Nucleation Of Islands Research Articles

2D island nucleation controlled by nanocluster diffusion during Si and Ge epitaxy on Si(1 1 1)-(7 × 7) surface at elevated temperatures

We have studied two-dimensional (2D) island nucleation at the initial stages of Si and Ge epitaxial growth near monatomic steps and on wide (up to 10 μm) terraces on the Si(1 1 1)-(7 × 7) surface in 500–750 °C interval by in situ ultrahigh vacuum reflection electron microscopy (UHV REM) and ex situ atomic force microscopy (AFM). Concentration of 2D islands nucleated on the terraces and depletion zone width near the steps have been measured as functions of deposition rate and substrate temperature. We have shown that GeSi and Si nanoclusters are dominating diffusion species providing surface mass transport on the Si(1 1 1)-(7 × 7) surface during Ge and Si epitaxial growth at temperatures above 500 °C and 600 °C, respectively; the activation energy of GeSi nanocluster diffusion has been first estimated at 1.3–1.4 eV. The kinetics of 2D island nucleation and growth near the steps (both for Si and Ge epitaxy) and on the wide terraces (for Ge epitaxy) is limited by surface diffusion only, and critical nucleus consists of i ≫ 1 nanoclusters.

Iodine Adsorption on Ni(110): 2D-Phase Transitions and NiI2 Growth

Scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy, and density functional theory calculations have been used to investigate I2 adsorption on the Ni(110) surface. Dissociative adsorption takes place rapidly to form a c(2 × 2) layer at an iodine coverage of 0.5 ML. DFT calculations show that hollow sites are energetically preferable for iodine adsorption on the Ni(110) surface. Increase of iodine coverage above 0.5 ML leads to the commensurate–incommensurate phase transition via nucleation of loops of domain walls and to their subsequent ordering into the striped superstructure. At saturation, a striped domain-wall phase degenerates into the uniaxially compressed quasi-hexagonal phase. Further iodine dosing leads to the nucleation and growth of 2D nickel iodide islands.

Growth and Dielectric Properties of Monoclinic Gd2O3 on Si(001)

Gadolinium oxide (Gd2O3) was grown at low temperatures (250 °C) on Si(001) using molecular beam epitaxy. The crystal structure was investigated with X-ray diffraction experiments, where the crystal structure changes from cubic to monoclinic depending on the oxygen supply. We propose that Gibbs-Thomson effect is responsible for the change in crystal structure due to the initial nucleation of nanometer-sized islands and its variation in diameter depending on the growth conditions. The dielectric properties of the monoclinic structure were investigated using Capacitance-Voltage measurements, where platinum was used as contact material. Dielectric constants larger than 20 were obtained, which is higher than for the cubic structure. The monoclinic Gd2O3 layers suffer from flat-band voltage instabilities, which is most likely due to defects located at the interface or near-interface region of the Gd2O3 and silicon. Forming gas annealing results in an improvement of the dielectric behaviour.

High quality 4H-SiC homo-epitaxial wafer using the optimal C/Si ratio

Epitaxial growth of 4H-SiC on 4° off-axis substrates has been performed under different C/Si ratio in order to improve the quality level of epitaxial layer. 4H-SiC homoepitaxial layer is grown in horizontal warm-wall planetary reactor. The growth rate, doping concentration, morphology defect of epitaxial layer are characterized using the Fourier transform infrared, Hg schottky contact and optical surface analyzer, respectively. Meanwhile, the epitaxial layer is etched by molten KOH and microscopy observation to understand conversion of BPDs dependence of C/Si ratio. It is suggested that the defect density in the epitaxial layer is increased under C-rich and Si-rich condition, since the increasing of 2D island nucleation and formation of silicon drops, respectively. Moreover, this low conversion ratio is considered to be induced by suppression lateral growth under the low C/Si ratio condition. As a result, epitaxial layer with high yield (99.7%) and extremely low basal plane dislocations density (＜0.05 cm−2) can be obtained by the optimization of C/Si ratio.

Etching of step-bunched Si(1 1 1) surface by Se molecular beam observed by in situ REM

Using in situ ultrahigh vacuum reflection electron microscopy, we have first visualized the evolution of step-bunched Si(1 1 1) surface during Se beam etching depending on substrate temperature T and terrace size required for survival of etching-induced vacancies. Below T ~ 630 °C, the etching proceeds in layer-by-layer mode via periodic nucleation of small 2D vacancy islands without noticeable motion of steps. However, in 630–830 °C interval, the etching of the 7 × 7 reconstructed Si(1 1 1) step-bunched surface is realized by both step flow and continuous nucleation and growth of 2D vacancy islands on wide terraces, which leads to the excavation of initially flat surface between step bunches. At T > 830 °C, the etching proceeds by step flow with preservation of initial surface morphology. We have found that the Si(1 1 1) surface etching by Se beam is characterized by 2.65 eV activation energy that limits etching kinetics in low-temperature range when the whole Si(1 1 1) surface is converted to disordered impurity-induced Si(1 1 1)“1 × 1”-Se phase.

CO Gas Induced Phase Separation in PtPb@Pt Catalyst and Formation of Ultrathin Pb Nanosheets Probed by In Situ Transmission Electron Microscopy

PtPb@Pt catalysts are very useful and widely applied in various industrial reactions. Here, the phase stability of such catalysts is compared in both CO gas and vacuum conditions at elevated temperatures using aberration-corrected in situ transmission electron microscopy (TEM). A Pt aggregation process takes place affected by CO gas, which results in direct exposure of the PtPb core to CO. A phase separation process, in which Pb atoms are stripped off the original PtPb@Pt nanoparticles, is unambiguously identified in CO gas. At initial stages, the as nucleated Pb islands are amorphous. Once the ultrathin Pb islands reach ≈3.5 nm or higher, they suddenly became crystalline. The interaction between Pb and CO gas stabilizes the ultrathin Pb nanosheets, resulting in the formation of a large quantity of Pb nanosheets and Pb-depleted PtPb0.08 nanoparticles. In sharp contrast, when heated up in a vacuum, the PtPb@Pt catalyst remains intact. The results of this study shine light on the "toxic" effect of CO that results in failures of many Pt-based catalysts and discloses formation mechanism of ultrathin Pb nanosheets.

"Dual-Template"-Directed Synthesis of Bowl-Shaped Mesoporous Platinum Nanostructures.

Asymmetric bowl-shaped metals make up an important class of nanostructured materials that exhibit great potentials in catalysis, energy storage, and biomedical applications. Introducing the mesopores within the framework of bowl-shaped metals would further increase the surface area and thus increase the utilization efficiency of metals and the availability of the (electro)catalytically active sites. In this work, a one-pot surfactant-templated aqueous synthesis is developed to fabricate nanosized asymmetric platinum (Pt) bowl-shaped mesoporous nanospheres (BMSs) withgood purity and uniformity. Amphiphilic dioctadecyldimethylammonium chloride acts as the "dual-template" surfactant that drives the anisotropic nucleation and growth of mesoporous Pt islands with cylinder micelles along the curved surface of the vesicles, resulting in the formation of the Pt BMSs with a bowl-shaped morphology and mesoporous structure. The diameter of the Pt BMSs can also be tailored by changing the reduction kinetics during the synthesis. More interestingly, the BMSs are able to interconnect into a more sophisticated structure of one-dimensional nanochains by increasing the K2PtCl4 amount added. This novel synthetic protocol brings a higher hierarchy into asymmetric bowl-shaped metals, rendering the metal BMSs with more accessible (electro)catalytically active sites. Because of the unique morphology and structure, the Pt BMSs show enhanced electrocatalytic activity and stability toward the hydrogen evolution reaction with respect to a commercial Pt/C catalyst.

Characterising submonolayer deposition via visibility graphs

We use visibility graphs as a tool to analyse the results of kinetic Monte Carlo (kMC) simulations of submonolayer deposition in a one-dimensional point island model. We introduce an efficient algorithm for the computation of the visibility graph resulting from a kMC simulation and show that from the properties of the visibility graph one can determine the critical island size, thus demonstrating that the visibility graph approach, which implicitly combines size and spatial data, can provide insights into island nucleation and growth processes.

Non-classical nucleation and collective diffusion in epitaxially grown ultrathin films

Nucleation phenomena are very important in nature because they control the formation of novel phases which can have technologically important properties and applications. LEEM experiments in real time are described for island nucleation in Pb/Si(1 1 1), that show unusually fast, defect-free large Pb islands. The experiments confirm that this growth is due to the collective motion of the wetting layer and provide real time information about the rate of mass transport. Review of other experimental systems where similar unusually fast nucleation processes are present can lead to a better understanding of these intriguing processes, so predictability of the growth conditions (temperature, flux rate, coverage) can be attained.

Electromigration Effect on Vacancy Islands Nucleation on Si(100) Surface during Sublimation

By means of in situ ultrahigh vacuum reflection electron microscopy, the nucleation of vacancy islands on wide terraces of the Si(100) surface is investigated. The temperature dependence of the displacement of a vacancy island nucleation center is determined in the process of heating a sample with a dc electric current. On the basis of a theoretical model, the effective electric charge of addimers is estimated in the direction across dimer rows of the surface. The effective charge has a positive sign and does not exceed 15 units of the elementary charge in the temperature range of 1020–1120°C.

On the performance of vertical MoS2 nanoflakes as a gas sensor

Despite their potential applications, a limited number of studies for synthesizing vertical MoS2 nanoflakes especially via CVD have been reported so far, which generally involve tedious complex- and/or multi-step growth processes. In this study, direct synthesis of vertical MoS2 nanoflakes grown on the SiO2/Si substrate during a rapid sulfidation process by CVD method has been reported. Material characterization was performed using Raman spectroscopy, XRD and FE-SEM. The XRD results indicated the dominant phase of 2H–MoS2 within the synthesized layers. The characteristic distance between the two dominant peaks of E12g and A1g in the Raman spectra confirms the multi-layered structure for grown nanoflakes. Based on the experimental results, the growth mechanism has been explained considering nucleation and growth of two-dimensional islands, followed by coalescence of these islands. Subsequently, in the final stage, standing nanoflakes grow vertically. The vertical MoS2 nanoflakes film forms n-channel for back-gated FET gas sensor, of which gas sensing performance towards ethanol and methanol vapors have been studied. These structures with an increased number of edge sites presented high and fast responses to ethanol and methanol at 1 and 10 ppm concentrations and showed significant potential and promising application as a gas sensor transistor.

Formation of Twinned Graphene Polycrystals.

Liquid metals have been widely used as substrates to grow graphene and other 2D materials. On a homogeneous and isotropic liquid surface, a polycrystalline 2D material is formed by coalescence of many randomly nucleated single-crystal islands, and as a result, the domains in a polycrystal are expected to be randomly aligned. Here, we report the unexpected finding that only 30°-twinned graphene polycrystals are grown on a liquid Cu surface. Atomic simulations confirm that the unique domain alignment in graphene polycrystals is due to the free rotation of graphene islands on the liquid Cu surface and the highly stable 30°-grain boundaries in graphene. In-depth analysis predicts 30 types of possible 30°-twinned graphene polycrystals and 27 of them are observed. The revealed formation mechanism of graphene polycrystals on a liquid Cu surface deepens our fundamental understanding on polycrystal growth and could serve as a guideline for the controlled synthesis of 2D materials.

Polar edges and their consequences for the structure and shape of hBN islands

The ionic component of the strong bond in hexagonal boron nitride (hBN) has been grossly disregarded in literature. Precisely this quantity is demonstrated to govern the shape of monolayer hBN islands grown at high temperatures. HBN zigzag edges are charged and energetically less favorable than the neutral armchair edges, in contrast to those of the purely covalent graphene. Nucleation of hBN islands occurs exclusively on either the inner or the outer corners of substrate steps. Taking into account the charge at edges of hBN islands offers a powerful framework to understand the nucleation of the islands and their orientation with respect the founding steps, as well as various equilibrium shapes, including prominently a right-angled trapezoid. BN dimers are identified as basic building blocks for hBN. A surprisingly strong interaction between hBN and the pre-existing steps on the moderately reactive Ir(1 1 1) substrate is uncovered. Localized charges are probably relevant for all 2D-materials lacking inversion symmetry.

Confined step-flow growth of Cu intercalated between graphene and a Ru(0 0 0 1) surface

By comparing the growth of Cu thin films on bare and graphene-covered Ru(0 0 0 1) surfaces, we demonstrate the role of graphene as a surfactant allowing the formation of flat Cu films. Low-energy electron microscopy, x-ray photoemission electron microscopy and x-ray absorption spectroscopy reveal that depositing Cu at 580 K leads to distinct behaviors on both types of surfaces. On bare Ru, a Stranski–Krastanov growth is observed, with first the formation of an atomically flat and monolayer-thick wetting layer, followed by the nucleation of three-dimensional islands. In sharp contrast, when Cu is deposited on a graphene-covered Ru surface under the very same conditions, Cu intercalates below graphene and grows in a step-flow manner: atomically-high growth fronts of intercalated Cu form at the graphene edges, and extend towards the center of the flakes. Our findings suggest potential routes in metal heteroepitaxy for the control of thin film morphology.

Evolution of Micropits on Large Terraces of the Si(111) Surface during High-Temperature Annealing

The transformation of micropits on large terraces of the Si(111) surface containing no vicinal atomic steps has been investigated by in situ ultrahigh-vacuum reflection electron microscopy upon thermal annealing of the substrate in the range of 1200–1400°C. A procedure for the formation of micropits on large terraces of the Si(111) surface with the application of focused-ion-beam (Ga+) technology has been proposed. It has been found that the micropit decay kinetics varies upon reaching the critical radius Rcrit, which is caused by the activation of nucleation of two-dimensional vacancy islands on the micropit bottom. A theoretical model describing variations in the lateral sizes of the micropit both before and after reaching Rcrit has been proposed. Based on analysis of the found temperature dependence of the nucleation frequencies of two-dimensional vacancy pits on the micropit bottom, the effective energy of nucleation of a vacancy island has been determined to be 4.1 ± 0.1 eV.

Laser heat treatment of martensitic steel and dual-phase steel with high martensite content

Laser heat treatment of galvanised steels with a martensite content superior to 80% were performed on a 1 cm wide area over an extensive temperature range [620 K - 1350 K]. The material softening induced is studied through uniaxial tensile testing and SEM microstructural observation. A treatment temperature close to Ac3 yields a massive increase in the ductility of the specimens while reducing the mechanical strength. This change in mechanical properties is associated with the nucleation of new austenite islands and the vanishing of the initial martensite laths. The results presented in this paper pave the way to localised variations of the strength-ductility trade-off, which could be useful for several industrial applications, particularly for enabling plastic forming or stamping of the martensitic steel sheets at low temperature.

Semipolar (112¯2) GaN Films Improved by in situ SiNx Pretreatment of m‐Sapphire Substrate Surface

The authors report on growth and characterization of semipolar () GaN films improved by an in situ SiNx pretreatment of m‐sapphire substrate surface. Formation of SiNx and Ga‐rich surface at the initial growth stages is evidenced by X‐ray photoelectron spectroscopy. With the SiNx pretreatment, the GaN nucleated island density decreases due to the reduction of nucleation density by the SiNx mask. X‐ray diffraction and Raman analyses show reduction of threading defect density for the SiNx pretreated GaN films. Cathodoluminescence studies show enhanced D0X emission and suppressed defect‐related emissions. The improvements in crystalline quality and optical properties of semipolar GaN films with the SiNx pretreatment are attributed to the reduction in nucleated island density and coalescence boundaries, and enhancement of the +c island sidewall facet growth under Ga‐rich growth conditions.

Coherent X-ray spectroscopy reveals the persistence of island arrangements during layer-by-layer growth

Understanding surface dynamics during epitaxial film growth is key to growing high quality materials with controllable properties. X-ray photon correlation spectroscopy (XPCS) using coherent x-rays opens new opportunities for in situ observation of atomic-scale fluctuation dynamics during crystal growth. Here, we present the first XPCS measurements of 2D island dynamics during homoepitaxial growth in the layer-by-layer mode. Analysis of the results using two-time correlations reveals a new phenomenon - a memory effect in island nucleation sites on successive crystal layers. Simulations indicate that this persistence in the island arrangements arises from communication between islands on different layers via adatoms. With the worldwide advent of new coherent x-ray sources, the XPCS methods pioneered here will be widely applicable to atomic-scale processes on surfaces.

On the Mechanism of the Vapor–Solid–Solid Growth of Au-Catalyzed GaAs Nanowires

The mechanism of the vapor–solid–solid growth of Au-catalyzed GaAs nanowires in the temperature range of 420–450°C is investigated. For the first time, the effect of elastic stresses caused by a difference in the atomic densities of the catalyst and nanowire material on the solid-phase nucleation rate is considered. By assuming that the growth of the GaAs nucleus at the catalyst–nanowire interface is limited by the As-diffusion flux in the catalyst, it is shown that vapor–solid–solid growth can be implemented through the polycentric-nucleation mode in the temperature range under consideration. The intensity of the nucleation of coherent islands upon vapor–solid–solid growth is shown to be higher than the intensity of nucleation in the case of vapor–liquid–solid growth because a low interphase surface energy is implemented at coherent solid–solid conjugation. It is proved that the nucleation of Au-catalyzed GaAs nanowires by the vapor–solid–solid mechanism is possible only when GaAs-island growth proceeds due to As diffusion along the catalyst–nanowire interface.

GaN‐Based Nanorods/Graphene Heterostructures for Optoelectronic Applications

The insulating character of sapphire, meltback etching of Si, bulk and surface defects prevented the efficient integration of GaN nanostructures in optoelectronic devices. Here, it is demonstrated that graphene can simultaneously serve as an electrical bottom contact, a chemically inert buffer layer, and a superior lattice and thermal matched growth substrate. Vertically aligned, high crystal quality GaN nanorods (NRs) without bulk defects such as threading dislocations and with only a mild strain at the NRs’ base are grown by metal‐organic vapor‐phase epitaxy on defect‐free graphene using nanometer‐sized AlxGa1−xN nucleation islands. Here no influence of the supporting substrate on the GaN epitaxy is observed. However, at defects in graphene the effects of dangling bonds and the underlying substrate, presumably through nanoholes in graphene, on the properties of GaN NRs are visible. It is also shown that surface defects in InxGa1−xN/GaN NRs from planar films produced by etching of the defective material can be effectively passivated with only 10 nm alumina deposited by atomic layer deposition. This is confirmed by the increase in electroluminescence measured on finished devices with graphene top contact. These results can potentially lead to new material combinations including graphene, GaN, and other relevant semiconductors like Si toward yet unexplored device concepts.