Electrochemical Gate-Controlled Charge Transport in Graphene in Ionic Liquid and Aqueous Solution

Challenges for Field-Effect-Transistor-Based Graphene Biosensors

Owing to its outstanding physical properties, graphene has attracted attention as a promising biosensor material. Field-effect-transistor (FET)-based biosensors are particularly promising because of their high sensitivity that is achieved through the high carrier mobility of graphene. However, graphene-FET biosensors have not yet reached widespread practical applications owing to several problems. In this review, the authors focus on graphene-FET biosensors and discuss their advantages, the challenges to their development, and the solutions to the challenges. The problem of Debye screening, in which the surface charges of the detection target are shielded and undetectable, can be solved by using small-molecule receptors and their deformations and by using enzyme reaction products. To address the complexity of sample components and the detection mechanisms of graphene-FET biosensors, the authors outline measures against nonspecific adsorption and the remaining problems related to the detection mechanism itself. The authors also introduce a solution with which the molecular species that can reach the sensor surfaces are limited. Finally, the authors present multifaceted approaches to the sensor surfaces that provide much information to corroborate the results of electrical measurements. The measures and solutions introduced bring us closer to the practical realization of stable biosensors utilizing the superior characteristics of graphene.

Advances in solar energy harvesting integrated by van der Waals graphene heterojunctions

Graphene has garnered increasing attention for solar energy harvesting owing to its unique features. However, limitations hinder its widespread adoption in solar energy harvesting, comprising the band gapless in the molecular orbital of graphene lattice, its vulnerability to oxidation in oxidative environments, and specific toxic properties that require careful consideration during development. Beyond current challenges, researchers have explored doping graphene with ionic liquids to raise the lifespan of solar cells (SCs). Additionally, they have paid attention to optimizing graphene/Si Schottky junction or Schottky barrier SCs by enhancing the conductivity and work function of graphene, improving silicon's reflectivity, and addressing passivation issues at the surface/interface of graphene/Si, resulting in significant advancements in their power conversion efficiency. Increasing the functional area of graphene-based SCs and designing efficient grid electrodes are also crucial for enhancing carrier collection efficiency. Flaws and contaminants present at the interface between graphene and silicon pose significant challenges. Despite the progress of graphene/Si-based photovoltaic cells still needs to catch up to the efficiency achieved by commercially available Si p-n junction SCs. The low Schottky barrier height, design-related challenges associated with transfer techniques, and high lateral resistivity of graphene contribute to this performance gap. To maximize the effectiveness and robustness of graphene/Si-based photovoltaic cells, appropriate interlayers have been utilized to tune the interface and modulate graphene's functionality. This mini-review will address ongoing research and development endeavors using van der Waals graphene heterojunctions, aiming to overcome the existing limitations and unlock graphene's full potential in solar energy harvesting and smart storage systems.

Structure evolution at the gate-tunable suspended graphene-water interface

Graphitic electrode is commonly used in electrochemical reactions owing to its excellent in-plane conductivity, structural robustness and cost efficiency1,2. It serves as prime electrocatalyst support as well as a layered intercalation matrix2,3, with wide applications in energy conversion and storage1,4. Being the two-dimensional building block of graphite, graphene shares similar chemical properties with graphite1,2, and its unique physical and chemical properties offer more varieties and tunability for developing state-of-the-art graphitic devices5-7. Hence it serves as an ideal platform to investigate the microscopic structure and reaction kinetics at the graphitic-electrode interfaces. Unfortunately, graphene is susceptible to various extrinsic factors, such as substrate effect8-10, causing much confusion and controversy7,8,10,11. Hereby we have obtained centimetre-sized substrate-free monolayer graphene suspended on aqueous electrolyte surface with gate tunability. Using sum-frequency spectroscopy, here we show the structural evolution versus the gate voltage at the graphene-water interface. The hydrogen-bond network of water in the Stern layer is barely changed within the water-electrolysis window but undergoes notable change when switching on the electrochemical reactions. The dangling O-H bond protruding at the graphene-water interface disappears at the onset of the hydrogen evolution reaction, signifying a marked structural change on the topmost layer owing to excess intermediate species next to the electrode. The large-size suspended pristine graphene offers a new platform to unravel the microscopic processes at the graphitic-electrode interfaces.

Electrostatic Gating of Monolayer Graphene by Concentrated Aqueous Electrolytes

Electrostatic gating using electrolytes is a powerful approach for controlling the electronic properties of atomically thin two-dimensional materials such as graphene. However, the role of the ionic type, size, and concentration and the resulting gating efficiency is unclear due to the complex interplay of electrochemical processes and charge doping. Understanding these relationships facilitates the successful design of electrolyte gates and supercapacitors. To that end, we employ in situ Raman microspectroscopy combined with electrostatic gating using various concentrated aqueous electrolytes. We show that while the ionic type and concentration alter the initial doping state of graphene, they have no measurable influence over the rate of the doping of graphene with applied voltage in the high ionic strength limit of 3-15 M. Crucially, unlike for conventional dielectric gates, a large proportion of the applied voltage contributes to the Fermi level shift of graphene in concentrated electrolytes. We provide a practical overview of the doping efficiency for different gating systems.

The First-Water-Layer Evolution at the Graphene/Water Interface under Different Electro-Modulated Hydrophilic Conditions Observed by Suspended/Supported Field-Effect-Device Architectures

Interfacial water molecules affect carrier transportation within graphene and related applications. Without proper tools, however, most of the previous works focus on simulation modeling rather than experimental validation. To overcome this obstacle, a series of graphene field-effect transistors (GFETs) with suspended (substrate-free, SF) and supported (oxide-supported, OS) configurations are developed to investigate the graphene-water interface under different hydrophilic conditions. With deionized water environments, in our experiments, the electrical transportation behaviors of the graphene mainly originate from the evolution of the interfacial water-molecule arrangement. Also, these current-voltage behaviors can be used to elucidate the first-water layer at the graphene-water interface. For SF-GFET, our experimental results show positive hysteresis in electrical transportation. These imply highly ordered interfacial water molecules with a separated-ionic distributed structure. For OS-GFET, on the contrary, the negative hysteresis shows the formation of the hydrogen-bond interaction between the interfacial water layer and the SiO₂ substrate under the graphene. This interaction further promotes current conduction through the graphene/water interface. In addition, the net current-voltage relationship also indicates the energy required to change the orientation of the first-layer water molecules during electro-potential change. Therefore, our work gives an insight into graphene-water interfacial evolution with field-effect modulation. Furthermore, this experimental architecture also paves the way for investigating 2D solid-liquid interfacial features.

A sprayed graphene transistor platform for rapid and low-cost chemical sensing

We demonstrate a novel and versatile sensing platform, based on electrolyte-gated graphene field-effect transistors, for easy, low-cost and scalable production of chemical sensor test strips. The Lab-on-PCB platform is enabled by low-boiling, low-surface-tension sprayable graphene ink deposited on a substrate manufactured using a commercial printed circuit board process. We demonstrate the versatility of the platform by sensing pH and Na⁺ concentrations in an aqueous solution, achieving a sensitivity of 143 ± 4 μA per pH and 131 ± 5 μA per log₁₀Na⁺, respectively, in line with state-of-the-art graphene chemical sensing performance.

High-intensity vector signals for detecting SARS-CoV-2 RNA using CRISPR/Cas13a couple with stabilized graphene field-effect transistor

False detection of SARS-CoV-2 is detrimental to epidemic prevention and control. The scalar nature of the detected signal and the imperfect target recognition property of developed methods are the root causes of generating false signals. Here, we reported a collaborative system of CRISPR-Cas13a coupling with the stabilized graphene field-effect transistor, providing high-intensity vector signals for detecting SARS-CoV-2. In this collaborative system, SARS-CoV-2 RNA generates a "big subtraction" signal with a right-shifted feature, whereas any untargets cause the left-shifted characteristic signal. Thus, the false detection of SARS-CoV-2 is eliminated. High sensitivity with 0.15 copies/μL was obtained. In addition, the wide concerned instability of the graphene field-effect transistor for biosensing in solution environment was solved by the hydrophobic treatment to its substrate, which should be a milestone in advancing it's engineering application. This collaborative system characterized by the high-intensity vector signal and amazing stability significantly advances the accurate SARS-CoV-2 detection from the aspect of signal nature.

Facile preparation of covalently functionalized graphene with 2,4-dinitrophenylhydrazine and investigation of its characteristics

This article reports a fast and easy method for simultaneously in situ reducing and functionalizing graphene oxide. 2,4-Dinitrophenylhydrazine hydrate salt molecules are reduced by graphene oxide by reacting with oxide groups on the surface and removing these groups, and 2,4-dinitrophenylhydrazone groups are replaced with oxide groups. The synthesized materials have been investigated using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and UV absorption. Also, the morphology has been examined with a scanning electron microscope (SEM) and Brunauer-Emmett-Teller (BET) analysis. The result of the photocurrent response and electrochemical behavior of the samples through cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) have been analyzed to investigate the effect of physical and chemical changes compared to graphene.

Ion-Selective Membrane-Coated Graphene-Hexagonal Boron Nitride Heterostructures for Field-Effect Ion Sensing

An intrinsic ion sensitivity exceeding the Nernst-Boltzmann limit and an sp ² -hybridized carbon structure make graphene a promising channel material for realizing ion-sensitive field-effect transistors with a stable solid-liquid interface under biased conditions in buffered salt solutions. Here, we examine the performance of graphene field-effect transistors coated with ion-selective membranes as a tool to selectively detect changes in concentrations of Ca²⁺, K⁺, and Na⁺ in individual salt solutions as well as in buffered Locke's solution. Both the shift in the Dirac point and transconductance could be measured as a function of ion concentration with repeatability exceeding 99.5% and reproducibility exceeding 98% over 60 days. However, an enhancement of selectivity, by about an order magnitude or more, was observed using transconductance as the indicator when compared to Dirac voltage, which is the only factor reported to date. Fabricating a hexagonal boron nitride multilayer between graphene and oxide further increased the ion sensitivity and selectivity of transconductance. These findings incite investigating ion sensitivity of transconductance in alternative architectures as well as urge the exploration of graphene transistor arrays for biomedical applications.

Reversible hydrogenation restores defected graphene to graphene

Graphene as a two-dimensional material is prone to hydrocarbon contaminations, which can significantly alter its intrinsic electrical properties. Herein, we implement a facile hydrogenation-dehydrogenation strategy to remove hydrocarbon contaminations and preserve the excellent transport properties of monolayer graphene. Using electron microscopy we quantitatively characterized the improved cleanness of hydrogenated graphene compared to untreated samples. In situ spectroscopic investigations revealed that the hydrogenation treatment promoted the adsorption ofytyt water at the graphene surface, resulting in a protective layer against the re-deposition of hydrocarbon molecules. Additionally, the further dehydrogenation of hydrogenated graphene rendered a more pristine-like basal plane with improved carrier mobility compared to untreated pristine graphene. Our findings provide a practical post-growth cleaning protocol for graphene with maintained surface cleanness and lattice integrity to systematically carry a range of surface chemistry in the form of a well-performing and reproducible transistor.

Impact of electrostatic doping on carrier concentration and mobility in InAs nanowires

We fabricate dual-gated electric double layer (EDL) field effect transistors based on InAs nanowires gated with an ionic liquid, and we perform electrical transport measurements in the temperature range from room temperature to 4.2 K. By adjusting the spatial distribution of ions inside the ionic liquid employed as gate dielectric, we electrostatically induce doping in the nanostructures under analysis. We extract low-temperature carrier concentration and mobility in very different doping regimes from the analysis of current-voltage characteristics and transconductances measured exploiting global back-gating. In the liquid gate voltage interval from -2 to 2 V, carrier concentration can be enhanced up to two orders of magnitude. Meanwhile, the effect of the ionic accumulation on the nanowire surface turns out to be detrimental to the electron mobility of the semiconductor nanostructure: the electron mobility is quenched irrespectively to the sign of the accumulated ionic species. The reported results shine light on the effective impact on crucial transport parameters of EDL gating in semiconductor nanodevices and they should be considered when designing experiments in which electrostatic doping of semiconductor nanostructures via electrolyte gating is involved.

Gold nanoparticle-mediated non-covalent functionalization of graphene for field-effect transistors

Since its discovery, graphene has attracted much attention due to its unique electrical transport properties that can be applied to high-performance field-effect transistors (FETs). However, mounting chemical functionalities onto graphene inevitably involves the breaking of sp2 bonds, resulting in the degradation of the mechanical and electrical properties compared to pristine graphene. Here, we report a new strategy to chemically functionalize graphene for use in FETs without affecting the electrical performance. The key idea is to control the Fermi level of the graphene using the consecutive treatment of gold nanoparticles (AuNPs) and thiol-SAM (self-assembled monolayer) molecules, inducing positive and negative doping effects, respectively, by flipping the electric dipoles between AuNPs and SAMs. Based on this method, we demonstrate a Dirac voltage switcher on a graphene FET using heavy metal ions on functionalized graphene, where the carboxyl functional groups of the mediating SAMs efficiently form complexes with the metal ions and, as a result, the Dirac voltage can be positively shifted by different charge doping on graphene. We believe that the nanoparticle-mediated SAM functionalization of graphene can pave the way to developing high-performance chemical, environmental, and biological sensors that fully utilize the pristine properties of graphene.

DNA-Gated Graphene Field-Effect Transistors for Specific Detection of Arsenic(III) in Rice

As one of the most toxic forms of arsenic, inorganic As(III) is easy to accumulate in rice, leading to severe public health problems. Effective control of As(III) requires the development of fast analytical methods for its detection with high sensitivity and specificity. Toward this end, in this work, we report the fabrication of an As(III) electrochemical sensor based on a solution-gated graphene transistor (SGGT) platform with a novel sensing mechanism. The gold gate electrode of the SGGT was modified with DNA probes and then blocked with bovine serum albumin (BSA). The specific interaction between As(III) and gold disrupted the adsorption states of DNA probes, redistributing surface charges on the gate electrode, further leading to potential drop changes at the interfaces of the gate electrode and graphene active layer. This new mechanism based on DNA-charge-redistribution-induced SGGT current responses (denoted as "DNA-SGGT") was found to greatly improve the selectivity of the sensor: the response of DNA-SGGT to As(III) was effectively enhanced fourfold, while to other interfering cations, it was significantly reduced. The optimized sensor showed a detection limit as low as 5 nM with superior selectivity to As(III). The as-prepared DNA-SGGT-based sensor has also been successfully applied to the detection of As(III) in practical rice samples with a high recovery rate, showing great potential for heavy metal detection in many types of food samples.

Super-Nernstian pH Sensor Based on Anomalous Charge Transfer Doping of Defect-Engineered Graphene

The conventional pH sensor based on the graphene ion-sensitive field-effect transistor (Gr-ISFET), which operates with an electrostatic gating at the solution-graphene interface, cannot have a pH sensitivity above the Nernst limit (∼59 mV/pH). However, for accurate detection of the pH levels of an aqueous solution, an ultrasensitive pH sensor that can exceed the theoretical limit is required. In this study, a novel Gr-ISFET-based pH sensor is fabricated using proton-permeable defect-engineered graphene. The nanocrystalline graphene (nc-Gr) with numerous grain boundaries allows protons to penetrate the graphene layer and interact with the underlying pH-dependent charge-transfer dopant layer. We analyze the pH sensitivity of nc-Gr ISFETs by adjusting the grain boundary density of graphene and the functional group (OH-, NH₂-, CH₃-) on the SiO₂ surface, confirming an unusual negative shift of the charge-neutral point (CNP) as the pH of the solution increases and a super-Nernstian pH response (approximately -140 mV/pH) under optimized conditions.

Modulation Doping via a Two-Dimensional Atomic Crystalline Acceptor

Two-dimensional nanoelectronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Our Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations establish the large work function and narrow bands of α-RuCl₃ enable modulation doping of exfoliated single and bilayer graphene, chemical vapor deposition grown graphene and WSe₂, and molecular beam epitaxy grown EuS. We further demonstrate proof of principle photovoltage devices, control via twist angle, and charge transfer through hexagonal boron nitride. Short-ranged lateral doping (≤65 nm) and high homogeneity are achieved in proximate materials with a single layer of α-RuCl₃. This leads to the best-reported monolayer graphene mobilities (4900 cm²/(V s)) at these high hole densities (3 × 10¹³ cm^-2) and yields larger charge transfer to bilayer graphene (6 × 10¹³ cm^-2).

Graphene field-effect transistors as bioanalytical sensors: design, operation and performance

Graphene field-effect transistors (GFETs) are emerging as bioanalytical sensors, in which their responsive electrical conductance is used to perform quantitative analyses of biologically-relevant molecules such as DNA, proteins, ions and small molecules. This review provides a detailed evaluation of reported approaches in the design, operation and performance assessment of GFET biosensors. We first dissect key design elements of these devices, along with most common approaches for their fabrication. We compare possible modes of operation of GFETs as sensors, including transfer curves, output curves and time series as well as their integration in real-time or a posteriori protocols. Finally, we review performance metrics reported for the detection and quantification of bioanalytes, and discuss limitations and best practices to optimize the use of GFETs as bioanalytical sensors.

Moving Electrons Purposefully through Single Molecules and Nanostructures: A Tribute to the Science of Professor Nongjian Tao (1963-2020)

Electrochemistry intersected nanoscience 25 years ago when it became possible to control the flow of electrons through single molecules and nanostructures. Many surprises and a wealth of understanding were generated by these experiments. Professor Nongjian Tao was among the pioneering scientists who created the methods and technologies for advancing this new frontier. Achieving a deeper understanding of charge transport in molecules and low-dimensional materials was the first priority of his experiments, but he also succeeded in discovering applications in chemical sensing and biosensing for these novel nanoscopic systems. In parallel with this work, the investigation of a range of phenomena using novel optical microscopic methods was a passion of his and his students. This article is a review and an appreciation of some of his many contributions with a view to the future.

Nonzero Wavevector Excitation of Graphene by Localized Surface Plasmons

Electrochemical surface-enhanced Raman scattering measurements of single layer graphene provide unique information on resonant excitation induced by localized surface plasmons under controlled electron or hole doping. The highly confined electromagnetic field from the LSPs of the Au nanodimer structures prepared on defect-free graphene can generate holes and electrons of the electrochemical potentials beyond the limit of far-field light illumination. The electrochemical in situ SERS spectra prove nonzero wavevector excitation through the observation of normally forbidden Raman bands in graphene. The present findings point to a novel approach to breaking the limit of optoelectronic interactions and photochemical reactions of graphene and other semiconductors.

Ion Sensing with Solution-Gated Graphene Field-Effect Sensors in the Frequency Domain

Here, we examine the concept of frequency domain sensing with solution-gated graphene field-effect transistors, where a sine wave of primary frequency (1f) was applied at the gate and modulation of the power spectral density (PSD) of the drain-source current at 1f, 2f, and 3f was examined as the salt in the gate electrolyte was switched from KCl to CaCl₂, and their concentrations were varied. The PSD at 1f, 2f, and 3f increased with the concentration of KCl or CaCl₂, with the PSD at 1f being the most sensitive. We further correlated these changes to the shift in Dirac point. Switching the graphene substrate from oxide to hexagonal boron nitride, led to an improved device-to-device reproducibility and a significant reduction of noise, which translated to a higher signal-to-noise ratio and resolution in sensing salt concentrations. The signal-to-noise ratio at 1f was found to be a logarithmic function of KCl or CaCl₂ concentration in the 0.1 to 1000 mM range.

Fermi level-tuned optics of graphene for attocoulomb-scale quantification of electron transfer at single gold nanoparticles

Measurement of electron transfer at single-molecule level is normally restricted by the detection limit of faraday current, currently in a picoampere to nanoampere range. Here we demonstrate a unique graphene-based electrochemical microscopy technique to make an advance in the detection limit. The optical signal of electron transfer arises from the Fermi level-tuned Rayleigh scattering of graphene, which is further enhanced by immobilized gold nanostars. Owing to the specific response to surface charged carriers, graphene-based electrochemical microscopy enables an attoampere-scale detection limit of faraday current at multiple individual gold nanoelectrodes simultaneously. Using the graphene-based electrochemical microscopy, we show the capability to quantitatively measure the attocoulomb-scale electron transfer in cytochrome c adsorbed at a single nanoelectrode. We anticipate the graphene-based electrochemical microscopy to be a potential electrochemical tool for in situ study of biological electron transfer process in organelles, for example the mitochondrial electron transfer, in consideration of the anti-interference ability to chemicals and organisms.

Changing the Blood Test: Accurate Determination of Mercury(II) in One Microliter of Blood Using Oriented ZnO Nanobelt Array Film Solution-Gated Transistor Chips

The measurement of ultralow concentrations of heavy metal ions (HMIs) in blood is challenging. A new strategy for the determination of mercury ions (Hg²⁺ ) based on an oriented ZnO nanobelt (ZnO-NB) film solution-gated field-effect transistor (FET) chip is adopted. The FET chips are fabricated with ZnO-NB film channels with different orientations utilizing the Langmuir-Blodgett (L-B) assembly technique. The combined simulation and I-V behavior results show that the nanodevice with ZnO-NBs parallel to the channel has exceptional performance. The sensing capability of the oriented ZnO-NB film FET chips corresponds to an ultralow minimum detectable level (MDL) of 100 × 10^-12 m in deionized water due to the change in the electrical double layer (EDL) arising from the synergism of the field-induced effect and the specific binding of Hg²⁺ to the thiol groups (-SH) on the film surface. Moreover, the prepared FET chips present excellent selectivity toward Hg²⁺ , excellent repeatability, and a rapid response time (less than 1 s) for various Hg²⁺ concentrations. The sensing performance corresponds to a low MDL of 10 × 10^-9 m in real samples of a drop of blood.

Electrical Double Layer of Supported Atomically Thin Materials

The electrical double layer (EDL), consisting of two parallel layers of opposite charges, is foundational to many interfacial phenomena and unique in atomically thin materials. An important but unanswered question is how the "transparency" of atomically thin materials to their substrates influences the formation of the EDL. Here, we report that the EDL of graphene is directly affected by the surface energy of the underlying substrates. Cyclic voltammetry and electrochemical impedance spectroscopy measurements demonstrate that graphene on hydrophobic substrates exhibits an anomalously low EDL capacitance, much lower than what was previously measured for highly oriented pyrolytic graphite, suggesting disturbance of the EDL ("disordered EDL") formation due to the substrate-induced hydrophobicity to graphene. Similarly, electrostatic gating using EDL of graphene field-effect transistors shows much lower transconductance levels or even no gating for graphene on hydrophobic substrates, further supporting our hypothesis. Molecular dynamics simulations show that the EDL structure of graphene on a hydrophobic substrate is disordered, caused by the disruption of water dipole assemblies. Our study advances understanding of EDL in atomically thin limit.

Two-Dimensional Graphene Family Material: Assembly, Biocompatibility and Sensors Applications

Graphene and its chemically exfoliated derivatives—GO and rGO—are the key members of graphene family materials (GFM). The atomically thick crystal structure and the large continuous π conjugate of graphene imparts it with unique electrical, mechanical, optical, thermal, and chemical properties. Although those properties of GO and rGO are compromised, they have better scalability and chemical tunability. All GFMs can be subject to noncovalent modification due to the large basal plane. Besides, they have satisfying biocompatibility. Thus, GFMs are promising materials for biological, chemical and mechanical sensors. The present review summarizes how to incorporate GFMs into different sensing system including fluorescence aptamer-based sensors, field-effect transistors (FET), and electrochemical sensors, as well as, how to covalently and/or non-covalently modify GFMs to achieve various detection purpose. Sensing mechanisms and fabrication strategies that will influence the sensitivity of different sensing system are also reviewed.

Free radical sensors based on inner-cutting graphene field-effect transistors

Due to ultra-high reactivity, direct determination of free radicals, especially hydroxyl radical (•OH) with ultra-short lifetime, by field-effect transistor (FET) sensors remains a challenge, which hampers evaluating the role that free radical plays in physiological and pathological processes. Here, we develop a •OH FET sensor with a graphene channel functionalized by metal ion indicators. At the electrolyte/graphene interface, highly reactive •OH cuts the cysteamine to release the metal ions, resulting in surface charge de-doping and a current response. By this inner-cutting strategy, the •OH is selectively detected with a concentration down to 10-9 M. Quantitative metal ion doping enables modulation of the device sensitivity and a quasi-quantitative detection of •OH generated in aqueous solution or from living cells. Owing to its high sensitivity, selectivity, real-time label-free response, capability for quasi-quantitative detection and user-friendly portable feature, it is valuable in biological research, human health, environmental monitoring, etc.

Optical Imaging of Charges with Atomically Thin Molybdenum Disulfide

Mapping local surface charge distribution is critical to the understanding of various surface processes and also allows the detection of molecules binding to the surface. We show here that the optical absorption of monolayer MoS₂ is highly sensitive to charge and demonstrate optical imaging of local surface charge distribution with this atomically thin material. We validate the imaging principle and perform charge sensitivity calibration with an electrochemical gate. We further show that binding of charged molecules to the atomically thin material leads to a large change in the image contrast, allowing determination of the charge of the adsorbed molecules. This capability opens possibilities for characterizing impurities and defects in two-dimensional materials and for label-free optical detection and charge analysis of molecules.

Slippery and Sticky Graphene in Water

Understanding modulation of water molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials. Here, we examine normal and shear forces on supported single-layer graphene using atomic force microscopy and find that the electrolyte composition affects the molecular slippage of nanometer thick films of aqueous electrolytes along the graphene surface. In light of the shear-assisted thermally activated theory, water molecules along the graphene plane are very mobile when subjected to shear. However, upon addition of an electrolyte, the cations can make water stick to graphene, while ion-specific and concentration effects are present. Recognizing the tribological and tribochemical utility of graphene, we also evaluate the impact of this behavior on its frictional response in the presence of water. It appears that the addition of an electrolyte to pure water causes a reduction of the thermal activation energy and of the shear-activation length at several concentrations, both results conversely affecting the friction force. Further, this work can inspire innovation in research areas where changes of the molecular slippage through the modulation of the doping characteristics of graphene in liquid environment can be of use, including molecular sensing, lubrication, and energy storage.

Gate-Free Hydrogel-Graphene Transistors as Underwater Microphones

A perfect impedance match from water-rich hydrogels to an oceanic background makes hydrogel microphones ideal for long-distance, underwater acoustic reception with zero reflection. A novel hydrogel-graphene transistor is thus designed to work under a gate-free mode, in which a sheet of graphene directly converts mechanical vibrations from a microstructured hydrogel into electrical current. This work shows that the quantum capacitance of graphene plays an important role in determining the shift of the Fermi level in graphene and subsequently the amplitude of the current signal. Once employed underwater, this device provides a response to sound waves with high stability, low noise, and high sensitivity in a much-needed low-frequency domain.

Transfer-Free, Large-Scale Growth of High-Quality Graphene on Insulating Substrate by Physical Contact of Copper Foil

High-quality, large-area, single-layer graphene was directly grown on top of a quartz substrate by a low-pressure chemical vapor deposition (CVD) process using Cu vapor as a catalyst. In this process, continuous generation and supply of highly concentrated Cu vapor is the key to the growth of large-scale, high-quality graphene. It was achieved by direct physical contact, or "touch-down," of a Cu foil with an underlying sacrificial SiO₂ /Si substrate, and the target quartz substrate was placed on top of the Cu foil, eventually having a quartz/Cu/SiO₂ /Si sandwich structure. To establish the reaction mechanism, a test growth was performed without the quartz substrate, which revealed that Cu is diffused through the SiO₂ layer of the sacrificial SiO₂ /Si substrate to form liquid-phase Cu-Si alloy that emits massive Cu vapor. This Cu vapor catalyzes thermal decomposition of supplied CH₄ gas.

Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review

Gas sensors are devices that convert a gas volume fraction into electrical signals, and they are widely used in many fields such as environmental monitoring. Graphene is a new type of two-dimensional crystal material that has many excellent properties including large specific surface area, high conductivity, and high Young’s modulus. These features make it ideally suitable for application for gas sensors. In this paper, the main characteristics of gas sensor are firstly introduced, followed by the preparation methods and properties of graphene. In addition, the development process and the state of graphene gas sensors are introduced emphatically in terms of structure and performance of the sensor. The emergence of new candidates including graphene, polymer and metal/metal oxide composite enhances the performance of gas detection significantly. Finally, the clear direction of graphene gas sensors for the future is provided according to the latest research results and trends. It provides direction and ideas for future research.

Characterization of the Ionic Liquid/Electrode Interfacial Relaxation Processes Under Potential Polarization for Ionic Liquid Amperometric Gas Sensor Method Development

Electrochemical amperometric sensors require a constant or varying potential at the working electrode that drives redox reactions of the analyte for detection. The interfacial redox reaction(s) can result in the formation of new chemical products that could change the initial condition of the electrode/electrolyte interface. If the products are not inert and/or cannot be removed from the system such that the initial condition of the electrode/electrolyte interface cannot be restored, the sensor signal baseline would consequently drift, which is problematic for the continuous and real-time sensors. By setting the electrode potential with the periodical ON-OFF mode, electrolysis can be forestalled during the off mode which can minimize the sensor signal baseline drift and reduce the power consumption of the sensor. However, it is known that the relaxation of the structure in the electrical double layer at the ionic liquid/electrode interface to the steps of the electrode potential is slow. This work characterized the electrode/electrolyte interfacial relaxation process of an ionic liquid based electrochemical gas (IL-EG) sensor by performing multiple potential step experiments in which the potential is stepped from an open circuit potential (OCP) to the amperometric sensing potential at various frequencies with different time periods. Our results showed that by shortening the sensing period as well as extending the idle period (i.e., enlarge the ratio of idle period versus sensing period) of the potential step experiments, the electrode/electrolyte interface is prone to relax to its original state, and thus reduces the baseline drift. Additionally, the high viscosity of the ionic liquids is beneficial for electrochemical regeneration via the implementation of a conditioning step at zero volts at the electrode/electrolyte. By setting the working electrode at zero volts instead of OCP, our results showed that it could further minimize the baseline drift, enhance the sensing signal stability, and extend the functioning lifetime of a continuous IL-EG oxygen sensor.

Fast water transport in graphene nanofluidic channels

Superfast water transport discovered in graphitic nanoconduits, including carbon nanotubes and graphene nanochannels, implicates crucial applications in separation processes and energy conversion. Yet lack of complete understanding at the single-conduit level limits development of new carbon nanofluidic structures and devices with desired transport properties for practical applications. Here, we show that the hydraulic resistance and slippage of single graphene nanochannels can be accurately determined using capillary flow and a novel hybrid nanochannel design without estimating the capillary pressure. Our results reveal that the slip length of graphene in the graphene nanochannels is around 16 nm, albeit with a large variation from 0 to 200 nm regardless of the channel height. We corroborate this finding with molecular dynamics simulation results, which indicate that this wide distribution of the slip length is due to the surface charge of graphene as well as the interaction between graphene and its silica substrate.

Near room temperature chemical vapor deposition of graphene with diluted methane and molten gallium catalyst

Direct growth of graphene integrated into electronic devices is highly desirable but difficult due to the nominal ~1000 °C chemical vapor deposition (CVD) temperature, which can seriously deteriorate the substrates. Here we report a great reduction of graphene CVD temperature, down to 50 °C on sapphire and 100 °C on polycarbonate, by using dilute methane as the source and molten gallium (Ga) as catalysts. The very low temperature graphene synthesis is made possible by carbon attachment to the island edges of pre-existing graphene nuclei islands, and causes no damages to the substrates. A key benefit of using molten Ga catalyst is the enhanced methane absorption in Ga at lower temperatures; this leads to a surprisingly low apparent reaction barrier of ~0.16 eV below 300 °C. The faster growth kinetics due to a low reaction barrier and a demonstrated low-temperature graphene nuclei transfer protocol can facilitate practical direct graphene synthesis on many kinds of substrates down to 50-100 °C. Our results represent a significant progress in reducing graphene synthesis temperature and understanding its mechanism.

Oxygen evolution reaction dynamics monitored by an individual nanosheet-based electronic circuit

The oxygen evolution reaction involves complex interplay among electrolyte, solid catalyst, and gas-phase and liquid-phase reactants and products. Monitoring catalysis interfaces between catalyst and electrolyte can provide valuable insights into catalytic ability. But it is a challenging task due to the additive solid supports in traditional measurement. Here we design a nanodevice platform and combine on-chip electrochemical impedance spectroscopy measurement, temporary I-V measurement of an individual nanosheet, and molecular dynamic calculations to provide a direct way for nanoscale catalytic diagnosis. By removing O₂ in electrolyte, a dramatic decrease in Tafel slope of over 20% and early onset potential of 1.344 V vs. reversible hydrogen electrode are achieved. Our studies reveal that O₂ reduces hydroxyl ion density at catalyst interface, resulting in poor kinetics and negative catalytic performance. The obtained in-depth understanding could provide valuable clues for catalysis system design. Our method could also be useful to analyze other catalytic processes.

Electrocatalysis offers important opportunities for clean fuel production, but uncovering the chemistry at the electrode surface remains a challenge. Here, the authors exploit a single-nanosheet electrode to perform in-situ measurements of water oxidation electrocatalysis and reveal a crucial interaction with oxygen.

Abrupt p-n junction using ionic gating at zero-bias in bilayer graphene

Graphene is a promising candidate for optoelectronic applications. In this report, a double gated bilayer graphene FET has been made using a combination of electrostatic and electrolytic gating in order to form an abrupt p-n junction. The presence of two Dirac peaks in the gating curve of the fabricated device confirms the formation of a p-n junction. At low temperatures, when the electrolyte is frozen intentionally, the photovoltage exhibits a six-fold pattern indicative of the hot electron induced photothermoelectric effect that has also been seen in graphene p-n junctions made using metallic gates. We have observed that the photovoltage increases with decreasing temperature indicating a dominant role of supercollision scattering. Our technique can also be extended to other 2D materials and to finer features that will lead to p-n junctions which span a large area, like a superlattice, that can generate a larger photoresponse. Our work creating abrupt p-n junctions is distinct from previous works that use a source-drain bias voltage with a single ionic gate creating a spatially graded p-n junction.

High-quality graphene via microwave reduction of solution-exfoliated graphene oxide

Efficient exfoliation of graphite in solutions to obtain high-quality graphene flakes is desirable for printable electronics, catalysis, energy storage, and composites. Graphite oxide with large lateral dimensions has an exfoliation yield of ~100%, but it has not been possible to completely remove the oxygen functional groups so that the reduced form of graphene oxide (GO; reduced form: rGO) remains a highly disordered material. Here we report a simple, rapid method to reduce GO into pristine graphene using 1- to 2-second pulses of microwaves. The desirable structural properties are translated into mobility values of >1000 square centimeters per volt per second in field-effect transistors with microwave-reduced GO (MW-rGO) as the channel material and into particularly high activity for MW-rGO catalyst support toward oxygen evolution reactions.

Noninvasive Measurement of Electrical Events Associated with a Single Chlorovirus Infection of a Microalgal Cell

Chlorovirus Paramecium bursaria chlorella virus 1 (PBCV-1) contains a viral-encoded K(+) channel imbedded in its internal membrane, which triggers host plasma membrane depolarization during virus infection. This early stage of infection was monitored at high resolution by recording the cell membrane depolarization of a single Chlorella cell during infection by a single PBCV-1 particle. The measurement was achieved by depositing the cells onto a network of one-dimensional necklaces of Au nanoparticles, which spanned two electrodes 70 μm apart. The nanoparticle necklace array has been shown to behave as a single-electron device at room temperature. The resulting electrochemical field-effect transistor (eFET) was gated by the cell membrane potential, which allowed a quantitative measurement of the electrophysiological changes across the rigid cell wall of the microalgae due to a single viral attack at high sensitivity. The single viral infection signature was quantitatively confirmed by coupling the eFET measurement with a method in which a single viral particle was delivered for infection by a scanning probe microscope cantilever.

Highly stable and ultrasensitive chlorogenic acid sensor based on metal–organic frameworks/titanium dioxide nanocomposites

Metal–organic frameworks/titanium dioxide nanocomposites were utilized as novel electrode materials for ultrasensitive chlorogenic acid determination with improved stability.

Quantifying the effect of ionic screening with protein-decorated graphene transistors

Liquid-based applications of biomolecule-decorated field-effect transistors (FETs) range from biosensors to in vivo implants. A critical scientific challenge is to develop a quantitative understanding of the gating effect of charged biomolecules in ionic solution and how this influences the readout of the FETs. To address this issue, we fabricated protein-decorated graphene FETs and measured their electrical properties, specifically the shift in Dirac voltage, in solutions of varying ionic strength. We found excellent quantitative agreement with a model that accounts for both the graphene polarization charge and ionic screening of ions adsorbed on the graphene as well as charged amino acids associated with the immobilized protein. The technique and analysis presented here directly couple the charging status of bound biomolecules to readout of liquid-phase FETs fabricated with graphene or other two-dimensional materials.

Electrolyte-gated graphene Schottky barrier transistors

A new device architecture for flexible vertical Schottky barrier (SB) transistors and logic gates based on graphene-organic-semiconductor-metal heterostructures and ion gel gate dielectrics is demonstrated. The devices show well-behaved p- and n-type characteristics under low-voltage operation (<1 V), yielding high current densities (>100 mA cm(-2) ) and on/off current ratios (>10(3) ).

Enhanced transmission modulation based on dielectric metasurfaces loaded with graphene

We present a hybrid graphene/dielectric metasurface design to achieve strong tunable and modulated transmission at near-infrared (near-IR) frequencies. The proposed device is constituted by periodic pairs of asymmetric silicon nanobars placed over a silica substrate. An one-atom-thick graphene sheet is positioned over the all-dielectric metasurface. The in-plane electromagnetic fields are highly localized and enhanced with this metasurface due to its very low Ohmic losses at near-IR wavelengths. They strongly interact with graphene. Sharp Fano-type transmission spectrum is obtained at the resonant frequency of this hybrid configuration due to the cancelation of the electric and magnetic dipole responses at this frequency point. The properties of the graphene monolayer flake can be adjusted by tuning its Fermi energy or chemical potential, leading to different doping levels and, equivalently, material parameters. As a result, the Q-factor and the Fano-type resonant transmission spectrum of the proposed hybrid system can be efficiently tuned and controlled due to the strong light-graphene interaction. Higher than 60% modulation in the transmission coefficient is reported at near-IR frequencies. The proposed hybrid graphene/dielectric nanodevice has compact footprint, fast speed, and can be easily integrated to the current CMOS technology. These features would have promising applications to near-IR tunable filters, faster optical interconnects, efficient sensors, switches, and amplitude modulators.

Modification of Polyaniline-Based Gas Sensor by Electrophoretic Deposition of Metal Nanoparticles in Ionic Liquids

Nanoparticles synthesized in various ionic liquids (ILs) were immobilized by electrophoretic deposition (EPD) at the surface of a gas sensor made of thin polyaniline (PAni) film. We used pulsed DC voltage to overcome electrochemical treatment in IL-based electrolytes. In spite that EPD is commonly used for synthesis of nanoparticle films or coatings, here we just functionalized the surface of PAni by scattered nanoparticles. Immobilized nanoparticles were observed by SEM imaging and dynamic responses of gas sensors functionalized by different nanoparticles (Ni, Ni-Fe and Ag-Cu) were compared. Using the EPD technique, sensitivity or selectivity of a gas sensor based on PAni can be improved easily.

Ultrarapid and ultrasensitive electrical detection of proteins in a three-dimensional biosensor with high capture efficiency

The realization of a high-throughput biosensor platform with ultrarapid detection of biomolecular interactions and an ultralow limit of detection in the femtomolar (fM) range or below has been retarded due to sluggish binding kinetics caused by the scarcity of probe molecules on the nanostructures and/or limited mass transport. Here, as a new method for the highly efficient capture of biomolecules at extremely low concentration, we tested a three-dimensional (3D) platform of a bioelectronic field-effect transistor (bio-FET) with vertically aligned and highly dense one-dimensional (1D) ZnO nanorods (NRs) as a sensing surface capped by an ultrathin TiO2 layer for improved electrolytic stability on a chemical-vapor-deposited graphene (Gr) channel. The ultrarapid detection capability with a very fast response time (∼1 min) at the fM level of proteins in the proposed 3D bio-FET is primarily attributed to the fast binding kinetics of the probe-target proteins due to the small diffusion length of the target molecules to reach the sensor surface and the substantial number of probe molecules available on the largely increased surface area of the vertical ZnO NRs. This new 3D electrical biosensor platform can be easily extended to other electrochemical nanobiosensors and has great potential for practical applications in miniaturized biosensor integrated systems.