Smooth Side Research Articles

Large-scale growth of millimeter-long single-crystalline ZnS nanobelts

Millimeter-long single-crystalline hexagonal ZnS nanobelts were grown on specific locations on a wafer scale. This is the first time that the millimeter-scale ZnS nanobelt has been synthesized. The longest nanobelts are about 3 mm. The as-grown nanobelts were characterized by means of field emission scanning electron microscopy, X-ray powder diffraction, high-resolution transmission electron microscopy, and selected area electron diffraction. The results indicate that the ultra-long nanobelts are pure single-crystalline hexagonal ZnS. There are two kinds of ZnS nanobelts existing in the products. One is the nanobelts that have two smooth sides and grow along the [0 0 1] longitudinal direction, and the other is the nanobelts that have one smooth side and one saw-teeth-like side, namely nanosaws, and grow along the [2 1 0] longitudinal direction. A vapor–liquid–solid mechanism is suggested for the lengthwise growth of the ZnS nanobelts (nanosaws) and a vapor–solid mechanism for the side direction growth of the saw-teeth of the nanosaws.

Heat Transfer and Friction Factor in a Square Channel With One, Two, or Four Inclined Ribbed Walls

An experimental study was carried out to investigate the heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with 45 deg inclined ribs on one, two, or four walls. Either two opposite walls or all four walls in the channel were heated. Tests were performed for Reynolds numbers (Re) ranging from 7600 to 24,900, the pitch to rib height ratio (P∕e) of 8.0, the rib height to channel hydraulic diameter ratio (e∕Dh) of 0.0667, and the channel aspect ratio of 1.0. The results show that the local Nusselt number and friction factor increase with the number of ribbed walls. With one ribbed wall, the Nusselt numbers on the ribbed side (B) were 50% and 63% greater than those on the adjacent smooth sides (L∕R) and the opposite smooth side (T), respectively. The Nusselt numbers, when the two opposite walls of a four-wall ribbed channel are heated, are found to be 1.49–1.52 times greater than those obtained when all four walls are heated.

Unusual corrugated nanowires of zinc oxide

We report an unusual morphology of ZnO nanowires with a hexagonal cross-section and corrugated side walls. Nanowires grow along the [0 0 0 1] direction and possess side walls built predominantly with facets of {1 0 1¯ 1} and {1 0 1¯ 1¯} families. Such a morphology deviates dramatically from the well-known growth habit of ZnO nanowires that involves smooth side walls represented by {1 0 1¯ 0} or {1 1 2¯ 0} facets with the lowest surface energy. The formation of corrugated nanowires is attributed to the lateral growth activated by the high vapor supersaturation and the presence of stacking faults.

THE NEW CONCEPTS AND NEW DEVELOPMENTS IN HTE THEORY OF POLYMER CRYSTALLIZATION

Recently a debate about the initial crystallization process which has not been the hotspot for a long time since the theory proposed by Hoffman- Lauritzen (LH) dominated the field arose again. For a long time the Hoffman-Lauritzen model was always confronted by criticism,and some of the points were taken up and led to modifications, but the foundation remained unchanged which deemed that before the nucleation and crystallization the system was uniform. In this article the classical nucleation and growth theory of polymer crystallization was reviewed, and the confusion of the explanations to the polymer crystallization phenomenon was pointed out. LH theory assumes that the growth of lamellae is by the direct attachment of chain sequences from the melt onto smooth lateral sides.

Optimal adaptive solution of initial-value problems with unknown singularities

The optimal solution of initial-value problems in ODEs is well studied for smooth right-hand side functions. Much less is known about the optimality of algorithms for singular problems. In this paper, we study the (worst case) solution of scalar problems with a right-hand side function having r continuous bounded derivatives in R , except for an unknown singular point. We establish the minimal worst case error for such problems (which depends on r similarly as in the smooth case), and define optimal adaptive algorithms. The crucial point is locating an unknown singularity of the solution by properly adapting the grid. We also study lower bounds on the error of an algorithm for classes of singular problems. In the case of a single singularity with nonadaptive information, or in the case of two or more singularities, the error of any algorithm is shown to be independent of r.

Ni electroplating on a resist micro-machined by proton beam writing

In this paper we report fabrication of high-aspect-ratio micro-structure of Ni by electroplating, using a micro-machining technique of resists using proton beam writing (PBW) at Japan Atomic Energy Agency (JAEA). A micro-structure of 5 μm thick PMMA was fabricated by exposure using PBW at 1.7 MeV and by development. A Ni structure was then formed by electroplating on the micro-structure of PMMA. Vertical and smooth side walls observed by a scanning electron microscope (SEM) indicate that PBW can be a versatile tool for fabrication of resists and metal microstructure in combination with electroplating. The electroplated Ni structure can be used as a resolution standard, which enabled us to focus the proton beam down to 130 nm.

Proton beam writing: a platform technology for nanowire production

P-beam writing (proton beam writing) is a unique direct write 3D nano-lithographic technique which has been developed at the Centre for Ion Beam Applications (CIBA), in the Physics Department of the National University of Singapore. This technique employs a focused MeV proton beam which is scanned in a predetermined pattern over a resist (e.g. PMMA, SU-8 or HSQ), which is subsequently chemically developed. In e-beam writing as well as p-beam writing the energy loss of the primary beam is dominated by energy transfer to substrate electrons. Unlike the high energy secondary electrons generated during e-beam writing the secondary electrons induced by the primary proton beam have low energies (typically less than 100 eV) and therefore a limited range, resulting in minimal proximity effects. The low proximity effects exhibited by p-beam writing coupled with the straight trajectory and high penetration of the proton beam enables the production of high aspect ratio, high density 3D micro- and nano-structures with well defined smooth side walls to be directly written into resist materials. These structures can be used as templates to electroplate metallic nanowires.

Harmonic vibrations of an elastic body with rigid circular unilaterally exfoliated inclusion

We solve the problem of determination of the stressed state near a thin circular rigid inclusion in an unbounded elastic matrix vibrating under the action of harmonic forces applied at the center of the inclusion. One side of the inclusion is coupled with the matrix and the other smooth side is exfoliated. The displacements in the matrix are described by the discontinuous solutions of the Lame equations, which enables us to reduce the boundary-value problem to a system of singular integral equations for the functions of the jumps of displacements and stresses on the inclusion. The indicated system of equations is solved by the collocation method by using special quadrature formulas for singular integrals. It is shown that the stress concentration sharply increases for a certain loading frequency.

Large Eddy Simulation of Turbulent Channel Flow with 3D Roughness Using a Roughness Element Model

Large eddy simulation of turbulent channel flow with dense and small 3D roughness elements is carried out using a roughness element model. Profiles of mean Reynolds stress, mean velocity and rms velocity as well as turbulent structures near the wall are obtained. The shear stress in the rough wall is larger than that in the smooth wall side and the rough wall has a larger influence on the channel flow. Profiles of mean streamwise velocity near the wall have logarithmic velocity distributions for both smooth and roughness walls, while there is a velocity decrease for the rough wall due to larger fractional drag. All the three components of rms velocities in the rough wall region are larger than that in the smooth wall region, and the roughness elements on the wall increase turbulent intensity in all directions. The streak spacing and average diameter of near wall quasi-streamwise vortices increase with the presence of roughness elements on the wall and it is shown that the rough wall induces complex and strong streamwise vortices. Results of dense and small 3D roughness elements in both turbulent statistics and structure, obtained with a relatively simple method, are found to be comparable to related experiments.

The Dependence of Demolding Characteristics on Side Wall Roughness of Mold in Thermal Imprint

Various Si molds with different side wall roughness were fabricated and the demolding forces for the thermal imprint to PMMA film were examined. The pattern size was the 2 μm half pitch line and space with the aspect ratio of about 2. The demolding force greatly depended on the side wall roughness of the mold. The demolding force normalized by the total side wall area of line patterns was 0.34 MPa for the smooth side wall mold.

Exposure parameters in proton beam writing for hydrogen silsesquioxane

In proton beam writing (PBW) a focused MeV proton beam is scanned in a predetermined pattern over a resist (e.g. PMMA, SU-8 or HSQ), which is subsequently chemically developed. In e-beam writing as well as p-beam writing the energy loss of the primary beam is dominated by energy transfer to substrate electrons. Unlike the high energy secondary electrons generated during e-beam writing the secondary electrons induced by the primary proton beam have low energy and therefore a limited range, resulting in minimal proximity effects. The low proximity effects exhibited by p-beam writing coupled with the straight trajectory and high penetration of the proton beam enables the production of high aspect ratio, high density 3D micro and nanostructures with well defined smooth side walls to be directly written into resist materials. This property together with the stability and focusing power of the end station ensures even exposures with nm smoothness and allows fabrication of details down to the 20nm level. In this paper, we present results like contrast and sensitivity for PBW using, hydrogen silsesquioxane (HSQ) and XR-1541, both are non-C based resists. Unlike PMMA and SU-8 resist HSQ shows aging effects, requiring optimized processing parameters in PBW.

New approach of Nano-Selective Area Growth (NSAG) for a precise control of GaN nanodots grown by MOVPE

Nanodots arrays of GaN have been successfully grown on GaN template substrates using nano-selective area growth (NSAG). The substrates used in NSAG were partially covered by dielectric masks in which nano-apertures have been patterned by electron-beam nanolithography and reactive ion etching. The growths were performed by low pressure metal organic vapour phase epitaxy (MOVPE) using 100% N 2 as carrier gas. TMGa and NH 3 were used as sources of gallium and nitrogen, respectively. The layers were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Perfect selectivity of GaN on the masked substrate has been obtained. The nanodots grown in the nano-apertures are well shaped and homogenous with smooth surface side walls.

Self-Organization of 3D Triangular GaN Nanoislands and the Shape Variation to Hexagonal

We report on the self-organization of large-scale uniform aligned three-dimensional (3D) GaN islands with distinct triangular (0001) and smooth side facets and the shape variations of the (0001) facets from triangular to hexagonal during metalorganic vapor-phase epitaxy (MOVPE) growth of GaN films on Si-rich SiNx patterned sapphire substrates. The triangular island shaping during the recrystallization processes of GaN nucleation layers (NLs) can be attributed to the enhanced diffusion and regrowth anisotropy. The island shape transition from triangular to hexagonal in the early stages of high-temperature growth of GaN epilayers is due to the gas-phase transport dominating growth mechanism and the limited diffusion length of edge adatoms compared with the increased island size.

One-dimensional gallium nitride micro/nanostructures synthesized by a space-confined growth technique

Hexagonal GaN prismatic sub-micro rods and cone nanowires have been synthesized in a large scale by a novel and controllable space-confined growth method. The as-synthesized GaN products are highly crystalline with a wurtzite structure. The prismatic rods have lengths of 15∼20 μm and diameters of 400∼500 nm enclosed by hexagonal smooth side surfaces and a pyramidal end. And the cone nanowires have average diameters of 150∼200 nm and lengths up to several tens of μm with a cone tip. The photoluminescence (PL) studies reveal prominent band-gap UV emission properties of GaN products and narrow FWHM, indicating the excellent luminescent performance and high crystal quality. For field emission characteristic of GaN nanowires, the turn-on field is about 9.5 V/μm and the current density reaches 1.0 mA/cm2 at an electric field of 18 V/ μm. The contrast experiments indicate a novel growth control can be achieved by using a graphite tube as reaction vessel. The sealed graphite tube combined with metallic initiator is greatly responsible for large-scale and uniform preparation of GaN prismatic rods and cone nanowires. Highly symmetric GaN hexagonal micropyramids are grown on a bare Si substrate. The growth mechanism and the control function of the graphite tube are demonstrated. These low-dimensional structures not only enrich semiconducting GaN family, but also are good building blocks for optoelectronic devices.

Fabrication of nanofluidic devices utilizing proton beam writing and thermal bonding techniques

The fabrication of polymer lab-on-a-chip systems for applications in Chemistry and Biology is one of the envisaged niche areas for the Proton Beam Writing (PBW) technique developed at the Centre for Ion Beam Applications (CIBA). Utilizing a highly focused beam of MeV protons, well-defined nanostructures with smooth and straight side walls have been directly written in a 500 nm to 10 μm thick PMMA layer spin coated onto a Kapton substrate. By subsequently thermally bonding the fabricated structures to bulk PMMA and carefully peeling off the Kapton, nanostructures can be attached to bulk PMMA. Finally, by bonding a PMMA sheet to the bottom side of the structure, an integrated PMMA device with enclosed multiple high aspect ratio nanochannels can be realized. Preliminary experiments conducted in order to test this polymeric device indicate good fluidic properties. The nanochannels can be easily filled with dye solution using both pressure and capillary action in the case of hydrophilic solutions.

New resists for proton beam writing

To explore the full capabilities of proton beam writing (PBW) as a lithographic tool it is important to investigate potential new resist materials. In PBW the interactions of the protons with the resist are comparable to the electron interactions with the resist in electron beam writing. In both techniques the induced secondary electrons will modify the molecular structure of the resist, therefore electron beam resists are potential candidates for PBW. Here we discuss resist properties such as contrast and sensitivity of two new negative resists for PBW. The first resist is a spin-coatable TiO 2 resist for which sub 10 nm resolution has been reported using electron beam writing. In PBW smooth side walls have been observed for this resist. Despite a relative low sensitivity of this resist for PBW (8000 nC/mm 2) it has potential applications in the area of integrated optical components such as waveguides and gratings because of its high refractive index. WL-7154 is a UV-sensitive negative resist and shows high sensitivity for PBW (4 nC/mm 2). This resist could function as a mold for Ni electroplating to fabricate Ni stamps for nanoimprint- and soft-lithography.

Fabrication of Si mold with smooth side wall by new plasma etching process

A new silicon etching process for imprint mold fabrication is developed. A special sample bed, which is coated by teflon film, is placed on the RF bias electrode in the conventional ICP etcher. Silicon is etched by mixture gas plasmas of Ar, SF 6 and C 4F 8 (or CHF 3). The silicon side etching can be reduced by use of the teflon coated sample bed. When the switching process is used, fine and deep silicon patterns with smooth silicon surface can be obtained for the teflon coated sample bed. The silicon pattern of 0.1 μm wide with the aspect ratio of 30 can be fabricated. A silicon mold is fabricated by the new etching process. A fine pattern of 0.2 μm wide can be transferred to a polymethylmethacrylate (PMMA) film by the thermal imprinting.

Differential variational inequalities

This paper introduces and studies the class of differential variational inequalities (DVIs) in a finite-dimensional Euclidean space. The DVI provides a powerful modeling paradigm for many applied problems in which dynamics, inequalities, and discontinuities are present; examples of such problems include constrained time-dependent physical systems with unilateral constraints, differential Nash games, and hybrid engineering systems with variable structures. The DVI unifies several mathematical problem classes that include ordinary differential equations (ODEs) with smooth and discontinuous right-hand sides, differential algebraic equations (DAEs), dynamic complementarity systems, and evolutionary variational inequalities. Conditions are presented under which the DVI can be converted, either locally or globally, to an equivalent ODE with a Lipschitz continuous right-hand function. For DVIs that cannot be so converted, we consider their numerical resolution via an Euler time-stepping procedure, which involves the solution of a sequence of finite-dimensional variational inequalities. Borrowing results from differential inclusions (DIs) with upper semicontinuous, closed and convex valued multifunctions, we establish the convergence of such a procedure for solving initial-value DVIs. We also present a class of DVIs for which the theory of DIs is not directly applicable, and yet similar convergence can be established. Finally, we extend the method to a boundary-value DVI and provide conditions for the convergence of the method. The results in this paper pertain exclusively to systems with “index” not exceeding two and which have absolutely continuous solutions.

Largely-tunable wideband Bragg gratings fabricated on SOI rib waveguides employed by deep-RIE

A largely-tunable wideband Bragg grating on a silicon-on-insulator (SOI) rib waveguide loaded with Au/Cr heater is presented. A deep first-order Bragg grating with smooth etched side wall has been successfully etched by deep reactive-ion etching (deep-RIE). As a result, a wide bandwidth of about 4 nm at -10 dB transmission-level was obtained with a tuning range of about 18 nm using the thermo- optic effect.

選択成長マイクロファセットを用いたGaN系面発光型レーザー

GaN microfacets have been grown by selective-area metalorganic vapor phase epitaxy (SA-MOVPE) and applied to GaN-based surface-emitting lasers. The crystal orientation of the microfacets was strictly controlled by the shapes of the mask-openings and growth conditions so that vertical and inclined microfacets were obtained. Hexagonal microprisms (HMPs) of GaN with 5-50μm in diameters have extremely smooth vertical side walls of GaN {10 1 0} microfacets. These GaN HMPs lase by optical pumping and have the inscribed hexagonal optical paths. Additionally, GaN-based horizontal cavity surface-emitting lasers (HCSELs) were successfully fabricated. The GaN-based HCSELs have Fabry-Perot cavity and outer micromirrors, which consist of SA-MOVPE grown {11 2 0} vertical and {11 2 2} inclined microfacets, and lased at room temperature by pulsed current injection. The laser beam emitted laterally from a Fabry-Perot cavity {11 2 0} vertical micromirror is reflected by a {11 2 2} inclined outer micromirror so as to be directed upward. The GaN-based HCSEL has a current-confining structure because the incorporation probability of p-type dopants (Mg) is strongly influenced by facet orientation (growth direction).