Surface Potential Modulation Research Articles

Exploring electron energy dependencies in the formation of surface charge on ZnO crystals

This study delves into the phenomena surrounding the emergence of negative charge accumulation on the surface of zinc oxide (ZnO) crystals upon exposure to low-energy electron irradiation within the range of E = 0 ÷ 600 eV. We demonstrate the critical significance of the primary electron energy in dictating the initial processes initiated at the ZnO surface. Energy thresholds governing both the genesis and elimination of surface charge, along with the formation of oxygen vacancies on the ZnO surface, are elucidated. Our findings reveal that irradiation instigates distinct dissociative pathways contingent upon electron energy, engendering diverse physical and chemical transformations via bond dissociation, desorption, and atomic and molecular restructuring of the surface. These insights serve as pivotal groundwork towards advancing the fabrication of epitaxial films, facilitating the controlled generation of two-dimensional structures predicated on surface potential modulation.

Ultrafast Negative Capacitance Transition for 2D Ferroelectric MoS2/Graphene Transistor.

Negative capacitance gives rise to subthreshold swing (SS) below the fundamental limit by efficient modulation of surface potential in transistors. While negative-capacitance transition is reported in polycrystalline Pb(Zr0.2Ti0.8)O3 (PZT) and HfZrO2 (HZO) thin-films in few microseconds timescale, low SS is not persistent over a wide range of drain current when used instead of conventional dielectrics. In this work, the clear nano-second negative transition states in 2D single-crystal CuInP2S6 (CIPS) flakes have been demonstrated by an alternative fast-transient measurement technique. Further, integrating this ultrafast NC transition with the localized density of states of Dirac contacts and controlled charge transfer in the CIPS/channel (MoS2/graphene) a state-of-the-art device architecture, negative capacitance Dirac source drain field effect transistor (FET) is introduced. This yields an ultralow SS of 4.8mV dec-1 with an average sub-10 SS across five decades with on-off ratio exceeding 107, by simultaneous improvement of transport and body factors in monolayer MoS2-based FET, outperforming all previous reports. This approach could pave the way to achieve ultralow-SS FETs for future high-speed and low-power electronics.

Surface potential modulation for improved mechanical energy harvesting and sensing in 3D printed biopolymer thermoelectret

This study presents the development of a special class of electret known as thermoelectret, made with polylactic acid (PLA), prepared via 3D printing technology followed by a heat-assisted corona discharged technique. We show that the bimodal surface polarity (i.e., ± 1 kV) is possible to achieve over the top and bottom surfaces of 3D printed PLA thermoelectret that enable a wide range of potential applications, such as mechanical energy harvesting to wearable biomedical sensors. Besides this, the enhanced surface potential leads to a giant longitudinal piezoelectric coefficient (d33) of 350 ± 15 pC/N, which is typically 10 times higher than the best-known piezoelectric polymer, polyvinylidene fluoride-trifluoro ethylene (PVDF-TrFE). To utilize this benefit, a 3D-printed thermoelectret-based mechanical energy harvester (3D-TEH) has been developed that delivers an outstanding power density of ⁓ 25 mW/m2, which makes it a suitable candidate for the self-power pressure sensor. As a proof of concept, we have demonstrated that 3D-TEH is capable of recording the bone-joints motions, coughing action, and human foot-pressure distribution that signify its potential application as a self-powered bio-medical sensor.

Molecular dipole regulated surface potential and ferroelectric characteristics in nanoconfined P(VDF-TrFE) architectures

Molecular dipole orientation is crucial to efficiently utilize the piezo- and ferroelectric polarization in polymers. Modern electronic applications of these polymer demand switching of dipoles with low voltage, which requires the dipole orientation to be perpendicular to the substrate (edge-on) rather than parallel (face-on). The 0D, 1D and 2D confinement of polymer architectures into nanoparticles, nanofibers, and thin films is a vital approach to control dipole orientation in ferroelectrics. Our findings show that for 1D (d33 ∼ 18 ± 2.2 pm/V) and 2D nanostructures (d33 ∼ 15 ± 1.5 pm/V) of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)), the molecular dipoles are edge-on, whereas for 0D, the dipoles are face-on, which would govern the ferroelectric and piezoelectric behaviour of these confined structures. The subsequent observation concludes that altering the dipole orientation allows precise modulation of surface potential and work function. The work functions of the 0D, 1D and 2D nanostructures are 4.56 ± 0.03 eV, 4.93 ± 0.51 eV and 4.68 ± 0.18 eV, respectively. The tunability of surface potential opens up the possibility of utilizing these nanostructures in single-material triboelectric series. Density functional theory (DFT)-based computations substantially support these experimental findings. This study provides a thorough understanding of the effect of molecule dipole orientation on piezo- and ferroelectric properties of P(VDF-TrFE) nanostructures.

Experimental demonstration of the strong impact of plasma excitation frequency range on electronic properties of silicon nitride/GaAs interfaces

Passivation/encapsulation of III–V electronic and photonic circuits and devices is often performed using plasma-enhanced chemical vapor deposited SixNy with a standard high frequency excitation of 13.56 MHz. The hypothesis of a possible higher passivation process efficiency for low frequency SixNy deposition on GaAs has been suggested by comparison with published work on high frequency. In this work, we report a direct experimental demonstration of the effect of the RF plasma source frequency on electronic properties of GaAs/SixNy interfaces during a passivation process. GaAs substrates from the same wafer are processed in the same reactor using the same plasma conditions except for the plasma excitation frequency and MIS structures are fabricated to probe interface electronic properties. Comparing interfaces obtained with a plasma source frequency below or over the ion transit frequency (∼ 2 MHz) shows drastic difference in electrical behavior. While Fermi level is pinned when a high frequency plasma (13.56 MHz) is used, a good modulation of surface potential is observed with a low frequency plasma (380 kHz), indicating an interface states density lower by orders of magnitude. X-ray photoelectron spectra analysis is performed in order to investigate GaAs/SixNy interface formation in relation with plasma excitation frequency. We show that a short exposure to the low frequency plasma results in the almost complete removal of native oxides species. To the contrary, a significant amount of oxides remains on structures treated by the high frequency technique. The low frequency plasma technique could be a better alternative to the standard high frequency, as it is less reliant on wet chemical treatments like ammonium sulfide passivation and could allow for processing at lower temperatures.

Surface Potential Modulation in Boronate-Functionalized Magnetic Nanoparticles Reveals Binding Interactions: Toward Magnetophoretic Capture/Quantitation of Sugars from Extracellular Matrix.

Phenylboronic acids (BAs) are important synthetic receptors that bind reversibly to cis-diols enabling their use in molecular sensing. When conjugated to magnetic iron oxide nanoparticles, BAs have potential for application in separations and enrichment. Realizing this will require a new understanding of their inherent binding modes and measurement of their binding capacity and their stability in/extractability from complex environments. In this work, 3-aminophenylboronic acid was functionalized to superparamagnetic iron oxide nanoparticles (MNPs, core diameter 8.9 nm) to provide stable aqueous suspensions of functionalized particles (BA-MNPs). The progress of sugar binding and its impact on BA-MNP colloidal stability were monitored through the pH-dependence of hydrodynamic size and zeta potential during incubation with a range of saccharides. This provided the first direct observation of boronate ionization pKa in grafted BA, which in the absence of sugar shifted to a slightly more basic pH than free BA. On exposure to sugar solutions under MNP-limiting conditions, pKa moved progressively to lower pH as maximum capacity was gradually attained. The pKa shift is shown to be greater for sugars with greater BA binding affinity, and on-particle sugar exchange effects were inferred. Colloidal dispersion of BA-MNPs after binding was shown for all sugars at all pHs studied, which enabled facile magnetic extraction of glucose from agarose and cultured extracellular matrix expanded in serum-free media. Bound glucose, quantified following magnetophoretic capture, was found to be proportional to the solution glucose content under glucose-limiting conditions expected for the application. The implications for the development of MNP-immobilized ligands for selective magnetic biomarker capture and quantitation from the extracellular environment are discussed.

(Invited) Sustainable Platinum Group Metal (PGM) Recycling of Proton Exchange Membrane Fuel Cells and Electrolyzer Cells (PEMFCs & ECs) As a Vital Step Towards Truly Renewable and Green Energy Conversion Technologies

Owing to their strong catalytic nature and high robustness under various conditions, platinum group metals (PGMs) are state-of-the-art electrocatalysts for proton exchange membrane fuel cells and electrolyzer cells (PEMFCs & ECs) of superior performance and long stability. However, PGM supplies involving limited natural reserves, energy intensive mining process and related environment issues raise increasing concerns. As an indispensable (though insufficiently developed) aspect of the PEM technologies, sustainable PGM recovery and reuse can fundamentally tackle those concerns and serve as a strong support to further secure widespread commercialization of the technologies.Due to the popular use of perfluorinated sulfonic acid as proton conduction component, PGMs in PEMFCs&ECs are not suitable for recycling through the traditional pyrometallurgical route, which operates at a temperature over 1500°C. Burning of the electrodes releases huge amount of emission, which is not only harmful to the environment and human, but also corrosive to the process infrastructure.An alternative PGM recycling process using a hydro-electrochemical route1, 2 reveals many advantages, including high extraction efficiency, environmental friendliness, recovery of multiple components, low demand for operation conditions & maintenance, etc. Adopting the intrinsic degradation mechanisms of the PGM nanoparticles in combination with surface potential modulation, dissolution behavior of typical PEMFC&EC electrocatalysts have been systematically investigated. Under optimal parameters, a recovery efficiency of over 99% was successfully achieved for platinum electrocatalyst. Moreover, new catalysts prepared from the recycled precursor demonstrated competitiveness to that of commercial equivalents.In project 3R, as a joint effort of fuel cell industries, recycling companies and academics supported by Danish Energy Agency, the achievements document a valuable milestone towards truly renewable and green energy conversion technologies.AcknowledgmentFinancial support from Danish Energy Agency EUDP project 3R, Nr. 64019-0551; Innovation Fund Denmark, InnoExplorer program, Nr. 9122-00112; Danish ESS lighthouse on hard materials in 3D, SOLID, Grant number 8144-00002B; Energi Fyns Udviklingsfond.Corresponding authorShuang Ma Andersen: mashu@igt.sdu.dkReference Sharma, R.; Gyergyek, S.; Andersen, S. M. ChemSusChem 2018, 11, (21), 3742-3750.Sharma, R.; Rode Nielsen, K.; Brilner Lund, P.; Bredmose Simonsen, S.; Grahl-Madsen, L.; Ma Andersen, S. ChemElectroChem 2019, 6, (17), 4471-4482.

Surface potential modulation as a tool for mitigating challenges in SERS-based microneedle sensors

Raman spectroscopic-based biosensing strategies are often complicated by low signal and the presence of multiple chemical species. While surface-enhanced Raman spectroscopy (SERS) nanostructured platforms are able to deliver high quality signals by focusing the electromagnetic field into a tight plasmonic hot-spot, it is not a generally applicable strategy as it often depends on the specific adsorption of the analyte of interest onto the SERS platform. This paper describes a strategy to address this challenge by using surface potential as a physical binding agent in the context of microneedle sensors. We show that the potential-dependent adsorption of different chemical species allows scrutinization of the contributions of different chemical species to the final spectrum, and that the ability to cyclically adsorb and desorb molecules from the surface enables efficient application of multivariate analysis methods. We demonstrate how the strategy can be used to mitigate potentially confounding phenomena, such as surface reactions, competitive adsorption and the presence of molecules with similar structures. In addition, this decomposition helps evaluate criteria to maximize the signal of one molecule with respect to others, offering new opportunities to enhance the measurement of analytes in the presence of interferants.

Chemically Induced Surface Potential Modulation at Pd|Al2O3|Graphene Field Effect Transistors: Implications for Enhanced H2 Sensing

Chemically Induced Surface Potential Modulation at Pd|Al₂O₃|Graphene Field Effect Transistors: Implications for Enhanced H₂ Sensing

Defect Engineering of Out-of-Plane Charge Transport in van der Waals Heterostructures for Bi-Direction Photoresponse.

Defects are ubiquitous in two-dimensional (2D) transition-metal dichalcogenides (TMDs), generated by the initial growth- or the postprocessing. However, the defects may play negative roles in the photoelectronic properties of TMDs due to the reduction of in-plane transport of carriers. In this work, we demonstrate that the Se-vacancy defects in MoSe2 side of the van der Waal heterostructure is able to switch direction of out-of-plane charge transport. Photoresponse spectra showed defect density enable modified surface potential of MoSe2-x, leading to the barrier reverse between graphene and MoSe2-x and switches of the photoresponse from the negative to the positive. This unexpected property stemmed from appearance of midgap states by defects at heterostructure, as demonstrated by the density functional theory calculation and scanning tunneling microscope results. MoSe2-0.2/graphene heterostructure has a broadband response ranging from 450 to 1064 nm and exhibits comparable or higher positive responsivity (5.4 × 103 A/W to -15.3 × 103 A/W at 632.8 and 5.7 × 103 A/W to -1.2 × 103 A/W at 1064 nm) to the negative one of the pristine MoSe2/graphene. Based on defect-engineered heterostructures, we construct optoelectronic OR and AND logic devices with a broadband operation. Our work elucidates an alternative avenue to tailor the out-of-plane charge transport in TMD-based heterostructure through defects, and potentially invokes applicable utilization for 2D photodetectors and optoelectronic logic gates.

Capacitance–Voltage Technique Based on Time Varying Magnetic Field for VDMOSFET—Part I: Concept and Implementation

In this article, we report a new capacitance–voltage technique ${C}$ ( ${V}$ ) to investigate the interface proprieties of MOS devices. This technique is based on surface potential modulation using time varying (ac) magnetic field. It is experimentally validated on vertical double diffusion MOSFET (VDMOSFET). Data show that the surface potential is modulated using external ac magnetic field. In fact, applying an ac magnetic field with dc voltage sweeping of VDMOSFET gate ( ${V}~_{G}$ ), we have been able, for the first time, to modulate the surface potential and measure the gate–source ( ${C} _{\text {GS}}$ ) and gate–drain ( ${C} _{\text {GD}}$ ) capacitances as a function of gate voltage ( ${V}~_{G}$ ) for P- and N-type transistors. Therefore, it offers a powerful tool to characterize different regions of the transistor and extract their technological and electrical parameters.

Capacitance–Voltage Technique Based on Time Varying Magnetic Field for VDMOSFET—Part II: Measurements and Parameter Extractions

In part I, we have proposed a new capacitance–voltage technique ${C}$ ( ${V}$ ), which is simultaneously based on external ac magnetic field for surface potential modulation and on dc voltage to sweep the gate voltage. In part II, we describe the parameter extractions of vertical double diffusion metal–oxide–semiconductor field effect transistor (VDMOSFET) devices, combining the capacitance characteristics of gate–source, ${C}_{\text {GS}}$ ( ${V}_{G}$ ) and gate–drain ${C}_{\text {GD}}$ ( ${V}_{G}$ ), measured using the above cited technique, with those measured conventionally. In doing so, we have been able to extract semiconductor capacitance ( ${C}_{\text {SC}}$ ), interface trap capacitance ( ${C}_{\text {it}}$ ), doping profile concentration ${N}$ ( ${x}$ ), and interface trap density ${D}_{\text {it}}$ ( ${E}$ ) of different regions of VDMOSFET.

Threshold Voltage Modeling of Double Gate-All-Around Metal-Oxide-Semiconductor Field-Effect-Transistors (DGAA MOSFETs) Including the Fringing Field Effects

In this paper, an analytical threshold voltage model of double gate-all-around metal-oxide-semiconductorfield-effect-transistors (DGAA MOSFETs) including the fringing field effects is developed. The total fringing capacitance arising due to induced fringing fields in the device is divided into inner, outer and bottom fringing capacitance. A simple expression for each fringe capacitance is developed individually. The 3-D Poisson's equation has been solved in the channel region using the parabolic potential approximation method to develop the surface potential expressions. The effects of fringing capacitances of inner and outer gates which causes charge induction in the source/drain regions have been incorporated within the developed surface potential expressions. The change in potential due to these induced charges of source/drain region along the channel is formulated and added with the developed surface potential expression at both surfaces. The obtained modified surface potential equations have been utilized to derive the expression of the threshold voltage of the device. The performance of the proposed model has been compared with the previously developed model of DGAA MOSFET structure without High-k dielectrics. The effects of variation of device parameters on the threshold voltage have been also analyzed. The accuracy of the proposed model has been verified by numerical simulation results obtained by a device simulator VTCAD from Cogenda Int.

The effect of a modified surface anchoring potential on the cholesteric–nematic transition

As it is known, the Rapini-Papoular potential is suitable for description of anchoring energy between the liquid crystal and bounding surfaces in the case of small deviations of the director from the easy orientation axis. We use modified surface potential that can describe the anchoring energy at large deviations of the director more adequately. The interaction effect of a cholesteric liquid crystal with a bounding surfaces on the nature and features of cholesteric-nematic phase transitions induced by an external magnetic field is studied. We determine the conditions for existence of three different phases in a cholesteric liquid crystal layer with a finite homeotropic anchoring at the boundaries. The dependencies of the threshold fields of orientational transitions between the phases of a liquid crystal on the material parameters of the cell are calculated. Landau expansions of the total free energy of cholesteric near the transition points are constructed. We determine the tricritical values of the anisotropy parameter of the surface anchoring, which characterize the nature of transitions between the phases of a cholesteric liquid crystal.

Enhanced luminescence from InGaN/GaN nano-disk in a wire array caused by surface potential modulation during wet treatment

Here we have demonstrated the profound impact of surface potential on the luminescence of an array of InGaN/GaN nano-disk in a wire heterostructure. The change in surface potential is brought about by a combination of dry and successive wet-processing treatments. The photoluminescence (PL) properties are determined as a function of size and height of this array of nano-disks. The observed characteristics are coherently explained by considering a change in quantum confinement induced by the change in surface potential, quantum-confined Stark effect, exciton binding energy and strain relaxation for varying surface potential. The change in hole bound state energy due to parabolic potential well near the side-wall is found to be the dominating factor. The PL peak position, full width at half-maximum, strain relaxation and integrated PL intensity are studied as a function of incident power and temperature. The devices demonstrate higher integrated PL intensity and slope efficiency.

The effects of bacteria-nanoparticles interface on the antibacterial activity of green synthesized silver nanoparticles.

Neutralization of bacterial cell surface potential using nanoscale materials is an effective strategy to alter membrane permeability, cytoplasmic leakage, and ultimate cell death. In the present study, an attempt was made to prepare biogenic silver nanoparticles using biomolecules from the aqueous rhizome extract of Coptis Chinensis. The biosynthesized silver nanoparticles were surface modified with chitosan biopolymer. The prepared silver nanoparticles and chitosan modified silver nanoparticles were cubic crystalline structures (XRD) with an average particle size of 15 and 20nm respectively (TEM, DLS). The biosynthesized silver nanoparticles were surface stabilized by polyphenolic compounds (FTIR). Coptis Chinensis mediated silver nanoparticles displayed significant activity against E.coli and Bacillus subtilus with a zone of inhibition 12±1.2 (MIC=25μg/mL) and 18±1.6mm (MIC=12.50μg/mL) respectively. The bactericidal efficacy of these nanoparticles was considerably increased upon surface modification with chitosan biopolymer. The chitosan modified biogenic silver nanoparticles exhibited promising activity against E.coli (MIC=6.25μg/mL) and Bacillus subtilus (MIC=12.50μg/mL). Our results indicated that the chitosan modified silver nanoparticles were promising agents in damaging bacterial membrane potential and induction of high level of intracellular reactive oxygen species (ROS). In addition, these nanoparticles were observed to induce the release of the high level of cytoplasmic materials especially protein and nucleic acids into the media. All these findings suggest that the chitosan functionalized silver nanoparticles are efficient agents in disrupting bacterial membrane and induction of ROS leading to cytoplasmic leakage and cell death. These findings further conclude that the bacterial-nanoparticles surface potential modulation is an effective strategy in enhancing the antibacterial potency of silver nanoparticles.

Effect of annealing on charging properties of electret fibers

In the present work, effect of annealing on charging and surface morphology of isotactic polypropylene (PP) filaments was reported. To enhance the surface crystallinity of the filaments, samples were annealed at 70 and 110 °C for 12 h. As expected, fiber surface crystallinity increased upon annealing which led more stable electret fibers. However, the change in the initial surface potential was not so remarkable. The same trend was also observed in filaments containing a commercial nucleating agent, NA11 (sodium 2,2′-methylene-bis(4,6-di-tertbutylphenyl)-phosphate). Surface crystallinity was analyzed using the intensity of specific peaks obtained from attenuated total reflection infrared spectroscopy analysis. The surface charge characteristics of fibers were determined using a modified surface potential decay test.

Performance Improvement of Poly-Si Tunnel FETs by Trap Density Reduction

In this brief, the tunnel FETs (TFETs) with a polycrystalline-Si (poly-Si) channel have been demonstrated, and the performance of the poly-Si TFET shows a significant improvement by the reduction of interface trap states ( $N_{\mathrm {{it}}}$ ) near the conduction band edge. The ON-state current ( $I_{\mathrm{\scriptscriptstyle ON}}$ ) conduction mechanism, band-to-band tunneling, of poly-Si TFETs is strongly affected by the band bending of poly-Si channel. The N2 plasma surface treatment before the gate dielectric deposition can produce a plasma-induced interfacial layer to reduce $N_{\mathrm {it}}$ obviously, which greatly enhances the surface potential modulation by the gate and improves the $I_{\mathrm{\scriptscriptstyle ON}}$ value of poly-Si TFETs $\sim 3.7\times $ . The OFF-state current ( $I_{\mathrm { {min}}}$ ) attributed to the trap-assisted tunneling (TAT) can also be reduced by a factor of $\sim 40$ %, because of the passivation of grain boundary trap ( $N_{\mathrm {trap}}$ ) of the poly-Si channel film. Consequently, the ON/OFF current ratio is enhanced from $9.42\times 10^{5}$ to $6.13\times 10^{6}$ . In addition, the subthreshold swing, $I_{{\mathrm{\scriptscriptstyle ON}}}$ , and $I_{\mathrm { {min}}}$ of poly-Si TFET exhibit superior short-channel effect immunity, which shows good feasibility for implementing high packing density of poly-Si thin-film transistors, such as 3-D nonvolatile memory and pixel driving device.

Dopamine Detection Based on Low-Voltage Oxide Homojunction Electric-Double-Layer Thin-Film Transistors

Quick and accurate detection of dopamine (DA) is of great significance for routine analysis and biomedical diagnosis. Here, low-voltage indium-zinc-oxide (IZO) homojunction electric-double-layer (EDL) transistors using nanogranular SiO2 electrolytes as the gate dielectrics are proposed for DA detection. A DA detection limit of 0.1 nM with good selectivity is obtained at a low voltage of 0.8 V. The DA sensing mechanism can be attributed to the modulation of surface potential of the indium-tin-oxide (ITO) gate electrode by preferential binding of DA molecule to phenylboronic acid. Such low-voltage oxide-based EDL transistors have potential application in biochemical sensors.

Instability and Surface Potential Modulation of Self-Patterned (001)SrTiO3 Surfaces

The (001)SrTiO3 crystal surface can be engineered to display a self-organized pattern of well-separated and nearly pure single-terminated SrO and TiO2 regions by high temperature annealing in oxidizing atmosphere. By using surface sensitive techniques we have obtained evidence of such surface chemical self-structuration in as-prepared crystals and unambiguously identified the local composition. The contact surface potential at regions initially consisting of majority single terminations (SrO and TiO2) is determined to be smaller for SrO than for TiO2, in agreement with theoretical predictions, although the measured difference below 100 meV is definitely smaller than theoretical predictions for ideally pure single-terminated SrO and TiO2 surfaces. These relative values are maintained if samples are annealed in UHV up to 200 degrees Celsius. Annealing in UHV at higher temperature (400 degrees Celsius) preserves the surface morphology of self-assembled TiO2 and SrO rich regions, although a non-negligible chemical intermixing is observed. The most dramatic consequence is that the surface potential is reversed. It thus follows that electronic and chemical properties of (001)SrTiO3, widely used in oxide thin films growth, can largely vary before growth starts in a manner strongly dependent on temperature and pressure conditions.