Work Function Changes Research Articles

Boron‐Rich Boron Nitride Nanotubes as Highly Selective Adsorbents for Selected Diatomic Air Pollutants: A DFT Study

AbstractBoron‐rich boron nitride nanotubes (BN‐BNNTs), which have boron antisite defects (BN), adsorb gas molecules more favorably as compared to their pristine counterparts because of the localized states of antisites. Using computational chemistry methods, the structural, adsorptive, and electronic properties of selected diatomic air pollutants (CO, NO, and SO) on BN‐BNNT (8,0), with a particular focus on the antisites, are investigated. It is found that CO adsorbs on BN with 180° angle (∠BNCO) while NO and SO adsorb with 142° angle (∠BNNO) and 116° angle (∠BNSO), respectively. This difference is ascribed to the repulsive interaction originated from lone pair electrons on N and S. Adsorption energy of CO, NO, and SO molecules dominates over that of O2 and N2 and is independent of their radii of BN‐BNNTs. The practical capacity of these pollutant molecules is calculated to be ≈5 mmol g–1 (14 wt%) under ambient conditions. Therefore, our results show that BN‐BNNT can be used as a highly selective adsorbent for diatomic air pollutants. BN‐BNNT as sensing materials in terms of change in bandgap and work function through the adsorption is also discussed.

Adsorption behaviour of CF4 and COF2 gas on the GaN monolayer doped with Pt catalytic: A first-principles study

GaN is an n-type semiconductor with a wide band gap and has broad application prospects in the field of gas detection. This paper uses the first-principles to study the adsorptions of five typical decomposition gases (CF4, COF2, CO2, O2 and N2) of SF6 alternative gas C4F7N by Pt doped GaN (Pt-GaN) monolayer and explores the feasibility of its application in the detection of decomposition components of C4F7N. The results show that the adsorption capacity of Pt-GaN to O2 (-0.263 eV) is greater than the other four gases (<-0.800 eV), and the adsorption mechanism of Pt-GaN as a resistive sensor is analysed in combination with its charge density difference and density of states represented by CF4 and COF2. On the other hand, the theoretical response values of these gases are obvious, amongst them, the theoretical recovery time of CF4 is almost 0, and the theoretical recovery time of COF2, CO2 and O2 at 498 K is only 2.2 s, 0 s and 0.149 s, respectively, indicating that Pt-GaN has industrial advantages in gas detection. After the five gases are adsorbed, the trend of work function change is different, which provides the possibility for Pt-GaN to be used as a field-effect transistor sensor to selectively detect gases All the content in this article provides theoretical guidance for Pt-GaN as a gas-sensitive material to realize the detection of C4F7N decomposition components.

Effects of Defect on Work Function and Energy Alignment of PbI2: Implications for Solar Cell Applications

Two-dimensional (2D) layered lead iodide (PbI2) is an important precursor and common residual species during the synthesis of lead-halide perovskites. There currently exist some debates and uncertainties about the effect of excess PbI2 on the efficiency and stability of the solar cell with respect to its energy alignment and energetics of defects. Herein, by applying the first-principles calculations, we investigate the energetics, changes of work function and the defective levels associated with the iodine vacancy (VI) and interstitial iodine (II) defects of monolayer PbI2 (ML-PbI2). We find that the PbI2 has a very low formation energy of VI of 0.77 and 0.19 eV for dilute and high concentration, respectively, reflecting coalescence tendency of isolated VI, much lower than that of vacancies in other 2D materials like phosphorene. Similar to VI, a low formation energy of II of 0.65 eV is found, implying a high population of such defects. Both defects generate in-gap defective levels which are mainly due to the unsaturated chemical bonds of p-orbitals of exposed Pb or inserted I. Such rich defective levels allow the VI and II as the reservoir or sinks of electron/hole carriers in PbI2. Our results suggest that the remnant PbI2 in perovskite MAPbI3 (or FAPbI3) would play dual opposite roles in affecting the efficiency of the perovskite: (1) Forming Schottky-type interface with MAPbI3 (or FAPbI3) in which the built-in potential would facilitate the electron-hole separation and prolong the carrier lifetime; (2) Acting as the recombination centers due to the deep defective levels. To promote the efficiency by the Schottky effect, our work reveals that the II defect is favored, and to reduce the recombination centers the VI defect should be suppressed. Our results shall be beneficial in improving strategies for the related optoelectronics applications.

From a bistable adsorbate to a switchable interface: tetrachloropyrazine on Pt(111).

Virtually all organic (opto)electronic devices rely on organic/inorganic interfaces with specific properties. These properties are, in turn, inextricably linked to the interface structure. Therefore, a change in structure can introduce a shift in function. If this change is reversible, it would allow constructing a switchable interface. We accomplish this with tetrachloropyrazine on Pt(111), which exhibits a double-well potential with a chemisorbed and a physisorbed minimum. These minima have significantly different adsorption geometries allowing the formation of switchable interface structures. Importantly, these structures facilitate different work function changes and coherent fractions (as would be obtained from X-ray standing wave measurements), which are ideal properties to read out the interface state. We perform surface structure search using a modified version of the SAMPLE approach and account for thermodynamic conditions using ab initio thermodynamics. This allows investigating millions of commensurate as well as higher-order commensurate interface structures. We identify three different classes of structures exhibiting different work function changes and coherent fractions. Using temperature and pressure as handles, we demonstrate the possibility of reversible switching between those different classes, creating a dynamic interface for potential applications in organic electronics.

Strain engineering of electronic structure and mechanical switch device for edge modified Net-Y nanoribbons

Net-Y is a new two-dimensional carbon structure, which has attracted research interest recently. Here, we study the relevant AB-type ribbons with edge modification, focusing on their strain controlling effects on their electronic structure and device characteristics. Intrinsic ribbons are metallic, but hydrogen or oxygen termination can transform them into semiconductors. Applying strain can effectively control the band gap size, resulting in a transition from an indirect band gap to a smaller direct band gap under appropriate strain, favorably to light absorbing. Strain can also change the work function of ribbons, especially for compressive strains, the work function is lowered significantly, which is beneficial to the improving of the field emission behaviors of ribbons. The analysis demonstrates that the change in band gap size is closely related to the variation of bonding and non-bonding composition between atoms with strain, while the change of work function is due to the variation of the attraction force and repulsion force between atoms upon strain. More interestingly, the strain can significantly regulates the &lt;i&gt;I&lt;/i&gt;-&lt;i&gt;V&lt;/i&gt; characteristic of device based on related ribbons. Therefore, a strain-gated mechanical switch with a very high current switching ratio &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;on&lt;/sub&gt;/&lt;i&gt;I&lt;/i&gt;&lt;sub&gt;off&lt;/sub&gt; can be obtained by making it reversibly work between the “on” and “off” states with stretching and compressing ribbons, which is of great significance in developing the logic circuits for flexible wearable electronic devices.

Computational investigation of van der Waals corrections in the adsorption properties of molecules on the Cu(111) surface.

Here, we report a computational investigation on the role of the most common van der Waals (vdW) corrections (D2, D3, D3(BJ), TS, TS+SCS, TS+HI, and dDsC) employed in density functional theory (DFT) calculations within local and semilocal exchange-correlation functionals to improve the description of the interaction between molecular species and solid surfaces. For this, we selected several molecular model systems, namely, the adsorption of small molecules (CH3, CH4, CO, CO2, H2O, and OH) on the close-packed Cu(111) surface, which bind via chemisorption or physisorption mechanisms. As expected, we found that the addition of the vdW corrections enhances the energetic stability of the Cu bulk in the face-centered cubic structure, which contributes to increasing the magnitude of the mechanical properties (elastic constants, bulk, Young, and shear modulus). Except for the TS+SCS correction, all vdW corrections substantially increase the surface energy, while the work function changes by about 0.05 eV (largest change). However, we found substantial differences among the vdW corrections when comparing its effects on interlayer spacing relaxations. Based on bulk and surface results, we selected only the D3 and dDsC vdW corrections for the study of the adsorption properties of the selected molecules on the Cu(111) surface. Overall, the addition of these vdW corrections has a greater effect on weakly interacting systems (CH4, CO2, H2O), while the chemisorption systems (CH3, CO, OH) are less affected.

Quantitative electronic structure and work-function changes of liquid water induced by solute.

Recent advancement in quantitative liquid-jet photoelectron spectroscopy enables the accurate determination of the absolute-scale electronic energetics of liquids and species in solution. The major objective of the present work is the determination of the absolute lowest-ionization energy of liquid water, corresponding to the 1b1 orbital electron liberation, which is found to vary upon solute addition, and depends on the solute concentration. We discuss two prototypical aqueous salt solutions, NaI(aq) and tetrabutylammonium iodide, TBAI(aq), with the latter being a strong surfactant. Our results reveal considerably different behavior of the liquid water 1b1 binding energy in each case. In the NaI(aq) solutions, the 1b1 energy increases by about 0.3 eV upon increasing the salt concentration from very dilute to near-saturation concentrations, whereas for TBAI the energy decreases by about 0.7 eV upon formation of a TBAI surface layer. The photoelectron spectra also allow us to quantify the solute-induced effects on the solute binding energies, as inferred from concentration-dependent energy shifts of the I− 5p binding energy. For NaI(aq), an almost identical I− 5p shift is found as for the water 1b1 binding energy, with a larger shift occurring in the opposite direction for the TBAI(aq) solution. We show that the evolution of the water 1b1 energy in the NaI(aq) solutions can be primarily assigned to a change of water's electronic structure in the solution bulk. In contrast, apparent changes of the 1b1 energy for TBAI(aq) solutions can be related to changes of the solution work function which could arise from surface molecular dipoles. Furthermore, for both of the solutions studied here, the measured water 1b1 binding energies can be correlated with the extensive solution molecular structure changes occurring at high salt concentrations, where in the case of NaI(aq), too few water molecules exist to hydrate individual ions and the solution adopts a crystalline-like phase. We also comment on the concentration-dependent shape of the second, 3a1 orbital liquid water ionization feature which is a sensitive signature of water–water hydrogen bond interactions.

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.

Heterojunction Engineering and Ideal Factor Optimization Toward Efficient MINP Perovskite Solar Cells

AbstractThe pursuit of high power conversion efficiency (PCE) and cost‐effective perovskite solar cells (PSCs) has spawned many innovative device structure designs. Compared with the traditional NIP type PSCs, electron transport layer (ETL) free PSCs have attracted growing attention due to their enormous potential in large area, low‐cost flexible application. However, there is still a lack of in‐depth understanding of the energy level arrangement indium tin oxide (ITO)/perovskite interface, resulting in poor device performance. Here, a metal/insulator/n‐type perovskite/p‐type spiro‐MeOTAD (MINP) structure is proposed to elaborate on the influence of the apparent work function and contact barrier change of the metal–semiconductor (MS) and metal–insulator–semiconductor (MIS) junction on the carrier transfer and collection. Common and easily available 5‐amino‐valeric acid is inserted into the ITO/perovskite interface to form an insulating dipole layer and to ensure quasi‐ohmic contact at the front interface. Consequently, the device changes from Schottky/PN cascade heterojunction type to a single PN heterojunction device with its ideal factor decreasing from 3.35 to 2.05. Accordingly, the champion device achieves 19.37% PCE with significantly increased Voc and FF compared to the pristine device. This work provides a facile and effective method to improve the application potential of novel ETL‐free PSCs.

How much does surface polymorphism influence the work function of organic/metal interfaces?

Molecules adsorbing on metal surfaces form a variety of different surface polymorphs. How strongly this polymorphism affects interface properties is a priori unknown. In this work we investigate how strongly the surface polymorphism influences the interface work functions for various organic/metal interfaces. To evaluate the whole bandwidth of possible polymorphs, we perform full theoretical structure search, probing millions of polymorph candidates. All of these candidates might be observed in reality, either by kinetic trapping or by thermodynamic occupation. Employing first-principles calculations and machine learning we predict and analyze the work function changes for those millions of candidates for three physically distinct model systems: the weakly interacting naphthalene on Cu(111), the strongly interacting anthraquinone on Ag(111), and tetracyanoethylene, which undergoes a reorientation from lying to standing polymorphs on the Cu(111) surface. These thorough investigations indicate that kinetic trapping of flat-lying molecules can lead to work function differences of a few hundred meV. If the molecules also reorientate, this can increase to a change of several eV. We further show that the spread in work function decreases when working in thermodynamic equilibrium, but thermally occupied phases still lead to an intrinsic uncertainty at elevated temperatures.

High-temperature oxidation and reduction of the inverse ceria/Cu(111) catalyst characterized by LEED, STM, nc-AFM and KPFM

The inverse catalyst ‘cerium oxide (ceria) on copper’ has attracted much interest in recent time because of its promising catalytic activity in the water–gas-shift reaction and the hydrogenation of CO2. For such reactions it is important to study the redox behaviour of this system, in particular with respect to the reduction by H2. Here, we investigate the high-temperature O2 oxidation and H2 reduction of ceria nanoparticles (NPs) and a Cu(111) support by low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). After oxidation at 550 °C, the ceria NPs and the Cu(111) support are fully oxidized, with the copper oxide exhibiting a new oxide structure as verified by LEED and STM. We show that a high H2 dosage in the kilo Langmuir range is needed to entirely reduce the copper support at 550 °C. A work function (WF) difference of △ϕ rCeria/Cu–Cu ≈ −0.6 eV between the ceria NPs and the metallic Cu(111) support is measured, with the Cu(111) surface showing no signatures of separated and confined surface regions composed by an alloy of Cu and Ce. After oxidation, the WF difference is close to zero (△ϕ Ceria/Cu–Cu ≈ −0.1…0 eV), which probably is due to a WF change of both, ceria and copper.

Dopant-Tunable Ultrathin Transparent Conductive Oxides for Efficient Energy Conversion Devices

HighlightsDopant-tunable transparent conductive oxide (≤ 50 nm) fabricated via electric-field-driven metal implantation (m-TCOs; m= Ni, Ag, and Cu) is demonstrated.The m-TCOs exhibit ultrahigh transparency, low sheet resistance, and broad work function tunability, leading to outstanding performance in various optoelectronic devices.The work function change is attributed to the interstitial metal atoms that provide the empty d-orbital, resulting in the shift of the Fermi level.

Flexible Organic Thin-Film Transistors With High Mechanical Stability on Polyimide Substrate by Chemically Plated Silver Electrodes

Conventionally, silver (Ag) electrodes for organic thin-film transistors (OTFTs) are prepared by evaporated method at high temperature. In this work, we report a new methodology to fabricate the Ag source/drain electrodes of flexible OTFTs arrays by chemically plated technique on polyimide substrate at room temperature. The indacenodithiophene-co-benzothiadiazole (IDT-BT) OTFTs with chemically plated electrodes obtain the saturation mobility of 0.25 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}\text{V}^{-1}\text{s}^{-1}$ </tex-math></inline-formula> . Furthermore, compared with high-temperature thermal evaporation method, the subthreshold swing of the IDT-BT OTFTs decreases from 1.97 to 0.89 V/dec with chemically plated Ag source/drain electrodes. The smaller threshold voltage and contact resistance are also obtained, owing to the change of work function. The bending tests indicate that the electrical properties of the devices maintain unchanged during tensile bending at a radius up to 1.4 mm. Moreover, the electrical property of the devices remains stable at a tensile bending cycle of 1000 cycles at a radius of 2.5 mm. This proposed methodology has great potential for flexible and wearable electronic devices.

Nitrogen-Containing Gas Sensing Properties of 2-D Ti2N and Its Derivative Nanosheets: Electronic Structures Insight.

In this work, the potentials of two-dimensional Ti2N and its derivative nanosheets Ti2NT2(T=O, F, OH) for some harmful nitrogen-containing gas (NCG) adsorption and sensing applications have been unveiled based on the quantum-mechanical Density Functional Theory calculations. It is found that the interactions between pure Ti2N and NCGs (including NO, NO2, and NH3 in this study) are very strong, in which NO and NO2 can even be dissociated, and this would poison the substrate of Ti2N monolayer and affect the stability of the sensing material. For the monolayer of Ti2NT2(T=O, F, OH) that is terminated by functional groups on surface, the adsorption energies of NCGs are greatly reduced, and a large amount of charges are transferred to the functional group, which is beneficial to the reversibility of the sensing material. The significant changes in work function imply the good sensitivity of the above mentioned materials. In addition, the fast response time further consolidates the prospect of two-dimensional Ti2NT2 as efficient NCGs’ sensing materials. This theoretical study would supply physical insight into the NCGs’ sensing mechanism of Ti2N based nanosheets and help experimentalists to design better 2-D materials for gas adsorption or sensing applications.

A Density Functional Theory study for adsorption and sensing of 5-Fluorouracil on Ni-doped boron nitride nanotube

In this research, the use of Ni-doped (8,0) boron nitride nanotube (Ni-BNNT) as both a sensor and an adsorbent for 5-Fluorouracil (5-FU) molecule was investigated by Density Functional Theory (DFT) method. The B3LYP method with 6-31G(d,p) basis set have been utilized. Six different adsorption configurations have been studied theoretically. After 5-FU adsorption on Ni-BNNT, the adsorption energy values were calculated as negative values in all configurations. Adsorption energy (ΔE) value and adsorption enthalpy (ΔH) value reached −0.75 eV and −0.78 eV values, respectively. Moreover, Gibbs free energy changes were computed to be negative values in all configurations and thus it was determined that the process could occur spontaneously. Charge transfer occurred between all configurations of Ni-BNNT and the 5-FU molecule. The HOMO-LUMO gap decreased in the NiN-BNNT (Ni-doped instead of N) structure, while it increased in the NiB-BNNT (Ni-doped instead of B) structure. The Ni-BNNT structure assistances from a recovery time as a sensor for 5-FU drug molecule. Moreover, the workfunction change occurred somewhat in all configurations, but it was calculated that there was more change (16.22%) in the NiN–O1-BNNT configuration. In addition, solvent (water) effect was also examined. Consequently, Ni-doped (8,0) BNNT structure can be used as both a sensor and an adsorbent for 5-FU molecule at room temperature.

Graphene-substrate decoupling by S segregation. A LEEM/LEED study

The growth of graphene on Ni(110) is studied with low energy electron microscopy, low energy electron diffraction (LEED) and associated methods at several sulfur coverages obtained by surface segregation and compared with growth on the clean surface. On the clean surface graphene grows nearly epitaxial with small azimuthal alignment fluctuations. The segregated S reduces the interaction between graphene and Ni, which results in the formation of several azimuthal orientations. The reduced film-substrate interaction is evident in the details of the LEED pattern, in the work function change and the energy dependence of the specular reflected intensity. This study clarifies the differences between earlier studies of graphene growth on Ni(110) and suggests using impurity surface segregation as means for decoupling of graphene from the substrate.

Observing Electrochemical Reactions on Suspended Graphene: An Operando Kelvin Probe Force Microscopy Approach

AbstractAn electrochemical micro‐reactor sealed with a single‐layer graphene (SLG) membrane is demonstrated that allows straightforward measurement with established scanning probe microscopies. SLG serves as a working electrode which separates the liquid electrochemical environment from the ambient to enable direct energy‐level determination. Kelvin probe force microscopy (KPFM) thereby reveals the shifts in Fermi‐level of suspended SLG under electrochemical reaction conditions in aqueous alkaline media. Polymer‐free transfer to create suspended SLG minimizes contributions to doping related to any support or contaminants, such that changes in work function (WF) relate predominantly to the electrochemical system under study. These WF changes are rationalized in the context of a simple model of electrochemical gating, providing insight into the interplay between electronic and electrochemical doping (through redox of water) of suspended graphene. Further changes in WF are attributable to the reversible functionalization of graphene during the oxygen evolution reaction. Mechanical changes in the suspended graphene in the form of bulging also occur, which are attributed to electro‐wetting of graphene induced by charge‐carrier doping.

Completely foldable electronics based on homojunction polymer transistors and logics.

An increase in the demand for completely foldable electronics has motivated efforts for the development of conducting polymer electrodes having extraordinary mechanical stability. However, weak physical adhesion at intrinsic heterojunctions has been a challenge in foldable electronics. This paper reports the completely foldable polymer thin-film transistors (PTFTs) and logic gate arrays. Homojunction-based PTFTs were fabricated by selectively doping p-type diketopyrrolopyrrole-based semiconducting polymer films with FeCl3 to form source/drain electrodes. The doping process caused a gradual work function change with depth, which promoted charge injection to semiconducting regions and provided a low contact resistance. In addition, the interfacial adhesion in the PTFTs was improved by interfacial cross-linking between adjacent component layers. The electrical performance of the resulting PTFTs was maintained without noticeable degradation even after extreme folding, suggesting that the proposed fabrication strategy can further be applied to various semiconducting polymers for the realization of foldable electronics.

Adsorption of Carbon Dioxide on Mono-Layer Thick Oxidized Samarium Films on Ni(100).

Studies of adsorption of CO on nanoscopic surfaces are relevant for technological applications in heterogeneous catalysis as well as for sorption of this important greenhouse gas. Presently, adsorption of carbon dioxide on pure and oxidized thin samarium layers near mono-layer thickness on Ni(100) has been investigated by photoelectron spectroscopy and temperature programmed desorption. It is observed that very little CO adsorb on the metallic sample for exposures in the vacuum regime at room temperature. For the oxidized sample, a large enhancement in CO adsorption is observed in the desorption measurements. Indications of carbonate formation on the surface were found by C 1s and O 1s XPS. After annealing of the oxidized samples to 900 K very little CO was found to adsorb. Differences in desorption spectra before and after annealing of the oxidized samples are correlated with changes in XPS intensities, and with changes in sample work function which determines the energy difference between molecular orbitals and substrate Fermi level, and thus the probability of charge transfer between adsorbed molecule and substrate.

Adsorption and coadsorption of H and Li on Ag(100) surface: DFT studies including dispersion correction

We report on the results of DFT calculations of H, Li adsorption and Li+H coadsorption on Ag(100) surface. By using the GGA–PBE functional and DFT+d3, we obtain the adsorption energies, coadsorption energies, structural parameters, work function change and dipole moments. Our GGA–PBE calculations show that H prefers the bridge site at 0.25 and 0.50 ML coverages but hollow site at 0.75 and 1.00 ML coverages. With the DFT+d3 functional, we find that H is most stable at bridge site at 0.25 ML coverage, however, it prefers the hollow site at 0.50 ML, 0.75 ML and 1.00 ML coverages. For Li adsorption, both GGA–PBE and DFT+d3 functionals show that Li is most stable at hollow site. For Li and H coadsorption, we find that Li at the hollow site with H at the nearest-neighbor bridge site as the most energetically favorable configuration. The evolution of local electronic structures of the adsorbates and Ag(100) surface has been analyzed in terms of local density states. Our calculations show a decrease in the work function when Li and H are coadsorbed on the Ag(100) surface which suggests a possible transfer of electron from the Li-H adsorbates to the Ag(100) surface.