Variation Of Surface Potential Research Articles

Effect of Physicochemical Surface Properties of Silicon-Substituted Hydroxyapatite on Angiogenesis.

Synthetic hydroxyapatite (HA) is a widely studied bioceramic for bone tissue engineering (BTE) due to its similarity to the mineral component of bone. As bone mineral contains various ionic substitutions that play a crucial role in bone metabolism, the bioactivity of HA can be improved by adding small amounts of physiologically relevant ions into its crystal structure, with silicate-substituted HA (Si-HA) showing particularly promising results. Nevertheless, it remains unclear how distinct material characteristics influence the bioactivity due to the intertwined nature of surface properties. A coculture methodology was optimized and applied for in vitro quantification of the biological response. Initially, HA and Si-HA samples were produced and characterized. To compare the bioactivity of the samples, a method was developed to measure interactions in an increasingly complex environment, first including fibronectin (FN) adsorption and subsequently cell adhesion in mono and coculture using primary human osteoblasts (hOBs) and human dermal microvascular endothelial cells (HDMECs), with and without FN precoating. An experimental set-up was designed to assess to what extent different surface features of the samples contribute to the induced biological response. An 8-nm gold sputter coating was applied to eradicate the electrochemical differences and polishing and abrading was used to reduce the differences in surface topographies. Overall, 1.25 wt% Si-HA exhibited most nanoscale variations in surface potential. In terms of bioactivity, 1.25 wt% Si-HA samples induced the highest osteoblast attachment and vessel formation. Additionally, in vitro vessel formation was established on Si-HA surfaces using a hOB:HDMEC cell ratio of 70:30 and a methodology was established that enabled the assessment of the relative effect of topographical and electrochemical features induced by silicon substitution in the HA lattice on their bioactivity. It was found that the difference in the amount of protein attached to HA and 1.25 wt% Si-HA after 2 h was affected by topographical differences. Conversely, electrochemical differences induced different vessel-like structure formation in coculture with a FN precoating. Without an FN precoating, both topographical and electrochemical differences dictated the differences in angiogenic response. Overall, 1.25 wt% Si-HA surface features appear to induce the most favorable protein adsorption and cell adhesion in mono and coculture with and without FN precoating.

Inducing Circularly Polarized Single-Photon Emission via Chiral-Induced Spin Selectivity.

One of the central aims of the field of spintronics is the control of individual electron spins to effectively manage the transmission of quantized data. One well-known mechanism for controlling electronic spin transport is the chiral-induced spin-selectivity (CISS) effect in which a helical nanostructure imparts a preferential spin orientation on the electronic transport. One potential application of the CISS effect is as a transduction pathway between electronic spin and circularly polarized light within nonreciprocal photonic devices. In this work, we identify and quantify the degree of chiral-induced spin-selective electronic transport in helical polyaniline films using magnetoconductive atomic force microscopy (mcAFM). We then induce circularly polarized quantum light emission from CdSe/CdS core/shell quantum dots placed on these films, demonstrating a degree of circular polarization of up to ∼21%. Utilizing time-resolved photoluminescence microscopy, we measure the radiative lifetime difference associated with left- and right-handed circular polarizations of single emitters. These lifetime differences, in combination with Kelvin probe mapping of the variation of surface potential with magnetization of the substrate, help establish an energy level diagram describing the spin-dependent transport pathways that enable the circularly polarized photoluminescence.

Ambipolar charge transfer of larger fullerenes enabled by the modulated surface potential of h-BN/Rh(111)

A detailed understanding of how molecules interact with two-dimensional materials, particularly concerning energy level alignment and charge transfer processes, is essential to incorporate functional molecular films into next-generation 2D material-organic hybrid devices. One of the major challenges in integrating molecular films in field-effect transistors is facilitating ambipolar charge transport, which is often hindered by the large electronic gap of the organic layers. This work compares the adsorption site-dependent energy level alignment of C60, C70, and C84 fullerenes induced by the spatial variation of the electrostatic surface potential of the h-BN/Rh(111) Moiré superstructure. As the size of the fullerenes increases, the HOMO-LUMO gap shrinks. In the case of C84, we find an intrinsic charge transfer from the substrate to the fullerenes adsorbed in the Moiré pore centers, rendering them negatively charged. The electric field effect-induced charging of neutral fullerenes and discharging of intrinsically negatively charged fullerenes are investigated using scanning tunneling spectroscopy, non-contact atomic force microscopy, and Kelvin probe force spectroscopy. Our findings show that on metal-supported h-BN, the LUMO level of C84 is sufficiently close to the Fermi energy that it can be neutral or 1e− negatively charged depending on slight variations of the electrostatic potential. The findings propose a path to make ambipolar charge transfer accessible and efficient by circumventing the need to overcome the fullerenes’ electronic gap.

Electroactive nanofiltration membranes based on carbon nanostructures for enhanced desalination performance

Recent advancements in materials science has paved the path for creating nanofiltration membranes that are both electrically conductive and suitable for desalination purposes. These smart membranes offer a powerful approach to regulate ion transport and fouling through surface potential variation. Herein, we report the development of an electrically responsive nanofiltration membrane through a novel process involving the physical entrapment of carbon nanostructures (CNS) within a regenerated cellulose network, followed by crosslinking with glutaraldehyde and casting. We investigated different crosslinking conditions to achieve optimal desalination results. Electron microscopy techniques such as, TEM and SEM, were utilized to confirm the successful incorporation of CNS within the cellulose network. Consequently, the resulting membrane exhibited remarkable electrical conductivity (6044 ± 6.404 S/m), which was exploited to improve membrane performance in desalination. Our results showed that applying an external voltage of −1 V increased the rejection rates of NaCl from 43.8 % to 54.6 %, and Na2SO4 from 75.6 % to 81.7 %, with insignificant effect on the permeate flux. This enhancement was attributed to the increased Donnan potential difference resulting from the polarization-induced charges generated on the membrane when a negative bias was applied. Moreover, the membrane demonstrated reversible ion transport modulation, excellent recyclability, and self-cleaning capabilities. Overall, our findings highlight the potential of these electroactive nanofiltration membranes for desalination and other ion separation applications.

Control of intrinsic polarity for work function modulation of polyvinylidene fluoride crystalline phases

Ferroelectric polymers with high flexibility and inherent piezo- and pyro-electric properties have gained tremendous importance for next-generation wearable electronics. In this context, we investigate the intrinsic polarity mediated work function modulation in α-, γ-, and β-crystalline phases of a ferroelectric polymer, namely, polyvinylidene fluoride. A wide range of surface potentials (i.e., −5 to −70 V) were observed depending upon the crystalline polymorph and their surface morphologies. For example, upon nucleation of electroactive γ- and β-phases, a reduction in spherulite size is observed in comparison to its α-counterpart. Ultraviolet photoelectron spectroscopy was employed to realize the effect of surface potential on the valence bands spectrum. In particular, the work function of the non-electroactive α-phase (φα ∼ 5.09 eV) significantly increased when it is converted into the electroactive γ (φγ ∼ 5.99 eV) and β (φβ ∼ 7.39 eV) phases. The advantage of surface potential variation is shown by synergistic charge generation as a result of contact electrification of single active material-based polar interfaces with different work functions.

Multifractality in Surface Potential for Cancer Diagnosis.

Recent advances in high-resolution biomedical imaging have improved cancer diagnosis, focusing on morphological, electrical, and biochemical properties of cells and tissues, scaling from cell clusters down to the molecular level. Multiscale imaging revealed high complexity that requires advanced data processing methods of multifractal analysis. We performed label-free multiscale imaging of surface potential variations in human ovarian cancer cells using Kelvin probe force microscopy (KPFM). An improvement in the differentiation between nonmalignant and cancerous cells by multifractal analysis using adaptive versus median threshold for image binarization was demonstrated. The results reveal the multifractality of cancer cells as a new biomarker for cancer diagnosis.

Understanding the unique S-scheme charge migration in triazine/heptazine crystalline carbon nitride homojunction

Understanding charge transfer dynamics and carrier separation pathway is challenging due to the lack of appropriate characterization strategies. In this work, a crystalline triazine/heptazine carbon nitride homojunction is selected as a model system to demonstrate the interfacial electron-transfer mechanism. Surface bimetallic cocatalysts are used as sensitive probes during in situ photoemission for tracing the S-scheme transfer of interfacial photogenerated electrons from triazine phase to the heptazine phase. Variation of the sample surface potential under light on/off confirms dynamic S-scheme charge transfer. Further theoretical calculations demonstrate an interesting reversal of interfacial electron-transfer path under light/dark conditions, which also supports the experimental evidence of S-scheme transport. Benefiting from the unique merit of S-scheme electron transfer, homojunction shows significantly enhanced activity for CO2 photoreduction. Our work thus provides a strategy to probe dynamic electron transfer mechanisms and to design delicate material structures towards efficient CO2 photoreduction.

High-accuracy characterization of pyroelectric materials: A noncontact method based on surface potential measurements

The characterization of pyroelectric materials is essential for the design of pyroelectric-based devices. Pyroelectric current measurement is the commonly employed method, but can be complex and requires surface electrodes. Here, we present noncontact electrostatic voltmeter measurements as a simple but highly accurate alternative, by assessing thermally-induced pyroelectric surface potential variations. We introduce a refined model that relates the surface potential variations to both the pyroelectric coefficient and the characteristic figure of merit (FOM) and test the model with square-shaped samples made from PVDF, LiNbO3 and LiTaO3. The characteristic pyroelectric coefficient for PVDF, LiNbO3 and LiTaO3 was found to be 33.4, 59.9 and 208.4 [Formula: see text]C m[Formula: see text]K[Formula: see text], respectively. These values are in perfect agreement with literature values, and they differ by less than 2.5% from values that we have obtained with standard pyroelectric current measurements for comparison.

Design and analysis of high-performance double-gate ZnO nano-structured thin-film ISFET for pH sensing applications

This paper reports a high-performance double-gate zinc-oxide thin-film ion-sensitive field-effect transistor (DG-ZnO-ISFET) for pH sensing applications. The sensing mechanism of DG-ZnO-ISFET and a simulation model for pH sensing analysis are demonstrated herein. Various aspects of the device are elucidated through the study of channel conductance, electron density, energy band, and surface-potential profiles. The DG-ZnO-ISFETs enhance minor surface potential variations by capacitive coupling across the dielectrics of the top and bottom gates. Further, it operates by altering the bottom-gate voltage (VBG), obviating the need for a reference electrode system. The device depicts enhanced voltage sensitivity of 205.57 mV/pH, which is substantially 3.48 × greater than the Nernst limit sensitivity. In addition, the DG-ZnO-ISFET renders a maximum current sensitivity of 10.82 mA/mm.pH at VBG=5V. Furthermore, it exhibits high linearity in both voltage and current sensitivity, with the coefficient of determination (R2) values of 0.984 and 0.964, respectively. With their increased sensitivity and linearity, DG-ZnO-ISFETs have the potential device for next-generation biosensors.

Surface potential variation across (hk1) and non-(hk1) grain boundaries of antimony triselenide

Electronic materials that can tolerate defects or create self-passivated interfaces expand the horizons to more sustainable electronic devices. Two heuristic models suggest that (i) antimony triselenide is a defect tolerant semiconductor (to certain point defects) and that (ii) the boundaries of grains with a specific orientation are self-passivated. Yet, experimental support or scrutiny of these models is lacking. Partly because measuring a passive quality of a material is perplexing. A possible way forward is to measure defect activity in correlation with other experimental evidence for defect presence, for example, by relating the grain boundaries’ (GBs) nanostructure and electronic quality. One measure of the electronic quality is the magnitude of the surface potential variation across grain boundaries—a high (charged) defect concentration and low defect tolerance typically leads to a significant surface potential variation. On the other hand, direct observation of defects (e.g., by electron microscopy) and a small surface potential variation indicates that the material is defect tolerant. In parallel, a comparison of the surface potential variation across grain boundaries in films with grains of different orientations provides a qualitative measure of the self-passivation of material because only certain crystallographic surfaces are expected to be self-passivated. Herein we report that regardless of the grains’ mutual respective orientation, the grain boundaries of antimony triselenide have a relatively benign electronic quality. This is evident in a shallow surface potential variation across grain boundaries (10–50 mV; 195 GBs were investigated). Yet, a high-resolution TEM investigation reveals that a variety of defects is prevalent at and near the GBs. Additionally, we found that GBs in films with distinct (hk1) orientation indeed have a lower surface potential variation than grains with (hk0) orientation. These findings support both heuristic models and provide a practical way to scrutinize defect tolerance in other polycrystalline semiconductors.

Electrostatic effect due to patch potentials between closely spaced surfaces

The spatial variation and temporal variation in surface potential are important error sources in various precision experiments and deserved to be considered carefully. In the former case, the theoretical analysis shows that this effect depends on the surface potentials through their spatial autocorrelation functions. By making some modification to the quasilocal correlation model, we obtain a rigorous formula for the patch force, where the magnitude is proportional to $\frac{1}{{a}^{2}}{(\frac{a}{w})}^{\ensuremath{\beta}(a/w)+2}$ with $a$ the distance between two parallel plates, $w$ the mean patch size, and $\ensuremath{\beta}$ the scaling coefficient from $\ensuremath{-}2$ to $\ensuremath{-}4$. A torsion balance experiment is then conducted, and we obtain a 0.4 mm effective patch size and 20 mV potential variance. In the latter case, we apply an adatom diffusion model to describe this mechanism and predicts a ${f}^{\ensuremath{-}3/4}$ frequency dependence above 0.01 mHz. This prediction meets well with a typical experimental results. Finally, we apply these models to analyze the patch effect for two typical experiments. Our analysis will help to investigate the properties of surface potentials.

Ferrotronics for the creation of band gaps in graphene

We experimentally demonstrate a simple graphene/ferrolectric device, termed ferrotronic (electronic effect from ferroelectric) device in which the band structure of single-layer graphene is modified. The device architecture consists of graphene deposited on a ferroelectric substrate which encodes a periodic surface potential achieved through domain engineering. This structure takes advantage of the nature of conduction through graphene to modulate the Fermi velocity of the charge carriers by the variations in surface potential, leading to the emergence of energy minibands and a band gap at the superlattice Brillouin zone boundary. Our work represents a simple route to building circuits whose functionality is controlled by the underlying substrate.

Hierarchical Colloidal Self-Assembly on Lattice-Mismatched Moiré Patterns.

Extending atomic epitaxy concepts to colloidal systems for realizing functional surface structures has recently piqued scientific interest. Akin to the growth of ordered metal clusters on graphene moiré, spatially ordered colloidal crystals have been realized on soft lithographically fabricated moiré patterns. In addition to moiré periodicity, lattice misfit strain can bring about a further level of hierarchy in colloidal self-assembly, although its role in self-organization remains unexplored. Here, we demonstrate the self-organized growth of micrometer-sized colloidal pyramid arrays with lateral order extending over millimeter length scales on lattice-mismatched moiré patterns. By probing the film growth dynamics with single-particle resolution, we uncovered the interplay between lattice misfit strain and topographically varying surface potential within the moiré unit cell, which significantly alters the nucleation process. We also show that the structural organization of colloids within moiré regions primarily depends on the moiré angle, and by tuning it, multiple levels of hierarchy can be achieved.

Electrochemical detection of fluoride ions in water with nanoporous gold modified by a boronic acid terminated self-assembled monolayer.

Nanoporous gold (npAu) is a perfectly suited platform for the electrochemical detection of minor amounts of chemical species in solution due to its high surface-to-volume ratio. By surface-modification of the self-standing structure with a self-assembled monolayer (SAM) of 4-mercaptophenylboronic acid (MPBA) it was possible to create an electrode very sensitive towards fluoride ions in water, also suitable for mobile use in future sensing applications. The proposed detection strategy is based on the change in the charge state of the boronic acid functional groups of the monolayer, induced by fluoride binding. The surface potential of the modified npAu sample reacts fast and sensitively to stepwise F- addition, showing highly reproducible, well-defined potential steps with a detection limit of 0.2 mM. Deeper insight into the reaction of fluoride binding on the MPBA modified surface was gained by electrochemical impedance spectroscopy. The proposed fluoride sensitive electrode exhibits a favorable regenerability in alkaline media, which is of central importance for future applications considering environmental as well as economical aspects.

Contributions of Ti-xTa cold spray composite interface to in-vitro cell growth

Surface charge of biomaterials is one of the most influential parameters on regulating the complex processes of cell responses in tissue engineering. This study explores the contributions of x Ta ( x ​= ​5; 10; 30 in wt %) interface with the Ti as matrix on the in-vitro cell growth when such composites were produced through cold spray additive manufacturing. Preliminary results reveal that formation of intimate contact between deposited Ti and Ta splats provides meaningful differences in work function, results in an estimated surface potential variation around 50 ​mV, that ultimately influence cell growth. Increasing mass fraction of Ta in the chosen cold sprayed Ti- x Ta composites was beneficial to initial cell attachment and proliferation upon the surface. Electrochemical response of cold sprayed coatings indirectly proves that Ta may act as anode and Ti performs as cathode in the electrochemical cells with possible surface charge gradient that allow to design and adapt biomaterials surfaces to a specific application. Understanding the mechanism of cell growth upon the surface of cold sprayed Ti- x Ta composites will contribute to design of biomaterial surface for promising osseointegrity in biomedical applications. • Cold spray establishes a Ti/Ta electrolytic cell with work function variations in guiding cell performance.

Recent Advances in In Situ/Operando Surface/Interface Characterization Techniques for the Study of Artificial Photosynthesis

(Photo-)electrocatalytic artificial photosynthesis driven by electrical and/or solar energy that converts water (H2O) and carbon dioxide (CO2) into hydrogen (H2), carbohydrates and oxygen (O2), has proven to be a promising and effective route for producing clean alternatives to fossil fuels, as well as for storing intermittent renewable energy, and thus to solve the energy crisis and climate change issues that we are facing today. Basic (photo-)electrocatalysis consists of three main processes: (1) light absorption, (2) the separation and transport of photogenerated charge carriers, and (3) the transfer of photogenerated charge carriers at the interfaces. With further research, scientists have found that these three steps are significantly affected by surface and interface properties (e.g., defect, dangling bonds, adsorption/desorption, surface recombination, electric double layer (EDL), surface dipole). Therefore, the catalytic performance, which to a great extent is determined by the physicochemical properties of surfaces and interfaces between catalyst and reactant, can be changed dramatically under working conditions. Common approaches for investigating these phenomena include X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning probe microscopy (SPM), wide angle X-ray diffraction (WAXRD), auger electron spectroscopy (AES), transmission electron microscope (TEM), etc. Generally, these techniques can only be applied under ex situ conditions and cannot fully recover the changes of catalysts in real chemical reactions. How to identify and track alterations of the catalysts, and thus provide further insight into the complex mechanisms behind them, has become a major research topic in this field. The application of in situ/operando characterization techniques enables real-time monitoring and analysis of dynamic changes. Therefore, researchers can obtain physical and/or chemical information during the reaction (e.g., morphology, chemical bonding, valence state, photocurrent distribution, surface potential variation, surface reconstruction), or even by the combination of these techniques as a suite (e.g., atomic force microscopy-based infrared spectroscopy (AFM-IR), or near-ambient-pressure STM/XPS combined system (NAP STM-XPS)) to correlate the various properties simultaneously, so as to further reveal the reaction mechanisms. In this review, we briefly describe the working principles of in situ/operando surface/interface characterization technologies (i.e., SPM and X-ray spectroscopy) and discuss the recent progress in monitoring relevant surface/interface changes during water splitting and CO2 reduction reactions (CO2RR). We hope that this review will provide our readers with some ideas and guidance about how these in situ/operando characterization techniques can help us investigate the changes in catalyst surfaces/interfaces, and further promote the development of (photo-)electrocatalytic surface and interface engineering.

Design of Core Gate Silicon Nanotube RADFET with Improved Sensitivity

The prominent feature of Silicon nanotube MOSFET for using RADFET application is its high Ion/Ioff ratio, minimal leakage current and less sensitive to short channel effects. Due to the above features the radiation behaviour of the device is studied to check for the applicability of a RADFET. Here both uniform and non-uniform irradiation characteristics are analysed. The focus of this study is on electrical characteristics and sensitivity, which is measured as a variation of threshold voltage of radiated and unirradiated device. It was found that on irradiation, the surface potential variation is high for 40 nm channel length hence the analysis is conducted for the same. It was proven to be successful, as the device achieves high Ion/Ioff ratio of the order 1013 and a sensitivity of 2.26 mv Gy−1. The obtained results are compared with DG RADFET and JL DG RADFET and it shows that Core gate Silicon nanotube RADFET has better electrical characteristics and sensitivity. The simulations are performed in Silvaco 3 D Atlas TCAD simulation software.

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys.

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin probe, both of which measure the Volta potential difference (VPD) between an oscillating probe tip and a sample surface by applying a nulling voltage equal in magnitude but opposite in sign to the tip-sample potential difference. By scanning a conductive KPFM probe over a sample surface, nanoscale variations in surface topography and potential can be mapped, identifying likely anodic and cathodic regions, as well as quantifying the inherent material driving force for galvanic corrosion. Subsequent co-localization of KPFM Volta potential maps with advanced scanning electron microscopy (SEM) techniques, including back scattered electron (BSE) images, energy dispersive spectroscopy (EDS) elemental composition maps, and electron backscattered diffraction (EBSD) inverse pole figures can provide further insight into structure-property-performance relationships. Here, the results of several studies co-localizing KPFM with SEM on a wide variety of alloys of technological interest are presented, demonstrating the utility of combining these techniques at the nanoscale to elucidate corrosion initiation and propagation. Important points to consider and potential pitfalls to avoid in such investigations are also highlighted: in particular, probe calibration and the potential confounding effects on the measured VPDs of the testing environment and sample surface, including ambient humidity (i.e., adsorbed water), surface reactions/oxidation, and polishing debris or other contaminants. Additionally, an example is provided of co-localizing a third technique, scanning confocal Raman microscopy, to demonstrate the general applicability and utility of the co-localization method to provide further structural insight beyond that afforded by electron microscopy-based techniques.

Surface potential and local conductivity measurements of micropatterned aromatic monolayers covalently attached to n-Si(111) via Si–C and Si–O bonds

The surface potentials and local conductivity of self-assembled monolayers (SAMs) formed using aromatic molecules covalently bonded to n-type silicon (111) via Si–C and Si–O bonds were measured using Kelvin probe force microscopy (KPFM) and conductive AFM (CAFM). Surface potential measurements were done using micropatterned SAMs with hexadecyl SAM as a reference to eliminate surface potential variations due to the cantilever tips. Micropatterning was conducted via vacuum ultraviolet photolithography at λ = 172 nm. Ellipsometry, X-ray photoelectron spectroscopy, static water contact angle and atomic force microscopy tests show that the aromatic SAMs were well-organized despite the short molecular lengths of the precursors. KPFM results show that Si–C bonded SAMs have higher surface potentials compared to Si–O SAMs, which is in agreement with dipole moments estimated by Molecular Orbital Package semi-empirical computations. CAFM scans showed conductive domains for the aromatic SAM regions, and Si–O SAMs exhibited a higher current than Si–C SAMs.

Physically and chemically smooth cesium-antimonide photocathodes on single crystal strontium titanate substrates

The performance of x-ray free electron lasers and ultrafast electron diffraction experiments is largely dependent on the brightness of electron sources from photoinjectors. The maximum brightness from photoinjectors at a particular accelerating gradient is limited by the mean transverse energy (MTE) of electrons emitted from photocathodes. For high quantum efficiency (QE) cathodes like alkali-antimonide thin films, which are essential to mitigate the effects of non-linear photoemission on MTE, the smallest possible MTE and, hence, the highest possible brightness are limited by the nanoscale surface roughness and chemical inhomogeneity. In this work, we show that high QE Cs3Sb films grown on lattice-matched strontium titanate (STO) substrates have a factor of 4 smoother, chemically uniform surfaces compared to those traditionally grown on disordered Si surfaces. We perform simulations to calculate roughness induced MTE based on measured topographical and surface-potential variations on the Cs3Sb films grown on STO and show that these variations are small enough to have no consequential impact on the MTE and, hence, the brightness.