Tantalum Oxide Research Articles

Study of waveguide background at visible wavelengths for on-chip nanoscopy.

On-chip super-resolution optical microscopy is an emerging field relying on waveguide excitation with visible light. Here, we investigate two commonly used high-refractive index waveguide platforms, tantalum pentoxide (Ta2O5) and silicon nitride (Si3N4), with respect to their background with excitation in the range 488-640 nm. The background strength from these waveguides were estimated by imaging fluorescent beads. The spectral dependence of the background from these waveguide platforms was also measured. For 640 nm wavelength excitation both the materials had a weak background, but the background increases progressively for shorter wavelengths for Si3N4. We further explored the effect of the waveguide background on localization precision of single molecule localization for direct stochastic optical reconstruction microscopy (dSTORM). An increase in background for Si3N4 at 488 nm is shown to reduce the localization precision and thus the resolution of the reconstructed images. The localization precision at 640nm was very similar for both the materials. Thus, for shorter wavelength applications Ta2O5 is preferable. Reducing the background from Si3N4 at shorter wavelengths via improved fabrication will be worth pursuing.

Growth behavior of tantalum oxide nanotubes during constant current anodization

The standard method for growing tantalum oxide (Ta2O5) nanotubes is the constant voltage anodization mode. Although the constant current anodization mode is another feasible approach, the corresponding mechanism is not yet clear. In the present work, Ta2O5 nanotubes have been prepared by constant current anodization in an electrolyte containing HF. The results show that Ta2O5 nanotubes ~ 18 μm in length can be obtained with a very rapid growth rate within 2 min at a current density of 125 mA cm−2. In addition, the effects of current density, anodizing time, concentration of F− ions and electrolyte temperature on the morphology and growth rate of Ta2O5 nanotubes were studied. This work proposes an effective way of preparing tantalum oxide nanotubes under constant current anodic oxidation conditions, and paves the way toward large-scale production of Ta2O5 nanotubes.

Cell Adhesion Characteristics on Tantalum Pentoxide Gate Insulator for Cultured-Cell-Gate Field-Effect Transistor.

Understanding the interaction between living cells and a tantalum pentoxide (Ta2O5) gate electrode is important for controlling cell adhesion and functions when developing a cultured-cell-gate field-effect transistor biosensor. In this study, we evaluate the cell adhesion characteristics of the Ta2O5 membrane without or with a polydopamine (pDA) coating for chondrocytes, which is expected as a treatment for improving biocompatibility. As a result, the native and pDA-modified Ta2O5 membranes are shown to have the appropriate surface tension (35-40 dyn/cm) for the adhesion of chondrocytes owing to the contribution of surface tension to not only the nonspecific adsorption of serum proteins as the scaffold of chondrocytes but also the maintenance of the conformation of serum proteins. In particular, the serum proteins adhere more efficiently to the native Ta2O5 membrane than to the pDA-modified ones owing to the relatively smaller surface tension of the native Ta2O5 membrane; as a result, the proliferation and production of extracellular matrix (ECM) proteins such as collagen and proteoglycans by chondrocytes are clearly enhanced on the native Ta2O5 membrane. Thus, the native Ta2O5 membrane shows superior performance for the chondrocyte culture on it compared with the pDA-modified ones.

The Effect of Temperature on the Purity of Nano-Scale Tantalum Powder Produced from Its Scrap by Reaction with Magnesium and Calcium

In this study, Ta powder was produced from Ta scarp via chemical processes using Mg and Ca powders. At first, Tantalum scraps were converted to Tantalum oxide (Ta2O5) at 1100˚C in an oxygen atmosphere. Tantalum oxide was reduced to Tantalum powder with Mg in a vacuum environment at 950 to 1200˚C for 3 hours. The obtained Ta powders further were reacted with Ca at 950˚C for 5 hours in a vacuum atmosphere. The powders were analyzed through X-ray diffraction patterns (XRD), scanning electron microscopy (SEM), as well as oxygen measurement. The results show that the average particles size of the produced Ta powders is about 58 nm with oxygen contents of 250 ppm.

Influence of europium (Eu) doped tantalum oxide nanoparticles (TaOx NPs): A potential contrast agent

We report a one of a kind study to develop Europium doped Tantalum oxide nanoparticles for XEOL imaging. X-ray excited optical luminescence (XEOL) is an emerging alternative to in-vivo optical imaging to visualize deep tissues. Therefore, a proper selection of host material is essential to generate high luminescent XEOL probes to acquire molecular information about diseases. Herein, Tantalum oxide nanoparticles (TaOx NPs), a well-known biocompatible CT contrast agent, is used as the host material. In this article, we present the experimental investigation of X% of Europium doped into TaOx nanoparticles (Eu: TaOx NPs) as a potential contrast for XEOL imaging. XEOL measured with TaOx and X% Eu: TaOx NPs showed blue and reddish-orange luminescence when exposed to X-rays. By changing the X-ray voltages between 50 and 200 keV, the intensity of reddish-orange luminescence of X% Eu: TaOx NPs increases gradually. Based on the results, we propose that the enhanced reddish-orange luminescence from Eu: TaOx NPs is due to the energy transfer from host material and direct radiative decay from luminescent centre.

Chemoresistive Nanostructured Metal-Oxide Sensors for Human Tumor Tissue and Blood Exhalations Analysis

Early-stage tumor detection is one of the most coveted goals for the world research community. An effective prevention, associated to reliable screening protocols, is crucial to favor a early diagnosis, allowing doctors and surgeons to intervene with high probabilities of success on tumor affected patients. The main purpose of this study is cancer detection, exploiting the volatile organic compounds (VOCs) produced by cancer cells as tumor biomarkers [1][2], by investigating tumor tissue and blood samples. Tumor biomarkers exhalation is attributed to two main cell involving biological mechanisms: altered metabolism and cellular membrane peroxidation [3]. This experimental work was carried on using an innovative, fast-responding and reliable patented device, named SCENT B1 [4], entirely designed and assembled in the Sensor Laboratory of the University of Ferrara. It hosts an array of four specific metal-oxide (MOX) sensors able to detect gases in low concentration (up to 10 ppm) with high stability and repeatability, chosen after many tests on different types of biological samples (feces, blood, cell cultures, etc.) [5,6]; the chosen sensors are based on different mixtures of tin, titanium, tungsten, niobium, tantalum and vanadium oxides (ST25Au, W11, STN, TiTaV). Moreover, SCENT B1 is gifted of a power supply and sensor signal transduction electronic boards, a pneumatic air system (necessary to direct the VOCs contaminated air from the sample chamber to the sensors) and an ad hoc management and data acquisition software (LSS4) [6]. The sensor output signal is a voltage directly proportional to the sensor conductance, depending on the chemical redox reactions taking place on the surface of the sensor sensing material. The sensor response is: R=Vgas/Vair , where Vgas and Vair are the sensor voltage in gas presence and in dry air after the steady state achievement respectively [6].Sensor responses were further elaborated, using different statistical approaches as principal component analysis and receiver operating characteristics methods.Measurements have been performed on colorectal cancer and healthy tissues surgically removed from the same operatory piece (directly and far enough from tumor mass respectively) (Figure 1a), and on blood samples collected from tumor affected subjects and healthy ones as controls (Figure 1b). with the future aim of extending the study to other tumor types. All four sensors gave larger responses (although with different amplitudes) to the tumor tissue with respect to the healthy one. Smaller and reproducible responses were given by the breeding ground (DMEM) only, confirming that it does not alter the measurements. The results are consistent with the stronger and altered metabolism of tumor cells, leading them to emit a larger amount of volatile biomarkers with respect to the healthy ones;.All the four sensors hosted by SCENT B1 proved to be capable of distinguishing between healthy and tumor tissue and blood samples, although with different discrimination power. The encouraging results of this feasibility study are the basis of a new in-depth study, which includes also a follow-up protocol based on post surgery blood monitoring of patients. Our future aims foresee the extension of this study to other tumor types and the obtainment of the SCENT B1 clinical validation as oncologic screening device. Figure 1

Tantala Kerr nonlinear integrated photonics

Integrated photonics plays a central role in modern science and technology, enabling experiments from nonlinear science to quantum information, ultraprecise measurements and sensing, and advanced applications such as data communication and signal processing. Optical materials with favorable properties are essential for nanofabrication of integrated photonics devices. Here we describe a material for integrated nonlinear photonics, tantalum pentoxide ( T a 2 O 5 , hereafter tantala), which offers low intrinsic material stress, low optical loss, and efficient access to Kerr nonlinear processes. We utilize > 800 n m thick tantala films deposited via ion beam sputtering on oxidized silicon wafers. The tantala films contain a low residual tensile stress of 38 MPa, and they offer a Kerr index of n 2 = 6.2 ± 2.3 × 10 − 19 m 2 / W , which is approximately a factor of 3 higher than silicon nitride. We fabricate integrated nonlinear resonators and waveguides without the cracking challenges that are prevalent in stoichiometric silicon nitride. The tantala resonators feature an optical quality factor up to 3.8 × 10 6 , which enables us to generate ultrabroad bandwidth Kerr soliton frequency combs with low threshold power. Moreover, we characterize tantala waveguides by supercontinuum generation from low-energy, ultrafast seed pulses. These demonstrations characterize future application directions with tantala integrated nonlinear photonics.

N-doped reduced graphene oxide anchored with δTa2O5 for energy and environmental remediation: Efficient light-driven hydrogen evolution and simultaneous degradation of textile dyes

Precipitation assisted facile hydrothermal method has been developed for anchoring nitrogen-doped reduced graphene oxide (NRGO) with tantalum pentoxide (TO) denoted as TO@NRGO. Synthesized materials were characterized using spectroscopic, optical and microscopic techniques and confirm transforming TO from orthorhombic to hexagonal phase (δ) upon anchoring with NRGO. TO@NRGO, TO and NRGO have been evaluated for photocatalytic hydrogen evolution, degradation of Methylene blue (MB) and Rhodamine B (RhB). The enhanced photocatalytic activity was observed in TO@NRGO nanocomposite compared to TO and NRGO due to decreased bandgap (2.5 eV) and increased surface area (312 m2 g−1). TO@NRGO evolved 19,500 µmol g−1 of hydrogen for 3 h. TO@NRGO showed better degradation efficiency of 94 and 88% at a time of 100 min for MB and RhB, respectively. The parameters involved in photocatalytic dye degradation, like the effect of pH, catalyst dosage, and initial concentration of dyes, have been carefully optimized to achieve maximum performance of the catalyst. The stability and reusability of TO@NRGO are good and managed to degrade dyes effectively even after the 5th run. Thus, TO@NRGO could serve as a choice of material in resolving the issues related to energy and the environment.

Solar energy-driven C−H activation of methanol for direct C−C coupling to ethylene glycol with high stability by nitrogen doped tantalum oxide

Direct photocatalytic coupling of methanol to ethylene glycol (EG) is highly attractive. The reported photocatalysts for this reaction are all metal sulfide semiconductors, which may suffer from photocorrosion and have low stability. Thus, the development of non-sulfide photocatalysts for efficient photocatalytic coupling of methanol to EG and H2 with high stability is urgent but extremely challenging. Herein, the first metal oxide photocatalyst, tantalum-based semiconductor, is reported for preferential activation of C−H bond within methanol to form hydroxymethyl radical (•CH2OH) and subsequent C−C coupling to EG. Compared with other metal oxide photocatalysts, such as TiO2, ZnO, WO3, Nb2O5, tantalum oxide (Ta2O5) is unique in that it can realize the selective photocatalytic coupling of methanol to EG. The co-catalyst free nitrogen doped tantalum oxide (2%N-Ta2O5) shows an EG formation rate as high as 4.0 mmol gcat−1 h−1, about 9 times higher than that of Ta2O5, with a selectivity higher than 70%. The high charge separation ability of nitrogen doped tantalum oxide plays a key role in its high activity for EG production. This catalyst also shows excellent stability longer than 160 h, which has not been achieved over the reported metal sulfide photocatalysts. Tantalum-based photocatalyst is an environmentally friendly and highly stable candidate for photocatalytic coupling of methanol to EG.

Highly Material Selective and Self‐Aligned Photo‐assisted Atomic Layer Deposition of Copper on Oxide Materials

AbstractThere is a growing need for bottom‐up fabrication methods in microelectronic industry as top‐down, lithography‐based methods face increasing challenges. In Photo‐assisted atomic layer deposition (Photo‐ALD), photons supply energy to the deposition reactions on the surface. Here, a process and patterning for Photo‐ALD of copper is reported, with inherently selective, self‐aligned film growth without any photomasking or additive layers. Highly conductive and pure copper films are selectively deposited on tantalum oxide for over hundred nanometers of film thickness, while no copper deposits on silicon or aluminum oxide. On anatase titanium dioxide, copper deposition is crystal‐facet selective. Selective deposition of a metal is realized on oxides, which has been especially challenging for ALD. This study indicates that the growth mechanism is closely related to photocatalysis: the photons interact with the material under the growing copper film, enabling the inherent selectivity. The findings provide promising material engineering schemes for microelectronics, photocatalysis, and photovoltaics.

Raman spectroscopic and theoretical studies on the structure of Ta (V) fluoro and oxofluoro complexes in molten FLiNaK

Combined with Raman spectroscopy and theoretical calculations, the tantalum fluoride and oxyfluoride ions in molten FLiNaK were studied. Firstly, the Raman spectra of K2TaF7 were recorded from 298 to 973 K and no structural evolution was observed. The coexistence of TaF72– and TaF83– anions was confirmed in K2TaF7 and molten FLiNaK-K2TaF7 mixtures. With the addition of Li2O (3 mol%), the terminal oxo ligands TaOF52– and TaOF63– were formed at the expense of TaF72– and TaF83– with the release of free F– ions, respectively. When the Li2O content increased to 13 mol%, two new oxyfluorides TaO2F2– and TaO2F32– were observed that contained two non-bridging oxygens. As the content of Li2O further increased to 40 mol%, insoluble solid tantalum oxide species like β-Ta2O5 was found, which indicates the transformation of intermediate oxyfluorides formed by the reaction of TaF72– and TaF83– fluorides with O2– in molten FLiNaK to oxides.

Effect of Adsorbed Water Molecules on the Surface Acidity of Niobium and Tantalum Oxides Studied by MAS NMR

Niobium oxides and tantalum oxides are typical solid acid catalysts that exhibit high activity and high water-resistance in the aqueous environment. To understand their acid catalytic performance, ...

Carbon coating and oxygen vacancies render superior Li-ion storage of crystalline Ta2O5 by enhanced pseudocapacitance

Tantalum pentoxide (Ta2O5) possesses enormous potentials as high-performance anodes for lithium-ion batteries owing to its high specific capacity and abundant sources. Herein, various crystalline Ta2O5 nanomaterials were fabricated by annealing the precursors of (NH4)2Ta2O3F6 mesocrystals or polydopamine coated (NH4)2Ta2O3F6 composites in Ar or air atmospheres, which were synthesized with a hydrothermal process and followed the in-situ coating of polydopamine in aqueous solution. Particularly, the products obtained in Ar possess abundant oxygen vacancies. When evaluated as anodes for LIBs, Ta2O5@C-Ar with carbon coating and oxygen vacancies exhibits superior lithium storage properties including remarkably high reversible capacity, high rate capability (166 mAh g−1 at 2 A g−1), and excellent cyclic stability (up to 1000 cycles at 1 A g−1 with 0.02% capacity fading per cycle), in comparison with Ta2O5-Ar and Ta2O5-Air electrodes. Kinetics analysis based on cyclic voltammograms (CVs) discloses that the higher lithium storage capacity and superior rate capability of Ta2O5@C-Ar are primarily attributed to fast surface capacitive kinetics, which stem from large active surface area, ultrasmall Ta2O5 nanosheets with conductive carbon layer, and abundant oxygen vacancies. This strategy of constructing electrode materials with conductive carbon coating and oxygen vacancies provides a promising avenue for engineering advanced anodes with high specific capacity and excellent rate capability.

Strong coloration of nanoporous tungsten oxides by in-vacuo lithiation for all-solid-state electrochromic devices

Electrochromic windows allow for an improved management of solar heat gains in buildings and enhanced visual comfort of building users. All-solid-state electrochromic devices can be especially durable. During their fabrication lithium has to be inserted, preferably in-vacuo. For such a dry lithiation process it is essential to determine to which extent sufficiently thick electrochromic layers can be colored and with which efficiency. In this study, we focus on the dry lithiation of sputtered nanoporous tungsten trioxide films and the induced optical response. The porosity of the WO3 films was in the order of 18 %, as determined from variable angle spectroscopic ellipsometry and effective medium approximation. Films with a thickness above 1 µm were dry-lithiated yielding a deep coloration, with a lithiation efficiency LE up to 1.5⋅106 cm2⋅mol−1. Hereby the light transmittance was reduced from 85.7% to 2.0%, with the spectral transmittance approaching 0% in the range from 600 nm to 1500 nm. These results suggest that lithium can diffuse over a large distance in nanoporous WO3. The columnar morphology and nanoporous structure of the thin films deposited at high working pressure is beneficial for the displacement of the lithium ions. All-solid-state devices were prepared using in-vacuo lithiation, tantalum pentoxide as a solid electrolyte and nickel oxide as a counter electrode. During cycling the solar transmittance varied between 8 % and 52 % and the light transmittance from 7.8% to 53.1%, the latter with a coloration efficiency CE up to 63 cm2⋅C−1. Accordingly, switching times of 17 s for coloration and 13 s for bleaching were determined. The gained insights are of high practical relevance for the fabrication of all-solid-state electrochromic devices.

Higher order mode supercontinuum generation in tantalum pentoxide (Ta2O5) channel waveguide

We fabricated tantalum pentoxide (Ta2O5) channel waveguides and used them to experimentally demonstrate higher-order mode supercontinuum (SC) generation. The Ta2O5 waveguide has a high nonlinear refractive index which was in an order magnitude of 10–14 cm2/W and was designed to be anomalously dispersive at the pumping wavelength. To the best of our knowledge, this is the first time a higher-order mode femtosecond pump based broadband SC has been measured from a nonlinear waveguide using the phase-matching method. This enabled us to demonstrate a SC spectrum spanning from 842 to 1462 nm (at − 30 dB), which corresponds to 0.83 octaves, when using the TM10 waveguide mode. When using the TE10 mode, the SC bandwidth is slightly reduced for the same excitation peak power. In addition, we theoretically estimated and discussed the possibility of using the broadband higher-order modes emitted from the Ta2O5 waveguide for trapping nanoparticles. Hence, we believe that demonstrated Ta2O5 waveguide are a promising broadband light source for optical applications such as frequency metrology, Raman spectroscopy, molecular spectroscopy and optical coherence tomography.

Reliability and Failure Mode in Solid Tantalum Capacitors

The reliability and failure modes in surface mount Solid Electrolytic and Polymer Tantalum capacitors were investigated using the parts manufactured with conventional technology and flawless technology (F-Tech) that suppresses typical defects such as crystalline inclusions in the amorphous matrix of the tantalum oxide dielectric. The accelerated tests were performed; failure rates were calculated based on the cumulative percent of failed parts vs time and acceleration factor. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and thermo-gravimetric/deferential scanning calorimetry analysis (TGA/DSC) were included in the investigation to analyze chemical and structural transformations in the capacitors failed in the accelerated tests. Results show strong impact of technology on reliability and failure mode including the lowest failure rate and no wear-out failure mode in Polymer Tantalum capacitors manufactured with F-Tech. No ignition and burning tantalum were found in the Solid Electrolytic Tantalum capacitors that failed short. Limiting temperatures and protecting the encapsulating epoxy compound from ignition and burning at normal conditions of the accelerated testing take place via ablation process—heat consuming irreversible structural and chemical transformations in the material such as carbonization process.

APPLICATION OF THE ELECTROSTATIC FIELD OF ELECTRET IN THE SURGICAL TREATMENT OF PATIENTS WITH GONARTHROSIS

A significant role in the pathogenesis of osteoarthritis (OA) belongs to the violation of the natural bioelectrogenesis of bone and cartilage tissue. External correction of bioelectric processes in joint structures by means of an electrostatic electret field is pathogenetically justified and has practical significance in the surgical treatment of degenerative diseases in orthopedics. The article presents an analysis of the results of surgical treatment in 45 patients with stage 2-3 gonarthrosis aged 30 to 70 years using electretes based on tantalum oxide. The results of treatment were evaluated based on the dynamics of the clinical and functional index WOMAC, data from clinical and radiological studies. The positive effect of the electrostatic field of electret in OA was manifested by a decrease in clinical symptoms of the disease, increased functional activity of patients, restraining degenerative changes in joint structures, and improving the quality of daily life during the follow-up period. The positive effect of the electrostatic field of electret is based on the activation of repair of bone-cartilage structures, which allowed to slow down the degradation of subchondral bone and articular cartilage in gonarthrosis.Clinical use of electret stimulators significantly effectively improves joint function which reduces the risk of progression of degenerative changes in the joint especially in patients with the second stage of the disease. The clinical use of electronic stimulators significantly effectively improves joint function which reduces the risk of progression of degenerative changes in the joint especially in patients with the second stage of the disease.The use of electretes in the treatment of patients with stage 3 gonarthrosis can improve the function of the joint for a long time more (2 years) and postpone the endoprosthesis operation.

Refractive Indexes and Spectroscopic Properties to Design Er3+-Doped SiO2-Ta2O5 Films as Multifunctional Planar Waveguide Platforms for Optical Sensors and Amplifiers.

This paper reports on the news about refractive index measurements and spectroscopic features of thin films, which can be applied as optical planar waveguides, focusing on their manufacturing processes, designs, and possible applications as optical amplifiers and sensors. Er3+-doped SiO2–Ta2O5 planar waveguides, with Si/Ta ratios of 90:10, 80:20, 70:30, 60:40, and 50:50, were prepared by a soft sol–gel process. Multilayer films were deposited by the dip-coating technique onto 10 μm SiO2–Si (100) p-type silicon and Si (100) silicon easily and successfully. The mechanisms of the densification process, porosity, and hydroxy group or water molecule occurrence have been accompanied by m-line and vibrational spectroscopy analyses. The thickness and refractive index values were used to understand better the influence of temperature and annealing time on the densification of the bulk films and the reduction of the pore volume as the tantalum oxide concentration increases. The refractive index shows the density of the films, and by the atomic force microscopy (AFM) technique, the films showed low surface roughness, achieving relatively high light confinement within the waveguide structure, and negligible optical loss due to surface scattering. Nanoparticle crystallization of Ta2O5 with size distribution ranging from 2.0 to 15.0 nm embedded in SiO2 was observed with size depending on annealing time and tantalum concentration. Intense and broadband emission positioned at 1550 nm, which is attributed to the 4I13/2 → 4I15/2 transition of Er3+ ions, was observed for all planar waveguides under excitation at 271, 272, and 278 nm. Depending on the porosity degree, the adsorption of H2O molecules occurs, changing the refractive index and contributing to the deactivation of excited states of Er3+ ions, making them an optical platform for use as an optical sensor for different species. Besides, the densified waveguides containing 20 or 30 mol % Ta exhibit high potential for applications as broadband optical amplifiers for wavelength division multiplexing (WDM), optical sensing, or augmented reality.

Initial electrical properties of tantalum oxide resistive memories influenced by oxygen defect concentrations

The initial electrical properties of tantalum oxide resistive memory were investigated using four metal electrodes (TiN, Ti, Ta, and Al) and two kinds of tantalum oxide with different amounts of intrinsic oxygen vacancies. The initial resistance depended on the electrode material. This indicated that oxygen scavenging by the electrodes contributed to a reduction in the resistance. However, the resistance change depended on the intrinsic oxygen vacancy concentration introduced during the tantalum oxide deposition. The forming voltage also depended on the electrode metal. For the device with an Al electrode, a clear aluminum oxide layer was identified at the electrode–insulator interface, which was hypothesized to be the origin of the high forming voltage. All factors concerning the oxygen vacancies, i.e. intrinsic vacancies introduced via film deposition and extrinsic vacancies caused by the electrode scavenging effect, influenced the initial state of tantalum oxide, and thus, its switching performance as a resistive memory.

Effect of High-κ Dielectric Layer on 1/f Noise Behavior in Graphene Field-Effect Transistors

We report the 1/f noise characteristics at low-frequency in graphene field-effect transistors that utilized a high-k dielectric tantalum oxide encapsulated layer (a few nanometers thick) placed by atomic layer deposition on Si3N4. A low-noise level of ~ 2.2 x 10^(-10) Hz-1 has been obtained at f = 10 Hz. The origin and physical mechanism of the noise can be interpreted by the McWhorter context, where fluctuations in the carrier number contribute dominantly to the low-frequency noise. Optimizing fabrication processes reduced the number of charged impurities in the graphene field-effect transistors. The study has provided insights into the underlying physical mechanisms of the noise at low-frequency for reducing the noise in graphene-based devices.