A Comparative Scanning Tunneling Microscopy Study of Physisorbed Linear Quadrupolar Molecules: C2N2 and CS2 on Au{111} at 4 K

Effect of head-group chemistry on surface-mediated molecular self-assembly

Surface molecular self-assembly is a fast advancing field with broad applications in sensing, patterning, device assembly, and biochemical applications. A vast number of practical systems utilize alkane thiols supported on gold surfaces. Whereas a strong Au-S bond facilitates robust self-assembly, the interaction is so strong that the surface is reconstructed, leaving etch pits that render the monolayers susceptible to degradation. By using different head group elements to adcust the molecule-surface interaction, a vast array of new systems with novel properties may be formed. In this paper we use a carefully chosen set of molecules to make a direct comparison of the self-assembly of thioether, selenoether, and phosphine species on Au(111). Using the herringbone reconstruction of gold as a sensitive readout of molecule-surface interaction strength, we correlate head-group chemistry with monolayer (ML) properties. It is demonstrated that the hard/soft rules of inorganic chemistry can be used to rationalize the observed trend of molecular interaction strengths with the soft gold surface, that is, P>Se>S. We find that the structure of the monolayers can be explained by the geometry of the molecules in terms of dipolar, quadrupolar, or van der Waals interactions between neighboring species driving the assembly of distinct ordered arrays. As this study directly compares one element with another in simple systems, it may serve as a guide for the design of self-assembled monolayers with novel structures and properties.

Modification of Ag(111) surface electronic structure via weak molecular adsorption of adenine measured with low temperature scanning tunneling microscopy and spectroscopy

Low temperature scanning tunneling microscopy and spectroscopy have been used to resolve modifications to the Ag(111) surface electronic structure due to the weak adsorption of the nucleobase adenine. Differential conductance spectroscopy recorded at 15 K reveals an upward energetic shift of the surface state native to Ag(111) from a band edge of -67 meV on the clean surface to +82.5 meV recorded over adenine islands. Differential conductance images show the impact of adenine domains on the density of available states as a function of energy relative to the uncovered Ag terraces as well as free-electron-like scattering in the adenine domains. Dispersion of the parallel wave vector of scattered electrons in the adenine domains is compared with the dispersion for electron scattering in bare silver and the ratio of effective masses for electrons in those bands is 1.1+/-0.2. It is hypothesized that this shift occurs due to a combination of effects brought on by the adsorption of adenine including dielectric screening of the first image potential.

Dimethyl sulfide on Cu{111}: molecular self-assembly and submolecular resolution imaging

The literature contains many studies of thiol-based, self-assembled monolayers (RSH); however, thioethers (RSR) have barely begun to be explored, despite having the potential advantages of being more resistant to oxidation and allowing for the control of self-assembly parallel to the surface. This paper describes a low-temperature scanning tunneling microscopy investigation of dimethyl sulfide on Cu{111}. Previous work on the adsorption of dibutyl sulfide on Cu{111} revealed that intermolecular van der Waals interactions directed the parallel ordering of dibutyl sulfide molecules in linear rows. Upon annealing to 120 K, small dibutyl sulfide domains reordered into very large, ordered domains free of defects. The current study reveals the effect of the shorter alkyl chain length of dimethyl sulfide on both the rate of diffusion and the packing structure of the molecule. At a medium surface coverage and at 78 K, it was found that dimethyl sulfide is mobile and forms large, ordered islands without the 120 K annealing that was required for dibutyl sulfide to arrange. Also, the molecular packing structure evolves from quadrupole-quadrupole interactions and results in a perpendicular arrangement of neighboring molecules instead of the parallel arrangement observed for dibutyl sulfide. We show high-resolution images of the dimethyl sulfide islands in which submolecular features are revealed. These high-resolution data allow us to propose a structural model for the adsorption site of each dimethyl sulfide molecule within the ordered structures. These results demonstrate that the length of the alkyl side chain is an important factor in determining how thioethers self-assemble on metal surfaces.

Novel orientational ordering and reentrant metallicity in K(x)C(60) monolayers for 3 < or = x < or = 5

STM studies on K(x)C(60) monolayers reveal new behavior over a wide range of the phase diagram. As x increases from 3 to 5 K(x)C(60) monolayers undergo metal-insulator-metal reentrant phase transitions and exhibit a variety of novel orientational orderings, including a complex 7-molecule, pinwheel-like structure. The proposed driving mechanism for the orientational ordering is the lowering of electron kinetic energy by maximizing the overlap of neighboring molecular orbitals. In insulating (metallic) K(x)C(60) this gives rise to orbital versions of the superexchange (double-exchange) interaction.

Substrate-mediated intermolecular interactions: a quantitative single molecule analysis

Long-range intermolecular interactions mediated by the surface are believed to be responsible for many effects in surface science, including molecular ordering, formation of nanostructures, and aligning reactive intermediates in catalysis. Here, we use scanning tunneling microscopy to probe the weak substrate-mediated interactions in benzene overlayers on Au{111} at 4 K. Using an automated procedure to monitor single molecule motion, we are able to quantify the substrate-mediated interaction strength. We explain quantitatively both the kinetics of the benzene motion and the thermodynamics that determine the packing structures benzene adopts in this system in light of these substrate-mediated interactions.

Benzene on Au{111} at 4 K: Monolayer Growth and Tip-Induced Molecular Cascades

Low-temperature scanning tunneling microscopy has been used to characterize the various structures of submonolayer and near-monolayer coverages of benzene (C6H6) on Au[111] at 4 K. At low coverage, benzene is found to adsorb preferentially at the top of the Au monatomic steps and is weakly adsorbed on the terraces. At near-monolayer coverage, benzene was found to form several long-range commensurate overlayer structures that depend on the regions of the reconstructed Au[111] surface, namely a (radical 52 x radical 52)R13.9 degrees structure over the hcp regions and a (radical 133 x radical 133)R17.5 degrees "pinwheel" structure over the fcc regions. Time-lapse imaging revealed concerted cascade motion of the benzene molecules in the (radical 133 x radical 133)R17.5 degrees pinwheel overlayer. We demonstrate that the observed cascade motion is a result of concerted molecular motion and not independent random motion.