Stripe-patterned Substrates Research Articles

A new surface design for molecular combing: A dissipative particle dynamics study

Molecular combing facilitates the investigation of single DNA molecules with a moving water–air interface to immobilize DNA molecules on a solid surface. In this study, we use dissipative particle dynamics to model the three-phase system with complex fluids. We visually demonstrated the DNA deposition process and quantitatively described the degree of DNA linearization. Then, we study the effect of the substrate property on the combing results. Finally, we propose a chemical heterogeneous stripe-patterned substrate that can improve the linearization of deposited DNA chains.

Design and Fabrication of CNT-Based ${E}$ -Gun Using Stripe-Patterned Alloy Substrate for X-Ray Applications

We report the enhancement of carbon nanotube (CNT) field emitters by growing them directly on stripe-patterned alloy substrates. For a gate voltage of 1300 V, CNT field emitters grown on full and stripe-patterned alloy substrates achieved respective emission currents of 0.22 and 2.32 mA, corresponding to tenfold performance enhancement. Performance enhancement is attributed to the reduced screening effect (as a result of the spacing between strips) during field emission and a damage-free patterning process. To verify the ability of the stripe-patterned CNT field emitters to generate X-rays, we designed and assembled an electron gun through brazing. With the open type X-ray system, an X-ray image of human teeth was successfully obtained, verifying the potential of the stipe-patterned CNT emitters in X-ray applications.

Many-body dissipative particle dynamics simulation of the anisotropic effect of droplet wetting on stripe-patterned heterogeneous surfaces

The droplet anisotropic wetting on chemically heterogeneous stripe-patterned substrates after impact is studied by using many-body dissipative particle dynamics in this work. After a low-velocity impact on substrates with different length ratio (ratio of stripe width to the initial droplet size) and Cassie area fraction, the droplet can show various shapes and contact angles in parallel and orthogonal directions to the stripes. The elongation of the droplet can be increasingly evident when increasing the length ratio. Also, the contact angles at both directions follow the Cassie-Baxter predicted values well at low length ratio while deviate from them at high length ratio. Both the capillary and kinetic effects have a significant influence on the anisotropic wetting. When impact with higher velocity, the droplet leaves some residues on the hydrophilic stripes of the substrates. Surprisingly, the residues for substrates with certain length ratio (≤0.316) are distributed in circular regions with almost identical radii, indicating the spreading stages of them are independent on the surface properties and dominated by the kinetic effect. When the droplet reaches a stable state, the main body shows anisotropic wetting behavior, indicating the capillary effect dominates the wetting instead of the kinetic effect at the retraction stage.

AlGaN/GaN high-electron-mobility transistor with distributed gate grown on stripe-patterned Si(111) substrate

We have successfully fabricated an AlGaN/GaN high-electron-mobility transistor with a distributed gate (DG-HEMT) on a stripe-patterned Si substrate. With the help of the stripe pattern, GaN film with low defect density could be deposited by two-step growth. The striped AlGaN/GaN structure could be obtained naturally by stopping the epitaxy process before coalescence. The DG-HEMT fabricated on the striped pattern layout shows good performance. The output characteristics were enhanced from 297 to 337 mA/mm, because the high quality of GaN grown on the patterned substrate can reduce the number of defects. In addition, the drain current was not decreased because the heat problem was reduced in the DG structure.

Instabilities of Layers of Deposited Molecules on Chemically Stripe Patterned Substrates: Ridges versus Drops.

A mesoscopic continuum model is employed to analyze the transport mechanisms and structure formation during the redistribution stage of deposition experiments where organic molecules are deposited on a solid substrate with periodic stripe-like wettability patterns. Transversally invariant ridges located on the more wettable stripes are identified as very important transient states and their linear stability is analyzed accompanied by direct numerical simulations of the fully nonlinear evolution equation for two-dimensional substrates. It is found that there exist two different instability modes that lead to different nonlinear evolutions that result (i) at large ridge volume in the formation of bulges that spill from the more wettable stripes onto the less wettable bare substrate and (ii) at small ridge volume in the formation of small droplets located on the more wettable stripes. In addition, the influence of different transport mechanisms during redistribution is investigated focusing on the cases of convective transport with no-slip at the substrate, transport via diffusion in the film bulk and via diffusion at the film surface. In particular, it is shown that the transport process does neither influence the linear stability thresholds nor the sequence of morphologies observed in the time simulation, but only the ratio of the time scales of the different process phases.

Patterned Substrates of Nano-Graphene Oxide Mediating Highly Localized and Efficient Gene Delivery

A facile approach was developed to fabricate patterned substrates of nano-graphene oxide, demonstrating highly localized and efficient gene delivery to multiple cell lines in a substrate-mediated manner. The GO substrates served as a valid platform to preconcentrate PEI/pDNA complexes and maintain their gradual releasing for a relatively long period of time. Our approach allowed successful gene delivery in selected groups of cells on the stripe-patterned GO substrates, without transfecting their neighbor cells directly cultured on glass. These GO substrates exhibited excellent biocompatibility and enabled effective gene transfection for various cell lines including stem cells, thus promising important applications in stem cell research and tissue engineering.

Quantum dot growth on a stripe-pattern

We study the development of the Asaro-Tiller-Grinfel'd instability on a thin strained film on top of a stripe-patterned substrate and the subsequent growth of self-organized quantum dots. We use a continuum model describing the evolution equation enforced by surface diffusion. We compute the elastic energy up to the first non-linear order and investigate the long time dynamics which describes the dot growth. We find different island locations depending on the substrate wavelength and thickness. As found in experiments, the instability long-time dynamics leads to islands located either on top or in the bottom of the pattern.

AlN/air distributed Bragg reflector by GaN sublimation from microcracks of AlN

We report the fabrication of a four-period AlN/air distributed Bragg reflector (DBR) by in-situ GaN sublimation from microcracks of AlN. The GaN/AlN multilayer structure was grown on a stripe-patterned and dot-patterned Si substrate, and subsequently annealed at 1200°C in H2 and NH3 atmosphere. Microcracks were observed on the surface and side face of the A1N/air DBRs. We achieved an AlN/air structure by the decomposition of GaN from the microcracks of AlN. Partially crack-free AlN layers were observed over a 5×5μm area in the c-plane of AlN/air DBRs. The root mean square (RMS) values of the surface and back surface of stripe-patterned AlN/air DBRs were 0.94 and 3.3nm, respectively. The relative reflectivity was measured using a He–Cd laser (442nm). In some areas, stripe-patterned and dot-patterned AlN/air DBRs showed a high reflectivity of 83.7% and 90.7%, respectively, at the wavelength of 442nm.

Electroless deposition of Au around electrochemically evolved hydrogen bubbles

In this report, we use stripe-patterned substrates and TEM grids as working electrodes to demonstrate that electrochemically evolved hydrogen bubbles can serve as templates and reducing agents for electroless deposition of Au from a Na 3Au(SO 3) 2 electrolyte to form hollow particles. This new hollow nanoparticle formation mechanism, which is termed as bubble template synthesis, has the potential to become a general route to the fabrication of various metallic hollow nanoparticles. We have also found that the addition of Ni 2+ ions in the electrolyte has an effect on the particle formation process. Electrodeposited metallic Ni enhances the hydrogen evolution and therefore the hollow particle formation. It also leads to an improved size distribution of the hollow particles.

In situ Control of Si/Ge Growth on Stripe-Patterned Substrates Using Reflection High-Energy Electron Diffraction and Scanning Tunneling Microscopy

Si and Ge growth on the stripe-patterned Si (001) substrates is studied using in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). During Si buffer growth, the evolution of RHEED patterns reveals a rapid change of the stripe morphology from a multifaceted “U” to a single-faceted “V” geometry with {119} sidewall facets. This allows to control the pattern morphology and to stop Si buffer growth once a well-defined stripe geometry is formed. Subsequent Ge growth on “V”-shaped stripes was performed at two different temperatures of 520 and 600°C. At low temperature of 520°C, pronounced sidewall ripples are formed at a critical coverage of 4.1 monolayers as revealed by the appearance of splitted diffraction streaks in RHEED. At 600°C, the ripple onset is shifted toward higher coverages, and at 5.2 monolayers dome islands are formed at the bottom of the stripes. These observations are in excellent agreement with STM images recorded at different Ge coverages. Therefore, RHEED is an efficient tool for in situ control of the growth process on stripe-patterned substrate templates. The comparison of the results obtained at different temperature reveals the importance of kinetics on the island formation process on patterned substrates.

Deposition of undoped indium oxide thin films on stripe-patterned substrates by spray CVD

Undoped indium oxide (In 2O 3) films were deposited on stripe-patterned Si substrates by an original spray chemical vapor deposition (spray CVD) using ethanol solution of indium (III) chloride. The step coverage of the films increased as the deposition temperature decreased. The grooves with 0.18 μm openings were successfully deposited at 253 °C. The step coverage of undoped indium oxide films were a similar tendency though resistivity of that showed the tendency opposite to the ITO film. Deposition mechanism of the undoped indium oxide films on stripe-patterned Si substrates was similar to that of the ITO films. The lowest resistivity of the films was 4.1×10 −3 Ω cm for the 260 nm-thick film deposited at 280 °C. The resistivity increased at higher temperature.

Growth and Characterizations of GaN-Based LEDs Grown on Wet-Etched Stripe-Patterned Sapphire Substrates

The material and electrical properties of GaN-based light-emitting diodes (LEDs) grown on wet-etched stripe-patterned substrates were investigated. Footprint-like patterns, located directly above the inclined groove sidewalls, were found on the as-grown LED surface. Cross-sectional transmission electron microscopy (TEM) showed that ‘tumor’-like structures with poor crystal quality were initiated on the inclined sidewalls, seeding dislocation bundles in the subsequently grown crystal. The high dislocation density slowed down the growth above the inclined sidewall, resulting in the uneven morphology. The fabricated devices showed over 30% enhancement in light output power as a result of improvements in both internal and extraction efficiencies.

Complex laterally ordered InGaAs and InAs quantum dots by guided self-organized anisotropic strain engineering on shallow- and deep-patterned GaAs (311)B substrates

Self-organized anisotropic strain engineering guided on shallow- and deep-patterned GaAs (311)B substrates is exploited for formation of complex laterally ordered architectures of connected InGaAs quantum dot (QD) arrays and isolated InAs QD groups by molecular beam epitaxy. The combination of strain and step engineerings on shallow stripe-patterned substrates transforms the periodic spotlike arrangement of the InGaAs QD arrays and InAs QD groups (on planar substrates) into a zigzag arrangement of periodic stripes which are well ordered over macroscopic areas on zigzag mesa-patterned substrates. In contrast, the formation of slow-growing facets on deep-patterned substrates produces QD-free mesa sidewalls, while InGaAs QD arrays and InAs QD groups form on the GaAs (311)B top and bottom planes with arrangements modified only close to the sidewalls depending on the sidewall orientation. The QDs on the shallow- and deep-patterned substrates exhibit excellent optical properties up to room temperature. Therefore, the concept of guided self-organization demonstrated on shallow-patterned (due to steps) and deep-patterned (due to facets) substrates is highlighted for creation of complex architectures of laterally ordered QDs for future quantum functional devices.

Ordered InAs Quantum Dots with Controllable PeriodsGrown on Stripe-Patterned GaAs Substrates

GaAs (001) substrates are patterned by electron beam lithographyand wet chemical etching to control the nucleation of InAs quantum dots(QDs). InAs dots are grown on the stripe-patterned substrates by solidsource molecular beam epitaxy. A thick buffer layer is deposited on thestrip pattern before the deposition of InAs. To enhance the surfacediffusion length of the In atoms, InAs is deposited with low growthrate and low As pressure. The AFM images show that distinctone-dimensionally ordered InAs QDs with homogeneous size distribution arecreated, and the QDs preferentially nucleate along the trench. With theincreasing amount of deposited InAs and the spacing of the trenches, anumber of QDs are formed beside the trenches increased. Thedistribution of additional QDs is long-range ordered, always along thetrench rather than across the spacing regions.

The Optical Characteristics of Epitaxial Lateral and Vertical Overgrowth of GaN on Stripe-Patterned Si Substrate

GaN thin layers were grown on stripe-patterned silicon (111) substrates with an AlN buffer layer by using metal organic chemical vapor deposition. The Si substrates were structured with various pattern sizes of 2.95, 7.85, and 18.2 μm by using a conventional photolithography and inductively coupled plasma etching process. The GaN layer on the stripe-patterned Si extends vertically and laterally during the growth. Photoluminescence (PL) spectra show a band-edge emission at 3.37 eV corresponding to the exciton recombination and a blue one around 2.8 eV corresponding to the transition associated with intrinsic defects such as oxygen and hydrogenated gallium vacancies. From the μ-PL spectra, the intensity of the band-edge emission from the laterally overgrown region of GaN layer is stronger than that from the vertical one, and the blue emission is not observed for the lateral one. Especially, when the pattern size was 7.85 μm, a 7.02-fold increase in PL intensity for the laterally overgrown region was obtained in comparison with that for the vertical one. We conclude that the enhancement of the optical properties for lateral overgrowth of GaN layers depends on the pattern size of the Si substrate and results from a decrease in the dislocation density.

Guided quantum dot ordering by self-organized anisotropic strain engineering and step engineering on shallow-patterned substrates

Complex laterally ordered architectures of InGaAs quantum dot (QD) arrays are created by self-organized anisotropic strain engineering guided through steps generated on shallow mesa-patterned GaAs (3 1 1)B substrates. On stripe-patterned substrates the well-ordered, spot-like arrangement of ordered QD groups on planar, unpatterned substrates is transformed into a zigzag arrangement of periodic stripes which become straight, well ordered, and connected over macroscopic distances on zigzag mesa-patterned substrates. These findings highlight the concept of guided self-organized QD ordering for formation of complex QD architectures, establishing the building blocks for future quantum functional devices.

Study of nucleation positions of InAs islands on stripe-patterned GaAs (1 0 0) substrate

By combination of prepatterned substrate and self-organized growth, InAs islands are grown on the stripe-patterned GaAs (1 0 0) substrate by solid source molecular beam epitaxy. Four [ 0 1 1 ¯ ] stripe-patterned substrates different in pitch, depth, and sidewall angle, respectively, are used in this work. The surface morphology obtained by atomic force microscopy shows that the InAs quantum dots can be formed either on the ridge or on the sidewall of the stripes near the bottom, depending on the structure of the stripes on the patterned substrate. The mechanism determining the nucleation position of the InAs dots is discussed. The optical properties of the InAs dots on the patterned substrates are also investigated by photoluminescence.

Complex quantum dot arrays formed by combination of self-organized anisotropic strain engineering and step engineering on shallow patterned substrates

One-dimensional (In,Ga)As quantum dot (QD) arrays are created on planar singular, vicinal, and shallow mesa-patterned GaAs (100) substrates by self-organized anisotropic strain engineering of an (In,Ga)As∕GaAs quantum wire (QWR) superlattice template in molecular beam epitaxy. On planar singular substrates, highly uniform single QD arrays along [0−11] are formed. On shallow [0−11] and [011] stripe-patterned substrates, the generated type-A and -B steps distinctly affect the surface migration processes which are crucial for QWR template development, i.e., strain-gradient-driven In adatom migration along [011] and surface-reconstruction-induced Ga∕In adatom migration along [0−11]. In the presence of both type-A and -B steps on vicinal substrates misoriented towards [101], the direction of adatom migration is altered to rotate the QD arrays. This establishes the relationship between self-organized anisotropic strain and step engineering, which is exploited on shallow zigzag-patterned substrates for the realization of complex QD arrays and networks with well-positioned bends and branches, exhibiting high structural and optical quality.

MBE growth of GaAs/AlGaAs quantum well on a patterned GaAs (0 0 1) substrate

A GaAs/AlGaAs quantum well (QW) was grown on a stripe-patterned (0 0 1) GaAs substrate with conventional molecular beam epitaxy. A mesa structure was formed with (0 0 1) facet on the top and (1 1 1) facet on the sides. It was found that the thickness of the QW layer on the (0 0 1) facet is controlled by the inter-surface diffusion of Ga adatoms from the (1 1 1) side facets. The behavior was investigated as a function of V/III ratio and the growth temperature.

Ordered droplet formation by thin polymer film dewetting on a stripe-patterned substrate

The dewetting process of thin polystyrene (PS) film on flat and stripe-patterned substrates is presented. Different dewetting processes were observed when the thin PS films annealed at above the glass transition temperature on these different kinds of substrates. The final dewetting on the flat substrate led to formation of polygonal liquid droplets, while on the stripe-patterned substrate, the droplets were observed to align at the centers of the stripes. A possible explanation for the dewetting process on the stripe-patterned substrate is proposed.