Decrease In Work Function Research Articles

Defect tuned SnO2 nanolayer coated TiO2 1-D core-shell structure for enhanced overall solar water splitting

The production of O 2 at TiO 2 surface during the photoelectrochemical water splitting is the bottleneck of the reaction process indicating the need for surface engineering of TiO 2 . Here, 1-D TiO 2 was coated with the defect tuned SnO 2 nanolayer (via sputtering) for enhanced solar water splitting activity. The sputtering strategy allows oxygen vacancy tuning of SnO 2 with complete coverage on TiO 2 . Importantly, the optimized SnO 2 /TiO 2 system exhibited the record photocurrent of 1.85 mA cm −2 at 1.23 V vs. RHE, incident photon to current efficiency of 95% at 350 nm, and H 2 and O 2 production with ∼90% of faradaic efficiency. Band structure analysis indicated a decrease in work function along with an upward shift of Fermi level and conduction band at the electrolyte interface after SnO 2 coating. The present study demonstrates the utilization of oxygen vacancy in SnO 2 for the surface engineered electrodes for facile O 2 production during solar water splitting.

High-efficiency planar heterojunction perovskite solar cell produced by using 4-morpholine ethane sulfonic acid sodium salt doped SnO2

Perovskite solar cells (PSCs) have become a promising photovoltaic (PV) technology. Meanwhile, developing an electron transport layer (ETL) has been an effective way to promote the power conversion efficiency (PCE) of PSCs. Here, a 4-morpholine ethane sulfonic acid sodium salt (MES Na+) doped SnO2 ETL is utilized in planar heterojunction PSCs. The results show that the MES Na+ doped ETL can improve the crystallinity, and absorbance of perovskite films, and passivate interface defects between the perovskite film and SnO2 ETL. The doping effect accounts for the enhancement of conductivity and the decreasing work function of SnO2. When 10 mg mL−1 MES Na+ was added to the SnO2 precursor solution, the device showed the best performance Jsc, Voc, and FF of the PSCs values, which were 23.88 mA cm−2, 1.12 V and 78.69%, respectively, and the PCE was increased from 17.43% to 21.05%. This doping ETL strategy provides an avenue for defect passivation to further increase the efficiency of perovskite solar cells.

Revealing the correlation between gas selectivity and semiconductor energy band structure derived from off-stoichiometric spinel CdGa2O4

Herein, the spinel CdGa2O4 and off-stoichiometric spinel CdGa2O4 (CdO-CdGa2O4) with 3D inverse opal spherical structure have been synthesized by the combination of ultrasonic spray pyrolysis technology and self-assembled hard-template method. The CdGa2O4, with band gap of 4.14 eV and baseline resistance of Mohm-level, exhibited excellent selectivity and high response to 1 ppm formaldehyde. On the other hand, the CdO-CdGa2O4 composites (Cd:Ga atom ratio = 1.2:2), with band gap of 2.46 eV and baseline resistance of kohm-level, exhibited best sensing performance to 1 ppm NO2 with negligibly low cross-responses (Rair/Rgas < 3.5) to any VOCs. Due to the semiconductor band gap is narrowed, and the conducting channel depending on the agglomeration of surface loaded CdO nanocrystals, the gas selectivity has been changed and the baseline resistance of off-stoichiometric spinel CdGa2O4 was reduced by many orders of magnitude. Meanwhile, the improvement of gas response can be attributed to ordered bimodal-porous structure, and the n-n heterojunctions induced decrease in work function and enhancement in chemisorbed oxygen. Accordingly, the proposed correlation between gas selectivity change and band gap modulation can open new route for solving the shortcoming of poor selectivity of metal oxide semiconductor gas sensor.

Impact of edge functionalization on electron field-emission characteristics of carbon nanotubes: A theoretical approach

A theoretical approach using first-principle calculations of the density of states (DOS) of functionalized carbon nanotubes (CNTs) is carried out to understand their enhanced electron field emission (FE) properties. The theoretical functionalization of CNTs is perceived using hydrogenation (H/O–H) and oxidation methods. In order to investigate the enhancement in the FE properties of functionalized CNTs, both single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) are studied. The work function is observed to be more decreased in case of zigzag SWCNT (φ = 3.32 for oxygenated SWCNT) as compared to others SWCNT. The dependence of DOS on zigzag/armchair edge functionalization has also been investigated to reveal the impact of DOS on FE. The observed results indicate that edge functionalization of CNTs leads to significant changes in the band structure, DOS, and work functions. The shifting of DOS away from the Fermi level and decrease in work function is evidenced owing to the reduction in the curvature effect of CNT. Also, due to enhanced DOS near the Fermi level, making them a better proposition for FE applications. Our calculations provide an in-depth insight into the functionalization mechanism to understand the enhanced FE properties from them.

Hydroxylation of the Zn terminated ZnO(0 0 0 1) surface under vacuum conditions

Under vacuum conditions, the polar surface ZnO(0 0 0 1) evolves in time forming a layer with H and additional O which changes the top layer stoichiometry and other properties such as its work function. In this work we present a study of the evolution of the ZnO(0 0 0 1) surface in ultra-high vacuum performed with time of flight direct recoil spectroscopy to detect the adsorbed H and O, plus other standard techniques including photoelectron spectroscopy and low energy electron diffraction. The experimental results are complemented with density functional theory (DFT) calculations for surfaces perfectly terminated and with triangular defects plus O adatoms. We show that the ZnO surface reaches a stable condition at an OH coverage of around 0.25 ML. We also studied the effect of increasing the partial pressure of H2 and of H2O on the layer growth. We observe that during the formation of the OH layer the work function decreases. Analysis of this variation of the work function with DFT reveals the role of the O adatoms in the adsorption process. Finally we show that the OH coverage decreases continuously with the increase of the sample temperature, without showing a well-defined desorption peak, which is in agreement with earlier predictions of the thermodynamics evolution. We believe that the present results contribute to clarify the behavior of the ZnO(0 0 0 1) surface under vacuum and will be useful in future works, particularly where H cannot be detected by standard techniques.

Perovskite semiconductor-engineered cascaded molecular energy levels in naturally-sensitized photoanodes

Naturally-sensitized photoanodes in dye-sensitized solar cells (DSSCs) are promising alternatives to enhance photoabsorption, electron excitation/injection, but voltage loss remains a challenge. Here, we focus on understanding the cascading of energy levels in perovskite semiconductor cosensitized naturally-sensitized photoanodes to leverage forward charge transport addressing the voltage loss arising from ITO/TiO2 heterojunction's built-in potential. The β-carotene-sensitized TiO2 photoanode modified with methylammonium lead iodide (MAPbI3) co-sensitizer causes an upward shifting in TiO2 Fermi level (EF). This phenomenon is predominantly attributed to increased initially injected electrons due to low MAPbI3 bandgap and high visible-light absorption. Enhanced charge separation and injection mechanisms at the TiO2/MAPbI3 interface increase the effective density-of-states (DOS >2.46 × 1021 cm−3) in the TiO2 conduction band (CB) and hence decrease its work function to 4.82 eV. The decrease in TiO2 work function suppressed CB bending at ITO/TiO2 heterojunction, which minimized the photoinduced electrostatic potential barrier up to 13.1%. The reduced Schottky barrier (φSBH<0.52 eV) only allows electrons tunneling, while inhibited back-electron transport reduced both current leakage and voltage loss yielding in high open-circuit voltage (Voc increase by 120%) and power conversion efficiency (PCE increase by 240%). The MAPbI3 incorporation also broadened photoanode absorbance by 2-fold, paving the way towards perovskite semiconductor cosensitization to avoid voltage loss from bio-integrated photoanodes for photovoltaic and other optoelectronic and photonic applications. Future works will focus on studying the series resistance, cathode electrode coating uniformity, recombination kinetics, solid electrolytes, as well as other aspects typically related to increasing the photocurrent levels of this β-carotene-sensitized solar cell.

γ-In2S3 nanosheets-composed flowerlike nanostructure doped by Al3+ ions with optimal electronic structure and decreased work function of γ-In2S3 for CO2 electroreduction to formate

It is an urgent imperative to convert CO2 molecules into valuable chemicals or fuels for mitigating the greenhouse effect and energy dilemma. Herein, γ-In2S3 nanosheets-composed flowerlike nanostructure doped by tunable amount of Al3+ ions was fabricated by a one-step solution-phase avenue for efficient CO2ER to formate. The particular nanosheets-composed flowerlike nanostructure exhibited good gas-permeable behavior, enlarging the capacity of CO2 adsorption and accelerating the mass-transport efficiency. The Al dopant in γ-In2S3 optimized the electronic structure of γ-In2S3, which enhanced the carrier density, conductivity, and exposed active sites. Besides, Al doping reduced the work function of γ-In2S3, which brought about the fast Faradaic process of CO2 activation and speeded up the subsequent reduction reaction. The optimal Al4.8%-In2S3 displayed the maximal formate Faradaic efficiency (FEHCOO–) of 91.2% at −1.0 V and maintained with a high FEHCOO–> 80% in a potential range of −0.9 to −1.2 V. Moreover, it exhibited a high formate energy efficiency (EEHCOO–) of 58.5% at −1.0 V and kept with a high EEHCOO–> 50% in a potential range of −0.9 to −1.2 V. Besides, Al4.8%-In2S3 delivered the partial current density of 23 mA cm−2 at −1.0 V for formate and increased to 31.7 mA cm−2 at −1.2 V. It also maintained over 90.8% of FEHCOO– and the current value decayed less than 2% after 14 h continuous electrocatalysis at −1.0 V, exhibiting remarkable operational stability in CO2ER. This work optimized electronic structure of metal chalcogenides by heterocations doping toward highly active and selective CO2ER to formate and beyond.

Effects of transition metal doping on surface properties and resistance to Cl− adsorption of α-Al2O3

Under the aqueous environment simulated by VASPsol, structures and work functions of surfaces, adsorption behavior of Cl− on the pure α-Al2O3 (0001) and fourth period transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) doped α-Al2O3 (0001) slabs have been calculated by Vienna Ab-initio Simulation Package (VASP) based on density functional theory (DFT), which ultimately affect the pitting resistance of the target surface. The results evidence that the interlayer spacing of the first and the second atomic layers and characteristic Al-O bond lengths of doped surfaces showed a trend of increasing first and then decreasing with the incremental nuclear charges of doped atoms, which is contrary to the change of work functions and the adsorption energy of Cl− on the corresponding surfaces. These changes are correlative and essentially depend on the variations of the surface charge distribution and surface reactivity. The doping of Sc, Ti, V, Cr, Mn, Fe, Co and Ni is proved to reduce the stability of the Al2O3 surface and makes it easier for Cl− to adsorb on the surface. The doping of Cu and Zn increases the minimum adsorption energy of Cl− on the surface by 0.07 and 0.09 eV, while the surface reactivity was not reduced. The loosening of the upper structure and the decrease of work function are the precursors of surface deterioration.

Fabrication of Large-Area Molybdenum Disulfide Device Arrays Using Graphene/Ti Contacts.

Two-dimensional (2D) molybdenum disulfide (MoS2) is the most mature material in 2D material fields owing to its relatively high mobility and scalability. Such noticeable properties enable it to realize practical electronic and optoelectronic applications. However, contact engineering for large-area MoS2 films has not yet been established, although contact property is directly associated to the device performance. Herein, we introduce graphene-interlayered Ti contacts (graphene/Ti) into large-area MoS2 device arrays using a wet-transfer method. We achieve MoS2 devices with superior electrical and photoelectrical properties using graphene/Ti contacts, with a field-effect mobility of 18.3 cm2/V∙s, on/off current ratio of 3 × 107, responsivity of 850 A/W, and detectivity of 2 × 1012 Jones. This outstanding performance is attributable to a reduction in the Schottky barrier height of the resultant devices, which arises from the decreased work function of graphene induced by the charge transfer from Ti. Our research offers a direction toward large-scale electronic and optoelectronic applications based on 2D materials.

Gas sensing of fibrous red phosphorene towards inorganic air pollutants: Insights from first-principles calculations

Environmental protection and public security make atmospheric monitoring as an important challenge. Two dimensional materials with unique physical and chemical properties are ideal candidates as gas-sensing materials. Here, we report the adsorption characteristics of fibrous red phosphorene based on first-principles calculations. The calculated adsorption energies, Bader’s charge transfer, electronic structures reveal that the adsorption of NO, NO2, O3 and SO2 leads to significant changes in electronic structures, which is suitable for gas sensing. The decrease in work function reveals that fibrous red phosphorene is also a field-effect transistor candidate for the specific detection of NO. Among considered gas molecules, only the sensing performance of NO can be enhanced by applying external in-plane strain. The calculation results could provide an atomistic insight and a better understanding of sensing properties of fibrous red phosphorene.

Substrate-morphology driven tunable nanoscale artificial synapse

ABSTRACT Two-terminal memristive devices are considering as a potential candidate to mimic human brain functionality to enable artificial intelligence. Particularly, two-terminal nanoscale devices are regarded as a promising solution for implementing bio-synapses due to their small dimensions, extremely compact, and low power to operate neuromorphic functions. Here, we demonstrate that the nanoscale charge transport and resistive switching behavior of VOx thin film can be tuned by modulating the substrate morphology. Particularly, the device prepared with flat-Si shows totally distinguished behavior in comprising of reactive ion-etched-Si substrates. Interestingly, conductive atomic force microscopy current maps revealed the electric field inhomogeneity due to a change in substrate morphology. A reliable bipolar resistive switching behavior of the corresponding etched devices have been demonstrated. Due to an increase in the etching time of substrate, an increase in active area and decrease in work function was observed. Further, nanoscale synaptic functions were generated from the corresponding devices, showing a strong conduction path at preferential bright spots of the particular devices. Moreover, finite element simulations confirm the modulation in generation of localized current conduction in particular etched devices by applying tip voltages. These findings represent a new way to generate nanoscale artificial synaptic functions.

Interfacial Charge Transfer Induced Electronic Property Tuning of MoS2 by Molecular FunctionalizationSupported by the National Natural Science Foundation of China (Grant No. 22002031), the Natural Science Foundation of Zhejiang Province (Grant No. LY18F010019), and the Innovation Project in Hangzhou for Returned Scholar.

The modulation of electrical properties of MoS2 has attracted extensive research interest because of its potential applications in electronic and optoelectronic devices. Herein, interfacial charge transfer induced electronic property tuning of MoS2 are investigated by in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy measurements. A downward band-bending of MoS2-related electronic states along with the decreasing work function, which are induced by the electron transfer from Cs overlayers to MoS2, is observed after the functionalization of MoS2 with Cs, leading to n-type doping. Meanwhile, when MoS2 is modified with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4-TCNQ), an upward band-bending of MoS2-related electronic states along with the increasing work function is observed at the interfaces. This is attributed to the electron depletion within MoS2 due to the strong electron withdrawing property of F 4-TCNQ, indicating p-type doping of MoS2. Our findings reveal that surface transfer doping is an effective approach for electronic property tuning of MoS2 and paves the way to optimize its performance in electronic and optoelectronic devices.

Enhanced solar water splitting of an ideally doped and work function tuned {002} oriented one-dimensional WO3 with nanoscale surface charge mapping insights

Overcoming the limitations and understanding the surface charge characteristics of WO3 is essential for achieving efficient photoelectrochemical (PEC) water splitting. Here, we propose an ideal dopant Y to overcome the limitations and engineer WO3 properties and work function with nanoscale surface charge insights for the first time. The doping of Y in WO3 yields, {002} crystal facet oriented 1-D morphology, decrease the bandgap and work function with upward conduction band shift and improves bulk and surface charge transport/transfer properties. The 1.14 at% Y doping shows a record photocurrent of ∼2.25 and 4.85 mA cm−2 (with hole scavenger) at 1.23 V vs RHE with the increased faradaic O2 production efficiency and upward conduction band shift allowing H2 evolution with >95 % of faradaic efficiency. Importantly, nanoscale surface charge mapping was performed, revealing a decrease in work function and the improved charge dynamic insights leading to the enhanced solar water splitting efficiency.

Effect of waiting time of indium tin oxide electrode in entry chamber on organic light-emitting diodes

To obtain efficient organic light-emitting diodes (OLEDs), the degradation in the surface properties of the indium tin oxide (ITO) electrode during the device fabrication should be prevented. In this study, we investigated the effect of the waiting time of ITO in the entry chamber before the transfer to the deposition chamber on the device performances of OLEDs. The current density and luminance of the OLED significantly decreased owing to the reduced hole injection with the increase in the waiting time. The origin of the reduced hole injection was estimated to be (1) the formation of trap sites at the interface between ITO and hole-transport layer and (2) decreased work function of ITO by an interface dipole. These phenomena could be attributed to the contamination of the ITO surface by the waiting under a relatively low vacuum and room light illumination.

Negative Hydrogen and Deuterium Ion Density in a Low Pressure Plasma in Front of a Converter Surface at Different Work Functions

Negative ion sources of neutral beam injection (NBI) systems for future fusion devices like ITER (“The Way” in Latin) rely on the surface conversion of hydrogen (or deuterium) atoms and positive ions to negative ions in an inductively coupled plasma (ICP). The efficiency of this process depends on the work function of the converter surface. By introducing caesium into the ion source the work function decreases, enhancing the negative ion yield. In order to study the isotope effect on the negative ion density at different work functions, fundamental investigations are performed in a planar ICP laboratory experiment where the work function and the negative ion density in front of a sample can be simultaneously and absolutely determined. For work functions above 2.7 eV, the main contribution to the negative hydrogen ion density is solely due to volume formation, which can be modeled via the rate balance model YACORA H−, while below 2.7 eV the surface conversion become significant and the negative ion density increases. For a work function of 2.1 eV (bulk Cs), the H− density increases by at least a factor of 2.8 with respect to a non-caesiated surface. With a deuterium plasma, the D− density measured at 2.1 eV is a factor of 2.5 higher with respect to a non-caesiated surface, reaching densities of surface produced negative ions comparable to the hydrogen case.

Enhanced Efficiency and Mechanical Robustness of Flexible Perovskite Solar Cells by Using HPbI3 Additive

High efficiency and mechanical stability are in great demand for commercial applications of the flexible perovskite solar cells (PSCs) in portable and wearable electronics. Herein, power conversion efficiency (PCE) and mechanical robustness of the methylammonium (MA)‐free flexible PSCs are simultaneously enhanced by incorporating a HPbI3 additive with optimal content in the perovskite precursor solution. The HPbI3 additive facilitates an improved morphology and crystallinity, as well as a decreased work function of the perovskite films, leading to improved device performance. As a result, PCEs of 20.1% and 18.74% are obtained for the rigid and flexible PSCs, respectively, which are among the best results for inverted MA‐free PSCs. The flexible PSCs maintain 95% and 70% of the initial PCE value after 1000 and 5000 bending cycles, respectively, at a bending radius of 4 mm. The current result reveals that using the HPbI3 additive is a universal strategy to enhance performance of the flexible PSCs effectively and promote the development of the perovskite‐based photovoltaics.

Ionically Gated Small-Molecule OPV: Interfacial Doping of Charge Collector and Transport Layer.

We demonstrate an improvement in the performance of organic photovoltaic (OPV) systems based on small molecules by ionic gating via controlled reversible n-doping of multi-wall carbon nanotubes (MWCNTs) coated on fullerene electron transport layers (ETLs): C60 and C70. Such electric double-layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning of the electronic concentration in MWCNTs and the fullerene planar acceptor layers, increasing it by orders of magnitude. This leads to the decrease of the series and increase of the shunt resistances of OPVs and allows use of thick (up to 200 nm) ETLs, increasing the durability of OPVs. Two stages of OPV enhancement are described upon the increase of gating bias Vg: at small (or even zero) Vg, the extended interface of ILs and porous transparent MWCNTs is charged by gating, and the fullerene charge collector is significantly improved, becoming an ohmic contact. This changes the S-shaped J-V curve via improving the electron collection by an n-doped MWCNT cathode with an ohmic interfacial contact. The J-V curves further improve at higher gating bias Vg due to the increase of the Fermi level and decrease of the MWCNT work function. At the next qualitative stage, the acceptor fullerene layer becomes n-doped by electron injection from MWCNTs while ions of ILs penetrate into the fullerene. At this step, the internal built-in field is created within OPV, which helps in exciton dissociation and charge separation/transport, increasing further the Jsc and the fill factor. The ionic gating concept demonstrated here for most simple classical planar small-molecule OPV cells can be potentially applied to more complex highly efficient hybrid devices, such as perovskite photovoltaic with an ETL or a hole transport layer, providing a new way to tune their properties via controllable and reversible interfacial doping of charge collectors and transport layers.

Charge Transfer Doping of 2D PdSe2 Thin Film and Its Application in Fabrication of Heterostructures

AbstractPalladium diselenide (PdSe2) is an emerging 2D material with exotic optical and electrical properties and widely tunable layer dependent band gap in the infrared regime. The ability to further tune the electronic properties of PdSe2 via doping is of fundamental importance for a wide range of electronic and optoelectronic device applications. Here, surface charge transfer doping of chemical vapor deposition grown p‐type PdSe2 thin film using benzyl viologen (BV) molecules is reported. The electrical transport measurements of the PdSe2 device show an increase in resistance by ≈1700 percent from 2.1 MΩ for the pristine sample to 36.2 MΩ upon BV doping, revealing electrons are transferred from BV molecules to PdSe2 resulting in an n‐doping. Raman characterization shows a red‐shift and broadening of A3g characteristic peak for the doped sample, while X‐ray photoelectron spectroscopy shows a negative shift in Pd 3d and Se 3d binding energies confirming n‐doping by BV. Kelvin force probe microscopy measurements show a ≈0.3 eV decrease in work function for doped PdSe2, consistent with the n‐doping by BV molecules. A selective doping of PdSe2 channel is implemented for the fabrication of lateral heterojunction device which shows good current rectifying behavior with a rectification ratio of up to ≈55.

Phosgene Gas Sensing of Ti2CT2 (T = F−, O−, OH−) MXenes

AbstractThe phosgene gas sensing properties of experimentally produced 2D titanium carbide MXenes with surface terminations (Ti2CT2: T = F−, O−, OH−) are studied by first‐principles calculations. The effect of Ti and C vacancy defects, which are frequently created in synthesizing MXenes, on the structural, mechanical, electronic, and gas adsorption properties are studied to analyze the phosgene gas sensing performance of Ti2CT2 MXenes. Pristine and defective Ti2C, Ti2CF2, and Ti2C(OH)2 show metallicity, making them ideal for gas sensing. Ti (C) vacancy defects in Ti2CO2, Ti2CF2, and Ti2C(OH)2 cause an increase (decrease) in work function, resulting in enhanced interaction between the MXene surface and a gas molecule. When a phosgene gas molecule is exposed on the surface of MXenes, only Ti2C(OH)2 shows stable adsorption and charge transfer. An ultra‐low recovery time, the time between adsorption and desorption of a phosgene gas molecule, is achieved for both pristine and defective Ti2C(OH)2 with Ti vacancy. The results elucidate that a Ti2C(OH)2 based sensor has a high potential for efficient reversible phosgene detection.

Ab-initio characterization of B4C3 monolayer as a toxic gases sensing material

The gas sensing behavior of different gas molecules was investigated by means of First-principles calculations based on density-functional theory. Due to the unique properties of the recently predicted B4C3 monolayer, it is widely to be studied and utilized towards the future promising applications of nano-electronics. We study the adsorption behavior of various gas molecules such as NO, H2S, CO, NH3 and SO2 on B4C3 sheet based on the adsorption energy, conductivity, recovery time, work function and charge transfer. The results are compared with other well-known 2D materials based gas sensors like BC3, silicene and germanene and it was found that B4C3 exhibit a preferable gas sensing performance. Charge transfer analysis depicts that CO, NO, and SO2 molecules behave as the charge acceptors whereas H2S and NH3 molecules act as the charge donors. It was also found that a 99% change in the band structure of B4C3 monolayer after the adsorption of NO shows an excellent increase in the conductivity. The calculated work function of B4C3 monolayer after the adsorption of gas molecules decreases for H2S, NH3, SO2 and NO while an increase was observed in the case of CO. This decrease in work function shows an inclination of B4C3 monolayer towards the gas sensing applications. The variation in bandgap affects the sensitivity and selectivity of B4C3 that exhibits its applications for utilization in gas sensing applications.