Double-layer Height Research Articles

Importance of Thickness in Human Cardiomyocyte Network for Effective Electrophysiological Stimulation Using On-Chip Extracellular Microelectrodes

We have developed a three-dimensionally controlled in vitro human cardiomyocyte network assay for the measurements of drug-induced conductivity changes and the appearance of fatal arrhythmia such as ventricular tachycardia/fibrillation for more precise in vitro predictive cardiotoxicity. To construct an artificial conductance propagation model of a human cardiomyocyte network, first, we examined the cell concentration dependence of the cell network heights and found the existence of a height limit of cell networks, which was double-layer height, whereas the cardiomyocytes were effectively and homogeneously cultivated within the microchamber maintaining their spatial distribution constant and their electrophysiological conductance and propagation were successfully recorded using a microelectrode array set on the bottom of the microchamber. The pacing ability of a cardiomyocyte's electrophysiological response has been evaluated using microelectrode extracellular stimulation, and the stimulation for pacing also successfully regulated the beating frequencies of two-layered cardiomyocyte networks, whereas monolayered cardiomyocyte networks were hardly stimulated by the external electrodes using the two-layered cardiomyocyte stimulation condition. The stability of the lined-up shape of human cardiomyocytes within the rectangularly arranged agarose microchambers was limited for a two-layered cardiomyocyte network because their stronger force generation shrunk those cells after peeling off the substrate. The results indicate the importance of fabrication technology of thickness control of cellular networks for effective extracellular stimulation and the potential concerning thick cardiomyocyte networks for long-term cultivation.

Step structure and surface morphology of hydrogen-terminated silicon: (0 0 1) to (1 1 4)

We have determined the equilibrium step structures and surface morphology for the whole range of monohydride-terminated (0 0 1)-terrace-plus-step silicon surfaces using scanning tunneling microscopy. The transformation in the equilibrium Si surface morphology caused by H-termination can be categorized into three different regimes delineated by the types of steps present on the clean surfaces. On nominal Si(0 0 1), the single-layer height B-type steps (S B) are mostly non-rebonded and rougher after H passivation. On surfaces dominated by double-layer height B-type steps (D B), such as Si(1 1 11), the non-rebonded D B and S B steps show a lower formation energy. Measurements on post-annealed surfaces indicate that the D B step formation energies strongly depend on H chemical potential. Smoother morphologies are observed following H-termination of surfaces oriented approximately between (1 1 7) and (1 1 4). This effect is quite apparent on Si(1 1 5), where the monohydride surface exhibits large (1 1 5)-(2 × 2) domains, a structure not observed on the clean surface. All of these structural modifications result directly from a change in the relative energies of the possible single- and double-layer step configurations.

Element specific surface reconstructions of islands during surfactant-mediated growth on Si (111).

The early stages of surfactant- (As, Sb) mediated homoepitaxial growth on Si (111) are examined by scanning tunneling microscopy and extensive ab initio calculations of Si(n) clusters (n</=12). The results reveal the different microscopic behavior of the two surfactants: On As-covered Si (111), one exclusively finds two-dimensional islands with double-layer height which show the (1 x 1) terrace structure. On Sb-covered Si (111), the islands show two different reconstructions: at the rim of the islands a (1 x 1) structure appears, while in the center the (square root 3 x square root 3) terrace structure is observed.

From bilayer to trilayer Fe nanoislands onCu3Au(001)

Self assembly on suitably chosen substrates is a well exploited root to control the structure and morphology, hence magnetization, of metal films. In particular, the Cu3Au(001) surface has been recently singled out as a good template to grow high spin Fe phases, due to the close matching between the Cu3Au lattice constant (3.75 Angstrom) and the equilibrium lattice constant for fcc ferromagnetic Fe (3.65 Angstrom). Growth proceeds almost layer by layer at room temperature, with a small amount of Au segregation in the early stage of deposition. Islands of 1-2 nm lateral size and double layer height are formed when 1 monolayer of Fe is deposited on Cu3Au(001) at low temperature. We used the PhotoElectron Diffraction technique to investigate the atomic structure and chemical composition of these nanoislands just after the deposition at 140 K and after annealing at 400 K. We show that only bi-layer islands are formed at low temperature, without any surface segregation. After annealing, the Fe atoms are re-aggregated to form mainly tri-layer islands. Surface segregation is shown to be inhibited also after the annealing process. The implications for the film magnetic properties and the growth model are discussed.

Thermal relaxation of isolated silicon pyramids on the Si([formula omitted])2×1 surface

Silicon mounds fabricated on the Si(100)2×1 surface by a tip of a scanning tunneling microscope (STM) have been observed by STM at substrate temperature of 770 K. The shape of the mound is a quadrangular pyramid with facets of regular array of steps with double layer height. For all the step, the dimer rows are perpendicular to the step edges, so called the DB step. Just after the fabrication, the pyramid begins to decay immediately layer-by-layer. During decay, the DB steps move scarcely, but only the SB steps at the bottom layer move due to attachment and detachment of atoms which are detached from the upper layers. For a single layer mound, the aspect ratio of the mound oscillates between about 2.5 and 1.5.

The temperature-dependent growth of Fe submonolayer film on Au(001) studied by barrier height imaging using scanning tunneling microscopy

The growth of Fe submonolayer films on Au(001) at various substrate temperatures (300–500 K) was investigated by scanning tunneling microscopy (STM). STM topographic and barrier height images show that the growth mode and morphology of the films markedly depend on growth temperature. Fe islands with a monolayer height nucleate and grow on flat terraces of Au(001) at 300 K. However, Fe islands with double-layer height and small pinholes were observed on terraces at 353 and 393 K, and step edges were decorated by Fe dendrites. A clear element-specific contrast between Fe islands and Au substrate was observed below 400 K. At elevated growth temperature, above 400 K, a segregated Au layer was observed. The mechanism of the surface segregation of Au is also discussed in terms of the surface diffusion through the pinholes.

D(A) steps and 2D islands of double layer height in the SiGe(001) system

The surfaces of step graded, partially relaxed Si(1-x)Ge(x)/Si(001) buffers were studied by scanning tunneling microscopy. The surface slips along <110> forming the crosshatch pattern, consisting of bunches of D(A) steps of double layer height. The D(A) steps are present in regions of large surface gradients close to the slips, as well as in planar regions between the slips. These regions are also characterized by the appearance of 2D islands of double layer height. The observations can be explained by assuming the strain due to the misfit dislocations to be locally anisotropic. Anisotropic misfit strain and efficient strain relaxation by the ( 2x8) Ge reconstruction were identified as the main factors causing the unusual step structure.

Evolution of Morphology During Etching of Si

ABSTRACTThe step morphology of clean Si surfaces has been studied under conditions of thermal etching in the temperature range 950 – 1250°C. Kinetic-bunching of steps is caused by direct current in the step-down direction around 950°C. By comparing the rate of thermal decay of these structures with and without direct current, the electromigration force causing this step bunching is estimated to be due to an effective charge of less than or approximately 0.01 electron units. Around 1150°C, step-bunching is caused by direct current in the step-up direction. By analysis of the patterns of step structure, the effective charge of the driving force is found to be approximately -0.1 electron units. Oxygen-induced etching of Si(001) and Si(111) has been studied in the temperature range of 700 – 900 °C, and at a pressure of 5 × 10−7 torr, conditions under which the surface is etched by the desorption of SiO. On Si(001), the original narrow distribution of double-layer height steps is preserved during the oxygen-etching process. On Si(111), the original narrow distribution of mixed single- and triple-layer height steps changes dramatically during oxygen-etching, leaving wide terraces of flat (111) surface separated by regions of high step density. At low etching temperatures (700°C), the steps remain straight within the step bunches and retain their distinct character as single- and triple-height steps. However, following higher temperature etching, the steps begin to merge into facets in the vicinity of defect structures. Following etching at the highest temperatures studied (815 and 830°C), the pinning action of the defect structures becomes apparent, and the pinned step-bunches become identifiable as (113) facets.

Step structures during MBE growth of GaAs and AlGaAs films on vicinal GaAs(110) surfaces inclined toward (111)B

We have investigated step structures during molecular beam epitaxial (MBE) growth of GaAs and AlGaAs films on vicinal GaAs(110) surfaces inclined at 6° toward (111)B using reflection high-energy electron diffraction. We find that during AlGaAs growth at temperatures above 630°C, the surfaces contain regular arrays oof with double-layer height. At temperatures below 610°C, the surface breaks up into two types of regions: one consisting of wide (110) facets without steps, the other consisting of arrays of steps with a high step density. During GaAs growth at temperatures above 580°C, all the surfaces contain well-ordered arrays of step with double-layer height. Based on the results, we discuss the formation mechanism of the quantum well wire-like structures reported previously.

Electronic structure of a stepped silicon surface

The surface has an average (111) orientation with up and down steps of a double-layer height. The step edges run along the (110) direction and result in atoms with either one or two dangling bonds. The calculation is carried out in a simple linear-combination-of-atomic-orbitals approach and self-consistency is rudimentarily achieved. Density of states projected at various atoms and electronic charge densities are presented. The Fermi level is found to be lowered by 0.3 eV with respect to the corresponding value for the (111) perfect surface, in good agreement with experiment.