In Situ Study of Hydrogenation of Graphene and New Phases of Localization between Metal–Insulator Transitions

Abstract

Monolayer graphene synthesized by chemical vapor deposition was subjected to controlled and sequential hydrogenation using RF plasma while monitoring its electrical properties in situ. Low-temperature transport properties, namely, electrical resistance (R), thermopower (S), Hall mobility (μ), and magnetoresistance (MR), were measured for each sample and correlated with ex situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak hydrogenation, the transport is seen to be governed by electron diffusion, and low-temperature transport properties show metallic behavior (conductance G remains nonzero as T → 0). For strong hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (G → 0 as T → 0). Weak localization (WL) behavior is seen with a negative MR for weakly hydrogenated graphene, and these WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly hydrogenated graphene.

Electronic Transport in Graphene with Aggregated Hydrogen Adatoms

“…For hydrogenated graphene, a model based on massless Dirac fermions with δ-function point potentials confirms this prediction of the unitary class, though in 2D systems localization lengths can be strongly energy-dependent and, eventually, very large [13]. However, no unanimous consensus has been reached since experiments on hydrogenated graphene point towards metal-insulator transition, theoretically justified by the presence of electron-hole puddles (2D percolation class) [14][15][16][17].…”

“…For hydrogenated graphene, a model based on massless Dirac fermions with δ-function point potentials confirms this prediction of the unitary class, though in 2D systems localization lengths can be strongly energy-dependent and, eventually, very large [13]. However, no unanimous consensus has been reached since experiments on hydrogenated graphene point towards metal-insulator transition, theoretically justified by the presence of electron-hole puddles (2D percolation class) [14][15][16][17].Early works treating finite concentrations of resonant impurities in graphene assumed that the total scattering cross section deviates little from the incoherent addition of the individual cross sections, for example in the Boltzmann equation framework [18]. This picture is valid for low defect concentrations, low charge-carrier densities, and random adatom distributions.…”

Role of H Distribution on Coherent Quantum Transport of Electrons in Hydrogenated Graphene

“…These results suggest that low temperature transport measurements could help with detecting the presence of a magnetic state in weakly hydrogenated graphene. Several experimental techniques pointed towards the possible occurrence of a metal-insulator transition in hydrogenated graphene, as a function of hydrogenation [30,31]. These results have been interpreted in terms of the crossover between different quantum transport regimes, triggered by hydrogenation-induced disorder [32,33].…”

Synthesis, characterization, and electronic properties of novel 2D materials : transition metal dichalcogenides and phosphorene.

“…Graphene is a prime example. By functionalization of graphene using nitrogen in RF plasma, a metal-insulator transition was found in graphene [148]. Heterostructures of graphene have been synthesized to take advantage of its long range transport and massless Dirac fermions but overcome its zero band gap [137,149].…”

“…Using a very precise control of precursor material in CVD, it has been shown that graphene can be doped with nitrogen [154]. Plasma assisted CVD [148] and chemisorption [155] have similar characteristic . In order to introduce atoms in to the lattice of a system, one can increase the energy of the dopant atoms.…”