Optimized Surface Treatment Research Articles

Schottky contacts on ultra-high-pressure-annealed GaN with high rectification ratio and near-unity ideality factor

We investigate the electrical characteristics of Ni Schottky contacts on n-type GaN films that have undergone ultra-high-pressure annealing (UHPA), a key processing step for activating implanted Mg. Contacts deposited on these films exhibit low rectification and high leakage current compared to contacts on as-grown films. By employing an optimized surface treatment to restore the GaN surface following UHPA, we obtain Schottky contacts with a high rectification ratio of ∼109, a near-unity ideality factor of 1.03, and a barrier height of ∼0.9 eV. These characteristics enable the development of GaN junction barrier Schottky diodes employing Mg implantation and UHPA.

Improvement of polymer electrolyte fuel cell performance by gas diffusion layer with relatively hydrophilic surface

To improve the performance of polymer electrolyte fuel cells under high current density operation, the flooding problem in gas diffusion layers (GDLs) should be solved. In this study, a novel GDL with a relatively hydrophilic surface, consisting of hydrophilic and hydrophobic pores, which yielded a wettability distribution in the thickness direction, was proposed to achieve smooth water transport. First, we fabricated GDLs with surfaces partially hydrophilized by Nafion, and we demonstrated that the cell performance improved with this treatment. Cell performance tests suggest that a GDL with optimized surface treatment could reduce the water accumulation compared with the hydrophobic GDL used in this study; it could also improve cell performance owing to low mass transport losses. To verify the surface treatment-induced improvement in the oxygen diffusivity of the GDL, the water transport in the GDL was simulated using a lattice Boltzmann method. The results showed that the water accumulation in the GDL with surface treatment was small because the water flow was limited to the thin relatively hydrophilic region. Consequently, the oxygen diffusivity was higher than that of the hydrophobic GDL. Therefore, the flooding problem can be suppressed by the surface treatment, which yields a relatively hydrophilic surface.

Tailored Interfaces in Fiber-Reinforced Elastomers: A Surface Treatment Study on Optimized Load Coupling via the Modified Fiber Bundle Debond Technique

The interface between the reinforcement and surrounding matrix in a fibrous composite is decisive and critical for maintaining component performance, durability, and mechanical structure properties for load coupling assessment, especially for highly flexible composite materials. The clear trend towards tailored solutions reveals that an in-depth knowledge on surface treating methods to enhance the fiber–matrix interfacial interaction and adhesion properties for an optimized load transfer needs to be ensured. This research aims to quantify the effect of several surface treatments for glass fibers applied in endless fiber-reinforced elastomers with pronounced high deformations. Due to this, the glass fiber surface is directly modified with selected sizings, using a wet chemical treatment, and characterized according to chemical and mechanical aspects. For this purpose, the interfacial adhesion performance between fibers and the surrounding matrix material is investigated by a modified fiber pull-out device. The results clearly show that an optimized surface treatment improves the interface strength and chemical bonding significantly. The fiber pull-out test confirms that an optimized fiber–matrix interface can be enhanced up to 85% compared to standard surface modifications, which distinctly provides the basis of enhanced performances on the component level. These findings were validated by chemical analysis methods and corresponding optical damage analysis.

Bonding repair of CFRP based on cold plasma treatment surface modification

The surface pretreatment before bonding repair determines the microscopic physical and chemical properties of the composites, and directly affects the bonding strength and maintenance effect. In this paper, the surface of the Carbon Fiber Reinforced Polymer (CFRP) was pretreated by Cold Plasma Treatment (CPT) under 2.5 times air atmosphere (0.25 MPa) to achieve the best interface micro-performance for bonding repair. Based on the surface microstructure and surface activity test, the single lap test was used to evaluate the bonding repair strength after surface modification. The results show that the optimized surface treatment parameters of the output power, the plasma flame-core distance, and the process time were 800 W, 7 mm, and 7 s, respectively. It is found that no damage to the fiber and the surface-free energy of the pretreated composites (800 W, 7 mm, 7 s) reaches 51.65 mN/m under the optimized process, which is higher than those of the untreated original value 43.83 mN/m and the manual-grinding treatment value 45.73 mN/m. The bonding strength of CFRP after CPT treatment was increased to 11.31 MPa, which was obviously better than the bonding strength of 7.11 MPa without pretreated specimen and the bonding strength of 8.14 MPa after manual-grinding treatment. The failure mode of samples after CPT treatment was cohesive failure, while those of manual-grinding treated and original untreated CFRP samples exhibited thin layer cohesive failure (TLC) and adhesion failure (ADH), respectively.

Optimized surface treatment of aerospace composites using a picosecond laser

In this work, a design of experiments (DoE) approach was used to determine optimum laser surface treatment conditions. Aerospace structural carbon fiber reinforced composites (Torayca® T800H/3900-2) were laser treated using a pulsed laser source with a wavelength of 355 nm, nominal pulse width of 10 ps, average power up to 7 W, and spot size of approximately 20 μm. The laser power, frequency, scan speed, and number of passes (1 or 2) were varied. The adherends were characterized using a handheld water contact angle device, scanning electron microscopy (SEM), optical profilometry, and laser induced breakdown spectroscopy (LIBS). Bonded specimens were mechanically tested using a modified double cantilever beam (DCB) method. The response models obtained from the DoE indicated that surface characteristics of adherends and failure modes of bonds were most sensitive to the average laser power. Below about 200 mW, adhesive failure was observed. For laser powers greater than about 600 mW, surface resin was largely removed from the fiber bed, and the failure mode was predominantly fiber tear. Cohesive failure (∼100%) was observed for a wide range of laser parameters indicating that laser treatment can be a robust means of surface preparation.

Improved Device Performance of Solution-Processed Single-Wall Carbon Nanotube Transistors by a Patterning Technique Using a Selective Surface Treatment.

During the fabrication processes for single-wall carbon nanotube thin-film transistors (SWCNT-TFTs), the impurities of organic residues such as photoresist and developer can be induced, which affects the charge transport. As a result, solution-processed SWCNT-TFTs exhibit poor and non-uniform device performance regardless of the intrinsic electrical characteristics. Here, we demonstrate a patterning technique using a selective surface treatment with solution-processed hydrophobic fluorocarbon copolymer in SWCNT-TFTs. By using the difference of wettability in a selective area, a channel region in SWCNT-TFTs can be patterned without the conventional photolithography and etching process. Furthermore, the optimized surface treatment results in denser random networks of SWCNTs in the channel patterned by such technique, compared to the dropcasted SWCNT. The statistical results of the key device metrics such as mobility and threshold voltage extracted from 30 SWCNT-TFTs conclusively prove the improved device performance of SWCNT-TFTs fabricated by such pattering technique. We believe that this work can provide a promising route to stimulate the process innovation of fabrication for high performance solution-processed electronics based on SWCNT random networks.

Scattering by a cylinder covered with an arbitrary distribution of impedance and application to the optimization of a tramway noise abatement system

A semi-analytical solution for the two-dimensional scattering of a line source by a cylinder with an arbitrary distribution of surface impedance and its image with respect to a vertical baffle is derived. This description is used to model the shadowing due to a low-height semi-cylindrical noise barrier close to a tramway. After validation against the boundary element method, this solution is used in a gradient-based optimization approach of the admittance distribution to maximize the broadband insertion loss in a given receiver zone. First, a hypothetical but passive distribution is found, showing an improvement of more than 20dB(A) with respect to a purely rigid barrier. Second, a feasible optimized surface treatment made of a porous layer and a micro-perforated resonant panel is proposed, with an improvement of 14dB(A) with respect to an entirely rigid barrier and 8dB(A) with respect to a uniform absorbent barrier. The optimization provides an automatic way of tuning the resonant panel so that the attenuation is enhanced in the frequency band where the source has the most spectral content. The benefit of using a non-uniform admittance distribution is evaluated in this idealized context to be about 8dB(A).

Orthogonal Growth of Horizontally Aligned Single-Walled Carbon Nanotube Arrays

Direction-controlled growth of horizontally aligned single-walled carbon nanotubes (SWNTs) on r-plane sapphire substrates and their alignment mechanisms are demonstrated. On a flat r-plane substrate, anisotropic nanotube−substrate interaction is known to align SWNTs parallel to the [1101] direction of the sapphire. We find that the introduction of a slight miscut (−1° inclined to the [1101] direction) on the substrate changed the SWNT growth direction by 90°, aligning perpendicular to the [1101] direction. This dramatic change of the growth direction is explained by the contribution of newly proposed one-dimensional surface atomic rows and/or atomic steps appeared on the r-plane. Annealing the substrate in hydrogen atmosphere prior to SWNT growth recovers the original nanotube growth direction, while annealing in air deteriorates the alignment. The direct growth of an orthogonally aligned SWNT array is achieved through optimized surface treatment. Site-selective directional control of aligned SWNTs ...

Enhancement of cell radiation sensitivity by pegylated gold nanoparticles

Biocompatible Au nanoparticles with surfaces modified by PEG (polyethylene glycol) were developed in view of possible applications for the enhancement of radiotherapy. Such nanoparticles exhibit preferential deposition at tumor sites due to the enhanced permeation and retention (EPR) effect. Here, we systematically studied their effects on EMT-6 and CT26 cell survival rates during irradiation for a dose up to 10 Gy with a commercial biological irradiator (Eaverage = 73 keV), a Cu-Kα1 x-ray source (8.048 keV), a monochromatized synchrotron source (6.5 keV), a radio-oncology linear accelerator (6 MeV) and a proton source (3 MeV). The percentage of surviving cells after irradiation was found to decrease by ∼2–45% in the presence of PEG-Au nanoparticles ([Au] = 400, 500 or 1000 µM). The cell survival rates decreased as a function of the dose for all sources and nanoparticle concentrations. These results could open the way to more effective cancer irradiation therapies by using nanoparticles with optimized surface treatment. Difficulties in applying MTT assays were also brought to light, showing that this approach is not suitable for radiobiology.

Acid aqueous solution treatment method in AZ31 magnesium alloy

In order to impart metallic luster to AZ31 magnesium alloy, the conditions of surface treatment were optimized, and the formative mechanism of metallic luster on the surface was inferred from the results of surface analysis. The target of this study was the surface of AZ31 magnesium alloy that was polished by emery paper (#2000) . When the optimal acid aqueous solution treatment was applied, more than 60% of the magnesium in the film was oxidized. In addition, the thickness of the film was about 55 nm. It was confirmed that hydrogen generation was inhibited under the optimized surface treatment conditions. This optimization was performed by examining oxidant addition and the temperature of the acid aqueous solution, and metallic luster was successfully imparted to the alloy. In the optimal acid aqueous solution treatment, it was found that the reaction rate of magnesium oxide formation was faster than that of the dissolution of magnesium from the film composed of magnesium oxide on the surface. For surface treatment conditions in which a dull surface was obtained, it is speculated that the surface became rough due to the generation of hydrogen.

Covalent functionalization of NiTi surfaces with bioactive peptide amphiphile nanofibers

Surface modification enables the creation of bioactive implants using traditional material substrates without altering the mechanical properties of the bulk material. For applications such as bone plates and stents, it is desirable to modify the surface of metal alloy substrates to facilitate cellular attachment, proliferation, and possibly differentiation. In this work we present a general strategy for altering the surface chemistry of nickel–titanium (NiTi) shape memory alloy in order to covalently attach self-assembled peptide amphiphile (PA) nanofibers with bioactive functions. Bioactivity in the systems studied here includes biological adhesion and proliferation of osteoblast and endothelial cell types. The optimized surface treatment creates a uniform TiO 2 layer with low levels of Ni on the NiTi surface, which is subsequently covered with an aminopropylsilane coating using a novel, lower temperature vapor deposition method. This method produces an aminated surface suitable for covalent attachment of PA molecules containing terminal carboxylic acid groups. The functionalized NiTi surfaces have been characterized by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and atomic force microscopy (AFM). These techniques offer evidence that the treated metal surfaces consist primarily of TiO 2 with very little Ni, and also confirm the presence of the aminopropylsilane overlayer. Self-assembled PA nanofibers presenting the biological peptide adhesion sequence Arg-Gly-Asp-Ser are capable of covalently anchoring to the treated substrate, as demonstrated by spectrofluorimetry and AFM techniques. Cell culture and scanning electron microscopy (SEM) demonstrate cellular adhesion, spreading, and proliferation on these functionalized metal surfaces. Furthermore, these experiments demonstrate that covalent attachment is crucial for creating robust PA nanofiber coatings, leading to confluent cell monolayers.

Surface structure modification on the gas separation performance of carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes for separating gases were deposited on porous Al 2O 3 disks using CH 3COCH 3+CH 4, by a remote inductively coupled-plasma (ICP) chemical vapor deposition (CVD). The new method for preparing membranes with a high H 2/N 2 selectivity without any reduction of the permeance was introduced by combining the surface treatment with high-energy ion bombardment and subsequent high-temperature pyrolysis study. The H 2/N 2 selectivity reached 50 with an extremely high permeance of approximately 1.6×10 −6 mol m −2 s −1 Pa −1 at 423 K. The O 2/N 2 selectivity reached 2.5 and the O 2 permeance was approximately 2×10 −7 mol m −2 s −1 Pa −1 at 398 K. Short and optimized surface treatment periods were required to ensure high efficiency. Without pyrolysis, surface treatments alone markedly reduced the H 2 and N 2 permeances but did not affect selectivity. Besides, without any surface treatment, pyrolysis alone markedly increased the H 2 and N 2 permeances, but did not improve selectivity, because the desorption of carbon left large pores. Surface treatment must be combined with pyrolysis to enhance simultaneously the permeance and selectivity of CMS membranes, in a manner very different from that associated with conventional pore-plugging mechanism in typical CVD.

Effects of surface treatments and annealing on carbon-based molecular sieve membranes for gas separation

A new method in preparing carbon-based molecular sieve (CMS) membranes for gas separation has been proposed. Carbon-based films are deposited on porous Al 2O 3 disks using hexamethyldisiloxane (HMDSO) by remote inductively coupled plasma (ICP) chemical vapor deposition (CVD). After treating the film with ion bombardment and subsequent pyrolysis at a high temperature, carbon-based molecule sieve membranes can be obtained, exhibiting a very high H 2/N 2 selectivity around 100 and an extremely high permeance of H 2 around 1.5 × 10 −6 mol m −2 s −1 Pa −1 at 298 K. The O 2/N 2 selectivity could reach 5.4 with the O 2 permeance of 2 × 10 −7 mol m −2 s −1 Pa −1 at 423 K. During surface treatments, HMDSO ions were found to be more effective than CH 4, Ar, O 2 and N 2 ions to improve the selectivity and permeance. Short and optimized surface treatment periods were required for high efficiency. Without pyrolysis, surface treatments alone greatly reduced the H 2 and N 2 permeances and had no effect on the selectivity. Besides, without any surface treatment, pyrolysis alone greatly increased the H 2 and N 2 permeances, but had no improvement on the selectivity, owing to the creation of large pores by desorption of carbon. A combination of surface treatment and pyrolysis is necessary for simultaneously enhancing the permeance and the selectivity of CMS membranes, very different from the conventional pore-plugging mechanism in typical CVD.

Studies on modifications of ITO surfaces in OLED devices by Taguchi methods

The influences of various surface treatments on indium-tin-oxide (ITO) anodes on the performance of OLED devices are investigated by the Taguchi Method. These surface treatments include chemical and mechanical processes under different conditions. In particular, we find that the chemical treatment producing ultra-smooth ITO film surfaces does not provide the highest efficiency and luminescence. The implications of the optimized surface treatment conditions from Taguchi Methods are discussed through device testing.

OBIC studies on 6H-SiC Schottky rectifiers with different surface pretreatments

The influence of different surface treatments on the device characteristics of Schottky rectifiers was investigated. Prior to the formation of Schottky barrier contacts the SiC surface was either thermally annealed in hydrogen, etched using O2 or CF4/Ar/H2 as reactive gases or Ar sputtered. Differences in reverse current densities of several orders of magnitude for different surface treatments were observed. The effective device area was determined by the optical beam induced current (OBIC) technique. The results confirm that the effective device area is given by the area of the metal contact and the additional area of the surface charge induced depletion layer. Our results indicate the importance of an optimized surface treatment in order to control the quality and the area of the surface region for achieving optimum device performance.

Optimization of low temperature surface treatment of GaAs crystal

This paper reports the effects of a low temperature surface treatment under AsH 3 overpressure on the GaAs regrown interface quality prepared by photostimulated molecular layer epitaxy. The regrown diode I–V characteristics are investigated as functions of treatment temperature, AsH 3 pressure and treatment time. Optimized surface treatment conditions enable a good regrown interface to be obtained even at a lower temperature of ∼480°C than the conventional high temperature treatment at ∼600°C. The surface treatment mechanism is also discussed in combination with the results of X-ray photoemission spectroscopy and quadrupole mass analysis.

Effects of Sulfide Passivation on the Performance of GaAs MISFETs with Photo-CVD Grown P3N5 Gate Insulators

Accumulation-mode and depletion-mode GaAs metal-insulator-semiconductor field-effect transistors (MISEETs), with sulfur-treatment and a photochemical vapor-deposited- P3N5 gate insulator, have been successfully fabricated. The devices have good linearity, low hysteresis in current-voltage characteristics, and the instability of the current less than 22 percent for the period of 1.0-1.0×104 s. The effective electron mobility and extrinsic transconductance of the FETs at room temperature are about 1300 cm2/V·s and 1.41 mS/mm for the accumulation-mode, and about 4500 cm2/V·s and 4 mS/mm for the depletion-mode, respectively. Capacitance-voltage (C-V) characteristics and Auger electron spectroscopy (AES) analysis for different sulfur-treatment conditions are discussed. The atomic concentration ratios of sulfur and oxygen to arsenide on GaAs surfaces and GaAs metal-insulator-semiconductor (MIS) interface properties are critically dependent on sulfur pretreatment conditions, and the optimum sulfur-treatment temperature is determined to be about 40° C. The minimum density of interface trap states for an Al/P3N5/GaAs MIS diode with the optimized surface treatment is about 4.3×1010 cm-2 eV-1.

Selective MLE growth of GaAs and surface treatment for ideal static induction transistor (ISIT) application

This paper reports the effects of low temperature surface treatment on the GaAs regrown interface quality prepared by photo-stimulated molecular layer epitaxy. The regrown diode characteristics are investigated as functions of treatment temperature, exposing AsH 3 pressure and duration. Optimized surface treatment gives a good regrown interface even at low temperatures of ∽ 480°C compared with conventional high temperature treatment at ∽ 600°C. The surface treatment mechanism is also discussed in combination with the results of X-ray photo-emission spectroscopy (XPS) and quadrupole mass analysis (QMS). Low temperature surface treatment is successfully applied to the ISIT fabrication with a channel length of 1800-100 Å having a few monolayer p + barrier layer thickness, which shows the DC transconductance g m as high as 1500 mS/mm with good reproducibility.

Laser surface alloying of ni film on al-based alloy

Based on SIMS analyses, X-ray diffraction patterns, scanning and transmission electron microscopy observations and chemical analyses at various scales, the mechanisms of LASER surface alloying of Ni coating of Al-based alloy have been investigated. The formation of Al 3Ni primary dendrites in the case of the low thickness of the initial Ni coating is reported and a double dendritic mode is observed in the case of the thicker coating, Al 3Ni 2 followed by Al 3Ni dendrites. A segregation in Si has been observed just below the exposed surface and related to the dendritic solidification mode. The optimized surface treatment parameters have been experimentally determined as a function of the initial thickness of the Ni coating.