2D electron systems (2DESs) in functional oxides are promising for applications, but their fabrication and use, essentially limited to SrTiO3 -based heterostructures, are hampered by the need for growing complex oxide overlayers thicker than 2 nm using evolved techniques. It is demonstrated that thermal deposition of a monolayer of an elementary reducing agent suffices to create 2DESs in numerous oxides.

We report the existence of confined electronic states at the (110) and (111) surfaces of SrTiO 3 . Using angle-resolved photoemission spectroscopy, we find that the corresponding Fermi surfaces, subband masses, and orbital ordering are different from the ones at the (001) surface of SrTiO 3 . This occurs because the crystallographic symmetries of the surface and subsurface planes and the effective electron masses along the confinement direction influence the symmetry of the electronic structure and the orbital ordering of the t 2g manifold. Remarkably, our analysis of the data also reveals that the carrier concentration and thickness are similar for all three surface orientations, despite their different polarities. The orientational tuning of the microscopic properties of two-dimensional electron states at the surface of SrTiO 3 echoes the tailoring of macroscopic (e.g., transport) properties reported recently in LaAlO 3 =SrTiO 3 (110) and (111) interfaces, and is promising for searching new types of two-dimensional electronic states in correlated-electron oxides.Two-dimensional electron gases (2DEGs) in transitionmetal oxides (TMOs) present remarkable phenomena that make them unique from a fundamental viewpoint and promising for applications [1,2]. For instance, heterostructures grown on the (001) surface of SrTiO 3 , a TMO insulator with a large band gap of approximately 3.5 eV, can develop 2DEGs showing metal-to-insulator transitions [3], superconductivity [4], or magnetism [5,6]. Recently, 2DEGs at the (111) and (110) interfaces of LaAlO 3 =SrTiO 3 were also reported [7]. The latter showed a highly anisotropic conductivity [8] and a superconducting state spatially more extended than the one at the (001) interface [9]. Interestingly, theoretical works have also predicted that exotic, possibly topological, electronic states might occur at interfaces composed of (111) bilayers of cubic TMOs [10-13], as two (111) planes of transition-metal ions form a honeycomb lattice, similar to the one found in graphene. In this context, the discoveries that 2DEGs can also be created at the bare (001) surfaces of SrTiO 3 [14-16] and KTaO 3 [17,18], and more recently at the (111) surface of KTaO 3 [19], open new roads in the fabrication and study of different types of 2DEGs in TMOs-in particular, using surface-sensitive spectroscopic techniques, which give direct information about the Fermi surface and subband structure of the confined states. The origin of the confinement is attributed to a local doping of the surface region due to oxygen vacancies and/or lattice distortions.Here we show that new types of 2DEGs can be directly tailored at the bare (110) and (111) surfaces of SrTiO 3 . Imaging their electronic structure via angle-resolved photoemission spectroscopy (ARPES), we find that their Fermi surfaces, subband masses, and orbital ordering are different from the ones of the 2DEG at the SrTiO 3 ð001Þ surface [14,15] and the ones predicted for the bulk, being thus uniquely sensitive to the confining crystallographic directi...