial growth mechanism of 2D Cr3Te4 on mica.

Two-dimensional (2D) magnetic materials are of wide research interest owing to their promising applications in spintronic devices. Among them, chromium chalcogenide compounds are some of the limited available systems that present both high stability in air and high Curie temperatures. Epitaxial growth techniques based on chemical vapour deposition (CVD) have been demonstrated to be a robust method for growing 2D non-layered chromium chalcogenides. However, the growth mechanism is not well-understood. Here, we demonstrate the epitaxial growth of Cr3Te4 nanoplates with high quality on mica. Atomic-resolution scanning transmission electron microscopy (STEM) imaging reveals that the epitaxial growth is based on nanosized chromium oxide seed particles at the interface of Cr3Te4 and mica. The chromium oxide nanoparticle exhibits a coherent interface with both mica and Cr3Te4 with a lattice mismatch within 3%, suggesting that, as a buffer layer, chromium oxide can release the interfacial strain, and induce the growth of Cr3Te4 although there is a distinct oxygen-content difference between mica and Cr3Te4. This work provides an experimental understanding behind the epitaxial growth of 2D magnetic materials at the atomic scale and facilitates the improvement of their growth procedures for devices with high crystalline quality.

Nonisothermal cellular automata simulation of two-dimensional snow crystal growth.

Natural snowflakes exhibit remarkable diversity with a fascinating sixfold symmetry and fractal structure. Efforts to discover the snow crystal growth pattern and mechanism behind it have been made in the past 50 years. Among those, cellular automata (CA) have made certain progress. To investigate the influence of latent heat release in snow crystal growth and have a deeper understanding of crystal growth characteristics, an improved CA model combined with vapor diffusion and heat diffusion is proposed to investigate the snowflake growth process in a two-dimensional and nonisothermal field in this paper. Simulation results show that obtained snow crystal growth behavior is consistent with previous experimental observations and satisfies the crystal growth dynamic theory. Latent heat release impacts the small structure of snow crystals, and it could have a more significant influence on snowflake morphology when vapor diffusion is slow and heat diffusion is dominating.

Effect of metastable phase on the crystallization and mechanical properties of MgO-Al2O3-SiO2 glass-ceramics without nucleating agents

Glass-ceramics without nucleating agents usually undergo surface crystallization, which deteriorates the overall performance of the products. In this paper, we evaluated the effects of the metastable MgAl2Si3O10 crystalline phase on the crystallization behavior of a MgO–Al2O3–SiO2 (MAS) glass without nucleating agents and mechanical properties of the glass-ceramics obtained. The results demonstrated that the precipitation of metastable MgAl2Si3O10 crystallites promotes the crystallization mechanism transformed from surface crystallization into volume crystallization with two-dimensional crystal growth. Furthermore, the grain size of MgAl2Si3O10 near the surface of the prepared glass-ceramics was larger than that of MgAl2Si3O10 inside, which helps to generate compressive stress and improves its mechanical properties. The glass-ceramics containing metastable MgAl2Si3O10 phase exhibited an enhanced hardness in the range of 7.6 GPa–9.5 GPa for indentation loads ranging from 2.94 N to 98 N, and indentation size effect behavior was observed in Vickers hardness tests of both MAS glass and glass-ceramics. The load-independent hardness values for MAS glass and glass-ceramics were reliably evaluated by the modified proportional specimen resistance (MPSR) model of 7.1 GPa and 7.6 GPa, respectively, with a high correlation coefficient of more than 0.9999. This work reveals the unexploited potential of the metastable phase in improving the crystallization ability and mechanical properties of glass-ceramics.

Explicit temperature coupling in phase-field crystal models of solidification

We present a phase-field crystal model for solidification that accounts for thermal transport and a temperature-dependent lattice parameter. Elasticity effects are characterized through the continuous elastic field computed from the microscopic density field. We showcase the model capabilities via selected numerical investigations which focus on the prototypical growth of two-dimensional crystals from the melt, resulting in faceted shapes and dendrites. This work sets the grounds for a comprehensive mesoscale model of solidification including thermal expansion.

Epitaxial growth of wafer-scale single-crystal transition metal dichalcogenide monolayers for future electronics

The growth of wafer-scale two-dimensional (2D) single crystals, especially transition metal dichalcogenides (TMDs), is significantly essential for various electronic applications. However, the growth of wafer-scale single-crystal TMD (sc-TMD) monolayers on commercial substrates still remains challenging. Recently, two works published in <i>Nature Nanotechnology</i> have independently reported a substrate-cutting strategy for epitaxial growth of wafer-scale sc-TMD monolayers, which might open up a new route for preparing wafer-scale sc-TMD monolayers on commercially scalable insulating substrates for future electronics.

Progress and perspective on the growth of two-dimensional single crystals

Progress and perspective on the growth of two-dimensional single crystals

Controlled growth of two-dimensional InAs single crystals via van der Waals epitaxy

Controlled growth of two-dimensional InAs single crystals via van der Waals epitaxy