Real-Time Measurements of Conductance Switching and Motion of Single Oligo(phenylene ethynylene) Molecules

Abstract

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...

Design and control of electron transport properties of single molecules

“…Despite of these recent advances, it remains a great challenge to actively control the electron transport in a molecule and to systematically change its behavior from one type to another since this requires not only precise control of molecular structure, but also accurate activation of different electron tunneling processes. Controlling electron transport at the molecular level has important consequences for many applications, such as molecular electronics (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), biosensors (27), and solar cells (28,29). For certain molecules, change of electron transport properties could take place due to their specific response to the change of molecular conformation or orientation (19)(20)(21)(22)(23)(24)(25), chemical reactions (26), and tautomerization (18).…”

“…Controlling electron transport at the molecular level has important consequences for many applications, such as molecular electronics (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), biosensors (27), and solar cells (28,29). For certain molecules, change of electron transport properties could take place due to their specific response to the change of molecular conformation or orientation (19)(20)(21)(22)(23)(24)(25), chemical reactions (26), and tautomerization (18). The latter experiment (18) has attracted considerable attention owning to the facts that the switching process involved does not result in drastic molecular conformation changes as often occurring in mechanical molecular switches induced by cis-trans isomerization of azobenzene (23)(24)(25), making the process potentially more relevant to applications in memory devices.…”

Role of Hydration on the Electronic Transport through Molecular Junctions on Silicon

“…49,53 RTS has also been observed in single molecular junctions (STM). [55][56][57][58] In these experiments, the current fluctuations were ascribed to stochastic modifications of the conformation of the S-Au link at the molecule/Au interface. [55][56][57] Recently, STM experiments showed that these two-level fluctuations are not observed for molecule linked on hydrogenated Si through a Si-C bond, 58 mainly because the binding energy is larger for Si-C than for Au-S bonds.…”

Electric Field Effect on the Adsorption State of Methylthiolate on Au(111)

“…Molecular-scale electronics is one of promissing technologies to overcome the device scale problem and conductance properties of single molecules have been intensively studied. Weiss and co-workers observed switching behavior of oligo (phenylene ethynylene) (OPE) molecules in alkanethiolate self assembled monolayers (SAMs) on gold by using scanning tunneling microscopy (STM) [1][2][3][4][5][6]. They demonstrated that the switching behavior can be controlled by the polarity of the applied elecric field between the STM tip and the substrate and the polarity dependence can be altered by changing the molecular dipole moment.…”