Doping Level Of Graphene Research Articles

Direct observation of a dispersionless impurity band in hydrogenated graphene

We show with angle-resolved photoemission spectroscopy that a new energy band appears in the electronic structure of electron-doped hydrogenated monolayer graphene (H-graphene). Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene's doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed of hydrogen 1s orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by x-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the pi* resonance.

High-Quality Graphene p−n Junctions via Resist-free Fabrication and Solution-Based Noncovalent Functionalization

An essential issue in graphene nanoelectronics is to engineer the carrier type and density and still preserve the unique band structure of graphene. We report the realization of high-quality graphene p-n junctions by noncovalent chemical functionalization. A generic scheme for the graphene p-n junction fabrication is established by combining the resist-free approach and spatially selective chemical modification process. The effectiveness of the chemical functionalization is systematically confirmed by surface topography and potential measurements, spatially resolved Raman spectroscopic imaging, and transport/magnetotransport measurements. The transport characteristics of graphene p-n junctions are presented with observations of high carrier mobilities, Fermi energy difference, and distinct quantum Hall plateaus. The chemical functionalization of graphene p-n junctions demonstrated in this study is believed to be a feasible scheme for modulating the doping level in graphene for future graphene-based nanoelectronics.

Control of Carrier Type and Density in Exfoliated Graphene by Interface Engineering

Air-stable, n-doped or p-doped graphene sheets on a chip were achieved by modifying the substrates with self-assembled layers of silane and polymer. The interfacial effects on the electronic properties of graphene were investigated using micro-Raman and Kelvin probe force microscopy (KPFM). Raman studies demonstrated that the phonon vibrations were sensitive to the doping level of graphene on the various substrates. Complementary information on the charge transfer between the graphene and substrate was extracted by measuring the surface potential of graphene flakes using KPFM, which illustrated the distribution of carriers in different graphene layers as well as the formation of dipoles at the interface. The Fermi level of single layer graphene on the modified substrates could be tuned in a range from -130 to 90 mV with respect to the Dirac point, corresponding to the doped carrier concentrations up to 10(12) cm(-2).

Effects of particle contamination and substrate interaction on the Raman response of unintentionally doped graphene

We investigated the inhomogeneities in the charge density of unintentionally doped graphene on SiO2 prepared by mechanical exfoliation. From the analysis of the G, D, and 2D phonon modes of the Raman spectra after displacing contaminants on graphene surface, and measuring the separation monolayer-substrate distance among zones with different doping levels, we deduce that the interaction with the substrate is the main cause of doping in graphene rather than particle contamination. In particular, we show how graphene doping levels vary within the same flake depending on the distance between graphene and the substrate.

Graphene on a Hydrophobic Substrate: Doping Reduction and Hysteresis Suppression under Ambient Conditions

The intrinsic doping level of graphene prepared by mechanical exfoliation and standard lithography procedures on thermally oxidized silicon varies significantly and seems to depend strongly on processing details and the substrate morphology. Moreover, transport properties of such graphene devices suffer from hysteretic behavior under ambient conditions. The hysteresis presumably originates from dipolar adsorbates on the substrate or graphene surface. Here, we demonstrate that it is possible to reliably obtain low intrinsic doping levels and to strongly suppress hysteretic behavior even in ambient air by depositing graphene on top of a thin, hydrophobic self-assembled layer of hexamethyldisilazane (HMDS). The HMDS serves as a reproducible template that prevents the adsorption of dipolar substances. It may also screen the influence of substrate deficiencies.

Aromatic Molecules Doping in Single-Layer Graphene Probed by Raman Spectroscopy and Electrostatic Force Microscopy

Electrostatic force microscopy (EFM) and Raman spectroscopy are used to study the effects of aromatic molecules on the doping level of single layer graphene (SLG). The EFM results demonstrate that the aromatic molecules with electron-donating group (n-doping) decrease the surface potential of the SLG films, while the molecules with electron-withdrawing groups (p-doping) increase their surface potential. Meanwhile, Raman spectra indicate that the aromatic molecules with electron-withdrawing groups make the SLG p-dope and upshifts in both Raman G and 2D frequencies. While the aromatic molecules with electron-donating groups n-dope the SLG and up-shift the 2D-but down-shift the G-band frequencies.

Theory of superconductivity for Dirac electrons in graphene

Phonon-exchange-induced superconducting pairing of effectively ultrarelativistic electrons in graphene is investigated. The Eliashberg equation obtained for describing pairing in the Cooper channel with allowance for delayed interaction are matrix equations with indices corresponding to the valence and conduction bands. The equations are solved in the high doping limit, in which pairing is effectively a single-band process, and in the vicinity of a critical quantum point of underdoped graphene for a value of the coupling constant for which pairing is an essentially multiband process. For such cases, analytic estimates are obtained for the superconducting transition temperature of the system. It is shown that the inclusion of dynamic effects makes it possible to determine the superconducting transition temperature, as well as the critical coupling constant for underdoped graphene, more accurately than in the static approximation of the BCS type. Estimates of the constants of electron interaction with the scalar optical phonon mode in graphene indicate that an appreciable superconducting transition temperature can be attained under a high chemical doping level of graphene.

The use of liquid helium-3 to obtain temperatures down to 0·3° K

Epitaxial Chemical Vapor Deposition growth of graphene on silicon carbide offers the maturity and reliability expected for large-scale fabrication of graphene-based devices. In particular, the ultimate challenge of graphene synthesis on SiC, i.e. quasi-free-standing monolayer graphene which comes through hydrogen atom intercalation of the sole buffer layer grown on the Si-face of SiC, offers high carrier mobility (as high as 6600 [cm2/Vs]) and electrical stability throughout the device processing cycle. In this report, we present extensive statistics of the electrical properties of QFS-monolayer graphene grown on 4H(0001) and 6H(0001) semi-insulating 10 mm × 10 mm substrates, being a result of 110 individual processes. The adopted explanation for the origin of the as-grown doping level in epitaxial graphene based on the spontaneous polarization of hexagonal SiC and its most up-to-date values is reaffirmed. We introduce the issue of the step-edge-induced offset voltage radial dependence and confront it with the morphological analysis of the average step edge height and terrace width, all related to the place of origin of a specific sample within a 4-inch SiC wafer. Finally, we conclude that within the step edge area QFS-monolayer graphene is statistically nearly half as resistive as the previously reported QFS-bilayer graphene.