Ion Beam Deposition and STM Analysis of Macromolecules on Solid Surfaces in UHV

Organic and biological macromolecules have attracted great interest within nanotechnology research due to properties that can be useful in future devices. Interesting functionalities such as optical activity, host-guest binding and molecular recognition are based on effects at the single molecular level. To study the properties and functions of molecules at submolecular scale, an atomically defined environment with low contamination is of great advantage. This can for instance be provided on a clean surface in ultrahigh vacuum (UHV), where surface sensitive analytical techniques and advanced nanostructure fabrication can be applied. Atoms and small organic molecules are easily transferred to a surface in vacuum by thermal sublimation. Many large, functional molecules, however, are nonvolatile and disintegrate at an elevated temperature, which hinders their application and investigation within the UHV environment. To explore the properties of nonvolatile molecules adsorbed at surface as well as the nanostructure fabricated from such molecules, electrospray ion beam deposition (ES-IBD), a technique capable of transferring nonvolatile molecules intact to surfaces in UHV, and in situ scanning tunneling microscopy (STM) are combined. A home-built ES-IBD setup allows molecular beam deposition with an unprecedented level of control: the ion beam composition is monitored by time-of-flight mass spectrometry (TOF-MS) prior to the deposition, molecular flux and coverage controlled via the ion beam current and the beam energy can be measured and adjusted. The modified surfaces are characterized in situ by STM, which reveals structural and electronic properties with submolecular resolution. Moreover, ex situ analysis by atomic force microscopy (AFM) allows to access meso-scale structural formation. In addition, Laser Desorption Ionization (LDI), Matrix-Assisted Laser Desorption Ionization (MALDI) and Secondary Ion Mass Spectrometry (SIMS) are performed to check the chemical composition and purity of the adsorbed materials. This work first introduces the principles of our experiments and reviews of the literature on molecular ion beam deposition. In the following, the experimental method is characterized in detail using Rhodamine 6G, an organic dye, as a model system. It is shown that homogeneous layers of adsorbed, intact molecules are formed on the surface and single molecule adsorbates are observed by STM. An investigation by SIMS and LDI reveals that the kinetic energy of the molecular ions upon collision with the surface determines whether intact or fragment ions are deposited. Molecular layer growth was studied using cluster ion beams of the nonvolatile surfactant SDS, resulting in an arrangement of the molecules typical for surfactants in a head-to-head and tail-to-tail manner based on the structural motive. Crystalline structures are observed by AFM resembling inverted bilayer-membranes formed on graphite and SiOx surfaces. A similar structural feature revealed by STM is a flat-laying double row of the molecules on Cu(100) surface showing the robustness of the structural motive. The properties of single molecular functional systems are studied on two types of large functional molecules: macrocycle complexes and proteins. Macrocyclic host-guest complexes of the three cations: Na+, Cs+ and H+ trapped in the cavity of dibenzo-24-crown-8 (DB24C8) are formed in the electrospray solutions and transferred intact to the surface. Characterization by STM and density functional theory calculations shows the structure of the complexes with the alkali ion present in the cavity. The shape of the complex at the surface is also determined. Cytochrome c, an electron transfer protein is deposited on surfaces in UHV as an ion beam of high or low charge-state. STM reveals unfolded strands with distinguishable submolecular structural features after the deposition of high charge states. When low charge states are deposited, two significantly different structures are observed. Folded proteins appear as globular features of 2-3 nm diameter, while unfolded proteins are imaged as at strands. On Au(111) the unfolded strands fold into compact two-dimensional patches. Additionally, the variation in kinetic energy upon deposition and elevated surface temperature are also studied regarding the change in conformation of the adsorbed proteins. The results of this thesis open up new opportunities for the creation of functional nanostructures on surfaces. Both the preparation of layers and thin films as well as the controlled immobilization of single functional molecules were demonstrated with ES-IBD and characterized at submolecular resolution in situ by STM. A great variety of nonvolatile substances from small to large functional molecules with different binding motives like covalent, complex coordination or hydrogen bonds, is compatible with ESI and thus can be used as building blocks to construct new materials in a well-controlled manner.

Electrospray Ion Beam Deposition of Complex Non-Volatile Molecules

Gordon Rinke

The rational synthesis of novel materials requires the control over the arrangement of matter in order to meet the desired properties for applications and devices. Ultimately, control means to define the place of each atom and determine its chemical state, as well as being able to confirm the result in a measurement. Control at the atomic level can be achieved with scanning tunneling microscopy (STM), both in imaging and atomic manipulation. The latter, however, can never be used in meso- or even macroscopic material synthesis. Self-ordering phenomena determines atomic control in material synthesis as well, observed for instance in protein folding or in molecular beam epitaxy (MBE) technology. In both cases the atomically determined arrangement of the atoms in the structure is controlled only by environmental parameters that steer the self-ordering phenomena. In this thesis I show that electrospray ion beam deposition (ES-IBD) or ion soft-landing is a material processing technique, which combines and even extends the level of control offered by MBE. It is demonstrated that a vast range of complex organic materials including peptides and proteins is now available to ultrapure vacuum processing and moreover completely novel schemes of controlling self-ordering processes are provided. In-situ STM is applied to investigate the structure formation of complex molecules deposited by ES-IBD on well-defined, clean, and atomically flat surfaces in ultrahigh vacuum at the greatest detail. This allows us to explore the novel control features that are intrinsic to the ES-IBD deposition process like online coverage monitoring, deposition energy control, and mass-selection to select charge states or reactive species. The molecules studied here include organic molecular salts, dye molecules, reactive polymer building blocks, and finally peptides and proteins. Crystalline layer-by-layer as well as island growth was observed and revealed the equivalence to conventional MBE growth. On the basis of these deposition experiments the crucial influence of clusters in the ion beam for high material flux was discovered. Furthermore, it was found that the chemical state of the molecule is a key factor for the deposition result as chemical reactions can be induced or the self-assembly behavior can be altered. Alongside the capability to handle also extremely reactive molecules, the feasibility to control chemical reactions occurring as a result of an external stimulus to a specifically selected reactive species is demonstrated. Employing the control parameters that are specific to ES-IBD like charge state selection and deposition energy to complex biomolecules opens up new perspectives for vacuum deposition. In addition to the self-assembly governed by the molecule-substrate and molecule-molecule interaction, it was possible to actively steer the structure formation of proteins by influencing their stiffness and their reactivity through their charge state as well as by the deposition energy, parameters intrinsic to the ES-IBD method. Hence, electrospray ion beam deposition is indeed a tool to prepare well defined surface coatings at highest level of control and complexity. To be relevant for industrial applications, the performance of the system has to be further improved, most importantly in terms of deposition rate. In general, ES-IBD enables new approaches for the growth of functional coatings but also serving as perfect sample preparation tool for the characterization of complex molecules on the atomic scale by scanning probe or more general surface science methods.

EPFL2013

Single-molecule-level characterization of supramolecular systems at surfaces

Dietmar Payer

This thesis deals with the self-organization of individual building blocks, specifically organic molecules and metal atoms, into supramolecular structures on metal surfaces under ultra high vacuum conditions (UHV). Self-organization of supramolecular systems is a promising candidate for a bottom-up approach in the fabrication of complex structures with specific properties. This process is steered by interactions between functional groups of the individual building blocks and is of apparent interest to gain a detailed understanding of the influence and the behavior of these functional groups enabling intermolecular binding. In the first part of the thesis, we investigate hydrogen bonded structures on metal surfaces. Of especial interest is the possibility of the formation of ionic hydrogen bonds, a special class of hydrogen bond which connects a charged donor (acceptor) and a neutral acceptor (donor) and is important in biological systems. By combined Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) measurement we show the deprotonation of carboxylic functions in the molecules, forming carboxylate groups involved in intermolecular binding. With our complementary experimental and theoretical examinations we clearly show that the carboxylate groups are indeed still charged although adsorbed onto a conducting metal surface and that the observed structures can be computationally reproduced by using these charges in classical molecular dynamic calculations. Our investigations clearly reveal that all requirements for the formation of ionic hydrogen bonds in these samples at metal surfaces under solvent free conditions are fulfilled. In the second part, the confinement of surface state electrons in self-assembled hydrogen-bonded structures is investigated. The examined structure contains two-dimensional cavities, with different inner diameters in the periodic arrangement and in domain boundaries. Inside the cavities we detect a spatially localized modification in the local density of states (LDOS) of the Ag(111) surface state electrons. The energy of the peak correlates with the cavity area as predicted by a simple "particle in a box" model. This clearly shows that the surface state electrons form a confined state inside the cavities. Furthermore we examine self-assembled periodic structures as templates for binding single macrocyclic molecules. The macrocyclic molecules can be deposited by sublimation, as STM-topographs with submolecular resolution reflect the shape of the intact molecules and reveal a coverage dependent aggregation behavior. We show that it is possible to fabricate a structure in which at most one macrocyclic molecule per pore can be found in a defined binding position. In the last part of the thesis we investigate the influence of individual metal atoms on the chemical activity and the self-assembly process of molecules on metal surfaces. First, we demonstrate the high chemical reactivity of these metal atoms. In our experiments we address the chemical reactivity of static active sites, like kinks and step edges, and of dynamic active sites, like the metal atoms, separately. Doing this we show that a non-negligible amount of the chemical reactivity of a metal surface is due to this dynamic adatom gas. Furthermore the influence of metal atoms on the structure formation of organic molecules is investigated. We show that two-dimensional structures based on a complex formation between metal centers and ligands can be formed and that the coordination number is independent of the substrate symmetry. Furthermore, we show the possibility that on metal surfaces one can prepare three-fold coordinated Fe-centers, a coordination number which is rarely seen in three dimensional chemistry. In the last part we study catenanes, which consist of two interlocked macrocyclic molecules. Main interest is on the confirmation of a structural change of the catenanes adsorbed on a metal substrate. For our experiments we use the fact that the catenanes are designed to "trap" a Cu-atom by complex formation. As this reaction is associated with a structural change and a change in intermolecular interaction, we can verify a reaction of catenanes adsorbed onto a surface with Cu adatoms and therefore a structural alteration by simply imaging the formed structures. Our measurements show the possibility of depositing large molecules like catenanes by sublimation and of their potential for use as molecular machines adsorbed onto metal surfaces.

EPFL2007

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.

EPFL2005