Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.

/pdf/raman-spectroscopy-as-a-versatile-tool-for-studying-the-54mhwmcfzi.pdf

Raman spectroscopy as a versatile tool for studying the properties of graphene

Graphene based materials: Past, present and future

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

/pdf/science-and-technology-roadmap-for-graphene-related-two-4q9m7czlkc.pdf

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities for a given defect density strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an inter-defect distance ~3nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.

Quantifying defects in graphene via Raman spectroscopy at different excitation energies

We present a Raman study of Ar+-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.

/pdf/quantifying-defects-in-graphene-via-raman-spectroscopy-at-ni0uoxanyv.pdf

Quantifying defects in graphene via Raman spectroscopy at different excitation energies.

We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E(2g) optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G(+) and G(-), by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D(') bands also redshift but do not split for small strains. We study the Gruneisen parameters for the phonons responsible for the G, D, and D(') peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance.

/pdf/uniaxial-strain-in-graphene-by-raman-spectroscopy-g-peak-2blddncxbq.pdf

Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Grüneisen parameters, and sample orientation

In this paper, we have sandwiched a single-atom-thick sheet of graphene, a 2-D allotrope of carbon that is exciting tremendous research interest, between two ferromagnetic electrodes. Graphene has already been shown to support spin polarized conduction, when spin polarized electrons were injected into graphene sheets along the plane using magnetic electrodes. Here, we study spin polarized conduction perpendicular the plane of the graphene in conduction perpendicular to plane (CPP) geometry. It was found that graphene was sufficient to reduce the exchange coupling between the magnetic electrodes. We also found that in a NiFe/Au/Graphene/NiFe stack, the graphene channels the spin current perpendicular to the plane and thus produces an enhanced MR effect compared to a simple stack without the graphene. A significant anisotropy was found in the observed magnetoresistance in the cross electrode geometry employed.

/pdf/graphene-in-multilayered-cpp-spin-valves-2i3ngy1idl.pdf

T. M. G. Mohiuddin

Papers

Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Grüneisen parameters, and sample orientation

Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Grüneisen parameters, and sample orientation

Graphene in Multilayered CPP Spin Valves