Tailoring the Doping Mechanisms at Oxide Interfaces in Nanoscale

Here, we demonstrate the nanoscale manipulations of two types of charge transfer to the LaAlO₃/SrTiO₃ interfaces: one from surface adsorbates and another from oxygen vacancies inside LaAlO₃ films. This method can be used to produce multiple insulating and metallic interface states with distinct carrier properties that are highly stable in air. By reconfiguring the patterning and comparing interface structures formed from different doping sources, effects of extrinsic and intrinsic material characters on the transport properties can be distinguished. In particular, a multisubband to single-subband transition controlled by the structural phases in SrTiO₃ was revealed. In addition, the transient behaviors of nanostructures also provided a unique opportunity to study the nanoscale diffusions of adsorbates and oxygen vacancies in oxide heterostructures. Knowledge of such dynamic processes is important for nanodevice implementations

Broadband Terahertz Generation and Detection at 10 nm Scale

Terahertz (0.1–30 THz) radiation reveals a wealth of information that is relevant for material, biological, and medical sciences with applications that span chemical sensing, high-speed electronics, and coherent control of semiconductor quantum bits. To date, there have been no methods capable of controlling terahertz (THz) radiation at molecular scales. Here we report both generation and detection of broadband terahertz field from 10 nm scale oxide nanojunctions. Frequency components of ultrafast optical radiation are mixed at these nanojunctions, producing broadband THz emission. These same devices detect THz electric fields with comparable spatial resolution. This unprecedented control, on a scale of 4 orders of magnitude smaller than the diffraction limit, creates a pathway toward THz-bandwidth spectroscopy and control of individual nanoparticles and molecules

Electron–Lattice Coupling in Correlated Materials of Low Electron Occupancy

In correlated materials including transition metal oxides, electronic properties and functionalities are modulated and enriched by couplings between the electron and lattice degrees of freedom. These couplings are controlled by external parameters such as chemical doping, pressure, magnetic and electric fields, and light irradiation. However, the electron–lattice coupling relies on orbital characters, i.e., symmetry and occupancy, of t_2g and e_g orbitals, so that a large electron–lattice coupling is limited to e_g electron system, whereas t_2g electron system exhibits an inherently weak coupling. Here, we design and demonstrate a strongly enhanced electron–lattice coupling in electron-doped SrTiO₃, that is, the t_2g electron system. In ultrathin films of electron-doped SrTiO₃ [i.e., (La_0.25Sr_0.75)­TiO₃], we reveal the strong electron–lattice–orbital coupling, which is manifested by extremely increased tetragonality and the corresponding metal-to-insulator transition. Our findings open the way of an active tuning of the charge–lattice–orbital coupling to obtain new functionalities relevant to emerging nanoelectronic devices

Tailoring LaAlO₃/SrTiO₃ Interface Metallicity by Oxygen Surface Adsorbates

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

Imprint Control of BaTiO₃ Thin Films via Chemically Induced Surface Polarization Pinning

Surface-adsorbed polar molecules can significantly alter the ferroelectric properties of oxide thin films. Thus, fundamental understanding and controlling the effect of surface adsorbates are crucial for the implementation of ferroelectric thin film devices, such as ferroelectric tunnel junctions. Herein, we report an imprint control of BaTiO₃ (BTO) thin films by chemically induced surface polarization pinning in the top few atomic layers of the water-exposed BTO films. Our studies based on synchrotron X-ray scattering and coherent Bragg rod analysis demonstrate that the chemically induced surface polarization is not switchable but reduces the polarization imprint and improves the bistability of ferroelectric phase in BTO tunnel junctions. We conclude that the chemical treatment of ferroelectric thin films with polar molecules may serve as a simple yet powerful strategy to enhance functional properties of ferroelectric tunnel junctions for their practical applications

Sharpened VO₂ Phase Transition via Controlled Release of Epitaxial Strain

Phase transitions in correlated materials can be manipulated at the nanoscale to yield emergent functional properties, promising new paradigms for nanoelectronics and nanophotonics. Vanadium dioxide (VO₂), an archetypal correlated material, exhibits a metal–insulator transition (MIT) above room temperature. At the thicknesses required for heterostructure applications, such as an optical modulator discussed here, the strain state of VO₂ largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO₂ films by employing an intermediate template layer with large lattice mismatch to relieve the interfacial lattice constraints, contrary to conventional thin film epitaxy that favors lattice match between the substrate and the growing film. A combination of phase-field simulation, in situ real-time nanoscale imaging, and electrical measurements reveals robust undisturbed MIT dynamics even at preexisting structural domain boundaries and significantly sharpened MIT in the templated VO₂ films. Utilizing the sharp MIT, we demonstrate a fast, electrically switchable optical waveguide. This study offers unconventional design principles for heteroepitaxial correlated materials, as well as novel insight into their nanoscale phase transitions