Atom-by-atom analysis of small nanoparticles using high-field methods

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Nanoparticles are among the most researched topics in current materials science due to their unique properties. In this project, we aim to make atomic resolution, single atom chemical analysis available for particles smaller than 10 nm and use it to answer fundamental questions regarding elemental distributions and structure in a size range where many particle systems exhibit their most outstanding performance.Chemical analysis of these very small particles is currently challenging, especially when information on a single atom basis is required. Atom probe tomography could be used to perform single atom analysis of these particles if they can be field evaporated in a controlled manner. Attempts preparing field emitters from particles using deposition in ambient conditions have been successful for particles > 10 nm but limited in data quality and throughput. Using UHV deposition, imaging of Au clusters down to a size of 1 nm was already successful 20 years ago, but limited to pure Au clusters produced inside the system.In this project, we will combine high field methods, in particular field-electron (FEM) and field-ion microscopy (FIM) with atom probe tomography to investigate nanoparticles smaller than 10 nm and reveal their chemical makeup. This will be done by constructing a novel analysis setup where FEM/FIM are combined with an ultra-high vacuum (UHV) electrospray injection device. This allows for the ultra-clean deposition of particles onto a UHV clean metal tip substrate, required for high quality single atom analysis of nanoparticles. The process will be precisely controlled by the observation of the arrival of single particles on the tip via FEM/FIM. This allows for structural analysis of the particle using the field ion microscope. We will also assess the projection laws and evaporation sequence during the field evaporation of the particles both theoretically through simulations and experimentally to lay the groundwork for precise 3D atom probe analysis. We will then transfer the so-fashioned particle field emitter tips into a dedicated atom probe, where we will analyse the distribution of the chemical elements within particles with single atom sensitivity and sub-nm 3D resolution. This technique will then be applied to the analysis of metallic core-shell nanoparticles for catalysis, doped semiconductor nanoparticles for solar energy conversion and intermediates of nonclassical crystallization processes obtained by a crystallization processes mimicking biomineralization in vivo.In the second phase of the project, we will focus on some of the fundamental questions surrounding nanoparticle synthesis: the early stages of nucleation and the role of internal interfaces. Atom probe measurements will be used to distinguish agglomeration from Ostwald ripening in a variety of systems used in catalysis and to assess the extent of interfacial/surface segregation of deliberately added elements as well as synthesis residues.