Probing buried carbon nanotubes within polymer–nanotube composite matrices by atomic force microscopy

Identification of nanoparticles and nanosystems in biological matrices with scanning probe microscopy

Identification of nanoparticles and nanosystems into cells and biological matrices is a hot research topic in nanobiotechnologies. Because of their capability to map physical properties (mechanical, electric, magnetic, chemical, or optical), several scanning probe microscopy based techniques have been proposed for the subsurface detection of nanomaterials in biological systems. In particular, atomic force microscopy (AFM) can be used to reveal stiff nanoparticles in cells and other soft biomaterials by probing the sample mechanical properties through the acquisition of local indentation curves or through the combination of ultrasound-based methods, like contact resonance AFM (CR-AFM) or scanning near field ultrasound holography. Magnetic force microscopy can detect magnetic nanoparticles and other magnetic (bio)materials in nonmagnetic biological samples, while electric force microscopy, conductive AFM, and Kelvin probe force microscopy can reveal buried nanomaterials on the basis of the differences between their electric properties and those of the surrounding matrices. Finally, scanning near field optical microscopy and tip-enhanced Raman spectroscopy can visualize buried nanostructures on the basis of their optical and chemical properties. Despite at a still early stage, these methods are promising for detection of nanomaterials in biological systems as they could be truly noninvasive, would not require destructive and time-consuming specific sample preparation, could be performed in vitro, on alive samples and in water or physiological environment, and by continuously imaging the same sample could be used to dynamically monitor the diffusion paths and interaction mechanisms of nanomaterials into cells and biological systems. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Subsurface imaging of carbon nanotube networks in polymers with DC-biased multifrequency dynamic atomic force microscopy

The characterization of dispersion and connectivity of carbon nanotube (CNT) networks inside polymers is of great interest in polymer nanocomposites in new material systems, organic photovoltaics, and in electrodes for batteries and supercapacitors. We focus on a technique using amplitude modulation atomic force microscopy (AM-AFM) in the attractive regime of operation, using both single and dual mode excitation, which upon the application of a DC tip bias voltage allows, via the phase channel, the in situ, nanoscale, subsurface imaging of CNT networks dispersed in a polymer matrix at depths of 10-100 nm. We present an in-depth study of the origins of phase contrast in this technique and demonstrate that an electrical energy dissipation mechanism in the Coulomb attractive regime is key to the formation of the phase contrast which maps the spatial variations in the local capacitance and resistance due to the CNT network. We also note that dual frequency excitation can, under some conditions, improve the contrast for such samples. These methods open up the possibility for DC-biased amplitude modulation AFM to be used for mapping the variations in local capacitance and resistance in nanocomposites with conducting networks.

Characterization of multi-walled carbon nanotube-polymer nanocomposites by scanning spreading resistance microscopy

Nanocomposites of aligned multi-walled carbon nanotubes (CNTs) embedded in a polymer matrix yield a unique combination of thermal and electrical properties and mechanical strength. These properties are intimately related to the composite nanostructure and to the growth and processing conditions. The alignment of the tubes, the filling fraction and the contact junction between the nanotubes are key parameters controlling the composite electrical conductivity. For this purpose, a full description of the composite nanostructure is required. Among the non-destructive scanning probe techniques, scanning spreading resistance microscopy is found to be a powerful technique in identifying the carbon nanotubes with true nanometer resolution, thus competing with SEM and TEM imaging. Additionally, the technique provides valuable information about the electrical conduction mechanism within the composite structure. Indeed, by using a controlled contact force and an appropriate model of conduction at the nanoscale, the tip-CNT contact resistance, the CNT intrinsic resistance and the CNT-epoxy-CNT resistance junction are evaluated. This latter is found to be the factor controlling the overall electrical conductivity of the composite.

Subsurface characterization of carbon nanotubes in polymer composites via quantitative electric force microscopy

Subsurface characterization of carbon nanotubes (CNTs) dispersed in free-standing polymer composite films was achieved via quantitative electric force microscopy (EFM). The effects of relative humidity, EFM probe geometry, tip-sample distance and bias voltage on the EFM contrast were studied. Non-parabolic voltage dependence of the EFM signal of subsurface CNTs in polymer composites was observed and a new mechanism was proposed taking consideration of capacitive coupling as well as coulombic coupling. We anticipate that this quantitative EFM technique will be a useful tool for non-destructive subsurface characterization of high dielectric constant nanostructures in low dielectric constant matrices.

A quantitative assessment of carbon nanotube dispersion in polymer matrices

Quantifying the nature and extent of the dispersion of nanomaterials in polymer matrices is the important first step in understanding the relationship between the nanoscale structure and the bulk scale functional performance of nanocomposites. We present here a methodology for using scanning electron microscope images of nanocomposites taken under high accelerating voltages to quantify four parameters that relate to the dispersion of the nanomaterial. This image analysis methodology is general and applicable to images from other microscopes as well. The analysis performed here was done on representative local areas of six samples to determine the effects of processing conditions, matrix chemistry, and carbon nanotube composition on the level of dispersion. Future work will involve expanding this analysis to rapidly cover larger areas and reducing the data in a manner that is similar to the approach of small angle scattering studies.