Undoped Graphene Research Articles

Nonperturbative model of harmonic generation in undoped graphene in the terahertz regime

We present a formalism to calculate the nonlinear terahertz response of monolayer graphene using a two-band tight-binding model of graphene. We develop the dynamic equations for the electron density matrix in the length gauge to calculate the interband and intraband carrier dynamics at terahertz frequencies. Using the calculated nonlinear interband and intraband current densities, we obtain the nonlinear transmitted and reflected terahertz fields. We find that when the conditions are such that the interband and intraband current densities are comparable, strong generation of odd harmonics is observed. We examine the response of undoped graphene as a function of a wide variety of system parameters, including operating temperature, Fermi velocity, pulse duration, and terahertz central frequency. We find that the response is generally most sensitive to the operating temperature and the terahertz central frequency. In particular, changing the ambient temperature from 10 to 100 K reduces the third harmonic field by a factor of 20, while increasing the terahertz central frequency from 0.5 to 5 THz results in an increase in the ratio of the generated third harmonic field to the fundamental of the reflected field from 19% to 49%. The very strong dependence on the temperature and central frequency is found to be due to the strong dependence of the nonlinear generation on the strength of the interplay between the intraband and interband dynamics. Our work demonstrates that, under the right conditions of low temperature and moderate THz field central frequency, third harmonic generation should be experimentally observable in undoped monolayer graphene at moderate THz field amplitudes of only 1 kV cm−1.

Spin-valley quantum Hall phases in graphene**Project supported by the National Natural Science Foundation of China (Grant Nos. 11447218, 11274059, 11404278, and 11447216).

We theoretically investigate possible quantum Hall phases and corresponding edge states in graphene by taking a strong magnetic field, Zeeman splitting M, and sublattice potential Δ into account but without spin–orbit interaction. It was found that for the undoped graphene either a quantum valley Hall phase or a quantum spin Hall phase emerges in the system, depending on relative magnitudes of M and Δ. When the Fermi energy deviates from the Dirac point, the quantum spin-valley Hall phase appears and its characteristic edge state is contributed only by one spin and one valley species. The metallic boundary states bridging different quantum Hall phases possess a half-integer quantized conductance, like e2/2h or 3e2/2h. The possibility of tuning different quantum Hall states with M and Δ suggests possible graphene-based spintronics and valleytronics applications.

Enhancement of Near-Infrared Light Graphene Interaction by Nanobeam Resonator

We propose that strong enhancement of light graphene interaction at telecommunication wavelengths can be obtained by combining graphene with one-dimensional photonic crystal nanobeam resonator. The enhancement mechanism originates from the combined effect of Fabry-Perot-like resonance and the low group velocity of the guided Floquet-Bloch mode at the frequencies near the band edge. Simulation results show the absorption of undoped graphene with a length of $6.5~\mu \text{m}$ is as high as 50%. When the Fermi energy level of graphene is raised from 0 to 0.8 eV, the transmission has a modulation depth of 7 dB. As a novel alternative, the structure we proposed may provide potential applications in the on-chip integrated photodetectors, optical modulators, and nonlinear optics.

Computational Study of Hybrid Nanomaterial/Insulator/Silicon Solar Cells

We present a computational study on hybrid nanomaterial-insulator-silicon solar cells where single-walled carbon nanotube or graphene forms the emitter as well as top conducting electrode on n-type crystalline silicon having a thin interfacial tunnel oxide. The effects of nanomaterial doping and tunnel oxide thickness on cell characteristics are modeled. Similar to bulk emitters, cell efficiency could be increased by chemical doping (p-type) of the nanomaterial. Unlike bulk, nanomaterial could get electrostatically doped (n-type) due to its low quantum capacitance, by the surface charge density in silicon. For chemically undoped graphene on lightly ( $10^{{16}}$ /cm $^{{3}}$ ) doped silicon, efficiency loss due to the electrostatic doping effect is $\sim 11$ %. A moderate p-type chemical doping (0.2 eV shift in Fermi level) of graphene reduces the aforementioned loss to $\sim 2$ %. The electrostatic doping effect in carbon nanotube-based cells is relatively small and independent to nanotube’s chemical doping. For tunnel oxide thickness $\ge 2$ nm, photogenerated carrier accumulation at silicon/oxide interface considerably enhances the electrostatic doping effect. The effect of tunnel oxide thickness variation on fill factor and open circuit voltage is shown to be qualitatively similar to standard bulk metal–insulator–silicon solar cells. Our model predicts an optimal oxide thickness of $\sim 1$ nm which confirms the experimental reports.

Atomic-scale diffractive imaging of sub-cycle electron dynamics in condensed matter.

For interaction of light with condensed-matter systems, we show with simulations that ultrafast electron and X-ray diffraction can provide a time-dependent record of charge-density maps with sub-cycle and atomic-scale resolutions. Using graphene as an example material, we predict that diffraction can reveal localised atomic-scale origins of optical and electronic phenomena. In particular, we point out nontrivial relations between microscopic electric current and density in undoped graphene.

RKKY interaction in bilayer graphene

We study the RKKY interaction between two magnetic impurities located on the same layer (intralayer case) or on different layers (interlayer case) in undoped bilayer graphene (BLG) in the four-bands model, by directly calculating the Green functions in the eigenvalues and eigenvectors representation. Our results show that both intra- and interlayer RKKY interactions between two magnetic impurities located on the same (opposite) sublattice are always ferromagnetic (antiferromagnetic). Furthermore we find unusual long-distance decay of the RKKY interaction in BLG. The intralyer RKKY interactions between two magnetic impurities located on the same sublattice, JAnAn(R) and JBnBn(R), decay closely as 1/R6 and 1/R2 at large impurity distances respectively, but when they are located on opposite sublattices the RKKY interactions exhibit 1/R4 decays approximately. In the interlayer case, the RKKY interactions between two magnetic impurities located on the same sublattice show a decay close to 1/R4 at large impurity distances, but if two magnetic impurities be on opposite sublattices the RKKY interactions, JA1B2(R) and JB1A2(R), decay closely as 1/R6 and 1/R2 respectively. Both intra- and interlayer RKKY interactions have anisotropic oscillatory factors which for intralayer case is equal to that for single layer graphene (SLG). Our results at weak and strong interlayer coupling limits reduce to the RKKY interaction of SLG and that of BLG in the two-bands approximation respectively.

Iodine doped graphene as anode material for lithium ion battery

I-doped graphene is synthesized by a facile heat treatment method and used as anode material for lithium ion battery. The doped graphene exhibits high reversible capacity (1690mAhg−1 at 100mAg−1), good cyclability (retaining 92.6% reversible capacity after 200 cycles) and excellent rate performance compared with undoped graphene. The superior electrochemical performance of the I-doped graphene is explained by the change of graphene lattice, defects and positive charge density introduced by the doping of I atoms.

Tuning the Magnetic Properties of Carbon by Nitrogen Doping of Its Graphene Domains.

Here we present the formation of predominantly sp(2)-coordinate carbon with magnetic- and heteroatom-induced structural defects in a graphene lattice by a stoichiometric dehalogenation of perchlorinated (hetero)aromatic precursors [hexachlorobenzene, C6Cl6 (HCB), and pentachloropyridine, NC5Cl5 (PCP)] with transition metals such as copper in a combustion synthesis. This route allows the build-up of a carbon lattice by a chemistry free of hydrogen and oxygen compared to other pyrolytic approaches and yields either nitrogen-doped or -undoped graphene domains depending on the precursor. The resulting carbon was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, photoelectron spectroscopy (XPS), and SQUID magnetometry to gain information on its morphological, chemical, and electronic structure and on the location of the nitrogen atoms within the carbon lattice. A significant lowering of the magnetization was observed for the nitrogen-doped carbon obtained by this method, which exhibits less ordered graphene domains in the range of approximately 10-30 nm as per TEM analysis compared to the nondoped carbon resulting from the reaction of HCB with larger graphene domains as per TEM and the presence of a 2D mode in the Raman spectra. The decrease of the magnetization by nitrogen doping within the sp(2)-coordinate carbon lattice can be attributed to an increase in pyrrole-type defects along with a reduction in radical defects originating from five-membered carbon ring structures as well as changes in the π-electron density of edge states.

Anomalous spectral features of a neutral bilayer graphene.

Graphene and its bilayer are two-dimensional systems predicted to show exciting many-body effects near the neutrality point. The ideal tool to investigate spectrum reconstruction effects is angle-resolved photoemission spectroscopy (ARPES) as it probes directly the band structure with information about both energy and momentum. Here we reveal, by studying undoped exfoliated bilayer graphene with ARPES, two essential aspects of its many-body physics: the electron-phonon scattering rate has an anisotropic k-dependence and the type of electronic liquid is non-Fermi liquid. The latter behavior is evident from an observed electron-electron scattering rate that scales linearly with energy from 100 meV to 600 meV and that is associated with the proximity of bilayer graphene to a two-dimensional quantum critical point of competing orders.

Fermi energy-dependence of electromagnetic wave absorption in graphene

Undoped graphene is known to absorb 2.3% of visible light at a normal angle of incidence. In this paper, we theoretically demonstrate that the absorption of 10–100 GHz of an electromagnetic wave can be tuned from nearly 0 to 100% by varying the Fermi energy of graphene when the angle of incidence of the electromagnetic wave is kept within total internal reflection geometry. We calculate the absorption probability of the electromagnetic wave as a function of the Fermi energy of graphene and the angle of incidence of the wave. These results open up possibilities for the development of simple electromagnetic wave-switching devices operated by gate voltage.

Spin-dependent Seebeck effect and spin caloritronics in magnetic graphene

We investigate the spin-dependent thermoelectric effects in magnetic graphene in both diffusive and ballistic regimes. Employing the Boltzmann and Landauer formalisms we calculate the spin and charge Seebeck coefficients (thermopower) in magnetic graphene varying the spin splitting, temperature, and doping of the junction. It is found that while in normal graphene the temperature gradient drive a charge current, in the case of magnetic graphene a significant spin current is also established. In particular we show that in the undoped magnetic graphene in which different spin carriers belong to conduction and valence bands, a pure spin current is driven by the temperature gradient. In addition it is revealed that profound thermoelectric effects can be achieved at intermediate easily accessible temperatures when the thermal energy is comparable with Fermi energy $k_BT\lesssim \mu$. By further investigation of the spin-dependent Seebeck effect and a significantly large figure of merit for spin thermopower $\mathcal{Z}_{\rm sp}T$, we suggest magnetic graphene as a promising material for spin-caloritronics studies and applications.

Effect of Nitrogen Doping on the Migration of the Carbon Adatom and Monovacancy in Graphene

Nitrogen-doped graphene (N-graphene) has important implications in graphene-based devices and catalysts. Nitrogen incorporation into graphene via postsynthetic treatment is likely to produce a non-negligible amount of defects and bond disorders, and the resulting nitrogen content is usually dominated by graphitic N and pyridinic N. To understand the kinetic stability of doped N and the effect of doped N on the self-healing of monovacancy in graphene, we have performed density functional theory calculations to study the adsorption and migration of an adsorbed C atom on undoped and N-doped graphene with and without a monovacancy (MV). The effects of N doping and hydrogenation on the migration of a MV in graphene are also studied. Our results suggest that the graphitic N doped in the vicinity of MV is kinetically unstable, and it could be transformed into a pyridinic N due to the migration of MV when N-graphene is through high-temperature annealing. The presence of a C adatom would easily repair the vacancy ...

Nitrogen-doped graphene modified electrode for nimodipine sensing

A simple and sensitive label-free electrochemical biosensor for nimodipine was fabricated by using nitrogen-doped graphene (NGE) modified electrode. The surface morphology and electron transfer behavior of the modified electrodes were studied by scanning electron microscopy and electrochemical impedance spectroscopy, respectively. The electrochemical behaviors of nimodipine at the modified electrodes were investigated by using cyclic voltammetry. On the basis of the excellent electrocatalytic activity of NGE, an obvious cathodic peak of nimodipine could be caused at the NGE modified glassy carbon electrode (GCE), moreover, the electrochemical signal was greatly enhanced compared with the undoped graphene modified GCE. Under the optimized conditions, the present sensor showed excellent performances for nimodipine detection including wide linear range of 0.02–3μM, low detection limit (9.1nM), good selectivity to the general coexisted interferences, etc. The proposed biosensor was also successfully applied to validate its capability for the analysis of nimodipine in tablets. The work would promote the potential application of NGE as an excellent material in fabricating electrochemical sensors for chemical or biochemical analysis.

One-step thermal-treatment route to fabricate well-dispersed ZnO nanocrystals on nitrogen-doped graphene for enhanced electrochemiluminescence and ultrasensitive detection of pentachlorophenol.

Heteroatom doping enables graphene with novel properties and thus may broaden the potential of graphene-based materials. In this paper, novel ZnO-nanocrystal-decorated nitrogen-doped graphene (N-GR) composites were prepared through a one-step thermal-treatment route using glycine as the nitrogen source. ZnO nanocrystals with a size about 8 nm were well-dispersed and tightly anchored on the N-GR sheet. Compared with ZnO-nanocrystal-decorated undoped graphene, the ZnO/N-GR nanocomposites could not only enhance the electrochemiluminescence (ECL) intensity by 4.3-fold but also moved the ECL onset potential positively for ∼200 mV. All these results could be ascribed to the presence of nitrogen in graphene which decreased the barrier of ZnO nanocrystals reduction. Furthermore, the ECL sensor based on ZnO/N-GR nanocomposites was fabricated for the ultrasensitive detection of pentachlorophenol (PCP). This recyclable and eco-friendly sensor has excellent performances including wide linear range (0.5 pM to ∼61.1 nM), low detection limit (0.16 pM, S/N=3), good selectivity, and stability, which is a promising sensor for practical application in environment analysis.

Controlled Synthesis of Nitrogen-Doped Graphene from a Heteroatom Polymer and Its Mechanism of Formation

Heteroatom polymeric precursor with an aromatic substituent (polyurethane acrylate) was designed and found to generate nitrogen-doped graphene at 1000 °C. Upon thermal degradation, a nitrogenous fragment was liberated in situ. As a consequence, nitrogen-doped graphene was formed in a single step. Further, this methodology was extended for studying the effect of annealing nonheteroatom polymer with an aromatic substituent (styrene butadiene random copolymer) under similar conditions. The isolated product demonstrated undoped graphene-like structure. The mechanism of nitrogen-doped graphitization was studied by annealing different heteroatom polymer units with an aliphatic and aromatic framework in a chemical vapor deposition reactor. The effect of precursor heteroatom polymer structure and annealing temperature in the presence of copper catalyst on the efficacy of formation of nitrogen-doped graphene was examined. The nitrogen-doped graphene exhibited semiconducting behavior over semimetallic behavior of the undoped graphene. Moreover, the nitrogen-doped graphene displayed better electrochemical properties compared to the undoped variety. These advances in the synthesis of graphitic structures can be readily extended for the fabrication of many advanced materials with controlled structure and properties.

A general solid-state synthesis of chemically-doped fluorescent graphene quantum dots for bioimaging and optoelectronic applications.

Graphene quantum dots (GQDs) have attracted increasing interest because of their excellent properties such as strong photoluminescence, excellent biocompatibility and low cost. Herein, we develop a general method for the synthesis of doped and undoped GQDs, which relies on direct carbonization of organic precursors in the solid state.

Functionalized graphene in quantizing magnetic field: The case of bunched impurities

Resonant scattering at the atomic absorbates in graphene was investigated recently in relation with the transport and gap opening problems. Attaching an impurity atom to graphene is believed to lead to the creation of unusual zero energy localized electron states. This paper aims to describe the behavior of the localized impurity-induced levels in graphene in a quantizing magnetic field. It is shown that in the magnetic field the impurity level effectively hybridizes with one of the n=0 Landau level states and splits into two opposite-energy states. The new hybridized state is doubly occupied, forming a spin-singlet and reducing the polarization of a Quantum Hall ferromagnet in undoped graphene. Taking into account the electron-electron interaction changes radically the spectrum of the electrons surrounding the impurity, which should be seen experimentally. While existing publications investigate graphene uniformly covered by adatoms, here we address a possibly even more experimentally relevant case of the clusterized impurity distribution. The limit of a dense bunch of the impurity atoms is considered, and it is shown, how such a bunch changes the spectrum and spin polarization of a large dense electron droplet surrounding it. The droplet is encircled by an edge state carrying a persistent current.

High harmonic generation in undoped graphene: Interplay of inter- and intraband dynamics

We develop a density-matrix formalism in the length gauge to calculate the nonlinear response of intrinsic monolayer graphene at terahertz frequencies. Employing a tight-binding model, we find that the interplay of the interband and intraband dynamics leads to strong harmonic generation at moderate field amplitudes. In particular, we find that at low temperature, the reflected field of undoped suspended graphene exhibits a third harmonic amplitude that is 32% of the fundamental for an incident field of 100 V/cm. Moreover, we find that up to the seventh harmonic and beyond are generated.

Synthesis of nitrogen-doped graphene supported Pt nanoparticles catalysts and their catalytic activity for fuel cells

In this work, we have developed an efficient approach to prepare the nitrogen-doped graphene supported Pt nanocomposites (Pt/N-G). Nitrogen-doped graphene (N-G) was prepared by pyrolysis of graphene oxide with cyanamide as nitrogen containing precursor at different temperatures, which led to high and controllable nitrogen contents. Subsequently, the Pt nanoparticles were dispersed over N-G surface by modified chemical polyol reduction process. The morphology and nanostructure of as-prepared catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), which verified that the Pt nanoparticles were uniformly dispersed over N-G surface with a narrow size distribution. The functional groups on the surface of the catalysts were investigated by the Fourier transform infrared spectra (FT-IR), while the elemental composition and nitrogen bonding configurations in N-G were further evaluated by X-ray photoelectron spectroscopy (XPS). Furthermore, electrochemical properties are studied by cyclic voltammetry (CV). The Pt/N-G catalysts showed the superior electrocatalytic activity toward methanol oxidation compared to that of Pt loaded on undoped graphene (Pt/G). The results suggested that N-G could be used as an effective catalyst support for fuel cell application.

Synthesis of phosphorus-doped graphene and its wide potential window in aqueous supercapacitors.

Phosphorus-doped (P-doped) graphene with the P doping level of 1.30 at % was synthesized by annealing the mixture of graphene and phosphoric acid. The presence of P was confirmed by elemental mapping and X-ray photoelectron spectroscopy, while the morphology of P-doped graphene was revealed by using scanning electron microscopy and transmission electron microscopy. To investigate the effect of P doping, the electrochemical properties of P-doped graphene were tested as a supercapacitor electrode in an aqueous electrolyte of 1 M H2 SO4. The results showed that doping of P in graphene exhibited significant improvement in terms of specific capacitance and cycling stability, compared with undoped graphene electrode. More interestingly, the P-doped graphene electrode can survive at a wide voltage window of 1.7 V with only 3 % performance degradation after 5000 cycles at a current density of 5 A g(-1), providing a high energy density of 11.64 Wh kg(-1) and a high power density of 831 W kg(-1).