Atomic Terraces Research Articles

Graphene growth by molecular beam epitaxy using a solid carbon source

AbstractThe direct elaboration of graphene by molecular beam epitaxy (MBE) has been studied, using a solid carbon source. Successful growth has been achieved on both the carbon and silicon terminated faces of silicon carbide substrates in the temperature range 1000–1100 °C, as confirmed by low energy electron diffraction (LEED) and X‐ray photoemission spectroscopy (XPS) analysis. Atomic force microscopy (AFM) observations show that the initial substrate structure, i.e. flat atomic terraces and half‐period high steps, remains almost unaffected during the growth, contrary to what is observed following the graphitization process.

Vacancy Mediated Diffusion at Surface-Confined Atomic Intermixing

The present study deals with diffusion behavior of adsorbed atoms on stepped crystal surfaces. In volume-immiscible systems, two-dimensional (2D) atomic intermixing at epitaxial interface could be completely blocked on step-free surface domains. This is a result of high diffusion barrier for direct atomic exchange between adsorbed layer and substrate. In that case, diffusion takes place exclusively across the steps of atomic terraces. In such systems, dynamic competition between energy gain by mixing and substrate strain energy results in diffusion scenario where adsorbed atoms form alloyed stripes in the vicinity of terrace edges. At high temperatures, the stripe width increases and finally completely destroys the terraces. This process leads to formation of alloyed 2D atomic islands on top of pure substrate layer. The atomistic Monte Carlo simulations reveal vacancy-mediated mechanism of diffusion inside atomic terraces as a result of spontaneous generation of vacancies at high temperature. Being in agreement with recent experimental findings, the observed surface-confined alloying opens up a way various surface pattern to be configured at different atomic levels on the crystal surface.

Dominance of broken bonds and nonbonding electrons at the nanoscale

Although they exist ubiquitously in human bodies and our surroundings, the impact of nonbonding lone electrons and lone electron pairs has long been underestimated. Recent progress demonstrates that: (i) in addition to the shorter and stronger bonds between under-coordinated atoms that initiate the size trends of the otherwise constant bulk properties when a substance turns into the nanoscale, the presence of lone electrons near to broken bonds generates fascinating phenomena that bulk materials do not demonstrate; (ii) the lone electron pairs and the lone pair-induced dipoles associated with C, N, O, and F tetrahedral coordination bonding form functional groups in biological, organic, and inorganic specimens. By taking examples of surface vacancy, atomic chain end and terrace edge states, catalytic enhancement, conducting-insulating transitions of metal clusters, defect magnetism, Coulomb repulsion at nanoscale contacts, Cu(3)C(2)H(2) and Cu(3)O(2) surface dipole formation, lone pair neutralized interface stress, etc, this article will focus on the development and applications of theory regarding the energetics and dynamics of nonbonding electrons, aiming to raise the awareness of their revolutionary impact to the society. Discussion will also extend to the prospective impacts of nonbonding electrons on mysteries such as catalytic enhancement and catalysts design, the density anomalies of ice and negative thermal expansion, high critical temperature superconductivity induced by B, C, N, O, and F, the molecular structures and functionalities of CF(4) in anti-coagulation of synthetic blood, NO signaling, and enzyme telomeres, etc. Meanwhile, an emphasis is placed on the necessity and effectiveness of understanding the properties of substances from the perspective of bond and nonbond formation, dissociation, relaxation and vibration, and the associated energetics and dynamics of charge repopulation, polarization, densification, and localization. Finding and grasping the factors controlling the nonbonding states and making them of use in functional materials design and identifying their limitations will form, in the near future, a subject area of "nonbonding electronics and energetics", which could be even more challenging, fascinating, promising, and rewarding than dealing with core or valence electrons alone.

Fabrication of PbTiO3 Nanocrystals on Atomically Flat Sapphire Substrates by Sputtering

In order to perform the growth position control, PbTiO3 (x=0) (PTO) nanocrystals were deposited on atomically flat and non-atomically flat -Al2O3 substrates by RF magnetron sputtering. The atomically flat substrates with atomic steps and terrace are expected to induce the lateral growth along a terrace between the surface steps. In the case of the atomically flat, the nanocrystals got lined up along the atomic steps, and uniform nanocrystals were obtained on surface in same pitch.

Active Sites for the Oxygen Reduction Reaction on the Low and High Index Planes of Palladium

Active sites for the oxygen reduction reaction (ORR) have been studied on the low index planes, n(100)-(111) and n(100)-(110) series of Pd with the use of hanging meniscus rotating disk electrode (RDE) in 0.1 M HClO4 saturated with O2. The Kotekey−Levich plots give a linear line with a slope of about 4, showing four electron reduction for the ORR on Pd electrodes, as is the case of Pt. Activity of ORR, which is estimated by the reduction current density at 0.90 V (RHE) jORR,0.90V, gives the following order on the low index planes: Pd(110) < Pd(111) < Pd(100). This order is completely opposite to that of Pt in 0.1 M HClO4: Pt(100) < Pt(111) < Pt(110). Specific activity of Pd(100) is nearly three times as high as that of Pt(110) at 0.90 V (RHE). The values of jORR, 0.90 V increase linearly with the increase of the terrace atom density on n(100)-(111) and n(100)-(110) series of Pd. This fact supports that (100) structure is the active site for ORR on Pd. ORR activity does not depend on the step structure of n(100)-(111) and n(100)-(110) series.

How r-Plane Al2O3 Surface Modifications Impact the Growth of Epitaxial (001) CeO2 Thin Films

We present evidence that it is the presence or absence of atomic terraces with a specific crystallographic orientation on the (102) Al2O3 surface that promotes growth of single-crystal (001) CeO2 films over polycrystalline (111) CeO2 films. The CeO2 film nucleates so that the [010] and [100] directions of the film align parallel and perpendicular to the terrace edges. In the absence of terraces, multidomain (111) CeO2 films result in which the in-plane orientation of the two domains are rotated by 85.71°, so that a [110] CeO2 direction aligns parallel to either the $$ \left[ {\bar{2}21} \right] $$ or $$ \left[ {\bar{2}\bar{4}1} \right] $$ Al2O3 direction.

Magnetic second-harmonic generation from interfaces and nanostructures

Magneto-optic techniques provide non-contact and non-destructive characterization of magnetic materials. This includes embedded magnetic nanostructures, which are accessible due to the large penetration depth of optical radiation. The linear magneto-optic Kerr effect is widely used in the growth and characterization of ultra-thin magnetic films and can show monolayer sensitivity. Nonlinear magnetic second-harmonic generation (MSHG) is a more difficult and expensive technique but, uniquely, can measure the surface and interface magnetism of centrosymmetric magnetic films with sub-monolayer sensitivity. MSHG is briefly reviewed and examples from high symmetry interfaces and nanostructures described. Low symmetry structures are more difficult to characterize, however, because of the large number of tensor components that may contribute to the signal. An important class of low symmetry systems exploits vicinal substrates to grow aligned magnetic nanostructures by self-organization. These structures have a high proportion of magnetic step or edge atoms relative to the terrace atoms, and the overall magnetic response is expected to contain significant contributions from these different magnetic regions. It is shown that contributions from these different regions can be identified using normal incidence (NI) MSHG. This new approach is used to determine hysteresis loops from Au-capped Fe monolayers grown on a vicinal W(1 1 0) substrate. Temperature-dependent studies of the MSHG contrast also allow Curie temperatures to be determined. This experimental procedure and phenomenology opens up low symmetry magnetic interfaces and aligned nanostructures to characterization by MSHG.

Improved luminescence efficiency of InAs quantum dots grown on atomic terraced GaAs surface prepared with in‐situ chemical etching

AbstractObservation of the enhanced luminescence efficiency of InAs quantum dots (QDs) grown on atomically controlled GaAs surfaces is reported. With the trisdimethylaminoarsenic (TDMAAs) in‐situ surface etching process, formation of atomic steps and terraces on GaAs surfaces were clearly observed. InAs QDs grown on the processed GaAs surfaces showed the clear dependence of QDs size, density and optical characteristics on the surface properties, i.e., the increase of the QDs height and diameter the decrease of the QDs density. About 6‐times enhancement of photoluminescence efficiency which has the peak around 1550‐nm wavelength was observed by growing InAs QDs on atomically controlled GaAs surfaces. This is due to the migration enhancement of InAs during thegrowth the QDs. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of substrate annealing on the quality of pulsed laser deposited Zn 1− xMg xO thin films

The annealing effects of sapphire substrate before deposition on the quality of epitaxial Zn 1− x Mg x O thin films grown by pulsed laser deposition are reported. Our Experimental results indicate that the surface quality of Zn 1− x Mg x O thin films and hexagonal columnar growth is improved on the annealed sapphire substrate at high temperatures due to formation of atomic terraces on the substrate surface. The photoluminescence signals also increases with the increasing annealing temperature of the substrate.

Atomically Flat Silicon Surface and Silicon/Insulator Interface Formation Technologies for (100) Surface Orientation Large-Diameter Wafers Introducing High Performance and Low-Noise Metal–Insulator–Silicon FETs

Technology to atomically flatten the silicon surface on (100) orientation large-diameter wafer and the formation technology of an atomically flat insulator film/silicon interface are developed in this paper. Atomically flat silicon surfaces composed of atomic terraces and steps are obtained on (100) orientation 200-mm-diameter wafers by annealing in pure argon ambience at 1200 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C for 30 min. Atomically flat surfaces with various terrace widths and step structures are observed by atomic force microscopy. It is found that the atomic terrace width changes widely with an off angle of the wafer surface from the (100) lattice plane. It is also found that the direction of the off angle significantly affects the atomically flat surface morphology, i.e., when the directions of the off angles are parallel to the <110> directions, the step structure is composed of alternating pairs of straight and triangular steps. When the directions of the off angles are parallel to the <100> directions, the step structure is composed of only straight steps. By precise control of the off angle and the direction toward the <100> directions for a 200-mm-diameter silicon wafer, we have succeeded in fabricating an atomically flat surface with straight atomic steps and a very uniform terrace width of 140-150 nm on the entire surface of a large-diameter silicon wafer. Furthermore, it is found that only radical-reaction-based insulator film formation technology, such as oxidation utilizing oxygen radicals carried out at a low temperature (400 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C ), preserves the atomic flatness of the insulator film/silicon interface. Finally, when MOSFETs are fabricated with an atomically flat interface, they exhibit near ideal subthreshold swing factors, with much smaller fluctuation, extremely lower 1/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> noise, and higher MOS dielectric breakdown field intensity compared with MOSFETs fabricated with conventional technologies.

Data Analysis Technique of Atomic Force Microscopy for Atomically Flat Silicon Surfaces

A data analysis technology of atomic force microscopy for atomically flat silicon surfaces has been developed. Atomically flat silicon-surfaces composed of atomic terraces and steps are obtained on (100) orientation 200 mm diameter wafers by annealing in pure argon ambience at 1,200°C for 30 minutes. Atomically flat silicon surfaces are lead to improve the MOS inversion layer mobility and current drivability of MOSFETs and to decrease the fluctuations in electrical characteristics of MOSFETs. It is important to realize the technology that evaluates the flatness and the uniformity of atomically flat silicon surfaces. The off direction angle is calculated by using two straight edge lines selected from measurement data. And the off angle is calculated from average atomic terrace width under assumption that height difference between neighboring terraces is equal to the step height, 0.135 nm, of (100) silicon surface. The analyzing of flatness of each terrace can be realized by converting the measurement data using the off direction angle and the off angle. And, the average roughness of each terrace is about 0.017-0.023 nm. Therefore, the roughness and the uniformity of each terrace can be evaluated by this proposed technique.

Atypical grain growth for (2 1 1) CdTe films deposited on surface reconstructed (1 0 0) SrTiO 3 substrates

The (1 0 0) SrTiO 3 substrate has emerged as the oxide substrate of choice for the deposition of a wide variety of materials. The substrate's unavoidable miscut leads to a step-terrace morphology when heated to high temperatures. This morphological transition is accompanied by an atomic scale repositioning of the uppermost terrace atoms, the nature of which is strongly dependent on the substrate temperature and ambient atmosphere used. Here, we report the deposition of CdTe films on the as-received and reconstructed surfaces of (1 0 0) SrTiO 3. The as-received substrate gives rise to a [1 1 1] CdTe film with four equally distributed in-plane grain orientations. The surface reconstruction, on the other hand, gives rise to an unprecedented reorientation of the film's grain structure. For this case, a [2 1 1] CdTe film emerges having twelve unevenly distributed in-plane orientations. We attribute the film's grain structure to an atomic scale surface reconstruction, with the anisotropic distribution of grain-types arising from a preferential formation due to the step edges.

Pseudogap formation in the metallic state of La0.7Sr0.3MnO3 thin films

We report on scanning tunneling microscopy and spectroscopy (STM/S) studies of epitaxial La0.7Sr0.3MnO3 (LSMO) thin films on a lattice matched (001) La0.3Sr0.7Al0.35Ta0.35O9 substrate for both as-grown and annealed films. In contrast to the as-grown films, the films annealed at 800 °C in air show atomic terraces with spectra that develop a gaplike structure with cooling. We show that the gap structure can be attributed to the predicted pseudogap in the manganites. Unlike several previous reports, we did not find electronic inhomogeneities in LSMO by STM/S.

High quality AlN grown on SiC by metal organic chemical vapor deposition

Growth conditions for AlN in two dimensional (2D) and three dimensional (3D) growth modes were explored on SiC using metal organic chemical vapor deposition. High quality AlN layers were obtained by alternating between 3D and 2D growth modes, referred to as modulation growth (MG). Long parallel atomic terraces without step terminations were observed in atomic force microscopy (AFM) scans of MG AlN, indicating a reduced dislocation density. X-ray diffraction rocking curves yielded full widths at half maximum (FWHM) of 86 and 363arcsec for the (002) and (102) reflections, respectively, giving further evidence of low dislocation density in the film. 3D-2D MG also releases some of the tensile strain in the AlN film, enabling the growth of thick, crack-free AlN on SiC substrates.

Site-specific adsorption of metallic and biological nanoparticles on nanostructured silicon surfaces

The (photo)electrochemical preparation of nanostructures on single crystalline Si surfaces is described and surfaces are characterized by tapping mode atomic force microscopy (TM-AFM), Kelvin probe microscopy (KPM), high-resolution electron energy loss spectroscopy (HREELS), and synchrotron radiation photoelectron spectroscopy (SRPES). The H-terminated Si(111) surface and the step-bunched Si surface that exhibits multiatomic bilayer (BL) steps are prepared. HREELS demonstrates the low contamination levels achieved with the used in-system combined electrochemistry/surface analysis apparatus. With a miscut in the direction, the surfaces prepared show a zigzag structure of the atomic terraces. These unique features provide the substrate for the (electro)deposition of platinum and for the immobilization of enzymes. Pt deposition, of relevance for the development of photovoltaic or fuel-generating solar cells, occurs predominantly at the edges of steps. On step-bunched surfaces, where KPM shows negative charging along the step edges and SRPES evidences the presence of an accumulation layer, the Pt nanoislands are considerably smaller than on H-terminated surfaces that are in flat band situation before deposition. SRPES analysis of the chemical and electronic condition after electrodeposition shows silicon oxide formation on both surfaces, suboxidic species, and the typical asymmetric Pt 4f core level lines. The results are discussed based on the details of the Pt deposition process at Si surfaces. As biomolecule, the enzyme reverse transcriptase (RT) of the avian myeloblastosis virus (AMV) was used. Immobilization is observed in TM-AFM experiments at the negatively charged step edges of the step-bunched surface. It is attributed to a superposition of attractive van der Waals and electrostatic interactions. The latter is related to the rather large Debye length of the carrier solution (∼4 nm) and the overall positive charge of the RT where the pH of the carrier solution (pH 7) is smaller than that for the isoelectric point (IP) of 8.3.

Plasmon Resonance in Silver Nanoparticles Arrays Grown by Atomic Terrace Low-Angle Shadowing

The plasmonic properties of aligned silver nanoparticles grown by atomic terrace low-angle shadowing (ATLAS) were studied by optical transmission spectroscopy. ATLAS is a novel nanofabrication technique that combines the advantages of self-assembly with control of the nanostructure features. The silver nanoarrays were optically characterized and distinct optical responses with strong polarization dependence were obtained indicating the existence of a strong near-field coupling between the particles. We found that different morphologies gave rise to a similar optical response due to the local contributions to the overall response. Numerical model based on discrete dipole approximation was used to give a qualitative insight into the studied phenomena.

Lipid Bilayer Membrane with Atomic Step Structure: Supported Bilayer on a Step-and-Terrace TiO2(100) Surface

The formation of a supported planar lipid bilayer (SPLB) and its morphology on step-and-terrace rutile TiO 2(100) surfaces were investigated by fluorescence microscopy and atomic force microscopy. The TiO 2(100) surfaces consisting of atomic steps and flat terraces were formed on a rutile TiO 2 single-crystal wafer by a wet treatment and annealing under a flow of oxygen. An intact vesicular layer formed on the TiO 2(100) surface when the surface was incubated in a sonicated vesicle suspension under the condition that a full-coverage SPLB forms on SiO 2, as reported in previous studies. However, a full-coverage, continuous, fluid SPLB was obtained on the step-and-terrace TiO 2(100) depending on the lipid concentration, incubation time, and vesicle size. The SPLB on the TiO 2(100) also has step-and-terrace morphology following the substrate structure precisely even though the SPLB is in the fluid phase and an approximately 1-nm-thick water layer exists between the SPLB and the substrate. This membrane distortion on the atomic scale affects the phase-separation structure of a binary bilayer of micrometer order. The interaction energy calculated including DLVO and non-DLVO factors shows that a lipid membrane on the TiO 2(100) gains 20 times more energy than on SiO 2. This specifically strong attraction on TiO 2 makes the fluid SPLB precisely follow the substrate structure of angstrom order.

Optical second-harmonic spectroscopy of Au(887) and Au(443) surfaces

In order to investigate the electronic states of step sites on Au surfaces, we have observed the reflected optical second-harmonic (SH) intensity from Au(887) and Au(443) surfaces in ultrahigh vacuum with a normal incident excitation light beam, as a function of the photon energy and the incident and SH light polarizations. The second-order surface nonlinear susceptibility elements observed in this measurement configuration were ${\ensuremath{\chi}}_{xxx}^{(2)}$, ${\ensuremath{\chi}}_{xyy}^{(2)}$, and ${\ensuremath{\chi}}_{yxy}^{(2)}$, where $x$ and $y$ are defined as the $[11\overline{2}]$ and $[\overline{1}10]$ directions, respectively. The step edges lie in the $y$ direction. The ratios of the nonlinear susceptibility elements $|{\ensuremath{\chi}}_{xyy}^{(2)}|/|{\ensuremath{\chi}}_{xxx}^{(2)}|$ and $|{\ensuremath{\chi}}_{yxy}^{(2)}|/|{\ensuremath{\chi}}_{xxx}^{(2)}|$ were different on the two surfaces in the photon energy range from $2\ensuremath{\hbar}\ensuremath{\omega}=2.5$ to 3.3 eV. The deviation of the SH response from that of an ideal $3m$ symmetric Au(111) surface was found to be larger for the Au(443) surface than for the Au(887) surface. This deviation is attributed to the atomic steps created by the miscut of the samples. In order to analyze the observed SH spectra, we calculated the electronic states of a Au(554) slab using a density-functional theory. We found that the low-energy onset of the SH intensity caused by the steps can be qualitatively interpreted by referring to the calculated partial density of the $d$-electronic states of the step and terrace atoms on the Au slab.

Initial sticking probability of O 2 on Cu(4 1 0)

We present here a supersonic molecular beam investigation of the initial sticking probability ( S 0) for the O 2/Cu(4 1 0) system. Over the temperature range between 130 and 800 K adsorption occurs dissociatively and S 0 increases up to ∼0.7 with beam energy, indicating that the process is activated. S 0 is larger for angles corresponding to molecules impinging on the step rises, implying that defects are more reactive than terrace atoms. The saturation value of S 0 is however lower than for the parent low Miller index surfaces; this indicates that the reactivity at nanosized terraces is reduced compared to extended Cu(1 0 0) faces. A precursor mediated adsorption path is observed at the lowest translational energy ( E i ) below 150 K, as in the case of O 2/Cu(1 1 0) and at variance with O 2/Cu(1 0 0). At low T and higher energy and for all E i at T > 150 K, adsorption occurs directly, yielding a sticking probability independent of surface temperature.

Platinum Deposition onto Si Single Crystal Surfaces I. Chemical, Electronic, and Morphological Surface Processes under Depletion Conditions

The initial stages of Pt electrodeposition from solutions on Si(111) surfaces were investigated by standard electrochemical techniques and atomic force microscopy. It was found that under depletion conditions the reduction of Pt species shows a potential dependence that results from the discharging of surface states. The Pt deposition rate increases with the surface state density. Injection of holes from the redox level leads to formation of an ultrathin silicon oxide layer, which takes place simultaneously as a chemical process. Capacitance–voltage measurements show that the potential-dependent Pt deposition increases the concentration of surface states, related to the solvolytic oxidation of Si surface defect sites. The topographical analysis of the surface shows initial Pt deposition on the tips of atomic terraces that extends to deposition along step edges for higher coverage. Also, step bunching is observed with a step height increase to 2–5 nm.