Epitaxial Rotation Research Articles

In situ manipulation of the active Au-TiO2 interface with atomic precision during CO oxidation.

The interface between metal catalyst and support plays a critical role in heterogeneous catalysis. An epitaxial interface is generally considered to be rigid, and tuning its intrinsic microstructure with atomic precision during catalytic reactions is challenging. Using aberration-corrected environmental transmission electron microscopy, we studied the interface between gold (Au) and a titanium dioxide (TiO2) support. Direct atomic-scale observations showed an unexpected dependence of the atomic structure of the Au-TiO2 interface with the epitaxial rotation of gold nanoparticles on a TiO2 surface during carbon monoxide (CO) oxidation. Taking advantage of the reversible and controllable rotation, we achieved in situ manipulation of the active Au-TiO2 interface by changing gas and temperature. This result suggests that real-time design of the catalytic interface in operating conditions may be possible.

Unlocking the Single‐Domain Epitaxy of Halide Perovskites

AbstractThe growth of epitaxial semiconductors and oxides has long since revolutionized the electronics and optics fields, and continues to be exploited to uncover new physics stemming from quantum interactions. While the recent emergence of halide perovskites offers exciting new opportunities for a range of thin‐film electronics, the principles of epitaxy have yet to be applied to this new class of materials and the full potential of these materials is still not yet known. In this work, single‐domain inorganic halide perovskite epitaxy is demonstrated. This is enabled by reactive vapor phase deposition onto single crystal metal halide substrates with congruent ionic interactions. For the archetypical halide perovskite, cesium tin bromide, two epitaxial phases, a cubic phase and tetragonal phase, are uncovered which emerge via stoichiometry control that are both stabilized with vastly differing lattice constants and accommodated via epitaxial rotation. This epitaxial growth is exploited to demonstrate multilayer 2D quantum wells of a halide‐perovskite system. This work ultimately unlocks new routes to push halide perovskites to their full potential.

Origin of Moiré structures inC60on Pb(111) and their effect on molecular energy levels

Low-energy electron diffraction LEED indicates that the monolayer structure of C60 on Pb 111 comprises two coexisting incommensurate structures with nonsymmetry epitaxial rotations near 20° relative to the Pb 111 lattice. These structures are observed in scanning tunneling microscopy STM as Moire superstructures having periods of about 46 A and 34 A. The Moire images and LEED patterns are consistent with two higher-order commensurate HOC structures that were identified using the hexagonal number sequence method. These structures are close to predictions from the Novaco-McTague theory of epitaxial rotation, assuming a weakly corrugated substrate potential. As a consequence of the fullerenes within the Moire structures having different local environments, the energetic alignment of the molecular resonances is also modulated, with shifts measured by tunneling spectroscopy of up to 20 meV.

Magnetic distorted spiral structure of Nd at 10 K

Nd has been suggested to have the unusual2q sinusoidalstructure below TN = 19.9 K. However, here will be presented an analysis of neutron scatteringdata, which shows that a flat distorted spiral with aspect ratio 1:3 atTN/2 = 10 K providesa better fit and a physically more appealing model. It turns into a striped (sinusoidal) structure only atT2 = 19.1 K, veryclose to TN, where the non-magnetic stripes are formed. A simple mean field theorydemonstrates that this transition may be driven by a minute anisotropy. Amodel of randomly placed singlet pairs á la Anderson resonance-valence-bond(RVB) phase is discussed. The observed splitting up of satellite peaks belowT2 is discussed in terms of an epitaxial rotation model. The results are discussed in relation toprevious models.

Adsorption geometry ofCu(111)−Csstudied by scanning tunneling microscopy

Using scanning tunneling microscopy at low temperatures we investigated cesium adsorbed on $\mathrm{Cu}(111)$. At low coverages we observe a hexagonally ordered Cs adsorption layer with a mutual adsorbate distance of $1.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. This distance is discussed in terms of a commensurate adsorption superstructure which is stabilized by long-range adsorbate interactions mediated by $\mathrm{Cu}(111)$ surface state electrons. Intermediate coverages are characterized by incommensurate superstructures which are rotated with respect to the substrate lattice. The rotation angle varies with coverage and follows a trend which is consistent with models of epitaxial rotation. With increasing coverage the adsorption layers are found to rotate toward alignment with the substrate.

Electrochemical growth of copper on well-defined n-Si(1 1 1):H surfaces

Monohydride-terminated n-Si(1 1 1) electrodes were used as substrates for the electrodeposition of Cu from 0.1 M H 2SO 4 + 0.1 mM CuSO 4. In situ scanning tunneling microscopy, ex situ atomic force microscopy and scanning electron microscopy revealed that (a) the nucleation starts preferentially at surface defects such as etch pits or step edges, and (b) the deposition occurs by a Volmer–Weber growth mode. In situ surface X-ray diffraction on this system, reported for the first time, revealed the structure and the epitaxial relationship of the copper deposit on the Si substrate. Under the X-ray beam, the Cu deposit could be stabilized only with a significant overpotential. Cu clusters grow with the (1 1 1), (1 1 0), and (0 0 1) planes oriented parallel to the Si(1 1 1), with the first two orientations being dominant. Within the Si(1 1 1) plane, Cu clusters can be found in any rotational orientation around the Si(1 1 1) surface normal. The intensity distribution can be described as powder rings. This distribution is, however, peaked along specific directions. For the Cu clusters with Cu(1 1 0)∥Si(1 1 1) it is found that in-plane Cu〈1 1 1〉∥Si〈1 1 0〉. The peaks in the distribution of the Cu clusters with Cu(1 1 1)∥Si(1 1 1) reveal that these Cu clusters are basically rotated by 30° around the Si(1 1 1) surface normal but exhibit additionally a ±4.5° epitaxial rotation around the mean, high symmetry direction.

Molecular Dynamics Study of Cu Thin Film Deposition onβ-Ta

AbstractMolecular Dynamics simulations were performed to study Cu film deposition on β-Ta. Three different β-Ta surfaces were used, two being atomically flat, and one resulting from Ta on Ta growth. We find that the Cu films develop a (111) texture with vertical grain boundaries between grains having different epitaxial relations with the β-Ta substrate. The epitaxial rotation angles were determined, as 5.2° and 10-13°, and the resulting strain reductions in the Cu films were identified. The effects of the substrate differences on the interfacial Ta/Cu intermixing and the epitaxy and grain boundary structure of the films are discussed.

Thermal and Structural Properties of N2 or CO on Ag(111)

Low-energy electron diffraction (LEED) measurements were made for N2 and CO adsorption on Ag(111) at temperatures (T) >25 K. The results indicate that at coverages up to a saturated monolayer, N2 and CO each forms a rotated incommensurate hexagonal structure. The first layer of CO is somewhat more strongly bound to the surface than that of N2, and has a monolayer lattice spacing that is ∼5% smaller. This result is interpreted as being due to a more perpendicular orientation of the CO molecules. The isobaric expansion coefficient for the CO monolayer is considerably smaller than that of N2, which may also be related to a more perpendicular orientation of CO molecules. The epitaxial rotation angle measured for N2 is close to the Novaco-McTague prediction for the lowest temperatures measured; the angle for CO, on the other hand, is very different from the predictions. No additional order was noted upon adsorption of the N2 second layer. Adsorption of a CO second layer, however, produces an oblique and mutual...

Epitaxial rotation of two-dimensional rare-gas lattices on Ag(111)

Low-energy electron-diffraction results from rare gases adsorbed on Ag(111) are presented. On the clean surface, the rare-gas overlayer lattices are always aligned with the substrate step direction, but when a small amount of a different adsorbate is put on the surface first, the overlayer grows in a rotated orientation at a nonsymmetry angle which depends on the rare gas and its exact lattice spacing and on the preadsorbate. These lattice rotation angles are not those predicted by the Novaco-McTague (NM) theory for epitaxial rotation, nor those predicted by any other model for epitaxial rotation. Molecular statics calculations for Xe/Ag(111) produce results in agreement with the NM prediction, not the experimental results. We speculate that atoms or molecules preadsorbed at step sites play an active role in the equilibrium rotation angles, possibly by changing the corrugation of the charge density in their vicinity.

Structural defects and epitaxial rotation of C60 and C70(111) films on GeS(001)

A transmission electron microscopy study of epitaxial C60 and C70 films grown on a GeS (001) surface is presented. The relationship between the orientation of the substrate and the films and structural defects in the films, such as grain boundaries, unknown in bulk C60 and C70 crystals, are studied. Small misalignments of the overlayers with respect to the orientation of the substrate, so-called epitaxial rotations, exist mainly in C70 films, but also sporadically in the C60 overlayers. A simple symmetry model, previously used to predict the rotation of hexagonal overlayers on hexagonal substrates, is numerically tested and applied to the present situation. Some qualitative conclusions concerning the substrate-film interaction are deduced.

Phase diagrams and rotated incommensurate phases of K, Rb, and Cs adsorbed on Ag(111).

Submonolayers of potassium, rubidium, and cesium on Ag(111) form hexagonal overlayers which increase in density uniformly as coverage is increased. There are no indications that these overlayers form condensed (island) structures at any submonolayer coverages. The stability of the overlayers increases with coverage, with melting temperatures changing from about 40 K at 0.11 coverage to 250 K at 0.33 coverage for potassium, with somewhat higher temperatures for Rb and Cs. The overlayers are in general incommensurate, but at certain coverages form structures which are commensurate with the substrate. The commensurate structures formed at temperatures down to 35 K are the (2$\sqrt{3}$\ifmmode\times\else\texttimes\fi{}2$\sqrt{3}$)R30\ifmmode^\circ\else\textdegree\fi{} structure for Cs and Rb, and the p(3\ifmmode\times\else\texttimes\fi{}3), (\ensuremath{\surd}7\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}7)R19.1\ifmmode^\circ\else\textdegree\fi{}, p(2\ifmmode\times\else\texttimes\fi{}2), and ($\sqrt{3}$\ifmmode\times\else\texttimes\fi{}$\sqrt{3}$)R30\ifmmode^\circ\else\textdegree\fi{} structures for K, Rb, and Cs. At certain densities the hexagonal overlayer structures are rotated with respect to the substrate. The rotation behavior at 35 K follows a general trend which is largely consistent with various models of epitaxial rotation. The rotation angles are temperature dependent, usually rotating toward alignment with the substrate (0\ifmmode^\circ\else\textdegree\fi{}) as the temperature is increased. Some of the data are consistent with the mean-field theory describing rotational epitaxy. \textcopyright{} 1996 The American Physical Society.

The rotational disorder of Pb/W(112)

Using high-resolution low-energy electron diffraction, we have observed the rotational disorder of Pb/W(112) during MBE growth. Pb atoms grow on W(112) substrate in a typical 3D fashion, forming Pb(111) islands as the thickness of the Pb layers increases. While the surface normal of the Pb(111) islands aligns with that of the W(112), the lateral lattice orientation within the Pb(111) plane is not unique with respect to the W(112) substrate. Depending on the growth rate, the Pb overlayer may have different morphologies. At the lower deposition rate, random orientations as well as six discrete orientations are observed. At the medium deposition rate, four discrete orientations are observed. At the higher deposition rate, only one orientation is observed but with certain lateral deviations. These orientations can be attributed to the substrate geometry, lattice matching at the interface and the symmetry principle for epitaxial rotation.

Theory and simulation of epitaxial rotation: Light particles adsorbed on graphite.

We present a theory and Monte Carlo simulations of adsorbed particles on a corrugated substrate. We have focused on the case of rare gases and light molecules, H 2 and D 2 , adsorbed on graphite. The competition between the particle-particle and particle-substrate interactions gives rise to frustration phenomena. From our model predictions for the epitaxial rotation angle of the adsorbed phases are determined. Our results extend and unify previously known descriptions. We have studied as a function of temperature and coverage the phase diagrams, especially the intermediate phases appearing between the commensurate and incommensurate phase for the adsorbed systems

Theory and Monte Carlo simulation of adsorbates on corrugated surfaces

Phase transitions in systems of adsorbed molecules on corrugated surfaces are studied by means of Monte Carlo simulation. Particularly, we have studied the phase diagram of D2 on graphite as a function of coverage and temperature. We have demonstrated the existence of an intermediate γ-phase between the commensurate and incommensurate phase stabilized by defects. Special attention has been given to the study of the epitaxial rotation angles of the different phases. Available experimental data is in agreement with the simulations and with a general theory for the epitaxial rotation which takes into account defect stabilized situations.

Theory and Monte Carlo simulation of adsorbates on corrugated surfaces

Phase transitions in systems of adsorbed molecules on corrugated surfaces are studied by means of Monte Carlo simulation. Particularly, we have studied the phase diagram of D 2 on graphite as a function of coverage and temperature. We have demonstrated the existence of an intermediate γ-phase between the commensurate and incommensurate phase stabilized by defects. Special attention has been given to the study of the epitaxial rotation angles of the different phases. Available experimental data is in agreement with the simulations and with a general theory for the epitaxial rotation which takes into account defect stabilized situations.