title: 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.
authors.affiliation: [1] Department of Chemistry, [2] Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Nakanishi
authors.forename: Kyle J M
authors.initials: KJ
authors.lastname: Bishop
authors.forename: Bartlomiej
authors.initials: B
authors.lastname: Kowalczyk
authors.forename: Abraham
authors.initials: A
authors.lastname: Nitzan
authors.forename: Emily A
authors.initials: EA
authors.lastname: Weiss
authors.forename: Konstantin V
authors.initials: KV
authors.lastname: Tretiakov
authors.forename: Mario M
authors.initials: MM
authors.lastname: Apodaca
authors.forename: Rafal
authors.initials: R
authors.lastname: Klajn
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart
authors.forename: Bartosz A
authors.initials: BA
authors.lastname: Grzybowski
mesh_terms: D004553:Electric Conductivity; D006046:Gold / Q000737:chemistry; D008027:Light; D053768:Metal Nanoparticles / Q000737:chemistry* / Q000528:radiation effects*; D008956:Models, Chemical; D010777:Photochemistry / Q000295:instrumentation*; D012834:Silver / Q000737:chemistry; D020349:Surface Plasmon Resonance; D013696:Temperature
chemical_list: D012834:Silver; D006046:Gold