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

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

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

Etch and Print: Graphene-Based Diodes for Silicon Technology

The graphene-silicon junction is one of the simplest conceivable interfaces in graphene-integrated semiconductor technology that can lead to the development of future generation of electronic and optoelectronic devices. However, graphene's integration is currently expensive and time-consuming and shows several challenges in terms of large-scale device fabrication, effectively preventing the possibility of implementing this technology into industrial processes. Here, we show a simple and cost-effective fabrication technique, based on inkjet printing, for the realization of printed graphene-silicon rectifying devices. The printed graphene-silicon diodes show an ON/OFF ratio higher than 3 orders of magnitude and a significant photovoltaic effect, resulting in a fill factor of ∼40% and a photocurrent efficiency of ∼2%, making the devices suitable for both electronic and optoelectronic applications. Finally, we demonstrate large-area pixeled photodetectors and compatibility with back-end-of-line fabrication processes

2D transition metal dichalcogenides nanosheets as gate modulated cold electron emitters

We report a detailed investigation of the field emission properties of transition metal dichalcogenides, namely MoS 2 and WSe 2 , taking advantage of an experimental setup realized inside a scanning electron microscope equipped with nano-manipulated probe-tips, used for positioning a tip-shaped anode at a nanometric distance from the emitting surface. For n-type WSe 2 monolayer on Si/SiO 2 substrate, we show that electrons can be extracted also from the flat part of the flake with a current intensity up to few nanoamperes. More interestingly, we demonstrate that the field emission current can be modulated by the back-gate voltage that controls the n-type doping of the WSe 2 monolayer. Similarly, we demonstrate that monolayer MoS 2 flakes are suitable for gate-controlled field emission devices, opening the way to the development of new field emission transistors based on ultrathin materials

Spatially Resolved Photo-Response of a Carbon Nanotube/Si Photodetector

Photodetectors based on vertical multi-walled carbon nanotube (MWCNT) film-Si heterojunctions are realized by growing MWCNTs on n-type Si substrates with a top surface covered by Si3N4 layers. Spatially resolved photocurrent measurements reveal that higher photo detection is achieved in regions with thinner MWCNT film, where nearly 100% external quantum efficiency is achieved. Hence, we propose a simple method based on the use of scotch tape with which to tune the thickness and density of as-grown MWCNT film and enhance device photo-response

Direct contacting of 2D nanosheets by metallic nanoprobes

We present a simple and fast methodology to realize metal contacts on two-dimensional nanosheets. In particular, we perform a complete characterization of the transport properties of MoS2 monolayer flakes on SiO2/Si substrates by using nano-manipulated metallic tips as metallic electrodes directly approached on the flake surface. We report a detailed experimental investigation of transport properties and contact resistance in back-gated field-effect transistor in which the Si substrate is used as the gate electrode. Moreover, profiting from the n-type conduction as well as the high aspect ratio at the edge of the MoS2 flakes, we also explored the possibility to exploit the material as a field emitter. Indeed, by retracting one of the metallic probes (the anode) from the sample surface, it has been possible to switch on a field emitted current by applying a relatively low external electric field of few tens of Volts for cathode-anode separation distance below 1µm. Experimental data are then analyzed in the framework of Fowler-Nordheim theory and its extension to the two-dimensional limit