Tribological properties of vertically aligned carbon nanotube arrays

Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1 mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction μ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of μ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect μ after a preconditioning by a few sliding cycles. However, a strong decrease of μ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.     

Surface properties of vertically aligned carbon nanotube arrays

This study focuses on the structural changes of vertically aligned carbon nanotube (CNT) arrays while measuring their adhesive properties and wetting behaviour. CNT forests grown by chemical vapor deposition with a height of ~ 100 µm, an outer CNT diameter of ~ 10 nm and a density of the order of ~ 10¹⁰ CNTs/cm² show an average adhesion of 4 N/cm² when pressed against a glass surface. The applied forces lead to the collapse of the regular CNT arrays which limits their reusability as functional dry adhesives. Goniometric water contact angle (CA) measurements on CNT forests show a systematic decrease from an initial value of ~ 126° to a final CA similar to highly orientated graphite. Environmental scanning electron microscopy shows that this loss of hydrophobicity is due to an evaporation induced compaction of CNTs together with the loss of their vertical alignment. We observe the formation of cellular patterns for controlled drying.     

Alcohol-assisted rapid growth of vertically aligned carbon nanotube arrays

Millimeter-to-centimeter scale vertically aligned carbon nanotube (VACNT) arrays are widely studied because of their immense potential in a range of applications. Catalyst control during chemical vapor deposition (CVD) is key to maintain the sustained growth of VACNT arrays. Herein, we achieved ultrafast growth of VACNT arrays using Fe/Al₂O₃ catalysts by ethanol-assisted two-zone CVD. One zone was set at temperatures above 850 °C to pyrolyze the carbon source and the other zone was set at 760 °C for VACNT deposition. By tuning synthesis parameters, up to 7 mm long VACNT arrays could be grown within 45 min, with a maximal growth rate of ∼280 μm/min. Our study indicates that the introduction of alcohol vapor and separation of growth zones from the carbon decomposition zone help reduce catalyst particle deactivation and accelerate the carbon source pyrolysis, leading to the promotion of VACNT array growth. We also observed that the catalyst film thickness did not significantly affect the CNT growth rate and microstructures under the conditions of our study. Additionally, the ultralong CNTs showed better processability with less structural deformation when exposed to solvent and polymer solutions. Our results demonstrate significant progress towards commercial production and application of VACNT arrays.     

Fabrication of vertically aligned ferroelectric polyvinylidene fluoride mesoscale rod arrays

We have fabricated vertically aligned ferroelectric PVDF mesoscale rod arrays comprising β and γ phases using a 200 nm diameter anodized aluminum oxide (AAO) as the porous template. We could synthesize the ferroelectric phase in mesoscale rod forms by combining the well‐established recipe for crystallizing the β phase using dimethyl sulfoxide (DMSO) at low temperature and template‐guided infiltration processing for the rods using AAO. We measured the dimensions of the PVDF rods by scanning electron microscopy and identified the polymorph phases by X‐ray diffraction and Fourier transform infrared spectroscopy. The length of the rods varied from 3.82 μm to 1.09 μm and the diameter from 232 nm to 287 nm when the volume ratio between DMSO and acetone changed from 5 : 5 to 10 : 0. We obtained well‐defined piezoresponse hysteresis loops for all rods with remnant piezoresponse ranging from 2.12 pm/V to 5.04 pm/V and coercive voltage ranging from 2.29 V to 2.71 V using piezoresponse force microscopy. Our results serve as a processing platform for flexible electronic devices that need high capacitance and piezoelectric functionalities such as flexible memory devices or body energy harvesting devices for intelligent systems. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3842–3848, 2013     

Field emission from in situ-grown vertically aligned SnO2 nanowire arrays

Vertically aligned SnO₂ nanowire arrays have been in situ fabricated on a silicon substrate via thermal evaporation method in the presence of a Pt catalyst. The field emission properties of the SnO₂ nanowire arrays have been investigated. Low turn-on fields of 1.6 to 2.8 V/μm were obtained at anode-cathode separations of 100 to 200 μm. The current density fluctuation was lower than 5% during a 120-min stability test measured at a fixed applied electric field of 5 V/μm. The favorable field-emission performance indicates that the fabricated SnO₂ nanowire arrays are promising candidates as field emitters.     

Growth and characterization of vertically aligned centimeter long CNT arrays

Record-long (21.7 mm) vertically aligned MWNT arrays were synthesized using water-assisted thermal CVD process. The catalyst lifetime was maintained for 790 min at optimized experimental condition. The growth of the centimeter long CNT was observed by real time photography at different growth conditions. The growth length increased linearly with increasing growth time followed by a sudden growth end. The ratio of ethylene and H₂ concentration as well as the water and ethylene concentration was studied and optimized which led to prolong catalyst lifetime. Transmission electron microscope images confirmed that most CNTs were double wall and the number of wall distribution was uniform along different height position. Raman spectra showed that the I_D/I_G ratio remained constant at the 3 studied positions along the CNT. TGA demonstrated negligible impurity incorporation in the CNT array.     

Growth termination mechanism of vertically aligned centimeter long carbon nanotube arrays

The mechanism of abrupt growth termination for growing vertically aligned centimeter long CNT arrays in chemical vapor deposition process was studied. We found that the growth length increases linearly with increasing growth time and the CNT growth terminates abruptly for all the experimental conditions applied. We investigated the change of particle size distribution, particle number density and the catalyst composition on the surface of the substrate with the exposure time and temperature in the reactor. For this study X-ray Photoelectron Spectroscopy (XPS), High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM) were employed. Focused Ion Beam (FIB) was used for preparation of the STEM samples. The experimental results demonstrated that the formed catalyst nanoparticles undergo compositional and morphological changes during the heating step of the substrate, when the temperature of the reactor is rising from ambient to the growth temperature, and during annealing at the set growth temperature. Most of the changes in the catalyst composition and morphology were related to metal diffusion into neighboring substrate layers during heating up to the growth temperature. Based on the results, a new scenario of abrupt termination mechanism for growing centimeter long CNT arrays using an iron–gadolinium binary catalyst was proposed.     

Polarization-dependent optical reflection from vertically aligned multiwalled carbon nanotube arrays

Optical reflection from an array of vertically aligned multiwalled carbon nanotubes (MWCNTs) acting as a black body absorber was investigated. We present the novel results of the experimental analysis of specular reflection involving the different light polarization configurations and wavelengths from visible and near infrared range. It was shown that with the increasing incidence angle, reflectance rises dramatically and is highly dependent upon the polarization state of the incoming light. Our results indicate that popular antenna model commonly used for explaining optical properties of MWCNTs must be applied with care when referenced to nanotube arrays.     

Design parameters for enhanced photon absorption in vertically aligned silicon nanowire arrays

Superior photon absorption in ordered nanowire arrays has been demonstrated recently. However, systematic studies are still missing to explore the limits of their implementation as functional photonic devices. With emphasis on silicon nanowires, we investigated the effects of nanowire diameter, length, morphology, and pitch on the photon absorption within the visible solar spectrum based on simulations. Our results reveal that these parameters are crucial but disclose a path to improve the absorbance drastically.

PACS

78.40.Fy; 78.67.Uh; 78.67.-n     

Buckling initiation and displacement dependence in compression of vertically aligned carbon nanotube arrays

Carbon nanotube (CNT) arrays have shown the remarkable ability to react as foam-like structures and exhibit localized buckling coordinated within specific regions. Here, we report on the low-cycle compression of bulk vertically aligned CNT arrays to observe initiation and growth of the buckling as a function of compressive strain. A critical strain is found above which the buckling region length increased and below which it remained at or below the applied strain. As previously observed, the buckling region of the CNT array propagates from the surface where growth occurred, which, in the test specimen, is a free surface and later receives compressive contact by a polished silicon substrate. The results are corroborated with nanoindentation on the surfaces, which indicate a stiffening of the near surface with increasing applied strain. Observation and results of the buckling region nature are important for applications of nanotube arrays as energy absorbing cushions, tunable dampers, thermal contacts, or in sliding contact.     

Fluffy carbon nanotubes produced by shearing vertically aligned carbon nanotube arrays

Fluffy carbon nanotubes (CNTs), which are cotton-like macroscopic structures, are obtained by simple high-speed shearing of vertically aligned CNT (VACNT) arrays. The fluffy CNTs are composed of CNT bundles with a diameter of several micrometers, and have an extremely low apparent density of 3-10 g/L. A requisite for their formation is the alignment of CNTs in the initial array. The shear between the rotor and the arrays tears the arrays along the axial direction and this results in their dispersion into low density fluffy CNTs.     

Thermal and electrical conductivity of tall, vertically aligned carbon nanotube arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays up to ∼6 mm high with an array density of 0.06 g cm⁻³ have been grown by chemical vapor deposition. Thermal conductivities (κ) and electrical conductivities (σ) were determined from 5 K to 390 K. The range for κ at 300 K is 0.5-1.2 W m⁻¹ K⁻¹ along the tube growth direction, with the shortest array having the highest κ, and an order of magnitude lower in the direction perpendicular to the tubes. The same trends also were evident for electrical conductivity, i.e., decreasing values with increasing array height and conductivity an order of magnitude lower in the perpendicular direction. Values of σ ranged from 7 to 14 S cm⁻¹ along the array at 300 K. The Seebeck coefficient is ∼20 μV K⁻¹ at 300 K. The effective Lorentz number indicates that thermal conductivity in the carbon nanotube arrays is phonon dominated over the full temperature range.     

Harvesting of large,substrate-free sheets of vertically aligned multiwall carbon nanotube arrays

Continuous multiwall carbon nanotube (MWCNT) synthesis, using chemical vapor deposition (CVD) of a xylene/ferrocene feedstock, produces MWCNT arrays comprised of dense stands of mutually collimated MWCNTs, each with their axis normal and anchored to the growth substrate. A method for non-destructively removing relatively large-scale MWCNT arrays, intact, from the growth substrate is herein reported. This harvesting technique yields a free-standing MWCNT array sheet composed of vertically aligned MWCNTs. Slight inter-MWCNT entanglement along their axes, inherent to the growth process, provides sufficient rigidity for handling and manipulation. Lastly, the integrity of the growth substrates is not compromised, and can be reused.     

Length dependent foam-like mechanical response of axially indented vertically oriented carbon nanotube arrays

The axial compressive mechanical response of substrate-supported carbon nanotube (CNT) arrays with heights from 35 to 1200 μm is evaluated using flat punch nanoindentation with indentation depths to 200 μm. The compressive behavior is consistent with that of an open-cell foam material with array height playing a role similar to that of occupation density for traditional foam. Mechanical yielding of all arrays is initiated between 0.03 and 0.12 strain and arises from localized coordinated plastic buckling. For intermediate CNT array heights between 190 and 650 μm, buckle formation is highly periodic, with characteristic wavelengths between 3 and 6 μm. Buckle formation produced substantial force oscillations in both the compressive and lateral directions. The compressive elastic modulus of the arrays is obtained as a continuous function of penetration depth and attains a value between 10 and 20 MPa for all arrays during mechanical yield. A qualitative model based upon concepts of cellular foam geometry is advanced to explain the observed CNT buckling behavior.     

Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper

We report the synthesis of periodic arrays of carbon nanotubes (CNTs) with different densities on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. At a growth pressure of 8 torr and temperature of 520 °C, vertically aligned bamboo-like CNTs were formed with a catalyst particle on the tip. Electrical properties of CNTs with different densities were investigated for the possible applications in field emission (FE). The investigation of FE properties reveals a strong dependence on the density of CNTs. Experimental results show that NSL patterned low density CNTs exhibit better field emission properties as compared to the high density CNTs. Low-density CNTs exhibit lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs results in the deterioration of the FE properties due to the screening of the electric field by the neighboring CNTs.     

Radial growth of vertically aligned carbon nanotube arrays from ethylene on ceramic spheres

Vertically aligned carbon nanotube (VACNT) arrays grown on ceramic spheres are obtained from ethylene using a floating catalysis process. The exhaust gas mainly contains light gaseous hydrocarbons, which decreases the contamination at the outlet of the reactor. Linear synchronous growth of the VACNT arrays is demonstrated and the morphology evolution of VACNT array grown on spheres is shown. The VACNT arrays on the spheres crack radially into a flower-like structure when the length of CNT is above 400 μm. The VACNT arrays grown on spheres still possess good flowability even when the length of the array reaches 1100 μm after a 2-h growth at 800 °C. The arrays on the spheres show good alignment, high purity and good graphitization. Meanwhile, with a decrease in temperature, the diameter of CNTs in the array correspondingly decreases, the distribution becomes narrower, and the growth rate decreases. The apparent activation energy is 180 ± 8 kJ/mol, indicating that ethylene is a good carbon source for fast and continuous radial growth of millimeter VACNT arrays on ceramic spheres.     

Effect of catalyst pattern geometry on the growth of vertically aligned carbon nanotube arrays

We report the effect of catalyst pattern geometry on the growth behaviour of carbon nanotube (CNT) vertical arrays. Larger patterns are seen to produce longer CNT arrays. We show that this is predominantly related to the pattern size dependence of the number of walls and relate this to the local availability of carbon feedstock species. In addition, the vertical alignment of CNT pillar arrays is seen to depend on the pattern design, in particular the relationship between the pillar dimension and the inter-pillar spacing.     

Friction and adhesion properties of vertically aligned multi-walled carbon nanotube arrays and fluoro-nanodiamond films

Sliding friction and adhesion properties of vertically aligned multi-walled carbon nanotube (VAMWCNT) arrays and fluoro-nanodiamond (F-ND) films on glass substrate have been quantitatively investigated in current study using atomic force microscopy. It was found that VAMWCNT arrays result in lower friction compared to F-ND films. Friction forces were also found to be consistently higher in nitrogen environment than in ambient environment for both samples and a surface chemistry based hypothesis was proposed. However, no apparent dependence of relative humidity was found on adhesion forces for both F-ND and VAMWCNT samples, indicating lack of correlation between nanoscale adhesion and friction. The implications from current study for designing movable components in micro- and nanoelectromechanical system devices were also discussed.     

Where physics meets chemistry: Thin film deposition from reactive plasmas

Functionalising surfaces using polymeric thin films is an industrially important field. One technique for achieving nanoscale, controlled surface functionalization is plasma deposition. Plasma deposition has advantages over other surface engineering processes, including that it is solvent free, substrate and geometry independent, and the surface properties of the film can be designed by judicious choice of precursor and plasma conditions. Despite the utility of this method, the mechanisms of plasma polymer growth are generally unknown, and are usually described by chemical (i.e., radical) pathways. In this review, we aim to show that plasma physics drives the chemistry of the plasma phase, and surface-plasma interactions. For example, we show that ionic species can react in the plasma to form larger ions, and also arrive at surfaces with energies greater than 1000 kJ?mol^–1 (>10 eV) and thus facilitate surface reactions that have not been taken into account previously. Thus, improving thin film deposition processes requires an understanding of both physical and chemical processes in plasma.