The Photoelectric Response of Gold‐Nanoparticle Monolayers

Abstract

Self-assembled arrays of metal nanoparticles, due to their novel properties, have been applied to optoelectronic devices, [1][2][3] electronic devices, [4,5] and the enhancement of linear and nonlinear optical responses.[6] Surface plasmon resonance (SPR), as one of the metal-nanoparticle properties, is caused by the coherent motion of the conduction-band electrons under an electromagnetic field [7,8] and often has a key role in the above-mentioned applications as it can amplify the local field. The electron-transport properties in metal-nanoparticle arrays [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] have also been explored and show Ohmic or Coulombic characteristics under various interparticle coupling strengths, [17] temperatures, [16][17][18][19] and local environments of the nanoparticles. [20] Recently, Jin et al. [26] have combined SPR with the electronic conductivity of a water-soluble Au colloid monolayer, which was formed by chemical absorption on an Al electrode coated with aluminum oxide. The results showed that SPR absorption of the Au-nanoparticle monolayer evidently caused the enhancement of current.However, the topology of the nanoparticle arrays, [13,21,22] such as voids and disorder, has an important influence on electron transport. Therefore, we believe that the topology of the nanoparticle monolayer would affect the process of their photoconductive behavior. Hydrophobic nanoparticles offer distinct advantages over hydrophilic ones in that the monolayer topology of the former can be well controlled by the solvent evaporation method or Langmuir-Blodgett (LB) technique. Herein, we report the synthesis of monodisperse 3 nm gold nanoparticles coated with n-C 12 H 25 SH (see transmission electron microscopy (TEM) images in the Supporting Information, Figure S1) by the modified method of Lin et al. [27] with distearyldimethylammonium bromide (DTAB). Using these hydrophobic particles, we fabricated monolayers with various degrees of coverage (q) by controlling the solvent evaporation time (t evp ), where q is defined as the percentage of the area occupied by nanoparticles in a unit area. The experimental results showed that the electron conduction of the hydrophobic gold-nanoparticle monolayers ( Figure S2) at low bias clearly increased [and hence the resistance decreased ( Figure S3)] with light irradiation. This change in photoresponse had a linear relationship with the degree of coverage of the gold-nanoparticle monolayers. Monochromic light experiments showed that the photoresponse effect arose from SPR absorption of gold nanoparticles in the monolayers.Gold-nanoparticle monolayers prepared during various t evp are investigated with TEM and UV/Vis spectroscopy. Figure 1 shows that the degree of coverage and the SPR peak clearly depend on solvent evaporation time: the longer the evaporation time, the higher the degree of coverage. At the same time, the SPR peak shifts from 544 to 560 nm with increasing degree of coverage. This finding is attributed to the increment in the die...

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