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Prof. Chris Leighton
September 7 @ 11:00 am - 12:00 pm
Affiliation: University of Minnesota, Department of Chemical Engineering and Materials Science
Recently, electrolyte gating techniques employing ionic liquids have proven highly effective in
tuning very large carrier densities at material surfaces. These electrolytes enable electric double
layer transistor operation, the large capacitances (10’s of μF/cm 2 ) generating electron/hole
densities up to 10 15 cm -2 , i.e., significant fractions of an electron/hole per unit cell in most
materials. This is sufficient to induce and control electronic phase transitions, generating much
excitement. Many uncertainties remain, however, including the true doping mechanism
(electrostatic vs. electrochemical), the relation between 2D surface and bulk chemical doping,
the role of disorder, the challenge of in operando characterization, and the universality of the
approach. In this seminar I will review our work applying electrolyte gating with solid “ion gels”
[1-3] to complex oxide materials (e.g., ultrathin epitaxial La1-x Srx CoO3-δ ), mostly focused on
electrical control of magnetism. Our findings greatly clarify electrostatic vs. electrochemical
response, resulting in a picture where electrostatic gating vs. oxygen vacancy formation can be
understood and predicted based on bias polarity, and the enthalpy of formation and diffusivity
of oxygen vacancies [4,5]. This is achieved through development of in operando probes, such as
synchrotron X-ray diffraction, and neutron reflectometry . Control of ferromagnetism is then
demonstrated in both electrochemical and electrostatic modes. Working in electrostatic mode,
and guided by theory , we demonstrate reversible electrical control of Curie temperature
over a 150 K window . This record electrostatic Curie temperature shift is achieved via gate-
induced cluster percolation, enabling optimized control of ferromagnetism.
Work supported primarily by the University of Minnesota NSF MRSEC.
 S. Wang, M.-J. Ha, M. Manno, C.D. Frisbie and C. Leighton, Nat.
Commun. 3, 1210 (2012).
 W. Xie, S. Wang, X. Zhang, C. Leighton and C.D. Frisbie, Phys. Rev.
Lett. 113, 246602 (2014).
 W. Xie, X. Zhang, C. Leighton and C.D. Frisbie, Adv. Electron. Mater. 3,
 J. Walter, H. Wang, B. Luo and C. Leighton, ACS Nano 10, 7799 (2016).
 J. Walter, G. Yu, B. Yu, A. Grutter, B. Kirby, J. Borchers, Z. Zhang, H.
Zhou, T. Birol, M. Greven, and C. Leighton, Phys. Rev. Mater. 1, 071403(R)
 P.P. Orth, R.M. Fernandes, J. Walter, C. Leighton and B.I. Shklovskii,
Phys. Rev. Lett. 118, 106801 (2017).
 J. Walter, T. Charlton, H. Ambaye, M. Fitzsimmons, P.Orth, R.
Fernandes, B. Shklovskii and C. Leighton, submitted (2018).