Large Work Function Difference Research Articles

Modification of the Highly Conductive PEDOT:PSS Layer for Use in Silver Nanogrid Electrodes for Flexible Inverted Polymer Solar Cells.

Silver nanogrid based flexible transparent electrode is recognized as the most promising alternative to ITO electrode for organic electronics, owing to its low production cost and excellent flexibility. Typically, a highly conductive thin film coating layer, such as highly conductive PEDOT:PSS (HC-PEDOT:PSS) is usually deposited onto the Ag-grid electrode to smooth the surface and to minimize the sheet resistance. In this paper, we found that inverted flexible polymer solar cells with structure of Ag-grid/HC-PEDOT:PSS/ZnO/photoactive layer/MoO3/Al generally exhibits strong S-shaped J-V curves, which could be eliminated by light-soaking treatment. Kelvin probe force microscope (KPFM) measurement proved that a large work function (WF) difference (0.70 eV) between HC-PEDOT:PSS and ZnO is the main reason for the formation of S-shape. White light soaking of the Ag-grid/HC-PEDOT:PSS gradually decreased the WF of HC-PEDOT:PSS from 5.10 to 4.60 eV, leading to a reduced WF difference between HC-PEDOT:PSS and ZnO from 0.70 to 0.38 eV. Such a WF difference decrease was believed to be the working mechanism for the light-soaking effect in this flexible device. Based on this finding, the HC-PEDOT:PSS solution was then modified by doping with polyethylenimine (PEI) and aqueous ammonia. The modified PEDOT:PSS film is characteristic of adjusting WF through varying PEI doping concentrations. By using such a modified PEDOT:PSS layer, light-soaking-free flexible inverted polymer solar cell with a power conversion efficiency of 6.58% was achieved for PTB7-Th:PC71BM cells. The current work provides a useful guideline for interfacial modification for Ag-grid based flexible electrode.

High performance asymmetric supercapacitor twisted from carbon fiber/MnO2 and carbon fiber/MoO3

Fiber-shaped supercapacitors are promising energy storage devices for future flexible and portable electronics. In this study, we report a fiber-shaped asymmetric supercapacitor (ASC) device made with the metal oxides having a large work function difference, directly grown on a flexible and conductive carbon fiber (CF) substrate. Specifically, carbon fiber/MnO2 (CF/MnO2) and carbon fiber/MoO3 (CF/MoO3) were fabricated using a simple electrodeposition method. The all-solid-state fiber-shaped ASC device was then assembled with CF/MnO2 as the positive electrode and CF/MoO3 as the negative electrode. The large work function difference between the metal oxides and the high conductivity of the CF substrate provided the ASC device with remarkable performance. In particular, it exhibited capacitance of 4.86 mF cm−2 and a wide operating voltage window of 2.0 V, which resulted in an excellent energy density of 2.70 μWh cm−2 and a power density of 0.53 mW cm−2. Also, it readily tolerated 3000 cycles of electrochemical testing and extreme mechanical deformation. Consequently, the outstanding performance and stability of the fiber-shaped ASC device shows great potential for future energy storage systems.

A Numerical Study on Graphene Nanoribbon Heterojunction Dual-Material Gate Tunnel FET

In this letter, a tunnel FET (TFET) combining both graphene nanoribbon (GNR) heterojunction (HJ) and gate work function (WF) engineering is studied with the numerical simulation. The lowest subthreshold swing is smaller than 15 mV/decade and the ON state current ( $I_{{\mathrm{\scriptscriptstyle ON}}}$ ) reaches to $1.7~\mu \text{A}$ with device width smaller than 3 nm. The tunnel width is well reduced with the dual-material gate (DMG) structure boosting the ON current. The channel with a wide energy gap ( $E_{g}$ ) GNR can effectively reduce the leakage current at OFF state when its length is 13 nm. It is found out that large WF difference between the gate and the GNR not only effectively enhances the tunneling effect but also leads to an electron quantum well hindering the regulation capability of the gate. This effect can be well reduced with large WF gate material on the top of the HJ region. The numerical simulation reveals that the GNR HJ-DMG TFET is a good candidate for low power applications.

Self-powered triboelectric aptasensor for label-free highly specific thrombin detection

An aptamer-based triboelectric biosensor is developed for a highly specific, label-free and self-powered detection of thrombin. For the first time, intermolecular recognition interactions are used to develop a selective nanosensor based on triboelectric effect. Positively charged Au nanoparticles (Au NPs+) with large difference in triboelectric polarity and work function are assembled onto Al film to increase the electrical output of triboelectric nanogenerators (TENG). Modification of anti-thrombin aptamers on the Au NPs+-assembled TENG affords the triboelectric nanosensor highly selective toward thrombin, even in clinical samples because of specific binding affinity between aptamers and thrombin unlike random DNA-modified TENGs with undetectable response. A 0.41nM limit of detection is achieved, which is directly demonstrated by the number of commercial LED lights without any supporting equipment such as power source and electrometer. Our study demonstrates an innovative and unique approach toward the self-powered and label-free detection of thrombin for rapid and simple in-field analysis.

Manifestation of axion electrodynamics through magnetic ordering on edges of a topological insulator

Because topological surface states of a single-crystal topological insulator can exist on all surfaces with different crystal orientations enclosing the crystal, mutual interactions among those states contiguous to each other through edges can lead to unique phenomena inconceivable in normal insulators. Here we show, based on a first-principles approach, that the difference in the work function between adjacent surfaces with different crystal-face orientations generates a built-in electric field around facet edges of a prototypical topological insulator such as Bi2Se3. Owing to the topological magnetoelectric coupling for a given broken time-reversal symmetry in the crystal, the electric field, in turn, forces effective magnetic dipoles to accumulate along the edges, realizing the facet-edge magnetic ordering. We demonstrate that the predicted magnetic ordering is in fact a manifestation of the axion electrodynamics in real solids.

Evolution of the SrTiO3-MoO3 Interface Electronic Structure: An in Situ Photoelectron Spectroscopy Study.

Modifying the surface energetics, particularly the work function of advanced materials, is of critical importance for a wide range of surface- and interface-based devices. In this work, using in situ photoelectron spectroscopy, we investigated the evolution of electronic structure at the SrTiO3 surface during the growth of ultra-thin MoO3 layers. Because of the large work function difference between SrTiO3 and MoO3, the energy band alignment on the SrTiO3 surface is significantly modified. The charge transfer and dipole formation at the SrTiO3-MoO3 interface leads to a large modulation of work function and to apparent doping in SrTiO3. The measured evolutions of electronic structure and upward band bending suggest that the growth of ultra-thin MoO3 layers is a powerful tool with which to modulate the surface energetics of SrTiO3, and this surface engineering approach could be generalized to other functional oxides.

Large work function difference driven electron transfer from electrides to single-walled carbon nanotubes.

A difference in work function plays a key role in charge transfer between two materials. Inorganic electrides provide a unique opportunity for electron transfer since interstitial anionic electrons result in a very low work function of 2.4-2.6 eV. Here we investigated charge transfer between two different types of electrides, [Ca(2)N](+)·e(-) and [Ca(24)Al(28)O(64)](4+)·4e(-), and single-walled carbon nanotubes (SWNTs) with a work function of 4.73-5.05 eV. [Ca(2)N](+) · e(-) with open 2-dimensional electron layers was more effective in donating electrons to SWNTs than closed cage structured [Ca(24)Al(28)O(64)](4+) · 4e(-) due to the higher electron concentration (1.3 × 10(22) cm(-3)) and mobility (∼ 200 cm(2) V(-1) s(-1) at RT). A non-covalent conjugation enhanced near-infrared fluorescence of SWNTs as high as 52%. The field emission current density of electride-SWNT-silver paste dramatically increased by a factor of 46,000 (14.8 mA cm(-2)) at 2 V μm(-1) (3.5 wt% [Ca(2)N](+) · e(-)) with a turn-on voltage of 0.85 V μm(-1).

Improved electrode performance in gallium nitride LEDs

Light-emitting diodes based on III-nitride semiconductors have made their way into applications such as backlighting for displays, and solid-state white lighting. High-power LEDs with efficacies larger than conventional fluorescent lamps are now commercially available, bringing the world closer toward the realization of energy-efficient solid-state lighting.1, 2 However, despite significant breakthroughs in light extraction and internal quantum efficiencies, the formation of low resistance, transparent ohmic contact to the highly resistive p-GaN (gallium nitride) layer continues to be a challenge.3 Modern commercial LEDs employ either translucent nickel-gold or indium tin oxide (ITO) as a transparent conductive electrode. Nevertheless, the low transparency of a nickel-gold stack—as well as increasing concerns over the soaring prices and future demand for indium—have initiated a search for an alternative material.4 In addition, next-generation LEDs require transparent conductive electrodes to be flexible (unlike ITO, which is brittle), cheap, and compatible with large-scale manufacturing methods. Graphene, the most recently discovered form of carbon, offers exceptional characteristics such as high transparency, mechanical flexibility, and superior thermal and electrical conductivities.5, 6 Despite its comparatively high sheet resistance (Rs/, recent work has shown that graphene can be applied to the p-GaN layer in GaN-based LEDs as a transparent conductive electrode in place of ITO.7 However, because of the large difference in work function (ˆ), integrating pristine graphene directly with p-GaN forms a Schottky barrier height (SBH, q®B ) at the interface. A large SBH is undesirable because it can create low contact resistance, which leads to strong current crowding and a high operating voltage. Figure 1.Ultraviolet photoelectron spectra of pristine and doped—with 10 and 20mM solutions of gold (III) chloride (AuCl3)—multilayer graphene (MLG) films. The inset bar diagram shows the variation in work function versus AuCl3 concentration. a.u.: Arbitrary units.

Assessment of Transparent Conducting Zinc Oxide as a Tunneling Contact to p-GaN

ABSTRACTUse of aluminum alloyed zinc oxide (AZO) as a transparent contact to p-GaN has received significant attention for GaN/InGaN light emitting diodes applications. Reports show that AZO as deposited on p-GaN forms a Schottky contact given the large work-function difference between AZO and p-GaN [1]. However, utilization of a thin nickel layer inserted between the AZO and p-GaN can result in an ohmic contact [2], given that Ni forms an ohmic contact to p-GaN upon annealing. Here, we undertake simulation studies of this AZO/Ni interface as a function of Nickel layer. Simulation studies of the transport mechanism in this contact indicate the likelihood of a tunneling junction at the AZO/Ni interface, thus the entire AZO/Ni forming an ohmic contact to p-GaN. Potential to better control injected current density uniformity utilizing such a contact structure is also discussed.

Surface transfer hole doping of epitaxial graphene using MoO3 thin film

Synchrotron-based in situ photoelectron spectroscopy investigations demonstrate effective surface transfer p-type doping of epitaxial graphene (EG) thermally grown on 4H–SiC(0001) via the deposition of MoO3 thin film on top. The large work function difference between EG and MoO3 facilitates electron transfer from EG to the MoO3 thin film. This leads to hole accumulation in the EG layer with an areal hole density of about 1.0×1013 cm−2, and places the Fermi level 0.38 eV below the graphene Dirac point.

Temperature-dependent contact resistivity of the nonalloyed ohmic contacts to p-GaN

Temperature-dependent contact resistivity of nonalloyed Pd/Pt/Au contacts to p-GaN films as well as temperature-dependent sheet resistivity of p-GaN films has been investigated in order to understand anomalously low contact resistivity (∼10−4 Ω cm2) considering the large work-function difference between the Pd and p-GaN. As the measured temperature decreases from 300 to 100 K, the contact resistivity increases by more than one order of magnitude. In addition, the sheet resistivity increases linearly with exp(To/T)1/4, implying variable-range hopping conduction via deep-level defects (DLDs). The density of the DLDs in the p-GaN films is estimated to be over 1019 cm−3, which suggests that the carriers may flow from the Pd directly to the dense DLDs, resulting in the anomalously low contact resistivity.

Organic semiconductor interfaces: Discrimination between charging and band bending related shifts in frontier orbital line-up measurements with photoemission spectroscopy

Gaq 3 is a promising luminescent organic semiconductor for applications in organic light emitting diodes. The frontier orbital alignment at the tris (8-hydroxyquinolinato) gallium (Gaq3)/Pt organic Schottky contact was determined by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. A Gaq3 thin film was deposited in several steps on a previously Ar+ sputtered pure Pt foil. After each growth step, the sample was characterized by XPS and UPS. The combination of XPS and UPS measurements allows the precise evaluation of the interface dipole independent from the simultaneously occurring band bending at the interface and charging artifacts. The measurements show that the Pt/Gaq3 interface has a strong dipole of 0.71 eV indicating the transfer of negative charge from Gaq3 to Pt. Due to the large work function difference between Pt and Gaq3, strong band bending occurred. At Gaq3 coverages higher than 128 Å strong charging shifts occurred in the overlayer related emission lines which were identified by measuring the high binding energy cutoff (secondary edge) of both the XP and UP spectra. Due to the several magnitudes difference between the x-ray and ultraviolet source photon intensities, differences between the high binding energy cutoff positions of both measurements allow to pinpoint charging shifts with high sensitivity. In order to investigate the possible influence of radiation damage to the Gaq3 film, a single growth step control film of Gaq3 of the same thickness as in the multi-step orbital line-up procedure was deposited. The comparison of XPS peak intensities and positions suggest that no significant chemical alteration occurred during the photoemission spectroscopy measurements of the multi-step deposition experiment. However, the doping level of the layer may have changed due to the radiation exposure during the measurements.

Energy level alignment at Alq/metal interfaces

The energy level alignment for both Mg/8-hydroxyquinoline aluminum (Alq) and Au/Alq interfaces has been determined by the ultraviolet photoemission measurements. For both interfaces, the difference between the Fermi level and the low-energy edge of the highest occupied molecular orbital (HOMO) is around 1.7 eV. This implies that the Fermi level with respect to the HOMO edge of Alq is independent of the work function of Mg and Au despite a large difference in the metal work function. A Fermi level alignment model is proposed, invoking a charge transfer between the metal and Alq and the formation of a dipolar layer at the metal/Alq interface.

Observation of Gold Thin Film on Cesium Graphite Intercalation Compound by Scanning Tunneling Microscopy

Thin film on stage-1 cesium graphite intercalation compound (Cs-GIC) which is known to be of very low work function has been studied using scanning tunneling microscopy (STM). After a small amount of Au was evaporated onto a Cs-GIC cleaved under ultrahigh-vacuum conditions, its surface was observed. At first, many islands with a diameter of several nanometers were found. These islands, however, gradually disappeared from the scanned region during observation. Our results suggest that the interaction of Au islands with Cs-GIC is weak, in spite of the large difference in work function between them.

Epitaxial ferroelectric thin films

Microstructure of epitaxial ferroelectric/conductive oxide heterostructures on LaAIO3(LAO) and Si substrates have been studied by conventional and high resolution transmission electron microscopy. The epitaxial films have a wide range of potential applications in areas such as non-volatile memory devices, electro-optic devices and pyroelectric detectors. For applications such as electro-optic devices the films must be single crystal and for applications such as nonvolatile memory devices and pyroelectric devices single crystal films will enhance the performance of the devices. The ferroelectric films studied are Pb(Zr0.2Ti0.8)O3(PLZT), PbTiO3(PT), BiTiO3(BT) and Pb0.9La0.1(Zr0.2Ti0.8)0.975O3(PLZT).Electrical contact to ferroelectric films is commonly made with metals such as Pt. Metals generally have a large difference in work function compared to the work function of the ferroelectric oxides. This results in a Schottky barrier at the interface and the interfacial space charge is believed to responsible for domain pinning and degradation in the ferroelectric properties resulting in phenomenon such as fatigue.

Schottky barrier heights and interface chemistry in Ag, In, and Al overlayers on GaP(110)

We have carried out a study of the chemical reaction of silver, indium, and aluminium layers with cleaved GaP(110) surfaces using photoemission with synchrotron radiation. Core level photoelectron spectra show that silver and indium overlayers do not cause an interface reaction with GaP(110). The deposition of Al, on the other hand, leads to an extensive exchange reaction which also proceeds at low temperature, although influenced by changes in overlayer growth morphology. Surface band bending induced by the metallic overlayers was investigated as a function of deposition for n- and p-type material. In contrast to earlier findings, almost identical Schottky barrier heights for In and Ag deposition are obtained, despite the large difference in work function between these two metals. Results for Al also suggest that a small range of pinning positions is responsible for the Schottky barrier heights for junctions of these metals with GaP(110). We find that large peak shifts due to a surface photovoltage induced by the photoemission light source affect the determination of the Schottky barrier heights. This and other possible reasons for the discrepancy with earlier work are discussed.

Electrical characteristics of hydrogen implanted silicon Schottky diodes having large difference in metal work function

Hydrogen ion implantation was carried out on Schottky diodes having large difference in metal work function, Ti/p-Si and PtSi/p-Si diodes. Current-voltage (I-V) measurements showed that, following ion implantation, Ti/p-Si diodes exhibited rectifying characteristics; in contrast, no significant rectifying behavior was observed in the PtSi/p-Si diodes. These results showed a dependence of the implantation effects upon metal overlayer work functions. Consequently, the observations did not seem to indicate the occurrence of Fermi level pinning due to a highly damaged near-surface region after ion implantation, as previously suggested. Capacitance-voltage (C-V) measurements revealed a decrease in the diode capacitances along with a significant reduction of acceptor concentration following the implantation. In general, the results suggest that ion implantation alters the electrical characteristic of the diodes mainly by creating defects in the semiconductor depletion region. All such defects act as recombination centers giving rise to a deviation of the electrical characteristics from the normal behavior. They do not play a role in producing a Fermi level pinning.

Diffusion and adsorption of titanium oxide on platinum as related to strong metal-support interactions

Investigations by field emission microscopy show that two distinguishable Ti oxide layers are formed on rounded Pt crystallites by diffusion from an oxygen-deficient TiO 2 vapor deposit. Layer I, appearing at 700 K, covers all high Miller index areas, [110], and the vicinal areas of [100]. Layer II is formed at 800 K. In contrast to Layer I, it exhibits a layer boundary. Layer II covers all crystallographic areas except [111]. When it breaks up upon heating above 800 K, numerous high work function islands (~2 × 10 10 cm −2) are formed in the high Miller index areas. Because of the large work function difference between these islands and their surroundings, very strong electrostatic fields (patch fields) are created. The islands grow at elevated temperatures and disappear at about 1100 K. Above this temperature, Ti oxide is adsorbed only in the vicinal areas of [111] (1100 to 1200 K) and on [100] (1200 to 1240 K). The possible influence of the patch fields on chemical surface reactions is discussed.

An assessment of the suitability of rf sputtered amorphous hydrogenated Si as a potential solar cell material

We review several material properties thought to be important for the production of high efficiency amorphous semi-conductor Schottky barrier solar cells, with particular reference to a systematic study made in this laboratory of amorphous Si:H alloys prepared by rf sputtering. These requirements include: (a) an optical gap which efficiently absorbs the solar spectrum in a thin film, (b) a Fermi level position close to the conduction band for a large work function difference between metal and semiconductor, (c) high photoconductivity, (d) long carrier migration lengths, and (e) a low overall density of gap states. Using our data on these electronic properties, we show that we can satisfy these conditions somewhat by depositing at an argon pressure of 20 mTorr. Furthermore, we demonstrate, using a simple Schottky barrier structure, that significant improvements in device performance are achieved, particularly in open circuit voltage, as the above properties are optimized. Other factors which might still limit the performance of the device are discussed, and from an examination of the temperature dependence of the short circuit current, we conclude that the most likely to be important is the “effective” diffusion length for valence band holes.