Carbon Nanotubes for Electronics and Energy

Ever since their discovery, carbon nanotubes have been touted as a new material for the future and a correspondingly lengthy list of possible applications are often cited in the literature. This excitement for carbon nanotubes is a result of their richly varying physical, electronic and optical properties, where it is possible to have single, double and multiple carbon walls with each wall potentially being either semiconducting or metallic and possessing unique optical transitions covering the ultraviolet to infrared spectral range. However, to date the realization of many of the proposed applications has been hindered by exactly the characteristic that made carbon nanotubes so attractive in the first place, namely the inherent inhomogeneity and varying properties of as-prepared or grown material. In order to become a true advanced material of the future, methods to prepare carbon nanotubes with defined length, wall number, diameter, electronic and optical property are necessary. Additionally, such methods to sort carbon nanotubes must afford high purity levels, be amenable to large-scale preparation and be compatible with subsequent integration into device architectures. In this work these issues are addressed with the use of gel based sorting techniques, which with the use of an automated gel permeation system allows for the routine preparation of milligram quantities of metallic and semiconducting carbon nanotubes, chirality pure single walled carbon nanotubes and even double walled carbon nanotubes sorted by their outer-wall electronic type. Having developed techniques to prepare large quantities, methodologies to control the order and orientation of this 1 D nanomaterial on the macro scale are developed. Inks of carbon nanotubes with liquid crystal concentrations and aligned films thereof are developed and this newfound control over the electronic and structural property opened the door for energy related applications. For example the use of thin films as the transparent electrodes in silicon:carbon nanotube solar cells or as the light harvesting layer in combination with fullerenes with the goal of creating an all carbon solar cell. Likewise on the few nanotube level the unique optical transitions of different nanotube chiralities are used in the fabrication of nanoscale photosensitive elements

Asymmetry of resonance Raman profiles in semiconducting single-walled carbon nanotubes at the first excitonic transition

Carbon nanotubes are one-dimensional nanoscale systems with strongly pronounced chirality-dependent optical properties with multiple excitonic transitions. We investigate the high-energy G mode of semiconducting single-walled nanotubes of different chiralities at first excitonic transition by applying resonant Raman spectroscopy. The G mode intensity dependence on excitation energy yielded asymmetric resonance Raman profiles similar to ones we reported for the second excitonic transition. We find the scattering efficiency to be strongest at the incoming Raman resonance. Still, the degree of asymmetry is different for the first and second transitions and the first transition profiles provide a narrower line shape due to longer exciton lifetimes. The overall scattering efficiency is up to a factor of 25 times more intense at first excitonic transition, compared to the second transition. The fifth-order perturbation theory, with implemented phonon scattering pathways between excitonic states, excellently reproduced experimental data

Carbon Nanotubes for Photovoltaics: From Lab to Industry

The use of carbon nanotubes (CNTs) in photovoltaics could have significant ramifications on the commercial solar cell market. Three interrelated research directions within the field are crucial to the ultimate success of this endeavor; 1) separation, purification, and enrichment of CNTs followed by 2) their integration into organic solar cells as a photosensitive element or 3) in silicon solar cells as a hole selective contact. All three subtopics have experienced tremendous growth over the past 20 years and certainly the performance of the silicon‐based cells is now rapidly approaching that of those on industrial production lines. With a view to these three research areas, the purpose of this Progress Report is to provide a brief overview of each field but more importantly to discuss the challenges and future directions that will allow CNT photovoltaics to move out of the research lab and into end user technology. These include efforts to upscale CNT purification, improvements in power conversion efficiency, increased light absorption, the identification of new material combinations, passivation strategies, and a better understanding of charge separation and energy transfer within these systems

Ternary PM6:Y6 Solar Cells with Single‐Walled Carbon Nanotubes

An organic solar cell with single-walled carbon nanotubes (SWCNTs) in the photoactive layer is typically a type II heterojunction with the semiconducting SWCNTs acting as the electron donor and C60 or other fullerene derivatives as the acceptor. Herein, the performance of solar cells consisting of (6,5) SWCNTs combined with C60 and three nonfullerene acceptors is evaluated in a bilayer architecture. SWCNTs are then combined with the donor/acceptor PM6:Y6 in a ternary mixture and both bulk heterojunction and bilayer devices are fabricated. The SWCNTs are found to extend the light absorption of PM6:Y6 solar cells into the infrared but their use must strike a balance between the SWCNT concentration to enhance light absorption and solvent-induced changes to the morphology of the active layer

Excitonic Resonances in Coherent Anti-Stokes Raman Scattering from Single-Walled Carbon Nanotubes

In this work we investigate the role of exciton resonances in coherent anti-Stokes Raman scattering (er-CARS) in single walled carbon nanotubes (SWCNTs). We drive the nanotube system in simultaneous phonon and excitonic resonances, where we observe a superior enhancement by orders of magnitude exceeding non-resonant cases. We investigated the resonant effects in five $(n,m)$ chiralities and find that the er-CARS intensity varies drastically between different nanotube species. The experimental results are compared with a perturbation theory model. Finally, we show that such giant resonant non-linear signals enable rapid mapping and local heating of individualized CNTs, suggesting easy tracking of CNTs for future nanotoxology studies and therapeutic application in biological tissues

Excitonic Resonances in Coherent Anti-Stokes Raman Scattering from Single Wall Carbon Nanotubes

In this work we investigate the role of exciton resonances in coherent anti-Stokes Raman scattering (er-CARS) in single walled carbon nanotubes (SWCNTs). We drive the nanotube system in simultaneous phonon and excitonic resonances, where we observe a superior enhancement by orders of magnitude exceeding non-resonant cases. We investigated the resonant effects in five $(n,m)$ chiralities and find that the er-CARS intensity varies drastically between different nanotube species. The experimental results are compared with a perturbation theory model. Finally, we show that such giant resonant non-linear signals enable rapid mapping and local heating of individualized CNTs, suggesting easy tracking of CNTs for future nanotoxology studies and therapeutic application in biological tissues.Comment: 17 pages, 6 figure

Front and Back‐Junction Carbon Nanotube‐Silicon Solar Cells with an Industrial Architecture

In the past, the application of carbon nanotube-silicon solar cell technology to industry has been limited by the use of a metallic frame to define an active area in the middle of a silicon wafer. Here, industry standard device geometries are fabricated with a front and back-junction design which allow for the entire wafer to be used as the active area. These are enabled by the use of an intermixed Nafion layer which simultaneously acts as a passivation, antireflective, and physical blocking layer as well as a nanotube dopant. This leads to the formation of a hybrid nanotube/Nafion passivated charge selective contact, and solar cells with active areas of 1–16 cm$^{2}$ are fabricated. Record maximum power conversion efficiencies of 15.2% and 18.9% are reported for front and back-junction devices for 1 and 3 cm$^{2}$ active areas, respectively. By placing the nanotube film on the rear of the device in a back-junction architecture, many of the design-related challenges for carbon nanotube silicon solar cells are addressed and their future applications to industrialized processes are discussed

How to recognize clustering of luminescent defects in single-wall carbon nanotubes

Semiconducting single-wall carbon nanotubes (SWCNTs) are a promising material platform for near-infrared in vivo imaging, optical sensing, and single-photon emission at telecommunication wavelengths. The functionalization of SWCNTs with luminescent defects can lead to significantly enhanced photoluminescence (PL) properties due to efficient trapping of highly mobile excitons and red-shifted emission from these trap states. Among the most studied luminescent defect types are oxygen and aryl defects that have largely similar optical properties. So far, no direct comparison between SWCNTs functionalized with oxygen and aryl defects under identical conditions has been performed. Here, we employ a combination of spectroscopic techniques to quantify the number of defects, their distribution along the nanotubes and thus their exciton trapping efficiencies. The different slopes of Raman D/G+ ratios versus calculated defect densities from PL quantum yield measurements indicate substantial dissimilarities between oxygen and aryl defects. Supported by statistical analysis of single-nano- tube PL spectra at cryogenic temperatures they reveal clustering of oxygen defects. The clustering of 2–3 oxygen defects, which act as a single exciton trap, occurs irrespective of the functionalization method and thus enables the use of simple equations to determine the density of oxygen defects and defect clusters in SWCNTs based on standard Raman spectroscopy. The presented analytical approach is a versatile and sensitive tool to study defect distribution and clustering in SWCNTs and can be applied to any new functio- nalization method

Dielectric Screening Inside Carbon Nanotubes

Dielectric screening plays a vital role for the physical properties in the nanoscale and also alters our ability to detect and characterize nanomaterials by optical techniques. We study the dielectric screening inside of carbon nanotubes and how it changes electromagnetic fields and many-body effects for encapsulated nanostructures. First, we show that the local electric field inside a nanotube is altered by one-dimensional screening with dramatic effects on the effective Raman scattering efficiency of the encapsulated species for metallic walls. The scattering intensity of the inner tube is two orders of magnitude weaker than for the tube in air, which is nicely reproduced by local field calculations. Secondly, we find that the optical transition energies of the inner nanotubes shift to lower energies compared to a single-walled carbon nanotubes of the same chirality. The shift is higher if the outer tube is metallic than when it is semiconducting. The magnitude of the shift suggests that the excitons of small diameter inner metallic tubes are thermally dissociated at room temperate if the outer tube is also metallic and in essence we observe band-to-band transitions.Comment: main: 19 pages, 6 figures supporting: 8 pages, 3 figure