Step Edge Region Research Articles

Continuous network structure of two-dimensional silica across a supporting metal step edge: An atomic scale study

The network structure of a silica bilayer film at a monolayer-bilayer transition and across a supporting metal step edge was studied at the atomic scale by scanning tunneling microscopy. The ring size distribution, ring-ring distances, and height profiles are analyzed across the step edge region. Density functional theory proposes two models to explain the observed network structure: a pinning of the lower layer to the substrate and a carpetlike mode. The results indicate a continuous coverage of the silica bilayer film across the step edge.

Sapphire substrate off-angle and off-direction dependences on characteristics of AlGaN-based deep ultraviolet light-emitting diodes

We investigated dependences of sapphire substrate off-angle from 0.2° to 3.0° and off-direction (a-axis or m-axis) on the device characteristics of the AlGaN quantum-well deep ultraviolet LEDs emitting 280–290 nm. We found that the terrace regions of the LEDs showed a lower threading dislocation density, while the step-edge regions of the LEDs contain more threading dislocations, more Ga, and thicker well widths. The emissions from the terrace regions of the LED with the off-angle were dominant at higher current injection. Finally, the light output power from the LED with the m-axis 1.0° off was the highest among the samples. We then developed a model to explain the above optical properties of our deep ultraviolet LEDs with off-angles.

Infrared emissivity and microwave transmission behavior of flaky aluminum functionalized pyramidal-frustum shaped periodic structure

Composites or structures with adequate infrared reflection and microwave transmission are favorable from practical point of view. In this paper, oriented flaky aluminum filled pyramidal-frustum shaped periodic coatings was developed, and the effect of area of flaky aluminum powder filled coating on the infrared emissivity as well as microwave transmission behavior was intensively studied combined experiments with simulation. Results shown that the increments of oriented flaky aluminum filled coating could effectively reduce infrared emissivity. After spraying flaky aluminum filled periodic structures on the microwave absorbing material, the frequency corresponding to minimal reflection loss is inclined to shift towards lower frequency. Numerical simulation results shown that reflection coefficient gradually increases with increasing in frequency and area ratio of flaky aluminum filled coating. Besides, there exists intrinsic attenuation of flaky aluminum filled pyramidal-frustum shaped periodic coating, which is probably attributed to the diffuse scattering within step edge region.

Spatially resolved scanning tunneling spectroscopy of single-layer steps on Si(100) surfaces

Single-layer steps at Si(100) surfaces/interfaces present significant challenges to the quantitative characterization of buried dopant devices as well as the accurate imaging and relocation of fabricated quantum structures. We demonstrate the detailed spatially resolved scanning tunneling spectroscopy study across monolayer step edges on Si(100) surfaces and quantitative determination of the local density of state distributions and behavior of the band gap at step edges. The influence on the local electrostatic environment due to step edge states has been quantified while accounting for the effects of scanning tunneling measurement conditions. The dangling bond states on Si(100) surfaces are utilized as a fingerprint to quantify the local band bending landscape and to make corrections to the experimentally observed surface state energy levels and band gap values at the step edge regions. We observe a significant band gap narrowing behavior along a rebonded single-layer type B step edge on a degenerately boron-doped $p$-type Si substrate.

A Reversible Dendrite-Free High-Areal-Capacity Metallic Lithium Electrode

A reversible dendrite-free high-areal-capacity metallic lithium electrode (MLE) is pivotal to develop beyond lithium ion batteries-LIBs (aqueous Li-air or soluble cathodes, Li-S, Li-O2, etc.), whose practical specific energy strongly depends on the realization of the high-areal-capacity of reversible MLEs, and to develop post-LIBs without safety concerns as well. Recent advances of reviving MLE studies further shed new light on the key role of electrolytes. [ 1 ] In this study, a novel electrolyte has been prepared by the complexation of lithium cations supplied by lithium bis(fluorosulfonyl)imide (LiFSI) and tetraethylene glycol dimethyl ether (G4) with the co-solvent of 1,3-dioxolane (DOL). [ 2 ] The matrix has been disclosed to enable MLE to deliver a high-areal-capacity at elevated temperature in our latest publication. The composition of LiFSI-2G4 (the molar ratio of G4 to Li+ is 2) is found to be a solvate ionic liquid (SIL) with appealing attributes of suppressing lithium dendrite formation at 60 oC, possibly due to the fact that the high fraction of surfactant-role G4 in the solid-electrolyte interphase (SEI) film and elevated temperature could lower the Ehrlich-Schwoebel barrier to facilitate adatoms migration down the descending step that favor two-dimensional layer-by-layer lithium deposition mode.[ 3 ] To make this SIL work at ambient temperature, decreases in the viscosity and interfacial resistance are of great importance. The addition of DOL into LiFSI-2G4 has been demonstrated to lead to the decrease in the viscosity but the absence of alteration of the solvation manner of Li+.[ 4 ] Herein, we first unveil the significant impact of the introduction of cyclic voltammetry (CV) premodulation, prior to the lithium plating onto Cu or Li working electrode, on lithium electrodeposition process and surface film chemistry. With CV premodulation, a Li/Li cell can be cycled at 5.0 mA cm-2 for 100 cycles, and a Cu/Li cell can be cycled at 5.0 mA cm-2 for 450 cycles with a perfect Coulombic efficiency (CE) of 100% unless taking into account of the tiny loss of lithium source during CV premodulation, where both cells exhibit the exceptional high-areal-capacity of 12 mAh cm-2 at 25 oC. In-situ optical observation clearly demonstrated that two-dimensional layer-by-layer dendrite-free growth mode is the prevailing growth mode of lithium electrodeposits in the SIL of LiFSI-2G4 using DOL as the co-solvent with the assistance of CV premodulation. Post-mortem analyses (XPS, SEM, EDX) indicated that the outer G4-derived SEI films along the planar lithium particles, that can retard the surge of lithium ions at the preferential spots due to the strong binding energy of G4 and Li+ (ca. -2.8 eV for nLi +:nG4=1:2) at high current densities, should be subjected to a redistribution process from the top-centered region to the step-edge region along initial planar lithium particles upon CV premodulation, which render lithium adatoms to deposit in the two-dimensional layer-by-layer growth mode because of the decline in the Ehrlich-Schwoebel barrier. The combination of a novel SIL and CV premodulation paves a new avenue to developing practical MLEs for beyond or Post LIBs. To the best of our knowledge, MLE is first reported to achieve a high specific areal-capacity of 12 mAh cm-2 at 5 mA cm-2 and ambient temperature by the combination of a novel SIL and the CV premodulation technique. Details of this study will be presented in a paper under the peer-review. [1] J. Qian, J.-G. Zhang et al., Nat Commun 2015, 6; b) Y. Lu, L. A. Archer et al., Nat Mater., 2014, 13, 961-969; c) W. Li, Y. Cui et al., Nat Commun., 2015, 6. [2] D. Aurbach, J. Power Sources., 2000, 89, 206-218. [3] a) H. Wang, N. Imanishi et al., ChemElectroChem 2015, 2, 1144-1151; b) Q. Chen, K. Geng, K. Sieradzki, J. Electrochem. Soc., 2015, 162, A2004-A2007. [4] H. Wang, N. Imanishi et al., in The 56th Battery Symposium in Japan, Nagoya, 2015, 3A03.

Micro-Raman spectroscopy of graphene grown on stepped 4H-SiC (0001) surface

Graphene grown by chemical vapor deposition on 4H-SiC (0001) was studied using micro-Raman spectroscopy and atomic force microscopy (AFM). AFM revealed that the graphene structure grown on on-axis substrates has a stepped morphology. This is due to step bunching, which results from etching in hydrogen as well as from the process of graphene formation itself. It was shown by micro-Raman spectroscopy that the properties of graphene present on step edges and on terraces are quite different. Graphene on terraces is uniform with a relatively small thickness and strain fluctuations. On the other hand, graphene on step edges has a large thickness and strain variations occur. A careful analysis of micro-Raman spatial maps led us to the conclusion that the carrier concentration on step edge regions is lowered when compared with terrace regions.

Characterization of Mo/Si multilayer growth on stepped topographies

Mo/Si multilayer mirrors with nanoscale bilayer thicknesses have been deposited on stepped substrate topographies, using various deposition angles. The multilayer morphology at the step-edge region was studied by cross section transmission electron microscopy. A transition from a continuous- to columnar layer morphology is observed near the step-edge, as a function of the local angle of incidence of the deposition flux. Taking into account the corresponding kinetics and anisotropy in layer growth, a continuum model has been developed to give a detailed description of the height profiles of the individual continuous layers. Complementary optical characterization of the multilayer system using a microscope operating in the extreme ultraviolet wavelength range, revealed that the influence of the step-edge on the planar multilayer structure is restricted to a region within 300 nm from the step-edge.

Structural characterization of the N-terminal mineral modification domains from the molluscan crystal-modulating biomineralization proteins, AP7 and AP24.

The AP7 and AP24 proteins represent a class of mineral-interaction polypeptides that are found in the aragonite-containing nacre layer of mollusk shell (H. rufescens). These proteins have been shown to preferentially interfere with calcium carbonate mineral growth in vitro. It is believed that both proteins play an important role in aragonite polymorph selection in the mollusk shell. Previously, we demonstrated the 1-30 amino acid (AA) N-terminal sequences of AP7 and AP24 represent mineral interaction/modification domains in both proteins, as evidenced by their ability to frustrate calcium carbonate crystal growth at step edge regions. In this present report, using free N-terminal, C(alpha)-amide "capped" synthetic polypeptides representing the 1-30 AA regions of AP7 (AP7-1 polypeptide) and AP24 (AP24-1 polypeptide) and NMR spectroscopy, we confirm that both N-terminal sequences possess putative Ca (II) interaction polyanionic sequence regions (2 x -DD- in AP7-1, -DDDED- in AP24-1) that are random coil-like in structure. However, with regard to the remaining sequences regions, each polypeptide features unique structural differences. AP7-1 possesses an extended beta-strand or polyproline type II-like structure within the A11-M10, S12-V13, and S28-I27 sequence regions, with the remaining sequence regions adopting a random-coil-like structure, a trait common to other polyelectrolyte mineral-associated polypeptide sequences. Conversely, AP24-1 possesses random coil-like structure within A1-S9 and Q14-N16 sequence regions, and evidence for turn-like, bend, or loop conformation within the G10-N13, Q17-N24, and M29-F30 sequence regions, similar to the structures identified within the putative elastomeric proteins Lustrin A and sea urchin spicule matrix proteins. The similarities and differences in AP7 and AP24 N-terminal domain structure are discussed with regard to joint AP7-AP24 protein modification of calcium carbonate growth.

Preparation of the atomically straight step-edge Si (111) substrates as templates for nanostructure formation

ABSTRACTWe report on the experimental discovery that the distribution of kinks along steps on vicinal Si(111) surfaces depends on the direction of the dc current passed along the steps for resistive annealing. The as-cleaned Si(111) surface miscut ∼1° towards [112] has a small (<3°) unavoidable azimuthal deviation, which produces a number of kinks along the step-edges. When the azimuthal misorientation is from [112] towards [110] [110], dc current flowing in the direction [110] [110] climbing up the kinks straightens the step-edges as opposed to the current flowing in the opposite [110] [110]direction. During annealing around 800°C, the dc current in the direction climbing up the kinks straightens the steps. The up-climbing current direction transports and concentrates the kinks in a region outside the template area, leaving a kink-free atomic step-edge region as an ideal template for a variety of nanostructure formations. The straight step edges produced in this manner have uniform atomic configuration known as U(2, 0).

Reduction of blocking artifact in block-coded images using wavelet transform

We propose a simple yet efficient method which reduces the blocking artifact in block-coded images by using a wavelet transform. An image is considered as a set of one-dimensional signals, and so all processing including the wavelet transform are one-dimensionally executed. The artifact reduction operation is applied to only the neighborhood of each block boundary in the wavelet transform at the first and second scales. The key idea behind the method is to remove the blocking component which reveals stepwise discontinuities at block boundaries. Each block boundary is classified into one of shade region, smooth edge region, and step edge region. Threshold values for the classification are selected adaptively according to each coded image. The performance is evaluated for 512/spl times/512 images JPEG coded with 30:1 and 40:1 compression ratios. Experimental results show that the proposed method yields not only a PSNR improvement of about 0.69-1.06 dB, but also a subjective quality nearly free of the blocking artifact and edge blur.

Gypsum growth in the presence of background electrolytes studied by Scanning Force Microscopy

Background electrolytes in aqueous solutions can strongly influence the growth rate of gypsum, which can be easily shown from bulk growth experiments. However, such experiments cannot be used to decipher the actual mechanism unambiguously. We have used Scanning Force Microscopy in order to characterize the microtopography of the gypsum (010) surface, to determine the dominate step-generating process, and to investigate the influence of NaNO3 and NaCl as background electrolytes on the growth kinetics of monolayer steps. Due to surface defects, the step density can be inhomogeneous, resulting in variability in bulk growth rate data. Two-dimensional nucleation (birth and spread) has been found to be the dominant process that forms new steps on the (010) surface. Surface nuclei may be bounded by [100] and [001] steps, judging from the shape and orientation of the smallest observed surface islands during growth. In addition, ab initio molecular orbital calculations show that [100] and [001] steps on the (010) surface of gypsum have nearly identical energies. Nevertheless, well beyond the nucleation stage, growth occurs on the (010) surface via the advance of monolayer steps (representing one growth slice, 7 A in height) parallel to [101] and [001]. Steps parallel to [101] migrate faster than [001] steps, in agreement with ab initio calculations which show that [101] steps have a considerably higher energy than [001] steps. The step displacement velocity of the fast growth direction parallel to [101] is increased in the presence of NaCl, relative to NaNO3, as background electrolytes, whereas the slow growth direction parallel to [001] shows no background electrolyte effect. The different growth kinetics for the two major growth directions in the presence of different electrolytes can be explained by the difference in step energies. On the basis of our data, it appears very likely that a specific interaction between [101] steps and Cl− in solution results in an increased dehydration frequency for hydrated Cat2+ near [101] step-edge regions relative to NO3− containing supersaturated solutions.

Electromagnetic and microstructural characterization of YBa2Cu3O7 step edge junctions on (001) LaAlO3 substrates

Sharp and straight step edges on (001) LaAlO3 (LAO) substrates were ion milled by using an electron beam defined amorphous carbon thin film mask. YBa2Cu3O7 (YBCO) thin films patterned to narrow strips across the step edges gave high quality Josephson junctions. Their current–voltage (I–V) curves could be well described by the resistively shunted junction model with or without excess current. By varying the YBCO film thickness over a fixed step height, the critical current density (jc) of the junction could be changed by several orders of magnitude. For junctions with high jc, typical IcRn (product of critical current and normal resistance) values of around 100 μV at 77 K and more than 1 mV at 4.2 K were obtained. Some excess current was observed. For junctions with low jc, the dependence of Ic on an applied magnetic field was strong even at low temperatures. The Ic showed a main peak in the center and well-defined periods as a function of applied magnetic field. The minimum Ic value suppressed by the magnetic field was about 20% of its maximum value at 4.2 K. Junctions with low jc usually showed hysteretic I–V curves at low temperatures. The McCumber constant βc fell in the range of 0.8–2. Fiske and flux-flow resonances were observed for some junctions. The shunting capacitances of the junctions were estimated from the McCumber constant βc, Fiske resonances, and flux-flow resonances. A shunting capacitance value per unit area of 12–35 fF/μm2 was obtained. High resolution cross-sectional transmission electron microscopy was used to study YBCO films grown across straight and wavy step edges. Two 90° tilt boundaries were formed at the edge of a step. The top and bottom YBCO films had their c axis oriented normal to the (001) plane of LAO. In the edge region, the c axis of the YBCO film was tilted by 90°, the a axis was normal to the (001) plane of LAO, and the b axis was lying along the step edge. For wavy step edges, second phase particles usually appeared in the YBCO film along the step edge region. Defects were found on the surface of the LAO substrate along the wavy step edge region. These defects might act as nucleation centers for the second phase particles in the YBCO film.

Microstructure of YBa2Cu3O7-δ/Ag Bilayers Grown In Situ on Step-Edge SrTiO3 Substrates

AbstractThe microstructure of YBa2Cu3O7-δ(YBCO)/Ag bilayers sputter deposited in situ on step-edge SrTiO3 (STO) substrates, was carefully examined by transmission electron microscopy (TEM). Considerable variation in the YBCO film growth morphology is found near steps, including film protrusions beyond the step edges and film growth on slopes. Lattice images recorded near steps unveil a high density of crystalline defects in the film. An increased density of defects is also found near the substrate interface for film grown on etched STO surface. However, these defects are confined to the interface region and do not propagate beyond the scale of the STO surface roughness. Comparison of TEM lattice images of YBCO(100)/Ag and YBCO(001)/Ag junctions from the same specimen unveils a distinct difference in the interfacial microstructure of those two junctions. Whereas the former exhibits a sharp crystalline interface, the latter typically features a thin ( ∼ 20 Å ) interfacial layer of amorphous material. The YBCO film morphology and the high density of defects in the step edge region uncovered in this study suggest that manufacture of reproducible and uniform YBCO/Ag (Au)/YBCO (SNS) Josephson junctions using the step-edge technique, will prove a difficult task.