We introduce the concept of an XPS 'Topofactor', which can be used in conjunction with the XPS 'Thickogram', to provide overlayer thicknesses on topographic samples of known geometry. The concept is essentially simple; analysis is performed with the sample normal directed towards the XPS analyser, the equivalent planar thickness is calculated from the Thickogram and the Topofactor applied to the result to provide the actual thickness. The Topofactor is thus defined as the ratio of the true overlayer thickness to the apparent overlayer thickness obtained by assuming the sample is ideally planar. Within this paper we describe how Topofactors can be calculated and consider cylinders (fibres) and spheres (particles) in some detail. For spheres, a Topofactor of 0.67 is a useful average value and for cylinders, a Topofactor of 0.79 is useful, (i.e. ∼2/3 and 4/5, respectively) both values would typically provide overlayer thicknesses within 10% of the actual value. An analytical description of cylindrical and spherical Topofactors to within 1% error is provided which accounts for variations in thickness and relative electron attenuation lengths. The Topofactors are useful for macroscopic and microscopic samples, but not for nanoscopic samples such as nanofibres and nanoparticles. In this case, we provide an analytical model that can predict the relative XPS signals from the core and the shell of nanomaterials to an error that is typically better than 10%.
The candela, the SI (syste`me internationale) unit for optical radiation, has been one of the base units since the inception of the system. The latest definition was in 1979, when it was linked to the derived unit, the watt. Advances in optical technology and the needs of the communication sector suggest that it is timely that consideration be given to redefining the candela in terms of fundamental quantum optical entities, i.e. photons. Validation of this approach will require comparison against the most accurate conventional technique, cryogenic radiometry. A definition in terms of photon number and the requirements for demonstrating equivalence with existing techniques is discussed, together with new possibilities which would result from further improvements in accuracy. Work being carried out at the National Physical Laboratory (NPL) towards these goals is described, drawing on developments of photon-counting calibration techniques and low temperature measurements, and research into single photon sources and detectors.
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