The reduction of graphene oxide

Most electronic devices exploit the electric charge of electrons, but it is also possible to build devices that rely on other properties of electrons. Spintronic devices, for example, make use of the spin of electrons. Valleytronics is a more recent development that relies on the fact that the conduction bands of some materials have two or more minima at equal energies but at different positions in momentum space. To make a valleytronic device it is necessary to control the number of electrons in these valleys, thereby producing a valley polarization. Single-layer MoS(2) is a promising material for valleytronics because both the conduction and valence band edges have two energy-degenerate valleys at the corners of the first Brillouin zone. Here, we demonstrate that optical pumping with circularly polarized light can achieve a valley polarization of 30% in pristine monolayer MoS(2). Our results, and similar results by Mak et al., demonstrate the viability of optical valley control and valley-based electronic and optoelectronic applications in MoS(2) monolayers.

Valley polarization in MoS2 monolayers by optical pumping

We provide a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures. A salient feature of our review is a critical comparison between carrier transport in graphene and in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells, inversion layers) so that the unique features of graphene electronic properties arising from its gap- less, massless, chiral Dirac spectrum are highlighted. Experiment and theory as well as quantum and semi-classical transport are discussed in a synergistic manner in order to provide a unified and comprehensive perspective. Although the emphasis of the review is on those aspects of graphene transport where reasonable consensus exists in the literature, open questions are discussed as well. Various physical mechanisms controlling transport are described in depth including long- range charged impurity scattering, screening, short-range defect scattering, phonon scattering, many-body effects, Klein tunneling, minimum conductivity at the Dirac point, electron-hole puddle formation, p-n junctions, localization, percolation, quantum-classical crossover, midgap states, quantum Hall effects, and other phenomena.

/pdf/electronic-transport-in-two-dimensional-graphene-32oou47sy4.pdf

Electronic transport in two-dimensional graphene

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

Graphene, a two-dimensional, single-layer sheet of sp(2) hybridized carbon atoms, has attracted tremendous attention and research interest, owing to its exceptional physical properties, such as high electronic conductivity, good thermal stability, and excellent mechanical strength. Other forms of graphene-related materials, including graphene oxide, reduced graphene oxide, and exfoliated graphite, have been reliably produced in large scale. The promising properties together with the ease of processibility and functionalization make graphene-based materials ideal candidates for incorporation into a variety of functional materials. Importantly, graphene and its derivatives have been explored in a wide range of applications, such as electronic and photonic devices, clean energy, and sensors. In this review, after a general introduction to graphene and its derivatives, the synthesis, characterization, properties, and applications of graphene-based materials are discussed.

Graphene-Based Materials: Synthesis, Characterization, Properties, and Applications

Spin transport experiments in graphene, a single layer of carbon atoms ordered in a honeycomb lattice, indicate spin-relaxation times that are significantly shorter than the theoretical predictions. We investigate experimentally whether these short spin-relaxation times are due to extrinsic factors, such as spin relaxation caused by low impedance contacts, enhanced spin-flip processes at the device edges, or the presence of an aluminum oxide layer on top of graphene in some samples. Lateral spin valve devices using a field-effect transistor geometry allowed for the investigation of the spin relaxation as a function of the charge density, going continuously from metallic hole to electron conduction (charge densities of $n\ensuremath{\sim}{10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}2}$) via the Dirac charge neutrality point $(n\ensuremath{\sim}0)$. The results are quantitatively described by a one-dimensional spin-diffusion model where the spin relaxation via the contacts is taken into account. Spin valve experiments for various injector-detector separations and spin precession experiments reveal that the longitudinal $({T}_{1})$ and the transversal $({T}_{2})$ relaxation times are similar. The anisotropy of the spin-relaxation times ${\ensuremath{\tau}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\tau}}_{\ensuremath{\perp}}$, when the spins are injected parallel or perpendicular to the graphene plane, indicates that the effective spin-orbit fields do not lie exclusively in the two-dimensional graphene plane. Furthermore, the proportionality between the spin-relaxation time and the momentum-relaxation time indicates that the spin-relaxation mechanism is of the Elliott-Yafet type. For carrier mobilities of $2\ifmmode\times\else\texttimes\fi{}{10}^{3}--5\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }{\text{cm}}^{2}/\text{V}\text{ }\text{s}$ and for graphene flakes of $0.1--2\text{ }\ensuremath{\mu}\text{m}$ in width, we found spin-relaxation times on the order of 50--200 ps, times which appear not to be determined by the extrinsic factors mentioned above.

/pdf/electronic-spin-transport-in-graphene-field-effect-52lpo6cu4y.pdf

Electronic spin transport in graphene field-effect transistors

Spin relaxation in graphene is investigated in electrical graphene spin valve devices in the nonlocal geometry. Ferromagnetic electrodes with in-plane magnetizations inject spins parallel to the graphene layer. They are subject to Hanle spin precession under a magnetic field B applied perpendicular to the graphene layer. Fields above 1.5 T force the magnetization direction of the ferromagnetic contacts to align to the field, allowing injection of spins perpendicular to the graphene plane. A comparison of the spin signals at B = 0 and B = 2 T shows a 20% decrease in spin relaxation time for spins perpendicular to the graphene layer compared to spins parallel to the layer. We analyze the results in terms of the different strengths of the spin-orbit effective fields in the in-plane and out-of-plane directions and discuss the role of the Elliott-Yafet and Dyakonov-Perel mechanisms for spin relaxation.

/pdf/anisotropic-spin-relaxation-in-graphene-z85d4hfq1r.pdf

Anisotropic spin relaxation in graphene.

Quantization of the current flowing across a nanometre-scale constriction in graphene is usually destroyed through charge-scattering from rough edges and impurities. But by using high-quality suspended samples and a constriction whose length is shorter than its width, conductance quantization in graphene has now been demonstrated.

/pdf/quantized-conductance-of-a-suspended-graphene-1wbxgmpp0i.pdf

Quantized conductance of a suspended graphene nanoconstriction

We measure spin transport in high mobility suspended graphene (μ ≈ 10(5)cm(2)/(V s)), obtaining a (spin) diffusion coefficient of 0.1 m(2)/s and giving a lower bound on the spin relaxation time (τ(s) ≈ 150 ps) and spin relaxation length (λ(s) = 4.7 μm) for intrinsic graphene. We develop a theoretical model considering the different graphene regions of our devices that explains our experimental data.

/pdf/spin-transport-in-high-quality-suspended-graphene-devices-3jw1zxelv7.pdf

Spin Transport in High-Quality Suspended Graphene Devices

Spin transport in graphene carries the potential of a long spin-diffusion length at room temperature However, extrinsic relaxation processes limit the current experimental values to $1--2\text{ }\ensuremath{\mu}\text{m}$ We present Hanle spin precession measurements in gated lateral spin valve devices in the low to high (up to ${10}^{13}\text{ }{\text{cm}}^{\ensuremath{-}2}$) carrier density range of graphene A linear scaling between the spin-diffusion length and the diffusion coefficient is observed We measure nearly identical spin- and charge diffusion coefficients indicating that electron-electron interactions are relatively weak and transport is limited by impurity potential scattering When extrapolated to the maximum carrier mobilities of $2\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }{\text{cm}}^{2}/\text{Vs}$, our results predict that a considerable increase in the spin-diffusion length should be possible

/pdf/linear-scaling-between-momentum-and-spin-scattering-in-2ye2zmnx3d.pdf

A. Veligura

Papers

Electronic spin transport in graphene field-effect transistors

Anisotropic spin relaxation in graphene.

Quantized conductance of a suspended graphene nanoconstriction

Spin Transport in High-Quality Suspended Graphene Devices

Linear scaling between momentum and spin scattering in graphene