Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles

Abstract

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.

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Figure 1: Functionalized nanoparticles for photoconductive films.
Figure 2: Photoconductance and inverse photoconductance of irradiated nanoparticle films.
Figure 3: Energy diagrams and temperature dependence of responses.
Figure 4: Conductance of mixed films and theoretical predictions.
Figure 5: Modulation and ‘switching’ of photoconductance.

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Acknowledgements

We thank M. Ratner, G. C. Schatz and R. van Duyne for discussions and advice. This work was supported by the Alfred P. Sloan Fellowship and the Dreyfus Teacher-Scholar Award (to B.A.G.).

Author Contributions H.N. performed the experiments, and collected and analysed the data; K.J.M.B., A.N., E.A.W. and B.A.G. developed the theoretical model; B.K. and R.K. synthesized nanoparticles and thiols; K.V.T. and M.M.A. helped with the construction of the Faraday cage and with data analysis; J.F.S. planned synthesis and helped with the interpretation of results; and B.A.G. conceived the experiments, analysed results, and wrote the paper.

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Correspondence to Bartosz A. Grzybowski.

Supplementary information

Supplementary Information

This file contains Supplementary Notes (incorporating Figures S1-S7): (1) Further experimental details, (2) Discussion of the origins of negative activation energy, (3) Transport model of photoconductance in NP arrays, (4) Further comments regarding plasmonic effects; and Supplementary References. (PDF 1070 kb)

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Nakanishi, H., Bishop, K., Kowalczyk, B. et al. Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles. Nature 460, 371–375 (2009) doi:10.1038/nature08131

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