Layer-separable and gap-tunable topological insulators.

The way in which states of a topological insulator (TI) transform from monolayer to bulk is an important issue for applications in spintronics. However, unlike graphite, most layered materials are difficult to exfoliate. Using first-principles calculations, we predict that thallium selenide (TlSe) will be a layered TI with rather weak interlayer coupling and thus it should be exfoliated easily. The evolution of the topological states can also be driven by doping with indium (In) atoms or applying lateral strains. A comparison of TlSe and ß-InSe shows that the influences of structural and chemical components on the electronic structures determine the topological phase. Effects of van der Waals interactions in this TlSe material are also addressed.

Tuning molecule-substrate coupling via deposition of metal adatoms.

Organic-inorganic hybrids constitute an important class of functional materials. The fundamentals at the molecular levels are, however, relatively unexplored. PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) is a colorant and extensively applied in organic-based optoelectronic devices. PTCDA/Cu(111) and Fe-PTCDA/Cu(111) metal-organic hybrid monolayers are studied by low temperature scanning tunneling microscopy and spectroscopy (STS) and density functional theory (DFT). The former exhibits Moiré pattern-modulated molecular density of states while the latter adapts a commensurate adlattice. Both imaging and spectroscopic results suggest a strong hybridization between PTCDA molecules and Cu(111) substrate. Weak PTCDA-Cu(111) coupling can be obtained by the introduction of Fe adatoms. Compared to PTCDA/Cu(111), STS spectra of Fe-PTCDA/Cu(111) exhibit a higher energy and sharper features of the frontier orbitals. Together with the DFT results, we found that the PTCDA-Cu(111) coupling is attenuated by the presence of Fe adatoms and Fe-PTCDA coordination.

Highly Active NiO-Ni(OH)2 -Cr2 O3 /Ni Hydrogen Evolution Electrocatalyst through Synergistic Reaction Kinetics.

High activity catalysts for hydrogen evolution reaction (HER) play a key role in converting renewable electricity to storable hydrogen fuel. Great effort has been devoted to the search for noble metal free catalysts to make electrolysis viable for practical applications. Here, a non-precious metal oxide/metal catalyst with high intrinsic activity comparable to Pt/C was reported. The electrocatalyst consisting of NiO, Ni(OH)2 , Cr2 O3 , and Ni metal exhibits a low overpotential of 27, 103, and 153 mV at current densities of 10, 100, and 200 mA cm-2 , respectively, in a 1.0 m NaOH electrolyte. The activity is much higher than that of NiOx /Ni or Cr2 O3 alone, showing the synergistic effect of NiOx /Ni and Cr2 O3 on catalyzing HER. Density functional theory calculations shows that NiO and Cr2 O3 on Ni surface lower the disassociation energy barrier for breaking H-OH bond, while Ni(OH)2 and Cr2 O3 create preferred sites on Ni surface with near-zero H* adsorption free energy to promote H* to gaseous H2 evolution. These synergistic effects of multiple-oxides/metal composition enhance the disassociation of H-OH and the evolution of H* to gaseous H2 , thus achieving high activity and demonstrating a promising composition design for noble metal free catalyst.

Hydrogen Evolution Driven by Photoexcited Entangled Skyrmion on Perovskite Ca2Nan-3NbnO3n+1 Nanosheet.

A magnetic skyrmion is a topologically stable state with potential applications for realizing the next-generation spintronic devices. Here, we demonstrate the real-space observation of skyrmions in Dion-Jacobson phase perovskite, Ca2Nan-3NbnO3n+1- (CNNO), nanosheets by using optical injection. The CNNO4 and CNNO6 nanosheets exhibit weak ferromagnetics, while the CNNO5 nanosheet is superparamagnetic. The magnetic skyrmion can be clearly observed in those 2D nanosheets in the absence of the external magnetic field. First-principles calculations and micromagnetic simulations predict that the magnetic skyrmions in CNNO nanosheets is Néel-type with a diameter of 11-15 nm, in corresponding to the experiments. Our findings provide insights for developing room-temperature skyrmions in CNNO nanosheets for skyrmionic water-splitting performance in future energy generation and quantum computing devices.

Efficacious CO2 Photoconversion to C2 and C3 Hydrocarbons on Upright SnS-SnS2 Heterojunction Nanosheet Frameworks.

In this work, SnS-SnS2 heterostructured upright nanosheet frameworks are constructed on FTO substrates, which demonstrate promising photocatalytic performances for the conversion of CO2 and water to C2 (acetaldehyde) and C3 (acetone) hydrocarbons without H2 formation. With post annealing in designated atmospheres, the photocatalytic activity of the SnS-SnS2 heterostructured nanosheet framework is critically enhanced by increasing the fraction of crystalline SnS in nanosheets through partial transformation of the SnS2 matrix to SnS but not obviously influenced by improving the crystallinity of the SnS2 matrix. DFT calculations indicate that transformed SnS possesses the CO2 adsorption sites with significantly lower activation energy for the rate-determining step to drive efficient CO2 conversion catalysis. The experimental results and DFT calculations suggest that the SnS-SnS2 heterojunction nanosheet framework photocatalyst experiences Z-scheme charge transfer dynamic to allow the water oxidation and CO2 reduction reactions occurring on the surfaces of SnS2 and SnS, respectively. The Z-scheme SnS-SnS2 heterostructured nanosheet framework photocatalyst exhibits not only efficient charge separation but also highly catalytic active sites to boost the photocatalytic activity for CO2 conversion to C2 and C3 hydrocarbons.

Interfacial effects on leakage currents in Cu/α-cristobalite/Cu junctions.

As the miniaturization trend of integrated circuit continues, the leakage currents flow through the dielectric films insulating the interconnects become a critical issue. However, quantum transport through the mainstream on-chip interfaces between interconnects and dielectrics has not been addressed from first principles yet. Here, using first-principles calculations based on density functional theory and nonequilibrium Green's function formalism, we investigate the interfacial-dependent leakage currents in the Cu/α-cristobalite/Cu junctions. Our results show that the oxygen-rich interfaces form the lowest-leakage-current junction under small bias voltages, followed by the silicon-rich and oxygen-poor ones. This feature is attributed to their transmission spectra, related to their density of states and charge distributions. However, the oxygen-poor interfacial junction may conversely have a better dielectric strength than others, as its transmission gap, from -2.8 to 3.5 eV, is more symmetry respect to the Fermi level than others.

Ab Initio and Theoretical Study on Electron Transport through Polyene Junctions in between Carbon Nanotube Leads of Various Cuts.

In this study we look into the interference effect in multi-thread molecular junctions in between carbon-nanotube (CNT) electrodes of assorted edges. From the tube end into the tube bulk of selected CNTs, we investigate surface Green's function and layer-by-layer local density of states (LDOS), and find that both the cross-cut and the angled-cut armchair CNTs exhibit 3-layer-cycled LDOS oscillations. Moreover, the angled-cut armchair CNTs, which possess a zigzag rim at the cut, exhibit not only the oscillations, but also edge state component that decays into the tube bulk. In the case of cross-cut zigzag CNTs, the LDOS shows no sign of oscillations, but prominent singularity feature due to edge states. With these cut CNTs as leads, we study the single-polyene and two-polyene molecular junctions via both ab initio and tight-binding model approaches. While the interference effect between transport channels is manifested through our results, we also differentiate the contributions towards transmission from the bulk states and the edge states, by understanding the difference in the Green's functions obtained from direct integration method and iterative method, separately.

Sr-Doped Cubic In2O3/Rhombohedral In2O3 Homojunction Nanowires for Highly Sensitive and Selective Breath Ethanol Sensing: Experiment and DFT Simulation Studies.

In recent years, it is urgent and challenging to fabricate highly sensitive and selective gas sensors for breath analyses. In this work, Sr-doped cubic In2O3/rhombohedral In2O3 homojunction nanowires (NWs) are synthesized by one-step electrospun technology. The Sr doping alters the cubic phase of pure In2O3 into the rhombohedral phase, which is verified by the high-resolution transmittance electron microscopy, X-ray diffraction, and Raman spectroscopy, and is attributable to the low cohesive energy as calculated by the density functional theory (DFT). As a proof-of-concept of fatty liver biomarker sensing, ethanol sensors are fabricated using the electrospun In2O3 NWs. The results show that 8 wt % Sr-doped In2O3 shows the highest ethanol sensing performance with a high response of 21-1 ppm, a high selectivity over other interfering gases such as methanol, acetone, formaldehyde, toluene, xylene, and benzene, a high stability measured in 6 weeks, and also a high resistance to high humidity of 80%. The outstanding ethanol sensing performance is attributable to the enhanced ethanol adsorption by Sr doping as calculated by DFT, the stable rhombohedral phase and the preferred (104) facet exposure, and the formed homojunctions favoring the electron transfer. All these results show the effective structural modification of In2O3 by Sr doping, and also the great potency of the homojunction Sr-doped In2O3 NWs for highly sensitive, selective, and stable breath ethanol sensing.

Nearly spherical CoP nanoparticle/carbon nanosheet hybrids: a high-performance trifunctional electrocatalyst for oxygen reduction and water splitting.

Developing active multifunctional electrocatalysts composed of earth-abundant and cheap elements is an urgent demand in energy conversion applications. This study presents a facile approach for the scalable synthesis of nanostructured cobalt phosphide embedded in carbon nanosheets (CoP NPs/CNSs). The hybrid structures show highly efficient trifunctional electrocatalytic activities toward the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) under alkaline condition. The catalytic performances, which are remarkably superior to those of the previously reported CoP nanostructures enclosed by single or a few low index facets, can be attributed to the nearly spherical shape of the CoP nanoparticles with many more exposed crystal planes. Density functional theory (DFT) computations are performed to investigate the facet effects of CoP on electrocatalytic activity, and they reveal the relatively low overpotentials of (101) facets towards the OER and the free energy of water dissociation (ΔG H2O) and adsorbed H intermediates (ΔG H*) of (311) toward the HER being close to thermoneutral. This work is expected to inspire the design and fabrication of multifunctional and high-efficiency electrocatalysts by selectively exposing specific crystal planes.

GdFe0.8Ni0.2O3: A Multiferroic Material for Low-Power Spintronic Devices with High Storage Capacity.

Multiferroic materials are strong candidates for reducing the energy consumption of voltage-controlled spintronic devices because of the coexistence of ferroelectric (FE) and magnetic orders in a single phase. In this article, we present a new multiferroic perovskite, GdNixFe1-xO3 (GFNO), produced via sputtering on a SrTiO3 substrate. The proposed GFNO is FE and canted antiferromagnetic (AFM) within a monoclinic framework at room temperature. The FE polarization of the GFNO is up to 37 µC/cm2. When capped with a Co layer, the resulting heterostructure exhibits voltage-controlled magnetism (VCM). The heterostructured device exhibits two distinct features. First, its VCM depends on the magnitude as well as the polarity of the applied bias, thereby doubling the number of available magnetic readout states under a fixed voltage. Furthermore, the magnetic order of the device can be controlled very effectively within ±1 V. These two characteristics satisfy the requirements for low-power and high-storage technology. Theoretical analysis and experimental results indicate the importance of Ni dopant in regulating the polarity-dependent multiferroicity of this gadolinium ferrite system.

Conductance Switching in Single-Peptide Molecules through Interferer Binding.

Detection of bioprocess-interfering metal ions and molecules is important for healthcare, and peptide single-molecule junctions have shown their potential toward sensing these targets efficiently. Using first-principles calculations, we investigate the conductance of Cys-Gly-Cys and cysteamine-Gly-Gly-Cys peptide junctions, and the effect of its change upon copper-ion (Cu2+) or bisphenol A (BPA) binding. The calculated conductance of the peptides and the Cu2+-peptide complexes agrees well with the experimental data and that of the BPA-bond peptides is further predicted. Our analyses show that the conductance switching mainly comes from the structure deformation of the peptide caused by Cu2+ binding or from the new conduction channel added by BPA binding. Our results suggest that the cysteamine-Gly-Gly-Cys junction can recognize Cu2+ and BPA better than the Cys-Gly-Cys one does.

Engineering the interlayer exchange coupling in magnetic trilayers.

When the thickness of metal film approaches the nanoscale, itinerant carriers resonate between its boundaries and form quantum well states (QWSs), which are crucial to account for the film's electrical, transport and magnetic properties. Besides the classic origin of particle-in-a-box, the QWSs are also susceptible to the crystal structures that affect the quantum resonance. Here we investigate the QWSs and the magnetic interlayer exchange coupling (IEC) in the Fe/Ag/Fe (001) trilayer from first-principles calculations. We find that the carriers at the Brillouin-zone center (belly) and edge (neck) separately form electron- and hole-like QWSs that give rise to an oscillatory feature for the IEC as a function of the Ag-layer thickness with long and short periods. Since the QWS formation sensitively depends on boundary conditions, one can switch between these two IEC periods by changing the Fe-layer thickness. These features, which also occur in the magnetic trilayers with other noble-metal spacers, open a new degree of freedom to engineer the IEC in magnetoresistance devices.

Spin-Dependent Molecule Symmetry at a Pentacene-Co Spinterface.

Incorporating spin-polarized scanning tunneling microscopy (SP-STM) measurements and first-principles calculations, we resolve spin-polarized states and consequent features in a pentacene(PEN)-Co hybrid system. Symmetry reduction of PEN clarifies the PEN adsorption site and the Co stacking methods. Near the Fermi energy, the molecular symmetry is spin-dependently recovered and an inversion of spin-polarization in PEN with respect to Co is observed. The experimental findings and calculation results are interpreted by a pz-d hybridization model, in which spin-dependent bonding-antibonding splitting of molecular orbitals happens at metal-organic spinterfaces.

Mapping polarization induced surface band bending on the Rashba semiconductor BiTeI.

Surfaces of semiconductors with strong spin-orbit coupling are of great interest for use in spintronic devices exploiting the Rashba effect. BiTeI features large Rashba-type spin splitting in both valence and conduction bands. Either can be shifted towards the Fermi level by surface band bending induced by the two possible polar terminations, making Rashba spin-split electron or hole bands electronically accessible. Here we demonstrate the first real-space microscopic identification of each termination with a multi-technique experimental approach. Using spatially resolved tunnelling spectroscopy across the lateral boundary between the two terminations, a previously speculated on p-n junction-like discontinuity in electronic structure at the lateral boundary is confirmed experimentally. These findings realize an important step towards the exploitation of the unique behaviour of the Rashba semiconductor BiTeI for new device concepts in spintronics.

Digitized charge transfer magnitude determined by metal-organic coordination number.

Well-ordered metal-organic nanostructures of Fe-PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer.

Recognizing nucleotides by cross-tunneling currents for DNA sequencing.

Using first-principles calculations, we study electron transport through nucleotides inside a rectangular nanogap formed by two pairs of gold electrodes which are perpendicular and parallel to the nucleobase plane. We propose that this setup will enhance the nucleotide selectivity of tunneling signals to a great extent. Information from three electrical probing processes offers full nucleotide recognition, which survives the noise from neighboring nucleotides and configuration fluctuations.

Effect of electrode orientations on charge transport in alkanedithiol single-molecule junctions.

Using first-principles calculations based on the density functional theory and the nonequilibrium Green's functions approach, we study the charge transport in Au-alkanedithiol-Au single-molecule junctions with different electrode orientations and molecular lengths. We attribute the recently measured high-/low-conductance in these heterostructures to two distinct electrode orientations, [100] and [111], which can control the electrode-molecule coupling as well as the tunneling strength by way of diverse band structures. Our detailed analysis on the transmission spectra suggests that even a single alkanedithiol junction can serve as a double quantum-dot system to yield tunable quantum interference.

Current-driven oscillations and time-dependent transport in nanojunctions.

Resonance inelastic conduction in molecular-scale electronics can be used to channel energy into a given mode of the molecular component to generate a desired motion. Dependence of the conductance properties on the molecular configuration, in turn, leads to a time-modulated current whose temporal properties are subject to control. We use an ab initio nonequilibrium formalism and the example of Au-C60-Au junctions to illustrate the strongly correlated phenomena of current-driven dynamics and time-dependent conductance in nanoelectronics, noting implications to, and potential applications in, several disciplines.