Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.

We show spin lifetimes of 12.6 ns and spin diffusion lengths as long as 30.5 µm in single layer graphene non-local spin transport devices at room temperature. This is accomplished by the fabrication of Co/MgO-electrodes on a Si/SiO2 substrate and the subsequent dry transfer of a graphenehBN-stack on top of this electrode structure where a large hBN flake is needed in order to diminish the ingress of solvents along the hBN-to-substrate interface. Interestingly, long spin lifetimes are observed despite the fact that both conductive scanning force microscopy and contact resistance measurements reveal the existence of conducting pinholes throughout the MgO spin injection/detection barriers. The observed enhancement of the spin lifetime in single layer graphene by a factor of 6 compared to previous devices exceeds current models of contact-induced spin relaxation which paves the way towards probing intrinsic spin properties of graphene.

We present a new fabrication method of graphene spin-valve devices which yields enhanced spin and charge transport properties by improving both the electrode-to-graphene and graphene-to-substrate interface. First, we prepare Co/MgO spin injection electrodes onto Si ++ /SiO 2 . Thereafter, we mechanically transfer a graphene-hBN heterostructure onto the prepatterned electrodes. We show that room temperature spin transport in single-, bi-and trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion lengths reaching 10 µm combined with carrier mobilities exceeding 20, 000 cm 2 /Vs.

We investigate spin and charge transport in both single and bilayer graphene non-local spin-valve devices. An inverse dependence of the spin lifetime τs on the carrier mobility µ is observed in devices with large contact resistance area products (RcA > 1 kΩµm 2 ). Furthermore, we observe an increase of τs with increasing RcA values demonstrating that spin transport is limited by spin dephasing underneath the electrodes. In charge transport, we measure a second contact-induced Dirac peak at negative gate voltages in devices with larger RcA values demonstrating different transport properties in contact covered and bare graphene parts. We argue that the existence of the second Dirac peak complicates the analysis of the carrier mobilities and the spin scattering mechanisms.Graphene has drawn strong attention because of measured spin diffusion length of some µm at room temperature. While most spin transport devices only exhibit spin lifetimes up to several hundred picoseconds at room temperature [1][2][3][4][5][6][7][8][9] there are only few reports with spin lifetimes above one nanosecond.10-12 Nevertheless, all experimental values of the spin lifetimes are some orders of magnitude shorter than theoretically predicted 13,14 indicating that in present devices spin transport is limited by extrinsic sources of spin scattering. These include spin-orbit coupling by adatoms, edge effects and ripples.6,10,13,15-18 Additionally, spin scattering may result from the underlying substrate or the spin injection and detection contacts. 14,19,20 The importance of the latter might be indicated by recent electron spin resonance (ESR) experiments on graphene nanoribbons and small flakes that were only weakly coupled to the substrate and had no electrodes.21,22 Interestingly, the measured spin lifetimes of localized spin states are at least 200 ns while the estimated spin lifetimes of conduction electrons are 30 ns, which is larger than any reported values from electrical Hanle spin precession measurements.In this Rapid Communication, we investigate the influence of MgO barriers on spin and charge transport properties by fabricating both single layer (SLG) and bilayer graphene (BLG) non-local spin-valve devices with variable contact resistance area products R c A of the MgO/Co electrodes. We explore the relationship between spin lifetime τ s and charge carrier mobility µ in SLG and find a similar 1/µ dependence as seen in previous spin transport studies on exfoliated bilayer graphene (BLG) devices.12 This dependence is only seen in samples with R c A > 1 kΩµm 2 . In fact, we observe that devices with long τ s additionally exhibit a second Dirac peak in charge transport, which stems from the electrodes. This contact-induced Dirac peak overlaps with the Dirac peak of the bare graphene which complicates the analysis of the carrier mobility and thus complicates a clear assignment of the dominant spin scattering mechanism in graphene. For devices with low R c A we find an overall strong decrease of τ s showing that transparent contacts yie...

By successive oxygen treatments of graphene non-local spin-valve devices we achieve a gradual increase of the contact resistance area products (RcA) of Co/MgO spin injection and detection electrodes and a transition from linear to non-linear characteristics in the respective differential dV -dI-curves. With this manipulation of the contacts both spin lifetime and amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small RcA values. With increasing RcA values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co/graphene interaction by improving the oxide barrier during oxygen treatment.