Session W22: Focus Session: Carbon Nanotubes and Related Materials: Thin Films & Networks

Assembly, physics, and application of highly electronic-type purified semiconducting carbon nanotubes in aligned array field effect transistors and photovoltaics

Recent advances in (1) achieving highly monodisperse semiconducting carbon nanotubes without problematic metallic nanotubes and (2) depositing these nanotubes into useful, organized arrays and assemblies on substrates have created new opportunities for studying the physics of these one-dimensional conductors and for applying them in electronics and photonics technologies. In this talk, I will present on two topics that are along these lines. In the first, we have pioneered a scalable approach for depositing aligned arrays of ultrahigh purity semiconducting SWCNTs (prepared using polyfluorene-derivatives) called floating evaporative self-assembly (FESA). FESA is exploited to create FETs with exceptionally high combined on-conductance and on-off ratio of 261 $\mu$S/$\mu$m and 2$x$10$^{5}$, respectively, for a channel length of 240 nm. This is 1400$x$ greater on-off ratio than SWCNT FETs fabricated by other methods, at comparable on-conductance per width of 250 $\mu$S/$\mu$m, and 30-100$x$ greater on-conductance per width, at comparable on-off ratio of 10$^{5}$-10$^{7}$. In the second, we have discovered how to efficiently harvest photons using semiconducting SWCNTs by driving the dissociation of excitons using donor/acceptor heterojunctions. The flow of energy in SWCNT films occurs across a complex energy landscape, temporally resolved using two-dimensional white light ultrafast spectroscopy. We have demonstrated simple solar cells driven by SWCNT excitons, based on bilayers between C60 and ultrathin (5 nm) films of SWCNTs that achieve a 1{\%} solar power conversion efficiency (7{\%} at the bandgap). High internal quantum efficiency indicates that future blended or multijunction cells exploiting multiple layers will be many times more efficient.   

Performance Enhanced Photoconductive Channels Based on (Carbon Nanotube)-(CdS Nanowire) Hybrid Nanostructures

Previous researches showed that the performance of photoconductive channels based on individual CdS \textit{nanowires} (NWs) is improved compared to the CdS \textit{bulk} or \textit{thin-film}-based channels. However, the assembly of a single NW is usually complex and time-consuming. Herein, we report a high-performance photoconductive channel based on CNT--CdS NW hybrid nanostructures, which can be easily prepared on both flat and curved substrates. In our experiments, the CNT-network-based channel was fabricated using a directed assembly method. We found that our channels exhibited much larger photocurrent and faster photoresponse than those of previously reported CNT or CdS NW-based channels. In addition, we fabricated the channels on a curved surface.   

Impact of SWCNT characteristics and processing on the performance of nanotube-silicon solar cells

Single-wall carbon nanotubes (SWCNTs) sorted by length, electronic type and chirality are used to understand the influence of SWCNT characteristics on the performance of SWCNT-silicon solar cells. Solution-processed SWCNT films are deposited on n-doped silicon substrates and p-doped to yield photovoltaic devices that are competitive in terms of photo-conversion efficiency, fill factor and open-circuit voltage. The temperature dependence of the dark-current and the transient reverse-bias recovery technique are used to clarify the nature of the devices as a function of SWCNT type and chirality as well as device manufacturing method, where the latter ranges from the vacuum filtration of aqueous colloidal suspensions to the flow-deposition of super-acid solutions.   

15{\%} Power Conversion Efficiency from a Gated Nanotube/Silicon Nanowire Array Solar Cell

Despite their enhanced light trapping ability the performance of silicon nanowire array solar cells have, been stagnant with power conversion efficiencies barely breaking 10{\%}. The problem is understood to be the consequence of a high photo-carrier recombination at the large surface area of the Si nanowire sidewalls. Here, by exploiting 1) electronic gating via an ionic liquid electrolyte to induce inversion in the n-type Si nanowires and 2) using a layer of single wall carbon nanotubes engineered to contact each nanowire tip and extract the minority carriers, we demonstrate silicon nanowire array solar cells with power conversion efficiencies of 15{\%}. Our results allow for discrimination between the two principle means of avoiding front surface recombination: surface passivation and the use of local fields. A deleterious electrochemical reaction of the silicon due to the electrolyte gating is shown to be caused by oxygen/water entrained in the ionic liquid electrolyte. While encapsulation can avoid the issue a non-encapsulation based solution is also described.   

Carbon nanotube fiber based flexible, lightweight and broadband photodetector

Ultrabroadband absorption properties of carbon nanotubes (CNTs) make them attractive materials for solar cell and photodetector applications. In particular, CNT fibers, which have the unique properties of flexibility and high mechanical strength combined with excellent electrical and optical properties, hold the promise as flexible, broadband photodetectors with inherent polarization sensitivity. Here, we explore the optoelectronic properties of high-performance multifunctional fibers of CNTs towards the development of lightweight, flexible, and broadband photodetectors. We present a photothermoelectric-effect-based flexible CNT-fiber photodetector fabricated using a novel technique. The spatial variation of doping in CNT fibers creates a Seebeck coefficient gradient, leading to a photothermoelectric signal. The current-voltage characteristics of the fiber photodetector produced polarization-sensitive short-circuit currents and open-circuit voltages in response to light in a wide wavelength range, from the visible to the far-infrared. In the terahertz frequency range, the device showed responsivities as high as 2.1 mA/W.   

Polarization-dependent terahertz spectroscopy of macroscopic films of aligned single-wall and multiwall carbon nanotubes

The light absorption properties of carbon nanotubes are strongly anisotropic, especially in the terahertz (THz) region of the electromagnetic spectrum due to their inherently one-dimensional intraband carrier dynamics. Macroscopic films of aligned carbon nanotubes are thus ideal for developing high-performance, low-cost THz polarizers. Here, we present results of polarization-dependent time-domain THz spectroscopy studies of large-area films of aligned single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs) in a frequency range of 0.15-1 THz by varying the polarization of the incident beam with respect to the carbon nanotube alignment direction. The nematic order parameter ($S$), the extinction ratio (ER), and the degree of polarization (DOP) were calculated to establish the performance of the films as polarizers. We found the $S$ of the SWCNT film to be 0.96. The ER of the SWCNT was found to be -12 dB. The measured value of the $S$ for the MWCNT was 0.77, with an ER of -11 dB.   

Revisiting length-dependent Raman spectroscopy of single-wall carbon nanotubes using single chirality, length-refined populations

As-synthesized, single-wall carbon nanotube (SWCNT) populations contain a wide variety of diameter and length nanotubes. This structural diversity with its accompanying property variation, especially in optical properties, provides a great challenge to application and characterization for these materials. Given the tremendous effort to understand the diameter-dependent optical properties of SWCNTs, it is perhaps surprising that little recent work has re-examined the consequences of length, a correspondingly important factor in terms of characterization, to our understanding of the science. Early work demonstrated that for SWCNTs of mixed chirality, the variation in length drove a degradation at short lengths in the optical responses. Since then, identification that morphological impurities and defect density were highly correlated with length in singly sorted samples prompted a reexamination of those results. In this work we probe the length-dependent optical properties of SWCNTs using extremely high-quality SWCNT populations based on multiple aqueous-two-phase separations followed by size exclusion chromatography length-separations. Our results reveal significant and varying dependencies on properties including the G peak intensity and D/G ratio as measured by resonant Raman spectroscopy with the length of single species SWCNT populations, and in other optical properties including absorption and fluorescence.   

Wide dynamic range stretchable electrical interconnect using carbon nanotube sheets and elastomer

Stretchable electric conductor has been investigated for bendable electronics and wearable devices. Nanoscale conducting materials such as silver nanowires, gold nanoparticles, graphenes, and carbon nanotubes had been employed for stretchable conductor. Various structural designs like wrinkle, coil and even fabric had been adopted to reduce a direct stress applied to conducting materials for structural stability. Once such conducting materials are stretched, however, their resistance increased enormously because of the dimensional change related to Poisson ratio and the percolation based electrical conduction. In this study, we fabricated a stretchable conductor by combining carbon nanotube sheets and highly stretchable elastomer, which only has 10{\%} of resistance change while it is stretched up to 600{\%} strain. And we found out that the resistance change can be decreased less than 1{\%}, even though it is stretched up to 600{\%} strain, by using a proper capping method that prevents the contacts between folded regions of conducting materials. We expect that this method can help the practical usage of this stretchable conductor as a stretchable electrical interconnect applications.   

Enhanced Electrical Conductivity of Aluminum by Carbon Nanotube Hybrid Dilution

Carbon nanotubes (CNTs) have been recognized as potential candidate for reinforcements in lightweight metals. A composite consisting of CNTs embedded in an Al-matrix might work as an ultra-low-resistive material with the potential of having a room-temperature resistivity far below Al, Cu and Ag. While several advances have been made in developing Al-CNT composites, three major challenges: (1) interfacial bond strength between CNT and the Al matrix, (2) homogeneous dispersion of the CNTs in the Al matrix and impurity (CNTs) scattering centers, continue to limit progress in Al-CNT composites. Several conventional methods including powder metallurgy, melting and solidification, thermal spray and electrochemical deposition have been used to process Al and CNT to form composites. We present preliminary results that address these challenges and demonstrate the fabrication of easily drawable Al-CNT composites into wires of diameter $\le $ 1.0mm with $\sim$ 18{\%} $\pm$ 2{\%} reduction in the electrical resistivity of Al-CNT composite using CNT-hybrid as reinforcement and an inductive melting technique that takes advantage of the induced eddy current in the melt to provide \textit{in-situ} stirring.   

Carbon Nanotube Networks Reinforced by Silver Nanowires with Improved Optical Transparency and Conductivity

We have fabricated highly transparent and conductive free-standing nanocomposite thin film electrodes by adding silver nanowires (AgNWs) to dry-spun Multiwall Carbon Nanotube (MWNT) aerogels. This nanocomposite exhibits desirable properties such as high optical transmittance, excellent flexibility and enhanced electrical conductivity. The incorporation of the AgNWs to the MWNT aerogels was accomplished by using a spray coating method. The optical transparency and sheet resistance of the nanocomposite was tuned by adjusting the concentration of AgNWs, back pressure and nozzle distance of the spray gun to the MWNT aerogel during deposition. As the solvent evaporated, the aerogel MWNT bundles densified via surface tension which caused the MWNT bundles to collapse. This adjustable process was responsible in forming well defined apertures that increased the nanocomposite's transmittance up to 90 percent. Via AgNWs percolation and random interconnections between separate MWNT bundles in the aerogel matrix, the sheet resistance decreased from 1 K ohm/sq to less than 100 ohm/sq.   

Atomistic Simulations of Fluid Flow through Graphene Channels and Carbon Nanotubes

The transport of aqueous solutions in artificial nanopores is of both fundamental and technological interest. Recently, carbon nano-structured materials (fullerenes) have attracted a great deal of attention in nanotechnology. In fact, due to their large specific surface area, high thermal conductivity, extremely low surface friction and superior mechanical properties, graphene channels and carbon nanotubes (CNTs) are promising candidates to be implemented as fluid conduits in nanosystems. Performing Non-equilibrium Molecular Dynamics simulations, we study the transport of water-eletrolyte solutions inside single and multi-wall graphene channels and inside zig-zag and armchair CNTs of similar cross sectional area. In order to calibrate the force fields, we use dedicated criteria relevant to the hydrodynamics of the systems of interest. Different fluid driving mechanisms such as pressure fields, electro-osmosis and thermal gradients are evaluated. We conduct a detailed analysis of the transport efficiency of each system to impose similar volumetric flow rates. From the simulations, we extract density and velocity profiles to study the liquid structure, wall slippage and flow enhancement in order to compare the hydrodynamic performance of these two novel materials.   

Mechanism of H$_2$O Induced Conductance Changes in AuCl$_4$ Functionalized CNTs

Functionalized carbon nanotubes (CNTs) are promising candidates for nanoscale sensors due to their high surface-to-volume ratio and the fact that their properties are very sensitive to perturbations. We employ \textit{ab-initio} self-interaction corrected density functional theory combined with the non-equilibrium Green's function method to study the electronic and quantum transport properties of CNTs functionalized with AuCl$_4$ molecules. In particular, we investigate the electronic structure and characterize the conductance for different concentrations and configurations of randomly distributed AuCl$_4$ with and without the adsorption of H$_2$O molecules, and propose a mechanism that explains the origin of the recently observed resistivity changes of AuCl$_4$ functionalized CNTs upon H$_2$O adsorption. We find that the adsorption of H$_2$O shifts the highest occupied Cl and Au states down in energy and thereby reduces the scattering of the electrons around the Fermi level, hence enhancing the conductivity. Our results help facilitate the development of highly sensitive nanoscale H$_2$O vapor sensors based on AuCl$_4$ functionalized CNTs.   

Alternative nanostructures for thermophones

There is a large promise for thermophones in high power sonar arrays, flexible loudspeakers, and noise cancellation devices. So far, freestanding aerogel-like carbon nanotube sheets demonstrate the best performance as a thermoacoustic heat source. However, the limited accessibility of large size freestanding carbon nanotube sheets and other even more exotic materials published recently, hampers the field. We present here new alternative materials for a thermoacoustic heat source with high energy conversion efficiency, additional functionalities, environmentally friendly and cost effective production technologies. We discuss the thermoacoustic performance of alternative nanoscale materials and compare their spectral and power dependencies of sound pressure in air. The study presented here focuses on engineering thermal gradients in the vicinity of nanostructures and subsequent heat dissipation processes from the interior of encapsulated thermoacoustic projectors. Applications of thermoacoustic projectors for high power SONAR arrays, sound cancellation, and optimal thermal design, regarding enhanced energy conversion efficiency, are discussed.