Defect Density Of Graphene Research Articles

Energetics and Kinetics of Hydrogen Electrosorption on a Graphene-Covered Pt(111) Electrode.

The Angstrom-scale space between graphene and its substrate provides an attractive playground for scientific exploration and can lead to breakthrough applications. Here, we report the energetics and kinetics of hydrogen electrosorption on a graphene-covered Pt(111) electrode using electrochemical experiments, in situ spectroscopy, and density functional theory calculations. The graphene overlayer influences the hydrogen adsorption on Pt(111) by shielding the ions from the interface and weakening the Pt-H bond energy. Analysis of the proton permeation resistance with controlled graphene defect density proves that the domain boundary defects and point defects are the pathways for proton permeation in the graphene layer, in agreement with density functional theory (DFT) calculations of the lowest energy proton permeation pathways. Although graphene blocks the interaction of anions with the Pt(111) surfaces, anions do adsorb near the defects: the rate constant for hydrogen permeation is sensitively dependent on anion identity and concentration.

Metal-incorporated laser-induced graphene for high performance supercapacitors

High performance supercapacitors are promising to address the rising energy conversion and storage challenges. Revealing the role of heteroatom incorporation and nanoparticle anchoring in modifying the capacitive properties of graphene-based electrodes is scientifically important to boost the supercapacitor performance, but technically challenging. We demonstrate a simple, but efficient route to fabricate porous graphene with effective heteroatom incorporation and metal nanoparticles anchoring by laser direct writing metal-coated polyimide films. Silver (Ag), copper (Cu), titanium (Ti), and tellurium (Te) are introduced into the porous laser-induced graphene (LIG) as dopants and nanoparticles. Compared to Ti and Te, Ag and Cu exhibit high incorporation efficiency. Incorporation of Ag and Cu into the LIG sheets results in high level disorder and structure expansion, which contributes to the enhanced capacitive performance. All-solid-state micro-supercapacitors (MSCs) made from the metal-incorporated LIGs (M-LIGs) were assembled. The capacity performance of M-LIG-based MSCs exhibits a similar variation trend with respect to the metal type. Besides, the M-LIG-based MSCs exhibit excellent capacitance stability at a bending angle beyond 150°. The results suggest that the effective metal doping and the metal nanoparticle modification could elevate the structural defect density in graphene, therefore contributing to high carrier density and enhanced capacitive performance.

Transfer-Free CVD Growth of High-Quality Wafer-Scale Graphene at 300 °C for Device Mass Fabrication.

Direct chemical vapor deposition of graphene on semiconductors and insulators provides high feasibility for integration of graphene devices and semiconductor electronics. However, the current methods typically rely on high temperatures (>1000 °C), which can damage the substrates. Here, a growth method for high-quality large-area graphene at 300 °C is introduced. A multizone furnace with gradient temperature control was designed according to a computational fluid dynamics model. The crucial roles of the chamber pressure in the film continuity and hydrogen composition in the graphene defect density at low temperature were revealed. As a result, a uniform graphene film with the Raman ratio ID/IG = 0.08 was obtained. Furthermore, a technique of laminating single-crystal Cu foil as a sacrificial layer on the substrate was proposed to realize transfer-free growth, and a wafer-scale graphene transistor array was demonstrated with good performance consistency, which paves the way for mass fabrication of graphene devices.

Tracking Interfacial Changes of Graphene/Ge(110) During In-Vacuum Annealing

Graphene quality indicators obtained by Raman spectroscopy have been correlated to the structural changes of the graphene/Germanium interface as a function of in-vacuum thermal annealing. Specifically, it is found that graphene becomes markedly defected at 650 {\deg}C. By combining scanning tunneling microscopy, x-Ray Photoelectron Spectroscopy and Near Edge x-ray Absorption Fine Structure Spectroscopy, we conclude that these defects are due to the release of H_{2} gas trapped at the graphene/Germanium interface. The H_{2} gas was produced following the transition from the as-grown hydrogen-termination of the Ge(110) surface to the emergence of surface reconstructions in the substrate. Interestingly, a complete self-healing process is observed in graphene upon annealing to 800 {\deg}C. The subtle interplay revealed between the microscopic changes occurring at the graphene/Germanium interface and graphene's defect density is valuable for advancing graphene growth, controlled 2D-3D heterogeneous materials interfacing and integrated fabrication technology on semiconductors.

Macro copper-graphene composites with enhanced electrical conductivity

Composites demonstrating enhanced electrical conductivity compared to copper have been highly sought after for their advantages in efficient energy transport behavior. While such conductors have been demonstrated in 1D (nanowires) and 2D (films) samples, achieving similar behavior in 3D has been challenging owing to the limitations of the synthesis techniques used. In this paper, novel macro-scale 3D copper conductors were demonstrated with increased electrical conductivity through the addition of graphene. Hot extrusion was used to manufacture 12 AWG copper-graphene composite wires with varying graphene content and defect density. Graphene defect density was measured using Raman spectroscopy. Results showed that the electrical conductivity of composites with low defect density graphene increased as a function of graphene content. Comparatively composites with high defect density graphene demonstrated lower electrical conductivity. This study provides first-of-its-kind evidence of 3D metal composites whose bulk electrical performance has been enhanced using graphene additive in minute quantities (15 ppm). Further developments in this area are essential to achieve high performance composite conductors that can improve energy transport efficiency and pave way for industrial adoption of such materials in the future.

Effect of the defect densities of reduced graphene oxide network on the stability of lithium-metal anodes

Graphene is considered an ideal lithium metal host material to suppress dendrite growth and buffer volume changes. Although defects on graphene are generally considered to enhance its lithiophilicity, defects that promote unstable solid electrolyte interphase (SEI) growth are rarely noticed. To explore which defect density of graphene is the most ideal host material, different defect densities of graphene are prepared as Li-metal host materials through thermal reduction at different temperatures (200 °C, 400 °C, 800 °C, 1200 °C, 1600 °C and 2200 °C). We are the first to demonstrate that molten lithium can spread into a graphene matrix with any density of defects and that Li2C2 is formed in less-defective 2200 °C rGO/Li anode. The 2200 °C rGO/Li composite anode exhibits the best electrochemical performance in all rGO/Li anodes, and the symmetrical cell with 2200 °C rGO/Li electrode shows stable cycling for 1200 h with a small polarization voltage of 12 mV at 1 mA cm−2. Moreover, the 2200 °C rGO/Li || S full cell displays stable cycling performance even with a high sulfur loading of 9.4 mg cm−2; additionally, a high initial areal capacity of 9.7 mA h cm−2 can be realized.

Enhancing Structural Properties and Performance of Graphene-Based Devices Using Self-Assembled HMDS Monolayers.

The performance of graphene devices is often limited by defects and impurities induced during device fabrication. Polymer residue left on the surface of graphene after photoresist processing can increase electron scattering and hinder electron transport. Furthermore, exposing graphene to plasma-based processing such as sputtering of metallization layers can increase the defect density in graphene and alter the device performance. Therefore, the preservation of the high-quality surface of graphene during thin-film deposition and device manufacturing is essential for many electronic applications. Here, we show that the use of self-assembled monolayers (SAMs) of hexamethyldisilazane (HMDS) as a buffer layer during the device fabrication of graphene can significantly reduce damage, improve the quality of graphene, and enhance device performance. The role of HMDS has been systematically investigated using surface analysis techniques and electrical measurements. The benefits of HMDS treatment include a significant reduction in defect density compared with as-treated graphene and more than a 2-fold reduction of contact resistance. This surface treatment is simple and offers a practical route for improving graphene device interfaces, which is important for the integration of graphene into functional devices such as electronics and sensor devices.

Confocal laser scanning microscopy as a real-time quality-assessment tool for industrial graphene synthesis

For the industrial quality control (QC) of the chemical vapor deposition (CVD) graphene, it is essential to develop a method to screen out unsatisfactory graphene films as efficiently as possible. However, previously proposed methods based on Raman spectroscopy or optical imaging after chemical etching are unable to provide non-invasive and fast analysis of large-area graphene films as grown on Cu foil substrates. Here we report that the reflection mode of confocal laser scanning microscopy (CLSM) provides a high-contrast image of graphene on Cu, enabling the real-time evaluation of the coverage and quality of graphene. The reflectance contrast, Rc, was found to be dependent on the incident laser wavelength, of which the maximum was obtained at 405 nm. In addition, Rc decreases with increasing defect density of graphene. The dependence of Rc on the graphene’s quality and laser wavelengths were explained by the tight-binding model calculation based on the Fresnel’s interference formula. Thus, we believe that the reflection mode CLSM would be a very powerful quality-assessment tool for the mass production of CVD graphene films grown on Cu.

Effects of precursors’ purity on graphene quality: Synthesis and thermoelectric effect

A chemical vapor deposition method has been proven to produce large scale monolayer graphene. However, it is often reported that such graphene contains a varying amount of defects. In this work, methane precursors of different purities [99.99% (4-9G), 99.90% (3-9G), and 98.90% (2-9G)] were used. It is shown that the introduction of defects occurs during graphene growth. It has been attributed to the presence of trace oxygen molecules in the gas precursors. By controlling the amount of oxygen present, one is able to tune the defect density in graphene at will. It is purported that the oxygen reacts with methane to yield methanol and formaldehyde. The latter is oxidized to formic acid. As the graphene network expands, the alcohol and formic acid are incorporated as C—O and O—C=O functional groups. In turn, the graphene experiences an overall global tensile strain due to local bond distortion induced by the electronegative oxygen containing groups. Furthermore, the presence of N2 molecules impedes the proper coalescing of carbon-containing molecules for the formation of the sp2-rich carbon network. Electrical measurement conducted suggests that a high purity precursor (4-9G) induces the least amount of defects, which confers a high Seebeck coefficient (105.1 µV/K) and a low sheet resistance (58.3 Ω). In the case of a larger volume of oxygen in the precursor, electrical performance decreases generally.

N/S dual-doped graphene with high defect density for enhanced supercapacitor properties

N/S dual-doped graphene was prepared by one-pot process using graphene oxide as raw material and thiourea and urea as reduction-dopants. The field emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), Raman spectroscopy (Raman), nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and other means were used to characterize the microstructure and morphology of the samples. The electrochemical properties of the samples were tested by cyclic voltammetry, electrochemical alternating impedance and constant current charge-discharge techniques, and compared with graphene and nitrogen-doped graphene. Results show that the defect density of graphene can be increased more effectively by N/S dual doping than by nitrogen doping, and the contents of doped nitrogen and sulfur have a significant effect on the morphology and performance of the samples. The specific surface area of the best sample reaches 275.8 m2 g−1, and its conductivity is 477.6 S m−1. When the window voltage is −1.2-0 V, the best sample shows superior specific capacitance of 386.5 F g−1 and a high energy density of 69.6 Wh kg−1 at a scan rate of 10 mV s−1. At the current density of 10 A g−1, after 5000 constant current charge/discharge cycles, the specific capacitance retention rate is 94.5%, showing excellent cyclic stability.

Graphene electrical properties modulated by swift heavy ion irradiation

Changes in electrical properties of graphene devices induced by the energetic ion irradiation are very important for their application in harsh radiation environment. This paper presents the modulating behavior of electrical properties of graphene-based devices induced by swift heavy ions (SHIs). Graphene field effect transistors (GFETs) were irradiated by 1.79 GeV Ta ions and it was found that at lower fluence (109-1010 ions/cm2), the SHIs irradiation can effectively optimize the performance of GFETs, while at higher fluence (∼1011 ions/cm2), the electrical properties of the devices were significantly deteriorated after the irradiation process. The effective length and width of the graphene strip and irradiation fluence are the main factors that determine the improvement in performance of GFETs. Raman spectroscopy was employed to figure out the correlation between the initial defect density in graphene and changes in electrical performance of GFETs. It was shown that the competition among various factors such as the doping, local annealing and defect creation dominates the GFET performance. This work explores the best conditions for improving the electrical properties of GFETs and provides an important reference data for the utilization of graphene based irradiated devices in aerospace electronics.

Passivation of Germanium by Graphene for Stable Graphene/Germanium Heterostructure Devices

Graphene grown directly on Ge via chemical vapor deposition (CVD) can passivate the underlying Ge surface, preventing its oxidation in ambient air for at least months. However, the factors that govern oxidation of Ge coated with graphene have not been elucidated. We investigate the effect of graphene synthesis parameters and Ge surface orientation on passivation of Ge and correlate these data with the density and type of defects in graphene. Oxidation of Ge can be reduced by increasing the H2:CH4 flux ratio or decreasing the growth rate, which decrease the density of atomic-scale defects, such as point defects and grain boundaries, in graphene. Oxidation of graphene is concomitant with oxidation of Ge and occurs more readily when the density of atomic-scale defects is relatively high. Passivation of Ge, however, depends more strongly on Ge surface orientation, as Ge(110) oxidizes significantly less than Ge(001) or Ge(111), even at the same graphene defect density. These results provide a pathway for engineering high-quality graphene films on Ge, which may enable improved passivation of Ge and direct integration of graphene-based or hybrid graphene/Ge heterostructure devices on conventional semiconductor platforms.

Thermal stability of graphene in inert atmosphere at high temperature

To investigate the thermal stability of graphene during sintering process of ceramic, the effect of temperature on stability of single-layer graphene (SLG) in argon atmosphere was studied. The Raman D-band/G-band intensity ratio(ID/IG) and the blue shifts of G mode showed that, the graphene defect density gradually increased with the increase in heat treatment temperature (HTT). There was limited damage in the graphene when treated temperature lower than 800 °C. The causes of the defects were discussed. The defect types were mainly on-set defects. Boundary defects were detected when the temperature reached above 800 °C. Atom Force Microscope (AFM) results showed that the defects gradually extended from the boundary and holes in the layer to the edge and ultimately led to isolated graphene islands. The experiment results could be useful to study the changes and developing the holistic performance of graphene in high temperature sintering process.

Direct observation of unusual interfacial Dzyaloshinskii-Moriya interaction in graphene/NiFe/Ta heterostructures

Graphene/ferromagnet interface promises a plethora of new science and technology. The interfacial Dzyaloshinskii Moriya interaction (iDMI) is essential for stabilizing chiral spin textures, which are important for future spintronic devices. Here, we report direct observation of iDMI in graphene/Ni80Fe20/Ta heterostructure from non-reciprocity in spin-wave dispersion using Brillouin light scattering (BLS) technique. Linear scaling of iDMI with the inverse of Ni80Fe20 thicknesses suggests primarily interfacial origin of iDMI. Both iDMI and spin-mixing conductance increase with the increase in defect density of graphene obtained by varying argon pressure during sputter deposition of Ni80Fe20. This suggests that the observed iDMI originates from defect-induced extrinsic spin-orbit coupling at the interface. The direct observation of iDMI at graphene/ferromagnet interface without perpendicular magnetic anisotropy opens new route in designing thin film heterostructures based on 2-D materials for controlling chiral spin structure such as skyrmions and bubbles, and magnetic domain-wall-based storage and memory devices.

Defect formation in supported graphene irradiated by accelerated xenon ions

Raman spectroscopy and Monte-Carlo simulation studies for supported graphene irradiated by 160 MeV Xe ions are presented. Changes in the density and dominating types of defects with increasing fluence were observed. In order to analyze contribution of defect formation mechanisms, in which the substrate is involved, a comparative study was performed for graphene on SiO2/Si, copper and glass substrates. The major defining mechanisms were found to be atomic recoils and formation of defects induced by hot electrons. For graphene on copper, the impact of substrate recoil atoms was found to be greater comparing to graphene on silicon oxide and glass, where the recoils participated approximately equally. Moreover, a possibility of defect formation in graphene due to hot electrons generated in the substrate near the interface was noted. Finally, a linear dependence of air-induced doping on D and G peak intensity ratio that represents defect density in graphene was found. The study is useful for solving the long-standing controversy on major mechanisms of defect formation in irradiated graphene, as well as for graphene-based nanoelectronic device engineering.

Effects of carbon defects on ZnO nanorods directly grown on graphene

The correlation between carbon defects and the density of ZnO nanorods directly grown on graphene was studied. Thermal annealing was performed to vary the defect density of graphene on which ZnO nanorods were directly grown via hydrothermal synthesis. We found that ZnO nanorods were densely distributed on a highly defective graphene. Furthermore, specific defect sites were observed to provide upright ZnO nanorods. Raman spectroscopy revealed that the thermally induced defect corresponds to a carbon vacancy, which is expected to provide a reactive graphene surface where precursors can be easily attached to trigger the nucleation and further growth of ZnO nanorods. The local measurement of defects is believed to elucidate the key parameters for the growth of highly oriented ZnO nanorods.

Inductively coupled remote plasma-enhanced chemical vapor deposition (rPE-CVD) as a versatile route for the deposition of graphene micro- and nanostructures

Multiple layers of graphene thin films with micro-crystalline orientation and vertical graphene nano-sheets were grown on different substrates (i.e., polycrystalline nickel foil, Ni(111), highly oriented pyrolytic graphite) using a single-step process based on low-pressure remote Plasma-Enhanced Chemical Vapor Deposition (rPE-CVD). In contrast to previous studies, a novel basic approach to this technique including a new remote inductively coupled RF plasma source has been used to (i) minimize the orientational effect of the plasma electrical fields during the catalyst-free growth of graphene nano-sheets, (ii) warrant for a low graphene defect density via low plasma kinetics, (iii) decouple the dissociation process of the gas from the growth process of graphene on the substrate, (iv) tune the feedstock gas chemistry in view of improving the graphene growth, and (v) reduce the growth temperature as compared to conventional chemical vapor deposition (CVD). In order to study the various aspects of the rPE-CVD graphene growth modes and to assess the characteristics of the resulting graphene layers, Raman spectroscopy, XPS, SEM, and STM were used. The results give evidence for the successful performance of this new rPE-CVD plasma deposition source, that can be combined with in situ UHV-based processess for the production of, e. g., hybrid metal ferromagnet/graphene systems.

Enhancement of graphene thermoelectric performance through defect engineering

Thermoelectric properties of materials are typically evaluated using the figure of merit, ZT, which relies on both the electrical and thermal properties of the materials. Although graphene has a high thermoelectric power factor, its overall ZT value is quite low as it possesses extremely high thermal conductivity. Phonons are the main heat carrier in graphene, and therefore propagation of heat in the material may be modulated by introducing defects into the structure, resulting in reduced thermal conductivity. In this study, we investigate the effect of graphene defect density on the thermoelectric performance of graphene. The defects introduced into graphene by oxygen plasma treatment reduce its Seebeck coefficient as well as its electrical conductivity; as a result, the thermoelectric power factor declines with increasing defect density. However, at higher defect densities, the reduction in thermal conductivity dominates over the reduction in electrical conductivity and, consequently, graphene treated in this way is observed to possess ZT values of up to three times that of pristine graphene. Therefore, it may be concluded that introducing controlled amount of defects into graphene is an effective way of reducing its thermal conductivity, thereby enhancing the performance of graphene-based thermoelectric devices.

Surface functionalization of epitaxial graphene on SiC by ion irradiation for gas sensing application

In this work, surface functionalization of epitaxial graphene grown on silicon carbide was performed by ion irradiation to investigate their gas sensing capabilities. Swift heavy ion irradiation using 100MeV silver ions at four varying fluences was implemented on epitaxial graphene to investigate morphological and structural changes and their effects on the gas sensing capabilities of graphene. Sensing devices are expected as one of the first electronic applications using graphene and most of them use functionalized surfaces to tailor a certain function. In our case, we have studied irradiation as a tool to achieve functionalization. Morphological and structural changes on epitaxial graphene layers were investigated by atomic force microscopy, Raman spectroscopy, Raman mapping and reflectance mapping. The surface morphology of irradiated graphene layers showed graphene folding, hillocks, and formation of wrinkles at highest fluence (2×1013ions/cm2). Raman spectra analysis shows that the graphene defect density is increased with increasing fluence, while Raman mapping and reflectance mapping show that there is also a reduction of monolayer graphene coverage. The samples were investigated for ammonia and nitrogen dioxide gas sensing applications. Sensors fabricated on pristine and irradiated samples showed highest gas sensing response at an optimal fluence. Our work provides new pathways for introducing defects in controlled manner in epitaxial graphene, which can be used not only for gas sensing application but also for other applications, such as electrochemical, biosensing, magnetosensing and spintronic applications.

Manipulation of defect density and nitrogen doping on few-layer graphene sheets using the plasma methodology for electrochemical applications

In this study, we successfully manipulate the defect density and thickness (number of layers) of few-layer graphene sheets at low temperatures (≤300°C) using microwave plasma torch (MPT) coupled with plasma-enhanced chemical vapor deposition (PECVD) method. The graphene defects and thickness of 2-10 layers can be controlled by variation in the argon flow rate and pressure during graphene growth step. The quality and thickness of graphene sheets are characterized using Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). The nitrogen plasma has been demonstrated to effectively dope N atoms onto graphene sheets in which the N-doping content varies monotonously from 4.7 to 13.3 at%. The distribution of the N-doping types can be tuned by control of the graphene defect density with the ID/IG ratio increasing gradually from 0.3 to 0.8 and the nitrogen plasma power to generate multiple functionalities of the resultant materials. For example, the medium-quality graphene doped by high-power nitrogen plasma exhibits the highest electrocatalytic activity toward the oxygen reduction reaction (ORR) with a mean electron-transfer number of 3.94 which is comparable with that of platinum. The high-quality graphene doped by low-power nitrogen plasma shows the high activity and selectivity for simultaneously detecting uric acid, ascorbic acid, and dopamine because of the high content of the pyridinic-N structure.