Ferromagnetic stability optimization via oxygen-vacancy control in single-atom Co/TiO nanostructures.

Oxygen vacancies and their correlation with the nanomagnetism and electronic structure are crucial for applications in dilute magnetic semiconductors design applications. Here, we report on cobalt single atom-incorporated titanium dioxide (TiO) monodispersed nanoparticles synthesized using a thermodynamic redistribution strategy. Using advanced synchrotron-based X-ray techniques and simulations, we find trivalent titanium is absent, indicating trivalent cations do not influence ferromagnetic (FM) stability.

Micrometre-scale single-crystalline borophene on a square-lattice Cu(100) surface.

Borophene, a crystalline monolayer boron sheet, has been predicted to adopt a variety of structures-owing to its high polymorphism-that may possess physical properties that could serve in flexible electronics, energy storage and catalysis. Several borophene polymorphs have been synthesized on noble metal surfaces but for device fabrication larger single-crystal domains are typically needed with, ideally, weak borophene-substrate interactions. Here we report the synthesis of borophene on a square-lattice Cu(100) surface and show that incommensurate coordination reduces the borophene-substrate interactions and also leads to a borophene polymorph different from those previous reported.

Designing and controlling the properties of transition metal oxide quantum materials.

This Perspective addresses the design, creation, characterization and control of synthetic quantum materials with strong electronic correlations. We show how emerging synergies between theoretical/computational approaches and materials design/experimental probes are driving recent advances in the discovery, understanding and control of new electronic behaviour in materials systems with interesting and potentially technologically important properties. The focus here is on transition metal oxides, where electronic correlations lead to a myriad of functional properties including superconductivity, magnetism, Mott transitions, multiferroicity and emergent behaviour at picoscale-designed interfaces.

Tuning two-dimensional phase formation through epitaxial strain and growth conditions: silica and silicate on NiPd(111) alloy substrates.

Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here, the competition between 2D van der Waals (VDW) silica and 2D Ni silicate phases on NiPd(111) alloy substrates was systematically investigated experimentally as a function of Si surface coverage, annealing time and temperature, O partial pressure, and substrate composition and the results were compared with thermodynamic predictions based on density functional theory (DFT) calculations and thermochemical data for O. Experimentally, 2D Ni silicate was exclusively observed at higher O pressures (∼10 Torr), higher annealing temperatures (1000 K), and more prolonged annealing (10 min) if the substrate contained any Ni and for initial Si coverages up to 2 monolayers.

Strong Orbital Polarization in a Cobaltate-Titanate Oxide Heterostructure.

Through a combination of experimental measurements and theoretical modeling, we describe a strongly orbital-polarized insulating ground state in an (LaTiO_{3})_{2}/(LaCoO_{3})_{2} oxide heterostructure. X-ray absorption spectra and ab initio calculations show that an electron is transferred from the titanate to the cobaltate layers. The charge transfer, accompanied by a large octahedral distortion, induces a substantial orbital polarization in the cobaltate layer of a size unattainable via epitaxial strain alone.

Causes of ferroelectricity in HfO-based thin films: an ab initio perspective.

We present a comprehensive first principles study of doped hafnia in order to understand the formation of ferroelectric orthorhombic[001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal[100] grains can transform into orthorhombic[001] during thermal annealing when they are laterally confined by other grains. We show that among 0%, 2% and 4% Si doping, 4% doping provides the best conditions for the tetragonal[100] → orthorhombic[001] transformation.

Large-area single-crystal sheets of borophene on Cu(111) surfaces.

Borophene, a theoretically proposed two-dimensional (2D) boron allotrope, has attracted much attention as a candidate material platform for high-speed, transparent and flexible electronics. It was recently synthesized, on Ag(111) substrates, and studied by tunnelling and electron spectroscopy. However, the exact crystal structure is still controversial, the nanometre-size single-crystal domains produced so far are too small for device fabrication and the structural tunability via substrate-dependent epitaxy is yet to be proven.

Nature of Lone-Pair-Surface Bonds and Their Scaling Relations.

We investigate the (surface) bonding of a class of industrially and biologically important molecules in which the chemically active orbital is a 2 p electron lone pair located on an N or O atom bound via single bonds to H or alkyl groups. This class includes water, ammonia, alcohols, ethers, and amines. Using extensive density functional theory (DFT) calculations, we discover scaling relations (correlations) among molecular binding energies of different members of this class: the bonding energetics of a single member can be used as a descriptor for other members.

Controlling Mobility in Perovskite Oxides by Ferroelectric Modulation of Atomic-Scale Interface Structure.

Coherent and epitaxial interfaces permit the realization of electric field driven devices controlled by atomic-scale structural and electronic effects at interfaces. Compared to conventional field effect devices where channel conductivity is modulated by carrier density modification, the propagation of atomic-scale distortions across an interface can control the atomic scale bonding, interatomic electron tunneling rates and thus the mobility of the channel material. We use first-principles theory to design an atomically abrupt epitaxial perovskite heterostructure involving an oxide ferroelectric (PbZrTiO) and conducting oxide channel (LaNiO) where coupling of polar atomic motions to structural distortions can induce large, reversible changes in the channel mobility.

Single Atomic Layer Ferroelectric on Silicon.

A single atomic layer of ZrO exhibits ferroelectric switching behavior when grown with an atomically abrupt interface on silicon. Hysteresis in capacitance-voltage measurements of a ZrO gate stack demonstrate that a reversible polarization of the ZrO interface structure couples to the carriers in the silicon. First-principles computations confirm the existence of multiple stable polarization states and the energy shift in the semiconductor electron states that result from switching between these states.

How Correlated is the FeSe/SrTiO_{3} System?

Recent observation of ∼10 times higher critical temperature in a FeSe monolayer compared with its bulk phase has drawn a great deal of attention because the electronic structure in the monolayer phase appears to be different than bulk FeSe. Using a combination of density functional theory and dynamical mean field theory, we find electronic correlations have important effects on the predicted atomic-scale geometry and the electronic structure of the monolayer FeSe on SrTiO_{3}. The electronic correlations are dominantly controlled by the Se-Fe-Se angle either in the bulk phase or the monolayer phase.

Justifying quasiparticle self-consistent schemes via gradient optimization in Baym-Kadanoff theory.

The question of which non-interacting Green's function 'best' describes an interacting many-body electronic system is both of fundamental interest as well as of practical importance in describing electronic properties of materials in a realistic manner. Here, we study this question within the framework of Baym-Kadanoff theory, an approach where one locates the stationary point of a total energy functional of the one-particle Green's function in order to find the total ground-state energy as well as all one-particle properties such as the density matrix, chemical potential, or the quasiparticle energy spectrum and quasiparticle wave functions. For the case of the Klein functional, our basic finding is that minimizing the length of the gradient of the total energy functional over non-interacting Green's functions yields a set of self-consistent equations for quasiparticles that is identical to those of the quasiparticle self-consistent GW (QSGW) (van Schilfgaarde et al 2006 Phys.

Orbital Engineering in Nickelate Heterostructures Driven by Anisotropic Oxygen Hybridization rather than Orbital Energy Levels.

Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking LaTiO_{3}-LaNiO_{3}-3×(LaAlO_{3}), a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni 3d orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O K-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states.

Polarization-driven catalysis via ferroelectric oxide surfaces.

The surface chemistry and physics of oxide ferroelectric surfaces with a fixed polarization state have been studied experimentally for some time. Here, we discuss the possibility of using these materials in a different mode, namely under cyclically changing polarization conditions achievable via periodic perturbations by external fields (e.g.

Engineered Unique Elastic Modes at a BaTiO_{3}/(2×1)-Ge(001) Interface.

The strong interaction at an interface between a substrate and thin film leads to epitaxy and provides a means of inducing structural changes in the epitaxial film. These induced material phases often exhibit technologically relevant electronic, magnetic, and functional properties. The 2×1 surface of a Ge(001) substrate applies a unique type of epitaxial constraint on thin films of the perovskite oxide BaTiO_{3} where a change in bonding and symmetry at the interface leads to a non-bulk-like crystal structure of the BaTiO_{3}.

Intrinsic interfacial phenomena in manganite heterostructures.

We review recent advances in our understanding of interfacial phenomena that emerge when dissimilar materials are brought together at atomically sharp and coherent interfaces. In particular, we focus on phenomena that are intrinsic to the interface and review recent work carried out on perovskite manganites interfaces, a class of complex oxides whose rich electronic properties have proven to be a useful playground for the discovery and prediction of novel phenomena.

Orbital engineering in symmetry-breaking polar heterostructures.

We experimentally demonstrate a novel approach to substantially modify orbital occupations and symmetries in electronically correlated oxides. In contrast to methods using strain or confinement, this orbital tuning is achieved by exploiting charge transfer and inversion symmetry breaking using atomically layered heterostructures. We illustrate the technique in the LaTiO_{3}-LaNiO_{3}-LaAlO_{3} system; a combination of x-ray absorption spectroscopy and ab initio theory reveals electron transfer and concomitant polar fields, resulting in a ∼50% change in the occupation of Ni d orbitals.

Reversible modulation of orbital occupations via an interface-induced polar state in metallic manganites.

The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin ordering in metal oxides. Here, we use ab initio calculations to show that reversibly modulating the orbital populations on Mn atoms can be achieved at ferroelectric/manganite interfaces by the presence of ferroelectric polarization on the nanoscale. The change in orbital occupation can be as large as 10%, greatly exceeding that of bulk manganites.

Controlled doping of carbon nanotubes with metallocenes for application in hybrid carbon nanotube/Si solar cells.

There is considerable interest in the controlled p-type and n-type doping of carbon nanotubes (CNT) for use in a range of important electronics applications, including the development of hybrid CNT/silicon (Si) photovoltaic devices. Here, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope single-walled carbon nanotube (SWNT) thin films, using a simple spin coating process. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT.

Tuning the structure of nickelates to achieve two-dimensional electron conduction.

Metallic electronic transport in nickelate heterostructures can be induced and confined to two dimensions (2D) by controlling the structural parameters of the nickel-oxygen planes.

Mechanism for strong binding of CdSe quantum dots to multiwall carbon nanotubes for solar energy harvesting.

As hybrid nanomaterials have myriad of applications in modern technology, different functionalization strategies are being intensely sought for preparing nanocomposites with tunable properties and structures. Multi-Walled Carbon Nanotube (MWNT)/CdSe Quantum Dot (QD) heterostructures serve as an important example for an active component of solar cells. The attachment mechanism of CdSe QDs and MWNTs is known to affect the charge transfer between them and consequently to alter the efficiency of solar cell devices.

Modifying the electronic orbitals of nickelate heterostructures via structural distortions.

We describe a general materials design approach that produces large orbital energy splittings (orbital polarization) in nickelate heterostructures, creating a two-dimensional single-band electronic surface at the Fermi energy. The resulting electronic structure mimics that of the high temperature cuprate superconductors. The two key ingredients are (i) the construction of atomic-scale distortions about the Ni site via charge transfer and internal electric fields, and (ii) the use of three-component (tricomponent) superlattices to break inversion symmetry.

Dynamic evanescent phonon coupling across the La(1-x)Sr(x)MnO3/SrTiO3 interface.

The transport and magnetic properties of correlated La0.53Sr0.47MnO3 ultrathin films, grown epitaxially on SrTiO3, show a sharp cusp at the structural transition temperature of the substrate.

Interface-induced polarization and inhibition of ferroelectricity in epitaxial SrTiO₃/Si.

We use SrTiO₃/Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed (pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.