Epitaxial Phenomena Research Articles

Commensurate Assembly of C60 on Black Phosphorus for Mixed-Dimensional van der Waals Transistors.

2D crystals can serve as templates for the realization of new van der Waals (vdW) heterostructures via controlled assembly of low-dimensional functional components. Among available 2D crystals, black phosphorus (BP) is unique due to its puckered atomic surface topography, which may lead to strong epitaxial phenomena through guided vdW assembly. Here, it is demonstrated that a BP template can induce highly oriented assembly of C60 molecular crystals. Transmission electron microscopy and theoretical analysis of the C60 /BP vdW heterostructure clearly confirm that the BP template results in oriented C60 assembly with higher-order commensurism. Lateral and vertical devices with C60 /BP junctions are fabricated via a lithography-free clean process, which allows one to investigate the ideal electrical properties of pristine C60 /BP junctions. Effective tuning of the C60 /BP junction barrier from 0.2 to 0.5eV and maximum on-current density higher than 104 mA cm-2 are achieved with graphite/C60 /BP vertical vdW transistors. Due to the formation of high-quality C60 film and the semitransparent graphite top-electrode, the vertical transistors show high photoresponsivities up to ≈100 A W-1 as well as a fast response time under visible light illumination.

Epitaxy: a methodological approach to the study of an old phenomenon

We describe the use of an integrated methodological approach to study epitaxial phenomena.

Coalescence and epitaxial self-assembly of Cu nanoparticles on graphene surface: A molecular dynamics study

A detailed understanding of surface decoration and interconnecting technologies is essential in realizing high-performance functional devices incorporating metal nanoparticle/graphene nanohybrids. This study employs classical molecular dynamics (MD) simulations to investigate the formation of amorphous copper (Cu) layers on a graphene surface via the collision, coalescence and nucleation of individual Cu nanoparticles (NPs) at temperatures in the range of 300–1300 K. The results indicate that the coalescence and melting temperatures are both sensitive to the particle size and the presence of the substrate. Moreover, an epitaxial interaction is found between the Cu NPs and the graphene substrate, in which mobile Cu atoms are captured and dragged to the graphene surface to produce self-assembled NP layers via a nucleation process. A series of structural evolutions and phase transitions are revealed during thethermalization process of the NPs. Finally, the results show that the presence of the substrate and associated contact epitaxy phenomenon play a key role in governing the structural morphology and thermal behavior of the Cu NP-based thin film.

Strain relaxation and epitaxial relationship of perylene overlayer on Ag(110).

We present a room temperature STM study of perylene self-assembly on Ag(110) beyond the monolayer coverage regime. Coupling of the perylene aromatic boards yields π-π bonded stacks. The perylene stacks self-assemble into a continuous three-dimensional epitaxial overlayer of (3 × 5) symmetry. The self-assembly is driven by thermodynamic balance established under coupling of the intra- and intermolecular interactions and the molecule-substrate interaction all accommodating the short-range thermal motion of the constituent molecules. The balance bestows to the overlayer the unique ability to accommodate the underlying substrate morphology and to spread over the surface steps as a single structure preserving its lateral order and keeping epitaxial relationship with every surface terrace. The complete epitaxy is driven by (i) anchoring of half of the perylene stacks into specific adsorption sites on each terrace, (ii) interlacing of the perylene stacks across the steps within the entire H-bonded network, and (iii) relaxation of the overlayer strain via enhancement of the overlayer-specific vibrational modes and short-range thermal motion of the constituent molecules. This complete epitaxy phenomenon is described via (i) structural and statistical analysis of the molecularly resolved STM topographies, (ii) monitoring of the short-range molecular displacements under the strain relaxation, (iii) highlighting of specific intra-molecular and inter-molecular vibration modes through detailed analysis of HREELS spectra, and (iv) parametrization of the intermolecular interaction via pair potential calculation.

Thin (111) oriented CoFe2O4 and Co3O4 films prepared by decomposition of layered cobaltates

The formation and structural characterization of highly (111)-oriented Co3O4 and CoFe2O4 films prepared by a novel procedure from 00l-oriented NaCoO2 and Na(CoFe)O2 is reported. The Na(CoFe)O2 films were deposited on MgO, SrTiO3, LaAlO3, and Zr(Y)O2 single crystals with (100) and (111) orientations by chemical solution deposition method and crystallized at 700°C. Subsequently they were transformed into (111)-oriented spinel phase during post-growth annealing at 800–1000°C. Morphology and structure of the films was investigated by means of scanning electron microscopy and X-ray diffraction. While all spinel films exhibit pronounced out-of-plane orientation irrespective of substrate, the rate of in-plane orientation strongly depend on lattice misfit values. Different epitaxial phenomena ranging from true one-to-one epitaxy to the existence of many-to-one epitaxy involving two or more orientations were determined by full 3D texture analysis.

Templated quasicrystalline ordering of single elements and molecules

We have used quasicrystals as templates for the exploration of new epitaxial phenomena. Several interesting results have been observed in the growth on surfaces of the common Al-based quasicrystals [1]. These include pseudomorphic monolayers, quasiperiodically modulated multilayer structures, and fivefold-twinned islands with magic heights influenced by quantum size effects [1]. Here we present our recent works on the growth of various elements and molecules on a new substrate, icosahedral (i) Ag-In-Yb quasicrystal, which have resulted in various epitaxial phenomena not observed previously. The growth of Pb on the five-fold surface of i-Ag-In-Yb yields a film which possesses quasicrystalline ordering in three-dimension [2]. Using scanning tunneling microscopy (STM) and DFT calculations of adsorption energies, we find that lead atoms occupy the positions of atoms in the rhombic triacontahedral (RTH) cluster, the building block of the substrate, and thus grow in layers with different heights and adsorption energies. The adlayer–adlayer interaction is crucial for stabilizing the epitaxial quasicrystalline structure. We will also present the first example of quasicrystalline molecular layers. Pentacene adsorbs at tenfold-symmetric sites of Yb atoms around surface-bisected RTH clusters, yielding quasicrystalline order [3]. Similarly, C-60 growth on the five-fold surface of i-Al-Cu-Fe at elevated temperature produces quasicrystalline layer, where the growth is mediated by Fe atoms on the substrate surface [3]. The finding of quasicrystalline thin films of single elements and molecules opens an avenue for further investigation of the impact of the aperiodic atomic order over periodic order on the physical and chemical properties of materials.

Laser-deposited thin hafnium dioxide condensates: Electron-microscopic study

Thin films of hafnium dioxide (HfO2) have been obtained by pulsed laser ablation of hafnium targets in oxygen atmosphere and characterized by transmission electron microscopy and electron diffraction. Conditions ensuring the formation of an amorphous phase, tetragonal and monoclinic modifications of HfO2 have been determined. It is established that the crystalline phase is formed on orienting substrates at lower temperatures than on neutral ones. The phenomenon of epitaxy has been observed for tetragonal modification of HfO2. Annealing in air leads to crystallization of an initially amorphous film with the formation of a monoclinic HfO2 modification.

Transfer matrix solution of the Wako–Saitô–Muñoz–Eaton model augmented by arbitraryshort range interactions

The Wako–Saitô–Muñoz–Eaton (WSME) model, initially introduced in the theory of proteinfolding, has also been used in modeling RNA folding and some epitaxial phenomena. Theadvantage of this model is that it admits exact solution in the general inhomogeneouscase (Bruscolini and Pelizzola 2002 Phys. Rev. Lett. 88 258101) which facilitatesthe study of realistic systems. However, a shortcoming of the model is that itaccounts only for interactions within continuous stretches of native bonds oratomic chains while neglecting interstretch (interchain) interactions. But due tothe biopolymer (atomic chain) flexibility, the monomers (atoms) separated byseveral non-native bonds along the sequence can become closely spaced. Thisproduces their strong interaction. The inclusion of non-WSME interactions intothe model makes the model more realistic and improves its performance. In thisstudy we add arbitrary interactions of finite range and solve the new model bymeans of the transfer matrix technique. We can therefore exactly account for theinteractions which in proteomics are classified as medium and moderately long rangeones.

Surface reconstructions and relaxation effects in a centre-symmetrical crystal: the {00.1} form of calcite (CaCO3)

Reconstructing polar surfaces of crystals represents the pathway needed to obtain both surface stability and minimum of surface free energy. Knowing these equilibrium conditions is, in turn, necessary to interpret surface kinetics, especially when foreign adsorption or epitaxial phenomena are involved. Here we reconstructed the {00.1} dipolar surfaces of a centre-symmetrical crystal, calcite (CaCO3), either by removing one half of the ions in the outermost layer of the face, or by applying calcite to the octopolar Lacmann's model, already proposed for NaCl-like lattices. This second way respects the bulk symmetry of the crystal and was carried out by removing ¾ of the ions in the outermost layer and ¼ in the second to last one, respectively. Relaxed and athermal surface energies were determined at the empirical level and the octopolar CO3 terminated {00.1} form was the most stable. Moreover, calculation shows that the {00.1} form, along with the {10.4} and {01.2} rhombohedra, can enter the athermal equilibrium morphology of the calcite crystal. Finally, when recollecting our results on the reconstructed {01.2} form of calcite and {111} NaCl octahedron, it can be stressed that the bulk crystal symmetry has to be considered if one aims to achieve the self consistency of the surface reconstruction (Curie's principle).

The Structure Of Liquid Water; Novel Insights From Materials Research; Potential Relevance To Homeopathy

This paper provides an interdisciplinary base of information on the structure of liquid water. It begins with a synthesis built on the information base on the structure 5 of noncrystalline, inorganic, covalently-bonded condensed liquid phases, such as SiO2, S, Se, P, and H2O, which exists in the materials science literature. The data for water are analyzed through the prism of well-established algorithms in materials research: the connection of properties to structure; the pressure-temperature (P-T) phase diagrams; the phenomenon of epitaxy; the phenomenon of liquid-liquid phase separation; the stability of two phase colloids; and, the recently discovered effects of weak magnetic and electric fields on the structure of simple inorganic oxides. A thorough combing of the literature of the condensed matter properties reflecting structural features of essentially pure water obtained via the normal processes of preparing homeopathic remedies, provides another rich data base. The examination of these data through the standard materials science paradigms leads to the following conclusion: Many different structures of liquid water must exist within the range of observations and processes encountered near ambient conditions. A typical sample of water in these experimental ranges no doubt consists of a statistical-mechanical-determined assemblage of monomers and oligomers (clusters) of various sizes up to at least several hundred H2O units. The importance of the structural similarity of SiO2 and OH2 is very relevant to the structure of the latter as well as to the probability of epitaxy in controlling at least the region contiguous to the silicate glass surfaces of many common containers. The most distinctive feature of bonding in liquid water is not only the “well-known hydrogen bonds, but the necessary presence of a wide range” of van der Waals bonds between and among the various oligomeric (cluster) structural units. It is this range of very weak bonds that could account for the remarkable ease of changing the structure of water, which in turn could help explain the half-dozen well-known anomalies in its properties. In its subtler form, such weak bonds would also allow for the changes of structure caused by electric and

Present status and perspective of IBAD and PLD system in SRL and self-epitaxy in PLD-CeO 2 on IBAD seed layer

This paper reports the recent activity for the coated conductor in Superconductivity Research Laboratory (SRL), especially concerning to the ion beam assisted deposition (IBAD) and pulsed laser deposition (PLD) methods, which are considered to be the most promising from the recent achievements in terms of the length and critical current density. In SRL, we have recently introduced a reel-to-reel IBAD machine. The IBAD machine successfully brought about a textured Gd 2Zr 2O 7 layer on Hastelloy tapes. The degree of in-plane texturing, delta phi Δ ϕ, of the samples was 13° in a meter long conductor. This IBAD machine will be a strong tool to study a long YBCO coated conductor. The second recent result is the PLD-CeO 2 buffer layer on an IBAD tape. Being different from the conventional studies, the PLD-CeO 2 layer not only grew epitaxially but also developed its degree of texturing during the PLD deposition. The degree of texturing in the CeO 2 layer reached in the order of 2° and 5° on the IBAD tapes with Δ ϕ around 10° and 25°, respectively. The texturing was so fast that we obtained a Δ ϕ of 10° in 1 min and 5–6° in 6 min, starting from a thin IBAD substrate with a Δ ϕ of 24–27°. We can call this “self-epitaxy” in the meaning of using no assist like Ar ion in the IBAD method. This new epitaxy phenomenon resulted in a small misorientation angles between grains and then reduced the weak-link induced degradation of J c in the grain boundaries of the conductor. A PLD-YBCO layer on the above CeO 2 layer achieved a critical current density as high as 3.8 MA/cm 2 at 77 K and 0 T, which was not so largely degraded compared to the conventional coated conductors with the misorientation angles around 10° and J cs of 1–2 MA/cm 2. Therefore, “self-epitaxy” is strongly expected to accelerate a practical conductor development due to its rapidly and highly texturing characteristic with the possibility to weaken the weak-link problem inherent in oxide superconductors.

Epitaxial phenomena across length and time scales

AbstractThe morphological evolution of an epitaxial film results from atomistic processes such as adatom motion and step–adatom interactions asserting their influence over the macroscopic length and time scales of the growth front. Modelling epitaxial phenomena thus necessitates making a compromise between the detailed information provided by first‐principles methods and the computational flexibility afforded by methods such as Monte‐Carlo simulations and continuum equations of motion, in which atomistic processes are replaced by coarse‐grained effective kinetics. We will review the various approaches that are available for modelling epitaxial phenomena using the (001) surfaces of III–V compound semiconductors as a case study, with a view to making direct comparisons with experimental measurements and to establishing a methodology that is capable of incorporating all pertinent length and time scales. Copyright © 2001 John Wiley & Sons, Ltd.

Curvature walls and focal conic domains in a lyotropic lamellar phase

In the quasi-ternary CPCl/brine/hexanol lyotropic system, the interface of the \({L_\alpha }\) lamellar and L3 sponge phases displays a phenomenon of epitaxy: the layers of the lamellar phase tend to make a constant non-trivial angle with the interface. Thin samples of lamellar phase embedded in the sponge phase are thus submitted to oblique anchoring conditions and defects are created in the lamellar phase in order to satisfy the bulk lamellar ordering and the boundary conditions. We have studied small droplets of lamellar phase in the sponge phase. They do not exhibit the classic \({L_\alpha }\) defects (focal conic domains) but wall defects, which appear in order to satisfy the smectic elasticity and the boundary conditions. Moreover we show through experiments in controlled geometry that, even in the presence of focal conic domains, wall defects control the size and periodicity of the textures which are observed at the interface.

Level-set methods for the simulation of epitaxial phenomena

We introduce a model for epitaxial phenomena based on the motion of island boundaries, which is described by the level-set method. Our model treats the growing film as a continuum in the lateral direction, but retains atomistic discreteness in the growth direction. An example of such an ``island dynamics'' model using the level-set method is presented and compared with the corresponding rate equation description. Extensions of our methodology to more general settings are then discussed.

Island-Size Distributions for Submonolayer Epitaxy: Rate Equations and Beyond

AbstractThe scanning tunnelling microscope has revolutionized the quantitative analysis of epitaxial phenomena. This, in turn, has spawned a huge theoretical effort aimed at analyzing various aspects of the morphology of growing surfaces. One of the most important general approaches to have emerged from this effort is based on the application of scaling concepts to epitaxial island-size distributions in the regime of submonolayer coverage prior to coalescence. We first discuss the analytical basis for scaling solutions to rate equations. In the limit of irreversible aggregation, a solution is obtained in terms of the capture numbers which agrees with previous work. For reversible aggregation, we identify a new quantity that may be regarded as a continuous analogue of a critical island size. We then examine the influence of spatial correlations by introducing a method for modeling epitaxial phenomena in terms of the motion of island boundaries, which is implemented numerically using the level set method. This island dynamics model is continuous in the lateral directions, but retains atomic scale discreteness in the growth direction. Several choices for the island boundary velocity are discussed and computations of the island dynamics model using the level set method are presented.

Orientational Dependence of the Surface Free Energy of theLα−L3Interface

Geometrical, topological, and energetical properties on an interface between the lamellar Lα and the isotropic sponge phase L3 are considered. The sponge phase shows a characteristic interbilayer separation d3 that is larger than the interbilayer separation dα in the Lα phase. This feature controls the experimentally observed phenomenon of orientational epitaxy (Quilliet, C.; Blanc, C.; Kléman, M. Phys. Rev. Lett. 1996, 77, 522) that keeps the lamellar Lα bilayers tilted with respect to the interface. We discuss the concentration profile near the interface, the geometry of smooth matching between the bilayers of both phases and estimate the surface tension of the Lα−L3 interface at equilibrium orientation of layers. The energy needed to deviate the layers from the equilibrium orientation (“anchoring” energy) is calculated using the Frenkel−Kontorova dislocation model. The model also predicts faceting, which is observed experimentally. Both the surface tension and anchoring energies are of the order of K/dα, where K is the bend elastic constant. The role of layer undulation in tangential anchoring is discussed and illustrated experimentally. Finally, an outcome of the theory is that the orientation is related to the topological parameter of the sponge phase (coordination number of the passages introduced by Porte et al., J. Phys. (Paris) 1988, 49, 511).

Seeded Epitaxial Growth of PbTiO3 Thin Films on (001) LaAlO3 using the Chemical Solution Deposition Method

ABSTRACTEpitaxial PbTiO3(PTO) thin films were grown on (001) LaAlO3 (LAO) substrates by a two-step process from a mixed Pb-Ti double-alkoxide solution precursor. Scanning electron microscopy, x-ray diffraction, atomic force microscopy, and transmission electron microscopy were performed to characterize the microstructure of films and to evaluate the epitaxial phenomena. In the first step, very thin PTO films were deposited and then heat treated at 800 °C for lh. The films were broken up into isolated islands that have epitaxial relationship of [100](001)PTO∥[100](001)LAO. In the second step, another PTO thin films were deposited and then annealed at temperatures between 380 °C and 800 °C for 1h. In this seeded process, epitaxial PTO thin films, unobtainable when PTO thin films were deposited on the unseeded LAO substrates, could be grown. The islands formed in the first step act as seeds for the growth of epitaxial thin film in the second step.

Evidence of a Phenomenon of Epitaxy at the Interface between a Lamellar L alpha Phase and a L3 Sponge.

In the quasiternary lyotropic system CpCl $/$brine $/$hexanol, the interface between the lamellar ${L}_{\ensuremath{\alpha}}$ and the ``sponge'' ${L}_{3}$ phase adopts geometries characteristic of a tilted anchoring of the lamellar layers on the sponge phase. Quantitative measurements of this new phenomenon are performed at equilibrium. Additionally, the evolution of the characteristic distances ${d}_{\ensuremath{\alpha}}$ and ${d}_{3}$ of the two phases in the biphasic region with swelling variation was determined by x-ray scattering. Our results support the hypothesis of an epitaxial matching between the two phases.

The microscopic mechanism of surfactant epitaxy by molecular dynamics

Surfactant epitaxy has been expected to be a powerful method to improve thin-film growth from 3D island to layered growth, but the atomic level process of surfactant epitaxy has not been revealed yet. The purpose of this paper is to elucidate the phenomenon of surfactant epitaxy by molecular dynamic simulations. It is found that a smaller surface energy is necessary for the surfactant to be stable on the surface and that the difference of the atomic radii between the surfactant and the adatom plays an important role in the process of quick exchange of their positions.

Epitaxial growth of PbTiO3 thin films on (001) SrTiO3 from solution precursors

A mixed alkoxide liquid precursor was used to form epitaxial PbTiO3 thin films by spin-coating on cubic (001) SrTiO3 substrates. The films were heat-treated at temperatures between 380 °C/1 h and 800 °C/1 h. X-ray diffraction, atomic force microscopy, scanning and transmission electron microscopy were used to characterize the microstructure of the films and to evaluate the epitaxial phenomena. At ∼400 °C/1 h, a polycrystalline, metastable Pb-Ti fluorite crystallizes from the pyrolyzed amorphous precursor. At slightly higher temperatures (∼420 °C/1 h), the thermodynamically stable phase with the perovskite structure epitaxially nucleates at the film/substrate interface. A small number of epitaxial grains grow through the film toward the surface and consume the nanocrystalline fluorite grains. Coarsening of the perovskite grains leads to a reduction in mosaic spread during further heating. Pores, which concurrently coarsen with grain growth, produce a pitted surface as they disappear from within the film. At 800 °C/1 ha dense epitaxial PbTiO3 film with a smooth surface is observed. Parameters governing the formation of a- and c-domains are discussed as well as the small tilts of the domain axes away from the substrate normal.