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Interface-mediated Charge Transport


Impedance spectroscopy of Ag2S nanocrystal-GeS2 composites at 130°C shows drastic changes in electronic and ionic conductivity with varying nanocrystal size: 9.3 nm (left), 7.0 nm (middle), and 4.3 nm (right) [2].

In nanoscale systems, the density of interfaces becomes very large and can strongly influence or even dominate the charge transport properties of nanomaterials. The Milliron group studies charge transport in nanocrystal films and composites, seeking to understand how factors such as nanocrystal size/surface area, doping, polymer or inorganic glassy matrices, and processing techniques impact electron and ion motion. We adapt common electrical and electrochemical techniques, such as four-point probe and Hall Effect measurements, electrochemical impedance spectroscopy, and half-cell electrochemical measurements, to nanomaterial systems, and extend them with capabilities such as air-free measurements or in situ measurements while varying temperature or atmosphere composition [1].

One example of our recent efforts in this area is our study of mixed electronic-ionic charge transport in nanocomposites of Ag2S nanocrystals in a GeS2 matrix [2] using a combination of DC conductivity measurements and impedance spectroscopy. Here, we found that changing the Ag2S-GeS2 interface density by changing the Ag2S nanocrystal size provided systematic control over the composites’ electronic and ionic (silver ions) conductivities.


(Right) Schematic of charge transport in ligand-stripped PbSe nanocrystal films. Typically, PbSe films must be backfilled with elemental lead (a) or selenium (c) to achieve high electron or hole mobilities, respectively. By controlling the PbSe nanocrystal surface chemistry, hole and electron transport can be realized in porous PbSe nanocrystal films with lead-rich surfaces (b, holes) or selenium rich surfaces (d, electrons) [2]. (Left) When Nb-TiO2 nanocrystals were charged by insertion of lithium ions, strong modulation of visible transmittance was observed [5].

Other examples include measuring the influence of nanocrystal surface composition and chemistry on electronic transport through ligand-stripped semiconductor nanocrystal films, [3] and the role of surface vs. homogeneous tin doping in tin-doped indium oxide (ITO) [4]. In the case of ITO nanocrystals, we have found that surface doping increases the conductivity of nanocrystal films by 2-3 orders of magnitude, and we are continuing to study the mechanism responsible for this change. In Nb-TiO2 nanocrystal films, dynamic modulation of localized surface plasmon resonance in the NIR through capacitive charging was demonstrated with insertion of lithium ions [5].

Related papers:

[1] John Ephraim, Deanna Lanigan, Corey Staller, Delia J. Milliron, and Elijah Thimsen. “Transparent Conductive Oxide Nanocrystals Coated with Insulators by Atomic Layer Deposition,” Chem. Mater.,  28, (2016), 5549–5553 [pdf]

[2] RY Wang, R Tangirala, S Raoux, JL Jordan-Sweet, DJ Milliron. "Ionic and electronic transport in Ag2S nanocrystal - GeS2 matrix composites with size-controlled Ag2S nanocrystals" Adv. Mater. 24 (2012), 99-103 [pdf]

[3] E Rosen , AM Sawvel , DJ Milliron , and BA Helms. “Influence of Surface Composition on Electronic Transport Through Naked Nanocrystal Networks,Chem. Mater. 26, 7 (2014), 2214-2217 [pdf]

[4] SD Lounis, EL Runnerstrom, A Bergerud, D Nordlund, and DJ Milliron. “Influence of Dopant Distribution on the Plasmonic Properties of Indium Tin Oxide Nanocrystals,” J. Am. Chem. Soc.,  136, 19 (2014), 7110-7116 [pdf]

[5] Clayton J. Dahlman, Yizheng Tan, Matthew A. Marcus, and Delia J. Milliron. “Spectroelectrochemical Signatures of Capacitive Charging and Ion Insertion in Doped Anatase Titania Nanocrystals,” J. Am. Chem. Soc.,  137, (2015), 9160−9166 [pdf]