We report an oxygen surface adsorbates induced metal-insulator transition at the LaAlO3/SrTiO3 interfaces. The observed effects were attributed to the terminations of surface Al sites and the resultant electron-accepting surface states. By controlling the local oxygen adsorptions, we successfully demonstrated the nondestructive patterning of the interface two-dimensional electron gas (2DEG). The obtained 2DEG structures are stable in air and also robust against general solvent treatments. This study provides new insights into the metal-insulator transition mechanism at the complex oxide interfaces and also a highly efficient technique for tailoring the interface properties.

LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures and the 2D electron gas (2DEG) formed at the interface are another class of systems where surface effects play important roles. [9][10][11] In complex oxides, surface dissociation and migration of water molecules bear particular technological importance. [ 3,12,13 ] In LAO/STO heterostructures, effects of water on the interface electrical properties have been experimentally observed. [ 14 ] In addition, reversible interface metal-insulator transitions controlled by conducting atomic force microscope (c-AFM) were previously reported [15][16][17][18] and linked to the surface water layers formed in the air. [ 19 ] Despite of these interesting experimental fi ndings, clear understanding of the surface dynamics and the coupling mechanism between surface water layers with the interfacial 2DEG is still lacking. Without this knowledge, accurate interpretation of LAO/STO heterostructures performance in air and their potential device applications will be hindered. In this work, we have found that a partially dissociated water layer forms at the polar LaAlO 3 surface. The densities and distributions of adsorbed protons and hydroxyls strongly affect the structural and electrical properties of the LaAlO 3 /SrTiO 3 heterostructures. Consideration of the weakly adsorbed molecular water layers also turned out important. The hydrogen bond network formed from water molecules enables active surface proton hopping at room temperature and provides an effective pathway for engineering the interface properties from the surface. The multifaceted role of water revealed by this work is undoubtedly critical in understanding the unique characters of LaAlO 3 /SrTiO 3 interfaces, and can also provide valuable guidelines for the future development of oxides based surface engineering methodologies and device applications. Results and Discussion Water Dissociation on LAO Surfaces and Density Dependent Bonding EnergiesWater dissociation at AlO 2 terminated LAO surface is verifi ed by oxygen 1s state X-ray photoemission spectroscopy (XPS) spectra ( Figure 1 a). Besides the peak at 530.5 eV which is associated with oxygen atoms in the oxide lattice, a higher energy peak Electronic properties of low dimensional systems are particularly sensitive to surface adsorbates. Clear understanding of such phenomena can lead to highly effective and nondestructive material engineering techniques. In this work, water adsorption at the surface of LaAlO 3 /SrTiO 3 heterostructures is systematically studied. The saturation of surface dangling bonds by spontaneous water chemisorptions is found to be a main enabler of the formation of the interface 2D electron gas. In particular, when imbalanced distributions of water based ions, namely protons and hydroxyls, are generated, interface electron doping or depletion becomes surface adsorbates dominant and independent of the LaAlO 3 layer thickness. The investigations also reveal the importance of hydrogen bonding through molecular water layers, which provides an energeticall...

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