In our lab we use conductive-AFM (C-AFM) lithography to locally control the metal-insulator transition at the interface of LaAlO3/SrTiO3. The technique is reversible and allows for creation of a wide range of nanostructures at oxide interfaces simply by applying positive or negative voltages to the AFM tip and scanning across the LaAlO3 surface.
As illustrated above, we work with TiO2-terminated SrTiO3 with approximately 3 unit cells LaAlO3, a layer that is approximately 1.2nm. The LaAlO3/SrTiO3 interface is initially insulating. However, by applying a positive voltage with a c-AFM tip, we protonate the top surface and these ions attract electrons at the interface, thus locally inducing the metal-insulator transition at the interface[1,2]. Nanowires and other structures can be created at the interface by moving tip on the surface. The width of nanowires can be as thin as 2 nm by properly controlling the tip voltage, although typical widths are about 10 nm. This process is reversible. By applying a negative voltage to the AFM tip, the interface can restore insulating state and nanowires can be erased. With c-AFM lithography, we are able to create various nanostructures such as single-electron transistors (SETs), broadband THz emitters and ballistic electron waveguides.
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- C. Cen, S. Thiel, G. Hammerl, C. W. Schneider, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, "Nanoscale Control of an Interfacial Metal-Insulator Transition at Room Temperature," Nature Materials 7, 298 (2008).
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- Y. Ma, M. Huang, S. Ryu, C. W. Bark, C.-B. Eom, P. Irvin, and J. Levy, "Broadband Terahertz Generation and Detection at 10 nm scale," Nano Letters 13, 2884 (2013).
- M. Tomczyk, G. Cheng, H. Lee, S. Lu, A. Annadi, J. P. Veazey, M. Huang, P. Irvin, S. Ryu, C.-B. Eom, and J. Levy, "Micrometer-Scale Ballistic Transport of Electron Pairs in LaAlO3/SrTiO3 nanowires," Physical Review Letters 117, 096801 (2016).