Edward D. Boyes

On Low Voltage Scanning Electron Microscopy and Chemical Microanalysis

The current status and general applicability of scanning electron microscopy (SEM) at low voltages is reviewed for both imaging (low voltage scanning electron microscopy, LVSEM) and chemical microanalysis (low voltage energy-dispersive X-ray spectrometry, LVEDX). With improved instrument performance low beam energies continue to have the expected advantages for the secondary electron imaging of low atomic number (Z) and electrically non-conducting samples. They also provide general improvements in the veracity of surface topographic analysis with conducting samples of all Z and at both low and high magnifications. In new experiments the backscattered electron (BSE) signal retains monotonic Z dependence to low voltages (<1 kV). This is contrary to long standing results in the prior literature and opens up fast chemical mapping with low dose and very high (nm-scale) spatial resolution. Similarly, energy-dispersive X-ray chemical microanalysis of bulk samples is extended to submicron, and in some cases to <0.1 µm, spatial resolution in three dimensions at voltages <5 kV. In favorable cases, such as the analysis of carbon overlayers at 1.5 kV, the thickness sensitivity for surface layers is extended to <2 nm, but the integrity of the sample surface is then of concern. At low beam energies (E(0)) the penetration range into the sample, and hence the X-ray escape path length out of it, is systematically restricted (R = F(E(0)(5/3))), with advantages for the accuracy or elimination of complex analysis-by-analysis matrix corrections for absorption (A) and fluorescence (F). The Z terms become more sensitive to E(0) but they require only one-time calibrations for each element. The new approach is to make the physics of the beam-specimen interactions the primary factor and to design enabling instrumentation accordingly.

Environmental High Resolution Electron Microscopy in Materials Science

An environmental cell high resolution electron microscope (EHREM) has been developed for in-situ studies of dynamic gas molecule-solid interactions on the atomic scale. It allows access to metastable intermediate phases of materials, to both surface and bulk structural changes which are often interrelated, to reaction mechanisms for probing materials performance in reaction environments and to sequences of reversible microstructural and chemical development associated with their operation. Materials transported through air can be restored or recreated and samples damaged, e.g. by dehydration, by the usual vacuum environment in a conventional electron microscope can be preserved. A Philips CM30 high resolution transmission EM (HRTEM)/Scanning transmission EM (STEM) system has been extensively modified in our laboratory to add an environmental cell in the EM column which provides facilities for in-situ gas-solid reaction studies in controlled atmospheres of gas or vapor at pressures of 0–50 mbar, instead of the regular TEM high vacuum environment. The integrated new environmental cell capability is combined with the original 0.23 nanometer (nm) TEM lattice resolution with access to interatomic spacings, STEM imaging (bright field/annular dark field), electron diffraction and chemical microanalyses. Regular sample holders are used and include hot stages to >1000 °C. The learnings from well designed dynamic in-situ EM studies of reaction sequences between gases or vapors and solids can be considerable. Examples of in-situ applications include direct studies of dynamic reactions with chemically stabilized silica based ceramics, the formation of active vanadyl pyrophopsphate catalysts from their hemihydrate precursors for vapor phase oxidation of hydrocarbons and carbon nanostructures.

Nanotechnology in the Laboratory and in Society

The potential benefits of Nanotechnology in society are huge. Not only can we hope for novel materials and cleaner and leaner manufacturing processes but for effective help in cleaning up some of the excesses and pollution from the past. The field has been widely (some would say wildly) hyped, and the threat and promise of technically and commercially disruptive technologies have been enthusiastically promoted, including by those wishing to profit from speculative positions in the intellectual property. We are now in the hard slog phase of developing real and substantial applications through to commercial success. It has of course been widely publicized that the whole point of nanotechnology is new size related properties, and so it would be naïve and perhaps unwise, even irresponsible, to assume that all the results will be benign or better in the area of safety, health and the environment (SHE). Although many of them may well be just fine, in principle one must allow some of the novel possibilities could include negative consequences, and a responsible approach includes proper product and process stewardship assessments before new technologies are widely applied. These should include economic and technical total life cycle analyses (LCAs). Reinforcing the need for this, there is also in society a healthy skepticism and some inherent distrust of the new. This is especially the case where the benefits to the end use consumer are less than clear or are not equitably shared. This need not, and certainly should not, be the case with nanotechnology. The issues and concerns of the person in the street need to be addressed in helpful, respectful and non-judgmental ways. As might be expected, the science may not always be simple; for example with discontinuous size dependent properties reported. However, the potential importance of the new field is reflected in the careful and quite detailed assessments already launched by various governmental, quasi-governmental and non-governmental organizations, as well as in the insurance industry and in the more informed and enlightened commercial enterprises.