Reconfigurable lateral anionic heterostructures in oxide thin films via lithographically defined topochemistry

Laterally structured materials can exhibit properties uniquely suited for applications in electronics, magnetoelectric memory, photonics, and nanoionics. Here, a patterning approach is presented that combines the precise geometric control enabled by lithography with topochemical anionic manipulation of complex oxide films. Utilizing oxidation and fluorination reactions, striped patterns of $\mathrm{SrFe}{\mathrm{O}}_{2.5}/{\mathrm{SrFeO}}_3,\mathrm{SrFe}{\mathrm{O}}_{2.5}/{\mathrm{SrFeO}}_2\mathrm{F}$, and ${\mathrm{SrFeO}}_3/{\mathrm{SrFeO}}_2\mathrm{F}$ have been prepared with lateral periodicities of 200, 20, and 4 μm. Coexistence of the distinct chemical phases is confirmed through x-ray diffraction, optical and photoemission microscopies, and optical spectroscopy. The lateral heterostructures exhibit highly anisotropic electronic transport and also enable transience and regeneration of patterns through reversible redox reactions. This approach can be broadly applied to a variety of metal-oxide systems, enabling chemically reconfigurable lateral heterostructures tailored for specific electronic, optical, ionic, thermal, or magnetic functionalities.

Figure 1

Fabrication process for achieving oxide lateral heterostructures. A hard mask is lithographically defined on a film of $\mathrm{SF}{\mathrm{O}}_{2.5}$ (spin coating, exposure, developing, deposition, and lift-off). This is followed by either an ozone anneal to achieve the $\mathrm{SF}{\mathrm{O}}_{2.5}/{\mathrm{SFO}}_3$ heterostructure or a fluorination reaction to achieve the $\mathrm{SF}{\mathrm{O}}_{2.5}/{\mathrm{SFO}}_2\mathrm{F}$ heterostructure. A wet etch removes the mask.

Figure 2

Optical micrographs of (a) a 200-μm periodicity ${\mathrm{SFO}}_3/\mathrm{SF}{\mathrm{O}}_{2.5}$ heterostructure, (b) a 200-μm periodicity ${\mathrm{SFO}}_3/\mathrm{SF}{\mathrm{O}}_{2.5}$ heterostructure, and (c) a 4-μm periodicity ${\mathrm{SFO}}_3/\mathrm{SF}{\mathrm{O}}_{2.5}$ heterostructure. (d) A PEEM image measured at 690 eV, on the F $K$-edge, of a 20-μm periodicity ${\mathrm{SFO}}_3/{\mathrm{SFO}}_2\mathrm{F}$ heterostructure. Scale bars are included.

Figure 3

X-ray diffraction measured from (a) $\mathrm{SF}{\mathrm{O}}_{2.5}/{\mathrm{SFO}}_3$, (b) $\mathrm{SF}{\mathrm{O}}_{2.5}/{\mathrm{SFO}}_2\mathrm{F}$, and (c) ${\mathrm{SFO}}_3/{\mathrm{SFO}}_2\mathrm{F}$ heterostructures (bottom panels) with data from monolithic constituent phases shown in the top and middle panels for reference.

Figure 4

X-ray diffraction of ${\mathrm{SFO}}_3/{\mathrm{SFO}}_2\mathrm{F}$ heterostructure (purple) with superimposed simulated diffraction data of ${\mathrm{SFO}}_3$ (blue) and ${\mathrm{SFO}}_2\mathrm{F}$ (red).

Figure 5

Electronic resistivity as a function of temperature for monolithic insulating $\mathrm{SF}{\mathrm{O}}_{3\text{−}\delta }\left(\delta \approx 0.5\right)$ and metallic ${\mathrm{SFO}}_3$ (top), and resistance as a function of temperature for a 200-μm periodicity ${\mathrm{SFO}}_3/\mathrm{SF}{\mathrm{O}}_{2.5}$ heterostructure (bottom) measured perpendicular to the stripe direction (red, dotted) and parallel to the stripe direction (green, solid), with inset visualization of current direction in a film.

Figure 6

Optical absorption spectra of monolithic $\mathrm{SF}{\mathrm{O}}_{2.5}$ (top), ${\mathrm{SFO}}_3$ (middle), and of the 200-μm periodicity ${\mathrm{SFO}}_3/\mathrm{SF}{\mathrm{O}}_{2.5}$ heterostructure (bottom).

Figure 7

XRD data and optical microscope images of a 200-μm period $\mathrm{SF}{\mathrm{O}}_{2.5}/{\mathrm{SFO}}_2\mathrm{F}$ heterostructured film undergoing a sequence of oxidation and thermal reduction $\left(\sim 250{}^{\circ }\mathrm{C}\right)$ steps, demonstrating reconfigurable pattern behavior.