Project work / Master topics in the TEM Gemini Centre, Fall 2009

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1 Project work / Master topics in the TEM Gemini Centre, Fall 2009 As a physics project or diploma student in the transmission electron microscopy (TEM) group you can take an active part in one of the exciting research projects which require finest scale material characterisation. You work together with a PhD student, a SINTEF research team or one of our external collaborators to achieve a common goal. The work can have an applied character and be very practical, or theoretical to support experimental activities within the group. Also a combination of practical and theoretical work is a possibility. In all projects the TEM is used to understand the structure of a material down to the atomic level and relate this to important macroscopic properties. Examples of student projects which are available for the fall semester 2009 and in which you can participate in are: Developing and characterising of new aluminium alloys TEM characterisation of nanostructures of functional oxides (PbTiO 3, ZnO, CuO) TEM characterisation of semiconductor nanowires TEM characterisation of catalysts for future applications TEM characterisation of solar cell materials (Si, quantum dots, nanowires) These projects are described in more details at the end of this document. Earlier, several student projects have led to publications [1-6]. Due to high demand on the research facilities and the intensive supervising we want to give, we can take in max 4 students this semester. A collection of materials studied in the TEM Gemini centre during the last year: Collaboration partners in parentheses for each material. Aluminium precipitates (Hydro), Alanates for hydrogen storage (IFE, Hauback), Ferroelectric thin films (Tybell), Catalysis particles (Statoil,Cambridge University, Holmen), Aluminium surfaces(hydro, Nisancioglu) Silicon solar cells (REC, Lohne), PbTiO 3 nanorods (Grande, Einarsrud) and carbon nanotubes (Elkem).

2 We offer: Choice of a project that fits your interests and background. Training in operating advanced scientific equipment (for example TEM) or specialised simulation software. Weekly meetings with a supervisor during the project. Being part of a large and dynamic scientific consortium. Possibility in extending the project (to diploma/phd) or eventual summer job. You are encouraged to contact one of us if you like to hear more details on a specific project, other available projects, options in academia or industry after a diploma in TEM or possibilities to incorporate own research ideas related to TEM. For more information on the current activities within the group, group members or collaborators, equipment and recent publications, see the TEM Gemini Centre homepage: Contacts: Ton van Helvoort (Room D4-149, Tel , a.helvoort@ntnu.no) Randi Holmestad (Room D4-153, Tel , randi.holmestad@ntnu.no) John Walmsley (Room D4-113, Tel , john.walmsley@sintef.no) References: [1] Dheeraj DL, Patriarche G, Largeau L, Zhou HL, Hoang TB, Mosses AF, Grønsberg S, van Helvoort ATJ, Fimland B-O and Weman H, " Growth and characterization of wurtzite GaAs nanowires with defect-free zinc blende GaAsSb inserts", Nano Letters, 8, , [2] Rørvik P-M, Lyngdal T, Sæterli R, van Helvoort A T J, Holmestad R, Grande T and Einarsrud M-A, "Influence of volatile chlorides on the molten salt synthesis of ternary oxide nanorods and nanoparticles", Inorganic Chemistry, 47, , [3] Marioara CD, Andersen SJ, Birkeland A, Holmestad R, Orientation of Silicon Particles in a Binary Al- Si Alloy, Journal of Materials Science, , [4] Rørvik PM, Almli A, van Helvoort ATJ, Holmestad R, Tybell T, Grande T and Einarsrud MA PbTiO 3 nanorod arrays by self-assembly of nanocrystals, Nanotechnology, 19, , [5] Eberg E, Monsen AF, Tybell T, van Helvoort ATJ and Holmestad R, "Comparison of TEM specimen preparation of perovskite thin films by tripod polishing and conventional ion milling", Journal of Electron Microscopy 57, , [6] Sæterli R, van Helvoort ATJ, Wang G, Rørvik P-M, Tanem BS, Grande T, Einarsrud M-A and Holmestad R, "Detailed TEM characterisation of PbTiO 3 nanorods", Journal of Physics: Conference Series, 126, , 2008.

3 Development and characterization of new Al alloys In studies of light metal alloys there are challenges when it comes to establishing relations between the microstructure and the mechanical properties, as for example strength, hardness and ductility. In Al-Mg-Si- (Cu) alloys, which are industrially relevant due to their superior mechanical properties (high strength /weight ratio), the hardness increase is due to precipitation of nanometre-sized metastable phases that form from solid solution during heat treatment. We have to fully understand this precipitation in order to get the wanted properties (alloy design). The student project will consist of experimental testing of mechanical properties with different heat treatments, and complementary microstructure studies by TEM. This project is done in close collaboration with SINTEF and Norwegian Light Metal industry (Hydro, Steertec Raufoss and RTIM). Within this field there are possibilities for continuation as a PhD student. Contact persons: Randi Holmestad (randi.holmestad@ntnu.no), Calin Marioara (calin.d.marioara@sintef.no) Modeling of precipitates in Al alloys The impressive experimental knowledge derived within the group on the properties of Al- Mg-Si(-Cu) alloys has paved the way for a better theoretical understanding of these materials. Detailed theoretical knowledge on the connections between the various precipitates nucleating in the alloy and their influence on the host lattice will be extremely useful both for the creation of better structural materials, of considerable interest to industry, and the basic understanding of the precipitation. Even seemingly minor studies in this area will be connected to the very forefront of the field. Growth and evolution of the precipitates is intimately connected with the interaction with the host lattice, i.e., with the interface between precipitate and Al. The planned student project hence will focus on density functional theory based investigations (employing the plane wave based code VASP) of combined Al/precipitate systems, revolving around determination of stable interfaces, various stabilization mechanisms for a given interface (binding of vacancies e.g.), and the properties of the strain field within Al generated by the precipitate. Contact persons: Flemming Ehlers (flemming.ehlers@ntnu.no) and Randi Holmestad (randi.holmestad@ntnu.no).

4 TEM characterization of functional oxide nanostructures One dimensional nanostructures can be produced via simple and low-cost chemical production routes. In the Inorganic Materials and Ceramics group at the Department of Materials Technology at NTNU several different ferroelectric nanostructures are synthesised via chemical bottom-up approaches, for example BaTiO 3 and PbTiO 3. TEM is used to study single nanorods and eventually variations in crystal structure and orientation as function of process parameters. Such studies should contribute to the optimalisation of the process and the nanostructures for future applications. Contact persons: Randi Holmestad (randi.holmestad@ntnu.no), Ton van Helvoort (a.helvoort@ntnu.no), Mari-Ann Einarsrud (mariann.einarsrud@material.ntnu.no) and Tor Grande (tor.grande@material.ntnu.no ). TEM-characterization of heterostructured core-shell nanowires Figure 1. Dark field image of GaAs nanowire with GaAsSb insert. Image taken by diploma student Sondre Grønsberg. From Dheeraj DL et al., "Growth and characterization of GaAs nanowires with defectfree zinc blende GaAsSb inserts", Nano Letters, 8, , There are fascinating developments in synthesis and understanding of III-V nanowires (NWs) (diameter nm) produced at NTNU via molecular beam epitaxy. Previous project students have contributed by using the TEM to characterize pure GaAs NWs and GaAs NWs with GaAsSb inserts. At the moment characterization of GaAs core NWs with low defect densities and with an AlGaAs shell is most interesting. These heterostructures depict a dramatic increase in optical efficiency. NWs have interesting optical properties and "bandgap design" from atomically abrupt changes in crystal phase and/or crystal material is a target which comes in sight. In Autumn 2009 we start a large project on optimizing these NW structures for solar cell applications. In the project the student will characterize NWs using TEM. Size and shape variations with sub-nm precision will be established. The main aim of using TEM in this student s project is to determine the crystal phase, growth direction and eventual lattice defects in the NWs. All these parameters have a distinct effect on the desired properties of the NWs. Results from the student s project will contribute to further optimization of the growth and the understanding the physical properties of the NWs. Contact persons: Ton van Helvoort (a.helvoort@ntnu.no), Helge Weman ( helge.weman@iet.ntnu.no) and Bjørn-Ove_Fimland (Bjorn.Fimland@iet.ntnu.no).

5 TEM studies of catalysts for future applications In collaboration with the Department of Chemical Engineering we study different types of porous materials (substrates) with small metal particles which are used as catalysts in a broad range of catalytic processes. To understand the properties of the catalysts it is of crucial importance to know the size (down to a few nanometers) and the structure of the particles, in addition to their composition. We are now establishing a tomography technique which makes it possible to image these particles and substrates in 3D. We also have equipment to study these metal nano particles in reduced states, without exposing them to oxygen. We have in this project collaboration with Statoil-Hydro. Contact persons: John Walmsley (john.walmsley@sintef.no), Magnus Rønning (magnus.ronning@chemeng.ntnu.no) and Anders Holmen (anders.holmen@chemeng.ntnu.no ). TEM investigations of microstructure in silicon solar cell materials Supply of silicon is one of the bottlenecks of the photovoltaic industry. Cost and energy payback time for the silicon production can be significantly reduced through the so-called metallurgical route. The disadvantage is that the resulting material often contains higher level of impurities and crystal defects which reduce the efficiency of silicon solar cells, by introducing recombination centres for electrons and holes in the bulk. To improve the silicon wafer production, it is important to understand how dislocations are formed and where precipitates of other phases are nucleated and grown during solidification and cooling, and how this affects the lifetime. By combining light microscopy (LM), scanning electron microscopy (SEM) to find grain orientations, and electron beam induced current (EBIC), one can find areas with high and low efficiency in different parts of the silicon ingot. A combination of these techniques with TEM studies of grain boundaries, dislocations and particles in the same material, will a) EBIC, b) SEM c) TEM give a deeper understanding of the material and processes. The student will do the TEM in collaboration with others who will do/has done most of the other experiments. Contact persons: Randi Holmestad (randi.holmestad@ntnu.no), John Walmsley (john.walmsley@sintef.no), Heidi Nordmark (heidi.nordmark@sintef.no), Marisa Di Sabatino (Marisa.Di.Sabatino@sintef.no), Eivind Øvrelid (Eivind.J.Ovrelid@sintef.no), Otto Lohne ( otto.lohne@material.ntnu.no).

6 Synthesis and characterisation of CuO nanowires CuO is a narrow band gap p-type direct semiconductor and a major component in various high temperature superconductors and giant magnetoresistive materials. Nanowires of CuO are among others investigated with respect to their use as field emitters, solar cell components, anodes for Li ion batteries or superhydrophobic surface coatings. Single or bicrystalline CuO nanowires can be easily grown by the student at the start of the project by using a solid-vapour process based on elevated temperature oxidisation in air. The wires are about 100 to 200 nm in width and up to 20 µm in length. Some wires show twinning in the monoclinic phase. The student will do the TEM characterisation her/himself to determine wire sizes, crystal phase / defects and growth mechanism. To support the experimental work, some basic TEM simulation will be done on high resolution imaging and electron diffraction. A detailed knowledge of the nanostructure is needed as base for modifying (chemically or physically) the nanowire properties for different applications. Contact persons: Ton van H elvoort (a.helvoort@ntnu.no), Pavel Sikorski (pawel.sikorski@ntnu.no) and Florian Mumm (florian.mumm@ntnu.no ). TEM-characterisation of ZnO nanorods for 3 generation solar cells Hybrid inorganic/organic solar cells are promising candidates for achieving low-cost solar cell devices. Nanostructural control of the morphology of the inorganic acceptor bears potential for improving charge transfer and conversion efficiencies in such devices by creating direct charge carrier pathways and increasing surface area, respectively. ZnO as a wide band gap material is a promising inorganic acceptor material, because of its transparency (band gap 3.37eV) and its low price due to great abundance. Pulsed laser deposition (PLD) allows for controlled growth of ZnO thin films and nanostructures of various materials. However PLD growth mechanisms are not fully understood and microscopic investigations are needed to explain them. The project is part of a research project and is a collaboration between NTNU (Helge Weman s group) and SINTEF Materials and Chemistry. In the project, the student will characterize thin films and nanorods using TEM. Size and shape variations with sub-nm precision can be established. The main goal is to determine the crystal structure, growth direction and lattice defects. Results from the student s project will contribute to understand the growth mechanisms and to optimize the na nostructure growth. Contact persons: Cecile Ladam (cecile.ladam@sintef.no), Helge W eman (helge.weman@iet.ntnu.no), John Walmsley (john.walmsley@sintef.no), Per Erik Vullum (per.erik.vullum@sintef.no) rd

7 Incoherent imaging mode: understanding it s future potential Scanning transmission electron microscopy (STEM) in a TEM is a very powerful technique giving directly interpretable highest resolution images with Z-contrast and a perfect integration with analytical techniques for determining the composition or electronic structure of materials. An electron beam is focused down to the nanometer or atomic scale and scanned across the sample. The high angle scattered transmitted electrons are recorded for each scanned position. Recent developments in scanning electron microscopy (SEM) show that impressive STEM results can already be obtained at 30 kv. Hence, STEM will become more accessible due to the lower equipment cost, increased ease of use and the reduced beam damage at lower voltages of a SEM compared to TEM. Such equipment will become available in NTNU Nanolab. In this project a theoretical comparison will be made between STEM in a TEM and in a SEM. As the speed of the electrons used is different, the scattering processes within the sample and hence the detector set-up and resulting images are different between the TEM and SEM approach. Image contrast interpretation, quantification and resolution limit have to be react person: Ton van Helvoort (a.helvoort@ntnu.no). addressed in the SEM set-up to use these new possibilities to their max. In this project the practical work is limited and the focus lies on understanding the principles behind the technique. Cont TEM characterization by ECAP. of the nano-structures in CP-Ti processed Equal channel angular pressing, ECAP, as one of the most important Severe plastic deformation (SPD) methods has been proved a promising method to produce ulta-fine grained ( UFG) or nano-structured bulk materials, which excess much higher strength than their conventional counterparts. With its good biocompatibility and without toxic elements such as V and Al in the composition, commercially pure titanium (CP-Ti) is an ideal material for producing medical implants. However, due to its relatively low mechanical strength, the application of CP-Ti in the biomedical area has been much limited. With the aim to 0.5 µm significantly improve the strength of CP-Ti, SINTEF Materials and Chemistry in collaboration with NTNU, Department of Materials Science and Engineering, has started a research project funded by the Research Council, to produce UFG CP-Ti by ECAP processing. Within this project, TEM characterization of the nano structures including high angle grain boundaries (HAGB), low angle grain boundaries (LAGB), twin boundaries, stacking faults and dislocations in CP-Ti after ECAP is a very important part. TEM characterization is crucial in 5 nm order to understand the formation mechanism of the nano structures and to optimise the processing parameters of ECAP. Contact persons: John Walmsley (john.walmsley@sintef.no), Yanjun Li (yanjun.li@sintef.no) and Hans Jørgen Roven (hans.roven@material.ntnu.no),