Interfacial engineering of Bi2S3/Ti3C2Tx MXene based on work function for rapid photo-excited bacteria-killing

Interfacial engineering of Bi3.64Mo0.36O6.55/Ti3C2 Schottky junctions based on work function for efficient photoexcitation-assisted degradation of antibiotics.

Self-assembled monolayers (SAMs) have emerged as a powerful strategy for interfacial engineering in organic and molecular electronics, enabling control of surface properties such as wettability, adhesion and electrode work function (WF). The WF is a key parameter for charge injection, transport, and device performance. By adjusting molecular design, dipole orientation, and surface coverage, SAMs allow precise tuning the WF, optimizing energy-level alignment in devices such as organic solar cells, organic light-emitting diodes, and organic thin-film transistors. This review focuses on WF modulation of gold electrodes, a widely used material due to its chemical stability, high conductivity, and compatibility with thiol-based SAMs. We provide a comprehensive overview of thiol derived SAMs for gold surface modification, emphasizing their impact on WF as measured by kelvin probe force microscopy (KPFM), kelvin probe (KP), and ultraviolet photoelectron spectroscopy. Key parameters including molecular dipole, packing density, chain length, and terminal groups are discussed, along with the advantages of mixed SAMs for achieving precise WF control. These studies demonstrate that strategic molecular selection enables WF tuning across a broad range of 3.7-6.0 eV on gold surfaces. This review underscores the potential of SAMs as a versatile tool for advancing organic and molecular electronic through tailored interfacial engineering.

The present paper considers the application of the high temperature Kelvin probe for measurements of changes in work function (WF) of oxide materials during surface reactions at elevated temperatures and in controlled gas phase environments. The method is based on the determination of work function changes from measurements of the contact potential difference (CPD) between the Pt reference electrode and the studied specimen of a metal oxide. Experimental data on work function changes are reported for yttria-stabilised cubic zirconia (10 mol-%Y2O3) during oxidation and reduction experiments at 1173 K. It was found that changes in CPD exhibit good stability with time (2 mV drift over 15 h). The reproducibility of CPD changes during successive oxidation and reduction experiments remains within 0·5 mV. Good agreement was revealed between the obtained experimental data and the theoretical model describing the effect of oxygen partial pressure on the WF of both zirconia and the Pt electrode.

Energy structures of molecular semiconductors contacting metals under air studied by the diffusion potential measurements and the Kelvin probe technique

Metal corrosion is an electrochemical process typically controlled by features such as grain boundaries, defects, coating/metal interfaces, composition, and local chemical environments. Advances in characterization tools have progressed the means to resolve these features with hopes of better understanding corrosion initiation and progression. However, many localized techniques, such as SECM, SRET, and electrochemical microcells do not provide sub-micron spatial resolution, as well as require in-situ observation of the kinetic processes to investigate corrosion mechanisms. Scanning Kelvin probe force microscopy (SKPFM) has presented a non-destructive alternative to better understand corrosion mechanisms on the micro to nano-scale. SKPFM measures surface electronic properties wherein the Volta potential difference (VPD) between Kelvin probe and surface is collected. This Volta potential difference have been directly correlated to corrosion potentials, and thus provides strong predictability of the corrosion mechanisms seen on complex microstructures without degrading the material. When co-localized with elemental characterization provided from SEM/EDS, Volta potential differences can be correlated to elemental composition. A major drawback with SKPFM is the lack of reproducible results seen for materials’ measured Volta potential differences. There are many reasons for the inconsistency, including variations in experimental conditions (i.e. temperature and relative humidity), operation system and scanning technique, and sample surface conditions (i.e. crystallographic orientation, adsorbed species, roughness, and surface terminating atoms). This work focuses on another plausible factor to this VPD inconsistency by observing the effects of probe choice, as well as probe wear from regular probe usage. To understand these effects, it is important to distinguish that SKPFM spatially maps the Volta potential difference between the surface and the probe (Δψ), which can be linked to the difference in modified work functions between the surface and probe (Δφ). It can be seen from this relationship that any variation in the Kelvin probe's makeup or its wear characteristics would shift the observed Volta potential difference of the material without the material itself characteristically changing. This is why it is common practice to compare the changes in Volta potential differences seen in a singular image, and why comparing results directly to others' published work is notably difficult to accomplish. This work attempts to address the inconsistencies seen from published SKPFM results by developing a relative scaling of Kelvin probe work function. This can be accomplished by mapping relatively inert standards just prior to imaging the material of interest and calculating Volta potential differences between the material’s phases and the inert standard. Much like in common electrochemical experimentation, SKPFM will utilize the standard as a “reference electrode”. The relative work functions of these inert standards were approximated with first-principle calculations by density function theory (DFT). From this work, we hope that the discrepancies seen from SKPFM results can start to be resolved through improving the calculations of various material's relative Volta potentials. Figure 1

The scanning Kelvin probe (SKP) is a device that measures the difference in work function between a sample and probe tip on the instrument, and has proven to be a useful research tool over the past several decades. With this technique, the Volta potential difference, also known as the contact potential difference (CPD), can be determined which can characterize the corrosion, contamination, and coating condition of a surface of interest. The recently developed Field Deployable Scanning Kelvin Probe (FDSKP) allows for a variety of new uses as it allows the SKP technique to be used in a field environment rather than samples or test components being brought to a lab. However, additional challenges come with the increased portability and versatility of the FDSKP, as the set-up and tear down nature of a field deployable unit necessitates a rapid determination that the system is properly set up and functioning. A calibration electrode solves this problem while further demonstrating the underlying connections between electrical potential and the work function as measured by the SKP.The absolute potential difference, also called the Galvani potential (Φ), can be described as the sum of two components: the Volta potential (Ψ) and the dipole potential (χ)1. The Volta potential is directly related to the electrode potential and, therefore, corrosion potential2. This relationship has been used in previous methods of SKP calibration that included relating the Volta potential differences measured by the SKP to the electrode potentials of bare metals and the corrosion potentials of immersed metals3. The effort described here develops a new approach to SKP calibration by applying an external potential onto the calibration electrode surface to further modify the Volta potential and, therefore, the measured Volta potential difference between the polarized sample and the probe tip. The design of the calibration electrode includes flat conductive regions across which a potential can be applied. By applying an external potential to a sample, the measured work function is changed by that amount, thus sharp voltage changes between regions of different applied potential can be observed.The calibration electrode described here is a flexible Kapton substrate with vacuum deposited gold regions produced through a mask and UV lithography fabrication process (Figure 1). Three different geometric designs of matching shapes with a 10 um gap between them are present: bars, wedges, and a puzzle-piece type design. By electrically connecting one region from each of the three designs into two groups, a potential difference could then be applied between the two regions of each design group by means of a potentiostat or battery. With this applied potential, an SKP area scan was performed above the sample. The average work function for each of the two regions was determined. The difference in average work function correlates very well with applied potential across a range of voltage from -1 V to 1 V at steady-state. Similar results were recorded on both coated and uncoated electrodes. As the correlation between observed work function difference and applied potential is very good, this can be used to calibrate an SKP system and demonstrate its proper functioning. A system check can include determining that the measured voltage difference on the calibration electrode matches the applied potential. Additionally, the sharp distinction on the sample between regions of different work function can allow for the calibration and tuning of SKP parameters, such as electrometer and proportional integral derivative (PID), in order to optimize the system’s measurement capability. As SKP techniques move from the lab into the field, a calibration electrode is very useful as it will greatly assist in system set-up and facilitate parameter optimization to allow for better and clearer results. References Grunmeir, G., Juttner, K. and Stratmann, M. (2000) Novel Electrochemical Techniques in Corrosion Research. In R.W. Cahn, P. Haasen & E.J. Kramer (Eds.), Material Science and Technology: Corrosion and Environmental Degradation (Volume I, p. 285-382). Wiley-VCH.Leng, H. Streckel and M. Stratmann “The Delamination of Polymeric Coatings from Steel. Part I: Calibation of the Kelvin Probe and Basic Delamination Mechanism,” Corrosion Science 41 (1999): p. 547-578.B. Cook et al. “Calibration of the Scanning Kelvin Probe Force Microscope under Controlled Environmental Conditions,” Electrochimica Acta 66 (2012), p. 100-105. Figure 1

It is supposed that in two-stage electrostatic precipitation, electrostatic forces arising from contact potential differences are important in the adhesion of talc, silica, flyash, and similar materials. ThiS idea is based on observation indicating that these particles have surfaces with different work functions. If these electrostatic forces are dominant, a rough correlation should exist between adhesive force and charge transfer during a contact, and the magnitude of the charge transferred should be sufficient to account for the measured forces. Described are the design and construction of an apparatus to measure the adhesive force between small particles and the charge transferred during that contact, and the results of those measurements. Adhesive forces of 1O-9 N and charges of 11 electrons can be measured. Adhesive forces for a specific set of materials vary over an order of magnitude. Charge transfer is usually only tens of electrons except for some tests with gallium arsenide. No material tested shows any correlation between force and charge transfer, and in some cases the charge is too small to account for the measured force. The charge transferred during an adhesive contact does not appear to affect the adhesion in the materials tested.

The purpose of the research in this dissertation was to elucidate the intrinsic properties of how nanoparticles are different from bulk materials. This was done by mechanical and electronic studies of the properties of designed nanoparticles using advanced modes of atomic force microscopy. Information relating to the work functions, contact potential difference, Young’s Moduli, elasticity, and viscoelasticity can be investigated using state-of-the-art atomic force microscope (AFM) experiments. Subsurface imaging of polystyrene encapsulated cobalt nanoparticles was achieved for the first time using Force Modulation Microscopy (FMM) in conjunction with contact mode AFM. Previously prepared sample of polystyrene coated cobalt nanoparticles were studied. Tapping-mode AFM was used to evaluate the size of coated nanoparticles. Force modulation microscopy was used to visualize details of the outer polystyrene coating. Differences between the softer polystyrene outer coating and the harder cobalt nanoparticle core was visualized based upon the elastic and viscoelastic properties. Variances in sample elasticity were monitored via the amplitude channel that monitors the oscillation amplitude of the cantilever while scanning. Viscoelastic differences were mapped by the phase channel which provides information of the phase lag of the probe. The identification of designed nanoparticles based upon electrochemical properties was evaluated using the Kelvin Probe Force Microscopy (KPFM) mode of AFM. The contact potential difference between the tip and the sample is measured using an AC bias that is offset with a compensating DC bias while operating in either tapping-mode or non-contact mode AFM. The contact potential difference is more commonly referred to as the difference in work function between the tip and the sample. The work function of a material can be calculated using a reference material with a known work function. Cobalt nanoparticles and gold nanoparticles were imaged using KPFM and baseline experimental contact potential difference values were obtained. Thus far, co-deposition of a mixed nanoparticle solution led to inconclusive results as the experimental and theoretical contact potential difference values were calculated. However, future studies relating to this experiment are planned.

Specimens of electrolytic iron and electrolytic nickel were outgassed by intensive heat treatment in high vacuum and the contact potential difference between them measured by the Kelvin null method. An apparatus is described by means of which the photoelectric long wave limits could be determined on the same specimens and under identical conditions as used for the contact potential measurements. The photoelectric long wave limits were determined in one case by using filters in the path of the light and in a second case by the method of plotting the photoelectric sensitivity curves obtained using monochromatic illumination of the metals. The results remained unchanged between 300 and 750 hours of heating in the first case and between 400 and 600 hours in the second. The equilibrium value of the contact potential difference was Fe-Ni+0.21\ifmmode\pm\else\textpm\fi{}0.01 volt. The photoelectric long wave limits determined by the second method were 2620A\ifmmode\pm\else\textpm\fi{}10A for the iron and 2500A\ifmmode\pm\else\textpm\fi{}10A for the nickel. The corresponding work functions are 4.71\ifmmode\pm\else\textpm\fi{}0.02 volts for the iron and 4.93\ifmmode\pm\else\textpm\fi{}0.02 volts for the nickel. Using the method published by Fowler the work functions were 4.77 volts for the iron and 5.01 volts for the nickel. The difference in these work functions by either method is equal to the measured contact potential difference between the metals within the limits of error of the photoelectric measurements.

Kelvin probe and ultraviolet photoemission measurements of indium tin oxide work function: a comparison

Kelvin Probe (KP), a non-contact, non-destructive vibrating capacitor device, was used to measure the work function (WF) difference of thin Pt films, deposited on oxidized silicon substrates, with highly diluted H 2 gas, in ppm levels, in the presence of with and without relative humidity (RH). Response times were extracted from the behavior of WF shift as a function of H 2 concentration values. Measurements were compared for zero and non-zero RH conditions at a fixed temperature of 30°C. Changes in WF were evaluated by using HP VEE program, suitably modified for the present measurements. The events were executed step by step for every second time interval through an input formatted file. The data was recorded at each event and was analyzed for the shift in WF with respect to the H 2 concentration. The shift in the WF and the response time (τ 90 ) at different concentration levels for the Pt films is reported in this paper. The presence of humidity shows saturation of response time after 500 ppm whereas zero humidity conditions show a continuous reduction at higher concentration values.

Work function changes of Au were measured by Kelvin probe force microscopy (KPFM) in the nonpolar liquid decane. As a proof of principle for the measurement in liquids, we investigated the work function change of an Au substrate upon hexadecanethiol chemisorption. To relate the measured contact potential difference (CPD) during the chemisorption of alkanethiols to a change of the work function, the influence of physisorbed decane must be taken into account. It is crucial that either the work function of the scanning probe microscope (SPM) tip or the sample surface remains constant throughout the reaction, since both contribute to the CPD. We describe two routes for determining the work function shift of Au coated with a monolayer of alkanethiols: In the first route, the SPM tips were taken as reference surfaces (constant tip work function). For this approach, we used Au(111) surfaces and kept the SPM tip ex situ during the adsorption process. In the second route, structured surfaces with reactive and inert parts were studied by KPFM (constant reference work function). For this route, we prepared nanometer sized Au structures by nanosphere lithography on SiO(x) substrates. Now, the SiO(x) served as the inert reference surface. The shifts in the work function after exposure to the hexadecanethiol (HDT) solution were determined to be ΔΦ(Au+HDT,decane-Au,air) = -1.33 eV ± 0.07 eV (route I) and ΔΦ(Au+HDT,decane-Au,air) = -1.46 eV ± 0.04 eV (route II). Both values are in excellent agreement with the work function shifts determined by ultraviolet photoemission spectroscopy (UPS) reported in literature. The presented procedures of measuring work function changes in decane open new ways to study local reactions at solid-liquid interfaces.

Recent researches in thermal energy harvesting have revealed the remarkable efficiency of thermionic energy converters comprising very low work function electrodes. From room temperature and above, this kind of converter could supply low power devices such as autonomous sensor networks. In this type of thermoelectric converters, current injection is mainly governed by a mechanism of thermionic emission at the hot electrode which explains the interest for low work function coating materials. In particular, alkali metal oxides have been identified as excellent candidates for coating converter electrodes. This paper is devoted to the synthesis and characterization of potassium peroxide K2O2 onto silicon surfaces. To determine optimal synthesis conditions of K2O2, we present diagrams showing the different oxides as a function of temperature and oxygen pressure from which phase stability characteristics can be determined. From the experimental standpoint, we present results on the synthesis of potassium oxide under ultra high vacuum and controlled temperature. The resulting surface is characterized in situ by means of photoemission spectroscopy (PES) and contact potential difference (CPD) measurements. A work function of 1.35 eV is measured which and the expected efficiency of the corresponding converter is discussed. It is generally assumed that the decrease of the work function in the alkali/oxygen/silicon system, is attributed to the creation of a surface dipole resulting from a charge transfer between the alkali metal and oxygen.

A novel experimental technique was used for direct measurement of cathode material work function at elevated temperature. Dispenser cathodes were activated in a vacuum chamber and the work function determined from the contact potential difference between the cathode and a precisely calibrated probe tip. The experimental test setup allowed for cathodes to be activated and the work function measured without breaking vacuum. B-type, M-type, and other cathodes were activated and their work function tracked during the initial heating stage and during cooling after activation. The various cathode types showed a smooth increase in work function as the cathode cooled, but the B-type had a lower room temperature work function than other cathodes. It is suggested that residual gas in the chamber may have caused the rise in work function during cooling.

Polycrystalline Ni samples were irradiated by He and H ion beams of 1 MeV, 2 x 1016 ions/m2/s under a pressure of 1 x 10-4 Pa, and a He ion beam of 500 eV, 2 x1017 ions/m2/s under a working pressure of about 1 x 10-2 Pa. The work function (WF) change, extremely sensitive to various surface changes, was recorded using a Kelvin probe via measuring the contact potential difference between the probe and the sample. The results indicate that the irradiation of 500 eV He ions result in WF decrease at low fluence range, and then WF increase till saturation with increasing the fluence, while 1 MeV ions only induce WF decrease, then saturation. A surface model of loosely bound adsorbed layer plus native oxide layer on metals is presented to explain the observed phenomena. The nuclear stopping is responsible for the results in the case of 500 eV He+ irradiation that is powerful enough to sputter away the whole overlayer from the bulk surface. In MeV case, the electronic stopping plays a decisive role, which allows merely the topmost adsorbed layer to be removed partially by He and H ions of 1 MeV. Due to desorption of the topmost adsorbed layer, the WF decreases as a result of decrease of the surface dipoles towards the bulk. If the oxide layer is removed subsequently due to sputtering, the WF will increase.