Variation Of Surface Potential Research Articles

Surface studies and photoluminescence of GaN epitaxial layers grown by MOCVD technique for applications of solar cells and light emitting devices

This article presents theoretical and experimental results of n-type GaN semiconductor. In the first part, the variation of surface potential of GaN with varying carrier concentration in the range from 1015 to 1019 cm−3 is determined as well as the change in the electric field with variation of surface potential is also estimated. In addition, the change in the electric field (F) with effect of irradiation of laser beam on the GaN sample, while measuring photoluminescence, is found by developing appropriate theoretical models using known activation energies of traps, which are obtained by the experimental photoluminescence (PL) measurements. In the second part, the experimental results of n-type GaN such as growth by MOCVD, electrical, structural and optical properties are presented. The un-doped GaN thin films have been successfully deposited onto c-sapphire substrates by MOCVD technique with different recipes. It is observed that the carrier concentration of the films varies with varying deposition parameters. The PL studies are systematically carried out on these samples at low temperature. The correlation between PL results and the electrical data such as carrier concentration (n) or mobility (μ) is analyzed for GaN thin films. Based on the activation energies of donor–acceptor pair (DAP) and the yellow band emissions obtained from the PL measurements, the possible transition energy levels in the energy band are hypothetically proposed. The highly resolved phonon replicas in PL spectra are observed for high carrier density samples. The n and μ are found to be in the range of 1017–1018 cm−3 and 65–120 cm2/V−s, respectively. The XRD analysis reveals that the samples show preferred orientation and hexagonal structure. On the other hand, the nature of the samples changes from epitaxy to polycrystalline with increasing carrier concentration in the samples.

Improving lateral resolution of ambient electrostatic force microscopy by intermittent contact method

ABSTRACTElectrostatic force microscopy (EFM) including its derivative, Kelvin probe force microscopy, is widely used to investigate mesoscopic/nanoscopic conducting/semiconducting structures and electrical interactions. However, the resolution of EFM in ambient is largely limited by the applicable force-detection techniques. This work demonstrates that the lateral resolution of ambient EFM can be significantly improved using an intermittent contact method in conjunction with a resonant multi-frequency method for electrostatic force detection. A lateral resolution of sub-10 nm is obtained. The high-resolution contrast reflects the variation of surface potential in sample on a nanoscale. The mechanism, which is attributed to the decreased effective tip-sample distance, is elucidated by theoretical analysis. The newly developed technique can be easily integrated with existing techniques to herald the next generation of high-resolution probe techniques for various nanoscopic applications.

An incremental double-layer capacitance of a planar nano gap and its application in cardiac-troponin T detection

Surface potential is one of the most important properties at solid–liquid interfaces. It can be modulated by the voltage applied on the electrode or by the surface properties. Hence, surface potential is a good indicator for surface modifications, such as biomolecular bindings. In this work, we proposed a planar nano-gap structure for surface-potential difference monitoring. Based on the proposed architecture, the variance of surface-potential difference can be determined by electrical double layer capacitance (EDLC) between the nano-gap electrodes. Using cyclic voltammetry method, in this work, we demonstrated a relationship between surface potential and EDLC by chemically modifying surface properties. Finally, we also showed the proposed planar nano-gap device provides the capability for cardiac-troponin T (cTnT) measurements with co-existed 10µg/ml BSA interference. The detection dynamic range is from 100pg/ml to 1µg/ml. Based on experimental results and extrapolation, the detection limit is less than 100pg/ml in diluted PBS buffer (0.01X PBS). These results demonstrated the planar nano-gap architecture having potentials on biomolecular detection through monitoring of surface-potential variation.

Charging effects and surface potential variations of Cu-based nanowires

The present work reports charging effects and surface potential variations in pure copper, cuprous oxide and cupric oxide nanowires observed by electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). The copper nanowires were produced by wet synthesis, oxidation into cuprous oxide nanowires was achieved through microwave irradiation and cupric oxide nanowires were obtained via furnace annealing in atmospheric conditions. Structural characterization of the nanowires was carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. During the EFM experiments the electrostatic field of the positive probe charged negatively the Cu-based nanowires, which in turn polarized the SiO2 dielectric substrate. Both the probe/nanowire capacitance as well as the substrate polarization increased with the applied bias. Cu2O and CuO nanowires behaved distinctively during the EFM measurements in accordance with their band gap energies. The work functions (WF) of the Cu-based nanowires, obtained by KPFM measurements, yielded WFCuO>WFCu>WFCu2O.

B13-O-10Low Energy Secondary Electron Imaging for Various Semiconductors using Fountain Detector

Recently, the energy filtered secondary electron (SE) images give fruitful information of the specimen, not only morphology and composition but also surface potential, electrical field etc. Such modern detector has been developed for inlens SEM and outlens detectors were abandoned. We have developed low-pass secondary electron detector (LPSED) for outlens SEM and named “Fountain Detector (FD)”. This detector is composed of bias grid above and electron detector below the specimen. The bias voltage and the geometry of the detector limit the energy and acceptance angle of SE, respectively. The biggest advantage of FD detector is that we can easily trace the trajectories of the secondary electros. Fig. 1 shows the photograph of prototype FD. The subtraction of the images taken different bias voltages gives the energy filtered images. Using this detector, we have characterized various semiconducting materials. In this presentation we demonstrate the observations of surface potential variation of GaN islands on Si substrate and p-n junction of SiC wafer.

Influence of Surface Morphology on the Antimicrobial Effect of Transition Metal Oxides in Polymer Surface.

In this study, the physical properties of transition metal oxide surfaces were examined using scanning probe microscopic (SPM) techniques for elucidating the antimicrobial activity of molybdenum trioxide (MoO3), tungsten trioxide (WO3), and zinc oxide (ZnO) embedded into the polymers thermoplastic polyurethane (TPU) and polypropylene (PP). We utilized atomic force microscopy (AFM) in the contact imaging mode and its derivative single-pass Kelvin probe force microscopy for investigating samples that were presumably identical in their compositions, but showed different antimicrobial activity in bacterial adhesion tests. Our results revealed that surfaces with larger roughness and higher surface potential variation showed stronger antimicrobial activities compared to smoother and homogeneously charge-distributed surfaces. In addition, capacitance gradient (dC/dZ) measurements were performed to elucidate the antimicrobial activity arising from the different dielectric behavior of the transition metal oxides in this heterogeneous polymer surface. We found that the nano-scale exposure of transition metal oxides on polymer surfaces provided strong antimicrobial effects. Applications arising from our studies will be useful for public and healthcare environments.

Gate Tunnelling Current Model for Nanoscale MOSFETs with Varying Surface Potential

ABSTRACTAt nanoscale, the channel of metal oxide semiconductor field effect transistors (MOSFETs) acts like a potential well within which electron energy levels are quantized and a nonzero wavefunction is obtained at the oxide–semiconductor interface. As a result the gate tunnelling current has emerged a significant constraint in respect of scaling of ultrathin gate oxides. We have developed an analytical model considering varying surface potential with applied voltage for evaluating gate tunnelling current through thin dielectrics in nanoscale MOSFETs. The electron wavefunction has been calculated by treating the band profile in the channel as a triangular potential well. The tunnelling probability through the gate oxide has been evaluated using Jeffreys–Wentzel–Kramers–Brillouin approximation method. The tunnelling current density is estimated from the evaluated interface wavefunction along with the tunnelling probability. The results from the present model compare well with the published Mondal–Dutta model and the experimental data. The novelty of the present model lies in its simplicity and its analyticity requiring much less computational efforts for its implementation.

Use of Ultrananocrystalline Diamond as a Phase-contrast Aperture Material

Boron-doped “ultrananocrystalline diamond” (UNCD), available as a 2 μm-thick layer on a “diamond on insulator” wafer (Advanced Diamond Technologies, Romeoville, IL), is investigated as a potential material for making devices that can be placed in the back focal plane of the objective lens of a TEM, the goal being to produce high contrast for in-focus images of weak phase objects. The resistance of diamond to knock-on damage when irradiated, even at 300 keV, is an attractive feature. In addition, diamond (indeed, carbon in general) is not expected to form a native oxide that is thicker than a monolayer. The ultrananocrystalline form of diamond is favored over a microcrystalline form because unwanted, “patchy” variations in surface potential (due to differences in the work-function for different crystal facets) are expected to decay more rapidly, i.e. on a length scale comparable to the domain size. We believe, however, that care must be taken to compensate for the contact potential (Galvani potential) between the UNCD device layer and the supporting (boron-doped) silicon wafer (handle).

Growth from behind: Intercalation-growth of two-dimensional FeO moiré structure underneath of metal-supported graphene.

Growth of graphene by chemical vapor deposition on metal supports has become a promising approach for the large-scale synthesis of high quality graphene. Decoupling of the graphene from the metal has been achieved by either mechanical transfer or intercalation of elements/molecules in between the metal and graphene. Here we show that metal stabilized two-dimensional (2D)-oxide monolayers can be grown in between graphene and the metal substrate thus forming 2D-heterostructures that enable tuning of the materials properties of graphene. Specifically, we demonstrate the intercalation-growth of a 2D-FeO layer in between graphene and Pt(111), which can decouple the graphene from the metal substrate. It is known that the 2D-FeO/Pt(111) system exhibits a moiré-structure with locally strongly varying surface potential. This variation in the substrate surface potential modifies the interface charge doping to graphene locally, causing nanometer-scale variation in its work function and Fermi-level shifts relative to its Dirac point.

The effect of patch potentials in Casimir force measurements determined by heterodyne Kelvin probe force microscopy

Measurements of the Casimir force require the elimination of the electrostatic force between the surfaces. However, due to electrostatic patch potentials, the voltage required to minimize the total force may not be sufficient to completely nullify the electrostatic interaction. Thus, these surface potential variations cause an additional force, which can obscure the Casimir force signal. In this paper, we inspect the spatially varying surface potential of e-beamed, sputtered, sputtered and annealed, and template stripped gold surfaces with Heterodyne amplitude modulated Kelvin probe force microscopy (HAM-KPFM). It is demonstrated that HAM-KPFM improves the spatial resolution of surface potential measurements compared to amplitude modulated Kelvin probe force microscopy. We find that patch potentials vary depending on sample preparation, and that the calculated pressure can be similar to the pressure difference between Casimir force calculations employing the plasma and Drude models.

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.

Simplification of pseudo two dimensional battery model using dynamic profile of lithium concentration

A sophisticated BMS often requires an accurate yet simple battery model. Simplified models such as the single particle model (SPM), however, provide limited insight because lithium concentration variations over the cell thickness dimension are often neglected. In this study, we propose a simplified electrochemical model that, by reviving the lithium concentration variation across the cell thickness, provides significantly improved prediction power over the SPM. The lithium concentration profiles are first derived according to the electrochemistry-based pseudo-two dimensions (P2D) model under the steady-state assumption, they are then relaxed into a dynamic model. By employing steady-state concentration profiles coupled with averaged dynamics, the proposed model predicts electrolyte and solid surface concentrations (ce and css, respectively) and potential variations across the cell during both transients and stead-state conditions. The proposed model is validated by comparing the cell voltage, ce, and css concentration RMS errors with respect to the P2D model under pulse and constant current inputs. The simulation shows that the proposed model achieves at least 74%, 77%, and 65% RMS error reductions for cell voltage, css and ce, respectively, when compared to SPM in constant current simulation. Finally, the presented model is used to predict and detect local lithium plating, which the SPM is not capable of doing; our method yields predictions similar to those of the original P2D model at computational loads comparable to SPM.

Aluminium–copper–nickel thin film compositional spread: Nickel influence on fundamental alloy properties and chemical stability of copper

An Al–Cu–Ni thin film compositional spread was deposited by thermal evaporation and investigated in order to study the Ni influence on the overall properties. The chemical composition was detected by energy dispersive X-ray spectroscopy and showed a compositional spread of approximately 20at.% Ni. Decreasing the Ni content in the Al–Cu–Ni thin films resulted in an increased grain size and characteristic surface microstructure evolution. Scanning Kelvin probe measurements were performed to investigate the surface potential variation along the compositional gradient, and a distinct surface potential drop was observed between Al–Cu–7at.% Ni and Al–Cu–13at.% Ni. The results of the X-ray photoelectron spectroscopy surface analysis and Auger electron spectroscopy as well as the electrochemical investigations by cyclic voltammetry evidenced mainly the presence of Al2O3 but also CuO and Cu2O together with metallic Cu were clearly identified along the compositional gradient. Chemical dissolution experiments have shown that Ni is enhancing the chemical stability of Cu, excepting inside the compositional region between 7 and 13at.% Ni.

The Surface Potential Variation of Neural Stem/Progenitor Cells during Differentiation Process

Background: Stem cells have been extensively found in different tissues and organs. Understanding the characteristics of stem cells to maintain their stability may lead to find new ways to control stem cell differentiation. Methods: In this study, the surface potential variation of neural stem/ progenitor cells (NSPCs) during differentiation process was examined. Using cell electrophoresis for measuring the electrophoretic mobility of NSPCs, we showed the electrophoretic mobility of NSPCs could rapidly increase during the first hour of differentiation process. Moreover, an activation energy model describing the variation of the surface potential of NSPCs was proposed. Findings: When NSPCs need to change their phenotypes to differentiate, they require enough energy to process this phenotype alteration. Thus, NSPC differentiation could be enhanced or delayed by controlling electrophoretic mobility of cells, which provided a new vision for stem cell differentiation. Conclusions: This study provided an alternative to regulate the NSPC differentiation by controlling the cellular electrophoretic mobility.

Measurements of temporal and spatial variation of surface potential using a torsion pendulum and a scanning conducting probe

An electrostatic-controlled torsion pendulum combined with a scanning conducting probe is used to measure the temporal and spatial variations of the surface potential on test mass. The apparatus can work in static and scanning modes. Temporal variation of the surface potential can be measured with the static mode, and its voltage variation with a level of $15\text{ }\text{ }\ensuremath{\mu}\mathrm{V}/{\mathrm{Hz}}^{1/2}$ at 0.03 Hz has been achieved for a $5\ifmmode\times\else\texttimes\fi{}5\text{ }\text{ }{\mathrm{mm}}^{2}$ area. The spatial distribution of the surface potential can be measured with the scanning mode, and the surface potential distribution can be obtained at a level of $330\text{ }\text{ }\ensuremath{\mu}\mathrm{V}$ at 0.125 mm spatial resolution. The experimental results tell us that the apparatus provide a new way to investigate the charge distribution and its variation, which is very useful in helping to understand the physical mechanism of the surface charge distribution of an actual object.

Nanoscale surface potential variation correlates with local S/Se ratio in solution-processed CZTSSe solar cells.

Thin film solar cells made from Cu, Zn, Sn, and S/Se can be processed from solution to yield high-performing kesterite (CZTS or CZTSSe) photovoltaics. We present a microstructural study of solution-deposited CZTSSe films prepared by nanocrystal-based ink approaches using scanning probe microscopy (SPM) and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). We correlate scanning Kelvin probe microscopy (SKPM) maps of local surface potential with SEM/EDS images of the exact same regions of the film, allowing us to relate observed variations in surface potential to local variations in stoichiometry. Specifically, we find a correlation between surface potential and the S/(S + Se) composition ratio. In particular, we find that regions with high S/(S + Se) ratios are often associated with regions of more negative surface potential and thus higher work function. The change in work function is larger than the expected change in the valence band position with these small changes in sulfur, and thus the data suggest an increase in acceptor-like defects with increasing sulfur. These findings provide new experimental insight into the microscopic relationships between composition, structure, and electronic properties in these promising photovoltaic materials.

Quantitative non-contact voltage profiling of quasi one-dimensional nanoelectronic devices

Local electrical characterization tools, such as Electrostatic force microscopy (EFM), can provide local electrical information of nanoelectronic devices, albeit mostly qualitative. For example, EFM images are convolution of local surface potential, capacitance, and contact potential variations in the device. In this study, we demonstrate a calibration procedure to obtain quantitative local voltage distributions of quasi one-dimensional nanoelectronic devices based on carbon nanotubes and ZnO nanowires. By comparing the results with IV measurements of the same devices, we can obtain local electrical properties of devices such as contact resistance, intrinsic resistivity of the nanomaterial, and resistance of a defect.

Photogenerated charges and surface potential variations investigated on single Si nanorods by electrostatic force microscopy combined with laser irradiation

Photogenerated charging properties of single Si nanorods (Si NRs) are investigated by electrostatic force microscopy (EFM) combined with laser irradiation. Under laser irradiation, Si NRs are positively charged. The amount of the charges trapped in single NRs as well as the contact potential difference between the tip and NRs' surface is achieved from an analytical fitting of the phase shift - voltage curve. Both of them significantly vary with the laser intensity and the NR's size and construction. The photogenerated charging and decharging rates are obtained at a timescale of seconds or slower, indicating that the Si NRs are promising candidates in photovoltaic applications.

Theoretical determination of the impact of channel morphology and quantum size effects on the surface potential of nanocrystalline silicon thin film transistors

In this work, we propose an analytical model allowing the calculation of nanocrystalline silicon thin film transistors surface potential by considering a granular morphology of silicon nanocrystallites forming the channel. Results show that, according to the quantum effects on dielectric constant and band gap, the surface potential values are strongly related to the silicon crystallites structure in terms of size and geometry. The comparison of our results with existing research data shows a good agreement between the surface potential shapes and an interesting difference in the surface potential variations related to the morphology specificity considered in our theoretical approach.

Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure

Interfaces between organic semiconductors and dielectrics may exhibit interfacial electronic polarization, which is equivalently quantified as a contact potential difference (CPD), an interface dipole, or a vacuum level shift. Here we report quantitative measurements by scanning Kelvin probe microscopy (SKPM) of surface potentials and CPDs across ultrathin (1–2 monolayer) crystalline islands of the benchmark semiconductor pentacene thermally deposited on a variety of polymer dielectrics (e.g., poly(methyl methacrylate), polystyrene). The CPDs between the pentacene islands and the polymer substrates are in the range of −10 to +50 mV, they depend strongly on the polymer type and deposition temperature, and the CPD magnitude is correlated with the dipole moment of the characteristic monomers. Surface potential variations within 2 monolayer (3 nm) thick pentacene islands are ∼15 mV and may be ascribed to microstructure (epitaxial) differences. Overall, the microscopy results reveal both strong variations in i...