This dissertation focuses on the development of catalytic soft and probe lithography for nanofabrication on reactive self-assembled monolayers (SAMs). A general introduction was given in Chapter 1, which highlights the importance of investigating new patterning tools based on soft lithography and scanning probe microscopy. These techniques allow the generation of nanometer-size patterns with wide materials versatility. Finally, it gives a brief outline of the topics presented in the following chapters.
In Chapter 2, a literature overview on chemical transformation of functional groups of SAMs and monolayer-protected gold nanoparticles was given as an introduction to the work presented in Chapters 3 and 4. A short review on microcontact printing (µCP) was presented as a preface to Chapters 5, 6, and 7. Moreover, the use of scanning probe lithography for nanofabrication was covered as an introduction to Chapter 8.
In Chapter 3, the use of thioethers as ligands for the preparation of monolayer-protected gold nanoparticles (MPCs) has been presented. The obtained gold colloids were characterized by TEM and 1H NMR. The weak interaction between gold and thioethers leads to decreased stability of these colloids as compared to thiol-protected counterparts. However, when thioether ligands with multiple attachment points were used, for example a calixarene tetrathioether derivative, the obtained MPCs were as stable as thiol-protected gold colloids, thus widening the scope of ligands that can be applied in MPC preparation.
Chapter 4 describes the in-plane polymerization of SAMs of norbornenethiol and -thioether derivatives. MPCs were used as a model system for easy characterization of the reactivity of surface-bound reactants. By using the Grubbs’ catalyst, metathesis polymerization on gold nanoparticles has confirmed that the norbornene moieties maintained their activity when immobilized. The in-plane metathesis polymerization of the adsorbate on flat gold did not cause a change in thickness or order, as confirmed by electrochemistry. Electrochemical impedance spectroscopy showed that the layers became more resistant towards desorption owing to the formation of multiple attachment points and reduced solubility of the polymer as compared to the monomer.
Chapter 5 describes the selective insertion of single molecules in patterned SAMs. Coordination via palladium-phosphine interactions has been used to successfully grow single, isolated molecules into nanometer-sized objects, which can be detected by atomic force microscopy (AFM) by the attachment of the phosphine-functionalized MPCs to inserted Pd pincer molecules. Selective insertion of the pincer molecules has been obtained by using micropatterned substrates with varying chain length thiols prepared by microcontact printing. The gold nanoparticles only appeared in areas of C16SH or shorter, but not in areas of C18SH.
Patterning SAMs by diffusion-less microcontact printing was described in Chapter 6. Two approaches were presented for the catalytic preparation of patterned surfaces. In the first approach, microcontact printing of a catalytically active species has for the first time been performed on a preformed, densely packed, and chemically active trimethyl silyl ether (TMS) SAM. The sulfonic acid catalyst was immobilized on colloidal gold and used as ink in mCP. By mCP, the colloid-bound catalyst was transferred to a TMS adsorbate SAM causing hydrolysis of the TMS adsorbate in the contacted areas, thus providing patterns of different functional groups. In the second approach, direct catalytic µCP by has been performed for the first time without transfer of ink. Oxidized PDMS stamps were used for catalytic patterning by taking advantage of the acidity of the silicon oxide on the PDMS stamps. Catalysis took place in the contacted areas on preformed TMS and TBDMS SAMs. Unlike traditional µCP, which relies on the ink transfer from the stamp to the substrate, this process does not involve ink transfer, thus the pattern creation process is diffusion-less. Although a relatively long contact time was needed for pattern creation, submicron edge resolution was obtained.
In Chapter 7, various methods have been described for functionalization of micropatterned substrates. Several micropatterned self-assembled monolayers on gold have been prepared by microcontact printing: OH/CH3, OH/TMS, and OH/TBDMS. Another micropatterned substrate (hydrolyzed-TBDMS/TBDMS) was prepared by catalytic microcontact printing of the sulfonic acid-functionalized MPCs on the TBDMS SAM as described in Chapter 6. By physisorption of amino-functionalized polystyrene microbeads, both OH/TMS, OH/TBDMS, and hydrolyzed-TBDMS/TBDMS gave good pattern enhancements preferentially on the OH areas. For chemical functionalization, the results depended on the stability of the functional groups. For the OH/CH3 SAMs, a lot of reactions gave enhancement effects, such as surface-initiated polymerization and reaction with isocyanate derivative PDI. On the OH/TMS SAM, however, even under very mild conditions, reactions took place in all areas leading to the loss of the original patterns probably because of removal of TMS groups under these conditions. The OH/TBDMS substrates retained their patterns after reaction with PDI and hexylamine, indicating that this reaction could be potentially used for functionalization of patterns created by catalytic probe lithography. An insertion approach for pattern functionalization has been performed on the hydrolyzed-TBDMS/TBDMS substrate. Preferential insertion of a dendritic wedge with a thioether moiety was found in the hydrolyzed-TBDMS areas showing promise in further functionalizing these patterns.
In Chapter 8, catalytic probe lithography is described for nanofabrication at self-assembled monolayers. The same catalyst that has been used for the preparation of sulfonic acid-funtionalized MPCs as described in Chapter 6 was used to modify gold-coated AFM tips. Catalytic probe lithography was carried out by scanning TBDMS SAMs with catalytically active tips. The tips induced local hydrolysis of the silyl ether moieties in the scanned areas, thus creating patterned surfaces. The areas where reaction took place showed higher friction. This is in good agreement with a detailed friction study, thus confirming that the pattern formation was caused by tip-induced hydrolysis. The thinnest lines created so far are in the range of 28-35 nm, comparable to the resolution of existing techniques, such as dip-pen nanolithography.
The results presented in this thesis have shown that catalytic soft lithography and catalytic probe lithography are new patterning tools for nanofabrication at self-assembled monolayers. Monolayer-protected gold nanoclusters have been used as model systems for studying both immobilized catalyst and immobilized reactants. Catalytic microcontact printing of the colloid-bound catalyst on the reactive SAMs is shown to be the closest model that resembles the situation of functionalized probes for pattern creation. With the successful application of the reaction systems in catalytic probe lithography, it is anticipated that more versatile systems will be developed.