Two-dimensional Island Growth Research Articles

Pd/Co3O4(111) Interface Formation

The formation of the metal–oxide interface in the Pd/Co3O4(111) model catalyst was investigated by means of density functional theory (DFT), synchrotron radiation photoelectron spectroscopy (SRPES), and scanning tunneling microscopy (STM). The electronic metal–support interaction results in a substantial charge transfer at the interface yielding atomically dispersed Pd2+ species and partially oxidized Pdδ+ aggregates coupled with a partial reduction of Co3O4(111). Atomically dispersed Pd2+ species at the fcc site on the Co3O4(111) surface were found to be the most energetically favorable configuration. In comparison to the dispersed Pd2+ species, the formation of Pd dimers, trimers, and tetramers was found to be less favorable. The analysis of the Bader charges revealed a substantial net positive charge on Pd atoms in dimers, trimers, and tetramers which is consistent with the formation of partially oxidized Pdδ+ aggregates detected by SRPES. The analysis of the charge distribution in Co3O4(111) revealed a partial reduction of Co3+ to Co2+ cations in the first and second Co layers. According to DFT, Pdδ+ aggregates are prone to oxidation to PdO in the presence of O2 and H2O. The partially oxidized Pdδ+ and Pd4Ox aggregates form 1 to 2 monolayer thick clusters which serve as nuclei for the growth of metallic Pd0 nanoparticles. At high Pd coverage, Pd nanoparticles coalesce resulting in the growth of two-dimensional islands that densely cover the Co3O4(111) substrate.

Fast nonthermal processes in pulsed laser deposition

Pulsed Laser Deposition (PLD) is widely used to grow epitaxial thin films of quantum materials such as complex oxides. Here, we use in-situ X-ray scattering to study homoepitaxy of SrTiO$_3$ by energetic (e-) and thermalized (th-) PLD. We find that e-PLD suppresses the lateral growth of two-dimensional islands, which suggests that energetic particles break up smaller islands. Fast interlayer transport occurs for both e-PLD and th-PLD, implying a process operating on sub-microsecond timescales that doesn't depend strongly on the kinetic energy of the incident particles.

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.

Step Kinetics Dependent on the Kink Generation Mechanism in Colloidal Crystal Growth

Nucleation and growth of two-dimensional (2D) islands on a terrace are the dominant growth mechanisms of colloidal crystals whose particle interaction is attractive. The step velocity, vstep, of the 2D islands at various area fractions, ϕarea, and polymer concentrations, Cp, has been investigated. At low Cp (weak attractive interaction between particles), there is a nearly linear relationship between vstep and ϕarea, whereas it is parabolic for high Cp (strong attraction). Depending on the Cp, two manners of kink generation at a step are observed: 1D nucleation under strong attraction and association of mound formation under weak attraction. As a result of mound formation, abundant kinks are created at steps, resulting in a linear relationship between vstep and ϕarea, whereas the relationship is parabolic for step propagation associated with 1D nucleation. Though the kink site is the most favored site for particles to be incorporated into crystals, weak attractive interaction makes the step front site an ...

Epitaxial growth and structure of monolayer cerium oxide films on Rh(111)

We prepared monolayer cerium (Ce) oxide films on Rh(111) to investigate their growth and structure using scanning tunneling microscopy (STM), low-energy electron diffraction, X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. For quantitative analysis of Ce-oxide films, we used the combined techniques of XPS and Rutherford backscattering spectrometry to determine the concentration of Ce and O atoms. We prepared a monolayer (ML) Ce-oxide film by annealing a metallic Ce film at 0.3ML coverage in an oxygen atmosphere. A well-ordered Ce-oxide phase with a (4×4) unit cell was obtained. The epitaxially grown Ce-oxide film aligned along the 〈110〉 azimuthal direction of Rh(111). The number of Ce and O atoms in the (4×4) unit cell was estimated. The STM images indicated that the two-dimensional island growth of the p(4×4) phase with p3m1 symmetry can be explained using the missing Ce atoms model. A simulated STM image of the p(4×4) structural model was in good agreement with the experimental STM image. The formation of Ce-oxide films on Rh(111) at submonolayer coverage was discussed on the basis of the results of DFT+U calculations.

Evolution of planar defects during homoepitaxial growth of β-Ga2O3 layers on (100) substrates—A quantitative model

We study the homoepitaxial growth of β-Ga2O3 (100) grown by metal-organic vapour phase as dependent on miscut-angle vs. the c direction. Atomic force microscopy of layers grown on substrates with miscut-angles smaller than 2° reveals the growth proceeding through nucleation and growth of two-dimensional islands. With increasing miscut-angle, step meandering and finally step flow growth take place. While step-flow growth results in layers with high crystalline perfection, independent nucleation of two-dimensional islands causes double positioning on the (100) plane, resulting in twin lamellae and stacking mismatch boundaries. Applying nucleation theory in the mean field approach for vicinal surfaces, we can fit experimentally found values for the density of twin lamellae in epitaxial layers as dependent on the miscut-angle. The model yields a diffusion coefficient for Ga adatoms of D = 7 × 10−9 cm2 s−1 at a growth temperature of 850 °C, two orders of magnitude lower than the values published for GaAs.

Analysis of the Spiral Step Structure and the Initial Solution Growth Behavior of SiC by Real-Time Observation of the Growth Interface

Microscopic real-time observation of the solution growth interface of SiC using Fe–Si solvent at 1673 K was successfully performed to understand the interface morphology at the initial stage of solution growth. The growth interface was composed of a number of domains with step–terrace structures originating from either spiral growth of 4H-SiC or two-dimensional island growth of other polytypes. The validity of the measurements of the step structures by bright-field and interference images was confirmed by ex-situ atomic force microscopy. In addition, the encounter between a spiral growth domain and steps from another spiral growth domain was observed, and it was found that a certain step height is needed to stop spiral growth by covering the spiral center.

The impact of the surface on step-bunching and diffusion of Ga on GaAs (001) in metal-organic vapour phase epitaxy

We studied diffusion by measuring step-bunching, island spacing, and the transition from step-flow growth to two-dimensional island growth of (001) GaAs in metal-organic vapour phase epitaxy and correlated them with the surface reconstruction measured by reflectance anisotropy spectroscopy. The V/III ratio had a small effect, while the square root of the growth rate was anti-proportional to the diffusion length. The thermal activation energy was about 2.3 eV on terraces and 1.6 eV on domains at higher temperatures. Pronounced step-bunching coincided with large domains at the step-edges, causing smoother steps for the [11̅0] misorientation. This Ga-rich reconstruction at the step-edges is needed for the Schwoebel barrier to induce step-bunching. At higher temperatures of domains grow in size, the Schwoebel barrier reduces and nucleation becomes easier on this surface which reduces diffusion length and thus step-bunching.

STM study of the preparation of clean Ta(110) and the subsequent growth of two-dimensional Fe islands

This report deals with the preparation of a clean Ta(110) surface, investigated by means of scanning tunneling microscopy/spectroscopy as well as by low-energy electron diffraction and Auger electron spectroscopy. The surface initially exhibits a surface reconstruction induced by oxygen contamination. This reconstruction can be removed by annealing at high temperatures under ultrahigh vacuum conditions. The reconstruction-free surface reveals a surface resonance at a bias voltage of about −500mV. The stages of the transformation are presented and discussed. In a next step, Fe islands were grown on top of Ta(110) and investigated subsequently. An intermixing regime was identified for annealing temperatures of (550–590)K.

Stochastic model for the epitaxial growth of two-dimensional islands in the submonolayer regime

The diffusion-based growth of islands composed of clusters of metal atoms on a substrate is considered in the aggregation regime. A stochastic approach is proposed to describe the dynamics of island growth based on a Langevin equation with multiplicative noise. The distribution of island sizes, obtained as a solution of the corresponding Fokker–Planck equation, is derived. The time-dependence of island growth on its fractal dimension is analysed. The effect of mobility of the small islands on the growth of large islands is considered. Numerical simulations are in a good agreement with theoretical results.

Dynamic scaling and kinetic roughening of poly(ethylene) islands grown by vapor phase deposition

Vapor phase deposition of poly(ethylene) is shown to produce uncross-linked thin films consisting of linear (CH2)100 oligomers with narrow molar mass distribution of dispersity DM=1.10. Early stages of the film formation are characterized by the growth of two-dimensional compact islands of constant 7–8nm thickness. The transversal evolution of islands is studied in the context of the Dynamic Scaling Theory. The aggregation regime is found to be valid in a narrow range of coverage 0.1<θ<0.3. The critical island size is estimated to be i=1. Kinetic roughening of the growing front gives a set of the scaling exponents αloc=0.67, αs=0.85, β=0.33 (β=0.2 for the late stages of growth) and z=2.2 that does not fit into any of the known universality classes. Macromolecular relaxation at the island edges is suggested to explain the observed inconsistence.

Pentacene on graphene: Differences between single layer and bilayer

Using atomic force microscopy we have examined morphology of sub-monolayer thick pentacene layers, deposited on exfoliated single-layer and bilayer graphene, transferred onto SiO2. We observe two-dimensional island growth in the range of substrate temperatures from 10 to 60°C. We find that island size distributions on single-layer graphene are broader and centered at higher average island areas than on bilayer graphene. We have found activation energy for molecule aggregation of Ea=427±25 and Ea=537±46meV for single layer and bilayer graphene, respectively. We associate the observed differences in pentacene layer morphology with the influence of the dipole field that can be associated with the water layer at the graphene/SiO2 interface.

Oxynitride Formation Processes on Si(001) Studied by Means of Reflectance Difference Spectroscopy

We investigated nitric oxide (NO) adsorption processes on Si(001) at temperatures of 110–873 K at a pressure of 1.0×10-5 Pa by reflectance difference spectroscopy (RDS). The transition of the growth modes between Langmuir-type adsorption and two-dimensional oxide island growth was identified from the spectral line shape obtained with RDS, and from Arrhenius plots of the time constant for reaction in the growth of a monolayer oxynitride film. The decrease in the time constant at temperatures below 300 K compared to that at temperatures above 573 K suggested that trapping-mediated adsorption takes place at temperatures below 300 K.

Lattice Kinetic Modeling of the Anisotropic Growth of Two-Dimensional Islands on Barite (001) Surface

In this paper, we present simulations of twodimensional islands on the barite (001) face. These simulations were performed with MMonCa, a computer code based on the kinetic Monte Carlo technique. Our results are in excellent agreement with previous in situ atomic force microscopy (AFM) observations. Indeed, MMonCa is able to precisely reproduce both the thickness and the characteristic fan shape of barite (001) two-dimensional islands, which is defined by two straight steps parallel to the ⟨120⟩ directions and a curved step connecting them. An addition, MMonCa also simulates the orientation reversal of islands in successive growth monolayers. Fundamental for the adequate reproduction of the shape of barite (001) islands is the introduction of an anisotropy factor for the incorporation of growth units into crystallographically nonequivalent step edges. The results presented in this paper demonstrate that the consideration of simple crystallographic and geometrical constraints can be enough to provide a consistent explanation for the development of complex nanotopographies during the growth of crystals.

Time-evolution of oxidation states at the Ni(111) surface: O2 incident translational energy dependence

Metallic Ni is expected to be a substitution candidate for Pt and Pd in catalytic materials. Generally speaking, catalytic reactions take place on a metal oxide layer. In this study, therefore, the oxidation states of the Ni(111) surface, which were made by irradiation with a supersonic O2 molecular beam, were analyzed by soft x-ray photoemission spectroscopy with synchrotron radiation. The oxygen uptake curve and initial sticking rate were found to show remarkably strong dependence on the O2 incident energy for energies of up to 2.3 eV. The intermediate plateau seen in the oxygen uptake curve for low incident energies was found to disappear with increasing incident energy due to a change of the dissociative adsorption mechanism from a two-dimensional island growth model to a direct activated adsorption model. Due to this activated adsorption, the formation rate of NiO and peroxide nickel increased as compared to backfilling oxidation by O2 gas.

Nucleation and Island Growth of Alkanethiolate Ligand Domains on Gold Nanoparticles

The metal oxide cluster α-AlW(11)O(39)(9-) (1), readily imaged by cryogenic transmission electron microscopy (cryo-TEM), is used as a diagnostic protecting anion to investigate the self-assembly of alkanethiolate monolayers on electrostatically stabilized gold nanoparticles in water. Monolayers of 1 on 13.8 ± 0.9 nm diameter gold nanoparticles are displaced from the gold surface by mercaptoundecacarboxylate, HS(CH(2))(10)CO(2)(-) (11-MU). During this process, no aggregation is observed by UV-vis spectroscopy, and the intermediate ligand-shell organizations of 1 in cryo-TEM images indicate the presence of growing hydrophobic domains, or "islands", of alkanethiolates. UV-vis spectroscopic "titrations", based on changes in the surface plasmon resonance upon exchange of 1 by thiol, reveal that the 330 ± 30 molecules of 1 initially present on each gold nanoparticle are eventually replaced by 2800 ± 30 molecules of 11-MU. UV-vis kinetic data for 11-MU-monolayer formation reveal a slow phase, followed by rapid self-assembly. The Johnson, Mehl, Avrami, and Kolmogorov model gives an Avrami parameter of 2.9, indicating continuous nucleation and two-dimensional island growth. During nucleation, incoming 11-MU ligands irreversibly displace 1 from the Au-NP surface via an associative mechanism, with k(nucleation) = (6.1 ± 0.4) × 10(2) M(-1) s(-1), and 19 ± 8 nuclei, each comprised of ca. 8 alkanethiolates, appear on the gold-nanoparticle surface before rapid growth becomes kinetically dominant. Island growth is also first-order in [11-MU], and its larger rate constant, k(growth), (2.3 ± 0.2) × 10(4) M(-1) s(-1), is consistent with destabilization of molecules of 1 at the boundaries between the hydrophobic (alkanethiolate) and the electrostatically stabilized (inorganic) domains.

The 2-D growth of gold on single-layer graphene/Ru(0001): Enhancement of CO adsorption

The growth and morphology of two-dimensional (2-D) gold islands on a single-layer graphene supported on Ru(0001) have been studied by scanning tunneling microscopy (STM). Our findings show that gold exhibits 2-D structures up to a gold dosage of 0.75 equivalent monolayers, and that these 2-D gold islands are thermally stable at room temperature. Parallel polarization modulation infrared reflection absorption spectroscopic (PM-IRAS) and high resolution electron energy loss spectroscopic (HREELS) studies indicate that carbon monoxide (CO) adsorbs on these 2-D gold islands at 85 K, showing a characteristic CO stretching feature at 2095 cm − 1 for a saturation coverage of CO. The red shift of the CO stretching frequency compared to that on charge neutral gold is consistent with electron transfer from graphene to gold, i.e., an electron-rich gold overlayer. Preliminary data obtained by dosing molecular oxygen onto this CO pre-covered surface suggest that the 2-D gold islands catalyze the oxidation of CO.

Growth, Structure, and Stability of Ag on CeO2(111): Synchrotron Radiation Photoemission Studies

The growth and interfacial electronic properties of Ag on CeO2(111) thin films have been studied by synchrotron radiation photoemission spectroscopy (SRPES), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). Stoichiometric CeO2(111) thin films were grown on a Ru(0001) substrate. Ag grows as three-dimensional (3D) particles on the well-ordered CeO2(111) surface at 300 K with a number density of ∼1 × 1012 particles/cm2. When the CeO2(111) surface has a high density of defects, Ag initially populates these defect sites, leading to a two-dimensional (2D) island growth at low coverages followed by 3D islanding at high coverages. The binding energy of Ag 3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. No strong interaction between Ag and CeO2(111) is found. The CeO2(111) surface is slightly reduced upon Ag deposition, which can be ascribed to the reverse spillover of oxygen atoms from the Ag−CeO2 boundary to the Ag nanoparticles. The Ag particles on CeO2(111) experience significant sintering when the temperature increases before they desorb from the surface.

In Situ STM Study of Cu Electrodeposition on TBPS-Modified Au(111) Electrodes

The impact of 3,3′-thiobis(1-propanesulfonic acid, sodium salt) (TBPS) on Cu/Au(111) electrodeposition has been investigated by electrochemical methods and scanning tunneling microscopy (STM). Cyclic voltammetry and galvanostatic experiments indicate that Cu growth on Au(111) - which is known to be strongly kinetically hindered in additive-free, aqueous perchloric acid solutions - proceeds significantly faster in the presence of TBPS. The TBPS molecules either “float” on top of the growing film or become incorporated into the deposit. Complementary in situ STM studies show that Cu underpotential deposition (UPD) proceeds via two distinct mechanisms. One-dimensional growth of Cu stripes was observed between 0.05 and 0.35 for TBPS-modified Au(111) electrodes. Each stripe is composed of two or three parallel rows of Cu atoms oriented along the main crystallographic directions of the Au(111) substrate. An increase of the TBPS concentration near the solid/liquid interface restricts the Cu stripe growth to a narrow potential regime between 0.3 and 0.35 and two-dimensional Cu island growth becomes the favored growth mechanism. The latter fully dominates in TBPS-containing electrolyte. Cu growth in the overpotential deposition (OPD) regime results in a smooth Cu film with low surface roughness, in contrast to defect-mediated 3D island growth in additive-free electrolytes.

Elementary steps at the surface of ice crystals visualized by advanced optical microscopy

Due to the abundance of ice on earth, the phase transition of ice plays crucially important roles in various phenomena in nature. Hence, the molecular-level understanding of ice crystal surfaces holds the key to unlocking the secrets of a number of fields. In this study we demonstrate, by laser confocal microscopy combined with differential interference contrast microscopy, that elementary steps (the growing ends of ubiquitous molecular layers with the minimum height) of ice crystals and their dynamic behavior can be visualized directly at air-ice interfaces. We observed the appearance and lateral growth of two-dimensional islands on ice crystal surfaces. When the steps of neighboring two-dimensional islands coalesced, the contrast of the steps always disappeared completely. We were able to discount the occurrence of steps too small to detect directly because we never observed the associated phenomena that would indicate their presence. In addition, classical two-dimensional nucleation theory does not support the appearance of multilayered two-dimensional islands. Hence, we concluded that two-dimensional islands with elementary height (0.37 and 0.39 nm on basal and prism faces, respectively) were visualized by our optical microscopy. On basal and prism faces, we also observed the spiral growth steps generated by screw dislocations. The distance between adjacent spiral steps on a prism face was about 1/20 of that on a basal face. Hence, the step ledge energy of a prism face was 1/20 of that on a basal face, in accord with the known lower-temperature roughening transition of the prism face.