Noe T. Alvarez

Dry Contact Transfer Printing of Aligned Carbon Nanotube Patterns and Characterization of Their Optical Properties for Diameter Distribution and Alignment

A scalable and facile approach is demonstrated where as-grown patterns of well-aligned structures composed of single-walled carbon nanotubes (SWNT) synthesized via water-assisted chemical vapor deposition (CVD) can be transferred, or printed, to any host surface in a single dry, room-temperature step using the growth substrate as a stamp. We demonstrate compatibility of this process with multiple transfers for large-scale device and specifically tailored pattern fabrication. Utilizing this transfer approach, anisotropic optical properties of the SWNT films are probed via polarized absorption, Raman, and photoluminescence spectroscopies. Using a simple model to describe optical transitions in the large SWNT species present in the aligned samples, polarized absorption data are demonstrated as an effective tool for accurate assignment of the diameter distribution from broad absorption features located in the infrared. This can be performed on either well-aligned samples or unaligned doped samples, allowing simple and rapid feedback of the SWNT diameter distribution that can be challenging and time-consuming to obtain in other optical methods. Furthermore, we discuss challenges in accurately characterizing alignment in structures of long versus short carbon nanotubes through optical techniques, where SWNT length makes a difference in the information obtained in such measurements. This work provides new insight to the efficient transfer and optical properties of an emerging class of long, large diameter SWNT species typically produced in the CVD process.

Selective photochemical functionalization of surfactant-dispersed single wall carbon nanotubes in water.

Ultraviolet (UV) irradiation of single wall carbon nanotubes (SWCNTs) individually dispersed in surfactants leads to diameter and type-selective photohydroxylation of the nanotubes. Photohydroxylation of first semiconductor and then small diameter metallic SWCNTs was confirmed after 254 nm UV irradiation in acidic, neutral, and basic aqueous solutions at ambient and elevated temperatures. The increased oxygen content of the SWCNTs after UV irradiation, as detected by X-ray photoelectron spectroscopy, suggests that SWCNTs were hydroxylated by reaction with water. Attenuated total reflectance Fourier transform infrared analysis provides evidence of hydroxyl functional groups on their surface. This photochemical reaction is impeded by molecular oxygen and appears to involve a reactive intermediate generated in the vicinity of semiconducting SWCNTs. This represents a noncontaminating selective reaction in the liquid phase that uses an intrinsic property of the tubes.

Flexible Micro-Supercapacitor Based on Graphene with 3D Structure

Flexible micro-supercapacitors (MSCs) are constructed by 3D graphene from chemical vapor deposition. Without using any binder or metal current collector, the as-prepared 3D graphene MSC exhibits good flexibility, excellent cyclic life, and high areal capacitance of 1.5 mF cm-2 at a scan rate of 10 V s-1 . The electrochemical performance is further improved by oxygen plasma functionalization.

Beyond graphene foam, a new form of three-dimensional graphene for supercapacitor electrodes

Graphene foam (GF) is a three-dimensional (3D) graphene structure that has been intensively studied as an electrode material for energy storage applications. The porous structure and seamlessly connected graphene flakes make GF a promising electrode material for supercapacitors and batteries. However, the electrical conductivity of GF is still unsatisfactory due to the lack of macropore size (∼300 μm) control that hinders its applications. Previously we reported a new seamless 3D graphene structure – graphene pellets (GPs) – with well-controlled mesopore size (∼2 nm), high electrical conductivity (148 S cm−1) and good electromechanical properties that differ substantially from the known GF. Here we demonstrate that the obtained 3D graphene structure is an ideal scaffold electrode for pseudocapacitive materials and redox additive electrolyte systems. For example, after electrochemical coating with MnO2, the GP/MnO2 electrode showed specific and volumetric capacitance up to 395 F g−1 and 230 F cm−3 at 1 A g−1, respectively. When combined with a hydroquinone and benzoquinone redox additive electrolyte, the GPs showed a specific capacitance of 7813 F g−1 at 10 A g−1. Moreover, when the GP/MnO2 electrode was assembled with a GP/polypyrrole electrode, the obtained full cell showed good electrochemical performance with a maximum energy density of 26.7 W h kg−1 and a maximum power density of 32.7 kW kg−1, and a reasonable cycle life for practical application. The ease in material processing combined with the excellent electrical and electromechanical properties makes GPs promising for a variety of energy storage applications.

Aligned carbon nanotube/copper sheets: a new electrocatalyst for CO2 reduction to hydrocarbons

We controlled the morphologies of copper (Cu) nanostructure on aligned carbon nanotube (CNT) sheets, influencing the efficiency of the electrocatalytic reduction of CO2. Functionalized CNT sheets affected the pulsed electrodeposition of copper in terms of 3D growth, bonding, and electrochemical activity. CNT/Cu sheet electrocatalyst shows high performance in electrochemical reduction of CO2 to hydrocarbons at room temperature and atmospheric pressure. Reduction products were carbon monoxide (CO), methane (CH4), and ethylene (C2H4) gases. Carbon monoxide yields (178 μmol cm2 mA−1 h−1) and methane yields (346 μmol cm2 mA−1 h−1) at oxygen-plasma-treated CNT/Cu sheet electrodes were remarkably higher than other CNT/Cu and CNT sheets. Experimental results also show 3D morphology of copper growth on CNT sheets may play a critical role in hydrocarbon products from CO2.

Wet Catalyst-Support Films for Production of Vertically Aligned Carbon Nanotubes

A procedure for vertically aligned carbon nanotube (VA-CNT) production has been developed through liquid-phase deposition of alumoxanes (aluminum oxide hydroxides, boehmite) as a catalyst support. Through a simple spin-coating of alumoxane nanoparticles, uniform centimer-square thin film surfaces were coated and used as supports for subsequent deposition of metal catalyst. Uniform VA-CNTs are observed to grow from this film following deposition of both conventional evaporated Fe catalyst, as well as premade Fe nanoparticles drop-dried from the liquid phase. The quality and uniformity of the VA-CNTs are comparable to growth from conventional evaporated layers of Al(2)O(3). The combined use of alumoxane and Fe nanoparticles to coat surfaces represents an inexpensive and scalable approach to large-scale VA-CNT production that makes chemical vapor deposition significantly more competitive when compared to other CNT production techniques.

Radiation Performance of Polarization Selective Carbon Nanotube Sheet Patch Antennas

Carbon nanotube (CNT) sheet patch antennas are explored through simulation, fabrication, and measurement to evaluate the performance of the CNT material as an RF radiator. The thickness of the CNT sheet was found to have a significant impact on the radiation performance of the patch antenna due to the material skin depth, with an ~ 5.5-dB improvement to the realized gain achieved when the CNT sheet thickness was increased from 0.5 μm to 5 μm, likely due to lower surface impedance. The 5 μm-CNT sheet patch antenna exhibited 2.1-dBi total realized gain compared with 5.6-dBi realized gain for baseline copper patch antenna yielding a 3.5-dB reduction attributable to the material substitution. A unique polarization sensitivity behavior was seen by adjusting the alignment of the CNTs within the CNT sheet patch structure. Optimal RF performance was observed when the CNTs within the sheet material were aligned with the E-plane of the patch antenna. When the CNT alignment was orthogonal to that of the E-plane of the patch antenna, the realized gain was reduced by over 8 dB. The input reactance changes from inductive to capacitive due to the geometry and alignment of the CNTs within the patch.

Dendrimer-Assisted Self-Assembled Monolayer of Iron Nanoparticles for Vertical Array Carbon Nanotube Growth

Self-assembled monolayers (SAMs) of iron oxide nanoparticles have been prepared using carboxylic-acid-terminated dendrimers. The iron-containing SAM was used as the catalyst for growth of vertical arrays of carbon nanotubes (CNTs). This approach has the potential for producing diameter controlled CNTs from premade catalyst nanoparticles as well as large scale production of CNTs by chemical vapor deposition.

Polymer Coating of Carbon Nanotube Fibers for Electric Microcables

Carbon nanotubes (CNTs) are considered the most promising candidates to replace Cu and Al in a large number of electrical, mechanical and thermal applications. Although most CNT industrial applications require macro and micro size CNT fiber assemblies, several techniques to make conducting CNT fibers, threads, yarns and ropes have been reported to this day, and improvement of their electrical and mechanical conductivity continues. Some electrical applications of these CNT conducting fibers require an insulating layer for electrical insulation and protection against mechanical tearing. Ideally, a flexible insulator such as hydrogenated nitrile butadiene rubber (HNBR) on the CNT fiber can allow fabrication of CNT coils that can be assembled into lightweight, corrosion resistant electrical motors and transformers. HNBR is a largely used commercial polymer that unlike other cable-coating polymers such as polyvinyl chloride (PVC), it provides unique continuous and uniform coating on the CNT fibers. The polymer coated/insulated CNT fibers have a 26.54 μm average diameter-which is approximately four times the diameter of a red blood cell-is produced by a simple dip-coating process. Our results confirm that HNBR in solution creates a few microns uniform insulation and mechanical protection over a CNT fiber that is used as the electrically conducting core.

Carbon nanotube sensor thread for distributed strain and damage monitoring on IM7/977-3 composites

Laminated composite materials are used in applications where light weight is a key requirement. However, minor delamination damage in composites can propagate and lead to the failure of components. Failure occurs because delamination reduces the local bending stiffness and increases bending stress, which leads to the propagation of damage and eventual failure. These failures may be avoided if the damage could be detected early and repaired. Although many damage detection methods have been investigated, none are in widespread use today to prevent the failure of composites. This paper describes the use of carbon nanotube sensor thread to monitor strain and damage in composite materials. Sensor thread was bonded onto an IM7-laminated composite coupon to measure surface strain in a quasi-static uniaxial tensile test. The sensor thread was calibrated against a strain gage, which was also mounted to the coupon. The sensor thread measured the average strain over the length of the sample and indicated when the strain exceeded a nominal safe level. Sensor thread was also bonded to the surface of laminated composite panels in different patterns and detected, located and partially characterized the damage caused by multiple impacts to the panel. The new findings in this paper can be summarized as; (1) carbon nanotube sensor thread was tested as a distributed sensor for the first time on IM7/977-3 composites; (2) the sensor thread was found to monitor strain and detect damage in the composites with a potential sensitivity down to the micro-crack level; (3) the sensor thread was barely visible on the composite and did not add significant mass or affect the integrity of the composite; (4) the data acquisition system developed was simple and reliable.