Papers

AVS1997 Session SS+AS-MoM: Self-Assembled Monolayers and Solid-Liquid Interfaces

Monday, October 20, 1997 8:20 AM in Room B1/2

Monday Morning

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8:20 AM		SS+AS-MoM-1 Orientation and Order in Microcontact-Printed, Self-Assembled Monolayers of Alkane Thiols on Gold, Investigated with NEXAFS G. Hähner, D. Fischer, A. Marti, N.D. Spencer (ETH Zurich, Switzerland) Self-assembled monolayers (SAMs) have attracted significant interest over the last decade due to their ease of preparation and vast potential for applications. Thiols on gold, prepared by immersing the substrates into dilute ethanolic thiol solutions, have attracted particular attention ¹. Recently, Whitesides et al. have developed the so-called microcontact printing technique (µCP), which allows microstructured organic monolayers to be prepared ². In short, a stamp made of PDMS is "inked" with the thiol solution and subsequently, after drying, is brought into contact with the substrate. We have investigated such contact-printed layers of alkane thiols on gold with the synchrotron-based method of near edge x-ray absorption fine structure spectroscopy (NEXAFS) and compared them with layers prepared by the conventional immersion technique. While some surface analytical methods, such as contact angle measurement, can give indirect information about order in such films, NEXAFS allows the direct determination of the order and orientation of the alkane chains and is highly sensitive to certain inherent parameters of the adsorbed layer, such as the number of gauche conformations. Such defects play an important role in possible applications of these layers. We have studied the influence of various preparation conditions on order and orientation of the resulting films, in particular the effect of the solution concentration and the chain length ³. ¹L.H. Dubois and R.G. Nuzzo, Annu. Rev. Phys. Chem. 43 (1992) 437 ²A. Kumar and G.M. Whitesides, Appl. Phys. Lett. 63 (1993) 2002; A. Kumar, H.A. Biebuyck and G.M. Whitesides, Langmuir 10 (1994) 1498 ³D. Fischer, A. Marti, and G. Hähner, J. Vac. Sci. Technol. A, in press
8:40 AM		SS+AS-MoM-2 Ordered Thiophene, Methanethiol, and Benzeneselenol Monolayers on Au(111) formed by Gas- and Liquid-phase Deposition: New Insights into the Chemisorption Mechanism. M.H. Dishner, J.C. Hemminger, F.J. Feher (University of California, Irvine) The popular mechanism for SAM formation by thiols on Au(111) invokes deprotonation of the thiol (RSH) to form a thiolate on the surface (RS-Au); furthermore, it is believed that the thiols etch gold vacancy islands or "pits" in the surface. We have adsorbed thiophene (C₄H₄S) from ethanolic solution, deposited methanethiol from the gas phase, and formed benzeneselenol SAMs from solution, in order to probe the proposed mechanism. Our STM experiments show that thiophene forms well-ordered monolayers that exhibit a (2√19 x √3) R30° unit cell and gold vacancy islands. After annealing, the gold vacancy islands coalesce and merge with step edges. Rapid gas-phase dosing (10⁴ monolayers/sec) of methanethiol (CH₃SH) forms gold vacancy islands that are initially aligned along the stripes of the Au(111) (23 x √3) surface reconstruction, but disappear after annealing to yield ordered pit-free domains of methanethiol. Benzeneselenol deposition from solution also yields well ordered monolayers; however, chemisorption fails to produce gold vacancy islands - instead single layer high gold island protrusions are formed. These small islands are annealed to form much larger hexagonal gold islands. In light of our experiments with thiophene, methanethiol and benzeneselenol, we find it unlikely that chemisorption of organosulfur and organoselenium reagents results in the chemical etching of gold. Instead, our results suggest the gold vacancy islands and gold islands are the kinetic product of chemisorption. These structures are caused by the redistribution of gold atoms on the surface and the relaxation of the clean gold reconstruction.
9:00 AM		SS+AS-MoM-3 Nucleation and Growth Phenomena in Organic Amphiphile Monolayers G.E. Poirier (National Institute of Standards & Technology) Alkanethiols chemisorb strongly on noble metal surfaces. As amphiphiles, the molecules spontaneously align and terminate growth at one molecular monolayer. The film thickness, its electron transport properties, and its adhesive properties can be controlled by changing the length of the alkyl chain, its degree of saturation, and its terminal functional group. These monolayers are therefore desirable in biomimetic and biosensing applications. Using an ultra-high vacuum scanning tunneling microscope, we have investigated nucleation and growth phenomena during monolayer assembly. Monolayer islands nucleate preferentially in unfaulted stacking domains of the herringbone reconstructed Au(111) surface. Islands grow in plane with lateral expulsion of the Au bridging rows and compression of the dislocation elbows into denuded zones. Compression of the surface Au layer is relieved by creation of adatoms and vacancies. Vacancies heterogeneously nucleate islands at the defected herringbone elbows in denuded zones. These results demonstrate how interactions between the assembling amphiphiles and the Au surface reconstruction influence mesoscale aspects of the final monolayer structure.
9:20 AM		SS+AS-MoM-4 Forced Alignment of Adsorbed Organic Monolayers During STM Observation. F. Stevens, D. Buehner, T.P. Beebe, Jr. (University of Utah) Scanning tunneling microscopy (STM) has proven to be a powerful technique for observing monolayers of molecules self-assembled at a liquid-solid interface. Because STM allows molecular resolution images to be acquired in real space and near-real time, it allows the observation of domain boundaries and monolayer defects, as well as observation of dynamic behavior of the monolayer. However, the question of how much the STM perturbs the system under observation has often arisen. That is, are the structures and behaviors observed by STM the same as those present when the system is not being observed? This has proved to be a difficult question to address, as many of these phenomena can only be observed by STM. It has been found that the STM can definitely disturb adsorbed monolayers under some conditions (for example, by application of a high-voltage pulse, or by lowering the tunnel gap to a small value) but the effect on the monolayer of scanning under "normal conditions" has remained unclear. In recent experiments we have found that the scanning action of the STM tip clearly acts to align domains of octadecanol or octatriacontane in a reversible and repeatable manner. Furthermore, this effect is not seen when scanning the liquid crystal 8CB. These results show that with some systems, imaging by STM appears to perturb the observed monolayer, while in other systems it does not. This information is vital to interpretation of STM images, as observed features can be related to intrinsic properties of the monolayer only if the monolayer is not perturbed by the imaging process. Observing the effect of changing the scan direction provides a fairly simple test of whether an observed monolayer is being perturbed by the STM tip. Also, observing whether or not a monolayer is perturbed by the STM tip can give an indication of the balance between attractive and repulsive forces in the monolayer.
9:40 AM		SS+AS-MoM-5 Molecular Monolayer Dynamics: Atomistic Potentials and Ensemble Responses N.D. Shinn, T.A. Michalske (Sandia National Laboratories); C. Daly (Northeastern University); U. Landman (Georgia Institute of Technology) Beyond chemical modification, self-assembled monolayers (SAMs) offer the opportunity to engineer the mechanical properties of a surface or interface. How are the atomistic potentials that determine molecular structure and ordering manifested in the mechanical response of a mesoscopic ensemble? We are combining Acoustic Wave Damping (AWD) experiments and molecular dynamics simulations to elucidate mechanisms for energy storage and dissipation within alkane thiol SAMs. The AWD technique measures the elastic energy storage and dissipative loss within a SAM adsorbed onto the gold electrode of a quartz crystal transverse shear resonator. The shear modulus of the monolayer is obtained from the resonator frequency response data using an equivalent circuit model. Because the AWD experiment induces high-frequency shear deformation of the SAM without a contact probe, nanosecond molecular dynamics simulations are directly applicable for interpreting the measured shear modulus. These simulations have revealed that periodic lateral translation of the substrate induces motions other than simple molecular libration, thereby accounting for the anomalously low measured shear moduli. Varying the constituent molecule’s chain length alters the SAM’s storage modulus. Both experiments and simulations provide evidence that energy dissipation mechanisms are associated with defects within the two-dimensionally ordered layer. The role of intermolecular interactions is further explored by comparing solution-deposited SAMs with monolayers formed by adsorption from the vapor phase, thereby probing the nanomechanics during the self-assembly process. We emphasize the correlation of systematic experiments with representative simulations to establish a broadly applicable conceptual understanding of SAM mechanics. Supported by the DOE-BES (DE-AC04-94AL85000).
10:00 AM		SS+AS-MoM-6 Simulation of Interfaces using Molecular Dynamics M.R. Philpott (University of Singapore); J.N. Glosli, J.F. Belak (Lawrence Livermore National Laboratory); J.E. Roberts (IBM Almaden Research Center) Present day computer calculations range from electric double layer simulations on an office work station to crack propagation on parallel machines with thousands of nodes. The bottlenecks are better models for interatomic potentials, especially ones for the the making and breaking of chemical bonds in condensed matter. This talk begins with an overview of molecular dynamics methods and comments on technologies where modelling can help (polymers, drug design, magnetic recording, lithography). It will be illustrated with studies of electrochemical interfaces (no bond formation, simple potential with a long range component), and the structure and nanotribology of amorphous carbon films (complicated potential describing the chemistry of covalently bonded carbon-hydrogen networks) similar to that on your PC's hard disk. For electrochemical systems classical dynamics with constant N,V,T and either constant electric potential or constant surface charge (you cannot have both) is used to model electric double layers of simple aqueous electrolytes between charged metal surfaces and for comparisons with Gouy-Chapman theory. In simple calculations we model the surface as flat and featureless and in more advanced models we use surface potentials that describe the atomic topography and the tendency of water molecules to adsorb top site with oxygen down. The simulations show: compact, diffuse, surface oriented water layers and an identifiable bulk zone. The electric potential across the cell deviates considerably from Gouy-Chapman predictions due to finite size of atoms, water layering and electrostatic quadrupole effects. For the tribology systems we have investigated the growth, chemical composition, physical structure and tribology of amorphous C and CH films up to 10 nm thick. The Brenner potentials used in this work allow incident C atoms to bond to previously deposited atoms in ways determined by the bonding state of nearest and next nearest neighbors. Results for film composition with deposition energy, temperature and beam composition are described. This talk will end on a note of rampant uninhibited speculation about where the field will evolve in the next ten years. The electrochemical study was funded in part by the Office of Naval Research. The tribology study was funded by DOE Crada No. TC-297-92-C. All work was performed at LLNL and IBM Almaden.
10:40 AM		SS+AS-MoM-8 Effect of the Environment on the Molecular Orientation of Thiol Monolayers Investigated by Second Harmonic Generation (SHG) F. Eisert (University of Heidelberg, Germany); O. Dannenberger (University of Washington); M. Buck (University of Heidelberg, Germany) For the structural analysis of self-assembled monolayers techniques requiring UHV or at least ambient atmosphere are usually applied. For certain practical applications, however, the knowledge of their structure and behavior in contact to liquids is of fundamental interest. Thus the question of how structures derived from studies under non-liquid conditions can be transferred to layers in contact with liquids is of crucial importance. To address this topic an end group derivatised thiol 4-(12-mercaptododecyl)aminonitrobenzene (p-NO₂-C₆H₄-NH(CH₂)₁₂SH,NAT) self- assembled on gold substrates was investigated. SHG allows to probe film formation and molecular orientation both in situ and ex situ. Using different laser wavelengths the S-Au bond formation can be addressed independently from the nitro aniline endgroup (NA). The coverage dependent behavior of the SHG active endgroup can be studied by polarisation and phase sensitive detection. This allows to determine the orientation and shift of the resonance frequency of the NA moiety. For the completed monolayer under ambient atmosphere the nitroaniline end group is tilted by 51° from the surface normal. This result is in excellent agreement with independent IRRAS and NEXAFS investigations ¹. In contrast, when immersed in ethanol the orientation of the NA group adopts a significantly more upright position. This can be ascribed to an improved interaction of NAT with the environment, i.e. ethanol. Additional coverage dependent studies reveal a reorientation of the end group with increasing adsorption time. ¹Langmuir 12; 5330 (1996)
11:00 AM		SS+AS-MoM-9 Self-Assembled Monolayers of n-alkane Thiols: Investigation of Chain Length Dependence by Nonlinear Optical Vibrational Spectroscopy A. Lampert, F. Eisert, M. Buck, M. Grunze (University of Heidelberg, Germany) The chain length dependence of monolayers of n-alkane thiols (CH₃(CH₂)_mSH) adsorbed on evaporated polycrystalline gold and silver films was analyzed in the C-H stretching region by IR-vis sum frequency spectroscopy (SFS). Since SFS is sensitive to non-centro symmetric structures it yields information complementary to conventional IR spectroscopy, i.e. in contrast to IR spectroscopy the C-H spectrum of the completed films is dominated by the methyl stretch modes. The chain length was varied between m = 1 and m = 21. The stability of thiols as a function of chain length is addressed. On gold the vibrational spectra for chains shorter than m = 10 are sensitively dependent on the exposure to ambient atmosphere and thus an inert gas atmosphere is required for reproducible measurements. For both substrates the SFS signals of the methyl stretching modes alternate as a function of chain length. The magnitude of this variation is significantly larger on gold in agreement with the larger tilt angles. This so called odd-even effect is shifted by one methylene unit on gold in comparison with silver. The chain length dependent intensity variations of the modes agree with the interpretation that the sulphur adopts an sp³ hybridization on gold and an sp hybridization on silver. The orientation of the methyl groups determined by SFS is compared with the orientation of the methylene chains determined by IR. Assuming extended "all-trans" chains perfect agreement is obtained between the SFS and IR data for thiols on gold whereas systematic deviations are seen for silver. The SFS data for m=1 (ethane thiol) deviates significantly from m>1 and it is concluded that the methyl group is inclined to the substrate plane.
11:20 AM		SS+AS-MoM-10 Thermal Behavior of Alkylsiloxane Self-Assembled Monolayers on the Oxidized Si(100) Surface G.J. Kluth, M.M. Sung, M. Sander, R. Maboudian (University of California, Berkeley) The thermal behavior of alkyltrichlorosilane-based self-assembled monolayers on the oxidized Si(100) surface has been examined under ultrahigh vacuum conditions for different chain lengths. High resolution electron energy loss spectroscopy shows that the monolayers are stable in vacuum up to 740 K independent of chain length. Above 740 K the chains begin to decompose as indicated by the decreased intensity of C-H modes. Following this initial step, methyl groups attached to silicon atoms are observed, indicating that the Si-C bond remains intact. Furthermore, the water contact angle remains high even after decomposition has begun, indicating that the monolayer retains some degree of order. From these observations it is concluded that the chains decompose through C-C bond cleavage, resulting in the desorption of hydrocarbon fragments and a gradual reduction in chain length. The behavior of specially synthesized chains, in which the four carbon atoms at the end of the chain have been deuterated, confirms this desorption picture.
11:40 AM		SS+AS-MoM-11 The Influence of Surface Modifiers on the Kinetics of Methanol Electrooxidation at Platinum Surfaces T.D. Jarvi, S. Sriramulu, E.M. Stuve (University of Washington) Desirable features of the direct methanol fuel cell have generated interest in electrocatalytic oxidation of methanol. Despite considerable research in this area, insufficient methanol electrooxidation kinetics and poor selectivity to complete oxidation persist. Attempts to remedy these problems have primarily focussed on surface modification through electrodeposition of metallic adlayers, such as ruthenium. While electrochemistry provides a convenient means to form an adlayer, it does not, in general, offer good control over the deposition process or a definitive way to characterize the adlayer. In order to overcome these difficulties we have initiated ex-situ studies of well defined metallic adlayers on platinum surfaces. Physical vapor deposition in vacuum allows us to dose virtually any material of interest at a controlled rate. Once formed, the adlayer may be characterized by electron spectroscopies and transferred to the electrochemical cell to study methanol reaction kinetics. In this way, the enhancement or poisoning mechanism(s) of the reaction modifier may be mapped out by probing the methanol reaction kinetics as a function of modifier coverage. For example, our results indicate that adsorbed CO, an effective catalyst poison, begins to decrease reaction kinetics significantly at a coverage of only 0.04 ML. The CO modification results may be explained by defect control of the reaction or through an ensemble effect. The coverage dependent behavior of poisons and promoters will be compared.