Research Interests

Electron Doping Rare-Earth Nickelates

Rare earth nickelates exhibit a rich temperature-composition phase diagram involving charge, orbital and magnetic ordering. Recent experimental work on samarium nickelate (SmNiO3) has demonstrated that SmNiO3 can be doped with interstitial hydrogen in a reversible manner, resulting in a large increase in resistivity and band gap [1]. To elucidate this behavior, we use the first-principles density functional theory (DFT) + U method to study the effect of added electrons on the crystal and electronic structure of SmNiO3. Specifically, we relaxed starting structures obtained by adding electron concentrations of 1/4, 1/2, 3/4, and 1 electron per Ni. We find that the added electrons localize on the nickel sites resulting in a high spin Ni2+ configuration, leading to a large gap between the occupied and unoccupied eg orbitals. The electrons localize on the nickel regardless of how we add electrons to SmNiO3. We add electron either artifically by increasing the number of electrons in the calculation with a compensation postive background charge, or by addind an interstitial atom, such as hydrogen or lithium. In the latter case, an eletron is transferred from the atom to a nearby nickel.
F. Zuo, P. Panda, M. Kotiuga, J. Li, M. Kang, C. Mazzoli, H. Zhou, A. Barbour, S. Wilkins, B. Narayanan, M. Cherukara, Z. Zhang, S. K. R. S. Sankaranarayanan, R. Comin, K. M. Rabe, K. Roy, and S. Ramanathan, Habituation based synaptic plasticity and organismic learning in a quantum perovskite, Nature Communications 8, 240 (2017). link

Charge Transport in Molecular Junctions & Inorganic/Organic Interfaces

We use a combination of first-principles calculations and experiment to explain this change in transport properties through a shift in the local electrostatic potential at the junction caused by nearby conducting and solvent molecules chemically bound to the electrodes. This effect is found to alter the conductance of 4,4′-bipyridine-gold junctions by more than 50%. Moreover, we develop a general electrostatic model that quantitatively relates the conductance and dipoles associated with the bound solvent and conducting molecules. Our work shows that solvent-induced effects can be used to control charge and energy transport at molecular-scale interfaces.
(a) Transmission function of Au−BP−Au junction at a 0.8 BP molecule/nm2 coverage and Lorentzian fit. Inset: Au−BP−Au junction geometry. (b) Transmission function of Au−BP−Au junctions for different coverages ranging 0.2−1.4 BP molecule/nm2 near EF. (c) DFT + Σ conductance values of Au−BP−Au junctions (squares) compared to an electrostatics-based model (dashed line) as a function of coverage.

Conductance contours (10−5G0), from the electrostatic model, as a function of the ratio of BP to solvent on surface, Θs, and dipole of bound solvent, site density −0.8 BP molecules/nm2. Gray dashed line: dipole of trichlorobenzene (TCB). Inset: Schematic of dipole lattice model with conducting molecules (blue) and solvent (red), large arrows denote the molecule bridging the interfaces.
M. Kotiuga , P. Darancet, C. R. Arroyo, L. Venkataraman, and J. B. Neaton, Adsorption-Induced Solvent-Based Electrostatic Gating of Charge Transport through Molecular Junctions, Nano Letters 15, 4498 (2015). link
Contact: mkotiuga [at]