Correlating the Influence of Disulfides in Monolayers across Photoelectron Spectroscopy Wettability and Tunneling Charge-Transport

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   This crit. review provides an overall survey of the basic concepts and up-to-date literature results concerning the very promising use of gold nanoparticles (AuNPs) for medicinal applications. It includes AuNP synthesis, assembly and conjugation with biol. and biocompatible ligands, plasmon-based labeling and imaging, optical and electrochem. sensing, diagnostics, therapy (drug vectorization and DNA/gene delivery) for various diseases, in particular cancer (also Alzheimer, HIV, hepatitis, tuberculosis, arthritis, diabetes) and the essential in vitro and in vivo toxicity. It will interest the medicine, chem., spectroscopy, biochem., biophysics and nanoscience communities (211 refs.).
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   Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Patterning Self-Assembled Monolayers: Applications in Materials Science. Langmuir 1994, 10, 1498– 1511,  DOI: 10.1021/la00017a030

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   Langmuir (1994), 10 (5), 1498-511CODEN: LANGD5; ISSN:0743-7463.

   This paper describes an exptl. simple technique, based on stamping or contact printing, to pattern the adsorption of alkanethiolates on surfaces of gold, on scales from 0.2 to 100 μm, and illustrates the use of these patterned surfaces. An elastomeric stamp, fabricated from poly(dimethylsiloxane) (PDMS), was used to deliver alkanethiol to predetd. regions of the surface of an evapd. gold film. With this technique, org. surfaces patterned with well-defined regions exhibiting different chem. and phys. properties have been produced. This technique was used to pattern the adsorption of single and multiple self-assembled monolayers (SAMs) on a single substrate. This method of prepg. patterned surfaces has been used in a variety of applications, including the prepn. of well-defined, heterogeneous substrates for scanning probe microscopies, the formation of microelectrodes, the formation of microstructures of silicon, the prepn. of substrates for the study of condensation figures, and the prepn. of substrates for patterned formation of microcrystals.
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   Liao, W.-S.; Cheunkar, S.; Cao, H. H.; Bednar, H. R.; Weiss, P. S.; Andrews, A. M. Subtractive Patterning via Chemical Lift-Off Lithography. Science 2012, 337, 1517– 1521,  DOI: 10.1126/science.1221774

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   Science (Washington, DC, United States) (2012), 337 (6101), 1517-1521CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)

   Conventional soft-lithog. methods involving the transfer of mol. "inks" from polymeric stamps to substrates often encounter micrometer-scale resoln. limits due to diffusion of the transferred mols. during printing. The authors report a "subtractive" stamping process in which silicone rubber stamps, activated by oxygen plasma, selectively remove hydroxyl-terminated alkanethiols from self-assembled monolayers (SAMs) on gold surfaces with high pattern fidelity. The covalent interactions formed at the stamp-substrate interface are sufficiently strong to remove not only alkanethiol mols. but also gold atoms from the substrate. A variety of high-resoln. patterned features were fabricated, and stamps were cleaned and reused many times without feature deterioration. The remaining SAM acted as a resist for etching exposed gold features. Monolayer backfilling into the lift-off areas enabled patterned protein capture, and 40-nm chem. patterns were achieved.
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   A system for probing mol. recognition events at org. interfaces using fluorescent receptors is described. Receptors formed from the bis(2,6-diaminopyridine) amide of isophthalic acid are incorporated in mixed self-assembled monolayers (SAMs) of alkanethiols on gold and shown to interact with barbituric acid derivs. from soln. Individual parameters that affect the ability of receptors on surfaces to recognize ligands from soln. along with varieties of solvents for ligand solns. were examd.
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   A review. Life implies movement. Most forms of movement in the living world are powered by tiny protein machines known as mol. motors. Among the best known are motors that use sophisticated intramol. amplification mechanisms to take nanometer steps along protein tracks in the cytoplasm. These motors transport a wide variety of cargo, power cell locomotion, drive cell division and, when combined in large ensembles, allow organisms to move. Three types of cytoplasmic motor proteins are known: (1) myosins, which move on actin filaments, (2) dyneins, and (3) kinesins, which use microtubules as tracks. Motor defects can lead to severe diseases or may even be lethal. The basic principles of motor design and mechanisms have now been derived, and an understanding of their complex cellular roles is emerging.
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   Yao, H.; Miki, K.; Nishida, N.; Sasaki, A.; Kimura, K. Large Optical Activity of Gold Nanocluster Enantiomers Induced by a Pair of Optically Active Penicillamines. J. Am. Chem. Soc. 2005, 127, 15536– 15543,  DOI: 10.1021/ja053504b

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   Large Optical Activity of Gold Nanocluster Enantiomers Induced by a Pair of Optically Active Penicillamines

   Yao, Hiroshi; Miki, Kanae; Nishida, Naoki; Sasaki, Akito; Kimura, Keisaku

   Journal of the American Chemical Society (2005), 127 (44), 15536-15543CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   The authors have succeeded for the 1st time in prepg. a pair of Au nanocluster enantiomers protected by optically active thiols: D- and L-penicillamine (D-Pen and L-Pen). CD spectroscopy confirmed the mirror image relation between the D-Pen-capped and the L-Pen-capped Au nanoclusters, suggesting that the surface modifier acts as a chiral selector, and that the nanoclusters have well-defined stereostructures as common chiral mols. do. No CD signals could be obtained when the Au nanoclusters were synthesized by using a racemic mixt. (rac-Pen). These chiroptical properties were studied for the three sepd. fractions of each of the Au nanoclusters (D-Pen-capped, L-Pen-capped, or rac-Pen-capped clusters) by polyacrylamide gel electrophoresis (PAGE). Each fractioned component has the mean diam. of 0.57, 1.18, or 1.75 nm that was detd. by a soln.-phase small-angle x-ray scattering. With a decrease in the mean cluster diam., optical activity or anisotropy factors gradually increased. From the kinetic and the structural considerations, the origins of large optical activity of the Au nanocluster enantiomers are discussed.
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   Gautier, C.; Bürgi, T. Chiral N-Isobutyryl-Cysteine Protected Gold Nanoparticles: Preparation, Size Selection, and Optical Activity in the UV-vis and Infrared. J. Am. Chem. Soc. 2006, 128 (34), 11079– 11087,  DOI: 10.1021/ja058717f

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   Chiral N-isobutyryl-cysteine protected gold nanoparticles: preparation, size selection, and optical activity in the UV-vis and infrared

   Gautier, Cyrille; Buergi, Thomas

   Journal of the American Chemical Society (2006), 128 (34), 11079-11087CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

   We have prepd. gold nanoparticles covered with N-isobutyryl-L-cysteine and N-isobutyryl-D-cysteine, resp. These particles with a mean particle size smaller than 2 nm are highly sol. in water and are amenable to chiroptical techniques such as vibrational CD (VCD) and CD spectroscopy. D. functional theory shows that the VCD spectra are sensitive toward the conformation of the adsorbed thiol. Based on the comparison between the exptl. VCD spectrum and the calcd. VCD spectra for different conformers, a preferential conformation of the thiol adsorbed on the gold particles can be proposed. In this conformation the carboxylate group interacts with the gold particle in addn. to the sulfur. The particles could furthermore be sepd. according to their charge and size into well-defined compds. The optical absorption spectra revealed a well-quantized electronic structure and a systematic red-shift of the absorption onset with increasing gold core size, which was manifested in a color change with particle size. Some compds. showed basically identical absorption spectra as analogous gold particles protected with L-glutathione. This shows that these particles have identical core sizes (10-12, 15, and 18 gold atoms, resp.) and indicates that the no. and arrangement of the adsorbed thiol are the same, independent of the two thiols, which have largely different sizes. Some sepd. compds. show strong optical activity with opposite sign when covered with the D- and L-enantiomer, resp., of N-isobutyryl-cysteine. The origin of the optical activity in the metal-based transitions is discussed. The observations are consistent with a mechanism based on a chiral footprint on the metal core imparted by the adsorbed thiol.
10. 10

    Yeung, S. Y.; Ederth, T.; Pan, G.; Cicėnaitė, J.; Cárdenas, M.; Arnebrant, T.; Sellergren, B. Reversible Self-Assembled Monolayers (rSAMs) as Robust and Fluidic Lipid Bilayer Mimics. Langmuir 2018, 34, 4107– 4115,  DOI: 10.1021/acs.langmuir.8b00226

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    Reversible Self-Assembled Monolayers (rSAMs) as Robust and Fluidic Lipid Bilayer Mimics

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    Langmuir (2018), 34 (13), 4107-4115CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    Lipid bilayers, forming the outer barrier of cells, display a wide array of proteins and carbohydrates for modulating interfacial biol. interactions. Formed by the spontaneous self-assembly of lipid mols., these bilayers feature liq. cryst. order, while retaining a high degree of lateral mobility. Studies of these dynamic phenomena have been hampered by the fragility and instability of corresponding biomimetic cell membrane models. Here, we present the construct of a series of oligoethylene glycol-terminated reversible self-assembled monolayers (rSAMs) featuring lipid-bilayer-like fluidity, while retaining air and protein stability and resistance. These robust and ordered layers were prepd. by simply immersing a carboxylic acid-terminated self-assembled monolayer into 5-50 μM aq. ω-(4-ethylene glycol-phenoxy)-α-(4-amidinophenoxy)decane solns. It is anticipated that this new class of robust and fluidic two-dimensional biomimetic surfaces will impact the design of rugged cell surface mimics and high-performance biosensors.
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    Sethuraman, A.; Han, M.; Kane, R. S.; Belfort, G. Effect of Surface Wettability on the Adhesion of Proteins. Langmuir 2004, 20, 7779– 7788,  DOI: 10.1021/la049454q

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    Effect of Surface Wettability on the Adhesion of Proteins

    Sethuraman, Ananthakrishnan; Han, Mina; Kane, Ravi S.; Belfort, Georges

    Langmuir (2004), 20 (18), 7779-7788CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    Besides significantly broadening the scope of available data on adhesion of proteins on solid substrates, we demonstrate for the first time that all seven proteins (tested here) behave similarly with respect to adhesion exhibiting a step increase in adhesion as wettability of the solid substrate decreases. Also, quant. measures of like-protein-protein and like-self-assembled-monolayer (SAM)-SAM adhesive energies are provided. New correlations, not previously reported, suggest that the helix and random content (as measures of secondary structure) normalized by the mol. wt. of a protein are significant for predicting protein adhesion and are likely related to protein stability at interfaces. At. force microscopy (AFM) was used to directly measure the normalized adhesion or pull-off forces between a set of seven globular proteins and a series of eight well-defined model surfaces (SAMs), between like-SAM-immobilized surfaces and between like-protein-immobilized surfaces in phosphate buffer soln. (pH 7.4). Normalized force-distance curves between SAMs (alkanethiolates deposited on gold terminated with functional uncharged groups -CH3, -OPh, -CF3, -CN, -OCH3, -OH, -CONH2, and -EG3OH) covalently attached to an AFM cantilever tip modified with a sphere and covalently immobilized proteins (RNase A, lysozyme, bovine serum albumin, Ig, γ-globulins, pyruvate kinase, and fibrinogen) clearly illustrate the differences in adhesion between these surfaces and proteins. The adhesion of proteins with uncharged SAMs showed a general "step" dependence on the wettability of the surface as detd. by the water contact angle under cyclooctane (θco). Thus, for SAMs with θco < ∼66°, (-OH, -CONH2, and -EG3OH), weak adhesion was obsd. (>-4 ± 1 mN/m), while for ∼66 < θco < ∼104°, (-CH3, -OPh, -CF3, -CN, -OCH3), strong adhesion was obsd. (≤8 ± 3 mN/m) that increases (more neg.) with the mol. wt. of the protein. Large proteins (170-340 kDa), in contrast to small proteins (14 kDa), exhibit characteristic stepwise decompression curves extending to large sepn. distances (hundreds of nanometers). With respect to like-SAM surfaces, there exists a very strong adhesive (attractive) interaction between the apolar SAM surfaces and weak interactive energy between the polar SAM surfaces. Because the polar surfaces can form hydrogen bonds with water mols. and the apolar surfaces cannot, these measurements provide a quant. measure of the so-called mean hydrophobic interaction (∼-206 ± 8 mN/m) in phosphate-buffered saline at 296 ± 1 K. Regarding protein-protein interactions, small globular proteins (lysozyme and RNase A) have the least self-adhesion force, indicating robust conformation of the proteins on the surface. Intermediate to large proteins (BSA and pyruvate kinase-tetramer) show measurable adhesion and suggest unfolding (mech. denaturation) during retraction of the protein-covered substrate from the protein-covered AFM tip. Fibrinogen shows the greatest adhesion of 20.4 ± 2 mN/m. Unexpectedly, IgG and γ-globulins exhibited very little adhesion for intermediate size proteins. However, using a new composite index, n (the product of the percent helix plus random content times relative mol. wt. as a fraction of the largest protein in the set, Fib), to correlate the normalized adhesion force, IgG and γ-globulins do not behave abnormally as a result of their relatively low helix and random (or high sheet) content.
12. 12

    Samuel, B.; Zhao, H.; Law, K.-Y. Study of Wetting and Adhesion Interactions Between Water and Various Polymer and Superhydrophobic Surfaces. J. Phys. Chem. C 2011, 115, 14852– 14861,  DOI: 10.1021/jp2032466

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    Study of Wetting and Adhesion Interactions between Water and Various Polymer and Superhydrophobic Surfaces

    Samuel, Benedict; Zhao, Hong; Law, Kock-Yee

    Journal of Physical Chemistry C (2011), 115 (30), 14852-14861CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    The wetting and adhesion characteristics of 20 different surfaces were studied systematically by both static H2O contact angle (θ) and dynamic contact angle measurement techniques: sliding angle (α) and advancing (θA) and receding (θR) contact angles. These surfaces cover surfaces of all traits, from smooth and flat to rough and artificially textured. Fourteen of the surfaces are flat, and they range from molded plastic sheets to soln. coated polymer films to CVD polymd. polymer films and to self-assembled monolayers on Si wafers. The rest of the surfaces include 4 fluorosilane coated textured Si wafer surfaces and 2 natural surfaces derived from the front and back side of the rose petal. Static H2O contact angle data suggest that these surfaces vary from hydrophilic with θ at ∼71° to superhydrophobic with θ exceeding 150°. Plots of θ of these surfaces vs. α, (cos θR - cos θA), and the contact angle hysteresis (θA - θR) all yield scattered plots, indicating that there is little correlation between θ and α, (cos θR - cos θA) and (θA - θR). Since the later 3 parameters were mentioned to relate to adhesion semiempirically between a liq. droplet and the contacting surface, the present work demonstrates with generality that contact angle indeed does not relate to adhesion. This is consistent with a known but not well recognized fact in the literature. The authors study both the wetting and adhesion forces between H2O and these 20 surfaces on a microelectromech. balance (tensiometer). When the H2O drop 1st touches the surface, the attractive force during this wetting step was measured as the snap-in force. The adhesion force between the H2O drop and the surface was measured as the pull-off force when the H2O drop separates (retracts) from the surface. The snap-in force decreases monotonously as θA decreases and becomes zero when θA is >150°. The very good correlation is not unexpected due to the similarity between the wetting and the snap-in process. The anal. of the pull-off force data is slightly more complicated, and the quality of the H2O-surface sepn. depends on the surface adhesion. For surfaces that show strong adhesion with H2O, there is always a small drop of H2O left behind after the H2O droplet is pulled off from the surface. Despite this complication, the authors plot the pull-off force vs. α, (cos θR - cos θA) and (θA - θR), and found very little correlation. However, the pull-off force is found to correlate well to the receding contact angle θR. Specifically, pull-off force decreases monotonically as θR increases, suggesting that θR is a good measure of surface adhesion. Very interestingly, the authors also observe a qual. correlation between θR and the quality of the pull-off. The pull-off is clean, free of H2O residue after pull-off, when θR is >∼90° and vice versa. The implications of this work toward surface contact angle measurements and print surface design are discussed.
13. 13

    Ramachandran, S.; Tsai, B.-L.; Blanco, M.; Chen, H.; Tang, Y.; Goddard, W. A. Self-Assembled Monolayer Mechanism for Corrosion Inhibition of Iron by Imidazolines. Langmuir 1996, 12, 6419– 6428,  DOI: 10.1021/la960646y

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    Self-Assembled Monolayer Mechanism for Corrosion Inhibition of Iron by Imidazolines

    Ramachandran, Sunder; Tsai, Bao-Liang; Blanco, Mario; Chen, Huey; Tang, Yongchun; Goddard, William A., III

    Langmuir (1996), 12 (26), 6419-6428CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    Some of the most effective corrosion inhibitors for oil field pipeline applications are the oleic imidazoline (OI) class of mols. However, the mechanism by which OIs inhibit corrosion is not known. We report atomistic simulations (quantum mechanics and mol. dynamics) designed to elucidate this mechanism. These studies lead to the self-assembled monolayer (SAM) model for corrosion inhibition, which explains the differences in corrosion inhibition efficiency for various OI mols. The SAM model of OI inhibitors involves the following crit. elements: (i) The function of the OI is to form a self-assembled monolayer on the native oxide surface of iron; this serves a protective role by forming a hydrophobic barrier preventing migration of H2O, O2, and electrons to the Fe surface. (Ii) The imidazoline head group serves as a sufficiently strong Lewis base to displace H2O from the Lewis acid sites of the iron oxide surface. (Iii) These head groups self-assemble on the surface to form an ordered monolayer on the iron oxide surface. [3 × 3 for the (001) cleavage surface of α-Fe2O3.]. (I.v.) The long hydrophobic tail (e.g., 2-oleic acid) tilts to form a tightly packed hydrophobic monolayer. [For α-Fe2O3(001) the tilt angle is about 72° with respect to the surface normal.]. (V) This hydrocarbon tail must have a sufficient length to cover the surface. [For α-Fe2O3(001) the chain length must be 12 or more carbon atoms.]. (Vi) The hydrophobic tail and the pendent group (e.g., -CH2CH2NH2) must lead to an octanol/water partition coeff. (log P) below a crit. value (about 7) in order to rapidly form the monolayer. This SAM model should be useful in developing both alternative environmentally benign corrosion inhibitors and higher temp. corrosion inhibitors.
14. 14

    Nuzzo, R. G.; Allara, D. L. Adsorption of Bifunctional Organic Disulfides on Gold Surfaces. J. Am. Chem. Soc. 1983, 105, 4481– 4483,  DOI: 10.1021/ja00351a063

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    Adsorption of bifunctional organic disulfides on gold surfaces

    Nuzzo, Ralph G.; Allara, David L.

    Journal of the American Chemical Society (1983), 105 (13), 4481-3CODEN: JACSAT; ISSN:0002-7863.

    Stable, oriented, polyfunctional org. monolayers can be prepd. on Au surfaces by soln. adsorption of org. disulfides. Ellipsometry, wetting angle, and Fourier transform IR reflection spectroscopic measurements demonstrate that both polar and nonpolar org. functional groups can be placed at an ambient interface with known packing densities and selected orientations.
15. 15

    Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. Fundamental Studies of the Chemisorption of Organosulfur Compounds on Gold(111). Implications for Molecular Self-Assembly on Gold Surfaces. J. Am. Chem. Soc. 1987, 109, 733– 740,  DOI: 10.1021/ja00237a017

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    Fundamental studies of the chemisorption of organosulfur compounds on gold(111). Implications for molecular self-assembly on gold surfaces

    Nuzzo, Ralph G.; Zegarski, Bernard R.; Dubois, Lawrence H.

    Journal of the American Chemical Society (1987), 109 (3), 733-40CODEN: JACSAT; ISSN:0002-7863.

    Studies of the adsorption of methanethiol and MeSSMe on an Au(111) surface under ultrahigh vacuum conditions are described. Both adsorbates bind strongly, with the bonding of the disulfide being favored greatly. Under these conditions, the disulfide bond is dissocd. to give a stable surface thiolate. Adsorption of methanethiol does not involve cleavage of the S-H bond. The implications of these results for soln. adsorption expts. and the thermodn. characterizing monolayer formation are discussed.
16. 16

    Bain, C. D.; Evall, J.; Whitesides, G. M. Formation of Monolayers by the Coadsorption of Thiols on Gold: Variation in the Head Group, Tail Group, and Solvent. J. Am. Chem. Soc. 1989, 111, 7155– 7164,  DOI: 10.1021/ja00200a039

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    Formation of monolayers by the coadsorption of thiols on gold: variation in the head group, tail group, and solvent

    Bain, Colin D.; Evall, Joe; Whitesides, George M.

    Journal of the American Chemical Society (1989), 111 (18), 7155-64CODEN: JACSAT; ISSN:0002-7863.

    Long-chain alkanethiols, HS(CH2)nX, adsorb from soln. onto Au and form oriented, ordered monolayers. Monolayers exposing >1 functional group at the surface can be generated by coadsorption of ≥2 thiols from soln. In general, the ratio of the concns. of the 2 components in a mixed monolayer is not the same as in soln. but reflects the relative solubilities of the components in soln. and interactions between the tail groups, X, in the monolayer. Multicomponent monolayers do not phase-segregate into single-component domains large enough to influence the contact angle (a few tens of angstroms across) and also do not act as ideal two-dimensional solns. In the two-component system HS(CH2)nX/HS(CH2)nCH3 in ethanol, where X is a polar tail group such as CH2OH or CN, adsorption of the polar component is particularly disfavored at low concns. of the polar component in the monolayer. These isotherms may arise from poor solvation of the polar tail groups in the quasi-two-dimensional alkane soln. provided by the Me tail groups. From dil. solns. in alkanes, adsorption of HS(CH2)10CH2OH is strongly preferred over HS(CH2)10CH3, probably due to the stabilization afforded by intramol. H bonds between the OH tail groups. The wettability of mixed monolayers is not linear in the compn. of the surface. In a surface comprised of a polar and a nonpolar component, the polar component is more hydrophilic when its concn. in the monolayer is low than when the monolayer is composed largely of the polar component.
17. 17

    Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Comparison of Self-Assembled Monolayers on Gold: Coadsorption of Thiols and Disulfides. Langmuir 1989, 5, 723– 727,  DOI: 10.1021/la00087a027

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    Comparison of self-assembled monolayers on gold: coadsorption of thiols and disulfides

    Bain, Colin D.; Biebuyck, Hans A.; Whitesides, George M.

    Langmuir (1989), 5 (3), 723-7CODEN: LANGD5; ISSN:0743-7463.

    Ordered, org. monolayers were formed on Au slides by adsorption from EtOH of HS(CH2)10CH2OH, HS(CH2)10CH3, [S(CH2)10CH2OH]2, [S(CH2)10CH3]2, and binary mixts. of these mols. in which 1 component was terminated by a hydrophobic Me group and 1 by a hydrophilic alc. group. The compns. of the monolayers were detd. by XPS. Wettability was used as a probe of the chem. compn. and structure of the surface of the monolayer. When monolayers were formed in solns. contg. mixts. of a thiol and a disulfide, adsorption of the thiol was strongly preferred (∼75:1). The advancing contact angles of water and hexadecane on monolayers formed from solns. contg. mixts. of 2 thiols, a thiol and a disulfide, or 2 disulfides depend on the proportion of OH-terminated chains in the monolayer and are largely independent of the nature of the precursor species. This observation suggests that both thiols and disulfides give rise to the same chem. species (probably a thiolate) on the surface. This model is supported by the observation by XPS of indistinguishable S(2p) signals from monolayers derived from thiols and disulfides.
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    Ulman, A. Self-Assembled Monolayers of Thiols; Thin Films; Academic Press, 1998; Vol. 24.

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    Xue, Y.; Li, X.; Li, H.; Zhang, W. Quantifying thiol–gold interactions towards the efficient strength control. Nat. Commun. 2014, 5, 1– 9,  DOI: 10.1038/ncomms5348

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    Kong, G. D.; Byeon, S. E.; Park, S.; Song, H.; Kim, S.-Y.; Yoon, H. J. Mixed Molecular Electronics: Tunneling Behaviors and Applications of Mixed Self-Assembled Monolayers. Advanced Electronic Materials 2020, 6, 1901157,  DOI: 10.1002/aelm.201901157

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    Mixed Molecular Electronics: Tunneling Behaviors and Applications of Mixed Self-Assembled Monolayers

    Kong, Gyu Don; Byeon, Seo Eun; Park, Sohyun; Song, Hyunsun; Kim, Seo-Yeon; Yoon, Hyo Jae

    Advanced Electronic Materials (2020), 6 (2), 1901157CODEN: AEMDBW; ISSN:2199-160X. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A review. Mol. electronics promise electronic devices that are miniaturized to mol. and at. levels. Despite significant efforts made towards understanding and improving the electronic functions of devices based on homogeneous mol. systems, the roles, effects, and importance of mol.-level diln. and mixing in active components for device performance have only recently begun to be understood. Recent advances in mol. electronics based on heterogeneous active components, defined as mixed mol. electronics, are reviewed, with a particular focus on mol. tunnel junctions contg. mixed self-assembled monolayers (SAMs). Through a stepwise approach, the following is provided: a) a summary of the unique (supra)mol. and electronic structural features of mixed SAMs compared to homogeneous SAMs, b) a discussion on fundamental understanding of charge transport across mixed mol. systems, c) a comprehensive summary of functional mixed-mol. tunneling devices, and d) perspectives on the future direction of mixed mol. electronics.
21. 21

    Jin, J.; Kong, G. D.; Yoon, H. J. Deconvolution of Tunneling Current in Large-Area Junctions Formed with Mixed Self-Assembled Monolayers. J. Phys. Chem. Lett. 2018, 9, 4578– 4583,  DOI: 10.1021/acs.jpclett.8b01997

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    Deconvolution of Tunneling Current in Large-Area Junctions Formed with Mixed Self-Assembled Monolayers

    Jin, Junji; Kong, Gyu Don; Yoon, Hyo Jae

    Journal of Physical Chemistry Letters (2018), 9 (16), 4578-4583CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)

    Whereas single-component self-assembled monolayers (SAMs) have served widely as org. components in mol. and org. electronics, how the performance of the device is influenced by the heterogeneity of monolayers was little understood. This paper describes charge transport by quantum tunneling across mixed SAMs of n-alkanethiolates of different lengths formed on ultraflat template-stripped gold substrate. Elec. characterization using liq. metal comprising eutectic gallium-indium alloy reveals that the surface topog. of monolayer largely depends on the difference in length between the thiolates and is translated into distribution of tunneling c.d. As the length difference is more significant, more phase segregation takes place, increasing the modality of Gaussian fitting curves. Consequently, statistical anal. permits access to deconvolution of tunneling currents, mirroring the phase-segregated surface. The authors' work provides an insight into the role of surface topog. in the performance of mol.-scale electronic devices.
22. 22

    Kong, G. D.; Kim, M.; Cho, S. J.; Yoon, H. J. Gradients of Rectification: Tuning Molecular Electronic Devices by the Controlled Use of Different-Sized Diluents in Heterogeneous Self-Assembled Monolayers. Angew. Chem., Int. Ed. 2016, 55, 10307– 10311,  DOI: 10.1002/anie.201604748

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    Gradients of Rectification: Tuning Molecular Electronic Devices by the Controlled Use of Different-Sized Diluents in Heterogeneous Self-Assembled Monolayers

    Kong, Gyu Don; Kim, Miso; Cho, Soo Jin; Yoon, Hyo Jae

    Angewandte Chemie, International Edition (2016), 55 (35), 10307-10311CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Mol. electronics has received significant attention in the last decades. To hone performance of devices, eliminating structural defects in mol. components inside devices is usually needed. The authors herein demonstrate this problem can be turned into a strength for modulating the performance of devices. The authors show the systematic diln. of a monolayer of an org. rectifier (2,2'-bipyridine-terminated n-undecanethiolate) with electronically inactive diluents (n-alkanethiolates of different lengths), gives remarkable gradients of rectification. Rectification is finely tunable in a range of approx. two orders of magnitude, retaining its polarity. Trends of rectification against the length of the diluent indicate the gradient of rectification is extremely sensitive to the mol. structure of the diluent. Further studies reveal that noncovalent intermol. interactions within monolayers likely leads to gradients of structural defect and rectification.
23. 23

    Kong, G. D.; Jin, J.; Thuo, M.; Song, H.; Joung, J. F.; Park, S.; Yoon, H. J. Elucidating the Role of Molecule–Electrode Interfacial Defects in Charge Tunneling Characteristics of Large-Area Junctions. J. Am. Chem. Soc. 2018, 140, 12303– 12307,  DOI: 10.1021/jacs.8b08146

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    Elucidating the Role of Molecule-Electrode Interfacial Defects in Charge Tunneling Characteristics of Large-Area Junctions

    Kong, Gyu Don; Jin, Junji; Thuo, Martin; Song, Hyunsun; Joung, Joonyoung F.; Park, Sungnam; Yoon, Hyo Jae

    Journal of the American Chemical Society (2018), 140 (38), 12303-12307CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Interfacial chem. at org.-inorg. contact critically dets. the function of a wide range of mol. and org. electronic devices and other systems. The chem. is, however, difficult to understand due to the lack of easily accessible in-operando spectroscopic techniques that permit access to interfacial structure on a mol. scale. Herein, we compare two analogous junctions formed with identical org. thin film and different liq. top-contacts (water droplet vs eutectic gallium indium alloy) and elucidate the puzzling interfacial characteristics. Specifically, we fine-tune the surface topog. of the org. surface using mixed self-assembled monolayers (SAMs): single component SAM composed of rectifier (2,2'-bipyridyl-terminated n-undecanethiolate; denoted as SC11BIPY) is systematically dild. with nonrectifying n-alkanethiolates of different lengths (denoted as SCn where n = 8, 10, 12, 14, 16, 18). Characterization of the resulting mixed SAMs in wettability and tunneling currents with the two sep. liq. top-contacts allows us to investigate the role of phase segregation and gauche defect in the SAM//liq. interfaces. The results reported here show the difference in length between SC11BIPY and SCn is translated into nanoscopic pits and gauche-conformer defects on the surface, and the difference in contact force-hydrostatic vs user pressures-and hence conformity of contact account for the difference in wettability and rectification behaviors. Our work provides an insight into the role of mol.-electrode interfacial defects in performance of mol.-scale electronic devices.
24. 24

    Ben Amara, F.; Dionne, E. R.; Kassir, S.; Pellerin, C.; Badia, A. Molecular Origin of the Odd–Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?. J. Am. Chem. Soc. 2020, 142, 13051– 13061,  DOI: 10.1021/jacs.0c04288

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    24

    Molecular Origin of the Odd-Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

    Ben Amara, Fadwa; Dionne, Eric R.; Kassir, Sahar; Pellerin, Christian; Badia, Antonella

    Journal of the American Chemical Society (2020), 142 (30), 13051-13061CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Elucidating the influence of the monolayer interface vs. bulk on the macroscopic properties (e.g., surface hydrophobicity, charge transport, and electron transfer) of org. self-assembled monolayers (SAMs) chem. anchored to metal surfaces is a challenge. This article reports the characterization of prototypical SAMs of n-alkanethiolates on gold (CH3(CH2)nSAu, n = 6-19) at the macroscopic scale by electrochem. impedance spectroscopy and contact angle goniometry, and at the mol. level, by IR reflection absorption spectroscopy. The SAM capacitance, dielec. const., and surface hydrophobicity exhibit dependencies on both the length (n) and parity (nodd or neven) of the polymethylene chain. The peak positions of the CH2 stretching modes indicate a progressive increase in the chain conformational order with increasing n between n = 6 and 16. SAMs of nodd have a greater degree of structural gauche defects than SAMs of neven. The peak intensities and positions of the CH3 stretching modes are chain length independent but show an odd-even alternation of the spatial orientation of the terminal CH3. The correlations between the different data trends establish that the chain length dependencies of the dielec. const. and surface hydrophobicity originate from changes in the polymethylene chain conformation (bulk), while the odd-even variation arises primarily from a difference in the chem. compn. of the interface related to the terminal group orientation. These findings provide new phys. insights into the structure-property relation of SAMs for the design of ultrathin film dielecs. as well as the understanding of stereostructural effects on the elec. characteristics of tunnel junctions.
25. 25

    Chen, J.; Giroux, T. J.; Nguyen, Y.; Kadoma, A. A.; Chang, B. S.; VanVeller, B.; Thuo, M. M. Understanding interface (odd–even) effects in charge tunneling using a polished EGaIn electrode. Phys. Chem. Chem. Phys. 2018, 20, 4864– 4878,  DOI: 10.1039/C7CP07531F

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    Understanding interface (odd-even) effects in charge tunneling using a polished EGaIn electrode

    Chen, Jiahao; Giroux, Thomas J.; Nguyen, Yen; Kadoma, Atte A.; Chang, Boyce S.; VanVeller, Brett; Thuo, Martin M.

    Physical Chemistry Chemical Physics (2018), 20 (7), 4864-4878CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)

    Charge transport across large area mol. tunneling junctions is widely studied due to its potential in the development of quantum electronic devices. Large area junctions based on eutectic gallium indium (used in the form of a conical tip top electrode) have emerged as a reliable platform for delineating structure-property relationships. Discrepancies, however, arise from different tip-morphologies and fabrication techniques. It can be, therefore, challenging to make reliable conclusions based on mol. features. Of particular note is the discrepancy between the behaviors of hydrocarbons contg. odd and even numbered carbons across different EGaIn electrodes. Moreover, inconsistencies in tip roughness and oxide thickness can lead to more than a 100× increase in current densities with narrow distribution in data. Besides effects on the precision vs. accuracy of data, a theor. predicted length-dependent limit to observation of the odd-even effect has not been realized exptl. We developed a method to chem. polish the EGaIn tip to allow formation of smooth conformal contact due to re-establishment of liq. character at the point of contact though tension-driven reconstruction of a thin oxide layer. To evaluate the polished tip, we measured charge transport behavior across n-alkanethiolate SAMs and obsd. good correlation in the odd-even oscillation behavior to that obsd. from wetting studies. Since these mols. are homologues of each other, only differing in the orientation of the terminal CH2CH3 moiety, the odd-even effects are governed by orientation induced differences in the absences of SAM (gauche) defects. Comparison of obtained data with the literature shows significant difference between odd-numbered SAMs across Ag and Au.
26. 26

    Chen, J.; Chang, B.; Oyola-Reynoso, S.; Wang, Z.; Thuo, M. Quantifying Gauche Defects and Phase Evolution in Self-Assembled Monolayers through Sessile Drops. ACS Omega 2017, 2, 2072– 2084,  DOI: 10.1021/acsomega.7b00355

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    Quantifying Gauche Defects and Phase Evolution in Self-Assembled Monolayers through Sessile Drops

    Chen, Jiahao; Chang, Boyce; Oyola-Reynoso, Stephanie; Wang, Zhengjia; Thuo, Martin

    ACS Omega (2017), 2 (5), 2072-2084CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)

    Self-assembled monolayers (SAMs) are widely used in surface modifications, specifically in tuning surface chem. of materials. Structure and properties of SAMs have been extensively studied often with sophisticated tools, even for the simplest nalkanethiolate SAMs. In SAMs, esp. in linear n-alkanethiolates, properties are dependent on chain-length that is best manifested in the so-called odd-even effect, a simple yet not fully understood phenomenon. One main challenge is fully delineating the origin of length-dependent properties, which can be due to structure (ideal SAMs), defect evolution, or substrate-mol. effects. This study demonstrates that utilizing wetting behavior of polar (water) and non-polar (hexadecane) solvents on n-alkanethiolate SAMs formed on ultra-flat gold and silver surfaces, evolution of chain-length dependent gauche defects can be revealed and parameterized through a newly defined dimensionless no. (χ). The observation of odd-even effect in hydrophobicity, however, depends on the thiols chain length, and it was only obsd. on longer chain (> C8) mols. The trend in this odd-even effect demonstrate that there are three main transitions in nature of wetting, hence structure, across n-alkanethiols. From wetting with hexadecane, the role of dispersive components in wetting reveal that the SAMs are dynamic, which we attribute to rotations assocd. with previously reported evolution in gauche defects and changes in packing d. From re-expression of the Young-Dupre equation, therefore, we define a new dimensionless no. assocd. with mol. conformations, whose periodicity mirrors the energetics of Goodman's conformations of n-alkanes in unbound states and assocd. four- or two-twist turns. We therefore infer that the evolution in surface energy is largely due to mol. conformations and assocd. relaxations of the bound thiolates.
27. 27

    Rooth, M.; Shaw, A. M. pH-Controlled Formation Kinetics of Self-Assembled Layers of Thioctic Acid on Gold Nanoparticles. J. Phys. Chem. C 2007, 111, 15363– 15369,  DOI: 10.1021/jp075083l

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    27

    pH-Controlled Formation Kinetics of Self-Assembled Layers of Thioctic Acid on Gold Nanoparticles

    Rooth, Maxim; Shaw, Andrew M.

    Journal of Physical Chemistry C (2007), 111 (42), 15363-15369CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    The adsorption kinetics of thioctic acid (ToA) self-assembled monolayer (SAM) formation has been obsd. by monitoring small changes in the extinction of a gold nanoparticle surface, interrogated by evanescent wave cavity ring-down spectroscopy. A direct measurement of the adsorption kinetics for charged and neutral SAM formation has been made and modeled and shows significant differences in the lateral interactions between surface moieties: the interaction parameter for the charged species is 20 times larger than for the neutral species. Titrn. of ToA SAMs deposited from basic conditions demonstrate a pH-switchable surface with charge densities varying from 0.1 to 1.0 e nm-2. Similar measurements were performed from SAMs deposited under acidic conditions producing charge densities of 1.8 e nm-2. The surface potential varies between -44 and -198 mV for all SAMs, which produces an interfacial pH 0.8-3.4 units more acidic than the bulk and interfacial capacitances varying from 5.5 to 88 μF cm-2.
28. 28

    Han, S.; Park, H.; Han, J. W.; Yoshizawa, K.; Hayashi, T.; Hara, M.; Noh, J. Solvent Effect on the Formation of Octaneselenocyanate Self-Assembled Monolayers on Au(111). J. Nanosci. Nanotechnol. 2019, 19, 4795– 4798,  DOI: 10.1166/jnn.2019.16706

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    Solvent effect on the formation of octaneselenocyanate self-assembled monolayers on Au(111)

    Han, Seulki; Park, Heegun; Han, Jin Wook; Yoshizawa, Kaisel; Hayashi, Tomohiro; Hara, Masahiko; Noh, Jaegeun

    Journal of Nanoscience and Nanotechnology (2019), 19 (8), 4795-4798CODEN: JNNOAR; ISSN:1533-4880. (American Scientific Publishers)

    Solvent effect on the formation and electrochem. behavior of octaneselenolate (C8Se) self-assembled monolayers (SAMs) on Au(111) derived from the adsorption of octaneselenocyanate (C8Se-CN) mols. in various solvents at 363 K fort h was examd. by scanning tunneling microscopy (STM) and cyclic voitammetry (CV). STM imaging clearly revealed that the formation and structure of C8Se SAMs were markedly influenced by the polarity of solvent. C8Se SAMs formed in octane were composed of liq.-like disordered phase. In contrast, C8Se SAMs formed in DMF had ordered domains but not a uniform surface, whereas C8Se SAMs formed in ethanol had uniform surface and highly ordered domains with a (2 x 2 √13) structure. it was found that the structural quality of C8Se SAMs increased with increasing solvent polarity in the order of ethanol >DMF > octane. CV measurements also showed that blocking efficiency of C8Se SAMs for electrode reaction increased with increasing the structural quality of SAMs.
29. 29

    Ivashenko, O.; van Herpt, J.; Feringa, B.; Browne, W.; Rudolf, P. Rapid Reduction of Self-Assembled Monolayers of a Disulfide Terminated Para-Nitrophenyl Alkyl Ester on Roughened Au Surfaces During XPS Measurements. Chem. Phys. Lett. 2013, 559, 76– 81,  DOI: 10.1016/j.cplett.2012.12.060

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    Rapid reduction of self-assembled monolayers of a disulfide terminated para-nitrophenyl alkyl ester on roughened Au surfaces during XPS measurements

    Ivashenko, O.; van Herpt, J. T.; Feringa, B. L.; Browne, W. R.; Rudolf, P.

    Chemical Physics Letters (2013), 559 (), 76-81CODEN: CHPLBC; ISSN:0009-2614. (Elsevier B.V.)

    The application of multiple spectroscopic techniques to the characterization of self-assembled monolayers (SAMs) is essential for detg. whether the desired surface functionalization was achieved, the mols. are integer after self-assembly and well-packed on the substrate. The authors studied the use of surface enhanced Raman spectroscopy and XPS to characterize SAMs of a disulfide-functionalized nitrobenzene (i.e. 4-nitrophenyl-5-(1,2-dithiolan-3-yl)pentanoate). Electron-induced damage during acquisition of photoemission spectra of the monolayer occurs through redn. of the nitro group. Use of roughened bulk gold substrates to enhance the SERS sensitivity results in an increase in the rate at which the redn. proceeds.
30. 30

    Kong, G. D.; Yoon, H. J. Influence of Air-Oxidation on Rectification in Thiol-Based Molecular Monolayers. J. Electrochem. Soc. 2016, 163, G115– G121,  DOI: 10.1149/2.0091609jes

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    Influence of Air-Oxidation on Rectification in Thiol-Based Molecular Monolayers

    Kong, Gyu Don; Yoon, Hyo Jae

    Journal of the Electrochemical Society (2016), 163 (9), G115-G121CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)

    The effect of air oxidn. on mol. rectification in large-area junctions formed from a self-assembled monolayer (SAM) comprising 2,2'-bipyridyl terminated n-alkanethiolate is studied. Upon exposure to air, the rectification ratio decreases as a function of air-exposure time and disappears in ∼3 h. Structural analyses of the SAM using XPS spectra and wet electrochem., as well as several phys.-org. studies indicate that the decrease of rectification over time stems from structural degrdn., facilitated by light, of the thiolate anchoring group of the SAM.
31. 31

    Lee, L. Y. S.; Lennox, R. B. Electrochemical Desorption of N-Alkylthiol SAMs on Polycrystalline Gold: Studies Using a Ferrocenylalkylthiol Probe. Langmuir 2007, 23, 292– 296,  DOI: 10.1021/la061684c

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    Electrochemical Desorption of n-Alkylthiol SAMs on Polycrystalline Gold: Studies Using A Ferrocenylalkylthiol Probe

    Lee, Lawrence Yoon Suk; Lennox, R. Bruce

    Langmuir (2007), 23 (1), 292-296CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    The reductive voltammetric desorption of n-alkylthiol self-assembled monolayers (SAMs) was studied using ferrocenyldodecanethiol (FcC12SH) as a probe. A short (10 s) incubation in a 2 mM FcC12SH soln. labels the pre-existing defect sites present in a tetradecanethiol (C14S-Au) SAM. Addnl. defects in a C14S-Au SAM are then created by voltammetric cycling to reductive potentials. The effects of changing the desorption potential and the hold time at the desorption potential were studied by monitoring the change in the 2 characteristic FcC12S-Au peaks. These 2 peaks are assocd. with FcC12S-Au filling individual single site defects (peak I at 260 mV) and pinhole defects (peak II at 380 mV). The reductive desorption potential applied to a binary (FcC12S-/C14S-Au) SAM results in a partial desorption and a potential remixing of the phase-sepd. state.
32. 32

    Neuman, K. C.; Nagy, A. Single-Molecule Force Spectroscopy: Optical Tweezers, Magnetic Tweezers and Atomic Force Microscopy. Nat. Methods 2008, 5, 491– 505,  DOI: 10.1038/nmeth.1218

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    Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy

    Neuman, Keir C.; Nagy, Attila

    Nature Methods (2008), 5 (6), 491-505CODEN: NMAEA3; ISSN:1548-7091. (Nature Publishing Group)

    A review. Single-mol. force spectroscopy has emerged as a powerful tool to investigate the forces and motions assocd. with biol. mols. and enzymic activity. The most common force spectroscopy techniques are optical tweezers, magnetic tweezers and at. force microscopy. Here the authors describe these techniques and illustrate them with examples highlighting current capabilities and limitations.
33. 33

    Martínez, L.; Carrascosa, L. G.; Huttel, Y.; Lechuga, L. M.; Román, E. Influence of the Linker Type on the Au–S Binding Properties of Thiol and Disulfide-Modified DNA Self-Assembly on Polycrystalline Gold. Phys. Chem. Chem. Phys. 2010, 12, 3301– 3308,  DOI: 10.1039/b924504a

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    Influence of the linker type on the Au-S binding properties of thiol and disulfide-modified DNA self-assembly on polycrystalline gold

    Martinez, Lidia; Carrascosa, Laura G.; Huttel, Yves; Lechuga, Laura M.; Roman, Elisa

    Physical Chemistry Chemical Physics (2010), 12 (13), 3301-3308CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)

    We investigate the Au-S binding properties of thiol and disulfide-modified DNA on polycryst. gold by means of XPS in conditions close to dynamic processes of biosensors. The dependence of the immobilization period on the quality and d. of the self-assembly process of thiol (SH-(CH2)6-DNA), disulfide (DNA-CH2)6-SS-(CH2)6-DNA and DMTO-SS-(CH2)6-DNA) sulfur-modified oligonucleotide solns. (1 μM) that are employed for bioreceptor immobilization is analyzed. Two electronic components are found in the anal. of the S 2p core levels. One of them is clearly assocd. to thiolate formation, while the other can be assocd. to different origins. In order to identify the origin of this last component, a quantification of the non-specifically adsorbed species has been performed by rinsing the self-assembled monolayers (SAMs) with a mercapto hexanol (MCH) soln. It has been found that non-specifically adsorbed species contribute only partially to the appearance of this sulfur peak component in SAMs formed from disulfides. Electron bombardment was performed to study the evolution of this component as a consequence of surface degrdn. due to radiation effects. The results are also correlated with the possible presence of disulfides. We found that MCH is not stable during the measurements. The evolution of this compd. and the possible causes for this behavior are discussed.
34. 34

    Souto, M.; Yuan, L.; Morales, D. C.; Jiang, L.; Ratera, I.; Nijhuis, C. A.; Veciana, J. Tuning the Rectification Ratio by Changing the Electronic Nature (Open-Shell and Closed-Shell) in Donor–Acceptor Self-Assembled Monolayers. J. Am. Chem. Soc. 2017, 139, 4262– 4265,  DOI: 10.1021/jacs.6b12601

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    34

    Tuning the Rectification Ratio by Changing the Electronic Nature (Open-Shell and Closed-Shell) in Donor-Acceptor Self-Assembled Monolayers

    Souto, Manuel; Yuan, Li; Morales, Dayana C.; Jiang, Li; Ratera, Imma; Nijhuis, Christian A.; Veciana, Jaume

    Journal of the American Chemical Society (2017), 139 (12), 4262-4265CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    This Communication describes the mechanism of charge transport across self-assembled monolayers (SAMs) of two donor-acceptor systems consisting of a polychlorotriphenylmethyl (PTM) electron-acceptor moiety linked to an electron-donor ferrocene (Fc) unit supported by ultraflat template-stripped Au and contacted by a eutectic alloy of gallium and indium top contacts. The electronic and supramol. structures of these SAMs were well characterized. The PTM unit can be switched between the nonradical and radical forms, which influences the rectification behavior of the junction. Junctions with nonradical units rectify currents via the HOMO with a rectification ratio R = 99, but junctions with radical units have a new accessible state, a single-unoccupied MO (SUMO), which turns rectification off and drops R to 6.
35. 35

    Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Self-Assembled Monolayers of Thiols and Dithiols on Gold: New Challenges for a Well-Known System. Chem. Soc. Rev. 2010, 39, 1805– 1834,  DOI: 10.1039/b907301a

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    Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system

    Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C.

    Chemical Society Reviews (2010), 39 (5), 1805-1834CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)

    A review. Self-assembled monolayers (SAMs) of alkanethiols and dialkanethiols on gold are key elements for building many systems and devices with applications in the wide field of nanotechnol. Despite the progress made in the knowledge of these fascinating two-dimensional mol. systems, there are still several hot topics that deserve special attention to understand and to control their phys. and chem. properties at the mol. level. This crit. review focuses on some of these topics, including the nature of the mol.-gold interface, whose chem. and structure remain elusive, the self-assembly process on planar and irregular surfaces, and on nanometer-sized objects, and the chem. reactivity and thermal stability of these systems in ambient and aq. solns., an issue which seriously limits their technol. applications (375 refs.).
36. 36

    Inkpen, M. S.; Liu, Z.-F.; Li, H.; Campos, L. M.; Neaton, J. B.; Venkataraman, L. Non-Chemisorbed Gold-Sulfur Binding Prevails in Self-Assembled Monolayers. Nat. Chem. 2019, 11, 351– 358,  DOI: 10.1038/s41557-019-0216-y

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    Non-chemisorbed gold-sulfur binding prevails in self-assembled monolayers

    Inkpen, Michael S.; Liu, Zhen-Fei; Li, Haixing; Campos, Luis M.; Neaton, Jeffrey B.; Venkataraman, Latha

    Nature Chemistry (2019), 11 (4), 351-358CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)

    Gold-thiol contacts are ubiquitous across the phys. and biol. sciences in connecting org. mols. to surfaces. When thiols bind to gold in self-assembled monolayers (SAMs) the fate of the hydrogen remains a subject of profound debate-with implications for our understanding of their phys. properties, spectroscopic features and formation mechanism(s). Exploiting measurements of the transmission through a mol. junction, which is highly sensitive to the nature of the mol.-electrode contact, we demonstrate here that the nature of the gold-sulfur bond in SAMs can be probed via single-mol. conductance measurements. Critically, we find that SAM measurements of dithiol-terminated mol. junctions yield a significantly lower conductance than soln. measurements of the same mol. Through numerous control expts., conductance noise anal. and transport calcns. based on d. functional theory, we show that the gold-sulfur bond in SAMs prepd. from the soln. deposition of dithiols does not have chemisorbed character, which strongly suggests that under these widely used prepn. conditions the hydrogen is retained.
37. 37

    Zotti, L. A.; Kirchner, T.; Cuevas, J.-C.; Pauly, F.; Huhn, T.; Scheer, E.; Erbe, A. Revealing the Role of Anchoring Groups in the Electrical Conduction Through Single-Molecule Junctions. Small 2010, 6, 1529– 1535,  DOI: 10.1002/smll.200902227

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    Revealing the Role of Anchoring Groups in the Electrical Conduction Through Single-Molecule Junctions

    Zotti, Linda A.; Kirchner, Thomas; Cuevas, Juan-Carlos; Pauly, Fabian; Huhn, Thomas; Scheer, Elke; Erbe, Artur

    Small (2010), 6 (14), 1529-1535CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)

    A combined exptl. and theor. study is presented revealing the influence of metal-mol. coupling on electronic transport through single-mol. junctions. Transport expts. through tolane mols. attached to gold electrodes via thiol, nitro, and cyano anchoring groups are performed. By fitting the exptl. current-voltage characteristics to a single-level tunneling model, we ext. both the position of the MO closest to the Fermi energy and the strength of the metal-mol. coupling. The values found for these parameters are rationalized with the help of d.-functional-theory-based transport calcns. In particular, these calcns. show that the anchoring groups det. the junction conductance by controlling not only the strength of the coupling to the metal but also the position of the relevant mol. energy levels.
38. 38

    Zharnikov, M.; Grunze, M. Spectroscopic Characterization of Thiol-Derived Self-Assembling Monolayers. J. Phys.: Condens. Matter 2001, 13, 11333– 11365,  DOI: 10.1088/0953-8984/13/49/314

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    Spectroscopic characterization of thiol-derived self-assembling monolayers

    Zharnikov, M.; Grunze, M.

    Journal of Physics: Condensed Matter (2001), 13 (49), 11333-11365CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)

    A review. This article reviews recent progress in the spectroscopic characterization of aliph. and arom. thiol-derived self-assembled monolayers (SAMs) on noble metal substrates. Several complementary techniques such as near edge x-ray absorption fine structure spectroscopy, XPS, and IR reflection absorption spectroscopy were applied to study the balance between intermol. and adsorbate-substrate interactions, chem. identity of the headgroup, and absorption site homogeneity at the sulfur-metal interface. Whereas in the thioaliph. SAMs the headgroup-substrate interaction is a decisive factor for the structure and packing in these films, these parameters are mainly detd. by the intermol. interactions in the thioarom. films. Only one sulfur species could be detected in the S 2p HRXPS spectra of both aliph. and arom. SAMs suggesting binding of individual mols. as thiolates. Conclusions on the heterogeneity of the adsorption sites are derived and evidence that the studied films represent highly correlated mol. assemblies are presented.
39. 39

    Rodriguez-Douton, M. J.; Mannini, M.; Armelao, L.; Barra, A.-L.; Tancini, E.; Sessoli, R.; Cornia, A. One-Step Covalent Grafting of Fe4 Single-Molecule Magnet Monolayers on Gold. Chem. Commun. 2011, 47, 1467– 1469,  DOI: 10.1039/C0CC04583G

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    One-step covalent grafting of Fe4 single-molecule magnet monolayers on gold

    Rodriguez-Douton, Maria Jesus; Mannini, Matteo; Armelao, Lidia; Barra, Anne-Laure; Tancini, Erik; Sessoli, Roberta; Cornia, Andrea

    Chemical Communications (Cambridge, United Kingdom) (2011), 47 (5), 1467-1469CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)

    Iron(III)-based single-mol. magnets were covalently grafted on Au(111) in one step using 1,2-dithiolan-3-yl side groups. Reaction with the substrate quant. affords a monolayer of electronically intact clusters doubly linked to the surface via Au-S bonds, as demonstrated by a combination of STM, XAS/XMCD and XPS studies.
40. 40

    Kumar, S.; van Herpt, J. T.; Gengler, R. Y. N.; Feringa, B. L.; Rudolf, P.; Chiechi, R. C. Mixed Monolayers of Spiropyrans Maximize Tunneling Conductance Switching by Photoisomerization at the Molecule–Electrode Interface in EGaIn Junctions. J. Am. Chem. Soc. 2016, 138, 12519– 12526,  DOI: 10.1021/jacs.6b06806

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    Mixed Monolayers of Spiropyrans Maximize Tunneling Conductance Switching by Photoisomerization at the Molecule-Electrode Interface in EGaIn Junctions

    Kumar, Sumit; van Herpt, Jochem T.; Gengler, Regis Y. N.; Feringa, Ben L.; Rudolf, Petra; Chiechi, Ryan C.

    Journal of the American Chemical Society (2016), 138 (38), 12519-12526CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    This paper describes the photoinduced switching of conductance in tunneling junctions comprising self-assembled monolayers of a spiropyran moiety using eutectic Ga-In top contacts. Despite sepn. of the spiropyran unit from the electrode by a long alkyl ester chain, we observe an increase in the c.d. J of a factor of 35 at 1 V when the closed form is irradiated with UV light to induce the ring-opening reaction, one of the highest switching ratios reported for junctions incorporating self-assembled monolayers. The magnitude of switching of hexanethiol mixed monolayers was higher than that of pure spiropyran monolayers. The first switching event recovers 100% of the initial value of J and in the mixed-monolayers subsequent dampening is not the result of degrdn. of the monolayer. The observation of increased cond. is supported by zero-bias DFT calcns. showing a change in the localization of the d. of states near the Fermi level as well as by simulated transmission spectra revealing pos. resonances that broaden and shift toward the Fermi level in the open form.
41. 41

    Ivashenko, O.; van Herpt, J. T.; Feringa, B. L.; Rudolf, P.; Browne, W. R. UV/Vis and NIR Light-Responsive Spiropyran Self-Assembled Monolayers. Langmuir 2013, 29, 4290– 4297,  DOI: 10.1021/la400192c

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    41

    UV/Vis and NIR Light-Responsive Spiropyran Self-Assembled Monolayers

    Ivashenko, Oleksii; van Herpt, Jochem T.; Feringa, Ben L.; Rudolf, Petra; Browne, Wesley R.

    Langmuir (2013), 29 (13), 4290-4297CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    Self-assembled monolayers of a 6-nitro BIPS spiropyran (SP) modified with a disulfide-terminated aliph. chain were prepd. on polycryst. gold surfaces and characterized by UV/vis absorption, surface-enhanced Raman scattering (SERS), and X-ray photoelectron spectroscopies (XPS). The SAMs obtained are composed of the ring-closed form (i.e., spiropyran) only. Irradn. with UV light results in conversion of the monolayer to the merocyanine form (MC), manifested in the appearance of an N+ contribution in the N 1s region of the XPS spectrum of the SAMs, the characteristic absorption band of the MC form in the visible region at 555 nm, and the C-O stretching band in the SERS spectrum. Recovery of the initial state of the monolayer was obsd. both thermally and after irradn. with visible light. Several switching cycles were performed and monitored by SERS spectroscopy, demonstrating the stability of the SAMs during repeated switching between SP and MC states. A key finding in the present study is that ring-opening of the surface-immobilized spiropyrans can be induced by irradn. with continuous wave NIR (785 nm) light as well as by irradn. with UV light. We demonstrate that ring-opening by irradn. at 785 nm proceeds by a two-photon absorption pathway both in the SAMs and in the solid state. Hence, spiropyran SAMs on gold can undergo reversible photochem. switching from the SP to the MC form with both UV and NIR and the reverse reaction induced by irradn. with visible light or heating. Furthermore, the observation of NIR-induced switching with a continuous wave source holds important consequences in the study of photochromic switches on surfaces using SERS and emphasizes the importance of the use of multiple complementary techniques in characterizing photoresponsive SAMs.
42. 42

    Hamoudi, H.; Esaulov, V. A. Selfassembly of α,ω-Dithiols on Surfaces and Metal Dithiol Heterostructures. Ann. Phys. 2016, 528, 242– 263,  DOI: 10.1002/andp.201500280

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    Self-assembly of α,ω-dithiols on surfaces and metal dithiol heterostructures

    Hamoudi, Hicham; Esaulov, Vladimir A.

    Annalen der Physik (Berlin, Germany) (2016), 528 (3-4), 242-263CODEN: ANPYA2; ISSN:0003-3804. (Wiley-VCH Verlag GmbH & Co. KGaA)

    α,ω-Dithiols present an interesting case of mols. with two reactive terminal -SH groups (HS-R-SH) that allow their use as binders between different metallic entities. They have thus been used in mol. electronics conduction measurements, in "nanogap" electrodes of interest in plasmonics, as building blocks of more complex structures such as metal intercalated superlattices and in the formation of metalized org. thin films, including doped graphene type films. There exist however many problems, because the mols. may end up in undesirable configurations with both thiol terminals bound to the same metal particle/substrate or link with other mols. to produce "multi-mol." or "multilayer" structures. This report discusses various key questions on dithiol linking with metal surfaces and the quest of protocols of making problem free dithiol metal structures. It then describes the use of dithiols and their SAMs to produce various metal org. heterostructures useful for mol. electronics and formation of doped metalized org. thin films. We discuss the build up of these structures by self assembly and lithog., their chem. compn. and functional properties.
43. 43

    Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. High Resolution X-Ray Photoelectron Spectroscopy Measurements of Octadecanethiol Self-Assembled Monolayers on Au(111). Langmuir 1998, 14, 2092– 2096,  DOI: 10.1021/la971104z

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    High Resolution X-ray Photoelectron Spectroscopy Measurements of Octadecanethiol Self-Assembled Monolayers on Gold(111)

    Ishida, Takao; Hara, Masahiko; Kojima, Isao; Tsuneda, Satoshi; Nishida, Naoki; Sasabe, Hiroyuki; Knoll, Wolfgang

    Langmuir (1998), 14 (8), 2092-2096CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    We have measured high-resoln. XPS spectra of 1-octadecanethiol (ODT) self-assembled monolayers (SAMs) on a Au(111) surface in order to investigate the Au-S binding properties at the initial stage of SAM growth and after the desorption of the ODT from the Au surface. For the SAM prepd. by the 1 min immersion into the ODT soln., we found new sulfur species around 161 eV and assigned them to isolated sulfur without C-S cleavage. We also confirmed the presence of a similar 161 eV peak in the S(2p) spectra after desorption of the ODT mols. Furthermore, we obsd. a peak shift in the carbon 1s (C(1s)) peak, depending on the surface coverage of the ODT. In addn. to the C(1s) peak of 285.1 eV which might correspond to alkyl chains of densely packed ODT mols., another lower binding energy peak at 284.3 eV appeared after annealing. This lower C(1s) binding energy peak formation suggested that some of the alkyl chains for the remaining ODT mols. might be oriented parallel to the Au surface after annealing.
44. 44

    Cortés, E.; Rubert, A. A.; Benitez, G.; Carro, P.; Vela, M. E.; Salvarezza, R. C. Enhanced Stability of Thiolate Self-Assembled Monolayers (SAMs) on Nanostructured Gold Substrates. Langmuir 2009, 25, 5661– 5666,  DOI: 10.1021/la804251a

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    Enhanced Stability of Thiolate Self-Assembled Monolayers (SAMs) on Nanostructured Gold Substrates

    Cortes, Emiliano; Rubert, Aldo A.; Benitez, Guillermo; Carro, Pilar; Vela, Maria E.; Salvarezza, Roberto C.

    Langmuir (2009), 25 (10), 5661-5666CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    Degrdn. of thiolate self-assembled monolayers (SAMs) in ambient conditions and liq. environments seriously limits the fabrication of thiol-based devices. Here, we demonstrate that nanostructured gold exhibits higher resistance to SAM degrdn. and increased electrochem. stability against thiolate desorption in relation to polycryst. preferred oriented Au(111). The increased stability can be related to the presence of a large no. of defects, such as adatoms, vacancies, and steps where the thiolate binding energy is stronger than at terraces. The nanostructured Au is an interesting platform because it can be easily prepd., has surface enhanced Raman spectroscopy (SERS) activity, and exhibits a high signal/noise ratio for amperometric detection because of its large real surface area.
45. 45

    Watcharinyanon, S.; Nilsson, D.; Moons, E.; Shaporenko, A.; Zharnikov, M.; Albinsson, B.; Mårtensson, J.; Johansson, L. S. O. A Spectroscopic Study of Self-Assembled Monolayer of Porphyrin-Functionalized Oligo(phenyleneethynylene)s on Gold: The Influence of the Anchor Moiety. Phys. Chem. Chem. Phys. 2008, 10, 5264– 5275,  DOI: 10.1039/b802914h

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    A spectroscopic study of self-assembled monolayer of porphyrin-functionalized oligo(phenyleneethynylene)s on gold: the influence of the anchor moiety

    Watcharinyanon, Somsakul; Nilsson, Daniel; Moons, Ellen; Shaporenko, Andrey; Zharnikov, Michael; Albinsson, Bo; Maartensson, Jerker; Johansson, Lars S. O.

    Physical Chemistry Chemical Physics (2008), 10 (34), 5264-5275CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)

    Porphyrin-functionalized oligo(phenyleneethynylene)s (OPE) are promising mols. for mol. electronics applications. Three such mols. (1-3) with the common structure P-OPE-AG (P and AG are a porphyrin and anchor group, resp.) and different anchor groups, viz. an acetyl protected thiol, -S-COMe (1), an acetyl protected thiol with methylene linker, -CH2-S-COMe (2), and a trimethylsilylethynyl group, -C≡C-SiMe3 (3) were synthesized and the corresponding self-assembled monolayers (SAMs) on Au(111) substrates were prepd. The integrity and structural properties of these films were studied by XPS and near-edge x-ray absorption fine structure spectroscopy. Probably the films formed from 1 have a high orientational order with an almost upright orientation and dense packing of the mol. constituents, i.e. represent a high quality SAM. But mol. 2 formed disordered mol. layers on Au, even though the mol.-surface bonding (thiolate) is the same as in the case of mol. 1. Probably the methylene linker in mol. 2 has a strong impact on the quality of the resulting film, so that a well-ordered SAM cannot be formed. The silane system, 3, is also able to bind to the gold surface but the resulting SAM has a poor quality, being significantly disordered and/or comprised of strongly inclined mols. Probably the nature of the anchor group along with a possible linker is an important parameter which, to a high extent, predetermines the entire quality of OPE-based mol. layers.
46. 46

    Grumelli, D.; Cristina, L. J.; Maza, F. L.; Carro, P.; Ferrón, J.; Kern, K.; Salvarezza, R. C. Thiol Adsorption on the Au(100)-Hex and Au(100)-(1 × 1) Surfaces. J. Phys. Chem. C 2015, 119, 14248– 14254,  DOI: 10.1021/acs.jpcc.5b03931

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    46

    Thiol Adsorption on the Au(100)-hex and Au(100)-(1 × 1) Surfaces

    Grumelli, Doris; Cristina, Lucila J.; Maza, Flavia Lobo; Carro, Pilar; Ferron, Julio; Kern, Klaus; Salvarezza, Roberto C.

    Journal of Physical Chemistry C (2015), 119 (25), 14248-14254CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    Alkanethiol adsorption on the Au(100) surfaces was studied by using scanning tunneling microscopy, XPS, and electrochem. techniques. Adsorption of hexanethiol (HT) on the Au(100)-hex surface gave elongated Au islands following the typical stripes of the reconstruction. Ordered mol. arrays forming hexagonally distorted square patterns cover the stripes with surface coverage ≈0.33. However, HT adsorption on the Au(100)-(1 × 1) surface shows the absence of the elongated island and the formation of square mol. patterns with a surface coverage ≈0.44. The core level shift of thiolates adsorbed on the Au(100)-(1 × 1) and Au(111) is only 0.15 eV, suggesting that chem. rather than surface sites dets. the binding energy of the S 2p core level. These results are also important to complete the authors' knowledge of the chem. and surface structure for small thiolated AuNPs where the Au(100) together with the Au(111) are the dominant faces.
47. 47

    Park, J.-W.; Shumaker-Parry, J. S. Strong Resistance of Citrate Anions on Metal Nanoparticles to Desorption Under Thiol Functionalization. ACS Nano 2015, 9, 1665– 1682,  DOI: 10.1021/nn506379m

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    Strong Resistance of Citrate Anions on Metal Nanoparticles to Desorption under Thiol Functionalization

    Park, Jong-Won; Shumaker-Parry, Jennifer S.

    ACS Nano (2015), 9 (2), 1665-1682CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)

    Adsorbed citrates, probed by IR and XPS spectra, remained on the surface following thiol addn. to the gold nanoparticles (AuNPs), whereas acetoacetate anions were desorbed. XPS quant. anal. indicated that the surface d. of alkyl and aryl thiolates for AuNPs with an av. diam. of ∼40 nm was 50-65% of the value of a close-packed SAM on Au(111). A citrate/thiolate coadsorption model that describes this final mixed surface compn. was developed. Intermol. interactions between weakly coordinated oxyanions, such as polyprotic carboxylic acids, can lead to enhanced stability of the metal-ligand interactions, and this needs to be considered in the surface modification of metal nanoparticles by thiols or other anchor groups.
48. 48

    Cristina, L. J.; Ruano, G.; Salvarezza, R.; Ferrón, J. Thermal Stability of Self-Assembled Monolayers of N-Hexanethiol on Au(111)-(1 × 1) and Au(001)-(1 × 1). J. Phys. Chem. C 2017, 121, 27894– 27904,  DOI: 10.1021/acs.jpcc.7b05883

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    48

    Thermal stability of self-assembled monolayers of n-hexanethiol on Au(111)-(1 × 1) and Au(001)-(1 × 1)

    Cristina, Lucila J.; Ruano, Gustavo; Salvarezza, Roberto; Ferron, Julio

    Journal of Physical Chemistry C (2017), 121 (50), 27894-27904CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    Thermal desorption in an ultrahigh vacuum of n-hexanethiol (C6T) self-assembled monolayers (SAMs) prepd. from ethanolic solns. on Au(111) and Au(001) unreconstructed surfaces was investigated by XPS. The SAMs desorption was performed from room temp. (RT) to 380 K. We report that the hexanethiolate surface satn. coverage is bigger (∼0.4 ML) for the SAM on Au(001) than on Au(111) (∼0.33 ML). We identified a greater stability for C6T SAMs on Au(001). Large amts. of physisorbed species were found on preferred oriented (111) polycryst. Au at the low coverage regime at RT, while the SAM on the Au(001) single crystal at this conditions desorbs at a steady pace. At 340 K, both SAMs remain stable at the coverage expected for the lying-down phases that maximizes the van der Waals interactions. We observe that at higher temps. the carbon alpha-sulfur bond breaks, producing free S on both gold surfaces.
49. 49

    Rani, J. R.; Lim, J.; Oh, J.; Kim, D.; Lee, D.; Kim, J.-W.; Shin, H. S.; Kim, J. H.; Jun, S. C. Substrate and Buffer Layer Effect on the Structural and Optical Properties of Graphene Oxide Thin Films. RSC Adv. 2013, 3, 5926– 5936,  DOI: 10.1039/c3ra00028a

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    Substrate and buffer layer effect on the structural and optical properties of graphene oxide thin films

    Rani, J. R.; Lim, Juhwan; Oh, Juyeong; Kim, Dukhan; Lee, Dongil; Kim, Jung-Woo; Shin, Hyeon Suk; Kim, Jae Hun; Jun, Seong Chan

    RSC Advances (2013), 3 (17), 5926-5936CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)

    Graphene oxide (GO) thin films on various substrates show surprising variations of their structural and optical properties. These variations were also studied by depositing GO via introducing a gold nanoparticles buffer layer on quartz substrate. The effect of the substrate as well as buffer layer results in short range order crystn. in deposited GO films with an increase in inter-planar spacing. XPS anal. shows that GO undergoes redn. when spin coated on ITO/glass substrate. The deposited GO films exhibit luminescence emission, and the introduction of gold buffer layer results in a blue shift of the photoluminescent emission spectra. The GO on gold buffer layer shows almost const. optical absorption in the whole visible spectral region like graphene. The present study indicates that buffer layer effects and the interaction between different substrates and GO is strong enough to affect the oxygen linkages in GO which in turn changes its structural and optical properties, which may find potential application in graphene based optoelectronic device fabrication.
50. 50

    Ivashenko, O.; Logtenberg, H.; Areephong, J.; Coleman, A. C.; Wesenhagen, P. V.; Geertsema, E. M.; Heureux, N.; Feringa, B. L.; Rudolf, P.; Browne, W. R. Remarkable Stability of High Energy Conformers in Self-Assembled Monolayers of a Bistable Electro- And Photoswitchable Overcrowded Alkene. J. Phys. Chem. C 2011, 115, 22965– 22975,  DOI: 10.1021/jp206889y

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    Remarkable Stability of High Energy Conformers in Self-Assembled Monolayers of a Bistable Electro- and Photoswitchable Overcrowded Alkene

    Ivashenko, Oleksii; Logtenberg, Hella; Areephong, Jetsuda; Coleman, Anthony C.; Wesenhagen, Philana V.; Geertsema, Edzard M.; Heureux, Nicolas; Feringa, Ben L.; Rudolf, Petra; Browne, Wesley R.

    Journal of Physical Chemistry C (2011), 115 (46), 22965-22975CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)

    Although bistability of mol. switches in soln. is well established, achieving highly robust bistable mol. switching in self-assembled monolayers remains a challenge. Such systems are highly attractive as components in org. electronics and mol.-based photo- and electrochromic devices. Here the authors report a remarkably robust surface confined bis-thiaxanthylidene redox switch that shows excellent bistability, manifested in reversible changes in spectroscopic and electrochem. properties and in phys. properties such as water contact angle changes (ca. 30° difference in water contact angle between the two redox states of a bis-thiaxanthylidene self-assembled monolayer). The effect of surface immobilization of bis-thiaxanthylidene on its photochromic, thermal and electrochem. properties is described. Surface immobilization is achieved by incorporating thiol- and alkylsiloxy-terminated "legs" on one of the tricyclic arom. units. The mol. switch in its neutral and dicationic state, generated by bulk electrolysis, was characterized in soln., in the solid state and on surfaces, by UV/vis absorption, Fourier transform IR, x-ray photoelectron, and Raman spectroscopies and by cyclic voltammetry. In soln., the redox switching to the dicationic state is achieved by oxidn. at 1.2 V vs. SCE. Redn. of the dication at < 0.4 V results in initial formation of a highly unstable twisted conformation that reverts via a syn-folded conformational state to the most stable (anti-folded) conformer. Although the syn-folded state can be obtained by UV irradn. at < 0° C, the twisted conformation is not observable in soln., even at 200 K. Remarkably, in monolayers on electrodes this highly unstable form (which is generated by redn. of the dicationic state) is relatively stable even for several minutes. This stability is ascribed to the formation of densely packed monolayers in which the intermol. interactions provide for a substantial barrier to thermal interconversion of the various conformational states.
51. 51

    Wang, Z.; Dong, Y.; Li, H.; Zhao, Z.; Bin Wu, H.; Hao, C.; Liu, S.; Qiu, J.; Lou, X. W. (Enhancing Lithium-Sulphur Battery Performance by Strongly Binding the Discharge Products on Amino-Functionalized Reduced Graphene Oxide. Nat. Commun. 2014, 5, 5002,  DOI: 10.1038/ncomms6002

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    Enhancing lithium-sulphur battery performance by strongly binding the discharge products on amino-functionalized reduced graphene oxide

    Wang, Zhiyu; Dong, Yanfeng; Li, Hongjiang; Zhao, Zongbin; Wu, Hao Bin; Hao, Ce; Liu, Shaohong; Qiu, Jieshan; Lou, Xiong Wen

    Nature Communications (2014), 5 (), 5002CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)

    Lithium-sulfur batteries are one very appealing power source with high energy d. But their practical use is still hindered by several issues including short lifespan, low efficiency and safety concern from the lithium anode. Polysulfide dissoln. and insulating nature of sulfur are generally considered responsible for the capacity degrdn. However, the detachment of discharge products, i.e., highly polar lithium sulfides, from nonpolar carbon matrix (for example, graphene) has been rarely studied as one crit. factor. Here we report the strongly covalent stabilization of sulfur and its discharge products on amino-functionalized reduced graphene oxide that enables stable capacity retention of 80% for 350 cycles with high capacities and excellent high-rate response up to 4 C. The present study demonstrates a feasible and effective strategy to solve the long-term cycling difficulty for lithium-sulfur batteries and also helps to understand the capacity decay mechanism involved.
52. 52

    Wang, H.; Zhou, H.; Gestos, A.; Fang, J.; Niu, H.; Ding, J.; Lin, T. Robust, Electro-Conductive, Self-Healing Superamphiphobic Fabric Prepared by One-Step Vapour-Phase Polymerisation of Poly(3,4-Ethylenedioxythiophene) in the Presence of Fluorinated Decyl Polyhedral Oligomeric Silsesquioxane and Fluorinated Alkyl Silane. Soft Matter 2013, 9, 277– 282,  DOI: 10.1039/C2SM26871J

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    Robust, electro-conductive, self-healing superamphiphobic fabric prepared by one-step vapour-phase polymerization of poly(3,4-ethylenedioxythiophene) in the presence of fluorinated decyl polyhedral oligomeric silsesquioxane and fluorinated alkyl silane

    Wang, Hongxia; Zhou, Hua; Gestos, Adrian; Fang, Jian; Niu, Haitao; Ding, Jie; Lin, Tong

    Soft Matter (2013), 9 (1), 277-282CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)

    A robust, elec. conductive, superamphiphobic fabric was prepd. by vapor-phase polymn. of 3,4-ethylenedioxythiophene (EDOT) on fabric in the presence of fluorinated decyl polyhedral oligomeric silsesquioxane (FD-POSS) and a fluorinated alkyl silane (FAS). The coated fabric had contact angles of 169° and 156° resp. to water and hexadecane, and a surface resistance in the range of 0.8-1.2 kΩ .box.-1. The incorporation of FD-POSS and FAS into the PEDOT layer showed a very small influence on the cond. but improved the washing and abrasion stability considerably. The coated fabric can withstand at least 500 cycles of std. laundry and 10 000 cycles of abrasion without apparently changing the superamphiphobicity, while the cond. only had a small redn. after the washing and abrasion. More interestingly, the coating had a self-healing ability to auto-repair from chem. damages to restore the liq. repellency.
53. 53

    Bazylewski, P.; Boukhvalov, D. W.; Kukharenko, A. I.; Kurmaev, E. Z.; Hunt, A.; Moewes, A.; Lee, Y. H.; Cholakh, S. O.; Chang, G. S. The Characterization of Co-Nanoparticles Supported on Graphene. RSC Adv. 2015, 5, 75600– 75606,  DOI: 10.1039/C5RA12893E

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    The characterization of Co-nanoparticles supported on graphene

    Bazylewski, P.; Boukhvalov, D. W.; Kukharenko, A. I.; Kurmaev, E. Z.; Hunt, A.; Moewes, A.; Lee, Y. H.; Cholakh, S. O.; Chang, G. S.

    RSC Advances (2015), 5 (92), 75600-75606CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)

    The results of d. functional theory calcns. and measurements using XPS of Co-nanoparticles dispersed on graphene/Cu are presented. It is found that for low cobalt thickness (0.02-0.06 nm) the Co forms islands distributed non-homogeneously which are strongly oxidized under exposure to air to form cobalt oxides. At greater thicknesses up to 2 nm the upper Co-layers are similarly oxidized whereas the lower layers contacting the graphene remain metallic. The measurements indicate a Co2+ oxidn. state with no evidence of a 3+ state appearing at any Co thickness, consistent with CoO and Co[OH]2. The results show that thicker Co (2 nm) coverage induces the formation of a protective oxide layer while providing the magnetic properties of Co nanoparticles.
54. 54

    Tao, C.-a.; Wang, J.; Qin, S.; Lv, Y.; Long, Y.; Zhu, H.; Jiang, Z. Fabrication of pH-sensitive Graphene Oxide–Drug Supramolecular Hydrogels as Controlled Release Systems. J. Mater. Chem. 2012, 22, 24856– 24861,  DOI: 10.1039/c2jm34461k

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    Fabrication of pH-sensitive graphene oxide-drug supramolecular hydrogels as controlled release systems

    Tao, Cheng-an; Wang, Jianfang; Qin, Shiqiao; Lv, Yanan; Long, Yin; Zhu, Hui; Jiang, Zhenhua

    Journal of Materials Chemistry (2012), 22 (47), 24856-24861CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)

    A novel graphene oxide (GO)-based hydrogel with drug mols. as crosslinking agents was fabricated. Metformin hydrochloride (MFH), which has no arom. groups, but only has N-contg. functionalities, was chosen as a model drug. When a very small amt. of MFH was introduced into a GO soln., a supramol. assembled hydrogel was rapidly formed without any polymers or chem. additives. The gelation process can be influenced by the wt. ratio of MFH to GO. The driving forces were hydrogen bonding and electrostatic attraction. Both air-dried and freeze-dried samples of GO-MFH hydrogels were utilized for controlled drug release in water (pH = 7) and hydrochloric acid (pH = 1, 3 and 5). The release mechanism of capsulated MFH was Fickian diffusion according to the fitted results. Both the freeze-dried and air-dried samples released around 74% of the MFH in a strongly acidic medium after 70 h, but only 50% in a neutral soln. This pH-sensitivity makes it a potential candidate for the controlled release of drugs in the acidic environment of the stomach.
55. 55

    Haubner, K.; Murawski, J.; Olk, P.; Eng, L. M.; Ziegler, C.; Adolphi, B.; Jaehne, E. The Route to Functional Graphene Oxide. ChemPhysChem 2010, 11, 2131– 2139,  DOI: 10.1002/cphc.201000132

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    The Route to Functional Graphene Oxide

    Haubner, Kinga; Murawski, Jan; Olk, Phillip; Eng, Lukas M.; Ziegler, Christoph; Adolphi, Barbara; Jaehne, Evelin

    ChemPhysChem (2010), 11 (10), 2131-2139CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)

    We report on an easy-to-use, successful, and reproducible route to synthesize functionalized graphite oxide (GO) and its conversion to graphene-like materials through chem. or thermal redn. of GO. Graphite oxide contg. hydroxyl, epoxy, carbonyl, and carboxyl groups loses mainly hydroxyl and epoxy groups during redn., whereas carboxyl species remain untouched. The interaction of functionalized graphene with fluorescent methylene blue (MB) is investigated and compared to graphite, fully oxidized GO, as well as thermally and chem. reduced GO. Optical absorption and emission spectra of the composites indicate a clear preference for MB interaction with the GO derivs. contg. a large no. of functional groups (GO and chem. reduced GO), whereas graphite and thermally reduced GO only incorporate a few MB mols. These findings are consistent with thermogravimetric, X-ray photoelectron spectroscopic, and Raman data recorded at every stage of prepn. The optical data also indicate concn.-dependent aggregation of MB on the GO surface leading to stable MB dimers and trimers. The MB dimers are responsible for fluorescence quenching, which can be controlled by varying the pH value.
56. 56

    Lees, W. J.; Whitesides, G. M. Equilibrium constants for thiol-disulfide interchange reactions: a coherent, corrected set. J. Org. Chem. 1993, 58, 642– 647,  DOI: 10.1021/jo00055a016

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    Equilibrium constants for thiol-disulfide interchange reactions: a coherent, corrected set

    Lees, Watson J.; Whitesides, George M.

    Journal of Organic Chemistry (1993), 58 (3), 642-7CODEN: JOCEAH; ISSN:0022-3263.

    Equil. consts. (Keq) for the thiol-disulfide interchange reactions between dithiothreitol (DTT) and lipoic acid (14.2 ± 0.7), lipoic acid (Lip) and mercaptoethanol (13.3 M ± 1.0 M), and mercaptoethanol (ME) and glutathione (GSH or GSSG) (1.20 ± 0.10) were measured in D2O at pD 7.0 by 1H NMR spectroscopy. Two of these equil. consts. [DTT and Lip (21.3 ± 0.9), and Lip and ME (8.6 ± 0.7)] were also measured in D2O/CD3OD. These values are compared with those obtained by other methods. A coherent set of values for the equil. consts. between DTT or ME and thiols having a range of structures was assembled. The recommended value for the equil. const. between DTT and GSH is 210 M (Keq = [DTTox][GSH]2/([DTTred]-[GSSG])).
57. 57

    Vilan, A.; Aswal, D.; Cahen, D. Large-Area, Ensemble Molecular Electronics: Motivation and Challenges. Chem. Rev. 2017, 117, 4248– 4286,  DOI: 10.1021/acs.chemrev.6b00595

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    Large-Area, Ensemble Molecular Electronics: Motivation and Challenges

    Vilan, Ayelet; Aswal, Dinesh; Cahen, David

    Chemical Reviews (Washington, DC, United States) (2017), 117 (5), 4248-4286CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)

    A review. The authors review charge transport across mol. monolayers, which is central to mol. electronics (MolEl) using large-area junctions (NmJ). The authors strive to provide a wide conceptual overview of three main subtopics. First, a broad introduction places NmJ in perspective to related fields of research, and to single mol. junctions (1 mJ), in addn. to a brief historical account. As charge transport presents an ultrasensitive probe for the electronic perfection of interfaces, in the 2nd part ways to form both the monolayer and the contacts are described to construct reliable, defect-free interfaces. The last part is dedicated to understanding and analyses of current-voltage (I-V) traces across mol. junctions. Notwithstanding the original motivation of MolEl, I-V traces are often not very sensitive to mol. details and then provide a poor probe for chem. information. Instead the authors focus on how to analyze the net elec. performance of mol. junctions, from a functional device perspective. Finally, the authors shortly point to creation of a built-in elec. field as a key to achieve functionality, including nonlinear current-voltage characteristics that originate in the mols. or their contacts to the electrodes.
58. 58

    Weiss, E. A.; Kaufman, G. K.; Kriebel, J. K.; Li, Z.; Schalek, R.; Whitesides, G. M. Si/SiO2-Templated Formation of Ultraflat Metal Surfaces on Glass, Polymer, and Solder Supports: Their Use as Substrates for Self-Assembled Monolayers. Langmuir 2007, 23, 9686– 9694,  DOI: 10.1021/la701919r

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    Si/SiO2-Templated Formation of Ultraflat Metal Surfaces on Glass, Polymer, and Solder Supports: Their Use as Substrates for Self-Assembled Monolayers

    Weiss, Emily A.; Kaufman, George K.; Kriebel, Jennah K.; Li, Zhefeng; Schalek, Richard; Whitesides, George M.

    Langmuir (2007), 23 (19), 9686-9694CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)

    This paper describes the use of several methods of template stripping (TS) to produce ultraflat films of silver, gold, palladium, and platinum on both rigid and polymeric mech. supports: a composite of glass and UV-curable adhesive (optical adhesive, OA), solder, a composite of poly(di-Me siloxane) (PDMS) and OA, and bare OA. Silicon supporting its native oxide layer (Si/SiO2) serves as a template for both mech. template stripping (mTS), in which the metal film is mech. cleaved from the template, and chem. template stripping (cTS), in which the film-template composite is immersed in a soln. of thiols, and the formation of the SAM on the metal film causes the film to sep. from the template. Films formed on all supports have lower root-mean-square (rms) roughness (as measured by at. force microscopy, AFM) than films used as-deposited (AS-DEP) by electron-beam evapn. Monolayers of n-dodecanethiolate formed by the mTS and cTS methods are effectively indistinguishable by scanning tunneling microscopy (STM); molecularly resolved images could be obtained using both types of surfaces. The metal surfaces, before being cleaved, are completely protected from contact with the atm. This protection allows metal surfaces intended to support SAMs to be prepd. in large batch lots, stored, and then used as needed. Template stripping thus eliminates the requirement for evapn. of the film immediately before use and is a significant extension and simplification of the technol. of SAMs and other areas of materials science requiring clean metal surfaces.
59. 59

    Chiechi, R. C.; Weiss, E. A.; Dickey, M. D.; Whitesides, G. M. Eutectic Gallium-Indium (EGaIn): A Moldable Liquid Metal for Electrical Characterization of Self-Assembled Monolayers. Angew. Chem., Int. Ed. 2008, 47, 142– 144,  DOI: 10.1002/anie.200703642

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    Eutectic gallium-indium (EGaIn): a moldable liquid metal for electrical characterization of self-assembled monolayers

    Chiechi, Ryan C.; Weiss, Emily A.; Dickey, Michael D.; Whitesides, George M.

    Angewandte Chemie, International Edition (2008), 47 (1), 142-144CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Micrometer-sized conformal electrodes formed from Ga-25% In (EGaIn) are ideal for measuring the elec. properties of self-assembled monolayers. The image shows the bifurcation of the EGaIn between a metal needle and an Ag surface (not shown) at the point indicated by the two white arrows. The portion above the arrows will form the EGaIn electrode.
60. 60

    Wang, W.; Lee, T.; Reed, M. A. Mechanism of Electron Conduction in Self-Assembled Alkanethiol Monolayer Devices. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 035416,  DOI: 10.1103/PhysRevB.68.035416

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    Mechanism of electron conduction in self-assembled alkanethiol monolayer devices

    Wang, Wenyong; Lee, Takhee; Reed, M. A.

    Physical Review B: Condensed Matter and Materials Physics (2003), 68 (3), 035416/1-035416/7CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)

    Electron tunneling through self-assembled monolayers (SAM's) of alkanethiols is investigated using nanometer-scale devices. Temp.-dependent current-voltage measurements were performed on alkanethiol SAM's to distinguish between different conduction mechanisms. Temp.-independent electron transport is obsd., proving that tunneling is the dominant conduction mechanism of alkanethiols, as well as exhibiting an exponential dependence of tunneling current on the mol. length with a decay coeff. β. From the bias dependence of β, a barrier height ΦB of 1.39 ± 0.01 eV and a zero-field decay coeff. β0 of 0.79 ± 0.01Å-1 were detd. for alkanethiols.
61. 61

    Simeone, F. C.; Yoon, H. J.; Thuo, M. M.; Barber, J. R.; Smith, B.; Whitesides, G. M. Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions. J. Am. Chem. Soc. 2013, 135, 18131– 18144,  DOI: 10.1021/ja408652h

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    Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

    Simeone, Felice C.; Yoon, Hyo Jae; Thuo, Martin M.; Barber, Jabulani R.; Smith, Barbara; Whitesides, George M.

    Journal of the American Chemical Society (2013), 135 (48), 18131-18144CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Anal. of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SCn (with n = no. of carbon atoms) incorporated in junctions having structure AgTS-SAM// Ga2O3/ EGaIn leads to a value for the injection tunnel c.d. J0 (i.e., the current flowing through an ideal junction with n = 0) of 103.6±0.3 A·cm-2 (V = +0.5 V). This estn. of J0 does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths n = 1-18. This value of J0 is estd. under the assumption that values of the geometrical contact area equal the values of the effective elec. contact area. Detailed exptl. anal., however, indicates that the roughness of the Ga2O3 layer, and that of the AgTS-SAM, det. values of the effective elec. contact area that are ∼10-4 the corresponding values of the geometrical contact area. Conversion of the values of geometrical contact area into the corresponding values of effective elec. contact area results in J0(+0.5 V) = 107.6±0.8 A·cm-2, which is compatible with values reported for junctions using top-electrodes of evapd. Au, and graphene, and also comparable with values of J0 estd. from tunneling through single mols. For these EGaIn-based junctions, the value of the tunneling decay factor β (β = 0.75 ± 0.02 Å-1; β = 0.92 ± 0.02 nC-1) falls within the consensus range across different types of junctions (β = 0.73-0.89 Å-1; β = 0.9-1.1 nC-1). A comparison of the characteristics of conical Ga2O3/ EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technol. for phys.-org. studies of charge transport across SAMs.
62. 62

    Thuo, M. M.; Reus, W. F.; Simeone, F. C.; Kim, C.; Schulz, M. D.; Yoon, H. J.; Whitesides, G. M. Replacing -CH2CH2- With -CONH- Does Not Significantly Change Rates of Charge Transport Through AgTS-SAM//Ga2O3/EGaIn Junctions. J. Am. Chem. Soc. 2012, 134, 10876– 10884,  DOI: 10.1021/ja301778s

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    Replacing -CH2CH2- with -CONH- Does Not Significantly Change Rates of Charge Transport through AgTS-SAM//Ga2O3/EGaIn Junctions

    Thuo, Martin M.; Reus, William F.; Simeone, Felice C.; Kim, Choongik; Schulz, Michael D.; Yoon, Hyo Jae; Whitesides, George M.

    Journal of the American Chemical Society (2012), 134 (26), 10876-10884CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    This paper describes phys.-org. studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the mols. constituting the SAM. Replacing a -CH2CH2- group with a -CONH- group changes the dipole moment and polarizability of a portion of the mol. and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure AgTS-S(CH2)mX(CH2)nH//Ga2O3/EGaIn (TS = template stripped, X = -CH2CH2- or -CONH-, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (nonshorting) junctions (when compared to n-alkanethiolates of the same length). Probably synthetic schemes that combine a thiol group on one end of a mol. with a group, R, to be tested, on the other (e.g., HS∼CONH approx. R) using an amide-based coupling provide practical routes to mols. useful in studies of mol. electronics.
63. 63

    Ai, Y.; Kovalchuk, A.; Qiu, X.; Zhang, Y.; Kumar, S.; Wang, X.; Kühnel, M.; Nørgaard, K.; Chiechi, R. C. In-Place Modulation of Rectification in Tunneling Junctions Comprising Self-Assembled Monolayers. Nano Lett. 2018, 18, 7552– 7559,  DOI: 10.1021/acs.nanolett.8b03042

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    63

    In-Place Modulation of Rectification in Tunneling Junctions Comprising Self-Assembled Monolayers

    Ai, Yong; Kovalchuk, Andrii; Qiu, Xinkai; Zhang, Yanxi; Kumar, Sumit; Wang, Xintai; Kuehnel, Martin; Noegaard, Kasper; Chiechi, Ryan C.

    Nano Letters (2018), 18 (12), 7552-7559CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)

    This paper describes tunneling junctions comprising self-assembled monolayers that can be converted between resistor and diode functionality in-place. The rectification ratio is affected by the hydration of densely packed carboxylic acid groups at the interface between the top-contact and the monolayer. The authors studied this process by treatment with water and a water scavenger using three different top-contacts, eutectic Ga-In (EGaIn), conducting-probe at. force microscopy (CP-AFM), and reduced graphene oxide (rGO), demonstrating that the phenomena is mol. in nature and is not platform-speciffc. The authors propose a mechanism in which the tunneling junctions convert to diode behavior through the lowering of the LUMO, which is suffcient to bring it close to resonance at pos. bias, potentially assisted by a Stark shift. This shift in energy is supported by calcns. and a change in polarization obsd. by XPS and Kelvin probe measurements. The authors demonstrate light-driven modulation using spiropyran as a photoacid, suggesting that any chem. process that is coupled to the release of small mols. that can tightly bind carboxylic acid groups can be used as an external stimulus to modulate rectification. The ability to convert a tunneling junction reversibly between a diode and a resistor via an effect that is intrinsic to the mols. in the junction extends the possible applications of Mol. Electronics to reconfigurable circuits and other new functionalities that do not have direct analogs in conventional semiconductor devices.
64. 64

    Yoon, H. J.; Shapiro, N. D.; Park, K. M.; Thuo, M. M.; Soh, S.; Whitesides, G. M. The Rate of Charge Tunneling through Self-Assembled Monolayers Is Insensitive to Many Functional Group Substitutions. Angew. Chem., Int. Ed. 2012, 51, 4658– 4661,  DOI: 10.1002/anie.201201448

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    64

    The Rate of Charge Tunneling through Self-Assembled Monolayers Is Insensitive to Many Functional Group Substitutions

    Yoon, Hyo Jae; Shapiro, Nathan D.; Park, Kyeng Min; Thuo, Martin M.; Soh, Siowling; Whitesides, George M.

    Angewandte Chemie, International Edition (2012), 51 (19), 4658-4661, S4658/1-S4658/23CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    The authors characterize herein the rates of charge transport by tunneling across mols. - arrayed in self-assembled monolayers (SAMs) - contg. a common head group and body (HS(CH2)4CONH(CH2)2) and structurally varied tail groups (-R). These mols. are assembled in junctions with the structure template-stripped Ag/SAM//Ga2O3/eutectic GaIn. Changing the structure of R over a range of common aliph., arom., and heteroarom. org. tail groups does not significantly influence the rate of tunneling. In making these measurements, the authors use C12 and C18 alkanethiols as calibration stds. to allow comparison with results from other types of junctions. Structure of the tail group R was varied over a range of structures used typically in org. chem., while maintaining an approx. const. thickness of the tunneling barrier. The span of the rates of tunneling current was less than a factor of approx. five over the entire series. This very small response of the charge transport to the structure of R suggests that the rates of tunneling through SAMs are largely insensitive to differences in the electronic structures of R. Studies exploring the correlation between mol. structure and charge transport based on systematic phys.-org. measurements of the rate of charge transport over a wide range of structures are sparse.
65. 65

    Baghbanzadeh, M.; Belding, L.; Yuan, L.; Park, J.; Al-Sayah, M. H.; Bowers, C. M.; Whitesides, G. M. Dipole-Induced Rectification Across AgTS/SAM//Ga2O3/EGaIn Junctions. J. Am. Chem. Soc. 2019, 141, 8969– 8980,  DOI: 10.1021/jacs.9b02891

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    65

    Dipole-Induced Rectification Across AgTS/SAM//Ga2O3/EGaIn Junctions

    Baghbanzadeh, Mostafa; Belding, Lee; Yuan, Li; Park, Junwoo; Al-Sayah, Mohammad H.; Bowers, Carleen M.; Whitesides, George M.

    Journal of the American Chemical Society (2019), 141 (22), 8969-8980CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    This Article describes the relationship between mol. structure, and the rectification of tunneling current, in tunneling junctions based on self-assembled monolayers (SAMs). Mol. dipoles from simple org. functional groups (amide, urea, and thiourea) were introduced into junctions with the structure AgTS/S(CH2)nR(CH2)mCH3//Ga2O3/EGaIn. Here, R is an n-alkyl fragment (-CH2-)2 or 3, an amide group (either -CONH- or -NHCO-), a urea group (-NHCONH-), or a thiourea group (-NHCSNH-). The amide, urea, or thiourea groups introduce a localized elec. dipole moment into the SAM and change the polarizability of that section of the SAM, but do not produce large, electronically delocalized groups or change other aspects of the tunneling barrier. This local change in electronic properties correlates with a statistically significant, but not large, rectification of current (r+) at ±1.0 V (up to r+ ≈ 20). The results of this work demonstrate that the simplest form of rectification of current at ±1.0 V, in EGaIn junctions, is an interfacial effect, and is caused by a change in the work function of the SAM-modified silver electrode due to the proximity of the dipole assocd. with the amide (or related) group, and not to a change in the width or mean height of the tunneling barrier.
66. 66

    Kumar, S.; Merelli, M.; Danowski, W.; Rudolf, P.; Feringa, B. L.; Chiechi, R. C. Chemical Locking in Molecular Tunneling Junctions Enables Nonvolatile Memory With Large On–Off Ratios. Adv. Mater. 2019, 31, 1807831,  DOI: 10.1002/adma.201807831

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67. 67

    Qiu, L.; Zhang, Y.; Krijger, T. L.; Qiu, X.; Hof, P. v.; Hummelen, J. C.; Chiechi, R. C. Rectification of current responds to incorporation of fullerenes into mixed-monolayers of alkanethiolates in tunneling junctions. Chem. Sci. 2017, 8, 2365– 2372,  DOI: 10.1039/C6SC04799H

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    Rectification of current responds to incorporation of fullerenes into mixed-monolayers of alkanethiolates in tunneling junctions

    Qiu, Li; Zhang, Yanxi; Krijger, Theodorus L.; Qiu, Xinkai; Hof, Patrick van't; Hummelen, Jan C.; Chiechi, Ryan C.

    Chemical Science (2017), 8 (3), 2365-2372CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)

    This paper describes the rectification of current through mol. junctions comprising self-assembled monolayers of decanethiolate through the incorporation of C60 fullerene moieties bearing undecanethiol groups in junctions using eutectic Ga-In (EGaIn) and Au conducting probe AFM (CP-AFM) top-contacts. The degree of rectification increases with increasing exposure of the decanethiolate monolayers to the fullerene moieties, going through a max. after 24 h. We ascribe this observation to the resulting mixed-monolayer achieving an optimal packing d. of fullerene cages sitting above the alkane monolayer. Thus, the degree of rectification is controlled by the amt. of fullerene present in the mixed-monolayer. The voltage dependence of R varies with the compn. of the top-contact and the force applied to the junction and the energy of the lowest unoccupied π-state detd. from photoelectron spectroscopy is consistent with the direction of rectification. The max. value of rectification R = |J(+)/J(-)| = 940 at ±1 V or 617 at ±0.95 V is in agreement with previous studies on pure monolayers relating the degree of rectification to the vol. of the head-group on which the frontier orbitals are localized.
68. 68

    Jiang, L.; Yuan, L.; Cao, L.; Nijhuis, C. A. Controlling Leakage Currents: The Role of the Binding Group and Purity of the Precursors for Self-Assembled Monolayers in the Performance of Molecular Diodes. J. Am. Chem. Soc. 2014, 136, 1982– 1991,  DOI: 10.1021/ja411116n

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    Controlling Leakage Currents: The Role of the Binding Group and Purity of the Precursors for Self-Assembled Monolayers in the Performance of Molecular Diodes

    Jiang, Li; Yuan, Li; Cao, Liang; Nijhuis, Christian A.

    Journal of the American Chemical Society (2014), 136 (5), 1982-1991CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    This paper describes that the performance of mol. diodes based on self-assembled monolayers (SAMs) depends on the type of anchoring group and purity of the precursors of these SAMs. The SAMs were formed on ultrasmooth template-stripped silver (AgTS) surfaces, which served as the bottom-electrode, and a eutectic alloy of gallium-indium was used as the top-electrode. When these junctions incorporate SAMs S-(CH2)11Fc (≃ SC11Fc) derived from HSC11Fc, they are good mol. diodes and rectify currents with rectification ratios R (≃ |J(-1.0 V)|/|J(+1.0 V)|) of ∼1.0 × 102. Replacing the thiol by disulfide or thioacetate functionalities in the precursor resulted in mol. diodes with values of R close to unity. Cyclic voltammetry and angle resolved XPS indicated that the SAMs derived from the disulfide or thioacetate precursors have lower surface coverages and are more defective than SAMs derived from thiols. In the junctions these defective SAMs caused defects and increased the leakage currents. The purity of the thiol-precursor is also crucial: 3 or 5% of disulfide present in the thiol caused a 28 or 61% decrease in R, resp., and >15% of disulfide lowered R to unity, while the yield in nonshorting junctions remained unchanged. The authors' results show that the type of binding group, and the purity of the thiols, are crucial parameters in the exptl. design of mol. electronic devices to ensure optimal device performance by keeping leakage currents to a min.
69. 69

    Fracasso, D.; Valkenier, H.; Hummelen, J. C.; Solomon, G. C.; Chiechi, R. C. Evidence for Quantum Interference in SAMs of Arylethynylene Thiolates in Tunneling Junctions With Eutectic Ga–In (EGaIn) Top-Contacts. J. Am. Chem. Soc. 2011, 133, 9556– 9563,  DOI: 10.1021/ja202471m

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    Evidence for Quantum Interference in SAMs of Arylethynylene Thiolates in Tunneling Junctions with Eutectic Ga-In (EGaIn) Top-Contacts

    Fracasso, Davide; Valkenier, Hennie; Hummelen, Jan C.; Solomon, Gemma C.; Chiechi, Ryan C.

    Journal of the American Chemical Society (2011), 133 (24), 9556-9563CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    This paper compares the c.d. (J) vs. applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivs. of approx. the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liq. eutectic Ga-In (EGaIn) supporting a native skin (∼1 nm thick) of Ga2O3 as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheol. properties that distinguish EGaIn from other top-contacts; however, it may also have limited the max. values of J obsd. for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10-1A/cm2 at V = 0.4 V). For AC, however, J is 1 (using log avs.) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qual. agreement with gDFTB calcns. on single AC, AQ, and AH mols. chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0<|V|<0.4 V. The calcns. predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calcd. precisely. In this sense, the theor. predictions and exptl. conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theor. predictions on single mols. and the measurements on SAMs suggest that mol.-mol. interactions do not play a significant role in the transport properties of AC, AQ, and AH.
70. 70

    Neese, F. The ORCA Program System. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73– 78,  DOI: 10.1002/wcms.81

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    The ORCA program system

    Neese, Frank

    Wiley Interdisciplinary Reviews: Computational Molecular Science (2012), 2 (1), 73-78CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)

    A review. ORCA is a general-purpose quantum chem. program package that features virtually all modern electronic structure methods (d. functional theory, many-body perturbation and coupled cluster theories, and multireference and semiempirical methods). It is designed with the aim of generality, extendibility, efficiency, and user friendliness. Its main field of application is larger mols., transition metal complexes, and their spectroscopic properties. ORCA uses std. Gaussian basis functions and is fully parallelized. The article provides an overview of its current possibilities and documents its efficiency.
71. 71

    Neese, F. Software update: the ORCA program system, verison 4.0. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2018, 8, e1327,  DOI: 10.1002/wcms.1327

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