Fabrication and Characterisation of SAMs for Spin Crossover and Photocleavable Surfaces

This thesis investigates self-assembled monolayers (SAMs) of molecules of various complexity with a special focus on fabrication of surfaces that could exhibit an intrinsic active function, more precisely a capability to switch between two stable states upon external stimuli (spin crossover phenomenon) or to expose functional groups upon irradiation with light, i.e. photocleavage of a SAM. SAM formation of a complex novel molecule, lipoic acid ester of α-hydroxy-1-acetyl-pyrene (reagent 1), was studied in chapter 3. It contains i) a dithiolane headgroup capable of binding to gold surfaces via two sulfur atoms, and ii) a chromophore that makes it light sensitive (photocleavable) and leads to a deprotection of lipoic acid molecule upon exposure to soft UV (365nm). Reagent 1 successfully forms SAM1, but it interacts with the gold surface weaker than conventional thiol based SAMs, due to cross-linking of dithiolane headgroups. Nevertheless, SAM1 is a relatively stable monolayer and exerts a higher barrier against diffusion of copper ions towards gold surface in electrochemical deposition than SAMs of similarly complex molecules but with thiol headgroups or shorter alkanethiol SAMs terminated with carboxylic acid groups (COOH). SAM1 undergoes photolysis upon soft UV (365nm) irradiation, but only in the acidic catalyst 100mM HCl in isopropanol (IPA). Unexpectedly, this also leads to a removal of the resulting lipoic acid monolayer, thus ultimately leading to SAM1 with lower surface coverage, and changes packing and ordering. SAMs of alkanethiols terminated with COOH and of varying chain length were investigated in order to better understand the cause for the instability of lipoic acid monolayer. Loss of molecules from the surfaces was found to be a common issue in the COOH SAMs, however, the severity of the loss is strongly related to the initial SAM thickness. Thin SAMs like DTBA SAM yield huge (∼50%) loss, while thick SAMs like MUA SAM show no detectable loss. On the other hand, photo-patterned SAM1 produces especially high selectivity of Cu deposition between UV treated and non-treated regions, which is associated to this loss of surface coverage, packing and ordering. Surprisingly, reagent 1 also interacts with glass and silicon oxide surfaces to form hydrophobic films that exhibit green fluorescence under soft UV light. Such surfaces are photo-sensitive and can be photo-patterned in the air or in the acidic catalyst to produce non-fluorescent hydrophilic regions due to photo-bleaching of pyrene groups and photo-deprotection of lipoic acid molecules, respectively. The films and patterns stored under ambient conditions are detectable for at least 35 days. Formation of those films is associated with an interaction of pyrene group with adsorbates on the surfaces, while film growth is attributed to the cross-linking of dithiolane headgroups. SAM fabrication of metal complexes was explored in chapter 4. Two novel ligands L1 and L2, and their corresponding Fe(II) complexes C1 and C2, which can exhibit spin crossover (SCO) behaviour in bulk, were investigated. Both ligands successfully form SAMs. However, SAM L1 does not coordinate Fe(II), while its preformed complex C1 is not stable on Au surface and forms SAM L1 instead of SAM C1. In contrast, SAM L2 coordinates Fe(II) at nearly 100% yield, which leads to almost the same chemical composition as in a SAM of its preformed Fe(II) complex C2 (SAM C2). Although complex C2 with MeCN as the sixth exogenous ligand (SEL) exhibits low spin (LS) state in bulk at room temperature, only high spin (HS) state was detected in SAM C2. Complex C2 exhibits a unique property of changing its spins state in certain solvents, because a solvent molecule can easily displace the sixth exogenous ligand (SEL). However, rinsing SAM C2 with such solvents did not lead to a spin transition, and LS state was never observed for the SAM. This implies that the strength of the ligand field may need to be increased or SEL with a higher affinity coordinated to complex C2, in order to change the spin state by rinsing or to detect SCO in SAM C2. A long-chain alkanethiololigoethyleneglycol (LCAT-OEG) type molecule terminated with the azide group (reagent 2) was investigated for the facilitation of click chemistry on gold surfaces (chapter 5). Reagent 2 forms a good quality SAM (SAM2), and the concentration of reagent 2 in the SAM can be reduced in a controlled and predictable manner by the addition of LCAT-OEG-1 or LCAT-OEG-4 to the growth solution. QCMD measurements indicate that the whole surface of SAM2 successfully undergoes click reaction with cycloalkyne in aqueous solution without any catalyst. Finally, simple alkanethiol and aromatic type SAMs were investigated for use in surface-enhanced Raman spectroscopy (SERS) with nanoparticle-on-mirror configuration and for studying plasmonic systems, due to their ability to yield optimum precision over the control of thickness and dielectric function (chapter 5)

Generalized circuit model for coupled plasmonic systems.

We develop an analytic circuit model for coupled plasmonic dimers separated by small gaps that provides a complete account of the optical resonance wavelength. Using a suitable equivalent circuit, it shows how partially conducting links can be treated and provides quantitative agreement with both experiment and full electromagnetic simulations. The model highlights how in the conducting regime, the kinetic inductance of the linkers set the spectral blue-shifts of the coupled plasmon

Nanooptics of molecular-shunted plasmonic nanojunctions.

Gold nanoparticles are separated above a planar gold film by 1.1 nm thick self-assembled molecular monolayers of different conductivities. Incremental replacement of the nonconductive molecules with a chemically equivalent conductive version differing by only one atom produces a strong 50 nm blue-shift of the coupled plasmon. With modeling this gives a conductance of 0.17G(0) per biphenyl-4,4'-dithiol molecule and a total conductance across the plasmonic junction of 30G(0). Our approach provides a reliable tool quantifying the number of molecules in each plasmonic hotspot, here <200.We acknowledge financial support from EPSRC grant EP/ G060649/1, EP/I012060/1, EP/L027151/1, EP/K028510/1, ERC grant LINASS 320503. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. C.T. and J.A. acknowledge financial support from Project FIS2013- 41184-P from MINECO, ETORTEK 2014-15 of the Basque Department of Industry and IT756-13 from the Basque consolidated groups.This paper was originally published in Nano Letters under a CC-BY licence (F Benz, C Tserkezis, LO Herrmann, B de Nijs, A Sanders, DO Sigle, L Pukenas, SD Evans, J Aizpurua, JJ Baumberg, Nano Letters 2015, 15, 669−674