Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles

Abstract

In traditional photoconductors, the impinging light generates mobile charge carriers in the valence and/or conduction bands, causing the material's conductivity to increase. Such positive photoconductance is observed in both bulk and nanostructured photoconductors. Here we describe a class of nanoparticle-based materials whose conductivity can either increase or decrease on irradiation with visible light of wavelengths close to the particles' surface plasmon resonance. The remarkable feature of these plasmonic materials is that the sign of the conductivity change and the nature of the electron transport between the nanoparticles depend on the molecules comprising the self-assembled monolayers (SAMs) stabilizing the nanoparticles. For SAMs made of electrically neutral (polar and non-polar) molecules, conductivity increases on irradiation. If, however, the SAMs contain electrically charged (either negatively or positively) groups, conductivity decreases. The optical and electrical characteristics of these previously undescribed inverse photoconductors can be engineered flexibly by adjusting the material properties of the nanoparticles and of the coating SAMs. In particular, in films comprising mixtures of different nanoparticles or nanoparticles coated with mixed SAMs, the overall photoconductance is a weighted average of the changes induced by the individual components. These and other observations can be rationalized in terms of light-induced creation of mobile charge carriers whose transport through the charged SAMs is inhibited by carrier trapping in transient polaron-like states. The nanoparticle-based photoconductors we describe could have uses in chemical sensors and/or in conjunction with flexible substrates.

Enhanced Photoresponse of Conductive Polymer Nanowires Embedded with Au Nanoparticles

“…[ 45 ] Next, a 40 nm thick polymethylmethacrylate (PMMA) fi lm was Optoelectronic materials, such as, quantum dots, conductive polymer, nanocarbon materials, and plasmonic nanomaterials, used in photoresponsive and photovoltaic devices have aroused much attention due to their broad applications in solar energy harvesting, environmental monitoring, optical communication, and so on. [1][2][3][4][5][6][7][8][9][10] Among these, conductive polymers are particularly appealing because of their fl exibility, easy processability, and potential for low-cost fabrication combined with unique electrical, electronic, and optical properties similar to metals or semiconductors. [11][12][13][14] Moreover, it has been shown that one-dimensional (1D) conductive polymer nanowires, nanotubes, and nanofi bers exhibit signifi cantly improved physicochemical characteristics compared to their bulk counterparts, for instance, increased electrical conductivity, size-dependent excitation or emission, easier bandgap tunability, and coulomb blockade, which offer a promising opportunity to realize a wide variety of applications in optoelectronics.…”

Electron transport in a gold nanoparticle assembly structure stabilized by a physisorbed porphyrin derivative

“…A 4 nm diameter spherical gold nanoparticle contains approximately 1750 gold atoms 13 and has a surface area of approximately 50 nm 2 . From elemental analysis, a single gold nanoparticle should adsorb approximately 50 molecules of 2T ͑ratio of 2T:gold atoms=1:33͒.…”

“…1 Normal and inverse photoconductivity has been observed by irradiation using visible light close to the surface-plasmon resonance of the gold nanoparticles. 2 Spin-polarized molecular wires connecting the gold nanoparticles showed negative magnetoresistance behavior. 3 The electron transport properties of the assembled gold nanoparticles are strongly dependent on the assembly structure, as well as on the organic molecules connecting the nanoparticles.…”

A nanoscale Ti/GaAs metal-semiconductor hybrid sensor for room temperature light detection

“…Positive photoresistance ͑negative photoconductivity͒ has been observed in macroscopic semiconductor heterostructures 4 and nanoparticle films. 5 However, the nanoscale Ti/GaAs metal/semiconductor hybrid structures ͑MSHs͒ we describe here function via a fundamentally distinct principle; the photoinduced switching from ballistic 6 to diffusive transport of carriers. This results in a scaledependent positive photoresistance ͑SDPP͒ that increases with decreasing dimensions.…”