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(a) Chemical structure of thiol-functionalized OPE, 1,4-bis-(3-mercapto-phenylethynyl)benzene, ligand molecule. (b) Schematic drawing of the double-barrier tunneling junctions. Junction 1 is the tunneling barrier between STM-tip and Au core, which consists of a vacuum layer and OPE ligand, and junction 2 is the tunneling barrier between the Au core and Au(111) surface, which consists of only the OPE ligand.
Comparison between SEM and STM images of chemisorbed OPE-protected Au nanoparticles on Au(111) substrate. (a)–(c) and (d)–(e) are the SEM and STM images, respectively, of the sample prepared by immersion in a 0.16 nM DMF solution of OPE-protected Au nanoparticle for 10 min. (b) and (c) are magnified images of the square area (A and B, respectively) of (a). The size of (b) and (d) is 150 × 150 nm2 and the size of (c) and (e) is 100 × 100 nm2. The STM images for (d) and (e) were obtained with set-point currents of 20 pA and sample bias voltages of −1.8 V and 1.5 V, respectively.
Stochastic switching behaviors of OPE-protected Au nanoparticle, observed using STM. (a) STM images obtained with a sample bias voltage of −1.5 V, set point current of 10 pA, and image size of 35 × 35 nm2. (b) Successive STM images of single OPE-protected Au nanoparticle were acquired at 7 -, 15 -, 25 -, 32 -, 47 -, 59 -, and 80-min intervals from the image A. (c) is the cross-sectional profile of the Au nanoparticle in Fig. 3(b) (black solid lines and blue broken lines represent as A and B, respectively).
Tunneling current versus sample bias voltage characteristics in STM-tip/vacuum/OPE ligand/Au core/OPE ligand/Au(111). (a)–(f) show the linear plots and the corresponding semi-logarithmic plots. Solid lines represent the experimental results and broken lines represent the theoretical I(V) curves calculated using the orthodox theory. The values of the evaluated parameter are as follows: (a) C 1 = 0.039 aF, C 2 = 0.062 aF, R 1 = 71 GΩ, R 2 = 12 GΩ, and Q 0 = −0.06e; (b) C 1 = 0.053 aF, C 2 = 0.079 aF, R 1 = 194 GΩ, R 2 = 11 GΩ, and Q 0 = −0.023e; (c) C 1 = 0.084 aF, C 2 = 0.15 aF, R 1 = 44.6 GΩ, R 2 = 15 GΩ, and Q 0 = −0.15e; (d) C 1 = 0.056 aF, C 2 = 0.067 aF, R 1 = 32 GΩ, R 2 = 10 GΩ, and Q 0 = −0.061e; (e) C 1 = 0.045 aF, C 2 = 0.091 aF, R 1 = 21.9 GΩ, R 2 = 8 GΩ, and Q 0 = 0.147e; (f) C 1 = 0.068 aF, C 2 = 0.046 aF, R 1 = 17.5 GΩ, R 2 = 7.5 GΩ, and Q 0 = 0.114e, where e is the unit charge.
Set-point current (I set) dependence of tunneling resistances (a) R 1, and (b) R 2 for OPE-protected Au nanoparticles. Open and closed circles represent the evaluated values of R 1 and R 2, respectively. (a) Solid line indicates the power law R 1∝1/Iset when I set R 1 = 2 V. (b) The solid and the dashed lines show the power law relationship I set R 1 = 2 V (R 1/R 2 = 1) and I set R 2 = 0.04 V (R 1/R 2 = 50), respectively. The average values of R 2 for OPE-protected Au nanoparticles are 15 GΩ when I set ≤ 20 pA.
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