Katherine E. Moore

Separation of Single-Walled Carbon Nanotubes with a Gel Permeation Chromatography System

A gel permeation chromatography system is used to separate aqueous sodium dodecyl sulfate suspensions of single-walled carbon nanotubes (SWCNTs). This automated procedure requires no precentrifugation, is scalable, and is found to yield monochiral SWCNT fractions of semiconducting SWCNTs with a purity of 61-95%. Unsorted and resulting monochiral fractions are characterized using optical absorption and photoluminescence spectroscopy.

Double‐Walled Carbon Nanotube Processing

Single-walled carbon nanotubes (SWCNTs) have been the focus of intense research, and the body of literature continues to grow exponentially, despite more than two decades having passed since the first reports. As well as extensive studies of the fundamental properties, this has seen SWCNTs used in a plethora of applications as far ranging as microelectronics, energy storage, solar cells, and sensors, to cancer treatment, drug delivery, and neuronal interfaces. On the other hand, the properties and applications of double-walled carbon nanotubes (DWCNTs) have remained relatively under-explored. This is despite DWCNTs not only sharing many of the same unique characteristics of their single-walled counterparts, but also possessing an additional suite of potentially advantageous properties arising due to the presence of the second wall and the often complex inter-wall interactions that arise. For example, it is envisaged that the outer wall can be selectively functionalized whilst still leaving the inner wall in its pristine state and available for signal transduction. A similar situation arises in DWCNT field effect transistors (FETs), where the outer wall can provide a convenient degree of chemical shielding of the inner wall from the external environment, allowing the excellent transconductance properties of the pristine nanotubes to be more fully exploited. Additionally, DWCNTs should also offer unique opportunities to further the fundamental understanding of the inter-wall interactions within and between carbon nanotubes. However, the realization of these goals has so far been limited by the same challenge experienced by the SWCNT field until recent years, namely, the inherent heterogeneity of raw, as-produced DWCNT material. As such, there is now an emerging field of research regarding DWCNT processing that focuses on the preparation of material of defined length, diameter and electronic type, and which is rapidly building upon the experience gained by the broader SWCNT community. This review describes the background of the field, summarizing some relevant theory and the available synthesis and purification routes; then provides a thorough synopsis of the current state-of-the-art in DWCNT sorting methodologies, outlines contemporary challenges in the field, and discusses the outlook for various potential applications of the resulting material.

Unraveling the interplay of backbone rigidity and electron rich side-chains on electron transfer in peptides: The realization of tunable molecular wires

Electrochemical studies are reported on a series of peptides constrained into either a 310-helix (1–6) or β-strand (7–9) conformation, with variable numbers of electron rich alkene containing side chains. Peptides (1 and 2) and (7 and 8) are further constrained into these geometries with a suitable side chain tether introduced by ring closing metathesis (RCM). Peptides 1, 4 and 5, each containing a single alkene side chain reveal a direct link between backbone rigidity and electron transfer, in isolation from any effects due to the electronic properties of the electron rich side-chains. Further studies on the linear peptides 3–6 confirm the ability of the alkene to facilitate electron transfer through the peptide. A comparison of the electrochemical data for the unsaturated tethered peptides (1 and 7) and saturated tethered peptides (2 and 8) reveals an interplay between backbone rigidity and effects arising from the electron rich alkene side-chains on electron transfer. Theoretical calculations on β-strand models analogous to 7, 8 and 9 provide further insights into the relative roles of backbone rigidity and electron rich side-chains on intramolecular electron transfer. Furthermore, electron population analysis confirms the role of the alkene as a “stepping stone” for electron transfer. These findings provide a new approach for fine-tuning the electronic properties of peptides by controlling backbone rigidity, and through the inclusion of electron rich side-chains. This allows for manipulation of energy barriers and hence conductance in peptides, a crucial step in the design and fabrication of molecular-based electronic devices.

Separation of Double-Walled Carbon Nanotubes by Size Exclusion Column Chromatography

In this report we demonstrate the separation of raw carbon nanotube material into fractions of double-walled (DWCNTs) and single-walled carbon nanotubes (SWCNTs). Our method utilizes size exclusion chromatography with Sephacryl gel S-200 and yielded two distinct fractions of single- and double-walled nanotubes with average diameters of 0.93 ± 0.03 and 1.64 ± 0.15 nm, respectively. The presented technique is easily scalable and offers an alternative to traditional density gradient ultracentrifugation methods. CNT fractions were characterized by atomic force microscopy and Raman and absorption spectroscopy as well as transmission electron microscopy.

Aligned Carbon Nanotube Thin Films from Liquid Crystal Polyelectrolyte Inks

Single walled carbon nanotube thin films are fabricated by solution shearing from high concentration sodium nanotubide polyelectrolyte inks. The solutions are produced by simple stirring of the nanotubes with elemental sodium in dimethylacetamide, and the nanotubes are thus not subject to any sonication-induced damage. At such elevated concentrations (∼4 mg mL(-1)), the solutions exist in the liquid crystal phase and during deposition this order is transferred to the films, which are well aligned in the direction of shear with a 2D nematic order parameter of ∼0.7 determined by polarized absorption measurements. Compared to similarly formed films made from superacids, the polyelectrolyte films contain smaller bundles and a much narrower distribution of bundle diameters. After p-doping with an organic oxidizer, the films exhibit a very high DC electrical to optical conductivity ratio of σ(DC)/σ(OP) ∼ 35, corresponding to a calculated DC conductivity of over 7000 S cm(-1). When very thin (T550 ∼ 96%), smooth (RMS roughness, R(q) ∼ 2.2 nm), and highly aligned films made via this new route are used as the front electrodes of carbon nanotube-silicon solar cells, the power conversion efficiency is almost an order of magnitude greater than that obtained when using the much rougher (R(q) ∼ 20-30 nm) and less conductive (peak σ(DC)/σ(OP) ∼ 2.5) films formed by common vacuum filtration of the same starting material, and having the same transmittance.

Sorting of Double-Walled Carbon Nanotubes According to Their Outer Wall Electronic Type via a Gel Permeation Method

In this work, we demonstrate the application of the gel permeation technique to the sorting of double-walled carbon nanotubes (DWCNTs) according to their outer wall electronic type. Our method uses Sephacryl S-200 gel and yields sorted fractions of DWCNTs with impurities removed and highly enriched in nanotubes with either metallic (M) or semiconducting (S) outer walls. The prepared fractions are fully characterized using optical absorption spectroscopy, transmission electron microscopy, and atomic force microscopy, and the entire procedure is monitored in real time using process Raman analysis. The sorted DWCNTs are then integrated into single nanotube field effect transistors, allowing detailed electronic measurement of the transconductance properties of the four unique inner@outer wall combinations of S@S, S@M, M@S, and M@M.

The effect of a macrocyclic constraint on electron transfer in helical peptides: a step towards tunable molecular wires.

Two helical peptides, one constrained by a covalent side-chain staple, exhibit vastly different electronic properties despite adopting essentially the same backbone conformation. High level calculations confirm that these differences are due to the additional backbone rigidity imparted by the macrocyclic constraint.

Photocurrent Spectroscopy of (n, m) Sorted Solution-Processed Single-Walled Carbon Nanotubes

Variable-wavelength photocurrent microscopy and photocurrent spectroscopy are used to study the photoresponse of (n, m) sorted single-walled carbon nanotube (SWNT) devices. The measurements of (n, m) pure SWCNT devices demonstrate the ability to study the wavelength-dependent photoresponse in situ in a device configuration and deliver photocurrent spectra that reflect the population of the source material. Furthermore, we show that it is possible to map and determine the chirality population within a working optoelectronic SWCNT device.

Field emission from single-, double-, and multi-walled carbon nanotubes chemically attached to silicon

The chemical attachment and field emission (FE) properties of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs) chemically attached to a silicon substrate have been investigated. A high density of CNTs was revealed by atomic force microscopy imaging with orientation varying with CNT type. Raman spectroscopy was used to confirm the CNT type and diameter on the surfaces. The field emission properties of the surfaces were studied and both current-voltage and Fowler-Nordheim plots were obtained. The SWCNTs exhibited superior FE characteristics with a turn-on voltage (Eto) of 1.28 V μm−1 and electric field enhancement factor (β) of 5587. The DWCNT surface showed an Eto of 1.91 V μm−1 and a β of 4748, whereas the MWCNT surface exhibited an Eto of 2.79 V μm−1 and a β of 3069. The emission stability of each CNT type was investigated and it was found that SWCNTs produced the most stable emission. The differences between the FE characteristi...

Wool deconstruction using a benign eutectic melt

Wool fibre is deconstructed in a facile ‘top down’ fabrication process into functional, nano-dimensional α-keratin chains using a benign choline chloride-urea deep eutectic solvent (DES) melt. After breakdown, the keratin can be easily isolated from the DES mixture through dialysis, and freeze-dried to form a protein powder ready for subsequent processing for applications such as wound healing or animal feedstock. The process is simple, efficient, and environmentally friendly. It can potentially utilise what would otherwise be a waste stream, stemming from wool that is deemed unsuitable for the clothing industry, and at the same time providing an additional revenue source.