No abstract

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...

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 Fowler-Nordheim 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 temperatur...

A high throughput fabrication method of molecular junctions with a typical area of 0.005–0.01μm2 is presented. The small size is determined by one optical lithography step. The structure of junctions is metal-SAM-metal, where SAM is a self-assembled molecular layer with <105 molecules. The effect of attributes such as temperature, type of metal films, and molecular structure of the SAM on the I-V characteristics of the junctions is found to be in agreement with previous results and theoretical predictions. The prospect advantages of the junctions for future research are also mentioned.