Position Of Dirac Point Research Articles

Field-Effect Thermoelectric Hotspot in Monolayer Graphene Transistor.

Graphene is a promising candidate for the thermal management of downscaled microelectronic devices owing to its exceptional electrical and thermal properties. Nevertheless, a comprehensive understanding of the intricate electrical and thermal interconversions at a nanoscale, particularly in field-effect transistors with prevalent gate operations, remains elusive. In this study, nanothermometric imaging is used to examine a current-carrying monolayer graphene channel sandwiched between hexagonal boron nitride dielectrics. It is revealed for the first time that beyond the expected Joule heating, the thermoelectric Peltier effect actively plays a significant role in generating hotspots beneath the gated region. With gate-controlled charge redistribution and a shift in the Dirac point position, an unprecedented systematic evolution of thermoelectric hotspots, underscoring their remarkable tenability is demonstrated. This study reveals the field-effect Peltier contribution in a single graphene-material channel of transistors, offering valuable insights into field-effect thermoelectrics and future on-chip energy management.

Strong doping reduction on wafer-scale CVD graphene devices via Al2O3 ALD encapsulation

We present the electrical characterization of wafer-scale graphene devices fabricated with an industrially-relevant, contact-first integration scheme combined with Al2O3 encapsulation via atomic layer deposition. All the devices show a statistically significant reduction in the Dirac point position, Vcnp , from around +47 V to between −5 and 5 V (on 285 nm SiO2), while maintaining the mobility values. The data and methods presented are relevant for further integration of graphene devices, specifically sensors, at the back-end-of-line of a standard CMOS flow.

Topological phase diagram of optimally shaken honeycomb lattices: A dual perspective from stroboscopic and nonstroboscopic Floquet Hamiltonians

We present a direct comparison between the stroboscopic and nonstroboscopic effective approaches for ultracold atoms in shaken honeycomb lattices, focusing specifically on the optimal driving introduced by A. Verdeny and F. Mintert []. In the fast-driving regime, we compare the effective nonstroboscopic Hamiltonian derived through a perturbative expansion with a nonperturbative calculation of the stroboscopic Floquet Hamiltonian, obtained through a direct numerical approach. We find that the leading off-diagonal tunneling coefficients, which define the undriven model, retain their isotropic nature under the effect of the driving, and their values are independent of the computational approach used. This consistency ensures the robustness of certain topological properties, such as the position of Dirac points, as expected. Conversely, the next-to-leading diagonal tunneling coefficients, generated dynamically, can vary across different approaches without affecting the system topology. This suggests that the topological phase diagram is most conveniently represented in terms of the physical parameters characterizing the driving and directly accessible in experiments, rather than in terms of the tunneling parameters. Published by the American Physical Society 2024

Computationally Efficient Region-Wise Potential- Based Extremely Closed-Form Analytical Modeling of B/N Substitution Doped GFETs

A simplified region-wise potential-based analytical model is established for boron (B) or nitrogen (N) substitution doped graphene field-effect transistor (GFET). The closed-form direct analytical relation between graphene channel potential and applied bias condition is developed by imposing the effective approximation for Fermi–Dirac integral function in various regions of operation. The boundary for distinct GFET operating regime is separated based on the position of Dirac point and Fermi energy level with respect to applied bias condition. The semiclassical drift and diffusive transport model is utilized to obtain the drain current. In addition to that, the drift component, diffusion coefficient, and its corresponding current component for B and N substitution doped GFETs have been modeled individually and their influence on total current is discussed. The proposed region-potential-based analytical model has shown good agreement with numerically solved self-consistent model. Also, the physical insights in device design, such as threshold voltage and saturation drain potential model with respect to various doping/device parameters, have been examined.

Two band tunneling for a pnp junction in tetralayer graphene

We investigate the band structure and the tunneling of ABCA-tetralayer graphene (ABCA-TTLG) subjected to an external potential U 0 applied between top and bottom layers. Using the tight-binding model, including the nearest t and next-nearest-neighbor t ′ hopping, low-energy model and two-band approximation model we study the band structure variation along the lines Γ − M − K − Γ in the first Brillouin zone , electronic band gap near Dirac point K , transmission properties and conductance, respectively. Our results reveal that ABCA-TTLG exhibits markedly different properties as functions of t ′ and U 0 . We show that the hopping parameter t ′ changes the energy dispersion, the position of K and breaks sublattice symmetries. A sizable band gap is created at K , which could be opened and controlled by the applied potential U 0 . This gives rise to 1D-like van Hove singularities (VHS) in the density of states (DOS). Resonant electronic transmission through graphene-based a p n p junction is studied as a function of the incident wave vector, the width and height of the barrier with and without U 0 . The resonant features in the transmission result from resonant hole states in the barrier and strongly influence the conductance. Our results are numerically discussed and compared with the literature. • Quantum tunneling of ABCA-tetralayer graphene in an potential applied between top and bottom layers is investigated. • The hopping parameter acts by changing the energy dispersion and the position of Dirac point K, also breaks the sublattice symmetries. • A sizable band gap is created at K, which could be opened and controlled by the potential giving rise to 1D-like van Hove singularities (VHS) in the density of states. • The resonant features in the transmission result from resonant hole states in the barrier and strongly influence the conductance.

Liquid-gated Field-effect-transistor Based on Chemically Reduced Graphene Oxide for Sensing Neurotransmitter Acetylthiocholine

In this work, an enzymatic liquid-gated field-effect-transistor sensor based on chemically reduced graphene oxide film was develop for determination of acetylthiocholine in aqueous conditions. The device was designed with interdigitated electrode configuration and then manufactured by combining lithography and chemical vapor deposition techniques in clean room. Graphene oxide material (prepared by Hummer method) was chemically reduced using a strong reducing agent hydrazine, and then drop-casted onto the channel region. The results have demonstrated a successful reduction of graphene oxide with clearly shifting of 02 characteristic peaks comparing with graphene oxide. Consequently, the transfer curve of as-prepared reduced graphene oxide based transistor exhibits ambipolar characteristics with a V-shape. Acetylcholinesterase was immobilized on top of reduced graphene oxide film with the aid of glutaraldehyde trapping agent. It was found that the release of proton from enzymatic hydrolysis of acetylthiocholine has caused significant variation in charge concentration and mobility in the channel, thus generated a significant blue shift in position of Dirac point on ambipolar curve. The developed sensor exhibits good sensing performances with LOD of 250 µM in concentration range 0 – 0.8 mM.

Band gap opening and surface morphology of monolayer graphene induced by single ion impacts of argon monomer and dimer ions

Craters have been observed upon irradiation by single ion impacts of Ar monomer and Ar dimer ions with 35 keV/atom in monolayer graphene on copper at a fluence of 1 × 1012 atoms/cm2. The observed craters at the ion impacts are in the underlying copper substrate, which pull down the graphene layer conformally towards them, introducing disorder into graphene. The number density of craters per atom produced by Ar-monomers is higher than that of Ar-dimer ions. In the ion impact region, disorder is high, and graphene is metallic in both Ar-monomer and dimer irradiation. In the ion impact region of dimer irradiated graphene, local density of states is higher and there is a shift in Dirac point position by +0.07 eV and a bandgap opening of 0.25 eV in the ordered region ∼15 nm away from the ion impact. Microscopic ripples in graphene are not observed in the case of monomer irradiation while they are present in the case of dimer irradiation with a reduced wavelength. Isotropic compressive strain is introduced by the ion impacts and it is larger for Ar-dimer irradiation. The combined effects of compressive strain along with breaking the equivalence between A and B sublattice of graphene is attributed to the opening of the band gap in graphene.

Influence of the Electrolyte Salt Concentration on DNA Detection with Graphene Transistors.

Liquid-gated Graphene Field-Effect Transistors (GFET) are ultrasensitive bio-detection platforms carrying out the graphene’s exceptional intrinsic functionalities. Buffer and dilution factor are prevalent strategies towards the optimum performance of the GFETs. However, beyond the Debye length (λD), the role of the graphene-electrolytes’ ionic species interactions on the DNA behavior at the nanoscale interface is complicated. We studied the characteristics of the GFETs under different ionic strength, pH, and electrolyte type, e.g., phosphate buffer (PB), and phosphate buffer saline (PBS), in an automatic portable built-in system. The electrostatic gating and charge transfer phenomena were inferred from the field-effect measurements of the Dirac point position in single-layer graphene (SLG) transistors transfer curves. Results denote that λD is not the main factor governing the effective nanoscale screening environment. We observed that the longer λD was not the determining characteristic for sensitivity increment and limit of detection (LoD) as demonstrated by different types and ionic strengths of measuring buffers. In the DNA hybridization study, our findings show the role of the additional salts present in PBS, as compared to PB, in increasing graphene electron mobility, electrostatic shielding, intermolecular forces and DNA adsorption kinetics leading to an improved sensitivity.

Energy-Gap Opening Near the Dirac Point after the Deposition of Cobalt on the (0001) Surface of the Topological Insulator BiSbTeSe2

It is shown for the first time that Co subnanometer coatings deposited by molecular-beam epitaxy on the (0001) surface of the topological insulator BiSbTeSe2 at a temperature of 330°C open an energy gap in the spectrum of topological surface states in the region of the Dirac point with a shift of the Dirac-point position caused by the preliminary deposition of an adsorbate at room temperature. The gap width is 21 ± 6 meV. Temperature-dependent measurements in the range of 15–150 K show no changes in the energy-gap width.

Controllable p‐to‐n Type Conductance Transition in Top‐Gated Graphene Field Effect Transistor by Interface Trap Engineering

AbstractModulating the electronic property of graphene by doping is essential for its device and circuit applications. Unfortunately, controllable p‐ and n‐type doping in top‐gated graphene field effect transistor (GFET) is reported. Here, the O3‐based atomic layer deposited Al2O3 layer as top gate dielectric is chosen. The epoxide functional group formed in the O3 process serves as effective interface trap sites. As the sweeping range of top gate voltage increases, the Dirac point position of GFET moves from positive voltage to negative voltage. The shift of the Dirac point voltage indicates the doping transition of graphene from p‐type to n‐type. The gate voltage dependent doping can be attributed to the charge exchange between graphene and interface trap sites. Furthermore, a trap‐dependent charge model is proposed to explain the transport mechanism in the doping process. This approach is promising to produce the complementary p‐ and n‐type top‐gated GFET for multiple applications.

Design of the trinitrotoluene biosensor using polydiacetylene conjugated with peptide receptors coated on GR-FETs with colorimetric response

PurposeSmart biosensors that can perform sensitive and selective monitoring of target analytes are tremendously valuable for trinitrotoluene (TNT) explosive detection. In this research, the pre-developed sensor was integrated with biological receptors in which they enhanced the sensitivity of the sensor. This is due to conjugated polydiacetylene onto a peptide-based molecular recognition element (Trp-His-Trp) for TNT molecules in graphene field-effect transistors (GR-FETs) as biosensor that is capable of responding to the presence of a TNT target with a colorimetric response. The authors confirmed the efficacy of the receptor while being attached to polydiacetylene (PDA) by observing the binding ability between the Trp-His-Trp and TNT to alter the electronic band structure of the PDA conjugated backbones. The purpose of this paper is to demonstrate a modular system capable of transducing small-molecule TNT binding into a detectable signal. The details of the real-time and selective TNT biosensor have been reported.Design/methodology/approachFollowing an introduction, this paper describes the way of fabrication GR-FETs with conventional photolithography techniques and the other processes, which is functionalized by the TNT peptide receptors. The authors first determined the essential TNT recognition elements from UV-visible spectrophotometry spectroscopy for PDA sensor unit fabrication. In particular, the blue percentage and the chromic response were used to characterize the polymerization parameter of the conjugatedpbackbone. A continuous-flow trace vapor source of nitroaromatics (two, four, six-TNT) was designed and evaluated in terms of temperature dependence. The TNT concentration was measured by liquid/gas extraction in acetonitrile using bubbling sequence. The sensor test is performed using a four-point probe and semiconductor analyzer. Finally, brief conclusions are drawn.FindingsBecause of their unique optical and stimuli-response properties, the polydiacetylene and peptide-based platforms have been explored as an alternative to complex mechanical and electrical sensing systems. Therefore, the authors have used GR-FETs with biological receptor-PDAs as a biosensor for achieving high sensitivity and selectivity that can detect explosive substances such as TNT. The transport property changed compared to that of the field-effect transistors made by intrinsic graphene, that is, the Dirac point position moved from positive Vg to negative Vg, indicating the transition of graphene fromp-type ton-type after annealing in TNT, and when the device was tested from RT, the response of the device was found to increase linearly with increasing concentrations. Average shifting rate of the Dirac peak was obtained as 0.1-0.3 V/ppm. The resulting sensors exhibited at the limit ppm sensitivity toward TNT in real-time, with excellent selectivity over various similar aromatic compounds. The biological receptor coating may be useful for the development of sensitive and selective micro and nanoelectronic sensor devices for various other target analytes.Originality/valueThe detection of illegally transported explosives has become important as the global rise in terrorism subsequent to the events of September 11, 2001, and is at the forefront of current analytical problems. It is essential that a detection method has the selectivity to distinguish among compounds in a mixture of explosives. So, the authors are reporting a potential solution with the designing and manufacturing of electrochemical biosensor using polydiacetylene conjugated with peptide receptors coated on GR-FETs with the colorimetric response for real-time detection of TNT explosives specifically.

Effect of oxygen atoms on graphene: Adsorption and doping

Abstract Graphene is an excellent material, which can be used in aerospace field. However, it was reported that when graphene works in an oxygen-rich environment, its surface inevitably absorbs oxygen atoms. And oxygen atoms are doped when graphene has defects. In this study, we investigate the effect of adsorption and doping of oxygen atoms on the properties of graphene using first-principles calculations. We found that the adsorption of single oxygen atom can only change the position of Dirac point. However, the adsorption of multiple oxygen atoms may lead to band gaps. Meanwhile, when graphene has defects, oxygen atoms can repair its lattice. But oxygen atoms also localize some electrons, giving graphene a larger band gap than pure defective graphene. This study is helpful to evaluate the properties of graphene in an oxygen-rich environment. At the same time, it has certain significance for the application of graphene in band gap engineering.

Massless Dirac fermions in stable two-dimensional carbon-arsenic monolayer

We predict from DFT based electronic structure calculations that a monolayer made up of Carbon and Arsenic atoms, with a chemical composition (CAs3) forms an energetically and dynamically stable system. The optimized geometry of the monolayer is slightly different from the buckled geometric configuration observed for silicene and germanene. The results of electronic structure calculations predict that it is a semi-metal. Interestingly, the electronic band structure of this material possesses a linear dispersion and a Dirac cone at the Fermi level around the high symmetric K point in the reciprocal lattice. Thus, at low energy excitation (up to 105 meV), the charge carriers in this system behave as massless Dirac-Fermions. Detailed analysis of partial density of state suggests that the 2pz orbital of C atoms plays vital role in determining the nature of the states, which has a linear dispersion and hence the Dirac cone, around the Fermi level. Thus, the electronic properties of CAs3 monolayer are similar to those of graphene and other group IV based monolayers. In addition, we have also investigated the influence of mechanical strain on the properties of CAs3 monolayer. The buckled configuration becomes the planar configuration for a tensile strain beyond 18%. Our results indicate that the monolayer possesses linear dispersion in the electronic band structure for a wide range of mechanical strain from -12 to 20%, though the position of Dirac point may not lie exactly at the Fermi level. The linear dispersion disappears for a compressive strain beyond -12% & it is due to the drastic changes in the geometrical environment around C atom. Finally, we wish to point out that CAs3 monolayer belongs to the class of Dirac materials where the behaviour of particles, at low energy excitations, are characterized by the Dirac-like Hamiltonian rather than the Schrodinger Hamiltonian.

Topological Surface States of Dirac Fermions in n-Bi2Te3 –ySey Thermoelectrics

In n-Bi2Te3 and n-Bi2Te3 –ySey thermoelectrics, the surface states of Dirac fermions of the interlayer Van der Waals surface (0001) are studied by scanning tunneling microscopy and spectroscopy. The surface morphology and modulated line profiles of the tunneling microscopy images are determined by local distortions of the surface density of electronic states and depend on the composition. The Dirac point ED in the studied compositions is localized in the band gap and shifts to the top of the valence band with increasing Se content in n-Bi2Te3 –ySey alloys. The dependence between the parameters of the surface states of Dirac fermions (Dirac-point position, Fermi velocity, surface fermion concentration) and thermoelectric properties (Seebeck coefficient and power parameter) in the thermoelectrics under study is determined.

Dirac States of 2D Topological Insulators: Effect of Heterovalent Dopant-Content.

We have studied Bi2Se3 at its 2D-limit using scanning tunneling spectroscopy (STS). Bulk Bi2Se3 is a well-known topological insulator having gapless surface states. In the 2D limit, the interior of the material exhibits a band gap, whereas the periphery shows a gapless metallic state having a Dirac point. We demonstrate a method to tune the Fermi energy and hence the Dirac point of Bi2Se3 nanoplates through doping at the anionic site. For this purpose, STS measurements were carried out on the Bi2Se3 system. We have used bromide as a dopant, which turns the material to n-type in nature. As a result, STS studies infer that the Fermi energy (EF) shifted toward the conduction band and consequently the Dirac point could be found to move away from Fermi energy. Through STS measurements, we have demonstrated a correlation between the shift of Dirac point position and the dopant content. The size, shape, and compositions of Bi2Se3 nanoflakes and concentration of bromine in the doped nanostructures were determined using transmission electron microscopy, associated energy dispersive X-ray spectroscopy analysis, and X-ray diffraction.

Топологические поверхностные состояния фермионов Дирака в термоэлектриках n-Bi-=SUB=-2-=/SUB=-Te-=SUB=-3-y-=/SUB=-Se-=SUB=-y-=/SUB=-

Topological surface states of Dirac fermions in n-Bi2Te3−ySey thermoelectrics L.N. Lukyanova, I.V. Makarenko, O.A. Usov, P.A. Dementev Ioffe Institute, 194021 St. Petersburg, Russia Abstract In n-Bi2Te3 and n-Bi2Te3−ySey thermoelectrics, the surface states of Dirac fermions of the interlayer van der Waals plane (0001) were studied by scanning tunneling microscopy (STM) and spectroscopy. The surface morphology and modulated line profiles of the images in tunneling microscopy is determined by local distortions of the surface electron states density and depend on the composition. The Dirac point ED of the studied compositions is localized in the energy gap, and it shifts to the top of the valence band with increase of Se content in the solid solutions. The dependence between Dirac surface state parameters (the Dirac point position, the Fermi velocity, the fermion surface concentration) and thermoelectric properties (Seebeck coefficient and the power factor) is established for the thermoelectrics studied.

Effect of uni-axial and bi-axial strains and vertical electric field on free standing buckled germanene

In this work, the effects of small uni-axial and bi-axial strains and vertical electric field on the band dispersion and dielectric response of germanene are studied by density functional theory. The results show that by applying the vertical electric field a small band gap at the K point is opened in germanene. The created band gap increases with electric field. The vertical electric field only affects the optical spectra in the low energy range below 1 eV. The band structure of germanene under small biaxial strain maintains the feature of bands near the K point without applying the strain, including a pair of two linear dispersive bands crossing at the Fermi level and the Dirac point position. The CBM at the Γ point moves downward as the biaxial strain increases. By applying the small uniaxial strain, a direct gap at K point is observed and the value of band gap increases with increasing of strain. Imaginary part of dielectric function shows that the inter-band transitions are red-shifted for uni- and bi-axial tensile strains and are blue shifted for uni- and bi-axial compressive strains. The predicted tunability of the band gap and dielectric response by applying the strain and electric field may leads to a wide range of applications of germanene in electronic and optoelectronic devices.

Effect of In-Plane Biaxial Strain on the Dirac Cone in α-Graphyne and Band Gap in α-BNyne

Effects of strain on the electronic properties of α-graphyne and its BN analogues (α-BNyne) are investigated using density functional theory. It can be observed that, for both α-graphyne and α-BNyne sheets, under biaxial strain, the triple bonds show an insignificant change while the single bonds rise substantially. The band gap of α-BNyne decreases with in-plane biaxial strain. For α-graphyne under tensile strain, two linear bands cross at the Fermi level, the band energies still degenerate near the Fermi level and there is no shift in the position of Dirac points. Also, the tensile strain decreases the slope of the linear band. The results show that the in-plane biaxial strains do not affect the contribution of the out-of-plane pz orbitals for the Dirac cone. This insensitivity of the Dirac cone to such structural deformation suggests that α-graphyne is a promising two-dimensional material for future flexible electronic devices at the nanoscale.

Design and manufacture of TNT explosives detector sensors based on GFET

PurposeSmart sensors based on graphene field effect transistor (GFET) and biological receptors are regarded as a promising nanomaterial that could be the basis for future generation of low-power, faster, selective real-time monitoring of target analytes and smaller electronics. So, the purpose of this paper is to provide details of sensors based on selective nanocoatings by combining trinitrotoluene (TNT) receptors (Trp-His-Trp) bound to conjugated polydiacetylene polymers on a graphene channel in GFET for detecting explosives TNT.Design/methodology/approachFollowing an introduction, this paper describes the way of manufacturing of the GFET sensor by using investigation methods for transferring graphene sheet from Cu foil to target substrates, which is functionalized by the TNT peptide receptors, to offer a system which has the capability of answering the presence of related target molecules (TNT). Finally, brief conclusions are drawn.FindingsIn a word, shortly after graphene discovery, it has been explored with a variety of methods gradually. Because of its exceptional electrical properties (e.g. extremely high carrier mobility and capacity), electrochemical properties such as high electron transfer rate and structural properties, graphene has already showed great potential and success in chemical and biological sensing fields. Therefore, the authors used a biological receptor with a field effect transistor (FET) based on graphene to fabricate sensor for achieving high sensitivity and selectivity that can detect explosive substances such as TNT. The transport property changed compared to that of the FET made by intrinsic graphene, that is, the Dirac point position moved from positive Vg to negative Vg, indicating the transition of graphene from p-type to n-type after annealing in TNT, and the results show the bipolar property change of GFET with the TNT concentration and the possibility to develop a robust, easy-to-use and low-cost TNT detection method for performing a sensitive, reliable and semi-quantitative detection in a wide detection range.Originality/valueIn this timeframe of history, TNT is a common explosive used in both military and industrial settings. Its convenient handling properties and explosive strength make it a common choice in military operations and bioterrorism. TNT and other conventional explosives are the mainstays of terrorist bombs and the anti-personnel mines that kill or injure more than 15,000 people annually in war-torn countries. In large, open-air environments, such as airports, train stations and minefields, concentrations of these explosives can be vanishingly small – a few parts of TNT, for instance, per trillion parts of air. That can make it impossible for conventional bomb and mine detectors to detect the explosives and save lives. So, in this paper, the authors report a potential solution with design and manufacture of a GFET sensor based on a biological receptor for real-time detection of TNT explosives specifically.

First-principles study on electronic properties of stanene/WS2 monolayer

We present first-principles calculations to study the stability and electronic properties of stanene on WS2 hybrid structure. It can be seen that the stanene is bound to WS2 substrate with an interlayer distance of about 3.0 Å with a binding energy of −51.8 meV per Sn atom, suggesting a weak interaction between stanene and WS2. The nearly linear band dispersion character of stanene can be preserved with a sizeable band gap in stanene on WS2 hybrid structure due to the difference of onsite energy induced by WS2 substrate, which is more helpful to the on–off current ratio in the logical devices made of stanene/WS2. Moreover, the band gaps, the position of Dirac point with respect to Fermi level, and electron effective mass (EEM) of stanene on WS2 hybrid structure can be tuned by the interlayer distance, external electric field and strains. These results indicate that stanene on WS2 hybrid structure is a promising candidate for stanene-based field-effect transistor (FET) with a finite band gap and high carrier mobility.