MESA+ Institute for Nanotechnology

The central topic of this thesis is the deposition of metals on Self-Assembled Monolayers (SAMs). Metals are deposited in the form of submicron scale islands, nanometer scale clusters, and as supramolecular, organometallic coordination cages. Several SAMs on various substrates were prepared and analyzed for their quality and usefulness as metal deposition platform. Pulsed laser Deposition (PLD) is used as metal deposition technique in combination with nanosieve shadow masks for patterning. The combination can be used to deposit metal on organic SAMs in submicron patterns without damaging the SAM. Small clusters could be deposited on SAMs using PLD. The clusters were proven to be insulated from the substrate by the SAM using scanning tunneling microcopy (STM) and conducting probe atomic force microscopy (CP-AFM). Thin patterns of Pt, Pd and Au could be enhanced by electroless deposition (ELD) of Cu. Large cavitands, with thioether functionalities could be assembled on surfaces and analyzed as single molecules. Via organometallic coordination cage complexes could be assembled and disassembled on a single molecule scale, and monitored by atomic force microscopy (AFM).

In Chapter 2 a literature overview of the field of molecular electronics is given. The focus is on the use of SAMs in electronic devices and the electronic analysis of SAMs and surface-immobilized single molecules. The most important device architectures and measurement setups are discussed. Molecular electronic devices like SAM field-effect transistors (SAMFETs), diodes, switches and capacitors are described. Most recently the focus is on correct and reproducible measurements of electronic properties of molecules,
i.e. a form of ‘benchmarking’ of the field of molecular electronics.

In Chapter 3 the preparation and analysis of SAMs made from difunctional molecules is described. SAMs of 1,9-nonanedithiol (NDT) seem suitable for the incorporation into nanoscale electronic devices. XPS, electrochemistry (cyclic voltammetry and electrochemical impedance spectroscopy) and contact angle measurements indicate that the molecules are in an upright position on the surface and expose a free thiol group. 1,16-Hexadecanedithiol did not form ordered SAMs and most molecules were bound to the surface with both thiol groups. Biphenyldithiol and 1,4-di(phenylethynyl-4’-thioacetyl)benzene formed SAMs with a lower order although the molecules were only bound with one S-group to the surface. The insertion of conjugated molecules into decanethiol (DT) SAMs was investigated.

Chapter 4 describes the preparation of Au-SAM-Au sandwiches by Pulsed Laser Deposition (PLD) of Au through silicon nitride nanosieves on SAMs. Patterns of islands with submicron diameters could be made over areas of several square millimeters. Electrochemical Cu-deposition showed that on octadecanethiol (ODT) SAMs, roughly 15 % of these islands deposited at a pressure of 0.01 mbar are electrically insulated from the gold surface. If lower deposition pressures were used no insulated islands were obtained. On the thinner NDT SAMs no insulated islands could be obtained irrespective of what deposition pressure was used.

In Chapter 5 the deposition with PLD of nm size Au, Pd and Pt clusters is described. Clusters, with sizes depending on the deposition conditions were prepared on carbon membranes and analyzed with transmission electron microscopy (TEM). These clusters were also deposited on SAMs. Pd clusters were deposited on top of DT SAMs and it was shown with scanning tunneling microscopy (STM) that these particles were insulated from the Au substrate. A Coulomb blockade was observed for this system even at room temperature. Conducting probe atomic force microscopy (CP-AFM) also showed that gold clusters deposited on ODT and DT SAMs were insulated from the Au substrate by the SAM. The bias necessary to visualize the clusters in the current ( I ) images was higher for the thicker ODT SAM than for the DT SAM.

Chapter 6 shows AFM analyses of islands made from various metals, deposited on various SAMs on Au and SiO
2. The versatility of the PLD technique in combination with nanosieves and SAMs is demonstrated. The use of patterns of deposited metal clusters made by PLD through nanosieves and microsieves on SAMs for pattern replication by electroless deposition (ELD) of Cu is described. Patterns that were made using short PLD can be selectively enhanced with Cu. This method reduces the potentially damaging gas phase deposition step.

In Chapter 7 large cavitand molecules and Pd-containing cage complexes that are functionalized with thioether groups are inserted into mercaptoundecanol (MU) SAMs on gold. The inserted molecules and complexes can be detected by AFM as they protrude from the SAM. Inserted cage complexes can be disassembled using triethylamine as competing ligand for the Pd centers. Cages in solution can exchange with single surface bound cavitands forming a new “mixed” cage on the surface.

The results of the work described in this thesis show that PLD can be a valuable technique that can be used for the fabrication of metal-molecule contacts on SAMs. For larger electrode sizes, nanosieves can be used for patterning the deposited metal, while short direct deposition yields insulated clusters with nanometer sizes on SAMs. The preparation of metal-SAM-metal junctions with different dimensions might be useful in the electronic analysis of SAMs and single molecules and in the fabrication of small electronic devices. Single molecules that can be assembled and disassembled on a surface shows a potential data storage system with nanometer size bits, that can be written, read and erased by reversible supramolecular processes.