Private Colleges, State Aid, and the Establishment Clause

Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of similar to 60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene.Funding Agencies|NMS under the IRD Graphene Project (NPL); EMRP</p

Antibiotic Adverse Reactions and Drug Interactions

Magnetic force microscopy (MFM) offers a unique insight into the nanoscopic scale domain structures of magnetic materials. However, MFM is generally regarded as a qualitative technique and, therefore, requires meticulous calibration of the magnetic scanning probe stray field (Bprobe) for quantitative measurements. We present a straightforward calibration of Bprobe using scanning gate microscopy on epitaxial graphene Hall sensor in conjunction with Kelvin probe force microscopy feedback loop to eliminate sample-probe parasitic electric field interactions. Using this technique, we determined Bprobe ~ 70 mT and ~ 76 mT for probes with nominal magnetic moment ~ 1 × 10-13 and &gt; 3 × 10-13 emu, respectively, at a probe-sample distance of 20 nm.Funding Agencies|Concept Graphene project||IRD Graphene project||MetMags project|||CSD2010-00024|</p

A prototype of RK/200 quantum Hall array resistance standard on epitaxial graphene

Epitaxial graphene on silicon carbide is a promising material for the next generation of quantum Hall re- sistance standards. Single Hall bars made of graphene have already surpassed their state-of-the-art GaAs based counterparts as an RK/2 (RK = h/e^2) standard, showing at least the same precision and higher break- down current density. Compared to single devices, quantum Hall arrays using parallel or series connection of multiple Hall bars can offer resistance values spanning several orders of magnitude and (in case of parallel connection) significantly larger measurement currents, but impose strict requirements on uniformity of the material. To evaluate the quality of the available material, we have fabricated arrays of 100 Hall bars con- nected in parallel on epitaxial graphene. One out of four devices has shown quantized resistance that matched the correct value of RK/200 within the measurement precision of 1e-4 at magnetic fields between 7 and 9 Tesla. The defective behaviour of other arrays is attributed mainly to non-uniform doping. This result con- firms the acceptable quality of epitaxial graphene, pointing towards the feasibility of well above 90% yield of working Hall bars

Visualisation of edge effects in side-gated graphene nanodevices

Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of ,60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene

Quantum Hall devices on epitaxial graphene: towards large-scale integration

Quantum Hall devices have been used as the primary standard of electrical resistance for over two decades, and they are unlikely to be replaced in this role any time soon. The work presented in this thesis was being done towards the goal of establishing epitaxial graphene on silicon carbide as a new material of choice for these devices. Experiments on individual devices have already demonstrated that due to unique electronic properties of graphene and peculiarities of its interaction with the SiC substrate, quantum resistance standards based on epitaxial graphene can operate at higher temperatures, lower magnetic fields, or higher current densities, as compared to their state-of-the-art gallium arsenide counterparts. Here, we were aiming at developing the technology for reliable mass-production of the devices.One of the issues that we address is the carrier density control. We have found that photochemical gating, a technique which has previously been used for this purpose, becomes unreliable when the electron density needs to be lowered by more than 1016 m2. Instead, corona discharge can be used for efficient electrostatic gating, enabling us to sweep the carrier density from 4*1016 electrons*m-2 to 5*1016 holes*m^-2 and to observe the quantum Hall effect at low doping.The presence of bilayer patches in majority-monolayer samples is another important problem. We have observed both metallic and insulating behaviour of these patches while driving the monolayer into the quantum Hall regime. When the bilayer is metallic, we show that a patch completely crossing the Hall bar will break down the quantum Hall effect in a way that agrees with theoretical expectations. Further, we propose imaging these patches by optical microscopy as a way of avoiding them, by selecting substrates where the patches are sufficiently small and sparse. We demonstrate that, despite the optical contrast being less than 2%, the bilayer areas can be imaged in real time using digital post-processing. Also, we show that optical microscopy can be used to detect the steps that form on the SiC surface during graphene growth, and even measure their height: steps as low as 1.5 nm could be clearly seen.Finally, we have fabricated arrays of 100 Hall bars connected in parallel, devices which provide a low-ohmic quantum standard if every single Hall bar works correctly. We have chosen a substrate with a sufficiently low bilayer content, and adapted the geometry of the Hall bar to the shape of the patches. One out of for devices has performed correctly within the relative measurement precision of 10-4 in magnetic fields above 7 tesla. We see this as a confirmation that the quality of graphene was sufficiently high to enable ≥99% yield of working Hall bars