Characterization of the electric transport properties of black phosphorous back-gated field-effect transistors

We use thin layers of exfoliated black phosphorus to realize back-gated field-effect transistors in which the Si/SiO2 substrate is exploited as gate electrode. To prevent the detrimental effect of the air exposure the devices are protected by Poly(methyl methacrylate). We report the observation of an improved contact resistance at the interface between the layered material and the metal contact by electrical conditioning. We also demonstrate the existence of a hysteresis in the transfer characteristics that improves by increasing the gate voltage sweep range. Finally, we prove the suitability of such transistors as memory devices

Local Characterization of Field Emission Properties of Graphene Flowers

An experimental study about field emission properties of commercially available graphene flowers cloth is reported. Material characterization by means of x-ray diffraction, Raman spectroscopy, and x-ray photoemission spectroscopy confirms the formation of high quality vertical few-layers graphene nanosheets. A tip-anode setup is exploited in which nanomanipulated tungsten tip is used as the anode at controlled distance from the emitter in order to reduce the effective emitting area below 1 μm2, giving access to local characterization. A turn-on field as low as 0.07 V nm−1 and field enhancement factor up to 32 for very small cathode–anode separation distances is demonstrated, in the range 400–700 nm. It is also shown that the turn-on field increases for increasing distances, while the field enhancement factor decreases. Finally, time stability of the field emission current is reported, evidencing a reduction of the fluctuations for lower current levels

CO2 Compression At World's Largest Carbon Dioxide Injection Project

LectureCompressors for carbon dioxide (CO2) applications have been widely used in the O&G industry for urea production, enhanced oil recovery (EOR) and CO2 pipeline services for many years. New environmental challenges have re-focused the attention on increasing the efficiency of rotating equipment including optimization of the equipment configuration. To address these challenges, an electric motor driven CO2 compressor train has been designed for a large CO2 injection project in Western Australia. This compressor train was equipped with two compressor casings coupled through two gearboxes on either side of a double-ended variable frequency drive (VFD) motor. For the CO2 injection project, the risk of condensation inside the carbon steel pipeline between compressor discharge and the injection wells was identified as a concern. The mixture of liquid water and CO2 results in carbonic acid that may result in corrosion of the carbon steel pipeline. To mitigate this risk the water needs to be removed from the CO2 stream prior to entering the pipeline. To maximize the removal of water, the CO2 compressor intermediate stage (3rd stage) pressure is required to operate within a fixed pressure range. The compressor string with two gearboxes, two compressor casings and a VFD results in a complicated torsional system. In cases where torsional vibration frequencies coincide with resonance frequencies, large torsional deflections and internal stresses could be generated. Continuous operation under these conditions could result in fatigue failure. Free-vibration analysis and a forced-response analysis were conducted for an analytical review of rotor response to static and harmonic torsional loads. VFD motors generate pulsating torques; even if the pulsating torques are very small with respect to the main torque, they can excite compressor train resonances with potential shaft and/or couplings damages. Final validation of the compressor trains has been the Full Speed, Full Load test and ASME PTC10 Type 1 performance test campaign conducted in June 2011. During this testing, torque pulsation measurements has been conducted for validation of torsional analytical results and pulsation amplitudes under steady state and start-up conditions. The compressor train was designed to be modularized. This required the package’s overall dimensions to meet very stringent space constraints. The project engineering challenges include: ergonomic design focusing on improving maintenance and operations access, compressor supports stiffness analysis for installation on module steel work and compressor train operability assessment. High CO2 discharge pressures, high operating temperatures and the need to minimize fugitive emissions result in challenges for the dry gas seals system (DGS) and lead to the design of a highly customized DGS configuration

Temperature Dependence of Electrical Resistance in Graphite Films Deposited on Glass and Low-Density Polyethylene by Spray Technology

Graphite lacquer was simply sprayed on glass and low-density polyethylene (LDPE) substrates to obtain large area films. Scanning Electron Microscopy (SEM) images, Raman spectra, X Ray Diffraction (XRD) spectra and current-voltage characteristics show that at room temperature, the as-deposited films on different substrates have similar morphological, structural and electrical properties. The morphological characterization reveals that the films are made of overlapped graphite platelets (GP), each composed of nanoplatelets with average sizes of a few tens of nanometers and about forty graphene layers. The thermoresistive properties of the GP films deposited on the different substrates and investigated in the temperature range from 20 to 120 °C show very different behaviors. For glass substrate, the resistance of the film decreases monotonically as a function of temperature by 7%; for LDPE substrate, the film resistance firstly increases more than one order of magnitude in the 20–100 °C range, then suddenly decreases to a temperature between 105 and 115 °C. These trends are related to the thermal expansion properties of the substrates and, for LDPE, also to the phase transitions occurring in the investigated temperature range, as evidenced by differential scanning calorimetry measurements

Dominant n-type conduction and fast photoresponse in BP/MoS2 heterostructures

In recent years, van der Waals heterojunctions between two-dimensional (2D) materials have garnered significant attention for their unique electronic and optoelectronic properties and have opened avenues for innovative device architectures and applications. Among them, the heterojunction formed by black phosphorus (BP) and molybdenum disulfide (MoS2) stands out as a promising candidate for advanced optoelectronic devices. This study unravels the interplay between BP, MoS2, and Cr contacts to explain the electrical behavior of a BP/MoS2 heterojunction showing rectifying behavior with dominant n-type conduction, and a high ON/OFF current ratio around 104 at ± 20 V. The higher unexpected current observed when applying a negative bias to either MoS2 or BP side is elucidated by an energy band model incorporating a type II heterojunction at the BP/MoS2 interface with Cr forming a Schottky contact with MoS2 and an ohmic contact with BP. The BP/MoS2 heterojunction shows pronounced photoresponse, linearly dependent on the incident laser power, with a responsivity of 100 μA/W under white light at 50 μW incident power. Time-resolved photocurrent measurements reveal a relatively fast response with characteristic rise times less than 200 ms. This work demonstrates that BP/MoS2 van der Waals heterojunctions have unique electrical and photoresponse characteristics that are promising for advanced optoelectronic applications

Notulae to the Italian alien vascular flora: 17

In this contribution, new data concerning the distribution of vascular flora alien to Italy are presented. It includes new records and status changes from casual to naturalized for Italy or for Italian administrative regions for taxa in the genera Callianthe, Chamaecyparis, Chamaeiris, Cotoneaster, Erigeron, Freesia, Hemerocallis, Juglans, Kalanchoë, Ludwigia, Nassella, Paulownia, Physocarpus, Pistia, Saccharum, Setaria, and Vachellia. Nomenclatural and distribution updates, published elsewhere, and corrections are provided as supplementary material

Air Pressure, Gas Exposure and Electron Beam Irradiation of 2D Transition Metal Dichalcogenides

In this study, we investigate the electrical transport properties of back-gated field-effect transistors in which the channel is realized with two-dimensional transition metal dichalcogenide nanosheets, namely palladium diselenide (PdSe2) and molybdenum disulfide (MoS2). The effects of the environment (pressure, gas type, electron beam irradiation) on the electrical properties are the subject of an intense experimental study that evidences how PdSe2-based devices can be reversibly tuned from a predominantly n-type conduction (under high vacuum) to a p-type conduction (at atmospheric pressure) by simply modifying the pressure. Similarly, we report that, in MoS2-based devices, the transport properties are affected by pressure and gas type. In particular, the observed hysteresis in the transfer characteristics is explained in terms of gas absorption on the MoS2 surface due to the presence of a large number of defects. Moreover, we demonstrate the monotonic (increasing) dependence of the width of the hysteresis on decreasing the gas adsorption energy. We also report the effects of electron beam irradiation on the transport properties of two-dimensional field-effect transistors, showing that low fluences of the order of few e-/nm2 are sufficient to cause appreciable modifications to the transport characteristics. Finally, we profit from our experimental setup, realized inside a scanning electron microscope and equipped with piezo-driven nanoprobes, to perform a field emission characterization of PdSe2 and MoS2 nanosheets at cathode–anode separation distances as small as 200 nm