Strong anomalous optical dispersion of graphene: complex refractive index measured by Picometrology

Röling²,

Applied Physics Letters

et al. 2010

104

Imaging ellipsometry studies of graphene on SiO 2 / Si and crystalline GaAs are presented. We demonstrate that imaging ellipsometry is a powerful tool to detect and characterize graphene on any flat substrate. Variable angle spectroscopic ellipsometry is used to explore the dispersion of the optical constants of graphene in the visible range with high lateral resolution. In this way, the influence of the substrate on graphene's optical properties can be investigated.

“…Modeling our data results in a similar dispersion, as reported in Ref. 15, and shows the same trend as reported in Ref. 14.…”

Section: -supporting

confidence: 90%

mentioning

confidence: 99%

Imaging ellipsometry of graphene

Röling²,

Applied Physics Letters

et al. 2010

104

“…In order to understand the ablation mechanism, the timedependent temperature of graphene layer under the laser ir- [5,23], whereas the heat capacity of graphene was assumed to be close to bulk graphite in this modelling. The boundary conditions of the graphene on glass model were defined as periodic, thus a large system can be simulated by modelling a small part of it.…”

Section: Resultsmentioning

confidence: 99%

Ti:sapphire femtosecond laser direct micro-cutting and profiling of graphene

Zhang

Wang

et al. 2012

Appl. Phys. A

This paper reports the formation of uniform single layer micro-patterns of graphene on a glass substrate using direct femtosecond laser cutting. The cutting of graphene was achieved in air and argon. By translating the graphene sample with respect to the laser beam, continuous micro-channels were carved. The cutting geometry can be controlled by varying the laser fluence and the scanning path. Also, 1∼2 μm wide graphene micro-ribbons were hatched out. The ablation threshold of graphene was determined to be 0.16∼0.21 J/cm 2 . With the laser fluence higher than the ablation threshold, graphene was ablated rapidly and removed completely without damaging the glass substrate. Atomic force microscopy (AFM) and Raman spectroscopy have been used to confirm the ablation of graphene. Time domain finite difference modelling was employed to understand the thermal history of the laser ablation process.

“…However, such features appear after transferring the MLG on SiO 2 /Si substrate ( Figure S2). We note that the typical penetration depth of Raman laser into graphite is ~ 50-100 nm [21][22][23][24] and hence the Raman data in Figure 1b conveys information about the graphene layers away from Ni/MLG interface. These layers are free from any crystal defect as evidenced by the absence of Raman D peak mentioned above.…”

mentioning

confidence: 99%

Giant Current-Perpendicular-to-Plane Magnetoresistance in Multilayer Graphene as Grown on Nickel

2014

Strong magnetoresistance effects are often observed in ferromagnet-nonmagnet multilayers, which are exploited in state-of-the-art magnetic field sensing and data storage technologies. In this work we report a novel current-perpendicular-to-plane magnetoresistance effect in multilayer graphene as-grown on catalytic nickel surface by chemical vapor deposition. A negative magnetoresistance effect of ~10 4 % has been observed, which persists even at room temperature. This effect is correlated with the shape of the 2D peak as well as with the occurrence of D peak in the Raman spectrum of the as-grown multilayer graphene. The observed magnetoresistance is extremely high as compared to other known materials systems for similar temperature and field range, and can be qualitatively explained within the framework of "interlayer magnetoresistance" (ILMR).[Keywords: Graphene, Chemical Vapor Deposition, Raman Spectroscopy, Interlayer Magnetoresistance, Current-Perpendicular-to-Plane Transport] 2 Artificial layered structures often exhibit strong magnetoresistance (MR) effects that are exploited in various data storage and magnetic field sensing technologies 1 . Graphite is a naturally occurring layered material in which single graphitic layers (or "graphene") are stacked on each other. Graphene, epitaxially grown on ferromagnets (such as nickel), is particularly attractive for spintronics because such systems can potentially realize perfect spin filtering 2 and giant Rashba splitting 3 . However, CPP (current-perpendicular-toplane) MR properties of such layered graphene/ferromagnet structures are still largely underexplored. Here we consider multilayer-graphene (MLG) as-grown on nickel by chemical vapor deposition (CVD) and show that these structures exhibit large and nearly temperature-independent CPP-MR of ~ 10 4 % for a small magnetic field of ~ 2 kilogauss. This MR effect is correlated with the shape of the 2D peak and also with the occurrence of the D peak in Raman spectrum of as-grown MLG. These Raman features can be controlled by varying the CVD growth parameters. Such large negative CPP-MR, which persists even at room temperature, has hitherto not been reported in any graphitic system 4-14 . Figure 1a shows the device schematic. CVD growth of MLG is performed on 2 cm × 2 cm nickel (Ni) foils, which act as catalyst for graphene growth as well as bottom electrical contact. To ensure uniform current distribution 6 , the second contact is fabricated at the center of the top MLG surface using silver epoxy. Area of the top contact is ~ 1 mm 2 . As shown in Figure S1 (section I, Supplementary Information), the Ni substrate is polycrystalline with primarily (111) grains. Details of the fabrication process are provided in section I of Supplementary Information. Figure 1b shows a FESEM image of the as-grown large-area MLG on Ni. Raman spectra taken from three representative regions of this sample are shown in the top inset of Figure 1b. The top Raman spectrum (black line) is most commonly observed, with few occurren...