Alkanedithiols

Gharge Transport in Single Au Alkanedithiol Au Junctions Coordination Geometries and Conformational Degrees of Freedom. Journal of the American Chemical Society, 130, 318-326. 

Wang W, Lee T, Kretzschmar I, Reed MA (2004) Inelastic electron tunneling spectroscopy of an alkanedithiol self-assembled monolayer. Nano Lett 4(4) 643-646... 

Solomon GC, Gagliardi A, Pecchia A, Frauenheim T, Di Carlo A, Reimers JR, Hush NS (2006) Understanding the inelastic electron-tunneling spectra of alkanedithiols on gold. J Chem Phys 124(9) 094704-094710... 

Au-a,co-alkanedithiol-Au junctions represent one of the simplest molecular junctions which have been studied since 1971 [241]. These alkanedithiol-based junctions were developed as a model system and test bed for various experimental approaches, as well as for theoretical studies for exploring fundamental concepts of charge transport in molecular junctions. 

The nature of the conductance families reported for single alkanedithiol junctions is still under debate. Current interpretations are based on different contact geometries [64, 115, 242], different molecular conformations [64, 244], substrate roughness [245], or the control of tip movement [252],... 

The discrepancies in the reported conductance data of Au-alkanedithiol-Au junctions attracted our attention, and we decided to carry out an in-depth experimental study of the charge transport properties of Au-a,oo-alkanedithiol-Au molecule junctions in a non-conducting solvent. The combination with quantum chemistry ab initio simulations yielded a detailed view of this archetype of molecular junctions, and helped to resolve the puzzle on the role of microscopic geometries at the contacts and in the molecular conformation. 

Table 1 Summary of the conductance values of Au-a.oo-alkanedithiols-Au molecular junctions recorded in 1,3,5-trimethylbenzene [64] ...

Fig. 14 Experimental (a) and calculated (b) conductance values of Au-n-alkanedithiol-Au junctions vs number n of methylene units in a semilogarithmic representation. The three sets of conductance values - high (H), medium (M), and low (L) - are shown as squares, circles, and triangles. The straight lines were obtained from a linear regression analysis with decay constants (3n defined per methylene (CH2) unit. The conductances of many different, nonequivalent gauche isomers cover the window below the medium values in (b) [64]...

To provide further insight into the nature of multiple conduction states observed experimentally, DFT-based calculations of alkanedithiols coupled to Au electrodes were carried out. Calculations were performed for different configurations of an extended molecule composed of an n-alkanedithiol with variable chain length (n = 4... 10) bridged between two pyramids of 45-55 Au atoms (Fig. 15a-c). These clusters mimic the contact region of the gold electrodes. Molecular... 

Independent of the contact geometry, the calculations also demonstrated that the introduction of gauche defects resulted in a decrease of the bridge conductance by a factor of 10, as compared to an all-trans alkanedithiol chain (see Fig. 14b, triangles). Due to variations in the number and positions of gauche defects, as well as various contact geometries, the molecular junctions can exhibit conductance values up to two orders of magnitude below the conductance values of an all-trans conformation of the alkyl chain. 

Comparison of the experimental data with the trends predicted from quantum chemistry ab initio simulations demonstrated that the multiple conductance values of Au-alkanedithiol-Au junctions could be attributed to different Au-sulfur coordination geometries and to different conformations of the alkyl chain. In particular, the medium conductance corresponds to an all-trans conformation of the alkyl chain, with each sulfur atom coordinated in atop position to a single... 

The sequence of low conductance values L is attributed to isomers of alkanedithiols with gauche defects. This interpretation is also supported by the experimentally observed temperature dependence of L conductance values. The alteration of gauche states appears to be an activated process [244]. 

The invariance of IETS in an M-A-M junction vs an M-I-A-M device is exceptionally well demonstrated by the work of Reed [30], Figure 7 shows the Au-alkanedithiol-Au structure he used to create a single barrier tunnel diode. The IET spectra obtained from this device were stable and repeatable upon successive bias sweeps. The spectrum at 4.2 K is characterized by three pronounced peaks in the 0-200 mV region at 33,133, and 158 mV. From comparison with previously reported IR, Raman, and high-resolution electron energy-loss (HREEL) spectra of... 

Paulsson M, Krag C, Frederiksen T, Brandbyge M (2009) Conductance of alkanedithiol single-molecule junctions a molecular dynamics study. Nano Lett 9 117-121... 

Engelkes VB, Beebe JM, Frisbie CD (2004) Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols effect of metal work function and applied bias on tunneling efficiency and contact resistance. J Am Chem Soc 126 (43) 14287-14296... 

Some miscellaneous examples of QD synthesis using surfactant systems are described below. A layer-by-layer [206] structure of dithiol self-assembled monolayers (SAM) and CdS mono- and multilayer nanoparticles were fabricated on a gold substrate covered with alkanedithiol. SAMs were formed by an alternate immersion of the substrate into ethanolic solutions of dithiol, and dispersion of CdS nanoparticles (ca. 3nm in diameter), the latter of which was prepared in A0T/H20/heptane w/o microemulsions. 

Nakanishi T, Ohtani B, Uosaki K (1998) Fabrication and characterization of CdS-nanoparticle mono- and multilayers on a self-assembled monolayer of alkanedithiols on gold. J Phys Chem B 102 1571-1577... 

This gives a p value from 0.83 to 0.72 A-1 in the bias range from 0.1 to 1.0 volt, which is comparable to results reported previously. For example, Holmlin et al. reported a p value of 0.87 A-1 by mercury drop experiments,52 and Wold et al. have reported p of 0.94 A-1 and Cui et al. have reported p of 0.64 A-1 for various alkanethiols by using the conducting atomic force microscope technique.43 46 Recently Xu et al. reported a p of 0.8 A-1 (1.0 per methylene) for alkanedithiols.88... 

The conductance of SAMs formed from an alkanedithiol, a OPE dithiol, and an OPV dithiol was measured using a crossed-wire experimental apparatus, in which a SAM of the... 

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