Work Function Changes Research Articles

Growth of uniform Mg-doped p-AlGaN nanowires using plasma-assisted molecular beam epitaxy technique for UV-A emitters

In this manuscript, we have shown the growth and extensive structural and optical characteristic of the uniformly Mg-doped Al0.23Ga0.77N (UV-A region, λ ∼ 323 nm) nanowire. The Kelvin probe force microscopy was employed to determine the profile of holes in p-type AlGaN nanowires by measuring the work function changes induced by Mg incorporation. The influence of surface band bending on doping concentration has thoroughly been discussed. Our experiment confirms the homogeneous incorporation of Mg throughout the nanowire without any top surface Mg segregation. In this work, we have also demonstrated a comprehensive analysis of acceptor states induced thermal quenching behaviour in the optical transition of Mg-doped AlGaN nanowire. We propose a phenomenological model, based on the rate equation which confirms that achieving higher ‘hole’ (p-doping) concentration in AlGaN nanowire (>1018 cm−3) is more conducive than the planar counterpart if the growth of NWs is carried out at optimized process conditions. This rate equation-based model has also demonstrated the influence of sidewall surface passivation in those AlGaN nanowires.

How adsorbed H, O, OH, and Cl affect plain adsorption of imidazole on copper

In real environments, metal surfaces are never clean. Even during active dissolution in corrosion, they are likely covered with adsorbed species such as O, OH, H, and Cl. For this reason, we addressed how such species (hereinafter labeled as X(ads)) affect the adsorption bonding of imidazole—used as an archetypal model of azole corrosion inhibitors—on Cu(111). To this end, we performed a systematic high-throughput DFT study by considering over 400 different adsorption configurations, where the effects of coverage, the type of X(ads) species, and the distance between imidazole and X(ads) were scrutinized. Our calculations indicate that O and Cl enhance the adsorption bonding of imidazole, H has almost no effect, whereas OH either diminishes or has a negligible impact on the adsorption of imidazole. The effect of the X(ads) species on the imidazole adsorption usually diminishes with the increasing distance between X(ads) and imidazole and with decreasing X(ads) coverage. Three coadsorption effects of X(ads) on non-dissociative adsorption of imidazole were identified. The first stabilizing effect, operative for O and OH, is due to the hydrogen bond formation with the nearby adsorbed imidazole. The second stabilizing effect, relevant for O and Cl, is due to the X(ads) induced enhancement of the N–Cu bond between the nearby adsorbed imidazole and the surface; this effect enhances with increasing coverage of X(ads). The last effect is related to the X(ads) induced work function change and can be stabilizing (for O and Cl), negligible (for H), or destabilizing (in the case of OH) for adsorption of imidazole and explains why the impact of X(ads) on adsorption bonding of imidazole diminishes with the X(ads) coverage. Finally, if the coverage of X(ads) is too high, then the chemisorption of imidazole is prevented either sterically or due to the unavailability of free surface sites.

First-principles study of the contact resistance and optoelectronic properties of PdSe2/MoTe2 van der Waals heterostructure optoelectronic devices

In this study, density functional theory (DFT) calculations are carried out to investigate the contact barriers with different electrodes (Sc, Al, Ni, Pd), transport properties, and light-absorption characteristics of the PdSe2/MoTe2 van der Waals (vdW) heterostructure. The lower n-type Schottky barriers of Sc-PdSe2 and Pd-MoTe2 are demonstrated. The mechanism of interlayer contact of Sc, Al-PdSe2, and Pd, Ni-MoTe2 is confirmed by the following analysis. First, the interlayer charge transfer to form an electric dipole, and the presence of an interfacial dipole, is shown to lead to changes in the metal work function and the band structure off-sets of PdSe2, MoTe2, thus changing the Schottky barrier height (SBH). Second, from the interfacial density of states (DOS) analysis, we demonstrate the strong hybridization of metal interface atoms with monolayer 2D material interlayer atoms, explaining the generation of gap states. In the end, the heterostructure inter-TMDs charge-transfer optical transition mechanism of the MoTe2 VBM to the PdSe2 CBM is demonstrated from the electronic structure analysis of the PdSe2/MoTe2 heterostructure. The low contact resistance and high light absorption explain the excellent performance of PdSe2/MoTe2 heterostructure optoelectronic devices. This work can provide theoretical guidance for designing next-generation electronic and optoelectronic devices.

Ab Initio Studies of Work Function Changes of CO Adsorption on Clean and Pd-Doped ZnGa2O4(111) Surfaces for Gas Sensors

We performed first-principles calculations to study the adsorption of the CO molecules on both clean and Pd-doped ZnGa2O4(111) surfaces. The adsorption reaction and work function of the CO adsorption models were examined. The CO molecules on the clean and Pd-doped ZnGa2O4(111) surfaces exhibit maximum work function changes of −0.55 eV and −0.79 eV, respectively. The work function change of Pd-doped ZnGa2O4(111) for detecting CO is 1.43 times higher than that of the clean ZnGa2O4(111). In addition, the adsorption energy is also significantly reduced from −1.88 eV to −3.36 eV without and with Pd atoms, respectively. The results demonstrate ZnGa2O4-based gas sensors doped by palladium can improve the sensitivity of detecting CO molecules.

Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering.

ConspectusControlling the physical and chemical properties of surfaces and interfaces is of fundamental relevance in various areas of physical chemistry and a key issue of modern nanotechnology. A highly promising strategy for achieving that control is the use of self-assembled monolayers (SAMs), which are ordered arrays of rodlike molecules bound to the substrate by a suitable anchoring group and carrying a functional tail group at the other end of the molecular backbone. Besides various other applications, SAMs are frequently used in organic electronics for the electrostatic engineering of interfaces by controlling the interfacial level alignment. This is usually achieved by introducing a dipolar tail group at the SAM–semiconductor interface. Such an approach, however, also changes the chemical character of that interface, for example, affecting the growth of subsequent layers. A strategy for avoiding this complication is to embed polar groups into the backbones of the SAM-forming molecules. This allows disentangling electronic interface engineering and the nucleation of further layers, such that both can be optimized independently. This novel concept was successfully demonstrated for both aliphatic and aromatic SAMs on different application-relevant substrates, such as gold, silver, and indium tin oxide. Embedding, for example, ester and pyrimidine groups in different orientations into the backbones of the SAM-forming molecules results in significant work-function changes. These can then be fine-tuned over a wide energy range by growing mixed monolayers consisting of molecules with oppositely oriented polar groups. In such systems, the variation of the work function is accompanied by pronounced shifts of the peaks in X-ray photoelectron spectra, which demonstrates that electrostatically triggered core-level shifts can be as important as the well-established chemical shifts. This illustrates the potential of X-ray photoelectron spectroscopy (XPS) as a tool for probing the local electrostatic energy within monolayers and, in systems like the ones studied here, makes XPS a powerful tool for studying the composition and morphology of binary SAMs. All these experimental observations can be rationalized through simulations, which show that the assemblies of embedded dipolar groups introduce a potential discontinuity within the monolayer, shifting the energy levels above and below the dipoles relative to each other. In molecular and monolayer electronics, embedded-dipole SAMs can be used to control transition voltages and current rectification. In devices based on organic and 2D semiconductors, such as MoS2, they can reduce contact resistances by several orders of magnitude without adversely affecting film growth even on flexible substrates. By varying the orientation of the embedded dipolar moieties, it is also possible to build p- and n-type organic transistors using the same electrode materials (Au). The extensions of the embedded-dipole concept from hybrid interfaces to systems such as metal–organic frameworks is currently underway, which further underlines the high potential of this approach.

On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat‐Lying on Various Metals and Hetero‐Structures (Adv. Mater. Interfaces 18/2022)

Buried Interface Dipoles The formation of a buried interface dipole at a copper/2D cesium/organic hetero-interface, which can be manipulated by the outer organic monolayer is reported in article number 2200361 by Maximilian Dreher, Gregor Witte, and co-workers. By combined experimental and theoretical methods the contribution of the inner and outer interface dipoles could be deciphered and their influence on the overall work function shift identified.

Attenuation of Photoelectron Emission by a Single Organic Layer.

We report an in situ study of the thin-film growth of cobalt-phthalocyanine on Ag(100) surfaces using photoelectron emission microscopy (PEEM) and the Anderson method. Based on the Fowler–DuBridge theory, we were able to correlate the evolution of the mean electron yield acquired with PEEM for coverages up to two molecular layers of cobalt-phthalocyanine to the global work function changes measured with the Anderson method. For coverages above two monolayers, the transients measured with the Anderson method and those obtained with PEEM show different trends. We exploit this discrepancy to determine the inelastic mean free path of the low-energy electrons while passing through the third layer of CoPc.

On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat‐Lying on Various Metals and Hetero‐Structures

AbstractModifying the work function of metal electrodes by monolayers of molecules with specifically tailored electronic properties is a versatile tool, but such chemical modifications often also affect the adsorption geometry and packing density, making microscopic modeling difficult. Using scanning tunneling microscopy, it is shown that the recently synthesized partially fluorinated dinaphthothienothiophenes (DNTTs) adopt the same interface structure on different metal substrates independent of the degree of fluorination. Combining Kelvin probe measurements and density functional theory (DFT) calculations, a highest occupied molecular orbital (HOMO) pinning effect for such FxDNTTs on Au(111) and Ag(111) induced by collective electrostatic interactions in the monolayer is observed. Since the adsorption of weakly interacting molecules such as the FxDNTTs is not restricted to specific surfaces as is the case with SAMs, this concept is extended to metal substrates with quite different work function values. For a low work function surface such as Cs(110), a lowest unoccupied molecular orbital (LUMO) pinning effect is predicted at the theoretical level. Since such alkali metal surfaces are not experimentally accessible, a well‐defined Cs monolayer on Cu(100) as a low work function substrate is used instead. For this substrate, however, a variation is observed in the LUMO energies and the work function as a function of the degree of fluorination. This is attributed to the formation of a second interface dipole at the buried Cs/Cu interface, which is modulated with the degree of fluorination and competes with the dipole at the outer molecule/Cs interface. Such a second internal interface dipole, which can be modified by the top layer, has to be considered when going to more complex heterointerfaces.

Sensing properties of nonmetal doped blue phosphorene toward NO and NO2 molecules: A first‐principles study

AbstractFirst‐principles calculations based on density functional theory (DFT‐D2 method) are adopted to systematically investigate the structure stability and sensing properties of NO and NO2 adsorbed on single nonmetals (B, C, and Si) and double nonmetals (1B1C, 1C1Si, and 1B1Si) doped blue phosphorene. The results show the chemisorption of the gas molecules absorbed on single nonmetal doped blue phosphorene with large adsorption energy, charge transfer, and small adsorption distance. Similarly, for gas molecules absorbed on double nonmetal doped blue phosphorene, while NO interaction with 1C1Si co‐doped blue phosphorene is weak. There is a strong hybridization between gas molecules and doped substrates due to the enhancing interaction, resulting in an increasing adsorption ability for gas molecules. We find that the conductivity and work function change caused by nonmetal doping is the main reason for improving the sensitivity of gas molecules, which shows more possibilities for practical gas sensor applications. Therefore, nonmetal doped blue phosphorene provides a new direction for detecting NO and NO2 in the gas sensing field.

Enhancement of voltage controlled magnetic anisotropy (VCMA) through electron depletion

Recent advancement in the switching of perpendicular magnetic tunnel junctions with an electric field has been a milestone for realizing ultra-low energy memory and computing devices. To integrate with current spin-transfer torque-magnetic tunnel junction and spin–orbit torque-magnetic tunnel junction devices, the typical linear fJ/V m range voltage controlled magnetic anisotropy (VCMA) needs to be significantly enhanced with approaches that include new materials or stack engineering. A possible bidirectional and 1.1 pJ/V m VCMA effect has been predicted by using heavily electron-depleted Fe/MgO interfaces. To improve upon existing VCMA technology, we have proposed inserting high work function materials underneath the magnetic layer. This will deplete electrons from the magnetic layer biasing the gating window into the electron-depleted regime, where the pJ/V m and bidirectional VCMA effect was predicted. We have demonstrated tunable control of the Ta/Pd(x)/Ta underlayer's work function. By varying the Pd thickness (x) from 0 to 10 nm, we have observed a tunable change in the Ta layer's work function from 4.32 to 4.90 eV. To investigate the extent of the electron depletion as a function of the Pd thickness in the underlayer, we have performed DFT calculations on supercells of Ta/Pd(x)/Ta/CoFe/MgO, which demonstrate that electron depletion will not be fully screened at the CoFe/MgO interface. Gated pillar devices with Hall cross geometries were fabricated and tested to extract the anisotropy change as a function of applied gate voltage for samples with various Pd thicknesses. The electron-depleted Pd samples show three to six times VCMA improvement compared to the electron accumulated Ta control sample.

Adsorption of toxic H2S, CO and NO molecules on pristine and transition metal doped α-AsP monolayer by first-principles calculations

Using first-principles calculations based on density functional theory (DFT-D2 method), the adsorption behavior of pristine and transition metal (TM) atoms (Fe, Ni, Pd and Pt) doped α-AsP monolayer for toxic H2S, CO and NO molecules are systematically studied. The results show that TM dopants prefer to substitute As atoms with lower binding energy. TM doping significantly enhances the adsorption ability of the α-AsP monolayer for H2S, CO and NO molecules. Furthermore, there is a significant change in work function (WF) after H2S, CO and NO adsorption, meaning that the TM doped α-AsP monolayer is sensitive to gas molecules. Double NO can be stably adsorbed on the TM doped substrate plane, indicating that they are suitable for reversible sensor applications under practical circumstances. Our calculations can provide theoretical guidance to develop promising novel two dimensional V–V binary AsP-based gas sensors.

Gas sensing and capturing based on the C7N6 monolayer with and without metal decoration: A first-principles investigation

We have investigated the geometrics, energetics, charge transfer, electron localization function (ELF), electron difference density (EDD), work function, electronic and gas sensing and capturing properties of the pure and metal-decorated C7N6 monolayers toward gases like NO2, SO2, NO, NH3, H2O, H2S, CO, CH4, CO2 and N2 by means of first-principles calculations. It is discovered the pristine C7N6 monolayer is not suitable for gas sensing and capturing materials. The Li decoration on the C7N6 monolayer is energetically and kinetically favorable, and remarkably enhances the adsorption strength of toxic gases (NO2, SO2, NO, and NH3), which in turn significantly influences the electronic properties (particularly conductivity) of the Li-decorated monolayer with apparent charge transfer and changes in work functions. Based on the analysis of recovery time, adsorption strength, changes in electronic properties and work function, the capture capacities and the influence of humidity on the sensing and capturing properties of NO2, SO2, NO, and NH3 on the monolayer, the Li-decorated C7N6 monolayer would be a promising potential of highly sensitive gas sensing and capturing materials for NO and NH3 gases, and an effective scavenger of NO2 and SO2 in specific scenarios in dry environment that call for experimental evaluation.

2D KBr/Graphene Heterostructures-Influence on Work Function and Friction.

The intercalation of graphene is an effective approach to modify the electronic properties of two-dimensional heterostructures for attractive phenomena and applications. In this work, we characterize the growth and surface properties of ionic KBr layers altered by graphene using ultra-high vacuum atomic force microscopy at room temperature. We observed a strong rippling of the KBr islands on Ir(111), which is induced by a specific layer reconstruction but disappears when graphene is introduced in between. The latter causes a consistent change in both the work function and the frictional forces measured by Kelvin probe force microscopy and frictional force microscopy, respectively. Systematic density functional theory calculations of the different systems show that the change in work function is induced by the formation of a surface dipole moment while the friction force is dominated by adhesion forces.

Construction of Charge Transport Channels at the NiOx/Perovskite Interface through Moderate Dipoles toward Highly Efficient Inverted Solar Cells.

NiOx-based perovskite solar cells (PSCs) have attracted much attention because of their low fabrication temperature, suppressed hysteresis, and superior stability. However, the poor interfacial contacts between NiOx and perovskite layers always limit the progress of PSCs. Here, we applied 2-thiophenemethylamine (TPMA) as charge transport channels at the interface between NiOx and perovskite layers. The introduction of TPMA provides moderate dipole moment pointing to the perovskite side and effectively promotes the charge transportation. Meanwhile, TPMA anchorage also passivates the defect states at the surfaces of both NiOx and MAPbI3, which compensates the voltage loss due to the change in NiOx work function induced by the dipole. Thus, the device performance has been significantly enhanced in both electrochemical properties and power conversion efficiency. Our work has demonstrated a new way of improving current and voltage in the NiOx-based PSCs simultaneously through a moderate interfacial dipole moment toward highly efficient PSCs.

Controlling the metal work function through atomic-scale surface engineering

Adsorbate induced work function modification of Ni have been investigated by means of first-principles calculations. More specifically, the adsorption of Li, Na, Si, Zr, Pd, Pt, or Sn at various coverages on Ni low-index surface models have been considered. In the case of Sn, a more thorough investigation was performed comparing the adsorption as an overlayer structure with the case of surface alloy formation. Our calculations suggest that the most stable Sn@Ni configuration corresponds to a surface alloy, and here the Ni(100)c(2 × 2)-Sn, Ni(110)c(2 × 2)-Sn, and Ni(111)(3×3)R30∘-Sn surface alloys were found to display similar stability. Concerning the induced work function change, a different behaviour as a function of coverage was observed depending on the nature of the Sn@Ni surface model. Both overlayer adsorption and surface alloying were found to induce a work function decrease already at relatively low coverages (≈ 0.05 atom Å −2), regardless of the underlying surface orientation. However, while the work function obtained for stable surface alloys was found to monotonously decrease as the coverage increases, the work function for the stable overlayer structures goes through a minimum. For all investigated surface modifications, the change in work function was found to be consistent with the orientation of the charge transfer at the adsorbate–surface interface. The computed data in this work may serve as handles for experimental endeavours aiming to optimize properties of active materials through atomic-scale surface engineering.

The effect of oxygen-containing species on corrosion behavior of Ta (1 1 0) surface: A DFT study with an experimental verification

Adsorption of oxygen-containing species on the surface of tantalum (Ta) electrode significantly affects its electrochemical corrosion behavior. Density-functional theory (DFT) is employed to investigate the adsorption energies, structural properties and electronic structures of atomic oxygen (O) and molecular water (H2O) on Ta (110) surface. The adsorption behavior of H2O at room temperature is also studied based on ab initio molecular dynamics (AIMD). We find the passivation of Ta metal is mainly attributed to the strong adsorption of oxygen atoms. Thermodynamic results show that bulk Ta2O5 is easily formed at room temperature, which is the fundamental reason for the spontaneous passivation of Ta (110) surface. The formation of an oxygen monolayer (1.00 ML) on Ta (110) surface dramatically increases the work function, making the equilibrium potential of Ta electrode move in the positive direction, thus slowing down the corrosion rate of Ta metal. However, the adsorption of H2O causes a negative work function change, which promotes its anodic dissolution. The electrochemical impedance spectra (EIS) of tantalum foil in three different NH4F-methanol electrolytes (pure, 0.01 M water and oxygen saturated) shows that the charge transfer resistance increases in the sequence RH2O < Rpure < Roxygen, which can be well explained by the results of DFT calculations.

A computational study of the interaction of oxygenates with the surface of rutile TiO2(110). Structural and electronic trends

A variety of OH containing molecules in their different modes of adsorption onto the rutile TiO2(110) are studied by means of density functional theory. A special focus is given to ethanol, ethylene glycol and glycerol. The different species were analyzed with respect to the adsorption energy, work function, and atomic Bader charges. Our results show that dissociated adsorption is favored in all cases. Within these modes, the strongest binding is observed in the case of bidentate fully dissociated adsorption, followed by bidentate partially dissociated then the monodentate dissociated modes. The dependence is also noted upon charge transfer analysis. Species adsorbing with two dissociated OH groups show a negative charge which is roughly twice as large compared to those exhibiting only one dissociated group. In the case of molecular adsorption, we find a small positive charge on the adsorbate. The change in work functions obtained is found to be negative in all studied cases. We observe a trend of the work function change being more negative for glycerol (3 OH groups) followed by ethylene glycol (2 OH groups) and the remaining alcohols (1 OH group), thus indicating that the number of OH groups present is an important factor in regards to work function changes. For the complete series of adsorbates studied (methanol, ethanol, isopropanol, ethylene glycol, glycerol, hydrogen peroxide and formic acid) there is a linear relationship between the change in the work function and the adsorption energy for the molecular adsorption mode. The relationship is less pronounced for the dissociated adsorption mode for the same series.

Effective Work Functions of the Elements

Effective Work Functions of the Elements

Field emission arrays from graphite fabricated by laser micromachining

Laser microstructuring allows the manufacturing of conical shaped emitters on a graphitized carbon substrate without any additional treatment. Integral field emission measurements were performed in a diode configuration with a 50 μm mica spacer and a metalized Si-grid in a vacuum chamber at pressures of about 10−9 mbar. Emission currents up to 10 μA at a voltage of 1000 V (11 MV/m) with a positive current drift were observed for an array of 16 emitters. A strong modification of the emitter tip is noticeable after individual measurements. A comparable alteration of the surface is also achieved by an electrochemical treatment of the samples with a potassium hydroxide solution. However, the IV-measurements of such samples show not only higher onset voltages but also higher integral emission currents. For all arrays, an ideal Fowler–Nordheim (FN) behavior is only noticeable at applied voltages below 625 V (7 MV/m). Numerical calculations indicate that the deviation in the FN-plot for higher voltage values is caused by the change of the effective work function.

Intra-device gating effect in graphene electrode-based organic diodes

Current–voltage characteristics of graphene electrode-based thin-film organic diodes show non-saturating high reverse bias currents, unlike standard indium tin oxide-based devices. We show that this deviation is controlled by the intra-device self-gating electric field on graphene anode from the counter cathode through the organic thin-film device layer. This effect contributes to additional charge dynamics due to the quantum capacitance presented by the semi-metallic graphene in series to the semiconductor capacitance of the organic device layer. This leads to a dynamic workfunction change of the graphene electrode as a function of the applied voltage across the diode. The proposed self-gating phenomenon is captured using an analytical model established using the combination of current–voltage and capacitance–voltage relations in these devices based on drift and diffusion equations. The results of the analytical model agree well with the experimental measurements carried out with fabricated P3HT:PCBM based organic thin-film diodes with graphene anode and aluminum cathode. We further support these results with elaborate self-consistent numerical simulations, all showing an increased reverse bias current contributed by the self-gating effects in graphene devices. Our results, being obtained from fundamental device physics, are applicable to a broad range of other devices like light-emitting diodes, perovskite-based devices, and thin-film a-Si devices. • The work function of graphene changes when subject to an external electric field. • The field effects, when employed as an electrode in a thin-film organic device, can impart changes to the behavior of these devices. • A combinational approach contributed by the device layer and the graphene stack can map this behavioral change. • The results showed a non-saturating higher reverse currents in graphene electrode-based organic diodes.