Plasma-treated Graphene Research Articles

Thin and Uniform Atomic Layer Deposited ZrO2 Film on Functionalization Graphene

Graphene has been extensively studied as an outstanding material for flexible electronic devices and wearable biosensors due to its excellent mechanical flexibility, high carrier mobility, high surface to volume ratio and biocompatibility. In particular, for achieving graphene field-effect transistors (GFETs) based sensors of high sensitivity and stability, the deposition of thin and uniform high-k dielectric films on graphene is important. Among the dielectric films deposited through various deposition methods, atomic layer deposited (ALD) dielectric films have advantages for depositing ultra-thin and uniform films at relatively low temperature (<300℃) due to its surface-limited reaction mechanism. However, ALD dielectric films on graphene usually show non-continuous and rough surface morphology owing to inertness of the graphene basal plane. ALD relies on the chemisorption of precursor molecules with the surface functional groups, but strong sp2 carbon bonding of graphene basal plane prevents the facile nucleation of dielectric materials on the graphene surface during the ALD process. Thus, various techniques for functionalization of the graphene surface have been suggested. In this paper, we demonstrate the use of low-cost and scalable atmospheric oxygen plasma treatment to efficiently functionalize the surface of chemical vapor deposited (CVD) graphene with minimal structural and electrical degradation. We show the deposition of ultra-thin (< 10 nm) and uniform high-k dielectric films of ALD ZrO2 on the graphene without any seed layer after graphene functionalization through atmospheric oxygen plasma treatment. Water contact angle measurements and Raman spectroscopy show that graphene surface becomes hydrophilic due to dangling bond-type defects after atmospheric plasma treatment. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) show that ALD ZrO2 films on the plasma-treated graphene are deposited thin, uniform, and polycrystalline at low ALD process temperature of 150℃. It is notable that the plasma treatment affects not only the surface coverage but also the crystallinity of the ALD ZrO2 film on graphene at low temperature. Electrical characterization also reveals that atmospheric plasma treatment generates functional groups in graphene surface by shifting the Dirac voltage (Vdirac) of GFETs to positive voltage. However, after ALD process, the thin yet pin-hole free ALD ZrO2 films on the plasma-treated graphene reduces charge trapping sites in and around graphene, recovering the charge transport and Vdirac (~0V) in GFETs. The passivation effect of ALD is also revealed as shown by a dramatic enhancement in air stability of GFETs. The sensors based on GFETs of Vdirac close to 0V can operate at low voltages, which is beneficial for power limited portable or wearable devices. Consequently, we believe that uniform and crystalline ALD ZrO2 film on the atmospheric plasma treated graphene will find use in flexible graphene electronics and biosensors owing to its low process temperature and capacity to improve device performance and stability.

Layer-Dependent Photoelectrochemical Performance of Multi-Layer Graphene Catalysts on Silicon Photocathode

Development of sustainable energy sources is an urgent issue to meet growing demand in world energy consumption. Photoelectrochemical cells are used to split hydrogen and oxygen from water molecules to generate chemical fuels to satisfy our ever-increasing energy demands. However, it is a major challenge to design efficient catalysts to use in the photoelectrochemical process. Recently, research has focused on carbon-based catalysts, as they are non-precious and environmentally benign. Especially, graphene possesses excellent transmittance and superior intrinsic carrier mobility, thus there have been several attempts to use graphene as a catalyst. We are the first to investigate the use of monolayer graphene as an electrocatalyst for efficient hydrogen evolution reaction (HER), employing a Si substrate as a photocathode. The understanding of the correlated interaction between solid catalysts/electrode and liquid electrolyte during HER operation is also very important for efficient hydrogen production, however, few researches have been conducted before to study the interface between carbon catalysts/semiconductor electrode and aqueous solution. Here, we have explored to develop an enabling technology and design rules for the efficient and durable catalyst/electrode system. We prepared mono-, double-, tri-, and multilayer graphene on p-Si photocathodes and investigated the layer dependence of catalytic activity for HER. Interestingly, double-layer graphene/Si exhibits noticeably improved photon-to-current efficiency and modifies the band structure of the graphene/Si photocathode. Based on in-depth electrochemical and electrical analyses, the band structure of graphene/Si was shown to result in a much lower work function than Si, accelerating the electron-to-hydrogen production potential. Specifically, plasma-treated double-layer graphene exhibited the best performance and the lowest work function. We electrochemically analyzed the mechanism at work in the graphene-assisted photoelectrode. Atomistic calculations based on the density functional theory were also carried out to more fully understand our experimental observations. We believe that investigation of the underlying mechanism in this high-performance electrode is an important contribution to efforts to develop high-efficiency metal-free carbon-based catalysts for photoelectrochemical cell hydrogen production.

Fabrication of sub-nanometer pores on graphene membrane for ion selective transport.

The ability to sieve ions through nanopores with high throughput has significant importance in seawater desalination and other separation applications. In this study, a plasma etching process has been demonstrated to be an efficient way to produce high-density nanopores on graphene membranes with tunable size in the sub-nanometer range. Besides the pore size, the nanopore density is also controllable through adjusting the exposure time of the sample to argon or oxygen plasma. The plasma-treated graphene membranes can selectively transport protons, Na+ and Cl- ions. Density function theory calculations uncover that the sp3 and vacancy-type defects construct different energy barriers for different ions, which allow the defected graphene membrane to selectively transport ions. Our study indicates that oxygen plasma etching can be used as a very convenient and efficient method for fabricating a monolayer filtration graphene membrane with tunable sub-nanometer pores.

Piezoresistive Effect in Plasma-Doping of Graphene Sheet for High-Performance Flexible Pressure Sensing Application.

This paper presents a straightforward plasma treatment modification of graphene with an enhanced piezoresistive effect for the realization of a high-performance pressure sensor. The changes in the graphene in terms of its morphology, structure, chemical composition, and electrical properties after the NH3/Ar plasma treatment were investigated in detail. Through a sufficient plasma treatment condition, our studies demonstrated that plasma-treated graphene sheet exhibits a significant increase in sensitivity by one order of magnitude compared to that of the unmodified graphene sheet. The plasma-doping introduced nitrogen (N) atoms inside the graphene structure and was found to play a significant role in enhancing the pressure sensing performance due to the tunneling behavior from the localized defects. The high sensitivity and good robustness demonstrated by the plasma-treated graphene sensor suggest a promising route for simple, low-cost, and ultrahigh resolution flexible sensors.

Elastic and electronic properties of oxygen plasma-treated graphene sheets from first principles

Mechanical and electronic properties of oxygen plasma-treated graphene sheets are investigated using density functional theory (DFT). Oxygen plasma-treated graphene is modeled using a graphene sheet with adsorbed epoxide functional groups (C–O–C) on its one side. The most stable configurations of such oxidized graphene sheets with different O/C ratios ranging from 12.5% to 50% are then calculated. In the special case of O/C = 50% (fully oxidized surface), both single- and double-sided oxidation cases are considered. The elastic and electronic properties of the energetically most favorable configurations are evaluated under the tensile and compressive loads in harmonic range. For structures with high O/C ratios (O/C [Formula: see text] 25%), the elastic constants (modulus of elasticity and bulk modulus) are significantly smaller than those of graphene while for low O/C ratios (O/C [Formula: see text] 12.5%), these quantities are almost equal to the elastic constants of pristine graphene. We also found that the electronic bandgap of the oxidized sheets is increased under tensile loading.

Controllable Nanotribological Properties of Graphene Nanosheets

Graphene as one type of well-known solid lubricants possesses different nanotribological properties, due to the varied surface and structural characteristics caused by different preparation methods or post-processes. Graphene nanosheets with controllable surface wettability and structural defects were achieved by plasma treatment and thermal reduction. The nanotribological properties of graphene nanosheets were investigated using the calibrated atomic force microscopy. The friction force increases faster and faster with plasma treatment time, which results from the increase of surface wettability and the introduction of structural defects. Short-time plasma treatment increasing friction force is due to the enhancement of surface hydrophilicity. Longer-time plasma treatment increasing friction force can attribute to the combined effects of the enhanced surface hydrophilicity and the generated structural defects. The structural defects as a single factor also increase the friction force when the surface properties are unified by thermal reduction. The surface wettability and the nanotribological properties of plasma-treated graphene nanosheets can recover to its initial level over time. An improved spring model was proposed to elaborate the effects of surface wettability and structural defects on nanotribological properties at the atomic-scale.

Double-Layer Graphene Outperforming Monolayer as Catalyst on Silicon Photocathode for Hydrogen Production.

Photoelectrochemical cells are used to split hydrogen and oxygen from water molecules to generate chemical fuels to satisfy our ever-increasing energy demands. However, it is a major challenge to design efficient catalysts to use in the photoelectochemical process. Recently, research has focused on carbon-based catalysts, as they are nonprecious and environmentally benign. Interesting advances have also been made in controlling nanostructure interfaces and in introducing new materials as catalysts in the photoelectrochemical cell. However, these catalysts have as yet unresolved issues involving kinetics and light-transmittance. In this work, we introduce high-transmittance graphene onto a planar p-Si photocathode to produce a hydrogen evolution reaction to dramatically enhance photon-to-current efficiency. Interestingly, double-layer graphene/Si exhibits noticeably improved photon-to-current efficiency and modifies the band structure of the graphene/Si photocathode. On the basis of in-depth electrochemical and electrical analyses, the band structure of graphene/Si was shown to result in a much lower work function than Si, accelerating the electron-to-hydrogen production potential. Specifically, plasma-treated double-layer graphene exhibited the best performance and the lowest work function. We electrochemically analyzed the mechanism at work in the graphene-assisted photoelectrode. Atomistic calculations based on the density functional theory were also carried out to more fully understand our experimental observations. We believe that investigation of the underlying mechanism in this high-performance electrode is an important contribution to efforts to develop high-efficiency metal-free carbon-based catalysts for photoelectrochemical cell hydrogen production.

Beyond defect formation: Spectroscopic characterization of plasma-induced structural and electronic transformations in graphene

Recent research suggests plasma-induced hydrogenation is an efficient method for inducing a band-gap in graphene. To date, the characterization of plasma treatment-induced chemical changes is performed almost exclusively by Raman spectroscopy with the extent of hydrogenation presented as the evolution defect structures in the sp2 lattice of graphene. Alarmingly, almost no attention is given to the concurrent electronic modification. Here, x-ray induced Auger emission spectroscopy is utilized to better understand the effect of plasma treatment on the electronic properties of graphene beyond the formation of defects as determined by Raman spectroscopy. The results indicate the fine structure of the CKLL emission offers a suitable complement to Raman spectroscopy in assessing the extent of chemical and electronic changes induced by H2 plasma treatments. Significant changes to the D-value, defined as the distance between local maxima and minima in the CKLL Auger emission, are observed after only 30 s of treatment (p &amp;lt; 0.001), while the ID/IG ratio remains statistically equivalent (p = 0.441). The results indicate significant differences in the electronic properties of plasma-treated graphene are observed concomitant to sp2 defect structures normally attributed to hydrogenation.

Manipulating fluorescence quenching efficiency of graphene by defect engineering

We report on the manipulation of the fluorescence quenching of Rhodamine 6G (R6G) on graphene by defect engineering via hydrogen and Ar+ plasma treatments. The amount and nature of defects in graphene were estimated on the basis of the Raman intensity ratios I(D)/I(G) and I(D)/I(D′) of graphene. Results showed that the quenching factor (QF) gradually decreases from ∼40 to ∼4 and ∼12 for hydrogenated graphene (sp3 defects) and Ar+-plasma-treated graphene (vacancy-like defects), respectively, with different amounts of defects. Our results indicated that the fluorescence quenching efficiency of graphene is strongly dependent on the amount and nature of defects.

Functionalization of graphene by atmospheric pressure plasma jet in air or H2O2 environments

The functionalization of graphene, which deforms its band structure, can result in a metal-semiconductor transition. In this work, we report a facile strategy to oxidize single-layer graphene using an atmospheric pressure plasma jet (APPJ) that generates a variety of reactive plasma species at close to ambient temperature. We systematically characterized the oxygen content and chemical structure of the graphene films after plasma treatment under different oxidative conditions (ambient air atmosphere or hydrogen peroxide solution) by X-ray Photoelectron Spectroscopy (XPS). Plasma-treated graphene films containing more than 40% oxygen were obtained in both oxidative environments. Interestingly, prolonged irradiation led to the reduction of graphene oxides. N-doping of graphene also occurred during the APPJ treatment in H2O2 solution; the nitrogen content of the doped graphene was dependent on the duration of irradiation and reached up to 8.1% within 40min. Moreover, the H2O2 solution served as a buffer layer that prevented damage to the graphene during plasma irradiation. Four-point probe measurement revealed an increase in sheet resistance of the plasma-treated graphene, indicating the transition of the material property from semi-metallic to semiconducting.

The electrochemical 4-chlorophenol sensing properties of a plasma-treated multilayer graphene modified photolithography patterned platinum electrode

The present work describes the electrochemical 4-chlorophenol (4-CP) sensing properties of oxygen plasma treated multilayer graphene samples (MLG).

Temperature dependence of interface barrier height change and field emission studies of plasma-treated graphene films

The field emission properties of plasma-treated graphene films (PTGFs) have been investigated from 300 to 50 K. It is found that the turn-on field increases from 4 to 6.2 V/µm, and the current density decreases with decreasing temperature. On the basis of the semiconductor thermionic emission theory, the changes in field emission properties at low temperature can be successfully explained. The increase in interface barrier height at low temperature results in the changes in the field emission properties of PTGFs.

Adhesion and differentiation of human mesenchymal stem cells on plasma-functionalized graphenes with different feeding gases

Surface functionalisation of graphene layer has been investigated to improve the efficiencies in bio-applications such as stem cell differentiation or protein sensing. Plasma treatment can induce various functionalities on graphene surface simply by changing feeding gas. In this study, the surface chemistry of graphene is modified by plasma treatment using three different feeding gases of H2, N2, and O2. The effects of differential functionalisation are examined by the adsorption of fibronectin and the adhesion, proliferation, and differentiation of human mesenchymal stem cells. Fibronection adsorption is enhanced on the N2 plasma-treated graphene, but it is not clear on the others. hMSCs adhesion is broadened on all of the plasma-treated graphene substrates, especially on the N2 plasma-treated one. H2 plasma treatment enhances cell proliferation and osteogenic differentiation, and N2 and O2 plasma treatments show a tendency of heterogeneous differentiation to osteoblasts and myoblasts. Different responses of stem cell adhesion, proliferation, and differentiation depending on the plasma-treated graphene surface indicate the importance of highly specific surface functionality in stem cell engineering.

Controlling protein adsorption on graphene for cryo-EM using low-energy hydrogen plasmas.

Despite its many favorable properties as a sample support for biological electron microscopy, graphene is not widely used because its hydrophobicity precludes reliable protein deposition. We describe a method to modify graphene using a low-energy hydrogen plasma, which reduces hydrophobicity without degrading the graphene lattice. We show that the use of plasma-treated graphene enables better control of protein distribution in ice for electron cryo-microscopy and improved image quality by reducing radiation-induced sample motion.

Graphene의 플라즈마 표면 개질과 박테리아 셀룰로오스와의 결합성 검토

The study was focused to evaluate the possibility for combination membrane of bacterial cellulose (BC) and graphene with high electrical properties. BC with natural polymer matrix was known to have strong physical strength. For the combination of graphene with BC, the surface of graphene was modified with oxygen plasma by changing strength and time of radio waves in room temperature. Water contact angle of modified graphene grew smaller from to . XPS analysis showed that oxygen content after treatment increased from 2.99 to 10.98%. Damage degree of graphene was examined from ratio of Raman analysis. ratio of non-treated graphene (NTG) was 0.11, and 0.36 to 0.43 in plasma treated graphene (PTG), increasing structural defects of PTG. XRD analysis of PTG membrane with BC was same to BC only, indicating chemically combined membrane. In FT-IR analysis, 1,000 to 1,300 (C

Terahertz and optical study of monolayer graphene processed by plasma oxidation

We report on our terahertz and optical study of monolayer graphene grown by chemical vapor deposition and processed by plasma oxidation. The plasma oxidation induces oxygen-related defects, and the resulting disorder increases the sheet resistance of graphene as measured via terahertz spectroscopy. The excitonic absorption peak weakens considerably and blue shifts upon plasma oxidation, resulting in higher transmittance in both the visible and ultraviolet regions. Our oxygen plasma-treated graphene also exhibits a free-carrier doping effect as confirmed by the blue shift of the Raman G band.

Ultrasensitive Flexible Graphene Based Field-Effect Transistor (FET)-Type Bioelectronic Nose

Rapid and precise discrimination of various odorants is vital to fabricating enhanced sensing devices in the fields of disease diagnostics, food safety, and environmental monitoring. Here, we demonstrate an ultrasensitive and flexible field-effect transistor (FET) olfactory system, namely, a bioelectronic nose (B-nose), based on plasma-treated bilayer graphene conjugated with an olfactory receptor. The stable p- and n-type behaviors from modified bilayer graphene (MBLG) took place after controlled oxygen and ammonia plasma treatments. It was integrated with human olfactory receptors 2AG1 (hOR2AG1: OR), leading to the formation of the liquid-ion gated FET-type platform. ORs bind to the particular odorant amyl butyrate (AB), and their interactions are specific and selective. The B-noses behave as flexible and transparent sensing devices and can recognize a target odorant with single-carbon-atom resolution. The B-noses are ultrasensitive and highly selective toward AB. The minimum detection limit (MDL) is as low as 0.04 fM (10(-15); signal-to-noise: 4.2), and the equilibrium constants of OR-oxygen plasma-treated graphene (OR-OG) and ammonia plasma-treated graphene (-NG) are ca. 3.44 × 10(14) and 1.47 × 10(14) M(-1), respectively. Additionally, the B-noses have long-term stability and excellent mechanical bending durability in flexible systems.

Thinning and functionalization of few-layer graphene sheets by CF4 plasma treatment

Structural changes of few-layer graphene sheets induced by CF4 plasma treatment are studied by optical microscopy and Raman spectroscopy, together with theoretical simulation. Experimental results suggest a thickness reduction of few-layer graphene sheets subjected to prolonged CF4 plasma treatment while plasma treatment with short time only leads to fluorine functionalization on the surface layer by formation of covalent bonds. Raman spectra reveal an increase in disorder by physical disruption of the graphene lattice as well as functionalization during the plasma treatment. The F/CF3 adsorption and the lattice distortion produced are proved by theoretical simulation using density functional theory, which also predicts p-type doping and Dirac cone splitting in CF4 plasma-treated graphene sheets that may have potential in future graphene-based micro/nanodevices.PACS81.05.ue; 73.22.Pr; 52.40.Hf.

Photoelectron spectroscopy studies of plasma-fluorinated epitaxial graphene

Fluorination of graphene has emerged as an attractive approach toward manipulating its physical, chemical, and electronic properties. To this end, we have demonstrated the viability of sulfur hexafluoride plasmas to fluorinate graphene as a safer alternative to the commonly reported techniques of fluorination that include exposures to fluorine and xenon difluoride gas. Incorporation of fluorine moieties on graphene after SF6 plasma-treatment was confirmed by x-ray photoelectron spectroscopy. Modifications in the valence band states of graphene after plasma-treatment were characterized by ultraviolet photoelectron spectroscopy. Increase in work function of plasma-treated graphene demonstrates the ability of plasma-assisted fluorination to modify the electron emission characteristics of graphene. Raman spectroscopy reveals that the majority of carbon atoms in graphene retain their sp2 hybridization after the plasma-treatment.

Transition from Metallic to Semiconducting Behavior in Oxygen Plasma-treated Single-layer Graphene

ABSTRACTWe investigate the structural, optical and electrical properties of single-layer graphene exposed to oxygen plasma treatment. We find that the pristine semimetallic behavior of graphene disappears upon plasma treatment, in favour of the opening of a bandgap and the featuring of semiconducting properties. The metal-to-semiconductor transition observed appears to be dependent on the plasma treatment time. The semiconducting behavior is also confirmed by photoluminescence measurements. The opening of a bandgap in graphene is explained in terms of graphene surface functionalization with oxygen atoms, bonded as epoxy groups. Ab initio calculations of the density of states show more details about the oxygen–graphene interaction and its effects on the graphene optoelectronic properties, predicting no states near the Fermi level at increasing epoxy group density. The structural changes are also monitored by Raman spectroscopy, showing the progressive evolution of the sp2 character of pristine graphene to sp3, due to the lattice decoration with out-of-plane epoxy groups.