Au substrate

Fig. 46—XPS spectrum of [emim][Tf2N] thin film deposited on a polycrystalline Au substrate.

Fig. 4.4 Atomistic representation of successive steps in the ECALE synthesis of CdTe on an Au substrate. Observe the deposition and stripping of Te for assembling the correct atomic planes of the zinc blende structure. (Adapted from [27])...

Ultrathin films of CdS ranging in coverage from 25 to 200 ML were grown also by the previous method on Au substrates (of non-specified nature) and were characterized by quantitative Raman resonance [41], It was found that the electronic structure of the films in this coverage regime corresponds to that of bulk CdS. It was concluded also that ECALE does not involve growth by random precipitation of CdS onto the Au surface the thin deposited layers of the material were contiguous. 

The preparation of immobilized CdTe nanoparticles in the 30-60 nm size range on a Te-modified polycrystalline Au surface was reported recently by a method comprising combination of photocathodic stripping and precipitation [100], Visible light irradiation of the Te-modified Au surface generated Te species in situ, followed by interfacial reaction with added Cd " ions in a Na2S04 electrolyte. The resultant CdTe compound deposited as nanosized particles uniformly dispersed on the Au substrate surface. 

Figure 1.3 Field distributions along the Ag-tip surface and corresponding Ag-tip geometry. z = 0 corresponds to the Au-substrate. r/R is the normalized radius from the pointdirectly beneath the tip (R is the Rayleigh length R = /2n). Reprinted with permission from S. Klein, Electrochemistry, 71, 114 (2003). Copyright 2003, The Electrochemical Society of Japan.

SAMs of alkanethiols on an Au(l 11) surface are widely used to control surface properties, electron transfer processes and to stabilize nano-clusters [6, 7]. SAMs are formed by chemical bond formation between Sand Au when an Au(l 11) substrate is immersed in a solution containing several mM of alkanethiols for hours to days. Various functions have been realized by using SAM s of alkanethiols on Au substrates as listed in Table 16.1. 

Figure 20. I U) curves for Cg-Au (left) and Gal-Au (right) in H2O as a function of pH (adjusted with phosphate buffer). The numbers 1—4 in the Gal-Au data identify voltage plateaus. Cartoons of the experimental arrangements for measuring curves of individual nanoclusters in solution are shown at the top of each data column. The insulated STM tip, ligand-capped Au nanocluster and an octanethiol-coated planar Au substrate are shown. Length and shapes are not to scale. (Reprinted with permission from Ref. [35], 1998, American Chemical Society.)...

Several precautions were taken to ensure the immobilization chemistry. First, the sulfhydryl groups containing the macromolecular fraction was spectrophotometrically determined according to the literature [15]. We found that every set of 150 base pairs contained approximately one disulfide group. Since the DNA fragment used has hundreds of base pairs, each DNA strand seems to have one disulfide as its terminal group. Next, we made IR spectral measurements in a reflection-absorption (RA) mode [14b]. A freshly evaporated gold substrate was immersed into the DNA solution for 24 h at 5°C. The substrate was carefully rinsed with deionized water, dried under vacuum and was immediately used for the measurements. An Au substrate treated with unmodified, native sonicated CT DNA solution was also prepared as the control measurement. The / -polar-ized radiation was introduced on the sample at 85° off the surface normal and data were collected at a spectral resolution of 4 cm with 2025 scans. 

As described in the subsequent section, we have reported the use of an oligodeoxyribonucleotide having five successive phosphorothioates for modified electrode preparations [19]. The IR-RAS spectrum for an Au substrate treated with the oligodeoxyribonucleotide (Fig. 4) confirmed that the formed surface phase reflects the intended adsorbate. Recently, Willner et al. also reported the use of an oligodeoxynucleo-... 

To fulfill both the requirement of CFME for the practical applications and the necessity of Au substrate to assemble so-called promoters to construct the third-generation biosensor, Tian el al. have combined the electrochemical deposition of Au nanoparticles (Au-NPs) onto carbon fiber microelectrodes with the self-assembly of a monolayer on these Au-NPs to facilitate the direct electron transfer of SOD at the carbon fiber microelectrode. The strategy enabled a third-generation amperometric 02 biosensor to be readily fabricated on the carbon fiber microelectrode. This CFME-based biosensor is envisaged to have great potential for (he detection of 02" in biological systems [158],... 

It has become clear that the potentials needed to form atomic layers shift negatively as the semiconductor films grow, especially over the first 25 cycles. The most probable reason is formation of a junction potential between the Au substrate and the depositing compound semiconductor. 

Two examples from literature illustrate this approach nicely. Moore et al.114 assembled thiol-terminated long-chain S204-crown TTF onto Au and Pt surfaces by thiolato-metal bonds (see Figure 12). In the presence of various cations, most successfully Ag+, small differences were observed in the first oxidation potential (typically 60-80 mV). Similar responses were observed in solution state experiments with the same materials. The SAMs were stable when electrochemically cycled over the first oxidation wave but unstable when scanned beyond this point. Liu et al.115,116 prepared SAMs of 45 and 46 on Au substrate. Anchored to the solid surface by four Au S bonds per molecule, these SAMs were stable for hundreds of cycles over the full oxidation range. In response to the presence of Na+ both the TTF oxidation waves were shifted anodically by 55-60 mV. This observation was ascribed to either surface aggregation or cooperativity of neighbouring crown rings. 

An early measurement of current through a molecule was the report in 1995 of the resistance of a single C60 molecule, 1 (Fig. 6), deposited on an Au substrate and located and measured by an STM probe [59]. The conductivity was a respectable 18 nS. Most molecules studied as wires are more linear, with a coordinating atom at one or both ends. 

The bis(triphenylamine)-substituted fullerene 41 makes a dense and stiff mono-layer that transfers onto an Au substrate by LS but not by LB [105]. It rectifies with RR = 10 (Fig. 18f) and the RR does not decrease at all upon successive cycling [105],... 

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