Frequency Independent Behavior Research Articles

Waste Coffee Ground-Derived Porous Carbon for Neurochemical Detection.

We present an optimized synthetic method for repurposing coffee waste to create controllable, uniform porous carbon frameworks for biosensor applications to enhance neurotransmitter detection with fast-scan cyclic voltammetry. Harnessing porous carbon structures from biowastes is a common practice for low-cost energy storage applications; however, repurposing biowastes for biosensing applications has not been explored. Waste coffee ground-derived porous carbon was synthesized by chemical activation to form multivoid, hierarchical porous carbon, and this synthesis was specifically optimized for porous uniformity and electrochemical detection. These materials, when modified on carbon-fiber microelectrodes, exhibited high surface roughness and pore distribution, which contributed to significant improvements in electrochemical reversibility and oxidative current for dopamine (3.5 ± 0.4-fold) and other neurochemicals. Capacitive current increases were small, showing evidence of small increases in electroactive surface area. Local trapping of dopamine within the pores led to improved electrochemical reversibility and frequency-independent behavior. Overall, we demonstrate an optimized biowaste-derived porous carbon synthesis for neurotransmitter detection for the first time and show material utility for viable neurotransmitter detection within a tissue matrix. This work supports the notion that controlled surface nanogeometries play a key role in electrochemical detection.

Development of structured W/O emulsions with the use of only candelilla wax

AbstractWe investigated the development of water‐in‐oil (W/O) emulsions using only candelilla wax (CW), evaluating the effects of different water to CW oleogel ratios (40:60, 50:50, 60:40) and the CW concentration (0.75% to 3%). The emulsions were developed by shearing the systems with an ultra‐turrax type homogenizer (60 s at 25°C) at the different water to CW oleogel ratios. After 0 and 20 days of storage (25°C) the emulsions were evaluated through microscopy, rheology, water droplet diameter, emulsion stability, and x‐ray diffraction measurements. The results showed that at all water to CW oleogel ratios the surface‐active components of the CW (i.e., triterpenic alcohols, aliphatic alcohols, and fatty acids) stabilized the oil–water interface, while the n‐alkanes and long chain esters formed an oleogel in the oil phase. Independent of the storage time, all the CW emulsions showed frequency independent rheological behavior. However, after applying a strain within the plastic region the 40:60 and 50:50 emulsions formulated with 1.5% to 3% CW provided the higher elasticity and emulsion stability, even after two freeze‐thaw cycles. In particular, the 40:60 and 50:50 emulsions with 1.5% CW had a recovery profile similar to commercial mayonnaise. In contrast, independent of the CW concentration, the 60:40 emulsions showed the lowest recovery profile and highest emulsion instability. These results showed the CW as a multifunctional material capable to develop structured W/O emulsions at room temperature without adding surfactants. The structured W/O emulsions developed by CW could be useful in the formulation of trans‐free, stable low‐fat edible spreads.

Impedance Spectroscopy Analysis of (1-<i>x</i>)Ba<sub>0.06</sub>(Na<sub>1/2</sub>Bi<sub>1/2</sub>)<sub>0.94</sub>TiO<sub>3</sub>-<i>x</i>NaTaO<sub>3</sub> Ceramics

The conventional solid-state reaction method was used to prepare the lead-free solid-solution (1-x)Ba0.06(Na0.5Bi0.5)0.94TiO3-xNaTaO3; 0 ≤ x ≤ 1. X-ray diffraction analysis of all the compositions of the series confirmed the solid solution’s successful formation. The electrical impedance and conductivity of the material were studied using the impedance spectroscopy method at temperatures ranging from room temperature to 500○C. For the Ba0.06(Na0.5Bi0.5)0.94TiO3 rich compositions ac conductivity increased with frequency at all temperatures, whereas for the NaTaO3 rich compositions ac conductivity exhibited a frequency-independent behaviour. As the amount of NaTaO3 incorporated into the compound Ba0.06(Na0.5Bi0.5)0.94TiO3 was increased, a transition occurred in the electrical impedance, specifically, the impedance values shifted from low to high. A theoretical model based on correlated barrier hopping (CBH) has been used to explain the conduction mechanism in these solid solutions.

Microfluidic-Assisted Alignment of Graphene Nanosheets for Microfiber Formation

Recently, our lab developed graphene oxide microfibers for ultrasensitive neurochemical detection with fast-scan cyclic voltammetry (FSCV). Graphene-fiber microelectrodes provide a significant advance over traditional carbon-fiber microelectrodes due to their resistance to detrimental fouling, fast electron transfer kinetics, and frequency independent behavior. Despite our recent advancement, there still exists a gap in our understanding of how specific changes in the carbon chemical structure influences adsorption and interaction of various neurochemicals at the surface. Understanding the extent to which the carbon chemical structure influences electrochemical detection is critical for designing tailor-made electrodes for ultrasensitive analyte detection. From a molecular level point-of-view, the graphene sheet consists of two regions with different structure: i) the basal plane, consisting of conjugated sp2 hybridized carbon atoms, and characterized by an atomic flatness and low defect density; and ii) the edge plane, that forms sp3 sites with a high level of defects and a variety of functional groups and dangling bonds. It is generally believed that the electrochemical properties of the basal and edge plane are significantly different, however, there has been debate concerning the electrochemical activity at the basal and edge plane. The conventional consensus is that electron transfer kinetics is faster on the edge plane in comparison with the basal plane sites, while the opposite barrier of this debate provides evidence for fast electron transfer at pristine basal plane. Moreover, how these factors influence neurochemical detection, particularly at rapid scan rates, is still lacking due to the necessity of using microelectrodes with FSCV. To date, most studies analyzing the influence of basal vs. edge plane surfaces on electrochemical detection have used macroelectrodes like highly ordered pyrolytic graphite (HOPG) which are not amenable to FSCV detection due to the large capacitive currents obtained at macroelectrodes at scan rates consisting of hundreds of volts per second. Therefore, to address this existing knowledge gap in understanding how edge vs. basal planes influence subsecond neurochemical detection, the ability to develop microelectrodes with controlled surfaces is crucial. Here, I will discuss our labs latest effort in developing graphene oxide microfibers with controllable nanosheet alignment to study the influence of basal vs. edge plane availability on neurochemical detection at rapid scan rates. To assist in nanosheet alignment, we use microfluidic channels to assist in fluid-driven control of nanosheet orientation. Using microfluidics to help develop highly controllable electrode materials provides a critical advance in our ability to develop novel carbon surfaces for detection.

Graphene oxide fiber microelectrodes with controlled sheet alignment for sensitive neurotransmitter detection.

Here, we synthesized and characterized graphene oxide (GO) fiber microelectrodes with controllable nanosheet orientation to study the extent to which sheet alignment and orientation impacts electrochemical detection of neurochemicals. The alignment of the GO nanosheets was characterized by scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry. The electrochemical performance of GO microelectrodes and its suitability for subsecond detection of neurotransmitters was further evaluated by fast-scan cyclic voltammetry (FSCV). We have shown that the GO sheet alignment has a considerable effect on the electron transfer kinetics, frequency independent behavior, and detection suitability for specific neurotransmitters. Therefore, this fine-tuning aspect of the electrode surface for specific electrochemical detection should be taken into consideration for any future utilization of GO-based biological sensors.

Acoustofluidic lysis of cancer cells and Raman spectrum profiling.

The lysis of cancer cells inside a sessile droplet was performed using traveling surface acoustic waves (SAWs) without any chemical reagents. Raman spectrum profiling was then carried out to explore detailed cell-derived data. The Rayleigh waves formed by an interdigital transducer were made to propagate along the surface of an LiNbO3 substrate. Polystyrene microparticles (PSMPs) were used to establish mechanical cell lysis effectively, and gold nanoparticles (AuNPs) were added to enhance the Raman signals from the lysed cells by SAWs. The lysis efficiency was evaluated according to the size and concentration of the PSMPs in experiments where the frequency was varied. Lysis occurred mainly by mechanical collision using PSMPs in a high-frequency domain, and the lysis efficiency was improved by increasing the application time and the energy density of the SAWs. Raman signals from the lysed cells were greatly enhanced by nanogaps formed by the AuNPs, which were evenly distributed irrespective of the SAWs through the frequency-independent behavior of the AuNPs. Finally, detailed Raman spectra of MDA-MB-231, malignant breast cancer cells, were acquired, and various organic matter-derived peaks were observed. The 95% confidence region for cells subjected to lysis was more widely distributed than that of cells not subjected to lysis. The proposed SAW platform is expected to facilitate the detection of small quantities and to be applied in biomedical applications.

Topographic controls on active microwave behavior of mountain snowpacks

An uncalibrated distributed multiphysics snow model driven by downscaled weather forecasts (30-m, 15-min) was implemented as a Radar Observing System Simulator (ROSS) in Senator Beck Basin (SBB), Colorado to elucidate topographic controls on C-, X- and Ku-bands active microwave sensing of mountain snowpacks. Phase-space maps of time-evolving grid-scale ROSS volume backscatter show the accumulation branch of the backscatter-snow water equivalent (σ-SWE) hysteresis seasonal loop that is the physical basis for radar retrieval (direct inference) of SWE and snowpack physical properties. There is good agreement in the accumulation season (R2 ∼ 0.7) between Sentinel-1 and ROSS predictions corrected using average Sentinel-1 measurements under snow free conditions to estimate snow-ground backscatter, capturing well spatial patterns tied to elevation, slope, and aspect. Root Mean Square Deviations (RMSDs) do not exceed ±3.2 dB for ripening snowpacks in early spring and ±2.4 dB for dry snowpacks in the accumulation season when the mean absolute bias is <1 dB for all land-cover types with topographic slopes ≤ 30°. Grid-point RMSDs are attributed to the underestimation of snowfall on upwind slopes compounded with forecast errors for the weather near the ground. Like Sentinel-1, ROSS backscatter fields exhibit frequency-independent single-scaling behavior in the 60–150 m scale range for dry snowpacks in the accumulation season, while frequency-dependent scaling behavior emerges in the ablation season. This study demonstrates skillful physical modeling capabilities to emulate Sentinel-1 observations in complex terrain. Conversely, it suggests high readiness to retrieve snow mass and snowpack properties in mountainous regions from radar measurements at high-spatial resolutions enabled by SAR technology.

Dielectric Properties of Polyaryletherketone/CaCu3Ti4O12 Nanocomposite Films Fabricated via Cast Film Extrusion Process

Polymer composites prove to be the best choice for improving the dielectric properties of polymers. This work mainly focuses on studying the dielectric behavior of polyaryletherketone (PAEK), a high-temperature polymer, and its nanocomposite films with CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (CCTO) ceramics. The dielectric constant of PAEK improves significantly after the addition of CCTO nanoceramics and shows nearly frequency-independent behavior. The dielectric loss of these nanocomposites is low and is in the range of 0.004–0.006 at room temperature and 1 kHz. Dielectric properties exhibit very good temperature stability up to 150 °C. Activation energies calculated from both electric modulus and conductivity analyses show that the movement of charge carriers is responsible for both relaxation and conduction processes. The conductivity mechanism follows the correlated barrier hopping (CBH) model at low temperatures and the nonoverlapping small polaron tunneling (NSPT) model at high temperatures. Breakdown characteristics are analyzed using Weibull probability distribution. These PAEK/CCTO nanocomposites are proposed as potential materials for high-temperature dielectric applications.

Graphene-Fiber Microelectrodes for Ultrasensitive Neurochemical Detection

Here, we have synthesized and characterized graphene-fiber microelectrodes (GFME's) for subsecond detection of neurochemicals with fast-scan cyclic voltammetry (FSCV) for the first time. GFME's exhibited extraordinary properties including faster electron transfer kinetics, significantly improved sensitivity, and ease of tunability that we anticipate will have major impacts on neurochemical detection for years to come. GF's have been used in the literature for various applications; however, scaling their size down to microelectrodes and implementing them as neurochemical microsensors is significantly less developed. The GF's developed in this paper were on average 20-30 μm in diameter and both graphene oxide (GO) and reduced graphene oxide (rGO) fibers were characterized with FSCV. Neat GF's were synthesized using a one-step dimension-confined hydrothermal strategy. FSCV detection has traditionally used carbon-fiber microelectrodes (CFME's) and more recently carbon nanotube fiber electrodes; however, uniform functionalization and direct control of the 3D surface structure of these materials remain limited. The expansion to GFME's will certainly open new avenues for fine-tuning the electrode surface for specific electrochemical detection. When comparing to traditional CFME's, our GFME's exhibited significant increases in electron transfer, redox cycling, fouling resistance, higher sensitivity, and frequency independent behavior which demonstrates their incredible utility as biological sensors.

Omni-directional and broadband acoustic anti-reflection and universal acoustic impedance matching

Abstract The mechanism of the perfect anti-reflection of acoustic waves, regardless of frequency and incident angle, is presented. We show that reflections at a planar interface between two different acoustic media can be removed by adding a nonlocal metamaterial that compensates for the impedance mismatch. The properties required of a nonlocal metamaterial are explicitly specified through spatio-temporally dispersive mass density and bulk modulus. We analyze the characteristics of spatio-temporal dispersion according to the thickness of the matching layer. We discuss the issue of the total internal reflection caused by conventional matching layers and explain how our nonlocal matching layer avoids this. The practical design of our nonlocal layer using metamaterials is explained. The omni-directional frequency-independent behavior of the proposed anti-reflection matching layer is confirmed through explicit numerical calculation using the finite element method, and comparisons made to the conventional quarter-wave matching layer approach.

Dielectric Spectroscopy of PP/MWCNT Nanocomposites: Relationship with Crystalline Structure and Injection Molding Condition.

In this paper, we study the correlation between the dielectric behavior of polypropylene/multi-walled carbon nanotube (PP/MWCNT) nanocomposites and the morphology with regard to the crystalline structure, nanofiller dispersion and injection molding conditions. As a result, in the range of the percolation threshold the dielectric behavior shifts to a more frequency-independent behavior, as the mold temperature increases. Moreover, the position further from the gate appears as the most conductive. This effect has been associated to a modification of the morphology of the MWCNT clusters induced by both the flow of the molten polymer during the processing phase and the variation of the crystalline structure, which is increasingly constituted by γ-phase as the mold temperature increases. The obtained results allow one to understand the effect of tuning the processing condition in the frequency-dependent electrical behavior of PP/MWCNT injection-molded nanocomposites, which can be successfully exploited for an advanced process/product design.

Toward 40 GHz excitation of diamond-based HBAR

We have obtained an excitation of longitudinal bulk acoustic waves in a diamond-based High overtone Bulk Acoustic Resonator (HBAR) at microwave and enhanced frequency bands as EHF up to 40 GHz. As an effective piezoelectric transducer, an aluminum-scandium nitride film is employed. The frequency response of acoustic overtones excited in the HBARs with a different aperture in the 1.0 up to 40 GHz range is measured. Some peculiarities of HBAR's frequency response concerned with the thin film piezoelectric transducer are studied. It is established that the maximal magnitude of the quality factor varies near the value of about 10 000 within all the frequency bands investigated. The approximately frequency-independent Q-factor behavior can explain in terms the Landau–Rumer approximation describing the acoustic attenuation in a diamond. An estimation of the HBAR's quality parameter as Q × f ≈ 4 × 1014 Hz at 40 GHz is obtained. In our opinion, that value exceeds all known data among the given devices. The results obtained will be helpful in the development of microwave and EHF acoustic resonators as well as highly sensitive acoustic sensors. The development of the EHF band opens up the possibility of obtaining acoustic waves of submicrometer and nanometer scales (in various media) and implementing acoustic studies of micro- and nanoobjects.

Effects of Frequency on the Induced Fields of Deep Transcranial Magnetic Stimulation Systems: A Numerical Case Study

Transcranial Magnetic Stimulation (TMS) is a neurophysiological technique that is used as a noninvasive diagnostic and therapeutic tool for neurological conditions. This paper addresses the effects of frequency variation on magnetic and induced electric fields for deep brain stimulation scenario, considering the hippocampus region of the brain. A modified magnetic resonance coupling (MMRC) coil is designed and utilised to stimulate a realistic human head model at frequencies from 1 to 30 kHz, with constant and variable excitation current scenarios, while considering safety guidelines inside the brain. Sensitivity analysis is performed to quantify the reliability of field estimates. It is found that, although the maximum value of the induced electric field increases linearly with frequency, the general pattern of that field distribution inside the head demonstrates frequency-independent behaviour. Both depth of stimulation and field focality demonstrate a noticeable change with frequency below 12 kHz, while these attributes are almost saturated above this frequency band when using MMRC coil. It is also noted that, although increasing operating frequency can reduce the excitation current, the safety of the patient is prone to be jeopardized. Hence, to choose the optimum frequency range for the TMS system, several factors including, safety limits, stimulation threshold, desired magnetic flux density, power consumption, and target depth should be considered. This study provides a systematic approach to analyse the efficacy of deep TMS (dTMS) systems by making adequate trade-offs for the frequency selection to stimulate the targeted depth without exceeding safety limits, while using minimum possible power.

Mechanical, thermal stability and microstructural properties of emulsion-filled gels: Effect of sugar beet pectin/soy protein isolate ratio

The current study focused on how matrix ratio affected the gel properties and microstructures of sugar beet pectin (SBP)/soy protein isolate (SPI)-based gels with and without emulsion. Roles of emulsion stabilized by Tween 20 and different ratios of matrices in gelation induced by laccase and heat treatment were evaluated. The textural, rheological and thermal stability properties of either gels or gels containing emulsion were enhanced with increasing level of SBP and weakened with increasing concentration of SPI. With the increase of SBP concentration, the emulsion addition showed no significant influence on the textural parameters of gels. However, the thermal stability of gel was weakened because of the emulsion addition. The E/P212 showed improved textural and rheological properties compared with the gel without emulsion. All gels exhibited a frequency-independent behavior, and the water holding capacity of each gel was about 90%. Moreover, microstructure analysis indicated that the well-filled honeycomb gel was formed in E/P212. The gelation behavior was mainly dominant by SBP, and the underlying gelation mechanism was that the SBP was cross-linked to promote the gelatinization reaction and rate of SPI.

Low frequency dielectric behavior and AC conductivity of polymer derived SiC(O)/HfCxN1-x ceramic nanocomposites

The dielectric behavior of spark plasma sintered SiC(O)/HfCxN1-x nanocomposites synthesized through polymer derived ceramic route was investigated in the frequency range of 1 kHz to 1 MHz at room temperature. The nanostructural features revealed HfCxN1-x nanocrystals encapsulated in a nanometric thin layer of carbon dispersed uniformly in a SiC(O) matrix with segregated free carbon. The nanocomposites exhibited colossal permittivity values in the order of 103 at 1 kHz which reduced to 646 at 1 MHz. The interfacial polarization mechanism existing between complex nanostructural interfaces and the percolation of HfCxN1-x nanocrystals are believed to be responsible for the high permittivity values observed in the measured frequency range. The AC conductivity exemplified a frequency independent behavior at lower frequencies while at higher frequencies, the conductivity exhibited frequency dependence, indicating the existence of hopping type mechanism.

Modeling viscoelastic behavior in flexible multibody systems

Viscoelasticity plays an important role in the dynamic response of flexible multibody systems. First, single degree-of-freedom joints, such as revolute and prismatic joints, are often equipped with elastomeric components that require complex models to capture their nonlinear behavior under the expected large relative motions found at these joints. Second, flexible joints, often called force or bushing elements, present similar challenges and involve up to six degrees of freedom. Finally, flexible components such as beams, plates, and shells also exhibit viscoelastic behavior. This paper presents a number of viscoelastic models that are suitable for these three types of applications. For single degree-of-freedom joints, models that capture their nonlinear, frequency-dependent, and frequency-independent behavior are necessary. The generalized Maxwell model is a classical model of linear viscoelasticity that can be extended easily to flexible joints. This paper also shows how existing viscoelastic models can be applied to geometrically exact beams, based on a three-dimensional representation of the quasi-static strain field in those structures. The paper presents a number of numerical examples for three types of applications. The shortcomings of the Kelvin–Voigt model, which is often used for flexible multibody systems, are underlined.

A viscoelastic model for seismic attenuation using fractal mechanical networks

SUMMARY Seismic attenuation (quantified by the quality factor Q) has a significant impact on the seismic waveforms, especially in the fluid-saturated rocks. This dissipative process can be phenomenologically represented by viscoelastic models. Previous seismological studies show that the Q value of Earth media exhibits a nearly frequency-independent behaviour (often referred to as constant-Q in literature) in the seismic frequency range. Such attenuation can be described by the mathematical Kjartansson constant-Q model, which lacks of a physical representation in the viscoelastic sense. Inspired by the fractal nature of the pore fluid distribution in patchy-saturated rocks, here we propose two fractal mechanical network (FMN) models, that is, a fractal tree model and a quasi-fractal ladder model, to phenomenologically represent the frequency-independent Q behaviour. As with the classic viscoelastic models, the FMN models are composed of mechanical elements (spring and dashpots) arranged in different hierarchical patterns. A particular parametrization of each model can produce the same complex modulus as in the Kjartansson model, which leads to the constant-Q. Applying the theory to several typical rock samples, we find that the seismic attenuation signature of these rocks can be accurately represented by either one of the FMN models. Besides, we demonstrate that the ladder model in particular exhibits the realistic multiscale fractal structure of the saturated rocks. Therefore, the FMN models as a proxy could provide a new way to estimate the microscopic rock structure property from macroscopic seismic attenuation observation.

Self-Assembly of Symmetrical and Asymmetrical Alkyl Esters in the Neat State and in Oleogels

Saturated alkyl esters play an important role in determining the functional properties of the vegetable waxes used in the formulations of natural cosmetics and edible oleogels. We studied the relationship between the thermo-mechanical properties and crystal microstructure developed by saturated symmetrical (SE: 14:14, 16:16, 18:18, 20:20, and 22:22) and asymmetrical (AE: 18:14, 18:16, 18:20, 18:22) esters in the neat state and in oleogels. Additionally, we evaluated the effect of 1-stearoyl-glycerol (MSG; 0.5% and 1%) in the development of SE and AE oleogels. The X-ray and microscopy analysis in the neat state showed that SE self-assembled developing plate-like crystals, while AE developed acicular-like crystals. Microscopy analysis indicated that AE and SE followed similar crystallization behavior in the oleogels. The AE oleogels had higher elasticity (G’) than the SE oleogels. In both types of oleogels as the ester carbon number increased the oleogels’ G’ decreased and crystal size increased. The addition of just 0.5% MSG, particularly in the AE oleogels, limited the decrease in G’ as the ester carbon number increased, mainly because MSG decreased crystal size. The calorimetry results suggested that during cooling the MSG and the alkyl esters developed a co-crystal. Nevertheless, part of the MSG did not interact with the ester molecules and crystallized independently. These MSG crystals acted as active fillers of the microstructure formed by the co-crystals. The overall effect was that in comparison with the alkyl ester oleogels the alkyl ester oleogels with 0.5% and 1% MSG had higher G’ with frequency independent rheological behavior. This rheological behavior was particularly evident with the AE oleogels. Therefore, ester composition and molecular structure (i.e., symmetry or asymmetry) greatly influence its molecular self-assembly and subsequently the oleogels' thermo-mechanical properties. Studies using molecular mechanics modeling are underway to establish the mechanism for AE and SE self-assembly with and without MSG. The overall goal is to understand and control the crystallization of vegetable waxes for the development of functional edible oleogels.

Tailoring the performance of mechanically robust highly conducting Silver/3D graphene aerogels with superior electromagnetic shielding effectiveness

Multifunctional lightweight and mechanically robust aerogels have been shown potential interest in flexible electronics and electromagnetic interference (EMI) shielding applications. In this work, we demonstrate an environmental friendly template assist cost-efficient silver/3D graphene aerogels (Ag/GAs) by one step freeze-casting route and explore for conductive electrodes and shielding applications. The influence of Ag content on the structural, mechanical, electrical properties, and shielding performance of GAs and its EM interactions were studied. The Ag/GAs exhibited elastic behavior without evidencing plastic deformation under cyclic compressive loading. Broadband conductivity studies suggest the Ag/GAs exhibited superior frequency-independent behavior with electrical conductivity ~0.032 S/cm and consequently enhanced the EM shielding effectiveness up to ~32 dB. The efficient EMI shielding performance of the Ag/GAs is envisaged by connective dissipation as well as multiple reflections and scattering of EM waves by the effect of Ag nanofillers in the 3D-graphene network.

Analysis of Electrical Characteristics of Pd/n-Nanocarbon/p-Si Heterojunction Diodes: By C-V-f and G/w-V-f

Diamond films are candidate for a wide range of applications, due to their wide band gap, high thermal conductivity, and chemical stability. In this report, diamond-based heterojunction diodes (HJDs) were fabricated by growing n-type nanocarbon composite in the form of nitrogen-doped ultrananocrystalline diamond/amorphous carbon (UNCD/a-C:H:N) films onto p-type Si substrates. X-ray photoemission and the Fourier transform infrared spectroscopies were employed to examine the contribution of nitrogen atoms from the gas phase into the deposited films. The results indicate the incorporation of nitrogen atoms into the grain boundaries of UNCD/a-C:H film by replacing hydrogen atoms. The capacitance- (C-V-f), conductance- (G/ω-V-f), and series resistance-voltage characteristics of the fabricated Pd/n-(UNCD/a-C:H:N)/p-Si HJDs were studied in the frequency range of 40 kHz-2 MHz. The existence of interface states (Nss) and series resistance (Rs) were attributed to the interruption of the periodic lattice structure at the surface of the fabricated junction as well as the defects on the (UNCD/a-C:H:N)/Si interface. By increasing the frequency (≥500 kHz), the Nss reveals an almost frequency-independent behavior, which indicates that the charges at the interface states cannot follow ac signal at higher frequency. The obtained results demonstrated that the UNCD/a-C:H:N is a promising n-type semiconductor for diamond-based heterostructure diodes.