Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, ...depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the ID edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.
Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the ...graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.
The possibility of using a fast neutron beam with the associated alpha particle (AAP) technique in conjunction with a specially designed probe for carbon-to-oxygen (C/O) nuclear well logging was ...investigated. Measurements were performed to show the influence of borehole fluid, iron casing, and the cement around the casing in the C/O analysis. Tests were conducted by using fast neutron activation analysis (FNAA), as the conventional approach for C/O measurements in well logging and by using AAP. Analyzed samples were made from quartz sand and graphite powder mixed in different C/O ratios characteristic for the oil-containing formations. Diesel fuel was used as a simulant of the borehole fluid. Monte Carlo (MC) simulations were conducted and the results were compared with the experimental data. In both tests, the linear relation between the measured and real C/O ratios was obtained, but when the FNAA was used, the linear relation was significantly influenced mainly by the presence of the borehole fluid. Contrary to FNAA, the AAP technique resulted in a significant reduction of the background signal coming from the borehole fluid.
•A neutron probe for oil logging was developed with an outside diameter of 43 mm.•NP was equipped with a four-segmented alpha particle detector operable at high temperatures.•The possibility of ...measuring the C/O ratio as a function of the angle around the NP axis was shown.•Measurements were done on geological samples taken from Croatian oilfields.•Measurements on the high-temperatures show degradation of the signal coming from the gamma-ray detector.
A recently developed concept of a neutron probe (NP) -for C/O oil logging, based on associated alpha particle technique, has been developed and demonstrated by using a specially constructed NP. The NP contains a high-temperature alpha particle detector as well as a gamma detector with high energy resolution, suitable for a high-temperature environment. The probe was tested on samples collected from Croatian oil fields and on samples prepared with known C/O ratios. This NP in comparison to conventional neutron activation probes significantly reduces the background signal coming from the borehole and measures the C/O ratio depending on the azimuth angle around the borehole longitudinal axis. The time (position) resolution was found to be 2 ns enabling inspection of the C/O ratio in rock formations’ layers down to 10 cm thickness. The NP was successfully tested for different environmental temperatures up to 175 °C. The pulse amplitude degradation coming from the gamma detector was noticed to be a function of the temperature.
A neutron-based sensor has been developed able to measure the water cut and the chlorine content in crude oil. The sensor is based upon the use of a pulsed fast neutron source. Two different cases ...were studied. Case ‘‘A’’: chlorine detection with no water present and case ‘‘B’’: water cut measurements with no chlorine present. The minimum detection limits for case A and case B were found to be (62 ± 6) mg/L and (2 ± 2) vol%, respectively.
•A neutron sensor for on-line determination of water-cut and chlorine impurities in crude oil was developed.•Measurements were done on geological samples taken from Croatian oilfields and on standard samples prepared in the laboratory.•Since both chlorine and oxygen have similar gamma-ray lines at 6.1 MeV, these two were separated by choosing a different pulsing frequency for the neutron beam.•A similar setup (neutron probe) could be easily made for monitoring individual wells.
Particle computed tomography (pCT) is an emerging imaging modality that promises to reduce range uncertainty in particle therapy. The Bergen pCT collaboration aims to develop a novel pCT prototype ...based on the ALPIDE monolithic CMOS sensor. The planned prototype consist of two tracking planes forming a rear tracker and Digital Tracking Calorimeter (DTC). The DTC will be made of a 41 layer ALPIDE-aluminum sandwich structure. To enable data acquisition at clinical particle rates, a large multiplicity of particles will be measured using the highly-granular ALPIDE sensor. In this work, a first characterization of the ALPIDE sensor performance in ion beams is conducted. Particle hits in the ALPIDE sensor result in charge clusters whose size is related to the chip response and the particle energy deposit. Firstly, measurements in a 10 MeV 4He micro beam have been conducted at the SIRIUS microprobe facility of ANSTO to investigate the dependence of the cluster size on the beam position over the ALPIDE pixel. Here, a variation in cluster size depending on the impinging point of the beam was observed. Additional beam tests were conducted at the Heidelberg Ion-Beam Therapy Center (HIT) investigating the cluster size as a function of the deposited energy by protons and 4He ions in the sensitive volume of the ALPIDE. Results show the expected increase in cluster sizes with deposited energy and a clear difference in cluster sizes for protons and 4He ions. As a conclusion, the variation in cluster size with the impinging point of the beam has to be accounted for to enable accurate energy loss reconstruction with the ALPIDE. This does, however, not affect the tracking of particles through the final prototype, as for that only the center-of-mass of the cluster is relevant.
An accurate calculation of proton ranges in phantoms or detector geometries is crucial for decision making in proton therapy and proton imaging. To this end, several parameterizations of the ...range-energy relationship exist, with different levels of complexity and accuracy. In this study we compare the accuracy of four different parameterizations models for proton range in water: Two analytical models derived from the Bethe equation, and two different interpolation schemes applied to range-energy tables. In conclusion, a spline interpolation scheme yields the highest reproduction accuracy, while the shape of the energy loss-curve is best reproduced with the differentiated Bragg-Kleeman equation.
•Four parameterization models for calculation of the proton range are compared.•A sub-percent range calculation accuracy is obtained for all models above 90MeV.•The analytical models require few datapoints for convergence.•Using more than five datapoints, the spline-interpolated model is the most accurate.•The energy loss-curve is best represented using the Bragg-Kleeman equation.
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