AVS1996 Session SS+AS-ThM: Structure of Self Assembled Monolayers

Thursday, October 17, 1996 8:20 AM in Room 203A

Thursday Morning

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8:20 AM		SS+AS-ThM-1 Unexpected Structure and Properties of \omega\-substituted Thiol Monolayers on Gold G. Scoles, T. Leung, P. Fenter, P. Eisenberger (Princeton University) We use grazing incidence x-ray diffraction to study monolayers of \omega\-substituted thiol (1-undecenethiol) on a gold (111) surface, in which each thiol is terminated with a double bond. Surprisingly, the structure of these monolayers is found to be dramatically different from methyl-terminated (n-alkanethiol) monolayers. The unit mesh is 5 angstrom by 17.3 angstrom, as compared with 10 angstrom by 8.66 angstrom in the c(4\sr\3 x 2\sr\3)R30 structure of the alkanethiol monolayers. Moreover, the monolayers made of molecules that contain a double bond exhibit higher thermal stability. The domain size begins to increase at T /approx/ 140C, whereas the alkanethiol monolayers melt and start annealing at around 90C. These unexpected phenomena illustrate that the structures and properties of this class of materials (thiols on gold) depend strongly upon the interactions within the films. The structure of these films will be interpreted based on the properties of the molecular structure.
8:40 AM		SS+AS-ThM-2 Surface Characterization of Terminally Functionalized Alkanethiol Self-assembled Monolayers and Mixtures A. Belu, C. Tidwell (University of Washington); B. Tarasevich, D. Allara (Pennsylvania State University); B. Ratner (University of Washington) The influence of chemical functional groups at the surface of organic thin films has been evaluated using a systematic series of terminally functionalized alkanethiol self-assembled monolayers (SAMs) and their binary mixtures. TOF-SIMS, angle-dependant XPS, contact angle, IR and ellipsometry have been used to characterize surface properties. Homogenous SAMs of X(CH\sub 2\)\sub 15\SH on gold with X = -CH\sub 3\, -CH\sub 2\OH, -CO\sub 2\CH\sub 3\, -CO\sub 2\H and -SO\sub 4\H were studied as well as mixtures (3:1, 1:1, 1:3 targeted surface concentration) of the SAMs with functional groups -CH\sub 3\/-CO\sub 2\H, -OH/-COOH, and of methyl terminated SAMs with mixed alkane length CH\sub 3\(CH\sub 2\)\sub 15\SH/CH\sub 3\(CH\sub 2\)\sub 9\SH. ESCA, contact angle, IR and ellipsometry confirm the presence of the expected functionality of the homogeneous SAM surfaces and allow quantification of the components in the mixed SAM surfaces. With TOF-SIMS, intact molecular ions representative of the alkanethiols can be identified and and employed for the quantification of the mixed surfaces. Furthermore, methods of ion formation in static SIMS experiments can be probed by comparing the differently terminated SAMs. In addition, studies of biological interactions with the SAMs and mixtures are currently underway.
9:00 AM		SS+AS-ThM-3 The Surface Structure of Self-Assembled Fluorinated Thiolate Monolayers D. Castner (University of Washington); M. Tsao, J. Rabolt (University of Delaware) The surface structure of SAMs prepared from CF\sub 3\(CF\sub 2\)\sub 7\(CH\sub 2\)\sub 2\SH (F8H2), CF\sub 3\(CF\sub 2\)\sub 7\(CH\sub 2\)\sub 11\SH (F8H11), CF\sub 3\(CF\sub 2\)\sub 7\C(O)NH(CH\sub 2\)\sub 2\SH (F8), and F8H11/F8 mixtures were examined by XPS, static ToF-SIMS, NEXAFS, and FTIR. The XPS results were consistent with the fluorinated tails located at the SAM-vacuum interface and the sulfur atoms at the SAM-Au interface (thiolate species, S\sub 2p3/2\ BE = 162 eV) for all molecules. Changes in the XPS composition and C\sub 1s\ spectra with variation of the number of methylene and amide groups were used to show the preferential enrichment of F8H11 in the F8H11/F8 mixed SAMs. For example, a 1:1 solution mixture generated a 2:1 (F8H11/F8) mixed SAM. The ToF-SIMS fragmentation patterns of the fluorinated SAMs were noticeably more complex than observed for alkyl SAMs. At low masses (<200) C\sub x\F\sub y\ fragments from the fluorinated tails were observed. At masses above 200 many recombination fragments were detected (e.g., AuF\sub 2\). The negative Au\sub 2\(M-H) and Au(M-H)\sub 2\ fragments were the most informative for identification of individual thiolates. NEXAFS and FTIR results for the F8 and F8H2 SAMs were consistent with a vertical orientation of the molecules. The NEXAFS polarization dependence of the F8H11 SAM differed from the other two SAMs and indicated either a slight tilt or disorder of the molecules. This is probably due to a competetion in F8H11 molecule between the alkyl chain portion of the molecule which prefers a ca. 30\super o\ tilt and the fluorinated tail which prefers a vertical orientation.
9:20 AM		SS+AS-ThM-4 Scanning Tunneling Microscopy and Tunneling Spectroscopy of Self-assembled Monolayers of Polyoxometalates M. Kaba (University of Delaware); I. Song (Kangnung National University, Korea); M. Barteau (University of Delaware) Polyoxometalates (POMs) are d\super 0\ early transition metal oxygen-anion clusters that exhibit a wide range of molecular architectures, surface charge densities, and both photo- and thermal-redox behaviors. Varying in sizes from about one to a few nanometers, these molecular clusters have found applications as acid and oxidation catalysts, electrode functionalization agents, and anti-retroviral agents, among others. Using Scanning Tunneling Microscopy (STM), we have accurately resolved the molecular aniosotropies of various POMs and have shown that the POMs self-assembled from aqueous solutions into ordered, monolayer arrays on inert graphite supports. The array spacings were consistent with the molecular dimensions of the polyanions, but the packings of the anions were slightly different from those in the bulk structure. Since the STM failed to resolve the cations, their positions in the 2-D surface arrays were established from spatially-resolved Tunneling Spectroscopy (TS). Subsequently, the identity of the cation was used to control the spacing between the anions. Furthermore, we demonstrated the lability of the 2-D structures not only by the cation exchanges, but by pyridine adsorption onto the acid sites in the monolayers. TS measurements on individual POM molecules revealed current peaks, referred to as negative differential resistance (NDR), in their current-voltage (I-V) spectra. This phenomenom (NDR) has been explained by other investigators as a consequence of a double-barrier resonant tunneling structure or quantum well in which the electron transmission probability decreases with increasing applied potential at a resonance (or trap state energy) level. All of the POMs exhibited this behavior at different applied potentials. More recently, we have shown that the NDR voltages can be correlated to the reduction potentials of the POMs. More reducible POMs showed NDR in their I-V spectra at smaller applied potentials (negative sample biases).
9:40 AM		SS+AS-ThM-5 The Interfacial Structure of Self-assembled Alkanethiol Monolayers in Equilibrium with Water at Ambient Temperature T. Boland, D. Allara (Pennsylvania State University) The successful design of a biomaterial or a biosensor warrants the examination of the reactions at the sensor-biomolecule interface. Many of such reactions are based on molecular recognition events that are dependent on the equilibrium structure of the interface. In Situ external reflection-absorption FTIR is used to study the structure of pure and mixed alkanethiol self-assembled monolayers (SAMs) on gold at varying partial pressures of water (0-100% relative humidity). Specifically the structures of SAMs from methyl and COOH terminated hexadecanoic solutions were investigated. Simultaneously, quartz crystal microbalance (QCM) gravimetry is used to acquire water-SAM isotherms. Methyl terminated SAMs showed higher order at low relative humidity with decreasing order at humidities above 60%. At low humidity levels there is partial water coverage probably clustering around defects in the SAM, forcing the hydrophobic chains closer together. At high humidity levels, QCM indicating monolayer coverage, the terminal methyl groups are particularly disordered while the methylene backbone is only slightly disordered compared to its dry structure. The COOH terminated SAMs, contrarily, show only few changes in structure that reflect the interaction of the terminal group with water. This in situ FTIR technique is particularly useful in the analysis equilibrium structures of mixed SAM-water interfaces. The use of these model organic surfaces may lead to a better understanding of the influence of surface structure in molecular recognition systems.
10:00 AM		SS+AS-ThM-6 Temperature-Programmed Desorption of Chemical Probes from Self- Assembled Monolayers A. Vogt, T. Beebe, Jr. (University of Utah) In solution-phase formation of self-assembled monolayers (SAMs), a requirement for most systems is a solvent that is less reactive toward the substrate than the self-assembling molecule. Vapor-phase deposition does not require the use of a solvent, but solvent presence has been shown in the past to be very useful for forming well ordered SAMs. The solution-phase adsorption of alkanethiols on metal surfaces using a number of different solvents has been discussed in the literature, and solvents forming the most ordered monolayers have been identified. The most common solvent used for self-assembly of alkanethiols is ethanol, but its role in the formation of SAMs is not entirely understood. Temperature-programmed desorption (TPD) is a useful technique to obtain an understanding of chemical interactions that take place at surface of SAMs. In TPD, the activation energy of desorption and vibrational pre-exponential factor of an adsorbed molecule can be calculated using the Polyani-Wigner equation. Reactions resulting in different chemical states of the terminal groups of SAMs can also be observed in a TPD experiment, for example in the form of multiple desorption temperatures obtained for a particular mass and coverage. TPD has been used to study the adsorption and interaction of various molecules on solvent-free vapor- deposited SAMs. The penetration and reactivity of adsorbates (i.e. solvents) following solvent-free vapor-phase deposition may help explain the role of a particular solvent and more importantly the necessity of the solvent.
10:20 AM		SS+AS-ThM-7 A Combined SFG and NEXAFS Determination of Molecular Orientation of a Self-assembled Monolayer M. Yeganeh, J. Chen, S. Dougal, R. Polizzotti (Exxon Research and Engineering Co.) Previously, infrared-visible sum-frequency generation (SFG) was used to determine the interfacial molecular orientation of systems containing CH\sub 3 moieties. In these experiments the vibrational SFG intensities generated by the symmetric stretch of the CH\sub 3 group, were measured as a function of IR photon energy for ssp (s=SFG, s=visible, and p=IR) and sps polarization configurations. The information obtained from the spectra was used to infer the molecular orientation at the interface. The strength of the resonance signal generated by the symmetric stretch of CH\sub 3 for the sps polarization configuration strongly depends on the molecular orientation and is extremely small for tilt angles smaller than 40 Deg. This phenomena creates large uncertainty in the determination of the tilt angle. To avoid this uncertainty, we have shown that the asymmetric stretch of CH\sub 3 can be detected in the pss polarization configuration. We were able to detect splitting between the in-plane and out of plane stretch of this moiety. This single spectra was used for the determination of the tilt angle. The signal to noise ratio of this feature is always high and independent of the orientation of the molecule. To confirm the results using this new capability, we have measured the tilt angle of a stearic acid monolayer on sapphire substrate using both NEXAFS and SFG spectroscopy. The variation of the NEXAFS fluorescence-yield of the C-H\super * resonance feature as a function of the angle of incidence was used to determine the orientation of the steraic acid monolayer. The results (tilt angle=33. +/- 3.) obtained from these two independent techniques were in excellent agreement.
10:40 AM		SS+AS-ThM-8 Compositional Imaging of Patterned Self-assembled Monolayers using Scanning Probe Microscopies: Effects of Chain-Length, Packing, and Density G. Bar (Albert-Ludwigs University, Germany); S. Rubin, A. Parikh, B. Swanson, T. Zawodzinski (Los Alamos National Laboratory) We report the role of material properties such as elasticity, density, and packing in the compositional imaging of patterned self-assembled monolayers (SAMs) of n- alkanethiolates by scanning probe microscopies. We investigated the frictional and elastic properties of patterned SAMs of mixed chain length as well as spatially patterned SAMs of uniform chemical composition (single chain-length), but different packing and densities using lateral force and force modulation microscopies (LFM and FMM). Images of spatial variations in frictional properties and elasticity were obtained in all cases. The observed elasticity correlates directly with the corresponding friction images as well as with the spatial distribution of the two components on the surface. Further, friction between the tip and the sample domains is found to show a reversible dependence on the applied load. At low or moderate loads, the domains of the shorter component show high friction and elasticity in both cases. The above friction contrast reverses at higher loads, but reverts to the original upon reduction of the load. The observation of image contrast for patterned SAMs composed of identical chemical composition, but different packings reveals that packing and density variations play an important role in the image contrast formation in LFM. The results clearly prove that the chemical identity of the exposed end-group does not uniquely determine the frictional properties of monolayer films.
11:00 AM		SS+AS-ThM-9 Structural Correspondence between Layered, Crystalline Silver Alkanethiolates and Correspondent Self-assembled Monolayers A. Parikh, S. Rubin, G. Strouse, B. Swanson, T. Zawodzinski (Los Alamos National Laboratory) Existence of a strong structural correlation between the families of single monolayers of alkanethiolates on a planar silver surface (CH\sub 3\(CH\sub 2\) \sub n\ SH/Ag[111]) and the bulk synthesized, polycrystalline solids of silver alkanethiolates [CH\sub 3\(CH\sub 2\)\sub n\SAg]\sub m\ is shown using a combination of quantitative applications of transmission and polarized, external reflection infrared spectroscopies (TIRS and PER-IRS). The structural similarities shared by the two classes of materials probed, included chain-conformational order, packing density, and defect contents as functions of the length of the alkyl chain. The use of the optical response functions derived from the solid crystals to calculate the monolayer spectra in alkyl chain C-H stretching modes give the average chain-orientation at ~24-30 degrees from the surface normal for the highly ordered cases (n>10), in good agreement with those reported using independent techniques. Direct transferability of the optical response functions illustrated in the above calculations further imply a good correspondence between the two classes of materials in their dominant inter- and intramolecular interactions. The above results serve to show that the 3-D bulk analogs are useful structural models for the alkanethiolate monolayers.
11:20 AM		SS+AS-ThM-10 TOF-SIMS Investigation of Molecular Overlayers: Thiols on Gold D. Rading, R. Kersting (Physikalisches Institut der Universit\um a\t M\um u\nster, Germany); A. Benninghoven (Physikalisches Institut der Universit\um a\t, Germany) There is an increasing interest in analytical techniques for the characterization of molecular overlayers. TOF-SIMS (time-of-flight secondary ion mass spectrometry) has been successfully applied to that kind of analysis. The technique provides molecular information from the uppermost monolayer with high sensitivity. In order to get more systematic information about the secondary ion emission behavior of molecular overlayers, we carried out a detailed investigation of thiols. Self assembled layers on gold (chemisorption) as well as spin-coated films on silicon (physisorption) of alcanethiols with 4 to 18 C-atoms, thiosalicylic acid, and 6-thiopurine have been investigated. We determined secondary ion yields Y and damage cross sections \sigma\ for the most characteristic secondary ion species emitted from these overlayers. In addition, the disappearance temperature for the molecular ion signals were determined from TP-SIMS (temperature programmed secondary ion mass spectrometry) measurements providing information about the surface binding energy E\sub B\. We found that the damage cross section \sigma\ decreases with increasing binding energy E\sub B\. This relation between \sigma\ and E\sub B\ allows an estimation of the lateral energy distribution in the sputter region at the surface.