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S D Findlay

Publications and source records attributed to S D Findlay.

8 recordsLinked to original sources

Quantitative structure retrieval using scanning transmission electron microscopy.

A method is described that reconstructs the projected object potential using data recorded in the coherent imaging mode of a scanning transmission electron microscope. The technique is applicable in the presence of multiple scattering. It is not required that the thickness is known. Model examples exploring the nature of the data set required, the stability of the algorithm and the limitations on resolution are provided.

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Modelling imaging based on core-loss spectroscopy in scanning transmission electron microscopy.

Recent experimental realizations of atomic column resolution core-loss spectroscopy in the scanning transmission electron microscope have increased the importance of routinely modelling core-loss images. We discuss different approaches to wave function simulation and how they may be used in conjunction with the mixed dynamic form factor model to simulate images resulting from such inelastic scattering events. It is shown that, as resolution improves and in situations where the degree of thermal scattering is high, detailed quantitative comparisons will require the thermal scattering of electrons to be adequately modelled. Indeed, for sufficiently strong thermal scattering even qualitative interpretation may be affected: we give an example where this leads to a contrast reversal. We describe two methods suited to this purpose, the frozen lattice model and the scattering factor model, and explain how they may be combined with the mixed dynamic form factor approach.

Journal Article↗

Spectroscopic imaging of single atoms within a bulk solid.

The ability to localize, identify, and measure the electronic environment of individual atoms will provide fundamental insights into many issues in materials science, physics, and nanotechnology. We demonstrate, using an aberration-corrected scanning transmission electron microscope, the spectroscopic imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that the spectroscopic information is spatially confined around the scattering atom. Furthermore, we show how the depth of the atom within the crystal may be estimated.

Journal Article↗

Atomic-resolution electron energy loss spectroscopy imaging in aberration corrected scanning transmission electron microscopy.

The "delocalization" of inelastic scattering is an important issue for the ultimate spatial resolution of innershell spectroscopy in the electron microscope. It is demonstrated in a nonlocal model for electron energy loss spectroscopy (EELS) that delocalization of scanning transmission electron microscopy (STEM) images for single, isolated atoms is primarily determined by the width of the probe, even for light atoms. We present experimental data and theoretical simulations for Ti L-shell EELS in a [100] SrTiO3 crystal showing that, in this case, delocalization is not significantly increased by dynamical propagation. Issues relating to the use of aberration correctors in the STEM geometry are discussed.

Journal Article↗

Lattice-resolution contrast from a focused coherent electron probe. Part I.

To develop a Bloch wave framework for lattice-resolution contrast derived from coherent or incoherent scattering of an electron probe focused onto a crystal, boundary conditions which influence the propagation of an arbitrarily distorted coherent electron probe are addressed. These boundary conditions are particularly relevant for a probe focused within a unit cell, and lead to a general theory which hinges on Bloch wave excitation amplitudes being written as a function of beam position and focus. Whereas antisymmetric Bloch states are not excited for an incident plane wave at an exact zone axis orientation, these states may be strongly excited depending on probe focus and position within the unit cell. Equations for both coherent and incoherent lattice image contrast in scanning transmission electron microscopy are derived for any detector configuration in the Bloch wave framework. An equivalent expression amenable to evaluation via multislice techniques is also described. It is shown explicitly how mixed dynamic form factors for incoherent scattering should be taken into account for annular dark field or backscattered electron detectors, as well as for characteristic losses detected by X-ray emissions or by electron energy loss spectroscopy. A background contribution from "absorbed" electrons is included in the theory. The contribution of cross-talk from neighbouring columns to incoherent contrast is examined within the context of this theoretical framework.

Algorithms↗

Lattice-resolution contrast from a focused coherent electron probe. Part II.

In the previous paper, boundary conditions matching the probe to the crystal wave function in scanning transmission electron microscopy were applied by matching the whole wave function across the boundary. It is shown here how that approach relates to previous Bloch wave formulations using (phase-linked) plane wave boundary conditions for wave vectors implied by the range of transverse momentum components in the incident probe. Matching the whole wave function across the boundary, and including a suitably fine mesh in the reciprocal space associated with the crystal to allow matching of transverse momentum components within the probe, leads to a structure matrix A containing many elements which would normally be excluded for plane wave incidence. For perfect crystals, the A-matrix may be block diagonalised. This leads to a considerable increase in the computational efficiency of the model and yields important insights into the physics of convergent probes in perfect crystals-reciprocity in coherent imaging and the small aperture limit for coherent and incoherent contrast are considered. The numerical equivalence of the incoherent lattice contrast calculated in this Bloch wave method and the multislice method using mixed dynamic form factors will be demonstrated. Comparison between both these methods and the frozen phonon model, a prevalent multislice method for annular dark field simulation which has the theoretical advantage of handling double channelling, will be made.

Algorithms↗

Channelling effects in atomic resolution STEM.

A Bloch wave theory for incoherent scattering of an incident plane wave has proved successful in predicting the fine detail in 2-D zone axis channelling patterns formed by ADF, BSE and characteristic X-ray detection in beam rocking mode. A previously published example of polarity determination of GaAs by channelling contrast is compared with simulations in order to illustrate the applicability of the theory. Modification of boundary conditions for a focused coherent probe allows lattice-resolution incoherent contrast based on ADF and EELS detection as well as X-ray emissions to be catered for within a similar theoretical framework. Mixed dynamic form factors constitute an integral part of this theory, where quantum-mechanical phase is a core issue. Simulations of lattice-resolution ADF and EELS are discussed with reference to various zone axis projections of GaAs. Issues of single versus double channelling conditions, and local versus nonlocal interactions, are discussed in relation to X-ray, ADF and EELS detection.

Journal Article↗

Direct-to-consumer promotion of prescription drugs. Economic implications for patients, payers and providers.

Spending on outpatient prescription drugs in the US is accelerating rapidly. Although numerous factors are driving this trend, attention has recently focused on the role played by the marketing, promotion and advertising of pharmaceuticals, in particular direct-to-consumer (DTC) advertising. In 1997, the US Food and Drug Administration (FDA) issued a 'guidance' on such mass media promotion. The guidance altered existing FDA rules and effectively permitted pharmaceutical companies to promote prescription drugs on television and radio without giving detailed or even summary information on indications, efficacy or potential adverse effects. Since then, television commercials, in particular, and print advertisements in consumer magazines and newspapers have proliferated rapidly. Pharmaceutical companies spent $US1.8 billion on DTC advertising in 1999, a 40% increase over 1998. This spending in 1999 was heavily concentrated on about 50 drugs. Evidence is growing that DTC promotion of prescription drugs is: (i) alerting consumers to the existence of new drugs and the conditions they treat; (ii) increasing consumer demand for many drugs; (iii) contributing increasingly to the recent sharp increase in the number of prescriptions being dispensed; (iv) raising sales revenues; and, thus, (v) contributing to the higher pharmaceutical costs of health insurers, government and consumers. The public policy issues surrounding DTC advertisements centre on the following questions: (i) are the advertisements leading to the inappropriate clinical use of some drugs? (ii) are the advertisements inducing both consumers and physicians to choose more costly new brand-name drugs over less expensive, but equally effective, older brand or generic drugs? (iii) do television advertisements for prescription drugs contain a balanced amount of information on benefits versus potential adverse effects? and (iv) will the revenue benefits generated by DTC advertising cause pharmaceutical companies to focus more on developing products to treat prevalent but not life-threatening conditions, such as baldness, sexual dysfunction or memory loss? These questions are just beginning to be probed despite prescription drug spending, insurance coverage and payment policies having become major political issues in the US.

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