Formation Of Dipole Layer Research Articles

Understanding the impact of interface reaction on dipole strength at MgO/SiO2 and Y2O3/SiO2 interfaces

We investigated the variation in negative dipole strength at MgO/SiO2 and Y2O3/SiO2 interfaces when changing the annealing temperature. The reduction in negative dipole strength was observed after silicate formation. We found that the reactivity at the interfaces is one of the dominant factors of dipole layer formation at high-k/SiO2 interfaces for chemically reactive systems where cation migration should be taken into account as the driving force of the dipole layer formation.

Positive and negative dipole layer formation at high-k/SiO2 interfaces simulated by classical molecular dynamics

We show the electric dipole layer formed at a high-k/SiO2 interface can be explained by the imbalance between the migration of oxygen ions and metal cations across the high-k/SiO2 interface. Classical molecular dynamics (MD) simulations are performed for Al2O3/SiO2, MgO/SiO2, and SrO/SiO2 interfaces. The simulations qualitatively reproduce the experimentally observed flatband voltage (VFB) shifts of these systems. In the case of the Al2O3/SiO2 interface, a dipole layer is formed by the migration of oxygen ions from the Al2O3 side to the SiO2 side. By way of contrast, opposite dipole moments appear at the MgO/SiO2 and SrO/SiO2 interfaces, because of a preferential migration of metal cations from the high-k oxide toward the SiO2 layer in the course of the formation of a stable silicate phase. These results indicate that the migrations of both oxygen ions and metal cations are responsible for the formation of the dipole layer in high-k/SiO2 interfaces.

The investigation of chemical interaction and energy level alignment at Bepp2/Fe65Co35 interface

In a bilayer system of Bepp2–FeCo, the element content variation and chemical states of the Bepp2–FeCo interface were investigated using X-ray and ultraviolet photoelectron spectroscopy with Ar ion etching. Chemical reaction was observed for Co and Fe with Bepp2 at the interface. Ultraviolet photoelectron spectroscopy results showed a downward energy shift of −1.0eV at the interface. This behavior was attributed to the formation of an interface dipole layer. The hole injection barrier ΦpB was 2.0eV, and the electronic injection barrier ΦnB was 0.6eV. Moreover, only as the FeCo thickness is less than 3nm, an uniaxial anisotropy can be induced on the organic layer with the investigation of magnetic optical Kerr effect, this can be used as a multi-function devices.

Time-resolved photoemission study of the electronic structure and dynamics of chemisorbed alkali atoms on Ru(0001)

We investigate the electronic structure and photoexcitation dynamics of alkali atoms (Rb and Cs) chemisorbed on transition-metal Ru(0001) single-crystal surface by angle- and time-resolved multiphoton photoemission. Three- and four-photon photoemission (3PP and 4PP) spectroscopic features due to the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ resonances arising from the $ns$ and $np$ states of free alkali atoms are observed from $\ensuremath{\sim}2$ eV below the vacuum level in the zero-coverage limit. As the alkali coverage is increased to a maximum of 0.02 monolayers, the resonances are stabilized by formation of a surface dipole layer, but in contrast to alkali chemisorption on noble metals, both resonances form dispersive bands with nearly free-electron mass. Density functional theory calculations attribute the band formation to substrate-mediated interaction involving hybridization with the unoccupied $d$ bands of the substrate. Time-resolved measurements quantify the phase and population relaxation times in the three-photon photoemission (3PP) process via the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ resonances. Differences between alkali-atom chemisorption on noble and transition metals are discussed.

A Current-Induced Channel Organic Thin-Film Transistor

An organic thin-film transistor structure is proposed in which a channel is created through formation of an electric dipole layer at the semiconductor–insulator interface. The dipole is composed of mobile carriers, electrons, and holes and results from the modification of gate insulator to allow controlled amounts of gate current to flow in the ON-state. 2-D device simulations are used to show that the proposed structure offers the potential for obtaining significantly higher channel charge and current without requiring higher voltages. It is also shown that these advantages come while maintaining OFF-current and unity gain frequency comparable with conventional transistors.

Investigation of optimal hydrogen sensing performance in semiconducting carbon nanotube network transistors with palladium electrodes

The work function of palladium (Pd) is known to be sensitive to hydrogen (H2) via the formation of a surface dipole layer or Pd hydride. One approach to detect such a change in the work function is based on the formation of a Schottky barrier between Pd and a semiconductor. Here, we demonstrate a H2 sensor operable at room temperature by assembling solution-processed, pre-separated semiconducting single-walled carbon nanotube (SWNT) network bridged by Pd source/drain (S/D) electrodes in a configuration of field-effect transistors (FETs) with a local back-gate electrode. To begin with, we observed that the H2 response of the fabricated SWNT FETs can be enhanced in the linear operating regime, where the change in the work function of the Pd S/D electrodes by H2 can be effectively detected. We also explore the H2 responses in various SWNT FETs with different physical dimensions to optimize the sensing performance.

Effects of PTFE and PEDOT:PSS Double Buffer Layers on Flexible Polymer Solar Cells

We investigate the effects of polytetrafluoroethylene (PTFE) on organic photovoltaic (OPV) devices with structure of ITO/PTFE/PEDOT:PSS/P3HT:PCBM/PTFE/Al by inserting thermally evaporated PTFE films between indium-tin-oxide (ITO) and PEDOT:PSS layers, and also between P3HT:PCBM and Al, respectively. Significant improvement in terms of open-circuit voltage, Fill factor (FF), and thereby in its commensurate power conversion efficiency is achieved compared to devices with PEDOT:PSS and LiF buffer layers. The OPVs performance enhancement is attributed to the formation of an artificial dipole layer resulting from the rich, negatively charged fluorine that facilitates the charge extraction process.

Alcohol/water-soluble porphyrins as cathode interlayers in high-performance polymer solar cells

Three alcohol/water-soluble porphyrins, Zn-TPyPMeI: zinc(II) meso -tetra( N -methyl-4-pyridyl) porphyrin tetra-iodide, Zn- TPyPAdBr: zinc(II) meso -tetra[1-(1-adamantylmethyl ketone)-4-pyridyl] porphyrin tetra-bromide and MnCl-TPyPAdBr: man- ganese(III) meso -tetra[1-(1-adamantylmethyl ketone)-4-pyridyl] porphyrin tetra-bromide were employed as cathode interlayers to fabricate polymer solar cells (PSCs). The PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) and PCDTBT (poly[ N -9"- hepta-decanyl-2,7-carbazole- alt -5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)])-blend films were used as active layers in polymer solar cells (PSCs). The PSCs with alcohol/water-soluble porphyrins interlayer showed obviously higher power conversion efficiency (PCE) than those without interlayers. The highest PCE, 6.86%, was achieved for the device with MnCl- TPyPAdBr as an interlayer. Ultraviolet photoemission spectroscopic (UPS), carrier mobility, atomic force microscopy (AFM) and contact angle ( θ ) characterizations demonstrated that the porphyrin molecules can result in the formation of interfacial dipole layer between active layer and cathode. The interfacial dipole layer can obviously improve the open-circuit voltage ( V oc) and charge extraction, and sequentially lead to the increase of PCE.

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5eV and thermionic electron emission (TE) has been detected at temperatures less than 500°C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~5×10−5A/cm2 at 500°C. In contrast, the initial TE current density of a film aged for 18months was ~1.8×10−9A/cm2 at 500°C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3×10−4mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2h of exposure with the sample at 500°C, the surface exhibited a stable emission current density of ~2.3×10−6A/cm2 (an increase by a factor of ~1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5×10−8mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400°C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~200meV, and the TE intensity increased by a factor of ~100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.

Versatile electron-collecting interfacial layer by in situ growth of silver nanoparticles in nonconjugated polyelectrolyte aqueous solution for polymer solar cells.

Novel PEIE-Ag composites by in situ growth of silver nanoparticles in poly(ethylenimine)-ethoxylated (PEIE) aqueous solution are explored as an efficient interfacial layer for improving inverted polymer solar cells (PSCs) performance. The hybrid PEIE-Ag interfacial material is simple to fabricate only via ultraviolet irradiation with good water-solubility and unique film formation. The generated Ag nanoparticles can anchor in the PEIE polymer chains to form a conductive continuous interpenetrating network structure. Combining of the advantages of PEIE and Ag nanoparticles, the PEIE-Ag shows enhanced charge transport, electron selective and collection, and improved light-harvesting, mainly due to the surface plasmon resonance effect, better energy alignment induced by the formation of ideal dipole layer, as well as the improved conductivity. These distinguished interfacial properties result in the power conversion efficiency of inverted PSCs based on poly[4,8-bis(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) photoactive layer substantially improved up to 7.66% from 6.11%. Moreover, the device performance is insensitively dependent on the thickness of the PEIE-Ag interfacial layer, broadening the thicknesses selection window for interfacial materials. These results demonstrate that PEIE-Ag is a potential interfacial material compatible with roll-to-roll techniques and suitable for printed electronic devices.

(Invited) Molecular Dynamics Simulation of Dipole Layer Formation at High-k/SiO2 Interfaces

Electric dipole layer formation at high-k/SiO2 interface is reproduced by classical molecular dynamics simulation based on a simple two-body rigid ion model (1). The dipole layer was spontaneously formed by the migration of oxygen ions across the high-k/SiO2 interface. In case of Al2O3/SiO2, a part of oxygen ions of Al2O3 penetrated into the SiO2 side, resulting in the formation of a built-in potential of about 0.5 V. The opposite migration of oxygen ions, from SiO2 side to high-k oxide side, is also reproduced by using different potential parameters of ionic radius and effective charge. The simulation result suggests that the dipole is not merely formed by the oxygen density difference. Rather, oxygen ions are driven by some interatomic forces at the interface. We discuss the origin of the driving force of the oxygen migration in terms of the multipole moments around cations in the oxides.

Molecular Dynamics Simulation of Dipole Layer Formation at High-k/SiO2 Interfaces

Implementation of high-k/metal gate stack poses a challenge in controlling the threshold voltage (VTH) of MOSFET devices. Recent studies have shown that an electric dipole layer formed at high-k/SiO2 interface is responsible for anomalous VTH shifts. The origin of the dipole layer has been explained by several mechanisms, such as the areal density difference of O atoms [1], electronegativity difference [2], and contact induced gap state [3]. Among these models, the first model proposed by Kita and Toriumi [1] is noteworthy because of its simplicity and potential to be applicable to wide variety of high-k materials. The model states that the interface dipole is formed by the movement of negatively charged O ions from higher oxygen-density side to a lower one.Recently, we performed molecular dynamics (MD) simulations of high-k/SiO2 interfaces and succeeded in reproducing the O atom migration across the Al2O3/SiO2 interface [4]. The O ion moves one-sidedly from Al2O3 side to SiO2 one, but opposite movement never occurs. The simulation result implies that the O ion migration is induced by some drift forces, but not by a simple diffusion due to the O density gradient across the high-k/SiO2. In this paper, we discuss the driving force of the O ion migration across the high-k/SiO2 in terms of the multipole moment around oxide cation.The MD simulation was performed by a commercial simulation package SCIGRESS ME from Fujitsu Ltd. Al2O3/SiO2 interface model was constructed by sandwiching amorphous Al2O3 blocks in between amorphous SiO2 blocks as shown in Fig.1. The hetero-oxide structure was annealed by the isothermal-isobaric MD calculation for 100 ps, thermostated at 1000 K by a velocity scaling with keeping the pressure at atmospheric pressure. The Born-Mayer-Huggins potential is employed in the MD simulation.Figure 2(a) shows a charge density profile across the SiO2/Al2O3 interface. Electric dipole layer appears at the interface, and it is directed from the SiO2 side to Al2O3 side. Figure 2(b) shows the electric potential profile around the interface, which is obtained by solving one-dimensional Poisson’s equation. The built-in potential at SiO2/Al2O3 interface is about 0.5 V, which matches the experimental value of the flat-band voltage shift [1]. Thus the direction and magnitude of the dipole moment in our model agrees with the experimental result. Figure 3 shows the magnified image of the SiO2/Al2O3 interface, in which the O ions originated from Al2O3 are colored by green. It can be clearly seen that the O ion moves one-sidedly from Al2O3 side to SiO2 one.The simulation result indicates that the O ion is driven by some drift forces, but not by a simple diffusion due to the density difference at the interface. It is inferred that the O ions in Al2O3 are attracted to the strong positive charges of Si ions. The positive charges of Si ions must be canceled by the negative charges of O ions in SiO2, but at very close distance, there is an appreciable octupole moment around the Si ion which is located at the center of SiO4 tetrahedron (see Fig.4). On the other hand, there is an hexadecapole moment around the Al ion which located at the center of AlO6octahedron. The electrostatic field induced by the octupole moment around the Si ion is stronger than that of the hexadecapole moment around the Al ion, and thus O ions in the vicinity of the interface can be attracted to Si ions in specific directions. Opposite dipole formation at SiO2/Y2O3interface can also be explained by the difference in the multipole moments around Si and Y ions [4].The multipole moment around an oxide cation is relevant to its electronegativity of the O density of the oxide. Therefore, our “multipole moment induced O migration model” can be regarded as the unified model of the O density [1] model and the electronegativity model [2]. Acknowledgement This work is supported by JST-CREST, and a Grant-in-Aid for Scientific Research (B) from the MEXT Japan.

Photophysics of Voltage Increase by Photoinduced Dipole Layers in Sensitized Solar Cells.

Significant overpotentials between the sensitizer and both the electron and hole conductors hamper the performance of sensitized solar cells, leading to a reduced photovoltage. We show that by using properly designed type-II quantum dots (QDs) between the sensitizer and the hole conductor in thin absorber cells, it is possible to increase the open circuit voltage (Voc) by more than 100 mV. This increase is due to the formation of a photoinduced dipole (PID) layer. Photogenerated holes in the type-II QDs are retained in the core for a relatively long time, allowing for the accumulation of a positively charged layer. Negative charges are, in turn, injected and accumulated in the TiO2 anode, creating a dipole moment, which negatively shifts the TiO2 conduction band relative to the electrolyte. We study this phenomenon using a unique TiO2/CdSe/(ZnSe:Te/CdS)/polysulfide system, where the formation of a PID depends on the color of the illumination. The PID concept thus introduces a new design strategy, where the operating parameters of the solar cell can be manipulated separately.

Molecular dynamics study on the formation of dipole layer at high-k/SiO2 interfaces

We show that electric dipole layer at Al2O3/SiO2 interface is reproduced by classical molecular dynamics simulation with a simple two-body rigid ion model. The dipole layer was spontaneously formed by the migration of oxygen ions from Al2O3 side to SiO2 side. Built-in potential at the Al2O3/SiO2 is estimated to about 0.35 V, which roughly compares with the experimental value of the flat band voltage shift. Contrary, no significant dipole layer appeared at Y2O3/SiO2 interface. The simulation results are explained in terms of the difference in the magnitude of multipole moments around cations of these oxides.

Influence of gridded gate structure on gas sensing behavior of hydrogen

A gridded Pt/SiO2/Si MOS sensor for hydrogen detection has been fabricated on p-type ⟨100⟩ Si wafer having resistivity (1–6 Ω cm). The SiO2 and Platinum (Pt) gate thickness were kept about 10 nm and 35 nm. The performance of Pt gate MOS sensor was evaluated through C-V characteristics (capacitance vs voltage) upon exposure to H2 (250 ppm–4000ppm) at different frequencies (25 kHz and 50 kHz) in a closed chamber at air ambient atmosphere. The capacitance of the sensor decreases with increase in frequency as well as H2 gas concentration. The flat band voltage characteristics have been evaluated at different frequencies and concentrations. It decreases as the frequency and concentration of gas both increases. The maximum flat band voltage change was observed −0.6 V at 25 kHz. The sensor exhibits better sensitivity (∼88%) at low frequency (25 kHz). The high response of sensor is attributed to the side wall diffusion, increase in surface area caused by inner side wall and increase in porosity, increase in fixed surface state density, spill-over mechanism and change in interface state density on exposure of gas along with the formation of dipole layer.

Fabrication and characterization of gridded Pt/SiO2/Si MOS structure for hydrogen and hydrogen sulphide sensing

A gridded gate Pt/SiO2/Si MOS capacitor has been fabricated for detection of Hydrogen (H2) and Hydrogen Sulphide (H2S) gases. The MOS device was fabricated on P-type Si <100> (1–6 Ω cm) wafer with thermal oxide layer of thickness about 100 Å, whereas, Platinum (Pt) gate of ∼350 Å was deposited by thermal evaporation technique. The C–V (capacitance vs voltage) and G–V (conductance vs voltage) measurements have been performed for the evaluation of gas sensing behavior of fabricated MOS capacitor structure in H2 (250–4000 ppm) and H2S (1000–6000 ppm) gases at both room and 120 °C temperatures, in a closed chamber in air atmosphere. It has been observed that the value of capacitance decreases with increase in gas concentration. The fabricated MOS capacitor sensor has shown better sensitivity towards H2 (88.6%) at room temperature (∼25 °C) as compared to (∼45%) at 120 °C. Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) studies have revealed the porous nature of the deposited metal film. The side wall diffusion, spillover of Hydrogen into oxide layer, increase in fixed oxide charge density, increase in surface area caused by gridded structure, the formation of dipole layer and change in interface state density on gas exposure, may be the mechanisms of gas sensing for improved sensitivity of the fabricated MOS device.

Application of Biuret, Dicyandiamide, or Urea as a Cathode Buffer Layer toward the Efficiency Enhancement of Polymer Solar Cells

Three amino-containing small-molecule organic materials-biuret, dicyandiamide (DCDA), and urea-were successfully applied as novel cathode buffer layers (CBLs) in P3HT:PCBM bulk heterojunction polymer solar cells (BHJ-PSCs) for the first time, resulting in obvious efficiency enhancement. Under the optimized condition, the power conversion efficiencies (PCEs) of the CBL-incorporated BHJ-PSC devices are 3.84%, 4.25%, and 4.39% for biuret, DCDA, and urea, which are enhanced by ∼15%, ∼27%, and ∼31%, respectively, compared to the reference poly(3-hexylthiophene-2,5-diyl) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ-PSC device without any CBL. The efficiency enhancement is primarily attributed to the increases of both short-circuit current density (Jsc) and fill factor (FF), for which the enhancement ratio is found to be sensitively dependent on the molecular structure of small-molecule organic materials. The surface morphologies and surface potential changes of the CBL-incorporated P3HT:PCBM photoactive layers were studied by atomic force microscopy and scanning Kelvin probe microscopy, respectively, suggesting the formation of an interfacial dipole layer between the photoactive layer and Al cathode, which may decrease the energy level offset between the work function of Al and the lowest unoccipoed molecular orbital level (LUMO) of the PCBM acceptor and consequently facilitate electron extraction by the Al cathode. The difference in the enhancement effect of biuret, DCDA, and urea is due to their difference on the work function matching with P3HT:PCBM. Besides, the coordination interaction between the lone-pair electrons on the N atoms of the amino (-NH2) group and the Al atoms may prohibit interaction between Al and the thiophene rings of P3HT, contributing to the efficiency enhancement of the CBL-incorporated devices as well. In this sense, the different CBL performance of biuret, DCDA, and urea is also proposed to partially originate from the differences on their chemical structure, specifically the number of amino groups.

Nitrocatechol/ZnO Interface: The Role of Dipole in a Dye/Metal-Oxide Model System

The electronic properties of a prototype system suitable for dye-sensitized solar cell applications are investigated both experimentally and theoretically by means of electron spectroscopies (high-resolution electron energy loss spectroscopy, HREELS, and ultraviolet and X-ray photoemission spectroscopies, UPS and XPS) and first-principles density functional theory (DFT)-based calculations. The comparison of HREELS and UPS data with the DFT results allows the microscopic description of electronic structure modifications upon interface formation, and provides a quantitative evaluation of the ionization energy and electron affinity changes induced by functionalization: these variations can be associated to the electric dipole of the functional species and, thus, to the formation of an interface dipole layer.

Strong Schottky barrier reduction at Au-catalyst/GaAs-nanowire interfaces by electric dipole formation and Fermi-level unpinning

Nanoscale contacts between metals and semiconductors are critical for further downscaling of electronic and optoelectronic devices. However, realizing nanocontacts poses significant challenges since conventional approaches to achieve ohmic contacts through Schottky barrier suppression are often inadequate. Here we report the realization and characterization of low n-type Schottky barriers (~0.35 eV) formed at epitaxial contacts between Au-In alloy catalytic particles and GaAs-nanowires. In comparison to previous studies, our detailed characterization, employing selective electrical contacts defined by high-precision electron beam lithography, reveals the barrier to occur directly and solely at the abrupt interface between the catalyst and nanowire. We attribute this lowest-to-date-reported Schottky barrier to a reduced density of pinning states (~10(17) m(-2)) and the formation of an electric dipole layer at the epitaxial contacts. The insight into the physical mechanisms behind the observed low-energy Schottky barrier may guide future efforts to engineer abrupt nanoscale electrical contacts with tailored electrical properties.

8.9% Single‐Stack Inverted Polymer Solar Cells with Electron‐Rich Polymer Nanolayer‐Modified Inorganic Electron‐Collecting Buffer Layers

Enhanced power conversion efficiency (PCE) is reported in inverted polymer solar cells when an electron‐rich polymer nanolayer (poly(ethyleneimine) (PEI)) is placed on the surface of an electron‐collecting buffer layer (ZnO). The active layer is made with bulk heterojunction films of poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM). The thickness of the PEI nanolayer is controlled to be 2 nm to minimize its insulating effect, which is confirmed by X‐ray photoelectron spectroscopy and optical absorption measurements. The Kelvin probe and ultraviolet photoelectron spectroscopy measurements demonstrate that the enhanced PCE by introducing the PEI nanolayer is attributed to the lowered conduction band energy of the ZnO layer via the formation of an interfacial dipole layer at the interfaces between the ZnO layer and the PEI nanolayer. The PEI nanolayer also improves the surface roughness of the ZnO layer so that the device series resistance can be noticeably decreased. As a result, all solar cell parameters including short circuit current density, open circuit voltage, fill factor, and shunt resistance are improved, leading to the PCE increase up to ≈8.9%, which is close to the best PCE reported using conjugated polymer electrolyte films.