We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.

An earlier paper (Zatsarinny O and Froese Fischer C 2002 J. Phys. B: At. Mol. Opt. Phys. 35 4669) presented oscillator strengths for transitions from the 2p 2 3P term to high-lying excited states of carbon. The emphasis was on the accurate prediction of energy levels relative to the ionization limit and allowed transition data from the ground state. The present paper reports some refined transition probability calculations for transitions from 2p 2 3 P, 1 D, and 1 S to all odd levels up to 2p3d 3 P o . Particular attention is given to intercombination lines where relativistic effects are most important. 1 The customary unit cm −1 used here is related to the SI units of energy (joules) by 1 cm −1 = 1.986 445 61(34)× 10 −21 J [5].

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.

We use self- and directed assembly to pattern organic monolayers on the nanometre scale. The ability of the scanning tunnelling microscope to obtain both nanometre-scale structural and electronic information is used to characterize patterning techniques, to elucidate the intermolecular interactions that drive them and to probe the structures formed. We illustrate three successful approaches: (1) phase separation of self-assembled monolayers by terminal and internal functionalization, (2) phase separation of self-assembled monolayers induced by post-adsorption processing and (3) control of molecular placement by insertion into a self-assembled monolayer. These methods demonstrate the possibilities of patterning films by exploiting the intrinsic properties of the molecules. We employ these methods to prepare matrix-isolated samples to probe molecular electronic properties of single and bundled molecules.

A digital image tracking algorithm based on Fourier-transform cross-correlation has been developed to correct for instrumental drift in scanning tunneling microscope images. A technique was developed to eliminate cumulative tracking errors associated with fractional pixel drift. This tracking algorithm was used to monitor conductance changes associated with different conformations in conjugated molecular switch molecules and to trace the diffusion of individual benzene molecules on Ag{110}. Molecular motions have been tracked for up to 25 h (400 images) of acquisition time.

In order to understand the electronic properties involved in conductance switching of individual molecules, it is important to analyze and to understand the motions of the molecules and the substrate atoms to which they are bound. We and others have studied the conductance switching of oligo(phenylene ethynylene) (OPE) molecules isolated in host self-assembled monolayer (SAM) matrices 1-3 and their place-exchange up and down substrate step edges 4 using scanning tunneling microscopy (STM). Previously, conductance switching and motion of the OPE molecules were analyzed by determining the apparent height of each molecule in each STM frame, allowing us to characterize tens of molecules over hundreds of frames. 4,5 However, on the time scale of imaging, each OPE molecule is imaged for only 1-25 ms per image, depending on the image resolution. To broaden the dynamic range for measuring the switching and motion of OPE molecules and the host SAM, real-time (data obtained at 10 kHz), fixed lateral position topographic measurements have been acquired.Real-time motions have previously been studied using STM for single atoms and molecules using time-dependent tunneling measurements. 6 These studies, performed in ultrahigh vacuum, analyzed the dynamics of single atoms as they diffused or were laterally manipulated by blanking the STM current feedback loop and recording the tunneling current versus time. Here, we measured the real-time conductance switching and place-exchange for OPE molecules and the host SAM by recording height versus time (z vs. t) with the feedback loop actiVe, thereby minimizing drift and recording the topographic height of the molecule, enabling us to perform these measurements in ambient conditions. Sample preparation has been described previously. 1,2 Briefly, a dodecanethiolate SAM was adsorbed from a 1 mM solution onto a flame-annealed Au{111} substrate for 5 min. This short adsorption time forms SAM matrices containing relatively less order and more defect sites than typical SAMs, ultimately allowing for more stochastic switching and motion. 1 After adsorption, the SAM was rinsed with ethanol and dried under nitrogen. Nitro-functionalized OPE molecules (4-(2-nitro-4-phenylethynyl phenylethynyl)benzenethiol) were inserted for 1 min from a 1 µM solution into the preformed SAM under a nitrogen environment.During image acquisition, z versus t data were obtained by pausing the tip for 20 s over OPE molecules and the host SAM. Figure 1A displays a 500 Å × 500 Å imaged area of a SAM containing inserted nitro-functionalized OPE molecules that appear as protrusions from the SAM. 7 Several images were obtained over this area with z versus t measurements recorded in each image for each inserted OPE molecule (labeled 1-4 in Figure 1A) and over the host SAM (labeled a). The labels are offset from each molecule to show each molecule's location within the host SAM displayed between 4 and 5 s and place-exchange up and down the substrate step edge (∼2.3 Å height change), as displayed at several points. By tabulati...

We describe an annealing procedure for self-assembled monolayers (SAMs) that uses vapor-phase molecules to modify the local domain structure. Existing SAMs of decanethiolate on Au{111} were annealed using vapor-phase dodecanethiol molecules, so that the original and newly introduced molecules could be distinguished using scanning tunneling microscopy (STM). Molecules deposited from the vapor phase inserted at existing monolayer defect sites and domain boundaries, and at substrate step edges forming discrete network-like domains. The SAM molecular lattice can be preserved across molecular terrace boundaries between the decanethiolate and dodecanethiolate domains. Candidate molecular electronic component molecules were inserted from solution in the decanethiolate matrix as isolated molecules. These inserted molecules could then be surrounded by dodecanethiolate molecules introduced from the vapor phase, thus demonstrating a method for controlling the local environment of inserted molecules.

We have observed nitro-functionalized oligo(phenylene-ethynylene) molecules exhibiting motion up and down Au{111} substrate monatomic step edges within host self-assembled monolayers of n-alkanethiols, independent of previously observed conductance switching. Single molecules have been imaged with scanning tunneling microscopy to place-exchange reversibly between the top and bottom of monatomic substrate step edges.