Graphene Nanoelectronics Research Articles

Ultrahigh-mobility semiconducting epitaxial graphene on silicon carbide.

Semiconducting graphene plays an important part in graphene nanoelectronics because of the lack of an intrinsic bandgap in graphene1. In the past two decades, attempts to modify the bandgap either by quantum confinement or by chemical functionalization failed to produce viable semiconducting graphene. Here we demonstrate that semiconducting epigraphene (SEG) on single-crystal silicon carbide substrates has a band gap of 0.6 eV and room temperature mobilities exceeding 5,000 cm2 V-1 s-1, which is 10 times larger than that of silicon and 20 times larger than that of the other two-dimensional semiconductors. It is well known that when silicon evaporates from silicon carbide crystal surfaces, the carbon-rich surface crystallizes to produce graphene multilayers2. The first graphitic layer to form on the silicon-terminated face of SiC is an insulating epigraphene layer that is partially covalently bonded to the SiC surface3. Spectroscopic measurements of this buffer layer4 demonstrated semiconducting signatures4, but the mobilities of this layer were limited because of disorder5. Here we demonstrate a quasi-equilibrium annealing method that produces SEG (that is, a well-ordered buffer layer) on macroscopic atomically flat terraces. The SEG lattice is aligned with the SiC substrate. It is chemically, mechanically and thermally robust and can be patterned and seamlessly connected to semimetallic epigraphene using conventional semiconductor fabrication techniques. These essential properties make SEG suitable for nanoelectronics.

Tip Growth of Quasi-Metallic Bilayer Graphene Nanoribbons with Armchair Chirality.

Graphene nanoribbons (GNRs), quasi one-dimensional (1D) narrow strips of graphene, have shown promise for high-performance nanoelectronics due to their exceptionally high carrier mobility and structurally tunable bandgaps. However, producing chirality-uniform GNRs on insulating substrates remains a big challenge. Here, we report the successful growth of bilayer GNRs with predominantly armchair chirality and ultranarrow widths (<5 nm) on insulating hexagonal boron nitride (h-BN) substrates using chemical vapor deposition (CVD). The growth of GNRs is catalyzed by transition metal nanoparticles, including Fe, Co, and Ni, through a unique tip-growth mechanism. Notably, GNRs catalyzed by Ni exhibit a high purity (97.3%) of armchair chirality. Electron transport measurements indicate that the ultrathin bilayer armchair GNRs exhibit quasi-metallic behavior. This quasi-metallicity is further supported by density functional theory (DFT) calculations, which reveal a significantly reduced bandgap in bilayer armchair GNRs. The chirality-specific GNRs reported here offer promising advancements for the application of graphene in nanoelectronics.

Wet etching of gold on graphene for high-quality resist-free graphene surfaces

Wet etching of gold on graphene is challenging due to the weak adhesion of the resist mask to graphene. We report an operating procedure for alkali ion-free wet etching of gold on graphene using a mixture of hydrochloric and nitric acids (aqua regia) with a high lateral resolution down to 100 nm. We investigate the role of positive and negative resists, electron beam lithography (EBL) dose, hard-bake, oxygen etching, aging, and sensitivity to the etch parameters, such as the freshness of dilute aqua regia, etch time, and the order of etched samples. The negative-tone resist provides the best results. The over-dosed EBL exposure can enhance the resist adhesion, as hard-bake below the glass-transition temperature and well-defined wet etch of the resist-residua-free gold surface. We also present a cleaning procedure to avoid bubble formation after the hard bake. Our results demonstrate that wet etching of gold on graphene using aqua regia is a viable method for achieving high-quality resist-free graphene surfaces. This method has potential applications in graphene nanoelectronics and nanophotonics, where high-quality graphene surfaces are essential for device performance.

Current Transients in Graphene Electronics under Single‐Particle Irradiation

Low‐dimensional materials hold great promise in next‐generation electronics. However, the performance of such devices is susceptible to external perturbations such as irradiation due to the high exposure of constituent atoms to the environment. Herein, real‐time time‐dependent density‐functional theory at the tight‐binding level extended to open systems for electrons and Ehrenfest dynamics for ions is developed and used to explore the effects of single‐H irradiation on graphene electronics. The results show that the peak current displays distinct energy and site dependences, which are largely different from the dependences of the stopping powers. Charge‐density analysis shows that the current transients are driven by delocalized plasmonic excitation, in contrast to localized electronic excitation, which plays a crucial role in the stopping power. The site dependence of the transient current is determined by the electron density at the irradiation site and the ionic charges. These findings highlight the roles of lattice discreteness and electronic structures of materials, which have been overlooked in previous studies based on theoretical formulations and semiempirical models. Using the insights gained from the calculations and the dataset constructed under typical space‐irradiation conditions, the device responses of graphene nanoelectronics are modeled, laying the ground for device design in the space environment.

An epitaxial graphene platform for zero-energy edge state nanoelectronics

Graphene’s original promise to succeed silicon faltered due to pervasive edge disorder in lithographically patterned deposited graphene and the lack of a new electronics paradigm. Here we demonstrate that the annealed edges in conventionally patterned graphene epitaxially grown on a silicon carbide substrate (epigraphene) are stabilized by the substrate and support a protected edge state. The edge state has a mean free path that is greater than 50 microns, 5000 times greater than the bulk states and involves a theoretically unexpected Majorana-like zero-energy non-degenerate quasiparticle that does not produce a Hall voltage. In seamless integrated structures, the edge state forms a zero-energy one-dimensional ballistic network with essentially dissipationless nodes at ribbon–ribbon junctions. Seamless device structures offer a variety of switching possibilities including quantum coherent devices at low temperatures. This makes epigraphene a technologically viable graphene nanoelectronics platform that has the potential to succeed silicon nanoelectronics.

Effects of Er atoms on graphitization process and structural defects for epitaxial graphene

Thermal decomposition of SiC at high temperature usually brings about excessively fast Si sublimation and a very rough surface. In order to fabricate high-quality homogeneous epitaxial graphene on a SiC(0001) substrate, highly reactive erbium atoms are employed in this work. Scanning tunneling microscopy and Raman spectroscopy have been utilized to investigate the modulations of Er atoms on graphitization evolution and structural defects for graphene after annealing durations. Experimental results show that Er atoms pre-deposited on clean substrates can definitely enhance the surface graphitization of SiC and make graphene grow in a controllable way. The existence of Er layer is believed to break Si–C bonds at low temperature and to decrease the Si sublimate rate. It is also demonstrated that Er atoms can modify the type of structural defects in graphene, and the areal density of flower defects increases to 1.22 × 1012 cm−2, quadrupling that in pristine graphene. This work puts forward a fabrication method for epitaxial graphene with flower defects in high density and will enlighten some future applications of graphene in nanoelectronics, electron energy filtering, and chemical catalysis.

Reliable Fabrication of Graphene Nanostructure Based on e-Beam Irradiation of PMMA/Copper Composite Structure.

Graphene nanostructures are widely perceived as a promising material for fundamental components; their high-performance electronic properties offer the potential for the construction of graphene nanoelectronics. Numerous researchers have paid attention to the fabrication of graphene nanostructures, based on both top-down and bottom-up approaches. However, there are still some unavoidable challenges, such as smooth edges, uniform films without folds, and accurate dimension and location control. In this work, a direct writing method was reported for the in-situ preparation of a high-resolution graphene nanostructure of controllable size (the minimum feature size is about 15 nm), which combines the advantages of e-beam lithography and copper-catalyzed growth. By using the Fourier infrared absorption test, we found that the hydrogen and oxygen elements were disappearing due to knock-on displacement and the radiolysis effect. The graphene crystal is also formed via diffusion and the local heating effect between the e-beam and copper substrate, based on the Raman spectra test. This simple process for the in-situ synthesis of graphene nanostructures has many promising potential applications, including offering a way to make nanoelectrodes, NEMS cantilever resonant structures, nanophotonic devices and so on.

Graphene functionalized hybrid nanomaterials for industrial-scale applications: A systematic review

As of remarkable electrical, optical, magnetic, thermal and mechanical properties and large specific surface area, graphene belongs to two-dimensional material of sp2 hybridization carbon atoms, has attracted a lot of interest in the recent decades. For the enormous utilization of graphene in nanoelectronics, the production of high quality graphene in large scale is essential. The properties and applications of this 2-D carbon structure have opened up new possibilities for potential systems and devices. Due to its unique properties in relation to other carbon nanomaterials, research interest in this material grew quickly. Though there are numerous graphene generating techniques, complications in the production of large scale, highly ordered structures of graphene have triggered advanced research in this area leading to the development of a broad spectrum of graphene nanostructures. Utilities of graphene is often behaving as a composite or filler material with metals, metal oxides and polymers. The customization of graphene properties and surface chemistry is focused on controlling its performance in a polymer matrix device. Due to its unusual combination of excellent properties, graphene has evolved as one of the most innovative nanomaterials. ‘Graphene’ is so thick that not even helium, the smallest gas atom, can move through it and it is impermeable to gasses. The typical review overviews the current understanding of the properties of graphene and its derivatives such as: graphene oxide (GO), porous graphene and porous graphene oxide, graphene oxide incorporated liquid crystal (GO-LC), graphene scroll. The review emphasis on simple and effective methodologies for the development of graphene nanocomposite. Various components of smart synthesis techniques are illustrated. Furthermore systematic review explored, interesting applications diversities of graphene and its derivatives, especially in the field of electronics, energy storage were examined along with the potential prospects for their further development, key challenges and significant opportunities in the industrial potential scale.

First-Principles Electronic-Structure Study of Graphene Decorated with 4d-Transition Atoms

Adsorption configurations, electronic structures and net spins of graphene adsorbing 4d transition atoms are calculated by first-principles calculations to explore the magnetic modification of decorating metal atoms on graphene. Y, Zr and Nb atoms can be adsorbed on graphene sheet via ionic bonds with an evident charge transfer, while Mo, Tc, Ru and Rh atoms form covalent-like bonding with graphene carbon atoms due to orbital hybridization, as indicated by Mulliken atomic charges and electron density differences. The 4d-transition atoms can be adsorbed on a carbon-ring center and atomic-bridge with a high binding energy as the typical chemisorption, which leads to specific modifications in electronic-band character and magnetic properties by introducing electron-states near Fermi-level. By adsorbing 4d-transition atoms, the electronic structure of graphene will alter from a semi-metal to a metal character, and engender net spin magnetism from the spin-polarization in 5s and 4d orbitals of adsorption atoms. This paper provides a significant theoretical basis for further experimental explorations of the atom-decorated graphene in nanoelectronics.

Electronic Properties of Graphene Nanoribbons

Nowadays, graphene physics is aimed at finding a way to control the band gap of graphene, which is necessary for application of graphene in nanoelectronics. The purpose of this work is to investigate the systems that break an inversion symmetry of a hexagonal lattice. The electronic properties of armchair and zigzag graphene nanoribbons are obtained and the dependence of the electronic properties on the edge effects is shown.

Hexagonal Nanopits with the Zigzag Edge State on Graphite Surfaces Synthesized by Hydrogen-Plasma Etching

We studied, by scanning tunneling microscopy, the morphology of nanopits of monolayer depth created at graphite surfaces by hydrogen plasma etching under various conditions such as H$_2$ pressure, temperature, etching time, and RF power of the plasma generation. In addition to the known pressure-induced transition of the nanopit morphology, we found a sharp temperature-induced transition from many small rather round nanopits of ~150 nm size to few large hexagonal ones of 300-600 nm within a narrow temperature range. The remote and direct plasma modes switching mechanism, which was proposed to explain the pressure-induced transition, is not directly applicable to this newly found transition. Scanning tunneling spectroscopy (STS) measurements of edges of the hexagonal nanopits fabricated at graphite surfaces by this method show clear signatures of the peculiar electronic state localized at the zigzag edge (edge state), i.e., a prominent peak near the Fermi energy accompanied by suppressions on either side in the local density of states. These observations indicate that the hexagonal nanopits consist of a high density of zigzag edges. The STS data also revealed a domain structure of the edge state in which the electronic state varies over a length scale of ~3 nm along the edge. The present study will pave the way for microscopic understanding of the anisotropic etching mechanism and of spin polarization in zigzag nanoribbons which are promising key elements for future graphene nanoelectronics.

Strain-Guided Oxidative Nanoperforation on Graphene.

Increased applications of nanoporous graphene in nanoelectronics and membrane separations require ordered and precise perforation of graphene, whose scalablility and time/cost effectiveness represent a significant challenge in existing nanoperforation methods, such as catalytical etching and lithography. A strain-guided perforation of graphene through oxidative etching is reported, where nanopores nucleate selectively at the bulges induced by the prepatterned nanoprotrusions underneath. Using reactive molecular dynamics and theoretical models, the perforation mechanisms are uncovered through the relationship between bulge-induced strain and enhanced etching reactivity. Parallel experiments of chemical vapor deposition (CVD) of graphene on SiO2 NPs/SiO2 substrates verify the feasibility of such strain-guided perforation and evolution of pore size by exposure of varied durations to oxygen plasma. This scalable method can be feasibly applied to a broad variety of 2D materials (e.g., graphene and h-boron nitride) and nanoprotrusions (e.g., SiO2 and C60 nanoparticles), allowing rational fabrication of 2D material-based devices.

Single-layer graphene with electronic properties of a gated bilayer

We demonstrate that single layer graphene exhibits the electronic structure of a bilayer when it is connected to two gated bilayers. The energy gap characteristic for gated bilayer is induced in the single layer and it persists for distances separating the bilayers up to 20 nm. Most importantly, we report on the appearance of topologically protected gapless state localized in a single graphene layer when the distant bilayers differ in the stacking order. The one-dimensional current in the topological state flows in a single layer graphene, and this finding suggests a new exploitation of graphene in nanoelectronics.

A facile approach to direct growth of layer-tunable graphene on Ge substrates

Direct synthesis of graphene with tunable layer number on semiconductor has proven challenging. Synthesis of layer-tunable graphene on catalytic metal surfaces, in particular Cu and Ni, has shown great success. The growth methods on metal layers involve an inevitable transfer procedure, which degrades and contaminates the graphene. Thus, direct synthesis of graphene on semiconductor is necessary and profound. Here, a facile synthesis approach for direct growth of graphene on Ge(110) subtrate via ion implantation was reported. Interestingly, the graphene growth during annealing is not self-limiting and the thickness of graphene can be precisely controlled by the carbon ion implanted fluence. Moreover, a detailed growth process upon segregation was investigated, and the results show graphene nucleation tends to occur near the Ge atomic step edges. Furthermore, the facile approach involving the ion implantation can be adopted to investigate the growth process on semiconductor and to control the number of graphene layers, thus promoting the practical application of graphene in nanoelectronics.

Graphene : An Out Standing Material

In photovoltaics, research is aiming to investigate new materials able to push the efficiency limit for solar cells towards the highest values without increasing the fabrication cost. This paper presents a review about graphene material and its potential use in all technological fields. Due to its high conductivity, transparency and amazing properties. It seems that it has an important place in the next generation of solar cells instead of silicon or thin film based solar cells, researchers found diverse applications for graphene in nanoelectronics, aviation, industry, transport, biomedecine and others. This paper present a review about the state of art about the graphene material in photovoltaic solar cells where very interesting efficiencies were recorded.

Two-dimensional graphene electronics: current status and prospects

Graphene, a two-dimensional carbon material, is reviewed for how it is currently used in nanometer-scale electronics (nanoelectronics). The history of the discovery of graphene is briefly described. Theoretical work on graphene heterostructures is examined. The primary focus is on the practical use of graphene in nanoelectronics applications. Prospects for graphene and post-graphene nanoelectronics are discussed.

Review Article: Tunneling-based graphene electronics: Methods and examples

The authors present an overview of the main theoretical approaches used to describe tunnel processes in graphene nanoelectronics. Two currently central theoretical methods of calculating tunnel current, the Bardeen tunneling Hamiltonian approach and the method of nonequilibrium Green's functions, are reviewed in a pedagogical fashion. Several examples are used to illustrate the specific features of the methods. An application of both methods to the analysis of current flow in graphene tunnel field-effect transistors is discussed.

Effective Synthesis of Highly Oxidized Graphene Oxide That Enables Wafer-scale Nanopatterning: Preformed Acidic Oxidizing Medium Approach

Demand for rapid and massive-scale exfoliation of bulky graphite remains high in graphene commercialization and property manipulation. We report a procedure utilizing “preformed acidic oxidizing medium (PAOM)” as a modified version of the Hummers’ method for fast and reliable synthesis of graphene oxide. Pre-mixing of KMnO4 and concentrated H2SO4 prior to the addition of graphite flakes enables the formation of effectively and efficiently oxidized graphene oxide (EEGO) featured by its high yields and suspension homogeneity. PAOM expedites diffusion of the Mn-oxidants into the graphite galleries, resulting in the rapid graphite oxidation, capable of oxidizing bulky graphite flakes (~0.8 mm in diameter) that can not be realized by the Hummers’ method. In the scale-up tests, ten-time amount of graphite can be completely exfoliated by PAOM without need of extended reaction time. The remarkable suspension homogeneity of EEGO can be exploited to deposit ultra-flat coating for wafer-scale nanopatterning. We successfully fabricated GO optical gratings with well-defined periodicity (300 nm) and uniform thickness (variation <7 nm). The combination of the facile and potent PAOM approach with the wafer-scale patterning technique may realize the goal for massive throughput graphene nanoelectronics.

Graphene nanoelectronics

Graphene nanoelectronics

Fully integrated graphene electronic biosensor for label-free detection of lead (II) ion based on G-quadruplex structure-switching

This work presents a fully integrated graphene field-effect transistor (GFET) biosensor for the label-free detection of lead ions (Pb2+) in aqueous-media, which first implements the G-quadruplex structure-switching biosensing principle in graphene nanoelectronics. We experimentally illustrate the biomolecular interplay that G-rich DNA single-strands with one-end confined on graphene surface can specifically interact with Pb2+ ions and switch into G-quadruplex structures. Since the structure-switching of electrically charged DNA strands can disrupt the charge distribution in the vicinity of graphene surface, the carrier equilibrium in graphene sheet might be altered, and manifested by the conductivity variation of GFET. The experimental data and theoretical analysis show that our devices are capable of the label-free and specific quantification of Pb2+ with a detection limit down to 163.7ng/L. These results first verify the signaling principle competency of G-quadruplex structure-switching in graphene electronic biosensors. Combining with the advantages of the compact device structure and convenient electrical signal, a label-free GFET biosensor for Pb2+ monitoring is enabled with promising application potential.