Soft-landing electrospray ion beam deposition of sensitive oligoynes on surfaces in vacuum

Characterizing the complex structure of such molecules with highly resolving, vacuum-based methods like scanning probe microscopy requires their transfer into the gas phase and further onto an atomically clean surface in ultrahigh vacuum without causing additional contamination. Conventionally this is done via sublimation in vacuum. However, similar to biological molecules, large synthetic compounds can be non-volatile and decompose upon heating. Soft-landing ion beam deposition using soft ionization methods represents an alternative approach to vacuum deposition. Using different oligoyne derivatives of the form of R-1-(C equivalent to C)(n)-R-2, here we demonstrate that even sensitive molecules can be handled by soft-landing electrospray ion beam deposition. We generate intact molecular ions as well as fragment ions with intact hexayne parts and deposit them on clean metal surfaces. Scanning tunneling microscopy shows that the reactive hexayne segments of the molecules of six conjugated triple bonds are intact. The molecules agglomerate into ribbon-like islands, whose internal structure can be steered by the choice of the substituents. Our results suggest the use of ion beam deposition to arrange reactive precursors for subsequent polymerization reactions. (C) 2014 The Authors. Published by Elsevier B.V.

Electronic and magnetic interactions in single molecular and atomic contacts

Verena Katharina Schendel

This thesis encompasses an investigation of organic molecules on metallic substrates as well as a study of electronic and magnetic effects in atomic-sized palladium contacts. For this, a scanning tunneling microscope, operating at 6 K and ultra-high vacuum is used. The clean environment allows characterization of single molecules and atoms under well-defined conditions. Moreover, by examining at low-temperatures it is possible to study electronic and magnetic effects spectroscopically, which would otherwise be thermally obscured. The first part of this thesis focuses on studying organic molecules absorbed on metallic substrates. The molecules under investigation are thiophene derivatives of the archetypical organic semiconductor pentacene. The thiophene derivatives, tetracenothiophene (TCT) and anthradithiophene (ADT), contain either one or two thiophene groups at the terminal benzene rings. The dependence of the deposition temperature as well as the impact of different metallic substrates on their adsorption behavior is studied. When depositing the molecules on a Au substrate at room temperature, the molecules stay fully intact. In contrast, when deposited onto a Cu substrate, a breaking of the thiophene entity is observed. The breaking can be prevented by cooling the substrate to 200 K during deposition. Conductance measurements on the TCT molecules with an intact and broken thiophene entity reveal differences in their transport characteristics. The intact ones exhibit lower conductances than the broken ones. This can essentially be attributed to the fact that the latter ones bind covalently to the substrate, which facilitates transport. ADT deposited on Cu(111) can be reversibly and repeatedly interconverted between two distinct adsorption congurations associated with different conductances, and thus has been investigated as a molecular switch. The switching mechanism is related to a change between two different bonding configurations of the molecule to the substrate. The non-switched state ("off") can be ascribed to a strongly bound state and the switched state ("on") to a weaker bound configuration. The switching process is not only restricted to the location beneath the tip but also can be initiated within an extended area of about 100 nm in radius. This long-ranged addressing is enabled by means of surface state carriers of the Cu(111) surface. The non-local switching process is fully reversible, isomer selective and efficient over radial distances of about 100 nm. The second part of this work examines atomic-sized palladium (Pd) contacts. Pd exhibits outstanding properties owing to its high density of states (DOS) at the Fermi level. This renders Pd a "nearly ferromagnetic" material. Upon size confinement the DOS can be pushed to higher values and Pd can develop a net magnetization. Transport measurements were carried out for two distinct contact geometries; a tip-adatom and and a tip-surface contact geometry. Whereas for the tip-adatom contacts no magnetic state can be detected, measurements conducted for the tip-surface geometry might indicate the presence of a magnetic moment. The differences are attributed to the adsorption configuration of the single bridging atom in the junction. Spectra taken in point-contact with an adatom reveal a distinct inverted V-shaped feature. Presently we attribute this feature to paramagnons.

EPFL2015

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

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

Nicha Thontasen

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.

EPFL2011