Interfacial Charge Polarization Research Articles

Interfacial charge polarization in Co2P2O7@N, P co-doped carbon nanocages as Mott-Schottky electrocatalysts for accelerating oxygen evolution reaction

Developing heterogeneous non-noble metal electrocatalysts to modulate the valence-electron state near the Fermi level of metal centers is the pivotal for efficient oxygen evolution reaction (OER). Herein, we report a Mott-Schottky heterojunction electrocatalyst of the Co2P2O7@N, P co-doped carbon nanocages, in which the metallic N, P co-doping carbon layer as a co-catalyst can effectively modulate the overfilled Co center eg orbital occupation of the Co2P2O7 nanoparticles and stabilize the microstructure. The density functional theory (DFT) calculations also reveal the build-in electric field promoted local charge polarization in the heterojunction interface greatly boosts the targeted intermediate (OOH*) adsorption with a higher intrinsic activity. The as-obtained electrocatalysts manifest superior OER catalytic performance with an overpotential of only 310 mV at a current density of 50 mA cm-2 and negligible current loss for 100 h in 1.0 M KOH, much lower than the benchmark RuO2 (370 mV). This work demonstrates a heterointerface charge polarization concept to accelerate OER in efficient electrocatalysts for water splitting.

Direct observations of the charge behavior of a high-efficiency blue organic light-emitting diode under operating conditions using electric-field-induced doubly resonant sum-frequency-generation vibrational spectroscopy

We investigated the carrier behavior in a high-efficiency multilayer blue organic light-emitting diode (OLED) that employs the triplet–triplet annihilation (TTA) phenomenon using electric-field-induced doubly resonant sum-frequency generation (EFI-DR-SFG) vibrational spectroscopy. We observed enhancements of the EFI-DR-SFG signals—particularly the peaks at 1404, 1464, and 1601 cm−1—when we applied forward- or reverse-bias voltages to the blue OLEDs. The results of our EFI-DR-SFG analyses of the carrier behavior in an operating blue OLED showed changes in the peak intensities at 1404 and 1464 cm−1, indicating that the electrons accumulate at the electron-blocking layer/emission layer interface. Conversely, the changes in the intensities of the peaks obtained from the hole-transport materials are caused by the cancelation of the interfacial polarization charges at the hole-injection layer/hole-transport layer interface. These results demonstrate that the EFI-DR-SFG technique is a powerful and effective tool for directly investigating the carrier behavior in practical multilayer organic devices with complicated operating mechanisms.

Yolk-Shell Fe/Fe4 N@Pd/C Magnetic Nanocomposite as an Efficient Recyclable ORR Electrocatalyst and SERS Substrate.

A yolk-shell Fe/Fe4 N@Pd/C (FFPC) nanocomposite is synthesized successfully by two facile steps: interfacial polymerization and annealing treatment. The concentration of Pd2+ is the key factor for the density of Pd nanoparticles (Pd NPs) embedded in the carbon shells, which plays a role in the oxygen reduction reaction (ORR) and surface-enhanced Raman scattering (SERS) properties. The ORR and SERS performances of FFPC nanocomposites under different concentrations of PdCl2 are investigated. The optimal ORR performance exhibits that onset potential and tafel slope can reach 0.937 V (vs reversible hydrogen electrode (RHE)) and 74 mV dec-1 , respectively, which is attributed to the synergistic effects of good electrical conductivity, large electrochemically active areas, and strong interfacial charge polarization. Off-axis electron holography reveals that interfacial charge polarization could facilitate the ORR of Pd NPs and defective carbon simultaneously and the shell with low density of Pd NPs is easier to form strong interfacial charge polarization. Moreover, FFPC-3 with maximum EF of 2.3 × 105 results from more hot-spots, local positive charge centers to attract rhodamine 6G molecules, and magnetic cores. This work not only offers a recyclable multifunctional nanocomposite with excellent performance, but also has instructional implications for interfacial engineering for electrocatalysts design.

Selective thermal radiation at longitudinal optical phonon energy under geometric condition of metal-semiconductor mesa stripe structures

Selective thermal emission at approximately 284 cm−1 with a line width of 13 cm–1 at 628 K is obtained by heating the Au-GaAs mesa-type line and space structures. This emission demonstrates a characteristic polar radiation distribution of electric dipoles and a wavelength independent of the structure size or the direction of emission; thus, it is ascribed to thermally generated electric dipoles resonating with the longitudinal optical phonon energy. This dipole is formed by the interface polarization charges of metal-GaAs-metal stripes. These distinct features are different from the radiation properties of interface phonon polaritons or electronic systems such as intersubband transitions of quantum wells. It is estimated that the mesa height and GaAs space width ranging from hundreds of nanometers to several micrometers provide high generation efficiency of electric dipoles and resultant radiation at high temperatures.

Polar charges effect on multisubband electron mobility in the semiparabolic quantum wells based on AlN/AlGaN/AlN

This work presents multisubband electron mobility in the polarization semi-parabolic quantum wells (SPQWs) AlN/AlxGa1−xN/AlN. First, a theory of multisubband mobility between the lowest two subbands under a uniform external electric field is studied. By using the variation method, the one-dimensional Poisson and Schrödinger equations have been solved within a finite potential barrier model and a bent band figured by all confinement sources (realistic model). Then, we computed and discussed about the effective confining potential profile, the wave function and multisubband mobility. Our result shows that the positive interface polarization charges effect on the distribution of the two-dimensional electron gas (2DEG) in SPQWs so it has great influences on multisubband mobility.

Defective Carbon–CoP Nanoparticles Hybrids with Interfacial Charges Polarization for Efficient Bifunctional Oxygen Electrocatalysis

AbstractThe development of efficient catalysts for both oxygen reduction and evolution reactions (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)) is central to regenerative fuel cells and rechargeable metal–air batteries. It is highly desirable to achieve the efficient integration of dual active components into the catalysts and to understand the interaction between the dual components. Here, a facile approach is demonstrated to construct defective carbon–CoP nanoparticle hybrids as bifunctional oxygen electrocatalysts, and further probe the interfacial charge distribution behavior. By combining multiple synchrotron‐based X‐ray spectroscopic characterizations with density functional theory calculations, the interfacial charge polarization with the electrons gathering at the defective carbon surface and the holes gathering at the CoP surface due to strong interfacial coupling is revealed, which simultaneously facilitates the ORR and OER with remarkable bifunctional oxygen electrode activities. This work not only offers a bifunctional oxygen catalyst with outstanding performance, but also unravels the promoting factor of the hybrids from the view of interfacial charge distribution.

Atomic Layer Deposition and Performance of ZrO2-Al2O3 Thin Films

Thin mixed and nanolaminate films of ZrO2 and Al2O3 were grown by atomic layer deposition from the corresponding metal chlorides and water. The films were grown at 350°C in order to ensure ZrO2 crystallization in the as-deposited state. The relative thicknesses of layers in the structure of the nanolaminates were controlled in order to maximize the content of metastable polymorphs of ZrO2 that have higher permittivity than that of the stable monoclinic ZrO2. The multilayer films demonstrated interfacial charge polarization and saturative magnetization in external fields. The conductivity of the films could be switched between high and low resistance states by applying voltages of alternating polarity.

Complex impedance study on polymer-derived amorphous silicon carbonitride

Abstract Polymer-derived amorphous-SiCN pyrolyzed at 1200 °C was studied by impedance spectroscopy at different frequencies and temperatures. Obtained impedance spectra were analyzed by using equivalent circuit method. Results revealed that two relaxation effects were involved over the entire studied temperature range, which corresponded to free carbon phase at high frequency and the interface at low frequency. The impedance spectroscopy could be well-fitted by two parallel RC circuits in series. Interface behavior dominated the polarization relaxation process, which was controlled by the resistance as well as the capacitance of interface. All processes performed a 1/T1/4 behavior, which follows a three-dimensional (3D) random hopping mechanism. This paper further found that amorphous-SiCN ceramic materials underwent an interfacial charge polarization process over a frequency range from 0.05 Hz to 10 MHz at different temperatures from −50 to 300 °C.

High-Temperature-Superconducting Weak Link Defined by the Ferroelectric Field Effect

In all-oxide ferroelectric (FE) - superconductor (S) bilayers, due to the low carrier concentration of oxides compared to transition metals, the FE interfacial polarization charges induce an accumulation (or depletion) of charge carriers in the S. This leads either to an enhancement or a depression of its critical temperature depending on FE polarization direction.Here we exploit this effect at a local scale to define planar weak-links in high-temperature superconducting wires. This is realized in BiFeO3(FE)/YBa2Cu3O7(S)bilayers in which the remnant FE domain structure is written at will by locally applying voltage pulses with a conductive-tip atomic force microscope. In this fashion, the FE domain pattern defines a spatial modulation of superconductivity. This allows us to write a device whose electrical transport shows different temperature regimes and magnetic field matching effects that are characteristic of Josephson coupled weak-links. This illustrates the potential of the ferroelectric approach for the realization of high-temperature superconducting devices.

Colossal dielectric constant and interfacial charge polarization in a polymer-derived amorphous silicon carbonitride

Abstract The dielectric properties of a polymer-derived amorphous silicon carbonitride were measured at different frequencies and temperatures. The results revealed that the material underwent an interfacial-charge polarization process. This process resulted in an S-shaped frequency dependence of the dielectric constant that reached a colossal value of 2 × 104 at room temperature. With regard to the effect of the temperature on the dielectric properties of this material, the dielectric loss peak corresponding to the interfacial-charge polarization process shifted to higher frequencies with temperature by following a 3-dimentional (3D) hopping mechanism. The unique bi-phased amorphous structure of this material accounted for the observed results.

Linear intersubband optical absorption in the semiparabolic quantum wells based on AlN/AlGaN/AlN under a uniform electric field

The linear intersubband optical absorption in the polarization semiparabolic quantum wells (SPQWs) are investigated for typical AlN/AlxGa 1−x N/AlN. First, the one-dimensional Poisson and Schrödinger equations have been solved by the variational method within a finite potential barrier model and a bent band figured by all confinement sources (realistic model). Then, the intersubband optical absorption between the lowest two subbands has been theoretically studied under an uniform external electric field. Computed results including the effective confining potential profile, the wave function and the distribution of electron gas in quantum wells, intersubband optical absorption coefficient have been discussed. Our calculation shows that the positive interface polarization charges affect on the distribution of the two-dimensional electron gas (2DEG) in SPQWs so it has great influences on the total optical absorption coefficients significantly.

Maneuvering charge polarization and transport in 2H-MoS2 for enhanced electrocatalytic hydrogen evolution reaction

Semiconducting 2H-MoS2 with high chemical stability is a promising alternative to the existing electrocatalysts for the hydrogen evolution reaction (HER); however, the HER performance largely suffers from the limited number of active S sites and low mobility for charge transport. In this work, we demonstrate that the limitations of 2H-MoS2 for the HER can be overcome by forming hybrid structures with metallic nanowires. Taking the integration with Pd as a proofof- concept, we show with solid experimental evidence that the one-dimensional structure of metallic nanowires facilitates electron transport to active S sites, while the interfacial charge polarization between MoS2 and Pd increases the electron density of the S sites for improved activity. As a result, the hybrid structure exhibits a current density of 122 mA·cm-2 at -300 mV versus RHE and a Tafel slope of 44 mV·decade-1 with excellent durability, well exceeding the performances of bare 2H-MoS2 and metallic 1T-MoS2. This work provides insights into electrocatalyst design based on charge transport and polarization, which can be extended to other hybrid structures.

Boosting Photocatalytic Water Splitting: Interfacial Charge Polarization in Atomically Controlled Core-Shell Cocatalysts.

Platinum is a commonly used cocatalyst for improved charge separation and surface reactions in photocatalytic water splitting. It is envisioned that its practical applications can be facilitated by further reducing the material cost and improving the efficacy of Pt cocatalysts. In this direction, the use of atomically controlled Pd@Pt quasi-core-shell cocatalysts in combination with TiO2 as a model semiconductor is described. As demonstrated experimentally, the electron trapping necessary for charge separation is substantially promoted by combining a Schottky junction with interfacial charge polarization, enabled by the three-atom-thick Pt shell. Meanwhile, the increase in electron density and lattice strain would significantly enhance the adsorption of H2O onto Pt surface. Taken together, the improved charge separation and molecular activation dramatically boost the overall efficiency of photocatalytic water splitting.

Boosting Photocatalytic Water Splitting: Interfacial Charge Polarization in Atomically Controlled Core–Shell Cocatalysts

AbstractPlatinum is a commonly used cocatalyst for improved charge separation and surface reactions in photocatalytic water splitting. It is envisioned that its practical applications can be facilitated by further reducing the material cost and improving the efficacy of Pt cocatalysts. In this direction, the use of atomically controlled Pd@Pt quasi‐core–shell cocatalysts in combination with TiO2 as a model semiconductor is described. As demonstrated experimentally, the electron trapping necessary for charge separation is substantially promoted by combining a Schottky junction with interfacial charge polarization, enabled by the three‐atom‐thick Pt shell. Meanwhile, the increase in electron density and lattice strain would significantly enhance the adsorption of H2O onto Pt surface. Taken together, the improved charge separation and molecular activation dramatically boost the overall efficiency of photocatalytic water splitting.

Electron distribution in polar heterojunctions within a realistic model

We present a theoretical study of the electron distribution, i.e., two-dimensional electron gas (2DEG) in polar heterojunctions (HJs) within a realistic model. The 2DEG is confined along the growth direction by a triangular quantum well with a finite potential barrier and a bent band figured by all confinement sources. Therein, interface polarization charges take a double role: they induce a confining potential and, furthermore, they can make some change in other confinements, e.g., in the Hartree potential from ionized impurities and 2DEG. Confinement by positive interface polarization charges is necessary for the ground state of 2DEG existing at a high sheet density. The 2DEG bulk density is found to be increased in the barrier, so that the scattering occurring in this layer (from interface polarization charges and alloy disorder) becomes paramount in a polar modulation-doped HJ.

Extraction of net interfacial polarization charge from Al0.54In0.12Ga0.34N/GaN high electron mobility transistors grown by metalorganic chemical vapor deposition

AlxInyGa(1-x-y)N materials show promise for use in GaN-based heterojunction devices. The growth of these materials has developed to the point where they are beginning to see implementation in high electron mobility transistors (HEMTs) and light emitting diodes. However, the electrical properties of these materials are still poorly understood, especially as related to the net polarization charge at the AlInGaN/GaN interface (Qπ(net)). All theoretical calculations of Qπ(net) share the same weakness: dependence upon polarization bowing parameters, which describe the deviation in Qπ(net) from Vegard's law. In this study, direct analysis of Qπ(net) for Al0.54In0.12Ga0.34N/GaN HEMTs is reported as extracted from C-V, I-V, and Hall measurements performed on samples grown by metalorganic chemical vapor deposition. An average value for Qπ(net) is calculated to be 2.015 × 10−6 C/cm2, with just 6.5% variation between measurement techniques.

Surface band bending and band alignment of plasma enhanced atomic layer deposited dielectrics on Ga- and N-face gallium nitride

The effects of surface pretreatment, dielectric growth, and post deposition annealing on interface electronic structure and polarization charge compensation of Ga- and N-face bulk GaN were investigated. The cleaning process consisted of an ex-situ wet chemical NH4OH treatment and an in-situ elevated temperature NH3 plasma process to remove carbon contamination, reduce oxygen coverage, and potentially passivate N-vacancy related defects. After the cleaning process, carbon contamination decreased below the x-ray photoemission spectroscopy detection limit, and the oxygen coverage stabilized at ∼1 monolayer on both Ga- and N-face GaN. In addition, Ga- and N-face GaN had an upward band bending of 0.8 ± 0.1 eV and 0.6 ± 0.1 eV, respectively, which suggested the net charge of the surface states and polarization bound charge was similar on Ga- and N-face GaN. Furthermore, three dielectrics (HfO2, Al2O3, and SiO2) were prepared by plasma-enhanced atomic layer deposition on Ga- or N-face GaN and annealed in N2 ambient to investigate the effect of the polarization charge on the interface electronic structure and band offsets. The respective valence band offsets of HfO2, Al2O3, and SiO2 with respect to Ga- and N-face GaN were 1.4 ± 0.1, 2.0 ± 0.1, and 3.2 ± 0.1 eV, regardless of dielectric thickness. The corresponding conduction band offsets were 1.0 ± 0.1, 1.3 ± 0.1, and 2.3 ± 0.1 eV, respectively. Experimental band offset results were consistent with theoretical calculations based on the charge neutrality level model. The trend of band offsets for dielectric/GaN interfaces was related to the band gap and/or the electronic part of the dielectric constant. The effect of polarization charge on band offset was apparently screened by the dielectric-GaN interface states.

Phase-field modeling of switchable diode-like current-voltage characteristics in ferroelectric BaTiO3

A self-consistent model has been proposed to study the switchable current-voltage (I-V) characteristics in Cu/BaTiO3/Cu sandwiched structure combining the phase-field model of ferroelectric domains and diffusion equations for ionic/electronic transport. The electrochemical transport equations and Ginzburg-Landau equations are solved using the Chebyshev collocation algorithm. We considered a single parallel plate capacitor configuration which consists of a single layer BaTiO3 containing a single tetragonal domain orientated normal to the plate electrodes (Cu) and is subject to a sweep of ac bias from −1.0 to 1.0 V at 25 °C. Our simulation clearly shows rectifying I-V response with rectification ratios amount to 102. The diode characteristics are switchable with an even larger rectification ratio after the polarization direction is flipped. The effects of interfacial polarization charge, dopant concentration, and dielectric constant on current responses were investigated. The switchable I-V behavior is attributed to the polarization bound charges that modulate the bulk conduction.

Leakage mechanisms in InAlN based heterostructures

We propose a model for leakage currents in Schottky contacts on InAlN/GaN heterostructures based on two distinct tunneling mechanisms. Our modeling relies on structural parameters, in particular, InAlN dielectric constant, interface polarization charges and Schottky barrier height, which are experimentally determined in the first part of our work. The first leakage mechanism is dominant in heterostructures with very thin (≤7 nm) InAlN barriers and consists in tunneling assisted by a deep level located 1.7 eV below the InAlN conduction band edge. We provide experimental evidence for this level through photocapacitance measurements. The second mechanism is on the other hand dominant in thicker InAlN layers and is linked to the appearance of highly doped regions where direct tunneling through the whole InAlN barrier is significantly enhanced. We also show that the two mechanisms may coexist for InAlN layers of intermediate thickness. Our findings confirm a progressive degradation of the InAlN material quality as the layer is grown thicker.

Electrostatic correlations on the ionic selectivity of cylindrical membrane nanopores

We characterize the role of electrostatic fluctuations on the charge selectivity of cylindrical nanopores confining electrolyte mixtures. To this end, we develop an extended one-loop theory that can account for correlation effects induced by the surface charge, nanoconfinement of the electrolyte, and interfacial polarization charges associated with the low permittivity membrane. We validate the quantitative accuracy of the theory by comparisons with previously obtained Monte-Carlo simulation data from the literature, and scrutinize in detail the underlying forces driving the ionic selectivity of the nanopore. In the biologically relevant case of electrolytes with divalent cations such as CaCl2 in negatively charged nanopores, electrostatic correlations associated with the dense counterion layer in the channel result in an increase of the pore coion density with the surface charge. This peculiarity analogous to the charge inversion phenomenon remains intact for dielectrically inhomogeneous pores, which indicates that the effect should be observable in nanofiltration membranes or DNA-blocked nanopores characterized by a low membrane permittivity. Our results show that a quantitatively accurate consideration of correlation effects is necessary to determine the ionic selectivity of nanopores in the presence of electrolytes with multivalent counterions.