Changes In Surface Work Function Research Articles

Exploring metal-supported few-layer graphene for photocathode application through Cs/O activation process

Exploring a photocathode with features of visible-light response, high quantum efficiency, long life-span, easy availability and low cost is the desire of electron source applications. Here, we report the photoemission behavior of nickel-supported few-layer graphene through Cs/O activation process. We observe that this Cs/O-activated graphene-based photocathode can possess a cut-off response wavelength of 750 nm, a quantum efficiency over 0.1% at 350 nm, and a 1/e operating lifetime of over 50 h under continuous mW-level laser irradiation at a pressure of 10-7 Pa. Importantly, the photoemission capability after degradation can be completely restored again to its original maximum through the additional Cs deposition process. Moreover, the changes in surface composition and work function characterized by in-situ photoelectron spectroscopy demonstrate the low work function and excellent stability. This metal-supported graphene material with low-work-function surface provides a route to obtain robust photocathode working from ultraviolet to visible waveband.

Surface structure and work function change of pentafluorobenzeneselenolate self-assembled monolayers on Au (111)

The formation and surface structure of pentafluorobenzeneselenolate (PFB-Se) self-assembled monolayers (SAMs) on Au(111) prepared by solution and vapor depositions were examined by scanning tunneling microscopy (STM). STM observations showed that solution-deposited PFB-Se SAMs on Au(111) had only a disordered phase containing many dark areas with irregular shapes, regardless of the deposition time. However, it was found that the degree of structural order of PFB-Se SAMs was markedly improved by the vapor deposition method at 348 K. After short deposition for 5 min, PFB-Se SAMs on Au(111) consisted of two distinct regions: a bright protruding region and a dark region with partial ordered domains. Interestingly, PFB-Se SAMs after deposition for 1 h showed various structural features with three different contrasts: an ordered dark region, an ordered and disordered bright region, and a very bright region. The ordered domain is assigned to a (5 × √3) structure. The relatively uniform and nicely ordered PFB-Se SAMs with a (14 × √3) structure were formed after longer deposition for 2 h, resulting from further adsorption and structural rearrangements. Furthermore, Kelvin probe force microscopy measurements (KPFM) exhibited a significant difference in the work function of pentafluorobenzenethiolate and the PFB-Se SAM-modified gold surface by 0.36 eV due to the large difference in the domain and packing structures of both SAMs. Herein, STM and KPFM results will give new insights into the formation and structures of PFB-Se SAMs on Au(111) and further the fundamental understanding of the interfacial electronic properties of fluorinated aromatic backbone-based thiolate and selenolate SAMs.

Simulations of field emission from copper electrodes with inclusion of oxygen surface layer and work function changes based on first-principles calculations

Analysis of field emission requires the inclusion of the internal potentials that shape the electronic wavefunctions and tunneling probabilities; details of the work function that are dependent on material quality and defects; and the role of the density of states (DOS) that influences the electronic supply. Here, these factors are collectively included on the basis of density functional theory to obtain predictions of field-dependent electron tunneling current densities. Results are obtained in copper for three different orientations. The DOS is predicted to be broadened by an externally applied electric field. The (100) copper is shown to yield the largest current density, and the (111) orientation is the lowest. The presence of an oxide surface monolayer is shown to increase the work function, leading to the emission of current reductions. The technique is general and can be applied to other materials (e.g., carbon fibers) that have shown promise as cathode emitters.

Enhanced photo-stability of inverted organic solar cells via using polyethylenimine in the electron extraction layers

Abstract The influence of using Polyethylenimine (PEI) in the electron extraction layers (EELs) on the photo-stability of inverted organic solar cells (OSCs) is investigated. Results show that introducing PEI in the commonly used ZnO EELs, either mixed in the same layer with the ZnO (ZnO:PEI mixture) or in a separate layer in the form of a bilayer EEL configuration (ZnO/PEI), reduces the susceptibility of OSCs to photo-degradation with the mixture giving more significant stability enhancements. In comparison to the widely used ZnO EEL, the use of ZnO:PEI mixture is found to lead to ∼ 5× higher PCE stability under continuous UV irradiation, the latter being the main factor behind the limited stability of the cells under sunlight, primarily due to a more stable V o c and FF. Tests on electron only devices show that the higher photo-stability is mainly due to the role of the ZnO:PEI mixture in minimizing light-induced deterioration in electron collection efficiency at OSCs electron collection contacts under sunlight. Kelvin probe force microscopy (KPFM) measurements show that the use of ZnO:PEI helps maintain a more stable surface potential at the electron collection contact, hence the more stable V o c and FF. X-ray photoelectron spectroscopy (XPS) measurements reveal that the stability enhancement is likely associated with chemical interactions between PEI and ITO at the ITO/ZnO:PEI interface that produce a more electronically and energetically stable contact that is less susceptible to changes in surface potential or work function by UV illumination. Using PEI in the form of a neat layer next to the ITO in a bilayer EEL (PEI/ZnO) is found to provide a similar significant enhancement in photo-stability, further ascertaining the above conclusions. The findings not only reveal the potential of utilizing PEI in the EEL, but also shed light on some of the root causes of the limited photo-stability of OSCs and approaches to mitigate their effect.

Electrostatic memory in KTaO3

Perovskite oxide materials with strong spin–orbit coupling (SOC) have tremendous potential for next generation spin-electronic devices. In this work, we present the creation of artificial electrical domains on the conducting surface of KTaO3 (KTO) having strong SOC. A conductive atomic force microscopy tip has been used to induce extremely small electrically active charge domains on the surface of KTO. The versatility of creating such nanodomains is that the features can be written and erased with nanometer scale precision. The written signal (phase difference between the written and unwritten regions) on KTO has turned out to be much higher than that on other reported perovskite oxides. Kelvin probe force microscopy measurements are performed to probe the surface potential and work function changes in the regions of nano-electrical domains. Magnetic force microscopy measurements suggest generation of a magnetic field when electrostatic charge is written in a dipolar configuration which can be utilized in future nano-electric devices.

Operando study of Pd(100) surface during CO oxidation using ambient pressure x-ray photoemission spectroscopy

The surface chemical states of Pd(100) during CO oxidation were investigated using ambient pressure x-ray photoelectron spectroscopy and mass spectroscopy. Under the reactant ratio of CO/O2 = 0.1, i.e. an oxygen-rich reaction condition, the formation of surface oxides was observed with the onset of CO oxidation reaction at T = 525 K. As the reactant ratio (CO/O2) increased from 0.1 to 1.0, ∼ 90 % surface oxides remains on surface during the reaction. Upon the formation of surface oxides, the core level shift of oxygen gas phase peak was observed, indicating that change of surface work function. As CO oxidation takes places, i.e. making a transition from CO covered surface to the oxidic surface, the work functions of surface oxide on Pd(100) and Pt(110) display opposite behavior.

Universal Scaling Relationship To Screen an Efficient Metallic Adsorbent for Adsorptive Removal of Iodine Gas under Humid Conditions: First-Principles Study

Safe control and removal of radioactive iodine gases (I-129 and I-131) leaking from the accidents in chemical factories or nuclear industries are of importance because of their critical damage to the biosphere. We study the adsorptive removal of the off-gaseous iodine using transition metals of group 10 and group 11 under humid conditions. First-principles calculations enable to capture key adsorption natures of iodine and water molecules on the adsorbent surfaces. The underlying mechanism is analyzed by thermodynamic free energies, electronic structures, and surface work function changes. Our results unveil why silver metal shows notably outstanding efficiency for the iodine removal. We propose an innovative and insightful map to guide sorting out the best metal adsorbents and impregnants for dramatic improvement of the adsorptive removal of the radioactive iodine gas. Our study is useful for preventing critical risks from chemical and nuclear accidents.

Quantitative depth distribution analysis of elements in high alloy steel using MCs+-SIMS approach

Elemental composition analysis in alloy samples using secondary ion mass spectrometry (SIMS) suffers from matrix effects which hamper the analytical results. These matrix effects arise due to numerous factors like change in surface work function due to primary ion beam implantation, presence of reactive chemical species in the samples or on the sample surface, sample composition, chamber pressure etc. which changes the ionization probability of secondary ion species and hence produces non-linear effects in the production of secondary ions. In order to minimize these effects, Cs+ primary ion beam in conjunction with MCs+ (M being the element of interest) secondary ion monitoring (MCs+-SIMS) approach has been used. The MCs+-SIMS approach has been extensively used for quantitative depth distribution analysis in semiconductor samples but limited reports are available for the analysis of alloys sample. In the present work, the MCs+-SIMS approach has been utilized to determine the chemical composition and quantitative depth distribution of multiple elements (C, Si, Ni, Mn, Cr, Co, Mo and Fe) in high alloy steel samples. Quantitative depth distribution analysis revealed a non-homogenous distribution of elements on the steel surface which can be due to the presence of surface oxide layers. The minor and major constituents such as Cr, Mo; Mn, Ni; and Si; were determined with relative error of <2%, 6% and 5%, respectively. Surface distribution analyses of these elements were also carried out using both Cs+ and O2+ primary ion beams. An interesting and contrasting feature has been observed for surface distribution analysis carried out with MCs+ as compared to M+ secondary ion detection mode. It was observed that the surface distribution patterns of non-homogenous elements showed homogenous distribution patterns in case of MCs+ secondary ion monitoring approach. This anomaly can be attributed to the difference in formation mechanism of MCs+ and M+ secondary ions.

Piezotransistive GaN microcantilevers based surface work function measurements

Surface work function (SWF) measurements using a piezotransistive III–nitride cantilever has been demonstrated on multiple surfaces. The minimum detectable surface potential change of 10 mV was achieved with a signal to noise ratio of 3. This method was applied to determine the surface potential changes due to exposure of 5 ppm NO2 in graphene and In2O3 thin film, simultaneously with conductivity changes. The potentiometric measurements yielded 100 and 80 mV potential changes in SWFs of graphene and In2O3 respectively, which matches very well with experimental data published earlier indicating the efficacy of this readily miniaturizable measurement technique.

Spatial inhomogeneity and temporal dynamics of a 2D electron gas in interaction with a 2D adatom gas

Fundamental interest for 2D electron gas (2DEG) systems has been recently renewed with the advent of 2D materials and their potential high-impact applications in optoelectronics. Here, we investigate a 2DEG created by the electron transfer from a Ag adatom gas deposited on a Si(111) sqrt{{bf{3}}}{boldsymbol{times }}sqrt{{bf{3}}}-Ag surface to an electronic surface state. Using low-energy electron microscopy (LEEM), we measure the Ag adatom gas concentration and the 2DEG-induced charge transfer. We demonstrate a linear dependence of the surface work function change on the Ag adatom gas concentration. A breakdown of the linear relationship is induced by the occurrence of the Ag adatom gas superstructure identified as Si(111) sqrt{{bf{21}}}{boldsymbol{times }}sqrt{{bf{21}}}-Ag only observed below room temperature. We evidence below room temperature a confinement of the 2DEG on atomic terraces characterised by spatial inhomogeneities of the 2DEG-induced charge transfer along with temporal fluctuations. These variations mirror the Ag adatom gas concentration changes induced by the growth of 3D Ag islands and the occurrence of an Ehrlich-Schwoebel diffusion barrier of 155 ± 10 meV.

Dipole formation at organic/metal interfaces with pre-deposited and post-deposited metal

In organic electronic devices, the interfacial dipole at organic/metal interfaces is critical in determining the carrier injection or extraction that limits the performance of the device. A novel technique to enable the direct measurement of underburied dipoles is developed and demonstrated. By tilting the shadow mask by a small angle, metal atoms diffuse into the opening slit to form an ultrathin metal layer during the evaporation process. As the ultrathin metal layer cannot screen out the dipole-induced surface work function change, the dipole strength and direction at the organic/metal interface can be revealed. It was found that the polarity of the organic material, the Fermi-level pinning and the interface morphology all play important roles in dipole formation. By comparing the energy level shifts at the organic/pre-deposited metal and organic/post-deposited metal interfaces, the dipole formed by molecular interactions could be distinguished from the dipole formed by Fermi-level pinning.

Surface properties and work function changes induced by atomic oxygen adsorbed on HfC(1 1 1) surface

Hafnium carbide (HfC) is regarded as one of the most promising cathode materials for field emission. But the experimental results did not provide a detail picture of the surface properties. In this work, we perform an ab initio study of the surface energies and work functions for the (100), (110), (111), (210), (310) and (311) surfaces of hafnium carbide. For the polar surface of (111) and (311) plane, a new method is taken to calculate the surface energy of the different surface terminations. The results indicate that the Hf termination surface is most stable, which are consistent with the experimental results. Additionally, we focused in particular on oxygen atom induced work function changes on HfC(111) plane as a function of coverage. An unexpected decrease of the work function is found at low coverage, and a reasonable resolution for this anomaly is given based on the method of Roman et al.

Direct Synthesis of Graphene with Tunable Work Function on Insulators via In Situ Boron Doping by Nickel-Assisted Growth

Work function engineering, a precise tuning of the work function, is essential to achieve devices with the best performance. In this study, we demonstrate a simple technique to deposit graphene on insulators with in situ controlled boron doping to tune the work function. At a temperature higher than 1000 °C, the boron atoms substitute carbon sites in the graphene lattice with neighboring carbon atoms, leading to the graphene with a p-type doping behavior. Interestingly, the involvement of boron vapor into the system can effectively accelerate the reaction between nickel vapor and methane, achieving a fast graphene deposition. The changes in surface potential and work function at different doping levels were verified by Kelvin probe force microscopy, for which the work function at different doping levels was shifted between 20 and 180 meV. Finally, the transport mechanism followed by the Mott variable-range hopping model was found due to the strong disorder nature of the system with localized charge-carrie...

Influence of vacancy defect on surface feature and adsorption of Cs on GaN(0001) surface.

The effects of Ga and N vacancy defect on the change in surface feature, work function, and characteristic of Cs adsorption on a (2 × 2) GaN(0001) surface have been investigated using density functional theory with a plane-wave ultrasoft pseudopotential method based on first-principles calculations. The covalent bonds gain strength for Ga vacancy defect, whereas they grow weak for N vacancy defect. The lower work function is achieved for Ga and N vacancy defect surfaces than intact surface. The most stable position of Cs adatom on Ga vacancy defect surface is at T1 site, whereas it is at BGa site on N vacancy defect surface. The E ads of Cs on GaN(0001) vacancy defect surface increases compared with that of intact surface; this illustrates that the adsorption of Cs on intact surface is more stable.

Surface work function of indium tin oxide treated using plasma immersion ion implantation

Oxygen plasma immersion ion implantation (O-PIII) was introduced to modify the surface of indium tin oxide (ITO) thin films. X-ray photoelectron spectroscopy and Kelvin probe were employed to study the surface work function of the treated ITO films. The results showed that the surface work function of ITO can be further enhanced by O-PIII treatment based on an oxygen inductively coupled plasma (O-ICP). The change in surface work function correlated largely with the surface stoichiometry and the high work function should be attributed to the elimination of oxygen vacancies. The surface work function of the modified ITO films showed a rapid decreasing trend with a time-dependent curve immediately after O-ICP and O-PIII treatments. However, a stabler modification effect was achieved by O-PIII.

Effect of Ar+ ion etching treatment on the surface work function of Hg3In2Te6 wafer

Synchrotron radiation photoemission spectroscopy (SRPES) was employed to investigate the changes of surface work function of Hg3In2Te6 wafer under different Ar+ ion etching parameters. Hg concentration on the Hg3In2Te6 surface decreased with increasing the etching time. Meanwhile, the calculated work function and electron affinity of Hg3In2Te6 wafer also decreased with increasing the etching time. It was found that the decrease of work function of the Hg3In2Te6 wafer was mainly caused by the decrease of electron affinity which was closely related to the increase of electron concentration due to the escape of Hg2+ on the surface of Hg3In2Te6 wafer.

MCsn+-SIMS: An Innovative Approach for Direct Compositional Analysis of Materials without Standards

This review primarily deals with the compensation of ‘matrix effect’ in secondary ion mass spectrometry (SIMS) for direct quantitative analysis of materials using MCs+-SIMS approach. Emphasis has been given on exploring the formation mechanisms of MCsn+ (n = 1, 2,..) molecular ions (M denotes the element to be analyzed and Cs+ is the bombarding ion) emitted in the SIMS process. Following a brief introduction on SIMS, a study on MCsn+ molecular ions emitted from various metal targets under Cs+ primary bombardment has been discussed. These studies essentially concentrate on the secondary ion energy distributions of the emitted species under different bombarding energies and impact angles. We have discussed a systematic study on the effect of reactive species like oxygen and cesium on the emission of MCsn+ molecular ions. In all cases we have estimated the changes in local surface work function from the leading parts of the secondary ion energy distributions. The observations are explained in the light of surface work function changes at the sputtering sites. Formation mechanisms of MCsn+ molecular ions are explained in the framework of sputter-ion emission models. Suppression of matrix effects in the compositional analysis of molecular beam epitaxy (MBE) - grown SiGe alloy structures has been discussed in detail. We have essentially applied the MCs+-SIMS method for the minimization of ‘matrix effect’, which is routinely encountered in conventional SIMS analysis hindering quantification of materials. Under the impact of energetic Cs+ ions, both SiCs+ and GeCs+ ions have been monitored for SiGe alloy samples with Ge concentration between 0 and 72 at.%. Following optimization of various experimental parameters, we have shown that the matrix effects are practically suppressed for Ge content up to 72 at.% in SiGe samples. A procedure for the accurate quantification of Ge concentration in Si1−xGex alloys using this method has been presented. This novel methodology has successfully been applied in the depth profiling of Si/Ge multilayer structures grown by MBE technique.

Adjusted surface work function of InN films annealed at vacuum and at high-pressure N2 conditions

Annealing studies were performed to investigate the effects of heat treatment on InN thin films by changing the annealing condition from vacuum to high pressure N2. A significant variation of ∼400meV in the surface work function was observed for InN films. The basic principles of Kelvin probe measurement revealed that the surface band bending ESBB is crucial in investigating the significant changes of surface work function on the InN. The stoichiometric ratio imbalance of In and N was indirectly determined to be the main cause of the variation in band bending by analyzing the X-ray diffraction and energy dispersive X-ray measurements. Thus, the annealing treatment could be an effective method to adjust the surface work function of InN by changing the annealing condition from vacuum to high pressure N2.

Doping Mechanism and Electronic Structure of Alkali Metal Doped Tris(8-hydroxyquinoline) Aluminum

We investigated the electronic structures of alkali metal doped Alq3 molecules prepared by codeposition of Na and tris(8-hydroxyquinolinato)aluminum (Alq3) (Na:Alq3) using in situ synchrotron radiation photoelectron spectroscopy. The doping ratio of Na to Alq3 (RNa) was 0, 0.3, 0.6, 0.8, 2.7, 3.4, or 3.9. The work function, calculated from photoemission spectra, remained at 3.6 eV ± 0.03 eV for all samples, while the energy of the highest occupied molecular orbital increased from 2.2 to 2.85 eV as the doping ratio increased from RNa = 0 to RNa = 2.7. The work function and valence band spectra indicated that there is no band bending or surface work function change due to an alkali doping effect, in contrast to the findings of previous reports. The doped layer was composed of an n-type organometallic complex according to the analysis of O 1s and N 1s spectra. The N-type doping effects shown in the N 1s spectra of coevaporation samples were reflected in the schematic band diagram, so the energy difference between the Fermi level (EF) and the lowest unoccupied molecular orbital (LUMO) decreased by 0.64 eV. The schematic band diagram demonstrates that a monotonic shift of the LUMO toward EF was observed with increasing doping, which is in contrast to general n-type doping effects in inorganic semiconductors. Also, we experimentally observed increased electron transport characteristics of alkali metal doped Alq3. The operating voltage at 100 mA/cm2 decreased from 11.9 V (RNa = 0) to 9.6 V, and the luminance at 10.5 V increased from 3575 cd/m2 (RNa = 0) to 9675 cd/m2 when the Na:Alq3 film (RNa = 2.7) was inserted between Alq3 and LiF/Al.

Correlated conductivity and work function changes in epitaxial graphene

Correlation between conductance and surface work function (SWF) changes caused by molecular adsorption on epitaxial graphene on both faces of 6 H-SiC has been investigated. The SWF and conductance changes, explained on the basis of graphene band diagram, indicate C-face multilayer and Si-face few layer graphene behave as p and n-type sensing layers, respectively. A quantitative model correlating conductance and SWF changes has been proposed within the framework of Boltzmann transport theory. Our results further indicate that for epitaxial graphene, the charge interaction by the adsorbed molecules and related work function changes can be strongly influenced by the SiC substrate.