Quasi-free Graphene Research Articles

The effect of quasi-free graphene layer on the electrical transport properties of sandwich-like graphene / Co nanoparticles / graphene structure

The paper reports on the charge transport in the sandwich-like graphene / Co nanoparticles / graphene structure (G/CoNPs/G). It is fabricated by combining the electrochemical deposition of Co nanoparticles onto a single layer CVD graphene and their capping with another graphene layer. The isolated semi-spherical, surface oxidized Co NPs of 250 nm in diameter serve as a support for the top graphene layer and provide its suspended state confirmed by atomic force microscopy revealing periodic ripples on its surface. A high optical transparency of 92.4% is found for the G/CoNPs/G sandwich. The observed twofold reduction in the sheet resistance with a ∼ 1.5 times increase in the positive magnetoresistive effect for G/CoNPs/G compared to the pristine graphene is attributed to the suppression of carrier scattering due to the substrate. From the Hall effect measurements, a predominant role of the top graphene layer in the charge transport in G/CoNPs/G is established, with a significantly increased carrier mobility µ∼1800 cm2/(V·s). An elastic intervalley scattering length estimated from the magnetoresistance results is found to be three times increased in G/CoNPs/G, indicating that the top graphene layer is almost unaffected by the substrate. The proposed technology looks promising for possible optoelectronic and magneto-optical applications.

Effect of Electron–Phonon Interaction on the Conductivity and Work Function of Epitaxial Graphene

In the context of the cluster model of epitaxial graphene, the condition for a stepwise change in the charge of carbon atoms under the action of graphene–substrate electron–phonon interaction and an external electric field is derived. Such charge transfer brings about corresponding steps of the static conductivity and work function of epitaxial graphene. Numerical estimates for the case of weak coupling of quasi-free graphene with metal and semiconductor substrates are given.

Влияние электрон-фононного взаимодействия на проводимость и работу выхода эпитаксиального графена

AbstractIn the context of the cluster model of epitaxial graphene, the condition for a stepwise change in the charge of carbon atoms under the action of graphene–substrate electron–phonon interaction and an external electric field is derived. Such charge transfer brings about corresponding steps of the static conductivity and work function of epitaxial graphene. Numerical estimates for the case of weak coupling of quasi-free graphene with metal and semiconductor substrates are given.

Effect of intercalated hydrogen on the electron state of quasi-free graphene on a SiC substrate

To judge the role of intercalated hydrogen in the doping of quasi-free epitaxial graphene, two systems, specifically, (i) the graphene–NH-SiC{0001} and (ii) graphene–hydrogen single layer–NH-SiC{0001} structures (N = 4, 6) are considered. In case (i), the shift of the Dirac point of epitaxial graphene is induced by the electrostatic potential of spontaneous polarization of the substrate; in case (ii), the shift is due to the field of a double electrical layer formed because of the absorption of hydrogen atoms. It is shown that, in case (ii) compared to case (i), the concentration of holes in graphene is higher and the concentration of electrons is lower. This result is consistent with the currently available experimental data.

On the density of states of disordered epitaxial graphene

The study is concerned with two types of disordered epitaxial graphene: (i) graphene with randomly located carbon vacancies and (ii) structurally amorphous graphene. The former type is considered in the coherent potential approximation, and for the latter type, a model of the density of states is proposed. The effects of two types of substrates, specifically, metal and semiconductor substrates are taken into account. The specific features of the density of states of epitaxial graphene at the Dirac point and the edges of the continuous spectrum are analyzed. It is shown that vacancies in epitaxial graphene formed on the metal substrate bring about logarithmic nulling of the density of states of graphene at the Dirac point and the edges of the continuous spectrum. If the Dirac point corresponds to the middle of the band gap of the semiconductor substrate, the linear trend of the density of states to zero in the vicinity of the Dirac point in defect-free graphene transforms into a logarithmic decrease in the presence of vacancies. In both cases, the graphene-substrate interaction is assumed to be weak (quasi-free graphene). In the study of amorphous epitaxial graphene, a simple model of free amorphous graphene is proposed as the initial model, in which account is taken of the nonzero density of states at the Dirac point, and then the interaction of the graphene sheet with the substrate is taken into consideration. It is shown that, near the Dirac point, the quadratic behavior of the density of states of free amorphous graphene transforms into a linear dependence for amorphous epitaxial graphene. In the study, the density of states of free graphene corresponds to the low-energy approximation of the electron spectrum.

Ultrafast Charge Transfer at Monolayer Graphene Surfaces with Varied Substrate Coupling

The charge transfer rates of a localized excited electron to graphene monolayers with variable substrate coupling have been investigated by the core hole clock method with adsorbed argon. Expressed as charge transfer times, we find strong variations between ~3 fs (on graphene "valleys" on Ru(0001)) to ~16 fs (quasi-free graphene on SiC, O/Ru(0001), or SiO2/Ru). The values for the "hills" on Gr/Ru and on Gr/Pt(111) are in between, with the ratio 1.7 between the charge transfer times measured on "hills" and "valleys" of Gr/Ru. We discuss the results for Gr on metals in terms of hybridized Ru-C orbitals, which change with the relative Gr-Ru alignment and distance. The charge transfer on the decoupled graphene layers must represent the intrinsic coupling to the graphene empty π* states. Its low rate may be influenced by processes retarding the spreading of charge after transfer.

Study of the intercalation of graphene on Ni(111) with Cs atoms: Towards the quasi-free graphene

In this work we present an experimental study of the intercalation of the graphene/Ni(111) system by cesium atoms. The evidence of a complete alkali intercalation has been obtained by Auger and angle-resolved electron-energy-loss measurements. The changes in the electronic and vibrational properties demonstrate that a weakly bonded graphene layer can be synthesized on Ni(111) by means of Cs intercalation.

Graphene: Synthesis and features of electronic structure

Changes in the electronic structure of the valence band and the corresponding dispersion dependences of the electron states of graphene synthesized by a cracking catalytic reaction of propylene at the surface of Ni(111) thin layers and the subsequent intercalation of Au atoms underneath the formed monolayer of graphene have been measured and analyzed using angle-resolved photoelectron spectroscopy. It is shown that this technique of graphene synthesis causes the formation of a monolayer graphene cover over large areas with features of the electronic structure typical for a quasifree graphene: a linear dispersion of graphene π states in the region of the point of the Brillouin zone with the Dirac point localization near the Fermi level. It was established that contact between the graphene monolayer and the intercalated Au layer additionally causes a substrate-induced spin-orbital splitting of graphene π states, which also occurs in the region of dispersion-dependence linearity.

Ambipolar doping in quasifree epitaxial graphene on SiC(0001) controlled by Ge intercalation

The electronic structure of decoupled graphene on SiC(0001) can be tailored by introducing atomically thin layers of germanium at the interface. The electronically inactive (6 root 3 x 6 root 3)R30 degrees reconstructed buffer layer on SiC(0001) is converted into quasi-free-standing monolayer graphene after Ge intercalation and shows the characteristic graphene pi bands as displayed by angle-resolved photoelectron spectroscopy. Low-energy electron microscopy (LEEM) studies reveal an unusual mechanism of the intercalation in which the initial buffer layer is first ruptured into nanoscopic domains to allow the local in-diffusion of germanium to the interface. Upon further annealing, a continuous and homogeneous quasifree graphene film develops. Two symmetrically doped (n- and p-type) phases are obtained that are characterized by different Ge coverages. They can be prepared individually by annealing a Ge film at different temperatures. In an intermediate-temperature regime, a coexistence of the two phases can be achieved. In this transition regime, n-doped islands start to grow on a 100-nm scale within p-doped graphene terraces as revealed by LEEM. Subsequently, the n islands coalesce but still adjacent terraces may display different doping. Hence, lateral p-n junctions can be generated on epitaxial graphene with their size tailored on a mesoscopic scale. (Less)

Formation of quasi-free graphene on the Ni(111) surface with intercalated Cu, Ag, and Au layers

This paper reports on a study by angle-resolved photoelectron and low-energy electron energy loss spectroscopy of graphene monolayers, which are produced by propylene cracking on the Ni(111) surface, followed by intercalation of Cu, Ag, and Au atoms between the graphene monolayer and the substrate, for various thicknesses of deposited metal layers and annealing temperatures. It has been shown that the spectra of valence-band π states and of phonon vibrational modes measured after intercalation become similar to those characteristic of single-crystal graphite with weak interlayer coupling. Despite the strong coupling of the graphene monolayer to the substrate becoming suppressed by intercalation of Cu and Ag atoms, the π state branch does not reach at the K point of the Brillouin zone the Fermi level, with the graphene coating itself breaking up partially to form graphene domains. At the same time after intercalation of Au atoms, the electronic band structure approaches the closest to that of isolated graphene, with linear π-state dispersion near the K point of the Brillouin zone, and the point of crossing of the filled, (π), with empty, (π*), states lying in the region of the Fermi level, which makes this system a promising experimental model of the quasi-free graphene monolayer.