Atomically Abrupt Liquid-Oxide Interface Stabilized by Self-Regulated Interfacial Defects: The Case of $\mathrm{Al}/{\mathrm{Al}}_{2}{\mathbf{O}}_{3}$ Interfaces

Atomically Abrupt Liquid-Oxide Interface Stabilized by Self-Regulated Interfacial Defects: The Case of $Al / {Al}_{2} O_{3}$ Interfaces

Joongoo Kang, Junyi Zhu, Calvin Curtis, Daniel Blake, Greg Glatzmaier, Yong-Hyun Kim, and Su-Huai Wei

Phys. Rev. Lett. 108, 226105 – Published 30 May 2012

Abstract

The atomic and electronic structures of the liquid Al/(0001) $α − {Al}_{2} O_{3}$ interfaces are investigated by first-principles molecular dynamics simulations. Surprisingly, the formed liquid-solid interface is always atomically abrupt and is characterized by a transitional Al layer that contains a fixed concentration of Al vacancies ( $\sim 10 at . %$ ). We find that the self-regulation of the defect density in the metal layer is due to the fact that the formation energy of the Al vacancies is readjusted in a way that opposes changes in the defect density. The negative-feedback effect stabilizes the defected transitional layer and maintains the atomic abruptness at the interface. The proposed mechanism is generally applicable to other liquid-metal/metal-oxide systems, and thus of significant importance in understanding the interface structures at high temperature.

Received 17 November 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.226105

Authors & Affiliations

Joongoo Kang^1,*, Junyi Zhu¹, Calvin Curtis¹, Daniel Blake¹, Greg Glatzmaier¹, Yong-Hyun Kim², and Su-Huai Wei¹

¹National Renewable Energy Laboratory, Golden, Colorado 80401, USA
²Graduate School of Nanoscience and Technology (WCU), KAIST, Daejeon 305-701, Korea

^*joongoo.kang@nrel.gov

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Vol. 108, Iss. 22 — 1 June 2012

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Figure 1
Two-dimensional liquid Al layers at the interface with $α − {Al}_{2} O_{3}$ . (a) A snapshot of the liquid Al layers near the liquid-solid interfaces taken from a first-principles MD simulation at $T = 1000 K$ . Large and small balls represent Al and O atoms, respectively. (b) Atomic density plots, which are averaged in planes parallel to the interfaces, are shown for Al atoms (solid red line) and O atoms (dashed blue line) along the $z$ direction perpendicular to the interfaces. The position of the first oxygen layer at the left interface is set to zero. For comparison, a plane-averaged density plot of the Al atoms in a freestanding Al film is also presented (dotted black line). (c) A representative snapshot of the first liquid layer (denoted by 1) at $T = 1000 K$ , elucidating a disordered in-plane atomic structure. (d) Simulated lateral Al-Al radial distribution functions $g_{x y}$ of the first liquid layer versus the lateral position $r_{x y}$ .Reuse & Permissions
Figure 2
Intrinsic vacancies in the honeycomblike, transitional Al layer. (a) In-plane atomic ordering of the transitional Al layer at the interface. The O atoms in the O-terminated (0001) ${Al}_{2} O_{3}$ are also shown (Al: large balls, O: small balls). In (b), the Al chemical potential ( $μ_{Al}$ ) is presented as a function of the Al coverage ( $θ$ ) in the transitional layer with the liquid Al chemical potential, $μ_{Al} (liquid)$ , set to zero. (c) Schematics of possible relations between the coverage $θ$ and the chemical potential $μ_{Al}$ that correspond to different interface morphologies (see text). Filled and open dots represent stable and unstable solutions, respectively.Reuse & Permissions
Figure 3
Microscopic origin of the abrupt $Al / {Al}_{2} O_{3}$ interface. The charge density plot of the defect levels of the Al vacancies ( $V_{Al}$ ) at the interface: (a) top view and (b) side view. Large and small balls represent Al and O atoms, respectively. The Al atoms in the transitional layer are colored green. In (c), the total electronic density of states (DOS; solid line) of the $Al / {Al}_{2} O_{3}$ interface is shown. The Fermi level ( $E_{F}$ ) is set to zero. The shaded area represents the partial DOS projected on the O $2 p$ orbitals near the $V_{Al}$ , which is enlarged by a factor of 30 for better visibility. The schematic shown in the inset depicts the energy gain due to electron transfer from $E_{F}$ to the $V_{Al}$ acceptor level ( $ε_{a}$ ) just above the ${Al}_{2} O_{3}$ VBM ( $ε_{VBM}$ ) state. (d) The energy differences of the electronic levels, $E_{F} - ε_{a}$ and $E_{F} - ε_{VBM}$ , are plotted as a function of the Al coverage ( $θ$ ).Reuse & Permissions

Physical Review Letters

Atomically Abrupt Liquid-Oxide Interface Stabilized by Self-Regulated Interfacial Defects: The Case of Al/Al2O3 Interfaces

Joongoo Kang, Junyi Zhu, Calvin Curtis, Daniel Blake, Greg Glatzmaier, Yong-Hyun Kim, and Su-Huai Wei

Phys. Rev. Lett. 108, 226105 – Published 30 May 2012

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Atomically Abrupt Liquid-Oxide Interface Stabilized by Self-Regulated Interfacial Defects: The Case of $Al / {Al}_{2} O_{3}$ Interfaces