Properties Of Carbon Nanotube Arrays Research Articles

Investigation of Field Emission Properties of Carbon Nanotube Arrays of Different Morphologies.

This article presents, for the first time, a comparative analysis of the emission characteristics of large-area field-effect cathodes (LAFE) based on carbon nanotubes (CNTs) of various morphologies according to key parameters using a unique computerized technique. The work presents a description of a technology for creating various CNT arrays and their comprehensive structure characterization. All CNT arrays synthesized by the catalytic PECVD method on a silicon substrate showed a high degree of chemical purity under the presented technological conditions. In some cases, nanoisland films of Fe were used as a catalyst; in others, thin films of NiO were used, which were deposited on a silicon wafer by chemical vapor deposition (CVD) and atomic layer deposition (ALD), respectively. As a result of these studies, it turned out that an array with a thick CNT coating has good resistance to the action of strong electric fields, fairly good uniformity of distribution of emission centers, a fairly high selection current (2.88 mA/cm2 at 4.53 V/μm), and compliance with the normal current mode according to the "orthodox" test, which makes the morphology of such structures the most promising for further technological optimization of CNT-based cathodes for various practical applications.

Adhesion properties of carbon nanotube arrays for an adhesive foot of a space crawling robot

A new type of space crawling robot was proposed for spacecraft maintenance missions and a nanoscale microarray structure was designed as the robot foot structure. In this study, a carbon nanotube array structure was adopted as the adhered nanoscale microarray structure of the robot foot. A theoretical model of the carbon nanotube structure was established, and the influence of the structural parameters of the carbon nanotube array on its adhesion characteristics was analyzed. A force model of carbon nanotube arrays on different roughness surfaces and under different preloads was established using the discrete element software, EDEM. A simulation of the desorption process of the carbon nanotube array structure in the normal and tangential directions was conducted. The adhesion characteristics of the carbon nanotube structure were analyzed. On this basis, the carbon nanotube microarray structure was fabricated using chemical vapor deposition, and a force test system was established on the platform of a scanning electron microscope to test the adhesion force of the carbon nanotube array in a vacuum environment. The influence of preloading and roughness on the adhesion characteristics of the carbon nanotube array structure was analyzed.

Modification of the mechanical properties of carbon nanotube arrays using electron irradiation induced oxidation

This paper describes a method of altering the compliance and adhesion of nominally vertically aligned multi-walled carbon nanotube (CNT) arrays (“turfs”) using oxidation caused by electron irradiation from typical imaging conditions in a scanning electron microscope. A combination of electron microscopy and infrared spectroscopy demonstrates that typical imaging conditions lead to the deposition and further oxidation of amorphous carbon on CNT turfs. The elastic modulus of the turfs, as measured by nanoindentation, decreases from approximately 100 to 50 MPa after irradiation and exposure to ambient laboratory conditions. The adhesion between the diamond indenter tip and turf is effectively eliminated once the amorphous carbon coated the turfs. In addition, the CNT turf exhibits decreased energy dissipation capabilities and a decreased viscoelastic response during contact loading. This suggests that further research on characterization of CNT turfs should consider the possibility that electron beam exposure during imaging may impact the subsequently measured properties, and that to lessen these effects, imaging times and accelerating voltages should be minimized.

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.(1,2) These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.(2) Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.(3-5) The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.1,2 These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.2 Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.3-5 The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

Field Emission Properties of Carbon Nanotube Arrays on the Thickness-Controlled Flexible Substrate by the Pattern Transfer Process

A technigal with the polydimethylsiloxane (PDMS) solution infiltrated into the SiOx-coated CNTAs has been utilized to directly transfer the CNTAs away from the silicon substrate. The oxide coating layer was utilized to protect the morpholgy of as-grown patterned vertical aligmed carbon nanotube (CNTs) arrays. The high density plasma reactive ions etching (HDP-RIE) system was used to make the CNTs emerge from the surface of the flexible substrate and modify the crystallines of CNTs. After the protecting oxide was HDP-RIE-processed for 8 min, the emission current properties were enhanced to be 1.03 V/microm and 1.43 V/microm, respectively, for the turn-on field and the threshold field, as compared with 1.25 V/microm and 1.59 V/microm for the as-grown CNTs, accordingly. The Field Emission (FE) enhancement after dry etching could be attributed to the open-ended structures and better crystalline.

Effect of Dry Oxidation on the Performance of Carbon Nanotube Arrays Electrochemical Capacitors

ABSTRACTIn this study, the effect of dry oxidation on the electrochemical properties of carbon nanotube arrays is investigated. Oxygenated surface functional groups were introduced to the arrays by oxygen plasma treatment, where their surface concentrations were varied by controlling the exposure time. The finding presented herein shows an augmentation of nearly thirty times in term of specific capacitance when the arrays are oxidized. Similar behavior is also observed in the non-aqueous electrolytes where the specific capacitance of the oxidized carbon nanotube arrays is measured more than three times higher than that of the pristine ones. However, overexposure to oxygen plasma treatment reverses this effect. At such high oxidation level, the damage to the graphitic structure becomes more pronounced such that the capacitive behavior of the arrays is overshadowed by their resistive behavior. These findings are important for further development of carbon nanotube based electrochemical capacitors.

Reversible Tuning of the Wettability of Carbon Nanotube Arrays: The Effect of Ultraviolet/Ozone and Vacuum Pyrolysis Treatments

Among diverse types of synthetic materials, arrays of vertically aligned carbon nanotubes have attracted the most attention, mainly because of their exceptional mechanical, electrical, optical, and thermal properties. However, their wetting properties are yet to be understood. In this present study, oxygenated surface functional groups have been identified as a vital factor in controlling the wetting properties of carbon nanotube arrays. The results presented herein indeed show that a combination of ultraviolet/ozone and vacuum pyrolysis treatments can be used to vary the surface concentration of these functional groups such that the carbon nanotube array can be repeatedly switched between hydrophilic and hydrophobic.

Controlled Growth and Supercapacitive Behaviors of CVD Carbon Nanotube Arrays

Low-pressure chemical vapor deposition (LP-CVD) technique has been utilized for controlled growth of carbon nanotube (CNT) arrays on silicon wafers. The tube-diameters of CNTs and the number of graphene layers are controlled by varying the thickness of catalyst films. The catalyst particle density and the growth conditions such as the ambient gas and the local environment are all crucial for the formation of vertically aligned CNT arrays. The length of CNT arrays can be controlled by altering the growth time. In addition, the supercapacitive properties of CNT arrays with various morphologies growing on different current collectors have been investigated using a less corrosive 0.5 M Na2SO4aqueous solution as the electrolyte. Vertically aligned CNT arrays on Ti-Si substrate produce a higher capacitance compared to randomly oriented CNTs on the same current collector. Furthermore, Ni foam enables better utilization of active materials than Ti-Si substrate. CNT arrays electrodes fabricated by this simple, low cost approach demonstrate stable and consistent capacitor behaviors for a wide range of scan rates. Moreover, CNT arrays electrodes provide better platform for further integration with transitional metal oxide, via simple sputtering or electrodeposition technique, to enhance the supercapacitive performance.

Field emission properties of carbon nanotube arrays grown in porous anodic alumina

AbstractCarbon nanotubes (CNTs), with their excellent electronic properties and extremely high aspect ratio, represent an ideal material for building electron sources based on field emission. Fowler‐Nordheim equation describes quite successfully the field emission phenomenon, especially for single or isolated tips. However, some complications arise when populations of CNTs are considered, where collective effects and large variability in the emitters features influence the measured I–V characteristics. In this work, the emission properties of multi‐walled CNTs grown within ordered anodic alumina templates are investigated. These CNT matrices produce current densities up to some tens of mA/cm2, and the field enhancement factor for collective emission sources can be estimated. Such material can be modelled as an ordered and uniform array of emitters and a simulation of the electrostatic field on the emission tips can be done in order to evaluate the field enhancement factor and its dependence on various geometries. This allows comparing predictions from simulation and experimental measurements, in a direct way. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Tunable field emission properties of carbon nanotube arrays by engineering Fe catalysts

High-quality carbon nanotube (CNT) arrays composed of nanotubes with different diameters and wall numbers were synthesized by water-assisted chemical vapor deposition (CVD) from engineered Fe catalysts. Interestingly, the distribution of nanotube diameter and wall number broadened over 2.5 times as the catalytic Fe thickness increased. The mean diameter and wall number of nanotubes increased monotonically with the Fe thickness, while the calculated CNT area density within an array dropped about 32 times. Field emission measurements revealed that the turn-on voltage for CNT arrays decreased from 3.5 to 2.5 V/µm with the increased catalytic Fe thickness. It was believed that the screening effect from the proximity of neighboring nanotubes has a dominant influence than the CNT diameter on the resulting turn-on voltage.

Characterization and field-emission properties of carbon nanotube arrays in nanoporous alumina template and on blank Si substrate

Ordered carbon nanotube (CNT) arrays were synthesized within anodized aluminum oxide template by thermal decomposition of hydrocarbon precursor with hydrogen ambient at growth temperature as low as 500°C. Excess hydrogen in precursor mixture enables a steady supply of mobile hydrocarbon reactant which promotes facile solid-phase diffusion. The activation energy for CNT growth was determined to be 0.55eV, a number smaller than 1.02eV for similar precursor in nitrogen ambient. Moreover, CNTs grown in anodized aluminum oxide nanopores in this low temperature process were found to exhibit unusually high field-emission current of 100mA∕cm2 at 8V∕μm.

Investigation of field emission properties of carbon nanotube arrays defined using nanoimprint lithography

We demonstrate the use of nanoimprint lithography as an alternative low-cost fabrication route for the production of ordered arrays of individual carbon nanotube field emitters. A high emission site density of 4×105cm−2 was observed and is well within the specification of the cathode for a field emission display. The measured field enhancement values from the geometry of the nanotubes were in reasonable agreement with the values obtained through electrical measurements. We also show that the distribution of the field enhancement factor is Gaussian, indicative of the presence of well ordered arrays of carbon nanotube field emitters.

Field emission properties of carbon nanotube arrays through the pattern transferprocess

A process for transferring carbon nanotube (CNT) arrays from a silicon wafer to analumina substrate coated with Ag paste is proposed. A current density of up to325 mA cm−2 at an electricfield of 2.4 V µm−1 was achieved. The influence of the patterned size and the length of carbon nanotubes onthe field emission properties were investigated. Through this transfer method, the adhesionbetween the CNTs and the substrate is enhanced and the current density and turn-onvoltage are improved. The effects of the microstructure of the emitting sites at the CNT tipon the current density were also studied.

Effect of Gas Exposure on Field Emission Properties of Carbon Nanotube Arrays

Effect of Gas Exposure on Field Emission Properties of Carbon Nanotube Arrays