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Yoon, Hyo

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Yoon, Hyo
Author's Publications: search.filters.isAuthorOfPublication.a726e3e0-c316-471f-9566-9d9b2f0c6a07

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Now showing 1 - 7 of 7
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    The Rate of Charge Tunneling through Self-Assembled Monolayers Is Insensitive to Many Functional Group Substitutions
    (Wiley-VCH Verlag Berlin, 2012) Yoon, Hyo; Shapiro, Nathan D.; Park, Kyeng Min; Thuo, Martin M.; Soh, Siowling; Whitesides, George
    Insensitivity: A series of molecules containing a common head group and body as well as structurally varied tail groups (-R) has been used in junctions with the structure Ag/S(CH2)4CONH(CH2)2R//Ga2O3/EGaIn to study the rates of charge transport by tunneling. Changing the structure of R over a range of common aliphatic, aromatic, and heteroaromatic organic groups was found to not significantly influence the rate of tunneling (see plots; the dashed lines represent calibration standards).
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    Replacing –CH2CH2- with -CONH- Does Not Significantly Change Rates of Charge Transport Through AgTS-SAM//Ga2O3/EGaIn Junctions
    (American Chemical Society (ACS), 2012) Thuo, Martin M.; Reus, William F.; Simeone, Felice; Kim, Choongik; Schulz, Michael D.; Yoon, Hyo; Whitesides, George
    This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a −CH2CH2– group with a −CONH– group changes the dipole moment and polarizability of a portion of the molecule 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 (non-shorting) junctions (when compared to n-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS∼CONH∼R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics.
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    Fluorination, and Tunneling across Molecular Junctions
    (American Chemical Society (ACS), 2015) Liao, Kung-ching; Bowers, Carleen; Yoon, Hyo; Whitesides, George
    This paper describes the influence of the substitution of fluorine for hydrogen on the rate of charge transport by hole tunneling through junctions of the form AgTSO2C(CH2)n(CF2)mT//Ga2O3/EGaIn, where T is methyl (CH3) or trifluoromethyl (CF3). Alkanoate-based self-assembled monolayers (SAMs) having perfluorinated groups (RF) show current densities that are lower (by factors of 20–30) than those of the homologous hydrocarbons (RH), while the attenuation factors of the simplified Simmons equation for methylene (β = (1.05 ± 0.02)nCH2–1) and difluoromethylene (β = (1.15 ± 0.02)nCF2–1) are similar (although the value for (CF2)n is statistically significantly larger). A comparative study focusing on the terminal fluorine substituents in SAMs of ω-tolyl- and -phenyl-alkanoates suggests that the C–F//Ga2O3 interface is responsible for the lower tunneling currents for CF3. The decrease in the rate of charge transport in SAMs with RF groups (relative to homologous RH groups) is plausibly due to an increase in the height of the tunneling barrier at the T//Ga2O3 interface, and/or to weak van der Waals interactions at that interface.
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    Influence of Environment on the Measurement of Rates of Charge Transport across Ag TS /SAM//Ga 2 O 3 /EGaIn Junctions
    (American Chemical Society (ACS), 2014) Barber, Jabulani Randall; Yoon, Hyo; Bowers, Carleen; Thuo, Martin M.; Breiten, Benjamin; Gooding, Diana M.; Whitesides, George
    This paper investigates the influence of the atmosphere used in the fabrication of top electrodes from the liquid eutectic of gallium and indium (EGaIn) (the so-called “EGaIn” electrodes), and in measurements of current density, J(V) \((A/cm^2)\), across self-assembled monolayers (SAMs) incorporated into \(Ag/SR//Ga_2O_3\)/EGaIn junctions, on values of J(V) obtained using these electrodes. A gas-tight measurement chamber was used to control the atmosphere in which the electrodes were formed, and also to control the environment in which the electrodes were used to measure current densities across SAM-based junctions. Seven different atmospheres—air, oxygen, nitrogen, argon, and ammonia, as well as air containing vapors of acetic acid or water—were surveyed using both “rough” conical-tip electrodes, and “smooth” hanging-drop electrodes. (The manipulation of the oxide film during the creation of the conical-tip electrodes leads to substantial, micrometer-scale roughness on the surface of the electrode, the extrusion of the drop creates a significantly smoother surface.) Comparing junctions using both geometries for the electrodes, across a SAM of n-dodecanethiol, in air, gave \(log |J|mean = −2.4 \pm 0.4\) for the conical tip, and \(log |J|mean = −0.6 \pm 0.3\) for the drop electrode (and, thus, \(\Delta log |J| \approx 1.8)\); this increase in current density is attributed to a change in the effective electrical contact area of the junction. To establish the influence of the resistivity of the \(Ga_2O_3\) film on values of J(V), junctions comprising a graphite electrode and a hanging-drop electrode were compared in an experiment where the electrodes did, and did not, have a surface oxide film; the presence of the oxide did not influence measurements of \(log |J(V)|\), and therefore did not contribute to the electrical resistance of the electrode. However, the presence of an oxide film did improve the stability of junctions and increase the yield of working electrodes from ∼70% to ∼100%. Increasing the relative humidity (RH) in which J(V) was measured did not influence these values (across methyl \((CH_3)^-\) or carboxyl \((CO_2H)^-\) terminated SAMs) over the range typically encountered in the laboratory (20%–60% (RH)).
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    Omniphobic “R F Paper” Produced by Silanization of Paper with Fluoroalkyltrichlorosilanes
    (Wiley-Blackwell, 2013) Glavan, Ana; Martinez, R; Subramaniam, Anand; Yoon, Hyo; Nunes, Rui M. D.; Lange, Heiko; Thuo, Martin M.; Whitesides, George
    The fabrication and properties of “fluoroalkylated paper” (“\(R^F\) paper”) by vapor-phase silanization of paper with fluoroalkyl trichlorosilanes is reported. \(R^F\) paper is both hydrophobic and oleophobic: it repels water \((θ_{app}^{H2O} > 140^{\circ})\), organic liquids with surface tensions as low as \(28 \space mN \space m^{-1}\), aqueous solutions containing ionic and non-ionic surfactants, and complex liquids such as blood (which contains salts, surfactants, and biological material such as cells, proteins, and lipids). The propensity of the paper to resist wetting by liquids with a wide range of surface tensions correlates with the length and degree of fluorination of the organosilane (with a few exceptions in the case of methyl trichlorosilane-treated paper), and with the roughness of the paper. \(R^F\) paper maintains the high permeability to gases and mechanical flexibility of the untreated paper, and can be folded into functional shapes (e.g., microtiter plates and liquid-filled gas sensors). When impregnated with a perfluorinated oil, \(R^F\) paper forms a “slippery” surface (paper slippery liquid-infused porous surface, or “paper SLIPS“) capable of repelling liquids with surface tensions as low as \(15 \space mN \space m^{-1}\). The foldability of the paper SLIPS allows the fabrication of channels and flow switches to guide the transport of liquid droplets.
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    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
    (American Chemical Society (ACS), 2014) Yoon, Hyo; Bowers, Carleen; Baghbanzadeh, Mostafa; Whitesides, George
    This paper describes a physical‐organic studyof the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self‐assembled monolayer (SAM)‐based large‐area junctions of the form AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn. Here AgTS is a template‐stripped silver substrate, ‐M‐ and ‐T are “middle” and “terminal” functional groups, and EGaIn is eutectic galliumindium alloy. A range of uncharged polar groups (‐T), having permanent dipole moments in the range 0.5 < μ <4.5, were incorporated into the SAM. A comparison of the electrical characteristics of these junctions with junctions formed from n‐alkanethiolates 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 (β) and injection current (Jo) for SAMs terminated in non‐polar n‐alkyl groups, and polar groups, are statistically indistinguishable.
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    Replacing \(Ag ^{TS} SCH_2-R\) with \(Ag^{TS}O_2C-R\) in EGaIn-Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport
    (Wiley-Blackwell, 2014) Liao, Kung-ching; Yoon, Hyo; Bowers, Carleen; Simeone, Felice; Whitesides, George
    This paper compares rates of charge transport by tunneling across junctions with the structures \(Ag^{TS}X(CH_2)_{2n}CH_3 //Ga_2O_3 /EGaIn \space\) (n=1–8 and \(X= -SCH_2-\) and \(O_2C-\)); here \(Ag^{TS}\) is template-stripped silver, and EGaIn is the eutectic alloy of gallium and indium. Its objective was to compare the tunneling decay coefficient (\(\beta\), Å\(^{−1}\)) and the injection current (\(J_0\), A cm\(^{−2}\)) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates. Replacing \(Ag^{TS}SCH_2-R\) with \(Ag^{TS}O_2C-R\) (R=alkyl chains) had no significant influence on \(J_0\) (ca. \(3\times10^3 A cm^{−2}\)) or \(\beta\) (0.75–0.79 Å\(^{−1}\))—an indication that such changes (both structural and electronic) in the \(Ag^{TS}XR\) interface do not influence the rate of charge transport. A comparison of junctions comprising oligo(phenylene)carboxylates and n-alkanoates showed, as expected, that \(\beta\) for aliphatic (0.79 Å\(^{−1}\)) and aromatic (0.60 Å\(^{−1}\)) SAMs differed significantly.