| 1. |
- Czigany, Zs., et al.
(author)
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Imaging of fullerene-like structures in CNx thin films by electron microscopy, Sample preparation artefacts due to ion-beam milling
- 2003
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In: Ultramicroscopy. - 0304-3991 .- 1879-2723. ; 94:3-4, s. 163-173
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Journal article (peer-reviewed)abstract
- The microstructure of CNx thin films, deposited by reactive magnetron sputtering, was investigated by transmission electron microscopy (TEM) at 200kV in plan-view and cross-sectional samples. Imaging artefacts arise in high-resolution TEM due to overlap of nm-sized fullerene-like features for specimen thickness above 5nm. The thinnest and apparently artefact-free areas were obtained at the fracture edges of plan-view specimens floated-off from NaCl substrates. Cross-sectional samples were prepared by ion-beam milling at low energy to minimize sample preparation artefacts. The depth of the ion-bombardment-induced surface amorphization was determined by TEM cross sections of ion-milled fullerene-like CNx surfaces. The thickness of the damaged surface layer at 5° grazing incidence was 13 and 10nm at 3 and 0.8keV, respectively, which is approximately three times larger than that observed on Si prepared under the same conditions. The shallowest damage depth, observed for 0.25keV, was less than 1nm. Chemical changes due to N loss and graphitization were also observed by X-ray photoelectron spectroscopy. As a consequence of chemical effects, sputtering rates of CNx films were similar to that of Si, which enables relatively fast ion-milling procedure compared to carbon compounds. No electron beam damage of fullerene-like CNx was observed at 200kV. © 2002 Elsevier Science B.V. All rights reserved.
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| 2. |
- McCaffrey, J.P., et al.
(author)
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Surface damage formation during ion-beam thinning of samples for transmission electron microscopy
- 2001
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In: Ultramicroscopy. - 0304-3991 .- 1879-2723. ; 87:3, s. 97-104
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Journal article (peer-reviewed)abstract
- All techniques employed in the preparation of samples for transmission electron microscopy (TEM) introduce or include artifacts that can degrade the images of the materials being studied. One significant cause of this image degradation is surface amorphization. The damaged top and bottom surface layers of TEM samples can obscure subtle detail, particularly at high magnification. Of the techniques typically used for TEM sample preparation of semiconducting materials, cleaving produces samples with the least surface amorphization, followed by low-angle ion milling, conventional ion milling, and focused ion beam (FIB) preparation. In this work, we present direct measurements of surface damage on silicon produced during TEM sample preparation utilizing these techniques. The thinnest damaged layer formed on a silicon surface was measured as 1.5nm thick, while an optimized FIB sample preparation process results in the formation of a 22nm thick damaged layer. Lattice images are obtainable from all samples. Copyright © 2001 Elsevier Science B.V.
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| 3. |
- Stöger, M., et al.
(author)
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Separation of pure elemental and oxygen influenced signal in ELNES
- 2002
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In: Ultramicroscopy. - 0304-3991 .- 1879-2723. ; 92:3-4, s. 285-292
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Journal article (peer-reviewed)abstract
- The energy loss near edge structure (ELNES) of many elements is strongly influenced by the presence of oxygen or other elements at surfaces, grain boundaries, or in the bulk material. The presented investigation deals mainly with the influence of oxygen at the surface. A method for the separation of both, the pure bulk signal and the oxidized surface signal, was evaluated and tested on Al, Cu, Mg, and Si. A comparison of experimental data with ab initio bandstructure calculations and other proofs of the accuracy of ELNES separation are presented. Influences of error propagations were tested and are exemplarily given for Al and Si.
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| 4. |
- Ali, Hasan, 1985-, et al.
(author)
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An electron energy loss spectrometer based streak camera for time resolved TEM measurements
- 2017
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In: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 176, s. 5-10
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Journal article (peer-reviewed)abstract
- We propose an experimental setup based on a streak camera approach inside an energy filter to measure time resolved properties of materials in the transmission electron microscope (TEM). In order to put in place the streak camera, a beam sweeper was built inside an energy filter. After exciting the TEM sample, the beam is swept across the CCD camera of the filter. We describe different parts of the setup at the example of a magnetic measurement. This setup is capable to acquire time resolved diffraction patterns, electron energy loss spectra (EELS) and images with total streaking times in the range between 100 ns and 10 μs.
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| 5. |
- Ali, Hasan, 1985-, et al.
(author)
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Noise-dependent bias in quantitative STEM-EMCD experiments revealed by bootstrapping
- 2024
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In: Ultramicroscopy. - : Elsevier. - 0304-3991 .- 1879-2723. ; 257
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Journal article (peer-reviewed)abstract
- Electron magnetic circular dichroism (EMCD) is a powerful technique for estimating element-specific magnetic moments of materials on nanoscale with the potential to reach atomic resolution in transmission electron microscopes. However, the fundamentally weak EMCD signal strength complicates quantification of magnetic moments, as this requires very high precision, especially in the denominator of the sum rules. Here, we employ a statistical resampling technique known as bootstrapping to an experimental EMCD dataset to produce an empirical estimate of the noise-dependent error distribution resulting from application of EMCD sum rules to bcc iron in a 3-beam orientation. We observe clear experimental evidence that noisy EMCD signals preferentially bias the estimation of magnetic moments, further supporting this with error distributions produced by Monte-Carlo simulations. Finally, we propose guidelines for the recognition and minimization of this bias in the estimation of magnetic moments.
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| 6. |
- Ali, Hasan, 1985-, et al.
(author)
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Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS
- 2019
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In: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 196, s. 192-196
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Journal article (peer-reviewed)abstract
- The weak signal strength in electron magnetic circular dichroism (EMCD) measurements remains one of the main challenges in the quantification of EMCD related EELS spectra. As a consequence, small variations in peak intensity caused by changes of background intervals, choice of method for extraction of signal intensity and equally differences in sample quality can cause strong changes in the EMCD signal. When aiming for high resolution quantitative EMCD, an additional difficulty consists in the fact that the two angular resolved EELS spectra needed to obtain the EMCD signal are taken at two different instances and it cannot be guaranteed that the acquisition conditions for these two spectra are identical. Here, we present an experimental setup where we use a double hole aperture in the transmission electron microscope to obtain the EMCD signal in a single acquisition. This geometry allows for the parallel acquisition of the two electron energy loss spectra (EELS) under exactly the same conditions. We also compare the double aperture acquisition mode with the qE acquisition mode which has been previously used for parallel acquisition of EMCD. We show that the double aperture mode not only offers better signal to noise ratio as compared to qE mode but also allows for much higher acquisition times to significantly improve the signal quality which is crucial for quantitative analysis of the magnetic moments.
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| 7. |
- Ali, Hasan, 1985-, et al.
(author)
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Single scan STEM-EMCD in 3-beam orientation using a quadruple aperture
- 2023
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In: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 251
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Journal article (peer-reviewed)abstract
- The need to acquire multiple angle-resolved electron energy loss spectra (EELS) is one of the several critical challenges associated with electron magnetic circular dichroism (EMCD) experiments. If the experiments are performed by scanning a nanometer to atomic-sized electron probe on a specific region of a sample, the precision of the local magnetic information extracted from such data highly depends on the accuracy of the spatial registration between multiple scans. For an EMCD experiment in a 3-beam orientation, this means that the same specimen area must be scanned four times while keeping all the experimental conditions same. This is a non-trivial task as there is a high chance of morphological and chemical modification as well as non-systematic local orientation variations of the crystal between the different scans due to beam damage, contamination and spatial drift. In this work, we employ a custom-made quadruple aperture to acquire the four EELS spectra needed for the EMCD analysis in a single electron beam scan, thus removing the above-mentioned complexities. We demonstrate a quantitative EMCD result for a beam convergence angle corresponding to sub-nm probe size and compare the EMCD results for different detector geometries.
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| 8. |
- Angseryd, Jenny, 1979, et al.
(author)
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Quantitative APT analysis of Ti(C,N)
- 2011
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In: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 111:6, s. 609-614
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Journal article (peer-reviewed)abstract
- A specially produced Ti(C,N) standard material, with a known nominal composition, was investigated with laser assisted atom probe tomography. The occurrence of molecular ions and single/multiple events was found to be influenced by the laser pulse energy, and especially C related events were affected. Primarily two issues were considered when the composition of Ti(C,N) was determined. The first one is connected to detector efficiency, due to the detector dead-time. The second one is connected to peak overlap in the mass spectrum. A method is proposed for quantification of the C content in order to establish the C/N ratio. A correction was made to the major C peaks, C at 6 and 12 Da, with the 13C isotopes, at 6.5 and 13 Da, according to the known natural abundance. In addition, a correction of the peak at 24 Da, where C and Ti overlap, is proposed based on the occurrence of single/multiple events for respective element. The results were compared to the results from other techniques such as electron energy loss spectroscopy, chemical analysis and X-ray diffraction. After applying the corrections, atom probe tomography results were satisfactory. Furthermore, the content of dissolved O in Ti(C,N) was successfully quantified. © 2011 Elsevier B.V.
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| 9. |
- Boll, Torben, 1979, et al.
(author)
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An APT investigation of an amorphous Cr-B-C thin film
- 2015
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In: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 159, s. 217-222
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Journal article (peer-reviewed)abstract
- A magnetron sputtered amorphous Cr-B-C thin film was investigated by means of atom probe tomography (APT). The film is constituted of two phases; a Cr-rich phase present as a few nanometer large regions embedded in a Cr-poor phase (tissue phase). The Cr-rich regions form columnar chains oriented parallel to the growth direction of the film. It was found that the Cr-rich regions have a higher B:C ratio than the Cr-poor regions. The composition of the phases was determined as approximately 35Cr-33B-30C and 15Cr-40B-42C (at%), respectively. The results suggest that this type of nanocomposite films has a more complex structure than previously anticipated, which may have an importance for the mechanical and electrical properties.
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| 10. |
- Bäcke, Olof, 1984, et al.
(author)
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Enhanced thermal stability of a polymer solar cell blend induced by electron beam irradiation in the transmission electron microscope
- 2017
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In: Ultramicroscopy. - : Elsevier BV. - 1879-2723 .- 0304-3991. ; 173, s. 16-23
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Journal article (peer-reviewed)abstract
- We show by in situ microscopy that the effects of electron beam irradiation during transmission electron microscopy can be used to lock microstructural features and enhance the structural thermal stability of a nanostructured polymer:fullerene blend. Polymer:fullerene bulk-heterojunction thin films show great promise for use as active layers in organic solar cells but their low thermal stability is a hindrance. Lack of thermal stability complicates manufacturing and influences the lifetime of devices. To investigate how electron irradiation affects the thermal stability of polymer:fullerene films, a model bulk-heterojunction film based on a thiophene-quinoxaline copolymer and a fullerene derivative was heat-treated in-situ in a transmission electron microscope. In areas of the film that exposed to the electron beam the nanostructure of the film remained stable, while the nanostructure in areas not exposed to the electron beam underwent large phase separation and nucleation of fullerene crystals. UV–vis spectroscopy shows that the polymer:fullerene films are stable for electron doses up to 2000 kGy.
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