Increase In Surface Potential Research Articles

A Versatile Buffer Layer for Polymer Solar Cells: Rendering Surface Potential by Regulating Dipole

Spin‐coated film of poly(vinylidenefluoride‐hexafluoropropylene) (P(VDF‐HFP)) acts as a cathode/anode buffer layer in polymer solar cells (PSCs) with conventional/inverted device structures. Such devices show optimized performances comparable with the controlled device, making P(VDF‐HFP) a good substitute for LiF/MoO3 as a cathode/anode buffer layer. Ultraviolet photoelectron spectroscopy (UPS) and Kelvin force microscope (KFM) measurements show that increased surface potential of active layers improves cathode contact. In piezoresponse force microscopy (PFM) measurement, P(VDF‐HFP) responds to applied bias in phase curve, showing tunable dipole. This tunable dipole renders surface potential under applied bias. As a result, open‐circuit voltage of devices alters instantly with poling voltage. Moreover, positive poling of P(VDF‐HFP) together with simultaneous oxidation of Ag gradually improves performance of inverted structure device. Integer charge transfer (ICT) model elucidates improved electrode contacts by dipole tuning, varying surface potential and vacuum level shift. Understanding the function of dipole makes P(VDF‐HFP) a promising and versatile buffer layer for PSCs.

Numerical simulation of multi-combined effects of parameters on polymer charging characteristics due to electron irradiation

Charging characteristics of an insulator specimen due to non-penetrated electron irradiation have been attracting a great deal of attention in the fields such as scanning electron microscopy, electron probe analysis, and space irradiation. In this paper, we use a numerical simulation model based on Monte Carlo method for investigating the electron scattering. The elastic scattering is calculated with the Mott cross-section, and the inelastic scattering is simulated with Penn model and the fast secondary electron model according to the primary energy. The charge transport caused by the build-in electric field and charge density gradient is calculated with finite-difference time-domain method. Multi-combined effect of correlative parameters on charging characteristics is investigated by efficient multithreading parallel computing. During the irradiation, the landing energy of primary electrons decreases due to the negative surface potential, which makes the secondary electron yield increase. Variations of secondary electron current and sample current are presented to verify the validity of the simulation model by comparing with existing experimental results. Evolutions of leakage current, surface potential and internal space charge quantity are calculated under different conditions of incident electron current, primary energy and sample thickness. The results are presented in contour maps with different multi-parameter combinations, primary energy and sample mobility, primary energy and sample thickness, and primary energy and incident current. The balance state of charging will be determined by leakage current under conditions of a larger primary energy, sample mobility, incident current, or a less sample thickness, which is shown as the leakage current dominated mode. While in the cases of a lower primary energy, sample mobility, incident current, or a larger sample thickness, the balance state of charging is mainly dominated by secondary electron current, as the secondary electron current dominated mode. In other cases except the above two, the balance state will be determined by both leakage and secondary currents as the mixture mode. In the same mode, variations of charging characteristics with parameters are monotonic. When the change of a parameter makes the negative surface potential increase, the effect of this parameter on negative surface potential will be weakened, while the effects of other parameters on the negative potential will be enhanced. With the change of current dominated mode, the total charge quantity exhibits the local maximum with respect to the sample thickness, and the value of this maximum increases with primary energy. Moreover, the leakage current increases with incident current linearly. The presented results can be helpful for understanding regularities and mechanisms of charging due to electron irradiation, and estimating the charging intensity under different conditions of irradiation and sample material.

Surface potential dynamic characteristics of the insulating sample under high-energy electron irradiation

The surface potential and electron yield dynamic characteristics of an insulating thick sample under high-energy electron beam irradiation are obtained by combining the numerical simulation and experimental measurement. The numerical model takes into account the electron scattering, charge trapping, and charge transport. The results show that due to the electron scattering and transport, the space charge is weakly positive in the near surface and strongly negative inside sample; along the depth direction, the space potential decreases to a minimum value slowly, and then increases gradually and finally tends to zero; with the electron beam irradiation, the surface potential decreases to the negative kV magnitude gradually, and the total electron yield gradually increases to a stable value that is slightly less than unity. After stopping irradiation, the surface potential increases gradually, but charges are not eliminated completely. The surface potential decreases linearly with the increase of the beam energy, and increases with the increase of the incident angle, however it decreases slightly with the increase of the sample thickness.

Binary self-assembled monolayers modified Au nanoparticles as carriers in biological applications.

Gold nanoparticles (AuNPs) are good nonviral carriers because of their ease of synthesis and conjugation in biochemistry, and self-assembled monolayers (SAMs) provide a tunable system to change their interfacial properties. Using homogeneously mixed carboxylic acid and amine functional groups, a series of surface potentials and isoelectric points (IEPs) could be obtained and allow systematic study of the effect of surface potential. In this work, the result of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay revealed that binary-SAM modified AuNPs have high biocompatibility with HEK293T cells. The amount of AuNPs ingested by the cells was found to increase with increasing surface potential and the difference was also confirmed with a scanning transmission electron microscope. The ability of binary-SAM modified AuNPs as carriers was examined, and the plasmid deoxyribose nucleic acid (DNA)-containing eGFP reporter gene was used as the model cargo. Fluorescence imaging revealed that the transfection efficiency generally increased with increasing surface potential. More importantly, when the IEP of the AuNPs was higher than that of the environment of the endosome but lower than that of the cytoplasm, the plasmid DNA can be protected better and released more easily during the endocytosis process hence higher efficiency is obtained with 60% NH2 and 40% COOH in the binary-SAM.

DNA combing on low-pressure oxygen plasma modified polysilsesquioxane substrates for single-molecule studies.

Molecular combing and flow-induced stretching are the most commonly used methods to immobilize and stretch DNA molecules. While both approaches require functionalization steps for the substrate surface and the molecules, conventionally the former does not take advantage of, as the latter, the versatility of microfluidics regarding robustness, buffer exchange capability, and molecule manipulation using external forces for single molecule studies. Here, we demonstrate a simple one-step combing process involving only low-pressure oxygen (O2) plasma modified polysilsesquioxane (PSQ) polymer layer to facilitate both room temperature microfluidic device bonding and immobilization of stretched single DNA molecules without molecular functionalization step. Atomic force microscopy and Kelvin probe force microscopy experiments revealed a significant increase in surface roughness and surface potential on low-pressure O2 plasma treated PSQ, in contrast to that with high-pressure O2 plasma treatment, which are proposed to be responsible for enabling effective DNA immobilization. We further demonstrate the use of our platform to observe DNA-RNA polymerase complexes and cancer drug cisplatin induced DNA condensation using wide-field fluorescence imaging.

Effect of Surface Potential on NIH3T3 Cell Adhesion and Proliferation

Cell adhesion is central to many cell behaviors including survival, differentiation, and motility. With the recent development of biomaterials and medical instrumentation, cell behaviors on artificial biosurfaces have gained attention from the research community. Self-assembled monolayers (SAMs) are known for their excellent ability to modify surfaces. To achieve more precise control of surface properties, mixed 6-amino-1-hexanethiol and 6-mercaptohexanonic acid were deposited on a gold substrate, and in a physiological environment, arbitrary zeta potentials between −187 and +6 mV were obtained. This binary SAM system elucidated the effect of surface potential on the adhesion and proliferation of NIH3T3 cells cultured on these surfaces. Cell adhesion, density, morphology, and proliferation were investigated by optical, fluorescence, and scanning electron microscopes. It was found that increased surface potential promoted cell attachment; hence, the initial cell density increased. However, the apparent proliferation rate decreased with increasing surface potential due to contact inhibition between adjacent NIH3T3 cells at higher density. When the initial density was low and cells did not contact each other, surface potential had little or no effect on proliferation. A more positive surface potential also changed the cell shape from bipolar to spreading and allowed more cell–cell and cell–substrate interactions due to the enhanced cell adhesion.

Dust grain surface potential in a non-Maxwellian dusty plasma with negative ions

AbstractDust charging processes involving the collection of electrons and positive/negative ions in a non-equilibrium dusty plasma are revisited by employing the power-law kappa (κ)-distribution function. In this context, the current balance equation is solved to obtain dust grain surface potential in the presence of negative ions. Numerically, it is found that plasma parameters, such as the κ spectral index, the negative ion-to-electron temperature ratio (γ), the negative–positive ion number density ratio (α), and the negative ion streaming speed (U0) significantly modify the dust grain potential profiles. In particular, for large kappa values, the dust grain surface potential reduces to the Maxwellian case, and at lower kappa values the magnitude of the negative dust surface potential increases. An increase in γ and U0 leads to the enhancement of the magnitude of the dust grain surface potential, while α leads to an opposite effect. The relevance of present results to low-temperature laboratory plasmas is discussed.

Influence of Surface Potential on the Adhesive Force of Radioactive Gold Surfaces

Radioactive particles may acquire surface potential through self-charging, and thus can behave differently from natural aerosols in atmospheric systems with respect to aggregation, deposition, resuspension, and transport to areas surrounding a radioactive source. This work focuses on the adhesive force between radioactive particles and metallic surfaces, which relates to the deposition and resuspension of particles on surrounding surfaces. Scanning surface potential microscopy was employed to measure the surface potential of radioactive gold foil. Atomic force microscopy was used to investigate the adhesive force for gold that acquired surface charge either by irradiation or by application of an equivalent electrical bias. Overall, the adhesive force increases with increasing surface potential or relative humidity. However, a behavior that does not follow the general trend was observed for the irradiated gold at a high decay rate. A comparison between experimental measurements and calculated values revealed that the surface potential promotes adhesion. The contribution of the electrostatic force at high levels of relative humidity was lower than the one found using theoretical calculations due to the effects caused by enhanced adsorption rate of water molecules under a high surface charge density. The results of this study can be used to provide a better understanding of the behavior of radioactive particles in atmospheric systems.

The crystallization kinetics of potassium chloride in the presence of additives with common ions

In this study, the growth and dissolution rates of closely sized KCl crystals that were classified with respect to their separation voltages in an electrostatic separator were investigated in the presence of K2SO4, CH3COOK, NH4Cl and BaCl2 additives, having common cations or anions. Separation voltages were taken as a measure of surface potential. The experiments were carried out in a typical stagnant medium type single crystal measurement system.From experimental results it is observed that thread-like growth behaviors resulting from surface nucleation were dominant and dead zones where there was no growth or dissolution did always exist. For additives having common cation, it was found that the growth in the thread-like structure increases with the increase of surface potential. It was also found that, additives having common anions, initially led to decrease and neutralize the surface potential, then oppositely charged the crystal surface, when the crystal growth/dissolution behavior changes were considered. This study obviously proved that, crystal growth and dissolution behaviors can be controlled by changing the surface potentials of crystals by using additives having common ions.

Kelvin probe force microscopy of nanocrystalline TiO2 photoelectrodes

Dye-sensitized solar cells (DSCs) provide a promising third-generation photovoltaic concept based on the spectral sensitization of a wide-bandgap metal oxide. Although the nanocrystalline TiO2 photoelectrode of a DSC consists of sintered nanoparticles, there are few studies on the nanoscale properties. We focus on the microscopic work function and surface photovoltage (SPV) determination of TiO2 photoelectrodes using Kelvin probe force microscopy in combination with a tunable illumination system. A comparison of the surface potentials for TiO2 photoelectrodes sensitized with two different dyes, i.e., the standard dye N719 and a copper(I) bis(imine) complex, reveals an inverse orientation of the surface dipole. A higher surface potential was determined for an N719 photoelectrode. The surface potential increase due to the surface dipole correlates with a higher DSC performance. Concluding from this, microscopic surface potential variations, attributed to the complex nanostructure of the photoelectrode, influence the DSC performance. For both bare and sensitized TiO2 photoelectrodes, the measurements reveal microscopic inhomogeneities of more than 100 mV in the work function and show recombination time differences at different locations. The bandgap of 3.2 eV, determined by SPV spectroscopy, remained constant throughout the TiO2 layer. The effect of the built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far.

Effects of Chemical Treatments and Ultrathin Al2O3 Deposition on InAlN Surface Investigated by X-ray Photoelectron Spectroscopy

The effects of chemical treatments and Al2O3 deposition on the InAlN surface were investigated by X-ray photoelectron spectroscopy (XPS). Independent of the extent of native oxide removal, the In 4d core-level binding energy was the same for untreated, HCl-treated, and HF-treated InAlN. This indicated a strong pinning tendency of the Fermi level at the InAlN bare surface. However, a 300 meV decrease in the In 4d binding energy was observed after atomic layer deposition (ALD) of Al2O3, which indicated an increase in the negative surface potential at the InAlN surface. The reduction of positive charge at the InAlN surface is discussed.

Molecule counting with alkanethiol and DNA immobilized on gold microplates for extended gate FET

Several molecule counting methods based on electrochemical characterization of alkanethiol and thiolated single-stranded oligonucleotide (HS-ssDNA) immobilized on gold microplates, which were used as extended gates of field effect transistors (FETs), have been investigated in this paper. The surface density of alkanethiol and DNA monolayers on gold microplates were quantitatively evaluated from the reductive desorption charge by using cyclic voltammetry (CV) and fast CV (FCV) methods in strong alkali solution. Typically, the surface density of 6-hydroxy-1-hexanethiol (6-HHT) was evaluated to be 4.639molecules/nm2, and the 28 base-pair dsDNA about 1.226–4.849molecules/100nm2 on Au microplates after post-treatment with 6-HHT. The behaviors on surface potential and capacitance of different aminoalkanethiols on Au microplates were measured in 0.1mol/L Na2SO4 and 10mmol/L Tris–HCl (pH=7.4) solutions, indicating that the surface potential increases and the double-layer capacitance decreases with the length of carbon chain increased for the thiol monolayers, which obey a physics relationship for a capacitor. Comparably, a simple sensing method based on the electronic signals of biochemical reaction events on DNA immobilization and hybridization at the Au surface of the extended gate FET (EGFET) was developed, with which the surface density of the hybridized dsDNA on the gold surface of the EGFET was evaluated to be 1.36 molecules per 100nm2, showing that the EGFET is a promising sensing biochip for DNA molecule counting.

Analytical model of surface depletion in GaAs nanowires

Poisson's equation is solved to provide a comprehensive model of nanowire (NW) surface depletion as a function of interface state density, NW radius, and doping density. This model improves upon established theory by giving distinct solutions to the cases of full and partial NW depletion while implementing the charge neutrality level and accurate Fermi-Dirac statistics. To explain the underlying physics, key parameters were plotted as a function of both interface state density and NW radius, showing interesting features such as the lowering of the Fermi level in fully depleted NWs and marked increase in surface depletion width and built-in surface potential (relative to a planar film equivalent) in partially depleted NWs. Finally, examination of NW conductivity found that for NWs of radius acrit, the minimum NW radius before which the entire NW is depleted, conductivity can be reduced by up to 95% relative to bulk. Additionally, majority carrier inversion is predicted to occur in thin NWs.

Quantum effects in electron emission from and accretion on negatively charged spherical particles in a complex plasma

The authors have investigated the electron emissions (thermionic, electric field, photoelectric, and light induced field) from and electron accretion on a charged particle in a complex plasma, on the basis of a three region electrical potential model in and around a charged spherical particle in a complex plasma, characterized by Debye shielding. A continuous variation of the transmission coefficient across the surface of a particle (corresponding to emission and accretion) with the radial electron energy ɛr has been obtained. It is seen that the numerical values of the emission and accretion transmission coefficients [D(ɛr)] are almost the same. This is the necessary and sufficient condition for the validity of Saha’s equation for thermal equilibrium of a system of dust and electrons. This is in contrast to the earlier condition, which limited the range of validity of Saha’s equation to the range of the applicability of Born approximation. It is seen that D(ɛr) increases with increasing ɛr, increasing negative electric potential on the surface, decreasing radius, and deceasing Debye length. The electron currents, corresponding to thermionic, electric field, photoelectric and light induced field emission increase with increasing surface potential; this fact may have significant repercussions in complex plasma kinetics. Since numerically D(ɛr) is significantly different from unity in the range of ɛr of interest, it is necessary to take into account the D(ɛr)−ɛr dependence in complex plasma theory.

Green Emission from a Strain-Modulated InGaN Active Layer

Strain-induced quantum dots (QDs) like island formations are demonstrated to effectively suppress pits/dislocation generation in high indium content (26.8%) InGaN active layers. In addition to the strain redistribution in the QD-like islands, strain modulation on the InGaN active layers by using the GaN island capping is employed to form an increased surface potential barrier around the dislocation cores, which inhibits the carrier transport to the surrounding dislocations. Cathodoluminescence shows distinct double-peak emissions at 503 nm and 444 nm, corresponding to the In-rich QD-like emission and the normal quantum well emission, respectively. The QD-like emission becomes dominated in photoluminescence due to the carrier localization effect of In-rich InGaN QDs at relatively low “carrier injection current". Accordingly, green emission may be enhanced by the following origins: (1) reduction in pits/dislocations density, (2) carrier localization and strain reduction in QDs, (3) strain modulation by GaN island capping, (4) enhanced light extraction with faceted GaN islands on the surface.

Potential Steps at C60–TiOPc–Ag(111) Interfaces: Ultrahigh-Vacuum–Noncontact Scanning Probe Metrology

Nanoscale structure-electric potential relations in films of the organic molecular semiconductors C(60) and titanyl phthalocyanine (TiOPc) on Ag(111) have been measured under UHV conditions. Noncontact force methods were utilized to image domain structures and boundaries with molecular resolution, while simultaneously quantifying the local surface electric potential. Sensitivity and spatial resolution for the local potential measurement were first established on Ag(111) through direct observation of the electrical dipole and potential step, φ(step) = 10 ± 3 mV, of monatomic crystallographic steps. A local surface potential increase of 27 ± 11 mV occurs upon crossing the boundary between the neat Ag(111) surface and C(60) islands. Potential steps in binary C(60)-TiOPc films, nanophase-separated into crystalline C(60) and TiOPc domains, were then mapped quantitatively. The 207 ± 66 mV potential step across the C(60)-to-TiOPc domain boundary exhibits a 3.6 nm width that reflects the spatial resolution for electric potential across a material interface. The absence of potential asymmetry across this lateral interface sets the upper bound for the C(60)-TiOPc interface dipole moment as 0.012 e nm.

Closed-Form Model for High-k MOSFET Channel Parameters: Reflecting Non-Saturating Inversion Surface Potential, Gate Stack Traps, and Work Function Anomaly

Closed-form, textbook-appropriate equations have been derived for the drain current ID, the channel conductance gD, and the trans-conductance gm of high-k MOSFETs, incorporating high-k gate stack charges Qdi,gsc, non-saturating inversion surface potential increase Δφs,inv, and work-function difference ϕMS. These ab initio relations, developed without imposing any assumptions, provide a clear view of the degrading effects of the high-k gate stack charges, the non-saturating inversion surface potential, and the semiconductor-metal work function difference on ID, gD, and gm. Rational estimates have been made of the latter which illustrate the relative weights of each of the three non-ideal factors in the degradation of the channel parameters of the high-k gate stack. The degradation appears to be most severe for the channel conductance, followed by the drain current, and then the trans-conductance. The work-function anomaly does not directly affect the trans-conductance for which the major degrading factor is the non-saturating surface potential. Even for moderate drain voltages, the numerical estimates reveal the drain current versus the drain voltage relation to become more non-linear in the case of the high-k gate stack. Comparison with the available experimental high-k gate stack data supports the import of these equations.

Structure of rat skin after application of electret characterized by DSC

Polypropylene (PP) electrets with surface potential of −500V, −1000V and −2000V were prepared by constant voltage corona charging. The electrets were applied to excised rat skin for 2 hours respectively and then the skin samples were analyzed with the differential scanning calorimetry (DSC) technique to study the alteration of lipid organization of the skin. There were three peaks at 63°C, 82.7°C and 115.1°C in the DSC spectra for rat skin untreated, which have been assigned essentially to lipid, lipid-protein and protein alterations. For −500V electret treated-skin sample, only a single peak appeared at 79.1°C. With the increase of electret surface potential from −500V to −2000V, the transition temperature and peak areas at moderate decreased first and then increased. The negative electret could result in the transition of stratum corneum (SC) lipid from gel to liquid crystal and protein transition from α helix structure to β folding structure. The regulation action of electret to skin mircostructure presented an effect of "potential window".

Stabilization of oil-in-water emulsions by non-ionic detergents: the effect of polyoxyethylene chain length.

Abstract The stability of emulsions of anisole and chlorobenzene in water has been estimated by following microscopically the size distribution of emulsions as a function of time. The effect of increasing the length of the polyoxyethylene glycol chain of the hexadecyl ether non-ionic stabilizers from three units to nine units was determined. Electrophoretic measurements indicated that the higher stability obtained on increasing the glycol chain length is not due to an increasing surface potential: it is ascribed to an entropic effect. Stability data are tabulated as rates of coalescence (s−1) and results are presented relating these rates to zeta-potential, surface concentration and polyoxyethylene chain length. The results are discussed qualitatively and compared with the results obtained with cetomacrogol 1000 (Elworthy & Florence, J. Pharm. Pharmac., 1967, 19, 140S).

Resonance Frequencies of Electrically Charged Nanoparticles

We develop a procedure to analyze charged nanoparticle (NP) surface modes. Using the resonance condition derived by Rosenkrantz and Arnon, we obtain frequencies at which the electromagnetic (EM) radiation stimulates resonance over a wide range of modes. Our results confirm that the relation between the resonance frequencies and the excess surface charge can be described by a monotonically increasing function. Taking the derivative of this function, it is evident that the lower surface potentials have a greater influence on the resonance frequency. This effect decreases as the surface potential increases. Surface modes contribute to the surface energy of charged NPs, and for this reason, they can modify charged NP optical properties. We found that there is a strong dependence of the resonance frequencies on the electrically charged NP refractive index and surface potential. This dependence can play an important role in nonmetallic nanotechnological devices, such as attenuators and modulators in optical communication and optical detectors in biomedical sensors.