Photoconductivity null apparatus for the determination of minority carrier lifetime

A variable time constant exponential pulse generator used to null exponential photoconductivity decay signals is described. Circuitry using this generator for the direct readout of minority carrier lifetime of semiconductors is also presented.     

Spatially resolved fluorescence lifetime mapping of enzyme kinetics in living cells

Traditional cuvette‐based enzyme studies lack spatial information and do not allow real‐time monitoring of the effects of modulating enzyme functions in vivo. In order to probe the realistic timescales of steric modifications in enzyme–substrate complexes and functional binding–unbinding kinetics in living cells without losing spatial information, it is imperative to develop sensitive imaging strategies that can report enzyme kinetics in real time over a wide dynamic range of timescales. Here we present a multi‐photon excitation‐based, ultra‐fast photon detection using a streak camera and Laguerre expansion‐based fast deconvolution approach for achieving high spatio‐temporal resolution in monitoring real‐time enzyme kinetics in single cells. In particular, we report spatially resolved, nanosecond‐scale fluorescence dynamics associated with binding–unbinding kinetics of endogenous metabolic co‐factor nicotinamide adenine dinucleotide with enzymes in intact living cells. By monitoring real‐time kinetics of NAD(P)H–enzyme kinetics in primary hepatocytes isolated from young and aged mouse models, we observed that the mechanism of inhibition of mitochondrial respiration at complex I site is mediated by redistribution of free and protein‐bound nicotinamide adenine dinucleotide pools and that this equilibrium redistribution is affected by age‐related modifications in mitochondrial function. We describe unique advantages of Laguerre deconvolution algorithm in comparison with conventional lifetime analysis approaches. Non‐invasive monitoring of metabolic dysfunctions in intact animal models is an attractive strategy for gaining insight into the dynamics of tissue metabolism in health and in various metabolic syndromes such as cancer, diabetes and aging‐induced metabolic dysfunctions. Besides the example demonstrated above, we envisage that the proposed method can find applications in a variety of other situations where intensity‐based approaches fall short owing to spectroscopic artefacts.     

Cutting teeth in the SEM

An SEM was used to observe and record dental tissues as they were being cut. Sound human deciduous and permanent teeth were stored in 70% ethanol until required, then soaked in water, superficially dried and screwed to the SEM specimen stage through small drill holes made when they were wet. Many specimens were frozen and studied at cryogenic temperatures so that they would not become dehydrated. Edges used to cut the teeth included steel and tungsten carbide fashioned to resemble clinical cutting instruments, and diamond ultramicrotome knives and burs. The cutting tools were held either in a micromanipulator or a rigid tool post clamped to the specimen stage. The finest control was obtained by moving the specimen with the usual stage controls. SEM was conducted at 3 or 5 kV using TV speed scanning on the uncoated samples. All experiments were video-taped. 3-D control was difficult with a mono image and a real-time stereo system was therefore developed. Continuous, flowing sections of enamel could be obtained using diamond knives to cut the prism-free, surface zone tangentially. Thin sections of dentine, cement and bone curled up as they were cut, thus demonstrating permanent deformation. Subsurface enamel always fractured as it was cut, either locally as the tissue passed over the knife edge or tore out beneath the plane of the knife or by larger fragments cleaving off at larger distances ahead of the knife. Appearances were characteristic of prism orientation with respect to cutting direction. No anisotropy of cutting behaviour was found with dentine or bone: These tissues only fractured when thicker sections were taken. The SEM methods employed here can be usefully applied in the study of other materials.     

X-ray reflection-topography in the SEM

A simple arrangement is proposed, which permits the recording of x-ray topographical images of crystal surfaces inside a conventional SEM equipped with a removable metallic target. The capabilities of the method are demonstrated for the case of an Fe-target, where the characteristic radiation is Bragg-reflected from the surface of different LiF crystals.     

Dynamic Wear Tests in the SEM

Wear studies were conducted in the chamber of a scanning electron microscope (SEM) to permit direct observations of the wear processes. The sliding specimens had been metallographically polished and etched prior to test so that the effect of the alloy micro-structures could be observed. Test alloys included: low and high carbon steels, stainless steels, and bronze. As wear particles were generated, and became trapped in the interface, they produced surface damage which was in the form of material removed, or metal transfer to one of the sliding members. Video tape records were made at magnification up to 8000X and photomicrographs were taken at higher magnification to show the geometry of the wear particles.     

Electron-count imaging in SEM

We propose a method and a system for counting secondary electrons and for displaying micrographs from the SEM in real time. Evaluating the images obtained by the new system, we confirm that electron counting images have a higher signal to noise ratio than analogue images. This tendency is remarkable when the secondary electron signal is weak. We present the method and show experimental results obtained with our novel system for the detection of the secondary electron signal.     

In situ topography measurement in the SEM

An automatic technique for measuring heights in situ in the SEM based on a combination of stereo-metric and focusing methods has been developed. A pair of images of a surface element on the specimen is obtained by tilting the beam electrically in a manner such that the plane containing the tilt axis is coincident with the focal plane of the final lens. Cross-correlation is used to determine the parallax between the image pair which is then used to iteratively correct the height of the tilt axis by changing the lens focus. As a result, the lens focus tracks the specimen topography. With an appropriate specimen surface containing high resolution features for image correlation, the technique is capable of maintaining both its lateral and vertical resolutions over several decades of height displacement up to 100 μm. In an experimental system based on a commercial electron-optical column, spot, line profile, and three-dimensional measurements have been demonstrated.     

High resolution at low beam energy in the SEM: resolution measurement of a monochromated SEM

The resolution of secondary electron low beam energy imaging of a scanning electron microscope equipped with a monochromator is quantitatively measured using the contrast transfer function (CTF) method. High-resolution images, with sub-nm resolutions, were produced using low beam energies. The use of a monochromator is shown to quantitatively improve the resolution of the SEM at low beam energies by limiting the chromatic aberration contribution to the electron probe size as demonstrated with calculations and images of suitable samples. Secondary electron image resolution at low beam energies is ultimately limited by noise in the images as shown by the CTFs.     

扫描电子显微镜探头新进展

介绍了扫描电子显微镜（SEM）信号探头研究的最新成果。针对常见扫描电子显微镜缺陷而开发的新型探头不仅改善了仪器成像质量,也极大地扩展了仪器的使用范围,简化了样品的准备工艺过程。     

Cathodoluminescence and gaseous scintillation in the environmental SEM

A novel detection means for the environmental SEM (ESEM) is described. Certain gases, apart from being the environmental conditioning medium, can also act as a scintillator detector. All signals, such as secondary and backscattered electrons which can cause a particular gas to luminesce, can be detected. It is further concluded that the gas can act as a generalized detector device for all signal-gas reactions provided some suitable parameter can be monitored. New possibilities in the detection of specimen cathodoluminesce created by the ESEM are demonstrated.     

Colour micrographs for backscattered electron signals in the SEM

The backscattered electron (BSE) signals detected by a pair of detectors in the SEM can be used for producing colour micrographs. The image corresponding to any of these signals, or to a mixture of these signals, is assigned one primary colour and two or more of these images are superimposed onto the same colour frame. In addition, the mixing of signals from the gaseous detector device together with their use for colour imaging is also examined.     

Control of charging in low-voltage SEM

Charging of the specimen under electron beam irradiation is a common problem in scanning electron microscopy (SEM). It results in unstable imaging conditions and a loss in resolution due to defocus of the beam. In addition, it can cause permanent changes in some specimens from translocation of mobile ions under the influence of the induced electrostatic field. To minimize charging and its associated problems, the incident beam energy must be carefully chosen to be the value E₂ at which a dynamic charge balance is obtained. This article presents data on E₂ values for a variety of materials and demonstrates how E₂ is affected by the choice of angle of beam incidence.