Description: Publication date: Available online 5 April 2018 Source: Ultramicroscopy Author(s): G. ten Haaf, S.H.W. Wouters, D.F.J. Nijhof, P.H.A. Mutsaers, E.J.D. Vredenbregt The energy distribution of a high brightness rubidium ion beam, which is intended to be used as the source for a focused ion beam instrument, is measured with a retarding field analyzer. The ions are created from a laser-cooled and compressed atomic beam by two-step photoionization in which the ionization laser power is enhanced in a build-up cavity. Particle tracing simulations are performed to ensure the analyzer is able to resolve the distribution. The lowest achieved full width 50% energy spread is (0.205 ± 0.006) eV, which is measured at a beam current of 9 pA. The energy spread originates from the variation in the ionization position of the ions which are created inside an extraction electric field. This extraction field is essential to limit disorder-induced heating which can decrease the ion beam brightness. The ionization position distribution is limited by a tightly focused excitation laser beam. Energy distributions are measured for various ionization and excitation laser intensities and compared with calculations based on numerical solutions of the optical Bloch equations including ionization. A good agreement is found between measurements and calculations.

Description: Publication date: Available online 5 April 2018 Source: Ultramicroscopy Author(s): Yan Jun Li, Huanfei Wen, Eiji Arima, Yukinori Kinoshita, Hikaru Nomura, Zongmin Ma, Lili Kou, Yoshihiro Tsukuda, Yoshitaka Naitoh, Yasuhiro Sugawara, R. Xu, Z.H. Cheng We investigated a method to obtain a stable contrast mode on the TiO 2 (110) surface. The stable contrast rate is approximately 95% with a W-coated Si cantilever, which demonstrates that a stable tip apex plays an important role to obtain the real geometry of the surface during atomic force microscopy measurement. Information related to surface structure and tunneling current on the TiO 2 (110) surface can be obtained by the W-coated Si cantilever. It is possible to investigate the electronic structure and surface potential on the TiO 2 (110) surface with atomic resolution. In particular, the proposed method could be widely applied to investigate the catalytic activity and the mechanism of a catalytic reaction by a metal-coated tip in the future.

Description: Publication date: June 2018 Source: Ultramicroscopy, Volume 189 Author(s): Nathan D. Wallace, Anna V. Ceguerra, Andrew J. Breen, Simon P. Ringer Atom probe tomography is a powerful microscopy technique capable of reconstructing the 3D position and chemical identity of millions of atoms within engineering materials, at the atomic level. Crystallographic information contained within the data is particularly valuable for the purposes of reconstruction calibration and grain boundary analysis. Typically, analysing this data is a manual, time-consuming and error prone process. In many cases, the crystallographic signal is so weak that it is difficult to detect at all. In this study, a new automated signal processing methodology is demonstrated. We use the affine properties of the detector coordinate space, or the ‘detector stack’, as the basis for our calculations. The methodological framework and the visualisation tools are shown to be superior to the standard method of crystallographic pole visualisation directly from field evaporation images and there is no requirement for iterations between a full real-space initial tomographic reconstruction and the detector stack. The mapping approaches are demonstrated for aluminium, tungsten, magnesium and molybdenum. Implications for reconstruction calibration, accuracy of crystallographic measurements, reliability and repeatability are discussed.

Description: Publication date: Available online 30 March 2018 Source: Ultramicroscopy Author(s): Florian F. Krause, Dennis Bredemeier, Marco Schowalter, Thorsten Mehrtens, Tim Grieb, Andreas Rosenauer For simulation of transmission electron microscopic images and diffraction patterns, the accurate inclusion of thermal diffuse scattering by phonons is important. In the frozen phonon multislice algorithm, this is possible, if thermal displacements according to the realistic, quantum mechanical distribution can be generated. For pure crystals, quantum mechanical calculations based on DFT yield those displacements. But for alloys one is usually restricted to the Einstein approximation, where correlations between atoms are neglected. In this article, molecular dynamics simulations are discussed and used as an alternative method for displacement calculation. Employing an empirical Stillinger-Weber type potential, classical motion is used as an approximation for the quantum mechanical dynamics. Thereby, correlations and possible static atomic displacements are inherently included. An appropriate potential is devised for AlGaN by fitting to force constant matrices determined from DFT and elastic constants of AlN and GaN. A comparison shows that the empiric potential reproduces phonon dispersions and displacement expectations from DFT references. The validity for alloys is successfully demonstrated by comparison to DFT calculations in special quasirandom structures. Subsequently, molecular dynamics were used in multislice simulations of both conventional and scanning TEM images. The resulting images are in very good agreement with DFT based calculations, while a slight yet significant deviation from Einstein approximation results can be seen, which can be attributed to the neglect of correlations in the latter. The presented potential hence proves to be a useful tool for accurate TEM simulations of AlGaN alloys.

Description: Publication date: Available online 29 March 2018 Source: Ultramicroscopy Author(s): Michal Horák, Viktor Badin, Jakub Zlámal Standard 3D interpolation polynomials often suffer from numerical errors of the calculated field and lack of node points in the 3D solution. We introduce a novel method for accurate and smooth interpolation of arbitrary electromagnetic fields in the vicinity of the optical axis valid up to 90 % of the bore radius. Our method combines Fourier analysis and Gaussian wavelet interpolation and provides the axial multipole field functions and their derivatives analytically. The results are accurate and noiseless, usually up to the 5th derivative. This is very advantageous for further applications, such as accurate particle tracing, and evaluation of aberration coefficients and other optical properties. The proposed method also enables studying the strength and orientation of all multipole field components. To illustrate the capabilities of the proposed algorithm, we present three examples: a magnetic lens with a hole in the polepiece, a saturated magnetic lens with an elliptic polepiece, and an electrostatic 8-electrode multipole.

Description: Publication date: Available online 29 March 2018 Source: Ultramicroscopy Author(s): Megan Canavan, Dermot Daly, Andreas Rummel, Eoin K. McCarthy, Cathal McAuley, Valeria Nicolosi In-situ transmission electron microscopy is rapidly emerging as the premier technique for characterising materials in a dynamic state on the atomic scale. The most important aspect of in-situ studies is specimen preparation. Specimens must be electron transparent and representative of the material in its operational state, amongst others. Here, a novel fabrication technique for the facile preparation of lamellae for in-situ transmission electron microscopy experimentation using focused ion beam milling is developed. This method involves the use of rotating microgrippers during the lift-out procedure, as opposed to the traditional micromanipulator needle and platinum weld. Using rotating grippers, and a unique adhesive substance, lamellae are mounted onto a MEMS device for in-situ TEM annealing experiments. We demonstrate how this technique can be used to avoid platinum deposition as well as minimising damage to the MEMS device during the thinning process. Our technique is both a cost effective and readily implementable alternative to the current generation of preparation methods for in-situ liquid, electrical, mechanical and thermal experimentation within the TEM as well as traditional cross-sectional lamella preparation.

Description: Publication date: Available online 29 March 2018 Source: Ultramicroscopy Author(s): Svetlana Korneychuk, Bart Partoens, Giulio Guzzinati, Rajesh Ramaneti, Joff Derluyn, Ken Haenen, Jo Verbeeck A technique to measure the band gap of dielectric materials with high refractive index by means of energy electron loss spectroscopy (EELS) is presented. The technique relies on the use of a circular (Bessel) aperture and suppresses Cherenkov losses and surface-guided light modes by enforcing a momentum transfer selection. The technique also strongly suppresses the elastic zero loss peak, making the acquisition, interpretation and signal to noise ratio of low loss spectra considerably better, especially for excitations in the first few eV of the EELS spectrum. Simulations of the low loss inelastic electron scattering probabilities demonstrate the beneficial influence of the Bessel aperture in this setup even for high accelerating voltages. The importance of selecting the optimal experimental convergence and collection angles is highlighted. The effect of the created off-axis acquisition conditions on the selection of the transitions from valence to conduction bands is discussed in detail on a simplified isotropic two band model. This opens the opportunity for deliberately selecting certain transitions by carefully tuning the microscope parameters. The suggested approach is experimentally demonstrated and provides good signal to noise ratio and interpretable band gap signals on reference samples of diamond, GaN and AlN while offering spatial resolution in the nm range.

Description: Publication date: Available online 29 March 2018 Source: Ultramicroscopy Author(s): Guoqiang Han, Bo Lin Atomic force microscope (AFM) is an analytical instrument which is used to study the surface structure and morphology of materials. The AFM can measure and observe samples either in air or liquid environment. However, the standard AFM requires a long time to acquire accurate images and data. In our work, the compressive Sensing (CS) was applied in order to reduce the imaging time, lower the interactions between the probe and the sample, finally avoid sample damage in AFM. Three samples (PAA film, TGG1 grating and BOPP film) were used as the testing samples. Different image reconstruction algorithms (l1-ls, TVAL3, GPSR and IHT) were employed to reconstruct AFM image with different sampling rate. And various sampling patterns (Random Scan, Row Scan, SRM, Spiral Scan and Square-shape Scan) were used to obtain the undersampling data. A large number of experiments show that the choice of sampling pattern and image reconstruction algorithm has significant impact on the quality of the reconstructed images in AFM. Subsequently the reconstruction results of sample topographic images were analyzed and evaluated by the image quality indicators (PSNR and SSIM). The CS method can be used to obtain accurate images by reducing measurement data. It finally improves the measurement speed of AFM without cutting down the quality of AFM image.

Description: Publication date: May 2018 Source: Ultramicroscopy, Volume 188 Author(s): H. Schmid, E. Gilardi, G. Gregori, P. Longo, J. Maier, P.A. van Aken Epitaxial undoped and Gd 2 O 3 -doped ceria films were grown by pulsed laser deposition on (1 1 1) faced Y 2 O 3 -stabilized zirconia (YSZ). Highly localized cerium reduction at the film-substrate interfaces is revealed by atomically resolved valence EELS mapping using C s aberration-corrected scanning transmission electron microscopy. The chemical profiles reveal interdiffusion of Ce, (Gd), Y, Zr, forming an intermixing zone at the interface 7–9 (1 1 1) lattice planes wide. In its vicinity, the fraction of Ce 3+ raises gradually over 6–8 lattice planes from zero in the bulk ceria to ≈100% in one single plane at the interface. Beyond this plane the Ce 3+ fraction drops sharply within the YSZ substrate. In the vicinity of the interface systematic scan deflections are observed during EELS line scans. The advancing electron probe experiences a retarding force at the ceria side, and an accelerating force at the YSZ side, irrespective of the scan direction. This behavior is suggestive of coulombic interactions between the electron probe and a charged interface. This is interpreted as an indication of the presence of a space-charge situation at the YSZ/ceria interface, resulting from an excess negative charge at the ceria side (due to Ce 3+ cations) and an excess positive charge at the YSZ side (due to oxygen vacancies).

Description: Publication date: Available online 28 March 2018 Source: Ultramicroscopy Author(s): Yushu Shi, Wei Li, Sitian Gao, Mingzhen Lu, Xiaodong Hu An atomic force microscopy (AFM) scanning head is designed with the probe orthogonal scanning mode for metrological AFM to eliminate the curvature distortion. The AFM probe is driven by piezo stage and the scanning trajectory of the probe in 3 directions are orthogonal to reduce the cross coupling. A new optical lever amplification optical path is developed to eliminate the coupling error. The tracing lens and probe tip are moved as an integrated part. The AFM is operated at contacting mode. The step approach process of the probe tip is tested to the sample surface and the noise of the AFM head is analyzed. The response of the probe demonstrates a 0.5 nm resolution of the probe head in the z direction. Finally, the planar scanning performance of the scanning head is demonstrated compared with tube scanning AFM.