Dithiol molecules

Figure 20. A representation of the technique used in the mechanically controllable break junction for recording the current through a single molecule, (a) The gold wire was coated with a SAM of the molecular wires (b) and then broken, under solution (c), via extension of the piezo element under the silicon surface (see Figure 19). Evaporation of the volatile components and slow movement of the piezo downward (see Figure 19) permits one molecule to bridge the gap (d) that is shown, in expanded view, in the insert. The insert shows a benzene-1,4-dithiolate molecule between proximal gold electrodes. The thiolate is normally FI-terminated after deposition end groups denoted as X can be either FI or Au, the Au potentially arising from a previous contact/retraction event.

Ohgi T, Sheng H-Y, Nejoh H (1998) Au particle deposition onto self-assembled monolayers of thiol and dithiol molecules. Appl Surf Sci 130-132 919-924... 

The first molecular measurement using MCBs was reported in 1997 by Reed and coworkers [37], who measured the conductance of a single benzene-1,4-dithiol molecule (83a). This work was followed in 1999 by measurements on oligothiophene 105 by Bourgoin and coworkers [38]. 

Figure 5.59 Schematic of the multilayer formation of alternate 1,6-hexane dithiol molecules and Cu(ll) ions [87]...

Figure 5.2 Dithiol molecule placed between to gold electrodes.

A guided self-assembly of gold nanoparticles succeeded also on patterned GaAs surfaces. Stmctured Be-doped GaAs surfaces were first decorated with a monolayer of xylyl dithiol molecules. A uniform monolayer of dodecanethiol stabilized 5 nm Au particles, generated by spreading a hexane solntion... 

Haiss et al. have also developed an alternative approach that also employs an STM. When the STM tip is placed in elose proximity to a low coverage of the dithiol molecule, characteristie jumps are observed which are attributed to the attachment or detachment of the analyte moleeule to or from the STM tip. The molecular conductance is ealeulated from the applied tip-substrate bias and the value of the eurrent jumps. In order to improve aceuracy and also to distinguish between single moleeule and multiple attachment/detachment events the jumps are statistically rendered in the... 

According to Samanta et ol. [62], who used a GF-based method to calculate the transmission function of electrons across the dithiol ligand, the resistance R per molecule was determined by the Landauer formula, R = hl2e )IT(Eg), where T( g) is the transmission function, T exp[2(m g) /H]. For the dithiol molecule, the barrier height was estimated to be g 2.8eV, and the resulting resistance calculated as R 30GQ. 

The factor 2 accounts for the double occupancy of each orbital. This approach was used recently [169] to calculate current through a molecular species. Benzene 1,4-dithiolate molecule (as used in the experiment described in Ref. [67]), between two jelKum surfaces. The result demonstrates the large sensitivity of the computed current to the microscopic structure of the molecule-metal contacts. 

The preparation of multilayers based on the same or on different structural units may take advantage of the reactivity of the monolayers and particularly of the tail groups. Different strategies have been adopted as an example, dithiol molecules lead to multilayers thanks to the formation of an S-S bond between molecules on the surface [20-22]. The defects present in a single monolayer are inherited by the monomolecular layer anchored to its tail group. Hence, the degree of order of a multilayer is usually much lower with respect to the first monolayer oti the substrate. In the majority of cases, bi- or tri-layers are prepared, although few hundreds of micron thick multilayers can be also obtained. 

Fig. 4.6 Schematic illustration of the micro-gap electrodes which are connected with the network of organic dithiols and AuNPs. Shaded circles indicate AuNPs, and lines indicate organic dithiol molecules (Reprinted with permission from Ogawa et al. 2001, Copyright 2001 Elsevier)...

Varga and Pantelides presented a new method, based on the ab initio, density-functional theory, with which the I — V characteristics of such a system can be calculated. Subsequently, they used their approach to study two systems, i.e., a benzene-dithiolate molecule between two gold electrodes and a finite carbon nanotube between two aluminum electrodes. As typical examples we show in Fig. 51 the current, I, and the conductance, IjV, as a function of applied voltage, V, for the first example. That / is highly non-linear as a function of V is readily identified. The non-trivial current distribution in the molecule under an applied external potential is also seen in the figure. 

Fig. 51 (Top part) Calculated I — V characteristics as well as the conductance for a benzene-dithiolate molecule between two gold electrodes. (Bottom part) The x component of the current in the same system for a fixed value of V. The colour coding is given to the right. Reproduced with permission of American Physical Society from ref. 157.

The nanoparticles prepared in the reverse micellar systems need to be recovered from reverse micelles and immobilized onto stable supports [122, 123]. There have been several reports relating to the immobilization of nanoparticles from reverse micellar systems onto supports made from inorganic and organic materials. For example, dithiol molecules can also be used for the immobilization of metal and semiconductor nanoparticles. Colvin et al. have demonstrated the immobilization of CdS nanoparticles, prepared in the reverse micelles, onto the self-assembled dithiol monolayers on an Au substrate [124]. This approach was used to prepare an Au-CdS nanoparticle multilayer [125] and a CdS nanoparticle multiplayer on Au substrate [126]. Nakanishio et al. demonstrated the layer-by-layer self-assembly of CdS and ZnS nanoparticles on Au substrate, from a reverse micellar solution, using dithiol molecules [127, 128]. In these methods, the preparation of the CdS nanoparticles is completely separated from the immobilization procedure and hence the chemical and physical properties of the nanoparticles can be well controlled when using an appropriate reaction medium such as a reverese micellar system. 

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