Recently it was discovered that the structure of the molecule-metal interface in alkane thiol-based self-assembled monolayers (SAMs) is more complex that first believed. Thiols have been shown not only to lift the herringbone reconstruction of Au(111) but remove a significant fraction of the Au surface atoms. The etch pits formed by these vacancies are thought to be one of the weakest areas of the SAM films in terms of susceptibility to and degradation by oxidizing species.

 

In an effort to slightly weaken the molecule-metal interaction and prevent the formation of etch pits, we have chosen the study the interaction and assembly of trimethylphosphine (PMe3) on Au(111) using a scanning tunnelling microscope (STM). This is to our knowledge the first atomic-scale characterization of a phosphine species adsorbed on a metal surface.

 

At full monolayer coverage PMe3, the molecules formed a hexagonally packed layer which exhibited a (√7 x √7)R19o unit cell. The interaction between PMe3 and Au caused Au atoms to be ejected from the herringbone reconstruction, but not the substrate surface itself (as in the thiol case) and led to the formation of small Au islands. This effect manifested in the herringbone spacing increasing from that of native gold (6.33 nm) to 11.2 ± 0.9 nm. As this system was exposed to various annealing treatments, a fraction of the molecules desorbed, the Au islands coalesced, and the herringbones disappeared entirely, indicating that the underlying Au surface adopted a 1 x 1 reconstruction. The data indicate that the PMe3/Au island formation is a kinetically limited process.

Finally, we have developed a mathematical equation that gives the theoretical island coverage ( q ) as a function of the maximum island coverage ( qmax), the native herringbone spacing (xo) and the experimental herringbone spacing (x): q=qmax(1-xo/x).This should be useful in future studies of many types of SAMs on Au(111), or any similarly reconstructed surface.

Ultrathin insulating films on metal substrates are unique systems to use the scanning tunneling / atomic force microscope to study the electronic and structural properties of single atoms and molecules, which are electronically decoupled from the metallic substrate. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film [1]. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. Employing a low temperature tuning fork AFM the different charge states can be observed directly in the force signal[2]. In the case of STM of molecules on ultrathin insulating films the electronic decoupling allows the direct imaging and manipulation of molecular orbitals [3]. As we have recently demonstrated detailed structural information can be attained by Atomic Force Microscopy which leads to the direct imaging of the molecular geometry. I.e. the complete chemical structure of single molecules can be accessed in scanning probe microscopy [4].

 

1. J. Repp, G. Meyer, F.E. Olsson, M. Persson, ‘Controlling the Charge State of Individual Gold Adatoms’, Science 305, 493 (2004)

2. L. Gross, F. Mohn, P. Liljeroth, J. Repp, F. J. Giessibl , G. Meyer, “Measuring the Charge State of an Adatom with Noncontact Atomic Force Microscopy” Science, 324, 1428-1431 (2009)

3. P. Liljeroth, J. Repp, G. Meyer, Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules Science, 317, 1203-1206 (2007)

4. L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer ‘The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy’, Science, 325, 1110-1114 (2009)

 

 

State-resolved measurements of CH4 reactivity on Ni(111) and Ni(100) have yielded detailed insights into how methane’s vibrational energy promotes reactivity. To date, such measurements have relied on post-dose quantification of reaction products to measure reactivity as a function of methane’s translational energy, internal vibrational and rotational state, and surface temperature. Here, we describe a new detection scheme for measuring state-resolved reaction probabilities. This method uses a variation of the “King and Wells” molecular beam reflectivity method to improve reproducibility and significantly decrease the data acquisition time for state-resolved measurements. Rather than modulate the flux of molecules incident on the surface, we modulate laser excitation. In this way, the partial pressure change due to modulation reveals the difference in reactivity with and without laser excitation, which is the key quantity needed to obtain state resolved reaction probabilities. We demonstrate the method for methane incident on Ni(111) and show that it provides real-time coverage dependent reaction probabilities while decreasing data acquisition time and permitting a more expansive exploration of how energy and chemical identity influences reactivity at the gas-surface interface. We use this experimental method to extend our study of CH4 activation to additional vibrational states, including the 2ν4 bend overtone, and different surfaces. Comparing the reactivity of these states provides insight into key features of energy flow during the reaction, and data to assess the generality of non-statitical behavior in gas-surface reactivity.

---