Faceted Islands Research Articles

Mechanisms of Quasi van der Waals Epitaxy of Three-Dimensional Metallic Nanoislands on Suspended Two-Dimensional Materials.

Understanding structure at the interface between two-dimensional (2D) materials and 3D metals is crucial for designing novel 2D/3D heterostructures and improving the performance of many 2D material devices. Here, we quantify and discuss the 2D/3D interface structure and the 3D morphology in several materials systems. We first deposit faceted Au nanoislands on graphene and transition metal dichalcogenides, using measurements of the equilibrium island shape to determine values for the 2D/Au interface energy and examining the role of surface reconstructions, chemical identity, and defects on the grown structures. We then deposit the technologically relevant metals Ti and Nb under conditions where kinetic rather than thermodynamic factors govern growth. We describe a transition from dendritic to faceted islands as a function of growth temperature and discuss the factors determining island shape in these materials systems. Finally, we show that suspended 2D materials enable the fabrication of a novel type of 3D/2D/3D heterostructure and discuss the growth mechanism. We suggest that emerging nanodevices will utilize versatile fabrication of 2D/3D heterostructures with well-characterized interfaces and morphologies.

Dynamical evolution of Ge quantum dots on Si(111): From island formation to high temperature decay

Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in opto- and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real-time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550°C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll-like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600°C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600°C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111).

Micrometre-scale single-crystalline borophene on a square-lattice Cu(100) surface.

Borophene, a crystalline monolayer boron sheet, has been predicted to adopt a variety of structures-owing to its high polymorphism-that may possess physical properties that could serve in flexible electronics, energy storage and catalysis. Several borophene polymorphs have been synthesized on noble metal surfaces but for device fabrication larger single-crystal domains are typically needed with, ideally, weak borophene-substrate interactions. Here we report the synthesis of borophene on a square-lattice Cu(100) surface and show that incommensurate coordination reduces the borophene-substrate interactions and also leads to a borophene polymorph different from those previous reported. Micrometre-scale single-crystal domains formed as isolated faceted islands or merged together to achieve full monolayer coverage. The crystal structure of this phase has ten boron atoms and two hexagonal vacancies in its unit cell. First-principles calculations indicate that charge transfer, rather than covalent bonding, binds this two-dimensional boron to the Cu(100) surface. The electronic band structure of this material features multiple anisotropic tilted Dirac cones.

Electrodeposition of Cu(111) onto a Ru(0001) seed layer for epitaxial Cu interconnects

The electrodeposition of Cu onto epitaxial Ru(0001) seed layers was investigated from a sulfuric acid-based solution containing dilute copper(II) sulfate and chloride ions. Using galvanostatic deposition at −350 μA/cm2, Cu was deposited epitaxially onto a 30 nm-thick Ru(0001) seed layer, despite a compressive misfit strain between −6.9% and −8.3%, depending on the extent of strain relaxation of the Ru layer. However, rather than depositing as a single crystal, Cu grew as a bicrystal having a common out-of-plane orientation of Cu(111) and two equivalent in-plane orientations. The Cu grain size was large, on the order of micrometers, and the grain boundaries were identified as incoherent ∑3{211} twin boundaries. The Cu initially grew as isolated islands, coalescing into a contiguous film at thicknesses around 50 nm. The Cu film was rough, and thickness and coverage varied over the electrodeposited region. After the initial island growth, Cu void fraction and film roughness both decreased with thickness as the deposit transitioned into a planar film with nanometric islands growing on the film surface. However, at thicknesses exceeding 200 nm, anisotropic growth of large, faceted Cu islands on the planar Cu film again increased the surface roughness. The epitaxially deposited Cu bicrystal showed an improvement in resistivity when compared with polycrystalline Cu similarly electrodeposited onto a polycrystalline Ru seed.

Fabrication of spectrally sharp Si-based dielectric resonators: combining etaloning with Mie resonances.

We use low-resolution optical lithography joined with solid state dewetting of crystalline, ultra-thin silicon on insulator (c-UT-SOI) to form monocrystalline, atomically smooth, silicon-based Mie resonators in well-controlled large periodic arrays. The dewetted islands have a typical size in the 100 nm range, about one order of magnitude smaller than the etching resolution. Exploiting a 2 µm thick SiO2 layer separating the islands and the underlying bulk silicon wafer, we combine the resonant modes of the antennas with the etalon effect. This approach sets the resonance spectral position and improves the structural colorization and the contrast between scattering maxima and minima of individual resonant antennas. Our results demonstrate that templated dewetting enables the formation of defect-free, faceted islands that are much smaller than the nominal etching resolution and that an appropriate engineering of the substrate improves their scattering properties. These results are relevant to applications in spectral filtering, structural color and beam steering with all-dielectric photonic devices.

Artifact-free microstructures of the Cu–In reaction by using cryogenic broad argon beam ion polishing

Metallurgical systems containing substantial amounts of metallic indium have always been very challenging to investigate due to the difficulties involved with sample characterization caused by the substantial softness of indium. Such difficulties are overcome in this study, and artifact-free microstructures are successfully obtained for the first time by using cryogenic broad Ar beam ion polishing for the reactions between Cu and molten indium at 180 ℃. A two-phase layer consisting of Cu11In9 and In forms at the Cu/In interface. The growth of this layer follows parabolic kinetics, indicating that it is a diffusion-controlled process. Within the layer, Cu11In9 islands are larger near the In side than those near the Cu side, suggesting that coarsening of the Cu11In9 islands occurs. During the cooling process after the reaction, large faceted Cu11In9 islands heterogeneously precipitate over the two-phase layer. Simultaneously, nonfaceted Cu11In9 particles homogeneously precipitate within the In phase.

Coalescence dynamics of 3D islands on weakly-interacting substrates

We use kinetic Monte Carlo simulations and analytical modelling to study coalescence of three-dimensional (3D) nanoscale faceted silver island pairs on weakly-interacting fcc(111) substrates, with and without concurrent supply of mobile adatoms from the vapor phase. Our simulations show that for vapor flux arrival rates F < 1 monolayer/second (ML/s) coalescence manifests itself by one of the islands absorbing the other via sidewall facet migration. This process is mediated by nucleation and growth of two-dimensional (2D) layers on the island facets, while the supply of mobile atoms increases the nucleation probability and shortens the time required for coalescence completion. When F is increased above 1 ML/s, coalescence is predominantly governed by deposition from the vapor phase and the island pair reaches a compact shape via agglomeration. The crucial role of facets for the coalescence dynamics is further supported by a mean-field thermodynamic description of the nucleation energetics and kinetics. Our findings explain experimental results which show that two-dimensional film growth morphology on weakly-interacting substrates is promoted when the rate of island coalescence is suppressed. The present study also highlights that deviations of experimentally reported film morphological evolutions in weakly-interacting film/substrate systems from predictions based on the sintering and particle growth theories may be understood in light of the effect of deposition flux atoms on the energetics and kinetics of facet-layer nucleation during coalescence.

Epitaxial Growth of Cu2O on Ag Substrate by Electrodeposition

Cuprous oxide (Cu2O) is a p-type semiconductor with a direct bandgap of 2.1 eV and high hole mobility at ambient temperature. Recently, the importance of using an epitaxial film to fabricate optoelectronic devices to improve the performances has been pointed out [Mahenderkar et al., Science, 355, 1203–1206 (2017)]. Investigation of epitaxial growth of Cu2O on, mostly, MgO substrate by molecular beam epitaxy or pulsed laser deposition has been carried out for a decade. However, these techniques require high-quality substrates and expensive facilities. Electrodeposition, on the other hand, provides another option with ease of operation. However, the substrate has to be electrically conductive and possesses a low lattice mismatch to Cu2O. In this study, the epitaxial growth of Cu2O on Ag substrate by electrodeposition was studied by a combinatorial substrate approach mainly using the electron backscatter diffraction (EBSD) technique. The effect of substrate orientation on epitaxial growth can thus be explored efficiently. The lattice mismatch between Ag and Cu2O is 4.3 %. Prior to the deposition, the annealed, polycrystalline Ag substrate was electropolished and analyzed by EBSD over a marked region of about 500 x 500 μm2. Atomic force microscopy was used to characterize the surface morphology of the substrate grains of different orientations. Afterwards, Cu2O films were deposited galvanostatically on the substrates in an electrolyte containing copper sulfate and lactic acid. The crystal structure and orientation of the marked area was analyzed by EBSD again after electrodeposition. The orientation distribution of the films and the orientation relationship between the overlaid Cu2O and the Ag substrate were thus obtained. X-ray diffraction was also used for phase identification. Results indicated that the substrate grains exhibited similar surface roughness values of approximately 3 nm regardless the orientation. X-ray diffraction reults confirmed that the films deposited at different conditions contained solely the Cu2O phase. No diffraction peaks of Cu and CuO were found. At a low current density of 0.06 mA/cm2, the film has a rough surface composing of faceted islands of 5-20 mm in size. A portion of the islands exhibited the same orientation as the substrate grain. On the other hand, at a high current density of 0.25 mA/cm2, films with smooth surfaces were obtained. The Cu2O films all grew epitaxially on the Ag grains in two orientation relationships: a cube-on-cube one and a <100>/45o one. The origin of the <100>/45o orientation relationship was further studied and will be explained in detail. The deposited films were also analyzed by photoluminescence at room temperature and showed strong near band-edge emissions at 1.97-2.06 eV.

Large-area borophene sheets on sacrificial Cu(111) films promoted by recrystallization from subsurface boron

Borophene, an atomically thin covalently bonded boron sheet, has attracted great attention as a novel quantum material because of its structural tunability and potential utilization in flexible and transparent electronics. So far, borophene has been synthesized on silver or copper single crystals, but these substrates are small, very expensive, and unsuitable for study of transport properties or electronics applications. Here, we report synthesis of borophene on nanometer-scale thick Cu(111) films grown on sapphire. We have developed a process of enlarging faceted borophene islands, by repeated submersion of boron into copper at high temperature and resurfacing and re-crystallization at lower temperature. This discovery was enabled by real-time feedback from low-energy electron microscopy and diffraction. We demonstrate synthesis of borophene as faceted micrometer-size monocrystal islands or as full-monolayer sheets. The process is scalable to wafer size; moreover, Cu films could be sacrificed and sapphire reused. Our work opens the door for new experiments and brings applications one step closer.

Epitaxial Growth of Cu2O on Ag Substrate by Electrodeposition

In this study, the epitaxial growth of Cu2O on Ag substrate by electrodeposition was studied by a combinatorial substrate approach mainly using the electron backscatter diffraction technique. Cu2O films were deposited galvanostatically in an electrolyte containing copper sulfate and lactic acid at four current densities. Results indicated that the substrate grains exhibited similar surface roughness values of approximately 3-5 nm regardless the orientation. At a low current density of 0.060 mA/cm2, the deposited film has a rough surface composing of faceted islands of about 1 mm in size. No orientation relationship (OR) between the crystals and substrate grains can be identified. About 50% of the Cu2O crystals deposited at 0.125 mA/cm2 exhibits a cube-on-cube OR with their underlying grains, whereas those grew on a <100>//ND grain exhibiting a <100>/45° OR. With increasing the deposition current density to 0.250 mA/cm2, many substrate grains were deposited by a flat epitaxial layer having a <100>/45° OR. Only few <100>//ND grains are covered by a cube-on-cube epilayer.

In‐Situ Annealing of the (110) and (001) Surfaces of SrTiO3 Nanocuboids by High‐Resolution Transmission Electron Microscopy

Thermal annealing of SrTiO3 substrates is used in a wide range of applications, including films and nanoparticle growth, electronic devices, catalysis, and photocatalysis. To predict and control reaction kinetics during high temperature treatments, it is essential to understand dynamics in real time. For this purpose, in‐situ electron microscopy techniques are increasingly used, as structural changes can be monitored at the nanoscale. Herein, the authors report on the surface evolution of the (110) and (001) surfaces of SrTiO3 nanocuboids observed in‐situ during thermal treatment at 800 °C, using aberration corrected high‐resolution transmission electron microscopy. By establishing a threshold for the cumulative electron dose, below which only temperature induced growth occurs, the authors observe the formation of epitaxial, faceted TiO islands, in agreement with previously reported results on the thermal treatment of (001) surfaces of SrTiO3 in reducing environments. When surface evolution is a consequence of the combined effects of electron beam damage and temperature‐induced growth, structural modification of the cuboids is highly dynamic. In this case formation of faceted islands occurs rapidly in conjunction with sublimation of the cuboids at considerably larger speeds than in the case of thermal treatment.

Dynamics of faceted thin films formation during vapor deposition

In this study, an anisotropic phase-field model was established to simulate the growth of crystalline thin films during vapor deposition. The formation and evolution of characteristic surface with faceted morphologies were demonstrated, in accordance with the regularly obtained microstructure in the actual experiments. In addition, the influences of deposition parameters, such as the deposition rate and the interfacial energy, on the formation mechanism of the characteristic morphology were also illustrated. While a relatively low surface energy of substrate was regarded, the faceted islands were formed, owing to the anisotropic interfacial energy of thin films. In the condition of a high surface energy of substrate, the layered structures of deposited films were produced, which was corresponding to the Frank–van der Merwe growth mode. As various deposition rates were utilized in the numerical simulations, diverse surface morphologies were developed on the basis of the dominant mechanisms, correlating with the adatom diffusion and the deposition kinetics. According to the calculation results, it was observed that a surface character with flattened morphology was generally driven by the adatom diffusion, while the factor of the deposition kinetics was inclined to roughen the surface of thin films. These numerical simulations enhanced the knowledge of thin film growth and facilitated the progress of the vapor deposition technology for advanced applications.

Surface structure and electrochemical properties of platinum films grown on SrTiO3(100) substrates

We fabricated Pt films on SrTiO3 (STO)(100) using a DC-magnetron sputtering method to investigate the preferred orientation, surface structure, and electrochemical property. A film grown at 400 °C showed the two-dimensional polycrystalline features of Pt(111). Reflection high-energy electron diffraction (RHEED) showed diffraction patterns independent of the in-plane incident angle of the film. Films grown at 600 and 700 °C exhibited a preferred orientation of Pt(100) and (110). The films exhibited the morphology of faceted islands with roughness of several tens of nm, which consisted of two kinds of domains, namely a domain with preferred orientation of (100) and one with (110). The (100) and (110) domains had 45-degree twin boundaries, which were observed as V-shaped streaks by RHEED. The (100) domain was aligned in orientation of [011]Pt//[010]STO, which suggests that the binding strength of the (110) plane at the interface was larger than that of the (100). With a further increase in the growth temperature up to 750 °C, the film primarily showed a preferred orientation of (100) with an in-plane orientation of [010]Pt//[010]STO. The film also showed an island structure; however, atomic force microscopy revealed that the top was atomically flat.

Epitaxial Ni nanoparticles on CaF2(001), (110) and (111) surfaces studied by three-dimensional RHEED, GIXD and GISAXS reciprocal-space mapping techniques

The development of growth techniques aimed at the fabrication of nanoscale heterostructures with layers of ferroic 3d metals on semiconductor substrates is very important for their potential usage in magnetic media recording applications. A structural study is presented of single-crystal nickel island ensembles grown epitaxially on top of CaF2/Si insulator-on-semiconductor heteroepitaxial substrates with (111), (110) and (001) fluorite surface orientations. The CaF2 buffer layer in the studied multilayer system prevents the formation of nickel silicide, guides the nucleation of nickel islands and serves as an insulating layer in a potential tunneling spin injection device. The present study, employing both direct-space and reciprocal-space techniques, is a continuation of earlier research on ferromagnetic 3d transition metals grown epitaxially on non-magnetic and magnetically ordered fluorides. It is demonstrated that arrays of stand-alone faceted nickel islands with a face-centered cubic lattice can be grown controllably on CaF2 surfaces of (111), (110) and (001) orientations. The proposed two-stage nickel growth technique employs deposition of a thin seeding layer at low temperature followed by formation of the islands at high temperature. The application of an advanced three-dimensional mapping technique exploiting reflection high-energy electron diffraction (RHEED) has proved that the nickel islands tend to inherit the lattice orientation of the underlying fluorite layer, though they exhibit a certain amount of {111} twinning. As shown by scanning electron microscopy, grazing-incidence X-ray diffraction (GIXD) and grazing-incidence small-angle X-ray scattering (GISAXS), the islands are of similar shape, being faceted with {111} and {100} planes. The results obtained are compared with those from earlier studies of Co/CaF2 epitaxial nanoparticles, with special attention paid to the peculiarities related to the differences in lattice structure of the deposited metals: the dual-phase hexagonal close-packed/face-centered cubic lattice structure of cobalt as opposed to the single-phase face-centered cubic lattice structure of nickel.

Investigation of the properties and formation process of a peculiar V-pit in HVPE-grown GaN film

Hexagonal V-pits with an inverted pyramid shape appear on the surface of a GaN film after etching by hot H3PO4. A regular structure that is different from the V-pit morphology is observed in the cathodoluminescence image. The carrier concentration and stress distributions are nonuniform inside the V-pit. Furthermore, no dominant dislocations or defects are observed in the facets or bottom area of the V-pits. An island coalescence process is considered to contribute to the formation of such V-pits, with the coalesced island facets changing from {101m} to {112m} at a concave state for the different growth velocities.

Anisotropic invasion and its consequences in two-strategy evolutionary games on a square lattice.

We have studied invasion processes in two-strategy evolutionary games on a square lattice for imitation rule when the players interact with their nearest neighbors. Monte Carlo simulations are performed for systems where the pair interactions are composed of a unit strength coordination game when varying the strengths of the self-dependent and cross-dependent components at a fixed noise level. The visualization of strategy distributions has clearly indicated that circular homogeneous domains evolve into squares with an orientation dependent on the composition. This phenomenon is related to the anisotropy of invasion velocities along the interfaces separating the two homogeneous regions. The quantified invasion velocities indicate the existence of a parameter region in which the invasions are opposite for the horizontal (or vertical) and the tilted interfaces. In this parameter region faceted islands of both strategies shrink and the system evolves from a random initial state into the homogeneous state that first percolated.

Structure and Dynamics of Confined Alcohol-Water Mixtures.

The effect of confinement between mica and graphene on the structure and dynamics of alcohol-water mixtures has been studied in situ and in real time at the molecular level by atomic force microscopy (AFM) at room temperature. AFM images reveal that the adsorbed molecules are segregated into faceted alcohol-rich islands on top of an ice layer on mica, surrounded by a pre-existing multilayer water-rich film. These faceted islands are in direct contact with the graphene surface, revealing a preferred adsorption site. Moreover, alcohol adsorption at low relative humidity (RH) reveals a strong preference of the alcohol molecules for the ordered ice interface. The growth dynamics of the alcohol islands is governed by supersaturation, temperature, the free energy of attachment of molecules to the island edge and two-dimensional (2D) diffusion. The measured diffusion coefficients display a size dependence on the molecular size of the alcohols, and are about 6 orders of magnitude smaller than the bulk diffusion coefficients, demonstrating the effect of confinement on the behavior of the alcohols. These experimental results provide new insights into the behavior of multicomponent fluids in confined geometries, which is of paramount importance in nanofluidics and biology.

Van der Waals-Induced Chromatic Shifts in Hydrogen-Bonded Two-Dimensional Porphyrin Arrays on Boron Nitride.

The fluorescence of a two-dimensional supramolecular network of 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) adsorbed on hexagonal boron nitride (hBN) is red-shifted due to, primarily, adsorbate-substrate van der Waals interactions. TCPP is deposited from solution on hBN and forms faceted islands with typical dimensions of 100 nm and either square or hexagonal symmetry. The molecular arrangement is stabilized by in-plane hydrogen bonding as determined by a combination of molecular-resolution atomic force microscopy performed under ambient conditions and density functional theory; a similar structure is observed on MoS2 and graphite. The fluorescence spectra of submonolayers of TCPP on hBN are red-shifted by ∼30 nm due to the distortion of the molecule arising from van der Waals interactions, in agreement with time-dependent density functional theory calculations. Fluorescence intensity variations are observed due to coherent partial reflections at the hBN interface, implying that such hybrid structures have potential in photonic applications.

Anisotropy of electrical and magnetic transport properties of epitaxial SrRuO3 thin films

SrRuO3 (SRO) thin films with different thickness (2–70 nm) have been grown on (001), (110), and (111) SrTiO3 substrates. The (001)-SRO films (2–8 nm) exhibited smooth flat surfaces whereas the (110)- and (111)-SRO films featured a faceted island structure. Room temperature resistivity and residual resistivity are the lowest for the (111)-SRO films (30–70 nm). Over all thicknesses, we observed enhanced magnetization in the (111)-SRO films (∼4 μB/Ru) compared with that for the (001)- and (110)-SRO films (∼2 μB/Ru and ∼3 μB/Ru, respectively), suggesting a low-spin state t2g(3↑,1↓), high-spin state t2g(3↑)eg(1↑), and mixed low- and high-spin states for the (001)-, (111)-, and (110)-SRO films, respectively. The dependence of resistivity on temperatures near TC follows a power law with exponent β = 0.312 and β = 0.363 for the (110)- and (111)-SRO films, respectively. These critical exponents are consistent with magnetic data with scaling law M = C (TC − T)β. At low temperatures, dM/dT and dρ/dT show a linear relationship in the temperature range for the Fermi liquid. These results suggest that the intrinsic electrical and magnetic transport properties are coupled.

Generalized Muller-Kern formula for equilibrium thickness of a wetting layer with respect to the dependence of the surface energy of island facets on the thickness of the 2D layer.

Experimental results indicate a particular importance of such a value as the equilibrium thickness of the wetting layer during epitaxial growth according to the Stranski-Krastanow mechanism in systems with a lattice mismatch. In this paper the change in free energy during the transition of atoms from the wetting layer to the island in such systems is considered. Recent experimental results also show that the surface energy of the island's facets depends upon the thickness of the deposited material. So, in this paper the equilibrium thickness of the wetting layer, at which transition from 2D to 3D growth becomes energetically favorable, is calculated with the assumption that the specific energy of the island's facets depends upon the wetting layer thickness. In this approximation a new generalized Muller-Kern formula is obtained. As an illustration of the proposed method, an example of a numerical calculation according to the new formula for the material system of germanium on a silicon (001) surface is given. The result for the found equilibrium thickness of the wetting layer is rather unexpected since it differs from the value obtained in the bounds of the traditional Muller-Kern model.