Graphene Channel Research Articles

In-plane gate graphene transistor with epitaxially grown molybdenum disulfide passivation layers

We demonstrate in-plane gate transistors based on the molybdenum disulfide (MoS2)/graphene hetero-structure. The graphene works as channels while MoS2 functions as passivation layers. The weak hysteresis of the device suggests that the MoS2 layer can effectively passivate the graphene channel. The characteristics of devices with and without removal of MoS2 between electrodes and graphene are also compared. The device with direct electrode/graphene contact shows a reduced contact resistance, increased drain current, and enhanced field-effect mobility. The higher field-effect mobility than that obtained through Hall measurement indicates that more carriers are present in the channel, rendering it more conductive.

Andreev reflection between aluminum and graphene across van der Waals barriers

We present planar aluminum superconductor–graphene junctions whose hybrid interface is engineered for couplings ranging from tunneling to the strongly coupled regime by employing an atomically thin van der Waals tunneling barrier. Without the vdW barrier, we find Al makes strongly coupled contacts with the fully proximities graphene channel underneath. Using a large band gap hexagonal boron nitride (hBN) barrier, we find the junctions always remain in the weak coupling regime, exhibiting tunneling characteristics. Using monolayer semi-conducting transition metal dichalcogenides (TMDs) such as MoS2, we realize intermediate coupling with enhanced junction conductance due to the Andreev process. In this intermediate regime, we find that junction resistance changes in discrete steps when sweeping a perpendicular magnetic field. The period of the resistance steps in the magnetic field is inversely proportional to the junction area, suggesting the physical origin of our observations is due to magnetic-field-induced vortex formation in the planar junction.

Rationally designed graphene channels for real‐time sodium ion detection for electronic tongue

AbstractMonitoring taste‐inducing ions and molecules continuously in liquids or solutions is of great considerable matter for the realization of the electronic tongue (E‐tongue). Particularly from the five major tastes, the highly selective, sensitive detection of Na+ in real‐time is prioritized. Prioritization is due to the saltiness of food is the key ingredient in most meals. Nevertheless, existing Na+ detecting devices have relatively low performances of selectivity, sensitivity, and lack of on–off functions. Additionally, conventional devices significantly deteriorate in capacity due to repetitive usage or lifetime shortage by degradation of the sensing material. Herein, a graphene‐based channel was rationally designed by the facile decoration of Calix[4]arene and Nafion to address this issue. They act as a receptor and a molecular sieve, respectively, to enhance selectivity and sensitivity and elongate the life expectancy of the device. This device was merged with a microfluidic channel to control the injection and withdrawal of solutions to fulfill dynamic on–off functions. The fabricated device has highly selective, sensitive Na+ detection properties compared to other 10 molecule/ionic species. Dynamic on–off functions of the device were available, also possesses a long lifespan of at least 220 days. Additionally, it can precisely discriminate real beverages containing Na+, which can be observed by principal component analysis plot. These features offer the possibility of ascending to a platform for E‐tongues in near future.image

Competition between Hydration Shell and Ordered Water Chain Induces Thickness-Dependent Desalination Performance in Carbon Nanotube Membrane

Exploring new reverse osmosis (RO) membranes that break the permeability-selectivity trade-off rule is the ultimate goal in seawater desalination. Both nanoporous monolayer graphene (NPG) and carbon nanotube (CNT) channels have been proposed to be promising candidates for this purpose. From the perspective of membrane thickness, both NPG and CNT can be classified into the same category, as NPG is equivalent to the thinnest CNT. While NPG has the advantage of a high water flux rate and CNT is excellent at salt rejection performance, a transition is expected in practical devices when the channel thickness increases from NPG to infinite-sized CNTs. By employing molecular dynamics (MD) simulations, we find that as the thickness of CNT increases, the water flux diminishes but the ion rejection rate increases. These transitions lead to optimal desalination performance around the cross-over size. Further molecular analysis reveals that this thickness effect originates from the formation of two hydration shells and their competition with the ordered water chain structure. With the increase in CNT thickness, the competition-dominated ion path through CNT is further narrowed. Once above this cross-over size, the highly confined ion path remains unchanged. Thus, the number of reduced water molecules also tends to stabilize, which explains the saturation of the salt rejection rate with the increasing CNT thickness. Our results offer insights into the molecular mechanisms of the thickness-dependent desalination performance in a one-dimensional nanochannel, which can provide useful guidance for the future design and optimization of new desalination membranes.

Photoresponse of Graphene Channel in Graphene-Oxide–Silicon Photodetectors

Graphene-on-silicon photodetectors exhibit broadband detection capabilities with high responsivities, surpassing those of their counterpart semiconductors fabricated purely using graphene or Si. In these studies, graphene channels were considered electrically neutral, and signal amplification was typically attributed to the photogating effect. By contrast, herein, we show graphene channels to exhibit p-type characteristics using a structure wherein a thin oxide layer insulated the graphene from Si. The p-type carrier concentration is higher (six-times) than the photoaging-induced carrier concentration and dominates the photocurrent. Additionally, we demonstrate photocurrent tunability in the channel. By operating this device under a back-gated bias, photocurrent tuning is realized with not only amplification but also attenuation. Gate amplification produces a current equal to the photogating current at a low bias (0.2 V), and it is approximately two orders of magnitude larger at a bias of 2 V, indicating the operation effectiveness. Meanwhile, photocurrent attenuation enables adjustments in the detector output for compatibility with read-out circuits. A quantification model of gate-dependent currents is further established based on the simulation model used for metal–oxide–semiconductor devices. Thus, this study addresses fundamental issues concerning graphene channels and highlights the potential of such devices as gate-tunable photodetectors in high-performance optoelectronics.

Hot-electron resonant terahertz bolometric detection in the graphene/black-AsP field-effect transistors with a floating gate

We evaluate the terahertz (THz) detectors based on field-effect transistor (FET) with the graphene channel (GC) and a floating metal gate (MG) separated from the GC by a black-phosphorus (b-P) or black-arsenic (b-As) barrier layer. The operation of these GC-FETs is associated with the heating of the two-dimensional electron gas in the GC by impinging THz radiation leading to thermionic emission of the hot electrons from the GC to the MG. This results in the variation of the floating gate potential, which affects the source–drain current. At the THz radiation frequencies close to the plasmonic resonance frequencies in the gated GC, the variation of the source–drain current and, hence, the detector responsivity can be resonantly large.

Design of silicon-on-insulator field-effect transistor using graphene channel to improve short channel effects over conventional devices

Design of silicon-on-insulator field-effect transistor using graphene channel to improve short channel effects over conventional devices

Unexpected Behavior in Thermal Conductivity of Confined Monolayer Water

Monolayer water can be formed under extreme confinement and will present distinctive thermodynamic properties compared with bulk water. In this work, we perform molecular dynamics simulations to study the thermal conductivity of monolayer water confined in graphene channels, finding an unexpected way of thermal conductivity of monolayer water dependent on its number density, which has a close correlation with the structure of water. The monolayer water is in an amorphous state, and its thermal conductivity increases linearly with the area density when the water density is low at first. Then, the thermal conductivity increases as the number density of water rises, which is attributed to the formation of a crystal structure and the reduction of crystal defects as the number of water molecules increases. After reaching the zenith, the thermal conductivity decreases rapidly owing to the formation of a wrinkle structure of monolayer water with excessive water molecules, which weakens the phonon dispersion. Moreover, we further investigate the remarkable effects of the channel height on both the structure and thermal conductivity of monolayer water. In summary, this study demonstrates the close connection between the thermal conductivity of monolayer water and its structure, contributing to not only expanding the understanding of the thermodynamic property of nanoconfined water but also benefiting the engineering applications for nanofluidics.

Vacancy Assisted Bilayer Graphene Contact for Monolayer Graphene Channel Devices

Contact engineering of monolayer graphene channel FETs has been a fundamental limiting factor hindering its intrinsic benefits for applications such as THz electronics, quantum sensing, quantum computing/communication devices, etc. This experimental work presents a unique technique to achieve record low contact resistance using carbon vacancy-assisted bilayer graphene contact. The bilayer graphene contact with engineered carbon vacancies lowered the metal-graphene interfacial distance and enhanced the atomic orbital overlap, eventually lowering the contact resistance. The interfacial properties and orbital interactions are investigated using density functional theory and non equilibrium Green’s function based transport computations. The reduction in contact resistance is then experimentally validated using unique Kelvin probe structures with monolayer and bilayer contacts. The captured contact resistance is as low as ~36 Ω.μm, which is a record low contact resistance value reported to date.

Ultraviolet Photoresponse Enhancement of Graphene/Al x O y -NPs/MoS2 Heterostructure Photodetectors

Molybdenum disulfide (MoS2) has been widely used to fabricate the photodetectors in recent years due to its excellent optical properties. However, the narrow detection bandwidth of MoS2 films limits their applications. Here, a thin Al film was deposited on the surface of MoS2 and naturally oxidized to form the AlxOy nanoparticles (AlxOy-NPs) for the enhanced performance of graphene/AlxOy-NPs/MoS2 heterostructure photodetectors. The material analyses were performed to confirm the formation of AlxOy-NPs on MoS2 and its influence on light absorption. Additionally, the photoresponse of graphene/AlxOy-NPs/MoS2 heterostructure photodetectors was investigated, and found that the localized surface plasmon resonance (LSPR) was induced in the ultraviolet-A (UV-A) band (365 nm), increasing the photocurrent owing to the p-type nature of graphene channel with the recombination of excited electrons from AlxOy-NPs/MoS2 to enhance the photoresponse. The photodetectors with an AlxOy film of 0.5 nm presented a UV-A light detection capability with the photoresponsivity, quantum efficiency, and detectivity of 6.24 A/W, 2120%, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.79\times 10^{{10}}$ </tex-math></inline-formula> Jones, respectively, at the optical power of 10 mW/cm2. Thus, the proposed one-step growth method of AlxOy-NPs on MoS2 films to fabricate the graphene/AlxOy-NPs/MoS2 heterostructure photodetectors with excellent photoresponse can facilitate the development of novel 2-D material photodetectors in future.

Sensitive Room-Temperature Graphene Photothermoelectric Terahertz Detector Based on Asymmetric Antenna Coupling Structure

A highly sensitive room-temperature graphene photothermoelectric terahertz detector, with an efficient optical coupling structure of asymmetric logarithmic antenna, was fabricated by planar micro-nano processing technology and two-dimensional material transfer techniques. The designed logarithmic antenna acts as an optical coupling structure to effectively localize the incident terahertz waves at the source end, thus forming a temperature gradient in the device channel and inducing the thermoelectric terahertz response. At zero bias, the device has a high photoresponsivity of 1.54 A/W, a noise equivalent power of 19.8 pW/Hz1/2, and a response time of 900 ns at 105 GHz. Through qualitative analysis of the response mechanism of graphene PTE devices, we find that the electrode-induced doping of graphene channel near the metal-graphene contacts play a key role in the terahertz PTE response. This work provides an effective way to realize high sensitivity terahertz detectors at room temperature.

An Electrolyte-Gated Graphene Field-Effect Transistor for Detection of Gadolinium(III) in Aqueous Media

In this work, an electrolyte-gated graphene field-effect transistor is developed for Gd3+ ion detection in water. The source and drain electrodes of the transistor are fabricated by photolithography on polyimide, while the graphene channel is obtained by inkjet-printing a graphene oxide ink subsequently electro-reduced to give reduced graphene oxide. The Gd3+-selective ligand DOTA is functionalized by an alkyne linker to be grafted by click chemistry on a gold electrode without losing its affinity for Gd3+. The synthesis route is fully described, and the ligand, the linker and the functionalized surface are characterized by electrochemical analysis and spectroscopy. The as functionalized electrode is used as gate in the graphene transistor so to modulate the source-drain current as a function of its potential, which is itself modulated by the concentration of Gd3+captured on the gate surface. The obtained sensor is able to quantify Gd3+ even in a sample containing several other potentially interfering ions such as Ni2+, Ca2+, Na+ and In3+. The quantification range is from 1 pM to 10 mM, with a sensitivity of 20 mV dec-1 expected for a trivalent ion. This paves the way for Gd3+ quantification in hospital or industrial wastewater.

Strong perpendicular anisotropic ferromagnet Fe3GeTe2/graphene van der Waals heterostructure

Two-dimensional magnets offer a new platform for exploring fundamental properties in van der Waals (vdW) heterostructures and their device applications. Here, we investigated heterostructure devices of itinerant metallic vdW ferromagnet Fe3GeTe2 (FGT) with monolayer chemical vapor deposited graphene. The anomalous Hall effect measurements of FGT Hall-bar devices exhibit robust ferromagnetism with strong perpendicular anisotropy at low temperatures. The electrical transport properties measured in FGT/graphene heterostructure devices exhibit a tunneling transport with weak temperature dependence. We assessed the suitability of such FGT/graphene heterostructures for spin injection and detection and investigated the presence of FGT on possible spin absorption and spin relaxation in the graphene channel. These findings will be useful for engineering spintronic devices based on vdW heterostructures.

Nonlocal Spin Valves Based on Graphene/Fe3GeTe2 van der Waals Heterostructures.

With recent advances in two-dimensional (2D) ferromagnets with enhanced Curie temperatures, it is possible to develop all-2D spintronic devices with high-quality interfaces using 2D ferromagnets. In this study, we have successfully fabricated nonlocal spin valves with Fe3GeTe2 (FGT) as the spin source and detector and multilayer graphene as the spin transport channel. The nonlocal spin transport signal was found to strongly depend on temperature and disappear at a temperature below the Curie temperature of the FGT flakes, which stemmed from the temperature-dependent ferromagnetism of FGT. The spin injection efficiency was estimated to be about 1%, close to that of conventional nonlocal spin valves with transparent contacts between ferromagnetic electrodes and the graphene channel. In addition, the spin transport signal was found to depend on the direction of the magnetic field and the magnitude of the current, which was due to the strong perpendicular magnetic anisotropy of FGT and the thermal effect, respectively. Our results provide opportunities to extend the applications of van der Waals heterostructures in spintronic devices.

Fabrication of suspended graphene field-effect transistors by the sandwich method

A novel fabrication technique that can be used for making a series of suspended graphene field-effect transistors on Si-substrate is discussed. The electrical properties of graphene channel can be significantly degraded by defects and chemical residues between graphene and substrate. To minimize electrical degradation, a method of physically suspending graphene from the substrate has been considered while maintaining its structural integrity. To address this problem, we employed a sandwich method to fabricate a suspended GFET, realizing 76% device fabrication yield that is higher than those realized by the other methods. Furthermore, the degradation of electrical properties due to external factors decreased. As our method has a mechanically stable structure, it can be imposed to make electrical devices with various two–dimensional (2D) materials. Our method can also be applied to the engineering of future devices in various applications because a large amount of electrically clean samples can be manufactured at once.

Ultrasensitive detection of methamphetamine by antibody-modified transistor assay

Effective detection of methamphetamine (Met) requires a fast, sensitive, and cheap testing assay. However, commercially available methods require expensive instruments and highly trained operators, which are time-consuming and labor-intensive. Herein, an antibody-modified graphene transistor assay is developed for sensitive and minute-level detection of Met in complex environments. The anti-Met probe captured charged targets within 120 s, leading to a p-doping effect near the graphene channel. The limit of detection reaches 50 aM (5.0 × 10−17 M) Met in solution. The graphene transistor would be a valuable tool for Met detection effective prevention of drug abuse.

Boron Nitride-Graphene (BN-G) Bilayer as a Channel of Graphene Based Field Effect Transistor

According to the effect of the interlayer interaction of the boron nitride sheet on electronic properties, especially the energy band gap of the graphene sheet in the boron nitride-graphene (BN-G) bilayer, we propose a gapless graphene-based field effect transistor (FET). It is comprised of a boron nitride layer on top of graphene in the channel region. In this study, we investigate the transfer characteristic and output characteristic of the proposed device for different values of the interlayer distance of (BN-G) bilayer. Also, we compare the output results with simulated bilayer graphene channel FET. We find that the Ion/Ioff ratio in the proposed device shows a significant promotion compared to graphene bilayer channel FET. Our first-principles calculations show that by decreasing the inter-layer distance of (BN-G) bilayer, the energy gap increase which leads to a dipper Ioff current and an increase of Ion/Ioff ratio up to 104 for an inter-layer distance of 2.7 angstroms. Moreover, it is found that the proposed device output characteristic displays a very good saturation due to improved pinch-off of the channel.

Seed layer engineering for crack-free sol–gel Alumina deposition on GFETs

Low-cost and low thermal budget-based spin-coated sol–gel Alumina was explored as a dielectric/passivation layer for graphene field effect transistor (GFET). Post thermal annealing, the crack was observed in the sol–gel Alumina layer exactly above the graphene channel. The possible mechanism of crack could be graphene lateral restoring movement due to (i) the Thermal Expansion Coefficient (TEC) difference between graphene and adjacent layers and (ii) shrinkage stress generated during the solvent removal process. Based on the crack formation phenomenon, a combination of different annealing schemes (low thermal budget deep ultraviolet (DUV) annealing) and seed layer engineering (thickness and different deposition schemes) were carried out. Finally, a novel two-step seed layer deposition method with DUV annealing was proposed and demonstrated to resolve the crack issue successfully and also able to retain the Dirac point in the electrical characteristics.

Machine learning identification of atmospheric gases by mapping the graphene-molecule van der waals complex bonding evolution

Atmospheric doping of graphene, and the variation in atmospheric composition (e.g., humidity) makes graphene machine learning (ML) models limited to controlled environments (e.g., dry air), and unsuitable for atmospheric applications (e.g., non-invasive medical diagnosis). Herein, using nano-porous activated carbon for atmospheric passivation of the graphene channel, Extreme Gradient Boosting (XGBoost), K-nearest neighbors (KNN), and Naïve Bayes supervised ML models for atmospheric gas identification were developed, with model feature contribution studied using the Game theoretic approach, SHapley Additive exPlanations (SHAP). Unlike conventional graphene-based ML models which only monitor the gas adsorption induced doping and scattering without tuning voltage (Vt) modulation, in this work, the tuning voltage evolution of the gas adsorption induced van der Waals (vdW) complex doping and scattering characteristics was mapped out over time. The accuracy, precision, recall, and F1-score of the developed models in the environments: atmospheric ammonia, atmospheric acetone, ammonia in dry air, and acetone in nitrogen were 100% for XGBoost, 96%, 97%, 97%, 97% respectively for KNN, and 95%, 95%, 96%, 96% respectively for Naïve Bayes, even at single-digit ppb gas concentrations. Hence, our results advance the application of graphene based electronic nose models from conventional controlled environments to actual atmospheric sensing.

Graphene Channel Electron-Multiplying Charge-Coupled Pixel

Traditional electron-multiplying charge-coupled devices suffer from slow readout and large power consumption because of the complex driving circuitry implemented for the square clocking to serially transfer the charge during the readout process. At the same time, the photo-generated carriers are also prone to pre-amplification losses. Owing to the electrostatic capacitive coupling of the tunable graphene channel with the silicon photogate, an in-situ transient photoresponse study of the single graphene-oxide-silicon heterostructure driven into pre-avalanche condition by a fast sinusoidal gating signal is presented. This technique paves the way for futuristic sinusoidally driven, graphene-based, out-of-plane electron-multiplication charge-coupled devices with specific low-power surveillance and adaptive optics applications. The operating scheme of the device demonstrates the excellent capability for sensing short and small bursts of photons (25 nW). The maximum multiplication factor of ~8.5 and responsivity of 350 A/W are achieved. The operating equipment limitations and their respective solutions are also discussed.