Molecular wires, SAMs

Synthesis of Dipole-Possessing Molecular Wire SAMs to Control Schottky Barriers in Organic Electronic Devices... 

Figure 16. Protocol for inserting molecular wires into dodecanethiolate SAMs at grain boundaries. Relative con-...

Figure 17. Schematic of the lithographic patterning and replacement of conjugated molecules in an alkanethiol matrix, (a) Normal STM imaging of an alkanethiolate SAM with tip bias Vb. (b) SAM removal by applying a voltage pulse Vp to the substrate, (c) Carrying out the same voltage pulse as in (b), but under a solution of molecular wires (expanded structure at bottom) causes (d) insertion of the wires into the newly vacated site.

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.

A typical method for fabricating multiple complex layers is illustrated in Figure 2.11,12 First, an Au/mica or Au/ITO plate is immersed in a chloroform solution of tpy-AB-SS-AB-tpy (tpy=2,2 6, 2" -terpyridyl), providing Au-S-AB-tpy SAM on the plate. In the case of connecting the Fe(II) ion, the tpy-terminated plate is immersed in 0.1 M Fe(BF4)2 aq or (NH4)2Fe(S04)2 aq to form a metal complex. Subsequently, the metal-terminated surface is immersed in a chloroform solution of the ligand Lj or L2 to form a bis(tpy)iron structure (Fig. 2b). The latter two processes are repeated for the preparation of multilayered bis(tpy)iron (II) complex films with linear structures. When L3 is used instead of Lj or L2, the resulting molecular wires have a dendritic structure (Fig. 2c). 

We fabricated the modified ITO electrodes4 by a combination of SAM formation with a terpyridine derivative and stepwise metal-terpyridine coordination reactions in a similar manner as that described in the previous section (Fig. II).11,13 A cleaned ITO was immersed in a 0.1 M solution of 4-[2,2 6, 2"-terpyridin]-4 -yl-benzoic acid (tpy-BzA) in chloroform for 12 h to anchor the carboxyl group to ITO. Subsequently, the modified ITO was immersed in an aqueous solution of 0.1 M CoCl2, Fe(BF4)2 or Zn(BF4)2 for 2—3 h to form metal-terpyridine coordination reactions. Finally, the metal-coordinated ITO was immersed in a 0.1 M acetonitrile solution of a terpyridine-functionalized porphyrin, tpy-ZnTPP, providing the target molecular wires, [M-ZnTPP], on electrodes (Fig. 11). In addition, a cleaned ITO was immersed in a 0.1 M ethanol solution of carboxylate-functionalized porphyrin, C10ZnTPP, to afford a modified ITO as a reference. 

Unlike alkanethiol SAMs, there are few reports in the literature of studies on fundamental metal/SAM interactions when the monolayer is comprised of molecules with extended -conjugation that includes aromatic rings, as is the case of prospective molecular wires. This might also be a reflection of the considerably low number of fundamental studies on arenethiol SAMs. Figure 10 shows the structure of some of the molecules proposed as suitable components of a molecular electronic device. 

The syntheses of molecular wires that exhibit nearly linear I(V) curves are described in this section. Many of the molecular wires have previously been tested. In each case, the molecular wires bear one or more thioacetyl end groups that can be deprotected, during the self-assembled monolayer (SAM) formation step, to afford the free thiols for attachment to metallic probes. 

Figure 4.3 Protocol for inserting molecular wires into dodecanethiolate SAMs at grain boundaries and step edges. Relative conductance recording was done with a STM tip. The molecule at the bottom has also been used in this study.

With Reed, we further developed methods to insert the molecular wires at controlled rather than at random locations. By applying controlled voltage pulses to an alkanethiolate SAM under a solution of molecular wires (Figure 4.6), we were able to achieve the precise placement of molecular wire bundles (< 10 molecules/bundle) at progranuned positions (Figure 4.7)." ... 

In order to make reliable and measurements on groups of molecules as might ultimately be used in initial device structures, a testbed method was developed by Reed which he call the nanopore. The nanopore system consists of a small (30-50 nm diameter) surface of evaporated metal (which can vary, but most often Au or Pd) on which a SAM of the molecular wires or devices is permitted to form. An upper metal (usually Au or Ti) contact is then evaporated onto the top of the SAM layer making a sandwich of metal-SAM-metal through which I(V) measurements are recorded (Figure 4.10). Using such a small area for the SAM ( 10(X) molecules), we can probably form SAMs that are defect-free since the entire areas are smaller than the typical defect density of a SAM. This would eliminate electrical shorts that can occur if one had evaporated metal atop a SAM that is larger, for example, micron-sized. 

The hydroquinone-quinone redox couple was used in a device to electro-chemically switch between linear conjugation and cross-conjugation in the oligo(phenylenevinylene) (OPV) molecular wires 68 and 69, respectively (Scheme 9.11) [47]. After inserting the molecule in an octanethiolate self-assembled monolayer (SAM) on gold, an electrochemical potential was employed to switch between the quinone and hydroquinone. Electrochemical STM was used to control the oxidation state and to measure the electrical conductivity... 

A number of conjugated 2D pol3mers are electrically conducting and offer promise for application in molecular electronics, a field that pushes the goal of miniaturization to the ultimate extreme. Self-assembled monolayers (SAMs) can be assembled on a gold electrode and the other end of the molecular wire probed with a Au tip of a conductive atomic force microscope (CAFM) for the conductivity measurement. An organometallic example is shown in Fig. 13.8, where conductivity is much enhanced by the presence of the redox-active ferrocene units. ... 

The conductance of a molecular junction was measured in 1997.26 As shown in Figure 5.2, two Au wires were covered with SAMs of benzene-1,4-dithiol in THF. The wires were bent until they broke, and the broken ends were brought together in picometer increments... 

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