Effect of assisted ion energy on properties of silicon oxide thin film deposited by dual ion‐beam sputtering

A silicon oxide thin film barrier was prepared with different oxygen ion energies, and its chemical composition, surface morphology, optical, and barrier property related to the deposition condition were characterized and discussed. Our study showed that in O₂‐assisted process the stoichiometric silicon oxide thin film was obtained at a critical deposition condition of 100 eV oxygen ion energy. The thin film deposited at the critical condition showed the lowest surface roughness giving similar or higher optical transmittance than that of pure polycarbonate substrate. The boiling and tensile tests performed on the thin film deposited using assisted ions prior to the deposition process showed improvement in the adhesion between the oxide barrier layer and the polymer substrate. In addition, interface domination for improving the barrier properties of silicon oxide thin film was achieved through introduction of dual ion‐beam sputtering without pretreatment. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Transformation of chemical vapor deposited individual graphene crystal with oxidation of copper substrate

Here, we reveal the structural transformation process of as-synthesized individual graphene crystals with oxidation of a copper (Cu) foil. We found that the transformation of a graphene crystal with Cu oxidation is significantly different for the thermal annealing and room temperature long-term atmospheric oxidation. Annealing creates large cracks in an individual graphene crystal due to the thermal stress and strain created by rapid oxidation of Cu surface. The cracks are further enhanced with longer annealing duration enabling oxygen diffusion through cracks, thereby accelerating oxidization of Cu. Eventually, the graphene crystals are completely damaged, leaving behind the highly oxidized Cu surface. On the other hand, in case of room temperature long-term atmospheric oxidation, oxygen diffusion occurs underneath of a graphene crystal through the reactive edge without any large cracks formation. The graphene crystals decouple from Cu surface during the oxygen diffusion and oxidation process, however no structural deformation is observed. This finding shows the significant contrast of structural change of graphene crystal and oxidation behaviors of Cu surface with thermal annealing and room temperature atmospheric oxidation.     

Oxidation of Silicon, Silicon Carbide, and Silicon Nitride in Gases Containing Oxygen and Chlorine

Chlorine contamination accelerates the oxidation of silicon-based ceramics through the formation of volatile silicon chloride or silicon oxychloride species which degrade the protective character of the SiO₂ film. Accelerated attack may occur by active corrosion or formation of bubbles in the oxide layer. Si₃N₄ is much more resistant to this attack than either silicon or SiC. This resistance may be related to the presence of a thin silicon oxynitride layer below the SiO₂ scale which forms on Si₃N₄.     

Sol-Gel Alumina Coating for Improved Cyclic Oxidation Resistance of Silicon Nitride

A 25 nm thick α-alumina layer was deposited on a turbine-grade silicon nitride by sol-gel dip coating and subsequent heat treatment in air at 1200°C. This layer had a nanometer grain structure. Silicon nitride protected by this thin layer showed a significant improvement in oxidation resistance over its uncoated counterpart after 200 cyclic exposures in air at 1250°C. The oxide layer grown on the coated silicon nitride also exhibited superior surface morphology, compared with the uncoated silicon nitride.     

Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe₃O₄ supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.     

An advanced electrocatalyst with exceptional eletrocatalytic activity via ultrafine Pt-based trimetallic nanoparticles on pristine graphene

It has been a great challenge to directly deposit uniform metal particles onto pristine graphene due to its low surface energy and chemical inertness. Without any surfactant or functionalization, we have developed a unique synthesis of high-quality PtRuNi trimetallic nanoparticles supported on pristine graphene via a simple but effective supercritical route. Due to excellent wettability between supercritical carbon dioxide and the carbon surface, ultrafine metal particles are uniformly and firmly anchored on the graphene sheets. While well retaining its intrinsic structure and outstanding electronic conductivity, the pristine graphene with well-dispersed PtRuNi trimetallic nanoparticles shows significantly improved catalytic activity towards methanol oxidation, which is at least ten times higher than those of the commercial Pt/C and homemade Pt/XC-72 catalysts. The resulting trimetallic hybrid also exhibits high stability as compared to Pt and PtRu/pristine graphene composites and the reduced graphene oxide counterparts. In principle, the supercritical method can be applied to other metal nanoparticles in fabrication of high-performance graphene-based nano-catalysts.     

Fabrication mechanism of friction-induced selective etching on Si(100) surface

As a maskless nanofabrication technique, friction-induced selective etching can easily produce nanopatterns on a Si(100) surface. Experimental results indicated that the height of the nanopatterns increased with the KOH etching time, while their width increased with the scratching load. It has also found that a contact pressure of 6.3 GPa is enough to fabricate a mask layer on the Si(100) surface. To understand the mechanism involved, the cross-sectional microstructure of a scratched area was examined, and the mask ability of the tip-disturbed silicon layer was studied. Transmission electron microscope observation and scanning Auger nanoprobe analysis suggested that the scratched area was covered by a thin superficial oxidation layer followed by a thick distorted (amorphous and deformed) layer in the subsurface. After the surface oxidation layer was removed by HF etching, the residual amorphous and deformed silicon layer on the scratched area can still serve as an etching mask in KOH solution. The results may help to develop a low-destructive, low-cost, and flexible nanofabrication technique suitable for machining of micro-mold and prototype fabrication in micro-systems.     

A green approach for the reduction of graphene oxide by wild carrot root

A green approach for the reduction of graphene oxide (GO) using wild carrot root is reported in this work. It avoids the use of toxic and environmentally harmful reducing agents commonly used in the chemical reduction of GO to obtain graphene. The endophytic microorganisms present in the carrot root, reduces exfoliated GO to graphene at room temperature in an aqueous medium. Transmission electron microscopy and atomic force microscopy images provide clear evidence for the formation of few layer graphene. Characterization of the resulting carrot reduced GO by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy shows partial reduction of GO to graphene. Raman spectroscopy data also indicates the partial removal of oxygen-containing functional groups from the surface of GO and formation of graphene with defects.     

Reduction dependent wetting properties of graphene oxide

This study reports contact angle measurements of standard, diol and aromatic solvents on graphene oxide thin films thermally reduced in ultra-high vacuum up to 900 °C. The films were chemically and morphologically characterized using respectively X-ray photoemission spectroscopy and atomic force microscopy. The characterization shows that the wetting occurs in the chemically heterogeneous regime, namely the surface roughness (3 nm) does not influence the wetting properties of the samples. Zisman, Owens–Wendt and Neumann methods have been applied in order to calculate the surface free energy of the thin films showing that the Owens–Wendt method best fit the data trends. The surface free energy varies from 51 mN/m (pristine graphene oxide) to 39 mN/m (900 °C reduced graphene oxide). A correlation between the surface chemical composition, the surface free energy and its polar and dispersive components is reported, giving a rationale to the wetting properties of graphene oxide and reduced graphene oxide.     

Study of a thin anodic oxide on n-InP by photocurrent transient, capacitance measurements and surface analysis

Investigation of the anodic oxide on n-InP has been made with an approach coupling photocurrent transient and capacitance measurements in the dark, associated to chemical surface analysis by photoelectron spectroscopy (XPS). Growth, formation and stability of anodic oxides obtained on n-InP were investigated under illumination at pH 9. A potentiostatic mode has been preferred to explore the chemical evolution of the interface. This study shows that the decay of the photocurrent transients, the modification of the capacitances and the chemical evolution of the surface composition are totally related. The growth of a thin and stable oxide provides a blocking behaviour of the oxidised n-InP toward the photo-transfer. Moreover, our study evidenced a formation and growing of a mixed In-P oxide layer, in three stages.     

B4C--SiC/C复合材料氧化过程的TG/DTA分析

制备了Ｂ_４Ｃ－ＳｉＣ／Ｃ复合材料,对不同组成的复合材料在２０１５００℃升温氧化过程进行了ＴＧ／ＤＴＡ（热重和差热联用）分析。结果表明,复合材料具有不同的高温抗氧化性能,此差异可归因于以下几方面：复合材料的组成不同（包括陶瓷粒子的种类、含量）,不同种类的陶瓷粒子氧化转变成陶瓷氧化物的温度、速率不同,生成的陶瓷氧化物在高温下的物性（对基体材料的润湿性、粘度及流动性、挥发性和对氧的扩散系数等）不同。通过在炭基体中弥散Ｂ４Ｃ粒子可明显提高炭基体在８５０℃以下的抗氧化性。当复合材料中Ｂ４Ｃ含量较高而ＳｉＣ含量低时,样品表面将趋于形成Ｂ２Ｏ３较为富集的硼硅酸玻璃相,在１５００℃以下具有良好的氧化防护效果;当样品中ＳｉＣ含量较高时,在高温下（＞１２００℃）,随着部分Ｂ２Ｏ３的挥发和大量ＳｉＯ２的生成,样品表面将趋于形成ＳｉＯ２较为富集的硼硅酸盐玻璃相,在高达１４００℃以上仍具有良好的氧化防护效果     

Microstructure,oxidation and thermal shock resistance of graphene reinforced SiBCN ceramics

SiBCN ceramics were prepared using various volumes of graphene platelets (GPLs) as nanofiller. The effects of the nanofiller on microstructure, and oxidation and thermal shock resistance of as-sintered ceramics were investigated. The phase composition and microstructures were very similar for all investigated ceramics consisting primarily of β-SiC, BNC and small amounts of α-SiC with relatively homogeneously distributed 5–10 nm thick GPLs in the matrix. For SiBCN ceramics incorporating graphene as nanofiller, a porous oxide layer forms at 1500 °C and the oxidation behavior shows a linear kinetics by thickness measurement method. Gas evolution during heating lead to a passive oxidation behavior and weight loss. Graphene reinforced SiBCN ceramics exhibit thermal shock resistance superior to monoliths of the same material. The graphene distributed in SiBCN matrix can dissipate the energy of crack growth and acts as a stopper to cracks. The toughening mechanisms offered by graphene, including pull-out and bridging appear to aid in ameliorating thermal shock effects. Furthermore, the existence of a dense oxide surface layer retards oxygen diffusion into the inner matrix and heals surface pores and cracks, which also contributes to thermal shock resistance.     

Effects of thermal oxidation conditions of silicon electrodes on cathodic electrochemiluminescence of Ru(bpy)3 chelate

Hot electrons emitted from thin oxide film-coated heavily doped silicon electrodes by cathodic pulse polarization can induce electrochemiluminescence from luminophores. The intensity of electrochemiluminescence produced at the electrode surface is dependent on the features of thin oxide films formed by thermal oxidation. As a preliminary study, we investigated the effect of thermal oxide growth conditions on the intensity of electrochemiluminescence produced at these electrodes, such as oxidation atmospheres, oxidation temperature, oxidation time and pre-treatment of wafers, using ruthenium(II) tris-(2,2′-bipyridine) chelate as a model luminophore. Optimal oxidation conditions of heavily doped silicon electrodes were obtained for the generation of intense electrochemiluminescence at this kind of silicon electrodes.     

石墨烯量子点修饰柔性硅纳米线阵列用于NO2检测

将化学刻蚀法与金属辅助刻蚀法相结合,制备了形貌统一、分布均匀的高质量柔性硅纳米线阵列结构,用石墨烯量子点对其表面进行修饰,得到了表面稳定且具有强载流子传输能力的柔性石墨烯量子点/硅纳米线核?壳结构阵列,用其制备气敏设备检测NO2. 结果表明,基于该阵列的电阻式气敏设备对NO2的检测灵敏性及可重复性极高,检测浓度极限达20 mg/m3;不同弯曲度的柔性石墨烯量子点/硅纳米线阵列的气敏特性未大幅度降低,弯曲90o时响应电流峰值为未弯曲时的70%.     

Junction characteristics of chemically-derived graphene/p-Si heterojunction solar cell

Experimental and theoretical investigations on the heterojunction of silicon (Si) with chemically derived graphene have been presented. The stability study of graphene oxide and reduced graphene oxide (rGO) in aqueous medium were performed by visual observation and surface charge measurement. The detailed characterization by FT-IR, UV–Vis, and Raman spectroscopy exhibited the formation of rGO with a high optical band gap of 3.6 eV. The atomic force microscopy analysis for rGO sample revealed the formation of flakes with thickness ≈ 10 nm. The rGO was spin-coated on the p-Si substrate for fabrication of a heterojunction device, with the structure of rGO/p-Si. In the fabricated device, incident light was transmitted through the thin rGO film to reach the junction interface, generating photoexciton, and thereby a photo-conversion efficiency of 0.02% was achieved. The theoretical simulation of rGO/p-Si heterojunction device using solar cell capacitance simulation 1D software showed the efficiency of 1.32%. Such large deviations in efficiency between experiment and theory have been discussed in details.     

Substrate effects on the strain relaxation in GaN/AlN short-period superlattices

ABSTRACT: We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.     

Inkjet Printing Based Layer‐by‐Layer Assembly Capable of Composite Patterning of Multilayered Nanofilms

Surface modification involves developing a versatile thin film by combining the physical, chemical, or biological characteristics of the functional materials and can facilitate controlling material for desirable aims. Layer‐by‐layer (LbL) assembly can be used to create materials with controlled thicknesses and morphologies, diverse functionalities, and unique structures on any surface. However, despite the advantages of the LbL fabrication technique, there are limits to its application because it is a time‐consuming process and has difficulty controlling the shape of nanofilms. In addition, controlling the lateral organization is difficult because the preparation methods are based on one‐pot self‐assembly. In this study, a multilayered fabrication system is developed for the high‐throughput LbL assembly of nanofilms through inkjet printing. With various types of materials from synthetic polymer to graphene oxide to natural polymer and protein, the approach can tune the preparation of nanoscale multilayers with desired structures and shapes for specific applications on various substrates, including a silicon wafer, quartz glass, and cellulose‐based paper.     

High sensitivity surface enhanced Raman spectroscopy of R6G on in situ fabricated Au nanoparticle/graphene plasmonic substrates

Plasmonic gold nanoparticles (AuNP) with controllable dimensions have been fabricated in situ on graphene at moderately elevated temperature for high sensitivity surface enhanced Raman spectroscopy (SERS) of Rhodamine 6G (R6G) dye molecules. Significantly enhanced Raman signature of R6G dyes were observed on AuNP/graphene substrates as compared to the case without graphene with an improvement factor of 400%, which is remarkably greater than previous results obtained in ex situ fabricated SERS substrate. Simulation of localized electromagnetic field around AuNPs with and without the underneath graphene layer reveals an enhanced local electromagnetic field due to the plasmonic effect of AuNPs, while additional Ohmic loss occurs when graphene is present. The enhanced local electromagnetic field by plasmonic AuNPs is unlikely the dominant factor contributing to the observed high SERS sensitivity on R6G/AuNP/graphene substrate. Instead, the p-doped graphene, which is supported by the large positive Dirac point shift away from “zero” observed in AuNP/graphene field effect transistors, promotes SERS signals through enhanced molecule adsorption and non-resonance molecular–substrate chemical interaction.     

Low-damage direct patterning of silicon oxide mask by mechanical processing

To realize the nanofabrication of silicon surfaces using atomic force microscopy (AFM), we investigated the etching of mechanically processed oxide masks using potassium hydroxide (KOH) solution. The dependence of the KOH solution etching rate on the load and scanning density of the mechanical pre-processing was evaluated. Particular load ranges were found to increase the etching rate, and the silicon etching rate also increased with removal of the natural oxide layer by diamond tip sliding. In contrast, the local oxide pattern formed (due to mechanochemical reaction of the silicon) by tip sliding at higher load was found to have higher etching resistance than that of unprocessed areas. The profile changes caused by the etching of the mechanically pre-processed areas with the KOH solution were also investigated. First, protuberances were processed by diamond tip sliding at lower and higher stresses than that of the shearing strength. Mechanical processing at low load and scanning density to remove the natural oxide layer was then performed. The KOH solution selectively etched the low load and scanning density processed area first and then etched the unprocessed silicon area. In contrast, the protuberances pre-processed at higher load were hardly etched. The etching resistance of plastic deformed layers was decreased, and their etching rate was increased because of surface damage induced by the pre-processing. These results show that etching depth can be controlled by controlling the etching time through natural oxide layer removal and mechanochemical oxide layer formation. These oxide layer removal and formation processes can be exploited to realize low-damage mask patterns.     

Repeatable change in electrical resistance of Si surface by mechanical and electrical nanoprocessing

The properties of mechanically and electrically processed silicon surfaces were evaluated by atomic force microscopy (AFM). Silicon specimens were processed using an electrically conductive diamond tip with and without vibration. After the electrical processing, protuberances were generated and the electric current through the silicon surface decreased because of local anodic oxidation. Grooves were formed by mechanical processing without vibration, and the electric current increased. In contrast, mechanical processing with vibration caused the surface to protuberate and the electrical resistance increased similar to that observed for electrical processing. With sequential processing, the local oxide layer formed by electrical processing can be removed by mechanical processing using the same tip without vibration. Although the electrical resistance is decreased by the mechanical processing without vibration, additional electrical processing on the mechanically processed area further increases the electrical resistance of the surface.