High On−Off Conductance Switching Ratio in Optically-Driven Self-Assembled Conjugated Molecular Systems

Abstract

A new azobenzene-thiophene molecular switch is designed, synthesized, and used to form self-assembled monolayers (SAM) on gold. An "on/off" conductance ratio up to 7 x 10(3) (with an average value of 1.5 x 10(3)) is reported. The "on" conductance state is clearly identified to the cis isomer of the azobenzene moiety. The high on/off ratio is explained in terms of photoinduced, configuration-related changes in the electrode-molecule interface energetics (changes in the energy position of the molecular orbitals with respect to the Fermi energy of electrodes) in addition to changes in the tunnel barrier length (length of the molecules). First principles density functional calculations demonstrate a better delocalization of the frontier orbitals as well as a stronger electronic coupling between the azobenzene moiety and the electrode for the cis configuration over the trans one. Measured photoionization cross sections for the molecules in the SAM are close to the known values for azobenzene derivatives in solution.

Electronic and transport properties of azobenzene monolayer junctions as molecular switches

“…Nonetheless, for junctions with a physisorbed top contact, the cis configuration ("ON" state) exhibits a larger current for any given voltage than that of the trans configu- ration ("OFF" state), and the ON/OFF ratio of the current is more pronounced than that of the junctions with chemisorbed top contacts. These theoretical results for junctions with a physisorbed top contact are, therefore, in reasonable qualitative consistency with the reported measurements [8][9][10] . A quantitative matching of the ON/OFF ratio to the experiments is not expected because the length of the molecules in experiments are much longer than those in our study.…”

Thiolate Chemistry: A Powerful and Versatile Synthetic Tool for Immobilization/Functionalization of Oligothiophenes on a Gold Surface

“…Keywords: conjugation · cyclic voltammetry · molecular electronics · monolayers · thiophene have been prepared by covalent grafting of an iodo intermediate followed by a Suzuki coupling, [19a] whereas ferrocene redox probes have been introduced on biphenyl derivatives adsorbed onto glassy carbon by electrochemical activation. [19b] As reviewed recently, [20] SAMs that involve oligothiophenes have been widely investigated [3c, d, 21-31] and used for applications in molecular junctions, [25] diodes, [3c, d, f] switches, [26] field-effect transistors, [5d, e] mass spectrometry, [27] chemosensors, [28] or photovoltaic devices.…”

Molecular Nanoelectronics

“…A short spacer should favor the electron transfer rate through the junction and increases its conductance, while a longer spacer could improve the decoupling of the azobenzene moiety from the substrate, thus allowing a larger dynamic of the switching event, and thus a larger "on/off" conductance ratio. Recently, we reported the synthesis and the electrical properties of a new molecular switch in which the azobenzene moiety is linked to a bithiophene spacer and a short (4 carbon atoms) alkanethiol [149,150]. Such a design is expected to combine the benefit of a rather long spacer, while preserving a sufficiently high level of current due to the presence of electron-rich bithiophene unit (compared to a fully saturated spacer with the same length).…”

“…The analysis of these results using well-established electron transport models and molecular frontier orbitals from first principles DFT calculations indicates that this high photo-induced on/off ratio results from a synergistic combination of SAM thickness variation and modification of the energy offset between the lowest unoccupied molecular orbital (LUMO) and the electrode Fermi energy. Moreover, these azobenzene derivatives can switch their configuration with the top electrode deposited on the SAM [138] and they can have switching times ~ 1-10 µs comparable to molécules in solution [150], thus they are prone for solid-state molecular switch devices for low-demanding, low-cost applications.…”