Erin V. Iski

Experimental demonstration of a single-molecule electric motor

For molecules to be used as components in molecular machines, methods that couple individual molecules to external energy sources and that selectively excite motion in a given direction are required. Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions, but electrically driven motors have not yet been built, despite several theoretical proposals for such motors. Here we report that a butyl methyl sulphide molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunnelling microscope are used to drive the directional motion of the molecule in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. The direction and rate of the rotation are related to the chiralities of both the molecule and the tip of the microscope (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices.

Time-resolved studies of individual molecular rotors.

Thioether molecular rotors show great promise as nanoscale models for exploring the fundamental limits of thermally and electrically driven molecular rotation. By using time-resolved measurements which increase the time resolution of the scanning tunneling microscope we were able to record the dynamics of individual thioether molecular rotors as a function of surface structure, rotor chemistry, thermal energy and electrical excitation. Our results demonstrate that the local surface structure can have a dramatic influence on the energy landscape that the molecular rotors experience. In terms of rotor structure, altering the length of the rotors alkyl tails allowed the origin of the barrier to rotation to be more fully understood. Finally, time-resolved measurement of electrically excited rotation revealed that vibrational excitation of a C-H bond in the rotors alkyl tail is an efficient channel with which to excite rotation, and that the excitation is a one-electron process.

Mode‐Selective Electrical Excitation of a Molecular Rotor

Understanding and actuating the rotation of individual molecules on surfaces is a crucial step towards the development of nanoscale devices such as fluid pumps, sensors, delay lines, and microwave signaling applications. Recently a new, stable and robust system of molecular rotors consisting of thioether molecules (RSR) bound to metal surfaces has offered a method with which to study the rotation of individual molecules as a function of temperature, molecular chemistry, proximity of neighboring molecules, and surface structure. Arrhenius plots for the rotation of dibutyl sulfide yielded a rotational barrier of 1.2 0.1 kJ mol . While these results revealed that small amounts of thermal energy are capable of inducing rotation, thermodynamics dictates that thermal energy alone cannot be used to perform useful work in the absence of a temperature gradient. Therefore, for molecules to meet their full potential as components in molecular machines, methods for coupling them to external sources of energy that selectively excite the desired motions must be devised. Herein we describe a study of the electrical excitation of individual dibutyl sulfide (Bu2S) molecular rotors with electrons from a scanning tunneling microscope (STM) tip. Action spectroscopy was used to measure the effect of electron energy on the rate of rotation. The results revealed that tunneling electrons above a threshold energy excited a C H vibration in the rotor s alkyl tail that coupled selectively to rotation of the whole molecule. The Au ACHTUNGTRENNUNG{111} 22 p3 surface chosen for the study consists of domains of surface atoms with both hcp and fcc packing separated by narrow soliton walls with an intermediate packing structure. Therefore, molecular rotors in both areas were studied independently and the results were compared. For simplicity we only present data for fcc-adsorbed rotors in this communication (see Supporting Information for hcp data). Figure 1 a shows an STM image of an individual dibutyl sulfide molecular rotor on a Au ACHTUNGTRENNUNG{111} surface at 7 K. When imaging at 7 K under non-perturbative conditions (V=0.3 V, I= 10 pA), the molecules were static and appeared in STM images as crescent-shaped protrusions. Figure 1 b shows an image of a di-

Organic thin film induced substrate restructuring: An STM study of the interaction of naphtho[2,3-a]pyrene Au(111) herringbone reconstruction

The structural properties and the interaction strength of naphtho[2,3-a]pyrene (NP), a promising multifunctional organic material for optoelectronic devices, has been studied on Au(111) by means of scanning tunnelling microscopy. The perturbation of the native herringbone reconstruction of the pristine Au(111) surface was used to assess the interaction strength of the organic film with the surface. It was found that a moderate temperature treatment (500 K) of the NP film led to a new equilibrium structure, which dramatically perturbed the herringbone reconstruction. Our data suggest that organic-metal interfaces studied at room temperature or lower do not necessarily reflect the true equilibrium structures of the organic films, which are important in understanding the associated properties of organic thin film electronic devices. Interpretation of the self-assembled NP structure on Au(111) is discussed in conjunction with STM tip induced imaging effects which appear prevalent on these complex organic/meta...

Dynamics of Molecular Adsorption and Rotation on Nonequilibrium Sites

It is generally accepted that important events on surfaces such as diffusion and reactions can be adsorption site dependent. However, due to their short lifetime and low concentration in most systems, adsorbates on nonequilibrium adsorption sites remain largely understudied. Using low-temperature scanning tunneling microscopy, site-dependent adsorption is shown for the molecule butyl methyl sulfide, which is trapped in multiple metastable adsorption sites upon deposition onto a Au(111) surface at 5 K. As this molecule does not have enough energy to diffuse to its preferred adsorption site on the surface, it is possible to study the behavior of individual molecules in a variety of nonequilibrium sites. Here we present atomic-scale data of the same chemical species in three independent, metastable adsorption sites and equilibration to a single equilibrium site as a function of either electrical or thermal excitation. Butyl methyl sulfide exhibits distinctly different physical properties at all four adsorption sites, including rotational dynamics and appearance in scanning tunneling microscopy (STM) images. An energy profile is proposed for the adsorption and equilibration of these species, and a correlation is drawn between rotational barrier and adsorption energy.

Viewing and inducing symmetry breaking at the single-molecule limit.

Symmetry breaking by photons, electrons, and molecular interactions lies at the heart of many important problems as varied as the origin of homochiral life to enantioselective drug production. Herein we report a system in which symmetry breaking can be induced and measured in situ at the single-molecule level using scanning tunneling microscopy. We demonstrate that electrical excitation of a prochiral molecule on an achiral surface produces large enantiomeric excesses in the chiral adsorbed state of up to 39%. The degree of symmetry breaking was monitored as a function of scanning probe tip state, and the results revealed that enantiomeric excesses are correlated with the intrinsic chirality in scanning probe tips themselves, as evidenced by height differences between single molecule enantiomers. While this work has consequences for the study of two-dimensional chirality, more importantly, it offers a new method for interrogating the coupling of photons, electrons, and combinations of physical fields to achiral starting systems in a reproducible manner. This will allow the mechanism of chirality transfer to be studied in a system in which enantiomeric excesses are quantified accurately by counting individual molecules.

Carbamazepine on a carbamazepine monolayer forms unique 1D supramolecular assemblies

High-resolution STM imaging of the structures formed by carbamazepine molecules adsorbed onto a pseudo-ordered carbamazepine monolayer on Au(111) shows the formation of previously unreported 1-dimensional supramolecular assemblies.

Mechanistic modeling of erosion–corrosion for carbon steel

Abstract Carbon steel materials are widely used in the oil and gas production industry because of its availability, constructability, and relatively low cost. However, there are limits to the longevity of carbon steel because of its low corrosion resistance. In particular, carbon steel tubing and piping are susceptible to erosion–corrosion damage due to the erosive and corrosive nature of the produced fluid or gas. The combined effect of sand erosion and corrosion can be very significant. One form of erosion–corrosion of carbon steels occurs when entrained sand particles impinge the wall and remove part or all of a protective iron carbonate (FeCO3) scale allowing corrosion rates to increase to bare metal rates. This limitation raises the need for reliable prediction tools to properly design production facilities in terms of cost savings and safety. This chapter provides an overview of research and development in the area of erosion–corrosion modeling. The chapter discuses two erosion–corrosion prediction models that account for CO2 corrosion under iron carbonate-scale-forming conditions in the presence of sand. The chapter also offers discussion on FeCO3 scale formation and erosion resistance.

Comparison of Erosion Resistance of Iron Carbonate Protective Layer with Calcium Carbonate Particles Versus Sand

In oil and gas production, solid particles can be entrained in the produced fluid. While sand is widely considered as the most common source of solid particles, calcium carbonate particles can also be entrained in the flow, especially in carbonate formations. These entrained solid particlescan erode steel pipe surfaces and protective corrosion products, such as iron carbonate (FeCO3) scale that forms on the steel surface as a result ofthe CO2 corrosion process. The removal of protective layers can lead to high corrosion rates. Extensive research has previously been conducted to study the effect of sand erosion on removing protective iron carbonate scales. However, little is known about the erosion resistance of iron carbonate scale for calcium carbonate particles. The goal of the research presented in this paper is to study the erosion resistance of iron carbonate scale when eroded by calcium carbonate particles, and compare this erosion behavior with scale eroded by sand. Additionally, this research reports data and modeling that describe under what conditions removal of iron carbonate scale and the resulting erosion-corrosion are anticipated by solid particles such as sand or calcium carbonate. Results, for the conditions considered in this study, show that CaCO3 particles can cause considerable damage to iron carbonate scale leading to severe corrosion. For these conditions, sand was found to be more erosive than CaCO3 particles. Results from the erosion model developed in this study showed good agreement with current and previous experimental data.