Stephan Rauschenbach
The vacuum deposition of complex functional molecules and nanoparticles by thermal sublimation is often hindered due to their extremely low vapor pressure. This especially impedes the application of ultrahigh vacuum (UHV) based analytical and surface modification techniques for the investigation of these extremely interesting systems. On the other hand, specimen prepared under ambient conditions or in solution are typically not sufficiently well-defined and clean to allow a thorough and precise characterization. In order to bridge this technological gap, a novel ion beam source for controlled soft landing deposition in ultrahigh vacuum is constructed. The ion beam of nonvolatile particles is created by electrospray ionization (ESI). The deposition apparatus consists of six differential pumping stages designed to overcome the pressure difference of 13 orders of magnitude between the ambient pressure side, where ionization occurs, and the high or ultrahigh vacuum, where the deposition takes place. A variety of ion optical devices is employed to form, mass select and guide the ion beam through the pumping stages onto the deposition target. The ion beam is sampled from a supersonic expansion by a skimmer, collimated in a high pressure quadrupole ion guide, mass selected in a low pressure quadrupole ion guide and focused by electrostatic lenses. In order to have full control over all relevant parameters, the ion beam is characterized before the deposition by a linear time-of-flight mass spectrometer and a retarding grid energy detector. The flux, the composition and the kinetic energy of the ion beam can thus be measured and adjusted. The concept of ion beam deposition in high and ultrahigh vacuum is demonstrated by extensive mass spectrometric and deposition experiments. Many different types of ion beams, for instance composed of organic molecules, organic and inorganic ionically bound clusters, polymers and proteins, are created by ESI. Their properties are analyzed by mass spectrometry, with special focus on their behavior upon energetic collisions with a neutral gas, since these processes bear many similarities to collisions with a solid surface. Some of the ion beams are used for deposition. Ion beams of the protein BSA , of the dye molecule Rhodamine 6G (Rho6G), of organic ionic surfactant clusters composed of sodium-dodecyl sulfate (SDS) and of inorganic nanoparticles (gold colloids, carbon nanotubes, CdS nanorods and V2O5 nanowires), are deposited onto graphite and silicon oxide (SiOx) surfaces in high vacuum. The fluorescence of Rho6G is detected after its deposition, which is a proof for the destruction-free ion beam deposition, i.e. of a successful soft landing. For the other classes of deposited particles, diffusion on the surface and sometimes formation of nanostructures is observed. BSA forms fractal agglomerations on graphite, while it does not show any diffusion on SiOx surfaces. SDS forms flat, two dimensional islands on graphite and silicon. Finally it is demonstrated that large, inorganic nanoparticles (up to 106 µ) can be ionized and soft landed by the developed apparatus. Having proven the principle of low energy ion beam deposition for a wide variety of nonvolatile particles, the technique is now ready for being integrated with in-situ characterization techniques such as scanning tunneling and atomic force microscopy (STM, AFM). For this purpose, the ion beam deposition setup has been expanded by two vacuum chambers for sample preparation and analysis. Future experiments aim at the deposition and analysis of complex organic molecules in UHV, and at gaining a more detailed understanding of the soft landing process.
EPFL2008
