Free-standing Graphene Research Articles

Phonon anharmonicities in 7-armchair graphene nanoribbons

Graphene nanoribbons (GNRs) provide a potential alternative to overcome the shortcoming of gapless graphene and have been applied into next-generation nanoelectronic devices like FETs. The phonon anharmonicities in functional materials usually play a key role in the device performance. Here, temperature dependent Raman scattering measurements on high quality 7-armchair GNRs (7-AGNRs) on Au(111) synthesized by on-surface method were conducted in the temperature range from 80 to 520 K. The frequency of optical phonon G mode (collective acoustic phonon RBLM mode) linearly (nonlinearly) downshifts with temperature. The first-order temperature coefficient of G mode of 7-AGNR is calculated to be −0.026 cm−1K−1, almost twice that of freestanding graphene of −0.015 cm−1K−1 and half of that of CVD grown single layer graphene on Cu foils of −0.056 cm−1K−1. For RBLM, the anharmonicity is much weaker and attributed to the intrinsic quartic-phonon decay processes rather than the cubic-phonon ones. Our findings make a first step to the thermal properties of atomically precise GNRs.

Manufacturing Free-Standing Graphene Oxide/Carbon Nanotube Hybrid Papers and Improving Electrical Conductivity by a Mild Annealing Treatment

The present work aims to evaluate the manufacturing processes of free-standing graphene oxide (GO)/reduced graphene oxide(rGO)/carbon nanotube (MWCNT) papers-like and the effect on electrical conductivities after annealing treatment. The pure GO and the hybrid papers were obtained by vacuum filtration and submitted to mild thermal annealing at 400 °C under an argon atmosphere. Analyses comparing the carbon paper-like materials before and after thermal treatment were done by thermogravimetric analysis (TGA), Raman spectroscopy, scanning electron microscopy (SEM), and electrical conductivity measurements. The characterization results show the annealing process effect on the morphology of paper materials. The GO papers and the hybrid (GO/rGO/MWCNT) exhibited good free-standing structure properties. The SEM results suggest the formation of gas void due to the thermal treatment. TGA and Raman results showed the thermal reduction and restoration of the carbon sp2 network of the graphene oxide induced by thermal treatment. The thermal treatment was responsible for changing the almost insulator characteristic of free-standing pure GO paper, achieving an electrical conductivity of 752 S/m, and improving 45% of the free-standing hybrid carbon nanomaterials papers (GO/rGO/MWCNT) electrical conductivity, achieving the value of 1020 S/m.

Indirect Measurement of the Carbon Adatom Migration Barrier on Graphene

Although surface diffusion is critical for many physical and chemical processes, including the epitaxial growth of crystals and heterogeneous catalysis, it is particularly challenging to directly study. Here, we estimate the carbon adatom migration barrier on freestanding monolayer graphene by quantifying its temperature-dependent electron knock-on damage. Due to the fast healing of vacancies by diffusing adatoms, the damage rate decreases with increasing temperature. By analyzing the observed damage rates at 300-1073 K using a model describing our finite scanning probe, we find a barrier of (0.33 \pm 0.03) eV.

Superhydrophilic edge-rich graphene for the simultaneous and disposable sensing of dopamine, ascorbic acid, and uric acid.

A simple and rapid simultaneous sensing strategy of multiple biomarkers is of great importance but challenging in health diagnosis. In this study, a novel free-standing edge-rich graphene film (fs-ERG) was in situ fabricated via a facile chemical vapor deposition route on a porous Si3N4 substrate. The subsequent superhydrophilic modification of the fs-ERG not only makes it maintain the original abundant edge-rich sites, high conductivity, and hierarchical porosity, but also endows it with collective electrochemical characteristics. Thereafter, the superhydrophilic fs-ERG (S-fs-ERG) demonstrated a fast electron-transfer kinetics towards the oxidation of dopamine (DA), ascorbic acid (AA), and uric acid (UA), which promised a sensitive simultaneous electrochemical determination with low detectable limits of 0.1, 2.5 and 0.5 μM, respectively. Furthermore, this sensing electrode displayed high selectivity in the presence of co-existing interferences as well as excellent reproducibility, and thus performed well in DA, AA and UA detection in real samples. These superior sensing performance metrics combined with the low-cost and scalable fabrication of S-fs-ERG based electrodes bode well for their great potential for the simultaneous and disposable sensing of DA, AA and UA in practical application.

Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors.

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO2 gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm−3). Using the EMI-BF4 electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g−1 (delivering a gravimetric energy density of 85.6 W h kg−1), a volumetric capacitance of 94 F cm−3 (delivering a volumetric energy density of 44.7 W h L−1), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF4/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g−1 with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

Free-standing graphene oxide membrane works in tandem with confined interfacial polymerization of polyamides towards excellent desalination and chlorine tolerance performance.

We explored a unique concept in this study to develop a membrane containing a hierarchical porous architecture derived by etching a specific component from a demixed UCST blend as the support layer and a free-standing GO and a polyamide (PA) layer as functional surfaces. To selectively sieve ions and improve chlorine tolerance performance, three different strategies were proposed here. In the first case, the free-standing GO membrane was used as the active layer. In the second case, the free-standing GO was positioned in tandem with the PA layer formed in situ. In the third case, GO was added during the formation of the active PA layer in situ. The support layer with a gradient in pore sizes (realized by varying the composition in the blends) was fabricated via crystallization induced phase separation in a classical UCST system (PVDF/PMMA) and etching out the amorphous component (here PMMA). A gradient in the pore sizes was obtained by rationally stitching the various membranes obtained by varying the blends' composition. Pure water flux and rejection experiments were carried out to evaluate the performance of this composite membrane. This unique strategy resulted in excellent salt rejection (more than 95% for a monovalent ion), improved fouling resistance (more than 85%), excellent dye removal performance (more than 96% for a cationic dye), and outstanding chlorine tolerance performance and antibacterial activity. Thus, this study emphasizes that the free-standing GO membrane's positioning controls the membranes' overall performance.

Atomic Co decorated free-standing graphene electrode assembly for efficient hydrogen peroxide production in acid

A novel oxygen reduction reaction (ORR) electrode comprising isolated Co atom decorated vertically aligned graphene nanosheets is designed, which can enable the most energy-efficient, rapid acidic H2O2 production in a flow-cell reactor.

Evidence of synergistic electrocatalysis at a cobalt oxide–graphene interface through nanochemical mapping of scanning transmission X-ray microscopy

Free-standing graphene membranes are a promising candidate for use as in-situ environmental windows in X-ray (electron) microscopy. In this study, graphene membranes were used as the working electrode, and cobalt nanoparticles (Co-NPs) were grown directly on top of the graphene through electrochemical deposition for interfacial variation. The electronic structure and the chemical bonding states of the Co-NPs and the graphene materials were examined by using the high spatial resolution and element-specific properties of scanning transmission X-ray microscopy. X-ray absorption spectra of C, O, and Co revealed that the Co-NP size increased in accordance with the oxidation state (Co0/2+/3+), depending on the configuration of carbon bonding (C–C/C–OH/HO–C=O/O–C(O)–O/C=O–like state). We conducted a spectral comparison of the dipped graphene and the electrodeposited Co–graphene sample, which revealed an increase in C–OH formation before Co-NPs growth. In addition to electron transfer and electrochemical reduction, the oxidation evolution from C–OH to HO–C=O (or a defect) and the O–C(O)–O (or C=O) state paralleled the increase in Co-NPs size. We curve-fitted the results to demonstrate the reduction in chemical structure from mixing Co2+/3+ to Co3+/2+/0, and to explain the interfacial modulation and the unique metal Co0 layer on the surface of the Co-NPs. Our results provide information for the design of a reliable graphene window and offer an example for the interpretation of experimental X-ray (electron) microscopy.

Freestanding Graphene Fabric Film for Flexible Infrared Camouflage.

Graphene films, fabricated by chemical vapor deposition (CVD) method, have exhibited superiorities in high crystallinity, thickness controllability, and large‐scale uniformity. However, most synthesized graphene films are substrate‐dependent, and usually fragile for practical application. Herein, a freestanding graphene film is prepared based on the CVD route. By using the etchable fabric substrate, a large‐scale papyraceous freestanding graphene fabric film (FS‐GFF) is obtained. The electrical conductivity of FS‐GFF can be modulated from 50 to 2800 Ω sq−1 by tailoring the graphene layer thickness. Moreover, the FS‐GFF can be further attached to various shaped objects by a simple rewetting manipulation with negligible changes of electric conductivity. Based on the advanced fabric structure, excellent electrical property, and high infrared emissivity, the FS‐GFF is thus assembled into a flexible device with tunable infrared emissivity, which can achieve the adaptive camouflage ability in complicated backgrounds. This work provides an infusive insight into the fabrication of large‐scale freestanding graphene fabric films, while promoting the exploration on the flexible infrared camouflage textiles.

Facile preparation of graphene film and sandwiched flexible poly(arylene ether nitrile)/graphene composite films with high EMI shielding efficiency

From the industrial grade graphene nanosheet (GNS) powder, the flexible, ultra-thin and freestanding graphene films are prepared using ultrasonic-assisted dispersion and suction filtration. The sandwiched composites films are prepared by layer-by-layer assembly using poly(arylene ether nitrile) (PEN) as polymer matrix. The strong Π···Π interaction pulls the graphene sheets together in a face-to-face way and they assembly into flexible graphene films. High electromagnetic interference shielding effectiveness (EMI SE) is shown due to multiple internal reflection and absorption inside the porous graphene films. A 3 μm thick graphene film shows EMI SE over 20 dB, specific SE (SE/density/thickness, SSE/t) of 72967 dB·cm2/g and absorption ratio over 92%. Moreover, the highest EMI SE of 77 dB is reached by sandwiching 3-layers of graphene films with PEN, and the composites films exhibit high flexibility, high tensile strength over 40 MPa and glass transition temperature of 185 °C.

Freestanding multi-gate IZO-based neuromorphic transistors on composite electrolyte membranes

Brain-inspired neuromorphic computing would bring a breakthrough to the classical computing paradigm through its massive parallelism and potential low power consumption advantages. Introduction of flexibility may bring vitality to this area by expanding its application areas to such as wearable and implantable electronics. At present, the development of flexible neuromorphic devices makes it a choice with wide prospect for next-generation wearable artificial neuromorphic computing. In this study, a freestanding graphene oxide/polyvinyl alcohol composite solid electrolyte membrane is utilized as the gate dielectric and support material, and indium-zinc-oxide (IZO) neuromorphic transistors are fabricated on such membrane. Based on the in-plane gate modulation, many key synaptic plasticity behaviors have been successfully emulated, including excitatory postsynaptic current, paired-pulse facilitation, high-pass filtering, and spatiotemporal signal processing. Moreover, transition of the spiking logic and the superlinear and sublinear dendritic integration function are realized. Our results indicate that these freestanding IZO-based neuromorphic transistors may of great significance for future flexible anthropomorphic robots, wearable bionic perception.

Properties of free-standing graphene oxide/silver nanowires films and effects of chemical reduction and gamma irradiation

The need for stable, chemical resistant and conductive materials is on the rise in recent times. Graphene shows promising electrical conductivity and chemical stability, but the production of a continual, conductive layer is limited and expensive. Silver nanowires (AgNWs) are both conducive and economically viable, but they are sensitive to water and oxygen. In this study, graphene oxide (GO) and AgNWs were synthesized and combined in different mass ratios (3:1, 2.5:1.5, and 1:1) to obtain chemically stable composites with improved electrical properties. Composites were reduced using ascorbic acid. With the increase of AgNWs to GO mass ratio, the surface of the free-standing composite film improved: the root mean square roughness was lowered from 376 nm for GO to 168 nm for the composite with the mass ratio of GO:AgNWs 1:1, while the sheet resistance was lowered from 146 × 106 Ω/□ to 4 Ω/□. For the first time, the effects of gamma irradiation on the structure of the composites were studied. Doses of 15, 25, and 35 kGy were applied.

Graphene paper with electrodeposited NiCo2S4 nanoparticles as a novel flexible sensor for simultaneous detection of folic acid and ascorbic acid

We studied the production and electrochemical performance of NiCo2S4 electrodeposited reduced graphene oxide (rGO) paper for use as a freestanding flexible electrode for the simultaneous determination of folic acid (FA) and ascorbic acid (AA). Production of rGO paper was achieved through vacuum filtration of the aqueous dispersion of graphene oxide, followed by chemical reduction in hydrogen iodide solution. NiCo2S4/rGO paper material was fabricated with a simple approach using electrodeposition of hydrothermal synthesized NiCo2S4 nanoparticles directly on rGO paper. The physicochemical properties of as-prepared flexible NiCo2S4/rGO paper were characterized by means of field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray powder diffraction patterns. This freestanding paper electrode demonstrated high sensitivity, a wide linear range, and a low detection limit towards the simultaneous detection of FA and AA. NiCo2S4/rGO paper sensor, possessing easy preparation, low cost, and high flexibility, was tested in real samples with promising recoveries. This study exhibited that the approach of the electrodeposition of bimetallic sulfides on freestanding graphene paper substrates has great potential to develop high-performance flexible electrochemical sensors.

In-Situ Transmission Electron Microscopy Observation of Germanium Growth on Freestanding Graphene: Unfolding Mechanism of 3D Crystal Growth During Van der Waals Epitaxy.

Breakthroughs in cutting-edge research fields such as hetero-integration of materials and the development of quantum devices are heavily bound to the control of misfit strain during heteroepitaxy. While remote epitaxy offers one of the most intriguing avenues, demonstrations of functional hybrid heterostructures are hardly possible without a deep understanding of the nucleation and growth kinetics of 3D crystals on graphene and their mutual interactions. Here, the kinetics of such processes from real-time observations of germanium (Ge) growth on freestanding single layer graphene (SLG) using in-situ transmission electron microscopy are unraveled. This powerful technique provides a unique opportunity to observe new and yet unexplored phenomena, which are not accessible to the standard ex situ characterizations. Through direct observations, remote interactions are elucidated between Ge crystals through the graphene layer in double heterostructures of Ge/graphene/Ge. Notably, the data show real-time evidence of vertical Ge atoms diffusion through the graphene layer. This phenomenon is attributed to the remote interactions of Ge atoms through the graphene lattice, due to its interatomic interaction transparency. Additionally, key mechanisms governing nucleation and initial growth in graphene were systematically determined. These findings enlighten the growth mechanism of graphene and provide a new pathway for disruptive hybrid semiconductor-graphene devices.

Free-standing graphene aerogel with improved through-plane thermal conductivity after being annealed at high temperature

Both high through-plane thermal conductivity and low elastic modulus can reduce thermal interface resistance, which is important for thermal interface materials. The internal porous structure of graphene aerogel (GA) makes it to have a low elastic modulus, which results in its good compressibility. Also, the network structure of GA provides thermal conducting paths, which improve the through-plane thermal conductivity of GA. Annealing GA at 3000 °C helps to remove oxygen-containing functional groups and reduces defects. This greatly improves its crystallinity, which further leads to the improvement of its through-plane thermal conductivity and it has a low modulus of 1.37Mpa. The through-plane thermal conductivity of GA annealed at 3000 °C (GA-3000) was improved as the pressure increased and got to 2.93 W/ m K at a pressure of 1.13 MPa, which is 30 times higher than other graphene-based thermal interface materials (TIMs). These discoveries offer a novel approach for preparing excellent TIMs.

(Invited) Plasma Produced and Processed Multimaterials Based on Vertically Aligned Graphene Walls and Their Applications

The interest in new carbonaceous materials with large effective surfaces which are in addition highly conductive and stable is growing due to the downsizing of electrical devices and the demand for new low-cost materials. Carbonaceous materials show in in particular a great potential for applications in electrochemical devices, energy storage devices or biosensors. On the other hand, there is also an emerging need for new production methods which allow an environmentally friendly, low cost and scalable synthesis of these materials.Although there exists no miracle or unicorn method meeting all these demands, one special place in this race for new technologies are novel low temperature/cold plasma processes, which guarantee the synthesis and treatments of materials in dry processes which can go down to almost ambient temperatures.In this work, we present plasma synthesised and functionalized carbon-based nanomaterials, alone or enrobed in polymers, and their analysis. Moreover, the underlying production process will be analysed both experimentally and by means of simulations. The carbon nanomaterials are all produced by low temperature plasma procedures, and examples are given for different processes including catalyst-free or catalyst driven processes. All these processes result often in conductive nanostructures, like free-standing graphene, vertically aligned graphene nanosheets, thin films or nanoparticles. These materials are either pure carbons, hydrocarbons or nitrogen-containing carbons. In this presentation we will concentrate on nitrogen free and nitrogen containing vertically aligned graphene. Nitrogen introduction is obtained here either by the growth of the carbonaceous materials in a nitrogen-containing atmosphere or by means of plasma post-treatments. These post-treatment processes can lead to both doped and functionalized materials. A popular technique to simulate plasma-surface interactions is molecular dynamics (MD). MD simulations can provide insight into fundamental mechanisms like the formation of different kinds of bonds.The plasma produced graphene stuctures can be, (depending on the applications), additionally decorated with conductive nanoparticles or thin films. One example concerns the conformal coating or enrobing with plasma polymers, as for example plasmapolyaniline. The interplay between doping, functionalization together with enrobing/decoration is advantageous and results in different surface characteristics of interest which can be useful e.g. for the synthesis of storage devices.

Synthesis of freestanding few-layer graphene in microwave plasma: The role of oxygen

We systematically studied the role of oxygen in gas-phase synthesis of graphene in atmospheric hydrocarbon-fed microwave plasmas. Oxygen is introduced through the use of alcohols, and mixtures of ethylene and water. These reactants were contrasted with oxygen-free hydrocarbon reactants, including ethylene and toluene. Solid materials were collected at the plasma reactor exit and characterized. Gas-phase temperature and key species concentrations were measured using in situ Fourier-transform infrared absorption and emission spectroscopy inside the reactors. Ethanol resulted in pure few-layer graphene formation, in agreement with previous studies. In contrast, ethylene fed at the same flow rate produced a mixture of carbon allotropes. A shift towards graphene formation is observed when water is added to ethylene, or when the flow rate of ethylene is cut to half. Simulations suggest that reactants undergo rapid chemical reactions in the plasma front and the mixture composition in and immediately after the plasma is in chemical equilibrium. The primary factor that controls graphene growth appears to be the total amount of carbon available in the growth region. Oxygen, through CO formation, modulates the amount of acetylene and other growth species, while other factors require further study.

Microwave plasma-based high temperature dehydrogenation of hydrocarbons and alcohols as a single route to highly efficient gas phase synthesis of freestanding graphene

Understanding underlying processes behind the simple and easily scalable graphene synthesis methods enables their large-scale deployment in the emerging energy storage and printable device applications. Microwave plasma decomposition of organic precursors forms a high-temperature environment, above 3000 K, where the process of catalyst-free dehydrogenation and consequent formation of C2 molecules leads to nucleation and growth of high-quality few-layer graphene (FLG). In this work, we show experimental evidence that a high-temperature environment with a gas mixture of H2 and acetylene, C2H2, leads to a transition from amorphous to highly crystalline material proving the suggested dehydrogenation mechanism. The overall conversion efficiency of carbon to FLG reached up to 47%, three times as much as for methane or ethanol, and increased with increasing microwave power (i.e. with the size of the high-temperature zone) and hydrocarbon flow rate. The yield decreased with decreasing C:H ratio while the best quality FLG (low D/G–0.5 and high 2D/G–1.5 Raman band ratio) was achieved for C:H ratio of 1:3. The structures contained less than 1 at% of oxygen. No additional hydrogen was necessary for the synthesis of FLG from higher alcohols having the same stoichiometry, 1-propanol and isopropanol, but the yield was lower, 15%, and dependent on the atom arrangement of the precursor. The prepared FLG nanopowder was analyzed by scanning electron microscopy, Raman, x-ray photoelectron spectroscopy, and thermogravimetry. Microwave plasma was monitored by optical emission spectroscopy.

Structural stability and electron‐phonon coupling in two‐dimensional carbon allotropes at high electronic and atomic temperatures

Several two-dimensional carbon allotropes have been recently fabricated experimentally, triggering various innovative research and technological applications. The properties of such 2d materials under extreme conditions are to a large extent unknown. In this work, we study theoretically the effects of high electronic and atomic temperatures on free-standing graphene, pentaheptite, and biphenylene network. We analyse the limits of their structural stability with respect to thermal and nonthermal damage. With a state-of-the-art approach, we calculate the electronic heat capacity and the electron-phonon coupling parameter dependent on both electronic and atomic temperatures. The knowledge of these parameters may facilitate modelling of various two-dimensional carbon allotropes under laser pulse irradiation, improving understanding of their behaviour in experimental applications.

Frank-van der Merwe growth in bilayer graphene

Frank-van der Merwe growth in bilayer graphene