Terrace Nucleation Research Articles

Substrate Screening for the Epitaxial Growth of a Single-Crystal Graphene Wafer.

Epitaxial growth of a two-dimensional (2D) single crystal necessitates the symmetry group of the substrate being a subgroup of that of the 2D material. As a consequence of the theory of 2D material epitaxy, high-index surfaces, which own very low symmetry, have been successfully used to grow various 2D single crystals, while the rule of selecting the best substrates for 2D single crystal growth is still absent. Here, extensive density functional theory calculations were conducted to investigate the growth of graphene on abundant high-index Cu substrates. Although step edges are commonly regarded as the most active sites for graphene nucleation, our study reveals that, in some cases, graphene nucleation on terraces is superior than that near a step edge. To achieve parallel alignments of graphene islands, it is essential to either suppress terrace nucleation or ensure consistent orientations templated by both the terrace and step edge. In agreement with most experimental observations, we show that Cu substrates for the growth of single-crystalline graphene include vicinal Cu(111) surfaces, vicinal Cu(110) surfaces with Miller indices of (nn1) (n > 3), and vicinal Cu(100) surfaces with Miller indices of (n11) (n > 3).

Self-seeded growth of very large open-structured zeolite nanosheet assemblies with extraordinary micropore accessibility

Zeolite nanosheets (ZNs) offer improved micropore accessibilities and transport properties for enhanced molecular catalysis and separations. However, practical application of the ZN materials is hampered by the lack of efficient synthesis methods. Here, a ZN self-seeded method is demonstrated for single-step reproduction of flower-like assemblies of very large MFI ZN plates. The ZN plates are ∼60 nm-thick stacks of 4-nm-thick single-crystal sheets. The ZN flower growth involves terrace nucleation on the seed surfaces and subsequent ZN epitaxial growth in [010] orientation directed by a diquaternary agent. The open architecture of the assemblies prevented collapse and agglomeration of ZNs during thermal activation that effectively preserved the interconnected intra-sheet and inter-sheet micropore system. Thus, the ZN assemblies exhibited markedly enhanced molecular adsorption capacity and transport diffusivity for the probing xylene molecules as compared to the conventional crystals. The ZN assembly and its harvestable very large-sized ZNs have the potential for developing high-performance ZN adsorbents, catalysts, and molecular-sieve membranes. A facile self-seeded growth method reproduces zeolite nanosheet assemblies allowing nondestructive activation for high-performance adsorbents, catalysts, and molecular sieve membranes. • Established a new self-seeded method for reproducing MFI zeolite nanosheet (ZN) assemblies. • Revealed mechanisms of orthogonal ZN growth by defect-induced nucleation and epitaxial growth directed by diquaternary agent. • Demonstrated nondestructive activation of ZNs in the flower-like structure to preserve extraordinary micropore accessibility. • Characterized the interconnected multiscale micropore system that provides exceptional adsorption and transport properties.

Shear-driven motion of Mg {101¯2} twin boundaries via disconnection terrace nucleation, growth, and coalescence

Twin nucleation and growth are prevalent plastic deformation mechanisms in hexagonal close-packed metals such as Mg. For twin growth to occur, the interfaces that border the twinned domain must migrate and the kinetics associated with this process are yet to be fully explained. Thus, the objective of this study is to characterize the relationship between the kinetics of the ${10\overline{1}2}$ twin boundary in pure Mg in the stress-driven regime, and the nucleation, growth, and coalescence of disconnection terraces that serve as the mechanisms for migration. This problem is addressed via atomistic simulations adopting both two- (2D) and three-dimensional (3D) simulation geometries, and a model for the velocity of the ${10\overline{1}2}$ twin boundary as a function of temperature and shear stress is proposed. This study shows that the kinetics of ${10\overline{1}2}$ twin boundary migration must be addressed using 3D models, as 2D simulations do not properly capture disconnection terrace nucleation and growth processes, demonstrated via differences in activation volumes and energies. Importantly, simulations reveal an autocatalytic terrace nucleation mechanism as playing a role in twin growth, where nucleation of a new terrace is dependent on the growth of existing terraces on the twin plane.

Kinetic Growth Regimes of Hydrothermally Synthesized Potassium Tantalate Nanoparticles.

A general mathematical kinetic growth model is proposed on the basis of observed growth regimes of hydrothermally synthesized KTaO3 nanoparticles from electron microscopy studies on the surface morphology and surface chemistry. Secondary electron imaging demonstrated that there are two dominant growth mechanisms: terrace nucleation, where the surfaces are rough, and terrace growth, where surfaces are smooth. In the proposed model based upon standard step-flow growth, the rates of both mechanisms are established to be dependent on the chemical potential change of the growth environment-terrace nucleation dominates with larger negative chemical potential, and terrace growth dominates with smaller negative chemical potential. This analysis illustrates the importance of ending a synthesis in a regime of low negative chemical potential in order to achieve smooth well-faceted nanoparticles.

Structure and growth of Bi(110) islands on Si(111) 3×3−B substrates

The structure and growth of ultrathin Bi(110) islands were investigated on a Si(111)$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{\ensuremath{-}}B$ substrate by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Both even- and odd-layer-height islands nucleated on a one-monolayer-thick wetting layer. The islands preferred the even layer heights over the odd layer heights with an area ratio of 3:1. A weak, long-range corrugation was observed to overlap on the atomic arrangement at the top of the islands. The average distance between the peaks of the corrugation oscillated in accordance with the alternation of even and odd layer heights. Nucleation of single- and double-layer terraces occurred on the islands with even layer heights but not on those with odd layer heights. The unit cell of the single-layer terrace was aligned with that of the underlying even-layer-height island. The inequality in the height preference and the height-dependent oscillation of the corrugation suggested that the even- and odd-layer-height islands possessed different structures. The dominance and stability against terrace nucleation of the even-layer-height islands were consistent with the theoretically predicted stability of the paired layer-stacked black-phosphorus (BP)-like structure for ultrathin Bi(110) films. The alignment of the unit cell at the terrace on the island and STS spectra suggested a BP-like/bulklike/BP-like sandwich structure for the odd-layer-height Bi(110) islands.

The effects of annealing and growth temperature on the morphologies of Bi nanostructures on HOPG

We report on the growth of ultra-thin bismuth (Bi) films on the basal plane of highly ordered pyrolitic graphite (HOPG) substrates; we investigate the morphologies of films grown at room temperature and then annealed at high temperature, and the morphologies of Bi structures grown at high temperature. Films grown at room temperature nucleate flat islands on the HOPG terraces, and 1D nanorods from HOPG step edges, the islands and rods both have heights in the range 1–3 nm. During annealing, the flat islands break up into groups of aligned, ∼ 2.5 nm tall rods. For films grown at high temperature, terrace nucleation is almost nonexistent, and 1D structures grow from step edges, with heights up to 30 nm. Finally, we observe rods with distinctively different morphologies corresponding to (110) and (111) orientations, and infer a surface energy driven Bi(110) to Bi(111) orientation transition. We speculate that the dominant mechanism for the reorientation is coalescence.

Growth of atomically step-free surface on diamond {111} mesas

Diamond homoepitaxy by microwave plasma-enhanced chemical vapor deposition was investigated on {111} substrate. Growth at a low CH 4/H 2 ratio of 0.025% in a gas phase results in the formation of an atomically step-free surface over 10 × 10-µm 2 mesas of diamond {111} substrate, when there are no screw dislocations in the mesas. This was achieved through ideal lateral growth, in which two-dimensional terrace nucleation was completely suppressed. The application of the selective formation of the step-free surface and the lateral growth of diamond films will open the way for the realization of high-quality electronic devices using diamond.

Atomic Force Microscope Observation of Growth and Defects on As-Grown (111) 3C-SiC Mesa Surfaces

AbstractThis paper presents experimental atomic force microscope (AFM) observations of the surface morphology of as-grown (111) silicon-face 3C-SiC mesa heterofilms. Wide variations in 3C surface step structure are observed as a function of film growth conditions and film defect content. The vast majority of as-grown 3C-SiC surfaces consisted of trains of single bilayer height (0.25 nm) steps. Macrostep formation (i.e., step-bunching) was rarely observed, and then only on mesa heterofilms with extended crystal defects. As supersaturation is lowered by decreasing precursor concentration, terrace nucleation on the top (111) surface becomes suppressed, sometimes enabling the formation of thin 3C-SiC film surfaces completely free of steps. For thicker films, propagation of steps inward from mesa edges is sometimes observed, suggesting that enlarging 3C mesa sidewall facets begin to play an increasingly important role in film growth. The AFM observation of stacking faults (SF's) and 0.25 nm Burgers vector screw component growth spirals on the as-grown surface of defective 3C films is reported.

Silicalite Crystal Growth Investigated by Atomic Force Microscopy

Atomic force microscopy has been used to image the various facets of two morphologically distinct samples of silicalite. The smaller (20 microm) sample A crystals show 1 nm high radial growth terraces. The larger (240 microm) sample B crystals show growth terraces 1 to 2 orders of magnitude higher than the terraces on sample A with growth edges parallel to the crystallographic axes. Moreover, the terraces on the (010) face are significantly higher than the terraces on the (100) face - inconsistent with the previously proposed 90 degrees intergrowth structure. Sample A highlights that under certain synthetic conditions, silicalite grows in a manner akin to zeolites Y and A, via the deposition of layers comprising, in the case of silicalite, pentasil chains. It is probable that the rate of terrace advance is identical on the (010) and (100) faces, and it is the rate of terrace nucleation that dictates the overall growth rate of each facet and hence the relative size expressed in the final crystal morphology. Analysis of the growth terraces of sample B and detailed consideration of the structures of both MFI, and a closely related material MEL, lead to the proposal of a generalized growth mechanism for silicalite including the incorporation of defects within the structure. These defects are thought to be responsible for both the relative and the absolute terrace heights observed and may also explain the hourglass phenomenon observed by optical microscopy. The implications of this growth mechanism, supported by results of infrared microscopy, generate a new dimension to the continuing debate on the existence of intergrowths within one of the most important structures relevant to zeolite catalysis.

Multilayer cooperative sequential adsorption

Cooperative sequential adsorption is here extended to multilayer coverages. We discuss two different growth rules with cooperativity either restricted to only the first layer of coverage or applied in all layers. The unrestricted variant is considered in the case where lateral growth dominates over the nucleation of terraces. The limit of completely suppressed nucleation corresponds to a morphological transition to a flat interface from one governed by the Kardar–Parisi–Zhang equation. With the restricted growth rule we find interesting behavior resulting from a competition between lateral growth at the first layer and growth on the top of nucleated islands. There is an intermediate regime between random deposition at the submonolayer coverage and asymptotic random deposition behavior. In this regime the kinetic roughening can be studied by applying sequential adsorption rate equations for cluster lengths in the first layer, with an additional geometric argument.

Diffusion controlled growth of metallic nanoclusters at selected surface sites

We have investigated the growth of three-dimensional Ag particles at atomic steps on the surface of highly oriented pyrolytic graphite using a scanning electron microscope. By controlling the growth parameters the cluster growth was confined to the steps avoiding terrace nucleation. In this way quasi-one-dimensional chains of Ag nanoclusters of approximately 10 nm diam were produced. The results suggest the viability of an important new route to the creation of controlled nanoscale structures. A comprehensive surface study indicates that cluster mobility and coalescence play an important role in the growth mechanism on the steps. Evidence was also found that the graphite surface has several different types of surface steps. A quantitative analysis of the cluster distribution on the steps was performed, to investigate the nucleation and growth processes at temperatures from 50 to 205 °C.

Role of step and terrace nucleation in heteroepitaxial growth morphology: growth kinetics of CaF2/Si(111).

The thickness uniformity and the spatial distribution of lattice relaxation in thin ( $<$8 nm) Ca${\mathrm{F}}_{2}$/Si(111) films, observed with photoelectron spectroscopy and transmission electron microscopy, are seen to depend strongly on the initial nucleation kinetics. We develop a general model for heteroepitaxial growth that explains both these and literature results. Terrace or step nucleation leads to laminar films, although with different relaxation patterns; combined step and terrace nucleation leads to rough films due to different upper-layer nucleation rates on the differently sized islands.

Growth kinetics of CaF2/Si(111) heteroepitaxy: An x-ray photoelectron diffraction study.

Kinetic variations of the initial stages of CaF[sub 2] growth on Si(111) by molecular-beam epitaxy are studied with the [ital in] [ital situ] combination of x-ray photoelectron spectroscopy and diffraction. After the formation of a chemically reacted interface layer, the morphology of the subsequent bulk layers is found to be dependent on the substrate temperature and incident flux rate, as well as the initial interface structure. For substrate temperatures above [similar to]600 [degree]C, subsequent layers do not easily wet the interface layer, and a transition is observed from a three-dimensional island formation at low flux to a laminar growth following the coalescence of bilayer islands at higher flux. At lower substrate temperatures ([similar to]450 [degree]C), a different stoichiometry and structure of the interface layer leads to laminar growth at all fluxes, but with a different bulk nucleation behavior. Crystalline heteroepitaxy is not observed when growth initiates at room temperature, but homoepitaxy does proceed at room temperature if the first few layers are deposited at a high temperature. The different growth regimes are discussed in terms of a kinetic model separating step and terrace nucleation where, contrary to homoepitaxy, step nucleation leads to islanded growth.

Crystalline surfaces in helium

The particular properties of helium at low temperature recently allowed important progress in the general understanding of the structure and dynamics of crystal surfaces. Excellent agreement has been found between experimental results in helium 4 and the renormalization group theories of the roughening transition, which predict that it is an infinite order (Kosterlitz-Thouless) phase transition. The dynamic roughening of a crystal surface, i.e. the influence of a finite growth rate, has been particularly clarified. The dominant growth process of smooth faces close to their roughening transition has been shown to be the two-dimensional nucleation of terraces. In both helium 3 and helium 4, it also appeared possible to measure and analyze the three interface coefficients which describe the dissipation at rough crystal surfaces is non-equilibrium situations. The latter studies provide interesting information on the elementary processes (heat and mass flow) which occur in quantum liquids during their crystallization. A general review of all these phenomena is given.

The dynamic roughening of crystals

When a crystal grows at a non-zero velocity, its roughening temperature is lower than at equilibrium. A criterion for such a dynamic roughening is β 2/( aϱ SΔμ k B T)≈ r c /ξ≤1 where β is the step free energy, a the step height, ϱ S the crystal density, Δμ the difference in chemical potential (per unit mass) between the solid and the fluid phase, r c the critical radius for the nucleation of terraces and ξ the correlation length. The physical origin of this property is that the large scale fluctuations of the crystal surface see a lattice potential which is averaged to zero in time by the growth. From the point of view of the renormalization calculation, the dynamic roughening is a finite size effect which broadens the roughening transition. A good agreement has been found with measurements of the growth rate of helium 4 crystals. The case of helium 3 crystals shows that the disappearance of facets on slowly growing crystals may happen well below the true static roughening temperature if the coupling of the interface to the lattice is weak enough.

The Dynamic Roughness of Crystals

When a crystal grows at a non-zero velocity, its roughening temperature is lower than at equilibrium. A criterion for such a dynamic roughening is β2/(aϱsΔμkBT) ≈ rc/ξ ⩽ 1 where β is the step free energy, a the step height, ϱs the crystal density, Δμ the difference in chemical potential (per unit mass) between the solid and the fluid phase, rc the critical radius for the nucleation of terraces and ξ the correlation length. The physical origin of this property is that the large scale fluctuations of the crystal surface sees a lattice potential which is averaged to zero in time by the growth. From the point of view of the renormalization calculation, the dynamic roughening is a finite size effect which broadens the roughening transition. A good agreement has been found with measurements of the growth rate of helium 4 crystals. The case of helium 3 crystals shows that the disappearance of facets on slowly growing crystals may happen well below the true static roughening temperature if the coupling of the interface to the lattice is weak enough.

The roughening transition of a growing 4He crystal

We present a careful study of the growth of hexagonal close-packed 4He crystals near the roughening transition of their (0001) surfaces, at TR = 1.28 K. We confirm that the dominant growth mechanism in the smooth state (T < TR) is the two-dimensional nucleation of terraces of atomic height. Moreover, we discovered a new regime of growth, which takes place slightly below TR, for a surface state intermediate between smooth and rough. It exists in a temperature domain whose width increases with the applied driving force. All our experimental results are consistent with a new calculation based on the critical theory of roughening, which predicts a completely continuous or infinite-order transition. Precise values are deduced for the roughening temperature (TR = 1.28 ± 0.01 K) and the relative amplitude of the lattice potential (tc = 0.63 ± 0.13).