Complete Mapping of Thermodynamic Stability of Ternary Oxide SrTiO3 (001) Surface at Finite Temperatures.

The oxide surface structure plays a vital role in controlling and utilizing the emergent phenomena occurring at the interface of nanoarchitecture. A complete understanding of ternary oxide surfaces remains challenging due to complex surface reconstructions in various chemical and physical environments. Here a thermodynamic framework is developed to treat the stability of ternary oxide surfaces with finite temperature and chemical environments. Strontium titanate, as a representative ternary oxide, is used to establish the complete energy landscape of SrTiO3 (001) surface. The complete mapping yields a comprehensive understanding of various stable SrTiO3 surfaces with finite temperature and chemical potential or vapor pressure of the constituents, i.e., Sr (or Ti) metal and oxygen. This treatment also reveals a stable surface unknown yet with SrTi2O3 stoichiometry, which unveils the missing link between numerous previous experimental observations and the current understanding of SrTiO3 surface. Interestingly, the new surface shows an anisotropic surface-localized metallic state originating from the characteristic surface structure. The findings would provide a viable way to understand ternary oxide surfaces and further utilize SrTiO3 surfaces for oxide nanoarchitectures.

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3.

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

Geometrical Doping at the Atomic Scale in Oxide Quantum Materials.

Chemical dopants enabling a plethora of emergent physical properties have been treated as randomly and uniformly distributed in the frame of a three-dimensional doped system. However, in nanostructured architectures, the location of dopants relative to the interface or boundary can greatly influence device performance. This observation suggests that chemical dopants need to be considered as discrete defects, meaning that geometric control of chemical dopants becomes a critical aspect as the physical size of materials scales down into the nanotechnology regime. Here we show that geometrical control of dopants at the atomic scale is another fundamental parameter in chemical doping, extending beyond the kind and amount of dopants conventionally used. The geometrical control of dopants extends the class of geometrically controlled structures into an unexplored dimensionality, between 2D and 3D. It is well understood that in the middle of the progressive dimensionality change from 3D to 2D, the electronic state of doped SrTiO3 is altered from a highly symmetric charged fluid to a charge disproportionated insulating state. Our results introduce a geometrical control of dopants, namely, geometrical doping, as another axis to provide a variety of emergent electronic states via tuning of the electronic properties of the solid state.

Expeditiously Crystallized Pure Orthorhombic-Hf0.5Zr0.5O2 for Negative Capacitance Field Effect Transistors.

Ultralow-power logic devices are next-generation electronics in which their maximum efficacies are realized at minimum input power expenses. The integration of ferroelectric negative capacitors in the regular gate stacks of two-dimensional field-effect transistors addresses two intriguing challenges in today's electronics; short channel effects and high operating voltages. The complementary-metal-oxide-semiconductor-compatible Hf0.5Zr0.5O2 (HZO) is an excellent ferroelectric material crystallized in a noncentrosymmetric o-phase. The present work is the first to utilize pulsed laser deposition (PLD)-grown phase-pure ferroelectric HZO to achieve steep slope negative capacitance (NC) in field effect transistors (FETs). A dual-step growth strategy is designed to achieve phase-pure orthorhombic HZO on silicon and other conducting substrates. The room-temperature PLD-grown amorphous HZO is allowed to crystallize using rapid thermal annealing at 600 °C. The polycrystalline orthorhombic HZO is further integrated with atomic layer deposition-grown HfO2 to achieve a stable NC transition. The stack is further integrated into the molybdenum disulfide channel to achieve steep switching and a hysteresis-free operation of the resulting FETs. The subthreshold swings of the FETs are 20.42 and 26.16 mV/dec in forward and reverse bias conditions, respectively.

Design of Piezoelectric Actuator for Braille Module by Finite Element Method.

Piezoelectric actuators, exhibiting large displacement and torque, are attractive for use in a broad range of actuator applications. One of the important applications is a tactile device, e.g., refreshable Braille display. Piezoelectric actuation in the Braille module requires large torque and large displacement for the tactile sensing of the human finger. In this study, we design piezoelectric actuators for the tactile interface including a Braille cell by finite element method (FEM) simulations. FEM simulations allow us to capture the entire device physics and solve the coupled piezoelectricity/solid mechanics problem. We investigated the effect of the structure, geometric variation, and physical properties of the materials used on the displacement and torque force. The studied structure includes a trilayer bimorph consisting of piezoelectric and supporting layers, and a multilayer with multiple bimorphs. The simulation result provides a useful guide toward piezoelectric actuators of tactile function.

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 t2g and eg orbitals, so that a large electron-lattice coupling is limited to eg electron system, whereas t2g 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 t2g electron system. In ultrathin films of electron-doped SrTiO3 [i.e., (La0.25Sr0.75)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.

Oxygen Vacancy Linear Clustering in a Perovskite Oxide.

Oxygen vacancies have been implicitly assumed isolated ones, and understanding oxide materials possibly containing oxygen vacancies remains elusive within the scheme of the isolated vacancies, although the oxygen vacancies have been playing a decisive role in oxide materials. Here, we report the presence of oxygen vacancy linear clusters and their orientation along a specific crystallographic direction in SrTiO3, a representative of a perovskite oxide. The presence of the linear clusters and associated electron localization was revealed by an electronic structure represented in the increase in the Ti2+ valence state or corresponding Ti 3d2 electronic configuration along with divacancy cluster model analysis and transport measurement. The orientation of the linear clusters along the [001] direction in perovskite SrTiO3 was verified by further X-ray diffuse scattering analysis. Because SrTiO3 is an archetypical perovskite oxide, the vacancy linear clustering with the specific aligned direction and electron localization can be extended to a wide variety of the perovskite oxides.

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.

Reliable Piezoelectricity in Bilayer WSe2 for Piezoelectric Nanogenerators.

Recently, piezoelectricity has been observed in 2D atomically thin materials, such as hexagonal-boron nitride, graphene, and transition metal dichalcogenides (TMDs). Specifically, exfoliated monolayer MoS2 exhibits a high piezoelectricity that is comparable to that of traditional piezoelectric materials. However, monolayer TMD materials are not regarded as suitable for actual piezoelectric devices due to their insufficient mechanical durability for sustained operation while Bernal-stacked bilayer TMD materials lose noncentrosymmetry and consequently piezoelectricity. Here, it is shown that WSe2 bilayers fabricated via turbostratic stacking have reliable piezoelectric properties that cannot be obtained from a mechanically exfoliated WSe2 bilayer with Bernal stacking. Turbostratic stacking refers to the transfer of each chemical vapor deposition (CVD)-grown WSe2 monolayer to allow for an increase in degrees of freedom in the bilayer symmetry, leading to noncentrosymmetry in the bilayers. In contrast, CVD-grown WSe2 bilayers exhibit very weak piezoelectricity because of the energetics and crystallographic orientation. The flexible piezoelectric WSe2 bilayers exhibit a prominent mechanical durability of up to 0.95% of strain as well as reliable energy harvesting performance, which is adequate to drive a small liquid crystal display without external energy sources, in contrast to monolayer WSe2 for which the device performance becomes degraded above a strain of 0.63%.

Effect of laser fluence on electrical properties of (Sr0.75,La0.25)TiO3 thin films grown by pulsed-laser-deposition.

We have grown Sr0.75La0.25TiO3 (SLTO) thin films using pulsed laser deposition (PLD) with various laser energy fluences. We investigated the effect of energy fluence on the compositions of SLTO films. The stoichiometry of SLTO films was controlled by adjusting the laser energy density. At low energy densities below 1.0 J/cm2, SLTO films become non-stoichiometric with Ti deficiency. The Ti deficiency increases with decreasing the laser energy fluence. We have also investigated the effect of laser energy fluence on the electrical properties of the thin films. The electrical resistivity and carrier density intimately depend on the laser energy fluence as a result of the non-stoichiometry. After eliminating the effect of oxygen vacancies by post-annealing, the electrical properties are dependent on the cation stoichiometry in the oxide films.

Numerical and experimental study of actuator performance on piezoelectric microelectromechanical inkjet print head.

We have investigated the actuator performance of a piezoelectrically actuated inkjet print head via the numerical and experimental analysis. The actuator consisting of multi-layer membranes, such as piezoelectric, elastic and other buffer layers, and ink chamber was fabricated by MEMS processing. The maximum displacement of the actuator membrane obtained in the experiment is explained by numerical analysis. A simulation of the actuator performance with fluidic damping shows that the resonant frequency of the membrane in liquid is reduced from its resonant frequency in air by a factor of three, which was also verified in the experiment. These simulation and experimental studies demonstrate how much "dynamic force," in terms of a membrane's maximum displacement, maximum force and driving frequency, can be produced by an actuator membrane interacting with fluid.

Quantum confinement-induced tunable exciton states in graphene oxide.

Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology.

Selective semihydrogenation of alkynes on shape-controlled palladium nanocrystals.

A systematic study on the selective semihydrogenation of alkynes to alkenes on shape-controlled palladium (Pd) nanocrystals was performed. Pd nanocrystals with a cubic shape and thus exposed {100} facets were synthesized in an aqueous solution through the reduction of Na2PdCl4 with L-ascorbic acid in the presence of bromide ions. The Pd nanocubes were tested as catalysts for the semihydrogenation of various alkynes such as 5-decyne, 2-butyne-1,4-diol, and phenylacetylene. For all substrates, the Pd nanocubes exhibited higher alkene selectivity (>90 %) than a commercial Pd/C catalyst (75-90 %), which was attributed to a large adsorption energy of the carbon-carbon triple bond on the {100} facets of the Pd nanocubes. Our approach based on the shape control of Pd nanocrystals offers a simple and effective route to the development of a highly selective catalyst for alkyne semihydrogenation.

All graphene-based thin film transistors on flexible plastic substrates.

High-performance, flexible all graphene-based thin film transistor (TFT) was fabricated on plastic substrates using a graphene active layer, graphene oxide (GO) dielectrics, and graphene electrodes. The GO dielectrics exhibit a dielectric constant (3.1 at 77 K), low leakage current (17 mA/cm(2)), breakdown bias (1.5 × 10(6) V/cm), and good mechanical flexibility. Graphene-based TFTs showed a hole and electron mobility of 300 and 250 cm(2)/(V·s), respectively, at a drain bias of -0.1 V. Moreover, graphene TFTs on the plastic substrates exhibited remarkably good mechanical flexibility and optical transmittance. This method explores a significant step for the application of graphene toward flexible and stretchable electronics.

Hydrogen adsorption and carrier generation in LaAlO3-SrTiO3 heterointerfaces: a first-principles study.

Using the first-principles method, we investigate the hydrogen adsorption on the polar AlO(2) surface of LaAlO(3)-SrTiO(3) heterostructures with an n-type interface. It is found that the H atom is most stable when bound at the surface O atom. The adsorption energy for a given H density is lowered (i.e., stronger binding) with increasing LaAlO(3) thickness. The adsorbed H atom donates electrons to the conduction band of SrTiO(3), which results in the metallic conductivity of SrTiO(3). The charge transfer from H to SrTiO(3) significantly alters the electrostatic boundary condition and the Coulombic divergence inside LaAlO(3) completely disappears when one H atom is adsorbed every (2 × 1) surface unit cell. Our results also indicate that H(2) or H(2)O molecules exothermically dissociate and adsorb on the surface when the LaAlO(3) layer is thicker than certain critical values, suggesting that the H adsorption may play a role in the conducting properties of LaAlO(3)-SrTiO(3) heterostructures observed in experiment.

Oxygen vacancy clustering and electron localization in oxygen-deficient SrTiO(3): LDA + U study.

We find, using a local density approximation +Hubbard U method, that oxygen vacancies tend to cluster in a linear way in SrTiO(3), a prototypical perovskite oxide, accompanied by strong electron localization at the 3d state of the nearby Ti transition metal ion. The vacancy clustering and the associated electron localization lead to a profound impact on materials properties, e.g., the reduction in free-carrier densities, the appearance of characteristic optical spectra, and the decrease in vacancy mobility. The high stability against the vacancy migration also suggests the physical reality of the vacancy cluster.