Nature460, 371-375 (16 July 2009) | doi:10.1038/nature08131; Received 9 December 2008; Accepted 11 May 2009
Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles
Hideyuki Nakanishi1,2, Kyle J. M. Bishop2, Bartlomiej Kowalczyk1,2, Abraham Nitzan1,3, Emily A. Weiss1, Konstantin V. Tretiakov2, Mario M. Apodaca1, Rafal Klajn2, J. Fraser Stoddart1 & Bartosz A. Grzybowski1,2
Department of Chemistry,
Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA
School of Chemistry, Tel Aviv University, Tel Aviv, Israel
Correspondence to: Bartosz A. Grzybowski1,2 Correspondence and requests for materials should be addressed to B.A.G. (Email: grzybor@northwestern.edu).
In traditional photoconductors1, 2, 3, the impinging light generates mobile charge carriers in the valence and/or conduction bands, causing the material's conductivity to increase4. Such positive photoconductance is observed in both bulk and nanostructured5, 6 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)7, 8 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 states9, 10. The nanoparticle-based photoconductors we describe could have uses in chemical sensors and/or in conjunction with flexible substrates.
