Graphene Field Effect Transistor Devices Research Articles

(Nanocarbons Division SES Young Investigator Award) Surface Chemistry of Nanocarbon Field-Effect Transistors: From Fundamental Physics to Biosensing

Conductive carbon nanomaterials are particularly well-suited to form field-effect biosensors (bioFETs) because they are stable in aqueous media and can be chemically coupled to biomolecules. Their reduced dimensionality, either 2D (graphene) or 1D (carbon nanotubes), is key to their sensitivity: because they are entirely surfaces, these materials present remarkable electrical transport properties that are easily altered by microscopic surface interactions. Biochemical processes occurring in close proximity to their surface can therefore be transduced as changes in their electrical conductance. Selectivity for specific biochemical events, however, must be engineered via functionalization of the nanomaterial, i.e. by modifying its surface with bioactive or biorecognition elements (e.g., antibodies, aptamers, enzymes, polymer layer). Our group recently analyzed a corpus of studies presenting graphene field-effect transistors (GFETs) as bioanalytical sensors for the quantitative detection of biomarkers in the form of nucleic acids, proteins, ions or small molecules [1]. We found a striking variance in the reported sensing performance between these studies, that could not be explained by controlling for differences in graphene material, analyte type and for evolution over the years. Our hypothesis is that the lack of microscopic control on the biointerface is likely the most important source of variation in such bioFETs, highlighting the critical need to improve our knowledge and control of the functionalization of nanocarbon materials.In this talk, I will discuss our work investigating the interplay between surface chemistry and electrical properties in nanocarbon field-effect transistors. In the first part, I will focus on covalent additions using aryldiazonium salts. I will review what we have learned on the fundamental physics of this chemistry from experiments and simulations on carbon nanotube devices [2-5], in particular on its disruptive character because of the formation of sp 3 defects in the carbon lattice. More recently, we investigated this same chemistry on GFET devices and observed a heterogeneous response in their electrical conductance. Aiming to solve this, we developed a novel approach to precisely trigger and stop the reaction in situ using the applied gate voltage [6], enabling unprecedented control on the functionalization rate and yield on GFETs. In the second part, I will describe our ongoing efforts to characterize and control the biofunctionalization of GFETs through non-covalent pyrene derivatives, using a combination of electrical measurements, microscopy and spectroscopy techniques, and theoretical simulations. Finally, I will discuss our ongoing translational projects in oncology, on adapting nanocarbon-based bioFETs in cancer diagnostics applications.[1] Béraud et al, Analyst 146 (2), 403-428 (2021)[2] Bouilly et al, Applied Physics Letters 101 (5), 053116 (2012)[3] Bouilly et al, ACS nano 9 (3), 2626-2634 (2015)[4] Bouilly et al, Nano Letters 16 (7), 4679-4685 (2016)[5] Côté et al, Physical Chemistry Chemical Physics 24 (7), 4174-4186 (2021)[6] Bazan et al, Nano Letters 22 (7), 2635-2642 (2022)

Impact of Water-Water Correlation at Suspended Graphene/Water Interface Under Different Electro-Modulated

Graphene-based devices have extensive applications in aqueous environments. The molecular interactions across the graphene sheet significantly impact various devices, although few researches focused on elucidating the interface effects. In this study, we utilize trench structure to make graphene unilaterally and bilaterally suspended in water. The conduction status of both lateral and longitudinal currents (Id, Is, Ig) of graphene was monitored under two suspension scenarios to investigate the changes in interface evolution with the introduction of solvent-solvent interactions. For unilaterally suspended graphene, the current perpendicular to the graphene channel direction remains in the off state. For bilaterally suspended graphene, the current path is formed vertically between the graphene channel and the gate electrode, perpendicular to the direction of the graphene. This experimental setup illustrates the possibilities for exploring interfacial characteristics at the interface between 2D materials and solutions through practical experimentation.The detailed fabrication process of the graphene field-effect-transistor (GFET) device is depicted in Fig. 1. To fabricate the microchannel structure, a microfluidic pattern was generated through photolithography on the SiO2/Si substrate spun with a layer of photoresist. Then, the corresponding microchannel pattern with a thickness of 300 nm SiO2 and 30 μm Si layer was etched by reactive-ion etching (RIE) to form a trench capable of storing water. Afterward, Au electrodes were placed next to the microfluidic channel via evaporation utilizing a shadow mask. The second critical step involves the preparation of graphene. Graphene sheets were grown on a copper foil through the chemical vapor deposition (CVD) method. With the support of PMMA film, physically stacked graphene bilayer graphene was transferred to the target substrate. Ultimately, the suspended bilayer graphene was submerged in acetone for 24 hours to remove the PMMA layer and immersed in isopropanol (IPA) and DI water to clean the residues.According to this device structure, we successfully fabricate the suspension of graphene unilaterally or bilaterally in water. When introducing water through the trench, the bottom surface of the suspended graphene contacts with water molecules, while the upper surface of the suspended graphene is exposed to air. This configuration leads to a unilateral suspension in water (unilateral-suspension GFET). Conversely, when water is introduced through the trench and water is also added above the graphene channel, graphene comes into contact with water on both sides. This configuration results in its bilateral suspension in the water (bilateral-suspension GFET). The electronic transport characteristics of unilateral-suspension GFET and bilateral-suspension GFET were measured by Agilent semiconductor analysis B1500A. The gate voltage was applied to water through an Ag/AgCl electrode. Fig. 2 (a) displays the Raman spectroscopy of suspended graphene on GFET device. The most notable features of the Raman spectrum are the G peak around 1580 cm-1, and the 2D band around 2700 cm-1, which are typical characteristics of graphene. The ID/IG ratio of 0.19 indicates the good quality of the graphene layers. As shown in Fig. 2 (b), the scanning electron microscope (SEM) result of suspended graphene on the GFET device verifies the presence of large-area graphene. The diagram displays a clear demarcation line, facilitating the identification of graphene presence on the substrate. This can also imply the excellent quality of graphene with few defects.The transport behavior of unilateral-suspension GFET and bilateral-suspension GFET are measured individually, as shown in Fig. 3. The sum (ΔI) of The drain current behavior (Id) and source current behavior (Is), and gate current behavior (Ig) are recorded with changed gate voltages (from -1V to +1V) and constant VDS (VDS= 0.1V). For unilateral-suspension GFET, ΔI and Ig keep constant at zero, and there is no conduction path perpendicular to the graphene channel direction. For bilateral-suspension GFET, ΔI and Ig at positive gate voltage are opposite in direction and equal in absolute value, this indicates the formation of an obvious path between the channel and gate electrode. The apparent interfacial evolution is believed to change with the introduction of the other side of water.In conclusion, we compare the electrical characteristics shown by two different suspension situations of graphene-based field effect transistors. There are some interesting results of the conduction path relating to interfacial evolution. This result has potential for chemical sensing or battery application. Furthermore, it can contribute to fundamental research about the interface interaction between suspended graphene and water. Figure 1

Study of High-Energy Proton Irradiation Effects in Top-Gate Graphene Field-Effect Transistors

In this article, the effects of high-energy proton irradiation on top-gate graphene field-effect transistors (GFETs) were investigated by using 20 MeV protons. The basic electrical parameters of the top-gate GFETs were measured before and after proton irradiation with a fluence of 1 × 1011 p/cm2 and 5 × 1011 p/cm2, respectively. Decreased saturation current, increased Dirac sheet resistance, and negative drift in the Dirac voltage in response to proton irradiation were observed. According to the transfer characteristic curves, it was found that the carrier mobility was reduced after proton irradiation. The analysis suggests that proton irradiation generates a large net positive charge in the gate oxide layer, which induces a negative drift in the Dirac voltage. Introducing defects and increased impurities at the gate oxide/graphene interface after proton irradiation resulted in enhanced Coulomb scattering and reduced mobility of the carriers, which in turn affects the Dirac sheet resistance and saturation current. After annealing at room temperature, the electrical characteristics of the devices were partially restored. The results of the technical computer-aided design (TCAD) simulation indicate that the reduction in carrier mobility is the main reason for the degradation of the electrical performance of the device. Monte Carlo simulations were conducted to determine the ionization and nonionization energy losses induced by proton incidence in top-gate GFET devices. The simulation data show that the ionization energy loss is the primary cause of the degradation of the electrical performance.

Controllable Carrier Doping in Two-Dimensional Materials Using Electron-Beam Irradiation and Scalable Oxide Dielectrics.

Two-dimensional (2D) materials, characterized by their atomically thin nature and exceptional properties, hold significant promise for future nano-electronic applications. The precise control of carrier density in these 2D materials is essential for enhancing performance and enabling complex device functionalities. In this study, we present an electron-beam (e-beam) doping approach to achieve controllable carrier doping effects in graphene and MoS2 field-effect transistors (FETs) by leveraging charge-trapping oxide dielectrics. By adding an atomic layer deposition (ALD)-grown Al2O3 dielectric layer on top of the SiO2/Si substrate, we demonstrate that controllable and reversible carrier doping effects can be effectively induced in graphene and MoS2 FETs through e-beam doping. This new device configuration establishes an oxide interface that enhances charge-trapping capabilities, enabling the effective induction of electron and hole doping beyond the SiO2 breakdown limit using high-energy e-beam irradiation. Importantly, these high doping effects exhibit non-volatility and robust stability in both vacuum and air environments for graphene FET devices. This methodology enhances carrier modulation capabilities in 2D materials and holds great potential for advancing the development of scalable 2D nano-devices.

Interfacial Engineering of Quantum Dots–Metal–Organic Framework Composite Toward Efficient Charge Transport for a Short‐Wave Infrared Photodetector

AbstractShort‐wave infrared (SWIR) quantum dot (QD)‐converted photodetectors are one of the promising devices used in artificial intelligence and automatic navigation systems. However, compared to semi‐conductor‐type photodetectors, they suffer from poor chemical stability and weak electronic conductivity. In this study, a PbS QD and conductive metal–organic framework (MOF) hybrid composite is designed with SWIR detection capability. The Fourier‐transform infrared spectroscopy confirms the surface modification of PbS QDs with MOF. X‐ray absorption near‐edge structures spectra reveal the chemical bonding between MOF and PbS QDs. Furthermore, 2D grazing‐incidence small‐ and wide‐angle X‐ray scattering is utilized to unveil the particle stacking and the preferred orientation of the PbS@MOF thin film. By integrating the PbS@MOF thin film directly on top of the graphene field effect transistor (FET) device, the chemical stability and photoelectric properties of graphene FET are enhanced, and these are investigated using a focus ion beam with a high‐resolution transmission electron microscope. This study offers a strategy to simultaneously improve the chemical stability and the photoelectronic properties of the nanomaterials as well as contribute to the advancement of a QD‐converted SWIR photodetector.

Modelling and Optimization of the Differential Surface Capacitance in Electrolyte-Gated Graphene Field-Effect Transistors

Graphene field-effect transistors (GFETs) are a promising avenue for the detection of biomarkers, allowing to envision improvements in diagnostic approaches for different types of cancer and other diseases. To enable such applications, the detection metrics of the devices must be optimized, including their sensitivity, selectivity, and stability. In particular, the capacitance of the device is one of the key determinants of the sensitivity of the electrical current of graphene to nearby changes in electrical potentials. In this presentation, we present our approach to optimize the surface capacitance of electrolyte-gated GFETs, by investigating theoretically and/or experimentally the effect of specific design parameters, including the graphene area, the removal of the backgate, and the passivation of electrodes. On the theoretical side, a mathematical representation of the device capacitance was established as an equivalent circuit of capacitors modelling for the quantum capacitance of graphene, the backgate capacitance, and the capacitance of electrical double layers in the electrolyte. On the experimental side, microchips of GFET arrays were assembled using microfabrication techniques on two types of substrates: a standard Si++/SiO2 wafer in which the doped silicon layer is used as a backgate, and a pure non-conducting borosilicate glass wafer in comparison. The capacitance of the devices was then measured using a custom parallel electrical prober by stepping the applied voltage in the solution and analyzing the resulting current response through a standard RC direct current model. We will present our analyses by comparing the relative impact of electrode passivation, backgate usage, and graphene area on the capacitance of devices. These results will enable a rational design of GFET devices with improved sensitivity for their use in applications such as biological, chemical, and gas sensing.

Detection of Interleukin-6 Protein Using Graphene Field-Effect Transistor.

Universal platforms to analyze biomolecules using sensor devices can address critical diagnostic challenges. Sensor devices like electrical-based field-effect transistors play an essential role in sensing biomolecules by charge probing. Graphene-based devices are more suitable for these applications. It has been previously reported that Graphene Field-Effect Transistor (GFET) devices detect DNA hybridization, pH sensors, and protein molecules. Graphene became a promising material for electrical-based field-effect transistor devices in sensing biomarkers, including biomolecules and proteins. In the last decade, FET devices have detected biomolecules such as DNA molecules, pH, glucose, and protein. These studies have suggested that the reference electrode is placed externally and measures the transfer characteristics. However, the external probing method damages the samples, requiring safety measurements and a substantial amount of time. To control this problem, the graphene field-effect transistor (GFET) device is fabricated with an inbuilt gate that acts as a reference electrode to measure the biomolecules. Herein, the monolayer graphene is exfoliated, and the GFET is designed with an in-built gate to detect the Interleukin-6 (IL-6) protein. IL-6 is a multifunctional cytokine which plays a significant role in immune regulation and metabolism. Additionally, IL-6 subsidizes a variability of disease states, including many types of cancer development, and metastasis, progression, and increased levels of IL-6 are associated with a higher risk of cancer and can also serve as a prognostic marker for cancer. Here, the protein is desiccated on the GFET device and measured, and Dirac point shifting in the transfer characteristics systematically evaluates the device's performance. Our work yielded a conductive and electrical response with the IL-6 protein. This graphene-based transducer with an inbuilt gate gives a promising platform to enable low-cost, compact, facile, real-time, and sensitive amperometric sensors to detect IL-6. Targeting this pathway may help develop treatments for several other symptoms, such as neuromyelitis optica, uveitis, and, more recently, COVID-19 pneumonia.

Pure edge-contact devices on single-layer-CVD-graphene integrated into a single chip

We present a simple and cost-effective fabrication technique for on-chip integration of pure edge contact two-terminal (2T) and Graphene field effect transistor (GFET) devices with low contact resistance and nonlinear characteristics based on single-layer chemical-vapor-deposited (CVD) graphene. We use a smart print-based mask projection technique with a 10X magnification objective lens for maskless lithography followed by thermal evaporation of the contact material Cr-Pd-Au through three different angles (90° and ± 45°) using a customized inclined-angle sample-holder to control the angle during normal incidence evaporation for edge-contact to graphene. Our fabrication technique, graphene quality, and contact geometry enable pure metal contact to 2D single-layer graphene allowing electron transport through the 1D atomic edge of graphene. Our devices show some signatures of edge contact to graphene in terms of very low contact resistance of 23.5 Ω, the sheet resistance of 11.5 Ω, and sharp nonlinear voltage-current characteristics (VCC) which are highly sensitive to the bias voltage. This study may find application in future graphene-integrated chip-scale passive or active low-power electronic devices.

Bending Stability of Ferroelectric Gated Graphene Field Effect Transistor for Flexible Electronics.

In this work, we explored the potential of the ferroelectric gate of (Pb0.92La0.08)(Zr0.30Ti0.70)O3 (PLZT(8/30/70)) for flexible graphene field effect transistor (GFET) devices. Based on the deep understanding of the VDirac of PLZT(8/30/70) gate GFET, which determines the application of the flexible GFET devices, the polarization mechanisms of PLZT(8/30/70) under bending deformation were analyzed. It was found that both flexoelectric polarization and piezoelectric polarization exist under bending deformation, and their polarization direction is opposite under the same bending deformation. Thus, a relatively stable of VDirac is obtained due to the combination of these two effects. In contrast to the relatively good linear movement of VDirac under bending deformation of relaxor ferroelectric (Pb0.92La0.08)(Zr0.52Ti0.48)O3 (PLZT(8/52/48)) gated GFET, these stable properties of the PLZT(8/30/70) gate GFETs make them have great potential for applications in flexible devices.

Functionalisation of Graphene Sensor Surfaces for the Specific Detection of Biomarkers.

We report on a controllable and specific functionalisation route for graphene field-effect transistors (GFETs) for the recognition of small physiologically active molecules. Key element is the noncovalent functionalisation of the graphene surface with perylene bisimide (PBI) molecules directly on the growth substrate. This Functional Layer Transfer enables the homogeneous self-assembly of PBI molecules on graphene, onto which antibodies are subsequently immobilised. The sensor surface was characterised by atomic force microscopy, Raman spectroscopy and electrical measurements, showing superior performance over conventional functionalisation after transfer. Specific sensing of small molecules was realised by monitoring the electrical property changes of functionalised GFET devices upon the application of methamphetamine and cortisol. The concentration dependent electrical response of our sensors was determined down to a concentration of 300 ng ml-1 for methamphetamine.

Functionalisation of Graphene Sensor Surfaces for the Specific Detection of Biomarkers

AbstractWe report on a controllable and specific functionalisation route for graphene field‐effect transistors (GFETs) for the recognition of small physiologically active molecules. Key element is the noncovalent functionalisation of the graphene surface with perylene bisimide (PBI) molecules directly on the growth substrate. This Functional Layer Transfer enables the homogeneous self‐assembly of PBI molecules on graphene, onto which antibodies are subsequently immobilised. The sensor surface was characterised by atomic force microscopy, Raman spectroscopy and electrical measurements, showing superior performance over conventional functionalisation after transfer. Specific sensing of small molecules was realised by monitoring the electrical property changes of functionalised GFET devices upon the application of methamphetamine and cortisol. The concentration dependent electrical response of our sensors was determined down to a concentration of 300 ng ml−1 for methamphetamine.

Fabrication and Characterization of Epitaxial Graphene Field Effect Transistor Devices Based on a Monolithic Bottom Gate

Current electronics technology increasingly demands higher integration, flexibility, higher efficiency, and performance aspects such as compatibility with higher temperature operation of the semiconductor devices, which may find limitations when silicon is used. The superior intrinsic properties of SiC, eventually combined with the ability of growing monolithically epitaxial, high quality graphene on a SiC wafer (1), makes it a reliable alternative for some electronic applications, such as field effect transistors (FET), radio frequency (RF) power amplifiers, integrated circuits (IC), or sensors. In this work, we describe the fabrication and preliminary electrical characterizations of epitaxial graphene (EG) on a SiC substrate FET devices based on an alternative back gate architecture. We propose a new approach in which the FET device is built on a 4◦ off-axis cut, N+ doped 4H-SiC substrate (the back gate) with, on top of, it a 1μm semi-insulating homoepitaxial layer of SiC compensated with vanadium (the dielectric layer). EG will be used as FET conduction channel. Using this V-compensated dielectric layer is aimed to minimize effects on the FET characteristics such as from defects in the SiC crystal, especially below the FET active areas, which would have occurred when using ion implantation to create a buried gate. The EG film was grown by the high temperature Si sublimation method under an Ar ambient. Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were applied in the structural characterization of epigraphene. The lack of D-peak in Raman spectra, together with SEM and AFM images, indicate that high quality monolayer to few layer epitaxial graphene fully covering the SiC surface is deposited. The electrical characteristics of the EG channel-devices and the functionality of the bottom gate were examined with 2-probe and 4-probe method. The electrical properties of the FET devices were also investigated with 3 terminal configuration.

Performance Enhancement and Comparison of Graphene Field Effect Transistor Devices Coated with HMDS Layer

Graphene-based devices show good transfer characteristics, which depend upon the surface morphology of the material and substrate. During fabrication of the device, the substrate morphology is disturbed inappropriately by the surface contamination. In the present study, monolayer Graphene Field Effect Transistor (GFET) has been driven with hydrophobic Hexamethyl Disilazane (HMDS) layer. The HMDS layer is dehydrated before and after the exfoliated monolayer graphene, and the electrical characteristics were measured. The transfer curve of the HMDS coated graphene device demonstrates excellent FET characteristics and prevents contamination from the atmosphere under ambient conditions. Fabrication of GFET device on the hydrophobic substrate enhances the effective ambipolar behavior. It is beginning with an excellent platform and user-friendly device for biosensing applications.

Simulation of the Electrical and Thermal Properties of a Graphene Field Effect Transistor

In this paper, the electrical and thermal properties of Graphene field effect transistor (GFET) has been simulated by varying the width of graphene channel. Here, the electrical characteristics, like electron density, hole density, I-V Characteristics and charge carrier velocity profile in the channel region has been studied for three different values of graphene channel width: 1 nm, 2 nm and 5 nm. To analyse the thermal properties of the GFET device, the temperature profile of the graphene channel has been simulated for 100, 300 and 500K. After analysing the simulation of this characteristics, it is concluded that, both electrical and thermal properties of GFET can be improved by fabricating the channel with larger width in the GFET device.

Detection of Glial Fibrillary Acidic Protein in Patient Plasma Using On-Chip Graphene Field-Effect Biosensors, in Comparison with ELISA and Single-Molecule Array.

Glial fibrillary acidic protein (GFAP) is a discriminative blood biomarker for many neurological diseases, such as traumatic brain injury. Detection of GFAP in buffer solutions using biosensors has been demonstrated, but accurate quantification of GFAP in patient samples has not been reported, yet in urgent need. Herein, we demonstrate a robust on-chip graphene field-effect transistor (GFET) biosensing method for sensitive and ultrafast detection of GFAP in patient plasma. Patients with moderate–severe traumatic brain injuries, defined by the Mayo classification, are recruited to provide plasma samples. The binding of target GFAP with the specific antibodies that are conjugated on a monolayer GFET device triggers the shift of its Dirac point, and this signal change is correlated with the GFAP concentration in the patient plasma. The limit of detection (LOD) values of 20 fg/mL (400 aM) in buffer solution and 231 fg/mL (4 fM) in patient plasma have been achieved using this approach. In parallel, for the first time, we compare our results to the state-of-the-art single-molecule array (Simoa) technology and the classic enzyme-linked immunosorbent assay (ELISA) for reference. The GFET biosensor shows competitive LOD to Simoa (1.18 pg/mL) and faster sample-to-result time (<15 min), and also it is cheaper and more user-friendly. In comparison to ELISA, GFET offers advantages of total detection time, detection sensitivity, and simplicity. This GFET biosensing platform holds high promise for the point-of-care diagnosis and monitoring of traumatic brain injury in GP surgeries and patient homes.

UV Illumination as a Method to Improve the Performance of Gas Sensors Based on Graphene Field-Effect Transistors.

The ability to detect and recognize airborne chemical species is essential to enable applications in security, health, and environmental monitoring. Here, we report a sensing platform based on graphene field-effect transistor (GFET) devices combined with optical illumination for the detection of volatile compounds. We compare the change in resistance of GFET sensors upon exposure to analytes such as ethanol, dimethyl methylphosphonate (DMMP), and water vapors with and without the presence of a local illuminating ultraviolet (UV) light-emitting diode (LED). Our results show that UV illumination acts as a control knob for the electronic transport properties of graphene, increasing the device's response to ethanol, water, and DMMP, up to a factor of 54, and enabling ppb-level detection of DMMP at 800 ppb without chemical functionalization of the graphene layer. The sensing response can be optimized to reveal an analyte-specific interplay between the induced changes in carrier concentration and mobility of the GFET. These findings provide a pathway to enhancing the sensitivity of GFET sensors and a differentiation channel to improve their selectivity.

Novel graphene-based OPFET for optoelectronic applications

Graphene-based FET (Field Effect Transistor) has been widely explored for high speed optical applications such as photodetectors, terahertz modulators etc. On the other hand, graphene MESFET (Metal-Semiconductor Field Effect Transistor) which is not yet explored bears a thin line of distinction with respect to the graphene FET i.e. the oxide/insulator capacitance in graphene FET is replaced with dipole capacitance in graphene MESFET. This thin difference creates a significant variation in the characteristics of both the devices. In this paper, we comparatively analyze the photogating, high frequency amplification, inter-band, and intra-band performance of graphene FET and graphene MESFET devices for photodetection and terahertz modulation applications.

Optimization of oxygen plasma based etching of single layered graphene through Raman and FESEM characterization

Graphene field-effect transistors (GFETs) show high electron transfer rates, high charge-carrier mobility, low electrical noise levels and flexibility of surface functionalization, which are useful to realize the sensors for various applications. The key challenge is to develop the processes to fabricate a uniform array of GFETs devices. In this paper, single-layered graphene (SLG) was synthesized on copper (Cu) substrate using chemical vapor deposition (CVD) followed by its successful transfer over silicon (Si) substrate. To realize an array of graphene-based devices/ sensors, the process of oxygen plasma based clean etching of SLG was optimized. The Raman and Field Emission Scanning Electron Microscope (FESEM) characterizations ensure the complete etching of SLG in 240 s using 200 W, RF (13.56 MHz) power. Optimized SLG etching would help to fabricate an array of GFETs as a biochemical sensor platform.

A Compact Sensory Platform Based pH Sensor Using Graphene Field Effect Transistor.

A compact sensory platform has been fabricated using a graphene field effect transistor (GFET) to identify the biomolecules by pH sensing. The monolayer GFET is driven by an in-built top-gate for detecting the pH of the contacting buffer solution. The GFET device detects the effect of hydroxide ions on a graphite surface. Electrical characteristics of the device were measured after desiccating the buffer solution on the surface of the monolayer graphene. Electrically, the VDirac point shifted toward the positive direction when the pH value of the buffer solution is varied. The transfer curve of the device also moved in the positive direction with increasing pH values, indicating charge transfer from dopant molecules to the surface of graphene. The sensitivity of the device was estimated to be ~48.5 mV/pH. The fabrication of the compact GFET device with an in-built gate provides a platform for effective pH sensing with a user-friendly interface for biosensing applications.

Wafer-scalable chemical modification of amino groups on graphene biosensors.

Graphene's remarkable attributes make it suitable for application to biosensors for biomolecular recognition. Specific and precise target detection is realized by designing robust methods for immobilization of probe molecules, such as oligonucleotides, antibodies, receptors, and sugar chains, to a device surface. In this research, we developed a chemical modification method with a plasma treatment of amino groups on natural defects of graphene, which is compatible with a wafer-scalable semiconductor process, to prevent deterioration of the carrier mobility. The plasma treatment was optimized in terms of the efficiency of the amino radical generation, length of the mean free path, and reaction energy on graphene. The density of the modified amino groups on graphene was approximately 0.065 groups/nm2, and the change in the ΔId/ΔVg characteristic of the graphene field-effect transistor (FET) was negligible. DNA probes were then attached to the amino groups on the graphene FET. The target complementary DNA was detected at 1 nM after hybridization using the graphene FET devices. The plasma-assisted modification of the amino groups on the graphene surface was developed for immobilization of the DNA probes, and hybridization with the target DNA was demonstrated without deterioration of the carrier mobility.