Dirac Point Of Graphene Research Articles

Orthogonal Surface‐Enhanced Raman Scattering/Field‐Effect Transistor Detection of Breast and Colorectal Cancer‐Derived Exosomes using Graphene as a Tag‐Free Diagnostic Template

Cancer is one of the leading causes of high mortality worldwide, affecting most of the body organs including the breast and colon. Triple‐negative breast (TNBC) and colorectal cancers (CRC) usually present the poorest clinical outcomes and lowest disease‐free survival rates among the different cancer types. Novel biomarkers for early detection of TNBC and CRC have been proposed throughout the years, although the rate of success remains limited. Herein, a dual method for detection of cancer‐derived exosomes based on the measurement of the Raman spectra in graphene field‐effect transistors (GFETs) is proposed, thereby obtaining electrical signal metrics related to the variation of the Dirac point in graphene and optical features corresponding to representative bio‐molecular cancer structures in an orthogonal way. This pioneering method and classification routine can potentially be extended to other types of cancer, thus creating a unique and universal tag‐free diagnostic template for early cancer diagnostics.

Electron Transport Properties of Graphene/WS2 Van Der Waals Heterojunctions.

Van der Waals heterojunctions of two-dimensional atomic crystals are widely used to build functional devices due to their excellent optoelectronic properties, which are attracting more and more attention, and various methods have been developed to study their structure and properties. Here, density functional theory combined with the nonequilibrium Green's function technique has been used to calculate the transport properties of graphene/WS2 heterojunctions. It is observed that the formation of heterojunctions does not lead to the opening of the Dirac point of graphene. Instead, the respective band structures of both graphene and WS2 are preserved. Therefore, the heterojunction follows a unique Ohm's law at low bias voltages, despite the presence of a certain rotation angle between the two surfaces within the heterojunction. The transmission spectra, the density of states, and the transmission eigenstate are used to investigate the origin and mechanism of unique linear I-V characteristics. This study provides a theoretical framework for designing mixed-dimensional heterojunction nanoelectronic devices.

Topological Exact Flat Bands in Two-Dimensional Materials under Periodic Strain.

We study flat bands and their topology in 2D materials with quadratic band crossing points under periodic strain. In contrast to Dirac points in graphene, where strain acts as a vector potential, strain for quadratic band crossing points serves as a director potential with angular momentum ℓ=2. We prove that when the strengths of the strain fields hit certain "magic" values, exact flat bands with C=±1 emerge at charge neutrality point in the chiral limit, in strong analogy to magic angle twisted-bilayer graphene. These flat bands have ideal quantum geometry for the realization of fractional Chern insulators, and they are always fragile topological. The number of flat bands can be doubled for certain point group, and the interacting Hamiltonian is exactly solvable at integer fillings. We further demonstrate the stability of these flat bands against deviations from the chiral limit, and discuss possible realization in 2D materials.

Lévy flight for electrons in graphene: Superdiffusive-to-diffusive transport transition

In this work we propose an electronic L\'evy flight device, analogous to a recent optical realization. To that end, we investigate the transmission of electrons in graphene nanoribbons in the presence of circular electrostatic clusters, whose diameter follow a power-law distribution. We analyze the effect of the electrostatic clusters on the electronic transport regime of the nanoribbons, in terms of its diffusion behavior. Our numerical calculations show that the presence of circular electrostatic clusters induces a transition from L\'evy (superdiffusive) to diffusive transport as the energy increases. Furthermore, we argue that in our electronic L\'evy flight device, superdiffusive transport is an exclusive feature of the low-energy quantum regime, while diffusive transport is a feature of the semiclassical regime. Therefore, we attribute the observed transition to the chiral symmetry breaking, once the energy moves away from the Dirac point of graphene.

Interface design for the transport properties in asymmetric two-dimensional van der Waals multiferroic tunnel junctions

Multiferroic tunnel junctions (MFTJs) composed of ferroelectricity and ferromagnetism have attractive prospects in memory and computing device applications due to their low power consumption and nonvolatile switching. However, achieving both spin filter effect (SFE) and excellent tunnel electroresistance (TER) effect in two-dimensional (2D) van der Waals (vdW) MFTJs still remain challenging. Herein, using first-principles calculations, we investigate interface design for electronic and transport properties of asymmetric 2D vdW MFTJs that consist of CrI 3 , graphene, and ferroelectric Sc 2 CO 2 (containing Sc– P↑ and Sc– P↓ states). We find that such CrI 3/Sc– P↑/graphene MFTJs exhibit p -type Ohmic contact for Sc– P↑/graphene and SFE for CrI 3 / Sc– P↑ due to the transformation of CrI 3 from intrinsic semiconductor to half-metal. Simultaneously, the Dirac point of graphene at the Fermi energy has a big shift about 0.95 eV with the polarization reversal, suggesting that there is a giant barrier modulation. Moreover, the huge TER ratio about 1.7′10 5 % was also discovered by transport calculations. Our results provide an avenue for the design of future 2D vdW MFTJs. • 2D CrI 3 /Sc 2 CO 2 /graphene van der Waals MFTJ shows multiple nonvolatile resistance and high ratio of tunneling electroresistance. • Two heterointerfaces in CrI 3 / Sc– P↑/graphene exhibit Ohmic contact, while those in CrI 3 / Sc– P↓/graphene are Schottky contact. • Spin filter effect can take place in CrI 3 / Sc– P↑/graphene due to the transformation of CrI 3 from intrinsic semiconductor to half-metal. • Dirac point of graphene at the Fermi energy has a big shift about 0.95 eV with the polarization reversal.

Gap opening at the Dirac point of graphene on Cu(111): Hybridization versus sublattice symmetry breaking

Gap opening at the Dirac point of graphene on metals has been ascribed to hybridization, sublattice symmetry breaking, or combination of both. However, how and what to extent gap opens under the influence of these effects has been a question in controversy. In this paper, we explore gap opening in graphene on metals using a simple tight-binding (TB) model for a hypothetical heterobilayer, which mimics a system of graphene/metal. A theoretical framework for the gap opening is obtained using exact solutions to the TB-model Hamiltonian through small-parameter expansions. This result is formally similar to that obtained for an ad hoc model graphene with pseudospin asymmetry originating from the spin-orbit proximity effect, which is not considered in the present analysis but is taken over by more substantial effect of orbital-hybridization indispensable for the gap opening in graphene on metals. The present result reveals, for the first time, that sublattice symmetry breaking and hybridization asymmetry compensate each other and disruptively contribute to the gap opening at the Dirac point of graphene on metals. The picture is in marked contrast to the previous ones such as hybridization-induced sublattice symmetry breaking or an additive model, in which sublattice symmetry breaking and hybridization additively contribute to the gap opening. The results obtained by the present TB-model analysis are successfully used to interpret the results of ab initio density functional theory (DFT) calculations for graphene/Cu(111) and thereby, for the first time, to discriminate sublattice symmetry breaking and hybridization asymmetry explicitly. We also find, through these analyses of the DFT calculations, that hybridization asymmetry dominates gap opening at the Dirac point for large graphene-substrate separations (d), while sublattice symmetry breaking is dominant for small d, and both are balanced with each other in between to yield a small gap, which is practically closed in the system of graphene/Cu(111). Another point revealed by those analyses is that sublattice symmetry breaking and hybridization are necessarily in combination with each other and are together large or small depending on interface structures. All these results are expected to provide a useful insight into gap opening at the Dirac point of graphene in other systems including graphene/metal superstructures resulting from lattice mismatch, misorientation, intercalation, or other structural modifications.

The controllable electronic characteristics and Schottky barrier of graphene/GaP heterostructure via interlayer coupling and in-plane strain

Establishing the desired heterostructures by assembling suitable semiconductor materials has shown significant potential for applications in next-generation micro-nano electronic devices. In this present contribution, we demonstrate the geometric structure and electronic properties of graphene/GaP heterostructure by first-principles calculations. It is found that the heterostructure is characterized by weak interlayer coupling accompanying the stable layer spacing of 3.40 Å and binding energy of −39.35 meV, meaning that the interlayer is dominated by van der Waals (vdW) force. The electronic band structure of free-standing graphene and GaP monolayers are preserved well. Meanwhile, a tiny bandgap of approximatively 20 meV at the Dirac point of graphene is opened, which is attributed to the breakdown of sublattice symmetry. In the ground state, the Schottky contact of p-type is present with the n-type and p-type SBH of 1.71 eV and 0.10 eV, respectively, which can be effectively induced by imposing interlayer coupling as well as in-plane strains. Especially, the transition of p-Schottky contact to p-Ohmic contact occurs when the layer spacing decreases to 3.20 Å or the strain increases to +2 %. These theoretical results may offer potential guiding principle in future electronic devices.

Electric‐Field‐Tunable Bandgaps in the Inverse‐Designed Nanoporous Graphene/Graphene Heterobilayers

AbstractThe recent bottom‐up synthesis of atomically precise nanoporous graphene (NPG) offers a way of tuning graphene's properties by forming NPG/graphene (Grp) bilayers. Depending on the size, shape, and periodicity of the nanopores in NPG, the heterobilayers can exhibit various functionalities. This theoretical work presents an inverse design of NPG/Grp bilayers with electric‐field‐tunable bandgaps as a target property. The interlayer interaction in such heterobilayers can induce a bandgap in graphene either by breaking inversion symmetry (type I) or by moving and merging Dirac points of graphene (type II). The bandgap opening also requires electron–hole symmetry breaking induced by an applied perpendicular electric field, leading to two distinct, linear versus nonlinear, field dependences of the bandgap for the type‐I and type‐II cases, respectively. To translate the underlying physics of the bandgap opening in graphene into real atomic structures, the authors develop an inverse design method and find NPG/Grp bilayers with the target functionality. The field‐tunable bandgap in graphene, supported by first‐principles calculations for the inverse‐designed systems, holds promise for new types of graphene transistors.

Dirac Point Modulated Self-Powered Ultrasensitive Photoresponse and Color-Tunable Electroluminescence from Flexible Graphene/Metal–Organic Frameworks/Graphene Vertical Phototransistor

Integrated wearable optoelectronic devices synchronously exhibit contradictory phenomena of exciton generation and recombination with their control by an external electric field, and the mechanical strain associated with this has widespread applications. However, a demonstration of such single-miniaturized multifunctional wearable optoelectronics remains a daunting challenge. A versatile metal–organic framework (MOF) possesses tailorable outstanding optical properties through an inherent multiple charge-transfer mechanism between metal and ligand. Yet, its commercialization remains unsuccessful because of large porosity, poor conductivity, and deficient crystallinity. Graphene is honorable for its outstanding optoelectronic and mechanical behaviors. Combining excellent features of a versatile MOF with monolayer graphene, unprecedently, we report a self-powered ultrasensitive (external quantum efficiency ∼3 × 1010%) and ultrafast (response time ≈ 220 μs) wearable vertical phototransistor by utilizing a graphene/MOF/graphene/poly(vinylidene fluoride-co-trifluoroethylene) heterojunction on a flexible polydimethylsiloxane substrate through the formation of asymmetric Schottky junctions at MOF/graphene interfaces by a judicious incorporation of a piezo-sensitive poly(vinylidene fluoride) layer. Importantly, by the Dirac point modulation of graphene through a mechanical strain, we achieve optical strain sensors that can modify the amplitude and polarity of the photocurrent. Additionally, controlling the Dirac point of graphene and charge-transfer mechanisms of the MOF by a suitable electrical bias and strain, we demonstrate color-tunable broadband light emission from a flexible vertical phototransistor. Our unique discovery will enrich wearable integrated optoelectronics.

Electronic and Magnetic Properties of The Graphene/RE/Ni(111) (RE: La, Yb) Intercalation‐Like Interfaces: A DFT Analysis

AbstractThe effect of the rare‐earth (RE) metals (La and Yb) intercalation on the electronic and magnetic properties of the graphene/Ni(111) interface is studied using state‐of‐the‐art density functional theory calculations. In both systems, the intercalation of RE leads to the dramatic decrease of the magnetic moments of the Ni‐interface atoms and to the negligible moments of C‐atoms in a graphene layer, compared to the parent graphene/Ni(111) system. At the same time, the significant ‐doping of graphene together with a band‐gap opening is observed in both cases of the RE intercalation with a position of the graphene Dirac point reaching eV. Also the large density of states is found in the vicinity of the Fermi level () along the direction for the graphene‐derived states which can be attributed to the joint effect of the intercalated RE and interface Ni atoms. These factors – increased density of states at and absence of magnetism of C‐atoms in a graphene layer – indicate the possibility of the observation of the superconductive state in graphene in the considered RE‐based systems, which is important for the understanding of these and other electronics effects in the graphene‐based systems.

Anomalies at the Dirac Point in Graphene and Its Hole-Doped Compositions.

We study the properties of the Dirac states in SiC-graphene and its hole-doped compositions employing angle-resolved photoemission spectroscopy and density functional theory. The symmetry-selective measurements for the Dirac bands reveal their linearly dispersive behavior across the Dirac point which was termed as the anomalous region in earlier studies. No gap is observed even after boron substitution that reduced the carrier concentration significantly from 3.7×10^{13} cm^{-2} in SiC-graphene to 0.8×10^{13} cm^{-2} (5% doping). The anomalies at the Dirac point are attributed to the spectral width arising from the lifetime and momentum broadening in the experiments. The substitution of boron at the graphitic sites leads to a band renormalization and a shift of the Dirac point towards the Fermi level. The internal symmetries appear to be preserved in SiC-graphene even after significant boron substitutions. These results suggest that SiC-graphene is a good platform to realize exotic science as well as advanced technology where the carrier properties like concentration, mobility, etc., can be tuned keeping the Dirac fermionic properties protected.

Phononic linear and quadratic nodal points in monolayer XH (X=Si, Ge, Sn)

Topological phases in two-dimensional (2D) systems have been attracting tremendous attention since the discovery of graphene. Since the experimental probing could proceed in the whole phonon spectrum, intensive research effort has been devoted to the topological quantum phases in phononic systems. Via first-principles calculations, we predict that a family of 2D hexagonal materials, XH (X = Si, Ge, Sn), hosts ideal linear nodal points (LNPs) and quadratic phononic nodal points (QNPs). Specifically, the LNPs appear at the two inequivalent valleys, akin to the 2D Dirac point in graphene, connecting by an edge arc. The QNP is pinned at the Γ point, two edge states emerge from their projections. Remarkably, both LNPs and QNP enjoy an emergent chiral symmetry, we then show that they feature nontrivial topological charges. As a consequence, our work discusses the nodal points in the phonon spectrum of 2D materials and provides ideal candidates to study the topology for bosonic systems.

Dry and hydrated defective molybdenum Disulfide/Graphene bilayer heterojunction under strain for hydrogen evolution from water Splitting: A First-principle study

In search of exploiting cost-effective, highly stable, efficient, and greatly active electrocatalysts for hydrogen evolution reaction (HER), molybdenum disulfide-graphene heterostructures (MoS2/graphene) are promising, in contrast to noble metals. We used density functional theory (DFT) to examine the changes in the electronic structure of MoS2/graphene under compressive strain for structural defects in the MoS2 and graphene layers, as well as from hydration of the MoS2 layer. In the pristine MoS2/graphene heterostructure, a small bandgap (minigap) is opened at the graphene Dirac point, which is located above the Fermi energy, for dry and hydrated configurations. The presence of sulfur and carbon vacancies in the MoS2/graphene upshifts the Dirac point and widens the minigap. However, additional sulfur vacancies further widen or shrink the minigap depending on the location of the MoS2 conduction band bottom and the presence of defect bands at the Dirac point. The minigap tunability in the MoS2/graphene heterostructure could be engineered for producing efficient HER electrocatalysts and beyond. The Quantum Theory of Atoms in Molecules (QTAIM) reveals SC bond critical points, which correspond to van der Waals forces interactions in agreement with the Non-covalent Interaction (NCI) analysis. A correlation is drawn between the minigap and the electron density at the SC bond critical points.

Mechanical strain and electric-field modulation of graphene transistors integrated on MEMS cantilevers

This work proposes a structure which allows characterization of graphene monolayers under combined electric field and mechanical strain modulation. Our approach is based on a cantilever integrated into a two-dimensional graphene-based Field effect transistor (FET). This allows us to change graphene properties either separately or together via two methods. The first way involves electric field induced by the gate. The second is induction of mechanical strain caused by external force pushing the cantilever up or down. We fabricated devices using silicon-on-insulator wafer with practically zero value of residual stress and a high-quality dielectric layer which allowed us to precisely characterize structures using both mentioned stimuli. We used the electric field/strain interplay to control resistivity and position of the charge neutrality point often described as the Dirac point of graphene. Furthermore, values of mechanical stress can be obtained during the preparation of thin films, which enables the cantilever to bend after the structure is released. Our device demonstrates a novel method of tuning the physical properties of graphene in silicon and/or complementary metal-oxide-semiconductor technology and is thus promising for tunable physical or chemical sensors.

Detection of dielectric screening effect by excitons in two-dimensional semiconductors and its application

Atomically thin transition metal dichalcogenides (TMDCs) like MX<sub>2</sub> (M = W or Mo, X = S or Se) are well-known examples of two-dimensional (2D) semiconductors. They have attracted wide and long-lasting attention due to the strong light-matter interaction and unique spin-valley locking characteristics. In the 2D limit, the reduced dielectric screening significantly enhances the Coulomb interaction. The optical properties of monolayer TMDCs are thus dominated by excitons, the tightly bound electron-hole pairs. In this work, we briefly overview the history and recent research progress of optical spectroscopy studies on TMDCs. We first introduce the layer-dependent band structure and the corresponding modifications on optical transitions, and briefly mention the effects of external magnetic fields and the charge doping on excitons. We then introduce a novel sensing technique enabled by the sensitivity of excitons to the dielectric environment. The exciton excited states (Rydberg states) observed in monolayer TMDCs have large Bohr radii (> few nm), where the electric field lines between electron-hole pairs well extends out of the material. Hence the Coulomb interaction (which affects the quasiparticle band gap and exciton binding energies) in the monolayer TMDC is sensitive to the dielectric environment, making the excitons in 2D semiconductor an efficient quantum sensor in probing dielectric properties of the surroundings. The method is of high spatial resolution and only diffraction limited. We enumerate the applications of monolayer WSe<sub>2</sub> dielectric sensor in detecting the secondary Dirac point of graphene induced by the graphene-hBN superlattice potential, as well as the fractional correlated insulating states emerging in WS<sub>2</sub>/WSe<sub>2</sub> moiré superlattices. Meanwhile, a unified framework for describing the many-body interactions and dynamical screenings in the system is still lacking. Future theoretical and experimental efforts are needed for a complete understanding. Finally, we further discuss the perspectives and potential applications of this non-destructive and efficient dielectric sensing method.

Imaging Reconfigurable Molecular Concentration on a Graphene Field-Effect Transistor.

The spatial arrangement of adsorbates deposited onto a clean surface under vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors (FETs) exhibit reversible, electrically tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables the precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. Gate-tunable molecular concentration is explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. The molecular surface concentration is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.

Sensing Molecules with Metal-Organic Framework Functionalized Graphene Transistors.

Graphene is inherently sensitive to vicinal dielectrics and local charge distributions, a property that can be probed by the position of the Dirac point in graphene field-effect transistors. Exploiting this as a useful sensing principle requires selectivity; however, graphene itself exhibits no molecule-specific interaction. Complementarily, metal-organic frameworks can be tailored to selective adsorption of specific molecular species. Here, a selective ethanol sensor is demonstrated by growing a surface-mounted metal-organic framework (SURMOF) directly onto graphene field-effect transistors (GFETs). Unprecedented shifts of the Dirac point, as large as 15V, are observed when the SURMOF/GFET is exposed to ethanol, while a vanishingly small response is observed for isopropanol, methanol, and other constituents of the air, including water. The synthesis and conditioning of the hybrid materials sensor with its functional characteristics are described and a model is proposed to explain the origin, magnitude, and direction of the Dirac point voltage shift. Tailoring multiple SURMOFs to adsorb specific gases on an array of such devices thus generates a versatile, selective, and highly sensitive platform for sensing applications.

Coverage-dependent magnetic and electronic properties of graphene with Co adatoms

Decorating two-dimensional (2D) materials with transition-metal adatoms is an effective way to bring about new physical properties that are intriguing for applications in electronics and spintronics devices. Here, we systematically studied the coverage-dependent magnetic and electronic properties of graphene decorated by Co adatoms, based on first-principles calculations. We found that if the Co coverage is larger than 1/3[Formula: see text]ML, the Co atoms will aggregate to form a Co monolayer and then a van der Waals bilayer system between the Co monolayer and graphene forms. When the Co coverage is [Formula: see text][Formula: see text]ML, the Co adatom is spin-polarized with spin moment varying from 1.1 to 1.4[Formula: see text][Formula: see text]. The [Formula: see text] and [Formula: see text] orbitals of Co hybridize significantly with the [Formula: see text] bands of graphene, which generates a series of new bands in the energy range from [Formula: see text][Formula: see text]eV to 1[Formula: see text]eV with respect to the Dirac point of graphene. In most cases, the new bands near the Fermi level lead to topological states characterized by the quantum anomalous Hall effect.

Electronic structure of 2D quaternary materials and of their van der Waals heterostructures

The properties of the two dimensional quaternary compounds AgScP2Se6, AgBiP2Se6, CuBiP2Se6, and CuInP2S6 and the corresponding van der Waals heterostructures are studied using density functional theory. These compounds are dynamically stable, their electronic bandgaps range from 2.13 to 2.68 eV, and the positions of their band edges are suitable for their use for water splitting. Among the different heterostructures made from these monolayers, it is demonstrated that AgBiP2Se6/AgScP2Se6 and AgBiP2Se6/CuBiP2Se6 are type II heterostructures, which allow an efficient electron–hole separation. Then, we studied the electronic properties of the graphene/quaternary-2D-compound heterostructures: a shift of the graphene Dirac-point above the Fermi level is observed, which corresponds to a p-type heterostructure. Overall, this family of materials appears to be very promising for optical and electronic applications.

Spin-boson model of quantum dissipation in graphene: Nonlinear electrical response

Dissipation in the quantum solid of graphene is couched in the methodology of nonequilibrium statistical mechanics of an open quantum system. Of specific interest is the absorption accompanied by relaxation of energy because of an external frequency-dependent oscillatory electric field. At optical frequencies and for sizable amplitude of the electric field, the response is nonlinear, the treatment of which is simplified by means of a ``rotating wave approximation'' in which rapidly changing, off-resonance terms are omitted. The characteristic resonant frequency is what quantifies the tunneling between the valence and the conduction bands across the so-called Dirac point in graphene. The valence and the conduction states are mapped into the eigenstates of pseudo Pauli spin operators and the corresponding Hamiltonian, when embedded in a dissipative heat bath comprising the surrounding phonons and other electrons, makes possible a comprehensive analysis in terms of the much-studied spin-boson model of dissipative quantum statistical mechanics. A master equation for the density operator associated with this Hamiltonian yields rate equations for the mean population of the valence and the conduction states as well as the transition (matrix elements) between them. Further approximation of these rate equations allows contact with phenomenological treatments of the nonlinear optical conductivity. The present paper then provides a microscopic framework for investigating the response characteristics of a material of great topical interest using contemporary methods of quantum dissipation.