Large Work Function Research Articles

Parallel metal–insulator–metal diode with an ultrathin spin-coated hydrogen silsesquioxane insulating layer

In this study, we investigated a parallel metal–insulator–metal (MIM) diode with an ultrathin spin-coated hydron silsesquioxane (HSQ) layer. Ti and Au were adopted as the metal electrodes for the large work function difference. Conditions to obtain the ultrathin HSQ layer with a thickness of below 5 nm for tunneling were predicted and Ti/HSQ/Au diode devices with a parallel electrode arrangement were fabricated by using the conditions. The typical current–voltage characteristics of the fabricated diodes exhibited asymmetry of about 1.8 at 3.0 V. It was demonstrated that the dynamic zero bias resistance of the diodes was as low as about 8 MΩ. Based on the Simmons model, the estimated oxide-equivalent thickness of HSQ in the device was about 1.7 nm, which was in good agreement with the prediction. The good figures of merit of the fabricated diodes imply that the spin-coated ultrathin HSQ is very suitable for this application.

P-Type ohmic contact to MoS2via binary compound electrodes

Both n- and p-type ohmic contact to MoS2 can be obtained via different CuS surfaces, due to the weak metallicity and large work function variation of the CuS surfaces, and due to interface quasi-bonding between CuS and MoS2.

Functional groups and vertical strain regulate the electronic properties of Nb2NT2/MoTe2 heterojunction

In the current effort, a novel attempt to construct heterojunctions using Nb2N two-dimensional MXene materials with fascinating property and MoTe2 transition metal disulfide compounds is proposed. The effect of vertical strain on the electronic structure and interfacial contact properties of the heterojunction is investigated by first principles calculations. The O, F, and OH functional groups prefer to be adsorbed on top of the N atoms of the Nb2N MXene material. The configuration III of heterojunction has the lowest energy and stable model, and the equilibrium layer spacing is within the range of 3.061 Å to 3.328 Å, indicating that they belong to typical vdW heterojunctions. The large work function difference between the two components induces the transfer and rearrangement of charge. 0.12 e per cell spontaneously transfer from MoTe2 to Nb2NO2 layer. In contrast, the Nb2NF2 and Nb2N(OH)2 layers transfer 0.01 e and 0.21 e to the MoTe2 layer, respectively. n-type Ohmic contacts are formed in O terminal system, while p-type and n-type Schottky contacts are formed in F and OH terminal systems, respectively. Additionally, for vertical strain, with the decrease of the layer spacing, the p-type Schottky barrier height (SBH) decreases from 0.270 eV to 0.095 eV in former system, meanwhile the n-type Schottky barrier height in the latter system also decreases from 0.260 eV to 0.156 eV. These regulations of interest lays a theoretical foundation for bandgap engineering and Schottky junction preparation and opens a window into MXene-based heterojunction electron device.

Enhanced CO2 Photoreduction over Bi2Te3/TiO2 Nanocomposite via a Seebeck Effect

The activation of carbon dioxide (CO2) molecules and separation/transfer of photoinduced charge carriers are two crucial factors influencing the efficiency of CO2 photoreduction. Herein, we report a p-type Bi2Te3/commercial TiO2 (pBT/P25) nanocomposite for enhanced CO2 photoreduction. Upon light irradiation, a temperature gradient formed in pBT induces the Seebeck effect to build a thermoelectric field, which promotes the charge carriers’ separation/transfer. Additionally, pBT with a strong light absorption capacity generates the photothermal effect favoring the activation of CO2 molecules. In addition, the excellent electric conductivity and large work function render pBT an efficient cocatalyst for further improving the charge carriers’ separation/transfer. Owing to the synergistic enhancement effect of pBT on the activation of CO2 molecules and promotion of charge separation/transfer, we achieved the highest CO evolution rate over pBT(2)/P25 of 19.2 μmol·gcat−1·h−1, which was approximately 5.5 times that of bare P25. This work suggests that a thermoelectric material/semiconductor nanocomposite could be developed as an efficient photo-thermo-electro-chemical conversion system for enhanced CO2 reduction via promoting the charge carriers’ separation/transfer.

M3C2(OH)2 (M = Zr and Hf): An Ideal Electrode Material for sub‐5 nm 2D Field‐Effect Transistors

AbstractRealizing Ohmic contact is essential but challenging in the development of high‐performance 2D‐material‐based electronics. Here, using first‐principles calculations, the interfacial properties between 2D metallic M3C2T2 (M = Ti, Zr, Hf; T = O, F, OH) and 2D semiconductors are comprehensively investigated, taking group ΙV monochalcogenides in blue‐phosphorene phase as example, and the performance of short‐channel field‐effect transistors (FETs) with M3C2(OH)2 electrode is studied. The band alignments and the interface dipole analysis demonstrate that the M3C2(OH)2 and Ti3C2O2 electrodes form van der Waals interaction with the group ΙV monochalcogenides, conductive to weakening the Fermi level pinning and forming desired Ohmic contacts. Moreover, owing to the large work function of the Ti3C2O2, it realizes p‐type Ohmic contacts with 2D semiconductors. In addition, due to the favorable Ohmic contacts, the on/off ratio of the 5 nm gate‐length Zr3C2(OH)2–GeTe FET is around 105. Moreover, for the 3 nm gate‐length Zr3C2(OH)2–SnTe FET after adopting optimizing strategies, the on/off ratio increases from 102 to 106, and the subthreshold slope decreases from 125 mV dec‐1 to 58 mV dec‐1 below the thermionic limit (60 mV dec‐1). The results provide a theoretical guidance for achieving the intrinsic n‐type and p‐type Ohmic contacts and high‐performance FETs in 2D nanoelectronics.

High-performance aqueous potassium ion asymmetric supercapacitors based on tunable 2D transition metal oxides

Rechargeable aqueous potassium ion asymmetric supercapacitors are emerging as a promising alternative to lithium ion batteries due to their low cost and high safety. However, the large size of the potassium ions tends to lead to large deformations and even irreversible shattering of the active material during the insertion/extraction process. In this paper, amorphous/crystalline biphasic KxV2O5 (ac-KxV2O5) is synthesized by electrochemical deposition on three-dimensional nanoporous nickel tube collectors. The amorphous K0.25V2O5∙nH2O acts as a molecular column to maintain the large interlayer spacing of crystalline V2O5 bilayer, providing sufficient space to accommodate as much hydrated potassium ions as possible and their rapid transport, effectively alleviating structural damage to the host material. Benefiting from the large work function difference between V2O5 and MnO2, an aqueous potassium ion asymmetric supercapacitor is assembled using KxV2O5∙nH2O anode and KxMnO2∙nH2O cathode to obtain a volumetric energy of 32.7 mWh cm−3, which is superior not only to commercial energy storage devices but also to previously reported aqueous alkaline ion batteries and supercapacitors.

High output power density owing to enhanced charge transfer in ZnO-based triboelectric nanogenerator

We have fabricated triboelectric nanogenerarators (TENGs) using ZnO and polydimethylsiloxane (PDMS) in which ZnO growth temperature was varied in the range from 450 °Ct 750 °C. The output voltage/current of TENGs increases with increase of ZnO growth temperature up to 600 °C and then decreases with further increase of growth temperature up to 750 °C. TENG fabricated with ZnO grown at 600 °C exhibits maximum electrical output with peak to peak voltage of ∼210 V, current ∼95 μA and power density ∼8.8 mW/cm2 upon application of a periodic force of ∼30 N @ 6 Hz, which is higher than the power densities reported till date in the case of ZnO-based TENGs. This enhanced output power density can be mainly attributed to the large effective work function difference obtained between ZnO and PDMS. Electrical, photoluminescence and ultra-violet photoelectron spectroscopy measurements clearly suggest the dominant role of electron transport in the charge transfer between ZnO and PDMS. Practical applications of TENGs have been demonstrated by powering 38 LEDs and a stopwatch display. This study not only deepens the understanding of contact electrification between ZnO and PDMS, but opens up the immense potential of ZnO-based TENGs for possible vibration energy harvesting applications.

Realization of Double‐Gate Junctionless Field Effect Transistor Depletion Region for 6 nm Regime with an Efficient Layer

Herein, the authors suggest a junctionless field effect transistor with an embedded p‐type layer (EPL‐JLT) near the drain channel side, employing calibrated structure simulations to obtain a complete depletion region in a 6 nm channel length. The incorporation of a p‐type layer improves leakage current (I OFF) and subthreshold swing (SS) for a 6 nm regime structure at 5.9 eV work function (WF) while the ON current (I ON) diminishes a little. This considerable achievement in the leakage current enables obtaining multiple threshold voltages (V TH) by adjusting the gate WF. The goal aids in creating optimized structures, which is impossible in silicon JLFETs (Si‐JLT) due to their requirement for large WFs to obtain a depletion region even at higher channel lengths. The proposed device has a leakage current of 1 nA μm−1 even at a 5.1 eV WF. The scaling of the EPL‐JLT for different channel lengths is investigated.

50-µm thick flexible dopant-free interdigitated-back-contact silicon heterojunction solar cells with front MoOx coatings for efficient antireflection and passivation

We demonstrate experimentally a flexible crystalline silicon (c-Si) solar cell (SC) based on dopant-free interdigitated back contacts (IBCs) with thickness of merely 50 µm for, to the best of our knowledge, the first time. A MoOx thin film is proposed to cover the front surface and the power conversion efficiency (PCE) is boosted to over triple that of the uncoated SC. Compared with the four-time thicker SC, our thin SC is still over 77% efficient. Systematic studies show the front MoOx film functions for both antireflection and passivation, contributing to the excellent performance. A double-interlayer (instead of a previously-reported single interlayer) is identified at the MoOx/c-Si interface, leading to efficient chemical passivation. Meanwhile, due to the large workfunction difference, underneath the interface a strong built-in electric field is generated, which intensifies the electric field over the entire c-Si active layer, especially in the 50-µm thick layer. Photocarriers are expelled quickly to the back contacts with less recombined and more extracted. Besides, our thin IBC SC is highly flexible. When bent to a radius of 6 mm, its PCE is still 76.6% of that of the unbent cell. Fabricated with low-temperature and doping-free processes, our thin SCs are promising as cost-effective, light-weight and flexible power sources.

Encapsulating N‑doped graphite carbon in MoO2 as a novel cocatalyst for boosting photocatalytic hydrogen evolution

Generally, it is important to ameliorate the co-catalyst used in photocatalytic hydrogen evolution reactions (PHERs) to achieve efficient transfer and separation of photogenerated carriers, decrease the surface reaction energy barrier, and hence improve the photocatalytic activity. In this study, N-doped graphite carbon (GC) was introduced in situ to MoO2 to ensure the presence of well-dispersed active sites, lower the overpotential of hydrogen evolution, and further achieve high conductivity. Then, the MoO2/GC composite obtained was used as a co-catalyst of ZnIn2S4 (ZIS) in a PHER, resulting in a great improvement in the photocatalytic activity. Given the metallicity and large work function of MoO2/GC, a Schottky interface can form between MoO2/GC and ZIS, which accelerates the transmission of photogenerated electrons. As a result, the separation efficiency of photogenerated carriers improves, whereas the surface overpotential of PHERs clearly decreases for ZIS. This study proposes a new idea for exploiting efficient co-catalysts and promotes the wide and heavy use of carbon materials in the field of solar energy conversion.

Improved performance of flexible citrus resistive memory device through air plasma

Flexible natural material-based electronics have attracted considerable attention because it can be applied in wearable applications and bio smart electronics. Natural material citrus is used as the dielectric layer in this work to develop flexible resistive switching memory devices, with plasma ITO surface as the bottom electrode (BE) to investigate the effects of air plasma on device performances. The work function difference between the top electrodes (TE) and BE can be increased with plasma treatment. After optimization, the flexible citrus resistive memory device with a large work function difference between the TE and BE exhibits a good ON/OFF ratio of larger than 103, a low set voltage of around 0.76 V, uniform distribution of set voltages, small coefficients of variation of high resistance state, and low resistance state currents, and a long retention time of more than 104 s. The air plasma can also modify the ITO surface to make the surface more hydrophilic. Thus, the citrus film is easier to attach to ITO, which improves the bending performance of the device. The device under a bending radius of 4.9 mm showed no significant ON/OFF ratio changes when compared with that of the flat state. This information on the correlation between the plasma treatment time and the work function of the ITO electrode would be very useful in obtaining stable and uniform resistive switching properties in the flexible natural material-based resistive memory.

Optimization design of GaAs-based betavoltaic batteries with p–n junction and Schottky barrier structures

This paper presents the calculation model and the optimization design of the GaAs-based betavoltaic batteries with p–n junction and Schottky barrier structures. First of all, by using the Monte Carlo code, the transport process and energy deposition distribution of the source beta particles in the GaAs material are simulated. The relationships between the output parameters of batteries and the physical parameters of energy converter such as p–n junction depth, Schottky metal thickness, depletion region width and doping concentrations are discussed through the numerical calculation. For the GaAs p–n junction battery, the maximum output power density of 0.135 can be achieved when the junction depth is x j = 0.05 , the doping concentrations are and . Meanwhile, the short-circuit current density, open-circuit voltage, filling factor and energy conversion efficiency are 0.254 , 0.638 V, 83.3% and 2.63%, respectively. Among the selected metals of Au, Pd, Ni and Pt for the GaAs Schottky barrier diodes, the Pt-GaAs battery has the best output performance due to its large work function. The maximum output power density of 0.169 can be achieved when a 20 nm thick Schottky metal Pt is selected and the doping concentration is . The associated output parameters of the battery are 0.234 , 0.835 V, 86.5% and 3.29%, respectively.

Modulating Built-In Electric Field via Variable Oxygen Affinity for Robust Hydrogen Evolution Reaction in Neutral Media.

Work function strongly impacts the surficial charge distribution, especially for metal-support electrocatalysts when a built-in electric field (BEF) is constructed. Therefore, studying the correlation between work function and BEF is crucial for understanding the intrinsic reaction mechanism. Herein, we present a Pt@CoOx electrocatalyst with a large work function difference (ΔΦ) and strong BEF, which shows outstanding hydrogen evolution activity in a neutral medium with a 4.5-fold mass activity higher than 20 % Pt/C. Both experimental and theoretical results confirm the interfacial charge redistribution induced by the strong BEF, thus subtly optimizing hydrogen and hydroxide adsorption energy. This work not only provides fresh insights into the neutral hydrogen evolution mechanism but also proposes new design principles toward efficient electrocatalysts for hydrogen production in a neutral medium.

Modulating Built‐In Electric Field via Variable Oxygen Affinity for Robust Hydrogen Evolution Reaction in Neutral Media

AbstractWork function strongly impacts the surficial charge distribution, especially for metal‐support electrocatalysts when a built‐in electric field (BEF) is constructed. Therefore, studying the correlation between work function and BEF is crucial for understanding the intrinsic reaction mechanism. Herein, we present a Pt@CoOx electrocatalyst with a large work function difference (ΔΦ) and strong BEF, which shows outstanding hydrogen evolution activity in a neutral medium with a 4.5‐fold mass activity higher than 20 % Pt/C. Both experimental and theoretical results confirm the interfacial charge redistribution induced by the strong BEF, thus subtly optimizing hydrogen and hydroxide adsorption energy. This work not only provides fresh insights into the neutral hydrogen evolution mechanism but also proposes new design principles toward efficient electrocatalysts for hydrogen production in a neutral medium.

Improvement in Strain Sensor Stability by Adapting the Metal Contact Layer.

Research on stretchable strain sensors is actively conducted due to increasing interest in wearable devices. However, typical studies have focused on improving the elasticity of the electrode. Therefore, methods of directly connecting wire or attaching conductive tape to materials to detect deformation have been used to evaluate the performance of strain sensors. Polyaniline (PANI), a p-type semiconductive polymer, has been widely used for stretchable electrodes. However, conventional procedures have limitations in determining an appropriate metal for ohmic contact with PANI. Materials that are generally used for connection with PANI form an undesirable metal-semiconductor junction and have significant contact resistance. Hence, they degrade sensor performance. This study secured ohmic contact by adapting Au thin film as the metal contact layer (the MCL), with lower contact resistance and a larger work function than PANI. Additionally, we presented a buffer layer using hard polydimethylsiloxane (PDMS) and structured it into a dumbbell shape to protect the metal from deformation. As a result, we enhanced steadiness and repeatability up to 50% strain by comparing the gauge factors and the relative resistance changes. Consequently, adapting structural methods (the MCL and the dumbbell shape) to a device can result in strain sensors with promising stability, as well as high stretchability.

Tunable physical properties of Al-doped ZnO thin films by O2 and Ar plasma treatments

Al-doped ZnO (AZO) is a promising transparent conducting oxide that can replace indium tin oxide (ITO) owing to its excellent flexibility and eco-friendly characteristics. However, it is difficult to immediately replace ITO with AZO because of the difference in their physical properties. Here, we study the changes in the physical properties of AZO thin films using Ar and O2 plasma treatments. Ar plasma treatment causes the changes in the surface and physical properties of the AZO thin film. The surface roughness of the AZO thin film decreases, the work function and bandgap slightly increase, and the sheet resistance significantly decreases. In contrast, a large work function change is observed in the AZO thin film treated with O2 plasma; however, the change in other characteristics is not significant. Therefore, the results indicate that post-treatment using plasma can accelerate the development of high-performance transparent devices.

Fourteen percent efficiency ultrathin silicon solar cells with improved infrared light management enabled by hole‐selective transition metal oxide full‐area rear passivating contacts

AbstractThe present study investigates the application of hole‐selective transition metal oxide (TMO) layers (MoOx, V2Ox, and WOx) with silver (Ag) as full‐area rear contact to 22.5 μm‐thick low‐quality Cz p‐type c‐Si solar cells. Thin films of metal oxides are deposited directly on p‐type c‐Si by thermal evaporation at room temperature. The large work function of these TMOs creates strong accumulation at the interface with p‐type c‐Si, which allows only holes to transport and simultaneously suppress the interfacial recombination current density (J0) and contact resistivity (ρc). The current generation and losses of 22.5 μm‐thick solar cells with different hole‐selective TMO/Ag at the rear are simulated. The presence of TMO/Ag at the rear is found to significantly reduce parasitic light absorption at longer wavelengths which becomes more pronounced for ultrathin wafers, providing significant advantages over conventional Al contact. The best device performance was attained by the MoOx/p‐type c‐Si solar cells, demonstrating a considerably high efficiency (η) of 14% with Voc of 555 mV, FF of 76.0%, and Jsc of 33.2 mA/cm2. Furthermore, the present work is the first to employ MoOx, V2Ox, and WOx as rear contact in ultrathin p‐type c‐Si solar cells.

Efficiency limits of perovskite solar cells with n-type hole extraction layers

Inorganic materials, such as MoOx and V2Ox, are increasingly explored as hole transport layers for perovskite based solar cells. Due to their large work function and n-type nature, hole collection mechanisms with such materials are fundamentally different, and the associated device optimizations are not well elucidated. In addition, prospects of such architectures against the challenges posed by ion migration are yet to be explored—which we critically examine in this contribution through detailed numerical simulations. We find that, for similar ion densities and interface recombination velocities, ion migration is more detrimental for perovskite solar cells with n-type hole transport layers with much lower achievable efficiency limits (∼21%). The insights shared by this work could be of broad interest to critically evaluate the promises and prospects of n-type materials as hole transport layers for perovskite solar cells.

Al0.7Ga0.3N MESFET With All-Refractory Metal Process for High Temperature Operation

Ultrawide bandgap Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> N MESFETs with refractory Tungsten Schottky and Ohmic contacts are studied in 300-675 K environments. Variable-temperature dc electrical transport reveals large ON-state drain current densities for an AlGaN device: 209 mA/mm at 300 K and 156 mA/mm at 675 K in the ON-state (25% reduction). Drain and gate currents are only weakly temperature-dependent, suggesting potential for engineering temperature invariant operation. The ON-/ OFF-ratio is limited by OFF-state leakage through the gate, which is attributed to damage from sputter deposition. Future work using refractory metals with larger work functions that are deposited by electron beam deposition is proposed.

Study on a novel vertical enhancement-mode Ga 2 O 3 MOSFET with FINFET structure

A novel enhanced mode (E-mode) Ga2O3 metal–oxide–semiconductor field-effect transistor (MOSFET) with vertical FINFET structure is proposed and the characteristics of that device are numerically investigated. It is found that the concentration of the source region and the width coupled with the height of the channel mainly effect the on-state characteristics. The metal material of the gate, the oxide material, the oxide thickness, and the epitaxial layer concentration strongly affect the threshold voltage and the output currents. Enabling an E-mode MOSFET device requires a large work function gate metal and an oxide with large dielectric constant. When the output current density of the device increases, the source concentration, the thickness of the epitaxial layer, and the total width of the device need to be expanded. The threshold voltage decreases with the increase of the width of the channel area under the same gate voltage. It is indicated that a set of optimal parameters of a practical vertical enhancement-mode Ga2O3 MOSFET requires the epitaxial layer concentration, the channel height of the device, the thickness of the source region, and the oxide thickness of the device should be less than 5 × 1016 cm−3, less than 1.5 μm, between 0.1 μm − − 0.3 μm and less than 0.08 μm, respectively.