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The implementation of new 2-D materials based technologies in production processes requires the development of quality management tools. These have to be underpinned by appropriate measurements. Consequently there is a need for the development of the metrology for measurement methods, the development of certified reference materials (CRM) and finally standardization. This chain represents the ideal way to practically useful standards. The presentation will give an overview on the main players in the field and summarize the recent status of activities. At the highest level the metrology of chemical characterization of 2D materials is in the scope of the International Meter Convention, specifically the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology (CCQM). Pre-standardization is an activity field of the Versailles Project on Advanced Materials and Standards (VAMAS). Standardization is mainly addressed by addressed ISO Technical Committees. A summary on available CRMs relevant to the characterization of nano materials has been prepared by BAM. Examples from the work of BAM’s Division 6.1 “Surface Analysis and Interfacial Chemistry” showcasing the characterization of chemically modified graphene surfaces are given and specific needs for the development of metrology are addressed.
In the discussion the audience is invited to define specific needs which will be streamlined to the respective bodies!
In this talk sample prep/handling, instrument calibration and data acquisition methods with examples from XPS, Auger and SIMS will be addressed in terms of their contributions to the reproducibility of data delivered by the methods.
Active parties in the field are VAMAS TWA 2 “Surface chemical analysis” (http://www.vamas.org/twa2/index.html), ISO/TC 201 “Surface chemical analysis” (https://www.iso.org/committee/54618.html) and the Surface Analysis Working Group (SAWG) at the International Meter Convention (https://www.bipm.org/en/ committees/cc/wg/sawg.html). The tools to improve the reproducibility of spectra, depth profiles and images at these international platforms are inter-laboratory comparisons, validated SOPs, standards and certified reference materials (CRM) as well as uncertainty budgets and establishment of traceability chains. The last point is of specific importance because all the methods, XPS, Auger and SIMS, are not primary methods.
To address quantitative XPS, AES and SIMS results of relevant inter-laboratory comparisons organized by SAWG considering measurands as alloy surface composition and thickness of thin films will be introduced. These comparisons delivered results which are viewed to be benchmarking, some of them resulted in ISO/TC 201 standards. For quantitative XPS and AES the principal outline of an uncertainty budget will be discussed together with the audience. Another issue of quantitative XPS which definitely needs consideration are valid methods for a determination of the transmission function of the instruments and even for the emission angle in the respective experiments.
Concerning the field of depth profiling it has to be investigated together with the audience whether the ISO (or ASTM) standards we have are sufficient to guarantee comparable results. Having in mind the number of different sputter ion species available today and range of samples of interest (metals, semiconductors, organic films) this might be questionable. And, how do depth profiling by AR-XPS and variable excitation energy XPS compete here?
For imaging surface chemical analysis, the characterization of the imaging system is an issue to be investigated. Here the determination of lateral resolution is a relevant topic.
Finally, the future needs to develop metrology for new applications e.g., ambient-pressure XPS, bio samples, and core-shell nanoparticles, will be issues raised for a discussion with the audience.
A comprehensive characterization of plasma modified polymer surfaces or plasma-polymerized thin films needs access to parameters as
- concentration of saturated/unsaturated carbon species (e.g. aromaticity) or other double bonds as C=N or C=O,
- branching, and
- losses of crystallinty or other degrees of structural order.
Furthermore the complex ageing phenomena of plasma modified polymers/plasma-polymers and the measurement of an in-depth distribution of chemical species are challenges for the analyst. The talk will display selected examples where such challenges have been met by using advanced methods of surface chemical analyses as Photoelectron Spectroscopy with variable excitation energy (“SyncXPS”), X-ray Absorption Spectroscopy (NEXAFS) at C, N and O K-edges and Time-of-Flight Secondary Mass Spectroscopy (ToF-SIMS) combined with Principal Component analysis (PCA).
Ageing studies of plasma deposited organic films by surface chemical analysis (ESCA, ToF-SIMS, XAS)
(2006)
This talk was given within the scope of the SIMS-22 conference in October 2019 in Kyoto (Japan). It deals with the surface analytical investigation of nanoparticles by PCS-assisted ToF-SIMS. This technique is applicable to core-shell nanoparticles, in order to distinguish a complete encapsulation from an incomplete encapsulation of the core by the shell material. Furthermore, the depletion process of organic nanoparticle coatings caused by UV-weathering is investigated. Finally, the significance of grouping studies for nanomaterials research and risk assessment is demonstrated.
Graphene is a two-dimensional carbon network with unique properties. However, its low solubility, poor reactivity and the limited accessibility of a well-defined basal plane are major challenges for applications. An ideal method to overcome these problems is the covalent attachment of functional molecules to its surface which enable further reactive modifications for specific applications. There are several technologies for surface functionalization of graphene and related CNT materials. To get control on the functionalization process and to optimize the performance of the modified surfaces analytical tools for surface chemical characterization are required. X-ray absorption (NEXAFS) and photoelectron spectroscopy (XPS) have been identified to be rather powerful here [1-3]. Specifically, NEXAFS spectroscopy underpinned by quantum chemical spectrum simulations [4] is unique in a way to address changes of aromaticity and defect formation at the graphene surface during functionalization.
For relevant surface modification technologies, we present examples on how NEXAFS and XPS can do a good job. All presented modifications aim on the production of platforms for defined functional 2D nanomaterials, as for example multifunctional hybrid architectures. In detail, we investigated:
• A wet chemical method for covalent functionalization of graphene sheets by a one-pot nitrene [2+1] cycloaddition reaction under mild conditions. Here a reaction between 2,4,6-trichloro-1,3,5-triazine and sodium azide with thermally reduced graphene oxide (TRGO) results in defined dichlorotriazine-functionalized graphene sheets.
• Graphene and carbon nanotube functionalized by Vacuum-Ultraviolet (VUV) induced photochemical or r.f. cw low pressure plasma processes to introduce amino, hydroxy or brominated functionalities.
To underpin finger-print information delivered by C K-edge NEXAFS we studied the effects of selected point and line defects as well as chemical modifications for a single graphene layer model by density functional theory based spectrum simulations.
References
[1] P.-L. Girard-Lauriault et al., Appl. Surf. Sci., 258 2012 8448-8454, DOI: 10.1016/j.apsusc.2012.03.012
[2] A. Lippitz et al., Surf. Sci., 611 2013 L1-L7, DOI: 10.1016/j.susc.2013.01.020
[3] A. Faghani et al., Angew. Chemie (International ed.), 56 2017 2675-2679, DOI:10.1002/anie.201612422
[4] C. Ehlert, et al., Phys.Chem.Chem.Phys., 16 2014 14083-14095, DOI: 10.1039/c4cp01106f