Microfabrication of graphene devices used in many experimental studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepared on top of silicon wafers with a certain thickness of silicon dioxide. We study graphene's visibility and show that it depends strongly on both thickness of silicon dioxide and light wavelength. We have found that by using monochromatic illumination, graphene can be isolated for any silicon dioxide thickness, albeit 300 nm (the current standard) and, especially, approx. 100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, we quantitatively describe the experimental data without any fitting parameters.Comment: Since v1: minor changes to text and figures to improve clarity; references added. Submitted to Applied Physics Letters, 30-Apr-07. 3 pages, 3 figure

The exceptional electronic properties of graphene and its formidable potential in various applications have ensured a rapid growth of interest in this new material [1,2]. One of the most discussed and tantalizing directions in research on graphene is its use as the base material for electronic circuitry that is envisaged to consist of nanometer-sized elements. Most attention has so far been focused on graphene nanoribbons (see [3][4][5][6][7][8][9] and references therein). In this Letter, we report quantum dot (QD) devices made entirely from graphene, including their central islands (CI), quantum barriers, source and drain contacts and side-gate electrodes. We have found three basic operational regimes for such devices, depending on their size. For relatively large (submicron) CIs, size quantization plays an insignificant role, and our devices were found to operate as orthodox singleelectron transistors (SET) exhibiting periodic Coulomb blockade (CB) oscillations. The CB regime has been extensively studied previously using metallic and semiconducting materials [10,11] and, more recently, the first SET devices made from graphite [12] and graphene [1,13,14] were also demonstrated. The all-graphene SETs reported here are technologically simple, reliable and robust and can operate above liquid-helium temperatures T, which makes them attractive candidates for use in various charge-detector schemes [10]. For intermediate CI sizes (less than ∼100nm), we enter into the quantum regime, in which the confinement energy δE >10meV exceeds the charging energy E c . Such a strong quantization for relatively modest confinement is unique to massless fermions [1,2] and related to the fact that their typical level spacing δE ≈v F h/2D in a quantum box of size D is much larger than the corresponding energy scale ≈h 2 /8mD 2 for massive carriers in other materials (v F ≈10 6 m/s is the Fermi velocity in graphene, h the Planck constant and m the effective mass). This means that level splitting in graphene-based 100-nm devices should be tens and hundreds times larger than in typical semiconducting and metal QDs, respectively. This regime is probably most interesting from the fundamental physics point of view, allowing studies of relativistic-like quantum effects in confined geometries [15][16][17][18][19][20][21]. In particular, we have observed a strong level repulsion in QDs, which is a clear signature of quantum chaos (so-called "neutrino billiards" [15]). Conductance of our smallest devices is dominated by individual constrictions with sizes down to ∼1nm, which exhibit δE ∼0.5eV and a good-quality transistor action at room T. It is remarkable that these molecular-scale structures survive microfabrication procedures, remain mechanically and chemically stable and highly conductive under ambient conditions and sustain large (nA) currents. Our devices were made from graphene crystallites prepared by micromechanical cleavage on top of an oxidized Si wafer (300nm of SiO 2 ) [22]. By using high-resolution electron-beam lithography, we de...

It is widely assumed that the dominant source of scattering in graphene is charged impurities in a substrate. We have tested this conjecture by studying graphene placed on various substrates and in high-kappa media. Unexpectedly, we have found no significant changes in carrier mobility either for different substrates or by using glycerol, ethanol, and water as a top dielectric layer. This suggests that Coulomb impurities are not the scattering mechanism that limits the mean free path attainable for graphene on a substrate.

We report capacitors in which a finite electronic compressibility of graphene dominates the electrostatics, resulting in pronounced changes in capacitance as a function of magnetic field and carrier concentration. The capacitance measurements have allowed us to accurately map the density of states D, and compare it against theoretical predictions. Landau oscillations in D are robust and zero Landau level (LL) can easily be seen at room temperature in moderate fields. The broadening of LLs is strongly affected by charge inhomogeneity that leads to zero LL being broader than other levels.PACS numbers: 73.22. Pr, 81.05.ue One of the most celebrated consequences of the Dirac-like electronic spectrum of graphene is its zero LL centered at the neutrality point (NP) and shared by hole-and electron-like carriers [1]. Although the electronic properties become particularly interesting near the NP, it has proven difficult to probe this regime by transport measurements. The problem is not only the potential fluctuations that move the Dirac point spatially and average out interesting features [2]. The situation is additionally tangled because transport coefficients become nonmonotonic at the NP and sensitive to scattering details, even in high magnetic fields B [3]. Capacitance measurements provide an alternative. If graphene is incorporated in a capacitor as one of its electrodes, there appears a significant contribution into the total capacitance C due to the electronic compressibility. This contribution is often referred to as quantum capacitance C q = e 2 D and is a direct measure of the density of state D(E) =dn/dE at the Fermi energy E F (e is the electron charge; n the carrier concentration) [4,5].As for experimental studies of C q , graphene is unique for two reasons. First, it has an atomically thin body, which allows capacitors in which the classical, geometrical contribution plays a minor role so that C q can completely dominate the device's electrostatics. Second, D is a strong function of E F and, therefore, C q can be changed by applying gate voltage V g . This distinguishes graphene from conventional two-dimensional systems in which C q is usually a small and constant contribution that is difficult to discern experimentally [5]. Thanks to the V g dependence, several groups have already reported the observation of C q of graphene [6][7][8][9]. Their measurements showed the expected V-shape dependence centered at the NP. However, the electron and hole branches were often strongly asymmetric [6,7], contrary to expectations, and the absolute value of C q was either impossible to determine [8,9] or it disagreed with theory [6]. Most recently, capacitance measurements were also employed to prove the gap opening in double-gated bilayer graphene [10].In this work we report large (~100x100 μm 2 ) graphene capacitors with a thin (≈10 nm) dielectric layer and a high carrier mobility μ of ≈10,000 cm 2 /Vs maintained after the fabrication. In our devices, C q is no longer merely a correction but reaches ≈30% of the meas...