ABSTRACT
This article gives an overview of organometallic chemical-vapor-deposited (OM-CVD) metal layers and metal nanoparticles (NPs)on self-assembled monolayers (SAMs), which have been laterally patterned. Both enabling/disabling as well as growth and nongrowth resist patterning in SAMs will be mentioned. Simple methods like stamping of lateral SAM structures with as poly(dimethylsiloxane) (PDMS) stamp or more complex pattering technologies like focused ion beam (FIB) nanolithography will be discussed. Copper, palladium, platinum, gold, gallium nitride, and mercury are the metals/materials of interest, whereas the focus of this chapter lies on Au NPs, due to their promising application in biosensing. Nobel metal NPs show an interesting resonant absorption feature called localized surface plasmon resonance (LSPR). LSPR exhibits an absorption band in the visible, and its spectral position is extremely sensitive to environmental changes of the NP, both with respect to
the refractive index of surrounding materials as well as with respect to colloidal or clustered NP architectures. 5.1 Introduction
The adsorption of organosulfur compounds like thiols, sulfides, and disulfides on the surfaces of coinage metals (i.e., Cu, Ag, and Au) to fabricate so-called SAMs represents a chemisorption system with unique and interesting properties [1-4]. While the initial steps of the self-assembly process, including aspects of substrate characteristics [5, 6] and the structural properties of the resulting SAMs, are still matters of intense research effort, the focus of much activity is the deposition of further layers onto the SAMs. Both inorganic [7-14] organic [15-17] materials and living cells, such as bacteria [18], are under consideration. Here we focus on SAMs carrying a thiol moiety as the functional head group to allow coinage metal immobilization. Two immobilization schemes are considered, a) metal surfaces carrying a dithiol SAM and b) oxidic surfaces (glass or oxidized silicon wavers) with an SH-terminated silane SAM. These SH-modified surfaces allow the attachment of metallic ions [7, 8], metals (e.g., by conventional vacuum deposition technologies or electrochemical reduction of metallic ions in solution) [9-14], and the deposition of presynthesized Au NPs [19-24]. CVD is an alternative and commonly used technique for growing thin films of a variety of materials for a broad range of applications [25]. However, CVD is commonly known for the need of rather high operation temperature, typically well above 200°C. At these temperatures most organic materials will suffer in a form of destruction or at least chemical alteration. Effort has been put into the coating of prefabricated nano objects with thin films of silica, carbon, or gold by CVD methods [26-28]. Some work has been performed on growing Au NPs and nanorods via precursor-assisted CVD, for example, with aerosol-assisted CVD using hydrogen tetrachloroaurate(III) [29] or in the channel of mesoporous silica with dimethyl(hexafluoro-acetylacetoato)gold(III) [30]. Since recent years CVD of materials is also carried out on top of SAMs. This is due to the availability of organometallic (OM) precursors allowing CVD processes under mild thermal activation
(T ≈ 200°C for silanes and 60°C-70°C for thiols), which is compatible with most organic materials, critical with a SAM on gold or silver, and, to some extent, less critical with silanes on silica surface. The cleavage of the gold sulfur bond and desorption of the thiols occur at around 60°C in liquid and begin at 170°C in ultrahigh vacuum (UHV) [1, 31]. To achieve deposition onto well-organized, crystalline organic monolayers, the deposition temperature needs to be below the glass transition temperature of the SAM, typically below 120°C [31]. Copper deposition has been carried out successfully on 3-mer-captopropyltrimethoxysilane (MPTS) SAMs with Cu(II)bis-hexafluoroacetyl-acetonate, Cu+1(hexafluoroacetylacetonate) tri-methylvinylsilane, or (hexafluoroacetyl-acetonato) (vinyltrimethylsilane)copper(I) OM-CVD precursors, respectively [32-34]. Palladium and platinum OM-CVD was performed with bis (hexafluoroacetylacetonate) platinum (II), bis (hexafluoroacetylaceto-nato) palladium (II), and cyclopentadienyl allyl palladium directly on indium tin oxide (ITO) and SiO2/Si wafers with a patterned octade-cyltrichlorosilane (OTS) SAM as a resist and on 4,4’-biphenyldithiol SAMs on silver [35-39]. Aliganga et al. [40] have used liquid mercury at room temperature without the preparation of an OM precursor to deposit nanodroplets onto 1,8-octanedithiol SAMs with the aid of the mercury’s vapor pressure. GaN thin films were grown in solution with the help of the OM-CVD precursor Ga(N3)3NEt3 in toluene on 11-mercaptoundecanol SAMs. The -OH surface groups of the SAM reacted with the gallium precursor, yielding NH and covalent gallium-oxygen bonds [41]. Very successfully Au NPs have been grown with the help of the OM-CVD precursor trimethyl-phosphine gold methyl ((CH3)3PAuCH3) on dithiol SAMs on Ag and Au and on MPTS SAMs on glass, glass waveguides, and oxidized silicon wavers [38, 42-51]. The thermodynamically more labile precursors, as is typical for OM-CVD, offer a solution to the problem of coating temperature-sensitive surfaces. The particular interest in OM-CVD of Au on top of SAMs in general was initiated by the aim of depositing ultrathin layers of Au on top of a SAM as a keystep in the fabrication of integrated photonic devices designed to act as biosensors. Photonic waveguide devices with high sensitivity, as sensor transducers, are based on glass or silicon materials with oxidic surfaces and are incompatible with extended metal surfaces. These will cause high absorption, which leads to
unacceptably high waveguide losses and therefore to only short propagation and interaction lengths. In addition, many suppliers of fine chemicals and biomolecules offer sulfur-based compounds and biological recognition pairs, such as antibody-antigenes, with a binding ability to gold and silver, but not necessarily the adequate silane derivative for binding to an oxidic surface. Recent years have seen a tremendous impact on the Au NP optical response in biological assays, detection, labeling, and sensing, both in theoretical and in experimental work [53-58]. In particular the absorption feature of metal NPs, LSPR, plays an important role due to its sensitivity to environmental changes of the Au NPs. LSPR shifts by a few nanometers to a higher wavelength by coating the Au NPs; however, when the Au NPs are clustered into aggregates, a huge shift of up to 200 nm is observed. Also many prototype devices in molecular electronics have been proposed or demonstrated, which incorporate Au NPs as components, including single-electron transistors, single-electron charging devices, photonic switches, and quantum dots [59-63]. Taking the absorption losses of the optical devices into account and the phenomenal possibilities Au NPs offer in sensing technology, the aim is the deposition of only minute amounts of gold in the form of Au NPs on top of a waveguide, functionalized with MPTS. This will then allow creating a sensor platform, which can easily be functionalized with the help of the well-established SAM sulfur chemistry in the form of thiols, sulfides, and disulfides. Two device platforms were demonstrated with OM-CVD-grown Au NPs. The first was an optical Mach-Zehnder interferometer [52]. Here both arms of the silicon oxynitride interferometer were completely functionalized with MPTS and covered by about 10% with OM-CVD Au NPs. The Mach-Zehnder interferometer was operated outside the LSPR of the Au NPs; the Au NPs served as locations of the biorecognition SAMs only. Biotin-streptavidin and biotinylated antibody recognition was detected with a sensitivity for streptavidin of 20 ng/cm2. A DNA hybridization sensor [64] was built, taking advantage of the enhanced evanescent fields the Au NPs show when in LSPR [65]. Here first Au NPs were grown with an LSPR absorption maximum at 520 nm. After immobilization of biotin and successively streptavidin, a biotinylated catcher DNA half strand was attached to the bio-SAM via the free binding sites for biotin in the straptavidin layer.
