34 research outputs found
Odd-even effects in charge transport across n-alkanethiolate-based SAMs
This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions with the structure MTS/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on AuTS and AgTS demonstrated a statistically significant odd-even effect on AuTS, but not on AgTS, that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on AuTS with nodd and neven belonging to two separate sets, and while there is a significant difference between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0 ± 0.3 and log|J0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, ÎČ, for nodd and neven alkyl chains are indistinguishable (ÎČAu,even = 0.73 ± 0.02 Ă
-1, and ÎČAu,odd= 0.74 ± 0.02 Ă
-1). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of ÎČ and J0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates
The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in Ag TS S(CH 2 ) n M(CH 2 ) m T//Ga 2 O 3 /EGaIn Junctions
This paper describes a physicalâorganic studyof the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across selfâassembled monolayer (SAM)âbased largeâarea junctions of the form AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn. Here AgTS is a templateâstripped silver substrate, âMâ and âT are âmiddleâ and âterminalâ functional groups, and EGaIn is eutectic galliumindium alloy. A range of uncharged polar groups (âT), having permanent dipole moments in the range 0.5 < ÎŒ <4.5, were incorporated into the SAM. A comparison of the electrical characteristics of these junctions with junctions formed from nâalkanethiolates led to the conclusion that the rates of charge tunneling are insensitive to the replacement of terminal alkyl groups with terminal polar groups. The current densities measured in this work suggest that the tunneling decay parameter (ÎČ) and injection current (Jo) for SAMs terminated in nonâpolar nâalkyl groups, and polar groups, are statistically indistinguishable.Chemistry and Chemical Biolog
OddâEven Effects in Charge Transport across n -Alkanethiolate-Based SAMs
This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions with the structure MTS/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on AuTS and AgTS demonstrated a statistically significant oddâeven effect on AuTS, but not on AgTS, that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on AuTS with nodd and neven belonging to two separate sets, and while there is a significant difference between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0 ± 0.3 and log|J0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, ÎČ, for nodd and neven alkyl chains are indistinguishable (ÎČAu,even = 0.73 ± 0.02 Ă
â1, and ÎČAu,odd= 0.74 ± 0.02 Ă
â1). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of ÎČ and J0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates.Chemistry and Chemical Biolog
Rectification in Tunneling Junctions: 2,2âČ-Bipyridyl-Terminated n -Alkanethiolates
Molecular rectification is a particularly attractive phenomenon to examine in studying structureâproperty relationships in charge transport across molecular junctions, since the tunneling currents across the same molecular junction are measured, with only a change in the sign of the bias, with the same electrodes, molecule(s), and contacts. This type of experiment minimizes the complexities arising from measurements of current densities at one polarity using replicate junctions. This paper describes a new organic molecular rectifier: a junction having the structure AgTS/S(CH2)11-4-methyl-2,2âČ-bipyridyl//Ga2O3/EGaIn (AgTS: template-stripped silver substrate; EGaIn: eutectic galliumâindium alloy) which shows reproducible rectification with a mean r+ = |J(+1.0 V)|/|J(â1.0 V)| = 85 ± 2. This system is important because rectification occurs at a polarity opposite to that of the analogous but much more extensively studied systems based on ferrocene. It establishes (again) that rectification is due to the SAM, and not to redox reactions involving the Ga2O3 film, and confirms that rectification is not related to the polarity in the junction. Comparisons among SAM-based junctions incorporating the Ga2O3/EGaIn top electrode and a variety of heterocyclic terminal groups indicate that the metal-free bipyridyl group, not other features of the junction, is responsible for the rectification. The paper also describes a structural and mechanistic hypothesis that suggests a partial rationalization of values of rectification available in the literature.Chemistry and Chemical Biolog
Interactions between Hofmeister Anions and the Binding Pocket of a Protein
This paper uses the binding pocket of human carbonic anhydrase II (HCAII, EC 4.2.1.1) as a tool to examine the properties of Hofmeister anions that determine (i) where, and how strongly, they associate with concavities on the surfaces of proteins and (ii) how, upon binding, they alter the structure of water within those concavities. Results from X-ray crystallography and isothermal titration calorimetry show that most anions associate with the binding pocket of HCAII by forming inner-sphere ion pairs with the Zn2+ cofactor. In these ion pairs, the free energy of anionâZn2+ association is inversely proportional to the free energetic cost of anion dehydration; this relationship is consistent with the mechanism of ion pair formation suggested by the âlaw of matching water affinitiesâ. Iodide and bromide anions also associate with a hydrophobic declivity in the wall of the binding pocket. Molecular dynamics simulations suggest that anions, upon associating with Zn2+, trigger rearrangements of water that extend up to 8 Ă
away from their surfaces. These findings expand the range of interactions previously thought to occur between ions and proteins by suggesting that (i) weakly hydrated anions can bind complementarily shaped hydrophobic declivities, and that (ii) ion-induced rearrangements of water within protein concavities can (in contrast with similar rearrangements in bulk water) extend well beyond the first hydration shells of the ions that trigger them. This study paints a picture of Hofmeister anions as a set of structurally varied ligands that differ in size, shape, and affinity for water and, thus, in their ability to bind toâand to alter the charge and hydration structure ofâpolar, nonpolar, and topographically complex concavities on the surfaces of proteins.Chemistry and Chemical Biolog
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Introducing Ionic and/or Hydrogen Bonds into the SAM//Ga 2 O 3 Top-Interface of Ag TS /S(CH 2 ) n T//Ga 2 O 3 /EGaIn Junctions
Junctions with the structure AgTS/S(CH2)nT//Ga2O3/EGaIn (where S(CH2)nT is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga2O3. Introducing a series of Lewis acidic/basic functional groups (T = âOH, âSH, âCO2H, âCONH2, and âPO3H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm2, that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen- or ionic bonds at the T//Ga2O3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J0, for T = âCO2H, and T = âCH2CH3âtwo groups having similar extended lengths (in Ă
, or in numbers of non-hydrogen atoms)âsuggests that both groups make indistinguishable contributions to the height of the tunneling barrier.Chemistry and Chemical Biolog
Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)
Characterizing the MetalâSAM Interface in Tunneling Junctions
his paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag)TS/A(CH2)nH//Ga2O3/EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O2CR), and Au/acetylene (Au/CâĄCR), where R is an n-alkyl group. Values of J0 and ÎČ (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).Chemistry and Chemical Biolog