Alumina Buffer Research Articles

Controllable Preparation and Mechanism Study of Easy‐Peel High‐Density and Vertically Aligned Carbon Nanotube Forests

The stripping of carbon nanotube forests (CNTF) is an urgent problem in terms of its application in thermal management and semiconductor devices. The easy‐peel and vertically aligned CNTF is prepared in a two‐step process of high vacuum magnetron sputtering and chemical vapor deposition. Based on the thick alumina buffer layer, CNTF with heights of 100 μm, 300 μm, and 500 μm was prepared by adjusting the magnetron sputtering power. Through SEM observation and calculation, the corresponding densities were 2.5 × 109 cm−2, 6.4 × 109 cm−2, and 1.21 × 1010 cm−2, with an average curvature of 1.64 × 102 m−1, 1.35 × 102 m−1 and 0.53 × 102 m‐−1, respectively. The TEM, XPS, Raman, and TGA were used to investigate the growth mechanism of CNTF. The experimental results show that the catalyst particles annealed under high sputtering power refined with high density can grow a low‐defect, well‐oriented, and high‐density CNTF, and confirm the tip growth route. Further analysis shows that the easy‐peel properties of CNTF depend on the thickness of the alumina layer, the tip growth route, and the tight entanglement between the nanotubes. This paper provides technical guidance and support for the preparation, stripping, and application of monolithic CNTF.

Effect of catalyst support layers on emissivity of carbon nanotubes grown via floating catalyst chemical vapor deposition

We propose an efficient method of growing carbon nanotube (CNT) arrays on a variety of metals using chemical vapor deposition (CVD) assisted by a simple surface treatment of the substrate materials. The main feature of this method is the application of shot blasting with fine alumina shot to create the catalyst support layer that is required for the effective growth of CNTs on the various conductive materials, including the ultrahard metal tungsten. Energy dispersive X-ray spectroscopy shows that the shot blasting forms a noncontinuous layer where alumina nanoparticles are embedded as residue from the blasting media on the treated surfaces. Such a noncontinuous alumina layer can behave as the catalyst support layer, which is normally prepared via sputtering or a vacuum evaporation coating process. The influence of shot size on the characteristics of the nonuniform alumina buffer layer as well as the resultant multiwalled CNT arrays grown via CVD in conjunction with a floating iron catalyst are investigated. The results show that the morphology, crystallinity, and spectral emissivity of CNTs depend on the size of alumina shot used in the shot blasting treatment. The characteristics of the CNTs grown on various metal substrates with a discontinuous and continuous alumina buffer layers are compared with each other. These comparisons indicate that the degrees of hardness and the redox property of the substrates moderately affect the characteristics of CNTs on the discontinuous layer in contrast to the continuous layer.

Vertically aligned growth of small-diameter single-walled carbon nanotubes on flexible stainless steels by alcohol catalytic chemical vapor deposition with Ir catalyst on alumina buffer layer

We performed single-walled carbon nanotube (SWCNT) growth on flexible stainless-steel foils by applying alcohol catalytic chemical vapor deposition (CVD) using an Ir catalyst with an alumina buffer layer. When the alumina thickness was 90 nm, vertically aligned SWCNTs with a thickness of 4.6 μm were grown. In addition, Raman and transmission electron microscope results showed that the diameters of most SWCNTs were distributed below 1.1 nm. Compared with conventional CVD growth where Si wafers are used as substrates, this method is more cost effective and easier to extend for mass production of small-diameter SWCNTs.

Osmium catalyzed growth of vertically aligned and small-diameter single-walled carbon nanotubes by alcohol catalytic chemical vapor deposition

We showed that single-walled carbon nanotubes (SWCNTs) could be grown on SiO2/Si substrates by alcohol catalytic chemical vapor deposition (CVD) with an Os catalyst in a cold-wall CVD system. At a growth temperature of 700 °C, web-like SWCNTs were grown with diameters distributed below 0.9 nm. At a growth temperature of 800 °C, the SWCNT density increased and vertically aligned SWCNTs (VA-SWCNTs) 70 nm thick were grown on the SiO2/Si substrates after growth for 30 min. Raman and transmission electron microscope results showed that the diameters of the grown SWCNTs were distributed between 0.65 and 1.0 nm, and were much smaller than those of VA-SWCNTs obtained from conventional catalysts such as Fe and Co. Furthermore, compared with Ir, which has been reported to be a suitable catalyst for growth of small-diameter SWCNTs, we showed that Os could grow SWCNTs with even smaller diameters. Additionally, neither an alumina buffer layer nor cocatalyst were necessary to grow VA-SWCNTs with the Os catalyst. We consider that highly efficient suppression of aggregation and the relatively strong carbon-Os bond strength contributed to growth of small-diameter SWCNTs from the Os catalyst. Our studies are the first to report on Os as a catalyst for SWCNT growth, leading to SWCNTs with diameters less than ~1 nm. Hence, period 6 platinum-group metals are likely to be good catalysts for highly efficient growth of small-diameter SWCNTs.

High-contrast, highly textured VO2 thin films integrated on silicon substrates using annealed Al2O3 buffer layers

We present vanadium dioxide (VO2) thin films having high resistivity contrast with silicon substrates through use of crystallized alumina (Al2O3) buffer layers, engineered for this purpose. We first optimized the process by depositing VO2 onto C-plane sapphire substrates prior to alumina thin films. The latter of which were grown via atomic layer deposition on silicon substrates. We then applied rapid thermal annealing (RTA) to crystallize the alumina films. Scanning electron microscopy results indicated a thickness of 107 nm for each VO2 film, which yielded hot–cold resistivity contrast ratios of 9.76 × 104, 1.46 × 104, and 3.66 × 103, when deposited on the C-plane sapphire, the annealed buffers, and the as-deposited alumina buffers, respectively. Atomic force microscopy of the film surface roughness of the VO2 films indicated root mean squared roughness (Rq) of 4.56 nm, 6.79 nm, and 3.30 nm, respectively, for the films grown on the C-plane sapphire, annealed buffers, and as-deposited buffers. Finally, x-ray diffraction (XRD) of the VO2 films indicated the desired composition and strong (0h0)/(00h) texturing, when deposited on both the C-plane sapphire and the annealed alumina buffer layers. XRD results indicated a series of peaks corresponding to the α-Al2O3/C-plane sapphire, and an XRD analysis of the buffers alone confirmed crystallization of the buffer layer via RTA. The process defined in this paper produced a series of highly textured VO2 films making them most valuable for the integration of VO2 with silicon-based devices.

Large‐area spatial atomic layer deposition of amorphous oxide semiconductors at atmospheric pressure

AbstractIndium gallium zinc oxide (IGZO) is deposited using plasma‐enhanced spatial atomic layer deposition (sALD) on substrates as large as 32 × 35 cm2. Excellent uniformity and thickness control leads to high‐performing and stable coplanar top‐gate self‐aligned (SA) thin‐film transistors (TFTs). The integration of a sALD‐deposited aluminum oxide buffer layer into the TFT stack further improves uniformity and stability. The results demonstrate the viability of atmospheric sALD as a novel deposition technique for the flat‐panel display industry.

Sound velocities of the 23 Å phase at high pressure and implications for seismic velocities in subducted slabs

Dense hydrous magnesium silicates (DHMS) are believed to play an important role in transporting water back into the deep interior of the Earth. Recently, a new Al-bearing hydrous Mg-silicate, named the 23 Å phase [ideal composition Mg12Al2Si4O16(OH)14], was reported (Cai et al., 2015), which could be another important hydrous phase in subducting slabs. In this paper, we report new measurements of the compressional (P) and shear (S) wave velocities of the 23 Å phase under pressures up to 14 GPa at room temperature, using a bulk sample with a grain size of less than 20 μm and density of 2.947 g/cm3. The acoustic measurements were conducted in a 1000-ton uniaxial split-cylinder multi-anvil apparatus using ultrasonic interferometric techniques (Li et al., 1996). The pressures were determined in situ by using an alumina buffer rod as the pressure marker (Wang et al., 2015). A dual-mode piezoelectric transducer enabled us to measure P and S wave travel times simultaneously, which in turn allowed a precise determination of the sound velocities and elastic bulk (KS) and shear (G) moduli at high pressures. The velocities of this 23 Å phase (ambient VP = 7.53 km/s, VS = 3.72 km/s) are lower than those of phase A, olivine, pyrope, etc. (especially for S waves), yet the VP/VS ratio (from 2.02 to 1.94, decreasing with increasing pressure) is much higher compared to those mantle minerals (1.7–1.8). A fit to the acoustic data using finite strain analysis combined with the Hashin-Shtrikman (HS) averaging scheme yields: KS0 = 111.4(6) GPa, G0 = 41.0(2) GPa, and KS′ = 3.6, G′ = 1.8 for the bulk and shear moduli and their pressure derivatives. Using these data, the density and velocity profiles of the hydrous harzburgite and lherzolite were calculated along a warm slab P-T condition. The results suggest that a hydrous pyrolitic assemblage containing 23 Å phase should be distinguishable from a dry assemblage at the high pressure and temperature conditions relevant to Al-bearing subducted slabs.

Modifying the morphology and properties of aligned CNT foams through secondary CNT growth

In this work, we report for the first time, growth of secondary carbon nanotubes (CNTs) throughout a three-dimensional assembly of CNTs. The assembly of nanotubes was in the form of aligned CNT/carbon (ACNT/C) foams. These low-density CNT foams were conformally coated with an alumina buffer layer using atomic layer deposition. Chemical vapor deposition was further used to grow new CNTs. The CNT foam’s extremely high porosity allowed for growth of secondary CNTs inside the bulk of the foams. Due to the heavy growth of new nanotubes, density of the foams increased more than 2.5 times. Secondary nanotubes had the same graphitic quality as the primary CNTs. Microscopy and chemical analysis revealed that the thickness of the buffer layer affected the diameter, nucleation density as well as growth uniformity across the thickness of the foams. The effects of secondary nanotubes on the compressive mechanical properties of the foams was also investigated.

Effect of Alumina Buffers on the Stability of Top-Gate Amorphous InGaZnO Thin-Film Transistors on Flexible Substrates

Flexible top-gate amorphous InGaZnO thin-film transistors are fabricated on polyimide substrates. The effect of the alumina buffer layers on the device performance and stability is demonstrated using two types of atomic layer deposition reactant sources: 1) ozone and 2) water. Alumina buffers formed by water reactants have better barrier properties against the ambient than those formed by ozone. Furthermore, less charge trapping at sub-gap density-of-states occurs with higher film density of the buffer layer. Stability characteristics under negative bias temperature stress are enhanced by optimization of the buffer layer formation on flexible substrates.

Chemical Bath Deposition of Aluminum Oxide Buffer on Curved Surfaces for Growing Aligned Carbon Nanotube Arrays.

Direct growth of vertically aligned carbon nanotube (CNT) arrays on substrates requires the deposition of an aluminum oxide buffer (AOB) layer to prevent the diffusion and coalescence of catalyst nanoparticles. Although AOB layers can be readily created on flat substrates using a variety of physical and chemical methods, the preparation of AOB layers on substrates with highly curved surfaces remains challenging. Here, we report a new solution-based method for preparing uniform layers of AOB on highly curved surfaces by the chemical bath deposition of basic aluminum sulfate and annealing. We show that the thickness of AOB layer can be increased by extending the immersion time of a substrate in the chemical bath, following the classical Johnson-Mehl-Avrami-Kolmogorov crystallization kinetics. The increase of AOB thickness in turn leads to the increase of CNT length and the reduction of CNT curviness. Using this method, we have successfully synthesized dense aligned CNT arrays of micrometers in length on substrates with highly curved surfaces including glass fibers, stainless steel mesh, and porous ceramic foam.

RF-PECVD growth and nitrogen plasma functionalization of CNTs on copper foil for electrochemical applications

This work describes an efficient way to improve the adhesion, growth rate and density of CNTs on copper substrate using radio-frequency plasma enhanced chemical vapor deposition (RF-PECVD). The adhesion of an alumina buffer layer to the copper substrate is critical for the successful growth of CNTs. Hydrogen plasma was performed on the copper substrate to reduce copper oxide from the surface. The effect of two intermediate layers (Ti, Ni), as individual or in combination, between alumina and copper substrate on the CNT growth has been investigated. Furthermore, a nitrogen plasma treatment was carried out to functionalize the obtained CNTs. Electrochemical measurements were performed using CNTs grown on a copper substrate as electrodes and LiClO4 as electrolyte. The specific capacitance of the obtained electrodes increases from 49 up to 227Fg−1 for untreated and nitrogen-plasma treated CNTs at a scan rate of 10mVs−1, respectively.

High concentration of nitrogen doped into graphene using N2plasma with an aluminum oxide buffer layer

We performed plasma doping of nitrogen into single-layer graphene on SiO2. Using aluminum oxide as a buffer layer to reduce the plasma damage, up to 19.7% nitrogen was substitutionally doped into graphene. The nitrogen doping of graphene was confirmed by Raman and X-ray photoemission spectroscopy analyses. The n-doping property of the N-doped graphene was measured by Raman spectroscopy. Raman mapping was carried out to statistically confirm the Dirac cone shift of graphene resulting from the N-doping. The Dirac cone shift was directly measured by ultraviolet photoemission spectroscopy (UPS). The UPS result was consistent with the value calculated from the Raman G peak shift.

Performance improvement of organic light emitting diode with aluminum oxide buffer layer for anode modification

A thin Al2O3 insulating buffer layer deposited on indium tin oxide (ITO) anode by atomic layer deposition has been investigated for organic light-emitting diodes (OLEDs). With an optimal thickness of 1.4 nm and low density of structural defects of the Al2O3 film, the OLEDs current efficiency and power efficiency were simultaneously improved by 12.5% and 23.4%, respectively. The improvements in both current and power efficiency mean lower energy loss during holes injection process and better balanced charge injection. To understand the mechanism behind the enhanced performance of OLED by the buffer layer, a series of Al2O3 films of different thicknesses were deposited on ITO anode and characterized. The roughness, sheet resistance, and surface potential (EF′) of the Al2O3 modified ITO were characterized. Also, the properties of Al2O3 films were investigated at the device level. It is believed that the block of holes injection by the Al2O3 buffer layer makes more balanced carrier density in the emitting layer, thus enhances the current efficiency. Although less number of holes are injected into OLED due to the Al2O3 buffer layer, quantum tunneling through the ultra-thin buffer layer play an important role in contributing to the holes injection, which avoids crossing the interface barrier, resulting in less energy consumed and power efficiency enhanced.

Dip Coating of Nano Hydroxyapatite on Titanium Alloy with Plasma Assisted γ-Alumina Buffer Layer: A Novel Coating Approach

This paper reported a novel coating approach to deposit a thin, crack free and nano-structured hydroxyapatite (HA) film on Ti6Al4V alloy with Al 2 O 3 buffer layer for biomedical implants. The Al 2 O 3 buffer layer was deposited by plasma spraying while the HA top layer was applied by dip coating technique. The X-ray diffraction (XRD) and Raman reflections of alumina buffer layer showed α- to γ-Al 2 O 3 phase transformation; and the fractographic analysis of the sample revealed the formation of columnar grains in well melted splats. The bonding strength between Al 2 O 3 coating and Ti6Al4V substrate was estimated to be about 40 MPa. The presence of dip coated HA layer was confirmed using XRD, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis. The SEM images exhibited that HA top layer enveloped homogenously the troughs and crests of the underneath rough ( R a = 2.91 μm) Al 2 O 3 surface. It is believed that the novel coating approach adopted might render the implant suitable for rapid cement-less fixation as well as biocompatible for longer periods.

Rapid and scalable method for direct and indirect microstructuring of vertical aligned carbon nanotubes

Multi-walled carbon nanotube (MWCNT) films are grown by chemical vapor deposition (CVD) on silicon substrates using an alumina buffer and a Fe/Co layer as catalyst with different Fe/Co ratios. For both coatings, a scalable wet-chemical technique is applied. The highest CNT forests (100–125μm) are obtained with Fe content between 60 and 80wt.%. After deposition of the films, direct laser interference patterning (DLIP) method is used for fabricating micro patterns of the CNTs, using a frequency tripled Nd:YAG laser emitting 10ns pulses. Two different approaches for fabricating periodic MWCNT arrays are presented. The first approach is the direct patterning of the CNT layer itself (CNT-DLIP) obtaining well defined line-like structures with 10μm spatial period. By adjusting the laser fluence (from 150 to 250mJcm−2) and the number of laser pulses (from 1 to 20), the morphology (structure depth and line width) of the fabricated arrays can be varied. Thermal simulations of the CNT ablation process validate the experimental observations. The second approach involves indirect patterning of the CNTs, by fabricating line-like structures on the iron/cobalt catalyst layer (CAT-DLIP). In the last case, moderate energy densities (100mJcm−2) permitted to remove the catalyst layer locally at the interference maxima positions. By altering the number of laser pulses from 1 to 20 the line width can be tuned. The CNT forests are subsequently grown on the patterned catalyst.Using Raman spectroscopy, we demonstrate that in the case of direct patterning of the CNTs (CNT-DLIP) the chemical structure is preserved if a low number of laser pulses as well as moderate laser fluences are utilized. In the case of CAT-DLIP, the chemical structure of the grown CNTs on the patterned CAT-layer show a similar quality compared to the CNTs grown on the non-patterned CAT layer.

Fractal analysis and atomic force microscopy measurements of surface roughness for Hastelloy C276 substrates and amorphous alumina buffer layers in coated conductors

In coated conductors, surface roughness of metallic substrates and buffer layers could significantly affect the texture of subsequently deposited buffer layers and the critical current density of superconductor layer. Atomic force microscopy (AFM) is usually utilized to measure surface roughness. However, the roughness values are actually relevant to scan scale. Fractal geometry could be exerted to analyze the scaling performance of surface roughness. In this study, four samples were prepared, which were electro polished Hastelloy C276 substrate, mechanically polished Hastelloy C276 substrate and the amorphous alumina buffer layers deposited on both the substrates by ion beam deposition. The surface roughness, described by root mean squared (RMS) and arithmetic average (Ra) values, was analyzed considering the scan scale of AFM measurements. The surfaces of amorphous alumina layers were found to be fractal in nature because of the scaling performance of roughness, while the surfaces of Hastelloy substrates were not. The flatten modification of AFM images was discussed. And the calculation of surface roughness in smaller parts divided from the whole AFM images was studied, compared with the results of actual AFM measurements of the same scan scales.

Effect of Buffer Thickness on Single-Walled Carbon Nanotube Growth Using Aluminum Oxide Buffer Layer with Alcohol Gas Source Method

Aluminum oxide (Al2Ox) buffer layers were employed in carbon nanotube (CNT) growth using an alcohol gas source and Co catalyst. The CNT yield was strongly dependent on the Al2Ox buffer layer thickness. By optimizing the thickness of deposited Al, vertically aligned single-walled nanotubes (SWNTs) were grown on Si substrates, even under an ethanol pressure of 10(-1) Pa. Transmission electron microscopy (TEM) revealed that Co particle size was strongly dependent on Al thickness. Dense Co particles about 2-4 nm in diameter were formed at the optimal Al thickness. Surface flatness of oxide layers was relatively independent of oxide thickness, and could be easily maintained up to the onset of CNT growth. Thus, Al2Ox layers promote formation of nanosized Co particles suitable for SWNT growth, providing a significant enhancement to yield. At the higher Al thickness, the yield was reduced, which was due to the diffusion of Co particles into the Al2Ox layer. Therefore, control of Al thickness is important to obtain efficient CNT growth.

DIRECT GROWTH OF VERTICALLY ALIGNED CARBON NANOTUBES ON CU FOILS FOR APPLICATIONS IN LITHIUM ION BATTERIES

We report direct growth of vertically aligned carbon nanotubes (VCNTs) on Cu foils using thermal chemical vapor deposition and present the feasibility of possible applications as anode materials in lithium ion batteries. The VCNTs were vertically standing on the Cu foils which were covered with catalytic iron and alumina buffer layers. The growth temperature was 725°C and acetylene under atmospheric pressure was used as a hydrocarbon source. The VCNT grown had mean diameter of 5.8 nm and showed electrical ohmic contact to Cu foils. Electrochemical properties of Li ion battery cell using VCNT anode were examined and high capacitance was observed.

The role of aluminum oxide buffer layer in organic spin-valves performance

The electronic structures of the 8-hydroxyquinoline-aluminum (Alq3)/Al2O3/Co interfaces were studied by photoelectron spectroscopy. A strong interface dipole was observed, which leads to a reduction in the electron injection barrier. The x-ray photoelectron spectroscopy spectra further indicate that the Al2O3 buffer layer prevents the chemical interaction between Alq3 molecules and Co atoms. X-ray magnetic circular dichroism results demonstrate that a Co layer deposited on an Al2O3 buffered Alq3 layer shows better magnetic ordering in the interface region than directly deposited Co, which suggests a better performance of spin valves with such a buffer layer. This is consistent with the recent results from [Dediu et al., Phys. Rev. B 78, 115203 (2008)].

Development of High TCR Platinum Thin Films

High Temperature Coefficient of Resistance (TCR) platinum (Pt) thin films become increasingly attractive for devices with Micro Electro Mechanical Systems (MEMS) using high temperature. However, in annealing with high temperature, Pt thin films are degraded, and in comparison with bulk Pt, the TCR of Pt thin films are inferior. In this study, alumina (Al2O3) buffer layers were introduced between the silicon nitride (SiNx) films and Pt films. After annealing in the air, the structure of Pt thin films was observed by Scanning Electron Microscope (SEM) and analyzed by X-ray diffraction, and the depth-resolved elemental composition of interface between Pt thin films and Al2O3 buffer layers was observed by Secondary Ion Mass Spectrometry (SIMS). As the annealing temperature is higher, the grain size of Pt is larger. At the point of the largest grain size, the largest TCR could not obtain. In this paper, we discuss about the relationship between the structure of Pt thin film and annealing temperature, and about the effect of Al2O3 buffer layers.