O. E. Shmakova

University of Houston, Houston, TX, United States

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Publications (16)26.33 Total impact

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    ABSTRACT: The solution-phase thermal desorption of three series of self-assembled monolayers (SAMs) on gold generated from terminally perfluorinated alkanethiols was examined. Series 1 SAMs, F(CF2)x(CH2)11SH, where x = 1–10, consisted of a constant hydrocarbon segment length with an increasing fluorocarbon segment length. Series 2 SAMs, F(CF2)10(CH2)ySH, where y = 2–6, 11, consisted of a constant fluorocarbon segment length with an increasing hydrocarbon segment length. Series 3 SAMs, F(CF2)x(CH2)ySH, where x = 1–10 and y = 16 – x, consisted of both hydrocarbon and fluorocarbon segments in which the segment lengths were varied while holding the total chain length constant at 16 carbon atoms. SAMs from these three series were prepared and characterized using both ellipsometry and contact-angle measurements. The resultant SAMs were shown to be highly hydrophobic and oleophobic. The SAMs were heated in decalin (DC) and perfluorodecalin (PFD) at 80 °C for various periods of time to monitor their thermal stability when exposed to hydrocarbon versus fluorocarbon solvents. In general, SAMs derived from n-alkanethiols and terminally perfluorinated alkanethiols exhibited diminished thermal stabilities upon heating in a hydrocarbon solvent (DC) versus heating in a perfluorocarbon solvent (PFD). The thermal stability of the SAMs increased with increasing lengths of the CF2 or CH2 segments. We also examined the kinetics of thermal desorption of these SAMs. From these studies, SAMs composed of higher degrees of terminal perfluorination exhibited smaller rate constants for the initial stage (fast regime) of desorption. When compared with analogous alkanethiol SAMs, the terminally perfluorinated SAMs exhibited greater thermal stabilities in both DC and PFD. In addition, values of the rate constants for desorption of the alkanethiol SAMs were approximately double those of the terminally perfluorinated SAMs having similar chain lengths.
    The Journal of Physical Chemistry C. 09/2011; 115(40):19749–19760.
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 06/2010; 30(22).
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    ABSTRACT: Self-assembled monolayers (SAMs) formed from semifluorinated alkanethiols (SFATs) CF3(CF2)9(CH2)nSH (F10HnSH: n = 2, 11, and 17) on poly-crystalline Au and Ag were characterized by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and near edge X-ray absorption fine structure spectroscopy. SFATs were found to form highly ordered and densely packed SAMs on both substrates. The molecules are strongly bonded to the substrates via their sulfur head groups, in the same manner as conventional alkanethiol (AT) SAMs. The hydrocarbon (except for n = 2) and fluorocarbon parts of the adsorbed SFATs retain the expected planar zigzag and helical conformations of the respective bulk materials. The orientation of the fluorocarbon chains does not depend on the substrate. These entities are almost perpendicular to the substrate in F10H2S/Au and F10H2S/Ag and become slightly more tilted in SFAT SAMs with longer hydrocarbon moieties. However, the alkyl parts of the SFAT films exhibit tilt and twist angles that are similar to those of normal alkanethiol films on Ag and Au substrates despite the reduced packing density in the SFAT films as compared to normal AT SAMs. We suggest that the substrate-related differences in tilt and twist angles for both systems are associated with the different character of the head-group-substrate bonding on Au and Ag.
    Israel Journal of Chemistry (Online) 03/2010; 40(2):81 - 97. · 2.56 Impact Factor
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    ABSTRACT: Journal article
    Thin Solid Films 09/2007; · 1.87 Impact Factor
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    ABSTRACT: We report the formation and characterization of self-assembled monolayers (SAMs) derived from the adsorption of 4-mercaptophenylboronic acid (MPBA) on gold. For comparison, SAMs derived from the adsorption of thiophenol (TP), 4-mercaptophenol (MP), and 4-mercaptobenzoic acid (MBA) were also examined. The structure and properties of the SAMs were evaluated by ellipsometry, contact-angle goniometry, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Specifically, ellipsometry was used to assess the formation of monolayer films, and contact angle measurements were used to determine the surface hydrophilicity and homogeneity. Separately, PM-IRRAS was used to evaluate the molecular composition and orientation as well as the intermolecular hydrogen bonding within the SAMs. Finally, XPS was used to evaluate the film composition and surface coverage (i.e., packing density), which was observed to increase in the following order: TP < MP < MPBA < MBA. A rationalization for the observed packing differences is presented. The XPS data indicate further that ultrahigh vacuum conditions induce the partial dehydration of MPBA SAMs with the concomitant formation of surface boronic anhydride species. Overall, the analytical data collectively show that the MPBA moieties in the SAMs exist in the acid form rather than the anhydride form under ambient laboratory conditions. Furthermore, stability studies find that MPBA SAMs are surprisingly labile in basic solution, where the terminal B-C bonds are cleaved by the attack of hydroxide ion and strongly basic amine nucleophiles. The unanticipated lability observed here should be considered by those wishing to use MPBA moieties in carbohydrate-sensing applications.
    Langmuir 09/2007; 23(17):8866-75. · 4.38 Impact Factor
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    ABSTRACT: We demonstrate that it is possible to achieve exceptionally fine control over the anchoring of liquid crystals via the use of semi-fluorinated self-assembled monolayers of varying spacer length. We use this approach to map the detailed shape of an anchoring transition surface in thermodynamic phase space and to explore the links between anchoring and orientational wetting phenomena. These results allow one to design a substrate that will place a standard liquid-crystal film arbitrarily close to an anchoring transition between homeotropic and planar anchoring.
    EPL (Europhysics Letters) 01/2007; 59(3):410. · 2.26 Impact Factor
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    ABSTRACT: Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).
    Langmuir 03/2005; 21(3):933-6. · 4.38 Impact Factor
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    ABSTRACT: Self-assembled monolayers (SAMs) formed from alkyl-terminated, CF3-terminated, CF3CF2-terminated, and C10F21-terminated alkanethiolates on gold substrates were examined by low-energy (eV) ion−surface collisions to determine the impact of varying the degree of fluorination of the thiolate tail group. The fluorine-terminated SAMs are compared and contrasted to previously examined CF3-terminated Langmuir−Blodgett films. Polyatomic (M+• for benzene and pyrazine) and atomic (Mo+• and Cr+•) ions were collided with the different SAM films at various collision energies (20−70 eV). Data indicate that substitution of CH3 with CF3 as the terminal group has a substantial influence on the ion−surface interactions, including energy transfer (fragmentation), electron transfer (neutralization), and atom/group transfer (reaction). However, slight penetration into a depth of the film is apparent and illustrated with the formation of certain ion−surface reaction products and the intensities of those products. This penetration has a noticeable effect on low-energy ion−surface collisions. Also, results for a series of CF3(CH2)nS−Au films (where n = 12, 13, 14, 15) illustrate that ion−surface reactions vary with the orientation of the fluorine atoms that are present in the terminal group. The results from collisions of low-energy ions with these relatively well-defined surfaces allow further characterization of the collision events that have the potential to become valuable tools in surface analysis.
    04/2002;
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    ABSTRACT: Electron transport through CH3- and CF3-terminated self-assembled alkanethiols monolayers (SAMs) on Au(111) has been studied by Scanning Tunneling Microscopy (STM). Current-voltage (I-V) measurements by Scanning Tunneling Spectroscopy (STS) at precisely controlled positions of the structurally characterized lattice show no voltage gap for CH3-terminated decanethiols. In contrast, in the I-V-curves for CF3-terminated decanethiol monolayers, an asymmetric voltage gap of about 2V can be clearly observed. The latter result is discussed in terms of a model that addresses the influence of the position of the molecular dipole in the tunneling gap between the Sulfur coated substrate and the base metal STM tip. The results of lattice-position-resolved STS measurements will also be re-examined in the light of the above model and shown to be consistent with it. Finally, the relevance of this work to the interpretation of the working mechanisms of SAM-based electronic devices will also be discussed.
    03/2002;
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    ABSTRACT: The structure and the electronic properties of a series of CH3- and CF3-terminated alkanethiol monolayers on Au(111) have been studied by scanning tunneling microscopy (STM) and surface X-ray and helium scattering. At full coverage, the CH3-terminated monolayers form long-range ordered domains of a (Ð3xÐ3)R30o and a (2Ð3x3)R30o standing-up phase. By thermal desorption, distinct lying-down phases of intermediate density as well as the (pxÐ3) lying-down phase were generated. In contrast, the CF3-terminated monolayers at full coverage form a standing-up phase of hexagonal symmetry that exhibits no long-range order at room temperature. Even after annealing, the domain sizes are smaller by more than one order of magnitude compared to the CH3-terminated thiol monolayers. A comparison of the low-density lying-down phases suggests no measurable influence of the CF3-group on the ordering. The current-voltage dependence (I-V-curves) measured by scanning tunneling spectroscopy (STS) shows no voltage gap for CH3-terminated decanethiols. In contrast, in the I-V-curves for CF3-terminated decanethiol monolayers, an asymmetric voltage gap of about 2 V can be clearly observed. The latter results are discussed in terms of a microscopic model that includes the formation of an interfacial Coulomb barrier at the CF3/vacuum boundary. In addition, the effects of the tunneling conditions on the STM image contrast were examined. These studies demonstrate that the nature of the STM images and thus, the respective apparent lateral order of the films, strongly depends on the choice of the tunneling parameters.
    Surface Science 02/2002; 498(1). · 1.87 Impact Factor
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    ABSTRACT: The structure and the electronic properties of a series of CH3- and CF3-terminated alkanethiol monolayers on Au(111) have been studied by scanning tunneling microscopy (STM) and surface X-ray and helium scattering. At full coverage, the CH3-terminated monolayers form long-range ordered domains of a (√3×√3)R30° and a (2√3×3)R30° standing-up phase. By thermal desorption, distinct lying-down phases of intermediate density as well as the (p×√3) lying-down phase were generated. In contrast, the CF3-terminated monolayers at full coverage form a standing-up phase of hexagonal symmetry that exhibits no long-range order at room temperature. Even after annealing, the domain sizes are smaller by more than one order of magnitude compared to the CH3-terminated thiol monolayers. A comparison of the low-density lying-down phases suggests no measurable influence of the CF3-group on the ordering. The current–voltage dependence (I–V-curves) measured by scanning tunneling spectroscopy (STS) shows no voltage gap for CH3-terminated decanethiols. In contrast, in the I–V-curves for CF3-terminated decanethiol monolayers, an asymmetric voltage gap of about 2 V can be clearly observed. The latter results are discussed in terms of a microscopic model that includes the formation of an interfacial Coulomb barrier at the CF3/vacuum boundary. In addition, the effects of the tunneling conditions on the STM image contrast were examined. These studies demonstrate that the nature of the STM images and thus, the respective apparent lateral order of the films, strongly depends on the choice of the tunneling parameters.
    Surface Science - SURFACE SCI. 01/2002; 498(1):89-104.
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    ABSTRACT: We report the formation and subsequent topochemical polymerization of semi-fluorinated self-assembled monolayers (SAMs) containing diacetylene groups. The SAMs used were formed via the adsorption of the following derivatives:  CF3(CF2)n(CH2)16CCCC(CH2)10SH (n = 2, 3) and CF3(CF2)n(CH2)11CCCC(CH2)10SH (n = 9, 12) onto the surface of gold. The polymerization of the monolayers was initiated by UV irradiation and was monitored using resonance Raman spectroscopy. SAMs with short semi-fluorinated portions were found to polymerize more effectively than those with longer portions, indicating that steric factors become important with increasing perfluorinated chain length.
    Langmuir. 09/2001; 17(21).
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    ABSTRACT: We report a dramatic, concentration-dependent decrease in the rate of photo-oxidation of semiconducting polymers due to the addition of small amounts of metal nanoshells to the polymer. In each case, the nanoshell resonances are tuned to the triplet exciton-ground state energy of the polymer. The nanoshell dopants slow the oxidation rate yet do not affect the photoluminescent properties of the polymers to which they have been added. © 2001 American Institute of Physics.
    Applied Physics Letters 03/2001; 78(11):1502-1504. · 3.52 Impact Factor
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    ABSTRACT: Long-range interactions between self-assembled monolayers (SAMs) of semifluorinated alkanethiols have been studied by direct force measurements in water and aqueous NaCl solutions. SAMs prepared from three different thiols, with identical fluorinated head groups but varying hydrocarbon spacer lengths, were investigated: CF(3)(CF(2))(9)(CH(2))(x)SH, where x=2, 11, or 17. Force measurements show that the interactions in water and electrolyte solutions are composed of both double-layer interactions emerging from what appears to be charges adsorbed onto the surfaces and long-range "hydrophobic" attractions, in excess of the expected van der Waals forces. The three investigated thiols produce similar results in force measurements, though the contact angles with water are slightly different. The "hydrophobic" attraction has the form of step-like attractive discontinuities in the force profiles at separations ranging from 20 to 40 nm, caused by bridging of microscopic bubbles residing at the surfaces. The shape or range of these discontinuities are not significantly affected by replacement of the water with either 1 mM or 1 M NaCl solutions. The origin of the charges causing the electrostatic double-layer interaction is unclear, but some possible causes are discussed. Copyright 2001 Academic Press.
    Journal of Colloid and Interface Science 03/2001; 235(2):391-397. · 3.55 Impact Factor
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    ABSTRACT: Near-edge absorption fine structure (NEXAFS) measurements are used to probe the molecular orientation of semifluorinated (SF) mesogens, −(CH2)x(CF2)yF, which are attached to (i) the isoprene backbone of polyisoprene or a styrene-isoprene diblock copolymer (“soft” substrate), and (ii) a Au-covered solid substrate via a thiol link (“hard” substrate). The SF groups on both surfaces are oriented and on average are tilted from the sample normal. The tilt angle, τF-helix, of the fluorinated part of the SF group on each substrate is determined exclusively by the combination of x and y, increasing with increasing x and with decreasing y. Moreover, τF-helix is found to be independent of the surface topology (flat surfaces vs surfaces covered with holes or islands of the copolymer), casting solvent, and the architecture of the SF group (single vs 2-armed monodendron). Comparing the orientation of the SF groups on both substrates reveals that τF-helix is approximately 14° higher on the “soft” substrate.
    Macromolecules. 01/2000; 33:6068--6077.
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    ABSTRACT: This paper describes the synthesis of a series of alkanethiols containing perfluorinated terminal segments: F(CF2)m(CH2)nSH, where m=1, n=9–15; m=2, n=11–14; m=3, n=10–13; and m=4, n=9–12. Fluorinated alkyl iodides of the general formula F(CF2)m(CH2)nI, where m=1–4 and n=0 or 1, were added to long-chain ω-olefins that were functionalized at the α-terminus with a thioacetate group. The reactions proceeded in good yields under free radical conditions. Reduction of the resulting secondary iodides gave long-chain alkanethioacetates with perfluoroalkyl terminal segments. These intermediates were readily transformed into the corresponding terminally perfluorinated alkanethiols by acidic deprotection. The product thiols should find use in the generation of well-defined fluorinated interfaces using the self-assembled monolayer (SAM) technique.
    Journal of Fluorine Chemistry 01/1999; 93(2):107-115. · 1.94 Impact Factor