K Kangbo Lu

Electron Tomography on Micrometer-Thick Specimens with Nanometer Resolution

Transmission electron microscopy (TEM) is a well-established technique to explore matter down to the atomic scale. TEM tomography methods have been developed to obtain volume information at the mesoscopic dimensions of devices or complex mixtures of multiphase objects with nanometer resolution, but these methods are in general only applicable to relatively thin specimens with a few hundred nanometer thickness at most. Here we introduce an approach based on scanning TEM (STEM) tomography that pushes the resolution in three dimensions down to a few nanometers for several micrometer ultrathick specimens using a conventional TEM with 300 kV accelerating voltage, and we demonstrate its versatility for materials research and nanotechnology.

High-Angle Annular Dark Field Scanning Transmission Electron Microscopy on Carbon-Based Functional Polymer Systems

Two purely carbon-based functional polymer systems were investigated by bright-field conventional transmission electron microscopy (CTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For a carbon black (CB) filled polymer system, HAADF-STEM provides high contrast between the CB agglomerates and the polymer matrix so that details of the interface organization easily can be revealed and assignment of the CB phase is straightforward. For a second system, the functional polymer blend representing the photoactive layer of a polymer solar cell, details of its nanoscale organization could be observed that were not accessible with CTEM. By varying the camera length in HAADF-STEM imaging, the contrast can be enhanced between crystalline and amorphous compounds due to diffraction contrast so that nanoscale interconnections between domains are identified. In general, due to its incoherent imaging characteristics HAADF-STEM allows for reliable interpretation of the data obtained.

Annular dark-field scanning transmission electron microscopy (ADF-STEM) tomography of polymer systems

We have utilized bright-field conventional transmission electron microscopy tomography and annular dark-field scanning transmission electron microscopy (ADF-STEM) tomography to characterize a well-defined carbon black (CB)-filled polymer nanocomposite with known CB volume concentration. For both imaging methods, contrast can be generated between the CB and the surrounding polymer matrix. The involved contrast mechanisms, in particular for ADF-STEM, will be discussed in detail. The obtained volume reconstructions were analysed and the CB volume concentrations were carefully determined from the reconstructed data. For both imaging modes, the measured CB volume concentrations are substantially different and only quantification based on the ADF-STEM data revealed about the same value as the known CB loading. Moreover, when applying low-convergence angles for imaging ADF-STEM tomography, data can be obtained of micrometre-thick samples.

Tomographic imaging ultra-thick specimens with nanometer resolution

Transmission electron microscopy (TEM) is a well established and powerful technique to explore matter down to the atomic scale in two dimensions. Since real world objects have a three dimensional character the need to obtain volume information on nanometer scale is increasing especially on the mesoscopic dimensions of devices or complex mixtures of multiphase objects. TEM and scanning TEM (STEM) tomography methods have been developed in the past to fulfil this need, but these methods are limited to relatively thin specimens. In TEM this is caused mainly by the chromatic aberration and the dramatic increase of inelastic scattering in the sample, which leads ultimately to a strong resolution loss. Only high acceleration voltage TEM (above 1 million volts) allows for investigation of thicker specimens; this technique is, however, extremely expensive, far from being routine and highly destructive, in particular for organic matter. STEM tomography can minimize the effect of chromatic aberration, but is limited by the focal depth when large convergence angles are used. In this paper we establish a novel method based on dark field STEM tomography that pushes the resolution in all three dimensions down to a few nanometers for ultra-thick specimens of several micrometers at 300kV acceleration voltage.

Conductive carbon-nanotube - polymer nanocomposites prepared by latex technology

One bottleneck for using CNTs as fillers in polymer-based nanocomposites is that as-produced CNTs invariably exist as aligned aggregates or bundles that are tightly bound by an estimated interaction of 500 eV/µm of tube length for SWCNT or highly entangled in case of MWCNT. Therefore, the dispersion and incorporation of CNTs as individuals in polymer matrices is one of the most important and challenging tasks toward maximizing the translation of CNT properties to the nanocomposites.

Analysis of nano-composites based on carbon nanoparticles imbedded in polymers

Nowadays, polymer nano-composite is one of the most interesting research topics of polymer science and materials engineering, especially, when the nano-filler can bring polymer matrix special multifunctional properties. In this case, carbon nano-fillers such as carbon nanotubes (CNTs) and carbon black(CB) are excellent candidates for manufacturing high performance and conductive polymer nano-composites due to their unique properties.