An Electrospray Ion Source for Ultra-High Vacuum Deposition of Organic Molecules

Field: Chemistry (Physical Sciences)

During the course of this thesis a new instrument for the deposition of ions, produced by electrospray ionization, under ultra–high vacuum conditions was designed, assembled, and put into operation. The deposition techniques available under ultra–high vacuum conditions can usually not be applied to molecules with low vapour pressures, because most of these techniques are based on sublimation. The preparation techniques available under ambient conditions often do not come into consideration because of their lower degree of purity. Electrospray ionization is one possibility to transfer these molecules gently into the gas phase. Because the ions are created under ambient conditions in this process, a pressure difference of twelve orders of magnitude has to be bridged. For that reason five differentially pumped vacuum stages were set up. Several ion optical components guide the ion beam through these stages and ensure its purification. A quadrupole deflector separates the ions from neutral contaminations, originating from the ion source, and confines the ions’ kinetic energy. A quadrupole mass filter is used for mass selection during the deposition process. One main focus of the development was the design of an ion funnel, extending over two differentially pumped stages and achieving transmission of roughly ten percent. This ion funnel exhibits a higher efficiency of the differentially pumping than previous reported designs, because the inner diameter of its electrodes is smaller. The transmission properties of this funnel were investigated by detailed simulations. In parallel to the construction of the deposition instrument a low–temperature scanning tunnelling and atomic force microscope was purchased, which was adapted for the assembly with the ion source in cooperation with the manufacturer SPS CreaTec GmbH. The ion deposition is performed in the preparation chamber of the scanning tunnelling microscope system at a pressure of 1 × 10−9 mbar, ion currents up to 100 pA and impact energies below 10 eV. The apparatus is state of the art in these key features. Coverages of some ten percent of a monolayer–well suited for investigation by scanning tunnelling microscopy–can be prepared within a few hours. The prepared sample can be analysed without ever leaving the ultra–high vacuum. A number of different substances, most of them thermally unstable, were deposited on the surfaces of noble metal single crystals and studied by scanning tunnelling microscopy to prove the capabilities of the system. Investigations on the porphyrin MnTPPS show that purities of the molecular layers comparable to sublimation and pulsed injection can be achieved. The applicability on large fragile biomolecules was verified by experiments on deoxyribonucleic acid and bovine serum albumin. Experimenst on the ligands of metal–organic grid complexes confirm the suitability for small organic molecules. Ruthenium complexes, such as the investigated N3, are of great technological interest, due to their utilisation in dye–sensitized solar cell. To the best of our knowledge we present the first spectroscopic investigations on individual complexes of this kind.