Room Temperature Single Electron Charging in Gold Nanoparticle Networks Formed on Biopolymer Templates
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2×10-5 mol/l solution of poly-L-lysine-hydrobromide complex (54,000 amu) in 10/90% water/methanol was drop cast onto the electrodes that had been pre-cleaned using a UV/ozone dry process followed by a rinse in nanopure water. The hydrobromide was removed from the amine side chains of the biopolymer by submerging the cast film in a solution of 1% sodium hydroxide in water for about 20 hr. The 11-mercaptoundecanoic acid stabilized gold nanoparticles were synthesized from Schmid-Au55 nanoparticles[16] using ligand exchange.[9] TEM measurements showed that the radius of the metal core was 0.7 nm, with a variance of ±20%. The radius of the core and ligand shell together is estimated to be 2.1 nm. Nanoparticle decoration of the biopolymer was accomplished by placing a concentrated solution of the nanoparticles in dimethylsulfoxide onto the poly-Llysine film for about 20 minutes, after which it was rinsed in dimethylsulfoxide and then dichloromethane. From the molecular weight, the average length of the poly-L-lysine is about 30 nm. Each polymer can therefore accommodate a maximum of approximately seven nanoparticles. Current-voltage measurements were made in an electrically shielded vacuum chamber at room temperature.[3] Radio frequency (RF) electric fields could be applied by means of a dipole antenna placed close to the sample. Control I-V measurements were made at many stages of sample preparation. DISCUSSION Representative room temperature I-V behavior of the biopolymer - nanoparticle samples is shown in figure 1. All of the samples studied had non-linear I-V characteristics, with some having structure that was equally spaced in voltage. Control experiments showed that the I-V characteristics of the undecorated biopolymer had no structure and was almost indistinguishable from the behavior of the cleaned interdigitated electrodes. The structure is more easily seen in the conductance, as shown in figure 2. After subtraction of the measured conductance at zero bias, the I-V relationship is seen to 1200 800
Isd (fA)
400 0
-400 -800 -1200
-80 -60 -40 -20 0 20 40 60 80 V sd (V)
Figure 1. The I-V characteristic of a poly-L-lysine-nanoparticle sample.
22
dIsd/dVsd (fA/V)
20 18 16
600 Isd (fA)
24
0
-600
-80
0
Vsd (V)
80
14 12 10 8 -80 -60 -40 -20 0 20 40 60 80 Vsd (V)
Figure 2. Conductance features as a function of applied voltage. The inset shows the original I-V data with the measured conductance at zero bias removed. exhibit threshold behavior and current plateaus reminiscent of single electron behavior, as shown in the inset to figure 2. The threshold voltage VT varied from sample to sample. For the data shown in figure 2, VT is 12 ± 1 V and the conductance oscillations have a
spacing of ∆V = 25 ± 3 V. In general, for samples that show conductance oscillations the ratio ∆V/VT was close to two. Above threshold, the scaling I ∝ (V/VT-1)γ
(1)
was found to describe all sets of data, with γ = 1.2 ± 0.2, as illustrated in figure 3. Here the error includes the uncertainty in the current measurement and the sp
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