Electronic Properties Of Graphene Research Articles

First-principles investigation of the structure and electronic properties of graphene toward Li adsorption

ABSTRACT The first-principles calculation is performed to investigate the possibility of applying pristine, defected, N-doped, Al-doped and N-Al co-doped graphene in the feasible materials of lithium ion battery. Defected, monoatomic doped and diatomic doped graphene have different effects for Li adsorption. For monoatomic doped graphene, the adsorption energy of Al-doped graphene is the lowest, and its final adsorption distance is the shortest. Among the three types of diatomic doped graphene, N–Al para co-doped graphene is more suitable for the lithium ion battery anode material. Furthermore, the band structures of graphene absorbed with Li are analysed in detail, which indicates that pristine graphene adsorbed with Li belongs to physical adsorption while defected and doped graphene adsorbed with Li belongs to chemical adsorption. What’s more, Mulliken population analysis is applied to investigate the interaction between graphene and Li. According to the calculation results, it will provide a theoretical basis for the future application of graphene in lithium ion battery.

Synthesis and characterization of titania/graphene nanocomposite for application in photocatalysis

Titanium dioxide has been extensively investigated as a photocatalyst for water purification, presenting limitations such as recombination of electron-hole pairs generated by photons. The titania / graphene nanocomposites are promising materials to overcome these limitations due to the high specific area of graphene and unique electronic properties. In this work, an anatase-graphene nanocomposite was synthesized by a simple mixture assisted by ultrasound. Graphene was obtained by electrochemical exfoliation of graphite using the electrolysis technique. On the other hand, anatase was synthesized using the sol gel method. The obtained graphene, anatase and the nanocomposite material, were characterized with the X-ray diffraction technique (DRX), scanning electron microscopy (MEB) and transmission electron microscopy (MET). Using Raman spectroscopy, it was possible to verify that the graphite exfoliated correctly producing few layer-graphene. The lamellar nano-structure of the exfoliated graphite has crystallographic planes characteristic of graphite, graphene and graphene oxide. The presence of the anatase phase is shown in the diffraction spectrum of titania. The images obtained with SEM and TEM of the graphene sample show a layered lamellar structure and the TiO2 images show agglomerates of ellipsoidal nanoparticles. Obtained titania nanoparticles have a size of about 6 nm. Band gap value for such extremely low particle size nanocomposite is around 3.6 eV and presumably corresponds to the TiO2 (anatase) phase that completely surrounds the graphene. A nanocomposite model based on HRTEM observations is proposed. Considering the graphene electrical properties and the photocatalytic properties of TiO2, this nanocomposite promises to have applications in photocatalysis.

High density synthesis of topological point defects in graphene on 6H–SiC(000[formula omitted

The control of the physical properties of graphene is a forefront challenge for the development of applications based on this material. Defect engineering is a promising strategy to attain new properties by the insertion of well-defined defects in the carbon lattice. Among the many topological defects of graphene, the one that has the lowest energy per dislocation is a quasi-point-like defect, the so-called Flower Defect (FD). It is generally produced randomly during the graphene synthesis. The deliberate insertion of this defect is promising for the tuning of graphene electronic properties but necessitate to establish a synthesis route allowing to control the formation of flower defects. Here we report a controlled synthesis of graphene with FD using gold as an activator during the graphene synthesis from a 6H–SiC(0001¯) silicon carbide wafer. The synthesis consists of a two-step procedure. Creation of a proto-graphene template where one monolayer of gold is evaporated followed by synthesis close to 1200 °C. This leads to a high density of FD, around 1012cm-2. These results are supported by first-principles calculations showing that gold decreases the flower formation energy by a factor of 2. Scanning tunneling spectroscopy measurements reveal unexpected resonances close to the Fermi energy.

Structural and electronic properties of graphene and its derivatives physisorbed by ionic liquids

The interaction between ionic liquids (ILs) with 2D surface of graphene and its derivatives is significant in developing the better energy storage devices. Herein, the interaction of ILs such as [Emim]+[X]− and [Bmim]+[X]− (X = AlCl4, BF4, Cl and PF6) on graphene, crown ether graphene, graphenylene and graphyne surfaces are investigated using Density Functional Theory (DFT). We found that the physisorption of ILs on the 2D surface is mainly by the imidazolium ring and methyl group of cations driven by the van der Waals interaction. The ILs containing BF4− anion exhibit strong interaction with the considered 2D surfaces and the adsorption does not depend on the chain length of cations. The strong adsorption of ILs on the crown ether graphene sheet reveals the extent of influence of reactive element along with its distinct pore radius on adsorption of ILs compared to the adsorption of ILs on other graphene surfaces. The interaction between cations and BF4 anion forming CH…F bond is undisturbed even on interaction with the 2D surfaces. The minimal change in the anion-cation interaction with the 2D surface strengthens the adsorption. The charge transfer from ILs to 2D surface is found from NPA charge analysis and the bond strength between ILs-2D complexes were identified from topological analysis. The desorption of ILs is inferred from the thermochemical parameters of complexes.

Non-covalent Functionalization of Graphene to Tune Its Band Gap and Stabilize Metal Nanoparticles on Its Surface.

Controlling graphene conductivity is crucial for its potential applications. With this focus, this paper shows the effect of the non-covalent bonding of a pyrimidine derivative (HIS) on the electronic properties of graphene (G). Several G-HIS hybrids are prepared through mild treatments keeping unaltered the structures of both G and HIS. The attachment of HIS to G occurs by π–π stacking of the HIS-aromatic residue with the G surface. This partially blocks the pz electrons of G, giving rise to the splitting of both the valence and conduction bands. Moreover, the width of the splitting is directly related to the HIS content. This fact allows the fine-tuning of the band gap of G-HIS hybrids. Furthermore, HIS keeps its metal-complexing ability in the G-HIS hybrids. Taking advantage of this, a G-HIS–Cu(0) composite was prepared by H2 plasma reduction of a precursor of the G-HIS–Cu(II) type. G-HIS−Cu(0) contains Cu(0) clusters stabilized on the G surface due to interactions with the COO– functions of HIS. In an analogous hybrid, G-HIS–Au(0), the Au(0) NPs are also stabilized by COO– functions. This material, consisting of the coupling of Au(0) NPs and G-HIS, photocatalyzed water reduction under visible light radiation producing 12.5 μmol·g–1·h–1of hydrogen.

Oxygen intercalated graphene on SiC(0001): Multiphase SiOx layer formation and its influence on graphene electronic properties

Low-dimensionality materials are highly susceptible to interfaces. Indeed, intercalation of different chemical species in between epitaxial graphene and silicon carbide (SiC), for instance, may decouple the graphene with respect to the substrate due to the conversion of the buffer layer into a graphene layer. O-intercalation is known to release the strain of such 2D material and to lead to the formation of high structural quality AB-stacked bilayer graphene. Nonetheless, this interface transformation concomitantly degrades graphene electronic transport properties. In this work we employed different techniques in order to better understand the structure of the graphene/SiC interface generated by O-intercalation and to elucidate the origin of the poor electronic properties of graphene. Experimental results revealed the formation of a SiO2 rich layer with a defective transition layer in between it and the SiC, which is characterized by the existence of silicon oxycarbide structures. Scanning tunneling spectroscopy measurements revealed an extensive presence of electronic states just around the Fermi level all over the sample surface, which may suppress the charge carriers mobility around this region. According to theoretical calculations, such states are mainly due to the formation of silicon oxicarbides within the interfacial layer.

Micro-strain effect on electronic properties in graphene induced by silver nanowires

Strain in the lattice has a profound effect on the electrical and optoelectronic properties of the material. Here, we demonstrate the extent effect of micro-strain applied to graphene, induced by silver nanowires (AgNWs), on its lattice and electronic properties. In the typical process, we found that the micro-strain effect has critically expanded the lattice carbon-carbon bond length as confirmed by a dramatic red-shift in the Raman main character of graphene. For example, the G and 2D band vibration mode of graphene shifts as high as 11.31 and 13.47 cm −1 , respectively, that is from 1593.67 to 2687.46 cm −1 for unstrained graphene to 1583.36 and 2673.99 cm −1 for graphene under micro-strain induced by AgNWs with density 5 wire/100 μm 2 . Raman spectra also indicated that the shifting value is dependable on the nanowire's density. XPS analysis results also reveals that the changes in bond length directly cause a strong modification in its electronic state, altering its overall electronic properties. Enhanced electrical and optoelectronic properties is expected from this micro-strained graphene system. • Micro-strain effect induced by AgNWs cause modification in the electronic properties of graphene. • Significant adjustment in the graphene lattice obtained from micro-strain effect. • A large shifting in the electronic state binding energy under micro-strain is observed.

Band-gap tuning of graphene by Mg doping and adsorption of Br and Be on impurity: A DFT study

First principles DFT calculations have been performed to investigate the geometrical and electronic properties, and charge transfer mechanism of different impurities doped/adsorbed in graphene. The results indicate that Be and Mg doping significantly alter the structural and electronic properties of graphene. Studying different configurations of doping/adsorption of Mg, Be and Br we computed a variety of band-gap values, maximum being 0.48 eV. Badar charge analysis indicates that the substitution of impurities leads to valance charge transmission based upon the electronegativity of the atoms. The different band-gap values obtained in our investigations seem sufficient for use of graphene in optoelectronic and such devices where graphene is disable for its use to switch on/off devices.

Electronic Decoupling of Graphene from Copper Induced by Deposition of ZnO: A Complex Substrate/Graphene/Deposit/Environment Interaction

AbstractThis study presents experimental data of the interactions and reactions that occur during the early stages of the growth of ZnO on graphene supported on polycrystalline copper and the subsequent changes on the electronic properties of the graphene. The combination of substrate, graphene, and intercalated species (such as oxygen and water molecules) between graphene and copper due to air exposure, together to the evaporation of metallic zinc under oxygen atmosphere, induces the electronic decoupling of the graphene from copper by the formation of a nanometric layer of copper oxide. In particular, the final stage consists in the formation of a complex interface formed by ZnO/ZnO1−x/Zn/G/Cu2O/Cu. The role of each actor is discussed in terms of a galvanic corrosion reaction of the metallic substrate where the graphene is the cathode and the initial deposition of metallic zinc accelerates the kinetics of this reaction, after which ZnO grows on the metallic zinc initially deposited. In this manner, the electronic properties of graphene can be engineered by the combination and interrelation of substrates, environment, and new‐deposited materials, revealing a more complex and realistic picture for real fabrication processes. These results may help to improve the real applicability of graphene in mass production devices.

Contactless probing of graphene charge density variation in a controlled humidity environment

The electronic properties of graphene are highly sensitive to atoms and molecules adsorbed on its surface and to changes in its environment, such as temperature and humidity. In this paper, we examine the effect of humidity on the local carrier concentration of different types of graphene prepared by mechanical exfoliation, chemical vapour deposition and epitaxial growth on SiC, using in-situ Raman spectroscopy. We present a systematic and comparative study of the changes in Raman response using a vector analysis method to produce spatial maps of doping variation as a function of humidity. We also quantify the humidity induced carrier concentration change for different types of graphene. This study illustrates the effects of humidity on the electronic properties of graphene and provides a simple, contactless and quantitative method to directly evaluate water-induced doping effects in graphene, that are crucial for tailored device performance.

First-Principles Study of 3d Transition-Metal-Atom Adsorption onto Graphene Embedded with the Extended Line Defect.

A type of line defect (LD) composed of alternate squares and octagons (4–8) as the basic unit is currently an experimentally available topological defect in the graphene lattice, which brings some interesting modifications to the magnetic and electronic properties of graphene. The transitional-metal (TM) atoms adsorb on graphene with a line defect (4–8), and they show interesting and attractive structural, magnetic, and electronic properties. For different TMs such as Fe, Co, Mn, Ni, and V, the complex systems show different magnetic and electronic properties. The TM atoms can spontaneously adsorb at quadrangular sites, forming a metallic atomic chain along LD on graphene. The most stable configuration is the hollow site of a regular tangle. The TMs (TM = Co, Fe, Mn, Ni, V) tend to form extended metal lines, showing a ferromagnetic (FM) ground state. For the Co, Fe, and V atoms, the system is half-metal. The spin-α electron is insulating, while the spin-β electron is conductive. For the Mn and Ni atoms, Mn-LD and Ni-LD present a spin-polarized metal; for the Fe atom, Fe-LD shows a semimetal with Dirac cones. For Fe and V atoms, both Fe-LD and V-LD show spin-polarized half-metallic properties. And its spin-α electron is conducting, while the spin-β electron is insulating. Different TMs adsorbing on a graphene nanoribbon forming the same stable configurations of metal lines show different electronic properties. The adsorption of TMs induces magnetism and spin polarization. These metal lines have potential applications in spintronic devices and work as a quasi-one-dimensional metallic wire, which may form building blocks for atomic-scale electrons with well-controlled contacts at the atomic level.

Enhancement of electron transport and band gap opening in graphene induced by adsorbates

Impurities are unavoidable during the preparation of graphene samples and play an important role in graphene's electronic properties when they are adsorbed on graphene surface. In this work, we study the electronic structures and transport properties of a two-terminal zigzag graphene nanoribbon (ZGNR) device whose scattering region is covered by various adsorbates within the framework of the tight-binding approximation, by taking into account the coupling strength $\gamma$ between adsorbates and carbon atoms, the adsorbate concentration $n_i$, and the on-site energy disorder of adsorbates. Our results indicate that when the scattering region is fully covered by homogeneous adsorbates, i.e., $n_i=1$, a transmission gap opens around the Dirac point and its width is almost proportional to $\gamma^2$. In particular, two conductance plateaus of $G=2e^2/h$ appear in the vicinity of the electron energy $E=\pm \gamma$. When the scattering region is partially covered by homogeneous adsorbates ($0<n_i<1$), the transmission gap still survives around the Dirac point even at low $n_i$, and its width is firstly increased by $n_i$ and then declined by further increasing $n_i$; whereas the conductance decreases with $n_i$ in the regime of low $n_i$ and increases with $n_i$ in the regime of high $n_i$. While in the presence of disordered adsorbates whose on-site energies are random variables characterized by the disorder degree, the transmission gap disappears at low $n_i$ and reappears at relatively high $n_i$. Furthermore, the transmission ability of the ZGNR device can be enhanced by the adsorbate disorder when the disorder degree surpasses a critical value, contrary to the localization picture that the conduction of a nanowire becomes poorer with increasing the disorder degree. The physics underlying these transport characteristics is discussed. Our results are in good agreement with experiments.

A Simple and Robust Functionalization of Graphene for Advanced Energy Devices.

Efficient and selective methods for graphene functionalization are needed because they allow tuning of the graphene surface and electronic properties. To date, graphene has been functionalized using ionic bonds, π-π interactions, and covalent bonds. Graphene derivatives based on these methods have been used in various applications, but a new functionalization strategy that improves the properties of graphene is still needed. Herein, a new concept for graphene functionalization using halogenated graphene has been developed, in which brominated graphene is successfully functionalized by heteroatom-containing molecules to form onium bonds, such as pyridinium or ammonium. The counterion bromide is replaced with other anions, such as sulfate, by treating with sulfuric acid while retaining the molecules, which demonstrates the durable properties of onium bonding. To emphasize the advantages of this strategy for graphene functionalization, the performance for energy-related applications, such as biofuel cells, supercapacitors, and Li-ion batteries, is evaluated after introducing redox-active moieties onto graphene through onium bonding. This new graphene functionalization concept will provide a new approach to the design of tailor-made materials with targeted functions.

Band-gap engineering of graphene by Al doping and adsorption of Be and Br on impurity: A computational study

Graphene, being a gapless semiconductor, cannot be used in pristine form for optoelectronic applications such as solar cells. Therefore, it is necessary to tune a band-gap in pristine graphene. Density functional theory calculations have been performed to investigate the structural, electronic, and charge transfer mechanism of different impurities doped/adsorbed in graphene sheet. The results indicate that Al and Be doping can significantly modify the structural and electronic properties of graphene. Replacement of single C atom with Al atom induces a band-gap of 0.40 eV. Adopting different arrangements of Al, Be and Br employing doping and adsorption techniques we computed a variety of band-gap values, maximum value being 0.82 eV. Badar charge analysis indicates that the valence charge transmission takes place from less electronegative atoms towards atoms having higher electronegativity. The adsorption/doping of impurity atoms induce band-gap values in a significant wide range, which seems sufficient for its use in optoelectronic devices. Our results offer the opportunity to tune the electronic band structure of graphene for desired applications.

A first principle study of the structural, electronic, and temperature-dependent thermodynamic properties of graphene/MoS2 heterostructure.

After an initial assessment of the structural and electronic properties of graphene, monolayer MoS2, and graphene/MoS2 bilayer hetero-structure, the temperature-dependent thermodynamic properties of graphene/MoS2 bilayer hetero-structure are examined by using density functional theory calculations. The structure, bandgap, partial density of states, and thermodynamic properties of graphene, monolayer MoS2 and graphene/MoS2 system are investigated and analyzed. Findings from the present study are in good agreement with the previously reported theoretical and experimental studies. Monolayer MoS2 and graphene form a stable Van der Waals heterostructure owing to their negative binding energy, and the system acts as a zero-bandgap semiconductor. Debye temperature and the heat capacity of graphene, MoS2 monolayer, and graphene/MoS2 system are calculated from phonon dispersion relations to be 2100K, 600K, and 1400K, and 0.7J/g.K, 0.218J/g.K, and 0.46J/g.K, respectively. Introduction of graphene into the MoS2 semiconductor is, therefore, found to improve the overall thermodynamic properties of the composite as graphene preserved its superior thermal properties. The findings will be beneficial to calculate thermal conductivity of the graphene/MoS2 heterostructure for minimizing the temperature effect in electronic or optoelectronic devices.

Impact of covalent functionalization by diazonium chemistry on the electronic properties of graphene on SiC.

Plenty of strategies focused on covalent interaction have been developed to functionalize graphene's surface in order to employ it in a wide range of applications. Among them, the use of radical species including nitrene, carbene and aryl diazonium salts is regarded as a promising strategy to establish the covalent functionalization of graphene. In this work, we highlight the effect of diazonium chemistry on the electronic properties of graphene on SiC. On the basis of X-ray and synchrotron-based photoemission experiments, we were able to prove that 3,4,5-trimethoxybenzenediazonium (TMeOD) units, reduced and chemisorbed onto graphene using electrochemistry, preserve the electronic structure of the Dirac cone, through inducing a slightly additional n-type doping of graphene, as revealed by a downshift of the Dirac cone probed by angle-resolved photoemission experiments.

The alignment-dependent properties and applications of graphene moiré superstructures on the Ru(0001) surface.

The moiré superstructure of graphene on a lattice-mismatched metal substrate has profound effects on the electronic properties of graphene and can be used for many applications. Here, we propose to systematically tune the moiré superstructure of graphene on the Ru(0001) surface by rotating the graphene layer. Our study reveals two kinds of graphene moiré superstructures: (i) the ultra-flat graphene layers with height variations of less than 0.1 Å for rotation angles greater than 20° that have the same structural and electronic properties everywhere, and (ii) the highly corrugated graphene moiré superstructures with height variations from 0.4 to 1.6 Å for rotation angles less than 20°, whose electronic properties are highly modulated by the interaction with the substrate. Moreover, these rotated graphene moiré superstructures can serve as templates to produce matrices of size-tunable metal clusters from a few to ∼100 atoms. This study reveals the causes of the structural fluctuation of moiré superstructures of graphene on the transition metal surface and suggests a pathway to tune graphene's electronic properties for various applications.

Electronic properties of graphene with point defects (Review Article)

This review is an attempt to consistently examine the electronic spectrum of graphene containing defects (such as adsorbed atoms, substitutional atoms, vacancies) that can be adequately described using the Lifshitz model. Therefore, the known Hamiltonian of this model is chosen for the case of two-dimensional relativistic electrons, and the criteria for the appearance of an impurity resonance near the Dirac point are provided. The theory of concentration band structure transformation in graphene is outlined, from which it follows that a transport gap opens in the vicinity of the impurity resonance energy when a specific value of the impurity concentration is reached. Along the way, the question of whether or not it is possible (or impossible) for Dirac quasiparticles to become localized in such a spatially disordered system is analyzed. Based on this, it is feasible to explain and describe the recently observed in impure graphene phenomenon of metal-insulator transition, which turns out to be a direct consequence of the system’s Fermi energy falling into the domain of the transport gap. The concept of local spectrum rearrangement, which can also unfold as the defect concentration increases, is introduced and justified for graphene. We formulate the physical reasons why the minimum of graphene’s low-temperature conductivity dependence on the Fermi energy of electrons does correspond to the impurity resonance energy, and not the Dirac point, as it has been claimed in a number of theoretical and experimental studies. Furthermore, the mentioned minimum value proves to be not universal, but is dependent, instead, on the concentration of defects. The analytical considerations of the impurity effects are accompanied by numerical simulations of the examined system, and a complete correspondence between these two approaches is established as a result. In particular, the general scenarios of spectrum rearrangement, electron state localization, as well as of effects having a local nature, are confirmed.

Biomimetic Assembly of a Polydopamine Layer on Graphene as an Electron Gate for Fluorescent MicroRNA Detection in Living Cells.

A novel strategy was developed for microRNA detection based on the fluorescence quenching of polydopamine (PDA)-coated reduced graphene oxide (RGO) nanosheets (RGO@PDA). Compared with graphene oxide (GO), the reduction of GO and modification of the surface of RGO by PDA not only improve the stability, dispersity, biocompatibility, and cellular uptake without degeneration of the unique electronic properties of graphene, but also add an electron gate for harvesting electrons, as well as enabling efficient and forward electron transfer to avoid unwanted electron transfer and realize highly sensitive miRNA detection; thus a lower detection limit can be achieved in this sensing system. Remarkably, nanoprobes consisting of RGO@PDA and fluorescein-labeled single-stranded DNA can naturally enter cancer cells without the aid of transfection agents, as well as resisting enzymatic lysis and showing almost no effect on the cell viability. More importantly, intense and time-dependent fluorescence responses were observed from the important tumor marker microRNA-21 (miR-21) in living cells; thus suggesting that the proposed sensing platform shows great promise for applications in disease diagnosis and fundamental research into biochemistry.

Electric Double-Layer Gating of Two-Dimensional Field-Effect Transistors Using a Single-Ion Conductor.

Electric double-layer (EDL) gating using a custom-synthesized polyester single-ion conductor (PE400-Li) is demonstrated on two-dimensional (2D) crystals for the first time. The electronic properties of graphene and MoTe2 field-effect transistors (FETs) gated with the single-ion conductor are directly compared to a poly(ethylene oxide) dual-ion conductor (PEO:CsClO4). The anions in the single-ion conductor are covalently bound to the backbone of the polymer, leaving only the cations free to form an EDL at the negative electrode and a corresponding cationic depletion layer at the positive electrode. Because the cations are mobile in both the single- and dual-ion conductors, a similar enhancement of the n-branch is observed in both graphene and MoTe2. Specifically, the single-ion conductor decreases the subthreshold swing in the n-branch of the bare MoTe2 FET from 5000 to 250 mV/dec and increases the current density and on/off ratio by two orders of magnitude. However, the single-ion conductor suppressed the p-branch in both the graphene and the MoTe2 FETs, and finite element modeling of ion transport shows that this result is unique to single-ion conductor gating in combination with an asymmetric gate/channel geometry. Both the experiments and modeling suggest that single-ion conductor-gated FETs can achieve sheet densities up to 1014 cm-2, which corresponds to a charge density that would theoretically be sufficient to induce several percent strain in monolayer 2D crystals and potentially induce a semiconductor-to-metal phase transition in MoTe2.