Surface Dipole Research Articles

High-operating-temperature mid-infrared photodetectors via quantum dot gradient homojunction

Due to thermal carriers generated by a narrow mid-infrared energy gap, cooling is always necessary to achieve ideal photodetection. In quantum dot (QD), the electron thermal generation should be reduced with quantum confinement in all three dimensions. As a result, there would be a great potential to realize high-operating-temperature (HOT) QD mid-IR photodetectors, though not yet achieved. Taking the advantages of colloidal nanocrystals’ solution processability and precise doping control by surface dipoles, this work demonstrates a HOT mid-infrared photodetector with a QD gradient homojunction. The detector achieves background-limited performance with D* = 2.7 × 1011 Jones on 4.2 μm at 80 K, above 1011 Jones until 200 K, above 1010 Jones until 280 K, and 7.6 × 109 Jones on 3.5 μm at 300 K. The external quantum efficiency also achieves more than 77% with responsivity 2.7 A/W at zero bias. The applications such as spectrometers, chemical sensors, and thermal cameras, are also approved, which motivate interest in low-cost, solution-processed and high-performance mid-infrared photodetection beyond epitaxial growth bulk photodetectors.

Suppressing hydrogen bonds and controlling surface dipole: effective passivation for hydrophobic perovskite photoabsorber layers in solar cells

Wettability measurements with selected solutions reveal that passivation, which is vital for state-of the-art perovskite solar cells, is crucial to suppress hydrogen bonds for hydrophobic perovskite surfaces.

Control of the resistive switching voltage and reduction of the high-resistive-state current of zinc oxide by self-assembled monolayers.

We investigated the effect of self-assembled monolayer (SAM) modification of ZnO on the resistive switching behaviour by fabricating electrode-sandwiched devices (ITO/ZnO-SAM/Al). The resistive switching voltages of SAM-modified ZnO films were shifted from that of bare ZnO depending on the surface dipole induced by the SAMs. In particular, methylaminopropyl-substituted SAM-modified ZnO showed lower switching voltage (1.6 V) than bare ZnO (2.9 V). Moreover, the on/off ratio was also improved by SAM modification (from 102 to 104).

How to measure work functions from aqueous solutions.

The recent application of concepts from condensed-matter physics to photoelectron spectroscopy (PES) of volatile, liquid-phase systems has enabled the measurement of electronic energetics of liquids on an absolute scale. Particularly, vertical ionization energies, VIEs, of liquid water and aqueous solutions, both in the bulk and at associated interfaces, can now be accurately, precisely, and routinely determined. These IEs are referenced to the local vacuum level, which is the appropriate quantity for condensed matter with associated surfaces, including liquids. In this work, we connect this newly accessible energy level to another important surface property, namely, the solution work function, eΦliq. We lay out the prerequisites for and unique challenges of determining eΦ of aqueous solutions and liquids in general. We demonstrate - for a model aqueous solution with a tetra-n-butylammonium iodide (TBAI) surfactant solute - that concentration-dependent work functions, associated with the surface dipoles generated by the segregated interfacial layer of TBA+ and I- ions, can be accurately measured under controlled conditions. We detail the nature of surface potentials, uniquely tied to the nature of the flowing-liquid sample, which must be eliminated or quantified to enable such measurements. This allows us to refer aqueous-phase spectra to the Fermi level and to quantitatively assign surfactant-concentration-dependent spectral shifts to competing work function and electronic-structure effects, where the latter are typically associated with solute-solvent interactions in the bulk of the solution which determine, e.g., chemical reactivity. The present work describes the extension of liquid-jet PES to quantitatively access concentration-dependent surface descriptors that have so far been restricted to solid-phase measurements. Correspondingly, these studies mark the beginning of a new era in the characterization of the interfacial electronic structure of aqueous solutions and liquids more generally.

Recent Advances in In Situ/Operando Surface/Interface Characterization Techniques for the Study of Artificial Photosynthesis

(Photo-)electrocatalytic artificial photosynthesis driven by electrical and/or solar energy that converts water (H2O) and carbon dioxide (CO2) into hydrogen (H2), carbohydrates and oxygen (O2), has proven to be a promising and effective route for producing clean alternatives to fossil fuels, as well as for storing intermittent renewable energy, and thus to solve the energy crisis and climate change issues that we are facing today. Basic (photo-)electrocatalysis consists of three main processes: (1) light absorption, (2) the separation and transport of photogenerated charge carriers, and (3) the transfer of photogenerated charge carriers at the interfaces. With further research, scientists have found that these three steps are significantly affected by surface and interface properties (e.g., defect, dangling bonds, adsorption/desorption, surface recombination, electric double layer (EDL), surface dipole). Therefore, the catalytic performance, which to a great extent is determined by the physicochemical properties of surfaces and interfaces between catalyst and reactant, can be changed dramatically under working conditions. Common approaches for investigating these phenomena include X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning probe microscopy (SPM), wide angle X-ray diffraction (WAXRD), auger electron spectroscopy (AES), transmission electron microscope (TEM), etc. Generally, these techniques can only be applied under ex situ conditions and cannot fully recover the changes of catalysts in real chemical reactions. How to identify and track alterations of the catalysts, and thus provide further insight into the complex mechanisms behind them, has become a major research topic in this field. The application of in situ/operando characterization techniques enables real-time monitoring and analysis of dynamic changes. Therefore, researchers can obtain physical and/or chemical information during the reaction (e.g., morphology, chemical bonding, valence state, photocurrent distribution, surface potential variation, surface reconstruction), or even by the combination of these techniques as a suite (e.g., atomic force microscopy-based infrared spectroscopy (AFM-IR), or near-ambient-pressure STM/XPS combined system (NAP STM-XPS)) to correlate the various properties simultaneously, so as to further reveal the reaction mechanisms. In this review, we briefly describe the working principles of in situ/operando surface/interface characterization technologies (i.e., SPM and X-ray spectroscopy) and discuss the recent progress in monitoring relevant surface/interface changes during water splitting and CO2 reduction reactions (CO2RR). We hope that this review will provide our readers with some ideas and guidance about how these in situ/operando characterization techniques can help us investigate the changes in catalyst surfaces/interfaces, and further promote the development of (photo-)electrocatalytic surface and interface engineering.

Graphitic Carbon Nitride Nanosheets: Dual Functional Charge Selective Cathode/Anode Interface Layer for Polymer Solar Cells

Here, we report a systematic study proposing the utilization of graphitic carbon nitride nanosheets (g-C3N4 NS) thin film as a charge-selective cathode/anode interface layer for inverted as well as conventional polymer solar cells. The pristine ITO/g-C3N4 NS thin film-based electrodes were found to have a work function value of approximately 4.43 eV making them a potential candidate as a cathode interface layer for inverted devices. The work function value increased to ∼5.25 eV after UV-ozone treatment, befitting as an anode interface layer for conventional PSCs. The device with g-C3N4 NS as the interface layer exhibited similar short circuit current density for both inverted and conventional device architecture. This confirms the dual functionality of the g-C3N4 NS thin film in terms of charge-selectivity for efficient carrier collection. The overall power conversion efficiency for the two devices was approximately 5.60% and 6.05%, respectively. The UV-ozone treatment seems to alter the work function of g-C3N4 NS thin film by inducing surface dipoles due to the incorporation of oxygen atoms. This results in switching from electron-extracting behavior to hole extraction. Our results demonstrate the dual functional nature of g-C3N4 NS as an interface layer (CIL/AIL) for solution-processable polymer solar cells.

Origin and Regulation of Self‐Discharge in MXene Supercapacitors

AbstractMXene‐based supercapacitors are promising electrochemical energy‐storage devices due to their ultrahigh volumetric capacitance, high‐power characteristics, and excellent cyclability. However, they suffer from severe self‐discharging behavior while the underlying self‐discharging mechanism is still unclear. Here, the self‐discharge behavior of MXene‐based supercapacitors from surface electronic structure of MXenes is disclosed, and a novel method to mitigate it is proposed. A superficial engineering strategy based on bio‐thermal treatment is developed to effectively tailor surface electronic structure of Ti3C2Tx MXenes by eliminating hydroxyl terminations. With the evolution of surface electronic structure, as revealed by Kelvin probe force microscope and synchrotron radiation X‐ray absorption fine structure analysis, MXene‐based supercapacitors with common aqueous electrolytes show >20% decline in self‐discharge rate. This decline mechanism originates from the increased work function that induces higher zero‐charge potential after the removal of hydroxyl groups in MXenes. Meanwhile, the strengthened surface dipole leads to higher surface free energy between MXene and electrolytes. These two positive effects endow MXenes with weaker self‐discharge kinetics. Specifically, the activation‐controlled self‐discharge process is greatly suppressed. Illuminating the relevance between electronic structure and self‐discharge accompanying superficial engineering suppression strategy can guide to development of high‐performance energy storage devices.

An equivalent surface dipoles based electric field to magnetic field transformations for estimating radiated emissions

The new generation of communication devices operating in higher frequency bands is constantly pushing the complexity of field measurements for electromagnetic compliance and the design of field probes. These compliance are to be met for both electric and magnetic fields, of which if one set is available, the other can be estimated computationally using source reconstruction algorithms. The main objective of this paper is to refine the dipole-based source reconstruction algorithms using surface equivalence principle. Compared to earlier works, the algorithm is based on a novel way of placing a single dipole per point tangentially to a fictitious surface based on surface equivalence theorem instead of multiple orthogonal dipoles per point. The numerical results are compared with the state-of-art to demonstrate the effectiveness in obtaining accurate fields and locating the regions of the Device Under Test responsible for overshooting the interference limits.

Rapid, noncontact, sensitive, and semiquantitative characterization of buffered hydrogen-fluoride-treated silicon wafer surfaces by terahertz emission spectroscopy

Advances in modern semiconductor integrated circuits have always demanded faster and more sensitive analytical methods on a large-scale wafer. The surface of wafers is fundamentally essential to start building circuits, and quantitative measures of the surface potential, defects, contamination, passivation quality, and uniformity are subject to inspection. The present study provides a new approach to access those by means of terahertz (THz) emission spectroscopy. Upon femtosecond laser illumination, THz radiation, which is sensitive to the surface electric fields of the wafer, is generated. Here, we systematically research the THz emission properties of silicon surfaces under different surface conditions, such as the initial surface with a native oxide layer, a fluorine-terminated surface, and a hydrogen-terminated surface. Meanwhile, a strong doping concentration dependence of the THz emission amplitude from the silicon surface has been revealed in different surface conditions, which implies a semiquantitative connection between the THz emission and the surface band bending with the surface dipoles. Laser-induced THz emission spectroscopy is a promising method for evaluating local surface properties on a wafer scale.

Competition between oxygen and water molecules on SiO2/P-doped Si surface: The electrical dipole evolution on water/oxygen-adsorbed oxide surface

The performance of gas sensors is greatly influenced under humidity environment due to the dynamic correlation between water and oxygen molecules. In this work, we utilize the charge trapping characteristic to study surface gas adsorption and demonstrate the competitive adsorption behaviors between oxygen and water vapor from the perspective of electrical dipole by observing the evolution of second harmonic generation (SHG). By analyzing the trapping time constant of time-dependent SHG spectrum, we propose a dynamic model of the way water molecules weaken oxygen adsorption and develop water adsorption layer. Results show that the electric field created by charge trapping would further align the water molecules and enhance the SHG intensity. Here, a viewpoint of analyzing the net surface dipole change due to the adsorption–desorption on SiO2 surface to realize the energy preferable state is presented and provides a pathway for solving the water–sensor interaction problems in gas sensor.

3D simulations of strongly magnetized non-rotating supernovae: explosion dynamics and remnant properties

ABSTRACT We investigate the impact of strong initial magnetic fields in core-collapse supernovae of non-rotating progenitors by simulating the collapse and explosion of a $16.9\, \mathrm{M}_\odot$ star for a strong- and weak-field case assuming a twisted-torus field with initial central field strengths of ${\approx }10^{12}$ and ${\approx }10^{6}\, \mathrm{G}$. The strong-field model has been set up with a view to the fossil-field scenario for magnetar formation and emulates a pre-collapse field configuration that may occur in massive stars formed by a merger. This model undergoes shock revival already $100\, \mathrm{ms}$ after bounce and reaches an explosion energy of $9.3\times 10^{50}\, \mathrm{erg}$ at $310\, \mathrm{ms}$, in contrast to a more delayed and less energetic explosion in the weak-field model. The strong magnetic fields help trigger a neutrino-driven explosion early on, which results in a rapid rise and saturation of the explosion energy. Dynamically, the strong initial field leads to a fast build-up of magnetic fields in the gain region to 40 per cent of kinetic equipartition and also creates sizable pre-shock ram pressure perturbations that are known to be conducive to asymmetric shock expansion. For the strong-field model, we find an extrapolated neutron star kick of ${\approx }350\, \mathrm{km}\, \mathrm{s}^{-1}$, a spin period of ${\approx }70\, \mathrm{ms}$, and no spin-kick alignment. The dipole field strength of the proto-neutron star is $2\times 10^{14}\, \mathrm{G}$ by the end of the simulation with a declining trend. Surprisingly, the surface dipole field in the weak-field model is stronger, which argues against a straightforward connection between pre-collapse fields and the birth magnetic fields of neutron stars.

Intrinsic bitunable magnetism/polarity behavior in 2D Janus Cr2I3Y3 (Y = F, Cl, or Br) systems

Two-dimensional (2D) magnetoelectric (ME) materials with spontaneous magnetism and polarity have drawn a lot of research interest due to their potentially valuable physics and spintronic applications. One of the obstacles in the current study of 2D ME materials is to achieve flexible magnetism/polarity switches because of a fundamental contradiction between magnetism and polarity. In this work, we propose an intrinsic magnetism/polarity switchable 2D Janus structure Cr2I3Y3 (Y = F, Cl, or Br) with mechanical and thermodynamic stability. Janus Cr2I3Y3 monolayers show indirect-gap semiconductors with ferromagnetic ordering. Intrinsic vertical electric polarity (EP) in Janus Cr2I3Y3 structures is investigated by surface potential difference analysis and dipole calculations. We discover that there is an obvious ME coupling between magnetic anisotropy energy (MAE) and EP. MAE can be obviously tuned by rotating the orientations of spins within yz and zx vertical planes, and the easily distinguishable MAE on vertical planes has strong inherent relevance with vertical EP. Simultaneously, magnetic phase transition and magnetic moments can be manipulated by introducing vertical strain, which directly corresponds to the linear response of the EP strength. Our work provides a promising candidate for realizing the spintronic devices with multiple functionalities.

Machine Learning Assessment of the Impact of Global Warming on the Climate Drivers of Water Supply to Australia’s Northern Murray-Darling Basin

Droughts and long dry spells, interspersed with intense rainfall events, have been characteristic of the northern Murray-Darling Basin (NMDB), a major Australian agricultural region. The NMDB precipitation results from weather systems ranging from thunderstorms to larger scale events. The larger scale events exhibit high seasonal and annual rainfall variability. To detect attributes shaping the NMDB precipitation patterns, and hence net water inflows to the vast Darling River catchment area, numerous (45) possible attributes were assessed for their influence on rainfall trends. Four periods were assessed: annual, April–May (early cool-season), June–September (remaining cool-season), and October–March (warm-season). Linear and non-linear regression machine learning (ML) methods were used to identify the dominant attributes. We show the impact of climate drivers on the increasingly dry April–May months on annual precipitation and warmer temperatures since the early 1990s. The NMDB water supply was further reduced during 1992–2018 by the lack of compensating rainfall trends for the April–May decline. The identified attributes include ENSO, the Southern Annular Mode, the Indian Ocean Dipole, and both local and global sea surface temperatures. A key finding is the prominence of global warming as an attribute, both individually and in combination with other climate drivers.

High-Q out-of-plane Mie electric dipole surface lattice resonances in silicon metasurfaces.

Metasurfaces supporting surface lattice resonances (SLRs) with narrow linewidths and high quality factors have become an exciting platform for diverse applications. Here we numerically show, for the first time, that narrowband out-of-plane Mie electric dipole SLRs (ED-SLRs) can be excited together with the in-plane ED-SLRs and magnetic-dipole SLRs in periodic silicon disks under oblique incidence with TM polarization. Simulation results show that the out-of-plane ED-SLR can have four times larger quality factors than the in-plane one under the same excitation conditions, and can have distinct near-field distributions and dispersion relationships compared with the plasmonic counterpart in periodic metallic nanodisks. We further show that the out-of-plane ED-SLR can define a symmetry-protected bound state in the continuum (BIC) at normal incidence, which transits into a quasi-BIC when the excitation field symmetry is slightly broken by the small incidence angle. We expect this work will advance the engineering of Mie SLRs for applications in metasurface-based nanolasers, nonlinear optics, and optical sensing.

Preparation strategy for low-stress and uniform SiC-on-diamond wafer: A silicon nitride dielectric layer

Reducing the self-heating of SiC- and GaN/SiC-based high-powered devices by integrating diamond films offers promising performance enhancement of these devices. However, such a reduction strategy faces serious problems, such as diamond nucleation on SiC and stress accumulation greater than 10 GPa. In this work, a SiNx dielectric layer (∼50 nm) was coated onto the C polar face of a 4H–SiC wafer using microwave plasma chemical vapor deposition (MPCVD) to improve direct dense diamond nucleation and growth, significantly reduce the stress, and build Si–C(SiC)⋯Si⋯C(diamond) bond bridges. This SiNx thin layer, prepared by activating Si ions under Ar/N plasma during magnetron sputtering, gave rise to local Si3N4 crystal features and a low effective work function (EWF) for promoting surface dipoles with electronegative carbon-containing groups. In the H plasma environment during diamond growth, the local Si3N4 crystal was amorphized, and the N atoms escaped as a result of atomic H and the high temperature. At the same time, C atoms diffused into the SiNx and formed C–Si bonds (49.7% of the total C bonds) by replacing N–Si and Si–Si, thus increasing the direct nucleation density of the diamond grains. The diamond thin film grew rapidly and uniformly, with a grain size of approximately 2 μm in mixed orientation, and the stress of the 2-inch SiC-on-diamond wafer was extremely low (to ∼0.1–0.2 GPa). In comparison, partially connected diamond grains (>10 μm) on the bare SiC in the preferential (110) orientation resulted in a film with twin-grain features and significant stress, which was associated with the hexagonal lattice interface of 4H–SiC. These results are considered the material and surface/interface bases for actively controlling wafer fabrication and enhancing the heat dissipation of SiC and GaN/SiC electronics.

An equivalent surface dipoles based source reconstruction for estimating radiated emissions numerically from shielded PCBs

This work proposes an improved dipole-based source reconstruction method for PCB inside shielding enclosures. This method applies the surface equivalence principle in conjunction with Babinet’s principle for replacing the PCB and the slots of the shielding enclosures with the equivalent surface dipoles. The tangential electric fields are measured on a spherical surface around the shielded PCB structure in the propagating near field region. The unknown dipole coefficients are obtained by solving the inverse problem iteratively and then used to estimate the radiated emissions. The advantage of the proposed method is that it does not require field measurements in the reactive near field region and identifies the areas of the PCBs responsible for radiated emissions. The numerical results provide similar error performance to the discussed state of art techniques. The effect of noise in the estimation is also presented.

Sliding Schottky diode triboelectric nanogenerators with current output of 109 A/m2 by molecular engineering of Si(211) surfaces

Triboelectric nanogenerators (TENGs) are an autonomous and sustainable power-generation technology, seeking to harvest small vibrations into electricity. Here, by achieving molecular control of oxide-free Si crystals and using conductive atomic force microscopy, we address key open questions and use this knowledge to demonstrate zero-applied-bias current densities as high as 109 A/m2. Key to achieve this output, is to use a proton-exchangeable organic monolayer that simultaneously introduces a sufficiently high density of surface states (assessed as changes to carrier recombination velocities) coupled to a strong surface dipole in the form of a surface alkoxide anion (Si–monolayer–O−). We also demonstrate that the DC output of a Schottky diode TENG does not track the energy released as friction. This removes the complexity of controlling an unavoidable stick–slip motion, bypassing the requirement of aligning sliding motion and substrate topographical features. We reveal that there is no apparent correlation between the current of a static (biased) junction and the tribocurrent of the same junction when under motion and unbiased.

PSR J0952−0607: The Fastest and Heaviest Known Galactic Neutron Star

We describe Keck-telescope spectrophotometry and imaging of the companion of the “black widow” pulsar PSR J0952−0607, the fastest known spinning neutron star (NS) in the disk of the Milky Way. The companion is very faint at minimum brightness, presenting observational challenges, but we have measured multicolor light curves and obtained radial velocities over the illuminated “day” half of the orbit. The model fits indicate system inclination i = 59.°8 ± 1.°9 and a pulsar mass M NS = 2.35 ± 0.17 M ⊙, the largest well-measured mass found to date. Modeling uncertainties are small, since the heating is not extreme; the companion lies well within its Roche lobe and a simple direct-heating model provides the best fit. If the NS started at a typical pulsar birth mass, nearly 1 M ⊙ has been accreted; this may be connected with the especially low intrinsic dipole surface field, estimated at 6 × 107 G. Joined with reanalysis of other black widow and redback pulsars, we find that the minimum value for the maximum NS mass is (2.09 M ⊙) at 1σ (3σ) confidence. This is ∼ 0.15 M ⊙ heavier than the lower limit on implied by the white dwarf–pulsar binaries measured via radio Shapiro-delay techniques.

Characterization of apex structures of scanning tunneling microscope tips with field emission resonance energies

The apex structure of a scanning tunneling microscope (STM) tip consists of a base with radius of tens of nanometers and protrusion with atomic-scale sharpness. We characterized the tip base radius and sharpness on the basis of field emission resonance (FER) energies. We derived two quantities from the first- through sixth-order FER energies, which were related to tip sharpness and base radius. The base radius can remain unchanged while the sharpness varied, and the tips can have identical sharpness but different base radii. The base radius can significantly affect the peak intensities of FER, which corresponds to the mean lifetime of FER electrons, on a Ag(100) surface but not on those of FER on a Ag(111) surface. This difference results from the surface dipole layer and quantum trapping effect (QTE) on the Ag(100) surface which are greater than those on the Ag(111) surface.

Polarization-independent resonant lattice Kerker effect in phase-change metasurface

Resonant lattice Kerker effect in periodic resonators is one of the most interesting generalizations of the Kerker effect that relates to various fascinating functionalities such as scattering management and Huygens metasurfaces. However, so far this effect has been shown to be sensitive to the incident polarization, restricting its applications. Here, we report, for the first time, polarization-independent resonant lattice Kerker effect in metasurfaces composed of periodic Ge2Se2Te5 (GST) disks. For such a metasurface of square lattice, the spectrally overlap of the electric dipole and magnetic dipole surface lattice resonances can be realized by choosing an appropriate GST crystalline fraction regardless of the incident polarization. The operation wavelength and the required GST crystalline fraction can be conveniently tuned over large ranges since these parameters scale linearly with the disk size and the lattice period, greatly facilitating the design. Making use of the obtained resonant lattice Kerker effect, we realize a reconfigurable and polarization-independent lattice Huygens’ metasurface with a dynamic phase modulation of close to 2π and high transmittance. This work will advance the engineering of the resonant lattice Kerker effect and promote its applications in phase modulation and wavefront control.