No abstract

Current rectification, i.e., induction of dc current by oscillating electromagnetic fields, is demonstrated in molecular junctions at an optical frequency. The magnitude of rectification is used to accurately determine the effective oscillating potentials in the junctions induced by the irradiating laser. Since the gap size of the junctions used in this study is precisely determined by the length of the embedded molecules, the oscillating potential can be used to calculate the plasmonic enhancement of the electromagnetic field in the junctions. With a set of junctions based on alkyl thiolated molecules with identical HOMO-LUMO gap and different lengths, an exponential dependence of the plasmonic field enhancement on gap size is observed.

Metal quantum point contacts (MQPCs) with dimensions comparable to the de Broglie wavelength of conducting electrons reveal ballistic transport of electrons and quantized conductance in units of G(0) = 2e(2)/h. We measure the transport properties of 1G(0) Au contacts under laser irradiation. The observed enhancement of conductance appears to be wavelength-dependent, while thermal effects on conductance are determined to be negligible. For wavelengths that are not absorbed by Au, the results are consistent with a photoassisted transport mechanism in which conductance depends both on the electronic structure of the leads and on the interaction of the transporting electrons with oscillating electric fields originating from excitation of local plasmons. For wavelengths absorbed by Au, photoinduced mechanism is suggested to be the dominant transport mechanism. The results demonstrate optical control of ballistic transport in MQPCs and are also important for future interpretation of light effects on the conductance of single-molecule junctions.

Surface plasmons are coherent oscillations of conductive electrons that occur in a skin layer of metal and are capable of producing strong local electromagnetic fields in the near-field region.[1] Plasmons are imperative in surface-enhanced Raman [2][3][4][5] and fluorescence spectroscopy [6] as they significantly boost the sensitivity of these methods for the detection of dilute concentrations of analyte molecules. Plasmons can be coupled to molecular resonances, [7,8] or molecules can be exploited to control the properties of plasmons and the optical properties of nanoscale metal structures upon irradiation.[9] These studies, as well as several theoretical results, [10] suggest that plasmons should also affect the transport properties of molecular junctions. Several recently reported experimental approaches towards this goal are based on the average effect from a large number of junctions formed in ordered arrays of metal nanoparticles interlinked with molecules.[11] Herein we report the current response of individual well-defined molecular junctions to surface plasmons. The observed enhancement of current is explained by a photon-assisted tunneling mechanism."Suspended-wire" molecular junctions (SWMJs) were fabricated by trapping Au or Ag nanowires, which were capped with a self-assembled monolayer of either 1,9-nonanedithiol (C9) or decanethiol (C10) onto lithographically defined Au leads by using a dielectrophoresis technique (see Figure 1 and the Supporting Information). Figure 1 a shows representative I-V curves of junctions based on the two molecules. Transition voltage spectroscopy (TVS) and inelastic electron tunneling spectroscopy (IETS) measurements were taken in order to confirm the molecular nature of the junctions. TVS measurements are interpreted with a FowlerNordheim analysis, that is, plots of ln(I/V 2 ) versus 1/V, which reveal minimum points at transition-voltage (V T ) values that are characteristic of the molecules under investigation. [12] Figure 1 b shows typical TVS curves of junctions with C9 molecules. An average value of V T = (1.1 AE 0.07) V was calculated from all (C9 + C10) junctions. IETS measurements were taken at 5 K using a standard lock-in technique (Figure 1 c), which revealed typical alkane vibrations in both bias polarities.[13] The agreement of measured V T values with previous results, [14] and the lack of shift in the IETS peaks (within an error of AE 2 mV) prove that there is no potential divider in the suspended structures, that is, although the nanowires that are completely covered with a molecular layer could potentially form two molecular junctions in each SWMJ, only one junction per suspended nanowire (and a metal to metal contact on the other end) is formed.Laser irradiation of selected junctions under ambient conditions was carried out by using a microscope with maximum intensity of approximately 6 mW mm À2 and laser polarization parallel to the nanowires. Two wavelengths were used (see below): 781 nm (1.58 eV) and 658 nm (1.88 eV). We estimate the temperature inc...

Redox molecular junctions are promising systems for nanoelectronics applications, and yet they are still only marginally understood. The study of these systems has so far been conducted in solution, utilizing "electrolyte gating" to control their redox states and, as a result, their steady-state transistor-like conductance behavior. Here we explore redox junctions under vacuum at 77 K, and report real time detection of redox events in junctions of the type Au-6-thiohexanethiolferrocene-Au. Redox events are revealed as a two-level fluctuating signal in current-time traces with potential-dependent amplitude and frequency. Using a theoretical model for signals with a telegraph-like noise, the current-time traces are analyzed to extract the various molecular parameters which define the dynamics of the system. The presented method, which can be applied to other types of redox molecules, offers a new approach to study the unexplored territory of molecular dynamics in molecular junctions.