Functionalized CVD monolayer graphene for label-free impedimetric biosensing

Recent advances in large area graphene growth have led to many applications in different areas. In the present study, chemical vapor deposited （CVD） monolayer graphene supported on glass substrate electrochemical biosensing applications was examined as electrode material for We report a facile strategy for covalent functionalization of CVD monolayer graphene by electrochemical reduction of carboxyphenyl diazonium salt prepared in situ in acidic aqueous solution. The carboxyphenyl-modified graphene is characterized using Raman spectroscopy, X-ray photoelectron spectroscopy （XPS）, and atomic force microscopy （AFM）, as well as electrochemical impedance spectroscopy （hIS）. We also show that the number of grafted carboxyphenyl groups on the graphene surface can be controlled by the number of cyclic voltammetry （CV） scans used for electrografting. We further present the fabrication and characterization of an immunosensor based on immobilization of ovalbumin antibody on the graphene surface after the activation of the grafted carboxylic groups via EDC/NHS chemistry. The binding between the surface-immobilized antibodies and ovalbumin was then monitored using Faradaic EIS in [Fe（CN）6]^3-/4- solution. The percentage change of charge transfer resistance （Rct） after binding exhibited a linear dependence for ovalbumin concentrations ranging from 1.0 pg·mL^-1 to 100 ng·mL^-1, with a detection limit of 0.9 pg·mL^-1. Our results indicate good sensitivity of the developed functionalized CVD graphene platform, paving the way for using CVD monolayer graphene in a variety of electrochemical biosensing devices.

Investigation of multilayer domains in large-scale CVD monolayer graphene by optical imaging

CVD graphene is a promising candidate for optoelectronic applications due to its high quality and high yield.However,multi-layer domains could inevitably form at the nucleation centers during the growth.Here,we propose an optical imaging technique to precisely identify the multilayer domains and also the ratio of their coverage in large-scale CVD monolayer graphene.We have also shown that the stacking disorder in twisted bilayer graphene as well as the impurities on the graphene surface could be distinguished by optical imaging.Finally,we investigated the effects of bilayer domains on the optical and electrical properties of CVD graphene,and found that the carrier mobility of CVD graphene is seriously limited by scattering from bilayer domains.Our results could be useful for guiding future optoelectronic applications of large-scale CVD graphene.

Effects of substrate and transfer on CVD-grown graphene over sapphire-induced Cu films

We differentiated the effects of Cu films deposited on single crystalline a-,r-,and c-plane sapphire substrates upon graphene films synthesized with atmospheric pressure chemical vapor deposition(CVD).The data illustrate that the realization of high-crystalline Cu film is dependent not only on the crystallinity of underlying substrate,but also on the symmetric match of crystallographic geometry between metal film and substrate.We also systematically investigated the effects of PMMA removal on the Raman ID/IG and IG/I2D values of transferred graphene.The results reveal that different PMMA removal methods do not alter the ID/IG values;instead,the residue of PMMA increases the IG/I2D values and the thermal decomposition of PMMA leads to higher IG/I2D values than the removal of PMMA with acetone.The effects of PMMA removal on variations of the Raman spectra are also discussed.

Kinetics of defect formation in chemically vapor deposited （CVD） graphene during laser irradiation： The case of Raman investigation

The effect of laser irradiation on chemically vapor deposited （CVD） graphene was studied by analyzing the temporal evolution of Raman spectra acquired under various illumination conditions. The spectra showed that the normalized intensity of the defect-related peak increases with the square root of the exposure time and varies almost linearly with the laser power density. Furthermore, the hardness of graphene to radiation damage depends on its intrinsic structural quality. The results suggest that, contrary to the common belief, micro-Raman spectroscopy cannot be considered a noninvasive tool for the characterization of graphene. The experimental observations are compatible with a model that we derived from the interpretative approach of the Staebler-Wronski effect in hydrogenated amorphous silicon; this approach assumes that the recombination of photoexcited carriers induces the breaking of weak C-C bonds.

Ultraclean transfer of graphene by mechanically exfoliating polymer with modified crosslink density

The transfer of graphene from metallic substrates onto application-specific substrates is usually inevitable for the applications of high-quality graphene films derived from chemical vapour deposition(CVD)approaches.Commonly used to support the graphene films during the transfer,the coating of the polymer would produce the surface contaminations and hinder the industrially compatible transfer.In this work,through the thermal imidization of polyamide acid(PAA)to polyimide(PI)and tuning of the concentration of dangling chains,we achieved the ultraclean and crack-free transfer of graphene wafers with high electronic quality.The resulting contamination-free and hydrophilic surface also enabled the observed improved cell viability in a biomedical applications.By avoiding aqueous etching or the usage of strong bases,our proposed transfer method is industrially compatible for batch transfer of graphene films towards the real applications.

Graphene coatings for corrosion resistance of nickel and copper in acidic,alkaline and neutral environments

Graphene coatings have been reported to provide impressive corrosion resistance to nickel(Ni)and copper(Cu),because of remarkable characteristics of inertness and impermeablity of graphene.However,as the earlier investigations have generally been carried out in chloride environment,and it is important to understand the performance of graphene coating also in more aggressive environments such as acids and alkali.This study investigated the electrochemical corrosion behaviour of bare and graphene-coated(by chemical vapour deposition(CVD))Ni and Cu in 0.5 M H_(2)SO_(4),0.1 M NaCl and 0.5 M NaOH solutions.Electrochemical tests and post corrosion characterisation revealed the improvement in the corrosion resistance of Ni due to multilayer graphene coating to be similar in the three solutions,i.e.,the robustness of the barrier property of the multilayer graphene is largely unaffected by the aggressiveness of the corrosive environment.However,the improvement in corrosion resistance of bare Ni due to multilayer graphene is considerably greater(nearly 3 orders of magnitude)in the most aggressive of the test solutions(0.5 M H_(2)SO_(4)).The improvement is considerably less in 0.5 M NaOH because bare Ni develops a robust passive layer in highly alkaline solutions.The improvement in corrosion resistance of bare Cu is limited(within an order of magnitude)in the three solutions because Cu develops only 1-2 layers of graphene.

Wrinkle-free graphene with spatially uniform electrical properties grown on hot-pressed copper

The chemical vapor deposition （CVD） of graphene on Cu substrates enables the fabrication of large-area monolayer graphene on desired substrates. However, during the transfer of the synthesized graphene, topographic defects are unavoidably formed along the Cu grain boundaries, degrading the electrical properties of graphene and increasing the device-to-device variability. Here, we introduce a method of hot-pressing as a surface pre-treatment to improve the thermal stability of Cu thin film for the suppression of grain boundary grooving. The flattened Cu thin film maintains its smooth surface even after the subsequent high temperature CVD process necessary for graphene growth, and the formation of graphene without wrinkles is realized. Graphene field effect transistors （FETs） fabricated using the graphene synthesized on hot-pressed Cu thin film exhibit superior field effect mobility and significantly reduced device-to-device variation.

Chemical-sensitive graphene modulator with a memory effect for internet-of-things applications

Modern internet of things(IoTs)and ubiquitous sensor networks could potentially take advantage of chemically sensitive nanomaterials and nanostructures.However,their heterogeneous integration with other electronic modules on a networked sensor node,such as silicon-based modulators and memories,is inherently challenging because of compatibility and integration issues.Here we report a novel paradigm for sensing modulators:a graphene field-effect transistor device that directly modulates a radio frequency(RF)electrical carrier signal when exposed to chemical agents,with a memory effect in its electrochemical history.We demonstrated the concept and implementation of this graphene-based sensing modulator through a frequency-modulation(FM)experiment conducted in a modulation cycle consisting of alternating phases of air exposure and ethanol or water treatment.In addition,we observed an analog memory effect in terms of the charge neutrality point of the graphene,Vcnp,which strongly influences the FM results,and developed a calibration method using electrochemical gate-voltage pulse sequences.This graphenebased multifunctional device shows great potential for use in a simple,low-cost,and ultracompact nanomaterial-based nodal architecture to enable continuous,real-time event-based monitoring in pervasive healthcare IoTs,ubiquitous security systems,and other chemical/molecular/gas monitoring applications.

High current limits in chemical vapor deposited graphene spintronic devices

Understanding the stability and current-carrying capacity of graphene spintronic devices is key to their applications in graphene channel-based spin current sensors,spin-torque oscillators,and potential spin-integrated circuits.However,despite the demonstrated high current densities in exfoliated graphene,the current-carrying capacity of large-scale chemical vapor deposited(CVD)graphene is not established.Particularly,the grainy nature of chemical vapor deposited graphene and the presence of a tunnel barrier in CVD graphene spin devices pose questions about the stability of high current electrical spin injection.In this work,we observe that despite structural imperfections,CVD graphene sustains remarkably highest currents of 5.2×10^(8)A/cm^(2),up to two orders higher than previously reported values in multilayer CVD graphene,with the capacity primarily dependent upon the sheet resistance of graphene.Furthermore,we notice a reversible regime,up to which CVD graphene can be operated without degradation with operating currents as high as 108 A/cm^(2),significantly high and durable over long time of operation with spin valve signals observed up to such high current densities.At the same time,the tunnel barrier resistance can be modified by the application of high currents.Our results demonstrate the robustness of large-scale CVD graphene and bring fresh insights for engineering and harnessing pure spin currents for innovative device applications.

Quality assessment of graphene： Continuity, uniformity, and accuracy of mobility measurements

With the increasing availability of large-area graphene, the ability to rapidly and accurately assess the quality of the electrical properties has become critically important. For practical applications, spatial variability in carrier density and carrier mobility must be controlled and minimized. We present a simple framework for assessing the quality and homogeneity of large-area graphene devices. The field effect in both exfoliated graphene devices encapsulated in hexagonal boron nitride and chemical vapor-deposited （CVD） devices was measured in dual current-voltage configurations and used to derive a single, gate-dependent effective shape factor, t, for each device, β is a sensitive indicator of spatial homogeneity that can be obtained from samples of arbitrary shape. All 50 devices investigated in this study show a variation （up to tenfold） in β as a function of the gate bias. Finite element simulations suggest that spatial doping inhomogeneity, rather than mobility inhomogeneity, is the primary cause of the gate dependence of β, and that measurable variations of β can be caused by doping variations as small as 10^10 cm^-2. Our results suggest that local variations in the position of the Dirac point alter the current flow and thus the effective sample shape as a function of the gate bias. We also found that such variations lead to systematic errors in carrier mobility calculations, which can be revealed by inspecting the corresponding β factor.

Safe growth of graphene from non-flammable gas mixtures via chemical vapor deposition

Chemical vapor deposition has emerged as the most promising technique for the growth of graphene.However, most reports of this technique use either flammable or explosive gases, which bring safety concerns and extra costs to manage risk factors. In this article, we demonstrate that continuous monolayer graphene can be synthesized via chemical vapor deposition technique on Cu foils using industrially safe gas mixtures. Important factors, including the appropriate ratio of hydrogen flow and carbon precursor,pressure, and growth time are considered to obtain graphene films. Optical measurements and electrical transport measurements indicate graphene films are with comparable quality to other reports. Such continuous large area graphene can be synthesized under non-flammable and non-explosive conditions, which opens a safe and economical method for mass production of graphene. It is thereby beneficial for integration of graphene into semiconductor electronics.