Adam J. Blanch

Optimizing surfactant concentrations for dispersion of single-walled carbon nanotubes in aqueous solution.

The sonication-centrifugation technique is commonly used for dispersing single-walled carbon nanotubes (SWCNTs) in aqueous surfactant solutions. However, the methodologies and materials used for this purpose are widely varied, and few dispersive agents have been studied systematically. This work describes a systematic study into the ability of some well-known (and some less common) surfactants and polymers to disperse SWCNTs fabricated by two different techniques. UV-vis-NIR absorbance spectra of their supernatant solutions showed that the smaller ionic surfactants were generally more effective dispersants, with larger polymer and surfactant molecules exhibiting a reduced performance for ensembles of carbon nanotubes of smaller average diameter. Optimal surfactant concentrations were established for dispersions of carbon nanotubes produced by the electric arc method in aqueous solutions of sodium dodecylbenzene sulfonate, sodium deoxycholate, Triton X-405, Brij S-100, Pluronic F-127, and polyvinylpyrrolidone. This optimum value was determined as the point at which the relative concentration of nanotubes dispersed is maximized, before flocculation-inducing attractive depletion interactions begin to dominate. The aggregation state of carbon nanotubes dispersed in sodium dodecylbenzene sulfonate was probed by AFM at different stages of rebundling, showing the length dependence of these effects.

Accurate measurement of Atomic Force Microscope cantilever deflection excluding tip-surface contact with application to force calibration

Considerable attention has been given to the calibration of AFM cantilever spring constants in the last 20 years. Techniques that do not require tip-sample contact are considered advantageous since the imaging tip is not at risk of being damaged. Far less attention has been directed toward measuring the cantilever deflection or sensitivity, despite the fact that the primary means of determining this factor relies on the AFM tip being pressed against a hard surface, such as silicon or sapphire; which has the potential to significantly damage the tip. A recent method developed by Tourek et al. in 2010 involves deflecting the AFM cantilever a known distance from the imaging tip by pressing the cantilever against a sharpened tungsten wire. In this work a similar yet more precise method is described, whereby the deflection of the cantilever is achieved using an AFM probe with a spring constant much larger than the test cantilever, essentially a rigid cantilever. The exact position of loading on the test cantilever was determined by reverse AFM imaging small spatial markers that are milled into the test cantilever using a focussed ion beam. For V shaped cantilevers it is possible to reverse image the arm intersection in order to determine the exact loading point without necessarily requiring FIB milled spatial markers, albeit at the potential cost of additional uncertainty. The technique is applied to tip-less, beam shaped and V shaped cantilevers and compared to the hard surface contact technique with very good agreement (on average less than 5% difference). While the agreement with the hard surface contact technique was very good the error on the technique is dependent upon the assumptions inherent in the method, such as cantilever shape, loading point distance and ratio of test to rigid cantilever spring constants. The average error ranged between 2 to 5% for the majority of test cantilevers studied. The sensitivity derived with this technique can then be used to calibrate the cantilever spring constant using the thermal noise method, allowing complete force calibration to be accurately performed without tip-sample contact.

Calibration of atomic force microscope cantilevers using standard and inverted static methods assisted by FIB-milled spatial markers.

Static methods to determine the spring constant of AFM cantilevers have been widely used in the scientific community since the importance of such calibration techniques was established nearly 20 years ago. The most commonly used static techniques involve loading a trial cantilever with a known force by pressing it against a pre-calibrated standard or reference cantilever. These reference cantilever methods have a number of sources of uncertainty, which include the uncertainty in the measured spring constant of the standard cantilever, the exact position of the loading point on the reference cantilever and how closely the spring constant of the trial and reference cantilever match. We present a technique that enables users to minimize these uncertainties by creating spatial markers on reference cantilevers using a focused ion beam (FIB). We demonstrate that by combining FIB spatial markers with an inverted reference cantilever method, AFM cantilevers can be accurately calibrated without the tip of the test cantilever contacting a surface. This work also demonstrates that for V-shaped cantilevers it is possible to determine the precise loading position by AFM imaging the section of the cantilever where the two arms join. Removing tip-to-surface contact in both the reference cantilever method and sensitivity calibration is a significant improvement, since this is an important consideration for AFM users that require the imaging tip to remain in pristine condition before commencing measurements. Uncertainties of between 5 and 10% are routinely achievable with these methods.

The crystal chemistry of Al-bearing goethites: an infrared spectroscopic study

Abstract Aluminium substitution into goethite, α-FeOOH, has been studied systematically by infrared spectroscopy over the frequency interval 150 to 700 cm-1. A range of synthetic compositions, (Fe1-xAlx)OOH, 0 ≤ x ≤ 0.0857, plus end-member diaspore (AlOOH) were examined. The IR spectrum of FeOOH over the range of interest can be deconvoluted into 9 peaks: 670, 633, 497, 451, 409, 396, 360, 290 and 268 cm-1. With addition of Al, the spectra become broader and the frequencies of the modes shift to higher values. An effective line width parameter Δcorr was determined by autocorrelation analysis for each spectrum and there is a significant increase in the Δcorr for compositions with > 5.85 mol.% Al substitution, indicating that there is very significant structural strain associated with this solid solution at compositions where a significant number of Al sites will have an additional Al atom in the first cation coordination sphere. Trends in the values of Δcorr with composition mirrored published enthalpy of mixing values indicating that Al-goethite is metastable with respect to the goethite and diaspore end-members. The role of substitutional strain and possible partial order of Al is briefly discussed.

Simple and Rapid High‐Yield Synthesis and Size Sorting of Multibranched Hollow Gold Nanoparticles with Highly Tunable NIR Plasmon Resonances

Branched gold nanoparticles with sharp tips are considered excellent candidates for sensing and field enhancement applications. Here, a rapid and simple synthesis strategy is presented that generates highly branched gold nanoparticles with hollow cores and a ca.100% yield through a simple one-pot seedless reaction at room temperature in the presence of Triton X-100. It is shown that multibranched hollow gold nanoparticles of tunable dimensions, branch density and branch length can be obtained by adjusting the concentrations of the reactants. Insights into the formation mechanism point toward an aggregative type of growth involving hollow core formation first, and branching thereafter. The pronounced near-infrared (NIR) plasmon band of the nanoparticles is due to the combined contribution from hollowness and branching, and can be tuned over a wide range (≈700-2000 nm). It is also demonstrated that the high environmental sensitivity of colloidal dispersions based on multibranched hollow gold nanoparticles can be boosted even further by separating the nanoparticles into fractions of given sizes and improved monodispersity by means of a glycerol density gradient. The possibility to obtain highly monodisperse multibranched hollow gold nanoparticles with predictable dimensions (50-300 nm) and branching and, therefore, tailored NIR plasmonic properties, highlights their potential for theranostic applications.

Spring constant calibration techniques for next-generation fast-scanning atomic force microscope cantilevers.

As a recent technological development, high-speed atomic force microscopy (AFM) has provided unprecedented insights into dynamic processes on the nanoscale, and is capable of measuring material property variation over short timescales. Miniaturized cantilevers developed specifically for high-speed AFM differ greatly from standard cantilevers both in size and dynamic properties, and calibration of the cantilever spring constant is critical for accurate, quantitative measurement. This work investigates specifically, the calibration of these new-generation cantilevers for the first time. Existing techniques are tested and the challenges encountered are reported and the most effective approaches for calibrating fast-scanning cantilevers with high accuracy are identified, providing a resource for microscopists in this rapidly developing field. Not only do these cantilevers offer faster acquisition of images and force data but due to their high resonant frequencies (up to 2 MHz) they are also excellent mass sensors. Accurate measurement of deposited mass requires accurate calibration of the cantilever spring constant, therefore the results of this work will also be useful for mass-sensing applications.

Efficient attachment of carbon nanotubes to conventional and high-frequency AFM probes enhanced by electron beam processes

Carbon nanotubes are considered to be an ideal imaging tip for atomic force microscopy (AFM) applications, and a number of methods for fabricating these types of probe have been developed in recent years. This work reports the attachment of carbon nanotubes to AFM probes using a micromanipulator within a scanning electron microscope. Electron beam induced deposition and etching are used to enhance the quality and attachment of the carbon nanotube tip and improve the fabrication rate of the CNT AFM probes compared to existing techniques. The attachment process is also improved by using a mat of SWCNTs (buckypaper) as a CNT source, which simultaneously improves the ease of fabrication and rate of nanotube probe production. The aim of these improvements is to simplify and improve the attachment process such that these probes can be better and more widely used in applications that benefit from their unique properties. This improved process is then used to attach CNTs to the new generation of low-mass, high-frequency probes, which are designed for rapid AFM imaging. The ability of these probes to operate with CNT tips is demonstrated, and their wear-resistance properties were found to be significantly enhanced compared to unmodified probes. These wear-resistant probes imaging at high scan rates are proposed to be effective tools for increasing throughput in metrological analysis, particularly for imaging high-modulus surfaces with high roughness and high-aspect-ratio features.

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.

Surfactant concentration dependent spectral effects of oxygen and depletion interactions in sodium dodecyl sulfate dispersions of carbon nanotubes

Quenching of optical absorbance spectra for carbon nanotubes (CNTs) dispersed in sodium dodecyl sulfate (SDS) has been observed to be more pronounced at higher concentrations of the surfactant. The protonation-based quenching behavior displays wavelength dependence, affecting larger diameter nanotube species preferentially. Although absorbance may be recovered by hydroxide addition, pH measurements suggest that hydrolysis of SDS does not play a major role in the short term quenching behavior at high SDS concentrations. The degree of quenching is observed to correlate well with an increase in attractive depletion as SDS concentration is increased, while the extent of depletion is found to depend heavily on the concentration of preparation in comparison to the final SDS concentration. Attractive depletion in SDS is also found to be preferential for CNTs of larger diameter. It is proposed that depletion enhances the quenching effect due to close association of CNT-SDS complexes providing higher SDS densities on the CNT surface, leading to further oxidation. In addition, the quenching behavior in SDS is found to strongly suppress the optical and Raman signal from metallic nanotube species even at high pH. Displacement of SDS by sodium deoxycholate as a secondary surfactant is able to reverse the effects of protonation of metallic species, whereas hydroxide addition is only partially effective.

Improved Application of Carbon Nanotube Atomic Force Microscopy Probes Using PeakForce Tapping Mode

In this work PeakForce tapping (PFT) imaging was demonstrated with carbon nanotube atomic force microscopy (CNT-AFM) probes; this imaging mode shows great promise for providing simple, stable imaging with CNT-AFM probes, which can be difficult to apply. The PFT mode is used with CNT-AFM probes to demonstrate high resolution imaging on samples with features in the nanometre range, including a Nioprobe calibration sample and gold nanoparticles on silicon, in order to demonstrate the modes imaging effectiveness, and to also aid in determining the diameter of very thin CNT-AFM probes. In addition to stable operation, the PFT mode is shown to eliminate “ringing” artefacts that often affect CNT-AFM probes in tapping mode near steep vertical step edges. This will allow for the characterization of high aspect ratio structures using CNT-AFM probes, an exercise which has previously been challenging with the standard tapping mode.