Low-voltage High-resolution Electron Microscopy Research Articles

Successful application of Low Voltage Electron Microscopy to practical materials problems.

Low-voltage High-Resolution Electron Microscopy (LVHREM) has several advantages, including increased cross-sections for inelastic and elastic scattering, increased contrast per electron, decreased delocalization effects and reduced knock-on damage. Imaging at differing voltages has shown advantages for imaging materials that are knock-on damage sensitive. We show experimentally that different materials systems benefit from low voltage high-resolution microscopy. There are advantages for imaging single layer materials such as graphene at below the knock-on threshold; we present an example of imaging a graphene sheet at 40kV. We have also examined mesoporous silica decorated with Pd nanoparticles and carbon black functionalized with Pd/Pt nanoparticles. In these cases we show that the lower voltage imaging maintains the structure of the surrounding matrix during imaging, whereas aberration correction provides the higher resolution for imaging the nanoparticle lattice. Perhaps surprisingly we show that zeolites damage preferentially by ionization effects (radiolysis). The current literature suggests that below incident energies of 40kV the damage is mainly radiolitic, whereas at incident energies above 200kV the knock-on damage and material sputtering will be the dominant effect. Our experimental observations support this conclusion and the effects we have observed at 40kV are not indicative of knock-on damage. Other nanoscale materials such as thin silicon nanowires also benefit from lower voltage imaging. LVHREM imaging provides an excellent option to avoid beam damage to nanowires; our results suggest that LVHREM is suitable for nanowire-biological composites. Our experimental observations serve as a clear demonstration that even at 40keV accelerating voltage, LVHREM can be used without inducing beam damage to locate dislocations and other crystalline defects, which may have adverse effects on nanowire device performance. Low voltage operation will likely become the new mode of imaging for many electron microscopes, with the instrument being, in essence, tuned to extract all the information possible from each electron that transits the sample.

40keV atomic resolution TEM

Here we present the first atomic resolution TEM imaging at 40 keV using an aberration-corrected, monochromated source TEM. Low-voltage High-Resolution Electron Microscopy (LVHREM) has several advantages, including increased cross-sections for inelastic and elastic scattering, increased contrast per electron and improved spectroscopy efficiency, decreased delocalization effects and reduced knock-on damage. Together, these often improve the contrast to damage ratio obtained on a large class of samples. Third-order aberration correction now allows us to operate the TEM at low energies while retaining atomic resolution, which was previously impossible. At low voltage the major limitation to resolution becomes the chromatic aberration limit. We show that using a source monochromator we are able to reduce the effect of chromatic aberration and achieve a usable high-resolution limit at 40 keV to less than 1Å. We show various materials' examples of the application of the technique to image graphene and silicon, and compare atomic resolution images with electron multislice simulations.

The Molecular Structure of Polymer−Fullerene Composite Solar Cells and Its Influence on Device Performance

Understanding the nanomorphology of polymer-based solar cells is key to improving device efficiencies. A detailed study of the changes in polymer solar cell morphology at the molecular level and in real time under process conditions is presented. Using low voltage, high resolution electron microscopy (LVHREM) we observe the mechanisms of PCBM-rich domain migration in real time and provide evidence for vertical segregation within the bulk heterojunction and increased crystallinity of both P3HT and PCBM. The results of the study highlights the fact that P3HT:PCBM bulk heterojunction morphology should be viewed within the framework of a four-phase, three-dimensional, nonequilibrium system.

Immunogold staining of fibrinogen on biomaterial surfaces: the effect of labeling conditions

Fibrinogen was preadsorbed to six polymers of differing surface properties. Immunogold staining followed by low-voltage high-resolution electron microscopy was used to examine the goldbead surface distribution and the effects of the labeling conditions. The immunogold staining technique was successful in visualizing adsorbed fibrinogen within the monolayer regime. Fibrinogen was found to distribute in a rather homogeneous fashion on all surfaces at varying protein solution concentrations and staining times. The degree of staining of fibrinogen varied from polymer to polymer, indicating variable extents of extinction of antigenic epitopes upon adsorption. Bulk protein concentration, protein adsorption time, immunogold concentration, and immunogold labeling time were independently varied to determine the optimum fibrinogen staining conditions. Optimum staining (defined as having one monolayer of gold markers on a particular surface under constant adsorption conditions as visualized by electron microscopy) was determined on polyethylene using several 18-nm goldmarker concentrations. For polyethylene the optimum fibrinogen staining condition required a goldmarker concentration of 3.86 × 10 12 particles/ml for 30 min at room temperature. This resulted in a goldmarker density of 120 ± 3 markers/μm 2.