Graphene Devices Research Articles

Ultrasensitive biosensing platform for Mycobacterium tuberculosis detection based on functionalized graphene devices.

Tuberculosis (TB) has high morbidity as a chronic infectious disease transmitted mainly through the respiratory tract. However, the conventional diagnosis methods for TB are time-consuming and require specialists, making the diagnosis of TB with point-of-care (POC) detection difficult. Here, we developed a graphene-based field-effect transistor (GFET) biosensor for detecting the MPT64 protein of Mycobacterium tuberculosis with high sensitivity as a POC detection platform for TB. For effective conjugation of antibodies, the graphene channels of the GFET were functionalized by immobilizing 1,5-diaminonaphthalene (1,5-DAN) and glutaraldehyde linker molecules onto the graphene surface. The successful immobilization of linker molecules with spatial uniformity on the graphene surface and subsequent antibody conjugation were confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy. The GFET functionalized with MPT64 antibodies showed MPT64 detection with a detection limit of 1fg/mL in real-time, indicating that the GFET biosensor is highly sensitive. Compared to rapid detection tests (RDT) and enzyme-linked immunosorbent assays, the GFET biosensor platform developed in this study showed much higher sensitivity but much smaller dynamic range. Due to its high sensitivity, the GFET biosensor platform can bridge the gap between time-consuming molecular diagnostics and low-sensitivity RDT, potentially aiding in early detection or management of relapses in infectious diseases.

Effect of nanojunctions on thermal rectification in variable cross-section graphene based on local phonon resonance

Introducing variable cross-section structures to graphene is considered to be an important method for producing thermal rectification (TR). In our work, non-equilibrium molecular dynamics simulations of variable cross-section graphene with nanojunctions were conducted, and their effects on thermal-transport properties were systematically studied. The results show that the introduction of nanojunctions can induce local phonon resonance effects, and the type of nanojunctions can influence these effects. The use of inclined nanojunctions can weaken local resonance compared to the use of straight nanojunctions. According to the results, the local resonance can be improved by increasing the length of the inclined nanojunctions within a certain range. However, beyond a specific length, the local resonance weakens. Finally, the directional difference in the phonon spectral heat flux and the directional mismatch of the dispersion curves are used to explain the TR phenomenon in the model. It is also found that low-frequency phonons play a key role in the TR phenomenon through the phonon density of states. This work provides guidance for designing local phonon resonance-based thermal rectifiers by tailoring graphene devices.

High Gain Graphene Based Hot Electron Transistor with Record High Saturated Output Current Density

AbstractHot electron transistors (HETs) represent an exciting new device for integration into semiconductor technology, holding the promise of high‐frequency electronics beyond the limits of SiGe bipolar hetero transistors. With the exploration of 2D materials such as graphene and new device architectures, hot electron transistors have the potential to revolutionize the landscape of modern electronics. This study highlights a novel hot electron transistor structure with a record output current density of 800 A cm−2 and a high current gain α, fabricated using a scalable fabrication approach. The hot electron transistor structure comprises 2D hexagonal boron nitride and graphene layers wet transferred to a germanium substrate. The combination of these materials results in exceptional performance, particularly in terms of the highly saturated output current density. The scalable fabrication scheme used to produce the hot electron transistor opens up opportunities for large‐scale manufacturing. This breakthrough in hot electron transistor technology holds promise for advanced electronic applications, offering high current capabilities in a practical and manufacturable device.

Simultaneous dual-configuration van der Pauw measurements of gated graphene devices

The van der Pauw technique is the gold standard for measuring the sheet resistance of thin conductors by relying on the combination of two 4-point resistance measurements obtained in different current–voltage probe configurations. The presence of hysteresis and/or time-dependent effects will result in the van der Pauw technique producing inaccurate and misleading results. This limitation can be overcome by simultaneously measuring the two 4-point resistances at different frequencies with two lock-in amplifiers. In this work, we discuss in detail the fundamental requirements and practical considerations for the proper implementation of this measurement technique. Furthermore, we show that the relative deviation between sequential and simultaneous measurements is below 1% in the region of interest for a graphene-based field-effect transistor and 0.04% on average for resistive test structures. The simultaneous van der Pauw technique could therefore become a new standard for reliably assessing the electrical properties of 2D materials and devices.

Gate-tunable anomalous Hall effect in Bernal tetralayer graphene

Large spin-orbit coupling is often thought to be critical in realizing magnetic order-locked charge transport such as the anomalous Hall effect (AHE). Recently, artificial stacks of two-dimensional materials, e.g., magic-angle twisted bilayer graphene on hexagonal boron-nitride heterostructures and dual-gated rhombohedral trilayer graphene, have become platforms for realizing AHE without spin-orbit coupling. However, these stacking arrangements are not energetically favorable, impeding experiments and further device engineering. Here we report an anomalous Hall effect in Bernal-stacked tetralayer graphene devices (BTG), the most stable configuration of four-layer graphene. BTG AHE is switched on by a displacement field and is most pronounced at low carrier densities. The onset of AHE occurs in tandem with a full metal to a broken isospin transition indicating an orbital origin of the itinerant ferromagnetism. At lowest densities, BTG exhibits an unconventional hysteresis with step-like anomalous Hall plateaus. Persisting to several tens of kelvin, AHE in BTG demonstrates the ubiquity and robustness of magnetic order in readily available and stable multilayer Bernal graphene stacks—a new venue for intrinsic non-reciprocal responses.

Spontaneous broken-symmetry insulator and metals in tetralayer rhombohedral graphene.

Interactions among charge carriers in graphene can lead to the spontaneous breaking of multiple degeneracies. When increasing the number of graphene layers following rhombohedral stacking, the dominant role of Coulomb interactions becomes pronounced due to the significant reduction in kinetic energy. In this study, we employ phonon-polariton-assisted near-field infrared imaging to determine the stacking orders of tetralayer graphene devices. Through quantum transport measurements, we observe a range of spontaneous broken-symmetry states and their transitions, which can be finely tuned by carrier density n and electric displacement field D. Specifically, we observe a layer-antiferromagnetic insulator at n = D = 0 with a gap of approximately 15 meV. Increasing D allows for a continuous phase transition from a layer-antiferromagnetic insulator to a layer-polarized insulator. By simultaneously tuning n and D, we observe isospin-polarized metals, including spin-valley-polarized and spin-polarized metals. These transitions are associated with changes in the Fermi surface topology and are consistent with the Stoner criteria. Our findings highlight the efficient fabrication of specially stacked multilayer graphene devices and demonstrate that crystalline multilayer graphene is an ideal platform for investigating a wide range of broken symmetries driven by Coulomb interactions.

Highly Sensitive Hall Sensors Based on Chemical Vapor Deposition Graphene.

In this work, we demonstrate highly sensitive and scalable Hall sensors fabricated by adopting arrays of monolayer single-crystal chemical vapor deposition (CVD) graphene. The devices are based on graphene Hall bars with a carrier mobility of >12000 cm2 V-1 s-1 and a low residual carrier density of ∼1 × 1011 cm-2, showing Hall sensitivity higher than 5000 V A-1 T-1, which is a value previously only achieved when using exfoliated graphene encapsulated with flakes of hexagonal boron nitride. We also implement a facile and scalable polymeric encapsulation, allowing the performance of graphene Hall bars to be stabilized when measured in an ambient environment. We demonstrate that this capping method can reduce the degradation of electrical transport properties when the graphene devices are kept in air over 10 weeks. State-of-the-art performance of the realized devices, based on scalable synthesis and encapsulation, contributes to the proliferation of graphene-based Hall sensors.

Quantum Hall effect in graphene transferred by water-soluble transfer sheet and home-use laminator

For electronic device applications, large-area graphene grown on a metal catalyst by CVD requires a transfer to an insulating substrate. Previously, a transfer method that uses water-soluble polyvinyl alcohol and a laminator was proposed. The method is simple, harmless, and does not require advanced facilities. Based on this method, we fabricated fine patterns on graphene transferred onto a SiO2/Si substrate using lithography and investigated their transport properties. The carrier is controlled between the electrons and holes. The quantum Hall effect is observed at low temperatures. This method can be an option for graphene device production.

Instantaneous formation of covalently bonded diamond–graphite–graphene with synergistic properties

Diamond and graphene are the most widely used carbon allotropes and offer great potential for developing mechanical, electronic, energy-storage, and sensor applications. Their combination, especially interfacial covalent bonding, can impart excellent properties. However, achieving interfacial covalent bonding with superior performance using flexible and low-power strategies remains challenging. This study developed a novel instantaneous transformation method from diamond to graphene to prepare a new covalent structure of diamond–nano-graphite–graphene (CDGG). That is, a nanosecond-pulse laser induces sp3-to-sp2 instantaneous transformations from diamond to graphite in air, and the subsequent mechanical cleavage overcomes the weak van der Waals forces to achieve the final transformation of graphite to graphene. First, the key factors influencing laser-induced graphitization and mechanical cleavage were investigated, and a covalent carbon structure with multidirectional graphene was obtained. Furthermore, the mechanisms encompassing the lattice transformation, interface relationships, transformation time, and interface bonding were elucidated. The obtained new structure synergized the excellent properties of diamond, nano-graphite, and graphene, exhibiting superior lubrication, mechanochemical wear resistance, durability, and load-capacity. Compared to polished diamond, the obtained structure exhibited a significant decrease in the stable coefficient of friction by 49–59 % and a reduction of more than one order of magnitude in the relative wear rate under high friction against ferrous metals with a normal load of 1–9 N. Even under a heavy load of 100 N, it still exhibited superior lubrication and mechanochemical wear resistance. Finally, the preparation and patterning of covalent carbon structures were achieved on various diamond surfaces with high efficiency, environmental friendliness, and low power. This study is expected to broaden the scope of developing and applying diamond, diamond films, and graphene devices.

Experimental signatures of the transition from acoustic plasmon to electronic sound in graphene.

Fermi liquids respond differently to perturbations depending on whether their frequency is higher (collisionless regime) or lower (hydrodynamic regime) than the interparticle collision rate. This results in a different phase velocity between the collisionless zero sound and the hydrodynamic first sound. We performed terahertz photocurrent nanoscopy measurements on graphene devices, with a metallic gate close to the graphene layer, to probe the dispersion of propagating acoustic plasmons, the counterpart of sound modes in electronic Fermi liquids. We report the observation of a change in the plasmon phase velocity when the excitation frequency approaches the electron-electron collision rate that is compatible with the transition between the zero and the first sound mode.

Cleanliness of transferred graphene by acetone and acid

The cleanliness of transferred graphene is crucial for its application in various fields. The presence of residues of polymethyl methacrylate (PMMA) on the surface of transferred graphene has a negative impact on its transparency, transport, and other critical properties. In this study, we propose an alternative method to reduce such residues by using acetic acid instead of the commonly used acetone. The effects of treating graphene with acetic acid and acetone are comprehensively evaluated through optical microscopy, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results of this study demonstrate a significant reduction in surface residue when graphene is treated with acetic acid, compared to acetone treatment. Additionally, we applied this method to fabricate graphene-based supercapacitors and compared the results using acetic acid and acetone. This reduction in residue holds great promise for advancing the development of high-quality graphene devices.

Origin of electrical noise near charge neutrality in dual gated graphene device

This Letter investigates low frequency 1/f noise in an hBN encapsulated graphene device in a dual gated geometry. The noise study is performed as a function of top gate carrier density (nTG) at different back gate density (nBG). The noise at low nBG is found to be independent of top gate carrier density. With increasing nBG, noise value increases, and a noise peak is observed near charge inhomogeneity of the device. A further increase in nBG leads to a decrease in noise magnitude. The shape of the noise is found to be closely related to a charge inhomogeneity region of the device. Moreover, the noise and conductivity data near charge neutrality show clear evidence of noise emanating from a combination of charge number and mobility fluctuation.

Revealing flat bands and hybridization gaps in a twisted bilayer graphene device with microARPES

Controlling the electronic structure of two-dimensional materials using the combination of twist angle and electrostatic doping is an effective means to induce emergent phenomena. In bilayer graphene with an interlayer twist angle near the magic angle, the electronic dispersion is strongly modified by a manifold of hybridizing moiré Dirac cones leading to flat band segments with strong electronic correlations. Numerous technical challenges arising from spatial inhomogeneity of interlayer interactions, twist angle and device functionality have so far limited momentum-resolved electronic structure measurements of these systems to static conditions. Here, we present a detailed characterization of the electronic structure exhibiting miniband dispersions for twisted bilayer graphene, near the magic angle, integrated in a functional device architecture using micro-focused angle-resolved photoemission spectroscopy. The optimum conditions for visualizing the miniband dispersion are determined by exploiting the spatial resolution and photon energy tunability of the light source and applied to extract a hybridization gap size of eV and flat band segments extending across a moiré mini Brillouin zone. In situ electrostatic gating of the sample enables significant electron-doping, causing the conduction band states to shift below the Fermi energy. Our work emphasizes key challenges in probing the electronic structure of magic angle bilayer graphene devices and outlines conditions for exploring the doping-dependent evolution of the dispersion that underpins the ability to control many-body interactions in the material.

Valley-dependent transport in a mescoscopic twisted bilayer graphene device

We study the valley-dependent electron transport in a four-terminal mesoscopic device of the two monolayer graphene nanoribbons vertically stacked together, where the intersection forms a bilayer graphene lattice with a controllable twist angle. Using a tight-binding lattice model, we show that the longitudinal and transverse conductances exhibit significant valley polarization in the low energy regime for small twist angles. As the twist angle increases, the valley polarization shifts to the high energy regime. This arises from the regrouping effect of the electron band in the twisted bilayer graphene region. But for relatively large twist angles, no significant valley polarization is observed. These results are consistent with the spectral densities of the twisted bilayer graphene.

Interface effects on titanium growth on graphene

Poor quality interfaces between metal and graphene cause non-linearity and impair the carrier mobility in graphene devices. Here, we use aberration corrected scanning transmission electron microscopy to observe hexagonally close-packed Ti nano-islands grown on atomically clean graphene, and establish a 30∘ epitaxial relationship between the lattices. Due to the strong binding of Ti on graphene, at the limit of a monolayer, the Ti lattice constant is mediated by the graphene epitaxy, and compared to bulk Ti, is strained by ca. 3.7% to a value of 0.306(3) nm. The resulting interfacial strain is slightly greater than what has been predicted by density functional theory calculations. Our early growth stage investigations also reveal that, in contrast to widespread assumptions, Ti does not fully wet graphene but grows initially in islands with a thickness of 1–2 layers. Raman spectroscopy implies charge transfer between the Ti islands and graphene substrate.

Graphene versus concentrated aqueous electrolytes: the role of the electrochemical double layer in determining the screening length of an electrolyte

Understanding the performance of graphene devices in contact with highly concentrated aqueous electrolytes is key to integrating graphene into next-generation devices operating in sea water environments, biosensors, and high-density energy production/storage units. Despite significant efforts toward interpreting the structure of the electrochemical double layer at high concentrations, the interface between graphene-based materials and concentrated aqueous solutions has remained vaguely described. In this study, we demonstrate the use of graphene-based chemiresistors as a technique to indirectly quantify the experimental screening length of concentrated electrolytes that could clarify the interpretation of electrochemical measurements conducted at low ionic strength. We report a breakdown of the Debye–Hückel theory in the proximity of graphene surfaces at lower concentrations (10–50 mM) than previously reported for other systems, depending on cation size, dissolved oxygen concentration, and degree of graphene defectivity.

Electron-hole asymmetric magnetotransport of graphene-colloidal quantum dot device

Interfacing graphene with other low-dimensional material has gained attentions recently due to its potential to stimulate new physics and device innovations for optoelectronic and electronic applications. Here, we exploit a solution-processed approach to introduce colloidal quantum dot (CQD) to the bilayer graphene device. The magnetotransport properties of the graphene device is drastically altered due to the presence of the CQD potential, leading to the observation of AB-like oscillation in the quantum Hall regime and screening of the intervalley scattering. The anomalous magnetotransport behavior is attributed to the coulombic scattering introduced by the CQDs and is shown to be highly asymmetric depending on the polarity of the transport carriers. These results prove the potential of such flexible method for engineering microscopic scattering process and performance of the graphene device that may lead to intriguing device application in such hybrid system.

Chemically detaching hBN crystals grown at atmospheric pressure and high temperature for high-performance graphene devices

In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow to fabricate high carrier mobility graphene-hBN devices. By combining spatially-resolved confocal Raman spectroscopy and electrical transport measurements, we confirm the excellent quality of these crystals for high-performance hBN-graphene-based van der Waals heterostructures. The full width at half maximum of the graphene Raman 2D peak is as low as 16 cm−1, and the room temperature charge carrier mobilitiy is around 80 000 cm2/(Vs) at a carrier density 1 × 1012 cm−12. This is fully comparable with devices of similar dimensions fabricated using crystalline hBN synthesized by the high pressure and high temperature method. Finally, we show that for exfoliated high-quality hBN flakes with a thickness between 20 and 40 nm the line width of the hBN Raman peak, in contrast to the graphene 2D line width, is not useful for benchmarking hBN in high mobility graphene devices.

Enhancement of Underlayer Coverage in Sub‐Millimeter Bilayer Graphene by Modifying the Streamline Direction

The growth of large‐scale bilayer graphene (bi‐graphene) is significantly important for graphene‐based device fabrications. Chemical vapor deposition is usually used for the synthesis of high‐quality and large‐scale thin films including various monolayers and bilayers. However, a major challenge for bi‐graphene growth is the so‐called limited underlayer coverage, i.e., the faster growth of the top layer than the underlayer. Herein, using the circumfluence chemical vapor deposition, it is demonstrated that the underlayer growth can be greatly enhanced via optimizing the streamline, and high‐quality AB‐stacking sub‐millimeter bi‐graphene with underlayer coverage over 93% is achieved successfully. Raman spectroscopy and selected area electron diffraction confirm the high crystalline quality and uniformity of the as‐grown bilayers. The as‐fabricated field‐effect transistor using the bi‐graphene as the channel layer exhibits typical semiconductor transfer characteristics and a nonzero bandgap which is required for device applications. It is suggested that the optimized streamline design largely allows a reduction of difference in the edge growth kinetics between the top and bottom layers. Thus, in this work, a promising technical route is presented for the growth of large‐scale bi‐graphene with high underlayer coverage, beneficial for the development of functional graphene devices.

Machine Learning Attacks-Resistant Security by Mixed-Assembled Layers-Inserted Graphene Physically Unclonable Function.

Mixed layers of octadecyltrichlorosilane (ODTS) and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FOTS) on an active layer of graphene are used to induce a disordered doping state and form a robust defense system against machine-learning attacks (ML attacks). The resulting security key is formed from a 12×12 array of currents produced at a low voltage of 100mV. The uniformity and inter-Hamming distance (HD) of the security key are 50.0 ± 12.3% and 45.5 ± 16.7%, respectively, indicating higher security performance than other graphene-based security keys. Raman spectroscopy confirmed the uniqueness of the 10,000 points, with the degree of shift of the G peak distinguishing the number of carriers. The resulting defense system has a 10.33% ML attack accuracy, while a FOTS-inserted graphene device is easily predictable with a 44.81%ML attack accuracy.